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Polymer Conformation (polymer + conformation)
Selected AbstractsPhoto-induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar CellsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Marc Daniel Heinemann Abstract The photo-induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3-hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc-CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) are studied as well. The light-induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo-induced absorption (PIA) and light-induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo-excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long-term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc-CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed. [source] Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer BlendsADVANCED FUNCTIONAL MATERIALS, Issue 19 2009Christopher R. McNeill Abstract Here, studies on the evolution of photophysics and device performance with annealing of blends of poly(3-hexylthiophene) with the two polyfluorene copolymers poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,,2,,-diyl) (F8TBT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) are reported. In blends with F8TBT, P3HT is found to reorganize at low annealing temperatures (100,°C or below), evidenced by a redshift of both absorption and photoluminescence (PL), and by a decrease in PL lifetime. Annealing to 140,°C, however, is found to optimize device performance, accompanied by an increase in PL efficiency and lifetime. Grazing-incidence small-angle X-ray scattering is also performed to study the evolution in film nanomorphology with annealing, with the 140,°C-annealed film showing enhanced phase separation. It is concluded that reorganization of P3HT alone is not sufficient to optimize device performance but must also be accompanied by a coarsening of the morphology to promote charge separation. The shape of the photocurrent action spectra of P3HT:F8TBT devices is also studied, aided by optical modeling of the absorption spectrum of the blend in a device structure. Changes in the shape of the photocurrent action spectra with annealing are observed, and these are attributed to changes in the relative contribution of each polymer to photocurrent as morphology and polymer conformation evolve. In particular, in as-spun films from xylene, photocurrent is preferentially generated from ordered P3HT segments attributed to the increased charge separation efficiency in ordered P3HT compared to disordered P3HT. For optimized devices, photocurrent is efficiently generated from both P3HT and F8TBT. In contrast to blends with F8TBT, P3HT is only found to reorganize in blends with F8BT at annealing temperatures of over 200,°C. The low efficiency of the P3HT:F8BT system can then be attributed to poor charge generation and separation efficiencies that result from the failure of P3HT to reorganize. [source] Anomalous properties of spray dried solid dispersionsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2009Hisham Al-Obaidi Abstract The use of solid dispersions for oral dosage forms can increase the dissolution rate of poorly soluble drugs. Spray drying is one process that can be used to prepare solid dispersions. Spray dried solid dispersions of griseofulvin, poly[N -(2-hydroxypropyl)methacrylate] (PHPMA) and polyvinylpyrrolidone (PVP) were prepared from acetone and water. When methanol was substituted for water, the morphology, stability and dissolution properties of the solid dispersion changed dramatically. The glass transition temperature for the ternary solid dispersion (GF, PHPMA, and PVP) shifted from 83°C (acetone/water) to 103°C for the acetone/methanol system. These differences in the dispersions are thought to derive from conformational variations of the polymers in solution prior to spray drying. Both PHPMA and PVP formed globules in solution of a size range between 16 and 33 nm. The effect of drug and polymer concentration in solution (before spray drying) on the properties of the solid dispersion was studied. It was found that solid dispersions that were prepared using lower concentrations of drug and polymers in solutions resulted in the formation of particles that display a lower relaxation rate. This result supports the hypothesis that the polymer conformation may significantly change the properties of the solid dispersion. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4724,4737, 2009 [source] On/off switching on polymer conformationMACROMOLECULAR SYMPOSIA, Issue 1 2003Kenichi Yoshikawa Abstract The manner of folding transition from elongated coil to compact globule of single polymer chain is discussed. Based on theoretical consideration, it is argued the semi-flexible polymer chain exhibits large discrete transition on the level of individual single chains, whereas the transition looks continuous, or cooperative, on the ensemble of chains. As the experimental verification, in the present article thermodynamic and kinetic aspects of folding transition of single giant DNA molecules are described. It is shown that rich variety of nano-ordered structures are obtained from single DNA molecules through suitable setup of the experimental conditions. The stability of such nano-structures generated from single polymer chain is discussed in relation to the ordered compact structure with large number of chains in semi-dilute and concentrated polymer solutions. [source] Supramolecular selectivity of poly(ethylene oxide) in semi-crystalline polymer nanocompositesPOLYMER INTERNATIONAL, Issue 12 2007Li Zhou Abstract Semi-crystalline polymer nanocomposites were prepared using successive meltings and recrystallizations techniques by intercalation of small guest molecules such as 4-chlorotoluene (PCT), 4-bromotoluene (PBT) and 1,4-dibromobenzene (PDBB) into poly(ethylene oxide) (PEO) crystals. Differential scanning calorimetry, Fourier transform infrared spectroscopy and wide-angle X-ray diffraction experimental results show that supramolecular selectivity exists for the PEO,PDBB/PBT ternary system, while there is no supramolecular selectivity for PEO,PCT/PBT ternary nanocomposites. The interactions between PEO chains and small guest molecules have an important influence on the polymer conformation, which results in the dramatic difference in intercalation behavior. Copyright © 2007 Society of Chemical Industry [source] Thiophene-based ionic liquids: synthesis, physical properties, self-assembly, and oxidative polymerization,POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 10 2008Christopher T. Burns Abstract Preparation and polymerization of methylimidazolium-based ionic liquids (ILs) that incorporate a thiophene moiety at the terminus of a C10 alkyl chain are described. Both a bromide and nitrate salt of the amphiphilic thiophene IL self-assembles in water (albeit the nitrate to a lesser extent), adopting columnar mesophases. Polarized optical microscopy and small-angle X-ray scattering (SAXS) studies show that at low water content the IL,water binary mixtures form liquid crystalline mesophases possessing significant short-range ordering due to strong , interactions between adjacent thiophene moieties. At higher water content, the short-range ordering is lost, but long-range ordering persists up to ca. 45% (w/w) water. The chemical oxidative coupling of the nitrate monomer yields a highly water-soluble polymer. Electrochemical studies show that the polymer possesses a high oxidation potential (1.95,V) and thus, is resistant to chemical doping. In dilute aqueous solution, electronic absorption spectroscopy and X-ray scattering show the polymer adopts a random, coil-like conformational state. Slight improvement in the polymer conformation can be achieved by exchange of the counter anion. Copyright © 2008 John Wiley & Sons, Ltd. [source] Molecular Simulation Via Connectivity-altering Monte Carlo and Scale-jumping Methods: Application to Amorphous PolystyreneMACROMOLECULAR THEORY AND SIMULATIONS, Issue 7-8 2008Tim Mulder Abstract Well-equilibrated atactic-polystyrene (aPS) samples are obtained through the end-bridging Monte Carlo (EBMC) algorithm. A coarse-grained (CG) description of aPS is used; monomers are represented by two CG beads. The algorithm produces correct polymer conformations on all length scales, beyond the size of the CG beads. The code is very efficient, even though the acceptance of 0.001,0.005% is approximately 10,100 times lower than in the original EB code for PE. Systems of aPS of the order of 5000 monomers (50 chains of 100 monomers on average) can be equilibrated on all length scales within a week, in a single-processor run. The computer code is also adequate for simulations of other polymers that have the same regularity in their sequence of chemical groups and that are modeled at the same or at a coarser level of description. [source] Model of the influence of energetic disorder on inter-chain charge carrier mobility in poly[2-methoxy-5-(2,-ethylhexyloxy)- p -phenylene vinylene]POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 3 2009Petr Toman Abstract The theoretical model of the inter-chain charge carrier mobility in poly[2-methoxy-5-(2,-ethylhexyloxy)- p -phenylene vinylene] (MEH,PPV) doped with polar additive is put forward. The polymer chain states of a charge carrier were calculated by means of diagonalization of a tight-binding Hamiltonian, which includes disorder in both the local energies and transfer integrals. Consequently, the inter-chain charge carrier transport is taking place on a spatially and energetically disordered medium. Because it is believed that the additive does not significantly influence the polymer supramolecular structure, the polymer conformations were simplified as much as possible. On the other hand, the energetic disorder is rigorously described. The transfer rates between the polymer chains were determined using the quasi-classical Marcus theory. The model considered the following steps of the charge carrier transport: the charge carrier hops to a given polymer chain. Then, the charge carrier thermalizes to the Boltzmann distribution over all its possible states on this chain. After that, the charge carrier hops to any possible state on one of the four nearest neighboring chains. The results showed that the inter-chain charge carrier mobility is very strongly dependent on the degree of the energetic disorder. If the energetic disorder is doubled from 0.09 to 0.18,eV, the mobility decreases by two or three orders of magnitude. Copyright © 2008 John Wiley & Sons, Ltd. [source] Size-Dependent Spectroscopic Properties and Thermochromic Behavior in Poly(substituted thiophene) NanoparticlesCHEMPHYSCHEM, Issue 10 2004Naonori Kurokawa Dr. Nanoparticles are attractive nanomaterials, since they exhibit unique physical and chemical properties. We have found that poly(substituted thiophene) nanoparticles, ranging in size from several tens to hundreds of nanometers, exhibit size-dependent characteristic spectroscopic properties and thermochromic behavior, which can be explained by distorted polymer conformations in the surface layer (see graphic). [source] | |||||||||||||