Yan Chang, Kramer Patrick L, Yuan Rongfeng, Fayer Michael D
Department of Chemistry , Stanford University , Stanford , California 94305 , United States.
J Am Chem Soc. 2018 Aug 1;140(30):9466-9477. doi: 10.1021/jacs.8b03547. Epub 2018 Jul 20.
Polymeric hydrogels have wide applications including electrophoresis, biocompatible materials, water superadsorbents, and contact lenses. The properties of hydrogels involve the poorly characterized molecular dynamics of water and solutes trapped within the three-dimensional cross-linked polymer networks. Here we apply ultrafast two-dimensional infrared (2D IR) vibrational echo and polarization-selective pump-probe (PSPP) spectroscopies to investigate the ultrafast molecular dynamics of water and a small molecular anion solute, selenocyanate (SeCN), in polyacrylamide hydrogels. For all mass concentrations of polymer studied (5% and above), the hydrogen-bonding network reorganization (spectral diffusion) dynamics and reorientation dynamics reported by both water and SeCN solvated by water are significantly slower than in bulk water. As the polymer mass concentration increases, molecular dynamics in the hydrogels slow further. The magnitudes of the slowing, measured with both water and SeCN, are similar. However, the entire hydrogen-bonding network of water molecules appears to slow down as a single ensemble, without a difference between the core water population and the interface water population at the polymer-water surface. In contrast, the dissolved SeCN do exhibit two-component dynamics, where the major component is assigned to the anions fully solvated in the confined water nanopools. The slower component has a small amplitude which is correlated with the polymer mass concentration and is assigned to adsorbed anions strongly interacting with the polymer fiber networks.
聚合物水凝胶有广泛的应用，包括电泳、生物相容性材料、高吸水性材料和隐形眼镜。水凝胶的性质涉及被困在三维交联聚合物网络中的水和溶质的分子动力学，而这方面的特征尚不明确。在这里，我们应用超快二维红外（2D IR）振动回波和偏振选择性泵浦 - 探测（PSPP）光谱来研究聚丙烯酰胺水凝胶中水和小分子阴离子溶质硒氰酸盐（SeCN）的超快分子动力学。对于所研究的所有聚合物质量浓度（5%及以上），水和被水溶剂化的SeCN所表现出的氢键网络重组（光谱扩散）动力学和重取向动力学都明显慢于本体水。随着聚合物质量浓度的增加，水凝胶中的分子动力学进一步减慢。用水和SeCN测量得到的减慢幅度相似。然而，水分子的整个氢键网络似乎作为一个整体减慢，聚合物 - 水界面处的核心水群体和界面水群体之间没有差异。相比之下，溶解的SeCN确实表现出双组分动力学，其中主要组分归因于在受限水纳米池中完全溶剂化的阴离子。较慢的组分幅度较小，与聚合物质量浓度相关，归因于与聚合物纤维网络强烈相互作用的吸附阴离子。
