Physical and chemical adhesion of complex structured biomaterial particles: Pollen exine and cellulose nanocrystal
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Micro- and nano-particle adhesion plays an important role in many industrial fields, for example, in microelectromechanical systems (MEMS, affecting fabrication yield and operation), in printing (transfer toner particles onto substrates), in coating and paints applications (adhesion of powder paint), or in composite materials (reduce filler-filler adhesion and enhance filler-matrix adhesion). In all of these examples, the particle-particle and particle-substrate interactions govern many aspects of process and product design. These complex interactions are determined by material properties (for example, the Hamaker constant and elastic modulus), particle size and shape, surface properties (e.g., surface energy and roughness), external load, humidity and electrostatic charges. Particle-particle and particle-substrate adhesion have been the focus of intense study; however, they have not been fully understood. The purposes of this study are to 1) understand particle-particle adhesion mechanisms of complex structured microparticles, and 2) tailor particle-polymer matrix adhesion for enhanced mechanical performance of polymer composites. Two biomaterial particles, micro-sized pollen grains and nano-sized cellulose nanocrystals (CNCs), were used as model particles for these two purposes, respectively. In this work, the morphology effect of pollen exine on pollen-pollen interaction was characterized with atomic force microscopy to reveal the unique interaction mechanism. A hybrid interaction model was developed to capture the effect of the micro-structured morphology. Meanwhile, the elastic modulus of pollen exine was analyzed for the first time to understand the effect of mechanical properties on pollen-pollen interaction. In the second part of this work, cellulose nanocrystals (CNCs) were chemically modified and rendered acrylic functional groups on the surfaces. The particle-matrix interaction was tuned to utilize the reinforcing effect of the modified CNCs. Three different processing methods were developed to optimize the reinforcing benefits from the modified CNCs. Overall, this work presents new insights for effects of surface physical (morphology and mechanical property) and chemical properties on particle-particle and particle-matrix adhesion. Pollen grains were investigated as a model of complex structured microparticles, and a unique particle-particle interaction due to the complex morphology was revealed. The elastic moduli of pollen shells were firstly characterized with direct measurements. In the second part of this work, a chemical modification route was developed for cellulose nanocrystals to improve the particle-matrix adhesion and compatibility. The results of his work provided new considerations for material applications including paints, coatings, drug delivery, and composites, among others.