Chain conformation unit cell

Crystal Structure Crystal system Chain conformation Unit cell parameters ... 

This work is fully modem and Bernard Wunderlich (1920-1988), the leading polymer crystallographer of the late 20 century, would be proud of it even now Crystalline polymers can be discussed at many different levels repeating chain conformation, unit cell stmeture and overall space group of fiiU crystal. The polymorphic character of many polymer crystals was fully appreciated by Meyer. 

Form Crystal system Space group, lattice constants and number of chains per unit cell Molecular conformation Crystal density (g/cm )... 

Chains having identical conformations can be packed in different modes corresponding to different unit cells. 

For instance, in the three crystalline forms (a, 3, y) of i-PP the chains are always in the conformation of threefold helix (s(3/l)l symmetry) but are packed in different ways in monoclinic [24], hexagonal [25], and orthorhombic [26] unit cells, respectively. The X-ray diffraction spectra of unoriented samples in the crystalline forms a, P, y are reported in Fig. 6. 

This is, for instance, the case of the a and 5 forms of PVDF (also called form II and form lip, that is form II polarized), both contain chains with a TG+TG" conformation and have an orthorhombic unit cell with substantially identical dimensions [30, 31]. Detailed X-ray diffraction analyses have shown that the two chains in the unit cells are with the dipole vectors pointing in opposite directions in the a form, while they are in the same direction in the 8 form [32],... 

An analogous case, of identical chain conformations as well as of similar unit cell dimensions, have been described for the two crystalline forms of poly-p-phenylene terephtalamide [33-36] (better known with the trade name of Kevlar). The projections along the c axis of the packing of the chains proposed for the two forms [36] has been sketched in Fig. 8, corresponding to the localization of the chain axes in (0,0, z) and (1/2,1/2, z) for the more common polymorph, in (0, 0, z) and (1/2,0, z) for the other polymorph. 

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... 

According to a recent report, the unit cell of cellotetraose hemihydrate in single crystals contains two antiparallel chains, which are conformationally distinct—especially in the sugar geometries.74 However, all hydroxymethyl groups adopt similar gt orientations. Whether this oligosaccharide morphology can be implemented for cellulose II in fibers remains to be seen. 

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