Sono-mechanical activation-assisted proton-catalyzed self-polymerization of AMPS in water: Hydrogel formation and its application as a functional additive in ordinary Portland cement†

Abstract

A green, initiator-free method for polymerizing 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) was developed using low-frequency ultrasound (20 ± 3 kHz) in aqueous solution. Unlike conventional sonochemistry, where radical formation occurs via water sonolysis, the radical generation originates directly from the AMPS monomer, as verified by ESR, DPPH radical scavenging assays and NMR experiments. ESR experiments indicated the radical formation on AMPS at elevated temperature, which was lowered by cavitation-induced shear forces caused by low-frequency ultrasound. Water functions as a medium for proton-assisted activation, as no polymerization occurs in aprotic solvents, such as DMF and 1-decanol, as well as a hydrogen-bonded deep eutectic solvent (choline chloride: ethylene glycol in a 1:2 mol proportion). Comparative experiments showed that acrylamide, methacrylic acid, and (3-acrylamidopropyl)trimethylammonium chloride failed to polymerize ultrasonically, underscoring the unique reactivity of AMPS. AMPS also polymerized under microwave heating, albeit at a higher temperature, further confirming that cavitation-induced mechanical activation assists but does not solely govern radical formation. Mechanistic validation was obtained through model reactions involving acrylamide polymerization in the presence of methane sulfonic acid (1:1 mol ratio), and AMPS did not polymerize in its neutralized Na⁺ form. These results establish that the sulfonic acid group serves dual functions, as a proton donor and as a charge-balancing counterion, thereby enabling an acid-assisted, hydrogen-bond-directed radical polymerization pathway under sono-mechanical activation. The poly(AMPS) hydrogels were characterzied by FESEM, thermal analysis and swelling properties. The tunable swelling, governed by crosslinker density, was observed with water uptake decreasing from 1678 wt.% at 2 mol% MBA to 250 wt.% at 15 mol%. Lightly crosslinked gels exhibited self-healing behavior driven by reversible hydrogen bonding. GPC and viscosity analyses revealed simultaneous cavitation-induced depolymerization and radical polymerization, similar to ionizing radiation-based polymerization, though rapid crosslinking stabilized the hydrogel network. The incorporation of functional polymeric hydrogels into cementitious systems has emerged as an effective strategy for modifying hydration behavior, mechanical performance, and microstructural development. The cross-linked poly(AMPS) hydrogel was used in its calcium form (Ca-poly(AMPS)) as an additive in ordinary Portland cement, and its effect was evaluated through setting time, compressive strength, tensile strength, and heat of hydration.