Smectic polymers

As sparse as the dataset describing mainchain nematic LCP blends with conventional polymers is, it is rich compared to the almost non-existent data on the blending of other types of LCPs-side chain polymers, flexible spacer polymers, smectics, etc. 

Fig. 33. Self-dewetting of a polymer smectic film into droplets was monitored by tapping mode SFM. The micrographs were measured at room temperature after annealing of a 10 nm thick film at 115°C during different time intervals a - 0 min.b - 15 min.c- 2h,d - 4 h. The annealing temperature was 20 °C above the bulk isotropisation temperature. The polymer consists of a polymethylmethacrylate backbone with perfluorooctyl side chains [323]...

Figure 8. Schematic of Side Chain Polymers Smectic (bottom).

This type of material often has quite high solid-mesophase temperatures and is insoluble in most organic solvents. These types of polymers are usually nematic, reasonably thermally stable and have Tg temperatures above 0°C. de Gennes has predicted that for type polymers smectic phases should be obtained if the flexible segments are of equal length allowing lamellar organization. 

The theory of the cholesteric mesophase in polymer systems was examined in [132-135], and approaches to the theoretical analysis of the structure of polymer smectics are noted in [136-138]. However, the theory of polymer cholesterics and smectics has been elaborated to a much smaller degree than for nematics. 

The restricted size of the regions of LC ordering, in contrast to the macroscopic order which exists in low-molecular-weight liquid crystals, is a specific feature of polymer smectic phases [50, 51]. This follows from an analysis of the small-angle x-ray scattering and calculation of the onedimensional correlation functions Yi(x) ... 

Identification of nematic polymeric mesophases is a more complex problem than identification of polymer smectics. The structural data are usually limited to finding the absence of small-angle reflections in the x-rays of unoriented samples. The low enthalpy of the transition from the anisotropic to the isotropic phase (Table 6.9), close to the corresponding values characteristic of low-molecular nematic liquid crystals, and the absence of layered reflections indicate the one-dimensional type of ordering, although these data are insufficient for a complete description of the structure of nematic polymers, which can be both similar to and (Afferent from low-molecular-weight nematics. 

The formation of die fan-shaped textures typical of low-molecular-weight and polymer smectics is characteristic of some nematic polymers [40]. Such polymers should apparently have some elements of lay ed ordering which diffra from those known for low-molecular-weight cybotactic nematics. 

Fig. 14. Main-chain polymer Hquid crystal phases (a) nematic, (b) smectic.

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

No coherent threadline could be maintained and the extmdate flew off the windup as short, brittle, crystalline lengths. Not until many years later did other workers show that this polymer on cooling exhibits a mesophase transition directly from the isotropic melt to a smectic A phase. Good sources of information on Hquid crystals and Hquid crystal polymers are available (212—216). 

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

The effect of lateral methyl groups in the spacer on the phase behavior has been studied in several polybibenzoates [18,19] derived from poly(tetramethy]ene p,p bibenzoate), P4MB. The branched polymers display transition temperatures significantly lower than P4MB. Moreover, the substituents have a clear effect on the kind of mesophase formed. Thus, P4MB displays a smectic A mesophase, while the lateral methyl groups... 

PTEB-Q) to the annealed ones, owing to the presence of the crystalline phase. Moreover, the temperature of the peak increases with the annealing, as well as the broadness of the relaxation. These results suggest that the liquid crystalline phase gives raise to an a relaxation similar to that of amorphous polymers despite the existence of the two-dimensional order characteristic of smectic mesophases, and it changes following the same trend than that of semicrystalline polymers. 

Fig. 28. Room temperature 2H NMR spectra of the smectic liquid crystalline polymer (m = 6), oriented in its nematic phase by the magnetic field (8.5 T) of the NMR spectrometer with director ii parallel (left) and perpendicular (right) to the magnetic field...

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

Smectic liquid crystalline polymer 51 Solid echo 32 ---spectra 37, 39... 

The steric frustrations have also been detected in LC polymers [66-68]. For example, the smectic A phase with a local two-dimensional lattice was found by Endres et al. [67] for combined main chain/side chain polymers containing no terminal dipoles, but with repeating units of laterally branched mesogens. A frustrated bilayer smectic phase was observed by Watanabe et al. [68] in main-chain polymers with two odd numbered spacers sufficiently differing in their length (Fig. 7). 

We note that the bilayer smectic phase which may be formed in main-chain polymers with two odd numbered spacers of different length (Fig. 7), should also be polar even in an achiral system [68]. This bilayer structure belongs to the same polar symmetry group mm2 as the chevron structure depicted in Fig. 17b, and macroscopic polarization might exist in the tilt direction of molecules in the layer. From this point of view, the formation of two-dimensional structure of the type shown in Fig. 7, where the polarization directions in neighbouring areas have opposite signs, is a unique example of a two dimensional antiferroelectric structure. 

---