Impact of D-isomer content on the microstructure and mechanical properties of uniaxially pre-stretched poly(lactic acid)

Poly(lactic acid) with 2%, 4%, and 12% D-isomer content were selected to investigate the influence of stereoregularity on mechanical properties of uniaxially pre-stretched PLAs (ps-PLAs). The mobility of molecular chains was activated by pre-stretching because the network structures composed of cohesional entanglements were destroyed, leading to a brittle-ductile transition. The elongation at break increased to 145%, 175% and 190% at pre-stretching ratio of 0.5 for ps-PLA2, ps-PLA4 and ps-PLA12, respectively. The network structures of cohesional entanglements were easier destroyed in PLAs with more D-isomer due to the stereoregularity defects, leading to higher elongation at break. Then the elongation at break decreased with increasing pre-stretching ratio owing to the development of mesophase which might initiate crazing. The more D-isomer the less strain-induced mesophase resulting from the more active chain relaxation, and then the slower decrease of elongation at break. In addition, the modulus and strength of ps-PLAs were all increased with increasing pre-stretching ratio.