Periodicity along fiber axis

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Isotactic poly(methyl methacrylate), also, is an intricate case, resolved only after a 20-year debate. The repetition period along the chain axis is 10.40 A corresponding to S monomer units the entire cell contains 20 monomer units (four chains). At first, the stmcture was resolved as a 5/1 helix (183) with = 180° and 62 — 108° but no reasonable packing was found using this assumption. Further conformational calculations showed that helices like 10/1 or 12/1 should be more stable than the 5/1 helix. The structure was solved by Tadokoro and co-workers (153b) who proposed the presence of a double helix. Two chains, with the same helical sense and the same direction but displaced by 10.40 A one from the other are wound on each other, each chain having 10 monomer units per turn [i(10/l)] and a 20.80-A repeat period. As a result, the double helix has a 10.40-A translational identity period, identical to that found in the fiber spectmm. The conformational parameters are Of = 179° and 2 = -148°. Energy calculations indicate that the double helix is more stable by 4.4 kcal per-mole of monomer units than two isolated 10/1 helices, a result that is in line with the well-known capacity of this polymer to form complexes in solution (184). 

To shed more light on the structure of DNA, Rosalind Franklin and Maurice Wilkins used the powerful method of x-ray diffraction (see Box A-A) to analyze DNA fibers. They showed in the early 1950s that DNA produces a characteristic x-ray diffraction pattern (Fig. 8-14). From this pattern it was deduced that DNA molecules are helical with two periodicities along their long axis, a primary one of 3.4 A and a secondary one of 34 A The problem then was to formulate a three-dimensional model of the DNA molecule that could account not only for the x-ray diffraction data but also for the specific A = T and G = C base equivalences discovered by Chargaff and for the other chemical properties of DNA. 

Small angle x-ray scattering of the oriented films (samples B and C) showed a discrete interference along the fiber axis corresponding to a long period of 14.0 nm (Figure 3). The stretched film of sample D prepared with the same regeneration procedure used for sample B showed no orientation. This would be... 

Figure 10.39. Electron Micrograph of Fibrin. The 23-nm period along the fiber axis is half the length of a fibrinogen molecule. [Courtesy of Dr. Henry Slayter.]... 

The examined C/C composite was fabricated via. preformed yam method[6]. The reinforcing fiber, fiber volume fraction, stacking sequence, and dimensions of specimens of it are Toray M40, 50%, 0790°, and 30mm x 30mm x 3mm, respectively. In the laminated C/C composite, periodical cracks along the fiber axis direction, transverse cracks (TCs), frequently appear as shown in Fig. 1. The surface layers of the TCs especially affect characteristics of the coating on C/C composites. 

Monilethrix is a genetic anomaly in which hair fibers contain periodic constrictions along the fiber axis. Monilethrix hairs tend to fracture at these constrictions [63] and therefore must exhibit abnormal stretching behavior. Trichorrhexis nodosa is another abnormal condition, where hair fibers contain nodes at irregular intervals along the fiber axis. These nodes actually contain tiny fractures and tend to form broom like breaks [63] under stress. Therefore, nodosa hair fibers should also exhibit abnormal stretching behavior. Other hair shaft anomalies such as trichothiodystrophy and Menkes syndrome should also display abnormal stretching behavior consistent with the abnormal hair shaft condition associated with these diseases. 

Fig. 6. Idealized fiber diffiactogram firom X-ray or neutron scattering. An assembly of partially ordered blocks of microcrystallites diffracts to produce diffraction spots that are recorded on a flat film or an area detector. The periodicity of the macromolecular constituents within the microcrystallites is indicated by a series of diffracting lines having a regular spacing. The equator corresponds to the layer line 0, intersecting the nondiffracted central beam. The meridian is perpendicular to the equator and it lies parallel to the fiber axis. The spacing along the meridian provides information about the periodicity of the macromolecule and its hehcal symmetry. The so-called helical parameters, n and h, are directly related to the syrmnetry of the macromolecular chain n is the number if residues per turn, and h the projection of the residue on the helical axis.

In this section we present new results on the stabihty of the models of the nanofibers. Destabilization of the model can be achieved in several ways, which include an increase in the length of the periodicity. Lx, along the fiber axis, a decrease in the number of independent parent chains, or a decrease in the degree of polymerization of the parent chains. Each change causes the fiber to become thinner. At some point the fiber breaks up into individual droplets, because the spherical shape will minimize the ratio of surface area to volume in the system. 

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