In Situ Atomic Force Microscopy Observation of Folded-Chain Crystallization of Single Isolated Isotactic Poly(methyl methacrylate) Chains in Langmuir–Blodgett Monolayers

The real-time atomic force microscopy (AFM) observation of the crystallization of a folded-chain crystal at the stem-level will improve our understanding of the molecular crystallization mechanism. Previously, we observed the crystallization of isotactic poly(methyl methacrylate) (it-PMMA) in its Langmuir monolayer under high-humidity conditions at the stem-level (Ono et al. Macromolecules 2018, 51, 7629). However, due to the limited resolution of the AFM, the amorphous part of the chains could not be observed in the bulk monolayer. In this study, for the first time, the stem-level crystallization of a single isolated polymer chain into a folded-chain crystal has been clearly visualized in situ and in real-time by AFM. The sample was a single Langmuir monolayer deposited on mica, in which it-PMMA was solubilized as isolated chains in an it-oligo (MMA) monolayer, which was too small to crystallize owing to its low molecular weight. The isolated it-PMMA chain was crystallized into a folded-chain crystal comprising a double-stranded helix of the single it-PMMA chain under high-humidity conditions and was observed by using AFM. The nucleation occurred at any point in the chain, and the crystal grew by winding the residual part of the amorphous chain. Under the humidity condition at which the nucleation occurred frequently, multiple crystals were formed in a single chain to form a necklace-like crystal. Further, the stem-level growth of the crystal formed at the end of the amorphous chain was observed. Unexpectedly, the crystal grew not only on the side where the amorphous chain was attached but also substantially on the opposite side, where no amorphous chain was attached, clearly indicating that the chains had substantially slipped inside the folded double-stranded-helix crystal and crystallized on the opposite side. The crystallization behavior of a single isolated chain provides new insights into understanding polymer crystallization.