The structure of the interphase in polypropylene (PP)/multiwall carbon nanotube (MWNT) nanocomposite was characterized by thermal analysis and wide angle X-ray diffraction. The interphase was composed of non-covalently coated PP on acid functionalized MWNT (f-MWNT), where the volume of interphase increases with increased f-MWNT concentration from 1 wt% to 30 wt%. Larger lamella thickness, crystallinity, and smaller crystal size with respect to the bulk polymer were found at the interphase. Also, the thermal stability of the nanocomposite containing high volume fraction of interphase was demonstrated to be higher than that of the neat PP. Using designed thermal treatment, the interphase was successfully converted into a highly ordered crystalline structure with extended chain conformation. The understanding and control over the interphase formation at nanoscale afforded the observation of an unprecedented macroscopic rheological behavior where the viscosity and elastic modulus increased as temperature increased. A schematic showing polymer-CNT interaction at elevated temperature was proposed and verified. It is concluded that the constrained flow under high temperature originates from extended and immobile polymer chains at the interphase. (C) 2018 Elsevier Ltd. All rights reserved.