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At least 253 records · Page 14Linked to original sources
PubMed1990

Polymers for biodegradable medical devices. V. Hydroxybutyrate-hydroxyvalerate copolymers: effects of polymer processing on hydrolytic degradation.

The effects of melt processing on those properties of polyhydroxybutyrate-hydroxyvalerate copolymers that control hydrolytic stability were studied. Initial experiments using a non-thermal preparation technique enabled the relationship between initial molecular weight and rate of hydrolytic degradation to be established. This rate was conveniently expressed in terms of the time for 10% by weight of the sample to be eroded. By varying melt processing conditions, the range of effects of processing on sample molecular weight and molecular weight distribution were investigated. These effects were found to be quite dramatic, with a reduction to 50% of the initial molecular weight being readily achieved. Temperature, shear rate and dwell time interrelated to control the extent of thermal degradation achieved during processing. The principal effect of copolymer composition resulted from the fact that an increase in hydroxyvalerate content led to a reduction in melting point and enabled milder processing conditions to be used. Enhanced crystallinity, resulting from more favourable processing conditions reduced hydrolytic degradation rates. As a result, melt processing produced a complex interrelation of effects, all of which influenced the hydrolytic stability of the fabricated specimen. In general, molecular weight effects predominated.

Biodegradation, Environmental↗
PubMed2006

Methyl methacrylate monomer-polymer equilibrium in solid polymer.

OBJECTIVE: Poly(methyl methacrylate) (PMMA) is commonly processed in dentistry by thermally initiating the free-radical polymerization of methyl methacrylate (MMA). Residual MMA, a tissue irritant, is a concern. The concentration of MMA ([MMA]) versus time and temperature was studied to identify optimum processing conditions. MATERIALS AND METHODS: One hundred milligram portions of plain and dental PMMA powders were incubated (10-170 degrees C, 1-384 h), with and without 6.0 microL MMA added. After incubation, [MMA] was determined by GC. RESULTS: For plain PMMA alone, equilibrium was attained in about 100 h. The equilibrium data for log[MMA] versus 1/T was better fitted by a quadratic than a straight line, and formed an upper bound to the values of [MMA] when PMMA was incubated with MMA at temperatures > approximately 120 degrees C. The response surface for [MMA] versus log(time) and reciprocal temperature was fitted. An 'overshoot' in the equilibration process was identified, and postulated to be due to a rapidly formed intermediate of unknown chemistry. SIGNIFICANCE: Minimization of the residual MMA in acrylic denture bases prepared by processing a mixture of PMMA and MMA is important for reasons of mechanical properties and irritancy. The response surface mapped here allows direct identification of the optimum processing conditions.

Chemical Phenomena↗
PubMed2002

Characterisation of the mechanical properties of polymer films formed from aqueous polymer dispersions by creep testing.

The mechanical properties of films formed from an aqueous dispersion of polymethlymethacrylate (Eudragit NE30D) and as mixture with an aqueous dispersion of ethylcellulose (Aquacoat ECD30), have been assessed by applying creep tests at different temperatures, using a dynamic mechanical analyser. In the region of linear creep, the film prepared from 100% Eudragit was far less elastic than when 60% Aquacoat was present. In this region, when the applied stress was doubled, the strain response was doubled. In the non-linear region of behaviour, there is clear evidence that the mixed film is more elastic than the film containing only Eudragit.

Elasticity↗
PubMed1997

Polymer- versus silica-based separation media: elimination of nonspecific interactions in the chiral recognition process through functional polymer design.

A novel chiral stationary phase that contains a new chiral selector based on L-valine-3,5-dimethylanilide attached to monodisperse poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads has been prepared. The polymeric separation medium provides greatly enhanced enantioselectivities and reduced retention times when compared to the analogous silica-based chiral stationary phase in the separation of the enantiomers of 3,5-dinitrobenzamido derivatives of alpha-amino acids under normal-phase HPLC conditions. Separation factors (alpha) of up to 7 were achieved with the polymeric separation medium, which is also useful for the separation of large samples under overload conditions.

Chemistry Techniques, Analytical↗
PubMedDate not supplied

pH-sensitive cationic polymer gene delivery vehicle: N-Ac-poly(L-histidine)-graft-poly(L-lysine) comb shaped polymer.

Advancing biotechnology spurs the development of new pharmaceutically engineered gene delivery vehicles. Poly(L-histidine) ¿PLH¿ has been shown to induce membrane fusion at endosomal pH values, whereas PLL has a well documented efficacy in polyplex formation. Therefore, N-Ac-poly(L-histidine)-graft-poly(L-lysine) ¿PLH-g-PLL¿ was synthesized by grafting poly(L-histidine) to poly(L-lysine) ¿PLL¿. PLH-g-PLL formed polyplex particles by electrostatic interactions with plasmid DNA ¿pDNA¿. The mean particle size of the polyplexes was in the range of 117 +/- 6 nm to 306 +/- 77 nm. PLH-g-PLL gene carrier demonstrated higher transfection efficacy in 293T cells than PLL at all equivalent weight ratios with pDNA. The inclusion of chloroquine as an endosomolytic agent enhanced transfection for both PLL and PLH-g-PLL gene carriers. PLH-g-PLL enhanced beta-galactosidase expression compared to PLL, but still increased in efficacy when chloroquine was included.

Cell Line↗
PubMed2006

Coating of poly(p-xylylene) by PLA-PEO-PLA triblock copolymers with excellent polymer-polymer adhesion for stent applications.

Poly(p-xylylene) (PPX) was deposited by chemical vapor deposition (CVD) on stainless steel substrates. These PPX films were coated by solution casting of poly(lactide)-poly(ethylene oxide)-poly(lactide) triblock copolymers (PLA-PEO-PLA) loaded with 14C-labeled paclitaxel. Adhesion of PLA-PEO-PLA on PPX substrate coatings was measured using the blister test method. Excellent adhesion of the block copolymers on PPX substrates was found. Stress behavior and film integrity of PLA-PEO-PLA was compared to pure PLA on unexpanded and expanded stent bodies and was found to be superior for the block copolymers. The release of paclitaxel from the biodegradable coatings was studied under physiological conditions using the scintillation counter method. Burst release of paclitaxel was observed from PLA-PEO-PLA layers regardless of composition, but an increase in paclitaxel loading was observed with increasing content of PEO.

Adhesiveness↗
PubMed2006

Insights on polymer surfactant complex structures during the binding of surfactants to polymers as measured by equilibrium and structural techniques.

This tutorial review provides new insights into the binding interactions between anionic surfactant molecules and various macromolecules in solution. The systems are of inherent scientific interest because synergistic mixing between these two components leads to complexes commonly found in applications such as detergency, cosmetic products, rheology control, paint and pharmaceutical formulations. We describe how the basic foundations, which are prerequisite to characterize a given polymer/surfactant system are evaluated together with information on the binding mechanism and structure derived from several methodologies.

Anions↗
PubMed2002

Model of inversion of DNA charge by a positive polymer: fractionalization of the polymer charge.

We model one strand of DNA by a one-dimensional lattice (ODL) of negative charges and consider the problem of inversion of its charge by a positive polyelectrolyte (PE). In the neutral state of the ODL-PE complex, each of the ODL charges is locally compensated by a PE charge. When an additional PE molecule is adsorbed by ODL, its charge gets fractionalized into monomer charges of defects (tails and arches) on the background of the perfectly neutralized ODL. Defects spread all over the ODL, eliminating the self-energy of PE. For DNA this fractionalization mechanism leads to a substantial inversion of charge, a phenomenon which is widely used for gene delivery.

DNA↗