Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites

Linear low‐density polyethylene (LLDPE)/multiwalled carbon nanotube (MWNT) nanocomposites were prepared via melt blending. The morphology and degree of dispersion of nanotubes in the polyethylene matrix were investigated using scanning electron microscopy (SEM). Both individual and agglomerates of MWNTs were evident. The rheological behavior and mechanical and electrical properties of the nanocomposites were studied using a capillary rheometer, tensile tester, and Tera ohm‐meter, respectively. Both polyethylene and its nanocomposites showed non‐Newtonian behavior in almost the whole range of shear rate. Addition of carbon nanotubes increased shear stress and shear viscosity. It was also found that the materials experience a fluid‐solid transition below 1 wt% MWNT. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. With increasing nanotube content, the activation energy of flow increases. A decrease of about 7 orders of magnitude was obtained in surface and volume resistivity upon addition of 5 wt% MWNT. In addition, a difference between electrical and rheological percolation thresholds was observed. The results confirm the expected nucleant effect of nanotubes on the crystallization process of polyethylene. A slight increase in Young's modulus was also observed with increasing MWNT content.     

Rheological and Mechanical Characterization of Multi-Walled Carbon Nanotubes/Polypropylene Nanocomposites

The rheology and morphology of multi-walled carbon nanotube (MWNT)/polypropylene (PP) nanocomposites prepared via melt blending was investigated. The minor phase content of MWNT varied between 0.25 and 8 wt%. From morphological studies using a scanning electron microscopy technique a good dispersion of carbon nanotubes in the PP matrix was observed. The rheological studies were performed by a capillary rheometer, and mechanical properties of the nanocomposites were studied using a tensile and flexural tester. Both PP and its nanocomposites showed non-Newtonian behavior. At low shear rates the addition of MWNT content causes an increase in viscosity; however, viscosity is less sensitive to addition of MWNT content at higher shear rates. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. From this calculation it was concluded that the temperature sensitivity of nanocomposites was increased by increasing of nanotube content. An increase in tensile and flexural moduli and Izod impact strength was also observed by increasing the MWNT content. From rheological and mechanical tests it was concluded that the mechanical and rheological percolation threshold is at 1.5 wt%.     

A fluorescence study for the critical behavior of polymethylmethacrylate doped by multiwalled carbon nanotube (PMMA–MWNT) composite bulk gel systems

Polymethylmethacrylate (PMMA) doped by multiwalled carbon nanotube (MWNT) bulk gels were prepared with different amounts of MWNTs varying in the range between 1 and 20 wt%. Free-radical cross-linking copolymerization of PMMA–MWNT composite bulk gels was characterized by the steady-state fluorescence technique. Ethylene glycol dimethacrylate (EGDM) and pyrene (P _y ) were introduced as cross linker and fluorescence probe, respectively. Changes in the viscosity of the pregel solutions due to glass formation dramatically increased the P _y fluorescent intensities, which were used to study the glass transition of PMMA–MWNT composite gels for various MWNT contents. The fluorescence intensity of P _y is proportional to the average size of the glassy regions below and to the strength of the infinite network above the glass transition point. Observations around the glass transition point show that there are three regimes for MWNT concentration in which the gel fraction exponent, β, and the weight average degree of polymerization exponent, γ, differ drastically from percolation results, given in three dimensions as β=0.41 and γ=1.80.     

Electrospun PLA: PCL composites embedded with unmodified and 3-aminopropyltriethoxysilane (ASP) modified halloysite nanotubes (HNT)

Electrospinning is a simple and versatile fiber synthesis technique in which a high-voltage electric field is applied to a stream of polymer melt or polymer solution, resulting in the formation of continuous micro/nanofibers. Halloysite nanotubes (HNT) have been found to achieve improved structural and mechanical properties when embedded into various polymer matrices. This research work focuses on blending poly(ε-caprolactone) (PCL) (9 and 15 wt%/v) and poly(lactic acid) (PLA) (fixed at 8 wt%/v) solutions with HNT at two different concentrations 1 and 2 wt%/v. Both unmodified HNT and HNT modified with 3-aminopropyltriethoxysilane (ASP) were utilized in this study. Fiber properties have been shown to be strongly related to the solution viscosity and electrical conductivity. The addition of HNT increased the solution viscosity, thus resulting in the production of uniform fibers. For both PCL concentrations, the average fiber diameter increased with the increasing of HNT concentration. The average fiber diameters with HNT-ASP were reduced considerably in comparison to those with unmodified HNT when using 15 wt%/v PCL. Slightly better dispersion was obtained for PLA: PCL composites embedded with HNT-ASP compared to unmodified HNT. Furthermore, the addition of HNT-ASP to the polymeric blends resulted in a moderate decrease in the degree of crystallinity, as well as slight reductions of glass transition temperature of PCL, the crystallization temperature and melting temperature of PLA within composite materials. The infrared spectra of composites confirmed the successful embedding of HNT-ASP into PLA: PCL nanofibers relative to unmodified HNT due to the premodification using ASP to reduce the agglomeration behavior. This study provides a new material system that could be potentially used in drug delivery, and may facilitate good control of the drug release process.     

Morphological,Thermal, and Electrical Characterization of Syndiotactic Polypropylene/Multiwalled Carbon Nanotube Composites

In this work, syndiotactic polypropylene/multiwalled carbon nanotubes (MWCNT) nanocomposites, in various concentrations, were produced using melt mixing. The influence of the addition of MWCNT on the morphology, crystalline form, and the thermal and electrical properties of the polymer matrix was studied. To that aim, scanning electron microscopy, Raman spectroscopy, X-ray diffraction, differential scanning calorimetry, and dielectric relaxation spectroscopy were employed. Significant alterations of both the crystallization behavior and the thermal properties of the matrix were found on addition of the carbon nanotubes: conversion of the disordered crystalline form I to the ordered one, increase of the crystallization temperature and the degree of crystallinity, and decrease of the glass transition temperature and the heat capacity jump. Finally, the electrical percolation threshold was found between 2.5–3.0 wt.% MWCNT. For comparison purposes, the results of the system studied here are also correlated with the findings from a previous work on the isotactic polypropylene/MWCNT system.     

Steady Shear Rheological Behavior,Mechanical Properties,and Morphology of the Polypropylene/Carbon Nanotube Nanocomposites

Nanocomposites based on polypropylene (PP) and multiwall carbon nanotubes (MWNT) have been prepared through melt blending. Scanning electron microscopy (SEM) observations indicate that nanotubes were dispersed almost homogeneously throughout the matrix; however, some aggregates were also observed at high nanotubes loading. Rheological studies showed that at low shear rates, there is an increase in steady shear viscosity and shear stress of samples with increasing of nanotubes concentration. However, at high shear rates nanocomposites behave like pure PP. The activation energy of flow showed an increasing trend and has a maximum at 1wt% MWNT content. It was found that incorporation of nanotubes causes a remarkable decrease in surface and volume resistivity values of the polymeric matrix. The presence of CNTs improved the tensile and flexural properties of the polymeric matrix.     

Atomic mechanism of vitrification process in simple monatomic nanoparticles *

Glass formation in simple monatomic nanoparticles has been studied by molecular dynamics simulations in spherical model with a free surface. Models have been obtained by cooling from the melt toward glassy state. Atomic mechanism of glass formation was monitored via spatio-temporal arrangement of solid-like and liquid-like atoms in nanoparticles. We use Lindemann freezing-like criterion for identification of solid-like atoms which occur randomly in supercooled region. Their number grows intensively with decreasing temperature and they form clusters. Subsequently, single percolation solid-like cluster occurs at temperature above the glass transition. Glass transition occurs when atoms aggregated into this single percolation cluster are in majority in the system to form relatively rigid glassy state. Solid-like domain is forming in the center of nanoparticles and grows outward to the surface. We found temperature dependence of potential energy, mean-squared displacement (MSD) of atoms, diffusion constant, incoherent intermediate scattering function, radial distribution function (RDF), local bond-pair orders detected by Honeycutt-Andersen analysis, radial density profile and radial atomic displacement distributions in nanoparticles. We found that liquid-like atoms in models obtained below glass transition have a tendency to concentrate in the surface layer of nanoparticles. However, they do not form a purely liquid-like surface layer coated nanoparticles.     

In-situ elastomer composite based on fluorocarbon elastomer/liquid crystalline polymer blend

Blends of fluorocarbon elastomer (FKM) and liquid crystalline polymer (LCP) have been prepared by melt mixing technique. Processing studies indicated the decrease in the viscosity and the state of cure with the addition of 10 wt% LCP, and then increased at a higher rate with the addition of more LCP to the blend. The tensile strength values decreased at lower level of LCP. However, the modulus and tear strength values increased with higher increment of LCP content. From the X-ray diffraction measurements it has been observed that the crystalline structure of the FKM is greatly affected by the addition of LCP. The degradation temperatures from thermogravimetric analysis (TGA) suggested improved thermal stability of the fluorocarbon-LCP blends. From the dynamic mechanical analysis (DMA), it has been found that the glass transition temperature (T _g) of the blends increased with increase in LCP content. For the compositions of 10 wt% and 20 wt% LCP blends, enhancement in storage modulus is found above the glass transition of FKM. Under dynamic conditions the increase of LCP content restricts the matrix flow and hence cracks developed at the interface of the LCP fibrils and matrix.     

Fabrication and Properties of Carbon Nanotube and Poly(vinyl alcohol) Composites

Dispersion of carbon nanotubes in a polymer matrix is one of the most critical issues in carbon nanotube/polymer composites. In this paper we discuss the considerable improvement in the dispersion of multiwalled carbon nanotubes (MWNTs) in poly(vinyl alcohol) (PVA) matrix that was attained through gum arabic treatment. The mechanical properties of these MWNT/PVA composites show that only 2 wt% nanotube loading increases the tensile modulus by more than 130%.     

Carbon nanotube/polyaniline nanocomposites and their electrorheological characteristics under an applied electric field

A conducting polyaniline (PANI) was synthesized via an oxidative dispersion polymerization technique, using poly(vinyl alcohol) (PVA) as a polymeric stabilizer, in the presence of multi-walled carbon nanotubes (MWNT) purified in acidic solution, and dispersion stability of the MWNT in an aqueous solution of PVA was studied for different PVA concentrations. Their morphology was confirmed by a scanning electron microscope. Its electrorheological (ER) characteristics were also investigated by dispersing the PANI/MWNT composite particles in an insulating silicone oil. Its ER properties were examined using a rotational rheometer under varying applied DC electric field strengths, in which the ER fluid is generally composed of a suspension of conducting particles dispersed in an insulating fluid, which shows a rapid and reversible change in shear viscosity with an applied electric field. Synthesized PANI/MWNT composite particles are observed to enhance interparticular interactions, since the degree of polarization of PANI/MWNT composite particle increases with applied electric field strengths. The shear stresses of the PANI/MWNT nanocomposite based ER fluid increase with the electric field strength for a broad range of shear rates.     

Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes

Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM–MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM–MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point, t _gel for PAM–MWNTs composite gels showed that the gel fraction exponent β obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM–MWNT gel was also measured and found to be about 2.0, which agrees with a random resistor network.     

Oxygen Diffusion into Multiwalled Carbon Nanotube Doped Polystrene Latex Films Using Fluorescence Technique

This study examines the oxygen diffusion into polystyrene (PS) latex/multiwalled carbon nanotube (MWNT) nanocomposite films (PS/MWNT) consisting of various amounts of MWNT via steady state fluorescence technique (SSF). PS/MWNT films were prepared from the mixture of MWNT and pyrene (P)-labeled PS latexes at various compositions at room temperature. These films were then annealed at 170 °C above glass transition (T_g) temperature of PS. Fluorescence quenching measurements were performed for each film separately to evaluate the effect of MWNT content on oxygen diffusion. The Stern-Volmer equation for fluorescence quenching is combined with Fick’s law for diffusion to derive the mathematical expressions. Diffusion coefficients (D) were produced and found to be increased from 1.1?×?10^?12 to 41?×?10^?12 cm²s^?1 with increasing MWNT content. This increase was explained via the existence of large amounts of pores in composite films which facilitate oxygen penetration into the structure.     

Glass Transition Behavior and Dynamic Fragility of PMMA-SAN Miscible Blend-Clay Nanocomposites

An investigation of the segmental dynamics and glass transition behavior of a miscible polymer blend composed of poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) and its melt intercalated nanocomposite by dynamic mechanical analysis is presented. The principle goal was to address the effect of intercalation on local molecular structure and dynamics. The results showed that the intercalation of polymer chains in the galleries of organoclay (Cloisite 30B) led to a lower temperature dependence of the relaxation time (fragility) and activation energy of α-relaxation. Moreover, calculation of the distribution of the segmental dispersion showed a narrower dispersion in the glass transition region so that the Kohlrausch-Williams-Watts (KWW) distribution parameter (β_KWW) increased from 0.21 for neat PMMA to 0.34 for the 50/50 PMMA/SAN blend nanocomposite containing 3 wt% organoclay. Furthermore, the relaxation behavior of the blends showed a negative deviation from mixture law predictions based on the responses of the neat PMMA and SAN. These behaviors were attributed to the lack of specific interactions between the blend components (PMMA, SAN, and nanoclay layers) and the less cooperative behavior, i.e., less constraint for segmental relaxation, of the intercalated chains.     

A Study on Effect of Waviness on Mechanical Properties of Multi-Walled Carbon Nanotube/Epoxy Composites Using Modified Halpin–Tsai Theory

In order to improve the precision in prediction of mechanical properties of multi-walled carbon nanotubes (MWNTs)/epoxy composites, the effect of waviness of nanotubes is considered by applying a correction factor to a modified Halpin–Tsai equation. Data validation was carried out through the comparison of theoretical data with real mechanical test results obtained by the authors and in the literature. Tensile tests of various weight percents (wt%) of MWNT/epoxy composites were carried out to obtain the mechanical properties of MWNT/epoxy composites. Applicability of the proposed modification of Halpin–Tsai equation was endorsed by the experimental results and was found to be more accurate in comparison with results obtained from the same equation without considering the waviness effect.     

Nanocomposites based on polymer blends: enhanced interfacial interactions in polycarbonate/ethylene-propylene copolymer blends with multi-walled carbon nanotubes

In this article, the phase separation in the melt blended polycarbonate (PC) and ethylene propylene copolymer (EPC) has been studied with dynamic mechanical thermal analysis (DMTA) and scanning electron microscopy (SEM). Two glass transition temperatures on the tan δ curves were detected. This confirms the immiscibility of PC and EPC phases. Different content of multi-walled carbon nanotubes (MWCNTs) were added to the PC/EPC blends and the interfacial adhesion between MWCNTs and PC/EPC blend were shown using transmission electron microscopy (TEM). The MWCNTs were located in the PC phase and at the interfaces of PC and EPC phases. Moreover, the storage modulus (E′) of polymer blends was changed by the increasing content of EPC elastomer and MWCNTs. The value of E′ of PC decreased with an incorporation of EPC. While, along with an addition of MWCNTs in the PC/EPC blends an increase of E′ was visible. The strong interfacial interactions between the matrix and MWCNTs played the main role in increasing the values of the E′ of the nanocomposites.     

Aromatic Polyimide/MWCNT Hybrid Nanocomposites: Structure,Dynamics, and Properties

Two series of hybrid polyimide (PI)/multiwalled carbon nanotube (MWCNT) nanocomposites were prepared including COOH-functionalized or pristine nanotubes, and their structure, morphology and dynamics/mechanical properties at 20°C–500°C were studied using WAXD (Wide-angle X-ray diffraction), AFM (Atomic force microscopy), TEM (transmission electron microscopy), DSC (Differential scanning calorimetry), DMA (Dynamic mechanical analysis), CRS (creep rate spectroscopy) techniques, and stress–strain testing. The impact of nanofiller loadings of 0.125, 0.25, 0.5, or 1 wt% relative to PI was evaluated. Specific changes in the matrix morphology and different quality of nanotube dispersion in the nanocomposites with amorphous and semicrystalline matrices were determined. The best nanotube dispersion was observed in the composites with 0.5 wt% MWCNT-COOH. A peculiar high temperature dynamics, different for amorphous, and semicrystalline matrices, was revealed in these nanocomposites. The most dramatic changes in high temperature dynamics and a pronounced dynamic heterogeneity as well as substantially enhanced mechanical properties at room temperature were revealed in the case of a semicrystalline PI matrix. The results were treated in terms of the synergistic impact of nanotubes and matrix crystallites on dynamics in the intercrystalline regions of PI (“combined constrained dynamics effect”) and the peculiar interfacial dynamics.     

Influence of filler alignment in the mechanical and electrical properties of carbon nanotubes/epoxy nanocomposites

In this work, we report the mechanical and electrical properties of carbon nanotubes/epoxy composites prepared with aligned and randomly oriented nanotubes as filler. The samples are disks of 30 mm in diameter and 3 mm in thickness. To obtain the carbon nanotubes alignment, an external electric field (250 VAC; 50 Hz) was applied through the thickness of the sample during all the cure process. The AC electrical current was measured, during the cure, as a strategy to determine the optimum time in which the alignment reaches the maximum value. DC conductivity measured after the cure shows a percolation threshold in the filler content one order of magnitude smaller for composites with aligned nanotubes than for composites with randomly oriented filler (from 0.06 to 0.5 wt%). In the percolation threshold, the achieved conductivity was 1.4×10⁻⁵ Sm⁻¹. In both cases, aligned and randomly distributed carbon nanotube composites, the wear resistance increases with the addition of the filler while the Rockwell hardness decreases independently of the nanotubes alignment.     

Suppression of the unconventional metallic behavior by gate voltage in MWNT device

We observed an ambipolar behavior in multiwalled carbon nanotubes (MWNT) in a back-gate configuration, which allowed us to perform systematic inspection of the low-temperature transport properties against gate voltage. Power-law behaviors in temperature and bias-dependent conductance, disappeared when a high gate voltage was applied, and conductance became temperature- and bias independent. This indicates a gate-induced transformation from the unconventional to the normal metallic states in MWNT.     

Calorimetric study of polyethylene terephthalate)/poly(butylene terephthalate) blends

Poly(ethylene terephthalate)/poly(butylene terephthalate) blends [xPET/(l-x)PBT] were prepared by ultraquenching of the melt between two metal rolls rotating in a liquid nitrogen bath. Both DSC and WAXS studies indicate that immediately after preparation, the blends are amorphous regardless of the composition. Two glass transition temperatures are found for the as-quenched 0.5 PET/0.5 PBT blend. The activation energies determined from the dependence of each glass transition temperature on the heating rate are approximately the same. Furthermore, they are equal to the mean activation energy for the overall crystallization. This can be explained in terms of the percolation threshold theory.     

Effect of Spherical Nanoparticles on the Motion of Macromolecular Chains and Segments of Isotactic Polypropylene. I. Dynamic Mechanical and Thermal Properties

The role of spherical nano-CaCO₃ particles treated with 2 wt% and 6 wt% stearic acid (SA), respectively, on the motion of macromolecular chains and segments of isotactic polypropylene (iPP) was studied through the dynamic mechanical analysis and nonisothermal crystallization. Higher nucleation activity of the particles and more nucleating sites were achieved in the 6 wt% SA treated particle nanocomposites with respect to the 2 wt% SA counterpart. The increased nucleation efficiency caused high inhomogeneity and thus large mobility of the amorphous phase of iPP, which favored a low glass transition temperature (T_g ) in the nanocomposites. However, the spherical nanoparicles also spatially restrained the motion of macromolecular chains and segments, and the better the nanoparticles dispersed, the stronger the restriction was. Thus the glass transition temperature (T_g ) of the nanocomposites decreased with increasing filler loading but recovered at a certain particle concentration. At this filler content, the maximal α-transition temperature (T_α ) and the main melting peak temperature (T_m1 ) as well as the lowest degree of crystallinity (X_PP ) also occurred. This critical filler loading appeared at lower value (20 wt%) in 6 wt% SA treated nano-CaCO₃ composites with respect to 2 wt% SA counterpart (25%) due to the better dispersion of particles in the former. It was concluded that the mobility of the macromolecular chains and segments of iPP was dominated by the competition of the spatial confinement and nucleation effect of nano-CaCO₃ particles in the matrix.