Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter

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1 American Journal of Chemistry 2017, 7(6): DOI: /j.chemistry Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter Muhammad Ashfak Jahangir 1,*, Tareque Mahmud Rumi 1, Md Abdul Wahab 1, Md Iusuf Khan 2, Mohammad Arifur Rahman 1, Zubair Bin Sayed 3 1 Department of Textile Engineering, Southeast University, Dhaka, Bangladesh (Wuhan Textile University) 2 Port City International University, Chittagong, Bangladesh (Wuhan Textile University) 3 Port City International University, Chittagong, Bangladesh (Ahasanullah University) Abstract Mainly poly (lactic acid) (PLA) is a biocompatible thermoplastic aliphatic polymer. Among all of the biodegradable polymer polylactic acid (PLA) is very significant on biomedical and biocompatible field. For the selection of an appropriate non-hazardous solvent or solvent system it s important to determine the rheological properties and electrospinnability of the solution and the morphology of nanofibres. In this paper we have tried to investigate the solution of poly lactic acid (PLA) in various pure solvents and binary-solvent systems and their effect on different solution properties of nanofibre morphology and diameter. We observed the Nanofibre morphology by scanning electron microscopy (SEM). Compared to other solvent we found acetone/dimethylformamide solvent system gave the highest fibre productivity and finest defect-free nanofibres. Therefore, we also used various solvent ratio for more details. Also the polymer concentration in the solvent has great effect on nanofibre morphology and solution properties. Further investigation of the effect of the PLA concentration on the elastic (G ) and the plastic (G ) moduli showed a sudden increase of the elastic moduli (G ) at the critical chain entanglement concentration. The results showed that the solvent properties, boiling point, viscosity, conductivity and surface tension, have a significant effect on process productivity, morphology and diameter distribution of the PLA nanofibres. Keywords Electrospinning, Elastic modulus, Chain entanglements, Poly lactic acid, Nanofibres 1. Introduction PLA could be produced by the ring-opening polymerization of lactide. Various applications such as filtration systems [1], chemical and optical sensors [2], tissue engineering [3-5], wound healing [6] and release of drugs [7], could be possible by Electrospun nanofibres because of their high surface area to volume ratio, small pore size and high porosity. Electrospinning process needed high electric field [8] between a needle-tipped syringe containing the polymeric solution and a collector for the deposition of nanofibres. Following the basic procedure of electrospinning polymeric solution is electrically charged and forms a conical droplet at the needle tip. Due to electric forces which overcome the surface tension of the solution a polymeric jet is generated from the surface of the droplet and travels towards the collector. Between the needle and the collector, the solvent evaporates from the jet and consequently nanofibres can be collected [9]. Many * Corresponding author: mishumym@gmail.com (Muhammad Ashfak Jahangir) Published online at Copyright 2017 Scientific & Academic Publishing. All Rights Reserved parameters have effect on the morphology and diameter of the resultant nanofibers, solution properties, process parameters and ambient conditions [10-12]. For different type of application, the optimization of these parameters is significant. One of the main factors which affect the solution properties and consequently the electrospinnability of the solution considered the solvent or mixed- solvent system of polymer. On this study we focused on polymer concentration, conductivity and surface tension of the solution to find effect on the fibre morphology and diameter [13]. We found on the lab that bead-string morphology of produce fibre has a very significant effect on solvent properties and polymer i.e PS fibres density and boiling point. By increasing dielectric constant and dipole moment of solvent nanofiber productivity could be increased. Among different solvent methylene chloride (MC) was only able to dissolve the polymer on poly (caprolactone) (PCL) nanofibres. Also on the electrospinning process addition of dimethylformamide (DMF) to MC mobilize the process and production of nano fibre. Electrical conductivity of the PCL solution and DMF could decreased the viscosity that s why nano fibre could be collected. Similar result could be formed by using poly (vinyl chloride)

2 178 Muhammad Ashfak Jahangir et al.: Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter PVC using tetrahydrofuran (THF) and dimethylformamide (DMF) on the electrospinning process. DMF has polyelectrolyte behavior on poor solvent for PVC compared with THF. Smaller nanofibres could be collected by increasing DMF in the THF/DMF solution. The selection of appropriate solvent or solvent system depending on the type of polymer [22-24] are required to optimize the electrospinning process for the production of homogeneous nanofibrous mats. Chlorinated and fluorinated solvents could dissolve PLA (amorphous PDLA or crystalline PLLA). On the application in tissue engineering a low toxicity solvent system would be preferable when nanofibrous mats are produced to avoid any adverse reactions with cells. During the electrospinning another parameter i.e number of chain entanglements in the solution has a significant influence on fibre formation of polymeric solutions. A minimum level of chain entanglements and minimum concentration, is required for the production of continuous fibres [25, 26]. From different studies it has been showed that minimum polymer concentration which is known as entanglement concentration (Ce), is required for beaded fibre formation [27-29]. Below Ce only droplets of the polymeric solution are collected because there are no entanglements. The bead formation decreases on further increasing the polymer concentration. No fibres are produced at high concentration due to the high solution viscosity [15, 30]. From different studies we have already known that there are four different concentration regimes for polymer dissolved in a good solvent: dilute, semi dilute untangled, semi dilute entangled and concentrated. Polymer chains are separated by the solvent molecules in the dilute regime and only polymer droplets can be collected in the electrospinning process. Transition between the semi dilute untangled and the semi dilute entangled concentration represents by Ce. Uniform beaded nanofibres can be collected when C > Ce. It has been reported when the concentration is times Ce generally bead-free nanofibres could be produced [25, 31, 32]. Specific viscosity ηsp is proportional to C1.25 (Colby and his co-workers) in the semi dilute unentangled regime for a linear polymer in a good solvent and for the semi dilute entangled regime ηsp ~ C From this study our main aim is to produce PLA nanofibres using lower toxicity solvent systems. Also to show the relationship between solution electrospinnability and chain entanglements and the morphology of different solution properties, mean diameter of PLA nanofibres. 2. Materials and Methods 2.1. Materials Poly (lactic acid) Trade Name- PLA 4060D was purchased from Nature Works USA. It contains L-lactide content of around 88 weight% and its amorphous polymer has glass transition temperature (Tg) of C [36, 37]. All solvents were purchased from Chengdu Kelong Chemical Reagent Co., China. The specific solvents which we used in this study: acetone (AC), 1,4-dioxane (DX), tetrahydrofuran (THF), dichloromethane (DCM), chloroform (CHL), dimethylformamide (DMF) and dimethylacetamide (DMAc). Some properties of selected solvent have shown on Table Preparation of PLA Solutions We used acetone and another solvent (50/50 v/v) to obtain a 10% w/v polymer concentration to prepare the polymer solutions for electrospinning process by dissolving PLA in single solvents and binary-solvent systems. Then we added a pre weighted amount of polymer in a glass bottle for the preparation of all solutions. Then stirred the solution magnetically at room temperature (20-22 C) for 4 hours, for complete dissolution of the polymer. Then studied the most promising solvent system in detail. With concentrations 5, 7.5, 10, 12.5 and 15% w/v Pla could be dissolved to obtain the solutions Electrospinning Process We used a high voltage power supply to maintain electric field of -20 KV between a collector and a needle. We used 20-ml plastic syringe (Luer lock syringe) to the put the polymer solution and the polymer solution was pumped continuously at a rate of 0.1 ml min -1. We maintained vertical setup to connect the syringe and the needle (inner diameter 0.6mm) through polyethylene capillary tube. For the deposition of nanofibers to the collector we used rectangular copper plate covered with aluminium foil and located 15 cm from the needle tip. The polymer solution from the tip of the needle was collected with a Petri dish. During electrospinning, the collecting drum, which was 10 cm in diameter rotated at a surface speed of 20 cm min -1. During the electrospinning experiments room temperature was (~20 C). The collected specimen were dried in a vacuo for 7 days Characterisation of PLA Solutions A cone-plate measuring system (cone radius 40 mm, cone angle 1.59 degrees, truncation 56 μm) used here to measure the rheological properties of the PLA solutions at constant temperature (25 C). The oscillatory test was used to identify the storage (G ) and loss (G ) modulus of the solutions and to determine the solution viscosity rotational tests were used. The oscillatory measurements determined at a shear stress of 0.5 Pa between frequencies of 0.1 and 100 Hz and rotational tests ranged from 0.1 to 400 s -1 shear rate. We repeated three times measurements for each solution. Using a surface tension meter, we measured the surface tension of the PLA solutions. From each solution we have taken five measurements. To determine the electrical conductivity of the polymer solutions we used conductivity meter (Model 470) with a cell constant K = 1.07 and

3 American Journal of Chemistry 2017, 7(6): accuracy of ±0.5% Electrospun Fibre Analysis Field emission scanning electron microscopy (FEG-SEM, LEO 1530VP) has used to identify the morphology of electrospun PLA nanofibres using voltage of 5.0 kv. Gold/palladium sputter coater for each sample was coated for 2 minutes before observing the sample. We used image software (AxioVision) to measure the diameter of PLA nanofibers. To determine the mean nanofibre diameter 150 measurements for each sample were made. 3. Result and Discussion Single-solvent systems Figure 1. Single solvent systems on nanofibre morphology and their effect: scanning electron micrographs of PLA nanofibres from solutions of 10% (w/v) of PLA in: (a) acetone (10000x) 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)

4 180 Muhammad Ashfak Jahangir et al.: Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter Table 1. Basic parameters of different solvents (η is the viscosity, ε is the dielectric constant, σ is the surface tension, δ is the solubility parameter) Solvent Boiling point C η mpa s Electrical conductivity µs cm -1 ε σ mn/m δ -3/2 cal 1/2 cm Acetone - AC ,4-Dioxane - DX E Tetrahydrofuran - THF E Dichloromethane - DCM E Chloroform - CHL E Dimethylformamide - DMF E Dimethylacetamide - DMAc / Figure 2. Binary-solvent systems on nanofibre morphology and their effect: 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)

5 American Journal of Chemistry 2017, 7(6): hours needed to dissolve the poly lactic acid (PLA) in all solvents. Scanning electron micrographs Figure 1 shows the samples produced from electrospinning. Due to relatively high conductivity and dielectric constant, relatively low surface tension only acetone (AC) has produce a sufficient quantity of fibres to form a nanofibre mat. Figure 1(a) shows nanofibres bead string morphology with a mean nanofibre diameter of 757 nm. Continuous nanofibres could not produce by 1,4- dioxane (DX) and tetrahydrofuran (THF) only droplets were collected (Figure 1(b) and 1(c)), possible reason is low conductivity and their high surface tension. Samples produced from dichloromethane (DCM) and chloroform (CHL) has formed few nanofibres with many droplets Figures 1(c) and 1(d). Dimethylformamide (DMF) and dimethylacetamide (DMAc) could not produce nanofiber due to the higher dielectric constants: only droplets were collected as shown in Figures 1(f) and 1(g). DMF and DMAc as shown in Table 1 has high boiling point could be the possible reason that the polymeric jet did not have enough time to dry completely during its flight to the collector. The type of polymer/solvent system from which continuous fibres can be produced is greatly depends on the concentration the polymer solution. Binary-solvent systems In Figure 2 shows that nanofiber could be produced in addition of acetone to the other solvents (at a of ratio 50/50 v/v). Solvent systems AC/DX, AC/THF, AC/DCM and AC/CHL with bead-string morphology were found in Nanofibres as shown in Figures 2(a), 2(b), 2(c) and 2(d), respectively. Acetone/dimethylformamide and acetone/dimethylacetamide, as shown in Figures 2(e) and 2(f) could produce smooth defect-free nanofibres with a narrow diameter distribution. The boiling point of the second solvent in the mixed-solvent system increases the nanofibre diameter as shown in Figure 3. Different literature also found a similar correlation between fibre diameter for the electrospinning of poly(styrene) in m- cresol, toluene, dimethylformamide and tetrahydrofuran. Figure 2(c) and 2(d) shows that nanofibres collected from AC/DCM and AC/CHL led to porous appearance of nanofibers with the thinnest diameter (210 nm), as shown in Table 2. Stronger elongation forces are imposed to the jet and the high solution conductivity leads to the increase of surface charge of spinning jet resulting defect-free, uniform nanofibres with a narrow diameter distribution (small standard deviation). Due to the high boiling point of DMF these nanofibres were not completely dried when they reached the collector. Due to the higher dielectric constant of the second solvent (DMF, DMAc) the nanofibre diameter distribution are more homogenous samples with low standard deviation. Production of nanofibres with bigger diameter and wide diameter distribution Poly lactic acid solutions of AC/DCM and AC/CHL showed lower solution conductivity, as shown in Figure 3. Figure 3. Boiling point of the second solvent in binary solvent systems and effect of the mean nanofibre diameter (acetone, AC; dichloromethane, DCM; chloroform, CHL; 1,4-dioxane, DX; tetrahydrofuran, THF; dimethylformamide, DMF; dimethylacetamide, DMAc) Table 2. PLA solution properties. Basic physical properties of PLA solution with 10% w/v in single and binary-solvent systems (50/50 v/v ratio for binary-solvent systems) and mean diameter of nanofibres produced by the electrospinning process Solvent system Viscosity mpa s Surface tension mn m -1 Conductivity µs cm -1 Morphology and mean nanofibre diameter nm AC 102± ± ±275 DX 531± ± No nanofibres THF 280± ± No nanofibres DCM 167± ± No nanofibres CHL 372± ± No nanofibres DMF 155±9 38.3± No nanofibres DMAc 210± ± No nanofibres AC/DX a 305± ± Beaded fibres, 494±180 AC/THF a 221± ± Beaded fibres, 587±313 AC/DCM a 133± ± Beaded fibres, 1181±381 AC/CHL a 169± ± Beaded fibres, 679±296 AC/DMF a 252± ± Defect-free fibres, 210±36 AC/DMAc a 296± ± Defect-free fibres, 357±75

6 182 Muhammad Ashfak Jahangir et al.: Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter Effect of solvent ratio - acetone/dimethylformamide (AC/DMF) 10% w/v PLA solution with acetone/dimethylformamide as a dissolving solution was found to produce defect-free nanofibres with the smallest mean diameter and a narrow diameter distribution. Acetone and dimethylformamide were mixed in a range of (20:80, 40:60, 60:40, and 80:20 v/v) ratios and studies the effect of resultant PLA nanofibre morphology. Selected SEM images of the nanofibres has shown on Figure 4. The production of nano fibre by ratios 20/80 and 40/60 (Figure 4(a) and 4(b)) could not produce the nanofibres and only beads were collected. Viscosity, surface tension, and conductivity these solution properties of the PLA solutions were measured and are reported in Table 3. Using ratio 60/40 v/v the smallest and most uniform defect-free nanofibres produced. Smaller mean diameter (210 nm) and defect-free nanofibres could be produced by the solvent system AC/DMF at a ratio of 50/50 v/v. Figure 4. Solvent ratio (AC/DMF) on nanofibre morphology and their effect: scanning electron micrographs of PLA nanofibres from solutions of 10% (w/v) of PLA in acetone/dimethylformamide at a ratio of (v/v) (a) 0/100 (5000x), (b) 20/80 (5000x), (c) 40/60 (5000x), (d) 50/50 (15000x), (e) 60/40 (15000x) (f) 80/20 (15000x), (g) 100/0 (10000x) Table 3. Effect of solvent ratio (AC/DMF) Physical properties for different solvent ratios acetone/dimethylformamide and mean diameter of nanofibres produced by the electrospinning process at 10% w/v PLA solution AC/DMF v/v Viscosity mpa s Surface tension mn m -1 Conductivity µs cm -1 Mean nanofibre diameter nm 100/0 102± ± ±521 80/20 347± ± ±176 60/40 303± ± ±58 50/50 252± ± ±69 40/60 249±8 32.6± No 20/80 170± ± No 0/ ±9 38.3± No

7 American Journal of Chemistry 2017, 7(6): Figure 5. 10% PLA solution, solvent ratio acetone/dimethylformamide (AC/DMF) solutions viscosity and surface tension variation Table 3 shows the higher surface tension of the PLA solutions at AC/DMF ratios of 20/80 and 40/60 inhibits Nanofibre formation. Using a ratio of 50/50 v/v nanofibres were firstly produced. Surface tension and solution viscosity has shown on Figure 5. Increasing amount of acetone provides bigger nanofibres and higher standard deviation from the mean diameter. Viscosity, conductivity and surface tension effect on the produced fibre, is very difficult to separate on nanofibre morphology and diameter. Production of thinner and more uniform fibres with fewer defects [40, 41] is possible by increasing solution conductivity or charge density. By the application of an electric field the solution is exposed to greater tensile force and greater elongation comes with the polymeric jet. In competition with the viscosity of the solution the surface tension is the parameter. The production of nanofibres with fewer beads produce by reducing the surface tension. The jet becomes unstable and more beads appear on increasing the surface tension [13, 42]. However, application of a lower surface tension to get bead-free nanofibers is not a general rule, but depends on the type of solvent [13, 43]. Effect of polymer concentration In order to study effect on nanofibre morphology and diameter the concentration of PLA in the solvent system acetone/dimethylformamide (50/50 v/v) was varied from 5 to 15% w/v. Voltage of 20kV, flow rate 1 ml/h and over a time span of 10 minutes has used for all electrospun solutions. Maintained temperature and the relative humidity in the electrospinning chamber were 18 C and 47%. Figure 6 shows different polymer concentration for selected electrospun nanofibres and their SEM images. Figure 6. Polymer concentration on nanofibre morphology - scanning electron micrographs of PLA nanofibres from solutions of PLA in acetone/dimethylformamide at a ratio of 50/50 v/v at concentration (a) 5% w/v (2000x), (b) 7.5% w/v (5000x), (c) 10% w/v (15000x), (d) 12.5% w/v (15000x), (e) 15% w/v (15000x)

8 184 Muhammad Ashfak Jahangir et al.: Poly Lactic Acid (PLA) Fibres: Different Solvent Systems and Their Effect on Fibre Morphology and Diameter Table 4. Effect of polymer concentration. PLA solution in acetone/dimethylformamide (50/50 v/v) and mean diameter of nanofibres produced by the electrospinning process of PLA concentration and viscosity PLA concentration % w/v Solution Viscosity mpa.s Surface Tension mn m -1 Conductivity µs cm ±9 29± Mean nanofibre Diameter Few nanofibres, mainly beads ±7 29± ± ±7 29± ± ± ± ± ± ±206 Using the 5% w/v PLA solution we got few nanofibres with many beads as shown on figure 6. Insufficient chain entanglements are visible on beads formation. Using 7.5% w/v polymer concentration more nanofibres were produced even though a lot of defects were still present, as shown in Figure 6b. Beaded nanofibers are visible up to a concentration of 10% w/v. Defect-free fibres could be collected above 12.5% w/v. Incerased in the solution concentration increases viscosity of the solution as shown in the Figure 7. Competition between the surface tension and viscosity change in fibre morphology with increasing concentration. More chain entanglements increase in the polymer concentration results in, an increase in the solution viscosity, hence an increase in the viscoelastic force counterbalance the Coulombic stretching force and therefore resulting in nanofibres with fewer beads [41, 44]. the value previously reported [26] for linear polymer in good solvent (ηsp ~ C1.25), we used a binary solvent system (AC/DMF) is a fact. With the theoretical value (ηsp ~ C ) in the semi dilute entangled regime ηsp ~ C4.1 is not a good agreement. A log/log plot of the concentration dependence of the viscosity for PLA solution in the solvent system acetone/dimethylformamide 50/50 v/v has shown on Figure 7. Shear rate 1s -1 were taken as result. From the result we got to a concentration of 5.5% w/v solution viscosity is proportional to C0.6 and thereafter is proportional to C4.1. 6% w/v (marked Ce) is the boundary between the untangled and entangled regimes for change in slope from 0.6 to 4.1. A weaker dependence in the semi dilute untangled regime to the value previously reported [26] for linear polymer in good solvent (ηsp ~ C1.25), we used a binary solvent system (AC/DMF) is a fact. With the theoretical value (ηsp ~ C ) in the semi dilute entangled regime ηsp ~ C4.1 is not a good agreement. Figure 7. Acetone/dimethylformamide (50/50 v/v) PLA solution & effect of PLA concentration on viscosity A log/log plot of the concentration dependence of the viscosity for PLA solution in the solvent system acetone/dimethylformamide 50/50 v/v has shown on Figure 7. Shear rate 1s -1 were taken as result. From the result we got to a concentration of 5.5% w/v solution viscosity is proportional to C0.6 and thereafter is proportional to C4.1. 6% w/v (marked Ce) is the boundary between the untangled and entangled regimes for change in slope from 0.6 to 4.1. A weaker dependence in the semi dilute untangled regime to 4. Conclusions There are many kinds of bio-based and biodegradable polymers, among which one of the most promising is poly lactic acid (PLA), a biocompatible thermoplastic aliphatic polyester. PLA is a linear thermoplastic polyester produced by the ring-opening polymerization of lactide. We used electrospinning process to produce PLA nanofibres from PLA solution in various solvents and binary-solvent systems on this study. PLA nanofibre diameter and morphology and the effect of different solvent systems was the main focus of this study. Viscosity and concentration and its impact on the fibre formation for different polymer concentrations was utilized to find relations. We found that 10% w/v acetone produced PLA nanofibres from all PLA-solutions in single solvents system could produce defect free nano fibre. For the mixtures of acetone and another solvent we used binary solvent systems. Nanofibres could be collected on all the PLA solutions in binary solvent systems of 10% w/v. By scanning electron microscopy (SEM) we observed the diameter and the effect of different binary solvent systems on nanofibre morphology. Solvent systems acetone/1,4- dioxane,

9 American Journal of Chemistry 2017, 7(6): acetone/tetrahydrofuran, acetone/dichloromethane and acetone/chloroform forms beaded nanofibres, in addition acetone/dimethylformamide and acetone/dimethylacetamide solvent system could produce defect-free nanofibres. Higher electrical conductivity of the resulting PLA solution is a possible reason for this result. The binary solvent systems on the mean nanofibre diameter decreased as the boiling point of the second solvent in the binary- solvent system increased. Acetone/dimethylformamide (AC/DMF) on nanofibre morphology and diameter was also described on this paper. Solvent ratio 60/40 v/v used for thinner nanofibres with a narrow diameter distribution. Defect-free nanofibre could be produced on increasing the amount of acetone in the solvent system but the mean nanofibre diameter was found to increase. Solvent system AC/DMF at ratio 50/50 v/v. Here defect-free nanofibres could be produced on above 12.5% w/v PLA concentration. 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