E. B. Tarabukina

Hydrodynamic and conformational properties of a hyperbranched polymethylallylcarbosilane in dilute solutions

The hydrodynamic and conformational properties of a hyperbranched polymethylallylcarbosilane in hexane solutions have been studied by light scattering, sedimentation, translational diffusion, and viscometry. Fractions with M = (1–75) × 103 have been used in experiments. The solution behavior of the hyperbranched polymer significantly differs from the properties of both spherical particles and linear polymers. The shape of hyperbranched polymethylallylcarbosilane macromolecules differs from spherical—the asymmetry factor is p ≤ 1.5. Polymethylallylcarbosilane macromolecules in solutions are characterized by compact structure—the hydrodynamic radius is not higher than 4 nm at M = 75 × 103.

Synthesis and investigation of the solution behavior of graft block copolymers of polyimide and poly(methyl methacrylate)

Graft copolymers with a polyimide backbone and poly(methyl methacrylate) side chains are investigated in dilute chloroform and ethyl acetate solutions via the methods of molecular hydrodynamics and optics. Copolymer samples are prepared through the “grafting from” method via atom-transfer radical polymerization with a multicenter polyimide macroinitiator. In solutions of copolymers with low degrees of functionalization Z (40%), supermolecular structures are formed as a result of interactions between the polyimide backbones. In samples with Z → 100%, the backbone is well screened by side chains; therefore, molecular solutions are formed in both solvents. The hydrodynamic and conformational behavior of samples with high functionalization degrees changes after the transition from ethyl acetate to chloroform owing to the different thermodynamic qualities of the solvents with respect to the copolymer components. In both solvents, the backbone tends to avoid contact with a poor solvent. This effect is more pronounced in the case of ethyl acetate. Macromolecules of the studied graft copolymers are characterized by high equilibrium rigidities (>40 nm) that are 10 times higher than the corresponding characteristics of aromatic polyimides.

Effect of the length of branches on hydrodynamic and conformational properties of hyperbranched polycarbosilanes

The hydrodynamic and conformational properties of hyperbranched polycarbosilanes with different lengths of branches, namely, poly(methyl(allyl)carbosilane) containing three CH2 groups between branching centers and poly(methyl(undecenyl)carbosilane) whose branches are composed of 11 CH2 groups, have been studied in dilute solutions in hexane using the methods of molecular hydrodynamics and optics. Fractions with M < 17.5 × 104 have been used in experiments. The hydrodynamic properties of the above polycarbosilanes differ significantly from those of linear polymers since hyperbranched macromolecules are compact and their shape differs only slightly from spherical. The lengthening of chains between branching centers causes a change in the hydrodynamic characteristics, and the difference between hyperbranched polymers and dendrimers becomes more pronounced. As the length of branches increases, their conformation changes from an extended trans chain to a more or less bent rod.

Effect of fluorinated substituents on hydrodynamic and conformational properties of hyperbranched polycarbosilane in solutions

Fractions of hyperbranched polycarbosilane modified with fluorine-containing end groups are studied by the methods of molecular hydrodynamics and optics in dilute solutions in hexafluorobenzene, methyl tert-butyl ether, chloroform, tetrahydrofurane, and toluene. The results are compared with the data available for the original polymethylallylcarbosilane. The incorporation of fluorinated groups leads to a change in the hydrodynamic parameters of the hyperbranched polymer. In this case, in thermodynamically better solvents, the patterns of the molecular-mass dependence of intrinsic viscosity for fluorinated and original hyperbranched polycarbosilanes coincide since the macromolecules of the polymers under comparison are characterized by similar shapes and almost the same hydrodynamic sizes. In a thermodynamically worse solvent (toluene), the above-mentioned changes in hydrodynamic parameters and the character of the molecular-mass dependences are related to compaction of macromolecules and a decrease in their shape asymmetry. This tendency is probably associated with the selectivity of this solvent with respect to the fluorine atom.

Behavior of hyperbranched polymers in solutions

Hyperbranched polycarbosilanes are investigated by the methods of molecular hydrodynamics and optics. Dependences of the hydrodynamic and conformational properties of these polymers on their molecular mass, the length of linear chains between branching points, and the chemical structure of end groups are analyzed. The hydrodynamic behavior of hyperbranched polycarbosilanes is explained by the fact that the dimensions of their macromolecules are compact and their shape is close to spherical. The convolution of chains between branching points becomes more pronounced with an increase in their length and a decrease in the molecular mass of the polymer. When end fluorinated groups are incorporated into the hyperbranched polycarbosilane, in thermodynamically good solvents and θ solvents, hydrodynamic characteristics change apparently owing to a change in the density of macromolecules in solution. In a poor solvent, the compaction of fluorinated macromolecules and a reduction in their shape asymmetry are observed. At a fixed branching degree, the hydrodynamic properties of hyperbranched polymers depend on the structural regularity of their macromolecules: In terms of hydrodynamic properties, the hyperbranched polycarbosilane with a degree of branching of 1 and a random distribution of branching points within the volume of a macro-molecule is appreciably different from a dendrimer of the same chemical nature and is close to polycarbosilanes with a degree of branching of 0.5.

Synthesis and Investigation of Double Stimuli-Responsive Behavior of N-Isopropylacrylamide and Maleic Acid Copolymer in Solutions

Poly(N-isopropylacrylamide-co-maleic acid) [poly(NIPAAm-co-MA)] linear copolymer with comonomer molar ratio NIPAAm:MA = 94:6 was synthesized by free-radical copolymerization. The molar mass, MsD = 28,000 Da, of poly(NIPAAm-co-MA) was determined using hydrodynamic methods. The self-assembly of poly(NIPAAm-co-MA) in aqueous solution at a concentration of 0.015 g cm−3 within the pH interval from 1.8 to 10.6 and the temperature interval from 22 to 60°C was investigated by static and dynamic light scattering. The copolymer showed a double temperature and pH responsiveness. Three types of particles, namely, macromolecules (or unimers), micellar-like structures, and loose aggregates existed in the poly(NIPAAm-co-MA) aqueous solutions. The fraction of dissolved entities and the hydrodynamic radii of the micellar-like structures and loose aggregates depended considerably on temperature and pH. The temperature of the phase separation and the width of the phase separation interval increased with pH. An influence of pH on kinetic processes in poly(NIPAAm-co-MA) solutions was observed.

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.

Molecular mass characteristics and hydrodynamic and conformational properties of hyperbranched poly-L-lysines

Hydrodynamic and conformational properties of hyperbranched poly(amino acids) based on lysine have been studied by static and dynamic light scattering, velocity sedimentation, translational diffusion, and viscometry in dilute aqueous-saline solutions (0.2 M NaCl). The effects of synthesis conditions of hyperbranched poly(amino acids), modification of their end groups by histidine fragments, and incorporation of diacylated lysine residues between branching points of oligomers and polymers composed of lysine and glutamic acid on the molecular mass characteristics of the hyperbranched polymers have been ascertained. The hydrodynamic properties of the hyperbranched poly(amino acids) differ appreciably from the behavior of both linear polylysine and lysine dendrimers due to conformational features of their macromolecules.

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.

Behavior of thermosensitive graft copolymer with aromatic polyester backbone and poly-2-ethyl-2-oxazoline side chains in aqueous solutions

ABSTRACT Thermoresponsive graft copolymers with alkylene-aromatic polyester main chain and poly-2-ethyl-2-oxazoline side chains were synthesized. Two copolymer samples which differed in grafting density (0.5 and 0.7) were studied using dynamic and static light scattering and turbidimetry in aqueous solutions at concentration 0.0053 g cm−3. Hydrodynamic radii of scattering objects and their contribution to light scattering were obtained as a function of temperature in a wide temperature interval. Temperatures of phase separation were found out. Effect of grafting density on the copolymer behavior in aqueous solutions upon heating was determined. In particular, the phase separation temperature reduces with the decreasing grafting density.