Characterization by optical and electron microscopy

A distinctive structural feature of PpPTA and other para-aromatic polyamide fibers is the pleated sheet, which is shown in Fig. 6.13(c), (d) and (e). It was observed for the first time by Ballou in dark-field images taken from meridional reflections of the ED pattern. This phenomenon has been studied extensively by Dobb et The pleated sheet consists of parallel-oriented chains, which are hydrogen bonded along the direction of the crystallographic h-axis. They are usually oriented perpendicular to the surface of the filament, i.e. in the circular cross-section the fc-axes are directed preferentially along the radius, as shown in Fig. 6.14. The angle between adjacent planes of the pleat is about 170° and the distance between two pleats is about 250 nm, but both values may vary along the radius of the fiber cross-section. 

The pleated sheet can also be observed with optical polarization 

The lateral texture can be studied by measuring the lateral birefringence in a thin fiber cross section, or by registration of the 

Several models have been proposed for the structure of para-aromatic polyamide fibers. The simplest model, postulated by Northolt et a/., is based on observations by X-ray and electron diffraction and is depicted in Fig. 6.16(a). The fibre is considered as being 

Observed refractive indices of para-aromatic polyamide fibers and a PBO fiber radially lateral n tangentially lateral n i lateral n longitudinal. The results of Weeda were obtained by matching immersion liquids under the interference microscope, or from measuring the retardation. For PBO fiber, fi was estimated from a birefringence determination 

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