Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)

X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy have been used to study the well-known order-disorder transition (ODT) to the beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through combination of time-dependent and temperature-dependent 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 beta phase. Rapid thermal quenching is employed for PF8 studies of pure alpha phase samples while extended low-temperature annealing is used for improved beta phase formation. Low temperature PL studies reveal sharp Franck-Condon type emission bands and, in the beta 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 beta 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.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure

Computational fluid dynamics simulation and full-scale experimental model inter-comparison of the wind flow around a university campus

In this article, results obtained from two computational fluid dynamics solvers, WindSim and OpenFOAM, have been compared for the wind flow around the University of Mauritius’ campus for different wind directions, and a reference incident wind speed at diverse height above ground level. A grid resolution study is performed for both software and the mean differences of the two solvers with multiple turbulence models (standard k-ε, k-ε with Yap correction, and renormalization group k-ε) are analyzed with onsite measured data. The article concludes that the best results for the computational fluid dynamics simulation of the wind flow around buildings are obtained using OpenFOAM with k-ε turbulence model including Yap correction

Assessing temporal couplings in social–ecological island systems: historical deforestation and soil loss on Mauritius (Indian Ocean)

Temporal couplings, such as historical interactions between deforestation and soil loss, are responsible for the current state of a wide range of ecosystem services of the social–ecological system on Mauritius. Islands are suitable study sites for understanding temporal couplings and telecouplings because of their: (1) clearly defined physical boundaries, (2) finite local resources, and (3) relatively short human history. Six well-documented historical deforestation maps, starting from the first colonization of Mauritius in 1638, were used as input parameters to model two scenarios of cumulative soil loss, with and without deforestation, using the revised universal soil loss equation in a geographic information system. The scenarios show that historical deforestation since 1638 has resulted in a cumulative soil loss that drastically exceeds soil loss under a natural baseline scenario without deforestation. The adopted method illustrates to what extent the current state of the soil of a social–ecological system is negatively affected by past human–environment interactions. We suggest that potential negative impacts on insular societies are mitigated by telecouplings such as food, fuel, and fertilizer imports