Tailorable Manufacturing of Polymer Nanocomposites using Oscillating Magnetic Fields
Open Access
- Author:
- Spencer, Mychal
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 16, 2019
- Committee Members:
- Namiko Yamamoto, Dissertation Advisor/Co-Advisor
Namiko Yamamoto, Committee Chair/Co-Chair
Christine Dolan Keating, Committee Member
George A Lesieutre, Committee Member
Raymond Edward Schaak, Outside Member - Keywords:
- Nanocomposites
Nanoparticles
Magnetic assembly
Oscillating field
Polymer-matrix composites - Abstract:
- Hierarchical integration of nanoscale additives has the potential to improve properties of traditional aerospace composites, such as fiber-reinforced composites (FRPs). While traditional aerospace composites can supply high mass-specific properties, their transport properties and interlaminar mechanical properties can be still improved. Proper integration of nanofillers with advanced properties into polymer matrices, so-called polymer nanocomposites (PNCs), can contribute to transport property (electrical and thermal) improvement for electrostatic protection and heat dissipation, in addition to mechanical property improvement and unconventional, anisotropic, multi-functional properties. However, application of PNCs is currently limited due to two challenges: unknown structure-interface-property relationships and missing scalable and precise manufacturing methods. While the advanced properties of PNCs are designed based on the nanofillers’ properties, the PNC properties are often largely affected by nanofiller structures and their interface conditions (both polymer-nanofiller and nanofiller-nanofiller). Without tailored nanofiller structuring, specifically for the property of interest, property enhancement with nanofiller implementation has been observed to be smaller than theoretical estimation, or even negative, in the past. Thus, in this work, 1D assembly of nanofillers using magnetic fields of low frequency (< 1 Hz) is studied as a scalable and precise way to organize nanofillers, including the number of inter-nanofiller contacts. Oscillating magnetic fields are chosen over other active assembly approaches (such as electric, acoustic, etc.) because such assembly is non-contact, energy-efficient (approximately 100 G), fast (< 1 hour), and can achieve high periodicity and precise patterning over large areas. Unlike past magnetic assembly studies using pulsed fields that rely on weak thermal diffusion, with oscillating magnetic fields, lateral and transverse assembly mechanisms can be balanced by induced magnetic field gradients. The method also enables nanofiller organizational tuning, and therefore interface and interphase property control, even in highly viscous polymer matrices. As a model case, theoretical and experimental studies were conducted about the magnetic organization of superparamagnetic iron oxide nanoparticles (SPIONs). Using an image processing technique, assembly trends were quantified (such as size, length, width, and separation) against the assembly parameters (flux density, frequency, waveform, and volume fraction). By allowing enough time for the nanoparticles to respond to the oscillating field, nanoparticle assembly was controlled using a sinusoidal waveform at very low frequencies (< 0.1 Hz, 100 G): the assembly line length, width, and separation was increased by 43%, 59%, and 139%, respectively, when compared to those prepared using static fields of the same strength. Based on the above study on SPION assembly, large-sized PNCs (20 mm × 20 mm × 10 mm) were fabricated with ferrimagnetic maghemite within an aero-grade thermoset. Two fabrication methods with different surface modification and magnetic assembly processes were evaluated: dry-processed nanoparticles (surface modification only) assembled with the magnetic field of a solenoid coil pair, and wet-processed nanoparticles (surface modification and aggregate size control) assembled with the magnetic field of a Helmholtz coil pair. Nanoparticle structures were evaluated using optical microscopy and microCT scans, which were then correlated with the field conditions (static vs. oscillating and solenoid vs. Helmholtz coil), and nanoparticle conditions (dry- vs. wet-processed). Magnetic assembly with a sinusoidal field was observed to produce thicker nanoparticle lines, which enhanced transverse assembly, forming a continuous nanoparticle network and achieving percolation. Meanwhile, thinner nanoparticle lines, with fewer inter-nanoparticle contacts, were observed when assembled with a DC field. The above PNCs were characterized for anisotropic electrical and thermal conductivities. As for the PNC samples with dry-processed nanoparticles, transverse assembly with a sinusoidally oscillating field enhanced continuity of the assembly lines, decreasing the percolation threshold (0.15 vol% for sinusoidal field vs. 0.45 vol% for a DC field). On the other hand, due to the different mechanisms between thermal vs. electrical transport, the reduced number of inter-nanoparticle contacts formed with a DC field (thin lines) was observed to be more effective at enhancing thermal conductivity (80% enhancement with a DC field at 4.7 vol% vs. 20% enhancement with a sinusoidal field at 3.5 vol%). As for the PNC samples with wet-processed nanoparticles, while homogeneous dispersion was improved and inter-nanoparticle contacts could be controlled (DC vs. sinusoidal), discontinuity of the assembly lines led to no observed percolation (up to 4.0 vol%) and small thermal conductivity increases (15% enhancement with a 300 G DC field at 4.0 vol%). These results demonstrate that an oscillating magnetic field is useful in tailoring nanoparticle structures and interfaces/interphases in a highly viscous matrix for anisotropic property control and potentially as a solution to the scalable manufacturing of PNCs. From the above studies about magnetic assembly and structure-property relationships, a design space and corresponding guidelines were developed for the fabrication of polymer nanocomposites. I expect this work will provide a foundation for future investigations into the scalable manufacturing of PNCs with tunable transport properties. Further work is necessary in the future to overcome the limitations of this work: additional structure-interface-property relationship studies to supply more precise structure quantification and the integration of field oscillation to tailor PNCs of other types than those investigated in this work.