Correlation between the Interfacial Tension and Dispersed Phase Morphology in Interfacially Modified Blends of LLDPE and PVC

To study the phase size/interfacial tension relationship for LLDPE/PVC blends over a wide range, three modifiers of widely varying efficacy were chosen based on a cursory analysis of their acid−base interactions with the blend system. The modifiers used were poly(isoprene−4-vinylpyridine) (PIP−P4VP), poly(styrene−acrylic acid) (PS−PAA), and a hydroxyl-terminated polystyrene (PS−Si(CH 3 ) 2 OH). The ternary blend systems were rigorously characterized by tracking both interfacial tension and dispersed phase morphology evolution as a function of percent interfacial modifier. The interfacial tension was measured using the breaking thread technique, and the blends for morphology examination were prepared on a Brabender mixer, a predominantly shear mixing device. Both the interfacial tension and the morphology studies confirm that the best modifier was the PIP−P4VP followed by PS−Si(CH 3 ) 2 OH, and the PS−PAA copolymer did not have any influence on the system whatsoever. The area occupied per molecule at the PVC/LLDPE interface estimated from the emulsification curve (morphology data) for the poly(isoprene−4-vinylpyridine) is approximately 6 nm 2 per molecule. It is shown that PVC/LLDPE, due to the low interfacial tension of the unmodified blend, represents a case where the dynamic coalescence contribution to morphology development is essentially negligible. This has allowed the relationship between morphology and interfacial tension to be studied in a direct fashion. A comparison of the limiting values of morphology and interfacial tension, in the presence of the modifiers, compared to the unmodified system unambiguously indicates a close 1:1 relationship between the two in the absence of coalescence effects. This direct dependence between dispersed phase size and interfacial tension is observed for all three systems and is independent of the relative efficacy of the modifier itself. These results demonstrate a direct experimental confirmation of the interfacial tension/phase size relationship as predicted by Taylor theory.