Electrospinning of PVB Solved in Methanol and Isopropanol M. STENICKA 1,2, P. PEER-SVRCINOVA 3, P. FILIP 3, V. PAVLINEK 1,4, M. MACHOVSKY 1,4 1 Centre of Polymer Systems, University Institute Nad Ovcirnou 3685, 760 01 Zlin 2 Department of Polymer Engineering, Faculty of Technology Namesti T. G. Masaryka 275, 762 72 Zlin 3 Institute of Hydrodynamics, Academy of Sciences of the Czech Republic Pod Patankou 5, 166 12 Prague 6 4 Polymer Centre, Faculty of Technology Namesti T. G. Masaryka 275, 762 72 Zlin stenicka@ft.utb.cz Abstract: The aim of this contribution is to analyze polyvinylbutyral (PVB) solutions used for electrospinning. Specifically, it describes a suitability of various solvents for the process of electrospinning in which, under a strong electrostatic field, fibres are generated and deposited on a template as a non-woven sheet. The PVB was solved in methanol and isopropanol as 10 wt% solution. A rotational rheometer and a scanning electron microscope (SEM) were used for a characterization of the electrospinning process. Key-words: Electrospinning Polyvinylbutyral Methanol Isopropanol Rheology 1. Introduction Electrospinning is a process in which nano (micro) fibres are created from polymer solution or melt in the presence of electric field. The electric field is generated between a tip and a grounded collector by a high-voltage power supply. The drop of a polymer solution is stored on the tip, from which, in the presence of an electric field, a Taylor cone is created, and then a single fluid jet is ejected from the apex. As the charged jet travels in air, its diameter decreases due to simultaneous effects of the stretching of the jet, the evaporation of the solvent and high extension rates. A typical electrospinning process (Fig. 1) is described extensively in literature [1 4]. With respect to their small diameters, the electrospun fibres have a large specific surface; accordingly, the potential application of fibres is in various areas such as tissue scaffolds, filtration, nanocomposite materials and protective clothing [3, 5, 6]. The principle parameters affecting fibre formation include viscosity, conductivity, concentration, and surface tension of polymer solution, molecular weight, intensity of electric field strength, tip-to-collector distance, temperature and humidity. Fig. 1. Schematic sketch of the electrospinning apparatus. The viscosity of polymer solution has a significant impact on diameter of fibers; this parameter can be varied by molecular weight of polymer, ISBN: 978-1-61804-118-0 239
concentration of solution, type of solvent, temperature and additives. Ferry [7] has provided the extensive study of the linear viscoelastic properties of dilute polymer solutions. The effect of viscosity, conductivity and surface tension has been investigated by Fong et al. [8]. This contribution compares two types of solvents (good and poor) for the process of electrospinning from PVB solutions. 2. Experimental 2.1 Material The PVB was chosen for this experiment, which is non-toxic, odourless and environment friendly. The PVB (M w = 60 000 g/mol; Mowital, Kuraray Specialities Europe) was consecutively dissolved in methanol and isopropanol as 10 wt% solution (basic characteristics in Tab. 1) at 25 C. Table 1. Basic characteristics of the solvents used and PVB. Properties Methanol Isopropanol PVB Relative permittivity 32.7 19.9 3.6 [-] Specific conductivity 1.5 10 7 5.8 10 6 1.0 10 9 [S/m] Surface tension 22.12 21.38 [mn/m] Density [g/cm 3 ] 0.789 0.781 1.090 HSP δ D [MPa 1/2 ] δ p [MPa 1/2 ] δ Η 15.1 16.0 18.6 12.3 6.8 4.4 [MPa 1/2 22.3 17.4 13.0 ] HSP Hansen solubility parameter 2.3 Analysis of fibres The surface characteristics of the prepared nanofibre sheets were observed with a SEM Vega (Tescan, Czech Republic) with the accelerating voltage of 30 kv. For the viscosity measurements there was used a Bohlin Gemini CVOR 150 (bob and cup arrangement, diameters 25 and 27.4 mm). 3. Results The rheological measurement has a tight connection with solubility through the Hansen solubility parameters [9] δ D (representing energy from dispersion bonds between molecules), δ P (representing energy from polar bonds between molecules), and δ H (representing energy from hydrogen bonds between molecules) representing each molecule, see Table 1. These three parameters generate so-called Hansen space. The nearer two molecules are in this space, the more likely they are able to be dissolved into each other. This is exactly the situation for PVB solved in isopropanol that exhibits shorter distance in the Hansen space in comparison with methanol. The shear viscosities and the complex viscosities of solutions were measured in the ramp mode using a bob and cup arrangement. Fig. 2 depicts shear viscosity in comparison with complex viscosity of PVB dissolved in alcohols as 10 wt% solutions. PVB dissolved in good solvent isopropanol exhibits zero shear viscosity higher than PVB in poor solvent methanol. Shear viscosity and complex viscosity have almost the same values at the corresponding shear rate or frequency. Measurement of complex viscosity is preferable from macromolecular point of view. 2.2 Electrospinning of polymer solution The spinning conditions for each case are exactly the same, the fibres were obtained from the solutions at 25 kv with a tip-to-collector distance of 10 cm (= 250 V/mm) at 25 C. Fig. 2. Comparison of shear and complex viscosity vs. shear rate (open symbols) or frequency (solid symbols). Sample code: ( ) PVB in methanol, ( ) PVB in isopropanol. ISBN: 978-1-61804-118-0 240
Advances in Data Networks, Communications, Computers and Materials The viscoelastic spectra for both samples are shown in Figs. 2 4. Both samples present linear viscoelastic behaviour in a broad range of applied strains (Fig. 3). The dominance of the loss modulus over the storage one indicates that the macromolecular chains of PVB are disentangled (Fig. 4). The fact that the PVB dissolved in isopropanol reaches higher value than the PVB in methanol confirms its higher solubility (similar to steady shear). Fig. 3. The storage (solid symbols) and loss (open symbols) moduli vs. strain, linear viscoelastic regime. Symbols denoted as in Fig. 2. Fig. 5. SEM images of nanofibers sheet PVB in methanol. Fig. 4. The storage (solid symbols) and loss (open symbols) moduli vs. frequency. Symbols denoted as in Fig. 2. The quality of electrostatic spinnability of these solutions was evaluated using a SEM analysis. It was shown that with an increasing viscosity the spinnability of the respective solution deteriorates. The SEM images (Figs. 5 and 6) prove that the poor solvent is suitable for the process of electrospinning. On the other hand, the good solvent does not exhibit good spinnability. The average diameter of fibres created from PVB in methanol is 518 nm and standard deviation is 66 nm, see in Fig. 7. ISBN: 978-1-61804-118-0 241
Advances in Data Networks, Communications, Computers and Materials Absolute frequency [-] 40 PVB in methanol 10 wt% average 518.8 nm standard deviation 66.7 nm 30 20 10 0 0 200 400 600 800 1000 Fiber diameters [nm] Fig. 7. Histogram of nanofibers created from PVB dissolved in methanol. 4. Conclusion The electrospinning process is affected by a number of the parameters. This contribution focused on the impact of viscoelastic properties of the polymer solutions. It was shown that PVB dissolved in a poor solvent exhibited better spinnability (smooth fibres) in comparison with PVB in good solvent. Structure of nanofibres was documented by the SEM images. Acknowledgement This work has been supported by the Grant Agency of the Czech Republic, Grant No. P105/11/2342. Fig. 6. SEM images of nanofibers sheet PVB in isopropanol. References: [1] S. Ramakrishna, K. Fujihara, W.E. Teo, T.C. Lim, Z. Ma, An Introduction to Electrospinning and Nanofibres, World Scientific Publishing Co., Singapore, 2005. [2] A.L. Andrady, Science and Technology of Polymer Nanofibers, John Wiley & Sons, New Jersey 2008. [3] Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A Review on Polymer Nanofibers by Electrospinning and their Applications in Nanocomposites, Composites Science and Technology, vol. 63, 2003, pp. 2223 2253. [4] D.H. Reneker, A.L. Yarin, Electrospinning Jets and Polymer Nanofibers, Polymer, vol. 49, 2008, pp. 2387 2425. ISBN: 978-1-61804-118-0 242
[5] J. Doshi, D.H. Reneker, Electrospinning Process and Applications of Electrospun Fibres, Journal of Electrostatics, vol. 35, 1995, pp. 151 160. [6] P. Gibson, H. Schreuder Gibson, D. Rivin, Transport Properties of Porous Membranes Based on Electrospun Nanofibers, Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 187 188, 2001, pp. 469 481. [7] J.D. Ferry, Viscoelastic Properties of Polymers, Wiley, New York, 1980. [8] H. Fong, I. Chun, D.H. Reneker, Beaded Nanofibres Formed during Electrospinning, Polymer, vol. 40, 1999, pp. 4585 4592. [9] C.M. Hansen, Hansen Solubility Parameters: A User s Handbook (2 nd ed.), CRC Press, Taylor & Francis Group, USA, 2007. ISBN: 978-1-61804-118-0 243