Benchmarking the Elastic Modulus of Conjugated Polymers with Nanoindentation

The elastic modulus is a critical parameter for the design of conjugated polymers for wearable electronics and correlates with electrical and thermal transport. Yet, widely different values have been reported for the same material because of the influence of processing and measurement conditions, including the temperature, mode, direction, and time scale of deformation. Thus, results obtained via different methods are usually not considered to be comparable. Here, disparate techniques from nanoindentation to tensile testing of free-standing films or films on water, buckling analysis, dynamic mechanical thermal analysis, oscillatory shear rheometry, and atomic force microscopy are compared. Strikingly, elastic modulus values obtained for the same batch of regioregular poly(3-hexylthiophene) differ by a factor of less than four, which suggests that an approximate comparison is possible. Considering the small amount of material that is typically available, nanoindentation in combination with creep analysis is identified as a reliable method for probing the elastic modulus of films with widely different elastic moduli ranging from less than 0.1 GPa in the case of a polythiophene with oligoether side chains to several GPa for polymers without side chains. Since films can display anisotropic elastic modulus values, it is proposed that nanoindentation is complemented with an in-plane technique such as tensile testing to ensure a full characterization using different modes of deformation.