Introduction

The use of permafrost is considered as one option for C02 geological storage. From this view point two issues arise:

- depending on injection depth and site conditions the C02 may occur as individual and mixed gas hydrates, mineral and organo-mineral complexes (clathrates and carbonates) or in other forms;

- the long-term existence of underground C02 storage implies permafrost conservation and consolidation.

The present paper discusses cryogels - a promising material for underground works in permafrost regions. These are polymer gels generated due to freezing and subsequent thawing of aqueous polymer solutions. The more "freezing - thawing" cycles the cryogel is subjected to the better its mechanical properties: strength and elasticity increase and formation bond intensifies (Lozinsky, 1998, 2002). Cryogels, i.e. cryotropic gels, are

peculiar polymer solutions, with a higher critical solution temperature (Rebinder, 1979; Papkov, 1974). The spatial pattern of cryogels is formed during phase transformation and is stabilized with intermolecular bonds of different nature: covalent, coordination and ionic-electrostatic. This variable bond type generates the cryogel manifold and allows their properties to be varied significantly (Lozinsky, 1998, 2002; Sergeev et al., 1978).

Cryogels based on polyvinyl alcohol (PVA) are of great interest both scientifically and from an applied viewpoint. They are widely used in biotechnology, medicine, food industry and others (Sergeev et al., 1978; Lozinsky, 1998). In many respects this interest results from the excellent mechanical, diffusional and thermophysical properties of PVA cryogels, polymer availability, its non-toxicity and biocompatibility, as well as its relatively simple gelation technique involving freeze-thaw cycling. It is possible to regulate physico-chemical properties of cryogels within wide limits (Sergeev et al., 1978; Lozinsky, 1998) by varying polymer characteristics (molecular mass, chain-length distribution, content of residual o-acyl groups, tacticity and PVA concentration in the system), solvent composition, additive nature and cryogenic process conditions (temperature and freezing duration, thawing rate, number of over freezing cycles, etc.). In practice water is used as the basic solvent for PVA. PVA is resistant to oils and fats, aliphatic and aromatic hydrocarbons, weak acids and alkalis. In addition PVA exhibits universal adhesive and bonding properties and is also atmosphere and oxidation-resistant.

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