How Do Copolymer Compatibilizers Really Work?

Abstract

Most pairs of chemically different homopolymers of any useful molecular weight are thermodynamically immiscible. This is because any tiny repulsive interaction per monomer, multiplied by the large number of monomers in a typical polymer, overwhelms entropy of mixing (of the order of kBT per molecule where kB is the Boltzmann constant and T the temperature). Nonetheless blends of immiscible pairs of homopolymers are industrially ubiquitous. Two broad categories of such materials are rubber-toughened plastics and stiffened elastomers. Both are fine dispersions of one polymer in another, with the desirable properties of the resulting material (e.g., stiffness-toughness balance) depending crucially on its composite structure and the mechanical contrast between the two components (e.g., difference in moduli).Typically a fine dispersion, consisting of submicron particles, is necessary for good final properties. Also necessary is a strong interfacial bond between the particles and the matrix so that the resulting material does not fail at these interfaces. A typical volume fraction of the suspended component is perhaps 20–30%. Such polymer blends are prepared by two time-honored techniques (ancient, really, if one considers their antecedents in cookery): stirring and the use of polymeric surfactants—copolymers. The copolymers can be synthesized beforehand and added to a mixer where the components are stirred, but more often they are produced from the homopolymers during stirring by a grafting reaction, which somehow joins the two dissimilar homopolymers to form some sort of block copolymer.