Reversible Cross-linking, Microdomain Structure and Heterogeneous Dynamics in Thermal Reversible Cross-linked Polyurethane as Revealed by Solid-State NMR
writer：Zhang RC, Yu S, Chen SL, Wu Q, Chen TH, Sun PC, Li BH, Ding DT
keywords：Diels-Alder, T2 relaxometry, Low-field NMR, MQ NMR
source：期刊
specific source：J. Phys. Chem. B 2014, 118(4), 1126–1137
Issue time：2014年
Polyurethane material is widely utilized in industry and daily life due to its versatile chemistry and relatively handling ease. Here, we focused on a novel thermal reversible cross-linked polyurethane with comprehensive remarkable mechanical properties as reported in our recent work (Adv. Mater. 2013, 25, 4912). The microphase-separated structure and heterogeneous segmental dynamics was well revealed by T2 relaxometry experiments, which was also firstly be utilized to in-situ monitor the reversible crosslinking associated with Diels-Alder (DA) and Retro-Diels-Alder (RDA) reactions. On the basis of T2 relaxometry results, we determined the actual temperature of the (R)DA reaction as well as the corresponding activation energies of the motion of soft segments. Besides, the role of the temperature and crosslinker contents on the microdomain structure and dynamics are discussed in detail. It is found that the microphase separation is enhanced by the increase of temperature as well as the incorporation of crosslinkers. And the polyurethane samples are still thermal-stable even at a high temperature beyond the disassociation of the crosslinkages. Furthermore, Baum-Pines and three-pulse multiple-quantum NMR experiments are utilized to investigate the heterogeneous structures and dynamics of the mobile and rigid segments, respectively. Both the results obtained from the T2 relaxometry and multiple-quantum NMR experiments are in good agreement with the macroscopic mechanical properties of the polyurethane. Finally, it’s also well demonstrated that proton T2 relaxometry combined with multiple-quantum NMR is a powerful method to study the heterogeneous structures and dynamics of a multi-phase polymer system.