Adhesive Properties of Semiconducting Polymers: Poly(3-alkylthiophene) as an Ersatz Glue

The functionality and usability of p-conjugated (semi-conducting) 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 degrees). 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.