This is not an article about DNP spectroscopy. However, it shows how EPR spectroscopy and nitroxide spin labels can be used to characterize mobility of soft matter through shape analysis of the EPR spectrum. Since mobility and relaxation is directly connected I thought this article could also be of interest to the DNP community.
Miwa, Y. and K. Yamamoto, Simple and Highly Sensitive Measurement Method for Detection of Glass Transition Temperatures of Polymers: Application of ESR Power Saturation Phenomenon with Conventional Spin-Probe Technique. The Journal of Physical Chemistry B, 2012. 116(30): p. 9277-9284.
A combination of the microwave power saturation (MPS) method of electron spin resonance (ESR) and spin probing is proposed as a simple and practical technique for detecting the glass transition temperatures, Tg, of polymers with high sensitivity. Effects of the spin-probe size and concentration on the Tg value of polystyrene (PS) determined by MPS, Tg,ESR, were first evaluated. Spin-probed PS with four types of nitroxides, namely, di-tert-butyl nitroxide (DBN), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (BZONO), and 4?,4?-dimethyl-spiro(5α-cholestane-3,2?-oxazolidin)-3?-yloxy free radical (CHOL), having molecular weights of 144, 156, 276, and 473, respectively, and spin-labeled PS with TEMPO were prepared. The Tg,ESR values for the spin-probed PS with DBN, TEMPO, BZONO, and CHOL and spin-labeled PS were determined to 360, 363, 374, 374, and 375 K, respectively, within experimental uncertainties of 2 K, whereas the glass transition temperature determined by DSC, Tg,DSC, was 375 K for all samples. A significant decrease in Tg,ESR for small spin probes was shown to be due to decoupling between the mobilities of small spin probes and PS segments. Concerning the concentration, a decrease in the saturation factor, S, induced by shortening of the spin?spin relaxation time was observed for the spin-probed PS with CHOL when the concentration of CHOL was more than 1.0 wt %. Furthermore, Tg,ESR decreased slightly with increasing weight fraction of CHOL because of the ?plasticizer effect? of CHOL. However, the Tg,ESR and Tg,DSC values corresponded for each concentration. Thus, large spin probes, such as CHOL and BZONO, are appropriate for the determination of Tg,ESR values; the concentration of the spin probes does not affect the Tg,ESR value unless the overall Tg value is reduced by blending of excess spin probes. Finally, measurements of Tg,ESR in PS/silica composites containing more than 95 wt % silica are shown as an application example of the present method. Tg,ESR was clearly determined even for the PS/silica composites with 98 wt % silica; a decrease in Tg,ESR with increasing silica content was observed.