Effect of substrate and molecular weight on the stability of thin films of semicrystalline block copolymers

The thermal stability of the thin film morphology of two symmetric oxyethylene/oxybutylene block copolymers (E 76 B 38 and E 114 B 56 ) on mica and silicon was investigated via atomic force microscopy (AFM). It is found that morphological transition of E m B n thin films during melting is strongly dependent on the molecular weight of the diblock copolymers and their interaction with the substrate. For E 76 B 38 on mica, a single-layered structure transforms into a double-layered structure upon melting, but the same polymer on silicon retains a single-layered structure after melting and spreads quickly to wet-out the silicon surface. Conversely a longer polymer, E 114 B 56 , has a thin film on mica that does not change much after melting of the crystalline E block. A mechanism was proposed to explain the relative stability of E 76 B 38 and E 114 B 56 thin films upon melting. Internal stress is produced during melting and can be released along two directions. The release along the vertical direction is restricted by the energy barrier related to the segregation strength, and the release along the horizontal direction is dependent on the mobility of block copolymer related to the interaction between the block copolymer and the substrate. Domain size affects the release rate of the internal stress along the horizontal direction and thus the thermal stability of E m B n thin films. Switching between horizontal and vertical releases can be realized by controlling the domain size of the thin films.