Stanislav Patlazhan

Structural mechanics of semicrystalline polymers prior to the yield point: a review

The review focuses on the current studies of the deformation response and accompanying structural transformations of thermoplastic semicrystalline polymers subjected to uniaxial tension prior to the yield point. The mechanisms of strain-induced cavitation of amorphous layers and damages of crystalline lamellae are analyzed in line with novel results on the deformation behavior of solid polymers at temperatures exceeding the glass transition point. The coupling of viscoelastic and plastic deformation mechanisms with the small-strain structural transformations is critically discussed on the basis of the advanced theoretical modeling of mechanical properties of semicrystalline polymers.

Principles of Structural-Mechanical Modeling of Polymers and Composites 1

A general approach to the description of the mechanical properties of materials whose deformation is accompanied by considerable structural rearrangements is developed. As in the case of classical simulation of the deformation behavior of viscoelastic and elastoplastic bodies, the concept of basic structural-mechanical element that reflects the properties of a fragment of a continuous medium is introduced. The simplest version of two alternative structural states is considered. Equations that define the kinetics of transition between these states and the mechanical properties of the base structural-mechanical element include expressions for elastic strain, plastic flow, and structure evolution. It is assumed that structural transitions and plastic flow are effected by the thermofluctuation mechanism. It is shown that the array of elements characterized by an appropriate set of parameters can describe the specific features of deformation behavior and structure evolution for a wide range of materials, in particular, polymers and composites, at their early deformation stages.

Coupling of Nanocavitation With Cyclic Deformation Behavior of High-Density Polyethylene Below the Yield Point

The peculiarities of viscoelastic behavior of high-density polyethylene (HDPE) subjected to the uniaxial cyclic tensions and retractions below the yield point are studied. This required using three different deformation programs including (i) the successive increase in strain maximum of each cycle, (ii) the controlled upper and lower stress boundaries, and (iii) the fixed strain at the backtracking points. The experimental data are analyzed in a framework of the modified structure-sensitive model (Oshmyan , 2006, “Principles of Structural–Mechanical Modeling of Polymers and Composites,” Polym. Sci. Ser. A, 48, pp. 1004–1013) of semicrystalline polymers. It is supposed that increase in the interlamellar nanovoid volume fraction results in speeding-up the plastic flow rate while decreasing cavitation rate. Consequently, a proper fitting of the stress–strain cyclic diagrams is obtained for the applied deformation programs within the common set of model parameters. This makes it possible to reveal evolution of nanovoid volume fraction in HDPE during cyclic deformations.

Microstructure design of UHMWPE-based materials: Blending with graphite nanoplatelets

The main scope of this work was to develop a processing method to homogeneously distribute graphite nanoplatelets (GNP) within an ultra-high molecular weight polyethylene (UHMWPE) matrix. After this step, we estimated the toughness of the new nanocomposite material. Combining a sonication step, a micro-extrusion step and a hot-pressing step, we did not reach an optimal distribution state of the nanofillers. Nevertheless, the toughness of the nanocomposites evaluated by Charpy tester was higher than that of the reference UHMWPE (only processed by hot-pressing). We also found that our new processing procedure applied to neat UHMWPE leads to the higher toughness than that of the nanocomposite. Micro-extrusion appears as a promising processing tool for neat UHMWPE.

Orientation Effects in Hybrid-Polymer Mixtures

Composites based on melts of boron-oxide oligomer (BOO) and low-density polyethylene (LDPE) in the polyoxide-concentration range of 0–64 vol % were synthesized. The measurements of the thermomechanical and mechanical properties of the composites showed the incompatibility of the mixture components. The abnormal increase in the strength and the Young’s modulus of the LDPE/boron-oxide oligomer mixtures under the tension of molded composite specimens was registered in the range of 25–50 vol % polyoxide. The anomalies were explained as being due to polyoxide-fiber formation and confirmed by the electron-microscopy images. The abnormal changes in the differential pressure in a melt flow and the torque of an extrusion auger were observed in the same polyoxide-concentration range, which was explained by the polyoxide orientation in a melt flow and its planar structure. The chemical structure of boron-oxide oligomer exposed to extrusion mixing and its distribution within a molded specimen of the mixture were analyzed by IR spectroscopy. The opportunity to synthesize hydrolytically stable composites in a wide range of ratios owing to the polyoxide encapsulation in a polyethylene matrix was shown.