Relating Chemical Structure to Toughness via Morphology Control in

Jan 16, 2018 - Beijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing Technology and Business. University...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Relating Chemical Structure to Toughness via Morphology Control in Fully Sustainable Sebacic Acid Cured Epoxidized Soybean Oil Toughened Polylactide Blends Tong-Hui Zhao,† Wen-Qiang Yuan,† Yi-Dong Li,† Yun-Xuan Weng,*,‡ and Jian-Bing Zeng*,†,‡ †

School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China Beijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing Technology and Business University, Beijing 100048, China

‡

S Supporting Information *

ABSTRACT: The use of soybean oil or its derivatives to toughen polylactide (PLA) usually leads to limited toughening efficiency, due to the incompatibility between toughening agents and parent PLA. Herein, we report a dynamic vulcanization method to toughen PLA using sebacic acid cured epoxidized soybean oil (VESO), a fully sustainable and biodegradable component. A series of sebacic acid cured epoxidized soybean oil precursors (SEPs) were prepared with different carboxyl/epoxy equivalent ratio (R), which consequently dictates the chemical structure and the morphology of PLA/VESO blends after the dynamic vulcanization. We demonstrated that the chemical structure of VESO plays a critical role in the compatibility, morphology, and toughness of the PLA/VESO blends. By optimizing the R-value, supertoughened PLA blends can be obtained, as evidenced by the significant improvement in the tensile toughness (up to 150.6 MJ/m3) and the impact strength (up to 542.3 J/m). The results of the toughening mechanism from the morphology study confirm that the chemical structure of VESO is the key indicator of the toughening efficiency. For the PLA/VESO blends, at optimized R-value, the fracture energy can be dissipated efficiently through shear yielding of the PLA matrix induced by internal VESO cavitation to achieve supertoughness.

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INTRODUCTION The application of polylactide (PLA) is limited by its intrinsic brittleness.1−3 Many approaches have been designed to toughen PLA, and polymer blending is considered as the most efficient and costless way.4−7 The early studies used to use polymers or elastomers derived from petroleum to toughen PLA, which sacrificed the sustainability and biodegradability.8−12 Therefore, many efforts have been devoted to toughen PLA with renewable polymers, such as biomass-sourced polyesters, microbial polyesters, natural rubber and its derivatives, and biobased polyamide.7,13−18 In comparison with these renewable polymers, plant oils, a kind of inexpensive, green and sustainable feedstocks,19 provide a better candidate for use as a toughening agent. Among them, e.g., castor oil and soybean oil, have been tried to toughen PLA.20,21 Nevertheless, poor results were obtained due to the immiscibility between plant oils and PLA. Lowering the interfacial tension of the twocomponent, i.e., compatibilization strategy, therefore, has been developed. Specifically, block copolymers, e.g., poly(ricinoleic acid)−PLLA diblock copolymer20 and poly(isopropene-blactide) block copolymer,21,22 preferentially diffuse and segregate to either phase, lowering the interfacial energy and improving the adhesion of the two phases. If, on the other hand, the blend component can react with PLA, called reactive © XXXX American Chemical Society

blending, the toughening efficiency was improved more significantly.23,24 Although the reactive blending provides a powerful method, the toughening efficiency is still rather insufficient compared to other polymers toughened PLA blends.7 Therefore, the challenge remains to fabricate supertoughed PLA/plant oil blends. Previous investigations demonstrated that the toughening efficiency is strongly dependent on the phase morphology, interfacial compatibility, and the toughening mechanisms.4−7 For the typical “sea-island” PLA blends, super toughness was available when the particle size of the dispersed phase was in the range 0.7−1.1 μm and interfacial compatibilization occurred.25−28 Supertoughened PLA blends usually shared a similar toughening mechanism, i.e., cavitation induced extensive matrix shear yielding,26−29 which is the most efficient way for energy dissipation.30−33 It is noticed that many supertoughened PLA blends were fabricated by dynamic vulcanization,7,11,26−28 which is very powerful to tailor the phase morphology, interfacial compatibility, and toughening effect of the final blends.6,34 Besides these parameters, the chemical structure and Received: January 16, 2018

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DOI: 10.1021/acs.macromol.8b00103 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Reaction Time and Acid Value of SEP and Molecular Weight as well as Gel Fraction of PLA/VESO Blends blends neat PLA PLA/ESO PLA/VESO-0.2 PLA/VESO-0.3 PLA/VESO-0.4 PLA/VESO-0.5 PLA/VESO-0.6 PLA/VESO-0.8 PLA/VESO-1.0

timea (min)

acid valueb (mg of KOH g−1)

Mwc (×104 g mol−1)

PDIc

gel fractiond (wt %)

60 45 30 27 24 18 15

0.73 6.04 12.72 28.36 36.56 48.27 66.10 128.03

15.21 16.54 16.13 14.76 14.61 14.65 14.01 11.33 7.82

1.48 1.41 1.42 1.44 1.50 1.48 1.49 1.61 1.97

0 0 2.0 10.9 25.2 31.3 47.6 57.6 51.2

a

Reaction time for preparation of the precursors. bAcid value of related SEP determined by a standard titration method. cWeight-average molecular weight and polydispersity index of the isolated PLA. dGel fraction on the basis of the weight of precursors. structure and molecular weight analysis. The weight of the gel was measured as W2. The gel fraction (Gf) of VESO was calculated by

intrinsic properties of dispersed rubbery polymers could also affect the toughening effect.11,27,28,35,36 In this work, we report sebacic acid cured epoxidized soybean oil (VESO) toughened PLA blends fabricated by dynamic vulcanization of PLA with sebacic acid cured epoxidized soybean oil precursors (SEPs), and highlight the structure−property relationship of such blends. Depending on the carboxyl/epoxy equivalent ratio (R) in the preparation of the SEP precursors, VESO with different structures from dimer to branching and cross-linking were obtained. Then, the effects of VESO chemical structures on morphology, mechanical properties, and toughening mechanisms of the PLA/VESO blends were systematically investigated. By adjusting the chemical structure of VESO, fully sustainable and supertoughened PLA blends were obtained.

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Gf =

W2 × 100% wveso × W1

(1)

where wveso is the weight content (0.2) of VESO in the blends. Nuclear Magnetic Resonance (1H NMR) Spectroscopy. 1H NMR spectra were recorded on a Bruker AC-P 400 MHz spectrometer at room temperature with CDCl3 and tetramethylsilane (TMS) as a solvent and an internal chemical shift standard, respectively. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra were recorded on a RF-5301PC spectrophotometer (Shimadzu, Japan) in a range of wavenumbers from 4000 to 600 cm−1 with the resolution and scanning number of 4 cm−1 and 32 times, respectively. The samples for FT-IR analysis were prepared by pressing with KBr. Gel Permeation Chromatography (GPC). The molecular weight and polydispersity index were measured at 35 °C on a LC20 instrument (Shimadzu, Japan) with a waters Styragel HR4 column, a Rheodyne 7725i manual sample device, and an RID-20 refractive index detector. Monodisperse polystyrene was used as the standard, and tetrahydrofuran (THF) was used as the eluant and the solvent of PLA. The eluant flow rate and sample concentration were 1.0 mL min−1 and 0.25 mg mL−1, respectively. Scanning Electron Microscopy (SEM). SEM images were recorded on a JSM-6510LV (JEOL, Japan) scanning electron microscope at an accelerating voltage of 20 kV. The surfaces were sputtered with a layer of platinum before testing. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis was carried out on a TA Instruments DMA Q800 under a tensile mode from −70 to 130 °C with a heating rate and an oscillation frequency of 3 °C/min and 1 Hz, respectively. Mechanical Properties. Tensile properties were measured on a MTS E44 Universal Testing Machine. The experiment was carried out at room temperature with a crosshead speed of 10 mm/min. The gauge length between the two pneumatic clamps was 25 mm. Five measurements were performed for each sample, and the averaged result was reported. The notched Izod impact strength was measured on a Sansi ZBC7000 (Shenzhen, China) impact tester at room temperature in general accordance with ASTM D256. The averaged result from five measurements was reported for the samples.

EXPERIMENTAL SECTION

Materials. Polylactide (PLA 4032D) with a weight-average molecular weight (Mw) and a polydispersity index of 17.62 × 104 g mol−1 and 2.1 was procured from Natureworks. Epoxidized soybean oil (ESO) with 4.1 mol of epoxy group/ESO molecule was purchased from the Micxy Chemical Co., Ltd. (Chengdu, China). Sebacic acid (SA, 98.5%), 4-N,N-dimethylaminopyridine (DMAP, 99%), and Dchloroform with tetramethylsilane (TMS) as internal reference were obtained from the Micxy Chemical Co., Ltd. Ethanol and chloroform were obtained from Kelong Chemical Reagent Factory (Chengdu, China). All of the chemicals were used as received. Dynamic Vulcanization of PLA with SEP. The dynamic vulcanization of PLA with SEP was performed in a torque rheometer (Shanghai Kechuang, China) at 170 °C with a roller rotation rate of 80 rpm for 17 min. PLA was vacuum-dried at 80 °C for 12 h before processing. A series of PLA/VESO blends were prepared and designated as PLA/VESO-R, where VESO represents vulcanized SEP and R has the same meaning as that in SEP-R, namely, the carboxyl/epoxy equivalent ratio. For example, PLA/VESO-0.2 represents a blend prepared by dynamic vulcanization of PLA with SEP-0.2. The PLA/SEP weight ratio was fixed at 80:20 for all of the samples. Neat PLA and PLA/ESO (W/W, 80/20) were also processed with the same procedure. The products were injection-molded into a standard tensile bar (ASTM D638) and a notched Izod impact bar (ASTM D256) with a WZS10D MiniJet (Shanghai, China). The cylinder temperature and mold temperature were 180 and 40 °C, respectively. Gel Fraction Measurement. The gel fraction of the VESO in the PLA/VESO blend was measured via a solution method. The blend with a weight of W1 (∼1 g) was immersed in 30 mL of chloroform for 48 h to dissolve the non-cross-linking part. The insoluble part was collected through centrifugation and vacuum-dried at 80 °C for 24 h. After removal of insoluble gel, excessive ethanol, which can dissolve branched polymer of SA and ESO and the unreacted SA and ESO monomers, was added into the solution to precipitate PLA for

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RESULTS AND DISCUSSION Preparation and Characterization of PLA/VESO Blends. The procedure of dynamic vulcanization to prepare the PLA/VESO blends is similar to the previous work.37 We successfully prepared high performance and thermal processable VESO resin, where PLA is a minor component to just provide a skeleton. Inspired by that work, we hypothesized that the same dynamic vulcanization strategy can be duplicated to prepare the supertoughened PLA blends by simply reversing the PLA as the major phase. Nevertheless, the result is not that B

DOI: 10.1021/acs.macromol.8b00103 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Dependence of VESO Structures during the Curing Process of ESO and SA by Varying R

case. From the previous study, the Mw of PLA decreased drastically from 1.76 × 105 to ∼1.0 × 105 g mol−1 after direct dynamic vulcanization with ESO and SA, due to acidolysis of PLA in the presence of a high concentration of carboxyl groups. Therefore, we modified the craft that SEP precursors were prepared to reduce the carboxyl concentration. A series of SEP precursors with different R-values were prepared, where the reaction was taken at 180 °C under a N2 atmosphere for 15−60 min. The reaction was stopped before the formation of gel, depending on the carboxyl/epoxy equivalent ratio (R) (Figure S1, Supporting Information). The acid value of the precursor increases with an increase in R-value, as shown in Table 1. Reaction between ESO and SA with different R could generate different chemical structured VESO, as seen in Scheme 1. A dimer tends to be formed with R at 0.2. More branching and cross-linking structures form and cross-linking density increases with an increase in R-value. After obtaining SEP precursors, we then introduced them into the PLA matrix dynamic vulcanization, and established the role of chemical structure, tuned by varying the R, on the morphology of the blends. Figure 1 shows the development of

arises from the inadequate carboxyl group. The melt torques of PLA/VESO-0.3, PLA/VESO-0.4, PLA/VESO-0.5, and PLA/ VESO-0.6 increased apparently with increasing time, indicating the occurrence of dynamic vulcanization. The melt torques of the four samples almost leveled off after 17 min, which is indicative of the termination of vulcanization. PLA/VESO-0.8 and PLA/VESO-1.0 finished dynamic vulcanization quickly, due to the high dynamic vulcanization speed at high R-value. PLA/VESO-0.8 and PLA/VESO-1.0 exhibit much lower final melt torques than PLA/VESO-0.3, PLA/VESO-0.4, PLA/ VESO-0.5, and PLA/VESO-0.6, which can be ascribed to the more serious acidolysis of PLA matrix, due to the much higher acid value of SEP-0.8 and SEP-1.0. As listed in Table 1, the Mw of PLA in PLA/VESO-0.8 and PLA/VESO-1.0 decreases to 11.33 × 104 and 7.82 × 104 g mol−1, respectively, from 15.21 × 104 g mol−1 for neat PLA, whereas the reduce in Mw of PLA becomes less prominent with R-value of 0.3−0.6, due to the lower acid value of the precursors. It is interesting that the Mw of PLA in PLA/ESO and PLA/VESO-0.2 increases obviously compared to neat PLA, which can be attributed to the chain-link reaction between PLA and ESO, as the terminal hydroxyl and carboxyl groups of PLA are reactive toward the epoxy groups.38−40 No gel formed for neat PLA and PLA/ESO, as shown in Table 1. PLA/VESO-0.2 showed limited gel fraction with a value of 2.0 wt %. Apparently, the gel fraction of PLA/VESO exhibits an uptrend with increasing R-value, which is in accordance with the variation of gel fraction of the VESO versus R-value performed in a flask, as shown in Figure S1. The results indicate that VESOs with both branching and cross-linking structures are present when R > 0.2 and the content of cross-linked VESO increased with an increase in R-value, which is in accordance with our expectation of controlling the structure of VESO through R-value. Morphology. The morphology was investigated by SEM. Figure 2 shows the SEM images for the cryo-fractured surfaces of neat PLA, PLA/ESO, and PLA/VESO with different VESO structures. The neat PLA shows a very smooth surface (Figure 2a). PLA/ESO (Figure 2b) still shows an almost homogeneous morphology but with a much rougher surface than neat PLA, which is attributed to the plasticization of ESO to PLA, making the PLA matrix flexible to undergo some deformation during the cryo-fracture process. Agglomeration of ESO occurs locally (red arrow in Figure 2b), which may account for the relatively poor plasticization effect of ESO to PLA. PLA/VESO-0.2 (Figure 2c) shows a much rougher surface than PLA/ESO and

Figure 1. Torque versus time for dynamic vulcanization of PLA and SEP with different R-values.

melt torque as a function of mixing time for the dynamic vulcanization performed in a torque rheometer at 170 °C and 80 rpm. The melt torque of neat PLA and PLA/ESO (a directly blending sample) did not increase. PLA/VESO-0.2 shows a higher melt torque than PLA/ESO, due to the higher melt viscosity of SEP-0.2 compared to ESO. The melt torque of PLA/VESO-0.2 does not increase obviously with time, which C

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Figure 2. SEM images for the cryo-fractured surfaces of neat PLA (a), PLA/ESO (b), PLA/VESO-0.2 (c), PLA/VESO-0.3 (d), PLA/VESO-0.4 (e), PLA/VESO-0.5 (f), PLA/VESO-0.6 (g), PLA/VESO-0.8 (h), and PLA/VESO-1.0 (i).

Reaction Mechanism. The chemical reaction is complex with at least three types of reactions occurring during dynamic vulcanization of PLA with SEP precursors. First, dynamic crosslinking of SEP precursors took place, as evidenced by the formation of insoluble gels. Second, the chain-link reaction between terminal groups of PLA and epoxy groups of SEP (or ESO for PLA/ESO) endows PLA in PLA/ESO and PLA/ VESO-0.2 with higher Mw’s, compared to neat PLA after processing with the same procedure. Third, acidolysis of PLA occurred, as evidenced by the significantly reduced Mw’s of PLA after blending with high acid-valued SEP-0.8 or SEP-1.0. Both chain-link and acidolysis are able to generate VESO-g-PLA copolymers, locating at the interface of the dispersed VESO and the PLA matrix, to compatibilize the blends. To study the reaction mechanism deeply, we recorded the FT-IR spectra of neat PLA, the gels of the blends, and the VESO obtained by curing in a flask, as shown in Figure 3. The characteristic absorption for the −OH groups is observed at ∼3460 cm−1 for all of the gels and VESO (Figure 3a). The stretching vibrations for carbonyl of VESO and PLA appear at 1726 and 1745 cm−1, respectively, as shown in Figure 3b. The carboxyl stretching vibration for the gel of PLA/VESO-0.2 occurs at the same location with that of PLA, indicating PLA is the main component for the gel, which may be formed by cross-linking of PLA with SEP-0.2, as it contains many free epoxy groups. Apart from the main absorption at 1726 cm−1, a shoulder is observed at ∼1745 cm−1 for the gels of other PLA/ VESO blends, indicating that PLA was grafted onto the VESO

exhibits a quasi-layered and wrinkled phase-separated morphology, due to the reduced miscibility between PLA and VESO. Distinct phase-separated structure, with VESO particles dispersing uniformly in the PLA matrix, is formed when R is ≥0.3 (Figure 2d−i). However, the phase boundary between the dispersed VESO and the PLA matrix is unobservable for all of the blends, indicating a good compatibility, which decreased with increasing R-value, as evidenced by the gradually increasing size of dispersed particles.21,22,24 The pulling out of dispersed particles was observed for PLA/VESO-1.0 (red arrow in Figure 2i), indicating a poor interfacial adhesion between VESO-1.0 and the PLA matrix. The weight-average particle diameter (dw), a key morphology parameter determining toughening efficiency,25−28 of dispersed VESO in the PLA/VESO blends was measured from the SEM images by the software ImageJ with the method reported in our previous study.27,28,36 The values are 0.64, 0.74, 1.14, 1.61, 2.38, and 2.96 μm for the blends at R-values of 0.3, 0.4, 0.5, 0.6, 0.8, and 1.0, respectively. It is interesting to find that a large number of uniform cavities appear in the surfaces of PLA/VESO-0.3, PLA/VESO-0.4, and PLA/VESO-0.5, indicating that the VESOs underwent internal cavitation during the cryo-fracture process. The cavitation is hard to observe when R is ≥0.6. The difference arises from the structure of the VESOs, including the cross-linking density, where the VESO particles with lower R (the lower gel fraction) are softer and more flexible, making them easier to be deformed and to generate cavitation to terminate the craze. D

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Figure 3. FT-IR spectra (a) and enlarged spectra (b) of neat PLA, VESO, and the gels obtained from different PLA/VESO blends.

gels. In addition, another two absorption bands belonging to PLA are also observed at 865 and 754 cm−1 for all of the gels and the peak intensity decreases with increasing R-value, which indicates that acidolysis is less effective than chain-link with respect to grafting PLA chains onto VESO gels, accounting for the above-observed reducing compatibility with increasing Rvalue. We further recorded the 1H NMR spectra for neat PLA, isolated PLA from PLA/VESO-0.4, and ESO, as shown in Figure 4. Apart from the signals of neat PLA (δHa at 1.58 ppm and δHb at 5.18 ppm), some characteristic signals belonging to ESO, such as δHe (2.3 ppm), δHf (3.0 ppm), and δHg (4.25 ppm), are also observed for isolated PLA. There are no noncross-linked impurities in the isolated PLA, as the branched VESO is soluble in the precipitator ethanol. The single peak for the GPC curves (Figure S2) could also prove the absence of impurity with the isolated PLA. Thus, the NMR results again demonstrate that compatibilization occurred between PLA and VESO during dynamic vulcanization. According to the discussions, we proposed the interfacial compatibilization mechanism during the dynamic vulcanization of PLA with SEP precursors, as shown in Scheme 2. Mechanical Properties. Prior to mechanical properties analysis, the matrix degree of crystallinity (Xc) was investigated, as it would have some influences on the mechanical properties of the samples. The Xc’s of the injection-molded samples were calculated according to their heating scans (Figure S3), and the results are listed in Table 2. PLA/ESO shows the highest Xc, which is attributed to the promoted chain mobility after plasticization.41 The differences in the Xc values of different PLA/VESO blends are much smaller than those of PLA/ESO,

Figure 4. 1H NMR spectra of neat PLA (a), ESO (b), and isolated PLA (c) from PLA/VESO-0.4 blend.

despite the downtrend in crystallinity of the samples with increasing R-value. Figure 5 shows the stress−strain curves of neat PLA, PLA/ ESO, and PLA/VESO blends. The parameters including tensile strength (σ), Young’s modulus (E), and elongation at break (ε) are listed in Table 2. The brittle neat PLA shows the highest σ and E but the lowest ε with the values of 58.4, 1852 MPa, and 7%, respectively. The σ and E of PLA/ESO decrease drastically to 23.5 and 1294 MPa; meanwhile, the ε increases to 115%, which is attributed to the plasticization effect of ESO to PLA. The peeling off of the sample bar (Figure S4) occurred when breaking, which should result from the agglomeration of ESO in local areas (Figure 2b), leading to lower intermolecular interactions of PLA chains. PLA/VESO-0.2 shows a similar σ to PLA/ESO. However, both E and ε of PLA/VESO-0.2 are much lower than those of PLA/ESO, possible due to the reduced miscibility between PLA and VESO-0.2 combining with the reduced Xc of the PLA matrix. The peeling off of PLA/VESO0.2 sample bar also occurred during stretching, which may be correlated to the formed quasi-layered and wrinkled phaseseparated morphology. The VESO-0.2 may be rich in the interlayer space of the wrinkled PLA matrix, resulting in weak E

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Macromolecules Scheme 2. Interfacial Compatibilization Mechanisms for the Dynamic Vulcanization of PLA with SEP

Table 2. Thermal and Mechanical Properties of PLA/EBP Blends with Various Molar Ratios of SA samples

Xc (%)

PLA PLA/ESO PLA/VESO-0.2 PLA/VESO-0.3 PLA/VESO-0.4 PLA/VESO-0.5 PLA/VESO-0.6 PLA/VESO-0.8 PLA/VESO-1.0

8.7 32.7 25.2 22.9 21.3 20.5 20.3 18.5 19.7

σ (MPa) 58.4 23.5 23.7 33.2 34.4 32.9 32.7 30.3 30.4

± ± ± ± ± ± ± ± ±

0.8 0.9 1.4 0.6 0.2 0.7 0.7 0.6 1.2

ε (%)

E (MPa) 1852 1294 937 926 994 990 1096 1149 1274

± ± ± ± ± ± ± ± ±

190 52 83 36 150 63 96 80 133

7 115 64 629 445 426 328 324 36

± ± ± ± ± ± ± ± ±

1 27 5 33 45 68 43 22 11

TT (MJ/m3) 2.3 20.8 9.6 150.6 108.7 90.2 77.6 76.4 6.6

± ± ± ± ± ± ± ± ±

0.2 7.5 0.9 9.8 13.4 20.5 11.9 6.5 3.2

NIS (J/m) 34.1 92.3 100.1 483.5 542.3 394.9 143.8 62.5 59.1

± ± ± ± ± ± ± ± ±

2.1 7.2 17.6 4.7 8.6 35.7 20.6 3.4 1.7

interactions between the wrinkled layers, and therefore peeling off occurred through interlayer sliding. It is suprising to find that, with increasing R-value to 0.3, the ε of PLA/VESO-0.3 increases to 629%, which is ∼90 times higher than that of neat PLA. Such a high elongation at break has not been reported before for other elastomer toughened PLA blends with similar composition.4−7 Meanwhile, the σ increases to 33.2 MPa from 23.7 MPa of PLA/VESO-0.2, and the E only reduces very slightly. Thereafter, the elongation at break and tensile strength decreases gradually with further increasing R-value, which results from the gradually increasing dispersed VESO particle size and decreasing compatibility between the PLA matrix and the dispersed VESO. The ε maintains at high level with the value in the range 320−450% for PLA/VESO-0.4, PLA/VESO0.5, PLA/VESO-0.6, and PLA/VESO-0.8. The ε decreases drastically to 36% for PLA/VESO-1.0, which should be attributed to the lowest matrix Mw plus the largest dispersed particle size of the sample. The Young’s modulus PLA/VESO shows an uptrend with increasing R-value, which should be attributed to the increased degree of cross-linking and gel

Figure 5. Stress−strain curves of neat PLA, PLA/ESO, and PLA/ VESO blends with different SA/ESO ratios.

F

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Figure 6. SEM images for tensile fractured surfaces of neat PLA (a), PLA/ESO (b), PLA/VESO-0.2 (c), PLA/VESO-0.3 (d), PLA/VESO-0.4 (e), PLA/VESO-0.5 (f), PLA/VESO-0.6 (g), PLA/VESO-0.8 (h), and PLA/VESO-1.0 (i).

mechanism, as shown in Figure 6. Neat PLA shows a typical brittle fractured morphology with a very smooth surface (Figure 6a). Cavitation induced extensive matrix shear yielding occurred for both PLA/ESO and all PLA/VESO blends, in accordance with their ductile fracture behaviors. Internal VESO cavitation can be inferred from the absence of interlay gap and interfacial debonding induced VESO particle on the surfaces of the samples.3 Therefore, the tensile toughening of PLA/VESO blends follows the mechanism of internal cavitation induced matrix shear yielding, which is a very efficient way of energy dissipation.7,28 The matrix pulling off (red arrow in Figure 6c) is observed for PLA/VESO-0.2, which accounts for the low elongation at break during stretching. Matrix layers and interlayer gaps are observed for PLA/VESO-0.2 (red dashed circles in Figure 6c. VESO-0.2 should be rich in the interlayer gaps, reducing the interlayer interaction, thus leading to sliding between matrix layers during stretching to cause premature fracture. The size of cavities increases gradually with increasing R-value, which is in accordance with the variation of VESO particle size. The elongation at break correlates to the size of the formed cavities, because too large cavities are not stable enough during stretching and thus tend to break before well-developed matrix shear yielding, which explains the decreasing elongation at break of PLA/VESO blends with increasing R-value.

fraction. The tensile toughness (TT) was calculated by the integral area under the stress−strain curve. The values, as listed in Table 2, change similarly to the variation of elongation at break of the samples. PLA/VESO-0.3 shows the highest tensile toughness with a value of 150.6 MJ/m3, which is 65 times improvement compared to 2.3 MJ/m3 of neat PLA. The notched Izod impact strengths (NIS) of the samples were also investigated, and the results are listed in Table 2. Neat PLA shows an impact strength of 34.1 J/m. The impact strength of PLA/ESO increases to 92.3 J/m, which is slightly lower than that of PLA/VESO-0.2 with the value of 100.1 J/m. The impact strength first increases and then decreases with increasing R-value. Super toughness is achieved for PLA/ VESO-0.4 with a maximum impact strength of 542.3 J/m, which is ∼16 times improvement compared to neat PLA. The impact strength of the elastomer toughened PLA blend is strongly dependent on the particle size of dispersed elastomers. The optimum size (dw) for the highest toughening effect is calculated at 0.75 μm, as theoretically estimated and experimentally demonstrated in various studies.25−28 The VESO-0.4 shows a particle size of 0.74 μm, which is the same as optimum size to achieve the optimal toughening effect. Toughening Mechanism. Tensile Toughening Mechanism. The morphology for the tensile fractured surfaces of the samples was observed by SEM to study the tensile toughening G

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Figure 7. SEM images of stretched PLA/VESO-0.4 sample bar at different tensile stages (a, b, and c) as schematically indicated in part d.

Figure 8. SEM images for impact fractured surfaces of neat PLA (a), PLA/ESO (b), PLA/VESO-0.2 (c), PLA/VESO-0.3 (d), PLA/VESO-0.4 (e), PLA/VESO-0.5 (f), PLA/VESO-0.6 (g), PLA/VESO-0.8 (h), and PLA/VESO-1.0 (i).

PLA/VESO-0.4 as a typical example. Internal VESO cavities arrayed randomly in the overall scope at the initial stage of stretching (Figure 7a). The random cavities become deforming

The longitudinally cryo-fractured surfaces at different stretching stages and locations were observed by SEM (Figure 7) to further study the tensile toughening mechanism, with H

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Figure 9. SEM images for the surfaces at different zones (a, b, and c) of the unbroken joint of impact PLA/VESO-0.4 sample bar as schematically indicated in part d.

observed on the surfaces of PLA/VESO-0.3, PLA/VESO-0.4, and PLA/VESO-0.5. The supertoughened PLA/VESO-0.4 did not break completely during impact test, as shown in Figure 9d. The surfaces at different zones of the unbroken joint were observed by SEM (Figure 9) to study the impact-toughening mechanism. A large number of cavities array randomly at the initial stage (Figure 9a). The cavities deform and array orderly along the stretching direction (Figure 9b), and the matrix near the cavities begins to orientate and deform at the same time. The deformation and orientation of both cavities and the matrix develop drastically to highly orientated threadlike shapes before breaking (Figure 9c). According to the evolution of the morphology, we can speculate that the bulges on the fractured surface of PLA/ VESO-0.4 (Figure 8e) result from the plastic deformation of the PLA matrix and the depressions are formed by deformation of the cavities. PLA/VESO-0.3 and PLA/VESO-0.5 display a similar fractured surface morphology to PLA/VESO-0.4, which indicates that this group of PLA/VESO blends follow the same impact-toughening mechanism of internal cavitation induced matrix shear yielding, which is a very efficient energy dissipation mechanism under impact pattern.3,25 In contrast, more regular cavities are observed for the blends with R ≥ 0.6 (Figure 8g−i), which demonstrates much less developed deformation of both the cavities and the matrix under impact loading, corresponding to the gradually reduced impact strength. The premature breaking resulting from the lowered stability of enlarged cavities and reduced interfacial adhesion are responsible for the insufficient matrix plastic deformation under impact loading for PLA/VESO blends with R ≥ 0.6. Besides internal cavitation, debonding cavitation is also obviously observed on the surfaces of PLA/VESO-0.8 and PLA/VESO-1.0, as evidenced by the pulling out of dispersed particles. From the above discussion, we can conclude that the impact-toughening mechanism of PLA/VESO blends changes from internal cavitation promoted matrix shear yielding to internal & debonding cavitation induced matrix deformation for the VESO toughened PLA blends with increasing R-value.

and arraying orderly along the stretching direction at the onset necking zone (Figure 7b); meanwhile, the matrix near the cavities begins to orientate, attributing to the matrix shear yielding. Highly orientated threadlike cavities with much higher aspect ratio are formed at the thin necking zone (Figure 7c). Drastic matrix plastic orientation and deformation take place combining with the deformation of the cavities, which would undoubtedly lead to considerable tensile energy dissipation to exhibit high elongation at break and tensile toughness. The elongated cavities with hollow, microscaled, and cylinder-like shapes again indicate a tensile toughening mechanism of internal VESO cavitation promoted PLA matrix shear yielding for the blends with R ≥ 0.3. The levels of orientation and deformation for both the cavities and the PLA matrix at the necking zone of PLA/VESO-1.0 (Figure S5) are much lower than that of PLA/VESO-0.4, accounting for the lowest elongation at break of PLA/VESO-1.0. The less-developed cavity deformation and matrix shear yielding are attributed to the premature breaking resulting from the instability of largesized cavities from internal cavitation of the largest dispersed VESO-1.0 particles. Impact-Toughening Mechanism. Figure 8 shows the SEM images for the impact fractured surfaces of neat PLA, PLA/ ESO, and PLA/VESO blends with different R-values. The brittle neat PLA also shows a smooth impact fractured surface (Figure 8a). PLA/ESO (Figure 8b) and PLA/VESO-0.2 (Figure 8c) exhibit similar rough fractured surfaces, which is responsible for the higher impact strength of the two samples than neat PLA. Extensive matrix plastic deformation occurs for PLA/VESO blends with R-values of 0.3, 0.4, and 0.5 (Figure 8d−f), corresponding to their high impact strength. PLA/ VESO blends with higher R-values such as 0.6, 0.8, and 1.0 (Figure 8g−i) also exhibit some extent of matrix plastic deformation with obvious cavities and pulling out of dispersed particles observed on their surfaces, which is attributed to the increasing particle size of dispersed VESO and reducing interfacial compatibility with increasing R-value. It is noticed that no obvious cavities and pulling out phenomenon can be I

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CONCLUSIONS Fully sustainable PLA/VESO blends were fabricated by dynamic vulcanization of PLA with SA cured ESO precursors with different R-values. The morphology and mechanical properties of the PLA/VESO blends are dependent on the structure of VESO determined by R-value. The phase morphology changes from homogeneous for PLA/ESO (with R of 0) to phase-separated for PLA/VESO. The compatibility decreases and the dispersed VESO particle size increases with increasing R-value, due to the reducing chain-link reaction between PLA and VESO. Both the tensile toughness and the impact toughness first increase and then decrease with increasing R-value. The highly toughened blends are available with R-value of 0.3−0.5, attributed to the uniformly dispersed morphology with suitable interfacial adhesion. The elongation at break increases up to 629% and the impact strength increases up to 542.3 J/m, compared to 7% and 34.1 J/m of neat PLA, respectively. The tensile toughness follows a mechanism of internal cavitation induced matrix shear yielding regardless of the structure of VESO. The impact toughness for the blends with R-values of 0.3−0.5 also follows an internal cavitation induced matrix shear yielding mechanism, while that of the blends with R ≥ 0.6 follows the mechanism of internal combining debonding cavitation induced matrix shear yielding. This investigation demonstrates that the PLA/VESO blends with different VESO structures exhibit great viability as sustainable and biodegradable alternatives to petroleum-based polymers in widespread applications, where enhanced toughness over the performance of neat PLA is required.

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Hygiene and Safty of Plastics (Beijing Technology and Business University) (Grant No. SS201702), and Fundamental Research Funds for the Central Universities (XDJK2017A016 and XDJK2017C022).

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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.macromol.8b00103. Experiments for preparation of SEP precursors, gel fraction measurement, and isothermal curing of ESO and SA with different carboxyl/epoxy equivalent ratios in a flask at 180 °C and DSC measurements of the blends for degree of crystallinity calculation; development of gel fraction with time for isothermal curing of ESO and SA at 180 °C, GPC curves of isolated PLA from the blends, DSC heating scans of the injection-molded blends, digital photos of broken tensile bars, and SEM images of stretched PLA/VESO-1.0 sample bar at different tensile stages (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian-Bing Zeng: 0000-0003-1822-446X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (51673158), the Basic and frontier research project of Chongqing (cstc2017jcyjAX0426), the Opening Project of Beijing Key Laboratory of Quality Evaluation Technology for J

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