## 3 Results and Discussion
### Water-Uptake and Ion Exchange Capacity  
The water-uptake ability of the P(_bis_MEP)/BC nanocomposites was estimated by immersing the membranes in water during 48 h at room temperature. The _WU_ data (Table 4) shows the increase of this parameter from 79 \(\pm\) 6% to 155 \(\pm\) 8% with the increasing content of P(_bis_MEP) from 21 wt % to 50 wt %. Such an increase is probably due to the increasing concentration of highly hygroscopic acid groups in P(_bis_MEP), which are likely to interact electrostatically with the adsorbed water molecules. One observes that the value obtained for P(_bis_MEP)/BC_1 is lower than the _WU_ value of a pure BC membrane (121 \(\pm\) 11%). The difference, although statistically significant with respect to the series of membranes prepared in this work, is within the natural dispersion of the water-uptake determined in previous works for BC. For example, and restricting to our own results, we have reported values of 100% [20; 21] or even 165% [24]. Furthermore, both nanocomposites exhibit water-uptake values higher than those typically found for the commercial Nafion(r) ionomer (54%) [46], but lower than the corresponding values obtained for the cross-linked PMOEP/BC membranes prepared in our previous study [21].  
Table 4 also displays the IEC of the P(_bis_MEP)/BC nanocomposite membranes. This data demonstrate that IEC increased from 1.1 \(\pm\) 0.12 mmol g\({}^{-1}\) to 3.0 \(\pm\) 0.05 mmol g\({}^{-1}\) with the augment of P(_bis_MEP) content from 21 wt % to 50 wt % (P(_bis_MEP)/BC_1 and P(_bis_MEP)/BC_2, respectively). Regardless of the null IEC value of BC due to the absence of ionic groups, the results obtained for the nanocomposite membranes were expectable given the electrolytic nature of P(_bis_MEP) exhibiting  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Membrane \({}^{*}\)** & **Young’s Modulus/GPa** & **Tensile Strength/MPa** & **Elongation at Break/\%** \\ \hline BC & 10.2 \(\pm\) 2.21 & 222 \(\pm\) 47.0 & 4.1 \(\pm\) 0.93 \\ P(_bis_MEP)/BC\_1 & 6.22 \(\pm\) 0.95 & 47.8 \(\pm\) 9.4 & 0.68 \(\pm\) 0.30 \\ P(_bis_MEP)/BC\_2 & 2.68 \(\pm\) 0.41 & 23.5 \(\pm\) 3.3 & 1.1 \(\pm\) 0.24 \\ \hline \hline \end{tabular} \({}^{*}\)See Table 1 for membrane identification.  
\end{table}
Table 3: Tensile tests data: Young’s modulus, tensile strength and elongation at break of the P(_bis_MEP)/BC nanocomposites.  
an IEC of 3.5 \(\pm\) 0.02 mmol g\({}^{-1}\). The P(_bis_MEP)/BC_2 nanocomposite membrane with the highest IEC value will be the one exhibiting the higher protonic conductivity, since the IEC represents the quantity of ionizable groups that are responsible for the protonic conductivity of the membranes, as will be discussed in detail in the following section. Furthermore, the IEC values are in accordance with the _WUI_ capacity confirming that the latter parameter is also deeply correlated to the phosphate group of the polyelectrolyte. A similar trend was observed for the nanocomposites composed of cross-linked PMOEP (i.e., diprotic acidic phosphate bearing polymer, which is the monofunctional version (one polymerizable double bond) of _bis_MEP) and BC, whose IEC values were 1.95 mmol g\({}^{-1}\) and 3.38 mmol g\({}^{-1}\) for the nanocomposites containing 22 wt % and 48 wt % of the phosphate containing polymer, respectively [21]. These values are comparable to that of commercial membranes such as Nafion(r) (0.95-1.01 mmol g\({}^{-1}\), DuPont Co., Wilmington, DE, USA), fumasep(r) FKB and FKD (1.2-1.4 mmol g\({}^{-1}\), FuMA-Tech GmbH, Biettigheim-Bissingen, Germany), and JCM-II (1.8-2.9 mmol g\({}^{-1}\), Tingrun Co., Ltd., Beijing, China) [27].