Paracrystalline states

The term crystalline itself may be somewhat misleading when applied to CPs. Well-defined Bragg peaks can be obtained under less stringent conditions of order for instance, in the paracrystalline state [26] (see also Ref. 12, Vol. 1, p. 438), and this applied to all CPs, with the exception of PDAs, which form crystals in the usual sense of the word. 

The cellulose tension hypothesis presumes that the microfibrils are deposited in a partially crystalline or paracrystalline state and that during cell maturation they experience further crystallization and in so doing they contract axially. A conceptual... 

At this step, then, the differences between the effects of surface chemistry and secondary molecular motions as dictated by morphological order can be observed on thrombogenesis. In the case of the sterically ordered substrate, in conjunction with the subsurface cationic array, the sorbed pro-tein(s) can assume a paracrystalline state, and subsequently be subjected to further pertubations. However, in the case of the disordered substrate, the sorbed proteins can not assume a paracrystalline state and, therefore, will not, per se, be subjected to any further conformational changes. 

R. Hosemann, The paracrystalline state of synthetic polymers, Crit. Revs. Macromol. Sci. 1,351 (1972). 

Carbon Black is formed from the burning of gaseous or liquid hydrocarbons under conditions of restricted air access. According to electron micrographs taken with a phase contrast microscope, carbon black has a graphitelike microstructure with lattice distances of 0.35 nm. The layers lie parallel to the particle surface. Since discrete crystalline regions cannot be observed, the structure of carbon black is better described in terms of a paracrystalline state rather than a random distribution of graphite crystals. 

Ordinarily, there is a sharp distinction between the highly ordered state of a crystalline solid and the more random molecular arrangement of hquids. Crystalhne ice and liquid water, for example, differ from each other in this respect. One class of substances, however, tends so greatly toward an ordered arrangement that a melting crystal first forms a milky liquid, called the paracrystalline state, with characteristically crystalline properties. At higher temperatures, this milky fluid changes sharply into a clear liquid that behaves like an ordinary liquid. Such substances are known as liquid crystals. 

The paracrystalline state seems one of the most suited to biological functions, as it combines the fluidity and diffusibility of liquids while preserving the possibilities of "internal structure" characteristic of crystalline solids. As a matter of fact, this is indeed the case in a number of instances. 

Although the lipid bilayer structure is quite stable, its individual phospholipid and sterol molecules have some freedom of motion (Fig. 11-15). The structure and flexibility of the lipid bilayer depend on temperature and on the kinds of lipids present. At relatively low temperatures, the lipids in a bilayer form a semisolid gel phase, in which all types of motion of individual lipid molecules are strongly constrained the bilayer is paracrystalline (Fig. ll-15a). At relatively high temperatures, individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains. In this liquid-disordered state, or fluid state (Fig. 11—15b), the interior of the bilayer is more fluid than solid and the bilayer is like a sea of constantly moving lipid. At intermediate temperatures, the lipids exist in a liquid-ordered state there is less thermal motion in the acyl chains of the lipid bilayer, but lateral movement in the plane of the bilayer still takes place. These differences in bilayer state are easily observed in liposomes composed of a single lipid,... 

It is an unusual problem to construct a homogeneously distorted two-dimensional domain structure which allows us to cover the range of the hexagonal closely packed crystal up to a two-dimensional gas. For example, the concept of the paracrystal is based on the philosophy that every state of condensed matter has at least a micro-paracrystalline arrangement of segments within a distorted lattice of arbitrary symmetry with a defined coordination number. 

A second important event was the development by Hosemann (1950) of a theory by which the X-ray patterns are explained in a completely different way, namely, in terms of statistical disorder. In this concept, the paracrystallinity model (Fig. 2.11), the so-called amorphous regions appear to be the same as small defect sites. A randomised amorphous phase is not required to explain polymer behaviour. Several phenomena, such as creep, recrystallisation and fracture, are better explained by motions of dislocations (as in solid state physics) than by the traditional fringed micelle model. 

It is now generally accepted that the morphology of a polymer depends on the contributions of three different macro-conformations (a) the random coil or irregularly folded molecule as found in the glassy state, (b) the folded chain, as found in lamellar structures and (c) the extended chain. The fringed micelle (d) may be seen as mixture of (a), (b) and (c) (see Fig. 2.12) with paracrystallinity as an extreme. 

Insofar as small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe(lll), the paracrystallinity hypothesis will probably hold true. But the assumption that this will happen on a molecular level on each basic structural unit is not true. The unique texture and anisotropy of the ammonia catalyst is a thermodynamically metastable state. Impurity stabilization (structural promotion) kinetically prevents the transformation of platelet iron into isotropic crystals by Ostwald ripening [154]. Thus the primary function of alumina is to prevent sintering by acting as a spacer, and in part it may also contribute to stabilizing the Fe(lll) faces [155], [156], [298],... 

Characterization of polymer orientation is most often accomplished via X-ray techniques which are suited to crystalline and paracrystalline regions (i-d). However, semicrystalline polymers present a complex system of crystalline, amorphous, and intermediate pluses ( -d) and complete characterization of semicrystalline polymers can only be achieved by application of a variety of techniques sensitive to particular aspects of orientation. As discussed by Desper (4), one must determine the degree of orientation of the individual phases in semicrystalline polymers in order to develop an understanding of structure-property relationships. Although the amorphous regions of oriented and unoriented semicrystalline polymers are primarily responsible for the environmental stress cracking behaviour and transport properties of the polymers, few techniques are available to examine the state of the amorphous material at the submicroscopic level. 

WAXD and DSC were used to measure the amount of crystalline and amorphous phase in each of the samples. Figure 4 shows the crystaUinity of the samples as a function of draw ratio. The values of crystallinity fi-om DSC and WAXD are not in exact agreement however, the trends show no significant change in crystallinity due to solid state drawing. Similar results have been reported for semicrystalline polymers with degree of crystallinity measured by DSC, WAXD, NMR, and density (5,6,24), and the discrepancy between crystalMty values has been discussed in terms of either dilation of amorphous fi ee volume by crystal constraint (6,24) or presence of a paracrystalline intermedate phase (5,6),... 

At low degrees of crystallinity in the bulk state, a fringed micelle type of model (admitting the possibility of chain folding) may be appropriate, whereas at higher degrees of crystallinity, a paracrystalline type of model such as is depicted in Figure 1.7 may better reflect reality. 

The effect on bulk mechanical properties of these physical forces and states, especially paracrystallinity and hydrogen bonding, is to pseudo-... 

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