Transport Properties of Oxygen and Water Vapor In Biaxial Orientated Poly(L-lactic acid) Membranes

The relationships among chain orientation, crystalline structure, crystallinity, and oxygen and water vapor permeation properties of biaxial oriented poly(L-lactic acid) (BOPLLA) membranes were discussed in this study. The BOPLLA membranes were obtained from various treatments such as simultaneous and sequential biaxial stretching and then annealing with different lengths of time at 100, 130, or 160^oC, respectively. The crystal structure and lamellar behavior of BOPLLA membranes were determined by wide angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), respectively. The oxygen gas and water vapor permeability, diffusivity, and solubility were determined with a constant-volume variable-pressure permeation apparatus and the time-lag method. The results of WAXD indicated that increasing the annealing temperature and time will result in the increasing of crystallinity of BOPLLA. However, the in situ SAXS results revealed that the lamellar thickness of PLLA was maintained about the same as the annealing temperature increased from 100 to 130^oC, and a rapid increased from 130 to 160^oC possibly due to the α’- to α-form transition in the crystalline lattices. The α’- to α-form transition behavior can be also observed in the WAXD results of various membranes. In addition, the gas barrier property of the sequentially BOPLLA membranes is better than that of the simultaneously ones due to the orientated crystalline structure and higher crystallinity.