Crosslinking, polar group quantification, and dynamics of polyamide reverse osmosis membranes via NMR methods*

Establishing clear relationships between the chemical structure, water/ion transport, and water filtration performance in the salt selective polyamide (PA) layer of thin film composite (TFC) reverse osmosis (RO) is important for designing water filtration membranes, yet these relationships are difficult to establish. For instance, previous Ag⁺ probe Rutherford backscattering experiments by Coronell (https://doi.org/10.1021/es8002712) showed commercial polyamide membranes exhibit negligible amine populations, whereas ¹⁵N {¹H} NMR work by Qiu et al (https://doi.org/10.1016/j.memsci.2019.03.037) showed amines comprise ~10% of the nitrogen in a similar membrane.

 

In this talk, I will demonstrate a chemical separation method for purifying the PA from TFC membranes, the purity of which is quantified via ¹³C CPMAS NMR. Using this process, we report on four trimesoyl chloride (TMC)/isophthaloyl chloride (IPC)/metaphenylene diamine (MPD)-based TFC membranes in which the crosslink density was intentionally reduced by replacing trifunctional crosslinking TMC monomers with their linear IPC difunctional analog. The degree of crosslinking and fractions of unreacted carboxylic acid and amine polar groups are followed using ¹³C CPMAS NMR and FTIR. While the NMR shows a 2x decrease in crosslinking that causes a 30 % increase in salt passage, the addition of the difunctional analog leads to increased polar amine groups that reduce water permeance due to tighter binding of water in the membrane. Our results demonstrate that both crosslink density and polarity are important design criteria in RO membranes, and that ¹³C CPMAS is a powerful method for quantitatively monitoring such values. Furthermore, I will show results from ¹H field cycling and diffusometry NMR experiments in water-swollen samples from which we glean polymer segmental dynamics and water transport.