Poly crystal structure: proposed

Fig. 2.2 Most favored helical structures proposed for two crystal forms of poly(a-n-butyl-/ -L-aspartate) (7, R=Bu) [28]. (A) Model of a (P)-3.25i4-helix. (B) Model of a (P)-4is-helix...

The idea of stacking monoradicals with S = 1/2 and di- or poly-radicals with 5 > 1 in mixed crystals was proposed by Buchachenko (Buchachenko, 1979). If the chemical structures are almost identical, the two component radicals will have a chance to stack in alternation and the two spin sublattices having S = 1/2 and S> 1, respectively, will be formed with opposite orientation. 

Christ and Clark proposed a crystal structure for these compounds that is a derivative of that proposed for UO2F2 by Zacharisen. This structure consists of layers of 2-6 coordinated uranium in which the hexagonal dipyramidal poly-hedra share edges. These layers are basically hexagonal in symmetry with the uranyl ion axis normal to the sheet direction. Oxygen atoms in the sheet are displaced small distances above and below the plane of the uranium atoms to accommodate closer packing. The formula of this layer is [(U02)(0,0H)2]. Interlayer ions include monovalent or divalent cations and water molecules. The sheet is shown in Fig. 7(a). This structure has been reported for q -U02(0H)2 by Taylor. 

Another view has recently been proposed by Wegner.Naphthalene and other simple aromarics can be oxidize electrochemi-cally to form monomelic radial cat n salts (Ar. X ) which have conductivities of 10 to 10 s/cm. The crystal structures of these reveal that the aromatic moieties form stacks, along which the charges and the electrons are presumably delocalized. The structure is formally analogous to that deduced for oxidized (doped) polyacetylene in which the polyene chains are arranged in stacks. This leads to the idea that intermolecular delocalization is the important feature which leads to high conductivity. Other data are consistent with this rationale. Biphenyl and terphenyl radical cation salts have crystal structures very similar to that of oxidized (doped) poly(p-phenylene lO). In the older literature oligoanilines (26) are reported upon iodine treatment to yield conductivities up to 1 s/cm the aniline moieties are stacked in these materials as well. Poly(N-vinyl-carbazole) (27) forms radical cation structures by oxidation with... 

Figure I. Crystal structure models of atactic poly(vinyl alcohol) proposed by (a) Bunn( ) and (b) Sakurada et al. W Broken lines show hydrogen bonds. (Reproduced with permission from reference 7. Copyright 1997, Wiley-InterScience)...

The chain confonnidon of poly(trifluoroethylene) (PTrFE) crystal was first reported by Lando et iL to be a 3/1 helix [1). Tashiro et have studied the structure and ferroelectric transitioas of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) co-polymers and concluded that the crystal structure of PTVFE is the same as the cooled phase which has the all-trans conformation with skew bonds [2]. The latter structural mixiel permits the formatioa of the spontaneous polarization in TVFE. Lovlnger et al. carried out X-ray measurements on PTrFE and proposed the confoimation of an irregular succession TO. TC. and TT from the lattice spacing parallel to the carbon chain [3]. 

From studies in solution and in the sohd state, a number of structures both sheet-like [14] and helical [19, 21, 24—36] have been proposed over the years for these polymers. Poly(/9-alanine) 3 for example crystallizes as extended chains [14]... 

The melting of a crystalline-amorphous block copolymer of poly(tetrahydro-furan)-poly(isoprene) (PTHF-PI) was investigated using DSC by Ishikawa et al. (1991). They found a double melting peak, which was proposed to result from the semicrystalline structure of the crystalline PTHF layer, with less-ordered crystallites melting before those with well-ordered domains of chain-folded PTHF. Alternative explanations include fractionation of the polydisperse block copolymer or melting of crystals with different fold lengths. 

Much work has been reported on studying the structure of thermoset resins via SAXS, especially focussing on interpenetrating network polymers (IPNs), thermoset nanocomposites, rubber-modified thermosets and thermoset-thermoplastic blends. Most recently Guo et al, (2003) have examined the use of SAXS to monitor the nanostructure and crystalline phase structure of epoxy-poly(ethylene-ethylene oxide) thermoset-thermoplastic blends. This work proposes novel controlled crystallization due to nanoscale confinements. 

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