Rheology and molecular structure of polyethylene melts. 2. Effect of molecular structure on the viscoelastic characteristics

The viscoelasticity of low-density polyethylene is investigated as a function of molecular structure. The viscoelasticity is characterized by three numbers — the zero-shear viscosity, the characteristic relaxation time, and the index of the rate of fall of dynamic viscosity with deformation frequency. The molecular structure is also characterized by three numbers — , , g_w. The zero-shear viscosity is shown to depend on the polydispersity of the molecules and a corresponding analytic expression is obtained. The method of determining the characteristic relaxation time from the maximum of the frequency relaxation spectrum is justified and an expression giving the characteristic relaxation time as a function of the principal characteristics of the molecular structure is derived. It is shown that long-chain branching of the molecules has an incommensurably greater (several orders) effect on the rheological characteristics of the branched polymer than the polydispersity of the molecular weight. The effect of the polydispersity of the molecules of commercial low-density polyethylenes on the viscoelastic characteristics cannot be established without taking the branching of the molecules into consideration.For communication 1 see [1].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 120–127, January–February, 1976.     

Correlation of the effective and complex dynamic viscosities of viscose

The results of comparative measurements of viscosity of viscose in the steady-state flow and harmonic vibration regimes are presented. It is shown that in order to describe the dependence of the effective viscosity on the velocity gradients in the steady-state flow regime it is possible to employ the mathematical apparatus of the phenomenological theory used in dynamic investigations. The operating regimes of automatic viscometers intended for measuring the maximum Newtonian viscosity of viscose are determined.All-Union Scientific-Research Institute of Synthetic Fiber, Moscow Region. Translated from Mekhanika Polimerov, No. 4, pp. 756–759, July–August, 1973.     

Wear resistance of polyethylene

The effect of the molecular weight and branching of polyethylene on its wear resistance in direct contact with abrasive particles has been investigated. The abrasive wear resistance of the polyethylene increases with its molecular weight. Copolymers have better wear resistance than homopolymers."Plastolimer" Research and Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 542–544, May–June, 1971.     

Rheological characteristics of a low-molecular branched polyethylene melt near the melting point

Data are presented on the rheological characteristics of low-density polyethylene with near-critical molecular weight at 110°C. A comparison of the shear rate dependence of the viscosity in repeat measurements indicates that the material has a kinetic memory for isothermal, shear flow-initiated crystallization from the melt.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga; Plastpolimer Research and Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 1129–1132, November–December, 1973.     

Mechanical Properties of Blends of Low-Density Polyethylene with Chlorinated Polyethylene

The results of experimental investigation into the mechanical properties of blends of low-density polyethylene (LDPE) with chlorinated polyethylene (CPE) in tension are presented. The specimens of pure LDPE, CPE, and nine types of LDPE/CPE blends, with different content of components at 10 wt.% intervals, were examined. Data on the influence of blend composition on the tensile stress-strain diagram, elastic modulus, yield stress, breaking stress, and ultimate elongation are obtained. The results of investigations of creep are also reported. It is found that the creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 3, pp. 391–404, May–June, 2005.     

Viscosity of lignin-modified polypropylene determined by capillary viscometry

The properties of polypropylene containing 2, 5, 10, and 15% alkali sulfate lignin have been studied by the method of capillary viscometry at temperatures of 190, 230, and 250°C. It has been shown that introducing lignin, irrespective of the concentration, leads to an increase in the Newtonian viscosity of the melt and to an increase in the weighted-mean molecular weight. An analysis of the apparent relative activation energy of viscous flow shows that the greatest effect is obtained by introducing from 2 to 5% lignin. The results obtained permit a hypothetical conclusion concerning the mechanism of the crosslinking process (possible "ladder" structure).Mekhanika Polimerov, Vol. 3, No. 2, pp. 200–203, 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     

Study on the impact-shear viscosity of block polymers

The temperature dependence of the specific impact viscosity of several amorphous and amorphous-crystalline polymers as measured in tests by means of the authors' impact-cutting method is discussed. It is shown that this dependence agrees fairly well with the mechanical losses determined at the same interaction frequencies and, hence, is sensitive to the relaxation transitions in polymers (an enhanced impact stability corresponds to the transition regions). The similarity mentioned above and the correspondence between the temperature dependences of the density, the Poisson coefficient, and the elasticity modulus suggests that mechanical energy supplied is dissipated by a molecular-motion type including segment motion and relating to melting of the crystallites.S. M. Kirov Leningrad Institute of Textile and Light Industry. Translated from Mekhanika Polimerov, No. 2, pp. 226–230, March–April, 1972.     

Viscosity of melts obtained from polymers with a wide spread of particle size

By comparing existing relationships between the maximum viscosity and the molecular weight and between the effective viscosity and the shear velocity, the effect of the molecular-weight distribution of linear polymers on the shear velocity is analyzed.Scientific-Industrial Combine Plastopolimer. Scientific-Research Institute of Polymerized Plastics, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 753–756, July–August, 1972.     

Viscoelasticity of a curing epoxy composition in the "critical branching" region

The curing of an epoxy resin in the "critical branching" region has been investigated by mechanical and infrared spectroscopy. It is shown that the effective molecular weight between network points (M_c) during the polymerization of an actual system can be determined from the relaxation spectrum in the curing stage investigated.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 1, pp. 154–158, January–February, 1976.     

Optimal control of non-isothermal viscous fluid flow

For flow inside a four-to-one contraction domain, we minimize the vortex that occurs in the corner region by controlling the heat flux along the corner boundary. The problem of matching a desired temperature along the outflow boundary is also considered. The energy equation is coupled with the mass, momentum, and constitutive equations through the assumption that viscosity depends on temperature. The latter three equations are a non-isothermal version of the three-field Stokes–Oldroyd model, formulated to have the same dependent variable set as the equations governing viscoelastic flow. The state and adjoint equations are solved using the finite element method. Previous efforts in optimal control of fluid flows assume a temperature-dependent Newtonian viscosity when describing the model equations, but make the simplifying assumption of a constant Newtonian viscosity when carrying out computations. This assumption is not made in the current work.     

Enumeration of Wiener indices in random polygonal chains

The Wiener index of a connected graph (molecule graph) G is the sum of the distances between all pairs of vertices of G, which was reported by Wiener in 1947 and is the oldest topological index related to molecular branching. In this paper, simple formulae of the expected value of the Wiener index in a random polygonal chain and the asymptotic behavior of this expectation are established by solving a difference equation. Based on the results above, we obtain the average value of the Wiener index of all polygonal chains with n polygons. As applications, we use the unified formulae to obtain the expected values of the Wiener indices of some special random polygonal chains which were deeply discussed in the context of organic chemistry or statistical physics.     

Effect of antioxidants on the changes in the effective viscosity and tensile strength of polypropylene during processing and aging

The effective viscosity of an isotactic polypropylene (IPP) melt during processing and aging has been investigated together with the changes in the mechanical properties of stabilized IPP during aging. In the presence of polymeric antixoidants there is an increase in the effective viscosity of the IPP melt attributable to interaction with the degraded IPP molecules. During the aging of IPP articles the characteristic properties of polymeric antioxidants associated with their high molecular weight are not especially in evidence.Moscow Lomonosov Institute of Fine Chemical Technology. Translated from Mekhanika Polimerov, No. 5, pp. 938–941, September–October, 1970.     

Friction and rheological properties of resin-impregnated glass tape

The dependence of the dynamic coefficient of friction of resin-impregnated glass tape on the sliding speed over the mandrel surface and the viscosity of the resin is derived. The formulas obtained are confirmed by the results of experiments which indicate a linear dependence of the coefficient of friction on the sliding speed of the tape over a metal mandrel. Graphically, this dependence is expressed by straight lines with a common origin and a slope that increases with the viscosity of the resin. The coefficient of friction of glass tape sliding over a prewound layer at low speeds also has a linear dependence. At high speeds the dependence takes the form of a damped curve.Translated from Mekhanika Polimoerov, No. 1, pp. 166–170, January–February, 1976.     

Rheology and molecular structure of polyethylene melts 1. Melt identification from mechanical properties

It is proposed to use periodic deformation for the identification of the molecular structure of polyethylenes. By comparing the dependence of the complex and apparent viscosities on frequency and shear rate, respectively, it is shown that these functions are not equivalent for low-density polyethylene melts. The need for a numerical characteristic of the rheological functions, which should be related to the numerical characteristics of the molecular structure of the polymer, is established. The possibilities of using generalized relations for investigating the molecular structure are discussed. It is shown that the dynamic viscosity of polyethylene melts, described by a three-parameter equation, quite accurately reflects the viscoelastic properties of polyethylene melts and makes it possible, with the aid of the calculated function of the relaxation spectrum, to construct a frequency dependence of the modulus corresponding to the experimental data. Three numbers characterizing the viscoelasticity of polyethylene, which must be related with three molecular characteristics, are established.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 523–532, May–June, 1975.     

Effect of the composition and concentration of fibrous wastes from linen yarn production on the physicomechanical properties of polyolefin composites

An analysis is made of the feasibility of using wastes from the production of linen yarm (scutch, spinning and carding refuse) to modify low-density polyethylene. The effect of the composition and concentration of wastes on the processing properties (melt index), physicomechanical indices (tensile strength, elastic modulus in bending), and water resistance of composites based on low-density polyethylene is studied. It is found that the melt index decreases with an increase in the content of filler and that even a composite with a high filler content (40–50% by weight) maintains values of 0.2–0.3 g/10 min. The elastic modulus in bending increases with an increase in the content of waste, regardless of the nature of the latter. Tensile strength increases slightly and depends on the choice of filler. This result, combined with the reducation in the scatter of values of elastic modulus with an increase in filler concentration, is an indicator of the relatively high degree of heterogeneity of the systems that were studied. The water resistance of the composites decreases with an increase in filleer content. To reduce the heterogeneity of the systems, mixing should be intensified and modifiers such as stearic acid and polyisocyanates should be added.Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 3, pp. 408–416, May–June, 1997.     

On the theory of non-Newtonian polymer flow

The Eyring-Frenkel theory of viscosity of low-molecular liquids has been extended to solutions of high-molecular compounds. It is shown that there are flow units of different sizes in the system, their mean size being proportional to the molecular weight of the polymer. An expression is obtained for the non-Newtonian viscosity of polymer solutions. In the limiting case of high shear rates the viscosity of the solution coincides with that of the solvent. At low shear rates Flory's empirical relation for the viscosity of polymer solutions is theoretically obtained.Mekhanika Polimerov, Vol. 2, No. 5, pp. 779–784, 1966     

Effect of the molecular weight of polystyrene on the viscosity of concentrated solutions

The viscosity of solutions of polystyrene of various molecular weights (from 1.04 · 10² to 3.8 · 10⁵) in a poor solvent (decalin) and a good solvent (ethylbenzene) has been measured at temperatures from 15 to 70°C over a broad range of shear stresses from 10² to 10⁶ dyne/cm². The nature of the solvent has a considerable influence on the critical molecular weight and the absolute value of the viscosities of the solutions over the entire range of molecular weights and on the form of the flow curves of decalin solutions of polystyrene as a function of temperature. The heat of activation of viscous flow increases with increase in molecular weight and shear stress on the interval 20–80°C. The results obtained are explained in terms of the effect of the molecular weight of the polymer, the nature of the solvent, stress and temperature on structure formation in the solution and on the orientation of the macromolecules and structures in the flow process.Ural Gor'kii State University, Sverdlovsk. Translated from Mekhanika Polimerov, No. 5, pp. 920–926, September–October, 1970.     

Effect of orientation and molecular weight on the modulus of elasticity of polymer materials

The modulus of elasticity of a perfectly crystalline polymer is calculated as a function of the orientation of the crystallites. The calculations are based on the mechanics of a micro-inhomogeneous continuum. The dependence of the modulus of elasticity on crystallite orientation and molecular weight is calculated with reference to the example of crystalline kapron.Moscow-Lenin Pedagogical Institute. Problem Laboratory of Polymer Physics. Translated from Mekhanika Polimerov, Vol. 4, No. 6, pp. 1002–1007, November–December, 1968.