Olson Courtney M, Massari Aaron M
Department of Chemistry, University of Minnesota-Twin Cities , Minneapolis, Minnesota 55455, United States.
J Phys Chem A. 2018 Feb 15;122(6):1592-1599. doi: 10.1021/acs.jpca.7b12128. Epub 2018 Feb 3.
Fourier transform infrared and two-dimensional IR (2D-IR) spectroscopies were applied to polydimethylsiloxane (PDMS) cross-linked elastomer and a siloxane oligomer without solvent and swollen or dissolved in various solvents. The silicon hydride, which is covalently bound to the polymer chains, was the vibrational probe for the systems studied. There is almost an absence of vibrational solvatochromism in these systems. Frequency-frequency correlation functions obtained by 2D-IR spectroscopy show that the insensitivity of the FTIR spectra is due to overwhelming heterogeneity. However, the homogeneous contribution to the FTIR spectrum is smaller for the elastomer than the oligomer showing that the cross-linking process restricts the frequency fluctuations that are experienced by the hydride mode. The silicon hydride mode in a cross-linked, solvent-free PDMS film also exhibits spectral diffusion that must be due to polymer structural motions on the ultrafast time scale that are active above the glass transition temperature. Once solvents penetrate and swell the elastomer, the polymer likely continues to experience these polymer structural motions. However, we find that the vibrational dynamics are characteristic of the infiltrating solvents, showing that at least some fraction of the measured dynamics originate in solvent motions.
傅里叶变换红外光谱和二维红外（2D-IR）光谱被应用于聚二甲基硅氧烷（PDMS）交联弹性体以及一种硅氧烷低聚物，该低聚物在无溶剂状态下，以及在各种溶剂中溶胀或溶解的状态下进行研究。与聚合物链共价结合的硅氢化物，是所研究体系的振动探针。在这些体系中几乎不存在振动溶剂化显色现象。通过二维红外光谱获得的频率-频率相关函数表明，傅里叶变换红外光谱的不敏感性是由于压倒性的非均质性所致。然而，弹性体对傅里叶变换红外光谱的均匀贡献比低聚物小，这表明交联过程限制了氢化物模式所经历的频率波动。交联的、无溶剂的聚二甲基硅氧烷薄膜中的硅氢化物模式也表现出光谱扩散，这必定归因于在高于玻璃化转变温度时活跃的超快时间尺度上的聚合物结构运动。一旦溶剂渗透并使弹性体溶胀，聚合物可能会继续经历这些聚合物结构运动。然而，我们发现振动动力学是渗透溶剂的特征，这表明至少部分测量到的动力学源于溶剂运动。
