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HIGH MODULUS POLYMERS : THE PREPARATION
AND PROPERTIES OF ULTRA-HIGH MODULUS
POLYETHYLENE
I. Ward
To cite this version:
S TRUC TURE ET
PROPRIETES
MÉCA NIQUES
DES
POLYMÈRES
A
L'ÉTAT SOLIDE.
HIGH MODULUS POLYMERS
:
THE PREPARATION AND PROPERTIES
OF
ULTRA-HIGH MODULUS POLYETHYLENE
1. M. WARD
Department of Physics, University of Leeds, Leeds LS2 9JT, U.K.
Résumé.
-
Des polyéthylènes ultra-orientés, dont le module d'Young est comparable à celui du verre ou de I'aluminium, ont été produits par ultra-étirement et extrusion hydrostatique. Des règles générales ont été établies à présent pour produire ces matériaux ultra-orientés à partir de diverses structures initiales. Les polymères étirés se rapprochent le plus des matériaux composites. Des ponts intercristallins fournissent le renforcement et le haut module du matériau, tandis que la matrice orientée mais non cristalline donne naissance aux phénomènes de force de rétrécissement élevée, de point de fusion élevé et d'effets thermiques considérables.Abstract. - Ultra-highly oriented polyethylenes with Young's moduli comparable to glass and aluminium have been produced by superdrawing and hydrostatic extrusion processes in which the initial isotropie polymer is stretched thirty times or more. The plastic deformation process is sensi- tive to the molecular weight and molecular distribution of the polymer, the initial morphology and the drawing or extrusion conditions. Guidelines have now been established for the production of these ultra-highly oriented materials from a range of initial structures. Although the final drawn polymers al1 possess comparable high Young's modulus, other properties such as their viscoelastic and thermal behaviour may differ markedly. The drawn polymers are best regarded-as 'elaborate composite materials. Intercrystalline bridges provide reinforcement and hence high modulus, whereas oriented non-crystalline material gives rise to high shrinkage force and high melting temperatures with major superheating effects.
1. Introduction.
-
The recent discovery that ultra- high modulus linear polyethylene can be prepared by comparatively straightforward drawing [l] and hydro- static extrusion [2] has renewed interest in the plastic deformation behaviour of polymers. In addition, a new range of materials has been produced, with a novel range of structure and properties.2. The preparation of ultra-high modulus polyethy- lene by drawing.
-
There are three important factors which determine the drawing behaviour of linear polyethylene [l, 3,4]. These are :1) The molecular weight and molecular weight distribution of the polymer.
2) The morphologY of the sample before drawing, which relates to the initial crystallization conditions.
3) The temperature and rate of drawing.
Detailed studies have shown that the initial crys- tallization is critical for low molecular weight poly- mers. Factors such as the segregation during crys- tallization of low molecular weight material and the regularity of the lamellar fold surfaces are important in determining the rate of draw. We believe that drawing involves the extension of a network with crystalline lamellae and molecular entanglements acting as temporary junction points. For high mole- cular weight material, molecular entanglements play
a decisive role and the morphology of the sample is less important.
In al1 cases it is necessary to produce very high draw ratios if ultra-high modulus is to be achieved. The drawing process must be efective i.e. it must produce orientation and overall molecular alignment of the material, pulling out chain folds and molecular entanglements as appropriate. To a good approxi- mation there is a unique relationship between the Young's modulus of the oriented polymer and the draw ratio [l, 4,5,6], irrespeetive of molecular weight, initial thermal treatment and drawing conditions, providing that the latter have been optimized. It is important that high draw can be achieved in compa- ratively high molecular weight polymers provided that the draw temperature is correspondingly increas- ed [7]. The understanding gained has been sufficient to allow thè development of a satisfactory continuous drawing process for technological purposes as well as provide a wide range of new oriented materials for structural studies.
3. The hydrostatic extrusion process. - In parallel with the drawing process, it has also proved possible to produce ultra-high modulus oriented polymers in large unflawed sections by a hydrostatic extrusion process [2, 81. This work was initiated because of the belief that it would be favourable to produce high orientation in a stress field which is wholly compres-
C2-34 1. M. WARD
sive, so that tensile fracture of the polymer might be eliminated. and high orientation produced by the imposed deformation which takes place under condi- tions of plastic deformation. From a practical point of view, hydrostatic extrusion is important because it enables the production of large size samples for measurements of properties, especially mechanical anisotropy and thermal conductivity. It is also very instructive to attempt a detailed analysis of the mechanics of the process. This is simpler than for drawing, because the strain field is exactly specified by the die, whereas drawing is inhomogeneous and requires knowledge of the strain field in the neck which is less straightforward to obtain. It has therefore been possible to extend the analysis of drawing through a die (Hoffman-Sachs) to hydrostatic extru- sion of polymers [8, 91. The analysis is more compli- cated than for metals for three reasons :
1) As the polymer becomes oriented in its passage through the die, its yield behaviour changes due to intense strain-hardening.
2) The yield behaviour of a polymer is very depen- dent on strain rate.
3) The yield behaviour of a polymer depends on the hydrostatic component of stress (or in another representation on the normal stress on the shear plane as well as the shear stress).
By incorporating information from tensile drawing experiments and the redrawing of extruded samples it is possible to obtain the true-stress-true strain-strain rate relationship for a given polymer. The strain field in the die, which gives the strain and strain rate at any point, is calculable from the geometry of the die. A numerical integration procedure then enables the calculation of the extrudate velocity as a function of the extrusion pressure for a given extrusion ratio (the equivalent to draw ratio). The calculation requires assumptions regarding the pressure dependence of the yieid behaviour (and also friction). It has been shown that a reasonable fit can be obtained to the experi- mental data with satisfactory assumptions regarding this pressure dependence.
4. The mechanical behaviour of ultra-oriented poly-
ethylene. - 4.1 CREEP BEHAVIOUR. - Ultra-oriented polyethylene, although it can have a Young's modulus as great as glass or aluminium, is still a viscoelastic material. It shows creep and viscoelastic relaxation transitions similar in kind to the isotropie polymer and oriented linear polyethylene of lower draw ratio. The creep behaviour has been carefully examined [IO] because of its technological importance if these materials are to find engineering applications such as reinforcement of cement. It has been found that the creep is very non-linearly dependent on stress. Moreover the creep rate decreases markedly with increasing draw ratio, and at a given draw ratio, when the molecular weight of the polymer is increased. The
non-recoverable creep can be modelled by simple activated rate theory (the Eyring formulation) and moreover this plastic deformation process can be identified in the tnie-stress-true strain-strain rate curves as one of the mechanisms which operates in the drawing process [9]. It appears that drawing involves a low stress process (possibly shear of the interlamellar material) with a large activation volume ( - IO3
A)
and a high stress process. identical to the creep process, which has a much smaller activation volume ( N 10'A)
and relates to a more localized process such as the pulling out of a chain fold or a molecular entan- glement.4.2 ANISOTROPIC « ELASTIC » BEHAVIOUR. - It is also important to attempt to understand the very high Young's modulus of the oriented polymers, which is essentially a question of understanding the elastic behaviour. There are two relaxation transitions, the a and y relaxations [Il]. At
-
180 OC the polymer is below the y relaxation, and there is a plateau region where the modulus is nearly independent of tempe- rature, and can be as large as 160 GPa, which is 213 of the theoretical modulus. There is a similar plateau at-
-
100 O C , between the a and the y relaxations,the exact temperature range depending on the fre- quency of measurement. We have therefore attempted to mode1 these plateau moduli on the basis of struc- tural considerations which will now be discussed.
against 200
A).
There is no evidence for extended chain material. Although the melting point as deter- mined from DSC measurements increases markedly with increasing draw ratio, even approaching 145 OC in some cases, there are major superheating effects, and we attribute the high melting temperatures to these, and associate them with the high degree of orientation in the non-crystalline regions. The high orientation of the non-crystalline regions is also responsible for the very high shrinkage force at comparatively low temperatures (- 80 OC in somecases).
In summary, the key points from the structural studies show :
1) Fibrillar nature.
2) Very high crystalline orientation.
3) Some remaining periodic fluctuations.
4) Increases in crystal thickness with increasing draw ratio, but no extended chain material.
6 . Interpretation of high elastic modulus. - A priori,
the very high modulus could arise from two reasons : 1) A large number of taut tie molecules connecting the lamellar structure, which was formed after the destruction of the spherulitic texture. This is a series model with crystal lamellae and non-crystalline regions in series, the stress being homogeneously distributed across the lamellae.
2) Essentially extended chain crystals in parallel with the non-crystalline material. Here the stress is concentrated on the crystalline regions so that the crystal strain would be higher than in case (1) by a factor related directly to the crystallinity.
X-ray diffraction measurements on oriented samples under stress [12] have shown that model (2) is more correct than model (1). We- have found that crystal
moduli determined assuming homogeneous stress are less than the crystal modulus for an ideal sample with parallel lamellae by as much as a factor of 0.6. On the other hand, as we have emphasized, the struc- tural studies show that there is no extended chain material present. Moreover, there are increases in crystal thickness with high draw. We therefore propose that the increase in stiffness is due to intercrystal- line bridges (a continuous crystalline fraction). This explains the WAXS crystal thickness results. It also explains results of nitric acid etching experiments, where the molecular weight of the degraded material
has been determined by gel permeation chromato- graphy. It is possible, in fact, using a statistical model, to calculate the proportion of intercrystalline bridge material. If it is assumed that only the centre portion
of the intercrystalline bridge material is effective in transmitting stress throughout the structure, the situation is formally identical to that of a fibre rein- forced composite, with stress decay occurring at the ends of the intercrystalline bridges in a similar manner ta the stress decay at the ends of the fibres in the reinforced composite. The plateau moduli have been examined in the light of this model, using the related structure data, and a satisfactory understanding of the situation has been achieved.
It is therefore concluded that ultra-highly oriented polyethylene is like a reinforced composite. Inter- crystalline bridges are the reinforcing elements. and give rise to high stiffness and high thermal conduc- tivity. The matrix may contain some unconnected crystalline material but also oriented non-crystalline material which is important with regard to creep, shrinkage, the high shrinkage force and the high melting temperature. This approach also provides a satisfactory understanding of the X-ray crystal strain measurements.
7. Conclusion. - It is now possible to provide the principal guidelines for the preparation of ultra-high modulus polyethylene by drawing and hydrostatic extrusion. .Moreover, it can now be understood how these guidelines arise in terms of the effects of mor- phology and molecular weight on the plastic defor- mation behaviour.
C2-36 1. M. WARD
References
[l] CAPACCIO, G. and WARD, 1. M., Nature Phys. Sci. (1973) 143, U. K. Patent Appl. 10746173 filed 6.3.73 ; 'Polymer 15 (1974) 233 ; Polym. Eng. Sei. 15 (1975) 219.
[2] GIBSON, A. G., WARD, 1. M., COLE, B. N. and PARSONS, B., J. Mater. Sei. 9 (1974) 1193 ;
GIBSON, A. G. and WARD, 1. M., U.K. Patent Appl. 30823173 filed 28.6.73.
[3] CAPACCIO, G. and WARD, 1. M., Polymer 16 (1975) 239.
[4] CAPACCIO, G., CROMPTON, T. A. and WARD, 1. M., J. Polym. Sci. A 214 (1976) 1641.
[5] CAPACCIO, G., CHAPMAN, T. J. and WARD, 1. M., Polymer 16 (1975) 469.
[6] ANDREWS, J. M. and WARD, 1. M., J. Mater. Sci. 5 (1970) 411.
[7] CAPACCIO, G., CROMPTON, T. A. and WARD, 1. M., Polymer 17 (1976) 645.
[8] GJBSON, A. G., Ph. D. thesis, Leeds 1977. [9J COATES, P. D., Ph. D. thesis, Leeds 1976.
[IO] WILDING, M. A. and WARD, 1. M., Polymer (in press). [Il] SMITH, J. B., DAVIES, G. R., CAPACCIO, G. and WARD, 1. M.,
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[12] BRITT~N, R. N., JAKEWAYS, R. and WARD, 1. M., J. Mater.
