Structure Development upon Melt Drawing of Ultrahigh Molecular Weight Polyethylene:  Effect of Prior Thermal History

Abstract

The effect of thermal history on the melt drawing of ultrahigh molecular weight polyethylene (UHMWPE) reactor powder was studied. The samples for drawing were prepared by compression-molding of reactor powder at various temperatures above the melting point (T m). The drawing temperature (Td) was 150°C. It was found that the maximum achievable draw ratio at the optimum Td decreased from 60 to 23 when the prior-melt temperature increased from 160 to 230°C. The highly drawn films exhibited tensile moduli e58 GPa and strength e0.95 GPa at room temperature. Scanning electron microscopy (SEM) observations of the drawn films, etched by fuming nitric acid, revealed a characteristic "shish kebab" structure, as reported. Consistent with such morphology, differential scanning calorimetry (DSC) showed double melting endotherms at 134 and 143°C, corresponding to the "kebob" and "shish" components, respectively. The crystallinity evaluated by the total heat of fusion from the double peaks increased steadily with the draw ratio, and the increase was more rapid for the samples that were priormelted at a higher temperature and/or for a longer time. The formation of "shish" and "kebob" components and their crystal sizes were also significantly influenced by the prior-melting temperature and the elongation ratio. The efficiency of the draw, evaluated from the fraction of the "shish" component and the tensile properties vs draw ratio, was also interpreted from the differences of the prior-melt preparation conditions. The results suggest that the different level of entanglement formation, which was associated with the scale of segmental diffusion, affected significantly the resultant structure and properties.

Phase Development Mechanism during Drawing from Highly Entangled Polyethylene Melts

“…[4][5][6] Disentangling the initially entangled chains is a key for enabling such high performance. Recently, a less-entangled state of PE chains has been achieved by controlling the synthesis conditions, [11] but it can also be achieved by processing alone, even from conventional entangled melts, as suggested in this study.…”

“…The samples can then be successfully melt-drawn up to a maximum draw ratio of 45-50 at the optimum temperature of 150 8C. [4][5][6] It is also suggested the strain-induced crystallization with disentanglement occurs during the draw from the melt. [4] However, the phase development mechanism assumed above can not be experimentally confirmed because both crystalline reflection and amorphous scattering are too weak to detect using a conventional X-ray source, especially at the initial stage of melt-drawing.…”

“…This is a unique phenomenon, compared to the usual stress increase for the solid-state deformation of UHMW-PE. [4,6] Until approx. 150 s at around the midpoint of the plateau stress region, which corresponds to a draw ratio of 12, no crystalline diffraction is observed.…”

“…[9] The extended shish crystal formed during drawing from the melt exhibits the former spots, as previously reported for the melt-drawn UHMW-PE film. [5,6] The relative intensity change of each reflection is evaluated from the line profiles extracted along the equator for a series of the obtained WAXD patterns. Figure 2a duplicates these profiles as a function of processing time.…”

Structural arrangement of crystalline/amorphous phases of polyethylene‐block‐polystyrene copolymer as induced by orientation techniques

“…Such an etching procedure has been often applied for analyzing the drawn morphologies of PE homopolymer. 32 Figure 7 shows TEM (top) and SEM (bottom) images for the film swelled by toluene, subsequently drawn at room temperature up to DRs of 1.6 (left) and 3.2 (right). The initial films were isothermally crystallized at 90 o C. These images exhibit the gradual orientation of the cylinder along the drawing axis with increasing DR.…”