Liquid crystalline monomers

An additional advantage of the use of reactive, photopolymerisable liquid crystalline monomers with charge transfer or electroluminescent properties is the ability to generate circularly and linearly polarised light. This possibility... 

The order must then be frozen in before crystallisation occurs, since this would result in the formation of grain boundaries and a reduction in transport or emission efficiency. Device breakdown is also a possibility. The most efficient way to fix the liquid crystalline order is the formation of anisotropic networks by the polymerisation of reactive mesogens in the liquid crystalline state.Anisotropic polymer networks formed from the thermal or photoinitiated polymerisation of polymerisable, so-called photoreactive, liquid crystalline monomers have been used in a wide variety of electrooptic applications, see Chapter This is a more attractive approach than cross-linking... 

Self-standing nanostructured two-dimensional polymer films were prepared by in situ photopolymerization of ionic liquid crystalline monomer 11 that forms homeotropic monodomains of the smectic A phase on a glass plate (Figure 25.4). The film of 12 has a macroscopically oriented layered nanostructure as presented in Figure 25.5. 

Figure 25.4 Molecular structures of ionic liquid crystalline monomer 11 and polymer 12.

Gangadhara et al. have linked the cyanobiphenyl mesogen via a dicarbox-imide-group to an oxanorbornene ring system Vl-n, n=2-8 (see Fig. 4). Polymerization was carried out with Schrock type initiator 4. The dicarboximide linkage probably hindered the formation of LC phases even the introduction of relatively long spacers between the polymer backbone and the mesogen did not lead to liquid crystalline monomers or polymers [41]. 

Statistical copolymerization of SCLC-monomers with non-liquid crystalline monomers leads to dilution of the mesogenic units in the polymer, and (below a critical value) to the loss of the LC behavior of the polymer [47]. 

The introduction of perfluorinated groups generally favors microphase separation due to the immiscibility of fluorocarbons with hydrocarbons [66]. Norbornene derivatives with perfluorinated endgroups in the side chain were prepared by Wewerka et al. [67]. Monomer XII contained a relatively long (CF2)8-chain, separated via a long spacer (11 methylene-groups) from the norbornene, whereas monomer XIII has two relatively short (CH2)2(CF2)4-side chains (Fig. 11). Homopolymers and block copolymers were synthesized with one fluorinated monomer (XII or XIII) and one non-fluorinated non-liquid crystalline monomer (NBDE or COEN) with the Schrock-type initiators 4 and 5, respectively, leading to microphase-separated block copolymers. Table 9 and Table 10 summarize the physico-chemical properties of the homopolymers and block copolymers. 

Ferroelectric liquid crystalline monomers (Fig. 21, XXXVII) bearing two terminal vinyl groups were polymerized directly from their smectic A" liquid crystal phase using a Grubbs-type initiator. 

The investigation on oriented polymeric networks obtained by the photopolymerization of oriented low molecular weight species, as presented in this paper, has been carried out with a more or less conventional acrylate monomer. Already with this material an anisotropy in properties could be demonstrated. It is to be expected that even more pronounced effects can be obtained with monomers which have a strong tendency to alignment. Based on this idea we are now investigating liquid crystalline monomers in our laboratory. 

The above sections provide information about the synthesis of block copolymers using CRP techniques. In the first section, details about the nitroxide-based systems were presented. From the literature reports, it is clear that TEMPO and TEMPO-based analogs only produce block copolymers of St-based monomers successfully. Liquid crystalline monomers, sugar containing derivatives, and even silicon-based St monomers were successfully homopolymerized. Water soluble monomers like sodium 4-styrenesulfonate and 4-(dimethylamino) methylstyrene were successfully incorporated into block copolymers using TEMPO in aqueous media. However, there was evidence that even with some St derivatives, particularly those that were chloromethylated, side reactions occurred to pro-... 

PMMA styryl liquid crystalline monomer radical 135... 

Then the liquid crystalline monomers were polymerized under an ultraviolet irradiation. The system became a cholesteric gel. After heating the sample and removing the chiral molecules CB15, the helical structure remained. In Table 6.10, the cholesteric pitch P and reflective peak wavelength Amax are listed as a function of temperature at which polymerization occurs. The CB15 weighed 28% in the original mixture before the removal. 

Figure 16.3 Photopolymerizable Liquid Crystalline Monomers bone fixation screws, and artificial blood vessels.

Figure 16.5 Liquid Crystalline Monomers with Oxetane Units °...

Photopolymerizable liquid crystalline monomers are mostly based on acrylate groups that are introduced in the liquid crystal unit. Examples are shown in Figures 16.3 and 16.4. In addition, liquid crystalline monomers with pendent oxetane units, c.f. Figure 16.5 can be polymerized by ring-opening. Compositions are spin-coated and polymerization is initiated by UV light. 

Electrically switchahle holograms can be generated with formulations containing a liquid-crystalline monomer. A typical example is given in Chart 11.11. 

Along these lines of thinking block copolymers of styrene and 6-[4-(4-methoxyphenyl)phenoxy]hexylmethacrylate (MPPHM) were synthesized by anionic polymerization. Styrene was polymerized first in benzene using 5-BuLi as the initiator. After completion of polymerization the living PSIi chains were end-capped with diphenylethylene and the solvent was changed to THE Anhydrous liCl was introduced to the reactor, and the temperature was lowered to - 40 °C. At that temperature a solution of purified MPPHM in THE was introduced slowly. After complete reaction of the liquid crystalline monomer, polymerization was terminated with methanol . The procedure is outlined schematically in Scheme 9. 

Freely-suspended Films of Polymeric Liquid Crystals. The stabilization of freely-suspended films by using polymeric liquid crystals is obviously interesting and has been attempted previously. Unfortunately it seems to be extremely difficult to polymerize films of liquid crystalline monomers as these films were reported to always break during polymerization. It seems to be equally difficult to fabricate FS-films of polymeric liquid crystals in their smectic A and smectic C phases, most likely due to their enhanced viscosities. However, if one heats slightly into the isotropic phase it is possible to spread a film across an aperture which thins out to form a truly freely-suspended liquid crystal film after cooling into the smectic phases (57). Films of this type are homeotropic in the smectic A phase and show birefringence when cooled to the ferroelectric smectic C ... 

Photopolymerizable monomers and oligomers can be classified under four main groups including radical monomers and oligomers, unsaturated polyester resins, thiol-ene systems, and cationic monomers. In addition to these systems, particular photopolymerizable systems are also available such as expanding monomers, liquid crystalline monomers, and some other miscellaneous monomers. 

Table 4 Examples of expanding and liquid crystalline monomers Expanding monomers...

Liquid crystalline textures in polymeric systems can be obtained in two different ways. One way consists in inducing a specific texture in a liquid crystalline monomer phase. Often the forces acting on the sample will be sufficiently large to stabilize the texture during the polymerization. A texture may be obtained in this way even in polymers which do not display liquid crystalline phases. In the case of polymer 3 the polymerization in the nematic monomer phase under the influence of a magnetic field led to a polymer sample with high orientational order on a macroscopic scale. 

A homogenous polymerization was achieved by the polymerization of a nematic monomer to a nematic polymer where the temperature region of the liquid crystalline monomer phase overlapped the temperature region of the liquid crystalline polymer phase (see Table II). This polymerization process is influenced either by a precipitation or by a phase transition. 

Figure 17.5 Liquid crystalline monomers with oxetane units [25],...

Furthermore, not all polymerizable MLCs are precursors of PLCs. A polymer derived from an MLC may not be mesomorphic due to packing constraints introduced by the backbone of the chains and the change in entropy that attends linking the monomer units by covalent bonds. On the other hand, a non-liquid crystalline monomer may yield a PLC (due, in this case, to favorable entropic changes see Chapter 15). 

Recently, polymer networks by in situ polymerization of aligned low molar mass liquid crystalline monomers have been reported. By this technique, some interesting materials with molecular memory in the anisotropic state where the crosslinking took place can be made. This technique will be discussed in detail in Chapter 10. 

More recently, the possible use of liquid crystalline monomers (114), hy-perbranched polymers (115,116), inorganic-organic hybrid monomers (117-120), and sol-gel technology (121) has started to be examined for preparing improved... 

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