Quasielastic light scattering studies of swollen and stretched polymer gels

Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.