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In this work, we investigate the thermomechanical behavior of flax-fabric/epoxy composites by means of DMA experiments. Fabrics from flax fibers have undergone different chemical treatments: leaching, bleaching or mercerization. We study the influence of these chemical treatments on the dynamic mechanical relaxation spectra of the composites around the main relaxation mode. We construct master plots for the loss modulus for each sample using the time–temperature superposition (TTS) principle. The master curves were fitted with the Havriliak-Negami model and we show that both interface and interphase of composites are modified as a result of the chemical treatments. Best mechanical properties were found for mercerized treatment.

We propose and experimentally demonstrate a novel approach to a heralded single photon source based on spectral multiplexing (SMUX) and feed-forward-based spectral manipulation of photons created by means of spontaneous parametric down-conversion in a periodically-poled LiNbO3 crystal. As a proof-of-principle, we show that our 3-mode SMUX increases the heralded single-photon rate compared to that of the individual modes without compromising the quality of the emitted single-photons. We project that by adding further modes, our approach can lead to a deterministic SPS., Main text and supplementary material

The early stages of the cold crystallization process and the formation of a rigid amorphous phase as seen by the dielectric response of polylactide, PLA, and composites polylactide/carbon nanotubes, PLA/CNT, are studied here by broadband dielectric spectroscopy for CNT concentrations below percolation. The presence of precursors during the nucleation and crystallization process is demonstrated by the existence of a time shift between the decline in the number of mobile segments and the growth of a 3D ordered phase as seen by variable temperature wide angle X-ray scattering measurements. Also, the loss of the mobile amorphous phase is not justified by the slow lamellar growth in identical conditions. The variation of the molecular dynamics, either for short range reorientations or cooperative motions, is followed in both amorphous and semicrystalline states. The changes observed in the composites PLA/CNT show a moderate heterogeneous nucleating effect of the nanofiller and a sensitivity of the three subglass transitions to the chain ordering. The relaxation parameters of the segmental relaxation are not very sensitive to the presence either of lamellae or of the nanofiller.

After DC and AC conductivity measurements, percolation parameters, universality, and time-temperature-concentration superposition are evaluated in biocomposites based on poly(lactide) loaded with multiwalled carbon nanotubes either carboxylic, PLA/c-CNT, or pure but with a larger aspect ratio, PLA/LAR-CNT, in weight concentrations up to 7%. The polymer matrix is either 100% amorphous or cold-crystallized. Wide angle X-ray scattering is used to extract the crystallinity at each temperature. The nucleating effect for the cold crystallization of the PLA is greater for the LAR-CNT than for the carboxylic ones. DC and AC conductivities are measured in broad frequency and temperature ranges, from 1 mHz to 1 MHz and from 133 to 378 K, respectively. The critical percolation value increases for c-CNTs (from 1.8 to 2.4 vol%) upon cold crystallization of the PLA matrix as the formation of a conductive path is hindered by the existence of a crystalline phase, which is dominant over the reduction of the amorphous regions. No significant changes upon crystallization were found in the critical concentration value (0.6 vol%) for the LAR composites, where the conductive path is 2D. The universal scaling found here for different concentrations is lost when temperature varies. However, time-temperature superposition is present for each concentration of LAR or carboxylic CNTs. POLYM. COMPOS., 2013. © 2012 Society of Plastics Engineers

Dc and ac conductivities in a broad angular frequency, 2π × 10−3 s−1 < ω < 2π × 107 s−1, and temperature range, 133 K < T < 323 K, were measured in poly(e-caprolactone)/multiwalled carbon nanotube nanocomposites as a function of weight concentration (0 wt % ≤ p ≤ 3 wt %) . The universal dynamic response was observed at low p values, and the parameters were adjusted by working in several domains. Scaling laws predicted by the variation of dc conductivity as a function of p or characteristic frequency in percolation theory were followed and showed the formation of a 3D percolative cluster at concentration values as low as 0.3 wt %. The existence of tunneling conduction was proved, and its independence on temperature indicates that we are in the presence of a conventional tunnel effect among nanoparticles not yet in physical contact, through energy barriers made by the polymer chains. All these combined facts allowed construction of a master curve evidencing the time−temperature−composition superposition exi...

DC and AC electrical conductivity of bionanocomposites based on the immiscible polymer blend poly(epsilon-caprolactone)/polylactide (PCL/PLA, w/w 70/30), loaded with multiwall carbon nanotubes (CNT), were studied in a wide frequency range, 10(-3)or = for = 10(7) Hz from 143 to 313 K. The nanofiller concentration ranged from 0 to 4 wt % and it was shown to be selectively located in the PCL phase. The PCL crystallinity degree was not affected by the presence of CNT. The variation of the DC conductivity allowed the determination of the percolation threshold, p(c) = 0.98 wt %, and the critical exponent t = 2.2 of the scaling law. The linear dependence of log (sigma(DC)) versus p(-1/3) showed the existence of tunneling conduction among CNT not yet in physical contact. The temperature independent results indicated a conventional tunnel effect. The AC conductivity of the nanocomposites followed the predictions of the universal dynamic response and the s exponents were determined at low concentrations. Master curves are presented showing the length and temperature-time superpositions.

The complex dielectric modulus, M ∗ ( ω , T ), for poly( e -caprolactone) has been obtained by inverting the complex dielectric permittivity data, e ∗ ( ω , T ) recorded in a wide temperature and frequency range. The analysis of the dielectric loss modulus spectra M ″( ω , T ) have been performed by using a simulated annealing Monte Carlo procedure in order to extract the relaxation parameters for each relaxation mode. Besides the local modes γ and β, the segmental α mode associated to the glass transition, the conductivity and the interfacial polarization peaks could be analyzed with a better precision than by using the conventional non-linear least squares method in e ″ domain. The merging of the α and β modes at high temperatures is much better defined due to the less significant contribution of the ionic conductivity in M ∗ formalism.

Poly(α- n -alkyl β- l -aspartate)s, P n AALAs, are comb-like polymers with rod-like helical conformations which present crystallinity for long alkyl side chains ( n > 10). Dielectric spectroscopy experiments for P n AALAs (10 ⩽ n ⩽ 18) show the existence at low temperatures of two secondary modes corresponding to localized motions of the carbonyl group. At higher temperatures two cooperative mobilities coexist. The first corresponds to the segmental mobility of the amorphous side chains and has the characteristic of a polyethylene-like dynamic glass transition common to various series of nanophase segregated side chain polymers with different morphologies. A second segmental mode is attributed to the restricted rotational motion of the rigid rods which is hindered when the spacing between rods decreases. An extension to the results found for other polymer series with shorter lateral chains is presented, showing the tendency to approximate amorphous polyethylene dynamics in these confined nanodomains of increasing sizes.

Thermally stimulated depolarization currents, TSDC, wide-angle X-ray scattering, WAXS, differential scanning calorimetry, DSC, and polarized light optical microscopy, PLOM, have been used to examine poly(L-lactide)-b -poly( $ \upvarepsilon$ -caprolactone) diblock copolymers in a wide composition range. Both components are crystallizable and the miscibility in the amorphous phase has been determined from the behavior of the primary relaxations which are the dielectric manifestation of the glass transition, and also from the superstructural morphology revealed by PLOM and the compositional dependence of the melting points as determined by DSC. Distinct segmental mobilities in the amorphous phase which can be well resolved by TSDC are present; the $ \upalpha$ mode of the slower component shifts to lower temperatures as the PCL content increases while the glass transition of neat PCL is present for all compositions. A relaxation times bimodal distribution is apparent for PCL-rich copolymers. The composition dependence of the multiple glass transitions detected in these weakly segregated copolymers are predicted by the self-concentration model for a miscible blend made of components with a large Tg contrast.

We report entanglement swapping with time-bin entangled photon pairs, each constituted of a 795 nm photon and a 1533 nm photon, that are created via spontaneous parametric down conversion in a non-linear crystal. After projecting the two 1533 nm photons onto a Bell state, entanglement between the two 795 nm photons is verified by means of quantum state tomography. As an important feature, the wavelength and bandwidth of the 795 nm photons is compatible with Tm:LiNbO3-based quantum memories, making our experiment an important step towards the realization of a quantum repeater., Comment: Accepted in Phys. Rev. A

The effect of blending poly(styrene-co-maleic anhydride) with 14 mol% MA (PSMA14) with poly(ɛ-caprolactone) (PCL) is studied by calorimetry, X-ray diffraction, dielectric techniques, and microscopy. Variations in PCL crystallization, melting temperatures and crystallinity degree were observed. Additionally, the presence of a wide and asymmetric enthalpic step, whose position follows Fox predictions, demonstrates miscibility between the blend components. This is in agreement with the interlamellar location of PSMA14. Thermally stimulated depolarization techniques evidenced two segmental dynamics, with distinct compositional variations due to the local differences in composition perceived by each component as a consequence of chain connectivity. The two effective glass transition temperatures vary with composition, in agreement with the self-concentration model predictions when using the Kuhn's segment length for each component. From isothermal crystallizations a marked depression of the PCL equilibrium melting point ( T m o ) is observed in the blends together with an increase in the half-crystallization times, τ1/2, as the content of PSMA14 grows. PCL-rich blends exhibited ring-banded spherulites, with a ring periodicity that increased with crystallization temperature and with PSMA14 content. Preliminary results of the phase separation process are given from samples that were thermally treated under different conditions.

Polycarbonate/poly(e-caprolactone) (PC/PCL) blends are found to be miscible when extruded samples are studied without any further thermal treatment. PCL crystallizes in blends containing 60% or les...

New results of dielectric spectroscopy in semicrystalline nylon-6 samples with different moisture contents are presented by using a wide frequency (10-2−3 × 106 Hz) and temperature range (133−433 K). The dielectric absorption spectra in frequency domain are decomposed in Cole−Cole distributions corresponding to the local γ and β modes, two segmental α modes, and a high-intensity Maxwell−Wagner−Sillars relaxation. The presence of two segmental relaxations in the isothermal runs, attributed to the plasticized and the unplasticized α mode, is interpreted as the manifestation of two different length and time scales of cooperative motions. A quantitative comparison between the results obtained for the wet and dry samples, such as relaxation times variation, activation energies, Vogel−Tammann−Fulcher parameters, dielectric increments, and distribution widths, is presented for each mode and shows how the progressive drying of the sample during the experiment affects all these quantities.

Chain mobility in multiphase polymeric materials is studied by thermally stimulated depolarization currents and dielectric spectroscopy in poly(styrene)-b-poly(butadiene)-b-poly(c-caprolactone) triblock copolymers and poly(carbonate)/poly(c-caprolactone) blends. The variation in the relaxation time distribution of the dielectrically active components and in the number of orientable dipoles in the amorphous phases of these materials is interpreted as the result of the existence of a rigid amorphous phase constrained by the crystalline regions or in the case of the blends by a phase segregation which takes place when the crystallization process of the blend components advances. The mean relaxation times for the secondary and segmental motions are not affected when the phases are segregated.

Dielectric relaxations of polycarbonate (PC) and poly(e-caprolactone) (PCL) blends that were thought to be completely homogeneous in the amorphous phase from many previous calorimetric studies are performed by using the thermally stimulated depolarization currents technique. Samples kept for 18 months at room temperature were compression-molded and then either quenched or slowly cooled. Quenched samples evidenced a phase separation for the 20/80 PC/PCL composition and the existence of continuous concentration fluctuations for other blend compositions. The slowly cooled samples from the melt exhibited an increase of the crystallinity degree for both blend components, together with a segregation of a predominantly rich PCL phase and the appearance of high-temperature relaxations attributed to interfacial polarization. These findings are confirmed by new calorimetric results on the same blends and by transmission electron microscopy. The spherulitic growth rate also points to the coexistence of two phases wh...

Thermal sampling peaks recorded after windowing polarization are studied for the segmental mode in poly(e-caprolactone). Also, numerical decompositions of the global thermally stimulated depolarization current peak into pure Debye contributions are performed with direct signal analysis (DSA) and simulated annealing direct signal analysis procedures for Arrhenius and Vogel–Tammann–Fulcher (VTF) temperature dependences, respectively. It is found that the results between the experimental and the numerical procedures agree very well and the approximations made in the analysis of the experimental curves are thus validated, despite the unphysical values for the relaxation parameters found by both methods when using Arrhenius relaxation times. On the contrary, when VTF relaxation times are used for the numerical decomposition, agreement is found with the results of isothermal dielectric absorption as a function of frequency, together with reasonable values for reorientation energies, pre-exponential factors and ...

The behavior of the various relaxation modes in poly(e-caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3×10−3 to 1.8×109 Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, e″(ω), is analyzed by best fitting a sum of Cole–Cole distributions corresponding to the γ and β local modes and to the α relaxation which is the dielectric manifestation of the dynamic glass transition. The kinetic parameters of the three predominant relaxations are determined and relaxation plots describing the temperature dependencies of the relaxation times are given as a function of 1/T. These relaxation plots are insensitive within experimental errors, either to the molecular weight or to the water concentration. The hydration level (

The high temperature dielectric relaxation process, observed above the glass transition temperature in poly(ϵ-caprolactone) is studied by Thermally Stimulated Depolarization Currents experiments. The characterization is made by using the windowing polarization technique, decomposing the complex high temperature relaxation into Debye-like peaks. The profile of each peak is then analyzed by using a general kinetic order model, and the origin of the complex relaxation is discussed through the analysis of the parameters obtained from the best fit to the model. The distribution of the trap depths is obtained and their activation energies is found to vary from 0.8 to 1.2 eV. The space charge accumulation is mainly responsible for this relaxation mode in poly(ϵ-caprolactone).

Resumen La coexistencia de una mola hidatiforme completa y de un feto vivo es una entidad muy poco frecuente que clasicamente se ha asociado a un alto riesgo de aborto espontaneo, muerte intrauterina, parto pretermino, preeclampsia, hipertiroidismo y malignizacion. El mal pronostico de la gestacion y los riesgos maternos generan un dilema sobre la terminacion inmediata del embarazo en el momento del diagnostico o una conducta expectante. Presentamos el caso de una mola hidatiforme completa coexistente con un feto vivo que llego a termino sin presentar ninguna complicacion.

The existence of a distribution of relaxation times has been widely used to describe the relaxation function versus frequency in glass-forming liquids. Several empirical distributions have been proposed and the usual method is to fit the experimental data to a model that assumes one of these functions. Another alternative is to extract from the experimental data the discrete profile of the distribution function that best fits the experimental curve without any a priori assumption. To test this approach a Monte Carlo algorithm using the simulated annealing is used to best fit simulated dielectric loss data, ${\ensuremath{\varepsilon}}^{\ensuremath{''}}(\ensuremath{\omega}),$ generated with Cole-Cole, Cole-Davidson, Havriliak-Negami, and Kohlrausch-Williams-Watts (KWW) functions. The relaxation times distribution, $G(\mathrm{ln}(\ensuremath{\tau})),$ is obtained as an histogram that follows very closely the analytical expression for the distributions that are known in these cases. Also, the temporal decay functions, $\ensuremath{\varphi}(t),$ are evaluated and compared to a stretched exponential. The method is then applied to experimental data for $\ensuremath{\alpha}$-polyvinylidene fluoride over a temperature range $233 \mathrm{K}l~Tl~278 \mathrm{K}$ and frequencies varying from 3 MHz to 0.001 Hz. These data show the existence of two relaxation processes: the fast segmental ${\ensuremath{\alpha}}_{a}$ process associated with the glass transition and a ${\ensuremath{\alpha}}_{c}$ mode, which is slower and due to changes in conformation that can occur in the crystalline regions. The experimental curves are fitted by the simulated annealing direct signal analysis procedure, and the relaxation times distributions are calculated and found to vary with temperature. The decay function is also evaluated and it shows clearly its bimodal character and a good agreement with a KWW function with a temperature dependent $\ensuremath{\beta}$ for each mode. The relaxation plots are drawn for each mode and the Vogel-Tammann-Fulcher and Arrhenius parameters are found. The fragility parameter for polyvinylidene flouride (PVDF) is found to be 87, which characterizes this polymer as a relatively structurally strong material.

The analysis of Thermally Stimulated Depolarization Currents (TSDC) spectra in polymers requires the introduction of a distribution of relaxation times. Also, a decay of the induced polarization following a stretched exponential law can be used. Two different ways for introducing this effect in the TSDC equations are compared here together with the decomposition in elementary curves by a Simulated Annealing Monte Carlo procedure to find the distribution. It is found that the stretched exponential applied to computer generated curves corresponds in both cases to a nearly Debye process. Also the analysis of the α-peak of poly(e-caprolactone) with the three approaches are compared.

The analysis of thermostimulated currents by the fractional polarization procedure is used to understand percolative behaviors in conductor-insulator-like oil-resin mixtures. On the fundamental side oil-poor systems are particularly attractive. They offer for the first time the opportunity to apply a pure Debye-like dielectric treatment of the quasi-elastic dipolar relaxation since the involved dipoles are independent, associated with spatially separated bounded clusters. An unusual compensation phenomenon is observed in the sense that first it does not describe hierarchically correlated motions and secondly it is related to the conducting phase but exhibits characteristics of the insulating phase. This compensation phenomenon is interpreted within the framework of Fröhlich's approach of relaxation processes in biological materials, as significative of a coherent vibration resulting from Böse condensation effects. On the practical side, the surveillance of this compensation phenomenon appears to be a new way to follow the coalescence of conducting bounded clusters with aging.

The αa-mode (associated to the dynamic glass transition) in PVDF-α has been studied by Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Spectroscopy (DS) techniques. The distribution of relaxation parameters, reorientation energies, characteristic temperature, and preexponential factors of the Vogel–Tammann–Fulcher relaxation times have been precisely determined by using the Simulated Annealing Direct Signal Analysis applied to a partially discharged TSDC αa peak. This distribution has been used to predict the variation of the dielectric loss, e″(ω, T), in the temperature and frequency range where the DS measurements were made on the same material. The simulated e′(T, ω) for various ω, are compared to the experimental values. The width of the peak is always too low, due to the restricted distribution used for the generation of the curves. A relaxation map including the TSDC results is used to determine the relaxation time variation. In the limited frequency range where the AC DS experiments are performed (102 ≤ f ≤ 105 Hz) a master curve is drawn and the exponents of the frequency dependence are found at low and high frequency; also, a fitting to the Havriliak–Negami distribution is successfully performed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2483–2493, 1997

The molecular mobility of two semicrystalline polymers with flexible chains—chemically crosslinked low density polyethylene (XLPE) and poly(vinylidene fluoride) (PVDF)—has been investigated in the temperature range between room temperature (above their glass transition temperature) and their melting point. D.s.c. was used to characterize the conformational evolutions related to crystalline domains. Thermostimulated currents and creep were used to study the dynamics of these movements. For both polymers, the α-relaxation/retardation mode is located in the temperature range of a small endothermic phenomenon. These low temperature d.s.c. peaks are not related to any melting process but rather to conformational disorder in the crystal-amorphous interphase. The fractional polarization/stress technique was applied to obtain the distribution of relaxation/retardation times. A compensation phenomenon was found above the α-relaxation mode of β-PVDF and XLPE. It has been ascribed to conformational movements associated with an order-disorder phase transition in the vicinity of the melting point. These high temperature phases would correspond to the paraelectric phase of β-PVDF and to the hexagonal phase of XLPE. Insofar as the α-mode is liberating molecular mobility at the crystal-amorphous interphase, it appears as a precursor of the order-disorder transition.

A series of poly(alpha-n-alkyl beta-L-aspartates) which are nanophase self-assembled comb-like polymers has been studied by dielectric spectroscopy in a broad frequency range (10(-2) < or = nu < or = 3 x 10(6) Hz), with n-alkyls side chains of various lengths, 10 < or = n < or =18. In every member of the series the same relaxations were identified after the decomposition of the experimental isothermal trace in up to three peaks with relaxation times distributions. The strength, width and average relaxation time for all the relaxation modes were determined for each material. Besides the local low temperature, Arrhenius modes, two relaxation modes, alpha and alpha(PE), present a cooperative character whose dynamics are not affected by the side chains melting. The alpha(PE) relaxation is a polyethylene-like glass transition of the amorphous side chains and its dynamics is strongly dependent on the n value due to the increasing restrictions imposed by the self-assembled confinement. The strength of the alpha(PE) relaxation mode increases as the lateral chains loose their 2D order. The restricted chopstick motion of the rigid rods is thought to be the origin of the alpha mode; this motion is hindered at temperatures where the cage size decreases as a result of the increasing disorder with temperature.

A series of poly(alpha-n-alkyl beta-L-aspartates) which are nanophase self-assembled comb-like polymers has been studied by dielectric spectroscopy in a broad frequency range (10(-2)or = nuor = 3 x 10(6) Hz), with n-alkyls side chains of various lengths, 10or = nor =18. In every member of the series the same relaxations were identified after the decomposition of the experimental isothermal trace in up to three peaks with relaxation times distributions. The strength, width and average relaxation time for all the relaxation modes were determined for each material. Besides the local low temperature, Arrhenius modes, two relaxation modes, alpha and alpha(PE), present a cooperative character whose dynamics are not affected by the side chains melting. The alpha(PE) relaxation is a polyethylene-like glass transition of the amorphous side chains and its dynamics is strongly dependent on the n value due to the increasing restrictions imposed by the self-assembled confinement. The strength of the alpha(PE) relaxation mode increases as the lateral chains loose their 2D order. The restricted chopstick motion of the rigid rods is thought to be the origin of the alpha mode; this motion is hindered at temperatures where the cage size decreases as a result of the increasing disorder with temperature.

We present a different approach to obtain the distribution of relaxation times from dielectric spectroscopy (DS) and TSDC recorded current density. The method consists in decomposing the whole relaxation peak into a sum of elementary processes and it is based on the simulated annealing Monte Carlo procedure together with the direct signal analysis. It gives the contribution to the polarization of a set of N relaxation times which cover a chosen interval. The analysis of the DS curves in the frequency or time domain will lead to G(ln/spl tau/), while the TSDC analysis gives the N relaxation times whose superposition best reproduce the experimental profile, each of them characterized by a reorientation energy and by a preexponential factor of the Arrhenius or Vogel-Tammann-Fulcher, VTF, temperature dependent relaxation time.

Thermally stimulated depolarization currents, TSDC, experiments have been performed on a series of poly(styrene)-b-poly(butadiene)-b-poly(epsilon-caprolactone) triblock copolymers SBC with different proportions of the poly(epsilon-caprolactone) crystallizable block, PCL. The morphology of the segregated microphases varies with the PCL content and has been observed by transmission electron microscopy. The crystallinity of the PCL block is estimated by wide angle x-ray scattering, WAXS. The relaxation times distribution is extracted by a numerical decomposition of the TSDC spectra and it is shown that this distribution is not significantly changed on going from the homopolymer to the triblock copolymer with 16 wt % to 77 wt % of PCL in the original samples. Better segregation of the mesophase structure is reached when the samples are annealed at 413 K and important variations in the TSDC and WAXS spectra are observed as a result of the thermal treatment. For the S09B14C77 triblock copolymer the results obtained can be explained by postulating the existence of a rigid amorphous phase in the PCL block. Such rigid amorphous phase is located between the core-shell cylinders formed by the other blocks [with poly(styrene)(PS) as core and poly(butadiene)(PB) as shell] and is constrained by undulated lamellae of crystalline PCL material. In the case of S35B15C50 triblock copolymer, an important amount of diffuse PS-PCL interphase where the homopolymers are mixed must be present before annealing. The results for the material with the less abundant PCL block are explained as a result of the confinement in nanotubes of PCL surrounded by PB embedded in a vitreous PS matrix. Broadband dielectric experiments on these same materials confirm the results obtained by TSDC spectroscopy.

A new application of the simulated annealing Monte Carlo procedure is presented and applied to the extraction of the relaxation time distribution from dielectric spectroscopy either in time or frequency domain. This decomposition method named simulated annealing direct signal analysis~SADSA!, is applied to computer generated curves, e(t), e8(v), and e9(v), by using the most widely accepted empirical distributions. The discretized distribution fits exactly the analytical expression which can be evaluated in these cases for the set of parameters used in the simulation. Also, both distribution functions are found to be identical which proves that the method is certainly converging to the right solution in both cases. Experimental results on amorphous poly~aryl ether ether ketone! for e(t), e8(v), and e9(v) are analyzed with SADSA and the obtained relaxation time distribution is used to go from time to frequency domain and reciprocally. The results are compared to those obtained by assuming a Havriliak–Negami profile for the distribution function. © 2000 American Institute of Physics, Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICIT G97-000594). Comunidad de Madrid 07N/0063/1998! and to DGICYT (Grant No. PB 94-0049) Spain. FPI program of the Spanish Ministry of Science and Culture (MEC).