Formation of banded spherulites and the temperature dependence of the band space in olefin block copolymer

The formation of banded spherulites, which is a representative morphological feature for polymer crystalline aggregates, has attracted great interest during the past few decades. In this study, the crystalline morphologies of a type of olefin block copolymer (OBC) at different crystallization temperatures were observed systematically. It was found that banded spherulites formed at comparatively higher temperatures and the temperature dependence of the band space in OBC-banded spherulites could be divided into two regions: it firstly increased continuously with crystallization temperature between 115 and 119 °C; while beyond 120 °C, the changing tendency of the band space became unapparent and irregular. Scanning electron microscopy and atomic force microscopy confirmed that the alternative negative and positive bands could be attributed to the alternative flat-on and edge-on lamellae in the spherulites. Through analyzing the change of lamellar thickness and long period with temperatures, we speculated that the formation of the intriguing change of band space might be ascribed to the unbalanced surface stress, which was closely correlated to the amorphous layers of the OBC lamellae. We believe that this study contributes to understanding the relationship between the crystalline structure and banding phenomenon for semi-crystalline block copolymers.

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