Volume Transition of PNIPAM in a Nonionic Surfactant Hexagonal Mesophase

Abstract

We investigate the volume transition of a thermoresponsive polymer, poly(N-isopropylacrylamide), PNIPAM, in the presence of an aqueous solution of nonionic surfactant, C12E9. We combine turbidimetry with optical microscopy, NMR, and SAXS to follow the volume transition of the PNIPAM and the H1-isotropic transition of the surfactant/water system. Nonionic surfactants such as C12E9 are known to interact weakly with PNIPAM. Accordingly, we show that there is only a small change in the volume transition temperature for the PNIPAM in isotropic micellar solutions of C12E9, even for relatively high concentrations of C12E9. Interestingly, once the surfactant forms an H1 phase, there is a dramatic decrease in the coil−globule transition onset temperature. We believe that this behavior results from a competition between C12E9 in the H1 phase, and PNIPAM to associate with water. When PNIPAM in the H1 phase is cooled to low enough temperatures so as to be in the coil state, it locally disturbs the hexagonal phase ordering. Thus, we show that for PNIPAM in a weakly interacting surfactant matrix, it is the phase behavior of the matrix rather than the matrix chemistry that governs the coil−globule transition. Finally, we show that in a PNIPAM copolymer with a higher LCST we observe an interesting sequence of transitions in the surfactant phase: on cooling from a high temperature free-flowing turbid globular state (∼75 °C), we enter a free-flowing translucent coil phase (∼47 °C), then a turbid gel (∼25 °C) where the copolymer is collapsed in the H1 phase, and finally a low-temperature clear gel (∼5 °C) where the copolymer is in the expanded coil state.