Flow-Induced Reactive Self-Assembly

Abstract

Most polymer-polymer pairs are immiscible. However, if one polymer is properly dispersed within another, unique morphologies can be created with synergistic properties.1 For example, sheets of a low permeability polymer can greatly increase the barrier performance of a layered composite,2 while submicron size droplets of a rubbery phase can improve toughness 10-fold3 in a glassy or semicrystalline plastic. The key to these property improvements is creation and stabilization of the desired composite morphology. Intensive mixing as the polymers melt can create thin sheets, and subsequently fine droplets,4 of the minor component. However, these small droplets rapidly coalesce since they are far from thermodynamic equilibrium.4,5 The most successful strategy for stabilizing or compatibilizing these nonequilibrium structures is to fix complementary functional groups on the two polymers that can couple at the interfaces forming a graft or block copolymer during mixing.6 This copolymer provides a physical barrier against coalescence5,7-10 and also increases adhesion between the two phases, improving mechanical properties.11,12 In most previous studies and commercial processes graft or cross-linked copolymers were produced.6 Such branched and network structures tend to inhibit interfacial deformation, thereby restricting further area generation. We have prepared polymer chains with a single functional group on just one end. These terminally functional polymers can only form diblock copolymers. With a moderate chemical reactivity these functional groups produce relatively fine, stable droplets7 similar to results with reactively formed graft or crosslinked copolymers.3,5,6,9,13 However, we have recently discovered that very fast reaction rates lead to coupling of all the reactive chains when the mixture is shear processed. The resulting block copolymer self-assembles into a nanostructured morphology. Three different types of heterogeneous blends were studied in our experiments, each prepared with 70 wt % polystyrene (I) and 30 wt % of polyisoprene (II) (or polybutadiene). The first set of blends did not contain functional groups, while the second set involved a cyclic anhydride (III) attached to the polydiene,14 reacting with a primary aromatic amine (IV) at the terminus of the polystyrene.15 For the third set a primary aliphatic amine (V) was substituted for the aromatic amine of the second mixture.16 The blends were prepared by shear mixing in the molten state.17 The polydiene minor phase is expected to break up into droplets.4,5 Conversion of the reaction was determined by gel permeation chromatography (GPC) using a UV detector sensitive only to the polystyrene as it eluted from the columns. Since the polymers were relatively monodisperse,18 the amount of coupled chains could easily be distinguished and quantified.7 Phenyl isocyanate was added to the samples before dissolving in THF to quench all remaining amino groups. This prevented further reaction in solution and prevented adsorption of the amino groups to the chromatographic columns. At high temperatures a primary amine reacts essentially irreversibly with a cyclic anhydride to produce a cyclic imide plus water.19 The reaction of the aromatic amine with cyclic anhydride is relatively slow, while the reaction of the aliphatic amine is extremely fast compared to the blending process time scale. Comparative rates of reaction were measured by preparing homogeneous blends of reactive polystyrene containing the complementary functional groups in a stoichiometric ratio. The aromatic amine reaction reached a maximum conversion of 36% at 180 °C over the 20 min mixing time employed in these experiments. The aliphatic amine reaction reached essentially complete conversion to coupled chains in less than 1 min, i.e., when the first sample was removed from the mixer (see Figure 1a). Approximate rates of generation of block copolymer can be determined from the data in Figure 1a using the initial slopes. For the aromatic amine reaction we get