Electrospinning, as an efficient and versatile technique, has been used for the fabrication of polymer nanofibers in recent years. Various materials such as polymers, inorganic and hybrid materials, have been successfully electrospun into ultrafine fibers in solvent solution or in melt form. These fibers exhibit many outstanding features, such as diameters ranging from nanometers to micrometers, large surface area to volume ratios, various hierarchical structures and superior mechanical performance [1–6]. These features of the fibers can be influenced by many parameters which include: (1) the solution properties such as solution concentration, viscosity, elasticity, conductivity and surface tension; (2) governing variables such as injection rate, voltage and tip-to-collector distance (TCD); and (3) ambient parameters such as solution temperature, humidity and air velocity in the electrospinning chamber [7, 8].
Trans-polyisoprene (TPI) is an isomeric form of natural rubber. Natural rubber is the cis form, with chain bonds on the same side of the double bonds, and TPI is the transform, with chain bonds on opposite sides of the double bonds [9, 10]. Polymeric materials dissolved in a proper solvent can be subjected to the electrospinning process. Plastic polymers, having higher glass transition temperature (Tg) than room temperature, are relatively easy to be made into electrospun fibers, however rubbery polymers are difficult to be electrospun due to the low Tg . The shape of the fiber can be formed from a rubber solution by the electrospinning process, although it tends to be immediately destroyed by cold flow due to the lower Tg than room temperature. TPI has a lower Tg than that of ambient temperature. However, trans-1,4-polyisoprene can crystallize rapidly at temperatures below 60°C owing to its regular structure and at room temperature exists as a crystalline polymer characterized by greater hardness and tensile strength that are similar to the plastic polymers. TPI exists in polymorphs and the crystals exist in two different forms, i.e., high melting form and low melting form [12–15]. Because of the crystalline nature, TPI displays plastic behaviors quite different from the flexibility of natural rubber .
In this work, TPI was used probably for first time to generate nanofibers by the electrospinning method. We report the working conditions and optimized parameters for electrospinning TPI fibers. The morphology and physicochemical nature of TPI were also studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectrometry before and after being electrospun.
2 Materials and methods
TPI (Mw=81 K, Mooney viscosity=60) was obtained from Qingdao New Material Co. Ltd (China, Qingdao). Chloroform, and dimethylformamide (DMF), were obtained from Sigma-Aldrich (Seoul, South Korea).
2.2 Electrospinning TPI fibers
Figure 1 shows the electrospinning apparatus which has three components: a capillary tube with a pipette or needle of a small diameter, a metal collector and a high voltage supplier. A syringe with a metal needle was used as the solution reservoir. A drum-shaped counter electrode was located beside the reservoir. The electrospun fibers were collected on the tube, which was covered with aluminum foil, when a high voltage was supplied between the needle and the collector. TPI was dissolved in a double solvent, chloroform: DMF=9:1, adopted in this investigation due to the high conductivity of DMF. To prepare solutions with different weight percent concentrations, weighted amounts of polymers and solvents were mixed for several hours until homogeneous solutions were obtained. During electrospinning, a standard electrospinning apparatus was employed (NanoNC). The homogeneous TPI solution was delivered by a syringe pump with a stainless steel needle at a controlled flow-rate (1–8 ml/h) through tubing to the stainless steel needles (Hamilton, outer diameter=0.81 mm) with an applied high electrical voltage between 5 kV and 25 kV. Fibers were collected on a stainless steel tube covered with aluminum foil. The windup speed was maintained at 100 rpm. The distance between the capillary tip and the counter electrode TCD was 10–20 cm.
2.3 Characterization methods
The viscosities of the TPI solutions were measured at room temperature using a Brookfield digital viscometer (model HB-DV-II+ Pro) with a small sample adapter at 200 rpm.
The electrospun fibers were mounted on metal stubs by using conductive double-sided tape and sputter-coated with gold for a period up to 300 s. The morphologies were determined by the use of a scanning electron microscope (FE-SEM, Philips XL 30S, the Netherlands). The fiber diameter was measured from the SEM images, and eight images were used for each fiber sample. From each image, at least 20 different fibers and 100 different segments were randomly selected and their diameters were measured to generate an average fiber diameter by using Photoshop v8.0.
Infrared spectra of the electrospun fiber mats were obtained using a Fourier transform infrared spectrometer (SMART-APEX II ULTRA) in the 400–4000 cm-1 range.
1H-Nuclear magnetic resonance (NMR) spectra were recorded on a DRX-500 NMR spectrometer at 500 Hz, using deuterated chloroform (CDCl3) as the solvent and tetramethylsilane as an internal standard.
The overall crystallinity of the electrospun samples was measured using a differential scanning calorimeter (TA Instruments, Q200). The sample, of about 4 mg, was placed in a sealed aluminum pan, and the measurements were carried out using heating rates of 10°C/min. The fusion heat of the TPI crystals was taken as 140.82 J/g for calculations of the degree of crystallinity . The calculation was based on the heat of fusion value.
3 Results and discussion
3.1 Parameter investigation
The morphology of the TPI fibers is controlled by the materials parameters and process parameters, including solution concentration, applied voltage, distance, feeding rate and so on . Figure 2 shows the 1H-NMR spectrum of the pure TPI. It was found that 2.03 ppm and 1.95 ppm are the double methylene protons. The peak at 5.07 ppm is the specific of double bond proton on the trans-1,4-polyisoprene. In the spectrum, no single peak of the trans-1,2 and tans-3,4 structure was discovered. The chemical shift of the methyl proton is 1.56 ppm for the trans-1,4 structure and 1.64 ppm for the cis-1,4 structure . From the ratio of areas of the two peaks within the spectra, the trans-1,4 content of TPI was estimated to be 98.4%.
Herein, we prepared various concentrations (0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%) of TPI for the electrospinning of nanofibers. Figure 3 represents the relationship between the TPI concentration (wt%) and the solution shear viscosity in centipoises. It is obvious that the viscosity of the TPI solution was increased with the increasing concentration. At low concentration (0.50%–1%), deposition of homogeneous fiber was obtained. It was assumed that the conductivity and surface tension values are almost constant throughout TPI solutions tested at low concentrations. Therefore, viscosity has been a critical parameter for the successful preparation of the homogeneous fiber network. The effect of TPI solution concentration on the morphology and diameter of fibers was investigated by SEM photomicrographs, and the results are shown in Figure 4. At low concentrations (0.5%, 1%), smooth surfaces of homogeneous fibers were obtained. However, a portion of TPI nanofibers at 0.5% concentration were collapsed due to lack of proper evaporation of solvent in time (Figure 4A). Therefore, defect-free uniform fibers were observed mainly at 1 wt% (Figure 4C). The SEM photomicrographs for electrospun TPI with 2%, 3%, and 4% by weight are presented in Figures 4E, 4G, and 4I, respectively. The diameter of fibers at low concentration was found to be between 200 nm and 750 nm (Figures 4B and 4D), while the diameter with high concentration ranges from 900 nm to 3 μm (Figures 4F, 4H and 4J).
These results can be qualitatively understood in terms of the electrospinning process. As the electrified jet travels from the syringe tip to the collector, it undergoes a bending and stretching process , during which the electrified jet and therefore the resultant polymer fiber are elongated. During this time, the fiber is subject to two competing forces: the electrostatic force acts to elongate the fiber, while the viscoelastic force dampens this elongation. Lower viscosity solutions produce smaller diameter electrospun fibers; these solutions have a decreased viscoelastic restoring force and therefore exhibit an increased elongation as they travel from the syringe to the collector . In contrast, the larger diameter fibers were obtained by the high concentration solutions with high viscosity and surface tension. Finally, the concentration solution was set at 1 wt%.
Figure 5 shows the SEM images of TPI fibers collected at different distances between the capillary tip and the counter electrode. Figures 5B and 5C show that the fibers have a smooth surface. However, the diameter of the TPI fiber was not uniform (Figure 5C) when the distance between the needle and the collector increased to 20 cm, due to the electrostatic force acting on the fiber flow for a long time during the jet travelling to the collector. At the short distance, the fiber cannot be stretched in time and this led fibers to join together (Figure 5A).
The influence of different applied electrospinning voltages on the TPI fibers morphology is shown in the Figure 6. It is important to note that the applied voltage has an insignificant effect on the diameter of the fiber. The diameter of the TPI fiber was decreased slowly with the applied voltage increased (Figure 6). However, it can be seen that some small fibers with small diameters were fractured due to the high applied voltage that was found to generate a strong electrostatic force to stretch the fiber.
The injection rates of solutions are the main factors to determine the morphology and the diameters of TPI fibers. Figure 7 shows the SEM images of TPI fibers collected at different injection rates of TPI. The uniform diameter and smooth surface fibers were collected at flow rates of 2 ml/h and 4 ml/h (Figures 7B and 7C). The diameters of TPI fibers show less uniformity either at very low rates of injection or at very high flow rates (Figures 7A, 7D and 7E).
3.2 The crystallization property
Two crystalline forms, monoclinic (α) and orthorhombic (β), have been clearly identified for TPI. It was known the α form has a monoclinic unit cell with two chains, each containing two repeat units. The β form has an orthorhombic unit cell with four chains, each containing one repeat unit. Partially crystalline TPI can exist with only the α form, only the β form, or both crystal forms present. DSC scans of TPI and its fibers given in Figure 8 indicate the presence of both α and β, as represented by two distinguished peaks at Tendo=57°C and Tendo=49°C, respectively, with predominating β form in the pure TPI . The melting temperature of each peak was found to shift toward a higher temperature while the TPI was electrospun. The peak of α crystal becomes stronger. The sharp endothermal peak around 57°C represents the α form of electrospun TPI, while the peak for the β form (around 49°C) disappeared after electrospinning. The degrees of crystallinity before and after electrospinning, which were calculated from the DSC curves, were estimated as 20.1% and 30.25%, respectively. The degree of crystallinity was increased after electrospinning of TPI fibers. This suggests a certain transformation of crystalline forms in TPI, predominantly for a transformation from the β form to α form; however, there might be a possibility of a contribution from the molecular chains in the amorphous phase, which may rearrange into an ordered form of the crystalline phase based on thermodynamics tendency during the electrospinning process.
The possible reason behind this phenomenon might be the stretching force that was found to generate from the applied electric field of the jet flow of TPI during electrospinning process. It was evident that the spinning process was also related to the applied electric field; therefore the phase transformation resulted from both the spinning and the electric field process.
FTIR spectra for both pure and electrospun TPI are presented in Figure 9 and indicate all of the possible spectral responses for α and β crystalline forms. Pure TPI predominantly shows the β crystalline form as attributed by peaks at around 1212 cm-1 representing =C-H and H-C-C in plane bending . The peaks at around 997 cm-1, 978 cm-1 and 962 cm-1 regions indicate C-C stretching for the β form . It can be noted that the presence of a trace amount of the α form was confirmed from FTIR spectra of pure TPI by observing peaks at around 1260 cm-1 and 1280 cm-1 due to CH2 twisting .
By contrast, the FTIR spectrum for electrospun TPI primarily indicates the presence of mainly the α form. Two new sharp peaks for electrospun TPI observed at 1205 cm-1 and 863 cm-1 clearly show the predominant presence of the α form as attributed by =CH inplane bending, and out of plane bending, respectively . The observed spectral responses strongly support the significant transformation of crystallinity from the β form to the α form, as inferred from DSC analysis.
TPI fibers were successfully electrospun through an optimized process and probably reported for the first time. Smooth and uniform nanofibers can be generated at the optimum conditions: concentration 1%, voltage 15 kV, TCD 15 cm and injection rate 4 ml/h. The diameter of the TPI fibers increased with increase in concentration. The transformation of crystalline structure from β form to the α form was also evident along with electrospinning process.
The authors are grateful for the financial support for the project entitled “Structural control technology of high barrier elastomeric materials (2012-0759)”. Korea Ministry of Trade, Industry & Energy. The fellowship for PS is supported by Korea Ministry of Environment as “Converging Technology Project”.
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About the article
Published Online: 2014-07-23
Published in Print: 2015-01-01