摘要:

Polymer films containing dispersions of liquid crystal microdroplets have considerable potential for use in displays and other light control devices. These polymer-dispersed liquid crystal (PDLC) films operate by electric field control of light scattering, rather than by polarization control as in the case of twisted nematic systems. The scattering characteristics of the PDLC films are determined by the refractive indices of the polymer and liquid crystal and by the size of the microdroplets. We have found that it is possible to regulate the microdroplet size by controlling the droplet formation rate (i.e. the cure kinetics of the film). Using calorimetry and scanning electron microscopy, we determined the influence of cure kinetics on microdroplet size for epoxy-based PDLCs. We found that droplet size increased with increasing cure time constant. However, the relationship changed as cure temperature was varied, perhaps as a result of competing cure processes. We also determined the phase behaviour of the epoxy-based PDLCs. The liquid crystal acted as a plasticizer, depressing the glass transition temperature of the PDLC samples slightly below that of the pure epoxy. The temperature and enthalpy of the nematic to isotropic transition of the liquid crystal material in the microdroplets were both functions of cure temperature. From the transition enthalpy it was possible to estimate a, the fraction of liquid crystal contained in the droplets; we found that a decreased with increasing cure temperature, presumably as a result of greater liquid crystal solubility in the epoxy matrix at higher temperatures.

ISSN:0267-8292    

DOI:10.1080/02678298808086401

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A biaxial elastic model for the temperature dependence of Sc*helical pitch is proposed on the basis of a phenomenological theory of Sc*liquid crystals accompanied by a biaxial molecular ordering playing an important role for the anomalous behaviour of the helical pitch near the Sc*-SAtransition point. In the Sc*. phase ofp-(n-decyloxybenzylidene)-p-amino-(2-methylbuthyl)-cinnamate (DOBAMBC), a qualitative agreement with the observed helical pitch is found by choosing a few material parameters concerned with a biaxial elasticity and an anisotropy of the elastic constants.

ISSN:0267-8292    

DOI:10.1080/02678298808086402

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The temperature dependence of the three elastic constantskii(i= 1, 2, 3) and the twist viscosity γ1of two nematic side chain polyacrylates and one comparable low molecular weight compound have been measured by means of the Freedericksz effect. The change from the low to the high molecular liquid crystal causes a change of the ratiok33/k11from greater to less than unity, but the order of magnitude of the elastic constants remains the same. In contrast, the twist viscosity of the polymeric liquid crystal is about 1000 times greater in magnitude than that of a comparable low molecular weight liquid crystal. The activation energy for the viscosity of the polymer differs by a factor 3–4 from that of the low molecular weight liquid crystal. The elastic constants as well as the twist viscosity show a quadratic dependence on the order parameterSover a wide temperature range.

ISSN:0267-8292    

DOI:10.1080/02678298808086403

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The thermotropic liquid crystalline polymer VECTRA A900 (Celanese Corp.) shows a super-cooled liquid state in which it undergoes gradual crystallization and in which it maintains, when sheared, a high degree of molecular orientation. Both phenomena were studied by annealing the polymer 40 K above its melting temperature,Tm, and then cooling it quickly to the desired experimental temperature level,T.ForT>Tm, the sample is stable over a long time not showing significant changes. ForT<Tm, the polymer is in a super-cooled liquid state and undergoes gradual crystallization which can be observed as an increase in the complex modulus by several orders of magnitude. The rate of crystallization increases significantly with the intervalTm–T.Samples could be sheared in the initial stage of crystallization. The sheared samples showed a high degree of orientation in wide angle X-ray scattering patterns and large constrained recovery. Recovery was studied in two stages: the first recovery occurred at the shearing temperature and finished soon after the suspension of the shearing stress; the second recovery took place after the sample was completely remelted. The recoverable strains of both stages are severely temperature dependent and moderately time dependent. These observations are discussed with respect to the crystallization rate and the crystallinity of the super-cooled melt.

ISSN:0267-8292    

DOI:10.1080/02678298808086404

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The synthesis of homologues of 4-alkoxybenzylidene-4'-aminocinnamic acid 1-trifluoromethylalkyl esters is reported. Only three of the compounds exhibit smectic phases. Phases were identified using optical and electro-optic techniques together with X-ray diffraction studies. Some of the compounds, while not exhibiting liquid crystal phases, show crystalline polymorphism.

ISSN:0267-8292    

DOI:10.1080/02678298808086405

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A statistical mechanical theory is applied to study the equilibrium properties of discotic nematic liquid crystals. We report the calculation of thermodynamic properties for a model system composed of molecules interacting through angle-dependent pair potentials which can be broken up into rapidly varying short-ranged repulsions and weak long-range attractions. The repulsive interaction is represented by a repulsion between hard oblate ellipsoids of revolution and is a short-range, rapidly-varying, potential. The influence of attractive potentials, represented by dispersion and quadrupole interactions on a variety of thermodynamic properties is analysed. It is found that the thermodynamic properties for the discotic nematic-isotropic transition are highly sensitive to the form of effective one-body orientational perturbation potential. The discontinuity in the transition properties is more pronounced in the case of quadrupole interaction than for anisotropic dispersion interaction. A remarkable symmetry in the transition properties between prolate ellipsoids (ordinary nematic) and oblate ellipsoids (discotic nematic) is observed.

ISSN:0267-8292    

DOI:10.1080/02678298808086406

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Time-resolved light scattering studies have been undertaken for elucidating the dynamics of phase separation in aqueous HPC (hydroxypropyl cellulose) liquid-crystalline solutions. The HPC/water system phase separates during heating and returns to a single phase upon cooling. The phase diagram of thermally induced phase separation was subsequently established on the basis of cloud point measurements. For kinetic studies,T(temperature) jump experiments of 10 per cent aqueous HPC solutions were undertaken. Phase separation occurs in accordance with the spinodal decomposition mechanism. At lowTjumps or in reverse quenched experiments, the scattering maximum remains invariant as predicted by the linearized Cahn-Hilliard theory. However, at largeTjumps, the SD is dominated by non-linear behaviour in which scattering peaks move to low scattering angles. The latter process has been identified to be a coarsening mechanism associated with the coalescence of phase separated domains driven by a surface tension. A reduced plot has been established with dimensionless variablesQand t. It was found that the scaling law is not valid over the entire spinodal process. The time evolution of the scattering profiles of 10 per cent HPC solutions, following aTjump to 49°C, is tested with the scaling law of Furukawa. It seems that the kinetics of phase separation at 10 per cent solution resemble the behaviour of off-critical mixture.

ISSN:0267-8292    

DOI:10.1080/02678298808086407

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The first eleven members of the homologous series of α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)hexanes have been synthesized. The compounds are all enantiotropic nematogens and, with the exception of the heptyl, octyl and nonyl homologues, exhibit smectic phases. The thermal stability of the smectic A phase initially increases with the length of the terminal alkyl chain, passes through a maximum and then falls dramatically before disappearing. The smectic A phase subsequently reappears with the decyl homologue which has the highest smectic A-nematic transition temperature of the series. In order to understand this unusual behavior we have determined the entropies of transition for the compounds and we have measured the layer spacing of the smectic A phase for three of them.

ISSN:0267-8292    

DOI:10.1080/02678298808086408

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The sodium salt of the dinucleoside phosphate d(GpG), dissolved in water, forms two mesophases which have been characterized, by optical microscopy, as being of the cholesteric (columnar chiral nematic, N*) and hexagonal (M) type.

ISSN:0267-8292    

DOI:10.1080/02678298808086409

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0267-8292    

DOI:10.1080/02678298808086400

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor