Under confinement, unique features of crystallization arise, among which the slowing down and accelerating crystallization on cooling are both reported. The controversy can be resolved by performing isothermal crystallization experiments, taking into account temperature dependence of crystal nucleation and growth. The isothermal experiments of thin films were performed for the first time in this work in the wide range of thicknesses and wide range of temperatures, where it crystallizes. Fast scanning calorimetry (FSC, Flash DSC 1) was used to reach fast cooling and heating, avoiding crystallization and unwanted reorganization. Thin films of poly(ethylene terephthalate) (PET) were studied from 23 nm thickness with free surface and covered by a PS layer. Two-step crystallization was observed by FSC in films thinner than 60 nm at temperatures of crystallization below 140 °C. Evidence of strong free-surface-induced precrystallization was confirmed by atomic force microscopy (AFM). It was observed that highly mobile surface layer formed dendritic crystals first and then bulk layer formed spherulites. When we used thicker films or films crystallized at low undercoolings, no two-step crystallization was observed neither in FSC nor in AFM, presumably due to domination of bulk crystallization. Heterogeneous nucleation at the solid interface with crystalline poly(ether ether ketone) (PEEK) covering layer enhances crystallization at high temperatures. These results not only help to rationalize contradictory reports in the literature but also provide a strong methodology for study of crystallization and nanotechnology.

中文翻译：

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快速扫描量热法研究聚对苯二甲酸乙二醇酯薄膜两步结晶过程中自由表面与固体界面成核之间的相互作用

在限制条件下，出现了独特的结晶特征，其中报道了冷却时结晶速度的减慢和加速。考虑到晶体成核和生长的温度依赖性，可以通过进行等温结晶实验解决该争议。在这项工作中，薄膜的等温实验首次在较宽的厚度范围和较宽的温度范围内进行了结晶。快速扫描量热法（FSC，Flash DSC 1）用于快速冷却和加热，避免了结晶和不必要的重组。研究了聚对苯二甲酸乙二醇酯（PET）薄膜的厚度，其厚度为23 nm，具有自由表面并被PS层覆盖。通过FSC在低于140℃的结晶温度下在小于60nm的膜中观察到两步结晶。原子力显微镜（AFM）证实了强烈的自由表面诱导的预结晶。观察到，高迁移率的表面层首先形成树枝状晶体，然后本体层形成球晶。当我们使用较厚的薄膜或在低过冷度下结晶的薄膜时，在FSC和AFM中均未观察到两步结晶，这可能是由于块状结晶占主导。与结晶聚醚醚酮（PEEK）覆盖层的固体界面处的异相成核作用增强了高温下的结晶作用。