Abstract
The birefringence in steady shear flow was calculated for a triblock copolymer of an ABA type. The calculations were carried out for the Rouse model and the tube model. It was shown that the stress-optical law would not hold valid for a triblock copolymer. An apparent stress-optical coefficient Cσ=Δn sin 2χ/2σ was evaluated for the case where only the B segments were optically anisotropic and contributed to the birefringence. Here Δn is the birefringence, χ is the extinction angle, and σ is the shear stress. When the content of B component, ξ, was small, Cσ at the limit of low rate of shear was 1.5 times as large as the average stress-optical coefficient evaluated from ξ. The factor decreased and approached unity with increasing ξ. The quantity Cσ evaluated from the tube model decreased with increasing rate of shear. These features of birefringence, if confirmed experimentally, would prove that the contribution to stress of a polymer segment depends on its position along the chain and is the larger as it is closer to the center of the chain.
