Browsing by Subject "Copolymers"
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Item Open Access Accurate method for obtaining band gaps in conducting polymers using a DFT/hybrid approach(American Chemical Society, 1998) Salzner, U.; Pickup, P. G.; Poirier, R. A.; Lagowski, J. B.DFT calculations on a series of oligomers have been used to estimate band gaps, ionization potentials, electron affinities, and bandwidths for polyacetylene, polythiophene, polypyrrole, polythiazole, and a thiophene - thiazole copolymer. Using a slightly modified hybrid functional, we obtain band gaps within 0.1 eV of experimental solid-state values Calculated bond lengths and bond angles for the central ring of sexithiophene differ by less than 0.026 Å and 0.7° from those of the sexithiopnene crystal structure. IPs and EAs are overestimated by up to 0.77 eV compared to experimental bulk values. Extrapolated bandwidths agree reasonably well with bandwidths from band structure calculations.Item Open Access Conducting polymer composites: polypyrrole and poly (vinyl chloride-vinyl acetate) copolymer(John Wiley & Sons, Inc., 1997) Balci, N.; Bayramli, E.; Toppare, L.Composites of a polypyrrole (PPy) and poly (vinyl chloride-vinyl acetate) copolymer (PVC-PVA) were prepared both chemically and electrochemically. An insulating polymer was retained in the blend and the thermal stability of the polymer was enhanced by polymerizing pyrrole into the host matrix in both cases. The composites prepared electrochemically gave the best results in terms of conductivity and air stability. © 1997 John Wiley * Sons, Inc.Item Open Access Does the donor-acceptor concept work for designing synthetic metals? 2. theoretical investigation of copolymers of 4-(dicyanomethylene)-4H-cyclopenta[2, 1-b: 3, 4-b′]dithiophene and 3, 4-(ethylenedioxy)thiophene(American Chemical Society, 2002) Salzner, U.; Köse, M. E.Density functional theory (DFT) calculations were performed on oligomers of 3,4-(ethylenedioxy)thiophene (EDOT), 4-(dicyanomethylene)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CDM), and co-oligomers (CDM/ EDOT). Oligomer data were extrapolated to polymer values. Theoretical band gaps reproduce λmax from UV spectroscopy for PEDOT and are about 1 eV larger than electrochemical band gaps. λmax of PCDM/EDOT is predicted to be 0.42 eV smaller than that of PEDOT and 0.15 eV smaller than that of PCDM. PCDM/EDOT has a wide valence and an extremely narrow conduction "band". It is probably better not to refer to these localized states as a band at all. This rationalizes the mobility ratio of 500 between p-type and n-type charge carriers and the low n-type conductivity of PCDM/EDOT. The lack of dispersion of the conduction band is due to the very different EAs of EDOT and CDM.Item Open Access Electronic structure analysis and density functional study of an alternating donor/acceptor polymer(Bilkent University, 2001) Köse, Muhammet ErkanConducting polymers have attracted great attention for the last two decades because of their potential applications in many fields. One of the major goals for the scientists who are studying in this field is to synthesize conducting polymers with extremely low band gap. Such polymers would be intrinsically conducting, and thus eliminating the need for doping. Several powerful approaches are proposed to synthesize low band gap polymers. One of them is the donor/acceptor concept, which was proposed by Havinga and his co-workers. In their model, they attached regularly alternating electron withdrawing and electron donating groups to the carbon backbone. By connecting these groups, they aimed to decrease the lowest unoccupied molecular orbital (LUMO) level, and to increase the highest occupied molecular orbital (HOMO) level of the polymer respectively. By this way they claimed that one could obtain very low band gap conducting polymers by introducing the alternating donor/acceptor groups, where the electronegativity difference between these groups is highest. Huang and Pickup synthesized donor/acceptor copolymers of 3,4- ethylenedioxythiophene (EDOT) which is considered as electron donating group and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CDM) as electron withdrawing group. By changing the monomer ratio in the copolymers, they decreased the band gap to less than 0.16 eV, however, the intrinsic conductivity was about 10-3 S/cm. Although a very low band gap was achieved, the conductivities of copolymers were still low. To understand this behaviour and also to check the validity of donor/acceptor concept, we performed theoretical studies for these systems. Monomer through hexamer of EDOT, oligomers for 1:1 copolymer of EDOT and CDM were optimized using density functional theory. Ionization potentials, electron affinities, energy gaps, and band widths of the polymers were obtained by extrapolation. The lowest band gaps are calculated for copolymer poly-CDM-EDOT and poly-CDM. Band gaps of the polymers are found to agree well with the experimental values. When we combine two monomers (EDOT: electron-rich, CDM: electrondeficient) in one to one ratio and in different ratios in order to form co-oligomers, the highest occupied molecular orbitals of co-oligomers are averaged between the HOMO levels of parent homo-oligomers. Lowest unoccupied molecular orbitals of cooligomers are also averaged between the LUMO levels of parent homo-oligomers; however, averaged LUMO levels of co-oligomers are more close in energy to the LUMO level of CDM oligomer. We also found that poly-CDM and poly-CDMEDOT have very flat conduction bands. These narrow bands are attributed to the localized states, which decrease the mobility of n-type carriers in these bands consistent with the experimental findings. Overall, we concluded that donor/acceptor concept can be used to decrease the band gap while sacrificing the dispersion of valence and conduction bands. However, band broadening does not occur in alternating donor/acceptor polymers as donor/acceptor approach suggested.Item Open Access Liquid crystalline mesophases of pluronics (L64, P65, and P123) and transition metal nitrate salts ([M(H2O)6](NO 3)2)(American Chemical Society, 2005) Demirörs, A. F.; Eser, B. E.; Dag, Ö.The triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers, Pluronics (L64, P65, and P123), form liquid crystalline (LC) mesophases with transition metal nitrate salts (TMS), [M(H2O) n] (NO3)2, in the presence and absence of free water in the media. In this assembly process, M-OH2 plays an important role as observed in a TMS:CnEOm (C nEOm is oligo(ethylene oxide) nonionic surfactants) system. The structure of the LC mesophases and interactions of the metal ion-nitrate ion and metal ion-Pluronic were investigated using microscopy (POM), diffraction (XRD), and spectroscopy (FTIR and micro-Raman) techniques. The TMS:L64 system requires a shear force for mesophase ordering to be observed using X-ray diffraction. However, TMS:P65 and TMS:P123 form well structured LC mesophases. Depending on the salt/Pluronic mole ratio, hexagonal LC mesophases are observed in the TMS:P65 systems and cubic and tetragonal LC mesophases in the TMS:P123 systems. The LC mesophase in the water/salt/Pluronic system is sensitive to the concentration of free (H2O) and coordinated water (M-OH2) molecules and demonstrates structural changes. As the free water is evaporated from the H2O:TMS:Pluronic LC mesophase (ternary mixture), the nitrate ion remains free in the media. However, complete evaporation of the free water molecules enforces the coordination of the nitrate ion to the metal ion in all TMS:Pluronic systems. © 2005 American Chemical Society.