Structure and processing of polymer composites are essential for not only optimizing materials properties for high performance applications, but also making materials more environmentally sustainable. In the aerospace and automobile industries, the need for lightweight materials to increase fuel efficiency, while still boasting impressive mechanical properties drives innovation towards the manufacturing and use of more eco-friendly materials. In this dissertation, we concentrate on the processing and characterization of polymers reinforced with bio-based cellulose nanoparticles at an industrial scale. The successful incorporation of cellulose nanocrystals (CNCs) in polyamides, specifically polyamide 11 (PA 11) and polyamide 6 (PA 6), can result in a more sustainable material. However, unless polyamide nanocellulose composites can be produced at an industry scale, the use of traditional fillers like glass fibers will continue to be the industry standard despite their lack of being bio-based or providing comparable mechanical enhancement. Challenges of thermal stability, homogeneous dispersion, and moisture uptake have long served as the bottleneck to industrial-scale processing of these cellulose nanocomposites.
To overcome these challenges, various industrially viable pre-mixing techniques, such as planetary ball milling, roller blade mixing, and master batching, are developed herein to fabricate these materials for melt processing while retaining the thermal and mechanical integrity of the nanocomposites. In this dissertation, successful high-temperature processing of these nanocellulose composites is shown with resultant nucleated materials showing up to approximately 75 % reinforcement of PA 6 and up to 180 % of PA 11. The efficacy of cellulose nanocrystals as a nucleating agent and the respective crystallization kinetics of the nanocomposite at process-relevant conditions were explored using fast scanning calorimetry. It was found that the PA 11 composite would need at least 0.5 s above 100 °C in order to crystallize via heterogeneous nucleation and that in the heterogeneous regime, the nucleated samples exhibited much quicker peak crystallization times. This successful development and optimization of these processing methods and parameters taps into the immense potential cellulose nanomaterials have in creating high performance, environmentally sustainable materials.
Additionally, the use of these cellulose nanomaterials as templates for graphene oxide nanotubes is explored in this dissertation as well. Utilizing an inverting thermal degradation (iTD) method, the combustion kinetics of nanocellulose fibers were altered through surface hydrophobization and salt saturation to influence ignition propagation rate and ignition nucleating points. Samples were ignited at both slow and rapid rates, with flash burning of these nanofibers producing graphene oxide nanotubes through ignition of the bulk material and leaving behind the chemically altered surface structure of the nanofibers. Fiber aspect ratio, crystallinity, salt occlusion, ignition rate, and local oxygen availability proved influential in the resulting nanostructures, characterized through electron microscopy coupled with focused ion beam, solid state NMR, and XPS. Using this facile process and utilizing nanocellulose, the world's most abundant natural polymer, as the starting material, the work presented herein could have profound implications for the future production of environmentally sustainable carbon nanotubes.
In the biomedical industry, new materials and processing techniques are required to match the increasing demand for personalized treatment. Furthermore, bone implants that promote osseointegration and long-term retention has instigated the search for materials to replace the traditional metal or ceramic implants that typically come with a high risk of immune rejection. In this dissertation, a novel processing technique to fabricate porous poly(ether ether ketone) (PEEK) is developed and studied for its potential application as a bone replacement material.
A PEEK composite was created with the addition of nano hydroxyapatite (nHA), which has shown to improve the biocompatibility of the material. While hydroxyapatite-PEEK composites have previously been investigated for use in bone replacement, they have faced challenges with dispersion and ultimate mechanical tensile and compressive strength. The ethanol solvent-exchange based novel process presented herein produces a material with porous structuration that mimics the transition of cortical (compact) to cancellous (porous) bone while also having a homogeneous dispersion of hydroxyapatite throughout the material. With a density of 0.84 g cm−3 ± 0.18, this nanocomposite material exhibited compressive strength of up to 180 ± 15 MPa, which compares well to that of natural human cortical bone ranging from 100-230 MPa. This PEEK nanocomposite could provide a treatment alternative that displays a longer lifespan, lowers risk of immune rejection, and eliminates damage to surrounding tissue. The advancements in processing of nanocomposites presented in this work can greatly impact various industries from providing better medical care to reducing the carbon footprint of plastics. / Doctor of Philosophy / In the aerospace and automobile industries, the need for lightweight materials to increase fuel efficiency, while still boasting impressive mechanical properties drives innovation towards the manufacturing and use of more eco-friendly materials. This can be achieved through developing polymeric materials reinforced with nanoparticles derived from nature, specifically cellulose, the most abundant natural polymer. However, several challenges of processing at an industrial scale have served as a bottleneck to the commercial use of these environmentally friendly nanocomposite materials. In this work, various pre-mixing techniques are developed to fabricate these materials at an industrial scale while retaining the thermal and mechanical integrity of the nanocomposites. The successful development and optimization of these processing methods and parameters taps into the immense potential of cellulose nanocrystals (CNCs) as a reinforcement agent. Additionally, the use of these cellulose nanomaterials as templates for graphene oxide nanotubes is explored in this dissertation as well. Combustion kinetics of nanocellulose fibers were altered through surface modifications resulting in ignition of the bulk material, and retention of the surface structures of the nanofibers. Using this facile process and utilizing nanocellulose, the world's most abundant natural polymer, as the starting material, the work presented herein could have profound implications for the future production of environmentally sustainable carbon nanotubes. Furthermore, in the biomedical industry, new materials and processing techniques are required to match the increasing demand for personalized treatment. Furthermore, bone implants that promote integration with the body and long-term retention has instigated the search for materials to replace the traditional metal or ceramic implants that typically come with a high risk of immune rejection. To this end, polymeric implants, processed to mimic the structure and mechanical properties of bone, reinforced with nano hydroxyapatite (nHA), opens up the potential for improved interaction with the human body and ultimately longer implant retention. The advancements in processing of nanocomposites presented in this work can greatly impact various industries from providing better medical care to reducing the carbon footprint of plastics.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/107089 |
| Date | 24 June 2020 |
| Creators | Venkatraman, Priya |
| Contributors | Chemistry, Foster, Earl Johan, Long, Timothy E., Frazier, Charles E., Williams, Christopher Bryant, Barone, Justin Robert |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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