I. M. Ward

Preparation of ultra-high modulus linear polyethylenes; effect of molecular weight and molecular weight distribution on drawing behaviour and mechanical properties

Abstract A systematic investigation of the effect of molecular weight and molecular weight distribution on the cold drawing behaviour of linear polyethylene has been undertaken. In the molecular weight range studied, the natural draw ratio was very sensitive to the morphology of the initial material; spectacular effects on the natural draw ratio were observed provided that an optimum initial morphology was achieved. These effects can be related to both molecular weight and molecular weight distribution. The extensional modulus and melting behaviour of the drawn material was also examined. To a first approximation the extensional modulus related to the natural draw ratio, and at very high draw ratios (∼30) extremely high extensional moduli (∼700kbar) were obtained. The structure and properties of the drawn material did, however, also depend on the molecular weight and molecular weight distribution. In particular, when certain molecular weight requirements were satisfied, the oriented samples showed the presence of extended chain material. It does, however, appear that although differences in molecular weight and molecular weight distribution give rise to differences in extensional moduli, the presence of extended chain crystallization per se is not a necessary requirement for the production of high modulus material.

Review: The yield behaviour of polymers

Recent research on the yield behaviour of polymers is reviewed. Particular attention is given to the importance of the hydrostatic component of stress, the viscoelastic nature of the yield process, and the behaviour of oriented polymers.

The hot compaction of high modulus melt-spun polyethylene fibres

The production of solid section highly oriented polyethylene by compaction of melt-spun polyethylene fibres is described. Differential scanning calorimetry, X-ray diffraction and electron microscopy have been used to determine the structure of the compacted polymer. The essential feature of the process is shown to be selective surface melting of the fibres to form a polyethylene/polyethylene composite of very high integrity, yet maintaining a very high proportion of the strength and stiffness of the fibres.

Modelling of the energy absorption by polymer composites upon ballistic impact

Abstract In this paper we report on the development of a simple model for calculating the energy absorption by polymer composites upon ballistic impact. Three major components were identified as contributing to the energy lost by the projectile during ballistic impact, namely the energy absorbed in tensile failure of the composite, the energy converted into elastic deformation of the composite and the energy converted into the kinetic energy of the moving portion of the composite. These three contributions are combined in the model to determine a value for the ballistic limit of the composite, V0. The required input parameters for the model were determined by a combination of physical characterisation (for the physical and mechanical properties of the composites and the characteristics of the projectile) and from high-speed photography (for the size of the deformed region and the cone velocity). As the failure event usually occurred between two of a relatively small number of frames from the high-speed camera, the model predicted a range for V0. This range of V0 was compared with experimentally determined values for three composite systems: woven Nylon-66 fibres in a 50:50 mixture of phenol formaldehyde resin and polyvinyl butyral resin, woven aramid fibres in a similar matrix and Dyneema UD66 (straight gel-spun polyethylene fibres laid in a 0/90 fibre arrangement in a thermoplastic matrix). In all cases, the experimentally measured values of V0 were found to lie within the range predicted by the model. The size of the deformed region, formed through shear deformation, on the back face of the composite was found to relate directly to the in-plane shear modulus of the material. Perhaps the most surprising result was that the dominant energy absorbing mechanism was found to be the kinetic energy of the moving portion of the composites.

Dynamic mechanical behaviour and longitudinal crystal thickness measurements on ultra-high modulus linear polyethylene: a quantitative model for the elastic modulus

Abstract In this paper the dynamic mechanical behaviour of ultra-high modulus polyethylene is reported. The results are discussed in terms of a fibre reinforced composite model, in which the amount of fibre phase is related to the number of intercrystalline bridges determined from X-ray diffraction data. For both drawn and extruded materials a good correlation has been obtained between the plateau modulus at −50°C and the longitudinal crystal thickness. This correlation is given a quantitative interpretation in terms of the model and the increase in modulus with increasing deformation ratio is ascribed to an increase in continuity of the crystalline phase. The increase in tensile modulus on cooling through the γ-process is related primarily to the change in the tensile modulus of the amorphous phase. The fall in modulus at high temperature, on the other hand, indicates a fall in the shear modulus of the crystalline phase, which provides a satisfactory explanation of the α-process.

Infra-red and Raman spectra of poly(m-methylene terephthalate) polymers

Abstract A detailed comparison of the infra-red and Raman spectra of poly(trimethylene terephthalate) (3GT), poly(tetramethylene terephthalate) (4GT) and poly(ethylene terephthalate) (2GT) is reported. Particular attention has been given to the changes in the spectra which occur on crystallization. Additionally, the spectra of oriented 4GT samples have been examined under applied stress, because this produces a change in the structure of the crystalline regions. It is shown that the differences between the spectra of these polymers can be satisfacorily accounted for by changes in the conformation of the molecular chains involving both trans/gauche isomerism in the glycol sequence and the planarity of the terephthalate residue.

The strain-rate, temperature and pressure dependence of yield of isotropic poly(methylmethacrylate) and poly(ethylene terephthalate)

The yield behaviour of poly(methylmethacrylate) (PMMA) has been investigated in tension and compression over a range of testing temperatures and strain-rates. Both tensile and compressive yield stresses were found to increase monotonically with increasing strainrate and decreasing temperatures. Compressive yield stresses were in general found to be more dependent on strain-rate.The results of this investigation have been correlated with previous published data for the dependence of the torsional yield stress of PMMA on hydrostatic pressure. This was done by a modification of a theory proposed by Robertson which uses the internal viscosity approach to yield in glassy polymers. The modified theory clearly explains the temperature and strain-rate dependence of the yield stress and provides a quantitative explanation of the differences in behaviour between tension and compression in terms of the dependence of yield on the hydrostatic component of the applied stress.The tensile yield behaviour of isotropic amorphous poly(ethylene terephthalate) (PET) sheets has also been investigated over a wide range of temperatures and strain-rates. No torsion or compressive yield stresses are available because of the sheet form of the PET, but the results obtained in tension are shown to be fully consistent with the above theory, and with other published work.

An infra-red spectroscopic study of molecular orientation and conformational changes in poly(ethylene terephthalate)

Polarized infra-red (i.r.) spectroscopy has been used to study the changes in molecular orientation and conformation which occur on drawing poly(ethylene terephthalate) sheets. The i.r. data provided orientation functions 〈P2(θ)〉ir where θ is the angle between the molecular chain axis and the draw direction for these uniaxially oriented sheets. Excellent agreement was obtained between the i.r. orientation functions for absorption bands associated with benzene ring mode vibrations, and orientation functions obtained from optical birefringence. Infra-red orientation functions for absorption bands associated with the trans and gauche conformations of the glycol residue, were then combined with i.r. estimates of the trans/gauche concentrations to examine the changes occurring in drawing. For drawing at 80°C up to a draw ratio of about 3·5, there was good agreement with expectations based on the deformation of a rubberlike network.

Craze shape and fracture in poly(methyl methacrylate)

Abstract The shape of the primary craze at the tip of a crack has been studied by optical microscopy for two grades of poly(methyl methacrylate). The craze shapes are compared with the predictions of the Dugdale model for the plastic zone at a crack tip, and used to obtain a quantitative estimate of the craze stress and the effective stress intensity factor. The values of the stress intensity factor are then compared with those obtained directly from fracture toughness measurements.

A study of the adhesion of drawn polyethylene fibre/polymeric resin systems

The effect of plasma etching and chromic acid treatment on the surface adhesion of ultra-high modulus polyethylene fibres to an epoxy resin has been studied. The adhesion was determined from pull-out tests, and showed a significant improvement for both plasma and acid treatments. The mechanism of failure, however, appeared to be different in the two cases. Untreated and acid-treated monofilaments showed a fairly smooth surface and failure of the pull-out samples involved sliding along the monofilament/resin interface. Plasma treatment, on the other hand, produced a remarkable structure on the monofilament surface, into which resin penetrated to produce a mechanical keying between monofilament and resin. Failure in the pull-out test then involved rupture within the monofilament.