Carbon line widths

Fig. 15. Log-Log plot of aromatic carbon line-width (co1/2) as a function of the DVB mole-fraction, x, in the (Sty)i x(DVB)x sample...

Figures 6.30 and 6.31 present the same information for saturated hydrocarbons. In Figure 6.30, the saturated liquid state is on the lower part of the curve and in Figure 6.31 it is on the upper part of the curve. Below T y, the line width changes, indicating that the liquid probably does not flash below that level. Note that a line has been drawn only to show the relationship between the points a curve reflecting an actual event would be smooth. Note that a liquid has much more energy per unit of volume than a vapor, especially carbon dioxide. Note It is likely that carbon dioxide can flash explosively at a temperature below the superheat limit temperature. This may result from the fact that carbon dioxide crystallizes at ambient pressure and thus provides the required number of nucleation sites to permit explosive vaporization.

At higher temperatures kinetic line broadening and coalescence is observed (Fig. 8). Upon warming above the coalescence temperature of about 110 °C four lines for the aromatic methine carbons with decreasing line width are observable until decomposition takes place at about -70 °C (Fig. 8). From line shape analysis the energy barrier for the isomerization process is obtained as AG11 — 7.5kcal/mol.101... 

The 13C NMR spectra were measured regularly over 6 months (190 days) and the polymer structure became almost stable after this period. After 6 months, the relative intensities in the unsaturated carbon region were independent of CP time. The alkyl signal positions became constant after 11 days (Figure 38(b)). However, their line width gradually broadened to double the width after 6 months. 

C. J. Carman I guess it could be, but again I m not certain because there will be a distribution of chemical shifts which perhaps contributes to the line widths as well. In addition to the latter, other factors can contribute to line broadening. As was indicated by other speakers before me, conformational effects do play a role in polymer spectra, I m convinced of it. I m not certain of how to extract it but I m convinced it is there. In the center of the EPDM spectrum is the bulk of the structure which arises from the long runs of methylene carbons. There are several peaks which are very temperature dependent and very solution dependent. I am not sure how much of the line width is really due to molecular motion as opposed to distribution of chemical shifts. The shifts may differ from those found for the polymer when it is in solution. This is why I am a little hesitant to assign the broadening to crystallinity effects. 

In contrast to the spin-lattice relaxation parameters, which remain invariant, a sijbstantial broadening of the resonant lines occurs upon crystallization. The effect is relatively modest for cis polyisoprene at 0°C and 57.9 MHz, where comparison can be made at the same temperature. Here there is about a 50% increase in the linewidths upon the development of 30% crystallinity. Schaefer (13) reports approximately 3- to 5-fold broader lines (but they are still relatively narrow) for the crystalline trans polyisoprene relative to the completely amorphous cis polyisoprene at 40°C and 22.5 MHz. It is interesting to note in this connection that for carbon black filled cis polyisoprene the line-widths are greater by factors of 5-10 relative to the unfilled polymer. 

Adding the proton-free solvent carbon tetrachloride to polystyrene, one observes the relationship between line width and temperature as shown in Figures 5a and 5b. For the system polystyrene with 21.5% carbon tetrachloride the absorption curves for various temperatures are plotted against the field strength (2) in gauss. As the temperature decreases, the line broadens, as shown more clearly in... 

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