Mikhail Yu. Lavrentiev

A study of the different structural phases of the polymer poly(9,9′-dioctyl fluorene) using Raman spectroscopy

Abstract The polymer poly(9,9′-dioctyl fluorene) (F8) can be easily driven between a number of different structural phases by different thermal or solvent treatments. The phases which we have identified include (i) as-spin-coated, (ii) semi-crystalline, (iii) glassy, (iv) with an extended intrachain conformation. Atomic force microscopy has been used to investigate the surface structures associated with the different phases of the polymer and different solvents used to cast the film. Raman spectroscopy has been performed on these samples to understand how the polymer conformation changes with the phases. Variations in the relative intensity of the peaks and shifts in energy are observed. We have compared the F8 spectrum with a trimer of fluorene with butyl side chains and acrylate/hydroxy endgroups attached to the fluorene main chain via hexyl spacers. We identify a very small frequency dispersion in the vibrational modes connected to an increase of the conjugation length. Therefore, the assignment of the vibrational modes of the trimer is very important for understanding the F8 spectrum.

Phase stability of ternary fcc and bcc Fe-Cr-Ni alloys

The phase stability of fcc and bcc magnetic binary Fe-Cr, Fe-Ni, Cr-Ni alloys and ternary Fe-Cr-Ni alloys is investigated using a combination of density functional theory (DFT), Cluster Expansion (CE) and Magnetic Cluster Expansion (MCE). Energies, magnetic moments, and volumes of more than 500 alloy structures are evaluated using DFT, and the most stable magnetic configurations are compared with experimental data. Deviations from the Vegard law in fcc Fe-Cr-Ni alloys, associated with non-linear variation of atomic magnetic moments as functions of alloy composition, are observed. Accuracy of the CE model is assessed against the DFT data, where for ternary alloys the cross-validation error is smaller than 12 meV/atom. A set of cluster interaction parameters is defined for each alloy, where it is used for predicting new ordered alloy structures. Fcc Fe2CrNi phase with Cu2NiZn-like structure is predicted as the global ground state with the lowest chemical ordering temperature of 650K. DFT-based Monte Carlo (MC) simulations are used for assessing finite temperature fcc-bcc phase stability and order-disorder transitions in Fe-Cr-Ni alloys. Enthalpies of formation of ternary alloys calculated from MC simulations at 1600K combined with magnetic correction derived from MCE are in excellent agreement with experimental values measured at 1565K. Chemical order is analysed, as a function of temperature and composition, in terms of the Warren-Cowley short-range order (SRO) parameters and effective chemical pairwise interactions.

THEORETICAL INVESTIGATION OF THE LOW-LYING ELECTRONIC STRUCTURE OF POLY(P-PHENYLENE VINYLENE)

The two-state molecular orbital model of the one-dimensional phenyl-based semiconductors is applied to poly(para-phenylene vinylene). The energies of the low-lying excited states are calculated using the density matrix renormalization group method. Calculations of both the exciton size and the charge gap show that there are both

Density-matrix renormalization-group calculations of excited states of linear polyenes

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Electron–phonon interactions in poly(para-phenylene) oligomers

and

Density matrix renormalization group calculations of the low-lying excitations and non-linear optical properties of poly( para-phenylene)

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Breakdown of the adiabatic approximation in trans-polyacetylene

excitonic levels below the band threshold. The energy of the

11B-U-2 1A+G CROSSOVER IN CONJUGATED POLYMERS : THE PHASE DIAGRAM OF THE MOLECULAR-ORBITAL MODEL

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Monte Carlo simulations of Fe-Cr solid solution

exciton extrapolates to 2.60 eV in the limit of infinite polymers, while the energy of the

Cluster-Expansion-Based Monte Carlo Simulations of Fe-Cr Solid Solutions

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