I Deepu

Ambika Gopakumar

Deepu Ambika

Ambika Deepu

Gopakumar

Nanocellulose Based Functional Constructs for Clean water and Microwave Suppression

General Introduction

The contamination of water is one of the serious issue that we are facing globally. This is mainly due to the various pollutants like toxic textile dyes, heavy metals, bacteria, virus etc. which can cause severe adverse effects to human beings, aquatic life and the ecosystem. Generally these pollutants are coming from biological and physical process but the impact of water pollution depends on several factors like ecological impact of the pollutant, abundance of pollutant and the use of the water.

Unfortunately the most of the water pollution is derived due to the industries, agriculture and land-based activities of human. Considering the shortage of the usable water and the progressive pollution as a recognized global threat, the necessity to develop cutting-edge technology to address these major issues for water purification is very important for a clean environment. Membrane assisted water purification technology is one of such innovative approaches for creating a better healthier environment. .Thus, advanced technologies for filtration are an essential part of meeting the current and future needs for water innovations in the development of novel approaches to remove the minute contaminants like textile dyes from water. Considering the energy efficiency and ease of operation, membrane assisted filtration technology is anticipated to dominate among the water purification technologies. Among the water pollutants, toxic textile dye waste is an important pollutant to consider due to the emergence of the substantial number of modern textiles into the market. It is evident that after agriculture, the textile dyeing and finishing industry has generated a large water pollution problem. Among the common removal techniques, microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) using pressure driven technologies have received very much interest due to their high performance and low cost. Conventionally these membranes for water treatment are produced by phase inversion techniques, where it is very difficult to achieve uniform pore size and water flux. Here, the membranes produced via electrospinning has numerous advantages like higher effective porosity with continuously interconnected pores, high surface area etc. when compared with the membranes fabricated via phase inversion technique. Moreover, membranes fabricated via electrospinning have an excellent particle rejection property. These properties make electrospun membranes as an excellent platform for microfiltration applications, however the average porosity of the membranes fabricated from electrospinning is too big to eliminate minute contaminants like viruses and textile dyes via size exclusion. So in order to remove the minute contaminants like textile dye, an extra adsorption mechanism is important. One of the typical approaches to remove the toxic textile dyes from waste water is to enhance the electrostatic charges of the membrane to increase the adsorption capability.

Another problem that we are facing globally is the electromagnetic wave pollution arising due to outrageous usage of electronic gadgets operating at different frequencies. Extensive use of electrical and microelectronic systems for domestic, industrial and other application functioning in radio frequency band remains to be a severe concern. The electromagnetic radiations from such electronic equipment's affect the performance of other near or far region electromagnetic fields which is called electromagnetic interference (EMI), and is emerging to be a serious issue for circuit designers. For the safety of the sensitive circuits, light weight EMI shielding materials are very essential to shield the workspace from radiations emitted out from other electronic devices. EMI which can cause intolerable deterioration of equipment performance due to the unwanted radiated signals. If these concerns are not properly rectified, it may severely affects to the safety operation of many electronic devices. More over the electromagnetic waves from the sensitive electronic equipment's are reflected or absorbed by the human body. Thus misdirected radiations emitted out from the electronic devices may have adverse effect on human health. In the operation of microprocessor controlled devices, the use of high frequency signals may be emitted out of the equipment to the neighbouring environment can adversely affect the proper functioning of nearby equipment. So in order fix this issue, a proper shielding mechanism has to be given to the electronic devices to shield the both incoming and outgoing electromagnetic waves. Conventionally, metals were used to shield EM waves but are highly undesirable due to their inherent drawbacks such as higher weights, corrosive nature and difficulty of processing into intricate shapes. In this context, we need to address this insensible and unnoticed pollutant by designing electromagnetic shields based on conducting polymer nanocomposites thereby converting this undesirable energy into heat energy. Intrinsic conducting polymers (ICPs) like polyaniline (PANI) is an effective candidate for EMI shielding due to their tuneable conductivities, light weight and corrosion resistance as compared with typical metals. Moreover, the absorption dominant shielding characteristic of PANI compared to metals makes, PANI as an effective shielding material for many practical applications. This makes PANI as attractive alternative materials for EMI shielding. However, the existence of the conjugated system in PANI enhances the rigidity of the polymer molecule, resulting in poor film-forming capability. Cellulose nanofibers as an effective substrate have been proposed as a solution for this obstacle. By combining the cellulose nanofibers with PANI, the poor formability and frangibility of PANI can be alleviated to produce a conducting composite that is capable to shield electromagnetic radiations via absorption dominated mechanism.

The present study make use of nanocellulose as a sustainable solution for water purification and electromagnetic suppression. Cellulose nanofibers extracted from sunflower stalks were chosen to prepare membrane and flexible nanopapers for water purification and electromagnetic wave suppression respectively. Here we intended to produce a unique absorbent material from cellulose nanofibers (CNFs) via non-solvent assisted procedure using Meldrum's acid as esterification agent in order to enhance the absorption towards the toxic textile dye like crystal violet. In this work we also fabricated a series of conductive cellulose nanopapers via in-situ polymerization with polyaniline (PANI) to attenuate electromagnetic radiations specifically in a small bandwidth of the microwave frequency region (8.2 -12.4 GHz). We firmly believe that these finding offers a new platform for make use of cellulose nanofibers extracted from bio-mass for the water purification and electromagnetic wave suppression applications.

Chapter 1

Bibliographical Review 1. Introduction

Our society are increasingly demanding products made from renewable and sustainable resources that are biodegradable, non-petroleum based and have low environmental, animal/human health and safety risks. There is a growing trend to use bio-fibers as fillers and/or reinforces in plastics composites. Their flexibility during processing, highly specific stiffness, and low cost make them attractive to manufacturers. As a renewable material, cellulose and its derivatives have been widely studied, focusing on their biological, chemical, as well as mechanical properties. The materials based on cellulose and its derivatives have been used for more than 150 years in a wide variety of applications, such as food, paper production, biomaterials and pharmaceuticals. Cellulose is one of the most abundant biopolymers on earth, occurring in wood, cotton, hemp and other plant-based materials and serving as the dominant reinforcing phase in plant structures. Natural cellulose based materials (wood, hemp, cotton, linen, etc.) have been used by our society as engineering materials for thousands of years and their use continues today as verified by the enormity of the world wide industries in forest products, paper, textiles, etc.

In recent years, there is an increasing interest in the development of biodegradable and /or plant derived composite materials which sometimes referred to as "green composites because of the strong global demand for creating renewable sources. Its industrial use is mainly for making paper and cardboard but recently it has also attracted significant interest as a source of biofuel production. The scientists believe that cellulose nanofibers have a high potential to be used as transparent and extremely strong films in many different areas. Now a days, scientists are focused on the isolation, characterization, and search for applications of novel forms of cellulose, variously termed nanocrystals, whiskers, nanofibrils and nanofibers, such isolated cellulosic materials with one dimension in the nanometre range are referred to generally as nanocelluloses. These nanocelluloses combine important cellulose properties such as high specific strength and modulus, hydrophilicity, wide chemical modification capacity etc. with specific features of nanoscale materials. This is mainly caused due to the very large surface area of these materials. Nanocelluloses can be classified in three main subcategories namely 1) cellulose nanofibers (CNFs) 2) cellulose nanocrystals (CNCs) and 3) bacterial nanocellulose (BC). Surface functionalization on nanocellulose allows the tailoring of particle surface chemistry to facilitate self-assembly, controlled dispersion within a wide range of matrix polymers.

Cellulose , its sources, chemical composition and physical properties

The primary occurrence of cellulose is existing lignocellulosic material in forests, with wood as the most important source. Other cellulose-containing materials include agriculture residues, water plants, grasses and other plant substances and various sources of cellulosic fibers are shown in the Table 1.1. Besides cellulose, they contain hemicelluloses, lignin and a small amount of extractives. Chemical compositions of some typical cellulose-containing materials are shown in the Table 1.2. The most common sources of cellulose for industrial use are wood pulp and cotton linter.

Cellulose is a long chain of linked sugar molecules that gives wood its remarkable strength. It is the main component of plant cell walls, and the basic building block for many textiles and for paper. Cotton is the purest natural form of cellulose. Cellulose I is occurs in native cellulose found in nature, and it occurs in two allomorphs, Iα and Iβ. Cellulose II or regenerated cellulose emerges after recrystallization or mercerization with aqueous sodium hydroxide and it is the most stable crystalline form [2]. The major difference between these two forms of cellulose is the layout of their atoms: Cellulose II has antiparallel packing, whereas the chains in cellulose I run in a parallel direction [2]. Cellulose IIII and IIIII are

Various sources (Nano cellulosic fibers) References

Source

High pressure Homogenization:

This is the widely used method for the large scale production of cellulose nanofibers. This process has been extensively used by the dairy and food industry mainly to stabilize food emulsion. During this process, the cellulose fibers are pumped at high pressure and fed through a spring loaded valve assembly. As this valve opens and closes at a very fast rate, the fibers are exposed to a large pressure drop with shearing and impacting forces. This combination of forces will lead to a high degree of micro fibrillation of cellulose fibers. The extent of the cellulose fibrillation depends on the number of homogenization cycles and the applied pressure. The higher the pressure, the higher the efficiency of disruption per pass through the machine. Methods for producing micro fibrillated cellulose (MFC) were first reported by Herrick et al. [4]. They have passed the dilute cellulosic wood pulpwater suspensions through a mechanical homogenizer, in which a large pressure drop facilitated micro fibrillation. Li et al. extracted the cellulose nanofibers from de-pectinated sugar beet pulp by the combination of chemical treatments and high pressure homogenization. The diameter of the obtained cellulose nanofibers was from several nm to 70 nm. They showed that the crystallinity of nanofibers increased significantly after treatments [5]. Chen et al. also reported a method to fibrillate raw dried cotton fibers into individual cellulose nanofibers by chemical purification and pre-treatment by a high speed blender combined with high pressure homogenization (nanofibrillation). The resultant nanofibers were found to have a diameter of approximately 10-30 nm and high aspect ratios [6]. Cellulose nanofibers were isolated by Pelissari et al. from banana peels using a combination of chemical treatments, such as alkaline treatment, bleaching and acid hydrolysis. It was done by passing the suspensions of chemically treated fibers through a high pressure homogenizer [7].

Grinding

In grinding process, cellulose fibers are forced through a gap between two specially modified grooved discs, one is static and other is revolving at about 1,500 rpm. In this process, the cell wall structure is broken down by the high shearing forces and it resulting in nanosized individualized fibers. The extent of fibrillation is dependent upon the distance between the discs, the morphology of the disc channels and the number of passages through the grinder. Abe et al. obtained cellulose nanofibers with a uniform width of 15nm from wood by the grinding treatment in an undried state. They reported the nanocellulose fibre diameter with diameter of 15nm. This study demonstrated that grinding is a powerful quiet simple method for the production of cellulose nanofibers from plant fibers [8]. When homogenized cellulosic pulp was subjected to a grinder treatment by Iwamoto et al. the fibril bundles were further fibrillated and 10 repetitions of the grinder treatment resulted in uniform nanofibers 50-100 nm wide [9].

Cryocrushing:

Cryocrushing is an alternative method for producing nanofibers where fibers are frozen using liquid nitrogen, and then high shear forces are applied. This process involves the acting of the high impact forces on the frozen fibers, ice crystals exert pressure on the cell walls, causing them to rupture and then led to the formation of microfibrils. Cryocrushing combined with a high pressure fibrillation process was used by Wang et al. for the isolation of nanofibers with diameters in the range 50-100 nm from soybean stock [10]. Alemdar et al. also reported the extraction of cellulose nanofibers from wheat straw and soy hulls by mechanical treatment involving cryocrushing. They investigated the chemical composition, morphology, physical and thermal properties of the nanofibers to study their usability in bio-composite applications. They showed that the produced wheat straw nanofibers' diameters were in the range of 10-80 nm with lengths of a few thousand nanometres [11].

Chakraborty et al. also reported a novel technique to produce cellulose microfibrils through mechanical methods. The technique involved a combination of severe shearing in a refiner, followed by high-impact crushing under liquid nitrogen [12].

High Intensity Ultrasonic Treatments:

This process comprises of combination of chemical pre-treatment and high-intensity were obtained [14].

Steam Explosion process:

This process comprises of short time vapour phase cooking at temperatures in the range of 180-210°C, followed by explosive decompression and sudden release of pressure. In this process the cellulosic biomass is pressurized for a short period of time in an autoclave with water steam, and then explosively discharged to atmospheric pressure which results in a sudden disintegration of the starting material into a fibrous dispersed solid. This process results in a substantial breakdown of lignocellulosic structure, hydrolysis of the hemicellulose fraction, depolymerisation of the lignin components and defibrillation. treatment presents the advantage to disintegrate into microfibrils with small widths using a much lower input [23].

Acetylation

Acetylation is a chemical modification method, which introduce acetyl group (CH3-C (=O) -on to the surface of the nanocellulose as shown in the figure1.6, in order to convert the hydrophilic character of nanocellulose into hydrophobic [24][25][26]. The acetylation of cellulose depends on the accessibility and susceptibility of the OH groups in the amorphous and crystalline regions within the cellulose polymer chain [27]. The Acetylation has several advantages when compared to other chemical treatments for nanocellulose. Acetylation provides good dispersiblity of cellulose fibers in the polymer matrix during compounding, low costs, ease of use, shorter process duration and environmental friendliness. The main limitation of the nanocellulose in practical composite application is the self-agglomeration due to the presence of large surface hydroxyl groups, thereby resulting in crack or failure of the composites. Mukherjee et al. done surface acetylation of the nanocellulose using acetic anhydride and dimethyl formamide to reduce the agglomeration and they found that after acetylation the dispersion of acetylated nanocellulose had improved in PLA matrix [28].

Silyation

Silane is a chemical compound with chemical formula SiH4. Silanes are used as coupling agents to let glass fibers adhere to a polymer matrix, stabilizing the composite material. Silane coupling agents may reduce the number of cellulose hydroxyl groups in the fibre-matrix interface. In the presence of moisture, hydrolysable alkoxy group leads to the formation of silanols. The silanols then reacts with the hydroxyl group of the fibre, forming stable covalent bonds to the cell wall that are chemisorbed onto the fibre surface [29]. Therefore, the hydrocarbon chains provided by the application of silane restrain the swelling of the fibre by creating a cross-linked network due to covalent bonding between the matrix and the fibre. The reaction scheme is given in the figure 1.7. Silane coupling agents were also found to be effective in modifying natural fibre-polymer matrix interface and increasing the interfacial strength. Abdelmouleh et al. done the surface modification of cellulosic fibers using organo functional silane coupling agents in an ethanol/water medium.

They found the presence of SI-O-cellulose and Si-O-Si bonds on the surface of the cellulose and there by imparting the hydrophobic character to the cellulose fibers which was confirmed via water contact angle studies [30]. The simultaneous occurrence of cellulose hydrolysis and acetylation of hydroxyl groups have been reported. Fischer esterification of hydroxyl groups simultaneously with the hydrolysis of amorphous cellulose chains has been introduced as a viable one-pot reaction methodology that allows isolation of acetylated CNCs in a singlestep process as shown in the figure 1.8 [31,32]. This procedure can be applied to access various esters of nanocellulose as demonstrated by the authors. However, a strict control of the esterification degree using this method is quite tough. In a similar approach, Herrick et al. have proposed in the early development of nanocellulose, the simultaneous acetylation of NFCs during the mechanical shearing using as processing media a mixture of acetic acid and acetic anhydride with sulphuric acid as catalyst instead of water [33].

Polymer Grafting

Surface chemical modification of cellulose nanoparticles can be achieved by covalently attaching small molecules, as well as polymers. The general objective of this chemical modification is to increase the polar character of the nanoparticle and have a better compatibility with hydrophobic polymer matrices. Two main approaches can be used to graft polymers onto surfaces, i.e., "grafting onto" or "grafting from". The first method was extensively used for the fibers or the NCC particles and not for the NFC. The "grafting onto" approach consists of: (i) mixing the cellulosic nanoparticles with an existing polymer and a coupling agent to attach the polymer to the nanoparticle surface; or (ii) activating the cellulose substrates (or the polymer) and grafting one (or the other) onto the other one. The second strategy, i.e., "grafting from" approach, consists of mixing the cellulosic nanoparticles or the activated cellulosic nanoparticles with a monomer and an initiator agent to induce polymerization of the monomer from the nanoparticle surface.

Catonization

There are three kinds of methods to prepare cationic modified fibers. The first method is direct cationize fibers to get the cellulose fibre amine derivatives. The second method is cationic pre-polymer of short chain coupled to the fibre surface to obtain high surface charge density fibre. The third method is graft copolymerization of cationic polymers to the fibre; cationic monomer and neutral monomers are connected to the fibre surface by free radical copolymerization. All these three methods can make the fibre cationic, but the connection types of cationic and fibers are different, and finally the fibre morphology and performance are also different [36] There are few ways of introduction of cationic groups in cellulose by chemical reaction, for instance, attaching quaternary ammonium groups by using of The contamination of water is due to the excess amount of substance which can cause severe adverse effects to human beings, aquatic life and the ecosystem.

Generally the pollutants are coming from physical, biological and physical process but the impact of water pollution depends on several factors like ecological impact of the pollutant, abundance of pollutant and the use of the water. Unfortunately the most of the water pollution is derived due to the industries, agriculture and landbased activities of human. The figure 1.9 a clearly shows the distribution mode of water on earth and it is shown that the oceans contain around 97 % of all water and only remain 3% is available as fresh water. Out of the 3% fresh water 79% is confined in glaciers and icecaps as saline water at the same time the rest of water is produced via the water cycle. Figure 1.9 b shows that surprisingly the remaining freshwater, almost all of it exists as ground water and not accessible. Of all the freshwater on earth, only about 0.3 percent is contained in rivers and lakes and usable for human's daily life. Despite such limited capacity of usable water, the human kind is also polluting the water thereby minimizing the available fresh usable water [40]. Membrane technology is expected to continue to dominate the water purification technologies owing to its energy efficiency.

Next Generation filtration Media: Electrospun Membranes

There are several methods such as melt fibrillation [42], Island-in-sea [START_REF] Pike | Superfine microfiber nonwoven web[END_REF], and gas jet [START_REF] Reneker | Process and apparatus for the production of nanofibers[END_REF] etc. for producing nanofibers from high-volume production methods.

However, their usefulness is limited by combinations of restricted material ranges, possibly fibre assembly, and cost and production rate. Here, electrospinning has an advantage with its comparative low cost and relatively high production rate. The ability to form porous fibers through electrospinning means that the surface area of the fibre mesh can be increased tremendously, which property makes the electrospun membranes as promising candidate for filtration applications. High porosity, interconnectivity, micro scale interstitial space and large surface to volume ratio that nonwoven electrospun nanofiber meshes are an excellent material for membranes water filtration [START_REF] Ramakrishna | Electrospun nanofibers: solving global issues[END_REF].Fortunately, membrane separation is one of the most efficient methods to remove the contaminants from water, whose properties of porosity, surface charge, hydrophilic/phobic nature etc., play a vital role in filtration. Among them, electrospun three dimensional porous nanofibrous filtering membrane has received considerable attention because of their high porosity, good interconnectivity and high specific surface area [START_REF] Cao | Robust polyacrylonitrile nanofibrous membrane reinforced with jute cellulose nanowhiskers for water purification[END_REF] 1.

Nanocellulose as an sustainable solution for water purification

Nanocellulose can be tailored into novel and significantly improved physical and chemical properties. The unique features of nanocellulose include small diameter, high surface -to-volume ratio, abundant hydroxyl groups for easy functionalization, good mechanical properties and good chemical resistance. These excellent properties make nanocellulose as a huge potential candidate for waste water treatment. In an aqueous environment, the high surface area and high aspect ratio of nanocellulose are beneficial for the formation of an ultrafine three-dimensional network structures which can be explored for removal and absorption of various pollutants in the water. Since cellulose is hydrophilic in nature, cellulose has been used an antifouling hydrophilic coating to increase the flux of the membranes.

Cellulose tends to exhibit a high adsorption capacity for pollutant after suitable chemical modification on its surface with the aim of incorporating molecules that contain basic groups, particularly those that are rich in nitrogen, sulphur and oxygen [START_REF] Musyoka | Synthesis, characterization, and adsorption kinetic studies of ethylenediamine modified cellulose for removal of Cd and Pb[END_REF]. Cellulose nanofibers have been intensively studied due to their natural abundance, ease of functionalization and structural diversity [START_REF] Fernandes | Novel materials based on chitosan and cellulose[END_REF][START_REF] Peterson | Taming EMI in microprocessor systems: The authors propose a systematic approach to limiting the effects of electromagnetic interference[END_REF].

Nanotechnologies have been touted as having great potential for reducing costs and improving efficiency in pollution prevention, treatment and clean up [50].

Nanocellulose has generated interest in this area as an active sorbent material for contaminants and as stabilizer for other active particles. The easily functionalizable surface of nanocellulose allows for the incorporation of different chemical moieties that may enhance the binding and adsorption efficiency against the pollutants in water such as dyes, toxic heavy metals etc. Cellulose nanomaterial are a promising alternative adsorbent due to its high surface area-to-volume ratio, low cost, high natural abundance, and inherent environmental inertness. Moreover, CNFs easily functionalizable surface allows for the incorporation of chemical moieties that may increase the binding efficiency of pollutants to the CNFs. Carboxylation of CNFs is by far the most studied method for increasing their sorptive capacity. The sorption of organic contaminants has also been demonstrated with modified CNF matrices. The inherent hydrophilicity of CNFs can be reduced to improve the affinity of the material for hydrophobic compounds. The rapid progress made in the nanosciences, especially with nanocellulose; offer a potential new type of material for use in water treatment in the form of nanoparticles. Nanocellulose is efficient absorbent materials due to its high surface area to volume ratio, low cost, highly natural abundance and inherent environmental inertness. The manipulation of the surface chemistry of nanocellulose can be achieved by inclusion of both organic and inorganic functionalities; thereby it could acts as a water repellent such that it could remove oils that were spread on the surface of the water.

Nanocellulose based microfiltration membranes for water treatment has gained considerable interest in nowadays. The dimensions of nanocellulose and the strength of the material can be exploited in the fabrication of membranes for water treatment.

Membranes can greatly benefit from the high strength of nanocellulose. At low loadings of a few wt% nanocellulose can increase the tensile strength of polymer membrane up to 50% [START_REF] Sjöström | Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose[END_REF]52]. At low nanocellulose loading (< 4 wt %), membranes show increased porosity, larger pore size and greater surface hydrophilicity which lead to greater water permeability. The addition of nanocellulose to polymer membrane has generally been shown to increase membrane hydrophilicity, thereby reducing the antifouling problems of the membrane. All these properties such as membrane surface hydrophilicity, greater permeability, greater selectivity and greater resistance to bio-fouling make nanocellulose as a promising candidate for waste water treatment applications.

Cellulose nanocrystals (CNCs) based microfiltration membranes for water purification

The CNCs enhanced mechanical properties, low defects, high surface area to They had done silica nanoparticles removal and oil/water separation on the fabricated membranes. From the FESEM images shown in figure 1.15, from the top surface of the filtrated membrane they observed that the nanoparticles formed a thick film, but from the bottom surfaces of composite membrane surface there was no nanoparticles and they found that the fabricated membrane successfully removed the silica nanoparticles with 7-40 nm from the simulative waste water. They also reported that the composite membrane had the rejection ratio above 99.5% against the oil/water mixture. Moreover, they observed that the oil concentration of the filtrate was less than 5.5 ppm, which definitely satisfied with the environmental standard for waste water discharge (< 10 ppm) [START_REF] Cao | Robust polyacrylonitrile nanofibrous membrane reinforced with jute cellulose nanowhiskers for water purification[END_REF]. unacceptable deterioration of equipment performance due to the unwanted radiated signals [START_REF] Jana | Electromagnetic interference shielding by carbon fibre-filled polychloroprene rubber composites[END_REF]. If these issues are left unattended, it may cause severe damage to communication system and safety operation of many electronic devices. More over the EMI also causes a lot of health issues such as symptoms of languidness, insomnia; nervousness and headache on exposure to electromagnetic waves [START_REF] Yavuz | Electromagnetic applications of conducting and nanocomposite materials[END_REF][START_REF] Sittisart | Developing a Cost-Effective Composite Based on Electroless Nickel-Coated Cellulose Fibres for Electromagnetic Interference Shielding[END_REF][START_REF] Sienkiewicz | Biological effects of electromagnetic fields[END_REF].

In the operation of microprocessor controlled devices the use of high frequency signals may be transmitted out of the device to the surrounding environment can severely affect the functioning of nearby equipment. So in order to prevent this, an electronic devices must be shielded in such a way that both incoming and outgoing interferences should be filtered [START_REF] Trivedi | Shielding of electromagnetic interference using polyaniline[END_REF].

ICPs are new alternative candidates for EMI shielding applications due to their lightweight, corrosion resistance, ease of processing, and tuneable conductivities as compared with typical metals. More importantly, the dominant shielding characteristic of absorption other than that of reflection for metals render ICPs more promising materials in applications requiring not only high EMI shielding effectiveness but also shielding by absorption, such as in stealth technology.

Intrinsically conducting polymers (ICPs) are attractive alternative materials for EMI shielding.

EMI Shielding Theory

Shielding effectiveness is the ratio of impinging energy to the residual energy. When an electromagnetic wave pass through a shield, absorption and reflection takes place. Residual energy is part of the remaining energy that is neither reflected nor absorbed by the shield but it is emerged out from the shield. All electromagnetic waves consist of two essential components, a magnetic field (H) and an electric field (E) as shown in figure 1.17. These two fields are perpendicular to each other and the direction of wave propagation is at right angles to the plane containing the two components. The relative magnitude depends upon the waveform and its source. The ratio of E to H is called wave impedance. EMI shielding consists of two regions, the near field shielding region and far field shielding region. When the distance between the radiation source and the shield is larger than λ/2π (where λ is the wavelength of the source), it is in the far field shielding region. The electromagnetic plane wave theory is generally applied for EMI shielding in this region. When the distance is less than λ/2π, it is in the near field shielding and the theory based on the contribution of electric and magnetic dipoles is used for EMI shielding [START_REF] Wang | Intrinsically conducting polymers for electromagnetic interference shielding[END_REF]. Where E and H are electric and magnetic fields and the subscripts t and i refer to the transmitted and incident waves. E is measured in volts/m and H in amps/m. SE is a function of frequency [START_REF] Nalwa | Handbook of Organic Conductive Molecules and Polymers[END_REF].

The attenuation of an electromagnetic wave occurs by three mechanisms as shown in figure 1.18. They are Absorption (A), Reflection (R) and Multiple reflections (B).

Thus shielding effectiveness is the sum of all these three terms SE = A+ R+ B. Compounding with conductive fillers and 3) Intrinsically conducting polymers.

Conductive coating of plastics

Metallic plating [START_REF] Hochberg | Shielding for EMI and antistatic plastic resins with stainless steel fibres[END_REF][START_REF] Thornton | EMI shielding gasket[END_REF]

Cellulose nanopapers derived from bio-mass as an effective substrate for EMI Shielding applications

Intrinsically conductive polymers are promising candidates for EMI shielding applications due to their lightweight, corrosion resistance, ease of processing, and tuneable conductivities as compared with typical metals. More importantly, the dominant shielding characteristic of absorption other than that of reflection for metals render intrinsic conducting polymers more promising materials in applications requiring not only high EMI shielding effectiveness but also shielding by absorption, which is highly desirable in the current scenario. However, the existence of the conjugated system in conductive polymers enhances the rigidity of the polymer molecule, which resulted in the insolubility in most of the organic solvents. Moreover these polymers are not able to form a film by casting or other techniques. This is due to the fact that ability of such polymers to form H bonds is very low. In order to alleviate this problem, several works have been done in recent years to fabricate different conducting polymers composites by blending different polymers with excellent process ability. Among them, cellulose nanofibers from bio-mass as abundant renewable eco-friendly materials have gained considerable interest among scientific community. Cellulose nanofiber based papers or cellulose nanofiber suspensions give adequate H-bonding that films may be easily produced from the mixture of conducting polymers and cellulose nanofibers. By combining the cellulose nanofibers with conducting polymers, the poor formability and frangibility of conducting polymers can be alleviated to produce a conducting composite that is capable to shield electromagnetic radiations.

Very few works have been reported in this aspect.

Objectives of research

The major objectives of the research thesis are:

1) Extraction of cellulose nanofibers (CNFs) from sunflower stalks by steam explosion coupled with mild acid hydrolysis.

2) To produce a unique green adsorbent material from cellulose nanofibers (CNFs) via a non-solvent assisted procedure using Meldrum's acid as an esterification agent to enhance the adsorption toward positively charged crystal violet dyes.

3) Fabrication of polyvinylidene fluoride (PVDF) microfiltration membrane via Electrospinning process.

Materials

Sunflower stalks were procured from Umbria, Italy. The various chemicals used for the extraction of cellulose nanofibers are sodium hydroxide (NaOH), acetic acid (CH3COOH), sodium hypochlorite, and oxalic acid (H2C2O4), purchased from Sigma Aldrich, India. PVDF with Mw (2, 75000 g/mol), 2, 2-Dimethyl-1, 3 dioxane -4, 6-dione (98%) (Meldrum acid) and Iron oxide (Fe2O3) nanoparticles, 50-100nm particles size were procured from Sigma-Aldrich, Brazil, while Crystal violet dye (CV, C25 H30 N3 Cl, λmax =584nm, MM= 407.98g/mol) was procured from vetec (Brazil). Aniline, ammonium persulfate (APS) and hydrochloric acid (35%) were purchased from Sigma Aldrich Co. All other chemicals were procured from Sigma Aldrich and used as received.

Methods of Fabrication

This part gives a detailed description of the formulation used for the extraction of cellulose nanofibers from sunflower stalks and the preparation of nanocomposites membranes and papers with two different polymers, Polyvinylidene Fluoride (PVDF) and Polyaniline (PANI).

Extraction of Cellulose nanofibers via steam explosion coupled with Mild acid hydrolysis

The extraction procedure of cellulose nanofibrils (CNF) was done by a steam explosion treatment [1] that involved 1) alkali treatment with steam explosion; 2) bleaching and 3) mild acid hydrolysis coupled with steam explosion which is given in the figure 1. Initially the sunflower stalks were cut into small pieces with a grinder. A laboratory autoclave, model no: KAUC-A1 which can work with 137Pa was used for steam explosion treatment. 100g of grind piece of stalks were treated with 5wt% NaOH solution and kept in an autoclave with the pressure of 137 Pa with the temperature of 180°C in an autoclave for 1.5 hours. After that, a bleaching of the resultant alkali treated stalk sample was done by treating with 5wt% sodium hypochlorite solution for 1.5 hours. Bleaching was repeated six times until the residue become white in colour. After bleaching, the fibers were thoroughly washed, dried and subjected to mild acid hydrolysis using 5% oxalic acid under a pressure of 137Pa in an autoclave for 20 minutes. The pressure was released immediately and the process was repeated six times. The fibers were taken out, washed and dispersed in water and homogenized under continuous stirring for 6 hours and the resultant suspension became cellulose nanofiber aqueous suspension. The final product was washed with deionised water by successive centrifugations until neutralization.

Surface modification of CNFs using Meldrum's acid

Initially, the extracted cellulose nanofibers (CNFs) (1.3g) and Meldrum's acid (1.3g)

were mixed in a mortar and added to a round bottom flask provided with the reflux condenser and heated the whole mixture at 110°C for 4 hours in an oil bath with constant magnetic stirring. After the reaction, the resultant mixture was washed with de-ionized water and acetone to remove the non-reacted Meldrum's acid and then dried in an oven at 90° C for 1 hour.

Preparation of electrospun PVDF microfiltration membranes

17 wt% of PVDF pellets were dissolved in the solvent mixture of (DMF: Acetone) at a volume ratio of 60:40 at 50ºC. The PVDF nanofibrous electrospun membrane was prepared at 15 KV in a relative humidity of 60% at 28ºC at a feed rate of 1 mL/h with a constant tip to collector distance at 15 cm using electrospinning unit (HOLMARC: HO-NEV-02). After the electrospinning process, the electrospun membrane was placed in a hot air oven at 150ºC for 3 hrs to remove any residual solvent and to improve the membrane overall structural integrity before any characterization was performed.

Fabrication of PVDF/Meldrum modified CNFs based Nanofibrous Microfiltration (MF) Membranes

The electrospun PVDF membrane sample in the pattern of a circular shape with 2cm width was clamped in a Glass Filter Filtration Assembly (Holder-47mm), where 0.1wt% concentration of Meldrum acid modified CNFs or unmodified CNFs, pH 7.0 was impregnated into the PVDF electrospun membrane by applying pressure around 200 mm of Hg. Then the sample was heated at 100°C in an oven for 15 minutes to start the cross-linking reaction to immobilize CNF network. The thickness of the prepared membrane was calculated using thickness gauge (7301 MITUTOYO) and it was 170 ± 2 µm.

In-situ Polymerization of aniline on cellulose nanofibers

In this work, oxidative synthetic methodology [3] was employed for the synthesis of polyaniline. In order to prepare a set of PANI/CNF ratios of 0.2:1, 0.3:1, 0.5:1, 0.8:1, 0.9:1 and 1:1, different amounts of aniline monomer were dissolved in 40g of CNF suspension (1.15 mg/ml) at room temperature for 1 hour. Then the prepared solution was cooled down at 1°C in an ice bath, after which APS solution (dissolved in 40 ml of 1 M HCl) with APS/aniline 1.5 wt/wt was added. After 3 hrs, PANI/CNF suspension became dark green colour and suspended. Then the resulting aqueous mixture/ suspensions were washed with deionised water, acetone and 0.1 M HCl in order to obtain the aqueous suspension of PANI/CNF.

Fabrication of PANI/CNF flexible conductive papers

Vacuum filtration technique was used for the fabrication of PANI/CNF flexible composite papers. A filtration assembly with cellulose ester membrane (47 mm in diameter and 0.45 µm pore size) was employed for the vacuum filtration of the PANI/CNF suspension. After this the fabricated paper was washed several times with 1 M HCl to remove all the contaminants. Lastly, the fabricated PANI/CNF composite papers were peeled off from the filter and dried for 48 h.

Characterization Techniques

Transmission electron microscopy (TEM)

A JEOL JEM 2100 transmission electron microscope (TEM) was employed to evaluate the diameter of the prepared cellulose nanofibers (CNFs). Diluted CNFs suspension was sonicated for 7 minutes to avoid the agglomeration and directly dropped on the copper grid for characterization. The transmission electron microscopy of the extracted cellulose nanofibers from the sunflower stalks were conducted to investigate the dimensions, dispersion and morphology.

Scanning electron microscopy (SEM)

The morphology of PVDF electrospun membrane, PVDF/ Meldrum modified CNFs nanofibrous MF membranes, neat CNF paper and PANI/CNF flexible papers were examined using a SEM (CARL ZEISS, EVO MA10). The samples were sputtered with gold in an argon atmosphere. The accelerating voltage was 20 kV.

X-ray diffraction analysis (XRD)

X-ray diffraction patterns for prepared CNF, PANI and PANI/CNF samples were obtained with (SHIMADZU XRD-6000). The x-ray diffractograms were obtained at room temperature within in a 2ѳ range from 5 to 40° at a scan rate of 2° min -1 .XRD patterns were employed to check whether the Meldrum's acid modified CNFs preserves its cellulose I structure, for that XRD pattern of unmodified CNF and Meldrum's acid modified CNF was obtained with (SHIMADZU XRD-6000). The XRD patterns were collected from 2ѳ= 5-38° at a scan rate of 2° min -1 . The crystallinity index (CrI) of the CNF and Meldrum's acid modified CNF were determined by the Segal method as shown in equation 2.1 [4].

CrI = [(I002 -IAM)/I002] × 100 …………………………………….. (2.1)

In this equation, CrI expresses the relative degree of crystallinity, I002 is the maximum intensities of the (0 0 2) lattice diffraction at 2θ = 23°, and IAM is the intensity of diffraction at 2θ =18°. I002 represents both crystalline and amorphous regions, while IAM represent only the amorphous portion.

Thermogravimetric analysis (TGA)

The thermal stability of the Meldrum's acid modified CNF and CNF samples were determined by TGA measurements performed using a Mettler Toledo Thermogravimetric analyzer (TGA/SDTA 85-F). The amount of sample used for each measurement was 10 mg. All measurements were performed under nitrogen atmosphere with a gas flow of 100 mL min -1 by heating the material from room temperature to 800° C at a heating rate of 10 ° C min -1.

Conductimetic titration

The carboxyl group content present in the cellulose nanofibers before and after chemical modification with Meldrum's acid, were determined by conductimetic titration according to Tappi standard T-237 and adaptations of by Sjöström and Alen (1999) [5], as the following procedure: At first 0.5 g of dried cellulose nanofibers was added into 300mL of distilled water and dispersed by mechanical stirring. Then the material was filtered and added 300 mL of 0.1 M HCL and then stirred for one hour. After stirring, the material was filtered and washed with distilled water until the filtrate conductivity less than 5 S/cm. Then the filtered material was added into the mixture of 250 ml of 0.001M NaCl, and 1.5 mL of 0.1M HCl. Then the mixture was stirred and titrated with 0.05 M NaOH (concentration rigorously known), in the conductivimeter. Then plot a conductivity chart v/s volume of NaOH spent to calculate the amount of the carboxyl groups by using the equation 2.2.

Amount of carboxyl groups in (mmol/100g) = (B-A) X C ………………… (2.2) W

Where, A and B are the volumes (mL) of NaOH in the intersection curves of the graph.

C is the concentration of NaOH solution x 100 W is the weight of dried CNF and Meldrum's acid CNF employed.

Fourier transform infrared spectroscopy (FTIR)

The IR spectra of CNF powder, Meldrum's acid modified CNF, PANI powder and PANI/CNF flexible paper were recorded using (IR prestige-21, FOURIER TRANSFORM INFRARED SPECTRO PHOTOMETER, SHIMADZU). The KBr disk method was employed for obtaining the IR spectra of all samples from 400-4000cm -1 . The studies were carried out with a resolution of 2cm -1 and a total of 15 scans for each sample.

Water contact angle studies

Water contact angles of electrospun PVDF membrane, PVDF/unmodified CNF nanofibrous MF membrane and PVDF/Meldrum's acid modified CNF nanofibrous MF membrane was measured using SCA20 contact angle analyzer. We used 10µL of de-ionized water for the measurements. Three measurements were obtained for each sample and calculated the average value.

BET measurements

A Micrometrics Surface and Pore Size Analyzer (ASAP2020) was used to conduct N2adsorption-desorption experiments at 77 K. Before the experiment, the sample was degassed at 573 K for 18 h. The surface area was obtained using the BET model. BET measurements were employed to determine the surface area and average pore diameter of PVDF electrospun membrane and Meldrum modified CNFs based PVDF nanofibrous microfiltration membrane.

UV-Visible spectrophotometer

The nanoparticles removal and dye removal were monitored with (FEMTO Mod.

800X I) by transmittance, absorbance respectively. For the evaluation of the nanoparticles removal, the feed solution and filtrate were poured into PMMA disposable cuvette and transmittance was obtained from 300 to 900 nm while for the assessment of dye removal, the starting and filtrate solution were poured into the PMMA cuvette and absorbance was measured at 590 nm. To calibrate the transmittance and absorbance of the samples, the spectrum of a cuvette filled with water was employed as the reference. Ultraviolet-visible spectroscopy (UV-vis) procedure was applied for the determination of solute concentrations in the starting and final solutions.

Dye adsorption studies

The evaluation of the adsorption efficiency of the nanofibrous MF membranes was done by using the following method. Initially, 0.

Microfiltration performance of the nanofibrous MF membranes

The microfiltration test of the nanofibrous membranes was carried out in Glass Filter Filtration Assembly (Holder-47mm) with a filtration area of 9.6 cm 2 .The 0.2wt% of Fe2O3 nanoparticles with the size of (50-100 nm) was set as the feed solution. The microfiltration was conducted at a pressure of 200mm Hg.

Electrical conductivity measurements

Four probe methods using a Keitheley 2400 source meter equipped with a gold probe was employed for the DC conductivity measurements for the fabricated PANI/CNF paper. The tests were performed at the ambient atmospheric conditions.

Dielectric studies

The dielectric analysis of the PANI/CNF papers was done using a WAYNE KERR-6500B Precision Impedance Analyser with a frequency range of 30 Hz to 30 MHz.

High frequency dielectric measurements were done using a Novocontrol Alpha High-Resolution Dielectric analyser with 10 GHz and 15 GHz resonator having sample dimension of 3cm ×3cm. on the cellulose nanofibers such as TEMPO oxidation [1][2][3][4][5][6][7][8], acetylation [9][10][11][12],

Silyation [13], esterification [14,15] and catonization [16]. In most of the above mentioned surface modifications of cellulose nanofibers were done using toxic organic solvents. Among them TEMPO oxidation is attained a considerable attention due to its advantage of surface modification and fibrillation of fiber occurs simultaneously in single process. But an obvious disadvantage of this oxidation procedure is the use of TEMPO and sodium bromide, which both are associated with environmental issues. Moreover, the TEMPO reagent is very expensive to commercialize in the industry level. So there is every need to formulate new chemical design protocols on the surface treatment of cellulose nanofibers via solvent free procedure technique. In this context, we intended to produce a unique adsorbent material from cellulose nanofibers (CNFs) via non-solvent assisted method using Meldrum's acid as an esterification agent. To the extent of our knowledge, preparation of new green based surface modification of cellulose nanofibers (CNFs) via non-solvent assisted procedure using Meldrum's acid as esterification agent has not been addressed till date. This finding offers a new platform for the surface treatment of cellulose nanofibers using solvent free green technology.

Morphology of cellulose nanofibers (CNFs)

Transmission electron microscopy (TEM) was employed to investigate and measure

FTIR spectroscopy studies

The

XRD Studies

The XRD patterns of the neat CNFs and Meldrum's acid modified CNFs are shown in figure 3.4. Two characteristic peaks at 2ѳ = 16.3° and 2ѳ = 22.6º are observed from the XRD pattern of neat CNFs, which shows the cellulose type I structure [21].

From the X-ray diffraction pattern, it is clearly shown that the Meldrum's acid modification has been affected mainly by the surface hydroxyl groups of cellulose.

The XRD profiles of the Meldrum's acid modified CNF is representative of plantderived cellulose I structure, indicate that the cellulose type I structure is preserved even after the surface modification by Meldrum's acid. The crystallinity index (CrI)

of the Meldrum's modified CNFs and CNFs samples were evaluated by using Segal (1959) method, as given in Eq. ( 1) [22]. image, it can be seen that the fiber structure of the cellulose nanofibers are maintained well even after the surface modification of Meldrum's acid on cellulose nanofibers.

Thermo gravimetric analysis

Meldrum's acid modified CNF and CNF samples were thermo gravimetrically analyzed to compare the degradation characteristics.

Conclusion

This chapter has given the complete details about the surface modification of the cellulose nanofibers using Meldrum's acid and their characterizations. This modified water pollution due to dye waste is an important issue to study [1]. Biological processes, chemical processes, and the operation of electromagnetic radiation are common removal techniques [2][3][4][5][6]. From these mentioned methods, microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF) using pressure driven technologies have gained considerable interest due to their impressively high performance and low cost [7,8]. These conventional ultrafiltration, nanofiltration, and microfiltration membranes for water purification are produced by phase immersion methods like immersion precipitation and thermally induced phase separation, where the torturous path results in the low water flux. Electrospun membranes have some advantages over the membranes produced via phase immersion methods such as higher effective porosity with continuously interconnected pores, light weight, and high surface area [9,10]. Therefore, electrospinning is a promising technique for generating continuous fibers with diameters in the range of a few hundred nanometers [11]. They exhibit excellent mechanical strength, water flux, and particle rejection. These properties make electrospun membranes an excellent platform for microfiltration applications [12].

On the other hand, the average porosity of the electrospun membrane is too high to remove smaller objects like viruses and textile dyes by size exclusion [13,14]. So, to eliminate the minute particles like viruses and textile dyes, an additional adsorption mechanism is essential without affecting the permeability of electrospun MF membranes. One of the typical approaches to remove viruses or dyes from wastewater is to increase the electrostatic charges of the MF membrane to enhance the absorption capability [15]. PVDF (polyvinylidene fluoride) has earned great attention in recent years as a membrane material due to its excellent properties like high thermal stability, mechanical strength, and chemical resistance toward solvents, acids, and base. Compared to other fluoropolymers PVDF can quickly dissolve in most of the organic solvents. These outstanding properties make PVDF membranes have been used as membrane filters for water purification [16][17][18][19][20][21][22].

Nanomaterials play a significant role in developing new materials with high efficiency at low cost for water pollution. Among the nanomaterial's, cellulose nanofibers (CNFs) are one of the promising adsorbent materials for water purification due to their low cost, abundant hydroxyl groups, natural abundance, and ecofriendly nature. Moreover, the CNFs have the enormous amount of surface hydroxyl (OH) groups, which enables a lot of surface chemistries or incorporation of chemical groups that may increase the adsorption toward the various pollutants in water. Until now, a few extensive reports have been published in the area of water purification using nanocellulose by Ma et al. [3,5]. nanofibrous membrane reinforced with jute cellulose nanowhiskers, where they used TEMPO surface treatment for the cellulose nanowhiskers [24]. So there is every need to formulate new chemical design protocols on the surface treatment of cellulose nanofibers via a solvent free procedure technique.

In this context, we intended to produce a unique adsorbent material from cellulose nanofibers (CNFs) via non-solvent assisted method using Meldrum's acid as an esterification agent which can remove the dyes from water. To the extent of our knowledge, preparation of new green based adsorbent to adsorbing dyes from CNFs via a non-solvent assisted procedure using Meldrum's acid as esterification agent has not been addressed to date. This finding offers a new platform for the surface treatment of cellulose nanofibers using solvent free green technology.

Contact angle studies

The water contact angle measurements for the PVDF electrospun membrane,

Evaluation of Adsorption capacity of nanofibrous membrane against crystal violet dye

In this study, we used crystal violet (CV) dye as a model of positively charged particles to evaluate the adsorption efficiency of all membranes. To determine the adsorption efficiency, UV-vis spectroscopy was employed. They found that the thin composite films showed improved combination of flexibility and conductivity [13].

In this chapter we discussed about the fabrication of flexible, lightweight& highly conductive cellulose nanopapers via in-situ polymerization of aniline monomer on to cellulose nanofibers. The demonstrated paper exhibits good conductivity due to the formation of a continuous coating of polyaniline over the cellulose nanofibers during in-situ polymerization which is evident from SEM, FTIR and XRD analysis.

The free hydroxyl groups on the surface of nanocellulose fibers promptly forms intermolecular hydrogen bonding with PANI which makes the cellulose nanopapers even more robust. According to Maxwell's equations [21], a material's response to electromagnetic radiation is determined by the following parameters; electrical conductivity (σ), electrical permittivity (ε) and magnetic permeability (μ). The real part of permittivity representing the stored energy between the material and the fields while the imaginary parts indicate the dissipated energy. Materials with high dielectric constant can store a substantial amount of electric charge. A material with high dielectric constant and dielectric loss values would show high electromagnetic shielding effectiveness [22]. In the case of polymer nanocomposites, the dielectric constant mainly arises from matrix filler incongruity. Maxwell-Wagner-Sillars (MWS) theory declares that this mismatch results the interfacial polarization at the interface. Frequency dependent dielectric constant and loss factor of PANI/CNF papers with varying PANI ratios are shown in figure 5.9. The dielectric constant of all PANI/CNF papers (Figure 5.9a) decreases with increase in frequency. In the case of conducting polymer composites, the frequency dependency is related to the dissipation of charge at the filler-matrix interface. The material's inherent properties are mainly reflected at the higher frequency regions than surface issues related lower frequency region [23]. As shown in figure 5.9a, PANI/CNF (1:1) composite paper shows the maximum ε 1 value compared to other compositions. At 30 MHz, PANI/CNF (1:0.2) composite paper displayed ε' value of about 28 whereas PANI/CNF (1:1) composite paper showed ε' value of 36. This enhancement may be attributed to the decrease in the gap between fillers due to the high filler loading and because of the higher charge accumulation at MWS interface of PANI/CNF (1:1) paper. Figure 5.9b shows that the imaginary permittivity, ε 11 , increases with increase in PANI loading. Generally, in the case of nanocomposites, the conductive nanofiller networks acts as charge carriers and increase of these conductive networks results nanocomposites higher loss factor. As expected, PANI/CNF (1:1) paper display higher value of imaginary permittivity with a ε 11 value of 5.9 at 30

MHz.

The ac electrical conductivity, which is directly related to the loss factor (εʺ) is

given by the following equation.

σAC = 2 πfε0εʺ

Where f is the frequency in Hz and ε0 is the permittivity of free space (ε0= 8.854•10 - The CNF/PANI paper has showed good permittivity values even in the high frequency region of 15 GHz which makes it an ideal candidate for shielding EM waves at X band region.

Conclusion:

We reported herein a facile approach for the fabrication of highly processable PANI/CNF suspensions containing various aniline ratios. The PANI/CNF flexible composite papers exhibited high DC conductivity value of 3.14 × 10

Chapter 6

Electromagnetic Wave Attenuation Performance of PANI/CNF Nano Papers

Introduction

In our modern society the electromagnetic shielding of devices functioning in radio frequency band remains to be a severe concern. For the safety of the sensitive circuits, light weight EMI shielding materials are required to shield the workspace from radiations transmitted out from computers and telecommunication equipment.

Electromagnetic waves from the sensitive electronic equipment's are reflected or absorbed by the human body. Thus misdirected radiations emitted out from the electronic devices may have an adverse effect on human health [1]. Light weight, flexible polymer nanocomposite materials have been a key interest for EMI shielding and microwave absorption due to many advantages over traditionally used metal sheets [2][3][4]. Conducting polymers such as polyaniline (PANI), polypyrole (PPY) and polyacetylene (PA) are popular for EMI shielding. Among them, PANI has gained considerable interest due to the controllable electrical conductivity via simple protonation/deprotonation process, good environmental stability [5]. Thus various applications like sensors, biomedical devices, opto-electronics, electro chromic devices and fuel cells, PANI has been explored [6][7][8][9][10][11]. Furthermore, PANI possess a fascinating electromagnetic interference shielding mechanism by absorption, leading as a tremendous material especially for military purposes [5].

PANI occurs in various forms and the particular forms find diverse specific technological applications. Leucoemeraldine is the completely reduced form of PANI, which has been used in electro chromic devices. Emeraldine is the insulating form of the PANI, which is the 50% reduced and 50% oxidized form of polyaniline.

This emeraldine form has been used in sensors for HCl gas. The doping of the emeraldine form results in the conducting PANI, which has been used in various applications like sensors, EMI shielding and electro chromic devices. Unlike other conducting polymers like polypyrrole, polythiophene and polyfuran, the hetro atoms in the PANI contributes to π-bond formation thereby become conducting. This makes PANI as a unique conductive polymer among the conductive polymers [12].

However, the existence of the conjugated system in PANI enhances the rigidity of the polymer molecule, resulting in poor film-forming capability. This is due to the fact that the ability of such polymers to form H bonds is very low, so thereby it is difficult to form films by casting and drying from such dispersions. To fix this issue, extensive work has been done in recent years to fabricate various PANI composites by blending diverse polymers with excellent mechanical properties and process ability. A variety of polymeric materials with excellent physical properties have been used as good supports for PANI, such as cellulose, rubbers, plastics, and textiles [13][14][15][16]. Among them, cellulose nanofibers as abundant renewable ecofriendly materials have drawn more and more attention [17,18]. Cellulose nanofibers as a substrate have been proposed as a solution for this obstacle.

Cellulose nanofiber based papers or cellulose nanofiber suspensions give adequate H-bonding that films may be easily produced from the mixture of conducting polymers and cellulose nanofibers. By combining the cellulose nanofibers with PANI, the poor formability and frangibility of PANI can be alleviated to produce a conducting composite that is capable to shield electromagnetic radiations.

Cellulose nanopapers (CNPs) are a class of promising functional constructs which can be exploited for varying technological applications ranging from flexible optoelectronic devices [19][20][21] to nonwoven porous membranes for water purification [22][23] and as fire retarding gas barrier films [24][25]

SE Total (dB) = SEA + SER + SEMR (1)

It is also well known that SEMR can be neglected if the absorption component is ≥10 dB or the shield thickness is greater than the skin depth of the samples where the multiple reflections can be neglected because the reflected wave from the internal surface will be absorbed by the shield [4]. Then, the total shielding effectiveness can be re-expressed as equation (2).

SE Total (dB) = SEA + SER (2)
The total shielding effectiveness, shielding due to absorption and shielding due to reflection can also be expressed in terms of scattering parameters as given in equations ( 3),( 4) & (5). at a thickness of about 0.080 mm in X band region. In contrary, we were able to achieve c.a -23 dB (nearly 99% attenuation) at 1.0 mm thickness in X band region, which is considered to be a suitable value for commercial device applications. Table 6.1, manifests some of the works reported on cellulosic based EMI shielding materials and their corresponding shielding effectiveness, thickness and operating frequency of the shields. . It is also evident from this data that achieving >20 dB shielding effectiveness in cellulosic substrates is quite a remarkable value which makes the fabricated PANI/CNF nanopapers as commercially viable shields.

Conclusion

Nevertheless, the PANI coating on the cellulose nanofibers is of paramount importance in attenuating the incident EM waves. The total shielding effectiveness of the composite nanopapers was found to be -16. confirmed that, the PANI/CNF flexible composite paper was a good promising candidate for attenuating electromagnetic radiations. We firmly believe that, this study will be an effective platform for developing green microwave attenuators in future.

6.4

researchers to discover the nanoscale cellulose structures that exhibit extraordinary properties. These cellulose nanostructures are naturally ranging from a few nanometres to microns with excellent mechanical properties. Cellulose nanomaterial's inherent fibrous nature and remarkable mechanical properties combined with low cost, biocompatibility and sustainable source, suggest huge potential as a component in water filtration membranes and green microwave attenuators in future.

In the present study, cellulose nanofibers were successfully isolated from renewable, the PANI/CNF flexible composite paper was a good promising candidate for attenuating electromagnetic radiations. We firmly believe that, this study will be an effective platform for developing green microwave attenuators in future.

As a future outlook for designing efficient filters based on fully eco-friendly materials like nanocellulose and its functionalized analogues will be a sustainable solution for water purification. In this work we proved that the membranes fabricated from PVDF and surface modified cellulose nanofibers will be a promising candidate for water purification. However these membranes cannot be commercialize at industrial scale. As a future step to commercialize these membranes we need to focus on fully green based cost effective membranes from cellulose by replacing high cost synthetic polymers. As an extension of the EMI shielding performance, we need to focus on multi-layered approach of cellulose nanopapers in combination with decorated magnetic structures can be effectively used for designing high performance ultra-light weight electromagnetic shields. The multilayer architecture has a significant role in attenuating electromagnetic radiations. The impedance of air shield interface and interlayer interface can be finally tuned to facilitate enhanced absorption of electromagnetic waves. Fabrication of porous aerogel structures based on cellulose and conducting polymers is also a key area to work in future. The porous architecture has a pivotal role in attenuating electromagnetic radiations as the intensity of electromagnetic waves remarkably reduces due to the multiple scattering occurring within the porous network.

5 .

 5 , its sources, chemical composition and physical properties 1.2 Cellulose nanofibers (CNFs) and Cellulose nanocrystals (CNCs) 1.3 Isolation and Extraction techniques for cellulose nanofibers ( CNFs) Water purification as a comprehensive challenge for future and electrospun nanofibers as an advanced solution 1.5.1. Next Generation filtration media: electrospun fibers 1.5.2 Nanocellulose as a sustainable solution for water purification 1.5.2.1. Cellulose nanofibers (CNFs) based microfiltration membrane for Water purification 1.5.2.2 Cellulose nanocrystals (CNCs) based Microfiltration membrane for Water Purification 1.6 EMI Shielding as a global challenge for future and Intrinsically conducting polymers based on cellulose nanopapers as an advanced solution 1.6.1 EMI Shielding Theory 1.6.2 Polymers as EMI shielding material 1.6.3 Conductive coating of plastics 1.6.4 Compounding with conductive fillers 1.6.5 Intrinsically conducting polymers (ICPs)

Chapter 3 :Chapter 4 :

 34 Surface modification of cellulose nanofibers via non solvent assisted procedure using Meldrum's acid-a unique methodology………………….78-91 3.1 Introduction 3.2 Morphology of cellulose nanofibers (CNFs) 3.3 Characterization of Meldrum's acid modified CNFs. 3.3.1 FTIR spectroscopy studies 3.3.2 XRD studies 3.3.3 Morphology of the Meldrum's acid modified CNF 3.3.4 Thermogravimetric Analysis 3.3.5 Determination of the amount of carboxyl groups in the Meldrum's Meldrum's Acid modified cellulose nanofiber based PVDF nanofibrous Microfiltration membrane for dye water treatment and nanoparticle removal……………………………………………………..92-107 4.1 Introduction 4.2 Morphology of cellulose nanofibers and PVDF electrospun membrane 4.3 Morphology of Modified CNFs based PVDF nanofibrous MF membrane 4.4 Surface area of nanofibrous MF membrane 4.5 Contact angle studies 4.6 Evaluation of adsorption capacity of nanofibrous membrane against crystal violet dye 4.7 Microfiltration performance of nanofibrous MF membrane against nanoparticles

  Natural fibres can be considered as naturally occurring composites consisting mainly of cellulose fibrils embedded in lignin matrix. The cellulose fibrils are aligned along the length of the fibre, shown in figure1.1, which renders maximum tensile and flexural strengths in addition to providing rigidity. The reinforcing efficiency of natural fibre is related to the nature of cellulose and its crystallinity. The structure and the morphology of cellulose have been the studied and the key subject for a large amount of work. Basically it is a polydisperse linear polymer consisting of Danhydroglucose (C6H11O5) repeating units joined by 1,4-b-D-glycosidic linkages at C1 and C4 position 1 shown in figure1.2[1].The degree of polymerization (DP) is around 10,000. Each repeating unit contains three hydroxyl groups. These hydroxyl groups and their ability to hydrogen bond play a major role in directing the crystalline packing and also govern the physical properties of cellulose.

Figure 1 . 1 :

 11 Figure 1.1: Arrangement of Fibrils, Microfibrils and Cellulose in Cell walls.

Fig1. 2 : 1 . 2

 212 Fig1.2: Chemical Structure of Cellulose.

Figure 1 . 3 : 1 . 3

 1313 Figure 1.3: Schematics of cellulose fibre structure with emphasis from the (a) cellulosic plant sources and (b) cellulose fibre to the (c) cellulose nanofibers (CNFs) bundles consisted of crystalline and amorphous regions and to the (d) cellulose nanocrystals (CNCs) [3]. Copyright (2015). Reproduced with permission from Elsevier Ltd.

  ultrasonication. The ultrasonic impact can gradually disintegrate the micron-sized cellulose fibers into nanofibers. Before ultra-sonication the plant fibers are being purified to prepare the cellulose fibers by mild acid hydrolysis followed by alkali treatment and bleaching treatment. This process comprises of the removal of non-cellulosic materials like lignin, hemicelluloses etc. After chemical pre-treatment, the purified cellulose fibers are soaked in distilled water. About 110 ml of solution containing chemical-purified cellulose fibers are then placed in an ultrasonic generator of 20-25 KHz in frequency. The ultrasonication is conducted for 30 min to isolate the nanofibers. Li et al. prepared nanocellulose fibers by pre-treating cellulose in NaOH/urea/Thiourea solution and then defibrillating the fibers by ultrasonication. They achieved the high yield of 85.4%. The obtained nanocellulose fibers were about 30 nm in diameter with cellulose II crystal structure. They possess high thermal stability with an onset of thermal degradation at 270 °C and a maximum degradation temperature of 370 °C[13]. Chen et al. reported that the cellulose nanofibers were individualized from poplar wood by explosive chemical pre-treatment and high-intensity ultrasonication. When the output power of ultrasonic treatment used for the chemical-purified cellulose fibers was greater than 1000W, cellulose nanofibers were 5-20 nm in width and several microns in length

Figure 1 . 5 :

 15 Figure 1.5: Scheme of TEMPO mediated oxidation mechanism of the hydroxymethyl groups of nanocellulose

Figure 1 . 6 :

 16 Figure 1.6: Chemical modification of nanocellulose with acetic anhydride and DMF (N, N-dimethyl formamide.

Figure 1 . 7 :

 17 Figure 1.7: Silyation of Nanocellulose

Figure 1 . 8 :

 18 Figure 1.8: Reaction scheme illustrating the simultaneous occurrence of cellulose hydrolysis and esterification of hydroxyl groups using a mixture of acetic and hydrochloric acid as example

Figure 1 . 9 :

 19 Figure 1.9: a) Distribution of earth's water and b) Scarcity of the water usable for human needs

Figure 1 . 10 :

 110 Figure 1.10: Proportion of Population by Country Using Improved Drinking Water Sources in 2008 Source: World Health Organization [41]

Figure 1 . 11 :

 111 Figure 1.11: Proportion of Population by Country Using Improved Sanitation Facilities in 2008 Source: World Health Organization [41]

1. 5 . 2 . 1

 521 Cellulose nanofibers (CNFs) based microfiltration membrane for water purification The dimensions of CNFs and their strength can be exploited in the fabrication of membranes for water treatment. Recently many studies have been reported on the CNFs based nanofibrous microfiltration membrane for water treatment. Wang et al. prepared CNFs based microfiltration membranes capable of removing bacteria, virus and heavy metals. They demonstrated a two-layered nanoscale PAN/ micro scale PET fibrous scaffold containing infused ultra-fine functional CNFs. They showed that the demonstrated membrane could simultaneously remove bacteria, viruses or toxic heavy metal ions. They found that, the demonstrated membrane with the addition of a substantial amount of charged groups (positively charged carboxylate groups or negatively charged amine groups) on the extremely large surface area of ultra-fine cellulose nanofibers was facilitated completely removing of bacteria (E.coli), reaching a log-reduction value (LRV) of 4 for MS2 virus removal shown in

  al. and they prepared ultrafine CNFs via TEMPO/NaBr/ NaClO oxidizing method and fabricated thin film nanofibrous composite (TFNC) membranes by using prepared ultrafine CNFs. They showed that CNFs top barrier layer surface appeared to be relatively smooth as shown by SEM given in figure1.12. This resulted in a lower tendency to trap unwanted materials on the CNFs barrier surface, thereby reducing the fouling problem.

Figure 1 . 12 :

 112 Figure 1.12: SEM images of (a) cross-sectioned view of TFNC membrane with cellulose nanofiber barrier layer; (b) top view of TFNC membrane; and (c) magnified cellulose nanofiber barrier layer of TFNC membrane [54].Copyright (2011). Reprinted with permission from American Chemical Society.

Figure 1 . 13 :

 113 Figure 1.13: Permeation flux and rejection ratio of TFNC membrane containing cellulose nanofiber barrier layer (0.10 ± 0.02 µm thickness, prepared from 0.05 wt% aqueous suspension) as a function of time; measurement carried out a constant pressure of 30 psi and temperature of 37º C based on ultrafiltration of oil/water emulsions. (The ultrafiltration performance of commercial PAN10 and PAN400 membranes was also included for comparison [54].Copyright (2011). Reprinted with permission from American Chemical Society.

  volume ratio; CNCs have been successfully added to a wide variety of microfiltration membranes for water purification. Additionally, CNCs are particularly attractive nanoparticles because they have low environmental, health and safety risks, are inherently renewable, sustainable and have the potential to be processed in industrial-scale quantities at low costs. Cao et al. reported robust poly acrylonitrile nanofibrous membrane reinforced with jute cellulose nanowhiskers for water purification. They developed a novel double layer of poly acrylonitrile (PAN) electrospun nanofibrous membranes (with an average fibre diameter of 173 nm) reinforced with TEMPO (2,2,6,6-tetra methyl piperdine-1-oxyl radical) selectively oxidized jute cellulose nanowhiskers (with a diameter in the range of 3-10 nm). They showed that the introduction of cellulose whiskers into the composite membrane had improved the mechanical properties, such as tensile strength increased to 10 MPa for single layer composite membrane and 14 MPa for double layer composite membrane, shown in figure 1.14. This can interpreted by the strong hydrogen bonding of cellulose nanowhiskers network formed on the surface of PAN nanofibrous electrospun membrane.

Figure 1 . 14 :

 114 Figure 1.14: Stress-strain curves for PAN ES films, PAN ES films reinforced with cellulose nanowhiskers and double layers of PAN ES films reinforced with cellulose nanowhiskers [46]. Copyright (2013). Reproduced with permission from Elsevier Ltd.

Figure 1 . 15 :

 115 Figure 1.15: FESEM images of PAN fibrous composite membrane after filtration of silica nanoparticles, top surface (a) and bottom surface (b) [46]. Copyright (2013). Reproduced with permission from Elsevier Ltd.

Figure 1 . 16 :

 116 Figure 1.16: (a) TEM image of cellulose nanowhiskers fabricated by the TEMPO/NaBr/NaClO oxidation method. (The inset shows the electron diffraction pattern.) (b) SEM images of cross-sectional views of PAN electrospun nanofibrous scaffold and (c) cellulose nanowhisker-modified PAN electrospun nanofibrous scaffold. Schematic representations of electrospun nanofibrous scaffolds: (d) scaffold before nanowhisker infusion; (e) infused nanowhiskers forming loose crosslinked mesh; and (f) nanowhiskers collapsed onto the scaffold, forming bundles [55]. Copyright (2012). Reprinted with permission from American Chemical Society.

Figure 1 . 17 :

 117 Figure 1.17: Electromagnetic radiation vector

Figure 1 . 18 :

 118 Figure 1.18: Attenuation of an electromagnetic wave by a shield.

  ICPs,[START_REF] Heeger | Semiconducting and metallic polymers: the fourth generation of polymeric materials[END_REF] such as polyaniline (PANI), polypyrrole (PPY), polythiophene (PTH), poly(p-phenylene-vinylene) (PPV) (Figure1.19) have been synthesized subsequently and studied extensively.

Figure 1 . 19 : 1 . 6 . 6 EMI

 119166 Figure 1.19: Molecular structure of common intrinsic conducting polymers

  Fugetsu et al. investigated electrical conductivity and electromagnetic interference shielding efficiency of carbon nanotube/ cellulose composite paper. They observed that the CNTs form a continuously interconnected network on the cellulose fibers. They found that, the paper containing 8.32 wt% CNTs was electrically conductive with a volume resistance of 5.3 • 10 -1 Ω cm. The fabricated composite paper was capable of shielding electromagnetic interference over the tested range of 15-40 GHz, particularly in range of 30-40 GHz, with absorption as the essential shielding mechanism [90]. Marins et al. prepared conducting bacterial cellulose (BC) membranes coated with a high proportion of polyaniline (PAni) were prepared through in situ oxidative polymerization of aniline on the surface of the BC in the presence of acetic acid as the protonating agent. They investigated the effect of two different oxidizing agents, ammonium persulfate (APS) or iron (III) chloride (FeCl3), on the mechanical performance, electrical conductivity, crystallinity, morphology and ability to absorb the electromagnetic radiations. They obtained the shielding effectiveness (SE) of -5 dB for BC/PANI membranes at 8.2-12.4 GHz [91]. Cellulose/ carbon nanotube composite aerogel for highly efficient electromagnetic interference shielding was developed by Huang et al. and they demonstrated a scaffold structure which was well designed from nanofibrillar networks to nanosheet networks by controlling the concentration of cellulose in the sodium hydroxide/urea solution. The obtained conductive aerogel was first reported as an electromagnetic interference shielding material; it exhibits an electromagnetic interference (EMI) shielding effectiveness of 20.8 dB at frequency range of 8.2-12.4 GHz. They concluded that this type of green conductive aerogel has the potential to be used as lightweight shielding material against electromagnetic radiation, especially for aircraft and spacecraft applications [92]. Similar work was reported by Jian et al. and they developed the environmentally-friendly hybrid aerogels consisting of cellulose and multi-walled carbon nanotubes (MWCNTs). In addition, they obtained high electromagnetic interference (EMI) shielding effectiveness (SE) value of 19.4 dB They concluded that the absorption-dominant shielding mechanism of the fabricated aerogel helps a lot to reduce secondary radiation, which was beneficial to develop novel eco-friendly EMI shielding materials [93].

4 ) 5 ) 6 ) 8 )

 4568 Fabrication of Meldrum's acid modified cellulose nanofiber-based PVDF microfiltration membrane via vacuum filtration technique and their characterizations. Evaluation of adsorption efficiency of Meldrum's acid modified cellulose nanofiber-based PVDF microfiltration membrane against positively charged dyes. Microfiltration performance of Meldrum's acid modified cellulose nanofiber-based PVDF microfiltration membrane against nanoparticles. 7) Insitu polymerization of aniline on cellulose nanofibers (CNFs) Fabrication of conductive polyaniline (PANI)/ CNFs nanopapers via vacuum filtration technique and their characterizations. 9) EMI shielding performance of PANI/CNF nanopapers. To accomplish the goals, various experimental techniques and equipment's will be used to characterize the composite membranes and papers such as Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), UV-visible spectro photometer, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, X-ray Diffraction (XRD) analysis, Pore size analyser, dielectric measurements and PNA Network analyser.

  This chapter covers the detailed information regarding the different materials, methods/techniques and experimental techniques employed for the different composite systems based on the cellulose nanofibers (CNFs) with polyvinylidene Fluoride (PVDF) and Polyaniline (PANI). It gives a detailed description of the formulation used for the extraction of cellulose nanofibers from sunflower stalks and the preparation of nanocomposites membranes and papers with two different polymers, PVDF and PANI. This chapter also deals with the fabrication procedure for the cellulose nanofiber based PVDF microfiltration membrane and selfsupported cellulose nanofibers based polyaniline conductive papers. This chapter also contains the different characterization techniques employed for the above mentioned nanocomposites systems.

Figure 1 :

 1 Figure 1: Scheme of the extraction procedure for the extraction of cellulose nanofibers from sunflower stalks [2]. Copyright (2016). Reproduced with permission from Elsevier Ltd.

  05 g of unmodified CNF based PVDF nanofibrous MF membrane, Meldrum's acid modified CNF based PVDF nanofibrous MF membrane and PVDF membrane was immersed in 25 ml of CV aqueous solution (10mg/L, pH 7.0) on a shaking bed for 10-240 minutes at ambient temperature. The adsorption efficiency of CV dye was measured as a function of time. The concentration of the CV dye adsorbed on to the membrane was obtained from the concentration change of the CV solution before and after the adsorption steps using UV-visible spectrophotometer by optical adsorption at 590 nm.

2. 4 .

 4 14 EMI shielding performance EMI shielding values of Fabricated PANI/CNF paper was measured using an Agilent E8362B (10 MHz -20 GHz) two port PNA Network analyser coupled to a Keycom waveguide to measure the scattering parameters at frequency range of 8.2 to 12.4 GHz (X band). The scattering parameters (S11, S12, S22 & S21) were recorded and the SE Total of the PANI/CNF nanopapers were calculated with standard equations. Introduction This chapter provides the details about the surface modification of cellulose nanofibers extracted from sunflower stalks using Meldrum's acid via esterification and their characterization. Until now, various surface modifications have been done

  the morphology and average fiber diameter respectively of the prepared CNFs from the sunflower stalks. From the TEM image figure3.1 it allows evaluating that the fibers are in the range of 5-10 nm in diameters. From this image, it is clearly evident that the combination of steam explosion and mild acid hydrolysis is an efficient technique to prepare cellulose nanofibers. Chirayil et al. and Cherian et al. extracted cellulose nanofibers from isorafibers and banana leaf fibers respectively via steam explosion technique. They obtained cellulose nanofibers with the average diameter of 10-20 nm and 15nm from isorafibers and banana fibers respectively[17, 18]. It was anticipated that during steam explosion technique the lingo cellulosic structure undergoes chemical decomposition[19].

Figure 3 . 1 3 . 3

 3133 Figure 3.1 TEM image and diameter frequency distributions of cellulose nanofibers extracted from sunflower stalks.

Figure. 3 . 2

 32 Figure. 3.2 Strategy for the preparation of the Meldrum's acid modified cellulose nanofiber.

  IR spectra of unmodified CNFs and Meldrum's acid modified CNFs are shown in figure 3.3. From this figure, it is clearly shown that the following changes have occurred in the IR spectra of Meldrum's acid modified CNFs when compared withCNFs. 1) The presence of the band at 2954 cm -1 , is regarded as the asymmetric stretching of methylene (CH2) groups, due to the interaction between the malonyl groups and CNFs. 2) Strong band emerging at 1748 cm -1 for Meldrum's acid modified CNFs, attributed to the presence of ester groups formed during the esterification reaction between the malonic acid and CNFs. 3) the band emerging at 1384cm -1 for Meldrum's acid modified CNFs, indicates the presence of carboxylate groups. From this FTIR spectrum, it can be concluded that the esterification reaction between Meldrum's acid and CNFs was successful[20].

Figure. 3 . 3

 33 Figure. 3.3 FTIR spectra of Meldrum modified CNF and CNF

Figure. 3 . 4 X 3 . 3 . 3 Figure 3 . 5 :

 3433335 Figure. 3.4 X-ray diffraction pattern of Meldrum modified CNF and CNF

From the figure 3 . 6

 36 TGA plot, it can be clearly seen that the Meldrums'acid modified CNF starts to degrade in the range of 150-200° C, whereas in the case of neat CNF degradation starts after 220° C. This difference in the degradation profile could be attributed to the lower hydrogen bonding interactions in the Meldrum's acid modified CNF, because -OH groups are used up during the esterification between the Meldrum's acid and neat CNF. Moreover in the case of modified CNF, the organic moieties protruding outside can act as the initiation sites for degradation.

Figure 3 . 6 : 3 . 3 . 5

 36335 Figure 3.6: TGA analysis of Meldrum's acid modified CNF and CNF

Figure. 3 . 5

 35 Figure. 3.5 Conductimetic titration of extracted CNF and Meldrum's acid modified CNF

Meldrum's acid

  cellulose nanofibers were used for the fabrication of nanofibrous microfiltration membrane. From the FTIR spectra of the CNFs and Meldrum's acid modified CNFs, it can be concluded that the esterification reaction between Meldrum's acid and cellulose nanofibers was successful. From the x-ray diffraction pattern, it was clearly shown that the Meldrum's acid modification has been affected mainly on the surface hydroxyl groups of cellulose nanofibers. The crystallinity index values of both cellulose nanofibers and Meldrum's acid modified CNFs was calculated and it was 78% and 63% respectively. The decrease in the crystallinity index of Meldrum's acid CNFs compared to CNFs was due to the surface modification of CNFs by Meldrum's acid. Conductimetic titration analysis was employed to calculate the carboxylate content of the extracted cellulose nanofibers and Meldrum's acid modified cellulose nanofibers. From the conductimetic titration analysis of both CNFs and Meldrum's acid CNFs, it was found that the carboxylate content increased from 8.75 mmol/l00 g for CNF, to 71.35 mmol/100g for the Meldrum's acid modified CNF. The increased amount of carboxyl groups proves the effective esterification reaction of Meldrum's acid on the surface of the cellulose nanofibers.All plants and animals must have water to survive. With the evolution of various industries, water pollution is one of the biggest problems that the world faces. After agriculture, the textile dyeing and finishing industry has generated a large water pollution problem as most of the chemicals coming from these industries are highly toxic and directly or indirectly affect human health. Since the development of advanced modern textiles and the arrival of new synthetic dyes to the market daily,

  Most of the works based on nanocellulose in water treatment were reported by the Chemistry Department of Stony Brook University, USA. They demonstrated nanocellulose based membranes for the removal of positively charged dyes, bacteria, and viruses from water. They used (2, 2, 6, 6-tetramethylpiperidin-1-yl) oxyl (TEMPO) oxidation surface treatment for cellulose nanofibers for almost all of their studies. An obvious disadvantage of this oxidation procedure is the use of TEMPO and sodium bromide, which both are associated with environmental issues. Moreover, the TEMPO reagent is very expensive to commercialize in the industry level. Another work was done by Qiao et al., and they removed cationic dyes using carboxylate functionalized cellulose nanocrystals. They used pyridine as a solvent medium for the carboxylation of cellulose nanocrystals, which is highly toxic in nature [23]. Cao et al. removed silica nanoparticles from the water using poly-(acrylonitrile)

4. 2

 2 Morphology of PVDF electrospun membrane.

Figure 4 .

 4 Figure 4.1 showed the SEM images of PVDF electrospun membrane. From the

Figure 4 . 1 4 . 3

 4143 Figure 4.1 SEM image and diameter frequency distributions of PVDF electrospun membrane

Figure 4 .

 4 Figure 4.2 showed the SEM images of the top view of the modified CNFs based

Figure 4 . 2 4 . 4

 4244 Figure 4.2 SEM image of the top view of Meldrum's acid modified CNFs based PVDF nanofibrous MF membrane (The uncoated membrane part is displayed at the right side).

  Meldrum's acid, modified CNFs based PVDF membrane and unmodified CNFs based PVDF membrane were measured. The water contact angle obtained for the PVDF electrospun membrane, Meldrum's modified CNFs based PVDF membrane and unmodified CNFs based PVDF membrane were 142.6°, 123° and 121° respectively. The water contact angle for both modified CNFs and unmodified CNFs membranes were decreased to 123° and 121° respectively which indicates the enhancement of the wettability of both nanofibrous membranes compared to the PVDF electrospun membrane. It can be noticed that there was the only slight difference between the contact angles of both nanofibrous membranes. The water contact angle measurements confirmed that the fouling problem of the PVDF electrospun membrane could be decreased to some extent by the coating of modified CNFs and unmodified CNFs on to the surface of PVDF electrospun membrane.

Figure 4 .

 4 3 illustrates the adsorption efficiency, of 10mg/L CV by PVDF membrane alone, unmodified CNFs based PVDF membrane and Meldrum's modified CNFs based PVDF membranes towards crystal violet (CV) dye as a function of time. From the figure it was clearly shown that, with the 10mg/L of CV aqueous solution, CV adsorption of PVDF electrospun membrane and unmodified CNFs based PVDF membrane was around 1.368 and 2.948 mg/g of the membrane, whereas it was 3.984 mg/g of the membrane by Meldrum's acid CNFs based PVDF membrane. These observations were comparable with those of Ma et al. who evaluated the adsorption of positively charged CV dye was 3.8mg/g of the membrane using poly (acrylonitrile) (PAN) scaffold nanofibrous membrane with cellulose nanowhiskers [3]. It was found that the adsorption efficiency of Meldrum's modified CNFs based PVDF nanofibrous MF membrane was higher than that of PVDF electrospun membrane and unmodified CNFs based PVDF microfiltration membrane. This enhancement in the dye adsorption of the Meldrum's modified CNFs based PVDF membrane can be justified by the high electrostatic attraction between the positively charged CV dyes and carboxylate groups (negatively charged) on the Meldrum's acid modified CNFs (mechanism shown in figure 4.4). The adsorption of CV on to Meldrum's acid modified CNFs based PVDF membrane reachedequilibrium after 3 h. Therefore, it can be concluded that the adsorption efficiency (using 10mg/L CV aqueous solution) of the Meldrum's modified CNFs based PVDF membrane was about two to three times higher than that of PVDF electrospun membrane. Therefore, the demonstrated membrane is expected to be a very promising adsorbent for dye wastewater treatment and dynamic adsorption in practical applications.

Figure 4 . 3

 43 Figure 4.3 The adsorption capacity of PVDF membrane alone, PVDF/ unmodified CNF membrane and PVDF/ modified CNF membranes against time.

Figure 4 . 4 4 . 7

 4447 Figure 4.4 Schematic representation of crystal violet removal by Meldrum's acid modified CNFs based PVDF nanofibrous MF membrane and its mechanism

Figure 4 . 5 , 4 .

 454 Figure 4.5, 4.6 and 4.7 show the SEM images of the PVDF membrane alone,

Figure 4 . 5

 45 Figure 4.5 SEM images of the top surface (a) and the bottom surface (b) of PVDF electrospun membrane after the filtration of Fe2O3 nanoparticles.

Figure 4 . 6

 46 Figure 4.6 SEM images of the top surface (a) and the bottom surface (b) of unmodified CNFs based PVDF electrospun membrane after the filtration of Fe2O3 nanoparticles.

Figure 4 . 7

 47 Figure 4.7 SEM images of the top surface (a) and the bottom surface (b) of Meldrum's acid modified CNFs based PVDF electrospun nanofibrous membrane after the filtration of Fe2O3 nanoparticles.

Figure 4 . 8 8 Conclusion

 488 Figure 4.8 UV-visible spectra of feed and filtrate solution of Fe2O3 nanoparticles for Meldrum's acid modified CNFs based PVDF membrane

5. 2 Figure 5 . 1 :

 251 Figure 5.1: Strategy for the in-situ polymerization of aniline monomers onto cellulose nanofibers The scheme in figure 5.1, illustrates the insitu polymerization of aniline on the

Figure 5 . 2 :

 52 Figure 5.2: Interaction between CNF and PANI inset photographs of PANI/CNF paper.

Figure 5 . 3 :Figure 5 . 5 : 5 . 3 . 3 Figure 5 . 6 :Figure 5 . 7 :

 53555335657 Figure 5.3: Schematic showing the preparation of PANI/CNF composite paper

Figure 5 .

 5 Figure 5.6(a) illustrates the SEM image of neat CNF paper and figure 5.6(b)

Figure 5 . 8 :

 58 Figure 5.8: Images of (a) Fabricated PANI/CNF paper (b) conductivity demonstration of PANI/CNF paper.

12 F

 12 /m). As shown in figure5.9c, σAC increases with increase in frequency and PANI content. Contrast to DC electrical conductivity, AC conductivity arises from the charge accumulation and interfacial polarization. Significant amount of free charges present in the PANI/CNF (1:1) paper than lower PANI contents donates to the conductivity by hopping mechanism[24]. This may lead to the formation of conducting network among PANI chains. At 30 MHz, PANI/CNF (1:1) composite paper shows σAC value of 0.009S/m. Since the frequency of interest for the cellulose nanopapers was mostly X band region, the dielectric response of the CNF/PANI papers was also studied at 10 GHz and 15 GHz respectively. The dielectric properties of PANI/CNF (1:1) composite paper was further evaluated at higher frequencies of 10 GHz and 15 GHz (fig 5.10).The ε' value (1.66 for 10 GHz and 1.26 for 15 GHz) shows the same decreasing trend with respect to frequency dispersion whereas εʺ value (0.47 for 10 GHz and 0.58 for 15 GHz) increases with respect to frequency. At the highest frequency of electromagnetic waves, the dipoles cannot follow the field variations. This will decrease the permittivity because of the decreased charge accumulation.

Figure 5 . 10 :

 510 Figure 5.10: Dielectric properties of CNF: PANI (1:1) flexible paper at a frequency

  S12 are the forward transmission coefficient & reverse transmission coefficient respectively, S11 is the forward reflection coefficient were obtained from the vector network analyser (VNA). The SE Total values of the PANI/CNF nanopapers of varying thickness were measured by placing a 30 mm* 30 mm samples inside a waveguide and measurements were taken after calibrating the VNA.The variation of total shielding effectiveness and the corresponding shielding by absorption and reflection components of the fabricated PANI/CNF papers in X band (8.2-12.4 GHz) is given in figures 6.1& 6.2. A total SEvalue of c.a -23 dB was observed for PANI/CNF paper at 1.00 mm thickness. In all the EMI measurements, CNF to PANI ratio was 1:1 as this composition has shown a maximum DC conductivity value of 3.14x10 -1 S/cm. However, it was also observed that as the PANI ratio was increased, nanopapers exhibited poor flexibility and were found to be brittle in nature. It is also evident that the effect of forming a continuous network of PANI on to the surface of cellulose nanofibers has played a pivotal role in enhancing the electrical conductivity and attenuating electromagnetic radiations.Nevertheless, the secondary interactions such as the inter molecular hydrogen bonding between hydroxyl groups of the nanocellulose and the nitrogen of the PANI has been attributed as the key factor for achieving >99% attenuation even at a very low thickness of 1.0 mm. The basic mechanism of EM wave attenuation of PANI is by absorption mechanism but because of its poor film forming characteristics its application as EM shields is very limited. Thus the combined effects of cellulose nanofibers and PANI has resulted in formation of flexible and robust nanopapers which could be used as EM shields inside electronic devices such as terrestrial communication systems, aerospace communications & radar which operates mostly in X band frequencies. From figure 6.3, it is clearly shown that the PANI/CNF composite paper exhibited a predominant attenuation of microwave radiation by absorption mechanism. The flexible PANI/CNF composite papers demonstrated an attenuation of 87% by absorption and 13 % by reflection mechanism at 8.2 GHz, which is highly desirable as it prevents any further electromagnetic pollution and provides a noise free workspace. A cartoon illustrating the plausible shielding mechanisms of the fabricated PANI/CNF nanopapers is illustrated in figure 6.4. The incident EM waves are simultaneously reflected and absorbed by PANI/CNF nanopapers with an enhanced shielding by absorption mechanism in X band region. There are only few reports on the use of cellulose nanofibers in combination with conducting polymers for use as EMI shielding materials. Marins et al. made use of bacterial cellulose (BC) membranes in-situ polymerised with aniline monomer to form a coating of PANI on the surface of BC membranes. The overall shielding effectiveness of such membranes was found to be -5 dB (nearly 63.38% attenuation)

Figure 6 . 1 :Figure 6 . 2 :Figure 6 . 3 :

 616263 Figure 6.1: EMI shielding effectiveness of PANI/CNF flexible paper at (a) 0.25 mm & (b) 0.50 mm thickness in X band (8.2 -12 GHz)

Figure 6 . 4 :

 64 Figure 6.4: Schematic illustration of EMI shielding interference of demonstrated PANI/CNF flexible paper.

  3 dB and -23 dB at 0.75 mm and 1.0 mm paper thickness respectively at 8.2 GHz. The attenuation of EM waves was governed by a predominant absorption mechanism (c.a 97%) and very less reflection mechanism (c.a 13%). Such nanopapers have given a new arena for designing green microwave attenuators which can shield the incoming radiation by absorption dominated mechanism. Additionally, the demonstrated PANI/CNF flexible paper can be potentially used as material for plethora of applications such as flexible electrodes, sensors, electrically conductive and flexible films and other flexible paper based devices. The EMI shielding measurements of the demonstrated paper

  abundant and low-cost natural source, sunflower stalks by steam explosion coupled with mild acid hydrolysis. This technique comprises of alkaline treatment, bleaching, acidic steam treatment and homogenization. Morphological investigation was performed using transmission electron microscopy (TEM). Comparing the results with other reports, one can conclude that the nature and concentration of acid is of high importance with regard to the morphology of the resulting nanofibers. The first chapter is an updated survey of literature covering the field of cellulose nanofibers, various extraction techniques and its surface modifications. This chapter also describes the applicability of cellulose nanofibers in water purification membranes and electromagnetic interference systems. At the end of this chapter the main objectives are explained. The material details, experimental techniques and characterization methods employed in this study are presented in the second chapter.The third chapter explains about the surface modification of the extracted cellulose nanofibers via non-solvent assisted method using Meldrum's acid. From literature survey regarding the surface modification of cellulose nanofibers, one can concluded that the most of the surface modifications of cellulose nanofibers were done using toxic organic solvents. In this context, we intended to produce a unique adsorbent material from cellulose nanofibers (CNFs) via non-solvent assisted method using Meldrum's acid as an esterification agent. To the extent of our knowledge, preparation of new green based surface modification of cellulose nanofibers (CNFs) via non-solvent assisted procedure using Meldrum's acid as esterification agent has not been addressed till date. This finding offers a new platform for the surface treatment of cellulose nanofibers using solvent free green technology. Techniques such as FTIR, XRD and conductimetic titration method have been employed for the characterization of these Meldrum's acid modified cellulose nanofibers. From the FTIR spectra of the CNFs and Meldrum's acid modified CNFs, it can be concluded that the esterification reaction between Meldrum's acid and cellulose nanofibers was successful. From the x-ray diffraction pattern, it was clearly shown that the Meldrum's acid modification has been affected mainly on the surface hydroxyl groups of cellulose nanofibers. From the conductimetic titration analysis of both CNFs and Meldrum's acid CNFs, it was found that the carboxylate content increased from 8.75 mmol/l00 g for CNF, to 71.35 mmol/100g for the Meldrum's acid modified CNF. The increased amount of carboxyl groups proves the effective esterification reaction of Meldrum's acid on the surface of the cellulose nanofibers. The dye absorption studies and nanoparticle removal from water was carried out for the fabricated Meldrum's acid modified cellulose nanofiber based PVDF nanofibrous microfiltration membrane. Both CNF-based PVDF membranes (Meldrum's acid modified CNFs and CNF) were prepared by successive coating of modified and unmodified CNFs on to the surface of a PVDF electrospun membrane. All the demonstrated membranes showed high filtration capacity against the Fe2O3 nanoparticles. With the 10 mg/L of crystal violet (CV) aqueous solution, CV adsorption of PVDF electrospun membrane, and unmodified CNF-based PVDF membrane was around 1.368 and 2.948 mg/g of the membrane respectively, whereas it was 3.984 mg/g of the membrane by Meldrum's acid CNF-based PVDF membrane. This results was comparable with the reported values in the existing literature. The demonstrated Meldrum's acid modified CNF based PVDF was proven to be the efficient media that can concurrently eliminate the Fe2O3 nanoparticles and CV dyes from the water.Cellulose has been recognized as good matrix/substrate for biodegradable batteries, sensors and actuators. Cellulose nanofiber based papers or cellulose nanofiber suspensions give adequate hydrogen bonding that films may be easily produced from the mixture of conducting polymers and cellulose nanofibers. In this context, in-situ polymerized polyaniline (PANI)/cellulose nanofibers (CNFs) nanopapers with different PANI ratios were fabricated by vacuum filtration method. HCl and ammonium persulfate (APS) were employed as dopant and oxidant respectively.The PANI/CNF flexible composite papers exhibited high DC conductivity value of 3.14 × 10 -1 S/cm with a PANI: NFC ratio of 1:1. SEM images revealed that in-situ polymerization of PANI has formed a continuous network on the surface of the cellulose nanofibers. This formation of continuous network was attributed to the secondary forces of interaction via hydrogen bonding which was confirmed from FTIR studies. . At 30 MHz, PANI/CNF (1:0.2) composite paper displayed dielectric constant (ε') value of about 28 whereas PANI/CNF (1:1) composite paper showed ε' value of 36. Moreover PANI/CNF (1:1) paper display higher value of imaginary permittivity with a dielectric loss (ε '' ) value of 5.9 at 30 MHz.In the sixth chapter comprises of the electromagnetic wave attenuation performance of the fabricated PANI/CNF papers. The total shielding effectiveness of the composite nanopapers was found to be -16.3 dB and -23 dB at 0.75 mm and 1.0 mm paper thickness respectively at 8.2 GHz. The attenuation of EM waves was governed by a predominant absorption mechanism (c.a 97%) and very less reflection mechanism (c.a 13%). Such nanopapers have given a new arena for designing green microwave attenuators which can shield the incoming radiation by absorption dominated mechanism. Additionally, the demonstrated PANI/CNF flexible paper can be potentially used as material for plethora of applications such as flexible electrodes, sensors, and electrically conductive and flexible films and other flexible paper based devices. The EMI shielding measurements of the demonstrated paper confirmed that,

Table 1 . 2 :

 12 Chemical composition of some typical cellulose containing materials. Copyright 2012. Modified with permission from the Elsevier.

		Cellulose %	Hemicellulose %	Lignin%
	Hard wood	43-47	25-35	16-24
	Soft wood	40-44	25-29	25-31
	Bagasse	40	30	20
	Coir	32-43	10-20	43-49
	Corn cobs	45	35	15
	Corn stalks	35	25	35
	Cotton	95	2	1
	Flax (retted)	71	21	2
	Flax (unretted)	63	12	3
	Hemp	70	22	6
	Jute	71	14	13
	Kenaf	31	26	18
	Ramie	76	17	1
	Sisal	73	14	11
	Sunn	80	10	6
	Wheat straw	30	50	15
	Banana fibers	63-64	19	5

1.3.7 Enzymatic Pre-treatments:

  Deepa et al. reported the extraction of the cellulose nanofibers from the banana plant by the steam explosion process in an

	autoclave [15]. Chirayil et al. employed the steam explosion process for extracting This process enables the manufacture of cellulose nanofibers with significantly
	cellulose nanofibers from isora fibre in an autoclave. This technique involves reduced consumption [19]. In nature, cellulose is not degraded by a single enzyme
	alkaline treatment, bleaching, acidic steam treatment and homogenization. The but a set of cellulases are involved. These can be classified as A-and B-type
	results showed that the prepared cellulose nanofibers have nanofibrillar network like cellulases, which are able to attack highly crystalline cellulose and C-and D-type
	structure with high crystallinity and good thermal stability [16]. Cherian et al. also cellulases, which generally require some disorder structure in order to degrade
	reported that the steam explosion process was successful for the extraction of cellulose. Paakko et al. and Henriksson et al. reported that endoglucanase pre-
	cellulose nanofibrils from pineapple leaf fibres for the first time. Steam explosion treatment facilitates disintegration of cellulosic wood fiber pulp into cellulose
	coupled with mild acid treatment on the pineapple leaf fibres was found to be nanofibers [19-20]. Another work was done by Tibolla et al. and they isolated
	effective in the depolymerisation and defibrillation of the fibre to produce cellulose nanofibers from banana peel bran using chemical treatment (involves
	nanofibrils of these fibres [17]. alkaline treatment, bleaching and acid hydrolysis) and enzymatic treatment (alkaline
	1.3.6 Electrospinning: treatment and hydrolysis with xylanase). Nanofibers produced from chemical
	Electrospinning is quite simple method operates on the principle that a solution is treatment and enzymatic treatment had an average diameter of 10.9 and 7.6 nm and
	extruded -electrospun under the action of a high electric field. Once the voltage is a length of 454.9 and 2889.7 nm respectively. They showed that the nanoparticles
	high enough, a charged stream of matter is ejected following a rather complicated isolated from both of the treatments have potential application as reinforcing
	loop. During this process, the solvent evaporates leaving behind randomly oriented elements in composites [21].
	nanofibers accumulating on the collector. Cellulose nanofibers have been produced
	by using electrospinning technique by dissolving cellulose fibers in solvents, such as
	ethylene diamine, with a salt selected from the group consisting of potassium
	thiocynate, potassium iodide. Only few fundamental studies are currently dedicated
	to this method. Ma et al. prepared cellulose acetate (CA) nanofibers with fibre
	diameter ranging from 200 nm to 1 µm by electrospinning technique and solvent
	employed was acetone/DMF/trifluoroethylene (3:1:1) mixture [18].

1.4 Surface chemistries on nanocellulose

	heterogeneous& homogeneous conditions. Due to high crystallinity cellulose can be
	only dissolved in limited solvents, so many modifications have been conducted in
	heterogeneous conditions. Since chemical reactions occur only at the surface layer
	in heterogeneous conditions, the gross structure of the cellulose sample can largely
	maintained. Because of a natural advantage of an abundance of hydroxyl groups at
	the surface of cellulose Nano particles, different chemical modifications have been
	done such as esterification, catonization, Acetylation, oxidation, silylation, polymer
	grafting, etc. shown in figure 1.4.
	One of the drawback of nanocellulose with polar surfaces is poor dispersiblity/
	compatibility with non-polar solvents or resins. Therefore applications largely
	limited to aqueous or polar systems. To overcome this problem it is necessary to
	reduce the entanglement of the fibrils to improve their dispersion in matrix by
	chemical or surface modification. Cellulose has 3 alcoholic hydroxyl groups in each
	of its AGUs & chemical modification can be exclusively performed on these
	hydroxyls with practical relevance. Chemical modification can be conducted both in

Figure 1.4: Common modification chemistries on nanocellulose surfaces 1.4.1 TEMPO Oxidation

	This Procedure involves the application of a stable nitroxyl radical, the 2, 2, 6, 6-
	tetra methyl piperidine-1-oxyl (TEMPO) in the presence of NaBr & NaOCl. In this
	process, carboxylic acid groups at the surface of fibrillated cellulose and catalytic
	amounts of TEMPO & NaBr were dissolved in polysaccharide solution at pH 10-11
	& oxidation was started by addition of NaClO solution as a primary oxidant. The C6
	primary hydroxyls of cellulose can be entirely & selectively converted to C6 sodium
	carboxylate groups by TEMPO-mediated oxidation. TEMPO-mediated oxidation of
	nanocrystals obtained from acid hydrolysis of cellulose fibres was first reported by
	Araki et al. and they prepared a sterically stabilized cellulose microcrystal
	suspension by grafting of polyethylene Glycol (PEG) using TEMPO oxidation
	procedure [22]. In TEMPO oxidation newly installed carbonyl groups that imparted
	negative charges at the cellulose Nano crystal surface & thus induced electro static
	stabilization as shown in the figure1.5. Another work was reported by Saito et al.
	and they prepared homogeneous suspensions of invidualized microfibrils from
	TEMPO-catalysed oxidative of native cellulose. They found that, after treatment the
	native cellulose were readily disintegrated into long individual microfibers by a
	treatment with wearing blender, yielding transparent and highly viscous
	suspensions. They concluded that, compared to the conventional technique which
	disintegrate cellulose fibers into microfibrillar suspensions using mechanical
	treatment, a combination of TEMPO-mediated oxidation and homogenizing

5 Water purification as a comprehensive challenge for future and electrospun nanofibers as an advanced solution

	aqueous suspensions of nanocellulose with unexpected thixotropic gelling
	properties.
	2,3-epoxypropyl trimethylammonium chloride (EPTMAC) [37-39]. One end of the
	cationic reagent is quaternary ammonium salt, showing cationic character, and the
	other end of the structure is an epoxy group. Hasani et al. introduced epoxy propyl
	trimethyl ammonium chloride (EPTMAC) on the surface of nanocellulose, so that
	the nanocellulose was imparted positive charges [38]. Such surface catonization
	procedure was conducted through a nucleophilic addition of the alkali-activated
	cellulose to epoxy groups of EPTMAC. This modification resulted in the stable

1.

Table 1 .

 1

	PET/PAN layer thickness (µm)	Max. pore size (µm)	Mean flow pore size (µm)	Cellulose nanofiber loading (mg/cm 2 )	LRV for E.coli bacteria	LRV for MS2 virus
	100+40	0.78± 0.03 0.66 ± 0.01	0	6	0
	100+ 40	0.73± 0.01 0.38 ± 0.01 0.2 ± 0.05*	6	0
	100+ 40	0.78± 0.01 0.32 ± 0.01 0.2 ± 0.05**	6	>4
	*Cellulose nanofibers without PVAm grafting

3 and had the adsorption capability of 100mg chromium (Cr) (VI) or 260 mg lead Pb (II) per gram of cellulose nanofiber, while maintaining a high permeation rate (1300L/m 2 h/Psi). They concluded that the demonstrated membranes were unique as they could simultaneously remove toxic contaminants (virus, heavy metals) from polluted water with relatively large pores as well as bacteria by size exclusion [53]. ** Cellulose nanofibers with PVAm grafting.

Table 1.3: Retention of MS2 of composite membranes [53]. Copyright (2013). Reproduced with the permission from Elsevier Ltd. Another CNFs based nanofibrous microfiltration membrane was fabricated by Ma et

  Another interesting work was reported by Ma et al. and they demonstrated a multi-layered nanofibrous microfiltration (MF) membrane system with high flux, low pressure drop and high

retention capability against both bacteria and bacteriophages (a virus model) by impregnating ultrafine cellulose nanowhiskers (diameter about 5 nm) into an electrospun polyacrylonitrile (PAN) nanofibrous scaffold supported by poly (ethylene terephthalate) (PET) non-woven substrate. They reported that young's modulus and ultimate tensile strength for the cellulose nanowhiskers based nanofibrous membrane were 375± 15 & 14.3±0.4 MPa, respectively, which were about two times higher than those (i.e., 226± 20 & 8.5 ± 0.3 MPa) of the unmodified membrane containing only electrospun scaffolds. This was due to the stabilization of the electrospun scaffold by the formation of cross-linked cellulose nanowhiskers mesh, in agreement with the SEM images shown in figure 1.16.

  used for metalizing the plastic surface. There are different coating techniques have been employed to make conductive coating on the plastics. Applications of ICPs and their composites (or blends) owing to the higher electrical conductivity of these polymers in doped states have been extensively studied subsequently by many researchers.[START_REF] Pomposo | Polypyrrole-based conducting hot melt adhesives for EMI shielding applications[END_REF][START_REF] Desai | Electrospinning nanofibers of PANI/PMMA blends[END_REF][START_REF] Koul | Newer compatible conducting polymeric composites for elimination of electromagnetic radiations[END_REF].

	dB and they concluded that such a CNT-PS foam composite can be used
	commercially as a shielding material against electromagnetic radiation. The primary
	They are Foil laminates and tapes, Ion plating, Vacuum metallization, Zinc flame EMI shielding mechanism of such CNT-PS foam composites was demonstrated and
	spraying, Zinc arc spraying, Cathode sputtering, Conductive paints, Electro less ascribed to the reflection of electromagnetic radiation [71]. Yousefi et al. developed ICPs are termed organic polymers that possess the electrical, electronic, magnetic,
	plating and Electroplating [66-68]. Main disadvantage of the above mentioned a self-aligned RGO/epoxy nanocomposites with highly anisotropic mechanical and and optical properties of a metal while retaining the mechanical property, process
	techniques are, these process are secondary to the moulding operation and hence electrical properties for EMI shielding application. The large rGO sheets and their ability, etc. Since the first ICP, polyacetylene (PA), was successfully synthesized in
	demand additional surface preparation and special equipment. alignment into a layered structure play an important role in achieving a dielectric 1977 by Shirakawa et al. [74] great interests have been aroused and a series of
	polycaprolactone/ MWNT nanocomposites foams. Carbon nanotubes were dispersed
	within PCL by melt blending and by co-precipitation. They found that the PCL
	composites with very low vol% of MWNTs had high EMI shielding efficiency in
	the range of 60-80 dB [70]. Yang et al. developed a novel carbon nanotube -

and coating on plastics are well established techniques for decorative, automotive appliance and plumbing products. However, metallized plastics are also used for conductive applications. For EMI shielding purpose following methods are 1.6.4 Compounding with conductive fillers Conventional polymers are excellent electrical insulators having resistivity in the range of 10 15 -10 18 Ω cm and it is possible to make it conductive by the incorporation of a conducting material. Several extensive studies have been done to enhance the conductivity of the polymers by incorporating the conductive nano fillers like, graphene, carbon nanotubes, carbon black, carbon nano fibers etc. Li et al. investigated electromagnetic interference shielding of single -walled carbon nanotube epoxy composites. Their results indicated that the SWNTs can be used as effective light weight EMI shielding materials. They found that the epoxy composites with 15 wt % of SWNT have the highest EMI shielding effectiveness with 49 dB at 10 MHz [69]. Thomassin et al. investigated the EMI reduction of polystyrene (PS) foam composite for EMI shielding. They found that even at low concentration of CNT (7wt%) the composite had an efficient EMI in the range of 20 constant of over 14000 with 3 wt% of rGO at 1 kHz. They found that the demonstrated nanocomposites had very high dielectric constant and can be served as an effective EMI shielding materials due to their high charge absorbing characteristics with a remarkable EMI SE of 38 dB [72]. Zhang et al. fabricated graphene-PMMA nanocomposites by blending and then foamed by using an environmentally benign subcritical CO2 foaming technique for EMI shielding application. They found that the graphene-PMMA foam with a low graphene loading of 1.8 vol % exhibited not only a high conductivity of 3.11 S/m, but also a good EMI shielding efficiency of 13-19 dB at the frequencies from 8 to 12 GHz. The EMI shielding efficiency was mainly attributed to the absorption rather than the reflection in the investigated frequency range. Moreover they concluded that, this work provides a promising methodology to fabricate tough and lightweight graphene-PMMA nanocomposites microcellular foams with superior electrical and EMI shielding properties [73]. 1.6.5 Intrinsically conducting polymers (ICPs) Since the discovery of ICPs in the late 1970s [74] EMI shielding, as well as electrostatic discharge, have been well projected and evaluated by many early papers [75-78].

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Table 6 . 1 :

 61 Shielding effectiveness data for some cellulosic based nanocomposites for EMI Shielding application.

	SI No	Polymer/filler combination	SETotal (dB)	Thickness (mm)	Frequency	References
	1	Cellulose/CNT composite paper	-20	0.45	15-40 GHz	[30]
		Bacterial
	2	cellulose/PANI	-5	0.080	8.2-12.4 GHz [31]
		membrane
	3	Cellulose/CNT aerogel	-20	2.5	8.2-12.4 GHz [32]
	4	Cellulose/CNT aerogel	-19.4	-	8.2-12.4 GHz [33]
	5	CNF/PANI paper	-16.3	0.75	8.2-12.4 GHz (This work)
	6	CNF/PANI paper	-23	1.0	8.2-12.4 GHz (This work)

Nanocellulose Based Functional Constructs for Clean water and Microwave Suppression Deepu AmbikaGopakumar 2017

Conclusion

This chapter has given an insight to all the experiment details, characterization techniques, and different studies for various nano composites used in this thesis.

Strategy and Fabrication of in-situ Polymerized PANI/CNF Nano papers

Introduction

Among the conducting polymers, PANI gained a considerable interest due to its presence of the chemically flexible -NH-group in the polymer backbone which not only take part in the protonation/deprotanation but also contributes to π -bond formation [1], thus ensuring greater environmental stability. PANI can be obtained either by chemical or electrochemical method and does not involve any special precautions [2][3][4]. Although it is easily synthesised in bulk, its use is restricted due to its unprocessability by normal melt or solution techniques. But the presence of conjugated system in PANI leads to the rigidity of the polymer molecule, resulting in poor film-forming capability. This due to the lack of the ability to form hydrogen bonds and thereby it is difficult to prepare a film by casting and drying. In order to rectify this problem, various studies has been done to fabricate various PANI composites by blending it with various polymers. Cellulose has been recognized as good matrix/substrate for biodegradable batteries, sensors and actuators [5][6][7].

Cellulose nanofiber based papers or cellulose nanofiber suspensions give adequate hydrogen bonding that films may be easily produced from the mixture of conducting Chapter 7

Conclusions and Perspectives

This thesis investigates the applicability of cellulose nanofibers in water purification and EMI shielding. Due to environment and sustainability problems, this century has witnessed remarkable achievements in green technology through the development of eco-friendly bio-composites. Bio-composites are most suitable materials profound in nature for their use in various fields due to their eco-friendly advantages. It is known that recently there has been a substantial interest in the industrial applications of containing bio-fibre. For environmental awareness and the international demand for green technology, bio-nanocomposites have the huge potential to replace present petrochemical based materials. The beneficial engineering properties of bio-fibers are specific strength, low density, high toughness, and good thermal properties as compared to most of the synthetic polymers, reduced tool wear, respiratory irritation and ease of processing. Bio-fibers find many applications in bio medical, food packaging, geotextile, architecture, composites and automotive transportation and general system.

Among the bio-fibers cellulose is the most prevalent natural polymer and has greatly impacted the world and society in broad reaching areas. It was only within the last century that advanced processing and analytical instrumentation allowed