主链可逆缔合对高分子拓扑受限及结晶行为的影响

Crystallization is a fundamental issue of polymer physics. The particularity of polymer crystallization is the collective diffusion of chain segments, which originates in the covalent bonding of monomers. The reduction of the crystallization kinetics due to the collective diffusion has been realized, and chain dynamics of single chain has been introduced to describe this effect. Nevertheless, the chain conformation can be restricted when crystallization proceeding. The subsequent chain diffusion changes from the tube reptation of single chain to the conformational adjustment of the entanglement network. This topological constraint is the physical origin of the semicrystalline character of polymer, while the way how it affects crystallization is not straightforward. Given the difficulties in both the description and control of entanglement state, a more direct way to investigate this topological effect is to change the connection between monomers. In this proposal, the terminal group of PEG with moderate molecular weights will be modified to form ligand-metal coordination, which introduces reversible associations in the apparent long chain or network. The dynamic coordination makes the apparent topological constraint stronger and larger in scale when crystallization becomes faster. By combination of rheological investigation and in-situ tracking of crystallization (e.g. by synchrotron radiation X-ray scattering), the strength of topological constraint and corresponding crystallization behavior are available. With these informations, the mechanism of how topological constraint affects crystallization can be revealed. This work is important for a deeper comprehension of the relation between topological constraint and crystallization of polymer.

结晶是高分子物理的基本问题之一。高分子结晶的特殊性在于共价键连接的重复单元间需要协同扩散才能排入晶格。现有研究已充分认识到协同扩散导致的结晶变慢，并引入单链松弛动力学进行描述。然而，晶体的形成会导致部分链构象锁死，使链运动从单链的管道蛇行转变为相互制约的缠结网络构象调整。这种拓扑受限效应是高分子半晶性的根源，但其作用方式却并不明确。由于缠结描述和调控的困难，从更基本的长链特性入手，通过改变链段间连接方式来研究拓扑受限对结晶的影响无疑更为直接。本项目拟利用端基修饰的中等分子量聚乙二醇进行端基配位，形成主链含有可逆缔合的高分子量聚合物。由于缔合的动态特性，缔合长链感受到的受限强度和尺度随结晶动力学加快而增加。结合体系流变性质和同步辐射X射线散射等原位结晶研究，可以分别给出拓扑受限程度及相应结晶行为，进而分析拓扑受限对结晶的影响机理。本项目对于深化缠结及拓扑受限与结晶行为关系的研究有重要价值。

链间缠结的存在使得高分子往往为半结晶性。然而，现有结晶理论并没有充分考虑非晶部分，而是将结晶过程简化为近邻折叠片晶的生长。这一简化无疑影响了结晶过程的精确描述，也制约了材料结构性能的设计。本项目针对缠结对结晶影响这一重要问题，开展了以下研究：1）三联吡啶端基修饰的聚乙二醇（PEG）的结晶行为与配位金属离子的关系。结果表明体系粘度（表观链长）随金属离子与三联吡啶结合强度的增加而增加。同时，PEG片晶厚度随温度的变化逐渐受到抑制。这些改变可能来源于更多缠结的存在。2）聚己内酯在混合溶剂中的结晶。实验发现结晶时缠结含量越高，非晶区厚度越大，结晶度越低。此外，利用浓度与缠结含量的普适标度关系，建立了半定量的缠结含量与结晶参数的关系。3）辐照交联聚丙烯的拉伸诱导结晶。结果表明交联部分与自由链的松弛速率差异可能导致自由链在应力作用下的受迫松弛，从而引起结晶动力学的反常减慢和大尺寸子母晶的形成。除结晶之外，项目还研究了环氧树脂通过交联网络拓扑结构的控制实现自增韧，发现通过控制链增长型交联反应中的链转移速率，可以降低最终的交联密度，增加缠结含量，显著提高材料韧性。以上结果对认识缠结在结晶过程中的作用及控制材料性能方面有积极的探索意义。