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Abstract
PTFE colloidal dispersion is one of the primary product of PTFE end-used materials. PTFE colloidal particles are highly crystalline, and generally, are ellipsoidal in shape. These ellipsoidal particles undergo the flow-induced orientation, and take on flow-induced birefringence in a shear field. When a dispersion is unstable a shear field can induce an increase in aggregation rate. In this dissertation, how these rods orient, and how these colloidal particles aggregate in shear fields are studied.
A shear field can induce an ellipsoidal particle to orient. The degree of the orientation of such a rod can be described by an order parameter, and determined from the measurement of its birefringence signal. When the volume fraction of particles exceeds a critical value, a spontaneous phase transition will occur. In 1930's, Peterlin calculated the degree of the orientation of a single rod. In 1980's, Doi & Edwards calculated the degree of the orientation of rods in a concentrated solution and predicted the occurrence of a spontaneous phase transition. In this dissertation, the degree of the orientation of the rods with various aspect ratio are experimentally determined, and compared to the theories by Peterlin, and Doi & Edwards. A spontaneous phase transition is found when the rod concentration is higher than a critical value. And a simple model is proposed to describe the orientational phenomenon, and to predict the spontaneous phase transition.
A shear field can induce aggregation both in the dispersion bulk and at the air/water interface if a system is unstable. The shear-induced aggregation in a bulk has been well described by DLVO theory (Derjaguin, Landau, Verwey and Overbeek). However, PTFE colloidal particles are air-wet, and behave quite differently at the air/water interface. At the air/water interface, colloidal particles will partially lose their repulsive interaction, and DLVO theory will not be valid any more. In this dissertation, both the aggregation rates at an air/water interface and in the PTFE dispersion bulk are experimentally determined at various shear rates and temperatures. It is found that the area of the air/water interface plays a prominent role in its stability, and that the addition of oil could greatly improve the stability of PTFE dispersions. It is also found that both the zeta potential and surface charge density are increased after adding oil. Based on the calculated interfacial energies between bulk PTFE and various oils, it is shown that the oil would completely wet PTFE in the presence of water. Based on the FTIR spectra, it is shown that the oil would wet PTFE colloidal particles. Models are proposed to explain the enhanced stability.
