A Study on the Properties of Poly ( Styrene-co-Methyl Methacrylate ) / Silica Aerogel Nanocomposites Prepared via in situ SR & NI ATRP

Simultaneous reverse and normal initiation technique for atom transfer radical copolymerization (SR&NI ATRP) of styrene and methyl methacrylate in the presence of hexamethyldisilazane (HMDS)-modified silica aerogel nanoparticles (H-SAN) and investigating the effect of addition of H-SAN on the copolymerization and thermal properties of the products are discussed. Some unique features of the H-SAN are evaluated using nitrogen adsorption/desorption isotherm, SEM and TEM. Conversion and molecular weight determinations were carried out using GC and SEC respectively. Adding of H-SAN by 3 wt% results in decrement of conversion from 94 to 73%. In addition, molecular weight of copolymer chains decreases from 19500 to 16000 g.mol-1. However, polydispersity index values increases from 1.29 to 1.65. Linear increase of ln(M0/M) with time for all the samples shows that copolymerization proceeds in a living manner. Increasing thermal stability of the nanocomposites is demonstrated by TGA. DSC results show a decrease in glass transition temperature from 70.3 to 63.5 °C by addition of 3 wt% H-SAN.


Introduction
5][6] For example, polymer layered silicate nanocomposites present improvements in various properties (such as strength, thermal expansion coefficient, barrier, flammability resistance, environmental stability, and etc) in comparison with unmodified polymer. 4,7Due to the unique features of the nanocomposites, they can be employed in diverse applications such as electronics, adhesives, and automotive fields. 8In situ polymerization, solution blending and melt intercalation are three famous methods for the preparation of polymer nanocomposites. 9Incorporation of tailor-made polymers (organic phase) for the preparation of nanocom-posites is an attractive topic since the molecular features of the organic phase can be tailored and some specific properties of the resulting hybrid can be dictated. 10erogels are highly porous materials with surface areas between 500 and 1600 m 2 /g and densities between 0.002 and 0.8 g/cm 3 .6][17] Large surface area, high degree of porosity, very low bulk density, very low thermal conductivity, high optical transmission, low refractive index, and low sound velocity are some unique properties of the silica aerogels that make them appropriate potential candidate for many applications (e.g.][20][21] Application of various initiation techniques can be considered as one the main advantages of ATRP over other the controlled living radical polymerization (CLRP) methods.3] Schematic mechanism of SR&NI ATRP is presented in figure 1.
][26][27][28] Application of various initiation techniques for ATRP in the presence of different nano-fillers (e.g.0][31] In this study, pristine silica aerogel nanoparticles were modified by HMDS as a surface modifier agent and hydrophobic silica aerogel nanoparticles were obtained.Abundant advantages of SR&NI ATRP were employed to synthesize well-defined poly (styrene-co-methyl methacrylate) nanocomposites.Investigating the effect of H-SAN addition on the polymerization parameters (conversion, molecular weight, and PDI values) are carefully performed.By these results it can be concluded that increment of H-SAN in the polymerization media how can affect the polymerization kinetics (positive effect of negative effect).In addition, thermal properties of the prepared nanocomposites are studied in detail.By these results it can be comprehended that addition of H-SAN in the polymer matrix how can improve thermal features of the nanocomposites in comparison with the neat polymer.

Synthesis of Hydrophilic and HMDSmodified Silica Aerogel Nanoparticles
Pristine and H-SAN were prepared according to the procedure as reported previously. 20,32

3. SR&NI Atom Transfer Random Radical Copolymerization of Styrene and Methyl Methacrylate
Typical batch of copolymerization was run at 110 °C with the molar ratio of 200:1:0.2:0.2:0.12 for [M]:[EBiB]:[CuBr 2 ]:[PMDETA]:[AIBN] giving a theoretical molecular weight of 20457 g.mol -1 at final conversion.At first, styrene (0.073 mol, 8.34 ml), methyl methacrylate (0.058 mol, 6.2 ml), CuBr 2 (0.17 mmol, 0.039 g), PMDETA (0.17 mmol, 0.03 ml) and anisole (10 ml) were added to the reactor.Then, it was degassed and back-filled with nitrogen three times, and then left under N 2 with stirring at room temperature.After that, reaction temperature was increased to 110 °C during 5 min.Subsequently, predeoxygenated solution of AIBN (0.1 mmol, 0.017 g) in the styrene (0.043 mol, 5 ml) and predeoxygenated EBiB (0.87 mmol, 0.12 ml) were injected into the reactor to start the copolymerization reaction.At the beginning of the reac- tion, copolymerization media was green and gradually changed to brown.Samples were taken at the end of the reaction to measure the final conversion.

4. Preparation of Poly (Styrene-comethyl Methacrylate)/Silica Aerogel Nanocomposites by in Situ SR&NI ATRP
For preparation of nanocomposites, a desired amount of H-SAN was dispersed in 7 ml of styrene and the mixture was stirred for 17 hours.Then, the remained 1.34 ml of styrene was added to the mixture.Subsequently, copolymerization procedure was applied accordingly.Designation of the samples with the percentage of modified silica aerogel nanoparticles is given in table 1.

5. Characterization
Materials porosity was characterized by N 2 adsorption/desorption curves obtained with a Quntasurb QS18 (Quntachrom) apparatus.The surface area and pore size distribution values were obtained with the corrected BET equation (Brunauer-Emmett-Teller).Surface morphology of the samples was examined by Scanning Electron Microscope (SEM, Philips XL30) with acceleration voltage of 20 kV.Also, transmission electron microscope (TEM, FEG Philips CM) with an accelerating voltage of 200 kV was used.Gas chromatography (GC) is a simple and highly sensitive characterization method and does not require removal of the metal catalyst particles.GC was performed on an Agilent-6890N with a split/splitless injector and flame ionization detector, using a 60 m HP-INNOWAX capillary column for the separation.The GC temperature profile included an initial steady heating at 60 °C for 10 min and a 10 °C/min ramp from 60 to 160 °C.The samples were also diluted with acetone.The ratio of monomer to anisole was measured by GC to calculate monomer conversion throughout the reaction.Size exclusion Chromatography (SEC) was used to measure the molecular weight and molecular weight distribution.A Waters 2000 ALLI-ANCE with a set of three columns of pore sizes of 10000, 1000, and 500 Å was utilized to determine polymer average molecular weight and polydispersity index (PDI).Thermal gravimetric analysis (TGA) was carried out with a PL thermo-gravimetric analyzer (Polymer Laboratories, TGA 1000, UK).Thermograms were obtained from ambi-ent temperature to 700 °C at a heating rate of 10 °C/min.Thermal analysis were carried out using a differential scanning calorimetry (DSC) instrument (NETZSCH DSC 200 F3, Netzsch Co, Selb/Bavaria, Germany).Nitrogen at a rate of 50 mL/min was used as purging gas.Aluminum pans containing 2-3 mg of the samples were sealed using DSC sample press.The samples were heated from ambient temperature to 220 °C at a heating rate of 10 °C/min.

Results and Discussion
Nitrogen adsorption/desorption isotherm of the H-SAN indicate that this material can be classified as a mesoporous type according to the IUPAC classification (figure 2). 32 According to extracted data from nitrogen adsorption/desorption isotherm, BET surface area and average pore diameter of the synthesized aerogel (modified silica aerogel nanoparticles) are calculated 958.67 m 2 /g and 4.05 nm respectively.
TEM and SEM images of the H-SAN are displayed in figure 3.According to these images, H-SAN present spongy structure and the particles are relatively mono-dispersed.In addition, average diameter of the particles is estimated around 25 nm.During the copolymerization process, monomer conversion with time can present various behaviors.Thus, examining the variation of conversion with time for the copolymerization system can be considered as an important parameter.Variation of monomers (MMA and St) conversion versus time for the neat poly (styrene-co-methyl methacrylate) and its different nanocomposites are presented in figure 5.According to the results, by proceeding of the reaction, an increment in the conversion is obtained.Also, copolymerization of MMA and St without the modified silica aerogel nanoparticles results in higher conversion in various time steps.On the other hands, by increasing modified silica aerogel nanoparticles content lower monomer conversions are obtained (conversion: RSMAC 3< RSMAC 2< RSMAC 1<PRSM).These results indicate that presence of H-SAN can result in negative effect on the copolymerization rate and final conversion of the monomers.4][35] ii) Restriction of the mobility: due to the porous structure of the H-SAN, mobility of the radicals and growing macro-radicals in the solution can be restricted and therefore copolymerization rate and monomers conversion can be decreased.2][43] iii) Irreversible reactions: It is demonstrated that growing radicals in the copolymerization media can irreversibly react with various functional moieties.][35] Variation of number-averaged molecular weight (Mn) with conversion during the copolymerization is represented in figure 6. Theoretical line in this figure is displayed to present theoretical Mn values for the various conversions.As it can be expected, in the neat copolymer and different nanocomposites by increasing conversion copolymer chains with higher Mn were obtained.According to the results, PRSM has higher Mn values in comparison with the different nanocomposites (RSMAC 1-3).On the other hands, addition of the H-SAN in the copolymerization media results in decrement of Mn of the products.By increasing H-SAN content, more decrement in Mn is observed (Mn: PRSM > RSMAC 1 > RSMAC 2 > RSMAC 3).4][35] By precise evaluation of the results an appropriate agreement between experimental and theoretical Mn values are observed that this agreement can be considered as a suitable evidence for demonstration of the living nature of the copolymerization system.
Variation of PDI values with conversion for the neat copolymer and its various nanocomposites is displayed in figure 7. Since the copolymerization system (SR&NI ATRP) is a member of CLRP family, it could be predicted that PDI values present a decrement by proceeding of the reaction (by increasing of conversion).Although addition of nano-fillers can affect CLRP equilibrium, they can not change the living nature of the system and therefore a decrement in PDI values by increasing conversion are observed in the pure copolymer (PRSM) and its different nanocomposites (RSMAC 1-3).According to the figure 7, PDI value of the PRSM is lower than RSMAC 1-3.In the nanocomposites, by increasing modified silica aerogel nanoparticles content higher PDI values are obtained (PDI: PRSM< RSMAC 1< RSMAC 2< RSMAC 3).Increment of PDI values by increasing H-SAN content can be attributed to the impurity role of the H-SAN.][35] Figure 8 shows pseudo-first-order kinetic plots for the synthesized nanocomposites and the neat copolymer.
Results indicate that the slope of the curves (PRSM and RSMAC 1-3) remains fairly constant in the whole of the copolymerization and therefore it can be concluded that steady-state kinetics for all of the samples is obtained.4] Results also indicate that by increasing H-SAN content a considerable decrement in the copolymerization rate will be obtained.
SEC traces demonstrate that SR&NI ATRP of MMA and St in the presence of H-SAN results in copolymer with broaden molecular weight distribution.On the other hands, absence of the H-SAN in the copolymerization media leads to copolymer with lower PDI values.][35] According to the extracted data in table 2, by addition of 3 wt% of the H-SAN monomer conversion decreases from 94% to 73%.Meanwhile PDI values present an increment from 1.29 to 1.65. (1) where, [M] 0 and [EBiB] 0 are initial concentration of the monomers and ATRP initiator, respectively.Conversion is denoted by Conv.and the symbol of the average molecular weight of the monomers is M w .
TGA thermograms of the PRSM and RSMAC 1-3 in the temperature window of 25-700 °C in two different styles are displayed in figure 10.Char values at 700 °C are also presented in this figure.As expected, by increasing H-SAN content, char values of the prepared nanocomposites should be increased (char values: PRSM< RSMAC 1< RSMAC 2< RSMAC 3).
Figure 10 indicates that thermal stability of the PRSM is lower than various nanocomposites.Also, it can be concluded that by increasing H-SAN content, more im-  provements in thermal stabilities can be obtained (thermal stability: PRSM< RSMAC 1< RSMAC 2< RSMAC 3).Except the main degradation step in the TGA graphs (at the temperatures above 350 °C) that is attributed to the degradation of the synthesized copolymer and its nanocomposites, some other degradation can be also recognized.This degradation in the lower temperatures is ascribed by evaporation of water molecules and volatile materials such as residual monomers and HMDS.
Extracted data from TGA are graphically illustrated in figure 11.Degradation temperature of the samples versus amount of degradation is employed to show that addition of H-SAN in the copolymer matrix, results in an improvement of thermal stabilities of the nanocomposites (T X : temperature threshold at which X% of neat copoly-mer and its nanocomposites is degraded).Char values at 700 °C are also reported in the figure 10.
TGA results demonstrate that incorporation of H-SAN in the copolymer matrix leads to improvements in thermal stabilities.This improvement can be ascribed by using inorganic nature of the H-SAN.Modified silica aerogel nanoparticles as inorganic nano-filler can result in higher thermal stabilities of the RSMAC 1-3 in comparison with the PRSM. 36Absorption of the introduced gas (in oxygen atmosphere) in the porous structure of the incorporated nano-filler (H-SAN is classified as mesoporous materials) may be considered as another factor for increment of thermal stabilities of the nanocomposites in comparison with the PRSM. 33mprovement in thermal stability may be ascribed by inorganic nature of the incorporated nano-filler.H-SAN as inorganic nano-filler that are incorporated and dispersed within the copolymer matrices can result in improvement of thermal stability of the nanocomposites in comparison with the neat copolymer. 36Moreover, since the synthesized nano-filler is a member of mesoporous materials family it can absorb introduced gas (in O 2 atmosphere) can also results in increment of thermal stability of the nanocomposites.Finally, because of the three-dimensional network structure of the H-SAN, these nanoparticles can not be appropriately dispersed and therefore can not significantly improve thermal stabilities of the nanocomposites. 37valuation the effect of the H-SAN on the chain confinement and also determination of glass transition temperature (T g ) are performed by DSC analysis.DSC curves of the PRSM and RSMAC 1-3 are presented in figure 12.According to the figure 12, an obvious inflection in the cooling path are occurred which shows T g values of the PRSM and RSMAC 1-3.In addition, amorphous structures of the synthesized samples can be also concluded.Extracted T g values of the samples from DSC graphs (in the cooling path) are summarized in table 3. also responsible for more segment mobilities of the copolymer chains which in turn results in T g reduction.Moreover, according to the table 2, by increasing H-SAN in the copolymer matrix an increment in PDI value and also a decrement in Mn are observed.1] Reduction of Tg value by incorporating alumina nanoparticles as a nano-fillers was also reported by Schadler et al. 42

Conclusions
Tailor-made random poly (styrene-co-methyl methacrylate) and its different nanocomposites in the presence of H-SAN were synthesized by in situ SR&NI ATRP.Porous and spongy structure, spherical shape and high surface are some unique properties of the prepared H-SAN.SR&NI ATRP of styrene and methyl methacrylate in the presence of H-SAN results in a decrease of conversion from 94 to 73%.Moreover, molecular weight of the random copolymer chains decrease from 19757 to 16268 g.mol -1 and PDI values increase from 1.29 to 1.65.Improvement in thermal stability of the nanocomposites and decreasing T g values from 70.3 to 63.5 °C was also obtained by addition of 3 wt% of the H-SAN.According to the table 3, PRSM presents higher Tg value in comparison with the other samples (RSMAC 1-3).By incorporating of the H-SAN in the copolymer matrix a decrement in Tg value was happened (Tg: RSMAC 3<RS-MAC 2<RSMAC 1< PRSM).Decrement in Tg values by adding H-SAN in the copolymer matrix is related to the reduction of the packing of the copolymer chains.By dispersing H-SAN within the copolymer matrix, packing of the copolymer chains will be reduced and therefore copolymer chains can obtain more segment mobilities in which results in decrement of Tg values.In addition, due to the three-dimensional network structure of the H-SAN these nanoparticles can not appropriately dispersed within the copolymer matrix and therefore some free volume spaces for the dangling copolymer chains may be created.This is

Figure 2 .
Figure 2. Nitrogen adsorption/desorption isotherm of the H-SAN

Figure 3 .Figure 4 .
Figure 3. TEM (A) and SEM (B) images of the H-SAN

Figure 8 .
Figure 8. Kinetic plots with respect to the reaction time Figure 9. SEC traces of the neat random poly) styrene-co-methyl methacrylate) and its nanocomposites

Table 1 .
Designation of the samples

Table 2 .
Molecular weights and PDI values of the extracted copolymers resulted from SEC traces