A series
of alkyl-substituted indacenodithiophene (alkyl-IDT) semiconducting
donor–acceptor polymers were designed by DFT to have varying
degrees of backbone planarity and synthesized via direct arylation
polymerization (DArP). These polymers exhibit weak intermolecular
interactions, a glass transition temperature (T
g) below room temperature, and low degrees of crystallinity
from XRD measurements. Despite this, the field-effect mobilities (μ)
of these polymers are relatively high (0.06–0.20 cm2 V–1 s–1) with mobility increasing
with increasing backbone planarity. Because of the weak intermolecular
interactions, the polymers exhibit low elastic moduli (E
f) of less than 450 MPa. The polymer with the most twisted
backbone exhibits high ductility with a crack-onset strain (CoS) over
100%. These structure–property relationship studies provide
useful guidelines for designing semiconducting polymers with high
mobility, low stiffness, and high ductility enabling applications
in stretchable electronics.
Poly(indacenodithiophene-benzothiadiazole)
has received significant
interest because of its exceptional hole mobility despite its near-amorphous
thin-film morphology and brittleness at low M
n. In comparison, poly(indacenodithiophene-benzopyrollodione)
(PIDTBPD) has a lower hole mobility but is exceptionally ductile at
similar M
n. Herein, we synthesize random
indacenodithiophene (IDT) copolymers with varying amounts of incorporated
benzothiadiazole and benzopyrollodione (BPD), which introduces varied
degrees of backbone twist to each respective polymer system. This
allows us to elucidate how the BPD monomer introduction leads to conformational
and morphological changes that influence the crack onset strain (CoS)
and hole mobility of these near-amorphous IDT copolymers and the rates
by which each material property responds to sequentially larger BPD
incorporation. Results of density functional theory calculations suggest
that BPD introduction does not lead to significant differences in
backbone linearity between the studied polymers, and grazing incidence
wide-angle X-ray scattering demonstrates that the degree of crystallinity
within thin films is not significantly altered. It does, however,
lead to a more varied circular distribution of the hexadecyl side
chains around the polymer backbone. With increasing BPD incorporation,
a crossover point between CoS and hole mobility emerges. At this crossover
point, a random copolymer with 30% BPD introduction displays increased
CoS and an average hole mobility value equal to that of the PIDTBPD
system, suggesting that hole mobility is more sensitive to torsion
along the polymer backbone, while the response of the CoS is relatively
delayed. The data also suggest that the increase in CoS with increasing
BPD content does not arise because of differences in rigidity but
because the more circular distribution of the side chains makes polymer
chains with sufficient BPD content better able to flow.
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