Beyond Stretchability: Strength, Toughness, and Elastic Range in Semiconducting Polymers

Essentially all research done to date on the mechanical properties of polymeric semiconductors (e.g., for organic photovoltaics and thin-film transistors) has had the underlying goal of increasing the "stretchability": that is, the deformability and softness. However, softness is the wrong characteristic for many of the applications envisioned for organic semiconductors, including touch screens, chemical sensors, and many distributed sources of solar energy at risk of damage by indentation, scratching, and abrasion. A focus on modulus and ultimate extensibility-i.e., properties characteristic of "stretchability".at the expense of strength, toughness, and elastic range-i.e., properties characteristic of hardness and resilience-leaves many potentially lucrative applications on the table. For example, in the field of organic photovoltaics, applications in which materials can be integrated into surfaces already modified by human artifacts (e.g., rooftops, roads, and painted outdoor surfaces) comprise a much greater potential source of renewable energy than the niche uses envisioned for highly ductile devices (e.g., portable and wearable solar cells). Here, we examine the published mechanical behavior of a range of pi-conjugated (semiconducting) polymers (both donor-acceptor polymers and homopolymers) and investigate some of the molecular characteristics associated with strength, toughness, and elastic range. In particular, we extract these quantities from published measurements performed using pseudo-free-standing tensile tests ("film-on-water," FOW). The principal criterion for inclusion in our analysis is that at least one characteristic of the molecular structure (e.g., side-chain length, regioregularity, degree of polymerization, length of aliphatic spacer units, and ratio of semiconducting to insulating blocks in copolymers) is varied systematically by chemical synthesis. In doing so, it is possible to isolate the effects of these aspects of the chemical structure on the strength, toughness, and elastic range, even if these relationships were not reported in the primary literature.