This paper is concerned with a skin-core structure in poly (ethylene terephthalate) (PET) fiber drawn soon after immersion into hot water at 75-90°C. The drawing conditions for generating this structure are examined and the generating mechanism is explained in terms of the hot drawing mechanism reported by one of the authors. This skin-core structure is composed of two parts: one is the outer layer of low molecular orientation and the other is the inner part of high molecular orientation.
The birefringence of amorphous PET fibers upon hot drawing depends upon drawing temperature and rate. At higher temperature, their birefringence change with drawing is lower when the drawing rate is fixed. Moreover, their birefringence very little increases with drawing under the condition of “super drawing”.
On the other hand, the glass transition temperature of amorphous and unoriented PET fibers is 70°C and lowered by about 20°C by immersion into water, so that the birefringence of the specimens drawn in water is much smaller than those drawn in dry state at the same temperature. Therefore, the distribution of birefringence is developed by drawing of amorphous PET fibers having the distribution of water content. This is the reason why the birefringence is lower in the outer layer than in the inner part when the sample is drawn soon after immersion into hot water. Moreover, when the super-drawing takes place in the outer layer of the sample, but not in the inner part, a distinct layer structure is developed in the sample drawn to higher draw ratio.