ON THE ELECTROSPINNING OF PVB SOLUTIONS

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1 ON THE ELECTROSPINNING OF PVB SOLUTIONS Petra SVRCINOVA a, Petr FILIP a, Daniela LUBASOVA b a Institute of Hydrodynamics, Acad. Sci. Czech Rep., Pod Patankou 5, Prague 6, Czech Republic, svrcinova@ih.cas.cz b Technical University of Liberec, Fac. of Textile Engng, Dept. of Nonwovens, Studentska 2, Liberec, Czech Republic Abstract Extension rheological characteristics belong to the principal description of polymer jet formation during a process of electrospinning. However, in spite of nearly absence of shear deformation of polymer solutions during the process of their spinnability, it seems that shear electrorheological characteristics can serve as an indicator of good or bad quality of electrospun templates. The experiments were carried out with four polyvinylbutyral (PVB) solutions where as the solvents were consecutively used methanol, ethanol, isopropanol, and butanol in concentration of 10 wt% at 25 C. Shear viscosities in absence (η 0 ) and presence (η) of electric field were measured using an Anton Paar rotational rheometer MCR 501 equipped with an electrorheological cell C-PTD200/E (bob and cup arrangement). The electrospun fibres were obtained from the solutions at 30kV with a tip-to-collector distance of 10 cm (=300 V/mm). The surface characteristics of the prepared nanofibre sheets were observed with a scanning electron microscope Vega TS It was shown that the good solvents of PVB (butanol, isopropanol) exhibit an almost constant curve of η/η 0 vs. electric field strength, whereas the viscosity curves for the poor solvents (methanol, ethanol) exhibit enhancement. This enhancement corresponds to better quality of electrospun templates (obtained for polymer solutions of PVB with poor solvents). Keywords: electrospinning; elongational viscosity; polyvinylbutyral (PVB) 1. INTRODUCTION The process of electrospinning is based on electrical charge the application of which enables drawing microor nano-fibres from a polymer solution or melt. The electric field is generated between a tip and grounded collector by a high-voltage power supply. The drop of polymer solution is stored on a tip, from where there is created a Taylor cone in presence of electric field, and then a single fluid jet is ejected from the apex. As the charged jet travels in air, its diameter decreases due to high extension rates and simultaneous effect of stretching of the jet and evaporation of the solvent. A typical electrospinning process (Fig.1) is described extensively in the literature [1-4]. With respect to their small diameters, the electrospun fibres have a large specific surface and the potential application of fibres is in various areas such as tissue scaffolds, filtration, nanocomposite material and protective clothing [3,5,6]. Viscosity, conductivity, concentration, surface tension of polymer solution, molecular weight, intensity of electric field strength, tip-to-collector distance, temperature, humidity range among the principal parameters affecting fibre formation. The viscosity of polymer solution has an impact on diameter of fibres, morphology and path of jet, this parameter can be varied by molecular weight of polymer, concentration of solution, type of solvent, temperature and additives. The effect of viscosity, conductivity and surface tension has been investigated by Fong et al. [7]. They demonstrated that a usage of the polymer solution with low viscosity results in the formation of beads caused by the capillary breakup of the jet during the electrospinning due to dominating surface tension. Higher viscosity favours the formation of fibres without beads, however if viscosity of solution is too high, electrospinning is no longer possible. Wang et al. [8] performed elongation rheology

2 measurements with polymer solutions and they found that a slower rate of capillary thinning is expected to correlate with better spinnability of the polymer solution. Chuangchote et al. [9] applied several different solvents to electrospinning of poly(vinyl pyrrolidone) fibres and studied an influence of concentration and viscosity of solutions on a fibre diameter. Drew et al. [10] demonstrated that the effect of viscosity on fibre morphology is independent of polymer concentration (for the values exceeding a minimum concentration). They showed that electrospun fibres of polyethylene oxide can be successfully fabricated consisting only of 25 wt% polymer, the remainder of the fibre weight consisted of titanium dioxide. Fig. 1. Sketch of an electrospinning apparatus. Winslow [11,12] was the first who started research of rheological behaviour of liquids under a presence of electric field. Especially viscosity η as a main characteristic of so-called electrorheological (ER) fluids is studied. Generally, ER fluids are suspensions for which viscosity in presence of electric field increases intensively. Using quasielastic light scattering (QELS) Price et al. [13] analyzed the effect of external electric field on the dynamics of polymer chains. They found that in presence of an electric field there is a polarisation effect which is dependent on a difference in permittivity between the polymer segments and solvent. Wang and Huang [14] applied dynamic light scattering (DLS) for a qualitative comparison when a non-polar polymer (polystyrene) is solved either in polar or non-polar solvent. They revealed that in the latter case the effects from the externally applied electric field were negligible. This motivated Chen et al. [15] who carried out molecular dynamics simulation to predict the effects of external electric field on the diffusion dynamics of a polar or non-polar chain in polar or non-polar solvents. This contribution describes the suitability (characterised by structure and dynamics of polymer chains in a solution) of various polyvinylbutyral (PVB) solutions for the process of electrospinning. As a criterion shear rheological behaviour of these materials is taken into account, viz. a course of a curve viscosity ratio η/η 0 vs. electric field strength E. The symbols η and η 0 represent shear viscosities of a solution in the presence and absence of an electric field, respectively. In spite of high elongation rates that are imposed on jets during electrospinning, the present work focuses on a correlation between shear rheology of a polymer solution and its electrostatic spinnability. 2. EXPERIMENTAL The polyvinylbutyral (PVB) was chosen for this experiment, the commonly used polymer as the mesopore template, which is non-toxic, odourless and environment friendly. PVB (M w =60,000 g/mol; Mowital, Kuraray Specialities Europe (KSE)) was consecutively dissolved in methanol, ethanol, isopropanol, and butanol as

3 10wt % solution (basic characteristics in Table 1) at 25 C. For viscosity measurements there was used an Anton Paar rotational rheometer MCR 501 equipped with an electrorheological cell C-PTD200/E (bob and cup arrangement, diameter of 17 mm). The electrospun fibres were obtained from the solution at 30kV with a tip-to-collector distance of 10 cm (=300 V/mm). The surface characteristics of the prepared nanofibre sheets were observed with a scanning electron microscope Vega TS Table 1. Basic characteristics of the solvents used and PVB. Relative permittivity [-] Specific conductivity [S/m] Surface tension [mn/m] Density [g/cm 3 ] Methanol * Ethanol * Isopropanol * Butanol * PVB * The shear viscosities of all solutions were measured in the ramp mode using bob and cup arrangement, diameter 27mm. Fig.2 depicts shear viscosity of PVB dissolved in alcohols as 10 wt% solutions. PVB dissolved in good solvents (butanol, isopropanol) show shear thinning behaviour at high shear rates >100s -1 and their zero shear viscosities are higher. On the contrary, for poor solvents (methanol, ethanol) the viscosities are constant, zero shear viscosities are lower. For each solution there were carried out the experiments relating the viscosity ratio η/η 0 to the electric field strength E (Fig.3). The good solvents of PVB exhibit an almost constant curve of η/η 0 vs. E, whereas the viscosity curves for the poor solvents exhibit enhancement. Shear viscosity η [Pa.s] wt % PVB solutions, T=25 C Isopropanol Butanol Ethanol Methanol Shear rate γ [s -1 ] Fig. 2. Shear viscosity of PVB solutions (absence of electric field strength).

4 2 Viscosity ratio η/η wt% PVB solutions, T=25 C. Shear rate γ= 6.58 s -1 Methanol Ethanol Isopropanol Butanol Electric field strength E [V/mm] Fig. 3. Dependence of viscosity ratio η/η 0 on electric field strength for PVB solutions. Consequently, the quality of electrostatic spinnability of these materials was evaluated using SEM analysis. For 10 wt% PVB solutions the fibres were exhibited for ethanol and methanol only (Fig.3). It was shown that with increasing curve η/η 0 vs. E the spinnability of the respective solution improves. Based on the above findings it was shown that, unlike butanol and isopropanol, ethanol and methanol as the solvents of PVB are suitable for electrospinning. a) b) c) d) Fig. 4. SEM micrographs of PVB electrospun from 10 wt% solution: a) PVB in butanol, b) PVB in isopropanol, c) PVB in ethanol, d) PVB in methanol. 3. RESULTS AND DISCUSSION Impact of the solvents on electrostatic spinnability is validated through the SEM images. There were obtained no fibres for PVB dissolved in butanol and isopropanol (good solvents), whereas for PVB dissolved in ethanol and methanol (poor solvents) there were developed fibre sheets, see Fig.4. The shape of the polymer chain depends on solubility of polymer in a solvent. If the individual polymer chains in good solvent are free to uncoil and stretch, they exhibit relatively extended conformations, viscosity of solution is high. On the contrary when polymer chains stay coiled and grouped together into microscopic clusters then this topology reflects in lower viscosity. If the external electric field is applied, the polymer chains uncoil and stretch, therefore the viscosity of solution increases and the process of electrospinning is much easier. If the polymer chain in good solvent is uncoiled and stretched then in presence of electric field polymer chain will not react.

5 For electrorheological fluids a rapid and reversible change in viscosity is observed after application of an electric field. In this case there was observed a moderate increase of viscosity of a polymer solution, it is dependent on the time and intensity of treatment of an electric field. Accordingly it is possible that a polymer chain changes its shape uncoils and stretches. Presumably, this movement is very important for the process of electrospinning. 4. CONCLUSION The electrospinning process is affected by a number of the parameters. This contribution focused on the impact of an electric field on shear viscosity of the polymer solutions. It was shown that PVB dissolved in a poor solvent exhibited viscosity enhancement in the presence of an electric field. From this effect it can be deduced that these solvents are suitable for electrospinning. The electrorheological properties of the individual solvents play a substantial role in the process of electrospinning. It was found the correlation between shear viscosity and electrostatic spinnability. Structure of nanofibres was documented by the SEM images. ACKNOWLEDGEMENT The authors wish to acknowledge GA AS CR for the financial support of Grant No. A REFERENCES [1] Ramakrishna,S., Fujihara,K., Teo,W.E., Lim,T.C., Ma,Z. An Introduction to Electrospinning and Nanofibres. World Scientific Publishing Co., Singapore, [2] Andrady,A.L. Science and Technology of Polymer Nanofibers. John Wiley & Sons, New Jersey, [3] Huang,Z.-M., Zhang,Y.-Z., Kotaki,M., Ramakrishna,S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Comp.Sci.Tech. 63 (2003), [4] Reneker,D.H., Yarin,A.L. Electrospinning jets and polymer nanofibers. Polymer 49 (2008), [5] Doshi,J., Reneker,D.H. Electrospinning process and applications of electrospun fibers. J.Electrostat. 35 (1995), [6] Gibson,P., Schreuder-Gibson,H., Rivin,D. Transport properties of porous membranes based on electrospun nanofibers. Colloids and Surfaces A: Physicochemical and Engineering Aspects, (2001), [7] Fong,H., Chun,I., Reneker,D.H. Beaded nanofibres formed during electrospinning. Polymer 40 (1999), [8] Wang,M., Hsieh,A.J., Rutledge,G.C. Electrospinning of poly(mma-co-maa) copolymers and their layered silicate nanocomposites for improved thermal properties. Polymer 46 (2005), [9] Chuangchote,S., Sagawa,T., Yoshikawa,S. Electrospinning of poly(vinyl pyrrolidone): Effects of solvents on electrospinnability for the fabrication of poly(p-phenylene vinylene) and TiO 2 nanofibers. J.Appl.Polym.Sci. 114 (2009), [10] Drew,C., Wang,X., Samuelson,L.A., Kumar,J. The Effect of Viscosity and Filler on Electrospun Fiber Morphology. J.Macromol.Sci., Part A - Pure Appl.Chem. A40 (2003), [11] Winslow,W.M. Method and means for translating electrical impulses into mechanical force. 1947, U.S. Patent 2,417,850. [12] Winslow,W.M. - Induced fibrillation of suspensions. J.Appl.Phys. 20 (1949), [13] Price,C., Deng,N., Llyond,F.R., LI,H., Booth,C. Studies of polystyrene solution in an electric field: viscosity and dynamic light scattering. J.Chem.Soc., Faraday Trans. 91 (1995), [14] Wang,C.H., Huang,Q.R. Effects of external electric field on semidilute polymer solutions probed by dynamic light scattering. J.Chem.Phys. 106 (1997), [15] Chen,C.-L., Hua,C.-Y., Wu,C.-R. Computer simulation of polymer diffusion under external electric field. Macromol.Theory Simul. 10 (2001),