Nature and mechanism of friction of rubberlike polymers in different physical states

We have investigated the frictional properties of crosslinked butadiene-nitrile and butadiene-styrene copolymers and natural rubber in friction against polished steel under vacuum conditions in the temperature interval from –200 to +150° C, which embraces the glassy and high-elastic states, as well as the transition region between them. The temperature dependence of polymer friction is characterized by two maxima, a principal and a low-temperature maximum. The principal maximum, observed in the glass transition region, is not associated with the mechanical loss maximum observed in the polymers themselves. The temperature dependence of the force of friction is composed of three parts. In the high-elastic region there is an increase in the force of friction with fall in temperature, in accordance with the molecular-kinetic theory of friction of rubberlike polymers. In this region the nature of friction is associated with mechanical losses in the surface layer of polymer. The mechanical losses inside the polymer itself are unimportant. The deviation from the theoretical curve and the fall in the force of friction below a certain temperature in the transition region are chiefly associated with a decrease in the actual area of contact as the polymer passes into the glassy state. In the glassy region the friction is significantly determined by the mechanical losses in the polymer itself associated with the repeated elastic and forced-elastic deformation of the asperities in the layer of polymer in contact with the rigid surface. Therefore the low-temperature maximum is closely related to the mechanical loss maximum observed in the same temperature region in dynamic tests. Apart from this, the friction maximum is also associated with the increase in the forces of adhesion and the reduction of the actual area of contact at temperatures at which a forced-elastic mechanism of compression of the polymer asperities is not realized.Mekhanika Polimerov, Vol. 3, No. 1, pp. 123–135, 1967     

Mechanical properties of polyethylene and teflon over a broad range of strain rates

Tensile load-extension diagrams have been obtained for low- and high-density polyethylene and teflon. Conventional notions concerning the effect of strain rate on the properties of solids are found not to apply. The unconventional distribution of the family of - curves plotted for different strain rates is caused by the combination of high-elastic and plastic deformation associated with the extension of crystalline polymers. An attempt is made to correlate the data on a narrow range of strain rates with the aid of a model of a viscoelastic solid with variable relaxation time, and on a broader range by means of an equation incorporating the limiting dynamic diagram.Mekhanika Polimerov, Vol. 4, No. 1, pp. 45–52, 1968     

Long-time strength of high-elastic polymers

This article examines the question of the inadequacy of the Zhurkov equation for describing the laws of long-time strength of polymers, loading of which is accompanied by the development of high-elastic or induced-elastic deformations. On the basis of general considerations concerning the deformation dependence of the stress concentration coefficient and orientation hardening of polymers, a more general relation between static fatigue life and long-time strength is proposed.Mekhanika Polimerov, Vol. 1, No. 6, pp. 85–91, 1965     

Temperature dependence of E and tg δ for polymethylmethacrylate and polystyrene with different kaolin contents

The temperature dependence of the dynamic modulus of elasticity E and the loss tangent tg has been investigated for polymethylmethacrylate (PMMA) and polystyrene (PS) with different contents of kaolin filler (K), in the glassy state and in the region of incipient devitrification. It is concluded that supermolecular reinforcing structures appear in filled polymers in which the intermolecular forces are sufficiently strong.Mekhanika Polimerov, Vol. 3, No. 5, pp. 854–857, 1967     

Effect of the physical state on tearing of amorphous polymers

The temperature dependence of the basic tear characteristics of deformation and failure of SKS-85 copolymer in the range from Tg to T>T_f is examined. It is established that the total work done in tearing the polymer in the brittle state is determined by the work of deformation up to separation of the specimen into two parts. In contrast to this, the total work done in tearing the polymer in the high-elastic state is determined mainly by the work of formation of the tear surface.Mekhanika Polimerov, Vol. 1, No. 5, pp. 71–77, 1965     

Shear strength and laws of shear deformation of some cross-linked resins

The authors present the results of an experimental investigation and comparison with theory of certain laws of shear deformation and shear strength for rigid cross-linked resins. The possibility of describing the state of strain of homogeneous isotropic polymers by means of the generalized nonlinear Maxwell equation is considered. The results of an investigation of thin-walled tubular specimens of three cross-linked polymers based on epoxy resins and one linear polymer are offered as a check on the corresponding theoretical relations. The agreement between the theoretical and experimental results is perfectly satisfactory. Experimental values of the high-elastic constants in shear and tension have been determined. It is shown that the theoretical and experimental relations coincide with satisfactory accuracy. Certain experimental data on the comparative conditions of failure in shear and tension are presented.Mekhanika Polimerov, Vol. 2, No. 2, pp. 214–228, 1966     

Maximum force of friction in rubber-metal friction pairs under conditions of constant compressive deformation of the rubber

The maximum force of friction at the initial moment of slip has been investigated on rubber-metal friction pairs under conditions of constant compressive deformation of the rubber during transition from the high-elastic to the glassy state. Filled butadiene-nitrile rubber compounds were studied in the temperature range from +20 to –50°C. The temperature dependence of the maximum force of friction has a sharply expressed maximum near the glass transition temperature. As the temperature falls, the force of friction at first increases, in accordance with the molecular-kinetic theory. As the temperature continues to fall, in the transition region the maximum force of friction begins to rise more sharply owing to a sharp increase in the volume-mechanical friction component. The fall in the maximum force of friction below the glass transition point associated with a decrease in the deformed volume of rubber due to shrinkage and with the reduced mechanical loss factor.Mekhanika Polimerov, Vol. 3, No. 3, pp. 533–538, 1967     

Viscosity of non-Newtonian media in pure shear

On the basis of Geisekus's rheological equation of state the viscosity of a non-Newtonian fluid is investigated in relation to the particular mode of deformation (pure shear, axisymmetric deformation, simple shear), The intrinsic viscosity is calculated for pure shear, the following model being used: rigid ellipsoids of revolution uniformly distributed in an incompressible viscous Newtonian fluid. The dependence of the intrinsic viscosity on the parameter =(2/3)(q/D) (q is strain rate, D is the rotational diffusion coefficient) is obtained in specific form for various ratios of the ellipsoid semiaxes.Mekhanika Polimerov, Vol. 3, No. 5, pp. 927–932, 1967     

Temperature dependence of the strength of a woven composite in a plane state of stress

Conclusions On the basis of an analysis of test results obtained by typical patterns of simple momentary quasistatic loading at various temperatures within the 20–150°C range, we have found a family of strength surfaces for a woven organic plastic material in a plane state of stress in the reinforcement plane. These experimentally found strength surfaces can be approximated by the equation of a second-degree surface. It has been established that the range of safe states of stress in the stress space narrows nonuniformly with rising temperature; namely, this narrowing is accompanied by a shift of the center and a reorientation of the axes of the strength ellipsoid. We have revealed and described the temperature dependence of the components of the strength surface tensors involved in the strength criterion. The data can be used for predicting the strength of a composite material under consideration when the latter is subject to simple quasistatic loading patterns in the three-dimensional (₁₁, ₂₂, ₁₂) stress space in the reinforcement plane within a given test range of temperatures.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 452–457, May–June, 1978.     

Temperature-frequency dependence of the elastic constants of certain resins

The authors present the results of an experimental investigation of the elastic constants of certain crosslinked and linear polymers in the temperature range from –50 to +110° C and the frequency range from 10¹ to 10⁵ Hz. The elastic constants were determined by both quasi-static and dynamic methods. The range of application of the different methods are characterized with respect to a time scale. It is shown that when certain experimental conditions are observed (strain rate, frequency), the values of the constants obtained by the different methods almost coincide. On the basis of temperature measurements it is postulated that rigid epoxies have a secondary low-temperature dispersion region.Mekhanika Polimerov, Vol. 3, No. 1, pp. 60–65, 1967     

Viscoelastic properties and unstable flow of monodisperse polybutadiene solutions

The conditions governing the onset of unstable flow when concentrated solutions of monodisperse polybutadiene in -methylnaphthalene are extruded through a capillary have been investigated. The critical values of the parameters corresponding to these conditions have been found. It is shown that unstable flow of the "elastic turbulence" type is associated with transition of the system to the forced high-elastic state.Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 132–137, January–February, 1973.     

Effect of physicalstate on tearing of filled systems based on linear amorphous polymers

A study has been made of the temperature dependence of the service behavior, deformation, and tear rate of filled systems based on SKS-85. It is established that the introduction of fillers which enhance the strength of systems in the high-elastic state effect a loss of strength in the vitreous state. The observed reversal of the strengthening effect from positive to negative is explained by weakening of the adhesive strength of the bond between polymer and filler particles caused by the increase and concentration of shrinkage stresses during cooling of the specimens. This effect is called temperature inversion of the reinforcing action of fillers.Mekhanika Polimerov, Vol. 1, No. 6, pp. 98–102, 1965     

A comparison of the elastomeric properties of inorganic glasses and high polymers

The high-elastic properties and structural-mechanical characteristics of inorganic glasses and high-polymer materials are compared. Points of similarity and also differences in the mechanical behavior of organic and inorganic polymers in the high-elastic state are noted. Comparative estimates of the molecular weights of the chains in organic and inorganic polymer networks are given.Mekhanika Polimerov, Vol. 1, No. 3, pp. 21–28, 1965     

The problem of calculation of change in density of rubber during temperature-frequency (time) reduction

A graphical method is proposed for determination of the coefficient of vertical displacement of curves of the frequency dependence of viscoelastic functions of polymers during temperature-frequency (time) reduction. With respect to temperature, the dependence of the quantity ₀/ (where ₀ and are the densities of the material at the reduction temperature T₀ and the initial temperature T, respectively) is allowed for by use of one constant, the volume coefficient of the rmal expansion .Scientific-Research Institute for the Plastics Industry, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1136–1137, November–December, 1972.     

Characteristics of stretching of high-impact polystyrene and polycarbonate close to the glass transition temperature

A study has been made of the stress-strain characteristics of high-impact composites based on poly sty renes (PS) or polycarbonates (PC) in uniaxial stretching at temperatures close to the glass transition temperature of the matrix material Heterogeneity of the structure of the material at the microlevel, owing to the presence of a graft copolymer in the PS composites or the presence of polyethylene terephthalate (a reactive additive) in the PC, tends to lower the effective glass transition temperature as determined at high elongations. Weak interaction of the impact-strength modifiers with the matrix material (determining the heterogeneity of the structure at the macrolevel) results in a lower stability of uniform stretching in the region of the high-elastic state, but increases the values of the elongation at break in the transition region from the glassy state to the high-elastic state. From the standpoint of achieving high strength and deformability of high-impact composites during their processing, the structure should be organized so as to provide optimal heterogeneity at both the microlevel and macrolevel.Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 509–517, July–August, 1995.     

Generalized concentration dependence of the viscosity of concentrated polymer solutions

The construction of a generalized concentration dependence of the viscosity of concentrated solutions is analyzed. It is shown that there should be a single dependence for different polymers in different solvents of the form: (/_o)^1–2k=1+(1–2k)c[], where k is the Huggins-Martin constant, and [] is the intrinsic viscosity. Deviations from this relation may be observed in the presence of structure formation in the solution or when the experimental temperature is close to the glass-transition temperature of the system."Plastpolimer" Okhtinsk Research-Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 172–175, January–February, 1976.     

A rheological model of polymers and glass-reinforced plastics

Network polymers and the corresponding glass-reinforced plastics are investigated for a homogeneous uniaxial state of stress and constant temperature. A physical law relating the high-elastic strain and the stress in explicit form with once-determined structural constants is obtained for the damped (bounded) creep regime. The theoretical solutions are compared with the experimental data for a glass-reinforced plastic based on unsaturated polyester resin and glass mat reinforcement.Institute of Engineering Mechanics, Bulgarian Academy of Sciences, Sofia. Translated from Mekhanika Polimerov, No. 5, pp. 851–857, September–October, 1971.     

Rheological properties of rubbery polymers at low shear stresses

The rheological behavior of the rubbery amorphous polymers SKB-35 (sodium-butadiene rubber), SKN-26M(butadiene-nitrile rubber), and PIB-85 (polyisobutylene) has been investigated in relation to the creep process. The tests were conducted at low shear stresses, in the constant shear stress regime, on the temperature interval from 0 to 100°C using a parallel-plate viscometer. We have shown, for the first time, by a viscometric method that in the high-elastic state rubbery polymers have a particular equilibrium structure corresponding to each temperature. A reduction in temperature leads to molecular ordering processes associated with an increase in the viscosity of the polymer. A temperature rise leads to molecular disordering and a gradual fall in viscosity. At a given temperature the two processes converge to the same value of the equilibrium viscosity. The rate of the process of equilibrium structure formation is determined not only by the temperature but also by the nature of the polymer. The higher the polarity, the more slowly the equilibrium structure is established.Moscow Lenin State Pedagogical Institute, Problem Laboratory of Polymer Physics. Translated from Mekhanika Polimerov, No. 6, pp. 970–974, November–December, 1969.     

Elastic properties of certain polymer matrices at low temperatures

A study is made of the viscoelastic behavior of the polyimide DFO and polybenzoxasol (PBZ) at low temperatures. Pulsed ultrasound is used to measure ultrasonic velocity in polymers at the frequency f=5 MHz within the temperature interval 4.2–240 K. Data obtained from acoustic measurements is used to calculate the dynamic elastic modulus, shear modulus, compressive bulk modulus, Poisson's ratio, and certain thermophysical characteristics (Debye temperature, specific heat, coefficient of linear expansion, Grünheisen parameter). It is established that the investigated polymers have low dynamic moduli at 4.2 K. Their values change by 18–20% within the temperature range 4.2–240 K. The polymers also have a large free volume at the temperature of liquid helium. The results show that polyimide DFO and PBZ have good service properties at low temperatures.Paper presented at the IX International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Moscow State Academy of Automotive and Tractor Engineering, Russia. Translated from Mekhanika Kompozitmykh Materialov, No. 4, pp. 454–459, July–August, 1996.     

Thermal dependence of electric conductivity in thermoplastic composites

Thermal dependence of the electric conductivity of thermoplastic composites based on both amorphous (hiPS) and crystallized (PP) polymers is investigated in this study. Two types of carbon black fillers with different values of BET surface area were used as charge conductors. Composites based on crystallized polymer matrices indicate the sharp growth of electric resistivity just before the melting range. This maximum is followed by substantial decrease of resistance at T > T_melt. With the decrease of carbon black concentration both relative growth of resistance at the T T_melt and further dropping resistance at T > T_melt increase. Composites filled with particles of higher surface area are characterized by less pronounced matrix influence on thermal dependence of electric conductivity than composites filled with particles of lower surface area; this can be caused by more pronounced matrix/filler interaction in the first case. The range of temperatures at which the resistance increase occurs does not depend on the type of carbon filler and its concentration. Composites with amorphous matrices are characterized by distinct increase of resistance above glass transition. Thermal treatment of the sample significantly affects the initial values and intensity of the temperature dependence of the resistance.To be presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 526–532, July–August, 1995.