Electrical conductivity in iodine-doped ethyl cellulose

The electrical conductivity of solution-grown ethyl cellulose (EC) films, 5–30 μm thick, has been studied in the sandwich configuration (metal–EC–metal) as a function of iodine concentration from 0.5 to 5.0 wt% ratio. The studies were conducted in the temperature range 333–383 K, while the field was varied over the range (3.0–5.5) × 10⁴V/cm. Aluminium was used as the lower electrode, while the upper electrode was of Al, Ag, Cu, Au or Sn. Certain transient effects such as a large burst of current immediately after the application of field were observed. An attempt was made to identify the nature of the current by comparing the observed dependence on electric field, electrode material and temperature with the respective characteristic features of the existing theories of electrical conduction. The results show that the electrical conduction follows Ohm's law at lower fields, while at higher fields, space-charge limited current (SCLC) was observed. It was also found that Richardson–Schottky emission was responsible, to some extent, for the transport of charge carriers in the polymer. The conductivity of the films increased on doping with iodine. The dopant molecules are considered to act as additional trapping centes and provide links between the polymer molecules in the amorphous region, thus resulting in the formation of charge transfer complexes.     

Study of transient currents in discharging mode in pure and iodine doped ethyl cellulose films

A study of transient currents in discharging mode in vacuum aluminized pure and iodine doped ethyl cellulose (EC) films, approximately 20 μm in thickness, measured at various temperatures (303–353K) and applied fields (2.0 × 10⁴?4.5 × 10⁴ V/cm) is described. The order of current has been found to increase considerably with increase in poling temperature, field and iodine mixing. The currents have been found to show I ∝? t^?n (i.e. Curie–Von Schweidler law) time dependence with different slopes in the short and long time regions. The conductivity of the films is increased on doping with iodine. The dopant molecules are considered to act as additional trapping centres and provide a link between the polymer molecules in the amorphous region, thus resulting in the formation of a charge transfer complex. The results of the experiments and the mechanisms involved are discussed on the basis of available theories.     

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

DC 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.     

Pyroelectric behaviour of solution-grown cellulose acetate polymer films

The pyroelectric behaviour of cellulose acetate polymer films was studied in a vacuum of 10^?3 torr under a polarizing field strength range of 6.25 × 10⁵–50 × 10⁵ V/m and a polarizing temperature range of 304–406 K. The pyroelectric coefficient exhibited abnormal behaviour, starting initially on the negative side and then changing sign from negative to positive. Negative coefficients are believed to be due to dipolar orientation, whereas space charge polarization is responsible for positive pyroelectric coefficients.     

Transient and steady‐state conduction in ethyl cellulose (EC)–poly(methyl methacrylate) (PMMA) blends

Transient discharging currents and steady‐state conduction in solution‐grown ethyl cellulose (EC)–poly(methyl methacrylate) (PMMA) blends measured as a function of temperature (30–80 °C) and field strength (10–100 kV cm⁻¹) are reported. Transient currents are found to follow the Curie–VonSchweidler law, characterized by different slopes in short‐ and long‐time regions, having different decay constant values lying between 0.75–0.99, and 1.68–1.95. The corresponding activation energies are found to increase with time of measurement of discharge current. Isochronal characteristics (ie current versus temperature plots at constant times) constructed from the data seem to reveal a broad peak observed at 60 °C. The dependence of dark current at different temperatures (30–80 °C) in a metal (1)–EC–PMMA blend–metal (1)/(2) system on the applied voltage in the range 10–100 kV cm⁻¹ has also been studied; the current is found to be strongly temperature dependent. Dipole polarization and space charge resulting from trapping of injected charge carriers in energetically distributed traps and induced dipoles created because of the piling up of charge carriers at the phase boundary of the heterogeneous components of the blend are considered to account for the observed transient currents. The results of current–voltage measurement on blends are interpreted to show that the low‐field steady‐state conduction is ohmic in nature, and in high fields the charge carriers are generated by field‐assisted lowering of coulombic barriers at the traps and are conducted through the bulk of the material by a hopping process between the localized states by a Jonscher–Ansari Poole–Frenkel mechanism. The modified P–F barrier is calculated to be 1.89 × 10⁻¹⁹ J (1.18 eV), 1.92 × 10⁻¹⁹ J (1.20 eV) and 1.95 × 10⁻¹⁹ J (1.22 eV) for P₁, P₂ and P₃ blends, respectively. © 2000 Society of Chemical Industry     

Nanocomposites derived from cellulose acetate and highly branched alkoxysilane

Highly branched alkoxysilane (HB) units were prepared in situ via a Michael‐type reaction between pentaerythrithol triacrylate and aminopropyltriethoxysilane. These units were used as an inorganic component for the modification of cellulose acetate (CA) films using the sol–gel process. The thermal and dynamic‐mechanical behaviors, the morphology, and the dimensional stability of the modified CA films were analyzed. The siloxane‐modified CA films showed thermal stability similar to pure CA, but the residue content at 900°C increased with the addition of HB units. The morphology of these films was characterized by siloxane nanodomains dispersed in the CA matrix, with good interfacial adhesion between the phases. Moreover, the CA/siloxane nanocomposite films showed improved dimensional stability in comparison with CA, i.e., in the presence of HB, the dimensional change was reduced to around 50% of the value observed for pure CA. Finally, a complex dynamic‐mechanical behavior was obtained for the nanocomposite films, as a consequence of the heterogeneous morphology. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Dielectric behaviour of pure and nickel-doped cellulose acetate films

Pure and Ni (0·05%, 0·1% and 0·15%)-doped cellulose acetate (CA) films were formed on well-cleaned glass substrates using a solution growth technique. Metal–insulator–metal (MIM) sandwich structures of thin film capacitors were fabricated. Dielectric studies were carried out in the frequency range 1kHz to 10MHz in the temperature range 303–450K. The variation of capacitance and dielectric loss with temperature were studied at various frequencies. The variation of dielectric constants (ε′ and ε″) with frequency and temperature is discussed. An increase in capacitance with temperature was observed and this may be due to the chaotic oscillations of molecules in the polymer matrix. β (at lower temperature) and α (at higher temperature) relaxation loss peaks were observed. Doping results in the formation of charge transfer complexes/molecular aggregates in the polymer, which cause the shift of C_max value and the β and α relaxation loss peaks towards higher temperature. An appreciable dispersion of tan δ at lower frequency was noticed in all the samples studied. © 1998 SCI.     

Hybrids of cellulose acetate and sol–gel silica: Morphology,thermomechanical properties,water permeability,and biodegradation evaluation

Hybrids based on cellulose acetate (CA) and SiO₂ were prepared by hydrolysis of tetraethoxysilane (TEOS). More rigid films were obtained with an inorganic phase incorporation. The thermal stability of the hybrids was similar to pure CA. Composite membranes were prepared by casting of CA/TEOS mixtures onto a poly(vinylidene fluoride) support. The water permeation decreased with the incorporation of the inorganic phase. Hybrid membranes were able to retain solutes with a molar mass of ?9000 g/mol (?98% retention). Hybrids were submitted to biodegradation tests. The presence of the inorganic phase did not inhibit the growth of Thricoderma harzianum fungi. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2196–2205, 2002     

Electrokinetics and stability of a cellulose acetate phthalate latex

An experimental investigation is described on the surface electric characterization of a commercially available latex, Aquateric, composed of cellulose acetate phthalate polymer particles, and used in enteric-controlled drug release. Since the surface charge of dispersed systems is an essential parameter governing most of their behavior, it is of fundamental importance to characterize how that quantity changes in the different environments in which the colloids could be used. The experimental method used in this work is electrophoresis; we report measurements of electrophoretic mobility of the latex as a function of pH and ionic concentration in the dispersion medium. It is shown that the zeta potential of the polymer particles is negative for the whole pH range studied and increases with pH as the dissociation of surface acetic acid groups proceeds. A plateau value is found for pH > 5, corresponding to complete dissociation of available ionizable sites. The values of the electrophoretic mobility (μ_e) and the zeta potential (ζ) of Aquateric are also analyzed as a function of the concentration of 1-1 (NaCl) and 2-1 (CaCl₂) concentration. The anomalous surface conductance (associated to the mobility of counterions adsorbed in the inner part of the electric double layer of the particles) manifests in a maximum in the |μ_e|-NaCl concentration plot for 10⁻³M concentration. No such behavior is observed in the presence of CaCl₂ solutions, where only a decrease of the mobility with ionic strength is observed. The effect of AlCl₃ concentration on the mobility is also considered; it is found that at pH 2 aluminum ions adsorb on the particles and render them positively charged. When the pH of the suspensions is not maintained constant, the hydrolysis of aluminum gives rise to a less efficient control of the charge of the particles and no positive mobilities are observed. Electrophoretic mobility measurements as a function of pH at constant AlCl₃ concentration show an abrupt change of μ_e from negative to positive, interpreted as due to surface precipitation of Al(OH)₃. When the pH is further increased, a second charge reversal is found, corresponding to the isoelectric point (pH of zero zeta potential) of Al(OH)₃. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2721–2726, 1997     

Control of the crystal structure of microbial cellulose during nascent stage

The structure of the product, from an Acetobacter culture in the presence of Fluorescent Brightener, Direct Red 28, and Direct Blue 1, 14, 15 and 53, characterized by an X‐ray diffractormeter is a crystalline complex. On the other hand, solid‐state ¹³C‐NMR spectroscopy reveals that the product is noncrystalline. However, the X‐ray result of the product sample suggests that the dye molecule is included in the form of a monolayer between the cellulose sheets in the complex corresponding to the (11¯0) plane of microbial cellulose. But the celluloses regenerated from the Fluorescent Brightener product, the Direct Red 28 product, and the rest of the dye products are celluloses I, IV, and II, respectively. More specifically, the ¹³C‐NMR spectra revealed that the crystal types of cellulose from the Fluorescent Brightener and Direct Red 28 products are I_β and IV_I, respectively. Thus, the crystal structure of the product and the regenerated cellulose depends mainly on the position and number of the sulfonate groups in a direct dye and the interactions of the dye with the noncrystalline microbial cellulose in the nascent stage. The conformation and arrangement of the nascent cellulose chain changes when a direct dye adheres to it. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1726–1734, 2001     

Crystallization behaviour of cellulose acetate butylate/poly(butylene succinate)‐co‐(butylene carbonate) blends

The kinetics of the isothermal crystallization process from the melt of pure poly(butylene succinate)‐co‐(butylene carbonate) (PBS‐co‐BC) and its blends with cellulose acetate butylate (CAB) (10–30 wt%) was studied by differential scanning calorimetry (DSC) and the well‐known Avrami equation. In the blends, the overall crystallization rate of PBS‐co‐BC became slower with increasing CAB content. The equilibrium melting temperature ( ) of PBS‐co‐BC decreased with increasing CAB content, which was similar to that with other miscible crystalline/amorphous polymer blends. The slower crystallization kinetics of PBS‐co‐BC in the blends was explicable in terms of a diluent effect of the CAB component. By application of Turnbull–Fisher kinetic theory for polymer–diluent blend systems, the surface free energy (σ_e) of pure PBS‐co‐BC and of the blends was obtained, indicating that the blend with CAB resulted in a decrease in the surface free energy of folding of PBS‐co‐BC lamellar crystals. Copyright © 2006 Society of Chemical Industry     

Characterization and electrical properties of methyl‐2‐hydroxyethyl cellulose doped with erbium nitrate salt

X‐ray diffraction, infrared (IR), and electrical properties for pure and Er (NO₃)₃‐doped methyl‐2‐hydroxyethyl cellulose (MHEC) with concentrations of 0.5, 1, 2, 5, 7, and 10 wt % were studied. X‐ray analysis indicates that the addition of Er (NO₃)₃, which is a crystalline material, to MHEC at concentrations 10 and 13 wt % leads to the formation of crystalline phases in the amorphous polymeric matrix. The appearance of the bending mode ν₂ and the combination mode (ν₁ + ν₄) of Er (NO₃)₃ in the IR spectra of composite samples indicates the coordination of nitro group in the chains of MHEC. From the I–V characteristics, it was found that the charge transport mechanism in MHEC appears to be essentially space charge limited conduction, while the predominant mechanism in the composite samples is Poole–Frenkel. Values of both drift mobility (μ) and the charge carrier density (n) has been reported. The temperature dependence conductivity data has been analyzed in terms of the Arrhenius and Mott's variable range hopping models. Different Mott's parameters such as the density of states, N(E_F), hopping distance (R), and average hopping energy (W) have been evaluated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:2352–2361, 2006     

Electrical properties of composites in the vicinity of the percolation threshold

The electrical response of thermoplastic composites composed of carbon black and high‐density polyethylene near the electrical percolation threshold (p_c) has been investigated through the study of the volume resistivity and complex permittivity. The change in conductivity beyond p_c exhibited a critical exponent that was greater than predicted from percolation theory. Composites with carbon black contents slightly larger than p_c exhibited the greatest sensitivity in volume resistivity with temperature variations under the melting point of polyethylene. In addition, percolating composites with low carbon black contents exhibited significant “negative temperature coefficient” (NTC) effects and improvements in conductivity with annealing. Maxwell–Wagner interfacial polarization resulted in moderate increases in both the permittivity (ϵ′) and dielectric loss factor (ϵ″) below p_c, while at percolation, an abrupt and dramatic increase was observed for both components of the complex permittivity. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1573–1582, 1999