Chain orientation in sheared molten poly(ethylene oxide) fractions by measurement of infrared dichroism

Abstract

AbstractRheo‐infrared spectroscopy was used to study the development of orientation of molten narrow molar mass fractions of poly(ethylene oxide) [molar masses between 18,000 and 120,000 g/mol] during non‐Newtonian shear flow at shear rates between 2 and 270 s−1 and temperatures between 75 and 100°C. The steady state degree of orientation [expressed as the Hermans orientation function (fss)] reached a saturation level with increasing shear rate; fss increased with increasing molar mass (M) according to fss = C1 − C2/M (C1 and C2 are coefficients; the latter depended on shear rate and temperature). The coefficient C1 (fss) for a polymer with infinite molar mass took a universal value close to 0.05 for the temperatures and shear rates used. Under large shear stresses, the relationship between stress and orientation deviated markedly from linearity. The time to establish a steady state level of orientation was proportional to M1/2. The recovery of the isotropic state after the cessation of shear could initially be described by a simple exponential relaxation law: f ∝ e, where τρ is the relaxation time. The latter showed a weak molar mass dependence according to τr ∝ M0.6 and an Arrhenius temperature dependence with an activation energy of ∼60 kJ/mol. The relaxation of the shear stress after the cessation of shear was more rapid than the recovery of the isotropic state.