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Super Toughened and High Heat-Resistant Poly(Lactic Acid) (PLA)Based Blends by Enhancing Interfacial Bonding and PLA Phase Crystallization Ling Lin,† Cong Deng,*,†,‡ Gong-Peng Lin,† 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, Wangjiang Road 29, Chengdu, Sichuan 610064, China ‡ Analytical and Testing Center, Sichuan University, Wangjiang Road 29, Chengdu, Sichuan 610064, China ABSTRACT: Polycarbonate (PC) was incorporated into poly(lactic acid) (PLA) to prepare the high-PLA-content (≥66 wt %) PLA/PC blends with high impact strength and heat distortion temperature (HDT) via adding a compatibilizer to tailor the interfacial bonding and annealing to enhance the crystallization of PLA phase with the aid of an impact modifier. On the basis of the tensile test, scanning electron microscope, and transmission electron microscope observations, the change from the yielding behavior dominated by shear-yielding to a coexistence of shear-yielding and crazing should be the leading reason for the significant improvement of impact strength for the annealed PLA/PC blends. The increase of the ability to sustain the high local stress also played an important role in the improvement of impact strength. Meanwhile, the increase of HDT should be resulting from the formation of a rigid three-dimensional framework composed of rigid PC particles and PLA crystals formed in the PLA/ PC blends.

1. INTRODUCTION Because of renewability, biodegradability, and high mechanical strength, poly(lactic acid) (PLA) is considered as a promising substitute to some petrochemical-based polymers. However, the inherent brittleness of PLA seriously limits its wide applications. Many efforts have been made to improve PLA toughness. 1 Some research focused on increasing the elongation at break as much as possible, in which various plasticizers and polymers, such as lactide,2 oligomeric lactic acid,3 citrate esters,4 poly(ethylene glycol),5−7 adipates,8,9 poly(ethylene glycol)-succinate copolymer and poly(ethylene glycol)-succinate-L-lactide copolymer,10 poly(butyl acrylate),11 and poly(β-hydroxy-butyrate-co-β-hydroxyvalerate)12 were used as the toughening agents. Almost all the blends possess a high elongation at break, showing a significant improvement in the ductility. Some research paid much attention to improve the impact strength as much as possible, expecting to get a supertough material. Liu et al.13 prepared supertough PLA/ (EBA-GMA/EMAA-Zn) (the weight ratio is 80/20) blends via reactive blending, resulting in a blend with a notched izod impact strength over 800 J/m. Lu et al.14 applied a polyurethane elastomer prepolymer containing an isocyanate group to prepare a supertoughened PLA blend successfully via in situ reactive interfacial compatibilization. Feng et al.15 and Ma et al.16 also reported supertough PLA materials via blending PLA with POE-g-GMA and EVA containing different VA contents, respectively. Interestingly, several researchers revealed that promoting PLA crystallization made the impact strength of PLA blends increase greatly. Oyama et al.17 reported that the notched charpy impact strength of the as-prepared sample was 2−3 times higher than that of neat PLA; the impact strength significantly increased after annealing at 90 °C for 2.5 h, reaching 72 kJ/m2. Bai et al.18 further investigated the effect of © 2015 American Chemical Society

different crystallinities of PLA matrix on the impact strength of PLA/PCL blends, and the results showed that the impact strength linearly rose with an increase of the crystallinity of PLA in the case of no change in crystalline form. Here, the form refers to α, β, or γ form. In addition, the heat distortion temperature (HDT) of PLA also needs to be improved, besides its toughness, to expand its application in more fields. Blending PLA with polycarbonate (PC) is considered as one of the most promising methods to improve the impact property and HDT of PLA simultaneously due to its high impact strength and high HDT.19−23 Hashima et al.24 reported a supertough and high HDT PLA/PC/SEBS/ EGMA (40/40/15/5, weight ratio) alloy, which had a notched izod impact strength over 60 kJ/m2 and a HDT (0.455 MPa) of 94.5 °C. As a biobased polymer, the content of PLA in the PLA/PC blend should be as high as possible after fulfilling the requirements of different properties, which is favorable to minimizing the environmental burdens. How to improve simultaneously the toughness and heat resistance of PLA is a tough task at this condition. In this work, we found that the interfacial bonding and the crystallinity of PLA phase are very important to improve the impact strength and heat distortion temperature simultaneously; hence, the supertough and high HDT PLA/PC blends with over 66 wt % PLA were explored successfully by adding a compatibilizer to tailor the interfacial bonding and annealing to enhance the crystallization of PLA phase with the aid of an impact modifier. Here, it should be Received: Revised: Accepted: Published: 5643

January 8, 2015 May 7, 2015 May 12, 2015 May 12, 2015 DOI: 10.1021/acs.iecr.5b01177 Ind. Eng. Chem. Res. 2015, 54, 5643−5655

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Industrial & Engineering Chemistry Research

on a Netzsch TG-209-F1 thermogravimetric analyzer at a scanning range from 40 to 700 °C with a heating rate of 20 °C/ min and a dynamic nitrogen or air flow of 50 mL/min. Differential scanning calorimetry (DSC) was conducted on a DSC Q200 (TA, USA) under nitrogen flow at a heating rate of 10 °C/min. Sample used for DSC was picked up from a shell part of the injection-molded dumbbell specimen. X-ray diffraction (XRD) analysis was performed on a DX-1000 (Philips X, Netherland) X-ray diffractometer with Cu Kα radiation (wavelength, k = 0.154 nm) in the range of 2θ = 5− 40° and was operated at 40 kV and 25 mA with a scanning rate of 0.06°/s. Morphology was observed by an Inspect F SEM (FEI, Netherland) at an acceleration voltage of 5 kV. There were three types of fractured surfaces for SEM observation, which are type I, type II, and impact-fractured surfaces. Type I and II surfaces are illustrated in Figure 1. The rectangular specimens

noted that the content of PLA in the blend is much higher than that in the other PLA blends with the same impact strength and HDT. The different properties of the PLA/PC blends were surveyed via tensile, flexural, izod notched impact, and HDT tests, respectively. The thermal stability, the morphologies, and the crystalline structures of PLA/PC blends were investigated by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD), respectively. Finally, transmission electron microscope (TEM) was used to observe the yielding-necking or stress-whitening region formed in the tensile test to directly demonstrate the existence of crazing.

2. EXPERIMENTAL SECTION 2.1. Materials. PLA (4032D) was supplied by NatureWorks (USA). PC (2600) was purchased from Bayer (Germany). The compatibilizer, a copolymer of styrene and glycidyl methacrylate, with a brand name of ADR 4370S (coded as ADR), was obtained from Basf (Germany), and a core−shell impact modifier having a brand name of BPM 520 (coded as BPM) was bought from Dow chemical company (USA), in which the shell component is PMMA and the core is poly(butyl acrylate). 2.2. Preparation of Blends. PLA, PC, and BPM were dried at 80 °C for at least 10 h before being processed, but ADR was used as received. According to the designed formulation, PLA, PC, ADR, and BPM were melt extruded via a corotating twin-screw extruder (CTE20, Coperion Keya Machinery Co. Ltd., Nang-Jing, China) with a screw speed of 150 rpm and a barrel temperature of 190−230 °C. The extrudates were cooled, pelletized, and then dried in a vacuum oven at 80 °C for at least 10 h. Next, injection molding was carried out at a barrel temperature of 210−240 °C and a mold temperature of room temperature by an injection machine (MA1200/370, Haitian Plastic Machinery Ltd., China) to obtain the corresponding test specimens. Dumbbell specimens for the tensile test have the narrow section of 10 mm width and 4 mm thickness (GB/T 1040.2). Rectangular specimens for the flexural test (GB/T 9341) and the notched izod impact test (GB/T 1843) have the dimensions of 80 mm × 10 mm × 4 mm. The dimensions of rectangular specimens for the HDT measurement are 120 mm × 10 mm × 4 mm (GB/T 9341). In addition, the annealed samples were obtained via treating the injection molding samples in a vacuum oven for 6 h at 120 °C, which was chosen according to the peak temperature of the cold crystallization for neat PLA and its complete crystallization time. To ensure PLA crystallize as fully as possible, the annealing condition was set at 120 °C for 6 h. 2.3. Characterization. The uniaxial tensile test was performed on a universal test machine CMT4104 (Shenzhen SANS Testing Machine Co. Ltd., China) with a crosshead speed of 10 mm/min. The flexural test was also carried out on this machine at a speed of 5 mm/min. The izod notched impact test was performed by an impact tester ZBC1400-2 (Shenzhen SANS Testing Machine Co. Ltd., China) with a pendulum of 2.75 J after notching (V notch with an angle; a depth and a curvature radius at the apex of 45° of 2 mm and 0.25 mm, respectively). All these tests were conducted at room temperature. HDT was estimated by a HDT tester HDV2 (ATLAS, USA) at edgewise mode under a load of 1.82 or 0.455 MPa at a heating rate of 120 °C/h. HDT is the temperature at which the specimen deflection increases to 0.32 mm during the heating process. The thermogravimetric analysis (TGA) was performed

Figure 1. Schematic picture of the type I and II surfaces.

with the dimension of 80 mm (L) × 10 mm (W) × 4 mm (T) were used to fabricate these two surfaces, and the two kinds of surfaces were manufactured after a sample was immersed in liquid nitrogen at least for 3 h. Ultrathin slices with the thickness of about 100 nm were observed by a Tecnai G2 F20 S-TWIN TEM (FEI, USA) at an acceleration voltage of 200 kV, which were picked up from the samples obtained by injection molding before and after the tensile test. For the samples after the tensile test, the ultrathin slices were picked up from the yielding-necking or the stresswhitening region formed in the tensile test along the direction of about 45° to the strain direction. Before the samples were observed, they were stained with a 2 wt % osmium tetroxide aqueous solution.

3. RESULTS AND DISCUSSION 3.1. Fabrication of PLA/PC/ADR/BPM Blends. PC is used to improve the heat resistance of PLA in this work because of its high HDT. Figure 2a is the HDT test results of PLA/PC blends. As can be seen, whatever the samples are as-molded or annealed, the HDT increases after adding PC in comparison with neat PLA. Interestingly, an obvious increase of HDT was observed when samples were annealed; for instance, the HDT of the PLA70/PC30 blend increased to 75 °C from 60 °C after annealing. When the experimental HDT values are compared with the corresponding calculated values (the calculated value = x% × PC + (1 − x%) × PLA, in which the x% is the weight ratio of PC in a PLA/PC blend; “1 − x%” is the weight ratio of PLA in this PLA/PC blend; PC represents the value of HDT or impact strength for neat PC; PLA represents the value of HDT or impact strength for neat PLA), it can be seen that the experimental value is always lower than the corresponding calculated value, showing a poor interfacial bonding between 5644

DOI: 10.1021/acs.iecr.5b01177 Ind. Eng. Chem. Res. 2015, 54, 5643−5655

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Figure 2. HDT (a) and impact strength (b) of PLA/PC blends (the calculated value = x% × PC + (1 − x%) × PLA) and impact strength (c) and HDT (d) of PLA70/PC/ADR blends.

Figure 3. SEM micrographs of type I surfaces of PLA/PC blends: (a) PLA90/PC10, (b) PLA80/PC20, (c) PLA70/PC30, and (d) PLA60/PC40.

Figure 4. SEM micrographs of type I surfaces of PLA70/PC30/ADR blends: (a/a′) ADR 0 phr, (b/b′) ADR 0.1 phr, (c/c′) ADR 0.3 phr, and (d/ d′) ADR 0.5 phr.

decreases to the level adapting to the strain rate, the yielding is initiated. The corresponding stress is called yield stress σy. However, whether a polymer product can produce yielding depends on its fracture stress σb. Here, it should be noted that the σb and σy are not the corresponding parameters obtained from the tensile stress−strain curve. Only when σb > σy, the yielding can be initiated smoothly. Thus, the fact that the impact strength of the as-molded PLA70/PC30/ADR0.3 sample did not increase should be due to σb < σy. Certainly, this also can be explained by a yielding principle proposed by Nimmer.25,26 Namely, the ability to sustain the high local stress for the as-molded PLA70/PC30/ADR0.3 sample is not enough to prevent the fracture caused by the high local stress, so it displayed a brittle fracture. However, the impact strength and HDT increase obviously after adding ADR for the annealed

PLA and PC. In addition, it can be found that PC has low influence on the impact strength of PLA even though it owns a very high impact strength, as shown in Figure 2b. Similar to what occurred in HDT, the experimental impact strength is always lower than the corresponding calculated value. In order to improve the interfacial bonding between PLA and PC, a copolymer of styrene and glycidyl methacrylate (coded as ADR) was applied in this work. Figure 2c,d shows the impact and HDT tests results of PLA70/PC30/ADR blends. As can be seen, the impact strengths of the as-molded PLA70/PC30/ ADR samples have no apparent growth trend with an increase in the ADR. The reason might be explained as follows. Yielding is the prerequisite for the ductile fracture of a polymer product, and the segment relaxation time τ is related to stress σ: τ = τ0e(ΔE − aσ)/RT. Here, τ decreases with an increase in σ. After τ 5645


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Figure 5. SEM micrographs of type II surfaces of PLA70/PC30/ADR blends: (a/a′) ADR 0 phr, (b/b′) ADR 0.1 phr, (c/c′) ADR 0.3 phr, and (d/ d′) ADR 0.5 phr.

Figure 8. An ideal model of the “wet brush” state.

formed when the dosage of ADR is greater than or equal to 0.3 phr. To further reveal the formation of lamellar structure, the type II surfaces of PLA70/PC30/ADR were detected by SEM. Figure 5 indicates that the lamellas are distributed in the PLA matrix homogeneously. The reason for the formation of lamella-like structure (PC phase) should be related to the enhanced interfacial bonding between PLA and PC. For a blend, the formation of the lamella-like structure has a close relationship with the flow of the polymer fluid in the mold cavity. The distribution of pseudoplastic fluid displays the shape of the plug, as shown in Figure 6a. In the shear flow area, the flow rate increases gradually from the wall to the center. For the PLA70/PC30/ADR0.3 system, the softening PC particle dispersed in the PLA fluid; it was subjected to two forces along the flow direction, as shown in Figure 6b. F2 was caused by the fluid layer with slow flow rate while F1 resulted from another fluid layer with high flow rate. Therefore, the PC particle would be elongated at the flow direction due to the interaction between F1 and F2, as shown in Figure 6c. However, the lamella-like structure was not observed in the PLA70/PC30

Figure 6. Flow rate distribution of pseudoplastic fluid in a mold cavity (a) and the formation process of the lamella-like structure (b, c).

samples, and they exceed the corresponding calculated value greatly at 0.3 phr ADR, reaching 31.6 kJ/m2 and 86 °C, respectively. Particularly, the impact strength increases incredibly after annealing compared with the as-molded samples; for example, the impact strength of the PLA70/ PC30/ADR0.3 blend increased to 31.6 from 4.6 kJ/m2 after annealing. Obviously, the interfacial bonding between PLA and PC might be improved after incorporating ADR. To confirm the change of interfacial bonding, the morphologies of PLA/PC and PLA70/PC30/ADR blends were observed by SEM, and the micrographs are shown in Figures 3 and 4, respectively. It can be seen that the size of PC particles decreased with increasing ADR, which might be one reason for the improvement of impact strength and HDT after incorporating ADR. Interestingly, a lamella-like structure is

Figure 7. TEM photos of PLA70/PC30 (a, b) and PLA70/PC30/ADR0.3 blends (c, d). 5646


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Figure 9. Impact strengths of PLA/BPM blends (a) and PLA70/PC30/BPM blends (b) and HDT of PLA70/PC30/BPM blends (c).

phase as the “wet brush” state before annealing,27,28 as shown in Figure 8a. After annealing, most of the PLA blocks should still be in the PLA phase as the “wet brush” due to the same molecular structure. Certainly, a small amount of PLA blocks in the PLA-co-PC copolymer could crystallize because the annealing temperature is higher than the Tg of the PLA, which may lead to a little morphological change of a small amount of PLA blocks; however, the crystallinity of the PLA blocks should be very low, which cannot affect the existence of the PLA blocks as the “wet brush” at the interface between PLA and PC, as shown in Figure 8b. For the PC blocks in the PLAco-PC copolymer, PC has a Tg higher than 150 °C, and therefore, the segment motion of its molecular chains should be frozen at the annealing temperature of 120 °C. Thus, the obvious change of PC blocks in the PLA-co-PC copolymer should not occur during annealing and still existed in the interface between PLA and PC as the “wet brush”. On the basis of the analysis presented above, although the annealing temperature has a little effect on the crystallization of a small amount of the PLA blocks in the copolymer, most of both PLA and PC blocks in the PLA-co-PC copolymer should still be in the interface between PLA and PC as the “wet brush” state. Thus, the annealing effect on the interface between PC and PLA can almost be ignored, and it will not be considered in the subsequent discussion of the impact of annealing on different properties.

Table 1. Compositions of PLA/PC/ADR/BPM Blends sample code

I

II

III

PLA PC ADR BPM

90 10 0.3 5

80 20 0.3 5

70 30 0.3 5

blend. It can be deduced that the formation of the lamella-like structure should be related to the interfacial bonding between PLA and PC. To further confirm that the interfacial bonding between PLA and PC exists, TEM was performed in our work. Figure 7 shows the TEM micrographs of PLA70/PC30 and PLA70/PC30/ADR0.3 blends, in which it can be seen that there is a little difference between PLA70/PC30 and PLA70/ PC30/ADR0.3. For the PLA70/PC30 blend, the boundary of two phases is very clear, while it is very obscure for PLA70/ PC30/ADR0.3, implying that the enhanced interfacial bonding might be formed after adding the ADR. Moreover, the obscure part should belong to the PLA-co-PC or part of the ADR. Here, it should be noted that the effect of annealing on the interface between PLA and PC could almost be ignored. The reason can be explained as follows. Due to the same molecular weight and molecular structure between PLA matrix and PLA blocks in the PLA-co-PC copolymer, PLA blocks in the PLA-co-PC copolymer should be in the PLA phase as the “wet brush” state before annealing. Similarly, PC should also be in the PC

Figure 10. Impact strength (a), HDT-0.455 MPa (b), and HDT-1.82 MPa (c) of PLA/PC/ADR0.3/BPM5 blends. 5647


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Industrial & Engineering Chemistry Research Table 2. Tensile and Flexural Properties of PLA/PC/ADR0.3/BPM5 Blends samples PLA90/PC10/ADR0.3/BPM5 PLA80/PC20/ADR0.3/BPM5 PLA70/PC30/ADR0.3/BPM5 PLA PLA90/PC10/ADR0.3/BPM5 annealed PLA80/PC20/ADR0.3/BPM5 annealed PLA70/PC30/ADR0.3/BPM5 annealed PLA annealed

tensile strength (MPa) 61.1 57.8 56.9 70.4 59.3 56.4 57.0 74.8

± ± ± ± ± ± ± ±

0.5 0.2 0.3 0.3 0.2 0.3 0.1 0.4

flexural strength (MPa) 90.9 88.9 86.9 112.8 108.0 104.9 102.9 128.7

± ± ± ± ± ± ± ±

0.6 0.5 0.4 0.2 0.5 0.3 0.3 0.4

flexural modulus (MPa) 3105 2989 2792 3371 3612 3512 3383 3821

± ± ± ± ± ± ± ±

46 36 47 44 121 76 17 172

Figure 11. SEM micrographs of type I surfaces for PLA/PC/ADR0.3/BPM5 blends: (a/a′) PLA90/PC10, (b/b′) PLA80/PC20, and (c/c′) PLA70/ PC30.

Figure 12. SEM micrographs of type II surfaces for PLA/PC/ADR0.3/BPM5 blends: (a/a′) PLA90/PC10, (b/b′) PLA80/PC20, and (c/c′) PLA70/PC30.

For the sake of giving a super toughened PLA/PC blend, an impact modifier is very efficient to achieve the aim. In this work, a core−shell impact modifier marked as BPM was used. Figure 9a shows the impact test results of PLA/BPM blends. The impact strength increases with a raise in the loading of BPM for the as-molded system. Interestingly, the impact strength is obviously improved after annealing. For example, the impact strength of the PLA100/BPM10 blend increased to 54.3 from 16.1 kJ/m2 after annealing. Although BPM exhibited an excellent effect on the toughening of the PLA system, the

impact strength was not improved markedly for PLA70/PC30/ BPM blends, as presented in Figure 9b, showing that the poor interfacial bonding between PLA and PC seriously affected the toughening effect of BPM, particularly when samples were annealed. In addition, the HDTs of PLA70/PC30/BPM blends were also detected. As shown in Figure 9c, the HDT value decreases gradually with an increase in BPM. According to the results presented above, PC can improve the HDT of PLA; BPM shows an excellent effect on improving the impact property of PLA, and ADR plays a good role in 5648


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Figure 13. TG and DTG curves of PLA/PC/ADR0.3/BPM5 blends: N2 (a, b); air (c, d).

Table 3. TG and DTG Data of PLA/PC/ADR0.3/BPM5 Blends samples

T5% (°C)

Tmax (°C)

mass loss rate at Tmax (wt %/min)

PLA-N2 PLA90/PC10/ADR0.3/BPM5-N2 PLA80/PC20/ADR0.3/BPM5-N2 PLA70/PC30/ADR0.3/BPM5-N2

337.7 342.7 346.0 341.3

378.0 378.9 379.8 379.5

56.9 46.7 41.4 38.7

PLA-air PLA90/PC10/ADR0.3/BPM5-air PLA80/PC20/ADR0.3/BPM5-air PLA70/PC30/ADR0.3/BPM5-air

333.7 341.0 342.3 340.3

375.4 375.9 373.6 375.9

60.0 47.9 41.7 39.2

Figure 15. Representative ideal model of the change of PLA structure after annealing.

enhancing the interfacial bonding between PLA and PC. Therefore, combining PC, ADR, and BPM with PLA should be a good approach to improve the impact strength and heat resistance of PLA simultaneously. However, it needs to be noted that the dosage of BPM should be as less as possible because of its damage to the HDTs of PLA/PC blends. 3.2. Impact Property, Heat Resistance, and Thermal Stability of PLA/PC/ADR/BPM Blends. Table 1 lists the

compositions of PLA/PC/ADR/BPM blends; the impact and HDT tests results of these samples are shown in Figure 10. It can be seen that the impact strength increases gradually with a raise in PC dosage for the as-molded PLA/PC/ADR0.3/BPM5 blends, reaching 37.3 kJ/m2 after the content of PC gets to 30 wt % in the polymer matrix. Interestingly, similar to what occurred in PLA70/PC30/ADR and PLA/BPM blends, the

Figure 14. First DSC heating curves of PLA/PC/ADR0.3/BPM5 blends (a), XRD patterns of the PLA70PC30/ADR0.3/BPM5 blend (b), and crystallinity of PLA/PC/ADR0.3/BPM5 blends (c). 5649


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Figure 16. Representative ideal three-dimensional model for the inner structure of the annealed PLA/PC/ADR0.3/BPM5 blend.

Figure 17. SEM micrographs of the impact-fractured surfaces for PLA and PLA/PC/ADR0.3/BPM5 blends, as-molded: (a) PLA, (b) PLA90/PC10, (c) PLA80/PC20, and (d) PLA70/PC30; (a′), (b′), (c′), and (d′) are the corresponding SEM micrographs of the annealed samples.

Figure 18. Stress−strain curves of PLA/PC/ADR0.3/BPM5 blends: (a) PLA90/PC10, (b) PLA80/PC20, and (c) PLA70/PC30.

136 °C from 122 °C of the annealed PLA. However, under the load of 1.82 MPa, the improvement is not very significant, especially when the dosage of PC is lower than 30 wt %. Only for the sample with 30 wt % PC, a considerable increase appeared, reaching 82 °C from 65 °C of PLA. Similar to what occurred in PLA/PC blends, the obvious increase of HDT can also be observed after annealing. Taking the PLA70/PC30/ ADR0.3/BPM5 blend as an example, the HDT increased to 136 and 82 °C from 61 and 57 °C, respectively, for the corresponding load of 0.455 and 1.82 MPa.

impact strength achieved a significant increase after annealing. For instance, the impact strength of the PLA90/PC10/ ADR0.3/BPM5 blend increased to 42.6 from 9.1 kJ/m2 after annealing. Comparing the impact strengths of the annealed PLA/PC/ADR0.3/BPM5 blends with that of neat PLA, the impact strength of PLA increases significantly after incorporating PC, ADR, and BPM, reaching 7.2 times of PLA at least. Meanwhile, it can be seen that the HDT under the load of 0.455 MPa increases with increasing PC content for the annealed PLA/PC/ADR0.3/BPM5 blends, going up to 134− 5650


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Figure 19. Stress−strain curves of PLA70/PC30/ADR blends: (a) ADR 0.1 phr, (b) ADR 0.3 phr, and (c) ADR 0.5 phr.

Figure 20. Stress−strain curves of PLA/BPM blends: (a) BPM 5 phr, (b) BPM 10 phr, and (c) BPM 15 phr.

Compared with PLA composites shown in previous papers.24,29 The prepared PLA-based composite in this work has the comparative mechanical properties and heat resistance. Hashima et al.24 found that the impact strength of PLA40/ PC40/SEBS15/EGMA5 was 66 kJ/m2 at 40 wt % PLA, and its HDT was 95 °C. It should be noted that the PLA content is 40 wt % in the blend, which is much lower than 70 wt %. For the PLA/PC/ADR/BPM blend prepared in our work, the PLA content is about 70 wt %, and the results showed that the impact strength was near 60 kJ/m2. Its HDT was above 130 °C, so the PLA/PC/ADR/BPM blend prepared in this work has

Table 4. Widths and Thicknesses of the Annealed PLA/BPM Blends before and after the Tensile Test samples

width before test (mm)

thickness before test (mm)

width after test (mm)

thickness after test (mm)

PLA/BPM 5 phr PLA/BPM 10 phr PLA/BPM 15 phr

9.73 9.71 9.70

4.02 4.00 4.02

9.72 9.66 9.64

4.01 3.97 4.00

Figure 21. TEM micrograph of yielding-necking region for the as-molded PLA70/PC30/ADR0.3 blend (a) and stress-whitening region for the annealed PLA70/PC30/ADR0.3 blend (b). 5651


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Figure 22. TEM micrograph of yielding-necking region for the as-molded PLA/BPM15 blend (a) and stress-whitening region for the annealed PLA/ BPM15 blend (b1, b2, b3).

Figure 23. TEM micrograph of the yielding-necking region for the as-molded PLA70/PC30/ADR0.3/BPM5 blend (a) and the stress-whitening region for its annealed blend (b).

apparent advantages compared with the previous PLA-based composites. Table 2 shows the data of tensile and flexural properties of PLA/PC/ADR0.3/BPM5 blends. Before annealing, the tensile strength does not decrease with an increase in PC content but is maintained at the same level for the three PLA/PC/ADR0.3/ BPM5 blends. Its trend does not change after annealing, while the flexural strength and modulus decrease slightly with an increase in PC content for the three blends, but they increase after annealing. In addition, the morphologies of the PLA/PC/ADR0.3/ BPM5 blends were investigated by SEM. The corresponding micrographs including the type I and type II surfaces are shown in Figures 11 and 12. As can be seen, both the distribution of the dispersed phases and their particle sizes are quite homogeneous in the two directions. Similar to that existing

in PLA70/PC30/ADR blends, the lamella-like structure was also observed in the three samples. Figure 13 shows the TG and DTG curves of neat PLA and PLA/PC/ADR0.3/BPM5 blends, and the corresponding data are listed in Table 3. PLA/PC/ADR0.3/BPM5 blends have a similar change trend during heating under a N2 or air atmosphere. However, PLA/PC/ADR0.3/BPM5 blends have better thermal stability than neat PLA due to the higher T5% and the lower mass loss rate at Tmax. In a N2 atmosphere, the T5% of PLA was 337.7 °C, while the T5% values of PLA/PC/ ADR0.3/BPM5 blends were 342.7, 346.0, and 341.3 °C, respectively; in an air atmosphere, the T5% of PLA was 333.7 °C, while it increased, respectively, to 341.0, 342.3, and 340.3 °C for PLA/PC/ADR0.3/BPM5 blends. The mass loss rate at Tmax for PLA was about 60%/min in N2 or air, but it reduced to less than 50%/min for PLA/PC/ADR0.3/BPM5 blends. Obviously, PLA/PC/ADR0.3/BPM5 blends possessed better 5652


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movement of the segment in the PLA amorphous phase is limited. This situation is described concretely in Figure 15. Owing to the physical cross-linking of the crystal region, a soft three-dimensional network structure can be formed; undoubtedly, the bond between the amorphous and the crystal phase is very strong, so the segment motion in amorphous phase is seriously limited; meanwhile, the crystal phase also plays a good role in distortion resistance. Thus, the HDT increases significantly after annealing. As for PLA/PC/ADR0.3/BPM5 blends, the largest difference is that PC is incorporated. PC, as a polymer with a high Tg and a high HDT (about 130 °C under the load of 1.82 MPa), can also be considered as a rigid unit, so a rigid threedimensional network structure or a rigid three-dimensional framework, which can play an excellent role in distortion resistance, can be formed in these blends after annealing due to the presence of rigid PC particles and PLA crystals simultaneously. A model corresponding to the network formed in PLA/PC/ADR0.3/BPM5 blends is proposed in Figure 16. As shown in this figure, the solid and the dashed line represent a rigid connection. One PC particle could simultaneously be in two crystal zones while one crystal zone could fix two PC particles at least. Therefore, a rigid three-dimensional network structure can be formed. In addition, the presence of ADR can further enhance the sturdiness of this three-dimensional network structure because it can improve the interfacial bonding between PLA and PC, so the HDT was markedly improved for the annealed PLA/PC/ADR0.3/BPM5 compared with the as-molded one. Apart from HDT, the impact strength also achieved obvious improvement after annealing for PLA/PC/ADR0.3/BPM5 blends, as shown in Figure 10a. Figure 17 shows the impactfractured surfaces of PLA/PC/ADR0.3/BPM5 blends; it can be seen that, except for the PLA70/PC30/ADR0.3/BPM5 blend, the as-molded samples almost have the same characteristics as PLA, which is smoothness and no deformation, and obviously present the behavior of brittle fracture. However, the significant plastic deformation can clearly be observed for the annealed samples, which is the typical ductile-fracture characteristic. As mentioned previously, the change in impact strength caused by annealing should mainly be due to the increase of the crystallinity of PLA, which can be explained by a yielding principle proposed by Nimmer.25,26 According to this principle, the reason why the impact strength obviously increased after annealing is that the ability to sustain the high local stress was significantly improved due to the dramatic increase of crystallinity of PLA, so the plastic deformation was initiated and developed to the entire material, resulting in the ductile fracture. However, as can be seen from the stress−strain curves of the PLA/PC/ADR0.3/BPM5 blends shown in Figure 18, the as-molded samples also show the plastic deformation while their impact strengths are quite low; this is caused by the difference between tensile and impact test, which is that the stress increases slowly for the tensile test and quickly for the impact test. Comparing the stress−strain curves of the annealed PLA/ PC/ADR0.3/BPM5 blends with those of the corresponding asmolded blends, it can be seen that the stress of the annealed system does not decrease immediately after reaching the yielding point but is maintained at this level for a period of time, implying that the yielding behavior might be changed after annealing. Similarly, this phenomenon was also observed in PLA70/PC30/ADR and PLA/BPM blends, as displayed in

mechanical properties, heat resistance, and thermal stability than neat PLA. 3.3. Mechanism on the Improvements of HDT and Impact Strength after Annealing for the PLA/PC/ ADR0.3/BPM5 Blends. Similar to what occurred in the PLA/PC blends, the HDTs of the PLA/PC/ADR0.3/BPM5 blends show a close relationship with annealing, as shown in Figure 10b,c. Generally, annealing at a temperature higher than Tg may bring about the increase of crystallinity and the transformation of crystalline form for a semicrystalline polymer, both of which can affect the properties of this polymer. Figure 14a shows the first heating scans of the PLA/PC/ADR0.3/ BPM5 blends. Comparing the DSC curve of each annealed sample with that of the corresponding as-molded sample, all of them have the same characteristics of a single melting peak and almost the same melting point, indicating that the annealing did not induce a change in the crystalline form. Figure 14b shows the XRD result of the PLA70PC30/ADR0.3/BPM5 blend, which further demonstrates that no transformation of crystalline structure occurred. Meanwhile, the crystallinities of the PLA phase in the PLA/PC/ADR0.3/BPM5 blends were calculated according to the formula below. χc (%) = 100(ΔHm − ΔHc)/(ΔHf × WPLA )

where ΔHm and ΔHc are the melting and cold crystallization enthalpy during the first DSC heating scan, respectively; ΔHf is the heat fusion defined as the melting enthalpy of 100% crystalline poly(lactic acid), which is 93.7 J/g; 30 W PLA represents the weight fraction of PLA in a blend. The calculated data are shown in Figure 14c; it can be seen that the crystallinity obviously increases for all the annealed samples in comparison with the corresponding as-molded samples. The change caused by annealing should be mainly the increase of the crystallinity of PLA. However, it should be noted that the annealing temperature of 120 °C is lower than the Tg of PC (about 160 °C) and slightly higher than that of PMMA (about 105 °C); therefore, the change of the interfacial bonding between PLA and PC (or BPM) should be negligible after annealing. Essentially, the improvement of HDT after annealing results from the increase of the crystallinity of PLA. In order to explain the possible mechanism for the increase of HDT after annealing, several models are proposed, which are shown in Figures 15 and 16. It should be noted that the ideal model proposed here does not exist in neat PLA and its blends; it is just used to present the random rigid three-dimensional framework formed after annealing. Actually, HDT has a close relationship with the mobility of segment for a polymer, so everything favorable for the segment motion will decrease HDT and vice versa. PLA used in this work is a semicrystalline polymer with a quite low crystallization rate.31 Because the mold temperature is at room temperature, the as-molded PLA has low crystallinity. Since the arrangement of molecular chains in this case is not very orderly and compact, the segment motion can occur at a temperature lower than its Tg under the presence of a greater external force, which is the essential reason why the HDT of the as-molded PLA is lower than its Tg, as shown in Figure 10b,c. When annealed, part of the amorphous PLA transformed from an amorphous state to a semicrystalline state. Due to the orderly and compact arrangement of molecular chains, the crystal region can be considered as a rigid unit, so the 5653


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and rigid PLA crystals was formed in a sample, which played an excellent role in deformation resistance. DSC and XRD results illustrated that the change caused by annealing is only the increase of PLA phase crystallinity, resulting in improvement of the ability to sustain the high local stress, which lays the foundation for initiating plastic deformation or more intensive plastic deformation. The differences between the annealed samples and the as-molded samples in stress−strain curves and test phenomena imply the transformation of yielding behavior, which was confirmed via observing the impact-fractured surfaces using SEM and the yielding-necking/stress-whitening regions formed in the tensile test using TEM.

Figures 19 and 20. For PLA/BPM blends, the stress of the annealed system in the plastic deformation region is obviously higher than that of the as-molded system, and no necking was observed. However, stress-whitening during the tensile test indicated that the yielding behavior may be changed after annealing. Table 4 lists the width and thickness of the annealed PLA/ BPM blends before or after the tensile test. It can be seen that their differences before or after the test are very small, indicating that the volumes of these samples increased with the augment of their elongations during the tensile test, so a yielding behavior with crazing might occur for them.32 Because of the same characteristic in stress−strain curves for the annealed PLA/BPM blends, the yielding behavior may also contain crazing for the annealed PLA/PC/ADR0.3/BPM5 blends. Therefore, the yielding behavior of the as-molded PLA/ PC/ADR0.3/BPM5 blends should be dominated by the shear yielding, considering the significant yielding-necking observed in the tensile test and shear-flowing existing in impact-fractured surfaces; after annealing, the yielding behavior may contain not only the shear yielding but also the crazing, and the crazing should occupy a very important role. Thus, the dramatic increase in the impact strength of PLA/PC/ADR0.3/BPM5 blends was achieved after annealing. In order to prove the existence of crazing, the yieldingnecking and stress-whitening regions formed in the tensile test for PLA70/PC30/ADR0.3, PLA/BPM15, and PLA70/PC30/ ADR0.3/BPM5 blends were observed using TEM. Figure 21 shows the TEM micrographs of the as-molded and the annealed PLA70/PC30/ADR0.3. The results showed that, for the annealed PLA70/PC30/ADR0.3 blend, there are many crazes in the stress-whitening region, which are parallel to each other, while there is no obvious craze in the yielding-necking region for the as-molded PLA70/PC30/ADR0.3. For the PLA/ BPM15 blend, the crazes can be observed not only in the annealed sample but also in the as-molded sample, as shown in Figure 22. However, the crazes are not parallel to each other but disorderly in the stress-whitening region of the annealed PLA/BPM15 blend, which may be a combination of different types of crazes. Meanwhile this phenomenon was also observed in the stress-whitening region of the annealed PLA70/PC30/ ADR0.3/BPM5 blend (Figure 23). Therefore, from the viewpoint of the fracture mechanism, the yielding behavior resulting from the combination of different types of crazes occupied an important role in the fracture process of the annealed PLA/PC/ADR0.3/BPM5 blend, while the crazes were parallel or absent in the as-molded samples, which must be the leading reason for the great improvement of impact strength for the annealed PLA/PC/ADR0.3/BPM5 blend.

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: 86-(028)-85410259. E-mail: [email protected]. *Tel./Fax: 86-(028)-85410259. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (Grant No. 51121001 and 51421061) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1026) is sincerely acknowledged.

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REFERENCES

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4. CONCLUSION Incorporating PC into PLA is a very effective approach to improve its impact strength and heat distortion temperature simultaneously. The crystallization of the PLA phase is the essential factor to prepare a supertough and high heat-resistant PLA/PC biobased material. Meanwhile, a compatibilizer should be applied to improve the interfacial bonding between PLA and PC; a small amount of impact modifier also needs to be incorporated to further improve the impact strength. The reason why the HDT achieved such a large increase after annealing for the PLA/PC/ADR0.3/BPM5 blend is that a rigid three-dimensional framework composed of rigid PC particles 5654


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