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Control of structure and morphology of highly aligned PLLA ultrafine fibers via linear-jet electrospinning
Polymer 54 (2013) 6045e6051
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Control of structure and morphology of highly aligned PLLA ultrafine fibers via linear-jet electrospinning Zongyuan Liu a, Xiong Li a, Yin Yang a, Kai Zhang a, Xuefen Wang a, b, *, Meifang Zhu a, **, Benjamin S. Hsiao b a State Key Lab for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, PR China b Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2013 Received in revised form 25 July 2013 Accepted 21 August 2013 Available online 29 August 2013
In this work, poly(L-lactic acid) (PLLA) ultrafine fibers with different morphology and structure were fabricated by a novel linear-jet electrospinning method which relies on a conventional electrospinning set-up with continuous rotating drum. To control the morphology and structure of PLLA electrospun fibers, different solution systems and electrospinning conditions were investigated. Two PLLA solution systems (PLLA/DMF/CH2Cl2 and PLLA/CH2Cl2) with different concentration and conductivity were used for the electrospinning and their influences on the formation of the linear electrospinning jet were discussed. Two types of collecting patterns with aligned buckling and linear structure were achieved under the linear electrospinning jet. Highly aligned PLLA electrospun fibers with porous surface could be formed by using the highly volatile solvent CH2Cl2. Here, it should be emphasized that the diameter and surface porosity of such highly aligned PLLA electrospun fibers can be fine tuned by varying the winding velocity. The results of SEM images and polarized FTIR investigations verified that the as-spun PLLA porous surface fibers were highly aligned and molecularly oriented, leading to the enhanced mechanical performance as compared to the non-woven PLLA electrospun fibers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Linear-jet electrospinning Aligned Molecular oriented
1. Introduction Highly aligned ultrafine fibers with porous surface structures have become one of the most prominent materials for the applications in chemical, biomedical and electrical area in recent years [1]. Especially in the area of tissue engineering, the orientation of scaffolds is needed and has shown great promise in facilitating directed neurite outgrowth within cell and animal models along the aligned substrate [2]. Furthermore, the porous surface is also considered an important parameter that affects cell adherence, cell migration and neurite orientation, since it increases the specific surface area of the cell scaffolds. Here, the as-spun PLLA fibers possessed both good alignment and large specific surface area, which makes it an attractive scaffold for tissue engineering. Electrospinning has been known as a facile and versatile method for generating ultrafine fibers with diameters ranging from nano
* Corresponding author. State Key Lab for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, PR China. Tel.: þ86 21 67792860; fax: þ86 21 67792855. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Zhu). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.08.051
scale to micro scale made of various organic or inorganic materials [3e6]. In conventional electrospinning process, the polymer solution is ejected rapidly from a nozzle, accelerated by the high electrostatic force, and deposited randomly on a collector in the form of non-woven fabrics. Recent studies have reported various modified electrospinning techniques in order to prepare highly oriented electrospun fibers instead of non-woven fibers, such as collecting electrospun fibers on a dual grounded collection plate, collecting from a rotating disk, collecting from two opposite metallic needle spinnerets etc [7e10]. As these methods focus on modifying the exterior conditions of electrospinning including collecting device, spinneret device or outer electric field distribution etc, here we proposed a facile approach that is devoted to larger-scale continuous collection of highly aligned PLLA fibers from conventional electrospinning devices by changing the necessary electrospinning solution parameters. One major problem that causes the random deposition of electrospun fibers on the collector is the presence of bending instability during the conventional electrospinning process. To overcome this barrier, in our previous work, a novel self-bundling electrospinning method [11,12] was introduced to generate macroscopically continuously aligned electrospun fiber yarns from a PLLA solution
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system (PLLA/CH2Cl2/DMF). In present study, we found that the bending instability of electrospinning jet can be suppressed by adjusting the PLLA solution concentration or system with a very significant increased viscosity and decreased conductivity, and a linear jet path without bending instability from optimized PLLA solution will be observed. In general, the linear path of electrospinning jet will generate the deposition of linearly aligned fibers on the collector. However, buckling patterns including coiled structures, folded structures and meandering structures were also obtained from this linear path of jet due to the variation of the collecting velocity. Detailed investigation of the buckling phenomenon has revealed the presence of longitudinal compressive forces acting on the impinging thread when the electrified jet deposited on the collector [13,14]. The transition from aligned buckling structures to aligned linear structures is also investigated in this work and discussed in this paper. For the porous surface structure of the electrospun polymer fibers, high volatility solvents including binary solvent system can induce phase separation and produce electrospun fibers with high surface porosity [15]. Megelski et al. [16] had discussed the mechanism of porous structures in details. They suggested that although the temperature was kept constant during the electrospinning process, evaporative cooling due to rapid solvent evaporation occurs as the jet traverses the distance between the nozzle and collector, and thus two major mechanisms, thermally-induced phase separation (TIPS) and vapor-induced phase separation (VIPS), were introduced to explain the formation of porous structure. And VIPS is considered to have a critical influence on the process of pore formation under their experimental conditions. In our work, the porous surface of highly aligned PLLA electrospun fibers can be successfully achieved by using volatile solvent CH2Cl2. In addition, different surface morphologies were investigated under various collecting speed. Smoother surface with highly aligned PLLA electrospun fibers can be obtained at higher winding speed, which will give rise to novel applications relying on anisotropism and surface properties such as sensors, cell scaffolds and drug carriers. 2. Experimental 2.1. Materials Poly-L-lactic acid (PLLA, identified as L9000) was a commercial granulate with an average molecular weight of 1.2 105 g/mol, a glass-transition temperature (Tg) of 60 C, and a melting temperature (Tm) of 170 C from Biomer of Krailling, Germany. N,N-Dimethy formamide (DMF) and Dichloromethane(CH2Cl2) were purchased from Shanghai Chemical Reagent Plant. PLLA was dried at 50 C under vacuum for 6 h before use. 2.2. Electrospinning PLLA was dissolved in CH2Cl2 pure solvent at room temperature to prepare PLLA solutions with various concentrations (10 wt%, 12 wt%). For comparison, PLLA was also dissolved in a mixture of CH2Cl2 and DMF (75/25 v/v) to make 10 wt%, 12 wt%, 14 wt% and 16 wt% solutions. To obtain linearly electrospinning jet, the typical parameters for electrospinning experiments for all PLLA solution systems were as follows: the applied electric voltage was 5 kV, the solution feed rate was 20 ml/min, the distance between the spinneret and the grounded drum was 15 cm, the room temperature was about 20 C, around 50% in the electrospinning environment. It should be noted that, the deposition patterns and the relative humidity (RH%) was of electrospinning PLLA fibers can be controlled by adjusting the linear velocity of the collecting drum. In this
experiment, the chosen velocity is 1.3 m/s (250 r/min), 2.6 m/s (500 r/min), 5.2 m/s (1000 r/min), respectively. 2.3. Characterization Conductivity measurement was carried out by a conductivity tester (DDSI-308A, Shanghai Precision Instrument Co.) at room temperature. The morphology of the electrospun sample was examined by scanning electron microscopy (SEM, JSM-5600LV, Japan). All of the samples were spurted with gold and observed under a working voltage of 10 kV, and the diameter of fibers was measured from the SEM image using image analysis software. The photograph of the electrospinning jet motion was taken by a common digital camera (Olympus E380D, Japan). The molecular orientation of the obtained highly aligned PLLA electrospun fibers was investigated by FTIR spectroscopy using a Nicolet NEXUS-670 FTIR spectrometer having a resolution of 4 cm1 with a polarized wire-grid. The ultrathin fiber layers were ripped off from the asspun highly aligned fibers as well as the non-woven electrospun fibers as testing samples for the polarized IR testing. For mechanical properties testing, the prepared aligned ultrafine PLLA electrospun fibers as well as the nonwoven membrane were examined by using a DXLL-2000 universal materials testing machine (Chemical engineering equipment company, Shanghai) at 10 mm/s crosshead speed at room temperature. Three specimens were measured for each sample. Each specimen was rolled up like a tow to reach test accuracy requirement for tensile testing machine and the specimen sizes were 40 mm 15 mm 0.04 mm (length width thickness). 3. Results and discussion 3.1. Formation of the linear jet In our previous study, a facile self-bundling electrospinning method for preparation of aligned semi-hollow PLLA electrospun fibers from homogeneous polymer-solvent-nonsolvent system (PLLA/CH2Cl2/DMF) was introduced [11]. For PLLA/CH2Cl2/DMF solution system, the electrospinning process meets in conventional electrospinning with severe extent of bending instabilities with 10 wt% PLLA concentration. As can be seen from Fig. 1a, there was obviously a short straight segment followed by a typical bending instability in the path of the jet. When PLLA concentration increased to 12 wt%, the straight part of the path was elongated and the bending part of the path was restricted into very thin line (Fig. 1b). As the concentration further increased to 16 wt%, the bending part disappeared totally and then a steady linear jet path was established, as shown in Fig. 1c. Interestingly, for PLLA/CH2Cl2 system without containing nonsolvent DMF, the linear jet path can be observed when the solution concentration was in the range of 10w12 wt% (Fig. 2a and b). It is believed that the solution concentration, viscosity and conductivity may play an important role in the formation of such linear jet path. Further investigations will reveal that the origin of this phenomenon is predominantly caused by the viscosity and conductivity of electrospinning solution. For the influence of viscosity, as reported by other researchers [16], the thickness of the electrospinning jet has a strong dependence on the polymer solution viscosity, and the viscosity increases with the increase of polymer concentration due to the higher numbers of polymer chain entanglements. Thus the higher solution viscosity enables a thicker electrospinning jet due to a harder electrospinning stretching on the jet surface. As a result, the jet path generated a longer straight segment and continued to elongate until it is thin enough before the electrical energy is able to cause the jet path to coil. In the PLLA/CH2Cl2/DMF system, when the polymer
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compared to the conductivity of DMF which is 8.23 ms/cm. This reveals that the conductivity of the electrospinning solution plays a key role in influencing the Coulomb repulsion of the charge. Reneker et al. [17] believe that the electric bending coils are caused by the Coulomb repulsive forces and the uniformly distributed charges carried with the jet. The quantity of the electric charges has a close relationship with the solution conductivity and more charges can be carried on the jet surface at higher solution conductivity [18]. As the solution conductivity decreased, the quantity of electric charges carried on the jet surface decreased, results in a smaller Coulomb repulsive force present on the jet surface and the bending instability will be suppressed. 3.2. Aligned PLLA ultrafine electrospun fibers via linear-jet electrospinning
Fig. 1. Photographs of the generated jet electrospun from a PLLA/CHCl2/DMF solution system (75/25 CH2Cl2/DMF) with different polymer concentrations (a) 10 wt%, (b) 12 wt%, and (c) 16 wt%.
concentration increased to 16 wt%, a very long straight segment established since the electrical energy is unable to cause bending instability of the jet. In addition, the changes in conductivity of PLLA solution also affect on the formation of the linear straight jet. As can been seen from Figs. 1 and 2, when the PLLA concentration was set at 12 wt%, the very thin linear jet path had already formed in the PLLA/ CH2Cl2 system without DMF. While in the PLLA/CH2Cl2/DMF system, the electrospinning jet continued to be coiled until the concentration increased to 16 wt%. Table 1 listed the conductivities of PLLA solutions with/without the addition of DMF. With the addition of DMF into the solution, the conductivity increased from 0.51 ms/cm to 26.5 ms/cm, since pure CH2Cl2 has a lower conductivity 0.24 ms/cm as
From the results depicted above, choosing the appropriate solution systems and concentration will give rise to the suppression of the bending instability so that the regularly arranged collection of PLLA fibers can be controlled by a linear straight jet. Here, two kinds of PLLA ultrafine electrospun fibers were fabricated via this linear-jet electrospinning (as shown in Fig. 3) by adjusting the winding velocity of the collecting drum. The typical SEM images of electrospun fibers were shown in Fig. 4. For 12 wt% PLLA/CH2Cl2 solution system, when the collecting drum was motionless, the PLLA fibers deposited randomly and a non-woven fibrous membrane was obtained as shown in Fig. 4a. The buckled patterns of small twists with folded structure were formed as the straight linear jet impinging onto a collecting drum when the winding velocity was 1.3 m/s (250 r/min) (Fig. 4b). However, when the winding velocity increased to 2.6 m/s (500 r/min), the collected PLLA fibers presented a linearly arranged morphology instead of buckling patterns (Fig. 4c). The only difference is the winding velocity, indicating that winding velocity plays a key role in forming these two different morphologies. Tao Han et al. [13] have studied this kind of phenomenon and discussed the velocity, diameter, density and viscosity of the electrospinning jets and their impact on the formation of buckling patterns. It is believed that the buckling patterns are resulted from the presence of a force imbalance caused by longitudinal compressive forces acting on the interface when charged jet impinging onto collector moving at a constant velocity. Additionally, R. Kessick et al. [19] also revealed another mechanism for the formation of such buckling pattern. They believed that the Coulomb repulsive force on the jet surface is initially balanced by the viscoelastic restoring force of the polymer jet. But when the polymer jet impinges on a conductive collector, a force imbalance emerges since a fraction of the charge carried by the jet is transferred to the conductive collector, thus the polymer jet undergoes a structural change and contracts into the observed buckling patterns in order to restore equilibrium. For the viscous PLLA/CH2Cl2 solution system, it can be explained by the relationship between jet impinging speed and the collecting drum winding speed [20]. When the winding speed is lower than the jet impinging speed, the force imbalance arises at the interface of PLLA electrified jet and conductive collecting drum, then the jet starts to perform buckling process. If the winding speed increases to be very similar or higher than the jet impinging speed, the collecting drum will generate a stretching force suppressing the longitudinal compressive forces on the jet surface so that linearly arranged PLLA electrospun fibers were obtained. For the 16 wt% PLLA/CH2Cl2/DMF system, the generated linearly jet path also produced aligned buckling patterns on the collector with the low winding speed (1.3 m/s). As can be seen in Fig. 4c, the buckling patterns presented a very regular helical structure. But the increased solution conductivity by adding DMF in the solution
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Fig. 2. Photographs of the generated jet electrospun from a PLLA/CH2Cl2 solution system with different polymer concentrations (a) 10 wt%, (b) 12 wt%.
system gives rise to an enhanced Coulomb repulsive force on the jet surface resulting in a tighter buckling pattern, so that a higher winding speed is needed to restore the force balance when it impinges on the collector. As shown in Fig. 4d, it is very hard to stretch the buckles to enable a linearly arranged fibers with the winding speed increased to the 5.2 m/s (1000 r/min, the highest speed of the collecting device in our lab). However, it is also believed that the transition from such helical structure to the linearly arranged electrospun fibers can be observed with high enough take-up speed. It also should be noted that the winding velocity of the collecting drum not only controls the deposited morphology of the regularly arranged PLLA fibers, but also had impacts on the surface structure of the fibers.
3.3. Porous surface structure Based on the above experimental observation, the collected PLLA fibers will present the aligned arranged patterns from 12 wt% PLLA/CH2Cl2 solution system via linear-jet electrospinning method. And the buckling patterns will be substituted with the linearly arranged fibers when the collecting velocity increased from 1.3 m/s to 2.6 m/s. Interestingly, the surface structure of the linearly arranged PLLA fibers can be varied with the different collecting velocities as shown in Fig. 5. As can be seen that, the surface structure of PLLA fibers from porous surface (Fig. 5a) to smooth surface (Fig. 5b) is observed with the winding speed increased from 2.6 m/s to 5.2 m/s, and the average fiber diameter decreases from 4.5 mm to 1.8 mm simultaneously. To understand the mechanism of the microstructure change on the PLLA surface, a schematic regarding the procedure is illustrated in Fig. 6. In our previous report, the phase separation was the main reason to induce the porous structure formation [11]. In this work, when the electrified jet is ejected from nozzle, it accelerates towards the grounded collector and elongates rapidly. Within milliseconds the surface area of the jet is increased dramatically and this leads to an increased rate of solvent evaporation. Because of the high volatility of CH2Cl2, the evaporative cooling due to rapid solvent evaporation could lead to formation of thermodynamically unstable system, which results in a phase separation including polymer-rich and polymer-poor phases on the
fiber surface, as shown in the schematic Fig. 6a. As the fiber solidifies, the concentrated phase remains, whereas the polymer poor phase forms pores on the fiber surface (Fig. 6c). The process described above is the so-called “thermally induced phase separation” (TIPS) and it is one of the phase separation mechanisms widely discussed in the literature [21e24]. In addition, the occurrence of breath figures is another possible mechanism to explain the formation of porous surface described by Srinivasarao et al. [25]. The evaporative cooling leads to the condensation of moisture in the air and therefore grows water droplets on the fiber surface. As the jet dries, the water droplet evaporates and leaves a pore on the surface. But the breath figure was not considered as the principal factor in this case since the air humidity of the ambient environment is controlled below 50% in our laboratory. Furthermore, the porosity also can be controlled by varying the winding speed of the collecting drum. When winding speed increased to 5.2 m/s (1000 r/min), the as-spun linearly arranged PLLA fiber had a smaller diameter (around 3 mm in diameter) and a smoother surface with smaller and sparser pores. This could be explained that the intense stretching force generated by high winding speed has an impact on the phase separation and porous surface formation. The intense stretching force will accelerate the phase separation. Meanwhile, large volume of solvent will be extruded from the solution jet to the surface and evaporate rapidly. In this case, the phase-separated region would not have enough time to form distinct polymer-rich phase and polymer-lean phase. Even if the phase separations transform into a spot of polymer-rich and polymer-poor phases, the intense stretching force on the jet surface will also make the pores elongated and closed along the fiber axis which is apparent from Fig. 6d. It is also believed that by further increasing the winding speed of the drum, the obtained PLLA fiber will continue to decrease in diameter and surface porosity if a high-speed collecting instrument is applied.
Table 1 The conductivities of various concentrations of PLLA solution with/without the addition of DMF. Concentration (wt%)
DMF:CH2Cl2 (v/v)
Conductivity (ms/m)
10 12 10 12 16
0:100 0:100 25:75 25:75 25:75
0.46 0.51 29.1 26.5 23.2
Fig. 3. Photograph of the linear-jet electrospinning process with a photograph of the yielded PLLA fiber in the insert.
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Fig. 4. SEM images of PLLA fibers electrospun by the linear-jet electrospinning method from 12 wt% PLLA/CH2Cl2 solution system with 0.0 m/s (a, 0 r/min), 1.3 m/s (b, 250 r/min), 5.6 m/s (c, 1000 r/min) winding velocity, and from 16 wt% PLLA/CH2Cl2/DMF solution system (75/25 CH2Cl2/DMF) with 1.3 m/s (d, 250 r/min), 5.6 m/s (e, 1000 r/min) winding velocity.
3.4. Orientation investigation and discussion The molecular orientation of the linearly arranged PLLA fibers was also investigated by polarized FTIR. The FTIR spectra from parallel and perpendicular to the highly oriented and non-woven electrospun PLLA fibers were shown in Fig. 7a and b, respectively. In Fig. 7a, the distinct difference in absorbance intensities of the highly oriented PLLA fibers electrospun from 12 wt% PLLA/CH2Cl2 solution system was observed by this polarized measurement. When
the electric vector is parallel to the fiber axis, the bands attributed to the CeOeC stretch at 1080 cm1, the O]CeC stretch at 1180 cm1 and the eCH3 variable angle at 1365 cm1 and 1450 cm1 have distinct higher absorbance intensities than that is perpendicular to the fiber axis. However, in Fig. 7b, there is no marked difference in absorbance intensities when the electric vector is parallel or perpendicular to the non-woven PLLA fibers electrospun from the same PLLA system (12 wt% in CH2Cl2), which shows an isotropic property in the non-woven PLLA electrospun fibers. The absorbance
Fig. 5. SEM images of PLLA fibers with different surface structure electrospun from 12 wt% PLLA/CH2Cl2 solution system: surface image of the as-spun fibers with (a) 2.6 m/s (500 r/ min) and (b) 5.2 m/s (1000 r/min) winding speed, the insert image show the enlarged parts of the corresponding images.
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Z. Liu et al. / Polymer 54 (2013) 6045e6051 Table 2 Dichroic ratios at different bands.
Fig. 6. Schematic illustration of the sequential procedure for the formation of the two kinds of surface structure of the as-spun PLLA electrospun fibers, in which the small yellow balls represent the polymer-poor phase: (a) Initial phase separation occurred on the jet surface; (b) Solidification process of the jet influenced by solvent evaporation and surface stretching force; (c) Pores remained on the surface due to polymerpoor phase evaporation; (d) Pores might be closed due to the stretching force generated by high winding velocity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
intensities discrepancy between Fig. 7a and b can be attributed to the molecular orientation of polymer backbones along the polymer axis. The dichroic ratio from IR absorption is used to characterize the degree of molecular orientation. The dichroic ratio for a particular absorbance is defined by D ¼ A///At , where A// andAt represent the
Wavenumber (cm1)
A//
At
R
1080 1180 1365 1450
1.70 1.41 0.59 0.68
1.11 0.90 0.38 0.45
1.53 1.56 1.55 1.51
electric vector direction of the polarizer is oriented parallel or perpendicular to the fiber direction, respectively. Table 2 listed the calculated dichroic ratios at different bands and it is found that the R is very close to around 1.5 which indicated a good anisotropic property of the highly aligned PLLA fibers due to a significant amount of uniaxial orientation of the polymer chains. It is reported that, during conventional electrospinning process, the orientation of polymer chains was hardly observed in the as-spun fibers [26]. The strain rate in the conventional electrospinning process is very high and obvious birefringence of the electrified jet was observed, but most of the stretching occurs before the complete solidification of the jet, so that the molecular chain still had enough time to dissipate and relax the molecular orientation after deposited on the collector. In this work, however, the high mechanical winding of collecting drum contributes to the formation of molecular orientation. Additionally, the use of volatile CH2Cl2 and a higher concentration of polymer solution accelerate the solidification process of the electrospinning jet, and therefore a high degree of molecular orientation in the highly aligned PLLA fibers will be preserved instead of the random orientation in the non-woven ones.
3.5. Mechanical performance For mechanical enhancement, a high degree of molecular orientation and crystallization in the electrospun fibers is particularly important. In Fig. 8, curves a and b show the typical stressestrain curves of the as-spun linearly arranged PLLA fibers with different collecting speed (5.2 m/s and 2.6 m/s), respectively, from the 12 wt% PLLA/CH2Cl2 solution. For comparison, the non-woven PLLA fibers electrospun from the same solution system is also tested and shown in the curve c. As can been seen, the prepared PLLA non-woven membrane had a weak tensile strength (3 Mpa at break) with 118% elongation at break. In contrast, the highly aligned PLLA fibers exhibited a significantly higher tensile strength and larger
Fig. 7. Polarized FT-IR spectra of as-spun PLLA electrospun fibers from 12 wt% PLLA/ CH2Cl2 solution system: (a) highly aligned PLLA fibers; (b) non-woven PLLA fibers.
Fig. 8. Stressestrain curve of as-spun PLLA fibers from 12 wt% PLLA/CH2Cl2 solution system: (a) highly aligned PLLA fiber, 1000 r/min; (b) highly aligned PLLA fiber, 500 r/ min; (c) non-woven PLLA fiber.
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elongation: 13.9 Mpa and 211% for 5.2 m/s collecting speed, 4.5 Mpa and 188% for 2.6 m/s collecting speed, respectively. This result indicated that the alignment and higher molecular orientation of the PLLA fibers yielded by the linear-jet electrospinning method contributes to the enhancement of mechanical properties. Additionally, with increased collecting speed from 2.6 m/s to 5.2 m/s, an obvious increase of tensile strength from 4.5 Mpa to 13.9 Mpa can been seen from curve a and b, which indicated that the higher molecular orientation and lower surface porosity of the fibers due to higher collecting speed enhanced the mechanical performance. 4. Conclusion In summary, continuous highly oriented PLLA electrospun fibers with linear alignment or with helical alignment were successfully prepared by novel linear-jet electrospinning method from PLLA/ CH2Cl2 solution system with or without the addition of DMF, respectively. It is a very facile method to prepare large-scale continuous uniaxially oriented electrospun fibers via conventional electrospinning set-up. The results indicated that the higher concentration and lower conductivity of the polymer solution contribute to the formation of the single linear jet path during the electrospinning process. Different surface structures of PLLA highly oriented fibers electrospun from PLLA/CH2Cl2 system can be controlled by varying the winding velocity of the collecting drum, and the phase separation caused by the high volatile CH2Cl2 is the key factor leading to the microstructure formation. The highly oriented PLLA electrospun fibers with different surface structures are expected to find potential application in microelectronics, tissue engineering and optics. Acknowledgments This work was supported by National Science Foundation of China (51273042, 20874009, 21174028), Innovation Program of
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Shanghai Municipal Education Commission, National 863 Program of China (2012AA030309), Program of Changjiang Scholars and Innovative Research Team in University (IRT1221), Fundamental Research Funds for the Central Universities and 111 project (111-2-04). References [1] Greiner A, Wendorff JH. Angew Chem Int Edit 2007;46:5670e703. [2] Wang HB, Mullins ME, Cregg JM, McCarthy CW, Gilbert RJ. Acta Biomater 2010;6:2970e8. [3] Reneker DH, Chun I. Nanotechnology 1996;7:216e23. [4] Li D, Xia Y. Adv Mater 2004;16:1151e70. [5] Huang ZM, Zhang YZ, Kotaki M, Ramakrishna S. Compos Sci Technol 2003;63: 2223e53. [6] Bayley GM, Mallon PE. Polymer 2012;53:5523e39. [7] Sawawi M, Wang TY, Nisbet DR, Simon GP. Polymer 2013;54:4237e53. [8] Li D, Wang YL, Xia Y. Adv Mater 2004;16:361e6. [9] Xu CY, Inai R, Kotaki M, Ramakrishna S. Biomaterials 2004;25:877e86. [10] Pan H, Li L, Hu L, Cui X. Polymer 2006;47:4901e4. [11] Zhang K, Wang XF, Yang Y, Wang LL, Zhu MF, Hsiao BS, et al. J Polym Sci Pol Phys 2010;48:1118e25. [12] Wang XF, Zhang K, Zhu MF, Yu H, Zhou Z, Chen YM, et al. Polymer 2008;49: 2755e61. [13] Han T, Reneker DH, Yarin AL. Polymer 2007;48:6064e76. [14] Liu K, Ertley CD, Reneker DH. Polymer 2012;53:4241e53. [15] Luo CJ, Nangrejo M, Edirisinghe M. Polymer 2010;51:1654e62. [16] Megelski S, Stephens JS, Chase DB, Rabolt JF. Macromolecules 2002;35: 8456e66. [17] Reneker DH, Yarin AL. Polymer 2008;49:2387e425. [18] Uyar T, Besenbacher F. Polymer 2008;49:5336e43. [19] Kessick R, Tepper G. Appl Phys Lett 2004;84:4807e9. [20] Hellmann C, Belardi J, Dersch R, Greiner A, Wendorff JH, Bahnmueller S. Polymer 2009;50:1197e205. [21] Chen JS, Tu SL, Tsay RY. J Taiwan Inst Chem E 2010;41:229e38. [22] Pavia FC, Carrubba VL, Piccarolo S, Brucato V. J Biomed Mater Res A 2008;86A: 459e66. [23] Matsuyama H, Teramoto M, Kuwana M, Kitamura Y. Polymer 2000;41: 8673e9. [24] Song SW, Torkelson JM. J Membr Sci 1995;98:209e22. [25] Srinivasarao M, Collings D, Philips A, Patel S. Science 2001;292:79e83. [26] Reneker DH, Yarin AL, Zussman E, Xu H. Adv Appl Mech 2007;41:43e195.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Control of structure and morphology of highly aligned PLLA ultrafine fibers via linear-jet electrospinning Zongyuan Liu a, Xiong Li a, Yin Yang a, Kai Zhang a, Xuefen Wang a, b, *, Meifang Zhu a, **, Benjamin S. Hsiao b a State Key Lab for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, PR China b Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2013 Received in revised form 25 July 2013 Accepted 21 August 2013 Available online 29 August 2013
In this work, poly(L-lactic acid) (PLLA) ultrafine fibers with different morphology and structure were fabricated by a novel linear-jet electrospinning method which relies on a conventional electrospinning set-up with continuous rotating drum. To control the morphology and structure of PLLA electrospun fibers, different solution systems and electrospinning conditions were investigated. Two PLLA solution systems (PLLA/DMF/CH2Cl2 and PLLA/CH2Cl2) with different concentration and conductivity were used for the electrospinning and their influences on the formation of the linear electrospinning jet were discussed. Two types of collecting patterns with aligned buckling and linear structure were achieved under the linear electrospinning jet. Highly aligned PLLA electrospun fibers with porous surface could be formed by using the highly volatile solvent CH2Cl2. Here, it should be emphasized that the diameter and surface porosity of such highly aligned PLLA electrospun fibers can be fine tuned by varying the winding velocity. The results of SEM images and polarized FTIR investigations verified that the as-spun PLLA porous surface fibers were highly aligned and molecularly oriented, leading to the enhanced mechanical performance as compared to the non-woven PLLA electrospun fibers. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Linear-jet electrospinning Aligned Molecular oriented
1. Introduction Highly aligned ultrafine fibers with porous surface structures have become one of the most prominent materials for the applications in chemical, biomedical and electrical area in recent years [1]. Especially in the area of tissue engineering, the orientation of scaffolds is needed and has shown great promise in facilitating directed neurite outgrowth within cell and animal models along the aligned substrate [2]. Furthermore, the porous surface is also considered an important parameter that affects cell adherence, cell migration and neurite orientation, since it increases the specific surface area of the cell scaffolds. Here, the as-spun PLLA fibers possessed both good alignment and large specific surface area, which makes it an attractive scaffold for tissue engineering. Electrospinning has been known as a facile and versatile method for generating ultrafine fibers with diameters ranging from nano
* Corresponding author. State Key Lab for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, PR China. Tel.: þ86 21 67792860; fax: þ86 21 67792855. ** Corresponding author. E-mail addresses: [email protected] (X. Wang), [email protected] (M. Zhu). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.08.051
scale to micro scale made of various organic or inorganic materials [3e6]. In conventional electrospinning process, the polymer solution is ejected rapidly from a nozzle, accelerated by the high electrostatic force, and deposited randomly on a collector in the form of non-woven fabrics. Recent studies have reported various modified electrospinning techniques in order to prepare highly oriented electrospun fibers instead of non-woven fibers, such as collecting electrospun fibers on a dual grounded collection plate, collecting from a rotating disk, collecting from two opposite metallic needle spinnerets etc [7e10]. As these methods focus on modifying the exterior conditions of electrospinning including collecting device, spinneret device or outer electric field distribution etc, here we proposed a facile approach that is devoted to larger-scale continuous collection of highly aligned PLLA fibers from conventional electrospinning devices by changing the necessary electrospinning solution parameters. One major problem that causes the random deposition of electrospun fibers on the collector is the presence of bending instability during the conventional electrospinning process. To overcome this barrier, in our previous work, a novel self-bundling electrospinning method [11,12] was introduced to generate macroscopically continuously aligned electrospun fiber yarns from a PLLA solution
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system (PLLA/CH2Cl2/DMF). In present study, we found that the bending instability of electrospinning jet can be suppressed by adjusting the PLLA solution concentration or system with a very significant increased viscosity and decreased conductivity, and a linear jet path without bending instability from optimized PLLA solution will be observed. In general, the linear path of electrospinning jet will generate the deposition of linearly aligned fibers on the collector. However, buckling patterns including coiled structures, folded structures and meandering structures were also obtained from this linear path of jet due to the variation of the collecting velocity. Detailed investigation of the buckling phenomenon has revealed the presence of longitudinal compressive forces acting on the impinging thread when the electrified jet deposited on the collector [13,14]. The transition from aligned buckling structures to aligned linear structures is also investigated in this work and discussed in this paper. For the porous surface structure of the electrospun polymer fibers, high volatility solvents including binary solvent system can induce phase separation and produce electrospun fibers with high surface porosity [15]. Megelski et al. [16] had discussed the mechanism of porous structures in details. They suggested that although the temperature was kept constant during the electrospinning process, evaporative cooling due to rapid solvent evaporation occurs as the jet traverses the distance between the nozzle and collector, and thus two major mechanisms, thermally-induced phase separation (TIPS) and vapor-induced phase separation (VIPS), were introduced to explain the formation of porous structure. And VIPS is considered to have a critical influence on the process of pore formation under their experimental conditions. In our work, the porous surface of highly aligned PLLA electrospun fibers can be successfully achieved by using volatile solvent CH2Cl2. In addition, different surface morphologies were investigated under various collecting speed. Smoother surface with highly aligned PLLA electrospun fibers can be obtained at higher winding speed, which will give rise to novel applications relying on anisotropism and surface properties such as sensors, cell scaffolds and drug carriers. 2. Experimental 2.1. Materials Poly-L-lactic acid (PLLA, identified as L9000) was a commercial granulate with an average molecular weight of 1.2 105 g/mol, a glass-transition temperature (Tg) of 60 C, and a melting temperature (Tm) of 170 C from Biomer of Krailling, Germany. N,N-Dimethy formamide (DMF) and Dichloromethane(CH2Cl2) were purchased from Shanghai Chemical Reagent Plant. PLLA was dried at 50 C under vacuum for 6 h before use. 2.2. Electrospinning PLLA was dissolved in CH2Cl2 pure solvent at room temperature to prepare PLLA solutions with various concentrations (10 wt%, 12 wt%). For comparison, PLLA was also dissolved in a mixture of CH2Cl2 and DMF (75/25 v/v) to make 10 wt%, 12 wt%, 14 wt% and 16 wt% solutions. To obtain linearly electrospinning jet, the typical parameters for electrospinning experiments for all PLLA solution systems were as follows: the applied electric voltage was 5 kV, the solution feed rate was 20 ml/min, the distance between the spinneret and the grounded drum was 15 cm, the room temperature was about 20 C, around 50% in the electrospinning environment. It should be noted that, the deposition patterns and the relative humidity (RH%) was of electrospinning PLLA fibers can be controlled by adjusting the linear velocity of the collecting drum. In this
experiment, the chosen velocity is 1.3 m/s (250 r/min), 2.6 m/s (500 r/min), 5.2 m/s (1000 r/min), respectively. 2.3. Characterization Conductivity measurement was carried out by a conductivity tester (DDSI-308A, Shanghai Precision Instrument Co.) at room temperature. The morphology of the electrospun sample was examined by scanning electron microscopy (SEM, JSM-5600LV, Japan). All of the samples were spurted with gold and observed under a working voltage of 10 kV, and the diameter of fibers was measured from the SEM image using image analysis software. The photograph of the electrospinning jet motion was taken by a common digital camera (Olympus E380D, Japan). The molecular orientation of the obtained highly aligned PLLA electrospun fibers was investigated by FTIR spectroscopy using a Nicolet NEXUS-670 FTIR spectrometer having a resolution of 4 cm1 with a polarized wire-grid. The ultrathin fiber layers were ripped off from the asspun highly aligned fibers as well as the non-woven electrospun fibers as testing samples for the polarized IR testing. For mechanical properties testing, the prepared aligned ultrafine PLLA electrospun fibers as well as the nonwoven membrane were examined by using a DXLL-2000 universal materials testing machine (Chemical engineering equipment company, Shanghai) at 10 mm/s crosshead speed at room temperature. Three specimens were measured for each sample. Each specimen was rolled up like a tow to reach test accuracy requirement for tensile testing machine and the specimen sizes were 40 mm 15 mm 0.04 mm (length width thickness). 3. Results and discussion 3.1. Formation of the linear jet In our previous study, a facile self-bundling electrospinning method for preparation of aligned semi-hollow PLLA electrospun fibers from homogeneous polymer-solvent-nonsolvent system (PLLA/CH2Cl2/DMF) was introduced [11]. For PLLA/CH2Cl2/DMF solution system, the electrospinning process meets in conventional electrospinning with severe extent of bending instabilities with 10 wt% PLLA concentration. As can be seen from Fig. 1a, there was obviously a short straight segment followed by a typical bending instability in the path of the jet. When PLLA concentration increased to 12 wt%, the straight part of the path was elongated and the bending part of the path was restricted into very thin line (Fig. 1b). As the concentration further increased to 16 wt%, the bending part disappeared totally and then a steady linear jet path was established, as shown in Fig. 1c. Interestingly, for PLLA/CH2Cl2 system without containing nonsolvent DMF, the linear jet path can be observed when the solution concentration was in the range of 10w12 wt% (Fig. 2a and b). It is believed that the solution concentration, viscosity and conductivity may play an important role in the formation of such linear jet path. Further investigations will reveal that the origin of this phenomenon is predominantly caused by the viscosity and conductivity of electrospinning solution. For the influence of viscosity, as reported by other researchers [16], the thickness of the electrospinning jet has a strong dependence on the polymer solution viscosity, and the viscosity increases with the increase of polymer concentration due to the higher numbers of polymer chain entanglements. Thus the higher solution viscosity enables a thicker electrospinning jet due to a harder electrospinning stretching on the jet surface. As a result, the jet path generated a longer straight segment and continued to elongate until it is thin enough before the electrical energy is able to cause the jet path to coil. In the PLLA/CH2Cl2/DMF system, when the polymer
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compared to the conductivity of DMF which is 8.23 ms/cm. This reveals that the conductivity of the electrospinning solution plays a key role in influencing the Coulomb repulsion of the charge. Reneker et al. [17] believe that the electric bending coils are caused by the Coulomb repulsive forces and the uniformly distributed charges carried with the jet. The quantity of the electric charges has a close relationship with the solution conductivity and more charges can be carried on the jet surface at higher solution conductivity [18]. As the solution conductivity decreased, the quantity of electric charges carried on the jet surface decreased, results in a smaller Coulomb repulsive force present on the jet surface and the bending instability will be suppressed. 3.2. Aligned PLLA ultrafine electrospun fibers via linear-jet electrospinning
Fig. 1. Photographs of the generated jet electrospun from a PLLA/CHCl2/DMF solution system (75/25 CH2Cl2/DMF) with different polymer concentrations (a) 10 wt%, (b) 12 wt%, and (c) 16 wt%.
concentration increased to 16 wt%, a very long straight segment established since the electrical energy is unable to cause bending instability of the jet. In addition, the changes in conductivity of PLLA solution also affect on the formation of the linear straight jet. As can been seen from Figs. 1 and 2, when the PLLA concentration was set at 12 wt%, the very thin linear jet path had already formed in the PLLA/ CH2Cl2 system without DMF. While in the PLLA/CH2Cl2/DMF system, the electrospinning jet continued to be coiled until the concentration increased to 16 wt%. Table 1 listed the conductivities of PLLA solutions with/without the addition of DMF. With the addition of DMF into the solution, the conductivity increased from 0.51 ms/cm to 26.5 ms/cm, since pure CH2Cl2 has a lower conductivity 0.24 ms/cm as
From the results depicted above, choosing the appropriate solution systems and concentration will give rise to the suppression of the bending instability so that the regularly arranged collection of PLLA fibers can be controlled by a linear straight jet. Here, two kinds of PLLA ultrafine electrospun fibers were fabricated via this linear-jet electrospinning (as shown in Fig. 3) by adjusting the winding velocity of the collecting drum. The typical SEM images of electrospun fibers were shown in Fig. 4. For 12 wt% PLLA/CH2Cl2 solution system, when the collecting drum was motionless, the PLLA fibers deposited randomly and a non-woven fibrous membrane was obtained as shown in Fig. 4a. The buckled patterns of small twists with folded structure were formed as the straight linear jet impinging onto a collecting drum when the winding velocity was 1.3 m/s (250 r/min) (Fig. 4b). However, when the winding velocity increased to 2.6 m/s (500 r/min), the collected PLLA fibers presented a linearly arranged morphology instead of buckling patterns (Fig. 4c). The only difference is the winding velocity, indicating that winding velocity plays a key role in forming these two different morphologies. Tao Han et al. [13] have studied this kind of phenomenon and discussed the velocity, diameter, density and viscosity of the electrospinning jets and their impact on the formation of buckling patterns. It is believed that the buckling patterns are resulted from the presence of a force imbalance caused by longitudinal compressive forces acting on the interface when charged jet impinging onto collector moving at a constant velocity. Additionally, R. Kessick et al. [19] also revealed another mechanism for the formation of such buckling pattern. They believed that the Coulomb repulsive force on the jet surface is initially balanced by the viscoelastic restoring force of the polymer jet. But when the polymer jet impinges on a conductive collector, a force imbalance emerges since a fraction of the charge carried by the jet is transferred to the conductive collector, thus the polymer jet undergoes a structural change and contracts into the observed buckling patterns in order to restore equilibrium. For the viscous PLLA/CH2Cl2 solution system, it can be explained by the relationship between jet impinging speed and the collecting drum winding speed [20]. When the winding speed is lower than the jet impinging speed, the force imbalance arises at the interface of PLLA electrified jet and conductive collecting drum, then the jet starts to perform buckling process. If the winding speed increases to be very similar or higher than the jet impinging speed, the collecting drum will generate a stretching force suppressing the longitudinal compressive forces on the jet surface so that linearly arranged PLLA electrospun fibers were obtained. For the 16 wt% PLLA/CH2Cl2/DMF system, the generated linearly jet path also produced aligned buckling patterns on the collector with the low winding speed (1.3 m/s). As can be seen in Fig. 4c, the buckling patterns presented a very regular helical structure. But the increased solution conductivity by adding DMF in the solution
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Fig. 2. Photographs of the generated jet electrospun from a PLLA/CH2Cl2 solution system with different polymer concentrations (a) 10 wt%, (b) 12 wt%.
system gives rise to an enhanced Coulomb repulsive force on the jet surface resulting in a tighter buckling pattern, so that a higher winding speed is needed to restore the force balance when it impinges on the collector. As shown in Fig. 4d, it is very hard to stretch the buckles to enable a linearly arranged fibers with the winding speed increased to the 5.2 m/s (1000 r/min, the highest speed of the collecting device in our lab). However, it is also believed that the transition from such helical structure to the linearly arranged electrospun fibers can be observed with high enough take-up speed. It also should be noted that the winding velocity of the collecting drum not only controls the deposited morphology of the regularly arranged PLLA fibers, but also had impacts on the surface structure of the fibers.
3.3. Porous surface structure Based on the above experimental observation, the collected PLLA fibers will present the aligned arranged patterns from 12 wt% PLLA/CH2Cl2 solution system via linear-jet electrospinning method. And the buckling patterns will be substituted with the linearly arranged fibers when the collecting velocity increased from 1.3 m/s to 2.6 m/s. Interestingly, the surface structure of the linearly arranged PLLA fibers can be varied with the different collecting velocities as shown in Fig. 5. As can be seen that, the surface structure of PLLA fibers from porous surface (Fig. 5a) to smooth surface (Fig. 5b) is observed with the winding speed increased from 2.6 m/s to 5.2 m/s, and the average fiber diameter decreases from 4.5 mm to 1.8 mm simultaneously. To understand the mechanism of the microstructure change on the PLLA surface, a schematic regarding the procedure is illustrated in Fig. 6. In our previous report, the phase separation was the main reason to induce the porous structure formation [11]. In this work, when the electrified jet is ejected from nozzle, it accelerates towards the grounded collector and elongates rapidly. Within milliseconds the surface area of the jet is increased dramatically and this leads to an increased rate of solvent evaporation. Because of the high volatility of CH2Cl2, the evaporative cooling due to rapid solvent evaporation could lead to formation of thermodynamically unstable system, which results in a phase separation including polymer-rich and polymer-poor phases on the
fiber surface, as shown in the schematic Fig. 6a. As the fiber solidifies, the concentrated phase remains, whereas the polymer poor phase forms pores on the fiber surface (Fig. 6c). The process described above is the so-called “thermally induced phase separation” (TIPS) and it is one of the phase separation mechanisms widely discussed in the literature [21e24]. In addition, the occurrence of breath figures is another possible mechanism to explain the formation of porous surface described by Srinivasarao et al. [25]. The evaporative cooling leads to the condensation of moisture in the air and therefore grows water droplets on the fiber surface. As the jet dries, the water droplet evaporates and leaves a pore on the surface. But the breath figure was not considered as the principal factor in this case since the air humidity of the ambient environment is controlled below 50% in our laboratory. Furthermore, the porosity also can be controlled by varying the winding speed of the collecting drum. When winding speed increased to 5.2 m/s (1000 r/min), the as-spun linearly arranged PLLA fiber had a smaller diameter (around 3 mm in diameter) and a smoother surface with smaller and sparser pores. This could be explained that the intense stretching force generated by high winding speed has an impact on the phase separation and porous surface formation. The intense stretching force will accelerate the phase separation. Meanwhile, large volume of solvent will be extruded from the solution jet to the surface and evaporate rapidly. In this case, the phase-separated region would not have enough time to form distinct polymer-rich phase and polymer-lean phase. Even if the phase separations transform into a spot of polymer-rich and polymer-poor phases, the intense stretching force on the jet surface will also make the pores elongated and closed along the fiber axis which is apparent from Fig. 6d. It is also believed that by further increasing the winding speed of the drum, the obtained PLLA fiber will continue to decrease in diameter and surface porosity if a high-speed collecting instrument is applied.
Table 1 The conductivities of various concentrations of PLLA solution with/without the addition of DMF. Concentration (wt%)
DMF:CH2Cl2 (v/v)
Conductivity (ms/m)
10 12 10 12 16
0:100 0:100 25:75 25:75 25:75
0.46 0.51 29.1 26.5 23.2
Fig. 3. Photograph of the linear-jet electrospinning process with a photograph of the yielded PLLA fiber in the insert.
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Fig. 4. SEM images of PLLA fibers electrospun by the linear-jet electrospinning method from 12 wt% PLLA/CH2Cl2 solution system with 0.0 m/s (a, 0 r/min), 1.3 m/s (b, 250 r/min), 5.6 m/s (c, 1000 r/min) winding velocity, and from 16 wt% PLLA/CH2Cl2/DMF solution system (75/25 CH2Cl2/DMF) with 1.3 m/s (d, 250 r/min), 5.6 m/s (e, 1000 r/min) winding velocity.
3.4. Orientation investigation and discussion The molecular orientation of the linearly arranged PLLA fibers was also investigated by polarized FTIR. The FTIR spectra from parallel and perpendicular to the highly oriented and non-woven electrospun PLLA fibers were shown in Fig. 7a and b, respectively. In Fig. 7a, the distinct difference in absorbance intensities of the highly oriented PLLA fibers electrospun from 12 wt% PLLA/CH2Cl2 solution system was observed by this polarized measurement. When
the electric vector is parallel to the fiber axis, the bands attributed to the CeOeC stretch at 1080 cm1, the O]CeC stretch at 1180 cm1 and the eCH3 variable angle at 1365 cm1 and 1450 cm1 have distinct higher absorbance intensities than that is perpendicular to the fiber axis. However, in Fig. 7b, there is no marked difference in absorbance intensities when the electric vector is parallel or perpendicular to the non-woven PLLA fibers electrospun from the same PLLA system (12 wt% in CH2Cl2), which shows an isotropic property in the non-woven PLLA electrospun fibers. The absorbance
Fig. 5. SEM images of PLLA fibers with different surface structure electrospun from 12 wt% PLLA/CH2Cl2 solution system: surface image of the as-spun fibers with (a) 2.6 m/s (500 r/ min) and (b) 5.2 m/s (1000 r/min) winding speed, the insert image show the enlarged parts of the corresponding images.
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Z. Liu et al. / Polymer 54 (2013) 6045e6051 Table 2 Dichroic ratios at different bands.
Fig. 6. Schematic illustration of the sequential procedure for the formation of the two kinds of surface structure of the as-spun PLLA electrospun fibers, in which the small yellow balls represent the polymer-poor phase: (a) Initial phase separation occurred on the jet surface; (b) Solidification process of the jet influenced by solvent evaporation and surface stretching force; (c) Pores remained on the surface due to polymerpoor phase evaporation; (d) Pores might be closed due to the stretching force generated by high winding velocity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
intensities discrepancy between Fig. 7a and b can be attributed to the molecular orientation of polymer backbones along the polymer axis. The dichroic ratio from IR absorption is used to characterize the degree of molecular orientation. The dichroic ratio for a particular absorbance is defined by D ¼ A///At , where A// andAt represent the
Wavenumber (cm1)
A//
At
R
1080 1180 1365 1450
1.70 1.41 0.59 0.68
1.11 0.90 0.38 0.45
1.53 1.56 1.55 1.51
electric vector direction of the polarizer is oriented parallel or perpendicular to the fiber direction, respectively. Table 2 listed the calculated dichroic ratios at different bands and it is found that the R is very close to around 1.5 which indicated a good anisotropic property of the highly aligned PLLA fibers due to a significant amount of uniaxial orientation of the polymer chains. It is reported that, during conventional electrospinning process, the orientation of polymer chains was hardly observed in the as-spun fibers [26]. The strain rate in the conventional electrospinning process is very high and obvious birefringence of the electrified jet was observed, but most of the stretching occurs before the complete solidification of the jet, so that the molecular chain still had enough time to dissipate and relax the molecular orientation after deposited on the collector. In this work, however, the high mechanical winding of collecting drum contributes to the formation of molecular orientation. Additionally, the use of volatile CH2Cl2 and a higher concentration of polymer solution accelerate the solidification process of the electrospinning jet, and therefore a high degree of molecular orientation in the highly aligned PLLA fibers will be preserved instead of the random orientation in the non-woven ones.
3.5. Mechanical performance For mechanical enhancement, a high degree of molecular orientation and crystallization in the electrospun fibers is particularly important. In Fig. 8, curves a and b show the typical stressestrain curves of the as-spun linearly arranged PLLA fibers with different collecting speed (5.2 m/s and 2.6 m/s), respectively, from the 12 wt% PLLA/CH2Cl2 solution. For comparison, the non-woven PLLA fibers electrospun from the same solution system is also tested and shown in the curve c. As can been seen, the prepared PLLA non-woven membrane had a weak tensile strength (3 Mpa at break) with 118% elongation at break. In contrast, the highly aligned PLLA fibers exhibited a significantly higher tensile strength and larger
Fig. 7. Polarized FT-IR spectra of as-spun PLLA electrospun fibers from 12 wt% PLLA/ CH2Cl2 solution system: (a) highly aligned PLLA fibers; (b) non-woven PLLA fibers.
Fig. 8. Stressestrain curve of as-spun PLLA fibers from 12 wt% PLLA/CH2Cl2 solution system: (a) highly aligned PLLA fiber, 1000 r/min; (b) highly aligned PLLA fiber, 500 r/ min; (c) non-woven PLLA fiber.
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elongation: 13.9 Mpa and 211% for 5.2 m/s collecting speed, 4.5 Mpa and 188% for 2.6 m/s collecting speed, respectively. This result indicated that the alignment and higher molecular orientation of the PLLA fibers yielded by the linear-jet electrospinning method contributes to the enhancement of mechanical properties. Additionally, with increased collecting speed from 2.6 m/s to 5.2 m/s, an obvious increase of tensile strength from 4.5 Mpa to 13.9 Mpa can been seen from curve a and b, which indicated that the higher molecular orientation and lower surface porosity of the fibers due to higher collecting speed enhanced the mechanical performance. 4. Conclusion In summary, continuous highly oriented PLLA electrospun fibers with linear alignment or with helical alignment were successfully prepared by novel linear-jet electrospinning method from PLLA/ CH2Cl2 solution system with or without the addition of DMF, respectively. It is a very facile method to prepare large-scale continuous uniaxially oriented electrospun fibers via conventional electrospinning set-up. The results indicated that the higher concentration and lower conductivity of the polymer solution contribute to the formation of the single linear jet path during the electrospinning process. Different surface structures of PLLA highly oriented fibers electrospun from PLLA/CH2Cl2 system can be controlled by varying the winding velocity of the collecting drum, and the phase separation caused by the high volatile CH2Cl2 is the key factor leading to the microstructure formation. The highly oriented PLLA electrospun fibers with different surface structures are expected to find potential application in microelectronics, tissue engineering and optics. Acknowledgments This work was supported by National Science Foundation of China (51273042, 20874009, 21174028), Innovation Program of
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Shanghai Municipal Education Commission, National 863 Program of China (2012AA030309), Program of Changjiang Scholars and Innovative Research Team in University (IRT1221), Fundamental Research Funds for the Central Universities and 111 project (111-2-04). References [1] Greiner A, Wendorff JH. Angew Chem Int Edit 2007;46:5670e703. [2] Wang HB, Mullins ME, Cregg JM, McCarthy CW, Gilbert RJ. Acta Biomater 2010;6:2970e8. [3] Reneker DH, Chun I. Nanotechnology 1996;7:216e23. [4] Li D, Xia Y. Adv Mater 2004;16:1151e70. [5] Huang ZM, Zhang YZ, Kotaki M, Ramakrishna S. Compos Sci Technol 2003;63: 2223e53. [6] Bayley GM, Mallon PE. Polymer 2012;53:5523e39. [7] Sawawi M, Wang TY, Nisbet DR, Simon GP. Polymer 2013;54:4237e53. [8] Li D, Wang YL, Xia Y. Adv Mater 2004;16:361e6. [9] Xu CY, Inai R, Kotaki M, Ramakrishna S. Biomaterials 2004;25:877e86. [10] Pan H, Li L, Hu L, Cui X. Polymer 2006;47:4901e4. [11] Zhang K, Wang XF, Yang Y, Wang LL, Zhu MF, Hsiao BS, et al. J Polym Sci Pol Phys 2010;48:1118e25. [12] Wang XF, Zhang K, Zhu MF, Yu H, Zhou Z, Chen YM, et al. Polymer 2008;49: 2755e61. [13] Han T, Reneker DH, Yarin AL. Polymer 2007;48:6064e76. [14] Liu K, Ertley CD, Reneker DH. Polymer 2012;53:4241e53. [15] Luo CJ, Nangrejo M, Edirisinghe M. Polymer 2010;51:1654e62. [16] Megelski S, Stephens JS, Chase DB, Rabolt JF. Macromolecules 2002;35: 8456e66. [17] Reneker DH, Yarin AL. Polymer 2008;49:2387e425. [18] Uyar T, Besenbacher F. Polymer 2008;49:5336e43. [19] Kessick R, Tepper G. Appl Phys Lett 2004;84:4807e9. [20] Hellmann C, Belardi J, Dersch R, Greiner A, Wendorff JH, Bahnmueller S. Polymer 2009;50:1197e205. [21] Chen JS, Tu SL, Tsay RY. J Taiwan Inst Chem E 2010;41:229e38. [22] Pavia FC, Carrubba VL, Piccarolo S, Brucato V. J Biomed Mater Res A 2008;86A: 459e66. [23] Matsuyama H, Teramoto M, Kuwana M, Kitamura Y. Polymer 2000;41: 8673e9. [24] Song SW, Torkelson JM. J Membr Sci 1995;98:209e22. [25] Srinivasarao M, Collings D, Philips A, Patel S. Science 2001;292:79e83. [26] Reneker DH, Yarin AL, Zussman E, Xu H. Adv Appl Mech 2007;41:43e195.