Structure/property relations of graphene oxide/epoxy nanocomposites: tailoring the particle surface chemistry for enhanced electrical and thermal performance

Abstract

In this study, graphene oxide (GO) of various surface chemistry configurations were characterized and then utilized as epoxy fillers with a main objective of enhancing the electrical and thermal performance of the matrix, without compromising the mechanical properties.
The initial step of the study was to distinguish and establish the chemical pathways through which the surface chemistry of highly oxidized GO interacts with the crosslinking reactions of the matrix. For this, GO was produced with acidic oxidation, based upon potassium permanganate (KMnO4) and then characterized via Raman, thermogravimetric analysis (TGA) and X-ray spectroscopy (XPS), which revealed increased graphitic disorder and oxygen-based functionalities decorating the lattice. Afterwards, the GO was dispersed within the epoxy matrix via a solvent-based method, to give nanocomposites containing up to 2 wt.% of GO, a filler content that is sufficient for filler/matrix chemical interactions. The excess of epoxide groups in the system, associated with the GO surface chemistry, was confirmed with Fourier transform infrared spectroscopy (FTIR). These additional moieties react with the hardener consequently, displacing the reaction stoichiometry away from the optimum. The result of this is a change in the macromolecular architecture, which was revealed through the dielectric secondary relaxations. Furthermore, during post-curing (> 100 oC), hydroxyl groups on the GO surface react with residual epoxide groups through etherification reactions, to give a marked increase in the glass transition temperature (Tg). These reactions lead to increased filler/matrix interfacial interactions and contribute to increased tensile performance. In addition, post-curing serves to partially reduce the defect content of the GO lattice which, in turn, slightly increases the electrical conductivity of the system.
After establishing the chemical pathways of the GO/epoxy reactions and demonstrating the inefficient features of GO in enhancing the electrical and thermal properties of epoxy, an alternative surface chemistry should be sought. Thus, the second step of this study was to introduce an single-step synthetic route for the production of moderately oxidized GO (mGO), which would: allow enhanced electrical and thermal properties; maintain epoxy compatibility; ensure no adverse influence on the epoxy curing reactions and require potentially simplified material processingstrategies. This route included replacement of the KMnO4 with chromium trioxide (CrO3) as the oxidizing agent. The mGO was then characterized and contrasted with the previously synthesized GO and a commercially available low-oxygen graphitic product (edge-oxidized GO, eGO). Raman spectroscopy, TGA and XPS demonstrated a moderate level of oxidation and a reduced carbon defect content, compared to the GO and the eGO. Subsequently, the eGO and mGO were incorporated into the epoxy via a scalable high-speed mixing method and the respective nanocomposites were contrasted. Transmission and scanning electron microscopy showed a fine dispersion/exfoliation for the mGO and poor compatibility for the eGO which drastically affected the aspect ratio of the respective platelets. It was revealed that the mGO/epoxy interactions include slight perturbation of the epoxy crosslinking, albeit only at high filler contents (> 12 wt.%), while the eGO did not react with the matrix at all. Ultimately, the mGO led to a low electrical percolation threshold (Pt) of ~1 wt.%; a maximum increase in electrical conductivity of about eight orders of magnitude and a maximum thermal conductivity increase of 200% compared to the unfilled epoxy, while the tensile performance of the system was not compromised. Conversely, the eGO/epoxy systems showed poor behaviour, with a Pt of ~10 wt.% and a maximum thermal conductivity increase of 150%, while the tensile performance was rapidly compromised. Those effects were attributed to the fact that mGO displays mildly oxygenated graphitic lattice - not only on the peripheral (as in the case of eGO) but also on the basal plane.
Upon the single-step production of moderately oxidized GO surface chemistry, the possibilities of further improvements in terms of electrical and/or thermal performance had to be explored. Thus, the final step of this study was to graft various amino-terminated moieties onto the surface of mGO in an attempt to modify, furtherly, the interfacial interactions with the epoxy matrix. For this, the mGO was functionalised with two bifunctional molecules: poly(propylene glycol) bis(2-aminopropyl ether) of different molar masses (termed d230 and d4000 accordingly) and a trifunctional trimethylolpropane tris[poly(propylene glycol), amine terminated] reagent, termed t440. The grafting process was revealed to be successful via Raman spectroscopy, TGA and XPS, and the resulting functionalised (fGO) systems were termed d230/fGO, d4000/fGO and t440/fGO. It was shown that the grafting included typical epoxide-amine reactions that potentially increase the disorder onto the graphitic lattice, while the elevated temperatures of the process served to slightly reduce the initial mGO oxygen content. Afterwards, the abovementioned three fGO systems were incorporated into the epoxy where it was demonstrated by differential scanning calorimetry (DSC) that the presence of the grafted moieties affected the local fGO/matrix interfacial interactions and slightly perturbed the epoxy curing reactions. X-ray diffraction (XRD) revealed reduced graphitic stacking with increased reagent molecular mass, which eventually led in reduced Pt (0.5 wt.% with the usage of the d4000 reagent). Furthermore, the maximum electrical conductivity of the respective nanocomposites appeared to be slightly increased with increasing reagent molecular weight, an effect also related to the limited platelet stacking. For the same, potentially, reasons the thermal conductivity of the fGO-containing systems was adversely affected at low filler contents.

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Identifiers

ORCID for Orestis Vryonis: orcid.org/0000-0002-2862-4494

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

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