New low molar mass organosiloxanes with unusual ferroelectric properties

A new low molar mass chiral organosiloxane mesogen and its racemic analogue have been synthesized and their mesomorphic and ferroelectric properties investigated. The chiral derivative, denoted A*B, exhibits one tilted enantiotropic ferroelectric smectic C mesophase over a broad temperature range, with very high tilt angles and moderate spontaneous polarization (36° and 19 nC cm^-2 at 20°C). The achiral siloxane derivative, denoted A*B, exhibits one broad enantiotropic smectic C phase. Preliminary electro-optic measurements indicate that the spontaneous polarization is weakly dependent on temperature between 10°C and 50°C, the latter being the S*_c to isotropic phase transition. The tilt angle and layer spacing are temperature independent, and current response times of less than 200 μs were measured at 25°C for fields of 10 V μ^-1. These results are discussed in comparison with those for side chain polymer liquid crystal structures and other low molar mass ferroelectric materials.     

Ferroelectric and antiferroelectric low molar mass organosiloxane liquid crystals

The present paper is a study of the ferroelectric and antiferroelectric behaviour in low molar mass organosiloxane liquid crystal materials classed as mono-mesogens (AB type) and bi-mesogens (ABA type). A systematic study of series of materials all based on the same chiral mesogenic moiety is presented. The mesogen is a biphenylyl benzoate with a halogen X attached laterally to the benzoate ring which is closest to the chiral centre. Three homologous series, where the halogen is either fluorine, chlorine or bromine, are investigated. The number of silicon atoms in the linkage or end group is varied between two and nine. It is found that the materials have a broad (40 C) ferroelectric or antiferroelectric phase with a high P s (90nC cm2). The siloxane moiety forces the tilt angle to a temperature independent value close to 45. The antiferroelectric order is observed only in bi-mesogens with an odd number of silicon atoms in the siloxane link. The antiferroelectric order is attributed to the conformation of the molecule rather than to antiferroelectric interactions between layers.     

The preparation and properties of low molar mass liquid crystals possessing lateral alkyl chains

The first twelve members of three new mesogenic homologous series have been characterized, each of which are composed of molecules possessing lateral alkyl chains. The n-alkyl 2,5-bis-(4-n-hexyloxy-benzoyloxy)-benzoates are purely nematic whereas both nematic and smectic behaviour is observed for the 4-cyano-4'-biphenylyl 3'-n-alkyloxybenzoates and the 4-cyano-4'-biphenylyl 3',4'-n-alkyloxybenzoates. The transitional properties of these series are similar to those of the analogous conventional mesogens and hence, can be rationalized without making any special assumptions concerning the conformational distribution of the lateral alkyl chain.     

Electroclinic behaviour of polymer doped low molar mass ferroelectric liquid crystals

Abstract

Recently there has been much interest in the electroclinic effect due to its potential application in high speed optical modulators and grey scale displays. In present materials, however, the electroclinic effect is very dependent on temperature and falls off rapidly with increasing temperature. This limits the useful device operating range. In this paper the electroclinic behaviour of new, side chain polymer doped, low molar mass ferroelectic liquid crystal mixtures is reported. By measuring the rate of change of the electroclinic coefficient with temperature it is shown that the temperature range over which the electroclinic effect exists increases with polymer concentration. Data are also presented on electroclinic response times.     

Electroclinic behaviour of polymer doped low molar mass ferroelectric liquid crystals

Recently there has been much interest in the electroclinic effect due to its potential application in high speed optical modulators and grey scale displays. In present materials, however, the electroclinic effect is very dependent on temperature and falls off rapidly with increasing temperature. This limits the useful device operating range. In this paper the electroclinic behaviour of new, side chain polymer doped, low molar mass ferroelectic liquid crystal mixtures is reported. By measuring the rate of change of the electroclinic coefficient with temperature it is shown that the temperature range over which the electroclinic effect exists increases with polymer concentration. Data are also presented on electroclinic response times.     

Hybrid materials made from polymeric betaines and low molar mass salts

The blending of polyzwitterions with low molar mass salts, in particular with salts containing bulky, organic ions is studied. In many cases, homogeneous blends are obtained using stoechiometric amounts of salt, and in selected cases, the amounts of salt which can be admixed is even substantially higher. In contrast to inorganic salts, bulky organic salts are efficient plasticizers. Although the fundamental interaction in such blends is of electrostatic nature, the miscibility depends in a subtle way on specific interactions between particular ionic groups as well as on the molecular structures of polymer and salt.     

Antiferroelectric and ferrielectric liquid crystalline low molar mass materials and polymers

Novel liquid crystalline Low Molecular Mass (LMM) materials bearing two chiral lactate groups, as well as compounds of like structure to MHPOBC have been synthesized. All the LMM compounds exhibit the SmC*A (antiferroelectric) and/or SmC*gamma (ferrielectric) phases. The mesogens have been incorporated as pendant groups on polymer backbones of three different natures to prepare side chain liquid crystalline copolysiloxanes, homopolysiloxanes and homopolyacrylates. The investigation of the polymers by means of miscibility studies showed that the antiferro- and ferri-electric phases are strongly destabilized in the coposiloxanes and homopolysiloxanes, while in the homopolyacrylates a large temperature range mesophase is found to be miscible with the SmC*A phase. The physical properties of the mesophases and their stability, both for the LMM materials and the polymers, are presented and discussed.     

Structure formation in doped discotic polymers and low molar mass model systems

Doping of low molar mass materials or polymers, possessing disc-like units, with electron acceptors leads to the stabilization of columnar discotic phases or even to the induction of such phases in compounds which either display a nematic discotic phase or only an amorphous phase in the absence of the electron acceptor. The induced columnar phase corresponds frequently to a hexagonally ordered one. We have observed, however, in addition the induction of new columnar phases such as the rectangularly ordered (D^ro) and the columnar nematic phase (N^c). The enhancement of the tendency towards the formation of columnar phases is a consequence of electron acceptor—electron donor complex formation. Using a model proposed by de Jeu to describe the induction of smectic phases by complex formation we are able to account qualitatively for the experimental findings.     

Invited Lecture. Smectic A polymorphism from low to high molar mass systems

The polymorphism of smectic A phases in low molecular weight (L.M.W.) liquid crystals is associated with strong anomalies in the period defining the layers. The smectic A phases of liquid-crystalline comb-like polymers also have various modes of spacing. However, the behaviour of these polymer phases shows some peculiarities compared with L.M.W. compounds, especially due to the main chain which takes part in the smectic arrangement. To specify further the S_A polymorphism in high molar mass systems, the use of side chain polymers with partial fixation appears to be very promising: either taking advantage of better compatibility and lower viscosity in order to describe binary diagrams with L.M.W. mesogens, or through the partial insertions of long polar side groups known to generate anomalies of periodicity as for L.M.W. compounds.     

Collective and molecular dynamics in low molar mass and polymeric ferroelectric liquid crystals

By use of a novel sample preparation technique, it is possible to measure with one sample cell the collective and molecular dynamics (10^-1 Hz-10⁹ Hz) of macroscopically (in bookshelf geometry) oriented ferroelectric liquid crystals. Below 10⁶ Hz, two ferroelectric modes, Goldstone- and soft-mode, are observed, being assigned to fluctuations of the phase and amplitude of the helical superstructure respectively. Between 10⁶ and 10⁹ Hz, one dielectric loss process exists, the β-relaxation which originates from the librational motion (hindered rotation) of the mesogen around its molecular long axis. This process does not split or broaden at the non-ferroelectric-ferroelectric phase transition and it has an Arrhenius type temperature dependence. In comparing a racemic mixture with a chiral sample, it performs a similar frequency and temperature dependence. The experimental findings for the β-relaxation are in qualitative contrast to the predictions of the (generalized) Landau expansion of the free energy at the non-ferroelectric-ferroelectric phase transition. The experiment also leads to a modified understanding for the molecular origin of ferroelectricity in FLCs.     

Invited Lecture. Smectic A polymorphism from low to high molar mass systems

The polymorphism of smectic A phases in low molecular weight (L.M.W.) liquid crystals is associated with strong anomalies in the period defining the layers. The smectic A phases of liquid-crystalline comb-like polymers also have various modes of spacing. However, the behaviour of these polymer phases shows some peculiarities compared with L.M.W. compounds, especially due to the main chain which takes part in the smectic arrangement. To specify further the S_A polymorphism in high molar mass systems, the use of side chain polymers with partial fixation appears to be very promising: either taking advantage of better compatibility and lower viscosity in order to describe binary diagrams with L.M.W. mesogens, or through the partial insertions of long polar side groups known to generate anomalies of periodicity as for L.M.W. compounds.     

Odd-even effect in miscibility of main-chain liquid crystal polymers with low molar mass nematogens

Qualitative phase diagrams were constructed using the contact method for binary mixtures of several chemically-distinct low molar mass nematogens (LMMN) with a main chain liquid crystal polymer (TPB-x) which has a mesogenic group, 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxy phenyl) butane, separated by flexible alkyl spacers of variable length, x. Several interesting effects were observed. TPB-x was found to exhibit an odd-even variation in miscibility in the nematic state (2n + 1 = miscible, 2n=immiscible) with 4'-pentyl-4-cyanobiphenyl (5CB), but not with 4'-pentyloxy-4-cyanobiphenyl (5OCB) in which most polymers were completely miscible. On prolonged isothermal annealing in the biphasic region in 5CB, TPB-2n exhibited spherulitic crystallization of the liquid crystal polymer. These observations are shown to be qualitatively consistent with a modification of the Flory-Huggins theory by Brochard et al.     

Study of the surface and interfacial tensions in systems containing a low molar mass liquid crystal

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4'-n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of different volumes ranging from 5 to 11 mm³. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.     

Study of the surface and interfacial tensions in systems containing a low molar mass liquid crystal

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4'-n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of different volumes ranging from 5 to 11 mm³. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.     

Induction of the chiral nematic phase in hydrogen-bonded blends of smectic copolymers and low molar mass dopant

A new approach for the preparation of chiral nematic materials is described. The induction of a chiral nematic phase in hydrogen-bonded blends of smectic comb-shaped LC copolymers containing alkyloxy-4-oxybenzoic acid fragments with a low molar mass chiral dopant (a derivative of pyridine) was observed.     

Counterion and solvent effects on the dilute solution viscosity of polystyrene ionomers

We present an analysis of data on the intrinsic viscosity [η] of sulfo-polystyrene ionomers in several solvents for a variety of sulfonation levels and counterions. For solvents of low dielectric constant, 2 < ε < 18, [η] decreases from the base polymer value [η]₀ with increasing substitution level. This behavior was attributed to intramolecular association of ionic dipoles. The ratio [η]/[η]₀ was found to depend on a single reduced variable α_Aα_Sx, where x is the fractional substitution, α_A depends only on the counterion, and α_S ∝ ε^?1 depends only on the solvent. For solvents of high dielectric constant, 36 < ε < 47, [η] increases approximately as x³, and counterion effects are small. This behavior was attributed to ionic dissociation, giving rise to a polyelectrolyte effect. Implications of the low ε results are discussed in relation to association-induced gelation behavior and possible generalizations of the reduced variables approach.     

Induction of the chiral nematic phase in hydrogen-bonded blends of smectic copolymers and low molar mass dopant

A new approach for the preparation of chiral nematic materials is described. The induction of a chiral nematic phase in hydrogen-bonded blends of smectic comb-shaped LC copolymers containing alkyloxy-4-oxybenzoic acid fragments with a low molar mass chiral dopant (a derivative of pyridine) was observed.     

The crystallinity and miscibility of syndiotactic polystyrene blends after mixing with low molar mass liquid crystal or commercial lubricant

The quantities of the crystallinity of syndiotactic polystyrene (SPS) blended with another polymer in the group of poly(α-methyl styrene), poly(n-butyl methacrylate) or poly(cyclohexyl acrylate) with or without the additives were measured by X-ray diffraction and calculated by Ruland’s method. The SPS was synthesized by using metallocene catalyst and modified-methylaluminoxane as cocatalyst. The additive of low molar mass liquid crystal chemical (cyclohexyl-biphenyl-cyclohexane (CBC33)) or lubricant (glycerol monostearate (GMS)) was individually added to the blends of SPS in order to investigate the effects on the crystallinity of the blended SPS. From the experimental results, it was found that the percent crystallinities of the blends decreased with decreasing the percent of SPS in the blend because of the dilution of SPS. The depression of the percent crystallinity was in the order of PaMS > PCHA > PBMA according to the compatibility with SPS. The addition of GMS or CBC33 slightly decreased the percent crystallinity of the pure SPS. The addition of GMS impeded the depression of the SPS crystallinity in the blends, because their percent depression from pure SPS is similar (at around 25%) regardless to the components of the blends. The blends with added CBC33 have the similar depression of crystallinity as the pure blends because of the low concentration of CBC33 and the good compatibility of CBC33 with the SPS.