Kinetics of pressure-induced phase separation in polymer solutions: A time- and angle-resolved light scattering study

A unique high-pressure experimental system which permits simultaneous measurement of temperature, pressure and scattered light intensities during phase separation in polymer solutions has been developed. The system is operable at pressures up to 70 MPa and temperatures up to 473 K. The scattering cell is made of two flat sapphire windows with a path length of 250 {dollar}mu{dollar}m, and the scattered light intensities are monitored over the angle range from 0 to 30{dollar}spcirc{dollar} with a data acquisition rate of 3.2 ms/scan. The phase separations can be induced by pressure quenches at various rates up to 2000 MPa/sec.; Experiments were conducted with polystyrene/methylcyclohexane solutions at critical and off-critical concentrations. The solutions were subjected to a series of pressure quenches with different penetration depth into the region of immiscibility and the subsequent evolution of phase separation was monitored by angular and the time evolution of the scattered light intensities. Pressure quenches in solutions at critical polymer concentrations led to spinodal decomposition, which was displayed by a maximum (i.e., the spinodal ring) in the angular dependence of the scattered light intensities. The duration of the ring collapse was observed to depend on the quench depth and ranged from 3 to 160 seconds for quench depth in the range from 2 to 0.5 MPa, respectively. Pressure quenches in solutions at off-critical concentration led to phase separation by nucleation and growth for shallow quenches, as reflected by the absence of a maximum in the angular variation of the scattered light intensities, but to spinodal decomposition for deep quenches.; The characteristic wavenumber q{dollar}rmsb{lcub}m{rcub}{dollar} corresponding to the maximum scattered light intensity I{dollar}rmsb{lcub}m{rcub}{dollar} was observed to be non-stationary and moved toward lower wavenumbers with time for all quenches leading to spinodal decomposition. For the data corresponding to the early stage of phase separation, calculations of the apparent diffusivity based on the linear theory gave values in the range of 10{dollar}sp{lcub}-8{rcub}{dollar} to 10{dollar}sp{lcub}-10{rcub}{dollar} cm{dollar}sp2{dollar}/sec. For the data corresponding to the late stage of phase separation, power law approximation was used to analyze the evolution of q{dollar}rmsb{lcub}m{rcub}{dollar} and I{dollar}rmsb{lcub}m{rcub}.{dollar} Also, dynamic scaling hypotheses were tested for the structure factor.