Poly(L-lactide): V. Effects of storage in swelling solvents on physical properties and structure of poly(L-lactide)

The effects of storage at 25 °C in swelling solvents having different solubility parameter (δs) values of 16.8-26.0 J0.5 cm-1.5 on the physical properties and structure of as-cast poly(L-lactide) (PLLA) films was investigated by the degree of swelling (DS), differential scanning calorimetry (DSC), and tensile tests. It was found that PLLA film shows durability to swelling solvents having δs values much lower or higher than the value range of 19-20.5 J0.5 cm-1.5 and that the polymer solubility parameter (δp) for PLLA is in the value range of 19-20.5 J0.5 cm-1.5. The decrease in the glass transition temperature (Tg) and tensile properties and the increase in melting temperature (Tm) and crystallinity (xc) were larger for PLLA films swollen in solvents having a high DS at 7 days (DS7 days). The slight increase in Tm and xc for PLLA films after swelling in solvents with high DS7 days values was due to the crystallization of PLLA that occurred during swelling, while the small increase in Tg and elongation at break (εB) for PLLA films after immersion in the solvents having low DS7 days values was ascribed to stabilized chain packing in the amorphous region. The Tg, εB, and Young's modulus of the PLLA films after swelling in the solvents varied in the ranges of 47-57 °C, 4-8%, and 55-77 kg/mm2, depending on their DS7 days or δs values.