Ultraslow Thermoresponse in Hydrogels Synthesized with Radiation Technique

Hydrogels, which were synthesized by simultaneously occurring processes of radiation-induced polymerization and cross-linked reactions of acryloyl-l-proline methyl ester (APM) at 0 °C in aqueous alcohol solutions, exhibit an ultraslow volume change in response to variations in external temperature. The shrinking speed of the hydrogels prepared in aqueous alcohol is 1000 times slower than that of the hydrogel prepared in pure water; e.g., the hydrogel prepared in aqueous 1-propanol solution shrunk in 6 months. It is revealed that the ultraslow thermoresponse consists of quick- and slow-shrinking periods; the ratio between the two periods depends on the alcohol molecules in the solution. The generation of a hydroxyl group in the polymer network by the coupling reaction between -hydroxyalkyl radical of alcohols and -radical of proline moiety was confirmed by GC/MS analysis of the radiation products. The hydrogen bonding domain containing the hydroxyl groups is proposed to be responsible for the ultraslow response.