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.     

Evolution of morphology in high molecular weight polyethylene during die drawing

Deformed high molecular weight polyethylene (HMWPE) rod, formed by die drawing at 115C, was cleaved longitudinally at liquid nitrogen temperature and the cleaved surface was etched by the permanganic etching technique, A series of etched surfaces of HMWPE sections of variable draw ratio (1–13) was analysed by scanning electron microscopy (SEM), The evolution of crystalline structure in HMWPE during die drawing was observed directly. In undrawn HMWPE, the spherulites were made up of sheaf-like lamellae and scattered within an amorphous phase. During die drawing, first, microscopically inhomogeneous deformation occurred and the spherulites aligned along the drawing direction; then at a draw ratio of about 7, local melting occurred, the spherulites disintegrated and the sheaf-like lamellae oriented, followed by strain-induced recrystallization and the growth of the lamellae; finally, at a draw ratio of about 12, plastic deformation of the lamellae occurred and microfibrils were formed by drawing the lamellae.     

Deformation and transformation mechanisms of poly(vinylidene fluoride) (PVF2)

The deformation process and the accompanyingα→β phase transformation of poly(vinylidene fluoride), drawn at 82 to 90 and 130° C, has been characterized by electron microscopy and X-ray and electron diffraction. Micronecking occurs at both draw temperatures, fibrils and mosaic blocks being drawn off the edges of the micronecks. The degree of phase transformation, at the same elongation, is dependent on the draw temperature; at 130° C the majority of the sample remains in theα phase at the natural draw ratio. The phase transformation at both draw temperatures accompanies the transformation from lamellar (block) structure to fibril.     

The plastic deformation of ultra-high molecular weight polyethylene

Gel-spun filaments of different initial morphologies have been subjected to controlled drawing at elevated temperatures. The drawn samples have been examined by high-resolution scanning electron microscopy. The deformation mechanism at temperatures up to 120° C is very similar to crazing, especially in the case of unoriented gel-spun filaments. Filaments exhibiting a shish-kebab morphology offer the opportunity of examining the deformation of elementary fibrils in a quantitative way. The transformation of individual lamellae into fibrils is the initial deformation mode, which is followed by slip of fibrils at a later stage. This is concluded from a comparison of experimental data and model calculations of the maximum draw ratio. Drawing at 144° C results in the formation of globular aggregates of lamellae, with a characteristic long period of 40 nm. This long period persists until all the globules have been converted, by micronecking, into aggregate fibrils of extended-chain character. On a molecular scale, the various processes can be described as the temperature-dependent flow behaviour of an entanglement network.     

The tensile deformation of flax fibres as studied by X-ray scattering

Small and wide-angle X-ray scattering experiments with in-situ deformation of dry flax fibres have been carried out. An increase in the (200) peak intensity during deformation has been attributed to strain-induced crystallisation of the cellulose microfibrils, and provides evidence that the non-crystalline cellulose chains are initially oriented. However, no change in the equatorial small-angle streak (from cellulose microfibrils), the meridional reflection (from a crystalline/non-crystalline repeat along the fibre), or the microfibril orientation was seen.     

Microstructural changes induced in linear polyethylene by plastic deformation (rolling)

Mechanical spectrometry and differential scanning calorimetry analysis completed by wideangle X-ray diffraction and density measurements were performed on linear polyethylene in order to characterize the influence of a plastic deformation (rolling) on the microstructure. Thus, rolling performed at room temperature results in the breaking of the thicker lamellae. The changes in the microstructure in polyethylene induced by rolling are in agreement with those suggested by other authors. A phenomenological model, applied to the relaxations exhibited by polyethylene, allows quantitative analysis of the microstructural changes due to plastic deformation. Thus, rolling induces an increase in the interactions between phases through an increase in the number of tie molecules connecting the broken crystallites. Moreover, rolling results in an increase of crystallographic defect concentration within the crystalline phase and this plastic deformation induces phase transformation from orthorhombic to monoclinic lattice, which is preferentially developed for thicker lamellae.     

Plastic deformation of polypropylene

In a wide range of drawing temperatures (20 to 150° C) and draw-ratios (5 to 20) the axial long periodL _T is a unique function of temperature. It is completely independent of the long period of the starting material which was varied within wide limits (125 to 350 Å). The gradual transformation of the long period observed at small draw-ratio ( between 1 and 5) from the valueL ₀ of an undrawn sample to the limiting valueL _T>L ₀ of highly drawn polypropylene ( 5) could be demonstrated on samples withL ₀ larger thanL _T. The change of long period is abrupt, indicating a discontinuous step in the transformation from the original microspherulitic into the fibre structure.These results may be interpreted in the same manner as in the case of polyethylene. During plastic deformation in the neck the lamellae are broken into small folded chain blocks which are then incorporated into the microfibrils, the basic building element of the fibre structure. The work of deformational forces must so mobilise the chains in the blocks that they get rearranged with a new long period, corresponding to the temperature of drawing.Paper I: The limiting axial long period of drawn polypropylene,J. Polymer Sci. A2 in press.On leave of absence from the Institute of Chemical Physics, Rocosalano, CSIC, Madrid, Spain.     

Phenomenological aspects of the double yield of polyethylene and related copolymers under tensile loading

The double yield point is shown to be a common feature to polyethylene and ethylene copolymers, regardless of the crystallinity level. Particular attention has been paid to the influence of draw temperature and strain rate which unambiguously indicate a combination of two thermally activated rate processes. Various thermal treatments have been investigated in order to check the influence of the crystal thickness distribution and the chain topology on the yield behaviour. Isothermal crystallization at high temperature is shown to have little effect compared with variations of crystallinity, temperature and strain rate in the case of compression-moulded samples. On the other hand, a strong effect has been observed in the case of solution crystallization which is well known to affect the chain-folding topology. The results are fairly consistent with the previous proposal by Takayanagi that (1) two processes govern the plastic deformation of the crystalline lamellae in semi-crystalline polymers, and (2) these processes are closely related to the viscoelastic relaxations in the crystal. The crystalline lamellae may deform plastically through sliding of crystalline blocks (brittle process) and/or homogeneous shear (ductile process). In order to account for the dependency of the brittle-to-ductile transition on the copolymer structure and crystallization method, a molecular model is put forward on the basis of the chain topology concepts borrowed from our former investigations on the tensile drawing and the melting behaviour of ethylene copolymers.     

Fabrication of nanostructured pearlite steel wires using electropulsing

This work reports the refinement of pearlite structure into nanostructure using electropulsing. Nanostructured pearlitic steel wires possess nanoscale lamellae or nanoscale grain microstructures. Fabrication of nanostructures by severe plastic deformation and lamellar to grain transformation have been investigated. It is suggested that an aligned pearlite structure is preferred in severe plastic deformation. The lamellar to grain transformation is controlled by diffusion of carbon within cementite and also from cementite to ferrite phases. Carbon mobility is changed by mechanical, thermal and electrical states. The interface between nanoscale sub-grains in the ferrite phase has considerable carbon content. Numerical calculations and experimental observations demonstrated these mechanisms.     

An infra-red spectroscopic and X-ray diffraction study of cold-drawn high density polyethylene samples

Three samples of high-density polyethylene, one linear, the second with one ethyl branch per thousand carbon atoms and the third with a comparable concentration of butyl branches, cold drawn at 60° C to elongations of 300, 800 and 1600%, have been examined by infra-red spectroscopy and wide- and small-angle X-ray diffraction. The results are consistent with, and give additional information about, the Peterlin model of plastic deformation. The change in crystallinity with draw ratio has been measured by infra-red spectroscopy and a wide-angle transmission X-ray diffraction method that minimizes the effect of the uniaxial orientation of the drawn specimens. Wide-angle measurements by reflection proved to be unreliable. Changes in the relative concentrations of methylene groups in crystalline regions, in gauche conformations and in tie chains in amorphous regions, and the alignment of tie molecules, have been followed by unpolarized and polarized infra-red spectroscopy. Small-angle X-ray diffraction measurements show that microvoids, with average dimensions of 250 and 80 Å parallel to and perpendicular to the draw direction respectively, are formed during the transition from the lamellar to the fibrillar structure.     

The adiabatic fracture of thermoplastic fibres

The development of methods for measuring true stresses and strains in thermoplastics and of models for representing the results, makes it possible to predict polymer performance in a number of ways. Recently this method was used to study the stability of the tensile deformation of high-density polyethylene under adiabatic conditions. It was proposed that at high strain rates, thermomechanical softening would render the plastic deformation process unstable, promoting localised deformation and fracture. In this paper, the isothermal extension process measured at different temperatures is assumed to be stopped and then restarted after different draw ratios have been attained, as in the drawing of a fibre. In this way the effect of draw ratio on fibre tensile properties can then be predicted. It is shown that, with fast deformation under adiabatic conditions, the softening effect due to the increase in temperature exceeds the opposing influence of strain hardening so that the nominal stress is predicted to fall continuously with increased strain. This leads to a ductile fracture process, which, in a fibre, can generate mushroom shaped blobs of polymer at the broken ends. This effect has previously been reported by Hearle and co-workers. The applicability of the model to different type of fibre is considered.     

Formation of bimodal crystal textures in polypropylene

Electron microscopy, selected area electron diffraction, X-ray diffraction, and dynamic-mechanical testing have been used to study flow-crystallized and hot drawn isotactic polypropylene. As a result of these investigations, it was found that bimodal crystal textures can apparently be formed by at least two different treatments, but the corresponding morphologies are completely different. Flow-induced crystallization was observed to result in a microstructure of lamellae oriented perpendicular to the flow direction, while hot drawing of polypropylene films above a critical temperature produced a morphology of microfibrils lying parallel to the draw direction. Below this critical temperature, drawing produced a fibrillar morphology having only a typical unimodal fibre texture. As a result of information obtained here, a mechanism involving epitaxial deposition of chain segments onto growing lamellae is concluded to be responsible for formation of the bimodal crystal texture in flow-crystallized material.     

Structural model of mechanical properties and failure of crystalline polymer solids with fibrous structure

The failure of an axially strained polymer solid having a fibrous structure is caused by formation, coalescence, and growth of microcracks up to critical size crack, which then propagates catastrophically through the cross-section of the sample. The primary candidates for microcrack formation are the ends of microfibrils where the material connection by tie molecules to the rest of the sample is almost completely interrupted. The opening of microcracks and sliding motion of fibrillar elements ruptures locally the most strained taut tie molecules and, thus, produces radicals detectable by ESR. But, chain rupture is the consequence and not the cause of displacement of the strong fibrillar elements. It also does not substantially affect the load carrying properties of the sample which mainly depend on the lateral autoadhesion of microfibrils and fibrils and on their quasi-viscous resistance to axial displacement. Hence, one has to reject the completely inadequate models trying to base the observed load-elongation curve of such samples on the load carrying properties of those tie molecules which are eventually ruptured upon straining. Some examples of these models are treated explicitly.     

Comparing the surface and internal structure of polypropylene fibres using advanced microscopy techniques

This paper highlights the importance of both surface and internal (bulk) structure of polypropylene (PP) melt extruded monofilament fibres and the dependence of structure on processing conditions. Gravity spun and as-spun fibres showed similar spherulitic surface structure but Wide Angle X-ray Scattering (WAXS) results indicated that the overall fibre crystallinity was contrasting for the two fibre types. From analysis of longitudinal and transverse fibre cross sections using Scanning Probe Microscopy (SPM) and Environmental Scanning Electron Microscopy (ESEM) it was found that gravity spun fibres showed a shish-kebab type structure in contrast to the macrofibrillar internal structure of the as-spun variant. In situ tensile testing gave powerful evidence to suggest that deformation in the necking region for the gravity spun fibres was due to the composite behaviour of the spherulitic surface and the internal shish-kebab structure.     

Correlation of hardness and microstructure in unoriented lamellar polyethylene

The stress-strain diagrams of a series of melt-crystallized polyethylene samples with a varying number of chain defects have been investigated. The elastic modulus,E, and the surface hardness,MH, markedly decrease with increasing number of defects. The mechanical behaviour of the lamellar structure of PE modulated by a major exclusion of chain defects from the crystals is discussed in the light of Takayanagi's two-phase model. The data suggest thatE is very sensitive to the fraction of tight crystalline bridges between lamellae. The correlation found betweenE andMH emphasizes, in addition, the different and complementary role played by the amorphous layer in each mechanical test. In the former case one measures the elastic deformation of the layer reinforced by tie molecules. In the latter test the plastic deformation under compression of the lamellae sandwiched between noncrystalline layers is contemplated. In both cases the influence of the number of defects drastically affects the nature of the crystalline lamellae and surface layer and consequently substantially modifies both types of properties.     

Imaging of polyethylene films by diffraction contrast

The technique of crystalline diffraction contrast imaging of lamellae in spherulitic and oriented thin films of polyethylene is illustrated for both conventional transmission and scanning transmission electron microscopY. Bright-field “ghost” imaging permits real space crystallography of the specimen and reveals the occurrence of variable chain inclination in a given lamellar preparation. N beam annular dark-field scanning transmission microscopy is useful for distinguishing between curved lamellae and mosaic blocks as well as for the direct imaging of the amorphous regions between lamellae.     

Atomic force microscopy of plastically deformed polyethylene subjected to tensile deformation at varying strain rates

Abstract

The surface damage induced during tensile deformation of polyethylene at different strain rates was studied by atomic force microscopy (AFM) operated in tapping mode, before and subsequent to uniaxial tensile plastic deformation. Atomic force microscopy revealed striking differences in the deformed microstructures up to the nanoscale range. The surface of undeformed polyethylene was characterised by ribbonlike fibrils of width 0·25 νm and surface features of height about 20–60 nm. Fibrils were considered to consist of microfibrils of width 0·03–0.04 νm. Small scan (30 × 30 nm) AFM images provided details of microfibrils containing chains of molecules of ~ 0·5 nm wide. Tensile deformation in the plastic region involved stretching of fibrils and microfibrils resulting in the formation of surface openings. The ability of the ribbonlike surface fibrils and microfibrils to stretch, merge, and acquire an oriented and flat structure increased with increase in strain rate in the uniaxial tensile test. Also, with increase in strain rate the chains of molecules unfold and align to produce an oriented and elongated structure. The impact of deformation on amorphous regions could only be observed at high strain rates.     

Fibrillar structure of superdrawn polyoxymethylene fibres

A fibre structure of superdrawn polyoxymethylene fibres was examined by scanning electron microscopy. The fibre had a skin-core structure and was characterized by a double fibrillar network which was formed from a frame network of trunk fibril screens parallel to the fibre axis, a sub-network of branch fibrils inside the frame, thin cross-fibrils connected to the network and longitudinal void-chains connected to the cross-fibrils between the fibril screens. The frame fibrillar network originated in the undrawn spherulite network, trunk and branch fibrils originated in a bundle of lamellar crystallites and cross-fibrils probably resulted from the interlamellar amorphous phase. The fibrillar network was also varied in the radius direction, possibly due to the diversities of the undrawn spherulitic morphology and the deformation mode at necking.     

Effect of uniaxial deformation on the optical scattering losses of a semicrystalline fluorocopolymer

Both small- and wide-angle light scattering as well as transmission measurements have been used to investigate the optical scattering losses of a vinylidene difluoride-tetrafluoroethylene-hexafluoropropylene copolymer crystallized from the melt. The main origin of the scattering loss is the wide-angle light scattering from the spherulitic superstructure. Uniaxial deformation transforms this structure into a fiber morphology. The attenuation of fibers has been measured for light propagating both parallel and perpendicular to the orientation axis. For both directions, the attenuation decreases with increasing draw ratio. Annealing of the fibers while keeping their ends fixed is an effective method to reduce the attenuation further, to a low value as close to that of the melt.     

Direct electron microscopic observation of transcrystalline layers in microfibrillar reinforced polymer-polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight ratio of PET/LDPE = 50/50. Dog bone samples with MFC structure were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy. SEM observations on cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils. By means of TEM on stained ultrathin slices one observes the formation of transcrystalline layers of LDPE matrix on the surface of the PET microfibrils. In these layers the crystalline lamellae are aligned parallel to each other and are placed perpendicularly to the fibril surfaces. This is in contrast to the bulk matrix where the lamellae are quasi-randomly arranged.