Electron-microscopical and electron diffraction study of the deformation of thin films of spherulitic polyvinylidenefluoride

Abstract

It is well known that spherulites are the most common and at the same time the most complex structural form produced in crystallizable polymers when they are crystallized from solution or the melt [1]. Naturally therefore research workers have been interested in recent years in study of the structure of spherulites, in the mechanism of their deformation and in their structure in the orientated state, using for this purpose the methods of optical and electron microscopy, and optical, X-ray and electron diffraction (wide and low angles), etc. According to some authors [2–4] the deformation of spherulites involves breakdown of the original structure and formation of a new fibrillar structure, depending on the conditions of deformation. In papers by Kargin and his collaborators it is shown that in addition to the above mechanism deformation of spherulites can occur in stages with breakdown of more complex structures while the simple structural elements are preserved [5, 6]. A considerable degree of reversible deformation of spherulites is also possible under certain conditions [5]. All this shows that depending on the conditions of deformation, the chemical structure of the polymer and its previous treatment, the deformation of spherulites can follow different patterns. Because of the complexity of this problem neither the mechanism of deformation of spherulites [7] nor the structure of orientated crystalline polymers [8] have yet been completely clarified. These problems have been examined most thoroughly in a recent series of papers by Peterlin and his collaborators [9–18] and by Kabayashi and Nagasawa [19], in which the deformation of single crystals, thin spherulitic films and block specimens of polyethylene (PE) was studied. The occurrence of periodicity along fibrils in block specimens and thin films of PE was observed in the electron microscopy by Peterlin and his collaborators, who to increase the contrast made use of the methods of etching with fuming nitric acid and shadowing of thin films and replicas. Only in fibrils drawn from PE single crystals was it possible to observe fairly clear periodicity [20], In the same specimens, after they had been annealed at temperatures above 120 electron diffraction analysis showed a long period corresponding to the periodicity observed in the electron microscope. The long periods first found by Basset and Keller [21] by electron diffraction in the electron microscope, in stretched block specimens of PE (500–5000 Å) scarcely correspond to the real amorphous-crystalline structure of the polymer, and are obviously due to unsuitable choice of the etching conditions. We have shown previously [22] that it is possible to observe in the electron microscope, and by low-angle diffraction, long periods in stretched and then annealed spherulitic films of polyvinylidenefluoride (PVF 2) that have not received any additional treatment (shadowing, etching etc.) for the purpose of increasing the contrast in the electron microscope. The present paper presents the results of an electron-microscopical and electron diffraction (low and wide angles) study of thin spherulitic films of PVF 2, stretched at various temperatures and also subsequently annealed.