Gas permeation properties of poly(lactic acid) revisited

Pure gas permeation through solution-cast amorphous poly(lactic acid) (PLA) films is investigated using the time-lag method. New data on permeability, diffusivity and solubility for N2, O2 and CO2 are reported. At 30 °C, N2 permeability, diffusivity and solubility in PLA (98.7% L, 1.3% D) are 0.05 Barrer, 2.4 × 10−8 cm2/s, and 2.2 × 10−4 cm3 (STP)/cm3 (polymer) cmHg. The measured activation energy of N2 permeation is 34.6 kJ/mol. For O2, the corresponding values are 0.26 Barrer, 5.7 × 10−8 cm2/s, 4.9 × 10−4 cm3 (STP)/cm3 (polymer) cmHg and 24.0 kJ/mol. The values for CO2 are 1.10 Barrer, 4.4 × 10−9 cm2/s, 0.025 cm3 (STP)/cm3 (polymer) cmHg and 18.0 kJ/mol. These new measurements of the gas permeation properties of poly(lactic acid) (PLA) show considerable disagreement with previously reported results [H.J. Lehermeier, J.R. Dorgan, J.D. Way, Gas permeation properties of poly(lactic acid), J. Membr. Sci. 190 (2001) 243–251] obtained using a continuous flow apparatus. The effect of PLA optical compositions on permeation is also investigated and found to be consistent with the earlier report; the L/D ratio has no effect on the permeation properties provided the material is quenched into an amorphous state. The ability to generate diffusivity and solubility data along with the greater sensitivity of pressure measurement over gas composition argues in favor of the use of the time-lag apparatus for pure gas permeability measurements and suggests that the revised PLA permeation values reported here should be adopted.