Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume
Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume
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DOI：
10.1021/acs.macromol.3c00968
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发表时间：
2023-08-01
期刊：
影响因子：
5.5
通讯作者：
Won, You-Yeon
中科院分区：
文献类型：
--
作者：
Kim, Seyoung;Lee, Minhwan;Won, You-Yeon
嵌入聚合物基质中的荧光分子转子可以探测聚合物基质动力学的局部变化（例如界面/表面附近聚合物的玻璃化转变），这些变化无法通过宏观测量获得。然而，对于如何分析荧光数据，荧光强度实际上代表什么性质，以及什么是玻璃化转变测量的适当/最佳转子，几乎没有共识。通过对模型荧光转子法呢基-（2-羧基-2-氰基乙烯基）-久洛尼定（FCVJ）的实验，以及对其它类型分子转子的文献数据的重新分析，我们研究了光激发转子的弛豫过程与基体聚合物自由体积变化之间的相关性。研究了FCVJ掺杂聚苯乙烯（PS）、聚（乙酸乙烯酯）（PVAc）和聚（甲基丙烯酸异丁酯）（PiBMA）材料在其玻璃化转变温度（T（g））范围内使用自由体积模型成功地建模，该模型基于Loutfy的原始理论，该理论将转子探针的荧光与基质的自由体积相关联，以及Lipson和白色提出的总自由体积量的修正定义（基于局域相关晶格模型的自由体积），它包括振动自由体积和过剩自由体积。原子分子动力学模拟支持PS单体振动运动的时间尺度与文献中已知的转子内部旋转的内在时间尺度（皮秒量级）相当，因此转子的旋转是由单体硬核体积的摩擦力（而不是振动体积的摩擦力）控制的。对FCVJ负载的聚苯乙烯-聚乙二醇（PS-PEG）胶束在水中的测量表明，PS-PEG胶束的PS核心结构域具有较宽的玻璃化转变温度分布，基于自由体积模型的定量分析能够估计T（g）的实际范围。这些结果使我们得出结论，具有快速非辐射（旋转）弛豫和大体积旋转亚组（如FCVJ）倾向的荧光分子转子在转子的荧光和基质的自由体积之间表现出强耦合，因此是用于T（g）测量的有用探针。
Fluorescent molecular rotors embedded in polymer matricescan probethe local changes in the dynamics of the polymer matrix (such as theglass transition of polymers near interfaces/surfaces) that are notaccessible from macroscopic measurements. Yet, there is little consensusas to how the fluorescence data should be analyzed, what propertythe fluorescence intensity actually represents, and what are appropriate/optimalrotors for glass transition measurements. By experimentation witha model fluorescent rotor, farnesyl-(2-carboxy-2-cyanovinyl)-julolidine(FCVJ), and also re-analysis of data available in the literature forother types of molecular rotors, we investigated the correlation betweenthe relaxation processes of photo-excited rotors and free volume changesin matrix polymers. The temperature dependences of the fluorescenceintensities from FCVJ-doped polystyrene (PS), poly(vinyl acetate)(PVAc), and poly(isobutyl methacrylate) (PiBMA) materials across theirglass transition temperatures (T (g)) weresuccessfully modeled using a free volume model which is based on theoriginal theory of Loutfy that relates the fluorescence of a rotorprobe to the free volume of the matrix, and the revised definitionof the total free volume quantity proposed by Lipson and White (theLocally Correlated Lattice model-based free volume) that includesboth the vibrational free volume and the excess free volume. Atomisticmolecular dynamics simulation supports that the timescale of the vibrationalmotion of PS monomers is comparable to the intrinsic timescale ofa rotor's internal rotation known in the literature (on theorder of picoseconds), and thus the rotation of a rotor is controlledby the friction from the hard-core volume (not by the friction fromthe vibrational volume) of the monomers. Measurements on FCVJ-loadedpolystyrene-poly(ethylene glycol) (PS-PEG) micellesin water suggest that the PS core domain of the PS-PEG micellehas a broad distribution of glass transition temperatures; a quantitativeanalysis based on the free volume model enabled to estimate the actualrange of T (g). These results allow us toconclude that fluorescent molecular rotors with a tendency towardrapid non-radiative (rotational) relaxation and bulky rotating subgroups(such as FCVJ) exhibit a strong coupling between the fluorescenceof the rotor and the free volume of the matrix and are thus usefulprobes for T (g) measurements.