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 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.     

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. 46. physicochemical properties and microstructure of poly(vinyl alcohol) cryogels formed from polymer solutions in mixtures of dimethyl sulfoxide with low-molecular-mass alcohols

Poly(vinyl alcohol) (PVA) cryogels (PVACGs) are obtained and studied. The PVACGs are formed by freezing–defrosting of polymer solutions in dimethyl sulfoxide (DMSO) or its mixtures with one of the first members of the series low-molecular-mass aliphatic alcohols (methanol, ethanol, n-propanol, and n-butanol). PVA content in these solutions is 100 g/L, while the concentration of an aliphatic alcohol is varied in a range of 0.44–2.55 mol/L depending on its nature. The polymer solutions are subjected to the cryogenic treatment at temperatures 30, 40, or 50°C lower than the crystallization temperature of DMSO (+18.4°C). The frozen samples are defrosted at a heating rate of 0.03°C/min. It is shown that, in a certain range of lowmolecular-mass alcohol content in an initial system, its cryogenic treatment yields coarse-pored heterophase cryogels that have higher rigidity and heat endurance than those of DMSO–PVA cryogels. It has been shown that polymer cryoconcentration and phase separation play important roles in the formation of a cellular microstructure and an increase in the rigidity and heat endurance of PVACGs obtained in the presence of low-molecular-mass alcohols.     

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.     

Study of cryostructuring of polymer systems: 28. Physicochemical properties and morphology of poly(vinyl alcohol) cryogels formed by multiple freezing-thawing

Macroporous viscoelastic poly(vinyl alcohol) (PVA) cryogels are prepared from aqueous concentrated (80–120 g/l) PVA solutions subjected to 1–5 cycles of cryogenic treatment (freezing at ?20°C for 19 h and subsequent thawing at a rate of 0.3°C/min). Shear moduli and fusion temperatures of corresponding samples are determined and the structure of thin sections is studied by optical microscopy with subsequent processing and analysis of images obtained. The previously described effect of a substantial increase in the rigidity and thermal stability of PVA cryogels resulted from the repeated freezing-thawing cycles is confirmed. The largest (jumpwise) changes in the physicochemical characteristics of such gels and their macroporous morphology take place after the second cycle of cryogenic treatment. Moreover, depending on the PVA concentration in the initial solution, the mean cross section of micropores increases by a factor of 2–3 and the total porosity of cryogel rises by a factor of 1.5–2; i.e., the imperfection of material increases. Nevertheless, this negative (from view-point of the integral properties of cryogel) effect is completely overpowered by processes of additional structuring, which result in the strengthening of polymer phase proceeding during the repeated freezing-thawing cycles.     

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.     

Study of cryostructuring of polymer systems: 31. Effect of additives of alkali metal chlorides on physicochemical properties and morphology of poly(vinyl alcohol) cryogels

Macroporous viscoelastic poly(vinyl alcohol) (PVA) cryogels were prepared from aqueous PVA solutions containing additives (0–1.2 mol/l) of alkali metal chlorides (LiCl, NaCl, KCl, CsCl) by cryogenic treatment (freezing at −20°C for 12 h and subsequent thawing at a rate of 0.03°C/min). Shear moduli and fusion temperatures of corresponding samples were determined and the structure of thin sections was studied by optical microscopy with subsequent processing and analysis of images obtained. It was shown that the rigidity, heat endurance, and mean pore sizes of formed cryogels monotonically decrease with increasing content of chaotropic lithium chloride. In the case of other three salts, the dependences of rheological characteristics of cryogels on the concentration of low-molecular-weight electrolyte were extreme due to the competition between factors that promote and prevent PVA cryotropic gelation. At the same time, fusion temperatures of gel samples increased steadily with increasing content of these salts. Microscopic studies revealed substantial (by factor of two to three) decrease in macropore sizes even at low content of salt compared to mean cross sections of pores in cryogel containing no additive; morphometric analysis of obtained images makes it possible to reveal the linear correlations between the rheological characteristics of cryogels formed in the presence of LiCl and the sizes of their macropores.     

Study of cryostructuring of polymer systems. 33. Effect of rate of chilling aqueous poly(vinyl alcohol) solutions during their freezing on physicochemical properties and porous structure of resulting cryogels

Poly(vinyl alcohol) cryogels are obtained and investigated. The cryogels are prepared by freezing an initial polymer solution (100 g/L) at chilling rates of 1.85, 0.3, 0.03, or 0.003°C/min followed by defrosting the frozen (?20°C/12 h) preparations at heating rates of 0.3, 0.03, or 0.003°C/min. It is shown that a noticeable influence of the chilling rate on the rigidity and heat endurance of the gel matrix is observed at very slow regimes of chilling (on the order of thousandths of degree centigrade per minute). One of the reasons for a reduction in the rigidity of resulting cryogels and the fusion enthalpy of the nodes in their supramolecular networks is the formation of a homo-phase hydrogel during the long-term exposure of the concentrated polymer solution to low temperatures prior to the onset of ice crystallization. The effect of the regime of chilling the polymer solution being frozen on the porous structure of cryogels resulting from the thawing of the preparations is ambiguous due to the multifactor character of this effect; therefore, in many cases, well-defined structure-property correlations are not observed.     

Study of cryostructuring of polymer systems. 42. Physicochemical properties and microstructure of wide-porous covalently cross-linked albumin cryogels

Chemically cross-linked wide-porous protein cryogels have been obtained by freezing aqueous bovine serum albumin (BSA) solutions (30–50 g/L) at–15,–20, or–25°C in the presence of water-soluble carbodiimide (CDI) as a coupling agent. It has been shown that the gel-fraction yield and the swelling extent of the polymer phase of the formed spongy matrices depend primarily on the initial concentration of albumin and the amount of CDI added to a system, while the morphometric characteristics of the porous microstructure of the BSA cryogels are mainly determined by the temperature of the cryogenic treatment. The sizes of macropores in the obtained cryogels range from ≈50 to ≈200 μm. A high-sensitivity differential scanning calorimetric study of the conformational state of protein macromolecules incorporated into the spatial network of the polymer phase (gel walls of macropores) of the cryogels has shown that, during the cryotropic gelation, the native structure of albumin globules is subjected to “cold denaturation,” and the partly unfolded conformation of the protein is, simultaneously, fixed by intermolecular covalent cross links.     

A study of the microstructural and diffusion properties of poly(vinyl alcohol) cryogels containing surfactant supramolecular aggregates

Surfactant-containing poly(vinyl alcohol) (PVA) cryogels have been prepared by drying and reswelling hydrogel patches, previously obtained by the freeze/thaw procedure, in decyltrimethylammonium bromide (C10TAB) aqueous solutions. The microstructural and diffusive properties of the resulting material have been characterized by a combined experimental strategy. Gravimetric measurements show that the cryogel maximum swelling is not affected by the surfactant. The surfactant concentration within the cryogel, measured by ion chromatography, is the same as that in the rehydrating surfactant solution. Electron paramagnetic resonance (EPR) spin-probe and small-angle neutron scattering (SANS) measurements show that surfactant self-aggregation in the gel is similar to that in water, occurring at the same critical concentration and resulting in the formation of micellar aggregates whose structure is not affected by the cryogel polymeric scaffold. However, both the micelle intradiffusion coefficients, measured by PGSE-NMR, and the spin-probe correlation times, measured by EPR, indicate that dynamic processes in the hydrogel are much slower than in bulk water. A quantitative analysis of these results suggests that the cryogel polymer-poor domains, in which surfactant molecules are solubilized, have an average dimension of approximately 0.1 microm. Interestingly the experimental data also show that the polymer-poor phase contains more polymer than expected, suggesting that the spinodal decomposition, which occurs during the freezing step of cryogel preparation, is not complete or prevented by ice formation.     

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.     

Effect of solvent for vinyl acetate polymerization on microstructure of poly(vinyl alcohol)

The solution polymerization of vinyl acetate was carried out in several solvents at 0 to 100°C, using 2,2′-azobisisobutyronitrile as initiator. For the resulting poly(vinyl alcohol) (PVA), iodinecoloration, 1,2-glycol structure and tacticity were observed. The pentad tacticity of PVA was estimated from its methine carbon spectra by means of ¹³C-FTNMR spectrometer. Iodine-coloration ability of PVA varied markedly with the type of polymerization solvent and decreased in the following order: phenol > aq. phenol > methyl alcohol > ethyl acetate > DMSO, ethylene carbonate. The syndiotactic fraction in PVA also decreased with polymerization solvent in the same order as that of iodine coloration, while 1,2-glycol content of PVA was not almost affected by polymerization solvent except for phenol and aq. phenol. In solution polymerization performed, effect of polymerization temperature on tacticity was less than that of solvent.     

Reduction of silver ions to silver with polyaniline/poly(vinyl alcohol) cryogels and aerogels

The macroporous conducting polymer cryogels were prepared by the oxidation of aniline hydrochloride in the frozen aqueous solutions of poly(vinyl alcohol) at ? 24 °C. Corresponding polyaniline aerogels supported with poly(vinyl alcohol) have been obtained after thawing of cryogels followed by freeze-drying. Silver was deposited on the composites using the ability of polyaniline to reduce silver ions after the immersion in silver nitrate solutions. Swollen cryogels were coated only on the surface with macroscopic silver particles due to the closed-pore structure in cryogels and limited penetration of silver ions into macropores. The diffusion of silver ions to freeze-dried aerogels was better and further improved by vacuum treatment. Silver microcubes were produced in the pores, the weight fraction of silver in dry composites being typically several per cent, a maximum 13 wt%. The conductivity of the aerogels compressed to pellets depended on the processing and the highest value was 0.27 S cm^?1. The aerogels containing silver were characterized in detail with Raman spectroscopy.     

Reaction of poly(vinyl alcohol) with formaldehyde and polymer stereoregularity. Model compounds

The reaction of stereoisomers of pentane-2, 4-diol and heptane-2, 4, 6-triol with formaldehyde was investigated as a model for the formalization reaction of poly(vinyl alcohol) in order to determine effect of the stereochemical configuration of the polyol molecules on the reaction. The isotactic (meso) diol portion reacted with formaldehyde to give cis-formal several times faster than did the syndiotactic (dl) diol portion to give trans-formal at 30–80°C. In the reaction of heterotactic (meso-dl) triol which provides both the isotactic and syndiotactic diol portions in a molecule, the proportion of trans-formal in the total formal decreased as the reaction proceeded. This shows that the formation of cis-formal is also favored thermodynamically to a greater extent, and hence the intramolecular migration of trans-formal to cis-formal did occur during the reaction. The rates of hydrolysis of formals of the diols were compared with those of the triols in order to see the effect of a hydroxyl group adjacent to the formal ring on the reaction. No appreciable rate difference was observed between the dimer and trimer models both in cis- and trans- formals. Therefore it was deduced from these results that the increase of the rate of hydrolysis of poly(vinyl formal) with the increase of hydroxyl groups along the polymer chain is a characteristic of macromolecules that is not observed in the low molecular weight models.     

About the compatibility of polymer components in polymer complexes based on poly(acrylamide) and poly(vinyl alcohol)

The issue of applying the usual concepts of polymer compatibility to nonstoichiometric PVA/PAA mixtures of chemically complementary poly(vinyl alcohol) and poly(acrylamide), which form in water solution InterPC (intermolecular polymer complex) stabilyzed by H‐bonds, and PAA to PVA graft copolymers (PVA‐PAA_N) with different grafted chains number N, that are IntraPC (intramolecular polymer complexes) is discussed. PVA and PAA are compatible on molecular level. At the same time PVA/PAA mixture (50/50 W/W) is characterized by heterogeneous structure consists of InterPC with ϕ_char=9g_PVA/g_PAA and the excess of unconnected PAA. In the case of IntraPC, yet, only PVA‐PAA_N, where N=25, is characterized by a single glass transition temperature (Tg). At larger values of N separate PAA domains form giving rise to the corresponding Tg. These results are discussed in view of IntraPC structure peculiarities as a function of N investigated by IR spectroscopy.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). II. Polymerization of vinyl acetate in the presence of crosslinked poly(vinyl alcohol)

It is a common view that poly(vinyl acetate) has many branches at the acetyl side group, but that the corresponding poly(vinyl alcohol) has little branching. In order to study the branching in poly(vinyl acetate) and poly(vinyl alcohol) which is formed by chain transfer to polymer, the polymerization of ¹⁴C-labeled vinyl acetate in the presence of crosslinked poly(vinyl acetate), which was able to be decrosslinked to give soluble polymers, was investigated at 60°C and 0°C. This system made it possible to separate as well as to distinguish the graft polymer from the newly polymerized homopolymer. Furthermore, the degree of grafting onto the acetoxymethyl group and onto the main chain were estimated. It became clear that, in the polymerization of vinyl acetate, chain transfer to the polymer main chain takes place about 2.4 times as frequently at 60°C as that to the acetoxy group and about 4.8 times as frequently at 0°C.     

Structuring in poly(vinyl alcohol)-potassium iodide-iodine systems: Small-angle X-ray-scattering study

The sizes of supramolecular density fluctuations in gels and solid films obtained during formation of iodine inclusion compounds in an aqueous solution of poly(vinyl alcohol) are studied via small-angle X-ray scattering. It is shown that, in the case of iodine-containing poly(vinyl alcohol) samples prepared through drying of iodine-containing gels, several diffraction peaks that are related to the long-range order in the packing of structural fragments manifest themselves. The presence of the peaks indicates periodic density fluctuations in the packing of polymer chains with a period of 1.34 nm and a coherence length of 18.0 × 0.5 nm. The equidistance of peaks in units of scattering-vector modulus makes it possible to suggest the formation of layered systems, that is, systems with a single well-defined repeat period, during gel drying. Wet samples of the iodine-containing gel based on poly(vinyl alcohol) do not reveal any ordering. Heterogeneities in the structure of iodine-free gels may be regarded as voluminous structures with free cavities that survived after the extraction of iodine. These cavities in gels may be again filled with iodine or other complementary molecules.