A study on the relationship between preparation condition and properties/performance of polyamide TFC membrane by IR, DSC, TGA, and SEM techniques

Abstract

High-performance thin-film composite polyamide membranes for inorganic solute separation were prepared by the interfacial polymerization of trimesoyl chloride (TMC) with diethylenetriamine, 1,3-cyclohexanebis(methylamine), 2,3-diaminopyridine (DAP), m-phenylenediamine (MPD), piperazine (PIP) or a mixture of MPD and PIP/DAP, on the surface of a reinforced microporous polyethersulfone (PES) membrane support. The polyamide skin layers were isolated by dissolving the PES support in dichloromethane and the characteristics of the free skin layers were studied by infrared spectroscopy (IR), differential scanning calorimetry, thermogravimetric analysis (TGA), and scanning electron microscope (SEM) techniques. It was attempted to correlate the observed flux and rejection properties of the membranes with their preparation conditions such as reactant concentrations, reaction time, curing temperature etc. The glass transition temperatures (Tgs) of PES and polyamide blends are dependent on the concentration of the monomers (TMC/diamine) that are used for polyamide skin formation. An increasing trend in Tg values was observed with an increase in the concentration of TMC/diamine. The TGA thermograms show that the initial decomposition temperatures of the PES and polyamide blends are in the range of 330–400°C, which is about 20–90°C lower than that of PES (420°C). SEM images display that the top polyamide skin surface presents a honeycomb-like structure and the cross-section image of the membranes clearly displays the presence of a nodular structure that arises from the dense polyamide skin. In between the nodules, there are pore channels that traverse up to the polyester support. The stability of a polyamide skin layer strongly depends on the preparation conditions such as concentrations of the reactants and skin layer formation conditions such as the reaction time. A higher concentration of the acid chloride (TMC > 0.5%) results in the formation of a low molecular weight amide–acid [HOOC-(Ar-COHN-X-NHOC-Ar)n-COOH] skin layer, whereas a higher concentration of amine (Amine > 4%) produces a low molecular weight amide–amine [H2N-(X-NHOC-Ar-COHN-X)n-NH2] skin layer, as observed by the IR spectra, which are unstable for performance evaluation. The performance, i.e. rejection and water flux, of the composite membranes is strongly dependent on the chemical nature of the reactants used for the formation of polyamide skin, and on the skin layer preparation conditions such as concentrations of the reactants, curing time, and curing temperature. The relationship between the preparation conditions of polyamide skin and performance of the membranes was measured using 2,500 ppm feed solutions of NaCl and Na2SO4 at an operating pressure of 10 kg/cm2. Membranes having a wide range of performances ranging from reverse osmosis to nanofiltration to ultrafiltration can be obtained due to the synergistic effects by utilization of aromatic and aliphatic amine mixtures for the formation of polyamide skin layer.