Solid-State NMR Molecular Dynamics Characterization of aHighly Chlorine-Resistant Disulfonated Poly(arylene ether sulfone) Random Copolymer Blended with Poly(ethylene glycol) Oligomers for Reverse Osmosis Applications

Abstract

We have investigated the dynamics-transport correlations of a chlorine-resistant polymeric system designed as a next-generation reverse osmosis (RO) membrane material by solid-state NMR spectroscopy. A random disulfonated poly(arylene ether sulfone) copolymer in the potassium salt (-SO(3)(-)K(+)) form (BPS-20K) was blended with poly(ethylene glycol)s (PEGs) for improving water permeability. Blended BPS-20K/PEG membranes maintained the intrinsic chlorine-resistant property of BPS-20K, with a somewhat reduced salt rejection. The dynamic characteristics of BPS-20K/PEG blends studied by the spin-lattice relaxation time (T(1)) and rotating frame spin-lattice relaxation time (T(1)ρ) indicated correlations with the observed water uptake and permeability. (1)H T(1) measured on the polymer's aromatic phenylene rings and (1)H T(1)ρ measured on the oxyethylene (-CH(2)CH(2)O-) units of PEG were sensitive to the morphological changes, due to the blending of PEGs, induced in the mixed matrices. Membranes made of BPS-20K/PEG blends, with a lower molecular weight and higher amount of PEGs, that exhibited higher water permeability also provided shorter (1)H T(1) and T(1)ρ relaxation times. PEGs behaved as a plasticizer in the BPS-20K matrix, providing shorter (1)H T(1) times and therefore shorter motional correlation times in the nanosecond regime. (1)H T(1)ρ data have indicated the formation of networks among different polymeric chains via K(+)-oxyethylene ion-dipole interactions. Other properties that exhibit ad hoc correlations with the observed T(1) and T(1)ρ times include density, glass transition temperature, and salt rejection. Additionally, the ring flip motions measured on the hydrophobic phenylene rings did not reveal any correlations to the molecular weight and amount of PEGs blended, suggesting that the blending of PEG molecules modifies only the ionic domains of the BPS-20K polymer matrix.