Michael Winokur - Academia.edu (original) (raw)
Papers by Michael Winokur
Crystal Growth & Design, 2007
... Hyeun-Seok Cheun, Michael J. Winokur, and Andrew P. Monkman . ... Brown, SD; Bouchenoire, L.... more ... Hyeun-Seok Cheun, Michael J. Winokur, and Andrew P. Monkman . ... Brown, SD; Bouchenoire, L.; Bowyer, D.; Kervin, J.; Laundy, D.; Longfield, MJ; Mannix, D.; Paul, DF; Stunault, A.; Thompson, P.; Cooper, MJ; Lucas, CA; Stirling, WG J. Synchrotron Rad.2001, 8, 1172−1181. ...
The local molecular structure of amorphous HBr-doped polyaniline has been probed using convention... more The local molecular structure of amorphous HBr-doped polyaniline has been probed using conventional and differential anomalous scattering in conjunction with a single chain modelling analysis. Initial results identify a probable Br-ion site along the polymer chain approximately 3.3A above (or below) the average polyaniline molecular plane and situated, on average, 3.7A from the closest neighboring amine nitrogen.
The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in bot... more The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in both its base (EB) and salt (ES) forms has been studied using x-ray diffraction (XRD) techniques and these data reinforce the general belief that this local structure is extremely sensitive to the specific processing history. PANI powders precipitated as ES immediately following synthesis lead to reasonably crystalline materials whereas EB films cast from n-methyl pyrrolidine (NMP) solutions containing hydrogen bonding inhibitors are fully amorphous. Dedoping, by further treatments of ES with NH40H solutions, or doping, by immersion of EB in aqueous HF solutions, lead to further changes in the local molecular structure. Many of these structural phases bear a limited resemblance to the previously proposed structures for HCl-doped PANI powder and films [1,2]. The role of water hydration in the structural evolution is also examined and significant structural variations are found to occur in ES at radically different time scales.
ABSTRACT We present a rigorous method for assessing the influence of nematic microdomains within ... more ABSTRACT We present a rigorous method for assessing the influence of nematic microdomains within conjugated polymer (CP) thin films on CP optical spectroscopy. Nematic alignment, a characteristic widely seen in these optically active polymers, creates an environment in which overlap between the chromophore absorption and emission bands can lead to large scale self-absorption of the pi-pi(*) emission band even in moderately thin films. This nematic order also affects simple absorption measurements. Steady-state optical emission and absorption measurements of the blue emitting polymer poly[bis(2-ethylhexyl)fluorene] are used to demonstrate the impact of these effects.
... J. Maron? and M. J. Winokur* Department of Physics, University of Wisconsin, Madison, Wiscons... more ... J. Maron? and M. J. Winokur* Department of Physics, University of Wisconsin, Madison, Wisconsin 53706 BR Mattes Los Alarnos National Laboratory, Los Alamos, New Mexico 87545 Received September 23, 1994; Revised Manuscript Received March 15, 1995@ ...
Physical Review B, 1996
... Sci. Forum 91, 363 1992 . 13 D. Chen, MJ Winokur, M. Masse, and FE Karasz, Phys. Rev. B 41, 6... more ... Sci. Forum 91, 363 1992 . 13 D. Chen, MJ Winokur, M. Masse, and FE Karasz, Phys. Rev. B 41, 6759 1990 . 14 MJ Winokur, P. Davis, and D. Chen, in Chemical Physics of Intercalation II, Vol. ... Rev. Lett. 64, 3031 1990 ; HA Mizes and EM Conwell, Phys. Rev. ...
Macromolecules, 1998
... Abstract. The structural ordering within crystalline polyaniline (PANI) powders as emeraldine... more ... Abstract. The structural ordering within crystalline polyaniline (PANI) powders as emeraldine (ie, at 50% oxidation) in both its base (EB) and halogen acid doped salt (ES) forms has been studied using X-ray diffraction. ... PANI sample, a, b, c, α, β, γ, density (g/cm 3 ). ...
Synthetic Metals
The local molecular structure of amorphous HBr-doped polyaniline has been probed using convention... more The local molecular structure of amorphous HBr-doped polyaniline has been probed using conventional and differential anomalous scattering in conjunction with a single chain modelling analysis. Initial results identify a probable Br-ion site along the polymer chain approximately 3.3A above (or below) the average polyaniline molecular plane and situated, on average, 3.7A from the closest neighboring amine nitrogen.
Physical Review Letters, Jun 1, 1986
A stacking transformation with unusual structural and thermal hysteresis is reported in stage-2 a... more A stacking transformation with unusual structural and thermal hysteresis is reported in stage-2 alkali-metal-intercalated graphites. A sequence of one-dimensional higher-order commensurate phases appears to form a stepwise-continuous pathway between fcc and hcp intercalant structures as the temperature is increased. The presence of correlated impurities, in the form of disordered in-plane domain walls, can account for instabilities in the sequence of
We discuss initial results on spin-polarized injection in polymeric light-emitting diode structur... more We discuss initial results on spin-polarized injection in polymeric light-emitting diode structures. We use half-metallic LaCaMnO and LaSrMnO as the hole injector, PEDOT/PSS as an interfacial layer, MEH-PPV as the light emitting ogranic, and Ca/Al as electron-injector. The magnetic oxides La(Ca,Sr)MnO are highly spin-polarized at the Fermi energy, resulting in changes in the excitonic spin configurations in the polymer layer.
Phys Rev B, 2003
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to st... more X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.
The structures of conjugated organic polymer poly(p-phenylenevinylene) (PPV) films intercalated b... more The structures of conjugated organic polymer poly(p-phenylenevinylene) (PPV) films intercalated by SbF5 and H_2SO4 are reported. Equatorial x-ray diffraction data from samples prepared using chemical and vapor methods have been obtained and compared with previously proposed and new structural models. These newer model structures qualitatively resemble the layered motif previously suggested, but the detailed structural differences are quite pronounced. Non-equatorial scattering data will also be presented that shows the existence of three dimensional ordering within these p-type doped PPV films. This work supported by NSF Grant No. DMR-9631575.
J Luminesc, 2007
We investigate localized exciton states in π-conjugated polymers with finite torsion. The localiz... more We investigate localized exciton states in π-conjugated polymers with finite torsion. The localized states are associated with a perturbed transfer integral for which the magnitude of cosine of the torsion angle exceeds the magnitude of the corresponding cosine of the unperturbed system. The localized state energy is calculated as a function of the ratio of the perturbed to unperturbed transfer integrals. Particular attention is paid to the optically active symmetric localized states, and the effective oscillator strength, or square of the absolute magnitude of the transition dipole moment, is calculated as a function of the energy. The relation of the theory to recent optical studies of poly(di-noctylfluorene) (PF8) is discussed. phone (608) 265-4035; fax (608) 265-2334
Aps March Meeting Abstracts, Mar 1, 1998
The structures induced by the larger, anion-radical forming molecular (p-type) dopants (e.g. H_2S... more The structures induced by the larger, anion-radical forming molecular (p-type) dopants (e.g. H_2SO_4, AsF_5) are generally thought to be dominated by the creation of two-dimensional (2D) galleries of guest anions separated by a single stacked layer of the positively charged polymer host chains. This layered phase is often cast in analogy to that of quasi-2D intercalation compounds whereby a stage-1 layer compound occurs by direct insertion of the guest specie between preexisting stacks of the polymer chains. In the case of conducting polymers, and especially in PPV which has appreciable axial ordering within the b-c plane, the natural inclination is to present a model in which the molecular anion layer opens galleries perpendicular to the [100] plane of the PPV host. In this talk x-ray scattering data and modeling analysis of H_2SO_4-doped PPV are presented which suggests that the intercalation process actually involves a counterintuitive reconstruction in which the PPV chains undergo a disruptive sequence of translational displacements with insertion of layers along the orthogonal [010] direction. This surprising mechanism may also explain why higher order stages are not generally observed and why the intercalation process often produces layered materials with drastically reduced peak widths and coherence lengths. (This work supported by NSF Grant No. DMR-9631575.) ^Current affiliation: Argonne National Laboratory, Argonne IL
Polysilanes form a diverse family of silicon backboned polymers that are well-known for their ext... more Polysilanes form a diverse family of silicon backboned polymers that are well-known for their extensive variations in thermochromic behavior, main-chain conformations, and interchain packing. Crystalline poly(di-methylsilane) (PdmS) and poly(methyl-n-propylsilane) (PmpS) are generally reported to exist in a state characterized by an all-trans backbone conformation. The main chain conformations of the longer alkane derivatives are, as yet, unknown. Conventional x-ray diffraction studies in combination with structure factor refinements of PdmS and PmpS data suggest that, while the average structure nominally approximates that of an all-trans conformation, there is evidence for large Si-Si-Si-Si dihedral displacements away from a 180^circ trans-planar conformation. Pair-distribution function analysis of scattering data from PmpS, poly(methyl-n-butylsilane) and poly(methyl-n-hexylsilane) at high q, (which averages over crystalline, hexagonal columnar and amorphous regions) is also most consistent with models that include large average deviations from main chain planarity. (This work supported in part by NSF Grant No. DMR-9631575.)
Crystal Growth & Design, 2007
... Hyeun-Seok Cheun, Michael J. Winokur, and Andrew P. Monkman . ... Brown, SD; Bouchenoire, L.... more ... Hyeun-Seok Cheun, Michael J. Winokur, and Andrew P. Monkman . ... Brown, SD; Bouchenoire, L.; Bowyer, D.; Kervin, J.; Laundy, D.; Longfield, MJ; Mannix, D.; Paul, DF; Stunault, A.; Thompson, P.; Cooper, MJ; Lucas, CA; Stirling, WG J. Synchrotron Rad.2001, 8, 1172−1181. ...
The local molecular structure of amorphous HBr-doped polyaniline has been probed using convention... more The local molecular structure of amorphous HBr-doped polyaniline has been probed using conventional and differential anomalous scattering in conjunction with a single chain modelling analysis. Initial results identify a probable Br-ion site along the polymer chain approximately 3.3A above (or below) the average polyaniline molecular plane and situated, on average, 3.7A from the closest neighboring amine nitrogen.
The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in bot... more The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in both its base (EB) and salt (ES) forms has been studied using x-ray diffraction (XRD) techniques and these data reinforce the general belief that this local structure is extremely sensitive to the specific processing history. PANI powders precipitated as ES immediately following synthesis lead to reasonably crystalline materials whereas EB films cast from n-methyl pyrrolidine (NMP) solutions containing hydrogen bonding inhibitors are fully amorphous. Dedoping, by further treatments of ES with NH40H solutions, or doping, by immersion of EB in aqueous HF solutions, lead to further changes in the local molecular structure. Many of these structural phases bear a limited resemblance to the previously proposed structures for HCl-doped PANI powder and films [1,2]. The role of water hydration in the structural evolution is also examined and significant structural variations are found to occur in ES at radically different time scales.
ABSTRACT We present a rigorous method for assessing the influence of nematic microdomains within ... more ABSTRACT We present a rigorous method for assessing the influence of nematic microdomains within conjugated polymer (CP) thin films on CP optical spectroscopy. Nematic alignment, a characteristic widely seen in these optically active polymers, creates an environment in which overlap between the chromophore absorption and emission bands can lead to large scale self-absorption of the pi-pi(*) emission band even in moderately thin films. This nematic order also affects simple absorption measurements. Steady-state optical emission and absorption measurements of the blue emitting polymer poly[bis(2-ethylhexyl)fluorene] are used to demonstrate the impact of these effects.
... J. Maron? and M. J. Winokur* Department of Physics, University of Wisconsin, Madison, Wiscons... more ... J. Maron? and M. J. Winokur* Department of Physics, University of Wisconsin, Madison, Wisconsin 53706 BR Mattes Los Alarnos National Laboratory, Los Alamos, New Mexico 87545 Received September 23, 1994; Revised Manuscript Received March 15, 1995@ ...
Physical Review B, 1996
... Sci. Forum 91, 363 1992 . 13 D. Chen, MJ Winokur, M. Masse, and FE Karasz, Phys. Rev. B 41, 6... more ... Sci. Forum 91, 363 1992 . 13 D. Chen, MJ Winokur, M. Masse, and FE Karasz, Phys. Rev. B 41, 6759 1990 . 14 MJ Winokur, P. Davis, and D. Chen, in Chemical Physics of Intercalation II, Vol. ... Rev. Lett. 64, 3031 1990 ; HA Mizes and EM Conwell, Phys. Rev. ...
Macromolecules, 1998
... Abstract. The structural ordering within crystalline polyaniline (PANI) powders as emeraldine... more ... Abstract. The structural ordering within crystalline polyaniline (PANI) powders as emeraldine (ie, at 50% oxidation) in both its base (EB) and halogen acid doped salt (ES) forms has been studied using X-ray diffraction. ... PANI sample, a, b, c, α, β, γ, density (g/cm 3 ). ...
Synthetic Metals
The local molecular structure of amorphous HBr-doped polyaniline has been probed using convention... more The local molecular structure of amorphous HBr-doped polyaniline has been probed using conventional and differential anomalous scattering in conjunction with a single chain modelling analysis. Initial results identify a probable Br-ion site along the polymer chain approximately 3.3A above (or below) the average polyaniline molecular plane and situated, on average, 3.7A from the closest neighboring amine nitrogen.
Physical Review Letters, Jun 1, 1986
A stacking transformation with unusual structural and thermal hysteresis is reported in stage-2 a... more A stacking transformation with unusual structural and thermal hysteresis is reported in stage-2 alkali-metal-intercalated graphites. A sequence of one-dimensional higher-order commensurate phases appears to form a stepwise-continuous pathway between fcc and hcp intercalant structures as the temperature is increased. The presence of correlated impurities, in the form of disordered in-plane domain walls, can account for instabilities in the sequence of
We discuss initial results on spin-polarized injection in polymeric light-emitting diode structur... more We discuss initial results on spin-polarized injection in polymeric light-emitting diode structures. We use half-metallic LaCaMnO and LaSrMnO as the hole injector, PEDOT/PSS as an interfacial layer, MEH-PPV as the light emitting ogranic, and Ca/Al as electron-injector. The magnetic oxides La(Ca,Sr)MnO are highly spin-polarized at the Fermi energy, resulting in changes in the excitonic spin configurations in the polymer layer.
Phys Rev B, 2003
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to st... more X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the β phase in poly(9,9-(di-n,noctyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperaturedependent measurements. The ODT is well described by a simple Avrami picture of one-dimensional nucleation and growth but crystallization, on cooling, proceeds only after molecular-level conformational relaxation to the so called β phase. Rapid thermal quenching is employed for PF8 studies of pure α phase samples while extended low-temperature annealing is used for improved β phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the β phase, two distinguishable vibronic sub-bands with energies of approximately 199 and 158 meV at 25 K. This improved molecular level structural order leads to a more complete analysis of the higher-order vibronic bands. A net Huang-Rhys coupling parameter of just under 0.7 is typically observed but the relative contributions by the two distinguishable vibronic sub-bands exhibit an anomalous temperature dependence. The PL studies also identify strongly correlated behavior between the relative β phase 0-0 PL peak position and peak width. This relationship is modeled under the assumption that emission represents excitons in thermodynamic equilibrium from states at the bottom of a quasi-one-dimensional exciton band. The crystalline phase, as observed in annealed thin-film samples, has scattering peaks which are incompatible with a simple hexagonal packing of the PF8 chains.
The structures of conjugated organic polymer poly(p-phenylenevinylene) (PPV) films intercalated b... more The structures of conjugated organic polymer poly(p-phenylenevinylene) (PPV) films intercalated by SbF5 and H_2SO4 are reported. Equatorial x-ray diffraction data from samples prepared using chemical and vapor methods have been obtained and compared with previously proposed and new structural models. These newer model structures qualitatively resemble the layered motif previously suggested, but the detailed structural differences are quite pronounced. Non-equatorial scattering data will also be presented that shows the existence of three dimensional ordering within these p-type doped PPV films. This work supported by NSF Grant No. DMR-9631575.
J Luminesc, 2007
We investigate localized exciton states in π-conjugated polymers with finite torsion. The localiz... more We investigate localized exciton states in π-conjugated polymers with finite torsion. The localized states are associated with a perturbed transfer integral for which the magnitude of cosine of the torsion angle exceeds the magnitude of the corresponding cosine of the unperturbed system. The localized state energy is calculated as a function of the ratio of the perturbed to unperturbed transfer integrals. Particular attention is paid to the optically active symmetric localized states, and the effective oscillator strength, or square of the absolute magnitude of the transition dipole moment, is calculated as a function of the energy. The relation of the theory to recent optical studies of poly(di-noctylfluorene) (PF8) is discussed. phone (608) 265-4035; fax (608) 265-2334
Aps March Meeting Abstracts, Mar 1, 1998
The structures induced by the larger, anion-radical forming molecular (p-type) dopants (e.g. H_2S... more The structures induced by the larger, anion-radical forming molecular (p-type) dopants (e.g. H_2SO_4, AsF_5) are generally thought to be dominated by the creation of two-dimensional (2D) galleries of guest anions separated by a single stacked layer of the positively charged polymer host chains. This layered phase is often cast in analogy to that of quasi-2D intercalation compounds whereby a stage-1 layer compound occurs by direct insertion of the guest specie between preexisting stacks of the polymer chains. In the case of conducting polymers, and especially in PPV which has appreciable axial ordering within the b-c plane, the natural inclination is to present a model in which the molecular anion layer opens galleries perpendicular to the [100] plane of the PPV host. In this talk x-ray scattering data and modeling analysis of H_2SO_4-doped PPV are presented which suggests that the intercalation process actually involves a counterintuitive reconstruction in which the PPV chains undergo a disruptive sequence of translational displacements with insertion of layers along the orthogonal [010] direction. This surprising mechanism may also explain why higher order stages are not generally observed and why the intercalation process often produces layered materials with drastically reduced peak widths and coherence lengths. (This work supported by NSF Grant No. DMR-9631575.) ^Current affiliation: Argonne National Laboratory, Argonne IL
Polysilanes form a diverse family of silicon backboned polymers that are well-known for their ext... more Polysilanes form a diverse family of silicon backboned polymers that are well-known for their extensive variations in thermochromic behavior, main-chain conformations, and interchain packing. Crystalline poly(di-methylsilane) (PdmS) and poly(methyl-n-propylsilane) (PmpS) are generally reported to exist in a state characterized by an all-trans backbone conformation. The main chain conformations of the longer alkane derivatives are, as yet, unknown. Conventional x-ray diffraction studies in combination with structure factor refinements of PdmS and PmpS data suggest that, while the average structure nominally approximates that of an all-trans conformation, there is evidence for large Si-Si-Si-Si dihedral displacements away from a 180^circ trans-planar conformation. Pair-distribution function analysis of scattering data from PmpS, poly(methyl-n-butylsilane) and poly(methyl-n-hexylsilane) at high q, (which averages over crystalline, hexagonal columnar and amorphous regions) is also most consistent with models that include large average deviations from main chain planarity. (This work supported in part by NSF Grant No. DMR-9631575.)