Miscibility and compatibility in polymer blends is a topic of great academic and industrial importance. This is because miscibility and compatibility contribute to morphology, properties, and performance. Miscibility results in one phase; compatibility creates a disperse phase with size and stability determined by interfacial interactions. Miscible polymer properties are averaged similar to a plasticizer polymer, and compatible polymers retain properties of each component, such as toughening or reinforcement. With miscible polymer blends the continuous phase dominates properties; the disperse phase contributes via stress transfer. This chapter revisits the criteria for miscibility or compatibility in polymer blends and the contributors of compatibility compared with miscibility and incompatibility. Development of copolymers and their blending with thermosets and thermoplastics result in complex two-phase morphologies. The dynamics of phase separation observed in polymer blends leading to different morphologies and the criteria for phase separation is explained. A nanometer-dispersed phase requires strong interfacial interactions to stabilize the large interfacial area, and this is favored by rapid spinodal phase separation compared with size diminution by high shear. Nanoblends open up a new arena for polymer blends, and research shows that nanoblends have outstanding optical and mechanical properties.

Keywords

Miscibility; Polymer blends; Compatibilizer; Flory–Huggins theory; Cross-linking; Nanoblends

1.1 Introduction

1.2 Polymer–Polymer Miscibility Theory

1.2.1 Macromolecular Solubility

1.2.2 Binodal Phase Separation

1.2.3 Spinodal Phase Separation

1.2.4 Nanophase Separation

1.2.5 Blends of Semicrystalline Polymers

1.3 Incompatible Polymer Blends

1.4 Miscible Polymer Blends

1.4.1 Interactions and Miscibility

1.4.2 Loss of Desirable Properties

1.5 Cross-Linking of Miscible Polymer Blends

1.5.1 Cross-Linking of the Major Phase

1.5.2 Cross-Linking of the Minor Phase

1.6 Compatible Polymer Blends

1.6.1 Compatible Polymers

1.6.2 Shear-Induced Colloid Formation

1.7 Nanophase Blends

1.7.1 Properties of Nanoblends

1.8 Conclusion

References

1.1 Introduction

The art of mixing different materials was known to mankind from the Bronze Age. Concrete, metal alloys, and fiber composites that are considered to be typical examples were introduced [1]. In the early stages of polymer industry the major polymers used were wood, natural rubber, and gutta-percha along with natural fibers such as cellulose, protein fibers, and leather. The year 1846 witnessed the first polymer blend (natural rubber blended with gutta-percha) which was reported in the patents of Hancock and Parkes [1]. A single rotor masticating machine was used for the blending process. This was followed by the slow development of blending technology. In the first half of the twentieth century great progress in the polymer industry led to the development of a wide range of new polymers. Later the depletion of economic ways to develop new monomers, and the fact that newly developed monomers gave rise to polymers with intermediate properties as compared with the existing polymers, led to the development of polymer blending [2]. The last 80 years show an exponential increase in the number of polymer blend patents and literature; the number almost doubled after 1993 [3]. Thus, polymer blends are a class of materials in which at least two polymers are combined together resulting in a new material with different physical properties [4].

The important advantages of polymer blending can be summarized as:

Polymer blends can broadly be classified into three categories:

The main differences and the important conditions for miscibility are shown in Table 1.1.

The success of developing a polymer blend depends on the effectiveness with which the following inherent limitations are overcome [4].

When a polymer is made by the union of two or more different “mers” (the smallest repeat unit of a polymer chain) it is called a copolymer.

Copolymers include:

In a polymer blend the interactions between the different monomers are predominantly interactions such as hydrogen bonding, Van der Waals interaction, or dipole–dipole interactions, whereas in a copolymer the interaction is predominantly covalent in nature. In a copolymer the structural arrangement of molecular units plays an important role in properties such as melting temperature, glass transition temperature, modulus, and tensile strength. In a random copolymer these properties are averaged; a block copolymer gives superiority in individual properties [5]. The variation of the properties for polymer blends is shown in Figure 1.1.

Polymer blends are called “compatible” or “incompatible” depending on their resultant properties (Figure 1.1). Compatible blends exhibit fine phase morphology resulting in good physical properties. Usually the chances of getting synergistic properties are high in a compatible blend. Incompatible blends are fully immiscible and have poor mechanical properties.

1.2 Polymer–Polymer Miscibility Theory