CPM Seminar

DLS and XPCS in Thermosensitive Gels

Erik Geissler

Laboratoire de Spectrométrie Physique
Université J. Fourier de Grenoble

Intense efforts have been deployed over the past two decades to investigate a class of polymer gels that undergo volume phase transition (VPT). At a VPT, under an appropriate stimulus such as a change in temperature, solvent composition or external field, the swollen gel collapses with expulsion of the solvent. Proposed applications of such systems are generally either biomedical (e.g., drug release) or else either optical (smart windows) or mechanical (microfluidic valves). One of the more popularly studied polymers in this class is poly(N-isopropyl acrylamide) (PNIPA), which, in pure water, exhibits a VTP at 34°C.

In the swollen state below 34°C PNIPA gels are transparent and are amenable to investigation by standard dynamic light scattering (DLS) methods. When illuminated by a beam of coherent light the gels generate a static speckle pattern caused by long range variations in the polymer concentration that are frozen in by the cross-links. On a local scale, however, network chains belonging to different neighbouring nodes mutually interpenetrate, with the result that they are indistinguishable. As it is impossible to tell whether a given subchain is spatially confined or not, the local motion of the chains is effectively ergodic. The ergodicity is illustrated by the invariance of the measured intensity of the osmotic fluctuations with respect to the position of the detector in the speckle field, i.e., independent of the degree of heterodyning [1]. Ensemble averaging is therefore unnecessary. The collective diffusion coefficient, D_C, which determines the rate of swelling and deswelling of the gel, is also invariant with respect to the speckle field. At 20°C, D_C ≅ 4×10^-7 cm²/s.

When heated above 34°C the gels promptly turn white and start to deswell. The extremely slow rate of deswelling, however, suggests a bubble-like intermediate structure in which the water decomposes into droplets, each encased in an impermeable polymer skin. SAXS measurements indicate a skin thickness of ca. 10 nm [2]. The short penetration depth due to the micron-sized droplets precludes DLS observations of the internal structure of the gel. X-ray photon correlation spectroscopy (XPCS) measurements on this system reveal two time constants, one of the order of 100 s, and a second relaxation of the order of thousands of seconds. The physical origin of these relaxation processes will be discussed.

[1] - K. László, K. Kosik, C. Rochas and E. Geissler Macromolecules 36, 7771 (2003).
[2] - K. László, K. Kosik, C. Rochas and E. Geissler Macromolecules 37, 10067 (2004).