Polyamide Membrane Surface Research Articles

Preparation of surface-modified stimuli-responsive polymeric membranes by plasma and ultraviolet grafting methods and their riboflavin permeation

Acrylic monomers were grafted on the surface of commercial polyamide membranes using plasma polymerization and ultraviolet irradiation techniques. Graft reaction was confirmed by X-ray photoelectron spectra. Grafting yield increased with plasma and ultraviolet exposure time. Permeation of riboflavin through graft membranes was investigated. Poly(acrylic acid- g-amide) membranes showed a decrease in permeability of riboflavin at pH 4–5, while poly(methacrylic acid- g-amide) membranes exhibited decreased permeability at pH 6–7 regardless of the grafting methods. N-isopropylacrylamide-grafted polyamide membrane showed a temperature-dependent permeation of riboflavin.

The Origin of Large Pores on Aromatic Polyamide Membrane Surfaces

Linear polymer solutions are represented as suspensions of macromolecule spheres. Macromolecules concentrate at the interface during the membrane-making procedure. Permanent contacts are made between adjacent macromolecules. The macromolecules coalesce with each other to some extent, which causes a reduction in the interstitial void areas. The magnitude of an interstitial void is dependent on the size, packing arrangement, and degree of coalescence of the surface macromolecules. During membrane use, permeation occurs only through the interstitial voids in the skin layer. The interstitial void model is applied to some aromatic polyamide solutions and their resultant reverse osmosis/ultrafiltration membranes. The skin layer of such a membrane is composed of a monolayer of predominantly close-packed solution macromolecules that have partially coalesced with each other. Larger pores in the skin monolayer occur if interstitial voids are formed by surrounding by four or more macromolecule spheres. This paper compares casting composition and structure of the larger pores, according to the interstitial void model. The origins and magnitudes of the large pores are determined to be dependent on the casting solution composition and structure and on the membrane-making procedure. The practical utility of the interstitial void model is for design purposes. Membrane performance can be related to the precursor solution composition and/or the membrane-making procedure.

Morphology of the rejecting surface of aromatic polyamide membranes for desalination

In highly productive reverse osmosis membranes heterogeneous structures of high specific volume are developed in the rejecting layer by two quite different methods, both of which appear to involve fractal processes. In the asymmetric system the membrane specific volume is increased by adding to the polyamide solution a molecularly compatible component that is readily hydrolyzed and extracted from the concentrated surface layer, itself probably quenched without phase inversion, to develop a volume in substantial excess over the percolation limit. Available information about the factors and functionality of the condensation reaction used to form the thin-film composite structures indicates a non-equilibrium disordered aggregation of dense, but functional, polyamide particles to create a thin layer where every colloidal particle is surrounded by passages for transport. While percolation theory suggests that the asymmetric membrane is formed by the disordered aggregation of open spaces, the thin-film composite seems formed by a similar random and non-equilibrium aggregation of dense, hyperbranched polyamide particles.

EFFECT OF NONSOLVENT ADDITIVES ON THE PORE SIZE AND THE PORE SIZE DISTRIBUTION OF AROMATIC POLYAMIDE RO MEMBRANES

Abstract In this paper discussions are made on the effect of nonsolvent swelling agents on the average pore size and pore size distributions at the surface of polyamide membranes which result from casting solutions involving above nonsolvent swelling agents. The size of the polymer aggregate in the film casting solution and the size of polymer network pores are correlated to physicochemical data of ions which constitute the electrolytes used as nonsolvent swelling agents. As such ionic properties the charge density and the free energy of transition of ions from polyamide phase to water phase were considered. The validity of the correlation is limited in a range of casting solution composition where the polymer concentration in the casting solution is close to the limiting concentration of polymer at the phase boundary and the molar ratio of the nonsolvent swelling agent to the amide group involved in the polyamide polymer is equal to or slightly more than 0.7.

Diffusion of inorganic acids in polyamide films

The comparative characteristics of the diffusion of aqueous solutions of nitric and sulfuric acid into polyamide films have been presented. Diffusive transfer has been characterized by a stationary diffusion coefficient and by a retardation time. The influence of membrane thickness on the non-stationary diffusion parameters has been shown. This effect can be related to the relaxation process taking place on the surface of polyamide membrane. The process leads to a change of limiting boundary diffusion conditions with time. A relation between stationary and non-stationary diffusion coefficients has been demonstrated.

Reverse osmosis separations of some organic and inorganic solutes in aqueous solutions using aromatic polyamide membranes

AbstractThe reverse osmosis separations of some ethers, ketones, aldehydes, monocarboxylic acids, and inorganic salts in single‐solute (55–454 ppm) aqueous solution systems using aromatic polyamide membranes have been studied at 250 psig. It was found that reverse osmosis separation was essentially a function of steric parameter for ethers, and of both polar and steric parameters for ketones, aldehydes, and alcohols. Solute separations for monocarboxylic acids passed through a minimum at a pKa value of ∼4.8. The values for the free‐energy parameter for Li+, Na+, K+, Rb+, Cs+, and F−, Cl−, Br−, I−, and IO3− ions have been calculated for the polyamide membranes used. The above values for cations are negative and those for anions are positive, suggesting that the polyamide membrane surface behaves as if it is positively charged. Further, the data show that the polyamide membrane material is only about 40% as polar as that of the cellulose acetate membrane material studied earlier.