Crystalline phase polyphosphazenes

A number of polyphosphazenes of repeat unit [-PRR N-] also exhibit liquid-crystalline phases [166-168]. It is certainly intriguing that apparently the only classes of flexible chains that extensively exhibit liquid-crystalline phases are the polysiloxane and polyphosphazene semi-inorganic polymers. 

It is intriguing that even some flexible siloxane polymers form mesomorphic (liquid-crystalline) phases.34 139-166 Some illustrative data are given in Table 4.2. Both poly(diethylsiloxane) and pol y(di-n-propylsiloxane) show two crystalline modifications as well as a mesomorphic phase. (The other major class of semi-inorganic polymers, the polyphosphazenes, are also relatively flexible, and show similarly interesting behavior.)10167... 

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. 

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. 

Polyphosphazenes are partially crystalline polymers whose degree of crystallinity is a function of the conditions of preparation of the samples. The glass transition temperature of the amorphous phase is determined by the type of substituents at the phosphorus atom. The Tg for a series of polyphosphazenes in which the existence of mesophases has been reliably established [42] are reported in Table 4.4. The two-stage transition of the crystalline phase through the mesomorphic phase into an isotropic melt was established in [41-42]. The values of the temperatures of the phase transitions for a sCTies of polyphosphazenes are reported in Table 4.4. In polyaryloxyphosphazenes, T,- are close to. 

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... 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

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