More often than not, the mechanical properties (e.g., impact and tensile properties) of immiscible polymer blends are very poor owing to the lack of adhesion between the constituent components, which originates from strong repulsive thermodynamic (segmental) interactions. Therefore, in the past, a great deal of effort (Barlow and Paul 1984; Fayt and Teyssie 1989; Fayt et al. 1981, 1987, 1989; Gupta and Purwar 1985; Ouhadi et al. 1986a; Park et al. 1992; Schwarz et al. 1988, 1989; Srinivasan and Gupta 1994; Traugott et al. 1983) has been made to improve the mechanical properties of two immiscible polymers by adding a third component (e.g., a block copolymer). In this chapter, we confine our attention primarily to the situations where a nonreactive third component is added to two immiscible homopolymers in order to improve their mechanical properties. A polymer blend consisting of two immiscible homopolymers (say, A and B) has a very narrow interface, as schematically shown in Figure 4.1, because they have strong repulsive segmental interactions giving rise to a positive value of the Flory–Huggins interaction parameter (χ), i.e., χAB > 0. Helfand and Tagami (1971, 1972) derived the following expression relating the interfacial thickness d of a pair of immiscible homopolymers of infinite molecular weight to χ: . . . d = 2b/(6χ)1/2 (4.1). . . where the Kuhn length b is assumed to be the same for both components. They also derived an expression for the interfacial tension γ between two immiscible homopolymers: . . . γ = (χ/6)1/2bρokBT . . . in terms of χ, where kB is the Boltzmann constant, T is the absolute temperature, and ρo is the reference density (the inverse of monomeric volume of a reference component). Equation (4.1) indicates that the interfacial thickness between two immiscible homopolymers will be larger when the extent of repulsive segmental interactions is less, and Eq. (4.2) indicates that the interfacial tension between two immiscible homopolymers will be lower when the extent of repulsive segmental interactions is less.
Influence of Osmotic Pressure on Nanostructures in Thin Films of a Weakly-Segregated Block Copolymer and Its Blends with a Homopolymer