F. J. Baltá-Calleja

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.

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.

Microhardness and surface free energy in linear polyethylene: The role of entanglements

The microhardness of a series of melt crystallized samples of linear polyehtylene was investigated in a wide range of molecular weights. The x-ray long period was analyzed to study the variation of the hardness-derived constantb as a function of molecular weight (Mη). It is pointed out thatb offers a measure of the hardness depression due to the finite thickness of the lamellar crystals. The data obtained show that the increase and final leveling-off (forMη 200 000) ofb withMη parallels the concurrent increase of the surface free energy, as derived from DSC experiments. Results are discussed using the concept og chain folded lamellae as thermodynamically stable non-homogeneous microphases. Comparison of experimental and calculated data supports the view that the number of molecular entanglements, segregated onto the defective surface boundary of the heterogeneous crystals influence the shearing mechanism within the “mesocrystals” and thereby control the yield behavior of the material.

Role of thermal history on quiescent cold crystallization of PET

Abstract The cold crystallization of poly(ethylene terephthalate) (PET) has been studied as a function of the initial structure of the glass using density, microhardness, wide angle X-ray scattering, small angle X-ray scattering and DSC measurements. Glassy PET samples varying from slightly crystalline to completely amorphous phase were investigated. Results reveal that differences in the inner structure of the starting glassy material induce different crystallization rates from the glassy amorphous state. Thus, it is observed that crystallization rate decreases with the increasing cooling rate used to quench the samples. Results have been analyzed using the Kolmogroff–Avrami–Evans theory. A good agreement between theoretical and experimental data is obtained providing accurate values for kinetic constants. The different crystallization rates obtained are explained in terms of differences in nucleation density.

On the Origin of the Multiple Melting Behavior in Poly(ethylene naphthalene-2,6-dicarboxylate): Microstructural Study as Revealed by Differential Scanning Calorimetry and X-ray Scattering

In this article a study on the melting behavior and microstructure of semicrystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) prepared by crystallization from the glass under different annealing conditions is presented. The influence of the annealing temperature (Ta), annealing time (ta), and the heating rate (Rh) at which Ta is reached on the endothermic behavior of the samples was investigated by means of differential scanning calorimetry (DSC). A dual melting behavior appeared for low Rh values (2 deg · min−1) within the range of 145 °C < Ta < 250 °C and 1 min ≤ ta. ≤ 16 h. Samples subjected to fast heating rates (Rh = 200 deg · min−1) to reach a Ta ≥ 230 °C showed DSC traces in which a transition is observed from three peaks to a single melting peak when ta increases in the 30–240 min range. On the basis of the DSC results, PEN samples were prepared displaying single or dual endothermic behavior. The microstructure of these samples was studied by wide (WAXS) and small-angle X-Ray scattering (SAXS) techniques. The SAXS data were analyzed using the correlation function and interface distribution function formalisms, respectively. In samples with a single melting behavior, microstructural parameters such as the long spacing, the amorphous and the crystalline phase thicknesses are consistent with a lamellar stacking model in which the thickness distributions of both phases are almost the same. For samples exhibiting two melting endotherms, a dual lamellar model, which is in agreement with the experimental results is proposed.

Simultaneous measurements of small angle x-ray scattering, wide angle x-ray scattering, and dielectric spectroscopy during crystallization of polymers

A novel experimental setup is described which allows one to obtain detailed information on structural and dynamical changes in polymers during crystallization. This technique includes simultaneous measurements of small angle-wide angle x-ray scattering and dielectric spectroscopy (SWD). The capabilities of the technique have been probed by following in real time the crystallization process of a model crystallizable polymer: poly(ethylene terephthalate). By performing these experiments, simultaneous information from both, the amorphous and the crystalline phase is obtained providing a complete description of changes occurring during a crystallization process. The SWD technique opens up new promising perspectives for the experimental study of the relation between structure and dynamics in materials science.

Induction time for cold crystallization in semi-rigid polymers: PEN and PEEK

The appearance of long range density fluctuations in two isotropic semi-rigid polymers (PEN and PEEK), during the induction period prior to crystallization, when heating above the glass transition temperature, Tg, has been demonstrated by simultaneous wide- and small-angle X-ray scattering techniques. The parallel use of dielectric spectroscopy, to characterize the segmental dynamics of the amorphous polymers polymer above Tg, and of X-ray scattering, to estimate the induction period for cold crystallization, reveals that segmental mobility of the supercooled melt is a key factor in controlling the development of the pre-crystalline nanostructures.

Thermal anisotropy of polymer carbon fiber composites as revealed by photodeflection methods

Thermal diffusivity for carbon fiber composites was measured using different fiber types and polymer matrices. Photothermal testing was performed in the various directions parallel and perpendicular to the carbon fiber axis by different photothermal configurations. By focusing the laser beam with a spherical lens, local inhomogeneities of the composite surface in the range of 10 μm are distinguished. When focusing is done with the aid of a cylindrical lens an averaging over larger scales of the photothermal deflected signal takes place. The results for various carbon fiber materials are discussed in terms of thermal diffusion lengths and thermal diffusivity values. It is shown that the thermal photodeflection method is suitable for measuring anisotropy in oriented carbon fiber composites.

Diamagnetic susceptibility of solid and liquid paraffins

Abstract Experimental values for the diamagnetic susceptibility of a series of mono-disperse n-paraffins ranging between 6 and 50 carbon atoms in the solid (melt and solution crystallized) and in the liquid state are reported. From the dependence of the molecular susceptibility, ScHM, on the molecular weight, information about the intermolecular interactions between adjacent chain molecules and on the arrangement of the methyl end-groups is obtained. The ScHM values of melt- and solution-crystallized paraffins are, within experimental error, indistinguishable from each other. For C12H26 and C44H90 the specific susceptibility ScHM /M rises at the melting point within a few °C and reaches a plateau which characterizes the liquid state II. For the paraffin C24H50 an intermediate plateau between the melting point and the final true liquid state is observed and is called liquid state I. After cooling below the melting point TM, the ScHM value of the equilibrium state can be obtained after a short time only fro...