Dynamics in Poly(n-alkyl methacrylates): A Neutron Scattering, Calorimetric, and Dielectric Study

Combining neutron diffraction, neutron spin echo, differential scanning calorimetry, and dielectric spectroscopy, we have investigated the structure and dynamics of poly(n-butyl methacrylate) (PBMA) and poly(n-hexyl methacrylate) (PHMA). Signatures of the occurrence of a glass transition associated with the freezing of the intermolecular correlations within alkyl nanodomains are present in the structural data. Exploiting isotopic labeling, neutron scattering has revealed collective dynamics at the main-chain and side-group levels for both polymers and the self-motions of hydrogen atoms in the side groups of PHMA, adding valuable microscopic information to comprehensive relaxation maps and putting the relaxation results into a perspective. Moreover, we find exotic dynamical behavior for the side groups, characterized by extremely stretched (nearly logarithmic-like) decays of the correlation functions. For PHMA, a complete dynamical decoupling of side-group dynamics from the main-chain motions is found. The side groups of this polymer show an extremely "strong" temperature dependence of the structural relaxation time and much faster characteristic times for self than collective motions. The analogies found between the self-motions of the side-group H atoms in PHMA and the gamma-relaxation process in semicrystalline polyethylene (PE) strengthen the picture of confined PE-like dynamics within alkyl nanodomains. We discuss possible origins for the observed phenomenology.