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     

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 and characterization of nylon 6/foliated graphite electrically conducting nanocomposite

In this study, the nylon 6/foliated graphite (FG) electrically conducting nanocomposites with a low percolation threshold of less than 0.75 vol % have been prepared via an in situ polymerization approach in the presence of FG nanosheet filler. Based on laser counting, scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterization techniques, the structures and morphologies of the nanoscale filling particles and the resulting nanocomposites were examined. Using percolation theory, the conductivity behavior of the nanocomposite samples were modeled and analyzed. Through the use of mean‐field and excluded volume approaches, it was demonstrated that the experimentally observed percolation threshold values could be approximately estimated, and a correlation between the percolation threshold and the aspect ratio of FG particles could be quasi‐quantitatively established. Also, preliminary studies on the effects of FG nanosheets on the thermal properties of the host nylon 6 were performed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2844–2856, 2004     

Morphological study on water‐borne conducting polyaniline‐poly(ethylene oxide) blends

Conducting polyaniline‐poly(ethylene oxide) blends were prepared from their aqueous solutions. The blends displayed an electrical conductivity percolation threshold as low as 1.83 wt % of polyaniline loading. As demonstrated by scanning electron microscopy, polarized optical microscopy, and wide‐angle X‐ray diffraction studies, the conducting polyaniline took a fibrillar morphology in the blend, and it existed only in the amorphous phase of poly(ethylene oxide). A three‐phase model combining morphological factors instead of a two‐phase model was proposed to explain the low‐conductivity percolation threshold. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 605–612, 2002; DOI 10.1002/polb.10114     

Comparative analyses of the electrical properties and dispersion level of VGCNF and MWCNT: Epoxy composites

The electrical properties and dispersion of vapor‐grown carbon nanofibers (VGCNF) and multiwalled carbon nanotubes (MWCNT)—epoxy resin composites are studied and compared. A blender was used to disperse the nanofillers within the matrix, producing samples with concentrations of 0.1, 0.5, and 1.0 wt % for both nanofillers, besides the neat sample. The dispersion of the nanofillers was qualitatively analyzed using scanning electron microscopy, transmission optical microscopy, and grayscale analysis. The electrical conductivity and the dielectric constant were evaluated. The percolation threshold of MWCNT epoxy composites is lower than 0.1 wt % while for VGCNF lies between 0.1 and 0.5 wt %. The difference on the dispersion ability of the two nanofillers is due to their intrinsic characteristics. Celzard's theory is suitable to calculate the percolation threshold bounds for the VGCNF composites but not for the MWCNT composites, indicating that intrinsic characteristics of the nanofillers beyond the aspect ratio are determinant for the MWCNT composites electrical conductivity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Thermoelectric behavior of organic thin film nanocomposites

Organic thin film nanocomposites, prepared by liquid‐phase exfoliation, were investigated for their superior electrical properties and thermoelectric behavior. Single‐walled carbon nanotubes (SWNT) were stabilized by intrinsically conductive poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) in an aqueous solution. The electrical conductivity (σ) was found to increase linearly as 20 to 95 wt % SWNT. At 95 wt % SWNT, these thin films exhibit metallic electrical conductivity (～4.0 × 10⁵ S m^?1) that is among the highest values ever reported for a free‐standing, fully organic material. The thermopower (S) remains relatively unaltered as the electrical conductivity increases, leading to a maximum power factor (S²σ) of 140 μW m^?1 K^?2. This power factor is within an order of magnitude of bismuth telluride, so it is believed that these flexible films could be used for some unique thermoelectric applications requiring mechanical flexibility and printability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Low electrical percolation threshold in poly(ethylene terephthalate)/multi-walled carbon nanotube nanocomposites

Poly(ethylene terephthalate) (PET)/multi-walled carbon nanotube (MWCNT) nanocomposites were prepared by three different methods: in-situ polymerization technique (I-S), direct mixing in the melt (DM) and dilution of a 0.5 wt.% masterbatch, synthesized via in-situ polymerization, using melt mixing (MB). The morphology of the resulting nanocomposites was examined using scanning and transmission electron microscopy and their electrical properties were characterized by ac conductivity measurements. The I-S series of samples exhibited an extremely low electrical percolation threshold (p_c ≈ 0.06 wt.%), as compared to values of similar systems previously mentioned in literature. The MB series showed a comparable p_c value (p_c: 0.05-0.10 wt.%), whereas the investigation revealed a higher p_c in the DM series (p_c: 0.10-0.20 wt.%). Finally, selected concentrations of samples were prepared using OH-functionalized MWCNT, following the I-S procedure. The conductivity of these samples was found to be lower than that of samples with non-functionalized MWCNT.     

Rheology,electrical conductivity,and the phase behavior of cocontinuous PA6/ABS blends with MWNT: Correlating the aspect ratio of MWNT with the percolation threshold

Multiwalled carbon nanotubes (purified, p‐MWNT and ～ NH₂ functionalized, f‐MWNT) were melt‐mixed with 50/50 cocontinuous blends of polyamide 6 (PA6) and acrylonitrile–butadiene–styrene in a conical twin‐screw microcompounder to obtain conductive polymer blends utilizing the conceptual approach of double‐percolation. The state of dispersion of the tubes was assessed using AC electrical conductivity measurements and melt‐rheology. The rheological and the electrical percolation threshold was observed to be ～ 1–2 wt % and ～ 3–4 wt %, respectively, for blends with p‐MWNT. In case of blends with f‐MWNT, the rheological percolation threshold was observed to be higher (2–3 wt %) than p‐MWNT but the electrical percolation threshold remained almost same. However, the absolute values were significantly lower than blends with p‐MWNT. In addition, significant refinement in the cocontinuous morphology of the blends with increasing concentration of MWNT was observed in both the cases. Further, an attempt was made to understand the underlying concepts in relation to cocontinuous morphologies that how the geometrical percolation threshold which adversely suffered because of the attrition of tubes under prolonged shear contributed further in retaining the rheological percolation threshold. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1619–1631, 2008     

Electrical properties study of multi-walled carbon nanotubes/hybrid-glass nanocomposites

Electrical properties of multi-walled carbon nanotubes (MWNTs)/hybrid-glass nanocomposites prepared by the fast-sol–gel reaction were investigated in light of percolation theory. A good correlation was found between the experimental results and the theory. We obtained a percolation threshold ? _c  = 0.22 wt%, and a critical exponent of t = 1.73. These values are reported for the first time for a silica-based system. The highest conductivity measured on the MWNT/hybrid-glass nanocomposites was σ ≈ 10^?3(Ω cm)^?1 for 2 wt% carbon nanotube (CNT) loading. The electrical conductivity was at least 12 orders of magnitude higher than that of pure silica. Electrostatic force microscopy and conductive-mode atomic force microscopy studies demonstrated conductivity at the micro-level, which was attributed to the CNT dispersed in the matrix. It appears that the dispersion in our MWNT/hybrid-glass system yields a particularly low percolation threshold compared with that of a MWNT/silica-glass system. Materials with electrical conductivities described in this work can be exploited for anti-static coating.     

Conductivity,proton percolation,and gelation during the network growth in a flexible chain diepoxide and diamine mixture

The conductivity of a stoichiometric mixture of diglycidyl ether of 1,4‐butanediol and 1,6‐hexamethylene diamine has been studied during its polymerization at several temperatures where the ultimate product is a crosslinked gel. The decrease in the dc conductivity, σ₀, with the polymerization time, t, fits an equation for bond percolation, σ₀ ∼ [(t_gel‐t)/t_gel]^p, and yields a gelation time, t_gel which agrees with the t_gel determined from the viscosity and shear modulus measurements. It is proposed that as one covalent bond forms on chemical reaction, an indeterminable number of intermolecular H‐bonds in the structure vanish, and protonic conduction is disrupted. Thus, as the original H‐bond network gives way to a covalently bonded network, the mechanical rigidity increases, and protonic conductivity decreases. The gel point is reached when the increase in the number of covalent bonds brings the liquid's state up to its rigidity percolation threshold, and the decrease in the number of H‐bonds brings it down to its electrical percolation threshold. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 122–126, 2000     

Synthesis and characterization of C60/polyaniline composites from interfacial polymerization

C₆₀/polyaniline (PANI) nanocomposites have been synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in the presence of C₆₀ by using an interfacial reaction. When compared with the pure PANI nanofibers from the similar process, the diameter of the obtained C₆₀/PANI nanofibers was increased because of the encapsulation of C₆₀ into PANI during aniline polymerization, which resulted from the charge‐transfer interactions between C₆₀ and aniline fragment in PANI. In addition, the resulting C₆₀/PANI nanocomposites synthesized from the low initial C₆₀/aniline molar ratio (less than 1:25) showed the homogenous morphology composed of fiber network structures, which has an electrical conductivity as high as 1.1 × 10^?4 S/cm. However, the C₆₀/PANI nanocomposites from the higher initial C₆₀/aniline molar ratio (more than 1:15) showed the nonuniformly distributed morphology, and the electrical conductivity was decreased to 3.5 × 10^?5 S/cm. Moreover, the C₆₀/PANI nanocomposites from the interfacial reaction showed a higher value of electrical conductivity than the mechanically mixed C₆₀/PANI blends with the same C₆₀ content, because of the more evenly distributed microstructures. FTIR, UV–vis, and CV data confirmed the presence of C₆₀ and the significant charge‐transfer interactions in the resultant nanocomposites, which was responsible for the morphology development of the C₆₀/PANI and the variation of the electrical conductivity. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Linear low‐density polyethylene nanocomposites by in situ polymerization using a zirconium‐nickel tandem catalyst system

A series of linear low‐density polyethylene (LLDPE) nanocomposites containing different types of nanofiller (TiO₂, MWCNT, expanded graphite, and boehmite) were prepared by in situ polymerization using a tandem catalyst system composed of {Tp^Ms}NiCl ( 1 ) and Cp₂ZrCl₂ ( 2 ), and analyzed by differential scanning calorimetry, dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). Based on these analyses, the filler content varied from 1.30 to 1.80 wt %. The melting temperatures and degree of crystallinity of the LLDPE nanocomposites were comparable to those of neat LLDPE. The presence of MWCNT as well as boehmite nucleated the LLDPE crystallization, as indicated by the increased crystallization temperature. The DMA results showed that the presence of TiO₂, EG, and CAM 9080 in the LLDPE matrix yielded nanocomposites with relatively inferior mechanical properties compared to neat LLDPE, suggesting heterogeneous distribution of these nanofillers into the polymer matrix and/or the formation of nanoparticle aggregates, which was confirmed by TEM. However, substantial improvement in the storage modulus was achieved by increasing the sonication time. The highest storage modulus was obtained using MWCNT (1.30 wt %). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3506–3512     

Electrical conductivity of PUR/MWCNT nanocomposites in the molten state, during crystallization and in the solid state

The electrical conductivity of a nanocomposite constituted of multiwalled carbon nanotubes (MWCNT) dispersed in a semicrystalline polyurethane matrix, is investigated during cooling from the melt to the solid state. The same percolation threshold, ?_c = 0.85 wt.%, is obtained in the molten and in the solid state, although the exponent t of the percolation equation is significantly higher in the solid state. A remarkable increase of the conductivity during crystallization is observed for nanocomposites of MWCNT content above ?_c, but for contents below ?_c the conductivity decreases. Combined conductivity and PVT results, lead us to discard the hypothesis of an increase of the density of the conductive network (associated with volume shrinkage) as being the cause of the conductivity enhancement during crystallization. Instead, the analysis of the parameters of the percolation equation for the molten and the solid state, suggests a transition from a less effective conductive network to a more performing three dimensional network.     

Why does the electrical conductivity in PEDOT:PSS decrease with PSS content? A study combining thermoelectric measurements with impedance spectroscopy

We have investigated the electrical transport properties of poly(3,4‐ethylenedioxythiophen)/poly(4‐styrene‐sulfonate) (PEDOT:PSS) with PEDOT‐to‐PSS ratios from 1:1 to 1:30. By combining impedance spectroscopy with thermoelectric measurements, we are able to independently determine the variation of electrical conductivity and charge carrier density with PSS content. We find the charge carrier density to be independent of the PSS content. Using a generalized effective media theory, we show that the electrical conductivity in PEDOT:PSS can be understood as percolation between sites of highly conducting PEDOT:PSS complexes with a conductivity of 2.3 (Ωcm)^?1 in a matrix of excess PSS with a low conductivity of 10^?3 (Ω cm)^?1. In addition to the transport properties, the thermoelectric power factors and Seebeck coefficients have been determined. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Fundamental study of crystallization,orientation, and electrical conductivity of electrospun PET/carbon nanotube nanofibers

The morphology, structure, and properties of polyethylene terephthalate (PET)/Carbon Nanotubes (CNT) conductive nanoweb were studied in this article. Nanocomposite nanofibers were obtained through electrospinning of PET solutions in trifluoroacetic acid (TFA)/dichloromethane (DCM) containing different concentrations and types of CNTs. Electrical conductivity measurements on nanofiber mats showed an electrical percolation threshold around 2 wt % multi‐wall carbon nanotubes (MWCNT). The morphological analysis results showed smoother nanofibers with less bead structures development when using a rotating drum collector especially at high concentrations of CNTs. From crystallographic measurements, a higher degree of crystallinity was observed with increasing CNT concentrations above electrical percolation. Spectroscopy results showed that both PET and CNT orientation increased with the level of alignment of the nanofibers when the nanotube concentration was below the electrical percolation threshold; while the orientation factor was reduced for aligned nanofibers with higher content in CNT. Considerable enhancement in mechanical properties, especially tensile modulus, was found in aligned nanofibers; at least six times higher than the modulus of random nanofibers at concentrations below percolation. The effect of alignment on the mechanical properties was less important at higher concentrations of CNTs, above the percolation threshold. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2052–2064, 2010     

Influence of multiwall carbon nanotubes on morphology and electrical conductivity of PP/ABS blends

Polypropylene (PP) and acrylonitrile‐butadiene‐styrene (ABS) blends with multiwall carbon nanotubes (MWNT) were prepared by melt mixing. PP/ABS blends without MWNT revealed coarse co continuous structures on varying the ABS content from 40 to 70 wt %. Bulk electrical conductivity of the blends showed lower percolation threshold (0.4–0.5 wt %) in the 45/55 co continuous blends whereas the percolation threshold was between 2 and 3 wt % in matrix‐particle dispersed morphology of 80/20 blends. Interestingly, droplet size was observed to decrease with addition of MWNT above percolation threshold in 80/20 blends. Further, the bulk electrical conductivity was found to be dependent on the melt flow index of PP. The non‐polar or weakly polar nature of blends constituents resulted in the temperature independent dielectric constant, dielectric loss, and DC electrical conductivity. Rheological analysis revealed the formation of 3D network‐like structure in 80/20 PP/ABS blends at 3 wt % MWNT. An attempt was made to understand the relationship between rheology, morphology, and electrical conductivity of these blends. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2286–2295, 2008     

Conductive mechanism of polymer/graphite conducting composites with low percolation threshold

Preparation of the conducting composites of polystyrene/expanded graphite via in situ polymerization and investigation of the conductive mechanism were carried out. They are characterized by high conductivity and a low percolation threshold. The electrical conductivity reached 10^?2 S/cm with 3.0 vol % expanded graphite content, whereas the percolation threshold was 1.0 vol %. Optical micrographs revealed the heterogeneous distribution of the graphite particles and the formation of a conductive network in the polymer matrix. A model of primary particle was proposed to interpret the conductive phenomena. The primary particle is the basic conductive unit in the composites that comprises three of the following parts: the graphite particle, the compact‐adsorbed layer, and the wrapping shell. Our model was also used to explain the experimental data in our previous studies on nylon‐6/expanded graphite composites. A low percolation threshold of conducting composites can be also explained according to the model of the primary particle. Furthermore, the theoretical line of conductivity versus primary particle content calculated from the revised Flory's theory fits the experimental data well. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 954–963, 2002     

Isotactic polypropylene/carbon nanotube composites prepared by latex technology: Electrical conductivity study

Several series of nanocomposites were prepared using a latex-based process, the main step of which consisted of mixing an aqueous suspension of exfoliated carbon nanotubes (CNTs) and a polymer latex. In the present work, a systematic study on the electrical properties of fully amorphous (polystyrene - PS) as well as semi-crystalline (isotactic polypropylene - iPP) nanocomposites containing either single-wall (SWCNTs) or multi-wall carbon nanotubes (MWCNTs) has been conducted. Percolation thresholds as low as 0.05 wt.% or 0.1 wt.% were observed for SWCNT/iPP and MWCNT/iPP nanocomposites, respectively. The formation of a conductive percolating network at such a low CNT concentration is favored by the high intrinsic conductivity and the low viscosity of the polymer matrix. The electrical percolation threshold of the iPP-based system was found to be lower than its rheological percolation threshold. Beyond the percolation threshold, MWCNT-based nanocomposites generally exhibited higher conductivity levels than those based on SWCNTs, most probably due to the higher intrinsic conductivity of the MWCNTs as compared to that of the SWCNTs. These excellent electrical properties, associated with the strong nucleating effect of the CNTs reported earlier [1] and [2], render this type of nanocomposites extremely attractive from a technological point of view.     

Changes During Storage of Electrically Conductive Blends Polyaniline–Poly(Ethylene‐co‐vinyl Acetate)

Abstract

The electrical conductivity behavior of polyaniline–poly(ethylene‐co‐vinyl acetate) (PANI–EVA) blends was variable and dynamic during their storage. It was shown that the apparent concentration of the intrinsically conductive polymer at which a conductivity jump of the blends occurs (Φ _c ) is not a constant value over time. The electrical conductivity of the films of low PANI content (below 2.5 wt.%) increased by several (ca. 5) orders of magnitude. It was found that the PANI phase undergoes a flocculation process subsequently resulting in the formation of conductive pathways and a continuous network. Besides, the shape of percolation curves was found to change during storage of the films. Decreased conductivity deviations were registered for blends of low PANI content (<2.5 wt.%), indicating that an improvement (or decreasing number of defects) of the conductive pathways took place within the bulk of the insulating EVA matrix. These results and observed phenomena are discussed by means of the interfacial model for electrically conductive polymer blends. They supported the dispersion/flocculation phase transition within similar composite materials. The phase separation and conductivity jump are attributed to the interfacial interactions between the polymeric constituents. It was shown that the microstructure of the blends consists of highly ordered PANI paths embedded in the insulating EVA matrix. Long fibrils of PANI and interconnected fractal‐like networks were observed. It was found that the sizes of the PANI domains also varied during storage of the films. Due to the spontaneous flocculation of the primary PANI particles, conductive pathways are formed at extremely low percolation threshold (Φ _c , loading level ca. 5 × 10^?3 wt. fraction). Thus, an important property of the conductive constituent, namely its solid‐state rearrangement, was proved. This PANI self‐organization is also interpreted according to the interfacial model of polymer composites. On the other hand, the competition between self‐organization of the complex of PANI with dodecylbenzenesulfonic acid and crystallization of EVA matrix has resulted in structural changes and formation of continuous conductive networks within the blends, responsible for their significantly increased conductivity.     

Effects of processing conditions on rheological,thermal, and electrical properties of multiwall carbon nanotube/epoxy resin composites

We report on the effect of processing conditions on rheology, thermal and electrical properties of nanocomposites containing 0.02–0.3 wt % multiwall carbon nanotubes in an epoxy resin. The influence of the sonication, the surface functionalization during mixing, as well as the application of external magnetic field (EMF) throughout the curing process was examined. Rheological tests combined with optical microscopy visualization are proved as a very useful methodology to determine the optimal processing conditions for the preparation of the nanocomposites. The Raman spectra provide evidence for more pronounced effect on the functionalized with hardener compositions, particularly by curing upon application of EMF. Different chain morphology of CNTs is created depending of the preparation conditions, which induced different effects on the thermal and electrical properties of the nanocomposites. The thermal degradation peak is significantly shifted towards higher temperatures by increasing the nanotube content, this confirming that even the small amount of carbon nanotubes produces a strong barrier effect for the volatile products during the degradation. The ac conductivity measurements revealed lower values of the percolation threshold (pc) in the range of 0.03–0.05 wt %. CNTs for the nanocomposites produced by preliminary dispersing of nanotubes in the epoxy resin, compared to those prepared by preliminary functionalization of the nanotubes in the amine hardener. This is attributed to the higher viscosity and stronger interfacial interactions of the amine hardener/CNT dispersion which restricts the reorganization of the nanotubes. The application of the EMF does not influence the pc value but the dc conductivity values (σ_dc) of the nanocomposites increased at about one order of magnitude due to the development of the aforementioned chain structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011