Nb2O5-Based Oxide Ceramics and Single Crystals-Investigation of Dielectric Properties

The behavior of enhancements of dielectric constant of Nb₂O₅ through small substituents of TiO₂ is of interest. The ceramics of these solid solutions were prepared by the conventional mixed oxide method. Single crystal fibers were grown by the Lase r-Heated Pedestal Growth (LHPG) technique, which is considered to be a powerful tool for rapid growth of high-melting temperature oxides and incongruent melting compositions. The dielectric properties as function of temperature (-180°C to 180°C) and frequency (1 KHz to 1 MHz) were measured. The direct-current (dc) electric field dependence of the dielectric constant has been studied. The results show the strong dielectric dispersion which exists in a large frequency range which implies that the relaxation process involved is not of a Debye type. A relaxation distribution can be therefore expected. The influence of the cluster size dispersion is one assumption. Thus, the data of dc bias field dependence of the dielectric constant has been analyzed by the modified Devonshire relation including a cluster term giving the fitted parameters: cluster sizes distribution and their polar cluster polarization.

^aOn leave from the Petroleum and Petrochemical College, Chulalongkorn University, Thailand.     

Modified Porous Clay Heterostructures by Organic–Inorganic Hybrids for Nanocomposite Ethylene Scavenging/Sensor Packaging Film

Porous clay heterostructures (PCHs) were prepared by the surfactant‐directed assembly of mesostructured silica within the two‐dimensional galleries of clays. PCH is an interesting material for use as an entrapping system (for example, as an ethylene scavenger) because of its high surface area with uniform and specific pore sizes. In the present work, the PCH was synthesized within the galleries of bentonite by the polymerization of tetraethoxysilane (TEOS) in the presence of surfactant micelles. In addition, mesoporous clay was modified by an organic–inorganic hybrid material through the co‐condensation reaction of TEOS with the functional groups (methyl and thiol) designated as hybrid organic–inorganic PCH (HPCH) and mercaptopropyl functionalized PCH (MPPCH), respectively. The synthesized PCH, HPCH and MPPCH were blended with polypropylene (PP) to produce PCH/PP, HPCH/PP and MPPCH/PP for ethylene scavenging blown films. All nanocomposite films were evaluated as ethylene sensors by measuring the conductivity changes by the attachment time with the ethylene gas. According to the surface characterization, the specific surface areas of modified PCHs increased from 31 to about 500 m²/g. From the ethylene adsorption results, the PCH, HPCH and MPPCH show higher efficiency in adsorbing ethylene gas than those of bentonite because of the non‐polar property of the modified functional groups. Subsequently, the electrical conductivity of the nanocomposite films decreased when they react with the longer attachment time to the ethylene gas, and the largest conductivity drop resulted from the MPPCH/PP nanocomposite films. Copyright © 2011 John Wiley & Sons, Ltd.     

Effective surface treatments for enhancing the thermal conductivity of BN‐filled epoxy composite

To improve the thermal conductivity of BN‐filled epoxy composite, admicellar polymerization was used to coat polystyrene and polymethyl methacrylate on the BN surface to improve the interfacial adhesion in the composite. The treated surface was characterized by FTIR and contact angle measurements. The results show that the admicellar treatment led to improved wettability of epoxy resin on the treated surface. Thermal conductivity of the composite increased from 1.5 W/mK for untreated BN to 2.69 W/mK when the admicellar‐treated BN was used, indicating improvement in the interfacial adhesion between BN and epoxy resin in the composite. The mechanical properties of the composite also improved significantly. The surfactant : monomer molar ratio of 1 : 10 was found to be the optimum condition for the admicellar polymerization process. The solubility parameter concept was used to explain the difference in the effectiveness of polystyrene and polymethyl methacrylate. When compared to the more conventional silane treatment, admicellar treatment was found to be more effective in improving the interfacial adhesion between the BN particles and epoxy resin. SEM micrographs of the fractured surface of the composite further confirm the improvement in the interfacial adhesion after the admicellar treatment. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Wood flour-high density polyethylene composites: Influence of silanization and esterification on mechanical properties

Silanization and esterification are strategies used to treat wood flour (WF) to produce surface functionalized hydrophobic WF leading to an improvement in dispersion and compatibility between wood phase and polymer phase. Silanization involves functionalization of alkyl groups by coupling trimethoxy (propyl) silane (MPS), and esterification functionalizes WF with ester groups, using acetic anhydride (Ac). Modified WF was incorporated into recycled high-density polyethylene (HDPE) to form reHDPE/mod-WF composite system. Both modifications produced highly hydrophobic WF surfaces which improved the dispersion in the reHDPE matrix; resulting in a significant difference in impact strength, specifically at 20 wt% filler, from 74.5 Jm⁻¹ to 146.3 Jm⁻¹ and 113.5 Jm⁻¹, that is, up to 96% and 52% for MPS-WF and Ac-WF, respectively. However, filler agglomeration higher than 20 wt% reduces the composite impact strength. The results herein demonstrate that alkyl-functionalized WF show excellent dispersion in the reHDPE system and is the preferred technique to improve system mechanical resilience as compared to esterification.     

Synthesis and characterization of magnetic porous clay heterostructure

Magnetic porous clay heterostructure (magnetic PCH) was successfully synthesized using a simple precipitation method of applying magnetite onto a PCH surface. X-ray techniques were used to confirm the presence of magnetite in the composite. The magnetite particles, as investigated by the transmission electron microscopy, were spherical nanoparticles (~12.07 nm). The magnetic PCH exhibited characteristics of mesoporous material type IV, similar to PCH. Significant enhancement of the magnetic and dielectric properties in the high frequency range was also observed.     

Mesophase Structure‐Enabled Electrostrictive Property in Nylon‐12‐Based Poly(ether‐block‐amide) Copolymers

To search for alternative electrostrictive polymers and to understand the underlying mechanism, the structure‐ferroelectric/electrostrictive property relationship for nylon‐12‐based poly(ether‐b‐amide) multiblock copolymers (PEBAX) is investigated. Two PEBAX samples are studied, namely, P6333 and P7033 with 37 and 25 mol.% of soft poly(tetramethylene oxide) (PTMO) blocks, respectively. In both samples, poorly hydrogen‐bonded mesophase facilitates electric field‐induced ferroelectric switching. Meanwhile, the longitudinal electrostrictive strain (S₁)–electric field (E) loops are obtained at 2 Hz. Different from conventional poly(vinylidene fluoride‐co‐trifluoroethylene) [P(VDF‐TrFE)]‐based terpolymers, uniaxially stretched nylon‐12‐based PEBAX samples exhibit negative S₁, that is, shrinking rather than elongation in the longitudinal direction. This is attributed to the unique conformation transformation of nylon‐12 crystals during ferroelectric switching. Namely, at a zero electric field, crystalline nylon‐12 chains adopt a more or less antiparallel arrangement of amide groups. Upon high‐field poling, ferroelectric domains are enforced with more twisted chains adopting a parallel arrangement of amide groups. Meanwhile, extensional S₁ is observed for P6333 at electric fields above 150 MV m^?1. This is attributed to the elongation of the amorphous phases (i.e., amorphous nylon‐12 and PTMO). Therefore, competition between shrinking S₁ from mesomorphic nylon‐12 crystals (i.e., nanoactuation) and elongational S₁ from amorphous phases determines the ultimate electrostriction behavior in stretched PEBAX films.