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Generation of multihollow structured poly(methyl methacrylate) fibers by electrospinning under pressurized C[O.sub.2].

INTRODUCTION

Poly(methyl methacrylate) (PMMA, [[[C.sub.5][H.sub.8][O.sub.2]].sub.n]) is a synthetic resin produced by the polymerization of methyl methacrylate. It is a clear, colorless polymer available commercially in both pellet and sheet form. PMMA is a linear, amorphous, synthetic, nonbiodegradable polymer with a high transparency. Owing to its physical properties like good tensile strength and hardness, high rigidity, high surface resistivity, good insulation properties, and thermal stability it has become a key polymer for mechanical and optical applications. In addition, PMMA is easily available and synthesizable [1].

There are several techniques for the fabrication of composite fibers from polymers, such as drawing, template synthesis, phase separation, self-assembly, and electrospinning [2], In the drawing process, a micropipette with a diameter of a few micrometers is used to produce fibers in the nano-microscale. As the drawing technique uses only the polymers with good viscoelasticity as precursors, its application is limited. The template synthesis technique uses a template to produce a desired material or structure. For the case of nano-microfibers formation, the template refers to a metal oxide membrane with through thickness pores of nano-microscale diameter. However, continuous production of fibers cannot be achieved using this technique. In the phase separation technique, polymer is dissolved in a solution followed by gelation, extraction using a different solvent, freezing, and drying to produce porous materials with diameters in the nano-microscale. This process is simple and does not require sophisticated equipment; however, the process is time-consuming and can use only a few polymers for the feed process. Similarly, the self-assembly technique is also time-consuming and is limited only to a few selected polymer configurations. The most effective technique to produce nano-microfibers is the electrospinning technique. In this technique, nano-microfibers are produced by an electrically charged polymer solution or polymer melt. Low current and high voltage generated by a direct current power supply is used in this process to draw fibers from and the polymer melt. During the process, the charged polymer solution/melt is drawn out of a needle and is converted into a jet moving toward a collector. Due to its feasibility and versatility, the electrospinning technique is capable of producing nanofibers with various structures such as core-shell, bicomponent, and hollow and porous structures.

It is already known that hollow fibers have potential applications in microfluids, photonics, and energy storage [3-5]. Generally, hollow fibers are produced by template-assisted techniques or by using a coaxial capillary. In template processes, hollow fibers are fabricated by electrospinning and are then covered with a precursor material, which is processed by various deposition methods. Next, the core of the electrospun fibers is removed by selective dissolution or thermal degradation to yield hollow fibers. This process works well only for relatively short fibers as the long ones suffer from the problem of overlapping and entanglement. Flexible templates also lead to interconnections between the resulting fibers [6-8], In the coaxial process, two solutions are used as precursors and are subjected simultaneously using a spinneret with two coaxial capillaries to generate core/shell fibers. The core of these fibers is then selectively removed and hollow fibers are produced [9-12], In this work, multihollow nanomicrofibers were produced from PMMA solution by electrospinning under pressurized C[O.sub.2]. Under supercritical conditions, C[O.sub.2] has gas-like viscosity and diffusivity, and liquid-like density and solvating properties. These special properties make C[O.sub.2] an excellent solvent for various applications. In polymer processing, supercritical C[O.sub.2] is utilized as a solvent, an anti-solvent, or a plasticizer for microcellular foaming, particle production, polymer blending, obtaining polymer composites, and impregnation of polymers [13]. The main advantage of using supercritical C[O.sub.2] as a media for polymer processing is its poor reactivity toward the functional groups present in the polymers. This leads to a reduction in the glass transition temperature, reduction of melting temperature in some polymers, and polymer swelling resulting in notable changes in polymer processing. When electrospinning was conducted under high temperatures and high C[O.sub.2] pressures, dry electrospun fibers with hollow and porous morphologies were produced by simple depressurization of the chamber toward the end of the experiments [14].

EXPERIMENTAL

Materials

PMMA (MW 1,00,000) was purchased from Sigma-Aldrich and used as received. Dichloromethane (DCM; 99.0%), which was used as a solvent, was obtained from Wako Pure Chemical Industries and was used as purchased without further purification. For making the polymer solution, PMMA was dissolved in DCM to achieve concentrations of 10, 15, 20, and 25 wt%.

Experimental Setup and Procedure

The apparatus consisted of a nonconductive polyether ether ketone (PEEK) autoclave (AKICO PEEK) including cartridge heaters coupled with an electric fan, a high voltage (HV) power supply (Matsusada Precision HARb-30Pl), a high pressure pump (JASCO PU-1586), a high pressure syringe pump (Harvard Apparatus PHD-Ultra 4400), a back-pressure regulator (BPR; SCF-Bpg, JASCO SCF-001), and a stainless steel syringe with a volume of 8 mL. The PEEK autoclave consisted of two stainless steel flanges and a PEEK vessel (6.0 cm id; 15.0 cm od; 20.0 cm length). One flange was connected to a high voltage power supply and was used as an anode, and the other flange was used as a cathode to collect fiber products. The tip-to-collector distance was 8 cm. The experiment was conducted as follows: initially, a nonconductive PEEK vessel was heated to a desired temperature in the range 27-37[degrees]C (in fact, the temperature range was 27-41[degrees]C). A thermocouple was inserted in the PEEK vessel and contacted directly with the space of the PEEK vessel to control the temperature of the electrospinning process. K-type thermocouples were also fixed to the PEEK vessel walls to measure the radial temperature distribution. Once the desired temperature was reached, C[O.sub.2] was pumped into the PEEK vessel through the PEEK capillary tube at a desired pressure (4-6 MPa). A BPR was used to maintain a constant pressure. The polymer solution was injected into the PEEK vessel upon the attainment of the desired conditions. The high-pressure stainless steel syringe that was placed in the high-pressure syringe pump was used to inject the polymer solution via the PEEK capillary tube. The flow rate of the polymer solution was 0.05 mL [min.sup.-1]. At the same time, a high voltage power supply was applied to generate an electrostatic force. Although the applied voltage was found to have a significant impact on the diameters of the obtained fibers, the effect of the polymer concentration on the same was more pronounced. Hence, an applied voltage of 15 kV was used in this study. The experiments were carried out for 15 min using a tip with an inner diameter of 0.5 mm. The depressurization of C[O.sub.2] was performed by adjusting BPR around 2 min. In order to provide reliable results, each experiment was conducted two to four times. The morphology of the electrospun fibers was observed using a scanning electron microscope (SEM; JEOL JSM-6390LV) after gold coating and the fiber diameter was measured from the SEM images using image analyzer software (Image J 1.42).

RESULTS AND DISCUSSION

Electrospun Products Under Ambient Temperature

Like the electrospraying process, the electrospinning process also applies an electric potential to a polymer solution to produce a jet that accelerates from the capillary tip to the collector. The concentration of the polymer solution plays an important role in the formation of the nano-microfibers in the electrospinning process. The polymer solution, which acts a precursor for the formation of nano-microfibers, must have a concentration high enough to produce polymer entanglements. Figure 1 shows the SEM images of the electrospun fibers obtained from PMMA solutions with various concentrations. As seen from the SEM images, different morphologies were obtained. It is well known that the polymer concentration in the solution affects its viscosity as well as surface tension [15, 16]. This is the reason why polymer concentrations have significant effects on the morphologies of the nanofibers produced by electrospinning process. At polymer concentrations of 10 and 15 wt% the formation of beads and bead-strings was predominant. At these conditions, the PMMA solutions might have low viscosity and high surface tension. Therefore, the jet during its trajectory was divorced due to high surface tension resulting in the production of polymer droplets instead of fibers when a high voltage was applied. An increase in the PMMA concentration results in an increase in the viscosity and an increase in the surface tension of the solution. This results in the entanglement of polymeric chains, which subsequently leads to the formation of uniform bead-free nano-microfibers. Figure 1 clearly shows that bead-free nano-microfibers were obtained when the PMMA concentration increased to 20 and 25 wt%. Touny et al. [16] showed that at higher concentrations the viscosity of the solution increases thus leading to a decrease in electrostatic repulsion. A decrease in electrostatic repulsion leads to a decrease in the drawing stress in the jet thus leading to the formation of fibers in nano-microrange.

Electrospun Products Under Pressurized C[O.sub.2]

As mentioned above, polymer solution concentration is one of the important parameters affecting the formation of nano-microfibers in electrospinning process [2], This parameter has a large influence on the bending instability, which plays a vital role in electrospinning process. The bending instability occurred through loops spirally when the applied high voltage beats the surface tension of polymer solution droplet to form and sustain the jet from the tip to collector. During this process, the polymer solution jet elongates to the fiber collector and the polymer solvent evaporates followed by the polymer solidification leading to the formation of nano-microfibers on the fiber collector. The electrospinning process included evaporation and solidification was significantly affected by environment conditions such as temperatures and pressures. Figure 2 shows the SEM images of the electrospun products obtained from the PMMA solutions with varying concentrations under pressurized C[O.sub.2]. These figures show that at low PMMA concentrations the electrospun products consisted mainly of beads and were fiber-free. An increase in the PMMA concentration resulted in the formation of a mixture of beads and fibers. Further increase in the PMMA concentration resulted in the formation of continuous bead-free fibers. This phenomenon is very similar to that observed from the SEM images of the electrospun products formed at various PMMA concentrations without using pressurized C[O.sub.2] (Fig. 1). As explained before, when the PMMA concentration in the solution becomes high enough (viscous) to stabilize the jet, the PMMA solution is severely elongated and the solvent evaporates to form nano-microfibers, which upon solidification are deposited on the collector. As shown in Fig. 1, bead-free nano-microfibers were obtained when the PMMA concentration was 20 wt% under ambient conditions. On the contrary, beaded fibers and beads were prominent when the electrospinning process was performed under pressurized C[O.sub.2] with the same PMMA concentration (see Fig. 2). When the electrospinning process was carried out under pressurized C[O.sub.2], the PMMA solvent evaporated during jet travels and the remaining solvent diffused via the PMMA matrix and evaporated once the fibers reached the fiber collector. In other words, as electrospinning involves rapid solvent evaporation and phase separation due to jet traveling, solvent vapor pressure critically determines both the evaporation rate and drying time. Hence, solvent volatility plays an important role in the production of nano-microfibers by influencing the phase separation process [17-20]. This result indicated that the solvent might be removed almost completely from the PMMA solution due to an increase in the solubility of the PMMA solvent in C[O.sub.2], which subsequently assists its evaporation rate. At this condition, C[O.sub.2] had enough affinity to dissolve a portion of DCM. This affinity can be tuned by adjusting the temperature and pressure. In most cases, an increase in C[O.sub.2] pressure could enhance the amount of DCM removed [21].

Hollow-Structured Electrospun Products

Although most polymers are not soluble in C[O.sub.2] under supercritical conditions, the solubility of dense C[O.sub.2] in many amorphous polymers is as high as that of typical liquid swelling agents. The glass transition temperature ([T.sub.g]) of polymer-C[O.sub.2] systems such as PMMA-C[O.sub.2] decreases linearly with gas pressure or gas concentration in polymers. C[O.sub.2] has an advantage of being a nonpermanent swelling agent and thus aids the formation of solvent-free end products. Next, the interaction of C[O.sub.2] and PMMA also affects the mechanical and physical properties of PMMA [22]. Though DCM as a solvent is removed at the end of the process, it enhances the solubility of PMMA in C[O.sub.2] [23]. Figure 3 shows the SEM images of electrospun products obtained from 25 wt% PMMA solution under various conditions. At a pressure of 4 MPa, the electrospun products mainly consisted of entangled-beads or bead-strings. At these conditions, a fast mutual diffusion into and out of the solution drop containing PMMA as a solute occurred when tiny droplets of the PMMA solution produced by a needle were introduced into a high pressure vessel containing dense C[O.sub.2] because of the high solubility of DCM in dense C[O.sub.2]. Since a mass transfer occurred between the PMMA solvent (DCM) and the compressed anti-solvent (C[O.sub.2]), phase separation and precipitation of the PMMA solute also occurred. The PMMA solution underwent crystallization or precipitation from the liquid solution region into the solid-fluid region through the liquid-solid and liquid-fluid two-phase regions and the solid-liquid-fluid three-phase region. When the droplets of PMMA solution approached supersaturation, a very rapid phase change occurred to form a solid phase and PMMA precipitated on the surface of the fiber collector as electrospun beads. Beautiful electrospun bead-free fibers were generated when the pressure of C[O.sub.2] was increased to 5 and 6 MPa. Here also the PMMA solution droplets underwent a phase separation and precipitation process to form a PMMA solid as they did when the pressure of C[O.sub.2] was maintained at 4 MPa. As the C[O.sub.2] pressure increased, the amount of C[O.sub.2] in the system also increased thus affecting the C[O.sub.2] composition in the electrospinning vessel. Since higher pressures are required to maintain the solubility of C[O.sub.2] in the polymer solvent, in most cases, the cloud point pressure of polymer-solvent-C[O.sub.2] system rises with increasing amounts of C[O.sub.2] [21]. In this work, the cloud point was not measured. On the other hand, increasing C[O.sub.2] composition in the system also leads to a reduction in the solubility of C[O.sub.2] in polymer solvent (DCM). Therefore, bead-free fibers were obtained when the electrospinning process was conducted at higher pressures (5 and 6 MPa). In addition, the thermal diffusivity of C[O.sub.2] at 5 and 6 MPa was lower than that at 4 MPa and strongly influenced the mass transfer process between the solvent DCM-anti-solvent (C[O.sub.2]). This means that the diffusion of a species via a pressurized C[O.sub.2] medium at 4 MPa may occur at a faster rate than that at 5 and 6 MPa at the same temperature.

It has already been mentioned earlier that most of polymers are not soluble in supercritical C[O.sub.2]; hence, C[O.sub.2] is generally used as an anti-solvent, which is suitable for polymer recrystallization or precipitation in the form of nano-microparticles. Similarly, when a polymer is dissolved in an organic solvent and subjected as an electrospinning feed while using dense C[O.sub.2] as an anti-solvent, C[O.sub.2] will diffuse rapidly into the polymer-solvent liquid jet and extract it [19]. As a result, nascent electrospun products are obtained in dry conditions. As shown in Fig. 3, the electrospun products from PMMA solution were also dry conditions when the electrospinning process was performed under pressurized C[O.sub.2] at 4 and 6 MPa. Interestingly, when the electrospun beads and bead-free fibers of PMMA were cut cross-sectionally they seem to have hollow morphologies. C[O.sub.2] easily dissolved in DCM under dense conditions and large amounts of C[O.sub.2] might dissolve into the DCM-rich liquid phase. On the contrary, C[O.sub.2] is a very poor solvent for most polymers such as PMMA at these conditions. Nevertheless, this poor solubility of PMMA in C[O.sub.2] is advantageous for the separation of PMMA from DCM. In this work, the electrospinning process was conducted in batches at fixed operating pressures of C[O.sub.2]. C[O.sub.2] dissolved into the PMMA-DCM solution and affected a phase separation that seems to occur through spinodal decomposition. The interaction between C[O.sub.2] and the PMMA-DCM solution resulted in quick evaporation of DCM followed by the formation of phase boundaries and finally the electrospun products. The beads or fibers rapidly construct a skin after DCM was evaporated from the surface of the nascent fiber of PMMA. Simultaneously, C[O.sub.2] also substitutes DCM in the PMMA solution and promotes the swelling of PMMA. By simple depressurization of C[O.sub.2] toward the end of the experiments, C[O.sub.2] could be easily removed from the electrospun beads or fibers of PMMA thus resulting in the hollow morphologies of electrospun products. It could be said that the presence of C[O.sub.2] in the PMMA-DCM solution and low interfacial tension of the PMMA-DCM-C[O.sub.2] phase relative to the C[O.sub.2]-DCM phase led to the formation of beads or fibers with hollow morphologies when depressurization was performed at the end of the process.

Glass Transition Temperature

Clearly, the C[O.sub.2]-polymer interaction in polymer processing under supercritical conditions resulted in the sorption and swelling of polymers by C[O.sub.2]. One of the important effects of the polymer processing under supercritical C[O.sub.2] is the reduction in [T.sub.g] of polymers. This occurs when the sorption of C[O.sub.2] in the polymer solution is sufficient to overcome the normal of their [T.sub.g]. [T.sub.g] is a very important property of polymers at which a polymer transitions from a hard, glassy material to a soft, rubbery material [2], This temperature can also be defined as the temperature boundary for the mobility state of the polymer molecules. When the temperature of a polymer is lower than its [T.sub.g], it behaves in a brittle manner and is termed a glassy polymer; at temperatures above [T.sub.g], the polymer becomes soft or rubbery and the polymer molecular chains gain sufficient thermal energy to slide. Figure 4 shows the electrospun products obtained from 25 wt% PMMA solution when the electrospinning process was carried out at around the trend line of glass transition temperature of PMMA at varying C[O.sub.2] pressures. As shown in Fig. 4, the morphologies of electrospun products were affected by the pressure of C[O.sub.2] at these conditions. At 4.5 MPa and 28[degrees]C, the hollows morphology of electrospun beads-string looked like glass-like brittle behavior. As the C[O.sub.2] pressure was increased to 5 MPa, the hollow electrospun fibers seemed to exhibit a glass-rubber like behavior. Rubber-like behavior of hollow electrospun products was observed when the electrospinning process was carried out at a temperature of 29[degrees]C under 5.5 and 6 MPa pressurized C[O.sub.2]. These results clearly show that a change in operating parameters such as C[O.sub.2] pressure had a large influence on the morphologies (crack or hollow) and the thermal behavior of the so-obtained electrospun products. These results also confirmed that the [T.sub.g] of PMMA was changed when compressed C[O.sub.2] penetrates into the space between the PMMA chains to alter the chain mobility and free volume of PMMA [22, 24-26], Noto et al. [24] investigated the effect of high pressure C[O.sub.2] on the structure of PMMA with Fourier-transform infrared (FT-IR) spectroscopy at temperatures in the range 293-353 K. They reported that spectral changes are observed for PMMA where the vibrational band at 1680 [cm.sup.-1] disappears with increasing pressure of C[O.sub.2].

Depressurization Time

Supercritical C[O.sub.2] is commonly used as a porogen to produce pores within a polymer matrix. The method utilizes unique proper ties of supercritical C[O.sub.2] such as the combination of liquid-like densities (high solvent power) with gas-like viscosities (high diffusion rates). When a polymer is plasticized by saturation in supercritical C[O.sub.2] followed by C[O.sub.2] depressurization at a constant temperature, porous polymer foams are formed. Figure 5 shows the SEM images of the electrospun products obtained from 25 wt% PMMA solution when the electrospinning was carried out at temperatures in the range 28-29[degrees]C and pressures in the range 5-6 MPa with 5-20 s depressurization time. It was observed that electrospun products with hollow morphology were formed at these conditions. In gas polymer foaming technique, generally, fast depressurization results in higher supersaturation leading to a greater nucleation rate, thus leading to the formation of small pores [25]. It can be said that the average pore size and porosity decreased as the depressurization rate increased. In this work, it seemed that varying the depressurization time had no significant effect on the morphology of electrospun products at the same electrospinning pressure. Therefore, the size of their hollows was not observed. As shown in Fig. 5, electrospun products with single-hollow morphology were predominantly generated when the electrospinning was conducted at 5 MPa and 29[degrees]C with 5-20 s depressurization time. Conversely, electrospun products with multihollow morphology were obtained at 6 MPa and 28[degrees]C with the same depressurization time. It can be explained as follows. When the polymer foaming process was carried out under pressurized C[O.sub.2], the saturation of C[O.sub.2] into a polymer could occur either in a glassy or rubbery state. The thermodynamic stable state of polymer-C[O.sub.2] could be achieved by changing the pressure and/or temperature. Nucleation and pore growth mainly occurs in the rubbery zone, i.e., at a temperature near [T.sub.g] of an amorphous polymer. When C[O.sub.2] escapes from the polymer matrix in a glassy state, the porous structure so produced is stabilized. According to [T.sub.g] line of PMMA-C[O.sub.2] system [27], the electrospinning system will be in a glassy state when the experiment was conducted at 5 MPa and 29[degrees]C, while at 6 MPa and 28[degrees]C the electrospinning system moves to a rubbery state. Due to these conditions, different hollow morphologies of electrospun products are obtained. In addition, due to the shape memory effect, the shape memory polymer layer recovers the deformation that takes place in the rubbery state during depressurization.

CONCLUSIONS

The formation of PMMA electrospun products by electrospinning under pressurized C[O.sub.2] at 15 kV was studied at temperatures and pressures in the range 27-37[degrees]C and 4-6 MPa, respectively. The PMMA solution was prepared with a concentration of 10, 15, 20, and 25 wt% PMMA using DCM as a solvent. At low PMMA concentrations, the formation of electrospun products with beads and bead-strings was predominant. At 5 MPa, C[O.sub.2] seemed to have enough affinity to dissolve a portion of DCM to assist its. Under pressurized C[O.sub.2], the electrospun products with hollow core fibers and diameters around 4-16 pm were generated. The results showed that the change of operating parameters had a strong influence on the morphologies (crack or hollow) of the electrospun products. Conversely, variation in depressurization time had no significant effect on the morphology of the electrospun products at the same electrospinning pressure.

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Wahyudiono, (1) Koichi Okamoto, (1) Siti Machmudah, (2) Hideki Kanda, (1) Motonobu Goto (1)

(1) Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

(2) Department of Chemical Engineering, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo, Surabaya 60111, Indonesia

Correspondence to: M. Goto; e-mail: mgoto@nuce.nagoya-u.ac.jp

Contract grant sponsor: Precursory Research for Embryonic Science and Technology Program of the Japan Science and Technology Agency.

DOI 10.1002/pen.24302
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Author:Wahyudiono; Okamoto, Koichi; Machmudah, Siti; Kanda, Hideki; Goto, Motonobu
Publication:Polymer Engineering and Science
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Date:Jul 1, 2016
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