Miguel A. López-Manchado

Graphene filled polymer nanocomposites

Graphene has attracted the attention of a growing number of scientists from several disciplines due to its remarkable physical properties and chemical functionalisation capabilities. This review presents an overview of graphene/polymer nanocomposites discussing preparation, properties and potential applications. The challenges and outlook of these emerging polymer nanocomposites are also discussed.

Functionalized graphene sheet filled silicone foam nanocomposites

In this article we report the successful manufacture of a novel functionalized graphene sheet (FGS)/silicone porous nanocomposite. Both the cellular microstructure and the properties of the porous nanocomposite were investigated in detail. The thermal properties show great stability and heat dissipation efficiency, highlighting their potential in applications with intense thermal requirements. Additionally, compression measurements indicate that there was a favourable interaction between the graphene nanosheets and the polymer.

Use of butylamine modified graphene sheets in polymer solar cells

We describe a facile method to use soluble chemically derived few-layer graphene sheets (GSs) as part of a transparent electrode for the preparation of polymer solar cells. Chemically functionalized GSs were obtained by first covalently attaching fluorine and then exposing the obtained fluorinated graphene sheets to an aliphatic amine at room temperature. Scanning electron microscopy, atomic force microscopy and UV-Vis analyses confirmed that transparent graphene sheets with an average thickness of 0.7–0.9 nm had been obtained. Their application in a polymer solar cell is demonstrated. Such functionalized graphene, which is cheap and easily prepared, is expected to be used as hole acceptor material in polymer photovoltaic applications.

Crystallization kinetics of polypropylene: 1. Effect of small additions of low-density polyethylene

In order to analyse the effect of small additions (up to 20 wt%) of low-density polyethylene (LDPE) on the crystallization kinetics and thermodynamics of an isotactic polypropylene (iPP), morphological studies of iPP/LDPE blends have been carried out. The results have shown that at LDPE percentages above 10% in the blend, the LDPE aggregates pass over the spherulitic limits and go into the intraspherulitic domains of the PP. The half-crystallization times (τ12) as well as the Avrami exponent (n) and the chain folding energy for PP crystallization (σe) increase markedly at LDPE percentages above 10% in the blend. The isothermal radial growth rate, G, of the PP in the blends is one-third of the plain polymer, although it remains constant at any blend composition. However, the overall kinetic rate constant seems to be influenced by the presence of the melt LDPE phase. In all cases, and according to the obtained results, an instantaneous nucleation takes place.

Crystallization kinetics of polypropylene: II. Effect of the addition of short glass fibres

The effect of short glass fibres on the polypropylene (PP) cyrstallization kinetics and thermodynamics has been investigated. Although at high fibre percentages in the composite the spherulitic limits are not clearly defined, the presence of crystalline entities is evident. A marked decrease of the half time of PP crystallization, τ12, as well as a sensible increase of the overall crystallization rate, Kn, has been observed in the presence of fibres. However, at any crystallization temperature, a maximum of τ12 is reached at 20% glass fibre content in the composite followed by a continuous decrease as fibre percentage increases. A slight increase in the Avrami exponent as temperature increases is obtained although it is relatively independent of composite composition. In all cases an instantaneous nucleation takes place, a nucleant effect of fibres is evident, and no transcrystallinity has been observed with these short glass fibres. Fibres give rise to a marked increase in the crystallinity of the samples. The chain folding energy for PP crystallization (σe) decreases with fibre content, and a minimum is observed at 20% fibre content.

Thermal conductivity of carbon nanotubes and graphene in epoxy nanofluids and nanocomposites

AbstractWe employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states. PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.

Functionalised graphene sheets as effective high dielectric constant fillers

A new functionalised graphene sheet (FGS) filled poly(dimethyl)siloxane insulator nanocomposite has been developed with high dielectric constant, making it well suited for applications in flexible electronics. The dielectric permittivity increased tenfold at 10 Hz and 2 wt.% FGS, while preserving low dielectric losses and good mechanical properties. The presence of functional groups on the graphene sheet surface improved the compatibility nanofiller/polymer at the interface, reducing the polarisation process. This study demonstrates that functionalised graphene sheets are ideal nanofillers for the development of new polymer composites with high dielectric constant values.PACS: 78.20.Ci, 72.80.Tm, 62.23.Kn

Poly(lactic acid)/natural rubber/cellulose nanocrystal bionanocomposites Part I. Processing and morphology

PLA/NR/cellulose nanowhisker composites were prepared using three types of cellulose nanocrystals (CNC), i.e. unmodified CNC obtained from acid hydrolysis of microcrystalline cellulose and two surface modified CNC. The two modification reactions, consisting on the grafting of long alkyl chains and of PLA chains onto the cellulose nanocrystals were carried out in order to facilitate the incorporation of the nanocrystals in the PLA/NR blend. A novel processing method was optimized combining solvent casting and extrusion in order to obtain a homogeneous dispersion of the nanofillers in the blend. The CNC modifications determined their location in the PLA/NR blend and influenced its morphology.

Poly(lactic acid)/natural rubber/cellulose nanocrystal bionanocomposites. Part II: properties evaluation.

The crystallization, mechanical and biodegradation properties of poly(lactic acid)/natural rubber/cellulose nanocrystals (CNC) bionanocomposites were evaluated. Three types of CNC were used in this study, one unmodified (CNC), long alkyl chain grafted CNC (C18-g-CNC) and PLA grafted CNC (PLA-g-CNC). The CNC modifications determined the affinity of the nanocrystals toward the polymers and reflected on the ultimate properties. Interestingly, PLA-g-CNC acted as a nucleating agent for the PLA matrix in the bio-based PLA/NR blend. Good mechanical properties were reported, as the bionanocomposites maintained a high elongation at break for a concentration up to 3 wt.% of cellulose nanocrystals. Moreover, the disintegration study confirmed that the materials completely disintegrated after one month in compost.

Towards materials with enhanced electro-mechanical response: CaCu3Ti4O12-polydimethylsiloxane composites

We describe a straightforward production pathway of polymer matrix composites with increased dielectric constant for dielectric elastomer actuators (DEAs). Up to date, the approach of using composites made of high dielectric constant ceramics and insulating polymers has not evidenced any improvement in the performance of DEA devices, mainly as a consequence of the ferroelectric nature of the employed ceramics. We propose here an unexplored alternative to these traditional fillers, introducing calcium copper titanate (CCTO) CaCu3Ti4O12, which has a giant dielectric constant making it very suitable for capacitive applications. All CCTO–polydimethylsiloxane (PDMS) composites developed display an improved electro-mechanical performance. The largest actuation improvement was achieved for the composite with 5.1 vol% of CCTO, having an increment in the actuation strain of about 100% together with a reduction of 25% in the electric field compared to the raw PDMS matrix.