A Study of Cryostructuring of Polymer Systems. 53. The “Abnormal” Character of Variations in the Properties of Chitosan-Containing Composite Poly(vinyl alcohol) Cryogels upon Repeated Freezing–Defrosting

Complex cryogels have been obtained by freezing mixed aqueous solutions of poly(vinyl alcohol) (PVA) and chitosan hydrochloride (ChHC) at –20°С followed by incubation of frozen samples for 12 h and defrosting by heating at a rate of 0.03°С/min. Treatment of the cryogels with ammonia vapor has transformed ChHC into chitosan base, which has the form of a dispersed filler incorporated into the continuous phase of the obtained composite cryogels. Variations in the elasticity modulus of these composites upon repeated freezing–defrosting have been revealed to have an “abnormal” character. Unlike the tendency toward a substantial increase in this parameter well-known for various unfilled PVA cryogels, the rigidity of the obtained chitosan-containing composites remains almost unchanged for, at least, four subsequent cycles of the cryogenic treatment. It has been shown that one of the main reasons for this is the presence of low-molecular-weight electrolytes in a composite cryogel—NH4OH, which results from neutralization of ChHC with ammonia and exhibits a salting-in effect, and NH4OH, which can deprotonate OH groups of PVA and chitosan, thereby leading to the appearance of negatively charged units in polymer chains and, as a consequence, to their repulsion. When water-soluble components, i.e., the sol fraction, are washed from such a chitosan-containing composite cryogel, its susceptibility to the strengthening effect of repeated freezing–defrosting cycles is restored. The influence of the repeated cryogenic treatment on the physicomechanical and thermomechanical properties of these and a number of model unfilled and composite cryogels and the morphology of their macropores has been studied.