Photopolymerization, smectic phases

The following protocols (6-10) describe the synthesis of some cholesterol-based acrylates and their photopolymerization in an aligned cholesteric phase. The protocols utilize a modification of a system previously described by Shannon. 5 6 ip ie absence of a diacrylate comonomer, the cholesteric phase produced initially on copolymerization is not stable and reverts to a smectic phase on a single cycle of heating and cooling. In the presence of the diacrylate the first-formed phase is stable. This is one example of how crosslinking can stabilise the liquid crystal phase in liquid crystalline elastomers, others include, the so-called, polymer-stabilized liquid crystals and those described in the later protocols. 

For application to thin film transistors (TFTs), which can operate electronic papers, high carrier mobility exceeding 0.01 cm2 V-1 s-1 is necessary. For this purpose, nematic semiconductors with low molecular order, resulting in relatively low carrier mobility, are not suitable. For application of thin films of semiconductors to TFTs, stabilization of the highly ordered smectic phases, which exhibit high carrier mobility, by photopolymerization may be desirable. Kreouzis et al. studied the carrier transport properties of photopolymer-izable phenylnaphthalene, diphenylbithiophene, and quaterthiophene derivatives having an oxetane moiety or l,4-pentadien-3-yloxy in their alkyl side chain (Fig. 21) [107,108],... 

Photopolymerization performed in smectic phase or initially in an isotropic phase, followed by conversion to a biphasic medium... 

Charge carrier mobility measurements of the mesogens 9 and 10 (Figure 10) were made in different states the spin-coated films, the films that were slowly cooled from the isotropic to the smectic phase, and the films after crosslinking. Whereas alignment improved the field effect mobility, the photopolymerization reaction reduced the values of 2 x 10" and 5 x 10 cm s" approximately five times. ... 

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. 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

For this compound, which forms a smectic C phase at room temperature, an electron mobility of 1.5x10 cm s was reported. By virtue of its reactive groups, this diene compound can be photopolymerized to form a polymeric network [45]. 

Figure 10.17 (a) The birefringence of an oriented one-comb PLC (polyacrylate obtained by photopolymerization of a surface aligned MLC) as a function of temperature. The various phases (K, crystalline S, smectic N, nematic and I, isotropic) are shown in the graph, (b) The order parameter of the same polymer (open symbol) in its nematic state as a function of reduced temperature, T/T., where Tj is the isotropization temperature. Data for the monomer (filled symbol) are shown in the same graph, ((a) and (b) from data of Broer et al [111].)... 

Monomer B5A exhibits two enantiotropic smectic mesomorphic phases (Se66Sb75I). The S phase does not crystallize even when cooled down to -20 "C. In this study, we will concentrate on the photopolymerization of B5A at room temperature (Sg phase) and the detailed structural characterization of the resulting polymeric network (subsequently called PB5A-RT). The synthesis and structural characterization for monomer B5A will be presented in a separate paper and only summarized results are presented here for comparison with its polymer. Monomer B5A in the smectic E phase has an orthorhombic structure with unit cell dimensions, a=6.609 A, b=7.704 A and c=51.92 A. It adopts a tilted molecular bilayer structure within the unit cell in the c direction. Within each smectic layer the molecules are tilted, making an angle of about SO with the smectic layer normal (or c direction). 

Monomer B5A can be photopolymerized at room temperature in its Sg phase to near completion with little volume change. The smectic liquid crystalline structure is retained and locked into the resulting polymeric network upon polymerization. This can be attributed to the topochemical control exerted by the LC monomer matrix on the as-formed polymer a quasi-topochemical polymerization reaction is involved (B5A-RT and PB5A-RT has almost the same unit cell parameters). Fixation of the LC structure is realized through cross-linking, which severely restricts molecular motion and relaxation. The LC structure is maintained up to very high temperatures. This could extend the application temperature of... 

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