A thesis submitted to the School of Graduate Studies in partial fulfilment of the requirements of the Master of Applied Science degree / This thesis investigated structure-function relationships of materials designed to mimic the plant cell wall by comparing their mechanical properties measured using strain-induced buckling methods. The plant cell wall mimics are submicrometer-thick films composed of cellulose nanocrystals (CNCs) and various types of xyloglucan (XG), a common plant hemicellulose. Our goal was to establish links between film composition/architecture and elastic modulus, to better understand the interactions between plant cell wall components and their influence on mechanical properties.
Three buckling methods for measuring mechanical properties of supported films were compared. All methods involved compressing a thin film deposited onto a shape memory polymer or an elastomeric substrate, through thermal shrinking or mechanical compression, respectively. Two thermal shrinking methods (constrained in one axis or unconstrained) and one compression method (using a mechanical strain stage) were used. Based on the mismatch of mechanical properties between the film and the substrate, the rigid thin film “buckles” upon compression to dissipate strain. The resulting surface wrinkle sizes are characteristic of the mechanical properties of the thin film. A Fourier analysis algorithm with Gaussian curve fitting was optimized to extract wrinkle sizes accurately and reproducibly from microscopy images to reliably quantify the elastic moduli of thin films.
To select the most precise strain-induced buckling method, model layer-by-layer (LbL) thin films composed of CNCs and polyethylene imine were tested. All three buckling methods precisely quantified the elastic moduli of the films and helped us build connections between the mechanical properties and the film composition. Elastic moduli determined were 15-44 GPa (depending on composition) and films up to 350 nm-thick were tested. Based on sensitivity analyses, however, unconstrained thermal shrinking proved to be the most robust method for calculating the elastic modulus. We believe these buckling methods may find widespread use in the characterization and surface structuring of thin films for applications in biosensors, flexible electronics, point-of-care diagnostics, and for studying plant cell wall mimics.
Using the unconstrained thermal shrinking method, plant cell wall mimics were constructed using LbL thin film assembly with various concentrations of CNCs and XG. Three types of XG were compared: (1) unmodified XG, (2) XG with a fraction of the galactosyl residues removed (degalactosylated), and (3) a fragmented lower molecular weight XG. It was inferred that molecular weight impacts the stiffness of XG-CNC based on adsorption conformation of XG onto CNCs, where lower molecular weight XG results in a higher modulus film (27 ± 1 vs. 19 ± 2 GPa). As well, saccharide residues of XG, specifically galactosyl, impact XG’s capacity for self-association and interaction with CNCs, because saccharide residues hinder association through their glucan backbone. This is evidenced by the higher elastic moduli calculated for degalactosylated XG-CNC films (75 ± 6, GPa), compared to native XG-CNC films (19 ± 2 GPa). This work highlights the importance of material structure as it relates to overall performance and therefore function in natural systems, such as the plant cell wall. These studies contribute to a greater understanding of the mechanical properties of the plant cell wall and serve as a basis to extend bio-based and biomimetic materials to applications such as drug delivery, packaging, and coatings. / Thesis / Master of Applied Science (MASc) / The plant cell wall boasts impressive mechanical properties, balancing seemingly opposing properties of structural strength with flexibility. These natural materials have been a source of inspiration for new material design, but the phenomena that govern interactions between components and how their structures translate into function, have yet to be fully understood. In this work, we have constructed thin multilayered films to mimic the plant cell wall, composed of cellulose nanocrystals (rod-shaped nanoparticles derived from plant cellulose) and xyloglucan (a common hemicellulose “glue”). When the films on flexible supports are compressed, they buckle into wrinkled surface patterns that can be used to calculate their mechanical properties. This investigation compares three buckling methods and supports the notion that the mechanical performance of the plant cell wall is strongly dependent on the structure of the different components and the way they interact.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/25543 |
| Date | January 2020 |
| Creators | Stimpson, Taylor |
| Contributors | Cranston, Emily, Moran-Mirabal, Jose, Chemical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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