Gabriel Tkacik

Thermodynamic analysis of a membrane-forming system water/N-methyl-2-pyrrolidone/polyethersulfone

Abstract Thermodynamic analysis of a three-component, membrane-forming system: water (H 2 O; 1), N -methylpyrrolidone (NMP; 2), polyethersulfone (PES; 3) is described. The polymer used in the study was characterized by size exclusion chromatography, NMR spectroscopy and differential scanning calorimetry. The analysis indicated a linear, amorphous polymer with M w = 46400 g/mol and M n = 24700 g/mol. Information on thermodynamics of mixing of water and NMP was compiled from selected literature sources. Negative values of excess volume, enthalpy and entropy of mixing indicate solution nonideality, assumed to be due to formation of hydrogen-bonded complexes with favored stoichiometry NMP(H 2 O) 2 . The excess Gibbs free energy and the Flory-Huggins interaction parameter χ 12 were found to be positive, with concentration-dependent χ 12 in the range of 0.79-1.25. Light scattering and refractive index measurements were carried out with PES/NMP solutions. Analysis of scattered light intensities in the concentration range 5-30 weight percent polymer yielded a concentration-dependent χ 23 in the range of 0.36-0.55. For the polymer-nonsolvent interaction, only indirect estimates of χ 13 can be made from light scattering measurements with ternary H 2 O/NMP/PES solutions and from literature data on water sorption by PES. A value of χ 13 around 1.5 seems to be physically realistic. A theoretical phase diagram predicted from the Flory-Huggins theory with χ 12 = 1.0, χ 23 = 0.5, χ 13 = 1.5 agrees very well with measured ternary cloud point curves and tie-lines. Surface of Gibbs free energy computed for the ternary system as a result of this analysis will be used in studies of component mobilities, phase transformation kinetics and mechanism as well as other phenomena involved in membrane formation.

Characterization of porous sublayers in uf membranes by thermoporometry

Abstract Three ultrafiltration membranes, characterized previously by flux/rejection measurements, scanning electron microscopy and nitrogen sorption/desorption, were studied by thermoporometry (low-speed differential scanning calorimetry). Scanning speed effects were first investigated in the range 15 K/hr to 0.5 K/hr. Thermograms were then obtained at cooling rate of 1 K/hr and pore volume distribution curves were calculated. Obtained “pore” radii were found to be widely distributed between 10 and 70 nm with detectable maxima around 15-35 nm. These values are roughly one order of magnitude higher than those previously estimated for radii of functional pores (UF, SEM) in the same membranes. Like nitrogen sorption/desorption, thermoporometry seems to be probing void space in porous sublayers below membrane skins.

Component mobility analysis in the membrane-forming system water/n-methyl-2-pyrrolidone/polyethersulfone

Abstract Component mobilities (diffusion coefficients) were evaluated in the membrane-forming system water-N-methyl-2-pyrrolidone-polyethersulfone (H2O(1)-NMP(2)-PES(3)). Two types of solvent-nonsolvent binary diffusion coefficients were obtained. In the 1-2 binary mixture D12=5-7X10−6 cm2/sec. In agreement with observations of other authors, macromolecular diffusion coefficients measured by quasi-elastic light scattering (QELS) were found to exhibit anisotropy in semidilute and concentrated solutions. In the 2-3 binary, fast mode coefficients were: DC23=3-7X10−7 cm2/sec and the slow mode coefficients were: DCM23=3X10−9-8X10−12 cm2/sec. Fast and slow QELS modes were identified with cooperative (C) and center-of-mass (CM) motions of polymer chains, respectively. Friction factors were calculated for both modes. Concentration dependence of friction factors was found to be strikingly different for the two modes. In ternary mixtures, the fast-mode mobility was found to decrease with nonsolvent concentration, while the slow mode (where detectable) was unaffected by addition of nonsolvent. Mobility of PES in other solvent and nonsolvent systems was also investigated.