ИСТИНА

Development of polymeric materials employing reversible "dynamic" bonds is currently attracting great attention, since they can be designed to adapt to external stimuli in a controlled manner, reversibly changing their shape, mechanical properties, etc. Strong responsiveness due to breaking / restoration of dynamic bonds makes such materials promising for practical applications in various fields. For instance, borate cross-linked polyol hydrogels are used in hydraulic fracturing technology in oil production. In a hydraulic fracturing operation, high viscoelasticity of the gel at rest is crucial to suspend proppant, but reversible reduction of viscoelasticity during gel injection into the formation is necessary. In order to employ such gels in high temperature reservoirs, the strength of cross-links should be increased, since single borate ion cross-links are too weak due to their low energy leading to insufficient rheological properties. An approach of this work to increase the mechanical strength of hydrogels while maintaining their dynamic nature and self-repair ability consists in the use of both single and multifunctional dynamic cross-links for polyvinyl alcohol (PVA) [1]. As single cross-links, we used borate ions reacting with hydroxyl groups of the polymer and forming dynamic covalent bonds. As multifunctional cross-links, we obtained 25-nm silica nanoparticles (NPs) modified on the surface by phenylboronic acid, which also reacts with hydroxyl groups of the polymer, similar to borate ions. The modification was made by a two-step process: first, silica NPs were reacted with (3-glycidyloxypropyl)trimethoxysilane, so that epoxy groups were introduced at the surface; second, epoxy-modified silica was reacted with 3-aminophenyl boronic acid. The number of phenylboronate moieties on the surface of one nanoparticle was varied from 5 to 40. Due to the dynamic nature of the cross-links of both types, the gels retained their viscoelastic behavior and reversibility after flow at various concentrations of borate (0-30 per macromolecule) and modified silica NPs (0-4 per macromolecule). At the same time, the introduction of multifunctional NP cross-links resulted in a significant increase of viscoelasticity. Cross-linking with nanoparticles is more effective than single cross-linking with borate ions, since they are able to cross-link several polymer chains simultaneously. It is shown that an optimal content is 1-2 NPs per macromolecule, and one NP can be modified by only a few (ca. 10) phenylboronic groups to be an effective cross-linker. Dual dynamically cross-linked gels obtained in this work are promising for applications as fracturing fluids with enhanced properties in oil recovery. Acknowledgment: the work was financially supported by the Russian Science Foundation (project 19-73-20133).