Interfacial coarsening of ternary polymer blends with partial and complete wetting structures

Abstract

The coarsening of polymer mixtures is an important route towards major morphology modification in multiphase polymer systems. To date however the coarsening of ternary systems has not been significantly examined. In this study the phase coarsening mechanism via annealing for partial wetting, and complete wetting morphologies in ternary polymer blends is characterized. This is a route towards the examination of interfacial coarsening in polymer blends since ternary partially wet systems involve the presence of interfacial droplets while completely wet ternary systems are comprised of a complete interfacial layer. A partial wetting type of morphology is obtained for polybutylene succinate (PBS)/poly(lactic acid) (PLA)/polycaprolactone (PCL). Three different compositions for that system with composition ratios of ϕ(PBS/PLA) = 1.5; ϕ(PBS/PLA) = 3; and ϕ(PBS/PLA) = 10 are prepared to show the effect of the concentration of the self-assembled PLA droplets located at the interface of PBS/PCL. As the concentration of PLA decreases, the growth rate of the PLA phase during the annealing process sharply decreases due to a significant increase of the “surface to volume ratio” of the PLA droplets required in order to cover the interface. In this case, due to the short inter-droplet distances between PLA droplets at the interface, coalescence is controlled by the drainage time. This mechanism is confirmed by the observation of a linear relationship between the third power of droplet size and annealing time. For the 37.5%PBS/12.5%PLA/50%PCL blend, the conservation of interfacial-angles confirms that the annealing time has no effect on the angle values between phases, as predicted by Harkins spreading theory.The annealing process for complete wetting is studied at four compositions for an HDPE/PS/PCL blend where the PS phase is located as a continuous layer at the interface of co-continuous HDPE/PCL. In 33.3%HDPE/33.3%PS/33.3%PCL after 30 min of annealing, the PS phase thickness increases 49 times from 2.3 μm to 112 μm. Even for very low concentrations of 3%PS, a high coarsening rate of 0.0039 μm/s is observed. This sharp linear increase in PS phase size implies a capillary pressure mechanism and an impeded growth of Tomotika-like capillaries for all three phases that cause a confinement effect for coarsening of the middle PS phase.