The structure and properties of ultra-high modulus polymers☆

Published on Jan 1, 1979in Polymer Science U.s.s.r.
· DOI :10.1016/0032-3950(79)90385-X
I.M Ward1
Estimated H-index: 1
(University of Leeds)
Source
Abstract
The first successful preparation of homogeneous continuous high modulus polyethylene was carried out at Leeds University in 1972. Compression moulded sheets of comparatively low molecular weight polyethylenes were drawn 30 times to give thin tapes with initial moduli ∼ 70 GNm−2. Though these materials were of high modulus, they showed comparatively poor creep and strength properties. Supsequently, it has proved possible to extend the range of polyethylenes that can be highly drawn, and high modulus products have been obtained with considerably enhanced properties. Moreover, guidelines have been established for the development of small scale manufacture of fibres and tapes on a continuous basis. In addition, similar materials may be made in large sections by hydrostatic extrusion. It has been clearly established that high draw is essential in the preparation of high modulus by solid phase deformation. The preparation of the material is also affected by the initial structure (molecular weight, chemical composition and morphology) as well as by the drawing conditions (temperature, strain rate). Moreover, although the initial modulus depends only on the draw ratio, other properties such as melting behaviour, creep and strength depend on the initial structure and drawing conditions. Other interesting properties of these materials include increased melting points, a high thermal conductivity (comparable with stainless steel), high shrinkage force and a very high resistance to chemical attack (associated with very low permeability to gases). A wide range of structural techniques have been applied to examine these materials, including wide angle and small angle X-ray diffraction, laser Raman spectroscopy, broad line nuclear magnetic resonance and nitric acid etching followed by gel permeation chromatographic analysis of the degradation products. The results obtained suggest that although the end product features a periodicity corresponding to an initial lamellar structure formed in the early stages of stretching, high degrees of deformation are attributable to the formation of a large number of intercrystallite bridge bonds. The highly drawn polymer accordingly represents a continuous crystal phase, which serves as a strengthener for the structure as a whole. It is the degree of crystallinity, which may be determined from the X-ray data, which would account for a high elastic modulus and for thermal conductivity. A special study was made of the creep and of the recovery behaviour, since these are most significant properties for possible applications of ultra-high modulus PE fibres as reinforcing materials. It can be demonstrated that the creep consists of a recoverable part and an irrecoverable part and is satisfactorily described as an Eyring activated rate process. The link between creep and drawing can be established and leads to a comprehensive view of the plastic deformation processes. It has also proved possible to achieve high modulus in polypropylene fibres by high draw, although the results are less spectacular because of the much lower theoretical modulus of this material, and the highest modulus values obtained are only ∼ 20 GNm−2.
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#1B. Brew (University of Leeds)H-Index: 1
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Abstract The tensile drawing behaviour of polyoxymethylene has been studied, with particular reference to the production of ultra-high modulus oriented material. The influence of molecular weight and draw temperature, and the comparative effectiveness of single-stage and two-stage drawing processes have also been examined. In addition to the molecular weight range it appears that both draw temperature and drawing rate must be specified within very narrow limits, if ultra-high modulus material is...
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#1M.A. Wilding (University of Leeds)H-Index: 3
#2I. M. Ward (University of Leeds)H-Index: 88
Abstract The tensile creep and recovery of oriented linear polyethylene (LPE) monofilaments have been studied for a range of samples of different structure. Starting with a comparison of samples of different draw ratio prepared from one grade of polymer, the measurements were extended to examine the effects of molecular weight. Although the viscoelastic behaviour is markedly non-linear it was found valuable to model the creep and recovery at each level of stress by a simple linear solid represen...
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#1R. N. Britton (University of Leeds)H-Index: 1
#2R. Jakeways (University of Leeds)H-Index: 1
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Attempts have been made to determine the longitudinal crystal modulus for linear polyethylene by studying the change in the 0 0 2 reflection with load for a series of uniaxially oriented films. The specimens examined covered a wide range of draw ratios, and included a number of ultra high modulus films. In low draw ratio specimens, the assumption of homogeneous stress gave a value for the crystal modulus in good agreement with previous work. The results for the ultra high modulus specimens, on t...
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#1D. L. M. Cansfield (University of Leeds)H-Index: 1
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Following the discovery that linear polyethylene can be drawn to very high draw ratios to produce oriented fibres and films with ultra-high initial moduli, a similar study has been undertaken for polypropylene. In particular, the modulus/draw ratio relationship has been obtained for a range of polymers of different molecular weight and molecular weight distribution. The effects of thermal history and draw temperature were studied, and it was shown that under optimum conditions material with an i...
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#1G. Capaccio (University of Leeds)H-Index: 2
#2T. A. Crompton (University of Leeds)H-Index: 4
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The drawing behavior of a series of linear polyethylene homopolymers with weight-average molecular weight (Mw) ranging from 67,800 to ∼3,500,000 and variable distribution (Mw/Mn = 5.1−20.9) has been studied. Sheets were prepared by two distinct routes: either by quenching the molten polymer into cold water or by slow cooling below the crystallization temperature (∼120°C) followed by quenching into cold water. When the samples (2 cm long) were drawn in air at 75°C using a crosshead speed of 10 cm...
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#1J. B. Smith (University of Leeds)H-Index: 1
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The dynamic mechanical properties of a number of ultra-highly drawn polyethylenes have been studied over a wide range of temperature. It is shown that the materials possess low temperature Young's moduli as high as 1.6 Mbar, a figure which approaches the theoretical and experimental values for the c-axis crystalline modulus of this polymer. The α and γ relaxation processes are still clearly discernible even at highest drawn ratios (ca. 35) and a quantitative analysis of the results, using struct...
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Abstract The effect of molecular weight on the cold drawing behaviour of melt crystallized linear polyethylene has been studied. It is shown that the draw ratio achieved under comparable conditions rises with decreasing M w , very high draw ratios (∼36) being possible for optimum morphology of the undrawn polymer. The yield behaviour was also examined, and it is shown that the yield stress is affected in a complex fashion by both crystallization conditions and molecular weight. These results are...
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#1G. Capaccio (University of Leeds)H-Index: 3
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Recent work is presented on the plastic deformation of linear polyethylenes. This work demonstrates the importance of molecular weight and initial morphology in determining the drawing behavior, and shows that by appropriate choice of these two factors a substantial increase in draw ratio and hence stiffness can be achieved over conventionally-oriented polyethylenes. Under optimum conditions a modulus of approximately 700 Kbar was obtained at a draw ratio of ∼30. These very high modulus material...
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