Showing papers on "Polymer published in 2002"

Use of Ionic Liquids for π-Conjugated Polymer Electrochemical Devices

Abstract: π-Conjugated polymers that are electrochemically cycled in ionic liquids have enhanced lifetimes without failure (up to 1 million cycles) and fast cycle switching speeds (100 ms). We report results for electrochemical mechanical actuators, electrochromic windows, and numeric displays made from three types of π-conjugated polymers: polyaniline, polypyrrole, and polythiophene. Experiments were performed under ambient conditions, yet the polymers showed negligible loss in electroactivity. These performance advantages were obtained by using environmentally stable, room-temperature ionic liquids composed of 1-butyl-3-methyl imidazolium cations together with anions such as tetrafluoroborate or hexafluorophosphate.

Ultrapermeable, Reverse-Selective Nanocomposite Membranes

Abstract: Polymer nanocomposites continue to receive tremendous attention for application in areas such as microelectronics, organic batteries, optics, and catalysis. We have discovered that physical dispersion of nonporous, nanoscale, fumed silica particles in glassy amorphous poly(4-methyl-2-pentyne) simultaneously and surprisingly enhances both membrane permeability and selectivity for large organic molecules over small permanent gases. These highly unusual property enhancements, in contrast to results obtained in conventional filled polymer systems, reflect fumed silica-induced disruption of polymer chain packing and an accompanying subtle increase in the size of free volume elements through which molecular transport occurs, as discerned by positron annihilation lifetime spectroscopy. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing.

Carbon black filled conducting polymers and polymer blends

Abstract: The objective of this article was to review the use of carbon black (CB) as a conductive filler in polymers and polymer blends. Important properties of CB related to its use in conducting polymers are discussed. The effects of polymer structure, molecular weight, surface tension, and processing conditions on electrical resistivity and physical properties of composites are discussed. Several percolation models applicable to CB/polymer blends are reviewed. Emphasis is placed on recent trends using polymer blends as the matrix to obtain conducting composites at a lower CB loading. A criterion for the distribution of CB in polymer blends is discussed. © 2002 Wiley Periodicals, Inc. Adv Polym Techn 21: 299–313, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10025

Single-Molecule Optomechanical Cycle

Abstract: Light-powered molecular machines are conjectured to be essential constituents of future nanoscale devices As a model for such systems, we have synthesized a polymer of bistable photosensitive azobenzenes Individual polymers were investigated by single-molecule force spectroscopy in combination with optical excitation in total internal reflection We were able to optically lengthen and contract individual polymers by switching the azo groups between their trans and cis configurations The polymer was found to contract against an external force acting along the polymer backbone, thus delivering mechanical work As a proof of principle, the polymer was operated in a periodic mode, demonstrating for the first time optomechanical energy conversion in a single-molecule device

Fabrication of Carbon Multiwall Nanotube/Polymer Composites by Shear Mixing

Abstract: The dispersion of nanotubes in polymer matrices has been investigated as a means of deriving new and advanced engineering materials. These composite materials have been formed into fibers and thin films and their mechanical and electrical properties determined. The nanotube concentration at which conductivity was initiated (the percolation threshold) varied with host polymer. In poly(propylene), this was as low as 0.05 vol.-%, while higher concentrations were required for polystyrene and particularly for ABS. There was a small increase in elastic modulus and decrease in tensile strength at low nanotube loading, but as the concentration was increased there was a progressive increase in both strength and stiffness.

Morphological and mechanical properties of carbon-nanotube-reinforced semicrystalline and amorphous polymer composites

Abstract: In this work, multiwalled carbon nanotubes were investigated as potential mechanical reinforcement agents in two hosts, polyvinyl alcohol (PVA) and poly(9-vinyl carbazole) (PVK). It was found that, by adding various concentrations of nanotubes, both Young’s modulus and hardness increased by factors of 1.8 and 1.6 at 1 wt % in PVA and 2.8 and 2.0 at 8 wt % in PVK, in reasonable agreement with the Halpin–Tsai theory. Furthermore, the presence of the nanotubes was found to nucleate crystallization of the PVA. This crystal growth is thought to enhance matrix-nanotube stress transfer. In addition, microscopy studies suggest extremely strong interfacial bonding in the PVA-based composite. This is manifested by the fracture of the polymer rather that the polymer-nanotube interface.

Stable conjugated polymer electrochromic devices incorporating ionic liquids

Abstract: Electrochemical synthesis of conjugated polymers in ionic liquids, achievement of electroactivity and electrochroism of conjugated polymers in ionic liquids, and the use of the resulting conjugated polymers for the fabrication of electrochromic devices incorporating ionic liquids (28) as electrolytes are described.

Kinetic Study and New Applications of UV Radiation Curing

Abstract: Highly crosslinked polymers can be readily synthesized by photoinitiated polymerization of multifunctional monomers or functionalized polymers. The reaction can be followed in situ by real-time infrared (RT-IR) spectroscopy, a technique that records conversion versus time curves in photosensitive resins undergoing ultrafast polymerization upon UV exposure. For acrylate-based resins, UV-curing proceeds with long kinetic chains (7700 mol/radical) in spite of the high initiation rate. RT-IR spectroscopy proved very valuable in assessing the influence of various parameters, such as initiation efficiency, chemical structure of the telechelic oligomer, light intensity, inhibitory effect of oxygen, on polymerization kinetics. Interpenetrating polymer networks can be rapidly synthesized by means of UV irradiation of a mixture of difunctional acrylate and epoxy monomers in the presence of both radical and cationic-type photoinitiators. The same UV technology can be applied to crosslink solid polymers at ambient temperature, which bear different types of reactive groups (acrylate and vinyl double bonds, epoxy ring). UV radiation curing has been successfully used to produce within seconds weathering resistant protective coatings, high-resolution relief images, glass laminates and nanocomposites materials. Photoinitiated crosslinking polymerization.

Ordered nanoporous polymers from polystyrene-polylactide block copolymers.

Abstract: Nanoporous polystyrene monoliths were prepared from polystyrene−polylactide (PS−PLA) block copolymers that form hexagonally packed nanocylinders of PLA in a PS matrix. A morphology diagram was developed to determine the range in composition and molecular weight over which this morphology existed. Macroscopic alignment of these materials gave anisotropic monoliths that were subjected to mild degradation conditions leading to the chemical etching of the PLA. The resulting nanoporous monoliths consisted of a polystyrene matrix containing hexagonally close-packed, oriented, and continuous nanoscopic channels (pore size was tunable through synthesis or blending) lined with chemically accessible hydroxyl functional groups. Both the precursors and the porous materials were analyzed moleculary (size-exclusion chromatography and proton nuclear magnetic resonance spectroscopy) and structurally (small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry). In addition, the surfa...

Studies of polymer surfaces by sum frequency generation vibrational spectroscopy

Abstract: Recently, sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to study surfaces of polymer materials. This review summarizes the significant achievements in understanding surface molecular chemical structures of polymer materials obtained by SFG. It reviews in situ detection at the molecular level of surface structures of some common polymers in air, surface segregation of small end groups, polymer surface restructuring in water, and step-wise changed polymer blend surfaces. Studies of surface glass transition and surface structures modified by rubbing, plasma deposition, UV light irradiation, oxygen ion and radical irradiation, and wet etching are also discussed. SFG probing of polymer surfaces provides valuable insights into the relations between polymer surface structures and surface properties, which will assist in the design of polymer materials with desired surface properties.

Molecular dynamics simulation of a polymer melt with a nanoscopic particle

Abstract: We perform molecular dynamics simulations of a bead−spring polymer melt surrounding a nanoscopic particle. We explore the effect of the polymer/nanoparticle interactions, surface-to-volume ratio, and boundary conditions on both the structure and dynamics of the polymer melt. We find that the chains near the nanoparticle surface are elongated and flattened and that this effect is independent of the interaction for the range of interactions we study. We show that the glass transition temperature Tg of the melt can be shifted to either higher or lower temperatures by tuning the interactions between polymer and nanoparticle. A gradual change of the polymer dynamics approaching the nanoparticle surface causes the change in the glass transition. The magnitude of the shift is exaggerated by increasing fraction of surface monomers in the system. These behaviors support a “many-layer”-based interpretation of the dynamics. Our findings appear applicable to systems in which surface interactions dominate, including b...

Isotactic propylene copolymers, their preparation and use

Abstract: Unique copolymers comprising propylene, ethylene and/or one or more unsaturated comonomers are characterized as having: at least one, preferably more than one, of the following properties: (i) 13C NMR peaks corresponding to a regio-error at about 14.6 and about 15.7 ppm, the peaks of about equal intensity, (ii) a B-value greater than about 1.4 when the comonomer content of the copolymer is at least about 3 wt%, (iii) a skewness index, S?ix?, greater than about -1.20, (iv) a DSC curve with a Tme that remains essentially the same and a Tmax that decreases as the amount of comonomer in the copolymer is increased, and (v) an X-ray diffraction pattern that reports more gamma-form crystals than a comparable copolymer prepared with a Ziegler-Natta catalyst. These polypropylene polymers are made using a nonmetallocene, metal-centered, heteroaryl ligand catalyst. These polymers can be blended with other polymers, and are useful in the manufacture of films, sheets, foams, fibers and molded articles.

Cyclodextrin-threaded conjugated polyrotaxanes as insulated molecular wires with reduced interstrand interactions

Abstract: Control of intermolecular interactions is crucial to the exploitation of molecular semiconductors for both organic electronics and the viable manipulation and incorporation of single molecules into nano-engineered devices. Here we explore the properties of a class of materials that are engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4'-diphenylene vinylene) or polyfluorene through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects that red-shift and partially quench the luminescence. Our approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport. We used the polymers to prepare single-layer light-emitting diodes with Ca and Al cathodes, and observed blue and green emission. The reduced tendency for polymer chains to aggregate allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning force microscopy.

Star-shaped, dendrimeric and polymeric triarylamines as photoconductors and hole transport materials for electro-optical applications

Abstract: Review: Recent developments in the synthesis and application of home conducting oligometric and polymeric triarylamines are reviewed. The materials are classified as Star-shaped molecules, Spiros and dendrimers, Side-chain polymers, and Main-chain polymers and copolymers. This paper concentrates on the research reults of our group on the synthesis of a variety of such compounds, their structure-property relationship and their application in devices like organic light emitting diodes, solar cells and photorefractive systems. The thermal properties and elctronic properties of these compounds were varied by changing the chemical structure and nature of substituents. In the case of low molecular weight starshaped molecules the glass transition temperature could be increased to above 140°C by suitable structural design. Similarly, for polymeric triarylamines the variation of glass transition temperature was achieved over a wide range from 92 to 237°C. This is especially necessary for the wide spectrum of applications of these materials as hole conductors in low-T g photorefractive composites to high-T g materials in OLEDs. Moreover, the electronic energy levels and the band gao in these compounds can be manipulated to optimize the hole injection or electron transfer or emission properties or even photocurrent generation to make them suitable for various applications. Especially, the concept of copolymerization with other functional monomers results in multifunctional copolymers with good hole injection and transport porperties. The polymer networks involving triarylamine structures are not included in this Review, because this constitutes the subject-matter of insoluble hole transport materials and will be published elsewhere.

The formation of polymer vesicles or "peptosomes" by polybutadiene-block-poly(L-glutamate)s in dilute aqueous solution.

Abstract: Polybutadiene-block-poly(l-glutamate) copolymers were made by anionic polymerization and subsequent ring-opening polymerization of N-carboxyanhydrides and were characterized by NMR, IR, SEC, and circular dichroism. These polymers, when appropriately designed, form so-called “polymersomes” or “peptosomes”, vesicles composed of modified protein units. The size and structure of the vesicles are determined by dynamic light scattering, small-angle neutron scattering, and freeze-fracture electron microscopy. It is also shown that the size of the peptosomes does not depend on the pH; that is, the solvating peptide units can perform a helix−coil transition without serious changes of the vesicle morphology.

Hypercrosslinked polymers: basic principle of preparing the new class of polymeric materials

Abstract: Hypercrosslinked polystyrene networks have been recognised since the 1970s. They were prepared by extensive crosslinking of linear polystyrene chains in a strongly solvating media. Generally, the basic principle of obtaining hypercrosslinked polymers consists in the formation of a rigid highly solvated three-dimensional network. Owing to the high rigidity and reduced degree of chain entanglement, such “expanded” networks are characterized by loose chain packing, i.e., high free volume, and the unique ability to swell in both good solvents and non-solvents. In addition to the above polymer-analogues transformation of polystyrene, the article expands the synthesis techniques to polymerization and polycondensation procedures and also involves other types of polymers and monomers. Typical examples of preparing hypercrosslinked networks include the crosslinking of linear chains of polysulfone and polyarylates, copolymerization of styrene with a large amount of divinylbenzene, self-condensation of p -xylylene dichloride. All resulting materials differ substantially from classical polymers of the same chemical nature and comprise a new class of hypercrosslinked polymeric materials. Some polymers described in the literature were shown to also belong to the hypercrosslinked family.

Surfactants and Polymers in Drug Delivery

Abstract: Introduction Micelles Liquid Crystalline Phases Liposomes Microemulsions Emulsions Aerosols, Bubbles, and Foams Polymer Solutions and Gels Polymer Particles Degradation of Surfactants and Polymers in Drug Delivery Drying of Formulations Containing Surfactants and Polymers Index

Photopolymerizations of Thiol−Ene Polymers without Photoinitiators

Abstract: A thiol monomer is shown to copolymerize with vinyl ether, allyl, acrylate, methacrylate, and vinylbenzene monomers. These thiol−ene polymerizations are photoinitiated without the use of photoinitiator molecules. It is seen that the polymerization proceeds more readily when initiatorless samples are irradiated with light centered around 254 nm as compared to 365 nm light. To demonstrate resistance to oxygen inhibition, thin polymer films of 3−15 μm are polymerized while exposed to ambient air. Without photoinitiator molecules present, light is attenuated only by the monomer and polymer. This feature leads to greater penetration of ultraviolet light and allows for the polymerization of extremely thick polymers. Thick cures of up to 25 in. are obtained using a thiol−vinyl ether system.

Rare-earth doped polymers for planar optical amplifiers

Abstract: Optical waveguide amplifiers based on polymer materials offer a low-cost alternative for inorganic waveguide amplifiers. Due to the fact that their refractive index is similar to that of standard optical fibers, they can be easily coupled to existing fibers with low coupling losses. Doping the polymer with rare-earth ions that yield optical gain is not straightforward, as the rare-earth salts are poorly soluble in the polymer matrix. This review article focuses on two different approaches to dope a polymer waveguide with rare-earth ions. The first approach is based on organic cage-like complexes that encapsulate the rare-earth ion and are designed to provide coordination sites to bind the rare-earth ion and to shield it from the surrounding matrix. These complexes also offer the possibility of attaching a highly absorbing antenna group, which increases the pump efficiency significantly. The second approach to fabricate rare-earth doped polymer waveguides is obtained by combining the excellent properties of SiO2 as a host for rare-earth ions with the easy processing of polymers. This is done by doping polymers with Er-doped silica colloidal spheres.