Percolation-to-droplets transition during spinodal decomposition in polymer blends, morphology analysis

Abstract

Phase separation kinetics of the off-critical mixture of polystyrene and poly(methylphenylsiloxane) is studied by the time-resolved light scattering and optical microscopy. The results from the light scattering experiments are correlated with the images obtained by the optical microscopic observation in order to find characteristic features of the scattering intensity during the percolation-to-droplets morphology transition. At the beginning of the spinodal decomposition process only a bicontinuous network is present in the system and the light scattering intensity has only one peak. The network coarsens and at the same time small droplets appear in the system resulting in a growth of the scattering intensity at very small wave vectors. When the large network starts to break up into disjoint elongated domains a second peak in the scattering intensity appears. Finally, both peaks merge into a single peak at zero wave vector, indicating a complete transformation of elongated domains into spherical droplets of variable sizes. The comparison of the direct microscopic observations with the light scattering spectra shows that the process of breaking up of the bicontinuous network starts when the growth of the first peak, corresponding to the bicontinuous pattern, becomes very slow (essentially pinned down).

A framework for interpreting the coarsening and structural evolution of two co‐continuous immiscible viscous fluids

“…Experimental studies also show that linear coarsening does not continue indefinitely for all co‐continuous fluid systems. A decrease in the rate of coarsening beyond a critical domain size has been found for a phase separated system and for a number of mechanically mixed systems . This coarsening behavior is illustrated in Figure (solid line; larger dashed lines will be explained later) where the initial linear coarsening is called the primary coarsening regime, and the subsequent slower nonlinear coarsening is called the secondary coarsening regime.…”

“…The evidence to support this hypothesis is actually based on the observations of Demyanchuk and coworkers and of Lopez‐Barron and Macosko at the terminal transition at . Although Demyanchuk found that spherical droplets formed at a lower phase volume fraction of approximately 0.33, at a slightly higher phase volume fraction of approximately 0.35–0.36, stable “elongated” domains (observed in two dimensions) formed.…”

“…At low phase volume fractions, there is clear evidence that a large number of small spherical droplets of size similar to the preexisting co‐continuous structure are formed spontaneously at the primary coarsening transition . These droplets coexist with the remaining co‐continuous structure which continues to coarsen in the secondary coarsening regime.…”

Dynamics of Phase Separation in Polymer Blends Revisited: Morphology, Spinodal, Noise, and Nucleation

“…Finally, we would like to point out that CHC model with FHG free energy is not suitable to describe the final stages of phase separation without proper account of hydrodynamic flows 52–54. It has recently been demonstrated55–57 in experiments and that in late stages of phase separation when the bicontinuous network breaks into droplets (which must occur either due to the finite size effect or due to the wetting properties of the bounding walls) the system evolves according to the coalescence‐induced coalescence mechanism 52–54. In this mechanism any process of domains coalescence induces hydrodynamic flow, which pushes other domains towards each other and induces further coalescence events.…”

Phase Separation in Binary Polymer/Liquid Crystal Mixtures: Network Breaking and Domain Growth by Coalescence-induced Coalescence

“…Finally, a bicontinuous network starts to break and the elongated domains appear in the system which later transforms into the spherical droplets minimizing the free interfacial energy, Figure 3c (150 min). Interestingly (ref 5), breaking of the network shows up in the LS measurements ( Figure 2) by the appearance of two peaks (one coming from the dense system of droplets and one from the bicontinuous network). Very quickly both peaks merge into one peak, which quickly shifts into zero wave vectors.…”