J. Phys. Chem. C 2010, 114, 21201–21213
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A Solution Selection Model for Coaxial Electrospinning and Its Application to Nanostructured Hydrogen Storage Materials Zeynep Kurban,† Arthur Lovell,*,†,‡ Stephen M. Bennington,†,‡ Derek W. K. Jenkins,§ Kate R. Ryan,| Martin O. Jones,‡,| Neal T. Skipper,† and William I. F. David‡,| London Centre for Nanotechnology, UniVersity College London, Gower Street, London, WC1E 6BT, United Kingdom, ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Science and InnoVation Campus, Didcot, Oxfordshire, OX11 0QX, United Kingdom, Micro and Nano Technology Centre, STFC Rutherford Appleton Laboratory, Harwell Science and InnoVation Campus, Didcot, Oxfordshire, OX11 0QX, United Kingdom, and Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3JA, United Kingdom ReceiVed: August 19, 2010; ReVised Manuscript ReceiVed: September 27, 2010
Coaxial electrospinning was used to encapsulate the complex hydride ammonia borane in polystyrene to improve its properties as a hydrogen storage material. A solvent selection system was developed by using the Hansen solubility parameters to facilitate the choice of compatible solvents for core and shell. This enabled systematic optimization of the parameters needed for successful coelectrospinning. This approach has general application for any multiphase electrospinning system, including ones where the core is highly conducting or nonpolymeric. The resulting fiber morphologies depend strongly on the degree of miscibility of core and shell solutions. Fibers spun from immiscible core-shell solutions had a classic coaxial structure. Fibers produced from semimiscible core-shell solutions were highly porous, with inclusions extending through the fiber and an ordered radial and longitudinal distribution of nanoscale pores on the fiber surface. We suggest that this type of porosity may be due to an instability created in the nonaxisymmetric modes at the core-shell interface, resulting in intrusion of the core into the shell polymer. These controllably porous structures have numerous potential applications including materials templating or drug delivery. In the porous fibers, the temperature of the first hydrogen release of ammonia borane is reduced to 85 °C. This result suggests a nanostructured hydride, but a large mass loss indicates that much of the ammonia borane is expelled on heating. The coaxial fibers, in contrast, appear to encapsulate the hydride successfully. The coaxial and porous fibers alike showed no significant release of borazine, suggesting two different suppression mechanisms for this impurity. 1. Introduction Electrospinning was patented by Cooley and Morton in 19021,2 and has been used industrially to make filters since the late 1930s. However, it was only during the past decade that the technique was adopted by the growing nanotechnology industry as a cheap, scalable way to produce fibers with diameters ranging from tens of nanometers to micrometers.3-6 An electric field is used to pull a fine jet of a viscoelastic solution (often a polymer solution) from a feed nozzle or roller surface, from which an ultrathin filament precipitates as the solvent evaporates. Successful control of the resulting fiber morphology and diameter is governed by a delicate balance between the process and solution parameters.7 A more complex variant of this technique, called coelectrospinning (or coaxial electrospinning),8-12 typically uses two concentric nozzles to form fibers with a core material encapsulated within a different shell material, which must be polymeric or viscoelastic.8 Coelectrospinning can produce multiphase submicrometer structures in an efficient, scalable, one-step synthesis, such that it has already gained attention for a variety of applications including biomedicine (tissue engineer* To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +44 1235 446967. Fax: +44 1235 445720. † London Centre for Nanotechnology, University College London. ‡ ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot. § Micro and Nano Technology Centre, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot. | Inorganic Chemistry Laboratory, University of Oxford.
ing, controlled drug release systems),9,13,14 filter systems, catalysis, optical applications (waveguides), or as nanocables for microelectronics.8 This strong commercial potential continues to drive basic research into the mechanisms of the process. Our motivation for the current research is to investigate coelectrospinning as a means to encapsulate metal or complex hydrides inside a sheathing nanofilament of hydrogen-permeable polymer for hydrogen storage applications. There is an increasing urgency to find stable and lightweight hydrogen storage materials for clean transport applications.15 Promising options being investigated include Mg-based alloys,16-18 metal-borohydrides,19 alanates,20,21 and amido-boranes,22 but many of these materials suffer from poor dehydrogenation kinetics and high enthalpies of formation which require high operation temperatures (typically >200 °C). These fall outside the -40 to 85 °C range preferred for fuel cell operation. Additionally, some of these materials release impurities that may poison fuel cells, some have a low degree of reversible hydrogenation and hence recyclability, and many are unstable in air. Coelectrospinning offers the potential to overcome many of these challenges: by nanostructuring a hydride we can overcome thermodynamic and kinetic limitations,23 and by encapsulating it we can contain the spent material for regeneration and protect it from oxidation. For our initial investigation we have chosen ammonia borane (AB), NH3BH3, as the hydride core material. AB is a waxy solid at room temperature and contains one of the highest absolute hydrogen densities (19.6 wt %) of any complex hydride.22,24 Two thirds of this can be obtained by
10.1021/jp107871v 2010 American Chemical Society Published on Web 11/16/2010
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decomposition at temperatures up to 150 °C, closer to the target temperature range than other complex hydrides such as the alanates.24 AB has a number of properties that make it suitable for coelectrospinning, such as high solubility in many organic and inorganic solvents (e.g., water) and its stability in air. This makes the materials handling significantly less difficult. However, the dehydrogenation process has not been shown to be easily reversible. The nanostructuring of AB in solid state matrices such as mesoporous silica SBA-1525 and carbon cryogels26 has been shown to change the decomposition pathways of AB favorably, by lowering the desorption temperature, and suppressing the formation of fuel cell-poisoning impurities such as borazine.22 However, the scaffold adds extra weight, and regeneration of the spent AB remains a challenge. We hope to address these problems more effectively through encapsulation of AB in polymer nanofibers. This would retain the improved decomposition route, supplying a fuel stream of pure hydrogen through a lightweight polymeric sheath which confines the other reaction products. The polymer shell would retain its nanostructure following desorption, increasing the potential cyclability of the system. For the sheath polymer, we require a material that is lightweight, with a high permselectivity in favor of hydrogen gas. It should be stable at temperatures up to ∼200 °C so thermolysis of the AB can take place without decomposition of the polymer. Ideally it should also be cheap and easily available in bulk. Polystyrene (PS) was chosen for the initial investigation because it has good H2 permeability, of 23.8 barrer, and a melting point of 240 °C.27 The use of coelectrospinning for encapsulating and nanostructuring hydrides for hydrogen storage applications, to our knowledge, has not been reported previously. This study reports our first experiments using this method, identifying some of the problems that may be encountered when coelectrospinning highly polar, nonpolymeric core materials such as AB, which impose severe restrictions on the choice of shell solutions that can be used. In the literature, the effect of many of the parameters that control the physical spinning mechanisms of a compound solution set, such as the solution viscosity ratio, interfacial tension, solution conductivity, solvent vapor pressure, and degree of protrusion of the core nozzle outside the shell nozzle, are relatively well reported.8,28,29 Many papers discuss the importance of the miscibility of the core and shell solutions, but little has been done to quantify the effect of the interaction between the core and shell solutions (e.g., solution miscibility) for the control of the success of the spinning and the morphology of the fibers, and we discuss this in section 3. We report here on a systematic method for selecting solvents based on the use of Hansen solubility parameters (HSPs),30 which enables efficient identification of compatible core-shell solutions with parameters optimized for successful coelectrospinning. In this paper we describe the solution preparation and discuss how the core and shell solution properties and their physical and chemical interaction can affect the morphology of the resultant fibers. Our method, which can be used for all multiphase electrospinning applications, provides a more systematic approach to selecting solvents and solution combinations than more empirical methods. 2. Coaxial Electrospinning Process In our coelectrospinning system two solutions, one a precursor for the core and one for the shell, are delivered independently through the concentric nozzles with the flow rates controlled
Kurban et al.
Figure 1. Cut-away schematic of the coaxial electrospinning setup. The coaxial fiber has been halved to show the core-shell structure.
by two separate syringe pumps. When a high enough electric field is applied, electrostatic repulsions between the surface charges overcome the surface tension of the droplet. The outer fluid is drawn into a point, known as the Taylor cone,31 and a jet issues from its tip. If the outer fluid is sufficiently viscous compared to the inner fluid and there is enough interfacial stress then the inner fluid is also drawn into a cone and a coaxial jet is formed.32 After traveling a short distance the jet undergoes a bending instability, which causes it to spiral, stretch, and thin, finally forming a nanofiber in core-sheath configuration once a large proportion of each solvent has evaporated, as illustrated schematically in Figure 1. In a stable spinning regime, a continuous fiber is produced, and gram-scale quantities of material can be generated rapidly. As for single-phase electrospinning, fiber formation can only occur if the shell solution consists of molecular chains of significant concentration and length to entangle during the spinning process. For coaxial spinning there is no requirement for the core material to be viscoelastic. Many groups have reported results with liquids such as mineral oil8 and olive oil,33 and in this paper we report on work with a nonviscoelastic solution of ammonia borane. This enables a wide variety of materials, either in melt or solution form, to be encapsulated and structured, with properties of the shell chosen to enhance their functionality. In some cases, there are benefits in adding some polymeric materials to the core solution in that it can stabilize the fibers and prevent buckling and collapse as the core solution evaporates.34 Like its single phase variant, coelectrospinning is a nonlinear process that is governed by a large number of solution and process parameters. The solution parameters include polymer
Model for Coaxial Electrospinning molecular weight, viscosity, electrical conductivity, surface tension and solvent dielectric constant, and vapor pressure. The process parameters, which depend on the solution system being used, include the voltage applied between the nozzle and collector, solution feed rate, ambient temperature and humidity, diameter of nozzle, and nozzle-to-collector distance. In coaxial electrospinning, achieving the correct balance between the core and shell solution parameters, i.e., controlling the solution chemistry and hence interaction between the two solutions, is important for ensuring entrainment of the core in the shell solution and production of fibers with the core-shell configuration. The details for achieving this are given in the next section. 3. Solvent Selection for Coaxial Electrospinning It is clear that obtaining a good core-shell structure is most likely if the solutions of the core and the shell are immiscible, as they will then remain phase-separated throughout the spinning process. However, several groups have demonstrated that polymeric core-sheath nanofibers can also be fabricated by coelectrospinning combinations of miscible polymer solutions. Sun et al.11 report the formation of core-shell polymeric fibers using different concentrations of the same solution, polyethylene oxide in water-ethanol mixture, in the core and shell nozzles. They postulate that the characteristic time of the bending instability is around 1 ms, and is significantly smaller than the characteristic time of “diffusion spreading of a sharp boundary” between two miscible polymer solutions, so that it should be possible to spin miscible solutions of two polymers before they mix. However, this misses the fact that the residence time in the Taylor cone is typically of the order of 1 s, which is considerably longer than the diffusion time of two miscible polymer solutions. Yu et al.35 also report the fabrication of coaxial fibers with miscible core-shell solution combinations, polyacrylonitrile (PAN)/dimethylformamide (DMF) and PANco-PS/DMF. They report that while the use of the same solvent in the core and shell enables lower interfacial tension, it is important for the polymer not to precipitate at the fluid interface near the nozzle. Alternatively, Li et al.36 report that mixing between the solutions could occur if they were miscible. They report obtaining hollow nanofibers from immiscible solution combinations of poly(vinyl pyrrolidone) (PVP) and Ti(OiPr)4 in ethanol as shell solution and mineral oil as the core, once the oil is extracted from the core; however, they were unable to produce hollow fibers when miscible PVP-ethanol solution was used in the core instead. McCann et al.37 report that the immiscibility of solvents in their experiments was critical to the production of well-defined core-sheath nanofibers. Both groups report obtaining fibers with porous structures when miscible solution combinations are used. We studied a range of different solution combinations as defined by our solution selection model;miscible, semimiscible, and immiscible;and investigated the effect of solution miscibility on fiber morphology. The degree of miscibility is a necessary but not a complete guide to whether coaxial spinning is possible; one must also control the relative scale of a number of other parameters.8 At the nozzle tip, a stable, liquid Taylor cone is needed to enable a consistent and continuous core-shell structure to form. In a normal case for spinning compound (core-shell) solutions, the outer solution is chosen to have a higher conductivity than the core; in this case the driving force on the fiber comes from the electrostatic forces on the charge on the other surface of the shell. Stable spinning then depends on the outer shell solution drawing out the inner core solution through shearing and contact forces, “viscous dragging”, and “contact friction”, which keep
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21203 the inner solution confined to the core while the fibers are being stretched.8,11,36 For this reason the shell solution should be electrospinnable on its own and according to the review by Moghe and Gupta8 the following conditions must be satisfied: 1. The shell solution must have a higher viscosity than the core solution such that the viscous force imparted by the shell on the core is enough to overcome the interfacial tension. 2. The shell solution must have a higher electrical conductivity than the core solution such that the surface charge density is high enough to cause the elongational force. 3. The shell solution must have a higher flow rate than that of the core, so that the viscous drag applied by the sheath solution is sufficient to confine the core solution within the cone.38 4. The vapor pressure of both solutions must be sufficiently low and comparable so both the core and shell dry at similar rates to stop fibers collapsing. 5. The two solutions must possess low interfacial tension to prevent large stresses at the core-shell interface reducing fiber stability and to produce a stable Taylor cone.35,38 3.1. Solution Selection Using Hansen Solubility Parameters. The Hansen solubility parameters are widely used in industries such as paints and coatings where understanding and controlling solvent/polymer interactions are vital for optimization of polymer dissolution in solvents. The solubility of materials depends on entropy and on the nature of the interactions between the solvent and solute molecules. These interaction strengths are quantified and tabulated for many polymers and solvents as the polar (δp), dispersive (δd), and hydrogen bonding (δh) component of the Hansen solubility parameters in units of MPa1/2.30 These three parameters can be treated as coordinates for a point in a three-dimensional (3D) space, known as the Hansen space. The fourth HSP is a radius of interaction (R0), defining a solubility sphere around these coordinates. R0 is calculated empirically for each solute.30 If the Hansen coordinates of a potential solvent (or solvent blend) lie within the solubility sphere of a solute, such as a polymer, then the solvent would be expected to dissolve the solute. Ra gives the distance of the solvent from the center of the solute solubility sphere through the equation:
Ra2 ) 4(δpd - δsd)2 + (δpp - δsp)2 + (δph - δsh)2
(1)
where δds, δps, and δhs are the HSPs of a given solvent and δdp, δpp, and δph are HSPs of a solute marking the center of the Hansen solubility sphere.30 The dispersion term is described as being twice as important as the polar- and hydrogen-bonding terms and hence more weight is put on this in eq 1.30 The relative energy difference (RED) of the system is expressed as RED ) Ra/R0; if RED < 1, the molecular interactions are alike and the system will dissolve, if RED ≈ 1 then the system will partially dissolve, and if RED > 1 the system will not dissolve, with progressively higher values of RED suggesting progressively less favorable interactions.39 If one does not have an estimate for R0, then solvents can be ranked by Ra, with the smaller Ra indicating the better solvents. Since a 3-D graphical presentation is not easy to produce, a more practical method that involves the use of a twodimensional (2-D) plot was instead used in this investigation. In this model the polar (δp) and dispersive (δd) HSPs are combined to give a new parameter δv ) (δd2 + δp2)1/2, which is plotted against δh to produce a 2-D graphical representation of the solubility space. This is a good approximation of the 3-D
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Figure 2. Two-dimensional (δv vs δh) diagram showing the extent of solubility of polystyrene (PS) [b] in various chosen solvents. The solvents and solvent blends used in this study are marked with triangles [black and red, respectively]; sMS-1,2 ) 7:1.2,1 mass ratio of toluene: DMF; IS-1)3:1:1 volume ratio of toluene:DCE:PF; IS-2 ) 7:2:1 volume ratio of DCE:NB:PF. Those that are nonsolvents for PS but good solvents for AB are marked with a cross [×]. Other potentially good solvents for PS are marked with diamonds ((), these include chloroform, diethyl ether, benzene, ethyl benzene, p-diethylbenzene, tetrahydrofuran, styrene, o-xylene, chlorobenzene, o-dichlorobenzene, and carbon disulfide.
model as thermodynamic considerations led by Bagley et al. are reported to show that the effects of δd and δp display close similarity.40 The solubility region for a given material is again delimited by a circle with a radius R0. In the case of polystyrene, the coordinates of the center of the circle correspond to the solubility parameters δv ) 18 and δh ) 5 MPa1/2 and R0 ) 5,40 as shown by Figure 2; the circle appears elliptical due to the axis scales. To identify those solvents that can dissolve PS, the HSPs of a selection of solvents are plotted on the 2-D graph and any within the Hansen solubility circle are considered good solvents for PS. Although the exact HSPs for AB are unknown, the polar nature of AB gives it an affinity for solvation in polar solvents, such as water, DMF, and DMSO. Using this plot we were able to choose core-shell solvent combinations that were miscible, semimiscible, or immiscible. Water, for example, has a very high δh parameter (42.3 MPa1/2) and lies far from the PS solubility circle (Figure 2). It is therefore expected to be immiscible with those solvents in the circle and a nonsolvent for PS. Thus, it was used as a solvent for AB for the immiscible AB-PS solution combination. This method has the added advantage of efficiency and increased practicality over other solvent selection methods, such as the Teas graph method,41 which rely on time-consuming empirical analyses to identify good solvents for a polymer. 3.2. Solution Preparation. It is highly unlikely that any two pure solvents will have the correct miscibility as well as the correct core/shell ratio of conductivity, dielectric constant, viscosity, and vapor pressure necessary for coaxial electrospinning. Almost invariably mixtures of solvents will have to be used to obtain the required ratios. In this section we describe how to predict the properties of solvent mixtures to create the optimum solution sets for any given core and shell material. To assist in this solvent selection process, a solvent matrix containing all relevant parameters (such as HSPs, RED value for PS, conductivity, dipole moment, polarity, dielectric constant,
Kurban et al. and boiling point)30,42,43 was constructed; these values are tabulated separately in Tables 1 and 2 for the pure solvents and the solvent blends used in this study. Electrical conductivity was measured with a Jenway 4510 conductivity meter at room temperature; other values are standard. From this matrix we were able to determine the miscible, semimiscible, and immiscible solution set combinations used in the study. 3.2.1. Hansen Solubility Parameters. The specific solvent ratio used in the mixtures was chosen to keep the overall HSPs of the solvent mixture within the Hansen circle. The HSPs of the mixtures are calculated by using δnmix ) ∑iaiδni, where n represents the parameter type (p, d, or h) and ai is the volume fraction of solvent i. 3.2.2. Viscosity. Solution viscosity, η, is particularly important for the shell, partly to provide sufficient shear strength so that the electrostatic forces pull the whole fiber evenly, but also to suppress the linear instabilities that would otherwise cause the fiber to break up into droplets or form beaded structures.44 While solution viscosity depends on a combination of factors such as the effectiveness of the solvent at dissolving the polymer and average molecular weight of the polymer Mw and polymer concentration, varying only the latter was sufficient for controlling the viscosity due to its exponential dependence on polymer concentration:
ηspec ) exp([η]c) where c is the concentration and [η] is the limiting (intrinsic) viscosity, which is dependent on the dimension of the isolated polymer molecule and defined as:
[η] ) KMwa where K and a are empirically determined constants that are characteristic of the polymer-solvent system at a given temperature. For our purposes we used a concentration of 20 wt % PS, as this was found to be the lowest concentration of PS that produced stable nonbeaded fibers in the single phase. Since it is not a polymer, the AB concentration has little effect on the core solution viscosity, which was very low compared to that of the shell. 3.2.3. Electrical ConductiWity. Most of the liquids identified as good solvents for PS had very low electrical conductivities, making them unsuitable for electrospinning in their pure form. Toluene, which is immiscible with water and a nonsolvent for AB, was selected as the main shell solvent. However, we were not able to spin a PS-toluene solution despite trying a wide range of PS concentrations and voltages as high as 30 kV. Solvent conductivity can often be increased through the addition of ionic or protonic salts. The ability of a solvent to dissociate a salt and hence the increase in its conductivity is dependent on its dielectric constant (i.e., its polarity); solvent conductivity is shown to increase with dielectric constant at different salt concentrations.45,46 Toluene has a very low dielectric constant and thus dissolution of salts was unsuccessful. Despite this, by using a binary or ternary solvent system we were able to selectively change the dielectric constant and hence increase shell-solution conductivity to enable fiber drawing without rendering the shell solution incompatible (or miscible) with the core. The dielectric constant of the mixtures (εmix) was calculated by using εmix ) ∑iaiεi, where ai is the volume fraction of solution i.
Model for Coaxial Electrospinning
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TABLE 1: Hansen Solubility Parameters of Solvents and Solvent Mixtures and Their Miscibility with PS, with a PS Interaction Radius of R0 ) 540 and Ra Calculated with Eq 1 solvent
δd
δp
δv
δh
Ra
RED ) Ra/R0
PS-miscibility
polystyrene, PS N,N-dimethylformamide, DMF dimethyl sulfoxide, DMSO toluene, Tol 1,2-dichloroethane, DCE pyridine formic acid, FA water pyridium formate, PF 7:1 mass ratio Tol:DMF 3:1:1 vol ratio Tol: DCE:PF 7:2:1 vol ratio DCE:NB:PF
17.6 17.4 18.4 18 16.6 19 14.3 15.6 16.65 17.91 17.45 17.28
6.1 13.7 16.4 1.4 8.2 8.8 11.9 16 10.35 3.16 4.55 8.49
18.63 22.15 24.65 18.05 18.51 20.94 18.6 22.35 19.6 18.19 18.03 19.25
5 11.8 10.2 2 0.4 5.9 16.6 42.3 28.4 3.4 6.96 3.94
0 10.21 11.65 5.63 5.44 3.99 14.55 38.8 23.86 3.41 2.52 2.69
0 0.8 0.92 0.44 0.43 0.31 1.15 3.05 1.88 0.27 0.2 0.21
yes yes yes yes yes no no no yes yes yes
TABLE 2: The Solvent Parametersa Used for Solvent Selection (Solution Preparation) for Electrospinning42,43 solvent/solution
electrical conductivity at rt (S/cm)
dipole moment (D)
dielectric constant
boiling point (°C)
heat of vaporisation (J/mol)
vapor pressure at 20 °C (atm)
water DMF DMSO toluene nitrobenzene DCE pyridine formic acid pyrdinium formate 7:1 Tol:DMF 3:1:1 Tol:DCE:PF 7:2:1 DCE:NB:PF
5.5 × 10-3 6.0 × 10-8 2.0 × 10-9 8.0 × 10-16 2 × 10-10 4.0 × 10-11 5.0 × 10-8 5.5 × 10-3 1.49 × 10-2 6.0 × 10-8 2.19 × 10-4 4.39 × 10-4
1.87 3.8 3.96 0.4 4 1.8 1.41
78.54 36.7 46.6 2.4 34.8 10.42 12.3 58 27.15 6.69 15.12 8.07
100 153 189 111 211 83.5 115.2 100
40 790 47 600 52 900 38 060 40 590 33 910 40 200 23 100
2.75 × 10-2 2.24 × 10-2 3.54 × 10-4 2.46 × 10-2 1.39 × 10-3 8.37 × 10-2 1.74 × 10-2 1.31 × 10-1 2.19 × 10-2 2.10 × 10-2 5.22 × 10-2 3.15 × 10-2
a
Vapor pressure was calculated as discussed in section 3.2.4.
TABLE 3: Shell and Core Solution Mixtures Used To Make the Immiscible, Semimiscible, and Miscible Core and Shell Solution Combinationsa shell solution
core-shell solution set miscible semimiscible immiscible
composition
core solution viscosity at 20-22 °C (cP)
conductivity at 20-22 °C (S/cm) -7
MS-1 MS-2 sMS-1 sMS-2 IS-1
20 wt % PS in DMF
281
9.6 × 10
18 wt % PS in 7:1.2 mass ratio Tol:DMF 20 wt % PS in 7:1 mass ratio Tol:DMF 20 wt % PS in 3:1:1 vol ratio Tol: DCE:PF
254 526
