The diffusivity, solubility, and permeability ofwater vapor and nitrogenwere systematically investigated for Vacuum
Ultraviolet (VUV)-irradiated poly(lactic acid) (PLA) membranes by a xenon excimer lamp at 172 nm. The
photon was completely absorbed in the surface membrane, and the VUV irradiation did not depend on the
color center and bulk structure of the PLA membrane. Additionally, surface hydrophilicity increased in the irradiation
time range of 0–10 min and settled at a steady state in the irradiation time range of 30–100 min. We discovered
the surface photo-oxidation, scission reaction, and crystallization of PLA membranes by VUV irradiation.
The PLA chemical structure changed as chain cleavage and formation of C_C double bonds and hydroperoxide
OH at newly formed chain terminals through a Norrish II mechanism. The water vapor permeability of VUVirradiated
PLA membrane for 60 min was equal to that of unirradiated PLA membranes because the water
vapor diffusivity reduction for surface crystallization counteracted the solubility enlargement for surface hydrophilicity.
The temperature dependence onwater vapor permeability of VUV-irradiated PLAmembrane at lowrelative
feed pressure showed that diffusivity-controlled behavior and that, at high relative feed pressure,
diffusivity- and solubility-controlled behavior.