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     

Nanotube foam prepared by gelatin gel as a template

It has been known that simply mixing single-walled carbon nanotubes (SWCNTs) in an aqueous solution of gelatin disperses SWCNTs well for a period of months. Gels made from the gelatin-SWCNT mixture are also stable and have a clear, black color. Scanning electron microscopy shows that gelatin molecules self-organize into a foamlike structure. All SWCNTs are embedded in the gelatin film that makes up the foam walls. Those SWCNTs belonging to one face of the foam cell cannot approach other faces to make van der Waals contact. Thus, the foam structure is associated with stabilization of the SWCNT dispersion. The gelatin can be removed thermally while maintaining the foam structure to give a sponge made of nanotube foams. This highly porous solid is electrically conducting and mechanically stable and can be used as a structural frame for composite materials.     

Processing and Final Properties Improvement of Polyolefin-Sepiolite and Carbon Nanofibre Nanocomposites

Summary: In this work polypropylene (PP) nanocomposites with different nanofillers (sepiolites and carbon nanofibres) have been produced, processed by injection moulding and fibre spinning and analyzed in terms of mechanical properties improvements. Different concentrations of both fillers were used in nanocomposites preparation. The influence of nanofiller type and amount on mechanical properties were analyzed and discussed for each process studied. This study was completed with a basic morphological characterization in order analyze the nanofiller dispersion, distribution and orientation in the nanocomposites. The results achieved show that it is possible to obtain a good dispersion and distribution of the each kind of nanofillers with conventional processing methodologies when the nanofiller concentration is small. Moreover the nanocomposites obtained had better properties than the starting polymers, showing that sepiolite and carbon nanofiller are able to provide an important contribution to the improvement of mechanical properties of the materials analyzed, enlarging the final application possibilities of PP based products.     

Investigation of thermally stimulated charge relaxation mechanism in SiO<Subscript>2</Subscript> filled polycarbonate nanocomposites

The thermally stimulated charge relaxation properties of polycarbonate (PC) filled with SiO₂ nanofiller were studied by means of thermally stimulated discharge current (TSDC). The nanocomposite samples were further characterized by UV–vis spectroscopy, scanning electron microscopy, energy dispersive X-ray spectra, and differential scanning calorimetry (DSC) techniques to investigate the dispersion of nanofillers in polymer matrix and glass transition temperature. All pristine and nanocomposites samples of thickness about 25 μm were prepared using solution mixing method. The suitable weight percentage of SiO₂ nanofillers has been chosen to prevent the nonuniform dispersion. TSDC measurement of PC (Pristine) and PC+ (7% SiO₂) shows the single peak, while TSDC characteristic of other nanocomposites are showing two peaks. The higher temperature TSDC peak of pristine and nanocomposites samples is originated due to the charge relaxation from shallower and deeper trapping sites, however, low temperature peak is caused by dipolar relaxation of charge carriers. Since the position of higher temperature TSDC peak is generally an analysis of glass transition temperature of polymer/polymer nanocomposites. The authors have observed that the temperature of this peak is almost same as the T _g measured by DSC with 0 to ±5% variation. This article presents the deeper understanding of charge relaxation mechanism caused by SiO₂ nanofillers in polycarbonate.     

Preparation and dielectric properties of polyimide foams containing crosslinked structures

In order to obtain cellular materials with low dielectric properties, crosslinked polyimide foams were prepared using 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA), 4,4′‐oxydianiline (ODA) and 2,4,6‐triaminopyrimidine (TAP) as monomer via a poly(ester‐amine salt) precursor process. The structures of the precursors and the polyimide foams were characterized by thermogravimetric analysis (TGA) and FT‐IR, while the morphologies of the polyimide foams were viewed from scanning electron microscopy (SEM) measurements. The results revealed that the poly(ester‐amine salt) precursor containing TAP could successfully be converted to a crosslinked polyimide foam with relatively uniform cell structure. Also, the crosslinking of TAP improved the mechanical properties of foams in comparison with the non‐crosslinking systems. With increasing content of TAP, the dielectric constants of the polyimide foams decreased gradually. For the foam with TAP molar ratio at 15%, the dielectric constant was as low as 1.77 at the frequency of 10 kHz. Though the thermal resistance decreased slightly for crosslinked foams, the decomposition temperatures were still maintained above 520°C. Copyright © 2006 John Wiley & Sons, Ltd.     

Evidence of a monoclinic-like amorphous phase in composites of LDPE with spherical, fibrous and laminar nanofillers as studied by infrared spectroscopy

Well-dispersed nanocomposites of LDPE with spherical silica and laminar and fibrous silicates have been prepared by melt compounding with nanofiller compositions ranging from 50 to 5 wt%. Spatial dispersion and size domains of the aggregates in the composites series have been evaluated by electron microscopies (SEM and TEM). The methylene rocking (700-740 cm⁻¹) and bending (1400-1480 cm⁻¹) modes of LDPE in these composites were studied by FTIR spectroscopy. When the nanofiller is present in a 40 or 50 wt%, the amorphous phase of polyethylene adopts a monoclinic arrangement. This arrangement is due to the confinement induced by the nanofillers on the polymer matrix. When the fibrous silicate or the spherical silica are used as nanofillers, a dilution of the concentrated composite or annealing bring about a relaxing of the amorphous structure of the polymer, and the monoclinic-like conformations disappear. When the nanofiller is a laminar silicate, dilution or annealing only partly eliminate the monoclinic-like structure, which remains in all cases an important fraction of the polymeric matrix.     

Open-pore biodegradable foams prepared via gas foaming and microparticulate templating

Open-pore biodegradable foams with controlled porous architectures were prepared by combining gas foaming and microparticulate templating. Microparticulate composites of poly(epsilon-caprolactone) (PCL) and micrometric sodium chloride particles (NaCl), in concentrations ranging from 70/30 to 20/80 wt.-% of PCL/NaCl were melt-mixed and gas-foamed using carbon dioxide as physical blowing agent. The effects of microparticle concentration, foaming temperature, and pressure drop rate on foam microstructure were surveyed and related to the viscoelastic properties of the polymer/microparticle composite melt. Results showed that foams with open-pore networks can be obtained and that porosity, pore size, and interconnectivity may be finely modulated by optimizing the processing parameters. Furthermore, the ability to obtain a spatial gradient of porosity embossed within the three-dimensional polymer structure was exploited by using a heterogeneous microparticle filling. Results indicated that by foaming composites with microparticle concentration gradients, it was also possible to control the porosity and pore-size spatial distribution of the open-pore PCL foams.     

Real Time Monitoring of Morphologic and Mechanical Properties of Polymer Nanocomposites During Extrusion by near Infrared and Ultrasonic Spectroscopy

Extrusion is one of the most applied technologies for the processing of polymer nanocomposites for applications in automotive, electrical and packaging industrial sectors. These nanostructured materials have advantages in comparison to traditional polymer materials, so that properties like tensile strength and modulus, barrier and surface properties, electrical properties and flame retardancy will be improved. There is a need to control amount and dispersion of the nanofillers in the polymer matrix during melt processing and to control the influence of the processing conditions on the nanocomposite formation. For an adequate real time characterization it is necessary to measure directly in the extruder. Spectroscopic methods and Ultrasonic measurements are outstanding methods for this kind of in-line monitoring. This paper deals with the real time determination of the dispersion and the impact strength of polymer nanocomposites in the melt during extrusion by Ultrasonic measurements and NIR spectroscopy. These in-line measurements were correlated with off-line rheological measurements, transmission electron microscopy and mechanical test measurements by multivariate data analysis. The polymers used are polypropylene and polyamide 6. As nanofillers we used different modified layered silicates. We determined the degree of exfoliation as an indicator for the dispersion of the nanofiller in the polymer matrix for different layered silicates and at different process conditions.     

Synthesis and characterization of polyethylene/clay–silica nanocomposites: A montmorillonite/silica‐hybrid‐supported catalyst and in situ polymerization

A novel approach to the preparation of polyethylene (PE) nanocomposites, with montmorillonite/silica hybrid (MT‐Si) supported catalyst, was developed. MT‐Si was prepared by depositing silica nanoparticles between galleries of the MT. A common zirconocene catalyst [bis(cyclopentadienyl)zirconium dichloride/methylaluminoxane] was fixed on the MT‐Si surface by a simple method. After ethylene polymerization, two classes of nanofillers (clay layers and silica nanoparticles) were dispersed concurrently in the PE matrix and PE/clay–silica nanocomposites were obtained. Exfoliation of the clay layers and dispersion of the silica nanoparticles were examined with transmission electron microscopy. Physical properties of the nanocomposites were characterized by tensile tests, dynamic mechanical analysis, and DSC. The nanocomposites with a low nanofiller loading (<10 wt %) exhibited good mechanical properties. The nanocomposite powder produced with the supported catalyst had a granular morphology and a high bulk density, typical of a heterogeneous catalyst system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 941–949, 2004     

Is latex surface charge an important parameter for foam stabilization?

We describe the facile production of highly stable foams stabilized solely by cationic polystyrene latex particles. Three model polystyrene latexes were synthesized using either a cationic 2,2'-azobis(2-diisobutyramidine) dihydrochloride (AIBA) or an anionic ammonium persulfate (APS) radical initiator: a 724 +/- 81 nm charge-stabilized cationic polystyrene latex [AIBA-PS], an 800 +/- 138 nm sterically stabilized cationic latex prepared using a poly(ethylene glycol) monomethacrylate macromonomer [PEGMA-AIBA-PS], and a 904 +/- 131 nm charge-stabilized anionic polystyrene latex [APS-PS], respectively. The effect of particle surface charge, latex concentration, and solution pH on foam stability was studied in detail. The PEGMA-AIBA-PS latex proved to be the best foam stabilizer even at relatively low latex concentrations (3.0 wt %), with long-term foam stabilities being obtained after drying. The AIBA-PS latex also produced stable foams, albeit only at higher latex concentrations. However, the APS-PS latex proved to be an ineffective foam stabilizer. This is believed to be primarily due to the anionic surface character of this latter latex, which prevents its adsorption at the anionic air-water interface. This hypothesis is supported by the observation that the AIBA-PS latex no longer acts as an effective foam stabilizer above its isoelectric point (pH 7.04). Scanning electron microscopy studies revealed the formation of well-defined latex bilayers within dried foams, which indicates that the wet air bubbles are stabilized by latex monolayers prior to drying. However, little or no long-range ordering of the latex particles was observed on the surface of the bubbles, which is presumably related to the latex polydispersity.     

Control of the morphology of electrodeposited three-dimensional copper foam by tuning the pressure

Three-dimensional(3D) copper foams have been formed by electrodeposition at different nitrogen pressures and examined by scanning electron microscopy.The results indicate that an increase in system pressure leads to a decrease of the pore size of the copper foam due to the suppressed coalescence of hydrogen bubbles,while the thickness of the copper foam increases with decreasing pressure.Also,the walls around the pores on the copper foam consist of copper dendrites,and the copper dendrites are made up of copper grains with sizes less than 1 μm.The average sizes of the copper grains decrease with increasing system pressure.It has been demonstrated that copper foams with controllable 3D structure formed by electrodeposition at different pressures are comparable to those obtained by electrodeposition at normal pressure in the presence of specific additives.     

Mixed Dimensional Nanostructure (UiO-66-Decorated MWCNT) as a Nanofiller in Mixed-Matrix Membranes for Enhanced CO2/CH4 Separation

Mixed-matrix membranes (MMMs) with combination of two distinct dimensional nanofillers (such as 1D-3D, 2D-3D, or 3D-3D, etc.) have drawn special attention for gas separation applications due to their concerted effects on gas permeation and mechanical properties. An amine-functionalized 1D multiwalled carbon nanotube (NH₂-MWCNT) with exceptional mechanical strength and rapid gas transport was crosslinked with an amine-functionalized 3D metal-organic framework (UiO-66-NH₂) with high CO₂ affinity in a Schiff base reaction. The resultant crosslinked mixed-dimensional nanostructure was used as a nanofiller in a polysulfone (PSf) polymer matrix to explore the underlying synergy between 1D and 3D nanostructures on the gas separation performance of MMMs. Cross-sectional scanning electron microscopy and mapping revealed the homogenous dispersion of UiO-66@MWCNT in the polymer matrix. The MMM containing 5.0 wt. % UiO-66@MWCNT demonstrated a superior permeability 8.3 Barrer as compared to the 4.2 Barrer of pure PSf membrane for CO₂. Moreover, the selectivity (CO₂/CH₄) of this MMM was enhanced to 39.5 from the 28.0 observed for pure PSf under similar conditions of pressure and temperature.     

Preparation of magnetic polyurethane rigid foam nanocomposites

Super paramagnetic Fe₃O₄@SiO₂ nanoparticle was incorporated into polyurethane rigid foams in order to prepare new corresponded magnetic nanocomposite foams via one-shot method. The core–shell-structured nanoparticles were prepared by sol–gel method and characterized by transmission electron microscopy, X-ray diffraction, as well as Fourier transform infrared spectroscopy techniques. Magnetic nanoparticles were used up to 3 % in the foam formulations and the samples prepared successfully. Thermal, mechanical, and magnetic properties of nanocomposites were studied and the results showed superior properties in comparison with pristine foams.     

Improving piezoelectric and pyroelectric properties of electrospun PVDF nanofibers using nanofillers for energy harvesting application

In this study, it was aimed to increase the piezoelectric and pyroelectric properties of electrospun polyvinylidene fluoride (PVDF) nanofibers simultaneously by using specific nanofillers. Graphene oxide (GO), graphene, and halloysite nanotubes with different concentrations (0, 0.05, 0.4, and 1.6% wt/wt) were combined with PVDF solution and were fabricated in the form of nanofibers through electrospinning. Pyroelectric properties of samples were measured by submerging sealed samples in hot water (360°K) and ice (270°K). The piezoelectric properties of the samples were evaluated through bending tests. The microstructural, mechanical, and thermal properties of the electrospun PVDF nanocomposite were investigated using scanning electron microscope, Instron instrument, and thermogravimetric analysis, respectively. To further support the experimental observations for generating electric voltage in the bended nanogenerator, the PVDF nanogenerator (PNG) was also modeled by a finite element analysis based on the theory of linear piezoelectricity using COMSOL Multiphysics simulation software. Experimental results showed that adding nanofillers could improve the piezoelectric and pyroelectric properties of all samples, associated with the increment of β‐phase in the nanofibers. It was concluded that adding nanofillers could increase pyroelectricity about 50% more than piezoelectricity in pristine PVDF nanofiber web. The PNG containing 1.6 wt% GO showed the highest efficiency in terms of piezoelectricity and pyroelectricity. In addition, the results showed that the ratio of piezoelectric to pyroelectric coefficients was constant (~1.5) and it was independent of the nanofiller type and content. The effect of external force and vibration frequency on the output voltage was also investigated. Increasing the compressive force and vibration frequency caused a greater output voltage. Finally, the fabricated nanogenerator was integrated on insole and elbow to investigate its energy harvesting capabilities from body movement.     

Study of slabstock flexible polyurethane foams based on varied toluene diisocyanate isomer ratios

The morphological features of three flexible slabstock polyurethane foams based on varied contents of 2,4 and 2,6 toluene diisocyanate (TDI) isomers are investigated. The three commercially available TDI mixtures, that is, 65:35 2,4/2,6 TDI, 80:20 2,4/2,6 TDI, and 100:0 2,4/2,6 TDI were used. The foams were characterized at different length scales with several techniques. Differences in the cellular structure of the foams were noted with scanning electron microscopy. Small‐angle X‐ray scattering was used to demonstrate that all three foams were microphase‐separated and possessed similar interdomain spacings. Transmission electron microscopy revealed that the aggregation of the urea phase into large urea‐rich regions decreased systematically on increasing the asymmetric TDI isomer content. Fourier transform infrared spectroscopy showed that the level of bidentate hydrogen bonding of the hard segments increased with the 2,6 TDI isomer content. Differential scanning calorimetry and dynamic mechanical analysis (DMA) were used to note changes in the soft‐segment glass‐transition temperature of the foams on varying the diisocyanate ratios and suggested that the perfection of microphase separation was enhanced on increasing the 2,6 TDI isomer content. The preceding observations were used to explain why the foam containing the highest content of the symmetric 2,6 TDI isomer exhibited the highest rubbery storage modulus, as measured by DMA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 258–268, 2003     

Fabrication and characterization of biodegradable poly(butylene succinate-co-adipate) nanocomposites with halloysite nanotube and organo-montmorillonite as nanofillers

Biodegradable poly(butylene succinate-co-adipate) (PBSA)-based nanocomposites were successfully prepared. A commercial halloysite nanotube (HNT) and an organo-montmorillonite (denoted as 15A) served as reinforcing fillers. Scanning electron microscopy and transmission electron microscopy results confirmed the nano-scale dispersion of HNT and 15A in the composites. Differential scanning calorimetry results showed that 15A served as nucleating agent for PBSA crystallization, but HNT hardly affected the nucleation of PBSA. Both nanofillers assisted the isothermal crystallization of PBSA, with 15A demonstrating superior efficiency. Melting behavior study suggests that the presence of HNT or 15A hampered the melting-recrystallization process of the originally less stable crystals during heating scans. Thermogravimetric analyses revealed that 15A enhanced the thermal stability of PBSA in air environment, but HNT caused a decline at high loadings. The rigidity of PBSA, including Young’s/flexural moduli, evidently increased after the addition of HNT or 15A, with 15A showing higher enhancing efficiency than HNT at similar loadings. The flexural modulus increased up to 94% with 20 wt% in HNT and up to 48% with 5 wt% 15A loading. The rheological property measurements confirmed the achievement of pseudo-network structure at 5 wt% 15A loading, whereas the HNT-included system did not develop a network structure.     

Multifunctional elastomer nanocomposites with functionalized graphene single sheets

We demonstrate the use of functionalized graphene sheets (FGSs) as multifunctional nanofillers to improve mechanical properties, lower gas permeability, and impart electrical conductivity for several distinct elastomers. FGS consists mainly of single sheets of crumbled graphene containing oxygen functional groups and is produced by the thermal exfoliation of oxidized graphite (GO). The present investigation includes composites of FGS and three elastomers: natural rubber (NR), styrene–butadiene rubber, and polydimethylsiloxane (PDMS). All of these elastomers show similar and significant improvements in mechanical properties with FGS, indicating that the mechanism of property improvement is inherent to the FGS and not simply a function of chemical crosslinking. The decrease in gas permeability is attributed to the high aspect ratio of the FGS sheets. This creates a tortuous path mechanism of gas diffusion; fitting the permeability data to the Nielsen model yields an aspect ratio of ～1000 for the FGS. Electrical conductivity is demonstrated at FGS loadings as low as 0.08% in PDMS and reaches 0.3 S/m at 4 wt % loading in NR. This combination of functionalities imparted by FGS is shown to result from its high aspect ratio and carbon‐based structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Urea hard segment morphology in flexible polyurethane foam

A series of flexible polyurethane slabstock foam samples were prepared with varying water content and studied using transmission electron microscopy (TEM), video-enhanced optical microscopy (VEM), and small-angle X-ray scattering (SAXS). A new TEM sample preparation technique was developed in which the foam is impregnated with water, frozen, and microtomed, and the polyether soft segment is selectively degraded in the electron beam. Structures of two size scales were detected. A texture with grains (“urea aggregates”) 50–200 nm in size was imaged using both VEM and low-magnification TEM for foams with formulations containing more than 2 pphp water. For the first time, images of urea hard segment microdomains in polyurethane foam (approximately 5 nm in size) were obtained using high-magnification TEM. A microdomain spacing of approximately 6–8 nm was estimated from the SAXS scattering profiles. Glycerol was added to one of the formulations in order to modify the urea microphase separation and to give insight into morphology development in molded polyurethane foam systems. No structure was observed in low-magnification TEM images of the glycerol-modified foam, although smaller structures (hard segments) were detected at high magnification and by SAXS. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 573–581, 1998     

Impact of coated calcium carbonate nanofillers and annealing treatments on the microstructure and gas barrier properties of poly(lactide) based nanocomposite films

Amorphous poly(lactide) (PLA) and nanocomposite films were prepared from melt‐blending with precipitated calcium carbonate nanofillers (PCC). Nanocomposites based on uncoated PCC (PCC‐UT), stearic acid coated PCC (PCC‐S), and poly(ε‐caprolactone) coated PCC (PCC‐P) were investigated for an inorganic content fixed to 8 wt %. Using coated nanofillers allowed preserving both PLA average molar mass and thermal stability while enhancing the nanofiller dispersion state. Poly(ε‐caprolactone) was identified as the best coating for optimized morphology and thermal properties. Maxwell law accurately described the increase in oxygen barrier properties observed for the nanocomposites based on PCC‐S. A modified Maxwell law was proposed to take account of the additional increase in barrier properties evidenced for the PLA/PCC‐P nanocomposites and assigned to the particularly strong compatibility between PCL and PLA. Different annealing conditions were investigated to respectively study the impact of physical ageing and PLA crystallization on gas permeability. Different extents of physical ageing did not significantly modify the oxygen transport properties. However, a high permeability decrease was observed for the semicrystalline nanocomposites with respect to the amorphous reference PLA film. Finally, the gain in barrier properties was shown to result from both contribution of the nanofillers and the crystalline phase. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 649–658     

Role of interface in dispersion and surface energetics of polymer nanocomposites containing hydrophilic POSS and layered silicates

Three different hydrophilic nanofillers--natural and synthetic layered silicate as well as octaammonium polyhedral oligomeric silsesquioxane (POSS)--were incorporated into polyamide-6 by a solution-mixing method. The surfaces of the resulting polymer nanocomposites were characterized by X-ray diffraction, polarized optical microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and contact angle measurements. All polymer nanocomposites displayed enhancement in surface hydrophilicity as well as increase in surface free energy due to surface enrichment of the nanofillers. The degree of enhancement was found to depend on both nanofiller type and dispersion state. Interfacial interactions in the form of hydrogen bonding played an important role in affecting the dispersion state of the layered silicates. Exfoliated layered silicates caused a larger increase in hydrophilicity than aggregated layered silicate. On the other hand, aggregated POSS molecules were able to induce a large increase in hydrophilicity. Significant spreading of water was also observed on surfaces containing POSS molecules. Surface models have been proposed to explain these phenomena.