Localized picosecond-scale process in glassy poly(methyl methacrylate) far below Tg

Abstract

We carried out incoherent and coherent inelastic neutron scattering on glassy poly(methyl methacrylate) in a wide scattering vector range up to 4.9 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ with a high-energy resolution of 0.16 meV to investigate low-energy modes: the so-called boson peak and the picosecond-scale process. First, we studied the basic features of the two modes in the incoherent measurements and found that they are similar to those reported for many glass-forming materials. In the coherent scattering measurements, focusing on the Q dependence of the inelastic scattering intensity, we found that the boson peak and picosecond-scale process modes do not move in phase at a length scale of \ensuremath{\sim}7 \AA{}, while the motional coherency is preserved between neighboring side chains in a polymer. These observations suggest that the boson peak and picosecond-scale process are localized modes on two monomers within a polymer chain. The intrachain localization becomes more pronounced with increasing the excitation energy.