Exploration of the Critical Structural Parameter of Polyamide Decisive for Permeability Based on SWRO Membrane Regulated by Environmental-Friendly Solvent Gamma-valerolactone

The intrinsic structure of the aromatic polyamide (APA) layer which is decisive for permeability and permselectivity of the state-of-art thin film composite (TFC) reverse osmosis (RO) membranes should be analyzed precisely in-depth. In this work, seawater reverse osmosis (SWRO) membranes with different APA microstructures are prepared by introducing the environmental-friendly solvent Gamma-valerolactone (GVL) into the organic phase to modulate the diffusion rate of m-phenylenediamine (MPD), thereby altering the packing density and morphology of APA aggregates. Small-angle X-ray scattering is utilized to obtain the fractal dimension of APA aggregates which quantitatively reflects the irregularity and complexity of the APA aggregates based on the principle of statistics. Confirmed by the cross-section images, fractal dimensions of APA aggregates transform from pseudo-1-dimension to pseudo-2-dimension structures due to the weakened folding property and the enhanced structural flexibility of the APA colloid particles, along with the looser stacking morphology of the APA aggregates. By correlation analysis, the fractal dimension possessed strong linear and positive correlation with water permeability coefficient A (R2=0.9945), together with strong linear and negative correlation with water-salt permselectivity A/B (R2=0.9896). As a demonstration, parameter of fractal dimension was supposed to be the critical structural parameter which played the most important role in water permeability of SWRO membranes, and the permeability and the permselectivity of SWRO membranes could only be simultaneously improved on the basis of the fractal dimension infinitely close to 1-dimension with the nodular and linear structures of the APA aggregated mostly maintained.