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Title:Growth dynamics, charge density, and structure of polyamide thin-film composite membranes
Author(s):Matthews, Tamlin
Director of Research:Cahill, David G.
Doctoral Committee Chair(s):Cahill, David G.
Doctoral Committee Member(s):Mariñas, Benito J.; Leal, Cecilia; Braun, Paul V.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
reverse osmosis
Rutherford backscattering spectrometry
charge density
Abstract:The main objectives of this dissertation are: (1) developing a method to study the in situ growth dynamics of the interfacial polymerization of polyamide for thin-film composite reverse osmosis membranes using diffuse reflectance spectroscopy, Rutherford backscattering spectrometry (RBS), and atomic force microscopy (AFM); (2) quantifying the effects of trimesoyl choride (TMC) and m-phenylenediamine (MPD) monomer concentrations on the polyamide charge concentration in the near-surface region by X-ray photoelectron spectroscopy (XPS) and in the bulk by RBS; and (3) proposing a technique for measuring polyamide structures using wide-angle X-Ray scattering (WAXS). While developing these objectives, the goal is to characterize the polyamide layer formed on the polysulfone support, without physical or chemical removal, so that it is close to its native form, which has been used in industrial reverse osmosis applications. Growth dynamics by diffuse reflectance spectroscopy was developed for the polymerization of polyamide on porous polysulfone supports using varying concentrations of m-phenylenediamine (MPD) in water of 0.1– 100 g/L with a fixed concentration of trimesoyl chloride (TMC) in hexane of 1 g/L, and varying TMC concentrations of 0.1–10 g/L with a fixed MPD concentration of 20 g/L. A relationship was developed between diffuse reflectance and polyamide thickness. The diffuse reflectance data shows that ~50% of the polyamide thickness is produced in < 2 s for all TMC concentrations studied and for MPD concentrations >2 g/L. All studied concentrations of TMC at a fixed 20 g/L MPD concentration produced a polyamide thickness of ≈120 nm. Polyamide thickness increases from ≈10 to 110 nm with increasing concentration of MPD at 1 g/L TMC. The roughness measured with AFM increases with increasing MPD concentration but decreases with increasing TMC concentration. At MPD concentrations <0.5 g/L, polyamide does not grow on top of the polysulfone. The charge density of polyamide layers arises from unpolymerized free amine and carboxylic groups contributing positive and negative charges, respectively. The negative charge groups from carboxylic acid were tagged with Ag+. Using the same concentration ranges as the growth dynamics study, the charge densities were characterized in the bulk by RBS and in the near-surface by XPS. With increasing concentration of MPD, the charge density in the near-surface region is constant and ≈0.3 M, due to constant surface contact with the carboxylic acid containing TMC monomer. The charge density decreases from 0.3 M to 0.1 M in the polyamide bulk with increasing MPD concentration. TMC showed a 30× increase in charge density from 0.02 to 0.61 g/L in the bulk polyamide between 0.1 g/L TMC and 10 g/L TMC. The near-surface charge density also increases with increasing TMC concentration. Charge density was determined in the bulk RBS on membranes at varying pH between 3.5 and 10.5. These membranes show a good fit to a two pKa system, except the highest TMC concentration studied of 5 g/L which followed a one pKa system. Fitting the pH data using the pKa system shows that the total concentration of carboxylic acid groups decreases from 0.42 to 0.20 M with increasing MPD concentration. The decreasing carboxylic acid content is due to a higher concentration of MPD monomers. The total concentration of carboxylic acid groups increases with from 0.05 to 0.51 M with increasing TMC concentration. The concentration of TMC has a large effect on the charge density with the highest pH of 10.5 resulting in the highest measured charge density for each concentration increasing from 0.04 M to 0.55 M for 0.1 g/L to 5 g/L TMC. Grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) were successfully used to study the supported polysulfone ultrafiltration membrane and polyamide on polysulfone reverse osmosis membrane. In the GISAXS region we used the Guinier approximation in the very low-Q region of 0.001 Å^(-1)< Q < 0.006 Å^(-1) to determine a radius of gyration for the polymer chains of 83 Å for polysulfone and 80 Å for polyamide on polysulfone. We also observed linear regions between 0.011 Å^(-1) < Q < 0.016 Å^(-1) fit to I(Q) α Q^(-1.8), which corresponds to mass fractal clusters of the size d ~ 570 Å. Linear regions in GISAXS of I(Q) α Q^(-3.7) for polysulfone and Q^(-3.6) for polyamide on polysulfone were observed, which correspond to the Porod regime for smooth internal polymer interface sizes between 392.7 nm < d < 523.6 nm. The size of the interface is larger for higher incidence angles, which penetrate deeper into the porous structure of polysulfone. For loose, unsupported polyamide, a broad correlation peak centers at 1.34 Å^(-1), which corresponds to the average molecular spacing, or void size, of d = 4.7 Å.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Tamlin Matthews
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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