摘要:

Polymer dispersed liquid crystal thin films have been deposited on glass substrates by the processes of polymerization and solvent evaporation induced phase separation. The electron and the optical polarization microscopies of the films reveal that PDLC microdroplets formed during the process of phase separation near the top surface of the film remain exposed and respond to shear stress due to air or gas flow on the surface. Optical response of the film to an air flow-induced shear stress input on the free surface has been measured. Director orientation in the droplets changes with the applied shear stress leading to time varying transmitted light intensity. Director dynamics of the droplet for an applied step shear stress has been discussed from free energy considerations. Results on the measurement of light transmission as a function of the gas flow parameter unambiguously demonstrate the potential of these systems for use as boundary layer and gas flow sensors.

ISSN:0267-8292    

DOI:10.1080/02678299308027651

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The influence of dopants on the threshold electric field for switching anti-ferroelectric liquid crystals was determined from the optical response curve obtained by using a triangular wave. The effect of dopant concentration on the transition temperatures of the mixtures with a host material was also investigated. The threshold electric field was diminished depending on (i) the chemical structure of the additive and (ii) increasing amounts of the additive. The upper limits of the temperature region of the S*CAphases also decreased with increasing amount of additive, and these phases disappeared at 40 mol% of additive in all cases. Compound (C) is the most effective with respect to the threshold electric field. It is a two ring compound and has a chiral part similar to that of the host antiferroelectric liquid crystal compound.

ISSN:0267-8292    

DOI:10.1080/02678299308027652

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A fast electrooptic modulation in a polymer waveguide using a ferroelectric liquid crystal has been proposed. In this device, the surface stabilized ferroelectric liquid crystal and the soft mode ferroelectric liquid crystal are used as an active material on the passive polymer waveguide, and electrooptic switching is realized by controlling the total reflection at the polymer waveguide-liquid crystal interface. The response time is of the order of several microseconds. The analogue electrooptic modulation in the waveguide is realized using the field induced linear molecular tilt of the electroclinic effect in the soft mode ferroelectric liquid crystal.

ISSN:0267-8292    

DOI:10.1080/02678299308027653

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The electro-optical properties of a ferroelectric liquid crystal in thin surface stabilized cells with bookshelf geometry were studied with respect to the temperature and to the frequency with alternating amplitude and waveform of the electric field applied. Using a previously proposed model of director reorientation, the measurements were simulated, and a consistent description of the electro-optical behaviour of the system achieved. The fit of the simulations to the measurements provided material constants for the compound investigated and these are discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308027654

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The synthesis and properties of new mesomorphic ferroelectrics composed of nonchiral polyphilic molecules are described. Polyphilic molecular units are made up of a sequence of chemical fragments differing by their chemical nature. The segregation between unlike species is used to favour a polar packing. Ferroelectricity is demonstrated by the measurement of an acoustically induced piezoelectric response and the determination of repolarization currents. It is the first time that a ferroelectric repolarizable mesophase is obtained with non-chiral molecular units. The mechanism of ferroelectricity is therefore different from that found in conventional chiral smectic C.

ISSN:0267-8292    

DOI:10.1080/02678299308027655

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Proton spin-lattice relaxation studies were carried out in the SAand S*Cphases of the liquid crystal CI IPNOC using both conventional and fast field cycling NMR techniques.T1dispersion curves were obtained at two different temperatures for each mesophase covering frequencies from 102to 3 × 108Hz. In both mesophases theT1data can be described assuming the presence of three different relaxation mechanisms, namely local molecular rotations, molecular self-diffusion and collective motions. The self-diffusion constantD1was evaluated for several temperatures and the activation energy associated with the diffusion process was obtained. The expected contribution of the soft-mode for the spin-lattice relaxation could not be separated from the contribution of other collective motions. The correlation times associated with the rotations around the molecular long axis and with the fluctuations of this axis were evaluated for both the SAand the S*Cphases.

ISSN:0267-8292    

DOI:10.1080/02678299308027656

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The non-linear dielectric relaxation spectroscopy has been recently developed and applied to soft materials such as polymers. We have applied this new method to the S*Cphases of some ferroelectric liquid crystals. Under a weak AC electric field, the original and third order harmonic frequency components of electric displacement are proportional to the first and third powers of the applied electric field, respectively. The linear spectrum obtained from the original frequency component shows the relaxation of Debye type and the third order non-linear spectrum shows the relaxation with an extended form of Debye type to the non-linear case. The third order non-linear dielectric increment is found to be negative, which implies that the dielectric non-linearity of the liquid crystal in the S*Cphase is due to the saturation of molecular dipole moments induced by the applied electric field. The temperature dependence of the linear and third order non-linear spectra in the S*Cphase are also studied. Both spectra do not change their forms much through the whole temperature range of the S*Cphase. In the vicinity of the SA–S*Ctransition temperature, the critical behaviour is more remarkable in the third order spectrum than in the linear one.

ISSN:0267-8292    

DOI:10.1080/02678299308027657

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We report measurements on the temperature variation of the electroclinic coefficient (e) of three members of a homologous series and a commercial ferroelectric mixture. The pure compounds haveP/θ ratios which are an order of magnitude larger than that of the commercial mixture. This is reflected in the temperature variations of the electroclinic coefficient. Plots ofe−1versus temperature indicate that in all the four cases the Landau theory gives a good description of the critical divergence ofeas the S*C–SAtransition point is approached. We have made a detailed analysis of the data on one compound using the Garoff-Meyer model to confirm that the critical behaviour for the soft mode is indeed well described by the Landau theory.

ISSN:0267-8292    

DOI:10.1080/02678299308027658

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We present the analysis of static properties of a simple phenomenological model of antiferroelectric liquid crystals, capable of explaining some different sequences of phase transitions found experimentally. Stability analysis shows the existence of three simply modulated phases. Results are summed up in phase diagrams with dependence on the parameters of the model. The existence of doubly modulated incommensurate phases is predicted in parts of the phase diagrams.

ISSN:0267-8292    

DOI:10.1080/02678299308027659

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The electromechanical properties of a compound exhibiting an antiferroelectric phase were investigated. An electric field induced antiferroelectric-ferroelectric transition could be detected by vibration analysis. In the antiferroelectric state the vibrations almost vanish while in the ferroelectric state the electromechanical effect has the same magnitude as in the chiral smectic C* phase.

ISSN:0267-8292    

DOI:10.1080/02678299308027660

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor