Kushner Douglas I, Hickner Michael A
Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
ACS Macro Lett. 2018 Feb 20;7(2):223-227. doi: 10.1021/acsmacrolett.7b01004. Epub 2018 Feb 2.
The humidity-induced physical aging, or structural relaxation, of spin-cast Nafion thin films on gold, carbon, and native oxide silicon (n-SiO) substrates was examined using spectroscopic ellipsometry (SE). Physical aging rates, β, were calculated from the change in measured sample thickness, , upon exposure to controlled humidity. Three Nafion films, = 188, 57, and 27 nm, deposited on gold substrates demonstrated an increased β with decreasing thickness due to confinement. The Nafion film on n-SiO, = 165 nm, also showed a humidity-induced aging, while a Nafion film deposited on carbon, = 190 nm, exhibited no measurable humidity-induced aging. The reported rate of aging was related to the strength of the polymer/substrate interactions during film formation. Strong interactions between Nafion and the gold and n-SiO substrates anchored the thin film to the substrate during film formation, resulting in a nonequilibrium as-cast film and subsequent relaxation upon exposure to water vapor until complete plasticization. Weak interactions between the carbon substrate and Nafion resulted in fully relaxed as-cast films which displayed no relaxation upon hydration.
使用光谱椭偏仪（SE）研究了旋涂在金、碳和原生氧化硅（n-SiO）衬底上的Nafion薄膜的湿度诱导物理老化或结构弛豫。物理老化速率β是根据在受控湿度条件下测量的样品厚度变化计算得出的。沉积在金衬底上的三种厚度分别为188、57和27 nm的Nafion薄膜，由于受限效应，随着厚度减小β增大。沉积在n-SiO上厚度为165 nm的Nafion薄膜也表现出湿度诱导老化，而沉积在碳上厚度为190 nm的Nafion薄膜未表现出可测量的湿度诱导老化。所报道的老化速率与成膜过程中聚合物/衬底相互作用的强度有关。Nafion与金和n-SiO衬底之间的强相互作用在成膜过程中将薄膜固定在衬底上，导致形成非平衡的铸态薄膜，随后在暴露于水蒸气时弛豫，直至完全增塑。碳衬底与Nafion之间的弱相互作用导致形成完全弛豫的铸态薄膜，其在水合时不发生弛豫。
