Kinetic and Thermodynamic Control of the Selective Localization of Carbon Black at the Interface of Immiscible Polymer Blends

Abstract

Several strategies are reported that allow carbon black (CB) particles to be selectively localized at the interface of polyethylene/polystyrene (PE/PS) blends used as models of two-phase polyblends. A first general approach relies upon a kinetic control, i.e., the choice of such processing conditions that the CB particles are “immobilized” at the polyblend interface at least for a workable period of time. The dry premixing of the two powdery polymers and CB particles followed by compression molding is the first valuable kinetic control. A second one can be implemented during the melt blending of the immiscible polymers, which makes this strategy more attractive. Actually, CB is first dispersed in the melted polymer with which it less strongly interacts. Upon the addition and melting of the second polymer, CB particles are thermodynamically driven to this second polymer phase. The conducting particles are observed to accumulate at the polyblend interface at a rate and for a period of time that depend on the rheology of the polyblend under the processing conditions. The thermodynamically controlled localization of the CB particles at the two-phase polyblend interface is certainly the most efficient strategy, even though it is not the most general one. In this respect, CB particles have been oxidized in such a way that they cover a large range of pH. Depending on this surface property, the CB particles are spontaneously localized either in one polymer phase or at the interface. Actually, the selective localization of the CB particles changes from the PS phase to the interface and finally to the PE phase, when the pH of these particles is increased from 2.4 to 7.0.The selective localization of the CB particles at the polyblend interface is most interesting when the polymer phases are co-continuous, since then the conducting particles can percolate at a volume fraction as small as 0.002−0.003. Furthermore, this selective modification of the polyblend interface improves the stability of the phase morphology against coalescence when the material is annealed.