Mixed-crystal infrared studies of chain-folding in polyethylene single crystals: Low-temperature and molecular weight studies

Abstract

Infrared spectra of 40 PEH/1 PED mixed crystals have been obtained at low temperature as a function of the molecular weight of the PED. The band contour in the CD 2 bending region was resolved into crystalline and noncrystalline components by a least-squares fitting of Gaussian-plus-Lorentzian bands. The splittings of the crystalline component were analyzed in terms of the theory for finite chains of coupled oscillators, with the help of normal coordinate calculations for certain structures. Together with some results on n -paraffin mixed crystals, our studies show that (i) the spectroscopic observations are associated with interactions of crystalline stems of a single molecule; (ii) the observed crystalline splittings are incompatible with an essentially “switchboard” model of chain folding; (iii) adjacent reentry folding predominates, with folding along single (110) and (200) planes occurring; and (iv) for high molecular weights of PED double (110) fold planes prevail. These results are seen to be consistent with the morphologies known to be associated with the conditions of crystallization.