Polyethylene linewidths

The polyethylene linewidths, as illustrated in Fig. 1, clearly indicate that a constant value is attained in the melt between 1-8 X 10. Proton T2 measurements at 150°C indicate that there is a change in slope in this quantity as a function of molecular weight at about = 6 x 10. ( ) The critical molecular weight as determined from bulk viscosity measurements for this polymer has been given as 2 x 10 ( ) and 3.8 x 10. (5 ) These values are very close to those which would be deduced from the linewidth measurements. For polydimethyl siloxane the break in the proton T2-molecular weight curve occurs at about M = 5 x 10. (37) is about 2.5 x 10 from viscosity measurements. ( ) Fig. 5... 

The linewidth-temperature relation of the polyethylene oxide samples are given in Fig. 5. Despite the large differences in molecular weight, these samples have about the same linewidth, 300-350 Hz, in the crystalline state at 25°C. They all also possess a spherulitic type of morphology. The influence on the linewidth of the different types of supermolecular structures... 

These conclusions are further generalized by the more extensive data presented in Fig. 7 for polyethylene oxide and poly-trimethylene oxide. The continuous nature of the Ti function for both these polymers over a large temperature range is quite definite and is emphasized by the detailed data in the vicinity of the respective melting temperatures. This is true even for the polyethylene oxide samples where discontinuities in the linewidth are clearly indicated in Fig. 7. Obviously, the type of segmental motions which contribute to the two different relaxation pareim-eters are influenced quite differently by the presence of crystallinity. 

These results are reminiscent of and very similar to those reported here for the relaixation parameters of polyethylene oxide as illustrated in Figs. 5 and 7. We have found that the s are continuous with teit5>erature over a very wide molecular weight range. However, as is shown in Fig. 5, there are discontinuities in linewidths at the melting temperatures for the low molecular weight samples, but the linewidths are continuous... 

In the present work the limiting value of the linewidths for polyethylene oxide increases from 135 Hz in the melt above 70°C, to the range 300-350 Hz in the crystalline state at room temperature. As is indicated in Table I, the resonant linewidths for linear polyethylene increase substantially upon crystallization and attain values in the range 500-900 Hz at 45°C and 57.9 MHz. 

As has been emphasized previously (IJ), the level of crystallinity is not the major determinant of the linewidth in the semicrystalline state. Rather the supermolecular structure or morphology is a major factor in governing the magnitude of the linewidth. Structural factors and crystallization conditions under which low density (branched) polyethylene forms... 

The inability to "burn" a narrow hole in the polyethylene spectrum is an indication that the "natural homogeneous linewidth"... 

D. Axelson These were obtained for branched polyethylene and were similar to the melting point curves. The conditions were such that either direction gave identical linewidths. 

D. Axelson For the polyethylenes, at least, there is a major effect of morphology on linewidth. This is going to make more difficult a detailed description of the dynamics of the low frequency motion relative to a completely amorphous polymer. 

In the high resolution measurements by CPMAS NMR, the information about the molecular mobility and dynamics are involved in the intensity and the linewidth of each peak. Figure 7.21(a) shows the VT CPMAS spectra of polyethylene at temperatures between -120 and 90° [20]. The spectra at temperatures from -50 to -I20°C are expanded in Fig. 7.21(b). At room temperature, the spectrum consists of peaks for crystalline and amorphous phases. As the temperature is increased from 25 to 90°C, the intensity of peak A is enhanced. This increment of amorphous peaks arises from two effects. One is that the amount of the amorphous phase increases. The other is the increment of the CP efficiency for the amorphous phase which means that molecular mobility is increased by temperature. Both factors enhance the peak intensity of the amorphous phase. 

The zero point of the chemical shift is arbitrarily fixed. The relative ratios of the principal values are fixed as for the principal values for crystalline polyethylene listed in Table 1. (B) Ou = c 22< 0 33- The thinner lines indicate the theoretical lineshapes according to eqns (14) and (15) and the solid lines are obtained from the theoretical lines by convoluting the Lorentzian distribution function with a linewidth of 2 ppm. 

In the solid state, polymer chains can be (more or less) randomly coiled or arrayed in an ordered state, and these amorphous and crystalline domains give rise to different signals in the solid-state NMR spectrum of a bulk material. The broad-line NMR spectrum of a semicrystalline polymer such as polyethylene has already been introduced (Sec. II.H.2 and Fig. 30). The linewidth difference is caused by the more restrictive molecular motion... 

In contrast, motion at high frequencies in solids is usually of small amplitude thereby resulting in a substantially reduced spectral density in the Larmor frequency region.As such, the values for carbons in solids may vary between a few seconds to minutes and even hours, e g, the carbon spin-lattice relaxation time for crystalline polyethylene is ca. 700 seconds at ambient temperature Clearly, obtaining a spectrum can be time consuming if indeed not prohibitive in the case of multi-line spectra The situation is further aggravated by the fact that linewidths in the solid spectrum may be... 

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