Multiple shape memory behavior of highly oriented long-chain-branched poly(lactic acid) and its recovery mechanism.

The shape memory effect of highly oriented long-chain-branched poly(lactic acid) (LCB-PLA) prepared through solid-phase die drawing technology was studied by comparison with PLA. When the recovery temperature increased from 60°C to 120°C, for PLA, only one-step recovery at about 80°C can be observed and the recovery ratio was below 21.5%, while, for LCB-PLA, multiple recovery behavior with high recovery ratio of 78.8% can be achieved. For oriented PLA, the recovery curve of the final sample showed the same trend with that of sample suffering just free drawing; while for oriented LCB-PLA, the recovery curve of the final sample showed the same trend with that of sample suffering just die drawing. After shape recovery, the mechanical properties of LCB-PLA showed a linear downward trend with the recovery temperature. Together with amorphous phase, the oriented mesomorphic phase, which formed during solid die drawing, can act as switching domains. And thus, upon heating, the chain segment of amorphous phase relaxed at first and triggered the first macroscopical shape recovery, leading to the decrease of long period (Lac) and the thickness of the amorphous layer (La ). Then, with further increasing temperature, the oriented mesomorphic phase gradually relaxed resulting subsequently multi-shape recovery, and the Lac and the La further decreased. Therefore, by regulating the recovery temperature of oriented LCB-PLA, the shape recovery ratio and mechanical strength can be controlled effectively, and thus the self-reinforced and self-fastening effect can be achieved simultaneously for PLA as bone fixation material.