It is meaningful to analyze the functions of scCO2on theimproved phase dispersion of the PP/PS blends prepared via reac-tive extrusion. First, the change in viscosity ratio of PP and PS dueto the plasticizing effect of scCO2could improve the phase dis-persion if the viscosity ratio was getting closer to one [27–29]. Inthis study, with scCO2in thermal blending, the phase dispersionof PP/PS6 to PP/PS8 was improved relative to that of PP/PS1. Thus,it should be reasonable to infer that with scCO2in reactive extru-sion, the phase dispersion of the PP/PS blends could be improvedas well. This could probably be one reason why PP/PS3 to PP/PS5showed better phase dispersion than PP/PS2 did, which was definedas scCO2-assisted phase dispersion via viscosity change (functionPS5.1 of scCO2). Second, the presence of scCO2promoted the yield ofinterfacial modifiers, and thus improved the interfacial adhesion.This could probably be the other reason to interpret the better phasedispersion of PP/PS3 to PP/PS5 relative that of PP/PS2, which wasdefined as scCO2-promoted in situ compatibilization (function 2of scCO2). Thereby, the presence of scCO2in the in situ compat-ibilization could help improve the phase dispersion of the PP/PSblends through the two functions of scCO2mentioned above. Any-way, a primary in situ compatibilization existed in PP/PS3 to PP/PS5as well as in PP/PS2, and its effect on phase dispersion could beobserved through the improved phase dispersion of PP/PS2 relativeto that of PP/PS1. A basic thermal blending also existed in all thePP/PS blends prepared via different approaches. Thus, in additionto a basic thermal blending and a primary in situ compatibilization,the two functions of scCO2were also combined factors that couldcontribute to the improvement of phase dispersion of the PP/PSblends prepared via in situ compatibilization with scCO2. Thosefactors together could explain the better phase dispersion of thenew approach relative to the other ones.