Laser‐Raman study of the longitudinal acoustic mode in polyethylene

Abstract

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.

On the use of Raman‐active longitudinal acoustic modes in the study of polyethylene lamellar structures

“…For all solution crystals studied here, two weak band features appeared on the low-frequency side. Similar bands have also been reported by Olf et al 15 Their origin is unknown. It was observed in this work that their positions vary with that of the LAM.…”

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

“…As pointed out by Boyd41 for viscoelastic relaxations, it is likely that chain stems in the crystal do not move as rigid units but rather undergo a gradual displacement due to migration of a conformational defect. For PE, a 180° chain twist accompanied by a c /2 compressive strain has been reported to be the more probable defect for molecular transport in the crystal 42–46. Such a defect is able to move along the stable all‐trans planar conformation of the crystalline chain stem, from one fold surface of the crystal to the opposite one.…”

“…2) is equal to the crystallographic parameter a . ν t can be approximated to the frequency of the crystalline relaxation (ν c ), which is precisely associated with the activation of chain twists in the crystal phase 41–45. One may, therefore, assume where E a is the apparent activation energy of the relaxation process.…”

Low-frequency Raman spectroscopic study of ionomers

“…This value is consistent with the value derived from SAXS · measurements. 23 For the annealed samples, the chain lengths calculated from the Raman frequency are also shown in Figure 3. Since the frequency and the intensity of the LAM can be disrupted by conformational and configurational disorder, 23 .24 the existence of this mode in a random copolymer is interesting.…”