Polymer-Polymer Interfaces

Polymer-polymer interfaces play an important role in the properties of polymer blends. Most polymer pairs are immiscible, and the interfaces dividing polymer pairs are thin and weak. The presence of such weak interfaces between immiscible phases in blends makes the material very brittle. It is essential to have a fundamental understanding of the structures and properties of polymer–polymer interfaces in addition to knowing many practical methods for compatibilizing polymer blends to achieve the desired properties. While the polymer–polymer interfaces of materials of interest to industry are often very complicated, it is nevertheless helpful to understand the physics of polymer–polymer interfaces of simple model systems.
Definition of the Surface and Interface Tension Coefficients
The surface tension is the reversible work required to create a unit surface area at constant temperature (T), pressure (P) and composition (n)
?i = (?G / ?A) T, P, n
where ?i is the surface tension coefficient of the substance i, G is Gibbs’ free energy of the system, and A is the surface area. In immiscible liquids, interactions between components are located at the physical boundary creating the interface. The energy required to reversibly separate the two liquids is expressed as the work of adhesion:
W= ?1 + ?2 - ?12
Where ?1 and ?2 are surface tension coefficients of neat components and ?12 is the interfacial tension coefficient between the liquids 1 and 2.
Importance of the Interfacial Properties in Polymer Blends
The structure and morphology of immiscible blends depends on many factors among which the flow history and the interfacial properties are the most important. At high dilution and at low flow rates the morphology of polymer blends is controlled by three dimensionless microrheological parameters:
1. The viscosity ratio, ? = ?1 / ?2
Where ?1 is the viscosity of the dispersed liquid and ?2 that of the matrix.
2. The capillarity number*, ? = ?12 d / ?12
Where ?12, and d are respectively the shear stress, and the drop diameter.
*Capillary number : represents the relative effect of viscous forces versus surface tension acting across an interface between a liquid and a gas, or between two immiscible liquids

3. The reduced time, t* = t y / ?
Where t is deformation time and y is the rate of shear.
Thus, the interfacial and rheological properties are the keys for the morphology development in polymer blends, which in turn is the controlling factor for their performance. To improve performance of immiscible blends, usually they need to be compatibilized.
There are three aspects of compatibilization:
(1) Reduction of the interfacial tension that facilitates fine dispersion.
(2) Stabilization of morphology against its destructive modification during the subsequent high stress and strain processing (e.g., during the injection molding).
(3) Enhancement of adhesion between phases in the solid state, facilitating the stress transfer, hence improving the mechanical properties of the product.
Compatibilization can be carried out either by adding a compatibilizer to a polymer blend, or prepared during the reactive processing or blending. During the latter process the compatibilizing species are chemically formed in situ, directly across the interface.
Theoretical Aspects of the Interface
In the case of binary immiscible polymer blends mixing two polymers usually results in an immiscible system, characterized by a coarse, easy to alter morphology, and poor adhesion between the phases. These blends have large size domains of dispersed phase and poor adhesion between them. As a result, their performance is poor and irreproducible. In particular the impact strength, maximum strain at break, and the yield strength are affected.
Most incompatible polymers show interfaces in the range of 1–50 nm, depending on compatibility. During the mixing process, the incompatible polymers interpenetrate at temperatures above the glass transition. The interfacial width generally increases in time, reaching an equilibrium value, which is determined by the Flory–Huggins interaction parameter ?. From a detailed knowledge of the interaction parameter one can expect to obtain a better understanding of polymer miscibility. Furthermore, the ?-parameter is also correlated to the interface width which allows to tailor specific interfaces for special uses. For polymer blends the free Gibbs mixing energy can be described by a lattice model. The single A- and B-units are considered to have the same volume and they occupy the cells of a regular lattice. In general the free mixing energy can be formulated like ?G = ?H – T?S
Here ?G is the free mixing energy, ?H the mixing enthalpy, T the temperature and S the mixing entropy. The free mixing energy is thus consisting of two parts, one is the enthalpic part and the second is the entropic part. Following the approach of Flory and Huggins, the two contributions can be expressed by: