Effects of molecular weight and crystallization temperature on the morphology formation in asymmetric diblock copolymers with a highly crystalline block

The morphology formed in asymmetric poly(epsilon-caprolactone)-block-polybu tadiene (PCL-b-PB) copolymers has been investigated by differential scannin g calorimetry (DSC), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) as a function of total molecular weight M-n (8000 less than or equal to M-n less than or equal to 62000) and crystallization temperature T-c (-20 less than or equal to T-c less than or equal to 45 de grees C). All the copolymers have a cylindrical or spherical microdomain st ructure in the melt with the highly crystalline PCL block inside. In PCL-b- PB copolymers with M-n less than or equal to 19000, the SAXS result showed the morphological transition (microdomain structure --> lamellar morphology ) at each T-c by the crystallization of PCL blacks. The repeating distance of the lamellar morphology increased significantly with increasing T-c as u sually observed in the crystallization of homopolymers. The PCL crystallini ty chi(c) (i.e., the weight fraction of crystallized PCL blocks against tot al PCL blocks in the system) was 0.58-0.79, comparable to that of PCL homop olymer, In PCL-b-PB copolymers with M-n greater than or equal to 44000, on the other hand, the morphology did not change by the crystallization at any T-c, and chi(c) was extremely reduced (chi(c) < 0.15). The morphology form ed in PCL-b-PB with M-n = 30000 was a combination of above two cases; the r epeating distance was independent of T-c and equal to that of the microdoma in structure but the morphological transition was confirmed by SAXS and TEM . The relationship between the stabilities of the microdomain structure (or M-n) and lamellar morphology (or T-c) is qualitatively discussed.