In-Situ Observation of Phase Development during Melt-Drawing of Ultra-High Molecular Weight Polyethylene Using Synchrotron Radiation
Hiroki UEHAEA, Tomoyuki YOSHIDA, Syozo MURAKAMI, Masaru KOTERA
Department of Chemistry, Gunma University
JAPAN
Keywords: Melt Drawing, UHMW-PE, Synchrotron
Phase development during drawing from ultra-high molecular weight polyethylene (UHMW-PE) melt was analyzed by in-situ wide-angle X-ray diffraction (WAXD) measurement using synchrotron radiation. The sample films were prepared by compression molding of UHMW-PE reactor powder, having a viscosity average MW of 4x106, at 180oC and 30 MPa with an appropriate amount of antioxidants. The obtained melt-crystallized film was drawn up to draw ratio (DR) of ~10 within a chamber equipped with synchrotron beam line. Draw was made at the temperature (Td) of 140-150oC, which is well above the melting peak temperature of the sample film (135oC). A series of WAXD patterns were continuously recorded on high-resolution CCD camera during drawing. Exposure time was 1.5 second for each pattern recoded with a time interval of 5 second for date storage. During in-situ WAXD measurement upon melt drawing, the stress/strain curves were also recorded by weight cell equipped with drawing device on the synchrotron beam line. At the initial strain stage of draw, the stress/strain curve exhibits the plateau region, and none of the crystalline reflection appears on a series of in-situ WAXD patterns. However, as the sample draw proceeds, both oriented orthorhombic and hexagonal reflections simultaneously appear at the same DR around 5, where the stress/strain curve still shows the plateau deformation. With further increasing the strain up to DR of ~8, the intensity of the hexagonal reflection peak begins to decrease and subsequently that of the usual orthorhombic ones increase. Correspondingly, the rapid rise of draw stress, because of strain-hardening behavior, was recorded. A cooling after draw brings the complete phase transition from the hexagonal one to the stable orthorhombic one. The reflection peaks of such a high-temperature hexagonal phase were recognizable even at Td=140oC, which is 10oC below the reported transition temperature (150oC) from the orthorhombic to hexagonal phases.