Nature of the infrared absorption bands disappearing during the melting of polypropylene

Abstract

THE infrared spectra of isotactic polypropylene (PP) contain a number of bands (e.g. 840, 998 and 1168 cm -1) which practically disappear from the spectra of melted samples. The literature does not contain a single viewpoint regarding the nature of these bands. Some authors [1, 2] consider these to be associated with the unilateral order existing along the polymer chain, i.e. that their presence in the spectrum is only indirectly connected with the existence of threedimensional crystal structures. Two facts speak in favour of this theory: 1. The bands are present in the spectra of smeetic PP [1] which, on the basis of X-ray data, is amorphous and apparently consists of stretched spirals not incorporated in crystalline bundles. 2. I t was found during the calculation of the isotactic PP vibration spectra [3, 4], carried out to isolate the molecular coils, that normal vibrations exist (which is normally a combination of deformation oscillations of CH3-, CH~and CH-groups with C--C main-chain valence oscillations), and that these have frequencies similar to those actually observed as disappearing from the spectrum on melting. There are other authors, however [5, 6], who think that these bands appear as a result of chain interactions in the PP crystals. Support for this assumption is the fact that tempered PP samples, earlier thought to have a smectic structure [1], actually contain very small 1)13 crystals so that the X-ray spectrum is strongly distorted by peak broadening and mutual overlap of these peaks [7]. I t must be pointed out, regardless of the various interpretations of the nature of these bands, that an essential conditions for their appearance in the spectrum is the presence of long, isotaetie coil segments in the PP molecule, which are normally able to form crystals. The disappearance of the particular bands during polymer melting seems to be associated with a shortening of the equilibrium length of the isotactic PP coil segment in the melt, so that it becomes insufficient for the realization of the bands due to the interaction of vibrations. The PP produced in the presence of stereospeeific catalysts is made up of statistical Dand L-stereo-isomeric polymer units. The probability of an iso-