(361a) Influence of Interfacial Tension on the Morphology of Polymer Blends

In this contribution, the influence of interfacial tension on the morphology of polymer blends is experimentally investigated. The importance of a changing interfacial tension, based on diffusion of the polymeric chains of the components, on coalescence is highlighted. The results are compared with the drop size evolution calculated from a drainage model, using either an immobile, partially mobile or fully mobile interface.

Three different component materials are selected: Polybutene (PB, Indopol H-25, BP Chemicals, UK, MW = 635), Polybutadiene (PBD, Ricon 134, Sartomer, MW = 8000) and polydimethylsiloxane (PDMS, UCT, MW = 62500). The material combinations are PB/PDMS and PBD/PDMS and the blend concentrations are 1%wt.

Measurements of interfacial tension as a function of time are carried out using a temperature controlled pendant/sessile drop apparatus. The time dependent drop shape is fitted using the Gauss ? Laplace equation where interfacial tension is the fitting parameter. The blend morphology is studied with a well-defined flow history using a shear cell with temperature control. Small Angle Light Scattering (SALS) measurements are used to obtain the time evolution of the droplet average radius by applying the Debye-Bueche theory. In addition, optical microscopy is used to monitor the time evolution of the morphology in an independent manner.

Our results show that the polymer pairs used in this work cannot be considered fully immiscible and, to a certain extend, diffusion is always present. It is shown that the interfacial tension changes significantly in time when the molecular weight of the dispersed phase is lower than the molecular weight of the continuous phase. This change is caused by the migration of short molecules from the drop phase into the matrix phase. It is observed that the larger the molecular weight asymmetry across the interface, the more the interfacial tension is eligible to changes. When reversing the phases, a similar molecular weight asymmetry across the interface is still present. However, the behavior of the interfacial tension with time becomes different. Instead of a mere change, the interfacial tension shows a fast reduction and finally evolves towards a steady-state value. This phenomenon can be correlated to a fast saturation of the inclusions. Finally, the impact of the behavior of the interfacial tension on the flow-induced morphology is examined by means of optical microscopy and SALS experiments. It is observed that depending on the type of interfacial response, the morphology development in the early stages is dominated by either diffusion or coalescence. Moreover, it is shown that existing sharp-interface drainage models do not apply for these types of blends.