Showing papers on "Polymer published in 2005"

Dispersion and alignment of carbon nanotubes in polymer matrix: A review

Abstract: Polymer/carbon nanotube (CNT) composites are expected to have good processability characteristics of the polymer and excellent functional properties of the CNTs. The critical challenge, however, is how to enhance dispersion and alignment of CNTs in the matrix. Here, we review recent progress and advances that have been made on: (a) dispersion of CNTs in a polymer matrix, including optimum blending, in situ polymerization and chemical functionalization; and (b) alignment of CNTs in the matrix enhanced by ex situ techniques, force and magnetic fields, electrospinning and liquid crystalline phase-induced methods. In addition, discussions on mechanical, thermal, electrical, electrochemical, optical and super-hydrophobic properties; and applications of polymer/CNT composites are included. Enhanced dispersion and alignment of CNTs in the polymer matrix will promote and extend the applications and developments of polymer/CNT nanocomposites.

Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes.

Abstract: To realize polymer electrolytes with high ionic conductivity, we exploited the high ionic conductivity of an ionic liquid. In situ free radical polymerization of compatible vinyl monomers in a room temperature ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI), afforded a novel series of polymer electrolytes. Polymer gels obtained by the polymerization of methyl methacrylate (MMA) in EMITFSI in the presence of a small amount of a cross-linker gave self-standing, flexible, and transparent films. The glass transition temperatures of the gels, which we named "ion gels", decreased with increasing mole fraction of EMITFSI and behaved as a completely compatible binary system of poly(methyl methacrylate) (PMMA) and EMITFSI. The temperature dependence of the ionic conductivity of the ion gels followed the Vogel-Tamman-Fulcher (VTF) equation, and the ionic conductivity at ambient temperature reached a value close to 10(-2) S cm(-1). Similarly to the behavior of the ionic liquid, the cation in the ion gels diffused faster than the anion. The number of carrier ions, calculated from the Nernst-Einstein equation, was found to increase for an ion gel from the corresponding value for the ionic liquid itself. The cation transference number increased with decreasing EMITFSI concentration due to interaction between the PMMA matrix and the TFSI(-) anion, which prohibited the formation of ion clusters or associates, as was the case for the ionic liquid itself.

Handbook of Biodegradable Polymers

Abstract: Polyglycolide and Polylactide. Poly (p-dioxanone) and its Copolymers. Polycaprolactone. Polyhydroxyalkanoates. Poly (propylene fumarate). Poly (ortho esters). Other Polyesters. Polyanhydrides. Biodegradable Polyphosphazenes for Biomedical Applications. Poly (alklcyanoacrylates). Degradable Hydrogels. The Poloxamers: Their Chemistry and Medical Applications. Degradable Polymers Derived from the Amino Acid L-tyrosine. Natural Modified Polysaccharides. Oxidized Cellulose: Chemistry, Processing and Medical Applications. Gelatin. Collagen: Characterization, Processing and Medical Applications. Fibrinogen and Fibrin: Characterization, Processing and Medical Applications. Transductional Elastic and Plastic Protein-based Polymers as Potential Medical Devices. Genetically Engineered Protein Polymers. Surface Characterization of Bioerodible Polymers Using XPS, SIMS and AFM. Mechanisms of Polymer Degradation and Elimination. Non-medical Biodegradable Polymers: Environmentally Degradable Polymers.

Influence of the Degree of Ionization on Weak Polyelectrolyte Multilayer Assembly

Abstract: The effects of charge density on weak polyelectrolyte multilayer growth were investigated by assembling several strong polyelectrolytes with either poly(allylamine hydrochloride) (PAH) or poly(acrylic acid) (PAA) to keep one polymer fully charged while varying the degree of ionization of the other. The degree of ionization of the weak polyelectrolytes in solution and in a multilayer film was controlled by pH adjustment and quantified by Fourier transform infrared spectroscopy analyses. In all cases, the bilayer thickness exhibited a dramatic change from molecularly thin bilayers (case of two fully charged polymers) to much thicker bilayers when the degree of ionization of the weak polyelectrolyte decreased below 70−90% charged units. We also confirmed and quantified earlier observations that both the charge density and the effective pKa of a weak polyelectrolyte can change substantially from solution-state values when the polymer is incorporated into a multilayer film. In addition, it was found that the d...

Photoinduced Plasticity in Cross-Linked Polymers

Abstract: Chemically cross-linked polymers are inherently limited by stresses that are introduced by post-gelation volume changes during polymerization. It is also difficult to change a cross-linked polymer's shape without a corresponding loss of material properties or substantial stress development. We demonstrate a cross-linked polymer that, upon exposure to light, exhibits stress and/or strain relaxation without any concomitant change in material properties. This result is achieved by introducing radicals via photocleavage of residual photoinitiator in the polymer matrix, which then diffuse via addition-fragmentation chain transfer of midchain functional groups. These processes lead to photoinduced plasticity, actuation, and equilibrium shape changes without residual stress. Such polymeric materials are critical to the development of microdevices, biomaterials, and polymeric coatings.

Catalyst-transfer polycondensation. mechanism of Ni-catalyzed chain-growth polymerization leading to well-defined poly(3-hexylthiophene).

Abstract: We studied the mechanism of the chain-growth polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene (1) with Ni(dppp)Cl2 [dppp = 1,3-bis(diphenylphosphino)propane], in which head-to-tail poly(3-hexylthiophene) (HT-P3HT) with a low polydispersity is obtained and the Mn is controlled by the feed ratio of the monomer to the Ni catalyst. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that the HT-P3HT uniformly had a hydrogen atom at one end of each molecule and a bromine atom at the other. The reaction of the polymer with aryl Grignard reagent gave HT-P3HT with aryl groups at both ends, which indicates that the H-end was derived from the propagating Ni complex. The degree of polymerization and the absolute molecular weight of the polymer could be evaluated from the 1H NMR spectra of the Ar/Ar-ended HT-P3HT, and it was found that one Ni catalyst molecule forms one polymer chain. Furthermore, by reaction of 1 with 50 mol % Ni(dppp)Cl2, the chain initiator was fou...

Synthesis of Monodisperse Silica Particles Coated with Well-Defined, High-Density Polymer Brushes by Surface-Initiated Atom Transfer Radical Polymerization

Abstract: Monodisperse silica particles (SiPs) of diameter between 100 and 1500 nm were surface-modified in a mixture of ethanol/water/ammonia with a newly designed triethoxysilane having an atom transfer radical polymerization (ATRP) initiating site, (2-bromo-2-methyl)propionyloxyhexyltriethoxysilane. The surface-initiated ATRP of methyl methacrylate (MMA) mediated by a copper complex was carried out with the initiator-fixed SiPs in the presence of a “sacrificial” (free) initiator. The polymerization proceeded in a living manner in all examined cases, producing SiPs coated with well-defined PMMA of a target molecular weight up to 480K with a graft density as high as 0.65 chains/nm2. These hybrid particles had an exceptionally good dispersibility in organic solvents. Transmission electron microscopic and atomic force microscopic observations of their monolayers prepared at the air−water interface revealed that they formed an ordered 2-dimensional lattice extending throughout the monolayer.

Selective Nucleation and Growth of Metal−Organic Open Framework Thin Films on Patterned COOH/CF3-Terminated Self-Assembled Monolayers on Au(111)

Abstract: Metal-Organic open Framework coordination polymers (MOFs) are a fascinating class of materials. We demonstrate the selective growth of patterned thin films of MOF-5 and the subsequent loading with a palladium organic compound, which acts as a precursor for the deposition of palladium nanoclusters inside the cavities of the MOF-5 material.

Controlled synthesis of nonspherical microparticles using microfluidics.

Abstract: The controlled synthesis of nonspherical microparticles using microfluidics processing is described. Polymer droplets, formed by shearing a photopolymer using a continuous water phase at a T-junction, were constrained to adopt nonspherical shapes by confining them using appropriate microchannel geometries. Plugs were obtained by shearing the polymer phase at low shear rates, while disks were obtained by flattening droplets using a channel of low height. The nonspherical shapes formed were permanently preserved by photopolymerizing the constrained droplets in situ using ultraviolet light. Monodisperse plugs and disks of different lengths and diameters were obtained by varying the flow rates of the two phases.

Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA−Clay Nanocomposite Hydrogels

Abstract: The process of forming the unique organic/inorganic network structure of nanocomposite hydrogels (NC gels) was studied through changes in viscosity, optical transparency, X-ray diffraction, and mechanical properties. It was concluded that, during the preparation of the initial reaction solutions, a specific solution structure was formed from monomer (NIPA) and clay, where NIPA prevents gel formation of clay itself, and initiator (KPS) is located near the clay surface through ionic interactions. In subsequent in-situ free-radical polymerization, it was observed that the viscosity increased markedly during NC gel syntheses and in a manner similar to that in OR gel syntheses. Also, NC gels with different polymer contents exhibit characteristic two-step changes in the stress−strain curves, which correspond to the primary network formation and subsequent increase of cross-link density. These are because the polymerization proceeds on the clay particles which are relatively immobile, and clay platelets act as e...

Protein Resistance of Titanium Oxide Surfaces Modified by Biologically Inspired mPEG−DOPA

Abstract: In the present study, we have utilized X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (ELM), and optical waveguide lightmode spectroscopy (OWLS) to examine the surface adsorption and protein resistance behavior of bio-inspired polymers consisting of poly(ethylene glycol) (PEG) conjugated to peptide mimics of mussel adhesive proteins. Peptides containing up to three residues of 3,4-dihydroxyphenylalanine (DOPA), a key component of mussel adhesive proteins, were conjugated to monomethoxy-terminated PEG polymers. These mPEG−DOPA polymers were found to be highly adhesive to TiO2 surfaces, with quantitative XPS analysis providing useful insight into the binding mechanism. Additionally, the antifouling properties of immobilized PEG were reflected in the excellent resistance of mPEG−DOPA-modified TiO2 surfaces to protein adsorption. Measurements of mPEG−DOPA and human serum adsorption were related in terms of ethylene glycol (EG) surface density and serum mass adsorbed and demonstrated a thre...

Biomimetic anchor for surface-initiated polymerization from metal substrates.

Abstract: In this paper, we demonstrate the first use of a catecholic initiator for surface-initiated polymerization (SIP) from metal surfaces to create antifouling polymer coatings. A new bifunctional initiator inspired by mussel adhesive proteins was synthesized, which strongly adsorbs to Ti and 316L stainless steel (SS) substrates, providing an anchor for surface immobilization of grafted polymers. Surface-initiated atom transfer radical polymerization (SI-ATRP) was performed through the adsorbed biomimetic initiator to polymerize methyl methacrylate macromonomers with oligo(ethylene glycol) (OEG) side chains. X-ray photoelectron spectroscopy, surface FT-IR, and contact angle analysis confirmed the sequential grafting of initiator and polymer, and ellipsometry indicated the formation of polymer coatings of up to 100 nm thickness. Cell adhesion experiments performed with 3T3-Swiss albino fibroblasts showed substantially reduced cell adhesion onto polymer grafted Ti and 316L SS substrates as compared to the unmodified metals. Moreover, micropatterning of grafted polymer coatings on Ti surfaces was demonstrated by combining SI-ATRP and molecular assembly patterning by lift-off (MAPL), creating cell-adhesive and cell-resistant regions for potential use as cell arrays. Due to the ability of catechols to bind to a large variety of inorganic surfaces, this biomimetic anchoring strategy is expected to be a highly versatile tool for polymer thin film surface modification for biomedical and other applications.

Electrospinning of nanofibers with core-sheath, hollow, or porous structures

Abstract: Recently there has been a great deal of progress in the production of one-dimensional nanostructures with core-sheath structures from a rich variety of materials. Here we highlight the use of electrospinning in the fabrication of polymeric, ceramic, and composite nanofibers with core-sheath, hollow, or porous structures, as well as the efforts made to improve their morphological homogeneity, functionality, and device performance.

Synthesis of Poly(Lactic Acid): A Review

Abstract: Poly(lactic acid), a bio‐degradable polymer, has been studied extensively during the past 15 years. This paper presents a review on poly(lactic acid) (PLA) with focus on its stereochemistry, synthesis via ring‐opening polymerization, reaction kinetics and thermodynamics, synthesis of low molecular weight polymer, a continuous process for production of PLA from lactic acid, and blends. The different polymerization mechanisms, which have been proposed in the literature, are also summarized. Various catalyst systems, solvents, and reaction temperature and time give products of an entire range of molecular weights, ranging from a few thousand to over a million. Modeling and simulation of the ring‐opening polymerization of PLA is also discussed.

A modular and supramolecular approach to bioactive scaffolds for tissue engineering

Abstract: Bioactive polymeric scaffolds are a prerequisite for the ultimate formation of functional tissues. Here, we show that supramolecular polymers based on quadruple hydrogen bonding ureido-pyrimidinone (UPy) moieties are eminently suitable for producing such bioactive materials owing to their low-temperature processability, favourable degradation and biocompatible behaviour. Particularly, the reversible nature of the hydrogen bonds allows for a modular approach to gaining control over cellular behaviour and activity both in vitro and in vivo. Bioactive materials are obtained by simply mixing UPy-functionalized polymers with UPy-modified biomolecules. Low-molecular-weight bis-UPy-oligocaprolactones with cell adhesion promoting UPy-Gly-Arg-Gly-Asp-Ser (UPy-GRGDS) and the synergistic UPy-Pro-His-Ser-Arg-Asn (UPy-PHSRN) peptide sequences are synthesized and studied. The in vitro results indicate strong and specific cell binding of fibroblasts to the UPy-functionalized bioactive materials containing both UPy-peptides. An even more striking effect is seen in vivo where the formation of single giant cells at the interface between bioactive material and tissue is triggered.

Increasing the conductivity of crystalline polymer electrolytes

Abstract: Polymer electrolytes consist of salts dissolved in polymers (for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows1,2,3,4,5. For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer:salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6:LiXF6 (X = P, As, Sb); however, the conductivities were relatively low6,7. Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6- anions in the crystal structure with isovalent N(SO2CF3)2- ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-xIx ionic conductors8. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

Bio-based polymers and composites

Abstract: Preface About the Authors 1. Overview of Plant Polymers: Resources, Demands, and Sustainability 2. Plant Materials Formation and Growth 3. Isolation and Processing of Plant Materials 4. Polymers and Composite Resins from Plant Oils 5. Composites and Foams from Plant Oil-Based Resins 6. Fundamentals of Fracture in Bio-Based Polymers 7. Properties of Triglyceride-Based Thermosets 8. Pressure-Sensitive Adhesives, Elastomers, and Coatings from Plant Oil 9. Thermal and Mechanical Properties of Soy Proteins 10. Soy Protein Adhesives 11. Plastics Derived from Starch and Poly (Lactic Acids) 12. Bio-Based Composites from Soybean Oil and Chicken Feathers 13. Hurricane-Resistant Houses from Soybean Oil and Natural Fibers 14. Carbon Nanotube Composites with Soybean Oil Resins 15. Nanoclay Biocomposites 16. Lignin Polymers and Composites Index

Functionalization of single-walled carbon nanotubes with well-defined polystyrene by "click" coupling.

Abstract: Covalent functionalization of alkyne-decorated SWNTs with well-defined, azide-terminated polystyrene polymers was accomplished by the Cu(I)-catalyzed [3 + 2] Huisgen cycloaddition. This reaction was found to be extremely efficient in producing organosoluble polymer-nanotube conjugates, even at relatively low reaction temperatures (60 °C) and short reaction times (24 h). The reaction was found to be most effective when a CuI catalyst was employed in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene as an additive. IR spectroscopy was utilized to follow the introduction and consumption of alkyne groups on the SWNTs, and Raman spectroscopy evidenced the conversion of a high proportion of sp2 carbons to sp3 hybridization during alkyne introduction. Thermogravimetric analysis indicated that the polymer-functionalized SWNTs consisted of 45% polymer, amounting to approximately one polymer chain for every 200−700 carbons of the nanotubes, depending on polymer molecular weight. Transmission electron microscopy an...

Design and synthesis of N-maleimido-functionalized hydrophilic polymers via copper-mediated living radical polymerization: a suitable alternative to PEGylation chemistry.

Abstract: A series of alpha-functional maleimide polymethacrylates (M(n) = 4.1-35.4 kDa, PDi = 1.06-1.27) have been prepared via copper-catalyzed living radical polymerization (LRP). Two independent synthetic protocols have been successfully developed and the polymers obtained in multigram scale, with an 80-100% content of maleimide reactive chain ends, depending on the method employed. A method for the synthesis of amino-terminated polymers, starting from Boc-protected amino initiators, has also been developed, as these derivatives are key intermediates in one of the two processes studied in the present work. The alternative synthetic pathway involves an initiator containing a maleimide unit "protected" as a Diels-Alder adduct. After the polymerization step, the maleimide functionality has been reintroduced by retro-Diels-Alder reaction, by simply refluxing those polymers in toluene for 7 h. These maleimido-terminated materials, poly(methoxyPEG((475))) methacrylates and poly(glycerol) methacrylates, differ for both the nature and size of the polymer side branches and showed an excellent solubility in water, a property that made them an ideal candidate for the synthesis of new polymer-(poly)peptide biomaterials. These functional polymers have been successfully employed in conjugation reactions in the presence of thiol-containing model substrates, namely, reduced glutathione (gamma-Glu-Cys-Gly) and the carrier protein, bovine serum albumin (BSA), in 100 mM phosphate buffer (pH 6.8-7.4) and ambient temperature.

Influence of the molecular weight of poly(3-hexylthiophene) on the performance of bulk heterojunction solar cells

Abstract: The efficiency of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells dramatically depends on the molecular weight of the donor polymer P3HT. Only for P3HTs with a number-average molecular weight (Mn) of >10 000 have high power conversion efficiencies of >2.5% been reached. This behavior is caused by distinctly reduced charge carrier (hole) mobility in the donor phase of the devices built from lower Mn P3HT samples. The reduced performance of such devices is related to a reduced intermolecular ordering (π-stacking) of the P3HT phase. These findings highlight the important role of the molecular weight in semicrystalline semiconducting polymers for the device performance and may help in the further development of novel, efficient, lower band gap donor polymers for photovoltaic applications.

Peptidomimetic polymers for antifouling surfaces

Abstract: Peptidomimetic polymers comprising one or more DOPA moieties and related coatings and composites.