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 20,729 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Mesoporous aluminophosphate materials with variable amounts of zirconium have been synthesized at room temperature using a nonionic surfactant tri-block copolymer (PEO(20)PPO(70)PEO(20)) as the structure-directing agent. Powder X-ray diffraction of as-synthesized and calcined AlPO and ZrAlPO mesoporous materials shows a single broad peak near 2theta = 2.5 degrees, indicative of the average pore-pore correlation distance. Electron probe microanalysis shows that the ratio of P/Al in the powders is approximately 0.5, far lower than 1.0 for an ideal aluminophosphate framework. XRD, TEM, and N2 adsorption data indicate that the calcined samples consist of wormlike tubular materials having surface areas >350 m2/g and pores in the mesopore range. Electron spin resonance (ESR) studies of the gamma-irradiated and evacuated ZrAlPO samples show signals due to Zr3+ that increase with Zr content in addition to signals due to framework defects (i.e., V centers) and H atoms. The line shape and g values observed for Zr3+ are best explained as arising from a trivalent zirconium ion situated at the framework tetrahedral sites.
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PMID:Synthesis of mesoporous aluminophosphate (AlPO) and investigation of zirconium incorporation into mesoporous AlPOs. 1685 10

Acidic surfactants, single- and bi-2-methylheptanol polyethenoxy ether phosphate esters, H2PO3(OCH2CH2)nOCH2CH2CH2CH2CH2CH(CH3)2 (u-MHPEPE) and HPO3[(OCH2CH2)nOCH2CH2CH2CH2CH2CH(CH3)2]2 (d-MHPEPE), where n approximately 4, were synthesized. Phase behavior of u- and d-MHPEPE (u- and d-MHPEPE mixtures were abbreviated as MHPEPE) mixtures in aqueous solutions and vesicle formation were determined. Surface tension measurements showed that u-MHPEPE and MHPEPE have low surface tensions at critical micelle concentrations. gamma(cmc)=29.0 mNm(-1) and cmc=16.0 mmolL(-1) for u-MHPEPE, MHPEPE has two transition points suggesting the mixtures of u- and d-MHPEPE with gamma(cmc1)=30.5 mNm(-1) and cmc1=4.0 mmolL(-1), and gamma(cmc2)=27.3 mNm(-1) and cmc2=42.0 mmolL(-1). These values, specific gamma(cmc), are much lower than those of traditionally cationic or anionic surfactants such as cetyltrimethylammonium bromide (CTAB, gamma(cmc)=37.1 mNm(-1) at cmc=0.92 mmolL(-1)) and sodium dodecyl sulfate (SDS, gamma(cmc)=39.0 mNm(-1) at cmc=8.1 mmolL(-1)). Rich phase behavior was observed with increasing MHPEPE concentration, an isotropic L(1)-phase (micelle solution), an unstable emulsion-region (with time, the samples separate into two-phase), a transparently bluish and birefringent Lalpha-phase up to 200 mmol L(-1) with unilamellar and multilamellar vesicles. These unilamellar and multilamellar vesicles were demonstrated by using staining transmission electron microscopy (staining-TEM), which were compared to freeze-fracture TEM (FF-TEM). The vesicle-phase is stable for at least 1 year. Vesicle formation possibly could be explained in harmonization of the hydrophobic force of acidic surfactant tails, the hydrogen bonding (H-bonding) and the electrostatic interaction among polar headgroups of PEO ether phosphate ester. Phase transition from the flow birefringent unilamellar vesicles induced by addition of HCl, NaCl, NaOH, and increasing temperature has been observed. Surprisingly, for u-MHPEPE or d-MHPEPE in water, we just observed L1-phase (micelle solution) with increasing u-MHPEPE or d-MHPEPE concentration.
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PMID:A promising system of mixed single- and double-short-tailed PEO ether phosphate esters: phase behavior and vesicle formation. 1688 36

Homocentric ZnO nanobundles with pyramidlike and hexagonal prism shapes were synthesized in colloidal systems formed by PEO-PPO-PEO amphiphilic block copolymers. The prism- and pyramidlike ZnO crystals were produced by L64 and F68, respectively, which may be attributed to the different growth rates of various crystal facets. It was proposed that the two processes for crystallization, including nucleation and crystal growth, happened in the macromolecular micelles under hydrothermal conditions. The room-temperature photoluminescence spectra of the ZnO products showed sharp ultraviolet emission located around 390 nm originating from the radiative recombination of free excitons. The sharp emission, with a half-maximum of about 8 nm, gave a powerful attestation that the sample was of high crystal quality, which was consistent with the SEM and TEM observations. The single ultraviolet emission is important for the application of ZnO-based materials in the electronic and photonic realms.
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PMID:Hydrothermal synthesis of ZnO nanobundles controlled by PEO-PPO-PEO block copolymers. 1716 9

In this study, temperature-responsive magnetite/polymer nanoparticles were developed from iron oxide nanoparticles and poly(ethyleneimine)-modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer. The particles were characterized by TEM, XRD, DLS, VSM, FTIR, and TGA. A typical product has an approximately 20 nm magnetite core and an approximately 40 nm hydrodynamic diameter with a narrow size distribution and is superparamagnetic with large saturation magnetization (51.34 emu/g) at room temperature. The most attractive feature of the nanoparticles is their temperature-responsive volume-transition property. DLS results indicated that their average hydrodynamic diameter underwent a sharp decrease from 45 to 25 nm while evaluating the temperature from 20 to 35 degrees C. The temperature-dependent evolution of the C-O stretching band in the FTIR spectra of the aqueous nanoparticles solution revealed that thermo-induced self-assembly of the immobilized block copolymers occurred on the magnetite solid surfaces, which is accompanied by a conformational change from a fully extended state to a highly coiled state of the copolymer. Consequently, the copolymer shell could act as a temperature-controlled "gate" for the transit of guest substance. The uptake and release of both hydrophobic and hydrophilic model drugs were well controlled by switching the transient opening and closing of the polymer shell at different temperatures. A sustained release of about 3 days was achieved in simulated human body conditions. In primary mouse experiments, drug-entrapped magnetic nanoparticles showed good biocompatibility and effective therapy for spinal cord damage. Such intelligent magnetic nanoparticles are attractive candidates for widespread biomedical applications, particularly in controlled drug-targeting delivery.
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PMID:Temperature-responsive magnetite/PEO-PPO-PEO block copolymer nanoparticles for controlled drug targeting delivery. 1798 60

A new poly(ethylene oxide)-tetraphenylalanine polymer-peptide conjugate has been prepared via a "click" reaction between an alkyne-modified peptide and an azide-terminated PEO oligomer. Self-assembled nanotubes are formed after dialysis of a THF solution of this polymer-peptide conjugate against water. The structure of these nanotubes has been probed by circular dichroism, IR, TEM, and SAXS. From these data, it is apparent that self-assembly involves the formation of antiparallel beta-sheets and pi-pi-stacking. Nanotubes are formed at concentrations between 2 and 10 mg mL(-1). Entanglement between adjacent nanotubes occurs at higher concentrations, resulting in the formation of soft hydrogels. Gel strength increases at higher polymer-peptide conjugate concentration, as expected.
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PMID:Soft hydrogels from nanotubes of poly(ethylene oxide)-tetraphenylalanine conjugates prepared by click chemistry. 1916 Dec 73

This communication reports the detailed fabrication of electrodes and solid polymer electrolyte with ionic liquid (IL) as an electrolyte for dye sensitized solar cell (DSSC). Thick porous TiO2 film has been obtained by spreading and sintering TiO2 colloidal paste using "doctor blade" and characterized by SEM, TEM and XRD. The polymer electrolyte was PEO:KI/I2 incorporated with 1-ethyl 3-methylimidazolium thiocyanate (EMImSCN) as IL. Dispersal of IL in the polymer electrolyte improved the ionic conductivity and cell efficiency.
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PMID:Nanocrystalline porous TiO2 electrode with ionic liquid impregnated solid polymer electrolyte for dye sensitized solar cells. 1919 36

We report on the aggregation behavior of asymmetric amphiphilic poly(ethylene oxide)-block-poly(n-butyl acrylate) (PEO-PnBA) diblock copolymers into vesicles and micelles in aqueous media. These copolymers are composed of a hydrophilic PEO block of a constant chain length of 113 monomer units and longer soft hydrophobic PnBA blocks of 163 and 243 units, respectively. This study presents the first insight into the solution properties of such copolymers examined by dynamic light scattering (DLS), cryogenic and conventional transmission electron microscopy (cryo-TEM, TEM), field-flow fractionation (FFF), and turbidity measurements. Whereas the block copolymer with the shorter PnBA block forms crew-cut micelles, for the polymer with the longer hydrophobic block, the morphology of the aggregates strongly depends on concentration. Besides crew-cut micelles, we observed very peculiar vesicular structures, in particular entrapped, multilamellar, and multicompartment vesicles. Although PnBA has a low glass transition temperature, the aggregates formed in water are kinetically frozen.
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PMID:Self-assembly of asymmetric poly(ethylene oxide)-block-poly(n-butyl acrylate) diblock copolymers in aqueous media to unexpected morphologies. 1932 May 21

Block copolymer microphase separation in the bulk is coupled to amphiphilic block copolymer self-assembly at the air-water interface to yield hierarchical Langmuir-Blodgett (LB) structures combining organization at the meso- and nanoscales. A blend of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (Mn=141K, 11.4 wt % PEO) and polystyrene-b-poly(butadiene) (PS-b-PB) (Mn=31.9K, 28.5 wt % PB) containing a PS-b-PB weight fraction of f=0.75 was deposited at the air-water interface, resulting in the spontaneous generation of aggregates with multiscale organization, including nanoscale cylinders in mesoscale strands, via evaporation of the spreading solvent. The resulting features were characterized in LB films via AFM and TEM and at the air-water interface via Langmuir compression isotherms. Blends containing lower PS-b-PB contents formed mesoscale aggregate morphologies of continents and strands (f=0.50) or mesoscale continents with holes (f=0.25), but without the internal nanoscale organization found in the f=0.75 blend. The interfacial self-assembly of pure PS-b-PB at the air-water interface (f=1) yielded taller and more irregularly shaped aggregates than blends containing PS-b-PEO, indicating the integral role of the amphiphilic copolymer in regulating the mesoscale organization of the hierarchically structured features.
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PMID:Block copolymer strands with internal microphase separation structure via self-assembly at the air-water interface. 1946 88

The self-assembly of linear poly(ethylene oxide)-b-poly(glycerol monomethacrylate)-b-poly[2-(diisopropylamino)ethyl methacrylate] (PEO-b-PG2MA-b-PDPA) triblock copolymer into pH-responsive cross-linkable nanostructures in both organic and aqueous media is reported. Light scattering (LS), electron transmission microscopy (TEM), and nuclear magnetic resonance spectroscopy (NMR) techniques revealed that spherical particles with a core-shell architecture originated upon direct copolymer dissolution in THF, with PG2MA middle blocks occupying the nucleus, and PEO + PDPA segments forming the external layer. The hydroxylated core could be conveniently reticulated to form core cross-linked (CCL) micelles, which swelled without dissociating in presence of water at pH < pK(a) of amino groups. In the absence of stabilizing mechanisms (cross-links), the aggregates first disassembled in response to changes in the solvent selectivity due to water addition and eventually self-assembled again into spherical particles with a three-layered core-shell-corona structure. While pH-responsive PDPA segments were located at the core, PG2MA and PEO blocks composed the inner shell and corona, respectively. The interactions that facilitate micelle existence were reinforced by covalent cross-links in the PG2MA inner shell. Thus, depending on both the solution pH and the presence of cross-links, micelles exhibiting either pH-triggered or diffusion-controlled release mechanisms could be prepared. The encapsulation of enough amounts of guest molecules that interact strongly with the core-forming block led to the formation of cylindrical micelles. These results demonstrate that at least five different types of aggregates can be prepared from this versatile triblock copolymer, thus emphasizing the great potential of combining macromolecular design and sample manipulation strategies to devise functional nanostructures.
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PMID:Aggregation of a versatile triblock copolymer into pH-responsive cross-linkable nanostructures in both organic and aqueous media. 1960 85

A vesicle system is described that possesses a pH-induced "breathing" feature and consists of a three-layered wall structure. The "breathing" feature consists of a highly reversible vesicle volume change by a factor of ca. 7, accompanied by diffusion of species into and out of the vesicles with a relaxation time of ca. 1 min. The triblock copolymer poly(ethylene oxide)(45)-block-polystyrene(130)-block-poly(2-diethylaminoethyl methacrylate)(120) (PEO(45)-b-PS(130)-b-PDEA(120)) was synthesized via ATRP. Self-assembly into vesicles was carried out at a pH of ca.10.4. The vesicle wall was shown by cryo-TEM to consist of a sandwich of two external ca. 4 nm thick continuous PS layers and one ca. 17 nm thick PDEA layer in the middle. As the pH decreases, both the vesicle size and the thickness of all three layers increase. The increase of the thickness of the intermediate PDEA layer arises from the protonation and hydration, but the swelling is constrained by the PS layers. The increase of the thickness of the two PS layers is a result of an increasing incompatibility and an accompanying sharpening of the interface between the PS layers and the PDEA layer. Starting at a pH slightly below 6, progressive swelling of the PDEA layer with decreasing pH induces a cracking of the two PS layers and also a sharp increase of the vesicle size and the wall thickness. By pH 3.4, the vesicle size has increased by a factor of approximately 1.9 and the wall shows a cracked surface. These changes between pH 10.4 and 3.4 are highly reversible with the relaxation time of ca. 1 min and can be performed repeatedly. The change in the wall structure not only increases dramatically the wall permeability to water but also greatly expands the rate of proton diffusion from practically zero to extremely rapid.
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PMID:"Breathing" vesicles. 1972 30


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