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In a previous study we showed that a unique combination of high stiffness and high viscoelastic loss could be achieved by filling a polystyrene matrix with rigid inorganic spheres coated with a thin (∼200 nm) layer of a viscoelastic material. The sandwiching of this ‘lossy’ layer between the two rigid components was found to give a significant amplification of the tanδ loss peak associated with this material, without significantly compromising the sample stiffness. This was an experimental validation of the effect originally proposed by Gusev using finite element numerical studies. Following on from this, in the current study we have developed this concept further and shown that a similar amplification of viscoelastic loss can be achieved by incorporating rigid, but uncoated, particles into a phase separated matrix blend of polystyrene (PS) and a polystyrene/polyisoprene/polystyrene triblock co-polymer (SIS). The inspiration for this choice of the PS/SIS blend as the matrix came from some previous work where we studied, and modelled, the viscoelastic properties of these materials. In this work we show that in the filled PS/SIS blends, the loss amplification effect can been seen for different PS/SIS ratios, for different SIS polymers with different glass transition temperatures and also for glass fibres as well as for spherical particles. The key to seeing this effect is the fact that the SIS rubber phase was found to form a thin coating on the surface of the embedded particles during processing, effectively producing a surface coating layer on the particles (as well as phase separating within the PS matrix). As with our previous studies, it is shown that the experimentally measured effects are closely predicted by numerical micromechanical modelling based on the measured bulk properties of the three discrete components.

This paper is concerned with the prediction of the viscoelastic properties of rubber filled polymer blends. The question asked was as follows. Can the temperature dependent viscoelastic properties of phase separated polymer blends be adequately predicted using only a rational two phase micromechanics based analytical model with no empirical fitting parameters? In particular can this be achieved using only a knowledge of the individual bulk phase properties and the blend microstructure, but without any further detailed polymer physics knowledge such as the presence of an interphase region or any additional nanoscale structures within the separated rubber phase with the properties different from those of the two bulk phases? Blends of a polystyrene matrix containing phase separated rubber inclusions (a polystyrene-polyisoprene-polystyrene triblock polymer (SIS)) were manufactured in a range of blend fractions (up to 20 vol % of the triblock co-polymer). Experimental measurements, for the storage modulus G′ and the loss tangent tanδ, of both the individual phases and the blends, were made using dynamic mechanical tests over a range of temperatures from −50 to +70 °C. Numerical predictions, of the same properties, were first obtained using the generalised self-consistent Christensen and Lo model which uses a simple representative volume element (RVE) of an isolated sphere of the minority rubber component in a surrounding sheath of polystyrene matrix embedded in a homogeneous effective medium. The agreement between the Christensen and Lo model and the experimental measurements, for G′ and tanδ, was found to be excellent for rubber contents up to 10%. For a 20% rubber content, an improved prediction was obtained by altering the RVE to include the observed effect of having a polystyrene central core in a number of the dispersed rubber zones at this rubber fraction, using the Herve and Zaoui generalization of the Christensen and Lo model. Although conjoined (and therefore non-spherical) zones became more prevalent at the highest rubber content, use of the Tandon and Weng model showed that this shape anisotropy would not be expected to affect the viscoelastic properties.

The properties of self-reinforced single polymer composites produced by the Leeds hot compaction process are highly dependent on the compaction temperature as well as the constituent oriented elements used to produce the compacted sheets. In this paper, the variation in tensile mechanical properties of uniaxial hot compacted sheets manufactured from drawn polypropylene (PP) tapes with change in compaction temperature have been investigated, for a range of different draw ratio tapes. It is shown that there is a measureable difference between the optimum compaction temperatures required for obtaining the highest modulus and strength in the compacted sheets. The compaction temperature required to achieve the maximum tensile modulus was seen to increase with increasing draw ratio. The compaction temperature to obtain the maximum tensile strength was found to be both independent of the draw ratio and a few degrees higher than that for obtaining the maximum modulus. Peak modulus and peak tensile strength was shown to be dependent on the draw ratio of the drawn tape. Differential scanning calorimetry measurements on the compacted sheets were also performed in order to investigate the change in crystalline structure with compaction temperature and draw ratio. This has shown that the changes in structure within the oriented phase (i.e. tapes) during the compaction process itself are directly related to the final properties of the hot compacted sheets.

Carbon fibre composites possess excellent mechanical properties, but suffer from brittleness. Hybridisation with self-reinforced polypropylene (SRPP) is a promising strategy to introduce ductility into carbon fibre-reinforced polypropylene (CFRPP). The present work demonstrates how different damage mechanisms in these hybrid composites change as a function of the carbon fibre volume fraction, the directionality of CFRPP and SRPP and their relative layer thickness. Multiple fractures of the CFRPP layers or “fragmentation” is achieved by optimising these parameters. This leads to a ductile hybrid composite with a gradual failure development. publisher: Elsevier articletitle: Introducing ductility in hybrid carbon fibre/self-reinforced composites through control of the damage mechanisms journaltitle: Composite Structures articlelink: http://dx.doi.org/10.1016/j.compstruct.2015.04.069 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: Composite Structures vol:131 pages:259-265 status: published

This paper compares and contrasts the properties of self-reinforced polyamide 12/carbon fibre hybrid composites made by three different hybridisation routes, termed intra-yarn, intra-layer and inter-layer. The starting point for each route was to manufacture layers of woven cloth (containing both components), from which the hybrid composites were manufactured using the Leeds hot compaction technique. In all cases, a carbon fibre volume fraction of around 8% was the target. On balance, the intra-layer hybrids had the best combination of properties, although all three hybridisation routes yielded interesting results. This intra-layer hybrid configuration showed a significant increase in tensile modulus and strength, bending modulus and strength and penetration impact energy compared to a pure self-reinforced polyamide sheet. The only negative aspect was a reduction in the tensile failure strain from 11 to 2%, whereas the ductility in bending was unaffected by the incorporation of the carbon fibres. publisher: Elsevier articletitle: The influence of the hybridisation configuration on the mechanical properties of hybrid self reinforced polyamide 12/carbon fibre composites journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2016.12.029 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved. ispartof: Composites A, Applied Science and Manufacturing vol:95 pages:141-151 status: published

Single polymer composites (SPC) are finding increasing use due to their novel combination of lightweight and high toughness. This paper describes an investigation into the production and properties of hybrid single polymer composites (SPC) made from co-mingled tows of carbon fibres and nylon 12 (PA12) oriented multifilaments, using the Leeds hot compaction process. The aim of the study was to investigate ways to increase the stiffness of the SPC (by the addition of carbon fibres) without significantly compromising ductility and impact performance which a feature of SPCs. Experiments showed that the optimum processing temperature required for the production of the hybrid composites using the hot compaction technique depended on both the arrangement of the hybrid tows and the fraction of carbon fibres. For 22% volume fraction of carbon fibres, a well consolidated sample could be made at a temperature of 176C, 2C below the temperature at which major crystalline melting of the oriented PA12 fibres, and loss of molecular orientation, occurs. For braided cloth made from the same co-mingled tows, a higher temperature of 178C was required to give a well consolidated sample. A combination of wide angle x-ray (WAXS) and differential scanning calorimetry (DSC) measurements showed that this temperature was too close to the melting point of the PA12 multifilaments. For co-mingled tows containing 13% carbon fibres it was found that a well consolidated sample could be made from braided cloth using a lower temperature of 175C. At this temperature, sufficient melted and recrystallised matrix material was produced to consolidate the structure and produce a homogeneous composite. This temperature was 3C below the temperature at which substantial crystalline melting occurred, giving a usable temperature processing window. In tension the hybrid samples were found to fail in a brittle manner, with the whole sample breaking on failure of the carbon fibres. The hybrid samples were found to show ductile behaviour in bending as long as a substantial proportion of the molecular orientation of the PA12 fibres was retained.

To improve the toughness of carbon fibre composites, unidirectional carbon fibre prepregs were hybridised with highly oriented polypropylene (PP) tapes. The latter tapes are used in self-reinforced PP composites (SRPPs) with high toughness, but relatively low stiffness. The tensile behaviour of intralayer hybrids of oriented PP tapes and CFPP prepreg tapes was investigated by changing the layup and weave pattern, and by adding interleaved PP films. While stiffness and strength of these hybrids was decreased compared to CFPP, their ductility significantly increased by adding oriented PP tapes. The parallel behaviour of the two constituent materials was caused by delaminations, which developed after the CFPP failure and spreads over the entire sample. This behaviour was modelled and proven to be independent of layup and weave pattern. Interleaved films, which are often necessary for thermoformability, limited the delamination development by increasing the interlayer bonding and yielded a lower ultimate failure strain.

Recent research on the mechanical properties of hot-compacted polypropylene composites is reported. It is shown that incorporation of new high performance polypropylene tapes leads to significant improvements in tensile and flexural properties. The high temperature properties of these new composites are outstanding, showing higher stiffness and very low creep under permanent load. Comparisons are presented with both chopped glass-reinforced and continuous glass-reinforced polypropylene composites.

Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and −2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient α in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, α decreased from about 80 × 10 −6 °C −1 (for the isotropic PP) to 5 × 10 −6 °C −1 for the highly drawn PP filled with softwood.

It is shown that the incorporation of interleaved films has major advantages for the production of polyethylene single polymer composites by the process of selective melting (termed hot compaction). The key issue is to choose a compaction temperature which melts the minimum amount of the original oriented elements whilst achieving acceptable bonding within the compacted structure. Utilising an interleaved film, excellent interlayer peel strengths can be achieved at lower compaction temperatures giving greater retention of the oriented fraction of the original fibres or tapes and a wider processing window. For example, using a very high modulus, ultra-high molecular weight, polyethylene tape, together with an interleaved film, resulted in an in-plane modulus of 25 GPa, an in-plane strength of 500 MPa, and an interlayer strength of >10 N/10 mm. These are amongst the highest values reported for a single polymer composite. Other important factors have been investigated including fabric weave style and whether it is better to use fibres or tapes as the oriented reinforcement.

Macroscopic deformation and failure modes of polyolefines are reviewed in terms of deformation and failure models based on the craze initiation and propagation model of Kramer–Berger and the craze–crack transition model of Kramer–Brown. Although these models were formulated for amorphous polymers they are also valid for semi-crystalline polymers. The important role of the underlying molecular entanglement network in this approach is reflected by the strain hardening behaviour which is shown to be a robust measure for predicting slow crack growth performance. The polymer network response explains the experimentally observed presence of two Brittle–Ductile transitions, one at low temperature or high strain rates, linked with chain scission which dominates crazing, the other at elevated temperatures or low strain rates which involves disentanglement crazing. The relation between these two Brittle–Ductile transitions and the major transition temperatures for molecular mobility such as the glass transition and the crystal α relaxation temperature are discussed. Valid strategies for increasing the crack propagation resistance in polyolefines are reviewed. Finally an outlook for further research to complement the present knowledge base is formulated.

Recent work has shown that the interlayer adhesion of single-polymer composites produced by the Leeds hot compaction process can be significantly improved by either introducing interleaved films of the same polymer between the layers of woven oriented tapes that make up the composite or by the incorporation of carbon nanofibres (CNF) into the oriented tapes prior to hot compaction, or by a combination of these two approaches to produce a synergistic effect. Following on from these findings, we have now investigated the changes in the interlayer peel strength caused by incorporating other micron and nano-scale fillers into polypropylene films which are interleaved into the interlayer region of the hot compacted composite. The filler initially chosen for this study was talc, but a number of other fillers have also been investigated. Scanning electron microscopy has been used to image the peel surfaces and to undertake a preliminary study of the effects of particle size and shape on the peel strength.

The current work is a major extension of two very different studies carried out previously to investigate factors that affect the peel strength of single-polymer composites produced by the Leeds hot compaction process. First, it was found that the peel strength was significantly increased by introducing interleaved films, of the same polymer, between the layers of woven oriented tapes that make up the composite. Secondly, it was shown that incorporation of carbon nanofibres (CNF) into the oriented tapes prior to hot compaction could also increase peel strength. In the present study we have investigated the amalgamation of these two approaches, in particular to see if there are synergistic advantages in the combination. Samples were produced with and without interleaved films, and with and without carbon nanofibres, located either in the oriented polypropylene tapes, in the interleaved film or in both. Maximum peel strength was achieved with the combination of the interleaved film and the incorporated nanofibres, but importantly this could be achieved with the CNF located only in the film. This has significant processing and performance advantages as the incorporation of CNF into the oriented tapes tends to limit the drawability of the polypropylene due to internal voiding around the particles. Scanning electron micrographs of the hot compacted composites show a strong correlation between the observed damage on the peel surfaces and the measured peel loads. It is shown that the peel load is dependent on the fraction of melted matrix at the interface and hence the interleaved films give additional matrix material at this point. It is also shown that the incorporation of CNF promotes fibrillation, and so increases the amount of energy absorbed during peeling.

Oriented wood polymer composites (WPC) have been prepared by the Leeds die drawing process. Softwood and hardwood powder were used at 40% weight concentration (32% volume concentration) and in both cases materials with significantly increased stiffness (from 1.9 to 8.2 GPa) and strength (from 13 to 127 MPa) were obtained. Although the moduli of the drawn filled composites were lower than the equivalent unfilled polypropylene, the specific moduli, which take into account the lower density of the die drawn materials due to void formation were very similar. The type of wood particles and the use of polypropylene grafted with maleic anhydride had only a marginal influence on the mechanical properties of the die drawn composites. The morphology of the wood composites was studied by electron microscopy.

The Cox–Krenchel micromechanical model was applied to give predictions for the tensile moduli of isotropic and oriented wood polymer composites (WPC). The oriented WPC were produced by the Leeds die-drawing process using polypropylene filled with softwood and hardwood powders. The wood particles were extracted from the composites to determine their density and aspect ratio by dissolving in hot decalin. To measure particle shape and size, image analysis was employed. These experimental parameters were then introduced to the Cox–Krenchel model which was found to give prediction of tensile modulus in very good agreement with the experimental values.

Microinterferometry (MIF), wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and an Instron tensile tester (ITT) were used to determine the correlation between optical and structural properties of polypropylene (PP) fibers. For the purpose of the study a set of as-spun isotactic PP fibers were extruded by melt spinning at different draw-down ratios (DDR). The birefringence (Dn), degrees of orientation, degree of crystallinity (v), Young’s modulus (Ee), and tenacity (s) were determined for PP fibers at the different DDR. An equiangular orientation of PP fibers at particular DDR was predicted experimentally, and the transverse modulus (Et) was estimated for the tested fibers. Empirical formulae were developed for correlating the fiber birefringence with some of the studied structural properties of PP fibers. POLYM. ENG. SCI., 50:200–208, 2010. a 2009 Society of Plastics Engineers

In this paper we describe the production of a polypropylene (PP)/carbon nanofibre (CNF) nanocomposite, and subsequent characterisation of the structure and properties of the nanocomposite material at various stages of blending. Dispersion of the CNF throughout the matrix PP was monitored by scanning electron microscopy (SEM), and analysis of the lengths of the individual CNF was estimated using dynamic light scattering (DLS). This latter technique enabled a comparison to be made between the measured Young's modulus of the material and that predicted by micromechanical modelling, using the fibre length as determined by DLS. The temperature performance of the nanocomposite material was determined, and this behaviour has also been modelled.

This paper describes a new route for the manufacture of single polymer composites by combining the processes of hot compaction and film stacking. Assemblies of oriented fibres and tapes, were produced with and without an interleaved film of the sample polymer type, over a range of processing temperatures. Of particular interest was to choose temperatures so that in some cases only the interleaved film was melted and in other cases both the film and the fibre surfaces were melted, allowing a traditional film stacking process, a traditional hot compaction technique and the combination of both to be compared. The results showed that the combination of film stacking with hot compaction gave a better overall balance of mechanical properties and a wider temperature window for processing compared to a standard hot compaction procedure without a film. The combination process also gave much better wetting of all the oriented elements compared to a traditional film stacking process (where only the film is melted) due to the partial melting of all the fibre surfaces.

A key factor which limits the production speed of the polymer die drawing process is the premature fracture of the material on exit from the die. In this paper, the growth of damage in the material during the die drawing process has been studied using a combination of thermoplastic finite element analysis and structural characterisation by means of scanning electron microscopy and small angle X-ray scattering for the specific case of die drawing of polyoxymethylene. It is demonstrated that special profiled dies offer a more beneficial strain rate distribution than the conventional conical dies and allow higher production speeds to be obtained. Voids grow in the material as a result of the tensile stresses pertaining near the die exit and then, crazes appear from within the material at a critical stress level leading ultimately to final fracture. The results suggest that although the crazes initiate at a critical stress, the extent of crazing at the maximum draw ratio obtained (∼13) is independent ...

In this paper, we describe the application of the constrained rolling process to produce highly oriented polyacetal bars with enhanced mechanical properties. In this process, the heated polymer billet is deformed in a channel formed in the circumference of the bottom roll that provides lateral constraint to the material as it deforms. It is a process that has attracted interest due to its capability to produce thick cross-sectional oriented products continuously and at moderate production speeds. Here the focus is on two commercial grades of polyoxymethylene (a) Delrin® 100 and (b) Tarnoform® 300. Tarnoform®, unlike Delrin®, is a copolymer. The compression behaviour of these grades has been investigated in a plane strain channel die to determine the optimum constrained rolling conditions. Samples were then rolled to different reduction ratios close to but below the crystalline melting temperature of the two grades. The modulus and strength increased almost linearly with reduction ratio. Rolled Delrin® exhibited higher modulus and strength than Tarnoform®. Under impact loading, with the initial notch perpendicular to the rolling direction, the fracture process was incomplete for both resins with the specimens exhibiting a hinge type break. Structural investigations of the rolled samples were carried out by wide and small angle X-ray diffraction. The structures produced were very similar to those produced in plane strain compression test. The pole figures from the (100) reflection suggest that the c axes of the POM crystals are oriented along the rolling direction while ab planes showed clustering of orientation of (100) normals in six directions. SAXS patterns from the rolled samples with the X-ray beam parallel to the force direction showed two-point patterns that suggest the transformation of the spherulitic morphology to a fibrillar structure in this direction. However, perpendicular to the rolling direction, four-point patterns were obtained that suggest cooperative kinking of the lamellae during deformation to produce a chevron-like structure. The enhancement in properties as a result of molecular orientation suggests that these materials can have major commercial applications.

Interference polarizing (Pluta and Interphako) microscopes have been used to measure the birefringence and refractive indices during the cold-drawing of isotactic poly(propylene) (iPP), poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) fibres. The intrinsic birefringence (Δn0) and refractive indices (niso, and ) of iPP, PTFE and PVDF fibres were interferometrically predicted using affine and pseudo-affine models. In terms of these intrinsic values, a new way of calculating the orientation parameter is discussed. A selection of microinterferograms is given for illustration.

The elastic constants of poly(L-lactic acid) (PLLA) crystals are reported on the basis of a commercial software package and the published crystal structure of the α form. A chain modulus of 36 GPa and a shear modulus of 3 GPa have been obtained for cylindrically symmetric aggregates of perfectly oriented crystals. The helical conformation of the PLLA molecule reduces the stiffness in the chain axis direction because bond rotation plays a significant role in the deformation. X-ray crystal strain measurements suggest that shear of the crystal parallel to the helix axis is the easiest mode of deformation, in agreement with the expectations obtained from the low shear modulus of 3 GPa obtained from the theoretical calculations. A combination of small- and wide-angle X-ray scattering, differential scanning calorimetry, dynamic mechanical thermal analysis, and shrinkage measurements has been used to characterize the structure that develops and the crystal transformation that occurs during fiber processing. The structure that develops during processing very much depends on the crystal transformation, and a structural model is proposed for fibers at different degrees of plastic deformation. The transformation of the α crystal into the p form and vice versa is governed primarily by shear along the helix axis because the chains must shear past each other during the crystal transformation, disrupting the lamellar packing.

Polymer composites are usually either stiff or tough, but seldom both. Intralayer hybrids of carbon fibre and self-reinforced polypropylene (PP) do offer the potential to achieve a unique combination of toughness and stiffness. In these hybrids, the bonding between carbon fibre prepregs and PP tapes is a crucial parameter. For a weak bonding, the 20% ultimate tensile failure strain and high penetration impact resistance of self-reinforced PP were maintained. For a strong bonding, the ultimate tensile failure strain was strongly reduced, but the flexural performance was improved. For a homopolymer PP matrix in the prepregs, the weak bonding between fibre and matrix caused the penetration impact resistance to reduce according to a linear rule-of-mixtures. For a maleic anhydride modified PP matrix however, the strong fibre-matrix bonding greatly reduced the penetration impact resistance. These results provide new insights into designing hybrid composites with a unique balance of stiffness and failure strain. publisher: Elsevier articletitle: The importance of bonding in intralayer carbon fibre/self-reinforced polypropylene hybrid composites journaltitle: Composites Part A: Applied Science and Manufacturing articlelink: http://dx.doi.org/10.1016/j.compositesa.2015.06.017 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved. ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:76 pages:299-308 status: published

The Leeds die drawing process has been used to produce biaxially oriented polypropylene pipes with significant enhancement of hoop mechanical properties in terms of tensile modulus and tensile strength owing to a unique orientation of the α crystallites throughout their cross-section. Pendulum impact tests on these pipes show major improvements in impact resistance. Hydrostatic pressure tests were undertaken at room temperature 95 and 110°C. Major improvements in behaviour were found including more than an order of magnitude increase in lifetime at 95°C. Furthermore, the toughness was improved enormously leading to a brittle tough transition temperature below −70°C.

The elastic constants of poly(glycolic acid) (PGA) crystals are reported on the basis of a commercial software package and the published crystal structure of the polymer. Due to the planar zigzag conformation of the molecular chains, very high elastic anisotropy is found with a tensile chain modulus of 294 GPa and a longitudinal shear modulus for a fibre of 6 GPa. A combination of small and wide angle X-ray scattering and differential scanning calorimetry are used to characterise the structure of highly oriented PGA fibres. The combination of long period data, crystal size and crystallinity measurements suggests a structure similar to the Prevorsek model, with alternating crystalline and amorphous regions along the fibre axis, and layers of amorphous material in parallel. A parallel–series Takayanagi model, using the theoretically calculated chain modulus, is shown to give good agreement with the experimental data in a wide temperature range.

In this paper we describe the intrinsic deformation behaviour of polyoxymethylene in uniaxial tension, shear and plane strain compression at 160°C for strain rates from 10 -3 to 1 s -1 . In tension and shear, the deformation was determined by a novel video-controlled testing system. There was a significant evolution of volumetric strain in tension, indicating that damage mechanisms play a key role in the plastic deformation behaviour. Post-mortem analysis on the shear and compressed samples showed no evidence of dilatation. In uniaxial tension, significant strain hardening was observed at high strains while only a small increase of stress was observed in plane strain compression. In shear, the plastic deformation occurred at constant stress. For the specific case of die-drawing of polyoxymethylene, it has been shown with the aid of small angle X-ray scattering that voids are nucleated at the die-entrance by inhomogeneous shear deformation under negative (compressive) hydrostatic stress. The growth of the voids is then aided by the tensile stresses that are predominant in this process at die-exit. Extensive voiding occurred in the oriented sample produced at high strain rates, as revealed by the scanning electron microscopy, which affects the fracture strength and hence the viable production speeds of the die-drawing process.

The deformation behaviour of polyoxymethylene has been studied in plane strain compression at temperatures from 120 °C up to 165 °C and in uniaxial tension and simple shear at 160 °C for strain rates from 10−4 to 1 s−1. In uniaxial tension the stress–strain behaviour was determined by a novel video-controlled testing system. The measurements showed that there was a very significant evolution of volumetric strain, indicating that damage mechanisms play a key role in the plastic deformation behaviour. All tests showed similar deformation stages with a short region of visco-elastic behaviour followed by a rounded yield point. The von Mises equivalent yield stress for these tests showed a linear relationship with logarithmic strain rate, suggestive of an Eyring type thermally activated process. After yielding, all stress–strain curves showed a long plastic deformation regime, which in shear occurred at constant stress. In plane strain compression there was also only a very small increase in stress, in contrast to uniaxial tension where very significant strain hardening was observed at high strains, which is attributed to the onset of structural changes.

The mechanical properties and structure of uniaxially oriented polyoxymethylene (POM) produced by two solid-state processes, hydrostatic extrusion and die-drawing, are compared. In the former process there is no net component of tensile stress whereas in the latter case the sample is subjected to axial tensile stresses at the die-exit. The tensile nature of the stresses in die-drawing causes void formation and growth in the oriented sample whereas, in the case of hydrostatic extrusion, voids are suppressed due to the compressive stress fields. The mechanical properties of the oriented samples are compared together with relevant structural data, and their differences discussed.

Hydrolytic degradation studies have been undertaken on Maxon B, a bioresorbable block copolymer of polyglycolic acid (PGA) and polytrimethylene carbonate (TMC). Isotropic and oriented samples were studied by dynamic mechanical measurements over a wide range of temperatures. In addition to mechanical tests, water content and mass loss were also determined on the degraded samples. At early stages of degradation water content was the dominant factor and plasticisation lead to reductions in the glass transition temperatures of the PGA and TMC components. Orientation was shown to give significant improvements in the mechanical properties, including overall increases in modulus and an increase in the glass transition temperature of the PGA component, which is important for the behaviour at body temperature (37 °C). Oriented samples also showed significantly less reduction in mechanical properties on degradation. Simple one-dimensional Takayanagi models were used to provide useful insight into the understanding of the mechanical behaviour.

An analytical procedure is proposed and validated for predicting the elastic anisotropy and thermal expansion behaviour of a short fibre polymer composite, where both the fibre and the polymer matrix possess anisotropic material properties. The modelling strategy is to consider the fibre composite as an aggregate of units of structure, with an averaging scheme which takes into account the state of orientation. Validation of this strategy required the accurate determination of the unit properties and a satisfactory orientation averaging procedure. First, the unit properties were determined for very highly aligned samples with either an isotropic or an anisotropic fibre (glass or carbon) and either an isotropic or an anisotropic matrix (Nylon or liquid crystalline polymer). The fibre orientation and length distribution were determined by image analysis together with measurements of their elastic and thermoelastic properties. In combination with finite element calculations developed by Gusev, these results provided the basis for validation of appropriate analytical schemes to predict the unit properties. The orientation averaging procedures were validated by measurements on a model ribbed box component manufactured from a carbon fibre filled liquid crystalline polymer (anisotropic fibre and anisotropic matrix). Measurements of Young’s modulus and the coefficient of thermal expansion were combined with the determination of fibre orientation by image analysis. The key result was that if the fibre orientation level was high, the best prediction was to assume that the matrix orientation was identical to that of the fibres. For a lower degree of alignment, a better prediction was obtained by assuming that the matrix was isotropic.

Microindentation measurements are reported on a range of single polymer polyethylene (PE) composites, which are produced by hot compaction of high modulus PE fibers. It is possible to measure two hardness values, parallel and perpendicular to the fiber direction respectively, from which the microindentation anisotropy is defined. The hardness values relate to the instantaneous elastic recovery of the fibers, and the results show that the microindentation measurement is deforming a material volume below the surface of the sheets comparable to the dimensions of the fibers. It appears that the microindentation anisotropy approaches a limiting value with increasing fiber orientation, i.e., as the Young's modulus of the fibers increases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1659–1663, 2006

We describe a study of the hot compaction of woven nylon 6,6 multifilaments produced by a patented procedure, developed at the University of Leeds, for creating novel single-polymer composites. In this process, an assembly of oriented elements, often in the form of a woven cloth, is held under pressure and taken to a critical temperature so that a small fraction of the surface of each oriented element is melted, which on cooling recrystallizes to form the matrix of the single-polymer composite. This process is therefore a way of producing novel high-volume-fraction polymer/polymer composites in which the two phases are chemically the same material. Nylon is an obvious candidate material for this process because oriented nylon multifilaments are available on a commercial scale. The aim of this study was first to establish the conditions of temperature and pressure for the successful hot compaction of oriented nylon 6,6 fibers and second to assess the mechanical properties of the manufactured hot-compacted nylon sheets. A crucial aspect of this work, not previously examined in hot-compaction studies of other oriented polymers, was the sensitivity of the properties to absorbed water, with a significant change in the properties measured immediately after hot-compaction processing and 2 weeks later when 2% water had been absorbed by the compacted nylon sheets. As expected, the water uptake had a greater effect on those properties that depended on local chain interactions (e.g., the modulus and yield strength) and less effect on those properties that depended on the large-scale properties of the molecular network (e.g., strength). The only negative aspect of the properties of the hot-compacted nylon sheets was the elevated-temperature performance of the wet sample, with the modulus falling to a very low value at a temperature of 80°C. However, apart from the elevated-temperature performance, the majority of the measured properties of the hot-compacted nylon sheets were comparable to those of hot-compacted polypropylene and poly(ethylene terephthalate).

This paper describes the incorporation of carbon nanofibres (CNF) into polypropylene (PP) single polymer composites, materials where both the reinforcing phase and the matrix phase are PP. The CNF/PP composites were produced from an assembly of highly oriented tapes. The process of making the composites involves heating the tapes to a critical temperature such that a small fraction of the surface of each tape is melted; on cooling this recrystallises to form the matrix of the composite. The production of the composites required optimisation of three stages; incorporation of CNF into PP tapes, orientation of CNF/PP tapes by tensile drawing and hot compaction of the tapes. Results are presented to describe the research and findings in each of these key stages. Preliminary studies showed that the introduction of small amounts of carbon nanofibres (CNF) significantly improved the properties of isotropic PP. For example, 5% volume addition of CNF gave a 60% increase in the room temperature Young's modulus and a reduction of 35% in the thermal expansion coefficient. Moreover, the percentage enhancement of properties was greater at high temperatures where the stiffness of the PP is much reduced. These results can be very well understood in terms of conventional composite modelling. In unidirectional CNF/PP hot compacted composites the major improvements in mechanical behaviour are in the direction transverse to the orientation direction, where the CNF can make a proportionately greater contribution to the properties, and as shown by dynamic mechanical behaviour, this is most marked at high temperatures. Composite modelling based on uniform strain with appropriate allowance for the CNF aspect ratio predicts the behaviour extremely well. A very interesting result is that the peel strength of composites produced by hot compaction of woven CNF/PP shows a four-fold increase over woven PP composites and this is increased by another factor of two by the addition of a maleic anhydride compatibiliser. A further interesting result, of some practical significance, is that although the incorporation of CNF into PP causes voiding and some loss of molecular orientation during drawing, the hot compaction process closes and seals the voids, so that the original PP density is recovered.

Static tests and ultrasonic measurements (2.25 MHz) have been carried out on a series of composite laminates of glass fibres in a polypropylene matrix. A range of angle ply laminates were prepared for this study, with laminate angles θ of ±0, 10, 20, 30 and 40^∘. The high frequency measurements were made using the ultrasonic immersion technique, which allows the determination of a complete set of the elastic constants of a material. The relationship between the ultrasonically determined elastic constants of the angle ply laminates was found to be in excellent agreement with theoretical predictions, as previously validated for carbon fibre/epoxy angle ply laminates. A comparison between the ultrasonic and statically measured values was made for two of the angle ply laminates (θ = 0 and 20^∘). It was found that the static values were lower than those measured at ultrasonic frequency, particularly those constants that were more matrix dominated (for example the transverse moduli of the laminates). Measurements on a pure polypropylene sample at both testing frequencies confirmed that the change in matrix properties with frequency was the cause of this difference. The change in properties with test frequency is likely to be much larger in this system than in other composite materials because the glass transition temperature of polypropylene is close to ambient temperature. Dynamic mechanical tests (1 Hz) were carried out on a sample of pure polypropylene to assess this effect. We also give an appropriate method of estimating the dependence of glass transition temperature on frequency. The results for polypropylene are compared with those for other commonly used polymer matrix materials: epoxy resin, nylon and polyetheretherketone (PEEK): DMTA measurements were also made on these samples. The effect of test frequency on matrix properties, for the glass/PP laminates, was further investigated by examining the relationship of the Poisson's ratios with laminate angle using a mixture of ultrasonic experiments and theoretical predictions. Previously we have shown that the degree of anisotropy between the reinforcing fibre and the matrix phase is paramount in determining whether the material will show a negative Poisson's ratio at a critical laminate angle. The ultrasonic measurements carried out in this study on the glass/PP laminates showed a minimum in one of the Poisson's ratio at a laminate angle of 32°, but the value did not become negative. However, theoretical predictions showed that for a static frequency measurement (1 Hz), where the matrix is softer and hence the anisotropy of each laminate ply is higher, the laminate will show a negative Poisson's ratio with a minimum at a laminate angle of around 28°.

A range of flat hot-compacted single-polymer composite panels made from oriented polypropylene and polyethylene with differing dynamic modulus and damping capacity were freely suspended and subjected to mechanical excitation, allowing their acoustic frequency response over the audio bandwidth to be measured. The audio response over selected bands was correlated with the dynamic modulus and damping capacity measured in bending in these materials and compared with the response of a traditional composite material, namely, carbon-fiber-reinforced epoxy resin. Low damping and high dynamic modulus were found to result in relatively high output levels from the hot-compacted flat panels, which contrasted with the results previously measured on a traditional cone-shaped speaker made from a hot-compacted polypropylene material, which found high damping to be advantageous. The results of the current study on flat panels are explained in terms of mechanical impedance of the panels and their corresponding efficiency. It was concluded that the best flat-panel audio response came from compacted polyethylene sheets, which combined high stiffness, low density, and a low level of damping. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

The wear and mechanical properties of GUR 1020 (Perplas IMP 2000-2) Ultra High Molecular Weight Polyethylene (UHMWPE) subjected to gamma-irradiation in an atmosphere of acetylene, were evaluated for a range of processing conditions of irradiation, annealing and ageing. The results were compared with those obtained for the virgin UHMWPE material and material processed using conventional gamma-irradiation in nitrogen. Cross-linking produced by irradiation in acetylene, followed by subsequent annealing was found to be significantly more effective in improving the mechanical and wear properties of UHMWPE compared to when the material was irradiated in nitrogen. Gel fraction analysis on its own, while being able to detect the degree of cross-linking, was found to be insufficient in determining the effectiveness of the cross-links and the resulting mechanical properties of the UHMWPE material. The results suggest that gamma-irradiation in an atmosphere of acetylene may provide significant advantages over conventional UHMWPE processing and irradiation cross-linking techniques.

The discovery of methods to make highly oriented polymers has given tremendous stimulus to both basic polymer science and industrial developments in the period 1970 to the present. High modulus, hi...

The production and properties of single polymer composites is described, where the fibre and matrix phases are of identical chemical composition. The structure of the composites has been explored using mechanical tests, DSC and electron microscopy. Measurements of mechanical anisotropy, thermal expansion and impact behaviour are of especial interest. The commercialisation of the hot compacted materials is also described. Potential applications include the automotive industry, sports protection goods, loudspeaker cones and helicopter radomes.

A review is presented of recent research at Leeds University which has been directed at devising novel methods for the production of oriented polymer structures. First, the new hot compaction process for oriented fibre and tapes is described, together with its applications to polyethylene and polypropylene where there are a number of practical developments. Secondly, there is the use of hydrostatic extrusion to make load bearing oriented products from hydroxyapatite filled polyethylenes. The production routes include the application of high pressure annealing prior to hydrostatic extrusion and the preparation of high modulus polyethylene fibre/hydroxyapatite billets as the starting point. Finally, recent progress on die-drawing as a means to producing oriented monofilaments and biaxially oriented tubes is described, where the applications include polymer ropes, pipes for gas and water distribution and transparent cans for packaging.

We report the solid-state die-drawing of polypropylene and blends of polypropylene with a polyethylene elastomer to produce highly oriented products with enhanced mechanical properties. The blends showed an improvement in the drawability compared to the polypropylene homopolymer. The tensile modulus of the polypropylene homopolymer and the blends, along the draw direction, increased with draw ratio. In the transverse direction, the modulus of the homopolymer and the blends decreased with draw ratio as a result of anisotropy along the draw direction. The impact fracture behavior of the isotropic and the oriented samples was evaluated from noteched Charpy tests at 1 m/s over a wide range of temperatures. Linear elastic fracture mechanics were used to characterize the brittle fracture and the J-integral approach was used to characterize the semi-brittle and ductile fracture. In the isotropic state, the inclusion of the elastomer phase in the polypropylene matrix increased the toughness and also decreased the brittle-ductile transition temperature. The oriented sheets, drawn at 110°C to a draw ratio of 4, were tested with the initial notch parallel and perpendicular to the draw direction. When tested with the initial notch parallel to the draw direction, the toughness of the homopolymer and the blends decreased when compared with the isotropic material. The brittle-ductile transition temperature increased as a result of anisotropy. When tested with the initial notch perpendicular to the draw direction, the blends and the homopolymer showed considerable improvement in toughness compared to the isotropic state. Fracture along this direction is complicated, with subsidiary cracks propagating perpendicular to the main crack direction (which is parallel to the draw direction).

An analysis of the polymer die drawing process using full thermomechanically coupled finite element (FE) modelling is described. Strain rate and temperature dependent stress—strain data for polyoxymethylene (POM) have been obtained from uniaxial tensile tests and utilized in the numerical simulations. The FE model has been validated against die drawing experiments, and both refute the previously held assumption that ‘free’ drawing commences within the die as a result of separation of the polymer from the die wall prior to exit. Simulations have been employed using the fully validated model to study the influence of process parameters such as die half-angle, area reduction, die wall friction and drawing speed on the draw loads and the amount of free tensile drawing that takes place outside the die.

Creep studies were carried out on a range of homopolymers and copolymers of polyethylene with well-defined molecular weight and branch content. The creep data were analyzed in terms of two thermally activated processes acting in parallel and the effects of molecular weight and branch content are discussed. It is shown that increasing either the number-average molecular weight or the weight-average molecular weight gives improved creep behavior at all stress levels. The introduction of butyl branches leads to lower creep at low-stress levels but can give rise to higher creep at high stress. Plots of the equilibrium log10(strain rate) versus stress at fixed draw ratio (strain) can be used to define sections through a unique true stress/true strain/strain rate surface for each material. These creep results have an additional value in terms of the link between slow crack propagation (SCG) in polyethylene and fibril creep, confirming the proposal made elsewhere that SCG can be quantified in terms of creep to failure across the true stress/true strain/strain rate surface. © 2003 Wiley Periodicals, J Appl Polym Sci 89: 1663–1670, 2003

The Leeds die-drawing process has been used to make oriented sheets of toughened polypropylenes. Die-drawn oriented sheets were produced by drawing at 110°C to draw ratios of 4, 6, and 10. Comparative measurements have been undertaken of the plane stress fracture toughness at room temperature using the essential work of fracture method for isotropic and oriented polypropylene homopolymer and the two polypropylene blends containing 10 and 25% of a polyethylene-based elastomer. In the isotropic state, the blend containing 25% elastomer exhibited higher fracture toughness than the homopolymer and the 10% blend. The oriented sheets were tested both parallel (cracks perpendicular to the draw direction) and perpendicular (cracks parallel to the draw direction). For the latter case of cracks parallel to the draw direction, the fracture toughness of all the materials decreased with increasing draw ratio and up to a draw ratio of 4 the 25% blend exhibited higher fracture toughness than the other two materials. At higher draw ratios, however, the unfilled polypropylene was tougher than the blends. When tested parallel to the draw direction, all three materials failed with the cracks growing slowly initially followed by sudden rupture. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1336–1345, 2003

Highly oriented monofilaments were produced by a high-temperature die-drawing process followed by tensile drawing. It was shown that a successful high-speed process required high-quality melt-extruded rod. The mechanical properties and structure of the die-drawn products were investigated by means of tensile and bending tests, dynamic mechanical measurements, DSC, and X-ray diffraction. The bending modulus and the tensile strength increased with increasing draw ratio. It was also observed that at high draw ratios the γ-dispersion peak in the dynamic mechanical tan δ curve, which is associated with main chain micro-Brownian motion in the amorphous regions, diminishes, implying that these chains become taut. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1268–1278, 2003

The aim of this work was to establish the important parameters that control the hot compaction behaviour of woven oriented polypropylene. Five commercial woven cloths, based on four different polypropylene polymers, were selected so that the perceived important variables could be studied. These include the mechanical properties of the original oriented tapes or fibres, the geometry of the oriented reinforcement (fibres or tapes), the mechanical properties of the base polymer (which are crucially dependant on the molecular weight and morphology), and the weave style. The five cloths were chosen so as to explore the boundaries of these various parameters, i.e. low and high molecular weight: circular or rectangular reinforcement (fibres or tapes): low or high tape initial orientation: coarse or fine weave. A vital aspect of this study was the realisation that hot compacted polypropylene could be envisaged as a composite, comprising an oriented ‘reinforcement’ bound together by a matrix phase, formed by melting and recrystallisation of the original oriented material. We have established the crucial importance of the properties of the melted and recrystallised matrix phase, especially the level of ductility, in controlling the properties of the hot compacted composite.

The morphology of woven oriented polypropylene tapes and fibres has been studied both before and after processing by hot compaction. In this technique bundles of oriented tapes or fibres are subjected to suitable conditions of temperature and pressure so that just sufficient of each tape or fibre is selectively melted; on cooling, this material recrystallizes to bind the whole structure together. Three different polymers were studied, woven into various individual weave styles, in relation to optimum processing conditions. The weave style, the adhesion between neighbouring interfaces after compaction and the direction of crack propagation along the neighbouring interfaces on peeling were examined leading to clear correlations between the observed morphology and mechanical peel strength data.

Here we describe the development of a process, and the resulting mechanical properties, for hot-compacted sheets of woven poly(ethylene terephthalate) (PET) multifilaments. Investigation of the various processing parameters showed that a key aspect was the time spent at the compaction temperature, termed the dwell time. Molecular weight measurements, using intrinsic viscosity, showed that hydrolytic degradation occurred rapidly at the temperatures required for successful compaction, leading to embrittlement of the resulting materials with increasing dwell time. A dwell time of 2 min was found to be optimum because this gave the required percentage of melted material to bind the structure together, while giving only a small decrease in molecular weight. A combination of techniques, including mechanical tests, differential scanning calorimetry, and scanning electron microscopy, was used to examine the mechanical properties and morphology of the optimum compacted sheets. These tests reinforced the view from previous studies on hot-compacted polypropylene, of hot-compacted sheets as self-reinforced composites, whose behavior is a combination of the properties of the two components, that is, the original oriented multifilaments and the melted and recrystallized matrix. Other key findings from the research included a confirmation of the importance of obtaining high ductility in the melted and recrystallized phases, promoted by using a high molecular weight or by suppressing crystallinity during processing, and the proportionately high-impact performance of hot-compacted sheets, compared with that of other materials. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2223–2233, 2004

The deformation behavior of a particular grade of high-density polyethylene prepared in a number of different ways has been examined. The samples all have an initially isotropic crystalline texture but differ in the levels of network preorientation surviving as a legacy of the melt processing history. Measurements of shrinkage and shrinkage force have been used to characterize this network, and the results suggest that the density of active chains is much lower than found in the main molecular network operating during solid-state drawing. The subnetwork properties vary depending on the level of extrusion and rate of cooling experienced during processing, with those entanglements surviving in the oriented melt at the point of crystallization having a long relaxation time. The presence of this subnetwork can lead to the suppression of necking during subsequent solid-state deformation. The implication is that simple processing procedures can be devised to remove or control the instability associated with necking. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3135–3147, 2002

The failure mode of a number of polyethylenes has been studied under predominantly plane strain conditions. Square section samples, notched on all sides, have been tested in tension to failure over a range of crosshead speeds and temperatures. Integration under the subsequent load displacement curve has allowed the total energy, energy to peak load, and energy after peak load to be determined. The data have been analyzed in terms of the ratio of the energy after the peak load to the total energy. The results show that the material can change from brittle to ductile failure as a function of test speed. At a suitable temperature we have observed brittle-like failure at the highest and lowest test speeds and ductile failure at intermediate speeds. The resulting failure surface features correlate very strongly with the energy ratio analysis-flat smooth surfaces where low energy ratios are seen and large ductile tearing where high ratios are seen. The effect of molecular weight and polydispersity will be shown and possible failure mechanisms discussed.