O. V. Surov

Thermal stability of polyvinyl alcohol/nanocrystalline cellulose composites.

Thermal stability of polyvinyl alcohol/cellulose nanocrystals (PVA/CNCs) composites prepared with solution casting technique was studied. The PVA/CNCs composites were characterized by Fourier transform infrared spectrometry, X-ray diffraction, differential scanning calorimeter (DSC) and thermogravimetric (TG) analysis. Due to the presence of CNCs nanoparticles, thermal degradation of the composites occurs at much higher temperatures compared to that of the neat PVA. Thermal stability of the PVA/CNCs composites is maximally enhanced with CNCs content of 8-12 wt%. Some thermal degradation products of the PVA/CNCs composites were identified by mass spectrometric analysis. TG measurements with synchronous recording of mass spectra revealed that the thermal degradation of both CNCs and PVA in the composites with CNCs content of 8-12 wt% occurs simultaneously at a much higher temperature than that of CNCs or the neat PVA. However, with increasing CNCs content more than 12 wt% the thermal stability of the composites decreases. In this case, the degradation of CNCs comes first followed by the degradation of PVA.

Thermodynamic and structural aspects of sulfonamide crystals and solutions

The crystal structures of three sulfonamides with the general structure 4-NH(2)-C(6)H(4)-SO(2)NH-C(6)H(4/3)-R (R = 4-Et; 4-OMe; 5-Cl-2-Me) have been determined by X-ray diffraction. On the basis of our previous data and the results obtained a comparative analysis of crystal properties was performed: molecular conformational states, packing architecture, and hydrogen bond networks using graph set notations. The thermodynamic aspects of the sulfonamide sublimation process have been studied by investigating the temperature dependence of vapor pressure using the transpiration method. A regression equation was derived describing the correlation between sublimation entropy terms and crystal density data calculated from X-ray diffraction results. Also correlations between sublimation Gibbs energies and melting points, on the one hand, and between sublimation enthalpies and fusion enthalpies at 298 K, on the other hand, were found. These dependencies give the opportunity to predict sublimation thermodynamic parameters by simple thermo-physical experiments (fusion characteristics). Solubility processes of the compounds in water, n-hexane, and n-octanol (as phases modeling various drug delivery pathways and different types of membranes) were investigated and corresponding thermodynamic functions were calculated as well. Thermodynamic characteristics of sulfonamide solvation were evaluated. For compounds with similar structures processes of transfer from one solvent to another one were studied by a diagram method combined with analysis of enthalpic and entropic terms. Distinguishing between enthalpy and entropy, as is possible through the present approach, leads to the insight that the contribution of these terms is different for different molecules (entropy- or enthalpy-determined). Thus, in contrast to interpretation of only the Gibbs energy of transfer, being extensively used for pharmaceuticals in the form of the partition coefficient (log P), the analysis of thermodynamic functions of the transfer process provides additional mechanistic information. This may be important for further evaluation of the physiological distribution of drug molecules and may provide a better understanding of biopharmaceutical properties of drugs.

Nanocrystalline cellulose with various contents of sulfate groups.

Properties of films derived from aqueous nanocrystalline cellulose dispersions by water evaporation depend on concentration of sulfate groups. Namely type of thermodestruction and surface morphology change as a function of contents of sulfate groups. Surface roughness increases and water adsorption enhances with increasing sulfate groups content particularly at high relative pressure.

Thermogravimetric method used to determine the saturated vapor pressure in a wide range of values

Thermogravimetric method was used to study the evaporation of a number of solvents. The evaporation coefficients in the Langmuir equation were calculated in relation to the conditions of a thermogravimetric experiment (atmospheric pressure, carrier-gas) and nature of a substance under study.

Polymorphism of 4-tert-butylcalix[4]arene upon formation of n-hexane and acetonitrile complexes and thermal desolvation

Simultaneous analysis of X-ray powder diffraction patterns and DSC traces for 4-tert-butylcalix[4]arene-n-hexane and 4-tert-butylcalix[4]arene-acetonitrile compounds as functions of thermal history allows us to reveal the complex relationship between guest-free polymorphic products and desolvation of 4-tert-butylcalix[4]arene inclusion compounds.

Complexation of solvents and conformational equilibria in solutions of the simplest calix[4]arenes.

Structure optimization and calculation of electronic adsorption spectra of 25,26,27,28-tetrahydroxycalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene conformers have been performed by density-functional theory using hybrid B3LYP functional in cc-p VTZ and cc-p VDZ basis sets in Gaussian 09 package. Analysis of experimental UV-Vis spectra of solutions of 25,26,27,28-tetrahydroxycalix[4]arene, 4-tert-butylcalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene in various solvents has been carried out. It was shown that the ratio of absorption maxima at characteristic wavelengths at ca. 274 and 283 nm may be used to assess the extent of calix[4]arene/solvent interactions. The conclusion is drawn that spectral characteristics of calix[4]arenes are strongly affected by acid-base interactions.

Thermodynamic parameters of sublimation of calix[4]arenes

The temperature dependences of the vapor pressure of calix[4]arenes were determined by the Knudsen effusion method. Some calix[4]arenes can form congruently subliming intramolecular compounds with a solvent. The thermodynamic parameters of the sublimation of the compounds were calculated. Molecules of organic solvents incorporated in the cavity of calix[4]arenes stabilize the crystal lattice, increasing the enthalpy of sublimation. The electrostatic interactions presumably make a significant contribution to stabilization of the crystal lattice of stoichiometric complexes of calix[4]arenes with solvents.

Polyethylene oxide films reinforced by cellulose nanocrystals: Microstructure-properties relationship

Composite films of polyethylene oxide/cellulose nanocrystals (PEO/CNCs) comprising up to 75wt.% CNCs were obtained via solution casting. For the first time, investigations of the PEO/CNCs composite films over a wide composition range were carried out. Morphology, crystalline structure, thermal stability and the resulting reinforcing effect on the PEO matrix were studied. Polarizing optical microscopy (POM), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, Fourier transform infrared spectroscopy (FTIR), and tensile testing were used to examine the properties of the composites. The revealed extreme dependences of the composite properties refer to the CNCs content of 15-35wt.%. This composition range is characterized by destruction of PEO spherulites and appearance of a pronounced PEO fibrillar structure.

The Adsorption of Aqueous—Organic Mixture Components on Cellulose

The isotherms of composition changes were obtained for the cellulose—water—dimethylsulfoxide and cellulose—water—acetonitrile systems. The isotherms of sorption of aqueous—organic mixture components on cellulose were calculated. The heat effects of wetting of cellulose with water—dimethylsulfoxide and water — acetonitrile solutions were measured. The special features of sorption of the components on cellulose were shown to be related to the character of cluster formation in the binary systems under consideration.

Polymorphic conversions of 4-tert-butylcalix[4]arene upon the formation and thermal destruction of a complex with n-hexane

Complex polymorphic conversions of 4-tert-butylcalix[4]arene upon the formation and thermal destruction of a complex with n-hexane were revealed by combined analysis of powder X-ray pictures and data obtained by differential scanning calorimetry.