Rotating frame relaxation times

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

From the viewpoint of polymer applications, the full exploitation of the combined resolution/sensitivity enhancement techniques to obtain "high-resolution" spectra of rare-spin nuclei in solids requires variable temperature spinning capability. In this paper, we describe briefly a spinner assembly suitable for routine operation over a wide range of temperature at the full complement of spinning angles and report - C spectral data at low temperature on several polymers, including fluoropolymers. In addition, variable temperature spin-lattice and rotating frame relaxation times are reported for isotactic poly(propylene). 

To understand cross polarization and its many variants, we need several new concepts. These concepts include spinlocking (one element of this double-resonance pulse sequence), rotating-frame relaxation times, spin temperature and Curie s law, and finally, establishing contact, or mixing, between spin systems by means of applied rf. 

Fig. 11 Dependence of 1II rotating-frame relaxation times T p(H) on temperature for asr = as-...

The frequencies of motion to which NMR is sensitive in the solid state have been substantially extended to slow motion, by measuring the rotating-frame relaxation time Tj q [21] and the dipolar relaxation time TlD [22], This allows NMR studies to cover a range of molecular motions from 10 1 to 10n Hz. 

Sefcik et al. [3] studied quenched and annealed PET films with different crystallinities varying from 3-50%. They measured rotating-frame relaxation times of protons and carbons, Tip and Tip These parameters give information about molecular motions in the tens or hundreds of kilohertz range, which is a characteristic frequency range for many important motional processes in solid polymers. Sefcik et al. were particularly interested in relaxation times. Tip which, in principle, can provide information about molecular motions at specific sites within the polymer. A multiexponential behavior of the magnetization decay was observed. 

Relaxation parameters of interest for the study of polymers include 1) 13C and H spin-lattice relaxation times (T1C and T1H), 2) the spin-spin relaxation time T2, 3) the nuclear Overhauser enhancement (NOE), 4) the proton and carbon rotating-frame relaxation times (T p and T p), 5) the C-H cross-relaxation time TCH, and 6) the proton relaxation time in the dipolar state, T1D (2). Not all of these parameters provide information in a direct manner nonetheless, the inferred information is important in characterizing motional frequencies and amplitudes in solids. The measurement of data over a range of temperatures is fundamental to this characterization. 

The intensities of the lines(Figure 4a) are in the approximate ratio 5 5 1 for a, 3. and y carbons respectively and do not reflect the true 2 2 1 ratio. This must arise from a difference in rotating frame relaxation times for the protons and/or carbons such that the Cross-polarization dynamics of the a and 3 carbons are not the same as for the y carbon(5 ). Further, sidebands introduced by the sample rotatk>n(2.8 kHz) appear to low field of the main resonances, indicating that the inherent chemical shift anisotropy of the planar molecules is not averaging by thermal motion. These data are consistent with restricted overall motion and perhaps a preferential rotation about the C 2 symmetry axis of the pyridine. 

Longitudinal (spin-spin)-Ti(static)/Ti, (rotating frame) relaxation time... 

Fig. 1. Temperature dependence of the spin-lattice relaxation time (Tj), the rotating frame relaxation time (T,, ), and the spin-spin relaxation time (T ) for natural rubber. Measured at a radio frequency of 30 MHz and amplitude of 10 gauss (ref 9)...

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