Significant Melting Point Depression of Two-Dimensional Folded-Chain Crystals of Isotactic Poly(methyl methacrylate)s Observed by High-Resolution In Situ Atomic Force Microscopy

• Published in: The journal of physical chemistry. B Vol. 117; no. 18; pp. 5594 - 5605
• Main Authors: Takanashi, Yuma, Kumaki, Jiro
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 09.05.2013
• Subjects: Applied sciences; Atomic force microscopy; Crystallization; Crystals; Depression; Exact sciences and technology; Isotactic; Melting; melting point; mica; Microscopy, Atomic Force; Molecular weight; Organic polymers; Particle Size; Physicochemistry of polymers; Polymethyl Methacrylate - chemistry; Polymethyl methacrylates; polymethylmethacrylate; Properties and characterization; Protein Folding; Surface Properties; temperature; thermal properties; Transition Temperature; Two dimensional; Ultrathin films; Methyl methacrylate polymer; Atomic force microscopy; Melting; Molecular mass; Melting point
• ISSN: 1520-6106; 1520-5207; 1520-5207
• DOI: 10.1021/jp401284t

The properties of polymer ultrathin films are a subject of intense study from both practical and academic viewpoints. Previously, we found that upon compression, an isotactic poly(methyl methacrylate) (it-PMMA) Langmuir monolayer crystallized to form a two-dimensional (2D) folded-chain crystal, and the molecular image of the crystal with chain folding and tie chains was clearly visualized by atomic force microscopy (AFM). In the present study, the melting behaviors of the it-PMMA 2D crystals were successfully observed in situ by high-temperature AFM at the molecular lever for the first time. The chain–chain distances (∼1.2 nm) of the crystals were clearly resolved even at temperatures close to the melting temperatures (T m) of the 2D crystals. We found that the T m of the 2D crystals was at most 90 °C lower than the bulk crystals. The T m depression strongly depended on the molecular weight, while the molecular weight dependence of the bulk T m was negligible in the molecular weight regime studied. The T m depression also depended on the substrates, a slightly larger depression being observed on a sapphire substrate compared to that on a mica. The large T m depressions of the 2D crystals could not be explained by a simple Thomson–Gibbs argument, theoretical developments are necessary to understand the melting of the 2D crystals.