Time-temperature superposition applied to crosslinked polymers in creep with superimposed hydrostatic pressure

The creep of an epoxy composition under combined tension and hydrostatic pressure (up to 800 kgf/cm²) has been investigated. The applicability of time-temperature superposition to a complex state of stress is demonstrated. Hydrostatic pressure is shown to affect the deformability of crosslinked polymers in tension.Leningrad Technological Institute of the Refrigeration Industry. Plastpolimer Research-Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 541–544, May–June, 1976.     

Creep of fluoroplastic in shear with application of hydrostatic pressure

Conclusions 1. It has been shown that under a deformation regime with Vxy = const, with an increase in shear deformation the effect of the action of hydrostatic pressure rises. A considerable effect of pressure on the character of the shear diagram is observed in the pressure region above 500 kgf/cm².Paper presented at the Third All-Union Conference on Polymer Mechanics in Riga, 1976.Leningrad Technological Institute of the Refrigeration Industry. Okhtinsk Scientific-Manufacturing Union Plastpolimer Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 812–818, September–October, 1977.     

Friction properties of highly elastic materials at high contact pressures

A newly designed high-pressure tribometer has been employed to investigate the pressure dependence of the friction force of SKN-40 crosslinked butadiene-nitrile rubber in contact with a steel surface on the pressure range to 1200 kgf/cm² (20°C). Over the entire range of contact pressures the friction process is molecular-kinetic in nature and characterized by a linear dependence of the friction force on the logarithm of the sliding velocity. In the region of normal pressures up to 200 kgf/cm², where the effect of pressure on the friction force reduces to the formation of the actual contact area, the friction constant (proportionality factor relating the friction force and the actual contact area) is practically independent of the pressure. At pressures above 200–300 kgf/cm² the increase in the friction force at fixed actual contact area is attributable to the effect of pressure on the friction constant. The nature of this effect is related not with an increase in the chain-surface interaction energy (the activation energy does not increase), but with an increase in the forces of adhesion owing to the greater number of polymer chain-steel surface contacts on the actual contact area (increase in contact density).Lenin Moscow State Pedagogical Institute, Laboratory for Problems of Polymer Physics. Translated from Mekhanika Polimerov, No. 1, pp. 140–146, January–February, 1971.     

Investigation of the temperature change associated with the adiabatic compression of high-density polyethylene

The changes in the temperature of high-density polyethylene specimens during adiabatic compression and unloading have been experimentally investigated at various initial temperatures and at pressures up to 2500 kgf/cm². It is shown that the temperature rise depends nonmonotonically on the initial temperature, which is due to the temperature and pressure dependence of the volume expansion coefficient and thermal stability.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 416–419, May–June, 1976.     

Effect of external pressure on the physicochemical and mechanical properties of polymers

The behavior of crystalline polymers obtained by molding at high pressures has been investigated. External pressure can promote or retard polymer crystallization processes, depending on its intensity. Comparison of various physicochemical and mechanical properties of specimens obtained at different external pressures shows that the corresponding curves have two maxima: one at 200–800 kgf/cm² and another on the narrower interval between 1300 and 1500 kgf/cm². The position of the first maximum is associated with the molecular weight distribution and the flexibility of the polymer chain, and position of the second is determined by the chemical nature of the polymer. The data obtained help to make possible a rational choice of polymer processing methods in which pressure is involved.Mekhanika Polimerov, Vol. 4, No. 2, pp. 200–204, 1968     

Creep of linear unoriented polymers in tension,compression, and torsion

The creep of polymethyl methacrylate, polystyrene, and polyethylene in tension, compression, and torsion has been investigated over a broad interval of temperature below the glass transition point. It is shown that the creep criterion changes depending on the temperature. This is associated with changes in molecular mobility.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 24–28, January–February, 1970.     

The vibration-time analogy

The vibrocreep of low-density polyethylene has been investigated in nonisothermal deformation regimes on the frequency range 150–400 Hz at variable stress amplitudes from 3.1 to 5.8 kgf/cm². The validity of the time-vibration superposition principle has been tested.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 793–798, September–October, 1968.     

Electron-microscope study of cellulose and Capron fibers with different strenghts

Oriented polymer fibers whose strengths differ by a factor of two have been subjected to an electron-microscope study. Both cellulose (=40 and 75 kgf/mm²) and Capron (=80 and 170 kgf/mm²) fiber were investigated. Specimens were obtained by ultrasonic dispersion followed by negative contrasting of the dispersion products with FVK solution. It is shown that there is no correlation between the transverse dimensions of the microfibrils and the strength of the investigated fibers.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 364–366, March–April, 1971.     

Stress on the interatomic bonds near the surface of a polymer

The stresses on the interatomic bonds in the interior and at the surface of polyethylene terephthalate film in uniaxial tension have been determined by means of infrared transmission and reflection spectroscopy. The stresses are found using the shift in the frequency of the atomic vibrations produced by the application of a mechanical load. It is shown that the stresses on some bonds in the specimen reach 700 kgf/mm², i.e., approach their theoretical strength. The concentration of these bonds in a surface layer 1 µ thick is approximately an order greater than the concentration in the interior of the specimen.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 512–514, May–June, 1976.     

Comparison of the effect of ultraviolet radiation on polymer creep rate in a vacuum and in air

The effect of ultraviolet radiation on the creep rates of various polymers has been investigated in a vacuum and in air. It is shown that the ambient medium plays very little part in the processes leading to an increase in creep rate in the presence of ultraviolet radiation.Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 559–561, May–June, 1969.     

Effect of hydrostatic pressure on the tensile strength of polymer materials

The effect of hydrostatic pressure on the tensile strength of amorphous, crystalline, and thermosetting polymers has been investigated. The maximum hydrostatic pressure was 2000 kgf/cm². The surfaces of some specimens were protected from the ambient medium (oil). The tests showed that hydrostatic pressure improves the strength or high-elastic limit and Young's modulus of all the materials investigated. In the case of brittle materials, the increase in strength is greater if the surface is protected, whereupon the plasticity is also improved. Hydrostatic pressure produces important changes in the deformation behavior of crystalline polymers.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1043–1047, 1967     

Creep of polymers during tension under conditions of hydrostatic pressure

Hydrostatic pressure results in decrease in the creep rate of a polymer loaded by tensile stress. The decrease in the creep rate is associated with the increase in intermolecular interaction. The correlation found between the creep rate of material loaded by a constant tensile force and with change in the melting temperature of a polymer under pressure appears to be commensurate with change in intermolecular interaction.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 359–362, March–April, 1976.     

Strength of glass laminates at low temperatures and high strain rates

The tensile strengths of glass laminates of various compositions under standard, high-speed and impact loading have been experimentally investigated in relation to the effect of low-temperature and stress raisers. As the loading rate increases (from 0.7 to 253 · 10³ kgf/cm². · sec) and the temperature falls to –196° C, the effective stress concentration coefficient decreases, which indicates that the glass laminates will perform adequately under these conditions.All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 836–841, September–October, 1970.     

Study of the role of intermolecular interactions (hydrogen bonds) in the initial stage of orientation of polyvinyl alcohol and vinyl alcohol copolymers

The effect of hydrogen bond rupture on the process of destruction of the starting crystallite structure of polyvinyl alcohol and copolymers of vinyl alcohol and N-vinylpyrrolidone and -vinylcaprolactam has been investigated. The differential spectrum method is used to establish the frequency (3200 cm^–1) of the band corresponding to the vibrations of the bound hydroxyl groups; the temperature dependence of the optical density of the 3200 cm^–1 band has been recorded. The strength of the starting structure is calculated from the stress-strain diagram and its temperature dependence is plotted. It is concluded that the strength of the starting structure is chiefly determined by the presence of hydrogen bonds between the polymer macromolecules.Leningrad Kirov Institute of Textile and Light Industry. Translated from Mekhanika Polimerov, No. 2, pp. 327–329, March–April, 1971.     

Effect of hydrostatic pressure on the water absorption of epoxy polymers

The water absorption of epoxy polymers at hydrostatic pressures up to 1000 kgf/cm² has been studied. It has been shown that pressure very definitely reduces the water absorption value but essentially does not change the diffusion coefficient of water into the polymers under study. An explanation of the data obtained is given.     

Residual stress distribution in different types of adhesion joints

The results of a photoelastic investigation of the distributions of shrinkage and temperature residual stresses in glued joints and coatings are compared. The degree of nonuniformity of the residual stress distribution over the length and thickness is determined as a function of the scale factor. It is shown that the bulk of the residual stresses in metal joints glued with K-115 epoxy adhesive are temperature stresses, which may reach 130 kgf/cm². The shrinkage and temperature residual stress distributions are similar in character.Kucherenko Central Scientific-Research Institute of Building Structures. Translated from Mekhanika Polimerov, No. 4, pp. 738–742, July–August, 1971.     

Impact strength of high-density polyethylene with various molecular weight distributions

The Charpy impact strength of high-density polyethylene specimens prepared under low pressure with average molecular weight from 60·10³ to 1.5·10⁶ and differing substantially in molecular weight distributions has been studied at room temperature and at –190°C. It is shown that, both at room temperature and at temperature considerably below the glass-formation temperature, the impact strength of polyethylene in the range of molecular weights mentioned is determined mainly by the content of fractions with molecular weights about 10⁵ and more and does not depend on the width of the molecular weight distribution."Plastpolimer" Scientific-Research Association, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 919–921, September–October, 1973.     

Polyformaldehyde as an antifriction material

The antifriction properties of polyformaldehyde have been studied on the velocity interval from 4 · 10^–4 to 10 m/sec and at loads to 300 kgf/cm². Tests were conducted with and without lubrication on laboratory friction machines of various types and under service conditions. Steel, emery cloth, and polyformaldehyde were used as the second member of the friction pair. The values of the coefficients of friction, temperatures and performance limits for polyformaldehyde are compared with those for polycaprolactam.All-Union Scientific Research Institute of Railroad Transport, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 850–856, September–October, 1969.     

Durability of polymers as a function of loading regime during repeated loading with a small number of cycles

Specimens of oriented polycaproamide and unoriented polymethyl methacrylate subjected to uniaxial extension were used to study durability as a function of the number of loading cycles and the intervals between them in different segments of the creep curve. It was shown that the deformation hardening during the initial segment of the creep curve governs the ability of the specimen to withstand subsequent loading cycles. With a relatively low loading rate, deloading-loading cycles during the initial segment of the creep curve lead primarily to an increase in local stresses. During the second stage, repeated loading causes principally an increase in local heating. The interval between loading cycles has only a weak influence on durability.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 252–256, March–April, 1972.     

Reasons for the lowering of the lifetime of polymers under cyclic loading

A model is proposed which explains the lowering of the lifetime of polymers on transition from static to cyclic loading by the interaction of local stresses from an external load with residual stresses which arose after preceding loading cycles. The results predicted on the basis of the model have been checked experimentally on polymethyl methacrylate, and a good agreement has been shown.M. I. Kalinin Leningrad Polytechnic Institute. Translated from Mekhanika Polimerov, No. 2, pp. 279–283, March–April, 1976.