The problem of thermal expansion in polyethylene spherulites

Abstract

It is shown from simple considerations of the arrangement of crystal axes and from the known thermal expansion coefficients along these axes that a polyethylene spherulite would, on cooling from its crystallization temperature to room temperature, form radial voids (amounting to up to 4% of the volume) unless some reversible internal transformation prevents this. To clarify this ‘composite’ problem a co-ordinated investigation was carried out on changes in the macroscopic sample volume, in the lengths of the crystallographic axes and in the diameters of the individual spherulites with temperature. The results show that the changes in spherulite dimensions conform to those of the sample as a whole thus eliminating the necessity of void formation (which nevertheless appears to arise in thin film spherulites) leaving an inconsistency with the crystallographic data. It was noticed further that the thermally induced reversible changes in the spacing of the extinction bands did not conform to the changes in the spherulite diameter and in addition depended on the radius of the band. It is suggested that the reversible transformation needed to prevent voiding and to eliminate the above inconsistencies is also responsible for the changes in band spacing (possibly in the form of twisting and untwisting of the structure underlying the banding) and represents a novel, as yet unexplored, feature of spherulites.