Graft Architectural Effects on Thermoresponsive Wettability Changes of Poly(N-isopropylacrylamide)-Modified Surfaces

Abstract

Poly( N -isopropylacrylamide) (PIPAAm) exhibits a reversible, temperature-dependent soluble/insoluble transition at its critical temperature in aqueous media. When PIPAAm molecules are covalently attached to a solid surface, the graft configuration greatly affects the thermoresponsive wettability changes of PIPAAm-modified surfaces. Three types of temperature-responsive surfaces were prepared using PIPAAm grafts of different molecular architectures: PIPAAm terminally grafted surfaces, PIPAAm looped chain grafted surfaces using a copolymer of IPAAm and N -acryloxysuccinimide, and PIPAAm terminally grafted onto immobilized PIPAAm loops. These surfaces were prepared by changing the graft architecture as well as the density of PIPAAm chains to investigate temperature-responsive wettability changes. All surfaces showed temperature-responsive hydrophilic/hydrophobic surface property alterations demonstrated by observed large and discontinuous wettability changes. On both surfaces bearing terminally grafted PIPAAm, surface wettability changed dramatically over the range 32−35 °C, a temperature corresponding to the phase-transition temperature for PIPAAm in aqueous media. This implies that terminally grafted PIPAAm chains retain a highly mobile nature and respond rapidly to temperature changes. The loop-grafted surface showed relatively large wettability changes but had a slightly lower transition temperature (∼27 °C). This reduced transition temperature is likely due to restricted conformational transitions for this multipoint grafted PIPAAm. Combination of both loops and terminally grafted chains showed the largest surface free energy changes among three surfaces. We conclude that PIPAAm graft architecture strongly influences surface wettability responses to temperature changes due to differences in the dynamic motion of the grafted polymer chains.