Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA.
Soft Matter. 2018 Feb 14;14(7):1235-1246. doi: 10.1039/c7sm01805c.
Associating polymers are a class of materials with widely tunable macroscopic properties. Here, we investigate telechelic poly(dimethylsiloxanes) of several molecular weights (M) with different hydrogen bonding end groups. Besides the well-established increase of the glass transition temperature T with decreasing M, T remains unchanged as the end group varies from NH over OH to COOH. For the latter system, a 2nd T is found which indicates a segregated phase. In contrast, rheological measurements reveal a qualitative difference in the viscoelastic response of NH-terminated and COOH-terminated chains. Both systems show clear signs of end group association, but only the latter exhibits an extended rubbery plateau. All features observed in the rheology experiments have corresponding processes in the dielectric measurements. This provides insight into the underlying molecular mechanisms, and especially reveals that many end groups of the COOH-terminated chains phase segregate while a certain fraction forms binary associates and remains non-segregated. In contrast, the NH-terminated systems form only binary associates increasing the effective chain length, whereas the COOH-terminated system consists of two types of associates forming a crosslinked network. Remarkably, a single species of end group forms two qualitatively different types of associates: transient bonds which allow stress release by a bond-partner exchange mechanism, and effectively permanent bonds formed by a phase segregated fraction of end groups which are stable on the timescale of the transient mechanism.
遥爪聚合物是一类宏观性质可广泛调节的材料。在这里，我们研究了几种分子量（M）的带有氢键端基的末端官能化聚二甲基硅氧烷。除了众所周知的玻璃化转变温度 T 随着 M 的降低而升高之外，当端基从 NH 变为 OH 再变为 COOH 时，T 保持不变。对于后一种体系，发现了第二个 T，表明存在相分离。相比之下，流变学测量显示 NH 封端和 COOH 封端链的粘弹性响应存在定性差异。这两个体系都显示出端基缔合的明显迹象，但只有后者表现出扩展的橡胶状平台。流变学实验中观察到的所有特征在介电测量中都有相应的过程。这提供了对潜在分子机制的深入了解，特别是表明许多 COOH 封端链的端基发生相分离，而一部分形成二元缔合物且不发生相分离。相比之下，NH 封端体系仅形成增加有效链长的二元缔合物，而 COOH 封端体系由两种类型的缔合物形成交联网络。值得注意的是，单一类型的端基形成两种定性不同的缔合物：允许通过键交换机制释放应力的瞬态键，以及由相分离的端基部分形成的有效永久键，它们在瞬态机制的时间尺度上稳定。
