Effect of melting and crystallization on the conductive network in conductive polymer composites

Investigation on the effect of melting and crystallization of polypropylene (PP) on the conductive network of multi-wall carbon nanotubes (MWNTs) and carbon black (CB) in MWNT/PP and CB/PP composites is performed. The conductive networks formed by fillers with different aspect ratios (MWNTs and CB) are compared during melting and cooling experiments. The network is found to be deformed during melting and re-constructed again due to the re-agglomeration of fillers during isothermal annealing of the melt. Both deformation and re-construction of the network result in a substantial increase/decrease of the thermal resistivity of MWNT/PP and CB/PP composites. For the modelling of the dynamic network reformation three different approaches are tested: classic percolation theory, general effective medium theory (GEM) and Fournier equation.     

Thermal degradation and crystallization behavior of blend‐based nanocomposites: Role of clay network formation

The main goal of this study is to explicate the exact role of nanoclay particles on thermal degradation mechanism and crystallization behavior of blend‐based nanocomposites. Thermoplastic olefin (TPO) nanocomposites, as a simple model, were prepared via melt mixing in an internal batch mixer. X‐ray diffractometry (XRD) and transmission electron microscopy tests show that a relatively good dispersion of silicate layers was obtained in the system. On the addition of nanoclay, a remarkable reduction in rubber domain size was observed through scanning electron microscopy (SEM). Thermogravimetric analysis shows that nanoclay particles can retard thermal decomposition process. Thermal degradation kinetic studies, using Flynn–Wall–Ozawa method, reveal that addition of nanoclay contents higher than 1 wt % changes the mechanism of thermal degradation. A mechanism was proposed to explain this phenomenon based on SEM images of char residues. Non‐isothermal crystallization behavior of samples was investigated using differential scanning calorimeter. The unexpected reduction in crystallinity of TPO nanocomposites containing 5 wt % nanoclay was explained using rheometry analysis and attributed to the formation of stable percolated clay networks in this sample. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Processing-microstructure-property relationship in conductive polymer nanocomposites

The processing-microstructure-property relationship in conductive polymer nanocomposites was investigated. Nanocomposites of vapor grown carbon nanofiber (VGCNF)/high density polyethylene (HDPE) with different levels of nanofiber dispersion were formulated by changing the nanocomposites’ compounding temperature. Direct (SEM and optical microscopy) and indirect methods (linear viscoelastic properties) were used to characterize the dispersion of nanofiller. VGCNF aspect ratio before and after mixing was measured. Increasing processing temperature was found to increase the nanofiller agglomeration and reduce the breakage of nanofiller because of the decrease in the mixing shear stress and energy. The electrical and electromagnetic interference (EMI) shielding properties of the VGCNF/HDPE nanocomposites decreased with increase in processing temperature from 180 °C to 220 °C because the increase in the agglomeration of VGCNF was more significant than the preservation of the VGCNF aspect ratio. This finding does not mean that the increase in processing temperature will always lead to decrease in the electrical conductivity and EMI shielding properties for all polymer composites. For some composites, it is possible to preserve the filler aspect ratio enough so that the increase in agglomeration is less of a factor.     

Current-voltage characteristics of nanoplatelet-based conductive nanocomposites

In this study, a numerical modeling approach was used to investigate the current-voltage behavior of conductive nanoplatelet-based nanocomposites. A three-dimensional continuum Monte Carlo model was employed to randomly disperse the nanoplatelets in a cubic representative volume element. A nonlinear finite element-based model was developed to evaluate the electrical behavior of the nanocomposite for different levels of the applied electric field. Also, the effect of filler loading on nonlinear conductivity behavior of nanocomposites was investigated. The validity of the developed model was verified through qualitative comparison of the simulation results with results obtained from experimental works.     

A new approach for conductive network formation in electrospun poly(vinylidene fluoride) nanofibers

Conductive nanofibers of poly(vinylidene fluoride) (PVDF) filled with polyaniline (PANi)‐coated multi‐wall carbon nanotubes (MWCNTs) were fabricated using the electrospinning technique. PANi is an intrinsically conductive polymer. The addition of PANi‐coated MWCNTs to PVDF created short conductive strands on the surface of the nanofibers, facilitating the formation of a conductive network in the transverse direction of the nanofibers. Piezoelectricity along with electric conductivity makes these PVDF nanofibers promising for applications such as sensors and actuators. Electrospun PVDF nanofiber mats had higher piezoelectricity than melt‐processed samples produced using traditional polymer processing techniques, such as compression molding. Spectroscopic imaging techniques were employed to study the effects of the filler and processing conditions on the nanofiber structure. X‐ray diffraction, Fourier transform infrared spectroscopy and differential scanning calorimetry results indicated a large increase in the β‐phase crystals of the PVDF nanofibers. This higher content of β‐phase crystals enhanced the piezoelectricity of the nanofibers. © 2015 Society of Chemical Industry     

Role of multi-wall carbon nanotube network in composites to crystallization of isotactic polypropylene matrix

The composites (iPP/CNTs) made of isotactic polypropylene (iPP) and multi-wall carbon nanotubes (CNTs) were prepared by solution blending. To improve compatibility between CNTs and iPP and to enhance dispersion of CNTs in iPP matrix, CNTs were chemically modified by grafting alkyl chains. The chemically modified CNTs had about 6 wt% grafted alkyl chains. Rheological measurements indicated that CNTs caused gelation in iPP/CNTs due to CNT network formation and the critical gelation CNT concentration was about 7.4 wt%, which was considered to be high due to the low CNT aspect ratio in this study. Crystallization behaviors of iPP/CNTs were studied by using optical microscopy (OM) and differential scanning calorimetry (DSC). Radial growth rates of spherulites during isothermal crystallization of iPP/CNTs with CNT concentrations less than 2.0 wt% measured by using OM showed decreasing trends with increasing CNT concentration. Avrami analysis of the exothermic heat flow curves during isothermal crystallization of iPP/CNTs measured by DSC indicated that crystallization rates were accelerated when CNT concentrations were lower than the critical gelation concentration, because CNTs mainly functioned as nucleating agents for crystallization, while crystallization rates did not change obviously when CNT concentrations were higher than the critical gelation concentration, because CNT network could form and mainly functioned to provide restriction to mobility and diffusion of iPP chains to crystal growth fronts.     

Highly conductive graphene by low-temperature thermal reduction and in situ preparation of conductive polymer nanocomposites

Polydopamine-coated graphene oxide (DGO) films exhibit electrical conductivities of 11?000 S m(-1) and 30?000 S m(-1) upon vacuum annealing at 130 °C and 180 °C, respectively. Conductive poly(vinyl alcohol)/graphene and epoxy/graphene nanocomposites show low percolation thresholds due to the excellent dispersibility of the DGO sheets and their effective in situ reduction.     

Rheological studies on the quasi-quiescent crystallization of polypropylene nanocomposites

Isothermal crystallization of isotactic polypropylene (iPP)/organic montmorillonite (OMMT) binary nanocomposite and iPP/OMMT/poly(ethylene-co-octene) (PEOc) ternary nanocomposites has been investigated by polarized optical microscopy (POM), rheometry and scanning electron microscopy (SEM). At the stage of nucleation the heterogeneous nucleation effect of OMMT was much greater than the concentration fluctuation assisted nucleation effect in the ternary nanocomposite. Besides, PEOc played a role of inhibitor of OMMT nucleation agents at the nucleation stage because many of OMMT layers were distributed around PEOc-rich domains. At stage II of the crystal growth process, the entanglement effect of PEOc greatly affected the rheological response (storage modulus (G′) and its growth rate) due to the long side chains of PEOc component. In stage III of the growth process, OMMT layers and the entanglement of PEOc chains limited the motion of polypropylene chains. So the growth rate of G′ was slowed down. During the shrinkage and cooling process after isothermal crystallization, some fibril links between the spherulites, consisting of PEOc chains and iPP chains, were formed from the amorphous phases surrounding the spherulites.     

Neural network simulation for non-MSMPR crystallization

A neural network model has been developed for the simulation of steady state industrial crystallizers where, in general, the crystal size distribution cannot be described by simple mass and energy balances, i.e. they are non-MSMPR crystallizers. The model is based on fundamental equations of steady state suspension crystallization. The parameters in the nucleation rate have been chosen for the simulation of different chemicals. The particle size distribution of the product is expressed by the Rosin–Rammler equation. Different operating modes and deviations in crystal size distribution caused by the suspension being imperfectly mixed are presented by different values of modified Rosin–Rammler number. The ranges of variables in the neural network have been chosen based on data for industrial crystallizers. The dominant size of particle, and the productivity of the crystallizer can be predicted with input information. Thus, this neural network can be used for most chemicals and for different kinds of operating conditions. The results predicted with the neural network have been verified by solving the fundamental equations and by comparison with experimental data.     

Polymer matrix degradation and color formation in melt processed nylon 6/clay nanocomposites

Nylon 6 nanocomposites based on various quaternary alkyl ammonium organoclays were prepared by melt processing using a twin screw extruder. Dilute solution viscosity techniques were used to evaluate the level of polymer molecular weight degradation experienced during nanocomposite compounding; whereas colorimeter techniques were used to document color formation. In general, a significant reduction in nylon 6 matrix molecular weight was observed, which is believed to stem, in part, from reaction(s) between the surfactant of the organoclay and the polyamide chains. The level of degradation depends on both the type of nylon 6 material used and the surfactant chemistry in the organoclay. For a given organoclay, nanocomposites based on high molecular weight nylon 6 materials experience more matrix degradation, as well as color formation, than those based on low molecular weight materials; this is believed to arise from increased exposure of the organoclay surface to the nylon 6 owing to increased platelet exfoliation. Different organoclays lead to different levels of polymer degradation and color formation, depending upon the level of unsaturation present in the organic surfactant; the higher the number of double bonds the greater the degradation and the deeper the color formation. The primary mechanism of degradation is believed to be thermo-oxidative. Melt mixing of nylon 6 with model compounds, long-chain alkenes, shows that the same mode of degradation i.e. via double bonds can be replicated. In addition to unsaturation effects, the presence of hydroxyl-ethyl groups, opposed to methyl groups, in the organoclay surfactant, results in more color. Isothermal thermogravimetric analysis (TGA) was conducted on the organoclays to determine if thermal stability was a cause of molecular weight degradation; although, this relationship does not seem to exist, a direction correlation is observed between the organoclay degradation and nanocomposite modulus, or indirectly level of exfoliation. Use of antioxidant was found to reduce the amount of molecular weight loss. All evidence suggests that morphology and physical properties of nanocomposites formed from nylon 6 are not measurably affected by the reactions that lead to molecular weight degradation or color formation.     

PbWO4 formation during controlled crystallization of lead borate glasses

Lead borate glasses were prepared and then heat treated in order to obtain transparent glass-ceramics. Controlled crystallization of precursor lead borate glass at appropriate annealing temperature and time led to formation of the PbWO₄ crystallites. The observed broad blue emission band is related to the PbWO₄ crystallites. The influence of PbX₂ content (X=F, Cl, Br), PbF₂ concentration and lanthanide doping (Eu, Dy) on the excitation and emission spectra of lead borate glass-ceramics containing PbWO₄ phase was examined. The relationship between Pb–X bond and spectral line width of the blue emission can be successfully observed, when halogen X ions (X=F, Cl or Br) are also present in the distorted PbWO₄ crystallites.     

Destruction and formation of a conductive carbon nanotube network in polymer melts: In-line experiments

Investigations on electric conductivity and dielectric permittivity have been performed during melt processing of polycarbonate (PC) and polyamide 6 (PA6) containing different amounts of multi-walled carbon nanotubes (MWNT). For the experiments a measurement slit die containing two electrodes in capacitor geometry was flanged to the outlet of a twin-screw extruder. AC conductivity and the related complex permittivity were measured in the frequency range from 21.5 to 10⁶ Hz for different processing conditions (melt temperature and throughput) and after stopping the extruder. It was found that the conductivity dropped down to values typical for the matrix polymer when the extrusion started. After the extruder was stopped the conductivity shows an increase of up to eight orders of magnitude with time. This conductivity recovery in the rest time after mechanical deformation was found to be faster for increasing melt temperature or samples with higher CNT concentration. The increase of the conductivity in the quiescent melt is explained by reorganization of the conductive network-like filler structure, which was - at least partially - destroyed under mechanical deformation. The reformation kinetics of the conductive network after mechanical deformation is considered to be an agglomeration process, which can be approximated by a combination of cluster aggregation and percolation theory.     

Extended-chain and folded-chain crystallite formation during the crystallization of stretched crosslinked rubbers

The strain-induced crystallization phenomenon of a crosslinked polymer network was studied using statistical, thermodynamics. The basic approach was essentially that used by Flory (1), but in this work Flory's assumption of only a single crystalline phase (the extended-chain crystalline structure with chain parallel to stretch direction) was abandoned. The dimension of the crystalline vector-which is assumed to be parallel to the stretch direction and the percent crystallinity is taken as two independent variables instead of being treated as one single variable. A single variable treatment is inherent in the assumption of a single extended-chain crystallite. By use of the present approach, either the folded-chain crystallite or the extended-chain one is found to be thermodynamically stable depending on temperature, stretch ratio and molecular weight. The retractive force of a crystallized polymer network has been calculated. It is shown theoretically that the formation of an extended-chain crystallite will cause the retractive force to decrease, and that the formation of a folded-chain crystallite will cause the retractive force to increase in a reversible crystallization process.     

导电胶固化过程中导电网络形成的机理

研究了环氧树脂导电胶固化过程中电阻的变化与电极之间距离的关系,根据实验结果提出了如下观点：导电胶在固化过程中由不导电变为导电,是由于导电填料颗粒凝聚成为导电团簇并进一步形成导电网络。固化时体积收缩固然对电阻降低有一定贡献,但是和导电团簇形成相比它的重要性是第二位的。银粉颗粒凝聚形成导电网络与吸附在表面上的分散剂有密切的关系。     

Temperature and time dependence of conductive network formation: Dynamic percolation and percolation time

Temperature and time dependence of conductive network formation in vapor-grown carbon fiber (VGCF) filled high-density polyethylene (HDPE)/poly(methyl methacrylate) (PMMA), VGCF and ketjenblack (KB) filled HDPE/isotactic polypropylene (iPP) blends have been investigated. It is found that the filled conductive polymer composites are thermodynamically non-equilibrium systems, in which the conductive network formation is temperature and time dependent, a concept named as dynamic percolation is proposed. When the composites are annealed at a temperature above the melt point of polymer matrix, the dynamic process of conductive network formation can be monitored in a real time way. Such an in situ characterization method provides more interesting information about the dispersion of conductive particles in the polymer matrix. Furthermore, a thermodynamic percolation model is modified to predict the percolation time for VGCF and KB filled HDPE/iPP multi-phase systems during the annealing treatment, and it expresses experimental results well.     

Non-isothermal crystallization kinetics of polypropylene/MAP-POSS nanocomposites

Non-isothermal crystallization kinetics of polypropylene (PP)/methylacryloypropy polyhedral oligomeric silsesquioxanes (MAP-POSS) nanocomposites (PP/MAP-POSS) were investigated by DSC at various cooling rates. Jeziorny and Mo method were used to study the non-isothermal crystallization kinetics. The results show that the Jeziorny and Mo method are all successful in describing the non-isothermal crystallization kinetics of PP/MAP-POSS nanocomposites. The MAP-POSS can act a role of heterogeneous nucleation and increase the crystallization rate constant Z _c and decrease crystallization half time t _1/2, and the spherulite crystal size decreases, the inter-spherulitic action or crosslinking structure each other appear at the appropriate content. The DSC peak temperature T _p increase about 5 °C, t _1/2 reduce 0.21 min at 6 % content of MAP-POSS and heating rate of 10 °C/min. The MAP-POSS can also increase the mechanical property of PP/MAP-POSS nanocomposites, the tensile strength and impact strength increase from 12.97 to 19.93 MPa and from 33.2 to 52.6 kJ/m², respectively, at 4 % content of MAP-POSS. But the spherulitic crystal becomes larger and boundaries become clearer again; the macrophase separation will occur and mechanical properties decrease when more and more MAP-POSS was added. The nanocomposite has the best mechanical property at 4 % content of MAP-POSS.     

Morphology and crystallization behavior of the PP/PET nanocomposites

Nanocomposites containing polypropylene (PP), PET, and montmorillonite were prepared in a twin‐screw extruder. X‐ray diffraction, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, polarized optical microscopy, and differential scanning calorimetry were used to characterize the samples. Intercalated and exfoliated morphology were observed in the nanocomposites. The PET domains usually presented spherical shapes and they were the start point to PP crystallization. The average diameter and number of PET domains was evaluated. The influence of addition of PP‐MA as compatibilizer on PP/PET was investigated. The interconnected morphology was observed in the nanocomposite containing PP‐MA. The clay located predominantly in the interphase and in the PET phase. The crystallization process was monitored and the PET crystallization rate was slower in the nanocomposites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Polyhedral oligomeric silsesquioxane polyimide nanocomposites for color filters and flexible conductive films

Color filters and conductive films are widely used in spacecraft, while the lack of lightweight, flexibility, and atomic oxygen (AO) durability confine their potential applications in low earth orbit. In this study, a clear poly(amic acid) with an empirical 20 wt% polyhedral oligomeric silsesquioxane (POSS) solid content is designed for transparency and AO durability. Red, green, blue, and yellow dyes are reinforced with small amounts of 1–5 wt% in POSS polyimides for color filters. A silver nanowire network film is infiltrated onto the POSS polyimide for conductive film. Erosion depth upon hyperthermal AO exposure, surface morphology, surface chemistry, optical transparency, and conductivity have been systematically investigated. The erosion yields of all 20 wt% POSS polyimides decrease by an order of magnitude when subjected to 2.32 ± 0.05 and 2.39 ± 0.06 × 10²⁰ atoms cm⁻² AO fluences, as passivating SiO_x networks are formed on film surface. The small-amount dye additives into polyimides do not introduce obvious changes in AO durability and surface chemistry. The silver nanowire infiltrated POSS polyimide film shows a 65.7% transmittance at 550 nm and a sheet resistance of 8.50 ± 0.36 Ohm square⁻¹. This study reveals promising attempts of POSS-polyimide-based color filters and flexible conductive films for potential space applications.     

Curing dynamics and network formation of cyanate ester resin/polyhedral oligomeric silsesquioxane nanocomposites

Curing dynamics and network formation of cyanate ester resin (PT-30)/TriSilanolPhenyl polyhedral oligomeric silsesquioxane (POSS) nanocomposites were studied by means of differential scanning calorimetry (DSC), modulated temperature differential scanning calorimetry MTDSC), Fourier transform infrared (FTIR) and Raman spectroscopies. The incorporation of the POSS showed a strong catalytic effect (decrease in curing temperature and activation energy) on the curing reaction of PT-30. The activation energy of the PT-30 decreased with increasing POSS content. The most effective catalytic effect was observed at 5 wt% of the POSS. Both FTIR and Raman spectra monitored the formation of triazine (i.e. cyanurate) ring in the PT-30 and its nanocomposites with the POSS. Raman spectra revealed that the PT-30 resin preferentially reacted with -OH group in the POSS firstly to form a -O-(CNH)-O- bond, rather than react with itself to form the triazine rings, during the network formation of the PT-30/POSS nanocomposites. The strong catalytic effect of the POSS on the curing process of the PT-30 appears to be due to the formation of this -O-(CNH)-O- bond.     

Fatgiue behavior of metal matrix nanocomposites

Light-weight high-strength composites (particularly with aluminum and magnesium base alloys) have principal applications in a wide variety of fields ranging from automotive and aerospace structures to medical and energy applications wherein the materials undergo both static and dynamic (fatigue) loading conditions. Conventional metal matrix composites (MMCs), i.e. those filled by micro-sized reinforcements, have usually poor ductility and insufficient mechanical performance made them, therefore, unreliable to be used in some critical applications. Instead, those composites strengthened by nano-sized reinforcing agents, namely metal matrix nano-composites (MMNCs), have newly been developed in order to boost the mechanical properties. The current paper aims to study the fatigue behavior of the MMCs with a particular attention on recent investigations made on MMNCs. It is believed that the materials selection, microstructural features, manufacturing and processing parameters, etc. have a dominant influence on the fatigue response of MMNCs.