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991.
Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO3 and SrTiO3 . The segregation behavior for isovalent dopants (e.g., Sr2+ , Ca2+ , or Pb2+ at a Ba2+ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La3+ or Nd3+ at a Ba2+ site and Fe3+ or Ni3+ at a Ti4+ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline-earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series. 相似文献
992.
A proposed model for interfacial segregation in perovskites, with induced heterogeneous defect distributions, is extended here to account for the formation of internal boundary layer devices, such as positive temperature coefficient of resistance (PTCR) thermistors and internal boundary layer capacitors (IBLC). Boundary layer effects in doped BaTiO3 are attributed to factors which contribute to the formation of highly resistive boundary layers by a segregation-induced shift in donor incorporation and/or acceptor segregation, and the inhibiting action of segregated donors on boundary mobility and grain growth. The distribution of space charges, formed by electron transfer from conductive grains to resistive boundary layers, leads to the formation of impedance barriers in the grain-boundary vicinity. Depending on the grain size, and on relative size and spatial distribution of the space charge layer and the resistive layer, a transition from semiconducting properties to insulating properties may take place. This model accounts for the observed PTCR and IBLC phenomena. 相似文献
993.
Yoshihisa Kano Yuichi Katayama Saburo Akiyama Yasuaki Kitazaki Tomoyuki Kasemura 《The Journal of Adhesion》1997,62(1):75-92
The surface and bottom compositions of poly(2-ethylhexyl acrylate) (P2EHA)/poly-(vinylidene fluoride-co-hexafluoro acetone) [P(VDF-HFA)] blends were investigated by XPS analysis. In these blends, the enriched P(VDF-HFA) layer was observed from the top surface to a few nm depth, while the P2EHA component segregated at the bottom (the surface in contact with release liner). The probe tack values of the bottom side were remarkably larger than those of the surface side for the P2EHA/P(VDF-HFA) blends because the characteristic segregation structure was formed. We expected that the P2EHA/P(VDF-HFA) blends could be utilized as a non-backing pressure sensitive adhesive (PSA) tape. The effects of the film thickness of the blends and the molecular weights of P2EHA on the segregation structure were also examined. We thought that segregation behavior found in P2EHA/P(VDF-HFA) blends was caused by several factors, such as surface tension, miscibility, density, and rate of solvent casting. 相似文献
994.
995.
The morphology of polybutadiene containing polyurethanes has been studied by using small-angle light scattering (SALS), small-angle X-ray scattering (SAXS), phase contrast microscopy and electron microscopy. Domain structures having sizes of 1000, 100 and 10 nm are observed within these samples and are explained as the results of segregation on the molecular level, subchain level and segmental level, respectively. These phenomena are related to the molecular heterogeneity in chemical composition and average hard segment length. It has been found that because of the analogy in molecular structure with SBS block copolymers, segmented polyurethanes may form microdomains of soft and hard segments with more or less uniform size and regular arrangements in space. 相似文献
996.
The effect of siloxane chain length on surface segregation of poly(methyl methacrylate)‐grafted poly(dimethyl siloxane) (PMMA‐g‐PDMS)/poly(2‐ethylhexyl acrylate‐co‐acrylic acid‐co‐vinyl acetate) [P(2EHA‐AA‐VAc)] blends was investigated. The blends of PMMA‐g‐PDMS with P(2EHA‐AA‐VAc) showed surface segregations of PDMS components. The surface enrichments of PDMS in the blends depended significantly on the PDMS chain length. Also, this blend showed the gradient domain structure. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 375–380, 2003 相似文献
997.
Influences of contents and molecular weights of low‐density polyethylene (LDPE) on dioctyl phthalate (DOP) plasticization in the poly(vinyl chloride) (PVC) plastisol (PVC/DOP/AO = 100/30/6.5) were investigated using DMA and DSC. The plasticization effects of DOP on the PVC plastisol were found to decrease with increasing LDPE content. A negligible plasticization effect of DOP on the PVC plastisol was found when the LDPE content was equal to or higher than 75 parts per 100 parts by weight of LDPE and PVC together. Based on thermal fractionation experiments, a favorable interaction between LDPE and DOP was developed during melt blending of LDPE and the PVC plastisol. The present interaction enabled the incorporation of DOP into LDPE and decreased the plasticization effects of DOP on the PVC plastisol. A further decrease in the plasticization effects of DOP on the PVC plastisol by the presence of LDPE was found with increasing LDPE molecular weights. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2548–2555, 2002 相似文献
998.
A novel acrylic terpolymer with pendant perfluoropolyether (PFPE) segments has been synthesized and fully characterized. By hexamethylene diisocyanate functional groups PFPE monofunctional macromonomers have been grafted on a poly(butyl methacrylate-co-hydroxyethyl acrylate-co-ethyl acrylate) random terpolymer. Such grafted copolymer behaves like an interface-active material, since the perfluoropolyether segments in solvent cast films rearrange themselves at the air-polymer interface by surface segregation. In addition, blends of the above graft copolymer with acrylic base polymers (either the terpolymer itself or a commercial copolymer) have been examined in terms of surface segregation and fluorine enrichment of the external layers.The critical surface tension, γc, of solid films made of the neat graft copolymer as well as of the polymer blend has been evaluated by contact angle measurements and Zisman plots. Even a small addition (5 wt%) of the fluorinated copolymer to the acrylic component has been found very effective in lowering the surface tension. The outermost surface composition has been investigated by XPS technique, confirming the strong fluorine enrichment. Furthermore, SEM and EDX analyses have been performed on cross-sectioned films, showing that in the above polymer blends macrophase surface segregation has originated a thick layer made of fluorinated copolymer close to the air-polymer interface. 相似文献
999.
Constantin Ciobanu Xiaozu Han Constantin N. Cascaval Fengchun Guo Dan Rosu Leonard Ignat Gabriela Moroi 《应用聚合物科学杂志》2003,87(11):1858-1867
Three series of weakly crosslinked polyurethanes based on a hydroxyterminated polybutadiene, hydroxyterminated butadiene–acrylonitrile copolymer, and hydroxyterminated ethylene–adipate–maleate–fumarate copolymer were prepared while varying the hard segment content between 1.72 and 2.36 mol ? NH? COO? /1000 g polymer. Information on the microphase structure and the properties of the synthesized polyurethanes was obtained by differential scanning calorimetry, wide‐angle X‐ray scattering, and mechanical studies. Up to a urethane concentration of around 2 mol ? NH? COO? /1000 g polymer, there is a mixture of hard–hard and hard–soft segments. Above this concentration a large part of the hard–soft segments passes into hard–hard crystallites, liberating the soft segments. The best mechanical properties of the studied polyurethanes were found for a urethane concentration of around 2 mol ? NH? COO? /1000 g polymer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1858–1867, 2003 相似文献
1000.
Joshua A. Orlicki 《Polymer》2007,48(10):2818-2826
Hyperbranched polyethyleneimines were modified with methacrylated fluorosurfactants and aliphatic epoxides to provide a library of macromolecules with controlled chain ends and residual amine functionality. These materials were co-dissolved with a thermoplastic polyurethane-ether and the blends were subsequently deposited as films cast from solution. The surface chemistry of the cast films was determined using angle resolved X-ray photoelectron spectroscopy (AR-XPS) and Rutherford backscattering spectroscopy (RBS). Experimental results indicate that the modified hyperbranched polymers (HBPs) concentrate at the air-polymer interface. Furthermore, HBPs that were complexed to polyoxometalates (POMs) using electrostatic interactions also exhibited surface segregation in cast polymer films, resulting in ca. 10-fold increase of metal at the film surface relative to the known bulk concentration. Results from XPS and RBS examination of the films are consistent with surface segregation of the HBP-POM hybrids, exhibiting increased metal, fluorine, and nitrogen content near the surface of the film, as well as significant changes in wetting behavior. This study indicates that modified HBPs may be used to selectively transport inorganic species such as polyoxometalates to polymer film surfaces. 相似文献