Comparison of structure formation upon drawing of gel-cast and melt-crystallised UHMWPE films

Abstract

Structure and mechanical properties of ultra-high molecular weight polyethylene (UHMWPE) films of different draw ratios produced from gel-cast and meltcrystallised samples by multi-stage zone drawing have been studied in order to investigate the structural development and to clarify the role of interfaces between morphological units in the mechanical behaviour. The complex hierarchy of fibrillar structures generating upon neck formation is revealed. It is shown that the rearrangement of initial chain-folded crystallites into a microfibrillar structure occurs inside the volumes of supermolecular structures of larger sizes (spherulites in the melt-crystallised films and stacks of lamellae in the gel-crystallised films), the boundaries between the latter not having disappeared. Both the initial spherulites and stacks of lamellae with internally rearranged microfibrillar structure elongate upon drawing and comprise spindle-like units -macrofibrils -which are well resolved in scanning electron microscopy. Moreover, superfibrils have been revealed in the melt-crystallised films for the first time. They are, as suggested, the remnants of deformed and rearranged nascent polymer particles. Thus, the volume of the drawn films is considered to be full of interfaces. It is suggested that the majority of conformational defects, like chain ends, loops, tie molecules, entanglements, etc., are expelled from fibrillar crystallites in both intra-and interfibrillar regions. In addition to that, the interfibrillar spaces are crossed by tie molecules and micro-and macrofibrils. All of them hinder interfibrillar slip. The differences in creep and recovery of the investigated films are discussed in terms of density of packing and degree of connectivity of various scale fibrillar structural units. The lateral boundaries and the tips of fibrils are considered to be the weakest structural sites from which kink band formation and subsequent fracture of the loaded material begin.

Equal‐channel multiple angular extrusion of polyethylene

“…The behavior of plastic characteristics in the case of ECMAE differs from the observed one when traditional schemes of solid‐phase extrusion with typical continuous reduction of plasticity with increase in accumulated deformation 17, 34, 36. In the case of ECMAE, starting from definite ε, values ε y and ε b become constant.…”

“…With increasing ε, the increase in the mentioned properties becomes slower. This phenomenon is characteristic of ECMAE of semicrystalline polymers,15, 17 differing from such methods of solid‐phase processing as die‐drawn, hydrostatic extrusion, and rolling, where almost linear growth of these characteristics takes place with increase in the accumulated deformation up to limiting values of the deformation degree 32–34. It should be noted that in contrast to traditional methods of solid‐phase extrusion aimed at creation of high‐modulus states of crystallizing polymers, ECMAE determines balanced increase in modulus and strength.…”

“…It is known that after solid‐phase extrusion,34 when the process of rearrangement of the initial structure into a macrofibrillar structure finishes, further plasticity of extrudates depends on the degree of orientation of polymeric chains, structure of interfibrillar spaces, (i.e., by connectivity and density of packing of fibrillar structural units) and ability of all fibrillar morphological units to slip along one another. The last phenomenon is accompanied by intracrystalline slip, migration of defects through crystallites, an increase in orientation of molecular segments in disordered intra‐ and inter‐fibrillar regions and increasing amount of taut tie molecules.…”

Morphological Transformation in Polymer Composite Materials Filled with Carbon Nanoparticles: Part 1—SEM and XRD Investigations

“…The morphology of the drawn nanocomposite fibers (DR = 8) was also examined using the SEM method. It is well known [ 3 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ] that during high-temperature orientational drawing, the lamellae morphology transforms in the fibrillar one. As an example, the SEM micrograph of the internal structure of the fiber with the added nanodiscs is presented in Figure 6 .…”

Injection‐Molded Ultrahigh Molecular Weight Polyethylene Material with Improved Moldability for Artificial Joint Implants Design

“…[ 57 ] After that limit, additional stretching transpires only due to the slippage of the macrofibers along one another, accompanied by enhancement in orientation of molecular sections with arbitrary intrafibrillar and interfibrillar regions and increment in number of taut tie molecules; ergo, the structure ameliorates by dint of disentanglement of molecules and gauche‐trans transition occurring in tie molecules. [ 58 ] Slippage takes place when the applied load is adequate to tear tie molecules, overcome friction, create intracrystalline shear, and disentangle interpenetrated molecular chains. [ 59 ] Another phenomenon calling attention is the whitening of the specimens subjected to high strains, plastic deformation, due to the formation of kink bands which stem from the fracture of flimsiest macro‐ and microfibrils, and the release of the elastic energy stored within them as shown in Figure 7II‐c.…”