Preparation of a novel composition-gradient thermosensitive gel

The feasibility of a novel composition-gradient copolymer gel, in which the composition gradually changes with the distance, was examined. The slab-shaped copolymer gels of a thermosensitive primary component, N-isopropylacrylamide (NIPA) and an ionic secondary component, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) or acrylic acid (AA), were prepared between two substrates of hydrophilic glass and hydrophobic polytetrafluoroethylene (Teflon). In the NIPA-co-AMPS gels, prepared at 40 °C between glass and Teflon, the content of AMPS gradually decreased toward Teflon wall. The formation of the gradient composition can be attributed to the repulsion of hydrophilic AMPS to Teflon. The composition-gradient NIPA-co-AMPS gel bended and stretched reversibly without breaking in response to the change in temperature. On the other hand, in the NIPA-co-AA gels prepared at 40 °C between glass and Teflon, AA distributed homogeneously. The difference in the distributions of ionic components in NIPA-co-AA and NIPA-co-AMPS gels comes from the differences in the reactivities and interactions of the ionic components with NIPA. The conversion of ionic components and the rate of gelation of NIPA-co-AA are larger than those of NIPA-co-AMPS. The glass transition temperatures of NIPA-co-AA gels are higher than those of NIPA and AA gels. This fact is attributed to that NIPA and AA in the copolymer gels are strongly interacting by hydrogen bonding with amide and carboxylic groups. The essential points to prepare a composition-gradient copolymer gel are that in addition to the repulsion to a substrate the secondary component has little interaction with the primary component.