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

Abstract

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.