Dynamic light scattering in concentrated polyacrylamide solutions

Concentrated aqueous solutions of polyacrylamide were prepared by free radical polymerization, using ammonium persulfate as initiator, with different initiator/acrylamide mass ratios. The increase in initiator/acrylamide mass ratio resulted in two effects: a slight increase in conversion and a decrease in molecular weight (characterized as a decrease in intrinsic viscosity). Calculation of dimensionless concentration indicated that all polyacrylamide solutions were in the concentrated regime, in which solubilized macromolecules are described as highly interpenetrated solubilized coils. Intensity correlation function from dynamic light scattering experiments showed random motion could be bound to slow-mode and fast-mode relaxation processes. Fast-mode characteristic relaxation rate clearly seemed to be related to an Arrhenius-type activated process, while slow-mode characteristic relaxation rate had a more complex behavior, which was described as a consequence of two concurrent effects as temperature increased: decrease of macromolecular cluster sizes, resultant from breakage of intermacromolecular interactions, and increase in macromolecular dimensions, resultant from breakage of intramacromolecular interactions.