Swelling kinetics for a pH-induced latex-to-microgel transition

The kinetics of swelling of a series of six near-monodisperse, lightly cross-linked poly(2-vinylpyridine) latexes with mean diameters ranging from 380 to 1010 nm has been investigated by the pH jump method using a commercial stopped-flow instrument. These pH-responsive particles become substantially protonated at around pH 4.1, which leads to a rapid latex-to-microgel transition within a time scale of tens of milliseconds. The characteristic swelling time correlates linearly with the mean particle diameter, as predicted by the Tanaka equation. However, faster swelling is observed in the presence of added salt. This is contrary to the theory developed by Tanaka, which assumes that the relaxation of the polymer chains is the rate-limiting step. An alternative viewpoint, in which infusion of solvent determines the characteristic swelling time, satisfactorily explains the experimental observations and collapses most of the data, except for the largest microgels. This discrepancy is suggested to be due to the inaccurate sizing of these micrometer-sized swollen microgels by dynamic light scattering.