2: Recent Progress on Nafion-based Nanocomposite Membranes for
2.4 Preparation and Characterization of Nafion-based Nanocomposite
2.4.4 Various Nanoparticles Containing Nafion Composite Membranes
properties of the composite membranes may be the key to the performance of composite membranes.
Kim et al. have produced self-humidifying membranes by dispersing palladium (Pd) nanoparticles into the Nafion matrix.[95] These nanoparticles triggered water formation as a result of oxygen reduction and also methanol consumption by oxidation. This type of self-humidified membrane with a lower methanol cross-over exhibits a proton conductivity (C) and methanol permeability (P), i.e., P/C of 1.5, which is higher than that of neat Nafion membrane. In recent years, preparation and characterization of various other types of nanoparticles containing Nafion membranes have also been reported.[97–101]
2.5 Properties of Nafion-based Composite Membranes
Intrinsic properties of unmodified Nafion, such as lower thermal stability, poor mechanical properties, and high liquid permeability, puts limit on its applications. Nafion based nanostructured membranes consisting of nanofillers, such as clay, CNTs, silica or metal oxides, etc., frequently exhibit significant improvement of various properties when compared to those of neat polymer. Improvements generally include increased thermo-mechanical properties and thermal stability, and increased ion conductivity and liquid permeability.
2.5.1 Mechanical Properties
Dynamic mechanical analysis (DMA) has been used to study the temperature dependence of the storage modulus (G') of Nafion matrix on nanocomposite formation under different experimental conditions.[96] In general, Nafion shows two typical glass transition temperatures: –transition at about 120C and β–transition at about 20°C, which are assigned to the nonpolar backbone and polar clusters, respectively.[77] The DMA results of neat Nafion and its clay-containing nanocomposite membranes with two different clay loadings are presented in Figure 2.9.[57] It is clear from the figure that the nanocomposite membranes show dramatic increases in storage modulus. The nanocomposite containing 20 wt% clay shows a 6-time increase at low temperatures and orders of magnitude above 100°C. DMA results indicate that the
nanocomposite membranes are much stiffer and can withstand higher temperatures compared to the neat Nafion membrane. On the other hand, neat Nafion membranes show a high temperature α–transition characteristic of the polar group clusters at around 125°C. In contrast to neat Nafion membranes, the α–transition of the nanocomposite membranes shifts to much higher temperatures. For the nanocomposite membrane containing 20 wt% clay, the transition shifts to 215°C and the membrane becomes much weaker.
Figure 2.9 Storage modulus as a function of temperature of Nafion and Nafion–clay nanocomposites containing 10 and 20 wt% H+–MMT. Reproduced from Ref. [57a] with permission from Elsevier Science Ltd.
Kim et al.[96] studied the correlation between modulus and water uptake for the BPSHs [sulphonated poly(arylene ether sulphone) copolymers] and results showed that the former has approximately a seven times higher modulus than Nafion when water uptake was less than 30%, but the difference in modulus decreases as a function of increasing water uptake. This suggests that the higher modulus for the BPSHs after water uptake is a result of the stiffer aromatic backbone structure of the copolymer compared to the flexible perfluorocarbon structure of Nafion. The slow modulus decrease may be related to the semi-crystalline structure in Nafion.
These results are consistent with the previous stress-strain data of both systems under dry and wet conditions. The tensile strength and modulus of BPSHs were generally higher than those of Nafion resin and decreased with water uptake. Moreover, the elongation at break beyond the tensile yield of the copolymers increased with water uptake while a nearly constant elongation was observed in Nafion resin (see Fig. 2.10).[86, 87] The relatively small elongation change of Nafion under wet conditions might also be due to its semi-crystalline phase.
Figure 2.10 Tensile properties of Nafion/MMT nanocomposites: (a) stress–strain curves and (b) tensile elongation. Reproduced from Ref. [53] with permission from Elsevier Science Ltd.
The measurement of tensile properties is an efficient way to estimate the compatibility of Nafion matrix with organo-clays. Figure 2.10(a) shows the stress-strain curves of Nafion/MMT nanocomposite membranes.[53] It was reported that once nanolayer exfoliation has been achieved, the polymer/clay nanocomposites exhibit significantly improved properties compared with the neat polymer or the conventional composite.
The maximum strength and elongation at break for extruded Nafion membrane usually exceed 30 MPa and 200%, respectively. In the case of recast Nafion/MMT composite membranes, at a loading of 3 wt% clay, the strength increased more than 35% and the tensile elongation almost doubled than that of neat Nafion membrane. This might also result from the complete dispersion or exfoliation of the clay particles in the polymer matrix. As shown in Figure 2.11(b), the elongation at break of composite membranes reveals a positive deviation until filler content was 15 wt%. This is in good agreement with XRD results.
Figure 2.11 Young’s modulus of Nafion membranes filled with MWCNT, com. MWCNT–
COOH and lab. MWCNT–COOH as a function of the filler content. Reproduced from Ref. [74]
with permission from Elsevier Science Ltd.
Thomassin et al. measured the tensile properties of MWCNT-containing Nafion membranes.[74]
Figure 2.11 represents Young’s modulus of Nafion membranes filled with MWCNTs, comer.
MWCNTs–COOH, and lab MWCNT–COOH were used as a function of the filler content. [100]
The modulus was increased by 140% and 160% for MWCNTs contents of 1 and 2 wt% as compared to the neat Nafion, respectively. The reinforcing effect of MWCNTs should delay any membrane failure that occurs rapidly when Nafion is hydrated at a temperature higher than 80C.
The increase in the modulus was slightly higher in the case of lab. MWCNT–COOH, consistent with a better dispersion of MWCNTs in the Nafion matrix. Again, the shorter length of com.
MWCNT–COOH accounts for a lower increase in modulus observed in this case. However, in each case, Young’s modulus levels off at MWCNTs contents exceeding 2 wt%. This may be due to the increase aggregation of the tubes in the Nafion matrix.