Glassy state spin relaxation

Spin-spin relaxation times (T2) in polymer systems range from about 10-5 s for the rigid lattice (glassy polymers) to a value greater than 10-3 s for the rubbery or viscoelastic state. In the temperature region below the glass transition, T2 is temperature independent and not sensitive to the motional processes, because of the static dipolar interactions. The temperature dependence of T2 above Tg and its sensitivity to low-frequency motions, which are strongly affected by the network formation, make spin-spin relaxation studies suitable for polymer network studies. 

Another effect is the modulation of the rotational angular momentum of the ion and the interaction of this momentum with the spin angular momentum by means of its spin-rotational coupling. This mechanism (43) may be used to explain the absence of hyperfine splitting from vanadyl prophyrin in solution, whereas it is observed in the glassy state where spin-rotational relaxation will not be present. 

Masuda K, Tabata S, Sakata Y, Hayase T, Yonemochi E, Terada K (2005) Comparison of Molecular Mobility in the Glassy State Between Amorphous Indomethacin and SaUcin Based on Spin-Lattice Relaxation Times. Pharm Res 22(5) 797-805 Matsumoto T, Zografi G (1999) Physical properties of solid molecular dispersions of in-domethacin with poly (vinylpyrroUdone) and poly(vinylpyrrolidone-co-vinyl-acetate) in relation to indomethacin crystallization. Pharm Res 16(11) 1722-1728 Miyazaki T, Yoshioka S, Aso Y, Kojima S (2004) Ability of fxtlyvinylpyrrohdone and polyacrylic acid to inhibit the crystallization of amorphous acetaminophen. J Pharm Sci 93(11) 2710-2717 Miller D, Lechuga-BaUesteros D (2006) Rapid assessment of the structural reltixation behavior of amorphous pharmaceutical sohds effect of residual water on molecular mobility. Pharm Res 23(10) 2291-2305... 

In principal, resolution of Individual carbon resonances in bulk polymers, allows relaxation experiments to be performed which can be Interpreted in terms of main chain and side chain motions in the solid. In addition to the spin-lattice relaxation time in the Zeeman field, the spin-spin relaxation time and nuclear Overhauser enhancement, other parameters providing data on polymer dynamics include the proton and carbon spin-lattice relaxation times in the rotating-frame, T p, the cross-relaxation time Tqr, and proton relaxation in the dipolar field. Schaefer and Stejskal have carried out pioneering work in exploring polymer dynamics using solid-state NMR techniques. Measurement of T values in glassy PMMA at ambient temperature reveals that the a-CH3 carbon relaxes in <0.1s, the ester methyJL and methylene carbons in ca. Is and the two non-protonated (carbonyl and quaternary) carbons in ca. 10s. These results are consistent with the onset of internal reorientation of a-CH3 at this temperature relatively... 

For the individual interested in molecular motion, the important feature of spin-lattice relaxation (or other relaxation mechanisms) is the dependency on molecular motion to provide an efficient energy pathway for relaxation. Thus, molecular motions at the Larmor frequency for individual carbon atoms in a molecular framework may be mapped by T] measurements. Since the frequency of molecular motion is temperature dependent, additional thermodynamic and kinetic information may be obtained by measuring Tj values for different carbons over a range of temperatures. In the paper by Lyerla and coworkers in this volume, Tj measurements made for the first time over a range of low temperatures yielded specific information about motion in the backbone and side chains of a semi-crystalline and a glassy pol3nner. The data was taken at a Larmor frequency of 15.1 MHz for nuclei. It was also noted in this paper that the Tj values measured for the glassy polymer showed nonexponential behavior. As stated in the paper, this represented a distribution of Tj values due to the many different environments, and therefore the many different Tj mechanisms, present in the polymer. 

The inclusion of a variable temperature magic-angle spinning capability for solid state NMR spectroscopy makes feasible the investigation by relaxation parameters of structural and motional features of polymers above and below Tg and in temperature regions of secondary relaxations. Herein, we report variable temperature (50K to 323K) spectral data on semicrystalline poly(propylene) and glassy PMMA. Illustrative of the data are the Tj and results for isotactic... 

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