## 3 Results
### Thermal Effect  
The effect of the water absorption in the epoxy resin and composites was studied by DMTA. DMTA specimens were introduced in the climatic chamber and removed at selected periodic times. In all cases, the curves of loss of tangent (tan \(\delta\)) presented only one peak. A shoulder at low temperature was not observed in any case, this behavior has also been observed by other authors [5; 6]. The reason is the low moisture content absorbed by the studied system. Differences in T\({}_{\mathrm{g}}\) due to duplicate samples did not exceed 1-2 \({}^{\circ}\)C, while the standard deviation obtained in the storage modulus in the glassy state was higher, close to 10%. For this reason, the effect of moisture uptake on the mechanical properties was studied by flexural test. In spite of the good reproducibility of T\({}_{\mathrm{g}}\) measurements, the T\({}_{\mathrm{g}}\) decreases due to the fact that the plasticization effect was small, in account of the low water absorption content. For this reason, the T\({}_{\mathrm{g}}\) was measured at different aging times in order to obtain the tendency, which is shown in Figure 5.  
Figure 5: Decrease of glass transition temperature due to the water absorption.  
As produced, the glass transition temperature increases by the GNPs addition in a range of 4 or 5 \({}^{\circ}\)C, except for the composites reinforced with GNP4, whose Tg remains constant. Again, it is worthy to note that the initial Tg value of pristine resin is very high due to its high crosslinking degree, associated to its high curing temperature (140 \({}^{\circ}\)C). This could be associated to the rheological percolation [5; 17]. Nanofililer loads close to percolation hinder the mobility of polymer chain segments. The percolation threshold decreases with the increase of the specific surface area of nanofillers. GNP4 has lower specific surface area and higher thickness (Table 1), which implies that individual nanoparticles, due to their higher volume, are not as close as they are when using GNP1, GNP2 or GNP3 (see Figure 3c), limiting its influence on the polymer chain mobility and making glass transition temperature remain constant.  
The moisture absorption induces a Tg decrease. This is the best-known consequence of the water plasticization. As was expected, the Tg decrease is more noticeable at the beginning of the aging treatment and, after that, Tg remains nearly constant. This could be related to a catalytic effect. Absorbed water molecules can form hydrogen bonds (named Type II or bonded water) with some remaining oxirane groups, enhancing a postcuring reaction. In fact, this phenomenon could explain the dispersion of glassy storage modulus values. On the other hand, the plasticization effect of water should decrease the rigidity of the matrix, enhancing the chain mobility, but the resultant postcuring reaction would increase the crosslinking degree. Each of them provokes contrary effects on the storage modulus.  
The Tg of neat epoxy resin decreases 3.6 \({}^{\circ}\)C per each 1% in weight of moisture absorbed while this effect decreases in composites up to 2.5 \({}^{\circ}\)C. This means that the addition of graphene nanoplatelets shows numerous advantages related to the hydrothermal resistance: Decreasing the absorbed water content and the consequences of its plasticization.