WATER TRANSPORT IN POLYLACTIC ACID (PLA), PLA POLYCAPROLACTONE COPOLYMERS, AND PLA POLYETHYLENE-GLYCOL BLENDS/

Polylactic acid (PLA) is a hydrolytically degradable aliphatic polyest er, and water vapor permeability ability may have a significant influe nce on the rate of degradation. A method is devised to use bags prepar ed from PLA films and filled with molecular sieves to determine the wa ter vapor permeability in the polymer, its copolymers with caprolacton e, and blends with polyethylene glycol. The ''solution-diffusion'' mod el is used to determine the permeability parameters. These include the solubility coefficient, S, a measure of the equilibrium water concent ration available for hydrolysis and the diffusion coefficient, D, whic h characterizes the rate of water vapor diffusion into the film under specific conditions. Values of S and D at 50 degrees C and 90% relativ e humidity ranged from 400 x 10(-6) to 1000 x 10(-6) cm(3) (STP)/(cm(3 ) Pa) and 0.20 x 10(-6) to 1.0 x 10(-6) cm(2)/s, respectively. The S a nd D coefficients were also measured at 20 and 40 degrees C and compar ed to those of other polymers. The degree of crystallinity was found t o have little influence on the measured permeability parameters. The h eat of sorption, Delta H-s, and the activation energy of diffusion, ED , were used to show that the permeability process is best described by the ''water cluster'' model for hydrophobic polymers. Finally, the di ffusion coefficient is used to compare the rate of water diffusion to the rate of water consumption by ester hydrolysis. Results indicate th at hydrolytic degradation of PLA is reaction-controlled.