Influence of Acceptor Side-Chain Length and Conjugation-Break Spacer Content on the Mechanical and Electronic Properties of Semi-Random Polymers

Abstract

Conjugation-break spacers (CBS) have been shown to enhance the mechanical properties of conjugated polymers. In particular, incorporation of CBS units into semi-random polymers has revealed high ductility and low elastic moduli, attributed to the combined influence of the CBS units and the semi-random architecture. To further elucidate the structure–property relationships in these polymers, two new families of semi-random polymers are reported here. In the first, poly­(3-hexylthiophene)-based semi-random polymers incorporating diketo­pyrrolopyrrole (DPP) units were synthesized in which CBS units with 4–10 carbons were incorporated from 10 to 40% with an equivalent content of 2-decyltetradecyl-DPP (dtdDPP) to overcome solubility limitations previously observed with 2-ethylhexyl-DPP (ehDPP). These polymers had much higher solubility and could attain higher molecular weights, formed films with high integrity, and displayed extraordinary mechanical properties, with elastic moduli as low as 5.45 MPa and fracture strains as high as 398%. In the second family, the content of ehDPP was held constant at 10%, while the CBS content was varied from 10 to 50% (with an eight-carbon spacer) to deconvolute the influence of CBS and DPP content on mechanical properties. Polymer solubility, molecular weight, and processability were not shown to improve dramatically relative to the previous generation of ehDPP polymers with matched DPP and CBS content, but the mechanical properties of this series were quite notable, with elastic moduli as low as 4.08 MPa, an increase in toughness, and fracture strains as high as 432%. The extraordinary mechanical properties exhibited by these polymers can serve as a guide in the judicious selection of monomers and backbone architectures in the future synthesis of semiconducting polymers for flexible electronic applications.

Incorporation of hydrogen‐bonding units into polymeric semiconductors toward boosting charge mobility, intrinsic stretchability, and self‐healing ability

“…The elastic modulus of the blended thin films are significantly reduced by virtue of the excellent mechanical properties of elastomers. Alternatively, the intrinsic stretchability of conjugated polymers was explored by varying the structures of main chains, 12,72–75 and side chains, 76 as well as the molecular weights 77 . Herein, we focus on the discussions of the influence of H‐bonds on the tear resistance of conjugated polymers.…”

Backbone flexibility on conjugated polymer's crystallization behavior and thin film mechanical stability

“…[16][17][18] Recently, another approach, insertion of conjugation break spacers (CBS) into the backbone, has drawn increasing interests. 19 There are two major routes to incorporate CBS, one is through random copolymerization where the CBS unit is statistically distributed along the backbone; [20][21][22][23][24][25][26][27][28][29] the other, is engineering CBS into each repeating unit and blend the polymer with a fully conjugated counterpart. [30][31][32][33] Martens et al first investigated the hole transport in poly(pphenylene vinylene) derivatives using the former route in 2000.…”

“…23 Lipomi and coworkers focused on tuning E and crack-onset-strain (COS) by varying the alkyl CBS length, CBS concentration, and side chain length of a DPP conjugation core, they achieved COS as high as approximately 400% with E as low as 4 MPa. 29 However, despite the great stretchability, the charge mobility is unsatisfactory with the order of 10 À6 ~10 À4 cm 2 V À1 S À1 . 28,29 Mei and coworkers investigated the influence of CBS using the later route, that is, complementary semiconducting polymer blends (c-SPBs), 30,31 and showed its ability to retain charge transport characteristics and present tunable mechanical moduli when such polymers are blended with a fully conjugated counterpart.…”

Improved Mechanical Durability of High‐Performance OPVs Using Semi‐Interpenetrating Networks

“…[11] One approach to improve the mechanical properties is to engineer the molecular structure of conjugated polymers to improve flexibility. [18][19][20] For example, the incorporation of conjugation-break spacers on the backbone of polymers was shown to enhance mechanical robustness of polymer film. [19,20] Another approach is to fabricate OPVs based on blends of polymeric semiconductors, which are more mechanically robust than active layer blends containing a small molecular organic semiconductor.…”

“…[18][19][20] For example, the incorporation of conjugation-break spacers on the backbone of polymers was shown to enhance mechanical robustness of polymer film. [19,20] Another approach is to fabricate OPVs based on blends of polymeric semiconductors, which are more mechanically robust than active layer blends containing a small molecular organic semiconductor. [21] These strategies and other advances in the design of mechanically flexible organic electronic materials and devices are discussed in recent reviews.…”