Extended Chain Polyethylene Research Articles

Shear-induced martensitic transformations in crystalline polyethylene: Direct molecular-dynamics simulations

For the first time, we carry out molecular dynamics (MD) simulation of shear-induced martensitic phase transitions between the orthorhombic and non-orthorhombic (triclinic and monoclinic) phases of crystalline polyethylene (PE) in the framework of a realistic all atom model of the polymer. We show that the variation of the shear rate allows observing on a nano-sample both a strongly nonequilibrium phase transition occurring by random nucleation and irregular growth of a new phase ('civilian' way, for rapid deformations) and the coherent, or 'military', kinetics (generally considered as usual for martensitic transformations). We induce transitions from the orthorhombic to the triclinic phase according to two transformation modes observed in experiment on PE single crystals. Rapid deformation favors the transition directly to the triclinic phase, slow deformation - first to the intermediate monoclinic, and only then - to the triclinic phase. The second way corresponds to the experiment on extended chain PE. We explain this result and analyze the competition between different transformation and plastic deformation modes. Rotations of PE chains around their axes necessary for the transition between the orthorhombic and non-orthorhombic phases are executed by short twist defects diffusing along the chains. The transition between the monoclinic and triclinic phases occurs through half-chain-period translations of the chains along their axes, mostly collectively, as crystallographic slips.

Melting and van der Waals Stabilization of PE Single Crystals Grown from Ultrathin Films

We have obtained a series of single- and multilayered extended chain polyethylene (PE) crystals resting on a silicon dioxide substrate by crystallizing a low molar mass oligomer fraction of low polydispersity from an ultrathin film. Crystal thicknesses were determined from the height histograms of atomic force microscopy (AFM) images. For crystallization temperatures above 108 °C, the thickness of each layer was 16.9 ± 1.4 nm, close to the computed average extended length of the oligomer chain. The crystal melting was followed by a hot-stage optical microscopy (OM) in reflection. Different layers melt at different temperatures, due to differences in interfacial free energy depending on whether they are in contact with the substrate or another layer or have a free interface. The interfacial free energies of the different interfaces were obtained by the Gibbs–Thompson equation, and the difference between them was quantitatively related to the van der Waals stabilization energy resulting from placing a layer in contact with the substrate or with another layer. Our study demonstrates the importance of interfaces when considering the thermal behavior of ultrathin films of crystalline polymers and illustrates that a simple energetic concept can be used to explain the stabilization of crystals resting on a surface.

Rigid‐rod polymeric fibers

AbstractThis paper traces the historical development of high temperature resistant rigid‐rod polymers. Synthesis, fiber processing, structure, properties, and applications of poly(p‐phenylene benzobisoxazole) (PBO) fibers have been discussed. After nearly 20 years of development in the United States and Japan, PBO fiber was commercialized with the trade name Zylon® in 1998. Properties of this fiber have been compared with the properties of poly(ethylene terephthalate) (PET), thermotropic polyester (Vectran®), extended chain polyethylene (Spectra®), p‐aramid (Kevlar®), m‐aramid (Nomex®), aramid copolymer (Technora®), polyimide (PBI), steel, and the experimental high compressive strength rigid‐rod polymeric fiber (PIPD, M5). PBO is currently the highest tensile modulus, highest tensile strength, and most thermally stable commercial polymeric fiber. However, PBO has low axial compressive strength and poor resistance to ultraviolet and visible radiation. The fiber also looses tensile strength in hot and humid environment. In the coming decades, further improvements in tensile strength (10–20 GPa range), compressive strength, and radiation resistance are expected in polymeric fibers. Incorporation of carbon nanotubes is expected to result in the development of next generation high performance polymeric fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 100: 791–802, 2006

The path to “single-component composites”: An in situ X-ray study of melting and crystallization of extended-chain polyethylene fibers under pressure

“Single component” polymer composites, in which both the reinforcement and the matrix are made of the same polymer, are currently fabricated by subjecting oriented fibers to moderate pressure and temperature in a processing scheme termed “reversed pressure compaction”. In order to elucidate the nature of the transformations that lead from fibers to a monolithic composite, an X-ray diffraction study with a pressure cell was performed using synchrotron X-ray radiation (Beamline ID-11, ESRF). We observed a transition from the orthorhombic crystal to the meso-morphic hexagonal phase upon heating even at a low pressure (100bar). The hexagonal phase also reappears even upon cooling from the melt. These indicate that the path to high-performance composites from the oriented PE fibers occurs via sintering of the hexagonal phase. This phase allows enough chain mobility for the compaction process yet maintains the chain orientation to a large extent.

Heat release of polymer composites in fire

The relationship between heat release rate and other fire reaction properties of fibre reinforced polymer composite materials is investigated. The heat release rate and fire reaction properties of thermoset matrix composites reinforced with combustible fibres (aramid, extended-chain polyethylene) or non-combustible fibres (glass, carbon) were determined over a range of heat flux levels using the oxygen consumption cone calorimeter technique. The fire reaction properties that were measured were time-to-ignition, smoke density, carbon monoxide yield, carbon dioxide yield, mass loss rate and total mass loss. It is discovered that these reaction properties (apart from ignition time) are linearly related to the heat release rate for composites containing non-combustible fibres. When the reinforcement is combustible, however, the heat release rate only appears to be related to the carbon monoxide yield, mass loss rate and (in some cases) smoke density. This study clearly shows the importance of the relationship between heat release rate with smoke density and carbon monoxide yield, the two reaction properties that influence the survival of humans in fire.

Perforation of flexible laminates by projectiles of different geometry

This paper investigates the response of flexible laminates to ballistic impacts by projectiles of various geometries, namely, flat-ended, hemispherical, ogival (CRH 2.5) and conical (30° half-angle) projectiles. The laminate of interest is Spectra Shield ® comprising [0°/90°] extended chain polyethylene filaments embedded in a thermoplastic resin. Ballistic tests show that flat-ended projectiles cut the laminate through a shearing action, effectively punching a circular hole in the laminate whereas hemispherical projectiles perforate the laminates by stretching the Spectra filaments to failure resulting in a rectangular hole in the laminates. While the manner in which they are perforated are different, many similarities are observed in specimens perforated by flat ended and hemispherical projectiles such as the formation of a generator strip, the extent of delamination, the creasing of the laminate, tearing of the laminate at the edges, etc. Ogival and conical projectiles, on the other hand, perforate the laminates with minimal delamination and tearing of the specimens. Interestingly, the region of the specimens affected by the projectiles appears to increase in size instead of becoming more localised at higher impact velocities as often reported for most ballistic impacts events, including the ballistic perforation of woven fabric. This suggests flexible laminates are more effective in dissipating energy than woven fabric in the application of flexible armour.

Ballistic Impact Resistance of SMA and Spectra Hybrid Graphite Composites

The effect of adding small amounts of high strain hybrid components on the impact resistance of graphite epoxy composites subjected to projectiles traveling at ballistic velocities (greater than 900 ft/sec) has been studied. The hybrid components tested include superelastic shape memory alloy (SMA) and a high performance extended chain polyethylene (ECPE) known as Spectram. In all cases, the embedded SMA fibers were pulled through the graphite without straining to their full potential. It is believed that this is due to high strain rate effects coupled with a strain mismatch between the tough SMA and the brittle epoxy resin. However, a significant increase in energy absorption was found by adding ECPE and ECPE/SMA layers to the backface of the composite.

Simulation of the Structure and Properties of the Polyethylene Crystal Surface

The structures and thermodynamic properties of the (100), (010), and (110) lateral surfaces of extended-chain polyethylene crystals between 0 and 300 K were determined by minimization of vibrational free energy using consistent quasi-harmonic lattice dynamics. Slight rotations of the outermost chains from their corresponding orientations in the bulk were observed. These deviations from bulk structure were confined to the first three molecular layers (approximately 10 Å) at the surface. Surface free energy calculations for the crystal−vacuum interface found the (110) surface to be the most stable over the entire temperature range modeled, with free energies ranging from 95.4 erg/cm2 to 103.5 erg/cm2 at temperatures of 0 and 300 K, respectively. Surface free energies of the (100) and (010) surfaces were found to be at least 15% higher than the (110) surface, with the (100) surface slightly more stable than the (010) surface, over all temperatures considered. Surface free energy increases as the density of chains at the surface decreases. Intermolecular potential energy accounted for over 80% of the surface free energy at low temperatures. At higher temperatures, excess entropy accounted for almost half the surface free energy. The surface free energy was determined predominantly by deviations in lattice mode frequencies from those of the bulk.

Mechanical Characterization of Extended-Chain Polyethylene (ECPE) Fiber-Reinforced Composites

Composite materials reinforced with extended-chain polyethylene (ECPE) fibers are unlike typical stiff and brittle composite materials such as graphite/epoxy or fiberglass. The high ductility and energy absorption capacity of the ECPE reinforcing fibers gives these composites a unique mechanical response which makes them ideally suited for a variety of applications. However, this dissimilarity with more common materials requires special consideration of mechanical properties testing. In this paper, the mechanical behavior of ECPE-fiber-reinforced composites is investigated using standard composite test methods. Results of these tests are presented and discussed based on the properties of the ECPE reinforcing fibers and on the assumptions inherent in the test methods. ECPE/epoxy composites are characterized by high ultimate tensile strength, high tensile modulus, low shear modulus and strength, and viscoelastic response to loading. The highest available combination of fiber strength and strain-to-failure gives this material ductility and energy absorption capacity significantly higher than other common composite materials. Applications of ECPE composites are discussed.

Simulations of the Structure and Properties of the Polyethylene Crystal Surface

AbstractThe structures and thermodynamic properties of the (100), (010), and (110) lateral surfaces of extended-chain polyethylene crystals between 0 K and 300 K were determined by free energy minimization using consistent quasi-harmonic lattice dynamics. Slight rotations of the outermost chains from their corresponding orientations in the bulk were observed. These deviations from bulk structure were confined to the first three molecular layers (approximately 10 Å) at the surface. Surface free energy calculations found the (110) surface to be the most stable over the entire temperature range modeled, with free energies ranging from 95.4 erg/cm2 to 103.5 erg/cm2, at temperatures of 0 K and 300 K, respectively. Surface free energies of the (100) and (010) surfaces were found to be at least 15% higher than the (110) surface, with the (100) surface slightly more stable than the (010) surface, over all temperatures considered. Surface free energy increases as the density of chains at the surface decreases. The surface free energy at low temperatures was determined predominantly by intermolecular potential energy; at higher temperatures, excess entropy accounted for nearly half the surface free energy. Surface entropy was almost entirely due to lattice mode motions.

Air-plasma treated polyethylene fibres: effect of time and temperature ageing on fibre surface properties and on fibre-matrix adhesion

The effect of a low energy air-plasma treatment of extended chain polyethylene (ECPE) fibres (Spectra®900) on the adhesion with a matrix of epoxy resin has been studied. Surface energies of fibre and matrix were calculated by contact angles, measured with a Wilhelmy microbalance in different liquids, and the adhesion between fibres and the matrix was evaluated through a pull-out test. The results showed an increase in fibre-matrix adhesion by a factor of ca. 1.5 calculated by surface energy measurements, and by a factor of ca. 4 measured by the pull-out tests. Time (up to six months) and temperature (in the range 20–120 °C for 2 h) ageing caused some decrease in adhesion with respect to the values evaluated just after the fibre plasma treatment. The plasma treatment did not affect the fibre's mechanical properties.

Analysis of the temperature rise in the projectile and extended chain polyethylene fiber composite armor during ballistic impact and penetration

AbstractThe ultra‐high strength/modulus, extended chain polyethylene fiber (Spectra® fiber) composite has shown great potential as a lightweight armor material with its extraordinary capability of absorbing the kinetic energy of projectiles. But the relatively low melting point of this fiber (∼ 145°C) has caused concerns regarding the effect of temperature rise during the impact/penetration process on the performance as armor material. In this article, an analysis of temperature rise in projectile and the fiber composite during the impact/penetration process is described. Combining the simulation of impact deformation by finite element analysis and the simulation of temperature rise by a finite difference approximation of the related dynamic equations, the temperature rise caused by the projectile/composite interaction was estimated. Results show that there is a significant temperature rise at the projectile/composite interface due to the friction but that the short length of time involved in the process and the low thermal conductivity of Spectra fiber composite keep the temperature rise in a very small region (in the order of 0.001 cm) around the interface during the impact/penetration process. Consequently, the volume that is affected by the temperature rise is very small, in the order of total 0.001 cm3 around the projectile, and this is too small to generate any detectable effect on the armor performance. After the projectile is stopped, however, substantial heating of fiber composite can occur under specific conditions as the result of heat flow from the hot projectile embedded in the composite. This heating of fibers, however, is a postmortem effect and hence inconsequential to the ballistic performance of the composite armor.

New fiber‐reinforced polyurethane composites as a soft sonar window

A new polyurethane formulation has been developed for use as the acoustical window for a soft sonar dome. The polyurethane is based on a 3000 molecular weight difunctional polybutyleneoxyglycol (PBOG) softblock. In addition, a moderately high trifunctional PBOG was added to increase the cross‐link density for improving the polymer toughness. ‘‘Spectra,’’ an extended chain polyethylene fiber, was selected as the reinforcement fiber because it has excellent physical strength and is low in density. The lower density assists in matching the specific acoustical impedance of the composite to that of sea water. Fiber loadings up to 50% in thickness were used. Samples of this new composite were fabricated and their physical and acoustical properties were evaluated. Insertion loss of the test panel was measured in the USRD ATFI facility. The material was found to be acoustically transparent at the operating frequency range, and physically resilient to high impact loadings.

UV grafting of extended chain polyethylene fibres

A bench-scale continuous fibre treatment line has been built to modify polyethylene fibre surfaces using UV radiation-induced grafting. It is shown that grafting does indeed take place and that the grafts adhere well to the resin. Single-fibre pullout behaviour, interlaminar shear strength, and flexural behaviour were observed to improve considerably. The loss of tensile properties due to treatment was investigated and found to be small in the range of useful treatments. The interlaminar shear strength of the UV-treated fibre is as high as 38 MPa (5.5 ksi) as compared with the untreated fibre value of 10 MPa (1.5 ksi). The most important process parameters are identified and trends established.

Application of atomic force microscopy (AFM) to a block copolymer and an extended chain polyethylene

Application of atomic force microscopy (AFM) to a block copolymer and an extended chain polyethylene

Plasma surface modification of advanced organic fibres

Aramid and extended-chain polyethylene fibres have been treated in ammonia and oxygen plasmas in order to enhance adhesion to vinylester resins and thereby improve fibre/resin interfacial properties in composites made from these materials. For both aramid/vinylester and extended-chain polyethylene/vinylester composites, the plasma treatments result in significant improvements in interlaminar shear strength and flexural strength. Extended-chain polyethylene/vinylester composites also exhibit increased flexural modulus

Surface modification of extended chain polyethylene fibres to improve adhesion to epoxy and unsaturated polyester resins

AbstractExtended chain polyethylene fibres have been treated in ammonia and oxygen lo‐pressure gas discharges (plasmas) in order to enhance adhesion to epoxy and unsaturated polyester resins, respectively, and thus significantly improve fibre/resin interfacial properties in fibre‐reinforced polymer composites. Ammonia plasma treatment results in the incorporation of amine functional groups onto the fibre suface. The treated fibre surface has been analysed using XPS and spectrophotometric techniques. Extended chain polyethylene/epoxy composites made from ammonia, plasma‐treated fibres show a marked increase in interlaminar shear strength over composites made from untreated, corona‐treated or oxygen plasma‐treated fibres. The increase in fibre/resin adhesion after ammonia plasma treatment is confirmed by SEM observations of fracture surfaces, which show clean interfacial fracture surfaces in composites made from treated fibres. Fibres modified by oxygen plasma treatment contain a significant concentration of carbon‐oxygen functionalities, which contribute to the polarity of the surface and hence increase wet‐out by unsaturated polyester resins. The concentration and nature of carbon‐oxygen species on the fibre surface have been determined by XPS. Pull‐out tests on multifilament yarns embedded in a polyester resin confirm the high fibre/matrix adhesion achieved with the oxygen plasma‐treated fibres compared to corona‐treated or untreated fibres. Tensile properties of the fibres are reduced significantly after prolonged treatment in an oxygen plasma, while in an ammonia plasma the fibre strength is unaffected.

The hydrostatic extrusion of linear polyethylene at high temperatures and high pressures

An account is given of a series of hydrostatic extrusion experiments which has been carried out on linear polyethylene over a wide range of extrusion temperatures and environmental pressures. The results of pressure-to-pressure hot extrusions, which have been performed on melt crystallised polyethylene and also on pressure crystallised extended chain polyethylene, are discussed, with particular reference to the physical properties of the extruded products.

On the dissolution behaviour of extended chain polyethylene fibres

The influence of stress on the dissolution behaviour of extended-chain high molecular weight polyethylene fibres in p-xylene was investigated. Freely suspended in the solvent, the fibres dissolved at 119.5°, a temperature close to the equilibrium solubility temperature of 118.6° for perfect polyethylene crystals. However, when a stress of 0.4 GPa was exerted by straining the fibre 0.7%, it could withstand a temperature as high as 130° for at least three days. At still higher temperatures the induced stress relaxed completely, and dissolution immediately followed. These phenomena indicate that the fibre has a network structure. The cross-links are of a physical nature. Molecules are connected by topological defects such as entanglements, intertwinings and twist disclinations. These defects are trapped in crystallites; therefore the theory of Gee and Flory is applicable predicting that in such a system dissolution temperature of extended chain crystallites increases with stress. The required stress is transduced by tie molecules bridging the amorphous regions between crystallites. A study of dissolution under stress seems to be a direct method for the detection of topological defects such as entanglements.

Determination of crystallite size and lattice distortions in extended chain polyethylene and their change after oxydative degradation

High pressure crystallized linear polyethylene was treated with fuming nitric acid and of both samples treated and untreated the X-ray wide angle line widths of 18 hkl-reflections were analysed. The three axes and the orientation of the mean shape ellipsoid and thegki-values of the paracrystalline distortions were determined. The crystallite size perpendicular to the chain direction is up to 1650 A and parallel to the chain direction higher than 5000 A and is not be changed significantly by HNO3-treatment.