Rotational diffusion in polyacrylamide gels as a function of solvent quality

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Abstract
We have investigated the rotational motion of the dianion polyphenyl 2 (PP2) in polyacrylamide (PAA) gels by fluorescence anisotropy decay. A comparison of the dynamics of the probe molecule in various solvent mixtures of water–glycerol and water–methanol in solution as well as in the presence of the polymer matrix revealed a significant retardation of the reorientation with increasing PAA concentration. Having excluded specific interactions of PP2 with the gel matrix, we suggest that a modification of the solvent viscosity in the presence of the polymer is responsible for the alteration of the probe dynamics. The effect is strongly dependent on the solvent quality for the polymer component, i.e., more distinct under good solvent conditions (in pure water) than in a poor solvent (increasing glycerol content). Changing the crosslinker concentration from 0% to 4% we obtained fluid-like solutions as well as viscoelastic gels. However, the reorientational motion of PP2 was independent of the macroscopic constitution of the samples which means that on a microscopic scale the local environment for the probe molecule that determines its dynamics appears similar.