Study of cryostructuration of polymer systems: XIII. Some characteristic features of the behaviour of macromolecular thiols in frozen aqueous solutions

Thiol-containing derivatives of linear poly(acrylamide) (SH-PAAm) were synthesized through the free-radical copolymerization of acrylamide with bis-N-acryloyl-cystamine followed by the reduction of SS-bridges in the formed gel and separation of the solubilized polymeric products from the low-molecular admixtures. The polymer thus prepared, was used to study the peculiarities of the processes of cryotropic gel formation of water-soluble macromolecular thiols. It was shown that cryogenic treatment (freezing-frozen storage-thawing) of SH-PAAm water solutions containing soluble oxidants results in the formation of polymeric cryogels cross-linked with intermolecular disulphide bonds. The dependence of the oxidation rate of thiol groups on the temperature of a frozen system showed an extreme character due to the competition between the accelerating and decelerating factors; the major accelerating factor being an increase in the concentration of solutes in the unfrozen microphase after the crystallization of most part of the solvent. The position of the extreme points on the temperature axis and the absolute magnitude of the reaction rate were controlled, along with other factors, by the freezing mode used, i.e. by the thermal history of the system during its transition to the frozen storage conditions. The implementation of the low-temperature quenching procedure to the freezing of a polymer solution altered the respective kinetic curves giving rise to the appearance of lag-periods, whose duration depended on the particular temperature of frozen storage. With the use of H-2 NMR spectroscopy; the amount of unfrozen liquid microphase in macrofrozen solid samples was evaluated, and the time-dependent distinctions in the characteristics of these unfrozen inclusions were demonstrated for samples frozen using different methods. (C) 1999 Elsevier Science Ltd. All rights reserved.