S. G. Karpova

Structural-dynamic characteristics of matrices based on ultrathin poly(3-hydroxybutyrate) fibers prepared via electrospinning

Structural-dynamic analysis based on combined thermophysical and molecular mobility measurements via spin-probe ESR spectroscopy has been applied to films and fibrous matrixes based on poly(3-hydroxybutyrate). The dynamic behaviors of partially crystalline samples during deformation under conditions of electrospinning and cold rolling have been compared. The comparative results of the complex investigation of films and ultrathin fibers in the poly(3-hydroxybutyrate) matrix have shown that the electromechanical action leads to additional crystallization of the crystalline regions, spherulites, and lamellas in the polymer. The changes in the crystalline phase of the polymer are accompanied by an increase in the packing density of macromolecules in the intercrystalline space. With the use of the spin-probe ESR method, the effect of water and the oxidant ozone on the morphology of the amorphous phase of poly(3-hydroxybutyrate) ultrathin fibers has been determined. The measurements of the dynamics of spin-probe rotation in samples before and after cold rolling have shown that the additional orientation of poly(3-hydroxybutyrate) spherulites in a mechanical field results in stabilization of amorphous regions more resistant to the aggressive effects of ozone.

Microstructures and supramolecular structures of butadiene-nitrile rubbers: Paramagnetic-probe study

The chain microstructures and supramolecular structures of butadiene-nitrile rubbers are studied by ESR spectroscopy. On the temperature dependences of the rotational mobility (correlation time τc) of paramagnetic probes differing in size (2,2,6,6-tetramethylpiperidine-1-oxyl and 4-benzoate-2,2,6,6-tetramethylpiperidine-1-oxyl), relaxation transitions are observed. It is shown that there is a correlation between the Arrhenius parameters of rotational mobility of radicals and the copolymer composition and that different brands of rubbers differ in microstructure and supramolecular structure.

Nonwoven blend composites based on poly(3-hydroxybutyrate)–chitosan ultrathin fibers prepared via electrospinning

With the use of scanning electron microscopy, differential scanning calorimetry, and electron paramagnetic resonance, the structural–dynamic analysis of ultrathin fibrous matrixes based on poly(3-hydroxybutyrate) and blend composites of this polymer with chitosan is performed. It is shown that the addition of a small amount of chitosan causes change in the morphologies of the matrixes and leads to a marked increase in their melting enthalpies. It is found that the studied fibers contain amorphous regions with various morphologies. The dynamics of the spin probe TEMPO in these regions is investigated, and its change under the influence of increased temperature, an aqueous medium, and ozone is examined. The mechanism controlling the effects of chitosan, temperature, and an oxidative aggressive medium on the structuring of fibers is advanced.

Effect of temperature on the molecular mobility in polylactide

The effects of temperature on the molecular mobility in the amorphous phase and on the structural parameters of the crystalline phase of three brands of polylactide have been studied. It was found that annealing increases the melting temperature by 2–3°C and increases the degree of crystallinity by 2–6%. X-Ray analysis showed the possibility of formation of crystal structures of the α and β modifications in polylactide. The change in correlation time of radical rotation in amorphous regions of polylactide as a function of the history of thermal pretreatment of the samples was shown via the electron paramagnetic resonance method.

The effect of the type of solvent on the structural and dynamic parameters of polyurethane-styrene-co-acrylonitrile blends

The effect of solvents, such as THF, methyl ethyl ketone, ethyl acetate, and acetone, on the structures and molecular dynamics of polyurethane-styrene-co-acrylonitrile blends has been investigated with the use of spin-probe ESR spectroscopy, DSC, IR spectroscopy. There are slight changes in the values of molecular mobility, crystallinity, and elastic modulus of composites containing more than 50% polyurethane, and only at higher contents of the styrene-acrylonitrile copolymer are sharp changes in the structural and dynamic parameters observed in the presence of any solvent. In different solvents, the blend parameters improve in the following series: THF, methyl ethyl ketone, ethyl acetate, acetone. This outcome is determined by the thermodynamic affinities of the dissolved polymers for the solvent. It has been shown that the content of hydrogen bonds between nitrile and urethane groups (a band at 3344 cm−1) increases with the content of styrene-co-acrylonitrile in a blend. During reversible deformation, the molecular mobility of the radical increases and further tension is accompanied by reduced mobility in the oriented blends, independently of the solvent type.

The Structure of Polybutadienes and Butadiene-Acrylonitrile Copolymers

The paramagnetic probe method with the use of free radicals of different dimensions (2,2,6,6-tet-ramethyl-1-piperidinyloxy and 4-benzoate-2,2,6,6-tetramethyl-1-piperidinyloxy) has been employed to study the effect of the isomeric composition of butadiene units in polybutadienes and butadiene-acrylonitrile copolymers on the number and dimensions of ordered structures. The nature of density fluctuations and defective regions, that is, the regions in which the radicals are sorbed, has been ascertained. It has been shown that the ordered regions are composed of stereoregular chain fragments, while defective regions are enriched with butadiene isomers different from those present in prevailing amounts.

Influence of drug on the structure and segmental mobility of poly(3-hydroxybutyrate) ultrafine fibers

Ultrafine fibers of biodegradable natural polyester such as poly(3-hydroxybutyrate) containing dipyridamole at various concentrations as a drug are prepared by the electrospinning method. It is shown by scanning electron microscopy that the absence of dipyridamole or its low concentrations (from 0 to 1%) provide the complex morphology of fibers composed of cylindrical regions 1–3 μm in diameter and thickened spindle-like ones 5–7 μm in average diameter. An increase in the concentration of dipyridamole in fibers leads to disappearance of the latter regions, with the morphology being cylindrical. The features of the crystalline and amorphous structures of poly(3-hydroxybutyrate) and its mixtures with dipyridamole are examined via DSC and EPR probe techniques. It is shown that the addition of dipyridamole to the poly(3- hydroxybutyrate) polymer matrix results in a sharp increase in the crystallinity and a slowdown of the molecular mobility in amorphous regions of ultrafine fibers. The heat treatment (annealing) of fibers leads to a sharp increase in the polymer crystallinity and a reduction of the segmental mobility in intercrystalline regions of the initial poly(3-hydroxybutyrate) fibers and those containing 1% of dipyridamole. All results including the influence of the drug concentration on the shape of fibers and their dynamic characteristics agree well with the thermal and physical parameters and should be used in the design of therapeutic systems for targeted and sustained delivery of bioactive compounds.

The nature of sites of absorption of low-molecular-mass compounds by butadiene–acrylonitrile copolymers

The rotational mobilities of the paramagnetic probes TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and BZONO (4-benzoate-2,2,6,6-tetramethylpiperidine-1-oxyl) are studied via EPR spectroscopy and the equilibrium swelling of random and multiblock butadiene–acrylonitrile copolymers of various compositions and stereostructures of butadiene units in n-heptane, methyl acetate, and toluene are studied. The nature of defective regions sorbing low-molecular-mass compounds is ascertained. The rotational mobilities of the probes in defects of various structures are estimated. The sites of sorption are found to be identical in crosslinked and noncrosslinked elastomers. It is shown that the sorption of n-heptane and methyl acetate appears in the same defective regions as those of the TEMPO radical, while the sorption of toluene emerges in sorption sites where the value of free volume is sufficient for the sorption of the bulky radical BZONO. A decay in local molecular mobility in structural defects (“holes”) does not hinder the absorption of lowmolecular- mass compounds if the free volume is sufficient.

The effect of solvents on the structural-dynamic characteristics of films and nonwoven materials based on polyurethane, the styrene-acrylonitrile copolymer, and their blends

A structural-dynamic analysis combining thermophysical measurements and studies of segmental mobility via the ESR microprobe method is applied to films and nonwoven materials based on polyurethane, the copolymer of styrene and acrylonitrile, and blends of these polymers. The effect of solvents—such as tetrahydrofuran, ethyl acetate, and acetone—on the structures and molecular dynamics of films and matrixes based on PU and styrene-acrylonitrile copolymer ultrathin fibers is studied. It is shown that a solvent weakly affects the molecular dynamics of chains in the film and nonwoven polyurethane materials and has a strong effect on the molecular mobility in the films based on the styrene-acrylonitrile copolymer and blends with a high content of the copolymer. For the nonwoven material, this effect is insignificant. It is found that in both film and nonwoven materials mesomorphic structures are formed. An analysis of the dependences of the correlation time of probe rotation, τ, on the temperature and the polymer ratio reveals the presence of breaks at the melting temperatures of mesomorphic structures PU. This effect is the most pronounced at a 50: 50 mass ratio of the components for both film and nonwoven materials as a result of phase inversion. The spin-probe studies show that the oxidizer ozone influences the amorphous phase of the studied polymers. Measurements of the dynamics of probe rotation for film and nonwoven materials after ozonation demonstrate that, for both PU-based films and nonwoven materials, ozone has practically no effect on the molecular dynamics, whereas for the blends of these polymers and the styrene-acrylonitrile copolymer these changes in τ are significant.

Structural dynamic properties of nonwoven composite mixtures based on ultrafine tissues of poly(3-hydroxybutyrate) with chitosan

The structural dynamic parameters of ultrafine fibrous matrices of poly-3-hydroxybutyrate (PHB) and composite mixtures of PHB with chitosan were studied by differential scanning calorimetry, EPR spectroscopy, and scanning electron microscopy. The melting enthalpy of PHB fibers considerably increased when a small amount of chitosan was added. Amorphous regions with diverse morphology were found in the fibers under study. The dynamics of the TEMPO spin probe in these regions and its change in the fibers under the action of temperature, aqueous medium, and ozone was determined. The mechanism responsible for the effect of chitosan, temperature, and aggressive oxidating medium on the structure of ultrafine PHB fibers was suggested.