Liquid crystalline polymer, polyphosphazene

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. 

Bose, S., Pramanik, N., Das, C.K., Ranjan, A., and Saxena, A.K. (2010) Synthesis and effect of polyphosphazenes on the thermal, mechanical and morphological properties of poly(etherimide)/ thermotropic liquid crystalline polymer blend. Mater. Des., 31,1148-1155. 

Rath T, Kumar S, Mahaling RN, Mukherjee M, Das CK, Pandey KN, Saxena AK (2007) Flexible composite of PEEK and liquid crystalline polymer in presence of polyphosphazene. J Appl Polym Sci 104 3758-3765... 

Plummer CJG, Ziille B, Demarmels A, Kausch HH (1993) The stracture of filled and unfilled thermotropic liquid crystalline polymer injection moldings. J Appl Polym Sci 48(5) 751-766 Popa-Nita V, Gerlic I, Kralj S (2009) The influence of disorder on thermotropic nematic liquid crystals phase behavior. Int J Mol Sci 10(9) 3971-4008 Qin Y, Brydon DL, Mather RR, Wardman RH (1993) Fibres from polypropylene and liquid crystal polymer (LCP) blends 1. Effect of LCP concentration. Polymer 34(6) 1196-1201 Rath T, Kumar S, Mahaling RN, Mukherjee M, Das CK, Pandey KN, Saxena AK (2006) Flexible composite of PEEK and liquid crystalline polymer in presence of polyphosphazene. J Appl Polym Sci 104 3758-3765... 

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. 

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... 

A number of liquid crystalline polyphosphazenes with mesogenic side groups have been prepared (48—50). Polymers with nonlinear optical activity have also been reported (51). Polyphosphazene membranes have been examined for gas, liquid, and metal ion separation, and for filtration (52—54). There is interest in phosphazene—organic copolymers, blends, and interpenetrating polymer networks (IPNs) (55—61) to take advantage of some of the special characteristics of phosphazenes such as flame retardance and low temperature flexibility. A large number of organic polymers with cydophosphazene substituents have been made (62). 

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... 

Optical Materials. The polyphosphazene skeleton is electron-rich, which means that it provides a refractive index increment compared to conventional saturated organic backbones. In addition, the macromolecular substitution synthesis aUows highly unsaturated organic side groups to be linked to the skeleton in ways that allow the refractive index, the color, the liquid crystalline, and nonlinear optical characteristics of the polymer to be finely tuned. Thus, the use of these polymers in opto-electronic (photonic) switches and lens systems is a subject of growing interest. 

The two-step synthesis process shown in Figure 2 affords several possibilities for preparing new side-chain liquid crystal polymers. The polymerization process allows one to vary the molecular weight and molecular weight distribution (MWD), and potentially change the properties of the liquid crystalline state. Most of the polyphosphazenes reported in the literature, including the examples in this paper, are derived from the bulk uncatalyzed process this... 

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. 

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. 

Allcock HR, Cameron GC (1994) Synthesis of photocrosslinkable chalcone-bearing polyphosphazenes. Macromolecules 27 3131-3135 Andersson H, Gedde UW, Hult A (1992) Preparation of ordered, crosslinked and thermally stable liquid crystalline poly(vinylether) films. Polymer 33 4014—4018 BroerDJ (1996) Networks formed by photoinitiated chain cross-linking. In Crawford GP, Zumer S (eds) Liquid crystals in complex geometries, Taylor and Francis, London, pp 239-255 Broer DJ, Boven J, Mol GN, Challa G (1989) In-situ photopolymerization of oriented liquid-crystalline acrylates, 3. Oriented polymer networks from a mesogenic diacrylate. Makromol Chem 190 2255-2268... 

On the other hand, polyphosphazene 3.79 and its variants are non-crystalline. Then-ionic conductivities at room temperature are 1,000 times or more greater than that of polyethylene oxide).164 166 For battery type applications, 3.79 must be cross-linked lightly to prevent slow liquid-like flow, but this can be accomplished by radiation techniques without lowering the conductivity. An analogous type of polymer, with a poly-siloxane backbone and oligoether side groups, is being studied for similar applications. 

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