Dynamic light scattering from gels in a poor solvent

Abstract

Photon correlation spectroscopy measurements are reported of the collective motion diffusion coefficient Dc and the longitudinal elastic modulus Egel of polyacrylamide gels in the poor solvent region using a water : methanol (3 : 1 by volume) mixture as solvent. It is shown that the presence of cross-links hinders the movement of polymer self-entanglements, which are characteristic of θ solutions, and the ordinary gel breathing mode is observed with exponential autocorrelation functions whose relaxation rate is proportional to the square of the scattering vector K. Egel obeys a scaling law for concentrations c between 0.07 and 0.3 g cm-3 Egel ∝ C3.07±0.07 in agreement with neutron scattering measurements of the static coherence length in θ solutions. The diffusion coefficient of the collective motion in the gel Dc also follows a power law concentration dependence, but it is shown that this is accidental, since the coherence length as well as the solvent viscosity, both of which enter into the definition of Dc, are concentration dependent; in particular the solvent viscosity displays a free volume behaviour