Eunice Cunha

High performance free-standing films by layer-by-layer assembly of graphene flakes and ribbons with natural polymers

In this work, novel free-standing (FS) films based on chitosan, alginate and graphene oxide (GO) were developed through layer-by-layer assembly. First, GO was synthesized from graphite and multi-walled carbon nanotubes using a modified Hummers method, yielding oxidized graphene flakes (o-GFs) and oxidized graphene nanoribbons (o-GNRs), respectively, which were then characterized. Then FS films were produced and their morphological, thermal and mechanical properties, as well as the o-GF and o-GNR dispersion along the films were assessed. Their degradation and swelling profiles as well as their biological behavior were evaluated. Graphite and nanotubes were successfully oxidized and exfoliated forming stable suspensions that could be combined with chitosan (CHI) and alginate (ALG) solutions by layer-by-layer processing. The addition of o-GFs and o-GNRs resulted in rougher, hydrophilic FS films with significantly improved mechanical properties relative to CHI/ALG films. The presence of o-GFs or o-GNRs did not affect the thermal stability and the addition of o-GFs resulted in films with enhanced cytocompatibility. The results demonstrate the high potential of the GO reinforced films for biomedical applications, in particular o-GF films, for wound healing, and cardiac and bone engineering applications.

Self-Assembled Functionalized Graphene Nanoribbons from Carbon Nanotubes

Graphene nanoribbons (GNR) were generated in ethanol solution by unzipping pyrrolidine-functionalized carbon nanotubes under mild conditions. Evaporation of the solvent resulted in regular few-layer stacks of graphene nanoribbons observed by transmission electron microscopy (TEM) and X-ray diffraction. The experimental interlayer distance (0.49-0.56 nm) was confirmed by computer modelling (0.51 nm). Computer modelling showed that the large interlayer spacing (compared with graphite) is due to the presence of the functional groups and depends on their concentration. Stacked nanoribbons were observed to redissolve upon solvent addition. This preparation method could allow the fine-tuning of the interlayer distances by controlling the number and/or the nature of the chemical groups in between the graphene layers.

Biomedical films of graphene nanoribbons and nanoflakes with natural polymers

Graphene and its derivatives are promising as reinforcement for polymer nanocomposites. Additionally to their inherent outstanding mechanical properties, these nanoparticles may be functionalized to enhance their compatibility with the polymeric matrix and provide specific chemical and physical properties. In this work, new freestanding films (FS) based on chitosan (CHI), alginate (ALG) and functionalized graphene were developed using the layer-by-layer assembly. Suspensions of functionalized graphene nanoflakes (f-GF) and nanoribbons (f-GNR) were prepared from expanded graphite (EG) and multi-walled carbon nanotubes (MWNTs), respectively. The graphene nanoflakes and MWNTs were covalently functionalized using a 1,3-dipolar cycloaddition reaction that allowed the nanoparticles exfoliation. f-GNR and f-GF suspensions were characterized to demonstrate that graphene nanoflakes and MWNTs were successfully functionalized and exfoliated. Then, the layer-by-layer deposition of CHI, ALG and both types of functionalized graphene was investigated and FS films were produced. The morphology, thermal and mechanical characteristics of the produced FS films were assessed. Their degradation and swelling profiles as well as their biological behavior were evaluated. The incorporation of f-GF resulted in smoother films while the incorporation of f-GNR resulted in rougher films. When compared with the CHI/ALG bi-component films. Both graphene containing films remained hydrophobic. The graphene incorporation in the multilayered FS was estimated to be 1.7 wt% for f-GF and 2.5 wt% for f-GNR. The presence of functionalized graphene did not affect the thermal stability of the films, it increased the storage modulus and the dynamic mechanical response at 1 Hz and 37 °C, and decreased the electrical resistivity. The biological assays revealed cytocompatibility towards L929 cells when both f-GF and f-GNR were incorporated in the CHI/ALG matrix. In conclusion, these new f-GF and f-GNR reinforced FS films present great potential for use in biomedical applications such as films for wound healing or cardiac and bone engineering.

Production of cellulose nanofibers from Alfa grass and application as reinforcement for polyvinyl alcohol

ABSTRACT The work reported demonstrates an simple method of extracting cellulose nanofibers (CNF) from cellulose microfibers (CMF) obtained from the plant Stipatenacissima. Here, a method for the production of CNF from CMF extracted from Alfa grass by exfoliation in polyvinyl alcohol (PVA) solution, is demonstrated. The CMF were produced in powder form and exfoliated in PVA aqueous solution to produce composites with 2, 4, 5 and 10 wt-% of CNF. Scanning Electron Microscopy demonstrated exfoliation of CMF, dispersion of the CNF and wetting by the polymer. The composites were characterised by thermogravimetry, differential scanning calorimetry, X-ray diffraction and tensile testing. The addition of CNF to PVA reduced the crystallinity degree of PVA. The large increase of the Young’s modulus from 38 to 113% (relative to pure PVA) for composites with 2 to 10 wt-% of CNF incorporation is consistent with the extensive exfoliation of CMF into CNF and its excellent interface with PVA.

Water Dispersible Few-Layer Graphene Stabilized by a Novel Pyrene Derivative at Micromolar Concentration

The search for graphene or few-layer graphene production methods that are simple, allow mass production, and yield good quality material continues to provoke intense investigation. The present work contributes to this investigation through the study of the aqueous exfoliation of four types of graphene sources, which are namely graphite and graphite nanoflakes with different morphologies and geographical origins. The exfoliation was achieved in an aqueous solution of a soluble pyrene derivative that was synthesized to achieve maximum interaction with the graphene surface at low concentration (5 × 10−5 M). The yield of bilayer and few-layer graphene obtained was quantified by Raman spectroscopic analysis, and the adsorption of the pyrene derivative on the graphene surface was studied by thermogravimetric analysis and X-ray diffraction. The whole procedure was rationalized with the help of molecular modeling.

Melt mixing functionalized graphite nanoplates into PC/SAN blends

The authors gratefully acknowledge the financial support by Instituto de Polimeros e Compositos of the University of Minho and Fundacao para a Ciencia e Tecnologia (FCT) through Project PEst- C/CTM/LA0025/2013 (Strategic Project e LA 25-2013-2014). M. C. Paiva acknowledges the Leibniz Institute of Polymer Research Dresden for receiving her as guest researcher, and E. Cunha acknowledges FCT for PhD grant (SFRH/BD/87214/2012).