Thermodynamics of the pre-yield deformation behavior of glassy polymers: measurements with new deformation calorimeter

New deformation calorimeter was designed and built. The calorimeter is capable of measuring the differential heat flow between two cells containing a sample and a dummy. Deformation is performed in either compression or tension mode and ensures the equal forces applied to the sample and dummy in every moment during loading and unloading. The sensitivity of the calorimeter is <1 μW, and the time constant is about 3–4 min. High sensitivity of the instrument allows the measurement of very small deformation heats appearing at low strains (pre-yield strain region). Such data did not exist in polymer literature. Cylindrical samples with 4 mm diameter and 6 mm length were made of glassy polymers, such as polystyrene, (poly)methylmethacrylate, (poly)carbonate and cured epoxy-aromatic amine networks. Samples were deformed at room temperature by uniaxial compression up to strains 8–15% (which is slightly above upper yield point for polymers studied) with the loading rate 1 MPa/min. The same measurements with Cu single crystal were performed for comparison. The work and heat of deformation were measured and the internal energy stored in the deformed samples was calculated with the use of first law of thermodynamics. Total strain, work, heat and stored energy of deformation were split for the elastic (Hookean) and inelastic parts. It was found that in inelastic process all studied glassy polymers store internal energy which does not recover even after unloading of the samples. The parameter dUin/dWin, giving the fraction of the inelastic deformation work Win stored as the internal energy was measured as a function of strain. For annealed samples this parameter is close to 100% for small inelastic strains and decreases to 60% as deformation approaching to yield point. This supports the earlier formulated idea [Polym. Sci. 35 (11) (1993) 1819] that high level of energy storage is the characteristic feature of inelastic response of glassy substances. In contrast, the Cu single crystal does not show any energy storage during elastic and plastic deformation processes. Within experimental accuracy, all deformation work for Cu single crystal is converted to heat. Different deformation behavior of crystal and polymer glasses reflects the differences in deformation mechanisms for both types of the solids. Some details of the inelastic response mechanism for glassy solids based on measured results are discussed.