The dynamics of aqueous crosslinked poly(N-isopropylacrylamide) microgel particles under good solvency conditions

From a dynamic light scattering study of the concentration and temperature dependencies of weakly crosslinked poly(N-isopropylacrylamide) (PNIPAM) mic rogel particles in aqueous solutions it was found that the decay of the tim e correlation function is markedly slowed down with increasing microgel con centration and with decreasing temperature. The microgel particles exhibit an exponential decay in very dilute solutions or at higher temperatures; th is changes to a stretched exponential at higher concentrations or at lower temperatures. The observed slowing down occurrs over a relatively narrow co ncentration range, which has not been observed in some corresponding system s consisting of hard spheres. The time correlation function data were analy zed with the aid of a coupling model. The relaxation time that was calculat ed from this model increases with increasing concentration by a power law ( tau proportional to c(2.6)) in higher concentration or with decreasing temp erature, the result that is physically in accord with the change in the val ue of the stretched exponent. A wave vector dependence of the relaxation ti me was observed. The results have been interpreted as arising from the form ation of neighboring cages of the microgel particles. Over the concentratio n range studied, the apparent activation energy of the relaxation processes operating in the cage structures was found initially to increase with incr easing microgel concentration and then to approach a plateau value.