State Key Laboratory of Analytical Chemistry for Life Science, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China.
State Key Lab of Coordination Chemistry, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China.
J Am Chem Soc. 2022 Jan 26;144(3):1267-1273. doi: 10.1021/jacs.1c10575. Epub 2022 Jan 11.
An ultrathin surface layer with extraordinary molecular mobility has been discovered and intensively investigated on thin-film polymer materials for decades. However, because of the lack of suitable characterization techniques, it remains largely unexplored whether such a surface mobile layer also exists on individual polymeric nanospheres. Here, we propose a thermal-optical imaging technique to determine the glass transition () and rubber-fluid transition () temperatures of single isolated polystyrene nanospheres (PSNS) in a high-throughput and nonintrusive manner for the first time. Two distinct steps, corresponding to the glass transition and rubber-fluid transition, respectively, were clearly observed in the optical trace of single PSNS during temperature ramping. Because the transition temperature and size of the same individuals were both determined, single nanoparticle measurements revealed the reduced and increased of single PSNS on the gold substrate with a decreasing radius from 130 to 70 nm. Further experiments revealed that the substrate effect played an important role in the increased . More importantly, a gradual decrease in the optical signal was detected prior to the glass transition, which was consistent with a surface layer with enhanced molecular mobility. Quantitative analysis further revealed the thickness of this layer to be ∼8 nm. This work not only uncovered the existence and thickness of a surface mobile layer in single isolated nanospheres but also demonstrated a general bottom-up strategy to investigate the structure-property relationship of polymeric nanomaterials by correlating the thermal property ( and ) and structural features (size) at single nanoparticle level.
几十年来，人们一直在研究薄膜聚合物材料，发现并深入研究了具有非凡分子迁移率的超薄表面层。然而，由于缺乏合适的表征技术，人们在很大程度上仍不清楚这种表面迁移层是否也存在于单个聚合物纳米球上。在这里，我们首次提出了一种热光学成像技术，以高通量和非侵入的方式确定单个隔离聚苯乙烯纳米球（PSNS）的玻璃化转变（Tg）和橡胶-流体转变（Tf）温度。在升温过程中，在单个 PSNS 的光学轨迹中清楚地观察到两个明显的步骤，分别对应于玻璃化转变和橡胶-流体转变。由于确定了相同个体的转变温度和尺寸，因此单个纳米颗粒测量结果表明，随着半径从 130nm 减小到 70nm，单个 PSNS 在金衬底上的 和 减小。进一步的实验表明，衬底效应在 增加中起重要作用。更重要的是，在玻璃化转变之前检测到光学信号逐渐下降，这与具有增强分子迁移率的表面层一致。定量分析进一步表明该层的厚度约为 8nm。这项工作不仅揭示了单个隔离纳米球中表面迁移层的存在和厚度，而且还展示了一种通过将热性能（Tg 和 Tf）和结构特征（尺寸）相关联，在单个纳米颗粒水平上研究聚合物纳米材料的结构-性能关系的通用自下而上策略。
