Properties and morphology of poly( l -lactide) 4. Effects of structural parameters on long-term hydrolysis of poly( l -lactide) in phosphate-buffered solution

Hydrolysis of poly(l-lactide) (PLLA) films with different crystallinities (xc's), crystalline thicknesses (Lc's), and spherulite sizes, was studied in phosphate-buffered solution at 37°C. The films were prepared by altering the annealing or crystallization temperatures (Ta's) from the melt without or with quenching before annealing. The initial xc and Lc of PLLA films increased as they were prepared at higher Ta. The induction period until the start of a decrease of mass and tensile strength by hydrolysis became shorter as the initial xc of the PLLA films was higher, when compared at the same hydrolysis time. This indicates that PLLA films with higher initial xc underwent faster hydrolysis. This result could be explained in terms of the density of effective tie chains which should decrease with an increase in xc. It also seemed likely that higher xc and hence higher Lc produced more defects in the amorphous region, which promoted hydrolysis by enhancing water diffusion. The radius of spherulites had an insignificant effect on the hydrolysis of PLLA films in phosphate-buffered solution. A specific peak appeared at a low molecular weight around 1×104 in gel permeation chromatography spectra of hydrolyzed PLLA film, irrespective of the thermal history of the PLLA film. This suggested that the observed specific peak was ascribed to the component of one-fold in the crystalline region. The relationship between the melting temperature (Tm) and Lc was found to be Tm(K)=471[1−1.59Lc (nm)] for PLLA film hydrolyzed for 3 years.