## 3 Results
### Mechanical Effect  
Finally, as mentioned above, the modification of mechanical properties due to hydrothermal aging was also studied by DMTA. Table 3 collects the results obtained for epoxy resin and composites reinforced with different types of graphene at different aging states. The specimens were aged during 1 and 17 days, respectively. The water uptake on pristine resin induces an important increase of stiffness, a slight increase of mechanical strength and a decrease of elongation. These phenomena confirm that the postcuring reaction is catalyzed by the absorbed water, increasing the crosslinking degree. However, this effect \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Moisture Absorption (\%)** & **E (GPa)** & **\(\sigma\) (MPa)** & **\(\varepsilon\) (\%)** \\ \hline \multirow{3}{*}{**Epoxy resin**} & 0 & 2.24 \(\pm\) 0.24 & 146 \(\pm\) 20 & 5.7 \(\pm\) 1.3 \\  & 0.86 & 2.32 \(\pm\) 0.02 & 137 \(\pm\) 7 & 4.5 \(\pm\) 0.2 \\  & 1.55 & 3.61 \(\pm\) 0.44 & 166 \(\pm\) 29 & 3.5 \(\pm\) 0.3 \\ \hline \multirow{3}{*}{**GNP1/epoxy**} & 0 & 4.20 \(\pm\) 0.37 & 140 \(\pm\) 10 & 1.9 \(\pm\) 0.5 \\  & 0.92 & 3.94 \(\pm\) 0.02 & 120 \(\pm\) 2 & 2.1 \(\pm\) 0.1 \\  & 1.79 & 3.87 \(\pm\) 0.23 & 95 \(\pm\) 3 & 1.9 \(\pm\) 0.1 \\ \hline \multirow{3}{*}{**GNP2/epoxy**} & 0 & 4.30 \(\pm\) 0.80 & 111 \(\pm\) 4 & 2.6 \(\pm\) 0.4 \\  & 0.84 & 3.41 \(\pm\) 0.05 & 133 \(\pm\) 12 & 3.2 \(\pm\) 0.4 \\  & 1.49 & 3.39 \(\pm\) 0.09 & 127 \(\pm\) 20 & 2.9 \(\pm\) 0.3 \\ \hline \multirow{3}{*}{**GNP3/epoxy**} & 0 & 3.52 \(\pm\) 0.53 & 97 \(\pm\) 17 & 2.0 \(\pm\) 0.1 \\  & 0.80 & 3.48 \(\pm\) 0.03 & 113 \(\pm\) 2 & 2.4 \(\pm\) 0.1 \\  & 1.49 & 3.42 \(\pm\) 0.51 & 101 \(\pm\) 1 & 2.3 \(\pm\) 0.4 \\ \hline \multirow{3}{*}{**GNP4/epoxy**} & 0 & 3.33 \(\pm\) 0.0 & 136 \(\pm\) 5 & 2.7 \(\pm\) 0.1 \\  & 0.85 & 3.09 \(\pm\) 0.05 & 93 \(\pm\) 4 & 2.9 \(\pm\) 0.2 \\ \cline{1-1}  & 1.47 & 3.18 \(\pm\) 0.16 & 137 \(\pm\) 18 & 3.3 \(\pm\) 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of neat epoxy resin and composites at different aging states. is scarcely observed in composites. The stiffness and mechanical properties remain constant or suffer slight decreases by the moisture absorption. The decrease of mechanical properties could be associated to two different causes. Firstly, the water induces plasticization and also promotes the weak GNP-matrix interphase. Secondly, the slight increase of elongation on graphene/epoxy composite is clearly explained by the plasticization effect of the matrix. Initially, the mechanical behavior of non-aged samples is analyzed. The graphene addition induces an important increase of stiffness in spite of the high elastic modulus of the pristine resin. The addition of 0.5% GNPs induces an increase of the elastic modulus from 2.24 GPa to 3.94 GPa (76%) for GNP1/epoxy composites. The modulus increase of the composites seems to be proportional to the specific surface area of the graphene nanoplatelets, being the highest for GNP1 (76%) and the lowest for GNP3 and GNP4 (41%). In contrast, the mechanical strength and elongation at break markedly decrease with the graphene addition. This is mainly associated with the weak interphase, which makes necessary the incorporation of functional groups in the nanofiller in order to enhance a chemical interphase with the epoxy matrix [17; 18]. In contrast to the thermomechanical properties, the influence of moisture in mechanical properties of the neat epoxy resin and nanocomposites differ. In order to analyze it in depth, Figure 6 shows the variation of each flexural property as a function of the amount of absorbed moisture for neat epoxy resin and graphene/epoxy composites. In general, at the same moisture content, the mechanical properties of epoxy resin are significantly influenced by the presence of water in the network while the ones of composites are slightly influenced. Similar behavior was observed in the water uptake effect on the glass transition temperature.