Synthesis and Characterization of Copolymers: Tuning Material Properties of Polythiophenes and Epoxy-Thiol Polymer Networks, and Application of Low-Field NMR Spectroscopy to Multicomponent Polymer Systems
Type of DegreePhD Dissertation
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Society’s demands for more products and more energy places great strain on the industries responsible for producing them. Polymers present the opportunity to replace many of the “traditional” materials and open the door for advancement of technologies in ways previously unattainable. Polymers present many advantages over other materials as their inherent material properties are often easily altered, or tuned, via subtle yet distinct differences in their microstructures. Alteration of polymer microstructures and their effect on the resulting material properties is an ongoing investigation undertaken by many research groups. Timely and accurate characterization of these materials is critical to keep up with the hastening pace of the evolution of polymer science. This dissertation seeks to add to this discussion by providing insight into polythiophene copolymer microstructure-property relationships, reaction kinetics of an epoxy/thiol based crosslinked copolymer system, and application of low-field 1H NMR spectroscopy to compositional analysis of multicomponent polymer systems. As the demand for energy increases, the need for more efficient methods of energy generation is growing. Conductive polymers such as polythiophene and its derivatives present the opportunity to realize devices with tunable properties to aid in the recovery of the waste heat, which accounts for a large amount of the energy expended during energy generation. Poly(3-alkylthiophene)s have been widely studied for these application due to their well-controlled synthesis, typically enhanced solubility, and favorable optoelectronic and solid-state properties. Poly(3-hexylthiophene) is the most studied and utilized poly(alkylthiophene), however, other thiophene derivatives present alternative advantages such as enhanced charge transport, chemical stability, and a wide variety of functionalities. It is often desirable for a material to have a combination of these properties. Small changes to polythiophene microstructure, either by 3-position substitution or by copolymerization of thiophene derivatives, yields polymers with tunable properties and favorable combinations of homopolymer properties to enhance resulting device performance. This dissertation explores topics pertinent to polythiophene copolymer chemistries such as the tuning material properties via statistical copolymerization of 3-hexylthiophene with unsubstituted thiophene or 3-methoxy thiophene, and compositional drift of thiophene comonomers synthesized via Grignard metathesis. In addition, this dissertation explores the application of low-field 1H NMR spectroscopy to the compositional analysis of a variety of polyolefins. Nuclear magnetic resonance (NMR) spectroscopy is arguably one of the most widely used characterization techniques as it is capable of characterizing many critical aspects of polymer materials. Traditional, or high-field, spectrometers (spectrometers rated at >300 MHz) are the most widely used version of these instruments in contemporary macromolecular science as they produce high-quality data and are capable of detecting subtle but distinct differences in polymer architecture. However, high-field spectrometers have several critical requirements for operation including cryogenic fluids, specially-trained personnel, and large amounts of laboratory footage. Recently, low-field NMR spectrometers have started to take a foothold in the industry as a viable replacement for their high-field counterparts. The main drawback of low-field NMR spectrometers as compared to high-field instruments is the broader peaks often associated with the weaker magnetic fields. This problem is further exacerbated by the repeat units within polymer backbones as repetitive signals coalesce and widen the observed spectral peak further. To combat this issue, many experimental parameters can be tuned such as sample concentration and molecular weight. In addition, many instrumental parameters, such as the number of scans and relaxation delay, can be adjusted to more closely emulate a high-field experiment. Low-field NMR spectroscopy is utilized to accurately determine the microstructure and composition of mixed microstructure polyisoprenes, symmetric triblock copolymers, and polymer blends through a plethora of combinations of experimental and instrumental parameters to within 1-2% of a high-field NMR spectrometer (400 MHz). The ability of the low-field NMR spectrometer to accurately determine polyolefin microstructures and compositions will prove invaluable to both industry and academia as it will hasten research and analytical rates and also reduce the overall costs. The last subject discussed in this dissertation is the cure kinetics of an epoxy/thiol based crosslinked copolymer system intended to address environmental concerns with leakage of fracking wellbores. Hydrocarbons, brine, and other injected chemical components have the potential to leak from both operating and abandoned wells into the surrounding environment, contaminating resources such as groundwater supplies and ecosystems impacting human health. Mitigating the impact of leakage thereby requires solutions to extend wellbore lifetime and seal leakage pathways in these systems. Traditional strategies for intervening to end leakage in fractured wellbore systems rely on injection of solid particle slurries (> 100 microns) into the concrete to seal the fracture. But penetration into small cracks require commercially available ultra-fine (< 10 micron particles) cement technologies, that are significantly more expensive and have short set times, complicating injection operations. Two polymer systems were explored that address the potential hazard by sealing leakage pathways in fractured wellbore systems. As expected, the reaction rate of our system is highly dependent on curing temperature as are the resulting physical properties. A firm understanding of the relationships between system chemistry, cure kinetics and material properties will allow for tailoring the design of the polymer sealant to particular subsurface wellbore conditions.