Diels-Alder-based thermo-reversibly crosslinked polymers: Interplay of crosslinking density, network mobility, kinetics and stereoisomerism

Polymers crosslinked through thermo-reversible furan / maleimide Diels-Alder chemistry have been widely explored, since they stand as an ingenious design for reprocessable and self-healing thermosets and elastomers. For these polymeric products, crosslinking density plays a key role on the polymer thermo-reversibility. However, how this degree of network interconnectivity influences the kinetics of thermal reversibility has not yet been addressed. In order to tackle this problem, furan-grafted polyketones crosslinked by a bi-functional maleimide were prepared with different ratios between maleimide and furan groups. The thermo-reversible dynamics of the prepared polymers were then studied by rheology and differential scanning calorimetry. Here we show that, the thermo-reversible process occurs faster and at lower temperatures in polymers with lower crosslinking densities. Network mobility is responsible for such effect. It allows the formulations to rearrange their polymer network differently through the heating-cooling cycles. The results also point out that the crosslinking density is more relevant on the system reversible behavior than the stereoisomerism of the Diels-Alder adducts. Additionally, the polymer thermo-reversible features were shown to be dependent on its thermal history. This work impacts the development of reprocessable and self-healing crosslinked polymers, and the design of the corresponding reprocessing and healing procedures.通过热可逆呋喃（furan）/马来酰亚胺（maleimide）狄尔斯-阿尔德（Diels-Alder）化学交联的聚合物已被广泛研究，因其为可再加工、自修复热固性材料与弹性体提供了精巧的设计思路。对于这类聚合物制品，交联密度对其热可逆性能起着关键调控作用。然而，网络互连程度如何影响热可逆动力学这一问题，迄今尚未得到明确探讨。为解决这一难题，本研究通过调控双官能团马来酰亚胺与呋喃基团的投料比例，制备了一系列呋喃接枝聚酮交联聚合物。随后借助流变学（rheology）与差示扫描量热法（differential scanning calorimetry），对所制备聚合物的热可逆动力学行为展开了系统研究。本研究结果表明，交联密度更低的聚合物，其热可逆过程可在更低温度下更快发生。这一效应源于网络迁移率：其可使聚合物体系在加热-冷却循环中以不同方式重新排布其网络结构。研究还发现，相较于狄尔斯-阿尔德加合物的立体异构性，交联密度对体系可逆行为的影响更为显著。此外，聚合物的热可逆特性被证实与其热历史密切相关。本研究可为可再加工、自修复交联聚合物的开发，以及相应再加工与修复工艺的设计提供重要参考。