Effects of crystal structure of poly(β-propiolactone) blocks on the cooperative crystallization of a polyethylene-block-poly(β-propiolactone) diblock copolymer

The crystalline morphology of a double crystalline polyethylene-block-poly(β-propiolactone) diblock copolymer (PE-b-PPL) was examined using synchrotron small-angle X-ray scattering (SR-SAXS), where the PPL block crystallized into δ- or β-form according to the thermal history during sample preparation. Furthermore, the isothermal crystallization process of PE-b-PPL was pursued using simultaneous time-resolved SR-SAXS and wide-angle X-ray diffraction (SR-WAXD). The crystallization behavior of δPPL blocks (i.e., PPL blocks to crystallize into δ-form) was similar to that of PE blocks without any detectable induction time, so simultaneous crystallization of PE and δPPL blocks was observed on quenching from a lamellar microdomain structure (LMS), yielding a largely distorted LMS (with the standard deviation of each lamella thickness ∼ 36%). On the other hand, the βPPL blocks (i.e., PPL blocks to crystallize into β-form) started to crystallize after an appreciable induction time, showing sequential crystallization (i.e., advance crystallization of PE blocks and subsequent crystallization of βPPL blocks) to form a slightly distorted LMS (18%). Consequently, the final crystalline morphology was significantly different depending on the crystal structure of PPL blocks, though the crystallization started from an identical LMS. The crystallization mechanism of PE-b-PPL was discussed by considering the difference in crystallization kinetics between PE blocks and PPL (δPPL or βPPL) blocks.