Kinetics of pressure-induced phase separation (PIPS) in polystyrene+methylcyclohexane solutions at high pressure

Abstract

Kinetics of pressure-induced phase separation in polystyrene+methylcyclohexane solutions at high pressures (up to 25 MPa) have been studied as a function of polymer molecular weight (50 000 and 700 000), polymer concentration (in the range from 4 to 16% by mass) and the quench depth (in the range of 0.1–2 MPa), using time- and angle-resolved light scattering in a unique high-pressure cell with a path length of 250 μm. The results show that phase separation in solutions at critical polymer concentrations proceeds by spinodal decomposition which is displayed by a spinodal ring or a maximum in the scattered light intensities with angle. The time interval for the observation of spinodal ring was observed to depend on the quench depth. The ring collapse was observed to take place within the range of 3–160 s, shorter times being associated with deeper quenches. Phase separation in solutions at off-critical concentrations was observed to proceeded by nucleation and growth mechanism for shallow quenches (as reflected by the absence of a maximum in the angular variation of the scattered light intensities), but by the spinodal decomposition process for deep quenches. The characteristic wave number q m corresponding to the maximum scattered light intensity I m was observed to be non-stationary and moved to lower wave numbers with time for all quenches leading to spinodal decomposition. The time evolution of q m and I m were observed to obey the power law approximations q m∼ t − α and I m∼ t β . The exponents α and β were found to increase with the quench depth, while however, maintaining a β≅2 α relationship. The scaling characteristics of the structure factor were also analyzed. It was found that for a given quench depth the data at different times could be reduced to a single master curve when normalized with respect to the maximum in the scattered light intensity and the corresponding wave number. Calculations of the apparent diffusivity, based on the estimated values for the early stage characteristic wave number q m0, gave values around 10 −9 cm 2/s.