Spherulite formation from microphase-separated lamellae in semi-crystalline diblock copolymer comprising polyethylene and atactic polypropylene blocks

The characteristics of crystallization in a semi-crystalline block copolymer have been experimentally examined using small-angle X-ray scattering and small-angle light scattering. For this purpose, a flow-oriented polyethylene-block-(atactic polypropylene) diblock copolymer with Mw=113 kg/mol and a polyethylene volume fraction of 0.48 was used. First of all, the crystallization was found to be much suppressed as compared to that of polyethylene homopolymer (homo-PE), as evidenced by a crystallinity that was approximately one-third of homo-PE. Spherulite growth with rupturing the microphase-separated lamellae (micro-LAM) was found at crystallization temperatures in the range 95≤Tc≤101 °C. That is, spherulites can grow even if the crystallization is initiated in the spatially confining micro-LAM structure. However, it was further found that the micro-LAM structure did not co-exist with a spherulite. On the other hand, at much lower crystallization temperatures (Tc<95 °C), the micro-LAM structure was retained whereas no spherulites formed. The suppression of spherulite formation can be accounted for by an auto-decelerating effect of nuclei in a confining microdomain space.