Processing Approaches for Maintaining Multifunctionality in Advanced Thermoplastic Composites

Abstract

Multifunctional composites typically consist of a functional additive imparting multiple functionalities to a polymer matrix. The production of these composites is not trivial, as the functionality of the particles must be retained during production, and the polymer matrix and particle interactions can often reduce the effectiveness of the final product. When fundamental principles, including thermodynamic and hydrodynamic considerations, are utilized in the production of these materials, it is possible to create highly effective multifunctional composites with tremendous application potential. Common production methods for multifunctional composites include extrusion, solution casting, molding, and spinning techniques. Most of these techniques are used to create materials with a homogeneous microstructure, as thorough mixing is needed for the matrix material to successfully accept the functional particle. In solvent casting, this particle dispersion in the matrix is often controlled through a solvent exchange step, as particles must frequently be transferred from a suspension or colloid that is not miscible with the desired polymer matrix to a solvent that is. Another method to create well-dispersed particle composites is by creating a layered structure of two separate matrices. By embedding a functional particle into only one of these matrices, the particle density can effectively be controlled, and therefore the surface area to volume ratio is increased. This type of layered system is often produced through melt pressing or extrusion methods and requires a keen understanding of how to control the organization of a particle in the presence of two dissimilar matrices. Chapters 3 and 4 will jointly address the issue of transferring a functional particle from a colloid into a well dispersed thermoplastic matrix while also introducing a novel use for these materials through solution casting. The material studied consists of a modified cellulose nanocrystal (CNC) with additional carboxylated "hairs" called electrosterically stabilized nanocellulose crystals (ENCC). By embedding these particles in a thermoplastic polyurethane (TPU) matrix, a stimulated optical response is achievable through hydration of the bulk film. This phenomenon is reliant on the dispersion of the ENCC particles, as particle agglomeration will result in non-uniform opacity. Chapter 3 will address the issue with the solvent exchange of a charged tertiary mixture. ENCCs are highly hydrophilic while generally being stored in a colloid. This leads to difficulties when attempting to solvent exchange the colloid with a solvent that is miscible with both the ENCC and TPU. By modifying the existing solvent exchange process for CNC-TPU composites to account for the additional hydrophilic hairs of the ENCC, we were able to successfully transfer the ENCC from a water to DMF mixture and cast ENCC-TPU films of varying concentrations. Chapter 4 will further develop the understanding of these ENCC-TPU composites and the effects of ENCC particle dispersion on their mechanical and optical properties. The effects of particle dispersion on the mechanical and optical properties of ENCC-TPU films were probed through dynamic mechanical analysis (DMA) and ultraviolet-visible spectroscopy (UV-VIS) respectively. Chapter 5 will address the issue of producing a multilayered filament while containing a functional particle to one of the layered matrices, and the effects of multiple processing methods on the particle functionality. Melt pressing and multilayer extrusion were used to produce antiviral particle filled composites layered with a neat matrix. When the antiviral particles at the surface of the functional composite layer are rendered ineffective, the top layer of the multilayered system can be delaminated to expose a surface with fresh functional particles. The functional particles used in this study, cuprous oxide (Cu2O) and a polymer-based antiviral particle (AV1), are well studied antiviral agents. The antiviral performance can be measured pre and post extrusion giving insight into the effects of particle viability during processing. Through careful selection of the matrix materials, we were able to successfully produce multilayered systems consisting of alternating layers of neat polypropylene (PP) and a AV1-LDPE composite. Melt pressed samples showed minimal particle diffusion across the PP-LDPE interface, while the AV1 antiviral efficacy was only slightly reduced from 99.9% to 86.0% viability against COVID-19. We also produced multilayered filaments of the same layered system while also retaining uniform layering and minimal particle migration.