(114f) The Effect of Heat Treatment, Morphology and Crystallinity On Water Sorption in Polylactide (PLA)
In this research, the effect of morphology and crystallinity on water sorption in PLA via different heat treatment conditions was studied. 4032 PLA and 4060 PLA with a molecular weight of about 100,000 g/mol and two chemical ends consisting of hydroxyl group (-OH) called standard PLAs such as standard poly (L-lactide) 100k (PLLA100k or 4032 PLA) and standard poly (D,L-lactide) 100k (PDLLA100k or 4060PLA) commercially manufactured by NatureWorks were used as-received. Standard PLLA10k and PDLLA10k with a molecular weight of about 10,000 g/mol and the same chemical structure as standard PLLA100k and PDLLA100k were synthesized from L-lactide and D,L-lactice purchased from Sigma Aldrich. Henry's law constant was used to compare the water sorption level in all PLA samples. Water sorption in PDLLA showed a negligible effect of heat treatment but showed an effect of molecular weight. The difference in Henry's law constant calculated from sorption isotherm of PDLLA10k and PDLLA100k suggested that PDLLA100k absorbed 13% of water less than PDLLA10k did, which might be caused by a stronger effect of hydroxyl end group in low molecular weight PLA. Results in PLLA10k samples showed a strong effect of heat treatment on water sorption. Higher crystallinity PLLA10k gave higher water sorption. DSC experimental results showed an increase in the glass transition temperature of PLLA10k with increasing crystallinity, which is an indication of the formation of the rigid amorphous domain. However, heat treatment did not show a strong effect on water sorption in PLLA100k samples. Sorption isotherms in all tested PLLA100k samples were somewhat insensitive to crystallinity. Results from DSC experiments also showed almost no change in the glass transition temperature of PLLA100k under different heat treatment conditions.