Orientation of Thin Polyamide Layer-by-Layer Films on Non-Porous Substrates

The orientation of polyamide thin films on nonporous substrates was investigated using Fourier transform infrared (FTIR) spectroscopy and corroborated by near-edge X-ray absorption fine structure (NEXAFS) measurements. Polyamide films composed of m-phenylene diamine and trimesoyl chloride were synthesized via a layer-by-layer deposition process on both gold and silicon substrates, where the number of sequential monomer deposition steps (cycles) determines the overall polyamide film thickness. These films represent model materials for metrology development that can be applied to understand the selective polyamide layers in reverse osmosis (RO) membranes. Using the fingerprint region in the infrared spectrum for the m-phenylene diamine-trimesoyl chloride (MPD-TMC) system, peaks were identified that could be used as diagnostic modes for molecular orientation analysis. At extremely low cycle numbers, FTIR measurements suggest that the MPD-derived aromatic rings exhibited a tilt angle of similar to 6 degrees from the surface parallel, while the carbonyl group of the amide was similarly oriented with an average tilt angle of similar to 17 degrees The FTIR insights are in agreement with NEXAFS results, which indicate that the aromatic rings in the first-half layer-by-layer cycle are oriented parallel to the surface. Both FTIR and NEXAFS measurements further show high anisotropy in the materials at low cycle numbers. As the films approach 60 cycles (similar to 13 nm films), the films become isotropic and no orientation was detected.