Water uptake, distribution, and mobility in amorphous poly(D,L-lactide) by molecular dynamics simulation.

An explicit all-atom computational model for amorphous poly(lactide) (PLA) was developed. Molecular dynamics simulations of PLA glasses were conducted to explore various molecular interactions and predict certain physical properties. The density of a newly formed PLA glass aged for 100 ns at 298 K was 1.23 g/cm(3), close to the experimental range (1.24-1.25 g/cm(3)). The glass transition temperature (Tg = 364 K) was higher than experimental values because of the fast cooling rate (0.03 K/ps) in the simulation. The solubility parameter (20.6 MPa(1/2)) compared favorably to the literature. The water sorption isotherm obtained by relating the excess chemical potential of water in PLA to the Henry's law constant for water sorption was close to the experiment. At 0.6% (w/w), water molecules localize next to polar ester groups in PLA because of hydrogen bonding. Local mobility in PLA as characterized by the atomic fluctuation was sharply reduced near the Tg , decreasing further with aging at 298 K. The non-Einsteinian diffusion of water was found to correlate with the rotational β-relaxation of PLA C=O groups at 298 K. A relaxation-diffusion coupling model proposed recently by the authors gave a diffusion coefficient (1.3 × 10(-8) cm(2) /s at 298 K) which is comparable to reported experimental values.