Wolak J E, Jia X, White Jeffery L
Department of Chemistry, North Carolina State University, Campus Box 8204, Raleigh, North Carolina 27695-8204, USA.
J Am Chem Soc. 2003 Nov 12;125(45):13660-1. doi: 10.1021/ja037738b.
We report direct experimental observation of local conformational dynamics in a polymer chain at the calorimetric glass transition temperature Tg. Variable-temperature two-dimensional (2D) solid-state exchange NMR, at natural abundance, reveals segmental dynamics in pure polyisobutylene (PIB) occurring on a time scale of several seconds over the Tg range observed by DSC (203-208 K). To our knowledge, this is the first direct observation of molecular-level conformer interchange (trans-trans/trans-gauche/gauche-gauche) at the caloric glass transition temperature. Our results provide a chronologically accurate and pedagogically advantageous demonstration of molecular processes during a polymer phase transition, relative to traditional bulk mechanical and calorimetric techniques. More importantly, we use a miscible blend to demonstrate a general strategy for quantitative evaluation of configurational entropy changes via combination of temperature-dependent 2D exchange NMR and Adams-Gibbs theory. Our results on the Tg time scale are directly relevant to fundamental understanding of the Tg length scale, i.e., the dimension of cooperatively rearranging regions.
我们报告了在量热玻璃化转变温度Tg下对聚合物链中局部构象动力学的直接实验观察结果。在自然丰度下进行的变温二维（2D）固态交换核磁共振揭示了纯聚异丁烯（PIB）中的链段动力学，其发生时间尺度为几秒，跨越了DSC观察到的Tg范围（203 - 208K）。据我们所知，这是首次在量热玻璃化转变温度下对分子水平的构象异构体互换（反式 - 反式/反式 - 旁式/旁式 - 旁式）进行直接观察。相对于传统的本体力学和量热技术，我们的结果提供了聚合物相变过程中分子过程在时间顺序上准确且具有教学优势的展示。更重要的是，我们使用一种可混溶共混物来展示一种通过结合温度依赖的二维交换核磁共振和亚当斯 - 吉布斯理论来定量评估构型熵变化的通用策略。我们在Tg时间尺度上的结果与对Tg长度尺度（即协同重排区域的尺寸）的基本理解直接相关。
