Mario Hoyos

The development of electrical treeing in LDPE and its nanocomposites with spherical silica and fibrous and laminar silicates

Electrical treeing in LDPE and three LDPE nanocomposites, with spherical silica and fibrous and laminar phyllosilicates, has been studied. Electrical tests were performed at a 50 Hz frequency and voltages between 8 and 29 kV, and the time to inception of the first electrical partial discharges (TTI) of the electrical trees and the time to breakdown (TBD), related to the electrical stability of the insulator, were determined. Above 15 kV all the nanocomposites show longer inception times and shorter tree growth times than LDPE. It is proposed that both observations are caused by the modification of the polymer crystalline morphology induced by the presence of the fillers and by the development of a large number of interfacial structures, both organo–inorganic and amorphous–crystalline. Below 15 kV the TBD is increased in the nanocomposites with the laminar silicate because of tortuosity and the TTI is increased in the fibrous silicate containing a nanocomposite because of the LDPE crystalline morphology in the presence of the silicate. The nanosilica particles decrease the electrical stability in the whole voltage range by decreasing both TTI and TBD.

Superhydrophobic and highly luminescent polyfluorene/silica hybrid coatings deposited onto glass and cellulose-based substrates

Neat poly(9,9-dioctyl-9H-fluorene) (PFO) and composites of PFO and a modified organonanosilica P(7) at weight ratios 90/10, 70/30, and 50/50 have been employed to prepare fluorescent and superhydrophobic coatings by spraying onto three different substrates: glass, Whatman paper, and a filtration membrane of mixed cellulose esters. The water repellency of the coatings and their photophysical properties are therein studied. It is found that, irrespective of the substrate and the composite composition, all coatings remain fluorescent. In some of the coatings prepared, confined morphologies are created, which fluoresce with a wavelength distribution resembling that of an ordered planar β-phase. Among the coatings prepared in this work, those with a ratio PFO/P(7) of 50/50 are the ones with the strongest chain confinement and the highest surface roughness, being highly emissive at the β-phase wavelengths and also superhydrophobic. Depending on the substrate these materials are also tough and flexible (cellulose based substrates) or display a remarkable light transmittance (glass). A final merit of these multifunctional materials is the simplicity of the preparation procedure, adequate for large surfaces and industrial applications.

Effect of microstructure on the thermo-oxidation of solid isotactic polypropylene-based polyolefins

Abstract In the present work we aim to clarify the role of the microstructure and the crystalline distribution from the thermo-oxidation of solid isotactic PP (iPP) and ethylene-propylene (EP) copolymers. The effects of the content and quality of the isotacticity interruptions, together with the associated average isotactic length, on the induction time (ti) as well as on the activation energy (Eact) of the thermo-oxidation are analysed. Both parameters have been found to change markedly at an average isotactic length (n1) of 30 propylene units. While ti reaches a minimum when n1 is approximately 30 units, Eact increases quasi-exponentially as the number of units decreases from 30. This variation can be explained in terms of changes induced in the crystalline interphase, i.e. local molecular dynamics, which are closely linked to the initiation of the thermo-oxidation of isotactic PP-based polyolefins.

Triarylamine polymers of bridged phenylenes by (N-heterocyclic carbene)-palladium catalysed C–N coupling

A library of triarylamine copolymers with 2,7-fluorene, 3,7-dibenzo[b,d]thiophene and 2,7-carbazole units incorporated in the polymer backbone is reported. The polymers were synthesised by using C–N coupling of anilines and dibromoarenes, catalysed by N-heterocyclic carbene complexes of palladium. The properties of the polymers were tuned by changing the nature of the fused ring structure and the substitution of the pendant benzene of the arylamine. The use of these polymers as the charge transporting layer of an OFET is demonstrated with the highest mobility found for polymers derived from 2,7-dibromo-9,9′-dioctylfluorene and 4-methoxy-2-methylaniline.

Extended conjugation in poly(triarylamine)s: synthesis, structure and impact on field-effect mobility

Polytriarylamines with extended fused backbones are accessible by the coupling of anilines with dibromoarenes based on substituted indenofluorenes, diindenofluorenes, carbazoles and indolocarbazoles. The optical and electrochemical properties of these polymers show an increase in the HOMO energy levels and the onset of absorption on extending the length of the fused ring segment. The polymer derived from the indenofluorene unit shows the highest reported performance for a polytriarylamine in an OFET and this observation can be rationalized by DFT calculations of model oligomers that show higher calculated reorganization energies for the more extended diindenofluorene units.

Use of dynamic space charge measurements to differentiate charge injection and ionic dissociation in polypropylene

The time resolved fast pulsed electro-acoustic (PEA) method is used to investigate the space charge characteristics in an additive-free poly(propylene-ethylene) (PPE) copolymer in film form. The 190 mum-thick films were submitted to DC and low frequency AC stress for various durations at various temperatures. Upper electrode (UE) and lower electrodes (LE) are semi-conductive polyethylene and aluminum, respectively.