Thermo-optical analysis (TOA) as a tool of melting phenomena investigations

In the work, after preliminary discussion of the complexity of the phenomenon of melting chemical compounds, two basic research methods are presented: calorimetric methods and thermooptical method. The physical basis of the five main calorimetry techniques is now detailed (adiabatic calorimetry, differential thermal analysis – DTA, differential scanning calorimetry with heat compensation – DSC heat compensated, differential scanning calorimetry with heat flow – DSC heat flux and differential scanning calorimetry with temperature modulation – MDSC) and thermo-optical techniques used in phase transitions investigations. The advantages and disadvantages of these methods are shown in numerous examples and the accuracy attainable by the individual measuring techniques is compared

Distinguishing the Focal-Conic Fan Texture of Smectic A from the Focal-Conic Fan Texture of Smectic B

This publication presents methods of distinguishing the focal texture of the conical smectic phase A (SmA) and the crystalline smectic B phase (CrB). Most often, characteristic transition bars are observed in polarized light at the temperature point of the SmA–CrB phase transition. TOApy software transforms each image from a series of images recorded during POM observation to a function of light intensity versus temperature. Thermo-optical analysis is a powerful quantitative tool to notice this phase transition, but it has some limitations. The other applied method, the local binary pattern (LBP) algorithm, with high probability, detects differences between the textures of the conical focal fan of the SmA and CrB phases. The LBP algorithm is an efficient tool for texture classification

Prism coupler and microscopic investigations of DNA films

DNA is a polyelectrolyte capable of forming thin films with interesting optical properties. We investigated refractive indices and optical anisotropy of films of the native, sodium ion-based DNA (Na-DNA) and DNA bearing the cetyltrimethylammonium ion (DNA-CTMA) using a prism coupler technique. The light polarization direction was either parallel (nTE) or perpendicular to the surface plane of the films (nTM). The index values and the birefringence of DNA films vary considerably depending on the type of the counter-ion, the film fabrication method and the relative humidity (RH) of the environment. A high negative birefringence in films of Na-DNA, n TE-nTM = -0.03 at an RH ∼ 55 %, was measured in solution-cast films, indicating that the optically anisotropic DNA molecules are aligned in the plane parallel to the film surface. Refractive indices of DNA-CTMA thin films were smaller and more isotropic than those for films of Na-DNA polymer. The prism coupler reflectance curves showed a hysteresis of the index values when the RH of a DNA-CTMA film environment varied. Polarization microscopy studies showed liquid-crystalline textures at the edges of Na-DNA and DNA-CTMA films

The kinetics of the E-Z-E isomerisation and liquid-crystalline properties of selected azobenzene derivatives investigated by the prism of the ester group inversion

<div><p>Two new groups of azobenzene ester derivatives were synthesised: alkyl 4-[4-(nonyloxy)phenyl]diazenyl]benzoates and 4-[4-(nonyloxy)phenyl]diazenyl]phenyl alkanoates. All 35 presented homologues are mesogenic. Moreover, some of the above-mentioned compounds exhibit rich liquid-crystalline polymorphism likewise tetramorphism. During this investigation by the use of polarising optical microscopy, differential scanning calorimetry and X-Ray studies, six types of mesophases were detected: nematic, smectics (A, C, I, F) and G. Furthermore, due to the presence of the photosensitive azo moiety, the E–Z isomerisation reaction is possible. This process, which is initiated by the UV irradiation, causes significant changes in the UV-Vis absorption spectra of investigated compounds. However, the photoisomerisation is a reversible process and in the dark the thermal relaxation of Z isomer takes place. Based on the achieved data, the kinetic constants of the isomerisation and relaxation processes were calculated. It shows that conversion of the ester bond makes some changes in the optical properties. The shift of about 7 nm of the absorbance maximum was observed. Surprisingly, the inversion of the ester group has significant influence on the liquid-crystalline polymorphism replacing one mesophase (for benzoates) into four (for alkanoates).</p></div

Mechanism of Photochemical Phase Transition of Single-Component Phototropic Liquid Crystals Studied by Means of Holographic Grating Recording

Phototropic liquid crystals (PtLC) are a new class of materials possessing number of potential applications in photonics devices. However, so far a significant majority of PtLC materials has been realized by the doping a classical liquid crystal with a photochromic dye. The photochemical phase transition in such systems was investigated mainly by the monitoring of the changes in the transmittance. In this study, the photochemical phase transition of single-component phototropic liquid crystals was investigated using a holographic grating recording in combination with a polarized optical microscope. The <i>cis–trans</i> photoisomerization of compounds causes the isotropic-to-nematic (I–N) phase transition and so that the interference pattern can be mapped as a diffraction grating. The process of the grating build up was monitored by the first-order light diffraction, and simultaneously the area of the material exposed to the light was observed directly under a polarized microscope. The combination of the holographic technique with polarized optical microscopy has allowed to propose the mechanism of the I–N phase transition of LC compounds. It assumes three processes responsible for the grating formation. The results have a crucial importance in understanding the mechanism of photochemical phase transition of PtLCs, and thus they can be useful in construction of new optical devices