Broadband ac conductivity of conductor-polymer composites

The electrical conductivity of a composite model system formed by highly structured carbon black (CB) filled, within an amorphous polymer, poly(ethylene terephtalate) composite is studied. The dc conductivity as a function of CB content follows a scaling law of the type σ∝(p−pc)t yielding for the percolation concentration, pc=0.011 and for the exponent, t=2.17. The analysis of the temperature dependence of the conductivity suggests that for temperatures larger than 45 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating two CB aggregates. At lower temperatures, conductivity becomes temperature independent, which is typical of conventional tunneling. The frequency dependence of the conductivity is also studied between dc and 109 Hz. By the introduction of a shift factor ap, a procedure for the construction of a master curve based on a “time-length equivalence principle” is proposed. Finally, a model is introduced to describe the frequency dependence of the conductivity of CB-filled composites based on the behavior of charge carriers placed in a fractal object

Molecular dynamics of ferroelectric polymeric systems as studied by incoherent quasielastic neutron scattering

11 págs.; 15 figs.We have studied by incoherent quasielastic neutron scattering the molecular dynamics of the ferroelectric copolymers of vinylidene fluoride and trifluoroethylene with two different molar ratios. Thermal hysteresis effects accompanying the structural changes observed in these materials are found. In the temperature range investigated three motions were identified: (i) a vibrational motion of Debye-Waller type which shows increasing amplitude until the vicinity of the Curie temperature and then it follows a Q4 law, (ii) a slow jump diffusive motion of each chain in the restricted volume defined by other chains, thermally activated with an activation barrier of 8 kcal/mol, and (iii) a fast local diffusive motion with only some fraction of the chains concerned. © 1994 The American Physical Society.Grateful acknowledgments is due to DGICYT (Grant No. PB91-0355), to CICYT (Grant No. MAT92-148), to CICYT (Grant No. MAT94-0740E), to the Consejeria de Educacion de la Comunidad de Madrid (Grant No. 284/92) and to Fundacion Domingo Martinez (Grant No. 1992/93) for the generous support of this investigation.Peer Reviewe

Molecular dynamics of the α relaxation during crystallization of a glassy polymer: A real-time dielectric spectroscopy study

9 págs.; 12 figs.; 1 tab.The change of the α relaxation process occurring during isothermal crystallization of a glassy polymer, poly(aryl ether ketone ketone), has been followed in real time through measurement of the dielectric complex permittivity. The dielectric strength has been correlated with the degree of crystallinity derived from real time wide angle x-ray patterns using synchrotron radiation. The phenomenological Havriliak-Negami description has been used to analyze the changes of the dielectric strength, central relaxation time and shape parameters describing the relaxation, as crystallization proceeds. The evolution of the dielectric magnitudes with crystallization time has been interpreted in the light of the Schönhals-Schlosser model. According to this model, a restriction of the long scale motions of the polymeric chains as the material is filled in with crystals is observed. The derived dipole moment time correlation functions have been calculated for different crystallization times and fitted by the Kohlrausch-Williams-Watts function. The variation of the stretching parameters with crystallization time can be interpreted in terms of the coupling model assuming an increase of the intermolecular cooperativity of the α relaxation as the polymeric chains are constrained to move between crystalline regions. These results offer a contribution to the understanding of molecular dynamics of a glassy polymers during crystallization. © 1994 The American Physical Society.The authors are indebted to the CICYT (Grant No. MAT 94-0740E), Spain, for generous support of this investigation. J.M. thanks the Tempus Program (JEP 0644) for the support of his investigations.Peer Reviewe

DOPING OF NOVEL PHENYLENE DERIVATIVE POLYMERS RELATING TO ELECTRICAL CONDUCTIVITY

4 pags.3 figs.Poly-p-phenilene, prepared in laboratory, commercial poly-p-phenylene sulphide and two new high temperature thermoplastic polymers (polyarylethersulphone and polyaryletter ketone) have been doped by means of SbCl5. Measurements of electrical conductivity at room temperature carried out in situ as a function of doping time are reported

Phase changes in isotactic polypropylene measured by microhardness

2 pags, 1 fig

Microstructural changes in polyethylene-polypropylene blends as revealed by microhardness

5 pags., 4 figs., 2 tabs.Microhardness is used to examine the microstructural changes of a series of polyethylene (PE) /polypropylene (PP) blends in a wide composition range. This study complements previous hardness results obtained on high-density/low-density polyethylene systems. The use of isotactic polypropylene, as a blend component allows investigation of a material in which the hardness of the amorphous phase, contrary to PE, differs from zero. The influence of treatments such as crystallization of the PP-phase in the presence of molten PE, within the blend, or annealing the PE phase, while leaving the PP component unmodified, are discussed with reference to the additivity hardness values of the single components HPE and HPP. It is shown that the coexistence of the PP and PE phases inhibits the crystallization capability of one phase and modifies the annealing behaviour of the other phase leading, as a result, to depressed HPP and HPE values. The observed deviations of Hblend, throughout the composition range, from the additivity law of single components are quantitatively justified in the light of crystallinity changes of the PP phase and in terms of the population of modified lamellae of the PE component. © 1988 Chapman and Hall Ltd

Electrical Conductivity in Polymer-Carbon Fiber Composite materials

5 pags., 8 figs.Peer reviewe

Properties of SbCls-doped PPP/PPS composites Part 2 Surface mechanical behaviour

Microindentation hardness is used to characterize the surface mechanical behaviour of two series of sintered PPP/PPS composites in a wide range of compositions. This work complements a previous electrical conductivity study carried out to determine the insulator-conductor transition within these materials. The influence of annealing the PPP-phase at 400ºC before sintering the composites is discussed with reference to the additivity of hardness values of the single components PPP and PPS. The hardening of these composites upon doping with SbCIs is investigated in the light of the morphology of sintered materials. The elastic release of the material and the creep behaviour under the indenter are examined in terms of the porosity of the samples and of the dopant penetration within the material. In the case of compact materials, the hardness is found to increase due to a doping-induced surface hardening effect. The hardness increase in porous materials responds to an inclusion of the dopant species within the material volume.Peer reviewe

X-ray diffraction study of poly(ppheny1ene) Part 1. Variation of crystallite size with annealing temperature

A line profile analysis of the (hkO) reflections of sintered poly(p-phenylene) is presented. The wide angle X-ray diffraction maxima were analyzed to study the average size of the coherently diffracting domains as a function of annealing temperature TA. The results show that the lateral size of the coherently diffracting domains increases first slightly for TA 3OOOC. This is related to a rearrangement of rigid chains from disordered into the crystalline regions. The absence of SAXS supports the view of a nematic frozen-in liquid crystalline structure contributing to about 40% of the material.Peer reviewe

Structural implications on positron lifetimesin lamellar polyethylene with chain defects

7 pags., 4 figs.New positron lifetime data in a series of isothermally crystallized polyethylene pairs with known concentrations of chain defects, having a lamellar structure, have been measured. The materials were characterized by small‐angle and wide‐angle x‐ray scattering techniques. Positron lifetime data were computer analyzed and only three components could be resolved. It is shown that the annihilation mechanisms can be understood in terms of average distance between lamellar crystals (long period), crystal thickness, and chain defect concentration. The present results confirm, in consonance with the conclusions of other authors, the longest component to be due to ortho‐positronium pick‐off annihilation. The intermediate component may be ascribed to positronium trapped at the crystal‐amorphous interface. The application of a three‐state trapping model yields a lifetime of about 900 ps for this state. It is suggested that the shortest component may be due to free positron annihilation, para‐positronium self‐annihilation, and annihilation of positrons from a tightly bound state localized at the chain defects. An approximate calculation provides lifetime values around 250 ps for this short‐lived state.Peer reviewe