Growth dynamics of interfacially polymerized polyamide layers by diffuse reflectance spectroscopy and Rutherford backscattering spectrometry

The synthesis of polyamide films used as active layers in reverse osmosis membranes was studied by in-situ diffuse reflectance spectroscopy, Rutherford backscattering spectrometry (RBS), and atomic force microscopy (AFM). Aromatic polyamide layers were formed by interfacial polymerization 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. Polyamide growth dynamics were monitored in real-time by diffuse optical reflectance at λ=329 nm. A relationship was developed between diffuse reflectance and polyamide thickness. The diffuse reflectance data show 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 increased from ≈10 to 110 nm with increasing concentration of MPD at 1 g/L TMC. The roughness measured by AFM increased with increasing MPD concentration but decreased with increasing TMC concentration. At MPD concentrations <0.5 g/L, polyamide does not grow on top of the polysulfone.

► Study of growth dynamics and morphology of polyamide reverse osmosis active layers.
► >50% of the polyamide thickness was produced in the first 10 s of polymerization.
► Thickness increased with increasing MPD and was ≈120 nm for all TMC concentrations.
► Roughness increased with increasing MPD and decreased with increasing TMC.
► At MPD concentrations <0.5 g/L, polyamide grows within the polysulfone pores.