Tuning the Mechanical Properties of a Polymer Semiconductor by Modulating Hydrogen Bonding Interactions

Conjugation breakers
(CBs) with different H-bonding chemistries
and linker flexibilities are designed and incorporated into a diketopyrrolopyrrole
(DPP)-based conjugated polymer backbone. The effects of H-bonding
interactions on polymer semiconductor morphology, mechanical properties,
and electrical performance are systematically investigated. We observe
that CBs with an H-bonding self-association constant >0.7 or a
denser
packing tendency are able to induce higher polymer chain aggregation
and crystallinity in as-casted thin films, resulting in a higher modulus
and crack on-set strain. Additionally, the rDoC (relative degree of
crystallinity) of the stretched thin film with the highest crack on-set
strain only suffers a small decrease, suggesting the main energy dissipation
mechanism is the breakage of H-bonding interactions. By contrast,
other less stretchable polymer films dissipate strain energy through
the breakage of crystalline domains, indicated by a drastic decrease
in rDoC. Furthermore, we evaluate their electrical performances under
mechanical strain in fully stretchable field-effect transistors. The
polymer with the highest crack on-set strain has the least degradation
in mobility as a function of strain. Overall, these observations suggest
that we can aptly tune the mechanical properties of a polymer semiconductor
by modulating intermolecular interactions, such as H-bonding chemistry
and linker flexibility. Such understanding provides molecular design
guidelines for future stretchable semiconductors.