Processing a Supertoughened Polylactide Ternary Blend with High

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Processing a Supertoughened Polylactide Ternary Blend with High Heat Deflection Temperature by Melt Blending with a High Screw Rotation Speed Liang Deng,† Cui Xu,† Shuangshuang Ding,† Huagao Fang,‡ Xuehui Wang,*,† and Zhigang Wang*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei, Anhui 230009, P. R. China

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S Supporting Information *

ABSTRACT: A supertoughened ternary polylactide (PLA) blend with high heat deflection temperature was reported in this study. The ternary blend was composed of two major biocompatible and biodegradable polymers, namely, PLA and polyamide-11 (PA11), and one minor reactive elastomer, ethylene−acrylic ester−glycidyl methacrylate terpolymer (EGMA), as a toughener. Shear processing with a very high rotation speed was utilized to prepare the blend by melt blending. At the high screw rotation speed of 1000 rpm of the twin-screw extruder, the PLA/PA11/EGMA 40/40/20 ternary blend displayed simultaneous increases in tensile toughness (75.7 MJ/m3) and notched Izod impact strength (88.3 kJ/m2), nearly 33 times greater than those of PLA/PA11 50/50 binary blend. Moreover, a much higher heat deflection temperature was obtained for this supertoughened ternary blend as compared with neat PLA and PLA/EGMA blend without PA11, providing an economical and effective approach to make PLA-related materials with supertoughness and high heat deflection temperature.

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INTRODUCTION Polylactide (PLA), as a promising biobased polymer, has attracted much attention in recent years due to its renewability, biocompatibility, biodegradability, and excellent processability.1−7 However, due to its low entanglement density and high characteristic ratio (C∞), PLA suffers from an inherent drawback, notably brittleness, which restricts its large-scale commercial applications.8−12 The characteristic ratio, C∞, of amorphous PLA was reported to be about 12, implying that the PLA chain is rather stiff.11 PLA has also very low heat deflection temperature (HDT) due to its low glass transition temperature as compared with certain amorphous polymers and its much slower crystallization rate during processing as compared with certain semicrystalline polymers.13,14 To improve the toughness of PLA materials, the widely utilized cost-effective technique is reactive melt blending, which produces compatibilizers at the phase interface during melt mixing of PLA and certain elastomers.9,13,15−26 For example, Zeng et al. fabricated supertoughened PLA material through dynamic vulcanization of PLA with sebacic acid cured epoxidized soybean oil (VESO) precursors by optimizing the carboxyl/epoxy equivalent ratio (R).27 In their system, an interfacial reaction between epoxy group of VESO and the terminal group of PLA and a dynamic vulcanization in the elastomeric phase occurred simultaneously, resulting in distinct © XXXX American Chemical Society

improvements in the impact strength (up to 542.3 J/m) and tensile toughness (up to 150 MJ/m3).27 Wang et al. also prepared supertoughened PLA material through reactive melt blending of PLA with PEG-based diacrylate monomer (PEGDA), displaying improvements by 26 times in the notched Izod impact strength and 20 times in the tensile toughness at the optimum cross-linked PEGDA content.28 In our recent effort, a supertoughened PLA material was obtained by melt blending of PLA and ethylene−acrylic ester−glycidyl methacrylate random terpolymer (EGMA) at the composition of 80/20 at a low rotation speed of 80 rpm in a mixer,13 while the addition of some elastomers usually deteriorated the heat resistant property of PLA materials. On the other hand, due to its slow crystallization rate, PLA products are often in amorphous state during common extrusion and injection molding mode, showing a low heat deflection temperature (HDT) at about 65 °C, close to the glass transition temperature.8,29 Such types of PLA products with low HDT values are insufficient for application as daily necessities, because obvious shrinkage occurs when the applied Received: April 10, 2019 Revised: May 23, 2019 Accepted: May 27, 2019

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DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research environmental temperature is over 65 °C. Improving the crystallinity of PLA products surely will raise their heat deflection temperature for usage. Li et al. improved the heat deflection temperature by introducing intense shear flow induced sterecomplex crystals into PLA samples,29 which was carried out by a specific injection molding method, i.e., an oscillation shear injection molding (OSIM), but this method might be difficult to generalize for a conventional industrial application. In our recent report, the heat deflection temperature for the supertoughened PLA/EGMA 80/20 binary blend could be improved through post-thermal-annealing above the glass transition temperature.13 However, the post-thermalannealing process usually takes a long time because of the much lower cold crystallization rate of PLA matrix, which certainly is not favorable in industry. Alternatively, melt blending of PLA with another polymer component with a much higher heat deflection temperature is considered to be a cost-effective method, which can compensate for the disadvantage of PLA in this issue. Polyamide-11 (PA11) can be considered for this very purpose. PA11 is a biobased polymer, derived from biobased resources such as castor oil.30 An apparent advantage is that PA11 can crystallize in a much faster crystallization rate, with a high degree of crystallinity even in an injection molding cycle involving a high cooling rate. A high degree of crystallinity endows PA11 products with sufficient chemical resistance, high heat deflection temperature, and excellent dimensional stability.30,31 The melt blending of PA11 with PLA has been reported for improving PLA properties, including improvement in the heat deflection temperature.32−41 But the incorporation of PA11 in PLA had limited improvement in tensile toughness and impact strength because of immiscibility between these two components.41,42 Apparently a simultaneous improvement in both impact strength (toughness) and heat deflection temperature is a necessity for burgeoning applications of PLA-related materials. Taking a comprehensive consideration, we carried out the melt blending of PLA component with both EGMA and PA11 components in this work, aiming to improve the impact strength, toughness, and heat deflection temperature in the same ternary blend system. However, due to immiscibility between PLA and PA11 components, we have to apply a high shear process to produce the blend samples for this particular purpose. It has been reported that only a high shear process without introduction of any additives leads to the production of high performance PVDF/PA11 blends with significantly improved mechanical properties, which are inaccessible for the classical polymer blends.43 To the best of our knowledge, only sparse reports dealt with applying a high shear process to the melt blending of multicomponent PLA-related blends.33,34,44 In our work here, three typical screw rotation speeds of 150, 400, and 1000 rpm were applied to fabricate PLA/PA11/ EGMA ternary blends, respectively. First, we will report the results related to the tensile toughness and impact strength of these PLA blend samples and analyze the possible mechanisms for the impact strength improvement through thermal behavior and phase morphological observation. Then, heat deflection temperatures of these PLA blends and the effect of crystallinity on the HDT improvement will be analyzed thoroughly.

trade name Ingeo Biopolymer 4032D. The PLA sample had a density of 1.24 g/cm3 and a melt flow index of 7 g/10 min (210 °C, 2.16 kg). Gel permeation chromatography (GPC) provided a weight-average molecular mass of 252 kg/mol and a polydispersity index of 2.5 for PLA. Commercial grade biobased polyamide-11 (PA11) for this study was kindly provided by Arkema under the trade name Rilsan BMNO TLD. The PA11 sample had a density of 1.03 g/cm3 and a melt volume index (MVI) of 11 cm3/10 min (235 °C, 2.16 kg, 2 mm diameter). The weight-average molecular mass for PA11 was reported to be about 50 kg/mol.45 A commercial elastomer, ethylene−acrylic ester−glycidyl methacrylate random terpolymer (EGMA) for this study was also kindly provided by Arkema under the trade name LOTADERAX8900. The EGMA sample had contents of methyl acrylate and glycidyl methacrylate of 24 and 8%, respectively. The chemical structure of EGMA can be found in a previous report.20 High-temperature GPC provided a weight-average molecular mass of 197 kg/mol and a polydispersity index of 8.3 for EGMA. Melt Blending. PLA and PA11 were dried at 80 °C and EGMA was dried at 40 °C under vacuum for 12 h prior to melt blending. The melt blending of PLA, PA11, and EGMA was performed on a twin-screw extruder, a compounding machine (KraussMaffei ZE 25Ax43D UTXi-UG) for plastics purchased from KraussMaffei Berstorff GmbH, Germany. The extruder diameter and aspect ratio were 25.5 mm and 43/1, respectively. Detailed information on the extruder is provided in Table S1 in the Supporting Information. The temperature profile from the hopper to die was 40/180/185/190/195/200/ 200/200/200/200/200 °C. The highest screw speed for this compounding machine could reach 1200 rpm. Herein, we applied three screw speeds for making the PLA blend samples, respectively. The screw rotation speeds of 150, 400, and 1000 rpm correspond to average radial shear rates of 970, 2586, and 6466 s−1, respectively, around the area above the top part of the screw. PLA/PA11/EGMA ternary blends with mass ratios of 45/45/10 and 40/40/20 were mixed in molten state by applying the above-mentioned three screw rotation speeds, respectively. The throughput needs were 3, 8, and 12 kg/h for the screw rotation speeds of 150, 400, and 1000 rpm, respectively. The mixing times for which the extrudates had experienced in the extruder were about 3.3, 1.3, and 0.83 min for the screw rotation speeds of 150, 400, and 1000 rpm, respectively. The extrudates were quenched in a cold water bath and then pelletized and dried prior to thermal molding for various tests. In addition, PLA/PA11 50/50 binary blends with no addition of EGMA were prepared by applying the same procedure for comparison purposes. We note that the PLA/ EGMA 80/20 blend was not counted as a control because this composition was out of the line in the three-component phase diagram. In fact, the mechanical properties for PLA/EGMA 80/20 blends were not much affected by the applied screw rotation speeds. Tensile Deformation and Impact Property Tests. A Suns UTM2502 universal testing machine (Suns, Shenzhen, China) was used for the tensile property test. A crosshead speed of 10 mm/min was applied for the tests. The dumbbellshaped specimens were punched out from the molded sheets, which were hot pressed at 200 °C for 5 min and then quenched at room temperature. An XJUD-5.5 pendulum impact tester (JinJian-Test, China) was used for the notched Izod impact strength test. A rectangular specimen for the

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MATERIALS AND METHODS Materials. Commercially available polylactide (PLA) for this study was purchased from NatureWorks LLC under the B

DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research notched Izod impact strength test was 80 mm × 10 mm × 3 mm in size with a 45° V-shaped notch (upper radius of 0.25 mm and depth of 2.0 mm). For statistical data analysis, five specimens for each sample were tested. Both tensile and impact property tests were carried out at 23.5 °C. Thermal Behaviors. A TA Q2000 differential scanning calorimeter (DSC; TA Instruments, New Castle, DE) was used for recording the thermal behaviors of the blend samples. A heating rate of 10 °C/min was applied. The nitrogen flow rate was 50 mL/min. Phase Morphological Observation. A Hitachi HT-7700 transmission electron microscope (TEM) was operated at an accelerated voltage of 100 kV to record the phase morphology of the blends. The ultrathin sections of the blend samples were collected by using an ultramicrotome. The ultrathin sections with 80 nm thickness were stained by ruthenium tetroxide (RuO4) vapor for 4 h at room temperature prior to TEM observation. Wide-Angle X-ray Diffraction. Wide-angle X-ray diffraction (WAXD; GeniX 3D Cu Microspot, Xenocs SAS, France) was used to record the WAXD profiles of the blend samples to obtain crystallinity values.13 Dynamic Mechanical Analysis (DMA). A Netzsch dynamic mechanical analyzer (Netzsch, DMA 242E Artemis) was used for the DMA measurements. A strain of 0.03% in the tensile mode, a frequency of 1 Hz, a temperature range from −100 to 200 °C, and a heating rate of 3 °C/min were applied. Heat Deflection Temperature Test. A Vicat softening temperature (VST) ZWK-1302A (Suns, Shenzhen, China) was used to record the VST values according to method A120 in ISO 306:2013. A load of 10 N and a heating rate of 12 °C/6 min were applied. We also designed a homemade device to provide a comparable heat deflection temperature test. Dimethyl silicone oil in a beaker served as the temperature bath for the test. Two Hoffman clamps were used to hold the blend specimen. The distance between the two Hoffman clamps was kept steady by clamping a steel sheet with the same thickness as the blend specimen, parallel to the blend specimen in the initial position. The Hoffman clamps with the blend specimen and steel sheet were put into the dimethyl silicone oil bath. At room temperature, a standard weight of 20 g was laid on the blend specimen, and then the dimethyl silicone oil bath was heated at 2 °C/min with the bath temperature recorded by a thermometer. Digital photos of the blend specimen were taken during heating for examining the bending and deformation of the specimen.

Figure 1. Typical nominal stress−strain curves for (a) PLA/PA11 50/ 50, (b) PLA/PA11/EGMA 45/45/10, and (c) PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds. The corresponding screw rotation speed is marked near each curve for (b) and (c). The orange solid dot on the stress−strain curve in (c) indicates the yielding point.

screw rotation speeds, respectively. For PLA/PA11 50/50 binary blends a characteristic brittle deformation is clearly seen (Figure 1a), no matter which screw rotation speeds were utilized for their preparation. The values of elongation at break are all less than 9% with no obvious yielding observable prior to break. To increase the tensile toughness, a commercial elastomer, ethylene−acrylic ester−glycidyl methacrylate random terpolymer (EGMA, trade name LOTADER-AX8900) was taken into account. We note that, although EGMA is not biobased, the PLA/PA11/EGMA ternary blends still maintain high contents of the biobased components, which significantly reduces the product dependency on the petroleum resource. Parts b and c of Figure 1 show that the tensile deformation curves for both PLA/PA11/EGMA 45/45/10 and 40/40/20 blends prepared at the screw rotation speed of 150 rpm represent the characteristic of brittle break behavior, displaying the values of elongation at break at 17 and 21%, respectively, which are 2 times that for the PLA/PA11 50/50 blend. This means that introduction of an elastomer does not significantly enhance toughness at this condition. The normal screw rotation speed of less than 150 rpm for a general twin-screw extruder is apparently not sufficient for EGMA to approach its particular role in our mission. However, the blends behave as ductile materials if prepared at higher screw rotation speeds. At

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RESULTS AND DISCUSSION Simultaneous Improvements in Tensile Toughness and Notched Impact Strength. Both PLA and PA11 are biobased polymers with high tensile strength and Young’s modulus, but they vary in tensile toughness. PLA breaks at elongation values less than 8% in the tensile test as reported in our previous work,28 and according to the Ingeo Biopolymer 4032D Technical Data Sheet, its elongation at break is 6%, while PA11 exhibits yielding and necking, with elongation at break higher than 200% (curves not shown), consistent with another report.39 However, simple physical blending of PLA and PA11 cannot achieve the supertough property due to limited miscibility between PLA and PA11 components.36 Figure 1 shows the typical nominal stress−strain curves for PLA/PA11 50/50, PLA/PA11/EGMA 45/45/10, and 40/40/ 20 blends, which were prepared at 150, 400, and 1000 rpm C

DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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strength values were gained for all the specimens except PLA/ PA11 50/50 blends, as PLA/PA11 50/50 blends break extremely rapidly in the tensile test even before yielding occurs. Values of stress at break were taken to plot in Figure 2a instead. The results in Figure 2 show that the screw rotation speeds cause little effect on the Young’s modulus and yield strength for each blend system. The changes are subtle if not negligible, but the influences of EGMA content on the Young’s modulus and yield strength of the blends are obvious, with all the values dropping apparently with increasing EGMA content. The related results are clearly shown in Figure S2. The decreases in the Young’s modulus and yield strength with increasing EGMA content are close to being linear. The toughness of PLA-related materials is the property of most concern for their commercial applications. Two parameters can be used to represent the toughness of polymer materials: one of them is the tensile toughness obtained at a certain tensile deformation rate and the other is the notched Izod impact strength obtained at a rapid deformation rate. Figure 3 shows the changes in notched Izod impact strength

400 and 1000 rpm, the tensile deformation curves of PLA/ PA11/EGMA 45/45/10 blends exhibit distinct yielding and stable necking development, with the values of elongation at break at 44 and 121%, respectively. Moreover, the tensile deformation curves for PLA/PA11/EGMA 40/40/20 blends exhibit necking development and the values of elongation at break at 80 and 260%, respectively, plus their curves do not show distinct yielding points. The PLA/PA11/EGMA 40/40/ 20 blend prepared at the highest screw rotation speed of 1000 rpm eventually shows significant strain-hardening behavior when tensile deformation goes into a high strain range, which means the best tensile property can be obtained at the high screw rotation speed of 1000 rpm, demonstrating that a strong shear process plays a very important role in improving tensile properties of the melt blending system. Figure 2 further shows the changes of Young’s modulus, stress at break, and yield strength with screw rotation speed for

Figure 3. Changes of (a) notched Izod impact strength and (b) tensile toughness with screw rotation speed for PLA/PA11 50/50, PLA/PA11/EGMA 45/45/10, and PLA/PA11/EGMA 40/40/20 blends.

and tensile toughness with increasing screw rotation speed for PLA/PA11 50/50, PLA/PA11/EGMA 45/45/10, and PLA/ PA11/EGMA 40/40/20 blends. For the binary blends of PLA and PA11 with no addition of EGMA, the notched Izod impact strength values are lower than 3.0 kJ/m2, independent of the applied screw rotation speed, demonstrating an inherent brittleness of these blends. The tensile toughness values for PLA/PA11 50/50 blends are around 2.2 MJ/m3, independent of the applied screw rotation speed as well. The above result infers that an introduction of EGMA is prerequisite for toughness improvement. For PLA/PA11/EGMA 45/45/10 blend, the tensile toughness shows a linear increase with increasing screw rotation speed, from 4.1 to 42.8 MJ/m3, while the notched Izod impact strength values of the blend are still in the low level range with minor changes (7−12 kJ/m2) in the

Figure 2. Changes of Young’s modulus, E, and strength at break, σbreak, or yield strength, σ, with screw rotation speed for (a) PLA/ PA11 50/50, (b) PLA/PA11/EGMA 45/45/10, and (c) PLA/PA11/ EGMA 40/40/20 blends.

PLA/PA11 50/50, PLA/PA11/EGMA 45/45/10, and PLA/ PA11/EGMA 40/40/20 blends. The values of yield strength were obtained according to the procedure reported by Brooks et al.46 Figure S1 (in the Supporting Information) illustrates the change of strain/stress, ε/σ versus stress, σ for PLA/PA11/ EGMA 40/40/20 blend prepared at the screw rotation speed of 150 rpm, from which the yield strength and strain can be extracted. The yield point is shown on the stress−strain curve by an orange solid dot in Figure 1c. From this, the yield D

DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research applied screw rotation speed range, demonstrating limited improvement in toughness at the 10 wt % EGMA introduction. Therefore, further increase of EGMA content is certainly necessary. It can be seen from Figure 3 that for PLA/PA11/ EGMA 40/40/20 blend, the tensile toughness also shows an obvious increase with increasing screw rotation speed, from 3.8 to 75.7 MJ/m3, while the notched Izod impact strength values show an apparent increase with screw rotation speed, from 9.1 to 88.3 kJ/m2, demonstrating an obvious improvement in toughness at the 20 wt % EGMA introduction. It can be more clearly assured from these results that both of the highest tensile toughness and notched Izod impact strength values for the blends must be achieved by the 20 wt % EGMA introduction and by the application of a high screw rotation speed for the blend preparation. We emphasize here that the notched Izod impact strength (88.3 kJ/m2) of PLA/PA11/ EGMA 40/40/20 blend prepared at the screw rotation speed of 1000 rpm is far beyond the standard value of a supertoughness definition (35 kJ/m2) for PLA-related materials.8 The notched Izod impact strength of 88.3 kJ/m2 and tensile toughness of 75.7 MJ/m3 of PLA/PA11/EGMA 40/40/20 blend are about 34 times those of PLA/PA11 50/50 blend prepared at the same screw rotation speed of 1000 rpm, indicating an achievement of supertoughened mechanical property for this ternary PLA/PA11/EGMA blend material. Thermal Behaviors for PLA/PA11/EGMA Blends. In PLA/PA11/EGMA blends, the PLA and PA11 components are crystallizable. Therefore, the physical properties of the blends, such as the mechanical property and heat deflection temperature, are highly dependent on the glass transition temperature (Tg) and crystallinity in the solid state. DSC measurements were carried out to examine their thermal behaviors, focusing on the possible changes of Tg at different screw rotation speeds. Figure 4 shows DSC heat flow curves during heating scans for PLA/PA11/EGMA blends prepared at different screw rotation speeds. It is noticed that all the heat flow curves exhibit the same features: a clear glass transition for the PLA component at Tg3; an exothermal peak of the PLA component at the cold crystallization temperature, Tc, due to PLA cold crystallization; and two endothermal melting peaks of PLA (Tm1) and PA11 (Tm2) components when their crystals melt, which confirms that the PLA component in the prepared blends remains amorphous due to its slow crystallization rate and the PA11 component instead is in the crystallized state due to its fast crystallization rate even in a rapid cooling process. The crystallized PA11 component provides the basis for the high heat deflection temperature of the ternary blend as shown in Raising of Heat Deflection Temperature. The Tg1 and Tg2 marked on one curve in Figure 4b stand for the glass transition temperatures of the EGMA and PA11 components, respectively, which are intendedly elaborated under Phase Morphology of PLA/PA11/EGMA 40/40/20 Blend. It is considered that the mechanical properties, especially the notched impact strength and tensile toughness, are closely related to the Tg’s of the constituting components in the blends.13 The changes of Tg for PLA/PA11/EGMA blends are of particular interest, which reflect the compatibility among different components. Figure 5 shows the enlarged regional DSC heat flow curves for PLA/PA11/EGMA blends to clearly demonstrate the existence of Tg1 for EGMA and Tg2 for PA11 components in PLA/PA11/EGMA blends. Tg3 for PLA is also marked for comparison. To elucidate the effect of screw rotation speed on the compatibility among the components,

Figure 4. DSC heat flow curves at a heating rate of 10 °C/min for (a) PLA/PA11/EGMA 45/45/10 and (b) PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds.

the changes of Tg1 and Tg2 are shown in Figure 5c,f. It is interesting to find that Tg1 values for the introduced EGMA in these blends increase with increasing screw rotation speed, from −31.9 to −29.7 °C for the PLA/PA11/EGMA 45/45/10 blend and from −29.2 to −22.1 °C for the PLA/PA11/EGMA 40/40/20 blend. The increases in Tg1 for the EGMA component with increasing screw rotation speed are thought to be related to the gradually enhanced interfacial interactions between EGMA/PLA and EGMA/PA11 pairs. The more significant increase in Tg1 of the EGMA component in the PLA/PA11/EGMA 40/40/20 blend than in the PLA/PA11/ EGMA 45/45/10 blend infers that the EGMA component assists the mixing process in the extruder at the higher screw rotation speed due to its much higher viscosity than those of PLA and PA11 components (Figure S3). The more interesting result is that the Tg2 values show opposite changing trends with increasing screw rotation speed for the two blends. At lower EGMA content of 10 wt %, Tg2 increases from 39.7 to 40.2 °C, while at higher EGMA content of 20 wt % it decreases from 42.9 to 40.0 °C with increasing screw rotation speed. The increase of Tg2 for PA11 in the PLA/PA11/EGMA 45/45/10 blend might be due to the enhanced crystallinity of the PA11 component with increasing screw rotation speed, while the decrease of Tg2 for PA11 in the PLA/PA11/EGMA 40/40/20 blend is considered to be caused by favorable interfacial interactions between PA11 and EGMA components through the interfacial area increase assisted by increasing the screw rotation speed. This can be confirmed by TEM observation shown in Phase Morphology of PLA/PA11/EGMA 40/40/20 Blend. It is noted that the reactive compatibility might occur at the phase interfaces; however, its detection was not so straightforward at the moment because of the much less E

DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. Enlarged regional DSC heat flow curves for PLA/PA11/EGMA blends to demonstrate Tg’s (a, b, d, e) and changes of Tg for (c) EGMA (Tg1) and (f) PA11 (Tg2) components in PLA/PA11/EGMA blends with screw rotation speed.

interfacial layer quantity for approaching the detection limits of the spectroscopies such as Fourier transform infrared (FTIR). Some reports have confirmed the possible interfacial reactive compatibility for similar blend systems.16,32 Besides, Tg3 values for the PLA component in all the blends do not change much with screw rotation speed, indicating that PLA phase domains remain intact in the blend system. Note that Tg2 values for PA11 in PLA/PA11 50/50 blends are about 47.9 °C, which are independent of the screw rotation speed. The relatively lower Tg2 values of PLA/PA11/EGMA blends than that of PLA/ PA11 50/50 blend indicate that both EGMA component and screw rotation speed function through the melt mixing of EGMA and PA11 components.36 We emphasize here that the higher viscosity of the EGMA component might play a key role in assisting the rational dispersion of EGMA phase domains in the whole blend system. The changes of complex viscosity value as functions of angular frequency, ω, for EGMA, PA11, and PLA components as measured at 200 °C are shown in Figure S3. The EGMA component shows a much higher dynamic complex viscosity than both PA11 and PLA components do. For the higher rotation speed, a higher viscosity could be a beneficial factor for better dispersion of the dispersed phase domains. Phase Morphology of PLA/PA11/EGMA 40/40/20 Blend. TEM was further applied to observe the phase morphology of the blend samples having apparently enhanced impact strength, which is the most significant part of our work here, reflecting a combined effect of introduction of EGMA component and increase in the screw rotation speed, as shown in Figure 3. Figure 6 shows the typical TEM micrographs for

PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds. The phase domains for each component are pointed out by arrows of different colors. The olive, purple, and red arrows indicate PLA, PA11, and EGMA phase domains, respectively. The darkest area stands for the PA11 phase domain, which is characteristic of existent PA11 lamellar crystals, the bright one is the PLA phase domain, and the gray one is the EGMA phase, because EGMA can be homogeneously stained by RuO4. The phase domain assignment is apparent for PA11 because of its obvious lamellar crystals inside its phase domain, and the assignments for PLA and EGMA can be referred to our previous TEM observation on PLA/EGMA binary blend.13 It can be seen from Figure 6a that, at the low screw rotation speed of 150 rpm, the three components are arranged as an island−sea morphology as follows: PA11 serves as the matrix (sea, the dark color), PLA serves as the dispersed phase domain (island, the bright color), and EGMA serves as the adjunction region between the island and sea, surrounding the island. Overall, the three components are all isolated. Only a small amount of EGMA goes inside PLA phase domains. This phase morphology is quite consistent with that for the similar blend systems.36 The clear boundary between EGMA and PA11 indicates that they are not well-mixed at this mixing condition. Therefore, the PLA/PA11/EGMA 40/40/20 blend prepared at the screw rotation speed of 150 rpm behaves as a brittle material, with the notched Izod impact strength of 9.1 kJ/m2. When the blend was prepared at the screw rotation speed of 400 rpm, its phase morphology looked very different (Figure 6b), in which EGMA phase domains appear as F

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as reported in our previous publication.13 More interesting, when the screw rotation speed is raised to 1000 rpm, the phase morphology of the ternary blend looks completely different from those of the others. Figure 6c shows that PA11 phase domains have relatively smaller sizes than those in Figure 6a,b, PLA phase domains become smaller as well, and, more importantly, the EGMA phase domains exist as droplet-like in majority, serving as gaskets in between the PLA and PA11 phase domains. This type of disjunction distribution or interdigitate junction for the three phase domains is considered to be the major reason why the highest notched impact strength (88.3 kJ/m2) could be achieved for the PLA/PA11/ EGMA 40/40/20 ternary blend prepared at the highest screw rotation speed. It is noted that, for PLA/olefin block copolymer (OBC)/EGMA ternary blends prepared by reactive blending through high-shear extrusion,44 the high screw rotation speeds do not provide much obvious influence on the final mechanical properties, which is apparently much different from the result for PLA/PA11/EGMA ternary blends as we reported in this work. The reason for the difference might be that they did not employ a sufficient amount of the EGMA component. In another work the immiscible PLA/ PA11 blends were reactively blended by catalyzing the ester− amide interchange chemical reaction during high speed extrusions, for which the high screw rotation speeds of 1000 and 2000 rpm do not enhance the mechanical properties; instead, these two high screw rotation speeds decrease the mechanical properties, apparently because of the enhanced degradation of the polymer chains.33,34 Raising of Heat Deflection Temperature. Besides the tensile and impact mechanical properties, heat deflection temperature is another essential factor to expand the applications of PLA-related materials. In our previous work, it has been demonstrated that a post-thermal annealing could effectively increase the crystallinity of PLA/EGMA 80/20 blend, which facilitated the raising of the heat deflection temperature.13 However, post-thermal annealing takes extra processing time, not being an efficient way for a large-scale industrial application. In this work, an introduction of PA11 component to the blend system provides another facile approach to raise the heat deflection temperature, without imposing a post-thermal-annealing process anymore. The raising of the heat deflection temperature of PLA-based material is related to input of crystals in the blend, which have a high melting point. Figure 7 shows the WAXD intensity profiles for PLA/PA11 50/50 and PLA/PA11/EGMA 40/40/ 20 blends prepared at different screw rotation speeds. It can be seen that three apparent diffraction peaks appear at 7.4, 20.7, and 23.4°on the WAXD profiles, which can be attributed to the reflections of the (001), (200), and (210/010) lattice planes, respectively, for α′-form triclinic cell of PA11 crystals.47 The PLA component in these blends does not show any diffraction peaks, indicating that the PLA component remains in the amorphous state, consistent with the DSC heat flow curves shown in Figure 4. The absence of PLA crystallization is somehow due to its much slower crystallization rate and fast cooling rate when the sample was quenched from the molten state to room temperature. Figure S4 further shows the peak deconvolutions for the WAXD intensity profiles of PLA/PA11 50/50 and PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds. Peak deconvolutions of the WAXD profiles allow separation of the respective contributions of the amorphous and crystalline phases; thus, the crystallinity

Figure 6. TEM micrographs for PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds of (a) 150, (b) 400, and (c) 1000 rpm. The olive, purple, and red arrows indicate PLA, PA11, and EGMA phase domains, respectively.

droplets with small sizes, and are well-dispersed in both PLA and PA11 phase domains. The smaller sizes of EGMA are thought to be produced by high shear stress due to the increased screw rotation speed, recalling that the EGMA component as an elastomer has high viscosity (Figure S3). PLA phase domains seem to be much larger at this processing condition. Such a phase morphology usually led to a reasonably high impact strength (26.9 kJ/m2) for this blend, slightly lower than that of the PLA/EGMA 80/20 binary blend G

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Figure 7. WAXD intensity profiles for (a) PLA/PA11 50/50 and (b) PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds.

values for the blends can be obtained. For PLA/PA11 blends, the crystallinity values are 20.2, 23.0, and 23.4% for the respective screw rotation speeds of 150, 400, and 1000 rpm. The normalized crystallinity values are 40.4, 46.0, and 46.8%, respectively, by considering the PA11 mass content of 50 wt % in the binary blends. The slight increase in crystallinity with increasing screw rotation speed might be due to the increased PA11 chain mobility after the blend experienced increased shear stress, which very possibly caused some chain scissions.33 For PLA/PA11/EGMA 40/40/20 blends, the crystallinity values are 23.8, 21.2, and 21.4% for the respective screw rotation speeds of 150, 400, and 1000 rpm. The normalized crystallinity values are 59.5, 53.0, and 53.5%, respectively, by considering the PA11 mass content of 40 wt % in the ternary blends. The reduced crystallinity with increasing screw rotation speed is related to the different phase interfacial areas and interfacial interactions among the three phase domains. On one hand, the normalized crystallinity values of the PA11 component are higher for the ternary blends than that for the binary blends because the interface-assisted nucleation rate is higher for the former than for the latter.48−50 On the other hand, the reduced crystallinity values with increasing screw rotation speed for the ternary blends might be related to the greater separation effect of EGMA phase domains to the PA11 phase domains at higher screw rotation speeds, consistent with TEM observation shown in Figure 6. The above results indicate that the crystallinity values of the PA11 component in the blends are high enough to facilitate high heat deflection temperatures to the PLA-based materials. To further elucidate the effect of crystallization of the PA11 component on the heat deflection temperature, dynamic mechanical analysis (DMA) was carried out for the ternary blends. Figure 8 shows the changes of storage modulus, E′, loss modulus, E″, and loss tangent, tan δ, with temperature during DMA measurements for PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds. Three dropping stages can be found before 90 °C for the storage modulus, E′,

Figure 8. Changes of (a) storage modulus, E′, (b) loss modulus, E″, and (c) loss tangent, tan δ, with temperature during DMA measurements for PLA/PA11/EGMA 40/40/20 blends prepared at different screw rotation speeds.

during heating, which are clearly related to the glass transitions (Tg1, Tg2, and Tg3) of the three constituting components, EGMA, PA11, and PLA, respectively, as indicated by black arrows in Figure 8a. These three glass transitions can be more clearly demonstrated by changes of the loss modulus, E″, and the loss tangent, tan δ, with increasing temperature as shown in Figure 8b,c. The results are consistent with that reflected in DSC heat flow curves in Figure 5. The major storage modulus drop occurs in the temperature range from 65 to 82 °C for all the ternary blends, corresponding to the glass transition of the PLA phase domain, which also infers that PLA phase domains are in the amorphous state. It is predicted that the modulus dropping range could be removed if the PLA component crystallizes through a post-thermal annealing, corresponding to the cold crystallization of the PLA component, which can be clearly seen by the subsequent modulus increase in the DMA curves. With subsequently increasing the temperature from 93 to 115 °C, the storage modulus shows a rapid increase, corresponding to cold crystallization of the PLA phase. In the temperature range from 115 to 160 °C, the storage modulus values remain at a relatively high level (above 70 MPa). A rapid decrease in the storage modulus occurs from 160 to 184 °C, corresponding to melting of both PLA and PA11 crystals. During the whole heating process before melting of PLA and PA11 crystals, the ternary blends hold storage modulus values higher than 55 MPa, beyond the lowest point of the valley in the curves around 88 °C. The storage modulus holds high H

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Figure 9. Digital photos taken during heat deflection temperature test in dimethyl silicone oil bath for (a, left panels) neat PLA, (b, middle panels) PLA/PA11 50/50, and (c, right panels) PLA/PA11/EGMA 40/40/20 blends prepared at the high screw rotation speed of 1000 rpm. The standard weight was 20 g. Neat PLA sheet was prepared directly from raw material.

blend, the VST temperature is predicted to be raised by the same way. The measured VST value for the PLA/PA11/ EGMA 40/40/20 ternary blend is 162 °C, confirming the above prediction. The relatively lower VST for the ternary blend than for the PLA/PA11 binary blend is related to the EGMA component, which is a soft elastomer. Figure 9 shows selected digital photos taken during the heat deflection temperature test for neat PLA, PLA/PA11 50/50, and PLA/PA11/EGMA 40/40/20 blends prepared at the high screw rotation speed of 1000 rpm. The heat deflection of the sheet specimen was visually observed in dimethyl silicone oil bath with a precisely controlled temperature rise at a heating rate of 2 °C/min. The heat deflection temperature was judged by exposure of the rear steel sheet when the sheet specimen started to bend. The rear steel sheet was held by Hoffman clamps.13 From the photos taken at the raised temperatures in Figure 9, we can visually determine that the heat deflection temperatures for neat PLA, PLA/PA11 50/50 blend, and PLA/ PA11/EGMA 40/40/20 blend are 64, 170, and 164 °C, respectively. This result nicely agrees with the VST test. Once again, the above results confirm that the introduction of the PA11 component into the blend truly raises the heat deflection temperature due to its fast crystallization ability and appropriate high melting point.

values because PA11 crystals preexisting in the blend provide a supporting network frame for the other two components, where PLA phase domains become soft above its glass transition temperature. We note that, in the same temperature range, the PLA/EGMA 80/20 binary blend only holds storage modulus values of less than 6 MPa as reported in our previous work.13 Furthermore, PLA crystals formed through cold crystallization can further strengthen the network frame constructed by preexisting PA11 crystals with a high melting point, and this strengthened crystal network frame gives the highest storage modulus values above 137 MPa in the temperature range 115−124 °C, which certainly allows potential applications of this PLA-based ternary blend. On the whole, the above result demonstrates that the PA11 component indeed plays a crucial role in keeping a high heat deflection temperature for the PLA-based ternary blend material. To visually demonstrate the rise of the heat deflection temperature of the PLA/PA11/EGMA 40/40/20 blend as compared with that of neat PLA, the Vicat softening temperature (VST) values were measured and their heat deflections in hot dimethyl silicone oil bath were recorded. The VST value for neat PLA is 64 °C, close to the Tg of PLA. When PA11 is mixed with PLA, the VST value for the PLA/ PA11 50/50 blend is 171 °C; note that the melting peak temperature for PA11 in the blend is about 189 °C. We recall that PLA is in amorphous state and PA11 is in crystallized state due to their very different crystallization rates. If the PA11 component is introduced into the toughened PLA/EGMA

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CONCLUSIONS In this work, a supertough ternary blend together with high heat deflection temperature was prepared. The ternary blend I

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and 51473155). The project is also supported by the Open Research Fund of State Key Laboratory of Polymer Materials Engineering, Sichuan University (sklpme2017-4-06). We also appreciate the Open Research Funds of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.

consisting of polylactide (PLA), polyamide-11 (PA11), and ethylene−acrylic ester−glycidyl methacrylate terpolymer (EGMA) at a mass composition of 40/40/20 was prepared by a twin-screw extruder at a high screw rotation speed of 1000 rpm, yielding a shear-induced melt blending of PLA, PA11, and EGMA. Both the tensile toughness and the notched Izod impact strength of this ternary blend were significantly improved to 75.7 MJ/m3 and 88.3 kJ/m2 respectively, promoting it to a level beyond supertoughness for PLA-related materials. More importantly, the heat deflection temperature was significantly raised with the Vicat softening temperature (VST) value approaching 162 °C and the heat deflection temperature reaching 164 °C by visual deformation of the sheet specimens in a dimethyl silicone oil bath. Differential scanning calorimetry was utilized to examine the changes of glass transition temperature with increasing screw rotation speed for the blends, and transmission electron microscopy was used to observe the phase domain distributions for PLA, PA11, and EGMA components in the ternary blend, aiming to reveal the underlying toughening mechanism. The result illustrated that the improvement in toughness and impact strength was related to the unique disjunction distribution or interdigitate junction among three phase domains of PLA, PA11, and EGMA components, which could be obtained only at the highest screw rotation speed of 1000 rpm. The fast crystallization rate of the PA11 component in the ternary blend produced PA11 α′-form crystals in PA11 phase domains, which constructed the crystal network frame to sustain dimension stability against the heat deflection, thus providing the sufficiently high heat deflection temperature for applications in necessity. We emphasize here that the combination of blend composition, crystallization, and high stress shear process is crucial for the final performances of this ternary PLA blend system. By elevating the heat deflection temperature and increasing the mechanical toughness, the applications of PLA-related materials can be extended significantly.

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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/acs.iecr.9b01970.

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REFERENCES

Table containing mechanical specifications for extruder and plot for obtaining yield stress, mechanical properties versus EGMA content, dynamic complex viscosity for each component, and peak deconvolutions of WAXD intensity profiles (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 0551-63607703. Fax: +86 0551-63607703. E-mail: [email protected] (X.W.). *E-mail: [email protected] (Z.W.). ORCID

Huagao Fang: 0000-0001-9013-1079 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 J

DOI: 10.1021/acs.iecr.9b01970 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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