Hydrophobic Collapse Initiates the Poly(N-isopropylacrylamide) Volume Phase Transition Reaction Coordinate

The best-known examples of smart, responsive hydrogels derive from poly(N-isopropylacrylamide) (PNI-PAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNI-PAM hydrogels can undergo more than 50-fold volume changes within similar to 1 mu s intervals. Studies have tried to elucidate the molecular mechanism of these extraordinarily large responses. Nevertheless, the molecular reaction coordinates that drive the VPT remain unclear. Using visible nonresonance Raman temperature-jump spectroscopy, we determined the molecular ordering of this VPT. The PNIPAM hydrophobic isopropyl and methylene groups dehydrate with time constants of 109 +/- 64 and 104 +/- 44 ns, initiating the volume collapse of PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly slower, as our group previously discovered (360 +/- 85 ns). This determination of the ordering of the molecular reaction coordinate of the PNIPAM VPT enables the development of the next generation of super-responsive materials.