摘要:

AbstractAerogels are highly porous, nanostructured sol‐gel derived materials. In the nineteen sixties and seventies highly transparent and monolithic silica aerogels have mainly been used in Cherenkov detectors to determine the momentum of relativistic particles. After an increasing availability of granular SiO2‐aerogels from a BASF pilot plant in the eighties, applications of these materials in solar architecture (especially for daylighting purposes and transparent thermal insulations) have been investigated. Today, additionally research focuses on metalloxide‐, organic‐ and carbon‐aerogels. The applications range from acoustic impedance matching layers in ultrasonic transducers to subtrates for catalytic materials, transparent molds for melt casting, sensors, electrodes in supercapacitors and fuel cells as well as capture of cosmic dust and micrometeorits. Particularly interesting with respect to mass applications is the use of aerogel powder in the evacuated superinsulation of the BMW latent heat storage device which can be employed to defrost the windshield or to preheat the engine. A broad availability of cheap aerogels is expected from a production process based on subcritical drying (versus supercritical extraction in an autoclave) currently persued by H

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021102

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractProtein folding is one of the most challenging problems in science. How can polymers memorize and fold into unique conformations? How can they recognize molecules, catalyze chemical reactions, transfer molecular information, and create motions! The principle behind these mechanisms lies in the concept of thernodynamically stable phases of heteropolymers. Recent theories predict that the collapsed phase should be further classified into three phases: freely fluctuating like liquid, frozen in degenerate conformations, and frozen in a unique conformation. This yields a clue on how one can attempt creation artificial polymers capable to mimic some of the protein properties and functions. The reversible adsorption of target molecules is suggested as a primary means to achieve this goal. Target molecules with multiple adsorption sites play a twofold role. First, they mediate specific interactions between monomers and thus serve as “gluons”. Second, monitoring the adsorption provides the experimental possibility to test directly on monomer contacts, which is directly related to observation of the order parameter associated with heteropolymer freezing transition. A slight change in the backbone conformation alters the spacial arrangement of the group, allowing for reversible adsorption and release. Polymer gels are developed that can reversibly change their affinity to target molecules by orders of magnitude. The gels are made of copolymers of backbone monomers that can reorganize themselves through thermal volume phase transition and of monomers that can attract the target at multiple contact points. Further the gels “imprinted” with the target showed a marked increase in the affinity, thus mimicing a protein‐like ability to memorize and recognize certa

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021103

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe majority of synthetic polymer gels are formed by the covalent cross‐linking of linear or branched macro‐molecules using multi‐functional cross‐linking agents. Such gels are networks, or true macromolecules with (nominally) infinite molecular weight and consequently they swell rather than dissolve if immersed in a good solvent. However, there are also a whole class of materials called physical gels where non‐covalent crosslinks occur. These show similarities to covalent networks, but because the cross‐links are not permanent, they will show creep behaviour at very long times. Some of these form on cooling a heated solution, such as gelatin gels, while others form only on heating. Also, some gels are thermoreversible, while others are not.Physical gels can be formed from synthetic or biopolymers. In the latter case non‐covalent cross‐links often comprise more specific and complex mechanisms involving, rather than point‐like cross‐links, “junction zones” of known, ordered secondary structure such as multiple helices. Typically there is a specific, and often intricate, hierarchy of arrangements, which are more familiar to molecular biologists than to polymer physical chemists. In this paper we introduce viscoelastic techniques for characteristing physical gels, and then relate the properties to the underlying structure at the macromolecular and junction zone level. The parallels between synthetic and biopolymer gels will also be illustrated. Finally we describe recent work on the heat‐set gelation of globular proteins, and an attempt to relate the important parameter of the gelation time,tc, to both polymer conc

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021104

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractPorous networks of titanium dioxide have been produced using polymer gels as templates. Titanium dioxide is well known for its semiconductor and catalysis properties, and for being a very stable material. The porous polymer gels are produced in aqueous surfactant solutions. Following cleaning and solvent exchange procedures the titanium precursor can be incorporated within the gel and hydrolysed. The organic material is removed during calcination leaving the structured titanium dioxide. Both the rutile and anatase forms of titanium dioxide can be produced. The resulting crystal structure and the surface area of the oxide are important for determining its scope of application. These properties have been studied using electron microscopy, X‐ray diffraction and gas sorptio

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021105

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe sol‐gel transition of a novel glycol ester of ortho silicic acid is investigated by small angle neutron scattering technique. We measured the changes of scattering intensity as a function of time during gelation and show the formation of a fractal structure.When we mix our ester with an aqueous surfactant solution no precipitate is formed as observed in common template synthesis of mesoporous materials. Because of the water solubility of our silicate precursor it can be dissolved into the surfactant phase without phase separation. During hydrolysis of the precursor ethylene glycol is produced which does not affect the hydrophobic interaction of the surfactant.It is shown by contrast variation that surfactant does not affect the fractal dimension of the final silica gel. Measurements after gelation demonstrate the unchanged presence of surfactant micelle

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021106

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractTitanium species and substituted or unsubstituted zinc phthalocyanine (ZnPc) complexes exhibiting negative, positive or no charges are incorporated in mesoporous MCM‐41 materials by (i) hydrothermal synthesis in the presence of hexadecyltrimethylammonium chloride as surfactant and (ii) impregnation of pre‐synthesized calcined Si‐MCM‐41.The ordering of the hexagonal MCM‐41 structures is revealed from X‐ray diffractograms and nitrogen adsorption isotherms. The dispersion and the coordination of the Ti in the walls or in the pores is deduced from UV/Vis diffuse reflectance spectra (DR‐UV/Vis). The amount of Ti built into the walls of the MCM‐41 under maintenance of a perfect hexagonal structure is restricted to about 2 wt.‐%. In the pores up to 8 wt.‐% Ti can be fixed a silanol groups under molecular, i.e. non‐aggregated, dispersion.The degree of dispersion of ZnPc is concluded from the analysis of the Q‐band structure in the DR‐UV/Vis spectra. Predominantly non‐aggregated ZnPcs in a monomeric state can be obtained, (i) if the ZnPC is incorporated in the micellar structure of the surfactant molecules by adding it to the synthesis gel of the MCM‐41 or (ii) if the ZnPc is adsorbed to Ti‐MCM‐41 with Ti species in high dispersion in the pores. A tight contact between the donor ZnPc and the acceptor titanium oxide, enables an electron transfer and a quenchi

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021107

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractBy using the sol‐gel process, free base porphyrin doped silica and silica/titania, gels, monoliths, and glasses, with up to 20 wt.% titania, have been synthesised. The monoliths were characterised by BET isotherms, X‐Ray spectroscopy, and AFM spectroscopy. Absorption and fluorescence spectroscopic measurements indicate that, in Si and Si/Ti‐monoliths an equilibrium between the free base hematoporphyrin IX and its dicationic form exists. By additionally doping of pyrene as a donor dye, the energy transfer to hematoporphyrin IX as acceptor was investigated. Stem‐Volmer quenching of the fluorescence of pyrene was found in mo

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021108

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractSilica‐based inorganic networks combined with organic rare earth complexes should lead to a new class of luminescent materials, adding the chemical stability of silica to the strong UV absorption of organic ligands and the emissive properties of rare earth ions. This concept is demonstrated by the preparation of silica xerogels containing benzoic acid in its network and subsequent reaction (ion exchange) of these gels with terbium chloride solution. The product yields green luminescence similar to the one of solid terbium benzoate. Alternatively, complete rare earth complexes (example: europium thenoyltrifluoracetonate) may be immobilized in silica xerogels, yielding luminescent hybrid material

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021109

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractTemperature dependent EXAFS measurements have been carried out on 2‐mercaptoethanol‐stabilized CdTe nanocrystals. The technique allows to study the structural and dynamic properties of the CdTe core and the Cd‐SR (R = organic rest) shell separately. Based on the coordination numbers and the Cd/Te‐ratio determined by EXAFS, the formula [Cd54Te32(SCH2CH2OH)52]8−is suggested. The underlying structure is a tetrahedral CdTe core (zincblende) partially coated by a Cd‐SR su

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021110

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY

摘要:

AbstractThe gelation of TiO2‐sols, prepared by sol‐gel process, can be monitored by adding polyethylene glycol as polymer (sterical stabilisation). The forming of the inorganic TiO2network was examined by using the microprobe technique. Pyrene was doped in TiO2‐sols and its characteristic fluorescence spectroscopic changes were investigated during the gelation. The vibrational fine structure of its emission spectrum changes and the ratioI3/I1was determined and compared with homogenous solvents. By pulse fluorimetric measurements two different lifetimes of pyrene in TiO2‐sols wer

ISSN:0005-9021    

DOI:10.1002/bbpc.19981021111

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1998

数据来源: WILEY