Permeation Control through Porous Membranes Immobilized with Thermosensitive Polymer

Abstract

Permeation through a porous polycarbonate membrane, on which a thermosensitive polymer, poly(N-isopropylacrylamide), was immobilized, was investigated. For photoimmobilization of poly(N-isopropylacrylamide), photoreactive azidophenyl group was connected to the polymer either at a chain terminal or in side chains. The two types of derivatized polymers had different lower critical solution temperature (LCST). Prescribed amounts of the derivatized polymer were cast on the polycarbonate membrane and photoirradiated. When a small amount of polymer was used, a thin layer of immobilized polymer was not enough to cover pores of the polycarbonate membrane, while a thick gel layer of immobilized polymer was formed on the polycarbonate membrane to cover pores when a large amount of polymer was used. The former is represented by “porous membrane”, and the latter by “nonporous membrane”. The rate of water permeation through the porous membranes changed at different temperatures, although permeation through nonimmobilized membrane was independent of temperature. Water permeation through the porous membrane increased above the LCST of graft polymers. Hydraulic permeation through the nonporous membrane was not observed at any temperature. On the other hand, tryptophan permeation through the polymer-immobilized porous membrane became slower above the LCST, whereas that through the nonporous membrane became faster above the LCST. The permeation rate through the porous membrane was much higher than that through the nonporous membrane. The different temperature dependences of permeation can be explained as follows. In the case of the porous membrane, the graft chains expand below the LCST to close pores but contract at above the LCST to open pores. On the other hand, the nonporous membrane swells below the LCST to enhance diffusion of tryptophan and deswells above the LCST to reduce the diffusion. The present study demonstrated that the signal responsiveness of intelligent membrane can be controlled by the mode of device fabrication as well as by the nature of the sensoring unit.