Fusion and fission behaviors have been widely studied and have received a great deal of attentions in biology, chemical engineering and theoretical physics. They are helpful in understanding cell behaviors, developing artificial assemblies and producing multi-metal compounds.

In the past, people were able to realize the fusion and fission of lipids, surfactants, small organic molecules, polymeric micelles and vesicles by using salt, surfactants, ions, oxidants, reducing agents, ultraviolet rays and visible light. 

These methods can alter osmotic pressure between the liquid inside vesicles and the overall solution via changing the interaction within the double membranes or dissolving additives to integrate and split vesicles. A similar fusion and fission process can also be observed when producing alloys.

Nevertheless, reversible fusion and fission still remain a huge challenge. This is mainly because irreversible physical or chemical changes have taken place in the interface of monomers.

Research on reversible and controlled fusion and fission will greatly promote the development of responsive materials, and lead to considerable prospects in developing dynamic and deformable systems and customized fiber-featured structural materials.

To achieve reversible fusion and fission of artificial assemblies, a team led by professor Liu Yilun from the School of Aerospace Engineering at Xi'an Jiaotong University, together with a team led by professor Gao Chao from the Department of Polymer Science and Engineering at Zhejiang University, proposed a solvent-triggered morphology control strategy and realized reversible fusion and fission of graphene oxide–based fibers. The results were published in the journal Science on May 7.

Picture1: Fusion and fission of 100 graphene oxide (GO) fibers

Core-shell structure of graphene oxide (GO) fibers prepared by the wet-spinning method is a key factor in achieving reversible fusion and fission. The core-shell graphenes are closely arranged with a consistent orientation, which ensures the stability of the fiber swelling and restricts movements of internal GO sheets. Swollen fiber will form a loose porous structure internally, resulting in mass volume deformation. 

Under the stimulation of water and polar organic solvents, the topological structure of the shell of a single fiber is reversibly transformed between cylindrical and folded forms due to swelling or water loss. 

The fusion of multiple fibers has to go through three procedures: placing multiple dry fibers in a solvent and swelling them to bundles; pulling the fiber bundles away from the solvent at the same time so that the fibers gather together due to the existence of gas-liquid surface tension; the multiple fibers then shrink in the air because of water loss, and due to surface tension their shells come close to each other and are finally bonded together by hydrogen bonds to form a crude fiber consisting of multiple fine fibers. 

The fission of crude fiber also needs three procedures: placing the crude fiber in the solvent to make it swell and splitting it into fiber bundles; separating the bundles; and pulling the separated fibers out of the solvent and drying them independently (fusion-fission process of multiple fibers is shown in picture 1). Reversible fusion and fission can be achieved due to the repeatability of geometric deformation and the reconfigurable properties of hydrogen bonds.

Picture 2: Finite element simulation analysis for the fusion and fission process of two GO fibers

Finite element simulation research and theoretical analysis have studied the fusion and fission process of two fibers (shown in picture 2). For the fiber fusion process, the work reveals the shell instability process caused by water-loss shrinkage of the porous core-shell systems, and the capillary force-triggered fiber fold bonding mechanism. For the fiber fission process, it shows the interface separation driving force caused by the geometry of the swelling fiber (shown in picture 2A).

Picture 3: Presentation of applications

The properties of reversible fusion and fission have several potential applications.

First, they can produce crude fusion fibers with arbitrary adjustable diameters so that when the diameters increase, their mechanical performances do not fall. As structural materials, those fibers are expected to provide a mechanical advantage in the field of engineering. 

Second, flexible conversion between different fiber-based assembly structures is possible. For example, through controlled fusion and fission, more than 10,000 GO fiber-based assemblies are transformed between GO columns and GO nets, as well as between fused GO fibers and complicated assembly structures (picture 3A-F).

Third, by fusion and fission, GO fiber bundles can include and exclude various objects such as polyacrylonitrile short fibers (Picture 3G-J), sub-millimeter glass beads and polystyrene microspheres.

Moreover, ordinary fibers, including polyvinyl alcohol fibers, nylon, silk, stainless steel wires, glass fibers and basalt fibers, can also have the reversible fusion-fission characteristics with GO coating, which further expands the scope of related applications.

Paper link: http://science.sciencemag.org/content/sci/372/6542/614.full