Liquid crystals exhibit successively several fluids and/or viscous phases and therefore microscopical methods play an important role for their characterization. No really efficient routine dilatometric method exist for rather small samples. By simple addition of a Michelson's interferometer objective and a photo-multiplier to the basic equipment for the investigation of liquid crystals — i.e. a polarizing microscope associated to a heating and cooling stage — dilatometric investigations as well as studies of phase transitions are getting accessible. In fact, when a droplet of liquid crystal, set up in a spherical stamp, is lightened through the interferometer with a quasi monochromatic source, equal thickness circular fringes are observable on the surface of the sample. A thermal expansion of the droplet leads to displacements of these fringes. The principle for the measurement of the volume changes consists by recording, versus the temperature, the variations of the light intensity of the central fringe. The volume of the sample, at given temperatures, is determined from photomicrographs of the whole interference feature. This volume is lower than 0.1 mm3. Experiments on the pure compound octyl-cyanobiphenyl show that the method is convenient for the detection of the phase transition, even if the transformation is weakly first order asSA -N. Tested with a commercial ternary mixture the expansivity of phases and the transitions volumes changes can be deduced from experiments.
Authors:P. Martin, P. Godé, P. Villa, and G. Goethals
We studied the phase transition temperatures of a series of amphiphilic D-xylopyranose and D-xylofuranose derivatives in which the lipophilic part is an alkyl chain R (n-CnH2n+1), regiospecifically linked to D-xylose, at different positions, by Z which is an atom or a functional group (O, S, O–(CH2)3–S). The alkyl chain was moved from the C-1 to the C-5 position in the xylose moiety, thereby allowing us to compare directly
the phase transition temperatures of the individual materials. These compounds give thermotropic and/or lyotropic liquid crystals.
In some cases, we also observed solid–solid phase transitions.
Authors:R. H. Fattepur, K. H. Fattepur, and N. H. Ayachit
used extensively to measure thermodynamic functions like entropy and enthalpy. Especially, the studies on the temperature variation of dielectric dispersion and dielectric loss of liquidcrystals at microwave frequency can yield valuable information
Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4′-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases
Thermal properties of three ferroelectric liquid crystals, namely: 3-octyloxy-6-[2-fluor-4-(fluoroctyloxy)phenyl]-pyridine (FFP), 3-(3-fluor-nonyl)-6[4-heptyloxyphenyl]-pyridine (FNHPh-P), 4-[(S,S)-2,3-epoxyhexyloxy]phenyl 4-(decyloxy)benzoate (EHPhDB) were studied using heat flux differential scanning calorimetry method. All the transitions expected in this compounds, except the SmC*-SmA* and SmC*-S3 transitions, were detected in the DSC curves. The temperatures of the phase transitions and the enthalpy changes associated with them were determined. The transition from the liquid crystalline to the crystalline state showed significant hysteresis for all three compounds studied. The following transitions: SmA*-Is, SmG*-SmC* for FNHPh-P, N*-Is for EHPhDB, N*-Is and SmC*-N* for FFP also showed a small hysteresis basing on which first-order character of all the above transitions was implied. All three substances studied in this work are characterized by a complex polymorphism in the solid state.
Authors:Viorel Cîrcu, Ana S. Mocanu, Constantin Roşu, Doina Manaila-Maximean, and Florea Dumitraşcu
In recent years, there have been a lot of studies dealing with the design of multifunctional liquidcrystals because of their great potential in a wide range of applications like sensors, LCDs and OLEDs. Liquid
Authors:Daniela-Crina Marinescu, Elena Pincu, and Viorica Meltzer
Cholesteric liquidcrystals have many applications in advanced technologies such as direct measuring the temperature, displays, surface thermography and detection of atmosphere contamination, skin thermography, and
Positron annihilation lifetime technique was applied to study the electric field dependence of size, intensity and size distribution of free volumes in various liquid crystals negative nematic MBBA(N-(4-methoxybenzylidene)-4-n-butylaniline), positive nematic 5CB (4-cyano-4-n-pentylbiphenyl) and cholesteric mixture of MBBA and cholesteryl oleate. Positron annihilation decay curves were obtained in the direct and alternate electric field range from 0 to 120 V/mm, and annihilation curves were resolved into four lifetime components. The relation between the free volume parameters and the variation of molecular alignment is discussed for nematic and cholesteric liquid crystals.
Authors:D. Filip, C. Simionescu, D. Macocinschi, and I. Paraschiv
Miscibility in blends of semicrystalline polymers (poly(ethylene) adipate and poly(tetrahydrofuran)) and liquid crystal cholesteryl
palmitate was investigated by means of differential scanning calorimetry and polarizing optical microscopy. Some(concentration-dependent)
miscibility was found. A more pronounced miscibility exhibits the polyester-based blends probably due to the similar chemical
structure of the two components and stronger interactions between the two components.