The mechanical properties of Nafion are explained and modeled on the basis of Kafka’s general mesomechanical model and confronted with experimental results. In this approach, Nafion is looked upon as a composite consisting of three constituents: a crystalline Nafion, amorphous Nafion, and water. Taking into account the degree of hydration, its elastic, elastic-plastic, and hysteretic properties are discussed and modeled. It is shown how the interaction between the three constituents manifests itself on the macroscale.
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Abbreviations
- σ ij :
-
stress tensor
- δ ij σ :
-
isotropic part of σ ij (σ = σ ii /3)
- s ij :
-
deviatoric part of σ ij (σ ij − δ ij σ)
- ε ij :
-
strain tensor
- δ ij ε :
-
isotropic part of ε ij (ε = ε ii /3)
- e ij :
-
deviatoric part of ε ij (ε ij − δ ij ε)
- δ ij :
-
Kronecker’s delta
- E :
-
Young’s modulus
- ν:
-
Poisson’s ratio
- μ = (1+ν)/E :
-
deviatoric elastic compliance
- ρ = (1 − 2ν) / E :
-
isotropic elastic compliance.
- \( \overline{\Big|} \) :
-
overbar that relates the symbol | to its macroscopic value — the average in the representative volume element RVE
- |ω or | ω :
-
index that relates the symbol | to the ω -constituent — the average in the subvolume of RVE that is filled in by the ω -constituent
- ω = e :
-
elastic constituent in the general two-phase model
- ω = n :
-
inelastic constituent in the general two-phase model
- ω = a :
-
amorphous constituent in Nafion
- ω = c :
-
crystalline constituent in Nafion
- ω = w :
-
water comprised in Nafion
- ω = wa :
-
aggregate of two constituents in Nafion: of the amorphous Nafion with water
- | ij :
-
Einstein’s notation
\( {\varepsilon}_{ij}^{\prime }={\varepsilon}_{ij}-{\overline{\varepsilon}}_{ij}; \)
- δ ij ε′:
-
isotropic part of ε ′ ij
- e ′ ij :
-
deviatoric part of ε ′ ij
- σ ′ ij :
-
stress related to ε ′ ij similarly as σ ij is related to ε ij
- δ ij σ′:
-
isotropic part of σ ′ ij
- s ′ ij :
-
deviatoric part of σ ′ ij
- ν e {ν n }:
-
volume fraction of the elastic {inelastic} constituent in the general two-phase model
- ν a {ν w }:
-
volume fraction of the amorphous Nafion {of water} in the aggregate of amorphous Nafion with water
- V ω :
-
volume fraction of the ω - constituent in the total Nafion (ω = a, c, w, wa);
\( {R}_a^c=\frac{V_c}{V_a}; \)
- c a :
-
elastic limit of s a11 in the a -constituent
- C wa :
-
elastic limit of s wa11 in the wa -constituent
- n ω :
-
structural parameter (ω = a, c, w, wa)
- |Ω :
-
superscript that relates the symbol | to the respective Ω -specimen of Nafion
- Ω = H :
-
hydrated specimen
- Ω = D :
-
dry specimen
p = V H c η c μ c + V H wa η wa μ H wa
\( q=\frac{\mu_c}{V_c^H}\left(p\left|+{\eta}_c{\eta}_{wa}{\mu}_{wa}^H\right.\right) \).
- \( \overset{\cdot }{h} \) :
-
formal variable equal to 0 in elasticity and to \( \overset{\cdot }{\lambda } \) in plasticity
- | L :
-
value of | at the elastic limit.
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Acknowledgements
This work was supported by the Czech Science Foundation within projects P108/10/1296 and 103/09/2101. Acknowledged is also the support through the Institutional Project RVO: 68378297. D. Vokoun would like to thank Dr. M. Paidar for fruitful discussions.
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Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 50, No. 6, pp. 1065-1082 , November-December, 2014.
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Kafka, V., Vokoun, D. A Three-Scale Model of Basic Mechanical Properties of Nafion. Mech Compos Mater 50, 763–776 (2015). https://doi.org/10.1007/s11029-015-9466-y
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DOI: https://doi.org/10.1007/s11029-015-9466-y