Investigating structures and optical properties of monolayer films prepared from a photo-polymerizable surfactant in 2D
The overall objective of this PhD thesis research is to characterize, understand and ultimately control phase-separated structures in mixed films consisting of a perfluorinated fatty acid and a photopolymerizable surfactant. In these systems, film morphology, mechanical properties and spectroscopic properties are inter-related and this thesis explores these relationships. In this context the interaction between perfluorotetradecanoic acid (C13F27COOH, referred to as PF in this dissertation), and 10,12-pentacosadynoic acid (CH3(CH2)11−C≡C−C≡C−(CH2)8COOH, referred to as PCDA in this dissertation) has been studied in monolayers using a combination of surface and spectroscopic characterization techniques. To investigate the inter-relationship of the properties described above, film behavior under a variety of conditions, including behavior at different interfaces (solid-liquid, air-liquid), different film compositions and under different conditions of photoillumination and mechanical stress were explored. Thermodynamic and morphological studies of mixed monolayer surfactant films of PF and the photo-polymerizable diacetylene molecule, PCDA, were carried out. The films were prepared at the air-water interface and transferred onto solid supports such as a glass slides via Langmuir-Blodgett (LB) deposition technique. The presence of the perfluoroacid helped to stabilize the diacetylene surfactant monolayer in comparison with the diacetylene alone, allowing film transfer onto solid substrates without needing to add cations to the sub-phase or photo-polymerize the components prior to deposition. Addition of the perfluorocarbon to PCDA resulted in films with the photopolymer strands oriented perpendicular to the direction of the film compression in a Langmuir trough. This is in contrast with film structures formed from pure PCDA. Formation of these features could be explained by a two-step process that happened sequentially: first, the compression of monolayer with trough barriers while trying to maintain the surface pressure constant induces stress on the film surface; second, additional film buckling which was enhanced by the strong cohesion between PF and PCDA. Film compression data, supported by in situ fluorescence spectrophotometry, Brewster angle microscope imaging and atomic force microscope images of deposited films, supported this mechanism. Factors that controlled the orientation of the photopolymer fibers were also investigated. Fibers were found to consist of multiple strands, with each strand having a different orientation. Our investigation also revealed there was a preferred orientation for fibers in the film as a whole. The angle of approximately 60o to the direction of film compression during deposition from a Langmuir trough has been calculated with the help of dual-view, polarized fluorescence microscopy. This orientation was attributed to the mechanical stress exerted by the trough compression barriers coupled with rotation of the polymer fibers during film draining. The combination of Atomic Force Microscope (AFM) and fluorescence microscopy (FM) provided a thorough and comprehensive mapping of fundamental properties of mixed monolayer system, and enabled a quantitative determination of the degree of selectivity of the polymerization process.
AFM, Surface Chemistry, Surfactant, photopolymerization, Langmuir, Blodgett
Doctor of Philosophy (Ph.D.)