Kinetics of Glucose-Induced Swelling of P(NIPAM-AAPBA) Microgels

Rapid swelling is a major advantage of microgels over bulky gels, and chemical-induced swelling has been expected to occur on the same time scale as physically induced swelling. As an example, the kinetics of the glucose-induced swelling of poly(N-isopropylacrylamide-co-3-acrylamidophenylboronic acid) (P(NIPAM-AAPBA)) microgel was studied by turbidity. This process occurs on a time scale of 10(2) s, while the temperature-induced (de)swelling of PNIPAM microgels was reported to occur in time regime from 100 ns to tens of milliseconds. The slow glucose-induced swelling was attributed to the slow reaction between glucose and phenylboronic acid (PBA) groups, which was identified as the rate-determining step for microgel swelling. The rate constant of this reaction was further determined under various conditions and compared with that obtained in solution, using 3-aminophylboronic acid as low molecular weight analogue. The reaction is accelerated when the microgels are in a swollen state, while it is retarded when the microgels are shrunken, revealing different effects of the polymer network on the reaction kinetics. Although the swelling rate of P(NIPAM-AAPBA) microgel is limited by the slow reaction between glucose and PBA groups, it is still much faster than the macroscopic hydrogel beads with same components (similar to several hours).