Dynamics of Spinodal Decomposition in Polymer Solutions near a Glass Transition

Abstract

A model based on the Cahn−Hilliard formalism is used to simulate structure formation in thermally quenched polymer solutions in the vicinity of a glass transition. Simulation results for the system poly(methyl methacrylate)/cyclohexanol are correlated with light scattering and microscopy measurements recently reported in the literature. Good qualitative and quantitative agreement with experimental observations indicates the applicability of current free-volume and thermodynamic theories to quantify the dynamics near a glass transition. The model accurately predicts late-stage scaling exponents, and results show that the early stage of phase separation occurs on time scales ≪ 1 s. Pore growth rates based on zero-free-parameter calculations agree well with experimental observations. Simulations also show that quenching a phase-separated structure below a glass transition can result in the formation of a secondary droplet structure within the polymer-rich phase, prior to freezing-in of the preexisting morphology. Such secondary structures, which have also been observed experimentally, appear to have little or no effect on the calculated structure factor, which is also consistent with light-scattering measurements.

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

“…As shown from gray-scale intensity plots of the two-phase morphology depicted in Figure 7, the structure grows by one of two mechanisms, (1) coalescence, 48 where solvent-rich pores impinge on one another to form a larger pore, or (2) evaporation-condensation (Ostwald ripening), 49 where larger pores in the polymerrich matrix grow at the expense of smaller pores that are eventually reabsorbed into the surrounding matrix. As also found in our simulations on a binary polymer solution, 21 these two coarsening mechanisms may occur simultaneously. Figure 7 is an example of an off-critical quench on the polymer-rich side which results in the formation of a dispersed solvent-rich phase surrounded by an enriched polymer matrix.…”

“…21 The ternary mobilities, M ij , are given in terms of the binary friction coefficients, ij , by 29,25 M 11…”

“…The equality, M 12 ϭ M 21 , results from the definition of the component potentials and is not a general feature of ternary mobility coefficients.…”

“…As noted earlier, 21 care needs to be taken in the choice of the mesh size, ⌬x*, to avoid possible discretization artifacts such as mesh-size dependence and artificial pinning effects. Since the scaling parameter, a, is uniquely determined by eq.…”

“…16 -18 A few works have focused on polymer solutions. 19 -21 Whereas all these studies make qualitative comparisons between predictions from Cahn-Hilliard theory and experimental observations, our recent work on a binary polymer solution 21 has been the first to make quantitative predictions of the late-stage structure growth kinetics. Good agreement was found between calculated growth rates and growth rates determined from light scattering measurements.…”