Structural studies on gels from isotactic polystyrene

Abstract

The thermoreversible gelation phenomena exhibited by isotactic polystyrene (i-PS) was examined in depth, utilising a variety of experimental techniques. The primary aims were identification, relationship, description and visualisation of the morphologies corresponding to the two types of crystallization as diagnosed by the different crystal structures revealed by X-ray diffraction. While centred on electron microscopy the investigation used X-ray and electron diffraction, differential scanning calorimetry and Fourier transform infra-red spectroscopy in combination. A satisfactory correlation between all these techniques was established, including the important reassurance that the characteristics of the gel state are preserved on drying, a feature necessitated by most structure methods. Thei-PS gel system as a whole displays particularly clearly the distinction between gel forming and chain folded lamellar crystallization and the coexistence, morphological relationship and competition between the two crystallizable species. In addition, and most significantly, the gel crystals possess a crystal structure involving extended chains, close to an all-trans conformation, which are quite different to those obtained from conventionally crystallizedi-PS (3₁ helix). This former extended conformation in itself has become the starting point for new geometric and stereochemical considerations and in addition offers a convenient diffraction based fingerprint to delineate between the two modes of crystallization. Thus the conversion of the gel crystal structure to the conventional 3₁ helix structure on heating could be monitored. Further, that in the processes of heat annealing, remnants of these gel fibres initiate the development and dictate the orientation of a shish-kebab type platelet growth with the 3₁ helix crystal structure. This conversion process has been followed by electron microscopy and supplemented infra-red spectroscopy, low angle X-ray diffraction and differential scanning calorimetry. The different roles of fibres and platelets on stretching have been identified. Conditions for the existence of the gel crystals in their different variants are specified with relevance to ongoing arguments and discussions in the subject. Further, it was established that passing through the gel phase significantly enhanced the crystallization in the conventional crystal form making the normally slowly crystallizingi-PS into a fast crystallizable polyolefin.