A 3-D Printed Scaffold for Improving Coating Efficiency in Electrospun Membranes for Membrane Distillation Applications

This work focuses on the optimization of an existing initiated chemical vapor deposition (iCVD) process used for coating electrospun nanofiber membranes. Many of the more easily spinnable fibers are inherently hydrophilic, making them unsuitable for membrane distillation experiments. In MD, only water vapor is allowed to pass through a semi-permeable membrane while liquid water and salts are rejected. Chemical vapor deposition can be used to render these membranes hydrophobic and suitable for MD. Previous methods for coating these membranes involved securing the membrane to be coated parallel to the direction of the reactant gas flow, relying on the diffusion of reactants into the membrane to achieve coating. After coating of one side, the membrane would be flipped to repeat the procedure for the other side, demanding lengthy deposition times to ensure complete coverage. To improve this process, both in terms of coating time and reactant usage, a 3-D printed scaffold was designed that supports the membrane such that it is now normal to the gas flow, significantly increasing the reactant flux passing through the membrane itself, and taking advantage of the membraneâ??s permeability. This configuration allows polymerization on the forward facing surface of membrane, while also promoting penetration through the pores to polymerize on the rear face as well. After several design iterations, the scaffold was successfully able to cut the deposition time by 75% without sacrificing the integrity of the coating. The time required to achieve sufficient coatingâ??ignoring the time lost shutting down the process to flip the couponâ??has been reduced from 100 minutes per side to 50 minutes in total without ever stopping the coating process. Successful membrane distillation experiments have shown that systemic coating occurred throughout the nanofibers. Uncoated PAN membranes will wet out during MD experiments resulting in salt crossover. The coated membranes exhibited 100% salt rejection indicating complete coating of the hydrophobic polymer during the CVD experiment.