Temperature-Dependent Chain Structures during Solution-Grown Crystallization via Atomic Force Microscopy

Semi-crystalline polymers are widely utilized due to their favorable cost, and their crystallization behavior is significantly influenced by crystallization temperature. In this study, we employed AFM-based SMFS (single-molecule force spectroscopy) to investigate single-chain folding structures in Polyethylene Oxide (PEO) at various crystallization temperatures (T_c) to elucidate the temperature effect. We utilized force-volume (FV) mode for SMFS, which enables the measurement of single-chain information while simultaneously obtaining information about the surface morphology of the crystal. By comparing the morphological changes before and after SMFS, we can locate the position of the stretched single chain and observe the morphological changes after the single chain is stretched. Combined with the force-extension curves of a single polymer chain, we can infer information about the 3D structure of the molecular chains and the interchain structure in the crystal. Morphological and molecular-level structural data show that intermediate structures are more likely to form at low temperatures, while adjacent reentry structures form at high temperatures. Interestingly, temperature does not affect the degree of chain folding, i.e., the number of adjacent reentry folds. However, the aggregation structures formed by chain folding differ due to changes in surface free energy. This provides a new method for studying the structural changes of a single chain during polymer crystallization.