Plasticity of semicrystalline polyethylenes viewed through the prism of thermodynamics
Authors: Oleinik Eduard , Salamatina Olga , Rudnev Sergey , Bartczak Zbigniew , Galeski Andrzej 
Published: March 2012

AbstractThermodynamic characteristics such as mechanical work Wdef and heat Qdef of plastic deformation were measured at room temperature for several non‐oriented linear high and ultrahigh molecular mass polyethylenes (PEs). The characteristics were registered simultaneously at room temperature active uniaxial compressive loading in the strain interval εdef = 0–50% and rate 4 × 10−2 min−1. An isothermal Calvet‐type deformation calorimeter was used for the measurements. Changes of the internal energy ΔUdef stored by deformed samples were calculated from Wdef and Qdef according the first law of thermodynamics. It appears that all thermodynamic quantities linearly depends on degree of crystallinity χ = 0.5–0.9 (DSC) at conditions of the study. Such behavior of Wdef, Qdef, and ΔUdef had permitted an extrapolation of measured quantities to crystallinities χ = 0.0 (pure amorphous phase) and χ = 1.0 (pure crystalline phase) and determination of deformation thermodynamic characteristics for each of them. Both phases participate into Wdef. It appears that the work W, necessary to deform PE crystallites is considerably higher than W, the work necessary to deform the amorphous phase. At εdef ≤ 30% W is 3–4 times higher than W and about two times higher at higher strains. From W and W stress–strain curves for both phases of PE were withdrawn. Deformation heat of the amorphous phase Q is orders of magnitude lower than Q. It reflects the entropic nature of deformation of rubbery amorphous phase of PEs at low εdef. The Q originates from a friction during glide of dislocations trough crystallites. Interesting behavior shows the stored energy of cold work ΔUdef = f(χ). At strains εdef ≤ 30%, the stored energy ΔU is a little lower than ΔU. However, ΔU becomes higher than ΔU at εdef >30%. The ratio ΔUdef/Wdef = f(εdef) was constructed also. The ratio gives the fraction of Wdef, which is transformed into the stored energy of cold work ΔUdef at loading. Behavior of several materials: glassy polymers, PE, and crystalline metals were compared in terms of the ratio. At elastic process, the ratio ΔUdef/Wdef tends to unity for all the materials. Whole Wdef in this case is converted into ΔUdef. With εdef growth dissipative processes appear and deformation heat is evolved. The ratio tends to ΔUdef/Wdef < 1. Comparison of three mentioned above materials show, that critical stage of their deformation kinetics is nucleation of the inelastic strain carriers (dislocations in crystals, for example). Initiation is completed very early (at εdef ≤ εy, the yield strain) for crystalline metals and about 92–98% of the expended Wdef becomes converted into deformation heat at εdef ≥ εy. Plasticity proceeds differently in glassy polymers. Initiation stage continues in glasses for a high strain εdef level, usually higher than εy. The curve ΔUdef/Wdef = f(εdef) for PEs is located between curves characteristic for metals and glassy polymers. Initiation of PE plasticity becomes completed at εdef ≈ 10–20%. At εdef >30% the ratio does not depends on εdef and stay constant on the level 0.35–0.55 for different PEs. The nature of such saturation is not understood yet. Thermally stimulated recovery of residual strains εres stored in deformed and unloaded PEs at different εdef was measured also. The rate recovery curves dεres/dT show two separate peaks, one with maximum at Tm and the other with maximum much below Tm. Integration of both gives amount of εres accumulated in amorphous phase and crystallites of PE at different applied strains. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012