AVS 72 Session TF2-TuA: VSHOP Applications I, Membranes and Fluidics

The ability to control molecular transport through organic interfaces is crucial to a wide array of applications, including polymer and composite materials durability in harsh environmental conditions and membrane-based separations. In this talk, I will discuss how vapor-phase processing, specifically atomic layer deposition (ALD), vapor phase infiltration (VPI), andmolecular layer deposition (MLD) can be used to engineer organic interfaces with nanoscale precision for controlled molecular transport across them.

First, we utilized vapor-phase growth of inorganic materials on polymers and carbon fibers to enhance their durability under UV radiation and elevated temperatures. We showed that sequential VPI and ALD enable robust ZnO–polymer hybrid structures on poly(lactic acid) (PLA), where VPI creates subsurface and surface nucleation sites that promote conformal ZnO growth during ALD. While VPI or ALD alone provides limited protection, their combination yields polycrystalline ZnO coatings that reduce polymer photodegradation by more than two orders of magnitude, while preserving transparency in the visible range. In addition, we employed Al₂O₃ ALD on carbon fibers to reduce oxygen transport into the fibers and oxidative degradation at elevated temperatures. The Al₂O₃-coated fibers showed superior thermal durability and mechanical properties under these conditions.

Second, we used MLD to fabricate ion-selective polyamide membranes. MLD provides a unique route to synthesize ultrathin polyamide films with molecular-level control over thickness and chemistry. Using MLD, we grew ultrathin, ultrasmooth, and highly cross-linked aromatic polyamide selective layers directly on porous supports, achieving tunable, Angstrom-level controlled thickness in the 2-30 nm range. The MLD-based exhibited high water-ion and ion-ion selectivities. Beyond enhanced performance, MLD provides a powerful experimental platform for probing structure–composition–transport relationships with unprecedented precision. This approach opens new pathways toward rational design of polymer layers, building one molecular layer at a time.

Vapor phase infiltration (VPI) of metal oxides into polymeric membranes may offer a novel way to combine the benefits of polymeric and ceramic membranes and enable improved capabilities in water treatment membranes. In this paper, we investigate the infiltration of alumina into two types of membranes, namely polyamide nanofiltration (NF) membranes and commercial anion exchange membranes (AEMs). The infiltration is characterized by using X-ray photoelectron spectroscopy (XPS) to confirm the presence of metal oxide, quantify the amount of incorporation at the surface and as a function of depth, and probe the type of bonding taking place. Complementary SEM-EDX (Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy) and time of flight secondary ion mass spectrometry (ToF-SIMS) experiments show the distribution of the inorganic phase within the membranes. We find an increase in NaCl rejection from 90 to >95% for NF90 and MgSO4 rejection from 80 to 90% for NF270 membranes with minimal impact on water flux. For the AEMs, we find that the infiltrated membranes show improved resistance to degradation by chlorine without any changes in membrane resistance and permselectivity after infiltration. These results show that infiltration of metal oxides into polymeric membranes holds promise for improving the selectivity and stability of polymeric membranes without compromising other important performance metrics.

Membrane separations offer a vital pathway for recovering critical minerals such as lithium from magnesium-rich brines. However, conventional polymerization methods produce selective layers with poorly controlled molecular structure, resulting in low selectivity between Li⁺ and Mg²⁺. To overcome the limitations of solution-based polymerization, we used molecular layer deposition (MLD) to fabricate polyamide selective layers for nanofiltration (NF) and reverse osmosis (RO) membranes, using trimesoyl chloride/piperazine (NF) and trimesoyl chloride/m-phenylenediamine (RO) precursors. MLD-grown polyamide films exhibited structural advantages, including high crosslinking, high mass density, low surface roughness, morphological homogeneity, and precise thickness control. Inverse gas chromatography measurement of surface energy revealed that amine-terminated cycles were free of surface defects. MLD NF membranes deposited at 1.3 Å/cy achieved Li⁺/Mg²⁺ selectivity as high as 35, compared to 3 for commercial membranes, while maintaining comparable Li⁺ flux. While the Li⁺/Mg²⁺ selectivity of conventional NF membranes relied on Donnan exclusion arising from strong surface charges, MLD NF membranes achieved high selectivity with negligible surface charge through molecular sieving via 5 ± 1 Å ultramicropores. While conventional, solvent-based synthesis results in selective RO layers with overall thickness > 100 nm, MLD RO films grew at a rate of 2.4 Å/cy, enabling fabrication of functioning membrane films as thin as 2 nm. By probing the extreme low end of thickness, we improved water flux, but ultimately reached a performance limit, demonstrating that selective films with thickness below their Bjerrum length exhibited reduced salt selectivity due to decreased dielectric self-energy. These results establish a high-precision platform for probing and optimizing molecular transport in thin-film composite membranes and advancing membrane materials.

The application of cationic thin film coatings has gained interest as a method of producing antibacterial surfaces which can passively resist bacterial contamination and reduce the spread of infectious microbes. One common method of producing these cationic surfaces is through the exposure of surface amine groups to methyl iodide to produce quaternary ammonium compounds (QACs). These films are typically deposited from the liquid phase using spin coating, spray coating, or dip coating, but these coating methods can be difficult or impossible to implement effectively for high aspect ratio or enclosed microfluidic devices. Molecular layer deposition (MLD) is a vapor phase process involving the exposure of a surface to reactive precursors in a cyclic manner, forming a polymer film layer by layer in a highly precise and conformal way. To vaporize precursors, they must be neutrally charged, but the deposition of a polyurea film with tertiary or aromatic amine sites could be used as a substrate for post-synthetic surface quaternization with liquid or vapor-phase treatment with methyl iodide.

In this work, we investigate the formation of polyurea MLD chemistries based on low melting point 2,4-toluene diisocyanate (TDIC) combined with substituted diamines including bis(2-aminopropyl)(methyl)amine (BAPMA) and 2,6-diaminopyridine (DAP). These precursors enable incorporation of tertiary and aromatic nitrogen functionalities into polyurea thin films, providing reactive sites for subsequent in situ or ex situ quaternization. The resulting substituted polyurea films are investigated as candidate substrates for vapor-phase fabrication of cationic antimicrobial coatings compatible with microfluidic device architectures, characterizing their film thickness, composition, and surface charge before and after exposure to methyl iodide. These results demonstrate a new avenue towards the creation of films with MLD and a possible new approach toward the functionalization of surfaces with cationic coatings.