F. Yamamoto

Band pattern development in shear-oriented thermotropic copolyester extrudates

The mechanism of band pattern development in shear-oriented thermotropic copolyester extrudates is explained in terms of residual longitudinal tension. A liquid-crystalline copolyester consisting of 40 mol% poly(ethylene terephthalate) and 60 mol% p-oxybenzoate is extruded at 220°C in a wide shear-rate range without elongational deformation. These extrudates exhibit band patterns perpendicular to the extruded direction in the core layer after development of skin-core structures at high shear rates. The band pattern causes characteristic cone-shaped fracture surfaces after tensile tests. Moreover, in the core layer, cracks along band patterns are detected by optical and scanning electron microscope observations. These morphological characteristics are caused by a molecular orientation distribution in the radial direction, which also causes the distribution of thermal and mechanical properties. The mechanism of band pattern development is analysed by assuming that longitudinal tension is concentrated in the core layer during the cooling process. This stress is induced by differences in both Youngs modulus and longitudinal linear expansion coefficient between the skin and core layers. The quantity of the residual stress is estimated roughly by a thermal mechanical analysis.

Morphological changes of wide polyoxymethylene rod during a microwave heating drawing process

Abstract The microwave heating drawing process for producing a polyoxymethylene (POM) rod (2.5 mm in diameter) with a sonic modulus of ∼40 GPa has been analysed by investigating the changes in both orientation and thermal properties during drawing. During the initial crystalline deformation in the necking region, the lamellae are oriented perpendicular to the draw direction and are then unfolded into microfibrils. The crystalline orientation function reaches a high value (0.988) at a draw ratio of 6 immediately after necking. In the advanced ultra-drawing stage, the Young modulus increases gradually with increasing amorphous orientation. At the same time, the orientation distribution in the radial direction is caused by the temperature distribution induced in the radial direction of the rod. It is noted that fine adjustments of ambient temperature and microwave power are required to get ultra-high-modulus POM rods over 40 GPa with large crosssections.

High-temperature stability of optical transmission properties attained by liquid-crystal polymer jacket

Liquid crystal polyester (LCP) tight jackets, developed as secondary coatings on optical fibers, exhibit Youngs moduli of 10-20 GPa and linear expansion coefficients on the order of 10-6/°C. Because of their low linear expansion coefficients, and because they exhibit no thermal shrinkage, the LCP jackets cause a slight change in fiber strain on the order of 10-4percent/°C, or a change in the thermal coefficients of the transit time delay as low as 14-29 ps/km°C in an 80 to -60°C temperature range for tight-jacketed optical fibers. Furthermore, the LCP tight-jacketed optical fibers exhibit no excess loss in this temperature range.

High-modulus low-linear-expansion-coefficient loose-jacket optical fibers

Optical fibers loosely jacketed with high-modulus low-linear-expansion-coefficient polymers have been proposed, and the mechanical, thermal, and optical properties have been investigated. The loose-jacket tube is made of highly oriented polyoxymethylene (POM), which is produced by tensile-drawing the isotropic POM tube with dielectric heating. The oriented POM exhibits Youngs moduli 20-40 GPa and linear expansion coefficients of the order of 10^{-5}-10^{-6}\deg C-1. Owing to the low linear expansion coefficients of the material, the oriented POM loose-jacket optical fiber has no low-temperature excess losses, which is due to fiber bends caused by thermal contraction of the loose tube, in the -60-80\deg C temperature range.