Smectic phase monomer

To determine if this phenomenon is isolated to amorphous monomers, a liquid crystalline diacrylate (C6M) was polymerized in W7,W82 at temperatures corresponding to the two smectic phases as well as the isotropic phase. The polymerization rate for C6M is plotted as a function of time for representative temperatures in Figure 6. Again, the polymerization shows marked acceleration in the ordered smectic C phase and occurs much faster than the isotropic polymerization. As seen in the HDDA polymerizations, the smectic A rate also lies between the rates of the other two polymerization temperatures. 

There is only one example that fits into this section and it uses a spectacular way to form the crown ether in the polymer that is worth noting (Scheme 22). In contrast to all other procedures that use crown ethers pre-formed in the monomers, this example by Percec forms the crown ether [13]crown-4 during the polymerization reaction [59]. Cationic cyclocopolymerization of l,2-bis(2-ethenyloxyethoxy)ben-zene 37 with the mesogenic cyano biphenyl unit 38 gave copolymer 39 with a 1 1 ratio of 37 38 displaying a smectic phase. 

A change in rate and/or mode of propagation can be brought about by orientation of monomer molecules in the liquid crystalline state. For vinylo-leate in the smectic phase, a higher polymerization rate than in the isotropic phase was observed by Amerik and Krentsel [28]. A significant reduction in the polymerization rate of a relatively complex monomer [N-(p-acryloyl-oxybenzylidene)-p-methoxyaniline] in the nematic state was described by Perplies et al. [29], On the other hand, Paleos and Labes observed no change in the polymerization kinetics of a monomer, also of Schiff base character... 

The monomers XXIV-m-n were polymerized with initiator 4 in a monomer to initiator ratio of about 50. Table 16 summarizes the physico-chemical data for these polymers. Poly-(XXIV-m-n) exhibited smectic C mesophases, but these phases were only observed over a small temperature range and isotropization occurred shortly after the glass transition. It was not possible to identify the mesophase by polarized optical microscopy, but the smectic phase was confirmed by X-ray scattering experiments. Transition temperatures increased with the length of the carbon segments (n) but decreased with increasing siloxane segment (m). 

In these systems, the formation of both the nematic and smectic phases is attributed to a particular chain conformation implying that all the sub-units he parallel to each other to give an overall rod-hke shape, rather than a disk-hke or spherical shape. This is possible since the racemic AB2 monomer pos-... 

Another approach is that of polymerizing low molecular mass mesogens. For example, Bhowmik and coworkers polymerized viologen-based monomers and obtained liquid-crystalline ionomers which yielded smectic phases [84-87]. 

The synthesis of polymers capable of entering into a chiral nematic phase initially proved difficult as many of the acrylate and methacrylate comb-branch polymers to which a cholesterol unit was attached as a side chain tended to give a smectic phase. This was overcome by either copolymerizing the cholesterol-containing monomers with another potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety. Examples of both types are shown as structures VIII and IX. 

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 ... 

It seems that the longer mesogenic monomer, being more likely to form the smectic phase forms roughened and plate-like morphologies more readily. To explain why fibers (presumably nematic like) are formed at the lower temperature. 

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...

The expected preference of a statistical distribution of the mesogenic side chain has so far been only realized by the copolymerization in which n = 2 n.p = 12 and only if the monomer ratio is 1 1. Other compositions also led to smectic phases. The best proof for its cholesteric structure is its selective reflection of left-circular polarized light in the visible which indicates a left-handed cholesteric helix of the polymer. The structure has been established also by x-ray investigations (8). This copolymer is the first enantiotropic cholesteric polymer. 

No phase separation was observed when an isotropic monomer was polymerized to a nematic or smectic polymer. The nematic or smectic polymers are soluble in the isotropic monomer. When a certain conversion is obtained, a phase transition from isotropic to nematic or smectic phase takes place. This phase transition, caused by variation of the mole fraction of the monomer and the polymer, is comparable with a phase transition caused by varying the temperature at constant composition. 

The polymerization in the cholesteric state of this monomer is a heterogeneous phase reaction. The polymer formed is precipitated as a smectic phase, and as the conversion increases, the cholesteric monomer is dissolved in the isotropic system, and a homogeneous phase is formed. As the reaction proceeds further, the system becomes smectic ( 25) ... 

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