Viscoelastic coalescence of thermotropic liquid crystalline polymers: The role of transient rheology

Abstract

SynopsisThe coalescence in air of two polymeric drops into a single drop ͑also referred to as sintering͒ 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 flow.Remarkably the viscous coalescence model developed for Newtonian fluids ͑an extension of the Frenkel and Eshelby approach͒ agreed well under some conditions of temperature with coalescence data ͑i.e., observation of neck growth under a microscope͒. On the other hand, the extension of the Newtonian model to the viscoelastic case by incorporating the upper convected Maxwell model ͑UCM͒ assuming steady state stresses always underpredicted the rate of coalescence. The viscous neck growth model using the UCM constitutive equation was extended 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.

2D Encapsulation in Multiphase Polymers: Role of Viscoelastic, Geometrical and Interfacial Properties

“…Recent papers on ternary systems have suggested that composite droplet formation may be related to the viscosity of the dispersed components . Nemirovski et al studied the phase morphology of a number of three-component (thermoplastic/thermotropic) systems and suggested that the dispersed component A would encapsulate the dispersed component B (in a matrix C) when both the thermodynamics, expressed by means of a positive λ A/B spreading coefficient, and the kinetic effects, expressed by means of a dispersed phase viscosity ratio smaller than 1, acted cooperatively.…”

2D-Sintering Kinetics of Two Model Fluids as Drops

“…It was found that the steady-state UCM model does not describe well the sintering process of all viscoelastic materials. An effort was thus made to develop a model in which the transient rheology was taken into account. , It was concluded that the influence of viscoelasticity was limited to the first step of the coalescence process. For long times, the transient model converges to the Newtonian solution.…”

Sintering Of Polymers: Comprehension, Modeling And Application To Rotomolding Process

“…An effort has been made to develop a model in witch the transient rheology is taken into account [6,7]. An effort has been made to develop a model in witch the transient rheology is taken into account [6,7].…”