Crosslinking is a ubiquitous strategy in polymer engineering to increase the thermomechanical robustness of solid polymers but has been relatively unexplored in the context of π‐conjugated (semiconducting) polymers. Notwithstanding, mechanical stability is key to many envisioned applications of organic electronic devices. For example, the wide‐scale distribution of photovoltaic devices incorporating conjugated polymers may depend on integration with substrates subject to mechanical insult—for example, road surfaces, flooring tiles, and vehicle paint. Here, a four‐armed azide‐based crosslinker (“4Bx”) is used to modify the mechanical properties of a library of semiconducting polymers. Three polymers used in bulk heterojunction solar cells (donors J51 and PTB7‐Th, and acceptor N2200) are selected for detailed investigation. In doing so, it is shown that low loadings of 4Bx can be used to increase the strength (up to 30%), toughness (up to 75%), hardness (up to 25%), and cohesion of crosslinked films. Likewise, crosslinked films show greater physical stability in comparison to non‐crosslinked counterparts (20% vs 90% volume lost after sonication). Finally, the locked‐in morphologies and increased mechanical robustness enable crosslinked solar cells to have greater survivability to four degradation tests: abrasion (using a sponge), direct exposure to chloroform, thermal aging, and accelerated degradation (heat, moisture, and oxygen).

The functionality and usability of π-conjugated (semiconducting) polymers is dependent on the adhesive and interfacial properties of the solid film. Such properties are critical in devices incorporating semiconducting polymers because these layers serve both an active and structural role. They are load bearing in the sense that bending, stretching, scratching, and impact places stress within the semiconducting film at the interfaces with other layers in the device stack. Thus, these organic semiconductors must have good cohesive and adhesive properties despite being designed primarily for optoelectronic function (as opposed to mechanical stability). Here, we measure the effect of the alkyl side chain length on the mechanical and adhesive properties of poly­(3-alkylthiophene) (P3AT) using three different measurement techniques not often applied to conjugated polymers: nanoindentation (quasi-static and dynamic), a lap-joint shear test, and adhesive peel tests (90 and 180°). We performed these measurements alongside pseudo-free-standing (“film-on-water”) tensile tests commonly reported in the literature. We find a monotonic relationship between the length of the side chain and parameters associated with the storage of energy: decreased elastic modulus, strength, and resilience and increased elastic range, from the shortest to the longest side chain. However, we observed a maximum in toughness, fracture strain, and adhesive energy dissipation at A = heptyl or octyl, as well as differences in debonding behavior when P3AT films were deposited on top of a poly­(3,4-ethylenedioxythiophene):poly­(styrene sulfonate) (PEDOT:PSS) film. Notably, our findings suggest that an increase in the alkyl side chain length (beyond n = 8 for P3ATs) may be detrimental to adhesion and thus mechanical robustness.

The fracture behavior of concrete with steel and macropolypropylene hybrid fiber blends (HyFRC) is evaluated and compared with steel fiber reinforced concrete (SFRC). Fiber blends at identical volume fraction as SFRC are shown to produce an improvement in the fracture response. At small crack separation, immediately after cracking, higher cohesive stress is produced in HyFRC compared to SFRC. Polypropylene fibers in concrete contribute cohesive stresses at larger crack separation. Polypropylene fibers in the concrete matrix improve the efficiency of the steel fibers by mobilizing a higher resistance at the initiation of the pullout. The initial stiffness, peak load, and the residual frictional resistance of the steel fiber pullout are increased with the addition of the polypropylene fibers. The higher reinforcing efficiency of steel fibers in the presence of polypropylene fibers is due to improved fiber-matrix bond, which results in a higher cohesive stress at smaller crack openings.

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