Equipment develop and experiment study of the synchronization of electrospinning and electrospray

High-performance multi-functional nanomaterials have broad applications in various industrial fields. The assembling of nanofibers and nanoparticles prepared by electrospinning and electrospray is a typical structure for the functional combination of multi-materials. However, due to different solution properties and processing parameters, as well as the simultaneous mutual interferences, the controlling of the synchronization of electrospinning and electrospray technology is in an urgent demand. In this paper, an equipment was developed to realize the synchronization of electrospinning and electrospray technology, and multi-functional nanomaterials combining nanofibers and nanoparticles was achieved. Then, thermoplastic polyurethanes (TPU)/grapheneoxide (GO) and poly(lactic acid) PLA/polyaniline (PANI) membrane was prepared based on the self-developed equipment, verifying the process of the synchronization of electrospinning and electrospray.


Introduction
Recently, the use of nanoparticles in the modification of inorganic layers has attracted lots of attention, exhibiting excellent performance for multi-functional nanomaterials [1] .Electrospinning and electrospray technology are typical methods to prepare nanofibers and nanoparticles in various morphologies and characteristic dimensions.The combining of electrospinning and electrospray technology is an excellent way to prepare muti-functional composite membranes [2] , where the nanoparticles can enhance the membrane permeability, the resistance to biological pollution, and the photocatalytic properties.Electrospinning is used to prepare three-dimensional shaped materials [3] , and electrospray is used to product permeate nanoparticles into nanofibers uniformly to modify the nanofibers.Thus, the nanofibers can be served as the matrix, which can effectively prevent the agglomeration of electrospray nanoparticles to enhance the performance [4] .The composite membranes assembled with nanofibers and nanoparticles has been widely used in aerospace, military defense, communication, sensing, biological medical and other fields.
However, there are still some challenges on the preparation of functional nano-structures based on the combination of traditional electrospinning and electrospray.Up to present, there are several ways to combine the two technologies, like electrospray after electrospinning or electrospinning and electrospray simultaneously.While the two processes are carried out together, the electric interference between adjusted nozzles is serious, making it difficult for the combination of nanofibers and nanoparticles effectively.In this way, it is necessary to develop an integrated equipment combining the electrospinning and electrospray process together to meet the industrial requirements for the stable production of composite membrane assembled with nanofibers and nanoparticles.

Equipment functional design
Figure 1 is the schematic diagram of the synchronization of electrospinning and electrospray.The electrospray nanoparticles can be attached on the electrospinning nanofibers, so as to realize the preparation of thin membranes with multi-functional composite nano-structures.Several works have been done to develop the equipment to realize the electrospinning and electrospray joint regulation process, including the set-up of the hardware, the design of the whole prototype, the experiment verification, and the final integration prototype optimization.The overall system design of the equipment is mainly composed of the following modules: (1) Triaxial motion system module: This module is designed to control the movement of the collecting plate in the X, Y direction, and the electrospinning spinneret and the electrospray spinneret in the Z direction.
(2) Environment control module: This module mainly includes the electric field distribution, the temperature, the humidity and the air cleanliness of the environment.
(3) Liquid supply module: This module is designed to control the liquid supply mode, so as to meet the overall operation of the equipment.
(4) Voltage supply module: The module is designed to provide the electric field for the jet ejection.
(5) Software operation module: This module is designed for the user operation control interface.

Spinneret structure
The spinneret for electrospinning and electrospray plays a key role in the formation of nanofibers and nanoparticles, so it is necessary to design the internal structure of the spinneret.According to our previous research [5] , the assisted airflow around the nozzle can be used to extend and refine the nanofibers, while the air flow field is always limited by the gas pipe in a cylinder structure.To further improve the stable and uniform field strength distribution, a novel airflow-assisted spinneret structure is designed.As shown in Figure 3, the airflow-assisted spinneret mainly includes the liquid inlet, the liquid storage chamber, the gas inlet, the gas storage chamber, the airflow trough, and the liquid outlet, etc.The solution passes through the inlet to the reservoir, and the air flow arrives through the inlet for the storage of the solution and the supply of airflow.
When the system is working, the anode of the high voltage power supply is connected to the spinneret, and the grounded cathode is connected to the collecting plate.The liquid inlet of the spinneret is connected with the external liquid supply device, and the gas inlet of the spinneret is connected with the gas supply device.The collecting plate is placed under the spinneret.Under the joint action of the electric field force and the air flow field, nanofibers and nanoparticles can be deposited on the collection plate simultaneously.

Experiment verification 3.1. TPU/GO membrane
Several experiments have been carried out to verify the synchronization system of electrospinning and electrospray.During the experiment, the ambient temperature was 24 to 26℃, the humidity was 18% to 30% RH.
Firstly, TPU/GO membrane was prepared.16.6% TPU solution with the mixture solvent of N, N-Dimethylformamide and tetrahydrofuran (v:v=1:1) was used as the electrospinning solution.0.49% GO solution with the mixture solvent of N, N-Dimethylformamide and tetrahydrofuran (v:v=1:1) was used as the electrospinning solution.The electrospinning voltage was 10 kV, and the electrospray voltage was 5 kV.The collector rotation speed was 150 r/min.The SEM image of the prepared TPU/GO membrane was sampled as shown in Figure 4.It could be seen that the electrospray nanoparticles were attached to the nanofibers prepared by electrospinning, verifying the synchronization preparation process.

PLA/PANI membrane
PLA/PANI membrane was also prepared by the synchronization of electrospinning and electrospray, as shown in Figure 8% PLA solution with the mixture solvent of N, N-Dimethylformamide and dichloromethane (v:v=8:2) was used as the electrospinning solution, and PANI doped with camphor sulfonic acid in the N, N-Dimethylformamide solvent with mass fraction of 1%, 2%, 3% and 4% was used as the electrospray solution.The electrospinning voltage was 14 kV, and the electrospray voltage was 9 kV.The collector rotation speed was 150 r/min.The morphology of PANI is divided into total oxidation state, full reduction state, and intermediate oxidation state.Among them, only the intermediate oxidation state has conductive performance.The aniline monomer is doped by the oxidant perthiamine and camphor sulfonic acid, thus the conductivity is increased by 1 to 2 orders of magnitude.The conductivity of PANI solution with different mass fraction was tested, as shown in Figure 6.conductivity of the composite membrane increased with the increase of PANI content.It could be seen that the conductivity of composite membrane at 4% PANI mass fraction increased significantly compared with that at 3% PANI mass fraction, which was reached up to 10.18 μs/cm.

Figure 1 .
Figure 1.Schematic diagram of the synchronization of electrospinning and electrospray.

Figure 2 .
Figure 2. Composition diagram of the equipment system

Figure 3 .
Figure 3. Model diagram of the airflow-assisted spinneret

Figure 4 .
Figure 4. SEM image of the composite TPU/GO membrane

Figure 7 .
Figure 7.The membrane conductivity with different PANI concentrations