Viscoelastic Coalescence Of Thermotropic Liquid Crystalline Polymers: The Role Of Transient Rheology
The coalescence in air of two polymeric drops into a single drop (also referred to as sintering) was investigated for two thermotropic liquid crystalline polymers. Initial coalescence via elastic contact was ruled out based on the magnitude of the equilibrium compliance values and the process was, therefore, believed to be driven by surface tension and resisted by means of viscous)low. Remarkably the viscous coalescence model developed for Newtonian fluids (an extension of the Frenkel and Eshelby approach) agreed well under some conditions of temperature with coalescence data (i.e., observation of neck growth under a microscope). On the other hand the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model (UCM) assuming steady state stresses always underpredicted the rat( of coalescence. The viscous neck growth model using the UCM constitutive equation was extendcd to the transient stress case in order to incorporate the slow growth of viscosity at the startup of flow. The unsteady state UCM approach represented a qualitative improvement over the Newtonian and steady state UCM formulations because it predicted accelerated coalescence, relative to the Newtonian model, by increasing the relaxation time. However, the model was unable to quantitatively predict the experimental coalescence rates, as it overpredicted the acceleration of coalescence. (c) 2005 The Society of Rheology.