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

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EFFECT OF SOLVENT SYSTEMS ON ELECTROSPUN POLYMERIC FIBRES: PRELIMINARY STUDY ON POLY LACTIC ACID (PLA)

R.CASASOLA1, Dr. N.L. THOMAS2 and Dr. S. GEORGIADOU1

1Chemical Engineering Department, Loughborough University, Loughborough, UK 2 Materials Department, Loughborough University, Loughborough, UK

R.Casasola@lboro.ac.uk

Abstract: Electrospun poly lactic acid (PLA) nanofibres have attracted a lot of interest in the field of tissue engineering. The selection of an appropriate non-hazardous solvent or solvent system is important to define the electrospinnability of the solution and the production of nanofibres. In this study, poly lactic acid (PLA) solutions were prepared in various pure solvents and mixed-solvent systems: acetone, 1,4-dioxane, tetrahydrofuran, dichloromethane, chloroform, dimethylformamide and dimethylacetamide. Viscosity, conductivity and surface tension of each solution were measured. The effects of the solution properties on the morphology and diameter of the electrospun fibres were observed by scanning electron microscopy (SEM). Of the pure solvents, only acetone was found to produce a sufficient quantity of fibres to form a nanofibre mat. Of the mixed-solvent systems, acetone/dimethylformamide (50/50 v/v) gave higher fibre productivity and finer defect-free nanofibres. The mean diameter was about 200 nm. The results show that the solvent properties have a significant effect on both process productivity and morphology of the PLA nanofibres. 1. Introduction First patented by Formhals in 1934 [1], electrospinning is a simple, scalable, versatile and cost effective technique to produce continuous fibres from both synthetic and natural polymers [2-4]. Ultrafine fibres with diameter typically ranging from few hundred nanometres to a few micrometres exhibit outstanding properties such as high surface area to volume ratio, small pore size and high porosity. In the last 20 years, electrospinning has gained a lot of interest in several fields such as filtration system [5], chemical and optical sensors [6], tissue engineering [7-9], wound healing [10] and release of drugs [11]. The electrospinning process relies on the application of an electric field [12] between a needle-tipped syringe containing the polymeric solution and a collector for the deposition of nanofibres. Therefore the polymeric solution is electrically charged and a conical droplet is formed at the needle tip. As the electric strength overcomes the surface tension of the solution, a tiny polymeric jet is generated from the surface of the droplet and travels toward the collector. Between the needle and the collector, the solvent can evaporate from the jet and consequently nanofibres can be collected [13]. Morphology and diameter of the resultant nanofibres depend on many parameters that have been divided in three groups: solution properties, process and ambient parameters [14-16]. The optimization of these parameters is significant in order to obtain continuous nanofibres with specific morphology and well-definite physical and mechanical properties depending on the type of application. The selection of solvent or mixed-solvent system to dissolve the polymer is one of the main factors influencing the solution properties and consequently the electrospinnability of the solution. It has been shown that polymer concentration, conductivity and surface tension of the solution have an important effect on the fibre morphology and diameter [17]. Supaphol et al [18] investigated the effect of solvent properties on the productivity and morphology of polystyrene (PS) nanofibres. Six solvents with different properties, such as density, boiling point, solubility parameter and dielectric constant, were used to prepare PS solutions. They found that the diameter of PS fibers decreased with increasing density and boiling point and a large difference between the solubility parameters of the solvent and the polymer was responsible for bead-string morphology. Moreover the productivity of PS nanofibres increased with increasing dielectric constant and dipole moment of the solvents. Lee et al [19] studied the effect of different solvent systems on poly caprolactone (PCL) nanofibres. Only methylene chloride (MC) was able to dissolve the polymer. However the electrospinning process of the solution was often stopped due to the low boiling point of MC. Addition of dimethylformamide (DMF) to MC enhanced the electrospinning process and allowed the production of smaller nanofibres. They suggested that nanofibres could be collected because DMF decreased the viscosity and increased the electrical conductivity of the PCL solution. In a following study they found similar results [20]. The effects of the solvent

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composition on the morphology of electrospun poly (vinyl chloride) PVC fibres were investigated. A mixture of tetrahydrofuran (THF) and dimethylformamide (DMF) was used to prepare the PVC solution. They found that with increasing DMF in the THF/DMF solution, smaller nanofibres could be collected. DMF showed polyelectrolyte behaviour despite being a poor solvent for PVC in comparison with THF [20]. Therefore the optimization of the electrospinning process for the production of homogeneous nanofibrous mats involves the selection of an appropriate solvent or solvent system depending on the type of polymer. The main objective of this study is to investigate the effect of different solvent systems on the morphology and diameter of poly (lactic acid) (PLA) nanofibres. PLA is one of the most common synthetic polymers used for medical applications such as bone screws and sutures, due to its biodegradability and biocompatibility [21, 22]. Recently, electrospinning has gained a lot of attention in tissue engineering and regenerative medicine, mainly because nanofibrous structures can closely mimic the natural extracellular matrix (ECM). However, physical and biological properties similar to the ECM as well as minimal variation in nanofibre diameter and a bead-free morphology are requirements for a nanofibrous scaffold to be acceptable in tissue engineering [8, 11, 23]. 2. Materials and methods 2.1 Materials Poly (lactic acid) (PLA) was obtained from Nature Works LLC (PLA 4060D). All solvents were supplied by Fischer Scientific and used without further purification. Table 1 shows some properties of the selected solvents used in this study.

Solvent Boiling point Viscosity Dielectric constant Electri cal conductivity Surface tension

°C mPa·s (25°C) ε (20°C) S cm -1 mN/m (20°C)

Acetone - AC 56 0.308 20.60 5.00E-09 23.3

1,4-Dioxane - DX 101 1.177 2.21 5.00E-15 40.0

Tetrahydrofuran - THF 66 0.480 7.60 4.5E-05 28.0

Dichloromethane - DCM 40 0.449 9.10 4.30E-11 28.1

Chloroform - CHL 61 0.563 4.80 <1.0E-10 27.2

Dimethylformamide - DMF 153 0.920 36.70 6.00E-08 35.0

Dimethylacetamide - DMAc 166 1.960 37.80 / 43.7

Table 1. Properties of solvents used in this work [24]

2.2 Preparation of PLA solutions The polymer solutions for electrospinning were prepared by dissolving PLA in single solvents and mixed-solvent system of acetone and another solvent (50/50 v/v) to obtain a 10% w/v polymer concentration. Each solution was prepared using a heated stirrer (~40°C) until dissolution of the polymer was complete. 2.3 Electrospinning process A high voltage power supply was used to generate an electric field of 20kV between a collector and a needle. The polymer solution was placed in a 20-ml plastic syringe (Luer lock syringe, Sigma Aldrich) and a pump system (PHD ULTRATM, Harvard Apparatus) was used to feed a constant solution (1.0 ml/h) through the needle. A polyethylene capillary tube was used to connect the syringe and the needle (inner diameter 0.6mm) which was set up vertically. The collector was a rectangular copper plate covered with aluminium foil and located 15 cm from the needle tip for the deposition of nanofibres. The spinning time for each solution was fixed at 10 minutes. The electrospinning experiments were performed at room temperature (~20°C). Three nanofibre samples were prepared for each solution. 2.4 Characterisation of PLA solutions The rheological properties of the PLA solutions were measured at 25°C on a HAAKE VT550 rheometer equipped with cone-plate. Measurements were performed at a range of shear rates from 1 to 400 s-1. The viscosity measurements were repeated three times for each solution. The surface tension of the PLA solutions was measured by a surface tension meter (White Electric Instrument, DB2kS). Five measurements were taken for each solution. The electrical conductivity of polymer solutions was determined through the conductivity meter (Jenway Model 470).

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2.5 Electrospun Fibre Analysis The morphology of electrospun PLA nanofibres was characterized by field emission scanning electron microscopy (FEG-SEM, LEO 1530VP) using an accelerating voltage of 5.0 kV. Before observation, each sample was coated using a gold sputter coater for 2 minutes (SC7640, Emitech). The diameter of PLA nanofibres was measured using image software (AxioVision Rel 4.8). 150 measurements for each sample were considered to obtain an average value as well as the standard deviation. 3. Results Various PLA solutions had been prepared by dissolving PLA (10% w/v concentration) in pure solvents and mixed-solvent systems to investigate the effects of the solvent system on the morphology and diameter of PLA nanofibres. In the mixed solvent system acetone was chosen as the main solvent because of all single solvents only acetone allowed the production of PLA nanofibres. 3.1 Single-solvent systems Poly lactic acid (PLA) dissolved in all solvents within 3-4 hours. Figure 1 shows scanning electron micrographs of the fibres/beads collected from each solvent type. Of all the solvents, only acetone (AC) was found to produce a sufficient quantity of fibres to form a nanofibre mat. These nanofibres show bead string morphology with a mean nanofibre diameter of 757 nm, as shown in figure 1(a). 1,4-dioxane (DX) and tetrahydrofuran (THF) did not allow the production of continuous nanofibres, but many droplets were collected (Figure 1(b) and 1(c)). Coexistence of very few nanofibres and many droplets was observed in electrospun samples produced using dichloromethane (DCM) and chloroform (CHL). Despite the higher dielectric constant of dimethylformamide (DMF) and dimethylacetamide (DMAc), nanofibres could not be produced: only droplets were collected as shown in Figures 1(f) and 1(g). A possible explanation could be the high boiling point of DMF and DMAc as shown in table 1 and therefore the polymeric jet did not have enough time to dry completely during its flight to the collector.

(a)

(b) (c) (d)

(e) (f) (g)

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Figure 1. Scanning electron micrographs of PLA nanofibres from solutions of 10% (w/v) of PLA in (a) acetone (10 000x) with nanofibre diameter distribution, (b) 1,4-dioxane (5000x), (c) tetrahydrofuran

(5000x), (d) dichloromethane (5000x), (e) chloroform (5000x), (f) dimethylformamide (5000x) and (g) dimethylacetamide (5000x)

3.2 Mixed-solvent systems Addition of acetone to the other solvents (ratio 50/50 v/v) enhanced the electrospinning process allowing the production of nanofibres as shown in Figures 2. Nanofibres with bead-string morphology were found using the solvent systems AC/DX, AC/THF, AC/DCM and AC/CHL as shown in Figures 2(a), 2(b), 2(c) and 2(d), respectively. It seems that the nanofibre diameter decreases as the boiling point of the second solvent in the mixed-solvent system increases as shown in Figure 3. Smooth bead-free nanofibres with narrow diameter distribution were collected using the solvent system acetone/dimethylformamide and acetone/dimethylacetamide, as shown in Figures 2(e) and 2(f). Probably the synergistic effect of high dielectric constant of DMF and DMAc and low boiling point of acetone allows the production of homogeneous nanofibrous mats. The PLA solution in acetone/dimethylformamide produced nanofibres with the thinnest diameter (210 nm), probably due to the highest electrical conductivity of the solution (3.63 µS/cm) as shown in Table 2.

(a) (b) (c)

(d) (e) (f)

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Figure 2. Scanning electron micrographs of PLA nanofibres with nanofibre diameter distribution from solutions of 10% (w/v) of PLA in (a) acetone/1,4-dioxane (5000x), (b) acetone/tetrahydrofuran (4000x), (c)

acetone/dichloromethane (5000x), (d) acetone/chloroform (5000x), (e) acetone/dimethylformamide (20000x) and (f) acetone/dimethylacetamide (20000x)

Overall the addition of acetone in the mixed-solvent system reduced viscosity and surface tension but increased conductivity of the solutions. Probably the combined effects of higher electrical conductivity, lower surface tension and lower viscosity of the mixed-solvent solutions compared to the single solvent system allowed the production of PLA nanofibres. The solvent systems acetone/dimethylformamide (AC/DMF) and acetone/dimethylacetamide (AC/DMAc) allowed the production of bead-free nanofibres probably due also to the higher electrical conductivity of the resulting PLA solutions, as shown in Table 3.

Solvent system Viscosity Conductivity Surface tension

Pa·s µµµµS·cm-1 mN·m-1

AC 0.098 1.30 25.5

DX 0.531 0.03 36.8

THF 0.280 0.01 30.4

DCM 0.167 0.04 30.2

CHL 0.372 0.01 31.0

DMF 0.155 4.35 38.3

DMAc 0.210 2.69 37.7

AC/DX a 0.298 0.17 30.1

AC/THF a 0.221 0.40 26.9

AC/DCM a 0.133 0.62 27.6

AC/CHL a 0.169 0.65 27.3

AC/DMF a 0.226 3.63 30.6

AC/DMAc a 0.263 3.22 29.4

Table 3. Properties of 10% w/v PLA solutions in single and mixed-solvent systems (a ratio 50/50

v/v for mixed-solvent systems)

Figure 3. Boiling point of 2nd solvent in the mixed solvent system vs mean nanofibre diameter

It has been found that increasing solution conductivity or charge density allows the production of more uniform fibers and fewer droplets [25]. The solution is exposed to greater tensile force with the application of an electric field and therefore the polymeric jet is subjected to greater elongation. The surface tension is the parameter in competition with the viscosity of the solution. It has been found that reducing the surface tension enables the production of nanofibres with fewer beads. In fact, on increasing the surface tension, the jet becomes unstable and more beads appear on the nanofibre mats [26]. Nevertheless, the application of a lower surface tension to get bead-free nanofibres is not a general rule, but depends on the type of solvent [27]. 4. Conclusion In the present study, electrospinning was used to produce PLA nanofibres from PLA solution in various solvents and mixed-solvent systems. The main objective was to investigate the effects of the solvent system on the nanofibre diameter and morphology. Among the PLA-solutions in single solvents, only the PLA solution in 10% w/v acetone produced PLA nanofibres. For the mixed solvent systems, all the PLA solutions in mixed solvents of 10% w/v acetone and another solvent were spinnable and nanofibres could be collected. Larger nanofibres with a broad fiber diameter distribution were collected using the solvent systems acetone/1,4-dioxane, acetone/tetrahydrofuran, acetone/dichloromethane and acetone/chloroform. Among these solvent systems, it was found that the mean nanofibre diameter decreased as the boiling point of the second solvent in the mixed-solvent system increased. Bead-free nanofibres with narrow diameter distribution were collected using the solvent systems acetone/dimethylformamide and acetone/dimethylacetamide. Thinner nanofibres (210 nm) were produced with acetone/dimethylformamide, probably due to the higher electrical conductivity of the resulting PLA solution. The results obtained suggested and confirmed that the combined effects of viscosity, surface tension and conductivity play a significant effect of nanofibre morphology and diameter.

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