DEPENDENCE OF THE OBDD MORPHOLOGY ON DIBLOCK COPOLYMER MOLECULAR-WEIGHT IN COPOLYMER HOMOPOLYMER BLENDS

The ordered bicontinuous double-diamond (OBDD) morphology is known to depend strongly on the bulk composition of microphase-separated diblock copolymers and blends composed of a copolymer and a parent homopolymer. An additional factor that must be considered in blends is the homopolymer molecular weight relative to that of the corresponding block in the copolymer. In this work, transmission electron microscopy is employed to identify the morphologies which develop in a series of such blends, each composed of a symmetric poly(styrene-b-isoprene) (SI) diblock copolymer and a polystyrene homopolymer. The copolymers investigated here range in total molecular weight (M) from 20 000 to 180 000, and the corresponding homopolymers each possess a molecular weight equal to approximately M/4. While the OBDD morphology is generated in most of these blends, additional morphologies (e.g., lamellar catenoid, cylindrical, and disordered) are also observed and are found to depend on the magnitude of M. An experimental morphology diagram is presented for copolymer/homopolymer blends along the isopleth corresponding to a bulk composition of 66 vol %, styrene.