PC Blends with Different Strategies

Mar 13, 2014 - However, the thermal and mechanical properties of blends were improved noticeably upon the addition of nanoparticles. Finally, the mech...
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Compatibilization of PMMA/PC Blends with Different Strategies: Transesterification Catalyst versus Nanoparticles Shuting Xi, Yajiang Huang,* Qi Yang, and Guangxian Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering of China, Sichuan University, Chengdu 610065, China ABSTRACT: The efficiency of tin(II) 2-ethylhexanoate catalyst and hydrophilic silica nanoparticles in compatibilizing polycarbonate/poly(methyl methacrylate) (PMMA/PC) blends were compared in terms of the morphological, thermal, and mechanical properties. Both the catalyst and nanoparticles were found to refine the morphology of PMMA/PC blends. Although the blends with catalyst exhibited smaller phase size, they possessed more deteriorated mechanical performance and worse thermal properties than those of nanoparticle-filled ones due to the significant degradation during transesterification reactions. The addition of nanoparticles refined the morphology of PMMA/PC blends kinetically to a lesser extent. However, the thermal and mechanical properties of blends were improved noticeably upon the addition of nanoparticles. Finally, the mechanisms, advantages, and disadvantages of catalyst and nanoparticles in compatibilizing PMMA/PC blends were discussed and compared.

1. INTRODUCTION A blend of polycarbonate (PC) and poly(methyl methacrylate) (PMMA) is of great industrial and academic interest because this blend not only possesses the toughness and photodegradation stability of PC, but also bears the merit of PMMA in cost. However, the blending of PC and PMMA usually leads to phase-separated mixtures with poor interfacial adhesion and therefore inferior mechanical properties. For this reason, different compatibilization strategies, such as addition of premade copolymers and reaction compatibilization, have been proposed to promote the compatibility of the two phases.1−5 The transesterification reaction during melt blending, in which copolymers are formed in situ at the interface of blends, has long been recognized as an efficient method in improving the miscibility of polyester blends.6−10 Usually, the exchange reaction leads initially to the formation of block or graft copolyesters.11 As the reaction proceeds or more catalyst is added, however, copolyester molecules with increasing degree of randomness can form progressively.12,13 Residual catalysts contained in polymeric components after synthesis, such as those in poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), could also initiate the transesterification reaction.14,15 However, they usually need longer reaction times or higher blending temperatures, and the reaction rate is also slower compared to that for the same blends with added catalysts. The reaction process and the molecular structure of copolymers formed by the transesterification reaction are difficult to control since various factors, such as the type of catalysts, catalyst concentration, and mixing time, are involved in this complex process. Recently, nanoparticles (such as organic-modified clay, carbon black, and silica nanoparticles) with various shapes, sizes, and surface chemistries have also drawn great attention in compatibilizing polymer blends.16−18 The influence of nanoparticles on the transesterification in polyester blends is of particular interest19 since two distinct compatibilization mechanisms may work together to improve the propeties of © 2014 American Chemical Society

these reactive blends. Some studies have shown that nanoparticles may influence the transesterification reaction. It was reported that the transesterification reaction in thermotropic liquid crystalline polymer (TLCP)/PC blends was suppressed by hydrophobic silica nanoparticles20 which reduced the collision frequency between PC and TLCP molecules at the interface. The condensation reaction between the surface hydroxyl groups of hydrophilic silica nanoparticles and the hydroxyl end group of polyesters was also found to inhibit the transesterification of PET/PBT blends during melt mixing.19 As for PMMA/PC blends, Ray et al.21,22 found the size of the dispersed phase declined upon the addition of organoclay. The compatibilization mechanism was ascribed to the selective location of organoclay within the PMMA matrix, which significantly inhibited the droplet coalescence process, since no exchange reactions were detected. Singh et al.23 also found no graft copolymer formed in C15A nanoclay-filled PMMA/PC blends. Our previous studies have shown that fumed silica nanoparticles with different surface chemistries can refine the morphology of polyisobutene (PIB)/polydimethilsiloxane (PDMS) blends,24−26 polypropylene (PP)/polystyrene (PS) blends,27 and PS/poly(vinyl methyl ether) (PVME).28 However, the potentials of these silica nanoparticles in compatibilization of reactive polyester blends such as PMMA/PC blends have not been assessed. The purpose of this work was to present experimentally a comprehensive comparison of the compatibilization efficiency of the transesterification catalyst (namely reactive compatibilization) and nanoparticles (physical compatibilization) to PMMA/PC blends. The morphological, thermal, and mechanical properties were assessed using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Received: Revised: Accepted: Published: 5916

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(NMR) spectrometer (Avance II-600 MHz, Bruker) were used to investigate transesterification possibly occurring during melt blending.11 FTIR spectra were recorded at room temperature at a resolution of 4 cm−1. Part of the samples was immersed in acetone to remove the PMMA phase for at least 6 days before analysis. Samples for NMR were directly dissolved in the CDCL3 for study at room temperature. Tetramethylsilane was used as the internal standard. Tensile tests of samples were carried out using an Instron 4302 universal tensile test to determine the modulus, yield strength, and elongation at the break. Dumbbell-shaped specimens were compression molded and analyzed at a strain rate of 20 mm/min at room temperature. The values presented were the average of five individual measurements.

thermogravimetric analysis (TGA), and universal tensile testing, respectively. Fourier transform infrared spectroscope (FTIR) and 1 H nuclear magnetic resonance (NMR) spectrometer were used to determine the compatibilization mechanism in PMMA/PC blends upon the addition of catalyst or nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample preparation. Polycarbonate (PC) (K1300) and poly(methyl methacrylate) (PMMA) (IF 850) were purchased from Teijin Chemical Co. Ltd. (Japan) and LG Chemical (Korea), respectively. The parameters of materials are given in Table 1.PC was dried under vacuum at 80 Table 1. Characteristics of Polymer Components materials

Mw (g/mol)

Mw/Mn

Tg (°C)

PC PMMA

5.33 × 104 8.53 × 104

1.79 2.04

155.6 104.7

3. RESULTS AND DISCUSSION 3.1. Compatibilization Effect. Blend Morphologies. Figure 1 shows that PMMA/PC (20/80) blends with and

°C for 48 h, whereas PMMA was dried at 60 °C under vacuum for 12 h. Tin(II) 2-ethylhexanoate with a molecular weight of about 405.12 g/mol was purchased from Aldrich and used as the catalyst to promote the transesterification reaction in PMMA/PC blends. Hydrophilic fumed silica nanoparticles (Aerosil A200) with a primary diameter of 12 nm and specific surface area of 200 ± 25 m2/g, supplied by Degussa Corp, were used as fillers for PMMA/PC blends. A200 nanoparticles were dried under vacuum at 120 °C for 8 h prior to use. PMMA and PC were mixed with an internal mixer (XSS300) at 230 °C and 50 rpm for 6 min at a weight ratio of 20:80. For PMMA/PC blends filled with A200 nanoparticles, the nanoparticle content was 1, 3, and 5 wt %, respectively. As the catalyst has a high activity in inducing transesterification reaction in the blends, the catalyst content in the blends was 0.05, 0.10, 0.15, 0.2, 0.4, and 0.6 wt %. 2.2. Characterization. The morphology of PMMA/PC blends was characterized using a scanning electron microscope (SEM, JEOL SJM-5900VL) operated at an accelerating voltage of 20 KV. The specimen was first fractured in liquid nitrogen and then etched using acetone to remove the PMMA phase in order to create a better contrast. Then the sample was coated with a thin gold layer and investigated using SEM. The number average diameter of PMMA droplets was determined using the ImageJ (NIH, U.S.A.) software, according to the following equation, Dn =

∑ niDi /∑ ni

Figure 1. SEM images of PMMA/PC (20/80) blends in the presence of different contents of catalyst and A200 nanoparticles.

(1)

without tin(II) 2-ethylhexanoate catalyst or hydrophilic silica nanoparticles all exhibit heterogeneous phase-separated morphology. Pure PMMA/PC blend without catalyst shows large PMMA droplet size, and the size distribution is also broad. However, upon the addition of 0.2 and 0.4 wt % catalyst, there is a dramatic decrease in PMMA domain size. It is worth noting that the overall droplet size in the blend with 0.6 wt % catalyst is slightly larger than that of blend with 0.4 wt % catalyst. The droplet sizes in the A200-filled blends are significantly decreased up to 3 wt % nanoparticles compared to the unfilled blends. However, for the blend filled with 5 wt % A200 nanoparticles, the droplet size becomes larger compared to that in the 3 wt % nanoparticle-filled system, but it is still much smaller than the droplet size of neat blends. Figure 2 shows the number average diameters of the PMMA droplets versus

where ni represents the number of droplets with diameter Di. The glass transition temperatures (Tg) of materials were determined using a differential scanning calorimeter (DSC Q20, TA Instruments, U.S.A.) in the temperature range of 40− 200 °C under a nitrogen atmosphere with a heating rate of 10 °C/min. To eliminate the influence of sample preparation history, the Tg was taken from the second scan. The thermal stability of PMMA/PC blends with catalyst or nanoparticles was studied by a thermogravimetric analyzer (TGA 209F1, Netzsch). Samples were heated from room temperature to 800 °C under nitrogen atmosphere at a heating rate of 10 °C/min. Fourier transform infrared spectroscopy (FTIR; Nicolet 6700, Thermal Fisher, U.S.A.) with an attenuated total reflection accessory (ATR) and 600 MHz 1H nuclear magnetic resonance 5917

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Figure 2. Number average diameter of PMMA phase as a function of the (a) catalyst and (b) A200 nanoparticle content.

reducing the droplet size of PMMA/PC blends compared to that of A200 nanoparticles. Thermal Properties. Figure 4 shows the DSC results of PMMA/PC (20/80) blends with various contents of tin(II) 2ethylhexanoate catalyst and A200 nanoparticles. Table 2 summarizes the glass transition temperature (Tg) of corresponding blends. The Tg of PC and PMMA shift toward each other in the neat blends compared with the Tg of pure PC (155.6 °C) and PMMA (104.7 °C), indicating the existence of limited miscibility between PC and PMMA. The change in the Tg of PMMA-rich phase (elevated by ∼16 °C) is much larger than the decrease in the Tg of PC-rich phase (∼2.7 °C). This asymmetric change in Tg can be attributed to the relatively higher solubility of PC in PMMA phase.29 Upon the addition of 0.2 wt % catalyst, the Tg of two components shifts further toward each other. Eventually, they merge into a single one when the catalyst content increases to 0.4 and 0.6 wt %. However, the presence of a single Tg does not mean that these blends are completely miscible since an obvious two-phase structure is presented in these blends (Figure 1). For PMMA/ PC blends filled with A200 nanoparticles (Figure 4b), the Tg of both components shows no distinct difference from that of pure blend. This result suggests that the compatibilization mechanism in blends with A200 nanoparticles should be a kinetic one rather than a thermodynamic one.27,30 Also, the DSC results suggest that A200 nanoparticles may not induce a transesterification reaction in PMMA/PC blends. The TGA results of PMMA/PC blends with catalyst and nanoparticles are shown in Figure 5. The onset degradation

catalyst or nanoparticle concentration. As the catalyst content increases, the average size of PMMA droplets reduces from approximately 1.34 to 0.23 μm when 0.4 wt % catalyst is added. In contrast to the pristine blends, the dispersed phase size of nanoparticle-filled blends reduces to about 0.61 μm in the presence of 3 wt % A200 nanoparticles. The different efficiencies of catalyst and nanoparticles in morphological refinement can also be perceived according to the change in the optical transparency of compression-molded blend samples in Figure 3. The improved transparency of blend specimens with

Figure 3. Photographs of different PMMA/PC (20/80) blends prepared at 230 °C: (a) pure blends; (b) blends with 0.2 wt % catalyst; (c) blends with 3 wt % A200 nanoparticles. Please note that compression samples of blends with catalyst content higher than 0.2 wt % cannot be prepared due to their brittle nature, and corresponding photographs are therefore not provided here.

thickness about 0.5 mm upon the incorporation of catalyst and A200 nanoparticles clearly demonstrates that the characteristic size of the blend is refined and is comparable to the wavelength of visual light. However, the specimen with catalyst possesses a higher transparency than that of nanoparticle-filled ones. The above results reveal that the catalyst has a higher efficiency in

Figure 4. DSC results of PMMA/PC (20/80) blends with different loadings of (a) catalyst and (b) A200 nanoparticles. 5918

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Table 2. Glass Transition Temperature Tg (°C) of PC and PMMA in Neat and Compatibilized Blends filler content (wt %)

catalyst content (wt %)a PC PMMA a

component

blend

0.2

0.4

0.6

1

3

5

155.6 104.7

152.4 118.5

150.9 119.2

126.8 

126.1 

152.5 120.9

152.5 119.9

153.2 119.9

Note: “”represents that only one glass transition temperature is found.

Figure 5. Weight loss curves of PMMA/PC (20/80) blends upon heating at 10 °C/min in nitrogen atmosphere: (a) blends with catalyst; (b) blends with nanoparticles.

Table 3. TGA Data of PMMA/PC Blends filler content (wt %)

catalyst content (wt %)a Tonset Tp, PC Tp, PMMA a

0

0.2

0.4

0.6

1

3

5

363.4 510.4 377.4

364.6 476.5 380.6

350.9 436.4 −

349.7 435.1 −

366.3 509.2 381.8

368.7 516.7 378.7

367.5 517.9 380.6

Note: “−” shows that the blend exhibits one-step thermal decomposition behavior.

that A200 nanoparticles may locate mainly in the PC matrix. Therefore, the dramatic increase in the Tp of PC compared to that of the pristine blend may be related with the restricted chain dynamics or the absorption of volatile products during degradation in the presence of A200 nanoparticles.31 The above results suggest that the thermal stability of nanoparticle-filled PMMA/PC blends is much superior to that of blends with tin(II) 2-ethylhexanoate catalyst. Mechanical Properties. The effect of tin(II) 2-ethylhexanoate catalyst and A200 nanoparticles on the tensile properties of PMMA/PC blends is depicted in Figure 6. Figure 6a shows that pure PMMA/PC blends exhibit ductile behavior with a yield point. As blends with high contents of catalyst become very brittle and thereby are difficult to be compression molded into samples for mechanical measurements, only blends with 0.05, 0.1, and 0.15 wt % catalyst are considered here. With 0.05 wt % catalyst, the blends still exhibit ductile behavior. However, the blends in the presence of 0.1 and 0.15 wt % catalyst experience brittle failure. Conversely, ductile failure was always detected for blends with different silica nanoparticle loadings. Figure 6c displays that the elongation-at-break and modulus of blends increase slightly upon the addition of 0.05 wt % catalyst. However, as the catalyst loading increases to 0.10 wt % and 0.15 wt %, a large reduction in the elongation-atbreak of blends is observed. The diverse phenomena in blends with different catalyst loadings may be ascribed to the competitive contribution of copolymer compatibilization and catalyst-accelerated thermal degradation of the PC component.

temperatures (Tonset) determined as the temperature at which 5% weight loss occurs and the temperatures with maximum rate of weight loss (Tp) determined as the peak value of the first derivative of the TGA curves are presented in Table 3. The neat blends and blends with 0.2 wt % catalyst display two thermal decomposition processes, corresponding to that of PC and PMMA, respectively. The onset degradation temperature decreases dramatically from 363.4 to 349.1 °C with increasing catalyst contents. Specifically, upon the addition of only 0.2 wt % catalyst, the Tp of PC decreases significantly from 510.4 to 476.5 °C while the Tp of PMMA does not show much difference. This result suggests that the tin(II) 2-ethylhexanoate catalyst could dramatically accelerate the thermal degradation of PC.11 This situation becomes more obvious when the catalyst loading become higher. Moreover, only a single thermal decomposition step is found in blends with 0.4 wt % and 0.6 wt % catalyst. Similar accelerated thermal degradation of PC matrix should also occur during the melt mixing process, which may lead to materials with lower Tg and thus poorer mechanical properties. For PMMA/PC blends filled with nanoparticles, however, a two-step thermal decomposition mechanism is always observed in the whole nanoparticle concentration considered. Moreover, contrary to blends with catalyst, the Tonset of nanoparticle-filled blends retains nearly invariant for all nanoparticle contents. Also, the Tp of PC elevates gradually with increasing nanoparticle loading. At the same time, the Tp of PMMA in PMMA/PC blends nearly does not change. This result suggests 5919

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Figure 6. Typical stress−strain curves for PMMA/PC (20/80) blends with (a) tin(II) 2-ethylhexanoate catalyst and (b) A200 nanoparticles, and tensile properties of PMMA/PC (20/80) blends with various concentrations of (c) catalyst and (d) A200 nanoparticles.

For blends with 0.05 wt % catalyst, the former may be dominant, whereas for blends with higher catalyst loadings the latter, which will lead to a serious decline in the molecular weight of PC, may be decisive. Curves b and d of Figure 6 show that A200 nanoparticles can improve the tensile modulus and the elongation-at-break of PMMA/PC blends simultaneously up to 3 wt % nanoparticle loading. Especially, the elongation-atbreak increases noticeably from 25% to 50% upon the incorporation of 3 wt % nanoparticles. However, as the nanoparticle content further increases to 5 wt %, a dramatic reduction in the elongation-at-break is found. It is well-known that due to the large surface area and high surface energy of nanoscale particles, nanoparticles with high loadings tend to form large aggregates in polymer melts during mixing, which are usually detrimental to the mechanical properties of materials.32 The large reduction in the elongation-at-break of blends with 5 wt % nanoparticles should also stem from the poor dispersion of nanoparticles at high loadings. 3.2. Comparison of Compatibilization Mechanism. 3.2.1. Blends with Catalyst. The changes in morphological, thermal, and mechanical properties of PMMA/PC blends suggest that the catalyst possesses a high efficiency in compatibilizing this blend. To detect the possible transesterification between PC and PMMA during melt blending at 230 °C, FTIR and 1H NMR measurements were performed. Figure 7 depicts the FTIR spectra of PMMA/PC blends with and without catalyst together with their insoluble portion, which should be PC or PC-g-PMMA copolymer (if it exists). For comparison, FTIR spectra of neat PC and PMMA are also given. There exist two clearly distinguishable carbonyl stretching vibration peaks corresponding to PC (1769 cm−1) and PMMA (1722 cm−1) in PMMA/PC (20/80) blends. The

Figure 7. FTIR spectra of PC, PMMA, PMMA/PC (20/80) blend, blend with 0.4 wt % catalyst, and insoluble fraction of blend in acetone. The spectra shown are the enlarged part of the carbonyl region.

carbonyl stretching vibration peaks at 1722 cm−1 corresponding to PMMA disappeared after the PMMA phase was selectively extracted using acetone, indicating that no transesterification occurred in PMMA/PC blends when directly mixed at 230 °C. However, Figure 7 demonstrates that the carbonyl stretching vibration peaks at 1722 cm−1 still exist in the FTIR spectrum for the insoluble portion of blends with 0.4 wt % catalyst after extraction. These results suggested that transesterification reaction occurs between the carbonate and ester groups of PC and PMMA during melt blending, leading to the formation of PC-g-PMMA graft copolymers.33 The formation of graft copolymer could also be perceived by the change in the 1H NMR spectra of PMMA/PC blends upon the addition of catalyst as shown in Figure 8. Figure 8a shows 5920

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Figure 8. 1H NMR spectra for (a) pure PMMA, (b) PC, (c) PMMA/PC blends and (d) PMMA/PC blends with 0.4 wt % tin catalyst.

Figure 9. (a) FTIR spectra of PC, PMMA, PMMA/PC blend with 3 wt % A200 and its fraction insoluble in acetone. The spectra shows the magnification of the carbonyl region. (b) 1H NMR spectra for blends with 3 wt % A200 nanoparticles.

the 1H NMR spectra of neat PMMA, in which three different peaks at 0.841, 1.620, and 3.601 ppm corresponding to methyl (H(1)), methylene (H(2)), and methyl ester (H(3)) are exhibited, respectively. PC has two different proton environmental peaks at 1.680 and 7.1−7.3 ppm on the 1H NMR spectrum corresponding to the aromatic ring (H(1), H(2)) and the methyl group (H(3)) as shown in Figure 8b. The presence of some small peaks is ascribed to the influence of solvent. Figure 8c shows that the peak for the methyl ester proton of PMMA still locates at 3.601 ppm for the PMMA/PC blends. As no new peak is found in the 1H NMR spectra of PMMA/PC blends, the blending of PC and PMMA at 230 °C without catalyst should not lead to a transesterification reaction. This

result is consistent with previous reports that the transesterification in PMMA/PC blends took place only above 270 °C.34,35 As a result, the neat PMMA/PC (20/80) blend shows that the most coarsened morphology in Figure 1 is due to the immiscibility between components. However, for the PMMA/PC blends with catalyst (Figure 8d), the peak for methyl ester proton shifts from 3.601 to 3.619 ppm, implying that methyl ester of PMMA should participate in the exchange reaction during melt mixing. It has been suggested that the addition of tin(II) 2-ethylhexanoate catalyst in PMMA/PC blends would lead to the cleavage of PC chains and their subsequent linkage with the methyl ester group of the PMMA chain.33 The PC-g-PMMA graft copolymers formed at 5921

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the interface of PMMA/PC blends with catalyst during mixing can subsequently act as compatibilizers to reduce the interfacial tension of the blend, to facilitate the breakup of droplets, and to suppress the coalescence process between droplets via the steric repulsive mechanism or the retarded film drainage mechanism due to the stress gradient in the compatibilized interface.36 Therefore, the blend with catalyst exhibits a much finer droplet size compared with the pristine blends (Figure 1). It should be noted that the mechanical enhancement due to the compatibilization effect of catalyst could only be observed in the PMMA/PC blend with 0.05 wt % catalyst (Figure 6a). The noticeable deterioration in the mechanical properties of blends with 0.1 and 0.15 wt % catalyst and the brittle nature of blends with 0.4 and 0.6 wt % catalyst reveal the negative aspect of tin catalyst, namely the high possibility of its initiating the degradation of polymer chains (especially PC) during blending. The significant chain scission of the PC component caused by the transesterification reaction would largely reduce the melt viscosity of blends during melt mixing. In fact, the increase in the droplet size of blends with 0.6 wt % catalyst, as observed in Figure 1, suggests that the chain scission effect of the catalyst should dominate over the compatibilization effect of copolymers generated in situ during mixing. As lower shear stress caused by the PC matrix is involved in breaking up the PMMA droplets during mixing, larger PMMA droplets were produced in blends with 0.6 wt % catalyst. 3.2.2. Blends Filled with A200 Nanoparticles. As for the morphological refinement in PMMA/PC blends containing A200 nanoparticles, two possible reasons may be involved. The first one is related to the possible transesterification reaction in PMMA/PC blends in the presence of nanoparticles.21−23 Figure 9a shows that the carbonyl stretching vibration peak of PMMA at 1722 cm−1 disappears in the A200-filled PMMA/PC blend disposed using acetone. No new bands indicating copolymer structures of transesterification products were found in the spectra of this blend. Figure 9b shows that the peak for the methyl ester proton of PMMA in the 1H NMR spectra for the A200-filled blend is still located at 3.601 ppm. Thus, both the FTIR and 1H NMR results reveal that the possibility of a transesterification reaction between PC and PMMA during melt blending at 230 °C upon the addition of A200 nanoparticles can be excluded. Another possible reason for the morphological refinement in A200-filled PMMA/PC blends is the effect of selective distribution of nanoparticles on the phase dispersion process of blends during melt mixing. Theoretically, the localization of nanoparticles in polymer blends can be predicted by a wetting coefficient w12 defined as37 σ − σfiller − 2 w12 = filler − 1 σ12 (2)

Table 4. Surface Tension of Polymers and Nanoparticles at 230 °C surface tension material

γ

γd

γp

reference

PMMA (phase 1) PC (phase 2) A200

25.14 27.15 59.50

18.98 19.9 21.87

6.16 7.25 37.63

ref 40 ref 20 ref 41

blends. Therefore, A200 nanoparticles should locate mainly within the PC matrix. As the contrast between A200 nanoparticles and polymeric components is very low both in atomic force microscope (AFM) and transmission electron microscope (TEM), no direct proof about the preferential distribution of A200 nanoparticles is provided here. However, the preferential location of A200 nanoparticles within PC matrix can be supported by the improved thermal stability of the PC phase in PMMA/PC blends. The enriching of A200 nanoparticles in the PC matrix would increase the viscosity of the PC matrix phase due to the interaction between the surface of nanoparticles and PC chains and thus will be beneficial to the morphological refinement as observed in a blend filled with 1−3 wt % A200 nanoparticles. It is because a higher viscosity of matrix usually favors the droplet breakup process but is against the film drainage process during droplet coalescence.37 The larger droplet size in a blend with 5 wt % nanoparticles may be ascribed to the aggregation of A200 nanoparticles at high loadings, which thereby decreases their ability in refining the dispersed phase size. A similar phenomenon has also been reported in A200-filled PP/PS blends.27 It should be noted that the increase in the interfacial area of A200-filled blends due to the reduction in droplet size will lead to a higher collision frequency between PC and PMMA chains at the interface of blends. Therefore, the absence of a transesterification reaction in PMMA/PC blends with a refined morphology at 230 °C indicates that the reaction temperature is the dominant factor in determining the transesterification reaction rate. Simply increasing the contact area between PC and PMMA has negligible impact on the reaction process at this temperature. On the basis of the experimental results and related discussions described above, a schematic is constructed in Figure 10 to help the understanding of two different

with σ12 the interfacial tension between two polymers, σfiller−‑i the interfacial tension between nanoparticles and blends component. Nanoparticles should be distributed mainly in 2phase and 1-phase when the wetting coefficient is higher than 1 and lower than −1, respectively. In other conditions (−1 < w12 < 1), the nanoparticles would locate at the interface. Table 4 shows the values of surface tension of polymers and nanoparticles at 230 °C. The values of interfacial tension between two polymers and between polymer and A200 nanoparticles were calculated by using the harmonic-mean equation38 and Owens−Wendt equation.39 According to eq 2 the wetting coefficient was calculated to be 14.05 for the filled

Figure 10. Possible mechanism of catalyst and silica nanoparticles in compatibilizing PMMA/PC blends.

compatibilization strategies caused by tin catalyst and nanoparticles. In the presence of catalyst, the PMMA/PC blend undergoes a transesterification reaction during melt blending at 230 °C. PC-g-PMMA graft copolymers generated in situ at the interface of PMMA/PC blends reduce the interfacial tension. As a result, the droplet coalescence process is suppressed, and the breakup process is facilitated, leading to noticeably refined 5922

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the dispersed phase. However, the increase in the interfacial area between PC and PMMA due to the addition of A200 nanoparticles does not lead to a transesterification reaction between them during melt blending, suggesting that the compabilitization mechanism of A200 nanoparticles should be a kinetic one. Although A200-filled blends possess a larger droplet size than blends with catalyst, they display large improvements both in thermal and mechanical properties. The simultaneous addition of catalyst and nanoparticles does not produce a synergistic effect on the mechanical properties of blends.

morphologies. However, the catalyst also results in significant chain scission in the PC matrix which will lead to remarkable deterioration in the mechanical properties of blends, whereas A200 nanoparticles locate mainly within the PC matrix of PMMA/PC blends. The adsorption and restriction of nanoparticles on PC chains will increase the viscosity of the PC phase and act as a “compatibilizer” to kinetically refine the morphology of the blends. Unlike tin catalyst, A200 nanoparticles may not have a noticeable effect on the molecular weight and structure of the PC matrix according to the TGA and mechanical results. In a word, A200 nanoparticles are much better than the tin catalyst in compatibilizing PMMA/PC blends in view of the thermal and mechanical properties. The possibility of combining two kinds of compatibilization mechanisms, namely chemical compatibilization with catalyst and physical compatibilization with nanoparticles, to improve the mechanical properties of PMMA/PC blends is also evaluated. Figure 11 shows the stress−strain curve of

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (51373109, 51133005, 51121001), the Fundamental Research Funds for the Central Universities (2013SCU04A02), and the Innovation Team Program of Science and Technology Department of Sichuan Province (Grant 2013TD0013).

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REFERENCES

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Figure 11. Stress−strain curves for PMMA/PC (20/80) blends with a simultaneous addition of catalyst and A200 nanoparticles.

PMMA/PC blend with a simultaneous addition of 0.05 wt % catalyst and 3 wt % nanoparticles. It is found that this blend also displays a ductile failure behavior. The modulus and yield stress of this blend are quite similar to those of a blend with only 0.05 wt % catalyst. The elongation-at-break is between that of a blend with 0.05 wt % catalyst and a blend with 3 wt % nanoparticles. Therefore, it seems that the simultaneous addition of the tin catalyst and A200 nanoparticles does not produce a synergistic effect on the mechanical properties of blends.

4. CONCLUSIONS The transesterification catalyst [tin(II) 2-ethylhexanoate] exhibits a much higher efficiency than hydrophilic fumed silica nanoparticles (A200) in refining the morphology of PMMA/ PC blends during blending at 230 °C. Due to the promoted transesterification reaction between PC and PMMA, the dispersed phase size of blends with catalyst becomes much smaller compared to that of the neat PMMA/PC blends. However, blends with more than 0.1 wt % catalyst have poorer thermal and mechanical properties due to the noticeable reduction in the molecular weight of PC matrix caused by the transesterification reaction. For PMMA/PC blends filled with up to 3 wt % A200 nanoparticles, the droplet size is also dramatically reduced due to the preferential dispersion of A200 nanoparticles in the PC matrix which facilitates the breakup of 5923

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