Properties of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/Polycaprolactone Polymer Mixtures Reinforced by Cellulose Nanocrystals: Experimental and Simulation Studies.

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/polycaprolactone (PHBV/PCL) polymer mixtures reinforced by cellulose nanocrystals (CNCs) have been obtained. To improve the CNC compatibility with the hydrophobic PHBV/PCL matrix, the CNC surface was modified by amphiphilic polymers, i.e., polyvinylpyrrolidone (PVP) and polyacrylamide (PAM). The polymer composites were characterized by FTIR, DSC, TG, XRD, microscopy, BET surface area, and tensile testing. The morphological, sorption, thermal, and mechanical properties of the obtained composites have been studied. It was found out that with an increase in the CNC content in the composites, the porosity of the films increased, which was reflected in an increase in their specific surface areas and water sorption. An analysis of the IR spectra confirms that hydrogen bonds can be formed between the CNC hydroxyl- and the -CO- groups of PCL and PHBV. The thermal decomposition of CNC in the PHBV/PCL/CNC composites starts at a much higher temperature than the decomposition of pure CNC. It was revealed that CNCs can either induce crystallization and the polymer crystallite growth or act as a compatibilizer of a mixture of the polymers causing their amorphization. The CNC addition significantly reduces the elongation and strength of the composites, but changes Young's modulus insignificantly, i.e., the mechanical properties of the composites are retained under conditions of small linear deformations. A molecular-dynamics simulation of several systems, starting from simplest binary (solvent-polymer) and finishing with multi-component (CNC-polymer mixture-solvent) systems, has been made. It is concluded that the surface modification of CNCs with amphiphilic polymers makes it possible to obtain the CNC composites with hydrophobic polymer matrices.

Properties of polyacrylamide composites reinforced by cellulose nanocrystals.

In this work, a series of polyacrylamide/cellulose nanocrystal (PAM/CNC) composites with a wide range of compositions were prepared by a solution casting method. Subsequently, the influence of the PAM conformation on the behaviour of the PAM/CNC composites was studied. The microstructural, thermal, and mechanical properties of the PAM/CNC composites were also investigated, as well as their flocculation and dispersion behaviour. Thermal degradation of both CNC and PAM in the composites occurred simultaneously, at a much higher temperature than the degradation of the neat CNC. By TEM and SEM, PAM globule aggregates in the PAM/CNC composites were detected. With an increase in the medium acidity, the PAM globule aggregate size of in the composites decreased. Moreover, the composite films cast from high pH solutions (extended PAM conformation) exhibited superior strength properties than those cast from low pH solutions (contracted coil PAM conformation). The PAM globule conformation provided good re-dispersibility of the freeze-dried PAM/CNC composites by preventing aggregation of the CNC particles. The PAM globule adsorption onto the CNC particles caused CNC surface hydrophobization and a decrease in their surface charge, while maintaining high colloidal stability of the CNC suspensions. Furthermore, the CNC particles with adsorbed PAM were demonstrated to be useful as emulsifiers and compatibilisers.

Molecular Dynamics Simulation of Polyacrylamide Adsorption on Cellulose Nanocrystals.

Classical molecular dynamics simulations of polyacrylamide (PAM) adsorption on cellulose nanocrystals (CNC) in a vacuum and a water environment are carried out to interpret the mechanism of the polymer interactions with CNC. The structural behavior of PAM is studied in terms of the radius of gyration, atom-atom radial distribution functions, and number of hydrogen bonds. The structural and dynamical characteristics of the polymer adsorption are investigated. It is established that in water the polymer macromolecules are mainly adsorbed in the form of a coil onto the CNC facets. It is found out that water and PAM sorption on CNC is a competitive process, and water weakens the interaction between the polymer and CNC.

Water Effects on Molecular Adsorption of Poly(N-vinyl-2-pyrrolidone) on Cellulose Nanocrystals Surfaces: Molecular Dynamics Simulations.

Models of interaction between a poly(N-vinyl-2-pyrrolidone) macromolecule and a fragment of Iß-cellulose were built in a vacuum and water environment. The models were made to interpret the mechanism of interaction of the polymer and cellulose nanocrystals by the classical molecular dynamics method. The structural behavior of a poly(N-vinyl-2-pyrrolidone) macromolecule in water has been studied in terms of the radius of gyration, atom-atom radial distribution functions and number of hydrogen bonds. It was found that the polymer has a high affinity with the solvent and each monomer unit has on average 0.5 hydrogen bonds. The structural and energy characteristics of the polymer adsorption were investigated at different initial positions of the poly(N-vinyl-2-pyrrolidone) macromolecule relative to the cellulose fragment. It was observed that the polymer macromolecule was mainly adsorbed on the cellulose fragment in the globular form. Moreover, in the solvent the interaction of poly(N-vinyl-2-pyrrolidone) with the cellulose hydrophobic surface was stronger than that with the hydrophilic one. This study will show that the presence of water makes the interaction between the polymer and cellulose weaker than in a vacuum, and the polymer and cellulose mainly interact through their solvation shells.

Preparation and Characterization of Polyvinylpyrrolidone/Cellulose Nanocrystals Composites.

Composite films and aerogels of polyvinylpyrrolidone/cellulose nanocrystals (PVP/CNC) were prepared by solution casting and freeze-drying, respectively. Investigations into the PVP/CNC composite films and aerogels over a wide composition range were conducted. Thermal stability, morphology, and the resulting reinforcing effect on the PVP matrix were explored. FTIR, TGA, DSC, X-ray diffraction, SEM, and tensile testing were used to examine the properties of the composites. It was revealed PVP-assisted CNC self-assembly that produces uniform CNC aggregates with a high aspect ratio (length/width). A possible model of the PVP-assisted CNC self-assembly has been considered. Dispersibility of the composite aerogels in water and some organic solvents was studied. It was shown that dispersing the composite aerogels in water resulted in stable colloidal suspensions. CNC particles size in the redispersed aqueous suspensions was near similar to the CNC particles size in never-dried CNC aqueous suspensions.

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.

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.

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 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.