N-isopropylacrylamide grafted microporous polyethylene membrane

A temperature-sensitive N-isopropylacrylamide (NIPAAm) grafted polyethylene microporous membrane was prepared by the plasma-induced graft polymerization technique. The SEM observed that the poly(N-iso-propylacrylamide) (PNIPAAm) chains grafted on the membrane surface exhibited difference configurations around the lowercritical solution temperature (LCST). The XPS analysis indicated that the amide groups of the grafted PNIPAAm tended to distribute outwards when PNIPAAm chains were in the swelling state, while enveloped by the nonpolar main chains when in their shrinking state. The water permeation flux of the grafted membrane varied dramatically with a slight change in temperature around the LCST, which showed that the grafted PNIPAAm could act effectively as a chemical valve to control the on–off behavior of the membrane pores. The streaming potential method was used to evaluate the surface charge status of the grafted membrane at different temperatures and pH conditions. The distribution change of surface charges was also confirmed to be thermosensitive around the LCST, which is similar to the change of the water permeation flux. The grafted membrane acquired more negative charges over the LCST than that under the LCST, which is induced by the difference of specific anion adsorption due to the hydrophilicity–hydrophobicity transition of the grafted PNIPAAm.

The XPS and SEM analyses reveal that the grafted PNIPAAm chains on the PE membrane exhibit different configurations at the temperatures around the LCST. The nonpolar carbon chains of PNIPAAm tend to expose outwards to envelop the polar amide groups in the shrinking state, while the configuration converts to inverse in the swelling state. The water permeation flux across the NIPAAm grafted membrane shows strong temperature sensibility, and the grafted PNIPAAm chains could act as a chemical-valve to regulate the pore radius of the membrane pores by changing the temperature around the LCST. The electrical properties of the NIPAAm-grafted membrane were determined effectively by the streaming potential measurement, and . potentials were experimentally confirmed to be greatly thermosensitive, which is caused by the difference of the specific adsorption of anions on the grafted PNIPAAm chains due to the hydrophilicity–hydrophobicity transition. Acknowledgments The authors would like to thank the National Natural Science Foundation of China (No. 29876018) for financial support.