X-Ray Photon Correlation Spectroscopy Of Dynamics In Thermosensitive Gels

Temperature-sensitive hydrogels undergo a volume phase transition (VPT) when heated above a critical temperature T-c. For the poly(N-isopropyl acrylamide) (PNIPA)-water system, T-c.=34 degrees C. Below T-c the gels are transparent and highly swollen. On warming above T-c they promptly turn white and start to deswell. The rate of deswelling, however, can be orders of magnitude slower than that of swelling below T-c. The unstable intermediate structure above T-c, can retain the solvent and conserve the sample volume for may days, even with millimetre-sized samples. Light scattering observations of the internal structure of these gels above T-c are precluded by their strong turbidity. Small angle X-ray scattering measurements (SAXS), on the other hand, are less subject to multiple scattering as X-rays penetrate more easily into the bulk material. Conventional (incoherent) SAXS observations reveal intense scattering from smooth internal water-polymer interfaces with an estimated surface area of about 7 m(2)/g in the swollen gel. The dynamics in the off-equilibrium high temperature state, investigated by X-ray photon correlation spectroscopy (XPCS), displays a relaxation rate that is linearly proportional to the wavevector q, rather than to q(2) as in diffusion processes. The physical origin of this relaxation is consistent with jamming, a phenomenon that is common in other disordered systems.