Ken-Hsuan “Kirby” Liao Ph.D. Thesis Defense, September 19, 2012

Graphene Synthesis and Graphene/Polymer Nanocomposites

Ken-Hsuan “Kirby” Liao Ph.D. Thesis Defense
Chemical Engineering and Materials Science Department
September 19, 2012, University of Minnesota

Advisor: Chris Macosko
Professor and IPrime Director

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Graphene, a two-dimensional carbon sheet with single-atom thickness, has recently attracted significant interest due to its unique mechanical and electrical properties. It has been reported that incorporation of graphene in polymers can efficiently improve the materials’ electrical and mechanical properties. For reliable integration of graphene into practical graphene/polymer nanocomposites, it is essential to have a simple, reproducible and controllable technique to produce graphene on a large scale. We successfully developed a novel, fast, hydrazine-free, high-yield method for producing single-layered graphene. Graphene sheets were formed from graphite oxide by reduction with de-ionized water at 130 ºC. Over 65% of the sheets are single graphene layers. A dehydration reaction of exfoliated graphene oxide was utilized to reduce oxygen and transform C-C bonds from sp3 to sp2. The reduction appears to occur in large uniform interconnected oxygen-free patches so that despite the presence of residual oxygen the sp2 carbon bonds formed on the sheets are sufficient to provide electronic properties comparable to reduced graphene sheets obtained using other methods.

Cytotoxicity of aqueous graphene was investigated with Dr. Yu-Shen Lin by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methyl-thiazolyl-diphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. An appropriate alternate assessment, using the water soluble tetrazolium salt (WST-8) assay, reveals that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e. whether or not aggregation occurs) and mode of interaction with cells (i.e. suspension versus adherent cell types).

Ultralow percolation concentration of 0.15 wt% graphene, as determined by surface resistance and modulus, was observed from in situ polymerized thermally reduced graphene (TRG)/ poly-urethane-acrylate (PUA) nanocomposite. A homogeneous dispersion of TRG in PUA was revealed by TEM images. The aspect ratio of dispersed TRG, calculated from percolation concentration and modulus, was found to be equivalent to the reported aspect ratio of single-layered free standing TRG. This indicates TRG is mono-layer-dispersed in the matrix polymer.

How graphene/polymer nanocomposite glass transition temperatures (Tg) vary was investigated in this study. First we surveyed the literature. No changes in Tg were observed for graphene/polymer nanocomposites synthesized via physical blending processes such as solvent or melt blending, except aqueous blending. In contrast, chemical blending processes such as in situ polymerization or chemically modified fillers yielded significant Tg increases in graphene/polymer nanocomposites. We attribute these results to bonding interactions at the interfaces between matrix polymers and fillers. Physical blending processes cannot provide enough interaction at the interfaces, whereas chemical blending processes can yield strong interaction such as covalent bonds. Aqueous blending of graphene or graphene oxide nanocomposites with water soluble matrix polymers also cause Tg increases, even though the blending processes involve no chemical reactions. The reason for this exception is that hydrogen bonding forms between fillers (graphene oxide or reduced graphene) and water soluble matrix polymers. We then measured Tg in PMMA. We used isotactic PMMA (i-PMMA) and syndiotactic-rich atactic PMMA (a-PMMA) to make TRG/PMMA nanocomposites using solvent blending and in situ polymerization in order to investigate the stereo-regularity and processing effects on the Tg. A Tg increase was found in i-PMMA and in situ PMMA but not in a-PMMA. The results can be explained by the thin film confinement effect of polymer. We attribute the Tg increase to both a higher interaction density and a stronger hydrogen bonding at the interfaces.

We have studied the elastic modulus of graphene oxide with various oxygen content. We used in situ AFM nano-indentation to measure the influence of oxygen on the elastic modulus of graphene oxide with various carbon/oxygen (C/O) ratios. The results show that chemical reduction (lower oxygen contents) decreases the elastic modulus of graphene oxide. We speculate that chemical reduction of oxygen atoms of epoxy groups on graphene oxide surface removes the bridging effect between carbon atoms, which leads to more flexible sheets.