Investigation of the Nanomechanical Behaviors of Polymer Single Crystals by Atomic Force Microscopy Nanoindentation

Understanding the nanomechanical behavior of crystalline domains is crucial for establishing structure–property relationships in semicrystalline polymers. In this study, atomic force microscopy (AFM) nanoindentation on compliant polydimethylsiloxane (PDMS) substrates was used to quantify the Young’s modulus, fracture force, and deformation mechanisms of ultrathin polymer single crystals, including polyethylene (PE), poly(ethylene oxide) (PEO), poly(l-lactic acid) (PLLA), poly(ε-caprolactone) (PCL), and polyamide 66 (PA 66). Although several crystals showed comparable Young’s moduli, their fracture behaviors differed markedly. PE and PEO exhibited ductile deformation with permanent imprints and material pile-up, whereas PLLA, PCL, and PA 66 underwent brittle fragmentation. Cyclic loading–unloading further revealed pronounced crystal slip in PEO and hidden slip in thickness-stepped PE, while the other crystals remained mainly elastic before failure. These findings highlight force-activated chain dynamics as a key factor governing crystalline stem slip and the resulting brittle-to-ductile transition in polymer crystals.