In Situ Measure of Interfacial Tensions in Ternary and Quaternary Immiscible Polymer Blends Demonstrating Partial Wetting

This article reports on the use of the Neumann triangle method combined with a focused ion beam sample preparation technique and atomic force microscopy (NT-FIB-AFM) to measure interfacial tension ratios in partially wetted ternary and quaternary immiscible polymer blends prepared by melt processing. It is shown that PS/PP/HDPE, PS/PCL/PP, PLLA/PCL/PS, PMMA/PS/PP, and a quaternary blend system comprised of HDPE/PP/PS/PMMA all display a partial wetting morphology with a three-phase line of contact and that the interfacial tension ratios obtained by the NT-FIB-AFM approach compare well with results obtained by the classical breaking thread method. The HDPE/PP/PS/PMMA quaternary blend, in particular, is quite unique and displays a partial wetting morphology with spectacular PS/PMMA composite droplets located at the HDPE/PP interface. Furthermore, all of the above data generated for the ternary and quaternary systems also satisfy the Laplace equation. When 1% of an SEB diblock copolymer is added to the PS/PP/HDPE system, the Neumann triangle method reveals that the PS/HDPE interfacial tension decreases from 4.2 ± 0.6 to 3.3 ± 0.4 mN/m, with an estimated apparent areal density of 0.19 ± 0.07 molecule/nm 2 of copolymer at the PS/HDPE interface. The results presented in this paper show that it is possible to generate complex morphologies demonstrating partial wetting for a wide range of polymer blend systems with a relatively simple experimental approach. Furthermore, it allows the measurement of the interfacial tension ratios of a matrix-dispersed phase blend system examined in situ after melt processing. The apparent areal density of a copolymer interfacial modifier can also be estimated. This is an important result, since it is still a challenge to measure the variation of the interfacial tension as a function of the copolymer areal density in multiphase polymer blends.