Macromolecules, Vol.52, No.6, 2476-2486, 2019
Characterization of Hydrogen Bonding Formation and Breaking in Semiconducting Polymers under Mechanical Strain
Diketopyrrolopyrrole (DPP)-based donor-acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridinedicarboxamide (PDCA), inserted as end groups in alkyl side chains were prepared and investigated. In contrast to previously reported DPP polymers containing PDCA units as conjugation breakers along the polymer backbone, PDCA in the alkyl side chains readily produced almost quantitative formation of intermolecular H-bonding even at low PDCA unit content (<10 mol %) as shown by Fourier transform infrared spectroscopy (FTIR). The efficient intermolecular H-bonding was further supported by the appearance of a pronounced vibronic shoulder in the UV-vis spectra and a reduction of interlamellar spacing (from 24.02 to 22.87 angstrom) compared to the neat DPP polymer. Increasing mol % of PDCA units in side chains of DPP conjugated polymers also has a clear effect on the thermal and mechanical properties of the films as investigated by dynamic mechanical analysis (DMA). Polymers with a high loading of PDCA showed a linear increase in both tan delta intensity and temperature at which softening of film cross-linking occurs. In particular, at a comparable mol %, polymers with PDCA units along the conjugated backbone showed a lower transition intensity and on average a 10-20 degrees C higher temperature required for H-bonding breaking. FTIR coupled with crack onset measurements showed that H-bonding breaking during tensile deformation happens only at strains close to crack onset. All these observations suggest that molecular engineering of conjugated polymers bearing H-bonding units has a strong influence on microstructure, thermal and mechanical properties of solution processed films, and final energy dissipation mechanisms in stretchable electronics applications.