The best-known examples of smart, responsive hydrogels derive from poly(N-isopropylacrylamide) (PNIPAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNIPAM hydrogels can undergo more than 50-fold volume changes within ∼1 μs 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.

中文翻译：

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疏水性塌陷引发聚（N-异丙基丙烯酰胺）体积相变反应坐标

灵敏，反应灵敏的水凝胶的最著名例子来自聚（N-异丙基丙烯酰胺）（PNIPAM）交联的聚合物网络。这些水凝胶经历由温度，化学和/或环境变化触发的体积相变（VPT）。PNIPAM水凝胶可在约1μs的间隔内经历超过50倍的体积变化。研究试图阐明这些异常大反应的分子机制。然而，驱动VPT的分子反应坐标仍然不清楚。使用可见的非共振拉曼温度跳跃光谱，我们确定了该VPT的分子顺序。PNIPAM疏水性异丙基和亚甲基以109±64 ns和104±44 ns的时间常数脱水，从而引发了PNIPAM的体积崩溃。PNIPAM酰胺基团的随后脱水明显较慢，正如我们先前发现的那样（360±85 ns）。