Synthesis and characterisation of symmetric 4-armed star copolymers of PS capped with butadiene and PI: importance of the location of the interface

Abstract

This paper describes the synthesis of three well defined, near monodisperse, heteroarm star block copolymers named as PI 2(B 13PS) 2, PI 2(B 26PS) 2, and PI 2(B 52PS) 2 where PI and PS refer to polyisoprene and polystyrene, respectively, and B refers to butadiene. The subscript in B indicates the number of butadiene units. The morphologies determined by transmission electron microscopy of these polymers are discussed. The rheological properties were investigated and the results are compared with those of the related linear AB diblock and the hetero-4 arm star block copolymer PS 2PI 2 with simple architectures and no butadiene content. The polymer with the lowest number of butadiene units showed the highest T ODT (200 °C) while the polymer with the highest number showed the lowest T ODT value (186 °C). This is believed to be due, in part, to the ability of the PI arms to drag the small section of polybutadiene into their own territory causing the fluctuation of the interface from the connection point, and which also minimises the monomer correlation near the junction point. In addition, since the two PS arms of a star molecule are no longer constrained to occupy adjacent positions on the interface, the star begins to behave like a pair of PI–PS diblocks flexibly connected at their centres. Rheology also reveals the influence of a change of the linking on the linear viscoelastic properties. On increasing the amount of butadiene, a shoulder present in the time–temperature superposition (TTS) master curve of PS 2PI 2 having simple architecture is reduced in magnitude and almost disappears for the polymer having the highest number of butadiene units.