PC Blends Compatibilized and Toughened via Copolymers in

Jan 9, 2015 - The structures and comprehensive properties of PBT/PC blends were ... reaction between PBT and PC.7 Progressive transesterification...
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PBT/PC Blends Compatibilized and Toughened via Copolymers in Situ Formed by MgO-Catalyzed Transesterification Gong-Peng Lin, Ling Lin, Xiu-Li Wang,* Li Chen,* and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Due to the limited toughening effect of BPM520 (a commercial acrylic-based core−shell structure impact modifier) on poly(butylene terephthalate) (PBT)/polycarbonate (PC) blends, magnesium oxide (MgO) was used as a transesterification catalyst, and BPM520 as a toughening agent to prepare PBT/PC blends by extrusion and injection molding. The structures and comprehensive properties of PBT/PC blends were investigated. Results of thermogravimetric analysis, Fourier transform infrared analysis, differential scanning calorimetry, and rheological measurements showed that transesterification reactions occurred among PBT, PC, and BPM520. As a result, the impact property of the blends was improved significantly, and only 5 wt % of BPM520 was needed to make the PBT/PC blend possess excellent toughness in the presence of MgO, which suggested a very efficient toughening approach for PBT/PC blends.

1. INTRODUCTION Thanks to its good mechanical strength and toughness, strong dimensional stability, thermal resistance, and processing advantages, the market for poly(butylene terephthalate) (PBT) is growing quickly. However, neat PBT has poor notched impact strength, so it is necessary to significantly improve its notched impact strength to meet more field requirements. Blending two or more polymers has proved to be a useful way to overcome some drawbacks of both materials without significantly compromising their advantages.1−3 Blends of PBT and polycarbonate (PC) are of commercial interest because of their potential in complementarity. The amorphous PC provides impact resistance and toughness while the crystalline PBT provides enhanced chemical resistance and thermal stability. It is known that in PBT/PC blends, transesterification reactions 4−6 may occur between the components in the melt. And usually the PBT/PC blends may undergo three types of transesterification reactions: acidolysis (reaction between the carboxyl end groups of PBT and the carbonate groups of the PC), alcoholysis (reaction between the hydroxyl end groups of PBT and the carbonate groups of PC), and direct transesterification (reaction between the ester groups of PBT and the carbonate groups of PC). Generally, the direct transesterification is the major exchange reaction between PBT and PC.7 Progressive transesterification reactions lead to a transformation of the initial homopolymers into block copolymers and finally into random copolymers.7−10 And these formed substances (“copolymers”) can act as compatibilizer for PBT and PC.6,11,12 However, the copolymers formed via transesterification usually are limited, which made the interphase adhesion of PBT and PC poor. So the commercial PBT/PC blends present unsatisfactory mechanical properties, especially the notched impact strength, for example, their notched impact strength is less than 10 kJ/m2 when PBT is the dominant component. © XXXX American Chemical Society

Functionalized rubbers or elastomers, which usually have ester, maleic anhydride, and epoxy, etc., functional groups, are recognized as a kind of effective toughening agent for PBT/PC blends.13−16 It was reported that poly(ethylene-butyl acrylateglycidyl methacrylate copolymer) (PTW) could toughen PBT/ PC blends, and the notched impact strength of the blends increased from 58 to 538 J/m (about 70 kJ/m2) when the copolymer content approached 14 wt %.17,18 Yao et al.19 found that when only 15 wt % commercial epoxide-containing elastomers AX8900 (a random terpolymer of ethylene, acrylic ester and glycidyl methacrylate) was used, the notched impact strength of PBT/PC blends could be enhanced to 53 kJ/m2. Due to the occurrence of reactions between the epoxy groups of toughening agent and the carboxyl or hydroxyl terminal groups of PBT during the melt blending, these epoxidecontaining elastomers disperse in the matrix uniformly and improve the toughness of PBT/PC obviously. Besides the epoxide-containing elastomers, toughening agents with core−shell structure are often be used. Wu et al.20 used core−shell structured S-2001, whose core is copolymer of organosilicone and acrylic, shell is poly(methyl methacrylate) (PMMA), to toughen PBT/PC blends, and found that the notched impact strength of the blend could be increased to 67 kJ/m2 when S-2001 content was 15 wt %. PMMA has better compatibility with PC than PBT, so the PMMA is easy to be coated with PC and then uniformly disperses in the matrix of PBT.21 This made the interface strength and impact toughness of the blends are improved. The commercial acrylic-based core−shell structure impact modifier BPM520, whose shell is also PMMA, has shown an effective Received: October 13, 2014 Revised: January 3, 2015 Accepted: January 9, 2015

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DOI: 10.1021/ie504032w Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Compositions of PBT/PC Blends compositions number sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample sample

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

sample

PBT (g)

PC (g)

BPM520 (g)

MgO (g)

NaH2PO4 (g)

PBT/PC PBT/PC/5%BPM520 PBT/PC/10%BPM520 PBT/PC/15%BPM520 PBT/PC/20%BPM520 PBT/PC/5%BPM520/0.05%MgO PBT/PC/5%BPM520/0.1%MgO PBT/PC/5%BPM520/0.25%MgO PBT/PC/5%BPM520/0.5%MgO PBT/PC/5%BPM520/1%MgO PBT/PC/0.1%MgO PC/BPM520 PC/BPM520/MgO PBT/BPM520 PBT/BPM520/MgO (PBT/PC/MgO)/BPM520 (PBT/PC/MgO)/BPM520/NaH2PO4 (PC/BPM520/MgO)/PBT (PC/BPM520/MgO)/PBT/NaH2PO4

666.7 633.3 600.0 566.7 533.3 633.0 632.7 631.7 630.0 626.7 666.0 0.0 0.0 632.7 632.7 632.7 629.3 632.7 629.3

333.3 316.7 300.0 283.3 266.7 316.5 316.3 315.8 315.0 313.3 333.0 316.3 316.3 0.0 0.0 316.3 314.7 316.3 314.7

0.0 50.0 100.0 150.0 200.0 50.0 50.0 50.0 50.0 50.0 0.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0

0.0 0.0 0.0 0.0 0.0 0.5 1.0 2.5 5.0 10.0 1.0 0.0 1.0 0.0 1.0 1.0 1.0 1.0 1.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 0.0 5.0

toughening action for PLA and PLA-based blends.22,23 Juikar et al.23 reported when 10 wt % BPM520 was used, the notched impact strength of PC/PLA (80/10, mass ratio) blend could be enhanced to 49.8 kJ/m2 at room temperature, and retained a value of 36.6 kJ/m2 at 20 °C below zero. Although the functionalized elastomers or core−shell structure impact modifiers can act as effective toughening agent for PBT/PC blends, their dosage are usually nearly 15 wt %, which can make the blend possess desirable impact strength. Considering the cost performance, it is a very meaningful task to achieve the high impact strength with low amount of impact modifier for this important engineering plastic alloy. As mentioned in our previous part, transesterification reactions occur between PBT and PC and generate some copolymers, which act as compatibilizers for the PBT/PC blend. If a transesterification catalyst is added, the more copolymers may be formed, which can improve the compatibility of PBT and PC further. Besides this, it is reported that the addition of tin(II) 2-ethylhexanoate catalyst in PMMA/PC blends will lead to the cleavage of PC chains and their subsequent linkage with the methyl ester group of the PMMA chain.24 And this generated copolymer can act as a compatibilizer to reduce the interfacial tension of PC and PMMA.25,26 Therefore, if a suitable transesterification catalyst is added during the thermal processing, the amount of impact modifier such as BPM520 may be reduced although PBT/PC should possess high impact strength. In this paper, magnesium oxide (MgO), an effective catalyst for small molecule esterficaiton,27,28 is chosen as a transesterification catalyst for macromolecular transesterification. It is expected when a small amount MgO is added it can promote the transesterification reactions between PBT and PC or PC and the PMMA shell of BPM520. In order to confirm this, a series of experiment included FTIR, TGA, DSC, DMA, etc., have been carried out. Fortunately, all the results show that MgO is an effective catalyst for macromolecular transesterification, and when 0.1 wt % MgO is added, only 5 wt % BPM520 is needed to make PBT/PC blend possessing super toughness (the notched impact strength is 81.3 kJ/m2).

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(butylene terephthalate) (S600, F10NC010) was purchased from the DuPont Company. Polycarbonate (PC, Makrolon 3100) was purchased from the Bayer Company. PBT and PC were dried in a vacuum oven at 100 °C for 4 h prior to compounding. Acrylic-based impact modifier BPM520 (trade name, PARALOID, its Fourier transform infrared (FTIR) spectrum, and Tanδ vs temperature curve are shown in Figures S1 and S2) was supplied by Dow Chemical Company. Magnesium oxide (MgO), NaH2PO4, dichloromethane, methanol, phenol, and tetrachloroethane were supplied by the Kelong Chemical Co., Ltd. All of these materials were used without further purification. 2.2. Sample Preparation. Prior to the blending, PBT, PC, BPM520, MgO, or NaH2PO4 were dried at 100 °C in vacuum for 4 h. According to formulas (as shown in Table 1), the mixtures of different mass ratios were extruded by a corotating twin screw extruder (CTE20, Coperion Keya Machinery Co. Ltd., Nang-jing, China; L/D = 44, ϕ = 20.5 mm), operated at 190−240 °C with a screw speed of 150 rpm. Finally the extrudates were cut into pellets. Injection molding was carried out in a plastic injection machine (MA1200/370, Haitian Plastic Machinery Ltd., China) to obtain tensile (GB/T 1040.2), flexural (GB/T 9341), and impact (GB/T 1843) specimens. The sequence of temperature was 230, 240, 250, 240, and 230 °C for the five heating zones, and the injection pressure was set at 70 MPa. For all of the PBT/PC blends, the mass ratio of PBT and PC was constant at 2:1. The pellets obtained from the extrudates of PBT/PC blends were extracted in dichloromethane (CH2Cl2) for 72 h at room temperature. The insoluble part was filtered out and the solution was drop on the KBr-pellet, and then the CH2Cl2 was dried out under infrared lamp before FTIR analysis. After that, the insoluble part was further dissolved in a mixed solvent of phenol/tetrachloroethane (60/40, mass ratio), and the solution was precipitated by methanol to get the solid powder. Then, the solid powder was extracted in CH2Cl2 for 72 h at room temperature to make sure that no independent PC fraction was reserved. Finally, the insoluble part was obtained by filtration B

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Industrial & Engineering Chemistry Research Table 2. Mechanical Properties of PBT/PC Blends samplea sample sample sample sample sample sample sample sample sample sample sample a

1 2 3 4 5 6 7 8 9 10 11

tensile strength (MPa) 57.5 53.2 48.7 42.2 38.3 51.5 49.2 44.1 44.5 44.1 50.6

± ± ± ± ± ± ± ± ± ± ±

elongation at break (%)

0.2 0.3 0.2 0.3 0.3 0.1 0.3 0.1 0.4 0.3 0.3

37.1 45.3 120.1 155.7 151.2 99.2 254.8 205.9 194.7 249.3 30.4

± ± ± ± ± ± ± ± ± ± ±

13.0 6.7 7.6 12.7 19.3 15.1 20.3 16.7 9.1 10.1 2.1

flexural strength (MPa)

flexural modulus (MPa)

± ± ± ± ± ± ± ± ± ± ±

1695 ± 42 1624 ± 43 1553 ± 9 1555 ± 24 1410 ± 20 1581 ± 61 1588 ± 29 1571 ± 17 1571 ± 28 1516 ± 14 1632 ± 23

75.0 71.5 68.6 59.4 53.2 70.1 67.6 58.8 60.1 58.8 66.6

1.0 0.5 0.3 0.7 0.3 0.6 1.2 0.4 0.5 0.3 0.3

notched Izod impact strength (kJ/m2) 6.3 11.9 62.3 67.1 76.3 13.2 81.3 78.6 68.5 63.6 6.6

± ± ± ± ± ± ± ± ± ± ±

0.3 0.4 0.6 1.6 2.8 0.5 1.2 1.6 3.8 5.9 0.5

The composition is listed in Table 1.

and then was dried at 100 °C in vacuum for 4 h before FTIR analysis. The pellets obtained from the extrudates of PC/BPM520/ MgO blend were dissolved in a mixed solvent of phenol/ tetrachloroethane (60/40, mass ratio) completely, and then, methanol was added in order to get the solid powder. After that, the solid powder was extracted in CH2Cl2 for 72 h at room temperature to make sure that no independent PC fraction persisted in the insoluble part. The insoluble part was filtered out and the solution was dropped onto the KBr-pellet, and then the CH2Cl2 was dried out under an infrared lamp before FTIR analysis. The obtained insoluble parts were dried at 100 °C in vacuum for 4 h before FTIR analysis. 2.3. Characterization. The uniaxial tensile and flexural test were performed on a universal test machine CMT4104 (Shenzhen SANS Testing Machine Co. Ltd., China) with a crosshead speed of 50 and 20 mm/min, respectively. The notched Izod impact test was performed by an impact tester ZBC1400-2 (Shenzhen SANS Testing Machine Co. Ltd., China). The notches (depth 2 mm, mean radius 0.25 mm) were machined after injection molding. All these tests were conducted at room temperature. Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 F1 under nitrogen atmosphere (flow rate of 60 mL/min) at a heating rate of 20 °C/min, and the scanning scope was ranged from 40 to 700 °C. FTIR spectra were recorded on a Nicolet 6700 spectrophotometer by KBr-pellet. Dynamic mechanical analysis (DMA) was carried out on a DMA Q800 (TA) in tensile mode. The scans were carried out in single cantilever mode at a constant heating rate of 3 °C/min and at a frequency of 1 Hz from 0 °C to about 180 °C. Differential scanning calorimetry (DSC) was conducted on a DSC Q200 (TA) under nitrogen flow at a heating and cooling rate of 10 °C/min with a temperature range of 40−260 °C. Rheological measurements were carried out under a straincontrolled rheometer: ARES (advanced rheometrics expansion system) with a parallel-plate geometry (Φ= 25 mm) at 250 °C under a nitrogen atmosphere. Dynamic frequency sweep tests were carried out under a fixed strain amplitude of 5%, which was in the linear viscoelastic region, and a frequency range of 100−0.01 Hz. The cryogenically fractured surfaces of samples were observed by scanning electron microscopy (SEM, JEOL JSM5900LV) at an accelerating voltage of 20 kV. Before examination, the surfaces of all samples were sputter-coated with gold. For the etched images, the cryogenically fractured

surfaces was extracted in CH2Cl2 for 72 h at room temperature to make sure no independent PC fraction remained.

3. RESULTS AND DISCUSSION 3.1. Mechanical Properties of PBT/PC with Different Content of BPM520 and MgO. Just as we say in the introduction, a simple blend of PBT with PC alloy shows poor mechanical properties (shown in Table 2), especially the notched impact strength (6.3 kJ/m2). When impact modifier BPM520 is added, the notched impact strength and elongation at break increase, while the tensile strength, flexural strength, and flexural modulus decrease. As shown in Table 2, when the BPM520 content is 5%, the notched impact strength of PBT/ PC blend increases to 11.9 kJ/m2, which is almost twice as much as that of pure PBT/PC. Further increases the BPM520 content to 10%, 15%, or 20%, the notched impact strength of the blend increases greatly to 62.3, 67.1, and 76.3 kJ/m2, respectively. When MgO, a transesterification catalyst, was added, the mechanical properties of PBT/PC/5%BPM520 were influenced greatly. When 0.1% MgO is added, the notched impact strength increases from 11.9 kJ/m2 of PBT/PC/5%BPM520 to 81.3 kJ/m2, which is even higher than that of PBT/PC/20% BPM520 (76.3 kJ/m2). And at the same time, its elongation at break is higher than that of PBT/PC/20%BPM520 blend. When MgO content is 0.05%, the notched impact strength had a slight increase. Further increase MgO content, however, the notched impact strength of blend has a slight decrease. Besides, the tensile strength, flexural strength, and flexural modulus of PBT/PC/5%BPM520 decreased with the increase of MgO content. All the mechanical properties results show that 0.1% is an optimum MgO content for the PBT/PC/5%BPM520 blend. Why when only 0.1% MgO was added did the PBT/PC/5% BPM520 blend exhibit excellent notched impact strength? If only MgO was added, what happened for PBT/PC blend? It is found that when 0.1% MgO is added to the PBT/PC blend, the notched impact strength is just 6.6 kJ/m2, and the tensile strength, flexural strength, and flexural modulus decrease (shown in Table 1). This means that MgO is not an effective impact modifier for PBT/PC blend; however, it can enhance the interaction between PBT, PC, and BPM520. In order to illuminate this, a series of experiments have been carried out. 3.2. FTIR Analysis. First, we use FTIR spectrometer to investigate the interaction between PBT and PC. Wilkinson et al.29,30 reported that the PBT/PC blend changed its nature from block copolymer to random copolymer when the degree of transesterification increased. The random copolymer and C

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Figure 1. FTIR spectra of PBT/PC and PBT/PC/0.1%MgO blends soluble (a) and insoluble parts in CH2Cl2 (b).

pure PC can dissolve in CH2Cl2. Therefore, the PBT/PC and PBT/PC/0.1%MgO blends were extracted in CH2Cl2 to obtain PC and the transesterified random copolymer, and the transesterification between PBT and PC could be ascertained by observing the absorbance peak of CO in the FTIR curves. As shown in Figure 1, because of the different chemical environment of CO in PBT and PC, the FTIR characteristic band of CO in PBT is at 1711 cm−1, whereas in PC it is at 1774 cm−1. Figure 1a gives the FTIR results of PBT/PC and PBT/PC/0.1%MgO blends solute in CH2Cl2, and the spectra of pure PC is also provided for a direct comparison. As shown in Figure 1a, the FTIR curves of PBT/PC and PBT/PC/0.1% MgO exhibit strong absorption peak at 1774 cm−1 near the CO band of pure PC, and a smaller peak at 1720 cm−1. The existence of the peak at 1720 cm−1 proved that some PBT was incorporated into the PC macromolecular chain and some random copolymers formed in the blends. The insoluble fraction in CH2Cl2 was further extracted in C6H5OH/C2H2Cl4, which was the best solvent for PBT. Figure 1b gives the FTIR results of PBT/PC and PBT/PC/0.1%MgO blend solute in C6H5OH/C2H2Cl4, and the spectra of pure PBT is also provided for a direct comparison. Devaux et al.8 attributed the peak at 1720 cm−1 to the aliphatic ester group in the transesterification product, 1740 cm−1 to the aromatic ester, 1780 cm−1 to the aromatic carbonate, and 1770 cm−1 to the mixed aliphatic−aromatic carbonate. For the FTIR curves of PBT/PC and PBT/PC/0.1%MgO, the peaks at 1711 cm−1 are mainly contributed by pure PBT, and those at 1775 cm−1 are near the CO band of PC. Moreover, new peaks at 1752 cm−1 were observed, and these peaks were stronger with the addition of MgO to the PBT/PC blend. The existence of the peak at about 1775 cm−1 and especially the new peak at about 1752 cm−1 proved that some PC was incorporated into the PBT macromolecular chain and some copolymers (“PBT−PC”) formed in the blends. In order to check whether the BPM520 could react with PBT, PBT/BPM520 and PBT/BPM520/MgO blends were also prepared. Unfortunately, no appropriate solvent can be found to separate PBT and BPM520, so we cannot use FTIR to estimate this system. In order to prove whether a reaction existed between BPM520 and PC, the PC/BPM520/MgO blend (the composition is listed in Table 1) is also investigated by FTIR. Figure 2 gives the FTIR spectra of the soluble and insoluble fractions in CH2Cl2. The spectra of pure PC and BPM520 are also provided for comparison. From Figure 2, it

Figure 2. FTIR spectra of the PC/BPM520/MgO blend soluble and insoluble parts in CH2Cl2.

can be seen that there is a peak at 967 cm−1 in BPM520, and a similar peak is not observed in PC or soluble composition. The FTIR spectrum of the soluble fraction is as same as that of pure PC, which means that pure PC is dissolved in CH2Cl2. For the insoluble composition, not only the peak at 967 cm−1 is observed, but also all of the peaks contributed by pure PC exist. Because PC can dissolve in CH2Cl2 while BPM520 cannot, the existence of these peaks demonstrates that some PC has reacted with BPM520, which means that it did not dissolve in CH2Cl2. In other words, a reaction occurs between BPM520 and PC in the presence of MgO. 3.3. TG Analysis. The thermal degradation behaviors of PC, BPM520, and their blends under nitrogen atmosphere are shown in Figure 3, and the relative data are listed in Table S1. It is found that the T5% and char residue of PC/BPM520 were 419.0 °C and 19.0%, respectively, and the rate of Tmax and Tmax were 34.8% and 540.2 °C. When MgO is added, not only do the T5% and char residue decline to 406.2 °C and 13.6%, but also the rate of Tmax and Tmax is reduced to 22.7% and 470.8 °C. This indicates that an interaction exists between PC and BPM520 in the presence of MgO, which is in accord with FTIR results. D

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Figure 3. TG and DTG curves of PC, BPM520, and PC/BPM520/MgO blends.

Figure 4. TG and DTG curves of PBT, BPM520, and PBT/BPM520/MgO blends.

Although we cannot use FTIR to determine whether there is a reaction between PBT and BPM520, TGA analysis can be made. TG and DTG curves of PBT, BPM520, and PBT/ BPM520/MgO blends are shown in Figure 4, and the relevant data are also listed in Table S1. It is found that the thermal stability of PBT/BPM520 is similar to that of pure PBT. Even MgO is added, its thermal properties almost have no change. This illustrates that no reaction occurred between PBT and BPM520. 3.4. DSC Analysis. From the FTIR results (shown in Figure 1), it is determined that there occurs a transesterification reaction between PBT and PC, and this will reduce the crystallization ability of the PBT.31 From the DSC cooling curves of PBT and PBT/PC (Figure 5), we can see clearly that the crystallization peak of PBT/PC is much lower than that of PBT. Besides, we can also note that the DSC curve of PBT/ BPM520/MgO (sample 15) almost coincides with pure PBT, which indicates that the crystallization ability of PBT/ BPM520/MgO is equal to pure PBT. Combining the TGA and DSC results, we draw a conclusion that there is no reaction between PBT and BPM520. This result coincides with the report of Ambrosio et al.,32 in which he found that PMMA cannot react with PBT. The transesterification between PBT and PC will affect the melting and crystallization behavior of PBT/PC blends, which is investigated by DSC. Figure 6 shows the melting and crystallization behavior of PBT/PC/BPM520/MgO blends, and the data are listed in Table S2. The crystallization

Figure 5. DSC curves of PBT, BPM520, and PBT/PC/BPM520 blends at a cooling rate of 10 °C.

temperature (Tc) of all the blends declines and the cold crystallization temperature (Tcc) as well as the enthalpy value of cold crystallization (ΔHcc) increases, especially for the samples with MgO. This phenomenon is believed to originate from the reactions taking place between PBT and PC or PC and BPM520. The copolymers formed by transesterification, act as E

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Figure 6. DSC curves of PBT/PC/BPM520/MgO blends at a heating/cooling rate of 10 °C/min (primary circulation).

Newtonian behavior at low frequencies. The pure PBT/PC blend (sample 1) shows an increase in complex viscosity at low frequencies (yield) and this phenomenon also has been seen in samples 2, 6, and 7. The increase in complex viscosity is evidence for some reaction happening for the blends, which is also demonstrated by other researchers.34 But for the PBT/PC/ 0.1%MgO blend (sample 11), it shows a decrease in complex viscosity at low frequencies, which can be ascribed to the degradation of a PBT/PC blend resulting from the excessive transesterification of PBT and PC in the presence of MgO during the rheological measurement. But its complex viscosity is still higher than that of a pure PBT/PC blend in the most investigated frequency range due to the formation of more PBT−PC copolymer formed by the transesterification. In addition, for PBT/PC/BPM520 blends with or without MgO, their complex viscosity is also higher than that of pure PBT/PC blend at low frequencies, which is caused by the addition of the BPM520 elastomer. So, for the sample 2 without MgO, it has the highest complex viscosity at low frequencies. When 0.05% MgO along with 5%BPM520 is used to modify the PBT/PC blend, PC not only can react with PBT but also react with BPM520, and this reaction is still in progress in the rheological test. This means that sample 6 always has lower complex viscosity than that of sample 2. When 0.1% MgO is used, these transesterification reactions almost have completed during the twin-screw extrusion process, and the complex viscosity changes little at the lowest frequency. What is more, its complex viscosity curve is smoother than that of the PBT/PC/5%BPM520/0.05%MgO blend. This means that the compabilization has improved greatly during the processing via transesterification; therefore, this blend exhibits excellent notched Izod impact strength. 3.6. DMA Analysis. Figure 8 shows the DMA curves of PBT/PC/BPM520/MgO blends, and the data are listed in Table S3. Over the experimental temperature range (0−180 °C), PBT exhibits a glass transition temperature at 65.5 °C, and PC shows a glass transition peak at 166.1 °C. For the pure PBT/PC blend, two Tg peaks appear at 89.6 and 138.7 °C, respectively. The higher Tg peak can be attributed to the PBTrich phase, and the other Tg peak can be attributed to the PCrich phase. The Tg peaks of PBT and PC approaches each other, which indicates that the PBT/PC blend has partial miscibility resulting from their transesterification in the melt mixing.35−37 For the PBT/PC/0.1%MgO blend, the ΔTg value

interfacial agents to improve the compatibility of PBT/PC blends and increase the interphase adhesion, which hinder the process of crystallization of PBT. In addition, these copolymers are difficult to crystallize and also induce the lower crystallinity of the blends (Table S2). As we know, the crystallization ability of PBT has great relationship with the chemical resistance and thermal stability of the blend.31 From the table, we can see that the crystallinity of the PBT-phase in the blend just decreases a little except the blends with 0.1%MgO (sample 11). As far as PBT/PC/5%BPM520/0.1%MgO is concerned, its crystallinity is retained at 84.6% (decreasing from 32.4% of pure PBT to 27.4%), illustrating that it still possess good chemical resistance and thermal stability. 3.5. Dynamic Rheological Analysis. It is well-known that most polymer will show Newtonian behavior at low frequencies in rheological test, and if some reactions have occurred, their complex viscosity will deviate from that of a Newtonian fluid; therefore, rheological methods can be used to monitor this reaction.33 Curves of the viscosity vs the frequency at 250 °C for PBT/PC/BPM520/MgO systems are shown in Figure 7. For pure PBT and PC, they exhibit shear-thinning behavior at high frequencies and Newtonian behavior at low frequencies. But for the blends, their complex viscosity does not follow the

Figure 7. Frequency dependence of the complex viscosity (at 250 °C) for PBT/PC/BPM520/MgO blends. F

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Figure 8. DMA curves of PBT/PC/BPM520/MgO blends.

decreases from 49.1 to 25.8 °C, which means that the transesterification between PBT and PC is enhanced when MgO is added. For the PBT/PC blend modified by 5% BPM520 alone, the ΔTg value decreases from 49.1 to 44.5 °C, indicating that BPM520 can act as a compatibilizer for PBT/PC blend to some extent. When 0.05% MgO along with 5% BPM520 is used to modify the PBT/PC blend, the ΔTg value decreases further; however, two Tg peaks still exist. When the amount of MgO is increased to 0.1%, only an obvious peak at 90.3 °C is observed, indicating under this dosage of MgO, the PBT and PC phases are totally compatible. This excellent compatibility is ascribed to the higher transesterification degree between PBT, PC, and BPM520. 3.7. SEM Analysis. SEM was used to investigate the morphology of the blends. Figure 9 shows the SEM micrographs of cryo-fractured surfaces of the PBT/PC blends. The cryo-fractured specimens of the PBT/PC blends are extracted in CH2Cl2 to remove the PC on the surface, and their SEM micrographs are shown in Figure 10. Figure 9 (sample 1) displays the morphology of the PBT/PC blend without impact modifier, in which PBT is the continuous phase, and PC is the dispersed phase. From the figure, we can see that the interfacial adhesion is poor,38 and many holes with a smooth surface are formed. Obviously, these holes are formed after the PC domains are pulled off during fracture. What is more, it can be seen clearly from its etched SEM image (Figure 10, sample 1), many discontinuous holes are found due to the PC domains can be extracted easily in CH2Cl2. This illustrates that the compatibility between PBT and PC is not good even though there are transestification occurrence. When 5% BPM520 is added to the PBT/PC blend, the holes decrease a little; however, the interfacial boundary between PBT and PC still exists. When 0.1% transesterification catalyst MgO is added to the PBT/PC blend, the interfacial boundary between PBT and PC disappears, and the PC domains are hardly extracted by CH2Cl2 (Figure 10, sample 11). It is well-known that the compatibilizers tend to concentrate at the interface of the blend components, reducing the interfacial tension, preventing coalescence, and strengthening interfacial adhesion.39−42 The copolymers formed by the transestification can be regarded as a better candidate for interface modification.43 When 5% BPM520 together with 0.1% MgO is used in the PBT/PC blend, the micrograph of cryo-fracture surface is similar to that

Figure 9. SEM micrographs of cryo-fracture surfaces of the PBT/PC blends.

of PBT/PC/0.1% MgO blend. When the content of MgO decreases from 0.1% to 0.05%, the degree of the transesterification decreases, and the copolymer formed by transesterification is also reduced. Figure 10 (sample 6) shows that PC domains are easily extracted by CH2Cl2 and many holes are observed. This phenomenon indicates that the more copolymer is formed by transesterification the stronger interfacial adhesion between PBT and PC becomes.10 3.8. Intermolecular Interactions. From the above results we know that PC can react with both PBT and BPM520 in the presence of the transesterification catalyst MgO, so substances PBT−PC and “PC−BPM520”, and even “PBT−PC−BPM520” can be possibly in situ formed in the PBT/PC/5%BPM520/ 0.1%MgO blend. These copolymers can act as compatibilizers for PBT/PC blends. To verify this, the blends with different processing sequence are prepared and their mechanical properties are listed in Table 3. The blend samples 16 and 17 are prepared by two processes, i.e., PBT, PC, and MgO are first blended by a corotating twin screw extruder, which makes PC able to react with PBT. Then the extrudates are extruded with BPM520 with or without sodium dihydrogen phosphate (NaH2PO4), a transesterification inhibitor.44,45 Without NaH2PO4, the Izod impact strength of blend sample 16 is 82.6 kJ/m2, which is close to that of PBT/ PC/5%BPM520/0.1%MgO (shown in Table 2). But when NaH2PO4 is added into the blend, its Izod impact strength is only 12.3 kJ/m2, which is close to that of PBT/PC/5% BPM520. This means that when the transesterification between G

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interphase adhesion, the copolymer PBT−PC and even the copolymer PBT−PC−BPM520 formed in the interphase boundary of PBT and PC act as compatibilizers to improve interphase adhesion. So, the PBT/PC/5%BPM520/0.1%MgO blend shows excellent mechanical properties, especially desired impact strength.

4. CONCLUSION The aim of this article is to prepare a high impact PBT/PC blend (in which PBT is the dominant component) with low content of impact modifier via transesterification. For the pure PBT/PC blend, although the transesterification reaction can occur, the reaction degree is limited, which created low notched impact strength (6.3 kJ/m2). The addition of impact modifier BPM520 can improve the notched impact strength of a PBT/ PC blend to 76.3 kJ/m2; however, BPM520 with 20 wt % dosage should be used. Combining the results of FTIR, DSC, and TG measurements, it can be concluded that when the transesterification catalyst MgO is used, the transesterification occurs not only between PBT and PC, but also between PC and BPM520. And even the generated PBT−PC copolymer can continue to react with BPM520. Because the retention time during the thermal processing is short, the amount of MgO has a great effect on the transesterification. Only when its amount reaches 0.1 wt % can the transesterification almost complete during the preparation, which facilitates having only a few changes in complex viscosity at the lowest frequency. Furthermore, its complex viscosity curve is smoother and only one Tg (90.3 °C) is shown in the DMA curve. SEM images show that the compabilization has been improved greatly, and no obvious interfacial boundary of PBT and PC can be found. Therefore, the PBT/PC/5%BPM520/0.1%MgO blend exhibits excellent notched impact strength (81.3 kJ/m2).

Figure 10. SEM micrographs of cryo-fracture surfaces of the PBT/PC blends after extracted in CH2Cl2.

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PC and BPM520 is inhibited, only the PBT−PC copolymer cannot act as an effective compatibilizer. For blend sample 18, PC, BPM520, and MgO are first extruded, which made PC react with BPM520, and then the extrudates are secondarily blended with PBT. Via this processing procedure, not only PC−BPM520 but also PBT− PC copolymer are formed, which gave blend sample 18 excellent Izod impact strength (81.5 kJ/m2). However, when transesterification inhibitor NaH2PO4 is added, the transesterification between PBT and PC is inhibited, which made the Izod impact strength of blend sample 19 sharply decreased to 8.5 kJ/m2. This result illustrates that only PC−BPM52 also cannot act as an effective compatibilizer for PBT/PC blends. The above results demonstrate that only when PC reacts both with PBT and BPM520, to form both PBT−PC, PC−BPM520, and even PBT−PC-BPM520 copolymer, can PBT/PC blends show excellent mechanical properties, especially desired impact strength. The copolymer PC−BPM520, formed in the interphase boundary of PC and BPM520, enhances the

ASSOCIATED CONTENT

* Supporting Information S

The TG data of PBT, PC, BPM520, PC/BPM520, PC/ BPM520/MgO, PBT/BPM520 and PBT/BPM520/MgO are listed in supplementary Table S1. The DSC data of PBT, PBT/ PC, PBT/PC/5%BPM520, PBT/PC/5%BPM520/0.05%MgO, PBT/PC/5%BPM520/0.1%MgO, and PBT/PC/0.1%MgO are listed in supplementary Table S2. The DMA data of PBT, PC, PBT/PC, PBT/PC/5%BPM520, PBT/PC/5%BPM520/0.05% MgO, PBT/PC/5%BPM520/0.1%MgO, and PBT/PC/0.1% MgO are listed in supplementary Table S3. The FTIR spectrum and DMA curve of BPM520 are displayed in supplementary Figures S1 and S2, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Tel. and Fax: +86-28-85410755. E-mail address: xiuliwang1@ 163.com (X.-L. Wang)

Table 3. Mechanical Properties of PBT/PC/BPM520/MgO under Different Processing Sequence with or without NaH2PO4 samplea sample sample sample sample a

16 17 18 19

tensile strength (MPa) 49.1 52.9 48.0 52.3

± ± ± ±

0.4 0.2 0.2 0.2

elongation at break (%) 273.8 26.7 256.5 13.7

± ± ± ±

5.8 4.1 26.2 1.0

flexural strength (MPa) 66.4 71.3 68.3 73.9

± ± ± ±

0.9 0.9 0.5 0.5

flexural modulus (MPa) 1758 1800 1885 1941

± ± ± ±

42 40 25 38

Izod impact strength (kJ/m2) 82.6 12.3 81.5 8.5

± ± ± ±

1.2 0.4 1.1 0.9

The composition is listed in Table 1. H

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001) and the Excellent Youth Foundation of Sichuan (2011JQ0007).

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J

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