Pressure-induced phase separation in polymer solutions:: Kinetics of phase separation and crossover from nucleation and growth to spinodal decomposition in solutions of polyethylene in n-pentane

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- arid 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 he relatively short. The late stage of phase separation is entered within seconds. For quenches leading to spinodal decomposition the characteristic wavenumber qm corresponding to the scattered light intensity maximum I, 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) similar to t(-alpha) and I(m) similar to t(beta) with beta approximate to 2 alpha. In the intermediate and late stages, the domain size is found to grow from 4 to 14 mum 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) x 10(-12) m(2)/s and depend on the quench depth.