Showing papers on "Polymer published in 1995"

Principles of polymer chemistry

Abstract: Physical Properties and Physical Chemistry of Polymers.- Free-Radical Chain-Growth Polymerization.- Ionic Chain-Growth Polymerization.- Ring-Opening Polymerizations.- Common Chain-Growth Polymers.- Step-Growth Polymerization and Step-Growth Polymers.- Naturally Occurring Polymers.- Reactivity and Chemical Modifications of Polymers.- Polymeric Materials for Special Applications.

Synthesis and barrier properties of poly(ε‐caprolactone)‐layered silicate nanocomposites

Abstract: A new polymer-ceramic nanocomposite has been synthesized consisting of well-dispersed, two-dimensional layers of an organically modified mica-type silicate (MTS) within a degradable poly(e-caprolactone) matrix. A protonated amino acid derivative of MTS was used to promote delamination/dispersion of the host layers and initiate ring-opening polymerization of e-caprolactone monomer, resulting in poly(e-caprolactone) chains that are ionically bound to the silicate layers. The polymer chains can be released from the silicate surface by a reverse ion-exchange reaction and were shown to be spectroscopically similar to pure poly(e-caprolactone). Thick films of the polymer nanocomposite exhibit a significant reduction in water vapor permeability that shows a linear dependence on silicate content. The permeability of nanocomposite containing as low as 4.8% silicate by volume was reduced by nearly an order of magnitude compared to pure poly(e-caprolactone)

Graft copolymers that exhibit temperature-induced phase transitions over a wide range of pH

Abstract: There are many potential applications of 'intelligent' aqueous polymer systems in medicine, biotechnology, industry and in environmental problems. Many of these polymer systems undergo reversible phase transitions--for example, abrupt changes in volume--in response to external stimuli such as temperature, pH or the nature of the solvent. Most of the polymers studied previously are responsive to only one kind of stimulus. But for some applications, independent responsiveness to several factors, such as temperature and pH, may be required. Here we describe a polymer that undergoes marked solubility changes in water in response to temperature and/or pH changes. The polymer is prepared by grafting temperature-sensitive side chains onto a pH-sensitive backbone. We also find that block copolymers, in which the temperature- and pH-sensitive units alternate along the chain, show similar behaviour.

Intravascular stent and method

Abstract: A method for making an intravascular stent by applying to the body of a stent a solution which includes a solvent, a polymer dissolved in the solvent and a therapeutic substance dispersed in the solvent and then evaporating the solvent. The inclusion of a polymer in intimate contact with a drug on the stent allows the drug to be retained on the stent during expansion of the stent and also controls the administration of drug following implantation. The adhesion of the coating and the rate at which the drug is delivered can be controlled by the selection of an appropriate bioabsorbable or biostable polymer and the ratio of drug to polymer in the solution. By this method, drugs such as dexamethasone can be applied to a stent, retained on a stent during expansion of the stent and elute at a controlled rate.

Monte Carlo and molecular dynamics simulations in polymer science

Abstract: 1. Introduction. General Aspects of Computer Simulation Techniques and Their Applicaitons in Polymer Physics 2. Monte Carlo Methods for the Self-Avoiding Walk 3. Structure and Dynamics of Neutral and Charged Polymer Solutions: Effects of Long-Range Interactions 4. Entanglement Effects in Polymer Melts 5. Molecular Dynamics of Glassy Polymers 6. Monte Carlo Simulations of the Glass Transition of Polymers 7. Monte Carlo Studies of Polymer Blends and Block Copolymer Thermodynamics 9. Computer Simulations of Tethered Chains

Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity

Abstract: We demonstrate herein how conjugated polymers (molecular wires) can be used to interconnect (wire in series) receptors to produce fluorescent chemosensory systems with sensitivity enhancements over single receptor analogues. The enhancement mechanism in the polyreceptor materials is based on an energy migration scheme in which excitations, diffuse along the polymer backbone. Analyte binding produces trapping sites for the excitations which results in greatly attenuated emission intensity. Three different cyclophane-based receptor systems that bind paraquat were investigated. These systems are quenched by paraquat binding, and the quenching enhancements relative to a monomeric model compound were used to determine the efficiency of energy migration. Two polymers with related poly(phenyleneethyny1ene) structures were investigated, and the all-para system was found to exhibit more facile energy migration than the more electronically localized analogue that contained meta linkages. The para polyreceptor system was found to display a 65-fold enhancement in sensitivity to paraquat as compared to a model monoreceptor fluorescent chemosensor. However, we have determined that delocalization alone is not sufficient to produce facile energy migration, and the more delocalized polythiophenes appear to be less effective at energy migration than the para poly(phenyleneethyny1ene) material. Paraquat-induced fluorescent quenching studies on homologous polymers that lacked the cyclophane receptors were also performed. These results indicate that diffusive quenching by paraquat is enhanced by energy migration.

Control of protein-ligand recognition using a stimuli-responsive polymer

Abstract: Stimuli-responsive polymers exhibit reversible phase changes in response to changes in environmental factors such as pH or temperature. Conjugating such polymers to antibodies and proteins provides molecular systems for applications such as affinity separations, immunoassays and enzyme recovery and recycling. Here we show that conjugating a temperature-sensitive polymer to a genetically engineered site on a protein allows the protein's ligand binding affinity to be controlled. We synthesized a mutant of the protein streptavidin to enable site-specific conjugation of the responsive polymer near the protein's binding site. Normal binding of biotin to the modified protein occurs below 32 degrees C, whereas above this temperature the polymer collapses and blocks binding. The collapse of the polymer and thus the enabling and disabling of binding, is reversible. Such environmentally triggered control of binding may find many applications in biotechnology and biomedicine, such as the control of enzyme reaction rates and of biosensor activity, and the controlled release of drugs.

Kinetics of polymer melt intercalation

Abstract: the kinetics of polystyrene melt intercalation in organically modified mica-type silicates were studied using X-ray diffraction and transmission electron microscopy. By monitoring the change in the integrated intensity of the basal reflection of the silicate host, the rate of conversion from unintercalated to intercalated silicate was determined at various temperatures and for various molecular weights of polystyrene. Hybrid formation is limited by mass transport into the primary particles of the host silicate and not specifically by diffusion of the polymer chains within the silicate galleries. The activation energy of hybrid formation is similar to that previously measured for polystyrene self-diffusion in the melt, implying that the mobility of the polymer chains within the host galleries is at least comparable to that in the melt.

Dynamic Mechanical Study of the Factors Affecting the Two Glass Transition Behavior of Filled Polymers. Similarities and Differences with Random Ionomers

Abstract: The dynamic mechanical measurements on several polymers filled with very fine silica particles revealed that these composites exhibit two tan δ peaks. One was related to the usual polymer glass transition, while the other, occurring at a higher temperature, was assigned to the glass transition of regions containing chains of reduced mobility. Since many aspects of this behavior were found to be analogous to those of random ionomers, the results support the validity of the EHM model of ionomer morphology. The particle content, the number of monomer units interacting strongly with the surface of the particles, the thermal history of the sample, and the average MW of the polymer were all found to have a significant effect on the area and the maximum of the new tan δ peak. These effects are discussed in terms of a model which is based on the EHM ionomer model but takes into account the formation of tightly bound and loosely bond polymer chains around the filler particles.

Synthesis and application of modulated polymer gels.

Abstract: A class of environmentally responsive materials based on the spatial modulation of the chemical nature of gels is proposed and demonstrated here. The modulation was achieved by limiting the interpenetration of part of one gel network with another gel network. The gels so produced have an internally heterogeneous or modulated structure. Three simple applications based on the modulated gels are described here: a bigel strip, a shape memory gel, and a gel "hand." The bigel strip bends almost to a circle in response to a temperature increase or an increase in solvent concentration. The shape memory gel changes its shape from a straight line to a pentagon to a quadrangle as the temperature increases. These transitions from one shape to another are reversible. The gel "hand" in water can grasp or release an object simply by an adjustment of the temperature.

Micelle formation and solubilization of fluorescent probes in poly(oxyethylene-b-oxypropylene-b-oxyethylene) solutions

Abstract: Micellization of poly(oxyethylene-b-oxypropylene-b-oxyethylene) triblock copolymers (Pluronic polymers F68, P85, and F108) in aqueous solutions was studied, and critical micellization concentrations (cmc) were determined using surface tension measurements and fluorescent probes (pyrene, 1,6-diphenyl-1,3,5-hexatriene). The dependence of cmc on temperature was observed, and critical micellization temperatures characterizing temperature-dependent transitions of Pluronic unimers to multimolecular micelles were measured. The molecular characteristics of P85 and F108 micelles including their dimensions, molecular masses and surfactant aggregation numbers were determined using light-scattering and ultracentrifugation techniques. Depending on the type of Pluronic, the micelles had an average hydrodynamic diameter ranging from about 15 to about 35 nm, a molecular mass of about 200 kDa and aggregation numbers ranging from one to several dozens. The partitioning of fluorescent probes between aqueous and micellar phases was analyzed within the frame of a pseudophase model, and the partitioning coefficients were determined using the fluorescence data. The results are compared with previous reports and are discussed in relationship to the application of block copolymer micelles as microcontainers for drug delivery.

Active‐Filler‐Controlled Pyrolysis of Preceramic Polymers

Abstract: Manufacturing of bulk ceramic components from materials in the system Si-Me-C-N-O (Me = Ti, Cr, V, Mo, Si, B, CrSi2, MoSi2, etc.) from preceramic organosilicon polymers - such as poly(carbosilanes), poly(silazanes), or poly(siloxanes) - has become possible by incorporating reactive filler particles into the liquid or solid polymer pre-cursor. During pyrolytic decomposition of the polymer matrix, the filler particles react with carbon from the polymer precursor or nitrogen from the reaction gas atmosphere to form new (oxy)carbide or (oxy)nitride phases embedded in a nanocrystalline Si-O-C(-N) matrix. The selective expansion encountered in the filler phase reaction can be used to compensate for the polymer shrinkage upon pyrolytic conversion. The formation of a transient pore net-work between 400° and 1000°C is governed by the polymer decomposition as well as the filler particle reaction kinetics. Thus, the properties of the oxycarbonitride composite materials can be tailored by controlling the microstructures of the polymer-derived matrix phase, the filler network, and the residual porosity. Near-net-shape forming of bulk ceramic components, even with complex geometry, is possible, making novel applications of polymer-derived bulk materials in biomedical, electrical, and mechanical fields highly interesting.

Effect of the pressure drop rate on cell nucleation in continuous processing of microcellular polymers

Abstract: Microllular plastics are cellular polymers characterized by cell densities greater than 109 cells/cm3 and cells smaller than 10 μm. One of the critical steps in the continuous production of microcellular plastics is the promotion of high cell nucleation rates in a flowing polymer matrix. These high nucleation rates can be achieved by first forming a polymer/gas solution followed by rapidly decreasing the solubility of gas in the polymer. Since, in the processing range of interest, the gas solubility in the polymer decreases as the pressure decreases, a rapid pressure drop element, consisting of a nozzle, has been employed as a continuous microcellular nucleation device. In this paper, the effects of the pressure drop rate on the nucleation of cells and the cell density are discussed. The experimental results indicate that both the magnitude and the cell density are discussed. The experimental results indicate that both the magnitude and the rate of pressure drop play a strong role in microcellular processing. The pressure phenomenon affects the thermodynamic instability induced in the polymer/gas solution and the competition between cell nucleation and growth.

Interchain Excitations in Conjugated Polymers.

Abstract: The luminescence quantum yield is dramatically increased upon going from films to dilute blends or solutions of a soluble phenylenevinylene polymer. Nevertheless, direct measurements of the decay dynamics of the emissive excitons show that their lifetimes are not very different. Separation of the polymer chains suppresses interchain exciton formation, which otherwise competes favorably with the generation of emissive intrachain excitons in films. We discuss the consequences for the design of polymeric systems for optoelectronics.

Hybrid Organic-Inorganic Composites

Abstract: Some General Trends in the Area of Organic-Inorganic Composites Organic-Inorganic Hybrids with a Crystalline Polymer Matrix Inorganic-Protein Interactions in the Synthesis of a Ferrimagnetic Nancomposite Ion-Exchange Intercalation into the MPS[3 Layered Compounds: Design of Nanocomposites with Unusual Magnetic Electrical, and Nonlinear Optical Properties Preparation and Characterization of Nanocomposites of Poly(ethylene oxide) with Layered Solids Polymer-Clay Hybrids (Perfluorosulfonate Ionomer)-(Inorganic Oxide) Nanocomposites: Organic Modification of Surfaces of Silicon Oxide Nanoparticles Grown In Situ Nanostructured Organic-Inorganic Hybrid Materials Synthesized Through Simultaneous Processes Multiple Size Scale Structures in Silica-Siloxane Composites Studied by Small-Angle Scattering Composite Polymer Colloid Nucleated by Functionalized Silica Vinyl-Polymer-Modified Hybrid Materials and Photoacid-Catalyzed Sol-Gel Reactions Hybrid Organic-Inorganic Interpenetrating Networks A New Route to Polymer-Filled Glass: Hybrid Interpenetrating Networks with Appreciable Toughness Solidification of Colloidal Crystals of Silica Thermo-irreversible Gelation and Percolation-Based Mechanical Response via Metal-Olefin Coordination in Diene Polymers Hybrid Organic-Inorganic Silica Materials: Chemical Evidence for Organization in the Solid Hybrid Organic-Inorganic Materials: The Sol-Gel Approach Sol-Gel-Derived Silica-Siloxane Composite Materials: Effect of Reaction Conditions in Polymer-Rich Systems Hypervalent Spiro Polysiliconate and Polygermylate Ionomers: Novel Ladder and Network Materials Hexylene- and Phenylene-Bridged Polysiloxane Network Materials Structural Design of High-Performance Polymers for Sol-Gel Processing Preparation and Properties of High-Clarity Polyamide-Silica Hybrid Materials Preparation and Mechanical Properties of Polybenzoxazole-Silica Hybrid Materials Morphological Studies of Conductive Polyemers Deposited onto High-T[c Superconductors Novel Organic-Inorganic Composite Materials for Photonics Inorganic-Organic Hybrid Coatings for Metals and Glass Surfaces Surface Modification of Carbon Fibers for Advanced Composite Materials

Basic Properties of Polylactic Acid Produced by the Direct Condensation Polymerization of Lactic Acid

Abstract: Poly(lactic acids) with high molecular weights have been synthesized by direct condensation polymerization of lactic acid. These polymers have good mechanical properties and can be processed into products such as cups, film, and fiber, which can be used as compostable materials. This polymerization method can be applied to the synthesis of copolymers of lactic acid and other hydroxyacids. The properties of poly(lactic acid) and copolymers synthesized by the direct process are different from those of polymers obtained by the conventional lactide process.

Biotechnology of Breadmaking: Unraveling and Manipulating the Multi-Protein Gluten Complex

Abstract: Breadmaking is one of humankind's oldest technologies, being established some 4,000 years ago. The ability to make leavened bread depends largely on the visco-elastic properties conferred to wheat doughs by the gluten proteins. These allow the entrapment of carbon dioxide released by the yeast, giving rise to a light porous structure. One group of gluten proteins, the high molecular weight (HMW) subunits, are largely responsible for gluten elasticity, and variation in their amount and composition is associated with differences in elasticity (and hence quality) between various types of wheat. These proteins form elastomeric polymers stabilized by inter-chain disulphide bonds, and detailed studies of their structures have led to models for die mechanism of elasticity. This work has also provided a basis for direct improvement of wheat quality by transformation with additional HMW subunit genes.

"Domain" Coil-Globule Transition in Homopolymers

Abstract: The temperature-induced coil-globule transition has been studied in dilute aqueous solutions (with 200 mg/L SDS) for different fractions of poly(N-isopropylacrylamide) (PNIPAM) and poly-(N-isopropylmethacrylamide) (PNIPMAM) using scanning microcalorimetry, diffusion, and size-exclusion chromatography (FPLC). It has been shown that both these polymers undergo a coil-globule transition upon temperature increase. This transition is accompanied by cooperative heat absorption and a decrease of heat capacity, which makes it similar to the cold denaturation of globular proteins. The globule-coil transition is an all-or-none process only for the fractions with the lowest molecular weights (∼10 x 10 3 ) while fractions of larger molecular weights behave as if they consist of quasi-independent cooperative units, the domains. The number of domains in a macromolecule is proportional to the molecular weight of the polymer. This suggests that the domain character of cooperative transitions in large proteins does not, in principle, need evolutionary-selected amino acid sequences but can occur even in homopolymers.

Fluorescence studies of associating polymers in water : determination of the chain end aggregation number and a model for the association process

Abstract: Fluorescence probe experiments were carried out on aqueous solutions of urethane-coupled poly(ethylene oxide) polymers containing C 16 H 33 O end groups. These HEUR polymers associate in water, giving rise to a sharp increase in zero-shear viscosity with increasing concentration above 0.2-0.5 wt % polymer and a pronounced shear thinning at modest shear rates. At very low concentrations (a few ppm), the hydrophobic end groups of these polymers come together to form micelle-like structures. We are interested in the mechanism of the polymer association and in determining the number of hydrophobic groups N R that come together to form the micellar core. Fluorescence decay studies of pyrene excimer formation give values of N R close to 20, independent of polymer concentration. This N R value is a factor of 3 smaller than that found for typical nonionic micelles but larger than that inferred indirectly from different measurements on similar HEUR polymer systems. Steady-state fluorescence studies of intramolecular excimer formation in bis(1-pyrenyl)methyl ether (dipyme) solubilized in these polymers indicate that the micellar core is much more rigid than that of traditional surfactant micelles, with an estimated microviscosity an order of magnitude larger than that of sodium dodecyl sulfate micelles. A model is developed to accommodate these observations. In this model, the polymers form rosette-like micelles comprised of looped chains. At higher concentrations, larger structures are formed from aggregation of these micelles, held together by chains which bridge the micelles. The influence of dilution and of shear is to induce a bridge-to-loop transition, leading to a breakup of larger structures to smaller objects, micelles and smaller micelle aggregates.

Modeling of Luminescence Quenching-Based Sensors: Comparison of Multisite and Nonlinear Gas Solubility Models

Abstract: Quenching-based luminescence sensors generally are supported in organic or inorganic polymers and exhibit nonlinear Stem-Volmer quenching behavior. Two common explanations of the nonlinearity are either multisite binding or the nonlinear solubility properties of the analyte in the sensor. Both models have three fitting parameters. These two models are compared, and the merits of each are discussed. It is shown that while the underlying physical bases of the two models are radically different and chemically incompatible, the two models are mathematically equivalent for data fitting. The correspondence of parameters in the two models is given. The nonlinear solubility model is probably computationally slightly simpler to use, but the two-site model seems to better approximate the underlying chemistry of systems studied to date

Organically modified sol-gel sensors

Abstract: Sol-gel techniques can be used to make biosensors, waveguide sensors, and modified electrodes in a variety of configurations