Low-energy vibrational excitations in polymethylmetacrylate with IR and RAMAN spectroscopy

Abstract

As it is well known, depending mainly on the time-temperature scale at which they are studied, amorphous polymers exhibit viscoelastic behavior which involves different molecular motions, with very different dynamics. At low temperatures or short times, the molecular mobility is generally considered as localized and leads to a macroscopic mechanical or dielectric behavior, so called secondary relaxations. These motions are often described as rotations of lateral groups or as crankshaft motions of main chain segments. Though much work has already been published on the subject, the origin of these relaxations is still uncertain. In most currently manifestation molecular mobility in glassy polymers associated with low-energy excitation of the type: a) low-angular libration of monomer units of chain or molecules;1 b) correlated vibrations, manifesting as a boson peak (BP) in the IR and Raman spectra,2 and c) reorientation processes, manifesting as fast relaxation phenomena.3 The nature of BP is currently widely discussed. A number of authors believe that the appearance of additional quasi-local vibrational modes is associated with the existence of medium-order and cohesive inhomogeneity in disordered media.4 Others relate them to correlated vibrations: for example, in quartztorsional vibrations in a chain of several SiO4 tetrahedra.5 In a previous paper6 Based on our comparative study of IR and Raman spectra of these low-energy excitation plasticized polymethylmethacrylate (PMMA) were presumably attributed to the librational motion of the polymer chain segment the size in several monomer units. The purpose of this paper is to experimentally test this assumption.