Miscibility of Polymer Blends

Miscibility of Polymer Blends

Lec. 11 Miscibility of Polymer Blends…………………………………………..….Eng. Auda Jabbar Ms. C.


Miscibility of Polymer Blends

Miscibility: Capability of a mixture to form a single phase over certain ranges of temperature, pressure, and composition.

The miscibility term describes the homogeneity of polymer mixtures at some temperatures. Miscibility can be influenced by various factors such as morphology, crystalline phase, intermolecular interaction, and reduction of surface tension.


1. Whether or not a single phase exists depends on:

♦ The chemical structure

♦ Molar-mass distribution

♦ Molecular architecture of the components present.

2. The single phase in a mixture may be confirmed by:

♦ Light scattering

♦X- ray scattering

♦ Neutron scattering

The miscibility of two polymers is depending on the specific interactions between polymer chains. This can be explained by the factor of entropy in the following equation, which represents the second law of thermodynamics.

GM = HM - TSM

where, ΔG = change in free energy, ΔH = change in enthalpy, ΔS = change in entropy, T = absolute temperature.

For a homogeneous miscible blend the Gibbs free energy of mixing requires a negative value. For high molecular weight polymer blends, the gain in entropy is negligible. Hence, the free energy of mixing can only be negative if the heat of mixing is negative. This means that the mixing must be exothermic, which usually requires specific interactions between the blend components. These interactions may range from strongly ionic to weak and non-bonding, including hydrogen bonding, ion-dipole, dipole-dipole, and donor-acceptor interactions.

Based on the miscibility, three types of blends can be distinguished;

1- Completely miscible blends

2- Partially miscible blends

3- Fully immiscible blends

Partially miscible blends, in which a part of one blend component is dissolved in the other, exhibits normally good compatibility and fine phase morphologies. However, fully immiscible blends exhibit a coarse phase morphology having a sharp interface and a poor adhesion between both blend phases. This is the reason for often observed poor properties of immiscible blends, which strongly depend on the size and distribution of the phases.

Where: A: Immiscible system, B: Fully miscible system , C:Partially miscible system

Miscibility Window:

Range of copolymer compositions in a polymer mixture, at least one component substance of which is a copolymer, that gives miscibility over a range of temperatures and pressures.


1. Outside the miscibility window immiscible mixtures are formed.

2. The compositions of the copolymers within the miscibility window usually exclude the homopolymer compositions of the monomers

from which the copolymers are prepared.

3. The miscibility window is affected by the molecular weights of

the component substances.

4. The existence of miscibility windows has been attributed to an average force between the monomer units of the copolymer that leads to those units associating preferentially with the monomer

units of the other polymers

Methods of Investigating Miscibility:

Equation 1 provides an abstract definition of what miscibility means in terms of thermodynamics; from it the state of miscibility of a polymer pair cannot be obtained.

GM = HM - TSM…………………..(1)

In practice, the miscibility of a polymer pair is defined by the method

that is used to test it. In other words, it is defined in terms of degree of

dispersion, phase morphology or degree of interaction between the


The most common method to establish polymer miscibility is Differential Scanning Calorimetry (DSC), with which determination of the glass transition temperature (Tg) or the depression of the melting temperature allow one to obtain details of the mixing.

Completely miscible blends consist of one homogeneous phase. This type of blend exhibits only one glass transition temperature (Tg), which is between the Tg s of both blend components with a close relation to the blend composition.

In some cases, it is necessary to use other experimental techniques. The optical microscopy is used to study the spherulitic superstructure of

polymer crystals from the melt and explain the relationship between morphology and crystal growth rate.

In addition, Small-angle light scattering (SALS) and Small-angle X-ray

scattering (SAXS) are used to study the morphology of crystalline / amorphous polymer blends.

The miscibility of homopolymer/copolymer blends has been successfully

described by the binary interaction model. The most common specific intermolecular interactions occuring between two different polymer chains are: hydrogen bond, ionic bond and dipole-dipole interactions

In Table 1.1 are displayed several experimental methods used to characterize blends. These methods can be divided in three Categories:


Ex 1: Let me use an example to illustrate. Two polymers that do actually mix are polystyrene and poly(phenylene oxide).

As you can see, both of these polymers have aromatic rings. As you may know, aromatic rings like to stack up like little hexagonal poker chips. For this reason, these two polymers like to associate with each other. So they blend very nicely. There are a few other examples of polymer pairs which will blend. Here is a list of a few:

poly(ethylene terephthalate) with poly(butylene terephthalate)
poly(methyl methacrylate) with poly(vinylidene fluoride)

But most of the time, the two polymers you want to blend won't be miscible. So you have to play some tricks on them to make them mix. One is to use copolymers.

Polystyrene doesn't blend with many polymers, but if we use a copolymer made from styrene and p-(hexafluoro-2-hydroxyisopropyl) styrene, blending is a lot easier.

You see, those fluorine atoms are very electronegative, and they're going to draw electrons away from all the nearby atoms. This leaves the alcohol hydrogen very lacking in electrons, which means it is left with a partial positive charge. So that hydrogen will form strong hydrogen bonds with any group with a partial negative charge. Because of this, it's easy to form blends of this copolymer with polycarbonates, poly(methyl methacrylate), and poly(vinyl acetate)

Ex2: There's another way copolymers can be used to help polymers blend. Let's consider a random copolymer of styrene and acrylonitrile. This copolymer will blend with poly(methyl methacrylate) (PMMA). This is where it gets weird. PMMA won't blend with either polystyrene or polyacrylonitrile.

So why does the random copolymer blend with PMMA? The explanation is something like this: the styrene segments and the acrylonitrile segments of the random copolymer may not like PMMA, but they like each other even less. The styrene segments are non-polar, while the acrylonitrile segments are very polar. So, the styrene segments and the acrylonitrile segments blend into the PMMA to avoid coming into contact with each other.

Making Your Own Blends

Blends are usually made in two ways. The first way is to dissolve two polymers in the same solvent, and then wait for the solvent to evaporate. (presuming your two polymers are miscible).

While this method works fine in the laboratory, it could get expensive if you tried to do this industrially. Solvents aren't cheap, and if you're going to evaporate hundreds or thousands of gallons of them, you'll be paying a lot of money. Not to mention the effects on the environment of putting so much of your toxic solvents into the air, or the extra cost of recapturing all that solvent so it could be reused. So for making blends in large amounts, you heat the two polymers together until you're above the glass transition temperatures of both polymers. At this point they will be nice and gooey, and you can mix them together like a cake mix. This is often done in machines such as extruders. When your material cools, you'll have a nice blend, again, presuming your two polymers are miscible.