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

Abstract

Conjugation breakers (CBs) with different Hbonding 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.

A design strategy for high mobility stretchable polymer semiconductors

“…In stretchable electronics applications, our terpolymer semiconductors may be used under strain, which requires understanding the microstructure change in the stretched terpolymer semiconductor films. Several mechanisms can result in dissipating applied mechanical stress, such as chain stretching and alignment in amorphous domains, breaking aggregates and/or crystalline domains, rotation of monomer units, and breaking weaker bonding (i.e., hydrogen bonding or metal-ligand bonding) 32,34,[46][47][48] . We hypothesized that our terpolymers with low degree of crystallinity and lack of large crystalline domains may effectively dissipate applied mechanical stress without affecting their crystalline domains.…”

“…For example, charge carrier mobility is often reduced with increasing stretchability, which significantly limits the use of stretchable semiconducting polymers 30,31 . Even though significant progress has been made with the incorporation of dynamic bonding units, partial breakage of the conjugation makes it challenging to control the morphology, which makes it difficult to realize high mobility [32][33][34] . Several types of additives have been reported and have shown some promise.…”

Optimizing the self‐assembly of conjugated polymers and small molecules through structurally programmed non‐covalent control

“…Finally, these materials responded to bodily movements such as twisting and bending of joints that they were appended to. In a more recent study, Zheng et al assessed the correlation between strength of hydrogen bonding through conjugation breakers and polymer physical properties 106 . Stronger hydrogen bonding through urea groups yielded materials with higher flexibility, modulus, and charge mobility.…”

Metal–Ligand Based Mechanophores Enhance Both Mechanical Robustness and Electronic Performance of Polymer Semiconductors

“…[ 21–25 ] The strength of the dynamic bonds as well as number and position of the mechanophoric units were shown to be crucial factors for modulating the resulting mechanical properties. Recently, mechanophores based on hydrogen bonds have been employed in the backbone [ 26–28 ] or side chain [ 29 ] of PSCs as dynamic crosslinkers to facilitate thin film deformability. However, mechanophores usually break the conjugation of the polymer backbone, thereby reducing the charge carrier mobility.…”