Rheology and molecular structure of a polyethylene melt. 3. Relaxation spectra and characteristic relaxation time

By comparing the relaxation spectra respectively calculated on the basis of the iterative and approximation methods, it is confirmed that the latter may be used to describe the viscoelastic properties of polyethylene melts. A method is proposed for identifying the form of the frequency relaxation spectrum by three characteristic numerical quantities. The effect of the molecular-mass, the molecular-mass distribution, and the long-chain branching characteristics on the form of the frequency relaxation spectrum is considered. The question of the geometrical similarity of the relaxation spectrum is discussed, and it is shown that, in general, these spectra are not linearly similar. The asymmetry of the spectrum depends solely on the index representing the rate of fall of the dynamic viscosity with increasing frequency of deformation. Methods of determining the characteristic relaxation time without constructing a frequency relaxation spectrum are proposed; ways of determining the characteristic relaxation time from experimental data are indicated.For communication 2, see [10].Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 322–330, March–April, 1976.     

Thixotropy of filled-polyethylene melts subjected to periodic shear deformation

As a result of the action of periodic finite-amplitude shear deformations filled high-pressure polyethylene melts exhibit thixotropic properties. The effect of the previous deformation history on the frequency dependence of the complex dynamic shear modulus is investigated.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR Riga. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 927–931, September–October, 1968.     

Acoustic determination of molecular orientation in polymers

On the basis of the results of measurements of the ultrasonic propagation velocity in polyethylene terephthalate fibers it is shown that the Moseley formula is suitable for determining the molecular orientation of both amorphous and amorphouscrystalline polymers. The dependence of the molecular orientation factor of amorphous and crystalline specimens of polyethylene terephthalate on the draw temperature and draw ratio is investigated. The effect of polymer orientation on the ultrasonic shear-wave velocity in the fiber is studied.Kiev Branch, All-Union Scientific-Research Institute of Artificial Fibers. Translated from Mekhanika Polimerov, No. 1, pp. 26–34, January–February, 1976.     

Molecular destruction of polymers at the apex of a major crack

We employed infrared spectroscopy and infrared spectroscopy of deflected total internal reflection to study the rupture of polymer macromolecular chains (polyethylene and polypropylene) at the apex of a growing crack. It was shown that the break concentration at the crack surface is of the same order of magnitude as the total concentration of chemical bonds in the polymer; molecular rupture occurs not only at the apex itself but also at some distance from it. The dependence of the break concentration on the distance from the crack apex was measured. The observed patterns were related to the experimentally determined external-load distribution function for individual chemical bonds near the crack apex.I. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 621–625, July–August, 1972.     

Biaxial orientation of amorphous linear polymers

The dependence of the birefringence and orientation stress on the biaxial stretch ratio and orientation conditions has been experimentally investigated. The temperature dependence of these characteristics is explained in terms of the network structure of amorphous polymers. It is shown that the transformations of the supermolecular structures in the process of biaxial orientation depend on the orientation temperature — at higher temperatures better organized structures are formed. There is a formal relationship between the effect of orientation temperature on supermolecular structure formation and on the relaxation process responsible for the formation of a more thermally stable molecular network.Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 17–23, January–February, 1971.     

Moduli of elasticity of polymer melts

The physical significance of the so-called initial modulus, determined in measuring the rheological properties of thermoplastic melts from the initial slope of the time dependence of the stresses at given constant shear rate, is considered. It is shown that this quantity is related with the ratio of the shear and normal stresses.Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 534–538, May–June, 1969.     

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.     

Investigation of the dynamic fatigue of composite polymer materials

The dynamic fatigue of composite film materials has been studied as a function of the frequency and amplitude of deformation. The tests were carried out on a vibration apparatus with a frequency range from 10 to 600 cps. The test objects consisted of foil packaging with a polyethylene backing (foil-film).The dependence of the dynamic fatigue of foil-film on amplitude, frequency, and acceleration was studied. It has been shown that in all cases failure of the material is preceded by cracking of the foil and peeling of the foil from the polyethylene backing at the site of the cracks. The material fails as a result of puncture of the polyethylene backing by the broken edge of the cracked foil.Mekhanika Polimerov, Vol. 1, No. 5, pp. 90–94, 1965     

Effect of uv irradiation on the mobility of the molecular chains in polyethylene

Conclusions 1. A decrease in the mobility of molecular chains in polyethylene after UV irradiation was discovered by the spin probe method. This decrease is proportional to the time of irradiation.2. A discontinuity lacking in unirradiated polyethylene was found on the curve for the logarithm of the rotational correlation time of the spin probe in polyethylene as a function of the inverse temperature. It was shown that the nature of the discontinuity is related to a "thawing out" of the rotational motion of the chains in the crystalline phase.Institute of the Mechanics of Metal-Polymer Systems, Academy of Sciences of the Belorussian SSR, Gomel. Translated from Mekhanika Polimerov, No. 4, pp. 603–608, July–August, 1976.     

Deformation of elastic polyurethanes with a bimodal cellular structure

This article examines the cellular structure and mechanical properties of elastic foamed polyurethanes with a bimodal cellular structure (BFPUs). It is shown that the sizes of the oval cells (3–12 mm) in the elastic foamed polyurethenes that are studied are a tenth of an order greater than the sizes of the polyhedral cells (0.1–0.6 mm). Conversely, for rigid foamed polyurethanes, the polyhedral cells are more than a tenth of an order larger than the oval cells (microcells). The equations of the compression curve of BFFUs are found, and it is established that the deformation of BFPUs is determined by the deformation function of the cellular structure and the viscoelastic properties of the polymer matrix. The deformation functions of the cellular structure and the relaxation properties of BFPUs are determined. It is shown that choosing BFPUs with a cellular structure improves the comfort properties of the foamed material: the softness coefficient increased by 8–19% in the investigated case, while the support coefficient increased by 15–35%.Paper presented at the IX International Conference on the Mechanics of Composite Materials (Riga, 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 3, pp. 330–339, May–June, 1996.     

Viscoelastic characteristics of filled modified polyethylene melts

The effect of modifying polyethylene with maleic acid on the viscoelastic characteristics of filled melts has been experimentally investigated.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 558–560, May–June, 1971.     

Propagation of sound in oriented polymers

The propagation of sound in oriented polymers is considered. Starting from an assumption concerning the superposition of relaxation processes in polymers, expressions are obtained for the speed and absorption of sound as functions of the relaxation spectrum, molecular orientation, and crystallinity. It is shown that the orientation dependence of the acoustic properties is more distinctly expressed in the region of the glassy state.Scientific-Research Institute of Plastics, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 437–441, May–June, 1969.     

Rheology and molecular structure of polyethylene melts

Conclusions 1. The molecular structure indices of low-density polyethylene (LDPE), i.e., the weighted-average (wt.-av.) molecular weight, the number of branch points of long-chain branchings per molecule, and the polydispersity index, were calculated from the data of gel-permeation chromatography, assuming that the Mendelson—Drott hypothesis holds. They show a fairly close correlational relationship to the index of the mechanical characteristics of this melt.2. The correlation between the highest Newtonian viscosity and the molecular characteristics, taking into account the polydispersity index, is improved if the low-molecular-weight part of the molecular weight distribution MWD (M < 800) is neglected.3. The index of the exponential equation, which determines the dependence of the highest Newtonian viscosity of LDPE on the wt.-av. molecular weight, increases when the influence of the polydispersity is taken into account.4. Commercial samples of LDPE synthesized under similar technological conditions show a fairly good correlation between the polydispersity and the degree of branching of the molecule.5. From the highest Newtonian viscosity, the specific relaxation time of the LDPE, and the dependence of the degree of branching of the molecules on the polydispersity of the molecular weights, common for the given technology of the synthesis, it is possible to calculate the molecular structure indices of LDPE with satisfactory accuracy.For communication 3, see [3].The Walter Ulbricht Enterprise "Leunawerke," German Democratic Republic. Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. The Okhtinsk Scientific and Production Union "Plastpolimer," Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 283–293, March–April, 1977.     

Region of linearity of the deformation properties of contact-molded glass-reinforced plastic

A study was made of the creep and recovery of contact-molded glass-reinforced plastic in different directions in the plane of the sheet in uniaxial tension. On the basis of experimental data, the region of linearity of the deformation properties of the material under investigation was established. The influence functions in the relations of the theory of viscoelasticity were taken in the form proposed by Bronskii. The dependence of the parameters which determine these functions on direction was investigated. It is shown that, in practice, this dependence is inherent in only one of the parameters, and a relationship which is in satisfactory agreement with the experimental results is proposed to describe it.Moscow Institute of Chemical Machine Building. Scientific-Research Institute for Mechanics, M. V. Lomonosov State University, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 404–410, May–June, 1970.     

Predicting the performance of extruded low-pressure polyethylene from rheological characterstics

Characteristic features of the high-elastic stalling of the melts of modern types of low-pressure polyethylene is investigated. The possibility of predicting the performance of polyethylene pipes from measurements of the following rheological characteristics is demonstrated: melt viscosity, shear-rate gradient at the outset of high-elastic stalling, swelling of the extrudate, and the amplitude of self-sustained fluctuations in shear stress.Kazan' State Technical University, Tatarstan, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 4, pp. 558–563, July–August, 1996.     

Temperature - time analogy for thermorheologically complex polymeric materials

Conclusions 1. The viscoelastic properties of mixtures of crystalline polymers with elastomers have been studied as functions of temperature. It is shown that, for mixtures of PP with PIB and TEP, the concept of thermorheologically simple behavior is applicable only within a narrow interval of temperature.2. Parameters of the temperature—time analogy for thermorheologically complex materials have been calculated. It is established that, at temperatures above a characteristic temperature T*, a strong dependence of the correction coefficient on time and a strong dependence of the relaxation function on the ratio of components exist.3. It is shown that use of the concept of thermorheologically complex behavior for prediction of the long-term deformability allows analysis of the contributions of specific components to the viscoelastic behavior of the mixture in different temperature intervals.Paper presented at the Third All-Union Conference on Polymer Mechanics, Riga, November 10–12, 1976.Okhtinsk Scientific-Industrial Association "Plastpolimer," Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 614–620, July–August, 1977.     

Rheological characteristics of melts of low-molecular-weight branched polyethylene in the region of critical molecular weights

The viscosity and the first difference of the normal stresses have been measured at stationary shear rates from 5.68 to 4500 sec^–1 for melts of low-density polyethylenes obtained by thermal degradation of commercial polyethylene in a nitrogen atmosphere. In addition, the complex modulus has been measured on the angular frequency range from 0.396 to 198.5 sec^–1. Three regions of molecular weights differing with respect to the value of the exponent in the relation ₀ = kM have been established. At > normal stresses are observed in the region of shear rates corresponding to a linear dependence of the shear rate on the shear stresses.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga; Plastpolymer Research and Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 6, pp. 963–967, November–December, 1973.     

Infrared spectroscopic study of the prefracture state of rigid polymers

A correspondence has been established between the effects of stress and temperature on the half-width and the position of the maximum of the 972-cm^–1 absorption band for polyethylene terephthalate. It is shown that the prefracture state of polyethylene terephthalate is a softened state.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 520–523, May–June, 1976.     

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.     

Effect of modifying additions on the physicomechanical properties of composites based on polyethylene and wastes from linen yarn production

This article examines the feasibility of using coupling agents to alleviate the shortcomings characteristics of materials that contain natural fibers: low water resistance and a high degree of heterogeneity. A determination is made of the effect of additions of polyisocyanate and stearic acid on the fluidity of the melts and the strainstrength properties and water resistance of polyethylene composites containing mixtures of wastes from linen yarn production. It is shown that an addition of stearic acid significantly improves the dispersion of fibers in the composites, which in turn leads to a reduction in melt fluidity and an increase in elastic modulus in the high-filler-content region compared to composites that do not have additions of stearic acid. Additions of polyisocyanate appreciably increase the strength and water resistance of the given composites.Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 4, pp. 540–547, July–August, 1997.