Non-Universal Features in Uniaxially Extensional Rheology of Linear Polymer Melts and Concentrated Solutions: A Review

Rheological behavior of polymeric liquids has been considered to be universally determined by a few time-independent molecular parameters (such as the Rouse relaxation time and the entanglement number per chain) irrespective of the chemical structure of the polymer and the solvent (if any) and the polymer concentration. This universality has been supported by extensive experiments under shear conducted over 50 years and the successive theoretical development of coarse-grained molecular models. Nevertheless, recent experimental and theoretical studies have revealed the breakdown of this universality in the extensional flow behavior of both entangled and unentangled polymers. A key concept resulting from those studies is the reduction of segmental friction of highly oriented/stretched polymer chains in melts under fast extensional flow. The magnitude of this friction reduction changes with the chemical structure of the polymer and solvent as well as the polymer concentration even if the time-independent molecular parameters remain the same, which results in the non-universality in the extensional behavior. This review article summarizes these experimental and theoretical findings for linear polymers under uniaxially extensional flow, and discusses remaining problems in particular for the concept of friction. [Display omitted]