Rheological And ~1H Low-field Solid State NMR Study Of The Gelation Behavior In A Graphene/Diels-Alder Polymer Network

In recent years,thermally reversible cross-linking polymer networks exhibiting self-healing prepared by the Diels-Alder(DA)reactions has received extensive attention due to its excellent mechanical performance.As a novel two-dimensional nanometer material,graphene has high anisotropy and surface area,and is often used as filler in preparing nano-composite materials.As the development of nano-technology,we are becoming more and more concerned about the interesting behavior happened in the inorganic/organic interaction.In this study,we disperse the surfaced functionalized graphene with 3-aminopropyltriethoxysilane(KH-550)to a kind of reversible gelation by the DA reaction of four-arm furan and maleimide.We use rheology and 1H Solid State Low Field NMR to investigate the effects of the addition of Graphene and the heterogeneous structures.In rheology,we find that the addition of graphene cannot change the sol-gel transition point which was determined to be 85±1℃.Furthermore,gelation time decreases with increasing graphene concentration and curing temperature by using Time Resolved Mechanical Spectroscopy(TRMS).The activation energy of DA gelation with a solid content of 45 wt%is about 51 KJ/mol.The addition of graphene can shorten the gelation time by 15%,and does not affect relaxation exponent.It shows that the addition of graphene just speed up the DA reaction and increase the stiffness of gelation,but cannot change the gel long rage structure.We expand our research to probe into the segmental dynamics of the DA gelation by low-field NMR.According to MSE-FID signals,we find that gels relax faster and sols relax slower.We use Proton multiple-quantum NMR(MQ NMR)to detect the gel point,characterize composition,the dynamics of forming and final gels.The addition of graphene can shorten the gelation time and gelation time constant.After the subtraction of the mobile fractions,we get the normalized nDQ build-up curve for final gels.The corresponding Dres and a which indicate the residual dipolar couplings with the largest probability shows that the addition of graphene can enhance cross-linked density of DA gelation.In conclusion,we find that as a crosslinked point,the addition of graphene can limit the dynamic of polymer chain and increase the density of polymer nerwork by rheology and NMR.