Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Design of Supertoughened and Heat-Resistant PLLA/Elastomer Blends by Controlling the Distribution of Stereocomplex Crystallites and the Morphology Baogou Wu, Qingtao Zeng, Deyu Niu, Weijun Yang, Weifu Dong, Mingqing Chen, and Piming Ma*
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The Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China S Supporting Information *
ABSTRACT: Supertoughened and heat-resistant poly(L-lactide) (PLLA)/ elastomer blends were prepared by controlling the distribution of stereocomplex (sc) crystallites and the morphological change from sea−island structure to cocontinuous-like structure. To control the distribution of sc and the phase morphology, poly(D-lactide) (PDLA) was first blended with ethylene−vinyl acetate−glycidyl methacrylate elastomer (EVMG) to prepare EVMG/PDLA masterbatch comprising both free PDLA and EVMG-g-PDLA copolymers. The free and grafted PDLA would collaborate with PLLA to form sc in the matrix and at the interface, respectively, during subsequent melt-blending of the masterbatch with PLLA. Consequently, the in situ formed sc crystallites not only enhanced the interfacial adhesion but also increased the melt viscosity and crystallization rate of the PLLA matrix. The sc crystallites amount can be tuned via PDLA content to achieve designable properties and cocontinuous-like morphology, leading to highly improved toughness of PLLA; e.g., the notched impact strength of PLLA was increased by 84 times. Moreover, the PLLA/(EVMG/PDLA) blends exhibit both excellent impact toughness (>70 kJ/m2) and heat resistance (E′140°C > 130 MPa) after a simple annealing. This work provides an effective approach toward high performance PLA materials which may expand the application of PLA to more advanced domains.
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ene (ABS),17,18 polymerized soybean oil (PSO),19,20 and natural rubber (NR).21,22 However, most of the polymers exhibit unsatisfactory toughening effects due to poor compatibility and weak interfacial bonding, ascribed to inefficient chain entanglement density at the interface.23−25 Compatibilization can be performed by adding compatibilizers or by in situ reaction between two phases. For example, the compatibility between PLA and PBAT was improved by the in situ formation of PLA−PBAT copolymers, resulting in greatly enhanced tensile toughness,11 whereas the crystallization rate of PLA matrix was not obviously influenced. Recently, stereocomplexation between the enantiomeric PLLA and PDLA has emerged as an important strategy of tailoring the interfacial adhesion.26−32 In addition, the stereocomplex (sc) crystallites can survive in the PLLA melt as the melting point of sc crystallites is ∼50 °C higher than that of PLA homocrystallites (hc) and offer heterogeneous nucleation sites for PLLA homocrystallization.31−36 Even a small amount of PDLA in the PLLA matrix can dramatically accelerate the crystallization rate of matrix by forming sc in situ.31−33 Furthermore, sc could increase the melt viscosity of the PLA matrix due to the filler and sc network effect.3,33,37 He et al.26,27
INTRODUCTION In recent decades, bio-based and biodegradable polymers such as poly(lactide) (PLA), polyhydroxyalkanoate (PHA), and polycaprolactone (PCL) have received increasing attention in consideration of environmental and sustainability issues with respect to petroleum-based materials.1,2 Some obvious advantages like relatively low price, wide source, high mechanical strength, and easy processability make PLA promising to replace traditional petroleum-based plastics.3−5 However, the practical application of PLA as a commercial thermoplastic is far less widespread than expected, as it also has some limitations primarily due to its inherent brittleness and inferior heat-resistance (poor crystallization ability causes PLA difficult to crystallize in traditional melt processing like injection molding cycles, so melt-processed PLA articles remain amorphous at all times).6,7 The heat deflection temperature (HDT) of amorphous PLA is as low as 50−65 °C, while the HDT of high crystallinity PLA exceeds 100−120 °C.6−8 Therefore, a key point in improving the heat resistance of PLA products is to improve its crystallization rate and consequently a high(er) crystallinity. Many flexible polymers have been reported to conquer the brittleness of PLA, including PCL,9 poly(butylene adipate-coterephthalate) (PBAT), 10,11 poly(butylene succinate) (PBS),12,13 polyethylene (PE),14 polyurethane (PU),15 poly(ethylene-co-octene) (POE),16 acrylonitrile−butadiene−styr© XXXX American Chemical Society
Received: October 22, 2018 Revised: December 25, 2018
A
DOI: 10.1021/acs.macromol.8b02262 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of the reaction of PDLA melt grafting onto EVMG to form EVMG-g-PDLA copolymers. (b) Melt torque as a function of mixing time for EVM/PDLA and EVMG/PDLA blends. (c) A digital image of (A) EVM/PDLA blend and (B) EVMG/PDLA blend in xylene (statically placed for 72 h). (d) 1H NMR spectra of the EVMG/PDLA (20/12) blend before and after removing the free PDLA.
crystallites simultaneously, i.e., creating strong interfacial adhesion, increasing viscosity of matrix to form cocontinuous-like morphology, and expediting PLLA matrix crystallization. To control the distribution of sc crystallites, EVMG and PDLA were first melt blended as masterbatches (first step), resulting in the generation of EVMG-g-PDLA graft copolymers and subsequently melt blended with PLLA to fabricate PLLA/(EVMG/PDLA) blends (second step). Expectedly, the grafted PDLA and free PDLA chains would strongly collaborate with PLLA matrix to form sc distributed at the interface and the matrix, respectively. The morphology, mechanical properties, crystallization behavior, and heat resistance of the PLLA/(EVMG/PDLA) blends were systematically investigated.
prepared some novel core−shell nanoparticles such as cellulose nanocrystals-g-rubber-g-PDLA and silica-g-rubber-g-PDLA to toughen PLLA, and a high elongation at break was obtained due to the enhanced interfacial adhesion via stereocomplex interactions between PDLA segments and PLLA matrix. Bai et al.31,32 added poly(ethylene−methyl acrylate−glycidyl methacrylate)-g-PDLA copolymers into the PLLA matrix, leading to improved toughness and crystallization rate. However, the above blends did not achieve transformation from sea−island to cocontinuous-like structures since the amount of PDLA was too low (12 wt %. Effect of PDLA Content (x) on the Thermal Behaviors of L/(G/Dx) Blends. The crystallization behaviors of L/(G/
crystallites and stronger interfacial adhesion. On the other hand, the higher the PDLA content is, the more sc crystallites leading to higher matrix viscosity (see Figures S4 and S8). Thus, it facilitates the transition of the EVMG elastomer from the dispersed phase to the continuous phase. For example, the morphology of EVMG elastomers in the L/(G/D2) blend has shown a trend toward continuous phase transitions compared H
DOI: 10.1021/acs.macromol.8b02262 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Dx) blends were studied by using DSC and POM, as shown in Figure 9, 10, and the thermal parameters are listed in Table S1. As shown in Figure 9a,b, the crystallization rate of neat PLLA is slow with a weak crystallization peak at 99.5 °C, while the crystallization rate of the L/G blend is even slower because the grafting between PLLA and EVMG limits the mobility of PLLA chains. In contrast, the EVMG/PDLA masterbatches could sharply expedite the crystallization of the PLLA matrix. Even 1 wt % of PDLA increases the crystallization temperature of the L/(G/D1) blend to 110.7 °C without cold crystallization in the second heating process. The crystallization temperature of PLLA matrix in the L/(G/Dx) blends is increased with the PDLA content up to 12 wt % due to the nucleation effect of sc crystallites but drops off at higher PDLA content because excessive sc crystallites (network) hinders the mobility of PLLA chains.33 Meanwhile, double melting peaks of the L/(G/Dx) blends gradually merge into single peak with increasing PDLA content, indicating that the crystal turns to be more homogeneous.60 Isothermal crystallization properties of neat PLLA, L/G and L/(G/Dx) blends investigated by DSC are presented in Figure 9c,d. The half-life time of crystallization (t1/2) can be used to characterize the crystallization rate of semicrystalline polymers. A lower t1/2 indicates a higher crystallization rate. The t1/2 of PLLA and L/G blend is 28.8 and 34.8 min, respectively; however, it is shortened significantly once sc crystallites are formed in the L/(G/Dx) blends. For example, the value of t1/2 decreases to 2.4 min for L/(G/D1) blend and 1.3 min for L/ (G/D12) blend, indicating an efficient heterogeneous nucleating effect of sc crystallites. These results are agreement with the above nonisothermal crystallization behaviors. POM observations can more directly reflect the nucleating effect of sc on the PLLA matrix. As shown in Figure 10a,b, PLLA and the L/G blend exhibit a very long crystallization time (still large amorphous areas are visible after 40 min) and a low nuclei density. Consequently, few large spherulites with a diameter of 100−150 μm are observed. Impressively, the crystallization of L/(G/D2) and L/(G/D12) blends is completed within 3 and 2 min, respectively, due to the presence of sc (Figure 10c,d). Moreover, the crystal nuclei densities of the L/(G/D2) and L/(G/D12) blends are increased dramatically, and their average spherulite diameters are reduced to 8 and 5 μm, respectively. The above results obviously illustrate that the in situ formed sc can act as an effective nucleating agent to dramatically promote the crystallization of PLLA matrix, and the PDLA content for optimal crystallization rate is 12 wt %. Effect of Annealing on Heat Resistance and Mechanical Properties of L/G and L/(G/D12) Blends. To increase the crystallinity and the consequent heat resistance of the samples, a two-step compression method is designed in this part; i.e., the L/G and L/(G/D12) melts were first quenched to 100 °C for 2 min (annealing) and then quenched to room temperature. In particular, the annealed L/G and L/(G/D12) blends were denoted as A-L/G and A-L/(G/D12), respectively. In this study, DMA was used to evaluate the heat resistance of the materials. Figure 11 shows the storage modulus (E′) of the samples as a function of temperature. The L/G and L/(G/D12) blends without annealing (almost amorphous PLLA matrix) show steep reduction in E′ across the Tg, indicating quite poor heat resistance. Subsequently, because of the cold crystallization of the matrix, the E′ begins to rise when the temperature reaches 80 °C. The E′ of the L/G
Figure 11. Variations of storage modulus with increasing temperature for compression-molded L/G and L/(G/D12) blends with and without annealing at 100 °C for 2 min.
blend is only slightly increased after annealing because t1/2 of the L/G blend is more than 3 min at 100 °C and consequently limited crystallinity (Figures S10 and S11). In contrast, the E′ at 80 °C of L/(G/D12) is increased from 22 to 595 MPa after annealing under the same conditions, and the blend does not show cold crystallization during the heating process. These results manifest that the sc expedites the crystallization of PLLA, making the matrix completely crystallized during the short annealing, which is confirmed by DSC analysis (Figure S10). The A-L/(G/D12) blend has a higher crystallinity of hc (Figure S11), which together with the original sc crystallites can form a dense crystal network. The dense crystal network is capable of imparting a high modulus to the blend at elevated temperatures even the amorphous region has already softened;6,53,61 e.g., the E′ of the A-L/(G/D12) blend at 140 °C (E′140°C) is still higher than 130 MPa. In brief, high heat resistance of the A-L/(G/D12) blend can be obtained by short annealing at 100 °C. The improvement of heat resistance is strongly attractive for the practical applications of PLA materials. It is well-known that when the crystallinity of the PLLA matrix increases, the mechanical properties of the corresponding blend will change.9,52,53 Therefore, the mechanical properties of A-L/G and A-L/(G/D12) were tested, and the results are shown in Table 2. Clearly, the mechanical properties Table 2. Mechanical Properties of A-L/G and A-L/(G/D12) Blends samples A-L/G A-L/(G/ D12)
yield strength (MPa)
elongation at break (%)
notch impact strength (kJ/m2)
40.6 ± 1.8 41.6 ± 1.3
92 ± 8 82 ± 10
48.7 ± 1.5 70.8 ± 3.2
of the A-L/G blend varied little compared to the blend without annealing (see Table 1), which is due to the slightly varying crystallinity as described above. In contrast, the mechanical properties of the A-L/(G/D12) blend changed significantly. Fortunately, the impact toughness of the A-L/(G/D12) blend can still be higher than 70 kJ/m2, and the yield strength is slightly increased. Only the elongation at break has an obvious decrease but still reaches a relatively high value of 82%. In I
DOI: 10.1021/acs.macromol.8b02262 Macromolecules XXXX, XXX, XXX−XXX
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short, a supertoughened PLA material with high heat resistance was successfully prepared.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02262. Figures S1−S11 and Table S1 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.M.). ORCID
Weifu Dong: 0000-0002-7432-8362 Piming Ma: 0000-0002-4597-0639 Notes
The authors declare no competing financial interest.
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REFERENCES
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CONCLUSIONS In this work, a feasible strategy to fabricate supertoughened and heat-resistant PLLA-based materials has been developed via controlling distribution of stereocomplex crystallites and morphological evolution from sea−island structure to cocontinuous-like structure. The incorporation of EVMG improved the toughness of PLLA but inhibited PLLA crystallization as well. Interestingly, if EVMG is first compounded with PDLA at 230 °C to make a masterbatch (first step) and then blended with PLLA (second step), it can result in PLLA/(EVMG/PDLA) blends with not only superior toughness but also excellent crystallization capabilities. EVMGg-PDLA copolymers are generated in the first step, and the grafted PDLA chains would collaborate with PLLA matrix to form interface-located sc crystallites. Meanwhile, free PDLA can form sc with PLLA in the matrix as well. The controlled distribution of sc crystallites could simultaneously (i) enhance the interfacial adhesion, (ii) increase the melt viscosity of PLLA matrix to tailor phase morphology, and (iii) serve as efficient nucleating agents to expedite PLLA matrix crystallization. Consequently, the notched impact strength of the PLLA/(EVMG/PDLA) blend is 84 times and 1 time higher than that of neat PLLA and PLLA/EVMG blend, respectively. The performance of the PLLA/(EVMG/PDLA) blends is also dependent on the PDLA content. An optimal toughness and crystallization rate are both obtained at PDLA content of 12 wt % in the PLLA/(EVMG/PDLA) blend, and it possesses simultaneously excellent notched impact toughness (>70 kJ/ m2) and heat resistance (E′140°C > 130 MPa) after a short annealing. Therefore, this work offers a novel route for producing high-performance PLA-based materials that can be comparable to some of the petroleum-based engineering plastics.
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ACKNOWLEDGMENTS
This work is supported by the Excellent Youth Natural Science Foundation of Jiangsu Province (BK20170053), the National Natural Science Foundation of China (51873082, 51573074) and the Fundamental Research Funds for the Central Universities (JUSRP51624A). J
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