Swelling and Thermoresponsive Behavior of Linear versus\nCyclic Poly(<i>N</i>‑isopropylacrylamide) Thin Films

Abstract

Polymer topology and reduced dimensions\ncan have a significant\nimpact on the properties and phase transitions of polymeric films\nwith thicknesses below a few hundred nanometers. We study the impact\nof these effects in the case of thin films of cyclic and linear poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) exposed to water vapor.\nSpecifically, we monitor the swelling kinetics of the thin PNIPAM\nfilms, their interfacial interactions, and their LCST-type demixing\nphase transition, using white light interferometry and X-ray reflectivity.\nAs the film thickness decreases, the swelling ratio increases, presumably\ndue to the increasingly dominant effect of polymer/substrate interactions.\nThe time constants of the swelling process depend on both the film\nthickness and the PNIPAM topology. Consistent with earlier observations\nfor PNIPAM solutions, in thin swollen films of comparable concentration,\ncyclic PNIPAM exhibits a broader and thus less cooperative demixing\ntransition than the linear PNIPAM counterpart.