Controlling Structure and Properties in Biomatter via Physical Processing Pathways

Abstract

The increasing consumption of non-degradable plastics and their harmful effects on the environment urgently call for the design and fabrication of degradable and renewably sourced materials. Natural materials have been useful since ancient times for a variety of applications but were phased out in the last several decades in favor of petroleum-based polymers. Modern science, however, has shown that there is more to explore in the field of natural materials due to new understandings of the hierarchical structures and diverse molecules present in biomaterials. Due to concerns with plastic pollution, the effects of microplastics on human and ecological health, and a drive to reduce fossil fuel dependency, there is an increasing interest to harness the inherent properties of renewably-sourced biomaterials.Algae are promising candidates for bio-based materials as they can grow in a wide variety of aquatic environments, are already cultivated at industrial production rates as food and supplements, and have a high molecular complexity. Typically, algal materials are made from extracting specific molecules from the algae, converting those molecules into useful materials and letting the rest go to waste. Such methods destroy the hierarchical nano- and micro-structures that give biomaterials their inherent strength and toughness, and can generate chemical waste. Alternatively, making materials using the entire biological matter without extracting specific target components may allow for the retention of hierarchical interactions and structures and offers no-waste processing routes. In this thesis, I investigate the possibility of using whole, non-extracted biomass to replace conventional plastic products, addressing the challenges inherent in biopolymer processing that distinguish them from conventional petrochemicals. I start by investigating the potential of creating cohesive bioplastics from unprocessed algal biomatter with equipment currently used in the petrochemical polymers industry (Aim 1). Then, I use a model system of polysaccharides to understand how colloidal processing influences structure and properties in the solid state (Aim 2). Finally, I use the findings from the previous two Aims to control the structures and properties of foams made from whole algal biomatter and present a case study focusing on the applicability of algal foams as acoustically absorptive devices (Aim 3). In Aim 1, we analyze the mechanical and morphological differences of compression-molded bioplastics made from spirulina cells, scanning a range of temperature, pressure, and time conditions during fabrication, aiming to understand the structure property relationships in this new material class. Three-point bending and compression are used to analyze the mechanical properties, showing that as temperature increases, strength and stiffness increase until a point at which the material starts degrading and reducing in strength again. Scanning electron microscopy (SEM) is used to analyze the morphological properties, which shows a transition from a packed-powder morphology at low temperatures and pressures to a homogenous matrix morphology at higher temperatures and pressures. An optimal flexural strength of 26 MPa occurs at a temperature of 140 °C and 7 kN of pressing force, which gives an impressive compression strength of 76 MPa. Our mechanical testing reveals that in the optimal processing conditions those bioplastics fall well within the commercial plastics range of performance. Fourier Transform Infrared (FTIR) spectroscopy is used in conjunction with X-Ray Photoelectron Spectroscopy (XPS) to analyze the changes in bonding induced by the manufacturing process. The results indicate that the compression molding process induces changes in protein conformation and hydrogen bonding to create a homogenous matrix. Pre-treatment steps and additives are used to modify the mechanical performance of the materials, and the biodegradability of the pure spirulina bioplastics in soil is assessed, showing an ability for these bioplastics to be home compostable. In Aim 2, we design a binary biopolymer system that allows for studying the interactions between those components in the colloidal state, and subsequently, after lyophilization and characterization of the physical properties and structure of the resulting foams, reveals connections between colloidal processing and solid foam performance. The two key carbohydrates we select are representative of biopolymers found in algal and plant biomatter, cellulose and pectin. Green chemical treatments are used to modulate the electrokinetic interactions between those components. Zeta potential measurements show that treating bacterial cellulose with a citric acid-based deep eutectic solvent (DES) produces a more stable colloid without the use of harsh chemicals as is typically done. Small-angle X-ray scattering (SAXS) provides evidence that the DES treatment defibrillates the cellulose fibers, creating a denser network of fiber entanglements compared to non-treated cellulose. Rheological measurements show the effects of those network changes on the shear behavior of the colloids. The addition of pectin then further modifies the colloids, reducing the zeta potential and viscosity but improving the elasticity of the network. Foams are then created from the colloids by freeze drying, and mechanical testing and SEM are used to characterize the properties of the lyophilized colloids, which produce foams of entangled fibers. Adding DES moderately improves strength and stiffness while the addition of pectin completely changes the foam structure, reducing strength while improving the ability for the foams to recover strain. In Aim 3, we provide insights into the processing-structure-property relationship that determines the performance of whole algal foams in acoustic insulation applications as a case study. FTIR and X-ray Fluorescence (XRF) show distinct differences in algae composition, reflecting differences in species, growing condition, and processing conditions. Rheology is again used to characterize the algal colloids, showing that the differences in composition between the species contribute to differences in colloidal properties, with species higher in sulfur and starch creating both stiffer and more elastic colloids and gels than the other species. Sound absorption is measured via the two-microphone impedance tube method, showing excellent sound absorption at low to mid-range frequencies, with one species performing exceptionally well at higher frequencies. SEM and micro computed tomography (micro-CT) are used to assess the morphological differences between the foams and relate the morphologies to the sound absorption results. The CT data and acoustic data are combined using the Johnson-Champoux-Allard (JCA) model, which allows for standardized comparisons of the acoustic absorption performance of difference materials as well as the extraction of viscous air flow properties within the foams. Next, flammability testing shows that the processing and therefore the composition of the algae contribute to some species immediately self-extinguishing after being removed from a flame, while some will burn and smolder. Finally, Aim 3 presents biodegradation data, with a discussion on why typical polymer biodegradation tests as well as traditional CT characterization techniques require new methods and approaches to getting reliable data when used on whole-biomatter materials.