Space-resolved thermal properties of thermoplastics reinforced with carbon nanotubes

Composites comprising biobased poly(lactic acid) (PLA) and polyethylene (Bio-PE) were reinforced with multi-walled carbon nanotubes (MWCNTs). These nanocomposites were analyzed using space-resolved thermal analysis (TA) integrated with atomic force microscopy. The deflection temperature, which indicates thermal-induced expansion and thermal transitions of the composite, was monitored by nanoscale TA (nanoTA) utilizing the displacement of a cantilever in contact with the material. Results were compared to bulk electrical, mechanical and thermal properties. Electrical conductivity was detected at lower MWCNT loadings for PLA than for Bio-PE (at 2.5 vs. 5 mass%). Maximal electrical conductivity of 27 S m^?1 for PLA and 0.7 S m^?1 for Bio-PE-based samples was reached at 10 mass% MWCNT loading. Tensile behavior combined with thermogravimetric analysis indicated strong MWCNT–Bio-PE interactions, in contrast to PLA. The glass transition and melting temperature measured by differential scanning calorimetry (DSC) were not changed by the increase in MWCNT loading. Increased deflection temperature was registered by bulk heat deflection measurements on Bio-PE, but not for PLA. The thermal transitions obtained by nanoTA at the nanoscale were in the same temperature range as the first transitions observed upon temperature ramp in DSC (e.g., glass transition and melt temperatures of PLA and Bio-PE, respectively). Remarkably, thermal expansion was detected by nanoTA for PLA- and Bio-PE-based composites below electrical percolation threshold as well as an increase in PLA softening temperature. Space-resolved nanothermal analysis revealed thermal phenomena that are otherwise overlooked when bulk methods are applied.     

Graphite nanoplatelets/polymer nanocomposites: thermomechanical,dielectric, and functional behavior

Exfoliated graphite nanoplatelets (GNP)/epoxy resin nanocomposites were prepared and tested, varying the amount of the filler content. Systems’ morphology was investigated by means of scanning electron microscopy, while their thermal response was examined via differential scanning calorimetry (DSC). Broadband dielectric spectroscopy and dynamic mechanical thermal analysis were employed in order to characterize the produced systems. Static mechanical tests were also conducted at ambient. Reinforced systems exhibit improved performance under mechanical and electrical excitation. In particular, storage modulus increases systematically with GNP content. DSC results imply that glass transition temperature is not affected by the presence of GNP. Flexural modulus and storage modulus, as determined by static and dynamic mechanical tests, respectively, increased with filler content. Dielectric permittivity increases also systematically with GNP content. Recorded relaxation processes arise from the glass to rubber transition of the polymer matrix (α-mode), re-orientation of polar side groups of the polymer chains (β-mode), and interfacial polarization because of the accumulation of charges at the systems’ interface. Finally, the energy storing efficiency of the nanocomposites enhances with reinforcing phase in the examined frequency and temperature range. Optimum performance corresponds to the nanocomposite with maximum GNP loading.     

Optical and electrical properties of copper alumina nanoparticles reinforced chlorinated polyethylene composites for optoelectronic devices

The incorporation of transition metal oxide fillers into the polymer matrix through solution mixing polymerization imparts enhanced electrical and thermal properties. The present work focused on the optical properties, crystallinity, thermal stability, temperature-dependent conductivity, dielectric constant and modulus of chlorinated polyethylene/copper alumina (CPE/Cu–Al₂O₃) nanocomposites. Optical absorption measured using an ultraviolet–visible (UV–visible) spectrometer shows enhanced intensity and a blue shift for CPE/Cu–Al₂O₃ nanocomposites. The bandgap energy of CPE/Cu–Al₂O₃ nanocomposites was lower than pure CPE and minimum bandgap energy was recorded for a 7 wt% composites. The X-ray diffraction demonstrates that Cu–Al₂O₃ nanoparticles were uniformly introduced into the CPE matrix. Thermogravimetric analysis (TGA) manifests improved thermal stability of nanocomposites. Dielectric properties decrease with frequency, whereas AC conductivity increases with frequency, and both AC conductivity and dielectric properties increase with temperature. The maximum AC conductivity and dielectric constant were obtained for 7 wt % nanofiller loaded sample. For all systems, the activation energy for electrical conductivity decreases with rising temperatures. The experimental dielectric constant values of CPE nanocomposites were correlated with different theoretical models. The Bruggeman model was in good agreement with the experimental permittivity. The impedance experiments showed a decreasing trend with temperature, indicating the semiconducting nature of prepared nanocomposites.     

Improvement of electrical and material properties of epoxy resin/ aluminum nitride nanocomposites for packaging materials

This paper presents the influence of Aluminum Nitride (AlN) nanoparticles on the electrical and material properties of epoxy resin (EP). The EP/AlN nanocomposites with different concentrations of nano-AlN fillers are prepared. The dispersion of the nano-AlN particles in the composites is analyzed by a field emission scanning electron microscope (FESEM). The electrical properties are investigated by the space charge and DC conductivity measurements, whereas the material properties are studied by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that the homo-charge accumulation appears near both electrodes during the polarization, but there are limited negative charges left near both electrodes in the depolarization for the pure EP sample. There is no space charge accumulation in the 1 wt% and 2 wt% EP/AlN nanocomposites. The electric field distortion of the pure EP sample is 20%. Moreover, the electric field distortion initially decreases with the increase of the nano-AlN content, but it increases for the 2 wt% nano-AlN sample. Temperature has a dominant influence on the DC conductivity of the EP/AlN nanocomposites comparing to the pure EP. However, the DC conductivity of the nanocomposites becomes stable at high temperatures. It is also found that the weight loss of the samples decreases with the addition of the nano-AlN and the 1 wt% nano-AlN sample has the highest glass transition temperature. It is elucidated that the high apparent mobility and activation energy facilitate the space charge transport and suppressing the space charge accumulation. Furthermore, the nano-AlN filler can increase the trap level and trap energy density of the deep traps in the sample. The dielectric loss of the EP at high frequency is reduced with the content of 1 wt% nano-AlN. Furthermore, the addition of the nano-AlN can improve the thermal stability of the EP. The 1 wt% nano-AlN sample has the superior electrical insulation and material performance amongst the tested materials.     

Glass transition and polymer dynamics in silver/poly(methyl methacrylate) nanocomposites

Dynamic mechanical–thermal analysis (DMTA), differential scanning calorimetry (DSC), thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) were employed to investigate in detail glass transition and polymer dynamics in silver/poly(methyl methacrylate) (Ag/PMMA) nanocomposites. The nanocomposites were prepared by radical polymerization of MMA in the presence of surface modified Ag nanoparticles with a mean diameter of 5.6 nm dispersed in chloroform. The fraction of Ag nanoparticles in the final materials was varied between 0 and 0.5 wt%, the latter corresponding to 0.055 vol%. The results show that the nanoparticles have practically no effect on the time scale of the secondary β and γ relaxations, whereas the magnitude of both increases slightly but systematically with increasing filler content. The segmental α relaxation, associated with the glass transition, becomes systematically faster and stronger in the nanocomposites. The glass transition temperature T_g decreases with increasing filler content of the nanocomposites up to about 10 °C, in good correlation by the four techniques employed. Finally, the elastic modulus decreases slightly but systematically in the nanocomposites, both in the glassy and in the rubbery state. The results are explained in terms of plasticization of the PMMA matrix, due to constraints imposed to packing of the chains by the Ag nanoparticles, and at the same time, of the absence of strong polymer–filler interactions, due to the surface modification of the Ag nanoparticles by oleylamine at the stage of preparation.     

Preparation of polystyrene/MWCNT‐Valine composites: Investigation of optical,morphological, thermal,and electrical conductivity properties

The attempt of this research was to examine the effect of multiwalled carbon nanotube (MWCNT)‐Valine as efficient fillers on the thermal, optical, and electrical behaviors of polystyrene (PS). To reduce aggregation and obtain uniform spreading of fillers into the PS, at first, MWCNTs' surfaces were modified by Valine amino acid. Then, different contents of MWCNT‐Valine (0.5, 1, and 2 wt%) were added to PS by ultrasonication processes. The field emission scanning electron microscopy and transmission electron microscopy results showed a uniform distribution of modified MWCNTs into the matrix. The thermal properties of nanocomposites were improved by increasing nanofiller content. In addition, embedding of MWCNT‐Valine into the PS matrix increased the electrical conductivity of nanocomposites in comparison with pure PS.     

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     

Influence of organo-clay on electrical and mechanical properties of PP/MWCNT/OC nanocomposites

The electrical, thermal and mechanical properties of nanocomposites, based on polypropylene (PP) filled by multi-walled carbon nanotubes (MWCNTs) and organo-clay (OC), were studied with the purpose of finding out the effect of OC on the microstructure of MWCNTs dispersion and PP/MWCNT/OC composites. It was found that addition of organo-clay nanoparticles improved nanotube dispersion and enhanced electrical properties of PP/MWCNT nanocomposites. Addition of organo-clay (MWCNT/OC ratio was 1/1) reduced the percolation threshold of PP/MWCNT nanocomposites from ?_c = 0.95 vol.% to ?_c = 0.68 vol.% of carbon nanotubes, while the level of conductivity became 2–4 orders of magnitude higher. The DSC and DMA analyses have shown that the influence of organo-clay on the thermal and mechanical properties of material was not significant in composites with both fillers as compared to PP/OC. Such an effect can be caused by stronger interaction of OC with carbon nanotubes than with polymer matrix.     

Dynamic electrical thermal analysis on zinc oxide/epoxy resin nanodielectrics

The dielectric response of ZnO/epoxy resin nanocomposites was studied by means of dynamic electrical thermal analysis in the frequency range of 10^?1 to 10⁷ Hz, and over the temperature range of 30–160 °C, varying the content of the reinforcing phase. Scanning electron microscopy pictures were used for assessing the composites morphology and for examining the particles’ dispersion. The thermal properties of nanocomposites were examined by differential scanning calorimetry in the temperature range of 0–170 °C. Dielectric data were analyzed via dielectric permittivity and electric modulus formalisms. Recorded relaxation phenomena include contributions from both the polymeric matrix and the presence of the reinforcing phase. Processes related to the polymer matrix are attributed to the glass to rubber transition (α-relaxation) of the epoxy resin and local motions of polar side groups of the main polymer chain (β-relaxation). Finally, the slower process appearing at low frequencies and high temperatures, originates from interfacial phenomena due to the accumulation of unbounded charges at the system’s interface.     

Dielectric properties and molecular mobility of organic/inorganic polymer composites

Microstructure-dielectric properties relationship and molecular mobility of organic/inorganic polymer composites (OIPCs), consisting of polyurethane (PU) and sodium silicate (NaSi), were investigated in this work. Broadband dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) techniques were employed. Our interest was focused on the study of the glass transition mechanism and conductivity relaxation. The influence of the molecular weight of PU and inorganic phase content on the dielectric properties of the composites was of particular interest. Glass transition temperature shifts to higher temperatures with the addition of NaSi. The overall molecular mobility was found to increase in the composites, compared to the pure PU matrix. The results are more intense for the composites based on the PU with low molecular weight.     

Synergic effect in electrical conductivity using a combination of two fillers in PVDF hybrids composites

The objective of this work was to prepare novel conductive blends of poly(vinylidene fluoride) (PVDF) with polypyrrole (PPy) and to compare their performance with PVDF/multiwall carbon nanotube (MWCNT) composites and novel PVDF/PPy/MWCNT hybrid systems. All the compositions were prepared by melt mixing using a miniature mixer. The mixtures were characterized by Fourier transformed infrared (FTIR), wide angle X-ray diffraction (WAXD), thermogravimetric analyses (TGA), scanning and transmission electron microscopy (SEM and TEM, respectively) and volume electrical resistivity. For the binary PVDF/PPy and PVDF/MWCNT systems, percolation thresholds of 10 and 0.3 wt%, respectively, were found. In the hybrid systems, however, the percolation threshold for each filler was lower than in the binary systems, but the electrical conductivities were always much higher at all concentrations than the conductivities of the binary systems. Therefore, the addition of both fillers had a synergistic effect on the hybrid system conductivity, which was attributed to its morphology: the PPy increased the homogeneity of the MWCNT distribution and decreased the available free volume for the MWCNT; as a result the MWCNT rolled around the PPy particles bridging them through the PVDF matrix, increasing the quantum tunneling effect and thus, the electrical conductivity of the system.     

A comparative study on the AC/DC conductivity,dielectric and optical properties of polystyrene/graphene nanoplatelets (PS/GNP) and multi-walled carbon nanotube (PS/MWCNT) nanocomposites

Polystyrene/graphene nanoplatelets (PS/GNP) and polystyrene/multi-walled carbon nanotube (PS/MWCNT) nanocomposites were prepared through solution mixing processing. The effect of carbon filler (CF) (GNP or MWCNT) doping on the DC/AC electrical conductivity, dielectric characteristics and optical parameters (absorption coefficient, α and band gap energy, E_g) of nanocomposites were investigated and compared for similar doping concentrations. The observed behavior of the DC surface conductivity for PS/CF nanocomposites was explained according to the classical percolation theory, where the percolation thresholds (ϕ_c) for PS/GNP and PS/MWCNT nanocomposites were determined as 12.0 vol% and 3.81 vol% and the critical exponents (t) were calculated as 2.19 and 2.13, respectively. These results indicate that CFs create three dimensional CF network in PS matrix. The dielectric relaxation properties and the AC conductivity studied by means of Broadband Dielectric Spectroscopy (BDS) measurements, showed that the presence of carbon fillers significantly enhanced the capacitive/charge storage capabilities of the nanocomposites. The optical band gap energies (E_g) of PS/GNP and PS/MWCNT nanocomposites were obtained by using Tauc method. From applicative point of view, with their enhanced dielectric and AC conductivity properties of the PS/GNP and PS/MWCNT nanocomposites have the potential to be used in energy storage and electromagnetic interference (EMI) shielding applications.     

Morphology and thermal properties of polyhedral oligomeric silsesquioxane/polybutadiene nanocomposites prepared by anionic polymerization

The morphology and thermal properties of Allylisobutyl Polyhedral Oligomeric Silsesquioxane (POSS)/Polybutadiene (PB) nanocomposites prepared through anionic polymerization technique were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results of XRD, SEM and TEM showed that the aggregation of POSS in PB matrix occurred obviously, forming crystalline domains and the size of POSS particles increased with increasing POSS content. The DSC and TGA results indicated that the glass transition temperature (T _g) of the nanocomposites was significantly increased and the maximum degradation temperature (T _dmax) of nanocomposites was slightly increased compared with pure PB, implying an increase in thermal stability.     

共聚型氯醇橡胶锂离子聚合物电解质的制备和性能

以共聚型氯醇橡胶(ECO)为基体, 通过在基体中溶解不同浓度的LiCF₃SO₃制备了一系列聚合物电解质. 利用差示扫描量热技术(DSC)研究了该体系锂盐浓度对聚合物电解质玻璃化转变温度的影响, 用傅里叶变换红外光谱(FTIR)研究了体系内锂盐与聚合物基体的相互作用. 结果表明, 在相同锂盐浓度下, ECO基聚合物电解质的室温离子电导率比传统的聚环氧乙烷(PEO)基聚合物电解质提高了2个数量级, 并且体系电导率在升降温循环测试中没有弛豫现象产生. 这是由于ECO基体的非结晶性所致.     

Glass Transition Temperature Depression at the Percolation Threshold in Carbon Nanotube–Epoxy Resin and Polypyrrole–Epoxy Resin Composites

Summary: The glass transition temperatures of conducting composites, obtained by blending carbon nanotubes (CNTs) or polypyrrole (PPy) particles with epoxy resin, were investigated by using both differential scanning calorimetry (DSC) and dynamical mechanical thermal analysis (DMTA). For both composites, dc and ac conductivity measurements revealed an electrical percolation threshold at which the glass transition temperature and mechanical modulus of the composites pass through a minimum.

DC conductivity, σ_dc, as a function of the conducting filler concentration of the CNT– (▪) and PPy– (○) epoxy resin composites.     

Thermal history and enthalpy relaxation of an interpenetrating network polymer with exceptionally broad relaxation time distribution

Differential scanning calorimetry (DSC) of an interpenetrating network polymer of composition 25% polyurethane–75% poly(methyl methacrylate) shows a slowly increasing heat capacity, instead of the usual glass transition endotherm, whose onset temperature is not clearly discernible. On aging of the polymer at several temperatures between 193 and 333 K, an endothermic peak is observed whose onset is in the vicinity of the respective temperature of aging. The area under these peaks increases with increasing aging time at a fixed temperature. The effects are attributed to a very broad distribution of relaxation times, which may be represented by either a sum of discrete structural relaxation times of local network arrangement or by a nonexponential relaxation function which is equivalent to a distribution of relaxation times. In either view the vitrified state of the polymer can be envisaged as containing local structures whose own T_gs extend over a wide range of temperature. Aging decreases the enthalpy and produces an endothermic region which resembles an increase in C_p on heating because of relaxation of that local structure. The interpretation is supported by simulation of DSC scans in which the distribution of relaxation times is assumed to be exceptionally broad and in which aging introduced at several temperatures over a wide range produces endothermic effects (or regions of DSC scans) qualitatively similar to those observed for the interpenetrating network polymer. © 1994 John Wiley & Sons, Inc.     

Electrical transport properties of semiconducting lithium molybdate glass nanocomposites

We have reported the formation of lithium molybdate glass nanocomposites embedded with lithium molybdate nanophases from the x-ray diffraction and transmission electron microscopic studies. We have investigated the dc electrical conductivity in a wide temperature range for these glass nanocomposites, which exhibit semiconducting behavior. We have analyzed the dc electrical data in the light of polaronic conduction models of Mott and Schnakenberg. We have also studied ac electrical conductivity of these glass nanocomposites in wide temperature and frequency ranges. The experimental ac results have been analyzed with reference to various theoretical models based on quantum-mechanical tunneling and hopping over the barrier. We have observed that the temperature dependence of the dc conductivity is consistent with the polaronic hopping models, while the temperature and frequency dependence of the ac conductivity is consistent with the polaronic tunneling models.     

Electrical Properties Investigation in PA12/PANI Composites

Summary: Volume conducting PA-12 based composites powders were chemically prepared by in situ polymerization and aniline doping at room temperature. These kinds of polyamide / PANI composites were investigated regarding their electrical properties. Their ac and dc electrical properties measured in the frequency range of 10⁻²–10⁷ Hz are reported and the frequency dependence of electrical conductivity was investigated as a function of PANI concentration leading to the determination of the conductivity. The experimental conductivity was found to increase continuously with PANI content and explained by percolation theory with a relatively low percolation threshold of about 0.4 wt.%. The dielectric behavior of various PANI polymer composites has been characterized by the critical frequency ω_c (denoting the crossover from the dc plateau of the conductivity to its frequency dependent ac behaviour). Modelling the conductivity behavior versus volume fraction using Slupkowski approach has revealed that the considered parameters are not sufficient to describe the electrical conductivity behavior.     

Core–shell functionalized MWCNT/poly(m‐aminophenol) nanocomposite with large dielectric permittivity and low dielectric loss

Core–shell carboxyl‐functionalized multiwall carbon nanotube (c‐MWCNT)/poly(m‐aminophenol) (PmAP) nanocomposite were prepared through in‐situ polymerization of m‐aminophenol (m‐AP) in the presence of MWCNTs, and explicated as a dielectric material for electronic applications. The formation of thin PmAP layer on individual c‐MWCNT with excellent molecular level interactions at interfaces was confirmed by morphological and spectroscopic analyses. Here we conducted a comparative study of the dielectric performances of PmAP based nanocomposite films with pristine MWCNTs and c‐MWCNTs as fillers. Compared to PmAP/MWCNT nanocomposites, the PmAP/c‐MWCNT nanocomposites exhibited higher dielectric permittivity and lower dielectric loss. The well dispersed c‐MWCNTs in PmAP/c‐MWCNT nanocomposite produce huge interfacial area together with numerous active polarized centers (crystallographic defects), which in turn intensified the Maxwell‐Wagner‐Sillars (MWS) effect based on excellent molecular level interactions and thus, produce large dielectric permittivity (8810 at 1 kHz). The percolation threshold of PmAP/c‐MWCNT nanocomposites is found lower than that of the PmAP/MWCNT nanocomposites, which could be attributed to homogeneous distribution of c‐MWCNTs and strong c‐MWCNT//PmAP interfacial interactions in the nanocomposites. Copyright © 2016 John Wiley & Sons, Ltd.     

Isothermal crystallization kinetics and thermal behavior of poly(?-caprolactone)/multi-walled carbon nanotube composites

This study describes the preparation of poly(?-caprolactone) (PCL)/multi-walled carbon nanotube (MWCNT) composites by ultrasonically mixing the PCL and as-fabricated MWCNT in a tetrahydrofuran solution. The TEM images show that the MWCNT is well separated and uniformly distributed in the PCL matrix. Differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), X-ray diffraction (XRD) and polarized optical microscopy (POM) were used to investigate the isothermal crystallization kinetics, crystalline structure and thermal behavior of PCL and PCL/MWCNT nanocomposites. DSC isothermal results revealed that the activation energy of PCL extensively decreases with increasing MWCNT contents, suggesting that the loading of MWCNT into PCL matrix probably induced heterogeneous nucleation during crystallization processes. From TGA data, the addition of small amount of MWCNT into PCL matrix can improve the thermal stability of PCL matrix. TGA isothermal degradation data illustrate that the activation energy E_d of the composites is smaller than that of PCL. This phenomenon can be attributed to the incorporation of more MWCNT loading into PCL caused a decrease in the degradation rate and an increase in the residual weight for PCL/MWCNT nanocomposites.