Factors affecting electrical conductivity of carbon black-loaded rubber. I. Effect of milling conditions and thermal-oxidative aging on electrical conductivity of haf carbon black-loaded styrene–butadiene rubber

Styrene–butadiene rubber (SBR-1502) loaded with 50 phr carbon black (HAF) was prepared. The electrical conductivity of rubber vulcanizates was measured. It was found that milling conditions have a marked effect on the temperature dependence of the electrical conductivity σ(T) of test samples. In this case, the resistivity of prepared samples increases with remilling and has a minimum value on the σ(T) curve. Thermal-oxidative aging of such vulcanizates decreases the conductivity of the prepared samples.     

Effect of small additions of carbon nanotubes on the electrical conductivity of polyurethane elastomer

The effect of small (0.002–0.018 wt %) additions of single-walled carbon nanotubes on the dielectric properties and electrical conductivity of crosslinked polyurethane elastomer is studied in the temperature range of 133–453 K and the 10^?3 to 10⁵ Hz range of electric field frequencies. It is shown that the dependence of direct current conductivity σ _dc on temperature deviates significantly from the Arrhenius dependence and is described by the Vogel-Fulcher-Tamman equation σ _dc = σ _dc0exp{?DT ₀/(T ? T ₀)}, where T ₀ is the Vogel temperature and D is the strength parameter. A correlation is found between the nonmonotonic dependences of the glass transition temperature (T _g), D parameter, and σ _dc and the concentration of nanotubes with earlier results for their effects on the physicomechanical characteristics (strength and Young’s modulus) of these systems.     

Dielectric properties of films of Ag-ED20 epoxy nanocomposite synthesized in situ. Temperature dependence of direct current conductivity

The influence of silver myristate used as a precursor of silver nanoparticles on the direct current conductivity σ _dc of epoxy polymer within the concentration range of ≤0.8 wt % was investigated. The value of direct current conductivity was determined on the basis of analysis of the frequency dependence of complex permittivity within the frequency range of 10^?2–10⁵ Hz. The temperature dependence of σ _dc is composed of two regions. The dependence corresponds to the Vogel-Fulcher-Tammann empirical law σ _dc = σ _dc0exp{?DT ₀/(T-T ₀)} (where T ₀ is the Vogel temperature and D is the strength parameter) at temperatures higher than the glass transition temperature T _g. At the same time, T ₀ does not depend on the concentration of nanoparticles. The Arrhenius temperature dependence characterized by activation energy about 1.2 eV is observed at temperatures lower than T _g. The observed shape of the temperature dependence is related to the change in the mechanism of conductivity after “freezing” of ionic mobility at temperatures lower than T _g. The value of σ _dc is increased as the concentration of nanoparticles is raised within the temperature range of T > T _g. The obtained dependence of σ _dc on silver myristate concentration is similar to the root one, indicating the absence of percolation within the studied range of concentrations.     

Mechanism of dc electrical conductivity in poly(vinyl chloride)

Dc conductivity measurements were performed as a function of temperature on unplasticized poly(vinyl chloride) and on PVC plasticized with various amounts of dioctylphthalate. The conductivity curves consist of two or three straight-line segments denoted I, II, and III with increasing of temperature. The intersection of segments I and II occurs at the glass-transition temperature T_g. The slope in region I is independent of the DOP concentration, while the slope in region II decreases slowly with an increase in the amount of DOP. No dependence of the conductivity on the molecular weight was found. From the conductivity curves, activation energies were evaluated below and above T_g. These satisfactorily coincide with those determined by dielectric loss or by electrical transient phenomena. A dc conduction mechanism is proposed based on electronic hopping favored by the micro-Brownian motions responsible for dielectric losses. These motions involve smaller chain lengths below than above T_g. The experimental results are discussed and interpreted in terms of the proposed mechanism.     

Electrical properties of carbon black‐filled polymer composites

This work deals with the dielectric properties of conductive composite materials, which consist of thermoplastic polypropylene (PP) matrix filled with carbon black (CB). The CB concentration was systematically varied in a wide range. Our main interest is focused on the investigation of electrical conductivity mechanism and related percolation phenomena in these materials. To study the electrical and dielectric properties of composites we used broadband ac dielectric relaxation spectroscopy (DRS) techniques in a wide temperature range. By measurements of complex dielectric permittivity, ϵ*, the dependence of ac conductivity, σ_ac, and dc conductivity, σ_dc, on the frequency, the temperature and the concentration of the conductive filler was investigated. The behavior of this system is described by means of percolation theory. The percolation threshold, P_C, value was calculated to be 6.2 wt.% CB. Both, dielectric constant and dc conductivity follow power‐law behavior, yielding values for the critical exponents, which are in good agreement with the theoretical ones. Indications for tunneling effect in the charge carriers transport through the composites are presented. The temperature dependence of dc conductivity gives evidence for the presence of positive temperature coefficient (PTC) effect.     

Simultaneous kinetic and microdielectric studies of some epoxy–amine systems

Three reactive epoxy–amine systems based on diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DDS), 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA), and 4,4′-methylenebis [2,6-diethylaniline] (MDEA), were studied during isothermal curings at 140 and 160°C. The simultaneous kinetic and dielectric studies allow to express conductivity, σ, in terms of conversion, x, and of glass transition temperature, T_g. The conductivity, σ₀, of the initial monomer mixture and, σ_∞ of the fully cured network are measured. It is found that:

• The glass transition temperature, T_g, versus conversion, x, curves follows the equation of Di Benedetto modified by Pascault and Williams
• There exists a linear relation between log σ/log σ₀ and T_g.

So, it is possible to predict both kinetic and dielectric behaviors of these epoxy-amine systems by the knowledge of T_g0, ΔC_p0, and σ₀, respectively, glass transition temperature, heat capacity, and conductivity of initial monomer mixture, T_g∞ and ΔC_p∞, and σ_∞, respectively, glass transition temperature and heat capacity and conductivity of fully cured network. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2911–2921, 1998     

Effect of solvent and carbon black species on the rubber–carbon black interactions studied by pulsed NMR

The spin–spin relaxation time T₂ and the fraction of short T₂ component for composites of natural rubber with carbon black prepared under various conditions have been measured by pulsed NMR. Effect of swelling with a solvent (CCl₄), carbon black species (SAF, HAF, SRF) with different surface areas, and different initial carbon black loadings (35, 50, 70 phr) have been determined. Molecular motion in the rubber phase not in the immediate vicinity of the carbon black surface increases rapidly with increasing solvent concentration, yet it is still slightly restricted compared to rubber with solvent alone. On the other hand, molecular motion in the immobilized layer around carbon black and the fraction rubber in that layer are not affected by the solvent. This indicates strong restriction of molecular motion of polymers at the surface. For estimation of the thickness of the immobilized layer, the necessity of using an appropriate measure of surface area accessible to polymer molecules is pointed out. The degree of immobilization in the layer and the thickness of the layer do not vary appreciably with the nature of carbon black or the initial loading of carbon black.     

Nanosized carbon black as synergist in PP/POE-MA/IFR system for simultaneously improving thermal,electrical and mechanical properties

Nanosized carbon black (CB) was introduced into polypropylene/maleic anhydride-grafted polyolefin elastomer/intumescent flame retardant (PP/POE-MA/IFR) system to investigate the effect of nanofiller as synergist on thermal, electrical and mechanical properties of polymer composites. With 5 mass% CB into PP/POE-MA/IFR system (POFC5), the T_max (corresponded to the temperature at the maximum mass loss rate) under air was increased by 122.4 °C; its limited oxygen index was as high as 31.4%; its vertical burning rating (UL-94) reached V0, and the peak value of heat release rate was decreased to only 19% of neat PP in cone calorimeter testing. Moreover, PP composites exhibited good electrical conductivity with more than 1.6 mass% CB, which is a low loading level to reach the critical percolation concentration. In addition, a good balance on stiffness and toughness of PP composites was achieved; especially, Young’s moduli and impact strength of POFC5 were increased to 1.26 and 2.5 times in comparison with that of neat PP, respectively. These results indicated that CB was an effective synergist in multi-component PP composites to simultaneously improve thermal, electrical and mechanical properties.

     

Surface conductivity measurements for porous carbon electrodes

A method for measuring the surface conductivity in porous electrodes was developed. The knowledge of surface conductivity is very important for simulation and optimization of the capacitive deionization of aqueous solutions, which is a new promising and economical method for water desalination. The surface conductivity (SC) is the tangential (longitudinal) conductivity of a double electric layer (DEL). The SC involves the conductivity induced by the conductivity of the counterions of the surface groups in a highly disperse carbon electrode κ_sg and the surface conductivity κ_sch induced by the electrostatic charge of the inner surface of the electrode pores and the charge-compensating ions of the solution, i.e., the classical DEL. The value of κ_sg depends on the concentration of the surface groups C _sg on carbon, i.e., their exchange capacity, and κ_sch depends on the potential E. SC measurements were performed on activated porous carbon electrodes CH900 and SAIT. This SC measurement procedure is another method for studying DEL.     

The electrical conductivity of poly-p-xylylene + CdS nanocomposites

The dependence of electrical conductivity on the concentration of CdS (c) and temperature (T) over the temperature range 10–300 K was studied for poly-p-xylylene-CdS (PPX + CdS) nanocomposites prepared by solid-state cryochemical synthesis. The results were discussed in terms of the heterogeneous conductivity model including various charge transfer mechanisms in various nanocomposite regions. Under the illumination of a film with c > 11 vol % by a daylight lamp, the conductivity increased, and the σ(T) dependence was metallic in character at low temperatures. The photoconductivity of films at larger concentrations c was caused by the appearance of photoexcited electrons in CdS nanoparticles, the separation of charges at the nanoparticle-matrix boundary, and percolation effects in films. The PPX matrix was shown to actively participate in electrical conductivity; electrons in this matrix jumped between phenyl rings. The experimental dependence of dark conductivity σ(T) at temperatures from 150 to 300 K was analyzed using the Mott hopping conductivity model with variable jump lengths. The main points of the Mott theory of hopping conductivity were discussed. Original Russian Text ? I.A. Misurkin, S.V. Titov, T.S. Zhuravleva, I.V. Klimenko, S.A. Zav’yalov, E.I. Grigor’ev, S.N. Chvalun, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 534–540.     

Direct current conductivity studies on poly(3-methyl thiophene)

The direct current (dc) conductivity of poly(3-methyl thiophene) was measured in the temperature range of 77–300 K. The observed dc conductivity data were analyzed in the light of Mott’s variable range hopping model. Different Mott’s parameters such as characteristic temperature (T ₀), average hopping distance (R), average hopping energy (W), and density of states at the Fermi level (N [E _F ]) were evaluated. By taking the inverse of the coefficient of exponential decay of the localized states involved in the hopping process as 0.5 nm, a realistic value of density of states at the Fermi level (N [E _F ]) was obtained that agrees well with the values reported earlier for other conjugated polymers.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Oxygen-ionic conductivity

Total conductivity of Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with a cryolite structure is studied in the atmosphere with a low water vapor content under variation of the temperature (500 < T < 1000°C) and oxygen activity in the gas phase (10⁻¹⁸ < aO₂ < 0.21). Conductivity is divided into components. It is found that both the oxygen-ionic conductivity and the mobility of oxygen ions increase and the percentage of p-type electronic conductivity decreases at an increase in the concentration of oxygen vacancies. It is shown that compositions with a high strontium oxide content (0 ≤ x < 0.15) and accordingly high concentration of oxygen vacancies correspond to the maximum values of the oxygen-ionic conductivity and low activation energies. The Sr₆Ta₂O₁₁ and Sr_5.92Ta_2.08O_11.12 compositions in a wide range of aO₂ are characterized by negligibly low fractions of electronic conductivity component. For the compositions of x > 0.15, a transition occurs from the electrolytic region to mixed oxygen-hole conductivity character (at aO₂ = 0.21) at an increase in the oxygen activity.     

Electrical conductivity of small ions in polyacrylonitrile in the glass–transition region

The electrical conductivity process in a new class of ion-containing polymers—highly concentrated solid solutions of hydrated perchlorate salts in polyacrylonitrile (PAN)—is described (σ_dc = 10^?7?10^?2Ω^?1cm^?1). A low-ac instrument (70 cps) is used to measure electrical conductivity. We present a cryogenic system in which the temperature dependence of the conductivity is studied (78–340°K). The ionic character of the conductivity process is established. The conductivity both above and below the glass-transition (T_g) point is thermally activated with an activation energy of 0.7–0.9 e V for the glassy state (?_g) and 0.12–0.6 eV for the rubber-like state (?_r). The systems described exhibit a compensation effect between the pre-exponential factor for the conductivity in the glassy state σ_0g and the difference in activation energy ?_g – ?_r      

Ionic conduction in LiScn/Dimethylformamide/Poly(propylene oxide) system II. Mobility of lithium and thiocyanate ions

Ionic conductivity σ, shear viscosity η, and glass transition temperature T_g were measured on systems composed of lithiumthiocyanate LiSCN, N,N-dimethylformamide (DMF), and poly (propylene oxide) (PPO) over wide ranges of LiSCN concentration C and DMF content Φ. Using the dissociation constant of LiSCN reported in Part I, we have determined the concentration n of Li⁺ and SCN^? ions and then the mobility μ from σ. Data indicate that in the binary system of LiSCN/PPO, the σ versus C curve exhibits a maximum ca. C = 0.3 mol/L. In low C range, μ is independent of C but decreases with C in the range of C > 0.3 mol/L. Similar n dependence of μ is seen in the ternary systems containing DMF. The ratio of μ₀/μ (C) is lower than the ratio of viscosity η(C)/η₀ where μ₀ and η₀ indicate the values at infinite dilution of LiSCN. Thus the friction coefficient ?_ion for the translational diffusion of the ions is not proportional to the macroscopic viscosity. Relationship between μ and the monomeric friction ?_p for the segmental motion of the PPO chains is also discussed based on the data of T_g and the Williams-Landel-Ferry equation. ©1995 John Wiley & Sons, Inc.     

Thermal expansion,free volume,and molecular mobility in a carbon black-filled elastomer

The thermal expansion of a butadiene–styrene copolymer filled with carbon blacks differing tenfold in mean particle size (HAF and MT) was investigated. The glass transition was unaffected by MT and was raised only 0.2°C for every 10 parts per hundred by weight of polymer of HAF black added. The coefficient of expansion of the polymer component of the composite in the rubbery region was substantially unaffected by either carbon black, but decreased markedly with increasing black loading in the glassy state. These results suggest that free volume is not altered appreciably by the presence of the filler in the rubbery state, but expands with decreasing temperature below T_g. The latter effect is explained by dilatation due to stresses set up around filler particles, arising from differences in the expansion coefficients of filler and polymer, which are not relieved in the glassy state. The near invariability of T_g and of the rubbery fected by adsorption of polymer segments on the carbon black surface. A conservative rough estimate indicates that restriction of segmental motion is confined to a 30 Å layer around the particles in which T_g is elevated by only 10°C.     

An experimental investigation of the effect of strain on the electrical conductivity of a shape memory polymer

A thermally triggered shape memory polymer (SMP) was prepared by blending electrically conductive carbon black (CB) into the resin prior to curing. The CB filled composite can then be activated through resistive heating. With the aim of using such SMPs in reconfigurable structures and/or actuators, the effect of strain on the conductive nature of the SMP composite was investigated. The study has specifically focused on changes to conductivity in, i) the transverse direction during tensile elongation to assess the impact of the Poisson effect, and ii) in samples deformed in compression. The dynamic response characteristics of the electrically activated SMP were also tested to assess the feasibility of using the composite in tunable vibration damping applications. Findings have shown that the pattern of changes in the transverse conductivity, which is marked by an increase-decrease-increase sequence, resembles that seen in the axial direction. SEM imaging of the samples was performed along the axial and transverse axes of deformation and shows no anisotropy in the CB filler distribution. To demonstrate potential uses of a conductive SMP in the sub-T_g temperature range, a discussion of a vibration damping application has been included.     

Semiconducting poly(dicyanoacetylenes)

Poly(dicyanoacetylene) (PDCA) has been synthesized and characterized. The pristine polymer has EPR g-value, linewidth, unpaired spin concentration, spin—spin relaxation time (T₂), and room temperature dc conductivity (σ_RT) very similar to those of pristine cis-polyacetylene (PA), but shorter spin—lattice relaxation time (T₁). Saturation doping with iodine has little effect on most EPR characteristics of the polymer except for a slight increase in T₁. The doped PDCA has σ_RT value of only 5 X 10^-9 (Ω cm)^-1, indicating either low carrier concentration and/or carrier mobility. Partial cyclization of the nitrile groups by heating at 400°C of PDCA produces l-PDCA with significant increases in unpaired spin concentration and σ_RT but marginal effects on other properties. Saturation doping of l-PDCA with iodine increases σ_RT to 7 × 10^-3 (Ω cm)^-1 without appreciable changes in EPR characteristics. The dopants in both polymers can be removed by evacuation indicating only weak charge transfer interactions. The possible stereoelectronic contribution toward the property differences between the PDCA polymers and PA are discussed.     

Electrical conductivity of carbon black filled ethylene-vinyl acetate copolymer as a function of vinyl acetate content

The electrical conductivity of carbon black filled ethylene-vinyl acetate copolymer was measured as a function of carbon and vinyl acetate (VAc) content. For the composites whose matrices contain less than 32 wt% ofVAc content, a sharp break point of the relation between carbon content and conductivity was observed. The conductivity jumps as much as ten orders of magnitude at the break point. The critical carbon content corresponding to the break point can essentially be predicted by our previous model. This model was derived under certain assumptions, the most important of which is that when the interfacial excess energy introduced by carbon particles into the polymer matrix reaches a universal value,g ^*, the carbon particles begin to coagulate so as to avoid any further increase of energy and to form networks which facilitate electrical conduction. On the other hand, for the composites whose matrices contain more than 32 wt% ofVAc content, a sharp break of the relation between the carbon content and conductivity cannot be observed and conductivity increases continuously with increasingVAc content. In this region ofVAc content, carbon particles were dispersed well in theVAc rich matrices. This is because the presence of polar groups in aVAc component enhances its bonding to conductive fillers. In this case, the interfacial excess energy,g, seems to be the caseg0. Better dispersibility of fillers in this region ofVAc content can be shown from an electron micrograph (TEM).     

Effect of temperature and dopant concentration on the conductivity of samaria-doped ceria electrolyte

The conductivity, σ, of a samaria-doped ceria electrolyte is studied as a function of temperature and dopant concentration, x, which was from 5 to 30 mol%. It is shown that a maximum in σ versus x corresponds to a minimum in activation energy. It is found that the conductivity is completely due to oxygen vacancy conduction. The conductivity increases with increasing samaria doping and reaches a maximum for (CeO₂)_0.8(SmO_1.5)_0.2, which has a conductivity of 5.6×10^–1 S/cm at 800 °C. A curvature at T=T _c, the critical temperature, has been observed in the Arrhenius plot. This phenomenon may be explained by a model which proposed that, below T _c, nucleation of mobile oxygen vacancies into ordered clusters occurs, and, above T _c, all oxygen vacancies appear to be mobile without interaction with dopant cation. In addition, the composition dependences of both the critical temperature and the trapping energy are consistent with that of the activation energy. Electronic Publication     

Electric,dielectric, and dynamic mechanical behavior of carbon black/styrene‐butadiene‐styrene composites

The conductivity of styrene‐butadiene‐styrene block copolymers containing different amounts of extraconductive carbon black (CB) was investigated as a function of the mold temperature. The composites exhibited reduced percolation thresholds (between 1.0 and 2.0 vol % CB). The dynamic mechanical analysis characterization revealed that the glass‐rubber‐transition temperatures of both segments were not affected by the CB addition, although the damping of the polybutadiene phase displayed a progressive drop with an increase in the CB concentration. The normalized curves of tan δ/tan δ_max (where tan δ represents the value of the loss tangent at any measurement temperature and tan δ_max represents the loss tangent peak value at the corresponding temperature T_max) versus T/T_max (where T is the temperature and T_max is the maximum temperature), corresponding to both polystyrene and polybutadiene phases as well as the activation energy related to the glass‐rubber‐transition process, did not present any significant change with the addition of CB. The dielectric analysis revealed the presence of two relaxation peaks in the composite containing 1.5 vol % CB, the magnitude of which was strongly influenced by the frequency, being attributed to interfacial Maxwell‐Wagner‐Sillars relaxations caused by the presence of different interfaces in the composite. The mechanical properties were not affected by the presence of CB at concentrations of up to 2.5 vol %. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2983–2997, 2003