INTERFACIAL ACTIVITY OF GRAFT-COPOLYMERS IN BLENDS - EFFECT OF HOMOPOLYMER MOLECULAR-WEIGHT

The effect of varying the homopolymer molecular weight, corresponding to the minor phase, on the interfacial activity of graft copolymers in blends of homopolymers has been investigated. The system studied was polycarbonate/poly(methyl methacrylate) (PMMA) (65/35 w/w) blends containing polycarbonate/PMMA graft copolymers. The molecular weight of the polycarbonate homopolymer was increased from 3.6 to 52.4 kg mol-1, always maintained lower than that of the polycarbonate block (143.8 kg mol-1). Molecular weights of all other components were kept constant. Copolymer interfacial activity, in terms of its efficacy to reduce the size of the dispersed phase and create interface, deteriorated considerably with increasing polycarbonate homopolymer molecular weight. The interfacial area occupied by a single copolymer molecule did not change dramatically. This behaviour was observed with constant (10% w/w) and varying (1-40% w/w) copolymer content. In dilute solutions (1 x 10(-3) g cm-3) with selective solvents (toluene), the copolymer formed micelles consisting of approximately nine copolymer molecules with no evidence for significant swelling of the polycarbonate core by unreacted polycarbonate homopolymer. Thermodynamic parameters of aggregation showed the process to be enthalpy driven. Copolymer interfacial activity is discussed in terms of phase diagrams for these systems with increasing homopolymer molecular weight. Phase separation of copolymer and polycarbonate homopolymer from the PMMA-rich matrix as the system crosses the binodal has been argued to be the mechanism to account for the presence of stable fine polycarbonate-rich dispersions in a PMMA-rich matrix. The relative molecular weight of the polycarbonate homopolymer to that of the polycarbonate block controls the position of the critical copolymer-homopolymer concentration relative to that of the overall system.