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 studied. Differential scanning calorimetric (d.s.c.) measurements were performed on specimens of three glassy polymers: polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Materials were subjected to both a quenched and a well annealed heat treatment and subsequently deformed in compression to various levels of strain. Stress-strain curves and companion d.s.c. scans were compared. The well known enthalpy overshoot at T(g) was observed for the annealed samples, showing that ageing is accompanied by enthalpy relaxation. The annealed material was also found to require a higher stress to yield, and the additional work required to strain-soften the annealed polymer to the flow stress level of its quenched companion was found to correlate well with the area of the enthalpy overshoot of the annealed specimen. Inelastic deformation was found to increase the specific enthalpy of both annealed 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) exotherm was observed to develop with inelastic strain over a wide range of temperature. The pre-T(g) exotherm was found to evolve until essentially reaching a steady-state profile at approximately -25% strain. This evolution coincided with the strain-softening phenomenon observed in the corresponding stress-strain results. A pre- T(g) exotherm was also found to evolve with straining of the quenched material. Furthermore, the steady-state exotherms of the quenched and annealed materials were found to be nearly identical, as were their corresponding flow stress values after strain softening. Finally, a second, post- T(g) exotherm was found to develop with further straining beyond strains of -25%. This exotherm was found to increase with inelastic strain and coincided with the occurrence of strain hardening (due to chain orientation) in the materials. The presence of two distinct and separately evolving exotherms in the inelastically deformed polymers indicates the existence of two separate deformation resistances in glassy polymers, one related to the initial yield and strain-softening behaviour, and the other to the orientation-induced strain hardening of the material. The observation that the pre-T(g) exotherm is spread over a wide temperature range reflects the distributed nature of the structural state and may be quantified using a distribution in activation energy for the local rearrangements. The results therefore provide valuable information about the processes that must be accounted for in the development of accurate constitutive models of mechanical behaviour.