Probing in situ dynamics of polymeric thin films by label-free optical techniques
Sheppard, Gareth Ray
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In this dissertation, interfacial interactions are explored using label-free optical techniques to probe the dynamics within polymer thin films. Label-free detection of interfacial interactions can be used to directly study the changes in organic thin films without adversely influencing the system by labeling with target molecules. Surface plasmon resonance (SPR) and spectroscopic ellipsometry were used to investigate these interactions by monitoring changes in thickness and refractive index at an interface due to reactions on monolayers and within polymer thin films as well as to explore polymer- solvent effects. In order to facilitate the study of binding events particularly in organic solvents, thiolene-based microfluidic devices were coupled with surface plasmon resonance imaging (SPRI). By using commercially available materials, a robust microfluidic-SPRI device was produced using a simple fabrication scheme. This provided an integrated platform to investigate surface reactions in aqueous and organic solutions for high throughput, in situ analysis. Incorporation of microfluidics with SPRI allowed for direct control of the fluid dynamics within the areas of interest for interfacial tracking. A novel linear mixer design was used to generate a controlled concentration gradient in order to determine the changes in solvent refractive index due to the presence of polymer in solution. Combining the in situ gradient generator with SPRI, the refractive index increment of polymers in solution can be determined using a single image. A grating surface plasmon resonance sensor was created using blu-ray discs with a molecularly imprinted polymer. The 320 nm periodic structure on the blu-ray disc is an exemplary grating SPR sensor as it produces a single sharp plasmon when coated with a noble metal. Integrating the molecular sensing technique with grating coupled SPR has lead to the fabrication an inexpensive chemical sensor that is both selective for the imprinted target molecule and sensitive to minute refractive index changes within the polymer as binding occurs. In a parallel study, polymer-solvent effects in a thermo- responsive thin film hydrogel were investigated using spectroscopic ellipsometry. Measuring the film at multiple angles and wavelengths produces sufficient optical information to deconvolute the thickness and refractive index changes as the polymer collapses and swells in response to temperature changes.