Effects of Molecular Weight and Crystallization Temperature on the Morphology Formation in Asymmetric Diblock Copolymers with a Highly Crystalline Block

Abstract

The morphology formed in asymmetric poly(ε-caprolactone)-block-polybutadiene (PCL-b-PB) copolymers has been investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) as a function of total molecular weight Mn (8000≤Mn≤62000) and crystallization temperature Tc (−20≤Tc≤45°C). All the copolymers have a cylindrical or spherical microdomain structure in the melt with the highly crystalline PCL block inside. In PCL-b-PB copolymers with Mn≤19000, the SAXS result showed the morphological transition (microdomain structure→lamellar morphology) at each Tc by the crystallization of PCL blocks. The repeating distance of the lamellar morphology increased significantly with increasing Tc, as usually observed in the crystallization of homopolymers. The PCL crystallinity χc (i.e., the weight fraction of crystallized PCL blocks against total PCL blocks in the system) was 0.58—0.79, comparable to that of PCL homopolymer. In PCL-b-PB copolymers with Mn≥44000, on the other hand, the morphology did not change by the crystallization at any Tc, and χc was extremely reduced (xc<0.15). The morphology formed in PCL-b-PB with Mn=30000 was a combination of above two cases; the repeating distance was independent of Tc and equal to that of the microdomain structure but the morphological transition was confirmed by SAXS and TEM. The relationship between the stabilities of the microdomain structure (or Mn) and lamellar morphology (or Tc) is qualitatively discussed.