Jiashu Fan

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.

Exploring the crystallization-induced mesophase evolution in an olefin block copolymer through a rationally designed two-step isothermal crystallization strategy

The competition between crystallization and phase separation has always been a major concern for semi-crystalline block copolymers. In this work, in order to examine the mesophase evolution during crystallization in an olefin block copolymer (OBC), a “two-step isothermal crystallization” strategy was designed elaborately: the OBC samples were first isothermally crystallized at different temperatures (Tc) to create diverse crystallization histories; after melting, the second crystallization process was carried out to detect the degree of mesophase separation change during the first crystallization. It was quite interesting that the first crystallization at a lower Tc displayed little influence on the second crystallization process, while the first crystallization at a higher Tc could obviously accelerate the subsequent crystallization. In combination with the observation of the crystalline morphologies and phase morphologies in the melt, we speculated that hard blocks mainly crystallized separately during relatively rapid crystallization with a large quantity of nuclei; while crystallization-induced aggregation of hard blocks occurred during slow crystallization with a limited number of nuclei, changing the distribution of the hard blocks and accelerating the subsequent crystallization as a consequence. This work contributes to the understanding of the phase evolution during crystallization of OBC and such multi-block copolymers under different circumstances.