Direct Optical Visualization of Water Transport across Polymer Nano-films

Abstract

Gaining a detailed understanding of water transport behavior through ultra-thin polymer membranes is increasingly becoming necessary due to the recent interest in exploring applications such as water desalination using nanoporous membranes. Current techniques only measure bulk water transport rates and do not offer direct visualization of water transport which can provide insights into the microscopic mechanisms affecting bulk behavior such as the role of defects. We describe the use of a technique, referred here as Bright-Field Nanoscopy (BFN) to directly image the transport of water across thin polymer films using a regular bright-field microscope. The technique exploits the strong thickness dependent color response of an optical stack consisting of a thin (~25 nm) germanium film deposited over a gold substrate. Using this technique, we were able to observe the strong influence of the terminal layer and ambient conditions on the bulk water transport rates in thin (~ 20 nm) layer-by-layer deposited multilayer films of weak polyelectrolytes (PEMs).