Influence of extender oil composition on flowing property, thermal stability, and morphology of styrene-b-ethylene-co-butylene-b-styrene copolymer/polypropylene/oil blends

In this study, a series of styrene-b-ethylene-co-butylene-b-styrene copolymer (SEBS)/polypropylene (PP)/oil blends with different kinds of oil composition was developed through melt blending. The effect of oil with different composition and properties on its phase equilibrium and "redistribution" in multiphasic SEBS elastomer was systematically studied for the first time. Moreover, an integral influencing mechanism of oil composition on the structure and properties of SEBS/PP/oil blends was also put forward. The mineral oil was mainly distributed in ethylene/butylene (EB)/PP phase, which greatly enhanced the processing flowability of SEBS/PP/oil blends. With increasing oil C-N content, a redistribution of oil appeared and excess naphthenic oil (NO) entered the interphase of soft and hard phases. The dynamic mechanical thermal analysis (DMTA) analysis indicated that the polystyrene (PS) phase was plasticized, which also helped to improve the processing fluidity of blends. However, the plasticizing of physical cross-linking point PS resulted in a decrease in mechanical strength and thermal stability. Small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM) results showed that PS phase (45 nm to 55 nm) cylindrically distributed in EB/PP/oil matrix, the excess NO in the interphase enlarged the distance between PS phase and widen the escape channel for oil migration. At over 45% oil C-N content, the electron density difference between soft and hard phases reduced to the minimum, same as T-gPS, indicating a deeper plasticizing effect. The PS phase swelled and exhibited elastic behavior; thus, the force could be uniformly transferred between two phases. Importantly, a recover in strength and thermal stability was observed in O-5 blend. This work significantly filled the gap of studies in oil-extended thermoplastic elastomers (TPEs), exhibiting great theoretical guiding significance and application value.