Influence of Electrospinning Parameters on Fiber Diameter and ...

polymers Article

Influence of Electrospinning Parameters on Fiber Diameter and Mechanical Properties of Poly(3-Hydroxybutyrate) (PHB) and Polyanilines (PANI) Blends Ahmed M. El-hadi 1,2, * and Fatma Y. Al-Jabri 1 1 2

*

Department of Physics, Faculty of Applied Science, Umm Al-Qura University, Al-Abidiyya, P.O. Box, 13174, Makkah 21955, Saudi Arabia; [email protected] Higher Institute of Engineering and Technology, Department of Basic Science, El Arish, North Sinai 9004, Egypt Correspondence: [email protected]; Tel./Fax: +669541042942

Academic Editor: Naozumi Teramoto Received: 9 December 2015; Accepted: 24 February 2016; Published: 22 March 2016

Abstract: Random and oriented fibers of poly (3-hydroxybutyrate) (PHB) and their blends were manufactured using electrospinning using a co-solvent. The kind and the concentration of the co-solvent affected the diameter of electrospun fibers. The morphology, thermal analysis, and crystalline structure of electrospun fibers was studied using polarized optical microscop (POM), Differential scanning colametry (DSC), Scanning Electron Microscopy (SEM), Wide angle X-ray diffraction (WAXD), and FT-IR analysis. The diameter of the electrospun fibers decreases with increasing collector speed for the blends compared to pure PHB, which are about 6 µm in diameter. The fibers obtained from blends reduce to 2 µm. The aligned electrospun fiber mats obtained from pure PHB showed no signs of necking at different take-up speeds, but the blends show multiple necking. It was found by FT-IR that the peak intensity at 1379 cm´1 was lower by take up speed than in casting films; this peak is sensitive to crystallinity of PHB. The addition of polyanilines (PANIs) to (PHB) with a plasticizer decreases the diameter of the electrospun fiber. Keywords: electrospun fiber; conductive biopolymers; poly(3-hydroxybutyrate) (PHB); Polyanilines (PANI); mechanical properties

1. Introduction Electrospinning is a process that produces polymer fibers with diameters in the range between micrometer to nanometer dimensions with an applied high voltage. Higher electrical power is used to overcome the strength of the surface tension on the surface of the polymer solution, and a very thin charged jet is created. The jet forms a straight line, and the electrical forces elongate the jet 1000 times, making it very thin. In electrospinning, when an electrical voltage is applied between two electrodes, the droplets of liquid that are in the end of the needle form the so-called Taylor cone. The processing parameters that control the formation of the fiber are the solvent evaporation, the flow rate, the internal needle diameter, the distance from the needle to the collector, and the applied voltage [1]. In recent years many scientists have shown great attention to the use of electrospun PHB fibers in medicine to replace damaged or poorly grown nerves and tissue regeneration. PHB are natural polymers formed by using bacteria during fermentation. PHB is one group of polyhydroxyalkanoates (PHAs) that can be manufactured in larger quantities. PHB is a biopolymer, thermoplastic, non-toxic, has high crystallinity, and its physical properties are similar to polypropylene [2,3]. PHB can be applied

Polymers 2016, 8, 97; doi:10.3390/polym8030097

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Polymers 2016, 8, 97

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in biomedical requests, like surgical suture and wound dressings. However, the brittleness of PHB hinders this application [4]. PANI has good stability, good electrical conductivity, is biocompatible, and has low cost. PANI is difficult to dissolve in many solvents, so, it has poor processability. There is wide interest to develop biodegradable PHB, PANI, and plasticizers in the biomedical field to repair damaged nerves. Electrospun PHB fiber blends can be used in applications such as purification of water or air [5], biosensors [6], tissue engineering, wound healing, and release of drugs [7]. It can improve the physical properties of polymeric materials in many ways like a copolymers [8,9] or mixing the polymer with other polymers or addition plasticizers and lubricants [10–13]. The main goal is to preserve the main properties of the basic material with solving its disadvantages for use in the industry, while taking the economical approach and biodegradation process approach. The blend consists of two different polymers. Most of the polymers are classified into completely immiscible [14], partially miscible, or completely miscible. It is well known that the completely miscible blend [15] can improve the thermal and mechanical properties and biodegradation than another immiscible mixture [16]. In polymeric systems blends (full mixing) result in one glass transition temperature, where the interactions between the particles are stronger, like ionic bonds, dipole-dipole interactions, or hydrogen bonds [17,18]. The immiscible blends form two or more glass transition temperatures and the interactions between the particles is weaker, i.e., all mixing polymers components are in phase separation and each material keeps with its individual characteristics. Components that are miscible in the case of the solution, or molten state, but are separate structures in the solid state phase, include poly(3-hydroxybutyrate) and polyethylene oxide PHB/PEO [1,19], (poly(3-hydroxybutyrate)/poly(vinylidene fluoride) PHB/PVDF blend [20], and (poly(3-hydroxybutyrate)/poly(L-lactic acid) PHB/PLLA blend [21]. In addition, in the case of a miscible polymer blend by co-solvent, the resulting miscible mixture depends greatly on many factors, such as single solubility of polymers in a number of solvents, and the concentration of the polymer solution and the evaporation rate of the solvent. It is possible to get non-miscible or miscible polymer blends from the some polymers but not the same solvents. In this study, electrospun PHB fibers and their blends were successfully fabricated by electrospinning using different solvents. The main objective of this work is to manufacture (1) wound dressings of PHB with plasticizer; (2) the addition of PANIs + PHB + plasticizer to make material as conductive biopolymer fiber. The fiber can be used for the transfer the electrical signals from the brain to nerve system (replacing damaged nerves as a result of an accident or war) [1,22]. The aim of this work is to investigate the effect of electrospinning factors, such as applied voltage, inner needle diameter, and flow rate of the solution on fiber morphology. The structure and mechanical properties of the obtained fibers was analyzed by a tensile testing device and scanning electron microscopy (SEM). 2. Materials and Methods 2.1. Materials The molecular weight of PHB used is MW = 2.3 ˆ 105 g¨ mol´1 . The other materials used were PANI polyaniline salt and a plasticizer (tributyl O-acetylcitrate 98%), and solvents, such as chloroform (CF), dichloromethane (DCM), and dimethylformamide (DMF). All materials were purchased from Sigma-Aldrich, Germany. 2.1.1. Sample Preparation At first, we mixed PHB (80%) with a plasticizer (20%) in chloroform and left to dry to form a cast film, then dissolved it again with PANIs using a mixture of solvents (90% chloroform + 10% DMF or chloroform 75% and 25% dichloromethane). The investigated PHB/plasticizer/PANIs blends


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had weight ratios as follows: pure PHB (100/0/0), blend 1(80/20/0), blend 2 (79/20/1), blend 3 (77.5/20/2.5), blend 4 (75/20/4), and were prepared by a solution method. 2.1.2. Electrospinning Equipment PHB and its blends dissolved in different solvents at different concentrations and placed in a plastic syringe (5 mL) that connected to a needle with an inner diameter (ID) of 1.3 mm. Electrospinning was performed at room temperature with a home-made take-up rotating drum with variable speed and a high-voltage power supply from USA (model NO. ES60P-20W, Gamma High Voltage Research, Orlando, FL, USA). All fibers were pure PHB and their blends collected on the aluminum foil. A syringe-pump (No. BS-9000-USA, Braintree Scientific, Braintree, MA, USA) was used to feed the polymer solutions into the needle tip. The electrospun fibers were collected on a grounded collecting plate or rotating drum (homemade) with a maximum speed of 1100 rpm. 2.2. Sample Characterization 2.2.1. Thermal Analysis The thermal analysis of PHB and its blend were calculated by NETZSCH DSC 204 F1 Phoenix under a nitrogen atmosphere. The samples (about 5 mg weight) were heated from 25 ˝ C to 200 ˝ C, held for 1 min, and then quenched to ´50 ˝ C; the endo- and exothermal curves were then recorded. The glass transition temperature (Tg ) was obtained as the variation point of the heat capacity with a heating rate of 10 ˝ C/min and temperature ´50 to 200 ˝ C. 2.2.2. Wide Angle X-ray Diffraction (WAXD) Wide angle X-ray diffraction (WAXD). The measurements were carried out using a Panalytical X’pert PRO diffractometer (PANalytical B.V., Eindhoven, The Netherland) with Ni-filtered Cu Kα radiation with a wavelength of λ = 1.54178 Å over the 2θ range of 5–35˝ at 40 kV. WAXD data for pure PHB and their blends with additives were obtained at room temperature (~25 ˝ C) with a scan rate of 2˝ 2θ min´1 . Solution cast film samples were cut into rectangular pieces (4 cm2 ) and mounted on the matrix prior to analysis. 2.2.3. FT-IR Spectroscopy Infrared spectra of the investigated films, which were cut into rectangular pieces of 4 cm2 area, were recorded at room temperature and over the wavenumber range 550–4000 cm´1 using a Fourier Transform FT-IR 6100 Jasco (Jasco analytic instrument, Tokyo, Japan) spectrometer. 2.2.4. Scanning Electron Microscopy (SEM) The surface morphology of the PHB nanofibers and their blends was observed using a scanning electron microscope (JSM-6360LA, JEOL Co., Boston, MA, USA) at an accelerating voltage of 15 kV. The samples were sputter-coated with gold for 120 s to a thickness of 2–3 nm using a sputter coater (EMITECH K550X, Kent, UK). Images of several sample fibers were obtained using SEM to measure the fiber diameter. The samples are coated with gold. 2.2.5. Mechanical Properties Uniaxial tensile tests were made with dog-bone specimens (width 4.8 mm, length 22.25 mm, thickness 15–35 µm). All samples were tested in the tensile test device and cut in a dumbbell shape (Dumb Bell Ltd. SDL-100, DUMBBELL Co. Ltd., Saitama-Ken, Japan). Sample thickness was measured using a micrometer. Tensile tests were performed at room temperature with speed of 1 mm¨ min´1 , using a Shimadzu universal testing machine equipped with a 10 kN load cell and connected with a computer. Five specimens of each formulation were tested and the average values are reported. From the correlation between stress σ (in Pa) and elongation ε (in %), ε = (L0 ´ L)/L, L0 = original length,


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L = length after elongation. At the end, the fracture surface of the electrospun samples was studied Polymers 4 of 13 by SEM.2016, 8, 97 3. Results Resultsand andDiscussion Discussion 3.1. Fiber Morphology by Scanning Electron Microscopy (SEM) (SEM) SEM was performed after using the information from POM on how to get the best electrospun fibers thethe bestbest parameters determined previously. The SEM images that the electrospun fibers and andselect select parameters determined previously. The SEMshow images show that the fiber diameters in the range ofthe micrometers. Figure 1a,a'Figure shows1a,a' the morphologies of pure PHB electrospun fiberare diameters are in range of micrometers. shows the morphologies of electrospun fiber mats. After solvent evaporates, fibersevaporates, with a submicron-level are formed pure PHB electrospun fiberthemats. After the solvent fibers withdiameter a submicron-level on a collecting plate oron rotating collector at or different speeds. It isateasy to deform the fibers by random or diameter are formed a collecting plate rotating collector different speeds. It is easy to deform rotation which implies spinning, that the fiber is elastic and flexible. By using theand rotating collector, the the fibersspinning, by random or rotation which implies that the fiber is elastic flexible. By using fiber can be oriented The be fibers were formed without beads, andformed the average fiber diameter the rotating collector,(aligned). the fiber can oriented (aligned). The fibers were without beads, and was ~5 µm. We can see the orientation thecan fibers using different theusing rotating drum. the average fiber diameter was ~5 µm.ofWe see upon the orientation of thespeeds fibers by upon different A necking region was drum. not observed in pure PHB fibers thatPHB wereelectrospun collected atfibers the fixed speeds by the rotating A necking region waselectrospun not observed in pure that plate (random) or the rotation drum (aligned). were collected at the fixed plate (random) or the rotation drum (aligned).

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Figure 1.1. The Thesamples samples pure PHB 1 collected were collected on the aluminum foil following with the Figure of of pure PHB and and blendblend 1 were on the aluminum foil with the following die diameter, applied voltages, CF/DCM ratios, flow rate, and the distance between die diameter, applied voltages, CF/DCM ratios, flow rate, and the distance between needle andneedle target: −1 andwttarget: wt 20 %, kV, 1.3 75:25, mm, 20 kV,µm¨ 75:25, 0.185 20 cm, respectively; (a,a')atpure PHB at a 20 %, 1.320 mm, 0.185 h´1 , andµm⋅h 20 cm,, and respectively; (a,a') pure PHB a fixed target fixed target (random) and rotating drum with a speed of 1100 rpm with polymer concentrations of (random) and rotating drum with a speed of 1100 rpm with polymer concentrations of 20%; (b,b') blend (b,b') target blend(random) 1 at a fixed (random) and at aa speed rotating drum speed 1100 rpm with 120%; at a fixed andtarget at a rotating drum with 1100 rpmwith with apolymer concentrations polymer concentrations of 25%. of 25%.

Figure 1b,b' show the SEM images of blend 1 electrospun fiber mats that were collected using a Figure 1b,b' show the SEM images of blend 1 electrospun fiber mats that were collected using a fixed plate and rotating collector at different speeds. It is clear that the fibers appear smooth and that fixed plate and rotating collector at different speeds. It is clear that the fibers appear smooth and that the alignment of the fibers was enhanced at higher speed, i.e., upon increasing the take-up speed, the alignment of the fibers was enhanced at higher speed, i.e., upon increasing the take-up speed, there there is good alignment and the fiber diameter reduces. Figure 2 shows pure PHB electrospun fiber is good alignment and the fiber diameter reduces. Figure 2 shows pure PHB electrospun fiber mats mats with an average fiber diameter ~2 µm. It was found that, with increasing the collector speed, with an average fiber diameter ~2 µm. It was found that, with increasing the collector speed, the fiber the fiber diameter reduces. diameter reduces. Figure 2 shows the morphology of the electrospun fiber mats of pure PHB. Pure PHB have Figure 2 shows the morphology of the electrospun fiber mats of pure PHB. Pure PHB have diameters between 1.48 to 2.5 µm. It is manufactured by stretching the fiber using a rotating drum at diameters between 1.48 to 2.5 µm. It is manufactured by stretching the fiber using a rotating drum at 1100 rpm. The aligned electrospun fibers of pure PHB and showed no signs of necking during take1100 rpm. The aligned electrospun fibers of pure PHB and showed no signs of necking during take-up up at higher speed rotation compared with lower speed. Therefore, we use CF/DMF to obtain thinner fiber compared with using CF/DCM in the same sample. The average diameter of the fibers decreased significantly, and beads were not detected in pure PHB. Figure 3 shows blend 1 electrospun mats with average fiber diameters of ~2 µm, (using CF/DMF and rotating speed 1100 rpm). Therefore, we use CF/DMF to obtain a thinner fiber compared with


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Polymers 2016, 8, 97 5 of 13thinner at higher speed rotation compared with lower speed. Therefore, we use CF/DMF to obtain fiber compared with using CF/DCM in the same sample. The average diameter of the fibers decreased using CF/DCM in the same sample. The average diameter of the fibers decreased significantly. Beads significantly, and beadsinwere PHB. were not detected purenot PHBdetected and both in thepure blends use the co-solvent CF/DMF.


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using CF/DCM in the same sample. The average diameter of the fibers decreased significantly. Beads were not detected in pure PHB and both the blends use the co-solvent CF/DMF.

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(a) (c)

(b)

(b) (d)

Figure 2. The samples of pure PHB at different scales were collected on the aluminum foil with the

Figure 2. The samples of pure PHB at different scales were collected on the aluminum foil with the following polymer concentrations, die diameter, applied voltages, CF/DMF ratios, flow rate, and following polymer concentrations, diameter, CF/DMF ratios, flow rate, and −1, and 20 cm, distance between the needle anddie target: 20 wt. %,applied 1.3 mm,voltages, 20 kV, 90:10, 0.185 µm⋅h ´1 distancerespectively. between the target: 20 wt 1.3 mm, 20 and kV, (b,d) 90:10, µm¨ h with , and (a,c)needle at a fixedand target (random) with%, different scales, at a0.185 rotating drum a 20 cm, respectively. (a,c) atrpm. a fixed target (random) with different scales, and (b,d) at a rotating drum with a speed of 1100 speed of 1100 rpm. (c) (d) Figure 2. Theblend samples of pure PHB atmats different scales were collected on the aluminum foil with theCF/DMF Figure 3 shows 1 electrospun with average fiber diameters of ~2 µm, (using following polymer concentrations, die diameter, applied voltages, CF/DMF ratios, flow rate, and and rotating speed 1100 rpm). Therefore, we use CF/DMF to obtain a thinner fiber compared with distance between the needle and target: 20 wt. %, 1.3 mm, 20 kV, 90:10, 0.185 µm⋅h−1, and 20 cm, using CF/DCM in the same sample. The average diameter of the fibers decreased significantly. Beads respectively. (a,c) at a fixed target (random) with different scales, and (b,d) at a rotating drum with a were not detected in pure speed of 1100 rpm. PHB and both the blends use the co-solvent CF/DMF.

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Figure 3. The samples of blend 1 at different scales and speeds were collected on the aluminum foil with the following polymer concentrations, die diameter, applied voltages, CF/DMF ratios, flow rate, and distance between the needle and target: 25 wt %, 1.3 mm, 20 kV, 90:10, 0.185 µm⋅h−1 and 20 cm, (a,c) at fixed target (random); (b,d) at a rotating drum with a speed of 1100 rpm, respectively.

The aligned electrospun fibers of pure PHB (Figure 2) and blend 1 showed no signs of necking during take-up at higher speed rotation compared with lower speeds. The average electrospun fiber diameters of blend 1 were between randomly-oriented fibers in the (c) 2 and 2.5 µm. We compared the(d) Figure 3. The samples of blend 1 at different scales and speeds were collected on the aluminum foil

Figure 3. The samples of blend 1 at different scales and speeds were collected on the aluminum foil with the following polymer concentrations, die diameter, applied voltages, CF/DMF ratios, flow rate, with the following polymer concentrations, die diameter, applied voltages, CF/DMF−1ratios, flow rate, and distance between the needle and target: 25 wt %, 1.3 mm, 20 kV, 90:10, 0.185 µm⋅h and 20 cm, and distance between needle(b,d) andattarget: 25 wt %, with 1.3 mm, 20 kV, 90:10, 0.185 µm¨ h´1 and 20 cm, (a,c) at fixed target the (random); a rotating drum a speed of 1100 rpm, respectively. (a,c) at fixed target (random); (b,d) at a rotating drum with a speed of 1100 rpm, respectively.

The aligned electrospun fibers of pure PHB (Figure 2) and blend 1 showed no signs of necking during take-up at higher speed rotation compared with lower speeds. The average electrospun fiber diameters of blend 1 were between 2 and 2.5 µm. We compared the randomly-oriented fibers in the


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The aligned electrospun fibers of pure PHB (Figure 2) and blend 1 showed no signs of necking during take-up at higher speed rotation compared with lower speeds. The average electrospun fiber diameters of blend 1 were between 2 and 2.5 µm. We compared the randomly-oriented fibers in the same sample, which are about 3 µm in diameter, but the fibers obtained here have about two times smaller diameter. The decreased diameter of the electrospun fibers of PHB and their blends was attributed Polymers to higher take-up speed and the conductivity of DMF solvent. Fibers of pure PHB 2016, 8, 97 6 of 13 and blend 1 showed smooth surfaces. No necking region was observed in the fibers collected on the rotating same sample, which are about 3 µm in diameter, but the fibers obtained here have about two times drum (aligned fibers), i.e., the average diameter of the fiber was smaller at higher take-up speed if the smaller diameter. The decreased diameter of the electrospun fibers of PHB and their blends was fibers wereattributed spun using the take-up same polymer lower take-up to higher speed andat the conductivity of speed. DMF solvent. Fibers of pure PHB and blend 1 showed smooth surfaces.regions No necking region was observed in the fibers collected on the Figure 4 shows certain necking in electrospun fibers by collecting random and aligned rotating drum (aligned fibers), i.e., the average diameter of the fiber was smaller at higher take-up fibers of blend 1 (Figure 3) and blend 2 (Figure 4). Electrospun fibers obtained from blend 2 were much speed if the fibers were spun using the same polymer at lower take-up speed. thinner than the fibers obtained from pure PHB and blend 1 by using the same co-solvent, CF/DMF. Figure 4 shows certain necking regions in electrospun fibers by collecting random and aligned Smooth blend formed with diameters between 0.87fibers andobtained 2.5 µm,from approximately. fibers 2offibers blend 1are (Figure 3) and blend 2 (Figure 4). Electrospun blend 2 were In many much thinner the fibers obtained from By pure PHB and blend 1 by usingspeed, the same co-solvent, uses it is necessary to than develop aligned fibers. increasing the take-up thinner fibers can be CF/DMF. fibers are formed withthey diameters 0.87 and µm,aapproximately. formed. The fibersSmooth were blend fully2toughened before werebetween collected and2.5had smooth texture. PHB In many uses it is necessary to develop aligned fibers. By increasing the take-up speed, thinner fibers fibers show no necking regions; however, PHB blends 1 and 2 show a developed fiber neck at both can be formed. The fibers were fully toughened before they were collected and had a smooth texture. random and spinning. electrospun fibers showed surfaces.fiber The existence of a PHBrotation fibers show no neckingAll regions; however, PHB blends 1 and 2smooth show a developed neck at both random rotation spinning. All able electrospun fibers smooth surfaces. The existence of necking region meansand that the fiber is not to resist theshowed mechanical or electrical pull which would a necking region means the fiber is not able along to resistthe the fiber, mechanical or electrical pullmore which elastics, would flexible, lead to plastic deformation in that different positions i.e., the fibers are lead to plastic deformation in different positions along the fiber, i.e., the fibers are more elastics, it can deformat at higher speed. flexible, it can deformat at higher speed.

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Figure 4. The samples of blend 2 were collected on the aluminum foil with the following polymer

Figure 4. The samplesdie of diameter, blend 2 applied were collected on theratios, aluminum foil thebetween following concentrations, voltages, CF/DMF flow rate, andwith distance the polymer concentrations, dietarget: diameter, applied voltages, flow and distance between the needle and 25 wt %, 1.3 mm, 20 kV, 90:10,CF/DMF 0.185 µm⋅h−1ratios, , and 20 cm. (a) rate, at a fixed target (random); (b)target: at a rotating drum speed of (c) at 0.185 a rotating drum a speed of 490(a) rpm; at needle and 25 wt %,with 1.3a mm, 20380 kV,rpm; 90:10, µm¨ h´1with , and 20 cm. at (d) a fixed target rotating drum with a speed of 610 rpm; (e) at a rotating drum with a speed of 740 rpm, and (f) at a (random);a(b) at a rotating drum with a speed of 380 rpm; (c) at a rotating drum with a speed of rotating drum with speed 850 rpm. 490 rpm; (d) at a rotating drum with a speed of 610 rpm; (e) at a rotating drum with a speed of 740 rpm, and (f) at a rotating drum with speed 850 rpm.


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3.2. Mechanical Properties The mechanical properties of PHB and PANIs can be improved by the addition of a plasticizer. 7 of 13 Flexible elastic films or electrospun fibers with a significant reduction in stress and increased elongation can be obtained. It is known that PHB is brittle and rigid [2,3] and its elongation at break Properties (4%) 3.2. [3],Mechanical by using the rotation drum at a speed of 610 rpm; PHB did not give good elongation. Figure shows the properties stress–strain curves electrospun fibers ofby pure blend and blend 2. The5mechanical of PHB andfor PANIs can be improved the PHB, addition of a1, plasticizer. Flexiblestress elastic decreases films or electrospun a significant stress and2increased elongation The tensile from 29 fibers MPa with for pure PHB toreduction 26 MPa in for blend using a take-up speed canrpm, be obtained. is known PHB is brittle rigidfor [2,3]blend and its break (4%) [3], by that of 640 16 MPa Itfor blend 2that (random), and and 10 MPa 1 elongation (random).atThe results show using the rotation drum at a speed of 610 rpm; PHB did not give good elongation. the addition of 20 wt % plasticizer has a effect on the mechanical properties of the electrospun fibers Figure 5 shows the stress–strain curves for electrospun fibers of pure PHB, blend 1, and blend 2. of PHB. The elongation at break increased to 34%, while the tensile strength decreased to 10 MPa. The tensile stress decreases from 29 MPa for pure PHB to 26 MPa for blend 2 using a take-up speed of The addition of PANIs to PHB with plasticizer decreases the elongation at break to 15% and increases 640 rpm, 16 MPa for blend 2 (random), and 10 MPa for blend 1 (random). The results show that the the stress to 16 MPa. The presence of PANIs in blend 2 increases the tensile strength, enabling the use addition of 20 wt % plasticizer has a effect on the mechanical properties of the electrospun fibers of of thePHB. material in different applications. The elongation at break increased to 34%, while the tensile strength decreased to 10 MPa. The Figure 5 shows electrospun of blend 2 spun a high take-up of increases 610 rpm,the which addition of PANIs to PHB with fibers plasticizer decreases the at elongation at break velocity to 15% and demonstrated strength and in lower compared to electrospun fibersthe of use theofsame stress to 16 high MPa. tensile The presence of PANIs blendstrain 2 increases the tensile strength, enabling sample a fixed applications. target (random fibers). the collected material inatdifferent

Stress (MPa)


34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

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8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Strain (%)

Figure 5. Stress-strain curves ofof nanofibers: PHB with withaaspeed speed 610 rpm; blend 2 with a Figure 5. Stress-strain curves nanofibers:(a) (a)pure pure PHB ofof 610 rpm; (b) (b) blend 2 with a speed of 610 rpm; blend 2 (random);and and(d) (d) blend blend 11(random). speed of 610 rpm; (c) (c) blend 2 (random); (random).

Figure 6 shows theelectrospun tensile tests afterofcold drawing surfaces) of of the electrospun Figure 5 shows fibers blend 2 spun at(destroyed a high take-up velocity 610 rpm, whichfiber of blends 1 and 2 high using SEMstrength analysis. Itlower has been the electrospun have multiple demonstrated tensile and strainobserved comparedthat to electrospun fibers offibers the same sample collected at a fixedmultiple target (random fibers). necking. In addition, necking of the electrospun fibers happens along the fiber, which would Figure 6 shows the tensile tests after cold drawing (destroyed surfaces) of the electrospun lead to the change of a fibril structure as shown in Figure 6 at a high magnification. Variousfiber necking of blends 1 and 2 using SEM analysis. It has been observed that the electrospun fibers have multiple takes place only by collecting blends 1 and 2 at the fixed target (random). Small necking regions of necking. In addition, multiple necking of the electrospun fibers happens along the fiber, which would around 0.97, 1.15, and 2.15 µm were observed along each fiber. The mechanical behavior, in general, lead to the change of a fibril structure as shown in Figure 6 at a high magnification. Various necking depends on the fiber diameter. The electrospun fibers deformed via wide-ranging necking and plastic takes place only by collecting blends 1 and 2 at the fixed target (random). Small necking regions of deformation before breaking after cold drawing, as presented in Figure 6c,d. around 0.97, 1.15, and 2.15 µm were observed along each fiber. The mechanical behavior, in general, When the increased, the necking were along depends on tension the fiber was diameter. The electrospun fibersregions deformed via transmitted wide-ranginghomogeneously necking and plastic the fibers until the complete fiber mat broke (Figure 6c,d). The formation of the necks led to a decrease deformation before breaking after cold drawing, as presented in Figure 6c,d. in the fiber cross-sectional area. The fibers have some smaller regions that appear atalong the random When the tension was increased, the necking regions wereneck transmitted homogeneously the collector by tensile testing with different necking Necking transmits the fiberinuntil fibersor until the complete fiber mata broke (Figure 6c,d). length. The formation of the necks ledalong to a decrease the fiber cross-sectional the fiber completely fails. area. The fibers have some smaller neck regions that appear at the random collector or by tensile testing with a different necking length. Necking transmits along the fiber until the fiber completely fails.

Polymers 2016, 8, 97 Polymers 2016, 8, 97

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3.3. Thermal Analysis (DSC) DSC analyses were used to determine the melting and crystallization temperatures of PHB and its blends with PANIs and plasticizer. plasticizer. Figure 6a shows the DSC curves of the first heating, Figure 6b for cooling, and Figure 6c for the second heating. In the first heating curves, it is observed that pure ˝ C. PHB and its blend with plasticizer showed a melting temperature at approximately 151 and 169 °C. Two Two melting melting temperatures temperatures are arealso alsodetected detectedon onPHB PHBand andits itsblend. blend.

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(d)

Figure 6. 6. SEM of fractured fractured surfaces surfaces of of multiple multiple neck neck formations formations in in electrospun electrospun fibers fibers of of Figure SEM micrographs micrographs of blend 2 (a, b, and c) and blend 1 (d) after cold drawing, at different magnifications. blend 2 (a, b, and c) and blend 1 (d) after cold drawing, at different magnifications.

These double peaks are attributed to two lamellar thicknesses with different crystal structures These double peaks are attributed to two lamellar thicknesses with different crystal structures (the (the smallest melted first and the largest melted second). The melting temperatures of PHB with smallest melted first and the largest melted second). The melting temperatures of PHB with plasticizer plasticizer were lower than that for pure PHB. It was observed that the plasticizer has an effect on the were lower than that for pure PHB. It was observed that the plasticizer has an effect on the Tc and Tm Tc and Tm during the first heating, cooling, and second heating curves. By cooling, the Tc is shifted during the first heating, cooling, and second heating curves. By cooling, the Tc is shifted from 95 ˝ C for from 95 °C for PHB to 63 °C for blend 1, the Tc shifted to 81 °C for blend 2, 85 °C for blend 3, and PHB to 63 ˝ C for blend 1, the Tc shifted to 81 ˝ C for blend 2, 85 ˝ C for blend 3, and 94 ˝ C for blend 4. 94 °C for blend 4. Upon second heating there is no recrystallization peak for both PHB and its blends. Upon second heating there is no recrystallization peak for both PHB and its blends. There is no cooled There is no cooled crystallization temperature (Tcc) observed because the crystallization occurred crystallization temperature (Tcc ) observed because the crystallization occurred during cooling. This during cooling. This result shows that PANIs acted as nucleating agents. However, an improvement result shows that PANIs acted as nucleating agents. However, an improvement in crystallization in crystallization was found for blend 4 with 4 wt. % PANIs. The second melting temperature was was found for blend 4 with 4 wt % PANIs. The second melting temperature was shifted slightly to shifted slightly to higher temperatures compared to the first heating for blends 1, 3, and 4. This higher temperatures compared to the first heating for blends 1, 3, and 4. This occurred because the occurred because the crystallization conditions are different than the first preparation of the film. It crystallization conditions are different than the first preparation of the film. It is known that the glass is known that the glass transition temperature Tg from DSC is around 2–5 °C [1,2] for PHB. It is clear transition temperature Tg from DSC is around 2–5 ˝ C [1,2] for PHB. It is clear from Figure 7d that there from Figure 7d that there is one (Tg) for all blends with PANIs as few ratio between 1% and 4%, is one (Tg ) for all blends with PANIs as few ratio between 1% and 4%, meaning that it's a good mixed meaning that it's a good mixed (miscible). For this reason, we concluded that all blends were miscible (miscible). For this reason, we concluded that all blends were miscible systems. However, the Tg of systems. However, the Tg of PHB in blends preserve unchanged with weight fraction (1%–4%) of the PHB in blends preserve unchanged with weight fraction (1%–4%) of the PANIs at about ´7 ˝ C. The PANIs at about −7 °C. The PANIs can only be affected the crystallization process and serves as a PANIs can only be affected the crystallization process and serves as a nucleation agent. nucleation agent.


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Figure7. 7. DSC DSC curves curves for for pure pure PHB PHB and and its its blends blends (a) (a) first first heating; heating;(b) (b) cooling; cooling;(c) (c)second secondheating; heating;and and Figure (d) second heating to determine the glass transition temperature region. (d) second heating to determine the glass transition temperature region.

3.4. WAXD Analysis 3.4. WAXD Analysis Figure 8 shows the WAXD of electrospun fibers of pure PHB and their blends with different Figure 8 shows the WAXD of electrospun fibers of pure PHB and their blends with different take-up speeds. The characteristic reflection peaks of crystalline PHB at 2θ = 13.6 and 16.9° indicates take-up speeds. The characteristic reflection peaks of crystalline PHB at 2θ = 13.6 and 16.9˝ indicates the formation of PHB crystalline phase in the pure sample [1,2]. However, a certain reduction of the the formation of PHB crystalline phase in the pure sample [1,2]. However, a certain reduction of main reflection peaks at 2θ = 16.9° and some orientation at 2θ = 13.6°, as shown in Figure 8a, occurs because the crystalline structure of PHB deformed through the electrospinning process at higher takeup speeds. Similar results occur in blend 1 with different take-up speeds, as shown in Figure 8b.


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the main reflection peaks at 2θ = 16.9˝ and some orientation at 2θ = 13.6˝ , as shown in Figure 8a, occurs Polymersbecause 2016, 8, 97the crystalline structure of PHB deformed through the electrospinning process at higher 10 of 13 take-up speeds. Similar results occur in blend 1 with different take-up speeds, as shown in Figure 8b. peaks of of electrospun electrospun of pure PHB PHB (random), (random), blend 1 (random), blend 2 Figure 8c shows the WAXD peaks different take-up take-up speeds. speeds. The intensities of the crystalline peaks were (random), and blend 2 with different PANI content content and and by by varying varying the the take-up take-up speeds. speeds. The electrospun reduced by addition of different PANI pattern with small crystalline structures, which implies that the crystals in the fiber shows a broad pattern fibers are arenot not well developed. Incold the drawing cold drawing which the includes the of stretching of fibers well developed. In the process,process, which includes stretching electrospun electrospun fibers velocity, at a higher velocity, a broadening the peak was observed in the electrospun fiber fibers at a higher a broadening of the peakofwas observed in the electrospun fiber sample sample2),(blend 2), indicating a stretching change in the structure during the electrospinning (blend indicating a stretching and changeand in the structure during the electrospinning process. This process. This peak is of non-crystalline PANI. broad peak is broad of non-crystalline PANI. p u re v=25 v=38 v=49 v=61

P 0 0 0 0

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p u re B le n B le n B le n B le n

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Figure 8. ofof (a)(a) pure PHB with different take-up speeds (v), (b) 1 with1 Figure 8. WAXD WAXDof ofelectrospun electrospunfibers fibers pure PHB with different take-up speeds (v),blend (b) blend different take-up speeds, and (c) pure PHB, blends 1, 2, and blend 2 with different take-up speed (v). with different take-up speeds, and (c) pure PHB, blends 1, 2, and blend 2 with different take-up speed (v).

3.5. FT-IR Analysis

3.5. FT-IR Analysis The FT-IR spectrum of electrospun PHB fibers and their blends are shown in Figure 9 at different take-up speeds. They show a series of characteristic bands that are split into three bands around 1686, The FT-IR spectrum of electrospun PHB fibers and their blends are shown in Figure 9 at different −1, which correspond to the ester carbonyl group (1723 and 1686 cm−1 crystalline 1723, and 1742 cm take-up speeds. They show a series of characteristic bands that are split into three bands around 1686, −1 and 1742 cm amorphous). The main characteristic bands at 1278 cm−1 correspond to the –CH group of PHB. The band at 1278 cm−1 and 1379 cm−1 is sensitive to crystallinity of crystalline PHB. Figure 9a,b show a decrease in this peak with increasing take-up speeds, i.e., increasing the take-up speed of electrospun fibers leads to a decrease the crystallinity of PHB and their blends. Increased take-up speed leads to a lower degree of crystallization; this is clear in the Figure 9c, with blends 1 and 2, at


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1723, and 1742 cm´1 , which correspond to the ester carbonyl group (1723 and 1686 cm´1 crystalline and 1742 cm´1 amorphous). The main characteristic bands at 1278 cm´1 correspond to the –CH group of PHB. The band at 1278 cm´1 and 1379 cm´1 is sensitive to crystallinity of crystalline PHB. Figure 9a,b show a decrease in this peak with increasing take-up speeds, i.e., increasing the take-up speed of electrospun fibers leads to a decrease the crystallinity of PHB and their blends. Increased take-up speed leads to a lower degree of crystallization; this is clear in the Figure 9c, with blends 1 and 2, at Polymers 2016, 8, 97 11 of 13 a constant take-up speed of 490 rpm, and blends 3 and 4 as casting film. It was found that the peak ´ 1 intensity at 1383take-up cm was in rpm, the case of the3take-up 490 rpmfound thanthat in the film. a constant speedless of 490 and blends and 4 as speed casting at film. It was the casting peak ´1 −1 was The stretching 1452, 1379, 1223, and of 2887 thethan methyl weak peak intensitypeaks at 1383atcm less in the case the cm take-upcorrespond speed at 490to rpm in thegroup. castingAfilm. ´1methyl 1452,appears 1379, 1223, and 28873200 cm−1 correspond group. A weak peak for the The OHstretching group ofpeaks pureatPHB between and 4000 to cmthe , but the broad band of the OH −1, but the broad band of the OH ´ 1 for the OH group of pure PHB appears between 3200 and 4000 cm group centered at 3400 cm is attributed to the hydrogen bond between PHB and PANIs (blend 4 as a at 3400 cm−1 is attributed to the hydrogen bond between PHB and PANIs (blend 4 as casting group film).centered The intensity of OH increases with increasing PANIs content in the PHB matrix (blend 4 a casting film). The intensity of OH increases with increasing PANIs content in the PHB matrix (blend as a casting film). New peak appears at 1015 cm´−11 for NH2 for PANIs. Hydrogen bonding happens 4 as a casting film). New peak appears at 1015 cm for NH2 for PANIs. Hydrogen bonding happens between –NH2 –NH or –OH group of PANIs and C=O of PHB and plasticizer. between 2 or –OH group of PANIs and C=O of PHB and plasticizer. 1278

pure PHB (random) v=250 rpm v=380 rpm v=490 rpm v=610 rpm

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Figure 9. FT-IR spectrum of electrospun PHB and their blends in the infrared spectrum regions at

Figure 9. FT-IR spectrum of electrospun PHB and their blends in the infrared spectrum regions at different take-up speeds (a) from 400 to 1600 cm−1´1 of PHB and (a') from 1600 to 4000 cm−1 of PHB; ´1 (b) different take-up speeds (a) from 400 to 1600 cm of PHB and (a') from 1600 to 4000 cm of PHB; from 400 to 1600 cm−1 of blend 10; and (b') from 1600 to 4000 cm−1 of blend 10; (c) from 400 to 1600 ´1 ´1 of blend 10; (c) from 400 to (b) from 160010,cm and (b') fromcm1600 to 4000 −1 of to −1 of pure blends 14, 16,of 17,blend and (c')10; from 1600 to 4000 PHB cm and blends 1 and 2, at constant cm400 ´1 ´1 1600 cmtake-up of blends 10,490 14,rpm, 16, but 17, blends and (c') from 1600 to 4000 of pure PHB and blends 1 and 2, at speeds of 3 and 4 as a casting film.cm constant take-up speeds of 490 rpm, but blends 3 and 4 as a casting film.


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4. Conclusions Fibers of PHB and their blends have been fabricated by the electrospinning method using CF/DCM (75:25) and CF/DMF (90:10) as co-solvents. The addition of DMF demonstrated an increase in the solution conductivity after dissolving PHB in hot CF (90%) and then cold DMF (10%). It was found that the addition of cold DMF (10%) decreased the diameter of the fibers. The morphology, structure, and tensile properties of electrospun pure PHB and their blends were studied. SEM images showed that the average diameter of aligned electrospun fiber mats was smaller than those of random fibers. WAXD indicated that the crystallinity of the fibers decreased as the PANIs percentage increased compared to pure PHB, i.e., electrospun fibers of blends have low crystallinity, making them suitable for many applications. Necking of electrospun fibers of pure PHB was not observed upon collection at the fixed plate (random) or rotating drum (aligned). Fibers of blends 1 and 2 showed multiple necking and were observed at a stretching rate of 390–970 rpm, but at a higher speed of 1100 rpm there is no necking, implying that the fiber is aligned. WAXD and FT-IR analyses indicate that, upon increasing the take-up speed of electrospun fibers, the crystalline structure of PHB deformed and the crystals are not well developed. The FT-IR shows that a new peak appears at 1015 cm´1 for NH2 and 3400 for OH group of PANIs. The tensile stress decreases from 29 MPa for pure PHB to 26 MPa for blend 2 after a take-up speed of 640 rpm, 16 MPa for blend 2 (random), and 10 MPa for blend 1 (random). Supplementary Materials: The following are available online at www.mdpi.com/2073-4360/8/3/97/s1, Electrospinning is a suitable method to produce fiber. The tool consists of a pumping device, small needle, higher voltage (DC) 0–40 kV and two types of collectors: a fixed plate (random) and moving drum (homemade). Figure S1: Electrospinning on the rotating drum in our lab (homemade); Figure S2: Electrospinning at a fixed target to obtain random fibers; Figure S3: Fiber rolling after electrospinning processes on the frame and the method to collects by the frame for the dumb bell shape. Acknowledgments: The authors thankfully grant King Abdulaziz City for Science and Technology (KACST) for support of master student program (no. AT-35-225). Author Contributions: Ahmed M. El-Hadi planned the idea of the experiments. Ahmed M. El-hadi and Fatma Y. Al-Jabri advanced the idea and succeeded the project and the experiments. Conflicts of Interest: The authors declare no conflict of interest.

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