The kinetics of pressure-induced phase separation (PIPS) in solutions of polyethylene (M
w
= 108 000, PDI = 1.32) in n-pentane has been studied using time- and angle-resolved light scattering.
Controlled pressure quench experiments were conducted at different polymer concentrations (0.49, 0.97,
2.07, 2.8, 4.1, 5.0, and 10.8% 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 were 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”. The 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 new phase formation and growth is shown to be
relatively short. The late stage of phase separation is entered within seconds. For quenches leading to
spinodal decomposition, the characteristic wavenumber q
m corresponding to the scattered light intensity
maximum I
m is observed to be nonstationary, moving to lower wavenumbers after a very short elapsed
time. The growth of domain size is observed to follow power-law-type scaling with q
m ∼ t
-α and I
m ∼ t
β
with β ≈ 2α. In the intermediate and late stages, the domain size is found to grow from 4 to 14 μm
within 5 s. Analysis of the early stage of phase separation according to Cahn−Hilliard theory shows that
the diffusion coefficients are in the range of (1−9) × 10-12 m2/s and depend on the quench depth.