How Do Copolymer Compatibilizers Really Work?
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How Do Copolymer Compatibilizers Really Work? Scott T. Milner Introduction Most pairs of chemically different homopolymers of any useful molecular weight are thermodynamically immiscible.1 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.2-3 Two broad categories of such materials are rubbertoughened 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. The reason for stirring is evident: Droplets of the minority component tend to break up in a stirring flow. A simple scaling argument gives the size of the 38
MRS BULLETIN/JANUARY 1997
How Do Copolymer Compatibilizers Really Work?
Figure 1. At higher volume fractions ( = 0.2) of polystyrene in a mixture with poiyamide, large, polydisperse droplets are observed without the graft copolymer (a) while small, monodisperse droplets are produced with the graft copolymer, (b) similar in size to those observed at lower volume fractions.
ated when the homopolymers come into intimate contact at the interface. Because the homopolymers are strongly immiscible, a copolymer, once created, faces a barrier of many kBT when attempting to leave the interface. It is thus a good assumption that all the graft copolymer stays on the interface. Therefore if the amount produced could be measured, the coverage could be estimated from the observed droplet sizes and known volume fraction. Beck Tan, Tai, and Briber measured the amount of copolymer produced in the following way. They dissolved the blend in a common solvent and then separated the homopolymers by precipitation in a succession of nonsolvents. Without the copolymer, this separation of PS from PA would be complete. The copolymer of course must choose to follow the PS or the PA. Either way some PS will be found in the separ
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