## 3 Results and Discussion
### Mechanical Properties  
The mechanical properties of P(_bis_MEP)/BC_1 and P(_bis_MEP)/BC_2 nanocomposites were evaluated by typical tensile tests and the results of Young's modulus, tensile strength and elongation at break are summarized in Table 3. Data for the P(_bis_MEP) homopolymer could not be obtained given its low film-forming ability. Regarding the pure BC membrane, the attained results showed a Young's modulus of 10.2 \(\pm\) 2.21 GPa, tensile strength of 222 \(\pm\) 47.0 MPa and elongation at break of 4.1 \(\pm\) 0.93%, which reach agreement with values reported in literature [41].  
In the case of the nanocomposites, the Young's modulus and tensile strength increased with the increasing content of BC (50 wt % and 79 wt %), with the latter parameter experiencing a higher variation, particularly in the case of the nanocomposite containing 50 wt % of P(_bis_MEP). In fact, P(_bis_MEP)/BC_2 exhibits a tensile strength value of almost one order of magnitude lower than pure BC, whereas the Young's modulus is just four times inferior (Table 3). The impact is considerably lower in the case of the nanocomposite with 21 wt % polyelectrolyte (P(_bis_MEP)/BC_1), with the Young's modulus and the tensile strength attaining about 61% and 21.5% of the values of pure BC,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample \({}^{\star}\)** & **T\({}_{\text{d1}}\)/\({}^{\circ}\)C** & **T\({}_{\text{dmax1}}\)/\({}^{\circ}\)C** & **T\({}_{\text{dmax2}}\)/\({}^{\circ}\)C** \\ \hline P(_bis_MEP) & 230 & 289 & – \\ BC & 266 & 347 & – \\ P(_bis_MEP)/BC\_1 & 205 & 234 & 300 \\ P(_bis_MEP)/BC\_2 & 195 & 235 & 303 \\ \hline \hline \end{tabular} \({}^{\star}\)See Table 1 for sample identification.  
\end{table}
Table 2: TGA data of the P(_bis_MEP), BC and P(_bis_MEP)/BC nanocomposites.  
respectively. This outcome was expected given the superior mechanical performance of BC associated with the nanofibrils high aspect-ratio and highly ordered self-assembled nanostructure. The ultrafine nanofibrous network of pristine BC is indeed one of the major contributions of this biopolymer to the development of partially bio-based proton-exchange membranes for fuel cell applications. As an illustrative example of this feature, [PERSON] and co-workers [15] functionalized BC with carboxylic moieties, which drastically reduced the tensile strength and modulus of the carboxymethylated-BC biopolymer to values of about 413 kPa and 814 kPa, respectively. Furthermore, the inclusion of polyaniline (conducting polymer) in a carboxymethylated-BC network further decreased the mechanical performance of the composite gel membranes [16]. Worth noting is the fact that these P(_bis_MEP)-based nanocomposite membranes present a better mechanical performance than Nafion(r) (the reference perfluorosulfonic acid membrane) in terms of Young's modulus (0.25 GPa) and tensile strength (43 MPa) [42].  
On the other hand, the elongation at break of the nanocomposites is also lower when compared to the pure BC membrane (Table 3), probably due to a stronger constriction effect leading to an increase of the applied stress (with respect to the nominal stress) in the fracture region (with smaller cross-sectional area), going beyond the fracture point. In fact, the BC nanofibril network is known for exhibiting an auxetic behavior, i.e., negative Poisson ratio, for in-plane deformation [43]. This negative Poisson ratio implies that the network expands in the transverse direction when stretched [43; 44; 45]. The nanocomposite membranes are expected to display a conventional behavior with positive Poisson ratio, due to the fraction of P(_bis_MEP) and their lower crystallinity. The higher content of P(_bis_MEP) present in P(_bis_MEP)/BC_2 decreases the Young's modulus and tensile strength leading to a more elastic nanocomposite, in agreement with an elongation at break larger than for P(_bis_MEP)/BC_1.