O. N. Tretinnikov

IR investigation of hydrogen bonds in weakly hydrated films of poly(vinyl alcohol)

A system of hydrogen bonds in weakly hydrated PVA films containing up to ≤8.5 wt % water is investigated via IR spectroscopy. It is shown that water molecules bind to only part of the hydroxyl groups of the polymer that are available for hydration and form the first hydrating layer. In a completely dehydrated film, practically every hydroxyl group of PVA forms hydrogen bonds with two other hydroxyl groups and serves as both a proton donor and a proton acceptor. In the hydrated film, one to three water molecules directly bind with one hydroxyl group of PVA.

Detection and quantitative determination of the crystalline phase in poly(vinyl alcohol) cryogels by ATR FTIR spectroscopy

The molecular structure of poly(vinyl alcohol) in its cryogels obtained via a single freeze-thaw cycle of aqueous solutions of the polymer is investigated by ATR FTIR spectroscopy. By means of Fourier deconvolution and spectral subtraction, methods, it is found that the spectra of cryogels contain a hidden crystallinity band at 1144 cm−1 due to poly(vinyl alcohol). For poly(vinyl alcohol) films crystallized at different temperatures, a quantitative relationship is established between the relative intensity of absorption at a frequency of 1144 cm−1 in the spectra of the polymer and its degree of crystallinity estimated via wide-angle X-ray diffraction. In terms of this relationship, the degree of crystallinity of poly(vinyl alcohol) in the cryogels is determined from their ATR FTIR spectra. This parameter is found to be 6, 10, and 14% for the cryogels with PVA concentrations of 9, 17, and 29%, respectively. The obtained data suggest that the formation of the cryogels is accompanied not only by the appearance of polymer crystallites but also by a change in the system of hydrogen bonds between the polymer and water.

Effect of Alkali Halide Salt Additives on the Structure of Poly(vinyl alcohol) Films Cast from Aqueous Solutions

The molecular structures of poly(vinyl alcohol) films cast from polymer aqueous solutions (1 mol/L) containing 0.1 mol/L of LiCl, NaF, NaCl, KCl, CsCl, KBr, and KI salts are studied via FTIR spectroscopy. The addition of any of these salts except LiCl leads to an increase in the degree of crystallinity of poly(vinyl alcohol) in a film by a factor of 1.3–1.6. In contrast, LiCl significantly decreases the crystallinity of the polymer. It is found that, in the IR spectra of the films containing salt additives, the position of the maximum in the band of the stretching vibrations of OH groups of the polymer is shifted relative to its position in the IR spectra of the films free of salt additives. The magnitude and direction of this band shift depend on the types (anion or cation) and radii of ions comprising salts. The observed effects are interpreted in terms of existing ideas on the interactions of salt ions with the OH groups of water and other hydroxyl-containing molecules.

Photoinduced grafting polymerization onto the surface with the use of radiation of high-power ultraviolet light-emitting diodes

The photoinduced grafting polymerization of acrylic acid onto the surface of a polypropylene film under the action of radiation with a wavelength of 365 nm and an intensity of 8–193 mW/cm2 emitted by high-power UV light-emitting diodes is studied. Grafting is performed from a thin layer of the aqueous solution of the monomer onto the surface containing a photoinitiator (benzophenone) deposited from a solution in a volatile solvent. The amount of graft polymer is determined with the use of ATR FTIR spectroscopy. At an intensity of UV radiation of 193 mW/cm2, the time of attainment of grafting-polymerization saturation is decreased by a factor of 6 relative to this time at an intensity of 8 mW/cm2, typical for emitters based on mercury lamps. The rate of grafting polymerization in the studied UV-radiation-intensity range grows in proportion to the square root of intensity.

An attenuation total reflectance fourier transform spectroscopic study of crystallinity in the bulk and on the surface of aqueous and aqueous salt cryogels of poly(vinyl alcohol)

The crystallinity of poly(vinyl alcohol) in the bulk and on the surface of cryogels of this polymer prepared in the form of thick disks from solutions in pure water and in water containing potassium chloride is studied via attenuation total reflectance FTIR spectroscopy. The crystallinity values of the upper and lower surfaces of the samples are found from the attenuation total reflectance IR spectra of these surfaces. To determine the crystallinity in the bulk, a sample is cut in half in the middle of its thickness to measure the spectrum of the cut surface. It is found that, in the bulk of a salt-free cryogel, the content of the crystalline phase of the polymer is significantly higher (by a factor of 1.5) than that on the surface. Addition of a salt increases the degree of crystallinity in the bulk and does not change the crystallinity of the surface. Cryogelation mechanisms explaining the increased crystallinity in the bulk of the cryogel and the crystallizing effect of the salt additive are proposed.

Effect of Halide Salts of Alkali Metals on Crystallinity of Poly(vinyl Alcohol) Cryogels

The degree of crystallinity of poly(vinyl alcohol) in cryogels obtained by single freezing at–20°С followed by thawing of 13% aqueous solutions of the polymer bearing dissolved NaCl, KCl, CsCl, KBr, and KI in the concentration of 0.7 mol/kg is determined by attenuated total reflectance Fourier transform IR spectroscopy. It is established that the addition of NaCl, KCl, and CsCl to the poly(vinyl alcohol) solution leads to a substantial increase (by 1.5–1.7 times) in the degree of crystallinity in the cryogel prepared from this solution. The effect of KCl, KBr, and KI on the degree of crystallinity strongly depends on the salt anion. The replacement of the Cl– anion by the larger Br– anion reduces dramatically the crystallizing effect of the salt, while the even larger I– anion, in contrast, reduces rather than increases the degree of crystallinity relative to that of the cryogel without a salt. The effect of the salts on the crystallinity of poly(vinyl alcohol) cryogels is explained by the simultaneous action of two processes. One of them facilitates crystallization and consists in the strengthening of dehydration of poly(vinyl alcohol) owing to competition between the polymer molecules and the salt ions for the liquid water molecules during its freezing. The other process hampers crystallization and is connected with a reduction in the water freezing point under action of the salt ions.