Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan.
J Phys Chem B. 2013 May 9;117(18):5594-605. doi: 10.1021/jp401284t. Epub 2013 Apr 24.
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 (Tm) of the 2D crystals. We found that the Tm of the 2D crystals was at most 90 °C lower than the bulk crystals. The Tm depression strongly depended on the molecular weight, while the molecular weight dependence of the bulk Tm was negligible in the molecular weight regime studied. The Tm depression also depended on the substrates, a slightly larger depression being observed on a sapphire substrate compared to that on a mica. The large Tm 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.
聚合物超薄膜的性质是从实际和学术观点来看都是一个研究热点。以前，我们发现，在压缩过程中，等规聚甲基丙烯酸甲酯（it-PMMA）Langmuir 单层结晶形成二维（2D）折叠链晶体，并且晶体的分子图像具有链折叠和系链，可以通过原子力显微镜（AFM）清楚地可视化。在本研究中，首次在分子杠杆上通过高温 AFM 原位成功观察到 it-PMMA 2D 晶体的熔融行为。即使在接近 2D 晶体的熔融温度（Tm）的温度下，晶体的链链距离（约 1.2nm）也可以清楚地分辨出来。我们发现，2D 晶体的 Tm 比块状晶体低最多 90°C。Tm 降低强烈依赖于分子量，而在研究的分子量范围内，块状 Tm 的分子量依赖性可以忽略不计。Tm 降低还取决于衬底，与云母相比，蓝宝石衬底上观察到的降低略大。2D 晶体的大 Tm 降低不能用简单的 Thomson-Gibbs 论点来解释，需要理论发展来理解 2D 晶体的熔融。
