X-ray Photon Correlation Spectroscopy of Dynamics in Thermosensitive Gels

• Published in: Macromolecular symposia. 2007-09, Vol.256 (1), p.73-79
• Main Authors: Geissler, Erik, Kosik, Katalin, Fluerasu, Andrei, Moussaïd, Abdellatif, László, Krisztina
• Format: Article
• Language: English
• Subjects: deswelling; Diffusion rate; Dynamical systems; Dynamics; Gels; jamming; Photon correlation spectroscopy; poly(N-isopropyl acrylamide); SAXS; Scattering; small angle X-ray scattering; X-rays
• ISSN: 1022-1360; 1521-3900
• DOI: 10.1002/masy.200751008

Temperature‐sensitive hydrogels undergo a volume phase transition (VPT) when heated above a critical temperature Tc. For the poly(N‐isopropyl acrylamide) (PNIPA)‐water system, Tc. = 34 °C. Below Tc the gels are transparent and highly swollen. On warming above Tc they promptly turn white and start to deswell. The rate of deswelling, however, can be orders of magnitude slower than that of swelling below Tc. The unstable intermediate structure above Tc, 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 Tc 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 m2/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 q2 as in diffusion processes. The physical origin of this relaxation is consistent with jamming, a phenomenon that is common in other disordered systems.