Characterization of microstructure of poly(N-isopropylacrylamide)-grafted polycarbonate track-etched membranes prepared by plasma-graft pore-filling polymerization

Poly(N-isopropylacrylamide) (PNIPAM) was successfully grafted on the surfaces and in the pores of polycarbonate track-etched (PCTE) membranes by plasma-graft pore-filling polymerization method, and the microstructure of the PNIPAM-g-PCTE membrane was investigated systematically by employing XPS, SEM, FT-IR, AFM, contact angle instrument and water flux experiments. The results showed that, the grafted PNIPAM polymers were formed inside the pores throughout the entire membrane thickness, and there was not a dense PNIPAM layer formed on the membrane surface even at a pore-filling ratio as high as 76.1%. With the pore-filling ratio increasing, the pore diameters of PNIPAM-grafted membranes became smaller. When the pore-filling ratio was smaller than 44.2%, the pores of PNIPAM-g-PCTE membranes showed thermo-responsive gating characteristics because of the conformational change of grafted PNIPAM in the pores. On the other hand, when the pore-filling ratio was larger than 44.2%, the pores of membranes immersed in water were choked by the volume expansion of the grafted PNIPAM polymers, and the membranes did not show thermo-responsive gating characteristics any longer. The critical pore-filling ratio for choking the membrane pores was in the range from 30 to 40%. The contact angle of PNIPAM-g-PCTE membrane increased from 58.5° to 87.9° when the temperature changed from 25 to 40 °C. The thermo-responsive gating characteristics of the water flux of PNIPAM-g-PCTE membranes were mainly dependent on the pore size change rather than the variation of membrane/pore surface hydrophilicity.