Chain dynamics in poly(n-alkyl acrylates) by solid-state NMR, dielectric, and mechanical spectroscopies

A relaxation process was detected and quantified for the first time in various poly(n-alkyl acrylates) in the melt by a solid-state NMR method, NOE with dipolar filter. By comparison with dielectric spectroscopy and dynamic mechanical measurements carried out on the same samples, the relaxation process detected by NMR occurs at a higher temperature than the simple alpha-relaxation and on a longer time scale than the alpha beta-relaxation (cooperative alpha-relaxation). This relaxation process could be the isotropization of the main chain (by analogy with structurally similar polymethacrylates), but it is more probably a local relaxation within the alkyl side chain that is usually detected only at significantly lower temperatures. The fact that this side-chain motion is slower than the cooperative main-chain alpha-relaxation of restricted amplitude can be rationalized in the context of restricted amplitude main-chain motions in the local nanostructure present in the samples, as NMR and dielectric spectroscopy detect motions in different frames.