Calorimetric investigation of some polysaccharides subjected to high-pressure plastic deformation

Differential scanning calorimetry is used to establish that, at 20–200°C, an endothermic process proceeds in polysaccharides (cellulose, methyl cellulose, cellulose di- and triacetate, chitin, chitosan, starch, etc.) whose enthalpy is noticeably enhanced after plastic deformation of the polymers at 0.5–2.0 GPa. By the example of cellulose, it is shown that the process is associated with water present in the polymers. An increase in the enthalpy as a result of pressure treatment is due to both structural transformation of cellulose I into cellulose II and the intensification of intermolecular interactions in the polymers that is due to formation of a system of electric charges in the samples.     

Relevance of cage recombination in the plastic deformation of polymers

For 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.     

Plastic deformation of polymer blends with crystallizable components

The application of polymer blends depends mostly on their high ability to plastic deformation. Usually the studies of plastic deformation behavior include only the stress-deformation relationship and investigation of changes of morphology of the blends on the size level of inclusions. The energy absorption is also often considered. The presented, more rigorous, approach to plastic deformation of polymer blends containing a crystallizable component involves the studies of crystal plasticity and associated deformation of the amorphous phase. Plastic deformation of blends containing high-density polyethylene and isotactic poly(propylene) with other components were studied in two modes of deformation: 1. that avoids internal cavitation and 2. in tension with intense voiding. When no internal cavitation was involved, the plastic deformation was crystallographic in nature while the amorphous phase deformed in a manner to accommodate for the rotation and slips of the crystalline phase. Also the plastic deformation associated with intense voiding leads to a preferred orientation of certain crystallographic planes containing macromolecular chains which strongly suggests that the leading mechanisms in plastic deformation of blends are of crystallographic nature. The plastic deformation behavior depends very much on the glass transition of the amorphous component of the blend.     

Nominal and effective strains in severe plastic deformation processes

Severe plastic deformation (SPD) processes may be used for obtaining materials with a nanocrystalline structure and enhanced properties. Recent work on such methods is briefly reviewed, with special emphasis on processes resulting in only minor shape changes, i.e. multi-step forging (MSF), high-pressure torsion (HPT) and equal-channel angular extrusion (ECAE). The nominal strain levels achieved by these processes are discussed, taking into account various possible conditions of deformation (at constant or variable thickness for HPT, according to the exact shape of the die for ECAE). In actual operation, strain values may deviate from the calculated ones, particularly due to strain inhomogeneities (along the sample radius or through the thickness for HPT, longitudinal or transverse inhomogeneities in ECAE). The domains of application for these methods are considered.     

Thermostimulated processes in starch-bis(hydroxymethyl)propionic acid mixtures subjected to high-pressure plastic deformation

DSC and thermogravimetry are employed to study starch, bis(hydroxymethyl)propionic acid, and their mixtures of different compositions subjected to plastic deformation under pressures of 1 and 3 GPa. An endothermic peak with an enthalpy of 300 J/g observed in the thermogram of starch is most likely related to the breakage of hydrogen bonds. Treatment under pressure greatly reduces the enthalpy of this peak. For initial mixtures, the enthalpies of the endothermic peak of starch and the endothermic peak of melting of the acid are lower than those for individual components because of chemical interaction occurring between the mixture components during heating. The high-pressure treatment makes the decrease in the enthalpy of endothermic processes much more pronounced for both components.     

Simulation of plastic deformation in glassy polymers: Atomistic and mesoscale approaches

The mechanism of deformation in glasses is very different from that of crystals, even though their general behavior is very similar. In this study, we investigated the deformation of polycarbonate on the atomistic scale with molecular dynamics and on the continuum scale with a new simulation approach. The results indicated that high atomic/segmental mobility and low local density enabled the formation (nucleation) of highly deformed regions that grew to form plastic defects called plastic shear transformations. A continuum-scale simulation was performed with the concept of plastic shear transformations as the basic region of deformation. The continuum simulations were able to predict the primary and secondary creep behavior. The slope of the secondary creep depended on the interactions between the plastic shear transformations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 994-1004, 2005     

Structural approach to the study of deformation mechanism of amorphous polymers

A new microscopic procedure for the visualization of structural rearrangements in amorphous polymers during their deformation to high strains is described. This approach involves the deposition of thin (several nanometers) metallic coatings onto the surface of the deformed polymer. Subsequent deformation entails the formation of a relief in the deposited coating that can be studied by direct microscopic methods. The above phenomenon of relief formation provides information concerning the deformation mechanism of the polymer support. Experimental data obtained with the use of this procedure are reported, and this evidence allows analysis of the specific features of structural rearrangements during deformation of the amorphous polymer at temperatures above and below its glass transition temperature under the conditions of plane compression and stretching, uniaxial tensile drawing and shrinkage, rolling, and environmental crazing. This direct structural approach originally justified in the works by Academician V.A. Kargin appears to be highly efficient for the study of amorphous polymer systems.     

Tribological properties of an antifriction polymer modified by severe plastic deformation

The possibility of applying severe plastic deformation technologies to improving the tribological characteristics of semicrystalline antifriction polymers was studied. By the example of Nylon 6 subjected to equal-channel multiple angular extrusion, it was shown that severe plastic deformation favors a significant (more than three orders of magnitude) increase in the wear resistance of polymer and a 15–20% decrease in its friction coefficient. There are also increases in the maximum allowable contact pressure and temperature in the friction zone, at which the wear and the friction coefficients are stable.     

Heat and stored energy of plastic deformation of solid polymers and heterogeneous blends

Measurements of the mechanical work (A), the heat of deformation (Q) and differences between these quantities, i.e. the internal energy (U) stored in samples were performed under the unidirectional compression loading conditions by using constant temperature deformation calorimetry. It is shown for several glassy (PS, PC, PI-BD, PET, epoxy-amine network, ABS) semi crystalline (PBT, PET) polymers and blends (PC: ABS, PC: PBT), that 45–85% of the mechanical work of deformation is converted to internal energy stored in deformed samples U is quite high as compared with metals.

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Zusammenfassung Mittels Konstanttemperatur-Deformationskalorimetrie wurden bei gerichteter Kompressionsbelastung Messungen der mechanischen Arbeit (A), der Deformations-wärme (Q) und der Differenz beider Größen, d.h. der in den Proben enthaltenen inneren Energie (U) durchgeführt. Für einige amorphe Polymere (PS, PC, PI-BD, PET, Epoxy-Amine-Netzwerk, ABS), halbkristalline Polymere (PBT, PET) und Gemische (PC:ABS, PC:PBT) wurde gezeigt, daß 45–85 % der mechanischen Deformationsarbeit in den Proben als innere Energie gespeichert wird.

     

Anharmonicity and the elementary event of plastic deformation of amorphous polymers and glasses

The critical displacement of an excited atom (group of atoms) corresponding to the maximum in the interatomic attraction force plays an important part in the elementary event of plastic deformation of glassy solids. As a result of considerable departure of the excited kinetic unit from the equilibrium position and the nonlinearity of the interatomic interaction force, the microdeformation in the elementary event turns out to be a function of the degree of anharmonicity (Grüneisen parameter).     

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

The plasticity of semicrystalline polymers is analyzed in the framework of Young's dislocation model under the assumption of nucleation of screw dislocations from the lateral surface of the crystalline lamellae. It is proposed that the driving force for the nucleation and propagation across the crystal width of these screw dislocations relies on chain twist defects that migrate along the chains stems and allow a step‐by‐step translation of the stems through the crystal thickness. Such defects are identified as thermally activated conformational defects responsible for the so‐called crystalline relaxation. Dislocation kinetic equations are derived. Plastic flow rates attainable by dislocation motion in polyethylene and polypropylene are assessed with frequency–temperature data of the crystalline relaxation. Comparisons are made with experimental strain rates that enable homogeneous plastic deformation. In addition to temperature, the crystal lamellar thickness, which is a basic factor of the plastic flow stress in Young's dislocation model, is a major factor in dislocation kinetics through its influence on chain twist activation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 593–601, 2002; DOI 10.1002/polb.10118     

Effects of severe plastic deformation on the corrosion behavior of aluminum alloys

Effects of severe plastic deformation on the corrosion behaviors of Al alloys containing precipitates have been investigated. Al and its alloys were severely deformed by equal-channel angular pressing (ECAP) processes and the corrosion behaviors of the Al alloys were evaluated by means of potentiodynamic polarization in a neutral buffer solution containing 0.002 M chloride ion. Introduction of huge plastic deformation to both of Al-5.4 wt% Ni and Al-5 wt% Cu alloys increased pitting potential. In contrast, ECAP treatment of 4N pure Al resulted in a decrease in open circuit potential, slight increase of passive current and shift of pitting potential to the negative direction. The influence of the change in microstructures caused by severe plastic deformation was investigated. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical Reactivity of Metastable Materials, Warsaw, 17th-21st September, 2007     

Thermally induced low-temperature relaxation of plastic deformation in glassy organic polymers and silicate glasses

A deep analogy between the processes of low-temperature thermally induced relaxation of plastic deformation in amorphous polymers and inorganic glasses is observed. The results of the calculation of the activation energy and activation volume of this relaxation process in terms of the excited state model satisfactorily agree with the experimental data obtained for both epoxy polymer systems and sheet silicate glasses. This evidence allows us to conclude that the initial stage of macroscopic plastic deformation in glassy systems involves small critical displacements of excited atoms (groups of atoms) that are provided by local rearrangements of neighboring particles (entropy fluctuations). In the vicinity of the yield point, the number of excited atoms per unit volume induced by the action of mechanical stresses appears to be quite sufficient (10²⁶–10²⁷ m^?3) for promotion of a marked plastic deformation of glasses and preservation of appreciable amounts of internal energy.     

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     

Microstructures and mechanical properties of ultrafine grained pure Ti produced by severe plastic deformation

Microstructure evolution and mechanical behavior of ultrafine grained (UFG) commercially pure Ti produced by equal channel angular pressing (ECAP) were investigated. Repetitive pressings of the same sample were performed to six passes at 683 K, using the procedure designated as route B _c . After the sixth pass was finished, recrystallized grains were observed as similar as the fourth pass. The average size of the recrystallized grains was approximately 0.3 μm. The hardness value (H _v ) continuously increases with decreasing grain size. The H _v values are in good agreement with the other experimental data of Ti produced by severe plastic deformation processes. The similar slop k _H suggests that these microstructures have similar density of dislocations in the grains produced by the severe plastic deformation processes such as torsion straining, multiple forging, and ECAP. The grain size dependence of k _y in the present samples is 7.9  $ MPa\sqrt m $ . After six-pass ECAP, the ultimate tensile strength was increased by 60%. This is most likely due to considerable grain refinement through severe deformation by ECAP. The standard Hall–Petch relation for yield strength and hardness in the ECAPed Ti implies that the ECAPed Ti samples have similar texture and that the effect of grain size on strength may prevail over the effect of texture on the strength in Ti.     

Fundamental aspects of stress,deformation, and phase transitions in crystallizable polymers: Experiments with poly(ethylene terephthalate) in uniaxial stress fields

Interactions between rheology and fluid→solid transformations through crystallization are demonstrated through an experimental study of crystallization of polyethylene terephthalate in uniaxial stress fields. Effect of the strength of the stress field on the kinetics of crystallization is shown to diminish greatly beyond the initiation of crystallization. Consequences of crystallization in anisotropic stress fields in the generation of mechanical properties are also described. This experimental study points clearly to the deficiencies in current theoretical formulations of kinetics of crystallization.