Morphology and elasticity of polystyrene-block-polyisoprene diblock copolymers in the melt

Abstract

The influence of morphology on the viscoelastic properties of melts of microphase-separated polystyrene-block-polyisoprene (PS-b-PI) diblock copolymers was investigated in oscillatory shear and creep recovery experiments. By means of anionic polymerization, three PS-b-PI diblock copolymers with a narrow molecular weight distribution and different types of morphology (spherical, cylindrical and lamellar microstructure) were prepared. Linear viscoelastic shear oscillations and creep recovery experiments in shear were performed in order to determine the elastic and viscous properties of the diblock copolymers in the melt at small and large time scales. Our analysis reveals that melts of diblock copolymers are characterized by a pronounced elastic behavior leading to a relatively large recoverable deformation in creep recovery experiments. The elasticity of the diblock copolymers is also revealed by the appearance of the creep-ringing effect. Morphological investigations were carried out to establish relations between microstructure and melt elasticity. Since ordering phenomena take place in melts of diblock copolymers until an equilibrium morphology is achieved, the storage modulus G′ of diblock copolymer melts increases with time up to a steady-state value.