Study of Hybrid Cellulose Nanocrystals in Polymeric Nanocomposites
Date
2023-02-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-9740-5626
Type
Thesis
Degree Level
Doctoral
Abstract
Modern society's demand for biobased and sustainable materials is increasing annually. Environmental pollution related to petroleum-based plastics has motivated research into developing alternative bioplastics. These alternatives include carbohydrates such as starch, cellulose, pullulan, kefiran, and proteins. However, films from these biobased resources are brittle, moisture sensitive, and difficult to process for bioplastic applications. To address these challenges, various modification methods have been studied, including chemical modification and reinforcement by nanoparticles. Regarding nanoparticles, the exponential increase in metallic nanoparticle usage for various applications has raised concerns about their harmful effects on the environment. Thus, cellulose nanocrystals (CNCs) are being investigated as an alternative natural filler material. Cellulose is the most abundant biopolymer on Earth extracted from a wide range of resources, including wood, bacteria, and tunicate which is a marine animal.
In the past decades, academic and industrial communities have considered CNCs to develop biodegradable materials for a wide range of applications. The alignment and interaction between CNCs with different aspect ratios in a polymeric nanocomposite are less clear when used as a filler. In this thesis, the effects of magnetic field and shear force on the alignment of CNCs have been investigated. Two approaches to film development have been studied including multilayer and crosslinking to produce CNC-based films with functional properties. In these studies, the effect of hybrid i.e., a mixture of high aspect ratio CNCs (tunicate-based CNCs) with low aspect ratio CNCs (wood-based CNCs) on the improvement in the mechanical and physicochemical properties of CNC-based nanocomposites has been established. This overcomes the limitation of wood-based CNC for developing films with tunable properties. A deeper insight has been developed into the interaction between CNCs of different aspect ratios by understanding the change in morphology, surface properties, mechanical properties, and thermal properties. These works enhance the understanding of CNC and its application in developing functional materials.
In summary, the magnetic field alignment and shear force alignment of the CNCs improved the physicochemical and mechanical properties of the samples, with hybrid CNC shear force aligned films showing elastic moduli up to 2 GPa in the parallel force direction. The 2D-XRD results on the CNCs films showed, at lower low concentrations (ā1%), the calculated Hermans order parameters for TCNC with high ARs (ā63) were much higher than for WCNC with low ARs (ā12). In comparison with TCNC's Hermans order parameter, hybrid CNC films (HCNC) (1:1 ratio of
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WCNC:TCNC) displayed approximately similar order parameters. The chiral nematic CNC films successfully interlocked in a multilayer system while maintaining their structure with different visible color reflections. Through self-assembly, WCNC films displayed a chiral nematic structure. In order to develop biodegradable and functional materials, different reflection colors (red, green, and blue) were achieved and fabricated. When TCNC was added to WCNC (hybrid 1:1 ratio), the colors faded. This implies that the change in the average aspect ratio can interfere with the formation of the chiral nematic structure. CNCs in a hybrid configuration have been crosslinked with PVA polymer. The results showed a decrease in hydrophilicity and an increase in mechanical performance up to 1.76 GPa for the elastic modulus. The elongation at break also decreased by almost 70% with CNC reinforcement. XPS results confirmed the formation of covalent bonds for all nanocomposites except those reinforced with WCNC, which exhibited lower levels of crosslinker (MBA) and CāC formation. The percolation network in the hybrid configuration showed that the thermal properties and the chiral nematic structure of the CNC films can be altered with the aspect ratio of the CNCs. Films containing the higher aspect ratio CNCs showed a surface area as high as 91.9 m2/g which was significantly high. Cross-sectional SEM images demonstrated that WCNC films exhibit chiral nematic phases, which are also confirmed by their iridescent appearance. TCNC films exhibited almost twice the elongation at break when compared with WCNC films.
In this research, a comprehensive understanding of the intermolecular interaction between hybrid CNCs and their effects on the mechanical, thermal, moisture barrier, gas barrier, physicochemical, and optical properties was investigated. Overall, the study will support the development of innovative biobased materials.
Description
Keywords
Cellulose Nanocrystals, Nanocomposite films, Biodegradable and Functional Materials, Carbohydrate Polymers
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemical and Biological Engineering
Program
Chemical Engineering