Polymeric Melodies: Polyetherimide Modification-Based Acoustic Membranes and Sonochemical Degradation of Polystyrene

Abstract

The compatibilization of polymers for nanomaterials and the degradation of commodity plastics are two major areas in polymer chemistry. To achieve composite materials that meet the demands of modern technologies, polymers are modified using a vast array of chemistries, enabling the practical and effective deployment of nanomaterials. For example, graphene and carbon nanotubes hold significant promise for the advancement of computational power, energy storage, the reinforcement of commercial and construction materials, and in sensing technologies. However, these materials are often difficult to use independently and must be coupled with a substrate or embedded into a polymer matrix. To this end, several strategies for the generation of polymer/nanomaterial composites have been developed. These approaches have suffered setbacks regarding the full potential of the coupled nanofillers, as the predicted theoretical enhancements often fail to translate from the nanofiller to the matrix. Therefore, it is crucial to research strategies that can unlock the full potential of the materials destined to enable the next stage of technology. In this dissertation, we have modified polyetherimide (PEI) using a crosslinking approach to enable the reinforcement of a thin film membrane via carbon nanotube embedding. In addition to the generation of nanocomposites, effective chemical recycling of waste plastics has become paramount to solving the current pollution crisis. While most see discarded plastic as waste with no inherent value, the modern chemist sees a feedstock waiting to be mined for the valuable moieties and energy within. Judicious treatment of plastic waste can yield fuel sources, pharmaceutical scaffolds, and macromolecular plastics with new properties. However, true material circularity remains elusive due to the need for expensive catalysts, energy-intensive thermochemical processes, and the physical labor involved in collecting waste. In this dissertation, a catalytic, green process is explored to degrade polystyrene (PS) using acoustic energy. PS is degraded using ultrasonication into benzene using substantially lower amounts of catalyst compared to contemporary methods. Thin layers of polymer are often deposited onto single-layer graphene (SLG) to act as support substrates for large radius suspensions of SLG. The thickness of the polymer layer and the functionalities installed onto SLG are controlled to cater to specific applications. However, thin film polymer/SLG composites can be brittle and suffer from rapid degradation upon cycling. The material must be able to withstand a wider range of mechanical use to be commercially viable; therefore, polymer/SLG composites must be mechanically strong and robust. To improve the survivability and overall strength of a PEI/SLG composite, we installed crosslinking moieties onto PEI and introduced carbon nanotubes (CNTs) into the polymer matrix to increase the tensile strength. The CNTs are dispersed using sonication, and the films are spin-coated onto SLG on copper (SLG/Cu) from solutions comprised of functionalized PEI and CNTs in chloroform. The films are annealed to initiate crosslinking, and after removal of the Cu layer, freestanding thin films are recovered. The films show an increase in Young's modulus of up to 5.4 GPa with CNTs, along with a stiffer 2D modulus. The crosslinking of PEI in the presence of CNTs provides a new method for forming covalent linkages between a polymer and CNTs, achieving mechanically robust and reinforced thin films. Plastic waste pollution has only grown as a threat to public health. As plastic production continues to increase, recycling rates have not managed to curtail the inevitable accumulation of waste in the environment. Only 30% of all produced plastic is recycled in developed countries, with nearly non-existent programs in developing nations. While mechanical recycling and incineration enable the physical recycling and energy recovery from plastic waste, the reformed material is often mechanically compromised, and incineration releases toxic fumes into the atmosphere. To address this, chemical degradation and upcycling of plastics have been achieved using various catalysts. However, these processes require high catalyst loadings and intense conditions, lowering the efficiency and economic viability. Ultrasonication can overcome these limitations by leveraging the intense energy release during cavitation to drive a catalytic degradation of PS. Herein, we present the degradation of PS into benzene using ultrasonication with significantly lower molar amounts of aluminum chloride (AlCl3) than is required by thermochemical reactions. The reaction is rapid and able to produce quantitative yields of benzene from PS without any external sources of heat and pressure. Sonication not only requires much lower amounts of energy but also produces benzene in mild conditions, with the reaction proceeding at 0 °C. This work offers a more efficient, truly catalytic approach to the degradation of PS with AlCl3, increasing the economic viability of chemical recycling.