Modeling of the solid-liquid thermally induced-phase-separation (TIPS) membrane formation process.

摘要

The TIPS process is a pathway to the formation of a microporous membrane via phase separation caused by reducing the temperature of a homogeneous polymer solution. Solid-liquid (S/L) TIPS involves phase-separation into liquid and solid phases that form the pores and matrix, respectively, of the microporous structure. The major objective of this thesis is to develop a model that describes the phase separation process and captures the fundamental features of the S/L TIPS process. In addition, this thesis utilizes two experimental approaches to corroborate the model based upon measurements of real-time solidification and final membrane morphology. This model is the first to describe the complete microporous membrane-formation process.; Model equations developed for TIPS casting involve S/L phase separation due to a contact between a hot polymer solution and a planar boundary maintained below the solidification temperature. Allowance is made for cooling at the upper boundary of the hot solution. The model incorporates one-dimensional, unsteady-state heat and mass transfer. Furthermore, a scaling analysis technique that evaluates the relative importance of mass and heat transfer revealed that the mass transfer in most TIPS processes is negligible. Thus, the scaling analysis resulted in a simplified, heat-transfer model. This model is coupled with equations accounting for nucleation and growth of the crystalline solid phase. The resulting composite model is capable of predicting the instantaneous temperature profile, solidified region thickness, porosity and spherulite size.; Two polymer systems, polyvinylidene fluoride (PVDF)-dibutylphthalate (DBP) and isotactic polypropylene (iPP)/dotriacontane (C32H 66), were used for model validation. Microporous films were formed via TIPS processes for each system. The strategy employed for model validation involved the comparison of model predictions with data obtained using ultrasonic time-domain reflectometry (UTDR) to determine the instantaneous position of the interface between phase-separated and non-phase-separated phase during casting of PVDF/DBP films [Metters, 1996]. Also, the model predictions were compared to results obtained using scanning electron microscopy (SEM) and optical microscopy (OM) to determine the final spherulitic size of iPP/C 32H66 films. The latter experiments encompassed two different process (boundary) conditions: controlled cooling rate [Lloyd, 1993] and constant block temperature. The comparisons between model predictions and experimental results were conducted using trend analysis and statistical analysis methods. Results indicated that the model was able to account for a number of important trends including the growth of the phase-separated region. In addition, combined with a kinetic theory, the model can predict final spherulite sizes. Employing a rigorous statistical analysis technique, the model predictions for the spherulite size were compared with experimental measurements for both the controlled cooling rate and constant block-temperature process conditions. The comparison demonstrated that the model is corroborated for the controlled cooling rate condition for high concentrations and the constant block-temperature condition when the block-temperature is equal to or higher than room temperature. Consequently, the current model provides a thorough description of the complete TIPS membrane formation process and can help identify the process conditions that produce desired membrane morphologies.