Date of Award

5-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Kitchens, Christopher L

Committee Member

Ogale , Amod A

Committee Member

Hirt , Douglas E

Committee Member

Brown , Philip J

Abstract

Development of novel bio-based nanocomposites is fundamental to reduce the dependence on fossil resources and provide a sustainable future. In light of this global problem, cellulose nanocrystals (CNCs) have raised interest due to their remarkable mechanical properties. Adequate design of nanocomposites requires a fundamental understanding of how the structure affects the mechanical properties of the material. However, contradictory results in the literature have revealed a lack of comprehension of this relationship for CNC based composites. Consequently, this work provides an explanation to unusual results for CNC nanocomposites presented in the literature and demonstrates the ability to achieve property enhancements in biopolymer nanocomposites that are attributed to the nanoscale structure and self-assembly of CNCs.
The phase behavior and rheology of CNC aqueous suspension is first studied. The twisted shape of the cellulose crystals induces the transition of aqueous CNC suspensions from an isotropic fluid to a chiral nematic liquid crystal; at even higher concentrations a birefringent gel is obtained. The structural transformations can be identified from rheological measurements given that they affect the viscoelastic properties of the suspensions.
When the CNC aqueous suspensions are blended with sodium alginate solutions, the nanocrystals induce ordering of the alginate chains. These solutions were wet spun to form CNC - calcium alginate nanocomposite fibers. The twisted shape of the cellulose crystals induce the formation of a helical structure where the CNCs spiral around the fiber axis; this structural orientation is coincident with a reduced elastic modulus and an increased elongation at break. Until now this orientation had been observed only in native cellulose fibers and is deterministic of their unique mechanical properties. The presence of CNCs in the dope solution enables an increase in the stretching capacity during fiber spinning and an increase in the apparent jet stretch (JA). Wet spinning at the maximum possible JA retards the appearance of a spiral assembly of the CNC and consequently improves the mechanical properties of the fibers, which is expected with the addition of a rigid nanoparticle. Tensile testing at maximum JA showed a 38% increase in tenacity and a 123% increase in tensile modulus at 10 wt % CNC loading.
Given the importance of realistic measurements of CNC dimensions in suspension, a simple method to analyze static light scattering (SLS) data was developed. The applicability of Debye's expansion of the form factor was extended beyond the second order of the scattering vector. Simple analytical expressions were derived for the fourth and sixth moments of the distances within a nanoparticle that are valid for all shapes and can be used to solve for the average dimensions of monodispersed and polydisperse tri-axial nanorod populations. Without needing to solve for the dimensions, the moments provide information about aspect ratio regardless of the particle's geometry. This methodology can be easily applied to nanoparticles or macromolecules of complex shapes.

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