INTERFACIAL-TENSION COEFFICIENT FROM THE RETRACTION OF ELLIPSOIDAL DROPS

A new method for the determination of the interfacial tension coeffici ent between two immiscible fluids is proposed. The method is particula rly useful for the binary polymer blends. The deformed drop retraction method, DDRM, makes it possible to determine the dynamic interfacial tension coefficient, nu, from the time evolution of a distorted fluid drop toward its equilibrium form. Analysis of this interfacial tension -driven process led to a theoretical relation between the shape retrac tion rate and the system's geometrical and rheological characteristics . Measurements of either low viscosity model systems or high viscosity industrial polymer mixtures led to a good agreement with values obtai ned from the widely used breaking thread method. DDRM enables to measu re nu in polymeric blends of commercial interest-the high viscosity sy stems that would be very difficult to characterize by other techniques . Furthermore, for the first time it is possible to follow the time de pendence of the interfacial tension coefficient, thus unambiguously de termine the dynamic and equilibrium values of nu(12). For example, in low density polyethylene blends with polystyrene, LDPE/PS, nu decrease d with the polymer-polymer contact time, t(c), from nu = 6.9 mN/m at t (c) = 12 min, to nu = 5.2 mN/m at t(c) greater than or equal to 75 min -the latter may represent the true thermodynamic equilibrium value, nu (12). However, it is not clear whether such a reduction is exclusively due to the thermodynamically driven migration of chain-ends, low mole cular weight fractions and additives, or by the thermal degradation as well. The contact time dependence of nu explained some of the differe nces reported for the data obtained using different measurement techni ques, viz. pendant drop, capillary breakup, or ellipsoid retraction te chniques. (C) 1997 John Wiley & Sons, Inc.