pH-/Temperature-Triggered Gel Transition of Hyperbranched PEI-g-PDMAEMA as a Dual-Responsive Inhibitor for Clay
1School of Energy and Resources, China University of Geosciences (Beijing), Beijing 100083, China.
Abstract:
To mitigate clay hydration and wellbore instability during deepwater drilling, a pH/temperature dual-responsive graft copolymer, hyperbranched polyethylenimine-g-poly(2-(dimethylamino)ethyl methacrylate) (HPEI-g-PDMAEMA), was designed and synthesized via free radical polymerization. Optimized synthesis at an HPEI/DMAEMA mass ratio of 1:2 with 2.4% AIBN at 70 °C for 10 h yielded a grafting ratio of 35.2% and a molecular weight of 84.3 kDa. The copolymer exhibits a tunable lower critical solution temperature (LCST) of approximately 48 °C at pH 8, decreasing with increasing pH due to tertiary amine deprotonation. Zeta potential measurements confirm that the polymer retains a positive charge (+5 mV at pH 8) under weakly alkaline conditions, enabling strong electrostatic anchoring onto negatively charged clay surfaces. Above the LCST, dynamic light scattering reveals a sharp increase in hydrodynamic diameter from ~30 nm to >200 nm, confirming a hydrophilic-to-hydrophobic transition of PDMAEMA segments that drives the formation of a hydrophobically associated gel barrier. This thermally triggered gelation is fully reversible, as evidenced by repeated heating-cooling cycles with almost complete transmittance recovery. The gel barrier drastically reduces water uptake, with inhibition performance against clay swelling at 60 °C being 18.5 percentage points higher than that at 25 °C. Hot-rolling tests demonstrate that with only 1.5 wt% inhibitor, shale recovery reaches 94.1% at 150 °C (8.8 percentage points higher than unmodified HPEI) and remains above 60% even in 20 wt% CaCl2 or MgCl2 brines, highlighting exceptional resistance to divalent cations. Water contact angle on treated clay surfaces increases from 18.5° to 52.6°, confirming effective surface hydrophobization. This work provides a molecular-level gel-engineering strategy where pH governs electrostatic anchoring and temperature triggers reversible hydrophobic gelation, enabling on-demand switching of clay wettability and hydration resistance under high-temperature, high-salinity conditions.
