Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Effect of surface on the thermodynamic properties of polar liquids and liquid crystals trapped in a porous medium

The heat capacity of some polar liquids (2,6-lutidine, aniline and nitrobenzene) and liquid crystals (nitrophenyldodecyloxybenzoate and dodecylcyanobiphenyl) trapped in glass and nickel porous matrices have been measured by using a high-resolution adiabatic calorimeter. A decrease in the molar heat capacity of substances trapped in porous media compared with the bulk molar heat capacity was observed. A new phenomenon caused by the effect of the glass surface on the confined fluid was revealed. This phenomenon was manifested as an unusual behaviour of the heat capacity versus temperature curve (a ‘crevasse’ in a curve). In addition, we observed the nematic phase of dodecylcyanobiphenyl liquid crystal induced by the variable curvature of the glass matrix surface.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Abstract

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Molecular reorientation in the nematic and rotatory phases of 4,4'-di-n-pentyloxyazoxybenzene

Results of dielectric relaxation, quasielastic neutron scattering, calorimetric D.S.C. and preliminary X-ray measurements on the fifth member, POAOB, of the 4,4'-di-n-alkoxyazoxybenzene homologous series are presented. It has been found that POAOB exhibits two mesophases: a nematic (N) and an intermediate crystalline phase (CI) just below it. From comparison of the dielectric relaxation and quasielastic neutron scattering studies we can conclude that in the nematic phase the molecule as a whole performs rotational diffusion around the long axis (ω ~ 150ps) and at the same time the two moieties perform faster independent reorientations around the N-δ bonds (ω denotes a benzene ring) with δ ~ 5 ps. The intermediate crystal phase is identified as a solid uniaxial rotational phase in which fast molecular reorientations exist. It seems that the fast reorientations observed in the nematic phase to some extent survive to the crystal I phase. A model of molecular arrangements in the crystal I phase is proposed, and it explains the reduction of the dielectric increment observed on passing from the nematic phase to this phase.     

Textural and calorimetric investigations on a homologous series of mesogenic azocompounds complexed with palladium(II)

A comparative study of some mesogenic azobenzene compounds and their organometallic palladium(II) derivatives is reported. On the basis of the textural properties, calorimetric data and uniaxial order parameters (as deduced approximately from infrared spectroscopy), the main features induced by palladium complexation are (i) the transition temperatures increase, (ii) the number of ordered mesophases increases, (iii) the nematic uniaxial order parameter decreases, (iv) the textures are typical of highly ordered phases, and (v) an optical biaxiality appears in the nematic phase. The calorimetric data as well as the order parameters, if discussed in terms of molecular biaxiality result are interpreted at least qualitatively. Optical investigations show that macroscopical biaxial ordering can be achieved.     

Structural and dynamic properties of a new type of discotic nematic compounds

Thermodynamic, structural and dynamical properties of a new type of discotic compounds, a hydrocarbon without any heteroatoms, displaying a nematic discotic phase have been investigated by means of X-ray diffraction, electro-optical relaxation, and calorimetric studies. Of particular interest are the strength of the first order nematic—isotropic phase transition and the nature of the orientational fluctuations in the isotropic phase. The short range positional order was found to be biaxial in both the isotropic and the nematic phase. The isotropic phase displays strong pretransitional effects originating from orientational fluctuations in the neighbourhood of the transition to the nematic phase. The character of these pretransitional effects differs from that found for calamitic systems in that the number of correlated molecules g₂ is extremely large, of the order of 600 at the clearing temperature and the electro-optical relaxation time is very large, caused by the large value of g₂.     

Phase transitions,optical, dielectric and viscoelastic properties of colloidal suspensions of BaTiO3 nanoparticles and cyanobiphenyl liquid crystals

We report experimental studies on the phase transitions and physical properties of colloidal suspensions of BaTiO₃ nanoparticles and two cyanobiphenyl liquid crystals (4-pentyl-4?-cyanobiphenyl and 4-octyl-4?-cyanobiphenyl). From the differential scanning calorimetric measurements, we show that the nanoparticles have antagonistic effect on the isotropic to nematic and nematic to smectic-A phase transitions. The birefringence, dielectric anisotropy and splay elastic constant remain almost unchanged, whereas the bend elastic constant and rotational viscosity decrease considerably. The experimental results are discussed based on the possible contribution of BaTiO₃ nanoparticles and free surfactant molecules in the suspensions.     

Phase-transitions in Langmuir-Blodgett and cast films of a ferroelectric liquid crystal.

Langmuir-Blodgett films and cast films of a ferroelectric liquid crystal of sec-butyl-6-(4-(nonyloxy)benzoyloxy)-2-naphthoate have been fabricated. Their thermal behavior was investigated using infrared spectroscopy at elevated temperature combined with principal component analysis. The result shows a new phase transition from smectic A to nematic phase, compared to the phase sequence obtained by polarizing optical microscopy. Another solid transition of different isomeric crystals was also found, which was confirmed by calorimetric measurement.     

The advantages and disadvantages of direct and indirect calorimetry

Kleiber's definitions of what constitutes direct and indirect calorimetry are accepted as the beginning of a commentary on the advantages and disadvantages of direct and indirect calorimetry in which calorimetry is divided into a number of categories based on the kind of calorimetric measurement. For non-reaction calorimetry such as entropy determinations and differential scanning calorimetry, the only means of measurement is by direct calorimetry. For reaction calorimetry, a preference of direct over indirect calorimetry depends on the accuracy needed and the ability of the experimenter to define the system. The data necessary to correct the observed heat loss in direct calorimetry are often all that are needed to make an indirect calculation of the true heat loss. In general, because they are convenient and inexpensive to use, indirect calorimetric methods are preferable to direct methods. However, when possible, one method can be used to verify the results of the other.     

Influence of the chain length on the nematic-to-isotropic phase transition for the odd members of a highly non-symmetric pyrene-based series of liquid crystal dimers

This paper describes a detailed study of the nematic (N)-isotropic (I) phase transition in the homologous series of liquid crystal dimers, the α-(4-cyanobiphenyl-4?-oxy)-ω-(1-pyreniminebenzylidene-4?-oxy)alkanes (CBOnO.Py) by means of calorimetric and dielectric measurements as a function of temperature. It is concluded that for this transition, the latent heat or the entropy change decreases as the chain length of the odd dimers decreases, and this decrease is consistent with the observed tricritical behaviour.     

Luminescence behavior as a probe for phase transitions and excimer formation in liquid crystals: dodecylcyanobiphenyl

The luminescence behavior of a liquid crystal, dodecylcyanobiphenyl, as a function of temperature, is reported. The fluorescence intensity exhibits discontinuities at temperatures which correspond to the phase transitions: crystalline — smectic — isotropic. The results are interpreted in terms of singlet monomer and excimer formation.     

Thermal analysis of phase transitions in microemulsions

Differential scanning microcalorimetric measurements on phase transitions in water-oil-surfactant mixtures are presented, demonstrating that this method is highly sensitive towards small heat changes connected with structural transitions in the samples. The values for the latent heat of phase transitions are determined and the results are compared with predictions from mean field theory, emphasizing the role calorimetric experiments can play to identify the most important contributions to the free energy describing the mixtures. Doing this, the present status of the understanding of temperature dependent phase transitions in microemulsions is reviewed.     

A study of the nematic-smectic A-smectic C point in a binary liquid-crystalline mixture by adiabatic scanning calorimetry

An adiabatic scanning calorimetric technique has been used to investigate the nematic-smectic A-smectic C (NAC) multicritical point in mixtures of 4-n-pentyl-oxyphenyl-4'-n-octyloxybenzoate and 4-n-hexyloxyphenyl-4'-n-octyloxybenzoate. The NC transitions are weakly first order and the latent heat becomes zero at or very near the NAC point. Near the NAC point no thermal feature associated with the NA transitions could be observed. The heat capacity anomalies along the second order AC transition line and along the first order NC line indicate the presence of a tricritical point at or very near the NAC multicritical point. No thermal indications for a biaxial nematic phase were observed.     

The Specific Heat of Tetramethylammonium Salts

New measurements of the (N(CH₃ )₄ )₂ MnBr₄ specific heat by adiabatic calorimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are compared with previous calorimetric studies on similar tetramethylammonium bromide compounds. The thermodynamic behaviour of the tribromides and tetrabromides derivatives together with the influence on the phase transition parameters of the cation and halogen molecular substitutions are examined. The thermal relaxation experiments permit to study the behaviour of the crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions is used to describe the thermodynamic behaviour of some of these crystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Enthalpies of solution oft-butyl chloride and bromide and carbon tetrachloride and tetrabromide in alcohols

Standard enthalpies of solution of t-BuX and CX₄ (X=Cl and Br) in several alcohols are reported as measured by a calorimetric method. The corresponding transfer functions from the gas phase were calculated.     

Discontinuous ventilation in the rhinoceros beetle <Emphasis Type="Italic">Oryctes nasicornis</Emphasis>

Discontinuous gas exchange cycles (DGCs) are frequently observed with insects, i.e. oxygen take up and carbon dioxide release occur interrupted by periods of a few minutes up to many hours. The paper presents direct and indirect calorimetric experiments on DGCs of the scarabid rhinoceros beetle Oryctes nasicornis. A direct/indirect calorimetric experiment is presented. Total and specific heat production rates amount to 0.56mWand 0.42mWg⁻¹ in the first period without DGCs and to 0.43 mW (0.32 mW g⁻¹) in the second phase, resp. The mean DGC amplitude is 0.184 mW and thus between 33 and 66% of the total turnover.     

Self-organization of mesomeric-ionic hybrid heterocycles into liquid crystal phases: a new class of polar mesogens

The first mesoionic nematic and smectic A mesogens derived from sydnones that are characterized by both covalent and ionic features have been synthesized and evidenced by optical, calorimetric and X-ray diffraction studies.     

Strain induced homeotropic alignment in the smecticA phase of liquid crystals

We present a method to align liquid crystal (LC) samples in their SmecticA phase, useful to obtain permanent homeotropic single domains even in compounds where the nematic phase is absent. It is based on the application of a strain field, within the sample volume, which tends to align the molecules all towards a common direction. The study was carried out in samples of 4-decyl-4′-cyanobiphenyl (10CB), 4-octyl-4′-cyanobiphenyl (8CB) and n-hexyl-4′-n-pentyloxy biphenyl-4-carboxylate (65OBC) LCs, and the alignment efficiency was monitored by polarisation microscopy observations and by thermal diffusivity measurements.     

Diagnosis of Phase Shift in a Temperature-Modulated Calorimetric Method

In a temperature-modulated calorimetric method using the same apparatus as a standard differential scanning calorimeter, we have to pay attention to the thermophysical parameters of the apparatus, which cause phase shift in ac temperatures, such as heat capacity of base plate, heat capacity of a pan, thermal conductance between a heater and base plate, and thermal conductance between a pan and base plate. We performed the analysis of the thermal system of the apparatus with these parameters. Beside the theoretical consideration, we carried out heat capacity measurement in a wide range of modulation periods. We found that the experimental results were well-expressed in terms of these thermophysical parameters. This revised version was published online in July 2006 with corrections to the Cover Date.