Kamel Boudraa
email: boudraa.kamel@univ-saida.dz

Tewfik Bouchaour

Ulrich Maschke

Thermal analysis of interpenetrating polymer networks through

Keywords: Interpenetrating Polymer Network, Thermal analysis, Atomistic simulation, Glass transition

Introduction :

Atomistic molecular modelling are a very interesting tool to have preliminary information about blends, phase behavior and compatibility, which exist between polymers [START_REF] Wu | Atomistic molecular modelling of crosslinked epoxy resin[END_REF][START_REF] Cao | Atomistic studies of mechanical properties of graphene[END_REF]. They are an alternative solution to researchers who have not access to experimental. They have reached, recently [START_REF] Fredrickson | The theory of polymer dynamics[END_REF][START_REF] Nieminen | From atomistic simulation towards multiscale modelling of materials[END_REF][START_REF] Vvedensky | Multiscale modelling of nanostructures[END_REF][START_REF] Holzapfel | Constitutive modelling of arteries[END_REF][START_REF] Salhi | Recent advances in conjugated polymers for light emitting devices[END_REF][START_REF] Chodera | Markov state models of biomolecular conformational dynamics[END_REF][START_REF] Baghel | Polymeric Amorphous Solid Dispersions: A Review of Amorphization, Crystallization, Stabilization, Solid-State Characterization, and Aqueous Solubilization of Biopharmaceutical Classification System Class II Drugs[END_REF][START_REF] Gooneie | A review of multiscale computational methods in polymeric materials[END_REF], a very high level of sophistication, to the point of being able to predict easily several properties with a very high precision, thus anticipating the possible applications of the modelled systems. The choice of the modeling method depends on the system studied. Indeed, several methods are available; we can say the stochastic (Monte-Carlo simulations) or quantum (ab-initio) methods and the molecular dynamics method [START_REF] Valverde | Molecular Modelling: Principles and Applications[END_REF][START_REF] Rapaport | The Art of Molecular Dynamics Simulation[END_REF][START_REF] Paquet | Computational Methods for Ab Initio Molecular Dynamics[END_REF]. This last is used in when the studied system contains a considerable size of the molecular chains (more than 1300 atoms in a polymer chain) while for other methods the number of atoms is limited [START_REF] Trewin | Molecular Modelling for Beginners[END_REF][START_REF] Jahangirian | Simulation in manufacturing and business: A review[END_REF][START_REF] Fredrickson | The theory of polymer dynamics[END_REF][START_REF] Cheng | Molecular dynamics[END_REF][START_REF] Li | Challenges in multiscale modeling of polymer dynamics[END_REF]. Indeed, molecular dynamics is a deterministic method; the propagation of the system is done by applying the classical equations of the movement of the particles. This technique makes it possible to obtain so many structural characteristics as dynamic characteristics relating to the modeled system [START_REF] Heinecke | Molecular Dynamics Simulation[END_REF][START_REF] Raabe | Molecular Dynamics Simulations[END_REF].

The common of polymer material blending processes leads generally to the combination of thermodynamically incompatible compounds [START_REF] Yu | Polymer blends and composites from renewable resources[END_REF][START_REF] Utracki | Polymer blends handbook[END_REF]. Thus, the synthesis of interpenetrating polymer networks (IPNs) has become a consistent development path to obtain a polymer network that is stable over time [START_REF] Manias | Thermodynamics of polymer blends[END_REF]. This synthesis corresponds to the combination of crosslinked polymers of which at least one is synthesized in the immediate presence of the other. This allows combining within a single material the different properties of the two precursor networks while covering the deficiencies of each of them [START_REF] Sperling | Interpenetrating polymer networks[END_REF]. IPNs are dimensionally and thermally stable over time [START_REF] Xu | Organomontmorillonite-modified soybean oil-based polyurethane/epoxy resin interpenetrating polymer networks (IPNs)[END_REF][26][27][28] and may have improved properties of resistance to chemical and physical aging. One of the main interesting properties to consider when studying IPNs systems is their thermal properties. Recently, many scientific reports have been focused on synthesis [START_REF] Lipatov | Phase-Separated Interpenetrating Polymer Networks[END_REF][START_REF] Sperling | Interpenetrating Polymer Networks: An Overview[END_REF][START_REF] Shivashankar | A review on interpenetrating polymer network[END_REF][START_REF] Matricardi | Interpenetrating polymer networks polysaccharide hydrogels for drug delivery and tissue engineering[END_REF][START_REF] Dragan | Design and applications of interpenetrating polymer network hydrogels. A review[END_REF][START_REF] Lohani | Interpenetrating Polymer Networks as Innovative Drug Delivery Systems[END_REF][START_REF] Ligon-Auer | Toughening of photo-curable polymer networks: A review[END_REF][START_REF] Feig | Mechanically tunable conductive interpenetrating network hydrogels that mimic the elastic moduli of biological tissue[END_REF][START_REF] Rao | Mechanical and thermal properties of carbon black reinforced natural rubber/polyvinyl alcohol fully-interpenetrating polymer networks[END_REF][START_REF] Kumar | Advances in patented interpenetrating polymeric networks for biomedical applications[END_REF][START_REF] Sadakbayeva | Microstructured poly(2hydroxyethyl methacrylate)/poly(glycerol monomethacrylate) interpenetrating network hydrogels: UVscattering induced accelerated formation and tensile behavior[END_REF] and thermal properties of polymer blends [START_REF] Sen | Thermal properties of lignin in copolymers, blends, and composites: a review[END_REF][START_REF] Liu | Preparation and properties of polyethylene glycol based semi-interpenetrating polymer network as novel form-stable phase change materials for thermal energy storage[END_REF][START_REF] Zanjanijam | Rheological, mechanical and thermal properties of the PA/PVB blends and their nanocomposites: Structure-property relationships[END_REF][43][44], on the other hand on the observation, the characterization and the improvement of the interfaces and interphases present within such mixtures or of composites [START_REF] Wong | Ultra-Durable and Transparent Self-Cleaning Surfaces by Large-Scale Self-Assembly of Hierarchical Interpenetrated Polymer Networks[END_REF][START_REF] Zhong | Bioactive interpenetrating polymer networks for improving the electrode/neural-tissue interface[END_REF][START_REF] Park | A cross-linkable hole transport material having improved mobility through a semi-interpenetrating polymer network approach for solution-processed green PHOLEDs[END_REF][START_REF] Ren | Designing and Preparation of Fiber-Reinforced Composites with Enhanced Interface Adhesion[END_REF]. The relationships structure-property of interpenetrating polymer networks has sometimes been discussed in experiments, but without a theoretical study being accompanied in parallel. The originality of this work is due to the variety of systems studied according to the same type of model, but also to experimental measurements, which are confronted with modeling data (simulation). This type of study, for interpenetrating polymer networks, is little reported in the literature which encouraged us to start it. Indeed, we present experimental and theoretical study of synthesis of novel interpenetrating polymer networks based on Poly Hexyl-Ethyl-Acrylate (PHEA) and Poly Butyl-Acrylate (PBuA) networks. The choice of these two polymers is due to their various potential utilizations. Indeed, the 2-Ethylhexyl acrylate (HEA) is a very useful monomer for many industrial applications. It is marked essentially by his low glass transition temperature (Tg = -65°C) and has very good film formation property [START_REF] Haloi | Synthesis and characterization of poly(2-ethylhexyl acrylate) prepared via atom transfer radical polymerization, reverse atom transfer radical polymerization and radical polymerization[END_REF]. Poly(2ethylhexyl acrylate) (PHEA), are widely used in many applications [START_REF] Czech | Thermal stability of poly(2-ethylhexyl acrylates) used as plasticizers for medical application[END_REF]. The copolymers of PHEA with other acrylates, methacrylates, acrylonitrile, maleic acid esters, vinyl acetate, vinyl chloride, etc. find applications in adhesives, chemical intermediates, coatings, leather, plasticizers, plastics and textiles [START_REF] Haloi | Acrylic AB and ABA Block Copolymers Based on Poly(2-ethylhexyl acrylate) (PEHA) and Poly(methyl methacrylate) (PMMA) via ATRP[END_REF]. The Poly(n-butyl acrylate) (PBuA) can be used as a soft segment in thermoplastic elastomers due to its low glass transition temperature and durability [START_REF] Wang | Synthesis of Poly(n-butyl acrylate) Homopolymers by Activators Generated by Electron Transfer (AGET) ATRP Using FeCl3 6H2O/Succinic Acid Catalyst[END_REF]. It is a very ductile (~2000% elongation at break), sticky, colorless and transparent rubbery material [START_REF] Meng | Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA) blends: Structure and properties[END_REF]. In view of their complementary properties, blending PBuA with PHEA is a natural choice to improve properties including ductility without compromising transparency. The realization of IPN systems will be checked step by step by giving a detailed analysis of the swelling kinetics of the different networks in the reactive solution containing monomer and crosslinker and a detailed simulation of formation of these networks. Information about the glass transitions temperature of the resulting IPNs and TG/DTG curves are also presented and compared with their precursory networks.

Experimental part:

Monomers:

The monomers, n-butyl-acrylate (99%) and 2-hexyl-ethylacrylate (98%), designated (n-BuA) and (2-HEA) respectively, were obtained from Aldrich. 1,6-Hexanedioldiacrylate (HDDA), supplied by Cray Valley (France) is used as crosslinking agent. 2-hydroxy-2-methyl-1-phenyl-propane-1 (Darocur 1173) from Ciba-Geigy was used as a photo-initiator.

Sample preparation:

Prepolymers synthesis:

Mixtures of n-BuA/HDDA/Darocur 1173 and 2-HEA/HDDA/Darocur 1173 were prepared in the weight fractions 99/0.5/0.5 wt%. The initial mixtures were stirred mechanically for several hours, in order to ensure a uniform distribution of the monomer and the crosslinking agent over the sample volume, before they were cast in small flat sample holders made of Teflon. The samples were exposed under nitrogen atmosphere to a static UV lamp (Philips TL08) with a maximum emission wavelength at λ=365 nm and an intensity I 0 =1.5 mW cm -2 . The exposure time was fixed at 20 min, although 5 min were sufficient to achieve complete conversion of the monomer.

Prepolymers swelling study:

Investigations on swelling kinetics were carried out on PHEA and PBuA networks to see the compatibility which exists between the two prepolymers. The influence of the chemical nature of the monomer on the swelling behavior of the networks is examined. For each network, the used solvent is the reactive solution containing the other (Monomer/ Crosslinker/ Photoinitiator). Swelling behavior in terms of the swelling ratio Q in the case of an usual solvent is given as:

Swelling ratio = Q ( ) (1)
Where W s is the weight of the swollen network and W i is the weight of initially dried network. As the time proceeds, the two reactive solutions diffuse into the networks due to the concentration gradient resulting in the exponential swelling of the network. From the same swelling curve, it's observed that the two polymer networks have, practically, a similar physical behavior, that is to say they swell both in the same way and the same speed. Indeed, we note that they have almost the same swelling ratio with a slight increase on the side of the PHEA network and this is probably due to the chemical structure of the starting monomer forming the PHEA network which is large (6 carbons) compared to the n-BuA monomer. This means that the PHEA polymer network offers more space for the n-BuA monomer to diffuse in the network than in the opposite case.

We can also say that the two polymer networks have maximum swelling ratios (around 475 for the PBuA network and 488 for the PHEA), which means that each network can swell up to 4.8 times its initial volume.

With a small logic, each network contains the equivalent of 3.5 networks in addition to the initial network. In the theoretical part, we took this result into consideration with an approximation that each network contains the equivalent of three polymer networks plus the initial network. We added an intermediate network containing the same number of polymer networks to see the effect of the monomer on the physical behavior of the resulting IPNs.

IPNs synthesis:

Two kinds of sequential IPNs were synthesized. The first one consists of a PBuA network, swollen, until reaching equilibrium, in a solution containing the second monomer (2-HEA)/Crosslinker/Photoinitiator. The swollen sample was then exposed to UV irradiation (the same used before). The exposure time was fixed to 20 min to achieve complete conversion of all monomers in the precursor system. The resulting network will be called Ex-IPN1 (Experimental-IPN1). The second IPN was obtained with the same method described above, except that at first a PHEA network was formed, which was swollen in a second step, until reaching equilibrium, by a solution containing n-BuA, as mentioned before. The resulting network will be called Ex-IPN2.

TG measurement:

Investigations were made in a Perkin Elmer Pyris TGA instrument while heating the samples from 25 to 700°C at the rate of 10°C/min in nitrogen. Samples for thermal measurements were prepared by introducing approximately 10 mg of the sample into a TG pan. At least three duplicate samples having the same composition and prepared independently were used to check the reproducibility of the results. The polymer degradation temperature was determined from the midpoint of the transition range of the curves.

DSC measurement:

The Differential Scanning Calorimetry (DSC) technique was used for the study of polymer networks materials and in particular to detect the glass transitions temperature (T g ) [START_REF] Derouiche | Electro-Optical, and Dielectric Properties of Acrylic Tripropyleneglycol Based Polymer Network Systems Including LCs[END_REF][START_REF] Bouchikhi | Elaboration of Side-Chain Liquid-Crystalline Elastomers and Study of Their Swelling Behavior in Anisotropic Solvents[END_REF]. DSC measurements were performed on a Perkin Elmer Pyris Diamond calorimeter equipped with an Intracooler 2P system allowing cooling experiments. Samples for calorimetric measurements were prepared by introducing approximately 8 mg of the material into an aluminum DSC pan, which was sealed to avoid evaporation effects during the temperature treatment. A rate of 10°C/min (heating and cooling) was used in the temperature range from -75 to 150°C. The program consists first in cooling the sample followed by three heating and cooling cycles to take into account eventual thermal events related to the sample preparation history. The results presented in this work were obtained from the first heating ramps. In each case, at least three duplicates having the same composition and prepared independently were used to check the reproducibility of results.

Simulation details:

Simulation software:

The Culgi software (Chemistry Unified Language Interface from culgi.com) is used for the generation of the different systems, for the energy minimization, equilibrating systems and calculating the energy functional forms with the DREIDING Force Field [56]. The different macroscopic (volumetric) and microscopic (structural)

properties were done by the LAMMPS software package (Large-scale Atomic Molecular Massively Parallel Simulator from lammps.sandia.gov) and the results obtained from the latter are visualized and interpreted by the VMD software University of Illinois (http://www.ks.uiuc.edu/Research/vmd/) [START_REF] Humphrey | VMD -Visual Molecular Dynamics[END_REF][START_REF] Hsin | Using VMD: An introductory tutorial[END_REF].

Simulation procedure:

The methodology followed for the elaboration of the two Acrylate-based networks PBuA and PHEA and this in order to lead to the synthesis of sequential IPN networks is detailed. All the simulations have been carried out using DREIDING force-field [START_REF] Mayo | DREIDING: a generic force field for molecular simulations[END_REF][60] who is a simple, generic all-atom force field, developed by Mayo and coworkers at BioDesign Incorporated. DREIDING FF was developed to predict structures and dynamics of organic, biological, and main-group inorganic molecules. The generated structures with the builder included with the CULGI software do not exactly represent the equilibrate state of the system. Although excluded volume effects are taken into account, a certain degree of overlap still exists in the generated structures. Since the amount of interatomic overlap in the system depends on the density of the system, it is recommended to perform an energy minimization calculation followed by a few steps of molecular dynamics simulation with a very small time step on the structure to minimize this overlap, and the resulting high-energy hot spots in the structure, before any further simulation attempted [60]. For the final structures, minimization is carried out to reach the lowest energy state, and the MD simulation procedure was performed. The MD simulation follows the algorithm given in the Fig. 2. All systems were equilibrated for 100 ps with 0.1 fs time steps at (T = 298 °K, and P = 1 bar for NPT ensemble) with a frame output every 500 steps. The Berendsen thermostat and barostat [START_REF] Berendsen | Molecular dynamics with coupling to an external bath[END_REF] were used to maintain the temperature and pressure for all NVT and NPT simulations. Firstly, an NVT-MD simulation was performed to release any possible tensions. Secondly, an NPT-MD simulation is performed to bring the system back to experimental density. This protocol prevents the superimposition of segments or the formation of loops in the case of dense polymers. Finally, the simulation was carried out until the density of each system was stabilized. The non-bonded van der Waals and electrostatic interactions were truncated using atom based cut off distance of 12.5 A°. Periodic boundary conditions were implemented in all three principal directions.

Fig. 2 Summary of our process to equilibrate the different systems Generating prepolymer networks:

There are four points for generating the initial configurations of each polymer network. Initially, the polymeric repeat unit (RU) must be created and minimized (See Fig. 3). Then, this repetitive unit is used to generate a defined length polymer chain (160 RUs for PBuA and 130 for PHEA in our case). This latter is relaxed, in order to obtain the most stable configuration. The third phase consists in inserting the relaxed polymer chains together with one molecule of the crosslinker HDDA in a periodic homogeneous box with dimensions .

For the studied polymer networks, the confinement was carried out relative to the experimental approach.

Finally, the last step is to create bonds between the two polymer chain and the crosslinker in the box.

Generating interpenetrating polymer networks:

The creation of interpenetrating polymer networks is based on the main definition of these latters. The definition of sequenced synthesis stipulates that a network is synthesized in the immediate presence of the other one. That is, a network is created knowing that the first already exists. Given the complexity of the model, we have established an approximation based on the simple addition of polymer networks together in a simulation cube with different proportions to try to be as close as possible to the experiment and then we built the IPNs in the AmorphousBuilder module of the CULGI Software (See Fig. 4). The polymer networks were interpenetrated with each other due to the formation of physical entanglements between them without covalent bonds. In a molecular model, a force field defines interactions between atoms. A force field is determined by the functional form of the different interatomic interactions, and by the parameters used for these interactions. The DREIDING force field use functional forms that are given below [START_REF] Mayo | DREIDING: a generic force field for molecular simulations[END_REF][60]. Energies are generally expressed in Kcal mol -1 , lengths and distances in angstrom, angles in degrees (but angle force constants use radians instead of degrees). Units of force field parameters follow from this and the specific functional forms, but for completeness they are also given below.

Bond stretching

The bond stretching harmonic potential is given by:

( ) (2)
where r 0 is the equilibrium bond length expressed in angstrom and the bond stretching constant k r is expressed in kcal mol -1 °A-2 .

Angle bending

The angle bending harmonic potential is given by:

( ) (3)
where θ 0 is the equilibrium angle expressed in degrees and the angle bending constant k θ is expressed in kcal mol -1 rad -2 .

Angle bending potential in cosine form:

( ( )) (4)
Here, the angle bending constant has units of kcal mol -1 .

Angle bending potential in cosine form (DREIDING style):

For θ ≠ 180° ( ) (5)
For θ = 180°

( ) (6)
where θ 0 is the equilibrium angle and

( ) (7)
The angle bending constant has units of kcal/mol.

Torsion

Cosine series for torsional potential:

( ) ( ) ( ) (8)
The force constants

V 0 , V 1 , V 2 , V 3 have units of kcal mol -1 .
Torsional potential in cosine form (DREIDING style):

[ ( )] (9)
where n and Φ 0 determine the equilibrium angles. The force constant V 0 has units of kcal/mol.

Inversion

Inversion potential for planar configurations in cosine form (DREIDING style):

( ) (10)
The inversion constant u ψ is expressed in kcal mol -1 .

Van der Waals interactions

Lennard-Jones 12-6:2: The equation for an alternate form of the Lennard-Jones 12-6 potential, used by DREIDING, is given below:

( ) [( ) ( ) ] (11)
where r ij , ϵ ij , and σ ij denote the distance between the interacting sites, the Lennard-Jones well depth and the size parameter, respectively, for the pair of atoms i and j.

Here, r ij and σ ij are both expressed in A so that the constant ϵ ij has units of kcal mol -1 .

For interaction parameters between unlike pairs, the geometric combination rule for ϵ ij is given:

( ) (12)
For σ ij , the arithmetic and geometric combination rules are given by:  The arithmetic combining rule

( ) (13)
 The geometric combining rule

( ) (14)
The radial distribution function:

The Radial Distribution Function (RDF) measures the probability of finding a particle at distance r given that there is a particle at position 0. For small distances, the radial distribution is closely related to the packing of the molecules. Since at close distances the interatomic interactions are quite hard in nature, the distribution function shows a structure with regular peaks. At long distances, the layers become diffuse, and thus the probability of finding two atoms with a given separation is essentially a constant and proportional to the density. The RDF, denoted as g(r), is normalized by the density to provide a limit in g value of 1 at large values of r. There are two main types of RDF. One is that of a collection of atoms with itself (for example, just taking all atoms in a particular system) and the other is the RDF for atoms from two disjoint collections (for example, the RDF of a particular atom in one type of molecule with a particular atom in another type of molecule). The expression used to calculate the radial distribution function for (arbitrary) collections A and B is given by

( ) ( ) [( ) ( ) ] (15)
Where V is the total volume of the system, is the total number of pairs consisting of one bead from collection A and one from collection B, and ( ) is the number of AB atom pairs with distance between and (so with displacement vector with in a spherical shell with thickness ∆r around distance r). The expression in the second denominator is the volume of such a shell.

For the shell volume, the approximate expression 4πr 2 ∆r can be used and then simplify the expression by defining ( ). For the case where collection A is identical to collection B, the number of pairs becomes ( ), with N the number of beads in the collection. We then obtain the more familiar expression

( ) ( ) ( ) (16)

Results and discussions:

TG Results:

The curve corresponding to TG and DTG analysis are given in Fig. 5-a and Fig. 5-b respectively. The corresponding data are given in table 1. The inset of this figures shows the same results based on the range between 350 and 470 °C. It can be observed from our study that all the polymer networks have almost the same thermal behavior. Indeed, all the studied networks are stable up to 300 °C; a continuous mass loss in the range (340-460 °C) characterized by an important mass loss, and decomposes completely beyond 550 °C. The corresponding phenomena could be interpreted by a complete degradation of the materials (mass loss: 99.61%).

All products decompose in a single step. The kinetic aspect of the degradation depends on the compound of networks; Indeed, The Ex-IPN1 has a degradation kinetic similar to the one of PHEA network, and the Ex-IPN2 has a degradation kinetic similar to the one of PBuA network. From the DTG curve (Fig. 5-b) we can see that the peaks of the curves presented for the simple polymer networks are slightly wide in comparison to those presented by IPNs; this is probably due to the presence of some unreacted oligomers with the networks structure forming thereby an isolated blocks of polymer chains from the polymer network. These blocks begin to detach and degrade first by increasing the temperature followed by the skeleton of the sample. This behavior is less existed by increasing the density of the network leading us to the formation of IPNs, which confirms that we have stable IPNs in their structures by comparing them to the starting products. The thermal stabilities of the different polymer networks were evaluated from TG results checking temperature corresponding to 5%, 50%, and 95% of mass loss (T 5% , T 50% , and T 95% ). The start of degradation for all samples appeared to begin, with a light difference, at around the same temperature. After 50% of mass loss, the thermal behavior for the IPNs is found to be influenced by the nature of their precursory networks. Indeed, The Ex-IPN1 has a degradation behavior similar to the one of PHEA network, and the Ex-IPN2 has a degradation behavior similar to the PBuA network. This behavior continues until the last stage of total degradation.

DSC results:

The Thermal Analysis was carried out to know the phase separation behavior that can exist in the different polymer networks. Three samples from each system were studied. Once the results obtained, it is possible to draw the tangents to the curve of the heat flow and the glass transition temperature of the polymer was determined from the Half C p Extrapolated point of the transition range of the curves. For the analysis of the DSC, the study focused on the first heating, that is to say the first cycle of temperature variation (-70°C to 120°C). Having obtained similar curves for the three tests, we presented only one curve for composition. Apparently with the presence of the PBuA in the Ex-IPN1 structure, this decreasing of T g was expected because in the network IPN1 where the presence of the monomer 2-HEA is governing, this last tends to influence its behavior and thus to have glass transition temperature more or less close to that of the pure network PHEA, and vice-versa.

Atomistic simulation:

The generated atomistic structures have very high energies and cannot be served directly as starting configurations for molecular dynamics simulation. The minimization of a molecular model having a big size (number of atoms 36000) can converge towards a global minimum. But, to confirm this minimum reached, several minimizations must be made, starting from different initial conformations. The QM minimization method proposed by D. Sheppard et al. [START_REF] Sheppard | Optimization methods for finding minimum energy paths[END_REF] is employed to obtain the minimized structures. The principle idea of this method is to follow the force vector from an initial configuration to a stationary point of energy, while accelerating the system in the direction of the force (which mimics the actual dynamics). In addition to this acceleration, some depreciation is necessary to stop at the local minimum. This is usually done through a proportional and speed-dependent 'force' of damping. In QM mode, this damping procedure is replaced by a projection of the velocity along the force. The QM method can be coupled to a molecular dynamics integrator;

Euler integrator in this case [60]. The initial values of the potential energy of the different samples by DREIDING force field are Ep = 1.54E+07 kcal mol -1 , Ep = 3.18793e+07 kcal mol -1 , and Ep = 1.57867e+07 kcal mol -1 for the T-IPN1, T-IPN2 and T-IPN3 respectively. After performing the minimization process, the systems start to reduce its energies gradually according to the number of iterations until they reach a stable minimum energy (Table 2). The new values of the minimum energy are: Ep = 65066.001 kcal mol -1 after 8050 steps for T-IPN1, Ep = 5.95389e+04 kcal mol -1 after 1034 steps for T-IPN2 and Ep = 269226 kcal mol -1 after 2688 steps for T-IPN3. Thermal results:

Volumetric properties (Dilatometry technique):

One of the most popular theoretically approaches used to evaluate the compatibility between polymers is the glass transition temperature T g determination of the studied system and compare it with the experimental values of the component polymers. This theoretical one remains dependent on the chosen cooling rate for the simulation. Metalta and Soldera [START_REF] Metatla | Computation of densities, bulk moduli and glass transition temperatures of vinylic polymers from atomistic simulation[END_REF][START_REF] Soldera | Glass transition of polymers: Atomistic simulation versus experiments[END_REF] showed this effect by performing simulations on different polymer systems and found that the agreement between the two glass transitions, experimental and theoretical, is more improved at low cooling rates. It is evident that if these low cooling speeds lead to a better prediction, they lead to too long computational times. In our case, the theoretical measurements of the glass transition temperature for the different systems were performed step by step for each temperature by doing a cooling from 350 °K to 100 °K by increments of 25 °K. At each temperature, the constant NPT MD simulation was conducted for 300 ps and the conditions were kept the same as descripted in the simulation details section. During this procedure, the specific volume was calculated at each temperature and the glass transition temperature is determined by the intersection of the two slopes of the two curves corresponding to the process of heating and cooling as mentioned in Fig. 7. All the values obtained during this study for all the polymers are gathered in the table 3. The RDF is a concrete option to inspect the structural properties of the polymer systems providing a confirmed approach for the understanding of the atom interactions including the bonding or non-bonding ones. In the Fig. 8, we present the calculated RDF for the different systems used over this simulation and the inset of these figures

shows the same results based on the range between 0.8 and 3.5 °A. We note perfectly that the value of g (r)

fluctuates in its values as a function of r. For the different samples the value of g (r) is usually > 1 to about r = ~0.93 °A, ~1.14 < r > ~1.81 °A, and ~2.03 < r > ~2.32 °A before returning to 1 for values of r greater than 3.13

°A. This indicates that the distance to r between atoms is more frequent than expected for a completely random pattern, suggesting clustering. The values of g (r) < 1 indicate that the distances between atoms equal to r, are less frequent than expected for a completely random process. This suggests regularity. And for g (r) is zero for 0 < r > 1 °A, this extremely regular configuration has no distance between atoms.

We notice moreover that the intensities of the peaks are temperature dependants. As we can see in insets of the Fig. 8, the different picks presented by the g(r) curves at r =1.1°A are greater than the other picks at higher values of r. These later are decreasing in intensity from higher to lower temperatures. This effect looks to be produced by the variations in the density of polymer system with variation of temperature. We we took this effect into consideration and plot in the

Fig. 1

 1 Fig. 1 Swelling of the polymer networks in the reactive solution containing the monomers This figure constitutes a measure of the quantity of the reactive solution absorbed by the two different polymer networks. As the time proceeds, the two reactive solutions diffuse into the networks due to the concentration

Fig. 3 Fig. 4

 34 Fig. 3 Chemical structure of the RUs and generating polymer (Grey-carbon; Red-oxygen; White-hydrogen)

Fig. 5

 5 Fig. 5 TG (a) and DTG (b) Curves for the different Polymer networks

Fig. 6

 6 Fig. 6 represents the glass transition temperatures variation of the different networks. Only one single transition in the DSC curves for all the polymers is detected, especially for the two synthesized IPNs which indicates that the IPNs does not present phase separation. This would suggest that the different monomer 2-HEA and n-BuA are perfectly miscible. In addition, these figures informs us that the glass transition temperatures of both IPNs are close from/to each other and are controlled first by the quantity of the precursory polymer network present in the mixture. Indeed, the T g of the Ex-IPN1 shows a slight shift compared with the T g of the PHEA network.

Fig. 6

 6 Fig. 6 Experimental DSC Curves of the different systems at a heating rate of 10°C min -1

  second polymer network in simultaneity with the first structure with various percentages affects the resulting theoretical T g and passes the latter closer to that presented by the initial polymer network. Indeed, the T g value presented by the T-IPN3 which contains the same proportions of the polymer networks comes between the two values presented by the other T-IPNs which deviate towards the values presented by the predominant networks in the T-IPN. This behavior was observed during the experiment which confirms the good choice of the adopted model during the atomistic simulation.

Fig. 7

 7 Fig. 7 Evolution of the specific volume according to the temperature for the PHEA, PBuA, T-IPN1, T-IPN2 and T-IPN3 systems

  Fig. 9 the slope of the fitted lines to g(r) values at r=1.1 °A. As is shown, the slopes of the fits lines are crossed around a temperature estimated to be the T g . The values are presented in table 3 and they stay in good agreement with the experimental one. It should be mentioned is that T g values obtained from structural calculations (RDF) are more accurate and reflect the experiment more better than the calculations obtained by the dilatometry technique.

Fig. 8 Fig. 9

 89 Fig. 8 The radial distribution function for various pairs of atoms for the T-IPN1, T-IPN2 and T-IPN3 systems

Table 1

 1 Degradation Data of the different polymer networks

		T 5% /°C	T 50% /°C	T 95% /°C
	Ex-IPN1	328	400	438
	Ex-IPN2	344	414	448
	PBuA	344	411	449
	PHEA	344	402	442

Table 2

 2 minimization results of the different samples with QM method

	Polymer	Bond Stretching /kcal mol -1	Angle Bending /kcal mol -1	Torsion /kcal mol -1	Inversion/ kcal mol -1	VdW /kcal mol -1	Total Potential/ kcal mol -1
		Initial	3813.90	4276	1391.67	87.12	4.41484e+07	1.54e+07
	T-IPN1	Final	1710208.5	514967.4 2920.83	6911.72	7.8249 e +06	6.5066e+04
		Initial	3222.49	3458.03 1149.13	90.8557	3.18714e+07 3.18793e+07
	T-IPN2
		Final 1.16478e+06 381818	2460.4	9256.82	4.39558e+06 5.95389e+04
		Initial	3463.46	3923.68 1302.32	98.8054	1.57779e+07 1.57867e+07
	T_IPN3
		Final	34449.9	55967.6 2626.71	812.579	175369	269226

Table 3

 3 T g result values of the different systems with two determination methods (All the temperature values are As we can realize, although the values of the glass transition temperatures obtained from the proposed model are higher than the experimental values, they remain in good agreement with the latter. This shift in theoretical values is probably due to the cooling rate used during the simulation. It is also to be noted that the addition of a

	in /°C)
	T g /°C	IPN1	IPN2	IPN3	PBuA	PHEA
	Experimental	-65.24	-46.68	--	-45.25	-63.07
	Theoretical	Dilatometry RDF	-57 -56	-35 -49	-50 -55	-44 -45	-58 -61

-The author thank the members of CULGI ® for their continued support -This work was granted access to the HPC ressources of UCI-UABT "Unité de Calcul Intensif" of the University Abou bekr Belkaïd of Tlemcen financed by the DGRSDT "Direction Générale de la recherche Scientifique et du Développement Technologique"


Kamel Eddine Boudraa
email: boudraa.kamel@univ-saida.dz

Tewfik Bouchaour

Christophe Beyens

Ulrich Maschke

Tewfik Bouchaour

Novel interpenetrating polymer network composed of poly(Butyl Acrylates) and poly(Ethyl-HexylAcrylate)

Keywords: Interpenetrating polymer Networks, Swelling behavior, Polymer synthesis, thermal properties, Poly Hexyl-Ethyl-Acrylate, Poly Butyl-Acrylate

published or not. The documents may come

Novel interpenetrating polymer network composed of poly(butyl acrylates) and poly(ethyl-hexyl acrylate)

INTRODUCTION

Many researchers have studied the science of polymer gels; this is due to potential industrial applications of this kind of polymer systems. The attention was to focus on the innovation and development of new materials for these systems. The major shortcoming of this kind of polymer networks is their poor mechanical properties when they are in the swollen state. Consequently, reinforcement is needed for any future considered application. The increasing in the cross-linking density, co-polymerization with a more resistant system are among the ways to reinforce these systems, as well as the formation of interpenetrating polymer networks, IPNs.

An Interpenetrating Polymer Network, IPN, known as a blend of at least two polymers in a network form; One of them is synthesized and/or crosslinked in the immediate presence of the other(s). The synthesis of this new material class makes it possible to have in certain cases new materials with good performances in comparison with the initial polymer precursors [1][START_REF] Utracki | History of commercial polymer blends[END_REF]. These polymeric networks are synthesized by any known method of polymer synthesis. The most used approaches [START_REF] Sperling | Introduction to Physical Polymer Science[END_REF] for the synthesis of IPNs are: first, polymer network I is synthesized. Then, monomer II plus crosslinker and activator are swollen into network I and polymerized in situ.

The resulting IPN is called sequential IPN. In the second case, the monomers and/or pre-polymers plus crosslinkers and activators of both components are mixed, followed by simultaneous polymerization via noninterfering reactions. Typical syntheses involve chain and step polymerization kinetics. While both polymerizations proceed simultaneously, the reactions ratios are rarely identical. The notion of IPNs containing two polymers, or more, chemically different was developed and there are approximately 400 documents published each year on this type of material [START_REF] Thapliyal | Interpenetrating Polymer Networks[END_REF][START_REF] Xiao | Synthesis and characterization of photocrosslinkable gelatin and silk fibroin interpenetrating polymer network hydrogels[END_REF][START_REF] Koul | Interpenetrating polymer network (IPN) nanogels based on gelatin and poly (acrylic acid) by inverse miniemulsion technique: Synthesis and characterization[END_REF][START_REF] Guo | Hydrogels of collagen/chondroitin sulfate/hyaluronan interpenetrating polymer network for cartilage tissue engineering[END_REF][START_REF] Zhang | Polymer-Based Stimuli-Responsive Recyclable Catalytic Systems for Organic Synthesis[END_REF][START_REF] Slaughter | Dynamic Swelling Behavior of Interpenetrating Polymer Networks in Response to Temperature and pH[END_REF][START_REF]Interpenetrating Polymer Networks (IPN): Structure and Mechanical Behavior[END_REF][START_REF] Yao | Preparation and characterization of shape memory composite foams with interpenetrating polymer networks[END_REF][START_REF] Lai | Effects of Cross-Link Density on Structures and Properties of Dual-Sensitive Semi-Interpenetrating Polymer Networks Hydrogel Microspheres[END_REF][START_REF] Patel | Synthesis and characterization of interpenetrating polymer networks from transesterified castor oil based polyurethane and polystyrene[END_REF][START_REF] Jia | Synthesis and characterization of bio-based PA/EP interpenetrating network polymer as coating material for controlled release fertilizers[END_REF]. They are now promising candidate materials in engineering [START_REF] Zhang | Synthesis and unusual volume phase transition behavior of poly(Nisopropylacrylamide)-poly(2-hydroxyethyl methacrylate) interpenetrating polymer network microgel[END_REF], Drug Delivery [START_REF] Lohani | Interpenetrating Polymer Networks as Innovative Drug Delivery Systems[END_REF], biomedical and industrial fields [START_REF] Dragan | Design and applications of interpenetrating polymer network hydrogels. A review[END_REF].

In order to use interpenetrating polymer networks for practical applications, it is required to understand the different properties of these polymer networks. Swelling experiment are a simple and "low-cost techniques" to characterize a polymer network. The swelling measurements can be used for quality control and serve as an indexing tool for polymer systems with different levels of crosslinking. Other techniques can be used to characterize an interpenetrating polymer networks. Indeed, thermal analysis includes a whole series of techniques of characterization of materials based on the study of the variation of a physical property according to the temperature. They are thus primarily macroscopic approaches of the behavior of the materials, which involve considerations of the equilibrium thermodynamic states, the irreversible thermodynamic processes and kinetics, associated with state changes (transition phenomena) and relaxationnal phenomena that can accompany them.

The 2-Ethylhexyl acrylate (EHA) is a very useful monomer for many industrial applications. It is marked essentially by his low glass transition temperature (Tg = -65°C) and has very good film formation property [START_REF] Haloi | Copper catalyzed atom transfer radical copolymerization of glycidyl methacrylate and 2-ethylhexyl acrylate[END_REF][START_REF] Haloi | Synthesis and characterization of poly(2-ethylhexyl acrylate) prepared via atom transfer radical polymerization, reverse atom transfer radical polymerization and radical polymerization[END_REF]. Poly(2-ethylhexyl acrylate) (PEHA), are widely used in many applications [START_REF] Czech | Thermal stability of poly(2-ethylhexyl acrylates) used as plasticizers for medical application[END_REF]. The copolymers of PEHA with other acrylates, methacrylates, acrylonitrile, maleic acid esters, vinyl acetate, vinyl chloride, etc. find applications in adhesives, chemical intermediates, coatings, leather, plasticizers, plastics and textiles [START_REF] Bicak | Atom transfer graft copolymerization of 2-ethyl hexylacrylate from labile chlorines of poly(vinyl chloride) in an aqueous suspension[END_REF][START_REF] Haloi | Acrylic AB and ABA Block Copolymers Based on Poly(2-ethylhexyl acrylate) (PEHA) and Poly(methyl methacrylate) (PMMA) via ATRP[END_REF].

The Poly(n-butyl acrylate) (PABu) can be used as a soft segment in thermoplastic elastomers due to its low glass transition temperature and durability [START_REF] Matyjaszewski | Polymerization of n-Butyl Acrylate by Atom Transfer Radical Polymerization. Remarkable Effect of Ethylene Carbonate and Other Solvents[END_REF][START_REF] Wang | Synthesis of Poly(n-butyl acrylate) Homopolymers by Activators Generated by Electron Transfer (AGET) ATRP Using FeCl 3 6H 2 O/Succinic Acid Catalyst[END_REF]. It is a very ductile (~2000% elongation at break), sticky, colorless and transparent rubbery material [START_REF] Wen | Thermomechanical and optical properties of biodegradable poly(L-lactide)/silica nanocomposites by melt compounding[END_REF][START_REF] Meng | Transparent and ductile poly(lactic acid)/poly(butyl acrylate) (PBA) blends: Structure and properties[END_REF]. In view of their complementary properties, blending PABu with PEHA is a natural choice to improve properties including ductility without compromising transparency. This work is as part of the systematic studies undertaken in our laboratories to explore the physical properties of composite materials made up of interpenetrating polymers networks. The novelty in our work is the choice of the two monomers 2-Ethylhexyl acrylate and n-butyl acrylate with an objective to synthetize a new sequential interpenetrating polymer network and the extraction of different physical parameters issued from this synthesis.

To the best of our knowledge, experimental results on swelling kinetics of polymer networks in a solution containing these two kinds of monomers and crosslinker in order to form an interpenetrating polymer network as well as the swelling theoretical description have not been reported until now in literature. The realization of IPN systems will be checked step by step by giving a detailed analysis of the swelling kinetics of the different networks in the reactive solution containing monomer and crosslinker. The variation of the crosslinker amount allows obtaining either a loosely crosslinked network with more efficient swelling or more dense networks with limited swelling properties. In addition, different physical properties issued from this experience are also investigated given us useful information on IPNs formation. Information about the glass transitions temperature of the resulting IPNs and TGA thermograms are also presented and compared with their precursory networks.

EXPERIMENTAL SECTION:

Monomers:

The monomers, n-butyl-acrylate (99%) and 2-ethyl-hexylacrylate (98%), designated (n-ABu) and (2-EHA) respectively, were obtained from Aldrich. 1,6-Hexanedioldiacrylate (HDDA), supplied by Cray Valley (France) is used as crosslinking agent. 2-hydroxy-2-methyl-1-phenyl-propane-1 (Darocur 1173) from Ciba-Geigy was used as a photo-initiator.

Prepolymers synthesis:

Mixtures of n-ABu/HDDA/Darocur 1173 and 2-EHA/HDDA/Darocur 1173 were prepared in different weight fractions by varying the quantity of the monomer (n-ABu and 2-EHA) and HDDA, keeping the amount of Darocur 1173 constant (Monomer/HDDA/Darocur 1173=99/0.5/0.5 wt.-%, 97/2.5/0.5wt.-%, and 94.5/5/0.5wt.-%). The initial mixtures were stirred mechanically for several hours, in order to ensure a uniform distribution of the monomer and the crosslinking agent over the sample volume, before they were cast in small flat sample holders made of Teflon. The samples were exposed under nitrogen atmosphere to a static UV lamp (Philips TL08) with a maximum emission wavelength at λ=365nm and an intensity I0=1.5mW/cm2. The exposure time was fixed at 20 min, although 5 min were sufficient to achieve complete conversion of the monomer.

IPNs synthesis:

Two kinds of sequential IPNs were synthesized. The first one consists of a PABu network, swollen, until reaching equilibrium, in a solution containing the second monomer (2-EHA)/Crosslinker/Photoinitiator. The swollen sample was then exposed to UV irradiation (the same used before). The exposure time was fixed to 20 min to achieve complete conversion of all monomers in the precursor system. The resulting network will be called IPN1. The second IPN was obtained with the same method described above, except that at first a PEHA network was formed, which was swollen in a second step, until reaching equilibrium, by a solution containing n-ABu, as mentioned before. The resulting network will be called IPN2. As we used various quantities of the crosslinker, we collect in the table 1, all the networks used in this study.

Characterization:

Real-Time Infrared Spectroscopy: FTIR spectra of thin films (less than 10 µm) in the transmission mode were recorded at room temperature with a Perkin Elmer 2000 FTIR model (The number of scans was 16 with spectral resolution of 4 cm-1), before and after UV exposure. Each dose was applied once and the interval of time between the end of exposure and the infrared analysis was kept constant (below one minute). The experiments were repeated three times to validate the results and check if they are reproducible.

Prepolymers swelling study:

Investigations on swelling kinetics were carried out on PEHA and PABu networks to see the compatibility which exists between the two prepolymers. The influence of the crosslinking density and the chemical nature of the monomer on the swelling behavior of the networks were also examined. For each network, the used solvent is the reactive solution containing the other (Monomer/Crosslinker/Photoinitiator).

Swelling behavior in terms of the swelling ratio Q in the case of an usual solvent is given as:

Swelling ratio = Q s i i WW W   (1)
Where Ws is the weight of the swollen network and Wi is the weight of initially dried network.

Thermal analysis:

For the thermal analysis, the Differential Scanning Calorimetry (DSC) technique was used for the study our polymer networks systems and in particular, to detect the glass transitions temperature (Tg) [START_REF] Derouiche | Optical, Electro-Optical, and Dielectric Properties of Acrylic Tripropyleneglycol Based Polymer Network Systems Including LCs[END_REF][START_REF] Bouchikhi | Elaboration of Side-Chain Liquid-Crystalline Elastomers and Study of Their Swelling Behavior in Anisotropic Solvents[END_REF]. DSC measurements were performed on a Perkin Elmer Pyris Diamond calorimeter equipped with an Intracooler 2P system allowing cooling experiments. Samples for calorimetric measurements were prepared by introducing approximately 8 mg of the material into an aluminum DSC pan, which was sealed to avoid evaporation effects during the temperature treatment. A rate of 10°C/min (heating and cooling) was used in the temperature range from -75 to 150°C. The program consists first in cooling the sample followed by three heating and cooling cycles to take into account eventual thermal events related to the sample preparation history. The results presented in this work were obtained from the first heating ramps. In each case, at least three duplicates having the same composition and prepared independently were used to check the reproducibility of results.

ATG measurement:

Investigations were made in a Perkin Elmer Pyris ATG instrument while heating the samples from 25 to 600°C at the rate of 20°C/min in nitrogen. Samples for thermal measurements were prepared by introducing approximately 10 mg of the sample into an ATG pan. At least three duplicate samples having the same composition and prepared independently were used to check the reproducibility of results. The polymer degradation temperature was determined from the midpoint of the transition range of the thermograms.

RESULTS AND DISCUSSION

Real-Time Infrared Spectroscopy:

From the point of view of the spectroscopic characterization of the monomers, a FTIR-analysis spectroscopy of the monomers is carried out. The cells to be analyzed are prepared according to the physical state of the material to analyze at room temperature. The analysis by infrared spectroscopy was carried out every 5 minutes during 20 minutes. The absorption band of the FTIR spectrometer is included between 400 and 4000 cm -1 . We were interested in the spectrum between 700 and 1800 cm -1 for better representing the absorption bands. Since the crosslinking ratio does not have any influence on the final obtained spectrum, we represent in the figures of the FTIR only the spectra of the polymeric samples with a 0.5%wt. HDDA crosslinking ratio. Several absorption bands are available to monitor the polymerization/crosslinking processes and to evaluate the conversion of the acrylic double bonds of the polymers PEHA, PABu, IPN1 and IPN2. One of the most characteristic absorption bands which is quite often used in IR-analysis of acrylates are the one corresponding to the -CH=CHdeformation vibration (at 810 cm -1 and 1637 cm -l ). The calculation of the monomer conversion is made by considering the peak heights of these absorption bands. According to the Beer-Lambert law, the absorbance A of a species at a given wavelength is proportional to the molar concentration of this species. When a dose D is applied, its disappearance can be evaluated by the conversion ratio:

    () ( 0) (%) 100 1           D D A C A (2)
where (A λ ) D=0 is the height of the absorption band of the precursor system at the defined absorption band (810 or 1637 cm -1 , i.e., radiation dose D is zero) and (A λ ) D is the corresponding result for the system exposed to a dose D.

Prepolymers swelling study:

The Tanaka-Fillmore theory [START_REF] Tanaka | Spectrum of light scattered from a viscoelastic gel[END_REF][START_REF] Tanaka | kinetics of swelling of gels[END_REF][START_REF] Li | Kinetics of swelling and shrinking of gels[END_REF][START_REF] Shibayama | Volume phase transition and related phenomena of polymer gels[END_REF] is one of the pioneering theories used for describing the swelling process of polymer networks in different solvents and was used by different researchers to rationalize their experimental data [START_REF] Ganji | Theoretical Description of Hydrogel Swelling: A Review[END_REF][START_REF] Sivanantham | Swelling/deswelling of polyacrylamide gels in aqueous NaCl solution: Light scattering and macroscopic swelling study[END_REF][START_REF] Gabriel | Non-monotonic swelling of surface grafted hydrogels induced by pH and/or salt concentration[END_REF][START_REF] Jian | Coupled instabilities of surface crease and bulk bending during fast free swelling of hydrogels[END_REF]. In this theory, only the osmotic pressure due to the solvent penetration used to explain the network expansion during swelling is taken into account and therefore the distribution of solvent in the network is not considered. A criticism to this theory was introduced by Komori and Sakamoto [START_REF] Komori | On Tanaka-Fillmore's kinetics swelling of gels[END_REF], where the authors assumes that the gel network is not extended due to the osmotic pressure of the gel, but to a swelling pressure which is generated by the solvent excess penetrating in the polymer network that tends to contract. Indeed, in addition to the swelling pressure assumed as driving force to expand the polymer network, the polymer network elasticity is considered. These two models displayed a good agreements of the theoretical results obtained with the experiment. In our case, and to rationalize our experimental data, we consider an interesting approach given by H. Omidian [START_REF] Omidian | A model for the swelling of superabsorbent polymers[END_REF]. Indeed, in this model, based on the Voigt viscoelastic model presented in references [START_REF] Tschoegl | The Phenomenological Theory of Linear Viscoelastic Behaviour[END_REF][START_REF] Aklonis | Introduction to Polymer Viscoelasticity[END_REF],

the authors present an exploratory investigation of the influence of crosslinker on the ability for swelling and on the absorption ratio for the acrylic polymers. For this purpose, they associate the spring element with resistance to expansion by the polymer network and the dashpot element with resistance to permeation.

The figure 4 and figure 5 shows the swelling kinetics of both polymer networks at room temperature (T=21°C) in both reactive mixtures made up of monomer + HDDA + Darocur 1173 during 2880 minutes. The solid lines represent the theoretical fit obtained by fitting the experimental swelling data to the expression (3) using the Voigt viscoelastic model:

  1 exp t t QQ      (3)
Given by H. Omidian [START_REF] Omidian | A model for the swelling of superabsorbent polymers[END_REF][START_REF] Lin | Hydrogels in controlled release formulations: Network design and mathematical modelling[END_REF][START_REF] Liu | Modified acrylic-based superabsorbents with hydrophobic monomers: synthesis, characterization and swelling behaviors[END_REF], where Q t (g/g) is the swelling capacity at time t, Q ∞ (g/g) denotes the theoretical equilibrium solvent absorption, t (min) is the swelling time, and τ (min) stands for a rate parameter, denoting the time required to reach 63% of equilibrium solvent absorption. The results are listed in the table 2.

Q ∞ τ

PABu These curves constitute a measure of the quantity of the reactive solution absorbed by the two different polymer networks. As the time proceeds, the two reactive solutions diffuse into the networks due to the concentration gradient, the osmosis effect ceases and mixing dominates resulting in the exponential swelling of the network.

Further, as expected, by increasing the crosslinking degree of the first network the swelling of the monomer of the second network will be lower. For the polymer networks with lower crosslinker concentration, a high swelling ratio is observed, reaching approximately 5 times its initial weight. This ratio decreases when the rigidity of the polymer network is increased reaching thus approximately 2 times its initial weight for the networks having 2.5% wt. HDDA, to achieve approximately 1.2 times its initial weight for the networks having 5% wt. HDDA. The swelling time constant gives us information about the ability of the polymer network to swell that is to say that if a polymer network has a high swelling time constant value then its swelling is slow and spend more time to reach saturation. In the case of figure 6, where we present the time constants extracted from the fits to eq (3)

for the different systems, both time constants shows a linear decrease with increasing the crosslinking ratio of the different polymer networks. This was expected because it known that the highly crosslinked polymer network quickly reach equilibrium. The interesting observation here is that the swelling time constants of the PABu network are considerably higher than those presented for PEHA polymer network. The PABu network spends more time to swell than the PEHA network. This is due to the highest density which is reflected on the chains mobility of the PABu network. Thus, the penetration of the n-EHA monomer inside the PABu network is slow, that mean that the interactions between the PABu network and the n-EHA monomer are weak than in the other case. We perform an experimental determination of the mesh size, ξ, and the molecular weight of the polymer chains between crosslinks, M c , of the various systems studied here and we report in the table 3 and figure 7 the different values derived from swelling experience. The knowledge of these two parameters is crucial to understanding the nanostructure of the crosslinked network; they were reported in literature [START_REF] Canal | Correlation between mesh size and equilibrium degree of swelling of polymeric networks[END_REF][START_REF] Betz | Swelling behaviour, charge and mesh size of thermal protein hydrogels as influenced by pH during gelation[END_REF][START_REF] Kirchhof | Diels-Alder Hydrogels for Controlled Antibody Release: Correlation between Mesh Size and Release Rate[END_REF] by many researchers and can be related to the swelling ratio. For gels at highly swelling conditions (Q > 10), these parameters are given as:

    3/5 3/5 12 1 1 / 2 2 c c M Q M V             (4)
 is the specific volume of the polymer, V 1 is the molar volume of solvent, and 12

 is the polymer-solvent interaction parameter.

    1/2 1/2 1/3 2 1/3 2 2, 0 0 s r Q r    (5)
Here   1/2 2 0 r is the root-mean-squared end-to-end distance of network chains between two adjacent crosslinks in the unperturbed state. It can be determined using the following relationship [START_REF] Canal | Correlation between mesh size and equilibrium degree of swelling of polymeric networks[END_REF]:

    1/2 1/2 1/2 2 0 2 c n n r M r l C N l C M         (6)
Where C n is the Flory characteristic ratio (C n = 9,24 for the PolyABu and 9,5 for the PolyEHA), l is the bond length along the polymer backbone (l = 1,54 A°), N is the number of bonds between adjacent crosslinks, and M r is the molecular weight of the repeating units of the composed polymer (M r = 128,7 g/mol for the PolyABu and 184,28 g/mol for the PolyEHA). All these data were token from references [46][START_REF] Coelho | Synthesis of poly(2-methoxyethyl acrylate) by single electron transfer-Degenerative transfer living radical polymerization catalyzed by Na 2 S 2 O 4 in water[END_REF].

Combining Equations ( 5) and ( 6 3: Different values of the two parameters M c and ξ derived from swelling experience As we can see, the two parameters M c and ξ are principally affected by the degree of crosslinking of the two polymer networks. Indeed, they decrease with increasing the crosslinking ratio and became smaller for the highly crosslinked networks. That was expected because as we did introduce the crosslinker in the polymer network as we increase the density of the polymer network and thus the size of the chains decrease. The mesh size (ξ) is playing the role of obstacles for the reactive solution and consequently controlling the diffusion of the monomer solution in the network.

Thermal analysis results:

DSC measurements were performed to know the behavior of phase separation on the different polymer networks. The polymer glass transition temperature was determined from the Half Cp Extrapolated point of the transition range of the thermograms. The value for the glass transition is the average for several measurements realized for each composition. Only one single transition in the DSC thermograms for all the polymers is detected, especially for the two synthesized IPNs which indicates that the IPNs does not present phase separation. This would suggest that the different monomer 2-EHA and n-ABu are perfectly miscible. In addition, these figures clearly illustrate the glass transition temperatures for each polymer network and its direct dependence with the crosslinking density. Indeed, for the pure polymer network PABu and the pure polymer network PEHA, the glass transition temperature decreases with the decrease of the crosslinker. In other words, in the highly crosslinked polymer networks, the mobility of the polymer chains decreases which causes that Tg is quickly reached, opposing for the loosely crosslinked networks where the mobility of the chains is not reduced, which makes the Tg is small, as its mentioned in the table 4 where we gathered all the values obtained during this experiment for all the polymers networks. where the presence of the monomer 2-EHA is governing, by making references to the swelling curves discussed before, this last tends to influence its behavior and thus to have glass transition temperature more or less close to that of the pure network PEHA, and vice-versa. The thermo gravimetric analysis (TGA) was performed to determine thermal stability of the polymeric materials and to examine the influence of their structural differences on thermal stability. The corresponding curves are given in figure 10 and figure 11 and the corresponding data are given in table 5. These curves displays that all the studied networks decompose in one single step, they are stable up to 300 °C; shows a continuous weight loss in the range (340-460 °C) characterized by an important weight loss, and decomposes completely beyond 500 °C. The corresponding phenomena could be interpreted by a complete degradation of the materials (weight loss: 99.61%). It can be observed that the increase in the crosslinking ratio increase slightly the thermal stability of the sample. However, kinetic aspect of the degradation depends on the compound of networks; Indeed, The IPN1 has a degradation kinetic similar to the one of PEHA network, and The IPN2 has a degradation kinetic similar to the one of PABu network. The figure 11, where the DTG curves are given, the peak of the curves presented for the networks of polymers with a low crosslinking ratio is wide in comparison to the other peaks; this is probably due to the presence of some unreacted oligomers with the networks structure forming thereby an isolated blocks of polymer chains from the polymer network. This behavior does not exist by increasing the crosslinking degree, which confirms that one has stable polymer networks in their structures by increasing the crosslinking density. The thermal stabilities of the different polymer networks were evaluated from TGA thermograms checking temperature corresponding to 5%, 50%, and 95% of weight loss (T 5% , T 50% , and T 95% ). The start of degradation for all samples appeared to begin, with a light difference, at around the same temperature; however, the kinetic aspect of the early stage of degradation indicates the presence of chain blocks which have not reacted with the polymer network and which influences the kinetics of the first degradation steps. After 50% of weight loss, the thermal stability is found for all the polymer networks and thus an increase in the thermal stability with the increase of the crosslinking ratio.

%

QUALITATIVE ANALYSIS:

The synthesis of the acrylic interpenetrating polymer network enabled us to deduce from it that its precursory networks present a high and a perfect compatibility between them. This two chosen polymer networks are known for their high reactivity and similar properties.

An observation of the swelling curves shows that the polymer networks PEHA and PABu have very close swelling ratio for the networks having the same proportions of the crosslinker. The Q max decreases dramatically as we pass from the hard to the loose polymer network. For the loosely PEHA network, a slightly higher swelling ratio is presented, reverberating on the mesh size values. This behavior is expected and it is due primarily to the chemical structure of the two networks; The PEHA network present a longer polymer chain than PABu. As we increase the amount of the crosslinker, we observe that the PABu network presents the higher Q max values, reflecting on the mesh size values too. The PABu network swells slightly better than the PEHA network when increasing the crosslinker density. So we can say that the increase in the density increase the structure order too in the network and leads to a more stable network structure. The loosely polymer networks, absorb a huge amount of the reactive solution, which favors the formation of isolated chain blocks inside; this phenomenon is translated by a large peak in the DTG curves of the loosely networks. However, the higher crosslinking density leads us to obtain a harder polymer network and more stable in its structure; thus a lower swelling ratio. In other words, the increasing of crosslinking density will give chance to all monomers to react and be attached to the crosslinker and form a dense and homogeny network; this increasing causes an apparition of a resistive force of the polymer chains to elongation and consequently reducing the swelling ratio rather than polymer networks presenting lower crosslinking density.

Another interesting information issued from the swelling curves is that we can control the amount of the second monomer penetrating the first network just by controlling its swelling. If, for example, we desire to synthesize an IPN with a (50:50) ratio for each component, we swell the first network until it reaches 200% a swelling ratio even if this latter has not reached its swelling balance yet, differing in conventional IPN synthesis methods.

Another remark is that the increase in the percentage of the crosslinker leads to an increase of the glass transition of the IPN1 bringing it closer to that of the PABu network. The same phenomenon (but inversed) is observed in the case of the IPN2 in a less marked way. Indeed, the Tg of the IPN2 decrease this time by increasing the percentage of the HDDA crosslinker approaching that of the PEHA. This brings us to conclude, that each IPN network synthesized will be influenced mainly, in its properties, by the amount of the absorbed monomer until reaching equilibrium. For the loosely networks, the amount of the absorbed monomer will be the majority in the resulting IPN and therefore this last will be dependent, in its properties, of the properties of the absorbed monomer. This dependence is limited every time we increased the crosslinking ratio of each networks until (more or less) almost equal in the two highly crosslinked networks.

CONCLUSION:

In this work, we demonstrated a synthesis of a novel sequential interpenetrating polymer network, composed of poly(2-ethylhexyl acrylate) and poly(n-butyl acrylate) and we studied the physical properties by varying the crosslinker ratio. The conversion of double bonds of the acrylate monomers was monitored by FTIR spectroscopy in terms of radiation dose. The PEHA network shows the highest reactivity compared to the PABu network. The intensity of the peaks of absorption corresponding to IPNs becomes weaker from IPN1 to IPN2 with decreasing amount of the n-ABu in the network. An upper limit of the double bond conversion was reached near 95-98%. The compatibility that exists between these two networks is investigated by studying the swelling of each network in the reactive solution containing the mixture made up of monomer and crosslinker. The results

show high swellings ratio. Initially, the systems swelled exponentially to saturation over 48 h due to concentration gradient. These ratios are very close for the networks having the same proportions of the crosslinker and inform us that the composition of the IPNs is controlled by varying the crosslinker ratio in the initial mixture; higher crosslinking density leads us to obtain a harder interpenetrating polymer network, homogeny and more stable in its structure. The DSC results inform us that the glass transition temperature of the different studied systems is influenced by the amount of the monomers absorbed by the network and that all the resulted IPNs, especially the IPN with 0.5% crosslinker, does not present any phase separation. Finally, the TGA thermograms show that all the networks decompose completely beyond 550°C and it was found an increase in the thermal stability with the increase of the crosslinking ratio.

Fig. 1 :

 1 Fig. 1: FTIR spectra for the IPN1 0.5% wt. HDDA for 20 minutes at the absorption band 810 cm -1

Fig. 2 :

 2 Fig. 2: Acrylic double bond conversion (%) of IPN1, IPN2, PEHA and PABu prepared by UV curing using 0.5%wt. HDDA

Fig. 3 :

 3 Fig. 3: FTIR Spectra of different networks The FTIR spectra of the different systems are shown in Figure 3. (To clarify the differences, only the wavenumber range is shown). Absorption bands at 840 cm -1 and 1063 cm -1 (corresponding to the CH out-ofplane vibrations of C-CH3 and O-CH3), which are characteristic bands of PABu, are found in the IR spectra of IPN2 and IPN1. The intensity of the peaks of absorption corresponding to IPNs becomes weaker from IPN2 to IPN1 with decreasing amount of ABu in the network.

Fig. 4 :Fig. 5 :

 45 Fig. 4: Swelling of the PABu network in the reactive solution containing the monomer 2-EHA

Fig. 6 :

 6 Fig. 6: time constant versus the variation of the HDDA Crosslinker (The lines are guides for eyes)

Fig. 7 :

 7 Fig. 7: DSC thermograms for the prepolymer networks

Figures 7

 7 Figures 7 and 8 represent respectively the glass transition temperatures variation of the different prepolymer networks and for the Interpenetrating polymer networks IPN1 and IPN2 with various crosslinking densities.

Fig. 8 : 15 Table 4 :

 8154 Fig. 8: DSC thermograms for the IPNs with different crosslinking ratio

Fig. 9 :

 9 Fig. 9: Glass transition variation versus crosslinker HDDA percentage

Fig. 10 :

 10 Fig. 10: Thermogravimetric curves of the different systems

Fig. 11 :

 11 Fig. 11: Derivative thermogravimetric curves of the different systems

Table 2 :

 2 different parameters extracted from the fits to eq.[START_REF] Sperling | Introduction to Physical Polymer Science[END_REF]

		0.5 %	4,73 ± 0,08	206,19±16,88
	(wt.-% HDDA)	2.5%	2,11 ± 0,06	191,46±29,24
		5%	1,43 ± 0,04	165,89±22,56
		0.5%	4,79 ± 0,15	129,06±20,57
	PEHA (wt.-% HDDA)	2.5%	1,89 ± 0,03	120,02±8,37
		5%	1,1 ± 0,05	113,02±24,81

  ), one can easily calculate the mesh size of the polymer network.

		PABu		PEHA
		M c (g/mol)	ξ (°A)	M c (g/mol)	ξ (°A)
	0.5%	28900	770	30250	810
	2.5%	7540	300	6280	270
	5%	4000	190	2920	157
	Table

Table 5 :

 5 Degradation Data of the different polymer networks

		wt. HDDA	T 5% (°C)	T 50% (°C)	T 95% (°C)
		0.5 %	328	400	438
	IPN1	2.5 %	319	413	445
		5 %	338	421	450
		0.5 %	344	414	448
	IPN2	2.5 %	334	423	449
		5 %	336	424	452
		0.5 %	344	411	449
	PolyABu	2.5 %	349	425	456
		5 %	342	426	454
		0.5 %	344	402	442
	PolyEHA	2.5 %	339	410	442
		5 %	349	423	448

This work was supported by University Dr

CONFLICT OF INTEREST:

The authors declare that they have no conflict of interest.

References:

[1]. Chakrabarty D, (1988) Polymer Gels and Networks, Vol 6,[191][192][193][194][195][196][197][198][199][200][201][202][203][204]