Sb. Chun et Cd. Han, The role of the order-disorder transition temperature of block copolymer in the compatibilization of two immiscible homopolymers, MACROMOLEC, 32(12), 1999, pp. 4030-4042
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.