ENERGY-STORAGE DURING INELASTIC DEFORMATION OF GLASSY-POLYMERS

In this paper, aspects of the microstructural state of glassy polymers that evolve during physical ageing and inelastic deformation were stu died. Differential scanning calorimetric (d.s.c.) measurements were pe rformed on specimens of three glassy polymers: polystyrene (PS), polyc arbonate (PC) and poly(methyl methacrylate) (PMMA). Materials were sub jected to both a quenched and a well annealed heat treatment and subse quently deformed in compression to various levels of strain. Stress-st rain curves and companion d.s.c. scans were compared. The well known e nthalpy overshoot at T(g) was observed for the annealed samples, showi ng that ageing is accompanied by enthalpy relaxation. The annealed mat erial was also found to require a higher stress to yield, and the addi tional work required to strain-soften the annealed polymer to the flow stress level of its quenched companion was found to correlate well wi th the area of the enthalpy overshoot of the annealed specimen. Inelas tic deformation was found to increase the specific enthalpy of both an nealed and quenched specimens. In the annealed material, the enthalpy overshoot at T(g) was found to decrease with inelastic strain and was completely erased by about -20% strain. Simultaneously, a pre- T(g) ex otherm was observed to develop with inelastic strain over a wide range of temperature. The pre-T(g) exotherm was found to evolve until essen tially reaching a steady-state profile at approximately -25% strain. T his evolution coincided with the strain-softening phenomenon observed in the corresponding stress-strain results. A pre- T(g) exotherm was a lso found to evolve with straining of the quenched material. Furthermo re, the steady-state exotherms of the quenched and annealed materials were found to be nearly identical, as were their corresponding flow st ress values after strain softening. Finally, a second, post- T(g) exot herm was found to develop with further straining beyond strains of -25 %. This exotherm was found to increase with inelastic strain and coinc ided with the occurrence of strain hardening (due to chain orientation ) in the materials. The presence of two distinct and separately evolvi ng exotherms in the inelastically deformed polymers indicates the exis tence of two separate deformation resistances in glassy polymers, one related to the initial yield and strain-softening behaviour, and the o ther to the orientation-induced strain hardening of the material. The observation that the pre-T(g) exotherm is spread over a wide temperatu re range reflects the distributed nature of the structural state and m ay be quantified using a distribution in activation energy for the loc al rearrangements. The results therefore provide valuable information about the processes that must be accounted for in the development of a ccurate constitutive models of mechanical behaviour.