Fortman David J, Snyder Rachel L, Sheppard Daylan T, Dichtel William R
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States.
Department of Chemistry and Chemical Biology, Cornell University, Baker Laboratory, Ithaca, New York 14853, United States.
ACS Macro Lett. 2018 Oct 16;7(10):1226-1231. doi: 10.1021/acsmacrolett.8b00667. Epub 2018 Sep 20.
Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, cross-linked PHU resins that are not derived from isocyanates.
通过动态共价键交联的聚合物网络通常会牺牲传统热固性材料的坚固机械性能，以换取快速高效的可再加工性。聚氨酯因其优异的机械性能、广泛的应用和易于合成，是可再加工交联聚合物的有吸引力的材料，但其合成通常依赖于有害的异氰酸酯前体。由胺和环状碳酸酯衍生而来的聚羟基聚氨酯（PHU）是传统聚氨酯的有前途的替代品。即使在没有外部催化剂的情况下，PHU网络也可以通过转氨基甲酰化反应进行再加工，但这个过程在150°C以上的温度下需要数小时。我们通过引入动态二硫键，大大缩短了PHU网络的再加工时间。使用胱胺作为共聚单体得到的材料具有与其他刚性交联PHU相似的热稳定性和机械性能。尽管这些材料具有优异的机械性能，但它们表现出快速的应力松弛，在150°C下的特征松弛时间低至30秒。这种特性使得在高温压缩成型仅30分钟后，通过动态热机械分析（DMTA）测量，能够以定量恢复交联密度的方式进行再加工。引入二硫键是获得非异氰酸酯衍生的可再加工交联PHU树脂的一种有前途的方法。
