Department of Chemistry , University of Minnesota , 207 Pleasant Street SE , Minneapolis , Minnesota 55455-0431 , United States.
Department of Chemistry , Northwestern University , 2145 Sheridan Road , Evanston , Illinois 60208 , United States.
J Phys Chem B. 2019 Feb 14;123(6):1432-1441. doi: 10.1021/acs.jpcb.8b11489. Epub 2019 Jan 31.
Cross-linked polymers are used in many commercial products and are traditionally incapable of recycling via melt reprocessing. Recently, tough and reprocessable cross-linked polymers have been realized by incorporating cross-links that undergo associative exchange reactions, such as transesterification, at elevated temperatures. Here we investigate how cross-linked polymers containing urethane linkages relax stress under similar conditions, which enables their reprocessing. Materials based on hydroxyl-terminated star-shaped poly(ethylene oxide) and poly((±)-lactide) were cross-linked with methylene diphenyldiisocyanate in the presence of stannous octoate catalyst. Polymers with lower plateau moduli exhibit faster rates of relaxation. Reactions of model urethanes suggest that exchange occurs through the tin-mediated exchange of the urethanes that does not require free hydroxyl groups. Furthermore, samples were incapable of elevated-temperature dissolution in a low-polarity solvent (1,2,4-trichlorobenzene) but readily dissolved in a high-polarity aprotic solvent (DMSO, 24 to 48 h). These findings indicate that urethane linkages, which are straightforward to incorporate, impart dynamic character to polymer networks of diverse chemical composition, likely through a urethane reversion mechanism.
交联聚合物被广泛应用于各种商业产品中，传统上无法通过熔融再加工进行回收利用。最近，通过引入在高温下发生缔合交换反应（如酯交换）的交联键，实现了坚韧且可再加工的交联聚合物。在这里，我们研究了含有氨酯键的交联聚合物在类似条件下如何释放应力，从而实现其再加工。以端羟基星形聚（氧化乙烯）和聚（（±）-丙交酯）为原料，在辛酸亚锡催化剂的存在下，用亚甲基二苯基二异氰酸酯交联。低平台模量的聚合物表现出更快的松弛速率。模型氨酯的反应表明，交换是通过锡介导的氨酯交换进行的，而不需要游离羟基。此外，样品在低极性溶剂（1,2,4-三氯苯）中无法在高温下溶解，但在高极性非质子溶剂（DMSO）中很容易溶解（24 至 48 小时）。这些发现表明，易于引入的氨酯键通过氨酯反转机制赋予了具有不同化学组成的聚合物网络动态特性。
