Department of Organic Materials Science, Sandia National Laboratories, Albuquerque, NM 87185, USA.
Int J Mol Sci. 2020 Jul 22;21(15):5176. doi: 10.3390/ijms21155176.
NMR spectroscopy continues to provide important molecular level details of dynamics in different polymer materials, ranging from rubbers to highly crosslinked composites. It has been argued that thermoset polymers containing dynamic and chemical heterogeneities can be fully cured at temperatures well below the final glass transition temperature (T). In this paper, we described the use of static solid-state H NMR spectroscopy to measure the activation of different chain dynamics as a function of temperature. Near T, increasing polymer segmental chain fluctuations lead to dynamic averaging of the local homonuclear proton-proton (H-H) dipolar couplings, as reflected in the reduction of the NMR line shape second moment () when motions are faster than the magnitude of the dipolar coupling. In general, for polymer systems, distributions in the dynamic correlation times are commonly expected. To help identify the limitations and pitfalls of analyses, the impact of activation energy or, equivalently, correlation time distributions, on the analysis of H NMR temperature variations is explored. It is shown by using normalized reference curves that the distributions in dynamic activation energies can be measured from the temperature behavior. An example of the analysis for a series of thermosetting polymers with systematically varied dynamic heterogeneity is presented and discussed.
NMR 光谱学继续为不同聚合物材料的动力学提供重要的分子水平细节，范围从橡胶到高度交联的复合材料。有人认为，含有动态和化学不均匀性的热固性聚合物可以在远低于最终玻璃化转变温度 (T) 的温度下完全固化。在本文中，我们描述了使用静态固态 H NMR 光谱法来测量不同链动力学的活化作为温度的函数。在 T 附近，聚合物链段的波动增加导致局部同核质子-质子 (H-H) 偶极耦合的动态平均，这反映在 NMR 线宽二阶矩（）随着运动速度快于偶极耦合的幅度而减小。通常，对于聚合物体系，预计会存在动态相关时间分布。为了帮助识别分析的局限性和陷阱，研究了活化能或等效的相关时间分布对 H NMR 温度变化分析的影响。通过使用归一化参考曲线表明，可以从 温度行为中测量动态活化能的分布。介绍并讨论了一系列具有系统变化动态不均匀性的热固性聚合物的 分析示例。
