关键词: 细菌纤维素, 机械剥离, 纳米纤维膜, 加捻, 应变传感

Abstract: Bacterial cellulose (BC), as a suitable alternative to petroleum-based materials, has many inherent and unique properties such as biocompatibility, biodegradability, breathability and high-water holding capacity. But it is difficult to dissolve in common organic solvents because of its tight intramolecular and intermolecular hydrogen bonds. BC usually exists in the form of thin membranes, and the mechanical properties of BC dried membranes are poor. The current methods for preparing BC nanofiber membranes (NFMs) with BC fibers all inevitably destroy the original structure of BC. 
In this work, the hydrogen bonding between water and nanofibers in BC hydrogel membranes was weakened by the solvent replacement method, and the layer-by-layer peeling of BC dry membranes was achieved by hot-pressing drying combined with the top-down mechanical peeling method to produce high-strength BC-NFM, and BC fibers could be obtained by further twisting of NFMs. The morphology, structure and physicochemical properties of the BC dry membrane, NFMs and BC fibers were analyzed and studied by characterization means such as scanning electron microscopy, X-ray diffractometer, thermogravimetric analyzer, infrared spectrometer and tensile test. In addition, the strain sensing fiber BC/CNT can be achieved by embedding functional materials such as CNT into NFMs before twisting. The resistance change rate of the BC/CNT fiber obtained by this method can reach 2%. It is shown that the randomly distributed nanofibers on the surface of the BC dry membrane all have a network structure and exhibit a dense structure. As the mechanical peeling step proceeds, the nanofibers on the NFM surface become dispersed and the number of disordered nanofibers on the surface increases, which proves that NMP weakens the hydrogen bonds between the solvent and the BC nanofibers, thus facilitating the mechanical peeling of BC, and in turn leading to the appearance of microfibrils on the NFM surface. The crystallinity of all three mechanically exfoliated NFMs is lower than that of the dry BC membrane, and the crystallinity of 3rd-NFM is the smallest, demonstrating that the NMP treatment does not affect the BC crystal structure. The small-angle scattering patterns show that the arc diameter gradually becomes smaller with the increase of the stripping number, and the 3rd-NFM is the smallest, which proves that the stripping process breaks the hydrogen bonds inside BC and increases the disorder. The intensity of the tensile vibrational peak of the cellulose C−H bond decreases with the increase of the number of peeling, which proves that NMP can break the hydrogen bonds between BC molecules and form new hydrogen bonds with the hydroxyl groups in BC molecules. The NFMs with thickness in the range of 5.0 to 8.0 μm shows a maximum transmission of 23%, water absorption of 2,284% and tensile strength of 338.0 MPa, each of which is higher than that of the BC dry membrane. Compared with the maximum decomposition temperature of the BC dry membrane (359.7 °C), the main weight loss peak temperatures of all the three NFMs are reduced in the range of 333.7 to 339.5 °C, demonstrating the disruption of intermolecular and intramolecular hydrogen bonds of BC by NMP. Surface SEM images of BC fibers show that the 3rd-NFM-fiber has the smallest diameter and the tightest structure, proving that mechanical peeling effectively reduces the diameter of NFMs-fiber and enhances the structural denseness of the fiber. The monitoring of tiny human body movements by BC/CNT conductive fibers fully demonstrates their potential application in smart wearable devices. 
This paper provides scientific data for the preparation of BC-NFMs by top-down method of mechanical peeling, which provides new ideas for the development of high-strength NFMs.

Key words: bacterial cellulose, mechanical peeling, nanofiber membrane, twist, strain sensing