Competition in Crystallization between Different Components and Formation of Concentric Double Spherulites in Block Copolymers

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A unique morphology of spherulites, namely double spherulites like concentric circles, was observed for poly(epsilon-caprolactone)-block-poly(ethylene glycol)-block-poly(epsilon-caprolactone) (PCL-PEG-PCL) triblock copolymers, ABA-2 and ABA-3, with PCL compositions of 66 and 60 wt%, respectively. The inside and outside spherulites in the concentric spherulites were those of PCL and PEG, respectively. In formation of the conceniric spherulites, the spherulites of the PCL component first occurred necessarily and grew, and then the PEG spherulite developed from the growth front of the PCL spherulite. Any nucleation of the PEG spherulite prior to that of PCL was not observed. In the triblock copolymers, ABA-1 and ABA-4, with PCL compositions of 83 and 34 wt%, respectively, no concentric spherulites were observed but a usual single spherulite was formed ; in ABA-1 only PCL spherulites was formed while only PEG spherulite was observed in ABA-4. Two melting peaks appeared for ABA-2 and ABA-3 in which the concentric spherulites was observed. The SAXS curves also had two kinds of peaks due to alternating structure of the crystal lamella and amorphous layer for both ABA-2 and ABA-3. The long spacing estimated from the peak position suggested that the amorphous chains of the counter component existed between the crystal lamella.