Temperature sensitivity and electrokinetic behavior of a 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 lower-critical 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.