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At least 1,171 records · Page 65Linked to original sources
PubMed2007

Surface ATRP of hydrophilic monomers from ultrafine aqueous silica sols using anionic polyelectrolytic macroinitiators.

A convenient two-step route was developed to prepare new anionic ATRP macroinitiators from near-monodisperse poly(2-hydroxyethyl methacrylate) precursors by partial esterification with 2-bromoisobutyryl bromide, followed by esterification of the remaining hydroxyl groups using excess 2-sulfobenzoic acid cyclic anhydride. These new macroinitiators can be electrostatically adsorbed onto ultrafine cationic Ludox CL silica sols; subsequent surface polymerization of various hydrophilic monomers in aqueous solution at room temperature afforded a range of polymer-grafted ultrafine silica sols. The resulting sterically stabilized particles were characterized by dynamic light scattering, transmission electron microscopy, aqueous electrophoresis, FTIR spectroscopy, and elemental microanalyses.

Journal Article↗
PubMedDate not supplied

Coenzyme models. 10. Rapid oxidation of NADH by a flavin immobilized in cationic polyelectrolytes.

The title flavin rapidly oxidizes NADH, the observed enhancement being of 460--4650-fold compared with 3-methyltetra-O-acetylriboflavin. Analysis of the kinetic data at 30 degrees C established that the reaction proceeds according to the Michaelis-Menten kinetics at low ionic strength (mu = 0.02: Km = ca. 10(-5) M, kact. = 0.06-0.37 s-1) and according to the simple second-order kinetics at high ionic strength (mu = 0.3), the largest rate difference being 342-fold. On the other hand, the oxidation of 1-benzyl-1,4-dihydronicotinamide, a NADH model compound, was not accelerated by the use of the title flavin. It is concluded, therefore, that the formation of the polymeric flavin-NADH complexes due to the electrostatic interaction is responsible for the marked rate enhancement. This is the first example that the oxidation of NADH is facilitated in the nonenzymatic system.

Flavins↗
PubMed2006

Adsorption of polyelectrolyte molecules to a nanostructured monolayer of amphiphiles.

We investigate the conformation of atactic poly(sodium 4-styrene sulfonate) (PSS) molecules both in water-ethanol solutions and adsorbed to a template provided by a nanostructured monolayer of flat-lying octadecylamines on graphite. The conformations of the PSS molecules in these solutions undergo two conformational transitions, a re-entry transition at 40 vol % and a coil-globule conformational transition at 80 vol % ethanol. On the surface the conformations are characterized by two different length scales: on a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble.

Adsorption↗
PubMed1998

Studies in the setting of polyelectrolyte cements: part VII. The effect of divalent metal chlorides on the properties of zinc polycarboxylate and glass-ionomer dental cements.

A study is reported in which a zinc polycarboxylate and a glass polyalkenoate dental cement, were prepared from aqueous solutions of divalent metal chlorides, namely ZnCl2, CaCl2, MgCl2 and SrCl2, all at 1.0 mol dm-3 concentration, as well as from pure water. Calcium chloride was employed at additional concentrations, i.e. 2.0, 0.5 and 0.1 mol dm-3. As was previously found for monovalent salts, setting of the zinc polycarboxylate was speeded up and water uptake generally enhanced by the presence of the divalent metal salts. However, the divalent salts were found to reduce the compressive strength at 24 h (from 86 MPa to about 60 MPa). The glass polyalkenoate showed broadly similar effects to those observed in the presence of monovalent salts, with the setting time being increased, water uptake inhibited and compressive strength at 24 h reduced; however, by contrast, the working time was generally reduced. These results occur because the rate of the neutralization process is increased by the divalent salts, a consequence of the reduced pH of the poly(acrylic acid) caused by these salts. Infrared spectroscopy demonstrated interactions between the metal chlorides and poly(acrylic acid), with various chelate structures being apparent from the position of the asymmetric carbonyl stretch.

Journal Article↗
PubMed1990

The effect of physical aging on the dissolution rate of anionic polyelectrolytes.

The effect of physical aging on the dissolution and mechanical properties of hydroxypropyl methylcellulose phthalate (HPMCP) was investigated. Dissolution rate measurements were performed on films which, initially above the glass transition temperature, Tg, were quenched to a sub-Tg storage temperature, aged at that temperature for a period of time and then quenched again to 25 degrees C. Within the time scale of observation, reductions in the dissolution rate to a limiting value were observed. HPMCP was also found to age in the same storage temperature range as determined by a creep compliance technique. These mechanical results indicate a change in glass structure and show that a limiting density was approached. Parallel changes were observed in the dissolution rate studies suggest that dissolution rate is governed in part by glass density. Therefore, mechanical changes of glassy films can yield pharmaceutically relevant information about the extent of physical aging and serve as an indicator of the effect of aging on dissolution rate.

Chemical Phenomena↗
PubMed1992

Effect of loading on swelling-controlled drug release from hydrophobic polyelectrolyte gel beads.

The effect of oxprenolol HCl loading on the kinetics of polymer swelling and drug release from suspension-polymerized poly(methyl methacrylate-co-methacrylic acid) (PMMA/MAA) beads has been studied in detail. Within the range of variables studied, the polymer swelling rate increases with buffer pH and concentration. And an ionization-controlled swelling mechanism (analogous to the relaxation-controlled mechanism) mechanism) seems to become more rate-limiting at higher buffer concentrations. At oxprenolol HCL loading levels below 17.8%, the drug release and associated dimensional changes (in pH 7.4) exhibit an extended quasi-linear region despite the inherent limitation of spherical geometry. At higher loading levels, the drug release becomes faster and first-order in nature. This is apparently a result of the transition from a dissolved to a dispersed system above a percolation threshold (15-18% loading in the present study). As a result of competition from processes such as the reduction of bead dimension due to drug release and the dimensional increase due to polymer swelling and osmotic contributions from the drug, the transient bead diameter increases monotonically during drug release at loading levels up to 25.6%, whereas upon further increasing the drug loading, the bead diameter goes through a maximum during the early drug release, which eventually increases again as a result of the slow but continuous increase in polymer swelling due to further ionization. In all cases, both the drug release and the dimensional changes approach completion as the penetrating ionization fronts meet at the center, indicating a true swelling-controlled behavior.

Buffers↗