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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Supertoughened Polylactide Binary Blend with High Heat Deflection Temperature Achieved by Thermal Annealing above the Glass Transition Temperature Liang Deng, Cui Xu, Xuehui Wang, and Zhigang Wang* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *

ABSTRACT: Through thermal annealing above the glass transition temperature, a supertoughened binary blend with the highest notched Izod impact strength of 98 KJ/m2 was achieved, which was about 52 times of that of neat polylactide (PLA; 1.9 KJ/m2). The binary blend was composed of biocompatible and biodegradable PLA and ethylene−acrylic ester−glycidyl methacrylate terpolymer (EGMA) elastomer at the composition of 80/20 PLA/EGMA. For one toughened binary blend with the notched Izod impact strength of 94 KJ/ m2, its tensile elongation at break was kept above 120%. Moreover, this supertoughened binary blend also displayed a much higher heat deflection temperature for application. Thermal annealing induced crystallization of the PLA matrix in the blend, and a linear correlation between the notched Izod impact strength and crystallinity was revealed. The possible toughening mechanism for the PLA/EGMA 80/20 blend with thermal annealing was analyzed from the viewpoint of negative pressure effects, as imposed on EGMA elastomeric particles during the quench process and thermal annealing thereafter. Decreases of the glass transition temperatures for the EGMA elastomeric particles in the blend were observed for both the quench and thermal annealing processes, which originated from asymmetric thermal shrinkages between the EGMA elastomeric phase and PLA matrix phase. KEYWORDS: Elastomer, Biodegradable, Impact strength, Glass transition temperature, Tensile property

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INTRODUCTION

A widely utilized cost-effective technique to improve toughness of PLA is reactive blending, which produces compatibilizers at the phase interface during melt mixing of PLA and certain elastomers.13−26 For example, Fang et al. prepared supertough PLA materials through in situ reactive blending of PLA with PEG-based diacrylate monomer (PEGDA), showing improvements by a factor of 20 in the tensile roughness and by a factor of 26 in the notched Izod impact strength at the optimum CPEGDA content.14 Their highest notched Izod impact strength value was about 50 KJ/ m2. Liu et al. reported the preparation of supertoughened PLA ternary blends, consisting of PLA, elastomeric ethylene−butyl acrylate−glycidyl methacrylate terpolymer (EBA-GMA), and zinc ionomer of ethylene−methacrylic acid copolymer (EMAAZn). Effective interfacial reactions between the epoxy groups of EBA-GMA and the terminal groups of PLA were thought to be responsible for significant increase in the notched Izod impact

Being one of the most innovative biopolymers, polylactide (PLA) has been attracting much attention due to its excellent performances in renewability, ideal carbon cycle excluding petroleum resources, biocompatibility, and biodegradability.1−3 However, commercial PLA products made from polymerization of L-lactic acid are brittle and their toughness in the raw state is inadequate to withstand a high impact strength.4−8 On the other hand, a low heat deflection temperature (HDT) for PLA products is a headache for some typical applications, due to the low degree of crystallinity for PLA materials resulting from the low crystallization rate during an injection-molding cycle involving a high cooling rate and a relatively low mold temperature. Inadequacies in both toughness and heat deflection temperature restrict particular applications of PLA materials in long-term commercial uses, such as automotive and electronics.9,10 Therefore, in the past 2 decades, most research work in modifying PLA was trying to enhance toughness, especially impact performance, and to increase the heat deflection temperature of the PLA products.4,6,10−22 © XXXX American Chemical Society

Received: August 10, 2017 Revised: November 7, 2017

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DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering strength of the PLA materials.24 Oyama prepared high performance PLA blends by reactive blending of PLA with ethylene−acrylic ester−glycidyl methacrylate terpolymer (EGMA), and he obtained the highest notched Izod impact strength value of 72 KJ/m2. Note that the value for his neat PLA was about 1.8 KJ/m2.23 On the other hand, the low crystallization rate of PLA generally produces amorphous PLA after conventional extrusion and injection molding, which results in a low heat deflection temperature (HDT) of about 60 °C close to its glass transition temperature.15,23 Such kinds of PLA parts with low crystallinities show obvious shrinkage when the applied environmental temperature is over 60 °C, vividly indicating an unsatisfactory low heat deflection temperature for applications as daily necessities. Only the crystalline PLA phase with a high melting point of above 160 °C is known to impart sufficient mechanical properties for PLA usage.27 Therefore, the enhancement in crystallization for PLA is crucial to achieve improvement in HDT.10 Thermal annealing above the glass transition temperature is an effective way to modify the crystal forms, crystallinity, and crystal dimensions for semicrystalline polymers.10,28,29 The heat deflection temperature of PLA is considered to be effectively improved with increasing crystallinity. Although Oyama reported an obvious improvement in impact strength for PLA/EGMA blends when the crystallinity was increased,23 the mechanism for improvement of impact strength for PLA blends with a high crystalline PLA matrix, as compared with as-prepared amorphous PLA blends, is not clear so far. In the present work, a supertoughened PLA/EGMA 80/20 blend with the highest notched Izod impact strength of 98 KJ/ m2 was achieved by thermal annealing at a temperature above the glass transition temperature. When the blend underwent thermal annealing at 80 °C for 8 h, it showed not only a high notched Izod impact strength of 94 KJ/m2 but also an elongation at break above 120% with obviously enhanced tensile stress in the tensile test. This supertoughened blend also showed a much higher heat deflection temperature for applications. The effects of thermal annealing on the mechanical properties of the blend were investigated by using wide-angle X-ray diffraction technique, dynamic mechanical analysis, and transmission electron microscopy. The toughening mechanism is proposed from the viewpoint of negative pressure effects, which are related to the dropping of the glass transition temperature of EGMA elastomeric particles in the blend.30−32

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blend. We note that the mixing time of 8 min was experimentally confirmed to give out the maximum value in impact strength when the PLA/EGMA 80/20 blend was annealed. Therefore, we chose the mixing time of 8 min in the study. Thermal Annealing and Impact and Tensile Property Tests. The specimens of PLA/EGMA blend for thermal annealing and impact and tensile property tests were obtained by compression molding at 200 °C under vacuum using a homemade vacuum laminator. The specimens in the molten state from the vacuum laminator were quenched into water at room temperature. Then, the specimens were enclosed into an air-proof stainless steel mold for thermal annealing in a water bath set at different temperatures with temperature fluctuations less than 1.0 °C. The notched impact strength test was performed by using an XJUD-5.5 pendulum impact tester (JinJian-Test, China). The size of the rectangular specimen was 80 × 10 × 3 mm3 with a 45° V-shaped notch (upper radius of 0.25 mm and depth of 2.0 mm). The tensile property test was performed by using a Suns UTM2502 universal testing machine (Suns, Shenzhen, China) at a crosshead speed of 10 mm/min. Dumbbell-shaped samples were punched out from the molded sheets for the tensile property test. Impact and tensile property tests were all performed at room temperature of 23.5 °C. Five specimens for each sample were tested from the viewpoint of statistical data analysis. Differential Scanning Calorimeter. The heat flow curves during heating scan from 20 to 200 °C for the samples were measured by using TA Q2000 differential scanning calorimeter. The heating rate was 10 °C/min. A nitrogen atmosphere was applied during the measurement. Wide-Angle X-ray Diffraction. The degrees of crystallinity for the specimens were measured by using wide-angle X-ray diffraction (WAXD; GeniX 3D Cu Microspot, Xenocs SAS, France). WAXD patterns were taken with the aid of a semiconductor detector (Pilatus 100 K, DECTRIS, Swiss) with a resolution of 487 × 195 (pixel size of 172 μm) attached to a multilayered mirror (FOX 3D Cu 21-21, Xenocs SAS) with a focused Cu Kα X-ray source (GeniX 3D Cu Xenocs SAS), generated at 50 kV and 0.6 mA. The wavelength of Xray was 0.154 nm, and the sample to detector distance was 38.9 mm. The beam was focused onto the sample position with a spot size of 60 μm × 60 μm. Each WAXD pattern was collected in 10 s, and the background was subtracted. Dynamic Mechanical Analysis. Dynamic mechanical analysis (DMA) measurements in the tensile mode were carried out by using a Netzsch dynamic mechanical analyzer (Netzsch, DMA 242E Artemis). The DMA measurements were performed in the linear region with a strain of 0.03%. A frequency of 1 Hz, a heating rate of 3 °C/min, and a temperature range from −150 to 200 °C were applied. Heat Deflection Temperature Test. The heat deflection temperature test was performed by using a homemade apparatus. The water bath or dimethylsilicone oil bath in a beaker was used to provide the environmental temperatures for the test. Two Hoffman clamps were used to hold a blend specimen. A sheet steel with the same thickness as the blend specimen was clamped parallel to the specimen to sustain the distance between the Hoffman clamps. Standard weights were put on the blend specimen during the test. The bath temperatures were recorded by using a calibrated thermometer. The Vicat softening temperatures (VST) were also measured according to ISO-306 by using VST XRW-300HB (Chengdu Cots Scientific Instruments). The load of 10 MPa was applied. The area of indenter was 1 mm2. The heating rate was 120 °C/h. Transmission Electron Microscopy. The phase morphology of dispersed EGMA elastomeric particles in PLA matrix was examined by using transmission electron microscopy (TEM; Hitachi HT-7700) at an accelerated voltage of 100 kV. Ultramicrotome was used to collect sections with thickness of about 80 nm from the blend samples, and then these sections were stained by ruthenium tetroxide (RuO4) for 4 h at room temperature. For EGMA particle size analysis, at least 160 particles from each TEM micrograph were analyzed by using the image analysis software (ImageJ, U.S. National Institutes of Health (NIH)).

MATERIALS AND METHODS

Materials. Commercially available polylactide (Natureworks product PLA2003D) was purchased for this study. The PLA sample had a density of 1.24 g/cm3 and a melt flow index of 6 g/(10 min) (210 °C, 2.16 kg). Gel permeation chromatography measurement showed that the PLA sample had weight-averaged molecular mass of 296 kg/mol and polydispersity of 2.3. The chosen reactive elastomer (LOTADERAX8900, Arkema) was ethylene−acrylic ester−glycidyl methacrylate random terpolymer. The contents of methyl acrylate and glycidyl methacrylate in EGMA were 24 and 8%, respectively. Gel permeation chromatography measurement showed that the EGMA sample had weight-averaged molecular mass of 197 kg/mol and polydispersity of 8.3. Reactive Blending. PLA and EGMA were dried at 80 and 40 °C, respectively, under vacuum for 12 h prior to reactive blending. PLA and EGMA with a mass ratio of 80/20 were mixed at 200 °C for 8 min at a rotor speed of 80 rpm by using an XSS-300 torque rheometer (Shanghai Kechuang Co. Ltd.) to prepare the PLA/EGMA 80/20 B

DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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RESULTS AND DISSCUSSION Simultaneous Improvements in Notched Impact Strength and Tensile Toughness. At first, the thermal annealing effect on the change of notched Izod impact strength of the PLA/EGMA 80/20 blend was examined. Figure 1a

strength showed an obvious change with annealing time. Within the first 8 h annealing, the notched Izod impact strength showed a linear increase from 35 to 50 KJ/m2. Within a subsequent 4 h, the notched Izod impact strength showed a rapid increase from 50 to 84 KJ/m2. A further 12 h annealing only led to a linear increase of the notched Izod impact strength from 84 to 96 KJ/m2. When the thermal annealing temperature increased to 80 °C, 10 °C higher, the change of notched Izod impact strength with annealing time became much more significant. Just within 2 h, the notched Izod impact strength rapidly increased from 35 to 88 KJ/m2. The subsequent thermal annealing for up to 24 h led to an increase in the notched Izod impact strength from 88 to 98 KJ/m2. It is noticed that the notched Izod impact strength for neat PLA was just 1.9 KJ/m2.14 Therefore, the highest notched Izod impact strength for the thermally annealed blend is about 52 times of that for neat PLA. Tensile toughness was further examined by using mechanical tensile test. Figure 1b shows the nominal stress−strain curves for PLA/EGMA 80/20 blend thermally annealed at 80 °C for different annealing times. The result for thermal annealing at 80 °C is selectively shown here because the notched Izod impact strength for the blend changes significantly at this temperature. In Figure 1b, it can be apparently seen that the elongation at break basically decreases with increasing annealing time. For instance, PLA/EGMA 80/20 blend before annealing had an elongation at break of 320% and a tensile toughness of 99.4 MJ/m3; however, after annealing at 80 °C for 24 h, the elongation at break decreased to 60% and the tensile toughness decreased to 17 MJ/m3. The changes of elongation at break and tensile toughness with annealing time for the PLA/EGMA 80/ 20 blend annealed at 80 °C for different annealing times are shown in Figure S2. Those with annealing less than 8 h exhibited evident ductile fracture, with the elongation at break values kept above 130%. Although Oyama had obtained high performance PLA blends by reactive blending of PLA and EGMA, and the highest notched Izod impact strength of 72 KJ/ m2 for his best sample with the code annealed L80-200,23 the elongation at break for this best sample was just 12%. Another interesting output is an obvious increase of tensile strength from about 30 to 36 MPa at the plateau tensile region at the annealing time of 2 h and above. Up to now, we can conclude that simultaneous improvements in notched impact strength and tensile toughness can be achieved by choosing an appropriate thermal annealing temperature and annealing time for the PLA/EGMA 80/20 blend; for example, a thermal annealing for 2 h at the annealing temperature of 80 °C might be the best combination for the PLA/EGMA 80/20 blend, having the notched Izod impact strength of 88 KJ/m2 (46 times of that for neat PLA), the elongation at break of 188% (about 28 times of that for neat PLA), the tensile toughness of 67 MJ/ m3, and tensile strength at break of 39 MPa. Crystallinity Increases due to Thermal Annealing. The physical, mechanical, and thermal properties of PLA materials in the solid state are highly dependent on the crystallinity and crystalline morphology because PLA is a typical semicrystalline polymer. To elucidate the effect of thermal annealing on mechanical properties of the PLA/EGMA 80/20 blend, the crystalline structure and crystallinity were determined by using WAXD technique. Figure 2 shows the WAXD intensity profiles for PLA/EGMA 80/20 blend annealed for different annealing times at 70 and 80 °C, respectively. For the PLA/EGMA 80/20 blend without experiencing thermal annealing, no diffraction

Figure 1. (a) Changes of notched Izod impact strength as functions of annealing time for PLA/EGMA 80/20 blend annealed at 70 and 80 °C, respectively, and (b) nominal stress−strain curves for PLA/EGMA 80/20 blend annealed at 80 °C for different annealing times. The result for physical aging at 50 °C is enclosed in panel a.

shows the changes of notched Izod impact strength as functions of annealing time for PLA/EGMA 80/20 blends, which were annealed at 70 and 80 °C, respectively. The result for physical aging at 50 °C is enclosed in Figure 1a for comparison. As reported by Pan et al., chain conformational and microstructural rearrangements occur during the physical aging process.33 However, its effect on the notched Izod impact strength is minor, as it can be seen that the notched Izod impact strength only has a slight increase with increasing physical aging time at 50 °C. The glass transition temperature, Tg, for PLA/EGMA 80/20 blend was about 56 °C as determined by differential scanning calorimetry (DSC; Figure S1 in the Supporting Information). Therefore, even though the physical aging causes chain conformational and microstructural rearrangements, the physical aging at 50 °C does not bring about any significant microstructural changes for the blend. When the environmental temperature around the blend is raised above Tg of the blend, the blend can be considered in a thermal annealing process, because the polymer chains start to be active to involve in some obvious structural rearrangements, for example, a cold crystallization process, the typical one for semicrystalline polymers.34−37 When PLA/EGMA 80/20 blend was thermally annealed at 70 °C, the notched Izod impact C

DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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due to the limited macromolecular chain mobility at the low thermal annealing temperature.41 Therefore, the result in Figure 2a indicates that loose α′-form crystals are induced during the thermal annealing process at 70 °C. Figure 2b shows that at the thermal annealing temperature of 80 °C the crystallization of PLA in the blend follows a changing trend similar to that at 70 °C. The α′-form crystals with a loose 103 helical chain packing are induced at 80 °C as well because both the thermal annealing temperatures of 70 and 80 °C are much lower than 120 °C, the upper limit for the formation of α′-form crystals. However, the difference is also obvious between the cold crystallization processes at these two thermal annealing temperatures. The cold crystallization rate at 80 °C is much faster than that at 70 °C. Figure 2b demonstrates that when the PLA/EGMA 80/20 blend was annealed at 80 °C, a visible diffraction peak appeared at 2θ of 16.3° in the WAXD intensity profile within just 1 h and the diffraction peaks at 2θ of 16.3° and 18.6° developed significantly within just 2 h. The WAXD intensity profiles as shown in Figure 2 can be further analyzed by using a peak-fitting procedure.42−45 Representative peak deconvolutions of the WAXD intensity profiles for PLA/EGMA 80/20 blend samples with no thermal annealing and with thermal annealing at 80 °C for 8 h are shown in Figure 3a,b, respectively. Figure 3a presents a particular case, in which the sample was quenched and not annealed; only an amorphous phase existed. Note that two Gaussian curves must be combined to fit well the measured

Figure 2. WAXD intensity profiles for PLA/EGMA 80/20 blend annealed for different annealing times at (a) 70 and (b) 80 °C.

peaks can be seen in the WAXD intensity profile, illustrating that the as-prepared PLA/EGMA 80/20 blend remains amorphous due to its slow crystallization rate and rapid cooling rate upon quench from the molten state into water at room temperature. We note that neat EGMA is just an amorphous material, which does not show any diffraction peaks in its WAXD intensity profile (Figure S3). It can be seen from Figure 2a that a visible diffraction peak at 2θ of 16.3° appears in the WAXD intensity profile when PLA/EGMA 80/20 blend was annealed at 70 °C for 8 h, distinct diffraction peaks at 2θ of 16.3° and 18.6° appear when PLA/EGMA 80/20 blend was annealed at 70 °C for 12 h, and the diffraction peak intensities obviously increase when PLA/EGMA 80/20 blend was annealed at 70 °C for 24 h. The increasing diffraction peak intensities at 2θ of 16.3° and 18.6° indicate increasing crystallinity in PLA/EGMA 80/20 blend due to the prolonged cold crystallization process. The diffraction peaks located at 16.3° and 18.6° are attributed to the (110/200) and (203) lattice planes of PLA crystals.38,39 As reported in the literature, the formation of crystal forms of PLA shows obvious dependence on thermal annealing temperature.40,41 The formation of α′- and α-form crystals in PLA depends on the temperature for thermal annealing. At the thermal annealing temperature of 120 °C and above, the α-form crystals with a dense 103 helical chain packing can be induced, as indicated by the characteristic diffraction peaks at 2θ of 16.7° and 19.7°, respectively, whereas, at the thermal annealing temperature below 120 °C, the α′-form crystals with a loose 103 helical chain packing can be induced, as indicated by the characteristic diffraction peaks at 2θ of 16.4° and 18.5°, respectively, possibly

Figure 3. Peak deconvolutions of the WAXD intensity profiles for PLA/EGMA 80/20 blend samples with no thermal annealing (a) and with thermal annealing at 80 °C for 8 h (b). The open circle points represent the experimental data points. The blue curves represent the contribution of the amorphous phase, the green curves represent the contribution of the crystalline phase, and the magenta curves represent the sum of both the amorphous and crystalline phases. D

DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering WAXD data points, the open circles, possibly due to coexistence of two types of intermolecular chain distances in the blend.42−45 Figure 3b presents the peak deconvolution for the thermally annealed PLA/EGMA 80/20 blend sample, in which an additional two Gaussian curves represent the diffraction peaks relating to the α′-form crystal phase (green curves), coexisting with the two Gaussian curves relating to the amorphous phase (blue curves). The magenta curves represent the sum of the contributions from both the amorphous phase (blue curves) and crystalline phase (green curves), which overlap well with the measured total WAXD intensity profiles. The peak deconvolutions of the WAXD intensity profiles for all other thermally annealed PLA/EGMA 80/20 blend samples can be found in Figure S4 (at annealing temperature of 70 °C) and Figure S5 (at annealing temperature of 80 °C). It is noticed here that for PLA/EGMA 80/20 blend experiencing physical aging at 50 °C for at least 24 h, no crystalline diffraction peaks can be detected in their WAXD intensity profiles, which can be basically considered as in the amorphous state. Peak deconvolution of the WAXD intensity profile for the PLA/EGMA 80/20 blend provides the respective contributions of the amorphous phase and crystalline phase, which can be used to easily obtain the crystallinity in the annealed sample. Figure 4 shows the changes of crystallinity with annealing time

Figure 5. Change of notched Izod impact strength as a function of crystallinity for PLA/EGMA 80/20 blend samples annealed at 70 and 80 °C, respectively.

°C, respectively. As is expected, the notched Izod impact strength and crystallinity follow a linear relationship, for which the notched Izod impact strength increases almost linearly with increasing crystallinity. Even more interestingly, such a linear relationship can be simultaneously held for PLA/EGMA 80/20 blend samples, which were thermally annealed at 70 and 80 °C for different times. Fu et al. reported tailoring of the impact strength of PLA/PCL/TMC blends by controlling the crystallization of the PLA matrix and concluded that toughening of PLA/PCL blends became possible by increasing crystallinity of the PLA matrix.46 However, the highest notched Izod impact strength values they had obtained for PLA/PCL/ TMC blends associated with the highest crystallinity was still below 30 KJ/m2. Here, it is clearly shown that EGMA is an effective elastomer for enhancement of the impact strength of PLA, resulting in the notched Izod impact strength of 35 KJ/m2 for the PLA/EGMA 80/20 blend with no thermal annealing, whereas the same parameter for neat PLA is just 1.9 KJ/m2.14 The mechanism for the above toughening will be provided in a later section. WAXD measurement shows that PLA/EGMA 80/20 blend with no thermal annealing is in the amorphous state. Thermal annealing results in cold crystallization of PLA in the PLA/EGMA 80/20 blend, and it seems that increasing crystallinity leads to a linear increase in the notched Izod impact strength. Note that the notched Izod impact strength of the blend can reach above 90 KJ/m2 when the crystallinity increases just above 23%. It is apparently concluded that the much higher impact strength for the PLA/EGMA 80/20 blend can be potentially achieved by controlling the cold crystallization kinetics of the blend. Temperature Dependence of Storage Modulus and Heat Deflection Temperature. Besides the impact strength, heat deflection temperature is also a quite important parameter to evaluate the application of PLA materials. From the application viewpoint, the heat deflection temperature of PLA materials must be obviously improved. Thermal annealing has been demonstrated to effectively increase the crystallinity for the PLA/EGMA 80/20 blend, which brings about a potential to enhance the heat deflection temperature. To elucidate the effect of thermal annealing on heat deflection temperature for the blend, dynamic mechanical analysis (DMA) was carried out for the blend. Figure 6 shows the changes of storage modulus, E′, with increasing temperature for EGMA and PLA/EGMA 80/20 blend with no annealing and with annealing at 80 °C for 8 h,

Figure 4. Changes of crystallinity with annealing time for PLA/EGMA 80/20 blend samples annealed at 70 and 80 °C, respectively.

for PLA/EGMA 80/20 blend when thermally annealed at 70 and 80 °C, respectively. The increases in crystallinity with annealing time for the blend samples at both 70 and 80 °C follow a sigmoidal shape, exhibiting an initial slow increase to a rapid increase in crystallinity in the primary crystallization stage and approaching the second slow increase at the end of the primary crystallization stage. Figure 4 further confirms a rapid cold crystallization kinetics for the blend when thermally annealed at 80 °C, while it becomes much slower when the thermal annealing temperature decreases to 70 °C. Linear Dependence of Notched Izod Impact Strength on Crystallinity. Similar changing trends for the notched Izod impact strength and crystallinity with increasing thermal annealing time as shown in Figure 1a and Figure 4, respectively, infer that there is a certain correlation between the notched Izod impact strength and crystallinity for the PLA/EGMA 80/ 20 blend. It is quite interesting to plot the variation of notched Izod impact strength with crystallinity. Figure 5 shows the change of notched Izod impact strength with crystallinity for the PLA/EGMA 80/20 blend samples annealed at 70 and 80 E

DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. Changes of storage modulus, E′, with increasing temperature for EGMA and PLA/EGMA 80/20 blend with no annealing and with annealing at 80 °C for 8 h, respectively.

respectively. A singular storage modulus dropping stage can be seen for the EGMA sample, and the rapid modulus dropping is apparently related to the glass transition of EGMA, with Tg of about −17 °C. A similar glass transition can be distinctly seen for PLA/EGMA 80/20 blend. For both of them, decreasing Tg values can be seen as compared with that of neat EGMA. This is subjected to more discussion in a later section. Herein, the changes of storage modulus of the PLA matrix in the blend with increasing temperature are the main focus. The major storage modulus dropping in the temperature range from 55 to 82 °C occurs for the blend with no thermal annealing, apparently correlating to the glass transition of the PLA matrix. With further increasing temperature, the storage modulus shows a rapid increase from 107 to 137 °C due to cold crystallization of PLA and a rapid decrease from 137 to 153 °C due to melting of the formed PLA crystals. Therefore, for PLA/EGMA 80/20 blend there exists a wide temperature range between 82 and 107 °C, having much lower storage modulus values below 6 MPa, while the situation becomes a lot different for the annealed blend. The storage modulus dropping for PLA matrix is not straight. There are two dropping stages for the storage modulus, marked by three crossover temperature points, T1 (=66 °C), T2 (=89 °C), and T3 (=140 °C), determined by the crossover points of the dashed−dotted lines, overlapping with the storage moduli for each stage. Although the storage modulus shows a relatively less steep dropping from T1 to T2 because of the glass transition of the PLA matrix, the storage modulus still remains higher than 104 MPa before the temperature approached 100 °C because of the preformed PLA crystals in the blend due to thermal annealing. The preformed PLA crystals form a crystalline network, which can keep the storage modulus decrease rate being reasonably low from T2 to T3, sustaining a storage modulus value of 45 MPa at 140 °C. This result demonstrates that when PLA/EGMA 80/ 20 blend experiences thermal annealing at 80 °C for a sufficient time (2 h and above), the blend can have high storage moduli at the high temperatures (below the melting point of preformed PLA crystals). Overall, the annealed blend shows a much higher heat deflection temperature than the one with no thermal annealing, which is splendid for the application purpose. To demonstrate the difference in the heat deflection temperature between the PLA/EGMA 80/20 blend with and without thermal annealing, the heat deflection temperature test was performed in water or dimethylsilicone oil bath. Figure 7a

Figure 7. Digital photographs taken during the heat deflection temperature test in water bath (lower than 100 °C) and dimethylsilicone oil bath (higher than 100 °C) for PLA/EGMA 80/ 20 blend with no annealing (a, left panel) and with thermal annealing at 80 °C for 8 h (b, right panel). The standard weight was 20 g.

shows a series of digital photographs taken during the heat deflection temperature test in a water bath lower than 100 °C for PLA/EGMA 80/20 blend with no annealing. It can be seen that, in the water bath of 50 °C the specimen was parallel to the sheet steel held behind the specimen, and the specimen remained straight. The sheet steel was not visible at this temperature, indicating no deflection in the specimen. At 56 °C, the behind sheet steel became visible, indicating that the specimen in front of the sheet steel started to deflect. When the water bath temperature was 60 °C, the behind sheet steel was clearly exposed and the specimen also showed a clear deflection, and for the water bath temperature of 80 °C, the deflection of the specimen became so obvious that the whole setup of Hoffman clamps and sheet steel completely showed up. Therefore, it can be concluded that for PLA/EGMA 80/20 blend with no thermal annealing, the heat deflection temperature was about 56 °C, which is consistent with the glass transition temperature of the blend as measured by DSC (Figure S1). Figure 7b shows a series of digital photographs taken during the heat deflection temperature test in a water bath of 80 °C and in dimethylsilicone oil bath (higher than 100 °C) for PLA/ EGMA 80/20 blend with thermal annealing at 80 °C for 8 h. It indicates that in the water bath of 80 °C the specimen did not show any deflection at all. Even though the water bath temperature was increased to 100 °C, no deflection occurred for the specimen. Instead, the dimethylsilicone oil bath had to be used for the test. At both temperatures of 130 and 146 °C, the sheet steel behind the specimen was not exposed, inferring F

DOI: 10.1021/acssuschemeng.7b02751 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering that there appears to be no specimen deflection. Only when the temperature was set to 148 °C was the visible deflection starting to show. The above test results indicate that at the same test condition the annealed blend has much higher heat deflection temperature than the one with no annealing. Figure 8 shows a digital photograph for the specimens taken after the

Figure 9. TEM micrographs for PLA/EGMA 80/20 blend with no annealing (left panel, a, a′) and with thermal annealing at 80 °C for 8 h (right panel, b, b′). The micrographs at the bottom have higher magnification.

Figure 8. Digital photograph taken after the heat deflection temperature test for PLA/EGMA 80/20 blend with no annealing tested in the water bath of 80 °C (a) and PLA/EGMA 80/20 blend (b) with thermal annealing at 80 °C for 8 h tested in the dimethylsilicone oil bath of 130 °C. (Logo for Univeristy of Science and Technology of China reprinted with permission.)

shows the length distributions in the major and minor axis of an ellipse for EGMA particles in PLA/EGMA 80/20 blend with no annealing and with thermal annealing at 80 °C for 8 h, respectively. The average EGMA particle lengths for PLA/ EGMA 80/20 blend with no annealing are around 311 ± 18 nm in the major axis of an ellipse and 190 ± 18 nm in the minor axis of an ellipse. Then, the ratio of the length in the major axis to that in the minor axis of an ellipse is 1.64. The average EGMA particle lengths for PLA/EGMA 80/20 blend with thermal annealing at 80 °C for 8 h are around 245 ± 15 nm in the major axis of an ellipse and 165 ± 14 nm in the minor axis of an ellipse. Then, the ratio of the length in the major axis to that in the minor axis of an ellipse becomes 1.48. The EGMA particle sizes are much smaller than the particle sizes of the PCL component for toughnening PLA in PLA/PCL blends.47 The smaller particle sizes of EGMA particles can be attributed to interfacial reactions between the hydroxyl and carboxyl groups of PLA chains and the epoxy groups of EGMA. The interfacial reactions can be confirmed by FTIR spectra as shown in Figure S7. EGMA shows an absorption band at 910 cm−1, which can be ascribed to the epoxy groups, and this absorption band can still be seen when PLA and EGMA were mixed at the mass composition of 80/20 through solution blending, indicating an existence of only physical mixing of these two components. However, this absorption band disappears when PLA and EGMA were mixed through reactive blending, confirming an existence of interfacial reactions. A similar observation was reported in the literature.19 Furthermore, the interfacial reactions can help produce an interfacial layer, which appears as a thin layer with an intermediate gray color surrounding the dark EGMA particles, as shown in Figure 9a′,b′. Overall, not much obvious EGMA particle size changes are observed due to thermal annnealing, indicating that for PLA/EGMA 80/20 blend the significant improvement in notched Izod impact strength for the annealed sample is not related to the changes in the EGMA particle size. It has been reported that for PLA materials the calculated rubber particle

heat deflection temperature test: specimen “a” stands for the PLA/EGMA 80/20 blend with no annealing, tested in the water bath of 80 °C; specimen “b” stands for the PLA/EGMA 80/20 blend annealed at 80 °C for 8 h, tested in the dimethylsilicone oil bath of 130 °C. The huge distortion can be seen for the former one, and in contrast, there is no trace of distortion for the latter one. The results shown in Figures 7 and 8 are consistent with that shown in Figure 6. We note that similar results were observed when the applied weights were below 100 g. Furthermore, the Vicat softening temperatures (VST) for these two typical samples were measured according to ISO-306. VST was 65 °C for PLA/EGMA 80/20 blend with no annealing due to the amorphous phase of the PLA matrix, whereas VST became 122 °C when PLA/EGMA 80/20 blend was annealed at 80 °C for 8 h, indicating that the heat deflection temperature could be significantly improved. Toughening Mechanism for Notched Izod Impact Strength. It is well-known that the phase morphology, including the sizes of dispersed elastomeric phase domains, interfacial interactions, and interdomain distances, is a crucial factor that determines the mechanical property of the polymer blends. To examine any possible effect of the dispersed EGMA particle sizes, the difference in phase morphology between the annealed blend and the one with no annealing was examined. Figure 9 shows the typical TEM micrographs for PLA/EGMA 80/20 blend with no thermal annealing (left panel) and with thermal annealing at 80 °C for 8 h (right panel). A clear sea− island phase morphology is observed for both blend samples. EGMA phase domains appear as dark particles. The majority of EGMA particles with relatively large sizes take the shape of an ellipse, and this shape is apparently related to the shear effect in the melt processing. The particle sizes and distributions can be obtained by image analysis using ImageJ software. Figure S6 G

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ACS Sustainable Chemistry & Engineering sizes of 0.1−0.3 μm bring about the optimum impact strength improvement, phenomenologically consistent with our result.48 However, the particle sizes in diameter in the range 1−2 μm were observed for a sharp increase in impact strength as well for PLA blended with 20 wt % rubber.48 It will be demonstrated in a later section that the improvement in impact strength for the annealed blend is actually related to the glass transition temperature dropping for the EGMA component, which might be reflected in the insignificant EGMA particle size changes. For an impact strength test on PLA materials, the duration time of impact is short. In such a short time, the relaxation of PLA chains might be difficult to follow up on except that more flexible elastomeric polymer chains are blended into the PLA matrix. The impact toughening for blending PLA and an elastomer is attributed for improving the energy dissipation in the relaxation process. Herein, we try to apply the concept of negative pressure to elucidate the possible toughening mechanism for PLA/EGMA 80/20 blend with experiencing sufficient thermal annealing.30−32 For this purpose, the glass transition temperature change in the annealed blend is further disclosed in detail. Figure 10 shows the changes of loss tangent,

apparently higher Tg values from the DMA test than that from the DSC test (Figure S1) are rational because the DMA test discloses the dynamic process of the chain mobility. On the other hand, in the lower temperature range from −100 to 40 °C, the tan δ versus temperature curves for the three samples as shown in Figure 10b display one dominant peak for each sample, corresponding to the glass transition of the EGMA component. Neat EGMA shows a singular Tg at −17 °C, while PLA/EGMA 80/20 blend with no annealing shows a singular Tg at −31 °C. When EGMA is blended with PLA, there is a drop in Tg of about 14 °C, a significant drop of Tg for the EGMA component in the blend. The above drop in Tg for the EGMA component in the blend can be explained by the effect of negative pressure (i.e., thermal stress) as imposed on the dispersed EGMA elastomeric particles.30−32 Figure 11 schematically illustrates the generation

Figure 11. Schematic illustration of generation of negative pressure in the interfacial layer causing the dropping of glass transition temperature during the quench (from a to b) and thermal annealing (from b to c) processes.

of negative pressure in the interfacial layer between EGMA particles and PLA matrix during the initial quench and subsequent thermal annealing processes, which can eventually cause the drop of the glass transition temperature. It has been proposed that the negative pressure originates from the asymmetric thermal shrinkages in the polymer rubbery particle and matrix plastic phases during the cooling process for the blend preparation, which brings in an interfacial layer with a reduced density.30 The asymmetric thermal shrinkages in PLA/ EGMA 80/20 blend can be inferred by the much different linear thermal expansion coefficients between PLA and EGMA components. The linear thermal expansion coefficient for the PLA component was reported to be about 65 × 10−6 (mm/ mm)/°C,49,50 while the linear thermal expansion coefficients for the ethylene−methacrylic acid copolymers were reported to be (180−300) × 10−6 (mm/mm)/°C.51 It can be thought that the thermal shrinkage of the EGMA phase domains is much stronger than that of the PLA matrix, which produces an interfacial layer with an apparently reduced elastomeric density, as schematically illustrated in Figure 11b,c. Furthermore, panels a′ and b′ of Figure 9 eventually display the existence of such a kind of thin layer surrounding the EGMA particles. The

Figure 10. Changes of loss tangent, tan δ, with increasing temperature in a wide temperature range (a) and in the lower temperature range (b) for EGMA and PLA/EGMA 80/20 blend with no annealing and with annealing at 80 °C for 8 h, respectively.

tan δ, with increasing temperature for EGMA and PLA/EGMA 80/20 blend with no thermal annealing and with thermal annealing at 80 °C for 8 h. In the wide temperature range from −60 to 120 °C, the tan δ versus temperature curves for PLA/ EGMA 80/20 blend show one dominant peak, which corresponds to the glass transition of the PLA matrix, as clearly shown in Figure 10a. For the blend with no thermal annealing, the Tg value is 72 °C, while for the blend with thermal annealing at 80 °C for 8 h, the Tg value is 80 °C. The obvious increase in Tg is apparently due to the chain mobility confinement caused by PLA crystallization. It is noted that the H

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crystallization process in the blend produced the PLA crystals, which could form the crystalline network to improve the heat deflection temperature, which was disclosed by heat deflection temperature tests. The obvious enhancement in impact strength could be explained by the negative pressure effects, which were induced by mismatched thermal expansion coefficients of EGMA and PLA components during the quench process and by further volume shrinkage due to crystallization of the PLA matrix during the cold crystallization process. The negative pressure effects could be reflected in the glass transition temperature drop of EGMA elastomeric particles in the blend as disclosed by DMA test. Overall, thermal annealing could be a facile way to easily operate in industry for bringing the PLA/EGMA 80/20 blend from a regular material to a supertoughened one. By enlarging the heat deflection temperature range and the impact strength, we can extend the applications of PLA materials significantly.

thickness of this interfacial layer is estimated in the range of 20−60 nm. The negative pressure generated on the EGMA particles, in turn, could cause a dilation effect on the PLA matrix ligament between the EGMA particles, and the dilation could lead to a loose segment−segment aggregation, which is favorable for the local segmental motions, and therefore enhances the impact strength. The negative pressure developed during the quench process from the melt state (from a to b in Figure 11) controls the path of impact fracture, which accounts for impact strength improvement for PLA/EGMA 80/20 blend with no thermal annealing as compared with neat PLA. The further obvious improvement of impact strength for PLA/EGMA 80/20 blend with thermal annealing as compared with the one with no annealing follows the same mechanism (from b to c in Figure 11). As shown in Figure 10b, when PLA/ EGMA 80/20 blend was thermally annealed at 80 °C for 8 h, the Tg value becomes −35 °C. Therefore, there is a further decrease in Tg of about 4 °C, an obvious further drop of Tg for the EGMA component in the annealed blend. The decreased Tg value of the EGMA component in the annealed PLA/ EGMA 80/20 blend indicates a further enhanced negative pressure as imposed on EGMA particles. It is safe to say that the further drop in Tg originates from further PLA matrix shrinkage when PLA starts to crystallize.31,32 The PLA matrix shrinkage due to cold crystallization brings about some threadlike texture as can be seen in Figure 9b,b′. Up to now, from the viewpoint of negative pressure effect, the strong enhancement in impact strength can be explained for PLA/EGMA 80/20 blend as compared with neat PLA and for the annealed PLA/ EGMA 80/20 blend as compared with the one with no any thermal annealing. The negative pressure effect for PLA/ EGMA 80/20 blend originates from the mismatched thermal expansion coefficients of EGMA and PLA components during the quench process, and for thermally annealed PLA/EGMA 80/20 blend the cold crystallization of PLA matrix in the blend makes an additional contribution to the negative pressure effect. We emphasize here that the interfacial interaction between EGMA particles and PLA matrix also plays a key role on the formation of the interfacial layer with a reduced density; otherwise the voids could be produced at the interface instead.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02751. DSC heat flow curves and changes of elongation at break and tensile toughness, WAXD intensity profile for neat EGMA, peak deconvolutions of WAXD intensity profiles, length distributions in the major and minor axes of an ellipse for EGMA particles, and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 0551-63607703. Fax: +86 0551-63607703. E-mail: [email protected]. ORCID

Zhigang Wang: 0000-0002-6090-3274 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.W. acknowledges financial support from the National Natural Science Foundation of China (Grant Nos. 51673183 and 51473155). The project is also supported by the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Prof. Yongfeng Men at Changchun Institute of applied Chemistry, Chinese Academy of Sciences, is acknowledged for providing use of the WAXD facility.

CONCLUSIONS In this study, ethylene−acrylic ester−glycidyl methacrylate terpolymer (EGMA) elastomer was melt blended with polylactide (PLA) at the composition of 80/20, showing a notched Izod impact strength of 35 KJ/m2 and an elongation at break of 316%. When the PLA/EGMA 80/20 blend was thermally annealed at the temperatures (70 and 80 °C) above the glass transition temperature (56 °C) of the blend, the notched Izod impact strength could be significantly improved, reaching the highest value of 98 KJ/m2, about 52 times of that for neat PLA, while the elongation at break still remained above 120% when the thermal annealing time did not exceed 8 h (with notched Izod impact strength of 94 KJ/m2 at 8 h thermal annealing time), indicating that supertoughened PLA materials could be obtained simply through a combination of melt blending and subsequent thermal annealing. The best efficient combination of melt blending and thermal annealing for the PLA/EGMA 80/20 blend was thermal annealing at 80 °C for 2 h. Dynamic mechanical analysis (DMA) test disclosed that the PLA/EGMA 80/20 blend when experiencing thermal annealing could have a much wider heat deflection temperature range than the one with no thermal annealing. The cold

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K

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