南京大学王伟团队报道了单聚苯乙烯纳米微球分子迁移的光学成像。相关研究成果于2022年1月11日发表在《美国化学会杂志》。

几十年来，人们在高分子薄膜材料中发现了一种具有超级分子迁移能力的超薄表面层。然而，由于缺乏合适的表征技术，在单个聚合物纳米球上是否也存在这样的表面移动层仍有待进一步研究。

该文中，研究人员首次提出了一种热光成像技术，以高通量和非侵入方式测定单个孤立聚苯乙烯纳米球（PSN）的玻璃化转变（Tg）和橡胶-流体转变（Tf）温度。在温度渐变过程中，在单个PSN的光学轨迹中清晰地观察到两个不同的步骤，分别对应于玻璃化转变和橡胶-流体转变。由于同时测定了相同个体的转变温度和大小，单个纳米颗粒测量结果显示，在金基底上单个PSN的表观Tf降低，Tg增加，半径从130 nm减小到70 nm。

进一步的实验表明，基质效应在Tg的增加中起着重要作用。更重要的是，在玻璃化转变之前检测到光信号逐渐减少，这与具有增强分子迁移能力的表面层一致。定量分析进一步显示该层的厚度为～8nm。

该研究工作不仅揭示了单个孤立纳米球中表面移动层的存在和厚度，而且还展示了一种自下而上的策略，通过关联单个纳米颗粒的热性质（Tg和Tf）和结构特征（尺寸），研究聚合物纳米材料的结构-性质关系。

附：英文原文

Title: Optical Imaging of the Molecular Mobility of Single Polystyrene Nanospheres

Author: Shasha Liu, Mengqi Lv, Haoran Li, Sa Wang, Chengdong Feng, Xiaoliang Wang, Wenbing Hu, Wei Wang

Issue&Volume: January 11, 2022

Abstract: 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 (Tg) and rubber-fluid transition (Tf) 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 apparent Tf and increased Tg 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 Tg. 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 (Tg and Tf) and structural features (size) at single nanoparticle level.

DOI: 10.1021/jacs.1c10575

Source: https://pubs.acs.org/doi/10.1021/jacs.1c10575