Pre-ordering

Additional neutron scattering studies on different polymer systems could prove very important. Strobl [31,32,47,103] provides evidence that, for some polymers, lamellar crystallization is preceded by pre-ordering of the melt followed by formation of planar arrays of blocks. Investigating crystallization from the melt, Kaji and coworkers [25] find pre-ordering phenomena relating to orientational fluctuations of stiff polymer segments which, under appropriate conditions, determine phase separation prior to crystallization. 

D.A. Halliday, P.L. Burn, D.D.C. Bradley, R.H. Friend, O.M. Gelsen, A.B. Holmes, A. Kraft, J.H.F. Martens, and K. Pichler, Large changes in optical response through chemical pre-ordering of poly(/j-phcnylcnc vinylene), Adv. Mater., 5 40-43, 1993. 

As a consequence of either calculation, there must be shortcuts to arrive at the final and thus active conformation of a protein (usually only one conformation). It is known that the formation of secondary structure elements such as a-helices and jS-sheets proceeds preferentially and much faster than other parts of the protein the rapid formation of such elements pre-orders the amino acid chain and sets it up for faster formation of whole domains and then of the complete, native, cata-lytically active protein. 

At the end of the in vivo imaging session, animals are euthanized, tissues are dissected, and placed in a pre-ordered manner in a Petri dish cooled on a bed of ice. The dissection must be carried out quite rapidly (maximally 15 min) to ensure that the substrate luciferin remains in the tissue at sufficient concentration and the luciferase is not catabolized by postmortem biochemical events. 

Brains are rapidly dissected and sectioned by means of a brain matrix device and placed in a pre-ordered position on a Petri dish cooled on ice. 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

Although most of the initial polymers and copolymers are no longer available, these systems should help us to investigate the details and structural consequences of a pre-ordering of the polymer melt, or the formation of a precursor crystal in polymer crystallization. From the present and as yet still sketchy results, it appears however that (a) if a crystal phase results from the transformation of a liquid crystal phase (usually of hexagonal symmetry), different orientations of the crystal unit cell are likely, which is not observed in conventional polymer crystallization (for example, in polyethy-... 

Despite the limited number of R-matrices we have made available for reaction and the restriction procedures outlined above, the number of products generated is tremendous. In order to restrict these reactions along fairly identifiable patterns it was felt necessary to channel reactions along pre-ordered lines. 

Pre-order 80% of bulk materials normally supplied by the contractors in order to minimize mobilization time. 

The main reason is that the need for preconditioning cannot be foreseen. Acute myocardial infarction most frequently strikes unannounced. Thus, the application of preconditioning usually cannot be pre-ordered at will. Indeed, a body of evidence has accumulated that pre-infarction angina, which comes as close to preconditioning as possible, can be protective as regards limiting myocardial necrosis and protective against ventricular arrhythmias.4 10... 

Even at quiescent conditions the fundamental mechanisms of polymer crystallization, especially at an early stage, are still poorly understood [6-16]. For many years, nucleation and growth as a stepwise process has dominated the discussion [11,12]. In contrast to this view Strobl [13] proposed a multistage process to explain polymer crystallization, while others concluded on the basis of X-ray scattering data to a spinodal-assisted process [17-29]. Common to both views is that the crystallization is preceded by an ordered precursor (so-called pre-ordering). Clear structural information about such possible precursors - necessary to verify these hypotheses - is still scarce. As a resiJt, during recent years an important and still open debate has been going on about polymer crystallization. 

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