Abstract
The aging effect on the fine structure has been investigated of the undrawn PET fiber melt spun by means of differential thermal analysis (DTA), dilatometry, density, birefringence, load-extension and dynamic mechanical measurements.
The results obtained on the aged sample are as follows:
1. An endothermic peak in DTA apears at a lower temperature than T_??_, which suggests that a destruction of the fine structure originated by aging is producted at the very temperature.
2. An abrupt extraordinary voluminal expansion in dilatometry is observed in response to the temperature where the endothermic peak in DTA curve appears. It may be related to the volumeric relaxation which occurs in aging.
3. Necking stress increases with aging time and an yield point appears in a further drawing after the necking. It suggests the formation of stronger intermolecular bonds by aging.
4. A very small increase is observed in density. The results (1), (2) and (3) suggest that this small increase in density is due to the increase in the density of amorphous region but not due to the crystallization.
5. The location of β absorption maximum of mechanical tan δ shifts to the side of a lower temperature, which means that the local mode motion of COO groups is made easier by ageing. Birefringence decreases as the ageing time increases.
From these results the effect of aging on the fine structure of PET is considered to be explained as follows:
The volumeric relaxation is produced in an amorphous region, accompanying the increase in the intermolecular bonding energy. On the other hand, the decrease in restraint of a molecular chain accompanied by the volumeric relaxation gives a favorable effect on the local mode motion and shifts the β absorption maximum of tan δ.
