Enantiotropic liquid crystalline polymers

Institut fiir Organische Chemie, Universitat Mainz and Deutsches Kunststoff Institut, Darmstadt, West Germany 

In the past few years attempts to synthesize liquid crystalline polymers have started with a monomer in a liquid crystalline state. Three different types of polymer systems have been obtained  

Polymers with fixed liquid crystalline structure — if the polymerization temperature was below the glass ti ansition temperature ( ) of the polymer. These polymers exhibit a liquid crystalline structure in the solid glassy phase because the structure of the ordered monomer phase was frozen in by polymerization. By heating the polymer these frozen structures were irreversibly lost above the Tq An additional effect was observed starting from 

Until now there was no obvious correlation found between the monomer structure and the resulting pol qner phase. No.theorr retical structural conditions were described which would result in a liquid crystalline polymer with a definite ordered phase e.g. with a nematic a smectic or a cholesteric phase as in conventional liquid crystals. Although previous examples have established (8 9) the existence of enantiotropic liquid crystalline side chain polymers additional considerations are in order for a systematic synthesis of such polymers. 

In conventional LC phases the motion of the molecule is restricted only by the anisotropic interactions with its neighbors. This leads to the formation of the orientational long range order and in the case of smectic phases, to an additional lamellar sti ucture. However, completely different conditions normally exist in a liquid crystalline polymer. 

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Finkelmann, H., Ringsdorf, H. and Wendorff, J. H. Model considerations and examples of enantiotropic liquid crystalline polymers. Makromol. Chem. 179, 273 (1978)... 

Enantiotropic (Liquid Crystalline) Polymers Synthesis and Models... 

Table IV summarizes the results of polymerization experiments. In all cases enantiotropic liquid crystalline polymers were obtained. The homopolymers exhibit smectic phases. Contrary to our expectations smectic polymer phases are obtained also by copolymerization of the monomers in which the spacer length is n = 2 H2 = 6 and n = 6 n2 = 12. A similar result was published recently by Shibaev et al. (l)y based on...

H Finkelmann, H Ringsdorf and J H Wendorff. Model Considerations and examples of Enantiotropic Liquid Crystalline Polymers. Makromol.Chem. 179 273 (1978). 

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. 

Liquid crystalline polymers containing ferrocene in their side chains have been reported.235-240 Deschenaux used free-radical synthesis to prepare thermotropic liquid crystalline polymethacrylates containing ferrocene (Scheme 2.46).235 Polarized light microscopy showed that monomer 171 and its corresponding polymer 172 exhibited enantiotropic smectic A and C phases. 

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. 

Zhou et al. (1986) studied the transformation of monotropic to enantiotropic mesophase of the main-chain type liquid crystalline polymer, (3.15), in terms of the molecular weight effect. 

At about the same time, Percec and Nave (1987) also found that the transformation of the monotropic liquid crystalline polymers to the enantiotropic ones could be achieved by increasing molecular weight. The polymer studied by these authors has the following structural formula... 

All monomers are radically polymerizable. In accordance with the predictions derived from the model, these polymers exhibit enantiotropic liquid crystalline properties. Even the polymers prepared from isotropic monomers are liquid crystalline. Table II lists the phase transition and LC phases which were determined by DSC, x-ray investigations(10), and polarization microscopy. 

Case 1. Both monomeric structural unit and polymer display an enantiotropic mesophase. Upon increasing the molar mass from dimer to trimer, etc., Si decreases and therefore G increases. Beyond a certain molar mass Gi remains for all practical considerations constant. However, the slope of the increase of Tic-i is steeper than that of Tc- c> The difference between these two slopes determines the relative thermodynamic stabilities of the mesomorphic versus the crystalline phases at different polymer molar masses. As the molar mass is increased, the liquid crystalline regime is increased for main chain and side chain liquid crystalline polymers. 

Case 2. The structural unit displays a virtual or a monotropic mesophase the polymer displays a monotropic or an enantiotropic mesophase. The slope of the Tic-c versus molar mass (M) is steeper than that for Tc i, and as a consequence there will exist a critical value of M below which a mesophase is not observed. This effect has been observed in main chain and side chain liquid crystalline polymers. 

Let us first consider very briefly the influence of various parameters (i.e., nature of flexible spacer and its length, nature and flexibility of the polymer backbone and its degree of polymerization) on the phase behavior of a side chain liquid crystalline polymer. According to some thermodynamic schemes which were described elsewhere, the increase of the degree of polymerization decreases the entropy of the system and therefore, if the monomeric structural unit exhibits a virtual or monotropic mesophase, the resulting polymer should most probably exhibit a monotropic or enantiotropic mesophase. Alternatively, if the monomeric structural unit displays an enantiotropic mesophase, the polymer should display an enantiotropic mesophase which is broader. It is also possible that the structural unit of the polymer exhibits more than one virtual mesophase and therefore, at high molecular weights the polymer will increase the number of its mesophases. All these effects were observed with various polymer systems. ... 

Research into liquid-crystalline ferrocene sidechain polymers has been conducted by the research groups of Zentel and Deschenaux. One such liquid-crystalline polymer, polymethacrylate (18) and its monomer, showed enantiotropic smectic C and smectic A phases. This polymer also exhibited a weight average molecular weight of 100,000. ... 

Synthesis and mesomorphic properties of the first ferrocene-containing side-chain liquid crystalline polymers, 69, obtained by grafting either a 1,1 - or a 1,3-disubstituted ferrocene derivative functionalized by a vinyl group onto a polysiloxane have been reported [155J. These polymers showed enantiotropic SmC and/or SmA mesophases. 

Polymerization of organoiron monomers has also resulted in the production of liquid crystalline polymers containing ferrocene units in their sidechains. " Scheme 6 illustrates Deschenaux s free-radical synthesis of ferrocene functionalized thermotropic liquid crystalline polymethacrylates Monomer 31 and its corresponding polymer 32 exhibited enantiotropic smectic A and C phases. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

It should be assumed, however, that by analogy with the behavior of low-molecular-weight liquid crystals, there must be a temperature at which, in a pure polymer containing no solvent, transition from the liquid-crystalline to the isotropic state takes place. This transition must occur above the fusion temperature of the cyrstalline polymer (enantiotropic transition). Therefore, curves of compositions must, in principle, extrapolate... 

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