Phase Separation in Terms of Thermodynamics and Kinetics

Polymer-polymer systems exhibit phase behavior similar to other mixtures, such that an initially uniform system separates into two or more phases as a result of small change in thermodynamic variable. Two mechanisms can be envisioned to explain this phenomenon nucleation and growth (NG), and spinodal decomposition (SD). 

Nucleation is initiated by local fluctuations of concentration within a metastable region. The activation energy of nucleation depends on the value of the interface energy required to create a nucleus. The droplet grows by diffusion of macromolecules into the nucleate domains. The natural form of the phase separation through NG mechanism is the sea-island type. 

In the unstable region, the concentration fluctuations are delocalized and there is no thermodynamic barrier to phase growth. Thus, separations that take place spontaneously lead to long range phase separation. This process is called spinodal decomposition (SD). In this mechanism, decomposition starts with a co-continuous structure and gradually shifts to a droplet morphology because of the breakdown of the continuous structure [41]. 

In a dynamic and cross-linkable system, such as the curing of a thermoset that contains a thermoplastic, the phase separation is more complicated than nonreaction system. The phase separation is controlled by the competing effects of thermodynamics and kinetics of phase separation and cure rate of thermoset resin (i.e. time dependent viscosity of the system). 

The extent of phase separation can be measured directly by the scanning electron microscope (SEM), transmission electron microscope (TEM), optical microscope, and light or X-ray scattering technique. It is also investigated indirectly by measuring certain physical properties, such as glass transition temperature. 

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