Thermo-responsive hydrogels undergo volume phase transition (VPT). The temperature responsive VPT can be utilized for preparation of ‘smart’ materials. An ideal smart responsive material (SRM) would be triggered by a slight chemical or environmental change and respond in a desirable and reproducible way. Cross-linked Poly(N-isopropyl)acrylamide (PNIPAM) has one of the largest and fastest VPT, exhibiting a volume difference up to 30 fold within 1µs when temperature increases past the lower critical solution temperature (LCST). As a result, PNIPAM is a great candidate for developing SRM. However, even though PNIPAM is the most widely studied thermal responsive polymer, a deep understanding of the molecular mechanisms involved in the VPT is still lacking.
Our previous study found the time constant for dehydration of the amide group during VPT to be ~360 ns. In this study, a visible temperature jump (T-jump) Raman spectroscopy was constructed to monitor vibrational frequency of the isopropyl group and methylene backbone within PNIPAM during VPT. We found the VPT molecular mechanism occurs as the following: As the temperature elevates, the dehydration of the isopropyl groups occurs very quickly with a time constant ~68 ns, shortly after it is the dehydration of the polymer chains with a time constant ~104 ns. Finally, the dehydration of the amide group with a time constant ~ 360 ns
