Crystallization within melt ordered semicrystalline block copolymers: Exploring the coexistence of microphase-separated and spherulitic morphologies

The crystalline and microphase-separated morphology of semicrystalline bloc k copolymers are compared in bulk (0.5-1 mm) and thin films (5-0 mum), as c haracterized by transmission electron microscopy (TEM) and polarized light microscopy (PLM). Bulk films of a triblock copolymer of polystyrene, polybu tadiene, and polycaprolactone, (PS)(0.35)(PB)(0.15)(PCL)(0.5), where the su bscripts denote the mass fraction, show a lamellar-cylindrical microphase-s eparated morphology with a lamellar repeat of congruent to 62 nm, as observ ed by TEM. Bulk films of a diblock copolymer consisting of (PB)(0.2)(PCL)(0 .8) show an imperfect cylindrical microphase-separated structure by TEM coe xisting with crystalline lamellae of PCL. For both bulk specimens, the PLM shows a speckle pattern of birefringence that confirms the PCL is crystalli zed within the microphase-separated structure, but no large-scale spherulit es are observed by PLM. The microstructure in the as-cast thin films (5-10 mum) by TEM is lamellar-cylindrical for the triblock copolymer and cylindri cal for the diblock copolymer, as described above for the bulk specimens. U pon thermal treatment of these thin films, large and well-defined PCL spher ulites are observed by PLM for both block copolymers. TEM of the thermally treated triblock copolymer thin films shows that, after the formation of sp herulites, the microphase structure is no longer lamellar-cylindrical but e ntirely dominated by PCL lamellar crystals. In contrast, TEM of the thermal ly treated diblock copolymer thin films shows that the PCL lamellar crystal s coexist with the PB microphase-separated cylinders after the formation of PCL spherulites. Bulk specimens could not be induced to form spherulites u nder similar thermal treatment conditions.