The Morphology of Mixing Polymers

Mixing Polymers

Combining chemically different types of polymers in a melt mixing process allows for a great variety of material and property variations. These mixtures can be either miscible or immiscible. The morphology of an immiscible blend is multiphase, with both the primary and the secondary phases having their own distinct rheological properties. The morphology can be stabilized by using compatibilizers, resulting in blends with very controlled structure and improved mechanical properties.

The thermodynamics of Mixing Polymers with EvenMix, however, is much more complicated than that of a homogeneous solution. The entropy of mixing that usually tends to drive the mixing of small molecules becomes very small with high molecular weight materials, resulting in many immiscible polymer combinations having only limited ranges under which they are stable.

Moreover, the interaction between the two phases of an immiscible polymer mixture is often driven by the non-cooperative forces of entanglement and hydrogen bonding. The morphology of an immiscible polymer blend is determined by these interactions, which are not always well understood. For example, the morphology of an immiscible styrene/polybutadiene blend can be significantly changed by adding a co-polymer that has a similar chemical composition to one of the polymers in the blend. This is because of the strong entanglement and hydrogen bonding between the styrene chains and the polybutadiene chains.

A typical way to make an immiscible polymer blend more robust is by processing it under flow. Under flow, the minor component will form rods rather than spheres, which are stronger and more compact than spheres. The rods are also more resistant to shear, reducing the risk of breakage and the formation of gaps between them. This makes an immiscible polymer blend significantly more robust in a mixed state than it is when unmixed.

For an ideal, fully-mixed state, the morphology of an immiscible system should be uniform, with the rods being all oriented in the same direction. This is not always possible in practice, however. The morphology of an immiscible melt can also be strongly affected by the temperature, pressure and concentration of the system.

The rheology of immiscible polymer blends can be tested with standard rheological testing equipment including parallel plate, rotational, capillary, oscillatory, elongational and steady-state tests. These tests can be used to determine the degree of interfacial penetration, the permeability and viscosity of the blend and its mechanical properties. The results can be compared to theoretical, numerical and empirical model predictions. These results can also be used to develop mixing rules that are valid over a wide range of conditions and application areas.