Paracrystalline structure

Oriented material can be described in terms of a paracrystalline structure. On doping with AsFs, order is retained, with a new spacing developing perpendicular to the chain axis and a regular positioning of dopant along the chains 499>. 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

When tropomyosin solutions are dialyzed against divalent cations such as Mg " or Ca +, fibrous paracrystals are formed. Paracrystalline tactoids under these conditions have a characteristic axial repeat period of 40 nm (Cohen and Longley, 1966). The axial periodicity is formed by the alternating arrangement of wide bright (27 nm) and narrow dark (13 nm) bands, each of which is surrounded by clear boundary lines. Two sets of oppositely directed tropomyosin filaments are arranged in the paracrystalline structure and each boundary line corresponds to the position of... 

The model has been refined by two lines of study (Ohtsuki, 1974). The first refinement was made by analysis of the troponin-tropomyosin relationship in the paracrystalline structure (discussed in Section II,E,2). The analysis has confirmed that troponin lies approximately two-thirds of the molecular length (i.e., 27 nm) from one end of a filamentous tropomyosin molecule of 40-nm length. Another refinement was based on consideration of the arrangement of actin molecules in the thin filament. Corresponding molecules in two long-pitched strands of actin in the filament are shifted relative to each other by a distance of half the... 

When polypropylene is water-quenched under eertain conditions, a difference in crystalline order is obtained. In air-quench systems, yarns with a crystallinity of around 55% are obtained. The crystals are usually distributed throughout the yarn as lamellae and are normally in the monoclinic a-form. In the rapidly cooling water-quench process, a less-ordered smectic or paracrystalline structure is produced with a morphology that is basically fibrillar. Under certain drawing conditions, these smectic structures lead to high-strength fibers. 

There are several process requirements for the preparation of polypropylene staple with permanent three-dimensional helical curvature. Specifically, a rectangular spinneret-pack assembly is used to produce flow perturbation and to impart high internal stress. A specially designed cooling device cools the fiber quickly to form a paracrystalline structure in the fiber. The process principle is that the flow perturbed in the polypropylene melt creates internal stress on one side of the fiber section. Because of the stress memory of polypropylene, the internal stress difference at the interface of streamlined and perturbed flows can sustain in the fiber after it has been cooled and solidified. This leads to different crystal structures and shrink properties, and thus a fiber in the shape of a three-dimensional helix. 

In the two-step process, polypropylene resin of typically MFR = 35 g/min and isotacticity above 90% is spun at 280°C at a low speed of 200 m/min. The spun fiber, which has a structure of low-oriented hexagonal crystallinity, is drawn at a low temperature of 60°C over seven rolls. The drawn fiber, still having a paracrystalline structure, is drawn again at a higher temperature of 110°C 140°C to change the paracrystallinity into a highly oriented... 

PARACRYSTALLINE STRUCTURE OF POLYMER-CRYSTAL LAHICE DISTORTION INDUCED BY ELECTRON IRRADIATION. 

Granier, T, E.L. Thomas, and RE. Karasz. 1989. Paracrystalline structure of poly(p-phenylene vinylene). J Polym Sci Polym Phys Ed 27 469. 

It is noteworthy that other divalent metallic salts exert a similar effect on mitochondria of the basal infolding of epithelial cells of the distal nephron (Jasmin and Riopelle 1976). Piracetam that induced paracrystalline structures in the thyreocytes of old rats (Fig. 340) induced such an inclusion in a collecting duct epithelial mitochondrion (Fig. 315). 

The fibers produced from the polymer with low melt viscosity (MFI = 35 g/10 min) possess a paracrystalline structure with low orientation. The X-ray diffraction patterns of these fibers show halos indicating poor orientation. The overall molecular orientation as shown by the birefringence values decreases with increasing extrusion temperature. 

Cellulose is the major wall constituent in the supporting tissues of mature plant cells. The paracrystalline structure of cellulose, that results from energy minimization by the formation of inter- and intrachain hydrogen bonds, makes it mechanically the strongest known organic molecule on density basis (Niklas 1992). It is natural then that cellulose is the primary determinant of strength in structural tissues. 

The crystallization of oriented PET from the glassy state is known as strain-induced crystallization. The crystallization process involves rotation, alignment, and perfection of the internal order of the paracrystalline structures [15]. The purpose of many investigations have been to study the kinetics of strain-induced crystallization of PET during annealing of oriented samples [16,17],... 

Fig. 17.5. Physical structure of fibers (a) semicrystalline structure of poly(ethylene terephthalate), PET the broken lines are the borders of a fibril (b) paracrystalline structure of poly(p-phenylene terephthalamide), PPTA.

Fig. 17 Kratky plots of the partial scattering function under core contrast from the 5DH8HH (a) and 6HH10DH (b) samples at 0 = 2% at room temperature. The solid lines are fits with the paracrystalline structure factor for Q < 2.5 x 10 A"h For b a finite stapling of N = 8 could be determined...

A better product can be obtained if the precursor polymer contains some conjugated bonds which restrict the final structure and ensure extended conjugated-sequences. The most prominant example, is the Durham precursor-route to (equation 1). The precursor polymer is soluble in acetone, ethyl acetate, etc, and can be purified and cast to make films. The thermal elimination reaction produces cis-PA as the predominant form at 60 °C. Use of higher temperatures leads to higher proportions of trans-FA due to cis-trans isomerization. If the precursor film is unoriented the PA produced is amorphous. Stretching the precursor film during elimination leads to a highly oriented non-fibrous form of PA. trans-VA produced in this way exhibits a paracrystalline structure. 

From the width of the reflections of the wide-angle X-ray scattering (WAXS) fiber pattern the apparent size and the degree of perfection of the crystalline domains can be determined. In PpPTA the size and disorder parameters are strongly anisotropic. Normal to the chain axis the dimensions of the domains are 4-6 nm. Analysis of the meridional reflections shows that these fibers apparently have a paracrystalline structure with a longitudinal crystallite size of at least 25 nm and a lattice distortion parameter gu of 2-3%. Annealing at temperatures up to 600°C increases the lateral size to about 10 nm and the longitudinal size to about 80 nm, while gn decreases to about... 

Plastids of higher plants grown in darkness reach the stable form shown in Fig. 2 and are characterized by the presence of one or two crystal latticelike structures. These paracrystalline structures are called... 

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