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Helmut Cölfen

Publications and source records attributed to Helmut Cölfen.

At least 19 recordsLinked to original sources
PubMed2006

Synthesis of DL-alanine hollow tubes and core-shell mesostructures.

Three double hydrophilic block copolymers were used as crystal-growth modifiers of DL-alanine to generate amorphous precursor nanoparticles that undergo subsequent mesoscopic transformation to core-shell mesostructures and hollow tubes with quadratic cross-sections. The growth sequence can be stopped at various stages so that a series of intermediates between amorphous core- and crystalline-shell particles and tubes can be obtained. Time-dependent conductivity, TEM, SEM, and environmental scanning electron microscopy (ESEM) measurements were used to obtain a better understanding of the crystallization process, and a formation mechanism for the generation of the tubes is proposed. Na2SO4, NaCl, and NaNO3 as salts differ in their influence on the crystallization behavior of alanine by changing the solubility of alanine and by decreasing the stability of the intermediate particles. Core-shell mesostructures that formed in the dissolution-recrystallization process were captured as the transformation rate was decreased by the addition of copolymers or salts. Hollow tubes with quadratic cross-sections are the final product of the transformation process.

Alanine↗
PubMed2006

In situ investigation of complex BaSO4 fiber generation in the presence of sodium polyacrylate. 1. Kinetics and solution analysis.

Simple solution analysis of the formation mechanism of complex BaSO(4) fiber bundles in the presence of polyacrylate sodium salt, via a bioinspired approach, is reported. Titration of the polyacrylate solution with Ba(2+) revealed complex formation and the optimum ratio of Ba(2+) to polyacrylate for a slow polymer-controlled mineralization process. This is a much simpler and faster method to determine the appropriate additive/mineral concentration pairs as opposed to more common crystallization experiments in which the additive/mineral concentration is varied. Time-dependent pH measurements were carried out to determine the concentration of solution species from which BaSO(4) supersaturation throughout the fiber formation process can be calculated and the second-order kinetics of the Ba(2+) concentration in solution can be identified. Conductivity measurements, pH measurements, and analytical ultracentrifugation revealed the first formed species to be Ba-polyacrylate complexes. A combination of the solution analysis results and optical microscopic images allows a detailed picture of the complex precipitation and self-organization process, a particle-mediated process involving mesoscopic transformations, to be revealed.

Acrylic Resins↗
PubMed2006

In situ investigation of complex BaSO4 fiber generation in the presence of sodium polyacrylate. 2. Crystallization mechanisms.

The formation mechanisms of complex BaSO(4) fiber bundles and cones in the presence of polyacrylate sodium salt via a bioinspired approach at ambient temperature in an aqueous environment are reported. These complex organic-inorganic hybrid structures assemble after heterogeneous nucleation of amorphous precursor particle aggregates on polar surfaces, and the crystallization area can be patterned. In contrast to earlier reports, three different mechanisms based on the oriented attachment of nanoparticles were revealed for the formation of typical fibrous superstructures depending on the supersaturation or on the number of precursor particles. (A) High supersaturation (S > 2): large amorphous aggregates stick to a polar surface, form fiber bundles after mesoscopic transformation and oriented attachment, and then form a narrow tip through polymer interaction. (B) Low supersaturation (S = 1.02-2): only a few fibers nucleate heterogeneously from a single nucleation spot, and amorphous particles stick to existing fibers, which results in the formation of a fiber bundle. (C) Vanishing supersaturation (S = 1-1.02): nucleation of a fiber bundle from a single nucleation spot with self-limiting repetitive growth as a result of the limited amount of building material. These growth processes are supported by time-resolved optical microscopy in solution, TEM, SEM, and DLS.

Acrylic Resins↗
PubMed2006

Penetration coefficients of commercially available and experimental composites intended to infiltrate enamel carious lesions.

OBJECTIVES: Arresting of enamel lesions by infiltration with light curing composite resins might be a treatment alternative of future micro-invasive dentistry. So far only commercially available adhesives and fissure sealants have been used to infiltrate enamel lesions. Since these products have been optimized for adhesive purposes, the aim of this study was to develop optimized resins for the infiltration of enamel lesions and to measure their physical properties. METHODS: The penetration coefficients (PCs) of five adhesives and a fissure sealant as well as 66 experimental composite resins were determined. To establish the resins' PCs the viscosities, surface tensions, and contact angles to bovine enamel were measured. RESULTS: For the commercially available products PCs from 4.0 to 278.9 cm/s were measured. Four of these materials showed a good correlation with penetration depths obtained in a previous study. Experimental composites showed PCs from 0.2 to 474.9 cm/s. The addition of ethanol significantly increased the PCs due to a decrease of viscosity and contact angle. Highest PCs were found for mixtures containing TEGDMA, HEMA and 20% ethanol. SIGNIFICANCE: The knowledge about the PC of resin composites seems to be useful for the development of new materials optimized to infiltrate enamel lesions.

Animals↗
PubMed2006

Calcite mesocrystals: "morphing" crystals by a polyelectrolyte.

Crystallization of calcite from different concentrated calcium chloride solutions by the CO(2) vapor diffusion technique in the presence of polystyrenesulfonate (PSS) yields defined assemblies of nanoparticles with unusual morphology. From the typical calcite rhombohedra, the morphology can be systematically varied via rounded edges and truncated triangles to concavely bent lens-like shapes. Although these "crystals" are apparently well facetted as observed in light microscopy, electron microscopy analysis and BET isotherms reveal that the structures are highly porous and are composed of almost perfectly three-dimensionally aligned calcite nanocrystals, scaffolded to the final, partly curved structures. The formation of all mesostructures is discussed within the framework of a polymer-mediated structure-formation process, in which the polymer is acting in four different ways. The present model case also provides evidence for the importance of nonclassical mesoscopic processes in polymer-controlled crystallization in general.

Journal Article↗
PubMed2006

Morphosynthesis of alanine mesocrystals by pH control.

Crystallization of DL-alanine is studied as a single polymorph model case to analyze the different modes of crystallization of polar organic molecules in absence of any structure directing additives. Depending on supersaturation, which is controlled either by temperature or by pH, and in the absence of additives, crystallization by mesoscale assembly of nanoparticles is found over a wide range of conditions, leading to so-called mesocrystals. This supplements the classical molecule-based crystallization mechanism, which is identified at lower supersaturations and at pH values away from the isoelectric point (IEP). The resulting alanine crystals are characterized by SEM, XRD, and single-crystal analysis. Time-resolved conductivity measurements and dynamic light scattering of the reaction solutions reveal information about precursor structures and reaction kinetics. A formation mechanism is proposed for the alanine mesocrystals.

Journal Article↗
PubMed2006

Oriented attachment and mesocrystals: non-classical crystallization mechanisms based on nanoparticle assembly.

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as "non-classical crystallization", including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

Adsorption↗
PubMed2006

Synthesis of an anionically chargeable, high-molar-mass, second-generation dendronized polymer and the observation of branching by scanning force microscopy.

An efficient synthesis of a methacrylate-based, second-generation (G2) dendronized macromonomer and its free radical polymerization to the corresponding high-molar-mass G2 dendronized polymer are described. The molar mass is determined by gel permeation chromatography (GPC), light-scattering, and analytical ultracentrifugation and compared with values estimated from a scanning force microscopy (SFM) contour lengths analysis of individualized polymer strands on mica. The polymer carries terminal tert-butyl-protected carboxyl groups, the degree of deprotection of which with trifluoroacetic acid is quantified by NMR spectroscopy using the highest molar mass sample. SFM imaging of both protected (noncharged) and unprotected (charged) dendronized polymers on solid substrates reveals mostly linear chains but also some with main-chain branches. The nature of these branches is investigated and the degree roughly estimated to which they are formed. Finally, a synthetic model experiment is described which sheds some light on the aspect of whether chain transfer, a process that could lead to covalent branching, is of importance in the synthesis of the present dendronized polymers.

Journal Article↗
PubMed2006

Block-copolymer-controlled growth of CaCO3 microrings.

A novel way for directed solution growth of hollow superstructures of CaCO3 has been successfully developed on the basis of controlled self-assembly and polymer concentration gradients using a double-hydrophilic block copolymer with a hydrophobic modification as a directing agent. A formation mechanism of such rings is proposed on the basis of the formation of CaCO3 nanoparticles in unstructured block copolymer assemblies with subsequent aggregation of these primary nanoparticles. This leads to the formation of a polymer concentration gradient from the inside to the outside of the particle. As the polymer contains multiple chelating units, this leads to a selective dissolution of the center of the particle.

Calcium Carbonate↗
PubMed2006

Polymer-controlled crystallization of zinc oxide hexagonal nanorings and disks.

In this study, we have developed a novel route to the synthesis of ZnO nanorings, disks, and diskoidlike crystals on a large scale by a facile solution-based method by using polymers as crystal growth modifiers. The crystals precipitated with polyacrylamide (PAM) as the additive show ringlike morphology. A possible growth mechanism of the ZnO nanostructures based on typical polymer-crystals interactions in a mild aqueous solution is given. The polymer contains in the side chain a large number of amide ligands that are able to coordinate with Zn(2+) ions, that is, the otherwise just weakly exposed (001) face, leading to a lowering of surface energy and inhibition of growth along this direction and the formation of ringlike morphologies. While in the presence of carboxyl-functionalized polyacrylamide (PAM-COOH), nearly monodispersed disklike crystals were observed and finally evolved into diskoidlike microstructures with the reaction time prolonged. Polymer-directed crystal growth and mediated self-assembly of nanocrystals may provide promising routes to rational synthesis of various ordered inorganic and inorganic-organic hybrid materials with complex form and structural specialization.

Journal Article↗
PubMed2005

Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment.

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

Journal Article↗
PubMed2005

Amorphous layer around aragonite platelets in nacre.

We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.

Animals↗
PubMed2005

Dynamic and fluorescent nanoscale phenanthroline/terpyridine zinc(II) ladders. Self-recognition in unlike ligand/like metal coordination scenarios.

A new coordination kit, the HETTAP approach, is described, which allows one to prepare supramolecular heteroleptic metal phenanthroline/terpyridine racks and ladders in basically quantitative yield. Due to the dynamic nature of the nanoladder structures, a two-step double-transmetalation and scenarios with like metal/unlike ligand recognition were realized.

Crystallization↗
PubMed2005

Influence of the application time on the penetration of different dental adhesives and a fissure sealant into artificial subsurface lesions in bovine enamel.

UNLABELLED: Sealing of approximal enamel lesions by infiltration with low viscous resins seems to be a promising approach in non-operative dentistry and should bear advantages compared to remineralization or invasive treatment. OBJECTIVE: The aim of this in vitro study was to evaluate the penetration ability of five dental adhesives and a fissure sealant into initial enamel lesions for an application time of either 15 s or 30 s. METHODS: In each of 54 specimens of bovine enamel, three windows were demineralized for 14 days. Subsequently, two windows were etched with phosphoric acid for 5s in order to degrade the surface layer, whereas one window served as untreated control. The specimens were randomly divided into six groups and a fissure sealant as well as five different adhesives were applied onto the subsurface lesions and allowed to penetrate for either 15 or 30 s. Overlying material was wiped away and the resins were light cured. To visualize the penetrated resins and the remaining pore structures, the specimens were infiltrated with a low viscous fluorescent resin and studied using a Confocal Laser Scanning Microscope (CLSM). RESULTS: For Helioseal, Heliobond, Resulcin Monobond, and Excite an application time of 30s resulted in significantly higher (p < 0.05; t-test) penetration depths (47-105 microm) compared to 15s (29-49 microm). SIGNIFICANCE: Helioseal, Heliobond, Resulcin Monobond, and Excite are suitable for sealing artificial subsurface enamel lesions in vitro. After an application time of 30 s a significant deeper penetration could be observed for these materials.

Acid Etching, Dental↗
PubMed2005

Polymer-induced alignment of DL-alanine nanocrystals to crystalline mesostructures.

Crystallization of DL-alanine by cooling of a supersaturated solution in the presence of a chiral double-hydrophilic block copolymer poly(ethylene glycol)-block-poly(ethylene imine)-S-isobutyric acid (PEG4700-PEI1200-S-iBAc) yields crystal superstructures with an astonishing morphology. Although analysis by light microscopy reveals these crystals to be apparently well facetted, electron microscopy shows that they consist of three-dimensionally, well-aligned nanocrystals that are scaffolded to a so-called mesocrystal. This mesocrystal is formed by polymer-mediated structuration, and provides evidence for the importance of mesoscopic events in a typical crystallization process.

Journal Article↗