PVA pellets (molecular weight = 78000g/mol) were purchased from Merck Company, and PA6 pellets (molecular weight of 35000g/mol) were purchased from Sigma Aldrich Company. Formic acid with analytical grade prepared from Merck Company.
Instruments and sample preparation
One nozzle horizontal electrospinning (digital pump from DAWHA company with MS-2211 model, drum collector, voltage supply with a voltage range of 1-22kV) used to prepare PVA/PA6 nanofiber web. The electrospinning condition was 15wt%(50/50%wt), 9cm, 0.5ml/h, and 20kV. Electrospinning continued for two hours. Samples were cut from the resultant web, and then the samples were sent for solving PVA and floated in 70°C distilled water for two hours.
Samples characterizing
FESEM images of the nanofibers prepared from Seron Technology (South Korea, AIS2100 model) microscope. Nanofiber’s diameter was measured by Image J software. An average of 100 nanofiber diameters was used. Thickness and weight of the web measured by Insize digital microscope micrometer and digital balance with 0.0001 g accuracy. All the histograms are drawn with Minitab 19 software.
Morphology of the nanofibers
There is a bit difference between average diameters before and after the water etching (according to table 1) [6]. With higher resolution, the pores on the nanofibers were seen. In the electrospinning process because of applying, high tension to nanofibers [7-8], the cracks made on the nanofibers are like mesopores but after water treatment, they are reduced. So, cracks are the new way of making mesopores. From the FESEM results, it is clear that the porosity and the pore size have reduced after removing the PVA [4]. Unfortunately, the nanofibers did not get more porous after removing PVA.
Porosity and pore size measurement
The porosity of the web was measured by BET [5, 9], and results showed that the porosity was reduced by solving one part (according to table 2). Image processing shows the surface porosity, and BET shows the total porosity (only for mesopores which are not made usually in nanofibers webs). BET adsorption and deadsorption curves showed that the pores of the nanowebs are slit-like (table 3). BET showed that the nanoweb is not porous because it is for mesopores. The cracks increased the surface roughness of the nanoweb compared to my previous work [4] so, it is a new way for increasing the surface roughness. After water treatment, the cracks changed, and the removal of PVA resulted in lower porosity. Overall, the porosity reduced after water treatment. So, this work gave a new way for making higher surface roughness on the nanofibers and showed that compared to previous works using a hydrophilic polymer for hybrid nano webs or co-solution results in lower porosity. Comparing to my previous article, the specific surface area is lower, and it is for higher nanofibers diameters of this work. In p/p0 >0.6, the curves of adsorption and deadsorption are almost on each other, so the pores are open, but in my previous article was vice versa, and that means that the pores are near to micropores (<2nm) [4].
Measuring surface roughness and pore volume by Matlab
By using Matlab, surface roughness [10], and pore volume were measured. They decreased according to the results (Table 5). As you see in table 5, the threshold images show that the roughness of the nanofibers decreased after water treatment. Reduction in the surface roughness is for reducing the cracks.
Measuring the orientation of the nanofiber web
By using Matlab the orientation of the nanowebs measured, which the results are reported below (table 6). Water treatment had not any special effect on orientation degree, but same in other works, it has been more isotropy than before etching [6]. It happened because water replaces the nanofibers.