SEPARABILITY CRITERIA FOR ENTANGLED POLYSTYRENE SOLUTIONS USING FLOW BIREFRINGENCE

摘要

Step shear strain experiments were performed using several entangled polystyrene (PS) solutions to investigate factorability requirements of the non-linear relaxation modulus, G(t,γ) [≡σ_(12)(t,γ)/γ]. A phase modulated flow birefringence apparatus was used to measure optical equivalents of shear stress (n_(12)) and first normal stress difference (n_(11)-n_(22)) in a plane-Couette shear flow geometry. For all polymers studied, a separability time λ_k was identified beyond which the optical equivalent of G(t,γ) [B(t,γ) ≡ n_(12)/γ ≡ G(t,γ)xC] could be factorized into separate strain and time dependent functions. In every case, λ_k exceeded longest Rouse relaxation time τ_R and found to be of the order of terminal relaxation time τ_(d0). These findings could help explain previous experimental observations of delayed factorability and non-factorable relaxation moduli in well entangled polymer solutions and melts.