Plastic Deformation Of Polymers Research Articles

Adiabaticity Conditions for the Process of Stretching of Uniformely Deforming Polymers

Plastic deformation of polymers is accompanied by increasing temperature owing to the transformation of mechanical work for the deformation into heat. The effect of the strain rate on the heating of uniformly deforming polymer films is analyzed theoretically. A formula describing the heating value is derived. As in the case of a frontal spreading of the neck, an increase in the strain rate upon a uniform deformation results in the transition from an isothermal process to an adiabatic one. The rate of the transition to adiabatic strain conditions increases with a growth of deformation. As compared to the neck spreading process, the transition is shifted toward higher strain rates. At low elastic deformations and usual rate values, the strain proceeds in an adiabatic regime.

Effect of annealing temperature on interrelation between the microstructural evolution and plastic deformation in polymers

The mechanical loading induced flow of glassy polymers is triggered by the nucleation of shear transformation units, and strongly depends on the initial microstructural state of the material. Therefore, investigation of the possible relationship between the microstructural state variables and plastic deformation is required for a better understanding of the macroscopic response of this class of materials during large deformation. In this study, free volume content is considered as a state variable and thermal treatment is selected as a process through which the accelerated and forced evolution of the free volume can be imposed. For two well-known glassy polymers, poly(methyl methacrylate) and polycarbonate, the free volume content alteration upon annealing is monitored via positron annihilation spectroscopy, and the changes of the micro- and macromechanical properties are also obtained by utilizing nanoindentation technique and employing the homogeneous amorphous flow theory. The correlation between the microstructural state variable, that is, free volume, and the micromechanical state variable, that is, shear activation volume, is then investigated. The results reveal opposite direction of alterations of free volume and shear activation volume with annealing temperature. Accordingly, the possibility of the existence of an interrelation between these two state variables is critically discussed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1286–1297

Plastic deformation and cavitation in semicrystalline polymers studied by X-ray methods

This paper is aliterature review presenting the relation between the cavitation phenomenon and plastic deformation of polymers. The cavitation phenomenon, meaning the formation of numerous micro and nano size voids (cavities), is observed in many semicrystalline polymers deformed at temperatures above the glass transition. Cavitation occurs when crystals in the polymer are stronger than the amorphous phase. This means that the phenomenon may be controlled by applying an appropriate crystallization procedure. Thicker and less defected crystals usually grow when the time for crystallization is longer and it is easier to deform the amorphous phase with voiding. The cavitation process also depends on the deformation conditions. Faster stretching at lower temperatures is favorable for deformation with voiding. The cavities are observed only during tensile deformation as local triaxial straining is necessary for cavitation. Usually, cavities are formed when the strain is close to the yield point and voids have ananometer size. Larger, micrometer cavities dominate at higher strains, near the end of the deformation process.

Self-oscillating plastic deformation of polymers

Self-oscillating plastic deformation of polymers

Dimension of the local-plasticity zone as a correlation length of the structure of polymers under inelastic deformation

The structural meaning of the dimension rpl of the zone of local plastic deformation in polymers is determined. In simulating the polymer structure as a percolation cluster, the quantity rpl is the correlation length of the structure of the polymer deformed until inelastic deformations appear. This rule holds for both amorphous glassy polymers and amorphous-crystalline polymers and for various types of the indicated zones (crazing or shear).

On the micromechanisms of plastic deformation in semicrystalline polymers

Many different molecular processes that cause plastic deformation in polymers can be investigated by transmission electron microscopy (TEM). A short review of those deformation mechanisms is given in this paper using model morphologies and correlating them to the corresponding mechanical behavior.

Multidimensional solid state NMR: A unique tool for the characterisation of complex materials

AbstractA survey is given on new developments in multidimensional solid state NMR which evolves as an indispensable tool of materials characterisation. After a short introduction NMR is compared with other techniques, in particular X‐ray and neutron scattering. The unique information available by solid state NMR is demonstrated by specific examples of structural studies of chain conformation, advanced aspects of chain dynamics, phase separation and interfacial effects as well as non‐linear plastic deformation in polymers. Emphasis is placed on the new development of high resolution multiple quantum NMR spectroscopy of abundant nuclei in solids with first experimental examples on the location of hydrogen positions in organic solids, structure of inorganic glasses and chain order in polymer melts and elastomers.

Stick-slip in the scratching of styrene-acrylonitrile copolymer

Stick-slip process occurred during the scratch test of styrene-acrylonitrile copolymer. For the first time, a bamboo-like morphology of the scratch track corresponding to the stick-slip phenomenon was observed. A “joint” was formed during the stick stage and during slip a uniform “stem” was made. The period and amplitude of the stick-slip both increase with the vertical load and decrease with the driving speed. A theoretical model is constructed based on the stiffness of the system and the plastic deformation of polymer both in the vertical and horizontal directions. The model assumes no distinction between the coefficients of static and kinetic friction and gives quantitatively consistent results with experiments.

Plastic deformation of polymers with fibrous structure

The plastic deformation of fibrous material occurs primarily by a sliding motion of fibrils. To a first approximation, the displacement of their centers of mass can be well described by an affine transformation corresponding to the deformation of the bulk sample. Such a sliding motion of fibrils does not affect the morphology of the microfibrils. But by chain unfolding, it smoothes the surface inhomogeneities of the fibrils caused by microfibril ends which act as point defects of the microfibrillar lattice. This makes possible a more perfect lateral contact between adjacent fibrils and results in a steadily increasing resistance to plastic deformation. The sliding motion of fibrils produces a shear stress on skewed fibrils and this causes a slight shear displacement of microfibrils. But in spite of its smallness, this shear displacement enormously extends the interfibrillar tie molecules by chain unfolding and thus increases their fraction in the amorphous layers.

Relevance of cage recombination in the plastic deformation of polymers

AbstractFor a polymer in which permanent rupture of individual molecules is the rate‐limiting process for plastic deformation, the kinetics of chain‐end diffusion and secondary radical reactions should be compared with the kinetics of caged radical recombination in the calculation of activation parameters for plastic deformation. If mechanisms of cage escape are slower than those for cage recombination, the activation parameters for plastic deformation will differ from those for the initial bond‐breaking process. For the case of polyethylene deformed in the vicinity of 250°K, the critical thermally activated event appears to involve scission of the polymer molecule near the site of an abstracted hydrogen atom. For this system the dominant cage‐escape mechanism is diffusion, which is faster than either hydrogen abstraction or unzipping to the monomer. However, at low stresses the rate of cage recombination is expected to be higher than the rate of cage escape, so that the activation parameters for deformation should be the sum of those for chain scission and diffusion. The contribution of diffusion (ca. 15 kcal/mole) to the activation energy for deformation (E*, extrapolated to zero stress conditions) is relatively modest. However, the calculated molar activation volume for deformation V* increases by almost an order of magnitude, i.e., from ca. 10 to ca. 76 cm3/mole when diffusion is required. Consideration of experimental values of E* and V* for high molecular weight polyethylene indicates that, in the regime examined, chain scission plus chain‐end diffusion is required to effect plastic deformation.

プラスチックの塑性変形現象(塑性加工と材料特性小特集号)

プラスチックの塑性変形現象(塑性加工と材料特性小特集号)