A consensus
is emerging that mixed phases are present in bulk heterojunction
organic photovoltaic (OPV) devices. Significant insights into the
mixed phases have come from bilayer stability measurements, in which
an initial sample consisting of material pure layers of donor and
acceptor is thermally treated, resulting in swelling of one layer
by the other. We present a comparative study of the stability of polymer/fullerene
bilayers using two common OPV polymer donors poly(3-hexylthiophene),
P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)],
PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl
ester, phenyl-C71-butyric acid methyl ester, [60]PCBM bis-adduct,
and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry
to characterize the quasi-steady state behavior of the films, we find
that the polymer glass transition temperature (T
g) is critical to the bilayer stability, with no significant
changes occurring below T
g of the high T
g PCDTBT. Above the polymer T
g, we find the behavior is irreversible and most consistent
with swelling of the polymer by the fullerene, constrained by tie
chains in the polymer network and influenced by the rubbery dynamics
of the mixed region. The swelling varies significantly with the nature
of the fullerene and the polymer. Across the eight systems studied,
there is no clear relationship between swelling and OPV device performance.
The relationship between the observed swelling and the underlying
fullerene–polymer miscibility is explored via Flory–Rehner
theory.
New energetic compounds are designed to minimize their potential environmental impacts, which includes their transformation and the fate and effects of their transformation products. The nitro groups of energetic compounds are readily reduced to amines, and the resulting aromatic amines are subject to oxidation and coupling reactions. Manganese dioxide (MnO2) is a common environmental oxidant and model system for kinetic studies of aromatic amine oxidation. In this study, a training set of new and previously reported kinetic data for the oxidation of model and energetic-derived aromatic amines was assembled and subjected to correlation analysis against descriptor variables that ranged from general purpose [Hammett σ constants (σ(-)), pKas of the amines, and energies of the highest occupied molecular orbital (EHOMO)] to specific for the likely rate-limiting step [one-electron oxidation potentials (Eox)]. The selection of calculated descriptors (pKa, EHOMO, and Eox) was based on validation with experimental data. All of the correlations gave satisfactory quantitative structure-activity relationships (QSARs), but they improved with the specificity of the descriptor. The scope of correlation analysis was extended beyond MnO2 to include literature data on aromatic amine oxidation by other environmentally relevant oxidants (ozone, chlorine dioxide, and phosphate and carbonate radicals) by correlating relative rate constants (normalized to 4-chloroaniline) to EHOMO (calculated with a modest level of theory).
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