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Electrospraying technique under pressurized carbon dioxide for hollow particle production
Reactive and Functional Polymers 142 (2019) 44–52
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Electrospraying technique under pressurized carbon dioxide for hollow particle production Wahyudionoa, Hiroyuki Ozawaa, Siti Machmudahb, Hideki Kandaa, Motonobu Gotoa, a b
T
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Department of Materials Process Engineering, Nagoya University, Nagoya 464–8603, Japan Department of Chemical Engineering, Sepuluh Nopember Institute of Technology, Surabaya 60111, Indonesia
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospraying Hollow particles Pressurized CO2 Polyvinylpyrrolidone Electrohydrodynamic
Fabrication of polyvinylpyrrolidone (PVP) particles by an electrospraying process under pressurized carbon dioxide (CO2) used as an anti-solvent was studied. The process was carried out at a constant temperature with CO2 pressures ranging from 1 to 6 MPa. A fixed applied voltage (17 kV) was employed to generate electric field. PVP powder dissolved in dichloromethane (DCM) at concentrations of 4, 6, and 8 wt% was used as the starting material. Scanning electron microscopy observations indicated that the electrosprayed PVP particles produced at 1–3 MPa using 8 wt% PVP solutions have spherical morphologies. At a similar concentration, increasing the CO2 pressure up to 6 MPa resulted in particle strings and strings as major products. Interestingly, hollow core particles formed when ethanol was added as a secondary solvent for controlling the evaporation rate of DCM during the electrospraying process under pressurized CO2. The FT–IR spectra revealed that the structural properties of PVP did not change after the electrospraying process. This work demonstrates that electrohydrodynamic processes under pressurized CO2 are likely to be fruitful for the fabrication of organic polymers with hollow cores.
1. Introduction
methods have been used to fabricate nano and microscale particles, which include spray drying, solvent extraction/evaporation, coacervation, sol–gel-based polymerization, and supercritical fluid methods (use of a supercritical anti-solvent). However, these methods have some disadvantages such as expensive equipment, broad size-distribution of the produced materials, relatively large particle size, deterioration of the products due to the high-temperatures processes, and the difficulty of separating the organic solvent in the products [4]. Conversely, electrospraying is a simple and very versatile technique to produce nano and microscale particles from polymers in a single step [5–10]. The cost of this method is also low and it requires a low amount of solvents. In addition, electrospraying technique has some potential advantages. First, this technique may yield monodisperse droplets with sizes ranging from nanometers to micrometers. Free charges that are generated and then gathered on the surface of the droplet during the process do not influence the features of the polymers. Finally, the technique provides control over the size of the final products. Polyvinylpyrrolidone (PVP), also generally known as polyvidone or povidone, is a water-soluble polymer constructed from the monomer, N-vinylpyrrolidone. This polymer is commercially available at different average molecular weights, and similar to other types of polymers, its properties also vary according to its average molecular weight [11]. In general, the PVP dissolution rate decreases with increasing average
High-voltage electrohydrodynamic techniques are very powerful for generating and developing materials with the structural features required for tissue engineering applications, especially from polymers. Two such major techniques, namely electrospinning and electrospraying, employed for producing fibers and particles, respectively, utilize high electric fields. Electrospinning and electrospraying techniques work on the same principle with basic and very minor differences. Both techniques involve atomization processes and they employ an electrically charged jet of the polymer solution to generate nano and microscale fibers or particles [1–3]. When the polymer solution jet is accelerated and drawn with the aid of electric field, the stretched polymer solution jet may break. Then, droplets will be formed to generate nano and microscale particles or the stretched polymer solution jet will remain in the form of a filament to generate nano and microscale fibers. Eventually, the particles or fibers might be recovered after solvent evaporation. Similar to that of the electrospinning technique, the typical equipment of electrospraying consists of three main devices: a high voltage power supply to generate electric field, a syringe pump to deliver the polymer feed solution through a blunt–ended metal needle, and a grounded product collector to collect the electrospun products. Various ⁎
Corresponding author. E-mail address: [email protected] (M. Goto).
https://doi.org/10.1016/j.reactfunctpolym.2019.05.016 Received 2 February 2019; Received in revised form 27 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
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electrospraying experiments were carried out as follows: First, the non–conductive PEEK chamber was heated using an electric cartridge heater to the desired temperature of 30 ± 2 °C [30]. After the desired temperature was reached, CO2 was transported into the PEEK chamber through the PEEK capillary tube to the desired pressure (1–6 MPa). Before filling with CO2, the PEEK chamber was purged repeatedly (three times) with CO2 to replace the air with moisture. This purging may regulate the relative humidity of the PEEK chamber. Note that the relative humidity within the PEEK chamber was not measured during this study. A back–pressure regulator was used to maintain the experimental pressure. When the experimental conditions were reached, the polymer solution was injected into the PEEK chamber using a highpressure stainless steel syringe located in the high-pressure syringe pump via the PEEK capillary tube. The flow rate of the polymer solution was 0.05 mL/min. Subsequently, high-voltage power was supplied (17 kV) to generate electrostatic force. The operating condition was chosen based on the previously reported conditions [15–19,26]. In this work, the electrospraying time for each experiment was 15 min. In order to obtain more reliable results, each electrospraying experiment was carried out two to four times.
molecular weight, while the viscosity, adhesive power, and ability to construct complexes increases. This polymer also has low toxicity and good film-forming characteristics, and is therefore intensively used as an excipient and is mainly well-suited for the preparation of solid dispersions to enhance the dissolution rates of poorly water-soluble drugs in pharmaceutical applications [11–14]. In previous works, PVP was used as a starting material to generate nano and microscale fibers via an electrospinning process under pressurized carbon dioxide (CO2) [15–19]. This polymer was dissolved in dichloromethane (DCM) and directly injected into the electrospinning equipment under dense CO2. Under pressurized CO2 conditions, CO2 possesses gas-like diffusivity and viscosity, and liquid-like solvation ability and density. Owing to these special properties, CO2 can be used as an excellent solvent for a wide range of applications. CO2 has been employed as an anti-solvent, solvent, or microcellular foaming plasticizer for polymer blending, particle generation, impregnation of polymers, and generation of polymer composites under supercritical conditions [20–22]. The major advantage of employing CO2 as a medium for processing polymers, including PVP, is its poor reactivity with the functional groups of the polymers, and under the same conditions, CO2 has the ability to dissolve most organic solvents such as alcohols, acids, and other solvents with low volatility [23]. Tsivintzelis et al. [23] studied the phase composition of a CO2–DCM system under various conditions (temperatures and pressures) and found that CO2 has sufficient ability to carry a part of the DCM, and conversely, CO2 could not dissolve PVP up to 480 K and 290 MPa [24,25]. Owing to this phenomenon, the hollow fibers morphologies were successfully generated in nano- to micro-scale range from several polymers, with DCM as a solvent for the polymer under pressurized CO2 in a one-step process [15–19,26]. Further, hollow fibers or particles produced from polymers have attracted attention owing to their numerous applications such as in drug delivery, environmental protection, biotechnology, photonic crystals, materials science, and catalysis [27–29]. Several techniques (templating, self-assembly, and acid/alkali swelling) and diverse synthesis methods (precipitation polymerization, emulsion, dispersion, and suspension) have been proposed to produce these hollow fibers or particles. However, structural control with these techniques is difficult because they involve several steps of processing and pose some difficulties such as the template removal and the choice of a preferable core solution. Hence, in this work, in order to overcome the disadvantages of the common techniques and to accomplish the previously reported results [15–19,26], nano- and micro-scale particles were fabricated from a PVP solution by the electrospraying technique under pressurized CO2.
2.3. Characterization of the particles The collected electrospun products were inspected under a scanning electron microscope (SEM; Hitachi, S–4300, Japan) after gold coating (IB–3, Eiko Engineering, Japan). The diameter of the collected electrospun products was determined from the SEM images using the image analyzer software (Image J 1.42) [31]. To observe the functional groups of the electrospun products, the collected electrospun products at each operating condition were characterized using a Spectrum Two FT–IR spectrophotometer (PerkinElmer Ltd., England). This device was coupled through a standard optical system to the KBr windows and a universal attenuated total reflectance (UATR, P/N 10500 series, Specac) sampling accessory for spectral data collection with 4 cm−1 resolution. 3. Results and discussion 3.1. Electrospun products formed under ambient temperature Fig. 1 displays the SEM images of electrospun products generated from PVP solutions of various concentrations. The differences in the morphologies of electrospun products are obvious. At a low polymer feed solution concentration (4 wt%), apparently, the electrospun products have irregular morphologies and they merge with each other. In general, the viscosity of a polymer solution has a linear relationship with the concentration of the polymer solution, and it has a dominant effect on electrohydrodynamic processes, including the electrospraying process. A low viscosity of the polymer solution results in high surface tension of the polymer solution. Further, when jets of the polymer solution with low PVP concentration (4 wt%) reaches and lands on the electrospun collector without sufficient evaporation of the solvent, no solidified electrospun products can be generated on the electrospun collector. Levitt et al. [32] reported that wet electrospun products are generated when a low-viscosity polymer solution is fed as the starting material, because of the poor solvent evaporation during the electrohydrodynamic process. Pillay et al. [33] also explained that although the low-viscosity polymer solution may lead to the whipping mode during the electrohydrodynamic process, liquid droplets were still found in the electrospun product collector after they arrived and landed on it. As shown in Fig. 1(A–C), spherical PVP particles began to form from the PVP polymer solution as the PVP concentration increased, which implies an increase in the viscosity of the polymer solution and a decrease in its surface tension. At 6 wt% PVP feed solution concentration, PVP electrospun products composed of particles were clearly formed, although they seemed to have dimpled or shriveled
2. Experimental 2.1. Materials PVP (MW, 29,000; product no. 234257–100G) was brought from Sigma–Aldrich and used as received. The molecular weight of the starting material was not varied in this work. DCM (99.0%; product no. 132–06753) and ethanol (99.5%; product no. 052–03343) purchased from Wako Pure Chemical Industries were used as solvents; they were used without further purification. Based on previous reports [15–19,26], PVP solutions in DCM at 4, 6, and 8 wt% concentrations were used as the polymer feed solutions. Carbon dioxide (99%) was supplied by Sogo Kariya Sanso, Inc. Japan. 2.2. Experimental setup and methods The electrospraying apparatus consisted of a non–conductive PEEK chamber (AKICO PEEK), a high-voltage (HV) power supply unit (Matsusada Precision HARb–30P1), high-pressure pump (JASCO PU–1586), high-pressure syringe pump (Harvard Apparatus PHD–Ultra 4400), back–pressure regulator (AKICO HPB–450 SUS–316), and stainless steel syringe. The nozzle-to-collector distance was 8 cm. The 45
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Fig. 1. SEM images of electrospun products generated from PVP solutions of various concentrations at room temperature.
electrospun products [1–3]. Hence, apart from the nature of the polymer solution and the electrohydrodynamic process parameters, environmental aspects such as the relative humidity and temperature might also influence the morphology of the electrospun products because the latter influence the evaporation and solidification processes. Fig. 2 shows the SEM images of electrospun products generated from an 8 wt% PVP solution under pressurized CO2. At pressures between 1 and 3 MPa, the electrospun products mainly consisted of nano- and microparticles. The particles seem to have spherical morphologies, and the images in Fig. 2A, B, and C clearly show that no PVP polymer strings formed at these operating pressures. In fact, most of the PVP polymer particles have dimpled or shriveled morphologies. However, they seem to be of uniform sizes. The results indicate that the rapid evaporation rate of DCM used as a solvent for the polymer might have occurred, leading to the shrinkage of PVP particles. Tornello et al. [34] revealed that the solvent evaporation process in an electrospraying technique that involves a combination of heat and/or mass transfer processes may in general lead to the generation of particles with corrugated and collapsed morphologies. Upon increasing the operating pressure from 4 to 6 MPa, smooth PVP polymer fibers formed. At 4 MPa, the electrospun
morphologies. There was no string formation. However, nascent strings were found in the electrospun product collector when an 8 wt% PVP solution was used as the feed. This result suggests that at an increased PVP feed solution concentration the entanglement of PVP chains may be promoted to initiate and form electrospun fibers. Thus, an 8 wt% PVP solution was used as the starting feed for the ensuing electrospraying experiments under pressurized CO2. 3.2. Electrospun products formed under pressurized CO2 The setup of the apparatus and the electrohydrodynamic process for generating polymer fibers or particles on nano- and micro-scale from a polymer solution are known to be quite simple; in fact, this process is very intricate and involves the interaction of several physical instability processes. As mentioned above, the electrohydrodynamic process is based on the ability of the electric force to split a droplet and transform it into nano- and micro-scale units. Next, owing to the whipping instability it experiences during the time it passes from the metal needle to the electrospun collector, the charged polymer solution jet solidifies due to solvent evaporation to form nano- and micro-structured 46
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Fig. 2. SEM images of electrospun products generated from an 8 wt% PVP solution in DCM under pressurized CO2 at various pressures.
electrosprayed particles are converted into smooth electrospun fibers. As shown in Fig. 2E, an increase in the operating pressure (5 MPa) resulted in the generation of a mixture of beads and fibers. A further increase in the operating pressure to 6 MPa resulted in the generation of continuous bead-free fibers. However, the PVP electrospun fibers seem
products are composed of spherical particles with smooth fibers (Fig. 2E). This indicates that with the PVP feed solution concentration fixed, the surface tension of the PVP solution, which can affect the solvent evaporation rate, was altered to result in the observed morphologies of the electrospun products. As a result, the PVP
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to have flat (ribbon–like) morphology owing to the collapse of the rapidly solidified outer shell [17]. Judging from the results, although it is complicated to speculate a direct relationship between the morphology of the electrospun product and operating pressure, it could be stated that the surrounding environment might strongly influence the morphology of the electrospun products [3,15–19,26]. The results also indicate that the electrohydrodynamic system with PVP as the starting material and DCM as the polymer solvent can be isolated and operated in a closed chamber under dense CO2, whereby the temperature and relative humidity of the chamber might be modified to improve control over the drying process and to avoid contamination with undesired materials.
polymer solution increases markedly, and there is increased polymer chain–chain interactions, and the chain entanglement is increased. As noted before, an increase in the PVP concentration of the jet occurs during the DCM–ethanol evaporation process, and accordingly, spherical particles without chain entanglements are not generated effectively owing to this phenomenon. Apparently, Fig. 3B also indicates that the PVP electrospun products are seemingly in a wet condition. Apparently, the DCM–ethanol evaporation was incomplete, resulting in wet PVP particles or their partial dissolution in the electrospun product collector. Probably, the reason is as follows: When electrospraying is performed under CO2 environment, DCM–ethanol evaporates rapidly during the travel of the jet and the remaining DCM–ethanol diffuses through the PVP solute and evaporates once the particles reach the electrospun product collector. It is understood that CO2 is a very poor solvent under dense conditions for virtually all polymers and it has poor reactivity towards the functional groups present in polymers including PVP. Meanwhile, CO2 under the same condition has the ability to dissolve most organic solvents, including alcohols, acids, and other low-volatility solvents [23]. Tsivintzelis et al. [23] also reported that CO2 can accommodate a portion of the DCM–ethanol solvent under certain conditions from their study on the CO2–DCM–ethanol system phase composition under various conditions (temperature and pressure). Jiang et al. [38] also reported that CO2 might swell and dissolve DCM–ethanol under dense conditions, which makes DCM–ethanol a poor solvent for poly(lactic acid)–poly(ethylene glycol)–poly(lactic acid). Hence, it can be concluded that CO2 at 1 MPa might help the evaporation of DCM–ethanol from the PVP feed solution while the jet travels, although the amount of CO2 was not enough to accept a portion of these solvents. When the operating pressure was increased to 2 and 3 MPa, the PVP electrospun products were mainly composed of spherical particles. Nevertheless, the entangled particles and nascent strings were still formed and found in the electrospun product collector. Again, under these conditions, solvent (DCM–ethanol) evaporated rapidly from the polymer solution, resulting in an increase in the PVP concentration in the polymer solution jet. These results also suggest that the driving force for the PVP solidification in this process is the effect of anti–solvent induced by the CO2 solubilization, and the ability of CO2 solvent to dissolve substances is directly proportional to its density. The CO2 density increases with increasing pressure at a constant temperature. Furthermore, the PVP feed solution might have dispersed in dense CO2 when it was injected in the PEEK chamber, and the CO2 solvent might have been captured by the DCM–ethanol system. However, it seems that CO2 did not achieve the apt condition to solubilize the DCM–ethanol at pressures of 2 and 3 MPa [39,40]. At 4 and 5 MPa, the electrospun products obviously consisted of nano- and micro-scale PVP particles with spherical morphologies. No entangled particles or nascent strings were observed in the electrospun products. These results indicate that the DCM–ethanol solvent might have evaporated almost perfectly from the PVP solution, owing to the improvement of the DCM–ethanol solubility in CO2, which subsequently helps their evaporation rate. However, the thin ribbon-like structures surrounding PVP particles of small size were found in the PVP electrospun products when the electrospraying process was carried out at a pressure of 5 MPa. This thin ribbon-like structure might have originated from the collapse of quickly solidified outer shell of the jet of the PVP feed solution during the fast solvent evaporation. This phenomenon suggests that, when the electrospraying process is conducted under dense CO2, the DCM–ethanol solvents evaporate during the jet's trip in the system and the remaining DCM–ethanol diffuses through the PVP matrix and evaporates once the PVP particles land on the electrospun product collector [15–19,26]. On the contrary, PVP strings were predominantly formed and observed in the electrospun products when electrospraying was performed at a pressure of 6 MPa. Under this condition, the viscosity of the PVP feed solution might have been high, and thus the surface tension of the PVP feed solution is increased owing
3.3. Effect of ethanol addition Electrospraying is known to be a promising method to produce highly monodisperse fine particles in nano- and micro-scale from a polymer solution, when the precondition of a stable cone-jet formation is reached. Apart from this factor, there are many different operating parameters that affect a great number of aspects, and the complexity of the interdependence of these aspects should be untangled to reach the working condition for a successful electrohydrodynamic process. One of the important parameters of an electrohydrodynamic process is the solvent used to dissolve the polymer. The solvent selection is a crucial step because it determines the properties of the polymer feed solution during the electrohydrodynamic process. They include surface tension and conductivity that can affect the electrospun products. Hence, a combination of solvents was employed to meet all the requirements to generate the desired electrospun products, which might not be possible with the use of a single solvent [1,6,35–37]. Fig. 3 shows the morphologies of the electrospun products generated from an 8 wt% PVP solution in DCM–ethanol mixture at room temperature and under pressurized CO2. It shows that at room temperature (Fig. 3A), the electrospun products clearly consist of spherical PVP particles without any strings or fibers. Under this condition, the physical properties of the PVP solution, such as its conductivity, surface tension, and viscosity, might change due to the added co-solvent, ethanol [1,6,35–37]. Consequently, the stability of the jet of the PVP solution increased during the process to result in a successful electrospraying process. It should be noted that the physical properties of the PVP solution were not studied in this work. In addition, the results (Figs. 1D, E, and 3A) also indicate that two phenomena take place during the evaporation of the solvents from the polymer solution. They are the increase in the polymer feed solution concentration and the commencement of nascent string formation. Ethanol used as a co-solvent seems to provide control over the DCM evaporation rate, which can affect the skin generation from the PVP solution droplets due to the increased PVP concentration of the feed solution. Owing to this condition, the generation of nascent strings could be avoided during the electrospraying process. Nguyen et al. [6] suggested that the rapid evaporation of the solvent from the polymer solution during the electrospraying process may promote the increase in the polymer concentration between the core and surface of the jet. As a result, polymer particles containing strings would be observed in the electrospun product collector. As shown in Fig. 3B, the electrospun products obviously consist of entangled PVP particles when the electrospraying process is performed in CO2 environment under a pressure of 1 MPa. It is well known that one of the principal factors that affects the electrospraying process is the viscosity of the polymer feed solution, which usually depends on the molecular weight and concentration of the polymer. At a fixed molecular weight of the polymer, there are no or few polymer chain–chain interactions at a low concentration because the molecular balls of polymers are loose and might be isolated by the solvent. Therefore, there is no entanglement between the polymer molecules. As the concentration of the polymer feed solution increased, the viscosity of the 48
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Fig. 3. SEM images of electrospun products generated from an 8 wt% PVP solution in DCM–ethanol at room temperature and under pressurized CO2 at various pressures.
to the increased PVP feed solution concentration. As a result, the PVP chain entanglement increases and the generation of nano and microscale strings is promoted.
3.4. Hollow-structured electrosprayed products During the electrospraying process, the solvent evaporates from the polymer solution boundary of the jet of the polymer feed solution. Simultaneously, the solvent diffuses from the center to the boundary of 49
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Fig. 4. Solid and hollow structures of electrosprayed particles.
precipitation of the PVP solute from the PVP feed solution. Subsequently, the PVP feed solution solidifies or precipitates from the liquid solution zone to the solid fluid zone via the liquid–solid and liquid–fluid two-phase zones and the solid–liquid–fluid three-phase zone. When the PVP feed solution droplets approach supersaturation, a very quick phase shift occurs to generate a PVP solid phase, and then the PVP polymer precipitates as particle-free fibers on the electrospun collector. As shown in Fig. 4, the beautiful PVP electrosprayed particle product was also generated when the electrospraying process was carried out at a pressure of 4 or 5 MPa. Under this condition, as the operating pressure is increased to 4 or 5 MPa, the CO2 density increases, resulting in improved CO2 solvating power. As a result, the CO2 solubility in the DCM–ethanol–rich zone and the DCM–ethanol solubility in the CO2-rich gas zone are improved, leading to an increase in the concentration of CO2 in the PVP feed solution jets. Then, the PVP–DCM–ethanol solution leads to spinodal decomposition, producing a PVP–DCM–ethanol network solution accompanied with CO2-rich bubbles. As the jet moves to the electrospun product collector, the CO2rich bubble expands against the PVP-rich network and the bubble drives the PVP-rich phase radially outward, against the jet inner skin to generate a particle with a hollow core structure [18]. Accordingly, the results may point at the modulation of the thermal behavior of the polymer through the adjustment of the CO2 pressure at the constant temperature. However, note that the phase-equilibrium behaviors in relation to the different physical properties of the PVP polymer, DCM–ethanol solvent, and CO2 during the electrospraying experiments were not investigated. Despite the powerful technique and the simplicity of electrospraying for generating fine particles from a polymer feed solution, several process parameters restrict the productivity of this technique. In most cases, controlling the size and morphology of the electrospun products, which may affect their properties and the final application, are the most important parameters to be controlled in the electrospraying process. These two key parameters depend on the solution parameters (i.e.,
the polymer solution jet. Hence, these two processes may promote the formation of a polymer skin layer via the solidification process [41]. Owing to this phenomenon, the morphology of the formed particles is highly influenced by the physicochemical properties of the solvent used to prepared the polymer solution; therefore, a proper choice of solvents to dissolve the polymer at certain conditions represents a critical step to generate electrosprayed particles in nano- and micro-scale. In most cases, a polymer solvent with a high boiling point and low vapor pressure will drive the formation of small-sized particles with a smooth skin owing to lower polymer chain entanglement, while a polymer solvent with a low boiling point and high vapor pressure may lead to particles with a porous skin and even hollow structure owing to its faster evaporation rate [18,19,42–44]. The rapid solvent evaporation rate might also reduce the period that the polymer chains require to reform within the droplet during the quick solidification process. Fig. 4 shows high-magnification SEM images of the electrospun products generated from an 8 wt% PVP feed solution in DCM–ethanol at pressures of 3, 4, and 5 MPa, respectively. Obvious differences between the morphologies of the PVP electrosprayed particles are observed in the cross-sectional images. The PVP electrosprayed products generated at a pressure of 3 MPa clearly have a solid core while those produced at pressures of 4 and 5 MPa have a hollow-core structure. As mentioned before, under dense conditions, CO2 has the power to capture DCM–ethanol from the PVP feed solution jet, and it also dissolves in the PVP feed solution jet and induces the PVP–DCM–ethanol solution to phase separate. This may drastically promote the PVP jet skin formation within milliseconds, and the DCM–ethanol solvent is removed well on this time scale. Thereafter, the formation of PVP particle boundaries and structures by phase splitting might happen [18]. The following reasons may explain the observed features: at 3 MPa, the mutual diffusion entry and exit of the PVP feed solution drop take place during the electrospraying process. Mass transfer occurs between the DCM–ethanol system and dense CO2 acting as an anti-solvent, owing to the solvating power of CO2, to result in phase splitting and 50
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[3,45–47]. In addition, although CO2 is a very poor solvent for PVP, CO2 still possesses the capability to expand or to swell PVP up to several mass percentages under high pressures [48]. Another possibility is, under dense CO2 environment, the initial PVP feed solution droplet from a blunt-ended metal needle may swell due to the CO2 diffusion into the droplet during the electrospraying process. When the CO2 pressure in the electrospraying chamber is increased, the lifetime of the PVP feed solution droplet may decline because the droplet may shrink as the CO2–DCM–ethanol mixture evaporates to affect the solidification process. Simultaneously, a strong electric field is produced and accumulated at the blunt-ended metal needle. Hence, as the PVP feed solution starts to flow out from the blunt-ended metal needle, the shrunk droplet would be surrounded by the electrically charged ions. In general, when the electrostatic force is stronger than the surface tension of the polymer solution, the polymer solution may split from the blunt-ended metal needle to form a spray and will reach and land on the electrospun collector surface as particles. However, since electrospraying was carried under pressurized CO2, the rapid vitrification of the PVP feed solution jet skin occurs. At the same time, the mutual diffusion of CO2 on the PVP matrix also occurs to result in the expansion or swelling of the PVP electrosprayed particles [17,18,26,49]. This mutual diffusion seems to occur more intensively at higher CO2 pressures, and hence, the average diameter of the PVP particles increased with increasing CO2 pressure.
Fig. 5. Diameter distribution of electrosprayed particles generated from 8 wt% PVP in DCM–ethanol at room temperature and under pressurized CO2.
concentration, polymer molecular weight, solvent permittivity), the processing parameters (i.e., flow rate, applied voltage, distance between the needle and collector), and the environmental parameters (temperature, pressure, and relative humidity) [1–3]. In this work, although these parameters were not investigated intensively during the fabrication of PVP particles from the PVP feed solution by the electrospraying technique under pressurized CO2, the use of CO2 in the electrospraying chamber to replace air as the medium seems to significantly influence the size and morphology of the PVP electrosprayed particles, which are shown in Figs. 3, 4, and 5. Using the Image J 1.42 tool [31], at least 300 different PVP particles were chosen randomly from each image, which are represented in Fig. 3, and the PVP particle diameters were determined to estimate the particle size distribution. Fig. 5 shows the PVP particle diameter distribution for the electrospun products formed from 8 wt% PVP in DCM–ethanol at room temperature and under pressurized CO2. The average diameters of the PVP particles are 1.82 ± 0.69, 2.05 ± 0.73, 2.89 ± 1.24, 3.27 ± 2.12, and 4.52 ± 2.52 μm when the electrospraying experiments were carried out at room temperature and under pressurized CO2 at pressures of 2, 3, 4, and 5 MPa, respectively. The inner diameter of the blunt-ended metal needle was not varied during the electrospraying process. Clearly, bigger PVP particles were found in the electrospun product collector when the electrospraying process was carried out under pressurized CO2. At room temperature, the average diameter of the PVP particles is approximately 1.82 ± 0.69 μm, and it increases to 2.05 ± 0.73 μm when the electrospraying process is carried out under pressurized CO2 at a pressure of 2 MPa. The average diameter of the electrosprayed particles increased gradually with increasing CO2 pressure, and it could approach 4.52 ± 2.52 μm at 5 MPa CO2 pressure. This phenomenon may be attributed to the improvement in the CO2 solvating power to dissolve the DCM–ethanol solvents, which then influenced the PVP feed solution concentration. The increasing CO2 pressure may also help maintain and promote the CO2 solubility in the DCM–ethanol solvents. It is well known that solubility represents the equilibrium between a solvent and a solute, which is a key factor in the separation process involving dense CO2 as a medium. Naturally, this mutual relation may be useful in terms of separating the DCM–ethanol solvents from the PVP polymer during the electrospraying process, which can help and improve the evaporation of DCM–ethanol solvents. As informed above, with an increase in the evaporation rate of DCM–ethanol solvents, the concentration of the PVP feed solution and its viscosity increase during the electrospraying process. As a result, an increase in the CO2 pressure in this electrospun system will be followed by an increase in the diameter of the PVP particle product, with higher polydispersity
3.5. Characterization of electrosprayed products Although the solubility of PVP in CO2 under high pressure is typically quite low, interaction may still occur between CO2 and the functional groups of PVP. Normally, when the operating pressure is increased, the CO2 molecule is forced to insert between the PVP chains, resulting in increased adsorption of CO2 molecules [50–52]. Here, FT–IR spectroscopic analysis in the wavenumber range of 4000 to 400 cm−1 was employed to observe the possible modification and shifts in the PVP bands after the electrospraying process under pressurized CO2. Fig. 6 shows the FT–IR spectra of PVP raw material and PVP electrosprayed particles obtained by electrospraying at room temperature and under pressurized CO2. Some differences between the spectra of the PVP raw material and its electrosprayed particles were expected. As shown in Fig. 6, the FT–IR spectra of PVP and its electrosprayed particles are substantially similar. The spectra only differ in terms of the intensity of some peaks, indicating that the PVP electrosprayed particles generated by the electrospraying process at room temperature and under pressurized CO2 have functional groups identical to those of the
Fig. 6. FT–IR spectra of PVP raw material and PVP electrosprayed particles. 51
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PVP raw material. Nevertheless, the intensity of the band between 1654.92 and 1650.61 cm−1 in spectrum C is sharper than in spectra A and B. This suggests interaction between CO2 molecules and carbonyl functional groups that exist in the PVP structure. It seems that under high pressure, CO2 molecules diffuse strongly into PVP matrix and interact with its functional groups; hence, the H-bonding between the PVP functional groups and CO2 molecules is superior to interactions between the DCM–ethanol solvents and PVP carbonyl functional group. Accordingly, several potential interactions might occur between CO2 and PVP (i.e., sorption of CO2 by PVP, swelling of PVP by CO2, dissolution of PVP/CO2 in CO2/PVP) [52,53]. However, the FT–IR spectral characteristics of the PVP electrosprayed particles indicate that the electrospraying process under pressurized CO2 does not shift the bands of the PVP functional groups. Next, based on the results, it could be stated that the PVP electrosprayed particles generated by the electrospraying process under pressurized CO2 have same structure as the starting material.
[15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25]
4. Conclusions
[26]
Nano and microscale PVP particles were fabricated by an electrospraying process under pressurized CO2 at 17 kV at a temperature of 30 °C and pressures of 1–6 MPa. At a low PVP feed solution concentration (4 wt%), the PVP electrospun products have irregular shapes and merged with each other. On the contrary, increasing the PVP feed solution concentration (6 and 8 wt%) resulted in spherical particles as main products when electrospraying was performed at room temperature. At 1–3 MPa pressure and 8 wt% PVP solution concentration, the PVP electrospun products seemed to have spherical particle morphologies. At the same feed solution concentration, increasing the CO2 pressure up to 6 MPa resulted in particle strings and strings as main products. The addition of ethanol seemed to help control the DCM evaporation rate during the electrospraying process under pressurized CO2, resulting in hollow electrosprayed PVP particles. The results indicate that CO2 may expand or swell PVP particles up to several mass percentages under high pressure conditions, and consequently, the average diameter of the PVP electrosprayed particles increased with increasing CO2 pressure. The FT–IR spectral analysis indicated that the PVP structure did not change after the electrospraying process.
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Contents lists available at ScienceDirect
Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Electrospraying technique under pressurized carbon dioxide for hollow particle production Wahyudionoa, Hiroyuki Ozawaa, Siti Machmudahb, Hideki Kandaa, Motonobu Gotoa, a b
T
⁎
Department of Materials Process Engineering, Nagoya University, Nagoya 464–8603, Japan Department of Chemical Engineering, Sepuluh Nopember Institute of Technology, Surabaya 60111, Indonesia
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospraying Hollow particles Pressurized CO2 Polyvinylpyrrolidone Electrohydrodynamic
Fabrication of polyvinylpyrrolidone (PVP) particles by an electrospraying process under pressurized carbon dioxide (CO2) used as an anti-solvent was studied. The process was carried out at a constant temperature with CO2 pressures ranging from 1 to 6 MPa. A fixed applied voltage (17 kV) was employed to generate electric field. PVP powder dissolved in dichloromethane (DCM) at concentrations of 4, 6, and 8 wt% was used as the starting material. Scanning electron microscopy observations indicated that the electrosprayed PVP particles produced at 1–3 MPa using 8 wt% PVP solutions have spherical morphologies. At a similar concentration, increasing the CO2 pressure up to 6 MPa resulted in particle strings and strings as major products. Interestingly, hollow core particles formed when ethanol was added as a secondary solvent for controlling the evaporation rate of DCM during the electrospraying process under pressurized CO2. The FT–IR spectra revealed that the structural properties of PVP did not change after the electrospraying process. This work demonstrates that electrohydrodynamic processes under pressurized CO2 are likely to be fruitful for the fabrication of organic polymers with hollow cores.
1. Introduction
methods have been used to fabricate nano and microscale particles, which include spray drying, solvent extraction/evaporation, coacervation, sol–gel-based polymerization, and supercritical fluid methods (use of a supercritical anti-solvent). However, these methods have some disadvantages such as expensive equipment, broad size-distribution of the produced materials, relatively large particle size, deterioration of the products due to the high-temperatures processes, and the difficulty of separating the organic solvent in the products [4]. Conversely, electrospraying is a simple and very versatile technique to produce nano and microscale particles from polymers in a single step [5–10]. The cost of this method is also low and it requires a low amount of solvents. In addition, electrospraying technique has some potential advantages. First, this technique may yield monodisperse droplets with sizes ranging from nanometers to micrometers. Free charges that are generated and then gathered on the surface of the droplet during the process do not influence the features of the polymers. Finally, the technique provides control over the size of the final products. Polyvinylpyrrolidone (PVP), also generally known as polyvidone or povidone, is a water-soluble polymer constructed from the monomer, N-vinylpyrrolidone. This polymer is commercially available at different average molecular weights, and similar to other types of polymers, its properties also vary according to its average molecular weight [11]. In general, the PVP dissolution rate decreases with increasing average
High-voltage electrohydrodynamic techniques are very powerful for generating and developing materials with the structural features required for tissue engineering applications, especially from polymers. Two such major techniques, namely electrospinning and electrospraying, employed for producing fibers and particles, respectively, utilize high electric fields. Electrospinning and electrospraying techniques work on the same principle with basic and very minor differences. Both techniques involve atomization processes and they employ an electrically charged jet of the polymer solution to generate nano and microscale fibers or particles [1–3]. When the polymer solution jet is accelerated and drawn with the aid of electric field, the stretched polymer solution jet may break. Then, droplets will be formed to generate nano and microscale particles or the stretched polymer solution jet will remain in the form of a filament to generate nano and microscale fibers. Eventually, the particles or fibers might be recovered after solvent evaporation. Similar to that of the electrospinning technique, the typical equipment of electrospraying consists of three main devices: a high voltage power supply to generate electric field, a syringe pump to deliver the polymer feed solution through a blunt–ended metal needle, and a grounded product collector to collect the electrospun products. Various ⁎
Corresponding author. E-mail address: [email protected] (M. Goto).
https://doi.org/10.1016/j.reactfunctpolym.2019.05.016 Received 2 February 2019; Received in revised form 27 May 2019; Accepted 27 May 2019 Available online 28 May 2019 1381-5148/ © 2019 Elsevier B.V. All rights reserved.
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electrospraying experiments were carried out as follows: First, the non–conductive PEEK chamber was heated using an electric cartridge heater to the desired temperature of 30 ± 2 °C [30]. After the desired temperature was reached, CO2 was transported into the PEEK chamber through the PEEK capillary tube to the desired pressure (1–6 MPa). Before filling with CO2, the PEEK chamber was purged repeatedly (three times) with CO2 to replace the air with moisture. This purging may regulate the relative humidity of the PEEK chamber. Note that the relative humidity within the PEEK chamber was not measured during this study. A back–pressure regulator was used to maintain the experimental pressure. When the experimental conditions were reached, the polymer solution was injected into the PEEK chamber using a highpressure stainless steel syringe located in the high-pressure syringe pump via the PEEK capillary tube. The flow rate of the polymer solution was 0.05 mL/min. Subsequently, high-voltage power was supplied (17 kV) to generate electrostatic force. The operating condition was chosen based on the previously reported conditions [15–19,26]. In this work, the electrospraying time for each experiment was 15 min. In order to obtain more reliable results, each electrospraying experiment was carried out two to four times.
molecular weight, while the viscosity, adhesive power, and ability to construct complexes increases. This polymer also has low toxicity and good film-forming characteristics, and is therefore intensively used as an excipient and is mainly well-suited for the preparation of solid dispersions to enhance the dissolution rates of poorly water-soluble drugs in pharmaceutical applications [11–14]. In previous works, PVP was used as a starting material to generate nano and microscale fibers via an electrospinning process under pressurized carbon dioxide (CO2) [15–19]. This polymer was dissolved in dichloromethane (DCM) and directly injected into the electrospinning equipment under dense CO2. Under pressurized CO2 conditions, CO2 possesses gas-like diffusivity and viscosity, and liquid-like solvation ability and density. Owing to these special properties, CO2 can be used as an excellent solvent for a wide range of applications. CO2 has been employed as an anti-solvent, solvent, or microcellular foaming plasticizer for polymer blending, particle generation, impregnation of polymers, and generation of polymer composites under supercritical conditions [20–22]. The major advantage of employing CO2 as a medium for processing polymers, including PVP, is its poor reactivity with the functional groups of the polymers, and under the same conditions, CO2 has the ability to dissolve most organic solvents such as alcohols, acids, and other solvents with low volatility [23]. Tsivintzelis et al. [23] studied the phase composition of a CO2–DCM system under various conditions (temperatures and pressures) and found that CO2 has sufficient ability to carry a part of the DCM, and conversely, CO2 could not dissolve PVP up to 480 K and 290 MPa [24,25]. Owing to this phenomenon, the hollow fibers morphologies were successfully generated in nano- to micro-scale range from several polymers, with DCM as a solvent for the polymer under pressurized CO2 in a one-step process [15–19,26]. Further, hollow fibers or particles produced from polymers have attracted attention owing to their numerous applications such as in drug delivery, environmental protection, biotechnology, photonic crystals, materials science, and catalysis [27–29]. Several techniques (templating, self-assembly, and acid/alkali swelling) and diverse synthesis methods (precipitation polymerization, emulsion, dispersion, and suspension) have been proposed to produce these hollow fibers or particles. However, structural control with these techniques is difficult because they involve several steps of processing and pose some difficulties such as the template removal and the choice of a preferable core solution. Hence, in this work, in order to overcome the disadvantages of the common techniques and to accomplish the previously reported results [15–19,26], nano- and micro-scale particles were fabricated from a PVP solution by the electrospraying technique under pressurized CO2.
2.3. Characterization of the particles The collected electrospun products were inspected under a scanning electron microscope (SEM; Hitachi, S–4300, Japan) after gold coating (IB–3, Eiko Engineering, Japan). The diameter of the collected electrospun products was determined from the SEM images using the image analyzer software (Image J 1.42) [31]. To observe the functional groups of the electrospun products, the collected electrospun products at each operating condition were characterized using a Spectrum Two FT–IR spectrophotometer (PerkinElmer Ltd., England). This device was coupled through a standard optical system to the KBr windows and a universal attenuated total reflectance (UATR, P/N 10500 series, Specac) sampling accessory for spectral data collection with 4 cm−1 resolution. 3. Results and discussion 3.1. Electrospun products formed under ambient temperature Fig. 1 displays the SEM images of electrospun products generated from PVP solutions of various concentrations. The differences in the morphologies of electrospun products are obvious. At a low polymer feed solution concentration (4 wt%), apparently, the electrospun products have irregular morphologies and they merge with each other. In general, the viscosity of a polymer solution has a linear relationship with the concentration of the polymer solution, and it has a dominant effect on electrohydrodynamic processes, including the electrospraying process. A low viscosity of the polymer solution results in high surface tension of the polymer solution. Further, when jets of the polymer solution with low PVP concentration (4 wt%) reaches and lands on the electrospun collector without sufficient evaporation of the solvent, no solidified electrospun products can be generated on the electrospun collector. Levitt et al. [32] reported that wet electrospun products are generated when a low-viscosity polymer solution is fed as the starting material, because of the poor solvent evaporation during the electrohydrodynamic process. Pillay et al. [33] also explained that although the low-viscosity polymer solution may lead to the whipping mode during the electrohydrodynamic process, liquid droplets were still found in the electrospun product collector after they arrived and landed on it. As shown in Fig. 1(A–C), spherical PVP particles began to form from the PVP polymer solution as the PVP concentration increased, which implies an increase in the viscosity of the polymer solution and a decrease in its surface tension. At 6 wt% PVP feed solution concentration, PVP electrospun products composed of particles were clearly formed, although they seemed to have dimpled or shriveled
2. Experimental 2.1. Materials PVP (MW, 29,000; product no. 234257–100G) was brought from Sigma–Aldrich and used as received. The molecular weight of the starting material was not varied in this work. DCM (99.0%; product no. 132–06753) and ethanol (99.5%; product no. 052–03343) purchased from Wako Pure Chemical Industries were used as solvents; they were used without further purification. Based on previous reports [15–19,26], PVP solutions in DCM at 4, 6, and 8 wt% concentrations were used as the polymer feed solutions. Carbon dioxide (99%) was supplied by Sogo Kariya Sanso, Inc. Japan. 2.2. Experimental setup and methods The electrospraying apparatus consisted of a non–conductive PEEK chamber (AKICO PEEK), a high-voltage (HV) power supply unit (Matsusada Precision HARb–30P1), high-pressure pump (JASCO PU–1586), high-pressure syringe pump (Harvard Apparatus PHD–Ultra 4400), back–pressure regulator (AKICO HPB–450 SUS–316), and stainless steel syringe. The nozzle-to-collector distance was 8 cm. The 45
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Fig. 1. SEM images of electrospun products generated from PVP solutions of various concentrations at room temperature.
electrospun products [1–3]. Hence, apart from the nature of the polymer solution and the electrohydrodynamic process parameters, environmental aspects such as the relative humidity and temperature might also influence the morphology of the electrospun products because the latter influence the evaporation and solidification processes. Fig. 2 shows the SEM images of electrospun products generated from an 8 wt% PVP solution under pressurized CO2. At pressures between 1 and 3 MPa, the electrospun products mainly consisted of nano- and microparticles. The particles seem to have spherical morphologies, and the images in Fig. 2A, B, and C clearly show that no PVP polymer strings formed at these operating pressures. In fact, most of the PVP polymer particles have dimpled or shriveled morphologies. However, they seem to be of uniform sizes. The results indicate that the rapid evaporation rate of DCM used as a solvent for the polymer might have occurred, leading to the shrinkage of PVP particles. Tornello et al. [34] revealed that the solvent evaporation process in an electrospraying technique that involves a combination of heat and/or mass transfer processes may in general lead to the generation of particles with corrugated and collapsed morphologies. Upon increasing the operating pressure from 4 to 6 MPa, smooth PVP polymer fibers formed. At 4 MPa, the electrospun
morphologies. There was no string formation. However, nascent strings were found in the electrospun product collector when an 8 wt% PVP solution was used as the feed. This result suggests that at an increased PVP feed solution concentration the entanglement of PVP chains may be promoted to initiate and form electrospun fibers. Thus, an 8 wt% PVP solution was used as the starting feed for the ensuing electrospraying experiments under pressurized CO2. 3.2. Electrospun products formed under pressurized CO2 The setup of the apparatus and the electrohydrodynamic process for generating polymer fibers or particles on nano- and micro-scale from a polymer solution are known to be quite simple; in fact, this process is very intricate and involves the interaction of several physical instability processes. As mentioned above, the electrohydrodynamic process is based on the ability of the electric force to split a droplet and transform it into nano- and micro-scale units. Next, owing to the whipping instability it experiences during the time it passes from the metal needle to the electrospun collector, the charged polymer solution jet solidifies due to solvent evaporation to form nano- and micro-structured 46
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Fig. 2. SEM images of electrospun products generated from an 8 wt% PVP solution in DCM under pressurized CO2 at various pressures.
electrosprayed particles are converted into smooth electrospun fibers. As shown in Fig. 2E, an increase in the operating pressure (5 MPa) resulted in the generation of a mixture of beads and fibers. A further increase in the operating pressure to 6 MPa resulted in the generation of continuous bead-free fibers. However, the PVP electrospun fibers seem
products are composed of spherical particles with smooth fibers (Fig. 2E). This indicates that with the PVP feed solution concentration fixed, the surface tension of the PVP solution, which can affect the solvent evaporation rate, was altered to result in the observed morphologies of the electrospun products. As a result, the PVP
47
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to have flat (ribbon–like) morphology owing to the collapse of the rapidly solidified outer shell [17]. Judging from the results, although it is complicated to speculate a direct relationship between the morphology of the electrospun product and operating pressure, it could be stated that the surrounding environment might strongly influence the morphology of the electrospun products [3,15–19,26]. The results also indicate that the electrohydrodynamic system with PVP as the starting material and DCM as the polymer solvent can be isolated and operated in a closed chamber under dense CO2, whereby the temperature and relative humidity of the chamber might be modified to improve control over the drying process and to avoid contamination with undesired materials.
polymer solution increases markedly, and there is increased polymer chain–chain interactions, and the chain entanglement is increased. As noted before, an increase in the PVP concentration of the jet occurs during the DCM–ethanol evaporation process, and accordingly, spherical particles without chain entanglements are not generated effectively owing to this phenomenon. Apparently, Fig. 3B also indicates that the PVP electrospun products are seemingly in a wet condition. Apparently, the DCM–ethanol evaporation was incomplete, resulting in wet PVP particles or their partial dissolution in the electrospun product collector. Probably, the reason is as follows: When electrospraying is performed under CO2 environment, DCM–ethanol evaporates rapidly during the travel of the jet and the remaining DCM–ethanol diffuses through the PVP solute and evaporates once the particles reach the electrospun product collector. It is understood that CO2 is a very poor solvent under dense conditions for virtually all polymers and it has poor reactivity towards the functional groups present in polymers including PVP. Meanwhile, CO2 under the same condition has the ability to dissolve most organic solvents, including alcohols, acids, and other low-volatility solvents [23]. Tsivintzelis et al. [23] also reported that CO2 can accommodate a portion of the DCM–ethanol solvent under certain conditions from their study on the CO2–DCM–ethanol system phase composition under various conditions (temperature and pressure). Jiang et al. [38] also reported that CO2 might swell and dissolve DCM–ethanol under dense conditions, which makes DCM–ethanol a poor solvent for poly(lactic acid)–poly(ethylene glycol)–poly(lactic acid). Hence, it can be concluded that CO2 at 1 MPa might help the evaporation of DCM–ethanol from the PVP feed solution while the jet travels, although the amount of CO2 was not enough to accept a portion of these solvents. When the operating pressure was increased to 2 and 3 MPa, the PVP electrospun products were mainly composed of spherical particles. Nevertheless, the entangled particles and nascent strings were still formed and found in the electrospun product collector. Again, under these conditions, solvent (DCM–ethanol) evaporated rapidly from the polymer solution, resulting in an increase in the PVP concentration in the polymer solution jet. These results also suggest that the driving force for the PVP solidification in this process is the effect of anti–solvent induced by the CO2 solubilization, and the ability of CO2 solvent to dissolve substances is directly proportional to its density. The CO2 density increases with increasing pressure at a constant temperature. Furthermore, the PVP feed solution might have dispersed in dense CO2 when it was injected in the PEEK chamber, and the CO2 solvent might have been captured by the DCM–ethanol system. However, it seems that CO2 did not achieve the apt condition to solubilize the DCM–ethanol at pressures of 2 and 3 MPa [39,40]. At 4 and 5 MPa, the electrospun products obviously consisted of nano- and micro-scale PVP particles with spherical morphologies. No entangled particles or nascent strings were observed in the electrospun products. These results indicate that the DCM–ethanol solvent might have evaporated almost perfectly from the PVP solution, owing to the improvement of the DCM–ethanol solubility in CO2, which subsequently helps their evaporation rate. However, the thin ribbon-like structures surrounding PVP particles of small size were found in the PVP electrospun products when the electrospraying process was carried out at a pressure of 5 MPa. This thin ribbon-like structure might have originated from the collapse of quickly solidified outer shell of the jet of the PVP feed solution during the fast solvent evaporation. This phenomenon suggests that, when the electrospraying process is conducted under dense CO2, the DCM–ethanol solvents evaporate during the jet's trip in the system and the remaining DCM–ethanol diffuses through the PVP matrix and evaporates once the PVP particles land on the electrospun product collector [15–19,26]. On the contrary, PVP strings were predominantly formed and observed in the electrospun products when electrospraying was performed at a pressure of 6 MPa. Under this condition, the viscosity of the PVP feed solution might have been high, and thus the surface tension of the PVP feed solution is increased owing
3.3. Effect of ethanol addition Electrospraying is known to be a promising method to produce highly monodisperse fine particles in nano- and micro-scale from a polymer solution, when the precondition of a stable cone-jet formation is reached. Apart from this factor, there are many different operating parameters that affect a great number of aspects, and the complexity of the interdependence of these aspects should be untangled to reach the working condition for a successful electrohydrodynamic process. One of the important parameters of an electrohydrodynamic process is the solvent used to dissolve the polymer. The solvent selection is a crucial step because it determines the properties of the polymer feed solution during the electrohydrodynamic process. They include surface tension and conductivity that can affect the electrospun products. Hence, a combination of solvents was employed to meet all the requirements to generate the desired electrospun products, which might not be possible with the use of a single solvent [1,6,35–37]. Fig. 3 shows the morphologies of the electrospun products generated from an 8 wt% PVP solution in DCM–ethanol mixture at room temperature and under pressurized CO2. It shows that at room temperature (Fig. 3A), the electrospun products clearly consist of spherical PVP particles without any strings or fibers. Under this condition, the physical properties of the PVP solution, such as its conductivity, surface tension, and viscosity, might change due to the added co-solvent, ethanol [1,6,35–37]. Consequently, the stability of the jet of the PVP solution increased during the process to result in a successful electrospraying process. It should be noted that the physical properties of the PVP solution were not studied in this work. In addition, the results (Figs. 1D, E, and 3A) also indicate that two phenomena take place during the evaporation of the solvents from the polymer solution. They are the increase in the polymer feed solution concentration and the commencement of nascent string formation. Ethanol used as a co-solvent seems to provide control over the DCM evaporation rate, which can affect the skin generation from the PVP solution droplets due to the increased PVP concentration of the feed solution. Owing to this condition, the generation of nascent strings could be avoided during the electrospraying process. Nguyen et al. [6] suggested that the rapid evaporation of the solvent from the polymer solution during the electrospraying process may promote the increase in the polymer concentration between the core and surface of the jet. As a result, polymer particles containing strings would be observed in the electrospun product collector. As shown in Fig. 3B, the electrospun products obviously consist of entangled PVP particles when the electrospraying process is performed in CO2 environment under a pressure of 1 MPa. It is well known that one of the principal factors that affects the electrospraying process is the viscosity of the polymer feed solution, which usually depends on the molecular weight and concentration of the polymer. At a fixed molecular weight of the polymer, there are no or few polymer chain–chain interactions at a low concentration because the molecular balls of polymers are loose and might be isolated by the solvent. Therefore, there is no entanglement between the polymer molecules. As the concentration of the polymer feed solution increased, the viscosity of the 48
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Fig. 3. SEM images of electrospun products generated from an 8 wt% PVP solution in DCM–ethanol at room temperature and under pressurized CO2 at various pressures.
to the increased PVP feed solution concentration. As a result, the PVP chain entanglement increases and the generation of nano and microscale strings is promoted.
3.4. Hollow-structured electrosprayed products During the electrospraying process, the solvent evaporates from the polymer solution boundary of the jet of the polymer feed solution. Simultaneously, the solvent diffuses from the center to the boundary of 49
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Fig. 4. Solid and hollow structures of electrosprayed particles.
precipitation of the PVP solute from the PVP feed solution. Subsequently, the PVP feed solution solidifies or precipitates from the liquid solution zone to the solid fluid zone via the liquid–solid and liquid–fluid two-phase zones and the solid–liquid–fluid three-phase zone. When the PVP feed solution droplets approach supersaturation, a very quick phase shift occurs to generate a PVP solid phase, and then the PVP polymer precipitates as particle-free fibers on the electrospun collector. As shown in Fig. 4, the beautiful PVP electrosprayed particle product was also generated when the electrospraying process was carried out at a pressure of 4 or 5 MPa. Under this condition, as the operating pressure is increased to 4 or 5 MPa, the CO2 density increases, resulting in improved CO2 solvating power. As a result, the CO2 solubility in the DCM–ethanol–rich zone and the DCM–ethanol solubility in the CO2-rich gas zone are improved, leading to an increase in the concentration of CO2 in the PVP feed solution jets. Then, the PVP–DCM–ethanol solution leads to spinodal decomposition, producing a PVP–DCM–ethanol network solution accompanied with CO2-rich bubbles. As the jet moves to the electrospun product collector, the CO2rich bubble expands against the PVP-rich network and the bubble drives the PVP-rich phase radially outward, against the jet inner skin to generate a particle with a hollow core structure [18]. Accordingly, the results may point at the modulation of the thermal behavior of the polymer through the adjustment of the CO2 pressure at the constant temperature. However, note that the phase-equilibrium behaviors in relation to the different physical properties of the PVP polymer, DCM–ethanol solvent, and CO2 during the electrospraying experiments were not investigated. Despite the powerful technique and the simplicity of electrospraying for generating fine particles from a polymer feed solution, several process parameters restrict the productivity of this technique. In most cases, controlling the size and morphology of the electrospun products, which may affect their properties and the final application, are the most important parameters to be controlled in the electrospraying process. These two key parameters depend on the solution parameters (i.e.,
the polymer solution jet. Hence, these two processes may promote the formation of a polymer skin layer via the solidification process [41]. Owing to this phenomenon, the morphology of the formed particles is highly influenced by the physicochemical properties of the solvent used to prepared the polymer solution; therefore, a proper choice of solvents to dissolve the polymer at certain conditions represents a critical step to generate electrosprayed particles in nano- and micro-scale. In most cases, a polymer solvent with a high boiling point and low vapor pressure will drive the formation of small-sized particles with a smooth skin owing to lower polymer chain entanglement, while a polymer solvent with a low boiling point and high vapor pressure may lead to particles with a porous skin and even hollow structure owing to its faster evaporation rate [18,19,42–44]. The rapid solvent evaporation rate might also reduce the period that the polymer chains require to reform within the droplet during the quick solidification process. Fig. 4 shows high-magnification SEM images of the electrospun products generated from an 8 wt% PVP feed solution in DCM–ethanol at pressures of 3, 4, and 5 MPa, respectively. Obvious differences between the morphologies of the PVP electrosprayed particles are observed in the cross-sectional images. The PVP electrosprayed products generated at a pressure of 3 MPa clearly have a solid core while those produced at pressures of 4 and 5 MPa have a hollow-core structure. As mentioned before, under dense conditions, CO2 has the power to capture DCM–ethanol from the PVP feed solution jet, and it also dissolves in the PVP feed solution jet and induces the PVP–DCM–ethanol solution to phase separate. This may drastically promote the PVP jet skin formation within milliseconds, and the DCM–ethanol solvent is removed well on this time scale. Thereafter, the formation of PVP particle boundaries and structures by phase splitting might happen [18]. The following reasons may explain the observed features: at 3 MPa, the mutual diffusion entry and exit of the PVP feed solution drop take place during the electrospraying process. Mass transfer occurs between the DCM–ethanol system and dense CO2 acting as an anti-solvent, owing to the solvating power of CO2, to result in phase splitting and 50
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[3,45–47]. In addition, although CO2 is a very poor solvent for PVP, CO2 still possesses the capability to expand or to swell PVP up to several mass percentages under high pressures [48]. Another possibility is, under dense CO2 environment, the initial PVP feed solution droplet from a blunt-ended metal needle may swell due to the CO2 diffusion into the droplet during the electrospraying process. When the CO2 pressure in the electrospraying chamber is increased, the lifetime of the PVP feed solution droplet may decline because the droplet may shrink as the CO2–DCM–ethanol mixture evaporates to affect the solidification process. Simultaneously, a strong electric field is produced and accumulated at the blunt-ended metal needle. Hence, as the PVP feed solution starts to flow out from the blunt-ended metal needle, the shrunk droplet would be surrounded by the electrically charged ions. In general, when the electrostatic force is stronger than the surface tension of the polymer solution, the polymer solution may split from the blunt-ended metal needle to form a spray and will reach and land on the electrospun collector surface as particles. However, since electrospraying was carried under pressurized CO2, the rapid vitrification of the PVP feed solution jet skin occurs. At the same time, the mutual diffusion of CO2 on the PVP matrix also occurs to result in the expansion or swelling of the PVP electrosprayed particles [17,18,26,49]. This mutual diffusion seems to occur more intensively at higher CO2 pressures, and hence, the average diameter of the PVP particles increased with increasing CO2 pressure.
Fig. 5. Diameter distribution of electrosprayed particles generated from 8 wt% PVP in DCM–ethanol at room temperature and under pressurized CO2.
concentration, polymer molecular weight, solvent permittivity), the processing parameters (i.e., flow rate, applied voltage, distance between the needle and collector), and the environmental parameters (temperature, pressure, and relative humidity) [1–3]. In this work, although these parameters were not investigated intensively during the fabrication of PVP particles from the PVP feed solution by the electrospraying technique under pressurized CO2, the use of CO2 in the electrospraying chamber to replace air as the medium seems to significantly influence the size and morphology of the PVP electrosprayed particles, which are shown in Figs. 3, 4, and 5. Using the Image J 1.42 tool [31], at least 300 different PVP particles were chosen randomly from each image, which are represented in Fig. 3, and the PVP particle diameters were determined to estimate the particle size distribution. Fig. 5 shows the PVP particle diameter distribution for the electrospun products formed from 8 wt% PVP in DCM–ethanol at room temperature and under pressurized CO2. The average diameters of the PVP particles are 1.82 ± 0.69, 2.05 ± 0.73, 2.89 ± 1.24, 3.27 ± 2.12, and 4.52 ± 2.52 μm when the electrospraying experiments were carried out at room temperature and under pressurized CO2 at pressures of 2, 3, 4, and 5 MPa, respectively. The inner diameter of the blunt-ended metal needle was not varied during the electrospraying process. Clearly, bigger PVP particles were found in the electrospun product collector when the electrospraying process was carried out under pressurized CO2. At room temperature, the average diameter of the PVP particles is approximately 1.82 ± 0.69 μm, and it increases to 2.05 ± 0.73 μm when the electrospraying process is carried out under pressurized CO2 at a pressure of 2 MPa. The average diameter of the electrosprayed particles increased gradually with increasing CO2 pressure, and it could approach 4.52 ± 2.52 μm at 5 MPa CO2 pressure. This phenomenon may be attributed to the improvement in the CO2 solvating power to dissolve the DCM–ethanol solvents, which then influenced the PVP feed solution concentration. The increasing CO2 pressure may also help maintain and promote the CO2 solubility in the DCM–ethanol solvents. It is well known that solubility represents the equilibrium between a solvent and a solute, which is a key factor in the separation process involving dense CO2 as a medium. Naturally, this mutual relation may be useful in terms of separating the DCM–ethanol solvents from the PVP polymer during the electrospraying process, which can help and improve the evaporation of DCM–ethanol solvents. As informed above, with an increase in the evaporation rate of DCM–ethanol solvents, the concentration of the PVP feed solution and its viscosity increase during the electrospraying process. As a result, an increase in the CO2 pressure in this electrospun system will be followed by an increase in the diameter of the PVP particle product, with higher polydispersity
3.5. Characterization of electrosprayed products Although the solubility of PVP in CO2 under high pressure is typically quite low, interaction may still occur between CO2 and the functional groups of PVP. Normally, when the operating pressure is increased, the CO2 molecule is forced to insert between the PVP chains, resulting in increased adsorption of CO2 molecules [50–52]. Here, FT–IR spectroscopic analysis in the wavenumber range of 4000 to 400 cm−1 was employed to observe the possible modification and shifts in the PVP bands after the electrospraying process under pressurized CO2. Fig. 6 shows the FT–IR spectra of PVP raw material and PVP electrosprayed particles obtained by electrospraying at room temperature and under pressurized CO2. Some differences between the spectra of the PVP raw material and its electrosprayed particles were expected. As shown in Fig. 6, the FT–IR spectra of PVP and its electrosprayed particles are substantially similar. The spectra only differ in terms of the intensity of some peaks, indicating that the PVP electrosprayed particles generated by the electrospraying process at room temperature and under pressurized CO2 have functional groups identical to those of the
Fig. 6. FT–IR spectra of PVP raw material and PVP electrosprayed particles. 51
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PVP raw material. Nevertheless, the intensity of the band between 1654.92 and 1650.61 cm−1 in spectrum C is sharper than in spectra A and B. This suggests interaction between CO2 molecules and carbonyl functional groups that exist in the PVP structure. It seems that under high pressure, CO2 molecules diffuse strongly into PVP matrix and interact with its functional groups; hence, the H-bonding between the PVP functional groups and CO2 molecules is superior to interactions between the DCM–ethanol solvents and PVP carbonyl functional group. Accordingly, several potential interactions might occur between CO2 and PVP (i.e., sorption of CO2 by PVP, swelling of PVP by CO2, dissolution of PVP/CO2 in CO2/PVP) [52,53]. However, the FT–IR spectral characteristics of the PVP electrosprayed particles indicate that the electrospraying process under pressurized CO2 does not shift the bands of the PVP functional groups. Next, based on the results, it could be stated that the PVP electrosprayed particles generated by the electrospraying process under pressurized CO2 have same structure as the starting material.
[15] [16] [17] [18]
[19] [20] [21] [22] [23] [24] [25]
4. Conclusions
[26]
Nano and microscale PVP particles were fabricated by an electrospraying process under pressurized CO2 at 17 kV at a temperature of 30 °C and pressures of 1–6 MPa. At a low PVP feed solution concentration (4 wt%), the PVP electrospun products have irregular shapes and merged with each other. On the contrary, increasing the PVP feed solution concentration (6 and 8 wt%) resulted in spherical particles as main products when electrospraying was performed at room temperature. At 1–3 MPa pressure and 8 wt% PVP solution concentration, the PVP electrospun products seemed to have spherical particle morphologies. At the same feed solution concentration, increasing the CO2 pressure up to 6 MPa resulted in particle strings and strings as main products. The addition of ethanol seemed to help control the DCM evaporation rate during the electrospraying process under pressurized CO2, resulting in hollow electrosprayed PVP particles. The results indicate that CO2 may expand or swell PVP particles up to several mass percentages under high pressure conditions, and consequently, the average diameter of the PVP electrosprayed particles increased with increasing CO2 pressure. The FT–IR spectral analysis indicated that the PVP structure did not change after the electrospraying process.
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