An in situ Neumann triangle-focused ion beam-atomic force microscopy (NT-FIB-AFM) method has been used to measure modified PS/HDPE interfacial tensions in ternary PS/PP/HDPE blends prepared by melt mixing and demonstrating partial wetting. The ternary blend was modified with SEB, SB and SEBS copolymers Results related to the position of the PS droplet at the Interface show that a symmetrical diblock copolymer is somewhat more efficient in decreasing the interfacial tension compared to an asymmetrical one of similar molecular weight, while the SEBS triblock copolymer appears to have no effect at all Using the NT-FIB-AFM method, the lowest modified PS/HDPE interfacial tension is 3.0 +/- 0 4 mN/m for the symmetric diblock, compared to 4 2 +/- 0.6 mN/m (N = 34) for the unmodified Interface This corresponds to an apparent areal density in SEB copolymer equal to 0 16 +/- 0.03 molecules/nm(2), which is near reported saturation values. By varying the concentration of the copolymer, an emulsification curve reporting the value of the PS/HDPE modified interfacial tension as a function of the apparent areal density of the copolymer at the PS/HDPE interface has been obtained. The interfacial tension values obtained by the NT-FIB-AFM approach are significantly higher than the 0 5 +/- 0.2 mN/m (N = 3) result obtained by using the classical breaking thread method with the same materials. This discrepancy does not appear to be due to a poor migration of the copolymer to the PS/HDPE Interface, but could instead be attributed to the interfacial elasticity of the compatibilized interface, a phenomena that has not been accounted for so far in experimental studies on the morphology of compatibilized multicomponent polymer blends (C) 2010 Elsevier Ltd All rights reserved