Dielectric relaxation of vegetable-based polyurethane

Vegetable-based polyurethane (PU) was prepared in the thin film form by spin coating. This polymer is synthesised from castor oil, which can be extracted from the seeds of a native plant in Brazil called mamona. This polymer is biocompatible and is being used as material for artificial bone. The PU was characterised by dielectric spectroscopy in a wide range of frequency (10^–5 Hz to 10⁵ Hz) and by thermally stimulated discharge current (TSDC) measurements. The glass transition temperature (T _g = 39°C) was determined and using the initial rise method the activation energy was found to be 1.58 eV.     

Effect of Space Charge on Polarization Reversal Currents in a Vinylidene Fluoride/Trifluoroethylene Copolymer

Polyvinylidene fluoride and its copolymers with trifluoroethylene and tetrafluoroethylene are ferroelectric. Reversal of polarization in these polymers is known to occur by a rotation of the polar molecular chains, in response to an externally applied electric field. Recent evidence suggests that the dynamics of polarization reversal can be affected by movement of space charges in the amorphous phase of the polymers. If a 75/25 mol.% vinylidene fluoride/trifluoro-ethylene (VF₂/VF₃) copolymer is poled at elevated temperature, the subsequent polarization reversal currents show large asymmetry and significant shifts in successive reversals. The field required to reverse the polarization in such high-temperature poled material is significantly larger than the normal coercive field. The asymmetry in the reversal currents is consistent with the presence of local fields within the polymer, generated by space charge which accumulates during the poling process. The subsequent movement of this space charge causes the shift in successive reversal currents. The variation of reversal current with temperature indicates that the movement of space charge within the copolymer is a thermally activated process.     

Polarization fatigue in ferroelectric polymers

Ferroelectric materials are characterized by a spontaneous polarization which can be switched by an applied electric field. In many ferroelectrics the magnitude of the reversed polarization is observed to decrease when the polarization is switched many times. In ferroelectric polymers a significant decrease in reversed polarization may be observed after ~3000 cycles, and the reduction can be ~50% of the initial value. Infrared and X-ray observations show a conversion of the non-polar α crystal form to the ferroelectric β form in samples containing a mixture of the crystal phases. However there appear to be no crystalline changes of such a nature as would lead to fatigue. A decrease in dielectric loss at low frequencies is observed in all fatigued samples, suggesting that space charge trapping may be involved in the fatigue process     

Dielectric and related molecular processes in polymers

A comprehensive knowledge of the nature and origin of dielectric polarization and electrical conduction in viscoelastic polymers requires considerations of multiplicity of interactions of the polymer chains and their fine structure behavior. Molecular motions in polymers involve the concept of `reptation' in which a polymer chain moves in a fixed network of obstacles. The behavior of dielectric relaxation and electrical properties with ac and dc fields needs to be correlated with the viscoelastic molecular motions of the polymer chains     

Dielectric properties and spatial distribution of polarization inpolyethylene aged under ac voltage in a humid atmosphere

This paper reports the results of an investigation of the dielectric properties, growth of water tree density and spatial distribution of polarization in samples of non-stabilized LDPE. Samples were initially sand-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room temperature and 65°C). Aging both with and without the presence of electric field stress was studied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range 10^-5 to 10⁴ Hz. The results indicate a real movement of charges/ions in the polymer bulk which causes a deterioration of the electrical insulation properties even in the absence of AC fields. The presence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-modulation method) to determine the distribution of polarization, assuming mean polarization is zero and space charge is absent. In the absence of the external field there is a presence of ionic charges of opposite polarity, which increases with increasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after various aging times, and it was found that trees grown at room temperature are longer than those grown at 65°C. Also, the tree density was higher in specimens tested at room temperature     

Finite difference modeling of piezoelectric composite transducers with 0-3 connectivity

The mechanical and electrical properties of transducers are modelled by finite-difference representations of the relevant physical equations and the numerical problems are discussed. The theory is applied to a transducer consisting of a calcium-modified lead titanate (PTCa) ceramic mixed with the polymer P(VDF-TrFE), and comparison of /spl isin//sub 33/ with experimental values is found to be good. The introduction of ceramic into the mix has an immediate effect on the voltage, and s/sub 11/ and s/sub 12/. d/sub 31/ is not much affected by the presence of the ceramic until it reaches 70% of the mixture whereas d/sub 33/ changes sign rather quickly and then increases more slowly. A second transducer having the ceramic PZT5 mixed with P(VDF-TrFE) is also modelled and the characteristics of the 0-3 structure are compared with those of the 1-3 structure of the same components modelled in an earlier paper. The 1-3 construction brings the effect of the ceramic in much faster than the 0-3 construction. For example, at 50% ceramic, e/sub 33/, d/sub 3l/ and d/sub 33/ are roughly trebled compared to the 0-3 values.     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

Properties of side chain liquid crystal polyesters containing chiral groups in the main chain

The synthesis and investigation are reported for the phase behavior of a set of three liquid-crystalline side chain polyesters (SCLCPs), consisting of aromatic diacids and diols bearing cyano biphenyl moieties in the side chain. The materials contain a chiral center in the main chain and show a cholesteric phase indicative of a helical order with a preferred twist. In addition, they exhibit T_g's below room temperature and smectic bilayer phases. Depending on the substitution pattern of the aromatic core, S_c* or S_A* phases are observed. These polymers were synthesized in a four-step synthesis, starting from available materials. The dielectric relaxation behavior and the pyroelectric responses of the three SCLCPs tend to conform to their respective structure.