Properties and fabrications of ultra-thin polymer films and hierarchical composites are of great interest in packaging, electronics, separations, and other manufacturing fields. However, due to the inherently fragile nature of ultra-thin polymer films, measuring their properties has proven difficult. Additionally, variables controlling thin polymer patterns (e.g. substrate wetting property) and composites (weight percent of particulates in matrix) formation have not been fundamentally well understood. Within this spectrum, fundamental understanding of formation mechanisms of these patterns and composites are needed. Additionally, a new characterization technique is required to be able to measure the mechanical properties of fabricated composites and thin films.
The pattern (Chapter 2) and composite (Chapter 3) formation presented in this thesis is based upon flexible blade flow-coating, an evaporative self-assembly method. The impact of substrate wetting, varying from being hydrophobic (water advancing contact angle 113°) to hydrophilic (water advancing contact angle 27°), on polymer pattern formation is examined here (Chapter 2). We observe a variety of polystyrene structures including dots, hyper-branched patterns, stripes and lines that can be deposited on substrates with a range of wetting properties. We propose the mechanism for these pattern formations as a balance between Marangoni instability and solute absorption. When adding quantum dot nanoparticles into the polymer (poly(methyl methacrylate) solution in the flow-coating process on hydrophilic substrates, we could obtain free-standing hierarchical nanocomposite films with alternating line and film structures (Chapter 3). The ability to guide assemblies of nanoparticles and polymers in designated areas in one step via flow-coating, provides new understanding of the flow competition of mixing two components which are both on the nanometer scale. Additionally, we introduce a method designated for ultra-thin film tensile testing (Chapter 4). We demonstrate the capability of this method by stretching two-dimensionally macroscopic, yet nanoscopically thin, polymer films on the surface of water. Through laser tracking of the force and displacement on the film, we characterize the full stress-strain response of brittle (polystyrene), ductile (polycarbonate), and rubbery (cross-linked polydimethylsiloxane) polymer thin films. In the brittle (polystyrene) films, we observe a precipitous decrease in mechanical properties (Young’s modulus, strain at failure, and nominal stress at failure) for film thicknesses approaching the size of an individual polymer chain (~ 25 nm) yielding insights into polymer chain entanglement theory. To verify our hypothesis in polymer chain entanglement theory for determining failure properties of thin polymer films, we further study the molecular weight effect (853, 490, 137 and 61.8 kg/mol) of polystyrene on failure properties (Chapter 5). We compare maximum tensile strain, maximum tensile stress, and modulus respectively as a function of molecular weight as well as film thickness. We support our hypothesis on polymer inter-chain entanglements theory in thin polymer films by this molecular weight study. This thesis provides direct measurements of failure properties of ultra-thin films. These findings have important implications for the design of materials used in wide range of applications, as well as for the pursuit of new fundamental understanding of polymer physics in confined states
