Nanocomposites of NR/SBR Blend Prepared by Latex Casting Method: Effects of Nano-TiO2 and Polystyrene-Encapsulated Nano-TiO2 on the Cure Characteristics, Physical Properties, and Morphology

Nanocomposites of 80/20 (w/w) natural rubber (NR)/styrene butadiene rubber (SBR) blend with four loadings of either nanosized titaniumdioxide (nTiO 2 ) or polystyrene-encapsulated nTiO 2 (PS-nTiO 2 ), ranging from 3 to 9 parts byweight per hundred of rubber (phr), were prepared by latex casting method. The PS-nTiO 2 synthesized via in situ differential microemulsion polymerization displayed a core-shell morphology (nTiO 2 core and PS shell) with an average diameter of 42 nm. The cure characteristics (scorch time, cure time, and cure rate index), mechanical properties (tensile properties, tear strength, and hardness), thermal stability, glass transition temperature, and morphology of the prepared nanocomposites were quantified and compared. The results showed that the cure characteristics of all the nanocomposites were not significantly changed compared to those of the neat NR/SBR blend. The inclusion of an appropriate amount of either nTiO 2 or PS-nTiO 2 into the NR/SBR blend apparently improved the tensile strength, modulus at 300% strain, tear strength, hardness, and thermal stability but deteriorated the elongation at break of the nanocomposites. Based on differential scanning calorimetry, the glass transition temperature of all the nanocomposites was similar to that of the neat NR/SBR blend. Moreover, the morphology of the PS-nTiO 2 -filled rubber nanocomposites fractured surface analyzed by scanning electron microscopy showed an improvement in the interfacial adhesion between the rubber phase and the nanoparticles.

In this study, hybrid nanoparticles comprised of the nTiO 2 core and PS shell were synthesized via in situ differential microemulsion polymerization, which is an efficient process that can produce organic-inorganic hybrid nanoparticles with a core-shell morphology of much smaller diameter (<50 nm) than those obtained from the conventional emulsion polymerization, and requires much lower amount of surfactant than that used in the pristine microemulsion polymerization [6,27,28].However, prior to encapsulating, the nTiO 2 was modified by a polymerizable silane coupling agent aimed at bonding PS and nTiO 2 .This is due to the interaction between -OH groups on the nTiO 2 surface and methoxy groups of the silane compound.The modified nTiO 2 particles were then well dispersed in a styrene monomer, and the process was followed by the in situ differential microemulsion polymerization.The as-prepared hybrid nanoparticles were characterized by means of Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and particle size analysis.Finally, the effects of nTiO 2 and PS-nTiO 2 on the cure characteristics, mechanical properties, thermal stability, glass transition temperature ( g ), and morphology of the 80/20 (w/w) NR/SBR blend nanocomposites were investigated and compared.

Preparation of MPTMS-Modified nTiO 2 .
The MPTMSmodified nTiO 2 (MPTMS-nTiO 2 ) was prepared as follows: 4 g of MPTMS was dissolved in 800 mL of distilled water and the pH was then adjusted to 4.5 using acetic acid in a 1000 mL beaker while stirring for a minimum of 30 min until a clear homogeneous solution was formed.The dry nTiO 2 (13.4 g) was then added to the as-prepared MPTMS solution with stirring for an hour.Thereafter, the mixture was dried in an oven at 120 ∘ C for 12 h.

Synthesis and Characterization of PS-nTiO 2 .
The PS-nTiO 2 was synthesized by in situ differential microemulsion polymerization as follows: distilled water (60 mL), SDS (8 g), AIBN (0.12 g), and the as-prepared MPTMS-nTiO 2 (0.4 g) were mixed in a 500 mL Pyrex glass reactor, which was equipped with a double-jacket condenser, a nitrogen (N 2 ) gas inlet, and a dropping funnel for monomer feeding.The system was heated up to 70 ∘ C with stirring at 200 rpm using a magnetic stirrer under a N 2 atmosphere.Once the temperature reached 70 ∘ C, the styrene monomer (22.5 mL) was fed very slowly, in a dropwise manner over a period of 1.5 h.The reaction system was then maintained at 70 ∘ C with constant agitation for an additional hour and then cooled down to room temperature.
For characterizations, the obtained nanolatex was precipitated with methanol, filtered by Buchner funnel, washed well with distilled water, and then dried at 60 ∘ C for 12 h.The conversion of monomer (% conversion), solid content (% solid), and yield content (% yield) of the prepared PS-nTiO 2 were measured by gravity method and calculated by the following equations: where  0 ,  1 ,  2 ,  3 , and  4 are the weights (g) of PS-nTiO 2 , nTiO 2 , styrene monomer, nanolatex, and mixture, respectively.
The particle size and distribution of the prepared PS-nTiO 2 particles were investigated using a dynamic light scatter analyzer (DLS) with a Nano-series ZX analyzer.
The morphology of the nTiO 2 and PS-nTiO 2 was observed from the TEM images obtained from a Jeol JEM-2100 instrument operated with an acceleration voltage of 80 kV.The sample was stained with 1% (w/v) uranyl acetate for obtaining sufficient contrast.
Functional groups of the samples (nTiO 2 , MPTMS-nTiO 2 , and PS-nTiO 2 ) were analyzed from the FT-IR spectra obtained from the Nicolet 6700 over a frequency range of 4000 to 400 cm −1 .

Preparation and Characterization of Rubber Nanocomposites.
The NR and SBR lattices were mixed with curing ingredients (potassium oleate, potassium hydroxide, sulfur, zinc oxide, and zinc diethyl dithiocarbamate) and Wingstay L at an ambient temperature according to the recipe (based on dry weight) given in Table 1 and stirred at 150 rpm using a mechanical stirrer for 30 min.The obtained rubber blend was then mixed with either nTiO 2 or PS-nTiO 2 at 3, 5, 7, and 9 phr (based on dry weight) under constant agitation at 150 rpm for 30 min.The homogeneous latex was then kept overnight under mild stirring at 60 rpm for maturation.The obtained nanolatex was then cast as a sheet with a uniform thickness (20 × 20 × 0.15 cm 3 ) on a glass mold, allowed to air-dry for 24 h, and subsequently cured in an oven at 110 ∘ C according to the cure time ( 90 ) obtained from the moving die rheometer (MDR; A0225-rheo Tech MD + , Techpro) at 110 ∘ C with the rotating speed at 2 rpm.The obtained MDR rheographs also presented  s2 (time for incipient cure), and the curing rate The tensile test of the samples was carried out at an ambient temperature according to the ASTM D412 standard using an Instron testing machine (Series 5843) with a load cell of 1 kN and at a crosshead speed of 500 mm/min on the standard dumbbell-shaped specimen.At least five specimens were tested to obtain the average value for the tensile strength, modulus at 300% strain (M300), and elongation at break.
The tear strength was measured according to the ASTM D624 (die C) on an angle-shaped sample using the same Instron testing machine at a strain rate of 500 mm/min.At least five specimens were tested to obtain the average value.
Hardness was measured according to the ASTM D2240 using a durometer (Shore Instrument & Mfg.Co.) and expressed as Shore A hardness.
The thermal stability of the samples was evaluated by thermogravimetric analysis (TGA) on a Perkin Elmer Pyris 1 TGA instrument under a N 2 atmosphere at a heating rate of 20 ∘ C/min over the temperature range of 50-800 ∘ C. The thermograms displayed the temperatures for onset ( onset ), end set ( end set ), 50% weight loss ( 50% ), and maximum decomposition ( max ) and % char.
The  g of the samples was examined by differential scanning calorimetry (DSC, Mettler Toledo DSC 1 STAR e ) during the second heating scan from −70 ∘ C to 10 ∘ C under a N 2 atmosphere at a constant heating rate of 5 ∘ C/min.
The morphology of the cryogenic fractured surface (in liquid N 2 ) was observed using scanning electron microscopy (SEM; Jeol JSM-6480LV) under an accelerated voltage of 15 kV with a magnification of 500x.The fractured surface was sputter coated with a thin layer of gold under vacuum prior to analysis.

Results and Discussion
The obtained PS-nTiO 2 prepared by in situ differential microemulsion polymerization had the calculated % conversion, % solid, and % yield of about 94%, 22%, and 96%, respectively.The particle size distribution of the prepared PS-nTiO 2 , as obtained from the DLS analyzer, revealed a unimodal particle size distribution in which most of the particles fall within 17-86 nm diameter range and a mean nanoparticle diameter of 42 nm (Figure 1). Figure 2 shows representative TEM images of the bare nTiO 2 and PS-nTiO 2 particles.The nTiO 2 particles had an irregular but approximately spherical shape with a high degree of agglomeration (Figure 2(a)), due to the large surface area of nTiO 2 particles and the -OH groups on their surfaces as previously mentioned.In contrast, the core-shell structure (Figure 2(b)) with nTiO 2 particles (dark phase) as a core and PS (grey phase) as a shell can be observed after surface modification, suggesting that the nTiO 2 were successfully encapsulated by PS via the in situ differential microemulsion polymerization.It is assumed that the PS shell effectively suppressed or hindered the agglomeration of the hybrid nanoparticles.However, the encapsulation could be confirmed by analyzing the chemical structures of nTiO 2 and then compared with those of the PS-nTiO 2 using FT-IR analysis.
Figure 3 represents the FT-IR spectra of nTiO 2 , MPTMS-nTiO 2 , and PS-nTiO 2 in the range of 4000-400 cm −1 .The peak at 3309 cm −1 shown in the spectrum of nTiO 2 (Figure 3(a)) was contributed to the stretching and bending vibration of the -OH groups on the surface of the nTiO 2 particles, which were active sites for the reaction with methoxy groups of the MPTMS [19,21,23,24], while the broad peak at around 665 cm −1 was assigned to the vibration absorption peak of Ti-O and Ti-O-Ti bonds [19,21,23,24].By contrast, the additional peaks at 2923, 1726, 1641, 1138, and 1037 cm −1 seen in the spectrum of MPTMS-nTiO 2 (Figure 3(b)) corresponded to the stretching vibration of C-H, C=O, C=C, Si-O, and Ti-O-Si bonds, respectively [19][20][21][24][25][26], indicating the binding of methoxy groups in MPTMS to the -OH groups on the nTiO 2 surfaces to introduce double bonds onto the nTiO 2 surfaces, which can further react with the styrene monomer.For the PS-nTiO 2 (Figure 3(c)), the FT-IR spectrum shows the characteristic peaks of PS at 3024 (C-H arom), 2917 and 2850 (-CH 2 -CH 2 ), 1596 (C=C arom), 1492 and 1448 (-C 6 H 5 ), and 908 and 698 (-CH= arm) cm −1 , while the peaks at 1068 and 543 cm −1 were due to the vibration of Ti-O-Si and Ti-O-Ti bonds, respectively [19,21,23,24,26,27], indicating the existence of PS on the nTiO 2 particle surface.This confirmed the encapsulation of nTiO 2 particles by PS with the aid of MPTMS coupling agent through the free radical copolymerization of styrene monomers with methacrylate groups of MPTMS that chemically bonded with the nTiO 2 core via in situ differential microemulsion polymerization.
s2 and  90 obtained from the MDR rheographs and the CRI of the 80/20 (w/w) NR/SBR blend and its nanocomposites filled with 3, 5, 7, or 9 phr of the respective nanoparticle fillers (nTiO 2 and PS-nTiO 2 ) are summarized in Table 2.  s2 is a measure of the time to premature vulcanization while  90 is the optimum cure time of the vulcanizates.It can be seen that  s2 of all the nanocomposites was almost the same as that of the neat 80/20 (w/w) NR/SBR blend, indicating that the inclusion of either nTiO 2 or PS-nTiO 2 at a very low loading showed a little or no effect on  s2 of the nanocomposites.For the nTiO 2 -filled rubber nanocomposites,  90 increased by 3.2 and 3.6 min with increasing TiO 2 contents to 3 phr and 5 phr, respectively, compared to that of the neat NR/SBR blend.This is attributed to the absorption of accelerator by the -OH groups on the nTiO 2 surface, which consequently lowered the accelerator activity and slowed down the sulfur vulcanizing reaction, resulting in the delayed  90 of the nanocomposites [15,29].The existence of the surface -OH groups was previously confirmed by the FT-IR analysis.However, at higher loading levels of nTiO 2 (7 and 9 phr), the  90 values gradually decreased owing to the agglomeration of the nTiO 2 that covered some of the -OH groups and so lowered the amount of accelerator absorption.For the PS-nTiO 2 -filled rubber nanocomposites,  90 was slightly longer (1.7-2.9 min) than that of the neat NR/SBR blend and did not change significantly with increasing PS-nTiO 2 contents.In addition, the nTiO 2filled rubber nanocomposites exhibited a higher increase in  90 than the PS-nTiO 2 -filled rubber nanocomposites when compared at the same filler loading.This may be due to the PS shell that masked some of the -OH groups and also lowered the amount of accelerator absorption.The CRI of all the nanocomposites was slightly lower than that of the neat NR/SBR blend (Table 2), again due to the accelerator absorption by the nanofillers as previously mentioned.

Mechanical Properties.
The experimental data related to the mechanical properties (tensile strength, M300, elongation at break, tear strength, and hardness) of the 80/20 (w/w) NR/SBR blend and its nanocomposites filled with 3, 5, 7, and 9 phr of the respective nanoparticle fillers (nTiO 2 and PS-nTiO 2 ) are shown in Figures 4-6 and Table 3.It can be observed that all the nanocomposites have higher tensile strength, M300, tear strength, and hardness and lower elongation at break than the neat blend.This may be due to the reinforcing capability of nTiO 2 and PS-nTiO 2 caused by the binding force generated between the interface of nanoparticles and rubber matrix.The tensile strength of neat NR/SBR blend was 24.9 MPa.For the nTiO 2 -filled rubber nanocomposites, the tensile strength increased to the highest value (35.5 MPa) at 5 phr of nTiO 2 , which was 42.6% increase.The increased tensile strength was achieved because the nTiO 2 were finely dispersed in the rubber matrix and the stress was highly transferred across the interface of the nTiO 2 and  NR/SBR matrix.However, the tensile strength was considerably decreased to 27.5 MPa upon addition of nTiO 2 at 7 phr and then slightly decreased to 26.3 MPa at 9 phr (Figure 4(a)).This may be attributed to the self-agglomeration of nTiO 2 particles at high loadings that then lowered the filler-rubber interaction.In contrast, the tensile strength of the PS-nTiO 2filled rubber nanocomposites increased up to the highest value of 36.7 MPa (47.4% increase) at 3 phr of PS-nTiO 2 , followed by the substantial decrease to 28.5, 27.2, and 26.2 MPa at 5, 7, and 9 phr, respectively.The initial increment in the tensile strength at 3 phr filler was due to the PS molecules on the nTiO 2 surfaces that improved not only the dispersion of nTiO 2 particles but also the filler-rubber interfacial adhesion by chain entanglement of PS molecules of the PS-nTiO 2 particles and the SBR, which give rise to the additional crosslinks in the network structure that tend to hold themselves tightly.Thereafter, a reduction in the tensile strength at higher loading levels of PS-nTiO 2 (5-9 phr) may be due to the selfagglomeration of the excess PS-nTiO 2 particles located at the phase boundary of NR and SBR (because the PS-nTiO 2 particles were concentrated in the SBR phase), resulting in an incomplete network structure and poor crosslinking across the NR and SBR phase boundary.Similar trend was also observed for the tear strength as discussed later.
All nanocomposites containing the respective nanoparticle filler (nTiO 2 and PS-nTiO 2 ) exhibited a dose-dependent increase in the M300 over that of the neat 80/20 (w/w) NR/SBR blend with increasing filler levels (3-9 phr) from 3.5 MPa of the neat blend to 4.1-5.7 MPa (17-62.8%increase) for nTiO 2 and 4-4.9 MPa for PS-nTiO 2 (14.3-40% increase) (Figure 4(b)).This suggests that the incorporation of nanofillers even at a very low content can effectively reduce the mobility of rubber chains and increase the stiffness of the resulting nanocomposites.However, the increased M300 induced by the addition of nTiO 2 was slightly higher than that induced by the inclusion of PS-nTiO 2 .This could be because the PS-nTiO 2 -filled rubber nanocomposites possessed lower nTiO 2 content than the nTiO 2 -filled rubber nanocomposites when compared at the same nanofiller loadings.In addition, the PS shell could provide higher free volume in the rubber matrix and thus facilitated the segmental movement.Figure 4(c) shows a decrease in the elongation at break of the 80/20 (w/w) NR/SBR blend with increasing inclusion levels of the respective nanoparticle fillers (nTiO 2 and PS-nTiO 2 ).The elongation at break was decreased from 1717.6% of the neat NR/SBR blend to 1333.2-1629.4% (5.1-22.4% decrease) and 1523.6-1586.4% (7.6-11.3%decrease) by the inclusion of 3-9 phr of nTiO 2 and PS-nTiO 2 , respectively.This observation is expected since the stiff nanoparticles would act as restriction sites for the rubber chain movement and so reduce the elongation at break of the nanocomposites.However, the elongation at break of the neat NR/SBR blend is largely retained due to the very low loading of the nanofiller particles, and so the prepared vulcanizates were flexible nanocomposites.In addition, as the nanofiller loading was beyond 3 phr, the elongation at break of the PS-nTiO 2 -filled rubber nanocomposites was higher than that of the nTiO 2 -filled rubber nanocomposites when compared at the same filler loading.This could also be due to the lower nTiO 2 content and the higher free volume in the matrix of the PS-nTiO 2filled rubber nanocomposites that facilitated the segmental movement as mentioned above.
The tear strength of the 80/20 (w/w) NR/SBR blend and its nanocomposites containing 3-9 phr of nTiO 2 and PS-nTiO 2 exhibited the same trend with the tensile strength.All the nanocomposites have higher tear strength than the neat blend (Figure 5).The tear strength of the neat NR/SBR blend was about 77.5 N/mm.For the nTiO 2 -filled rubber nanocomposites, the tear strength increased to a maximum value (121.6 N/mm) at 5 phr, which was about 56.9% higher than that of the neat NR/SBR blend.However, the tear strength considerably decreased to 98 N/mm upon addition of nTiO 2 at 7 phr and then slightly decreased to 94.4 N/mm at 9 phr (Figure 5).In contrast, the tear strength of the PS-nTiO 2 -filled rubber nanocomposites increased up to the highest value of  130.6 N/mm (68.5% increase) at 3 phr of PS-nTiO 2 , followed by the substantial decrease in the tear strength to 100.7, 85.3, and 83.5 N/mm at 5, 7, and 9 phr, respectively.This may be due to the same reasons as previously discussed in the results of the tensile strength.
Finally, all the nanocomposites exhibited improved hardness as compared to the neat 80/20 (w/w) NR/SBR blend (Figure 6), and the hardness was increased with increasing loading of nanofillers as expected due to the restriction of segmental motion of the rubber chains.The hardness increased from 46.7 Shore A of the neat NR/SBR blend to 50.3-57.7Shore A (7.7-23.6%increase) and 55.7-65.5 Shore A (19.3-40.3%increase) by the inclusion of 3-9 phr of nTiO 2 and PS-nTiO 2 , respectively.As can be seen, the increased hardness induced by the addition of PS-nTiO 2 was much higher than that induced by the inclusion of nTiO 2 .This suggested that the nTiO 2 particles were embedded in the rubber matrix, while the PS-nTiO 2 particles can be located on the surface of the nanocomposites.

Thermal Behaviors.
The thermal stability in terms of  onset ,  50% ,  end set ,  max , and % char of the samples was studied by TG analysis.The representative TGA curves of the 80/20 (w/w) NR/SBR blend and its nanocomposites containing 3, 5, 7, and 9 phr of either nTiO 2 or PS-nTiO 2 are shown in Figure 7 and the degradation temperatures are listed in Table 4.The degradation of all samples occurred in one stage with the maximum weight loss temperature ( max ) at around 400 ∘ C as obtained from the derivative thermogravimetric (DTG) thermograms (not shown here).This indicated that both nTiO 2 and PS-nTiO 2 had no effect on the degradation mechanism of the NR/SBR blend but only affected the degree of degradation.However, the degradation temperatures revealed that the addition of either nTiO 2 or PS-nTiO 2  exhibited a dose-dependent increase in the thermal stability over that of the neat NR/SBR blend by shifting  onset ,  50% , and  end set towards higher temperatures.This may be caused mostly by the binding force between the interface of the nanofillers and rubber matrix and consequently restricted the mobility of the rubber chains.In addition, both nTiO 2 and PS-nTiO 2 have much higher thermal stability than both NR and SBR, thus consuming some of the heat and acting as a heat barrier in the thermal decomposition process [14,17,19,20].The nTiO 2 lost only 2.1 wt% at 800 ∘ C [20].As expected, the char formation of all the nanocomposites was higher than that of the neat NR/SBR blend (Figure 7).This is because the residue left in the system was mostly inorganic nanofillers.Moreover, the increased char formation may delay and reduce the thermal degradation of the samples by restricting the diffusion of volatile degradation products out of the samples [2,6,14,20].the presence of nanofillers had very little influence on  g of the nanocomposites.

Morphology.
The morphology of the 80/20 NR/ SBR blend and its two nanocomposite series was studied using SEM.Representative images of all the samples are shown in 9.All the images revealed a relatively rough and uneven fractured surface, indicating that the samples failed in a ductile manner, which is in good agreement with the result of the elongation at break.However, the PS-nTiO 2filled rubber nanocomposites exhibited rougher fractured surfaces (Figures 9(f)-9(i)) compared with the neat NR/SBR blend (Figure 9(a)) and the nTiO 2 -filled rubber nanocomposites (Figures 9(b)-9(e)).This may be due to the better interaction between the PS-nTiO 2 particles and the rubber matrix.

Figure 1 :
Figure 1: Particle size distribution of the PS-nTiO 2 prepared via in situ differential microemulsion polymerization.

Table 1 :
Formulation of the sulphur-curing system for the rubber compound (based on dry weight).

Table 2 :
Cure characteristics of the rubber compounds.