Termodinámica

 

Transición y estabilidad de fase de soluciones poliméricas en CO₂ supercrítico por turbidimetría

 

Phase transition and stability of polymers solutions in supercritical CO₂ by turbidimetry

 

C. H. Ortiz–Estrada¹*, J. G. Santoyo–Arreola¹, G. Luna–Bárcenas², I. C. Sanchez³ y R. C. Vásquez–Medrano¹

 

¹ Departamento de Ingeniería y Ciencias Químicas, Universidad Iberoamericana, Prolongación Paseo de la Reforma #880, Lomas de Santa Fe, México, D. F. 01219 México. * Autor para la correspondencia: E–mail: ciro.ortiz@uia.mx Tel. (55)–59504074. Fax. (55)–59504279

² Laboratorio de Investigación en Materiales, CINVESTAV Unidad Querétaro, Querétaro, Querétaro 76230 México.

³ Chemical Engineering Department, The University of Texas at Austin, Austin TX 78712, USA.

 

Recibido 11 de Septiembre 2007
Aceptado 22 de Noviembre 2007

 

Resumen

El comportamiento de fase de una solución polimérica en CO₂ supercrítico es un problema fundamental en el proceso de formación de partículas durante la nucleación en solución, donde el CO₂ es utilizado ya sea como solvente o antisolvente. En este trabajo, se determinó mediante turbidimetría, la transición y estabilidad de fase aplicando la medición del punto de nube y el monitoreo del tamaño de partícula durante la separación de fase inducida por presión, a partir de una solución homogénea en casos donde el CO₂ es solvente (sistema binario) o antisolvente (sistema ternario). Los sistemas estudiados fueron: poli(fluoroctil metacrilato) (PFOMA)–CO₂ y poliestireno (PS)–Tetrahidrofurano (THF)–CO₂, para diferentes condiciones de concentración del polímero, temperatura y presión en la región supercrítica del CO₂. Los resultados indican que el sistema PFOMA–CO₂ presenta una comportamiento LCST común para polímeros fluorados solubles en CO₂; para PS–THF–CO₂ el comportamiento UCST–LCST son observados que son altamente dependientes de la relación de concentración THF/CO₂.

El monitoreo del tamaño de partícula en solución, muestra las etapas de transición y estabilidad de fase de estable–metaestable–inestable, observándose la región espinodal. El fenómeno de nucleación aparece de manera importante una vez que la solución cruza la curva espinodal hacia la zona inestable, donde el crecimiento de las partículas es exponencial favoreciendo por lo tanto el mecanismo de coagulación, inducido por la supersaturación de la solución.

Palabras clave: dióxido de carbono supercrítico, estabilidad de fase, soluciones poliméricas, turbidimetría.

 

Abstract

The phase behavior of polymer solutions in supercritical CO₂ is a fundamental problem in particle formation process during the nucleation in solution, where the CO₂ is either used like as solvent or antisolvent. In this work, it was determined by turbidimetry, the phase transition and stability by cloud point measurements. Particle size was monitored during the pressure induced phase separation from a homogeneous solution in cases where the CO₂ is solvent (binary system) or antisolvent (ternary system). The studied systems were poly(fluorooctyl methyacrylate) (PFOMA)–CO₂ and polystyrene (PS)–tetrahydrofuran (THF)–CO₂, for different polymer concentrations, temperatures and pressures in the supercritical region of CO₂. The results indicate that the system PFOMA–CO₂ exhibits an LCST and for PS–THF–CO₂ a combination of UCST–LCST phase behaviours are observed that is highly dependent on the THF–to–CO₂ concentration ratio. By monitoring particle size in solution the transition stages and phase stability of stable–metastable–unstable states, which are associated with the spinodal region, are determined. The nucleation phenomenon appears once the solution crosses the spinodal curve toward the unstable area. In this region, the growth of the particles is exponential favouring coagulation that ultimately induces supersaturation.

Keywords: phase stability, polymers solution, turbidimetry, supercritical carbon dioxide.

 

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