Inelastic neutron and Raman scattering of poly(methyl methacrylate): Nanostructure of polymer glasses

Abstract

Low-energy excitations were investigated in amorphous poly(methyl methacrylate) by means of both neutron and Raman scatterings, in a wide temperature interval. The frequency dependence of the light-vibration coupling coefficient was deduced, especially at low temperature where the quasi-elastic contribution is not significant; it was not found to be linear. The vibrational density-of-states anomaly is interpreted in the frame of a non-continuous glassy nanostructure, which is characterized by a log-normal size distribution of domains.

Low-energy vibrational density of states of plasticized poly(methyl methacrylate)

“…The boson peak appears at an energy slightly lower than 2 meV for pure PMMA-H, as observed before [3], and higher for pure DBP (Figure 1). The relative intensity at low energy is higher for pure PMMA than for pure DBP, and decreases with plasticization.…”

“…Furthermore such arrangement is quasiperiodic with a period close to 4 nm. This inhomogeneity of the plasticized polymer was assumed to be due to the cohesion inhomogeneity of the glassy polymer without plasticizer: "More cohesive are separated by softer interdomain zones" [2,3]. The other interesting experimental result is the increase of the excess of low-frequency Raman scattering (LFRS) or Raman boson peak by plasticization: the boson peak is more intense than the one obtained by adding the LFRS of the two components (PMMA and DBP) according to the composition of the plasticized material.…”

“…It will be shown that the plasticization has no effect on the VDOS excess or neutron boson peak, and on the other hand that the low-frequency vibrations of PMMA drag along the motion of the DBP plasticizer. relaxational motions negligible at this temperature [3]. The scattering cross-sections were obtained after the usual standard calibrations by means of the vanadium runs and the removal of the empty cans contributions.…”

Dynamics in the Glassy State and Near the Glass Transition of Amorphous Polymers as Studied by Neutron Scattering

“…However, computer simulation of large fractal networks by Yakubo et al [66] showed that there are no excess excitations in the crossover region. It is often assumed that the origin of the boson peak is a confined phonon [46,47,67]. For amorphous polymers it was assumed that a glassy state has a non-continuous structure due to a stronger bonding between atoms in blobs than between atoms in different nearest-neighbor blobs.…”

The Raman coupling function in amorphous silica and the nature of the long-wavelength excitations in disordered systems

“…Though some attempts in this direction were made in the past [22], a definite conclusion as to the shape of C(ω) has not been reached [8,17,23], mainly due to the presence of "spurious" effects in RS (namely, the presence of quasi-elastic scattering (QES)…”