Tiberio A. Ezquerra

Confinement-Induced One-Dimensional Ferroelectric Polymer Arrays

This work demonstrates the use of wetting nanoporous alumina template with polymer solution to produce arrays of isolated poly(vinylidene fluoride) (PVDF) ferroelectric gamma-type nanorods supported within a nonpolar alpha-structure film. The method is based upon a crystal phase transition which occurs due to PVDF confinement within alumina nanoporous. The system was studied using scanning X-ray microdiffraction (micro-XRD) that allows the solid-solid phase transition from the alpha-nonpolar crystal form (bulk) to the gamma polar ferroelectric form (nanorod array) to be spatially resolved, as well as providing crystallinity and orientation information. The results reveal that the interaction between polymer chains and the porous membranes walls imposes a flat-on lamella growth along the nanorrods long axis, while improving crystal orientation.

Alternating-current electrical properties of graphite, carbon-black and carbon-fiber polymeric composites

Abstract Electrical conductivity measurements of graphite, carbon-black and carbon-fiber polymeric composites reported over a broad frequency range covering from d.c. to 109 Hz are comparatively discussed. The d.c. electrical conductivity data from carbon-black and graphite composites exhibit a conducting additive concentration dependence which can be explained on the basis of percolation theory. In both systems, tunneling conduction among particles appears as the predominant mechanism in the concentration range investigated. A frequency-dependent conductivity is observed which is stronger the lower the additive concentration. A modification of the percolation theory which includes the contribution of finite-size clusters is invoked to explain the frequency dependence of the conductivity. In carbon-fiber composites, the high fiber orientation gives rise to materials with higher electrical conductivity levels than those found for particulate composites. The high anisotropic conductivity additionally exhibits an almost absence of frequency dependence. This is explained by assuming the occurrence of a highly interconnected fiber network with almost an absence of electrical barriers.

Electrical transport in polyethylene-graphite composite materials

Abstract The transport mechanism in Poly(ethylene)-graphite composites has been investigated through the analysis of the dc conductivity and complex permitivity in the 10 2 –10 7 Hz frequency range. The study of this relatively simple system can be useful for the interpretation of transport in composite materials of higher complexity. A description of the transport mechanism in terms of tunneling conduction between adjacent particles is proposed to explain the observed experimental behaviour. Such a mechanism can be responsible for the observed dc conductivity deviations from the percolation theory predictions at low graphite concentrations.

Electrical conductivity of polyethylene-carbon-fibre composites mixed with carbon black

The study of electrical conductivity of high-density polyethylene-carbon-fibre composites mixed with different concentrations of carbon black is reported. The influence of the mixing procedure of the additives and material preparation is examined with regard to the conductivity values. The use of these two filler types in polyethylene composites combines the conducting features of both. Thus, while fibres provide charge transport over large distances (several millimetres), carbon black particles improve the interfibre contacts. Results are discussed with reference to simple electrical models. It is shown that for composites in which the segregated carbon black-polyethylene component lies above the percolation threshold the electrical interfibre contacts are activated through carbon black particle bridges, leading to a conductivity rise. This effect is more relevant in the case of shorter fibres. Processing of the material involving fibre orientation, such as in injection-moulding, decreases drastically the conductivity level reached.

Real time dielectric relaxation of poly(ethylene terephthalate) during crystallization from the glassy state

Abstract The crystallization of poly(ethylene terephthalate) has been followed in real time by measuring the dielectric complex permittivity. The measurements have been discussed by assuming the contribution to the dielectric losses of two types of amorphous phases: amorphous regions filling the interspherulitic space and amorphous regions located between crystalline lamellae within the spherulites. The deviation of the measurements from the calculated values assuming a simple two-phase model has been interpreted as being due to changes in the nature of the amorphous regions as crystallization proceeds. A phenomenological description of the experiments in terms of the Havriliak-Negami description permits the changes of the relaxation time distribution functions upon crystallization to be followed. The evolution with crystallization time of the derived dipole moment time correlation functions are discussed in the light of different models.

Influence of cross-linking on the segmental dynamics in model polymer networks

In an attempt to study the specific influence of cross-linking on the α relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10−1 Hz<F<105 Hz frequency range and in the 123 K<T<493 K temperature interval. The α relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the α-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme clearly shows that fragility increases as polymer network develops.In an attempt to study the specific influence of cross-linking on the α relaxation in polymer networks, a series of model heterocyclic polymer networks (HPN) with well-defined cross-link densities and constant concentration of dipolar units were studied. Model HPN systems were prepared by simultaneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl isocyanate (HI). These HPN systems were characterized by dielectric relaxation spectroscopy in the 10−1 Hz<F<105 Hz frequency range and in the 123 K<T<493 K temperature interval. The α relaxation in these systems depends on network density and shifts toward higher temperatures as the cross-link density increases for high HMDI/HI ratios. Discussion of the α-relaxation shape in light of recent models indicates that segmental motions above the glass transition systematically experience a growing hindrance with increasing degree of cross-linking. Description of the temperature dependence of relaxation times according to the strong–fragile scheme cl...

Assessment and formation mechanism of laser-induced periodic surface structures on polymer spin-coated films in real and reciprocal space.

In this work we evaluate the potential of grazing incidence X-ray scattering techniques in the investigation of laser-induced periodic surface structures (LIPSSs) in a series of strongly absorbing model spin-coated polymer films which are amorphous, such as poly(ethylene terephthalate), poly(trimethylene terephthalate), and poly(carbonate bisphenol A), and in a weaker absorbing polymer, such as semicrystalline poly(vinylidene fluoride), over a narrow range of fluences. Irradiation was performed with pulses of 6 ns at 266 nm, and LIPSSs with period lengths similar to the laser wavelength and parallel to the laser polarization direction are formed by devitrification of the film surface at temperatures above the characteristic glass transition temperature of the polymers. No crystallization of the surface is induced by laser irradiation, and crystallinity of the material prevents LIPSS formation. The structural information obtained by both atomic force microscopy and grazing incidence small-angle X-ray scattering (GISAXS) correlates satisfactorily. Comparison of experimental and simulated GISAXS patterns suggests that LIPSSs can be well described considering a quasi-one-dimensional paracrystalline lattice and that irradiation parameters have an influence on the order of such a lattice.

Assessment of femtosecond laser induced periodic surface structures on polymer films

In this work we present the formation of laser induced periodic surface structures (LIPSS) on spin-coated thin films of several model aromatic polymers including poly(ethylene terephthalate), poly(trimethylene terephthalate) and poly carbonate bis-phenol A upon irradiation with femtosecond pulses of 795 and 265 nm at fluences well below the ablation threshold. LIPSS are formed with period lengths similar to the laser wavelength and parallel to the direction of the laser polarization vector. Formation of LIPSS upon IR irradiation at 795 nm, a wavelength at which the polymers absorb weakly, contrasts with the absence of LIPSS in this spectral range upon irradiation with nanosecond pulses. Real and reciprocal space characterization of LIPSS obtained by Atomic Force Microscopy (AFM) and Grazing Incidence Small Angle X-ray Scattering (GISAXS), respectively, yields well correlated morphological information. Comparison of experimental and simulated GISAXS patterns suggests that LIPSS can be suitably described considering a quasi-one-dimensional paracrystalline lattice and that irradiation parameters have an influence on the order of such a lattice. Fluorescence measurements, after laser irradiation, provide indirect information about dynamics and structure of the polymer at the molecular level. Our results indicate that the LIPSS are formed by interference of the incident and surface scattered waves. As a result of this process, heating of the polymer surface above its glass transition temperature takes place enabling LIPSS formation.

Structure and properties of ferroelectric copolymers of poly(vinylidene fluoride)

The aim of this paper is to review the main structural features and properties which are related to the ferroelectric behavior of copolymers of poly(vinylidene fluoride) (PVF2). These are illustrated by describing the conformation, polymorphism and morphology of PVF2 and its copolymers. Copolymers of PVF2 and trifluoroethylene have special interest because of their prominent ferroelectric behavior as well as strong piezo- and pyroelectric activity. The review covers the study of the influence of molar composition on structure and the temperature induced ferroelectric to paraelectric phase changes. Advances and problems in X-ray diffraction results relating to changes in the long period, size of coherently diffracting domains and lattice spacings with temperature are discussed. A microstructural model to explain the transformation of paraelectric into ferroelectric crystals when cooling through the transition temperature is presented. Special attention is paid to the influence which the changes in structure exert upon mechanical properties. The dependence of polarization upon copolymer composition and temperature is analyzed. Dielectric spectroscopy is shown to be of great interest in showing transitions connected with reorientation of permanent dipoles. Finally, new results on molecular dynamics of these ferroelectric copolymers derived from incoherent neutron scattering are reported.

Molecular dynamics and microstructure development during cold crystallization in poly(ether-ether-ketone) as revealed by real time dielectric and x-ray methods

The isothermal crystallization process of poly(ether-ether-ketone) from the glass has been studied in real time by dielectric spectroscopy and x-ray scattering experiments. The combination of these two techniques revealed a complete picture of the crystallization processes from the point of view of both amorphous and crystalline phases. Analysis of results shows that the sample morphology consists of lamellar stacks, separated by rather broad amorphous regions. The lamellar stacks are highly crystalline (∼70%), as obtained from both dielectric and x-ray scattering measurements, and the amorphous phase within the stacks is constrained up to a level where no segmental relaxation is possible. The remaining amorphous phase, after completion of the primary crystallization process, still has a certain mobility, but it is significantly slower than the initial amorphous mobility. Dielectric data and x-ray results are found to be highly congruent.