Synthesis, Characterization, and 3D Printing of Conductive Polyhydroxybutyrate-Graphenic Nanomaterial Composites

• Author / Creator

  Li, Dan

• Advances in materials development are leading to the emergence of new classes of printable and inexpensive electronic devices, including sensors for healthcare monitoring. Most traditional sensors are built on rigid substrates, which restricts the use of these sensors. Research is driven by developing functional materials including conductors and composites that can respond to changes in their surroundings, and device fabrication techniques for patterning these materials on the micro scale. Among printable conductors, carbon nanomaterials including graphene, reduced graphene oxide (rGO), carbon nanotubes (CNTs) are highly conductive materials that can be printed from solution. However, these materials are comprised of rigid particles and tend to form brittle layers when printed without binders or additives, and their electrical properties are highly sensitive to moisture. A promising alternative is to incorporate carbon materials into polymer nanocomposites-consisting of conductive nanofiller embedded with an insulating, hydrophobic polymer matrix. The polymer matrix not only acts as a binder, but also provides mechanical support for the graphenic materials, allowing a variety of patterns and structures to be formed and maintained, while also protecting the conductive particles from moisture. Biopolymers, which are polymers that are derived from biological sources, can be introduced to this system as a more sustainable alternative to conventional polymers. In this thesis, conductive polymer composites comprised of the biopolymer polyhydroxybutyrate (PHB) and graphenic materials (i.e. rGO and CNTs) were formed through both solution processing and thermal mixing, with the aim of integrating these materials in conductive printable inks for flexible healthcare monitoring devices.
  
  A simple, solvent casting process was used to prepare the PHB/rGO composites using acetic acid as a food-safe solvent, and the effects of different reducing agents (sodium borohydride, hydrazine, and ascorbic acid (L-A.A)) have been systematically investigated. The L-A.A reduced rGO composites were found to be the most conductive composites, due to good dispersion, large particle size, and high intrinsic conductivity. The solvent method is compatible with many printing techniques such as direct ink writing and screen printing.
  The conductivity of PHB/rGO composites was tuned by varying the nanofiller concentration. The composites showed temperature-dependent resistivity, and in terms of sensing were found to have good selectivity to temperature with respect to both pressure and moisture. Flexible thin temperature sensors were developed by first printing silver electrodes and then depositing responsive composites on top of these electrodes. Stretchable sensors were fabricated through direct ink writing (customized 3D printing) of polydimethylsiloxane (PDMS) substrates on top of which a conductive composite ink was printed in a meandering pattern. Devices exhibited good sensitivity and stability, and thermal mapping was demonstrated using up to 12 × 12 array of sensing elements.
  Conductive PHB composites were made by an industrially-relevant thermal mixing process. Multi-walled carbon nanotubes (MWCNTs) were melt mixed in concentrations from 0.25 to 5 wt% using two types of polyhydroxybutyrate: one supplied by BRS (with some silicon impurities) and Biomer (PHB with plasticizers and nucleating agents), with a molecular weights of 190 kDa and 560 kDa, respectively. The extruded filaments were deposited in a layer by layer structure using a desktop 3D printer using a technique called ‘Fused Deposition Modeling’ (FDM), and the mechanical and electrical properties of printed designs were compared with hot compressed samples. 3D printed scaffolds with well-defined pore structure, tunable porosity, and high compressive modulus demonstrated that the composite materials have the potential for use in bone regeneration applications. The printed models showed geometrical and dimensional features close to the drawn model, reflecting the good printability of PHB composite materials. Overall, this work presents opportunities for the development of biopolymers that can be functionalized into printable composite inks in 2D and 3D printing for a variety of applications.

• Subjects / Keywords

  • Polyhydroxybutyrate-Graphenic Nanomaterial Composites

• Graduation date

  Spring 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-135z-hf23

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.