The Critical Role of Electron‐Donating Thiophene Groups on the Mechanical and Thermal Properties of Donor–Acceptor Semiconducting Polymers

Abstract

Organic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (Tg) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists in para‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record low Tg, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐Tg conjugated polymers for stretchable electronics.

Glass Transition Phenomenon for Conjugated Polymers

“…Similarly, the T g measured with glass fiber DMA goes up from −4.0 °C for P(DPPT) to 2.8 °C and 4.1 °C for P(DPPTT) and P(DPPTTT) with fused thiophene rings, respectively (Table ). However, AC‐chip calorimetry for P(DPPTTT) gives a T g of 22 °C, much higher than that from DMA, which is consistent with the conformational freedom of the structure . This suggests that extra caution needs to be paid when interpret the results from DMA .…”

An Intrinsically Stretchable High‐Performance Polymer Semiconductor with Low Crystallinity

“…The field of intrinsically stretchable polymer semiconductor has witnessed significant growth in recent years [23] . Various structural modification strategies have been reported, such as backbone engineering [24] [25] and side-chain engineering [26] [27] , which include flexible conjugation breakers [28] , longer and softer side chain [29] , or tuning the size and number of electron-donating thiophene groups [30] . The general design principle is to decrease the overall crystallinity of the polymer film while maintaining good electrical connections between aggregates, thus maintaining good charge transport.…”

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

“…PDPP‐T3 shows a small curvature at the vicinity of 0 °C. Zhang et al recently reported a T g of ~20 °C both from AC‐chip calorimetry and modified DMA . In general, the T g obtained from AC‐chip and DMA is 10–20 °C higher than that from conventional DSC .…”

“…Our recent review has briefly summarized several experimental techniques which have been used to measure the T g of CPs, including differential scanning calorimetry (DSC), AC‐chip calorimetry, dynamic mechanical analysis (DMA), and so forth . Among them, AC‐chip and DMA are successful in measuring T g of D–A CPs, whereas DSC, which has been widely used in nonconjugated and flexible polymers, often fails to detect T g of D–A CPs even with adequate sample mass . ( T g discussed herein refers to backbone T g .)…”