| When polymer devolatilization is conducted under certain conditions of reduced pressure, the vapor bubbles formed must be removed for effective devolatilization to occur. Until now, it was generally believed that these entrained bubbles rupture thereby releasing their contents while at the surface of the solution. In this dissertation, an alternate mechanism is proposed by which volatile components are extracted from polymer solutions when entrained bubbles are present. This mechanism is based on the idea that mass transfer by molecular diffusion occurs across the wall of a bubble that rests on the surface of the liquid and that the wall (which separates the bubble from the vapor space) thins due to drainage. The problem thus reduces to a hydrodynamic problem in which drainage of the bubble wall occurs through an extensional flow mechanism.; This dissertation is mainly concerned with the problem of the drainage and thinning of a bubble wall at the surface of a liquid pool. The theoretical analysis is focussed on the extensional thinning of the bubble wall that occurs due to gravitational effects and due to an imposed pressure difference across the bubble wall. The Jeffreys' number, Je, here defined as a measure of the relative importance of gravity driven thinning and pressure driven thinning, was found to be the critical parameter which governed the rate and nature of the thinning process. Je varies between 0 {dollar}<{dollar} Je {dollar}<{dollar} 1 with gravity dominating flow as Je approaches unity. Other parameters of importance are the initial size of the bubble and the magnitude of the pressure reduction that occurs. Experimental measurements of film thinning utilized a fiber optic probe to measure the thickness of the bubble wall as a function of time over a wide range of initial bubble sizes and Jeffreys' numbers.; The model presented in this dissertation forces a review of our understanding of foam devolatilization. Heretofore, foam devolatilization has been viewed as a catastrophic process whereas the release of volatiles from bubbles in polymeric solutions is caused by the relatively facile process of diffusion through the wall of a thinning bubble. |
