Polyphosphazenes crystalline structure

Polyphosphazene-Phthalocyanlne Structures. Thus, a related study was carried out with copper phthalocyanine units linked covalently to a poly(aryloxyphosphazene) (44). Non-polymeric copper phthalocyanine forms ordered stacked structures in the crystalline state. When... 

Crystallinity. Polyphosphazenes with a single substituent that is either small or rigid are semicrystalline. Their melting behavior consists of two first-order transitions, T(l) and Tm, separated by a 150-200°C gap. Optical microscopy showed that the crystalline structure was not lost at T(l), but rather existed as a mesophase until Tm was reached (2). Most fluoroalkoxy and aryloxy derivatives exhibited very high thermal stability with decomposition temperatures of 300-400°C. 

Schneider, N. S., Desper, C. R. and Beres, J. J. Mesomorphic structure in polyphosphazenes in A. Blumstein, ed. Liquid Crystalline Order in Polymers . Academic Press, New York, NY... 

First, we will consider the design of polyphosphazenes as side-chain liquid-crystalline materials.241 248 Side-chain liquid-crystalline polymers are a subclass of species described earlier as structure 3.72. Liquid crystallinity occurs when the rigid side groups become organized, usually in the semi-liquid state. The organization may be nematic (oriented but unlayered) or smectic (layered) as illustrated in Figure 3.25. 

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. 

Recent studies have extended the structure-property relationships in polyphosphazenes to include polymers that show interesting electrical or optical behavior in the solid state. These fall into three categories - (1) polymers that are good solid solvents for salts and which function as solid ionic conductors, (2) species that bear electronically active side groups, and (3) polymers that bear rigid organic units that generate liquid crystalline or non-linear-optical behavior. 

Second, the ability of completely aromatic polyesters to form mesomorphic structures is interesting. Obviously, in such cases, the appearance of nematic LC states seems most probable. Furthermore, some polybenzoates (3-8) show mesophases, which cannot be described in terms of the conventional classification of liquid crystals, with respect to their arrangement, and which are similar to mesomorphic structures of flexible-chain polyorganosiloxanes (9) and polyphosphazenes (10). Finally, studies of relaxations and phase transitions in rigid-chain LC polyesters, in particular, their molecular mobility in the solid state, i.e., below the melting temperature of crystalline phase, are of great interest. 

It has been found that some of the polyphosphazenes can exist in several crystalline polymorphs which only differ slightly in the packing of their chains. However, unlike PDES and PDFS, low temperature and hi temperature polymorphs in polyphosphazenes have not been found yet. Which kind of crystal structure modifications occur during the crystallization depends on the molecular weight, the... 

The morphology of the crystalline phase and the mesophase is correlated. They are both a function of the method of preparation of the samples, their past thermal history, and the defectiveness of the structure of the macromolecule [65]. Crystalline films of polyphosphazenes, polybistrifluoroethoxyphosphazene in particular, prepared from solution have a spherulitic structure which is preserved after melting of the crystalline phase to a total isotropic transition [41,53,57,62, 66]. On the contrary, optically anisotropic mesomorphic regions in the shape of rods in the microscope, which produce the beginning of crystals of the same... 

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