Deformation Of Polyethylene Research Articles

Recent Developments in Small-Angle X-Ray Scattering for the Study of Metals and Polymers

New applications of Small-Angle X-ray Scattering in Materials Science of metallic alloys and polymers are presental. The use of powerful X-ray ources like synchrotrous is shown to enable in-situ study of dynamic system changes. Examples are given m welding of precipitation hardening aluminium alloys, where precipitate microstructures in welded structures are studied with high resolution and simulated through non-isothermal heat treatments. The interest of in situ measurements is alse shown for polymeric systems. Demixion phenomena occuring in the DGEBA/MCDEA/PEI modified thermoset system far from the critical concentration is studied during the curing of the epoxy amide polymerization. A methodology showing evidence of a nucleation/grewth mechanism is presented. In addition, scattering measurements during deformation of polyethylenes is shown to yield the density of the molecules that are in direct relation to their long term properties.

The effect of strain rate, temperature, and molecular mass on the tensile deformation of polyethylene

AbstractConventional tensile testing applied to high density polyethylene can lead to erroneous impressions of the tensile response of the material due to a local reduction in cross section of the sample. Several workers have developed novel tensile testing techniques to measure the response of a small element as it deforms. The true stress true strain curve that results describes tensile deformation of the material in a geometry‐independent manner. Here, results from previous workers, together with some of our own, are interpreted in terms of the Haward‐Thackray spring‐dashpot model, in which the spring defines a strain hardening process according to the theories of high elasticity and the dashpot describes a strain‐independent viscous process. The effects that temperature, strain rate, and molecular mass have on each process are investigated. For a pipe‐grade, modified high density ethylene copolymer, sufficient data have been measured to interpret the effects of strain rate and temperature in accordance with an Eyring flow process, where the parameters for the two mechanisms are found to be similar.

A model for the large-strain deformation of polyethylene

A model for the large strain deformation of polyethylene under uni-axial tension is described. Based on observations made by other researchers, the model describes hardening at large strains by considering dislocation nucleation and reorientation of slip planes within the crystalline phase. The temperature and strain-rate sensitivity of this behaviour are predicted with reasonable accuracy in a pipe-grade high-density polyethylene. The model has only three adjustable parameters which are the crystal shear modulus, the Young's modulus and a critical dislocation length.

Plastic Deformation of Polyethylene Studied by Infrared Dichroism

Infrared dichroism measurements have been carried out to investigate the plastic deformation mechanisms of homogeneous and heterogeneous ethylene copolymers issued from the metallocene and the Ziegler–Natta catalyses, respectively. The two copolymers having the same density display significantly different plastic behavior that should result from different mechanisms of crystal slip. Such mechanisms are analyzed on the basis of the main crystal slip systems (100)[001] and (010)[001] having the higher shear compliances. Attention is focused on the Schmid factor, S, of the slip planes, which is the ratio of the resolved shear stress acting on each slip plane to the applied tensile stress. From the orientation functions of the three crystallographic axes of the orthorhombic unit cell, the Schmid factor of the two slip systems is determined as a function of plastic strain. The heterogeneous copolymer exhibits somewhat higher data for the (010)[001] slip, which suggest a preferred activation of this system during the yielding stage. In contrast, the homogeneous copolymer displays close values for the two slip systems, indicating that plastic flow is more likely governed by a combined activation of the two slip systems, and with eventual cross slip owing to the common [001] slip direction.

Deformation of the acetabular polyethylene liner and the backside gap

Polyethylene wear of acetabular components may cause osteolysis and aseptic loosening in total hip arthroplasty. Case reports of wear between the polyethylene liner and acetabular metal backing ( backside wear) have promoted speculation on the clinical importance of debris generation at this site and the potential importance of insert conformity. We examined 90 retrieved nonconforming acetabular components from a single manufacturer. Twenty-four liners (27%) showed polyethylene deformation corresponding to screw holes on the backside surface. The location of screw-hole deformations corresponded to the direction of polyethylene deformation (wear) on the articular surface in all cases. The polyethylene liners with backside deformations were significantly thinner than the liners without deformation ( P = .02). The presence of acetabular osteolysis did not significantly correlate with backside deformation, but there was a significant association between osteolysis and wear of the articular surface ( P = .0004). In this implant design, backside deformation (wear) may not be due to micromotion at the interface but to the viscoelastic nature of ultra-high-molecular weight polyethylene

Plastic deformation of polyethylene and ethylene copolymers: Part II Heterogeneous crystal slip and strain-induced phase change

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing with particular attention to the development of plastic instability. Heterogeneous crystal slip is suggested to take place when homogeneous crystal slip either is not allowed at the temperature and strain rate of the experiment or is exhausted owing to extension of the chain folds. The chain unfolding concomitant to the fragmentation of the crystalline lamellae is suspected to have a low strain hardening that is responsible for the phenomenon. Partial screw dislocations with a shorter Burgers vector than in the case of homogeneous slip are proposed to become operative because of the activation of 90 ° chain twists in the crystalline stems. Dissociation of dislocations into partials involves stacking faults in the orthorhombic structure that may turn into monoclinic structure through a martensitic-like transformation. Crystal slip is likely to concentrate in these faulty regions owing to the reduced molecular interactions and lower density. Two types of correspondence of the transformed monoclinic phase with the parent orthorhombic structure are observed. The modification of the chain-folding macroconformation as a function of the crystallinity of the materials is suspected to influence the transformation shear mode. © 1998 Chapman & Hall

Plastic deformation of polyethylene and ethylene copolymers: Part I Homogeneous crystal slip and molecular mobility

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing in parallel with the viscoelastic properties. Homogeneous plastic deformation is shown to take place at temperatures above the crystalline mechanical relaxation. The activation of homogeneous crystal slip is discussed in relation to the crystal lamella thickness and the molecular mobility of the crystalline chain stems. The thermally activated process of nucleation and propagation of screw dislocations that is proposed for the mechanism of the homogeneous crystal slip relies on the generation of 180° chain twists in the crystal stems of the sheared crystals. This kind of conformational chain defect is the basic link between the plastic and the viscoelastic properties of the materials. Homogeneous crystal slip can take place as long as the applied strain rate is consistent with the strain rate affordable by the screw dislocation propagation. The dependence on draw temperature of the crystal thickness in the fibre structure is ascribed to the stress-induced activation of 180° chain twists which allows an adjustment of the crystal thickness to the temperature of the experiment faster than an annealing treatment will. © 1998 Chapman & Hall

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation 3. Compression of polyethylene

The compression behaviour of a range of unoriented linear and branched polyethylenes was investigated using the technique of simultaneous small- and wide-angle X-ray scattering during deformation. Interlamellar deformation mechanisms, including simultaneous shear and rotation of lamellar stacks, were followed by crystallographic deformation mechanisms, including (100)[001] chain slip and a martensitic transformation. Immediate post-yield deformation behaviour involved a reconstruction of the lamellar microstructure, the deformation of which was responsible for strain hardening. The reconstruction process, which involved extensive lamellar rearrangement, was aided by the increased amount of amorphous material in the less crystalline linear low density branched polyethylenes. Branch length, however, was found to be unimportant.

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation—II. Cold drawing of linear polyethylene

The cold drawing behaviour of a range of unoriented linear polyethylenes was investigated using the technique of simultaneous small- and wide-angle X-ray scattering during deformation. The influences of molecular weight and thermal history were studied. The role of both variables was found to be highly important since by altering the percentage crystallinity they influenced the relative amounts of inter- and intra-lamellar deformation. The micromechanical deformation processes governing the different regions of the load-extension curve were found to be the same as for cold-drawn ethylene-α-olefin copolymers, demonstrating the fundamental similarity between all types of unoriented polyethylene PE.

Time resolved simultaneous small- and wide-angle X-ray scattering during polyethylene deformation: 1. Cold drawing of ethylene-α-olefin copolymers

The cold drawing behaviour of a range of polyethylene copolymers was investigated using the technique of simultaneous small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) during deformation. The influence of branch amount, branch length and lamellar population were studied. Branch length was found to be unimportant. Increasing branch content lowered the percentage crystallinity which resulted in an increase in the importance of interlamellar deformation mechanisms. Double yield points were observed in some samples. Molecular and lamellar deformation was totally reversible up to the first yield point, and was apportioned to the activation of fine chain slip. After the second yield point, for which coarse chain slip leading to lamellar fragmentation was held responsible, deformation was irreversible.

On the plastic behavior of homogeneous ethylene copolymers compared with heterogeneous copolymers

AbstractThe plastic behavior of homogeneous ethylene‐octene copolymers and heterogeneous ethylene‐butene copolymers has been compared on uniaxial tensile draw. True‐stress‐strain curves have been determined for various draw temperatures and strain‐rates. Significant differences in yield behavior and strain‐hardening have been revealed between the two types of materials, at equivalent crystallinity. In parallel, thermal analysis gave an indication that homogeneous copolymers have much narrow distributions of lamellae thicknesses together with lower values of the most probable thickness. Quantitative data from small‐angle X‐ray scattering support the latter point. An estimation of the surface free energy has revealed a more disordered chain‐folding topology for the homogeneous copolymers. Interpretation of the mechanical behavior is based on the framework of a previous model for the plastic deformation of polyethylene involving competition of homogeneous and heterogeneous crystal slip processes. The former process governs the ability of the material to develop macroscopically homogeneous plastic deformation; the second process is a precursor to necking. It is proposed that the nucleation of dislocations, which is the basic mechanism of the homogeneous crystal slip, is favored for homogeneous copolymers, owing to the lower value of the most probable lamellae thickness. More tie molecules and chain entanglements in the disordered amorphous phase may also favor stable homogeneous plastic deformation in the case of homogeneous copolymers.

Mesomechanics of plastic deformation and fracture of partially crystalline polyethylene

From the standpoint of physical mesomechanics, we have investigated plastic deformation mechanisms and the mechanical properties of partially crystalline polyethylene. We show that from the very beginning, plastic deformation occurs at the mesoscopic level. Fracture is preceded by fragmentation of the material. The observed stages of the process of plastic deformation of polyethylene are qualitatively similar to the stages of this process for metallic materials. The effect of electron bombardment on the mechanical properties of polyethylene is explained by the size reduction in the mesoscopic substructure formed on plastic deformation.

A simultaneous SAXS/WAXS and stress-strain study of polyethylene deformation at high strain rates

An experimental system has been commissioned which allows the collection of time-resolved simultaneous two-dimensional small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) and stress-strain data with a temporal resolution of 40 ms during the drawing of synthetic polymers. X-ray data collection is achieved via two CCD-based area detectors which are positioned in order to maximize the amount of scattering that can be observed from a particular sample. True stress-strain measurements are obtained from video extensometer and load cell measurements. The system was used at the Daresbury SRS to study the structural changes which occur in a sample of isotropic high-density polyethylene when subjected to an overall strain rate of ≈ 3 s −1. It is shown that the onset of both a partial stress-induced crystal phase change and micro-voiding in the sample can be directly correlated to the point of yield in the true stress-strain curve. © 1997 Elsevier Science Ltd.

Molecular orbital studies of polyethylene deformation

Young's modulus E for polyethylene in the chain direction is calculated with molecular orbital theory applied to n-alkanes C3H8 through n-C13H28 and analyzed with the cluster-difference method. Semiempirical CNDO, MNDO, and AM1 models and ab initio HF/STO-3G, HF/6-31G, HF/6-31G*, and MP2/6-31G* models are used. Cluster-difference results, when extrapolated to infinite chain length, give E in good agreement with moduli evaluated with molecular cluster or crystal orbital methods, provided minimal basis sets are employed. E decreases from 495 GPa (CNDO) to 336 GPa (MP2/6-31G*) as the level of theory is improved, consistent with established behaviors of the various models. Our calculations do not reproduce earlier molecular cluster or crystal orbital results, which gave E < 330 GPa. The most rigorous MP2/6-31G* model is known to overestimate force constants by ∼ 11%; the scaled modulus E = 299 GPa is in good accord with E = 306 GPa from recent calculations based on experimental vibration frequencies. © 1996 John Wiley & Sons, Inc.

A small-angle neutron scattering study of the plastic deformation of linear polyethylene

Small-angle neutron scattering (SANS) studies of a 5 vol% mixture of deuterated linear polyethylene with a protonated one was used to investigate the role of partial melting-recrystallization during the solid-state deformation of the polymer in pure shear. The extension of previous studies now gives data that cover the complete deformation range in a continuous manner. The quantity R 0, the ratio of the scattering at zero angle for a given strain to that at zero strain, decreases very rapidly in the strain range 0–2. Here only modest changes occur in the stress-strain curve. In the deformation region where the stress increases rapidly with strain, only a relatively small decrease occurs in R 0. The SANS results indicate that the major reorganization and homogenization occur at low strain. These results, coupled with the Flory-Yoon hypothesis of partial melting-recrystallization, explain the observations that for a ductile-type deformation the ultimate properties do not depend on the structural factors that define the initial undeformed state.

A Real-Time Simultaneous Small- and Wide-Angle X-ray Scattering Study of In-Situ Deformation of Isotropic Polyethylene

Small- and wide-angle X-ray scattering patterns were measured in real time during in situ deformation of isotropic polyethylene at room temperature. A linear low-density polyethylene (LLDPE) was compared with a high-density polyethylene (HDPE) of approximately the same molecular weight. SAXS showed that the start of cavitation in the HDPE coincided with the onset of a stress-induced martensitic transformation detected by WAXS at a low macroscopic strain (∼0.06). It is proposed that the transformation mechanism was the T2 2 mode. It was found that in the HDPE possible lamellar stack rotation occurred whereas in the LLDPE interlamellar shear was active. Altering the thermal history to change the degree of crystallinity and lamellar population altered the strain at which the martensitic transformation occurred. In the annealed LLDPE a higher crystallinity delayed the onset of the transformation by allowing a greater amount of deformation to occur via other deformation mechanisms. In annealed HDPE, however, the transformation was activated at lower strains since less deformation of the amorphous component was possible.

On the deformation of hard elastic polymers — applicability of the coil‐strand‐transition model

AbstractThe applicability of the coil‐strand‐transition model to describe the deformation of hard elastic polymers was studied by deformation and relaxation tests of hard elastic polyethylene and hard elastic polypropylene samples in different liquids. It was found that within the model the calculated values of the fibril diameter, the contribution of surface stress and the total stress are in agreement with measurements. The dominance of energetic contributions to the stress was also confirmed.

Viscoplastic Deformation of High-Density Polyethylene. The Influence of Temperature on the Strain Rate Effect.

Monotonic compressive loading tests and relaxation tests are conducted on high-density polyethylene (HD-PE) at the strain rates of 1.1×10-1 to 1.1×10-5s-1 and temperatures of 10, 25 and 40°C. The observed stress-strain responses are similar to those for steels at elevated temperature. The material constants of the viscoplasticity model based on overstress is determined for HD-PE at every temperature and the model is shown to successfully simulate such rate-dependent phenomena numerically. The examination of the stress-strain curves suggests that the strain-rate and temperature effects on the deformation curves can be expressed using a unified parameter. The viscoplastic behavior observed in the strain-rate cycling test at 25°C is reproduced in the similar test at 40°C designed for this parameter, and the phenomenon is verified in the numerical experiment by the overstress model.

Real-time in situ X-ray diffraction study of polyethylene deformation

A range of polyethylenes has been studied by X-ray diffraction during deformation. Changes in the crystalline orientation have been related to the load-extension curve. This was altered by changing the molecular weight, concentration of branches and strain rate. Crystalline orientation was first observed just after the maximum load, before any visible signs of yield. At about the same point, the monoclinic phase was observed in specimens that were subjected to a higher load and it disappeared when stress softening occurred. Initially the (110) orthorhombic reflection oriented to give four maxima, which implies tilted polymer chains and lamellae. Subsequently the (110) concentrated on the equator showing chains oriented parallel to the draw direction. The transition from tilted to fibre orientation occurred at different points on a nominal stress-strain curve but at approximately the same local strain, as estimated from the change in sample dimensions.

The application of mixed crystal i.r. spectroscopy to molecular deformation in polyethylene by rolling

The mixed crystal i.r. technique has been applied to polyethylene single crystal mats after rolling at room temperature to roll ratios up to 6.0. Reduction in the absorbance of the outer CD 2 bending components provides evidence of lamellar breakup. Model calculations indicate that the size of blocks which break away from the crystals is reduced to ∼36 A ̌ for a roll ratio of 6, while X-ray diffraction data show that crystallite reorientation occurs progressively with increasing roll ratio. Changes in small-angle X-ray scattering and the Raman longitudinal acoustic mode with rolling demonstrate that crystallite stacking periodicity is gradually lost and that crystal thickness remains unchanged.