Rashidi Vahid, Coyle Eleanor J, Sebeck Katherine, Kieffer John, Pipe Kevin P
Department of Mechanical Engineering, ‡Department of Materials Science and Engineering, and §Department of Electrical Engineering and Computer Science, University of Michigan , Ann Arbor, Michigan 48109, United States.
J Phys Chem B. 2017 May 4;121(17):4600-4609. doi: 10.1021/acs.jpcb.7b01377. Epub 2017 Apr 24.
Weak interchain interactions have been considered to be a bottleneck for heat transfer in polymers, while covalent bonds are believed to give a high thermal conductivity to polymer chains. For this reason, cross-linkers have been explored as a means to enhance polymer thermal conductivity; however, results have been inconsistent. Some studies show an enhancement in the thermal conductivity for polymers upon cross-linking, while others show the opposite trend. In this work we study the mechanisms of heat transfer in cross-linked polymers in order to understand the reasons for these discrepancies, in particular examining the relative contributions of covalent (referred to here as "bonding") and nonbonding (e.g., van der Waals and electrostatic) interactions. Our results indicate cross-linkers enhance thermal conductivity primarily when they are short in length and thereby bring polymer chains closer to each other, leading to increased interchain heat transfer by enhanced nonbonding interactions between the chains (nonbonding interactions being highly dependent on interchain distance). This suggests that enhanced nonbonding interactions, rather than thermal pathways through cross-linker covalent bonds, are the primary transport mechanism by which thermal conductivity is increased. We further illustrate this by showing that energy from THz acoustic waves travels significantly faster in polymers when nonbonding interactions are included versus when only covalent interactions are present. These results help to explain prior studies that measure differing trends in thermal conductivity for polymers upon cross-linking with various species.
弱链间相互作用被认为是聚合物中热传递的瓶颈，而共价键被认为能赋予聚合物链高导热性。因此，交联剂已被探索作为提高聚合物导热性的一种手段；然而，结果并不一致。一些研究表明聚合物交联后导热性增强，而另一些研究则显示出相反的趋势。在这项工作中，我们研究交联聚合物中的热传递机制，以理解这些差异的原因，特别是研究共价（这里称为“键合”）和非键合（例如范德华力和静电力）相互作用的相对贡献。我们的结果表明，交联剂主要在长度较短时提高导热性，从而使聚合物链彼此更靠近，通过增强链间的非键合相互作用（非键合相互作用高度依赖于链间距离）导致链间热传递增加。这表明增强的非键合相互作用，而非通过交联剂共价键的热传导途径，是提高导热性的主要传输机制。我们进一步通过表明，与仅存在共价相互作用时相比，当包含非键合相互作用时，太赫兹声波的能量在聚合物中传播得明显更快，来说明这一点。这些结果有助于解释先前关于用各种物质交联聚合物时测量到不同导热性趋势的研究。
