Impact of Backbone Rigidity on the ThermomechanicalProperties of Semiconducting Polymers with Conjugation Break Spacers
2020
There remains a lack
of fundamental understanding in the role of
backbone rigidity on the thermomechanical properties of conjugated
polymers. Here, we provide the first holistic approach to understand
the fundamental influence of backbone rigidity on an n-type naphthalene
diimide-based conjugated polymer, denoted by PNDI-Cx, through insertion
of a flexible conjugation break spacer (CBS). CBS lengths are varied
from fully conjugated with zero alkyl spacer (PNDI-C0) to a seven-carbon
alkyl spacer (PNDI-C7), with the CBS engineered into each repeat unit
for systematic evaluation. Solution small-angle neutron scattering
and oscillatory shear rheometry were employed to provide the first
quantitative evidence of CBS influence over conjugated polymer chain
rigidity and entanglement molecular weight (Me), demonstrating a reduction in the Kuhn length from 521 to
36 A for fully conjugated PNDI-C0 and PNDI-C6, respectively,
as well as a nearly consistent Me of ∼15
kDa upon the addition of CBS. Thermomechanical properties, such as
elastic modulus and glass-transition temperature, were shown to decrease
with an increasing length of CBS. An extraordinary ductility, upwards
of 400% tensile strain before fracture, was observed for high-molecular-weight
PNDI-C4, which we attribute to a high number of entanglements and
disruption of crystallization. Furthermore, the deformation mechanism
for PNDI-Cx was studied under strain through X-ray diffraction, polarized
UV–vis spectroscopy, and atomic force microscopy. Overall,
this work sheds light on the important role of backbone rigidity in
designing flexible and stretchable conjugated polymers.
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