Synthetic Mica: A Very Promising Substrate for Studying Solid Thin Films by Proton NMR
David L. VanderHart, Vivek M. Prabhu, Kristopher Lavery, Cindi L. Dennis
We demonstrate that a synthetic fluorophlogopite mica can be used as a proton-free, diamagnetic substrate for examining solid thin-film samples using conventional solid-state proton NMR experiments. The context of this work extends previous NMR studies on bulk samples to thin films. NMR was used to characterize the intimacy of mixing in bulk glassy photoresist formulations however, these materials are exclusively used as spin-coated thin films. Since observations for bulk samples do not necessarily carry over to thin films, we sought an NMR-friendly substrate for performing similar proton NMR experiments on glassy thin films. Proton detection and multiple pulse methods were chosen in order to maintain sensitivity. Two natural micas as well as the synthetic mica were each evaluated as possible substrates for spun-cast films in samples where the mica occupied over 98 % of the sample volume. However, it is seen that strong paramagnetism of the natural mica introduces strong spinning sidebands and linebroadening under magic angle spinning conditions. While we were able to demonstrate some reduction in these unwanted characteristics using oriented samples, the residual level of complication still caused too much ambiguity. The synthetic mica proved successful, showing no deterioration in resolution and only a slight increase in spinning sidebands. Investigations of the intimacy of mixing in a proof-of-concept commercial polymer blend and a more difficult photoresist blend, are reported. Yet, for a successful experiment designed to investigate the intimacy of mixing, one must pay scrupulous attention to excluding impurities during sample preparation and handling. In addition, one must be also careful to correct for other proton background signals associated with the empty probe. Finally, independent SQUID-based magnetic susceptibility measurements provides a firmer basis for our observations.
magnetic susceptibility, mica, nmr, proton, resolution, sensitivity, solids, spinning sidebands, thin film
, Prabhu, V.
, Lavery, K.
and Dennis, C.
Synthetic Mica: A Very Promising Substrate for Studying Solid Thin Films by Proton NMR, Journal of the American Chemical Society, [online], https://doi.org/10.1016/j.jmr.2009.08.010
(Accessed August 1, 2021)