FIGURE 2 The FTIR spectra of MMT, DABPPI, and MMT-PI (a ). The XRD spectra of MMT, DABPPI, and MMT-PI (b ). The cross-section (c ) and surface (d ) SEM images of MMT-PI.
Layered silicate, such as MMT, is reported to be impermeable to gas molecules,[40] which makes gas molecules difficult to penetrate because of its perfect crystal structure. Therefore, the addition of MMT to PI would create a physical obstacle to the diffusion of gas molecules in the composite. From the above SEM results, it could be inferred that it would be very difficult for gas to pass through the composite material with such a “brick-and-mortar” layered structure, which may greatly improve the gas barrier performance of the MMT-PI composite film.
The results of the gas barrier properties of DABPPI and MMT-PI are shown in Table 1 . Surprisingly, the WVTR of MMT-PI was significantly decreased. The value of WVTR decreased from 2.25 g/m2/day for DABPPI to 0.0185 g/m2/day for MMT-PI, a decrease of 99.18%. The results of previous studies have shown that the improvement in water vapor barrier properties of nacre-like composites is not very significant due to the hydrophilicity of the raw materials used. This concern is ameliorated by using PI as “mortar”. DABPPI is a kind of water-insoluble PI with relatively low water absorption. Combined with the dense lamellar structure, the diffusion of water molecules can be well hampered, thereby improving the water vapor barrier performance. The nacre-like structure has exceptional barrier properties for non-condensed, non-polar gases, like oxygen, which has also been confirmed by early studies.[41, 42] In this study, the oxygen permeability of MMT-PI was decreased conspicuously. Specifically, the DABPPI showed an OTR of 1944.69 cm3/m2/day/0.1MPa. While with the addition of 50% MMT, the OTR decreased to 7.98 cm3/m2/day/0.1MPa, a decrease of three orders of magnitude. With this simple method, MMT-PI composite film with noticeably improved water vapor and oxygen barrier properties can be achieved, which will provide a feasible strategy for the preparation of flexible high gas barrier films.
TABLE 1 The WVTR, OTR, and LOI of the DABPPI and MMT-PI