Water transport in polylactic acid (PLA), PLA/ polycaprolactone copolymers, and PLA/polyethylene glycol blends |
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Authors: | Georgette L Siparsky Kent J Voorhees John R Dorgan Kevin Schilling |
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Institution: | (1) Department of Chemistry and Geochemistry, Colorado School of Mines, 80401 Golden, Colorado;(2) Department of Chemical Engineering, Colorado School of Mines, 80401 Golden, Colorado;(3) Chronopol Inc., 4400 McIntyre Avenue, 80403 Golden, Colorado |
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Abstract: | Polylactic acid (PLA) is a hydrolytically degradable aliphatic polyester, and water vapor permeability may have a significant
influence on the rate of degradation. A method is devised to use bags prepared from PLA films and filled with molecular sieves
to determine the water vapor permeability in the polymer, its copolymers with caprolactone, and blends with polyethylene glycol.
The “solution-diffusion” model is used to determine the permeability parameters. These include the solubility coefficient,S, a measure of the equilibrium water concentration available for hydrolysis and the diffusion coefficient,D, which characterizes the rate of water vapor diffusion into the film under specific conditions. Values ofS andD at 50‡C and 90% relative humidity ranged from 400 × 10-6 to 1000 × 10-6 cm3 (STP)/(cm3 Pa) and 0.20 × 10-6 to 1.0 × 10-6 cm2/s, respectively. TheS andD coefficients were also measured at 20 and 40‡C and compared to those of other polymers. The degree of crystallinity was found
to have little influence on the measured permeability parameters. The heat of sorption, δHS, 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 diffusion coefficient is used to compare the rate of water diffusion to the rate of water consumption by ester
hydrolysis. Results indicate that hydrolytic degradation of PLA is reaction-controlled. |
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Keywords: | Polylactic acid polycaprolactone polyethylene glycol permeability diffusion solubility hydrolysis degradation water vapor |
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