Elena Yu. Melenevskaya

Delivery of fullerene-containing complexes via microgel swelling and shear-induced release

The absorption and release of poly(vinylpyrrolidone)-fullerene C60 complexes (PVP/C60) from a model microgel is studied. A dry microgel based on a chemically cross-linked sodium polyacrylate was swollen in the aqueous solutions of complexes which were afterwards released under shear stress. First, gel swelling degree in static conditions in the excess of PVP/C60 solutions was studied: the degree of swelling decreases with the increase in PVP/C60 concentration. While pure PVP is homogeneously distributed between the gel and the surrounding solution, a slight concentration of complexes outside the gel was recorded. It was attributed to PVP/C60 hydrophobicity leading to the decrease in the thermodynamic quality of fullerene-containing solution being gel solvent. The release of PVP/C60 solutions induced by shear was studied with counter-rotating rheo-optical technique and compared with PVP solution release under the same conditions. The amount of solution released depends on polymer concentration and shear strain. Contrary to pure PVP solutions in which rate of release decreases with the increase in polymer concentration, PVP/C60 complexes are released faster when fullerene concentration inside the gel is higher.

Chain Degradation during Dissolution of Polymer-Fullerene Nanocomposites as a Result of Interaction of Entangled Polymer Matrix with the Filler

Chain degradation of poly(α-methylstyrene) and polystyrene during dissolution of their nanocomposites with fullerene C60 in solvents of different quality with respect to fullerene was studied in detail by size exclusion chromatography and UV spectroscopy. Chain ruptures have been shown to arise during swelling of composites but only for samples with entangled polymer matrix. The data obtained confirm that the hindered mobility of chains because of interaction of the entangled matrix with fullerene is the only cause of degradation. Chain rupture leads to radical depolymerization accompanied by covalent binding of fullerene with the chain fragments, which results in changing of the polymer matrix structure. Chain degradation indicates deterioration of the mechanical properties of the polymers in the presence of C60. The possibility of chain degradation in polymer-filler nanocomposites under deformation with the simultaneous observation of an apparent reinforcement effect because of the addition of filler in the polymer matrix is discussed.

Effect of Centrifugal Field upon Hydrodynamic Characteristics of Fullerene C60 and Poly(N-vinylpyrrolidone) Complex in Aqueous Solutions

Abstract Velocity ultracentrifugation method was employed to study aqueous solutions of fullerene C60 and poly(N-vinylpyrrolidone) complex (PVP-C60). Strong dependence of sedimentation coefficient S 0 on ultracentrifuge rotor speed n was found. The S 0 value increases as the value of n decreases from that corresponding to isolated PVP-C60 macromolecules to the value that the clusters of PVP-C60 (registered by the light-scattering method) would have at n → 0. This dependence indicates that molar mass and size of clusters change depending on the centrifugal field applied.

Chain Degradation under Low-Intensity Sonication of Polymer Solutions in the Presence of Filler: Mechanism of Ultrasonic Degradation of Flexible Chain Macromolecules

Routine dispersion of fillers in polymer solutions in a usual ultrasonic cleaning bath has been shown to lead to chain degradation. It is the filler presence, only, that has been demonstrated to provoke chain degradation under low-intensity sonication. A critical analysis of the literature concerned with the effects arising from propagation of acoustic waves in a liquid and an experimental study of ultrasonic degradation of polymers in solution were carried out. Based on these results, the mechanisms of chain degradation were discussed. Our previously proposed universal mechanism of chain degradation in inhomogeneous hydrodynamic fields has been shown to explain the basic facts repeatedly confirmed over the years of studying the ultrasonic degradation: (i) the existence of a limiting molecular weight such that macromolecules with lower molecular weights are not subject to degradation and (ii) the dependences of degradation rate on polymer molecular weight, polymer concentration, and temperature.

Rheological Behavior of Poly(vinylpyrrolidone)/Fullerene C60 Complexes in Aqueous Medium

Aqueous solutions of complexes formed between poly(vinylpyrrolidone) (PVP), as a matrix polymer, and fullerene C60 were investigated. The effect of the external hydrodynamic field on the supermolecular assemblies formed by the complexes was analyzed. Despite the low content in the complexes (1.5 mass%), fullerene significantly modified the viscosity of aqueous PVP. Thus, the dynamic viscosity of the PVP/C60 complexes grew faster than that of the pure PVP upon increasing the PVP/C60 concentration. The difference in viscosities is especially pronounced for semidilute solutions. As a possible explanation, it is assumed that fullerenes act as crosslinks, in addition to the physical entanglements of the PVP macromolecules, which appear in the vicinity of the crossover concentration. Shear flow corresponding to the high shear rates destroyed fullerene‐induced intermolecular crosslinks in PVP/C60 solutions.

Thermal and Tribological Properties of Fullerene‐Containing Composite Systems. Part 3. Features of the Mechanism of Thermal Degradation of Poly‐(N‐Vinyl‐Pyrrolidone) and Its Compositions with Fullerene C60

By mass‐spectrometric thermal analysis (MTA) the thermochemical features of poly(N‐vinyl pyrrolidone) (PVP) and its compositions with fullerene C60 were studied. The mechanism of PVP thermal degradation was investigated; in particular the nature of the low‐temperature degradation (between 75 and 300°C) accompanied by output of pyrrolidone was explained as well as the influence of fullerene C60 on this mechanism. It was shown that during thermal degradation of copolymer PVP‐C60, there is a disappearance of the low‐temperature peaks of the output of pyrrolidone that is interpreted as an increase of the thermal stability of N‐vinyl‐pyrrolidone fragments in this product in comparison with their thermal stability in pure PVP.

The regioselectivity of fullerenols C60(OH)x determined by high-resolution solid-state13C and 1H NMR analysis

Abstract Fullerenols C60(OH)x with different number of OH-groups (3–12 per molecule) were obtained by using a specially developed two-step grafting procedure. The regioselectivity of bound to fullerene molecule hydroxyls was proved by solid-state 1H and 13C NMR. It has been found that the fullerenols with 2–3 OH groups per C60 contain some amount of non-converted residual t-Bu groups and the solo grafting of one hydroxyl to one bond occurs due to the structure of the intermediate product. Therefore, the fullerenols with a medium number of hydroxyls seem to be the most proper derivatives for further covalent conversions.