A NEW APPROACH TO TREATING PLASTIC STRAIN IN GLASSY-POLYMERS

A new deformation mechanism of glassy polymers is suggested. The appro ach is based on experimental data obtained in deformation calorimetric studies, residual strain epsilon(res) recovery rate measurements, the rmally stimulated creep, DSC, and other. Work and heat of deforming a sample were measured, and the concomitant variation of internal energy was calculated. A large number of glassy polymers and blends were exa mined. It was found that from the very start of the deformation, a rat her large fraction of the work is converted to internal energy of the polymer, which suggests that essential rearrangements of its structure occur. The model elaborated on in this study rests on the concept tha t the inelastic strain and steady-state plastic flow proceed in the po lymer structure containing small-scale plastic shear transformations ( PST), rather than in the initial polymer structure. The former structu re presents an excited metastable state. Formation of PST begins at th e very early stages of loading and reaches a steady-state regime at ep silon(def) = 20 - 35%. The PST constitute a major source of macroscopi c strain. At T < T(g), conformational rearrangements result from PST t ermination, rather than proceed as a direct stress-induced process. PS T are nonconformational shear moieties (that are not associated with a ny volume changes) surrounded by elastic stress fields. All energy sto red during the deformation is accumulated in these fields. PST may exi st only in a glassy polymer, but not in the rubberlike state. Deformat ion mechanisms of rubber and polymer glass are essentially different. Relaxation, physical aging, and molecular mobility in a deformed glass are closely related to nucleation and termination of the PST. In this study, strain behavior of glasses and crystals was compared. Because the shear displacements associated with individual PST are small, defo rmation of polymer glass requires that the concentration of PST be hig h and, hence, the magnitude of the energy stored is also high. The eff ects of PST nucleation and termination on macroscopic properties of po lymers were examined. Mass transfer during the deformation of glass pr oceeds via small-scale gamma, beta, and probably delta motions, rather than by segmental mobility. Mechanical losses in deformed samples are significantly higher than in the initial. Stress-induced nucleation o f new PST markedly increases the concentration of chain fragments invo lved in the motions in the glass. Nucleation of flaws and crazes in gl assy polymers was suggested to occur at the sites of high local PST co ncentration. The parameter describing the ability of material to dissi pate excess energy of a deformed body was introduced.