José M. González-Domínguez

The influence of a compatibilizer on the thermal and dynamic mechanical properties of PEEK/carbon nanotube composites

The effect of polyetherimide (PEI) as a compatibilizing agent on the morphology, thermal, electrical and dynamic mechanical properties of poly(ether ether ketone) (PEEK)/single-walled carbon nanotube (SWCNT) nanocomposites, has been investigated for different CNT loadings. After a pre-processing step based on ball milling and pre-mixing under mechanical treatment in ethanol, the samples were prepared by melt extrusion. A more homogeneous distribution of the CNTs throughout the matrix is found for composites containing PEI, as revealed by scanning electron microscopy. Thermogravimetric analysis demonstrates an increase in the matrix degradation temperatures under dry air and nitrogen atmospheres with the addition of SWCNTs; the level of thermal stability of these nanocomposites is maintained when PEI is incorporated. Both differential scanning calorimetry and synchrotron x-ray scattering studies indicate a slight decrease in the crystallization temperatures of the compatibilized samples, and suggest the existence of reorganization phenomena during the heating, which are favoured in the composites incorporating the compatibilizer, due to their smaller crystal size. Dynamic mechanical studies show an increase in the glass transition temperature of the nanocomposites upon the addition of PEI. Furthermore, the presence of PEI causes an enhancement in the storage modulus, and hence in the rigidity of these systems, attributed to an improved interfacial adhesion between the reinforcement and the matrix. The electrical and thermal conductivities of these composites decrease with the incorporation of PEI. Overall, the compatibilized samples exhibit improved properties and are promising for their use in industrial applications.

Solvent-Free Preparation of High-Toughness Epoxy−SWNT Composite Materials

Multicomponent nanocomposite materials based on a high-performance epoxy system and single-walled carbon nanotubes (SWNTs) have been prepared. The noncovalent wrapping of nitric acid-treated SWNTs with a PEO-based amphiphilic block copolymer leads to a highly disaggregated filler with a boosted miscibility in the epoxy matrix, allowing its dispersion without organic solvents. Although direct dispersion of acid-treated SWNTs results in modestly improved epoxy matrix mechanical properties, the incorporation of wrapped SWNTs produces a huge increase in toughness (276% improvement at 0.5 wt % loading) and impact strength (193% at 0.5 wt % loading) with no detrimental effect on the elastic properties. A synergistic effect between SWNTs and the block copolymer is revealed on the basis of tensile and impact strength results. Atomic force microscopy has been applied, obtaining stiffness mappings that identify nanostructure features responsible of the dynamic mechanical behavior. The electrical percolation threshold is greatly reduced, from 0.31 to 0.03 wt % SWNTs when block copolymer-wrapped SWNTs are used, and all the measured conductivity values increased up to a maximum of 7 orders of magnitude with respect to the baseline matrix (1 wt % wrapped-SWNTs loading). This approach provides an efficient way to disperse barely dispersible SWNTs without solvents into an epoxy matrix, and to generate substantial improvements with small amounts of SWNTs.

Covalent functionalization of MWCNTs with poly(p-phenylene sulphide) oligomers: a route to the efficient integration through a chemical approach

We report a new strategy to obtain high performance polymer-grafted multi-walled carbon nanotubes (MWCNTs). Chlorophenyl-functionalized MWCNTs, obtained through the in situ generation and reaction of diazonium compounds, were subjected to polymerization in the presence of Na2S and 1,4-dichlorobenzene in order to yield MWCNTs covalently functionalized with poly(p-phenylene sulfide) (PPS) oligomers. The MWCNT functionalization and the PPS oligomeric chain growth can be controlled throughout accessible experimental variables, including the possibility to carry out the whole process in a one-pot reaction. This represents an efficient and facile route to develop covalently grafted MWCNTs beyond those reported so far for the same polymer. These materials are promising fillers for the production of high performance PPS-based composite materials, due to the improvement of the filler–matrix compatibility. The manufacturing and characterization of some test samples show that oligomer-grafted MWCNTs induce the suppression of the intrinsic confinement effect imposed by the nanofiller, as for the observation of an increase in the PPS crystallinity. An outstanding increase in the PPS thermal stability and mechanical properties is also observed, as compared to bare MWCNTs, while leaving the electrical properties unharmed.

Single-walled carbon nanotubes covalently functionalized with polytyrosine: A new material for the development of NADH-based biosensors.

We report for the first time the use of single-walled carbon nanotubes (SWCNT) covalently functionalized with polytyrosine (Polytyr) (SWCNT-Polytyr) as a new electrode material for the development of nicotinamide adenine dinucleotide (NADH)-based biosensors. The oxidation of glassy carbon electrodes (GCE) modified with SWCNT-Polytyr at potentials high enough to oxidize the tyrosine residues have allowed the electrooxidation of NADH at low potentials due to the catalytic activity of the quinones generated from the primary oxidation of tyrosine without any additional redox mediator. The amperometric detection of NADH at 0.200V showed a sensitivity of (217±3)µAmM(-1)cm(-2) and a detection limit of 7.9nM. The excellent electrocatalytic activity of SWCNT-Polytyr towards NADH oxidation has also made possible the development of a sensitive ethanol biosensor through the immobilization of alcohol dehydrogenase (ADH) via Nafion entrapment, with excellent analytical characteristics (sensitivity of (5.8±0.1)µAmM(-1)cm(-2), detection limit of 0.67µM) and very successful application for the quantification of ethanol in different commercial beverages.

Grafting of a hydroxylated poly(ether ether ketone) to the surface of single-walled carbon nanotubes

A hydroxylated poly(ether ether ketone) (HPEEK) derivative has been covalently grafted onto the surface of acid-treated single-walled carbon nanotubes (SWCNTs) following two different esterification approaches. The hydroxylation degree of the HPEEK and the extent of the grafting reactions were determined by thermogravimetric analysis. Microscopic observations revealed an increase in the bundle diameter of the SWCNTs and the heterogeneous composition of the synthesized samples. Infrared, nuclear magnetic resonance and X-ray photoelectron spectra corroborated the grafting success, showing the appearance of signals associated with the ester group. The polymer-grafted SWCNTs display higher decomposition temperatures and a wider range of thermal degradation than the HPEEK. The esterification decreases the crystallization and melting temperature as well as the crystallinity of the HPEEK. The semicrystalline nature of the grafted samples was confirmed by X-ray diffraction analysis, which reveals a diminution in the crystal size of the polymer. Dynamic mechanical studies show an exceptional increase in the storage modulus and glass transition temperature of the polymer by the attachment to the SWCNTs. Slightly better thermal and mechanical properties are observed for the sample with the higher esterification degree. The HPEEK-grafted SWCNTs can be used as fillers to prepare PEEK nanocomposites with enhanced performance.

Influence of air oxidation on the surfactant-assisted purification of single-walled carbon nanotubes.

Arc discharge single-walled carbon nanotube (SWCNT) soot was treated under different experimental conditions including gas- and liquid-phase oxidation, heat treatment in an inert gas, and hydrogen gasification. Afterward, the samples were dispersed in a surfactant and centrifuged at a moderately high speed. Near-infrared spectra of all the dispersions were compared with that of raw SWCNT soot. The relative intensity of SWCNT characteristic spectral bands strongly increased for air-oxidized samples after centrifugation, while it did not substantially change for samples oxidized with nitric acid or reduced with hydrogen. The relative SWCNT spectral intensity was associated to the sample purity through the so-called purity index, which was calculated from the S(22) band transition of semiconducting SWCNTs. Air-oxidized samples experienced a 7-fold increase in the purity index during centrifugation, while it increased by only 2-3 times for nonoxidized samples. Air oxidation specifically improves the preferential stability of SWCNTs over carbonaceous impurities in the dispersions, leading to the highest purity index values reported so far.

Epoxy composites with covalently anchored amino-functionalized SWNTs: towards the tailoring of physical properties through targeted functionalization

Functionalization of single-walled carbon nanotubes (SWNTs) with covalently grafted amine moieties provides reactive fillers with potential for covalent anchoring to an epoxy matrix. Manufacturing and characterization of a high performance epoxy system reinforced with as-grown and aminated SWNTs are presented through four different approaches. Epoxy composite materials incorporating SWNTs aminated through sidewall addition reactions present enhanced mechanical, thermal and electrical properties, beyond the effect of unfunctionalized SWNTs. The functionalization pathways studied here lead to a composite with specific improvements in some of the physical properties of the epoxy matrix, which enables the tailored design of the composites properties through functionalization. The aminationviadiazonium reaction with 4-aminobenzylamine is especially effective in enhancing the tensile and impact properties of the epoxy composites (44% improvement in impact strength at 0.1 wt% loading) and leads to the highest increase in elastic modulus reported so far for the integration of aminated nanotubes into epoxy resin. Composites incorporating aminated SWNTs throughout the 1,3-dipolar cycloaddition reaction stand out for their thermo-oxidative stability and thermomechanical properties. The incorporation of as-produced arc-discharge SWNTs into the TGAP/DDS epoxy matrix leads to composite materials with the highest electrical conductivity among all the studied samples.

Reactive fillers based on SWCNTs functionalized with matrix-based moieties for the production of epoxy composites with superior and tunable properties

Composite materials based on epoxy matrix and single-walled carbon nanotubes (SWCNTs) are able to exhibit outstanding improvements in physical properties when using a tailored covalent functionalization with matrix-based moieties containing terminal amines or epoxide rings. The proper choice of grafted moiety and integration protocol makes it feasible to tune the composite physical properties. At 0.5 wt% SWCNT loading, these composites exhibit up to 65% improvement in storage modulus, 91% improvement in tensile strength, and 65% improvement in toughness. A 15 °C increase in the glass transition temperature relative to the parent matrix was also achieved. This suggests that a highly improved interfacial bonding between matrix and filler, coupled to improved dispersion, are achieved. The degradation temperatures show an upshift in the range of 40-60 °C, which indicates superior thermal performance. Electrical conductivity ranges from ~10(-13) to ~10(-3) S cm(-1), which also shows the possibility of tuning the insulating or conductive behaviour of the composites. The chemical affinity of the functionalization moieties with the matrix and the unchanged molecular structure at the SWCNT/matrix interface are responsible for such improvements.

Electrochemical sensing of guanine, adenine and 8-hydroxy-2′-deoxyguanosine at glassy carbon modified with single-walled carbon nanotubes covalently functionalized with lysine

This work reports the synthesis and characterization of single-walled carbon nanotubes (SWCNT) covalently functionalized with L-lysine (Lys) and the analytical performance of glassy carbon electrodes (GCE) modified with a dispersion of SWCNT-Lys (GCE/SWCNT-Lys) for the highly sensitive quantification of guanine, adenine and 8-hydroxy-2′-deoxyguanosine. Detection limits of 75, 195 and 97 nM were obtained for guanine, adenine and 8-hydroxy-2′-deoxyguanosine, respectively by voltammetric adsorptive stripping with medium exchange. GCE/SWCNT-Lys was successfully used for the detection of adenine and guanine oxidation after adsorption of salmon sperm-double stranded DNA. A clear definition of 8-hydroxy-2′-deoxyguanosine oxidation signal is observed even in the presence of large excess of guanine, adenine or salmon sperm-double stranded DNA.

Peptide-based biomaterials. Linking L-tyrosine and poly L-tyrosine to graphene oxide nanoribbons

Peptide-based biomaterials are being studied actively in a variety of applications in materials science and biointerface engineering. Likewise, there has been ongoing exploration over the last few decades into the potential biological applications of carbon nanomaterials, motivated by their size, shape, structure and their unique physical and chemical properties. In recent years, the functionalization of carbon nanotubes and graphene has led to the preparation of bioactive carbon nanomaterials that are being used in biomedicine as structural elements and in gene therapy and biosensing. The present study proposes different strategies for the bonding of l-tyrosine and the homopolypeptide poly-l-tyrosine to graphene oxide nanoribbons (GONRs). The covalent attachment of l-tyrosine was undertaken by amidation of the α-amine group of tyrosine with the existing carboxylic groups in GONR and by means of esterification through phenol nucleophiles contained in their side chains. In both cases use was made of protective groups to address the functionalization with the desired reactive groups. The linking of GONRs to the PTyr was attempted according to two different strategies: either by ester bonding of commercial PTyr through its phenol side groups or by in situ ring-opening polymerization of an N-carboxyanhydride tyrosine derivative (NCA-Tyr) with Tyr-functionalized GONRs. These biofunctionalized nanomaterials were characterized by Raman and infrared spectroscopies, X-ray photoelectron spectroscopy, thermogravimetric analysis, transmission electron microscopy, fluorescence and electrochemical techniques. On the basis of their properties, prospects for the potential utilization of the prepared hybrid nanomaterials in different applications are also given.