Recovery as a measure of oriented crystalline structure in poly(l-lactide) used as shape memory polymer

Semicrystalline poly(l-lactide), or PLLA, has been reported as possessing shape-memory effect [1], [2], [3], [4] and [5]; however, the mechanism responsible for the shape recovery has not been well addressed. Therefore, in this paper, we attempted to investigate the structural evolution in PLLA during the shape memory process by means of differential scanning calorimetry, dynamic mechanical analysis (DMA), optical birefringence and wide-angle X-ray diffraction measurements. The shape recovery behaviour and recovery stress as a function of stretch ratio and deformation temperature were quantified. Based on the present results, shape recovery decreases with increasing stretch ratio and deformation temperature. It is suggested that the shape recovery is associated with the disorientation of oriented amorphous chains formed during stretching and the drop in recovery is attributed to the strain-induced crystallization and the formation of highly oriented microfibrillar structure, hindering the mobility in the amorphous phase and thus impeding recovery. In particular, the validity of the stress-optical law is considered. It is shown that the stress-optical properties show divergence from the Gaussian theory for stretch ratio greater than ∼1.5.