Effects of irradiation temperature on swelling and shrinking kinetics of thermo-responsive gels prepared by radiation-induced polymerization

Acryloyl-l-proline methyl ester (A-ProOMe) gels were prepared by γ-ray irradiation of aqueous monomer solutions at temperatures ranging from 0 to 60°C. The gels with heterogeneously and homogeneously crosslinked structures were obtained by irradiation at temperatures higher and lower than the lower critical solution temperature (LCST) of approximately 14°C, respectively. The former gel had a faster shrinking rate than that of the latter gel. In contrast, the swelling rate of the gels changed drastically at irradiation temperatures between 20 and 30°C higher than the LCST. The irradiation temperature dependence of the responsive kinetics of the A-ProOMe gels was elucidated by observing the microscopic homogeneity of the gel network structure with a scanning electron microscope (SEM). These results clearly show that the responsive kinetics of the gels can be controlled by adjusting the irradiation temperature in relation to the homogeneity of the nanoscopically crosslinked structure and the microscopic gel network.