Novel phthalocyanine crystals as a conductive filler in crosslinked epoxy materials: Fractal particle networks and low percolation thresholds

Novel nanosized crystals of aquocyanophthalocyaninatocobalt (III) (Phthalcon 11) were used as a conductive filler in crosslinked epoxy materials. The crosslinked composite materials had a very low percolation threshold (φ_c ≈ 0.9 vol %). The relationship between the volume conductivity and the filler fraction follows the scaling law of the percolation theory and suggests that the conducting particle networks were formed by random percolation of primary aggregates. The occurrence of the low φ_c can be explained by the presence of a fractal Phthalcon 11 particle network formed from fractal aggregates during crosslinking. The position of the percolation threshold and the volume conductivity of these crosslinked materials were found to depend heavily on the processing conditions applied. These dependencies are explained in terms of specific particle–matrix interactions and the particle–particle interactions and by taking into account different mechanisms of particle network formation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 33–47, 2006     

Permanent antistatic phthalocyanine/epoxy nanocomposites - Influence of crosslinking agent, solvent and processing temperature

Cross-linked epoxy matrices containing small amounts of semi-conductive phthalocyanine (Phthalcon) nanoparticles were prepared using different crosslinking agents and processing temperatures. A starting mixture containing an optimum dispersion of these nanoparticles and with an almost equal and large Hamaker constant was always used. Nevertheless large differences in the relation between the volume conductivity σ_v and the particle concentration φ were found and this relation appeared to be sensitive to small changes in processing temperature and the application of a post-cure. Also the amine crosslinker chosen and the initial amount of solvent (catalyst) in the starting dispersion had a major effect. It was shown that these changes influence strongly the formation of and the final conductive fractal particle network morphology through the polymer matrix. During processing a local relaxation of the initially formed fractal particle network into another fractal particle network was often observed, which introduced or enlarged the amount of isolating material between the particles of the conductive network and changed the fractality and structure of the conductive backbone of the particle network. This local relaxation lowered the σ_v at each phthalcon concentration and enlarged φ_c by several orders of magnitude. The occurrence of local relaxation is dependent on the rate of viscosity change during the crosslinking of the polymer matrix components, the way the fractal conductive particle network is formed during processing (universal or non-universal) and the amount of solvent present. Local relaxation may even occur after the gel point of the polymer matrix. A severe post-cure may be needed to stop this local relaxation. To our knowledge local relaxation of a (fractal) nanoparticle network in a polymer matrix during processing is a new phenomenon, not reported before for polymer composites containing (conductive) nanoparticles.     

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     

Correlation between rheological,electrical, and microstructure characteristics in polyethylene/aluminum nanocomposites

Polyethylene (PE)/aluminum (Al) nanocomposites with various filler contents were prepared by a solution compounding method. We investigated the influence of the surface modification of Al nanoparticles on the microstructure and physical properties of the nanocomposites. The silane coupling agent octyl‐trimethoxysilane was shown to significantly increase interfacial compatibility between the polymer phase and Al nanoparticles. Rheological percolation threshold values were determined by analyzing the improvement in storage modulus at low frequencies depending on the Al loadings. Lower percolation threshold values were obtained for the composites prepared with the original nanoparticles than those prepared with the silane‐modified Al nanoparticles. A strong correlation between the time and concentration dependences of dc conductivity and rheological properties was observed in the different nanocomposite systems. The rheological threshold of the composites is smaller than the percolation threshold of electrical conductivity for both of the nanocomposite systems. The difference in percolation threshold is understood in terms of the smaller particle–particle distance required for electrical conduction when compared with that required to impede polymer mobility. It was directly shown by SEM characterization that the nanoparticle surface modification yielded better filler dispersion, as is consistent with our rheological and electrical analysis. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2143–2154, 2008     

Conductivity of crosslinked low surface energy epoxy coatings

A new and straightforward method has been studied to prepare crosslinked low surface energy semiconductive epoxy coatings. The low surface energy is obtained by adding a small amount of partially fluorinated bifunctional primary amine Jeffamine D230 crosslinker and the conductivity is achieved by adding a small amount of semiconductive nanosized Cobalt(III) phthalocyanine particles. The use of partially fluorinated crosslinker strongly influences the conductivity, the conductive particle network structure, and the network distribution in the coatings. Compared to coatings that are free of fluorine, variations in fractal dimension, percolation threshold, particle‐containing layer thickness, and conductivity level are observed as the amount of fluorinated species is varied. These differences can be explained by (local) differences in effective Hamaker constant, viscosity, curing rate, evaporation of the solvent, and presence or absence of polymer matrix between the particles in the network. Our results suggest that other crosslinked semiconductive low surface energy epoxy coatings can be realized in a similar manner, but careful optimization of processing conditions is required to obtain the desired conductivity levels at low filler concentration. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Chem 47: 366–380, 2009     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Synthesis and characterization of novel PDMS nanocomposites using POSS derivatives as cross‐linking filler

We report the synthesis and characterization of novel elastomeric nanocomposites containing polyhedral oligomeric silsesquioxanes (POSS) as both the cross‐linker and filler within a polydimethylsiloxane (PDMS) polymer matrix. These polymer composites were prepared through the reaction of octasilane‐POSS (OS‐POSS) with vinyl‐terminated PDMS chains using hydrosilylation chemistry. In addition, larger super‐POSS cross‐linkers, consisting of two pendant hepta(isobutyl)POSS molecules attached to a central octasilane‐POSS core, were also used in the fabrication of the PDMS composites. The chemical incorporation of these POSS cross‐linkers into the PDMS network was verified by solid‐state ¹H magic angle spinning NMR. Based on dynamic mechanical analysis, the PDMS nanocomposites prepared with the octafunctional OS‐POSS cross‐linker exhibited enhanced mechanical properties relative to polymer systems prepared with the tetrafunctional TDSS cross‐linker at equivalent loading levels. The observed improvements in mechanical properties can be attributed to the increased dimensionality of the POSS cross‐linker. The PDMS elastomers synthesized from the larger super‐POSS molecule showed improved mechanical properties relative to both the TDSS and OS‐POSS composites due to the increased volume‐fraction of POSS filler in the polymer matrix. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2589–2596, 2009     

Tuning the mechanical properties in model nanocomposites: Influence of the polymer‐filler interfacial interactions

This article presents a study of the polymer‐filler interfacial effects on filler dispersion and mechanical reinforcement in Polystyrene (PS)/silica nanocomposites by direct comparison of two model systems: ungrafted and PS‐grafted silica dispersed in PS matrix. The structure of nanoparticles has been investigated by combining small angle neutron scattering measurements and transmission electronic microscopic images. The mechanical properties were studied over a wide range of deformation by plate–plate rheology and uni‐axial stretching. At low silica volume fraction, the particles arrange, for both systems, in small finite size nonconnected aggregates and the materials exhibit a solid‐like behavior independent of the local polymer‐fillers interactions suggesting that reinforcement is dominated by additional long range effects. At high silica volume fraction, a continuous connected network is created leading to a fast increase of reinforcement whose amplitude is then directly dependent on the strength of the local particle–particle interactions and lower with grafting likely due to deformation of grafted polymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Electrically conductive nanocomposites with segregated structure based on poly(vinylidene fluoride-<Emphasis Type="Italic">co</Emphasis>-tetrafluoroethylene) and reduced graphene oxide

An approach was described to obtaining polymer composites with segregated structure that have high electrical conductivity at low concentrations of an electrically conductive filler. According to this approach, thin layers of electrically nonconductive nanodispersed graphene oxide are applied to the surface of polymer particles and conduction is produced by heat and chemical treatments. Hot pressing of the modified powder leads to combination of layers of the graphene-like filler to form a single electrically conductive network. For the first time, reduction of graphene oxide on the surface of polymer particles with hydrazine vapor at room temperature was performed. Comparison of the electrical conductivities of composites obtained by the thermal and chemical methods of graphene oxide reduction showed that the chemical reduction method gives composites with higher conductivities than the thermal method does. The maximum conductivity (0.5 S/m) was reached in a composite containing 0.6% chemically reduced graphene oxide.     

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     

Conductive ATO‐acrylate nanocomposite hybrid coatings: Experimental results and modeling

The electrical volume conductivity σ of antimony‐doped tin oxide (ATO)–acrylate nanocomposite hybrid coatings was investigated. The relation between σ and the volume filler fraction p was analyzed for the ATO‐acrylate coatings containing ATO nanoparticles grafted with different amounts of 3‐methacryloxy‐propyl‐trimethoxy‐silane coupling agent. Percolation thresholds were observed at very low filler fractions (1–2 vol %) for the coatings containing ATO nanoparticles with a low amount of surface grafting. A modified effective medium approximation (EMA) model was introduced. This model takes into consideration different distances between adjacent semiconductive particles in the particle network. The model elucidates how self‐arrangement of the particles influences the location of the percolation threshold in the log σ ? p plot. The modified EMA model can successfully explain the multiple transition behavior and the variable percolation thresholds found for the ATO‐acrylate nanocomposite hybrid coatings. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2147–2160, 2007     

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Dependence of silica particle sizes on network chain lengths,silica contents,and catalyst concentrations in in situ‐reinforced polysiloxane elastomers

Poly(dimethylsiloxane) networks were prepared by tetrafunctionally end‐linking hydroxyl‐terminated chains with tetraethoxysilane (TEOS). Molecular composites were then prepared by in situ sol–gel reactions on additional TEOS swelled into the networks, resulting in the formation of reinforcing silica fillers within the host elastomers. The amount of filler generated generally increased linearly with an increase in the TEOS swelling ratio, as expected. The silica particles formed were examined by small‐angle X‐ray scattering. Of particular interest were the relationships between particle size and molecular weight M_c of the network chains (mesh sizes), amount of filler introduced, and catalyst concentration. Particle sizes were smallest for the smallest values of M_c, possibly demonstrating constraining effects from the very short network chains. At fixed M_c and filler concentrations, higher catalyst concentrations gave larger particles. Increase in filler concentration generally had little effect on particle size at low and high loadings, but markedly increased sizes at intermediate levels (10–20 wt %), presumably caused by coalescence of the scattering entities into considerably larger aggregates. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1421–1427, 1999     

Synthesis and characterization of magnetoelectric polymer nanocomposites

Magnetoelectric polymer nanocomposite structures are synthesized using conducting polyaniline and nanosized BFO particles through in situ sol–gel polymerization. The effect of nanosized BFO in polyaniline matrix is studied. The SEM, XRD, VSM, FTIR, and UV–Vis studies were made to understand the morphology, crystalline structure, magnetic, and optical properties of PANI/BFO composites with various concentrations of nanofiller. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2418–2422, 2008     

Design and Preparation of Highly Filled Water‐Borne Polymer–Gibbsite Nanocomposites

Highly filled, high solids content, water‐borne polymer–Gibbsite nanocomposites are prepared with Gibbsite contents as high as 35 wt%. The polymer–Gibbsite nanocomposites are synthesised via conventional starved feed emulsion polymerization using negatively charged butyl acrylate‐co‐acrylic acid oligomers, which functioned as electrosteric stabilizers for the initial platelets and the subsequently formed latex particles. A simple mathematical model describing the amount of cooligomer required for the colloidal stability of the initial Gibbsite platelets and the subsequently formed particles was derived. This model was used to determine the reaction conditions required for obtaining colloidally stable nanocomposites with a targeted filler content. Cryogenic transmission electron microscopy characterization of the resulting nanocomposites with filler contents up to 20 wt% revealed fully encapsulated Gibbsite platelets and a mixed morphology of “muffin‐like” and encapsulated particles was obtained at higher filler contents.     

Influence of polymer particle size on the percolation threshold of electrically conductive latex‐based composites

Monodispersed copolymer emulsions, each with a different polymer particle size, were used to investigate the effect of particle size on the electrical and thermomechanical properties of carbon black (CB)‐filled segregated network composites. These emulsions were synthesized with equal moles of methyl methacrylate and butyl acrylate, with latex particle size ranging from 83 to 771 nm. The electrical percolation threshold was found to decrease from 2.7 to 1.1 vol % CB as the latex particle size was increased. Microstructural images reveal diminished latex coalescence, and improved CB segregation, with increasing latex particle size. In general, coalescence is shown to increase for all systems with increasing CB concentration. Furthermore, all systems exhibited a similar maximum electrical conductivity plateau of 0.7 S cm^?1, albeit at lower concentration for larger latex particle size. This ability to tailor percolation threshold with latex particle size provides an important tool for manipulating electrical and mechanical properties of polymer nanocomposites. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1547–1554, 2011     

Mean‐field theoretical analysis of brush‐coated nanoparticle dispersion in polymer matrices

The equilibrium dispersion of nanoparticles with grafted polymer chains into polymer matrices, of the same chemical structure as the brush, is studied through the device of mean‐field theory. Our results show that the disperion of brush‐coated nanoparticles into a matrix polymer is improved with (i) decreasing particle radius and (ii) increasing brush chain length. Both of these aspects can be understood based on the fact that, unlike the case of planar surfaces, homopolymer chains end‐grafted to spherical nanoparticle surfaces tangentially spread away from the surface thus alleviating the packing frustration that is created by the relatively high grafting densities. This permits significant brush/matrix overlap, even at high grafting densities, a regime that has only recently become experimentally available due to advances in polymer synthesis (i.e., the “grafting‐to” methods). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 351–358, 2008     

Highly Conductive Carbon Nanotube/Polymer Nanocomposites Achievable?

Carbon nanotubes (NT) have attracted growing interest in recent years as a conducting filler in the development of conductive polymer composites. However, most of experimental results show that the conductivity of NT/polymer composites is significantly lower than expected. Can NTs be an effective conductive filler for improving the electrical conductivity of polymers? In order to answer this question, a continuum model was constructed by introducing effective tunneling conduction in a non‐universal network for the prediction of electrical conductivity of NT/polymer composites. Based on this model, the effect of the microstructure of NT/polymer composites on conductivity was assessed particularly for NT/polyethylene, NT/polyimide, and NT/poly(vinyl alcohol) composites. NT contact resistance and tunneling resistance have significant influences on the conductivity. The effects of the potential barrier of polymer and the tortousity of single‐walled NTs on the conductivity were also analyzed. NTs cannot be considered as a valuable conductive filler for the development of highly conductive polymer composites unless the contact and tunneling resistances are reduced significantly.

     

Time dependence of piezoresistance for the conductor‐filled polymer composites

The piezoresistance and its time dependence of conductor‐filled polymer composites have been investigated. To reveal the origin of the time dependence of piezoresistance, the creep of the polymer matrix is also studied. Based on the interparticle separation change under the applied stress, a model has been developed to predict the piezoresistance and its time dependence. By analyzing this model, the influences of applied stress, filler particle diameter, filler volume fraction, matrix compressive modulus, potential barrier height, and the matrix creep behavior on the piezoresistance and its time dependence are interpreted quantitatively. These predicted results are compared with the experimental data obtained on the polymer composites filled with conductor fillers, and good agreements were obtained. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2739–2749, 2000