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 π-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-freestanding 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.

The mechanical properties of π-conjugated (semiconducting) polymers are a key determinant of the stability and manufacturability of devices envisioned for applications in energy and healthcare. These propertiesincluding modulus, extensibility, toughness, and strengthare influenced by the morphology of the solid film, which depends on the method of processing. To date, the majority of work done on the mechanical properties of semiconducting polymers has been performed on films deposited by spin coating, a process not amenable to the manufacturing of largearea films. Here, we compare the mechanical properties of thin films of regioregular poly(3-heptylthiophene) (P3HpT) produced by three scalable deposition processesinterfacial spreading, solution shearing, and spray coatingand spin coating (as a reference). Our results lead to four principal conclusions. (1) Spray-coated films have poor mechanical robustness due to defects and inhomogeneous thickness. (2) Sheared films show the highest modulus, strength, and toughness, likely resulting from a decrease in free volume. (3) Interfacially spread films show a lower modulus but greater fracture strain than spin-coated films. (4) The trends observed in the tensile behavior of films cast using different deposition processes held true for both P3HpT and poly(3butylthiophene) (P3BT), an analogue with a higher glass transition temperature. Grazing incidence X-ray diffraction and ultraviolet−visible spectroscopy reveal many notable differences in the solid structures of P3HpT films generated by all four processes. While these morphological differences provide possible explanations for differences in the electronic properties (hole mobility), we find that the mechanical properties of the film are dominated by the free volume and surface topography. In field-effect transistors, spread films had mobilities more than 1 magnitude greater than any other films, likely due to a relatively high proportion of edge-on texturing and long coherence length in the crystalline domains. Overall, spread films offer the best combination of deformability and charge-transport properties.

Transitioning toward more sustainable materials and manufacturing methods will be critical to continue supporting the rapidly expanding market for lithium-ion batteries. Meanwhile, energy storage applications are demanding higher power and...

The conductive polyelectrolyte complex poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) is ubiquitous in research dealing with organic electronic devices (e.g., solar cells, wearable and implantable sensors, and electrochemical transistors). In many bioelectronic applications, the applicability of commercially available formulations of PEDOT:PSS (e.g., Clevios) is limited by its poor mechanical properties. Additives can be used to increase the compliance but pose a risk of leaching, which can result in device failure and increased toxicity (in biological settings). Thus, to increase the mechanical compliance of PEDOT:PSS without additives, we synthesized a library of intrinsically stretchable block copolymers. In particular, controlled radical polymerization using a reversible addition–fragmentation transfer process was used to generate block copolymers consisting of a block of PSS (of fixed length) appended to varying blocks of poly­(poly­(ethylene glycol) methyl ether acrylate) (PPEGMEA). These block copolymers (PSS(1)-b-PPEGMEA(x), where x ranges from 1 to 6) were used as scaffolds for oxidative polymerization of PEDOT. By increasing the lengths of the PPEGMEA segments on the PEDOT:[PSS(1)-b-PPEGMEA(1–6)] block copolymers, (“Block-1” to “Block-6”), or by blending these copolymers with PEDOT:PSS, the mechanical and electronic properties of the polymer can be tuned. Our results indicate that the polymer with the longest block of PPEGMEA, Block-6, had the highest fracture strain (75%) and lowest elastic modulus (9.7 MPa), though at the expense of conductivity (0.01 S cm–1). However, blending Block-6 with PEDOT:PSS to compensate for the insulating nature of the PPEGMEA resulted in increased conductivity [2.14 S cm–1 for Blend-6 (2:1)]. Finally, we showed that Block-6 outperforms a commercial formulation of PEDOT:PSS as a dry electrode for surface electromyography due to its favorable mechanical properties and better adhesion to skin.

This paper demonstrates that a thin polymeric film (10−80 nm) can be continuously drawn from the meniscus of a nonpolar polymer solution at an air−water−fluoropolymer interface using a roll-to-roll process: "interfacial drawing". With this process, it is possible to control the thickness of the film by manipulating the concentration of the solution, along with the drawing velocity of the receiving substrate. We demonstrate the formation of thin films >1 m in length and 1000 cm 2 in area, using our custom-designed apparatus. Interfacial drawing has three characteristics which compare favorably to other methods of forming and depositing polymeric thin films. First, the films are solidified prior to deposition, which means that they can be used to uniformly coat nonplanar, rough, or porous substrates. Second, these films can be stacked into multilayered architectures without risk of redissolving the layer beneath. Third, for some materials, the process yields films with superior mechanical compliance for applications such as wearable or flexible devices, compared to films produced by spin-coating. We demonstrate the utility of interfacial drawing by forming thin films of various semiconducting polymers, including the active layers of all-polymer bulk heterojunction solar cells as well as barrier coatings. As part of these demonstrations, we show how floating polymeric films can be transferred easily to diverse substrates, including those with rough and irregular surfaces, such as textiles and fabrics.