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Preparation of Poly(methyl methacrylate)-Silica Nanoparticles via Differential Microemulsion Polymerization and Physical Properties of NR/PMMA-Si[O.sub.2] Hybrid Membranes.

Poly(methy methacrylate) (PMMA)-Si[O.sub.2] nanoparticles were prepared via differential microemulsion polymerization. The effects of silica loading, surfactant concentration, and initiator concentration on monomer conversion, particle size, particle size distribution, grafting efficiency, and silica encapsulation efficiency were investigated. A high monomer conversion of 99.9% and PMMA-Si[O.sub.2] nanoparticles with a size range of 30 to 50 nm were obtained at a low surfactant concentration of 5.34 wt% based on monomer. PMMA-Si[O.sub.2] nanoparticles showed spherical particles with a core-shell morphology by TEM micrographs. A nanocomposite membrane from natural rubber (NR) and PMMA-Si[O.sub.2] emulsion was studied for mechanical and thermal properties and pervaporation of water-ethanol mixtures. PMMASi[O.sub.2] nanoparticles which were uniformly dispersed in NR matrix, significantly enhanced mechanical properties and showed high water selectivity in permeate flux. Thus, the NR/PMMA-Si[O.sub.2] hybrid membranes have great potential for pervaporation process in membrane applications.

INTRODUCTION

The addition of inorganic fillers into polymer matrixes is well known as being a beneficial way to improve the polymer properties such as mechanical, thermal, optical, and electrical properties [1-4]. Polymer nanomaterials are widely used in many applications such as polymer-coated inorganic particles, solid barriers, hybrid membranes, and drug carriers [5-8] due to their good processability and properties. On the other hand, nanosilica is one of the most common inorganic fillers used in various fields as the silica is an effective reinforcement component in developing nanocomposites [9]. For polymer-silica composites, the difficulties in obtaining a well-dispersed silica in polymer matrix are due to the large quantity of hydroxyl groups on the surface of nanosilica and its high surface energy and polarity resulting in inferior compatibility and less stability between the polymer matrix and nanosilica; thus severe agglomeration and weak rubber-filler interactions occurred [10]. Grafting of polymer chains onto silica nanoparticles is one of the effective methods to increase the hydrophobicity of the particles and to improve interfacial interactions in nanocomposites resulting in better compatibility and dispersion of silica particles in the polymer matrix. It was found that the modification of nanoparticles through graft polymerization was a very effective method to increase the hydrophobicity of the nanoparticles that is beneficial to filler/matrix miscibility, and an improved interfacial interaction resulting from the molecular entanglement between the grafting polymer on the nanoparticles and the matrix polymer [11, 12].

Grafting polymer onto a silica surface by polymerization is characterized by many advantages, such as simple, low cost, easy control, and broader applicability. Polystyrene (PS)/silica composite particles have been successfully synthesized via soap-free emulsion polymerization and the incorporation of silica particles provided an enhancement in the thermal stability due to the strong interaction between silica and the polystyrene molecules [13]. Moreover, monodispersed polyisoprene (PIP)-Si[O.sub.2] nanoparticles were produced (20-60 nm) via differential microemulsion polymerization (DMP) and exhibited reduced nano-Si[O.sub.2] aggregation in the PIP matrix and the surfactant concentration used was around 3% based on monomer weight and PIP-Si[O.sub.2] has been used as an effective nano-filler in a NR latex [14]. In addition, from synthesis of poly(methyl methacrylate) (PMMA)/Si[O.sub.2] particles via emulsion polymerization, PMMA polymer could be grafted on the silica surface (37% grafting) and the silica particles became of lower aggregation [15]. Furthermore, hydrogenated polyisoprene (HPIP)-Si[O.sub.2] nanocomposites were synthesized via DMP followed by diimide hydrogenation, resulting in improved mechanical properties and exhibited a good retention of tensile strength after thermal aging and good resistance toward ozone exposure [16].

Since the polymer/silica nanocomposites not only have improved physical properties such as the mechanical properties and thermal properties of the materials, but also exhibit some unique properties, which have attracted strong interest in many industries. Besides common plastic and rubber reinforcement, one potential and practical application of this nanocomposite is in membrane separation. Pervaporation separates the liquid mixtures by the difference in the solubility and diffusivity of each liquid component within the membrane [17]. In order to achieve a good separation, the membrane must contain the active sites that interact strongly with the separated species. Silica nanoparticles in chitosan-silica complex membranes used in pervaporation dehydration of ethanol-water mixtures served as spacers between the polymer chains to provide extra space for water permeation, so as to bring about high permeation rates within the complex membranes [18]. The addition of silica nanoparticles to chitosan would enhance the selectivity and permeation flux of the complex membrane. A poly(vinyl alcohol) (PVA)-Si[O.sub.2] nanocomposite membrane with the incorporation of silica particles into PVA showed a high performance in pervaporative dehydration of an ethylene glycol (EG) aqueous solution [19]. The PVA-Si[O.sub.2] nanocomposite membrane exhibited desirable changes in the morphology and crystalline structure of the membranes, and the thermal stability and stability of the membranes in EG aqueous solution were significantly enhanced.

To produce inorganic-polymer hybrid particles, DMP is a challenging process using low surfactant concentration and yields nano-size particles and high conversion. This led to the motivation of our research on preparation of PMMA-Si[O.sub.2] nanoparticles via DMP. The effect of silica loading and surfactant concentration on monomer conversion, silica encapsulation efficiency and particle size were investigated. The prevulcanized hybrid membrane of a natural rubber latex and PMMA-Si[O.sub.2] nanoparticles was prepared and tested for pervaporation of water-ethanol mixtures.

EXPERIMENTAL

Materials

Nano-Si[O.sub.2] (Aerosil 200, particle size 12 nm) was supplied by Degussa (Thailand). Vinyl trimethoxysilane (VTS, [greater than or equal to] 98%, Sigma-Aldrich) and ammonia solution (25 wt% N[H.sub.4]OH, QReC) was used for silica surface modification. Methyl methacrylate (MMA, [greater than or equal to] 99%, Sigma-Aldrich) was washed with 10 wt% aqueous sodium hydroxide (NaOH, [greater than or equal to] 98%, QReC) to remove the inhibitor and dried over anhydrous magnesium sulfate (MgS[O.sub.4], [greater than or equal to] 98%, EMD). Ammonium persulfate (APS, [greater than or equal to] 99%, Ajax Finechem), sodium dodecyl sulfate (SDS, [greater than or equal to] 99%, Sigma-Aldrich), 1-pentanol ([C.sub.5][H.sub.11]OH, Ajax Finechem), and sodium bicarbonate (NaHC[O.sub.3], [greater than or equal to] 99%, EMD) were used as received. Deionized water was also applied for all polymerization processes.

For NR/PMMA-Si[O.sub.2] nanocomposite membrane preparation, natural rubber latex (NRL) with a total solid content of 60 wt% dry rubber content (DRC), zinc oxide (ZnO), zincdiethyl dithiocarbamate (ZDEC) as vulcanization accelerators and sulfur as vulcanizing agent were purchased from the Rubber Research Institute of Thailand. Ethanol ([C.sub.2][H.sub.5]OH, QReC) was applied for pervaporation of water-ethanol mixtures.

Synthesis of PMMA-Silica Nanoparticles

The nano-silica particles modified with VTS were prepared according to the literature [14]. PMMA-Si[O.sub.2] nanoparticles were prepared by DMP in a three-neck round-bottom flask equipped with magnetic stirrer and reflux condenser. As a typical synthesis, the modified Si[O.sub.2] was dispersed in deionized water with sonication in an ultrasonic bath for 1 h. Then, modified nano-Si[O.sub.2], SDS, APS, NaHC[O.sub.3], and deionized water (30 mL) were charged into the flask. Afterwards, the solution was deoxygenated by bubbling nitrogen gas for 30 min at room temperature. After that the solution flask was immersed in an oil bath at 75[degrees]C, the mixture of MMA monomer (14 mL) and 1-pentanol (0.2 mL) was added very slowly with continuous monomer dropping for 1 h. Finally, the reaction temperature was raised to 80[degrees]C to 85[degrees]C for an additional hour. The PMMA-Si[O.sub.2] emulsion was obtained, then particle size and particle size distribution (PSD) were measured. To measure the monomer conversion gravimetrically, the resulting emulsion was air-dried and the product was washed with acetone to remove residual monomer and free PMMA in the final product. The samples were dried in a vacuum oven at room temperature until constant weight was reached. Monomer conversion was calculated using Eq. 1:

Monomer conversion (%) = ([M.sub.0] - [M.sub.1)/[M.sub.2] x 100 (1)

where [M.sub.0], [M.sub.1] and [M.sub.2] are the mass of the composite particles, the charged silica particles and the charged monomer, respectively.

An acid etching method was used to determine the silica encapsulation efficiency. The composite sample was gradually added to an excess HF solution. The resulting dispersion was dried and the weight percent of the residue was determined gravimetrically. The silica encapsulation efficiency was calculated using Eq. 2:

Silica encapsulation efficiency (%Si encap eff) = [M.sub.ES]/[M.sub.S] x 100 (2)

where [M.sub.ES] and [M.sub.S] are the mass of encapsulated silica and total mass of Si[O.sub.2] in the system, respectively.

Characterization of PMMA-Si[O.sub.2] Nanoparticles

The particle size and PSD of the PMMA-Si[O.sub.2] latex were measured using a dynamic light scattering technique (DLS, Nanotrac 150 particle size analyzer) and reported as the number-average diameter ([D.sub.n]).

The chemical structure of VTS-Si[O.sub.2] and PMMA-Si[O.sub.2] was analyzed by FTIR spectroscopy (Perkin Elmer Spectrum RX I spectrophotometer). Infrared spectra were recorded in the region 4000 to 500 [cm.sup.-1], with a resolution of 0.5 [cm.sup.-1].

The microstructure of the grafted PMMA-Si[O.sub.2] was determined by [sup.1]H nuclear magnetic resonance (NMR) spectroscopy (Bruker 300 MHz spectrometer). The sample solution was prepared by dissolving 20 mg dried PMMA-SiCL in 1 mL of deuterated chloroform (CD[Cl.sub.3]) at room temperature.

The morphology of PMMA-Si[O.sub.2] nanoparticle was examined using a transmission electron microscope (TEM, JEOL JEM-2100) operating at an acceleration voltage of 80 kV. The sample latex was diluted 20 times with deionized water and then, the diluted solution was dropped on a 400-mesh copper grid at room temperature. After that the grid was stained with 1% Os[O.sub.4] prior to analysis to obtain sufficient contrast.

Preparation and Properties of PMMA-Si[O.sub.2] Filled NR Nanocomposite Membranes

For the preparation of NR/PMMA-Si[O.sub.2] nanocomposite membrane, the PMMA-Si[O.sub.2] nanoparticle latex at 10 wt% silica loading was selected to blend with natural rubber latex (NRL, total solid content of 60%) at various weight ratios (NR/PMMA-Si[O.sub.2] = 100/0, 90/10, 80/20, 70/30, 60/40) under a stirring rate of 450 rpm for 30 min at room temperature to form a good dispersion. ZnO (2 phr), ZDEC (1 phr), and sulfur (1.5 phr) were dropped into the system, and then, the mixture was heated to 60[degrees]C with constant stirring at 350 rpm for 2 h. Afterwards the nanocomposite latex was cast on a glass plate (9 cm x 9 cm x 3 mm) and dried at 70[degrees]C for 5 h. The membrane thickness was approximately 0.2 mm measured at five different points using a micrometer.

Thermogravimetric analysis (TGA) was performed with a thermal analysis instrument (Perkin-Elmer Pyris Diamond). A sample (10-20 mg) was placed into a platinum pan. The temperature was raised under a nitrogen atmosphere from room temperature to 800[degrees]C at a heating rate of 10[degrees]C/min with a nitrogen flow rate of 50 mL/min.

Differential scanning calorimetry (DSC, Mettler Toledo 822e) was used to measure the glass transition temperature (Tg). A sample was cooled to -100[degrees]C with liquid nitrogen and heated to 150[degrees]C at a heating rate of 10[degrees]C/min.

The mechanical properties of NR/PMMA-Si[O.sub.2] nanocomposite membranes were evaluated using a Universal Testing Machine (INSTRON 5566) at 500 mm/min of the cross-head speed according to ISO 37. All samples were cut into dumbbell-type specimens (type II) and the average of three measurements of the five specimens was obtained as the representative value.

Scanning electron microscopy (SEM, JSM-7610F) was used to investigate the fracture surface of NR/PMMA-Si[O.sub.2] nanocomposite membranes. The samples were fractured in liquid nitrogen and coated with gold by sputtering.

The contact angle of water was measured using a Standard Goniometer (Rame-Hart Model 200-F1). Water droplets were placed on NR/PMMA-Si[O.sub.2] composites and then, the dimensions of the droplets were examined using the software system. Each measurement was repeated three times and then evaluated for the final results.

Pervaporation

The pervaporation process was carried out using a plate and frame module made of stainless steel. The effective membrane area was 11.34 [cm.sup.2]. The membrane (NR/PMMA-Si[O.sub.2] = 90/10, 80/20, 70/30, 60/40) was put on a stainless steel porous support and contacted with the feed solution for 2 h by circulating the solution from a feed reservoir kept at room temperature. Then, the vacuum was applied to the permeate side and the pervaporation process was operated for an additional 3 h. Permeate was collected in cold traps while the liquid retentate was circulated back to the feed reservoir. The composition of permeate was determined using the calibration curve of the solution compositions versus their absorbance number.

The performance of the membrane for pervaporation was characterized from the total permeate flux, J (g/[m.sup.2]h). Total permeate flux was calculated using Eq. 3:

J = W/(A x t) (3)

where W, A, and t represent the total weight of permeate (g), the effective membrane area ([m.sup.2]), and the operating time (h), respectively.

RESULTS AND DISCUSSION

Characterization of PMMA-Si[O.sub.2] Nanoparticles

For the modified silica surface, the methoxy groups of the VTS coupling agent would be hydrolyzed and condensed with the silanol groups at the silica surface and then the MMA monomer was grafted on the silica surface. Figure 1 illustrates the FT-IR spectra of the VTS-Si[O.sub.2] and PMMA-Si[O.sub.2]. For silica modified with VTS, the absorption bands at 1,113, 805, and 470 [cm.sup.-1] were assigned to the Si-O-Si groups. The absorption peaks at 3,450, 2,926, and 2,850 [cm.sup.-1] corresponded to OH, CH, and C[H.sub.2] stretching of the VTS groups. The peaks at 1,634 and 1,387 [cm.sup.-1] were attributed to C=C stretching and C-H out of plain blending of the VTS group, respectively. These results indicated that VTS silane coupling agents could be bonded with silanol groups of silica to introduce a double bond on the silica surface. For PMMA-Si[O.sub.2] nanocomposite, the absorption peaks at 2,948, 1,731, and 1,449 [cm.sup.-1] were assigned to CH, C=O, and C[H.sub.3] stretching vibration of PMMA, respectively. All results obtained from FT-IR spectra confirmed that the silica nanoparticle has been successfully encapsulated by PMMA via DMP.

Additionally, Fig. 2 presents 'H-NMR spectra of PMMA-Si[O.sub.2] to identify the microstructure. The signal between 3.4 and 3.7 ppm corresponded to the protons of the methyl groups attached to the ester groups of the side chains. The peaks at 0.8 and 1 ppm referred to the protons of the methyl groups attached to the carbon of the backbone of the PMMA-Si[O.sub.2], and the peaks in the range of 1.6 to 2 ppm relate to the protons of the methylene groups from the backbone. The [sup.1]H-NMR spectra confirmed that the PMMA was grafted onto the silica surface.

The mechanism of PMMA-Si[O.sub.2] synthesis is proposed in Fig. 3. The modified silica, SDS (surfactant) and APS (initiator) were dispersed in deionized water to form a homogeneous solution. The surfactant produced the micelles in the solution in which the hydrophilic parts turn toward the aqueous phase and the hydrophobic parts form the core of organic phase. For DMP, the MMA monomer was dropwise fed into the solution and the initiator decomposed into free radicals in the aqueous phase that produced reactive monomer radicals on the silica surface and monomer molecules. Then, these monomer radicals were reacted with the other monomers to form the oligomeric radicals to produce the growing chains until termination resulted in the PMMA-Si[O.sub.2] nanoparticles. Therefore, the PMMA could be grafted onto the silica surface with core-shell morphology.

Effect of Parameters on PMMA-Si[O.sub.2] Preparation

For PMMA-Si[O.sub.2] synthesis using APS as initiator, the effect of initiator concentration on particle size and silica encapsulation efficiency is shown in Fig. 4. It can be seen that the particle size and silica encapsulation efficiency did not significantly change over the range of low initiator concentration (0.61-1.61 wt%). This phenomenon can be explained by two steps. First, oligoradicals were generated in the aqueous phase. Second, oligoradicals could continue to form new particles via the nucleation process. Due to the fact that these two processes controlled the reaction in each step [20]. At a low initiator concentration, the oligoradicals could grow less into particles with a small change size resulting in the particle size of 44 to 46 nm. This showed the similar results as the synthesis of polybutadiene (PB)-Si[O.sub.2] nanoparticles using potassium persulfate in which the particle size changed very slightly at the low initiator amount [21].

SDS was used as surfactant for encapsulation of nanosilica with PMMA at a concentration above the critical micelle concentration (CMC). The surfactant concentration had a significant effect on particle size and silica encapsulation efficiency as illustrated in Fig. 5a. The particle size decreased from 50.0 to 33.7 nm with an increase in the surfactant concentration from 3.34 to 10.34 wt% based on monomer. For DMP, after nucleation, the particles could grow and collide with each other. At low surfactant concentration (or not enough), after collision, the particles would merge together and become larger ones. Therefore, the surfactant concentration increased resulting in a smaller particle size. Furthermore, the silica encapsulation efficiency increased from 75.3% to 98.7% with an increase in the surfactant concentration. This is due to the surfactant being absorbed on the silica surface which then, provided the monomer swollen micelles resulting in the diffusion of hydrophobic monomer onto the silica surface [22]. On the other hand, at a higher surfactant amount, more surface of silica nanoparticles could be provided with monomer micelles which resulted in an increase in silica encapsulation efficiency. From the characteristics of the PMMA-Si[O.sub.2] latex as shown in Fig. 5b, the diameter of latex exhibited a trend of decreasing diameter and more transparent latex was produced with increasing surfactant concentration. Similar results were observed for the synthesis of PB-Si[O.sub.2] nanoparticles [21] and styrene butadiene copolymer (SBR)-Si[O.sub.2] nanocomposites [23].

Moreover, the effect of silica loading on particle size and silica encapsulation with PMMA is presented in Fig. 6a. The silica loading did not significantly affect the particle size of PMMA-Si[O.sub.2]. However, the silica encapsulation efficiency decreased from 81.1% to 29.7% with an increase in the silica loading from 5 to 20 wt% based on monomer. Due to the fact that the high amount of silica loading led to an increase in silica aggregation and decreased the encapsulated silica resulting in a low silica encapsulation efficiency. It showed a similar result as PIP-Si[O.sub.2], PB-Si[O.sub.2], and SBR-SiCF nanocomposites [14, 21. 23]. From the characteristics of PMMA-Si[O.sub.2] nanoparicles as shown in Fig. 6b, the latex did not clearly show a change of appearance with an increase in silica loading. Nevertheless, the DMP of PMMA on modified nano-Si[O.sub.2] could provide PMMA-Si[O.sub.2] nanoparticles with a monodispersion of silica in the PMMA latex. Therefore, this novel method could enhance the compatibility and dispersion of silica in the PMMA matrix, and reduce the silica-silica interaction resulting in a homogeneous PMMA-Si[O.sub.2] nanocomposite latex.

At the optimum condition (APS = 0.61 wt%, SDS = 5.34 wt%, Si[O.sub.2] = 10 wt%) for PMMA-Si[O.sub.2] synthesis by DMP, the high stability of the emulsion with a particle nano-size of 42.6 nm and silica encapsulation of 44.5% were obtained for further blending with NR to form NR/PMMA-Si[O.sub.2] nanocomposite membranes.

Morphology of PMMA-Si[O.sub.2] Nanoparticles and NRIPMMA-Si[O.sub.2] Hybrid Membranes

The morphology of PMMA-Si[O.sub.2] nanocomposites with different surfactant concentrations characterized by TEM is illustrated in Fig. 7. It was observed that the representative TEM micrographs exhibited a core-shell structure at different surfactant concentration, 3.34 wt%, 5.34 wt%, and 10.34 wt%, respectively. The darker areas in the center of particle represented the silica core and the brighter areas represented the PMMA encapsulated onto the silica surface as the shell. Interestingly, the thickness of the shell decreased with increasing surfactant concentration. This can be explained in that with increasing surfactant concentration, the grafting efficiency showed a decreasing tendency resulting in a reduction in the PMMA shell thickness. Nevertheless, the nano-silica particles were well dispersed in the latex and a narrow PSD was achieved for all PMMA-Si[O.sub.2] nanocomposites. It can be concluded that the PMMA-Si[O.sub.2] nanocomposites with core-shell structure have been successfully prepared via DMP which was similar to the synthesis of PIP-Si[O.sub.2] [14], PB-Si[O.sub.2] [21], and SBR-Si[O.sub.2] nanocomposites [23].

PMMA-Si[O.sub.2] emulsion at 10 wt% of silica loading was selected to blend with NR latex and the morphology of NR/ PMMA-Si[O.sub.2] nanocomposites at various PMMA-Si[O.sub.2] loading in nanocomposite membranes was characterized by SEM. The cross-sectional morphology of NR/PMMA-Si[O.sub.2] membranes with various PMMA-Si[O.sub.2] loading (NR/PMMA-Si[O.sub.2] ratio of 100/0, 80/20, and 60/40, equivalent to 0%, 2%, and 4% silica content) are shown in Fig. 8. When the PMMA-Si[O.sub.2] nanocomposite emulsion was blended with the NR matrix, the SEM images of NR/PMMA-Si[O.sub.2] (Fig. 8b and c) show good compatibility between the PMMA-Si[O.sub.2] nanofiller and the NR matrix. This provided evidence that the stronger interaction between PMMA-Si[O.sub.2] nanoparticles and NR latex resulted in a good dispersion of nanoparticles in the NR/PMMA-Si[O.sub.2] composite membranes compared with pure NR membrane.

Thermal, Mechanical, and Surface Properties of NR/PMMA-Si[O.sub.2] Hybrid Membranes

The effects of PMMA-Si[O.sub.2] loading at NR/PMMA-Si[O.sub.2] ratio of 100/0, 90/10, 80/20, 70/30, and 60/40 (equivalent to 0%, 1%, 2%, 3%, and 4% silica content in all nanocomposite membranes, respectively) on thermal, mechanical, and surface properties are summarized in Table 1. The Tg of NR/PMMA-Si[O.sub.2] nanocomposite membranes (-63.1[degrees]C to -63.8[degrees]C) at different blend ratios were about the same value as that of unfilled NR (-63.8[degrees]C) due to the low silica addition. The DSC thermograms of the nanocomposite membranes show a single Tg because of the good dispersion of the PMMA-Si[O.sub.2] emulsion in the NR latex. The encapsulation of silica (core) and PMMA (shell) enhanced the good compatibility and dispersion of silica in the NR latex resulting in the homogeneity of NR/PMMA-Si[O.sub.2] nanocomposite.

The initial decomposition temperature ([T.sub.id]) and maximum decomposition temperature ([T.sub.max]) of the unfilled NR and NR/ PMMA-Si[O.sub.2] nanocomposite membranes are presented in Table 1. The Tnmx of NR/PMMA-Si[O.sub.2] nanocomposite increased from 393.7[degrees]C to 395.2[degrees]C with an increasing PMMA-Si[O.sub.2] loading (1%-2.5% Si[O.sub.2]). This result implies that PMMA-Si[O.sub.2] nanoparticles could be uniformly dispersed in the NR latex resulting in the high thermal stability of the nanocomposite membranes [23]. From the DTG curves of the unfilled NR and NR/PMMA-Si[O.sub.2] nanocomposite membranes (Fig. 9), the main thermal decomposition of NR matrix (C-C chain bonds rupture and hydrogen transfer) was observed. Moreover, the degradation curves of NR/PMMA-Si[O.sub.2] samples are slightly shifted to a higher temperature with the addition of PMMA-Si[O.sub.2] into the NR latex because of the intertwining between one PMMA chain on the silica surface and another PMMA chain on a NR molecule [24-26].

Mechanical properties of PMMA-Si[O.sub.2] filled NR were investigated in terms of tensile strength, modulus at 300% strain and elongation at break. From Table 1, the tensile strength of NR/ PMMA-Si[O.sub.2] nanocomposite membranes with the addition of PMMA-Si[O.sub.2] at 10 wt% (1% Si[O.sub.2]) was slightly higher than the unfilled NR (23.0 MPa). This indicated that the PMMA-Si[O.sub.2] nanoparticles provide a reinforcing effect on natural rubber with a uniform dispersion of silica. In contrast, the tensile strength of nanocomposite membranes was decreased with addition of PMMA-Si[O.sub.2] loading at 30 to 40 wt% (3%-4% Si[O.sub.2]) due to silica aggregation and the low interaction between NR and PMMA-Si[O.sub.2] nanoparticles.

From Table 1, the modulus at 300% strain of PMMA-Si[O.sub.2] filled NR significantly increased with increasing silica concentration. For the blend ratio of 60/40, the modulus at 300% strain of NR/PMMA-Si[O.sub.2] was increased to 5.0 MPa, compared with unfilled NR (1.0 MPa). This is due to the high modulus of PMMA polymer and the reinforcement effect of silica (rigid particle) which reduced the flexibility of rubber chains, thus the material with higher silica content also exhibited higher modulus [27, 28]. The elongation at break of the nanocomposite membranes was decreased with increasing PMMA-Si[O.sub.2] content. It was observed that the sample with 4% silica content presented a low elongation at break (386%) compared with unfilled NR (894%) due to the presence of PMMA polymer as a brittle thermoplastic and the low strain at break [29]. Besides, the addition of silica filler restricted the flexibility of the rubber chains [24]. For the blend ratio of 80/20, a small silica loading (2%) gave a remarkable enhancement in the mechanical properties of membrane.

The water contact angles of the PMMA-Si[O.sub.2] nanocomposite membranes with different PMMA-Si[O.sub.2] loading are also presented in Table 1. The water contact angles decreased with an increase of PMMA-Si[O.sub.2] loading in the NR matrix. This confirmed that the hydrophilic properties of the PMMA-Si[O.sub.2] emulsion had an effect on the hydrophilic membrane surface. Moreover, the high dispersion of silica particles in the membrane could intervene in the tight packing of polymer chains and the diffusion of water molecules through the membranes are also easier [30, 31]. When the PMMA-Si[O.sub.2] loading increased from 0 to 40 wt% (Si[O.sub.2] content = 0-4 wt%), the contact angle of the filled NR surface decreased from 107.8[degrees] to 68.9[degrees]. This result indicated that the reactive hydroxyl groups of the PMMA-Si[O.sub.2] nanoparticle exhibited an enhanced effect on the hydrophilic surface of the NR composite films.

Pevaporation Performance

The separation of an ethanol-water mixture via pervaporation through the NR/PMMA-Si[O.sub.2] hybrid membrane was performed. Pervaporation experiments were carried out using a mixture of ethanol-water at 20 vol% ethanol concentration. The effect of PMMA-Si[O.sub.2] content in membrane on the total permeate flux is presented in Fig. 10. An increase of PMMA-Si[O.sub.2] content from 10 to 40 wt% in the membrane leads to an increase of permeate flux from 1,767 to 2,511 g/[m.sup.2]h. It can be explained that a higher PMMA-Si[O.sub.2] content in the membrane gave more reactive hydroxyl groups resulting in a stronger interaction between water molecules and the membrane. Therefore, more water molecules can pass and diffuse through the membrane [23]. Since the PMMA-Si[O.sub.2] emulsion composed of PMMA homopolymer, free silica, and PMMA encapsulated silica which PMMA was the hydrophilic polymer and silica particles had the hydrophilic groups (-OH) on the surface, adding PMMA-Si[O.sub.2] to NR latex can improve the hydrophilicity of membrane [30, 32]. These results are in accordance with a contact angle measurements as presented in Table 1. The water contact angle decreased with an increasing PMMA-Si[O.sub.2] loading in the composite membrane. This result indicated that the reactive hydroxyl groups of the PMMA-Si[O.sub.2] nanoparticles exhibited an enhanced effect on the hydrophilic surface of the composite film. Surprisingly, the results of the permeation measurement showed [greater than or equal to] 99.9 vol% water concentration in the composition of the total permeate flux. These results indicated that the highly dispersed PMMA-Si[O.sub.2] has an active surface, which could change the membrane structure, resulting in easier permeation of water molecules.

NR/PMMA-Si[O.sub.2] hybrid membranes at PMMA-Si[O.sub.2] content of 20 wt% (2 wt% silica) with good mechanical properties (tensile strength = 23.0 MPa, modulus at 300% strain = 2 MPa, elongation at break = 753%) was selected for separation of ethanol-water mixtures at various water concentrations (60%-100%) via pervaporation experiments. Figure 11 shows the effect of feed compositions on the total permeate flux through the nanocomposite membranes. A significant increase in permeability was observed with increasing feed water concentration and the permeate flux has [greater than or equal to] 99.9 vol% water concentration for all experiments. This result implies that the free volume in the membrane was increased with increasing feed water concentration resulting in highly permeate flux. Moreover, the volume of water molecule is smaller than that of ethanol molecule so that the water molecule can permeate freely [23, 30]. It is interesting to note that the physical properties of NR/PMMA-Si[O.sub.2] hybrid membranes could be improved resulting in a high potential for future applications in membrane separation technology.

CONCLUSIONS

PMMA-Si[O.sub.2] nanoparticles were synthesized via DMP. The silica loading, initiator, and surfactant concentration had an effect on particle size and silica encapsulation efficiency of PMMA-Si[O.sub.2] nanocomposites. A high monomer conversion of 99.9% and PMMA-Si[O.sub.2] nanoparticles with a size range of 30 to 50 nm were obtained at a low surfactant concentration (5.34 wt%). TEM micrographs exhibited a core-shell morphology of PMMA-Si[O.sub.2] nanoparticles. The NR/PMMA-Si[O.sub.2] hybrid membranes were made from a green polymer and used for pervaporation of ethanol-water mixtures. For mechanical properties of NR/ PMMA-Si[O.sub.2] membrane, the tensile strength and modulus of composites membrane were higher than unfilled NR. The membranes exhibited high permeate flux with increasing PMMA-Si[O.sub.2] content and feed water concentration, and high water selectivity for all experiments. Therefore, the PMMA-SiOz nanocomposites could be used as an effective material in future applications.

ACKNOWLEDGMENTS

The authors express their thanks to Dr. Khantong Soontarapa, Mr. Thammanoon Chanchemgpanich, and Ms. Pawinee Joungtangpiti for their assistance throughout this work.

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Dusadee Tumnantong, (1) Garry L. Rempel, (2) Pattarapan Prasassarakich (1)

(1) Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

(2) Department of Chemical Engineering, University of Waterloo, Ontario N2L 3G1, Canada

Correspondence to: P. Prasassarakich; e-mail: ppattara@chula.ac.th or G. L. Rempel; e-mail: grempel@uwaterloo.ca

Contract grant sponsor: Thailand Research Fund (through the Royal Golden Jubilee Project), Graduate School of Chulalongkorn University, Thailand Research Fund; contract grant number: IRG5780001; contract grant sponsor: Natural Sciences and Engineering Research Council of Canada (NSERC).

DOI 10.1002/pen.24611

Caption: FIG. 1. FT-IR spectra of VTS-Si[O.sub.2] (modified nano-silica) and PMMA-Si[O.sub.2]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. [sup.1]H-NMR analysis of PMMA-Si[O.sub.2] is carried out in CD[Cl.sub.3]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Formation mechanism of differential microemulsion polymerization of PMMA-Si[O.sub.2] nanoparticles. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Effect of APS concentration on ((a)) particle size, ((b)) %Si encap eff. Condition: M/[H.sub.2]O = 0.4, Si[O.sub.2] = 5 wt%, SDS = 5.34 wt% base on monomer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Effect of SDS concentration on ((a)) particle size, ((b)) %Si encap eff and characteristic of latex. Condition: M/[H.sub.2]O = 0.4, Si[O.sub.2] = 5 wt%, APS =0.61 wt% base on monomer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Effect of silica loading on ((a)) particle size, ((b)) %Si encap eff and characteristic of latex. Condition: M/[H.sub.2]O = 0.4, SDS = 5.34 wt%, APS = 0.61 wt% base on monomer. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. TEM micrographs and particle size distribution (PSD) of PMMA-Si[O.sub.2] with (a) SDS = 3.34 wt% (b) SDS = 5.34 wt% and (c) SDS = 10.34 wt%. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Cross-section SEM microscopy images of NR/PMMA-Si[O.sub.2] nanocomposite membranes with magnification of 5,000x: (a) pure NR, (b) NR/PMMA-Si[O.sub.2] (80/20), and (c) NR/PMMA-Si[O.sub.2] (60/40).

Caption: FIG. 9. DTG curves for pure NR and NR/PMMA-Si[O.sub.2] nanocomposites. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. Effect of PMMA-silica content in membranes on total permeate flux at 80 vol% water concentration in feed. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Effect of feed water concentration (vol%) on total permeate flux for NR/PMMA-Si[O.sub.2] nanocomposite membranes with PMMA-Si[O.sub.2] content of 20 wt%. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Thermal, mechanical, and surface properties of NR/
PMMA-Si[O.sub.2] nanocomposites membranes.

NR/PMMA-Si[O.sub.2]
(a) (wt/wt)                         100/0                90/10

Si[O.sub.2] content (b)
  (wt%)                               --                  1.0
Thermal properties
  [T.sub.g] ([degrees]C)            -63.8                -63.1
  [T.sub.id] ([degrees]C)           353.9                361.9
  [T.sub.max] ([degrees]C)          382.7                393.7
Mechanical properties
  Tensile strength (MPa)      23.0 [+ or -] 1.0    24.0 [+ or -] 1.1
  300% Modulus (MPa)          1.04 [+ or -] 0.02   1.32 [+ or -] 0.03
  Elongation at break (%)      894 [+ or -] 16      877 [+ or -] 22
Surface properties
  Contact angle ([degrees])   107.8 [+ or -] 3.7   95.5 [+ or -] 4.7
  Water droplet

NR/PMMA-Si[O.sub.2]
(a) (wt/wt)                         80/20                70/30

Si[O.sub.2] content (b)
  (wt%)                              2.0                  3.0
Thermal properties
  [T.sub.g] ([degrees]C)            -63.6                -63.5
  [T.sub.id] ([degrees]C)           362.0                360.7
  [T.sub.max] ([degrees]C)          394.5                394.8
Mechanical properties
  Tensile strength (MPa)      23.0 [+ or -] 0.1    11.1 [+ or -] 0.9
  300% Modulus (MPa)          2.04 [+ or -] 0.05   2.77 [+ or -] 0.11
  Elongation at break (%)      753 [+ or -] 12      593 [+ or -] 05
Surface properties
  Contact angle ([degrees])   83.1 [+ or -] 3.5    75.0 [+ or -] 2.3
  Water droplet

NR/PMMA-Si[O.sub.2]
(a) (wt/wt)                         60/40

Si[O.sub.2] content (b)
  (wt%)                              4.0
Thermal properties
  [T.sub.g] ([degrees]C)            -63.8
  [T.sub.id] ([degrees]C)           360.9
  [T.sub.max] ([degrees]C)          395.2
Mechanical properties
  Tensile strength (MPa)       7.3 [+ or -] 1.1
  300% Modulus (MPa)          4.97 [+ or -] 0.13
  Elongation at break (%)      386 [+ or -] 33
Surface properties
  Contact angle ([degrees])   68.9 [+ or -] 3.0
  Water droplet

(a) PMMA-Si[O.sub.2] preparation condition: M/[H.sub.2]O = 0.4,
Si[O.sub.2] = 10 wt%, SDS = 5.34 wt%, APS =0.61 wt% base on monomer.

(b) Silica content based on total rubber.
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Author:Tumnantong, Dusadee; Rempel, Garry L.; Prasassarakich, Pattarapan
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2018
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