Fabrication of hybrid nanocomposites with polystyrene and multiwalled carbon nanotubes with well-defined polystyrene via multiple atom transfer radical polymerization

The PS-grafted multiwalled carbon nanotubes (MWNTs) were produced by the bromo-ended PS (PS-Br) and pristine MWNTs in 1,2-dichlorobenzene at 110 °C for 72 h via atom transfer radical polymerization (ATRP). Bromo-ended PS (PS-Br) used as an initiator for the functionalization of MWNTs was synthesized with styrene by ATRP conditions using CuBr and N,N,N′,N′,N″-pentamethyldiethylenetriamine as catalyst. The PS-grafted MWNTs were fully characterized by ¹H-NMR, FT-IR, DSC, TGA, and SEM. The PS-grafted MWNTs were found to be highly soluble in a variety of organic solvents. The PS was chemically attached to the surfaces of MWNTs via ATRP approach, and the grafting amount of PS was 40–90%. From TGA and DSC measurements, the PS-grafted MWNTs were decomposed at lower temperature compared to that of PS-Br, and the functionalization of MWNTs increased the glass-transition temperature (T_g) of the grafted PS. The PS/PS-grafted MWNTs nanocomposites were prepared with PS and PS-grafted MWNTs by solution mixing in dimethylformamide (DMF). The resulting nanocomposites were found to be the homogeneous dispersion of PS-grafted MWNTs in PS matrix via aromatic (π–π) interactions between PS and PS-grafted MWNTs as determined by SEM and TEM.     

Influence of Stoichiometric Ratio and Mn-Doping on the Surface Morphology and Dielectric Properties of Perovskite (La,Sr)FeO3 Films

Lanthanum strontium ferrite (La_1−x Sr _x FeO₃, x = 0.1, 0.3, 0.5, 0.7) (LSF) films were prepared on platinized/silica/silicon (Pt/SiO₂/Si) substrates by a sol–gel method. The crystallization, surface morphology and dielectric properties of the LSF films have been studied. Among the samples, La_0.5Sr_0.5FeO₃ thin film have the most homogeneous, smooth surface and the best dielectric properties (ε _r  = 2881.58, tanδ = 1.20) at the frequency of 1 kHz. Mn-doping (y) in La_0.5Sr_0.5FeO₃ further improves the dielectric properties (ε _r  = 2922.91, tanδ = 1.11) of the film. Both ε _r and tanδ values of the Mn-doped (y = 0.02) La_0.5Sr_0.5FeO₃ films decreases with increasing the frequency. The biggest ε _r value is 3238.18 at the frequence of 0.02 kHz.     

Synthesis and Thermal,Structural, Dielectric,Magnetic and Magnetoelectric Studies of BiFeO3‐MgFe2O4 Nanocomposites

Spinel–perovskite magnetoelectric (ME) nanocomposites xMgFe₂O₄–(1?x)BiFeO₃, x = 0.1, 0.2, 0.3, and 0.4 were synthesized by sol‐gel method and characterized by differential thermal analysis, X‐ray diffraction analysis, dielectric and magnetic measurements. The samples were calcined at various temperatures and then the effect of annealing temperature on structural and magnetic properties was studied. From transmission electron microscopy, the average crystal size was found to be 30–50 nm. The magnetic behavior is found to be dependent on annealing temperature and magnesium ferrite content. The dielectric behavior with frequency and temperature has been modified with the induction of magnesium ferrite. The relative change of dielectric constant with magnetic field was observed in the nanocomposites. This relative change of magnetic field‐induced dielectric constant can also be expressed by Δε ~ γM² (where γ is magnetoelectric coupling coefficient).     

Effects of functionalized MWNTs with GMA on the properties of PMMA nanocomposites

The acid modification of multiwall carbon nanotubes (MWNTs) was performed by an HNO₃/H₂SO₄ solution. The glycidyl methacrylate (GMA) undergoing an opening‐ring was grafted onto the surface of acid‐modified MWNTs. The surface properties of MWNTs were investigated by Fourier transform infrared spectrometer (FTIR), Raman spectra, transmission electron microscopy (TEM), X‐ray diffraction, and thermogravimeric analysis. Then the MWNTs/ poly(methyl methacrylate) (PMMA) nanocomposites were prepared by in situ polymerization. The tribological and dielectric properties of nanocomposites were studied. As a result, GMA was grafted on the surface of MWNTs. The tribological and dielectric properties of MWNTs/ PMMA nanocomposites were improved as the content of the surface‐modified MWNT increased. The marked improvement in tribological and dielectric properties were attributed to the good dispersion of MWNTs that were bonded with C?C on the surface that participated in the polymerization of MMA. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of fast neutrons on dielectric properties of pure and gelatin doped-poly (vinyl alcohol) films

Cast thin films of pure and gelatin doped poly(vinyl alcohol) (PVA) with concentrations of 2, 5, 7, 10, and 15 wt% were prepared and subjected to fast neutron fluences in the range of 10⁵–10⁸ n/cm². The dielectric constant (ε′), dielectric loss tangent (tanδ) and a.c. electrical conductivity (σ_a,c) were investigated as a function of field frequency and temperature for all samples before and after irradiation. A clear glass transition peak was observed in ε′(T) and tanδ(T) curves. The irregular shift in the position of the glass transition temperature (T_g) in tanδ spectra with increasing neutron fluence can be attributed to either degradation or the predominance of the crosslinking process. The electric dipole moment and activation energy were calculated for pure and gelatin doped PVA samples before and after neutron irradiation.     

Dielectric Properties of Sol–Gel‐Derived Bi12SiO20 Thin Films

In this paper the dielectric properties of crack‐free, Bi₁₂SiO₂₀ thin films were investigated. The films were prepared on Pt/TiO₂/SiO₂/Si and corundum substrates using the sol–gel method. The formation of a pure Bi₁₂SiO₂₀ phase was observed at a temperature of 700°C. The Bi₁₂SiO₂₀ thin films, heat treated at 700°C for 1 h, had a dense microstructure with an average roughness (R_a) of 50 nm. The dielectric properties of the film were characterized by using both low‐ and microwave‐frequency measurement techniques. The low‐frequency measurements were conducted with a parallel capacitor configuration. The dielectric constant and dielectric losses were 44 and 7.5 × 10^?3, respectively. The thin‐film dielectric properties at the microwave frequency were measured using the split‐post, dielectric resonator method (15 GHz) and the planar capacitor configuration (1–5 GHz). The dielectric constant and the dielectric losses measured at 15 GHz were 40 and 17 × 10^?3, respectively, while the dielectric constant and the dielectric losses measured with the planar capacitor configuration were 39 and 65 × 10^?3, respectively.     

A novel route for well‐defined polystyrene‐grafted multiwalled carbon nanotubes via the radical coupling reaction

Multiwalled carbon nanotubes‐graft‐polystyrene (MWNTs‐g‐PS) was synthesized by atom transfer nitroxide radical coupling chemistry. MWNTs with 2,2,6,6‐tetramethylpiperidine‐1‐oxy (MWNTs‐TEMPO) groups was prepared first by esterification of 4‐hydroxy (HO)‐TEMPO and carboxylic acid group on the surface of MWNTs (MWNTs‐COOH); PS with bromide end group (PS‐Br) were then obtained by atom transfer radical polymerization using ethyl 2‐bromoisobutyrate as initiator and CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine as catalyst. The MWNTs‐TEMPO was mixed with PS‐Br and heated to 90°C in the presence of CuBr/PMDETA to form MWNTs‐g‐PS. The product was characterized by FTIR, NMR, TGA, and TEM. TEM indicates that the MWNTs are enveloped by the polymer molecules. The content of grafted polymers is 46.7% by TGA measurements when the number‐average molecular weight (M_n) of PS‐Br is 10,200 g/mol. The as‐prepared nanocomposites exhibit relatively good dispersibility in solvents such as CH₂Cl₂, THF, and toluene. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation and dielectric properties of poly(ε‐caprolactone) compounded with “bucky gels”‐like mixture

Poly(ε‐caprolactone) (PCL) was melt compounded with “Bucky gels”‐like mixture that prepared by grinding multiwalled carbon nanotubes (MWNTs) and ionic liquids (ILs). Raman spectrum showed the significant interaction between ILs and MWNTs. The dielectric behavior of PCL nanocomposites based on unmodified and IL‐modified MWNTs was studied from 40 Hz to 30 MHz. The addition of ILs significantly enhanced the dielectric property of PCL/IL/MWNT ternary nanocomposites, which was much higher than that of the sum of PCL/IL with PCL/MWNT binary nanocomposites. The dielectric properties of PCL/IL/MWNT nanocomposites were mainly influenced by ILs in low frequency and were dominated by MWNTs in high frequency. SEM results revealed that a more uniform and fine dispersion of MWNTs were achieved throughout the PCL matrix because of ILs. The addition of ILs in nanocomposites changed the crystallinity of PCL. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40231.     

Synthesis and characterization of polyimide/multi‐walled carbon nanotube nanocomposites

Polyimide/multi‐walled carbon nanotube (PI‐MWNT) nanocomposites were fabricated by an in situ polymerization process. Chemical compatibility between the PI matrix and MWNTs is achieved by pretreatment of the carbon nanotubes in a mixture of sulfuric acid and nitric acid. The dispersion of MWNTs in the PI matrix was found to be enhanced significantly after acid modification. The glass transition (T_g) and decomposition (T_d) temperature of PI‐MWNT nanocomposites were improved as the MWNT content increased from 0.5 to 15 wt%. The storage modulus of the PI/MWNT nanocomposites is nine times higher than that of pristine PI at room temperature. The tensile strength of PI doubles when 7 wt% MWNTs is added. The dielectric constant of the PI‐MWNT nanocomposites increased from 3.5 to 80 (1 kHz) as the MWNT content increased to 15 wt%. The present study demonstrates that enhanced mechanical properties can be obtained through a simple in‐situ polymerization process. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Electrical/dielectric properties and conduction mechanism in melt processed polyamide/multi-walled carbon nanotubes composites

The electrical and dielectric properties of polyamide 6 (PA6)/multi-walled carbon nanotubes (MWCNT) nanocomposites prepared by melt mixing were investigated by employing dielectric relaxation spectroscopy in broad frequency (10^?2–10⁶ Hz) and temperature ranges (from ?150 to 150 °C). Transmission electron microscopy revealed a good state of CNT dispersion in the polymeric matrix. The percolation threshold (p_c) was found to be 1.7 vol.% by using the dependence of both dc conductivity and critical frequency (f_c) from dc to ac transition on vol.% concentration in MWCNT. The actual aspect ratio of the nanotubes in the nanocomposites was calculated using a theoretical model (proposed by Garboczi et al.) and the obtained value was correlated with the p_c value according to the excluded volume theory. Additionally, the contact resistance (R_c) between the conductive nanotubes was found to be ～10⁵ Ω. Investigation of the temperature dependence of conductivity revealed a charge transport which is controlled by thermal fluctuation-induced tunneling for temperatures up to the glass transition. Finally, it was shown that the addition of nanotubes has no significant influence on the relaxation mechanisms of the PA6 matrix.     

Crystallinity of biodegradable polymers reinforced with functionalized carbon nanotubes

Well-dispersed multiwall carbon nanotubes (MWCNTs) were prepared by grafting poly(L-lactide-co-ε-caprolactone) (PLACL) biodegradable copolymer onto the sidewall of hydroxylated MWCNTs using oligomeric L-lactide (LA) and ε-caprolactone (CL). After preparation of MWCNT/PLACL composites, the effect of functionalized MWCNTs on crystallinity of PLACL was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and polarized light optical microscopy (POM). The surface functionalization effectively improved the dispersion and adhesion of MWCNTs which acted as reinforcing filler in the PLACL polymer matrix and hence improved the physical and thermomechanical properties of the nanocomposites. The glass transition temperature (T _g) and the crystallinity of nanocomposites decreased in comparison with those of neat PLACL when the concentration of functionalized MWCNTs in nanocomposites was 0.5 wt%. With further increment in concentration of functionalized MWCNTs, the T _g of composites increased until the T _g of neat PLACL, and also the crystallinity of composites increased. The functionalized MWCNTs have no significant effect on the melting point of nanocomposites. The MWCNTs acted as heterogeneous nucleation points and increased the lamella size and therefore the crystallinity of PLACL. Furthermore, the larger agglomerated clusters of both kinds of MWCNTs (i.e., MWCNT-grafted-PLACL and pristine MWCNTs) are more effective than small clusters as nucleation points for growing the spherulites.     

Preparation of La1?x Sr x CoO3 Oxide Electrode Material and its Influence on the Structure and Dielectric Performance of the Supported Ba1?x Sr x TiO3 Thin Films

Using the sol–gel method, La_1−x Sr _x CoO₃ (LSCO) electrode films were first fabricated on the Si (100) substrates, followed by the growth of Ba_1−x Sr _x TiO₃ (BST) thin films on the LSCO electrode film. The crystal structure and surface morphology of these films were characterized by XRD and SEM. The effects of Sr-doping and annealing temperature on the structure and electric resistivity of the LSCO films and the dielectric properties of the BST films were studied. Results show that the La_0.5Sr_0.5CoO₃ electrode annealed at 750 °C has the lowest electric resistivity, 1.1 × 10⁻³Ω cm. The relative permittivity of the La_0.5Sr_0.5CoO₃-supported BST films first increases and then decreases with Sr-doping. The relative permittivity of the BST film decreases while the dielectric loss increases with frequency. Among the studied BST films, Ba_0.5Sr_0.5TiO₃ has the largest relative permittivity and the smallest dielectric loss (95 and 0.1, respectively) when the frequency is 1 kHz.     

Effect of reaction time and temperature on the morphology and ion-exchange capacity of polystyrene beads sulfonated using different sulfonating agents

Monodisperse polystyrene (PS) beads 2.5 μm in size were synthesized by dispersion polymerization. The PS beads were chemically modified by three different sulfonating agents, namely, concentrated sulfuric acid, acetyl sulfate, and chlorosulfonic acid. When they were sulfonated using chlorosulfonic acid, the concentration of sulfonic acid (SO₃ ⁻) groups on the PS beads increased from 0.5 to 2.25 meq/g as the treatment time was increased from 10 to 360 min at 0 °C. As the reaction temperature was increased, the ion-exchange capacity (IEC) of the sulfonated PS beads increased. However, it can be seen that they lost their spherical shape. The PS surface was modified by the introduction of the SO₃ ⁻ groups to give an IEC of up to 2.28 meq/g, which provides 1.1 × 10¹⁰ SO₃ ⁻ groups per bead, without any deformation of their spherical shape.     

Studies on monobenzo 18-crown-6-ether substituted phenylenevinylene oligomer: synthesis,photoluminescent, and dielectric properties

Partially conjugated monobenzo 18-crown-6 substituted oligo (p-phenylenevinylene) (MB-OPV) was synthesized via Wittig route. Formation of the oligomer was characterized by spectral (FTIR, ¹H NMR, and ¹³C NMR), elemental and gel permeation chromatography analysis. The morphology of the oligomer film of one-dimensional (1D) and three-dimensional (3D) architectures were observed using atomic force microscopy. The oligomer showed excellent photoluminescence (PL) of bluish green emission at shorter wavelength of 542 nm. Stability of the polymer and glass transition temperature was determined by TGA/DSC. The dielectric properties such as dielectric constant and loss factor for the oligomer have also been studied with respect to change of frequency (50 Hz–5 MHz) and temperature (30–90 °C). The value of dielectric constant decreased with increasing frequency which indicates that the major contribution comes from orientation polarization. The value of dielectric constant increased with increasing temperature which is due to greater freedom of movement of the dipole molecular chains within the oligomer at high temperature.     

Degradation of the dielectric and piezoelectric response of β-poly(vinylidene fluoride) after temperature annealing

The effect of annealing temperature and time on the dielectric and piezoelectric response of poly(vinylidene fluoride), PVDF, was studied. The observed decrease in the value of the dielectric, ε′, and piezoelectric, d ₃₃, constants is related to depoling of the material and not to variations of the degree of crystallinity or the electroactive β-phase content. In a general way, the dielectric and piezoelectric responses decrease strongly in the first 4 h at a given temperature, in particular for temperatures higher that 80 °C, reaching stable values for longer annealing times. For most applications, the temperature of 100 °C will set the limit of suitable performance. Nevertheless, the material still retains a stable piezoelectric response of ca. 4 pC/N after reaching temperatures of 140 °C. The mechanisms behind the observed behavior are discussed.     

Effects of Mn-Doping on the Properties of BaBi4Ti4O15 Bismuth Layer Structured Ceramics

Bismuth-layer-structured (Ba_1−x Mn _x )Bi₄Ti₄O₁₅ (0.0 ≤ x ≤ 0.8) ceramics were prepared by a Sol–Gel method. The effects of the amount of Mn-doped on the phase structure, the dielectric as well as piezoelectric properties of BaBi₄Ti₄O₁₅ ceramics were studied. The X-ray diffraction results revealed that the introduction of Mn resulting in distortion of lattice, which contributes to the crystallization of the layered structure grains. The densification, dielectric and piezoelectric properties of the (Ba_1−x Mn _x )Bi₄Ti₄O₁₅ ceramics were significantly promoted by the Mn substitution of Ba. When the value of doping amount Mn is 0.4, the (Ba_0.6Mn_0.4)Bi₄Ti₄O₁₅ ceramic exhibited a high piezoelectric constant (d ₃₃ = 7.5 pC/N), a big relatively dielectric constant (ε _r  = 764.26) and a small dielectric loss (tanδ = 0.0124).     

Fabrication and electroanalytical characterization of label-free DNA sensor based on direct electropolymerization of pyrrole on p-type porous silicon substrates

Label-free DNA sensors based on porous silicon (PS) substrate were fabricated and electrochemically characterized. p-type silicon wafer was electrochemically anodized in an ethanolic hydrofluoric (HF) solution to construct a PS layer on which polypyrrole (PPy) film was directly electropolymerized. To achieve direct electropolymerization of PPy on PS substrate without pre-deposition of any metallic thin-film underlayer, a low resistivity wafer (0.01–0.02 Ω cm) was used. The rough surface of the PS layer allowed for a strong adsorption of the PPy film. Intrinsic negative charge of the DNA backbone was exploited to electrostatically adsorb 26 base pairs of probe DNA (pDNA) into the PPy film by applying positive bias. The pDNA was designed to hybridize with the target DNA (tDNA) which is the insertion element (Iel) gene of Salmonella enterica serovar Enteritidis. Dependence of peak current (i _p ) around 0.2 V vs Ag/AgCl on tDNA concentration and incubation time were shown from the cyclic voltammograms of PS/PPy + pDNA + tDNA substrates in a 0.01 M potassium perchlorate solution. Plot of i _p vs incubation time showed a reduction in current density (J) by ca. 29 μA cm⁻² every hour. Sensitivity obtained from a plot of i _p vs tDNA concentration was −166.6 μA cm⁻² μM⁻¹. Scanning electron microscopy (SEM) image of the cross-section of a PS/PPy + pDNA + tDNA multilayered film showed successful direct electropolymerization of PPy for a nano-PS DNA biosensor.     

Barium titanate‐based nanodielectrics of two chemically recyclable thermosets

In this work was investigated the effect of the addition of barium titanate (BaTiO₃) on electrical properties of two chemically recyclable thermosets, polyhemiaminal (PHA) and polyhexahydro‐s‐triazine (PHT), both fabricated from 4,4′‐oxydianiline (ODA), an ether derivative of aniline and paraformaldehyde. Thermal and mechanical properties as well as chemical recyclability of the two polymers and their nanocomposites/nanodielectrics were also investigated. In addition, a quantitative analysis was conducted of the nanoparticle dispersion in the PHA‐/PHT‐based BaTiO₃‐containing nanocomposites using transmission electron microscopy imaging and the nearest‐neighbor distance index and this index was used to analyze the investigated properties in connection with the proper mechanisms. Regarding the electrical properties for both neat polymers, conductivity values of the order of 10^?8 S m^?1 at 100 Hz were observed and dielectric constant values close to 2.80 for both polymers at 1 kHz. The addition of 0.5 wt% of BaTiO₃ ferroelectric nanoparticles increased by about 44% the dielectric constant (1 kHz) and conductivity (10² Hz) of the PHA‐based nanocomposite. PHA and PHT exhibited glass transition temperature (T_g) values in the range 125–180 °C. An increase of 7 °C in T_g was observed after the incorporation of 0.5 wt% of BaTiO₃ into PHA. Concerning the mechanical properties, values in the range 4.00–4.45 GPa for reduced modulus and 0.30–0.43 GPa for nanohardness for PHA and PHT polymers were observed. Independently of filler content or polymer matrix, both mechanical properties were enhanced after the addition of BaTiO₃. The chemical recycling of PHA/PHT and all nanocomposites in the initial ODA reagent after sulfuric acid treatment was successfully characterized using the NMR and Fourier transform infrared spectroscopic techniques. © 2018 Society of Chemical Industry     

Interactions Between an Anionic Fluorosurfactant and a PEO-PPO-PEO Triblock Copolymer in Aqueous Solutions

Surface tensions were determined for a mixture of an anionic fluorinated surfactant and a PEO-PPO-PEO triblock copolymer. The interactions between the two surfactant molecules in the mixed monolayer and the mixed micelle were studied through molecular interaction parameters (β ^σ, β ^M) and the molecule exchange energy (ε, ε _m). It was noted that synergism and strong attractive interactions took place between the anionic fluorinated surfactant and the triblock copolymer molecules in both mixed micelles and mixed monolayers, reflected by the interaction parameter values of between −10 and −18 for all mixtures investigated. Moreover, it can be seen from the value of (ε − ε _m) that when the mixture has a small amount of triblock copolymer, the formation of mixed micelle results in a greater decrease in energy than does the formation of a mixed monolayer. With an increase in the mole fraction of the triblock copolymer in the mixture, in order to obtain the lowest surface energy, surfactants tend to form mixed monolayers first, and then form mixed micelles.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.