Microwave synthesis of barium iron niobate and dielectric properties

A microwave synthesis was applied to prepare barium iron niobate (BFN) powders for the first time. Pure perovskite phase of BFN with a cubic symmetry was obtained when precursors were calcined at 850 °C for 14 h. Average particle size of pure BFN powder was estimated to be in submicron range. The examination of dielectric spectra indicated that BFN ceramics prepared by the present method exhibited rather high dielectric constant (∼ 30,000 at 300 °C for 1 kHz) with low dielectric loss (less than 0.6).     

Improvement of ultrathin polystyrene film stability by addition of poly(styrene-stat-chloromethylstyrene) copolymer: An atomic force microscopy study

A method to improve the stability of ultrathin polystyrene (PS) films on SiO_x/Si substrate is introduced. In this method, interfacial interactions between PS film and substrate are enhanced by addition of poly(styrene-stat-chloromethylstyrene(ClMS)) copolymer containing 5 mol% of ClMS group. The resulting slight structural modification of the copolymer does not cause phase separation in the polymer blend. On the other hand, the existence of polar ClMS groups provides anchoring sites on the polar SiO_x surface via dipolar interactions. In this study, ratios of the copolymers are varied from 0 to 40 wt.% in the thin films resulting in a systematic increase of the interfacial interactions. The dewetting behaviors of all films subjected to the same annealing conditions are explored via atomic force microscopy. The analyses of root mean square roughness and dewetting area as a function of annealing time and copolymer ratio provide information about the film stability. Our results indicate that blending small quantity of the copolymer with PS significantly increases the stability of ultrathin films.     

Self-assembling into interconnected nanoribbons in thin films of hairy rod poly(9,9-di(2-ethylhexyl)fluorene): Effects of concentration,substrate and solvent

This study utilizes atomic force microscopy to investigate the self-assembling behaviors from dilute solution into thin film of a well-known conjugated polymer, poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6). We have found that the structures of nanoscale aggregates depend on various experimental parameters including concentration, substrate and solvent. The self-assembling of PF2/6 from 0.05 mg/mL solution in toluene onto SiO_x/Si substrate results in the formation of interconnected nanoribbons with thickness and width of about 20 and 150 nm, respectively. Varying polymer concentration and type of substrate (SiO_x/Si or mica) significantly affects the nanoscale structures. The change of solvent to chlorobenzene, a slightly more polar solvent with slower evaporation rate, causes the growth of ribbon width to micron size with slight increase of the thickness. When the solvents with higher evaporation rate (i.e. chloroform and dichloromethane) are used, densely packed nanoribbons are obtained. Its width also grows to micron size. The measurements of UV/vis absorption and photoluminescence spectra detect some discrepancies in pattern, reflecting the variation of local chain organization within thin films prepared by using different solvents.     

Effects of dispersant concentration and pH on properties of lead zirconate titanate aqueous suspension

Lead zirconate titanate (PZT) aqueous suspensions were prepared at 60 wt.% solids loading using a commercial ammonium polyacrylate (APA) dispersant. Effects of the dispersant concentration on rheological behavior, dispersion and stability of PZT aqueous suspensions were investigated by means of zeta potential, viscosity and sedimentation height measurements. The results showed that, under suitable conditions, APA dispersant promoted particle dispersion and stabilization in PZT aqueous suspensions. For 60 wt.% solids loading suspensions, the dispersant concentration yielding the lowest viscosity was 0.5 wt.% based on PZT powder dried weight basis. Effects of pH on particle dispersion in the suspensions prepared with APA were studied by laser light scattering technique and scanning electron microscopy. The results showed an improvement in particle dispersion for the alkaline condition, which led to relatively low viscosity and highly stable suspension. Possible particle stabilization mechanisms at various pHs were discussed based on dissociation of the dispersant in water, polymer conformation and adsorption behavior of the dispersant on the particle surface.     

Photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) and its copolymer with anthracene in solvent–non-solvent: Roles of interchain interactions on the formation of non-emissive and emissive aggregates

This contribution investigates photophysical change of poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6) and its random copolymer with anthracene in solvent–non-solvent system. Isolated chains of the polymers in good solvent are driven to associate by addition of a poor solvent. The increase of interchain interactions upon decreasing solvent quality is found to cause the growth of a broad red-shift shoulder in absorption spectra of PF2/6. The measurements of photoluminescence (PL) spectra detect the increase of red-shift peaks at ∼440 nm and ∼525 nm. Detailed analyses of site-selective PL and PL excitation (PLE) spectra indicate the formation of emissive aggregates in this system. However, direct comparison of PLE and absorption spectra accompanied with the decrease of quantum yield suggests that different type of aggregates, the non-emissive one, also forms. The extent of interchain interactions dictates the existence of two types of aggregates. Our results signify the roles of interchain interactions on photophysical properties of PF2/6. Parallel studies of statistical copolymer of PF2/6 with anthracene groups (5 mol.%) observe drastic drop of the red-shift peak at ∼525 nm in PL spectra. The anthracenes exist as steric groups along the conjugated backbone, prohibiting the formation of emissive aggregates and/or excimers due to the increase of interchain proximity.     

Roles of polymeric dispersant charge density on lead zirconate titanate aqueous processing

Poly(acrylic acid) (PAA) and poly(acrylic acid-co-maleic acid) (PACM) were used as dispersants in preparation of lead zirconate titanate (PZT) aqueous suspensions. The effects of dispersant structure on particle stabilization were investigated through properties of the suspensions. Viscosity and sedimentation height measurements showed that addition of the dispersants improved particle stabilization. The dispersant concentrations to obtain the lowest viscosity were 0.4 wt% for PAA and 0.2 wt% for PACM based on powder dried weight basis. Furthermore, effects of pH were studied on the suspensions prepared with 0.2 wt% dispersants. Viscosity and sedimentation behaviors indicated the improvements of particle dispersion and suspension stability with an increasing pH. Particle dispersion revealed by laser light scattering and scanning electron microscopy supported an improvement of particle dispersion at alkaline pHs. Detailed analysis of these data indicated that the PACM exhibited higher dispersant efficiency for PZT aqueous suspension in all conditions. The results were discussed based on the concentrations of anionic –COO^? groups at various pHs and charge density along polymeric backbone of the dispersants.     

Influences of poly[(styrene)x-stat-(chloromethylstyrene)y]s additives on dewetting behaviors of polystyrene thin films: effects of polar group ratio and film thickness

This contribution investigates the addition of poly(styrene-stat-chloromethylstyrene (ClMS))s as dewetting inhibitors of polystyrene (PS) thin films with thicknesses ranging from 12 to 38 nm. The ClMS ratios in the copolymers are 5, 25 and 45 mol%. Atomic force microscopy and optical microscopy are utilized to follow morphological changes of blended PS/copolymer films upon annealing above their glass transition temperatures. We have found that thermal stability of the PS films is greatly improved when a small amount of the copolymers is added into the system. The polar ClMS groups provide anchoring sites with the polar SiO_x/Si substrate while the styrene segments favorably interact with the PS matrix. The effectiveness of the copolymers as dewetting inhibitors is also found to increase with mole ratio of ClMS group. While the stability of PS films is systematically improved upon addition of the highly substituted copolymers, using the copolymer with relatively low ratio of ClMS group could lead to the opposite result. This class of copolymers can be utilized for improving thermal stability of ultrathin PS films. The fundamental knowledge from this study is also important for designing or selecting structure of additives used to improve the stability of polymeric thin films.     

Chain organization and photophysics of conjugated polymer in poor solvents: Aggregates, agglomerates and collapsed coils

UV/vis absorption, excitation, and emission spectroscopies are utilized to explore the association behavior of conjugated polymer, poly[2-methoxy, 5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), in a homologous series of linear alcohols and solvent-nonsolvent systems. We demonstrate that the aggregation, agglomeration, and collapse of isolated chains, which all take place in the poor solvents, exhibit different optical signatures. The aggregate, in which chromophores interact electronically, causes a red shift of absorption and emission spectra while the weak interaction of chromophores in the agglomerates leaves electronic properties unaltered. The collapse of isolated chain is accompanied by a blue shift of the spectra. Energy transfer from excited chromophores of non-aggregated chains to the lower energy aggregates is significantly diminished in the system of alcohol solvents, allowing separate emissions from various chromophores to occur. The optical signatures and extent of energy transfer are used as tools to elucidate the association mechanism and chain organization of isolated chains into large particles upon decreasing solvent quality. The extreme collapse of isolated chains in the very poor solvents is found to inhibit the chain organization into aggregates.