THE STUDY OF POROUS NANOFIBRES MORFOLOGY MADE FROM PCL IN DEPENDENCE ON THE ELECTROSPINNING PARAMETRES AND SOLUTION COMPOSITION Eva MACAJOVÁ, Iva DUFKOVÁ, Pavel KEJZLAR Department of Material Science, Technical University of Liberec, Studentska 1402/2, 461 17 Liberec, Czech Republic Abstract The work deals with the influence of the composition solution and the parameters of electrospinning to the structure and morphology of PCL nanofibrous layers produced by method of electrospinning. The research is focused mostly on the study of pores size, porosity, diameters of fibers. Nanofibrous textiles are generated by electrospinning from the needle. The morphology of nanofibrous layers can be primarily affected by the composition of the solution polymer and by the integrity of used solvents. Keywords: Solvent, porous nanofibers, polymer, electrospinning. 1. ELECTROSPINNING In the industry polymer nanofibres can be produced by the use of an electrospinning method, in which electric powers affect polymerous solution or melt. Under appropriate conditions an electrically charged polymer solution will create very thin fibers by the use of the electrostatic field effect. The formation of fiber occurs between two oppositely charged electrodes, one of which is in contact with the liquid, the second electrode serves as a collector where nanofibrous layer is created. Charged liquid which is subjected to electrostatic forces opposite electrode is pulled and it forms very thin fibers. The diameter of fibers made by electrospinning may vary from tens of nanometers to micrometers. The diameter of fibers is most often in the range of 100-750 nm, depending on the type of polymer and external conditions of spinning process. Nanofibres produced by electrospinning have enormous potential in many fields especially in medicine, engineering, clothing industry, aerospace, energy etc. [1-5] 1.1 Porous nanofibres The most important advantages of nanofibrous layers are their high porosity and high specific surface, which is significant mainly for proliferation of cells on nanofibrous layers in tissue engineering and in the controlled release of drugs. They can be used in filtration processes or to increase catalytic reactions in the chemical industry. [5] Present work deals with creation of porous nanofibers, which is an interesting method of increasing their specific surface. Problems connected with the production of porous fibres by electrospinning are described for example in [6-9]. Electrospinning is influenced by the properties of the polymer solution, i.e. viscosity or surface tension. Morphology and diameter of nanofibres is affected by a selected solvent. [9] Production of porous fibers is from a practical point of view rather complicated and brings a number of issues that need to be solved. [9] One of the suitable polymers for application in medicine is for example polycaprolactone due to its biocompatibility. PCL is an inner ester which is prepared by catalytic. It is biodegradable and it can be
degraded by a hydrolysis of its ester linkages in physiological conditions. This is important for its biocompatibility with living organism and this is why this polymer is used mainly in biomedicine. [10] The main advantage of biocompatible nanofibres is the fact that they don t cause long-time stress for the human body. [10] 2. EXPERIMENT 2.1 The preparation of porous mikro/nano fibrous structure For the preparation of layers formed by porous micro/nanofibrous formations has been chosen electrospinning method the needle-like anode. The schema of the used apparatus is in Fig. 1. For the experiment were used 16% solution of PCL with a molecular weight Mw = 45000 g / mol, mixed in a solvent mixture of tetrahydrofuran (THF) and dimethylsulfoxide (DMSO). Pre-mixing ratios of solvents (THF : DMSO) were designed: 9:1 and 7:3. The observed parameters were: high voltage, distance from the collector, and proportioning (see tab. 1). Fig. 1: Electrospinning from the needle, 1 high voltage supply, 2 needle with polymer solution, 3 nanofibers on the way to the collector, 4 collector, 5 ground connection, 6 syringe pump.
Table 1: Experimental parameters. Concentration [ THF / DMSO] high Voltage [kv] Distance [cm] Proportioning [ml/h] 15 kv 15 cm 8 20 cm 8 25 cm 8 9:1 20 kv 15 cm 8 20 cm 8 25 cm 8 15 cm 8 25 kv 20 cm 8 25 cm 8 3 7:3 15 kv 17 cm 20 kv 17 cm 6 9 12 3 6 9 12 25 kv Tab. 1: Experimental parameters. 17 cm 3 6 9 12 2.2 Overall assessment The morphology of nanofibrous layers on the first and second testing series 9:1, 7:3 was assessed on the basis of images taken by a scanning electron microscope (SEM). In the first testing series, where the ratio of (THF : DMSO) was 9:1, at 15 kv and 15 cm distance to the collector, mixture of smooth nanofibers and porous micro-balls was formed (see Figure 2). The fiber diameter was around 150 nm, diameter of porous balls varies in the range of 5-25 µm; diameter of the pores was in the order of hundreds of nm. When was increased the high voltage and the collector distance, porosity of microspheres decreased and it began to create only wrinkled surface (see Figure 3 b), c), 4 b), c). The result of this experiment is that in the following measurements we can omit a distance from the collector.
Fig. 2. SEM image of electrospinned layer of 16% PCL with dissolvent THF:DMSO in a ratio 9:1 formed at 15 kv; Collector distance: a)15 cm; b) 20 cm; c) 25 cm. Fig. 3. SEM image of electrospinned layer of 16% PCL with dissolvent THF:DMSO in a ratio 9:1 formed at 20 kv; Collector distance: a) 15 cm; b) 20 cm; c) 25 cm. Fig. 4. SEM image of electrospinned layer of 16% PCL with dissolvent THF:DMSO in a ratio 9:1 formed at 25 kv; Collector distance: a) 15 cm; b) 20 cm; c) 25 cm. In the second testing series, the solvent concentration ratio of (THF : DMSO) was 7:3. In this measurement, following parameters were taken into account: high voltage and polymer solution dosing. The distance of the collector was selected as constant as mentioned previously (see Table 1). At voltage of 15 kv, 20 kv and 25 kv at a dose 3-12 ml/h it was formed mixture of smooth nanofibers, porous micro-beads and porous microfibers (Fig. 5). The fiber diameter was around 150 nm, diameter of porous beads was in the range of 3-40 µm and porous micro-fiber 1-6 µm. The diameter of the pores varied in the order of hundreds of nm to micro. Most of the pores were through the fiber. At high voltage of 25 kv and higher doses specifically at 6, 9 and 12 ml/h on the smooth surface were observed the remains of solvent.
Fig. 5. SEM image of electrospinned layer of 16% PCL with dissolvent THF:DMSO in a ratio 7:3 a) 15 kv, 6ml/h; b) 20 kv; 6ml/h; c) 25 kv; 6ml/h. Results The experiment is focused on the production of polycaprolactone nanofibrous layers. The first part deals with the preparation of test samples range with a respect to various parameters of the spinning process. The structure and porosity of micro / nanofibres is strongly influenced by a combination of many factors. Therefore various configurations of spun solution, various voltage, distance of collector and dosage were tested. The morphology of nanofibrous layers was evaluated with the help of images taken by a scanning electrone microscope, see Fig. 2-5. Based on the results of the first testing serie (THF:DMSO = 9:1), the optimal distance of collector was assigned as about 17 cm. With increasing voltage the production of porous micro-beads stopped and only wrinkled surface was created, see Fig. 3. b) c) and 4.b) c). In first testing serie (THF:DMSO = 9:1) it was not possible to obtain porous nanofibers, but only porous micro-beads were produced. It is necessary to have special conditions for the spinning process, such as change of temperature or relative humidity of air (impossible with this method) or change the dosing of the polymer. It was found that it is necessary to aim on various concentrations of the polymer solution. Further research will deal with testing line 7:3. Conclusions Based on the tests which were carried out, the most useful is the dissolvent prepared from mixture of (THF:DMSO) in 7:3. To get the optimal morphology of porous nanofibers it is necessary to choose a suitable collector distance and a corresponding voltage, which should assure production of nanofibers with a high porosity and high specific surface. ACKNOWLEDGEMENTS The research was supported by the SGS project Innovation in Material Engineering and by the project CxI CZ.1.05/2.1.00/01.0005
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