Impact of polyelectrolyte molecular weight on PEI/PSS multilayer formation and forward osmosis membrane performance

Polyelectrolyte (PE) molecular weight (M_w) is an insufficiently understood parameter in layer-by-layer (LbL) assembly and the performance of polyelectrolyte multilayer (PEM) membranes. Here, we systematically examined the role of hyperbranched polyethyleneimine (PEI) M_w on PEM growth, PEM membrane characteristics and corresponding forward osmosis (FO) performance of these membranes. PEM membranes using low- (0.6 kDa), medium- (25 kDa), and high-M_w (750 kDa) PEI as polycation in combination with high-M_w poly(sodium 4-styrenesulfonate) (PSS) (500-700 kDa) as polyanion were fabricated applying LbL on hydrolyzed polyacrylonitrile (HPAN) supports. Reflectometry analysis showed that increasing PEI M_w increases the adsorbed PE mass due to enhanced loop-and-tail formation. Molecular weight cut-off (MWCO) analysis demonstrated that M_w asymmetry between low-M_W PEI and high-M_w PSS results in incomplete HPAN pore coverage resulting in negligible salt rejection. In contrast, medium- and high-M_w PEI/PSS systems achieved complete HPAN pore coverage, showing MWCO values in the nanofiltration range. Medium-M_w PEI/PSS membranes with 6 BLs had a negative surface charge, correlated to PSS-termination, while similar membranes formed with high-M_w PEI resulted in a positive surface charge (i.e. PEI-termination) due to enhanced interchain entanglement. Higher swelling of PEI-terminated high-M_w PEI/PSS membranes induced similar water permeabilities with medium-M_w PEI/PSS membranes, offsetting the greater high-M_w PEI/PSS thickness. FO experiments confirmed that the exclusion mechanism of all PEI/PSS membranes is based on size when FO is performed with Pluronic® L-35 as a draw solution. Accordingly, medium- and high-M_w PEI/PSS membranes (complete pore coverage) exhibited improved water flux and reduced reverse solute flux compared to low-M_w systems.