Phase Boundaries and Copolymerization of Acrylic Acid + Butyl Methacrylate + Carbon Dioxide under High-Pressure Single- Fluid Initiation

RAMSES SEGUNDO MELEANS; JOANA E TASQUE; MIRIAM STRUMIA; FACUNDO MATTEA; JUAN M. GIUSSI; JUAN M. MILANESIO

Los cocos

Conferencia; VI Iberoamerican Conference on Supercritical Fluids; 2023

The use of pressurized fluids in precipitation polymerization offers the advantage of minimizing, or even eliminating, the need for a separation process, yielding almost pure polymers by a simple depressurization of the system [1]. Supercritical carbon dioxide (CO2) is the most used industrial pressurized fluid due to its low critical temperature (Tc = 31.1 °C) [2]. Pressurized CO2 can dissolve various vinyl monomers like acrylic acid (AAc) and methacrylic acid esters. For the proper design of supercritical polymerization reactors, it is crucial to consider the phase behavior of the initial reactive mixture. In this context, one of the main challenges is to experimentally determine the high-pressure phase scenario for the initial reactive system, to design and define the process variables that ensure the initiation of the polymerization in a single-fluid phase [3]. In this study, a variable volume high-pressure cell was employed to experimentally determine the bubble pressures of the ternary mixture CO2 + AAc + butyl methacrylate (BMA) with different BMA contents and temperatures, and with that information determine the phase boundaries of the ternary system. Then, the synthesis of acrylic acid-co-butyl methacrylate copolymers with different BMA concentrations were carried out in CO2. BMA acts as a hydrophobic associative group in the polymer chain, potentially enhancing inter- and intra- chain interactions and, consequently, changing the viscosity of aqueous polymer solutions, compared to polyacrylic acid solutions. In this study, the viscosities of the different solutions were measured using a rotational rheometer in buffered solutions at pH = 7 and in water at pH = 7 by adjusting the pH with sodium hydroxide. The incorporation of BMA in the polymer chain was analyzed using spectroscopy techniques like FTIR and 1H-NMR. At temperatures ranging from 60 to 100 oC, a maximum pressure of 140 bar was necessary to dissolve both comonomers in pressurized CO2 and initiate the polymerization in a single fluid phase. The polymers obtained in the reaction were white dry powders, and the results revealed that the hydrophobic monomer was effectively incorporated into the polymer chain. The obtained results also showed that increasing the BMA content from 4% to 8% w/w does not affect the solubility of the macromolecules in the solutions at pH = 7. However, an increase in viscosity was observed due to the presence of hydrophobic association effects as BMA incorporation increased.