In Situ AFM Observation of Folded‐Chain Crystallization of a Low‐Molecular‐Weight Isotactic Poly(methyl methacrylate) in a Langmuir Monolayer at the Molecular Level

Abstract

The formation of folded‐chain crystals (FCC) of a low‐molecular‐weight isotactic poly(methyl methacrylate) (it‐PMMA) in a Langmuir monolayer on mica is observed under high humidity by in situ atomic force microscopy at the molecular level. The size of the smallest FCC formed corresponds to the size of a crystal composed of a single chain (single‐chain crystal), and the crystals grow in a stepwise manner with steps comparable to a single chain. The crystal of it‐PMMA is composed of a double‐stranded helix, and taking into account the molecular weight dependence of the crystal growth, it is concluded that a single chain intramolecularly intertwines to form a double‐stranded helix quantitatively that further intramolecularly folds to form the single‐chain crystal, and the crystal grows with the unit of the single‐chain crystal. The crystallization process is also successfully observed in detail at the molecular level: an amorphous single chain is deposited and spread on a crystal growth plane as new stems to grow the crystal.

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

“…Thus, the 2D crystals that formed on the mica correspond to the single-crystal plane of the double-stranded helical crystal, as schematically represented by an ideal chain packing in Figure a. Some irregularity may also occur in the formation of the double-stranded helix and its folding (see Figure 2 in ref ). For each crystal, the molecular weight can be evaluated from the total stem length based on the crystal model and indicated in each image (469, 568, and 596 k).…”

“…Furthermore, the authors also observed a detailed molecular-level crystallization in which an amorphous single chain was deposited and spread on a crystal-growth plane as new stems to grow the crystal. 41 In previous observations of the crystallization of it-PMMA bulk monolayers, although the stem-level crystallization of folded-chain crystals comprising double-stranded helices could be observed, the individual amorphous chains that did not form thick double-stranded helices in the amorphous region could not be observed at the molecular level, owing to the limited resolution of AFM. We do not know the position at which the crystal nuclei formed in the amorphous chain or how the amorphous chain was incorporated into the crystal.…”

“…The authors observed various crystallization processes, such as the nucleation and growth of anisotropic nanonuclei, the cooperative slipping of the chain in a folded-chain crystal, and the formation and disappearance of unstable crystal nuclei smaller than the critical size. By examining the molecular weight dependence of crystallization, they found that the crystal growth did not proceed at a constant rate over time but proceeded stepwise with a single-chain crystal. , That is, after one chain intertwined to form a double-stranded helix, which folded into a folded-chain crystal, the next chain intertwined to form a double-stranded helix and folded into a folded-chain crystal on the existing crystal, and the crystal growth proceeded stepwise. The crystal mainly grew intramolecularly.…”

“…The intramolecular crystallization occurred in the somewhat entangled 2D monolayer. Furthermore, the authors also observed a detailed molecular-level crystallization in which an amorphous single chain was deposited and spread on a crystal-growth plane as new stems to grow the crystal …”

Critical Size and Formation Mechanism of Secondary Nuclei in Melt-Crystallized Polylactide Stereocomplex Crystals

“…By means of in situ X-ray scattering, Su et al reported that the sizes along the long and short axes of crystalline nanograins of poly­(9,9-di- n -octyl-2,7-fluorene) during the primary nucleation stage in a cold crystallization process were about 13 nm and 5 nm, respectively. Via real-time observation of the crystallization process of isotactic PMMA in a monolayer on a substrate via atomic force microscopy, Kumaki et al − observed the step growth manner of isotactic PMMA, namely, block by block with a time interval, rather than stem-by-stem growth with a constant rate. The size of the folded chain cluster, which they defined as crystalline nuclei, was independent of the molecular weight.…”