DETERMINATION OF THE DISTRIBUTION OF STRAIGHT-CHAIN SEGMENT LENGTHS IN CRYSTALLINE POLYETHYLENE FROM THE RAMAN LAM-1 BAND.

It is shown how the shape of the longitudinal acoustic vibration observed in the low-frequency region of the Raman spectra of crystalline polymers can be used to obtain a quantitative distribution of lengths of straight-chain segments associated with polymer lamellas. The procedure is demonstrated for a ″solid-state″ extrudate of polyehtylene and for a bulk-crystallized specimen of the same polymer. Equations relating the shape of the LAM-1 band to the shape of the distribution curve are given. The low intensity observed for the LAM-3 mode relative to LAM-1 is explained quantitatively without recourse to end effects. LAM-5 has been observed for bulk-crystallized polyethylene. For the extrudate we find the distribution of lengths of straight-chain segments to have a tail on the long-length side which is not present for the bulk-crystallized sample. The Raman technique is shown to provide new morphological data unattainable at present by other methods.