Facile synthesis of exfoliated and highly conductive poly(arylene disulfide)/graphite nanocomposites

Exfoliated graphite has been synthesized by first synthesizing H₂SO₄ intercalated compound in a H₂O₂‐H₂SO₄ mixture, followed by exfoliation under microwave irradiation. Poly(arylene disulfide)/graphite nanocomposites were then fabricated by absorbing cyclic(arylene disulfide) oligomers into the pores of exfoliated graphite. Subsequently, the nanocomposite precursor was subjected to heat treatment to carry out the in situ ring‐opening polymerization of the oligomers via free radical mechanism. The as‐prepared nanocomposite exhibited a exfoliated nanostructure as evidenced by transmission electron microscopy (TEM) observation. The nanocomposite with a very small amount of graphite, 5 wt%, possesses a highly electrical conductivity of 4 S/cm, therefore, many applications can be found as conductive materials. Copyright © 2004 John Wiley & Sons, Ltd.     

Polylactide/exfoliated graphite nanocomposites with enhanced thermal stability,mechanical modulus,and electrical conductivity

We have prepared a series of polylactide/exfoliated graphite (PLA/EG) nanocomposites by melt‐compounding and investigated their morphology, structures, thermal stability, mechanical, and electrical properties. For PLA/EG nanocomposites, EG was prepared by the acid treatment and following rapid thermal expansion of micron‐sized crystalline natural graphite (NG), and it was characterized to be composed of disordered graphite nanoplatelets. It was revealed that graphite nanoplatelets of PLA/EG nanocomposites were dispersed homogeneously in the PLA matrix without forming the crystalline aggregates, unlike PLA/NG composites. Thermal degradation temperatures of PLA/EG nanocomposites increased substantially with the increment of EG content up to ～3 wt %, whereas those of PLA/NG composites remained constant regardless of the NG content. For instance, thermal degradation temperature of PLA/EG nanocomposite with only 0.5 wt % EG was improved by ～10 K over PLA homopolymer. Young's moduli of PLA/EG nanocomposites increased noticeably with the increment of EG content up to ～3 wt %, compared with PLA/NG composites. The percolation threshold for electrical conduction of PLA/EG nanocomposites was found to be at 3–5 wt % EG, which is far lower graphite content than that (10–15 wt % NG) of PLA/NG composites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 850–858, 2010     

Thermal conductive PS/graphite composites

Polystyrene (PS) was compounded with graphite that possesses high thermal conductivity and layer structures, and the PS/graphite thermal conductive nano‐composites were prepared. Thermal conductivity of PS improved remarkably in the presence of the graphite, and a much higher thermal conductivity of 1.95 W/m K can be achieved for the composite with 34 vol% of colloidal graphite. The Maxwell‐Eucken model and the Agari model were used to evaluate the thermal conductivity of the composites. For the purpose of improving the interfacial compatibility of PS/graphite, realizing the exfoliation and nano‐dispersion of graphite in the PS matrix, three intercalation methods, including rolling intercalation, solvent intercalation, and pan milling intercalation, were applied to prepare the composites, and the morphologies, thermal conductivities, and mechanical properties of the composites were investigated. It should be noted that the one prepared by pan milling intercalation not only had excellent thermal conductivity but also much higher mechanical properties, resulting from a high degree of layer exfoliation of the graphite, the formation of the chain structure agglomerates of the graphite, and the creation of more conductive paths under the strong shear stress of pan milling. Copyright © 2008 John Wiley & Sons, Ltd.     

The role of interfacial compatibilization upon the microstructure and electrical conductivity threshold in polypropylene/expanded graphite nanocomposites

Attempts have been made to evaluate the effect of interface and degree of interfacial interaction upon electrical conductivity threshold in polypropylene/expanded graphite (PP/EG) nanocomposites, and dispersion state of graphite nanosheets. For this purpose, maleic anhydride grafted polypropylene (PPgMA) and maleic anhydride grafted EPDM (EPDMgMA) were used as compatibilizer. Nanocomposite samples containing 1–5 vol% of EG were prepared by melt mixing method using laboratory scale internal mixer. Characterization was carried out by using X‐ray diffraction (XRD), differential scanning calorimeter (DSC), thermo‐gravimetric analysis (TGA), scanning electron microscope (SEM), transmission electron microscope (TEM), and rheo‐mechanical spectroscopy (RMS). The conductivity measurements were carried out by using four point probe method according to ASTM D991. Results showed that the conductivity threshold is controlled by the extent of interfacial interaction between PP and EG. So, better conductivity was obtained using PPgMA as compatibilizer which causes higher level of interaction between PP and EG, and therefore better dispersion of the EG nanolayers in the polymer matrix. On the other hand, high levels of compatibilizers, especially EPDMgMA, caused formation of separated aggregates of EG shelled with the compatibilizer, which results in the reduction of conductivity of the nanocomposites. This finding has been verified by SEM, RMS, and conductivity measurements. Effects of EG nanolayers on crystalline structure and thermal decomposition temperature of the nanocomposites have also been investigated by DSC and TGA, respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermal conductivity anisotropy of expanded graphite/ chlorate salt composites for thermal energy storage

ABSTRACT

Expanded graphite (EG)/LiCl-NaCl phase change composites are prepared by aqueous solution method with different EG amount and forming pressure to enhance heat conduction for high-temperature latent heat thermal energy storage application. Their microstructure and thermal conductivity are characterized. Results indicate that the composites are uniform and the LiCl-NaCl eutectic is well dispersed in the graphite flakes. Thermal conductivity of the LiCl-NaCl can increase to as much as 40.51 W/(m·K), which is 46 times higher than that of pure eutectic salt. With forming pressure, the thermal conductivities of the samples show anisotropy because of a flattened irregular honeycomb network of graphite. Within certain limits, the greater the forming pressure is, the more pronounced the anisotropy performs. In addition, the formulas to calculate the thermal conductivity in the axial direction and the radial direction are given based on the average rotation angle φ of EG basal plane, and experimental data show that the formula in the radial direction is especially useful for calculating the thermal conductivity.     

Thermal conductivity of poly(ether ether ketone) and its short-fiber composites

The effects of crystallinity, orientation, and short-fiber filler on the thermal diffusivity D and thermal conductivity K of poly (ether ether ketone) (PEEK) have been studied. Below the glass transition, D increases by less than 10% as the crystallinity increases from 0 to 0.3. For amorphous PEEK, there is an abrupt drop in D at the glass transition (T_g ? 420 K). The drop is less prominent for the 30% crystalline sample and occurs at 20 K higher. At a draw ratio of 2.5, the axial thermal conductivity is 2.3 times higher while the transverse thermal conductivity is 30% lower than that of the unoriented material. For an injection-molded bar of carbon fiber reinforced PEEK, the variation of D with position along the width or thickness direction is found to correlate well with the fiber orientation. By regarding the injection-molded bar as a multidirectional laminate comprising a large number of unidirectional plies, the thermal conductivities along the longitudinal and transverse direction are calculated and found to agree closely with the experimental data. © 1994 John Wiley & Sons, Inc.     

Thermal conductivity of gel-spun polyethylene fibers

The thermal conductivities of unidirectional gel-spun polyethylene fiber-reinforced composites have been measured parallel (K∥?) and perpendicular (K⊥) to the fiber axis from 15 to 300K. The axial thermal conductivity K∥? varies linearly with volume fraction v_f of fiber, while the transverse thermal conductivity K⊥ follows the Halpin-Tsai equation. Extrapolation to v_f = 1 gives the thermal conductivity of gel-spun polyethylene fiber which, at 300K, has values of 380 and 3.3 mW cm^?1K^?1 along and perpendicular to the fiber axis, respectively. The axial thermal conductivity is exceptionally high for polymers, and is more than twice the thermal conductivity of stainless steel. This high value arises from the presence of a large fraction of long (> 50 nm) extended chain crystals in the fiber. Further improvement of up to a factor of 10 is possible if the length and volume fraction of the extended chain crystals can be increased. © 1993 John Wiley & Sons, Inc.     

Electrical conductivity of poly(ethylene terephthalate)/expanded graphite nanocomposites prepared by in situ polymerization

Nanocomposites based on poly(ethylene terephthalate) (PET) and expanded graphite (EG) have been prepared by in situ polymerization. Morphology of the nanocomposites has been examined by electronic microscopy. The relationship between the preparation method, morphology, and electrical conductivity was studied. Electronic microscopy images reveal that the nanocomposites exhibit well dispersed graphene platelets. The incorporation of EG to the PET results in a sharp insulator‐to‐conductor transition with a percolation threshold (?_c) as low as 0.05 wt %. An electrical conductivity of 10^?3 S/cm was achieved for 0.4 wt % of EG. The low percolation threshold and relatively high electrical conductivity are attributed to the high aspect ratio, large surface area, and uniform dispersion of the EG sheets in PET matrix. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Improved electrooxidation of phenol at exfoliated graphite electrodes

In the presented paper, we report on electrochemical oxidation of phenol occurring at exfoliated graphite (EG) in alkaline solution. The mechanism of the electrocatalytic reaction of phenol oxidation was modified on adding methanol to the phenol-containing electrolyte. Using the voltammetry method, the influence of methanol additive on cyclic behavior of EG electrode was examined. A particular attention has been paid to the first two cycles when an abrupt decrease in electrocatalytic activity of various electrode materials has been observed. The results obtained showed that in the presence of methanol EG, electrode preserves its electrocatalytic activity for a longer time of phenol oxidation. In the absence of methanol in a phenol/KOH electrolyte, the charge of phenol oxidation peaks decreases sharply on cycling, whereas in the presence of methanol, the observed drop is considerably inhibited. The anodic charge attained for the 15th cycle of phenol oxidation in methanol-admixed electrolyte is the same as that for the third cycle recorded in methanol-free electrolyte. The thermogravimetric analysis (TG), Fourier-transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) data showed that an improved electrocatalytic activity of EG can be accounted for by new chemical composition of oligomer film built on the EG surface with the participation of methanol and/or the products of its oxidation.Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005.     

A new process of fabricating electrically conducting nylon 6/graphite nanocomposites via intercalation polymerization

A new process was developed to fabricate electrically conducting nylon 6/graphite nanocomposites via intercalation polymerization of ϵ‐caprolactam in the presence of expanded graphite. The transition from an electrical insulator to an electrical semiconductor for nylon 6 occurred when the graphite volume content was 0.75, which was much lower than that of conventional conducting polymer composites. The electrical conductivity reached 10⁻⁴ S/cm when the graphite content was 2.0 vol %. The TEM microphotographs suggested that the low percolation threshold and the great improvement of electrical conductivity could be attributed to the high aspect ratio (width‐to‐thickness), the high expansion ratio in c axis of the graphite sheets and the homogeneous dispersion of the nanoscale graphite particles in the nylon 6 matrix. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1626–1633, 2000     

Fabrication of exfoliated graphite nanoplatelets-reinforced aluminum composites and evaluating their mechanical properties and corrosion behavior

Aluminum composites with different amounts of exfoliated graphite nanoplatelets particles were fabricated by powder metallurgy method. The mixture powders were consolidated at 520 MPa for 5 min and followed by pressureless sintering at 600 °C for 6 h. The mechanical properties of composites were evaluated by compression and hardness tests. The corrosion behavior in 3.5% NaCl solution was investigated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) measurements. The mechanical testing results showed that the maximum strength and Vickers hardness increase as a function of exfoliated graphite nanoplatelets content. Corrosion data indicated that the presence of exfoliated graphite nanoplatelets and the increase of its concentration raise the corrosion rate and reduce the polarization resistance of Al. SEM/EDX investigations revealed that the presence of exfoliated graphite nanoplatelets activates the corrosion of Al due to the occurrence of galvanic corrosion. SEM/EDX investigations confirmed the electrochemical measurements showing that the increase of exfoliated graphite nanoplatelets content increases the corrosion of Al.     

Thermal conductivity measurements using the flash method

Thermal diffusivity is the speed with which heat propagates through a material. It has a multitude of direct applications, such as determining heat transfer through brake pads at the moment of contact, etc., but more often it is used to derive thermal conductivity from the fundamental relationship tying it with specific heat capacity and density. Using a new multi-sample configuration system, and testing a reference sample adjacent to the unknown, specific heat capacity can be obtained parallel with thermal diffusivity. Thus, a single test yields thermal diffusivity and thermal conductivity with prior knowledge of density. The method is fast and produces results with high accuracy and very good repeatability. The sample size, 12 to 30 mm diameter and 2 to 5 mm thickness, is easy to handle and is well suited for a broad range of materials, even for composites, often a problem for other methods. Typical data on two polymers, Pyrex glass and Pyroceram 9606 are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal conductivity of high strength polyethylene fiber in low temperature

High strength polyethylene fiber (Toyobo, Dyneema® fiber, hereinafter abbreviated to DF) used as reinforcement of fiber‐reinforced plastics for cryogenic use has a high thermal conductivity. To understand the thermal conductivity of DF, the relation between fiber structure and thermal conductivity of several kinds of polyethylene fibers having different modulus from 15 to 134 GPa (hereinafter abbreviated to DFs) was investigated. The mechanical series‐parallel model composed of crystal and amorphous was applied to DFs for thermal conductivity. This mechanical model was obtained by crystallinity and crystal orientation angle measured by solid state NMR and X‐ray. Thermal conductivity of DF in fiber direction was dominated by that of the continuous crystal region. The thermal conductivity of the continuous crystal part estimated by the mechanical model increases from 16 to 900 mw/cmK by the increasing temperature from 10 to 150K, and thermal diffusivity of the continuous crystal part was estimated to about 100 mm²/s, which is almost temperature independent. The phonon mean free path of the continuous crystal region of DF obtained by thermal diffusivity is almost temperature independent and its value about 200 Å. With the aforementioned, the mechanical series‐parallel model composed of crystal and amorphous regions could be applied to DFs for thermal conductivity. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1495–1503, 2005     

Thermal conductivity epoxy resin composites filled with boron nitride

Boron nitride (BN) micro particles modified by silane coupling agent, γ‐aminopropyl triethoxy silane (KH550), are employed to prepare BN/epoxy resin (EP) thermal conductivity composites. The thermal conductivity coefficient of the composites with 60% mass fraction of modified BN is 1.052 W/mK, five times higher than that of native EP (0.202 W/mK). The mechanical properties of the composites are optimal with 10 wt% BN. The thermal decomposition temperature, dielectric constant, and dielectric loss increase with the addition of BN. For a given BN loading, the surface modification of BN by KH550 exhibits a positive effect on the thermal conductivity and mechanical properties of the BN/EP composites. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity K_par; and the drastic drop in the axial expansivity α_∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide-angle x-ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, K_par; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, α_par; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large-scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.     

Volume dependence of thermal conductivity and isothermal bulk modulus up to 1 GPa for poly(vinyl acetate)

The thermal conductivity λ and heat capacity per unit volume of poly(vinyl acetate) (260 kg mol⁻¹ in weight average molecular weight) have been measured in the temperature range 150–450 K at pressures up to 1 GPa using the transient hot-wire method, which yielded λ = 0.19 W m⁻¹ K⁻¹ at atmospheric pressure and room temperature. The bulk modulus K has been measured in the temperature range 150–353 K up to 1 GPa. At atmospheric pressure and room temperature, K = 4.0 GPa and (∂K/∂p)_T = 8.3. The volume data were used to calculate the volume dependence of λ, $g = - \left( {\frac{{\partial \lambda /\lambda }}{{\partial V/V}}} \right)_T .$ The values for g of the liquid and glassy states were 3.0 and 2.7, respectively, and g of the latter was almost independent of volume and temperature. Theoretical models can predict the value for g of the glassy state to within 25%. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1451–1463, 1998     

Study of Thermodestruction Processes in Polypropylene/exfoliated Graphite Composites

In the present paper the effect of the thermoexfoliated graphite (EG) content (0–0.310 parts by volume), constituent mixing conditions (a paddle mixer or a disk-and-screw extruder), annealing and aging on thermodestruction processes of polypropylene (PP)/exfoliated graphite composite materials (CM) by means differential thermal analysis and the thermogravimetric method have been investigated. The studies have shown that thermodestruction processes in CM based on PP and EG ran in air for expense of PP thermodestruction. The characteristics of thermodestruction in CM essentially depend on a regime of making and concentration of components. It is established that processes leading to the formation of less perfect crystalline structures or causing amorphization of PP (including formation of continuous space structures from EG particles, cluster traps) advances thermodestruction. The increase in crystallinity extent of PP favors to the decrease in thermodestruction processes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal behaviors of poly(ethylene terephthalate)/SiO2 nanocomposites prepared by cryomilling

The thermal behaviors of poly (ethylene terephthalate) (PET)/SiO₂ nanocomposites prepared by cryomilling were studied, by comparing with the cryomilled PET and the raw PET. Cryomilling induced amorphization of crystalline PET matrix and the decrease of PET molecular weight. Cryomilled PET/SiO₂ nanocomposites have two abnormal phenomena in the DSC 1st heating thermogram due to the stored energy induced by cryomilling. During DSC cooling process, the dispersed nanometer SiO₂ particles play a role of nucleating agent in PET matrix, and the hot crystallization ability of PET/SiO₂ nanocomposites improves a lot. Besides, the heat stability of cryomilled PET/SiO₂ nanocomposites improves more much during reheating. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1351–1356, 2006