Transparent poly(lactic acid) film crystallized by annealing beyond glass transition temperature

The effect of post-processing annealing beyond the glass transition temperature on optical transparency was studied using poly(lactic acid) (PLA) films containing a nucleating agent such as N,N′-ethylenebis(stearamide) (EBS) or N,N′-ethylenebis(12-hydroxystearamide) (EBHS). Although the PLA films without the nucleating agent became opaque after exposure to annealing, the film with an appropriate amount of the nucleating agent was found to be transparent (the haze value was lower than 10%). Since the crystallinity was enhanced by annealing, the film showed excellent heat resistance. As compared with EBS, moreover, EBHS had a better capability to produce a transparent film because the aggregates, which appeared during cooling from the molten state, were small. In the case of EBHS, a network structure of fibrous EBHS in molten PLA was confirmed, which was responsible for the reduced size of the PLA crystals and prevented spherulite formation.