Devolatilization of concentrated polymeric solutions in extensional flow

Previous investigations into polymer devolatilization processes have concluded that mass transfer rates in polymer solutions can be increased by increasing the surface area, decreasing the length scale for diffusion, or increasing the diffusion coefficient. These conclusions are limited by the premise that the devolatilization is controlled by molecular diffusion of the volatile component into the second phase, and there has been no effort to understand the role of convection in the devolatilization. In this dissertation the contribution of fluid convection in the devolatilization of a concentrated polymeric solution when the solution is subjected to extensional flow is identified and the model equations developed. The validity of the equations is verified with experimental data from polymer devolatilizations occurring in conjunction with two elongational deformation modes.; An enhancement to the mass transfer rate when compared to an undeformed sheet results by retaining the convective diffusion terms when solving the continuity equation of a volatile species in a viscous polymer. This enhancement is presented as a factor multiplied to the dimensionless time {dollar}{lcub}Dtsb{lcub}exp{rcub}{rcub}over{lcub}deltasbsp{lcub}0{rcub}{lcub}2{rcub}{rcub}{dollar} that results from solving the problem of unsteady mass transfer from a flat sheet. Since the convective terms contain an expression for the fluid velocity, the enhancement factor is a function of the flow kinematics, and as a result, the fluid rheology was found to affect the mass transfer. Model equations were developed for planar and uniaxial extensional flow continuous devolatilizations and enhancement factors were developed for various flow kinematics using Newtonian and Power-Law constitutive equations.; The model equations were compared with experimental data from two types of continuous isothermal elongational flow processes. A polyethylene/cyclohexane solution was devolatilized in a planar extensional flow process while a polystyrene/toluene solution was devolatilized in a fiber spinning process. The flow kinematics were independently measured for the fiber spinning experiment, and were observed to be a function of the stretch ratio. Both experiments agreed well with the model equations and support the premise that elongational flow improves the rate of interphase mass transfer.; By identifying an alternative mechanism for polymer devolatilization, this dissertation should serve as an origin for the design of new devolatilization processes that use elongational flow to achieve mass transfer rates previously thought to be unattainable in polymer processing.