COMPATIBILIZATION OF IMMISCIBLE POLYMERS
This post is based on information provided by (1) J. Bicerano, Prediction of Polymer Properties, third edition, Marcel Dekker, New York (2002) which is also the source for the image shown below; and (2) J. Bicerano, A Practical Guide to Polymeric Compatibilizers for Polymer Blends, Composites and Laminates, SpecialChem, December 2005.
The morphology of a polymer blend is the outcome of the competition between the drive towards thermodynamic equilibrium and the kinetic barriers that must be surmounted to achieve thermodynamic equilibrium. The morphology expected on the basis of the thermodynamic equilibrium phase diagram is attained in many instances. Kinetic barriers prevent thermodynamic equilibrium from being attained and instead “freeze in” a metastable morphology induced by the processing conditions in many other instances.
Miscibility is defined as the capability of a mixture to form a single phase over certain ranges of temperature, pressure, and composition. Most pairs of polymers are immiscible with each other. Even worse is the fact that immiscible polymers also tend to have less compatibility than would be required in order to obtain the desired level of properties and performance from their blends. Compatibility is defined as the capability of the individual component substances in immiscible blend to exhibit interfacial adhesion.
The compatibilization of immiscible polymers is an important, widespread, and difficult problem. There are two broad types of approaches for solving this problem: (1) Modification of processing conditions, for example by (a) increasing the processing temperature, and/or (b) Increasing the motor speed and/or improving the mixing by some other means. (2) Modification of blend formulation, for example by using additives such as (a) “standard” (premade) compatibilizers, (b) reactive compatibilizers, and/or (c) other substances (such as silica, carbon, or clay nanoparticles) that may manifest a compatibilizing effect under some conditions.
Some techniques [such as 1(a), 2(a) and 2(b), and perhaps in many cases also 2(c)] rely on thermodynamics to “break up” macrodomains and ensure “true” homogeneity of the system. Other methods [1(b) and 2(c)] rely on kinetics to “break up” domains constantly and force the system to remain “approximately” homogeneous in metastable morphologies with domain sizes not exceeding ~1 micron.
Several of these techniques are often combined in practice. For example, it is quite common to increase both the temperature and the shear rate during processing, while also including both a compatibilizer and/or other substances in the formulation.
It is difficult to prescribe a priori which method should be used for any particular problem. Each method has its own advantages and disadvantages. For example:
- If it were practically feasible, increasing the processing temperature to the point where two polymers become miscible would certainly solve thermodynamic incompatibility problems. However, this solution is impractical for many realistic systems in which the transition from a two-phase system to a one-phase system occurs far above the decomposition temperature of one or both components.
- Improving mixing can be relatively easy and straightforward, but the mixture can quickly phase separate into large droplets once shear (a kinetic factor) is removed.
- Compatibilizers (such as short chains of block copolymers or random copolymers) can reduce the interfacial tension to near-zero levels and promote mixing on the nanoscale. However, this effect is limited by the migration kinetics of compatibilizer molecules towards interfaces and can thus be very slow.
- Reactive compatibilizers rely on chemical reactions that take place during processing to attach themselves to immiscible polymers that are being blended and thus compatibilize them with each other. In practice, they can be either more or less effective than standard compatibilizers, depending on the choices of reactive groups and catalysts.
- The addition of compatibilizers of lower molecular weight sometimes leads to a dramatic worsening of various properties (such as stiffness, toughness, or flame retardancy) even if these additives improve the compatibility of the polymers in the blend.
- While the addition of nanoparticles may be a useful and interesting compatibilization method, more research is needed to elucidate its mechanism and expand its utilization.