The development of h Homogeneous nucleation rate model for thermoplastic foams based on a molecular partition function and Fickian diffusion
Department of Chemical Engineering, Chemistry and Environmental Science
Doctor of Philosophy
Knox, Dana E.
Barat, Robert Benedict
Loney, Norman W.
Volz, William E.
An improved homogeneous nucleation rate model for thermoplastic foams has been developed. This model does not rely on experimentally determined parameters and only uses pure component physical property data and a binary diffusion coefficient. This model, like those derived from classical nucleation theory, is made up of two parts, one that determines the size of the energy barrier in the nucleation process and one that estimates the forward rate of the process. A statistical-mechanical approach was used to create an energy term that is based on a molecular partition function. In this approach, the bulk phase (polymer and blowing, agent mixture) of the system is treated as a regular solution and the potential energy of this phase is estimated from regular solution theory. The rate component of the model is obtained by utilizing a diffusion-based model derived from Fick's law, Additional approaches including a diffusion only based approach, a fluctuation theory based approach, and a lattice model based approach were all unsuccessfully investigated.
The predictions obtained from the model have been compared to a polymethylmethacrylate/carbon system with limited success. Althoulgh the model results do not match the experimental data, there is a significant improvement over the results obtained from current models available in the literature. The data is limited to the one system described above as there was significant evidence of heterogeneous nucleation in most other systems identified in the literature.
Finally, the work also provides a comprehensive review of the literature on foam nucleation in thermoplastics. The review covers both homogeneous and heterogeneous models and looks at results obtained experimentally and theoretically. This review clearly identifies the need for an improved nucleation model that is not dependent on experimental parameters like the one developed in this work.
njit-etd2002-044 (248 pages ~ 15,254 KB pdf)
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Created August 12, 2003