Effect of initial crystallization on microstructure and mechanical properties of uniaxially pre-stretched poly(L-lactic acid)

Toughened and enhanced PLLA was acquired via uniaxially pre-stretching with pre-stretching ratio (PSR) of only 0.5. The effect of initial crystallization on microstructure and mechanical properties of pre-stretched PLLA (ps-PLLA) was investigated. With increasing initial crystallinity, the elongation at break (epsilon(b)) of ps-PLLA first increased and then decreased. The maximum epsilon(b) (153.3%) occurred at medium initial crystallinity. The destroyed cohesional entanglement and formation of orientation and mesophase induced by pre-stretching were major toughening mechanism. The network structure of cohesional entanglements was easier to be destroyed for PLLA with medium initial crystallinity owing to lower molecular chain density and less cohesional entanglement in amorphous region, and it was not easy to recover due to orientation and more mesophase, so the toughness was improved obviously. With further increasing initial crystallinity, the eb of ps-PLLA decreased owing to the reductive content of amorphous region and formation of crystalline continuous phase. The strength and modulus of ps-PLLA were enhanced after pre-stretching due to orientation and mesophase.