Article pubs.acs.org/Macromolecules
Mechanical Property Enhancement of Polylactide Nanofibers through Optimization of Molecular Weight, Electrospinning Conditions, and Stereocomplexation Xiwen Zhang, Ryohei Nakagawa, Kok Ho Kent Chan, and Masaya Kotaki* Department of Advanced Fibro-Science, Kyoto Institute of Technology, Gosyokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ABSTRACT: Aligned poly(L-lactide) (PLLA) electrospun nanofibers of different molecular weights (Mw = 100K, 300K, and 700K) were collected using a rotating disk at take-up velocities of 63, 630, and 1890 m/min. Structural development within the spun fibers was examined. Enhanced crystallinities were observed within the fibers spun at elevated take-up velocities, from the polymers with increased Mw. Mechanical properties were evaluated using the single nanofiber tensile test. Despite exhibiting remarkable crystalline and amorphous orientation, the fibers prepared from polymer of molecular weight, 700K, displayed a significant drop in tensile strength when a take-up velocity of 1890 m/min was used to stretch and align the fibers. The finding suggests that an optimum processing condition exists in the preparation of mechanically superior PLLA fiber electrospun from different molecular weight. This study has also collected aligned nanofibers electrospun from poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) blended solutions at take-up velocity of 630 m/min. Highest stereocomplexation activity was observed within the fibers spun from PLLA/PDLA blend solution with weight concentration 50/50 wt/wt. The tensile results highlighted that, among the various blend ratios spun with take-up velocity of 630 m/min, the highest tensile strength was observed for the fibers obtained at a blend ratio of 50/50 wt/wt. polymer chains into highly ordered structures. Zong et al.13 and Inai et al.14 highlighted decreases in crystallinity for nanofibers electrospun from poly(L-lactide) (PLLA), in comparison to solvent cast film. However, the differential scanning calorimetry (DSC) thermograms of electrospun PLLA fibers indicated the presence of cold crystallization events associated with the reorganization of the orientated molecular chains. Based on the understanding that PLLA is a slow crystallizing polymer, the molecules that were oriented by the shear forces during electrospinning were unable to fully crystallize due to drastic reduction in free volume as the solvent evaporates rapidly. The occurrence of molecular orientation is prevalent in high molecular weight polymers due to the delayed relaxation time of long polymer chains. The slow relaxation time allows the molecules to achieve higher orientation and undergo extension upon deformation, thus the onset of crystallization.15 One of the techniques to induce extensive mechanical stretching of macroscopically aligned electrospun fibers is via the use of a rotating disk or drum as the grounded collector.16−19 Enhanced molecular orientation and formation of stress-induced crystalline modifications in nylon-6, poly(oxymethylene), and poly((R)-3-hydroxybutyrate-co-(R)-3-hydrovalerate) (PHBV) were observed for fibers electrospun onto the collectors rotated at high take-up velocities.19−21
1. INTRODUCTION Electrospun nanofibers based on biodegradable and biocompatible polymers such as poly(hydroxybutyrate) (PHB), poly(lactide) (PLA), and poly(ε-caprolactone) (PCL) have been actively studied for medical and biological related applications such as temporary tissue scaffolds,1,2 sutures,3 and drug delivery carriers.4−6 In order to meet the requirements of certain applications such as tissue engineering scaffolds, the mechanical properties and enzymatic degradation behavior of the nanofiber mats have to be tunable.7,8 Modification of the nanofibers’ physical properties can be achieved by a variety of known approaches such as selecting appropriate materials,9 postprocessing treatment of the nanofibers, careful control of processing parameters,10,11 and utilization of specially designed apparatus during electrospinning.12 One of the motivations for the use of electrospinning to produce nanofibers lies in the efficient rate at which the polymer fibers can be fabricated. The mechanism behind the formation of nanofibers is the utilization of electrically induced shear force to draw and stretch polymer solution jets into fine fibers. As the charged solution jet ejects from the spinneret and transits to the grounded collector, the organic solvents within the jet composition are continuously removed via rapid evaporation. Upon deposition onto the collector, solid polymer fine fibers are obtained. It has been reported that the rapid rate at which the solvents are evaporated from the solution retards the organization of © 2012 American Chemical Society
Received: April 18, 2012 Revised: June 15, 2012 Published: June 25, 2012 5494
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placed on metallic stages before coating the surfaces with gold. A field emission scanning electron microscope (FESEM, Hitachi S4200), operated at an acceleration voltage of 15 kV, was used to observe the morphology of the nanofibers. The average fiber diameter of each sample was determined by using image analysis software (Image J) . Thermal characteristics of the PLLA as well as the blend nanofibers were evaluated by using a TA Instruments DSC 2920 apparatus. Samples were heated from 30 to 250 °C at a heating rate of 5 °C min−1 in a nitrogen environment. About 5−7 mg of the sample was sealed in an aluminum pan for the DSC measurements. The degree of crystallinity was obtained by using the equation25
In one of their pioneering works, Tsuji et al. have inferred that shear stresses acting on PLLA and PDLA enantiomeric blends during electrospinning could encourage stereocomplexation.22 For their study, the authors revealed that the development of stereocomplex crystallites had been encouraged by the presence of electrical shear forces induced by the application of high voltage during the electrospinning process. The report however did not highlight the exclusive formation of stereocomplex crystallites (absence of homocrystallites) within the electrospun fibers. Investigation into the preparation of highly stereocomplexed (rich in stereocomplex crystallites, in the absence of homocrystallites) electrospun nanofibers remains to be of recognized novelty due to the limited work performed in this aspect to present date. Given the knowledge that electrospinning is a fast process in which a charged polymer solution jet is continuously drawn from a fine spinneret, it is unclear whether the rapid solvent evaporation of the jet may provide sufficient time for the development of stereocomplex crystallites. This study aims to investigate the optimum conditions for the production of mechanically superior electrospun polylactide nanofibers via a two-part study. The first part defines some strategies on tuning the mechanical properties of PLLA nanofibers by varying molecular weight and take-up velocity. The second part is dedicated to optimizing stereocomplexation in the PLLA/PDLA blend nanofibers in order to obtain significant enhancement in nanofiber tensile properties.
Xc% = [(ΔHm − ΔHc)/93] × 100 where ΔHm is the melting enthalpy, ΔHc is the cold crystallization enthalpy, and the melting enthalpy of totally crystallized PLLA is 93 J/ g.25 X-ray diffraction (XRD) was performed under ambient condition using a Rigaku X-ray diffractometer (RINT-2100-FSL) with Cu Kα as the X-ray source and operated at 40 kV and 30 mA. The 2θ scanning angle was between 5° and 35°. Two-dimensional wide-angle X-ray diffractometer (2D-WAXD) was used to obtain the diffraction patterns via an imaging plate system with the graphite-monochromatized Cu Kα line as an incident X-ray beam (λ = 1.542 Å). Polarized Fourier transform infrared (PFTIR) spectra were collected by using a Perkin-Elmer Spectrum GX FTIR analyzer in transmission mode. Sixteen scans were signal-averaged with a resolution of 2 cm−1 at room temperature. In the parallel polarization, the direction of the electric vector of the incident beam is parallel to the fiber axis while in the perpendicular polarization; it is perpendicular to the fiber axis. 2.4. Single Nanofiber Tensile Test. A modified disk collector with gaps made in part of the disk’s circumference was used to collect single nanofiber. After short spinning time, the fibers deposited across the depressions were manipulated and transferred to a paper frame, after which the fibers were carefully removed under bright light, until a single fiber remained on the frame. The electrospun single nanofiber with a measured 10 mm gauge length was used as the tensile test specimen. Tensile test were performed at a strain rate of 2.5 mm min−1 using a Nanomechanical Testing System (Nano UTM, MTS Systems Corp.) with 500 mN load range and 50 nN load resolution. Ten samples were prepared under the same processing conditions for each tensile test. The fiber diameters were measured by FESEM observation of representative fiber segments not subjected to stress for calculation of tensile strength.
2. EXPERIMENTAL SECTION 2.1. Materials. PLLA with molecular weights (Mw) of 1 × 105 (PLLA-100), 3 × 105 (PLLA-300), and 7 × 105 g mol−1 (PLLA-700) were purchased from Polysciences, Inc. PDLA with Mw of 1.1 × 105 g mol−1 (PDLA-110) was also purchased from the same supplier. The polymer solution was prepared for electrospinning by dissolving PLLA and PLLA/PDLA blend into a mixture of organic solvents. PLLA-300 has been used for the preparation of blend fibers. The solvents comprising dichloromethane (DCM) and pyridine were mixed at a ratio of 60:40, respectively. To prepare electrospun nanofibers with consistent diameters, the concentration of the PLLA in the solvent has to be adjusted according to the PLLA Mw. The amount of polymer used to prepare the solutions was therefore fixed at 9, 4, and 1.75 wt % for PLLA-100, PLLA-300, and PLLA-700, respectively. Meanwhile, the PLLA/PDLA blends were prepared at blend ratios of 100/0, 80/20, 50/50, 20/80, and 0/100. The respective amounts of PLLA/PDLA blend polymer in the solution were 4, 3.61, 4.31, 6.98, and 11 wt %. All chemicals were used without further purification. 2.2. Electrospinning. The electrospinning process was performed using a commercially available setup, i.e., Nanon (MECC, Japan). The spinneret consists of a stainless metallic needle with an inner diameter of 0.2 mm, which is connected to a syringe containing the polymer solution. A voltage of 10 kV was applied to the polymer solution by connecting a high-voltage supplier to the spinneret, while the disk collector was grounded to create a potential difference. The feed rate of the syringe was fixed at 0.5 mL h−1 by using a programmable pump. The gap between the tip of the spinneret and the surface of the disk collector was 15 cm. Macroscopically aligned nanofibers were electrospun at disk rotational speeds of 100, 1000, and 3000 rpm, which correspond to the take-up velocities of 63, 630, and 1890 m min−1, respectively. The temperature during electrospinning is 25 °C, and the humidity is controlled in the range of 30−40%. To remove residual solvents which may be present within the nanofiber mats, the electrospun fibers were dried in the desiccator at room temperature for at least 24 h prior to postprocessing and characterization. 2.3. Structural Characterization of Electrospun Nanofibers. To observe the nanofibers under scanning electron microscope (SEM), the nanofiber mats were extracted from the disk collector and
3. RESULTS AND DISCUSSION 3.1. Structure and Mechanical Properties of PLLA Nanofibers. 3.1.1. Diameter of PLLA Nanofibers. The concentrations of the solutions prepared from different molecular weights were adjusted in order to obtain similar fiber diameters to be around 500 nm when electrospun at a constant take-up velocity of 63 m/min. Fiber diameters decreased with increasing take-up velocities for all three Mw by 20% in maximum (Table 1). The reduction in fiber diameters provides evidence of mechanical drawing imposed by the rotating disk collector on the fibers. 3.1.2. Effect of Take-Up Velocity on Thermal Characteristics of PLLA Nanofibers. The heating enthalpies obtained from DSC measurements for the nanofibers spun from various PLLA molecular weights and different take-up velocities can be found in Table 2. A shift in cold crystallization peak (Tc) from 84 to 73 °C was observed for PLLA-100 nanofibers when takeup velocity increased from 63 to 1890 m/min. Similar trends were also observed in both PLLA-300 and PLLA-700 nanofibers, although the extent of shifting is smaller with increasing PLLA molecular weight. This observation may be related to the enhanced molecular orientation in the 5495
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3.1.3. Effect of Take-Up Velocity on Molecular Orientation and Structure of PLLA Nanofibers. Figure 1 shows the 2D
Table 1. Fiber Diameters of PLLA Nanofibers with Different Take-Up Velocities Mw (g/mol)
take-up velocity (m/min)
100K
63 630 1890 63 630 1890 63 630 1890
300K
700K
fiber diameter (nm) 498 382 346 401 388 357 496 486 399
± ± ± ± ± ± ± ± ±
67 124 147 18 19 31 93 25 26
Table 2. Corresponding Thermal Properties of Electrospun PLLA Nanofibers with Different Take-Up Velocities Estimated by DSC Measurements mol wt (g/mol) 100K
300K
700K
take-up velocity (m/min)
Tc ΔHc (°C)a (J/g)b
63 630 1890 63
84 80 73 75
17 19 11 19
630
74
20
1890
71
9
63 630 1890
72 72 72
16 15 7
Tm (°C)c
ΔHm (J/g)d
ΔH (J/g)e
X (%)f
179 182 182 186, 189 185, 190 183, 191 185 185 184
55 56 57 62
38 37 46 43
41 40 49 46
62
42
45
64
55
59
60 62 59
43 48 52
46 52 56
Figure 1. WAXD pattern of electrospun PLLA nanofibers.
wide-angle X-ray diffraction (WAXD) patterns for aligned nanofibers collected from polymers with different molecular weights at various take-up velocities. The position of the reflection corresponds to 2θ = 16°, which was proposed to be α-form crystals. Enhanced diffraction intensity for the mentioned reflection suggests that PLLA fibers electrospun with increased Mw and take-up velocity exhibit higher crystallinity. A marked difference between the diffraction patterns of PLLA-300 and PLLA-700 nanofibers collected at a take-up velocity of 1890 m/min is the broadness of the diffraction lines. Broad diffraction lines observed on the patterns of PLLA-700 nanofibers collected at a take-up velocity of 1890 m/min suggest that the crystalline structure and size inside the nanofibers were different since they were subjected to nonuniform strain, with a possibility of high residual stresses being accumulated within the internal structure. This structure may be due to the nonuniform strain during the collection. In this part, 700K nanofibers crystalline structure has not been developed by high take-up velocity, so the tensile strength is lower than 700K fibers collected at lower take-up velocity. The presence of residual stresses in fibers had been reported to be unfavorable for the enhancement of mechanical properties.23 Polarized Fourier transform infrared spectroscopy (PFTIR) has been widely used in investigating the molecular chain orientation within the nanofibers.16,19,24 For FTIR spectra characterized by polarized infrared radiation, high absorbance intensity can be attained if the transition moment vector of the vibration coincides with the electric vector of the incident IR beam. The molecular orientation of aligned PLLA nanofibers collected from the rotating collector was characterized by using PFTIR. The molecular orientation can be attained quantitatively by determining the dichroic ratio, R = A∥/A⊥, where A∥ is the parallel-polarized infrared absorbance intensity and A⊥ is the perpendicular-polarized infrared absorbance intensity for a particular vibration component. For a nanofiber with randomly oriented molecules, R is equal to 1 while, for an anisotropic sample with polymer chains oriented along the fiber axis, R is equal to infinity. For this study, both vibration bands at 1265 and 1760 cm−1 associated with amorphous and crystalline carbonyl stretching, respectively, are examined for their dichroic characteristics. Since the transition moment vector of the carbonyl bond
a
Cold crystalline temperature. bCold crystallization enthalpy. cMelting temperature. dMelting enthalpy. eEnthalpy. fDegree of crystallinity.
amorphous phase as a consequence of extensive stretching induced by the mechanical drawing, especially at high take-up velocities. As a consequence, the oriented molecules would dissipate energy more readily in order to reorientate and form crystalline phases. However, the extent of shifting in Tc with increasing take-up velocity was much more apparent in PLLA100 than in PLLA-700 nanofibers, which is an indication that molecules orientation takes place easily when high molecular weight PLLA is present. As mentioned earlier, higher molecular weight polymers tend to have longer relaxation times due to their extensive entanglements, and therefore oriented molecules are able to retain for longer periods of time. Melting temperatures of PLLA-100 (179−182 °C) fibers were generally lower than PLLA-300 and PLLA-700 (184−185 °C), suggesting that the crystallites within lower molecular weight PLLA fibers were smaller. Initial crystallinity was calculated based on the subtraction of cold crystallization enthalpy from the melt enthalpy. The values of the various enthalpies can be found in Table 2. The increase in crystallinity for electrospun nanofibers collected at increased take up velocities is attributed to stress-induced crystallization. Comparing the fibers spun at take-up velocity of 1890 m/ min, both PLLA-300 and PLLA-700 exhibited similar crystallinities that are significantly higher than that in PLLA100. This phenomenon may suggest that the extent of allowable crystallization enhanced by shear stress has already reached saturation in PLLA-700 although the take-up velocity could play a part in inducing crystallization. 5496
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stretching is oriented close to perpendicular to the chain direction, the R value will be less than 1 if molecular chains are aligned along the fiber axis. Dichroic ratios of both the bands are listed in Table 3. A decrease in dichroic ratio was noted with Table 3. Polarized FT-IR of PLLA Nanofibers for Different Molecular Weight and Collected at Different Take-Up Velocities mol wt (g/mol) 100K
300K
700K
take-up velocity (m/min)
amorphous CO (wavenumber 1265 cm−1)
crystalline CO (wavenumber 1760 cm−1)
63 630 1890 63 630 1890 63 630 1890
1.34 1.28 1.13 2.13 1.31 1.25 1.13 1.10 0.98
1.19 1.13 1.10 1.75 1.12 0.96 1.05 1.04 0.86
increments in take-up velocity and Mw of polymers, indicating enhanced molecular chain orientation along the fiber axis. As such, high molecular weight PLLA-700 fibers electrospun at 1890 m/min exhibited lowest dichroic ratios for both crystalline and amorphous carbonyl bond stretching. 3.1.4. Optimization of Mechanical Properties of Single PLLA Nanofiber. Tensile properties of single PLLA nanofibers spun with polymers of different molecular weights were determined by using a nanotensile tester. Tensile stress−strain curves of the single PLLA nanofibers are compiled in Figure 2 while numerical values derived from the curves are summarized in Table 4. PLLA single nanofibers showed ductile stress−strain curves unlike the bulk PLLA especially at lower take-up velocities regardless of molecular weight. It is interesting to note that fracture behavior of PLLA under uniaxial loading can be modified in ductile manner when PLLA is spun into fibers via electrospinning. For fibers electrospun with PLLA-100 and PLLA-300, a common trend relating to the enhancement in tensile modulus and strength, coupled with drop in strain at break was observed with increasing take-up velocities. The increasing maximum tensile stress of both the PLLA-100 and PLLA-300 nanofibers with take-up velocity represents the ability of these nanofibers to undergo extensive strain hardening prior to fracture. Strain hardening is attributed to the gradual unfolding and slippage of the macromolecular chains in the nanofibers prior to fracture.25 It should be noted that the slippage of molecular chains in PLLA-100 would require less shear stress due to the presence of less molecular chain entanglements. As such, the PLLA-100 nanofibers especially those collected at high take-up velocities would exhibit very high strain hardening rates and would ultimately fail at high stresses and at low strains. Unlike the PLLA-100 nanofibers, the rate of strain hardening was lower in PLLA-300 nanofibers collected at similar take-up velocities and tended to exhibit higher strain at break during tensile tests. It could be deduced that a higher degree of molecular entanglements is present in polymers with larger Mw, which require more energy and shear stresses to unfold and realign. Since the nanofibers were subjected to substantial shear stresses when collected at high take-up velocities, the highly entangled molecules would likely unfold and align anisotropi-
Figure 2. Representative tensile stress−strain curves of single electrospun PLLA nanofibers.
Table 4. Tensile Properties of Single Electrospun PLLA Nanofibers Mw (g/mol)
take-up velocity (m/min)
100K
63 630 1890 63 630 1890 63 630 1890
300K
700K
tensile modulus (GPa) 4.1 3.6 7.8 2.5 3.1 5.2 1.5 4.6 3.6
± ± ± ± ± ± ± ± ±
1.7 1.2 2.0 0.8 0.2 2.2 0.2 2.0 0.3
tensile strength (MPa) 119 113 343 60 122 280 55 143 150
± ± ± ± ± ± ± ± ±
50 22 81 18 2 37 13 84 7
strain at break (mm/mm) 1.21 0.42 0.23 1.63 1.48 0.34 1.35 0.97 0.42
± ± ± ± ± ± ± ± ±
0.23 0.24 0.02 0.10 0.01 0.05 0.02 0.05 0.01
cally in the direction of the fiber axis. As such, the tensile strength of the nanofibers is bound to be high but with minimal deformability. Although a reduction in take-up velocity will lead to significant deterioration in nanofiber strength, the ductility of the fibers will tend to improve due to the alignment of the 5497
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electrospun fibers had previously highlighted the presence of internal stress within the crystallite. Low crystallization enthalpy coupled with the slow increase in crystallinity for the spun fibers, as reflected by DSC data, suggests that a bulk of the shear stress may have been directed to the deformation of the crystal segment. Upon extension during tensile tests, these deformed segments with high localized stress are subjected to chain slippage and breakage, which in turn progress into the onset of fiber fracture. From the above results, it can be deduced that the strength and ductility of the nanofibers can be optimized by controlling the take-up velocity as well as the PLLA Mw. It is clear that the mechanical properties of PLLA-300 can be easily manipulated by controlling the take-up velocity. In this case, nanofibers collected at high take-up velocities would almost certainly be much stronger but more brittle. On the other hand, the ductility of the fibers can be significantly improved by reducing the take-up velocity. It is thereby suggested that the take-up velocity of 630 m/min is the optimum condition among others that would provide the nanofibers with a good balance of high strength and ductility. 3.2. Spinning of Highly Stereocomplexed Nanofibers and Optimization of its Mechanical Properties. All PLLA/PDLA blend fibers with different blend ratios had been collected at the optimum take-up velocity of 630 m/min. The concentrations of the blend solutions were adjusted so that the average diameters of the blend fibers were similar. Figure 4
entangled molecules during tensile tests. However, it was observed that the realignment of entangled molecules was difficult when the Mw of the polymers are too high, such as in PLLA-700. When compared to PLLA-100 or PLLA-300 nanofibers, the PLLA-700 nanofibers exhibited inferior tensile modulus and strength, especially for nanofibers obtained at 1890 m/min. PLLA-700 nanofibers collected at lower take-up velocities would fracture at relatively high strains but with insignificant strain hardening even at the later stages of elongation. This characteristic suggests that although the entangled molecules would try to realign and stretch under tensile loading, the nanofibers would eventually fracture prior to the full extension of the entangled molecules or the tie chains. Another factor that could promote the elongation of the nanofibers is chain slippage within the crystallites. It is useful to note that with reference to the DSC data both PLLA-300 and PLLA-700 exhibited high crystallinity and increased crystallite sizes. Large crystallite sizes were reported to be likely to display brittle characteristics due to the high probability of defects being present within the crystals.26 In the aspect of tensile strength, PLLA-700 electrospun fibers exhibited the lowest strength among the fibers collected at 1890 m/min (Figure 3). It is reasonable to explain this phenomenon in relation to PLLA crystallites’ sensitivity to deformation27 and the development of residual stress within the fibers. Broad diffraction lines seen on WAXD pattern of the PLLA-700
Figure 4. DSC thermograms of nanofibers spun from PLLA/PDLA blends dissolved in DCM/pyridine at blend ratios (w/w) of (a) 20/80, (b) 50/50, and (c) 80/20 at take-up velocity of 630 m/min. Inset compares the thermograms for cast films prepared from PLLA/PDLA (50/50) that were dissolved in (d) DCM and (e) DCM/pyridine.
shows the DSC thermograms of nanofibers electrospun from PLLA/PDLA blends of various blend ratios and inset compares the thermograms for cast films prepared from PLLA/PDLA (50/50) that were dissolved in DCM and DCM/pyridine. Based on the knowledge that the melting temperature of PLA stereocomplex is at around 230 °C, the thermal data indicate that fully stereocomplexed nanofibers had been collected with the PLLA/PDLA = 50/50 blend ratio. This most favorable blend ratio for the formation of stereocomplex within electrospun fibers coincides with the optimum ratio reported in the literature for blend materials prepared via precipitation and solvent casting.28−30 To emulate the preparation method for cast films as highlighted by Tsuji et al., the solvent-cast film was annealed at 140 °C in vacuum prior to DSC character-
Figure 3. (a) Tensile modulus and (b) strength of PLLA nanofibers with different Mw at a function of take-up velocity. 5498
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ization.30 The single endothermic peak associated with the melting of homocrystallites at around 178 °C on the heating curve highlights the absence of stereocomplex crystallites in the blend film. This finding clearly demonstrates that both polymers PLLA and PDLA are not favorable of active complexation processes within dichloromethane solution. However, when pyridine was added to the blend solution for casting, the thermogram indicated an increased fraction of stereocomplex crystallites, with a small population of homocrystallites. Figure 5 shows the representative tensile stress−strain curves of PLLA/PDLA blend nanofibers prepared from various blend
Figure 5. Representative stress−strain curves for single electrospun blend fiber with different PLLA/PDLA blend ratios collected at 630 m/min take-up velocities.
ratios and collected at a take-up velocity of 630 m/min. Tensile modulus and tensile strength of nanofibers as a function of blend ratios are compiled in Figure 6. The tensile properties as a function of PDLA content are compiled in Table 5. Nanofibers spun from PLLA/PDLA = 50/50 blend ratio exhibited the highest tensile strength as compared to nanofibers of other blend ratios, which can be attributed to the presence of mechanically superior stereocomplex crystalline structure within the fibers. In case of stain at break, the elongation of the fibers during tensile test was shorter when increasing the PDLA content. This may be related to the polymer being composed of low molecular weight chains.
Figure 6. (a) Tensile modulus and (b)strength blend nanofibers at a function of PDLA content (take-up velocity of 630 m/min).
Table 5. Tensile Properties of Single Electrospun PLLA/ PDLA Blend Nanofibers with Different Blend Ratios and Take-Up Velocities
4. CONCLUSIONS The structure and mechanical properties of PLLA nanofiber spun from different polymer molecular weights and collected at various take-up velocities were examined. Because of stressinduced crystallization, the polymers exhibited enhancement in crystallinity with rapid take-up velocities. Moreover, improved crystallite orientation and elevated crystallinities were observed when nanofibers were spun from higher molecular weight polymers. Single nanofiber tensile test results indicate that higher take-up velocities led to higher tensile properties, with exception to PLLA 700K. The high molecular weight in PLLA 700K caused large distribution of crystallites, which induced residual stresses during high take-up velocities. Nanofibers with excellent strength and ductility can be obtained through the optimization of both the polymer molecular weight and the processing conditions, especially the take-up velocity. It is also
blend ratios (wt %)
take-up velocity (m/min)
100/0 80/20 50/50 20/80 0/100
630
tensile modulus (GPa) 3.1 2.4 10.5 6.6 4.5
± ± ± ± ±
0.2 1.3 2.4 0.1 2.9
tensile strength (MPa) 122 106 386 160 165
± ± ± ± ±
2 47 34 7 69
strain at break (mm/mm) 1.48 0.78 0.27 0.09 0.08
± ± ± ± ±
0.01 0.02 0.10 0.01 0.01
important to highlight that fully stereocomplex nanofibers have been successfully produced for the first time from high molecular weight PLLA/PDLA blends at a 50/50 wt/wt blend ratio. The work has successfully demonstrated that the use of appropriate solvents and the shear stresses induced during electrospinning can effectively promote exclusive formation of stereocomplexed crystallites within the nanofiber. Further stereocomplexation in the nanofibers was induced by increasing the take-up velocity to 1890 m/min, supporting the 5499
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(19) Lee, K. H.; Kim, K. W.; Pesapane, A.; Kim, H. Y.; Rabolt, J. F. Polarized FT-IR study of macroscopically oriented electrospun Nylon6 nanofibers. Macromolecules 2008, 41, 1494−1498. (20) Kongkhlang, T.; Tashiro, K.; Kotaki, M.; Chirachanchai, S. Electrospinning as a New Technique to Control the Crystal Morphology and Molecular Orientation of Polyoxymethylene Nanofibers. J. Am. Chem. Soc 2008, 130, 15460−15466. (21) Chan, K. H. K.; Wong, S. Y.; Li, X.; Zhang, Y. Z.; Lim, P. C.; Lim, C. T.; Kotaki, M.; He, C. B. Effect of Molecular Orientation on Mechanical Property of Single Electrospun Fiber of Poly[(R)-3hydroxybutyrate-co-(R)-3-hydroxyvalerate]. J. Phys. Chem. B 2009, 113 (40), 13179−13185. (22) Tsuji, H.; Nakano, M.; Hashimoto, M.; Takashima, K.; Katsura, S.; Mizuno, A. Electrospinning of Poly(lactic acid) Stereocomplex Nanofibers. Biomacromolecules 2006, 7, 3316−3320. (23) Smith, P.; Lemstra, P. J. Ultrahigh-Strength Polyethylene Filaments by Solution Spinning/Drawing, 2, Influence of Solvent on the Drawability. Makromol. Chem 1979, 180, 2983−2986. (24) Liu, Y.; Cui, L.; Guan, F.; Gao, Y.; Hedin, N. E.; Zhu, L.; Fong, H. Macromolecules 2007, 40, 6283−6290. (25) Termonia, Y.; Smith, P. Kinetic Model for Tensile Deformation of Polymers. 1. Effect of Molecular Weight. Macromolecules 1987, 20, 835−838. (26) Kinloch, A. J.; Young, R. J. Fracture Behavior of Polymers. Appl. Sci. Publ. 1985. (27) Hoogsteen, W.; Postema, A. R.; Pennings, A. J.; Brinke, G. T. Crystal Structure, Conformation, and Morphology of Solution-Spun Poly(L-lactide) Fibers. Macromolecules 1990, 23, 634−642. (28) Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S. H. Stereocomplex Formation between Enantiomeric Poly(lactides). Macromolecules 1987, 20, 906−908. (29) Tsuji, H.; Horii, F.; Hyon, S. H.; Ikada, Y. Stereocomplex Formation between Enantiomeric Poly(lactic acid)s. 2. Stereocmplex Formation in Concentrated Solutions. Macromolecules 1991, 24, 2719−2724. (30) Tsuji, H.; Hyon, S. H.; Ikada, Y. Stereocomplex Formation between Enantiomeric Poly(lactic acid)s. 3. Calorimetric Studies on Blend Films Cast from Dilute Solution. Macromolecules 1991, 24, 5651−5656.
established notion that high shear stress and orientation of molecular chain were able to promote stereocomplexation. Fully stereocomplexed nanofibers exhibited significantly higher ultimate tensile strength when compared to nanofibers formed from other PLLA/PDLA blend ratios or monotonic PLLA and PDLA.
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The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ma300289z | Macromolecules 2012, 45, 5494−5500
