- Email: [email protected]
MEH-PPV blends processed via electrospraying and electrospinning
Organic Electronics 15 (2014) 2993–2999
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Energy transfer and compatibility analysis of PVK/MEH-PPV blends processed via electrospraying and electrospinning Margarita Mondragón a,⇑, J. Uriel Balderas a, G. Lesly Jiménez a, Ma. Esther Sánchez-Espíndola b, Ciro Falcony c a
Sección de Estudios de Posgrado, Escuela Superior de Ingeniería Mecánica y Eléctrica (ESIME) Unidad Azcapotzalco del IPN, 02250 Mexico, D.F., Mexico Centro de Nanociencias y Micro y Nanotecnología (CNMN) del IPN, 07738 Mexico, D.F., Mexico c Departamento de Física, Centro de Investigación y de Estudios Avanzados (CINVESTAV) del IPN, 07360 Mexico, D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 11 March 2014 Received in revised form 8 August 2014 Accepted 23 August 2014 Available online 10 September 2014 Keywords: Förster energy transfer (FRET) Electrospraying Electrospinning Blend Compatibility
a b s t r a c t Solution polymer blends of a high molecular weight and a low molecular weight poly (9-vinyl carbazole) PVK with poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), at a fixed blend ratio of 95.5:0.5, were processed via electrospraying and electrospinning. SEM studies revealed that electrosprayed particles were produced when low molecular weight PVK was used, while electrospun fibers were successfully obtained from solutions containing high molecular weight PVK, over a concentration range of 4–10% (w/v). From the absorption spectra of the neat polymers it was determined that Urbach energy Eu increase and optical band gap Eg decreases due to the physical defects along the main chain introduced by these electrostatic processing methods. Photoluminescence spectroscopy revealed a particular applied voltage, which depends on concentration and molecular weight, where aggregation of PVK levels off. Luminescence quenching of MEH-PPV is also observed to increase with applied voltage consistent with possible energy transfer from shorter conjugation length segments to nearby longer conjugated segments. The ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL or PVKH) and the MEH-PPV (acceptor), ID/IA, exhibited minima at this particular voltage and then levels off, indicating not only maximum interpenetration and thus compatibility of both polymers but also maximum energy transfer. Hence, we demonstrate that compatibility and energy transfer can be optimized varying concentration and applied voltage during both electrospraying and electrospinning processes. Ó 2014 Elsevier B.V. All rights reserved.
1. 1.Introduction Within recent years, several technologies have focused on improving the color purity, brightness, lifetime and brightness of conjugated-polymer-based LEDs [1]. Conjugated polymers have attracted great interest both in industry and academia for electronic and optoelectronic ⇑ Corresponding author. Tel.: +52 55 57296300x64514; fax: +52 55 57296300x64493. E-mail address: [email protected] (M. Mondragón). http://dx.doi.org/10.1016/j.orgel.2014.08.040 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
applications because they often exhibit facile color tunability by chemical structure modification, processability, good charge transport properties, high quantum yield and large area fabrication [2,3]. Among these technologies blending of conjugated polymers has been considered an excellent alternative since novel properties and phenomena not found in the components can be obtained [4]. Despite this, relatively few studies have been carried out on blends of p-type (donor) and n-type (acceptor) conjugated polymers. If donor and acceptor molecules come into distances
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comparable with the Förster radius, the nonradiative energy transfer also called Förster resonance energy transfer (FRET) turns out to be efficient. The efficiency of the nonradiative energy transfer has provided detailed information on compatibility and therefore on phase separation in amorphous polymer–polymer blends [5,6]. Most of these studies have largely focused on polymer solutions, thin films, and bulk solid state; there are few experimental reports on electrospun nanofibers or films deposited by electrospray [7–10]. Conjugated polymer electrospun fibers and thin films of electrosprayed droplets have shown distinct electronic and optoelectronic properties as compared with spin coated films [11,12]. The principle behind electrospraying is the applied electric field that stretches the liquid meniscus at the tip of the nozzle. When the applied electric field is sufficiently high, the liquid meniscus will form a conical jet and further break into droplets due to electrostatic force. Electrospinning is an extension of the electrospray process that uses a high concentration solution, which is capable of producing fibers at micro- or nanometer dimensions [13]. In recent articles, energy transfer from PVK to MEH-PPV has been shown [14,15]. In this study we characterized optical and luminescent properties of PVK/MEH-PPV blends prepared via electrospraying and electrospinning with the objective of better understanding how these processes affect nonradiative energy transfer phenomena and thus polymer–polymer compatibility 2. Experimental 2.1. Materials The materials used were poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, Mw 40,000– 70,000), a high molecular weight poly(9-vinyl carbazole) (PVKH, Mw 1,100,000) and a low molecular weight poly(9-vinyl carbazole) (PVKL, Mw 25,000–50,000), from Aldrich. The solvent used was 1,2-dichloroethane (DCE, 99.8% HPLC grade), from Aldrich. For the electrospray process, PVKL/MEH-PPV (at a fixed blend ratio of 95.5:0.5) solutions were prepared at concentrations of 4, 6, 8 and 10% (w/v) using DCE. First, proper amount of PVKL was dissolved in DCE, at room temperature. Then, MEH-PPV was added to the PVKL solution under vigorous stirring for over 12 h. Electrospinning solutions were similarly prepared, but PVKH was used instead of PVKL. Additional solutions of neat MEH-PPV (1% w/v), neat PVKL (4% w/v) and neat PVKH (4% w/v) in DCE were prepared for the electrospray, electrospinning and spin coating processes. The spin coated films were prepared at a spin rate of 2800 rpm for 15 s and dried for 2 h at 60 °C in an oven. 2.2. Electrospray/electrospinning process The electrospray/electrospinning solutions were placed into a 3-ml syringe connected to a metallic 21 G needle. The needle was connected to a high-voltage power supply (RR30-5P, Gamma High Voltage Research) and the applied
voltage was varied from 14 to 28 kV, depending on solution concentration. The solutions were fed at 1 mL/h by a syringe pump (78-0100I Fisher Scientific). The materials were collected for about 5 min on a grounded aluminum foil, which was placed 17 cm from the needle tip. The samples were then dried for 2 h at 60 °C in an oven. All solutions were processed under the same conditions. 2.3. Characterization of as-prepared solutions and as-spun fibers A JEOL JSM-6100 scanning electron microscope (SEM) was used to investigate the morphology of the electrospun fibers and the electrosprayed thin films. Specimens were coated with gold/palladium and SEM micrographs were obtained using 15 kV secondary electrons. Transmission electron microscopy (TEM) images were achieved with a JEOL JEM-1010 operating at 60 kV. UV–vis absorption was recorded on a Varian Cary 50 UV–vis spectrophotometer. Photoluminescence emission (PL) and excitation (PLE) spectra were measured using a Jobin Yvon Horiba Fluoro Max-P spectrofluorometer. 3. Results and discussion 3.1. Morphology of the PVK/MEH-PPV films Fig. 1 shows the SEM images of the samples processed with PVKL/MEH-PPV/DCE and PVKH/MEH-PPV/DCE solutions at different concentrations and applied voltages, respectively. When the low molecular weight PVK (PVKL) was used, characteristic particles of the electrospray process were produced (Fig. 3A–C0 ). Correspondingly, fiber structures were successfully electrospun from the solutions containing the high molecular weight PVK (PVKH) (Fig. 3D–F0 ). The drawing of the polymer fluid jet into fine filaments or the breakup of the viscoelastic jet into submicron particles due to the electrostatic force depend on polymer concentration and molecular weight [16,17]. Even at the highest concentration used in this work (10% w/v) the loosely entangled polymer chains in the PVKL/MEHPPV/DCE solution were insufficient to produce fibers, as compared with the PVKH/MEH-PPV/DCE solutions. The average size of the electrosprayed particles increases with increasing polymer concentration (from 10 lm to 100 lm) and changes from particles of irregular shape to cup-shaped particles (Fig. 1A–C). The cup shell is formed by the polymer-rich phase and the hole formed by the evaporation of the solvent-rich phase [13]. In addition, higher applied voltages seem to prevent complete solvent evaporation process therefore; adjacent particles fused together and tend to form films on the collector (Fig. 1A0 –C0 ) [18,19]. On the other hand, since the viscosity of the solutions containing PVKH increases as solution concentration increases is plausible that the bead-on string structures change to uniform fibers of greater thickness due to the greater resistance of these solution to be stretched (Fig. 1D–F) [20]. Although an increase in electrical force can produce contradictory effects, in this case higher
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Fig. 1. SEM images of samples processed with PVKL/MEH-PPV/DCE solutions: A and A0 , 4%; B and B0 , 6%; C and C0 , 8%; and of samples processed with PVKH/ MEH-PPV/DCE solutions: D and D0 , 4%; E and E0 , 6%; F and F0 , 8%.
Fig. 3. Normalized PL emission spectra of spin coated and electrosprayed MEH-PPV thin films, at 1% w/v and different applied voltages.
Fig. 2. UV–vis absorption spectrum of electrosprayed MEH-PPV thin film and the PL emission spectra of electrosprayed PVKL films and electrospun PVKH fibers, at an applied voltage of (a) 14 kV and (b) 16 kV.
applied voltages facilitated thicker fibers most likely due to the ejection of more fluid in the jet (Fig. 1D0 –F0 ) [21,22]. 3.2. Optical characteristics of the polymers Fig. 2 shows the UV–vis absorption spectrum of neat MEH-PPV and the photoluminescence (PL) emission
spectra of pristine PVKH and PVKL, obtained from electrosprayed MEH-PPV and PVKL thin films and non-woven mats of electrospun PVKH fibers, at an applied voltage of 14 and 16 kV. We fail to prepare neat MEH-PPV fibers from solutions in DCE, even at much higher polymer contents, probably due to the molecular weight of the sample used. The emission band of the PVKL film (kmax = 460 nm) is broader and red-shifted relative to PVKH fibers (kmax = 418) at an applied voltage of 14 kV (Fig. 2a). However, at 16 kV the half-width and the kmax are both equal for PVKH and PVKL (Fig. 2b). The greater partial overlap of the emission spectrum of the PVKL with the absorption spectrum of the MEH-PPV compared to PVKH at 14 kV indicates that the probability for Förster resonance energy transfer (FRET) is higher in the electrosprayed films of the PVKL/ MEH-PPV blends than in the electrospun fibers of PVKH/ MEH-PPV blends. At higher voltages it seems not to be significant differences for FRET between the two systems (figures not shown).
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Table 1 Optical constants of the polymers. Applied voltage (kV)
14 16 18 20 spca a
MEH-PPV (espa)
PVKL (espa)
PVKH (spna)
kmax (nm)
Eg (eV)
Eu (meV)
kmax (nm)
Eg (eV)
Eu (meV)
kmax (nm)
Eg (eV)
Eu (meV)
517 517 517 517 503
2.00 2.00 2.00 2.00 2.06
81 81 108 103 76
345 345 345 345 345
3.27 3.27 3.26 3.25 3.32
170 166 170 255 36
345 345 345 345 345
3.29 3.18 3.18 3.18 3.39
145 228 257 263 39
spc = spin coated films; esp = electrosprayed films; spn = electrospun fibers.
An interesting feature is the red-shift of the PL spectra of PVKH and PVKL as voltage increases from 14 to 16 kV, where maxima level off at 478 nm. Further increase of applied voltage has not influence on PL spectra. These results indicate aggregation of the PVK molecules due to electrospray/electrospinning process conditions, as the absorption peak maxima is not shifted as shown in the next results. The optical band gaps Eg of neat polymers were determined by means of the Tauc equation used mainly for amorphous semiconductors [23,24]:
Ahm / ðhm Eg Þ
2
ð1Þ
where A is the absorbance, hm is the photon energy and Eg is the optical band-gap energy. The values of Eg were obtained from the intercept on the energy axis of the plots
(Ahm)1/2 vs hm and are given in Table 1, as well as the values of the absorption peak maxima (kmax). The width of localized states in the optical gap was estimated from the Urbach energy (Eu) [25]. The values of Eu calculated from the reciprocal gradient of the linear portion of the ln(A) vs. photon energy curves are given in Table 1. For comparison, kmax, Eg and Eu of their respective spin coated films are also shown. Interestingly, the electric potential applied during the electrospray and the electrospinning processes seems to introduce a significant number of structural defects in these three polymers as indicated by their larger Eu, compared with spin coated films. These defects produce localized states contributing to smaller apparent optical gap. In addition, the absorption peak maxima do not change for PVKH and PVKL with the processing conditions. However, a red-shift is observed in the absorption band of
Fig. 4. Normalized PL emission spectra of PVKL/MEH-PPV thin films from electrospray depositions, at different concentrations and applied voltages.
M. Mondragón et al. / Organic Electronics 15 (2014) 2993–2999
MEH-PPV thin films from electrospray deposition relative to spin coated films (from 503 to 517 nm), associated with increased conjugation. It might be concluded that under the influence of an electrical field the presence of the large pendant group in MEH-PPV has a large influence on achieving an extended conformation [26]. As a result of this, a 10 nm red-shift of the PL spectra of neat electrosprayed MEH-PPV films was observed (Fig. 3). 3.3. PL properties and miscibility of the blends Figs. 4 and 5 show the effect of increasing solid content and applied voltage on the photoluminescence (PL) of PVKL/MEH-PPV thin films from electrospray deposition and PVKH/MEH-PPV electrospun fibers, respectively. The PL spectra of the electrosprayed films obtained at the lowest applied voltage show two emission bands regardless of the solid content (Fig. 4a–d). The left band corresponding to the emission originated from the PVKL is red-shifted with increasing applied voltage until the main peak levels off at 473 nm due primarily to aggregation in the polymer caused by the electrospray process, as indicated before. However, this value is minor than the 478 nm found for the neat electrosprayed PVKL, revealing that for this low molecular weight polymer the presence of MEH-PPV helps to prevent aggregation. Remarkably, luminescence quenching of MEH-PPV (with a fixed maximum peak at 553 nm) is observed upon increasing applied voltage, suggesting energy transfer from
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shorter conjugation length segments to nearby longer conjugated segments [27,28]. The presence of various conjugations lengths might be ascribed to the introduction of physical defect along the main chain as indicated by the broadening of their absorption spectra. MEH-PPV emission is blue shifted from 587 nm to 553 nm, relative to the spin coated film, due to the decreasing aggregation of MEH-PPV by the presence of the PVK molecules [15]. Two emission bands are seen in the spectra of the electrospun fibers at the lowest voltage at each concentration (Fig. 5a–d). The band corresponding to the emission originated from the PVKH is red-shifted with increasing applied voltage but in this case the main peak levels off at same value (478 nm) found for the neat electrospun PVKH fibers. Correspondingly, the band due to MEH-PPV (also peaked at 553 nm) decreased upon increasing applied voltage, more likely due to energy transfer from shorter conjugation length to nearby longer conjugated segments of MEHPPV, as indicated for the electrosprayed films. The efficiency of the energy transfer is related to the compatibility of a blend containing a mixture of a donorlabeled polymer and an acceptor-labeled polymer. It will be substantially reduced if phase separation leads to a large increase in the distances between the donors and the acceptors [10]. Thus, the variation of the energy-transfer characterized by the ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL and PVKH) and the MEH-PPV (acceptor), ID/IA, was used to obtain information concerning the compatibility of the two polymeric species.
Fig. 5. Normalized PL emission spectra of PVKH/MEH-PPV fibers from electrospinning depositions, at different concentrations and applied voltages.
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Fig. 6 shows the ID/IA ratios vs the applied voltage of electrosprayed PVKL/MEH-PPV films and PVKH/MEH-PPV electrospun fibers, at different concentrations. Data can be classified in two levels: firstly the ratio ID/IA decreases indicating an increased energy transfer as a result of an increased compatibility. Secondly, as voltage increase the ID/IA ratio tends to level off meaning that energy transfer and compatibility no longer change significantly. The PL spectra (Figs. 4 and 5) indicate that the transition from the first to the second level is related to the applied voltage where the red-shift of PL spectra of the PVKs levels off. We interpret this to mean that, depending on the concentration, a certain applied voltage must be applied to maximize the interpenetration and thus compatibility of both polymers. And also, that a maximum in energy transfer occurs from this voltage attributed not only to FRET from PVKH and PVKL to MEH-PPV but also related to energy transfer from shorter conjugation length to nearby longer conjugated segments of MEH-PPV, for both electrosprayed films and electrospun fibers, which in turn influences the ratio ID/IA. The value of the applied voltage where the ratio ID/IA levels off occurred and also the value of the ratio itself
increase as concentration increases, since the two polymeric species cannot longer easily interpenetrate one another in a single phase. Interestingly, the ID/IA minimum seems to be distinctly lower in the PVKL/MEH-PPV than in the PVKH/MEH-PPV at 4% w/v, suggesting a higher level of blend miscibility in the electrosprayed films of PVKL/MEHPPV and thus a better energy transfer. This behavior results from the more random distribution of the chain in both polymers attained in the electrospray process, compared to electrospinning process. However, it has been reported that for a given degree of dispersion, the number of contacts can be enhanced by an extended chain conformation since it will increase the interpenetration of dissimilar chains [29]. 4. Conclusions Electrosprayed PVKL/MEH-PPV particles and electrospun PVKH/MEH-PPV fibers were successfully produced, over a concentration range of 4–10% (w/v). Both electrospray and electrospinning processes were shown to introduce the physical defects along the main chain of low molecular PVK, high molecular PVK and MEH-PPV. These processes also produce aggregation of the PVK molecules and extension of the MEH-PPV conjugation. PL spectra of the PVK/MEH-PPV blend evidences that a particular applied voltage, which depends on concentration and molecular weight, the red-shift of the band corresponding to PVK levels off. The quenching of the band corresponding to MEH-PPV continues quenching as voltage increases. The ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL or PVKH) and the MEH-PPV (acceptor), ID/IA, exhibited minima at this particular voltage and then tend to level off, indicating not only maximum interpenetration and thus compatibility of both polymers but also maximum energy transfer. Acknowledgments This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT) in the framework of the Project No. 168939 (project ‘‘CB-2011-01’’) and by the SIP-IPN. References
Fig. 6. ID/IA ratios of (a) PVKL/MEH-PPV thin films from electrospray depositions and (b) PVKH/MEH-PPV electrospun fibers, at different concentrations.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Energy transfer and compatibility analysis of PVK/MEH-PPV blends processed via electrospraying and electrospinning Margarita Mondragón a,⇑, J. Uriel Balderas a, G. Lesly Jiménez a, Ma. Esther Sánchez-Espíndola b, Ciro Falcony c a
Sección de Estudios de Posgrado, Escuela Superior de Ingeniería Mecánica y Eléctrica (ESIME) Unidad Azcapotzalco del IPN, 02250 Mexico, D.F., Mexico Centro de Nanociencias y Micro y Nanotecnología (CNMN) del IPN, 07738 Mexico, D.F., Mexico c Departamento de Física, Centro de Investigación y de Estudios Avanzados (CINVESTAV) del IPN, 07360 Mexico, D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 11 March 2014 Received in revised form 8 August 2014 Accepted 23 August 2014 Available online 10 September 2014 Keywords: Förster energy transfer (FRET) Electrospraying Electrospinning Blend Compatibility
a b s t r a c t Solution polymer blends of a high molecular weight and a low molecular weight poly (9-vinyl carbazole) PVK with poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), at a fixed blend ratio of 95.5:0.5, were processed via electrospraying and electrospinning. SEM studies revealed that electrosprayed particles were produced when low molecular weight PVK was used, while electrospun fibers were successfully obtained from solutions containing high molecular weight PVK, over a concentration range of 4–10% (w/v). From the absorption spectra of the neat polymers it was determined that Urbach energy Eu increase and optical band gap Eg decreases due to the physical defects along the main chain introduced by these electrostatic processing methods. Photoluminescence spectroscopy revealed a particular applied voltage, which depends on concentration and molecular weight, where aggregation of PVK levels off. Luminescence quenching of MEH-PPV is also observed to increase with applied voltage consistent with possible energy transfer from shorter conjugation length segments to nearby longer conjugated segments. The ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL or PVKH) and the MEH-PPV (acceptor), ID/IA, exhibited minima at this particular voltage and then levels off, indicating not only maximum interpenetration and thus compatibility of both polymers but also maximum energy transfer. Hence, we demonstrate that compatibility and energy transfer can be optimized varying concentration and applied voltage during both electrospraying and electrospinning processes. Ó 2014 Elsevier B.V. All rights reserved.
1. 1.Introduction Within recent years, several technologies have focused on improving the color purity, brightness, lifetime and brightness of conjugated-polymer-based LEDs [1]. Conjugated polymers have attracted great interest both in industry and academia for electronic and optoelectronic ⇑ Corresponding author. Tel.: +52 55 57296300x64514; fax: +52 55 57296300x64493. E-mail address: [email protected] (M. Mondragón). http://dx.doi.org/10.1016/j.orgel.2014.08.040 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
applications because they often exhibit facile color tunability by chemical structure modification, processability, good charge transport properties, high quantum yield and large area fabrication [2,3]. Among these technologies blending of conjugated polymers has been considered an excellent alternative since novel properties and phenomena not found in the components can be obtained [4]. Despite this, relatively few studies have been carried out on blends of p-type (donor) and n-type (acceptor) conjugated polymers. If donor and acceptor molecules come into distances
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comparable with the Förster radius, the nonradiative energy transfer also called Förster resonance energy transfer (FRET) turns out to be efficient. The efficiency of the nonradiative energy transfer has provided detailed information on compatibility and therefore on phase separation in amorphous polymer–polymer blends [5,6]. Most of these studies have largely focused on polymer solutions, thin films, and bulk solid state; there are few experimental reports on electrospun nanofibers or films deposited by electrospray [7–10]. Conjugated polymer electrospun fibers and thin films of electrosprayed droplets have shown distinct electronic and optoelectronic properties as compared with spin coated films [11,12]. The principle behind electrospraying is the applied electric field that stretches the liquid meniscus at the tip of the nozzle. When the applied electric field is sufficiently high, the liquid meniscus will form a conical jet and further break into droplets due to electrostatic force. Electrospinning is an extension of the electrospray process that uses a high concentration solution, which is capable of producing fibers at micro- or nanometer dimensions [13]. In recent articles, energy transfer from PVK to MEH-PPV has been shown [14,15]. In this study we characterized optical and luminescent properties of PVK/MEH-PPV blends prepared via electrospraying and electrospinning with the objective of better understanding how these processes affect nonradiative energy transfer phenomena and thus polymer–polymer compatibility 2. Experimental 2.1. Materials The materials used were poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, Mw 40,000– 70,000), a high molecular weight poly(9-vinyl carbazole) (PVKH, Mw 1,100,000) and a low molecular weight poly(9-vinyl carbazole) (PVKL, Mw 25,000–50,000), from Aldrich. The solvent used was 1,2-dichloroethane (DCE, 99.8% HPLC grade), from Aldrich. For the electrospray process, PVKL/MEH-PPV (at a fixed blend ratio of 95.5:0.5) solutions were prepared at concentrations of 4, 6, 8 and 10% (w/v) using DCE. First, proper amount of PVKL was dissolved in DCE, at room temperature. Then, MEH-PPV was added to the PVKL solution under vigorous stirring for over 12 h. Electrospinning solutions were similarly prepared, but PVKH was used instead of PVKL. Additional solutions of neat MEH-PPV (1% w/v), neat PVKL (4% w/v) and neat PVKH (4% w/v) in DCE were prepared for the electrospray, electrospinning and spin coating processes. The spin coated films were prepared at a spin rate of 2800 rpm for 15 s and dried for 2 h at 60 °C in an oven. 2.2. Electrospray/electrospinning process The electrospray/electrospinning solutions were placed into a 3-ml syringe connected to a metallic 21 G needle. The needle was connected to a high-voltage power supply (RR30-5P, Gamma High Voltage Research) and the applied
voltage was varied from 14 to 28 kV, depending on solution concentration. The solutions were fed at 1 mL/h by a syringe pump (78-0100I Fisher Scientific). The materials were collected for about 5 min on a grounded aluminum foil, which was placed 17 cm from the needle tip. The samples were then dried for 2 h at 60 °C in an oven. All solutions were processed under the same conditions. 2.3. Characterization of as-prepared solutions and as-spun fibers A JEOL JSM-6100 scanning electron microscope (SEM) was used to investigate the morphology of the electrospun fibers and the electrosprayed thin films. Specimens were coated with gold/palladium and SEM micrographs were obtained using 15 kV secondary electrons. Transmission electron microscopy (TEM) images were achieved with a JEOL JEM-1010 operating at 60 kV. UV–vis absorption was recorded on a Varian Cary 50 UV–vis spectrophotometer. Photoluminescence emission (PL) and excitation (PLE) spectra were measured using a Jobin Yvon Horiba Fluoro Max-P spectrofluorometer. 3. Results and discussion 3.1. Morphology of the PVK/MEH-PPV films Fig. 1 shows the SEM images of the samples processed with PVKL/MEH-PPV/DCE and PVKH/MEH-PPV/DCE solutions at different concentrations and applied voltages, respectively. When the low molecular weight PVK (PVKL) was used, characteristic particles of the electrospray process were produced (Fig. 3A–C0 ). Correspondingly, fiber structures were successfully electrospun from the solutions containing the high molecular weight PVK (PVKH) (Fig. 3D–F0 ). The drawing of the polymer fluid jet into fine filaments or the breakup of the viscoelastic jet into submicron particles due to the electrostatic force depend on polymer concentration and molecular weight [16,17]. Even at the highest concentration used in this work (10% w/v) the loosely entangled polymer chains in the PVKL/MEHPPV/DCE solution were insufficient to produce fibers, as compared with the PVKH/MEH-PPV/DCE solutions. The average size of the electrosprayed particles increases with increasing polymer concentration (from 10 lm to 100 lm) and changes from particles of irregular shape to cup-shaped particles (Fig. 1A–C). The cup shell is formed by the polymer-rich phase and the hole formed by the evaporation of the solvent-rich phase [13]. In addition, higher applied voltages seem to prevent complete solvent evaporation process therefore; adjacent particles fused together and tend to form films on the collector (Fig. 1A0 –C0 ) [18,19]. On the other hand, since the viscosity of the solutions containing PVKH increases as solution concentration increases is plausible that the bead-on string structures change to uniform fibers of greater thickness due to the greater resistance of these solution to be stretched (Fig. 1D–F) [20]. Although an increase in electrical force can produce contradictory effects, in this case higher
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Fig. 1. SEM images of samples processed with PVKL/MEH-PPV/DCE solutions: A and A0 , 4%; B and B0 , 6%; C and C0 , 8%; and of samples processed with PVKH/ MEH-PPV/DCE solutions: D and D0 , 4%; E and E0 , 6%; F and F0 , 8%.
Fig. 3. Normalized PL emission spectra of spin coated and electrosprayed MEH-PPV thin films, at 1% w/v and different applied voltages.
Fig. 2. UV–vis absorption spectrum of electrosprayed MEH-PPV thin film and the PL emission spectra of electrosprayed PVKL films and electrospun PVKH fibers, at an applied voltage of (a) 14 kV and (b) 16 kV.
applied voltages facilitated thicker fibers most likely due to the ejection of more fluid in the jet (Fig. 1D0 –F0 ) [21,22]. 3.2. Optical characteristics of the polymers Fig. 2 shows the UV–vis absorption spectrum of neat MEH-PPV and the photoluminescence (PL) emission
spectra of pristine PVKH and PVKL, obtained from electrosprayed MEH-PPV and PVKL thin films and non-woven mats of electrospun PVKH fibers, at an applied voltage of 14 and 16 kV. We fail to prepare neat MEH-PPV fibers from solutions in DCE, even at much higher polymer contents, probably due to the molecular weight of the sample used. The emission band of the PVKL film (kmax = 460 nm) is broader and red-shifted relative to PVKH fibers (kmax = 418) at an applied voltage of 14 kV (Fig. 2a). However, at 16 kV the half-width and the kmax are both equal for PVKH and PVKL (Fig. 2b). The greater partial overlap of the emission spectrum of the PVKL with the absorption spectrum of the MEH-PPV compared to PVKH at 14 kV indicates that the probability for Förster resonance energy transfer (FRET) is higher in the electrosprayed films of the PVKL/ MEH-PPV blends than in the electrospun fibers of PVKH/ MEH-PPV blends. At higher voltages it seems not to be significant differences for FRET between the two systems (figures not shown).
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Table 1 Optical constants of the polymers. Applied voltage (kV)
14 16 18 20 spca a
MEH-PPV (espa)
PVKL (espa)
PVKH (spna)
kmax (nm)
Eg (eV)
Eu (meV)
kmax (nm)
Eg (eV)
Eu (meV)
kmax (nm)
Eg (eV)
Eu (meV)
517 517 517 517 503
2.00 2.00 2.00 2.00 2.06
81 81 108 103 76
345 345 345 345 345
3.27 3.27 3.26 3.25 3.32
170 166 170 255 36
345 345 345 345 345
3.29 3.18 3.18 3.18 3.39
145 228 257 263 39
spc = spin coated films; esp = electrosprayed films; spn = electrospun fibers.
An interesting feature is the red-shift of the PL spectra of PVKH and PVKL as voltage increases from 14 to 16 kV, where maxima level off at 478 nm. Further increase of applied voltage has not influence on PL spectra. These results indicate aggregation of the PVK molecules due to electrospray/electrospinning process conditions, as the absorption peak maxima is not shifted as shown in the next results. The optical band gaps Eg of neat polymers were determined by means of the Tauc equation used mainly for amorphous semiconductors [23,24]:
Ahm / ðhm Eg Þ
2
ð1Þ
where A is the absorbance, hm is the photon energy and Eg is the optical band-gap energy. The values of Eg were obtained from the intercept on the energy axis of the plots
(Ahm)1/2 vs hm and are given in Table 1, as well as the values of the absorption peak maxima (kmax). The width of localized states in the optical gap was estimated from the Urbach energy (Eu) [25]. The values of Eu calculated from the reciprocal gradient of the linear portion of the ln(A) vs. photon energy curves are given in Table 1. For comparison, kmax, Eg and Eu of their respective spin coated films are also shown. Interestingly, the electric potential applied during the electrospray and the electrospinning processes seems to introduce a significant number of structural defects in these three polymers as indicated by their larger Eu, compared with spin coated films. These defects produce localized states contributing to smaller apparent optical gap. In addition, the absorption peak maxima do not change for PVKH and PVKL with the processing conditions. However, a red-shift is observed in the absorption band of
Fig. 4. Normalized PL emission spectra of PVKL/MEH-PPV thin films from electrospray depositions, at different concentrations and applied voltages.
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MEH-PPV thin films from electrospray deposition relative to spin coated films (from 503 to 517 nm), associated with increased conjugation. It might be concluded that under the influence of an electrical field the presence of the large pendant group in MEH-PPV has a large influence on achieving an extended conformation [26]. As a result of this, a 10 nm red-shift of the PL spectra of neat electrosprayed MEH-PPV films was observed (Fig. 3). 3.3. PL properties and miscibility of the blends Figs. 4 and 5 show the effect of increasing solid content and applied voltage on the photoluminescence (PL) of PVKL/MEH-PPV thin films from electrospray deposition and PVKH/MEH-PPV electrospun fibers, respectively. The PL spectra of the electrosprayed films obtained at the lowest applied voltage show two emission bands regardless of the solid content (Fig. 4a–d). The left band corresponding to the emission originated from the PVKL is red-shifted with increasing applied voltage until the main peak levels off at 473 nm due primarily to aggregation in the polymer caused by the electrospray process, as indicated before. However, this value is minor than the 478 nm found for the neat electrosprayed PVKL, revealing that for this low molecular weight polymer the presence of MEH-PPV helps to prevent aggregation. Remarkably, luminescence quenching of MEH-PPV (with a fixed maximum peak at 553 nm) is observed upon increasing applied voltage, suggesting energy transfer from
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shorter conjugation length segments to nearby longer conjugated segments [27,28]. The presence of various conjugations lengths might be ascribed to the introduction of physical defect along the main chain as indicated by the broadening of their absorption spectra. MEH-PPV emission is blue shifted from 587 nm to 553 nm, relative to the spin coated film, due to the decreasing aggregation of MEH-PPV by the presence of the PVK molecules [15]. Two emission bands are seen in the spectra of the electrospun fibers at the lowest voltage at each concentration (Fig. 5a–d). The band corresponding to the emission originated from the PVKH is red-shifted with increasing applied voltage but in this case the main peak levels off at same value (478 nm) found for the neat electrospun PVKH fibers. Correspondingly, the band due to MEH-PPV (also peaked at 553 nm) decreased upon increasing applied voltage, more likely due to energy transfer from shorter conjugation length to nearby longer conjugated segments of MEHPPV, as indicated for the electrosprayed films. The efficiency of the energy transfer is related to the compatibility of a blend containing a mixture of a donorlabeled polymer and an acceptor-labeled polymer. It will be substantially reduced if phase separation leads to a large increase in the distances between the donors and the acceptors [10]. Thus, the variation of the energy-transfer characterized by the ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL and PVKH) and the MEH-PPV (acceptor), ID/IA, was used to obtain information concerning the compatibility of the two polymeric species.
Fig. 5. Normalized PL emission spectra of PVKH/MEH-PPV fibers from electrospinning depositions, at different concentrations and applied voltages.
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Fig. 6 shows the ID/IA ratios vs the applied voltage of electrosprayed PVKL/MEH-PPV films and PVKH/MEH-PPV electrospun fibers, at different concentrations. Data can be classified in two levels: firstly the ratio ID/IA decreases indicating an increased energy transfer as a result of an increased compatibility. Secondly, as voltage increase the ID/IA ratio tends to level off meaning that energy transfer and compatibility no longer change significantly. The PL spectra (Figs. 4 and 5) indicate that the transition from the first to the second level is related to the applied voltage where the red-shift of PL spectra of the PVKs levels off. We interpret this to mean that, depending on the concentration, a certain applied voltage must be applied to maximize the interpenetration and thus compatibility of both polymers. And also, that a maximum in energy transfer occurs from this voltage attributed not only to FRET from PVKH and PVKL to MEH-PPV but also related to energy transfer from shorter conjugation length to nearby longer conjugated segments of MEH-PPV, for both electrosprayed films and electrospun fibers, which in turn influences the ratio ID/IA. The value of the applied voltage where the ratio ID/IA levels off occurred and also the value of the ratio itself
increase as concentration increases, since the two polymeric species cannot longer easily interpenetrate one another in a single phase. Interestingly, the ID/IA minimum seems to be distinctly lower in the PVKL/MEH-PPV than in the PVKH/MEH-PPV at 4% w/v, suggesting a higher level of blend miscibility in the electrosprayed films of PVKL/MEHPPV and thus a better energy transfer. This behavior results from the more random distribution of the chain in both polymers attained in the electrospray process, compared to electrospinning process. However, it has been reported that for a given degree of dispersion, the number of contacts can be enhanced by an extended chain conformation since it will increase the interpenetration of dissimilar chains [29]. 4. Conclusions Electrosprayed PVKL/MEH-PPV particles and electrospun PVKH/MEH-PPV fibers were successfully produced, over a concentration range of 4–10% (w/v). Both electrospray and electrospinning processes were shown to introduce the physical defects along the main chain of low molecular PVK, high molecular PVK and MEH-PPV. These processes also produce aggregation of the PVK molecules and extension of the MEH-PPV conjugation. PL spectra of the PVK/MEH-PPV blend evidences that a particular applied voltage, which depends on concentration and molecular weight, the red-shift of the band corresponding to PVK levels off. The quenching of the band corresponding to MEH-PPV continues quenching as voltage increases. The ratio of the intensity of the excitation spectra of the PVK (donor, both PVKL or PVKH) and the MEH-PPV (acceptor), ID/IA, exhibited minima at this particular voltage and then tend to level off, indicating not only maximum interpenetration and thus compatibility of both polymers but also maximum energy transfer. Acknowledgments This work was supported by the Consejo Nacional de Ciencia y Tecnología (CONACYT) in the framework of the Project No. 168939 (project ‘‘CB-2011-01’’) and by the SIP-IPN. References
Fig. 6. ID/IA ratios of (a) PVKL/MEH-PPV thin films from electrospray depositions and (b) PVKH/MEH-PPV electrospun fibers, at different concentrations.
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