Date of Award

Summer 8-22-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Committee Advisor

Carl P Tripp

Second Committee Member

William Gramlich

Third Committee Member

Douglas Gardner

Additional Committee Members

Alice Bruce

Barbara Cole

Abstract

One of the major problems facing humanity in the 21st century is the proliferation of petroleum-derived plastics in the environment. Polylactic acid (PLA), a bio-based polymer, is a promising alternative to petroleum-derived polymers. However, some of its properties such as low tensile strength and toughness limit its use in mainstream manufacturing industries. PLA may be reinforced with carbon or glass fibers; however, these synthetic reinforcements counteract the benefits of using a bio-based polymer. For this reason, cellulose nanofibers (CNFs), natural fiber reinforcements, may be used to strengthen PLA. CNFs have a hydrogel structure and are usually supplied as a 3 wt.% suspension, and thus, they must be dried prior to being incorporated into the PLA matrix. When CNFs are dried using evaporative methods, they agglomerate attributable to the high surface tension of water and the capillary forces exerted upon water removal. When CNFs agglomerate, they lose their nano-dimensionality and advantageous reinforcement properties. Thus, a method to dry CNFs and retain their nano-dimensional morphology is needed.

In this first two Chapters of this dissertation, we present a method to dry CNFs and to generate CNF-reinforced PLA composites using supercritical carbon dioxide (SC-CO2). Because SC-CO2 has zero surface tension, no capillary forces are exerted upon venting, and thus, we obtain disperse CNFs evenly distributed throughout the PLA matrix. With the addition of CNFs at 10 wt.% loading level, we obtain 23% and 6% increases in the tensile modulus and strength relative to neat PLA. These materials can also be used to generate CNF- reinforced PLA foams with enhanced compressive mechanical properties using SC-CO2 as the physical blowing agent, and this is presented in Chapter 3 of this dissertation. With the addition of CNFs at 2 wt.% loading level, we obtain 525% and 239% increases in the compression modulus and strength relative to neat PLA foams generated using the same foaming parameters.

Because CNFs are supplied as a 3 wt.% suspension, developing a method to concentrate or dry them to lower their transportation costs is important. In Chapters 4 and 5 of this dissertation, we present two different methods to dewater CNF suspensions using a dissolvable contacting particle and ultrasound-assisted SC-CO2 dewatering. Using the dissolvable contacting particle, we achieved a total solids content of up to 40 wt.% CNFs and the contacting particle was subsequently successfully removed. Using ultrasound-assisted SC-CO2 dewatering, we achieved a totals solids content of 20 wt.% CNFs. In both methods, the concentrated CNF suspensions were rehydrated back to 3 wt.% CNFs using manual mixing, and the CNFs retained the same fiber dimensions as in the original supplied 3 wt.% suspension.

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