Mechanisms of Plastic Deformation in Biodegradable Polylactide/Poly(1,4-cis-isoprene) Blends Marcin Kowalczyk, Ewa Piorkowska Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90 363 Lodz, Poland Received 13 May 2011; accepted 20 August 2011 DOI 10.1002/app.35489 Published online 5 December 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Polylactide (PLA), a main representative of biodegradable and made from renewable resources poly- mers, is surprisingly brittle at ambient temperature. In this article it is investigated how to increase its toughness by a strategy called ‘‘rubber toughening’’ using poly(1,4- cis-isoprene), a major component of natural rubber, which is immiscible with PLA, could be well dispersed in PLA matrix and is biodegradable. Immiscible blends of PLA with poly(1,4-cis-isoprene) were prepared by melt blend- ing and their properties were studied and optimized. Incorporation of as low as 5 wt % of rubber increased the strain at break of compression molded film during uniax- ial drawing, and also improved its tensile impact strength by 80%. The complex mechanism of plastic deformation in the blends leading to improvement of ductility and toughness was revealed. The rubbery particles initiated crazing at the early stages of deformation, as evidenced by transmission and scanning electron microscopy and also by small angle X-ray scattering. Crazing was imme- diately followed by cavitation inside rubber particles, which further promoted shear yielding of PLA. The sequence of those mechanisms was proven by micro- scopic investigation. All three elementary mechanisms acting in the sequence indicated are responsible for sur- prisingly efficient toughening of PLA by a major compo- nent of natural rubber. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 124: 4579–4589, 2012 Key words: polymer blends; mechanical properties; modification INTRODUCTION Polylactide (PLA), a biodegradable polymer, also produced from annually renewable resources, has the glass transition temperature (T g ) in the range of 50–60  C. As a consequence, PLA is stiff and brittle at room conditions. Although both optically pure poly(L-lactide) and poly(D-lactide) are crystallizable polymers, dimers of different chirality in the poly- mer chain decrease its ability to crystallize without a significant change in T g . Slowly crystallizing PLAs could be quenched below T g without noticeable crys- tallization and crystallize during subsequent heat- ing. 1,2 Crystallinity, if developed, slightly increases the modulus of elasticity and further decreases PLA’s ability to plastic deformation. 3 Significant engineering effort was made to improve PLA’s mechanical properties, mostly how- ever, without deep understanding of micromechan- isms involved. To modify the mechanical properties, PLA was blended with various substances which act as plasticizers 4–10 and with other polymers. 11–21 Blending PLA with other polymers, immiscible with PLA, can lead to a substantial improvement of draw- ability and impact strength without a decrease in T g . The important aspect of toughnening is retaining bio- degradability of the material. The phase separated PLA-based blends with biodegradable polymers: polyhydroxyalkanoate copolymer, 11 poly(butylene adipate-co-terephatalate), 12 poly(ether)urethane elasto- mer, 13 polyamide elastomer, 14 poly(butylene succi- nate-co-lactate), and poly(butylene succinate), 15 exhib- ited improved ductility and toughness. Increase of the strain at break during relatively slow drawing, at the rates below 20% per minute, required incorpora- tion of at least 10–20 wt % of the second component. Higher contents of modifiers, usually at the level of 20–40 wt %, were necessary to improve significantly the impact strength, which was accompanied by a marked decrease of the tensile strength and the elas- tic modulus. 12,13 Modification of PLA with polymers being neither biobased nor biodegradable, like ther- moplastic polyurethane, 17 acrylonitrile-butadiene-sty- rene copolymer, 19 and hydrogenated styrene-butadi- ene-styrene block copolymer (SEBS), 20 poly(ethylene- glycidyl methacrylate) (EGMA) 21 was also examined. To control the interface, reactive blending was employed 19–21 utilizing also additional reactive com- ponents: EGMA 20,21 and styrene-acrylonitrile-gly- cidyl methacrylate copolymer. 19 Indeed, reactive Correspondence to: E. Piorkowska (epiorkow@cbmm.lodz. pl). Contract grant sponsor: European Regional Development Fund, project BioMat, 2006-2008. Journal of Applied Polymer Science, Vol. 124, 4579–4589 (2012) V C 2011 Wiley Periodicals, Inc.