## 3 Results
### Single Lap Shear Tests  
Figure 3 shows the lap shear strength (LSS) of the SLS specimens for each condition. It was observed that the LSS strength was significantly affected by the aging conditions. In every condition, a significant decrease of the LSS was observed after 1 month of aging while, for most of the cases, a slight recovery of the LSS was noticed by increasing the aging time. This different behavior can be \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multicolumn{2}{c}{**Non-Aged \(T_{g}\) (\({}^{\circ}\)C)**} & \multicolumn{2}{c}{**2-Week-Aged \(T_{g}\) (\({}^{\circ}\)C)**} \\  & **1 st Scanning** & **2 nd Scanning** & **1 st Scanning** & **2 nd Scanning** \\ \hline Neat adhesive & 117.0 & 146.0 & 115.5 & 146.0 \\
0.00 SDS & 144.0 & 148.0 & 119.0 & 146.0 \\
0.25 SDS & 148.5 & 149.5 & 118.0 & 148.5 \\
1.00 SDS & 146.0 & 147.5 & 109.0 & 149.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Glass transition temperature for different in-bulk conditions. Figure 2: Schematics of combined effect of the water uptake (**left**) and amphiphilic behavior of sodium dodecyl sulfate (SDS) (**right**). explained by attending the water uptake results and also by the different chemical interactions inside the adhesive joint. First of all, it is necessary to understand the role of aging mechanisms due to water absorption. The water uptake generally induces the creation of weak hydrogen bonds, leading to a swelling of the polymer chains and causing, thus, a drastic reduction of the physical properties. This effect is called plasticization and is detrimental to the mechanical properties [41]. However, after this initial stage, a slight recovery is generally observed. This has already been shown in other studies [43; 48; 49] and can be explained by a change in the mechanism of water absorption. After reaching the water uptake saturation, longer aging times can induce a transformation of the weak hydrogen bonds into multiple chemical connections between the water molecules and the polymer chains, promoting an increase in secondary crosslinking, thus leading to a stiffening of the material and also an embrittlement, as observed in the examples given in Figure 4a. This cannot be confirmed by the \(T_{g}\) as there was only one measurement. Therefore, it is necessary to focus on the mechanical response of the adhesive joints, particularly on the displacement at failure. In the particular case of the neat adhesive, there was only a partial conversion of the epoxy matrix at the initial state, as the \(T_{g}\) was significantly below the maximum value (\(\sim\)150 \({}^{\circ}\)C). Therefore, the water uptake does not affect significantly the plastic properties of the epoxy matrix, as it presents a more prevalent plasticization effect at the initial state. This was confirmed by a slight variation of the displacement at failure with aging time, as shown in Figure 4b. However, in the case of CNT-doped adhesive joints, there are some different mechanisms. Here, the CNT addition itself and the dispersion state, dominated by the surfactant addition, play an important role in the aging mechanisms. As commented previously, a good dispersion implies a  
Figure 4: Mechanical effect of water uptake showing (**a**) a plasticization stage and (**b**) the displacement at failure. Figure 3: LSS of adhesive joints for each testing condition. high homogeneity of CNT distribution in the matrix, improving the barrier properties. This acts in an opposite way to the amphiphilic behavior of SDS. In addition, the SDS also induces variations in the chemical interactions between the epoxy matrix and the CNTs, leading to a more drastic reduction of the \(T_{g}\), as shown in Table 2. This was also observed by the displacement at failure shown in Figure 4b. In this case, the higher the SDS content, the higher the displacement at failure was. Moreover, it can be noted that the displacement at failure decreases after 2 months of aging.