Isothermal Immersion Technique Research Articles

Low frequency dielectric loss of nylon films from charging and discharging currents studies

Charging and discharging currents in Nylon-6, 6 pure and KCl doped polymer films were prepared by the isothermal immersion technique. The effect of field strength and charging temperature on the charging and discharging currents was studied from quasi steady state currents, activation energies were also calculated from discharging currents by the initial rise method and are in good agreement with the energies calculated from the steady state current data. The dielectric loss data was evaluated from the discharging currents data using Hamon's approximation method.

Growth, structure, dielectric and AC conduction properties of solution grown PVA films

Polyvinyl alcohol (PVA) films were deposited on pre-cleaned glass substrates using an isothermal immersion technique. Sandwich structures of the type Al-PVA-Al were formed to study the dielectric and conduction properties of PVA films. Aluminium electrodes were deposited by vacuum evaporation method. Thicknesses (d) of the dielectric films were measured by an electronic linear thickness measuring instrument (Tesatronic TTD20 model) and a gravimetric method and cross checked by a capacitance method. The dependence of the thickness of the PVA films on the immersion time, concentration and temperature of the solution had been studied. From the X-ray diffraction studies the structure of the deposited film was found to be amorphous. The dielectric properties were studied in the frequency range 1–30 kHz at various temperatures in the range 303–423 K. The capacitance, though dependent on frequency and temperature, was found to be almost invariant at room temperature for all frequencies. The dielectric constant ε′ of the films was found to be increasing with increase in thickness of the film. The loss factor ε″ was found to be increasing with increase in temperature up to the glass transition temperature (Tg) and decreasing above that (Tg). From AC conduction studies it was confirmed that the mechanism responsible for conduction is mostly due to ionic hopping. The films showed very high transmittance in the visible to near IR region.

Conduction mechanism in doped polymethyl methacrylate (PMMA) films

The electrical transport behaviour of ferrocene mixed poly (methyl methacrylate) (PMMA) films (≈ 20 µm in thickness) deposited by the isothermal immersion technique has been studied in the temperature range of 333–373 K and field from (2·0–4·0)×104 V/cm. It has been found that at higher fields and temperatures, the observed conduction behaviour could be consistently described by the Richardson-Schottky emission. The increase in current due to doping has been attributed to the formation of charge transfer complexes. The dopant molecules act as an additional trapping centre and provide a link between polymer molecules in amorphous region leading to the formation of charge transfer complex.

Pyroelectric behaviour of poly vinyl alcohol (PVA)-polyvinyl pyrrolidone (PVP) polymer blend films

The pyroelectric behaviour of PVP-PVA polyblend films of different weight ratios, grown by the isothermal immersion technique has been studied under different polarizing fields and temperatures in a vacuum of 10 −3 Torr. The pyroelectric coefficient g 3 is evaluated from the measured pyroelectric current. The pyroelectric behaviour of the blend showed a systematic variation with the ratio of the constituents, i.e. the pyroelectric coefficient was found to increase with increasing percentage of PVP in the blend suggesting good miscibility of the individual polymers in the blend. The effect of electrode materials of different work functions on the pyroelectricity was also studied. The observed pyroelectricity in the polymer blend films was considered to be due to both dipolar orientation and space charge polarization. Compared to other blend compositions, polymer blend I (PVP: PVA = 75:25) was suggested as the best electret having better pyroelectric properties.

Charging and discharging currents in poly(vinyl pyrrolidone) polymer thin films

Poly(vinyl pyrrolidone) (PVP) polymer films were prepared by the isothermal immersion technique. Transient currents (charging and discharging currents) in PVP thin films were measured in the temperature range 30–150 °C and field strengths of 2.3 × 10 6 − 23 × 10 6 V m −1. Activation energies were evaluated from quasi steady-state currents. The single relaxation peaks were observed from isochronal currents and low frequency dielectric relaxation. The activation energies were evaluated from both these methods and are almost similar. The polarization was attributed to dipolar mechanism.

Charging and discharging currents in polyvinylalcohol (PVA) polymer films

Charging and discharging currents in polyvinylalcohol (PVA) polymer films were measured over the temperature range 30–150°C and field strengths of (2.3–23) × 10 6 V m −1. Polymer films were prepared by the isothermal immersion technique. The effect of field strength and charging temperature on the transient currents was studied. Activation energies were evaluated from quasi steady state currents. A single relaxation peak was observed both from isochronal currents and low-frequency dielectric relaxation. Activation energies evaluated from both these methods are found to be in fairly good agreement. The polarization has been considered to be due to space charge origin along with some contribution from dipolar groups.

Transient currents in poly(vinyl alcohol)–poly(vinyl pyrrolidone) polymer blend films

AbstractTransient currents (charging and discharging currents) in poly(vinyl alcohol) (PVA)–poly(vinyl pyrrolidone) (PVP) polymer blend films were measured over the temperature range 30–150°C at field strengths of 2.32–23.2 × 106 Vm−1. Polymer films were prepared by the isothermal immersion technique. Activation energies were evaluated from quasi‐steady‐state currents. A single relaxation peak was observed both from isochronal currents and low frequency dielectric relaxation. Activation energies evaluated from these two methods are found to be in fairly good agreement. The polarization is considered to be due to space charge origin along with some contribution from dipolar groups. The maximum loss was observed in Sample I (PVA: PVP = 25:75), suggesting maximum heterogeneity in this blend ratio.

DC electrical conduction in solution‐grown pure and iodine‐doped poly(butylene terephthalate) films

AbstractDC electrical conduction in poly(butylene terephthalate) films sandwiched between planar aluminium electrodes was investigated. The films were grown by the isothermal immersion technique. The conduction process was studied in the temperature range 300–450K, with an applied electric field of 0.22 × 107−6.66 × 107 Vm−1. It was found that the Richardson‐Schottky type of conduction mechanism was dominant throughout the temperature range of study. The values of the Schottky field lowering constant β, the effective metalinsulator potential barrier and the activation energy were evaluated. The value of the activation energy (U = 0.7 eV) indicates the predominance of electronic conduction. The effect of impurities like iodine on electrical conduction was also investigated.

Thermally stimulated discharge current studies on styrene-methyl methacrylate copolymer films

Thermally stimulated discharge current (TSDC) studies on thin films of styrene-methyl methacrylate copolymer in the ratio 45:55 are reported. Films of thickness 7.36 μm were grown by the isothermal immersion technique. Only one current maximum was observed in the TSDC spectrum for polarizing fields < 20 × 10 5 V/m and two current maxima were observed at higher fields for all polarizing temperatures. Relaxation times and activation energies were evaluated using existing theories. The low-temperature peak was attributed to dipolar orientation and the high-temperature peak to glass transition relaxation.

Dielectric properties of polyacrylamide polymer films

Polyacrylamide polymer films of thickness 4.15 μm were prepared by isothermal immersion technique. The dielectric capacitance and dielectric loss of the films were studied as functions of frequency and temperature in the range 100 Hz|10 MHz and (300|450)K, respectively. Two dielectric loss peaks were observed in the dielectric loss spectra and were identified as β and α peaks. The β peak was attributed to the reorientation of dipoles and the α-peak was attributed to the deformations accompanied by large changes in the directions or locations of the dipoles.

Photo-electret studies of pure and doped polystyrene films

The photoelectret properties of pure and acrylic acid (AA) doped polystyrene (PS) films formed by an isothermal immersion technique have been studied. The photoelectrets were formed by simultaneous application of u.v. radiation for a short time period (2–10 min) and d.c. electric field (3–18 V). The depolarization current was obtained by reilluminating the photoelectret with the same radiation. It is found that the photo depolarization current increases with polarizing voltage and is greater for the doped samples than for the undoped samples, prepared under identical conditions. The current increases with the dopant concentration and with duration of polarization. The total charge, found by integrating the depolarization current-time curve of doped photoelectrets, tends to saturation at a certain voltage value. Doped PS gives better photoelectrets.

Spectroscopic studies of undoped and doped polystyrene films

Spectroscopic (i.r., u.v.) studies of undoped polystyrene and that doped with Acrylic Acid films (30μ thick), prepared by isothermal immersion technique, have been reported. It is found that in the case of i.r. spectra an additional absorption peak appears near 1420cm−1, whereas those present near 1300 and 980cm−1, as shoulders in the undoped sample, appear as sharp peaks in the doped samples. In the region 3300–2500cm−1 a broadening appears in the doped films which increases with the dopant concentration. In the case of u.v. spectra, out of four absorption regions present in the undoped films two bands merge and only three appear in the doped ones. The u.v. absoption increases with the dopant concentration. These results have been explained on the basis of a charge transfer complex formation due to the interaction between the dopant and the polymer matrix.

D.C. electrical conduction in solution-grown thin polyvinyl chloride films

The mechanism of conduction in polyvinyl chloride (PVC) films sandwiched between planar aluminium electrodes was investigated. The films were in the thickness range 500–1500Å and were obtained by the isothermal immersion technique. The conduction process was examined in a broad temperature range, 260–507 K, with applied electric fields ranging from 4.6 × 10 5 to 1.5 × 10 7 V cm −1. omegaic behaviour was observed in the temperature range 293–363 K at fieldsup to about 1.5 × 10 6 V cm −1. At low temperatures (below 325 K) and at high fields (greater than 1.5 × 10 6 V cm −1) the current transport is ascribed to a tunneling mechanism. At high temperatures (above 325 K) and at high fields (greater than 1.5 × 10 6 V cm −1) it is concluded that although the analytical form of the I–V characteristics is similar to that obtained for both Schottky emission and the Poole-Frenkel effect, the experimental value of β is incompatible with the theoretical values β S or β PF. Also, hysteresis and polarization effects were found in the I–V characteristics. The resistivity of the PVC films was about 10 13 ω cm at room temperature and at field of 10 7 V cm −1. The activation energy was 0.60 eV at temperatures above 325 K and at a field of 1.5 × 10 6 V cm −1.

Depolarization current studies of vinyl chloride-vinyl acetate copolymer thin films

Depolarization current characteristics of vinyl chloride-vinyl acetate (VC- VAc) copolymer thin films grown from solution by an isothermal immersion technique were studied as functions of the polarizing field (6.25 × 10 5−2.25×10 6 V m −, time (7.20 × 10 3−5.04×10 4 s), the temperature of polarization (305−425 K), the nature of the vacuum-evaporated electrode metals (gold, copper, silver and aluminium) used during polarization, the thickness of the copolymer fims (4000−10 000 Å) and the heating rates of depolarization (0.033−0.20 K s −1). Three relaxation processes at 340, 412 and 460 K were observed with activation energies of 0.59, 1.01 and 1.37 eV and relaxation times at 300 K of 3.66×10 3, 8.78×10 6 and 1.46×10 10 s respectively. The peak at 340 K is associated with orientation of the dipoles of the main polymer chain; that at 412 K is attributed to ionic movements in the amorphous region of the VC-VAc copolymer film; and that at 460 K is attributed to release of charge which is the resultant of the charge already present and that due to the injected charge carriers. The peaks at 340 and 412 K shift towards higher temperatures and their activation energies increase with increases in the polarizing time and temperature. This is attributed to a distribution of the relaxation times and activation energies and suggests that these two peaks are complex in nature and cannot be represented by a discrete level of activation energy having a single relaxation time.

Effect of electrode metals on the depolarisation current characteristics of PVAc thin films. IV

Depolarisation current characteristics of PVAc thin films grown from solutions by isothermal immersion technique have been studied as a function of polarising field, temperature and the nature of the vacuum deposited electrode metals-gold, silver, copper and aluminium-by the thermally stimulated depolarisation (TSD) current technique. The TSD current spectrum shows three individual relaxation processes, designated as beta , alpha and alpha ' in the order of their appearance with increasing temperatures. The alpha ' relaxation process is due to the space charge polarisation and is associated with the trapping of the charge carriers injected from the electrodes at the trap level of depth U=1.05 eV. It has been established that this relaxation process is a case of slow retrapping. The nature of the electrodes plays a dominant role in the polarisation mechanism of these films.

Epitaxial growth of solution grown polyvinylchloride (PVC) films

Thin films of polyvinylchloride (PVC) have been grown epitaxially from three different solvents onto the (001) plane of rocksalt crystals by the isothermal immersion technique. The ranges of values of the growth parameters (temperature and concentration of the solution and immersion time) over which epitaxial growth is obtained have been established in each case. Films grown from a benzene and acetone mixture show epitaxy only at temperatures of 58°C or more and with concentrations below 0.1 g/100 ml. The epitaxial relation is (010) [110] PVC∥(001) [110] NaCl. The crystallites in the epitaxial films are of pyramidal shape and the molecular chains are folded in a direction perpendicular to the substrate. Films grown from ethyl methyl ketone solution show epitaxy only at temperatures of 65°C or more for concentrations greater than or equal to 0.15 g/100 ml. Films grown from cyclohexanone solution show epitaxy over a wide range of temperatures and concentrations. The epitaxial relation is (001) [110] PVC∥(001) [110] NaCl and the crystallites are rod- or fibre-like. The molecular chains in these crystallites are folded in a direction parallel to the substrate. Our studies show that epitaxial growth occurs in the earliest stage of growth of the films and further growth results in a mixture of amorphous and polycrystalline regions. The mechanism of epitaxial growth is explained in terms of our model for the growth of polymer chains. Two possible configurations of the molecular chains in the unit cell of PVC on a (001) rocksalt surface are proposed to explain the two epitaxial orientation relations observed.

Thermally stimulated discharge currents in solution-grown thin polyvinyl butyral films

Thermally stimulated discharge (TSD) currents of polyvinyl butyral (PVB) thin films grown on glass substrates by isothermal immersion technique have been studied as a function of the polarization field, (750 to 25000 V/cm) and iodine doping concentration (0.5 to 0.9 g/100 ml of PVB solution). Polarized PVB films exhibit two glow discharge peaks, β - at 350 °K and α - at 430 °K corresponding to activation energies of 0.19 and 0.53 eV, respectively. It has been observed that the β-peak disappears at low polarizing fields (≈750 V/cm). The α-peak is attributed to the depolarization of the aligned dipoles connected to the main chain whereas the β-peak arises due to the local motion or twisting of the side groups connected to main chain. On doping with iodine, both unpolarized and polarized films exhibit TSD currents. This is attributed to the formation of charge transfer complexes between iodine and PVB chains where the dipoles may be aligned due to electrostatic interaction between the charge complexes. The disappearance of the β-peak at high field (≈10 kV/cm) in sharp contrast to the increasing and shifting to lower temperatures of the α-peak show that the complexes are formed with the side groups of the main chain. These conclusions regarding the charge complex formation have been further confirmed by UV and infrared optical absorption data on pure and doped PVB films.