Kinetics of pressure-induced phase separation (PIPS) in solutions of polydimethylsiloxane in supercritical carbon dioxide: crossover from nucleation and growth to spinodal decomposition mechanism

Abstract

The kinetics of pressure-induced phase separation (PIPS) in solutions of polydimethylsiloxane ( M w=94 300; PDI=2.99) in supercritical carbon dioxide have been studied using time- and angle-resolved light scattering. Controlled pressure quench experiments were conducted at different polymer concentrations (0.38, 0.9, 1.9, 2.5, 3.9, 5.5% by mass) to determine both the binodal and spinodal envelopes, and the critical polymer concentration. At each concentration, a series of rapid pressure quenches with different depths of penetration into the region of immiscibility was imposed and the time evolutions of the scattered light intensities were followed to determine the pressure below which the mechanism changes from ‘nucleation and growth’ to ‘spinodal decomposition’. This crossover is identified from the characteristic fingerprint scattering patterns associated with each mechanism. The spinodal decomposition process is characterized by the formation and evolution of a spinodal ring during phase separation that leads to a maximum in the angular variation of the scattered light intensity. The nucleation and growth mechanism is characterized by the absence of such a maximum and the continual decrease of the scattered light intensities with increasing angles. The time scale of PIPS is shown to be relatively short. The late stage of phase separation is entered within seconds. For quenches leading to spinodal decomposition, the characteristic wave number q m corresponding to the scattered light intensity maximum I m is observed to be non-stationary, moving to lower wave numbers after a very short elapsed time. The growth of domain sizes is observed to follow power-law-type scaling with q m ≈t −α and I m ≈t β with β≈2 α.