Characterization of Hydrogen Bonding Formation and Breaking in Semiconducting Polymers under Mechanical Strain

Abstract

Diketopyrrolopyrrole (DPP) based donor-acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridine dicarboxamide (PDCA), inserted as end groups in alkyl sidechains 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 sidechains 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 Å to 22.87 Å) compared to the neat DPP polymer. Increasing mol% of PDCA units in sidechains 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 crosslinking 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 °C to 20 °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 as well.

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

“…DSC measurements for PDPP‐T3 and PNDI‐C0, which have highly planar and rigid backbone structures, are presented in Figure (d,e), the step change in c p , thus the glass transition, cannot be readily seen from the DSC scans. A linear curve is observed and is expected based on various previous reports . PDPP‐T3 shows a small curvature at the vicinity of 0 °C.…”

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

“…Hydrogen bonding (H bonding) has been widely used to tune intermolecular interactions and self‐assembly structures by considering its relative bonding strength and directionality . On the basis of this consideration, as shown in Scheme and Figure , some of us incorporated urea groups in the side chains of DPP‐based polymers P175–P177 .…”

“…The semiconducting performances of the damaged devices can be almost fully recovered after a solvent and thermal healing treatment. Recently, they incorporated PDCA units into the side chains of P197–P200 . On the basis of FTIR studies, they concluded that the PDCA units in the side chains as in P197–P200 , are more favorable for the formation of H bonding compared with those in P194–P196 , in which the PDCA units are positioned in the backbones.…”

Random copolymerization of polythiophene for simultaneous enhancement of in‐plane and out‐of‐plane charge transport for organic transistors and perovskite solar cells

“…All synthesized polymers exhibited vibronic shoulders at around 600 nm, representing the existence of molecular ordering and aggregation of the SCPs in the film state. 36 Furthermore, P20 and P30, the random copolymers, exhibited distinct vibronic shoulders of similar intensity in comparison with the homopolymer P0, indicating that the former can form a highly ordered structure despite random copolymerization. However, the intensity of the vibronic shoulder of P10 was lower than that of the other polymers, suggesting not only that the interchain interaction between the polymers of P10 is weaker than in the others, but also that adding a small quantity of different monomer would disrupt the ordering of the polymer chains.…”