Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA.
J Phys Chem B. 2012 May 10;116(18):5501-15. doi: 10.1021/jp211831x. Epub 2012 Apr 30.
The vibrational spectrum of water near a thermo-sensitive polymer poly(N-isopropylacrylamide) (PNIPAM) undergoing conformational transition through the lower critical solution temperature (LCST) is calculated using molecular dynamics simulations. The characteristic structural features observed at the atomic scale for these proximal water molecules in a solvated polymer chain while undergoing the conformational transition are strongly correlated to their vibrational densities of states. Comparison of the vibrational spectrum below LCST for the proximal water with the vibrational spectrum obtained for bulk water reveals a significant fraction of the hydrogen bonding between the proximal water molecules and the polymer side groups. Hydrogen-bonded bridges of water molecules are formed between two adjacent and alternate monomers. This network of hydrogen bonding results in formation of locally ordered water molecules at temperatures below the LCST. Analysis of the simulation trajectories confirms the presence of a quasi-stable solvation structure near the PNIPAM. The calculated vibrational spectra for proximal water above the LCST suggest significantly reduced hydrogen bonding with the polymer and indicate a reduction in the structural stability of proximal water around a collapsed polymer chain. Systematic trends in the observed peak intensities and frequency shifts at the low- and high-frequency ends of the spectrum can be correlated with the structural and dynamical changes of water molecules below and above the LCST transition, respectively, for various polymer chain lengths. The simulations reveal that, compared to bulk water, the libration bands are blue shifted and OH stretch bands red shifted for water in proximity to PNIPAM with 30 monomer units below the LCST. The simulations suggest that vibrational spectra can be used as a predictive tool for quantifying atomic-scale structural transitions in solvation of thermo-sensitive polymers such as PNIPAM.
使用分子动力学模拟计算了在通过低临界溶液温度 (LCST) 进行构象转变的温敏聚合物聚 N-异丙基丙烯酰胺 (PNIPAM) 附近水的振动光谱。在溶剂化聚合物链中，这些近邻水分子在经历构象转变时观察到的原子尺度上的特征结构特征与它们的振动态密度密切相关。比较 LCST 以下近邻水的振动谱与获得的体相水的振动谱，揭示了近邻水分子与聚合物侧基之间存在相当一部分氢键。水分子的氢键桥在两个相邻和交替的单体之间形成。这种氢键网络导致在 LCST 以下温度下形成局部有序水分子。对模拟轨迹的分析证实了在 PNIPAM 附近存在准稳定的溶剂化结构。在 LCST 以上的近邻水中计算出的振动光谱表明与聚合物的氢键显著减少，并表明在聚合物链塌陷时近邻水分子的结构稳定性降低。在观察到的谱的低频和高频端的峰强度和频率位移的系统趋势可以分别与 LCST 转变以下和以上的水分子的结构和动力学变化相关联，对于不同的聚合物链长。模拟表明，与体相水相比，在 LCST 以下 30 个单体单元的 PNIPAM 附近，水的摆动带蓝移，OH 伸缩带红移。模拟表明，振动光谱可用作定量研究温敏聚合物（如 PNIPAM）溶剂化中原子尺度结构转变的预测工具。
