Search results for "polymers"
showing 10 items of 3567 documents
Unprecedented blue intrinsic photoluminescence from hyperbranched and linear polyethylenimines: Polymer architectures and pH-effects
2007
The intrinsic photoluminescence properties of hyperbranched polyethylenimines (PEIs) and their linear counterpart (LPEIs) have been studied in absence of any classical fluorescent probes. The comparison of the inherent fluorescence emission between hyperbranched polyethylenimines and their linear analogues demonstrates that linear polyamines are capable of producing strong intrinsic photoluminescence species having long excited lifetimes without need of having a tridimensional-branched structure. The creation of inherently fluorescent polymeric centers from hyperbranched and linear polyethyleimines can be modulated by specific chemical modification and oxidation of amine groups as well as b…
Switchable Supramolecular Crosslinking of Cyclodextrin-Modified Hyperbranched Polyethylenimine via Anthraquinone Dyes
2011
Two anthraquinone dyes, 5,8-di-N-adamantylamino-1,4-dihydroxyanthraquinone (AQ-OH) and 1,4-di-N-adamantylaminoanthraquinone (AQ-H), are synthesized. The combination of AQ-OH and AQ-H with hyperbranched polyethylenimine (PEI) bearing covalently attached β-cyclodextrin spontaneously formed host–guest complexes with hydrodynamic diameters of about 260 nm. Even enlarged structures of the trifunctional AQ-H were observed via DLS. Repeated measurements show that their assembly and disassembly is reversible with a critical point at 26 °C and therefore switchable via temperature. Also superstructures up to 100 μm were detected via TEM, cryo-TEM-, and SEM. Hence, a smart colored network of switchabl…
Characterization and Processability of Blends of Polylactide Acid with a New Biodegradable Medium-Chain-Length Polyhydroxyalkanoate
2015
In this work the processability and the mechanical performance of polylactide acid (PLA) based blends with a sample of a new biodegradable medium-chain-length poly (hydroxyalkanoate) (PHA) were investigated. The two polymers are incompatible with scarce adhesion and with the dispersed PHA particles size increasing with the PHA content. Rheological tests in shear flow indicate that adding PHA improves the processability of the matrix by increasing the content of this second component in the blend, as observed also for the torque curves, due to the very low viscosity of this new sample of PHA. The processability is only slightly worsened in non-isothermal elongational flow as the melt strengt…
Poly(S-ethylsulfonyl-l-homocysteine): An α-Helical Polypeptide for Chemoselective Disulfide Formation
2018
Homocysteine and cysteine are the only natural occurring amino acids that are capable of disulfide bond formations in peptides and proteins. The chemoselective formation of asymmetric disulfide bonds, however, is chemically challenging and requires an activating group combining stability against hard nucleophiles, e.g., amines, with reactivity toward thiols and soft nucleophiles. In light of these considerations, we introduced the S-alkylsulfonyl cysteines in our previous work. Here, we present the synthesis and ring-opening polymerization of S-ethylsulfonyl-l-homocysteine N-carboxyanhydrides. We demonstrate that the polymerization leads to narrowly distributed polypeptides (Đ = 1.1–1.3) wi…
Polymer chains confined into tubes with attractive walls: A Monte Carlo simulation
1994
A bead-spring off-lattice model of a polymer chain with repulsive interactions among repeating units confined into straight tubes of various cross sections, DT2, is studied by Monte Carlo simulation. We are also varying the chain length from N = 16 to 128 and the strength of a short-range attractive interaction between the repeating units and the walls of the tube. Longitudinal and perpendicular static linear dimensions of the chains are analyzed, as well as the density profile of repeating units across the tube. These data are interpreted in terms of scaling concepts describing the crossover between three-dimensional and quasi-one-dimensional chain conformations and the adsorption transiti…
Alkene epoxidations catalysed by Mo(VI) supported on Merrifield's polymer
1999
High conversion and epoxide selectivity from alkenes is achieved with tert-butyl hydroperoxide and a polystyrene-grafted Mo(VI) catalyst, with no noticeable leaching of Mo from the support.
Ferrocene-Containing Multifunctional Polyethers: Monomer Sequence Monitoring via Quantitative 13C NMR Spectroscopy in Bulk
2014
Ferrocenyl glycidyl ether (fcGE) and allyl glycidyl ether (AGE) are copolymerized via living anionic ring-opening polymerization to generate polyfunctional copolymers with molecular weights up to 40 300 g/mol and low molecular weight dispersities (Mw/Mn < 1.18). Copolymerizations were carried out in bulk at 100 °C and unexpectedly found to proceed without any isomerization of the allyl double bonds. The copolymerization behavior of fcGE and AGE was monitored by in situ quantitative 13C NMR kinetic measurements in bulk, evidencing the formation of random copolymers under these conditions, showing no gradient of comonomer incorporation. The redox-active behavior of the copolymers and homopoly…
Molecular dynamics of solid polymers as revealed by deuteron NMR
1983
Pulsed deuteron NMR spectroscopy is described, which has recently been developed to become a powerful tool for studying molecular dynamics in solid polymers. It is shown that by analyzing the line shapes of2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. By applying these techniques to selectively deuterated polymers, motional mechanisms involving different segments of the monomer unit can be monitored. In addition, motional heterogeneities in glassy polymers can be detected. The information a…
Cyclodextrins in Polymer Synthesis: Free Radical Copolymerization of Methylated β-Cyclodextrin Complexes of Hydrophobic Monomers with N-Isopropylacr…
2000
Methylated β-cyclodextrin (me-β-CD) was used to form water-soluble host/guest complexes with the hydrophobic monomers n-butyl methacrylate (1), cyclohexyl methacrylate (2), isobornyl acrylate (3), isobornyl methacrylate (4), and styrene (5), respectively. The free-radical copolymerizations of the water-soluble N-isopropylacrylamide (NIPAAm) with these complexed monomers were carried out in water at different molar ratios.
Evidence for chain transfer in the atom transfer radical polymerization of butyl acrylate
2000
Poly(butyl acrylate) (PBuA) of high molecular weight was synthesized by atom transfer radical polymerization (ATRP) in ethyl acetate. Whereas for low molecular weight polymers, a linear increase of the number-average molecular weight. M n , versus conversion and narrow molecular weight distributions indicate the suppression of side reactions, a downward curvature in the plot of M n versus conversion was observed for high molecular weights (M n > 50 000). This effect is explained by chain transfer reactions, leading to branched polymers. GPC measurements with a viscosity detector give evidence for the branched structure of high molecular weight polymers obtained in ATRP. In addition, transfe…