Cross-relaxation polymer motion

Table II highlights the 3C NMR relaxation times that reflect molecular motions and help define the physical properties of the cutin polymer. For those solid-like carbons that cross polarized, considerable motional freedom was evidenced for (CH2)n and CH2OCOR groups, on both MHz and kHz timescales, by the short values of Tj(C) and Tip(C), respectively. By contrast, the CHOCOR moiety was more restricted dynamically as judged from its long value of TX(C) low-frequency motions in particular were implicated by the strong dependence of Tip(C) on Bi. These latter groups...

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. 

An excellent review text has recently been published by Tycko outlining various experimental NMR approaches to molecular dynamics, both in small and macromolecules [85]. In terms of the application of these various methods to polymers, one can generally categorize the techniques as direct (e.g., 2D exchange, labeling and lineshape analysis) or indirect (e.g., relaxation, cross-relaxation) probes of molecular dynamics, and additionally, rank them in terms of whether they probe a specific, local type of motion or longer range, correlated chain dynamics. Finally, solid-state NMR... 

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... 

The simplest case of comb polymer is the H-shaped structure in which two side arms of equal length are grafted onto each end of a linear cross-bar [6]. In this case the backbones may reptate, but the reptation time is proportional to the square of Mj, rather than the cube, because the drag is dominated by the dumb-bell-like frictional branch points at the chain ends [45,46]. In this case the dependence on is not a signature of Rouse motion - the relaxation spectrum itself exhibits a characteristic reptation form. The dynamic structure factor would also point to entangled rather than free motion. 

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