E. A. Podorozhko

A study of cryostructuring of polymer systems. 34. Poly(vinyl alcohol) composite cryogels filled with microparticles of polymer dispersion

Macroporous filled and unfilled poly(vinyl alcohol) (PVA) cryogels are produced by cryogenic treatment (freezing at −20°C for 12 h followed by thawing at a rate of 0.03°C/min) of mixtures of an aqueous PVA solution and a full-component poly(vinyl acetate) (PVAc) dispersion or its individual components. The values of the elasticity modulus and fusion temperature are determined for obtained samples; their microstructure is studied by light microscopy of thin sections. It is shown that the effects that are induced by the incorporation of PVAc dispersion into the macroporous matrix of the PVA cryogel are due to the presence of both a discrete phase, i.e., solid PVAc microparticles, and ingredients of the liquid phase of the PVAc dispersion, mainly, urea. Therewith, the dispersed particles themselves serve as a reinforcing filler, i.e., increase the rigidity and (to a lesser extent) heat endurance of the cryogel, while urea, which possesses chaotropic properties and hinders the intermolecular hydrogen bonding of PVA chains, reduces the rigidity and heat endurance of the composites. As a result, the total effect is determined by the competition of differently directed influences of these components of PVAc dispersion and depends on its concentration in the resulting filled cryogel. It is also shown that PVAc microparticles are mainly entrapped in the gel phase of the macroporous matrix and form necklacelike aggregates, the cross-sectional areas and lengths of which depend on the degree of composite filling.

A study of cryostructuring of a polymer system. 39. Poly(vinyl alcohol) composite cryogels filled with chitosan microparticles

Composite macroporous poly(vinyl alcohol) (PVA) cryogels have been prepared by cryogenic treatment (freezing at −20°C for 12 h followed by defrosting at a rate of 0.03°C/min) of dispersions of chitosan particles swollen in an aqueous 120 g/L PVA solution. The elasticity moduli and fusion temperatures have been determined for corresponding samples, and their microstructure has been investigated by optical microscopic examination of their thin sections. The rigidity and heat endurance of the PVA-chitosan composite cryogels have been shown to increase with filler concentration. In addition, the rheological properties of initial dispersions have been studied and the appearance of adhesion contacts between discrete and continuous phases has been revealed already at the stage of dispersion preparation. The optical microscopic investigations of the morphological features of the initial dispersions and the microstructure of the composites have established a relatively uniform distribution of filler particles in the bulk of composite cryogels. It has been shown that chitosan particles incorporated into the matrix of the macroporous PVA cryogel are capable of sorbing heavy metal ions—in particular, bivalent copper ions—from aqueous solutions.

Study of cryostructuring of polymer systems. 32. Morphology and physicochemical properties of composite poly(vinyl alcohol) cryogels filled with hydrophobic liquid microdroplets

Cryogenic treatment (freezing at −20°C for 12 h followed by defrosting at a rate of 0.03°C/min) of decane, dodecane, or tetradecane emulsions in a poly(vinyl alcohol) solution (80 g/l) is employed to prepare composite cryogels containing microdroplets of liquid hydrophobic fillers entrapped into a macroporous hydrogel matrix. The effects of the type of a hydrocarbon, the degree of filling, and the addition of a surfactant (decaethylene glycol cetyl ether) on the physicomechanical properties, heat endurance, and morphology of the composites are studied. It is shown that, an increase in the content of liquid hydrophobic fillers within some range of their volume fraction enhances the rigidity of corresponding cryogels. Incorporation of the nonionic surfactant into the initial emulsions results in a complex dependence of the rigidity of the resulting composite cryogels on surfactant concentration and variations in the morphology of pores in the gel phase. At the same time, the heat endurance of all examined composite cryogels weakly depends on the type and concentration of the hydrocarbon fillers, as well as the presence of surfactant additives.

A Study of cryostructuring of polymer systems. 41. Complex and composite poly(vinyl alcohol) cryogels containing soluble and insoluble forms of chitosan, respectively

Complex macroporous poly(vinyl alcohol) (PVA) cryogels have been obtained by cryogenic treatment (freezing at–20°C for 12 h followed by defrosting at a rate of 0.03°C/min) of PVA–chitosan hydrochloride mixed solutions. The subsequent alkaline treatment of the cryogels has resulted in the transformation of the water-soluble salt form of chitosan into its insoluble basic form, which coagulates inside the bulk of the continuous phase of PVA cryogel into small particles with sizes of 2–5 µm. In the resulting composite cryogels, these particles play the role of an “active” filler, which increases the rigidity and heat endurance of the gel material. It has been shown that the sorption capacity of such chitosan particles entrapped into the bulk of composite cryogels with respect of bivalent copper ions is noticeably higher than the sorption capacity of ground chitosan particles incorporated as a discrete filler into the continuous phase PVA cryogels. The study of the properties of PVA–chitosan hydrochloride mixed solutions revealed that these polymers are, to a large extent, compatible with one another in a common solvent at a low ionic strength. Therefore, liquidliquid phase separation of these systems due to the thermodynamic incompatibility of macromolecules of different natures is observed only upon increasing the ionic strength by adding a low-molecular-mass salt (NaCl, 0.15 mol/L) to the solution.

A study of cryostructuring of polymer systems. 37. Composite cryogels formed from dispersions of poly(butadiene-co-styrene) latex in aqueous poly(vinyl alcohol) solution

Composite macroporous poly(vinyl alcohol) (PVA) cryogels have been prepared by cryogenic treatment (freezing at −20°C for 12 h followed by thawing at a rate of 0.03°C/min) of mixtures of an aqueous PVA solution and poly(butadiene-co-styrene) latex. The values of the elasticity modulus and fusion temperature have been determined for the obtained samples, and their microstructures have been studied by optical microscopy of thin sections. The rigidity and heat endurance of the filled cryogels have been shown to markedly increase with the fraction of the latex in the initial liquid disperse system. Moreover, it has been found that, beginning from some critical concentration of the latex in a mixture with the initial PVA solution, cryostructuring leads to the formation of two interpenetrating phases—a continuous phase of the hydrophilic PVA cryogel and a network of the hydrophobic phase that results from cryocoagulation of non-frost-resistant poly(butadiene-co-styrene) latex. The morphological features of this phase have been studied, and the sizes of macropores and the thicknesses of their walls have been determined with the help of scanning electron microscopy.

A study of cryostructuring of polymer systems. 43. Characteristics of microstructure of chitosan-containing complex and composite poly(vinyl alcohol) cryogels

The microstructure of complex and composite poly(vinyl alcohol) (PVA) cryogels containing water-soluble chitosan hydrochloride (ChHC) of dispersed particles of water-insoluble chitosan base (Ch), respectively, has been studied by optical microscopy and attenuated total reflection FTIR spectroscopy. The macroporous morphology of cryogels has been studied using preparations in the form of thin (~10 μm) sections and discs 1 mm thick. The introduction of non-gelling additives (NaCl and ChHC) into an initial PVA solution causes significant changes in the size and shapes of macropores in the complex cryogels formed by freezing–defrosting, as compared with the pores in the samples obtained under the same conditions without additives. The reasons for the changes are the process of phase segregation and the influence of low- and high-molecular-weight electrolytes on crystallization of ice, which plays the role of a porogen upon cryotropic gelation of aqueous PVA solutions. As a result of an alkaline treatment of the complex cryogels, which transforms ChHC into Ch, microcoagulation of chitosan yields discrete, almost spherical, particles with sizes of about 1–5 μm. IR spectral studies have shown that concentration gradients of the gelling and nongelling polymers arise along the thickness of the gel discs, with PVA concentration prevailing near the lower surface and ChHC or Ch concentration dominating near the upper surface of the disc.

A study of cryostructuring of polymer systems. 45. Effect of porosity of dispersed filler on physicochemical characteristics of composite poly(vinyl alcohol) cryogels

Composite cryogels simulating the properties of waterproof screens of hydraulic structures, such as protruding dykes and dams, have been obtained by a cryogenic treatment (freezing at –10…–30°C followed by incubation in the frozen state for 12 h and defrosting at a rate of 0.03°C/min) of suspensions of calcium-carbonate-containing (marble or coquina) or cellulose-containing (microcrystalline cellulose or sawdust) particles in aqueous poly(vinyl alcohol) solutions. Viscometric examinations of initial suspensions have shown that adhesion contacts arise between filler particles, as well as discrete and continuous phases, already at the stage of suspension preparation, thereby affecting the properties of resulting cryogels. This is most pronounced when high-porosity sawdust is used as a filler. It has been shown that all the dispersed materials used in the work are “active” fillers for poly(vinyl alcohol) cryogels, these fillers increasing the rigidity of the formed composites. Therewith, porous particles, into which the polymer solution can penetrate, are more efficient. The dependence of the composite rigidity on the temperature of the cryogenic treatment has, as a rule, a bell-shaped pattern with a maximum in the region of –20°C. Being tested for water permeability, the obtained model composite cryogels have exhibited pronounced antifiltration properties (the filtration coefficient is ≤(1–2) × 10–9 cm/s), thus indicating that such materials are promising for solving problems relevant to the protection of fascine hydraulic structures from erosion with snow water.

Novel immobilized biocatalyst for microbiological synthesis of pharmaceutical steroids

The steroid-transforming activity of free and immobilized cells of Pimelobacter simplex VKPM Ac-1632 entrapped in an operationally stable macroporous poly(vinyl alcohol) cryogel was studied. It was shown that the macroporous matrix of the carrier did not create any diffusional limitations for steroid access to the cells or the removal of the transformation product from them. The optimal conditions for the hydrocortisone 1,2-dehydrogenation into prednisolone by free and immobilized cells were elucidated. The immobilized biocatalyst was obtained in a granulated form and used in 32 successive cycles of steroid transformation. The average cycle duration was 45 min, and the prednisolone yield of during the first 20 cycles was 98%. It was established that the immobilized cells of the actinobacteria P. simplex retained high steroid-transforming activity over all of the transformation cycles. The physicochemical and diffusion characteristics of the polyvinyl alcohol gels and its granules were determined, and their high stability during repeated cycles of steroid transformation was shown. The results indicated that P. simplex immobilized cells represent an effective catalyst suitable for multiple use. Biomass consumption decreased upon its use, and product isolation, as well as culture storage, was much easier.