Mechanical and thermal properties of cryogels and foamed cryogels produced from aqueous solutions of poly(vinyl alcohol)

The effect of sodium chloride and technical-grade carbon (carbon black) on the mechanical and thermal properties of cryogels and foamed cryogels produced from homogeneous and heterogeneous (foamed) solutions of poly(vinyl alcohol) was studied.     

Study of Cryostructuring of Polymer Systems: 23. Composite Poly(vinyl alcohol) Cryogels Filled with Dispersed Particles Containing Ionogenic Groups

Composite poly(vinyl alcohol) cryogels containing particles of cross-linked dextran gels (Sephadexes and their ion-exchange derivatives with different ionogenic groups) as fillers were prepared and studied. It was shown that mechanical and thermal-physical properties of such composites were affected by the presence of ionogenic groups on the particles of dispersed fillers: as the ionite concentration increases, less rigid (than in the case of unmodified Sephadex particles) filled cryogels were formed. The properties of composite cryogels depend on the nature and concentration of counterion added with the ionite. Substantial increase in the rigidity and melting point of composites was observed when using strong anionites in the OH^–-form and strong cationites in the H⁺-form as fillers.     

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.     

Effect of polymer-precursor molecular mass on the formation and properties of covalently crosslinked chitosan cryogels

Features of cryotropic gelation in moderately frozen solutions of chitosan samples that have various molecular masses and that are covalently crosslinked with glutaraldehyde are studied. It is shown that the chain length of the macromolecular precursor, chitosan, affects the yield of the gel fraction; the swelling ability of the polymer phase of wide-pore cryogels; the dimensions of the gross capillary pores of cryogels; and, as a consequence, the hydrodynamic parameters of columns packed with a permeable continuous matrix based on the spongy gel material. The characteristics of the latter depend in a sophisticated manner on the molecular mass of the polymer used for formation of cryogels, a circumstance that is associated with competition of differently directed factors, specifically cryoconcentration effects, a strong increase in the viscosity of the reaction medium in the unfrozen liquid microphase, kinetic features of chain crosslinking at a high concentration of reagents, etc. Therefore, in each particular case, individual conditions for specific combination of the above-mentioned parameters of the process arise. As a result, a complex dependence corresponding is observed for the efficiency of cryostructuring of the chitosan-glutaraldehyde system and for the properties of the resulting cryogels.     

Protein chromatography by molecular imprinted cryogels

Abstract

Molecularly imprinted cryogels for protein recognition have received much attention in recent years with the development of supermacroporous polymer systems. Molecularly imprinted cryogels, which offer great advantages for the selective and effective purification and separation of biomacromolecules from their sources, have become an important candidate for affinity matrix. Cryogels with their macroporous structure give rise to use them in different applications, such as tissue engineering, protein recognition and separation fields and environmental issues. This review highlights the principles of molecular imprinting technology, the preparation and properties of cryogels, the recent developments and applications of molecularly imprinted cryogels especially in protein chromatography.     

Synthesis and characteristics of cryogels of chitosan crosslinked by glutaric aldehyde

The mechanism of cryotropic gelation in moderately frozen solutions of chitosan crosslinked by glutaric aldehyde is studied. Chitosan cryogels with large pores are synthesized at a low content of crosslinking agent in the reaction mixture or under conditions that do not lead to gelation at temperatures above 0°C. The dependences of the yield of gel fractions, the degree of swelling of the polymer phase of cryogels, and the hydrodynamic characteristics of cryogels on the temperature of synthesis are shown to be extremal. This result may be explained by the competition between the cryoconcentration of reagents in the nonfrozen liquid microphase, which assists the development of a crosslinked polymer network, and such factors as an increase in the viscosity of the reaction mixture and a reduced reagent mobility, which prevent gelation. The morphology of chitosan cryogels is studied, and the character of the macroporous structure of the samples prepared at different temperatures is shown to exert a stronger effect on the hydrodynamic characteristics of a cryogel than the degree of swelling of crosslinked polymers in the walls of its macropores.     

Cryogenic Designing of Biocompatible Blends of Polyvinyl alcohol and Starch with Macroporous Architecture

The present paper discusses synthesis, characterization, and blood compatibility studies of macroporous cryogels of PVA and starch. Biocompatible spongy porous hydrogels of polyvinyl alcohol–starch have been synthesized by repeated freezing–thawing methods and characterized by Infra red (FTIR) and environmental scanning electron microscopy (ESEM) techniques, respectively, to gain insights for structural and morphological features. The FTIR analysis of prepared cryogels indicated that starch was introduced into the network of cryogel possibly via formation of hydrogen bonds between the PVA and starch clusters. The “cryogels” were evaluated for their water uptake potentials and influence of various factors such as chemical architecture of the spongy hydrogels, pH and temperature of the swelling bath were investigated on the degree of water sorption by the cryogels. The hydrogels were also swollen in salt solutions and various simulated biological fluids. The biocompatibility of the prepared cryogels was judged by in vitro methods of blood–clot formation viz. percent haemolysis and protein (BSA) adsorption. The cryogels were also studied for their pores morphology and percent porosity and the effect of chemical composition on the extent of porosity was also investigated.     

Polymeric cryogels as a new family of macroporous and supermacroporous materials for biotechnological purposes

The review is devoted to a specific kind of polymeric gels formed as a result of non-deep freezing of solutions or colloidal sols of the corresponding precursors. These materials are termed as cryogels. They possess a series of unique properties, first of all, the specific character of porosity (macroporous cryogels with the pore cross-section from tenth fractions of μm to ∼10 μm and supermacroporous (gigaporous) cryogels with pores of tens and hundreds of μm) and are attractive from the viewpoint of biotechnological implementation. Approaches to the preparation of the so-called “smart” composites based on the cryogels are considered. The use of various cryogels as carriers of immobilized biocatalysts (enzymes, cells), matrices for wide-porous affinity sorbents and immunosorbents, and spongy scaffolds for 3D culturing of animal cells is discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 996–1013, May, 2008.     

Highly Elastic Super-Macroporous Cryogels Fabricated by Thermally Induced Crosslinking of 2-Hydroxyethylcellulose with Citric Acid in Solid State

Biopolymer materials have been considered a “green” alternative to petroleum-based polymeric materials. Biopolymers cannot completely replace synthetic polymers, but their application should be extended as much as possible, exploiting the benefits of their low toxicity and biodegradability. This contribution describes a novel strategy for the synthesis of super-macroporous 2-hydroxyethylcellulose (HEC) cryogels. The method involves cryogenic treatment of an aqueous solution of HEC and citric acid (CA), freeze drying, and thermally induced crosslinking of HEC macrochains by CA in a solid state. The effect of reaction temperature (70–180 °C) and CA concentration (5–20 mass % to HEC) on the reaction efficacy and physico-mechanical properties of materials was investigated. Highly elastic cryogels were fabricated, with crosslinking carried out at ≥100 °C. The storage modulus of the newly obtained HEC cryogels was ca. 20 times higher than the modulus of pure HEC cryogels prepared by photochemical crosslinking. HEC cryogels possess an open porous structure, as confirmed by scanning electron microscopy (SEM), and uptake a relatively large amount of water. The swelling degree varied between 17 and 40, depending on the experimental conditions. The degradability of HEC cryogels was demonstrated by acid hydrolysis experiments.     

Aligned/unaligned conducting polymer cryogels with three-dimensional macroporous architectures from ice-segregation-induced self-assembly of PEDOT-PSS

Porous conducting polymers are of great interest because of the huge potential to combine high surface areas in the dry state with physical properties relevant to organic electronics. Aligned or unaligned conducting polymer cryogels with 3D macroporous architectures have been prepared using the ice-segregation-induced self-assembly (ISISA) of different poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) freezing precursors as a dispersion or a formed hydrogel. The chemical composition and molecular structure of the resulting conducting polymer cryogels have been investigated by X-ray photoelectron spectroscopy and Raman spectroscopy, respectively. The morphologies of the PEDOT-PSS cryogels, together with their textural structures, have been revealed by scanning electron microscopy, mercury porosimetry, and nitrogen sorption tests. Processing PEDOT-PSS via ISISA endows the conducting polymers with novel properties, as demonstrated by a series of X-ray diffraction, differential scanning calorimetry, and electrical conductivity tests. These conducting polymer cryogels with aligned/unaligned macroporous architectures suggest the potential in the development of electronic components, tissue engineering, and next-generation catalytic and separation supports.     

Production of foamed cryogels by chemical gas generation in an aqueous solution of poly(vinyl alcohol)

Foamed cryogels were produced using the gas liberated in a redox reaction in an aqueous solution of poly(vinyl alcohol). The kinetics of the gas-generating reaction was studied. The mechanical and thermal characteristics of the foamed cryogels obtained were determined.     

Study of Cryostructuring of Polymer Systems: 25. The Influence of Surfactants on the Properties and Structure of Gas-Filled (Foamed) Poly(vinyl alcohol) Cryogels

Foamed poly(vinyl alcohol) (PVA) cryogels are studied. Such heterogeneous gel composites are formed as a result of the cryogenic treatment (freezing—storage in a frozen state—thawing) of water— PVA liquid foams in the absence and presence of surfactants. It is shown that the addition of ionic and nonionic surfactants to an aqueous PVA solution and its subsequent foaming result in the formation of liquid foam whose stability is lower than that of the foam prepared from an aqueous PVA solution in the absence of surfactant, i.e., surfactants cause a destabilizing effect on the foams containing PVA. Gas-filled PVA cryogels formed as a result of freezing—thawing of such foams contain large (up to ～180 μm) pores (air bubbles incorporated into the matrix of heterogeneous gel). Mechanical and thermal properties of cryogels depend on the nature and concentration of surfactants, as well as on the regime of cryogenic treatment. The rigidity of foamed PVA cryogels prepared in the presence of sodium dodecyl sulfate and cetyltrimethylammonium bromide ionic surfactants is lower and that in the presence of nonionic decaoxyethylene cetyl ether is higher than for equiconcentrated (by the polymer) foamed PVA cryogel containing no surfactant. Microscopic studies and the analysis of obtained images of cryogel structure demonstrate that the effect of surfactant on the morphology of freezing foam can be different, depending on the type of surfactant added to the initial system. This leads to foam-destabilizing effects such as the collapse, deformation, and coalescence of air bubbles; the failure of gel phase structure near the bubble surface; etc. However, the complete disintegration of the foamed structure is prevented by a very high viscosity of the unfrozen liquid microphase of a macroscopically solid sample and by the cryotropic PVA gelation that fixes the structure of partially destroyed foam.     

Effect of 1,4‐dioxane on synthesis of macroporous poly(N‐isopropylacrylamide) hydrogels

Thermosensitive Poly(N‐isopropylacrylamide) (PNIPA) hydrogels were synthesized by a free radical solution polymerization in three different ways. Normal hydrogels were prepared at room temperature and normal cryogels were prepared at subzero temperature. A cation surfactant dodecyl dimethyl benzyl ammonium bromide (DDBAB) was used during preparation of novel cryogels in freezing state. The response rates of normal hydrogels were very slow, whereas the rates of both normal and novel cryogels were very fast because of the macroporous structure of the cryogels. Mixed solvents which were composed of pure water and 1,4‐dioxane at various concentrations were used instead of pure water during the polymerization. The effects of the mixed solvent on morphology, swelling ratio, and deswelling/reswelling kinetics of the three kinds of hydrogels were investigated. For normal hydrogels and normal cryogels, there was no remarkable difference no matter the mixed solvent or pure water was used. However, the properties of the resulted novel cryogels were much different with the concentration of dioxane. Finally, the resulted hydrogels were used for concentrating emulsified paraffin. The different separation performance was attributed to the different structure of gel matrix. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6594–6603, 2008     

1H NMR Self-Diffusion Study of Morphology and Structure of Polyvinyl Alcohol Cryogels

The multicomponent self-diffusion of the polyvinyl alcohol (PVA) cryogels prepared by a freezing-thawing treatment of aqueous and water-DMSO solutions of PVA has been studied with the NMR FT-PGSE method. The temperature dependencies of the self-diffusion coefficients, Ds, for the PVA chains have a maximum at 45 degrees C due to the syneresis of cryogels. They are quite different from the monotonous increase of Ds for the aqueous solutions of PVA. Evaluated apparent activation energies, Ea, of the self-diffusion for the PVA chains in the PVA solutions and cryogels in D2O are practically the same and equal 22-24 kJ/mol below the crucial point. The proton spin-lattice relaxation times, T1, of the PVA chain also coincide with one another for solutions and cryogels. This means that molecular packing in cryogels depends mainly on the dimensions of the ice and polymer microcrystallites formed by freezing the solution. Above the crucial point polymer compartments become firmer, and the chain mobility somewhat reduces. The strength of cryogels also increases along with growing the DMSO contents and decreases by the BSA addition. For estimation of the cryogel morphology, effects of the restricted diffusion of both the water and PVA in a q-space have been taken into account. By the introduction of DMSO to cryogels the solvent filled pores become smaller, and channels become much shorter. The diameter of the PVA filaments is similar to those for all the cryogels, but the length of filaments with D2O is twice that for cryogels with a mixed solvent. Entrapment of BSA in the cryogel matrix by preparation leads to the increase of an average diameter of the water filled pores and destroys molecular packing the cryogel. Copyright 1999 Academic Press.     

Study of cryostructuring of polymer systems: 27. Physicochemical properties of poly(vinyl alcohol) cryogels and specific features of their macroporous morphology

Based on aqueous poly(vinyl alcohol) (PVA) solutions with different content of polymer having different molecular masses and chain tacticity, macroporous viscoelastic gels (PVA cryogels) are prepared in various regimes of freezing-storage in a frozen state-thawing. Shear modulus and fusion temperature of corresponding samples are measured; the structure of thin sections is studied by optical microscopy and the images are processed and analyzed. It is shown that the rigidity and heat endurance of cryogels rise with an increase in the concentration of initial PVA solution and a decrease in the rate of thawing. The influence of the temperature of cryogenic treatment and the PVA molecular mass has an extreme character. At the same time, the effect of the main parameters of cryotropic gelation on the macroporous morphology of PVA cryogels is manifested in the form of more complex dependences because of its multiple-factor character. Therefore, distinct structure-property correlations are not observed in many cases. Cluster analysis of the morphometric characteristics of cryogels in comparison with data on their rigidity makes it possible to classify these systems.     

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.     

Composite cryogel with immobilized concanavalin A for affinity chromatography of glycoproteins

Composite cryogels containing porous adsorbent particles were prepared under cryogelation conditions. The composites with immobilized concanavalin A (Con A) were used for capturing glycoproteins. Adsorbent particles were introduced into the structure in order to improve the capacity and to facilitate the handling of the particles. The monolithic composite cryogels were produced from suspensions of polyvinyl alcohol particles and porous adsorbent particles and cross‐linked under acidic conditions at sub‐zero temperature. The cryogels were epoxy activated and Con A was immobilized as an affinity ligand. Binding and elution of horseradish peroxidase (HRP) was studied in batch experiment and in a chromatographic setup. Increasing adsorbent concentration in composite cryogels will increase ligand density, which therefore enhances the amount of bound HRP from 0.98 till 2.9 (milligram enzyme per milliliter of gel) in the chromatographic system. The material was evaluated in 10 cycles for binding and elution of HRP.     

DNA adsorption via Co(II) immobilized cryogels

The separation and purification of important biomolecule deoxyribonucleic acid (DNA) molecules are extremely important. The adsorption technique among these methods is highly preferred as the adsorbent cryogels are pretty much used due to large pores and the associated flow channels. In this study, the adsorption of DNA via Co(II) immobilized poly(2-hydroxyethyl methacrylate-glycidyl methacrylate) [poly(HEMA-GMA)] cryogels was performed under varying conditions of pH, interaction time, initial DNA concentration, temperature, and ionic strength. For the characterization of cryogels; swelling test, Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), surface area (BET), elemental and ICP-OES analysis were performed. L-lysine amino acid was chosen as Co(II)-chelating agent and the adsorption capacity of cryogels was determined as 33.81 mg DNA/g cryogel. Adsorption of pea DNA was studied under the optimum adsorption conditions and DNA adsorption capacity of cryogels was found as 10.14 mg DNA/g cryogel. The adsorption process was examined via Langmuir and Freundlich isotherm models and the Langmuir adsorption model was determined to be more appropriate for the DNA adsorption onto cryogels.     

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. 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.