FIGURE 1 Fabrication process of the MMT-PI composite film.
The monolayer MMT was obtained in the first step, and the MMT colloid was yellow and transparent without precipitation, which shows that the MMT was completely stripped. Moreover, the TEM image shows that the size of the peeled MMT nanosheet was about 5 μm (Fig. S1 in Supporting Information), which has a reported thickness of about 1 nm,[35] and thus its aspect ratio of it was 5000. MMT colloid showed the Tyndall effect (Fig. S1 in Supporting Information), which proved that fully stripped MMT nanosheets were successfully obtained after high-power ultrasonic treatment.
Afterward, MMT colloid was mixed with a solution of PAAS. The mixture was temperature-programmed to obtain MMT-PI nacre-like composites by gravity-induced deposition. Fig.2a is the FTIR spectra of MMT, DABPPI, and MMT-PI. The FTIR of MMT has three characteristic peaks. Among them, the peaks at 3631 cm-1 and 3438 cm-1 are assigned to the -OH vibration on the surface of MMT, and the hydrogen bonding interaction between MMT and water molecules, respectively. And the spectrum displayed vibration bands at 1044 cm-1 for Si-O stretching. By observing the results of DABPPI and MMT-PI, the characteristic absorption peaks of the imide ring both appeared around 1771 cm-1 (C=O stretching vibration) and around 1370 cm-1 (C-N stretching vibration).[36, 37] Due to the existence of DABA, both DABPPI and MMT-PI have characteristic peaks at 1642 cm-1 (“Amide I”) and 1513 cm-1 (“Amide II”).[38] For MMT-PI, there are also absorption peaks at 3623 cm-1and 3351 cm-1, which are caused by the hydrogen bonding interaction between MMT and PI, respectively. Furthermore, the red shift of the peak caused by Si-O to 1013 cm-1 also proves the occurrence of hydrogen bonding interactions between them. All the above results indicated that due to the use of diamine DABA with an amide bond, the obtained PI could form strong interactions with the hydroxyl groups on the surface and sides of the MMT, thus facilitating the good dispersion of MMT in the PI matrix.
To explore the aggregate structure of the composite film, XRD was used, and the results are shown in Fig. 2b . No obvious diffraction peak was found for DABPPI, so it could be considered an amorphous polymer. For pristine MMT, the results are similar to those reported work,[39] the diffraction peak of the (001) plane appears at 2θ of 7.2o, and the correspondingd spacing is 1.23 nm. The XRD pattern of MMT-PI shows distinct differences compared to MMT. The peak of the (001) plane was shifted to 6.7 o (d spacing is 2.60 nm), which indirectly demonstrated the fact that the MMT was successfully exfoliated. Moreover, the increment of interlayer spacing between MMT slices may be due to the adhesion of PI chains to MMT. Meanwhile, its full width at half maxima (FWHM) narrows with the increase of its peak intensity, implying the existence of a strongly ordered laminated structure in the MMT-PI.
The ordered layered structure of MMT-PI can be further proved by the SEM. The MMT sheets presented the lay-flat arrangement with few defects due to uneven arrangements or aggregation (Fig.2c ). In MMT-PI composites, MMT sheets act as “brick” and PI chains act as “mortar”, together forming a so-called biomimetic nacre-like “brick-and-mortar” layered structure. MMT and PI are uniformly distributed, and they show no phase separation (as can be seen from Fig. S2 a, b , Supporting Information). The surface SEM results (Fig.2d ) show that there are no visible defects, and its surface uniformly covers MMT and PI. Similarly, the MMT and PI distribution is homogeneous and has no phase separation (Fig. S2 c, d in Supporting Information).