Volume Transition of PNIPAM in a Nonionic Surfactant Hexagonal Mesophase

We investigate the volume transition of a thermoresponsive polymer, poly( N -isopropylacrylamide), PNIPAM, in the presence of an aqueous solution of nonionic surfactant, C 12 E 9 . We combine turbidimetry with optical microscopy, NMR, and SAXS to follow the volume transition of the PNIPAM and the H 1 -isotropic transition of the surfactant/water system. Nonionic surfactants such as C 12 E 9 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 C 12 E 9, even for relatively high concentrations of C 12 E 9 . Interestingly, once the surfactant forms an H 1 phase, there is a dramatic decrease in the coil−globule transition onset temperature. We believe that this behavior results from a competition between C 12 E 9 in the H 1 phase, and PNIPAM to associate with water. When PNIPAM in the H 1 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 H 1 phase, and finally a low-temperature clear gel (∼5 °C) where the copolymer is in the expanded coil state.