Understanding interfacially-driven structural and flow properties of complex polymer solutions for sustainability applications

Abstract

Polymers are a diverse class of materials which are omnipresent in the modern world. The durability and ease of manufacture of polymers means that commodity plastics like polyethylene (PE) and polypropylene (PP) are used in a wide range of consumer and industrial products. While synthetic polymers have been produced for most of the past century, polymers are still being used in new ways for novel technologies. These technologies can utilize synthetic polymers, such as redox-active polymers used for electrochemical adsorbents to remove polymers from water, or natural polymers like cellulose, for use in microbeads for personal care products. However, one major limitation preventing the implementation of these polymer-based technologies is the difficulty in processing polymers to form different structures. Many polymers are formed into useful shapes, like coatings or beads, by solution processing. Generally, solution processing involves dissolution of a polymer, followed by shaping of the solution by a process like spraying, dripping, or emulsion, before finally removing the solvent to return the polymer to a solid form. However, many polymers are difficult to dissolve in most conventional solvents, and tend to be viscous or elastic when they are dissolved, preventing breakup into a droplets in a spray or flow through a nozzle to form drops. Additionally, even when polymers do dissolve in a solvent, they may preferentially adsorb to the interface of the fluid, resulting in the formation of films due to changes in humidity or solvent evaporation. This thesis explores the flows involved in processing two examples of interfacially-active polymer solutions, each aimed at producing a particular technology with applications in sustainability. In the first example, dispersions of polymers and carbon nanotubes (CNTs) in volatile solvents are coated onto a conductive substrate to produce electrochemical adsorbents capable of separating specific charged pollutants, such as heavy metals, from water. To produce these adsorbents in a replicable and scalable way, the CNT/polymer inks must be sprayed onto a surface, so a method to assess the suitability of these inks for spraying is needed. In the second example, solutions of natural polymers from biomass like cellulose are dissolved in an ionic liquid (IL) to create microbeads by two different methods - dripping and emulsion. The processability of these solutions is limited by their high viscosity, as well as the limited conditions under which cellulose is soluble; water in the air is absorbed by the hygroscopic IL/cellulose solution, causing the cellulose to precipitate. In order to study the processing of these systems, a technique is needed to induce the flows most relevant in dripping, spraying, and emulsions, while controlling for interfacial effects caused by evaporation of solvent and uptake of water. Because of the strong deformations induced by extensional flows in these processes, an extensional rheological technique provides the most relevant information for predicting the suitability of polymer solutions for processing in these ways. The first goal of this thesis was to implement a method for environmental control in extensional rheology. Environmental control of evaporation was introduced to a dripping-onto-substrate (DoS) instrument by using a transparent chamber with a sacrificial solvent chamber, which was used to generate an atmosphere saturated in solvent. This device was validated using model polymer solutions of polyethylene oxide (PEO), a polymer with clear elastic thinning behavior described by an extensional relaxation time λ_E. By comparing values of λ_E between freely evaporating and environmentally-controlled configurations of the instrument, the effects of evaporation on the extension of these solutions could be detected and quantified. Despite an over 20-fold increase in λ_E due to evaporation effects in some cases, the use of the environmental control chamber allowed the accurate measurement of solvent quality based on the scaling of λ_E with concentration, confirming the efficacy of environmental control for eliminating even very extreme evaporation effects. This environmentally-controlled DoS system was used to make proof-of-concept measurements of CNT/polymer inks, confirming that extensional rheology can inform the stability and processability of even very low-viscosity and volatile inks. Leveraging the environmental control system developed for control of evaporation in CNT/polymer inks, the second goal of this thesis was to use DoS to inform the processing of biomass solutions into microbeads. These solutions, made from cellulose and lignin dissolved in a mixture of IL and dimethyl sulfoxide (DMSO), were formed into microbeads by dripping into a solvent bath for precipitation before removing residual IL and DMSO by solvent extraction. The speed of this process and the size of the resulting beads were limited by the extensional viscosity of the solution moving through the nozzle. To assess the extensional behavior of these solutions, the environmental control chamber was used to limit hygroscopic effects, allowing for measurements to be made without absorption of water. Based on these measurements, extensional viscosity was reduced by the incorporation of lignin, the use of shorter cellulose chains, and the reduction of concentration. Most microbeads produced from this process were approximately spherical, with Young's moduli (E) around ~0.5 GPa and diameters around ~1 mm. However, bead shape, size, and stiffness could be tuned via biomass source, concentration, and degree of polymerization. In addition to solution composition, the size of the nozzle and the choice of precipitation and extraction solvents determined the structural and mechanical properties of the final beads. While the microbeads could be tuned in a number of ways, the dripping/precipitation process was generally too slow and produced beads too large to be useful for consumer applications, which typically require beads from 200-800 μm in diameter. To overcome the restrictions of the dripping/precipitation method, the final goal of this thesis was to develop and explore an emulsion/precipitation method for the production of smaller beads at scale. Based on the results from the dripping/precipitation method, this process involved mixing biomass solutions with various oils and precipitating by slowly dripping in ethanol, before removing residual IL by solvent extraction. Incorporation of lignin was successful up to 20% wt. of the total biomass, but at higher weight fractions, the emulsion was destabilized, resulting in significant aggregation of the beads. While this aggregation persisted in high lignin content beads, aggregation was prevented in pure cellulose beads by optimizing filtration and purification procedures to reduce exposure to humidity in the air. Finally, using dripping measurements, the interface between IL solution and oil was probed for various oils, showing that the roughness of the surface and the formation of spherical growths were determined by the adsorption of cellulose to the various oil interfaces. Based on the continued application of interfacial techniques coupled with environmental control, a scalable emulsion method was developed to produce beads with tunable surface morphology and varying composition and shape which can be optimized for specific consumer applications. Overall, this thesis used extensional rheology to establish relationships between processing and structure in two systems: CNT/polymer inks and biomass IL solutions. Fundamental advances were made in incorporating environmental control for extensional rheological measurements for systems with significant interfacial effects. Furthermore, specific insight was gained regarding the formulation, optimization, and scale up of electrochemical adsorbent and biomass microbead technology. Development of these technologies as well as other technologies relying on polymer solution processing can build off of this work for a more sustainable future.