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BY-NC-ND 3.0 license Open Access Published by De Gruyter October 8, 2016

Synthesis and Characterization of Poly (styrene-co-butyl acrylate)/Silica Aerogel Nanocomposites by in situ AGET ATRP: Investigating Thermal Properties

  • Khezrollah Khezri and Yousef Fazli EMAIL logo

Abstract

Hydrophilic silica aerogel nanoparticles surface was modified with hexamethyldisilazane. Then, the resultant modified nanoparticles were used in random copolymerization of styrene and butyl acrylate via activators generated by electron transfer for atom transfer radical polymerization. Conversion and molecular weight determinations were performed using gas and size exclusion chromatography respectively. Addition of modified nanoparticles by 3 wt% results in a decrease of conversion from 68 to 46 %. Molecular weight of copolymer chains decreases from 12,500 to 7,500 g.mol–1 by addition of 3 wt% modified nanoparticles; however, PDI values increase from 1.1 to 1.4. Proton nuclear magnetic resonance spectroscopy results indicate that the molar ratio of each monomer in the copolymer chains is approximately similar to the initial selected mole ratio of them. Increasing thermal stability of the nanocomposites is demonstrated by thermal gravimetric analysis. Differential scanning calorimetry also shows a decrease in glass transition temperature by increasing modified silica aerogel nanoparticles.

Introduction

Nanocomposites are one of the new classes of materials which are generally composed from incorporation of one or more nanofillers into the polymer matrix. Due to the incorporation of nanoscale fillers, nanocomposites exhibit behavior different from conventionally filled micro-composites [1, 2, 3]. Nanofillers belong to small size and high surface-to-volume ratio, and therefore can increase interfacial area between themselves and polymer matrix that results in more enhancements of several properties [2, 4]. Improvement in several properties of the nanocomposites (e. g. mechanical, thermal, and optical properties) as a resultant of nanofiller addition is clearly demonstrated [5, 6, 7]. Among various nanofillers, silica-based nanocomposites have attracted great attention in the recent decades. Silica/polymer nanocomposites have many potentially applications as aerospace materials, structural materials, electronic, and sensors [8].

Porous materials are categorized into three types based on their pore diameters which are namely: micropores, mesopores, and macropores. Moreover, porous materials with pore diameter less than 100 nm are generally named nanoporous materials [9, 10]. Silica aerogel is one of the nanoporous materials that consisting of a three-dimensional network of silica nanoparticles [11, 12]. Aerogels are unique porous nanostructured materials in which have a large surface area, high porosity, low bulk density, and extremely low thermal conductivity [13, 14, 15, 16]. Colloidal silica imprinting, microbead patterning, and presynthesized mesoporous silica scaffolding are three main methodsto synthesize pore structures of nanoporous materials[17].

Various methods for controlled radical polymerization (CRP) have been investigated that the most successful pathways are namely atom transfer radical polymerization (ATRP) [18], nitroxide-mediated polymerization (NMP) [19], and reversible addition-fragmentation chain transfer (RAFT) [20]. Among various CRP methods, ATRP has gained more attention due to its unique advantages such as: commercial availability of its reagents, application for various polymerization media and systems, ability to polymerization of different monomers and et cetera [18, 21, 22].

A review on literature indicates that some studies have been done on the synthesis and investigating properties of silica aerogel/polymer nanocomposites. Boday et al. have applied surface initiated ATRP to grow low polydispersities poly (methyl methacrylate) on silica aerogel to prepare of mechanically reinforced silica aerogel nanocomposites [23]. Sobani et al. have employed RAFT polymerization to synthesize well-defined polystyrene/silica aerogel nanocomposites. According to their report, double bond containing modifier was firstly attached on silica aerogel surface (surface modification) and the grafting through technique was applied to attach of polystyrene on the surface of silica aerogel [24]. Boday et al. have also prepared strong polycyanoacrylate/silica aerogel nanocomposites via chemical vapor deposition of cyanoacrylate monomers on amine-modified silica aerogels [25]. In addition, Costela at al. have applied polymer-filled nanoporous silica aerogel as hosts for highly stable solid-state dye lasers [26].

In this study, hydrophilic silica aerogel nanoparticles were modified with hexamethyldisilazane (HMDS). After that, organo-modified silica aerogel nanoparticles were incorporated as an additive in the activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) of styrene and butyl acrylate. Abundant advantages of the AGET ATRP were employed to synthesize well-defined poly (styrene-co-butyl acrylate) nanocomposites. By using this initiation technique, ATRP starts with less air-sensitive CuII complex and therefore copolymerization process can be facilitated. Effect of silica aerogel nanoparticles on the conversion, molecular weight (Mw), and polydispersity index (PDI) of the products is discussed in detail. Moreover, thermal properties of the neat poly (styrene-co-butyl acrylate) and its various nanocomposites are also studied.

Experimental

Materials

Styrene (St, Aldrich, 99 %) and butyl acrylate (BA, Merck, 99 %) were passed through an alumina filled column to remove inhibitors. Copper(II) bromide (CuBr2, Fluka, 99 %), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99 %), ethyl alpha-bromoisobutyrate (EBiB, Aldrich, 97 %), tin(II) 2-ethylhexanoate (Sn(EH)2, Sigma, 95 %), hexamethyldisilazane (HMDS, Merck), anisole (Aldrich, 99 %), tetrahydrofuran (THF, Merck, 99 %), neutral aluminum oxide (Aldrich, 99 %), tetraethoxysilane (TEOS, Merck), n-hexane (Merck), Ethanol (EtOH, Merck, 99 %), oxalic acid (Aldrich, >99 %), ammonium hydroxide (Tabriz Petrochemical Company) and were used as received.

Preparation of hydrophilic silica aerogel nanoparticles

Silica aerogel was prepared by a two-step, acid-base catalyzed, sol-gel polymerization followed by the ambient pressure drying as reported previously [24, 27].

Surface modification of silica aerogel nanoparticles with HMDS

Surface modification of hydrophilic silica aerogel nanoparticles was performed by immersing the wet gel in solution of n-hexane/hexamethyldisilazane in 50 °C for 24 h. The surface-modified wet gel was washed repeatedly in n-hexane in order to remove the unreacted surface modifier agents and dried at three time stages of 50 °C for 1 h, 150 °C for 2 h, and 200 °C for 1 h to obtain dry silica aerogel.

Random copolymerization of styrene and butyl acrylate via AGET ATRP and preparation of its nanocomposites

Random copolymerization of styrene and butyl acrylate was performed in a 150 ml three-neck lab reactor. A typical batch of copolymerization was run at 100 °C with the molar ratio of 150:1:1:1:0.5 for [Monomers]:[EBiB]:[CuBr2]:[PMDETA]:[Sn(EH)2]. At first, styrene (13.34 ml) and butyl acrylate (8.33 ml), anisole (10 ml), CuBr2 (0.26 g, 1.16 mmol) and PMDETA (0.24 ml, 1.16 mmol) were added into the reactor and the reactor was degassed and back filled with nitrogen three times and stirring was continued at room temperature. Subsequently, predeoxygenated solutions of reducing agent [Sn(EH)2, 0.18 ml, 0.58 mmol] and initiator [EBiB, 0.17 ml, 1.16 mmol] were injected and the reactor temperature was increased to 100 °C. After 17 h, copolymerization process was stopped by opening the reactor and exposing the catalyst to air. For preparation of nanocomposites, a desired amount of HMDS-modified silica aerogel nanoparticles was dispersed in 10 ml of styrene and the mixture was stirred for 21 h. Then, the remaining 3.34 ml of styrene was added to the mixture. Subsequently, copolymerization procedure was applied accordingly. Designation of the samples is given in Table 1.

Table 1:

Designation of the samples.

SampleMethod of preparationProportion of HMDS-modified silica aerogel nanoparticles (wt%)Dispersion time prior to the copolymerization (h)
RCSBAGET ATRP0
RCSBN 1In situ AGET ATRP121
RCSBN 2In situ AGET ATRP221
RCSBN 3In situ AGET ATRP321

Separation of copolymer chains from HMDS-modified silica aerogel nanoparticles and catalyst removal

For separating of copolymer chains from silica aerogel nanoparticles, nanocomposites were dissolved in THF. By high-speed ultracentrifugation (10,000 rpm) and then passing the solution through a 0.2 µm filter, copolymer chains were separated from silica aerogel nanoparticles. Subsequently, polymer solutions passed through an alumina column to remove catalyst species.

Characterization

Fourier transform infrared (FTIR) spectroscopy was performed by a Bruker FTIR spectrophotometer, within a range of 400–4,400 cm−1. Materials porosity was characterized by N2 adsorption/desorption curves obtained with a Quntasurb QS18 (Quntachrom) apparatus. Surface morphology of powder samples was examined by scanning electron microscopy (SEM, Philips XL30) and transmission electron microscopy (TEM, FEG Philips CM). The specimens were prepared by coating a thin layer on a mica surface using a spin coater (Modern Technology Development Institute, Iran). Gas chromatography (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. Size exclusion chromatography (SEC) was used to measure the molecular weight and molecular weight distribution. A Waters 2000 ALLIANCE with a set of three columns of pore sizes of 10,000, 1,000, and 500 Å was utilized to determine polymer average molecular weight and PDI. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker 300-MHz 1H NMR instrument with CDCl3 as the solvent and tetramethylsilane as the internal standard. Thermal gravimetric analysis (TGA) was carried out with a PL thermo-gravimetric analyzer (Polymer Laboratories, TGA 1000, UK). Thermal analysis was carried out using a differential scanning calorimetry (DSC) instrument (NETZSCH DSC 200 F3, Netzsch Co, Selb/Bavaria, Germany).

Results and discussion

During “surface modification reaction” an appropriate organic moiety attaches on the surface of nanoparticles by common organic reactions in which this procedure can improve the compatibility between the nanoparticles and polymer matrix. Therefore, hydroxyl groups on the surface of hydrophilic silica aerogel nanoparticles were replaced with hydrophobic groups of hexamethyldisilazane (HMDS).

FTIR spectra of the hydrophilic and HMDS-modified silica aerogel nanoparticles are represented in Figure 1. In the case of the hydrophilic silica aerogel nanoparticles, a broad peak in the range of around 3,500 cm–1 shows high intensity that is generally attributed to the absorbed water. In spite, HMDS-modified silica aerogel nanoparticles present low intensity peak in the same range that can demonstrate the majority of hydroxyl groups are reacted and converted to hydrophobic groups. Moreover, the peak around 1,620 cm–1, which is corresponded to the vibration of O-H groups in the water, is approximately disappeared in the HMDS-modified silica aerogel spectrum.

Figure 1: FTIR spectra of the hydrophilic and HMDS-modified silica aerogel nanoparticles.
Figure 1:

FTIR spectra of the hydrophilic and HMDS-modified silica aerogel nanoparticles.

Figure 2 represents nitrogen adsorption/desorption isotherm of the HMDS-modified silica aerogel nanoparticles. During nitrogen physisorption experiment, the synthesized sample reveals mesoporous materials behavior and shows IV isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification [28]. The N2 desorption cycle of the isotherm showed a hysteresis loop which can be attributed to the capillary condensation occurring in the mesoporous materials.

Figure 2: Nitrogen adsorption/desorption isotherm of the HMDS-modified silica aerogel nanoparticles.
Figure 2:

Nitrogen adsorption/desorption isotherm of the HMDS-modified silica aerogel nanoparticles.

According to extracted data from nitrogen adsorption/desorption isotherm, surface area of the synthesized aerogel (HMDS-modified silica aerogel nanoparticles) is calculated 662 m2/g. Also, average pore diameter is estimated around 3.2 nm.

Evaluation of size distribution and surface morphology of the HMDS-modified silica aerogel nanoparticles was performed by SEM. According to the images, HMDS-modified silica aerogel nanoparticles are frequently spherical shaped with diameter less than 50 nm. Also, as expected, aerogel structure that formed from silica nanoparticles is spongy. Figure 3 displays SEM images of the HMDS modified in two different magnifications. Although surface modification can reduce aggregation phenomena, some aggregated HMDS-modified silica aerogel nanoparticles can be observed by these images.

Figure 3: SEM images of the HMDS-modified silica aerogel in two different magnifications.
Figure 3:

SEM images of the HMDS-modified silica aerogel in two different magnifications.

Nanoporous and spongy structure of the HMDS-modified silica aerogel nanoparticles was evaluated by TEM. Figure 4 depicts TEM images of the HMDS-modified silica aerogel nanoparticles in two different magnifications.

Figure 4: TEM images of the HMDS-modified silica aerogel in two different magnifications.
Figure 4:

TEM images of the HMDS-modified silica aerogel in two different magnifications.

AGET ATRP is an appropriate route to circumvent these problems since it applies less oxygen sensitive reactants (CuBr2 in its initial components). AGET ATRP has all advantages of normal and RATRP and remains tolerant to oxygen during the operation. General mechanism of AGET ATRP is presented in Figure 5.

Figure 5: General mechanism for AGET ATRP.
Figure 5:

General mechanism for AGET ATRP.

As it can be seen, AGET ATRP employs reducing agents which are unable to initiate new chains. In this system, reducing agents are used to reduce transitional metal complex in the higher oxidation state (CuII/L). Then, generated activators participate in ordinary ATRP equilibrium. Some reducing agents such as Cu, [Sn(EH)2], hydrazine, and ascorbic acid have been used in AGET ATRP [29, 30]. Schematic presentation of AGET ATRP in the presence of HMDS-modified silica aerogel nanoparticles is sown in Figure 6.

Figure 6: General procedure for preparation of random poly (styrene-co-butyl acrylate)/HMDS-modified silica aerogel nanocomposites via in situ AGET ATRP.
Figure 6:

General procedure for preparation of random poly (styrene-co-butyl acrylate)/HMDS-modified silica aerogel nanocomposites via in situ AGET ATRP.

According to Figure 7, SEC traces of the neat poly (styrene-co-butyl acrylate) and other random copolymer chains are monomodal. Neat poly (styrene-co-butyl acrylate) reveals narrow distribution and low PDI value which successful AGET ATRP can be obviously demonstrated.

Figure 7: SEC traces of the neat random poly (styrene-co-butyl acrylate) chains and its nanocomposites with different HMDS-modified silica aerogel loading.
Figure 7:

SEC traces of the neat random poly (styrene-co-butyl acrylate) chains and its nanocomposites with different HMDS-modified silica aerogel loading.

According to the results, AGET ATRP of styrene and butyl acrylate without nanofiller results in appropriate control over the molecular weight and PDI value. Moreover, by adding HMDS-modified silica aerogel nanoparticles loading, the control over the copolymerization is diminished and conversion, Mn, and PDI values are influenced. By increasing 3 wt% of silica aerogel nanoparticles, conversion decreases from 68 to 46 %. Meanwhile, a decrease in Mn from 12,500 to 7,500 g.mol–1 is also occurred. PDI value but reveals a different behavior and increases from 1.1 to 1.4. Variation of kinetics parameters (conversion, Mn, and PDI) by adding silica aerogel nanoparticles can be attributed to two main reasons: (i) impurity role of the nanoparticles: Silica aerogel nanoparticles as an impurity can increase chain transfer and termination reactions of the propagating radicals, and therefore broadens molecular weight distribution of the resultant copolymers [31]. (ii) Mobility of growing radicals in the solution can be restricted by silica aerogel nanoparticles especially at higher loadings. Therefore, copolymerization rate and conversion value decrease by the addition of silica aerogel nanoparticles content [32]. Table 2 summarized extracted data from SEC analysis.

Table 2:

Molecular weights and PDI values of the random copolymers’ chains resulted from SEC traces.

SampleReaction time (h)Conversion (%)Mn (g.mol–1)Mw (g.mol–1)PDI
Exp.Theo.
RCSB176812,50011,80014,6001.1
RCSBN 1176210,80010,80013,7001.2
RCSBN 217519,7008,80013,0001.3
RCSBN 317467,5008,00011,1001.4

Theoretical molecular weight is calculated by using eq. (1):

(1)MnTheo=M0EBiB0×Conversion×Mm

According to Table 2, an appropriate agreement between the theoretical and experimental molecular weights can be considered as an appropriate evidence of controlled nature of the polymerization system.

Composition of the copolymer chains is evaluated using 1H NMR spectroscopy as a useful technique. Molar ratio of each monomer in the copolymer chains can be determined by integrating aromatic peaks area (SPh, 6.6–7.4 ppm, 5H) which corresponds to the phenyl ring of styrene and methylene near to the ester group of butyl acrylate (SM, 3.6–4.2 ppm, 2H) by using eqs (2) and (3):

(2)%St=SPh5SPh5+SM2×100
(3)%BA=SM2SPh5+SM2×100

Table 3 summarized the extracted data from 1H NMR spectroscopy analysis. According to the results, molar ratio of each monomer (styrene and butyl acrylate) in all the samples is approximately similar to the initial selected mole ratio of the monomers (St: ~67 % and MMA: ~33 %). According to the results, the percentage of styrene in the copolymer chains is somewhat higher than butyl acrylate in which it can be attributed to the higher mole fraction and reactivity ratio of the styrene (rSt: 0.74) in comparison with the butyl acrylate (rBA: 0.24) [33].

Table 3:

Extracted data from 1H NMR spectroscopy analysis.

SampleMole ratio (%)
StBA
RCSB6535
RCSBN 16733
RCSBN 26634
RCSBN 36931

Thermal stability of the neat random poly (styrene-co-butyl acrylate) and its different nanocomposites are evaluated by TGA. TGA thermograms of weight loss as a function of temperature in the temperature window of 30–700 °C in addition to their corresponding differential thermogravimetric (DTG) curves are represented in Figure 8.

Figure 8: (a) TGA and (b) DTG thermograms of the neat random poly (styrene-co-butyl acrylate) and its different nanocomposites.
Figure 8:

(a) TGA and (b) DTG thermograms of the neat random poly (styrene-co-butyl acrylate) and its different nanocomposites.

According to the results, thermal stability of the nanocomposites increased in comparison with the neat random copolymer. Also, by increasing HMDS-modified silica aerogel nanoparticles content, a slight increase in degradation temperatures was obtained. According to Figure 8(a), each thermogram presents some weight losses before the main degradation step. These weight losses can be attributed to the evaporation of water molecules that are absorbed on the surface of the silica aerogel nanoparticles and volatile materials (such as residual monomer, functionalities of silica aerogel nanoparticles, and low molecular weight oligomers) [34]. High thermal stability of the silica aerogel nanoparticles and also interaction between the silica aerogel nanoparticles and copolymer matrix are two main interpretations for a slight improvement in thermal stability of the nanocomposites in comparison with neat random copolymer [35]. Moreover, a slight improvement of thermal stability is also attributed to the three-dimensional network structure of the silica aerogel nanoparticles. Because of their structure, these nanoparticles cannot be properly dispersed and behaves as agglomerated nanoparticles which cannot significantly improve thermal stabilities of the nanocomposites [36]. Extracted data from DTG curves in combination with char values are also summarized in Table 4.

Table 4:

Extracted data from TGA and DTG thermograms for the neat random poly (styrene-co-butyl acrylate) and its nanocomposites.

SampleTGADTG (°C)
Char (%) at 650 °CStart pointPeak pointEnd point
RCSB3332408445
RCSBN 14.3334410445
RCSBN 25.5337414450
RCSBN 37.3337416451

Figure 9 represents DSC thermograms of the neat random poly (styrene-co-butyl acrylate) and its various nanocomposites. Since silica aerogel nanoparticles do not bear any transitions in this range of temperature, therefore only thermal transition of copolymers is observed. In these experiments, samples were heated from room temperature to 220 °C. Then, they were cooled to room temperature for distinguishing the phase conversion, Tg, and other irreversible thermal behaviors.

Figure 9: DSC Thermograms of the neat random poly (styrene-co-butyl acrylate) and its various nanocomposites.
Figure 9:

DSC Thermograms of the neat random poly (styrene-co-butyl acrylate) and its various nanocomposites.

According to the results of DSC analysis, structures of the synthesized samples are mainly amorphous and they have not gone through crystallization phenomenon since there is not any recrystallization peak. Table 5 summarized the extracted Tg values of the samples from DSC thermograms.

Table 5:

Tg values of the neat random poly (styrene-co-butyl acrylate) and its nanocomposites.

SampleMn (g.mol–1)PDITg (°C)
RCSB12,5001.142
RCSBN 110,8001.239
RCSBN 29,7001.334
RCSBN 37,5001.432

According to the results of Table 5, Tg value of the neat random copolymer is higher than all of the nanocomposites and a decrease in Tg values is observed by increasing silica aerogel nanoparticles content. Reduction of Tg values by increasing nanoparticles content may be attributed to the silica aerogel nanoparticles. These nanoparticles can reduce the packing of poly (styrene-co-butyl acrylate) chains and therefore increase the segments mobility’s which in turn results in Tg reduction. In addition, homogeneous dispersion of silica aerogel nanoparticles is restricted because of strong tendency of these nanoparticles to agglomerate (to reduce their surface energy). Therefore, some free volume spaces for the dangling poly (styrene-co-butyl acrylate) chains within matrices may be created which results in decrement of Tg values.

Conclusions

AGET ATRP of styrene and butyl acrylate was employed to synthesize tailor-made random poly (styrene-co-butyl acrylate) nanocomposites. Porous and spongy structure, spherical shape with diameter around 20 nm, and high surface area are some unique features of the synthesized HMDS-modified silica aerogel nanoparticles. AGET ATRP of styrene and butyl acrylate in the presence of HMDS-modified silica aerogel nanoparticles results in a decrease of conversion from 68 to 46 %. Moreover, molecular weight decreases from 12,500 to 7,500 g.mol–1 and PDI values increase from 1.1 to 1.4. 1H NMR spectra demonstrates that the molar ratio of each monomer in the copolymer chains is similar to the initial selected mole ratio of the monomers. Improvement in thermal stability of the nanocomposites and decreasing Tg values from 42 to 32 °C was also observed by incorporation of 3 wt% of HMDS-modified silica aerogel nanoparticles.

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Received: 2015-10-27
Accepted: 2016-7-16
Published Online: 2016-10-8
Published in Print: 2017-10-26

© 2017 Walter de Gruyter GmbH, Berlin/Boston

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