Enhancing iCVD Modification of Electrospun Membranes for Membrane Distillation Using a 3D Printed Scaffold

Abstract

Electrospun membranes have shown promise for use in membrane distillation (MD) as they exhibit exceptionally low vapor transport. Their high porosity coupled with the occasional large pore can make them prone to wetting. In this work, initiated chemical vapor deposition (iCVD) is used to modify for electrospun membranes with increased hydrophobicity of the fiber network. To demonstrate conformal coating, we demonstrate the approach on intrinsically hydrophilic electrospun fibers and render the fibers suitable for MD. We enable conformal coating using a unique coating procedure, which provides convective flow of deposited polymers during iCVD. This is made possible by using a 3D printed scaffold, which changed the orientation of the membrane during the coating process. The new coating orientation allows both sides as well as the interior of the membrane to be coated simultaneously and reduced the coating time by a factor of 10 compared to conventional CVD approaches. MD testing confirmed the hydrophobicity of the material as 100% salt rejections were obtained.

Keywords: electrospinning; initiated chemical vapor deposition; membrane distillation; nanofibers; water.

Figure 1

(a) Solidworks schematic of the support and (b) 3D printed support. The porous structure could be replaced with a solid, impermeable plate to serve as a control. Please note that this figure is best viewed in color.

Figure 2

Schematics of the initiated chemical vapor deposition (iCVD) chamber demonstrating the standard method (left) and the new approach using the scaffold (right).

Figure 3

SEM images at 10,000× magnification of (a) an uncoated electrospun PAN membrane and (b) a coated electrospun PAN membrane modified via the scaffold iCVD method. Note the scale bar of 5 μm.

Figure 4

XPS collected from PAN, PAN-TBPO, and PAN-PDVB samples.

Figure 5

Water flux performance over six hours for a PAN membrane modified in iCVD using the convective mesh support for 20 min. Experimental conditions: feed—5 M NaCl solution at 50 °C, permeate—DI water at 20 °C, flowrates—0.4 GPM, and no pressure difference.

Figure 6

Water flux comparison between iCVD deposition times of pDVB-coated PAN membranes using the 3D printed scaffold, a commercial Millipore PVDF membrane, and an electrospun PVDF membrane. The membranes were tested in direct contact membrane distillation (DCMD) for 6 h and flux values were averaged over that time. All experiments yielded a 100% rejection, which indicated no wetting occurred during the test. Experimental conditions: feed—5 M NaCl solution at 50 °C, permeate—DI water at 20 °C, flowrates—0.4 GPM, and no pressure difference.