Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2019-03, Vol.57 (6), p.343-352
• Main Authors: Zhang, Chi, Davies, Michael, Karan, Kunal
• Format: Article
• Language: English
• Subjects: Ellipsometry; Gold; Graphene; Hydrophilicity; Hydrophobicity; interfacial interactions; ionomers; Nafion; Platinum; Silicon dioxide; Substrates; Thermal expansion; Thermodynamic properties; Thickness; Thin films; Transition temperature
• ISSN: 0887-6266; 1099-0488
• DOI: 10.1002/polb.24792

ABSTRACT 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 ~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. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 343–352 Thin film studies have recently established that interfacial interactions of low‐crystallinity Nafion ionomers with different substrates influence the structure and properties. However, there are no simple methods or metrics to assess the favorability of these interactions. In this work, the authors demonstrate that Nafion thin film thermal expansion coefficients, determined using in situ thermal ellipsometry, show remarkable dependency on substrate hydrophilicity/hydrophobicity and may be useful for assessing the favorability of interfacial interactions.