Date of Award

Spring 5-5-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor

William M Gramlich

Second Committee Member

Douglas J Gardner

Third Committee Member

Barbara J W Cole

Additional Committee Members

Carl P Tripp

Mehdi Tajvidi

Abstract

Mechanically refined cellulose nanofibrils (CNFs) promise to be a high-volume, sustainable, nanoscale reinforcement for thermoplastic composites. They are currently held back by poor interfacial interactions with composite matrices, energy intensive drying, and drying induced fibril aggregation. In this dissertation, we explored how a grafting-through polymerization scheme modified the surface of CNFs with a wide variety of commodity polymers and overcame many of these technical challenges.

The first phase of the research was concerned with characterizing the unique morphology of these CNFs as a function of refinement energy. This characterization was employed to understand how the materials’ morphologies affected their interfacial interactions with porous substrates. In this work, optical, scanning electron, and atomic force microscopy were used to characterize the materials and mechanical testing was used to assess their interfacial interactions with porous model substrates. The second phase of the research explored how the grafting-through polymerization of commodity monomers occurred in the presence of methacrylated CNFs. Infrared spectroscopy measurements were used to explore the degree of grafting and microscopic analyses were employed to understand how these modifications affected the materials’ suspension morphology.

The final phase of the research looked at the modifications’ effects on drying behavior, surface energetics, and reinforcement ability in poly(lactic acid) (PLA). Scanning electron microscopy and inverse gas chromatography provided insights into how the grafted-polymer modifications improved the fibrillar morphology of spray-dried CNFs and increased their interfacial adhesion to PLA. Tensile testing and rheological characterization of composites made from these spray dried materials revealed their improved dispersion and network formation in the PLA matrix. Scale up of bench scale reactions to the pilot scale are demonstrated and 3D printing trials were conducted. Dramatic improvements in mechanical properties were seen for 3D printed samples modified with poly(N-isopropyl acrylamide). These improvements in mechanical properties were explored by dynamic mechanical analysis and tensile testing, revealing the effects of fibril alignment during printing.

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