Date of Award

Summer 8-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Dr. Sergei Nazarenko

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Dr. Trenton Gould

Committee Member 2 Department

Human Performance and Recreation

Committee Member 3

Dr. Scotty Piland

Committee Member 3 Department

Human Performance and Recreation

Committee Member 4

Dr. Derek Patton

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Dr. James Rawlins

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

The characterization of structure, thermal, gas transport, and free volume properties of two unique UV cured polymeric systems are studied and reported. In the initial pursuit of waterproof high water vapor transport membranes, it became apparent that the UV curing of fluorinated materials yielded routes to develop unique materials that fundamentally challenge conventional models for free volume and light gas transport behavior. UV-curing provides a means to rapidly “lock-in” morphologies that are accessible in the small molecule, monomer phase but rapidly become kinetically inaccessible when constraints such as covalent bonding and cross linking limit motion in the polymer system. This “locking-in” was exploited in Chapters II, III, IV, and V while Chapters VI and VII study this phenomenon.

While the motivation for this project stems from the ability to selectively transport water vapor over bulk water, similar principals are used in the selective transport of light gases. Light gases were extensively used in this work to probe the molecular structures of high permeability polymer networks. Probing of these molecular structures was supported by extensive free volume analysis using both volumetric and molecular probing of free volume properties.

The first chapter of this document outlines the basics of fluorinated, UV cured and thiol-ene materials, transport of gases in polymeric systems, and free volume. These topics are complimented by a discussion of the methods used throughout this work to study the phenomena described.

Chapter II gives brief overview of our research group’s historical modification of thiol-ene networks. Through the modification of a tetrafunctional thiol with various chemical moieties, our group has shown the ability to retain the glass transition temperature of a network while tuning the permeability of the system over three orders of magnitude through the use of hydrogen bonding groups, linear aliphatic, and linear perfluorinated modifications. UV-curing allows for the rapid development of a network where monomer functionality was retained, leading to networks with similar long range connectivity and short range differences in backbone spacing and free volume. Chapter III continues this work by exploring the modification of thiol-ene networks with silane groups. Chapter IV delves into the details of the modification procedure using perfluorinated acrylate moieties as the modification. By changing the length of linear perfluorinated acrylates bound covalently into the network, the repulsion of the network backbone by the fluorinated moiety increases. This phenomenon was evidenced by free volume, X-ray diffraction, DSC and pressure volume temperature data and analysis. The resulting “thermodynamic frustration,” caused by the incompatible fluorinated groups, increased the transport of light gases across 2.5 orders of magnitude in some cases and shows a stretching of the network backbone structure without impacting glass transition temperature.

Chapters VI and VII highlight the exploration of “switchable” fluorinated UV-cured acrylate side chain polymers. The rigid rod structure of side chain acrylates of a given length gives rise to a morphology that has a well-defined crystalline order and melting temperature. Melting of this ordered structure gives rise the “switching” behavior that can be utilized as a molecular valve for certain applications, activated using a thermal stimulus. The well-defined order of C8F17 -and C10F21 side chain acrylates was well characterized and shown to develop a mesophase upon UV-curing that was irreversible unless quenched using liquid nitrogen. The well-defined melting temperature of 72 °C was exploited in the study of gas transport properties that traversed this melting temperature, showcasing the permeability switching. Permeability gains across the melting of the ordered morphology, for several gases, were shown to be due to solubility increases alone. Contrary to standard two-phase systems where ordered phases increase tortuosity and therefore decrease diffusivity in the semicrystalline systems, these systems show no discernable switch in diffusivity across the transition temperature. Analysis of both the free volume hole size and volume contributions are reported. Furthermore, the side chain morphology lends itself to high He/H2 separation performance.

Available for download on Sunday, February 11, 2018

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