Interfacial interactions of Nafion ionomer with superhydrophilic (Pt, Au), hydrophilic (SiO2), and hydrophobic (graphene, octyltrichlorosilane [OTS]-modified SiO2) is investigated, using in situ thermal ellipsometry, by quantification of substrate- and thickness-dependent thermal properties of the ultrathin Nafion films of nominal thickness ranging 25-135 nm. For sub-50 nm thin Nafion films, the thermal expansion coefficient of films decreased in the order of most hydrophobic to most hydrophilic substrate: OTS > graphene > SiO2 > Au > Pt, implying weaker interpolymer and polymer-substrate interactions for films on hydrophobic substrates. Expansion coefficient of films on SiO2, graphene, and OTS-modified SiO2 decreased with thickness whereas that of films on Au and Pt substrates increased with thickness. Above similar to 100 nm of thickness, films on all substrates converged toward a common value representative of bulk Nafion. Thermal transition temperature was found to be higher for films on hydrophilic SiO2 than that for films on hydrophobic graphene and OTS-modified SiO2 but was not discernible for films on Au and Pt substrates. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 343-352