The phase transition of poly(N-isopropylacrylamide): the effect of aging

We study the effect of temporal and thermal aging of linear and hydrogel samples of poly(N-isopropylacrylamide) (PNIPAM). For this purpose, we designed a simple and inexpensive experimental device able to measure the optical density as a function of temperature. The samples were aged from 4 to 14 days at two temperatures: 24 and 37°C. We found that the phase transitions of linear PNIPAM samples shift to lower or higher temperatures, depending on the storage conditions, while the phase transitions of hydrogels always shift to higher temperatures. Structural differences between linear and hydrogel PNIPAMs and the formation of additional hydrogen bonds are behind these shifts.