Rheology for Better Sol-Gel Fiber and Film Formation
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RHEOLOGY FOR BETTER SOL-GEL FIBER AND FILM FORMATION C. W. MACOSKO, M. L. MECARTNEY, AND L. E. SCRIVEN Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Flow behavior of a liquid or suspension depends on how stress varies with strain rate, strainrate rotation, and strain history, as well as the progress of evaporation, extent of reaction, and degree of aggregation in sol-gel systems. Rheological methods suitable for measuring flow behavior are .summarized. Examples of measurements and microstructural observations by transmission electron microscopy made during gelation of four sol-gel systems are presented. The relation of rheological response to microstructure is discussed. INTRODUCTION During manufacturing of coated films continuously in a steady-state process like die coating or slot coating, the liquid being coated is first extended in one direction inside the die as the incoming flow is spread out into a layer proceeding to the exit slit. Then it is extended in a second direction in the deposition zone as the exiting flow is accelerated to the speed of the substrate being coated. Meanwhile it is narrowed in a third direction. The liquid is also sheared inside the die and in the deposition zone, because liquid does not slip at the die surfaces or the substrate surface. In drawing fibers, the liquid being processed is extended in one direction while being narrowed in the other two; it is sheared inside the passages and orifice of the die but very little within the fiber. The liquid's drawability, or spinnability, depends on its response to extension. Coating or spinning is followed by solidification by drying and reaction, which induce shrinkage, the converse of extension, and generally shear as well. Rheology
The ways that liquids and not-yet-rigid solids respond to extensional, or compressional, and shearing stresses, or strains, are the subject of rheology [1,2]. Rheology pertains not only to steady-state formation of films and fibers, but also to batch processing like spin coating and dip coating, which may be dominated by transient effects. The way that a liquid responds to shear and extension is a matter of its microstructure. When compact molecules of nanometer scale constitute a liquid, local rearrangement by thermal motions is ultrafast (e.g. nanoseconds and quicker) and so is relaxation into local thermodynamic equilibrium structure. Then viscosity, the ratio of resulting stress to the responsible shear rate or extension rate, depends only on temperature, pressure and composition, and not on deformation rate. The liquid is Newtonian. In contrast, when colloidal particles of sub-micron or micron scale densely populate a liquid suspension, Brownian movement is sluggish, thermodynamic equilibrium may be practically unattainable, and relaxation into local mechanical equilibrium structures is little faster than flow itself (e.g. time scales of fractions of a second and up) [3]. Then viscosity depends directly on deformation rate and is typically
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