ISSN:
0098-1273
Keywords:
Physics
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
A Raman band of low frequency, arising from an accordionlike vibration of all-trans \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} ({\rm CH}_2 \rlap{--})_n $\end{document} segments and previously observed in normal paraffins and in polyethylene single crystals, has now also been found in bulk and in cold-drawn polyethylene, both linear and branched. The accordionlike vibration, or longitudinal acoustic mode (LAM), in polyethylene is compared with the LAM in normal paraffins. Whereas the Raman bands corresponding to the third (LAM-3) and higher modes are quite intense in a long-chain paraffin such as n-C94H190, they are so weak in polyethylene as to be unobservable with the apparatus used. This is attributed to the presence of the chain fold in polyethylene. Of the two extreme structural models of the fold here discussed, namely the models of “tight folds” and of “loose loops,” only the latter seems capable of accounting for the weakness of LAM-3 and higher modes in polyethylene. A quantity called “nominal Raman length” is defined as the length of that all-trans n-paraffin that would have the same LAM-1 frequency as the polyethylene sample under consideration. The nominal Raman length is always greater than the average long spacing, deduced from discrete x-ray scattering at small angles after applying a Lorentz correction, and, after allowing for chain tilt, is found equal to the segment length between folds. This can be accounted for by both of the models mentioned. As a test of the theory of surface melting the frequency of the accordion vibration of annealed polyethylene single crystals was measured as a function of temperature up to the melting point; no frequency change with temperature was observable. On the basis of the naive idea that there is complete decoupling of the vibrations in the all-trans chain segment from the disordered (molten) surface layer, one would predict that upon surface melting and the concomitant shortening of the all-trans segment, the LAM-1 frequency should increase. A more careful analysis, taking into account the existence of coupling of the LAM to the surface layer, shows that the outcome of this experiment does not necessarily invalidate the idea of surface melting. Bulk polyethylene samples exposed to 60Co γ-radiation for doses up to 100 Mrad show a slight shift of the Raman band to lower frequencies, whereas no such shift was observed upon absorption of a swelling agent. A search, without success, was made for a longitudinal acoustic mode in polypropylene, poly(vinylidene fluoride), nylon 66, and polyoxymethylene.
Additional Material:
11 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1974.180120210
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