Water‐assisted mechanical testing of polymeric thin‐films

Impact of doping on the mechanical properties of conjugated polymers

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Real-time correlation of crystallization and segmental order in conjugated polymers

Modulating the segmental order in the morphology of conjugated polymers is widely recognized as a crucial factor for achieving optimal electronic properties and mechanical deformability. However, it is worth noting that the segmental order is typically associated with the crystallization process, which can result in rigid and brittle long-range ordered crystalline domains. To precisely control the morphology, a comprehensive understanding of how highly anisotropic conjugated polymers form segmentally ordered structures with ongoing crystallization is essential, yet currently elusive. To fill this knowledge gap, we developed a novel approach with a combination of stage-type fast scanning calorimetry and micro-Raman spectroscopy to capture the series of specimens with a continuum in the polymer percent crystallinity and detect the segmental order in real-time. Through the investigation of conjugated polymers with different backbones and side-chain structures, we observed a generally existing phenomenon that the degree of segmental order saturates before the maximum crystallinity is achieved. This disparity allows the conjugated polymers to achieve good charge carrier mobility while retaining good segmental dynamic mobility through the tailored treatment. Moreover, the crystallization temperature to obtain optimal segmental order can be predicted based on Tg and Tm of conjugated polymers. This in-depth characterization study provides fundamental insights into the evolution of segmental order during crystallization, which can aid in designing and controlling the optoelectronic and mechanical properties of conjugated polymers.

Unraveling the Stretch-Induced Microstructural Evolution and Morphology-Stretchability Relationships of High-Performance Ternary Organic Photovoltaic Blends

The stretchability and stretch-induced structural evolution of organic solar cells (OSCs) are pivotal for their collapsible, portable, and wearable applications, and they are mainly affected by the complex morphology of active layers. Herein, a highly ductile conjugated polymer P(NDI2OD-T2) is incorporated into the active layers of high-efficiency OSCs based on nonfullerene small molecule acceptors to simultaneously investigate the morphological, mechanical, and photovoltaic properties and structural evolution under stretching of ternary blend films with various acceptor contents. The structural robustness of the blend films is indicated by their stretch-induced structural evolution, which is monitored in real-time by a combination of in situ wide/small angle X-ray scattering. It is found that adding the soft P(NDI2OD-T2) can enhance the stretchability and structural robustness of ternary blend films by more entangled chains and tie chains to dissipate strain. Furthermore, the stretchability of the ternary blends can be superbly predicted by a 3D equivalent box model. This work provides instructive insight and guidance for designing stretchable electronics and predicting the stretchability of multicomponent blends.

Analyzing the Mechanical Properties of Free-Standing PACA Thin Films Using Microindentation Technique

Assessing the mechanical properties of materials is of fundamental relevance for their rational usage, but can be challenging with standard tensile testing for highly brittle polymers used, e.g., as coatings. Here, a procedure for the mechanical analysis of free-standing poly(alkyl cyanoacrylate) (PACA) films using microindentation has been explored. Rigid and transparent films from PACA with various side chain compositions were formed on top of square polymer frames by in situ polymerization. Under microscopic control, the free-standing films were analyzed using a microelectromechanical sensing system. By this procedure, decreasing Young's moduli E for increasing PACA side chain length and flexibility were determined with strain at break ε_B between 0.36% for poly(ethyl cyanoacrylate) and 4.6% for poly(methoxyethyl cyanoacrylate). Based on this successful application, the applied methodology may be relevant for characterizing various coating materials, which are otherwise hard to form as thin free-standing films, and using the data, e.g., in computationally assisted design and evaluation of hybrid material devices.

Influence of Side Chain Interdigitation on Strain and Charge Mobility of Planar Indacenodithiophene Copolymers

Indacenodithiophene (IDT) copolymers are a class of conjugated polymers that have limited long-range order and high hole mobilities, which makes them promising candidates for use in deformable electronic devices. Key to their high hole mobilities is the coplanar monomer repeat units within the backbone. Poly(indacenodithiophene-benzothiadiazole) (PIDT_C16-BT) and poly(indacenodithiophene-thiapyrollodione) (PIDT_C16-TPD_C1) are two IDT copolymers with planar backbones, but they are brittle at low molecular weight and have unsuitably high elastic moduli. Substitution of the hexadecane (C₁₆) side chains of the IDT monomer with isocane (C₂₀) side chains was performed to generate a new BT-containing IDT copolymer: PIDT_C20-BT. Substitution of the methyl (C₁) side chain on the TPD monomer for an octyl (C₈) and 6-ethylundecane (C_13B) afford two new TPD-containing IDT copolymers named PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B, respectively. Both PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B are relatively well deformable, have a low yield strain, and display significantly reduced elastic moduli. These mechanical properties manifest themselves because the lengthened side chains extending from the TPD-monomer inhibit precise intermolecular ordering. In PIDT_C16-BT, PIDT_C20-BT and PIDT_C16-TPD_C1 side chain ordering can occur because the side chains are only present on the IDT subunit, but this results in brittle thin films. In contrast, PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B have disordered side chains, which seems to lead to low hole mobilities. These results suggest that disrupting the interdigitation in IDT copolymers through comonomer side chain extension leads to more ductile thin films with lower elastic moduli, but decreased hole mobility because of altered local order in the respective thin films. Our work, thus, highlights the trade-off between molecular packing structure for deformable electronic materials and provides guidance for designing new conjugated polymers for stretchable electronics.