Effect of isothermal aging on Poly(N-isopropylacrylamide): Correlation between chemical structure and phase transition temperature property

Poly(N-isopropylacrylamide) (PNIPAm) is a stimuli-responsive polymer with wide applications as a scaffold in biomedical applications. This study presents the PNIPAm's thermoresponsive behavior during storage and its stability in potential pharmaceutical formulations as a drug delivery excipient. Therefore, the turbidity curves of linear PNIPAm aqueous solutions are investigated before and after its thermal oxidation under air at 120 degrees C and 130 degrees C. First, PNIPAm aqueous phase separation diagram is determined for unaged polymer and then compared to thermally aged samples at two distinct points (1 and 10%(w/w)). After isothermal aging, the transition temperature has increased with a shift up to 4 degrees C. The correlation between PNIPAm's chemical structure and phase transition temperature properties is illustrated in terms of chemical modifications, macromolecular changes as well as the polar/non-polar groups balance. It is observed that the water affinity and glass transition temperature (Tg) have increased at the macromolecular level. In addition, chains scissions are highlighted and seem to explain the shift of transition temperature. Chemical changes have revealed the presence of supplementary polar groups induced by aging. This multiscale study suggests a possible mechanism for the partial thermal oxidation based on all the observations.