Kinetically Controlled Formation of Multidimensional Poly(3,4

Article pubs.acs.org/cm

Kinetically Controlled Formation of Multidimensional Poly(3,4-ethylenedioxythiophene) Nanostructures in Vapor-Deposition Polymerization Oh Seok Kwon,†,§ Seon Joo Park,†,§ Hyun-Woo Park,‡ Taejoon Kim,‡ Minjeong Kang,‡ Jyongsik Jang,*,† and Hyeonseok Yoon*,‡ †

World Class University program of Chemical Convergence for Energy & Environment, School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, South Korea ‡ Alan G. MacDiarmid Energy Research Institute, Department of Polymer and Fiber System Engineering, Chonnam National University, Gwangju 500-757, South Korea S Supporting Information *

ABSTRACT: We establish, for the first time, the concept that the morphology of deposited polymer nanomaterials is highly affected by substrate curvature, as well as synthetic conditions. Nanonodules and nanorods can be grown on a nanofiber surface by controlling critical kinetic factors (temperature and pressure) during vapor deposition polymerization, leading to the formation of multidimensional polymer nanostructures. On the other hand, no remarkable nanostructures were generated on bulk flat substrate under the same conditions. Multidimensional poly(3,4-ethylenedioxythiophene) (PEDOT) nanofibers were fabricated successfully, and their hollow nanostructures, namely nanotubes, were also obtained by a core-etching process. It is expected that the multidimensional conducting polymer nanomaterials will have advantages when used as structures for superhydrophobic coatings, adhesion enhancement, separation, and energy conversion/storage. KEYWORDS: multidimensional nanostructures, conducting polymers, vapor deposition polymerization, nanofibers, nanotubes

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INTRODUCTION One-dimensional (1D) electronic materials with inherent nanoscale features are considered ideal components for developing unprecedented future technologies.1−6 Over past decades, enormous effort has been devoted to the fabrication of various 1D nanostructures.7−11 In particular, metallic and ceramic nanostructures have been fabricated extensively; their structures can be precisely controlled at the nanometer level and occasionally even at the atomic level12 using, for example, chemical vapor deposition13,14 or colloidal synthesis.15,16 Additionally, unique size- and structure-dependent characteristics have been identified in such inorganic nanomaterials. However, progress has been relatively slow in studying polymer nanomaterials. Polymers are composed of covalent bonds that are directional and thermosensitive, which makes the polymer structurally unstable at the nanometer level.17,18 Consequently, there are many technical difficulties in fabricating and manipulating polymer nanomaterials with controlled morphologies.19−22 Moreover, there is still a lack of a theoretical interpretation as to how polymer nanomaterials are generated through the polymerization process. Owing to their interesting electrical, optical, and mechanical properties, conducting polymers are considered as an alternative material to metals and inorganic semiconductors in future applications.21−26 Conducting polymers can be produced via vapor (e.g., oxidative CVD)27−30 and solution (e.g., micelle templating)31−34 phase synthesis. To the best of our knowledge, © 2012 American Chemical Society

however, there is only limited information on fabricating multidimensional polymer nanostructures in a controlled fashion so far.35−37 Here, we present a new bottom-up strategy for fabricating 1D conducting polymer nanofibers and nanotubes with unique surface substructures, specifically multidimensional poly(3,4-ethylenedioxythiophene) (PEDOT) nanostructures. The surface substructure was precisely controlled on the electrospun nanofibers, importantly depending on the curvature of the nanofiber template and polymerization kinetics.

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EXPERIMENTAL SECTION

Fabrication of Multidimensional PEDOT Nanostructures. Poly(methyl methacrylate) (PMMA) has polar groups that can interact with metal cations and is also soluble in various organic solvents. Thus, PMMA (1 g, Mw = 350 000, Aldrich) was dissolved in dimethyl formamide (DMF) at 70−80 °C, and PMMA nanofibers were electrospun from the PMMA/DMF solution. In the electrospinning process, the solution was injected through a stainless steel needle (22 gauge) that was connected to a high-voltage dc power supply (Nano NC 60 kV/2 mA). The solution was continuously fed through the nozzle connected with syringe pump (Kd scientific) at a rate of 12 μm min−1. High voltage (15 kV) was applied between the needle and the grounded collector (the distance was 15 cm). As a result, PMMA nanofibers were continuously ejected from the nozzle Received: June 25, 2012 Revised: October 2, 2012 Published: October 4, 2012 4088

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was used for field-emission scanning electron microscopy (FE-SEM) and EDS. The measurement of the electrical conductivity was carried out with a source-meter at ambient temperature by a four-probe method.

and accumulated on a cellulose substrate. The thickness of the nanofiber network film was controlled by adjusting the accumulation time. The resulting PMMA nanofibers were immersed into ferric chloride/methanol solution (40 mL). After drying under vacuum, the nanofibers were placed in the pressure-controllable reactor. Then, the 3,4-ethylenedioxythiophene (EDOT) monomer (Aldrich) was injected at a controlled reactor pressure and temperature, which resulted in the formation of PEDOT-coated PMMA nanofibers. The PEDOT nanotubular structures were obtained by dissolving the PMMA core with DMF solution. The final products were washed by suction filtration of water and methanol to remove residual reagents, and the energy-dispersive X-ray spectroscopy (EDS) analysis demonstrated that there were no impurities in the nanostructures. The four-probe conductivities of the nanostructures were 2 × 100 to 6 × 101 S cm−1. Instrumentation. Photographs of transmission electron microscope (TEM) were obtained with a JEOLJEM-200CX. A JEOL 3700

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RESULTS AND DISCUSSION Figure 1 outlines the synthetic procedure for multidimensional nanofibers and nanotubes under three representative vapordeposition polymerization (VDP) conditions. Polymer nanofibers were fabricated by electrospinning,38−41 and then conducting polymer was introduced onto the nanofiber surface through VDP.42−44 During this process, the electrospun nanofibers serve as both template and substrate for the growth or assembly of nanobuilding blocks (from oligomer to polymer). More specifically, PMMA nanofibers were accumulated on electrically conductive collectors by electrospinning (Figure 2a). The nanofibers were immersed in ferric chloride solution, leading to the adsorption of ferric ions on the PMMA nanofiber surface (Figure 2b). Because ferric ions are a potent Lewis acid, they form a chelate complex with lone electron pairs on the PMMA chains by coordination bonding. Subsequently, liquid monomers were vaporized at controlled pressures, and their chemical polymerization proceeded on the nanofiber surface with the adsorbed ferric ions, which resulted in the formation of a polymer sheath on the PMMA nanofiber. The core nanofiber was then removed by solvent etching, yielding a nanotubular structure. Conducting polymers such as PEDOT generally have strong interchain interaction, and PMMA dissolves well in DMF. Thus, the core nanofiber was readily etched without structural deformation of PEDOT shell. Electron microscopic analyses revealed that PEDOT layers with an average thickness of 20 nm were coated onto the PMMA nanofibers with an average diameter of 60 nm and the nanotubular structures were obtained successfully with no deformation after the etching process. Surprisingly, as shown in Figure 2e−h, the resulting nanotubes exposed unique surface substructures, such as nanoscale rods (NR, Figure 2e,f) and

Figure 1. Schematic illustration of the synthetic routes to multidimensional PEDOT nanostructures. PMMA nanofibers were accumulated on the collector by electrospinning, in which PMMA solution was supplied to the syringe tip at 5 μm min−1 and an electric field of 15 kV was applied between the tip and the grounded collector (distance, 15 cm). The PMMA nanofibers function as template as well as substrate for the growth of PEDOT under different synthetic conditions (temperature and pressure).

Figure 2. Multidimensional PEDOT nanostructures with unique surface substructures. PMMA nanofibers were accumulated on the collector by electrospinning, in which PMMA solution was supplied to the syringe tip at 5 μm min−1 and an electric field of 15 kV was applied between the tip and the grounded collector (distance, 15 cm). (a−h) The PMMA nanofibers function as template as well as substrate for the growth of PEDOT under different synthetic conditions (temperature and pressure). The morphologies of the resulting nanomaterials were characterized by FE-SEM and TEM (right top inset images): PMMA nanofibers (a) before and (b) after ferric ion adsorption; PMMA/PEDOT nanofibers with SL surface (c) before and (d) after core etching; PMMA/PEDOT nanofibers with NR surface (e) before and (f) after core etching; PMMA/PEDOT nanofibers with NN surface (g) before and (h) after core etching. The SEM and TEM images show the unique surface substructures. Additionally, the TEM images reveal the hollow interior of the nanotubular structures. 4089

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nodules (NN, Figure 2g,h), in addition to the smooth layer (SL, Figure 2c,d). To the best of our knowledge, there is no previous report on fabricating these unique multidimensional polymer nanostructures. Thus, key synthetic variables were further examined to determine how such morphologies were formed. Generally, the deposition of polymers in forming thin films is guided by a nucleation−growth mechanism. Specifically, the deposited nuclei grow on a substrate during polymerization until they become a polymer layer. During this process, kinetic and thermodynamic factors determine the morphology of the polymer layer deposited. Judging from our findings, oligomeric chain fragments consisting of EDOT rings are generated in the initial stage, the subsequent growth of which is highly dependent on synthetic conditions and substrate curvature. Because of the high curvature of the nanofiber, it is difficult to form a smooth polymer layer that completely covers the nanofiber surface via continuous chain growth. Thus, spontaneous assembling of EDOT oligomers could result in vertical growth rather than lateral growth on the nanofiber surface even though the interfacial tension between the EDOT oligomer and the PMMA is low. Under the same conditions, a control experiment was carried out using a flat PMMA film to demonstrate the effect of substrate curvature on the growth process of the polymer (see the Supporting Information, Figures S2 and S3). Although the polymer layer deposited on the flat surface was somewhat rugged, no remarkable features were observed, confirming the dependence of the chain growth on substrate curvature. On the other hand, the formation of the smooth layer on the nanofiber substrate was achieved under synthetic conditions that provided faster kinetics. The deposition of monomer vapor is enhanced by lowering the pressure and raising the temperature inside the reactor, which, in turn, leads to a rapid polymerization rate on the nanofiber surface. Such kinetic conditions are considered to create a local microenvironment capable of overcoming the geometric effect of the substrate. In fact, there are only a few reports in the literature on the kinetics of chemical polymerization of conducting polymers, mostly due to their insolubility in common solvents. Moreover, it is difficult to experimentally observe kinetic behavior during the VDP process that is carried out in a closed chamber with a controlled internal pressure. In this work, therefore, real-time measuring of the current flowing through the nanofiber substrate was attempted to monitor polymer formation during VDP, and a simple kinetic model was developed to estimate the polymerization rate therefrom. In the VDP process, monomer from the gas phase condenses on the substrate and is oxidized by the redox initiator, the adsorbed ferric ions. EDOT polymerization is believed to occur through monomer oxidation and the creation of radical cations, followed by a coupling reaction. The coupling reaction cyclically proceeds with the oxidation reaction, yielding a polymer when EDOT rings are close enough to react. Monomer disappears via the oxidation reaction, followed by the coupling reaction; the polymerization reaction continues as long as monomer and initiator are available. The rate of oxidation or coupling reaction is the sum of numerous individual oxidation and coupling steps.45 However, if the rates of oxidation (R1) and coupling (R2) reactions are independent of the length of the chain, then the rate of monomer depletion, which is synonymous with the rate of polymerization, can be written in the generic form −d[M ] = R1 + R 2 = k1[M ][I ] + k 2[M *]2 dt

Figure 3. Effect of kinetics on the formation of multidimensional PEDOT nanostructures. (a) Real-time changes in the resistance of nanofiber network films during VDP for PEDOT coating: the y-axis showing the change in current was normalized between [0,1] and the curve was fitted (lines) for the experimental data (symbols). A VDP chamber with an electrical feed-through was designed and used for the above measurements. (b) Calculated conversion curves for generating unique PEDOT surface substructures. The molar concentration ratios of initiator to monomer, [I]/[M], were 2.2, 6.7, and 10.0 for the substructure SL, NN, and NR, respectively. Additionally, the time at which the change in resistance was saturated in the curves in part a was determined as the polymerization time required for 90% conversion. (c) Rate constants calculated for different [I]/[M].

where I, M, and M * represent the redox initiator, the monomer, and the oxidized monomer, respectively. Considering that the monomer is stoichiometrically oxidized by the initiator of the polymerization, the integration of the above equation yields (see Supporting Information) [M(t )] α = [M ]0 1 − βe−4αk1t

(2)

where α=

(1) 4090

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Figure 4. Effect of polymerization kinetics on the surface morphology of multidimensional PEDOT nanostructures. FE-SEM images of PMMA/ PEDOT nanofibers obtained with different [I]/[M] and temperatures (insets: high-magnification images): the temperature increased from top line (60 °C) to bottom line (90 °C), and the [I]/[M] increased from the left to the right.

β=

0.5[I ]0 [M ]0

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CONCLUSIONS

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ASSOCIATED CONTENT

We created a new synthetic process that allows tailoring the surface morphology of 1D conducting polymer nanomaterials. Novel nanonodule and nanorod substructures were grown on the surface of electrospun polymer nanofiber substrate by controlling pressure and temperature. The kinetic model developed showed that slower polymerization kinetics were favorable to the growth of the unique substructures on the nanofiber surface. Several other potentially critical variables remain, which can affect the formation of the surface nanostructures, such as the reactivity of monomer and the interfacial tensions of monomer and substrate.46 Thus, there is an ongoing effort to develop a more generalized kinetic model. Lastly, it is expected that the multidimensional polymer nanostructures can have additional advantages when used as structures for superhydrophobic coatings, adhesion enhancement, separation, and energy conversion/storage.

The terms α and β are constants, and thus, it is possible to estimate the concentration of the monomer as a function of time. Figure 3a shows the real-time current changes in the nanofiber substrates during polymerization (the resistance was normalized to [0, 1]). There was no current flow until late in the middle period of the VDP, as expected, and the current began to flow at significantly higher conversions. The polymerization reaction starts with the injection of the monomer, and the degree of polymerization increases slowly with time. Therefore, the change in current can be correlated with the rate of polymerization. The time at which more than 90% of the monomer was consumed was determined from the curve, and the rate constant can be calculated from eq 2. The rate constants were 0.9 × 10−4, 1.1 × 10−5, and 5.7 × 10−6 M−1 s−1 for the surface substructure SL, NN, and NR, respectively. Figure 3b displays the conversion profile for the polymerization of each substructure, calculated as a function of time. These predictions support the hypothesis that the morphology of the polymer deposited on a nanosubstrate strongly depends on the reaction kinetics. Besides kinetic parameters such as temperature and pressure, the effects of initiator and monomer concentration were also examined. At the corresponding ranges, the calculated rate constants appeared to decrease with increasing molar concentration ratio of initiator to monomer (Figure 3c). The faster rate constants at lower concentration ratios are unfavorable to the formation of the unique surface substructures. Figure 4 shows the electron microscopic images of PEDOT/PMMA nanofibers prepared with the different concentration ratios of initiator to monomer. With decreasing concentration ratios, the population of unique substructures decreased considerably, and their morphology also became somewhat different. Although some of the surface substructures survived, this change was probably due to inhomogeneous deposition of the initiator on the nanofibers surface.

S Supporting Information *

Infrared spectra before and after PEDOT coating, FE-SEM images showing the effect of substrate curvature, details on the kinetic equation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.Y.), [email protected] (J.J.). Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest. 4091

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program (No. 2012R1A1A1042024), International S&T Cooperation Program (K20901002259-12E0100-01510) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), the NRF grant (No. 2011-0017125), and the World Class University (WCU) program through the NRF funded by the MEST (R31-10013).

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ABBREVIATIONS 1D, one-dimensional; PEDOT, poly(3,4-ethylenedioxythiophene); VDP, vapor-deposition polymerization; PMMA, poly(methyl methacrylate); SL, smooth layer; NN, nanonodule; NR, nanorod

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