Miscible blends common examples

Miscible blends are most commonly formed from elastomers with similar three-dimensional (Hansen, 1967a,b Hansen and Beerbower, 1971) solubility parameters. An example of this is blends from copolymer elastomers (e.g., ethylene-propylene or styrene-butadiene copolymers) of slightly different composition, or microstructure. When the forces between the components of the polymer blend are mostly dispersive, miscibility is only achieved in neat polymers with a very close match in Hansen s three-dimensional solubility parameter (Hansen, 1967a,b Hansen and Beerbower, 1971), such that small combinatorial entropy for high molecular weight elastomers can drive miscibility. 

Common examples of miscible blends are ethylene-propylene copolymers of different composition that result in an elastomer comprising a semicrystalline, higher ethylene content and an amorphous, lower ethylene content components. These blends combine the higher tensile strength of the semicrystaUine polymers and the favorable low temperature properties of amorphous polymers. Chemical differences in miscible blends of ethylene-propylene and styrene-butadiene copolymers can also arise from differences in the distribution and the type of vulcanization site on the elastomer. The uneven distribution of diene, which is the site for vulcanization in blends of ethylene-propylene-diene elastomers, can lead to the formation of two distinct, intermingled vulcanization networks. 

An excellent example of the role played by liquid-liquid phase separation in the ensuing crystallization is found in blends with syndiotactic poly(styrene).(77) Measurements of the glass temperature in mixtures with poly(2,6-dimethyl-l,4-diphenylene oxide) (PPO) indicate that the components are miscible in all proportions in the melt. However, mixtures of syndiotactic poly(styrene) with poly(vinyl methyl ether) represent partially miscible blends. When the poly(vinyl methyl ether) content exceeds 20% by weight, the melt separates into two liquid phases, one rich in syndiotactic poly(styrene), the other in poly(vinyl methyl ether). Thus, the two blends have a common crystallizing component. However, in one the crystallization takes place from a homogeneous melt in the other from one that is phase separated. The different melt structures profoundly affect the crystaflization kinetics. This can be seen when a comparison is made between the crystallization kinetics of syndiotactic poly(styrene) from a homogeneous or phase separated melt. 

The third component of such a blend, i.e., the solvent, and the kinetics of its removal can influence the resulting morphology. For example, if two miscible polymers are cast from a common solvent, one does not necessarily obtain a homogeneous mixture. A two-phase region can exist in the ternary phase diagram as shown in Fig. la, and as the solvent evaporates the composition may enter the two- phase region as shown by progressing from point A to point... 

Unfunctionalized PPE may be compatibihzed with immiscible PA by addition of functionalized polystyrene capable of forming copolymer with PA (Table 5.16). This is a common compatibilization strategy for PPE blends since both PS itself and functionalized polystyrenes with a relatively low level of functionality are miscible with PPE. The examples below include use of anhydride-, acid-, and epoxide-functionalized polystyrenes, all of which are capable of reacting with nucleophilic end-groups on PA to form a graft copolymer. 

Blending within the family of PO has, however, been more common [Plochocki, 1978]. Although they are usually immiscible with each other, there exists some degree of mutual compatibihty between them. The similarity of their hydrocarbon backbones and the closeness of their solubility parameters, although not adequate for miscibility, accounts for a relatively low degree of interfacial tension. Eor example, the solubility parameters of polyethylene, polyisobutylene, ethylene-propylene rubber and polypropylene are estimated to be 16.0, 15.4,... 

Miscibility of a polymer in fluid depends on the temperature, pressure, polymer concentration, molecular weight and molecular weight distribution, the polymer type and the nature of the solvent fluid under consideration. Many studies have been reported in the literature [1, 6, 7, 8, 20, 25-27]. The examples that are included in this chapter are selected from the measurements conducted in our laboratory. The examples have been chosen to be representative of miscibility of polymers in carbon dioxide, in organic solvents, and in mixtures of carbon dioxide + organic solvent. Examples are also included on the miscibility of blends of two different polymers in a common solvent. 

It is important to bear the fact in mind that when most polymers form a blend, they adopt a phase separation state which is determined by the laws of thermodynamics as was discussed in this chapter. To make a good polymer blend with controlled morphologies, the most commonly used strategy is to employ a block copolymer to act as intermediate . For example, a copolymer has two blocks A and B, block A needs to be miscible with polymer X and block B with polymer Y. 

As with the example presented for cosolvency in the case of polymer solutions in mixed solvents (Fig. 27), the origin of cosolvency for polymer blends in a common solvent can be interpreted as a dissection of a miscibility gap that would normally bridge the Gibbs phase triangle from one binary subsystem to the other binary system (here from 1/B to A/B) by special interactiMis between the completely miscible components (here 1/A). With the example of Fig. 32, the thermodynamic quality of the solvent for polymer A is almost marginal in this manner polymer B becomes completely miscible with certain solutions of polymer A in solvent 1. 

The combinatorial entropy is very small in polymer—polymer blends. Miscibility in polymer-polymer blends can normally only be achieved when the heat of mixing is negative. In molecular terms, that is accomplished by specific interactions between different molecules. The most common type of specific interaction is the hydrogen bond. Examples of... 

---