It
is challenging to realize both a fully conjugated rigid polymer
backbone and high molecular weight at the same time. Previously, we
reported a DPP-FT4 polymer with molecular weight up to 30 kDa. A new
design and synthesis was required to overcome this limitation. Here,
we report the successful synthesis of a conjugated semiconducting
polymer with tunable molecular weight over a wide range. Through molecular
design and synthesis control, our new polymer can be selectively prepared
with number-averaged molecular weight (M
n) ranging from approximately 20 to 100 kDa, realizing both high molecular
weight and high solubility at the same time. Four polymers within
this range were investigated, with particular emphasis on M
n of 50 kDa (P2) and 97 kDa (P4). The relationships between molecular weight and polymer
properties, molecular packing, and electrical behavior are explored
in detail. All the polymers in this series are fully soluble in nonchlorinated
solvents at room temperature, which is promising for large-area advanced
electronic device applications. The effect of molecular weight on
the charge-transport performance of our new polymer was investigated
using bottom-gate/top-contact field-effect transistor devices. Stable
device characteristics with high on/off ratios up to 107 were obtained. Of particular interest is the discovery that the
hole mobility of P2 (lower M
n) is higher than that of P4 (higher M
n). This is mainly due to morphological manipulation as
demonstrated by atomic force microscopy and grazing-incidence X-ray
diffraction.
Mechanical characterization of optical oxide thin films is performed using nano-indentation, and the results are explained based on the deposition conditions used. These oxide films are generally deposited to have a porous microstructure that optimizes laser induced damage thresholds, but changes in deposition conditions lead to varying degrees of porosity, density, and possibly the microstructure of the thin film. This can directly explain the differences in the mechanical properties of the film studied here and those reported in literature. Of the four single-layer thin films tested, alumina was observed to demonstrate the highest values of nano-indentation hardness and elastic modulus. This is likely a result of the dense microstructure of the thin film arising from the particular deposition conditions used.
During the fabrication of multilayer-dielectric (MLD) thin-film-coated optics, such as the diffraction gratings used in OMEGA EP's pulse compressors, acid piranha cleaning can lead to the formation of chemically induced delamination defects. We investigate the causes of these defects and describe a mechanism for the deformation and failure of the MLD coating in response to hydrogen peroxide in the cleaning solution. A fracture mechanics model is developed and used to calculate the crack path that maximizes the energy-release rate, which is found to be consistent with the characteristic fracture pattern observed in MLD coating delamination defects.
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