Molecular motion and distinctive features of the strength of thermotropic liquid-crystal oriented polymers

Nuclear magnetic resonance (NMR) is used to study molecular motion in two fully aromatic oriented liquid-crystal (LC) polymers (the copolymer with the brand name Vectra and poly-paraphenylphenyleneterephthalate) at high temperatures up to 610 K. Above the temperature of the thermotropic transition, a fine structure is observed in the NMR spectra analogous to structures seen previously only in studies of low-molecular-weight LC materials. Analysis of the results indicates that macromolecules in the LC state participate in cooperative types of motion. The relation between structural rearrangements and hardening during thermal processing is discussed, along with the role of distinctive features in the molecular motion in the mesophase. Fiz. Tverd. Tela (St. Petersburg) 40, 1173–1177 (June 1998) The authors dedicate this article to the memory of S. N. Zhurkov.     

Structural micromechanics of oriented polymers

To clarify whether the interfibrillar slippage occurs on plastic deformation of oriented polymers, flow creep of ultrahigh molecular weight polyethylene (UHMW PE) samples with various connectedness of microfibrils has been studied in a dead load mode at room temperature. The flow creep rate of melt-crystallized and gel-cast UHMW PE films drawn to various draw ratios, as well as of modified gel-crystallized samples (cross-linked/grafted or washed free of low molecular weight fraction) has been measured with the help of a unique laser interferometric technique (Doppler creep rate meter). The technique allows one to measure creep rates for deformation increments as small as 0.3 μ within an accuracy 1%. The interferometric technique enabled us to observe an extremely high variability of flow creep rate. It was recognized that the creep process accelerates or slows from time to time. A length of a loaded sample increased by multiple consecutive deformation jumps (or steps). The size distribution of the steps appeared to be controlled by the structure of interfibrillar regions. The influence of the latter on the variability of creep rate confirms a hypothesis that suggests a contribution of interfibrillar slippage to plastic deformation of oriented polymers. The observed phenomenon has been attributed to stick-slip motion of microfibrils and their aggregates sliding on each other under the action of applied stress. It was found that the creep rate decreases with increasing interfibrillar interaction.     

Direction-direction correlations of oriented polymers

We calculate the direction-direction correlations between the tangent vectors of an oriented self-avoiding walk (SAW). LetJ (x) andJ ^v (0) be components of unit-length tangent vectors of an oriented SAW, at the spatial pointsx and 0, respectively. Then for distances |x| much less than the average distance between the endpoints of the walk, the correlation function ofJ (x) withJ ^v (0) has, ind dimensions, the form . The dimensionless amplitudek(d) is universal, and can be calculated exactly in two dimensions by using Coulomb gas techniques, where it is found to bek(2)=12/25 ². In three dimensions, the -expansion to second order in together with the exact value ofk(2)in two dimensions allows the estimatek(3)=0.0178±0.0005. In dimensionsd4, the universal amplitudek(d) of the direction-direction correlation functions of an oriented SAW is the same as the universal amplitude of the direction-direction correlation functions of an oriented random walk, and is given byk(d)= ²(d/2)/(d–2) ^d .     

Microdeformational behavior of oriented semicrystalline polymers

A new experimental approach to the evaluation of microdeformational properties on the supermolecular level of highly oriented semicrystalline polymer is proposed. It is based on small-angle x-ray scattering measurements in situ during elastic and plastic tensile deformation of oriented samples along the orientation axes. The main idea of this approach consists of consideration of the competition of two processes occurring in uniaxial deformation: (1) intrafibrillar deformation of long periods; (2) mutual interfibrillar slip. A quantitative experimental criterion of inhomogeneity of intrafibrillar deformation is introduced. The combination of inhomogeneous fibril deformation and interfibrillar slip can explain all kinds of relationships between micro- (long period) and macrodeformations. The proposed approach was tested for several highly oriented polymer films and fibers.     

Molecular mobility and strengthening of oriented liquid-crystal polymers

Broad-line proton magnetic resonance has been used to study cooperative mesophase molecular motion in the example of a fully-aromatic oriented liquid-crystal polymer Vectra A950. The molecular mobility is found to decrease as a result of heat treatment, which is considered to be the reason for a higher tensile (rupture) strength owing to an increase in the activation energy for the fracture process. Fiz. Tverd. Tela (St. Petersburg) 41, 859–861 (May 1999)     

Mechanisms of reversible thermal deformation of oriented polymers

Reversible thermal deformation coefficients (TDCs) of oriented samples of a flexible-chain polymer (polyethylene) and of a number of rigid-chain polymers were measured in the longitudinal and transverse directions near room temperature. The same samples were used to measure the TDCs of crystallites by x-ray diffraction. The magnitudes of the TDCs of macroscopic oriented samples and of constituting crystallites and the characteristics of the thermal deformation of flexible-chain and rigid-chain polymers are compared. A conclusion is made that the mechanisms that determine thermal deformation in the longitudinal and transverse directions for the flexible-chain and rigid-chain polymers are different.     

Grain boundary influence on the mechanical behaviour of oriented bicrystals

It is shown through several experiments centred on dislocation transmission through a GB that relating macroscopic mechanical properties of a bicrystalline specimen to the atomic structure of the GB or to local dislocation reactions is not straightforward. Not only the long and short range stresses and the plastic properties of the two grains must be taken into consideration, but also the kinetics of events has to be taken into account to explain the final result.     

Initial stage of stress relaxation in oriented polymers

The initial stage of stress relaxation, i.e., a decrease of the stress with time in an extended solid at a fixed length of the specimen, has been studied in oriented fibers of a linear polymer, polypropylene, chosen as the object of investigation. Experimental data have been obtained, and the stress in the object from the beginning of tensile loading of the specimen has been analyzed. It has been found that the stress relaxation caused by thermal fluctuation conformational transitions in stretched chain polymer molecules occurs from the beginning of the stretching of the polymer fiber. The time dependence of the stress relaxation of polypropylene fibers in all initial stages of deformation of the fibers has been elucidated.     

Structural conditions of deformation and fracture of oriented crystalline polymers

Deformation of supermolecular structure elements of oriented crystalline polymers and nucleation of initial submicroscopic cracks induced by stress have been studied by the small-angle x-ray scattering technique. It is shown that the intrafibrillar amorphous interlayers have low strength and high deformability. The rupture of the weakest amorphous interlayers leads to nucleation of initial submicrocracks. The influence of submicrocracks on deformation around such cracks is revealed. The micromechanics of deformation and fracture of polymers is discussed.     

Micromechanics of fracture initiation in highly oriented polymers

Several analytical techniques have recently been used to investigate molecular bond rupture in materials. These are compared and discussed in terms of suggested molecular models of fracture in highly orientated polymer fibers and films.     

On one supramolecular mechanism of the nonlinear viscoelasticity of oriented polymers

A phenomenological model of the viscoelasticity of highly oriented polymer systems is developed based on the results of studying the relaxation of such systems (mainly, fibers of polyethylene terephthalate, polyamide-6, polyvinyl alcohol, and other polymers) in the loaded state. The effect of an applied load on their relaxation spectra agrees qualitatively with the deformation behavior of crystal-like bandles that are present in amorphous intercrystalline layers of the fibrillar supramolecular structure.     

Calculation of the intensity and dichroism of the IR absorption bands of oriented polymers

An algorithm for calculating the intensity and dichroism of polarized IR absorption bands of oriented polymers is proposed. The absorption curves in the IR spectrum of amorphous cis-1,4-polyisoprene and the dichroism of its absorption bands have been calculated. The polarization IR spectra of oriented amorphous cis-1,4-polyisoprene for a stretching ratio equal to 3 have been constructed. The calculated values of the absorption-band dichroism are in fairly good agreement with the experimental data.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 129–132, January–February, 2005.     

Interfaces between nanostructures and stepwise creep in highly oriented polymers

A comparative laser-interferometric study of steady creep in oriented ultrahigh-molecular-weight polyethylene films differing in the structure of interfaces between nanosized structural units has been carried out to gain a better understanding of the creep mechanism in oriented polymer materials. In contrast to conventional methods, laser interferometry permits measurement of creep rates from very small strain increments (0.3 μm) to within 1%. This technique made it possible to detect the stepwise nature of plastic deformation in creep. The data obtained suggest that the creep rate and its periodic changes are controlled by the structure of the interfaces, and that the plastic deformation itself occurs to a considerable extent through shear of nanosized structural units relative to one another by an “acceleration-deceleration” type. It is proposed that the “deceleration” phase is due to a glide resistance created by some “stoppers” having either physical or chemical nature, which become destroyed and reappear again in the course of creep. Fiz. Tverd. Tela (St. Petersburg) 41, 1788–1791 (October 1999)     

Dynamics of the strength of adhesive-bond joints between polymers and steel

We have investigated the time dependences of the strength of adhesive bonds between polymers (epoxy adhesive and polyamide) and steel 45 at various temperatures. These dependences have been determined based on the rates of two opposite processes, viz., an increase in strength due to the formation of coordination bonds between polymer molecules and Fe²⁺ ions and a decrease in strength due to ruptures of adhesive or polyamide molecules under the effect of internal stresses. The values of activation energy of these processes have been determined.     

Second moment of NMR spectra for oriented partially crystalline polymers

The theory of the second moment of NMR spectra of oriented partially crystalline polymers is developed in this paper. Two phase model of the structure of polymer is considered in deriving of second moment of NMR spectra. Possibilities of characterization of the degree of orientation of macromolecular chains separately in crystalline and noncrystalline regions of the polymer are discussed in this paper.     

Plastic coating of glass fibers and its influence on strength

Abstract

Sodium borosilicate glass fibers pulled from the melt were plastic coated “on-line” to preserve the fiber strength. By the use of tapered nozzles within the coating applicator the coating could be made concentric around the fiber, and a 40-μm concentric coating was found to be sufficient for the fiber to withstand a mild abrasion test with no deterioration in strength. Strength measurements made in ambient conditions showed that the fiber had a uniform breaking strain of 4% elongation. The same fiber measured at liquid nitrogen temperatures had a breaking strain of 14%, even after extensive handling, which is close to the theoretical glass cohesive strength.     

Plastic coating of glass fibers and its influence on strength

Sodium borosilicate glass fibers pulled from the melt were plastic coated “on-line” to preserve the fiber strength. By the use of tapered nozzles within the coating applicator the coating could be made concentric around the fiber, and a 40-μm concentric coating was found to be sufficient for the fiber to withstand a mild abrasion test with no deterioration in strength. Strength measurements made in ambient conditions showed that the fiber had a uniform breaking strain of 4% elongation. The same fiber measured at liquid nitrogen temperatures had a breaking strain of 14%, even after extensive handling, which is close to the theoretical glass cohesive strength.