Appearance of double spherulites like concentric circles for poly(ϵ-caprolactone)-block-poly(ethylene glycol)-block-poly(ϵ-caprolactone)

A unique morphology of spherulites, namely double spherulites like concentric circles, was observed for poly(ϵ-caprolactone)-block-poly(ethylene glycol)-block-poly(ϵ-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymers, ABA-2 and ABA-3, with PCL compositions of 66 and 60 wt%, respectively. The central and outer sections in the concentric spherulites were those of PCL and PEG, respectively. In formation of the concentric spherulites, the spherulites of the PCL component always appeared first 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 usual single-circle spherulites were formed; only PCL spherulites were formed for ABA-1 while for ABA-4 only PEG spherulites were observed. For ABA-2 and ABA-3 having the concentric spherulites, two melting peaks appeared and the WAXD patterns were a superposition of those for the PEG and PCL homopolymers. The SAXS curves for ABA-2 and ABA-3 also had two kinds of peaks due to the long spacings relative to the PCL and PEG crystal lamellae, respectively. The values of the long spacing estimated from the peak positions suggested that the PEG (or PCL) segments located between the crystal lamellae of PCL (or PEG) existed as amorphous segments.