Figure 5. OTR of (TA@LDH/PEO)50 hybrid films with different ratios of TA: LDH.
Hydrogen bonds are much weaker than electrostatic interactions but stronger than van der Waals forces. Therefore, hydrogen-bonded films have good tensile properties.27 Because hydrogen bonding is very important for mechanical properties, we regulated the ratio of TA to LDH. The OTR of four kinds of thin films with different ratios and layers were measured. Besides the (TA@LDH/PEO)50 film with TA: LDH of 3: 1, the samples with other proportions (1:1, 2:1 and 4:1) also exhibit the same change trend for OTR with increasing the bilayer number (Figure S4, Support Information).
No matter what the ratio of TA to LDH, the OTR of the films increased slightly after stretching (Figure 5). The oxygen resistance of the film is still good after stretching. However, with the decrease of LDH content in TA@LDH, the attenuation of the oxygen barrier is slower upon stretching. When the ratio of LDH nanosheets is 50%, the OTR of 120% stretching is 79% higher than that without stretching (from 99 to 177cm3/(m2·24h·0.1MPa)). When the ratio of LDH is 20%, the OTR of 120% stretching is only 23% higher than that without stretching (from 120 to 149 cm3/(m2·24h·0.1MPa)). These results show a higher content of TA is favorable to the film ductility due to the presence of elastic hydrogen network, although the incorporation of LDH nanosheet improves the barrier property.
Considering the balance between oxygen barrier and tensile properties, the best ratio of LDH is 25% (i.e. TA: LDH = 3:1). At 50-bilayer, the OTR is 110 cm3/(m2·24h·0.1MPa), and its oxygen barrier capacity is 28× higher than that of NR substrate. When the stretch ratio was 25%, the OTR increased by 12% to 124 cm3/(m2·24h·0.1MPa). The OTR is 128 cm3/(m2·24h·0.1MPa) at 50% stretching, which is 17% higher than that of the unstrained film. It still has good oxygen resistance at 100% stretching, with OTR increased by 20% to 133 cm3/(m2·24h·0.1MPa).
As shown in Table 1, the (TA@LDH/PEO)50 hybrid film has significant advantages over other films in published work. Although the oxygen barrier property of (TA@LDH/PEO)50 film is not the best, its tensile deformation ratio reaches 120%, which is not achieved by other works. And after 120% stretching, the permeability is only 29% higher than that without stretching. It can be found that increasing the tensile capacity needs to sacrifice part of the oxygen barrier capacity. Even so, our (TA@LDH/PEO)50 film shows simultaneously excellent tensile property and competitive oxygen barrier capability.
Table 1. Comparative study on gas barrier performance after stretching between our (TA@LDH/PEO)50 film and reported materials