Theoretical selection of solvent for production of electrospun PMMA fibers with wrinkled surfaces

Abstract

A Bagley graph is a 2D solubility chart that contains the Hansen solubility parameters of polymers and solvents to provide a systematic and theoretical method for selecting solvents during electrospinning. The interaction radius of the solubility circle of poly methyl methacrylate (PMMA) is 8.59 MPa^0.5. In the Bagley graph of PMMA, ethanol is located outside of the solubility circle and is therefore a non-solvent for PMMA. In the current study, ethyl acetate (EA) was chosen as the solvent for electrospinning PMMA to investigate the effects of variations in the applied voltage and solution concentration on spinnability and fiber morphologies. Statistical analysis showed that varying the applied voltage (14–20 kV) and solution concentration (6–8%) does not produce a significant difference in average fiber diameter. The surface of the PMMA fiber was irregularly wrinkled due to spinodal decomposition from thermal induced phase separation. Software developed with the Delphi 7.0 programming language was used to predict the liquid–liquid phase separation of mixing systems. Conclusively, the wrinkling was caused by the thermodynamic instability of the polymer solution undergoing rapid volatilization of EA.

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1. Introduction

An understanding of the physical properties of materials is essential to their utility in applications, moreover, electrospinning provides a bridge to learn the properties of fibers which transit from bulk materials to nanofibers.¹ Micro- and nanofibrous materials have been of immense interest because of their great potential in a number of applications including tissue engineering,^1,2 drug delivery,³ and catalysis.⁴ Recently, electrospinning has emerged as a reproducible and simple method for producing continuous fibers with diameters ranging from nanometers to several micrometers.^5,6 The basic principle of electrospinning is based upon the application of a high voltage to induce the formation of charged polymer jets, which are continuously stretched by electrostatic repulsions and then solidified to produce polymer fibers.^7–9 The process parameters (applied voltage, solution feed rate, tip-to-collector distance (TCD) etc.), solution properties (polymer molecular weight, solution concentration, solvent etc.) and the environmental conditions (temperature and relative humidity) of electrospinning significantly influence the fiber diameter, morphology and structure.^10,11 It has been reported that beads, beads-on-string, flat ribbon-liked fibers, cylindrical fibers, and branched fibers, called the fibers with primary structures, can be fabricated with electrospinning.^12,13 In general, electrospun nanofibers usually have a solid interior, circular cross section and smooth surface. Megelski et al.¹⁴ found that porous and wrinkled surfaces called secondary or hierarchical fiber structures emerged, when highly volatile solvents were used. The secondary structure remarkably changes the surface area of fibers, which is extremely important for functionalization of the fibers and could contribute to the expansion of industrial applications of electrospun materials.

It is generally believed that solvent properties influence not only polymer spinnability but also fiber morphology and diameter,^8,15 however, selecting a suitable solvent for electrospinning a given polymer is generally based on experience with similar polymer systems or on solubility models limited by the current physico-chemical database.¹⁶ Thus, solubility parameters are good indicators for choosing solvents for fabrication of nanofibers by electrospinning.^17–19 Ghorani et al.²⁰ chose N,N-dimethylacetamide (DMAc) and acetone as the mixing solvent for electrospinning cellulose acetate (CA) due to an appropriate similarity in solubility parameters of solvents and the polymer. Similarly, solubility parameters have been applied to choose the solvent for electrospinning other polymers, including poly methyl-phenyl-siloxane (PMPS)²¹ and ethyl cellulose.¹⁹ Solubility parameters can also facilitate the choice of compatible solvents for core and shell in coaxial electrospinning.^22,23 Lubasova et al.²⁴ put forward a theoretical approach for the choice of appropriate solvents for polyvinyl butyral (PVB) based upon Hansen solubility parameters (HSP). Previous work has abundantly used complicated Hansen 3D solubility graph. Other studies have utilized 2D graphs to reduce the complexity of choosing solvents, including Crowley's solubility maps and Lubasova's research which omitted one of Hansen's solubility parameters.²⁴ Compared with these approaches, Bagley graph benefits from having all three Hansen solubility parameters presented on a simple 2D graph to provide a more straightforward analysis of polymer and solvent compatibility.^22,25,26

PMMA, an amorphous, transparent thermoplastic polymer, has been utilized in light scattering and propagation materials,²⁷ bone implants and prosthetics,²⁸ biological adsorbents in water treatment,²⁹ due to its high transparency, heat resistance, compatibility with human tissue and freeze resistance. In recent years, PMMA has been electrospun with different solvents, including acetone,³⁰ dichloromethane (DCM),³¹ tetrahydrofuran (THF),¹⁴ N,N-dimethyl formamide (DMF)^29,32 and chloroform.³³ However, to the best of our knowledge, there are still few reports on a theoretical approach for choosing an appropriate solvent for electrospinning PMMA. In addition, little attention has been devoted to the PMMA/ethyl acetate (EA) system and formation of electrospun PMMA fibers with hierarchical structure. The objective of the present study is to introduce a 2D solubility graph, namely Bagley graph, to serve as a tool for choosing solvent systems for electrospinning a given polymer. Using this approach, EA was chosen as the solvent for electrospinning PMMA and the spinnability of the PMMA/EA system was investigated. Based on the Flory–Huggins model, software developed with the Delphi 7.0 programming language was used to predict the liquid–liquid phase separation of mixing systems. Thus, another objective of this work was to utilize the phase diagram to analyse the receivable and opportune mechanism of the formation of secondary structures on PMMA fibers.

2. Plotting of solubility graph

Hildebrand solubility parameter (one-dimensional solubility parameter)^34,35 fall short with particular materials and polar solvents, such as nitromethane and ethanol. Consequently, Hansen solved this problem by dividing the Hildebrand solubility parameter into three partial parameters: dispersion force component (δ_d), polar force component (δ_p), hydrogen bonding component (δ_h), called Hansen three-dimensional solubility parameters according to eqn (1) in units of MPa^0.5.^36,37 According to Hansen, an approximately spherical area of solubility may be fabricated in a three-dimensional sphere system of these three components. Solvents that lie within the solubility sphere of a given polymer are likely to dissolve the polymer. On the contrary, the poor solvents are located outside of the sphere. The radius of the polymer solubility sphere is an interaction radius, R, which has already been calculated empirically and is readily available.^36,38

As mentioned above, a Bagley graph containing all the components of HSP without imprecise omission was proposed to predict the solubility of PMMA in a range of solvent. Based on the similar component effects of dispersion force and polar force, a new parameter, δ_v, was put forward, as shown in eqn (2).²⁵ The interaction radius of the PMMA solubility sphere is 8.59 MPa^0.5.³⁶ All of the Hansen values used for this research were taken from the handbook written by Hansen,³⁶ listed in Table 1. The Bagley diagram of PMMA was constructed with a radius of 8.59 and is graphically represented as in Fig. 1. It is exhibited that only the point standing for ethanol lies outside the solubility sphere, indicating that PMMA and ethanol are immiscible. PMMA fibers have been previously electrospun from a range of different solvents, which verifies the solubility and spinnability of the systems, listed in Table 2. It is revealed that PMMA fibers with beads and uniform fibers with ribbon-shaped and circle cross section were obtained with different solvents. In addition, the molecular weight, solution concentration, and solvent have significant influences on the fiber diameter and morphologies. Our purpose for the selection of EA using the Bagley graph is to examine whether EA can dissolve PMMA and allow for subsequent electrospinning. Accordingly, EA was utilized to fabricate PMMA fibers by electrospinning. For the sake of determining the mechanism of hierarchical structure on fibers, all the electrospinning processes were performed at a temperature of 293 ± 3 K and relative humidity of 12 ± 2%.

δ² = δ_d² +δ_p² +δ_h² (1)

Table 1 Components of solubility parameters and properties of materials^36,39

	PMMA	DMF	DCM	Acetic acid	EA	Ethanol	THF	Chloroform
δd (MPa^0.5)	18.64	17.4	18.2	14.5	15.8	18.8	16.8	17.8
δp (MPa^0.5)	10.52	13.7	6.3	8.0	5.3	8.8	5.7	3.1
δv (MPa^0.5)	21.40	22.2	19.3	16.6	16.7	20.8	17.7	18.1
δh (MPa^0.5)	7.51	11.3	6.1	13.5	7.2	19.4	8	5.7
δ (MPa^0.5)	22.68	24.9	20.2	21.4	18.2	28.4	19.5	18.9
Boiling point (K)	—	426	313	391	350	351	339	334
Dielectric constant (293 K)	—	36.7	9.1	6.2	6.02	22.4	7.6	4.8
Electrical conductivity (S m^−1)	—	6 × 10^−8	4.3 × 10^−11	6 × 10^−9	1.0 × 10^−9	1.4 × 10^−9	4.5 × 10^−5	<1.0 × 10^−10
Dipole moment (Debye)	—	3.8	1.8	1.7	1.7	1.7	1.75	1.1

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Fig. 1 Bagley solubility graph of PMMA.

Table 2 Electrospinning PMMA with different solvents

Reference	PMMA		Solvents	Average fiber diameter (μm)	Fiber morphology
	M̄W (g mol^−1)	Concentration (%)
Piperno et al.^30	12 000	2	Acetone	2	Ribbon-shaped fibers with smooth surface
Megelski et al.^14	540 000	10	THF	10	Cylinder fibers with porous surface
Wang et al.^32	110 000	26	DMF	0.875	Ribbon-shaped fibers with smooth surface
Dayal et al.^31	350 000	10	DCM	—	Cylinder fibers with porous surface
Qian et al.^33	350 000	3.9	Chloroform	—	Fiber with beads and porous surface
	350 000	4.2	2,2,2-Trifluoroethanol	—	Cylinder fibers with porous surface
	350 000	3	1,1,1,3,3,3-Hexafluoro-2-propanol	—	Cylinder fibers with winkled surface
Liu et al.^28	120 000	30	THF–DMF (1 : 1, w/w)	0.97	Cylinder fibers with smooth surface
Bae et al.^29	350 000	21	DCM–DMF (1 : 1, w/w)	—	Cylinder fibers with porous surface

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3. Experimental work

3.1. Materials

Poly methyl methacrylate (PMMA, M̄_W = 500 000 g mol⁻¹) was purchased from Mitsubishi Rayon Polymer Nantong Co., Ltd. EA was purchased from Sinopharm Chemical Reagent Co., Ltd.

3.2. Electrospinning process

Solutions of different PMMA concentrations (6%, 7%, 8%) dissolved in EA without further purification and stirred at ambient temperature with sufficient stirring for 6 h. A self-assembled electrospinning apparatus consisted of a high voltage power supply (BGG DC high-voltage generator) purchased from the BMEI Co. Ltd. (Beijing, China) and a digitally controlled syringe pump purchased from Longer Pump Baoding Co., Ltd. was introduced to produce PMMA fibers.

During electrospinning, the PMMA solution for electrospinning was transferred into a 10 mL glass syringe with the syringe pump at a constant rate. The electrospun fibers were collected on a piece of aluminium foil placed at a given distance. All the electrospinning processes are performed under 293 ± 3 K and relative humidity of 12 ± 2%.

3.3. Characterization

The morphological appearance of the electrospun PMMA fibers was investigated visually with a scanning electron microscope (SEM), model JSM-6390. Each sample was coated with gold for 90 s prior to being observed under the SEM. The average diameter of PMMA fibers was determined by analyzing the SEM images by Image Pro Plus 6.0 (Media Cybernetics, USA).

4. Results and discussion

4.1. Influence of operation parameters

It is demonstrated that solution concentration and applied voltage play pivotal roles on fiber diameter and morphology in electrospinning.^40,41 Therefore, these two parameters were varied to investigate their effects on the spinnability of the PMMA/EA system and their influences on fiber morphology and structure. Given a constant applied voltage (18 kV), feed rate (1.2 mL h⁻¹) and TCD (15 cm), SEM of PMMA fibers from varied polymer concentration from 6–8% was exhibited in Fig. 2. It is demonstrated that solution concentration influenced electrospun fiber diameter, distribution and mat structure. This observations are consistent with the previous studies on electrospinning.^42,43 The most distinctive difference of the fibers shown in Fig. 2 is that the 6% concentration solution produced fibers with beads, whereas the 7% and 8% solutions produced continuous and uniform fibers with large diameter distribution (deviation of 1.74 μm and 1.32 μm, respectively). At the low concentration of 6%, the viscosity of solution is too low, consequently, beads on fibers emerged due to Rayleigh instability during the stretching of the spinning jet.⁴¹ The PMMA fibers obtained from the 7% concentration have a wide range of fiber diameters, from 1 μm to 9 μm with an average fiber diameter of 3.84 μm (Table 3). The higher concentration of 8% produced fibers with a diameter deviation less than that of 7% with an average diameter of 3.89 μm. The results of a paired t-test on concentration effects are listed in Table 3, which suggest that there is no statistically significant difference in fiber diameter when the solution concentration is increased from 7% to 8%. In addition, Fig. 2(d) provides a full and intuitive observation of the wrinkled structure on the surface of resultant PMMA fibers when electrospun with EA at a concentration of 8%.

Fig. 2 (a) SEM of PMMA fibers from 6%; (b) SEM of PMMA fibers from 7%; (c) SEM of PMMA fibers from 8%; (d) high magnification (×5000) SEM of PMMA fibers from 8%; parameters: voltage of 18 kV; distance between TCD of 15 cm; solution rate of 1.2 mL h⁻¹.

Table 3 Statistical assessment of effect of solution concentration on fiber diameter

Solution concentration (%)	Mean (μm)	Standard deviation	p-value of paired sample t-test
7	3.84	1.74	7% vs. 8% 0.898
8	3.89	1.32

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7% PMMA solutions were chosen to investigate the influence of applied voltage on solution spinnability. SEM of PMMA fibers electrospun at different voltages while all other parameters were kept constant are illustrated in Fig. 3. All of the fiber mats present a fibrillar structure without beads, however, portions of the fibers were redissolved by the non-volatilized solvent (marked by the white arrow in Fig. 3(a)). As the applied voltage increases, redissolving fibers mitigate and eventually disappear, which is demonstrated by a lack of fiber fusion. It is depicted that the fibers have irregularly wrinkled morphology with a cylindrical cross section and visibly varied diameter, seen in Fig. 3(c) and (d). Electrospinning involves excessive processing including solvent diffusion, solvent evaporation, heat transfer and mutual diffusion of polymer–solvent molecules besides operation variables.⁴⁴ The competition between solvent evaporation on the surface and solvent diffusion from the core towards the surface determines the section structure of electrospun fibers. With a high volatility solvent (DCM) in Qian's study,³³ the speed of solvent evaporation on the surface was faster than that of solvent diffusion, giving a ribbon-like cross section. Conversely, the surface evaporation of EA in the present study was slower than the solvent diffusion from the core, creating circle cross section. The influence of applied voltage on fiber diameter is exhibited in Table 4. The average fiber diameter is not remarkably changed with variations in the applied voltage, which reveals some similarities to the investigation of voltage effects on fibers by Lou et al.¹⁰ In addition, the paired sample t-test presented in Table 4 demonstrates that the p-values of 16 kV vs. 18 kV, 18 kV vs. 20 kV, and 16 kV vs. 20 kV are 0.262, 0.288, and 0.762, respectively, indicating that there is no statistically significant difference between fibers obtained from the different applied voltages. However, the distribution of fiber diameter narrowed with high voltage of 20 kV (standard deviation = 0.664, displayed in Table 4), which can be attributed to the uniform stretching promoted by the high electric field.

Fig. 3 SEM of PMMA fibers obtained by electrospinning solution with concentration of 7% at solution rate of 1.2 mL h⁻¹, TCD of 10 cm, applied voltage at: (a) 14 kV; (b) 16 kV; (c) 18 kV; (d) 20 kV.

Table 4 Statistical assessment of effect of applied voltage on fiber diameter

Applied voltage (kV)	Mean (μm)	Standard deviation	p-value of paired sample t-test
16	3.66	1.13	16 vs. 18 0.262
18	3.43	1.33	18 vs. 20 0.288
20	3.61	0.664	16 vs. 20 0.762

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4.2. Wrinkled surface formation mechanisms

The generation of hierarchical structures (pores, wrinkled surfaces, irregular grooves, etc.) on electrospun fibers has been attributed on different occasions to “breath figure”^45–47 arising from water condensation on polymer surface and phase separation^48,49 resulting from solvent-rich phase ultimate volatilization. Honarbakhsh et al.⁵⁰ considered that “breath figure” and thermally induced phase separation (TIPS) were mutually accounted for the irregular surface formation during electrospinning. In the breath figure model, the patterns form when water droplets condense on a cold surface and ordered in a hexagonal array,^45,51 after which the water volatilizes with the solvent and micron-sized pores are formed. However, in this study only wrinkles were obtained on the fiber surface. Additionally, gravity acts as the driving force to keep water attached to the surface, indicating that pores could be created only on the lower side of the fiber when electrospin horizontally. However, as shown in Fig. 2(d), the entire surface of the fibers was irregularly wrinkled, which is impossible to achieve even considering the fiber rotating in electrospinning. In summary, the breath figure model is not applicable in this system.

The formation of nano- and micro-structures has also attributed to phase separation in the process of membrane preparation. Two models for phase separation, called thermally induced phase separation (TIPS) and vapor induced phase separation (VIPS), are the most relevant mechanisms for hierarchical structure formation on fibers during electrospinning. In VIPS, phase separation of the polymer solution is induced by penetration of water vapor into the solution.⁵² Casper et al.⁴⁸ believed that TIPS and VIPS have a synergistic effect on formation of fiber structures during electrospinning of polystyrene (PS) with varied molecular weight and humidity. PMMA has been electrospun with different solvents and electrospinning parameters with the results summarized in Table 2. The properties of solvents have a profound effect on the evaporation rate and drying time which determine fiber hierarchical morphology. It is revealed that PMMA fibers electrospun from highly volatile solvents, including DCM, THF, and chloroform, had pores on their surface. The model of nucleation growth in TIPS, induced by the rapidly volatilizing of solvent, is attributed to the formation of pores on fibers. In Bae's study,²⁹ it was demonstrated that hierarchical structures of PMMA fibers emerge when the relative humidity was above 26%, and the formation of hierarchical structures was attributed to VIPS. VIPS is inadequate and essentially hindered by a dry atmosphere,⁵³ indicating that it is not a plausible mechanism for the generation of irregular surfaces at the low relative humidity of 12% in the current study. The evaporation speed of EA accelerates at lower humidity than our previous study, but the vapor pressure of EA (8.0 kPa at 289.6 K (ref. 54)) indicates that there is still adequate time for fiber stretching. It is reasonable to speculate that TIPS induced by rapid evaporation of solvent is responsible for the formation of the observed wrinkled PMMA fiber surfaces. During electrospinning, these situations of decreasing temperature and loss of solvent occur with EA evaporation, giving rise to thermodynamic instability of the PMMA solution, which is the driving force of phase separation. Afterwards, the solution separates into polymer-rich phase and solvent-rich phase, and the polymer-rich phase forms a solid fiber matrix while the solvent-rich phase leaves grooves by leaching solvent.

The mass transfer in TIPS is an essential aspect of phase behavior of polymer solutions, giving way to the evolution of varied fiber morphology. Dayal and coworkers^31,55 reported a theoretical study on the spatial and temporal evolution of fiber morphology with mass transfer by the Cahn–Hilliard phase field approach and Flory–Huggins free energy of mixing. Rong's group⁵⁶ developed software with Delphi 7.0 to improve predictions of the phase equilibrium of polymer systems using the Flory–Huggins model. This software was applied to predict several mixtures, such as poly lactic acid (PLA) polymer blends and poly ethylene (PE) and ethanol, revealing high consistencies with experimental results.^56,57 An understanding of the impact of the interaction parameters of PMMA and EA (χ_ij) in the Flory–Huggins model is required for predicting phase behavior of the binary system and calculating bimodal and spinodal lines. According to the regular solution theory, interaction parameters (χ_ij) can be evaluated with solubility parameters described by eqn (3),^58,59 where V_m is volume of lattice, and δ_t,i and δ_t,j are solubility parameters components of the polymer and solvent, respectively. For systems where dispersion forces dominate over polar and hydrogen-bonding forces, Lindvig et al.⁵⁸ expressed χ_ij with Hansen three-dimensional solubility parameters from eqn (4), where V_m is molar volume of solvent, and δ_d, δ_p and δ_h are the Hansen solubility parameters of polymer and solvent, respectively. Lindvig and coworkers^58,60 demonstrated that the interaction parameter described by eqn (4) is more suitable for the PMMA system than eqn (3).

In this study, the interaction parameter of PMMA and EA was calculated by eqn (4). The prediction result of the phase diagram by Rong's group using Flory–Huggins free energy of mixing model with software developed with the Delphi 7.0 programming language is represented schematically in Fig. 4. The detailed calculation and program compilation were described in Rong's article.^56,57 The phase diagram is divided into three regions, the stable homogeneous phase region, the metastable region and the unstable two-phase region defined by the binodal and spinodal curves. At temperatures above the critical point, the solution is homogeneous, as shown in Fig. 4. It is well known that spinodal decomposition (SD) and nucleation growth (NG) occur during liquid–liquid phase separation.³¹ Nucleation growth proceeds in the metastable region, resulting in isolated cellular pores on the polymer matrix, while spinodal decomposition occurs in unstable regions to produce irregularly wrinkled surfaces.⁶¹ During the electrospinning progress, the composition (Φ) raises instantaneously with the evaporation of EA. Simultaneously, the PMMA jet passes through the binodal and spinodal curves. The formation of wrinkled PMMA fiber surfaces is the result of the process of spinodal decomposition, shown in Fig. 4.

Fig. 4 A schematic representation of a binary phase diagram of PMMA/EA solution showing a liquid–liquid demixing gap.

By carefully analyzing the current results and considering previous work, a plausible mechanism of wrinkled surface formation is illustrated in Fig. 5. At a low relative humidity (12% in this study), air flow is created as the jet travels towards the collector. The vapor of EA saturates the nearby region of the jet–air interface, which effectively hinders the little water vapor in air from diffusing towards the interface. Simultaneously, the spinodal decomposition in TIPS caused by evaporation of solvent occurs, as shown in Fig. 5(b). Furthermore, the radial electrical charge aids the solvent in escaping from the fiber surface to accumulate the phase separation.^50,62 Eventually, solid fibers with wrinkled surfaces are obtained after EA volatilizes and the polymer solidifies. In the case of high humidity, water radially towards the fiber surface during electrospinning influences the interior fiber structure.⁵³

Fig. 5 Schematic representation of: (a) electrospinning process, (b) fiber evolution process.

5. Conclusions

Bagley graph, a 2D solubility graph, was used to choose a solvent for electrospinning PMMA. The center of the solubility circle in the Bagley graph is composed the solubility parameter components (δ_v, δ_h) of PMMA, and the radius of 8.59 MPa^0.5 is the interaction radius. Notably, only ethanol lies outside of the solubility circle, indicating that it is the non-solvent for PMMA. It is reported that PMMA fibers have been obtained by electrospinning from several solvents, including DCM, THF, and DMF. In this work, EA was selected as the electrospinning solvent to investigate the spinnability. Based on our previous work, the influence of applied voltage and solution concentration on fiber diameter and morphology was investigated. According to the statistical analysis, there was no significant variation in average fiber diameter with changes in applied voltage (14–20 kV) or solution concentration (6–8%). However, the surfaces of the PMMA fibers obtained in this work were irregularly wrinkled. The mechanism responsible for the wrinkled surface was elucidated with schematic of the proposed phase separation and fiber evolution process. The formation of wrinkled surfaces is due to the thermodynamic instability of the polymer solution undergoing the rapid volatilization of EA in low relative humidity. In summary, the thermally induced phase separation is responsible for the generation of wrinkled surfaces on PMMA fibers.

Acknowledgements

The authors would like to acknowledge the following financial supports: National Natural Science Foundation of China (no. 21376186), the Ministry of Education (Doctoral Special Research Foundation no. 20110201110032), China and the Fundamental Research Funds for the Central Universities (New Teacher Research Support Plan no. 08141002, International Cooperation Project no. 2011jdhz37 and Integrated Cross Project xjj2014136 in Xi'an Jiaotong University), Natural Science Basic Research Plan in Shaanxi Province of China (no. 2012JM2010), and the Ministry of Human Resources and Social Security of China (Sci. & Tech. Project for Overseas Scholars, no. 19900001).

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