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Electrospinning of methacrylate-based copolymers: effects of solution concentration and applied electrical potential on morphological appearance of as-spun fibers.

INTRODUCTION

In recent years, much interest has been paid to the development of and exploration for applying high electrostatic potentials in fabricating ultrafine fibers. This process is known as electrostatic spinning or electrospinning, with diameters being in sub-micrometer to nanometer range from materials of diverse origins [1-3]. This process involves the application of a strong electrostatic field over a conductive capillary attached to a reservoir containing a polymer solution or melt and a conductive collective screen. Upon increasing the electrostatic field strength up to a critical value but not exceeding it, charge species accumulated on the surface of a pendant drop destabilize the partially-spherical shape into a conical shape (commonly known as Taylor's cone). Beyond the critical value, a charged polymer jet is ejected from the apex of the cone (as a way for relieving the excess charges). The ejected charged jet is then carried to the collective screen via the electrostatic force. The Coulombic repulsion force is responsible for the thinning of the charged jet during its flight to the collective screen. The thinning of the charged jet is thought to diminish as soon as the charged jet is dry enough (so the viscosity of the polymeric solution or melt becomes too great).

Due to the possibilities for fabricating polymers in nanoscale, electrospinning is a unique technique for making ultrafine fibers that could be used as drug delivery vehicles [4,5]. Since electrospun fibers normally have diameters ranging from sub-micrometers down to nanometers, their surface area-to-volume ratios are very large. From a pharmaceutical point of view, various techniques, such as dry milling, wet grinding, air-jet milling, and wet milling, have been utilized in order to decrease the dimension of the controlled release vehicles to achieve the possible highest surface area-to-volume ratios. In the electrospinning process, drugs can be incorporated into the polymeric fibers simply by mixing a solution of drug formulation in a polymeric, spinning solution. Under the right conditions, electrospun fibers result. Controlled release of drugs from electrospun fibers has been of increasing interests in recent years [4,5], and the release characteristics should depend on interactions between polymer and drug pairs as much as on the sizes of the fibers.

For targeted release of oral medication, Rohm GmbH (Germany) developed methacrylate-based coating materials under the trade name Eudragit[R]. These methacrylate-based copolymers are different in their chemical characteristics, and they are able to dissolve in aqueous media at different pH levels. In the present study, three commercially-available methacrylate-based copolymers with different chemical characteristics were chosen and fabricated into ultrafine fibers using the electrospinning technique, with an aim for their future use as controlled-release vehicles of drugs. The effects of solution concentration and applied electrical potential on morphological appearance of the as-spun fibers were investigated.

EXPERIMENTAL

Materials

Methacrylate-based copolymers used in this work were poly(methacrylic acid-co-methyl methacrylate) (Eudragit[R] L100; [bar.M.sub.w] = 135,000 Da), poly(ethyl acrylate-co-methyl methacrylate-co-trimethyl-ammonioethyl methacrylate chloride) (Eudragit[R] RLPO; [bar.M.sub.w] = 150,000 Da), and poly(butyl methacrylate-co-(2-dimethylaminoethyl) methacrylate-co-methyl methacrylate) (Eudragit[R] EPO; [bar.M.sub.w] = 150,000 Da). These copolymers were produced commercially by Rohm GmbH (Germany) and generously donated by Pharmasant Laboratories (Thailand). These copolymers were chosen based on their diverse chemical characteristics (see Fig. 1) and, hereafter, are denoted E-L100, E-RLPO, and E-EPO, respectively. According to their chemical structure, E-L100 is a pH-dependent copolymer for targeted delivery in the jejunum where pH > 6.0; E-RLPO is a pH-independent copolymer for matrix formations; and E-EPO is a pH-dependent copolymer that is completely soluble in acidic media when pH [less than or equal to] 5.0 and it becomes swellable and permeable in aqueous media when pH > 5.0. Ethanol (EtOH), used as the solvent, was purchased from Labscan (Asia) (Thailand). EtOH was of analytical reagent grade.

[FIGURE 1 OMITTED]

Preparation and Characterization of Copolymer Solutions

Solutions of methacrylate-based copolymers were prepared by dissolving weighed amounts of each copolymer in EtOH to produce solutions with concentrations ranging between 10 and 35% w/v. The viscosity of the as-prepared solutions was measured using a Brookfield DV-III programmable viscometer, the surface tension was measured using a KRUSS DSA10-Mk2 drop-shape analyzer, and the conductivity was measured using an Orion 160 conductivity meter. All measurements were done at room temperature (about 25[degrees]C) and, particularly for surface tension measurement by the drop-shape analysis method, the atmosphere in the measuring chamber was saturated with vapor of the solvent to limit evaporation of the solvent from the pendant drop samples during each measurement.

Electrospinning of Copolymer Solutions and Characterization of As-Spun Fibers

The experimental setup for studying the effects of solution concentration and applied electrical potential on morphological appearance of the as-spun copolymer fibers is as follows. A solution of methacrylate-based copolymers with varying concentration between 10 and 35% w/v was placed in a 50-mL syringe. The syringe was clamped vertically to a PVC stand. A blunt-ended stainless-steel needle, used as a spinneret, was attached to the outlet of the syringe. A Gamma High Voltage Research D-ES30PN/M692 power supply was used to generate a high DC potential ranging between 7.5 and 22.5 kV across the needle (connected to the emitting electrode) and an aluminum foil that was used as a collector plate (connected to the ground electrode). The distance between the needle and the collector plate defines a collection distance, which was fixed at 15 cm. A constant flow of the solution through the needle was assured by a flow of pressurized nitrogen gas at the inlet of the syringe. The polarity of the emitting electrode was positive.

Characterization of As-Spun Fibers

The morphological appearance of the as-spun methacrylate-based fibers was investigated visually from scanning electron micrographs of a small section of the obtained electrospun webs using a JEOL JSM-4200 scanning electron microscope (SEM). The specimens for SEM observation were prepared by cutting an Al sheet covered with the as-spun webs, and the cut section was carefully affixed on an SEM stub. Each specimen was gold-coated using a JEOL JFC-1100E sputtering device before being observed under SEM. For each spinning condition, at least 50 to over 200 measurements for the fiber diameter (i.e., the width of lay-flat fibers) were recorded and statistically analyzed. Statistical analysis of the data obtained was carried out by constructing a histogram, from which an arithmetic mean and a standard deviation were obtained and reported.

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Characterization of As-Prepared Copolymer Solutions

Figure 2 shows viscosity values of the as-prepared co-polymer solutions as a function of solution concentration for all of the three methacrylate-based copolymers investigated. It should be noted that the viscosity values of both 30 and 35% w/v E-L100 solutions could not be measured, due to the highly viscous nature of the solutions. As expected, the solutions for a given copolymer type showed an increase in the viscosity value with increasing solution concentration. For a given solution concentration, the viscosity of E-L100 solution was much greater than those of E-RLPO and E-EPO solutions, despite the slightly lower [bar.M.sub.w] value that E-L100 exhibited in comparison with those of E-RLPO and E-EPO. According to the chemical structures shown in Fig. 1, the formation of intramolecular hydrogen bonding due to the interactions between carboxylic groups of different E-L100 molecules could be responsible for the observed highest viscosity among the three copolymer investigated. In addition, the viscosity of E-RLPO solution was slightly greater than that of E-EPO solution, despite the two copolymers having the same [bar.M.sub.w] value. According to Fig. 1, the presence of the chloride counter-ions could act as weak moieties linking different trimethylammonium groups of different E-RLPO molecules together, leading to the formation of weak network; hence, the observed higher viscosity of E-RLPO solution over that of E-EPO solution.

Figure 3 shows surface tension values of the as-prepared copolymer solutions as a function of solution concentration for all three methacrylate-based copolymers investigated. Obviously, the surface tension values for E-RLPO and E-EPO decreased very slightly with increasing solution concentration, and their values were very comparable (up to the solution concentrations of about 26% w/v). The observed decrease in the surface tension for E-RLPO and E-EPO could be the slight disruptive effect that the molecules of these copolymers had on the cohesiveness of EtOH molecules at the liquid/air interface. On the contrary, the surface tension value for E-L100 slightly increased with initial increase in the solution concentration, reached a maximum at the solution concentration of about 20% w/v, and decreased with further increase in the solution concentration. The observed slight increase in the surface tension at "low" solution concentrations could be a result of the formation of hydrogen bonding between EtOH and E-L100 molecules that could strengthen the cohesiveness of EtOH molecules at the liquid/air interface. However, when the solution concentration increased further, interactions between the carboxylic groups of E-L100 molecules became predominant, causing the strengthening effect to disappear.

[FIGURE 3 OMITTED]

Figure 4 shows conductivity values of the as-prepared copolymer solutions as a function of solution concentration for all three methacrylate-based copolymers investigated. Apparently, the conductivity values for E-L100 and E-RLPO appeared to be unaffected by changes in the solution concentration, while that for E-EPO decreased monotonically with increasing solution concentration. For any given solution concentration, the conductivity value for the solutions of E-RLPO appeared to be the highest, followed by that of E-L100 and E-EPO, respectively. According to the chemical structures shown in Fig. 1, the positive charge already present at the trimethyl-ammonium groups on E-RLPO molecules were the most likely reason for the observed highest conductivity of the resulting solutions, while partial ionization of the ionizable carboxylic groups on E-L100 molecules could be the reason for the observed second highest conductivity of the resulting solutions and, logically, E-EPO, which does not contain any charge nor ionizable moieties present on its molecules, should exhibit the lowest conductivity.

[FIGURE 4 OMITTED]

Electrospinning of As-Prepared Copolymer Solutions

Even though the solutions of all three methacrylate-based copolymers could be prepared in the concentration range of 10 to 35% w/v, only 10 to 20% w/v E-L100, 15 to 30% w/v E-RLPO, and 20 to 30% w/v E-EPO solutions were successfully electrospun into fibers. Tables 1 to 3 summarize some selected SEM images of electrospun products from methacrylate-based copolymer solutions to illustrate the effects of both the solution concentration and applied electrical potential on morphological appearance of the as-spun products. Due to the similarity in the results obtained for different copolymers, the effects of solution concentration and applied electrical potential on morphological appearance of the as-spun products were explained based on the results obtained for E-EPO (see Table 3).

Effect of Solution Concentration. For a fixed applied electrical potential, only discrete droplets with large size distribution were observed at the lowest concentration studied (i.e., 15% w/v). Solutions with low concentrations did not have enough chain entanglement to withstand both the electrostatic and Coulombic repulsion forces acting on an infinitesimal segment of an ejected, charged jet. Once the charged jet was broken up into smaller charged entities, surface tension worked to minimize the surface area of the broken jets and, hence, discrete spheres were formed as a result [6]. It should be noted that the sickle-shaped droplets were formed as a result of the evaporation of the solvent within the droplets.

Increasing the solution concentration to 20% w/v resulted in a combination of smooth and beaded fibers. At 20% w/v, the chain entanglement was high enough to prevent the charged jet from breaking up into discrete spheres. Further increase in the solution concentration to 25 and 30% w/v resulted in the formation of smooth fibers, because of the high enough chain entanglement to completely prevent the charged jet from breaking up. Interestingly, increasing polymer concentration also resulted in as-spun fibers with larger sizes. The increase in the fiber diameters was a result of the increased viscoelastic force that counteracts the Coulombic repulsion force that tries to stretch the charged jet, which, in turn, causes the jet to thin down [6].

Instead of the round fibers that are normally obtained in various other polymer systems, the as-spun E-EPO fibers were flat. The formation of flat, ribbon-like fibers could be a result of the rapid evaporation of the solvent (i.e., the boiling point of EtOH is 78[degrees]C). Due to the small size of the ejected, charged jet, the outer surface of the jet could "dry" much faster than its inner core. Since the time during which the jet was on flight to the target was very short, the inner core might not have enough time to "dry" completely. Once the jet deposited on the target, evaporation of the residual solvent continued. Since the outer surface of the depositing jets was already "dry", the evaporation of the solvent from the inner core could result in the collapse of the jets; hence, the flat fibers were formed [7].

Effect of Applied Electrical Potential. According to SEM images shown in Table 3, the effect of applied electrical potential on as-spun E-EPO products can be observed. At the lowest solution concentration investigated (i.e., 15% w/v), only discrete droplets were observed when the applied electrical potential of 7.5 kV was applied, while a combination of very small beaded fibers and discrete droplets were observed at 15 kV and only beaded fibers were observed at 22.5 kV. The results suggested that the electro-spinnability of the solution was increased with increasing applied electrical potential. Increased applied electrical potential led to an increase in the electrostatic force acting on an infinitesimal segment of an ejected, charged jet. Increased electrostatic force translated to a drastic decrease in the time of flight of an infinitesimal segment of the ejected, charged jet from the spinneret to the collector plate. Both the increase in the electrostatic force and the decrease in the time of flight of a jet segment resulted in a tremendous increase in the extensional rate during the linear deformation of the jet segment, leading to a tremendous decrease in the elongational viscosity of the solution that could help facilitate the fiber formation [8]. For other spinning conditions, no influence of the applied electrical potential on morphological appearance of the as-spun fibers was found (all of the fibers obtained were flat).

[FIGURE 5 OMITTED]

Effect of Spinning Condition on Average Diameter of As-Spun Fibers. Figures 5 to 7 illustrate average diameters for all of the methacrylate-based copolymer fibers obtained from various spinning conditions. The average diameters for all of the as-spun E-L100 fibers ranged between about 0.2 and 5.2 [micro]m; those for all of the as-spun E-RLPO fibers ranged between about 0.2 and 4.2 [micro]m; and those for all of the as-spun E-EPO fibers ranged between about 2.2 and 5.5 [micro]m. Generally, for a given applied potential, the average fiber diameter increased with increasing concentration of the spinning solutions. Increased solution concentration led to an increase in the viscoelastic force that counteracted the Coulombic stretching force [6]. For a given solution concentration, the average fiber diameter increased with increasing applied electrical potential. Increased applied electrical potential resulted in an increase in the electrostatic force that was responsible for the transport of the ejected, charged jet to the collective screen. The increased electrostatic force could result in an increase in the drawing rate of the jet from the spinneret.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

CONCLUSION

Three commercially-available methacrylate-based copolymers [i.e., poly(methacrylic acid-co-methyl methacrylate) (E-L100), poly(ethyl acrylate-co-methyl methacrylate-co-trimethyl-ammonioethyl methacrylate chloride) (E-RLPO), and poly(butyl methacrylate-co-(2-dimethylaminoethyl) methacrylate-co-methyl-methacrylate) (E-EPO)] were successfully electrospun into fibers using ethanol as the solvent. The effects of solution concentration and applied electrical potential on morphological appearance of the as-spun fibers were investigated. For a given applied electrical potential, increasing the concentration of the spinning solutions caused the morphology of the as-spun products to change from discrete droplets to a combination of beaded and smooth fibers and finally to completely smooth fibers. For a given spinning solution having a low concentration, increasing the applied electrical potential increased the electro-spinnability of the spinning solution. The average diameters for all of the as-spun fibers were found to range between about 0.2 and 5.5 [micro]m. Specifically, the average diameters for all of the as-spun E-L100 fibers ranged between about 0.2 and 5.2 [micro]m; those for all of the as-spun E-RLPO fibers ranged between about 0.2 and 4.2 [micro]m; and those for all of the as-spun E-EPO fibers ranged between about 2.2 and 5.5 [micro]m. Generally, for a given applied potential, the average fiber diameter increased with increasing concentration of the spinning solutions, and, for a given solution concentration, the average fiber diameter increased with increasing applied electrical potential.

REFERENCES

1. D. Li and Y. Xia, Adv. Mater., 16, 1151 (2004).

2. A. Frenot and I. S. Chronakis, Curr. Opin. Colloid Interface Sci., 8, 64 (2003).

3. Z.M. Huang, Y.Z. Zhang, M. Kotaki, and S. Ramakrishna, Compos. Sci. Tech., 63, 2223 (2003).

4. E. Kenawy, G.L. Bowlin, K. Mansfield, J. Layman, D.G. Simpson, E.H. Sanders, and G.E. Wnek, J. Controlled Release, 81, 57 (2002).

5. K. Kim, Y.K. Luu, C. Chang, D. Fang, B.S. Hsiao, B. Chu, and M. Hadjiargyrou, J. Controlled Release, 98, 47 (2004).

6. C. Mit-uppatham, M. Nithitanakul, and P. Supaphol, Macromol. Chem. Phys., 205, 2327 (2004).

7. S. Koombhongse, W. Liu, and D.H. Reneker, J. Polym. Sci. B. Polym. Phys., 39, 2598 (2001).

8. Z. Lewandowski, J. Appl. Polym. Sci., 79, 1860 (2001).

Varaporn Pornsopone

Petrochemistry and Polymer Science Graduate Program, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

Pitt Supaphol

Technological Center for Electrospun Fibers and The Petroleum and Petrochemical College, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

Ratthapol Rangkupan

Metallurgy and Materials Science Research Institute, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

Supawan Tantayanon

Department of Chemistry, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand

Correspondence to: P. Supaphol; e-mail: pitt.s@chula.ac.th; S. Tantayanon; e-mail: supawan.t@chula.ac.th

Contract grant sponsor: National Research Council of Thailand; contract grant number: 03009582-0002; contract grant sponsor: Chulalongkorn University through Ratchadapesek Somphot Endowment Fund; contract grant sponsor: Petroleum and Petrochemical Technology Consortium through a Thai government loan from Asian Development Bank (ADB); contract grant sponsor: Petroleum and Petrochemical College, Chulalongkorn University.
TABLE 1. Some scanning electron micrographs of as-spun E-L100 fibers
from solutions of E-L100 in EtOH. The collection distance was fixed at
15 cm.

 Solution concentration (% w/v)
Applied
potential (kV) 10 15 20

 7.5 Scale bar = Scale bar = Scale bar = 10 [micro]m
 1 [micro]m 5 [micro]m
15.0 Scale bar = Scale bar = Scale bar = 10 [micro]m
 1 [micro]m 10 [micro]m
22.5 Scale bar = Scale bar = Scale bar = 10 [micro]m
 1 [micro]m 10 [micro]m

TABLE 2. Some scanning electron micrographs of as-spun E-RLPO fibers
from solutions of E-RLPO in EtOH. The collection distance was fixed at
15 cm.

 Solution concentration (% w/v)
Applied
potential (kV) 15 20

 7.5 Scale bar = 1 [micro]m Scale bar = 1 [micro]m
15.0 Scale bar = 1 [micro]m Scale bar = 5 [micro]m
22.5 Scale bar = 1 [micro]m Scale bar = 5 [micro]m

 Solution concentration (% w/v)
Applied
potential (kV) 25 30

 7.5 Scale bar = 5 [micro]m Scale bar = 10 [micro]m
15.0 Scale bar = 5 [micro]m Scale bar = 10 [micro]m
22.5 Scale bar = 5 [micro]m Scale bar = 10 [micro]m

TABLE 3. Some scanning electron micrographs of as-spun E-EPO fibers from
solutions of E-EPO in EtOH. The collection distance was fixed at 15 cm.

 Solution concentration (% w/v)
Applied
potential (kV) 15 20

 7.5 Scale bar = 10 [micro]m Scale bar = 5 [micro]m
15.0 Scale bar = 10 [micro]m Scale bar = 5 [micro]m
22.5 Scale bar = 5 [micro]m Scale bar = 5 [micro]m

 Solution concentration (% w/v)
Applied
potential (kV) 25 30

 7.5 Scale bar = 10 [micro]m Scale bar = 10 [micro]m
15.0 Scale bar = 10 [micro]m Scale bar = 10 [micro]m
22.5 Scale bar = 10 [micro]m Scale bar = 10 [micro]m
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Author:Pornsopone, Varaporn; Supaphol, Pitt; Rangkupan, Ratthapol; Tantayanon, Supawan
Publication:Polymer Engineering and Science
Date:Aug 1, 2005
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