Macromolecular Rapid Communications
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The Enhanced Encapsulation Capacity of Polyhedral Oligomeric Silsesquioxane-based Copolymers for the Fabrication of Electrospun Core/Shell Fibers Adam J. P. Bauer, Tingying Zeng, Jianzhao Liu, Chananate Uthaisar, Bingbing Li*
This paper reports the use of polyhedral oligomeric silsesquioxane (POSS)-based copolymers to stabilize the core/shell interface for the facile fabrication of electrospun core/shell fibers. For the poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA)/poly(ε-caprolactone) (PCL) system, the bicontinuity of hybrid core/shell fibers can be tuned by controlling the phase separation of POSS-MMA/PCL in electrospinning solutions and therefore the size of PCL-inPOSS-MMA emulsion droplets. Our results demonstrate the enhanced encapsulation capacity of POSS-MMA copolymers as shell materials. Taking advantage of the rapid advancement of POSS-based copolymer synthesis, this study can potentially be generalized to guide the fabrication of various other POSS-based core/shell nano-/microstructures by using single-nozzle electrospinning or coaxial electrospinning. 1. Introduction Core/shell nano-/microfibers have been prepared by using coaxial electrospinning with a core/shell nozzle attached to
A. J. P. Bauer, J. Liu, Prof. B. Li Department of Chemistry, Science of Advanced Materials Doctoral Program,Central Michigan University, Mount Pleasant, MI 48859, USA E-mail: [email protected] C. Uthaisar Department of Physics, Science of Advanced Materials Doctoral Program, Central Michigan University, Mount Pleasant, MI 48859, USA T. Zeng Research Laboratory for Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Macromol. Rapid Commun. 2014, 35, 715−720 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
a double-compartment syringe.[1–4] It was also found that the single-nozzle electrospinning of emulsion solutions can produce core/shell fibers, for example, poly(ethylene oxide)fluorescein isothiocyanate-core/poly(ethylene glycol-block[5] L-lactic acid)-shell, polyaniline-core/polycarbonate-shell,[6] polyaniline-core/polystyrene-shell,[6] polyaniline-core/ poly(methyl methacrylate)-shell,[6] polyaniline-core/ poly(ethylene oxide)-shell,[6] and poly(methyl methacrylate)core/polyacrylonitrile-shell fibers.[7] Upon the formation of a core/shell fluid jet during electrospinning, the jet is immediately subject to capillary instabilities imposed at both the air/shell interface and the core/shell interface. The surface tension-induced Rayleigh-Plateau instability at the air/shell interface can be suppressed by the viscoelastic shell phase with sufficient chain entanglements, similar to single-component electrospinning. In the meantime, the instability generated at the core/shell interface determines
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the structural integrity of the inner phase (e.g., separated droplets or a continuous cylindrical core) and the chemical homogeneity of the interface. Because of the diverse applications of core/shell nano-/microstructures, research on tuning the interfacial instability becomes crucial for controllable and scalable fabrication. As traditional nanofillers, polyhedral oligomeric silsesquioxanes (POSS) have been used to enhance the mechanical and thermal properties of polymer materials.[8,9] More importantly, when POSS nanocages are covalently bonded to oligomers or polymer blocks, the resultant molecular hybrids exhibit interesting interfacial behaviors thanks to the low solid-state surface energy of POSS nanocages. The POSS-based molecular hybrids have therefore been studied as amphiphiles for supramolecular self-assembly[10–14] and for hierarchically structured thin films.[15–17] In contrast, being such an important family of copolymers that exhibit unique surface/interface properties, POSS-based copolymers have not been utilized to tune the interfacial instabilities during single-nozzle electrospinning and coaxial electrospinning, among other fiber fabrication methods. For instance, a potentially scalable fiber fabrication technique consisting of simultaneous centrifugal spinning and solution blowing was recently reported.[18] In this method, the air/liquid interfacial instability also plays a key role in the formation of a liquid jet and its stability during the subsequent spinning and gas blowing process. Thus, a good understanding of the surface and interfacial behavior of POSS-based copolymers in polymer liquid jets (e.g., liquid jets during electrospinning) becomes urgent, because, if achieved, it will provide significant insight into the facile fabrication of polymer fibers, including core/shell nano-/microfibers. Using the poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA)/poly(ε-caprolactone) (PCL) blend as a model system, this report, for the first time, demonstrates the superior encapsulation capacity of POSS-based copolymers as shell materials for fabricating core/shell fibers. By controlling the phase separation of POSS-MMA/ PCL, we were able to fabricate bicontinuous core/shell fibers with a PCL core and a POSS-MMA shell. When the POSS-MMA/PCL mass ratios were controlled above 40:60 and below 60:40, the resultant fibers were bicontinuous, stretchy, and of enhanced thermal stability. In the electrospinning solutions, the phase separation of POSS-MMA/ PCL resulted in PCL-rich emulsion droplets dispersed in a continuous POSS-MMA-rich phase that minimized the POSS-MMA/PCL interfacial energy. The POSS-MMA demonstrated a strong tendency to stabilize the twophase interface and therefore to suppress the interfacial capillary instability upon the stretching and drying of the liquid jet. The outermost layer of the as-spun fibers was naturally composed of POSS-MMA copolymers. The
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integrity of the PCL-rich core depends on the size of the PCL emulsion droplets. Increasing the POSS-MMA/PCL mass ratio to 60:40 and beyond resulted in smaller PCLrich droplets, giving rise to multiple discontinuous PCL cores encapsulated in the POSS-MMA shell. In the past decade, a large family of POSS-based copolymers has been produced, and the family is still expanding today. Our results can therefore be generalized to guide the fabrication of various other POSS-based core/shell nano-/microstructures by electrospinning.
2. Experimental Section Electrospun Fiber Preparation: Poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA with 25 wt% POSS; number-average molecular weight ( Mn ) = 2.2 × 104 g·mol−1 and weight-average molecular weight ( Mw ) = 5.7 × 104 g·mol−1; Sigma–Aldrich, St. Louis, MO), poly(ε-caprolactone) (PCL, Mn = 80 kg·mol−1, SigmaAldrich, St. Louis, MO), acetone (HPLC grade, Fisher Scientific, Waltham, MA), and chloroform (certified ACS reagent grade, Fisher Scientific, Waltham, MA) were used as received. Electrospinning solutions were prepared by dissolving POSS-MMA/PCL in a chloroform/acetone co-solvent (1:1, volume ratio). The solutions were gently stirred overnight and used immediately for electrospinning. The electrospinning set-up is schematically dipicted in Figure S1, and the processing parameters are summarized in Table S1 (see the Supporting Information). The electrospun fibers were collected on aluminium foil at ambient conditions. Characterization of Electrospun Fibers: Scanning electron microscopy (SEM, Hitachi VP-SEM S-3400N, Japan) was used to characterize morphology using the secondary electron-imaging function at 5–6 kV with a probe current of 6 × 10−11 amps and a working distance of 5–7 mm. Prior to imaging, samples were sputter coated with gold-palladium for 3 min to obtain contrast. The fiber diameters were measured by ImageJ 1.46 software to generate histograms of diameter distributions. Thermal analyses were performed using a differential scanning calorimeter (DSC Q2000, TA instruments, New Castle, DE) under a N2 gas flow at a scanning rate of 5 °C·min−1 (–80 to 150 °C). Proton nuclear magnetic resonance (1H NMR) spectra were obtained at 500 MHz using a Varian spectrometer with a tetramethylsilane (TMS) proton signal as the standard. X-ray photoelectron spectroscopy (XPS) was performed using on a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.69 eV).
3. Results and Discussion Figure 1A shows an optical microscopy (OM) image of an as-prepared 50:50 by mass POSS-MMA/PCL solution composed of 7 wt% POSS-MMA and 7 wt% PCL. The emulsion mainly contains droplets with diameters greater than 100 μm. The 1H NMR spectra were then recorded for the phase-separated solution. Figure 1(i), recorded for the top
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Figure 1. OM image (A) of a 50:50 by mass POSS-MMA/PCL solution (B). The 1H NMR spectra (i) and (ii) were recorded for samples pipetted from locations (i) and (ii) in the phase-separated solution (C).
layer, shows a 4.25:1 ratio of CH2 protons (–CH2 designated by # in the PCL structure, 4.057 ppm) to CH3 protons (–CH3 designated by * in the POSS-MMA structure, 3.586 ppm[21]). The calculated POSS-MMA/PCL mass ratio in the top layer is 15:85, and the PCL concentration in the top layer is about 11 wt%, assuming that equal masses for the top and the bottom layers concurred with complete phase separation. The actual PCL concentration in the top layer (core) could be slightly lower than 11 wt% because the lower solution density of PCL results in a slightly larger volume of the top layer, as shown in Figure 1C. As a 10 wt% PCL solution is required to produce bead-free, uniform PCL fibers, the viscoelastic force of emulsion droplets with ca. 11 wt% PCL can allow the formation of PCL core fibers. Meanwhile, according to Figure 1(ii), the calculated POSS-MMA/PCL mass ratio in the bottom layer is 78:22, which corresponds to 12 wt% POSS-MMA in the shell liquid phase that further suppresses the core/shell interfacial instability and therefore stabilizes the PCL core during electrospinning. It is worth mentioning that the fiber deposition rate is on the order of several tens of meters per second and that the draw ratio of an elongated liquid jet can reach up to 104.[22,23] Thus, the as-prepared emulsion is metastable during electrospinning. The morphologies of the electrospun fibers are shown in Figure 2A and B. The chemical constituents of POSS-MMA/ PCL fibers were confirmed using Raman (Figure S2) and 1H NMR spectra (Figure S3, see the Supporting Information). The actual POSS-MMA/PCL mass ratios in the fibers were determined by analyzing the characteristic chemical shifts
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of the two compounds in the 1H NMR spectra. The fibers prepared from the 50:50 by mass POSS-MMA/PCL solution contain 51:49 by mass POSS-MMA/PCL. After annealing at 85 °C for 96 h, a temperature below the glass transition tempearture, Tg, of POSS-MMA and above the melting tempearture, Tm, of PCL, the 51:49 by mass POSS-MMA/PCL fibers demonstrated structural integrity, as shown in Figure 2A′ and A′′, suggesting the existence of a structurally intact POSS-MMA shell. Figure 2B shows 19:81 by mass POSSMMA/PCL fibers (prepared from a 20:80 by mass solution) that exhibit cylindirical morphology similar to neat PCL fibers. After annealing at 85 °C for 96 h, the POSS-MMA/ PCL fibers melted and collapsed into films (Figure 2B′). The endotherms in Figure 2C demonstrated comparable Tm,PCL values for all fibers samples. Thus, the enhanced structural stability of the 51:49 by mass POSS-MMA/PCL fibers can be attributed to the encapsulation of the PCL phase in an intact POSS-MMA shell, which is thermally stable at 85 °C. Morphologies of POSS-MMA/PCL fibers with various blend ratios thermally annealed at different temperatures are shown in Figure S4 (Supporting Information). In contrast, poly(methyl methacrylate)(PMMA)/PCL fibers electropun from a 50:50 by mass solution melted and lost structural integrity upon annealing at 80 °C, though the glass transitions for neat PMMA and neat POSS-MMA occur within the same tempearature range (Figure S5). The PMMA/PCL fibers became mashed webs upon annealing at 85 °C (Figure S5C), suggetsing that the PMMA/PCL fibers are monolithic and that the enhanced encapsultaion capacity of POSS-MMA is atttributed to the POSS-containing block.
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Figure 2. SEM images of fibers prepared from solutions with POSS-MMA/PCL mass ratios of: A) 50:50 (7 wt% POSS-MMA and 7 wt% PCL), and, B) 20:80 (2.5 wt% POSS-MMA and 10 wt% PCL). The morphologies of the annealed fibers are shown in images (A′, A′′) and (B′), respectively. The endothermic peaks in (C) correspond to the Tm of the PCL-rich phase in the as-spun fibers. The compositions and mean diameters of the as-spun fibers are summarized in the table.
XPS analyses were further employed as a surface-sensitive probe of the first ca. 5 nm surface of the POSS-MMA/ PCL fibers. Figure 3 shows XPS high-resolution spectra of Si2p, C1s, and O1s, along with a table that summarizes the calculated atomic concentrations of Si, C, and O. The XPS results exhibit an atomic Si concentration of 4.25% for 51:49 by mass POSS-MMA/PCL fibers (prepared from the 50:50 by mass solution), comparable to that of 4.44% for neat POSS-MMA fibers, suggesting that the POSSMMA must be a major component of the outermost layer of the POSS-MMA/PCL fibers. Therefore, the XPS results are in agreement with the morphological observations in the thermal annealing experiments. The integrity of the PCL-rich core depends on the size of the PCL-in-POSS-MMA emulsion droplets in the electrospinning solution. For the 50:50 by mass POSS-MMA/PCL solution, the emulsion mainly contained droplets with diameters greater than 100 μm. According to the same theoretical prediction reported in a previous study,[7] if the core diameter is about 2 μm, a 100 μm diameter droplet is expected to produce ca. 0.25 m long intact core, which would still give rise to the bicontinuous structure of the resultant fibers. This hypothesis was confirmed by uniaxially stretching the composite fibers at elevated temperatures (i.e., temperatures close to 56 °C, the Tm of PCL; Figure 4A and B). The composite fibers were first annealed at 85 °C for 2 h to ensure a better separated core/shell interface, as depicted in Figure 4D. The sample was then removed from the oven and quickly uniaxially
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stretched to two times its original length before it cooled to room temperature. At an elevated temperature, the soft PCL core was easily stretched and the hard POSS-MMA shell was quickly broken due to the mechanical mismatch between the core and shell phases (Note: the neat POSS-MMA fibers are brittle (Figure S6) and can be easily broken, leaving a clear-cut cross-section (Figure 4C)). The intact PCL core phase thus observed was at least 25 μm long, as shown in Figure 4A, suggesting the bicontinuity of POSS-MMA/PCL fibers. The mean diameter (0.82 ± 0.21 μm) of the PCL-rich core and the thickness (0.40 ± 0.04 μm) of the POSS-MMA shell were further estimated by analyzing multiple SEM images similar to Figure 4A but of higher magnification. The PCL core diameter was expected to be higher than the estimated value, as these SEM images were taken for stretched fibers. Meanwhile, the mean diameter of as-spun fibers was found to be 2.68 ± 0.94 μm. Therefore, it is safe to conclude that the diameter of the core phase is ca. 1–2 μm, corresponding to 1–0.25 m long bicontinuous POSS-MMA/PCL fibers. The SEM images shown in Figure 4A were captured by taking advantage of the significant difference in the Tg of the PCL core and the POSS-MMA shell. When the 51:49 by mass POSS-MMA/PCL fibers were uniaxially stretched at room temperature, the core-shell structures can be still seen (Figure S7) and the POSS-MMA shell was apparently not detached from the PCL core. Furthermore, the stretchy nature of the 51:49 by mass POSS-MMA/PCL fibers originates from the bicontinuous nature of the core-shell
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Figure 3. XPS high-resolution spectra of Si2p, C1s, and O1s, and the experimental atomic concentrations for: A) neat PCL fibers, B) 51:49 by mass POSS-MMA/PCL fibers, and C) neat POSS-MMA fibers.
composite fibers. For instance, increasing the POSS-MMA/ PCL ratio to 70:30 by mass resulted in smaller PCL-rich droplets in the electrospinning solution, giving rise to a discontinuous PCL core phase encapsulated in the POSSMMA shell (Figure S8B). The resultant fibers were therefore not extendable. It is worthwhile to mention that a POSS-polystyrene (POSS-PS) copolymer was also examined to further demonstrate the encapsulation capacity of POSS-based copolymers. The fibers prepared from a 50:50 by mass POSSPS/PCL emulsion (Figure S9) can retain their structural
integrity upon annealing at 66 °C (Tm, PCL ≈ 56 °C; Tg, POSSPS ≈ 73 °C), implying that the PCL phase was encapsulated in the POSS-PS shell. The continuity of the PCL core can be further controlled by tuning the sizes of the PCL-in-POSSPS emulsion droplets.
4. Conclusions This report, for the first time, demonstrates the enhanced encapsulation capacity of POSS-based copolymers for
Figure 4. A, B) SEM images of uniaxially stretched 51:49 by mass POSS-MMA/PCL fibers. C) Cross-sectional morphology of neat POSS-MMA fibers. D) A schematic depiction of stretchy bicontinuous POSS-MMA/PCL fibers before and after thermal annealing.
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fabricating core/shell fibers. The mean diameter of the POSS-MMA/PCL fibers thus prepared was 2.68 ± 0.94 μm, with a POSS-MMA shell thickness of 0.40 ± 0.04 μm. Taking advantage of the expanding family of POSS-based copolymers, our study suggests a new route to encapsulate functional materials (e.g., nanoparticles, liquid crystals, monomers, etc.) in core/shell fibers that otherwise cannot be produced. This study also provides significant insight into the drug-eluting coatings of medical devices, where bioactive molecules are required to be encapsulated and protected from thermal and chemical destruction.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The authors would like to thank Philip Oshel for SEM training and CMU for the Early Career Investigator Award (613771) awarded to BL. Received: January 16, 2014; Revised: February 15, 2014; Published online: March 10, 2014; DOI: 10.1002/marc.201400032 Keywords: POSS-based copolymers; core/shell electrospinning; poly(ε-caprolactone) (PCL)
fibers;
[1] Z. Sun, E. Zussman, A. L. Yarin, J. H. Wendorff, A. Greiner, Adv. Mater. 2003, 15, 1929. [2] M. S. Kim, K. M. Shin, S. I. Kim, G. M. Spinks, S. J. Kim, Macromol. Rapid Commun. 2008, 29, 552. [3] C. S. Reddy, A. Arinstein, E. Zussman, Polym. Chem. 2011, 2, 835.
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[4] G. Kwak, G. H. Lee, S.-H. Shim, K.-B. Yoon, Macromol. Rapid Commun. 2008, 29, 815. [5] X. Xu, X. Zhuang, X. Chen, X. Wang, L. Yang, X. Jing, Macromol. Rapid Commun. 2006, 27, 1637. [6] M. Wei, J. Lee, B. Kang, J. Mead, Macromol. Rapid Commun. 2005, 26, 1127. [7] A. V. Bazilevsky, A. L. Yarin, C. M. Megaridis, Langmuir 2007, 23, 2311. [8] H. Pan, Z. Qiu, Macromolecules 2010, 43, 1499. [9] L. Zheng, R. J. Farris, E. B. Coughlin, Macromolecules 2001, 34, 8034. [10] L. Cui, J. P. Collet, G. Xu, L. Zhu, Chem. Mater. 2006, 18, 3503. [11] J. Miao, L. Zhu, J. Phys. Chem. B. 2010, 114, 1879. [12] B. Jiang, W. Tao, X. Lu, Y. Liu, H. Jin, Y. Pang, X. Sun, D. Yan, Y. Zhou, Macromol. Rapid Commun. 2012, 14, 767. [13] W. Zhang, B. Fang, A. Walther, A. H. E. Müller, Macromolecules 2009, 42, 2563. [14] B. H. Tan, H. Hussain, C. B. He, Macromolecules 2011, 44, 622. [15] R. Goseki, T. Hirai, Y. Ishida, M. Kakimoto, T. Hayakawa, Polym. J. 2012, 44, 658. [16] B. Ahn, T. Hirai, S. Jin, Y. Rho, K. Kim, M. Kakimoto, P. Gopalan, T. Hayakawa, M. Ree, Macromolecules 2010, 43, 10568. [17] Y. Tada, H. Yoshida, Y. Ishida, T. Hirai, J. K. Bosworth, E. Dobisz, R. Ruiz, M. Takenaka, T. Hayakawa, H. Hasegawa, Macromolecules 2012, 45, 292. [18] S. Mahalingam, M. Edirisinghe, Macromol. Rapid Commun. 2013, 34, 1134. [19] J. Liu, J. Fan, Z. Zhang, Q. Hu, T. Zeng, B. Li, J. Colloid Interface Sci. 2012, 394, 386. [20] J. Doshi, D. H. Reneker, J. Electrostat. 1995, 35, 151. [21] Y. Xue, H. Wang, D. Yu, L. Feng, L. Dai, X. Wang, T. Lin, Chem. Commun. 2009, 6418. [22] D. H. Reneker, A. L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 2000, 87, 4531. [23] M. Bognitzki, W. Czado, T. Frese, A. Shaper, M. Hellwig, M. Steinhart, A. Greiner, J. Wendorff, Adv. Mater. 2001, 13, 70.
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Communication
The Enhanced Encapsulation Capacity of Polyhedral Oligomeric Silsesquioxane-based Copolymers for the Fabrication of Electrospun Core/Shell Fibers Adam J. P. Bauer, Tingying Zeng, Jianzhao Liu, Chananate Uthaisar, Bingbing Li*
This paper reports the use of polyhedral oligomeric silsesquioxane (POSS)-based copolymers to stabilize the core/shell interface for the facile fabrication of electrospun core/shell fibers. For the poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA)/poly(ε-caprolactone) (PCL) system, the bicontinuity of hybrid core/shell fibers can be tuned by controlling the phase separation of POSS-MMA/PCL in electrospinning solutions and therefore the size of PCL-inPOSS-MMA emulsion droplets. Our results demonstrate the enhanced encapsulation capacity of POSS-MMA copolymers as shell materials. Taking advantage of the rapid advancement of POSS-based copolymer synthesis, this study can potentially be generalized to guide the fabrication of various other POSS-based core/shell nano-/microstructures by using single-nozzle electrospinning or coaxial electrospinning. 1. Introduction Core/shell nano-/microfibers have been prepared by using coaxial electrospinning with a core/shell nozzle attached to
A. J. P. Bauer, J. Liu, Prof. B. Li Department of Chemistry, Science of Advanced Materials Doctoral Program,Central Michigan University, Mount Pleasant, MI 48859, USA E-mail: [email protected] C. Uthaisar Department of Physics, Science of Advanced Materials Doctoral Program, Central Michigan University, Mount Pleasant, MI 48859, USA T. Zeng Research Laboratory for Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Macromol. Rapid Commun. 2014, 35, 715−720 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
a double-compartment syringe.[1–4] It was also found that the single-nozzle electrospinning of emulsion solutions can produce core/shell fibers, for example, poly(ethylene oxide)fluorescein isothiocyanate-core/poly(ethylene glycol-block[5] L-lactic acid)-shell, polyaniline-core/polycarbonate-shell,[6] polyaniline-core/polystyrene-shell,[6] polyaniline-core/ poly(methyl methacrylate)-shell,[6] polyaniline-core/ poly(ethylene oxide)-shell,[6] and poly(methyl methacrylate)core/polyacrylonitrile-shell fibers.[7] Upon the formation of a core/shell fluid jet during electrospinning, the jet is immediately subject to capillary instabilities imposed at both the air/shell interface and the core/shell interface. The surface tension-induced Rayleigh-Plateau instability at the air/shell interface can be suppressed by the viscoelastic shell phase with sufficient chain entanglements, similar to single-component electrospinning. In the meantime, the instability generated at the core/shell interface determines
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the structural integrity of the inner phase (e.g., separated droplets or a continuous cylindrical core) and the chemical homogeneity of the interface. Because of the diverse applications of core/shell nano-/microstructures, research on tuning the interfacial instability becomes crucial for controllable and scalable fabrication. As traditional nanofillers, polyhedral oligomeric silsesquioxanes (POSS) have been used to enhance the mechanical and thermal properties of polymer materials.[8,9] More importantly, when POSS nanocages are covalently bonded to oligomers or polymer blocks, the resultant molecular hybrids exhibit interesting interfacial behaviors thanks to the low solid-state surface energy of POSS nanocages. The POSS-based molecular hybrids have therefore been studied as amphiphiles for supramolecular self-assembly[10–14] and for hierarchically structured thin films.[15–17] In contrast, being such an important family of copolymers that exhibit unique surface/interface properties, POSS-based copolymers have not been utilized to tune the interfacial instabilities during single-nozzle electrospinning and coaxial electrospinning, among other fiber fabrication methods. For instance, a potentially scalable fiber fabrication technique consisting of simultaneous centrifugal spinning and solution blowing was recently reported.[18] In this method, the air/liquid interfacial instability also plays a key role in the formation of a liquid jet and its stability during the subsequent spinning and gas blowing process. Thus, a good understanding of the surface and interfacial behavior of POSS-based copolymers in polymer liquid jets (e.g., liquid jets during electrospinning) becomes urgent, because, if achieved, it will provide significant insight into the facile fabrication of polymer fibers, including core/shell nano-/microfibers. Using the poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA)/poly(ε-caprolactone) (PCL) blend as a model system, this report, for the first time, demonstrates the superior encapsulation capacity of POSS-based copolymers as shell materials for fabricating core/shell fibers. By controlling the phase separation of POSS-MMA/ PCL, we were able to fabricate bicontinuous core/shell fibers with a PCL core and a POSS-MMA shell. When the POSS-MMA/PCL mass ratios were controlled above 40:60 and below 60:40, the resultant fibers were bicontinuous, stretchy, and of enhanced thermal stability. In the electrospinning solutions, the phase separation of POSS-MMA/ PCL resulted in PCL-rich emulsion droplets dispersed in a continuous POSS-MMA-rich phase that minimized the POSS-MMA/PCL interfacial energy. The POSS-MMA demonstrated a strong tendency to stabilize the twophase interface and therefore to suppress the interfacial capillary instability upon the stretching and drying of the liquid jet. The outermost layer of the as-spun fibers was naturally composed of POSS-MMA copolymers. The
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integrity of the PCL-rich core depends on the size of the PCL emulsion droplets. Increasing the POSS-MMA/PCL mass ratio to 60:40 and beyond resulted in smaller PCLrich droplets, giving rise to multiple discontinuous PCL cores encapsulated in the POSS-MMA shell. In the past decade, a large family of POSS-based copolymers has been produced, and the family is still expanding today. Our results can therefore be generalized to guide the fabrication of various other POSS-based core/shell nano-/microstructures by electrospinning.
2. Experimental Section Electrospun Fiber Preparation: Poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methyl methacrylate)] (POSS-MMA with 25 wt% POSS; number-average molecular weight ( Mn ) = 2.2 × 104 g·mol−1 and weight-average molecular weight ( Mw ) = 5.7 × 104 g·mol−1; Sigma–Aldrich, St. Louis, MO), poly(ε-caprolactone) (PCL, Mn = 80 kg·mol−1, SigmaAldrich, St. Louis, MO), acetone (HPLC grade, Fisher Scientific, Waltham, MA), and chloroform (certified ACS reagent grade, Fisher Scientific, Waltham, MA) were used as received. Electrospinning solutions were prepared by dissolving POSS-MMA/PCL in a chloroform/acetone co-solvent (1:1, volume ratio). The solutions were gently stirred overnight and used immediately for electrospinning. The electrospinning set-up is schematically dipicted in Figure S1, and the processing parameters are summarized in Table S1 (see the Supporting Information). The electrospun fibers were collected on aluminium foil at ambient conditions. Characterization of Electrospun Fibers: Scanning electron microscopy (SEM, Hitachi VP-SEM S-3400N, Japan) was used to characterize morphology using the secondary electron-imaging function at 5–6 kV with a probe current of 6 × 10−11 amps and a working distance of 5–7 mm. Prior to imaging, samples were sputter coated with gold-palladium for 3 min to obtain contrast. The fiber diameters were measured by ImageJ 1.46 software to generate histograms of diameter distributions. Thermal analyses were performed using a differential scanning calorimeter (DSC Q2000, TA instruments, New Castle, DE) under a N2 gas flow at a scanning rate of 5 °C·min−1 (–80 to 150 °C). Proton nuclear magnetic resonance (1H NMR) spectra were obtained at 500 MHz using a Varian spectrometer with a tetramethylsilane (TMS) proton signal as the standard. X-ray photoelectron spectroscopy (XPS) was performed using on a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.69 eV).
3. Results and Discussion Figure 1A shows an optical microscopy (OM) image of an as-prepared 50:50 by mass POSS-MMA/PCL solution composed of 7 wt% POSS-MMA and 7 wt% PCL. The emulsion mainly contains droplets with diameters greater than 100 μm. The 1H NMR spectra were then recorded for the phase-separated solution. Figure 1(i), recorded for the top
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Figure 1. OM image (A) of a 50:50 by mass POSS-MMA/PCL solution (B). The 1H NMR spectra (i) and (ii) were recorded for samples pipetted from locations (i) and (ii) in the phase-separated solution (C).
layer, shows a 4.25:1 ratio of CH2 protons (–CH2 designated by # in the PCL structure, 4.057 ppm) to CH3 protons (–CH3 designated by * in the POSS-MMA structure, 3.586 ppm[21]). The calculated POSS-MMA/PCL mass ratio in the top layer is 15:85, and the PCL concentration in the top layer is about 11 wt%, assuming that equal masses for the top and the bottom layers concurred with complete phase separation. The actual PCL concentration in the top layer (core) could be slightly lower than 11 wt% because the lower solution density of PCL results in a slightly larger volume of the top layer, as shown in Figure 1C. As a 10 wt% PCL solution is required to produce bead-free, uniform PCL fibers, the viscoelastic force of emulsion droplets with ca. 11 wt% PCL can allow the formation of PCL core fibers. Meanwhile, according to Figure 1(ii), the calculated POSS-MMA/PCL mass ratio in the bottom layer is 78:22, which corresponds to 12 wt% POSS-MMA in the shell liquid phase that further suppresses the core/shell interfacial instability and therefore stabilizes the PCL core during electrospinning. It is worth mentioning that the fiber deposition rate is on the order of several tens of meters per second and that the draw ratio of an elongated liquid jet can reach up to 104.[22,23] Thus, the as-prepared emulsion is metastable during electrospinning. The morphologies of the electrospun fibers are shown in Figure 2A and B. The chemical constituents of POSS-MMA/ PCL fibers were confirmed using Raman (Figure S2) and 1H NMR spectra (Figure S3, see the Supporting Information). The actual POSS-MMA/PCL mass ratios in the fibers were determined by analyzing the characteristic chemical shifts
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of the two compounds in the 1H NMR spectra. The fibers prepared from the 50:50 by mass POSS-MMA/PCL solution contain 51:49 by mass POSS-MMA/PCL. After annealing at 85 °C for 96 h, a temperature below the glass transition tempearture, Tg, of POSS-MMA and above the melting tempearture, Tm, of PCL, the 51:49 by mass POSS-MMA/PCL fibers demonstrated structural integrity, as shown in Figure 2A′ and A′′, suggesting the existence of a structurally intact POSS-MMA shell. Figure 2B shows 19:81 by mass POSSMMA/PCL fibers (prepared from a 20:80 by mass solution) that exhibit cylindirical morphology similar to neat PCL fibers. After annealing at 85 °C for 96 h, the POSS-MMA/ PCL fibers melted and collapsed into films (Figure 2B′). The endotherms in Figure 2C demonstrated comparable Tm,PCL values for all fibers samples. Thus, the enhanced structural stability of the 51:49 by mass POSS-MMA/PCL fibers can be attributed to the encapsulation of the PCL phase in an intact POSS-MMA shell, which is thermally stable at 85 °C. Morphologies of POSS-MMA/PCL fibers with various blend ratios thermally annealed at different temperatures are shown in Figure S4 (Supporting Information). In contrast, poly(methyl methacrylate)(PMMA)/PCL fibers electropun from a 50:50 by mass solution melted and lost structural integrity upon annealing at 80 °C, though the glass transitions for neat PMMA and neat POSS-MMA occur within the same tempearature range (Figure S5). The PMMA/PCL fibers became mashed webs upon annealing at 85 °C (Figure S5C), suggetsing that the PMMA/PCL fibers are monolithic and that the enhanced encapsultaion capacity of POSS-MMA is atttributed to the POSS-containing block.
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Figure 2. SEM images of fibers prepared from solutions with POSS-MMA/PCL mass ratios of: A) 50:50 (7 wt% POSS-MMA and 7 wt% PCL), and, B) 20:80 (2.5 wt% POSS-MMA and 10 wt% PCL). The morphologies of the annealed fibers are shown in images (A′, A′′) and (B′), respectively. The endothermic peaks in (C) correspond to the Tm of the PCL-rich phase in the as-spun fibers. The compositions and mean diameters of the as-spun fibers are summarized in the table.
XPS analyses were further employed as a surface-sensitive probe of the first ca. 5 nm surface of the POSS-MMA/ PCL fibers. Figure 3 shows XPS high-resolution spectra of Si2p, C1s, and O1s, along with a table that summarizes the calculated atomic concentrations of Si, C, and O. The XPS results exhibit an atomic Si concentration of 4.25% for 51:49 by mass POSS-MMA/PCL fibers (prepared from the 50:50 by mass solution), comparable to that of 4.44% for neat POSS-MMA fibers, suggesting that the POSSMMA must be a major component of the outermost layer of the POSS-MMA/PCL fibers. Therefore, the XPS results are in agreement with the morphological observations in the thermal annealing experiments. The integrity of the PCL-rich core depends on the size of the PCL-in-POSS-MMA emulsion droplets in the electrospinning solution. For the 50:50 by mass POSS-MMA/PCL solution, the emulsion mainly contained droplets with diameters greater than 100 μm. According to the same theoretical prediction reported in a previous study,[7] if the core diameter is about 2 μm, a 100 μm diameter droplet is expected to produce ca. 0.25 m long intact core, which would still give rise to the bicontinuous structure of the resultant fibers. This hypothesis was confirmed by uniaxially stretching the composite fibers at elevated temperatures (i.e., temperatures close to 56 °C, the Tm of PCL; Figure 4A and B). The composite fibers were first annealed at 85 °C for 2 h to ensure a better separated core/shell interface, as depicted in Figure 4D. The sample was then removed from the oven and quickly uniaxially
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stretched to two times its original length before it cooled to room temperature. At an elevated temperature, the soft PCL core was easily stretched and the hard POSS-MMA shell was quickly broken due to the mechanical mismatch between the core and shell phases (Note: the neat POSS-MMA fibers are brittle (Figure S6) and can be easily broken, leaving a clear-cut cross-section (Figure 4C)). The intact PCL core phase thus observed was at least 25 μm long, as shown in Figure 4A, suggesting the bicontinuity of POSS-MMA/PCL fibers. The mean diameter (0.82 ± 0.21 μm) of the PCL-rich core and the thickness (0.40 ± 0.04 μm) of the POSS-MMA shell were further estimated by analyzing multiple SEM images similar to Figure 4A but of higher magnification. The PCL core diameter was expected to be higher than the estimated value, as these SEM images were taken for stretched fibers. Meanwhile, the mean diameter of as-spun fibers was found to be 2.68 ± 0.94 μm. Therefore, it is safe to conclude that the diameter of the core phase is ca. 1–2 μm, corresponding to 1–0.25 m long bicontinuous POSS-MMA/PCL fibers. The SEM images shown in Figure 4A were captured by taking advantage of the significant difference in the Tg of the PCL core and the POSS-MMA shell. When the 51:49 by mass POSS-MMA/PCL fibers were uniaxially stretched at room temperature, the core-shell structures can be still seen (Figure S7) and the POSS-MMA shell was apparently not detached from the PCL core. Furthermore, the stretchy nature of the 51:49 by mass POSS-MMA/PCL fibers originates from the bicontinuous nature of the core-shell
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The Enhanced Encapsulation Capacity of Polyhedral Oligomeric . . .
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Figure 3. XPS high-resolution spectra of Si2p, C1s, and O1s, and the experimental atomic concentrations for: A) neat PCL fibers, B) 51:49 by mass POSS-MMA/PCL fibers, and C) neat POSS-MMA fibers.
composite fibers. For instance, increasing the POSS-MMA/ PCL ratio to 70:30 by mass resulted in smaller PCL-rich droplets in the electrospinning solution, giving rise to a discontinuous PCL core phase encapsulated in the POSSMMA shell (Figure S8B). The resultant fibers were therefore not extendable. It is worthwhile to mention that a POSS-polystyrene (POSS-PS) copolymer was also examined to further demonstrate the encapsulation capacity of POSS-based copolymers. The fibers prepared from a 50:50 by mass POSSPS/PCL emulsion (Figure S9) can retain their structural
integrity upon annealing at 66 °C (Tm, PCL ≈ 56 °C; Tg, POSSPS ≈ 73 °C), implying that the PCL phase was encapsulated in the POSS-PS shell. The continuity of the PCL core can be further controlled by tuning the sizes of the PCL-in-POSSPS emulsion droplets.
4. Conclusions This report, for the first time, demonstrates the enhanced encapsulation capacity of POSS-based copolymers for
Figure 4. A, B) SEM images of uniaxially stretched 51:49 by mass POSS-MMA/PCL fibers. C) Cross-sectional morphology of neat POSS-MMA fibers. D) A schematic depiction of stretchy bicontinuous POSS-MMA/PCL fibers before and after thermal annealing.
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fabricating core/shell fibers. The mean diameter of the POSS-MMA/PCL fibers thus prepared was 2.68 ± 0.94 μm, with a POSS-MMA shell thickness of 0.40 ± 0.04 μm. Taking advantage of the expanding family of POSS-based copolymers, our study suggests a new route to encapsulate functional materials (e.g., nanoparticles, liquid crystals, monomers, etc.) in core/shell fibers that otherwise cannot be produced. This study also provides significant insight into the drug-eluting coatings of medical devices, where bioactive molecules are required to be encapsulated and protected from thermal and chemical destruction.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: The authors would like to thank Philip Oshel for SEM training and CMU for the Early Career Investigator Award (613771) awarded to BL. Received: January 16, 2014; Revised: February 15, 2014; Published online: March 10, 2014; DOI: 10.1002/marc.201400032 Keywords: POSS-based copolymers; core/shell electrospinning; poly(ε-caprolactone) (PCL)
fibers;
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