Reduced percolation thresholds of immiscible conductive blends

The DC conductivity of polymer blends composed of poly(ethylene‐co‐vinyl acetate) (EVA) and high density polyethylene (HDPE), where a conductive carbon black (CB) had been preferentially blended into the HDPE, were investigated to establish the percolation characteristics. The blends exhibited reduced percolation thresholds and enhanced conductivities above that of the individually carbon filled HDPE and EVA. The percolation threshold of the EVA/HDPE/CB composites was between 3.6 and 4.2 wt % carbon black, where the volume resistivity changed by 8 orders of magnitude. This threshold is at a significantly lower carbon content than the individually filled HDPE or EVA. At a carbon black loading of 4.8 wt %, the EVA/HDPE/CB composite exhibits a volume resistivity which is approximately 14 and 11 orders of magnitude lower than the HDPE/CB and EVA/CB systems, respectively, at the same level of incorporated carbon black. The dielectric response of the ternary composites, at a temperature of 23°C and frequency of 1 kHz, exhibited an abrupt increase of ca. 252% at a carbon concentration of 4.8 wt %, suggesting that the percolation threshold is somewhat higher than the range predicted from DC conductivity measurements. Percolating composites with increasing levels of carbon black exhibit significantly greater relative permittivity and dielectric loss factors, with the composite containing 6 wt % of carbon black having a value of ϵ′ ≈ 79 and ϵ″ ≈ 14. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1899–1910, 1999     

Carbon black‐filled polyolefins as positive temperature coefficient materials: The effect of in situ grafting during melt compounding

For the production of polymer‐based conducting composites serving as positive temperature coefficient (PTC) materials with lower room‐temperature resistivity and sufficiently high PTC intensity, carbon black has been pretreated with acrylic acid and some initiator and then melt‐mixed with low‐density polyethylene. Because of the in situ formation of covalent bonding at the filler/matrix interface, the distribution status and thermally induced displacement habit of the conductive fillers have changed accordingly. As a result, the electrical performance of the composites can be tailored as desired. The amount of acrylic acid and the treatment sequence of carbon black exert an important influence on the effectiveness of the modification. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 127–134, 2003     

Carbon black/alumina gel composite: Preparation by sol-gel process in the presence of polymer-grafted carbon black and its electric properties

The preparation of a novel carbon black/alumina gel composite by sol-gel reaction of aluminum isopropoxide (AIP) was investigated. When sol-gel reaction of AIP was carried out in the presence of untreated carbon black, a thin film of alumina gel was hardly obtained, because of the presence of aggregated carbon blacks. On the contrary, in the presence of poly(N-vinyl-2-pyrrolidone)-grafted, poly(2-methyl-2-oxazoline)-grafted, and poly(N,N-diethylacrylamide)-grafted carbon black, a deep-black, thin film of alumina gel, in which carbon blacks were uniformly incorporated, was obtained. Carbon black was incorporated into an alumina gel matrix by the hydrogen bond between carbonyl groups of the grafted polymer and the residual hydroxyl groups in the alumina gel. Electric resistance of the polymer-grafted carbon black/alumina gel composite was very sensitive to the vapor of solvents of the grafted polymer chains on carbon black surface: the electric resistance of the composite drastically decreased in humidity and N,N-dimethylformamide vapor, but not in n-hexane vapor. The logarithm of electric resistance of polymer-grafted carbon black/alumina gel composite linearly decreased in proportion to relative humidity. In addition, the electric resistance of poly(N,N-diethylacrylamide)-grafted carbon black/alumina gel composite under humidity decreased drastically at 32°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3591–3597, 1999     

A Links‐Nodes‐Blobs Model for Conductive Polymer Composites

A Links‐Nodes‐Blobs (L‐N‐B) model, based on the fractal and percolation concepts, is used to study the electrically conductive mechanism of conductive filler loaded polymer composites. The change in the conductivity of polymer composites during the mixing process can be explained as the competition between the breakdown of filler aggregates and the diffusion of ingredients of matrix material and impurities onto the surface of the filler. The value of the fractal dimension μ, which is the exponent in the power‐law relationship of the electrical conductivity σ = σ₀·(ϕ – ϕ_c)^μ, is calculated as 1.88. This value is close to the values obtained directly from experiments or from other simulations. The positive temperature coefficient (PTC) behavior in the conductivity of composite material is also explained by this model as the breakdown of the conductive filler network. If the thermo‐expansion induced strain is greater than the apparent on‐set strain ε_onset = mQ + 2 G/2d G·ε_b of the L‐N‐B model, a strong PTC effect would happen.     

Study on synthesis of poly(N‐isopropylacrylamide) grafted carbon black and temperature‐dependence conductivity of grafted carbon black/epoxy composites

Carbon black nanoparticle grafted with poly(N‐isopropylacrylamide) (CB‐g‐PNIPAAm) was synthesized by surface‐initiated atom transfer radical polymerization (SI‐ATRP). The temperature‐responsive behavior of CB‐g‐PNIPAAm was proved by temperature‐variable ¹H NMR. A temperature‐dependent conductive composite was prepared by blending CB‐g‐PNIPAAm with epoxy resin. The relationship between temperature and resistivity of the composite was studied: the composite exhibited a negative temperature coefficient (NTC) phenomenon. Possible mechanism for the NTC phenomenon was suggested. The results showed that resultant composites can be used in intelligent temperature‐switching. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1529–1535, 2008     

Electrical conducting behaviors in polymeric composites with carbonaceous fillers

Several kinds of polymer composites with carbonaceous fillers such as carbon black (CB), vapor‐grown carbon fiber (CF), and carbon nanotube (CNT) are prepared by a gelation/crystallization process or a melt mixing method. The electrical phenomena, changes of electrical conductivities with different filler's type, filler's concentration and temperatures, and the mechanism of electron transport in these carbon‐filled polymer composites are directly influenced by the geometric grain shape and aggregating morphology of the fillers dispersed in the polymer matrix. For the composites of CB and CF, long‐range macroscopic conduction are governed by the percolation phenomenon, the conduction is behaved through the conductive path formed by the conductors' contacting, and the thermal expansion changes the physical dimensions of the entire electrical network and leads to the changes in the electrical phenomenon. Microscopic conduction between conductive elements is influenced by the tunneling barrier or tunneling voltage, which varies with the temperature change, explaining the apparent observation of the temperature dependence of the composites. In comparison with fillers of CB and CF, the CNT performs unique electric properties for their nonspherical geometry and morphology as a three‐dimensional network (high structures), which has been visually proved by SEM photos in our former research, leading to the percolation threshold lower than 1% in the volume fraction and much less temperature dependence in its composites. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1037–1044, 2007     

Formation and structure of pachyman aggregates in dimethyl sulfoxide containing water

The aggregation of pachyman, β-(1 → 3)-D -glucan (M_w = 1.68 × 10⁵) from the Poria cocos mycelia, was investigated using static and dynamic laser light scattering (LLS) in dimethyl sulfoxide (DMSO) containing about 15% water, which leads to large aggregates. Both the time dependence of hydrodynamic radius and the angle dependence of the scattering intensity were used to calculate the fractal dimension (d_f) of the aggregates. The aggregation rate and average size of aggregates increase dramatically with increasing the polymer concentration from 1.7 × 10⁻⁴ g/mL to 8.6 × 10⁻⁴ g/mL, and with the decrease of the solvent quality, that is, water content from 13 to 15%. In the cases, the fractal dimensions change from 1.94 to 2.43 and from 1.92 to 2.54, respectively, suggesting that transforms of aggregation processes: a slow process called reaction-limited cluster aggregation (RLCA) to a fast process called diffusion-limited cluster aggregation (DLCA) in different polymer concentrations and water content. The fractal dimensions above 2 of the fast aggregation is larger than the 1.75 predicted for the ideal DLCA model, suggesting that the aggregation involves a restructuring process through the interchain hydrogen bonding interaction. There are no aggregates of pachyman in DMSO without water, but aggregates formed in the DMSO containing 15% water at 25°C as a compact structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3201–3207, 1999     

Anomalous small‐angle X‐ray scattering characterization of composites based on sulfonated poly(ether ether ketone), zirconium phosphates,and zirconium oxide

The morphology and distribution of zirconium oxide and zirconium phosphates in a matrix of sulfonated poly(ether ether ketone) (SPEEK) were investigated with anomalous small‐angle X‐ray scattering (ASAXS) and electron microscopy. ASAXS revealed that ZrO₂ was distributed in the SPEEK matrix in the form of nanoparticles smaller than 13 Å. A decrease in the conductivity suggested that the sulfonic groups were bound to the zirconium oxo species at the particle surface. Furthermore, two kinds of membranes containing zirconium phosphate were investigated. In one case, the phosphate was directly dispersed in the polymer solution for the casting of the membrane. In the other case, the phosphate was previously treated with n‐propyl ammonium and polybenzimidazole. From ASAXS data, the fractal dimension could be estimated. Mass‐fractal behavior was confirmed for the SPEEK membrane containing previously exfoliated zirconium phosphate, with aggregates of 6.3–165 Å. Surface‐fractal behavior was detected for membranes with untreated phosphates, with aggregates of 6.4–185 Å. The untreated phosphates caused an increase in the permeability, without changing the proton conductivity much. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 567–575, 2004     

Volume‐exclusion effects in polyethylene blends filled with carbon black,graphite, or carbon fiber

Conductive polymer composites possessing a low percolation‐threshold concentration as a result of double percolation of a conductive filler and its host phase in an immiscible polymer blend afford a desirable alternative to conventional composites. In this work, blends of high‐density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) were used to produce ternary composites containing either carbon black (CB), graphite (G), or carbon fiber (CF). Blend composition had a synergistic effect on electrical conductivity, with pronounced conductivity maxima observed at about 70–80 wt % UHMWPE in the CB and G composites. A much broader maximum occurred at about 25 wt % UHMWPE in composites prepared with CF. Optical and electron microscopies were used to ascertain the extent to which the polymers, and hence filler particles, are segregated. Differential scanning calorimetry of the composites confirmed that the constituent polymers are indistinguishable in terms of their thermal signatures and virtually unaffected by the presence of any of the fillers examined here. Dynamic mechanical analysis revealed that CF imparts the greatest stiffness and thermal stability to the composites. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1013–1023, 2002     

Preparation and electromagnetic interference shielding characteristics of novel carbon‐nanotube/siloxane/poly‐(urea urethane) nanocomposites

Poly(urea urethane) (PUU) with a poly(dimethylsiloxane) soft segment was synthesized. Different types of conductive fillers—carbon nanotube (CNT), silver‐coated carbon nanotube (CNT–Ag), and nickel‐coated carbon nanotube (CNT–Ni)—were blended with PUU to form partially conductive polymer composites. The results showed that highly conductive metals could improve the conductivity of CNTs significantly. When the filler contents were 3, 4, and 5 parts per hundred parts of resin (phr), the PUU/CNT composites possessed electromagnetic interference shielding effectiveness (SE) at 8.5, 28.4, and 26.0 dB as the electromagnetic wave frequencies were 12.3, 16.2, and 15.9 GHz, respectively. SE of the composites that contained CNT–Ni and CNT–Ag increased with the filler loading. At the same modified‐CNT loading, the CNT–Ni‐filled composites had a higher SE than those filled with CNT–Ag. Tensile stresses ranged from 5.7 to 15.6 MPa (a 177.3% increase) when the CNT concentration reached 8 phr. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 345–358, 2005     

An approach to one‐dimensional conductive polymer composites

A substantial approach to one‐dimensional (1D) electrically conductive composites was proposed which was based on the thermodynamic analysis of electric‐field‐induced particle alignment in a nonpolar thermoplastic polymer matrix. The process condition window was based on the real‐time exploration of dynamic percolation under different electric fields with carbon black (CB)‐filled polyethylene as a model. The CB content was the main factor of the process condition. Its upper limit was set as the critical percolation concentration at the thermodynamic equilibrium state without an electric field to eliminate the possibility of conductive network formation perpendicular to the electric‐field direction, whereas its lower limit the critical percolation concentration at the thermodynamic equilibrium state under a critical electric field (E*). A composite with CB content in this window, isothermally treated in an electric field not less than E*, showed conductivity in the electric‐field direction about 10⁵ times larger than that in the perpendicular direction. A 1D cluster structure in the direction of the electric filed was confirmed with scanning electron microscopy morphology observations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 184–189, 2005     

Dynamic mechanical and dielectric relaxation characteristics of microcellular rubber composites

An in‐depth study of the surface characteristics of novel conductive carbon black Ensaco 350G has been carried out using XPS and high‐resolution vacuum FTIR. Both methods showed the existence of oxygen containing surface groups like carboxyls, carbonyls, etc. Dynamic mechanical analysis and dielectric relaxation spectra of conductive carbon black (Ensaco 350G) reinforced microcellular EPDM composites were used to study the relaxation behavior as a function of temperature (?90 to +100°C) and frequency (100–10⁶ Hz). The effect of filler and blowing agent loadings on dynamic mechanical and dielectric relaxation characteristics has been investigated. The effect of filler and blowing agent loadings on glass transition temperature was marginal for all the composites (T_g value was in the range of ?37 to ?32°C), which has been explained on the basis of relaxation dynamics of polymer chains in the vicinity of fillers. The variation in the real and imaginary parts of the complex impedance with frequency has been studied as a function of filler and blowing agent loading. Additionally, an in‐depth study of the surface characteristics of the filler using XPS, high‐resolution vacuum FTIR and Raman spectra is also reported. Copyright © 2008 John Wiley & Sons, Ltd.     

ATRP of butyl acrylates from functionalized carbon black surfaces

The functionalization of carbon black surface with atom transfer radical polymerization (ATRP) initiating sites and subsequent ATRP of n‐butyl acrylate (n‐BA) and t‐butyl acrylate (t‐BA) from the surface of carbon black is reported. The polymerizations were carried out using CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as the primary catalytic system in anisole at 70 °C. The initiator density on carbon black surface was tuned and the effect of initiator density on the polymers grafted on the surface was illustrated. Polymerizations were also performed in the presence of a sacrificial initiator to indirectly monitor the molecular weight evolution of polymers formed in the system. Block copolymerization of t‐BA initiated from poly(n‐BA) grafted carbon black was conducted to achieve water‐dispersible carbon black composites after cleavage of the t‐butyl groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4695–4709, 2005     

Self‐assembled supramolecular microgels: Fractal structure and aggregation mechanism

Multifunctional molecules were designed to produce microgels with specific structures. Both static light scattering and dynamic light scattering were employed to determine the fractal dimension of the microgels. The protein, avidin, was strongly bound to four biotin moieties. Biotin was attached covalently to specifically engineered peptide nucleic acid (PNA) oligomers. Three designed DNA oligomers self‐assembled to produce a trifunctional three‐way junction (TWJ) with single‐stranded ends that were complementary to the PNA sequence. The sizes of the supramolecular aggregates were characterized by dynamic light scattering. The fractal dimension was obtained from the angular dependence of the scattered intensity when the microgels were large enough. When the microgels were formed via cooling from a temperature above the melting point of the PNA–DNA helices, reversible structures with a fractal dimension of approximately 1.86 were formed, which is consistent with a cluster–cluster aggregation mechanism. When the microgels were formed by the slow addition of biotinylated PNA bound to the TWJ to a solution of avidin at room temperature, the observed fractal dimension approached 2.6, which is consistent with a point–cluster aggregation mechanism. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3037–3046, 2003     

Viscoelastic properties of ionic polymer composites reinforced by soy protein isolate

Both linear and nonlinear viscoelastic properties of ionic polymer composites reinforced by soy protein isolate (SPI) were studied. Viscoelastic properties were related to the aggregate structure of fillers. The aggregate structure of SPI is consisted of submicron size of globule protein particles that form an open aggregate structure. SPI and carbon black (CB) aggregates characterized by scanning electron microscope and particle size analyzer indicate that CB aggregates have a smaller primary particle and aggregate size than SPI aggregates, but the SPI composites have a slightly greater elastic modulus in the linear viscoelastic region than the CB composites. The composite containing 3–40 wt % of SPI has a transition in the shear elastic modulus between 6 and 8 vol % filler, indicating a percolation threshold. CB composites also showed a modulus transition at <6 vol %. The change of fractional free volume with filler concentration as estimated from WLF fit of frequency shift factor also supports the existence of a percolation threshold. Nonlinear viscoelastic properties of filler, matrix, and composites suggested that the filler‐immobilized rubber network generated a G′ maximum in the modulus‐strain curves and the SPI formed a stronger filler network than the CB in these composites. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3503–3518, 2005     

Structure and dynamics of carbon black‐filled elastomers

The linear and nonlinear melt viscoelastic properties for a series of carbon black‐filled polymer composites were studied. Complementary tapping‐mode atomic force microscopy (AFM) studies were used to examine the dispersion and structural correlations of the filler particles in these composites. The low‐frequency dependence of the linear viscoelastic moduli gradually changes from liquidlike behavior for the unfilled polymer to pseudosolid character for composites with more than 9 vol % carbon black filler. The plateau modulus, inferred from the linear viscoelastic response, exhibits a somewhat discontinuous change at about 9 vol % filler. On the basis of the linear viscoelastic response, we postulate that the carbon black filler forms a continuous percolated network structure beyond 9 vol % filler, considerably lower than that expected from theoretical calculations for overlapping spheres and ellipsoids. We suggest that the lower threshold for percolation is due to the polymer mediation of the filler structure, resulting from the low functionality of the polymer and, consequently, few strong polymer–filler interactions, allowing for long loops and tails that can either bridge filler particles or entangle with one another. Furthermore, the strain amplitude for the transition from linear behavior to nonlinear behavior of the modulus for the composites with greater than 9 vol % filler is independent of frequency, and this critical strain amplitude decreases with increasing filler concentration. Complementary AFM measurements suggest a well‐dispersed carbon black structure with the nearest neighbor distance showing a discontinuous decrease at about 9 vol % filler, again consistent with the formation of a filler network structure beyond 9 vol % carbon black. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 256–275, 2001     

Individual strings of conducting carbon cones and discs in a polymer matrix: Electric field‐induced alignment and their use as a strain sensor

We demonstrate micromechanical strain sensors with integrated readout based on carbon nanocones and discs (CNCs) which are aligned into a string‐like formation using an alternating electric field and studied by AC impedance spectroscopy and electromechanical methods. The CNC particles are first dispersed into a polymer matrix with a particle fraction of 0.1 vol %. This value is well below the percolation threshold (～ 2 vol %), which suppresses particle aggregation and facilitates transparency allowing the use of an UV‐curable polymer. Alignment was carried out with a 1 kHz, 4 kV/cm electric field and is a consequence of dielectrophoretic effect. It develops in minutes and makes the initially insulating, nonaligned material conductive. This is followed by UV curing of the polymer matrix, which renders a solid state device. The stretching of the aligned strings in the cured polymer leads to a reversible piezoresistive effect, and a gauge factor of about 50 is observed. This is in a sharp contrast to CNC films with particle fraction above percolation threshold (13 vol %), which are conductive but not sensitive to stretching. The strings are Ohmic in nature and moreover show higher DC conductivity (22–500 S/m) compared to identically prepared carbon black strings (1–22 S/m). © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Filler–polymer interactions in both silica and carbon black‐filled styrene–butadiene rubber compounds

Bound rubber in a filled rubber compound is formed by physical adsorption and chemisorption between the rubber and the filler. Styrene–butadiene rubber (SBR) is composed of four components of styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units. Filler–polymer interactions in both silica and carbon black‐filled SBR compounds were studied by analyzing microstructures of the bound rubbers with pyrolysis‐gas chromatography. Differences in the filler–polymer interactions of the styrene, cis‐1,4‐, trans‐1,4‐, and 1,2‐units were investigated. The filler–polymer interactions of the butadiene units were found to be stronger than that of the styrene unit. The interactions of the cis‐1,4‐ and trans‐1,4‐units were stronger with carbon black than with silica, whereas the 1,2‐unit interacted more strongly with silica than with carbon black. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 439–445, 2001     

Thermosensitive graphene nanocomposites formed using pyrene‐terminal polymers made by RAFT polymerization

Thermosensitive graphene‐polymer composites have been prepared by attaching poly(N‐isopropylacrylamide) (PNIPAAm) onto the basal plane of graphene sheets via π‐π stacking. Pyrene‐terminated PNIPAAm was synthesized using reversible addition fragmentation chain transfer (RAFT) polymerization via a pyrene‐functional RAFT agent. Aqueous solutions of the graphene‐polymer composites were stable and thermosensitive. The lower critical solution temperature (LCST) of pyrene‐terminated PNIPAAm was measured to be 33 °C. When the pyrene‐functional polymer was attached to graphene the resultant composites were also thermosensitive in aqueous solutions exhibiting a reversible suspension behavior at 24 °C. Atomic force microscopy (AFM) analysis revealed that the thickness of a graphene‐PNIPAAm (M_n: 10,000 and PDI: 1.1) sheet was ～5.0 nm. The surface coverage of polymer chains on the graphene basal plane was calculated to be 7.2 × 10^?11 mol cm^?2. The graphene‐PNIPAAm composite material was successfully characterized using X‐ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR‐IR) spectroscopy, and thermogravimetric analysis (TGA). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 425–433, 2010     

Complementary effects of multiwalled carbon nanotubes and conductive carbon black on polyamide 6

This article introduces a newly innovative idea for preparation of superconductive ternary polymeric composites of polyamide 6 (PA6), conductive carbon black (CCB), and multiwalled carbon nanotubes (MWCNTs) with different weight ratios by a melt‐mixing technique. The complementary effects of CCB and MWCNTs at different compositions on rheological, physical, morphological, thermal, and dynamic mechanical and electrical properties of the ternary composites have been examined systematically. We have used a novel formulation to produce high‐weight fraction ternary polymer composites that show extremely higher conductivity when compared with their corresponding binary polymer composites at the same carbon loading. For example, with an addition of 10 wt % MWCNTs into the CCB/PA6 composite preloaded with 10 wt % CCB, the electrical conductivity of these ternary composites was about 5 S/m, which was 10 times that of the CCB/PA6 binary composite (0.5 S/m) and 125 times that of the MWCNT/PA6 binary composite (0.04 S/m) at 20 wt % carbon loading. The incorporation of the MWCNTs effectively enhanced the thermal stability and crystallization of the PA6 matrix in the CCB/PA6 composites through heterogeneous nucleation. The MWCNTs appeared to significantly affect the mechanical and rheological properties of the PA6 in the CCB/PA6 composites, a way notably dependent on the MWCNT contents. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1203–1212, 2010