Deformation and physical aging of rubber-modified polylactide graft copolymers

Abstract

With the goal of rubber-toughening polylactide (PLA), high PLA content graft copolymers containing a rubbery-backbone and randomly spaced polylactide arms were synthesized using a two-step "grafting from" approach to give poly(1,5-cyclooctadiene-co-5-norbornene-2-methanol-graft-lactide) [PCNL]. The reaction scheme allowed for independent control of the total backbone molar mass, the backbone molar mass between graft points (Mn,eff) and the weight fraction of PLA (wPLA). A series of graft copolymers, containing 80 to 99 wt.% PLA, were synthesized with Mn,eff ranging from 0.7 - 5.6 kg/mol. For an expected PLA graft molar mass of ~60 kg/mol, adventitious initiation of lactide was competitive with initiation by the macroinitiator, resulting in blends of PCNL and PLA. Graft copolymers where wPLA = 0.89 to 0.96, exhibited liquid-like packing of rubber domains in a matrix of PLA. The rubber domain radius (Rc) ranged from ~ 4 nm to 9 nm, and increased as a function of Mn,eff. In-situ small-angle x-ray scattering and tensile testing studies identified a new deformation phenomenon occurring prior to the yield point in both PLA and PCNL, independent of physical aging. We attribute the scattering evidence to micro-shear band and shear craze formation. Furthermore, while PLA deforms primarily by homogeneous crazing, rubber domain cavitation induced crazing was observed in PCNLs (wPLA = 0.95) with two different Rc (4 and 7 nm). Overall, the graft copolymer architecture seemed to have little to no effect on the physical aging kinetics of PLA. However, hetero-phase cavitation induced crazing was more prevalent for Rc = 7 nm, resulting in a more stable craze morphology that was not seen in PLA.