Cryostructuring of polymer systems. XXVI. Heterophase organic–inorganic cryogels prepared via freezing–thawing of aqueous solutions of poly(vinyl alcohol) with added tetramethoxysilane

Composite heterophase organic–inorganic hybrid cryogels of poly(vinyl alcohol) (PVA) containing silica constituents were prepared and studied. Such constituents were formed in the course of hydrolytic polycondensation (sol‐gel process) of tetramethoxysilane (TMOS) introduced in to the aqueous polymer solution prior to its freeze–thaw treatment. It was shown that moderate (over the range of ?15 to ?30°C) freezing, then frozen storage, and subsequent thawing of the water/PVA/TMOS systems resulted in the formation of macroporous composite cryogels filled with dispersed silica particles (discrete phase). The continuous phase of such gel materials represents the supramolecular PVA network, which is supposed to be additionally cured with the silicon‐containing oligomeric cross agents formed from TMOS in the course of hydrolytic polycondensation. The incorporated silica components influenced the morphology of cryogels. The effects of significant increase in gel strength and heat resistance with increasing TMOS concentration in the initial feed and with thawing rate decreasing have also been observed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Study of cryostructuration of polymer systems. XVII. Poly(vinyl alcohol) cryogels: Dynamics of the cryotropic gel formation

Freeze‐thaw treatment of concentrated (>5 g/dL) aqueous solutions of poly(vinyl alcohol) (PVA) (MW 115,000; DD ≈100%) resulted in the formation of opaque gels. The extent of such a cryostructuration process was exhibited in the rheological properties of similar PVA cryogels. The gels' strength depended on the initial polymer concentration in the solution to be frozen and on the conditions of a cryogenic influence. The key factor was the defrostation dynamics: the slower the thawing rate, the stronger the cryogel sample formed, provided other parameters of the process were identical. The observation for the kinetics of the freeze‐thaw–induced gel formation revealed the extreme character of the temperature dependence of the efficacy of PVA cryotropic gelation, the maximum point being in the vicinity of −2°C. It was shown that the effect of the strengthening of PVA cryogels prepared by means of a single‐cycle cryogenic treatment could be reached either with use of as slow as possible thawing regimes, or by the prolonged frozen storage of the samples at “high” subzero temperatures. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2017–2023, 2000     

Study of cryostructuration of polymer systems. XX. Foamed poly(vinyl alcohol) cryogels

Foamed poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze–thaw treatment of whipped PVA water solutions (polymer with MW of 69,000 Da and DD ～99 mol % was used), were obtained and their properties were studied. The rheological characteristics and macrostructure of these gel materials were controlled by the same factors as for the ordinary nonfoamed PVA cryogels (initial polymer concentration and freezing–thawing regimes) and also by the conditions of generation of fluid PVA foams. The study of the kinetics of the freeze–thaw‐induced gel formation of these foams revealed that the temperature dependence of the efficiency of cryotropic gelation showed a maximum at about ?1.5°C. The presence of low molecular weight admixtures in the initial polymer solution appears to be a rather important factor because the admixtures were capable of decreasing the stability of fluid PVA foams and weakening both foamed and nonfoamed cryogel samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1609–1619, 2001     

Cryostructuring of polymer systems. XXX. Poly(vinyl alcohol)‐based composite cryogels filled with small disperse oil droplets: A gel system capable of mechanically induced releasing of the lipophilic constituents

Composite heterophase poly(vinyl alcohol) (PVA) cryogels containing entrapped small droplets of Vaseline oil have been prepared and studied. Such oil‐filled cryogels were formed via freeze–thaw treatment of freshly prepared oil‐in‐water emulsions containing varied volume fraction of lipophilic phase, and the influence of the amount of this phase, as well as the effects of freezing conditions on the physicomechanical (shear moduli) and thermal (gel fusion temperature and fusion enthalpy) characteristics of resulting composites have been explored. It was shown that over certain range of PVA concentrations in aqueous phase and a range of volume fraction of the hydrophobic phase its microdroplets performed as “active” fillers causing an increase in both the gel strength and the heat endurance of composites. The light microscopy data on the morphological features of such filled PVA cryogels revealed the effect of diminution in size of oil droplets entrapped in the gel matrix as compared with the initial emulsions. This effect can be explained by the disintegrating action of crushing and shear stresses arising upon the system freezing and growth of ice crystals. The oil‐filled PVA cryogels were found to be capable of gradually releasing the lipophilic constituents (the Rose hips oil, in this case) in response to the cyclic mechanical compression. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Study of cryostructuration of polymer systems. XII. Poly(vinyl alcohol) cryogels: Influence of low-molecular electrolytes

The influence of the presence of low-molecular electrolytes in initial solutions of poly(vinyl alcohol) (PVA) on the results of cryotropic gelation of the polymer (gelation caused by the freezing thawing) and on the swelling characteristics of PVA cryogels prepared in a salt-free medium were studied. The reinforcing ability of the electrolytes with respect to the gel strength for both alkaline element cations and simple low-molecular anions has been shown to be in agreement with the positions of these ions in corresponding lyotropic (Hofmeister) series. Namely, the ions (chaotropic ones), which are capable to interfere the H-bonding, disturbed somewhat the cryotropic gel-formation of PVA and facilitated the marked additional swelling of cryogels preliminary prepared in pure water medium, whereas the ions (antichaotropic ones), which are capable to promote the H-bonding, caused the formation of reinforced cryogels and resulted in the shrinking of cryogels prepared without salt additives. More pronounced effects were observed for anions as compared with cations. Some anomalous swelling behavior of PVA cryogel in Cs⁺-containing solutions was supposed to be associated with the formation of weak chelates. In addition to antichaotropic inorganic salts like NaF, rather high salting-out effects in respect to PVA were exhibited by wellsoluble amino acid salts: glycine zwitter-ions, lysine monochlorohydrate, and monosodium aspartate. © 1996 John Wiley & Sons, Inc.     

Preparation and characterization of polyacrylamide cryogels produced from a high‐molecular‐weight precursor. II. The influence of the molecular weight of the polymeric precursor

Polyacrylamide gels and cryogels were prepared by the crosslinking reaction of polyacrylamide (a polymeric precursor) with glutaric aldehyde (a crosslinking agent) in liquid and moderately frozen aqueous media, respectively. Polymeric precursors of different viscosity‐ averagemolecular weights (0.3, 1, 3, and 9 MDa) were used. The molecular weight of the precursors, as well as the reaction temperature and concentration of the crosslinking agent, exerted a pronounced influence on the efficiency of gelation (gel fraction yield) and on the properties (swelling capability) and structural peculiarities of the resulting gels (reference samples) and cryogels. The highest efficacy was inherent in the cryotropic gelation process when the polymeric precursor had a molecular weight of about 3 MDa, whereas the implementation of polyacrylamides of lower (0.3 or 1 MDa) or higher (9 MDa) molecular weights diminished the gel formation efficiency. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Study of cryostructuring of polymer systems. XIX. On the nature of intermolecular links in the cryogels of locust bean gum

In this study of freeze–thaw‐induced gelation of aqueous locust bean gum (LBG) solutions we found that gel formation proceeded through at least two kinetic stages. Initially, rather weak spongy cryostructurates were formed as a result of freezing the system, but their strength and thermostability (fusion temperature) could be increased considerably by means of thawing out the system sufficiently slowly. Preparation of LBG cryogels in the presence of chaotropic (urea) and anti‐chaotropic (sodium sulfate) substances showed that the major mechanism responsible for stabilization of junction zones in these gel materials was hydrogen bonding. Therefore, use of the agents facilitating formation of hydrogen bonds enabled the gelation of LBG solutions without the need for synergistic polymers or cryogenic treatments. © 2000 Society of Chemical Industry     

Study of cryostructuration of polymer systems. XV. Freeze–Thaw‐induced formation of cryoprecipitate matter from low‐concentrated aqueous solutions of poly(vinyl alcohol)

Freeze–thaw treatment of low‐concentrated (<C*) aqueous solutions of poly(vinyl alcohol) (PVA) results in the formation of a cryoprecipitate fraction. It is shown that the efficiency of such a process (the yield of PVA cryoprecipitation) depends on the initial polymer concentration in the solution to be frozen and the conditions of a cryogenic influence. The key factor is defrostation dynamics: The slower the thawing rate, the higher the cryoprecipitation yield. The iodine‐staining method is employed for the quantitative analysis of PVA concentrations in the solutions under study and the necessity of the use of reduced (0–2°C) temperatures throughout such a PVA quantification is demonstrated. Observation of the kinetics of the freeze–thaw‐induced formation of cryoprecipitate matter reveals the extreme character of the temperature dependence of the efficacy of PVA macromolecule aggregation, the extreme point being situated in the vicinity of −2°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1978–1986, 1999     

Super‐macroporous dextran cryogels via UV‐induced crosslinking: synthesis and characterization

Cryogenic treatment and UV irradiation were exploited for the preparation of super‐macroporous cryogels from non‐modified high‐molar‐mass dextran. The photo‐crosslinking process was initiated by (4‐benzoylbenzyl)trimethylammonium chloride and N,N′‐methylenebisacrylamide (BAAm) was used as a crosslinking agent. Gel fraction yield and degree of swelling of the dextran cryogels were determined gravimetrically. Cryogel morphology and mechanical properties were studied using environmental scanning electron microscopy and dynamic rheological measurements, respectively. The effects of dextran concentration in the initial polymer solution, polymer molar mass and BAAm content on the crosslinking efficacy, physico‐mechanical properties and morphology of the cryogels were evaluated. The dextran cryogels were assessed as carriers of the model water‐soluble drug metoprolol. © 2017 Society of Chemical Industry     

Preparation and characterization by radiation of poly(vinyl alcohol) and poly(N‐vinylpyrrolidone) hydrogels containing aloe vera

In these studies, hydrogels for wound dressings were made from a mixture of aloe vera and poly(vinyl alcohol) (PVA)/poly(N‐vinylpyrrolidone) (PVP) by freezing and thawing, γ‐Ray irradiation, or a two‐step process of freezing and thawing and γ‐ray irradiation. We examined the physical properties, including gelation, water absorptivity, gel strength, and degree of water evaporation, to evaluate the applicability of these hydrogels for wound dressings. The PVA:PVP ratio was 6:4, the dry weight of aloe vera was in the range 0.4–1.2 wt %, and the solid concentration of the PVA/PVP/aloe vera solution was 15 wt %. We used γ radiation doses of 25, 35, and 50 kGy to expose mixtures of PVA/PVP/aloe vera to evaluate the effect of radiation dose on the physical properties of the hydrogels. Gel content and gel strength increased as the concentration of aloe vera in the PVA/PVP/aloe vera gels decreased and as radiation dose increased and the number of freeze–thaw cycles was increased. The swelling degree was inversely proportional to the gel content and gel strength. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1477–1485, 2003     

Study of cryostructurization of polymer systems. IX. Poly(vinyl alcohol) cryogels filled with particles of crosslinked dextran gel

The influence of deformable disperse filler on rheological and thermal properties of poly(vinyl alcohol) cryogels (obtained by freezing–thawing of concentrated aqueous solutions of the polymer) was studied. Spherical particles of crosslinked dextran gel—the Sephadex beads of two types: G-25 and G-100—were used as filler. It was found that an increase in the filler concentration from 0 to 20 wt% produced and increase in the composite cryogel strength; the greater reinforcement effect was observed with large-pore Sephadex beads into the cryogel matrix. An increase in the fraction of disperse phase also leads to an increase in fusion temperatures of these composites, such effect being determined mainly by the filler type. The filling of cryogels with G-100 beads improves the thermostability of composite samples to a higher extent as compared with the use of G-25 particles. Several possible reasons for the trends observed in rheological and thermal properties of Sephadexfilled PVA cryogels were revealed.     

Super-macroporous composite cryogels based on biodegradable dextran and temperature-responsive poly(N-isopropylacrylamide)

This contribution describes the fabrication of super-macroporous cryogels comprising an interpenetrating network of biodegradable dextran and temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) segments. Cryogels were prepared from nonmodified dextran and N-isopropylacrylamide (NIPAAm) via cryogenic treatment of their aqueous solutions and subsequent irradiation with Ultraviolet light. The weight ratio of the two precursors was varied in order to find out the proper conditions for producing cryogels of high gel fraction yield and favorable physico-mechanical properties. Temperature-responsive behavior and enhanced elastic modulus were established for cryogel materials containing ≥50 wt% PNIPAAm. Scanning electron microscopy study revealed an open-porous structure of cryogels below and above the temperature of volume phase transition. Such morphology featured a very quick response of the material to temperature changes. The capability of dextran/PNIPAAm cryogels for instant release of water-soluble substances was demonstrated as well.     

EFFECT OF DRYING METHOD ON MESOPOROSITY OF RESORCINOL–FORMALDEHYDE DRYGEL AND CARBON GEL

Resorcinol–formaldehyde hydrogels were synthesized by sol–gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution. RF cryogels, RF xerogels, and RF xerogels (MW gels) were respectively prepared from RF hydrogels by freeze drying, hot air drying, and microwave drying. Carbon cryogels, carbon xerogels and carbon MW gels were subsequently obtained by pyrolyzing RF drygels in an inert atmosphere. Freeze drying and microwave drying were effective to prepare mesoporous RF drygels and carbon gels. RF cryogels and carbon cryogels showed high mesoporosity over wide ranges of the molar ratio of resorcinol to catalyst (R/C) and the ratio of resorcinol to water (R/W) used in sol–gel polycondensation. Although RF xerogels had a few mesopores, carbon xerogels had no mesopores. RF MW gels and carbon MW gels showed mesoporosity if appropriate values of R/C and R/W were selected.     

Crystal violet dye sorption and transport in/through biobased PVA cryogel membranes

Cryogels based on poly(vinyl alcohol) [PVA] and three types of bioinsertions such as scleroglucan, cellulose microfibers, and zein, respectively, have been prepared using capacity of PVA to crosslink by repeated freezing–thawing cycles. The effect of the incorporation of biopolymers on the properties of PVA cryogel has been studied by using several techniques such as: scanning electron microscopy, differential scanning calorimetry, and Fourier transform infrared studies. The obtained biobased cryogel membranes were subjected to sorption and to diffusion experiments using Crystal Violet (CV), a dye commonly used in the textile industry and in medicine. Image analysis with CIELAB system was used both to monitor the cryogels loading with CV and to gain insight in the dye state into the gel, in correlation with the bioinsertion type and gels morphology. Dye diffusion but also sorption capacity of the cryogels was found to be closely related to the type of biopolymer. In this article the equilibrium (sorption isotherms) and transport properties (diffusion and permeability coefficients) of CV, in/through physical cross‐linked PVA hydrogel membranes with bioinsertions has been reported. The highest efficiency for the CV removal from aqueous solutions was obtained for the PVA/Scl cryogels. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41838.     

Swelling–deswelling kinetics of ionic poly(acrylamide) hydrogels and cryogels

A series of ionic poly(acrylamide) (PAAm) gels was prepared by free‐radical crosslinking copolymerization of acrylamide and N,N′‐methylenebisacrylamide in aqueous solutions. The gels were prepared both below and above the bulk freezing temperature of the polymerization solvent water, which are called as the cryogels and the hydrogels, respectively. The deswelling behavior of swollen gels in acetone as well as the reswelling behavior of the collapsed gels in water were investigated. It was shown that the cryogels respond against the external stimuli much faster than the hydrogels. The interior morphology of the cryogel networks exhibits a discontinuity and a two‐phase structure, compared to the continuous morphology of the hydrogel networks. Introduction of the ionic units in the network chains further increased the response rate of the cryogels. In contrast to these advantages of cryogels, they exhibit lower swelling capacities than the conventional hydrogels. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 319–325, 2006     

Synthesis of biodegradable hydroxyethylcellulose cryogels by UV irradiation

Biodegradable macroporous hydroxyethylcellulose (HEC) cryogels of good quality and high gel fraction yield (95%) were synthesized via a facile method. The latter involved a relatively fast preparation of homogeneous semidilute solution of polymer and photoinitiator, (4-benzoylbenzyl)trimethylammonium chloride, followed by freezing at a defined negative temperature, an extremely short UV irradiation and subsequent thawing. HEC cryogels were successfully prepared also by using H₂O₂ as a photoinitiator. The effects of the temperature of freezing, the HEC molecular weight and the concentration of HEC solution on the cross-linking efficiency, the swelling ratio and the enzymatic degradation of HEC cryogels were investigated. Due to the cryoconcentration phenomenon, cryogels are formed at substantially low initial concentrations of the studied polymers. The highest values of gel fraction yield are achieved in the 1-2 wt.% concentration range at −20 °C. As a rule, the higher the molecular weight, the greater the gel fraction yield of the resulting cryogels. Scanning electron microscopy (SEM) analysis reveals that the interior structure of HEC cryogels is completely different from the conventional HEC hydrogels. HEC cryogels undergo decomposition by the action of cellulase enzyme, however, due to their specific morphology, the rate of degradation is slower compared to the conventional HEC hydrogel of similar gel fraction yield.     

Intelligent gels and cryogels with embedded emulsions of various oils

Thermo‐responsive gels and cryogels with embedded microdroplets of Vaseline, olive, peanut, and linseed oils and their mixtures with hydrophobic dye Sudan 3 have been synthesized and studied. These composite gel matrices were obtained by the three‐dimensional copolymerization of N‐isopropylacrylamide and N, N'‐bis(acryloyl)cystamine in the presence of oil emulsions stabilized with sodium dodecylsulfate or Span 80. Polymerization was performed at room temperature for conventional gels and at ?15°C for cryogels. It was shown that all synthesized systems exhibit heat‐induced collapse at temperatures higher then 34°C. For conventional gels prepared at room temperature shrinking lasts within 20 to 80 min in accordance with gel composition. No squeezing of oil droplets was observed. In the case of cryogels, shrinking was accompanied by release of oils and response time was significantly shorter, about tens of seconds. Collapse character and release of lipophilic phase did not depend on the chemical nature of oils, dissolved compounds, and surfactant used for emulsion stabilization. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Turbidity investigation of the sol–gel transition in carrageenan gels under physiologic conditions

A spectrophotometer was used to measure the turbidity of a carrageenan gel as a function of temperature. The optical transmission of the gels was found to decrease as the gels undergo the sol–gel phase transition. The differential of transmission (I) with respect to temperature (T), dI/dT, exhibits peaks for both the cooling and the heating runs with the peak positions corresponding to temperatures of gelation and melting, respectively. The full-width at half-height of the dI/dT peak obtained from the heating curve is about 2.5 times broader than that from the cooling curve. This indicates that the melting of gels may involve multiple relaxation mechanisms. The area of the hysteresis loop covered by the cooling and the heating curves increases with a decrease in the scanning rate. The thermal cycling has little impact on the sol–gel transition in the gels. The experiments show that turbidity is a powerful tool for studying the sol–gel transition in carrageenan gels. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:29–35, 1998     

Tear force of physically crosslinked poly(vinyl alcohol) gels with different submicrometer‐scale network structures

Poly(vinyl alcohol) (PVA) gels can be easily prepared by either the freeze‐thawing (FT gel) method or by the cast‐drying (CD gel) method. Although the resulting nanostructured networks of the FT and CD gels are similar, their physical properties are quite different; while CD gels are transparent and elastic, FT gels are opaque and less elastic. Moreover, the tear energy of the FT gels is much greater than that of the CD gels, which is a direct result of micrometer‐scale differences in their network structures. In order to control the distribution of microcrystallites on nano‐ and micrometer scales, FT gels were prepared from PVA solutions with different water contents. As a result, the gel gradually became more transparent as the initial water content was decreased; and accordingly, the tear energy decreased. Tear resistance was improved in the case of FT gels by repeating the number of FT cycles, whereas with CD gels, this was achieved by increasing the gelation temperature. These results indicate that the microscopic network structures are characterized by a micrometer‐scale bundled‐polymer (fibril), which determines the tear energy of FT gels. Simple methods to control the fibril network structure of FT gels using a unidirectional freezing method are presented herein, with the swelling and mechanical properties of modified FT gels discussed in terms of their multiple‐scale network structures. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41356.     

Preparation and properties of PVA/PVP hydrogels containing chitosan by radiation

Radiation can induce chemical reactions to modify polymers even when they are in the solid state or at a low temperature. Radiation crosslinking can be easily adjusted by controlling the radiation dose and is reproducible. The finished product contains no residuals of substances required to initiate the chemical crosslinking, which can restrict its application possibilities. In these studies, hydrogels for wound dressing were made from a mixture of chitosan and polyvinyl alcohol (PVA)/poly‐N‐vinylpyrrolidone (PVP) by freezing and thawing, gamma‐ray irradiation, or combined freezing and thawing and gamma‐ray irradiation. The physical properties of the hydrogel, such as gelation, water absorptivity, and gel strength, were examined to evaluate the usefulness of the hydrogels for wound dressing. The PVA/PVP composition was 60:40, PVA/PVP–chitosan ratio was in the range 9:1–7:3, and the concentration of, PVA/PVP–chitosan as a solid was 15 wt %. A mixture of PVA/PVP–chitosan was exposed to gamma irradiation doses of 25, 35, 50, 60 and 70 kGy to evaluate the effect of irradiation dose on the physical properties of hydrogels. Water‐soluble chitosan was used in these experiment. The physical properties of the hydrogels, such as gelation and gel strength, were higher when the combination of freezing and thawing and irradiation were used rather than just freezing and thawing. The PVA/PVP–chitosan composition and irradiation dose had a greater influence on swelling than gel content. Swelling percent increased as the composition of chitosan in PVA/PVP–chitosan increased. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1787–1794, 2002