Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Luzinov, Igor

Committee Member

Mefford , Olin T

Committee Member

Urban , Marek

Committee Member

Kennedy , Marian

Abstract

The work in this dissertation is devoted to the synthesis and characterization of novel materials for off-line unattended sensing: shape-memory grafted nanofoams. The fabrication process and characterization of highly efficient, polymeric nanosensor element with the ability to selectively detect analytes and retain memory of specific exposure events is reported. These shape memory nanofoams could potentially act as efficient and highly sensitive coatings for evanescent waveguide-based optical monitoring systems. On exposure to specific analytes, the polymeric coatings locally change their internal structure irreversibly at the nanolevel, affecting the local optical properties such as refractive index. Currently, enrichment polymer layers (EPLs) are currently being used to detect of chemical vapors. EPLs are thin polymer films that can increase signal of an analyte through absorption. These films are designed to interact with analytes via chemical interactions while this analyte is present in the environment. Once the analyte is removed from the environment surrounding the EPL, these EPLs have no residual memory of the interaction(s). This dissertation will address this limitation in the field of chemical unattended sensing through the use of functionalized polymeric films that possess ability to retain memory of analyte exposure. Specifically, we will use chemically cross-linked gradient nanofoam as a material with built-in analyte-specific sensing properties. A novel method has been created to fabricate chemically functionalized shape memory nanofoams. First, a polymer film containing epoxy groups is deposited onto a substrate. Then, the film is cross-linked via reaction of the epoxy groups to create a non-soluble, yet swellable coating. This film is then treated with specific chemical substances capable of reacting with the epoxy functionalities. This procedure is necessary to convert the epoxy groups into various functional moieties. This process generates a chemically modified orthogonal gradient film whose composition is unique at each point across the surface of the film. Lastly, shape memory properties are imparted to the film by swelling it in a solvent with high affinity ('good') with the polymers and freeze-drying it under reduced pressure. Using this methodology, a shape memory nanofoam gradient film was produced that could 'remember' its original non-porous shape. This 'memory' occurs through the following process. Exposure of the foamed film to an analyte causes local plasticization of the film, making the polymer thickness decrease ('shrink'). Since the film possesses graded chemical composition, locations of the film interact in different ways to each analyte. Specifically, only certain regions in the film (which have specific thermodynamic affinity to the chemical) shrink, creating a unique thickness pattern targeted for each chemical. The resulting changes in local film morphology are irreversible and provide a permanent record or 'fingerprint' for the chemical event of interest. Ultimately, the films can be prepared on the surface of waveguide arrays to allow optical monitoring at different locations, for unattended sensing purpose. In conclusion, this dissertation provides the fundamental knowledge for synthesis of orthogonal chemical gradient nanofoam films. The methods are reported as a novel way of producing responsive polymer nanofoams for unattended sensing purposes. The results complement and add to the current state of the art materials used in the field of unattended sensing.

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