Abstract
PEN fibers spun at 4 km/min by melt-spinning were treated by high tension annealing (HTA) at $220^{\circ}C$. The as-spun PEN fibers and fibers treated by HTA at $220^{\circ}C$ were given various thermal history. The fine structure and physical properties of the PEN fibers were investigated with various techniques such as wide-angle X-ray diffraction, DSC thermograms, density, birefringence, dynamic viscroelasticity, tensile testing, etc$\ldots$ In case of the PEN fibers treated by HTA at $220^{\circ}C$, change with thermal history could not be detected from WAXD diagrams and DSC thermograms. In the case of dynamic viscoelasticity of PEN fibers treated by HTA, storage modulus decreased with thermal history. Also, birefringence and initial modulus of the PEN fibers treated by HTA decreased rapidly. Therefore, orientation of non-crystalline region is lowered. From tilting WAXD diagrams, in case of $6^{\circ}$ tilting, peaks became bigger with thormal history. Thus, thermal history affect not only the non-crystalline region but also the crystalline region. WAXD diagrams of PEN fibers at different temperatures from $23^{\circ}C\;to\;220^{\circ}C$ suggested that only d-spacing of $\beta$ form crystal increased. But the d-spacing of transformed $\alpha$ form crystal did not change. Also d-spacing returned to the original value on cooling.