On the Electrical Switching Properties of Polymeric Liquid Crystals

The properties of side chain polymeric liquid crystals in an electric field are described. The polymers investigated are (A) a polysiloxan-copolymer with Cl- and OCH₃-end groups in the mesogenic side chains and (B) a polyacrylic ester homopolymer with a CN-group in the mesogenic side chain.

Applying the Fréedericksz-effect, the rise times and the passive decay times are determined. With the two-frequency technique, the active decay time is also obtained. The order of magnitude for the active decay time is about one second. The active decay time is approximately constant in a temperature region about 15 K below the clearing point.     

Liquid Crystal Polymers: Conformation of Deuterated Parts by SANS

Comb like liquid crystal polymers are polymers on each monomer is grafted a hanging liquid crystal molecule. The bulk state has the classical liquid crystal phases (nematic, smectic); what is the conformation of a chain in these phases is the problem to solve with the help of small angle neutron scattering. The values of the global sizes R _|| and R _‖ of a chain (in the direction parallel and perpendicular to the director) are discussed as a function of the chemical species and the temperature. Strange results are discussed in terms of segregation of labelled parts.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Synthesis,structure, and mesomorphism of novel liquid crystalline acrylate monomers and polymers

A series of novel liquid crystalline monomers (M₁?M₈) and side chain polymers base polyacrylate backbone were synthesized. The chemical structures were characterized by FT-IR and ¹H-NMR spectra. The mesomorphism and thermal behavior was investigated by polarizing optical microscopy, differential scanning calorimetry, and thermogravimetric analysis. The relationships of structure and mesomorphism are discussed in detail. The eight monomers and their corresponding polymers all show enantiotropic nematic phase. With increasing the spacer length or flexibility of the terminal group, the melting temperature (T_m) and isotropic temperature (T_i) of the corresponding monomers and polymers all decreased. However, with increasing the rigidity of the mesogenic core, T_m and T_i of the corresponding monomers and polymers all increased. TGA showed that all the polymers obtained in this study had excellent thermal stability.     

Optical Anisotropy of Mesogenic Macromolecules with a Spacer in the Side Chain

Methods of translational diffusion, sedimentation, viscometry and flow and electric birefringence were used in the study of conformational properties of a number of comb-like polymers containing mesogenic side groups in dilute solution. Quantitative data on the degree of equilibrium rigidity of the main chain of these polymers were obtained by using modern theoretical concepts and hydrodynamic data. The value of optical anisotropy and the equilibrium and relaxation electro-optical properties of comb-like thermotropic macromolecules indicate that these macromolecules exhibit high intramolecular orientational and dipolar order. It is shown experimentally that in a comb-like molecule the increase in the distance between the mesogenic group and the main chain is accompanied by a sharp decrease in this intramolecular order.     

Surface Order Parameter of 4-n-Heptyl-4′-Cyanobiphenyl

We measured the order parameter near the interface between a liquid crystal and the solid substrate treated by several methods. The interfacial order parameter is different according to the surface treatment, and is larger than the bulk order parameter. Moreover, in the temperature above the clearing point, the interfacial order still has a finite value.     

Influence of the Orientational Order on the Optical Properties of Cholesteric Liquid Crystals

An orientational order parameter for cholesteric liquid crystals is defined in analogy to nematic mesophases. The influence of this parameter on the selective reflexion and the optical rotatory dispersion is described. Its relative change with temperature is evaluated from ORD measurements for an induced cholesteric soIution and is found to be similar to that known for nematic liquid crystals.     

The Preparation and Properties of the α,ω-bis(4,4′-Cyanobiphenyloxy)Alkanes: Nematogenic Molecules with a Flexible Core

The first twelve members of the homologous series of α,ω-bis(4,4′-cyanobiphenyloxy) alkanes have been synthesised. The compounds are nematogenic, although the mesophases for the first and third members of the series are monotropic. Both the nematic-isotropic transition temperature and the entropy of transition exhibit a pronounced dependence on the length of the flexible core; this is analogous to that found for main chain thermotropic liquid crystal polymers.     

Material Highlights: Liquid Crystal Polymers

Abstract

A key issue in the area of side chain liquid crystal polymers concerns the conformation of the backbone within the mesophase. In order to resolve this matter, Noirez and co-workers (Liquid Crystals, 1995, 18 129) have studied a wide range of polymers exhibiting both nematic and smectic behaviour using small angle neutron scattering. Their results combined with a survey of those in the literature reveal that if the polymer is solely nematogenic, then the backbone adopts a prolate conformation in which on average it lies along the director. By contrast, in a smectic phase the backbone adopts an oblate shape and on average lies perpendicularly with respect to the director. This arises from a microphase separation in which the backbones are distributed between the mesogenic layers. However, the backbones can cross the mesogenic layers although their ability t o do so depends on, amongst other factors, the degree of polymerization. If the polymer exhibits both nematic and smectic behaviour, then in the nematic phase the backbone adopts a slightly oblate shape resulting from smectic fluctuations.     

Breakdown of time–temperature superposition: From experiment to the coupling model and beyond

We studied chain and segmental relaxation in polyisoprene using broadband dielectric and light scattering spectroscopy. Our results demonstrate different temperature dependence of these processes (i.e. failure of the time–temperature superposition) in the entire temperature range studied. Segmental relaxation varies with temperature faster than chain relaxation at low temperatures while reverse is observed at high T. The transition between two regimes seems to occur around the dynamic crossover temperature. It appears that non-entangled and slightly entangled polymers exhibit the same behavior. The results are compared to predictions of the coupling model. We emphasize that current models cannot describe the data consistently.     

Optical and X-ray studies of a four component cyano liquid crystal mixture: Comparison of properties with that of a similar mixture

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano–end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano.     

Dielectric Spectroscopy of Side Chain Siloxane Liquid Crystal Copolymers

Side chain siloxane liquid crystal polymers (LCPs) are being investigated as potential optical storage media. Dielectric relaxation spectroscopy has been used to examine the real and imaginary components of permittivity as a function of frequency and temperature. The spectra obtained are discussed from the standpoint of both their molecular significance and their device implications. Spectra of liquid crystal homopolymers and a particular class of copolymers have been found to exhibit different features due to the larger free volume available to the mesogenic moieties in the copolymers. The relaxation behaviour of five LC copolymers is discussed and then two equivalent pairs of homopolymer and copolymer LCPs are considered in more detail. One pair shows anomalous relaxation behaviour and extra relaxation processes are attributed to internal reorientations.     

A simple model for cooperative and non-exponential processes in non-crystalline polymers

A quite simple Markov chain model is presented in an attempt to understand several characteristic properties of the structural relaxation in glass-former materials. A second order Markov model is proposed for a small region of the material. Only a few parameters are necessary for the definition of the material in the model equations namely the energy levels accessible to that small region and the transition probabilities between them. The later are considered dependent on the energy levels and the potential barrier heights between them, on temperature, on a reference transition rate parameter and on a cooperativity or memory parameter related to the interaction with the neighbour regions. Isothermal relaxations at different temperatures were simulated with selected values of the model parameters. As a result, non-exponentiality of energy relaxations and its temperature dependence appears directly related to the cooperativity parameter. Markov chain model shows also non-linearity and memory effects, and a transition between two different regimes in the dependence of the relaxation times with temperature.     

Physical Studies of Homologous 4-Cyanophenyl Esters of 4′- n-Alkylcinnamic Acids

The mean polarizabilities of homologous series of nematic liquid crystals of 4-cyanophenyl esters of 4′- n-alkylcinnamic acids are evaluated by using the MLδP method. From these polarizabilities the mean diamagnetic susceptibilities of the liquid crystals are also calculated. The orientational order parameters of these compounds are also estimated using the Vuks and Neugebauer methods. The variation of the order parameter with temperature is discussed.     

Surface Order Parameter and Polar Anchoring Energy in Nematic Liquid Crystals

The trends of temperature dependence of the surface order parameter and of the anchoring strength coefficient are theoretically evaluated in the Landau-de Gennes phenomenological theory for nematic liquid crystals. The cases of planar, homeotropic and tilted alignment at a planar limiting surface are considered. It is shown that the cases with surface order parameter greater than the bulk one or smaller than this can occur in function of the phenomenological coefficients in the surface free energy expression. The anchoring strength coefficient, W, diminishes when temperature approaches the nematic-isotropic point, after passing through a maximum in most of cases. The possibility of a non-zero anchoring strength at temperatures above the transition temperature is revealed. The obtained trends of W(T) describe well a large variety of experimental results from literature.     

The relationship between the second and fourth rank order parameters for liquid crystals composed of non-rigid molecules

The values of the fourth rank order parameter P₄ determined by Raman scattering experiments are invariably lower than those predicted by the Maier-Saupe theory of nematic liquid crystals. However this theory is only applicable to rigid particles whereas the nematogenic molecules studies contain flexible alkyl chains. We have estimated the influence of the alkyl chain on P₄ with the aid of a model which account successfully for the variation of the second rank order parameter P₂ along the chain. It is found that the values of P₂ and P₄ calculated for the CN direction in the 4-n-alkyl-4′-cyanobiphenyls are indistinguishable from those predicted by the Maier-Saupe theory. It would appear therefore that the low values observed for P₄ cannot be attributed to the non-rigidity of the alkyl chain.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Dielectric Relaxation Spectroscopy of Liquid Crystal in Nematogenic Mesophase

In the present study, we report the dielectric behavior of a high temperature nematic liquid crystal (NLC) in both the planar as well as homeotropic alignments. To understand the mechanism of such high temperature NLC, we have performed dielectric spectroscopy in frequency range 100 Hz to 10 MHz. The different dielectric parameters, viz., effective dielectric permittivity and dielectric anisotropy of the sample have been determined with the variation of frequency and temperature. It is found that the dielectric parameters depend on dipole moment and rotation of side chain of molecule. The phase transition of this high temperature NLC has been confirmed by the DSC study.     

Molecular Polarizabilities,Polarizability Anisotropies and Order Parameters of Nematic Liquid Crystals

Mean Molecular polarizabilities, polarizability anisotropies and order parameters are evaluated in members of three homologous series, using two theoretical models. The variation of polarizabilities and their anisotropies with number of carbon atoms in the alkyl chain is studied. The temperature dependence of order parameter is also presented. The results are compared with the reported values and the merits of the different models are discussed.     

Measurements of optical anisotropy of a calamitic lyotropic liquid crystal

Optical birefringence measurements for a calamitic lyotropic liquid crystal containing sodium lauryl sulphate/decanol/water are presented. A value of the order of 10^‐3 was obtained, much smaller than that of thermotropic liquid crystals. The order parameter temperature dependence is evaluated using an extrapolating method and taking into accounts the influence of the micelle shape. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)