Controlling Nanoscale Pore Size and Wall Composition in Polycarbonate Membranes via Atomic Layer Deposition and Sequential Infiltration Synthesis: Implications for High Water Permeance

Polymer membranes have a wide variety of applications, ranging from water treatment to energy storage. Many of these applications require precise control over the membrane porosity and surface chemistry. In this study, we explore the modification of isoporous polycarbonate (PC) track-etched membranes (PCTEs) by atomic layer deposition (ALD) and sequential infiltration synthesis (SIS) to tune the pore size and the pore wall surface chemistry. We first performed a detailed study of Al2O3 ALD and SIS in PCTE using a variety of in situ and ex situ measurements. We discovered that short precursor exposure times are critical to achieve conformal Al2O3 ALD in the PCTE nanopores, while longer precursor exposure times resulted in Al2O3 SIS within the bulk of the PC leaving the membranes brittle. Next, we tuned the PCTE pore size via Al2O3 ALD and studied its effect on the water contact angle and the water permeance. We found that the membranes became more hydrophilic, and the permeance decreased with increasing ALD Al2O3 cycles. Finally, we studied the effect of hydrophilic (SnO2)/hydrophobic (In2O3) ALD metal oxide coatings on membrane properties. We found that the most hydrophilic SnO2 showed the highest water flux and the least hydrophilic In2O3 showed the lowest water flux through the PCTE membranes. Our results demonstrate that ALD is an effective method to tune the surface and transport properties of PCTE membranes, but care must be exercised to avoid SIS and bulk modification of the polymer.