We have developed schemes to construct and characterize the
microstructure and
macroscopic properties of individual grain boundary defects in
extended-chain, conjugated polymers. Our
approach has been to take
[1,6-di(N-carbazolyl)-2,4-hexadiyne] (DCHD)
diacetylene monomer crystals
and introduce a single defect under specified boundary conditions.
Two monomer seed crystals are cut
from a precursor single crystal and then brought into close proximity
with one another. Monomer
bicrystals are created by a recrystallization step involving slow
evaporation of a DCHD solution. The
monomer bicrystals are then converted into polymer bicrystals through
thermal energy or by exposure
to high-energy radiation. We have found that the ability to retain
a cohesive interface between the crystals
after the solid-state reaction is a sensitive function of their
relative misorientation and the method of
polymerization. In general, small-angle grain boundaries remain
intact, while large-angle grain
boundaries are broken after polymerization. The geometrical
conditions required to obtain a coherent
interface are more stringent for radiation than thermal polymerization.
The macroscopic properties of
the polymer bicrystals are particularly sensitive to the geometry of
the interface. The efficiency of
photoconductive charge carrier transport across the grain boundary
decreases systematically with
increasing misorientation. The mechanical strength of the polymer
bicrystals also decreases with
increasing misorientation between crystals, with the fracture localized
to the engineered interface. Our
results are consistent with decreasing covalent bond connectivity of
the polymer chains across the interface
with increasing misorientation angle.