Deformation mechanisms in oriented high-density polyethylene

The deformation of samples of oriented high-density polyethylene has been analysed in terms of three principal deformation mechanisms,fibrillar slip, lamella slip andchain slip. From a study of small- and wide-angle X-ray diffraction patterns it is possible to deduce which mechanism or mechanisms are operating in particular cases. Material prepared in three different ways has been examined and it appears that in all three cases the primary mechanism for plastic deformation is [001] chain slip.In oriented and annealed material with a well-defined lamella crystal structure it has been possible to show that the recoverable elastic deformation is primarily due to reversible lamella slip. In this material plastic deformation by chain slip starts at a well-defined critical resolved shear stress of about 15 MNm^–2.Deformation of oriented unannealed material, in which the crystal structure is not so well-defined, appears to be more complicated. In material prepared by cold drawing some of the plastic strain may be accounted for by permanent lamella slip. Fibrillar slip does not appear to be a major deformation mechanism in any of the three materials.     

The structure of high-density polyethylene with a single-crystal texture

The structure of specially prepared high-density polyethylene with a single-crystal texture has been examined by wide-angle and small-angle X-ray diffraction and by electron microscopy. The bulk of the crystallites in the sample are found to be oriented within 8° of a perfect single-crystal texture. The small-angle X-ray diffraction patterns suggest that lamellar crystals are present oriented approximately perpendicular to the chain direction. Replicas taken from fracture surfaces indicate that the structure fractures cleanly parallel to (100) which, taken together with other evidence, suggests that molecules fold by adjacent re-entry in (100) planes. Etching in fuming nitric acid reveals a lamellar crystal structure which is consistent with that deduced from small-angle X-ray diffraction. Replicas which were taken of etched surfaces after deformation showed crystals which were tilted with respect to the chain direction. This is consistent with the deformation having taken place by [001] chain slip within the lamellae [1].     

The morphology,chain structure and fracture behaviour of high-density polyethylene

High-density polyethylene (HDPE) is being used more and more in critical long-term applications. For this reason it is important to have a strong understanding of those parameters which control the fracture behaviour of HDPE. In Part I of this work, fracture results were presented for eleven HDPE samples tested using a tensile testing machine. Such short-term tests do not accurately reflect the in-service loads on HDPE components, which tend to be low and static. It is, therefore, important to perform fracture tests under long-term static loads. The results of such testing are presented in this paper. The resistance to static fatigue was found to be most strongly dependent on molecular weight. Short branch concentration and short branch length were also found to exert an influence on the resistance to static fatigue. This result is similar to the findings presented in Part I of this work. However, there is some evidence that molecular weight influences fracture behaviour to a greater extent in the long-term tests. Notwithstanding, the similarity between the short-term and long-term results is important. It means that an early indication of the long-term performance of HDPE resins can be obtained from rapid comparative tests conducted using a tensile testing machine.     

The tearing of highly oriented linear polyethylene

The tearing behaviour of oriented linear polyethylene has been studied with particular emphasis on the effects of draw ratio, draw temperature and molecular weight distribution. In all cases the values of fracture surface energy were in the range 10² to 10⁴ Jm^–2. The differences between the low molecular weight grades and high molecular weight grades of linear polyethylene were large. At high molecular weight the energy for crack propagation parallel to the direction of orientation fell by a factor of approximately two as the draw ratio was increased from 10 to 20. At low and medium molecular weights, increasing the draw ratio above a value of 10 had no significant effect on the fracture surface energy. Observation of the fracture surfaces showed a simple relationship between the nature of the fracture surface and the value of the fracture surface energy. In general, the higher the value of fracture surface energy the less fibrillar is the fracture surface. Furthermore the fibrillar nature of the fracture surface is essentially determined by the void content of the material.     

The deformation of oriented high density polyethylene

Thin, single crystal-like textured films of HDPE were uniaxiaIly elongated at 56, 93 and 129°C along the chain direction. In all cases, the initial shish-kebab morphology was transformed via <001> crystal shear, chain slip and “defect” generation within the crystalline phase. Shear between shish-kebabs was observed at all temperatures and was identified by a rotation of lamellar normals away from the elongation direction. Craze-like structures developed at all temperatures as well, but only propagated laterally at temperatures above the alpha transition temperature of polyethylene. The evolution of the crystalline phase during deformation was imaged in detail by darkfield TEM. The generation of long (3μm), thin crystalline fibrils (“protofibrils”) of about 7 nm diameter indicated that the material had undergone strain induced crystallization. Lateral connections between the original kebabs were retained during drawing in many cases, and constituted tie fibrils between adjacent “craze” fibrils. The processes which occurred in thin films at temperatures above the alpha transition and which gave rise to long crystals provided insight into the generation of a continuous crystalline phase in bulk polyethylene.     

Crystallite size determination and identification of lamellar surfaces of microparacrystallites for uniaxially and doubly oriented polyethylene films

Low density polyethylene (PE) films stretched 4 x at 20° C and annealed at 100° C show the well-known SAXS four-point diagrams with a tilting angle of lamellae of 35°. The 7.5 nm thick lamellae consist of rod-like microparacrystallites (mPCs) of 25 nm x 7.2 nm lateral sizes; the long axes of the mPCs are turned around the c-axis by 31° from the b-axis. The mPCs of each lamella stack together laterally like monolayers of cigars. After rolling the molten film at room temperature and then annealing at 100° C, a doubly oriented film arises, each half of it, anterior or posterior, producing a monoclinic two-point diagram. These are superimposed in SAXS. The mPCs are oriented in the plane stress field so that their b-axis is orthorgonal to the stretching direction and parallel to the film surface; their long axes however are again turned as before, but now by 26°. Furthermore their a-axis is tilted around the b-axis by 8° and the lamellar basal surface tilted against the b-axis in the opposite direction by 40°. The line profiles of the SAXS reflections give evidence that statistical irregularities of the lamellar surfaces are correlated in the 8° tilted direction or along the chain axis with the neighbouring surfaces by means of ultrafibrillar properties of the lamellar bundles, e.g. ribbon-like microfibrillar details of the lamellae. These can be described quantitatively by applying the theory of paracrystals on the superlattice generated by the centres of the mPCs. The lamellar surfaces are approximately parallel to the {523} and {311} netplanes of mPCs for the uniaxially and doubly oriented films, respectively. The conventional theory of mesophases can never describe structures which combine lamellar and fibrillar properties.Dedicated to P. P. Ewald on the occasion of his 90th birthday.     

Epitaxial crystallization of linear low-density polyethylene on high-density polyethylene

The oriented crystallization of linear low-density polyethylene (LLDPE) on high-density polyethylene (HDPE) has been investigated by transmission electron microscopy. From morphology and electron diffraction, it is confirmed that epitactic growth of LLDPE lamellae on the HDPE crystals takes place with an adoption of the HDPE crystal thickness at the interface and a continuous thinning of the LLDPE lamellae in the interface. This revised version was published online in November 2006 with corrections to the Cover Date.     

Orientation-dependent mechanical properties and deformation morphologies for uniaxially melt-extruded high-density polyethylene films having an initial stacked lamellar texture

The mechanical properties and the associated plastically deformed morphologies of high density polyethylene films were investigated by tensile testing, wide-angle X-ray scattering and transmission electron microscopy. Uniaxially oriented films having a well-defined stacked lamellar morphology, both with and without row-nucleated structure were deformed at three angles, 0°, 45° and 90°, with respect to the original machine (extrusion) direction. A distinct orientation dependence of the mechanical properties was observed and this dependence has been related to the different morphologies developed during the plastic deformation processes. It was shown that lamellar separation, lamellar shear and lamellar break-up were the dominant initial deformation mechanisms for the respective 0°, 45° and 90° deformations. As a result, the 45° and 90° deformations generated a final microfibril morphology oriented along the stretch direction, while the 0° deformation resulted in broken blocks of crystalline lamellae. The presence of distinct row-nucleated crystalline fibrils in the initial structure stiffens the material in the 0° deformation; however, it significantly limits the ability of the materials to cold draw at the 90° deformation. Morphological models were proposed to explain the plastic deformation process for the different deformation angles, as well as for the deformation behaviour of semicrystalline polymers with an isotropic spherulitic morphology.     

Multiple kinks in lamellar linear polyethylene

Injection-moulded specimens made of linear polyethylene with preferred orientation of the molecular chains (c-axes) in the direction of injection were deformed in a bending test. Ultra-thin sections of the deformed material were made following chlorosulfonization. Examination using transmission electron microscopy revealed that deformational and superstructural elements can be elucidated simultaneously using this method. Deformation in the observed regions with multiple kink bands is found to be inhomogeneous and concentrated in two shearing processes: one along the borders of the kink band and the other within a kink band at an acute angle to the border of the band. It must be concluded from the formation of the lamellae in the kink bands that deformation, even within a kink band, is inhomogeneous and is predominantly concentrated in the shear zones, while the lamellar stacks lying between the shear zones appear under electron microscopy, to be nearly undeformed.     

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall     

The transverse compression of oriented nylon and polyethylene extrudates

A theoretical treatment is given for the total diametral compression of a transversely isotropic elastic cylinder under compression between parallel rigid plates under conditions of plane strain. Experimental results are presented for the compression of isotropic and oriented cylinders of nylon 6.6 and linear polyethylene (Rigidex). These results confirm that the theoretical treatment is valid to a good degree of approximation. In the case of linear polythylene, where data are presented for ultra-highly oriented extrudates, the results are of some interest with regard to the elastic properties of these unusual materials.     

Toughness of high-density polyethylene in shear fracture

This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II) of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF) concept. The results show that even under the Iosipescu loading, tensile deformation (mode I) is still involved in the fracture process, possibly due to the significant work hardening that HDPE develops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significant work-hardening may be involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE in mode II with its mode I counterpart.     

Double torsion fracture testing of high-density polyethylene

The fracture of polyethylene has been studied extensively using conventional testing geometries such as three-point bending (TPB) and single-edge notch tension (SENT). These geometries are of limited utility for studying crack growth, because the crack speed is constantly changing and the crack front is in the centre of the specimen. Double torsion (DT) is a fracture geometry that suffers neither of these disadvantages, yet has only received limited attention in the literature. Its use has been limited to highly brittle materials such as glass, ceramics, thermosetting plastics and PMMA. In contrast to these materials, high-density polyethylene (HDPE) is an inherently ductile polymer. Before the advantages of DT can be exploited for testing HDPE, it is first necessary to demonstrate the validity of DT fracture measurements performed on such a ductile material. In this paper it is shown that at moderate rates of loading and at temperatures below 0C, valid double torsion fracture results can be obtained for an ethylene 1-butene copolymer. A novel technique for specimen preparation and a simple method for accurately monitoring crack growth are also described.     

Time dependent recovery of oriented polyethylene

The recovery behaviour after creep of oriented linear polyethylenes has been studied over the temperature range 20–60 °C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It has been shown that in spite of significant differences in recovery strain level, the recovery kinetics are not affected over a wide range of the structural variations and experimental conditions. It is concluded that the time dependent recovery behaviour is consistent with a model where two thermally activated processes are acting in parallel. More exact values for the activation parameters for both processes of the model have been obtained by taking into account the time dependent distribution of the applied stress between these two processes.     

The distribution of creep activation energies in oriented polyethylene films

The “flow” creep in oriented polyethylene was studied using a Doppler velocity meter. The activation energy of the creep process was evaluated and it was established that there are three activation barriers for creep in the material studied. The most probable values, the scatter, and the relative concentration of these barriers in polyethylene samples were determined.     

The measurement of shear compliances for oriented polyethylene terephthalate sheet

Two techniques have been used to measure the shear compliances of one-way drawn uniplanar-axially-oriented polyethylene terephthalate (PET) sheet, draw ratio 51; a torsional method for all three compliancesS ₄₄,S ₅₅ andS ₆₆, and a recently-developed simple-shear method forS ₄₄ andS ₆₆. The two techniques gave values for the compliances in common which agreed very well. An examination of end effects in torsion has also been made. The determination of the three shear compliances complete the determination of all nine independent compliances for this PET sheet. The overall mechanical anisotropy has been considered in the light of existing structural information, and comparison made with the elastic constants of isotropic PET and a highly oriented PET fibre on the basis of the single-phase aggregate model.     

Fracture and tearing in oriented polyethylene

Measurements of energy for crack propagation (fracture surface energy) have been made on low-density and high-density polythenes both in the undrawn state and in different states of orientation produced by drawing under various conditions. Both cleavage and tear tests were employed. For the unoriented materials the values of fracture surface energies were in the range 10⁴ to 10⁵ J m^–2.With increasing orientation (represented by birefringence) the energy for crack propagation parallel to the direction of orientation fell by a factor of approximately 100. The differences between the low-density and high-density polymers, and between the different types of low-density polymers examined, were comparatively slight.Measurements of crack tip diameter showed a direct relation between this quantity and fracture surface energy. From their comparable studies of the tearing of rubbers Rivlin and Thomas have interpreted such a relationship as implying that the high values of fracture surface energy arise from the work required to deform the material in the crack tip up to the point of rupture. On this basis the reduction in fracture surface energy with increase in orientation is a direct result of the reduction in the diameter of the crack tip.     

Lamellar separation during the deformation of high-density polyethylene

Upon tensile straining at low deformation rates (=2×10^–5 sec^–1), spherulitic linear polyethylene behaves reversibly for extensions up to 40%. In stress relaxation experiments on unloaded specimens the stress increases with time. Samples kept under constant strain ( = 40%) over four months show macroscopic cracking. Microstructural investigation was performed using low- and wide-angle X-ray diffraction and transmission electron microscopy. These investigations reveal a very inhomogeneous deformation within those lamellar stacks for which the crystalline lamellae lie normal to the tensile axis. The deformation in that case is similar to what has been observed for elastic hard fibres. A two-mechanism model to explain the macroscopic observation on the basis of the microscopic observations is developed.