Central spot on the Hν-diffraction patterns of polymer samples containing spherulites

Abstract

On H ν -diffraction patterns of films containing spherulites, in addition to the typical four-petal pattern, scattering is often observed in the central region of the diffraction pattern (so-called central spot) [1–5]. The intensity of the central spot depends on the thickness and history of the sample and passes through a minimum four times when the film is rotated through 360° round a primary beam. Our observations indicate that for films obtained by rapid and irregular cooling of the melt, the intensity of central scattering is comparable with that of spherulitic reflexes. On reducing the rate of cooling the central spot became weaker and with fairly slow reduction in temperature and fairly thin films it disappeared completely. The spot also disappeares on annealing films. However, the central spot again appeared when the annealed specimen was deformed [2, 5]. The intensity of the central spot reached a maximum when the direction of elongation enclosed an angle of 45° with the axes of polaroids and (in the event of fairly low deformation) decreased to zero, when the direction of elongation enclosed an angle of 0 or 90° with the axes. Similar effects are also observed during the deformation of amorphous polymers [6] and are due to the fact that during deformation these materials, on the whole, become optically anisotropic. The well known theory developed by Kuhn and Gruhn [9] enables us to relate the intensity of scattering in the “forward” direction with segmental anisotropy of polymer molecules and their orientation in relation to the axis of elongation. For amorphous crystalline polymers, however, no quantitative relations were derived so far between the intensity of light passed through between the crossed polaroids and the molecular and supermolecular characteristics of the systems. Nevertheless, there are several experimental facts which support the view that during deformation the amorphous-crystalline polymers gradually acquire the properties of a uniaxial crystal and that anisotropy increases in proportion to elongation. Among these facts are: 1) periodicity in changing the intensity of light transmitted in proportion to elongation (Fig. 1 a); 2) direct observation of the macroanisotropy of polymers through a quartz prism (Fig. 1, b); 3) agreement (in first approximation) of the angular dependence of the intensity of transmission in crossed polaroids for uniaxially elongated fibres and films with the well known rule of transmission for uniaxial crystals. ▪ However, neither the similarity to an optically anisotropic crystal, nor the theory of scattering by a single particle (spherulite) [3, 4] explain the presence of central scattering when the axis of elongation is oriented along the plane of vibration of one of the polaroids. An attempt is made in this paper to give a possible explanation to this phenomenon.