A spectroturbidimetric study of the phase separation kinetics of polymer-supercritical fluid mixtures.

摘要

The objective of this research is to develop a fundamental understanding of the kinetics of phase separation and the microstructures produced via pressure perturbations in SW solvent-polymer solutions. To unambiguously interpret the growth mechanisms of the second phase, an understanding of the pretransitional, single-phase, characteristics of a polymer in an SW environment will be developed utilizing both experiment and polymer solution scaling theories. This research distinguishes itself from earlier studies of polymer solution phase separation by operating with an SCF-polymer solution rather than a liquid solvent-polymer solution. The “tunability” of the physico-chemical properties of a SW solvent provides a degree of flexibility not available with a constant-density liquid system. Extensive regions of chemical potential space can be effectively probed in an SCF-polymer solution by operating over a wide temperature and pressure range. Another important advantage of an SCF-polymer solution is that a near instantaneous, global pressure perturbation can be used to form the second phase with an SCF-polymer solution which is distinguished from temperature quench techniques used to traverse phase boundaries in constant-density, liquid solutions. The pressure quench is a flexible means for crossing a phase boundary and allows both the “starting” pressure and the “ending” pressure to be adjusted relative to the phase separation pressure.; Experimental data is presented in this dissertation on the kinetics of phase separation and the microstructures produced via pressure quenches in supercritical fluid (SCF) solvent-polymer solutions. The evolution of the second phase is characterized with a time-resolved multiple wavelength turbidimetric: technique used to determine the extinction coefficient of the phase separating system as a function of time and wavelength. The turbidity spectra are interpreted using Mie theory to determine the size and growth of the polymer-rich domains that are obtained after perturbing the system with a pressure quench. This technique is contrasted to time resolved small angle light scattering studies of pressure-induced phase separation. Original data are presented for several polyethylene-propane systems showing the impact of polymer concentration and molecular weight on the phase separation kinetics of an SCF-polymer mixture.