Advancements in polymers for application in electrochemical cells drive the requirement for a more thorough understanding of the deformation mechanisms and their interaction with humidity. Nanoindentation is a localized micro-scale testing alternative to the classical large-scale tensile test. The viscoelastic response of these polymeres, however, combined with higher compliance results in large scatters and a low repeatability. [1] This work combines tensile testing as a verification method, and advanced nanoindentation with post-indentation automatic image recognition to investigate thin polymer electrolyte membranes behaviour under stress (Fig. 1a).

Comparing results from different indenters, analysis methods and indentation protocols (Fig. 1b) shows a vast overestimation of the Young’s moduli obtained through the classical Oliver-Pharr [2] method in comparison to the value derived from tensile tests. A newly introduced method for thermoplastic polymeres was suggested by Mokhtari et al. [3] However, the severe creep leads to lack of repeatability and further errors. 

Moreover, different humidity levels were investigated to comprehensively understand the ambient conditions on the polymer’s mechanical properties. [4] A comparison between wetted and dried samples shows that the Young’s moduli and hardness are influenced.

The study investigates challenges of compliant polymer nanoindentation. We critically study corrective suggestions with regards to their applicability, reproducibility and accuracy. We delve deeper and develop the nanoindentation analysis by using automatic image recognition of the imprint image to supplement the depth-sensing contact area evaluation.