Influence of draw ratio on morphological and structural changes in hot-drawing of UHMW polyethylene fibres as revealed by DSC

Abstract

The melting behaviour of gel-spun, ultra-high molecular weight polyethylene fibres was investigated in an attempt to characterize their morphology after various stages of hot-drawing at 148 ~ In this drawing process a shish-kebab morphology is transformed into a smooth fibrillar structure. It was concluded that this transition initially proceeds by pulling elastically inactive loops, originally present in the folded chain lamellae of the shish-kebabs, taut between entanglements. Thereafter a considerable amount of entanglements is removed by pulling molecular chain ends through them, until ca. 2.5 entangelements per molecule remain in the ultimately drawn fibres. The fibrils in the fully drawn fibres were found to be composed of chain-extended orthorhombic crystallites with an average length of 70 nm, which are interrupted by defect regions (containing trapped entanglements and chain ends) of about 4 nm in length. If free shrinkage of the fibre was allowed, this structure had an equilibrium melting temperature of 140.5 ~ Upon constrained melting of the filaments, a solid-solid phase transition could be observed in the DSC thermograms (at a temperature of ca. 150 ~ associated with a transition of the chain-extended orthorhombic blocks in the fibrils into a hexagonal phase. The heat effect associated with this solid-solid transition in perfectly crystalline polyethylene (AHo_ h) was estimated to be 205 kJ/kg, whereas for the heat of fusion of the hexagonal phase (AHh _ m) a value of 81 kJ/kg was assessed.

The plastic deformation of ultra-high molecular weight polyethylene

“…X-ray investigations have shown the weight-average crystallite length to increase with increasing draw ratio [23]. In terms of our structural model presented above, this increase with X indicates that the newly formed fibrils contain longer crystallites than the globules from which these fibrils are drawn.…”

“…During the hot-drawing process, therefore, any conformational changes can be entirely attributed to the flow field generated by the deformation. The initial long period of 40 nm is not determined by the drawing temperature, but it is characteristic of aggregated shishkebab morphologies and related to the exceptionally large molecular weight between entanglements in the gel filament [22,23]. The observed invariance of the long period with the draw ratio indicates that the molecular conformation in the globular aggregates is not essentially changed during drawing.…”

SAXS experiments on voids in gel-spun polyethylene fibres

“…This fibre consists of a dense shish-kebab like structure with fibrillar backbones and much lamellar overgrowth as revealed by DSC experiments [25,39]. In Fig.…”

High-Performance Fibers

“…Polyolefin Fibers) is that they are made of essentially fully extended chains. A transition from shish-kebab to fibrillar structures during fiber drawing and development of chain extended conformation is described [104] and two melting points of two different polymorphs are observed [105,106]. The higher melting peak at 423 K is associated with the orthorhombic crystal structure.…”