Conformationally disordered structure

In a condis crystal cooperative motion between various conformational isomers is permitted. In the CD-glass this motion is frozen, but the conformationally disordered structure remains. For a condis crystal it is not necessarily expected that all possible conformations can be reached, but all conformations of the same type are involved in the condis crystal motion. If conformational isomers of low energy exist which leave the macromolecules largely in a parallel, extended, low energy conformation, conditions for the formation of condis crystals are given. The conformational changes involve more or less hindered rotations about backbone bonds or side chain bonds and are thus the some degree related to the orientational motion in plastic crystals. 

No. 23, 4th Nov.1996, p.7460-9 STRUCTURAL INVESTIGATION OF ORTHORHOMBIC-TO-HEXAGONAL PHASE TRANSITION IN POLYETHYLENE CRYSTAL. EXPERIMENTAL CONFORMATION OF THE CONFORMATIONALLY DISORDERED STRUCTURE BY X-RAY DIFFRACTION AND INFRARED-RAMAN SPECTROSCOPIC MEASUREMENTS Tashiro K Sasaki S Kobayashi M Osaka,University... 

In a condis crystal cooperative motion between various con- formational isomers is permitted. In the CD- glass this motion is frozen, but the conformationally disordered structure remains. 

Figure 2.113 indicates with an entropy plot that only the trans isomer of the 1,4-butadienes displays a stable mesophase. The crystals of the cis isomer have a more helical structure which packs less well in the crystal and needs much more space to rotate into a conformationally disordered structure. As a result of this different structure, the cis polymer melts at a lower temperature, and in one step. 

Keywords crystal structure, process variables, molecular weight, molecular weight distribution (MWD), polydispersity, copoljoners, nucleating agents, melt spinning, spinline, effects of molecular variables, effects of processing variables, tensile properties of filaments, birefringence, take-up velocity, conformationally disordered structure, amorphous orientation, crystalline orientation. 

Let us consider a structural limiting model, in which the polymer molecules, presenting a periodic conformation, are packed in a crystal lattice with a perfect three-dimensional order. Besides this limiting ordered model, it is possible to consider models of disordered structures having a substantially identical lattice geometry. 

This is, for instance, the case of PTFE, which at atmospheric pressure presents two reversible first-order transitions at 19 °C and 30 °C [67], In the transition at 19 °C the molecular conformation changes slightly, from a 13/6 to a 15/7 helix and the molecular packing changes from an ordered structure with a triclinic unit cell (corresponding to a positioning of the chain axes nearly hexagonal) toward a partially disordered structure (partial intermolecular rotational disorder) with a... 

The conformational disorder and packing structure of diazirine 91N2 is shown on the left of Scheme 7.25. The formation of azine 92 can easily be explained by the head-to-head crystallization of the chloro-diazirine, which experiences close N---C1 interactions that ultimately bring the carbene carbon close to the diazirine nitrogen... 

Chain Conformation, Crystal Structures, and Structural Disorder in Stereoregular Polymers... 

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

A particular kind of disorder, characterized by maintaining three-dimensional long-range periodicity only for some points of the structure, has been found in samples of syndiotactic polypropylene having a relatively low degree of stereoregularity.189 190 In these samples the chains present conformational disorder, which produces defects frozen in the crystals. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

Dobrosavljevic V, Stratt R (1987) Role of conformational disorder in the electronic-structure of conjugated polymers substituted polydiacetylenes. Phys Rev B 35 2781-2794... 

One of the most intriguing recent examples of disordered structure is in tomato bushy stunt virus (Harrison et ah, 1978), where at least 33 N-terminal residues from subunit types A and B, and probably an additional 50 or 60 N-terminal residues from all three subunit types (as judged from the molecular weight), project into the central cavity of the virus particle and are completely invisible in the electron density map, as is the RNA inside. Neutron scattering (Chauvin et ah, 1978) shows an inner shell of protein separated from the main coat by a 30-A shell containing mainly RNA. The most likely presumption is that the N-terminal arms interact with the RNA, probably in a quite definite local conformation, but that they are flexibly hinged and can take up many different orientations relative to the 180 subunits forming the outer shell of the virus particle. The disorder of the arms is a necessary condition for their specific interaction with the RNA, which cannot pack with the icosahedral symmetry of the protein coat subunits. 

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