Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Mechanical Engineering & Applied Mechanics

First Advisor

Kevin T. Turner

Abstract

Cellulose nanofibrils (CNFs) are a nanomaterial obtained from plant sources and have excellent mechanical properties, high aspect ratios, and biodegradability. As a reinforcing phase, CNFs have the potential to improve mechanical properties of polymer materials. The overarching objective of this thesis is to investigate the use of CNFs to enhance the strength andtoughness of polymers and traditional papers.

In the first part of this thesis, poly methyl methacrylate (PMMA) fibers are reinforced using CNFs to increase the strength and toughness. Fourier transform infrared (FTIR) spectroscopy is used to measure molecular orientation in fibers. Tensile tests and fiber-based fracture tests using edge-cracked fibers are used to quantify the enhancement of modulus, strength, and fracture toughness through the addition of CNFs to PMMA. Specifically, a 2× improvement in fracture toughness is observed at 1% wt. CNF content.

In the second part of the thesis, filter paper, which is a network of microscale cellulose fibers is infiltrated with CNFs to create all-cellulose sheets with heterogeneous mechanical properties. This is realized by printing and subsequent drying of an aqueous CNF solution and patterning of the infiltrated regions is used to engineer the strength and toughness.Single edge notch tension (SENT) tests are performed on the specimens to evaluate their fracture behavior. It is shown that geometric and elastic heterogeneity can be utilized to tune the toughness while maintaining or improving the strength.

Finally, to overcome limitations of SENT, a new experimental fracture specimen, the hinged rigid beam (HRB), was developed. The HRB eliminates the compressive stresses developed in conventional beam-bending fracture tests like the double cantilever beam method, thus making it suitable for testing thin materials such as paper. A mechanics model of the HRB was developed to allow critical strain energy release rate to be calculated from the measured force-displacement response and was validated via finite element analysis and experiments on thin PMMA sheets. This technique was used to characterize the toughness of several materials, including filter paper, copy paper and 2D lattices. Finally, the HRB was used to characterize and understand the fracture behavior of patterned nanocellulose infiltrated sheets.

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