Cooperative diffusionDof 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 ofDfor the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yieldedD=(3.4plusmn;0.5)times;10minus;6C0.76plusmn;0.03ecm2/s, in accord with a scaling prediction. For each of the gels prepared and having a givenCe, the uniaxial stretching was applied, and the valuesDwere investigated as a function of extension ratio agr; and direction bgr; with respect to the stretching direction. The results gaveD(agr;,Ce)=(3.4plusmn;0.5)times;10minus;6agr;xCyecm2/s, wherex=2/3 for bgr;=0deg; and minus;1/4 for bgr;=90deg;, andy=0.76, independent of agr; and bgr;. The results clearly indicate that upon increasing agr;,Dincreases 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.
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