The role of the order-disorder transition temperature of block copolymer in the compatibilization of two immiscible homopolymers

The important role that the order-disorder transition temperature (T-ODT) o f a block copolymer plays in the compatibilization of two immiscible homopo lymers is demonstrated, using the model ternary blend systems consisting of a polystrene-block-polybutadiene (SB diblock) copolymer and two immiscible homopolymers, polystyrene (hPS) and polyisoprene (hPI). For the study, SB diblock copolymers hating different microstructures were employed. We inves tigated via transmission electron microscopy (TEM) the morphology of the bl ends. We found that an SB diblock copolymer was very poorly distributed at the inter face between hPS and hPI in an hPS/hPI/SB ternary blend when the specimen was annealed at a temperature below the T-ODT of the block copolym er, while a more uniform distribution of the SB diblock copolymer was obser ved when a specimen was annealed at a temperature above its T-ODT. We have shown that the miscibility (or the interaction parameter) between the hPI a nd PB block in an SB diblock copolymer plays a decisive role in controlling the morphology at the interfaces between hPS and hPI. We conclude that a b lock copolymer must be designed, such that its T-ODT is below the targeted melt blending temperature, in order for the block copolymer to be able to a ct as an effective compatibilizing agent for two immiscible homopolymers. T his conclusion is supported further by investigating the tensile properties and morphology of ternary blends consisting of polypropylene (PP), hPS, an d polystyrene-block-poly(ethylene-co-1-butene)-block-polystyrene (SEBS trib lock) copolymer (Kraton G1650), which were prepared by melt blending at 200 degrees C in a batch mixer. That is, little improvement in the tensile pro perties of the ternary blends was observed when Kraton G1650 was added to P P/hPS binary blends. This observation is explained by a very poor distribut ion, observed by TEM, of Kraton G1650 at the interface between PP and hPS i n the ternary blend, This is attributed to the very high T-ODT, estimated t o be above 350 degrees C from currently held mean-field theory, of Kraton G 1650 compared to the melt blending temperature employed.