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

Kinetics of pressure-induced phase separation in polystyrene + methylcycloh exane solutions at high pressures (up to 25 MPa) have been studied as a fun ction of polymer molecular weight (50 000 and 700 000), polymer concentrati on (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 mu m. The results show that phase separation in solutions at critical polymer con centrations proceeds by spinodal decomposition which is displayed by a spin odal ring or a maximum in the scattered light intensities with angle. The t ime 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. Phas e separation in solutions at off-critical concentrations was observed to pr oceeded by nucleation and growth mechanism for shallow quenches (as reflect ed by the absence of a maximum in the angular variation of the scattered li ght intensities), but by the spinodal decomposition process for deep quench es. The characteristic wave number q(m) corresponding to the maximum scattered light intensity I,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 approxim ations q(m) similar to t(-alpha) and I-m similar to t(beta). The exponents alpha and beta were found to increase with the quench depth, while however, maintaining a beta similar or equal to 2 alpha relationship. The scaling c haracteristics of the structure factor were also analyzed. It was found tha t 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 sc attered 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. (C) 2 000 Elsevier Science Ltd. All rights reserved.