Ferroelectric liquid crystalline polymers ferroelectricity

Kawasaki, K., Kidera, H., Sekiya, T, and Hachiya, S., Molecular motion of ferroelectric liquid crystalline polymers, Ferroelectrics, 148, 233-243 (1993). 

The photoinduced flop of the polarization of ferroelectric liquid crystals showing a quick response was achieved by dissolving azobenzene moieties in the liquid crystals.114,11 Photocontrol of the switching behavior of ferroelectric liquid-crystalline polymers (33, Figure 14) was induced by admixing azobenzene derivatives.116... 

It is difficult to grow a good organic crystal film and a Langimur-Blogette film of up to 1 micron thickness. However, polymers have a wide choice and can be tailored to meet the above requirements. The polymers may be side chain liquid crystalline polymers, ferroelectric liquid crystalline polymers and amorphous polymers. Among them the side chain liquid crystalline polymers have drawn more attention. 

Ferroelectric side chain liquid crystalline polymers have been synthesized recently. Because of their non-central symmetry they don t need to be poled. This has led to the recent active research efforts in the field of ferroelectric side chain liquid crystalline polymers (Guglielminetti et al., 1990 Spassky et al., 1989 Kapitza et al., 1986). 

Shibaev et al. (1984) first synthesized a ferroelectric side chain polymeric liquid crystal. In the following years a lot of liquid crystalline polymers of such kind were synthesized. In early research studies techniques used to understand polymers and whether they showed the liquid crystal phase were limited so that the conclusion was ambiguous. It was only in 1988 when Uchida et al. (1988) measured the spontaneous polarization and the tilt angle that people became convinced that this side chain polymer in the literature (Shibaev et al., 1984) is indeed a ferroelectric liquid crystal. 

The side chain polysiloxane liquid crystalline polymers PI, P2 and P3 composed of the above three ferroelectric liquid crystals as the side groups exhibit the ferroelectric liquid crystal phase, their chemical formulae being respectively. 

It has been found experimentally that the spontaneous polarization Ps of side chain liquid crystalline polymers is more or less the same as that of small molecular mass liquid crystals, i.e., so far as the chemical formula of the side group of a side chain liquid crystalline polymer is the same as that of a small molecular mass liquid crystal. This phenomenon illustrates that either the side groups in the side chain liquid crystalline polymer or the small molecular mass ferroelectric liquid crystal in their Sc phase are aligned in same way. The dependence of Ps on temperature for the three small molecular mass ferroelectric liquid crystals LI, L2 and L3 and their polymer counterparts PI, P2 and P3 are depicted in Figure 6.40. 

The backbone affects the dynamic behavior of the ferroelectric liquid crystalline polymer. Sandwiching the two kinds of ferroelectric liquid crystals between two ITO-coated glass plates of 1.5 microns gap respectively, one constructs a SSFLC (surface stabilized ferroelectric liquid crystal) cell. The switch time between two optical states r is determined by... 

For the small molecular mass ferroelectric liquid crystal when reversing the polarity of the applied electric voltage the molecules rotate locally while their molecular mass centers don t necessarily move accordingly. But for side chain ferroelectric liquid crystalline polymers, as one of the side group ends is confined to backbone, shown in Figure 6.43, the polarity reversion must be accompanied by the movement of their mass centers, which causes a backflow in order to re-distribute the mass centers. Moreover, the side groups may collide with each other. The effect results in the displacement of the backbone. The above effects increase the difficulty of re-orientation and hence increase the viscosity. 

Endo et al. (1992) measured the optical transmission and the polarity-reverse current during the polarity reversion of a side chain ferroelectric liquid crystalline polymer. It was found that both parameters reached peak values at the same time. It was concluded that the rigid core of the side groups responsible for birefringence moves simultaneously with the dipole moment reversion and the latter contributes to the polarity reversion current. The FTIR experiment suggested that the backbone moves when the polarity is reversed. 

The molecule contains Si-0 bonds. FTIR suggested that the Si-0 bonds move when the side groups move. Hence, ferroelectric liquid crystalline polymers have higher rotational viscosities than small molecular mass ferroelectric liquid crystals. In Figure 6.44 the relation of rotational viscosity r/ and molecular weight Mw at 600 °C is plotted, rj increases as Mw increases and the quadratic law is observed. 

Boemlburg et al. (1991) first discovered the anti-ferroelectric liquid crystal phase in the chiral side chain liquid crystalline polymer, Sca phase. Several other research groups followed with more such side chain liquid crystalline polymers. Boemlburg et al. (1992) reported an anti-ferroelectric liquid crystal in the molecule... 

Scherwsky et al. (1989) first utilized a SSFLC display in terms of the ferroelectric liquid crystalline polymer. The polymer SSFLC display is fabricated on the ITO-coated plastic substrate. The display was 15 x 40 cm2 in area and had 100 x 300 pixels (Lagerwell, 1993). The display doesn t need the orientation layer which is essential in the conventional liquid crystal displays in order to anchor the liquid crystal molecules. By lightly bending the... 

If some of the side groups are substituted by dyes or fluorescent molecules, the display may work without polarizers. It is expected that large screen ferroelectric liquid crystalline polymer displays will come out in the near future. 

In principle, liquid crystalline polymers can be applied in displays. Unfortunately, the response of them to the external fields isn t satisfactory because their viscosity is greater than the small molecular mass liquid crystals by a few orders of magnitude. In fact, only when the temperature is near the glass transition temperature, can the response be measured in seconds. Apparently, this is far from the real requirement. One may mix the liquid crystalline polymer with small molecular mass liquid crystal for such a purpose, but the mixture doesn t show an advantage over the small molecular mass liquid crystal displays. The ferroelectric liquid crystalline polymer is an exception. It works with a very fast effect and can achieve a display with a response time of a few milliseconds or a fewr tens of milliseconds. 

Brodowsky HM, Boehnke UC, Kremer F, Gebhard E, Zentel R. 1999. Mechanical deformation behavior in highly anisotropic elastomers made from ferroelectric liquid crystalline polymers. Langmuir 15 274 278. 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

M. Mitsuishi, S. Ito, M. Yamamoto, H. Endo, S. Hachiya, T. Fischer, W. Knoll, Optical characterization of a ferroelectric liquid crystalline polymer studied by time-resolved optical waveguide spectroscopy. Macromolecules 31, 1565-1574 (1998)... 

Mesogenic groups can be incorporated into polymeric systems [7]. This results in materials of novel features like main chain systems of extraordinary impact strength, side-chain systems with mesogens which can be switched in their orientation by external electric fields or—if chiral groups are attached to the mesogenic units—ferroelectric liquid crystalline polymers and elastomers. The dynamics of such systems depends in detail on its molecular architecture, i.e. especially the main chain polymer and its stiffness, the spacer molecules... 

The molecule is a liquid crystalline polymer with chiral smectic C phase forming parts attached as side chains. The field required to switch the direction of polarization of the polymer is very low (0.3 MVm ). There is a lot of interest in liquid crystalline ferroelectric polymers, because of their possible use for fast-switching electro-optical devices. More information about ferroelectric liquid crystals can be found in references [36,37]. 

Shilov, S. V. Okretic, S. Siesler, H. W. Zentel, R. Oge, T., Fourier-Transform Infrared Study of the Switching Process in a Ferroelectric Liquid Crystalline Polymer. Macromol. Rapid Commun. 1995,16,125-130. 

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