BC pellicle

Fig. 10 SEM of the supramolecular structure of a BC pellicle, b disintegrated BC, c MFC. Reprinted with permission from [54]...

According to the impregnation approach, the BC pellicle was immersed in the chitosan solution for different periods of time. It was found that chitosan molecules would not only coat the BC surface but also penetrate through the BC fibrils. Chitosan was well incorporated with nanofibrils, resulting in a denser and more... 

In the comparison of BC and the BC-PVA composite, the crystallinity of BC in the composite was decreased by 50%. By immersing wet BC pellicles in polyvinyl alcohol [PVA] solution, PVA could penetrate the BC network structure and filled the pores. Acetal linkages were formed in the BC-PVA composites by a cross-linking reaction, which made PVA insoluble in water. However, the crystal destruction was observed after the cross-linking [84]. 

PVA could penetrate into the BC structure and enwrapped the BC fibrils. From the SEM images, it could be observed that the diameter of the composite fibrils was larger than that of unmodified BC [84]. However, the overall structure of BC and BC-PVA composite, fiber thickness, fiber distribution and three-dimensional orientations were quite similar [85]. The SEM examination of BC and BC-PVA revealed that interpenetrated networks could be formed by integrating PVA fiber into the original BC pellicle [85]. The drying method and conditions also have significant effects on the composite structure. The BC-PVA composites dehydrated by freeze-drying process [84] had a structure that was different from those dried in an incubator oven [85]. When PVA fibers are heated, they contract [86]. 

Although the fracture strain of PVA was very high, the fracture strain of the BC-PVA composite prepared hy immersing BC pellicle in PVA was much lower than that of PVA and close to that of BC. It was suggested that bonding between BC and PVA in the composite was extremely tight, resulting in a brittle composite [84]. 

After biosynthesis, BC peUicles are usually purified by washing with DI water and then treatment with diluted NaOH solution to ronove bacterial cells followed by a rinse with DI water. In wet state (never dried sheet), the SEM images of surface morphology of the modified BC pellicles by chitosan MW 30,000 and 80,000 are not significantly different from the unmodified BC as shown in Figure 6.5. 

They all display ultrafine fiber network structure of microfibrils of below 0.01 pm. The average water content of the BC and the modified BC pellicles is 98%-99%. After the pellicles are air-dried at room temperature (30°C), the pellicles become transparent thin films. The SEM image of the air-dried films and the re-swollen films are shown in Figures 6.6 and 6.7, respectively. 

Figure 2.2 Photographs of a static culture (with a visible BC pellicle in the interface air-culture medium) and a purified bacterial cellulose wet membrane produced in static conditions. Reproduced with permission from [29, 39].

---