How dilute are dilute solutions in extensional flows?

e investigate the concentration dependence of the characteristic relaxation time of dilute polymer olutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of ve different molecular weights 1.8 106 M 8.3 106 g/mol with concentrations spanning ve orders of magnitude were dissolved in two solvents of differing solvent quality diethylphthalate and oligomeric styrene . Optical measurements with a capillary breakup xtensional rheometer of the rate of filament thinning and the time to breakup in each fluid are sed to determine the characteristic relaxation time. A criterion for a lower sensitivity limit is ntroduced, in the form of a minimum concentration cmin necessary for experimental resolution of he effects of polymeric viscoelasticity. This criterion is validated by experiment and comparison to umerical calculations with a multimode bead-spring dumbbell model. These calculations also ationalize previous paradoxical observations of extensional thinning in fluid threads of ultradilute olymer solutions in which stress relaxation apparently occurred faster than predicted by the Zimm heory. Above this minimum sensitivity limit we show that the effective relaxation time of oderately dilute solutions 0.01 c /c* 1 in transient extensional flow rises substantially above he fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and bove the Zimm relaxation time computed from kinetic theory and intrinsic viscosity easurements. This effective relaxation time exhibits a power-law scaling with the reduced oncentration c /c* and the magnitude of the exponent varies with the thermodynamic quality of he solvent. The scaling of this “self-concentration” effect appears to be roughly consistent to that redicted when the dynamics of the partially elongated and overlapping polymer chains are escribed within the framework of blob theories for semi-dilute solutions. © 2006 The Society of heology. DOI: 10.1122/1.2357595