3.3.2. Swelling Ability

The swelling ability is an important factor in composite films and represents its water absorption resistance property and type of film use. The water absorption tendency of the film affects the swelling ability to a certain extent. Consequently, the water absorption of the film shows a trend similar to that of the swelling ability. As shown in Figure 4a,b, the swelling ability of both the films shows a trend that first increases and then decreases, and this was mainly attributed to the influence of functional groups. When the added amount of the inorganic filler (GO/AGO) was less than 0.1% (*w/w*), the hydrophilic functional groups in the inorganic filler resulted in temporary increase in the swelling ability of the film. However, when the amount was more than 0.1% (*w/w*), "blocking effect" caused by the increase of functional groups played the main role, leading to a decline in swelling [15,31]. Specifically, the area swelling ratio of films with 0.5% and 0.9% (*w/w*) AGO was significantly reduced, reaching 15% and 10%, respectively. Moreover, the area swelling ratio of the AGO/SPEEK film was lower than that of the GO/SPEEK film under the same loading, which was ascribed to the less hydrophilic amide groups introduced by AGO, and it narrowed the gap of the SPEEK polymer chain. Thereby, it can be concluded that the incorporation of AGO and SPEEK matrix can improve the usability of films for highhumidity food packaging.

**Figure 4.** Water permeability of AGO/SPEEK films. (**a**) Water uptake ratio, (**b**) area swelling rate, (**c**) water vapor permeability.

3.3.3. Water Vapor Permeability

WVP is one of the most important functional properties for a food packaging film [30]. Film with low WVP has a good water vapor barrier property, which can effectively reduce moisture transfer between the surrounding atmosphere and the food environment. As shown in Figure 4c, the SPEEK film has relatively high moisture permeability, which is attributed to the presence of abundant sulfonic acid groups that gather together to form

hydrophilic domains that generate hydrophilic transport channels [33]. The WVP result of AGO/SPEEK films was found to be similar to the results of WUR. One reason was that dispersed AGO constituted a barrier and hindered the migration of water molecules through the film, while the other was that AGO could extend the path of water molecules through the film, thereby delaying the time to penetrate the film [31]. However, it can be seen that the WVPs of AGO/SPEEK films were both above 50 × <sup>10</sup>−<sup>12</sup> <sup>g</sup>·cm/(cm2·s·Pa), which is higher than that of PE [34]. Therefore, the AGO/SPEEK films were found to be suitable for the preservation of fruits with high moisture content.
