E. F. Oleinik

Work, heat and stored energy in compressive plastic deformation of glassy polymers

Abstract For a number of organic glassy polymers with different chemical structures and immiscible blends, the mechanical work expended for deformation, the plastic deformation heat and the internal energy stored in deformed samples (stored energy of cold work) have been measured using different experimental techniques. A comparison of the measured quantities is made and suggestions concerning their nature are discussed. It is found that the fraction of expended mechanical work transformed into stored internal energy upon deformation is very high for all studied systems. This reflects the non-isostructural character of inelastic deformation in glassy polymers. A two-stage deformation mechanism is introduced and the experimental results are analyzed in a framework of the suggested mechanism.

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

Abstract 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 U in /d W in , giving the fraction of the inelastic deformation work W in 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.

Thermodynamics of inelastic deformation of amorphous and crystalline phases in linear polyethylene

Thermodynamic parameters (work W def and heat Q def) of inelastic deformation (uniaxial compression up to ɛdef = 50%) are measured for six samples of high-molecular-mass linear PE at room temperature under the regime of active loading. Energy excess ΔU def accumulated by the samples subjected to loading are calculated in terms of the first law of thermodynamics. All thermodynamic characteristics linearly increase with crystallinity χ of PE, thus making it possible to extrapolate their values to χ = 0 and 100% and to find the contributions of the amorphous and crystalline phases of the polymer to the overall thermodynamics of deformation. Both of the PE phases contribute to W def and ΔU def, while the crystalline phase alone contributes to heat Q def. At ɛ ≥ 30%, the energy contribution from the amorphous phase exceeds that from the crystalline phase. A comparison between the plastic behavior of PE crystals and glassy polymers demonstrates that PE crystallites are easier deformed (requires less work W def) than glassy polymers. At the same time, the amorphous phase of PE is harder to deform (requires more work W def and stores more energy ΔU def) than noncrystalline rubbers, apparently because of the deformation of tie chains. The thermodynamic characteristics of deformation are compared for three materials: crystalline metals, PE, and glassy polymers. The similarities and differences in their plastic behaviors are considered.

Computer simulation of rearrangements in chains of glassy polymethylene subjected at low temperature inelastic deformation

Molecular dynamics simulation of glassy polymethylene (PM) plastic deformation is performed up to ɛ = 30% in uniaxial compression regime at a temperature of 50 K, which is ∼140 K below Tg of the polymer. All atoms of PM chains are represented explisitly (all-atom model). Calculations were performed for two series of samples with different molecular mass distribution of chains: Samples have average degree of polymerization DP ≈ 212 with Mn ≈ 3000 and Mw ≈ 9500 (the first series) and DP ≈ 350, Mn ≈ 5000 and Mw ≈ 9500 (the second series). Each sample contains 12288 -CH2- monomeric units per computational sell. Nonaffine displacements of carbon atoms and conformational rearrangements in chains during deformation are visualized and analyzed. The transformation of relatively fragments of chains up to 16–20 monomer units length are basic structural units, non-conformational displacements of which controls plastic process. Relatively large nonaffine displacements are observed even in the range of low strains, which are usually interpreted as Hookean strains. In the range of yield tooth and steady plastic flow, the number of these displacements increases along with their amplitude. Conformational set of PM chains does not show a serious change during deformation. Analysis had shown that the number of conformational rearrangements of trans-gauche type in PM chains during deformation is small and such rearrangements do not play decisive role in the considered range of PM plasticity, even at ɛ > 15%, at the stage of the developed plastic flow.

Analysis of local rearrangements in chains during simulation of the plastic deformation of glassy polymethylene

A molecular-dynamics simulation of the low-temperature (∼100 K below T g) plastic deformation of glassy polymethylene (PM) was conducted. A model system consisting of 64 chains containing 100 CH2 groups (the united-atoms approach) in each computational cell with periodic boundary conditions was considered. The behavior of 32 such cells was considered. Each cell was subjected to an active isothermal uniaxial compression at a constant temperature of T def = 50 K to a strain of ɛ = 30%. An analysis showed that the inelastic deformation of glassy PM proceeded via nonaffine displacements (“gliding”) of chain fragments comprising 11–13 sites -CH2-. These displacements are correlated and directed mainly along chain axes. Only a small number of conformational rearrangements occur in chains during the deformation of the material. Conformational transitions add only small additional displacements to nonaffine atomic transformations. A free-volume analysis using Voronoi-Delaunay tessellation in the deformed polymer did not show its relation to local plastic rearrangements.

Inelastic deformation of glassy polyaryleneetherketone: Energy accumulation and deformation mechanism

The plastic deformation of glassy non-annealed polyaryleneetherketone (PAEK) was investigated via deformation calorimetry and thermally stimulated recovery of residual strain. Polymer samples were deformed at room temperature under uniaxial compression up to e def =–(40−50)% at a rate of 0.04 min−1. It was found that PAEK behaves in the deformation process similarly to many other glassy polymers: It stores internal energy excess at loading and contains two types of different inelastic strain carriers, namely the delayed elastic (e de) and plastic (e pl) strain carriers. The maximum level of the accumulated energy in PAEK reaches ≈ 8.3 J/g, which is close to those for glassy polystyrene and polycarbonate. Nearly all the deformation energy stored in PAEK is carried by the delayed-elastic strain. The carriers of plastic strain carry no extra energy or a very small amount of it. The inelastic deformation of glassy PAEK proceeds in two stages. The carriers of e de are nucleated at the first stage of the deformation process, and the carriers of e pl are nucleated at the second stage. It was shown that, during glassy-polymer loading, the molecular level structures carrying e pl never appear by themselves, but appear only as a result of spontaneous reorganization of e de. In other words, the plastic deformation appears in PAEK owing to the two-step process. This situation is typical for all glassy polymers.

Stepwise mechanism of the nucleation of plastic deformation in glassy polymers

The mechanism was studied for the nucleation of inelastic strain (its delayed elastic (anelastic) component εde and plastic component εp) during isothermal loading of specimens of organic glassy polymers of various chemical nature by uniaxial compression at Tdef < 0.6 Tg within the strain range εdef < 45–50%.

Effect of plastic deformation on the character of micro-brownian motions in glassy poly(methyl methacrylate)

Glassy PMMA samples are plastically deformed at room temperature in the uniaxial compression regime to residual strains of e res = 25%. Dielectric spectra of the initial and deformed samples are recorded via the method of broadband dielectric spectroscopy in the frequency range f = (5 × 10−4) − 107 Hz. The results are compared with the dynamic mechanical spectra of samples deformed under the same conditions. Dielectric and mechanical spectra are noticeably distorted by deformation. As a result, dielectric permittivity ɛ′ increases, shear modulus G′ decreases, and the intensity of dielectric β losses slightly increases, while dielectric and mechanical α losses increase appreciably. In addition, the “anomaly” of total dielectric Δɛtot and total mechanical ΔG tot dispersions (Δɛtot = ɛ0 − ɛ∞ ≈ Δɛα + Δɛβ and ΔG tot = G 0 − G ∞ ≈ ΔG α + ΔG β) occurs, that is, the polymer is transformed from the state with Δɛα ≪ Δɛβ and ΔG α Δɛβ and ΔG α > ΔG β. The described phenomenon is related to a strong gain in α dielectric and mechanical losses in the deformed material. It is found that α losses increase owing to an anelastic deformation component arising during glass loading. This component is responsible for an increase in the internal energy of the glass during its anelastic deformation. Possible causes of the observed effects are discussed.

Plasticity Mechanism for Glassy Polymers: Computer Simulation Picture

This review summarizes the data published over the past two and a half decades on the mechanism of plastic deformation of bulk isotropic linear glassy polymers in uniaxial tension, compression, and shear at low deformation temperatures (Тdef < 0.6Тg) and moderate loading rates. The main attention is focused on studies concerning the numerical computer simulations of plasticity of organic polymer glasses. The plastic behavior of glassy polymers at nano-, micro-, and macrolevels in the range of macroscopic strains up to ≈100% is discussed. Plasticity mechanisms are compared for organic, inorganic, metallic, polymer, and nonpolymer glasses with different chemical structures and types of interparticle interactions. The general common mechanism of plasticity of glassy compounds, the nucleation of plasticity carriers in them, and the structure of such carriers and their dynamics are covered. Within the framework of the common plasticity mechanism, the specific features of deformation in glassy polymers are analyzed. Specifically, the participation of conformational transformations in macromolecules in the deformation response of polymer glasses, change in intensity of the yield peak with the thermomechanical prehistory of the sample, and the role of van der Waals interactions in the accumulation of excess potential energy by the sample under loading are considered. The role of free volume and structural and dynamic heterogeneities in the plasticity of glasses is also discussed.