Dielectric Relaxation Spectroscopy
Molecular sub-monolayers (uniformly distributed molecules covalently bound to a substrate) enable an alternative approach to study glass transitions because these types of samples manifest different features than other glassy systems such as bulk molecular glasses, glassy polymers, or granular materials.- Molecular dynamics within sub-monolayer collections of surface-bound alkyl chains (substituted alkylsilanes) can be probed with highly sensitive narrow-band dielectric spectroscopy. In this system, a transition from independent dynamics to glassy motion is observed as the density is increased.
Our lab utilizes dielectric relaxation spectroscopy (DRS) to probe the molecular dynamics of small volume polymer samples and self-assembled monolayers (SAMs). Measurable motions include segmental reorientations, for example, those associated with glass transitions, as well as intermolecular rotations, like those typically seen in side chains of polymers. Samples are measured using interdigitated electrodes (IDEs) with an ultra-sensitive capacitance bridge. Capacitance and dissipation factor (tan(δ)) are measured as a function of sample temperature over a broad range (from ~4 K to ~400 K). Relaxation peaks are created in the measurement spectrum when a dipole-active motion within the sample occurs at approximately the same frequency of the applied alternating electric field (illustrated below). Utilizing a narrow band of frequencies (as opposed to the more common broad-band technique) enables extremely sensitive measurements to be made, and the samples dynamics can be tuned by changing the temperature. The IDE configuration allows measurement of very small volume samples, such as one-molecule-thick surface-mounted films (monolayers) of varying coverages, or small volumes of polymer.
- J. Crowe-Willoughby, D. R. Stevens, J. Genzer, and L. I. Clarke, "Investigating the molecular origins of responsiveness in functional silicone elastomer networks," Macromolecules 43, 5043 (2010). (journal) [paper]
- M. C. Scott, D. R. Stevens, J. R. Bochinski, and L. I. Clarke, "Dynamics within Alkylsiloxane Self-Assembled Monolayers Studied by Sensitive Dielectric Spectroscopy," ACS Nano 2, 2392 (2008). (journal) [paper]
- L. I. Clarke, D. Horinek, G. S. Kottas, N. Varaksa, T. F. Magnera, T. P. Hinderer, R. D. Horansky, J. Michl, and J. C. Price, "The dielectric response of chloromethylsilyl and dichloromethylsilyl dipolar rotors on fused silica surfaces," Nanotechnology 13, 533 (2002). (cover) (journal) [paper]
