Vapor Phase Infiltration for the Synthesis of Advanced Hybrid Materials

Abstract

Advanced hybrid materials consisting of inorganic components embedded into an organic polymer substrate have seen use in a variety of industries and applications, including semiconductors, energy, separations, and aerospace. One strategy for the scalable, solvent-free synthesis of hybrid materials is to use vapor phase infiltration (VPI). This technique is adapted from the sequential exposures of vapor-phase reactants used in atomic layer deposition (ALD), but with the emphasis of growing inorganics within a porous substrate instead of on a surface. Although VPI has shown tremendous potential for introducing beneficial properties into hybrid polymers, such as improved separation properties, resistance to etching, and conductivity, VPI has not yet seen industrial use, in large part due to being a relatively new technique. As a result, there remain a wide array of unexplored VPI processes and applications, requiring a deeper understanding into how VPI could be adapted to different systems. This dissertation aims to expand the library of VPI processes while discussing how VPI interacts with organic reactants and crystalline structures, what characterization strategies can be used to understand VPI modification, and how VPI can be used as a tool for introducing surface nucleation sites. The first half of this work explores VPI as a technique to grow metal-organic frameworks (MOFs) within polymers. The MOF ZIF-8 was first introduced into acrylonitrile butadiene styrene (ABS) polymer via a two-step reaction of zinc oxide from diethylzinc (DEZ) and water, followed by exposure to vapor-phase 2-methylimidazole (2-HmIM). After infiltration, a series of characterization steps were used to determine if MOF crystallization occurred within the bulk polymer, overcoming the compressive strain of the polymer. While time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling revealed the successful infiltration reaction between the reactants, determination of crystallization was made more difficult by the presence of surface MOF crystallites. To distinguish between surface crystallinity and bulk crystallinity, surface ZIF-8 layers were etched away in water, as confirmed by ToF-SIMS, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The detection of crystallinity via x-ray diffraction (XRD) after surface removal ultimately suggested the presence of crystalline ZIF-8 within the polymer interior. This work serves as a proof-of-concept for MOF infiltration and subsurface crystallization using only vapor phase processes. The second half of this work explores VPI as a method for introducing nucleation sites in various polymers, enhancing surface layer growth. Polymeric components on satellites in low Earth orbit (LEO) suffer from erosion due to atomic oxygen (AO) exposure, requiring protective coatings or layers to limit mass loss. In this work, I examined how VPI modification enhanced the nucleation and deposition of aluminum oxide on Kapton® polyimide, poly(methyl methacrylate) (PMMA), and polyvinylidene fluoride (PVDF), which serves to protect them from AO. Using ToF-SIMS and XPS to characterize infiltration profiles and surface composition, it was observed that VPI consistently nucleates thicker surface growth relative to ALD along with deeper infiltration into the bulk polymer. This additional aluminum oxide incorporation was found to reduce mass loss from AO compared to untreated and ALD-modified polymers, with varying improvements between polymers depending on their inherent ability to nucleate Al2O3. This work demonstrates the versatility of VPI as a tool for modifying diverse polymer structures, the ability of VPI to nucleate surface growth relative to ALD, and its practical use towards protecting polymers from erosion in harsh conditions. This work, along with the field of VPI as a whole, are summarized in a final discussion into future VPI chemistries and applications.