Effect of poly(vinyl alcohol) as an efficient crystallization-assisting agent on the enhanced crystallization rate of biodegradable poly(l-lactide)

Abstract

Fully biodegradable poly(L-lactide) (PLLA) and poly(vinyl alcohol) (PVA) blends were prepared through a solution and casting method at very low PVA contents of 0.5 and 1 wt%. The effect of PVA on the crystallization behaviors of PLLA was investigated in detail under different crystallization conditions. Both the nonisothermal melt and cold crystallization behaviors of PLLA have been significantly enhanced by PVA, as evidenced by the increase of the nonisothermal melt crystallization peak temperature and the decrease of the nonisothermal cold crystallization peak temperature, with respect to neat PLLA. The overall isothermal melt crystallization rates of PLLA in the PLLA/PVA blends are also obviously faster than that of neat PLLA at the same crystallization temperature; however, the crystallization mechanism remains unchanged for both neat and blended PLLA, regardless of crystallization temperature. The spherulitic morphology and growth rate study reveals that PVA not only obviously increases the nucleation density of PLLA spherulites but also increases the growth rates of PLLA spherulites in the PLLA/PVA blends to some extent, which both contribute to the enhanced overall isothermal crystallization rates of PLLA. Therefore, PVA is an efficient crystallization-assisting agent for the enhanced crystallization of PLLA, and it is probably the first biodegradable polymer to accelerate the crystallization of PLLA at a very low content by acting as both a nucleation-assisting agent and a spherulite growth-assisting agent. Despite the presence of PVA, both neat PLLA and the PLLA/PVA blends present the same crystal structures. A possible nucleation mechanism of PVA on the crystallization of PLLA was also discussed.