Crystallization within Melt Ordered Semicrystalline Block Copolymers: Exploring the Coexistence of Microphase-Separated and Spherulitic Morphologies

Abstract

The crystalline and microphase-separated morphology of semicrystalline block copolymers are compared in bulk (0.5-1 mm) and thin films (5-0 Im), as characterized by transmission electron microscopy (TEM) and polarized light microscopy (PLM). Bulk films of a triblock copolymer of polystyrene, polybutadiene, and polycaprolactone, (PS)0.35(PB)0.15(PCL)0.5, where the subscripts denote the mass fraction, show a lamellar-cylindrical microphase-separated morphology with a lamellar repeat of =62 nm, as observed 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 coexisting with crystalline lamellae of PCL. For both bulk specimens, the PLM shows a speckle pattern of birefringence that confirms the PCL is crystallized within the microphase-separated structure, but no large-scale spherulites are observed by PLM. The microstructure in the as-cast thin films (5-10 Im) by TEM is lamellar-cylindrical for the triblock copolymer and cylindrical for the diblock copolymer, as described above for the bulk specimens. Upon thermal treatment of these thin films, large and well-defined PCL spherulites are observed by PLM for both block copolymers. TEM of the thermally treated triblock copolymer thin films shows that, after the formation of spherulites, the microphase structure is no longer lamellar-cylindrical but entirely dominated by PCL lamellar crystals. In contrast, TEM of the thermally treated diblock copolymer thin films shows that the PCL lamellar crystals coexist with the PB microphase-separated cylinders after the formation of PCL spherulites. Bulk specimens could not be induced to form spherulites under similar thermal treatment conditions.