Study\non Intrinsic Stretchability of Diketopyrrolopyrrole-Based\nπ‑Conjugated Copolymers with Poly(acryl amide) Side Chains\nfor Organic Field-Effect Transistors

Abstract

The\ndevelopment of a π-conjugated polymer with hydrogen-bonding\nmoieties has aroused great attention because of the improved molecular\nstacking and the hydrogen-bonding network. In this study, PDPPTVT\n(diketopyrrolopyrrole-thiophenevinylenethiophene) and PDPPSe (diketopyrrolopyrrole-selenophene)\nalkylated with a carbosilane (SiC8) side chain and poly­(acryl amide)\n(PAM)-incorporated alkyl side chain were prepared, and their structure–performance\nand structure–stretchability correlation were evaluated. By\nincorporating the DPPTVT backbone and 0, 5, 10, or 20% PAM-incorporated\nalkyl side chain, the μ_h value could reach 2.0, 0.97,\n0.74, and 0.42 cm² V^–1 s^–1, respectively (<b>P1</b> to <b>P4</b>). The polymer\nwith the PDPPSe backbone and 5% PAM-incorporated alkyl side-chain\n(<b>P5</b>) exhibited the maximum μ_h value\nof 0.96 cm² V^–1 s^–1. By extending the PAM moiety from the backbone with alkyl spacers,\nthe solid-state packing and edge-on orientation can be properly maintained.\nSurprisingly, the PAM-incorporated alkyl side-chain can provide a\nhydrogen-bonding network serving as sacrificial bonding to mechanical\ndeformation. Therefore, the relevant changes in the crystallographic\nparameters including the crystalline size and the in-plane π–π\nstacking distance with a 100% external strain were less than 4 and\n0.8%, respectively, from <b>P1</b> to <b>P3</b>. Therefore, <b>P3</b> achieved an excellent stretchability while maintaining\nits molecular orientation and charge-transporting performance. Even\nwith 100% external strain, <b>P3</b> still provided an orthogonal\nμ_h over 0.1 cm² V^–1 s^–1. Moreover, by substituting the TVT moiety with the\nSe moiety, the ductility of the backbone can be further increased\nwhen the elastic modulus decreases from 0.80 to 0.36 GPa for <b>P2</b> to <b>P5</b>. The achieved high μ_h retention is over 20% after 500 stretching–releasing cycles\nwith a 60% external strain perpendicular to the channel direction\nfor the polymer composed of PDPPSe and 5% PAM content. The results\nmanifest that our newly designed DPP with the PAM-incorporated alkyl\nside chain provides a promising approach to promote the intrinsic\nstretchability of the π-conjugated polymers.