Composition dependence of crystallized lamellar morphology formed in crystalline-crystalline diblock copolymers

We have investigated the crystallized morphology formed at each temperature Tc (20 °C ≤ Tc ≤ 45 °C) in double crystalline poly(ɛ-caprolactone)-block-polyethylene (PCL-b-PE) copolymers as a function of composition (or volume fraction of PE blocks ϕPE) by employing small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. When PCL-b-PE with ϕPE ≤ 0.58 was quenched from a microphase-separated melt into Tc, the crystallization of PE blocks occurred first to yield an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers (PE lamellar morphology) followed by the crystallization of PCL blocks, where we can expect a competition between the stability of the PE lamellar morphology (depending on ϕPE) and PCL crystallization (on Tc). Two different morphologies were formed in the system judging from a long period. That is, the PCL block crystallized within the existing PE lamellar morphology at lower Tc (<30 °C) to yield a double crystallized alternating structure while it crystallized by deforming or partially destroying the PE lamellar morphology at higher Tc (>35 °C) to result in a significant increase of the long period. However, the temperature at which the morphology changed was almost independent of ϕPE. For PCL-b-PE with ϕPE ≥ 0.73, on the other hand, the morphology after the crystallization of PE blocks was preserved at every Tc investigated.