Liang Ying, Kitt Jay P, Minteer Shelley D, Harris Joel M, Korzeniewski Carol
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, USA.
Department of Chemistry, University of Utah, Salt Lake City, UT, USA.
Appl Spectrosc. 2021 Apr;75(4):376-384. doi: 10.1177/0003702820949129. Epub 2020 Oct 12.
Infrared and Raman spectroscopy techniques were applied to investigate the drying and aggregation behavior of Nafion ionomer particles dispersed in aqueous solution. Gravimetric measurements aided the identification of gel-phase development within a series of time-resolved spectra that tracked transformations of a dispersion sample during solvent evaporation. A spectral band characteristic of ionomer sidechain end group vibration provided a quantitative probe of the dispersion-to-gel change. For sets of attenuated total reflection Fourier transform infrared (ATR FT-IR) spectra, adherence to Beer's law was attributed to the relatively constant refractive index in the frequency region of hydrated - group vibrations as fluorocarbon-rich ionomer regions aggregate in forming the structural framework of membranes and thin films. Although vibrational bands associated with ionomer backbone CF stretching vibrations were affected by distortion characteristic of wavelength-dependent refractive index change within a sample, the onset of band distortion signaled gel formation and coincided with ionomer mass % values just below the critical gelation point for Nafion aqueous dispersions. Similar temporal behavior was observed in confocal Raman microscopy experiments that monitored the formation of a thin ionomer film from an individual dispersion droplet. For the ATR FT-IR spectroscopy and confocal Raman microscopy techniques, intensity in the water H-O-H bending vibrational band dropped sharply at the ionomer critical gelation point and displayed a time dependence consistent with changes in water content derived from gravimetric measurements. The reported studies lay groundwork for examining the impact of dispersing solvents and above-ambient temperatures on fluorinated ionomer transformations that influence structural properties of dispersion-cast membranes and thin films.
应用红外光谱和拉曼光谱技术研究了分散在水溶液中的Nafion离聚物颗粒的干燥和聚集行为。重量测量有助于在一系列时间分辨光谱中识别凝胶相的发展，这些光谱跟踪了分散样品在溶剂蒸发过程中的转变。离聚物侧链端基振动的特征光谱带为分散体到凝胶的变化提供了定量探针。对于衰减全反射傅里叶变换红外（ATR FT-IR）光谱组，遵守比尔定律归因于在水合基团振动频率区域中相对恒定的折射率，因为富含氟碳的离聚物区域聚集形成膜和薄膜的结构框架。尽管与离聚物主链CF拉伸振动相关的振动带受到样品内波长依赖性折射率变化的畸变特性的影响，但带畸变的开始标志着凝胶的形成，并且与Nafion水分散体的临界凝胶化点以下的离聚物质量百分比值一致。在共聚焦拉曼显微镜实验中观察到类似的时间行为，该实验监测了单个分散液滴中离聚物薄膜的形成。对于ATR FT-IR光谱和共聚焦拉曼显微镜技术，水的H-O-H弯曲振动带的强度在离聚物临界凝胶化点处急剧下降，并显示出与重量测量得出的含水量变化一致的时间依赖性。所报道的研究为研究分散溶剂和高于环境温度对影响分散浇铸膜和薄膜结构性能的氟化离聚物转变的影响奠定了基础。
