Ester Side Chain Functionalization Enhances Mechanical Properties of Poly(3-Hexylthiophene) while Maintaining High Hole Mobility

Regioregular poly­(3-alkylthiophene)­s (P3AT) are easy to synthesize conjugated polymers with good electrical properties, but they tend to be brittle, limiting their application. To improve their mechanical properties, we investigated incorporating ester groups in the side chains of P3AT six carbons away from the polymer backbone. Two random copolymer series were synthesized: poly­(3-alkylthiophene-2,5-diyl)-ran-(3-(6-pentanoatehexyl)­thiophene-2,5-diyl), where the alkyl is either n-hexyl (P3HT series) or n-dodecyl (P3DDT series). In both the series, the ester-functionalized side chain had a length similar to that of n-dodecyl and its content was varied from 0 to 100 mol %. The copolymer’s optical, thermal, structural, electrical, and mechanical properties were investigated. The effect of 10–25% ester content on copolymer film aggregation behavior was very different for each series: for the P3HT series, the side chains cocrystallized with the main chains into one crystal structure and behaved as one phase. As a result, incorporation of the longer ester-functionalized side chain greatly affected the thermal, morphological, and mechanical properties. For the P3DDT series, the side chains and main chains crystallize separately, and the n-dodecyl and ester-functionalized side chains appear to cocrystallize together. As a result, the main chain melting temperature decreases only slightly with the ester content, and the mechanical properties were not significantly improved with 10–25% ester. The best combination of mechanical robustness and charge carrier mobility was thus obtained for the P3HT random copolymer with ∼10% ester: a high fracture strain (29 ± 6%) combined with a high tensile strength (3.9 ± 0.6 MPa) resulted in a large toughness (90 ± 30 J/m3). This was achieved while maintaining the same high charge carrier mobility as P3HT of similar molecular weight (0.12 ± 0.01 cm2 V–1 s–1). Improved ductility was also shown by the thin film-on-elastomer technique. These results demonstrate that side chain modification can optimize both the mechanical and electrical properties of P3ATs when the side chains and the main chain behave as one phase.