Sidechain relaxation times

Figure 42. Histogram of relaxation time (r) distributions showing the number of atoms with a given value of t as a function of t (a) all atoms, solvent run (h) all atoms, vacuum run (c) backbone atoms, solvent run (d) sidechain atoms, solvent run.

Numerous multipulse and two dimensional experiments are used to identify unknown (co)polymers and these experiments are most useful in analyzing differences between copolymers or impurities in production problems. The Distortionless Enhancement via Polarization Transfer (DEPT) pulse sequence is frequently used to determine the carbon multiplicity (CH vs. CHj vs. CH3) of a carbon spectrum. Newmark showed excellent DEPT spectra could be obtained on sidechain groups in vinyl polymers or in typical polyesters and polyurethanes, but that it was difficult to observe good DEPT spectra (at 200 MHz) on backbone carbons, such as the ring carbons in cellulose acetate butyrate (CAB) with very short carbon spin-spin relaxation times (Tj) (8). 

In the case of the longer sidechain materials the degree of crystallinity that develops at room temperature after melting is 25% or less (see Figure 4). This lower degree of crystallinity is due to the high content of mobile alkyl component which extends from the backbone. The relatively high mobility of the sidechain carbons versus the low mobility of the backbone carbons was demonstrated by direct comparison of the spin lattice relaxation times (Ti) of the sidechain carbons compared to backbone carbons in samples with a Co sidechain content of 85%... 

To optimize force fields for long time scale motions Aliev et al. propose a new robust approach to use NMR spin-lattice relaxation times Ti of both backbone and sidechain carbons. This allows a selective determination of both overall molecular and intramolecular motional time scales. In addition they use motionally averaged experimental/ coupling constants for torsional FF parameters. The force constants in the FFs and the correlation times are fitted in an Arrhenius-type of equation. 

Poly(n-butyl acrylate). A study of the relaxation properties of PBA was initiated for several reasons. There are two backbone carbons with directly bonded protons thus the effect of the side chain on backbone motion might be determined. Also, the CH carbon should more directly reflect the distribution of correlation times necessary to begin analysis of alkyl sidechain motion. Finally, the lack of the additional chain-CH3 groups significantly loosens motional constraints in PBA. The effect of this on the overall dynamics of PBA was of interest. 

In summary, it appears from spectroscopic studies such as neutron scattering or NMR relaxation measurements which probe rotational water motions on a short time scale, 10 -10 s, and thus over a short distance range that, at the highest water contents, water mobility within the pore of an ionomeric membrane is not drastically different than bulk water mobility. However, as the water content of the membrane decreases, its mobility is increasingly hindered. The nanopore liquid in the membrane is essentially a concentrated acid solution and ion-water (as well as ion-ion) interactions will have significant influences on water motion. Intrusions of sidechains... 

One way to obtain dynamic results for longer time periods is to use stochastic dynamics methods (see Chapt. IV.D). As described in Chapt. IX.B.3, these have been used to simulate aliphatic sidechains in aqueous solution for periods of 100 ns or longer so that they permit adequate sampling of both the oscillations within a potential well and the transitions between wells.338 To provide an overview of the type of 13C NMR relaxation behavior that can... 

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