Amal Bendahou

Zoulikha Khouba

Tayeb Benabdallah

Ulrich Maschke
email: ulrich.maschke@univ-lille1.fr

Mesophase study of pure and doped cyanobiphenyl liquid crystals with salen type systems

Keywords: Schiff base, guest-host, Liquid Crystal, phase transition, nematic state, transition temperature, smectic state, texture of mesogenic systems

Mesophase study of pure and doped cyanobiphenyl liquid crystals with salen-type systems

Amal Bendahou, Zoulikha Khouba, Tayeb Benabdallah, Ulrich

Introduction

Liquid crystals (LCs) are an important class of materials, known for their anisotropic character, conferring them various physical properties. They are applied in many fields of display and visualization technology, such as liquid crystal displays (LCDs) and polymer dispersed liquid crystals (PDLCs) [START_REF] Mucha | Polymer as an important component of blends and composites with liquid crystals[END_REF][START_REF] Doane | Field controlled light scattering from nematic microdroplets[END_REF][START_REF] Grabchev | Fluorescent 3-oxybenzanthrone dyes in liquid crystalline media[END_REF]. Some LCs has useful semiconducting properties, suggesting many applications in the field of molecular electronics. They have the required characteristics for use as active layers in photovoltaic devices [START_REF] Schmidt-Mende | SelfOrganized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics[END_REF][START_REF] Archambeau | Organic solar cells with an ultra thin organized hole transport layer[END_REF]. LCs also possess have also several applications in data storage field, according to new holographic techniques [START_REF] Heilmeier | A new electric field controlled reflective optical storage effect in mixed liquid crystal systems[END_REF][START_REF] Matharu | Liquid crystals for holographic optical data storage[END_REF].

A large amount of work is focused on changing and improving the optical properties of LCs by chemical means such as synthesis, and physical way such as doping. The chemical method gives rise to a wide range of new LC molecules, having varied structures, containing different aliphatic chains, aromatic rings and different functional groups, and characterized by new nematic and/or smectic textures [START_REF] Cretu | Mononuclear Cu(II) complexes of novel salicylidene Schiff bases: synthesis and mesogenic properties[END_REF][START_REF] Kuo | Mesogenic heterocycles derived from quinoxaline Schiff Bases[END_REF][START_REF] Bagheri | Synthesis and properties of new liquid crystalline polyurethanes containing mesogenic side chain[END_REF].

Recently, dispersion of solutes in LCs seems to be the most suitable method for obtaining new mesogenic composite materials, known as guest-host systems, having new optical and physico-chemical properties. Among the many doping molecules, which have been subject of guest-host systems, one finds various dyes such as rhodamines and coumarin [START_REF] Zakerhamidi | Anisotropic and isotropic solvent effects on the dipole moment and photophysical properties of rhodamine dyes[END_REF][START_REF] Zakerhamidi | Effect of anisotropic and isotropic solvent on the dipole moment of coumarin dyes[END_REF], nanoparticules [START_REF] Tomylko | Dielectric and electro-optical properties of liquid crystals doped with diamond nanoparticles[END_REF] and carbone nanotubes [START_REF] Lisetski | Optical transmission of nematic liquid crystal 5CB doped by single-walled and multi-walled carbon nanotubes[END_REF][START_REF] Vimal | Thermal and optical study of semiconducting CNTs-doped nematic liquid crystalline material[END_REF][START_REF] Basu | Carbon nanotube dispersed liquid crystal: A nano electromechanical system[END_REF]. Due to their absorbance in the visible region, the dye doped LCs are the subject of several studies, in particular concerning intermolecular interactions of the dipole-dipole type, hydrogen bonding, aggregates and π-π stacking. These interactions are expressed in terms of absorption bands undergoing blue or red shifts, modification in their shape and/or intensity, during the passage from the isotropic phase to the nematic one, and vice versa, as well as the changing in their order parameter values [START_REF] Gilani | Solvatochromism and dichroism of fluorinated azoquinolin-8-ol dyes in liquid and liquid crystalline solutions[END_REF][START_REF] Ghanadzadeh | Solvent polarizability and anisotropy effects on the photophysical behavior of oxazine 1: An appropriatepolarizability indicator dye[END_REF].

Among the many systems cited in the literature, and employed as guests for mesogenic hosts, Schiff bases are not largely explored as doping molecules except in few reportsappearing in the literature [START_REF] Khouba | Dispersion of pentadentate Schiff bases and their transition metal complexes in liquid crystals[END_REF][START_REF] Khouba | Spectrophotometric study of liquid crystals containing pentadentate Schiff base type systems[END_REF][START_REF] Makwana | Study of binary systems and determination of latent transition temperatures (LTTs) of non mesogenic components[END_REF]. Unlike dyes, aromatic Schiff base systems are easy to prepare, and feature high thermal and photonic stabilities, allowing them to be used as innovative molecules in the guest-host study.Schiff baseThe structures of these molecules present a variety of functional groups, aromatic parts and aliphatic chains, conducting to different interactions with LCs. Moreover, it is known that Schiff bases play an important role in chemistry, especially in liquid-liquid extraction [START_REF] Hadjyoucef | Equilibrium studies on synergic liquidliquid extraction process of copper(II) from sulphate media with mixtures of some bidentate mono-Schiff bases and acyclic polyether non-ionic surfactant in chloroform[END_REF][START_REF] Zoubi | Solvent extraction of metal ions by use of Schiff bases[END_REF], coordination in solid and liquid states [START_REF] Khouba | Spectrophotometric investigation of interaction between iodine and pentadentate Schiff base ligands[END_REF][START_REF] Ghosh | Coordination of metalloligand [NiL] (H2L = salen type N2O2 Schiff base ligand) to the f-block elements: Structural elucidation and spectrophotometric investigation[END_REF][START_REF] Sivasubramanian | Iron(III)-Salen Complexes as Enzyme Models: Mechanistic Study of Oxo(salen)iron Complexes Oxygenation of Organic Sulfides[END_REF], catalysis and corrosion inhibition [START_REF] Cozzi | Metal-Salen Schiff base complexes in catalysis: practical aspects[END_REF][START_REF] Canali | Utilisation of homogeneous and supported chiral metal(salen) complexes in asymmetric catalysis[END_REF][START_REF] Negm | Corrosion inhibition efficiency of nonionic Schiff base amphiphiles of p-aminobenzoic acid for aluminum in 4N HCL[END_REF][START_REF] Agrawal | Schiff bases of ethylenediamine as corrosion inhibitors of zinc in sulphuric acid[END_REF], antibacterial and antifungal activities, etc. [START_REF] Taha | Syntheses, characterization, biological activity and fluorescence properties of bis-(salicylaldehyde)-1,3-propylenediimine Schiff base ligand and its lanthanide complexes[END_REF][START_REF] Golcu | Cd(II) and Cu(II) complexes of polydentate Schiff base ligands: Synthesis, characterization, properties and biological activity[END_REF][START_REF] Amer | Synthesis, spectral, antitumor and antimicrobial studies on Cu(II) complexes of purine and triazole Schiff base derivatives[END_REF][START_REF] Pahontu | Synthesis, Characterization, Crystal Structure and Antimicrobial Activity of Copper(II) Complexes with the Schiff Base Derived from 2-Hydroxy-4-Methoxybenzaldehyde[END_REF].

The motivation of the study comes from the fact that the salen Schiff base derivatives as well as their metal complexes are known for their high thermal and photonic stability, unlike some systems used as dopants (dyes by way of example). This makes it possible to envisage their use as innovative molecules in the guest-host study requiring a long exposure to the solar rays (solar panels for example) or under high temperatures. Such systems can therefore be used for the preparation of eutectic LCs whose nematic phase is characterized by a wide temperature range. This is all the more justified as the LCs doped with these solutes exhibit no phase separation in the chosen concentration range.

In this paper, spectral and thermalbehavior of two Schiff bases, namely,N,N'bis(salicylidene)ethylene-diamine(L1), N,N'-bis(5-chlorosalicylidene)ethylene-diamine (L2) (Figure 1) and two of their complexes, L1-iron(III) and L2-nickel(II), were studied in different isotropic and anisotropic media. The present work was undertaken in order to observe the effect of doping agents on the mesophase of pure 5CB and 8CB LCs. All the results obtained by UV-visible spectroscopy, differential scanning calorimetry and polarised optical microscopy were compared with those previously reported in the literature, concerning the same pure LCs.

Experimental

Materials and methods

All the isotropic solvents, with high purity (spectroscopic grade) were purchased from Sigma Aldrich. The pure nematic liquid crystals (5CB and 8CB) used in our experiments as anisotropic media are from Synthon Chemicals Germany.

The

Schiff bases N,N'-bis(salicylidene)ethylene-diamine(L1), N,N'-bis(5chlorosalicylidene)ethylene-diamine (L2) and their iron(III) and nickel(II) complexes were synthesized and purified according to the general method described in the literature [START_REF] Zolezzi | Syntheses and characterization of copper (II) complexes with Schiff-base ligands derived from ethylenediamine, diphenylethylenediamine and nitro, bromo and methoxy salicylaldehyde[END_REF][START_REF] Aranha | Synthesis, characterization, and spectroscopic studies of tetradentate Schiff base chromium(III) complexes[END_REF][START_REF] Hriharan | The Stereochemistry of Tetradentate Schiff Base Complexes of Cobalt(II)[END_REF],

and were used as guest materials. All systems were analyzed by infrared spectroscopy, gravimetric thermal analysis and nuclear magnetic resonance ( 1 H and 13 C).

It should be noted that only L1 Schiff base (98% purity) and its nickel complex (96% purity)

used in this study are commercially available (Alfa Aesar and Sigma Aldrich). The same ligands and some of their iron(III), cupper(II) and cobalt(III) complexes have also been the subject of several works in the literature [START_REF] Sivasubramanian | Iron(III)-Salen Complexes as Enzyme Models: Mechanistic Study of Oxo(salen)iron Complexes Oxygenation of Organic Sulfides[END_REF][START_REF] Rubavathi | Iron(III)-Salen-Catalyzed H2O2 Oxidation of Dibenzyl Sulphide[END_REF]. The results of their infrared, H 1 -NMR and C 13 -NMR are almost identical to those presented in this paper, which confirms the purity of our products [START_REF] Zolezzi | Syntheses and characterization of copper (II) complexes with Schiff-base ligands derived from ethylenediamine, diphenylethylenediamine and nitro, bromo and methoxy salicylaldehyde[END_REF][START_REF] Aranha | Synthesis, characterization, and spectroscopic studies of tetradentate Schiff base chromium(III) complexes[END_REF].

Preparation of the guest-host systems

After synthesis and characterization of the ligands and their complexes, the guest-host systems were prepared by dispersing a small quantity of each of these solutes in the LCs. The solute concentration range was situated is between 0.05 and 1.4 wt%. The systems are then placed under mechanical stirring in a temperature-controlled room (about 25°C) for about 48 hours to ensure their homogenization.

Techniques

Infrared Spectroscopy

The guest systems were analyzed by Fourier-transform infrared spectroscopy (FTIR spectroscopy) using a Bruker ALPHA spectrometer equipped with a Platinum ATR single reflection diamond attenuated total reflectance module (Perkin-Elmer, FTIR spectrometer Frontier). The number of accumulated scans was 16.

Nuclear Magnetic Resonance

1 H-NMR and 13 C-NMR spectra were acquired on a FT-NMR(800MHZ) Bruker instrument.

Deuterated Dimethylsulfoxide (with 0.06% Tetramethylsilan) was used as NMR solvent and was obtained from Eurisotop (Saclay, France) and used as received.

Thermal analysis

The thermal study of the solutes were performed using a Perkin Elmer thermogravimetric Analyzer Pyris1TGA from 20 to 900°C, at a heating rate of 10°C/min, under nitrogen atmosphere.

UV-visible spectroscopy

The electronic spectra of all systems have been recorded by using a Varian Cary 100 UVvisible double-beam spectrophotometer, equipped with a heating accessory for precise temperature control (± 0.1°C). The measurements were made as a function of temperature in Hellma quartz cells (100-QS) of 1 and 10mm thickness, for mesogenic and isotropic solvents respectively.

The UV-visible spectra for measuring the transition temperatures were obtained in the first heating mode, from 25 to 50°C, using the steps of 0,5 to 0,1°C and an appropriate hold time (1min) for each scan, to obtain a thermodynamic stable state, in the range of 200 to 800nm .

This ramp is so spread out over time that the system is permanently in a thermodynamic equilibrium, so that cooling or heating mode give exactly the same results and can be compared to those of DSC and POM.

The thermal study was undertaken by varying the temperature from 25 to 50°C, with steps of 0.5 to 0.1°C using appropriate hold time (1min) to obtain a thermodynamic stable state, in the range of 200 to 800 nm.

Differential Scanning Calorimetry (DSC)

DSC measurements were performed on a Perkin-Elmer PYRIS Diamond DSC apparatus.

Temperature and heat flow scales were calibrated with high purity indium standards. Samples of about 6mg were analyzed under nitrogen gas flow at a heating rate of 10°C/min, from -72 to 60°C, held at the latter for 2min, cooled back to -72°C, followed by an isothermal scan for 2min. The same steps were repeated during a second and a third heating/cooling cycle. The thermograms discussed here correspond to the second heating cycle.

Polarized Optical Microscopy (POM)

The polarized optical microscopy (POM) is a common technique that gives information on the transition temperatures and morphology of materials such as LCs and PDLC systems.

The POM measurements were realized on an Olympus BX-41, equipped with a heating/cooling stage Linkam LTS 350, a Linkam TMS 94 temperature control unit and a digital camera that can record images with high resolution, conjugated with a computer.

A droplet of the sample is placed on a microscopic slide of about 4-6µm and heated from room temperature (25°C) to a temperature of 50°C, so that the mixture became isotropic, then another slide was put on the first, and all is maintained at 50°C for about 5min, before cooling the sample down until the room temperature, at using a rate of 0.1°C/min.

During the first cooling mode operation, the transition temperatures of the systems are observed and the pictures of their textures are taken at the different states and temperatures.

Results and discussion

Characterization of the Schiff bases and their metal complexes

The Schiff bases L1 and L2, obtained as light yellow crystals, with 92 and 81% yield These values are much higher than the temperature range of our study (25-50°C) and ensure the thermal stability of our systems during the heat treatment.

Solvent effect on the absorption spectra of the systems in the visible region

The electronic spectra of all solutes in ethanol, dimethylsulfoxide (DMSO) and acetonitrile, as isotropic solvents, and in 5CB and 8CB as LCs, were obtained in the isotropic state, at 42°C

(figures 2 and 3). As shown in figure 2, the spectra of L1(figure 2a) and L2(figure 2b) present a single absorption band situated at 405-421nm, in ethanol and DMSO, reflecting a ketonic form of the two Schiff bases, in equilibrium with the enolic one, situated below 400nm, such

The iron(III) and nickel(II) complexes are characterized by the d-d transitions which are interpreted in terms of absorption band in the visible region of the spectra, at wider wavelengths (Figure 3) than those of free ligands (Figure 2). On the other hand, these systems are strongly colored so that a very low concentration of solute makes it possible to observe very clearly the absorption band of the visible and to better visualize the texture of the LC by optical microscopy (Figure 8).

Wavelength (nm) Absorbance

A single absorption band is obtained in each solvent and situated between 468 and 508nm, for L1-Fe complex, reflecting an octahedral geometry around the metal center. For L2-Ni complex, this band is transformed into a shoulder, in the same solvents, at higher frequency (536-550nm) and at 536nm in DMSO and ethanol for L2-Ni complex, reflecting a square planar geometry around Ni 2+ ion. In 5CB and acetonitrile, the band is transformed into a shoulder at the same frequency, referring to the same type of geometry as in ethanol and DMSO. In this case From these results, we can then conclude that the LCs act as the usual isotropic solvents toward the complexes, without specific interactions. [Solute ] wt %

The characteristic temperatures of all systems are gathered in the table 1tables 1 and 2. The T NI and T SN values are calculated using the following formulas: T NI = ½ (T N + T I ) and T SN = ½ (T S + T N0 ) [START_REF] Khouba | Dispersion of pentadentate Schiff bases and their transition metal complexes in liquid crystals[END_REF][START_REF] Grabchev | Synthesis and properties of fluorescent 1,8naphthalimide dyes for application in liquid crystal displays[END_REF], where T S and T N represent the last points of the levels of the smectic and nematic phases, respectively. As for T N0 and T I , they correspond respectively to the first points of the nematic and isotropic levels.

Differential scanning calorimetry

The pure LCs, 5CB and 8CB have been extensively studied in the literature by different techniques, especially differential scanning calorimetry (DSC), in their pure state, doped by different compounds (dyes, nanoparticules,..) and dispersed in polymer matrices [START_REF] Benmouna | Effect of Molecular Weight on the Phase Diagram and Thermal Properties of Poly(styrene)/8CB Mixtures[END_REF][START_REF] Perju | Polymer-dispersed liquid crystal composites for bioapplications: thermotropic, surface and optical properties[END_REF][START_REF] Park | Investigation of the photopolymerization-induced phase separation process in polymer dispersed liquid crystal[END_REF]. In this work, DSC analysis was applied for the same systems, to be compared to that of UVvisible spectroscopy. In addition, DSC measurements provide more information concerning the melting points and the enthalpies of the different phase transition. witch can be used for estimation of changes in the crystallinity of the LCs. All the results are calculated by Pyris Diamond DSC software.

The thermograms of L1-5CB and L1-8CB systems, resulting from the second heating cycle, are shown in figure 7 as examples. In DSC, the first heating cycle is used to erase the thermal history of the material, created as a result of the sample preparation. Before the first cycle, the material is not in its natural thermal state. The second cycle is then used because the sample has regained its basic thermal properties and hence the result is reproducible.

The method applied for determination of transition temperatures was from straight lines extended along the left-hand and right-hand parts of DSC tracks.

The nematic-isotropic transition temperatures of pure 5CB and 8CB, were respectively 37,0 and 41,9°C. The smectic-nematic transition of 8CB is observed at 34,7°C. L1-5CB 1,4wt-% and L2-5CB 0,7wt-% show T NI values at 34,3 and 35,9°C, respectively. As for 8CB systems, these transitions were observed towards 40,8 and 41,2°C for the higher concentration in L1 and L2. These values are of the same order of magnitude as those obtained by UV-visible as well as those from literature [START_REF] Sharma | Thermodynamics of activated phase transitions of 8CB: DSC and MC calorimetry[END_REF]. Comparing these values with those given in table 1, as well as those from the literature [START_REF] Sharma | Thermodynamics of activated phase transitions of 8CB: DSC and MC calorimetry[END_REF], we can conclude that in general, DSC results are higher than those obtained from UV-vis technique. A difference of about 1,1 to 2°C for all systems can be explained by the difference in their heating rate employed in each case : 10°/min in DSC and

Heat Flow (Mw)

Temperature (°C) steps of 0,1 and 0,5°C in UV-vis. In this later, the sample seems to be permanently in a thermodynamic equilibrium, so the precision given by UV-vis measurement of transition temperatures is better than of DSC.

It should be noted that these systems show the quasi-identical same thermal behavior observed in both DSC and UV-visible spectroscopy, even if their results are slightly different.

A regular decrease in transition temperatures as function of solute concentration was also observed, and proves to obey a linear law in such cases (Figure 6b as example). even if the transition temperature values of pure 5CB and 8CB show in DSC an increase of 1.7 and 1.1°C

respectively with regard to UV-visible spectroscopy. This can be related to the apparatus parameters used in each case. The melting peaks of pure 5CB and 8CB undergo a displacement of about 0,12 to 0,62°C, in the doped systems of 0,7 wt-%. The enthalpies of the Nematic-Isotropic transitions of the same pure LCs are of 2,01 and 3,02 J/g respectively, reflecting the loose of their orientational order to became an isotropic systems, by releasing a small amount of energy. In the doped

LCs, these values undergo a slight decrease in the doped systems which is of the order of about 0,2 J/g. Such a result shows that the orientational order crystallinity of the LCs is not strongly affected by the presence of these solutes, even at high concentration. The systems preserve, in general, the physical characteristics quasi-similar to those of the pure LCs, while acquiring new chemical and optical properties related to the structure of the solute inserted in the LC. This envisages their application in different fields of technology.

Polarized Optical Microscopy

The This shows that the presence of the solute in the LCs don't does not destroy its characteristic texture nor its crystallinity, even at high concentration. In addition, no phase separation was observed on our scale, justifying the homogeneous dispersion of all solutes in the two LCs. In addition, the results of the T NI and T SN obtained by UV-vis, DSC and POM for the doped LCs show that, in general, the nematic and/or smectic phases of these systems undergo a decrease of up to 2°C at maximum (for higher concentration). This makes it possible to assert that the LC character of these systems remains conserved. Phase separation phenomena were not detected in all cases considered here. Similarly, Schlieren and/or focal conic textures were clearly observed both in pure and doped LCs. The texture of these systems seems to be also conserved and further stabilized by the homogeneous insertion of the solute into the LC.

Figure 1 :

 1 Figure 1 : Chemical structures of the Schiff bases (L1 and L2) and the LCs (5CB and 8CB).

  respectively, were re-crystallized in ethanol. Their melting points are 126 and 157°C respectively. Their 1 H-NMR spectra (figures S1 and S2), recorded in d 6 -DMSO solvent exhibit a massif within the range 6.86-7.55ppm which is assigned to aromatic protons. The azomethine protons (-CH=N-) appeared as a singlet at 8.6ppm. The signals situated at 13.37-13.44ppm are characteristic of phenolic protons. The aliphatic protons appeared as singlet at 3.94ppm (s, 4H, -CH 2 -CH 2 -). The 13 C-NMR spectra of the Schiff bases (figures S3 and S4) show the signals of aliphatic carbons (-C-C-) situated at 58.46-58.72ppm and the aromatic ones at 116.43-132.32ppm. The azomethine and phenolic carbons appear respectively at 166.89 and 160.54ppm for L1 and at 165.74 and 159.53ppm for L2. IR spectra of the free ligands (figures S5 and S6) revealed the absorption bands of the azomethine groups (-CH=N-) at 1637-1632cm -1 as shown by Aranha and coll. [36]. These frequencies are shifted to the lower values in the spectra of their corresponding complexes which appeared at 1629 and 1623 cm -1 for L1-Fe (figure S7) and L2-Ni (figure S8) respectively. Such behaviour has often been encountered in the literature [38, 39] Such a result and suggests the coordination of the azomethine groups to the metal center. The thermal analysis (TGA) of the solutes, realized between 20 and 900°C, in inert atmosphere, show the degradation temperature values for L1, L2, L1-Fe and L2-Ni beyond 211, 222, 265 and 273°C, respectively (figures S9 to S12).

Figure 3 :

 3 Figure 3 : Electronic spectra of (a) [L1-Fe] and (b) [L2-Ni] complexes in different media, at 42°C.

3. 3

 3 Investigation of the phase transition of pure and doped liquid crystals using different techniques 3.3.1 UV-visible spectroscopy The study of the mesophase of pure and doped LCs, 5CB and 8CB, was first undertaken by UV-visible spectroscopy. This technique is still little exploited in the literature despite its simplicity and reliability [19]. The spectra were obtained in the range of the temperature between 25 to 50°C with step of 0.1°C around the phase transition. The concentration of the solutes varied in general from 0.05 to 1.4 wt-%. The metal complexes do not have the same solubility in LCs as their corresponding ligands, so the concentration effect of the complexes could not be studied and compared to that of the Schiff bases. The evolution of absorbance, measured at 600nm, as a function of temperature was plotted and presented in figures 4 and 5. The plots show distinguishable levels characterizing the different phases of the LC based system. These latter appear at different absorbance values which vanish in the isotropic state. In general, the increase in solute Schiff base concentration gradually decreases the T NI and T SN values of the different systems with regards of the pure LC. Contrary to the ligands, the complexes L1-Fe and L2-Ni show a slight increase in the T NI value of 5cb for the low solute concentration (0.05 wt-%), this inverse observed phenomenon requires a more detailed study in the future work.

Figure 4 :Figure 5 :

 45 Figure 4 : Variation of the absorbance at 600nm against temperature, of pure and doped 5CB by (a) L1 and (b) L2, at different concentrations.

Figure 6

 6 Figure 6 presents the variation of T NI values as function of concentration in of the solute, for L1-5CB and L2-8CB systems as examples, and obtained from both UV-visible and DSC techniques for comparison. In general, this evolution proves to be linear, justifying the homogeneity of the LC systems, which is interpreted by the regular dispersion of the solute molecules in the LCs. The slight deviation from the linearity observed for DSC data in the case of L1-8CB (Figure 6a), can be explained by the influence of apparatus parameters used in each case.

Figure 6 :

 6 Figure 6 : Variation of the T NI of (a) [L1-5CB] and (b) [L2-8CB] depending on the solute concentration, established by UV-visible spectroscopy.

  the UV-visible technique consists in the possibility of observing the fluctuation phenomenon around the nematic-isotropic transition of all systems. This can be interpreted, from their UV-visible spectra, in terms of the abrupt increase in absorbance and its drop to near zero, just before the passage to the isotropic state, in a relatively low temperature range of about 0,1 or 0,2°C. Such a phenomenon is at the origin of the discontinuous character of the N-I transition, both for the pure and doped LCs. in LCs in general.A fluctuation phenomenon was observed at the nematic-isotropic transition of all systems andinterpreted, in theirUV-visible spectra, by the abrupt increase of the absorbance just before the passage to the isotropic state.Such phenomenon can be interpreted by the discontinuous character of the phase transition, both for the pure and doped LCs. The fluctuation phenomenon has been largely discussed in the literature and studied by many different experimental techniques and theoretical models such as the Landau theory[START_REF] Stinson | Pretransitional phenomena in the isotopic phase of a nematic liquid crystal[END_REF] and using different experimental techniques[START_REF] Zink | A light-Scattering Study of pretransitional behavior around the isotropic-nematic phase transition in alkylcyanobiphenyls[END_REF][START_REF] Schrijver | Surface-induced pretransitional order in the isotropic phase near the isotropic-nematic phase transition[END_REF][START_REF] Nastishin | Pretransitional fluctuations in the isotropic phase of a lyotropic chromonic liquid crystal[END_REF][START_REF] Van Roie | Weakly first-order character of the nematic-isotropic phase transition in liquid crystals[END_REF].

Figure 7 :

 7 Figure 7 : Thermograms of (a) L1-5CB and (b) L1-8CB systems at different concentrations.

  texture and the phase transition temperatures of LC-based systems were observed by polarized optical microscopy, using in the first cooling cycle, using a rate of 0.1°C/min. The pictures shown in figures 8 and 9 were taken, at different temperatures, around the phase transition of the LC-based systems. The Schlieren and/or focal conic textures, characterising the nematic and/or smectic phases of pure 5CB and 8CB, are also observed in the doped LCs.

  The nematic-isotropic transition of pure 5CB and 8CB are observed at 35.4 and 40.5°C, respectively. These results are close to those obtained by UV-visible spectroscopy as well as those of the literature[START_REF] Marin | Chitosan as matrix for bio-polymer dispersed liquid crystal systems[END_REF].

Figure 8 :Figure 9 :

 89 Figure 8 : Polarising optical microscopy pictures of 5CB based systems. Magnification x50. Images using crossed polarizers. Isotropic state of pure 5CB (a). Schlieren texture of the nematic phase taken around the transitional phase of pure 5CB (b), L1-5CB 0,7 wt-% (c), L2-5CB 0,7 wt-% (d), [L1Fe]-5CB 0,05 wt-% (e) and [L2Ni]-5CB 0,05 wt-% (f).

  '-bis(5chlorosalicylidene)ethylene diamine (L2) and their metal complexes L1-Fe and L2-Ni were investigated and their structures elucidated by different analytical techniques. These systems b were used as novel doping agents for two LCs of the cyanobiphenyl type, namely, 5CB and 8CB. The study of the mesophase of pure and doped LCs by these solutes was carried out by UV-visible spectroscopy, equipped with a heating compartment. Such investigation is still rare in the literature, despite its simplicity and reliability. The insertion of the elaborated systems in the LCs proves to be homogeneous, even at high concentration (up to 1.4 wt %). This was justified by the linear evolution of their T NI and T SN as a function of the concentration in solute. The peculiarity of the UV-visible technique consists in the possibility of observing the fluctuation phenomenon around the nematic-isotropic transition, in terms of the abrupt increase in absorbance and its drop to near zero, in a relatively low temperature range of about 0.1 or 0.2°C.Such a phenomenon is at the origin of the discontinuous character of the N-I transition in LCs in general. On the other hand, the solute-solvent and solute-LC interactions were revealed by UV-visible spectra of the various solutes, carried out in ethanol, DMSO, acetonitrile, 5CB and 8CB at 42°C.The mode of absorption in 5CB, 8CB and acetonitrile, having the same cyanide group,is found to be virtually identical. The C≡N group seems to interact with the protons of the phenolic hydroxyls of the Schiff bases.

Table 1 :

 1 Characteristic temperatures of solute-5CB and solute-8CB systems, obtained by UVvisible spectroscopy.

	Solute-5CB	Wt-%	T NI	ΔT NI Solute-8CB Wt-%	T SN	T NI	ΔT NI
	Pure 5CB	0,00	35,3	-	Pure 8CB	0,00	32,9	40,8	-
	L1-5CB	0,17	35,0	0,3	L1-8CB	0,17	32,4	40,4	0,4
		0,33	34,9	0,4		0,36	32,0	40,0	0,8
		0,70	34,4	0,9		0,70	31,5	39,7	1,1
		1,00	34,2	1,1		1,00	30,6	39,1	1,7
		1,40	33,7	1,6
	L2-5CB	0,17	35,2	0,1	L2-8CB	0,17	32,2	40,3	0,5
		0,33	35,0	0,3		0,36	31,9	39,7	1,1
		0,70	34,6	0,7		0,70	31,3	39,4	1,4
	L1Fe-5CB	0,05	35

Acknowledgments

This research project was financially supported by the Grant Ministry of Higher Education and Scientific Research (MESRS, Algeria), as well as the CNRS Department. The authors gratefully acknowledge the help provided by Dr. Christophe Beyens, from UMET of Lille1 University, DSC analysis.

Absorbance Wavelength (nm)

has been observed for other Schiff bases [START_REF] Uzarevic | Keto-enol tautomerism inasymmetric Schiff bases derived from p-phenylenediamine[END_REF]. This band is transformed into a shoulder in 5CB, 8CB and acetonitrile, which contain a cyanide group (-C≡N). This can be explained by the passage of the intra-molecular interaction between OH and imine groups, in the case of ethanol and DMSO, to the inter-molecular interaction between the cyanide and OH groups, in the case of the LCs and acetonitrile.


Hayet Latifa Boudjema

Hayet Bendaikha

Ulrich Maschke

Green composites based on Atriplex halimus fibers and PLA matrix

Keywords: biodegradability, cellulose fibers, mechanical properties, PLA polymer, thermal properties

This work focuses on the potential use of cellulose fibers extracted from Mediterranean saltbush (Atriplex halimus) as a filler in polymeric matrix.The fully biodegradable composites were prepared from PLA as matrix and micro-cellulose fibers ranging from 0 to 15 wt%. The influence of the fiber content on the structure, mechanical, thermal and water absorption properties was evaluated. Mechanical results indicated that fibers acted effectively as reinforcement, increasing the tensile strength and the Young's modulus of PLA by 25% and 45 %, respectively. This is due to the good stress transfer betwen fibers and matrix through the strong interactions that have been evidenced by FTIR spectroscopy. The thermogravimetric analysis showed that PLA composites have a slightly lower degradation temperatures than the pure PLA but they still have favourable thermal stability. Water absorption measurements and biodegradability tests showed that the addidition addition of fibers accelerates degradation kineticks and confirm that the prepared composites are an environmentally safe material suited for different applications.

Introduction

The uncertainty of petroleum supply and environmental pollutions caused by nondegradable plastics have attracted world concerns over developing environmentally friendly materials from natural and renewable resources [START_REF] Zhang | Mechanochemical activation of cellulose and its thermoplastic polyvinyl alcohol ecocomposites with enhanced physicochemical properties[END_REF].

Polylactic acid is one of the most studied biodegradable polymers mainly because of its relatively high melting point, crystallinity and stiffness compared to other biopolymers. Currently, the majority of this linear aliphatic thermoplastic polyester is produced by polymerization of lactide, a cyclic dimer derived from lactic acid, which in turn is obtained by the fermentation of agricultural products such as corn, potato, cane sugar, and rice. In spite of its high biocompatibility and biodegradability, which make it a very good candidate for biomedical and packaging uses, a large number of its applications is still limited by its high production cost and brittleness [START_REF] Hapuarachchi | Multiwalled carbon nanotubes and sepiolite nanoclays as flame retardants for polylactide and its natural fibre reinforced composites[END_REF], [START_REF] Frone | Morphology and thermal properties of PLA-cellulose nanofibers composites[END_REF].

However, to achieve the performance needed for many engineering applications, the pure PLA is generally reinforced by different type of fibers or fillers. Recentely growing interest in the use of natural/bio-fibers as reinforcements or fillers for biodegradable polymers is noticed, thus creating a new class of materials known as fully ''biobased composites" [START_REF] Huda | Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: A comparative study[END_REF].

These natural fibers present many advantages compared to synthetic ones, such as natural availability, low cost, low density, good mechanical properties, eco-friendly and bio-degradability characteristics [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF], [START_REF] Frone | Morphology and thermal properties of PLA-cellulose nanofibers composites[END_REF]. They have even the functional capability to substitute or to compete glass fibers by having a higher specific strength and a similar specific[u1] modulus [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF], [START_REF] Bledzki | Composites reinforced with cellulose based fibers[END_REF]. The most common natural plants used in applications are bast fibers, such as hemp, jute, flax, kenaf, and sisal or agricultural crop residues such as oil palm, pineapple leaf, banana, and sugar palm. They can be obtained in abundance, low cost, and they are also renewable sources of biomass [START_REF] Akil | Kenaf fiber reinforced composites: A review[END_REF], [START_REF] Sahari | Natural fibre reinforced biodegradable polymer composites[END_REF].

Several research papers deal with the optimization of natural and man-made cellulose fibre-reinforced PLA composites. Some investigate biodegradability, others look for ways to improve temperature resistance or the composites' mechanical characteristics [START_REF] Graupner | Natural and man-made cellulose fibre-reinforced poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and application areas[END_REF]. In this study we have investigated the development of an environmental-friendly composite material prepared from polylactic acid (PLA) and cellulose fibers extracted from Mediterranean saltbush (Atriplex halimus) plant. This plant, commonly used as feed for animals, grows in the dry regions of North Africa and belongs to the Chenopodioideae family [START_REF] Boudjema | Composite material derived from biodegradable starch polymer and Atriplex Halimus fibers[END_REF]. The structure, mechanical and thermal properties of the prepared composites were determined by, FTIR spectroscopy, mechanical tensile tests, and thermogravimetric analysis. Biodegradability and permeability were also investigated.

Materials and methods

Materials

Matrix

Poly (lactic acid) (PLA), film grade was supplied by UMET laboratory (University of Lille 1, France). This biopolymer has a number-average molecular weight (M n ) of 94700 g/mol, weight-average molecular weight (M w ) of 179600 g/mol (determined by SEC analysis) and a density of 1.24 g/cm 3 .

Reinforcement

Cellulose microfibers were extracted from a saltbush (Atriplex halimus) plant (Figure 1), the details of the process have been reported earlier by us [START_REF] Boudjema | Composite material derived from biodegradable starch polymer and Atriplex Halimus fibers[END_REF]. We obtained two sizes of cellulose fibers: short fibers (SF: length 100 μm, diameter 80 m, aspect ratio 1.25) and long fibers (LF: length 1938 μm, diameter 430 μm, aspect ratio 4.5). The average length of the fibers was evaluated by microscopic observation. The chemical composition of the fibers was also estimated after various stages of chemical treatment using standard test methods (ASTM D1103-60 and ASTM D1104-56).

Chemicals

Dichloromethane (CH 2 Cl 2 ) has beenwas used to dissolve PLA and all reagents were supplied by Sigma Aldrich (France).

Composites processing

Solution-cast film samples of the biodegradables materials with 0, 5, 10 and 15 wt% fiber contents were prepared with a dry film thickness of around 100 μm and using dichloromethane as a solvent referring to many researchers [START_REF] Sanchez-Garcia | Morphology and barrier properties of solvent cast composites of thermoplastic biopolymers and purified cellulose fibers[END_REF]. 2 g of PLA was dissolved in 80 ml of dichloromethane CH 2 Cl 2 and stirred without heating for 30 min until complete dissolution of PLA, then proper amounts of the cellulose fibers were added and stirred for a further 3 min. subsequentlySubsequently, the mixture was cast onto Petri dishes to generate films after solvent evaporation at room temperature conditions.

The following terms (PLA) and (PLA-SF5,10,15),(PLA-LF5,10,15) are used to define the non-reinforced PLA film, and PLA composites containing short fibers (SF) and long fibers (LF) 5,10,15 % wt/wt of fibers to matrix.

Characterization[u2]

In this work, an Olympus BX41 optical light microscope with a capture camera (Micro Publisher 3.3 RTV) was used to examine the distribution of fibers in the matrix for both sides and to define the dimensions of the cellulose fibers.

Tensile tests were performed on a VERSA Test AFG100N tensile testing machine at a cross-head speed of 14mm/min. The test bars had a rectangular cross-section with dimensions of 6 mm×24 mm prepared by using a mold and press machine. The data were averaged over five to eight specimens. Thermogravimetric analysis (TGA) was performed using an ATG PYRIS 1 Perkin Elmer with specimen size of 5 to 10 mg. The isothermal tests were carried from 40 to 600°C at a heating rate of 20°C /min in the presence of nitrogen.

FT-IR measurements for different films were performed using a Perkin Elmer spectrometer ranging from 4000 to 400 cm -1 to identify different types of chemical bonds existing. The powdered samples and KBr (2 mg sample per 150 mg KBr) were pressed into a disk for FT-IR measurement.

Biodegradability of neat PLA and PLA/cellulose composites was evaluated by burying the samples in soil and measuring the weight loss, the soil media used was a mixture of organic hummus and sand. The samples were cut into 30x 30 mm squares, oven dried at 80°C to constant weight, weighted for the first time and each sample was placed in vessel containing soil. Exposure to the soil environment continued for 10 months, at predetermined time intervals, samples were taken out from soil, carefully washed with water and dried for weighing and returned again to the soil.

Water absorption of the PLA composites was also determined by placing different samples in an aqueous media. The rate (%) of absorbed water was calculated by: Water absorption (%) = x100

where W i is the weight of dried sample before immersion and W f represents the weight of dried material after water immersion.

Results and discussion

Morphology

The aim of microscopic examination was essentialy to investigate the influence of fiber microstructure on mechanical properties. The micrographs show that generally a relatively good dispersion was achieved in composites with low fiber contents even fibers were randomly incorporated. Figure 2A representing the PLA matrix shows a transparent film with a smooth surface at the exception of the presence of some air bubbles formed after solvent evaporation. The addition of fibers don't do not really affect optical properties of composites such as transparency which remained virtually unmodified compared to the neat PLA [START_REF] Sanchez-Garcia | Morphology and barrier properties of solvent cast composites of thermoplastic biopolymers and purified cellulose fibers[END_REF].

Figure 2B and C shows clear images of the cellulose fibers embedded in the biodegradable matrix, especially short fibers (SF) which were highly more homogeneous than the long fibers (LF). However, a phenomenon of agglomeration of fibers is observed in the formulations containing 15% of long fibers. This is probably due to the limited amount of polymer which is not sufficient to cover the full surface of the fibers [START_REF] Sanchez-Garcia | Morphology and barrier properties of solvent cast composites of thermoplastic biopolymers and purified cellulose fibers[END_REF], [START_REF] Nakagaito | Production of microfibrillated cellulose (MFC)-reinforced polylactic acid (PLA) nanocomposites from sheets obtained by a papermaking-like process[END_REF].

Mechanical analysis[u3]

Typical stress-strain curves of neat PLA and PLA composites with different fiber contents are shown in Figure 3. A sensitive growing on the tensile properties of composites was noticed with the progressive increments of cellulose fibers.

The modulus of the PLA was increased from 2.4 GPa to 3.1 GPa and 3.5 GPa with the addition of 15 wt% of short fibers and long fibers respectively, which corresponds to a 26% and 45% increase. Similarly for tensile strength, an increase of 40% and 50% was observed for PLA-LF15 and PLA-SF15 composites compared to pure PLA. In contrast, a sensitive decrease in the elongation at break from 15% to 10% for SF and to 6% for LF was noticed . These results correspond with many studies where generally we notice an improvement in mechanical properties of composites reinforced by cellulosic fibers or microcrystaline cellulose [START_REF] Jonoobi | Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion[END_REF], [START_REF] Bajwa | Characterization of bio-carbon and ligno-cellulosic fiber reinforced bio-composites with compatibilizer[END_REF] ], [START_REF] Suaduang | Effect of spent coffee grounds filler on the physical and mechanical properties of poly(lactic acid) bio-composite films[END_REF], [START_REF] Laaziz | Biocomposites based on polylactic acid and argan nut shell: Production and properties[END_REF].

Higher tensile elastic modulus refers to the rigid property of material that requires more stress to produce the elastic deformation when tensile force is applied [START_REF] Kamonwan | Effect of carbon-rich biochar on mechanical properties of PLA-biochar composites[END_REF]. The increase of elastic modulus could be attributed to strong intermolecular force between cellulose fibers and PLA molecules, consequently cellulose fibers particles restricted mobility of PLA polymer chain and resulted in enhancing the stiffness of the composite material [START_REF] Anwer | Comparison of the thermal, dynamic mechanical and morphological properties of PLA-Lignin & PLA-Tannin particulate green composites[END_REF], [START_REF] Nizamuddin | Synthesis and characterization of polylactide/rice husk hydrochar composite[END_REF].

The positive impact of the cellulose fibers essentielly in the composite strength showed that the Atriplex halimus fibers have good mechanical properties and that there are strong interactions between the PLA and the cellulose fibers [START_REF] Nakagaito | Production of microfibrillated cellulose (MFC)-reinforced polylactic acid (PLA) nanocomposites from sheets obtained by a papermaking-like process[END_REF]. However, the large standard deviations reported in Table 1 on the modulus and elongation at break values reported in Table 1 indicated that the fibers, especially long ones, were not homogeneously distributed.

Remarkably, the mechanical response of composites reinforced with SF is markedly superior than to composites with LF essentially due to their small size. This can confirm the proposal of Huda [START_REF] Huda | Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: A comparative study[END_REF], Osswald [START_REF] Osswald | Fundamental principles of polymer composites: processing and design[END_REF] who suggested that there is a critical fiber length[u4]; if this length is not required the matrix cannot effectively grip the fiber to take the strain, and the fibers will slip and be pulled out, instead of being broken under tension.

Moreover, Kamonwan [START_REF] Kamonwan | Effect of carbon-rich biochar on mechanical properties of PLA-biochar composites[END_REF] indicated that the filler size could have affected the stress distribution between the matrix and filler thus the better mixing or dispersion process of PLA/fibers composites needs to accomplish.

Thermal properties

TGA characterization allows to understand changes in the physical and chemical properties of composites measured as a function of increasing temperature through weight-loss.The results of this test can help to qualify composites for thermally sensitive applications.

The thermal stability of pure PLA and fiber-reinforced PLA composites was investigated with TGA.Thermogravimetric curves of Atriplex halimus fibers, neat PLA and PLA / fibers composites are shown in Figures 6 and7, respectively.

It is reported that PLA decomposes between 285 and 368°C, releasing cyclic oligomers, lactide molecules, CO and CO 2 [START_REF] Bajwa | Characterization of bio-carbon and ligno-cellulosic fiber reinforced bio-composites with compatibilizer[END_REF], [START_REF] Laaziz | Biocomposites based on polylactic acid and argan nut shell: Production and properties[END_REF]. The TG curve In our case of neat PLA shows a single step thermal degradation at 392°C.

In case of Atriplex halimus fibers, three-stage weight losses were observed; a first weight loss between 50°C and 130°C that can be ascribed to moisture loss, Second stage with a small hump in the temperature range from 250 to 300°C is characteristic of low molecular weight components, such as hemicelluloses. An other weight loss stage appearing at 358°C, is corresponded to the thermal degradation of cellulose and Final weight loss stage above 400°C is probably due to lignin decomposition suggested by other researchers at 450°C [START_REF] Laaziz | Biocomposites based on polylactic acid and argan nut shell: Production and properties[END_REF], [START_REF] Goriparthi | Effect of fiber surface treatments on mechanical and abrasive wear performance of polylactide/jute[END_REF].

For PLA/ fiber composites, the incorporation of Atriplex halimus fibers in PLA matrix has affected the thermal degradation temperature. The different composites showed a lower degradation temperature (less than 45°C) than that of PLA. Two-stage loss of mass was mainly observed for all composites; in the range from of 300°C to 400°C due to PLA depolymerization and a second at 350°C corresponding to the degradation of cellulose fibers. The thermal stability of composites decreased, which indicates that the distribution of fibers into PLA matrix is not regular with some agglomerations.

We can say that whatever the rate or the type of fiber, the maximum degradation temperatures of the composites is still around 365°C. Compared to the temperatures recorded by other studies varying between 320 à and 340°C, these values are very acceptable.

These lower degradation temperatures may be attributed to the decrease of molecular weight of PLA and sometimes to the low of affinity between the fibers/matrix [START_REF] Ragoubi | Effect of corona discharge treatment on mechanical and thermal properties of composites based on miscanthus fibres and polylactic acid or polypropylene matrix[END_REF], [START_REF] Sawpan | Effect of fibre treatments on interfacial shear strength of hemp fibre reinforced polylactide and unsaturated polyester composites[END_REF], [START_REF] Bhiogade | Degradation kinetics study of poly lactic acid (PLA) based biodegradable green composites[END_REF].

FT-IR analysis

Typical FT-IR spectra of the PLA and natural fiber reinforced PLA composites are presented in Figure 8. The spectrum of Atriplex halimus fibers is also represented for comparison.

The principal absorbance peaks obtained for the PLA are as follows: the peak around 3512 cm -1 was attributed to the hydroxyl (OH) stretching, the strong peaks at 2996 and 2943 were assigned to the asymmetric and symmetric mode of C-H stretching, respectively. The C=O stretching (from ester linkage) was observed at 1760 cm -1 , the peak at 1457 cm-1 was characterised by the CH 3 band, the C-H deformation appeared at 1383 cm -1 . the O-C asymmetric mode of the ester groups was observed at 1088 cm -1 , and the peaks appeared at 957 and 913 cm -1 were attributed to the rocking mode of CH 3 . Also the peaks at 864 and 753 cm -1 , can be referred to the amorphous and crystalline phases of the PLA, respectively [START_REF] Hapuarachchi | Multiwalled carbon nanotubes and sepiolite nanoclays as flame retardants for polylactide and its natural fibre reinforced composites[END_REF], [START_REF] Bhiogade | Degradation kinetics study of poly lactic acid (PLA) based biodegradable green composites[END_REF].

The spectral bands for Atriplex halimus fibers are at 3403 cm -1 (-OH intermolecular hydrogen bonds) related tot he presence of cellulose and hemicellulose, 2929 cm -1 (C-H stretching) present in hemicellulose composent, 1639 cm -1 (frequency of deformation vibration of CH 2 and CH groups are located), 1371 cm -1 (C-H band),1318 cm -1 (CH bending mode), 1281 cm -1 (C-O stretching mode of ester group) stretch of the acetyl group in lignins, 1110 cm -1 (C-O-C ether band), and 1051 cm -1 (C-O-C stretching vibration). Previous studies have confirme these trends the spectralof cellulosic materials [START_REF] Bajwa | Characterization of bio-carbon and ligno-cellulosic fiber reinforced bio-composites with compatibilizer[END_REF], [START_REF] Laaziz | Biocomposites based on polylactic acid and argan nut shell: Production and properties[END_REF], [START_REF] Kamonwan | Effect of carbon-rich biochar on mechanical properties of PLA-biochar composites[END_REF] .

The spectra of the fiber reinforced PLA composites have similar features to those of Pure PLA except for some changes due to the interaction with cellulose fibers; the OH stretching in the PLA/Fibers composite that appeared at 3437 cm -1 , was found to be different to that of the fibres (3496 cm -1 ) and the PLA (3512 cm -1 ). This indicates that in the PLA/Fibers composites, the OH groups (from fibres) formed hydrogen bonds with the carbonyl groups (C=O) of PLA as shown in Figure 8. A similar observation has also been reported for the PLA / rice starch composites by other researchers [START_REF] Ndazi | Characterization of hydrolytic degradation of polylactic acid/rice hulls composites in water at different temperatures[END_REF], [START_REF] Lee | Eco-composite from poly (lactic acid) and bamboo fiber[END_REF]. In addition, it was also evident that the intensity of C=O (1756 cm -1 ) stretching in the PLA / Fibers composites was significantly higher than that of the PLA.

This was due to the esterification between OH groups of Atriplex fibres and terminal carboxylic acid groups (COOH) of PLA. This agrees with the work reported by Semba et al [START_REF] Semba | The effect of crosslinking on the mechanical properties of polylactic acid/polycaprolactone blends[END_REF] who showed that the peak intensity of the C=O group increased due to the chemical reaction between the dicumyl peroxide and the PLA in a blend of PLA/polycaprolactone.

Water absorption

Figure 9 shows the percentage moisture absorption of the pure PLA and PLA/ fibers composites as a function of time of immersion at room temperature. The curve indicates that the water uptake behaviour of PLA composites obeys Fick's law [START_REF] Li | Further investigations on the hydrolytic degradation of poly(DLlactide)[END_REF], where A rapid absorption was observed within the first days of immersion then slows down to reach the equilibrium after 35 days of immersion.

Water absorption is generally attributed to several factors; firstly, to the presence of some microgaps or defects between the polymer chains. ; sSecondly, to the capillary processes through the gaps and flaws at the interface between cellulosic fibers and the PLA matrix. ; Thirdly thirdly to micro-cracks in the matrix formed during processing [START_REF] Laaziz | Biocomposites based on polylactic acid and argan nut shell: Production and properties[END_REF].

In our case this rapid water absorption[u5] at the beginning as water molecules penetrates into the fibers structure, mainly into capillaries as well as spaces between the fibers particles and the matrix.

The addition of long and short cellulose fibers increased the amount of absorbed water; from 0.9% for neat PLA and 1.6% for composites containing 15 wt% of LF. This can be attributed to the hydrophilic nature of the cellulose fibers and to the multiple hydroxyl groups which are available for interaction with the water molecules [START_REF] Ndazi | Characterization of hydrolytic degradation of polylactic acid/rice hulls composites in water at different temperatures[END_REF], [START_REF] Rudeekit | Determining biodegradability of polylactic acid under different environments[END_REF].

Biodegradability

The degradation of PLA is generally made by hydrolysis process which depends essentially on the permeability, the compostion of the polymer and the microbial activity of soil burial. As shown in Figure 9 the samples exhibited a slow weight loss in the first 20 days of soil burial, this period represents the time required for the hydrolysis step and which is confirmed by the water absorption results.

After 3 months of incubtaion incubation many changes in color and shape were observed, than then the sample become became fragile and brittle at the end of six months. After 9 months the they were already in pieces such that only a few parts could be handled. We can say that the necessary period for total degradtion of PLA by soil burial is estimated at 10 months [START_REF] Bhiogade | Degradation kinetics study of poly lactic acid (PLA) based biodegradable green composites[END_REF], [39].

PLA/fibers showed a higher weight loss than pure PLA with a rate of 92% and 90 % for composites containing 15 wt% of LF and SF due the fact the cellulose fibers are more hydrophilic than the PLA, which accelerates the hydrolysis process. Similar resulst results were reported for PVA/coconut-shell-powder composites [30].

Conclusions

The incorporation of cellulose fibres in a polymeric matrix is an interesting alternative to develop new materials with good mechanical properties and weak environmental impact. In this context we have investigated the use of new cellulose fibers extracted from Atriplex halimus plant as filler for PLA matrix.

The visual and morphological study of PLA composites showed that a relatively good dispersion was achieved except for somme aggregates in high content of longue fibers. That is why the short fibers were more effective than the long ones by giving better mechanical properties; with a fiber content of 15 % wt of short fibers the tensile strength and the young Young's modulus were raised by 25 % and 45 %, respectilvellyrespectively, while the elongation at break was sensibly reduced.

FTIR spectroscopy evidenced the changes in the structure of PLA by adding cellulose fibers and indicated the existence of interaction between (OH) groups of fibers and carboxilic group (COOH) present in PLA.

Furthermore, the addition of the fibers have has weakened the thermal stability and the water resistance of pure PLA by the fact that the cellulose fibers are more hydrophilic.

Biodegradation tests showed that the incorporation of fibers accelerate the kinetics of degradation, but the most suitable method for PLA composites at the end of thier their cycle of life is composting.

Finally, the prepared composites have potential for considerable growth with a green concept. Primary applications for these biocomposites include, for example, toys for children, furniture, hardware for electronic products, or one-way disposable products, and so on. Figure 3: The stress-strain curves for PLA composites (A) containing short fibers and (B) long fibers. Figure 4: The effect of long fibers content on the mechanical properties of PLA composites. Figure 5: The effect of the short fibers content on the mechanical properties of PLA composites. Figure 6 TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers.

Figure 2 :

 2 Figure 2: Optic micrographs of: (A) pure PLA, (B) PLA-SF10% biocomposite, (c) PLA-LF10% biocomposites.Figure3: The stress-strain curves for PLA composites (A) containing short fibers and (B) long fibers. Figure4: The effect of long fibers content on the mechanical properties of PLA composites. Figure5: The effect of the short fibers content on the mechanical properties of PLA composites. Figure6TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers. Figure 7: TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers.Figure 8: (a) FT-IR spectra for cellulose fibers, non-reinforced PLA and composites, (b): possible interactions between PLA and cellulose fibers.Figure 9: Weight loss curves of pure PLA, and PLA composites containing 15 % wt of short and long fibers and water absorption as function of storage time, type and the content of fiber.

Figure 7 :

 7 Figure 2: Optic micrographs of: (A) pure PLA, (B) PLA-SF10% biocomposite, (c) PLA-LF10% biocomposites.Figure3: The stress-strain curves for PLA composites (A) containing short fibers and (B) long fibers. Figure4: The effect of long fibers content on the mechanical properties of PLA composites. Figure5: The effect of the short fibers content on the mechanical properties of PLA composites. Figure6TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers. Figure 7: TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers.Figure 8: (a) FT-IR spectra for cellulose fibers, non-reinforced PLA and composites, (b): possible interactions between PLA and cellulose fibers.Figure 9: Weight loss curves of pure PLA, and PLA composites containing 15 % wt of short and long fibers and water absorption as function of storage time, type and the content of fiber.

Figure 8 :

 8 Figure 2: Optic micrographs of: (A) pure PLA, (B) PLA-SF10% biocomposite, (c) PLA-LF10% biocomposites.Figure3: The stress-strain curves for PLA composites (A) containing short fibers and (B) long fibers. Figure4: The effect of long fibers content on the mechanical properties of PLA composites. Figure5: The effect of the short fibers content on the mechanical properties of PLA composites. Figure6TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers. Figure 7: TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers.Figure 8: (a) FT-IR spectra for cellulose fibers, non-reinforced PLA and composites, (b): possible interactions between PLA and cellulose fibers.Figure 9: Weight loss curves of pure PLA, and PLA composites containing 15 % wt of short and long fibers and water absorption as function of storage time, type and the content of fiber.

Figure 9 :

 9 Figure 2: Optic micrographs of: (A) pure PLA, (B) PLA-SF10% biocomposite, (c) PLA-LF10% biocomposites.Figure3: The stress-strain curves for PLA composites (A) containing short fibers and (B) long fibers. Figure4: The effect of long fibers content on the mechanical properties of PLA composites. Figure5: The effect of the short fibers content on the mechanical properties of PLA composites. Figure6TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers. Figure 7: TGA (a) and DTG (b) scans for PLA composites containing 0, 5, 10, 15 % wt of short fibers.Figure 8: (a) FT-IR spectra for cellulose fibers, non-reinforced PLA and composites, (b): possible interactions between PLA and cellulose fibers.Figure 9: Weight loss curves of pure PLA, and PLA composites containing 15 % wt of short and long fibers and water absorption as function of storage time, type and the content of fiber.

Acknowledgements:

The authors would like to thank UMET Laboratory of the University Lille 1 (France) for providing the necessary materials and for technical support.