The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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ESTIMATION OF STRUCTURE OF WEB FROM ELECTROSPUN NANOFIBRES

J. MALAŠAUSKIENĖ and R. MILAŠIUS

Kaunas University of Technology, Department of Textile Technology, Lithuania jolanta.malasauskiene@stud.ktu.lt

Abstract: To produce nanofibres with uniform diameter is very sophisticated, because there are a lot of parameters which has influence on the diameter of nanofibres. The analysis of various works shows that the diameter of nanofibres always is distributed in different distributions. Its mean that to compare the averages values is not correct from mathematical statistic point. The average diameter of nanofibres is the main parameter for nanofibre characterization, but changes in average value do not suppose the changes in other characteristics, for example modal value or wide of the value distribution and etc. For this reason the new criterions are proposed. The percentage quantity and the modal value of the first distribution also the average diameter of two peaks of distribution (modal value and the value of the second highest peak) can be used for estimation of nonwoven structure, because these parameters can be used for comparing various webs of nanofibres. Therefore, these parameters allow to estimate how various parameters influence the structure of the web. Keywords: electrospinning, nanofibre, diameter, distribution. 1. Introduction Electrospinning is a simple process that spins ultrafine fibres of diameters ranging from 10 nm to several hundred nanometers. Electrospun webs from polymer solution have attracted significant attention because of their unique properties such as high porosity along with small pore sizes large surface area per mass ratio, flexibility and small diameter. Due to these characteristics nonwoven materials from nanofibres are used for medical applications, composite, filtration, and etc [1-2]. The diameter of nanofibres is one of the most important parameter related with elecrospinning process. Electrospinning process and the morphology of electrospun nanofibres, such as fiber diameter and uniformity, depend on the solution properties (solution concentration, solution viscosity, polymer type, polymer molecular weight, and surface tension), technological parameters (applied voltage, volume flow rate, distance between electrodes, and covering time of support material) and ambient conditions (temperature, humidity, and atmosphere pressure). Sometimes the influence of these parameters is visible very well, but not always. Many researches work in this area, but until this day there is no common agreement about the influence of the same parameter on the structure of nanofibres [1-6]. It is possible that the differences between the results occur because there is no common methodology for nanofibre diameter characterization. The analysis of various works shows that the web of nanofibres usually consists from nanofibres with different diameters. Sometimes the distributions of nanofibres diameter are closes to log normal distribution, but this distribution does not have the theoretical basic, because all nanofibres are manufactured at the same time. Describing the diameter of nanofibres by means of normal distribution, when the distribution of nanofibres differs from this one is not appropriate too. Therefore, it is difficult to compare the averages values when the character of dispersions of nanofibres differs [7-9]. Only the average value can not characterise nanofibres and the new criterions are needed. The goal of this article is to study the influence of the speed of support material on the structure of electrospun webs from PA 6.6 and to propose the new criterions for the quality of nonwoven structure estimation. The mathematical analysis and comparison with well known statistical distributions shows that distribution of nanofibres diameter is very close to compound distribution from several normal distributions. It means that the web in electrospinning process usually consists from several nanofibres with different diameter. The reasons of this phenomenon by different authors are explained in some ways [10-12].

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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Various polymers can be successfully electrospun into nanofibres. The most often polymers used in this process are nylon, polystyrene, polyacrylonitrile, PEO, water-soluble polymers and others. PA6.6 is a commonly polymer used due to its mechanical strength, thermal dimensional stability and higher moisture absorption. 2. Materials and methods 2.1 Materials The spinning solution was prepared by dissolving a weighed amount of PA6.6 granules in formic acid (85 %) under constant stirring for 12 hours to attain solution with concentration of 8 %. The solution was stirred at the temperature of the room. 2.2 Electrospinning technique The web of PA6.6 nanofibres was prepared by electrospinning equipment of “NanospiderTM” (Elmarco, Chech Republic). This equipment consists of rotating electrode to spin fibers directly from the polymer solution. The roller spinning electrode is partially submerged in polymer solution. A grounded collector electrode is fitted at the top of the spinner. By increasing electrostatic forces between electrodes, many Taylor cones from the polymer solution are formed on the rotating electrode. Only when the electrostatic charge overcomes the surface tension a charged jet of polymer solution ejected from the Taylor cone. A jet is moving toward upper electrode and lay on substratum material. During the experiments, webs from nanofibres were formed only using electrodes with tines. During all the experiments, the distance between electrodes was 13 cm; the applied voltage was 70 kV. The temperature of environment was T = 24 ± 2° C and the relative air humidity was φ = 50 ± 2 %. The speed of support material was changed. The first part of experiment was carried out when the speed of support material was v = 0.010 m/s, the time of covering was t = 25 s. The second: v = 0.006 m/s, t = 42 s, and the third: v = 0.002 m/s, t = 125 s.

2.3 Characterisation of nanofibre morphology The structure of electrospun PA6.6 web was observed by Scanning Electron Microscopy (SEM) SEM - FEI Quanta 200 (Netherlands). The diameter of nanofibres was measured using the analysis system LUCIA Image 5.0, with an accuracy ± 0.01 nm. All nanofibres were measured from 5 different SEM images. 3. Results The SEM images of the web at the different speed of support material are presented in Figure 1. It is evident that the speed of support, herewith the covering time has a significant influence on the structure of PA6.6 nanofibres. At the lowest speed, hense at the highest covering time, mostly nanofibres were formed on the support material. During the experiment 5 SEM images for each variant has been made. The 165 diameters of PA6.6 nanofibres for the first variant, 200 diameters of nanofibres for the second variant and 288 nanofibres for the thirst variant were found and measured.

a. b. c. Figure 1. SEM images of PA6.6 webs, when the speed of support material: a) v = 0.010 m/s (t = 25 s); b) v = 0.006 m/s

(t = 42 s); c) v = 0.002 m/s (t = 125 s)

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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After the analysis of experimental data it was observed that the speed of support material has not a siignifficant influence on the average diameter of PA6.6 nanofibres. The average diameter of the first, second and third parts of experiment is d̅ = 357 nm, d̅ = 373 nm and d̅ = 382 nm, respectively. The difference between average values is less than ± 3.5 %. For the deeper analysis of the structure of nowoven web, the frequency distributions of all parts of experiment are presented in Figure 2.

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Figure 2. Frequency distributions of PA6.6 nanofibres From Figure 2 it is evident that at higher covering time, hense at lower speed of support material thicker nanofibres were formed. In the range from 0 to 275 nm, 32.2 % of nanofibres was measured at the highest speed and only 21 % of nanofibres was measured at the lowest speed. Up to 325 nm, 50 % was measured at the highest speed of support material, while at the lowest speed 66 % of nanofibres was measured. It is possible that thicker nanofibres are formed when several nanofibres stick together and do not separate before reaching the upper electrode. Analyzing the histograms in Figure 2 we can observe that the distribution of the third part of experiment has only one obvious peak. For this reason this distribution is very close to normal distribution. However, the histograms of the first and of the second series of experiment have several peaks, so in both cases we can state that diameter of nanofibres is distributed by compoud distribution from several normal distributions. When the diameter of nanofibres is distributed in compound distribution, the average value of diameter can not estimate the structure of the web exactly. With the average value of nanofibres the modal value and the percentage quantity of the first distribution are proposed. The percentage quantity of nanofibres distributed in the first distribution is 32.2 % (first part of experiment). The percentage quantity of the second variant is only 24.04 %. Higher percentage quantity means more unique structure of the web. The modal value in both cases is 200 nm. Finally, the of distribution is proposed for estimation of nonwoven structure. From the histograms presented in Figure 2 the average diameter of two peaks has been calculated: dˈ = 375 nm (third part of experiment); dˈ = 350 nm (second part of experiment) dˈ = 250 nm (first part of experiment). According to all results it was decided that at the higher speed of support material more unique structure and thinner nanofibres are formed.

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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4. Conclusions

The distributions of electrospun PA6.6 nanofibres diameter are closes to compound distribution, consisted of several normal distributions.

At the lower covering speed more unique structure and thinner nanofibres are formed. The speed of support material does not have a significant influence on the average value of PA6.6

nanofibres diameter but have a significant influences on the structure of nanofibre web. The percentage quantity, the modal value of measurements of the first distribution and the average

diameter of two peaks of distribution can be used for comparison of nanofibres diameter. References [1] Mazoochi, T. et al: Investigation on the morphological characteristics of nanofiberous membrane as

electrospun in the different processing parameters, International Journal of Industrial Chemistry, 3 (2012) 2, 1-8, ISSN 2228-5547

[2] Banuškevičiūtė, A. et al: Formation of Thermoplastic Polyurethane (TPU) Nano/Micro Fibers by Electrospinning Process Using Electrode with Tines, Materials Science (Medžiagotyra), 17 (2011) 3 287-292, ISSN 1392-1320

[3] Zhang, C. et al: Study and Morphology of Electrospun Poly(vinyl alcohol) Mats, European Polymer Journal, 41 (2005) 3, 423-432, ISSN 0014-3057

[4] Adomavičiūtė, E.: Milašius, R.: The Influence of Applied Voltage on Poly(vinyl alcohol) (PVA) Nanofibre Diameter, Fibres &Textiles in Eastern Europe, 15 (2007) 5-6, 69-72, ISSN 1230-3666

[5] Dosunmu, O.O. et al: Electrospinning of Polymer Nanofibres from Multiple Jets on a Porous Tubular Surface, Nanotechnology, 17 (2006), 1123-1127, ISSN 0957-4484

[6] Mo, X.M. et al: Electrospun P(LLA-CL) Nanofibre: a Biomimetic Extracellular Matrix for Smooth Muscle Cell and Endothelial Cell Proliferation, Biomaterials, 25 (2004), 1883-1890, ISSN 0142-9612

[7] Leaf, G.A.V.: Practical Statistics for the Textile Industry: Part I, The Textile Institute, Manchester, 1984. [8] Ellison, C.J. et al: Melt Blown Nanofibers: Fiber Diameter Distributions and Onset of Fiber Breakup,

Polymer, 48 (2007) 3306-3316, ISSN 0032-3861 [9] Tsimpliaraki, A. et al: Optimizing the Nanofibrous Structure of Non-Woven Mats of Electrospun Bio-

Degradable Polymer Nanocomposites, Proceeding of Latest Advances in High Tech Textiles and Textile-Based Materials, 128-133, September 2009, Ghent, Belgium

[10] Malašauskienė, J.: Milašius, R.: Mathematical Analysis of the Diameter Distribution of Electrospun Nanofibres, Fibres &Textiles in Eastern Europe, 18 (2010) 6, 45-48, ISSN 1230-3666

[11] Malašauskienė, J.: Milašius, R.: Short-cut Method of Electrospun Nanofibres Diameter Distribution Estimation, Proceedings of Magic World of Textiles: 6th International Textile Clothing & Design Conference, Dragčevic, Z., 522–525, ISSN 1847-7275, October 2012, Faculty of Textile Technology, University of Zagreb, Zagreb,Croatia, Dubrovnik, 2012

[12] Malašauskienė, J.: Milašius, R.: Investigation and Estimation of Structure of Web from Electrospun Nanofibres, Journal of Nanomaterials, 2013, Article ID 416961, 6 pages, (2013), doi: 10.1155/2013/416961.