Fundamentals of Rheology

In the last section we showed the fundamentals of rheology, the theory that interrelates stress and strain, flow and deformation. This section deals with the practical behavior of solid polymers, namely, the mechanical properties. One has initially to differentiate between properties that are measured over short periods and those measured over long periods. Needless to say, the rate of measurement and the temperature dictate the type of performance and the magnitude of resulting data. 

The second chapter deals with the fundamentals of rheology and provides definitions of all the basic rheological parameters. It dwells on the non-Newtonian character of filled polymeric systems and explains the various anomalies that are encountered during the flow of viscoelastic materials. Various viscoelastic phenomena are depicted and these give an idea about the complexities involved in the flow of polymeric materials, which gets further complicated in the presence of fillers. 

Covas, J. A., et al., Rheological Fundamentals of Polymer Processing, Kluwer, 1995. 

The measurement of rheological properties of the PLFNCs in the molten state is crucial in order to gain a fundamental understanding of the nature of the processability and the structure-property relationships for these materials. 

The dispersion stability, rheology, and consolidation of numerous aqueous and non-aqueous Si3N4 suspensions have been studied extensively [251-257]. Recently a novel class of dispersants for Si3N4 powders in non-aqueous media has been designed and its interactions with the powder surface have been characterised systematically on the basis of surface chemistry and fundamentals of colloidal stabilisation [255, 258]. 

Dispersion behaviour in systems with liquid/liquid or liquid/gas interfaces (i.e. droplet or bubbles) has traditionally been described in terms of rheological properties, wetting properties, including contact angle and interfacial tensions, or phase behaviour and stability measurements. Direct force measurements provide a means to fundamentally probe the interactions between deformable interfaces that significantly impact the dispersion (or emulsion) behaviour. 

Thin solid films of polymeric materials used in various microelectronic applications are usually commercially produced the spin coating deposition (SCD) process. This paper reports on a comprehensive theoretical study of the fundamental physical mechanisms of polymer thin film formation onto substrates by the SCD process. A mathematical model was used to predict the film thickness and film thickness uniformity as well as the effects of rheological properties, solvent evaporation, substrate surface topography and planarization phenomena. A theoretical expression is shown to provide a universal dimensionless correlation of dry film thickness data in terms of initial viscosity, angular speed, initial volume dispensed, time and two solvent evaporation parameters. 

G Marrucci. Microrheological modelling. In JA Covas, JF Agassant, AC Diogo, J Vlachopoulos, K Walters, eds. Rheological Fundamentals of Polymer Processing . NATO ASI Ser. London Kluwer Academic Publishers, 1994,p 37... 

The stress in viscoelastic liquids at steady-state conditions is defined, in simple shear flow, by the shear rate and two normal stress differences. Chapter 13 reviews the evolution of both the normal stress differences and the viscosity with increasing shear rate for different geometries. Semiquantitative approaches are used in which the critical shear rate at which the viscosity starts to drop in non-Newtonian fluids is estimated. The effects of shear rate, concentration, and temperature on die swell are qualitatively analyzed, and some basic aspects of the elongational flow are discussed. This process is useful to understand, at least qualitatively, the rheological fundamentals of polymer processing. 

Luong, M.P. Reflections on Triaxial Testing, Rheology and Flowability, Silos, Fundamentals of Theory, Behaviour and Design Brown, C.J., Nielson, J., Eds. E. FN Spon London and New York, 1998 65-75Ch. 2.4. 

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