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-20A degrees D 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.