Synthesis and characterization of thermo-responsive polymer based on carboxymethyl chitosan and its potential application in water-based drilling fluid

Novel graft copolymer (CSGO) based on carboxymethyl chitosan (CMCS) with thermosensitive poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) side chain was synthesized through free radical polymerization . The highly grafted POEGMA chains on the amino groups of CMCS brought CSGO with thermo-responsive behavior with lower critical solution temperature (LCST) confirmed by turbidimetric, temperature sweep rheology and DSC tests. The potential application of CSGO in sodium bentonite (Na-Bent) dispersion was investigated through rheological, filtration and inhibition measurements, and the interaction mechanism between CSGO and Na-Bent was evaluated. The results showed that CSGO improved the rheological and filtration properties of Na-Bent dispersion before and after aging within 120 ℃, ascribing to combined results of aggregation and film-forming effects on bentonite particles. More interestingly, the thermal-induced hydrophilic-to-hydrophobic transition caused the association of Na-Bent/CSGO flocs and the formation of partially desorbed polymer aggregates, contributing to the thermo-thickening rheology, lower fluid loss and better shale dispersion inhibition ability at elevated temperature. Additionally, CSGO could effectively inhibit clay hydration swelling, following a Fick diffusion mechanism of water through the formed CSGO film on the clay core. Besides forming a film to minimize osmotic swelling, the mixed intercalation of CSGO into clay interlayer with a monolayer CMCS backbone and/or a monolayer POEGMA side chain could inhibit crystalline swelling to a certain extent. Therefore, the superior properties and the attractive high-temperature behaviors of CSGO highlighted its potential application in water-based drilling fluid with recycling values during circulation.