PhD Thesis Defence Presentations - Sandra Gkermpoura

   This present thesis demonstrates the preparation and investigation of nanocomposites in fiber form and composite ion gels ("soft" matter) using graphene as nanofiller. The preparation of nanocomposites fibers was achieved in low boiling solvent, chloroform (CHCl₃). Stable graphene dispersions were prepared by the liquid-phase exfoliation of pristine graphite in chloroform using pyrene-end functionalized PMMAs as dispersing agents. The non-covalent functionalization of the basal plane of graphene based on π-π interactions with the pyrene functionalized PMMAs leads to stable G/PMMA(Py)_nhybrids which can be used as nanofiller for preparing nanocomposites fibers.

   The polymers were synthesized using the ARGET ATRP polymerization and the click chemistry. The degree of quaternization was calculated according to the GPC and Uv-Vis analyses. It was found that the degree of quaternization for the linear pyrene end-capped PMMA, Py-PMMA-Py, was 83% and for pyrene functionalized star PMMAs bearing from three, (PMMA-Py)₃, until six pyrene groups, (PMMA-Py₂)_3, was 100% and 71.5%, respectively. The characterization of hybrid G/PMMA(Py)_n was studied with several techniques, such as UV-Vis spectrophotometry, TEM, SEM and Raman spectroscopy. The telechelic Py-PMMA-Py proved considerably a more efficient dispersing agent than the mono-functional Py-PMMA and the non-functionalized PMMA. Also, shear mixing & tip sonication proved to be an effective method for graphite exfoliation in comparison to other preparation routes. The graphene concentration, C_G, of hybrid G/PMMA(Py)₂ increased drastically in the range of 0.25-1.2 mg/mL, as the centrifugation rated decreases. Characterization of this hybrid reveals that most of graphene sheets was few-layer graphenes (2-5 layers, ~50%) with size in the order of 1μm. Also, the presence of pyrene functionalized star PMMAs led to stable few-layered graphene sheets in chloroform. The polymer (PMMA-Py₂)₃ proved to be a more efficient dispersing agent than its non quaternized analogue, (PMMA-Py)₃, as the characterization of the hybrids revealed. Too, the atomistic simulations showed excellent agreement with the experimental results and revealed the type of the various G/PMMA(Py)₂ and G/PMMA(Py)₃ conformations. More importantly, the atomistic simulations showed that the presence of pyrene functionalized PMMAs are responsible for the no-aggregation of graphene sheets after solvent evaporation during the composites’ preparation.

   As a next step, nanocomposite fibers PMMA/G_PMMA(Py)_n were successfully prepared through electrospinning using G/PMMA(Py)_n hybrid as nanofiller. The morphology of produced nanocomposite fibers is affected from nanofiller’s concertation. For low-concentration nanofiller, nanocomposite fibers show uniform morphology with no beads, as SEM microscopy reveals. Also, compared with the pure PMMA fibers, the nanocomposite PMMA/G-PMMA(Py)_n showed thinner fibers. The existence of graphene sheets in fibers was confirmed by TEM microscopy. The preliminary data of electrical properties of nanocomposite fibers were encourageable. Especially, for the nanocomposite fiber PMMA/G-PMMA(Py)₂ with graphene concentration 1 wt%, the conductivity σ_ac increases 7-8 orders magnitude comparing with the pure PMMA fibers. Also, improved mechanical properties of nanocomposites fibers of PMMA/G-PMMA(Py)₃was achieved.

   The last step of this study was the investigation of composite ion gels Polymer/Graphene in the ionic liquid [Bmim][PF₆]. The composite ion gels have been investigated in terms of influence of graphene sheets to their properties (mechanical and electrical). The P(BzMA-co-tBuMA)-b-P(MMA-b-EG-b-MMA)-b-P(BzMA-co-tBuMA) and PEO-PS-PEO were choosed for the preparation of composite ion gels. It was found that the formation of gels affects the improvement of gel’s properties by the presence of graphene. Also, with the addition of graphene, the electrical properties of the composite ion gels are evidently improved with the respect to that of polymeric ion gels. As a result, the PEO-PS-PEO/Graphene in [Bmim][PF₆] showed enhanced mechanical and electrical properties and thermal stability until to 100^oC.