Identification of slow dynamic processes in poly(n-hexyl methacrylate) by solid-state 1D-MAS exchange NMR

摘要

The molecular dynamics of the different molecular subunits in the amorphous polymer poly-(n-hexyl methacrylate), PnHMA, was investigated by recently developed solid-state 1D-MAS exchange NMR methods at temperatures close to the glass transition temperature. The molecular mobility of main-chain, side-chain, and carboxyl carbons could be assigned to different relaxation processes as identified by dielectric and mechanical relaxation spectroscopy as well as caloric methods. Information about both the time constants as well as the geometry of the processes could be obtained. It was found that the side chain contributes in the dynamic window of NMR exchange experiments (tau(c) approximate to 1 ms to 0.1 s) mainly to the beta process while the main chain and carboxyl group are found to contribute to both the beta and alpha processes. The data suggest that the contributions of side and main chain to the beta process are qualitatively different. All dynamic processes exhibit a wide distribution of correlation times. The proper separation of spin-exchange processes from the molecular dynamics is demonstrated. [References: 55]