Exploring the Physicochemical and Rheological Properties of Acrylic Acid - co - 2-Acrylamido-2-methylpropane Sulfonic Acid Copolymers Synthesized in Supercritical Carbon Dioxide

RAMSES SEGUNDO MELEANS; FACUNDO MATTEA; JUAN PADRÓ; MIRIAM C. STRUMIA; SÉVERINE CAMY; MATHIAS DESTARAC; JUAN M. MILANESIO

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Conferencia; VI Iberoamerican Conference on Supercritical Fluids; 2023

Acrylic acid (AAc) and 2-acrylamide-2-methylpropane sulfonic acid (AMPS) are vinylic monomers that can be polymerized using solution or precipitation techniques, utilizing either water or organic solvents. Depending on whether water or an organic solvent is used, suitable thermal initiators like azo compounds, peroxides, and persulfates, can be employed. After the reaction, precipitation and drying processes must be carried out to obtain a solvent-free polymer [1]. In precipitation polymerization, azo compounds are the preferred thermal initiators. At the beginning of the reaction, a homogeneous mixture of solvent, monomers, and initiators is prepared. As the reaction proceeds, the polymer precipitates, resulting in a polymerrich phase, that can be liquid or solid. An energy-intensive drying process is required at the end of the reaction. As an alternative approach, compressed solvents like carbon dioxide (CO2) can be used as solvents for precipitation polymerization [1]. Due to its adjustable density and solvent power, supercritical CO2 can effectively solubilize AAc and other vinylic monomers. An advantage of using CO2 lies in its capacity to yield high-purity end products via a straightforward depressurization of the system, avoiding precipitation or drying processes. Additionally, CO2 can be recycled by recompression after the reaction, thereby minimizing the presence of toxic organic solvents that could have an impact in human health [2]. CO2 is an inert solvent, showing no radical transfer reactions, making it a suitable choice for radical precipitation polymerization. Previous studies have proved that CO2 can be used in the synthesis of poly(acrylic acid) yielding higher average molecular weights compared to polymers synthesized in water or organic solvents [3]. This study focuses on the precipitation copolymerization of AAc with AMPS in supercritical CO2, with a reactive mixture ranging from 10 to 50 wt.% AMPS. The incorporation of AMPS into the polymer chain was investigated by means of Fourier Transform Infrared (FTIR) Spectroscopy and Proton Nuclear Magnetic Resonance (1H-NMR), the average molecular weight distributions and viscosity of the resulting polymeric solutions in water were examined by using Size Exclusion Chromatography (SEC) and rheology measurements, respectively. Additionally, the effect of variations of pH and salinity over the viscosity of the polymer solution was analyzed. The results showed the incorporation of AMPS and significant viscosity increases in water solution, particularly with AMPS contents up to 30 wt.%. Furthermore, the polymers displayed resistance to solution salinity as evidenced by the slight variation in the viscosity with increasing sodium chloride (NaCl), and calcium chloride (CaCl2) salt concentration.