DYNAMIC LIGHT-SCATTERING STUDY OF LINEAR POLYELECTROLYTE DIFFUSION INGELS

Dynamic light scattering (DLS) was used to study diffusion of Linear s odium poly(styrenesulfonate) (NaPSS), with molecular weights ranging f rom 35 000 to 1 200 000, in poly(acrylamide) (PA) gels made by copolym erizing acrylamide and N,N'-methylenebis(acrylamide) (BIS) in aqueous solvents of high and low ionic strengths. Laplace inversion of the cor relation functions shows that for the gels swollen in solvents with a high effective ionic strength two distinct, well-separated bands appea r in distributions of diffusion coefficients, corresponding to the col lective diffusion mode of the gel, D-g, and the tracer diffusion of th e probe polymer, D-p. The scaling laws D-p similar to M(p)(-2) and D-p similar to M(p)(-2.1), where M(p) is the molecular weight of probe Na PSS polymer, were found for 11 and 18 wt% gels, respectively. These re sults are close to the reptation prediction for the self-diffusion of neutral flexible polymers in gels. The dependence of D-p and D-g on th e concentration of the gel, c(g) was also measured, and scaling relati onships were observed with D-p similar to c(g)(-2.5) and D-g similar t o C-g(0.67). The dependence of D-p on the concentration of the probe p olymer c(p) did not obey a power law but showed behavior similar to th at predicted by random phase approximation theory for semidilute solut ions of polymer mixtures. In the case of gels swollen in deionized wat er, three different bands were observed where the slowest one was on t he long time measurement limit.