Extensional Rheology of Unentangled Linear Polymer Melts

Abstract

Under fast extensional flow, polymer chains are strongly oriented/stretched to exhibit highly nonlinear rheology. Strain-rate hardening due to the finite extensible nonlinear elasticity (FENE) was established several decades ago for entangled branch-on-branch polymers, and molecular models incorporating this FENE effect were developed on the basis of the concept of topological constraint for motion of mutually uncrossable chains. Nevertheless, for entangled linear polymers, recent experiments revealed that the hardening occurs in solutions but not in melts having the same entanglement density, which led to improvement of the models through incorporation of segmental friction (ζ) reduction in a stretched/co-aligned environment; ζ-reduction is absent in solutions because solvent molecules offer an isotropic environment for the polymer segments. On the basis of this knowledge, very recent studies focused on the extensional nonlinearities of the simplest material, unentangled melts of monodisperse linear polymers, to confirm that ζ-reduction occurs also in the absence of entanglement. Those studies further suggested an importance of flow-induced changes of the thermal Brownian force in addition to the FENE effect and ζ-reduction. This chapter outlines a theoretical framework needed to describe/understand these molecular aspects of unentangled melts and further discusses its consequence to the nonlinear rheology of entangled polymers