Semester

Summer

Date of Graduation

2005

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Rakesh Gupta.

Abstract

Light Emitting Devices (LEDs) containing conjugated polymers are fabricated using ink jet printing. A common problem in the processing of these materials is that the Newtonian viscosity of the polymer solution is not sufficient to describe the jetting performance, because the molecular weights and concentrations employed are such that the resulting solutions are elastic. These differences in fluid elasticity levels cannot be measured using traditional techniques like dynamic mechanical experiments or the first normal stress difference in shear, but they strongly impact the jetting behavior of the liquid. Here, a polystyrene-DECALIN system has been formulated, and the level of elasticity has been varied by varying the polymer concentration, molecular weight and molecular weight distribution. Although the shear viscosity of these solutions was as low as 3 mPa-s, elasticity levels (as quantified by the Trouton ratio) and differences in elasticity levels could be determined by making extensional viscosity measurements. The measurements themselves were made using a modified version of the extensional viscometer introduced by Agarwal and Gupta in 2002.;The jetting behavior of these liquids was examined with the help of two jetting set-ups, one located at the DuPont Experimental Station and the other that was locally-fabricated; both could supply a liquid drop on demand. It was found that a liquid with a high Trouton Ratio (higher molecular weight, higher concentration, use of a polydisperse polymer) was more likely to form a long tail and would be difficult to detach from the jet port; contrarily, a liquid with a low Trouton Ratio (a Newtonian liquid or one containing a low molecular weight polymer or a low concentration of monodisperse polymer) is more likely to form a short tail, give satellite drops and detach cleanly from the jet port. To obtain ideal jetting behavior, therefore, one needs a solution with a Trouton Ratio or fluid elasticity level that is high enough that satellite drops are not formed but is low enough that the drop separates easily and cleanly from the jet port.

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