Gary Ellis

Recent Advances in the Covalent Modification of Graphene With Polymers

Covalent binding of polymers to graphene represents an interesting alternative for the development of novel composite materials with a compendium of interfacial interactions. Through covalent linking, the concept of interface changes from a traditional view of interactions between components, such as van der Waals, hydrogen bonding, and so on, that is to say, at a polymer-filler interface, to a single compound concept where graphene forms an integral part of the polymeric chains. This feature article provides an overview of the strategies currently employed to functionalize graphene with polymers. We focus on the grafting-from and grafting-to methods used to bind polymers to graphene. The advantages and drawbacks, as well as the influence of each method on the final properties, are highlighted.

Rheological properties, crystallization, and morphology of compatibilized blends of isotactic polypropylene and polyamide

Blends of isotactic polypropylene (iPP) with the polyamide nylon-6 (N6), prepared by extrusion, were studied with a composition of up to 30% by weight polyamide. In the case of a 70/30 iPP/N6 blend, the influence of a compatibilizing agent based on polypropylene functionalized with maleic anhydride (PP-g-MA), with compositions of 1, 3, 5, and 10% by weight in polypropylene, was followed. The influence of the concentration of N6 and the compatibilizing agent on the rheological and thermal properties, and the morphology of the blends, was analyzed by monitoring the melt viscosity at different shear rates, differential scanning calorimetry, and polarized light microscopy. Vibrational spectroscopy was used to characterize the blends and to study the effect of the compatibilizing agent. The viscosity—composition curves for the iPP/N6 blends, in the composition and shear rate ranges analyzed, show a negative deviation from the additive rule, while the opposite trend is observed for the blends compatibilized with PP-g-MA. Important variations in the spectroscopic behavior was observed between compatibilized and noncompatibilized blends, which varied as a function of the compatibilizing agent concentration. The crystallization rates of iPP in the iPP/N6 blends, under both dynamic and isothermal conditions, are much greater than are those observed for pure iPP and are directly related to the nucleating activity of the polyamide. This effect is much smaller in the presence of the compatibilizing agent. The isothermal crystallization of the polyamide N6 in compatibilized blends is affected by the presence of iPP, reducing the crystallization rate due to the diluent effect of the polypropylene.

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.

Graphene functionalisation with a conjugated poly(fluorene) by click coupling: striking electronic properties in solution.

Graphene flakes covalently modified with a conjugated polymer, poly[(9,9-dihexylfluorene)-co-alt-(9,9-bis-(6-azidohexyl)fluorene)] (PFA), were efficiently synthesised by a Cu-catalysed Huisgen 1,3-dipolar cycloaddition between alkyne-modified graphene and an azide-functionalised polymer. Two approaches for the modification of graphene with alkyne groups were investigated (coupling with a diazonium salt generated in situ or an amidation reaction) and the optimum conditions determined. The success of the click-coupling approach was confirmed by FTIR, (1)H NMR, Raman, and X-ray photoelectron spectroscopy (XPS). The absorption and emission spectra of the click product show a strong solvent dependency.

A study of the autoxidation of some unsaturated fatty acid methyl esters using Fourier transform Raman spectroscopy

Abstract Near-infrared Fourier transform Raman and Fourier transform infrared spectroscopy have been used to investigate the chemical changes taking place during the curing reaction of several fatty acid methyl esters, which are used for modelling processes in the autoxidation of alkyd resin coatings. We have studied methyl oleate, methyl linoleate, and methyl linolenate in an attempt to monitor the degree of unsaturation within the fatty acid methyl esters (FAMES) during the complex autoxidation/polymerisation reaction that takes place once the paint system is coated onto a substrate and exposed to the atmosphere. The peaks around 1655 cm −1 have been assigned as follows: to the trans isomer at 1670 cm −1 , the cis isomer at 1655 cm −1 and the conjugated structure at 1640 cm −1 [B. Schrader, Raman/Infrared Atlas of Organic Compounds (2nd Edn), VCH, Weinheim (1989); J. K. Abenyega, M. Claybourn and G. Ellis, in preparations]. Raman spectra for the cure of methyl linoleate after 24 h show several interesting features, suggesting the formation of a highly conjugated cyclic structure. Current theories about the mechanism for the autoxidation of methyl linoleate make no mention of this aromatic product.

Carbohydrate Hydrogen‐Bonding Cooperativity − Intramolecular Hydrogen Bonds and Their Cooperative Effect on Intermolecular Processes − Binding to a Hydrogen‐Bond Acceptor Molecule

The high hydroxy (OH) group content in carbohydrates makes the study of carbohydrate OH···XH and OH···X H-bond energetics fundamental to understanding of carbohydrate recognition. There is, however, a relative lack of knowledge concerning the factors that allow a carbohydrate to participate in recognition events stabilised by intermolecular H bonds. We therefore present here a systematic study on the factors that determine the formation of a well-defined intramolecular H-bonding network between carbohydrate hydroxy groups, and its cooperative or anti-cooperative influence on selected intermolecular processes mediated by H bonds. With this in mind, we first determined the H-bonding networks of a series of carbohydrate derivatives − monoalcohols, 1,2- and 1,3-diols and amidoalcohols − by 1H NMR and FT-IR spectroscopy. The hydroxy groups of these compounds showed different abilities to form intramolecular H bonds, depending on their relative positions and configurations on the pyranose ring, and on the nature of the adjacent functional groups. It has also been shown that both the directionality and strength of the intramolecular H-bonding network of a carbohydrate govern the formation of cooperative or anti-cooperative H-bond centres, with consequent repercussions on the thermodynamics of the intermolecular H-bonding interactions of the carbohydrate in question. From this study, some general rules for the prediction of the intramolecular H-bonding network characteristics of a given carbohydrate and its influence on the energetics of intended intermolecular recognition processes have been inferred. The results presented here give a new perspective over understanding of the role of the H-bonding interactions in carbohydrate recognition and have fundamental implications for the rational design of glycoconjugates incorporating H-bonding motifs with geometrical and electronic complementarity to given receptor molecules.

Morphology and thermal properties of novel poly(phenylene sulfide) hybrid nanocomposites based on single-walled carbon nanotubes and inorganic fullerene-like WS2 nanoparticles

We report for the first time the preparation and characterization of novel poly(phenylene sulfide) (PPS) hybrid nanocomposites based on polyetherimide-modified single-walled carbon nanotubes (SWCNT-PEI). A traditional melt processing strategy was employed providing a versatile, economic and scalable route to the creation of new materials with improved properties. The influence of the presence and concentration of SWCNT-PEI on the structure, morphology, dynamic crystallization behaviour, thermal stability and thermal conductivity of PPS in PPS/SWCNT-PEI nanocomposites was investigated in detail. In the presence of SWCNT-PEI, the crystallization rate of the PPS matrix in the nanocomposites was lowered with respect to that of neat PPS, and the thermal stability and thermal conductivity of PPS increased with increasing SWCNT-PEI content. The combination of PPS/SWCNT-PEI with inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles offers an attractive route to combine the merits of organic and inorganic materials into novel hybrid nanocomposite materials. A synergistic improvement in the thermal properties of PPS/SWCNT-PEI nanocomposites was observed with the addition of IF-WS2. As previously observed in PEEK hybrid systems, the dispersion, morphology and thermal properties of PPS/SWCNT nanocomposites could be tuned by the introduction of small amounts of IF-WS2. The results obtained are very promising and suggest that the use of IF-WS2, a cheap and environmentally friendly reinforcing filler, can also provide an effective balance between performance, cost-effectiveness and processability of polymer/CNT nanocomposites based on the PPS matrix.

Thermal decomposition of technological polymer blends 1. Poly(aryl ether ether ketone) with a thermotropic liquid crystalline polymer

The thermal decomposition of blends of poly(aryl ether ether ketone), PEEK, with a thermotropic liquid crystalline polymer (LCP), VECTRA, was investigated using thermogravimetric analysis under dynamic conditions. The activation energies were obtained using integral and differential methods. The thermal analysis of the blends show that thermal stability is clearly affected with respect to the unblended materials, and it was also observed that the degradation process is accelerated by blending. In the case of VECTRA, the LCP is destabilized at high PEEK contents. The degrading polymer blends, at various degrees of conversion, were examined by Fourier transform infrared spectroscopy. The degradation mechanisms were analysed from the changes in the vibrational spectra. Blending appears to modify only the rate of degradation of the component polymers, the mechanisms remaining unchanged.

Synchrotron Infrared Microscopy Study of the Crystalline Morphology of the Interphase in Polypropylene/LCP‐Fiber Model Composites

Abstract The interphase of quiescent and sheared isotactic polypropylene–liquid crystalline polymer (iPP–LCP) fiber model composites has been examined with synchrotron infrared microscopy down to a lateral spatial resolution of 3 µm. By using the polarization of the beam, it is possible to distinguish between α and β transcrystalline (TC) phases. Structural evidence is presented for the presence of an α layer that nucleates the β TC phase in agreement with previous morphological studies.

Isothermal crystallisation of iPP/Vectra blends by DSC and simultaneous SAXS and WAXS measurements employing synchrotron radiation

Abstract The isothermal crystallisation behaviour and morphology of blends of isotactic polypropylene, iPP, and a liquid crystal polymer, Vectra A950, has been studied using differential scanning calorimetry, optical microscopy and simultaneous WAXS and SAXS in real-time measurements using synchrotron radiation. It has been observed that Vectra domains act as sites for the nucleation of iPP, and the rate of crystallisation is enhanced with increasing Vectra content in the blend. The presence of the α crystalline form in pure iPP, and both α and β forms for iPP in iPP/Vectra blends has been found. The SAXS patterns for iPP/Vectra blends containing β iPP are characterized by two different long period values that were related to the α and β lamellae. The secondary crystallisation mechanism has been investigated by SAXS/WAXS experiments. It is shown that, in contrast to primary crystallisation, secondary crystallisation of iPP is not affected by the presence of the thermotropic liquid crystalline polymer. As already known from pure iPP, the main process of secondary crystallisation is the growth of new lamellar stacks within remaining amorphous regions in the iPP spherulites.