Different surfactants are introduced to study the diameter and morphology transformation characteristics of electrospun nanofiber. Surfactants increase the net charge density and instability motion of charged jet. The instability motion provides a good way to stretch the charged jets into finer ones, by which the beaded structures are also prevented. Ultrafine nanofiber with average diameter less than 65 nm can be fabricated. The nanofiber diameter decreases with the increase of surfactant concentration in polymer solution. The nanofibers with anionic surfactant sodium dodecyl sulfate (SDS) have the smallest diameter. The cationic surfactant hexadecyl trimethyl ammonium bromide (HTAB) plays the best role to prevent the formation of beaded structures in nanofibers, and helps to increase the uniformity of electrospun nanofiber. The effects of surfactants on the nanofiber diameter and morphology have been studied, which would promote the industrial application of ultrafine polymeric nanofibers.
Ultrafine polymeric nanofibers have wide application in the fields of micro/nanosystem and flexible electronics. At present, quick and low cost fabrication are the main aspects for the development and industrial application of ultrafine polymeric nanofibers, which have attracted a lot of attentions in recent years [
During the electrospinning process, the high voltage is applied to stretch viscoelastic solution into the cone shape named as “Taylor Cone.” When the electrical field force overcomes the surface tension, a jet is ejected from the cone tip. The charge repulsion force that stems from the accumulated charges imports disturbances into the ejection and motion process of polymer jet. The instability motion is the important factor to stretch and thin the charged jet [
The morphology of electrospun nanofiber is an important factor to investigate the rheology behaviors of charged jet in the ejection process. With the inadequate stretching of liquid jet, there are usual beaded structures appearing along the electrospun nanofiber. The beaded structures stem from the competition surface tension of liquid jet [
In this work, different surfactants were introduced to decrease the nanofiber diameter and prevent the formation of beaded structures in electrospun nanofiber. The surfactants would increase the net charge density and enlarge the repulsion force among charged jets. Then, the instability motion was also enhanced by the charge repulsion fore. The transformation characteristics of nanofiber diameter and morphology were studied.
The electrospinning setup based on conventional pole-type nozzle configuration was built up in this work. The high voltage source (DW-P403-1AC, Tianjing Dongwen High Voltage Power Supply Plant, China) was used to provide electrical field between steel nozzle spinneret (inner diameter was 232
Polyvinylidene fluoride (PVDF, average molecular weight = 141,000 g/mol, DuPont, USA) solution was used as electrospinning solution. PVDF powder was added to the mixed blends of acetone (Sinopharm Chemical Reagent Co. Ltd., China) and N, N-dimethylformamide (DMF, Sinopharm Chemical Reagent Co. Ltd., China). The weight ratio of acetone to DMF in the blending solvent was 2 : 3. The PVDF concentration in the solution was 12 wt%.
Anionic surfactant sodium dodecyl sulfate (SDS), nonionic surfactants Triton X-100, and cationic surfactants hexadecyl trimethyl ammonium bromide (HTAB) were added to the PVDF solution to investigate the effect of surfactants on the diameter and morphology of electrospun nanofiber, respectively. All of these three surfactants were purchased from Sinopharm Chemical Reagent Co., Ltd. China, which were used without any further purification. The surfactant concentrations in the PVDF solution were 3.5 × 10−3 mol/L, 1.75 × 10−2 mol/L, and 3.5 × 10−2 mol/L, respectively.
In the experiment, the PVDF solution was transferred to the nozzle spinneret by the precision syringe pump at a flow rate of 200
Firstly, the characteristics of polymer solution with surfactants were tested. The viscosity of PVDF solution was 153 mPa·s, which was the same as the polymer solution without surfactant. Surfactants can increase the free charges in polymer solution and conductivity of solution. The conductivity of polymer solution increased from 138
The relationship between solution conductivity and surfactant concentration in the solution.
Then, PVDF solution without surfactants was used as electrospinning solution. Due to the fast evaporation of solvent, the charged jets cannot be stretched adequately into fine and uniform ones. Liquid jet would be shrank into beaded structure by the surface tension, as shown in Figure
Beaded PVDF nanofiber electrospun from PVDF solution without surfactants. (a) Electrospun beaded PVDF nanofiber. (b) Close view of beaded nanofiber with magnification of 22,000x.
The diameter distribution range of nanofiber gained from PVDF solution without surfactant.
The diameter and morphology of electrospun nanofiber was observed and measured by SEM. The average diameter was calculated from more than 50 data points in 10 samples.
And then, the effects of surfactant on the diameter and morphology were investigated by adding the surfactants to the polymer solution. The nanofibers electrospun form PVDF solution with surfactants of SDS, Triton X-100, and HTAB were shown in Figures
Electrospun nanofiber gained from PVDF solution with anionic surfactants of SDS. The concentration of SDS in the solution is (a) 3.5 × 10−3 mol/L, (b) 1.75 × 10−2 mol/L, and (c) 3.5 × 10−2 mol/L.
Electrospun nanofiber gained from PVDF solution with nonionic surfactants of Triton X-100. The concentration of Triton X-100 in the solution is (a) 3.5 × 10−3 mol/L, (b) 1.75 × 10−2 mol/L, and (c) 3.5 × 10−2 mol/L.
Electrospun nanofiber gained from PVDF solution with cationic surfactants of HTAB. The concentration of HTAB in the solution is (a) 3.5 × 10−3 mol/L, (b) 1.75 × 10−2 mol/L, and (c) 3.5 × 10−2 mol/L.
Then, the effects of surfactant concentration on the nanofiber diameter and distribution were studied. The relationships between nanofiber diameter and surfactant concentration in solution were shown in Figure
The relationship between nanofiber diameter and surfactant concentration in the solution: (a) SDS, (b) Triton X-100, and (c) HTAB.
The diameter distribution of nanofiber gained from PVDF solution with surfactants: (a) SDS: average diameter is 51.68 nm and standard deviation is 13.81 nm; (b) Triton X-100: average diameter is 63.91 nm and standard deviation is 11.09 nm; (c) HTAB: average diameter is 60.19 nm and standard deviation is 13.71 nm. The surfactant concentration in solution was 3.5 × 10−3 mol/L.
The diameter distribution of nanofiber gained from PVDF solution with surfactants: (a) SDS: average diameter is 40.71 nm and standard deviation is 10.02 nm; (b) Triton X-100: average diameter is 56.51 nm and standard deviation is 22.95 nm; (c) HTAB: average diameter is 45.55 nm and standard deviation is 15.15 nm. The surfactant concentration in solution was 1.75 × 10−2 mol/L.
The diameter distribution of nanofiber gained from PVDF solution with surfactants: (a) SDS: average diameter is 39.89 nm and standard deviation is 9.99 nm; (b) Triton X-100: average diameter is 40.62 nm and standard deviation is 10.24 nm; (c) HTAB: average diameter is 41.61 nm and standard deviation is 12.88 nm. The surfactant concentration in solution was 3.5 × 10−2 mol/L.
Different surfactants had played different roles in the rheology behaviors of charged jet. The nanofibers gained from PVDF solution with anionic surfactant SDS were shown in Figure
In this work, the anode of the high voltage source was connected to the steel nozzle spinneret. When injected from the spinneret, the liquid jet also carried away the positive charge accumulated on the spinneret. The cationic surfactant of HTAB would play a good way to provide excess positive charge to the liquid jet during the injection process. On the other hand, polymer solution with HTAB had the highest conductivity that would increase the free charges in solution. And then, the net charge density can be increased to enhance the instability motion of charged jet. Thus, the nanofiber electrospun from PVDF solution with cationic surfactant of HTAB had the smallest and least beaded structure in nanofiber.
Different surfactants were introduced to investigate the rheology behaviors of charged jet electrospun from PVDF solution. With the help of surfactants, net charge density in electrospinning jet was increased to enhance the charge repulsion force and the instability motion of charged jet. Charged jets can be stretched adequately into finer and uniform ones by the larger electrical field force. With the help of surfactants, electrospinning nanofiber with average diameter less than 65 nm can be fabricated, which was finer than nanofiber electrospun from PVDF solution without surfactant. The nanofiber diameter and diameter distribution range decreased with the increase of surfactant concentration in solution. Attributed to the larger net charge density, the cationic surfactant of HTAB would provide a great way to prevent forming beaded structures.
The effects of surfactant on the transform characteristics of nanofiber diameter and morphology were studied, which would provide a good way to promote the industrial application of polymeric nanofibers.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work is supported by theNational Natural Science Foundation of China (nos. 51035002 and 51305371), Major Science and Technology Projects of Fujian Province of China (no. 2012H6022), and Opening fund of Guangdong Provincial Key Laboratory of Micro-Nano Manufacturing Technology and Equipment (no. GDMNML2013-01).