Optimization of electrospinning of polycaprolactone in acetone, acetic acid and their mixtures

Transcription

1 Chapter-3 Optimization of electrospinning of polycaprolactone in acetone, acetic acid and their mixtures Summary: This chapter describes the optimization study of electrospinning of polycaprolactone (PCL). Two eco-friendly solvents, acetone and acetic acid were tried as the solvents to electrospin PCL. Various spinning parameters like polymer concentration, applied voltage, tip to collector distance and flow rate were adjusted to avoid bead formation and to get fibers with uniform fiber diameter and enough pore spacing for the cell migration. Clogging of the polymer at the tip of the needle is a major problem in electrospinning that hinders the scaling up of the process at mass scale. Use of combinations of acetone and acetic acid could successfully overcome the phenomenon of clogging. A part of this chapter has been published as Clogging Free Electrospinning of Polycaprolactone Using Acetic Acid/Acetone Mixture, Polymer-Plastics Technology and Engineering, DOI: /

2 82 Chapter Introduction The emergence of nanofibers has gained much importance in the past few decades due to the potential applications in the medical, engineering and defense fields. Many of the potential uses of nanofibrous membranes are due to high porosity, large surface area and small pore size distribution (Subbiah et al., 2005; Huang et al., 2003). Even though there are a number of methods for the fabrication of nanofibrous membranes, most successful method is the electrospinning process. Electrospinning is a robust method to produce continuous nanofibers from submicron diameter scale down to nanometer diameter scale through an electrically charged jet of polymer solution. In electrospinning, the elongation force is produced by the interaction of an applied electric field with electrical charge carried by the jet rather than by spindles and reels used in conventional spinning. Poly (ε-caprolactone) (PCL) has got lot of attention in biomedical applications due to its biocompatibility and biodegradability (Choi et al., 1999) and has been suggested for wide range (Allen et al., 2000; Madhaiyan et al., 2013; Hutmacher et al., 2001; Eldsäter et al., 2000). Moreover, PCL is a U.S. Food and Drug Administration approved polymer for implantable materials such as sutures. Electrospinning of PCL, its blends and composites has been tried by many workers for tissue engineering scaffolds (Xie et al., 2010; Venugopal et al., 2005; Meng et al., 2010; Yoshimoto et al., 2003; Reed et al., 2009). PCL can be electrospun with a number of solvents including, dichloromethane (DCM) (Erisken et al., 2008), trifluoroethanol (TFE) (Yang et al., 2008) and hexafluoro-2-propanol (HFP) (Nam et al., 2008). While considering the physical properties like fiber diameter and fiber morphology of the fiber mats obtained by using these solvents and their combinations are satisfactory. But toxic effects of these organic solvents to the

3 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 83 operator, environment and end users are quite dangerous. Investigators are trying to increase the desirability of electrospun materials for biomedical applications and reducing the environmental hazards during materials production (Khadka et al., 2010; Agarwal et al., 2011; Sill et al., 2008; Khadka et al., 2012; Khadka et al., 2011; Yoo et al., 2009). Acetone is a relatively ecofriendly and less-toxic solvent compared to other organic solvents, but there are only a few reports regarding the use of acetone in electrospinning of polymers especially of biocompatible polymers, such as PCL. Electrospinning of PCL in acetone was successfully done by Reneker et al. (2002) and recently by Bosworth et al. (2012). Tungprapa et al. (2007) and Son et al. (2004) have reported the use of acetone for electrospinning of cellulose acetate and found that tip of the needle was obstructed during electrospinning because of the fast evaporation of acetone. Glacial acetic acid has been tried by many workers as a relatively environmentally benign solvent for the electrospinning of PCL (Kanani et al., 2011; Chakrapani et al., 2012; Moghe et al., 2009). When glacial acetic acid alone was used as solvent, many microsphere-shaped beads and comparatively limited number of fine fibers were obtained (Kanani et al., 2011). Even though a lot of research is going on, scaling up of electrospinning process to a large scale is still a challenging task. This hinders the further integration of nanofibers into practical large-scale applications and limits current uses to FMCG (Fast Moving Consumer Goods) markets (Luo et al., 2012). Many modifications have been tried in the conventional electrospinning set up including multi-needle electrospinning. These schemes are practically inconvenient due to possible polymer clogging which occurs at the tip of the needle. In such a set up, a regular cleaning has to be done to each needle to prevent blockage of the nozzles during electrospinning, which makes

4 84 Chapter 3 the whole setup impractical at an industrial scale when thousands of needles are used simultaneously (Wang et al., 2012). Eventhough many other modifications like needleless electrospinning (Wang et al., 2009), use of circular cylindrical electrode (Wu et al., 2010), disc electrospinning setup (Zhou et al., 2009) and upward needleless spinning (Niu et al., 2012), spiral coil spinnerets (Wang et al., 2012) were also tried to improve the productivity; still solidification of polymer is a challenging obstacle. Not only in the industrial scale but also in the laboratory scale, clogging of polymer at the tip of the needle makes electrospinning research quiet laborious. The operator has to give continuous attention to manually remove the polymer deposit from the tip of the needle to avoid the stoppage of the process. The clogging of polymers at the tip of the needle is due to the rapid evaporation of the solvent soon after the ejaculation of the polymer which led to the formation of a highly viscous semi-solid at the spinneret (Kanjanapongkul et al., 2010). Applied voltage and polymer concentration were also affect clogging (Kanjanapongkul et al., 2010). This fast evaporation leads to the deposition of polymer at the tip and eventually leads to the blockage of the needle and further ejection of the polymer. Thus, clogging phenomenon is directly linked to the volatile nature of the solvent used. Since most of the solvents such as chloroform, dichloromethane, acetone etc. used for electrospinning of polymers are highly volatile, the clogging is a regular obstacle in electrospinning process. Eventhough a few solvents like formic acid, acetic acid or even water can be used for electrospinning of a few polymers, the fiber diameter and fiber morphology were not satisfactory. In this chapter, optimization of electrospinning of PCL using two solvents, acetone and acetic acid and their combinations in various ratios is

5 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 85 detailed. Effect of various spinning parameters such as flow-rate, voltage, tip to collector distance and applied voltage on fiber morphology and fiber diameter while using these solvents have also been studied. Effective reduction of clog formation while using these solvents has also been investigated. 3.2 Results and discussion Effect of solvents on the fiber diameter and morphology Fiber diameter and morphology of the fabricated membranes varied greatly as the solvent system changed. While using glacial acetic acid alone as the solvent, the observed fibers were very narrow in diameter. However, there were a lot of beads even at higher concentrations of PCL (Figure 3.1). Figure 3.1: SEM images of electrospun PCL membranes using glacial acetic acid as the solvent. 10 wt% PCL (a) and 15 wt% PCL (b) solutions electrospun at a tip to collector distance of 15 cm, an applied voltage of 20 kv and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution.

6 86 Chapter 3 In the case of membranes spun using 15 wt% PCL in glacial acetic acid, 80% of the individual fibers had a fiber diameter less than 250 nm. But the large number of beads present in the membrane limits its application potential and may make it mechanically unstable. Bead formation might be due to the capillary instability driven by surface tension of the polymer solution (Reneker et al., 2002). To overcome this surface tension sufficient viscoelasticity is needed and due to the lack of enough viscosity, the polymer tends to form beads in acetic acid solution. While using acetone alone as the solvent, bead formation is not so frequent as much as in the case of acetic acid. According to Fong et al. the phenomenon of bead formation is related to parameters like viscosity, surface tension, and evaporation rate. This may be the hypothetic reason for the reduced bead formation while using acetone due to the higher viscosity, lower surface tension, and faster evaporation rate (leaving less time for the uniform fibers to form beads) (Fong et al., 1999). In the case of PCL electrospun in acetone, the fibers formed were of higher diameter compared to acetic acid alone as the solvent (Figure 3.2). PCL electrospun using a 10 wt% polymer in acetone, 70% of the individual fibers were less than 500 nm in diameter whereas at 15 wt% the fiber diameter seemed to be increased. In the case of membranes fabricated using 10 wt% PCL solutions, the uniformity of fiber diameter was less that of the membranes fabricated using 15 wt% PCL solutions. This can also be explained in terms of increased viscosity at higher polymer concentrations which render higher elongation forces and leads to the formation of uniform fibers (Chowdhury et al., 2010).

7 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 87 Figure 3.2: SEM images of electrospun PCL membranes using acetone as the solvent. 10 wt% PCL (a) and 15 wt% PCL (b) solutions electrospun at a tip to collector distance of 15 cm, an applied voltage of 18 kv and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution. While using 1:1 acetone/acetic acid mixture as the solvent, most of the fibers were comparatively larger in diameter than both acetone and acetic acid alone used as solvent (Figure 3.3). More or less similar results were obtained at applied voltages 15 kv and 20 kv.

8 88 Chapter 3 Figure 3.3: SEM images of electrospun PCL membranes using acetone/ acetic acid in the ratio 1:1 as the solvent. 15 wt% PCL solution electrospun at an applied voltage 15 kv (a) and 20 kv (b), a tip to collector distance of 15 cm and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution. However, an increase in the tip to collector distance from 15 cm to 20 cm leads to the formation of more fine fibers (Figure 3.4). The ribbon like interconnected morphology of the obtained fibers is an indication of the deposition of wet fibers on the collector.

9 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 89 Figure 3.4: SEM images of electrospun PCL membranes using acetone/ acetic acid in the ratio 1:1 as the solvent. 15 wt% PCL solution electrospun at an applied voltage 15 kv (a) and 20 kv (b), a tip to collector distance of 20 cm and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution. At 7:3 acetone/acetic acid ratio, the fibers obtained have a more uniform fiber diameter than that of 1:1 ratio (Figure 3.5).

10 90 Chapter 3 Figure 3.5: SEM images of electrospun PCL membranes using acetone/ acetic acid in the ratio 7:3 as the solvent. 15 wt% PCL solution electrospun at an applied voltage 15 kv (a) and 20 kv (b), a tip to collector distance of 15 cm and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution. A noticeable change in the fiber diameter and uniformity in fiber diameter was observed while using a mixture of acetone and acetic acid in the ratio 3:7 (Figure 3.6 and Figure 3.7).

11 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 91 Figure 3.6: SEM images of electrospun PCL membranes using acetone/ acetic acid in the ratio 3:7 as the solvent. 15 wt% PCL solution electrospun at an applied voltage 15 kv (a) and 20 kv (b), a tip to collector distance of 15 cm and a flow rate of 1 ml/hour. Graphs represent the frequency of diameter distribution. The overall effect of the solvent on the fiber diameter is given in Figure 3.8. While using acetic acid alone as the solvent, the average fiber diameter is about 230 nm. But in the case of acetone alone, it is around 1300 nm. In an acetone/acetic acid ratio of 1:1, it is almost similar to that of acetone alone and is 1250 nm. While using 7:3 acetone/acetic acid mixture the fibers formed were of 1160 nm. Interestingly at 3:7 acetone acetic acid ratio, the average fiber diameter was very much decreased and is about 700 nm. These

12 92 Chapter 3 changes in fiber diameter can be explained in terms of the changes in the solution properties as a result of the changes in the ratio of acetone and acetic acid. Figure 3.7: SEM images of electrospun PCL membranes using acetone/ acetic acid in the ratio 3:7 as the solvent. 15 wt% PCL solution electrospun at an applied voltage 15 kv (a) and 20 kv (b), a tip to collector distance of 20 cm and a flow rate of 0.5 ml/hour. Graphs represent the frequency of diameter distribution.

13 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 93 Figure 3.8: Graph showing the effect of solvent system on the fiber diameter of electrospun polycaprolactone. In all cases 15 wt% PCL solution electrospun at an applied voltage of 20 kv, a tip to collector distance of 15 cm and a flow rate of 1 ml/hour. Most important solution parameters that might have high impact in the variation of fiber diameter are solution viscosity and conductivity. The viscosity and conductivity data of PCL solutions in acetone, acetic acid and their mixtures are shown in Table 3.1. In viscosity measurements, when acetone alone used as the solvent, by fixing the PCL concentration as 15%, viscosity of the solution was found to be 462 ± 1.4 mpa.s. When acetic acid alone used, the PCL solutions give comparatively very low viscosity (228 ± 1.3 mpa.s). But as the ratio of acetone to acetic acid increases to the higher values like 7:3, viscosity also increased considerably to that of acetic acid solution (386 ± 1.8 mpa.s) with extreme statistical significance (P < ).

14 94 Chapter 3 Similar behavior was observed for 1:1 mixture of acetone and acetic acid, however the obtained value was less than that of the 7:3 mixture of acetone and acetic acid (346 ± 1.2 mpa.s) with extreme statistical significance (P < ). The viscosity of PCL in 3:7 ratio of acetone and acetic acid was much less than that of acetone but slightly higher than acetic acid solution (263 ± 1.6 mpa.s). In this case, decrease in viscosity for the PCL solution in 3:7 ratio of acetone/acetic acid mixture with respect to that in acetone was almost less than 1.9 times. However, there were marginal variations in conductivity for various solvent combinations, they were not significant to explain the variation in fiber diameter and morphology. From the above results, it is plausible to assume that the reduction of viscosity of PCL solution in 3:7 ratio of acetone and acetic acid was the major reason for the observed reduction in fiber diameter. Highly viscous solutions will produce fibers with large diameters whereas very low viscosity leads to bead formation (Kanani et al., 2011). Viscosity of PCL solution in acetic acid alone was very low in order to produce continuous jet of polymer solution and hence resulted in bead formation (Maretschek et al., 2008). Table 3.1: Variation of conductivity and viscosity of PCL solution (15 wt%) in acetone, acetic acid and their mixtures. Sample solution in Conductivity (µs/cm) Viscosity (mpa.s) Acetone 4.5 ± ± 1.4 Acetic acid 3.9 ± ± 1.3 Acetone/acetic acid mixture 5.2 ± ± 1.8 (7:3) Acetone/acetic acid mixture 4.7 ± ± 1.2 (1:1) Acetone/acetic acid mixture (3:7) 4.1 ± ± 1.6

15 Optimization of electrospinning of polycaprolactone in acetone, acetic acid Effect of polymer concentration on the fiber diameter Solution concentration is one of the important parameters in electrospinning. Solution concentration and viscosity are two closely related factors. An increase in solution concentration always results in an increase in solution viscosity and in converse a decrease in solution concentration always leads to the decrease in solution viscosity (Henriques et al., 2009). If polymer concentration is too low, instead of fibers only beads will be formed due to breaking up of polymer jet into droplets (Maretschek et al., 2008). Electrospinning is suppressed at very high polymer concentrations since it prevents the flow of polymer solution continuously from the capillary tip. Figure 3.9: Graph showing the effect of polymer concentration on the fiber diameter of electrospun polycaprolactone in acetone, acetic acid and their mixtures (Acetone and acetic acid in the ratio 1:1, 7:3 and 3:7). In all cases a tip to collector distance of 15 cm, an applied voltage of 15 kv and a flow rate of 1 ml/hour maintained.

16 96 Chapter 3 From Figure 3.9, it is clear that while increasing the polymer concentration from 10 to 20 wt%, the fiber diameter is increasing. For the electrospun fibers produced with below 15 wt% solutions of PCL, beaded structures were observed. The variation in fiber diameter while using 10 wt% and 15 wt% of PCL is apparently negligible but at 20 wt% it is significant. In all polymer concentrations, fibers produced from acetic acid alone as the solvent has shown the smallest fiber diameter. The increase in fiber diameter at higher polymer concentrations is due to the increase in viscosity of the solution. Increase in concentration and viscosity lower the surface tension which in turn favours the formation of uniform fibers Effect of applied voltage on the fiber diameter The shape of the droplet at the syringe tip can be changed by the strength of the applied voltage and the resulting fiber morphology and aspect ratio were changed accordingly. Figure 3.10 shows the effect of applied voltage on the fiber diameter of electrospun polycaprolactone in acetone, acetic acid and their mixtures (1:1, 7:3 and 3:7). From the figure it is clear that while increasing the applied voltage, the fiber diameter seemed to be decreasing. Obtained results demonstrate that the effect of applied voltage on the fiber diameter can be explained in terms of the relationships among the three major forces, viz. the Coulombic, the viscoelastic and the surface tension forces, influencing the fiber diameters. At low applied potentials, the Coulombic force was not enough to overcome the surface tension resulted in as-spun fibers with large diameters. At moderate applied potentials up to 20 kv, all these forces were well balanced, resulting in fibers with narrow diameters. Increasing the applied voltage does increase the electrical force and creates smaller fiber diameters. Increasing the applied voltage, i.e., increasing

17 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 97 the electric field strength will increase the electrostatic repulsive force on the fluid jet which favours fiber formation (Chowdhury et al., 2010). Higher electrostatic repulsion force between capillary tip and collector which in turn provides higher drawing stress in the jet, hence making the bead size smaller and results in fine fibers (Lee et al., 2003). Figure 3.10: Graph showing the effect of applied voltage on the fiber diameter of electrospun polycaprolactone in acetone, acetic acid and their mixtures (Acetone and acetic acid in the ratio 1:1, 7:3 and 3:7). In all cases 15 wt% PCL solution electrospun at tip to collector distance of 15 cm and a flow rate of 1 ml/hour Effect of flow rate on the fiber diameter The flow rate of the polymer solution from the syringe also has an effect on fiber diameter and fiber morphology. While increasing the flow rate

18 98 Chapter 3 from 0.5 ml/hour to 1.5 ml/hour, there is a marginal increase in fiber diameter as shown in Figure Below 0.5 ml/hour, we could not observe any fiber formation and above 1.5 ml/hour resulted in the oozing of drops of the polymer from the tip of the needle without forming continous fibers. Figure 3.11: Graph showing the effect of flow rate on the fiber diameter of electrospun polycaprolactone in acetone, acetic acid and their mixtures (Acetone and acetic acid in the ratio 1:1, 7:3 and 3:7). In all cases 15 wt% PCL solution electrospun at tip to collector distance of 20 cm and an applied voltage of 20 kv. To a certain extent, an increase in the flow rate produces fibers with larger fiber diameters (Homayoni et al., 2009). A large increase in the flow rate causes fibers to be collected without sufficient solvent evaporation leading to flattened web-like appearance (Rutledge et al., 2001). The effect of flow

19 Optimization of electrospinning of polycaprolactone in acetone, acetic acid.. 99 rate on the diameter of the fibers obtained can also be explained in terms of the relationships between Coulombic, viscoelastic and surface tension forces which influence the fiber diameters. At a particular applied voltage, the electrostatic force, which carries a charged jet from the tip of the needle to the collector, may increase slightly when there is an increase in the feed rate. Only a small amount of polymer solution will be ejected from the capillary tip at lower flow rates, which lead to the formation of small droplet size. If the flow rate is high, a greater amount of solution volume will be ejected from the needle tip, results in the formation of a large droplet at the tip of the spinneret (Zargham et al., 2012). When the size of the droplet is too big to suspend at the tip of the spinneret, it either drops from the tip or is not completely carried away to the collector, which resulted in unstable jet and larger fibers. This ultimately resulted in an increase in the bead area and the diameters of the obtained fibers were increased with increase in flow rate Effect of tip to collector distance on fiber diameter Tip to collector distance has a direct role on jet flight time and electric field strength. A decrease in this distance shortens flight times and solvent evaporation time, and increases the electric field strength, which results in an increase of bead formation. Figure 3.12 shows the effect of tip to collector distance on fiber diameter. Decreasing the tip to collector distance resulted in the same result as increasing the voltage (Homayoni et al., 2005). At a tip to collector distance of 10 cm, the electrospun fibers with higher diameters were produced. While increasing the distance from the tip to collector, electrospun fibers with smaller diameter were produced. The wider gap allowed more time for the fluid jet to stretch fully and for the solvent to evaporate completely. When the gap is increased more, the collected fibers were partially dried before reaching the collector and fully stretched, and the fiber diameter was

20 100 Chapter 3 reduced. In all solvent ratios, the same trend could be observed even though the variation is negligible in the case of acetic acid alone as the solvent. Supaphol et al (2008) reported that the fiber diameter distribution decreased when the tip to collector distance increased from 5 cm to 15 cm. Figure 3.12: Graph showing the effect of tip to collector distance on the fiber diameter of electrospun polycaprolactone in acetone, acetic acid and their mixtures (Acetone and acetic acid in the ratio 1:1, 7:3 and 3:7). In all cases 15 wt% PCL solution electrospun at an applied voltage of 20 kv and a flow rate of 0.5 ml/hour Effect of solvents on clogging of polymer Figure 3.13 shows the reduction in clogging of polymer while using either acetic acid or a mixture of acetone and acetic acid as the solvents. While

21 Optimization of electrospinning of polycaprolactone in acetone, acetic acid using acetone alone as the solvent within 5 minutes, the polymer solution starts to clog on the tip of the needle (Figure 13 A(a)). Figure 3.13: Photographic image of the tip of the needle during the electrospinning of polycaprolactone in acetone (A), acetic acid (B) and acetone/acetic acid mixture (3:7) (C) at various time intervals 5 minutes (a), 15 minutes (b) and 30 minutes (c). Within 15 minutes of continuous electrospinning, a large amount of polymer was deposited at the tip of the needle and the spinning process partially hindered (Figure 13 A(b)). Flow of polymer was ceased after 30 minutes of spinning due to the blockage of spinneret and further electrospinning became impossible without removing the clumped polymer from the needle (Figure 13 A(c)). But in the case of electrospinning of PCL in acetic acid or in acetone/acetic acid mixtures, there was no clogging even after 30 minutes (Figure 13 B and Figure 13 C). The quantification of the polymer deposited at the tip of the needle after various time intervals gives a clear information about the effect of solvent system on the clog formation (Table 3.2).

22 102 Chapter 3 Table 3.2: Quantification of the clog formed at the tip of needle. Solvents used Weight of clog (mg) 5 min 10 min 15 min 30 min Acetone 5 ± ± ± ± 0.76 Acetic acid Acetone/acetic acid mixture (7:3) Acetone/acetic acid mixture (1:1) Acetone/acetic acid mixture (3:7) ± ± Results demonstrated that the use of acetone alone as the solvent for electrospinning resulted in clog formation even before 5 minutes of electrospinning. Electrospinning carried out using acetic acid alone as the solvent did not produce any clog. However from the morphology of the fibers (Figure 3.1), it is clear that membranes fabricated using acetic acid alone as the solvent were not satisfactory for any application due to large number of beads. Acetone/acetic acid mixture in the ratio 7:3 did not resulted in clog formation till 10 minute of spinning, however after 10 minutes of spinning, clog formation has been observed. At any point during electrospinning, when acetone/acetic acid miture in the ratio 1:1 or 3:7 were used, no clogging was observed. The clogging of polymers at the tip of the needle is due to the rapid evaporation of the solvent soon after the ejaculation of the polymer which led to the formation of a highly viscous semi-solid at the spinneret (Kanjanapongkul et al., 2010). This fast evaporation leads to the deposition of

23 Optimization of electrospinning of polycaprolactone in acetone, acetic acid polymer at the tip and eventually leads to the blockage of the needle and further ejection of the polymer. Thus, the clogging phenomenon has a direct correlation with the volatile nature of the solvent. Acetone is a highly volatile solvent and the fast evaporation of it from the PCL solution took place before the Taylor cone split into fibers which resulted in clogging. In comparison with acetone, acetic acid is less volatile and the polymer gets sufficient time to reach the collector before evaporation and thus there was no clogging. While taking a mixture of acetone and acetic acid, still the evaporation rate was sufficiently low to prevent clogging at the tip of the needle FTIR analysis Figure 3.14 shows the FTIR spectra of electrospun polycaprolactone using acetone, acetic acid and their mixtures as the solvents. There is no considerable variation in the IR peaks of electrospun PCL membranes while using acetone, acetic acid or their mixtures as solvent. The characteristic peaks of PCL are present in the spectra of all samples. An intense peak at 1724 cm -1 which is due to the presence of the ester carbonyl group that corresponds to the CO (stretching) in PCL polymer. The peak at 2862 cm -1 is due to the symmetric stretching of CH2 group and 2942 cm -1 is related to the asymmetrical stretching of CH2 group. The peak at 1294 cm -1 represents C O and C C stretching in the crystalline phase of PCL where as the peak 1160 cm -1 corresponds to C O and C C stretching in the amorphous phase of PCL. The peak at 1240 cm -1 can be assigned to the asymmetric COC stretching of PCL. All these results are comparable to the previous reports by other workers (Elzein et al., 2003; Elzubair et al., 2006).

24 104 Chapter Acetone Acetic acid 1240 % T 180 Acetone/Acetic acid, 1:1 120 Acetone/Acetic acid, 7:3 Acetone/Acetic acid, 3: Wave number (cm -1 ) Figure 3.14: FTIR spectra of electrospun polycaprolactone membranes fabricated using acetone, acetic acid and their mixtures. The FTIR results confirm that there is no change in the chemical structure of the PCL while using acetone, acetic acid or their mixtures as solvent for the electrospinning XRD analysis The crystallinity of the prepared membranes was characterized by X- Ray Diffraction (XRD). Figure 3.15 shows the XRD pattern of the sample indicating crystalline peaks. Results show that the XRD of PCL membrane contains three distinct reflections at the Bragg angles of about 21.4, 22.0 and 23.7, corresponding to the (110), (111) and (200) planes of the orthorhombic crystal structure respectively. The behavior of the dominant crystalline phase in the electrospun PCL membranes using various combinations of acetic acid and acetone can be predicted from the XRD patterns in order to indicate the change in dominant crystalline phase.

25 Optimization of electrospinning of polycaprolactone in acetone, acetic acid Lim et al (2008) have reported electrospinning of polycaprolactone nanofibers with smaller diameters having a higher degree of molecular orientation, crystallinity, stiffness, and strength. According to them nanofiber diameter and the resulting crystalline morphology is influenced by whether complete crystallization of polymer chains took place before or after the electrospinning jet has reached the collector. The former leads to the formation of smaller fibers with fibrillar structure and aligned lamellae, but, the latter leads to the formation of a misaligned lamellar structure. A similar trend can be observed here also. Intensity (a. u) Acetone/acetic acid= 7:3 Acetone/acetic acid= 3:7 Acetone/acetic acid= 1:1 Acetic acid alone Acetone alone Theta (Degrees) Figure 3.15: XRD analysis of electrospun PCL membranes electrospun using various solvents. In all cases 15 wt% PCL solution electrospun at an applied voltage of 20 kv, tip to collector distance of 15 cm and a flow rate of 0.5 ml/hour Tensile properties While considering the application of the fabricated membrane, it is most important to measure the tensile properties. The stress- strain curve of the material will give an overall idea about the tensile properties as shown in

26 106 Chapter 3 Figure Other mechanical properties like break stress, maximum elongation and modulus are given in Table 3.3. The membrane spun using acetone alone as the solvent got maximum break stress but the modulus seemed to be the lowest among all the samples tested in this study. While taking the maximum elongation, sample that were electrospun using acetic acid alone got less elongation at break. This is due to the fact that there were a lot of beads and these make it mechanically less stable. PCL membrane spun using 3:7 ratio of acetone and acetic acid got maximum elongation before the break. PCL membrane spun using 3:7 ratio of acetone and acetic acid showed highest toughness also Acetone/acetic acid, 3:7 Acetic acid Acetone/acetic Acid, 7:3 Acetone/acetic Acid/, 1:1 Acetone Stress (MPa) Strain (%) Figure 3.16: Stress- strain behavior of electrospun PCL membrane using various ratio of solvents. All cases, PCL concentration of 15 wt%, tip to collector distance of 20 cm, flow rate of 0.5 ml/h and applied voltage 15 kv were maintained.

27 Optimization of electrospinning of polycaprolactone in acetone, acetic acid Table 3.3: Mechanical properties of electrospun PCL membrane using various solvent ratios. All cases, PCL concentration of 15 wt%, tip to collector distance of 20 cm, flow rate of 0.5 ml/h and applied voltage 15 kv were maintained. Solvent Break stress Elongation at break (%) Tensile modulus Relative toughness (MPa) (MPa) Acetone 1.45 ± 0.23 Acetic acid 0.82 ± ± ± ± ± ± ± 6 Acetone and acetic acid in 1:1 ratio 0.99 ± ± ± ± 13 Acetone and acetic acid in 7:3 ratio 1.21 ± ± ± ± 16 Acetone and acetic acid in 3:7 ratio 1.36 ± ± ± ± Conclusion Electrospinning of PCL in acetone, acetic acid and their mixtures has been successfully optimized to obtain uniform fibers with less fiber diameter. The fiber morphology in terms of fiber diameter and frequency of beads, the acetone/acetic acid mixture has shown superiority over either acetone or acetic acid individually. 15 wt% PCL solution electrospun using acetone/acetic acid mixture in the ratio 3:7 as the solvent at an applied voltage of 15 kv, a tip to collector distance of 20 cm and a flow rate of 0.5 ml/hour has shown the best property in terms of fiber morphology and fiber diameter. A new combination of solvent system has been developed for the clogging free electrospinning of

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