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Toughening Glassy Poly(lactide) with Block Copolymer Micelles Tuoqi Li,† Jiuyang Zhang,† Deborah K. Schneiderman,‡ Lorraine F. Francis,† and Frank S. Bates*,† †
Department of Chemical Engineering and Materials Sciences and ‡Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Poly(lactide) (PLA), a compostable bioderived polyester, can be produced at a cost and scale that makes it an attractive replacement for nondegradable petroleum-derived thermoplastics. However, pristine PLA is brittle and unsuitable for use in applications where high impact strength and ductility are required. In this work we demonstrate that poly(L-lactide) (PLLA) in the glassy state can be toughened significantly via addition of an amphiphilic diblock polymer. Notably, a PLLA blend containing only 5 wt% poly(ethylene oxide)-b-poly(butylene oxide) (PEO−PBO) exhibited tensile toughness and notched Izod impact strength over an order of magnitude higher than neat amorphous PLLA without a significant reduction in transparency or elastic modulus. For a series of PLLA blends containing PEO−PBO of fixed composition (∼70% volume fraction PBO), the toughness was inversely related to the molar mass of the added modifier with the highest toughness observed for the blend containing the smallest diblock (∼7 kg/mol). Interestingly, at fixed composition and molar mass poly(L-lactide)-bpoly(butylene oxide) (PLLA−PBO) exhibited a substantial but reduced toughening efficiency compared to PEO−PBO. We attribute this difference to a change in the solubility parameter of the amphiphilc block. Using TEM, we show that the greatest toughening is observed when the diblock modifier forms small cylindrical micelles that are well dispersed in the PLLA matrix. This morphology is facilitated by a negative Flory−Huggins interaction parameter (χ) between PEO and PLLA. These insights suggest a new and versatile strategy for the facile and efficient toughening of brittle thermoplastics.
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copolymers with precisely tailored property profiles.27−32 However, these materials may be prohibitively expensive for single use applications such as food packaging. The key to designing inexpensive and tough PLLA materials may lie in fabricating blends. Many polymers including polycaprolactone,14,33 poly(butylene succinate),34 polyurethane elastomers,35 soybean oil derivatives,36 linear low density polyethylene (LLDPE),15,37 and acrylonitrile-butadiene-styrene (ABS)38 have been considered as modifiers for PLLA. Because the majority of these polymers are immiscible with PLLA, a compatibilizer must typically be added to stabilize the interfaces of the blend constituents.2 One limitation of polymer−polymer blends is that the constituents can form micron-sized features, reducing the transparency of material. To avoid this problem, others have proposed a versatile, simple, and potentially lowcost strategy to reinforce thermoplastics using block copolymer dispersions.39−41 Inspired by this strategy we have investigated low molar mass poly(butylene oxide)-containing diblock copolymers as modifiers to toughen PLLA. These diblocks are low cost and can be dispersed as nanoscaled micelles in PLLA using facile melt blending or solvent blending routes. As demonstrated below, these additives effectively toughen PLLA even at loadings lower
rowing concerns over pollution caused by nondegradable, petroleum-derived plastics have driven the development of more sustainable alternatives.1 By commercial volume the most successful example of a bio-based degradable polymer has been poly(lactide) (PLA). This degradable polyester currently can be produced with only 15−25% surcharge relative to poly(ethylene terephthalate), a price difference small enough to allow its use in commercial products such as textiles and disposable containers.2,3 Despite possessing a high tensile strength and elastic modulus, neat poly(L-lactide) (PLLA) is intrinsically brittle, which prevents its use in applications where toughness, impact resistance, and optical clarity are essential.3−5 PLLA shares many of the inherent limitations of the commodity thermoplastic poly(styrene) (PS). Many of the strategies that have been successfully employed to toughen polystyrene have also been investigated to alter the mechanical properties of PLLA. These methods include plasticization,6−9 copolymerization,10−13 blending with flexible polymers,14−17 addition of rigid fillers,18−20 and changing the polymer chain orientation.21 Addition of miscible low molar mass plasticizers (e.g., polyethylene glycol (PEG)22−25 and citrate esters6,26) can improve the flexibility of PLLA; however, a high loading (>10% by weight) is usually required, which can severely depress the glass transition temperature (Tg) of PLLA, reducing its already low upper service temperature limit.1,22 Copolymerization of lactide with other cyclic ester or lactone monomers can be used to generate statistical or block © XXXX American Chemical Society
Received: January 23, 2016 Accepted: February 17, 2016
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DOI: 10.1021/acsmacrolett.6b00063 ACS Macro Lett. 2016, 5, 359−364
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ACS Macro Letters
Figure 1. Chemical structures of block copolymer modifiers and poly(L-lactide) (PLLA).
than 5% by weight, without reductions in Tg, elastic modulus, or optical clarity. The chemical structures of the PLLA homopolymer (NatureWorks under the trade name Ingeo 2003D) and block copolymers used in this study are presented in Figure 1. As summarized in Table 1, a series of poly(ethylene oxide)-bpoly(butylene oxide) (PEO−PBO) diblocks identified as EB-1
Figure 2a−d. EB-1 generated small cylindrical micelles (PBO core diameter 100 °C) followed by melting (140 °C < Tm < 150 °C). For each of the EB/PLLA blends the melting transition was split into two distinct peaks with one higher and one lower than the melting point of neat PLLA (see Figure S1). We attribute this phenomenon to the influence of PEO on the mechanism of PLLA crystallization, which causes the formation of two different types of crystallized domains in the matrix.49,50 Notably, the LB diblock does not appear to affect the melting behavior of the PLLA matrix (Figure S1). The microstructure of the modified blends critically affects the tensile properties of these glassy materials. Representative engineering stress versus strain curves are presented in Figure 3a, and the associated yield strength (σy), elastic modulus (E), strain at break (εb), and toughness are summarized in Table 2. All blends modified with 5 wt% loading of diblocks exhibited improved toughness and ductility over the neat PLLA. The blend prepared using EB-1 exhibited the greatest improvement relative to neat PLLA with a 1300% increase in tensile toughness and a 2500% increase in εb. EB-2, EB-3, and LB were less effective in toughening the PLLA, similar to macrophaseseparated rubber modifiers as reported in the literature.51 At loadings of 5 wt%, none of the blends exhibited a significant drop in Young’s modulus. However, as is often observed with rubber toughening, the yield strength relative to neat PLLA decreased by 45% to 50%.52 A photograph of representative tensile specimens containing 5 wt% EB-1 before and after stretching is presented in Figure 3c. Before stretching the specimen was transparent and indistinguishable from neat amorphous PLLA. Upon deformation, the gauge region showed uniform whitening and necking. This opacity is caused by the formation of micron and submicron size holes, as shown by the TEM image in Figure 3b. We speculate that this porous structure is formed by cavitation of the rubbery core micelles, similar to what has been reported in block copolymer toughened thermoset epoxy.53,54 Under an applied tensile force, the micelles can cavitate, leaving nanosized voids in the PLLA matrix, which continue to grow in size and number with increasing strain. The effect of modifier concentration on mechanical performances was explored using blends containing EB-1. As shown in Figure 4a, relative to neat PLLA, adding 1.25% and 5 wt% EB-1 362
DOI: 10.1021/acsmacrolett.6b00063 ACS Macro Lett. 2016, 5, 359−364
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ACS Macro Letters
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sustainable glassy PLLA materials based on a facile processing route. Finally, controlled cavitation and void formation offer a new method for producing low density porous materials with a host of potential applications.57−59
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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/acsmacrolett.6b00063. The experimental methods and additional characterization data (Figures S1−S6 and Tables S1−S3) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-0819885 and the Center for Sustainable Polymers under Award Number CHE-1413862. T.L. and D.K.S. acknowledge support from the University of Minnesota Doctoral Dissertation Fellowship program. The authors extend their gratitude to Fang Zhou for assistance with TEM specimen preparation. We also appreciate help with impact strength test setup and specimen preparation from Dr. Chris M. Thurber, and Liangliang Gu. D.K.S. acknowledges Marc Hillmyer for support. Portions of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program.
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