Molecular mobility in glassy polymers upon plastic deformation

The complex storage modulus components G'(omega) and G "(omega) were studie d as functions of the frequency for the glassy polycarbonate (PC) and PMMA samples upon preliminary room-temperature plastic deformation by uniaxial c ompression to epsilon(def) approximate to 40-45%. The G'(omega) and G "(ome ga) values were measured at room temperature in the frequency range omega = 10(-2)-10(2) Hz. It was found that G'(omega) of the predeformed samples wa s lower than the modulus of the initial samples in the entire frequency ran ge studied, whereas the G "(omega) values markedly increased after deformat ion, especially in the low-frequency region. By partially annealing the res idual deformation at temperatures below the glass transition temperature, i t is possible to release the anelastic deformation component in PMMA and PC while retaining the elastic component. It is established that all changes in the G' and G " values observed in the frequency range studied are relate d completely to the anelastic component of the residual deformation. The pl astic deformation component does not affect the G' and G " values, although the amount of plastic deformation markedly exceeds the anelastic deformati on level. It is concluded that an increase in the molecular mobility in pre deformed polymeric glasses is due to the appearance of the plastic sheer tr ansformation (PST) regions that serve as elementary carriers of anelastic d eformation component in glassy polymers. It is suggested that this pattern is typical of all noncovalent low- and high-molecular-mass glasses, The PC samples were additionally characterized by positronium annihilation lifetim e spectroscopy (PALS). It was found that the appearance of anelastic deform ation component and its carriers (PST regions) is related to a small (2-3%) decrease in the free volume of the glass, while the size of the free volum e elements are virtually not affected by the deformation. Therefore, an inc rease in the molecular mobility and the deformation-induced softening of pr edeformed PC cannot be explained in terms of the free volume increase in th e polymeric glass upon deformation (which is possible for PMMA). Some physi cal factors responsible for the anelastic-deformation-induced activation of the molecular mobility in glassy polymers are considered. A general phenom enological scheme explaining the effect of anelastic deformation of glasses on the molecular mobility in these materials is proposed. A decrease in th e storage modulus G' in predeformed samples (deformation-induced softening) is probably related to the fact that PST regions are less rigid formations compared to the surrounding nondeformed glassy matrix. The PST accumulatio n in the course of deformation of a polymeric glass sample leads to a growt h in the concentration of "soft" inclusions and, hence, to a decrease int h e overall macroscopic rigidity of the system.