Pere Castell

Study of lanthanide triflates as new curing initiators for DGEBA

A new class of cationic initiators, lanthanide triflates, has been studied in the curing of diglycidyl ether of bisphenol-A (DGEBA). The kinetics of this process has been evaluated by the isoconversional method that has been proved as an excellent tool to predict the technical parameters of this process. The thermal stability of the crosslinked materials has been studied by thermogravimetric analysis (TGA) and their mechanical characteristics by dynamomechanic thermal analysis (DMTA).

Carbon Nanotube Effect on Polyaniline Morphology in Water Dispersible Composites

A straightforward, template-free chemical oxidative polymerization of aniline was used to prepare nanofibrillar polyaniline (nf-PANI) and a set of corresponding composites with multiwall carbon nanotubes (MWNTs). All the products showed remarkable water dispersibility since they are formed by hydrophilic particles of nanometric size. A comparative study performed on composites in a wide range of MWNT loadings has led to two main conclusions: on one hand, the presence of MWNTs affects neither the chemical structure nor the crystallinity of polyaniline. On the other hand, even small amounts of MWNTs have a significant effect on the morphology of polyaniline in composites. This effect is noticeable not only in electron microscopy images but also in the UV-vis absorbance of water dispersions and electrical conductivity behavior in the solid state. Competition between nucleation sites during polymerization is proposed as an explanation for these phenomena.

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.

Synthesis of New Epoxy Liquid-Crystalline Monomers with Azo Groups in the Central Mesogenic Core. Crosslinking with Amines

A new class of liquid-crystalline epoxy resins containing azomethine and azo groups was synthesized and structurally characterized. The liquid crystal behavior of these compounds was characterized by differential scanning calorimetry (DSC), hot-stage polarized optical microscopy (POM) and wide-angle X-Ray diffraction (WAXS). An aliphatic spacer between the mesogenic aromatic central core and the glycidyl groups favors the formation of different types of smectic mesophases with a wide range of stability. The LC resins were cured to obtain liquid crystal thermosets (LCTs). The cure was tested using aromatic primary diamines as curing agents and nematic-like networks were obtained. When tertiary amines were used catalytically, smectic C-like networks were retained when the spacer length had four methylene units or more. The polydomain ordered materials obtained were characterized by WAXS and their thermal stability was tested by thermogravimetric analysis (TGA).

Nanofibrilar Polyaniline: Direct Route to Carbon Nanotube Water Dispersions of High Concentration

Water dispersible nanofibrilar polyaniline (NF-PANI) provides a novel and direct route towards carbon nanotube water dispersions of high concentration. Carrying out the chemical synthesis of NF-PANI in the presence of carbon nanotubes (CNTs) results in an entirely nanostructured nanofibrilar polyaniline/carbon nanotube (NF-PANI/CNT) composite material that contains well segregated CNTs partially coated by NF-PANI. This new approach is simple, fast, and inexpensive, and enables the direct preparation of stable and homogeneous dispersions of the composites in water at concentrations up to 10 mg · mL(-1) , even for the highest CNT loadings of 50 wt.-% without the participation of surfactants or stabilizers.

Towards helical and Y-shaped carbon nanotubes: the role of sulfur in CVD processes

Sulfur is introduced as an additive in CVD processes for the production of carbon nanotubes over Co–Mo/MgO sol–gel catalysts employing two different approaches: for the first time as sulfonated Co–Mo/MgO sol–gel catalysts and, alternatively, as thiophene vapour during the CVD reaction process. On one hand, the use of sulfur compounds during the sol–gel catalyst preparation process generally leads to a significant modification of the matrix composition and the matrix–catalyst interaction. When employing a catalyst here containing sulfur compounds the induced matrix modifications yield enhanced growth of helical carbon nanotubes (HCNTs). On the other hand, adding sulfur in the form of thiophene vapour over non-sulfonated sol–gel catalysts, the growth of Y-shaped carbon nanotubes (YCNTs) is favoured depending on the flow conditions of the hydrocarbon source employed. The obtained results underline the general importance of the nature of the sol–gel matrix for the formation of HCNTs as well as of the local fluctuations in the gas phase for the growth of YCNTs. The findings are of importance for the fabrication of nanotube-based electronic devices.

Effect of Extrusion on the Mechanical and Rheological Properties of a Reinforced Poly(Lactic Acid): Reprocessing and Recycling of Biobased Materials

The aim of this research paper is to study the behaviour of a common used biopolymer (Poly(Lactic Acid) (PLA)) after several reprocesses and how two different types of additives (a melt strength enhancer and a nanoadditive) affect its mechanical and rheological properties. Systematic extraction of extrudate samples from a twin-screw compounder was done in order to study the effect in the properties of the reprocessed material. Detailed rheological tests on a capillary rheometer as well as mechanical studies on a universal tensile machine after preparation of injected specimens were carried out. Results evidenced that PLA and reinforced PLA materials can be reprocessed and recycled without a remarkable loss in their mechanical properties. Several processing restrictions and specific phenomena were identified and are explained in the present manuscript.

Crosslinking of trimellitimide glycidyl ester derivatives

The cure reaction of diglycidyl trimellitimide esters with 4-dimethylaminopyridine, diaminodiphenylmethane, and dicyandiamide as hardener was studied by differential scanning calorimetry (DSC). Isoconversional kinetic analysis was applied to nonisothermal DSC data and the dependences of activation energies on conversion degrees were obtained. Differences in the shape of these plots were found in accordance to the different kinetic schemes of the curing agents. The thermal properties of the final products evaluated by thermogravimetry do not show noteworthy differences.

Carbon nanotube composite materials: opportunities and processing issues

Funding from the Spanish Ministry of Education and Science (MEC) and the European Regional Development Fund (ERDF) under projects MAT 2006-13167-C02-01 and -02, MAT2007-66927-C02-01, is gratefully acknowledged. M.N. acknowledges the CSIC for a postdoctoral contract (I3PDR-6-02), financed by the European Social Fund. We like to thank the European Commission for the synchrotron project at the Soft Condensed Matter A2 beamline at HASYLAB (DESY-Hamburg, I-20080056 EC).

Improvement of mechanical properties of poly(lactic acid) by integration of sepiolite nanoclays: effect of ultrasonication on clay dispersion

Abstract Owing to the depletion of oil based resources, new environmentally friendly materials are in continuous development. Poly(lactic acid) (PLA), a thermoplastic polymer obtained from renewable resources, is being seriously considered as a replacement for petroleum based products. The aim of the present study is to enhance the mechanical properties of this biopolymer with the addition of a natural based filler, a functionalised sepiolite, and to ensure a good dispersion by applying ultrasonic waves throughout the compounding process. A good affinity between the polymer and the silicate filler is essential in the nanocomposite to attain the optimal reinforcement. Nanocomposites of PLA/sepiolite were processed by extrusion compounding containing different weight percentages of nanoreinforcement. An ultrasonic homemade device developed by the authors was coupled at die position and used to improve the dispersion of the nanoclays in the molten polymer. Nanocomposite materials were later injected to produce different specimens that were characterised. The effect on the mechanical properties of PLA reinforced with sepiolite was analysed by conducting flexural tests. Strain–stress curves were obtained, and their stiffness and toughness were compared for the different specimens. Micrographs (SEM) were examined to demonstrate the effectiveness of using ultrasounds to improve the dispersion of the nanoclays within the polymeric matrix. Viscosity under low shear rates was measured with a rotational parallel plate rheometer in order to monitor the changes induced on the rheological behaviour of the composites. It was demonstrated that the application of ultrasonic waves in composite compounding improves nanoclay dispersion, which results in an enhancement of the reinforcement of the fillers and decreases the viscosity of the composites during the process. These effects become more significant for high loads of reinforcement, especially for 10 wt-% sepiolite specimens.