A rheo‐infrared‐dichroism study of sheared molten polystyrene fractions

The development of orientation in molten narrow molar mass fractions of polystyrene during shear flow in the non‐Newtonian shear rate range (11–24 s−1) at temperatures between 225 and 275°C has been assessed by measurement of the infrared dichroism. Deviation from linearity between shear stress and orientation was found at high shear stresses in the steady state region. A sudden change in the shear rate caused a change in the shear stress that was one to two orders of magnitude‐state level of the Hermans orientation function (fss) was dependent on temperature, shear rate and molar mass (M): fss = C1–C2/M (where C1 and C2 are constants). The time to reach steady state on the inception of shear was proportional to M1/2. The recovery of the isotropic state after the cessation of shear followed initially a simple exponential law: f ∝ e−t/τ, where t is the time and τ is the relaxation time. The relaxation time showed only a weak molar mass dependence (τ ∝ M0.8), indicating that the decay of orientation was initially governed by processes other than reptation.