Investigating the Processing-Structure-Property Relationships of Auxetic Cellulose-Based Films
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Tao, Li
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Abstract
When stretched, few materials will expand in the direction perpendicular to the stretching direction. These materials are referred to as auxetic materials. Fiber networks of cellulose have been found to be auxetic, and the mechanism of the auxetic response in cellulose-based paper has been established. While the auxeticity of neat cellulose nanofibril (CNF) films prepared by vacuum filtration has been studied, the auxetic or non-auxetic behaviors of neat CNF films, neat cellulose nanocrystal (CNC) films, and CNF/CNC films prepared through drop casting have not been experimentally investigated. In this thesis, we show that out-of-plane auxetic response occurs in CNF films and CNF/CNC films, and the extent of this response is dependent on solids content and microstructure. All films are let air dry from 45 mL of suspension, with varying amounts of solids loading, followed by conditioning in 23°C and 50% relative humidity. Initial investigation into CNC films was unsuccessful. CNF films of all CNF solids loading showed various extent of auxetic response in the elastic region, of which 0.75 wt% CNF films exhibited the most consistent and highest extent of auxetic response. While some CNF/CNC films were auxetic in the elastic region, all CNF/CNC films were less auxetic than 0.75 wt% CNF films. Cross-sectional scanning electron microscopy (SEM) images combined with the auxetic testing results implied that low packing density and enough empty space in the internal structures of cellulose fiber networks might induce the auxetic response in CNF and CNF/CNC films. This thesis provides insights into the processing-structure-property relationships of auxetic cellulose-based films, which may have the potential to assist with the development of sustainable auxetic applications.
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Undergraduate Research Option Thesis