Search Results

You are looking at 1 - 10 of 26 items for :

  • "structural transition" x
  • Refine by Access: All Content x
Clear All

This paper presents investigations of phase and structural transitions occurring in water adsorbed on the surface of bovine serum albumin (BSA) and on the so-called ‘intelligentrs or ‘smart’ silica gel surface covered with a chemically bonded BSA phase. Cyclic changes of heat flow (HF) were observed in the samples studied during cooling and heating of the measuring cell of the differential scanning calorimetry (DSC) apparatus. These cyclic changes reflect structural transitions occurring in the water adsorbed on the surface at subambient and elevated temperatures. This is connected with cyclic changes (decay and reproduction) of ice-like structures existing in the adsorbed water layers. On the basis of quantitative investigations it appears that, depending on the direction of the cooling or heating process of the samples studied, the number of ice-like water structures in the surface film increases or decreases. It has been stated that the observed fluctuations occur spontaneously and suddenly in the whole volume of adsorbed water in different and not regular temperature ranges, especially at the ‘paradoxical effect’ temperatures.

Restricted access

Abstract  

The thermal behavior of hydrogels synthesized by solution polymerization between acrylamide, acrylic acid and diglycidyl acrylate (DGA) as a crosslinking agent was investigated. The structure of the hydrogel can be tightly controlled with the reaction temperature. This method produces a new type of hydrogels, which exhibit well defined structures at various scales of length simultaneously. These multi-structured hydrogels are hydrophilic, elastic, water insoluble, and soft polymers with an anisotropic optical response. The structure was observed by scanning electron microscopy (SEM), polarized light microscopy (PLM) and macroscopic visualization (CCD camera). In addition, structural transitions in the hydrogels were monitored by temperature modulated differential scanning calorimetry (TMDSC). Severe heating tests in an adiabatic oven were performed to analyze decomposition of the material. Fourier transform infrared (FTIR) spectroscopy was used to qualitatively analyze the hydrogels samples exposed to a sudden thermal treatment.

Restricted access

Abstract  

Differential scanning microcalorimetry and equilibrium thermohemolysis procedure were used to study the effect of acclimation temperature on thermally induced transitions and thermoresistivity of fish (trout) erythrocyte membranes. Strong correlation has been found between the rates and activation energies of erythrocyte thermohemolysis and acclimation temperature. Transition temperatures of five thermodynamically irreversible and one partially reversible transitions at about 87C as well as the overall shape of microcalorimetric curves of the erythrocyte ghosts do not vary with acclimation temperature. The results suggest an essential conservation of phospholipid microenvironment of membrane skeleton proteins despite the compensatory response in lipid composition of erythrocyte membrane bilayer.

Restricted access

Abstract  

This paper reviews our recent progress in determining the surface glass transition temperature, T g, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of T g. The T g of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to T g will be discussed.

Restricted access

Abstract  

The thermally induced structural transformation of a hydrogen-bonded crystal formed from an amphoteric molecule of 6-[2-methoxy-4-(pyridylazo)phenoxy]hexanoic acid MeO was studied using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction measurement (XRD). Crystal form of the hydrogen-bonded crystal was measured by single crystal four circle diffractometer (Mo-Kα radiation). As a result, the crystal of MeO was stabilized by many C–H⋅⋅⋅O hydrogen bonds, and the C–H⋅⋅⋅O hydrogen bonds were broken by thermal energy reversibly. After transformation the supramolecular architecture was composed of supramolecular polymer including free-rotation pentamethylene main chains.

Restricted access

Summary Thermally induced structural transformation of fibrous hydrogen-bonded molecular assemblage formed from an amphoteric pyridinecarboxylic acid of 6-[2-propyl- 4-(4-pyridylazo)phenoxy]hexanoic acid (C5PR) was studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetry (TG). The organized fibrous morphology formed in an aqueous solution was stable at temperatures below 150°C. The ordered crystalline solid phase (K1) of the original fibrous material altered to a disordered crystalline solid phase (K2) at 150°C and subsequently to an isotropic phase (I) at 172°C. In the isotropic state, the C5PR molecule was slowly decomposed by decarboxylation. Once the molecular assemblage was subjected to the mesophase by heating, another ordered crystalline solid phase (K3) appeared reversibly at 17°C. The heat budget analyses by DSC indicated that a conformational entropy change such as the side-chain propyl group and the main-chain pentamethylene unit in the hydrogen-bonded molecular assemblage took place between the two ordered crystalline solid phases K1 and K3.

Restricted access

Abstract  

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.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: G. A. Sharpataya, K. S. Gavrichev, A. V. Khoroshilov, V. N. Plakhotnik, and V. M. Gurevich

The heat capacity of cesium hexafluoroarsenate, CsAsF6, was measured from 300 to 850 K by differential scanning calorimetry. The results indicate that the structural transformation from the rhombohedral to cubic phase, occurring in the range 235-360 K, relates to gradual transition completed isothermally. The enthalpy of the transition is 6.015±0.186 kJ mol-1, and its entropy is 17.21±0.58 J K-1 mol-1Rln8, which suggests that this structural transition belongs to the order-disorder type. The thermodynamic properties of CsAsF6 were evaluated in the range 10-760 K.

Restricted access

Abstract  

Structural studies of powdered polyaniline (PANI) prepared in aqueous medium by the oxidative polymerization of aniline in the presence of dodecylbenzenesulfonic acid(DBSA) were performed by means of DSC and WAXS. The influence of the alkyl side-chains on the structure and crystallinity of the as-synthesized PANI—DBSA and on the structural transitions taking place in PANI upon washing and heating were investigated. It was found that DBSA induces crystallinity in the rigid matrix of PANI, and residual crystalline phases were also observed after the deprotonation of PANI—DBSA. For the first time, a melting peak and a relaxation transition of non-cross-linked PANI were registered.

Restricted access

The octahedral-tetrahedral structural transition of Co(py)2Cl2 was studied by high temperature reflectance spectroscopy and dynamic reflectance spectroscopy. The reversibility of the transition by standing at room temperature for 24 hrs was established.

Restricted access