Pw. Zhu et Dh. Napper, The dynamics of aqueous crosslinked poly(N-isopropylacrylamide) microgel particles under good solvency conditions, MACRO CH P, 200(4), 1999, pp. 783-790
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.