Showing papers on "Polymer published in 2000"

Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials

Abstract: This review aims at reporting on very recent developments in syntheses, properties and (future) applications of polymer-layered silicate nanocomposites. This new type of materials, based on smectite clays usually rendered hydrophobic through ionic exchange of the sodium interlayer cation with an onium cation, may be prepared via various synthetic routes comprising exfoliation adsorption, in situ intercalative polymerization and melt intercalation. The whole range of polymer matrices is covered, i.e. thermoplastics, thermosets and elastomers. Two types of structure may be obtained, namely intercalated nanocomposites where the polymer chains are sandwiched in between silicate layers and exfoliated nanocomposites where the separated, individual silicate layers are more or less uniformly dispersed in the polymer matrix. This new family of materials exhibits enhanced properties at very low filler level, usually inferior to 5 wt.%, such as increased Young’s modulus and storage modulus, increase in thermal stability and gas barrier properties and good flame retardancy.

Surfactant-assisted processing of carbon nanotube/polymer composites

Abstract: Interfacial interaction is one of the most critical issues in carbon nanotube/polymer composites In this paper the role of nonionic surfactant is investigated With the surfactant as the processing aid, the addition of only 1 wt % carbon nanotubes in the composite increases the glass transition temperature from 63 °C to 88 °C The elastic modulus is also increased by more than 30% In contrast, the addition of carbon nanotubes without the surfactant only has moderate effects on the glass transition temperature and on the mechanical properties This work points to the pathways to improve dispersion and to modify interfacial bonding in carbon nanotube/polymer composites

Polymer synthesis and processing using supercritical carbon dioxide

Abstract: Carbon dioxide is a clean and versatile solvent for the synthesis and processing of a range of materials. This review focuses on recent advances in polymer synthesis and processing using liquid and supercritical CO2. The synthetic techniques discussed include homogeneous solution polymerisation, precipitation polymerisation, dispersion and emulsion polymerisation, and bulk polycondensation. The formation of porous polymers and polymer blends is also considered, and the specific advantages of CO2 in these processes are evaluated in each case. The use of CO2 as a solvent for polymer processing is reviewed from a materials perspective, with particular attention being given to the formation of polymers with well defined morphologies. The variable solvent strength associated with supercritical fluids has been utilised in areas such as polymer fractionation and polymer extraction. Plasticisation phenomena have been exploited for the impregnation and heterogeneous chemical modification of polymeric materials. The formation of microcellular polymer foams by pressure induced phase separation is considered, as is the use of CO2 for polymer particle formation, spray coating, and microlithography. The aim of the review is to highlight the wide range of opportunities available to the materials chemist through the use of carbon dioxide as an alternative solvent.

Degradable Poly(β-amino esters): Synthesis, Characterization, and Self-Assembly with Plasmid DNA

Abstract: Poly(β-aminoesters) 1−3 were synthesized via the addition of N,N‘-dimethylethylenediamine, piperazine, and 4,4‘-trimethylenedipiperidine to 1,4-butanediol diacrylate. Polymerization proceeded exclusively via the conjugate addition of the secondary amines to the bis(acrylate ester). Polymers were isolated in up to 86% yields with molecular weights ranging up to 31 200 relative to polystyrene standards. The polymers degraded hydrolytically in acidic and alkaline media to yield 1,4-butanediol and β-amino acids 4a−6a and the degradation kinetics were investigated at pH 5.1 and 7.4. In general, the polymers degraded more rapidly at pH 7.4 than at pH 5.1. In initial screening assays, both the polymers and their degradation products were determined to be noncytotoxic relative to poly(ethylene imine), a polymer conventionally employed as a synthetic transfection vector. Polymers 1−3 interacted electrostatically with polyanionic plasmid DNA in water and buffer at physiological pH, as determined by agarose gel elec...

Structure and Properties of Poly(vinyl alcohol)/Na+ Montmorillonite Nanocomposites

Abstract: Poly(vinyl alcohol)/sodium montmorillonite nanocomposites of various compositions were created by casting from a polymer/silicate water suspension. The composite structure study revealed a coexistence of exfoliated and intercalated MMT layers, especially for low and moderate silicate loadings. The inorganic layers promote a new crystalline phase different than the one of the respective neat PVA, characterized by higher melting temperature and a different crystal structure. This new crystal phase reflects on the composite materials properties. Namely, the hybrid polymer/silicate systems have mechanical, thermal, and water vapor transmission properties, which are superior to that of the neat polymer and its conventionally filled composites. For example, for a 5 wt % MMT exfoliated composite, the softening temperature increases by 25 °C and the Young's modulus triples with a decrease of only 20% in toughness, whereas there is also a 60% reduction in the water permeability. Furthermore, due to the nanoscale d...

Siloxane Polymers for High-Resolution, High-Accuracy Soft Lithography

Abstract: We report the formulation of siloxane polymers for high-resolution, high-accuracy stamps for soft lithography. With this technique, a molecular, polymeric, or liquid ink is applied to the surface of a stamp and then transferred by conformal contact to a substrate. Stamps for this technique are usually made of a commercial siloxane elastomer with appropriate mechanical properties to achieve conformal contact but are incapable of printing accurate, submicrometer patterns. To formulate better stamp polymers, we used models of rubber-like elasticity as guidelines. Poly(dimethylsiloxane) networks were prepared from vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and junction functionality. The polymer formulations were characterized by strain at break as well as compression modulus and surface hardness measurements. This resulted in the identification of bimodal polymer networks having mechanical properties that allow the replication of high...

Supramolecular Polymer Materials: Chain Extension of Telechelic Polymers Using a Reactive Hydrogen-Bonding Synthon**

Abstract: Functionalizing the termini of low-molecular-weight telechelic polymers with strongly associating hydrogen bonding units (see Figure) results in a new set of supramolecular materials, as reported here. These materials possess the unique combination of polymer-like properties at room temperature and monomer-like properties at elevated temperatures.

Control of energy transfer in oriented conjugated polymer-mesoporous silica composites

Abstract: Nanoscale architecture was used to control energy transfer in semiconducting polymers embedded in the channels of oriented, hexagonal nanoporous silica. Polarized femtosecond spectroscopies show that excitations migrate unidirectionally from aggregated, randomly oriented polymer segments outside the pores to isolated, aligned polymer chains within the pores. Energy migration along the conjugated polymer backbone occurred more slowly than Forster energy transfer between polymer chains. The different intrachain and interchain energy transfer time scales explain the behavior of conjugated polymers in a range of solution environments. The results provide insights for optimizing nanostructured materials for use in optoelectronic devices.

Nanocomposites of polymers and metals or semiconductors: Historical background and optical properties

Abstract: Upon transmission of visible light through composites comprising of a transparent polymer matrix with embedded particles, the intensity loss by scattering is substantially reduced for particle diameters below 50-100 nm (nanoparticles, nanosized particles). As a consequence, related materials (nanocomposites) have found particular interest in optical studies. The first part of this article deals with a historical survey on nano-particles and nanocomposites and the importance of small particle sizes on their optical properties. The second part focuses on results from our laboratory concerning nanocomposites with extremely high or low refractive indices and dichroic nanocomposites and their application in bicolored liquid crystal displays (LCD). The inorganic colloids required for these studies (lead sulfide, iron sulfides, gold, and silver) were prepared in situ in presence of a polymer or isolated as redispersable metal colloids modified at the surface with a self-assembled monolayer (SAM) of an alkanethiol. The nanocomposites themselves were finally obtained by coprecipitation, spin coating, solvent casting or melt extrusion, with poly(ethylene oxide), gelatin, poly(vinyl alcohol) and polyethylene as matrix polymers.

History and recent progress in piezoelectric polymers

Abstract: Electrets of carnauba wax and resin have exhibited good stability of trapped charges for nearly 50 years. Dipolar orientation and trapped charge are two mechanisms contributing to the pyro-, piezo-, and ferroelectricity of polymers. Since the 1950s, shear piezoelectricity was investigated in polymers of biological origin (such as cellulose and collagen) as well as synthetic optically active polymers (such as polyamides and polylactic acids). Since the discovery of piezoelectricity in poled polyvinylidene fluoride (PVDF) in 1969, the pyro-, piezo-, and ferroelectricity were widely investigated in a number of polar polymers, such as copolymers of vinylidene fluoride and trifluoroethylene, copolymers of vinylcyanide and vinylacetate, and nylons. Recent studies involve submicron films of aromatic and aliphatic polyureas prepared by vapor deposition polymerization in vacuum and the piezoelectricity of polyurethane produced by the coupling of electrostriction and bias electric fields. Gramophone pickups using a piece of bone or tendon were demonstrated in 1959. Microphones using a stretched film of polymethyl glutamate were reported in 1968. Ultrasonic transducers using elongated and poled films of PVDF were demonstrated in 1972. Headphones and tweeters using PVDF were marketed in 1975. Hydrophones and various electromechanical devices utilizing PVDP and its copolymers have been developed during the past 30 years. This paper briefly reviews the history and recent progress in piezoelectric polymers.

Polymer layered silicate nanocomposites

Abstract: Polymers filled with low amounts of layered silicate dispersed at nanoscale level are most promising materials characterized by a combination of chemical, physical and mechanical properties that cannot be obtained with macro- or microscopic dispersions of inorganic fillers. Polymer layered silicate nanocomposites can be obtained by insertion of polymer molecules in the galleries between the layers of phyllosilicate. Here, hydrated alkaline or alkaline earth metal cations are hosted which neutralize the negative charge resulting from isomorphous substitutions of Mg or Al cations within the silicate. Insertion of polymer molecules to prepare “intercalation hybrids” can be carried out by replacing the water hydration molecules in the galleries by polymers containing polar functional groups, using the so called ion-dipole method. A more general technique involves compatibilization of the silicate by intercalation of an organic molecule, typically an organic alkylammonium salt, that replaces the cations in the interlayer galleries to form an organically modified layered silicate (OLS). The aliphatic chain of the OLS favors the intercalation of any type of polymer. Intercalated or delaminated polymer-silicate hybrids are obtained depending on whether the stack organization of the silicate layers is preserved or is lost, with single sheets being distributed in the polymer matrix. The methods currently used for preparing polymer layered silicate (PLS) nanocomposites are: in situ polymerization, from polymer solution, or from polymer melt. Although PLS nanocomposites have been known for a long time, it is the possibility of preparing them by melt intercalation of OLS in processing that is boosting the present interest in these materials and their properties. So far PLS nanocomposites have been characterized by X-ray diffractometry, transmission electron microscopy, differential scanning calorimetry, and NMR. Published results on PLS nanocomposites are reviewed concerning their characterization and properties with particular reference to fire retardant behavior.

Polymer, Metal, and Hybrid Nano‐ and Mesotubes by Coating Degradable Polymer Template Fibers (TUFT Process)

Abstract: ). Selective reaction with the silver atoms at the interface of the particles occurred and the coated particles were then extracted from reverse micelles. The powder of coated nanocrystals thus obtained was then dispersed in hexane, giving an optically clear solution. The size distribution was still rather large (30 %), and to reduce it a size-selected precipitation process [20] was used. In this process two solvents, such as hexane and pyridine, are mixed. The first is a good solvent and the second a poor solvent for the alkyl chains. With this solvent mixture, the larger coated particles flocculated whereas the smaller ones remain in the solution, thus providing size selection. By repeating this process several times, a homogenous clear colloidal solution of dispersed 4.3 nm nanocrystals is obtained. The concentration of nanocrystals with final size distribution around 13 % was controlled.

Impedance Spectroscopy Study of PEO‐Based Nanocomposite Polymer Electrolytes

Abstract: The addition of nanometric fillers (e.g., , ) to polymer electrolytes induces consistent improvement in the transport properties. The increase in conductivity and in the cation transference number is attributed to the enhancement of the degree of the amorphous phase in the polymer matrix, as well as to some acid‐base Lewis type, ceramic‐electrolyte interactions. This model is confirmed by results obtained from a detailed impedance spectroscopy study carried out on poly(ethylene oxide) [P(EO)]‐based polymer electrolyte samples with and without ceramic fillers. © 2000 The Electrochemical Society. All rights reserved.

Device performance and polymer morphology in polymer light emitting diodes: The control of thin film morphology and device quantum efficiency

Abstract: We present the results of a systematic study on how the processing conditions of spin casting affect the morphology of polymer thin films, and how the morphology affects polymer light-emitting diode (LED) performance. The absorption peaks of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) thin films, which reflects the conjugation of π electrons, are strongly correlated to the spin-casting conditions. At high spin speed, better conjugation is observed. In addition, the photoluminescence emission peak of MEH-PPV films at ∼630 nm has a strong correlation to polymer aggregation. By proper selection of organic solvents, polymer solution concentrations, and spin speeds, we are able to control the aggregation of the polymer chains. Subsequently, we are able to control the emission color and the quantum efficiency of the MEH-PPV LEDs by simply adjusting the spin-casting conditions. Although spin casting is the most commonly used technique for the preparation of polymer thin films, our fin...

Molecular mechanics of binding in carbon-nanotube–polymer composites

Abstract: Nanoscale composites have been a technological dream for many years. Recently, increased interest has arisen in using carbon nanotubes as a filler for polymer composites, owing to their very small diameters on the order of 1 nm, very high aspect ratios of 1000 or more, and exceptional strength with Young’s modulus of approximately 1 TPa. A key issue for realizing these composites is obtaining good interfacial adhesion between the phases. In this work, we used force-field based molecular mechanics calculations to determine binding energies and sliding frictional stresses between pristine carbon nanotubes and a range of polymer substrates, in an effort to understand the factors governing interfacial adhesion. The particular polymers studied were chosen to correspond to reported composites in the literature. We also examined polymer morphologies by performing energy-minimizations in a vacuum. Hydrogen bond interactions with the ∏-bond network of pristine carbon nanotubes were found to bond most strongly to the surface, in the absence of chemically altered nanotubes. Surprisingly, we found that binding energies and frictional forces play only a minor role in determining the strength of the interface, but that helical polymer conformations are essential.

Thermal Probe Measurements of the Glass Transition Temperature for Ultrathin Polymer Films as a Function of Thickness

Abstract: The glass transition temperature of polystyrene and poly(methyl methacrylate) films on polar and nonpolar substrates was measured as a function of thickness using a thermal probe in contact with a polymer film. Using a technique called local thermal analysis, heat loss into the film was monitored as the temperature of the probe was ramped from ambient temperature to temperatures as high as 200 °C. The glass transition temperature was determined from a change in slope in the heat loss versus temperature plot. The Tg of polystyrene on silicon oxide decreased by as much as 25 °C below the bulk value for films 13 nm thick. The same trend in the glass transition temperature was observed for polystyrene films on silicon oxide treated with hexamethyldisilizane (HMDS). The Tg of poly(methyl methacrylate) on silicon oxide increased by up to 7 °C above the bulk value for films 18 nm thick. For poly(methyl methacrylate) on silicon oxide treated with HMDS, the Tg decreased by 10 °C below the bulk value for films 21 n...

Molecular simulation of ultrathin polymeric films near the glass transition.

Abstract: Properties such as the glass transition temperature ( T(g)) and the diffusion coefficient of ultrathin polymeric films are shown to depend on the dimensions of the system. In this work, a hard-sphere molecular dynamics methodology has been applied to simulate such systems. We investigate the influence that substrates have on the behavior of thin polymer films; we report evidence suggesting that, depending on the strength of substrate-polymer interactions, the glass transition temperature for a thin film can be significantly lower or higher than that of the bulk.

Quantifying glass transition behavior in ultrathin free-standing polymer films

Abstract: We have used Brillouin light scattering to make a detailed study of the behavior of the glass transition temperature T(g) in ultrathin, free-standing polystyrene films. The glass transitions were experimentally identified as near discontinuities in the thermal expansion. The effects of film thickness, molecular weight, and thermal history on the measured T(g) values have been investigated. While the size of the glass transition effects was comparable for all molecular weights, a complicated M(n) dependence suggested a separation of the results into two regimes, each dominated by a different length scale: a low M(n) regime controlled by a length scale intrinsic to the glass transition and a high M(n) region, where polymer chain confinement induced effects take over.

Conversion of Some Siloxane Polymers to Silicon Oxide by UV/Ozone Photochemical Processes

Abstract: A variety of linear and cross-linked polysiloxanes are transformed into silicon oxide (SiOx) through the application of a recently developed room-temperature UV/ozone conversion process. Ozone and atomic oxygen, produced by exposure of atmospheric oxygen to ultraviolet radiation, remove organic portions of the polymers as volatile products and leave a thin silicon oxide surface film. The conversion rates differ for each polysiloxane studied and are related to differences in their chemical structures. X-ray photoelectron spectroscopy (XPS) measurements of atomic ratios indicate that UV/ozone treatment removes up to 89% of the carbon from the resultant surface film, leading to an overall stoichiometry close to that of SiO2. The binding energy of Si(2p) core level photoelectrons shifts from 101.5 eV for the polymer precursors to about 103.5 eV after UV exposure, consistent with the formation of silicon that is coordinated to four oxygen atoms. Ellipsometry measurements of apparent thickness changes during co...

Polymer Derived Engineering Ceramics

Abstract: Engineering ceramics in the system Si-O-C-N-B manufactured from preceramic silicon containing polymers gain in significance with increasing availability of advanced precursor materials such as poly(carbosilane), -(silazane), -(siloxane) or -(borosilazane). While high temperature resistant Si-C and Si-N ceramic fibers are already used to reinforce ceramic matrix composites (CMC) in air- and spacecraft structures, novel products such as coatings, tapes, foams, and complex shaped components for medium and low temperature applications in the fields of energy, environmental, transportation, and communication technologies become more important in the future. Preceramic polymers offer the possibility of using versatile plastic shaping technologies as well as advanced laminated object manufacturing techniques. Properties can be varied in a wide range by tailoring the molecular structure and composition of the precursor polymer and by loading the polymer with intert or reactive filler powders. Partial conversion of the organic polymer into organic/inorganic hybrid materials yields novel materials which exhibit properties between polymers and ceramics.

Detailed‐atomistic molecular modeling of small molecule diffusion and solution processes in polymeric membrane materials

Abstract: Atomistic molecular modeling techniques have proven to be a very useful tool for the investigation of the structure and dynamics of dense amorphous membrane polymers and of transport processes in these materials. As illustrations, the results of extensive atomistic molecular dynamics investigations on the transport of different small molecules in flexible chain rubbery and stiff chain glassy polymers are discussed. For this purpose bulk polymer models and interface models for liquid feed mixtures in contact with the upstream site of the respective membrane have been employed. A comparison between the static structure and the dynamic behavior of the free volume in the simulated flexible chain rubbery polymers and stiff chain glassy polymers reveals qualitative differences which are decisive for experimentally observable differences in the diffusion of small molecules in these materials. The simulation results for the interface models reflect important features of experimentally well characterized pervaporation processes.

Molecular weight characterization of poly(N-isopropylacrylamide) prepared by living free-radical polymerization

Abstract: A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105. For molecular weights below 4 × 104, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 105). Particular care is required in GPC sample preparation: it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <105, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10-(4.24±0.42) dL g-1...

Polymer Network–Stabilized Liquid Crystals

Abstract: The fabrication and properties of polymer network–stabilized liquid crystals, formed by polymerization of a small amount of a bifunctional photoreactive monomer dissolved in a liquid-crystalline phase, are reviewed. The polymer network morphology is strongly related to preparation conditions such as monomer content, polymerization temperature, and ultraviolet (UV) curing conditions. The transfer of anisotropic liquid-crystalline order onto the network is discussed in detail. The electro-optical performance of network-stabilized nematics, cholesterics, and ferroelectric smectics is largely dependent on the morphology of the network, as will be demonstrated with an emphasis laid on polymer-stabilized cholesteric textures (PSCTs). A general correlation between polymerization conditions, network morphology, and electro-optical behavior will be outlined and aspects concerning applications discussed.