Freeze-fracture, fibers

In this chapter, the results of past research are expanded because fiber cross sections were examined, rather than longitudinal views of fibers, and distributions of elements were obtained in addition to overall elemental spectra. Because the X-ray beam penetrates only a small distance into the surface of a sample (approximately 8-10 xm for a 25-kV excitation ), examination of a longitudinally mounted fiber produces elemental spectra of surface layers only. Such spectra may not be representative of the bulk of the fiber. In addition, this work improves upon past research in that the freeze-fracturing-freeze-drying EDS technique is suited to very small, fragile fiber samples (whether single fibers or small yam pieces), and is limited in size only in the operators ability to see and handle the samples. By using this procedure, compression of the fiber cross section and elemental redistribution are avoided. 

Comparison of the molecular length of CAB (ca. 38 A) and the characteristic lengths calculated from SAS data (vide ante) indicate that molecular pairs are involved in the columns of the solid state while, in gels, association of swollen columns might be involved. Micrographs of freeze-fractured and etched CAB gels (Fig. 17) show a 3-D network of fibrous bundles. The dimensions of the rectangular cross sections of the nontwisted fibers in dodecane, 209 x 104 A. and the twisted ones in 1-octanol, 263 x 82 A [481, correspond approximately to the cross-sectional areas determined by SANS in which a circular cross section model was employed. 

Figure 58. Freeze—fracture electron micrograph of compound 13 in (a) dichloromethane and (b) pyridine showing fiber network formation with helical textures.382 Scale bar represents 100 nm. (Reprinted with permission from ref 382. Copyright 2000 Wiley-VCH.)...

Figure 5.5 Cross section and surface of a microporous polysulfone sheet used in composite reverse osmosis membranes (a) total cross section of a polysulfone sheet cast on a nonwoven polyester fabric, then delaminated prior to freeze-fracture for SEM (note fiber trecks on backside of the sheet) (b) backside of sheet showing cellular structure, which extends through 85% of the sheet thickness (c) transition region from cellular to nodular structure near film surface (d) dense nodular structure at the surface (e) high magnification of the extreme top surface cross section (f) high magnification view of the surface structure showing tha texture of the top surface.

Research Centre (MERC) in collaboration with Remspec Inc. The ATR head consists of a three-inch zinc selenide (ZnSe) crystal, attached to the fiber optic cables, and is held in place by a mechanical seal housed within a plastic sleeve. The perforated stainless steel cover protects the ZnSe crystal, and the fibers are insulated to avoid freeze-fracture. The length of the crystal... 

Scanning electron microscopy images of freeze fractured hollow fiber membranes produced by dry-jet wet spinning from PEEKWC, a modified poly(ether ether ketone), were used to make pore size measurements as a function of different spinning conditions... 

Based on the experimental results it was proposed [99] that the fibers have a jelly-roll type strueture, i.e. it consists of concentric smectic layers. Although this model was basically supported by recent freeze fracture measurements [17] quenched fibers also show grooves (periodic modulation) of the layers. These grooves have been explained in terms of the polarization modulation, however they cannot account for the stability of the fibers. Presently we are working on a unified model that combines the layer modulation model with an additional out of plane polarization, which seems to be able to explain the stability of the fibers [104]. 

Figure 4 SEM photographs of fractured surfaces of PEI-TLCP blend fibers at the draw ratio of 1 (x 3000). The samples were fractured after freezing in liquid nitrogen. The amount of PEsl in the blends are (A) 0 phr, (B) 0.75 phr, (C) 1.5 phr, (D) 2.25 phr, (E) 3.75 phr, and (F) 7.5 phr. Source Ref. 11.

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