Rheology of Entangled Polymers Not Far above Glass Transition Temperature: Transient Elasticity and Intersegmental Viscous Stress

Abstract

This work studies the transient rheological responses of different entangled melts to startup deformation at unconventionally high rates, which can be accessed by working at sufficiently low temperature, e.g., at and below 130 °C for polystyrene. When the rate is as high as the second crossover frequency ωe observed from the small amplitude oscillatory shear (SAOS) data for storage and loss modulus G′ and G″, there is sizable viscous stress that dominates the initial mechanical response. It is shown based on the various polymer melts including PS, PMMA, SBR, PC, and PS mixtures that this viscous component of the stress cannot be neglected when characterizing such fast deformation in either extension or shear. This sizable frictional addition to the rubber elasticity component of the initial stress is founded to be preceded by solid-like deformation. The remarkable transient elasticity can be characterized by a modulus Ginterseg that grows well beyond the magnitude of the melt plateau modulus GN0 as the temperature lowers toward the glass transition temperature Tg. In the case of PS, Ginterseg reaches a level of 300 MPa at 110 °C and reduces to 2.0 MPa, i.e., 10GN0, at 130 °C. The initial elasticity is short-lived because the melts quickly transition to a state of viscous flow at a strain of just a few percent. The magnitude of the viscous stress at the solid-to-liquid transition defines a yield stress. This yield stress is found to scale with the applied rate in a power law, agreeing with the dependence of G″ on frequency ω from the SAOS data. Moreover, the intersegmental effect, i.e., the transient elasticity, is shown to also take place in unentangled and barely entangled PS melts as well as branched polyisoprene melts.