Dissertation -- Access Restricted On-Campus Only
Doctor of Philosophy (Ph.D.)
Hannes C. Schniepp
Graphene materials, exhibiting outstanding mechanical properties, are excellent candidates as reinforcement in high-performance polymer nanocomposites. In this dissertation, advanced atomic force microscopy (AFM) techniques are applied to study the nanomechanics of graphene/polymer nanocomposites, specifically the graphene/polymer interfacial strength and the stress transfer at the interface.;Two novel methods to directly characterize the interfacial strength between individual graphene sheets and polymers using AFM are presented and applied to a series of polymers and graphene sheets. The interfacial strength of graphene/polymer varies greatly for different combinations. The strongest interaction is found between graphene oxide (GO) and polyvinyl alcohol (PVA), a strongly polar, water-based polymer. On the other hand, polystyrene, a non polar polymer, has the weakest interaction with GO. The interfacial bond strength is attributed to hydrogen bonding and physical adsorption.;Further, the stress transfer in GO/PVA nanocomposites is studied quantitatively by monitoring the strain in individual GO sheet inside the polymer via AFM and Raman spectroscopy. For the first time, the strains of individual GO sheets in nanocomposites are imaged and quantified as a function of the applied external strains. The matrix strain is directly transferred to GO sheets for strains up to 8%. at higher strain levels, the onset of the nanocomposite failure and a stick-slip behavior is observed. This study reveals that GO is superior to pure graphene as reinforcement in nanocomposites. These results also imply the potential to make a new generation of nanocomposites with exceptional high strength and toughness.;In the second part of this dissertation, AFM is used to study the structure of silk proteins and the morphology of spider silks. For the first time, shear-induced self-assembly of native silk fibroin is observed. The morphology of the Brown Recluse spider silk is investigated and a novel silk/GO nanocomposite is proposed.;Finally, the growth, capacitance and frequency response of vertically oriented graphene sheets prepared by radio frequency plasma-enhanced chemical vapor deposition and used in electric double layer capacitors (EDLC) are presented. These capacitors exhibit the highest frequency response observed, to date, for carbon based materials, providing EDLC suitable for AC filtering. The results also suggest mechanisms other than surface area are operative in the double layer charge storage, such as a stronger polarization from graphene edges and vacancies.
© The Author
Cai, Minzhen, "Nanoscale Mechanical Characterization of Graphene/Polymer Nanocomposites using Atomic Force Microscopy." (2013). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539720323.
On-Campus Access Only