Terahertz Spectroscopy of Soft Condensed Matter: Single Methyl Branched n-Alkanes and Plastic-Crystals
Nickel, Daniel Vincent
Mittleman, Daniel M.
Doctor of Philosophy
Terahertz time-domain spectroscopy has long been recognized as a valuable tool for characterizing the far-IR range (0.1 THz to 10 THz) properties of innumerable condensed phase materials. Soft condensed matter, a broad subcategory of condensed matter, encompasses thermally and/or mechanically deformable phases such as liquids, liquid-crystals, plastic-crystals, and polymers. With comparatively more internal degrees of freedom and weaker atomic/molecular interactions, complex phase behavior and dynamics often emerge in these materials, which generally have not been explored in the THz range. Here, temperature-dependent THz-TDS is applied to three soft condensed matter systems. Using a custom designed reflection geometry spectrometer and a novel variable path length liquid sample cell, the isotropic polarizabilities of liquid phase methyl-branched n-alkanes and their linear structural isomers are characterized and compared in the THz range. In addition, the plastic-crystal’s succinonitrile and camphor are studied using transmission geometry THz-TDS. The THz range properties and dynamics characterized in these studies, from the structure-dependent isotropic polarizabilities of prototypical liquid hydrocarbons to the complex disorder/order transitions and correlated molecular dynamics of plastic-crystals, provide valuable fundamental insight into the behavior of soft condensed matter systems.