Gelation‐crystallization in isotactic polystyrene solutions and its implications to crystal morphology, to the origin and structure of gels, and to the chemical homogeneity of polyolefins

Abstract

Abstract As part of a wider study on the crystallization of isotactic polystyrene solutions it was observed that at sufficient concentrations (> 3–5%) gelation sets in below a certain (very high) supercooling in competition with the usual single crystal formation which in itself produces turbid suspensions. It was established that gelation is a form of crystallization (mode A) which must be of fringed micellar type to provide the connectedness as opposed to the chain folded lamellae (mode B) which gives rise to discrete particles. The gel crystals ( A ) display sharp melt endotherms and produce distinct x‐ray diffraction patterns both of which, however, differ decisively from those provided by crystals B , a distinction which can be preserved even after removal of the solvent. The melting points of A are significantly lower than those of B and the x‐ray diffraction patterns of A are incompatible with the recognized structure of polystyrene (3 1 helix) possessed by B ; they point to a broadly planar zig‐zag arrangement of the chain. This strongly suggests that we have blocks of chemically distinct sequences which could be syndiotactic or head‐to‐head tail‐to‐tail (presently with substantial support for the latter) which is responsible for the gel forming crystallization. However, so far the C 13 nuclear magnetic resonance (NMR) results do not provide the evidence for these distinct species but explanations for our observations on any other basis seem to lead to unsuperable difficulties from other points of view. Consequently, the paper is left open ended with the possibilities discussed. Amongst these the existence of a very few but long, chemically distinct sequences seems most attractive. The wider implications of the facts as they stand for crystal morphology (fringed micelles versus lamellae), for the origin and structure of gels in general, for the crystallization of block copolymers and for issues relating to chemical homogeneity (tacticity, head‐to‐head tail‐to‐tail) are discussed and preliminary effects are quoted which indicate that these issues may also be relevant to the usual atactic polymers.