1D 13 C direct polarization (DPMAS) experiments were carried out at 8-30 kHz spinning rates; recycle delays varied from 10 s to 2000 s and the SPINAL method was used to achieve the high-power (~175-180 kHz) 1 H decoupling. 4-32 transients were acquired for generating 1D 13 C DPMAS spectra, and chemical shifts were referenced externally to the methylene (-CH2-) group of adamantane (Sigma-Aldrich) set to 38.48 ppm. The carbon spin-lattice relaxation times (T1) were measured by the saturation recovery experiments with solid nanoparticles . All relaxation measurements were conducted at a nominal set temperature of 25°C. Reproducibility of the spectroscopic measurements was examined with replicate samples. The 13 C T1 relaxation data were fit using Origin (OriginLab, Northampton, MA) or SciDAVis software packages; for each carbon peak, the relaxation curve obtained using maximal peak height was fit by single-and double-exponential functions.