Polymeric Nanofibers - American Chemical Society

Chapter 12

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 16, 2014 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch012

Functional Polymer Nanofibers and Nanotubes via Electrospinning J. Zeng, Z. Sun, H. Hou, R. Dersch, H. Wickel, J. H. Wendorff, and A. Greiner Department of Chemistry and Scientific Center of Materials Science, Philipps-University of Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany

Functional polymer nanofibers were prepared by electrospinning of ternary polymer solutions (e. g. poly-L— lactide (PLA)) and low molecular weight metal compounds. These functionalizedfiberswere further modified by coating with poly(p-xylylene) (PPX) via chemical vapor deposition (CVD). Removal of PLA by decomposition or selective solvent extraction resulted in the formation of nanotubes with e. g. Pd-nanoparticles on the inside or outside of the tubes depending on the reaction conditions. Functional nanotubes available for numerous chemical modifications were obtained by the TUFT-process (Tubes by Fiber Templates). According to this process by PLA template fibers were coated with a mixture of poly(vinylalcohol) (PVA) and poly(acrylic acid) (PAA) followed by CVD of PPX as a protective coating. Heating of these composite fibers resulted in decomposition of PLA and chemical crosslinking PVA / PAA. Residual OH— groups were used as reactive sites for immobilization of functional small molecules such as anthracene derivatives. Co— electrospinning by coaxial jets of different materials resulted in fibers with concentric variation of materials concentration. For example, fibers with PLA shell and Pd core where obtained after co-electrospinning of PLA-chloroform and palladium diacetate - THF solutions followed by heating to 170 °C.

© 2006 American Chemical Society

In Polymeric Nanofibers; Reneker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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164 The preparation of polymer nanofibers and nanotubes is a challenge from academic as well as technical point of view. Polymer nanofibers and nanotubes can be prepared by methods involving self-organization or by template methods. A highly versatile method for the preparation of polymer nanofibers is electrospinning. Basic principles of electrospinning of polymer solutions have been described extensively in other contributions of this symposium series and related reviews (1) and therefore will be not discussed in detail here. Our research on electrospinning of polymer nanofibers is presently focused mainly on structure formation in electrospun fibers (2), on the reduction of fiber dimensions by systematic variation of processing parameters (3), the usage of electrospun fibers as templates for polymer nanotubes in the TUFT-process (4), on core/shell fiber preparation (5), and on the chemical modification of electrospun fibers, and on the search of new applications for functionalized electrospun nanofibers. In the context of this contribution it should be not unmentioned that other groups succeeded in the preparation of nanotubes involving electrospun fibers as well (6). Here, we want to give an overview on the work in our group on the preparation of functionalized polymer nanofibers and nanotubes via electrospinning and the chemical modification of electrospun nanofibers in order to present different conceptual approaches.

Results and discussion Particular advantage of electrospinning is that polymer fibers can be furnished with additional functionalities simply by additives in the medium used for electrospinning, by coating of electrospun fibers, or by usage of functional polymers. In course of our investigations we have functionalized electrospun polylactide (PLA) fibers by physical vapor deposition (PVD) with different metals such as aluminum, chromium, and gold. Poly(p-xylylene) (PPX) / metal composite-tubes were prepared by coating of metal-coated PLA fibers via chemical vapor deposition with PPX according the to the so-called Gorhamprocedure (7) and subsequent thermal decomposition of PLA template fibers. Inspection by transmission electron microscope (TEM) showed PPX tubes coated on the inside wall by the metals previously deposited on the template fibers (Figure 1). Functionalized polymer fibers can also be prepared by simultaneous spinning of the polymer material with a functional material, by spinning of a functional polymer, by postprocessing chemical treatment of electrospun polymer fibers, or by co-electrospinning. Polymer fibers can be loaded by metal compounds via electrospinning of polymer solutions, for example by electrospinning of a 1 : 1 mixture of PLA and silver acetate in dichloromethane solution. Silver is distributed all over the fibers as obvious from T E M micrograph (Figure 2a). According to the process shown schematically in Figure 2b polymer tubes with metal nanoparticles inside the tubes are readily available by electrospinnig of template fibers with metal compounds, followed by coating with the desired wall material, and subsequent degradation of the template core fibers accompanied by conversion of the metal



165 compounds to the corresponding elemental metals. Here, metal containing template fibers were obtained by electrospinning of PLA and palladium diacetate (PdOAc) (solution used for electrospinning: 3 % PLA and 3 % PdOAc in dichloromethane). These composite fibers were coated with PPX by CVD. Subsequent heating resulted in the formation of PPX tubes with Pd nanoparticles on the inside wall of the tubes (Figure 3). Energy dispersive (EDX) analysis and electron diffractometry of these fibers clearly showed the presence of palladium in these fibers (4b). High-resolution inspection of the "Pd-nanowires" showed the formation of Pd-nanoparticles with sizes centered at 10 nm (Figure 4) (4d). The particles are homogeneously distributed. Interestingly, the particles seem not to be agglomerated. Following the same concept PPX tubes were obtained with copper and silver nanoparticles (Figure 5). The sizes of Pd-nanoparticles as well as their positions depend significantly on the processing parameters. For example, heating of cut PLA-PdOAc-PPX composite fibers (cut by a scalpel perpendicular to long tube axis) for S hours in vacuum resulted in the formation of PPX-Pd-composite nanotubes with Pdnanoparticles also on the outside of the tubes (Figure 6a and Figure 6b). Otherwise uncut tubes of the same sample resulted in tubes with nanoparticles only inside the tubes (Figure 6c), which excludes the migration of Pd particles through the tube wall as possible explanation for the results with cut tubes. Electrospinning of polymers with tailor-made properties can result in fibers with corresponding properties. An example in case is shown for fluorescent electrospun fibers. Firstly, 9-Anthracence carboxylic acid was converted to 9carboxylic acid chloride according to scheme 1. Esterification of 9-carboxylic acid chloride and PVA (degree of hydrolysis 99 %) resulted in fluorescent P V A (2.5 mol % of OH groups of PVA were esterified) in DMF. Solution cast films showed strong blue fluorescence upon irradiation with UV-light. Fluorescence spectrum with excitation at 300 nm showed a maximum of light emission centered at 450 nm (Figure 7). Fluorescent fibers were obtained by electrospinning of 10 wt% P V A Anth solution in water (Fig. 8a). Inspection of the electrospun fibers by T E M showed smooth fibers with diameters in the range of 2 0 - 100 nm (Figure 8b). In contrast, electrospinning of PVA and other water-soluble polymers like polyethylene oxide under standard conditions results in beaded fibers (8). Following the TUFT-process, fluorescent tubes were prepared by reaction of OH-functionalized PPX tubes with fluorescent molecules according to scheme 2. Firstly, electrospun PLA template fibers were coated by PVA / PAA (dissolved in water 75 : 25 weight ratio) using airbrushing technique. The composite fibers were coated by PPX via C V D as protective layer in order to ensure OH-functionalization only inside the tubes. The PVA / PAA blend was stabilized by thermal crosslinking via esterification as described previously for bulk samples (9). PPX tubes with crosslinked PVA / PAA on the inside were developed by extraction of PLA with chloroform. The anthracenoyl chlorid was soaked into tubes with OH- groups inside and chemically linked to the inner tube wall by esterification. The final products of this procedure were fluorescent tubes, which maintained fluorescence even after several washing procedures with tetrahydrofurane and chloroform (Fig. 9a).



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Figure 1. TEM micrograph of unstained PPX tubes with aluminum (a, reported previously in Lit. (4a)) and gold (b) coating on the inside wall prepared by coating of PLA template fibers via PVD of corresponding metals followed by CVD of PPX and subsequent thermal decomposition of PLA. (Reproduced with permission from reference 4a. Copyright 2000 Wiley-VCH-Verlag.)

Figure 2. (a) TEM of unstained electrospun PLA /silver acetate fibers and (b) schematic process for the preparation ofpolymer tubes with metal nanoparticles via functionalized template fibers. (See page 11 of color inserts.)



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Figure 3. TEM ofPPX7 Pd tubes obtainedfrom electrospun PLA /PdOAc fibers after CVD of PPX, thermal decomposition of PLA and conversion of PdOAc to Pd. (Reproducedfrom reference 4b. Copyright 2002 American Chemical Society.)

Figure 4. High resolution TEM ofPPX / Pd tubes.



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Figure 5. SEM of electrospun PLA / Cu(OAc) fibers (a) and TEM of corresponding PPX/ Cu tubes (b), SEM of electrospun PLA / AgOAc fibers (c) and corresponding PPX/ Ag tubes (d). (Figure 5A is reproduced with permission from reference 5a. Copyright 2000 Wiley-VCH- Verlag.) 2

Figure 6. TEM of cut PPX-Pd composite tubes (a), high resolution TEM of PPXPd composite tubes (b), and TEM of uncut PPX-Pd tubes after heating to 385 ° for 5 hrs (a + b) and to 365for 3 hrs (c) (4d).



Scheme 1. Conversion of 9-anthracene carboxylic acid to 9-anthracene carboxylic acid chloride and esterification reaction with PVA

Wavelength(nm)

Figure 7. Fluorescence spectrum of solution cast films ofPVA-Anth (excitation wavelength 300 nm).

Figure 8, Fluorescence microscopy (a) and TEM (a) of electrospun PVA-Anth fibers. (See page 11 of color inserts.)



Scheme 2. Reaction ofPPX-PVA/PAA composite tubes with anthracenoyl chloride.

Figure 9. Fluorescence optical microscopy of PPX-PVA/PAA tubes functionalized by anthracene (a, magn. 480x) and necking-type defect (b). (See page 12 of color inserts.)

Of unknown origin are necking-type defects of the tubes which where observed with the fluorescent tubes by fluorescence microscopy (Figure 9b). A possible explanation could be post-processing mechanical damages by in proper handling but this is still speculation. A highly versatile method for the preparation of functional nanofibers is the co-electrospinning technique, which was recently published by several groups (5, 6b,c). Here, composite fibers with coaxial variation of concentration variation are prepared by coaxial spinning of jets. For example, composite fibers of polyethylene oxide (PEO, shell) and poly(dodecylthiophene) (PDT, core) were prepared by co-electrospinning of chloroform solutions (Figure 10a). Another example in case is co-electrospinning of PLA (shell, from chloroform solution) and PdOAc (core, from THF solution). Here, PLA / Pd composite fibers were obtained after conversion of PdOAc to Pd at 170 °C for 2 hours. Using the electrospinning technique it is possible obtain fibers from materials which otherwise do not form fibers by electrospinning.



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Figure 10. (a) TEM of an unstained sample of co-electrospun PEO (shell) and PDT (core) fiber and (b) TEM of annealed (170°C/2 h) unstained samples of co-electrospun PLA (shell) and PdOAc (core) (previously published in Lit. (8a).

Conclusions Electrospinning is a highly versatile method for the preparation of functional polymer nanofibers and nanotubes. Functional nanofibers were prepared by electrospinning of functional polymers, by electrospinning of mixtures of polymers and functional materials or by functionalization of polymer nanofibers via post-electrospinning coating. In addition, co-electrospinning offers many possibilities for new materials combinations. A l l of these methods are very general and offer manifold possibilities for the preparation of new materials for various field of applications including catalysis, special filtration, membranes, biomedical applications just to name a few. However, what is still missing is the placement of functionalities at particular locations along electrospun fibers, proper control of materials density gradients, as well controllable shape of electrospun fibers, which will be challenges for future work.

Acknowledgements The authors are indebted to Specialty Coating Systems, Indianapolis for the donation of parylene dimer and technical support, to Boehringer Ingelheim for the donation of PLA, and to Dr. A . Schaper and M . Hellwig for SEM and T E M support.

References 1.

For recent reviews see: a) Electrospinning and the formation of nanofibers; Fong, H.; Reneker, D. H. in: „Structure formation of polymeric fibers"; Salem, D. R.; Sussman, M. V., Eds.; Hanser 2000; p. 225. b) Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M . ; Ramakrishna, S. Comp. Sci. Techn. 2003, 63,


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2223. c) Polymer Nanofibers by Electrospinning; Dersch, R.; Greiner, A.; Wendorff, J. H . in: Dekker Encyclopedia of Nanoscience and Nanotechnology; Schwartz, J. A.; Contescu, C. I.; Putyera, K., Eds.; Marcel Dekker; New York 2004; p. 2931. a) Bognitzki, M . ; Frese, T.; Steinhart, M . ; Greiner, A.; Wendorff, J. H., Polymer Engineering and Science 2001, 41, 982. b) Bognitzki, M.; Czado, W.; Freese, T.; Schaper, A . ; Hellwig, M . ; Steinhart, M . ; Greiner, A . ; Wendorff, J. H., Adv. Mater. 2001, 13(1), 70. c) Dersch, R.; Liu, T.; Schaper, A . K.; Greiner, A.; Wendorff, J. H., J. Polym. Sci.: Part A: Polym. Chem. Ed. 2003, 41, 545. Jun, Z.; Hou, H.; Schaper, A . ; Wendorff, J. H.; Greiner, A., e-Polymers 2003, No. 9. a) Bognitzki, M . ; Hou, H.; Ishaque, M.; Frese, ,T.; Hellwig, M.; Schwarte, C.; Schaper, A.; Wendorff, J. H.; Greiner, A., Adv. Mater. 2000, 12, 637. b) Hou, H.; Zeng, J.; Schaper, A . ; Wendorff, J. H.; Greiner, A . , Macromolecules 2002, 35, 2429. c) Caruso, R. A.; Schattka, J. H.; Greiner, A., Adv. Mat. 2001, 13, 1577. d) Sun, Z.; Zeng, J.; Hou, H.; Wendorff, J. H.; Greiner, A., submitted. a)Sun, Z.; Zussman, E.; Yarin, A . L.; Wendorff, J. H.; Greiner, A., Adv. Mat. 2003, 15(22), 1929 a) Dong, H, Jones, W. E., Polymeric Materials: Science & Engineering 2002; 87, 273; b) Li, D.; Xia, Y., Nanoletters 2004, 4, 933; c) Loscertales, I. G.; Barrero, A., Marquez, M . ; M Spretz, R.; Velarde-Ortiz, R.; Larsen, G., J. Am. Chem. Soc. 2004, 126, 5376. Gorham, W. F., J. Polym. Sci. Part A-1 1966, 4, 3027. a) Ding, B.; Kim, H. Y . ; Lee, S. C.; Lee, D. R.; Choi, K. J., Fibers and Polymers 2002, 3, 73. b) Fong, H.; Chun, I.; Reneker, D. H., Polymer 1999, 40, 4585. Hassan, C. M . ; Peppas, A. N., Adv. Polym. Sci. 2000, 153, 37.


Polymeric Nanofibers - American Chemical Society

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