Controlling bubble density in MWNT/polymer nanocomposite foams by MWNT surface modification

Polymer nanocomposite foams are promising substitutes for polymeric foams. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Creating polymer foams with controlled foam morphology is of great importance for controlling foam properties. The foam morphology is influenced by the foaming conditions and filler properties. For carbon nanotube/polymer composite foams, dispersion state and aspect ratio of the carbon nanotubes have been shown to influence the bubble density and bubble size. In the current study, the influence of carbon nanotube surface chemistry on the bubble density of multi-walled carbon nanotube/poly(methyl methacrylate), MWNT/PMMA, nanocomposite foams was investigated. The surface of the MWNTs with controlled aspect ratio was covalently modified with glycidyl phenyl ether (GPE). Surface modified MWNT/PMMA nanocomposite foams were produced using a supercritical carbon dioxide foaming process. At constant MWNT concentration, the bubble density of polymer nanocomposite foams filled with GPE surface modified MWNT was found to be several times higher than that of polymer nanocomposite foams filled with nitric acid treated MWNT. After the MWNTs were modified with GPE, the surface chemistry of the MWNT became the dominant factor in determining the bubble density while the MWNT aspect ratio became less influential.     

Double-electrochromic coordination polymer network films

Formation and characteristic properties of organized double-electrochromic films consisting of electrochromic poly(4-(2,2':6,2″-terpyridyl)phenyliminofluorene) (P-1)-zinc ion complexes and electrochromic anions are reported. The anions are 2,2'-azino-bis(3-ethylbenzo-thiazoline-6-sulfonate) (ABTS) and poly((4-sulfonatophenyl)iminofluorene) (P-2). The films were prepared upon multiple sequential adsorption of P-1 and the zinc salts of ABTS and P-2 on solid supports using coordinative interactions between the Zn ions and the terpyridine (tpy) ligands. The ABTS and P-2 ions are incorporated in the films via electrostatic forces neutralizing the charge of the complexed divalent zinc (Zn(2+)) ions. The optical, electrochemical, and electrochromic properties of the films are described. Films consisting of the Zn ion complex of P-1 and ABTS are yellow in the neutral state and change their color to brownish gray and finally blue, if anodically oxidized at ～640 mV vs FOC. Films containing the Zn ion complex of P-1, with P-2 as a counterion, are yellow in the neutral state and change color to dark red and finally blue, if anodically oxidized at ～450 mV vs FOC. Compared with previously reported films of the Zn ion complex of P-1 with nonelectroactive hexafluorophosphate as the counterion, the new films exhibit faster response times, as well as higher contrast, and the colors in the oxidized state are modified. The films are stable under ambient conditions and might be useful as active layers in electrochromic devices.     

Morphology and optoelectronic properties of ZnO rod array/conjugated polymer hybrid films

Vertically aligned zinc oxide (ZnO) nanorods were grown on the ITO glass and then coated with the conjugated polymer poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) to make the hybrid films. Nanorods with different diameters were synthesized to study the influences of ZnO nanorod morphology and polymer infiltration on the photocurrent and optical properties of the hybrid films. Increasing the growth time leads to the formation of ZnO rod array with large rod diameter, large surface area and small inter-rod distance. Small inter-rod distance hinders the filling of DB-PPV into the porous ZnO rod microstructure and lowers the PN junction area. It leads to lower photocurrent of the hybrid film. The red shift of the photoluminescence spectra suggests that filling the polymer into the ZnO rod microstructure favors more planar molecular orientations of the conjugated polymers and leads to an increase in the effective conjugation length.     

Glucose and lactate biosensors based on redox polymer/oxidoreductase nanocomposite thin films

Glucose and lactate enzyme electrodes have been fabricated through the deposition of an anionic self-assembled monolayer and subsequent redox polymer/enzyme electrostatic complexation on gold substrates. These surfaces were functionalized with a negative charge using 11-mercaptoundecanoic acid (MUA), followed by alternating immersions in cationic redox polymer solutions and anionic glucose oxidase (GOX) or lactate oxidase (LAX) solutions to build the nanocomposite structure. The presence of the multilayer structure was verified by ellipsometry and sensor function characterized electrochemically. Reproducible analyte response curves from 2 to 20 mM (GOX) and 2-10 mM (LAX) were generated with the standard deviation between multiple sensors between 12 and 17%, a direct result of the reproducibility of the fabrication technique. In the case of glucose enzyme electrodes, the multilayer structure was further stabilized through the introduction of covalent bonds within and between the layers. Chemical cross-linking was accomplished by exposing the thin film to glutaraldehyde vapors, inducing linkage formation between lysine and arginine residues present on the enzyme periphery with amine groups present on a novel redox polymer, poly[vinylpyridine Os(bisbipyridine)2Cl]-co-allylamine. Finally, an initial demonstration of thin-film patterning was performed as a precursor to the development of redundant sensor arrays. Microcontact printing was used to functionalize portions of a gold surface with a blocking agent, typically 1-hexadecanethiol. This was followed by immersion in MUA to functionalize the remaining portions of gold with negative charges. The multilayer deposition process was then followed, resulting in growth only on the regions containing MUA, resulting in a "positive"-type pattern. This technique may be used for fabrication of thin-film redundant sensor arrays, with thickness under 100 angstrom and lateral dimensions on a micrometer scale.     

Investigation of the thermal and structural properties of single-molecule magnet/polymer nanocomposite films

The inclusion of manganese-based single-molecule magnets (SMMs) into solvent cast films of poly(methyl methacrylate) (PMMA) or polycarbonate (PC) was found to influence the thermal stability of these polymers. Examination of the thermal decomposition profiles of PMMA films by thermo-gravimetric analysis (TGA) established that increasing weight % of SMM results in both enhancement of the rate of decomposition initiated at “head-to-head” linkages along with suppression of the rate of decomposition initiated at vinylidene chain ends. In the case of PC films, the temperature at which the primary thermal decomposition occurs decreases with increasing weight % of SMM. The extent of these decomposition trends is correlated to the degree of SMM dispersal, as studied by transmission electron microscopy (TEM). Favourable interactions between the ligands coordinated to the SMMs and the polymer or solvent used in film preparation dictated the degree of SMM dispersal, with the ligand–polymer interactions being dominant on the nano-length scale (1–100 nm) and ligand–solvent interactions being dominant on the micro-length scale (>100 nm).     

Clay platelet partition within polymer blend nanocomposite films by EFTEM

Transmission electron microscopy (TEM) is the main technique used to investigate the spatial distribution of clay platelets in polymer nanocomposites, but it has not often been successfully used in polymer blend nanocomposites because the high contrast between polymer phases impairs the observation of clay platelets. This work shows that electron spectral imaging in energy-filtered TEM (EFTEM) in the low-energy-loss spectral crossover region allows the observation of platelets on a clear background. Separate polymer domains are discerned by imaging at different energy losses, above and below the crossover energy, revealing the material morphology. Three blends (natural rubber [NR]/poly(styrene-butyl acrylate) [P(S-BA)], P(S-BA)/poly(vinyl chloride) [PVC], and NR/starch) were studied in this work, showing low contrast between the polymer phases in the 40-60 eV range. In the NR/P(S-BA) and P(S-BA)/PVC blend nanocomposites, the clay platelets accumulate in the P(S-BA) phase, while in the P(S-BA)/PVC nanocomposites, clay is also found at the interfaces. In the NR/starch blend, clay concentrates at the interface, but it also penetrates the two polymer phases. These observations reveal that nanostructured soft materials can display complex morphochemical patterns that are discerned thanks to the ability of EFTEM to produce many contrast patterns for the same sample.     

Characterization of spray-coating methods for conjugated polymer blend thin films

We examine the characteristics and functionality of conjugated polymer thin films, based on blends of poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,NN′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT), using a spray-coating deposition technique suitable for large areas. The morphological properties of these blend films are studied in detail by atomic force microscopy (AFM) methods, showing that favourable results, in terms of layer deposition rate and uniformity, can be achieved using a 5:1 blend of o-dichlorobenzene and chlorobenzene as the solvent medium. A photoluminescence quenching efficiency of above 80 % is also observed in such blend films. As a feasibility study, prototypical photovoltaic devices exhibit open circuit voltages of up to 1.0 V under testing, and solar power conversion efficiencies in the 0.1–1 % order of magnitude; metrics which are comparable with those reported for spin-coated cells of the same active blend and device architecture.     

Dielectric relaxation and thermal studies on dispersed phase polymer nanocomposite films

Dispersed phase polymer nanocomposite films (PNC) based on PMMA–LiClO₄+ n-YSZ, has been prepared. The effect of filler concentration on dielectric constant, tanδ and ac conductivity has been observed. For each PNC films the activation energy for relaxation (E_τ) is almost same as the activation energy for ion conduction (E_a). The dc conductivity, the hopping frequency of charge carriers have been obtained at different temperature from the analysis of the ac conductivity data. For all the PNC films, the concentration of charge carriers has been calculated at different temperature using Almond–West formalism. The estimated activation energies for the dc conductivity and the hopping frequency are different, which indicates that the both charge carrier mobility and concentration contribute significantly to the ionic conductivity of polymeric electrolyte. Contribution of charge carrier mobility to the total conductivity has also been confirmed from the differential scanning calorimetry analysis. Improvement in thermal stability has been noticed with filler addition.     

Lead sulphide/phthalocyanine nanocomposite spun films

Composite lead sulphide/phthalocyanine thin films were produced for the first time by exposure of thin spun films of a lipophilic lead phthalocyanine derivative to H/sub 2/S gas. The formation of lead sulphide was confirmed by Raman and X-ray photoelectron spectroscopy measurements. UV-visible absorption and photoluminescence spectral measurements as well as a film morphology study using atomic force microscopy indicate the formation of lead sulphide clusters. From the blue spectral shift of the fundamental adsorption band, the size of the PbS clusters was estimated to be 2.24 nm.     

Pulse electrodeposited AgInS2 films

AgInS₂ films were pulse electrodeposited on tin oxide coated glass substrates at different duty cycles for the first time. The films were single phase with orthorhombic structure. Optical absorption measurements indicated a band gap in the range of 1.90–2.02 eV with decrease of duty cycle. Transmission spectra exhibited interference fringes. Using the envelope method, refractive index was calculated. Optical conductivity was evaluated from the absorption coefficient and refractive index data. Optical data was analyzed by the single-effective oscillator model.     

Hazardous industrial gases identified using a novel polymer/MWNT composite resistance sensor array

Hazardous industrial chemical gases pose a significant threat to the environment and human life. Therefore, there is an urgent need to develop a reliable sensor for identifying these hazardous gases. In this work, a silicon wafer microelectrode substrate for a resistance sensor was fabricated using the semiconductor manufacturing process. Conductive carbon nanotubes were then mixed with six different polymers with different chemical adsorption properties to produce a composite thin film for the fabrication of a chemical sensor array. This array was then utilized to identify three hazardous gases at different temperatures. Experimental results for six polymers for chemical gases, such as tetrahydrofuran (THF), chloroform (CHCl₃) and methanol (MeOH) at different temperatures, indicate that the variation in sensitivity resistance increased when the sensing temperature increased. The poly(ethylene adipate)/MWNT sensing film had high sensitivity, excellent selectivity, and good reproducibility in detecting all chemical agent vapors. Additionally, this study utilized a bar chart and statistical methods in principal component analysis to identify gases with the polymer/MWNT sensor.     

Synthesis and thermoelectric/electrical characterization of electrodeposited SbxTey thin films

Sb_xTe_y films were potentiostatically electrodeposited from acidic nitric baths at room temperature by controlling the applied potential. Near-stoichiometric Sb₂Te₃ thin films were obtained at applied potentials between ?0.15 and ?0.30 V vs. saturated calomel electrode (SCE). Post-annealing in a reducing environment resulted in an improvement in the crystal structure without the evaporation of the Te element. This result was indicated by a significant reduction in the electrical resistance and decrease in the FWHM of the main diffraction peaks. The power factor (σS²) increased from 44.2 to 372.1 μW/m K² after annealing at 473 K.     

Microstructural characterization of pulsed electrodeposited Au/Sn alloy thin films

Based on previous work that identified an electrodeposited composite, multi-layer structure as a viable method of producing eutectic Au/Sn alloys for solder applications, a study of individual phase formation was undertaken. The AuSn phase, because of its higher deposition current (>2.0 mA cm⁻²), has a much faster deposition rate than Au₅Sn, which is deposited at <1.0 mA cm⁻². AuSn formation is growth controlled, while Au₅Sn formation is nucleation controlled. The AuSn forms a continuous layer within 60 s with a grain size of 50–75 nm. Because of the high deposition current, the dominant formation mechanism is two-dimensional nucleation, resulting in a relatively rough surface finish. Au₅Sn, on the other hand, forms a continuous layer within 600 s with an average grain size of 200 nm. Because of the significantly lower deposition current, the dominant formation mechanism is lateral spreading instead of two-dimensional nucleation. The result is a very smooth finish on the deposit surface.     

ZnO films and nanorod/shell arrays electrodeposited on PET-ITO electrodes

In this work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. ZnO films with high optical transmittance were prepared from a zinc nitrate solution using a pulsed current technique with a reduced pulse time (3 s). The X-ray diffraction pattern of ZnO film deposited on PET-ITO electrode showed that it has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. ZnO nanorods were synthesized on electrochemical seeded substrate in an aqueous solution containing zinc nitrate and hexamethylenetetramine. In order to increase the stability of PET-ITO electrode to electrochemical and chemical stresses during ZnO nanorods deposition the surface of the electrode was treated with a 17 wt% NH₄F aqueous solution. Electrochemical stability of PET-ITO electrode was evaluated in a solution containing nitrate ions and hexamethylenetetramine. ZnO nanorod/shell arrays were fabricated using eosin Y as nanostructuring agent. Photoluminescence spectra of ZnO nanorod and ZnO nanorod/shell arrays prepared on the surface of PET-ITO electrode were discussed comparatively. By employing the 1.5 μm-length ZnO nanorod/shell array covered with a Cu₂O film a photovoltaic device was fabricated on the PET-ITO substrate.     

Growth and characterization of vacuum-deposited polyvinylcarbazole (PVK) films

The optical and infrared absorption spectra of polyvinylcarbazole (PVK) films deposited by vacuum evaporation show identical shape when compared with the corresponding spectra of solution-grown PVK films obtained by thermal evaporation of the solution of PVK and dichloromethane. This confirms the formation of PVK films on vacuum deposition. Electron spectroscopy for chemical analysis measurements indicate the presence of carbon and nitrogen in these films as expected from the structure of PVK. The scanning electron micrographs indicate that the surface of these vacuum-deposited films are quite smooth and remain so even after annealing them in the atmosphere at 373 K for 1 h.