Mechanistic Studies into Interfacial Interactions via Chemical Vapor Deposition Polymerization

Abstract

Chemical vapor deposition (CVD) polymerization is a widely used fabrication method for preparing substrate-independent thin film polymer coatings for a broad range of applications. Functional poly(p-xylylene) (PPX) coatings are specific examples of CVD polymer coatings, and they can be applied for surface functionalization and bio-conjugation. The first portion of this dissertation serves to explore the fundamental mechanism of area-selective CVD polymerization, which has a high potential to be utilized as one of the bottom-up processes. In this dissertation, we report a systematic study into the impact of thermodynamic processes on the area-selectivity of chemical vapor deposition (CVD) polymerization of functional [2.2]paracyclophanes (PCP). Adhesion mapping of pre-closure CVD films by atomic force microscopy (AFM) provided a detailed understanding of the geometric features of the polymer islands that form under different deposition conditions. Our results suggest a correlation between interfacial transport processes, including adsorption, diffusion, and desorption, and thermodynamic control parameters, such as substrate temperature and working pressure. This work culminated in a kinetic model that predicted both area-selective and non-selective CVD parameters for the same polymer/substrate ensemble (PPX-C + Cu). These findings are corroborated by STEM results indicating extensive reorientation of continuous CVD thin films on deposition-prohibited substrates at temperatures above 120 oC. Moreover, deposition on patterned substrates (Ru patterns on Si substrates) suggests that the area-selectivity is not affected by the surface geometry of hybrid substrates, such as the structure of patterns and feature/spacing sizes of patterns, supporting the application of area-selective CVD polymerization on 3-D materials. While limited to a focused subset of CVD polymers and substrates, this work provides an improved mechanistic understanding of area-selective CVD polymerization and highlights the potential for thermodynamic control in tuning area-selectivity. The second portion of this dissertation serves to extend the use of CVD-based reactive PPX coatings as a surface modification strategy to enhance biomolecule and biomaterial interaction. In this dissertation, we report a precise cell attachment method using CVD-initiated atom transfer radical polymerizations (ATRP), which provides a convenient access route to controlled radical polymerization on a wide range of different materials, to grow polyethylene glycol methacrylate (PEGMA) polymer brushes. This antifouling material shows the resistance of both protein and cell, promoting a high yield of cell attachment to the targeted sites. Moreover, this dissertation also demonstrates the use of CVD-based co-polymer coatings as intermediate layers to immobilize multiple biomolecules on substrates. CVD copolymer coating with designed functional groups was deposited on the biomaterial surface to selectively conjugate both viral vectors and peptides through chemical reactions. The ability to tether lentiviral vectors together with a mesenchymal stem cell (MSC)-binding peptide enhances cell communication among MSCs and increases cell binding and differentiation, providing a safe and efficient gene therapy delivery strategy.