Extended chain-type crystal

Moreover, if polyoxymethylene is recrystallized from nitrobenzene and the usual chain-folded lamellae of ca. 100 A thickness is treated with TXN in the presence of BF3, more perfect extended-chain-type crystals are formed. These facts confirm the conclusion formulated -by Wunderlich, that post-crystallization leads to chain-folded crystals because of kinetic restrictions, while crystallization in the polymerizing system gives thermodynamically more stable extended-chain crystals77). The formation of the thermodynamic product in polymerization is due to the growth of crystals at equilibrium polymerization conditions. Thermodynamically less stable crystals may redissolve as a result of depropagation, and crystals may thus grow further under equilibrium conditions. This (at least partly) eliminates kinetic restric-... 

DSC melting curves of (I) polyethylene extended chain-type crystals prepared by crystallization at 470 K under 500 MPa pressure and (II) folded chain-type polyethylene crystals prepared under atmospheric pressure... 

Several theories have been proposed to define the equilibrium structure of di-and triblock copolymers, one of whose components crystallizes.(242-244) Such theories should properly predict thermodynamic properties as well as equilibrium structure. However, common and central to all the theories is the basic assumption that the chains in the crystalline block are regularly folded in an adjacent re-entry array that leads to a smooth interface. The validity of this assumption for crystalline block copolymers needs to be carefully examined, in view of the experimental work that has been summarized earlier. This assumption is in contrast to homopolymers, where it has been established that the equilibrium condition requires extended chain type crystallites.(3) (See Chapter 2)... 

The formation of ECC is not only an extension of a portion of the macromolecule but also a mutual orientational ordering of these portions belonging to different molecules (intermolecular crystallization), as a result of which the structure of ECC is similar to that of a nematic liquid crystal. After the melt is supercooled below the melting temperature, the processes of mutual orientation related to the displacement of molecules virtually cannot occur because the viscosity of the system drastically increases and the chain mobility decreases. Hence, the state of one-dimensional orientational order should be already attained in the melt. During crystallization this ordering ensures the aggregation of extended portions to crystals of the ECC type fixed by intermolecular interactons on cooling. 

Hence, it can be concluded that the formed intermediate oriented phase is an indispensable stage preceding extended-chain crystallization so that this type of crystallization occurs in two stages the first stage involves formation of the oriented phase during melt deformation and the second formation of ECC from this phase on cooling. 

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

Proof that the anions of the substances mentioned really consist of long chains of P04 tetrahedra has been supplied by complete structural analyses of crystals. Four different types of such chain-like anions, differing in shape and inner periodicity, have so far been recognized. In the low temperature form of lithium polyphosphate, which has the same type of structure as lithium polyarsenate and diopside [Ca, Mg(Si03)2]x these are extended chains which (see Fig. 9a) as in enstatite have a Zweierperiodizit at (138) (see also ref. 66 in Table XVI). Rubidium polyphosphate has quite similar Zweierketten (55) (see Fig. 9b and ref. 63 in Table XVI) except that the P04 tetrahedra are somewhat rotated with respect to Form a. In the high... 

In crystal structures the two chains of insulin form highly ordered globular structures (8). Two main structural types form depending on crystallization conditions. In both structures the A-chains form two CC-helical segments, from residues Al—A8 and A13—A19, which are connected by a turn. In the structure referred to as the T-state, the B-chain contains two regions of extended chain, Bl—B8 and B21—B30, connected by an Ot-helix from B9-B19. In the R-state structure, the B-chain helix extends from Bl—B19. The crystallographic T-state structure best matches the solution structure of insulin determined by nmr (9), although the R-state can be induced in solution under the appropriate conditions. The surface of insulin which interacts with the insulin receptor includes the N- and C-termini of the A-chain and the C-terminus of the B-chain. 

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