Unaligned Conducting Polymer Cryogels with Three

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Aligned/Unaligned Conducting Polymer Cryogels with Three-Dimensional Macroporous Architectures from Ice-Segregation-Induced Self-Assembly of PEDOT-PSS Xuetong Zhang,* Chunyan Li, and Yunjun Luo* School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, PR China Received March 23, 2010. Revised Manuscript Received December 16, 2010 Porous conducting polymers are of great interest because of the huge potential to combine high surface areas in the dry state with physical properties relevant to organic electronics. Aligned or unaligned conducting polymer cryogels with 3D macroporous architectures have been prepared using the ice-segregation-induced self-assembly (ISISA) of different poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) freezing precursors as a dispersion or a formed hydrogel. The chemical composition and molecular structure of the resulting conducting polymer cryogels have been investigated by X-ray photoelectron spectroscopy and Raman spectroscopy, respectively. The morphologies of the PEDOT-PSS cryogels, together with their textural structures, have been revealed by scanning electron microscopy, mercury porosimetry, and nitrogen sorption tests. Processing PEDOT-PSS via ISISA endows the conducting polymers with novel properties, as demonstrated by a series of X-ray diffraction, differential scanning calorimetry, and electrical conductivity tests. These conducting polymer cryogels with aligned/unaligned macroporous architectures suggest the potential in the development of electronic components, tissue engineering, and next-generation catalytic and separation supports.

1. Introduction Conducting polymers are an extremely important class of organic materials that conduct electricity. The past two decades have witnessed a rapid growth in research on these polymers, represented by polyaniline, polypyrrole, and polythiophene, which has been driven by their unique electrochemical and electronic properties as well as by processing advantages of polymers relative to inorganic electronic materials.1,2 However, porous conducting polymers have been much less studied although they are of great interest because of the huge potential to combine high surface areas in the dry state with physical properties relevant to organic electronics.3 Theoretically, the inherent rigidity of most conducting polymers lends itself well to the generation of permanent microporosity (pore diameter of 50 nm) conducting polymers, especially for polyaniline and polypyrrole, have been prepared using chemical oxidation or electropolymerization of the corresponding monomers in the presence of organic molecules or colloid particles serving as templates.5-7 As for nonporous conducting polymers, the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT/PSS) complex (Figure 1a) becomes the most successful *Corresponding author. Tel and Fax: 86-10-68918591. E-mail: zhangxtchina@ yahoo.com (X.Zhang); [email protected] (Y.Luo). (1) H€using, N.; Schubert, U. Angew. Chem., Int. Ed. 1998, 37, 22–45. (2) Pierre, A. C.; Pajonk, G. M. Chem. Rev. 2002, 102, 4243–6424. (3) Cooper, A. I. Adv. Mater. 2009, 21, 1291–1295. (4) Jiang, J. X.; Su, F.; Trewin, A.; Wood, C. D.; Niu, H. A.; Jones, J. T.; Khimyak, Y. Z.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 7710–7720. (5) Bartlett, P. N.; Birkin, P. R.; Ghanem, M. A.; Toh, C. S. J. Mater. Chem. 2001, 11, 849–853. (6) Luo, X.; Killard, A. J.; Smyth, M. R. Chem.;Eur. J. 2007, 13, 2138–2143. (7) Fan, C.; Qiu, H.; Ruan, J.; Terasaki, O.; Yan, Y.; Wei, Z.; Che, S. Adv. Funct. Mater. 2008, 18, 2699–2707.

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in terms of commercial applications as a result of its merits such as a high dispersibility in water, excellent film-forming performance through conventional solution processing, and unique physical properties for the resulting films (high transparency in the visible range, high mechanical flexibility, and excellent thermal stability).8 The PSS in the complex has two general functions.9 The first function is to act as the source for the charge-balancing counterions, and the second one is to disperse PEDOT segments into an aqueous medium. So far, PEDOT-PSS acting as an excellent electrode material for batteries, particularly as a transparent electrode to replace indium tin oxide (ITO) in the optoelectronic devices, and as an antistatic coating in photographic films, has been developed.9 However, there have been no previous reports on the preparation and application of porous PEDOT-PSS. The ice-segregation-induced self-assembly (ISISA) process has been widely applied to the preparation of various scaffolds with highly sophisticated structures starting from low- or high-molecular-weight precursors, as well as colloid systems, as either an aqueous solution or suspension or a formed hydrogel.10-16 Actually, ISISA is a cryogenic process consisting of freezing the starting precursors, storage in the frozen state for a definite period of time, and defrosting these frozen precursors by simple thawing or freeze drying. The formation of typically hexagonal ice crystals during freezing causes every solute originally dispersed in the (8) Xia, Y.; Ouyang, J. Macromolecules 2009, 42, 4141–4147. (9) Kirchmeyer, S.; Reuter, K. J. Mater. Chem. 2005, 15, 2077–2088. (10) Gutierrez, M. C.; Ferrer, M. L.; del Monte, F. Chem. Mater. 2008, 20, 634– 648. (11) Zhang, H.; Hussain, I.; Brust, M.; Butler, M. F.; Rannard, S. P.; Cooper, A. I. Nat. Mater. 2005, 4, 787–793. (12) Mukai, S. R.; Nishihara, H.; Tamon, H. Chem. Commun. 2004, 874–875. (13) Nishihara, H.; Mukai, S. R.; Tamon, H. Carbon 2004, 42, 899–901. (14) Deville, S.; Saiz, E.; Nalla, R. K.; Tomsia, A. P. Science 2006, 311, 515–518. (15) Gutierrez, M. C.; Jobbagy, M.; Rapun, N.; Ferrer, M. L.; del Monte, F. Adv. Mater. 2006, 18, 1137–1140. (16) Ferrer, M. L.; Esquembre, R.; Ortega, I.; Mateo, C. R.; del Monte, F. Chem. Mater. 2006, 18, 554–559.

Published on Web 01/19/2011

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Figure 1. (a) Structural formula of conducting polymer PEDOTPSS and (b) typical digital photograph of the resulting PEDOTPSS dispersion (left) and hydrogel (right).

aqueous medium to be expelled to the boundaries between adjacent ice crystals, and subsequent high-vacuum sublimation of ice caused by freeze drying gives rise to a cryogel with macroporous structures characterized by “walls” of matter enclosing empty areas where ice crystals originally resided.10 In spite of the general applications of the ISISA process in preparing inorganic and organic cryogels with macroporous structures, control of the morphology of the resulting cryogels was only partial. Recently, the ISISA process has attracted considerable attention because of its significant progress11,17-20 in macropore orientation in the resulting scaffolds, which has been realized by combining this process with unidirectional freezing in liquid nitrogen to control the growth direction of the ice templates. To our knowledge, there have been no reports describing the manufacture of the conducting polymers with 3D microstructures by utilizing the ISISA process, although it was previously used to fabricate conducing scaffolds based on multiwalled carbon nanotubes21-23 and to fabricate nonconducting FeCl3/poly(vinyl alcohol) composites for the purpose of making conducting polypyrrole/poly(vinyl alcohol) scaffolds.24 In a previous study,25 we have presented the synthesis of organic conducting PEDOT-PSS aerogels with lightweight, large BET surface areas and hierarchically porous structures via supercritical CO2 drying metal ion cross-linked PEDOT-PSS (17) Vickery, J. L.; Patil, A. J.; Mann, S. Adv. Mater. 2009, 21, 2180–2184. (18) Zhang, H.; Cooper, A. I. Adv. Mater. 2007, 19, 1529–1533. (19) Zhang, H.; Long, J.; Cooper, A. I. J. Am. Chem. Soc. 2005, 127, 13482– 13483. (20) Gutierrez, M. C.; Garcı´ a-Carvajal, Z. Y.; Jobbagy, M.; Rubio, F.; Yuste, L.; Rojo, F.; Ferrer, M. L.; del Monte, F. Adv. Funct. Mater. 2007, 17, 3505–3513. (21) Gutierrez, M. C.; Hortig€uela, M. J.; Amarilla, J. M.; Jimenez, R.; Ferrer, M. L.; del Monte, F. J. Phys. Chem. C 2007, 111, 5557–5560. (22) Hortig€uela, M. J.; Gutierrez, M. C.; Aranaz, I.; Jobbagy, M.; Abarrategi, A.; Moreno-Vicente, C.; Civantos, A.; Ramos, V.; Lopez-Lacomba, J.; Ferrer, M. L.; del Monte, F. J. Mater. Chem. 2008, 18, 5933–5940. (23) Gutierrez, M. C.; Garcla-Carvajal, Z. Y.; Hortig€uela, M. J.; Yuste, L.; Rojo, F.; Ferrer, M. L.; del Monte, F. J. Mater. Chem. 2007, 17, 2992–2995. (24) Bai, H.; Li, C.; Chen, F.; Shi, G. Polymer 2007, 48, 5259–5267. (25) Zhang, X. T.; Chang, D. W.; Liu, J. R.; Luo, Y. J. J. Mater. Chem. 2010, 20, 5080–5085.

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supermolecular hydrogels. Herein, starting from different PEDOT-PSS precursors (dispersion/hydrogel) and using the ISISA process combined with unidirectional freezing, we present the fabrication of novel PEDOT-PSS cryogel monoliths with 3D macroporous architectures. It has been hereafter confirmed by SEM observations that the PEDOT-PSS cryogels made from dispersions have exhibited extra macropore alignment along the freezing direction whereas those made from hydrogels have not exhibited any apparent alignments. The molecular vibrational (Raman) spectrum has revealed that all of the resulting cryogels show the well-doped states of PEDOT with PSS, and X-ray powder diffraction (XRD) shows that, for the conjugated macromolecular chains within PEDOT-PSS cryogels, the interchain packing along a pseudo-orthorhombic R axis is more preferable than the interchain planar ring stacking. The work presented here, to the best of our knowledge, is the first detailed report on the direct process of conducting polymers with ISISA. More importantly, our work provides an efficient way for conducting polymers to control their macropore alignment by simple choosing an appropriate freezing precursor. In addition, our work also gives the inspiration for preparing any other organic conducting cryogels starting from colloid precursors of polymers such as polyaniline and polypyrrole. It could be envisioned that 3D macroporous architectures, together with their electro-optical properties, would cause these organic conducting cryogels to be applied in many fields with unexpected performance.

2. Experimental Section 2.1. Materials. 3,4-Ethylenedioxythiophene (EDOT) and poly(sodium 4-styrenesulfonate) (NaPSS, Mw = 70 000) were purchased from Sigma-Aldrich. Other analytical-grade chemicals, including oxidizing agents (Fe(NO3)3 3 9H2O, (NH4)2S2O8), were purchased from Beijing Chemical Reagents Company. All chemicals were used without further purification. 2.2. Synthesis Method. The PEDOT-PSS dispersion was synthesized according to the method reported elsewhere.26 In brief, 100 μL (0.9 mmol) of the EDOT monomer was initially diluted with 0.5 mL ethanol and then mixed with a NaPSS aqueous solution (0.37 g (1.8 mmol) of NaPSS dissolved in 6 mL of deionized water) via sonication to get a uniform milky dispersion, and then an oxidizing agent solution (0.21 g (0.9 mmol) of ammonium persulfate dissolved in 1.0 mL of deionized water) was added to above dispersion. The reaction took place at 50 C with magnetic stirring overnight. Afterwards, the resulting black mixture was dialyzed at least five times with deionized water to remove oligomers and other impurities. Ultimately, an 8.0 wt % PEDOT-PSS dispersion (8.0 wt % means the total mass fraction of PEDOT-PSS, as given below) was obtained by adjusting the mass concentration of the product with deionized water. The PEDOT-PSS hydrogel was synthesized according to a procedure reported elsewhere.27 Briefly, 150 μL (1.35 mmol) of an EDOT monomer was initially diluted with 0.5 mL of ethanol and then mixed with a NaPSS aqueous solution (0.56 g (2.7 mmol) NaPSS dissolved in 9.0 mL of deionized water) via sonication to form a uniform milky dispersion; finally the oxidizing agent solution (3.6 g (8.8 mmol) of Fe(NO3)3 3 9H2O dissolved in 2.0 mL of deionized water) was added to the above dispersion with brief vigorous magnetic stirring (60 s) and then was left undisturbed for at least 24 h to form hydrogel. The resulting ca. 8.0 wt % hydrogel was purified by exchanging the solvent at least once a week with a large amount of 0.1 M HCl (for the purpose of dissolving a small amount of the precipitated iron salt) and (26) Lefebvre, M.; Qi, Z.; Rana, D.; Pickup, P. G. Chem. Mater. 1999, 11, 262– 268. (27) Dai, T.; Jiang, X.; Hua, S.; Wang, X.; Lu, Y. Chem. Commun. 2008, 4279– 4281.

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Scheme 1. Fabrication of 3D Macroporpous Architectures of Conducting Polymer PEDOT-PSS Cryogels via an Ice-Segregation-Induced SelfAssembly Process with Unidirectional Freezinga

a (A) An example of the PEDOT-PSS freezing precursor held in a plastic centrifugal tube with diameter  height = 14  500 (mm2). (B) Unidirectional freezing of the PEDOT-PSS precursor via lowering the sample holder at a dipping rate of Vd = 3-50 mm/min vertically into liquid nitrogen. (C) Digital photographs of the resulting PEDOT-PSS monolithic cryogels obtained from a PEDOT-PSS dispersion (left) and a PEDOT-PSS hydrogel (right).

deionized water in sequence to remove low-molecular-weight components from the systems. Likewise, 5 and 10 wt % PEDOT-PSS hydrogels were obtained in a similar way.

2.3. Ice-Segregation-Induced Self-Assembly of PEDOTPSS. The ice-segregation-induced self-assembly of PEDOT-PSS precursors with unidirectional freezing is shown in Scheme 1. One of the synthesized PEDOT-PSS freezing precursors was held in a plastic centrifugal tube with dimension of diameter  height = 14  500 (mm2) and then vertically lowered into a cold source (liquid nitrogen) at a controlled rate (dipping rate Vd in the range of 3-50 mm/min) until all of the sample was immersed in the liquid nitrogen. The frozen samples were then freeze dried for 24 h and removed from the plastic centrifugal tube as intact monoliths. Cross sections and axial sections of the resulting cryogels were prepared by cutting them with a sharp razor blade. 2.4. Instrumentation. The morphology of the products was characterized using a ZEISS Supra 35 field-emission-gun scanning electron microscope at 5-10 kV. Samples for electron microscopy were prepared by cutting scaffolds with a sharp razor blade on a silicon wafer. X-ray powder diffraction data were collected with an X’Pert Pro MPD (PANalytical, The Netherlands) diffractometer using monochromatic Cu KR1 radiation (λ = 1.5406 A˚) at 40 kV and 40 mA. The diffraction patterns were optimized with a step length of 0.01(2θ) over an angular range of 5-70 (2θ) at a scanning speed of 0.01/s. X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Ultra spectrometer with a high-performance Al monochromatic source operating at 15 kV. XPS spectra were taken after all binding energies were referenced to the C 1s neutral carbon peak at 284.8 eV, and the elemental composition was determined from the ratio of peak areas after correction. Raman spectra were recorded on a Renishaw system 1000 with a 50 mW He-Ne laser operating at 632.8 nm with a CCD detector. Each final spectrum presented is an average of 10 spectra recorded in different regions randomly selected over the entire range of the sample. An AutoPore IV 9510 (Micromeritics, USA) was used for mercury intrusion porosimetry (MIP) experiments. The mercury pressure was increased from 0.5 to 60 000 psi (3.4 kPa-413.7 MPa), and the corresponding intruded volume was measured. The cumulative volume intruded was taken as the total pore volume. The incremental volume intruded at each pressure was used to estimate a pore size distribution for each sample. The mercury/vapor surface tension was taken to be 0.485 N m-1, and the assumed contact angle was 130. Nitrogen sorption measurements were performed with an ASAP 2010 (Micromeritics, USA) to obtain pore properties such as the BET-specific surface area, pore size distribution, and total pore volume. Before measurement, the sample was outgassed under vacuum at 250 C for ca. 10 h until the pressure was less Langmuir 2011, 27(5), 1915–1923

than 5 μmHg. A Keithley 4200 semiconductor characterization system was used to measure electrical conductivities of the samples via a two-probe method. DSC was carried out using a TA Instruments Q100 differential scanning calorimeter from -50 to 160 at a heating rate of 10 C/min with N2 protection. The weight of the samples is ca. 5 mg in all cases.

3. Results and Discussion The PEDOT-PSS freezing precursors of the dispersion or the formed hydrogel were prepared by solution chemistry. It has been well known that the 3,4-ethylenedioxythiophene (EDOT) monomer can be oxopolymerized by various oxidizing agents in the presence of PSS to form conducting polymer complex PEDOTPSS,26 and it was reported elsewhere27 that the formation of the PEDOT-PSS supermolecular hydrogel can be achieved by using various multivalent metal ions to cross link the PSS to form 3D networks driven by the electrostatic interaction between the metal ions and the anions attached to PSS. These facts prompt us to prepare the different PEDOT-PSS colloid precursors by simply using the different oxidizing agents during monomer oxidation. For example, if a PEDOT-PSS hydrogel was expected, then a stoichiometric excess of ferric nitrate could be used as the oxidizing agent as well as cross-linking agent to polymerize EDOT monomers in the presence of PSS, whereas if the PEDOT-PSS dispersion was expected, a stoichiometric amount of ammonium persulfate could be used as the oxidizing agent. According to this principle, we have successfully prepared the PEDOT-PSS complex with an initial molar ratio of PEDOT to PSS of 1:2, either as a dispersion or a formed hydrogel as shown in Figure 1b, by using different oxidizing agents. Less PSS in the composition would lead to the decreased stability of the resulting dispersions, and more PSS in the composition would lead to the decreased conductivity of the resulting cryogels. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used to investigate the effect of the oxidizing agents on the chemical composition and molecular structure of the resulting PEDOT-PSS colloid precursors. For convenience, all of the samples were first frozen and then freeze dried for spectroscopy investigations. These investigations also provide information on the chemical composition and molecular structure of the resulting PEDOT-PSS cryogels. Figure 2a shows XPS survey spectra of the resulting PEDOT-PSS cryogels obtained from the PEDOT-PSS hydrogels (sample 1) and dispersion (sample 2), respectively. The spectra of both samples show three main peaks located at 285, 533, and 164 eV DOI: 10.1021/la1044333

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Figure 2. (a) XPS survey spectra and (b) Raman spectra of the PEDOT-PSS cryogels obtained from the PEDOT-PSS dispersion and hydrogel, respectively.

as seen in Figure 2a, corresponding to elemental carbon, oxygen, and sulfur,28 respectively. The spectra of both samples show small peaks at 400 eV ascribed to the same element, nitrogen, but coming from the different sources. A small amount of nitrogen (ca. 1%) in sample 1 comes from oxidizing agent ferric nitrate, and that in sample 2 comes from oxidizing agent ammonium persulfate. This indicates that the oxidizing agents used during the polymerization were hardly completely removed by solvent exchange. Nearly the same amount (ca. 1%) of elemental iron exists in sample 1, one-third of which must serve as the counterions of the nitrate because the stoichiometric formula of ferric nitrate is Fe(NO3)3 and the rest of the iron serving as the counterions of the sulfonate attached to PSS because these cations are sometimes simply pulled in to maintain charge neutrality.26 However, for sample 2 no elemental iron appeared in the XPS spectra. These results, together with the fact that the formation of the hydrogels was never observed if Fe(NO3)3 was replaced by ammonium persulfate, indicate that a small amount of Fe3þ serves as the cross-linking agent during PEDOT-PSS hydrogel formation. Furthermore, for sample 2 there were two relatively strong peaks observed at 498 and 1070 eV, both of which are relevant to elemental sodium, whereas for sample 1 no corresponding peaks were apparently observed. Poly(sodium 4-styrenesulfonate), rather than PSS, was used to prepare PEDOT-PSS freezing precursors of the dispersion or the formed hydrogel. For sample 1, large amounts of 0.1 M HCl and H2O were used in sequence to purify the resulting hydrogel. During this process, the sodium ion in poly(sodium 4-styrenesulfonate) could be replaced (28) Nguyen, T. P.; Le Rendu, P.; Long, P. D.; De Vos, S. A. Surf. Coat. Technol. 2004, 180-181, 646–649.

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by a proton, thus we could not observe the corresponding elemental sodium peaks in the XPS spectrum. For sample 2, only a large amount of water was used to dialyze the resulting dispersion. During this process, the amount of sodium ion in poly(sodium 4-styrenesulfonate) could not be exchanged but remained, so we could observe the corresponding elemental sodium peaks in the XPS spectrum. Figure 2b shows the Raman spectra of the resulting PEDOT-PSS cryogels obtained from samples 1 and 2. Both spectra exhibit the well-doped characteristic of PEDOT: a strong vibrational Raman band centered at 1427 cm-1, which corresponds to the symmetric stretching mode of the aromatic CdC band and is related to the conjugation length in the cryogels, and three other important bands found at 1528, 1363, and 1258 cm-1 that are related to antisymmetric CR-CR and Cβ-Cβ stretching deformations as well as CR-CR interring stretching vibrations, respectively.28 The bands at 1563, 702, and 437 cm -1 can be assigned to asymmetric CRdCβ stretching, symmetric C-S-C deformation, and SO2 bending, respectively,29 and the bands at 990 and 573 cm-1 can be ascribed to oxyethylene ring deformation.30 The weak band at 1128 cm-1 can probably be assigned to the PSS component.31 The bands at 527 and 859 cm-1 are probably ascribed to a typical Raman vibration of -OCH2CH2O- groups attached to thiophene rings in the PEDOT.32 The absence of the peak at 1480 cm-1 that is related to the overoxidation of PEDOT33 confirms that the PEDOT-PSS cryogels are in a highly oxidized state and have not reached the overoxidized state. Combining the results from XPS with those from Raman spectra, we could say that different PEDOT-PSS colloid precursors, either as a dispersion or a formed hydrogel, could be facilely obtained by choosing an appropriate oxidizing agent between ammonium persulfate and ferric nitrate although both of them can be used to polymerize EDOT monomers into conducting polymers in the presence of PSS. To demonstrate that 3D macroporous structures could be obtained by processing various PEDOT-PSS precursors via ISISA, the PEDOT-PSS hydrogel was chosen in the first instance for investigation. The optimal mass concentration of PEDOT-PSS hydrogels for directional freezing is in the range of ca. 5.0-10.0 wt %. A lower concentration of hydrogels would lead to freeze-dried cryogels that are too fluffy to handle, and a higher concentration of hydrogels would lead to nonuniform cryogels. The typical photograph of the monolithic cryogel obtained from the PEDOT-PSS hydrogel with frozen directionally at a dipping rate of 5 mm/min is shown in Scheme 1 (part C), from which the top flat surface and coneshaped bottom of the cryogel can be easily recognized. The detailed morphology and structure of the resulting monolithic cryogel was observed by SEM as shown in Figure 3. It can be clearly seen that, on a large scale, the morphology of the resulting cryogels is quite uniform when the mass fraction of PEDOT-PSS to total freezing precursor is in the range of ca. 5.0-10.0 wt %. (Figure 3a,c), and closer observations in different areas disclose that the cryogels obtained from both ca. 5 wt % (Figure 3b) and 10 wt % (Figure 3d) are rich in interconnected open macropores. These open macropores would offer convenient channels for various ions to obtain fast mass transfer if such PEDOT-PSS cryogels could be used as (29) Schaarschmidt, A.; Farah, A. A.; Aby, A.; Helmy, A. S. IEEE LEOS Annu. Meet. Conf. Proc. 2009, 351–352. (30) Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. Macromolecules 1999, 32, 6807–6820. (31) Stavytska-Barba, M.; Kelley, A. M. J. Phys. Chem. C 2010, 114, 6822–6830. (32) Alloin, F.; Hirankumar, G.; Pagnier, T. J. Phys. Chem. B 2009, 113, 16465– 16471. (33) Yoon, H.; Hong, J. Y.; Jang, J. Small 2007, 3, 1774–1783.

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Figure 3. SEM images of conducting polymer cryogels obtained from (a, b) ca. 5 wt % and (c, d) 10 wt % PEDOT-PSS hydrogels.

Figure 4. SEM images of a (a) longitudinal section and (b-d) cross sections of the conducting polymer cryogels obtained from a ca. 8 wt % PEDOT-PSS aqueous dispersion.

electrochemical electrodes. In addition, the concentration of the hydrogels plays a significant role in determining the Langmuir 2011, 27(5), 1915–1923

morphology and structure of the obtained cryogels. For example, when the 5 wt % hydrogel is used, the resulting DOI: 10.1021/la1044333

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Figure 5. SEM images of a well-ordered domain with 3D macropore alignment within a conducting polymer cryogel obtained from a ca. 8 wt % PEDOT-PSS aquesous dispersion: (a) the whole domain. Arrow Z denotes the longitudinal direction, and arrows r1 and r2 denote the radial directions. Notice that Z ^ r1 ^ r2: (b) face Zr1, (c) face r1r2, and (d) face Zr2.

cryogel exhibits brushy macropores. When the 10 wt % hydrogel is used, the resulting cryogels exhibits smooth macropores. In comparison with agarose cryogels with the wellordered alignment of macropores obtained from its hydrogel precursor processed in the similar way,34 it was a pity that no obvious macropore alignments were observed by SEM for the resulting PEDOT-PSS monolithic cryogels obtained from their hydrogel precursor. This was further confirmed by electrical conductivity tests: there is no apparent difference between the axial and radial conductivity, both approaching 10-2 S/cm for the sample obtained from the 10 wt % hydrogel precursor, which is comparable to the value for the PEDOTPSS thin film26 The lack of an ordered arrangement of macropores in the PEDOT-PSS monolithic cryogels is probably ascribed to the fact that the PEDOT-PSS hydrogel skeleton has blocked the continuous growth of ice templates within the gel network. To achieve PEDOT-PSS cryogels with macropore alignment, the PEDOT-PSS aqueous dispersion without ice growth encumbrance was used to replace the PEDOT-PSS hydrogel before directional freezing. A typical photograph of the monolithic cryogel obtained from the PEDOT-PSS aqueous dispersion frozen directionally at a dipping rate of 5 mm/min is shown in Scheme 1 (part C), from which it can be explicitly seen that the top surface of the monolithic cryogel was intumesced to form a cone-shaped structure with a relatively sharp tip. This obviously resulted from supersaturated PEDOT-PSS being moved by the growing ice crystals along the longitudinal direction during the ISISA process. The structure and morphology of the resulting cryogels were investigated by SEM as shown in Figure 4a-d. There was no (34) Stokolsa, S.; Tuszynski, M. H. Biomaterials 2006, 27, 443–451.

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apparent difference in morphology between the inner and periphery parts of the resulting cryogels, and the well-ordered alignment of the macroporous structure could be observed along the longitudinal direction (i.e., the ice growth direction) for the resulting cryogels (Figure 4a), which resulted from phase separation that occurs during the directional freezing process.11 However, on a large scale no apparent alignments were observed from the cross section of the macroporous monolith (Figure 4b), and closer observations in different areas showed that there were several domains with random orientations over the whole macroporous monolith and that the boundaries of these domains could be easily recognized (Figure 4c), although all of these domains were well-aligned along the ice-growth direction. The reason for the random orientation of domains observed from the cross section of the macroporous monolith can be ascribed to the fact that several ice cores grew simultaneously into cubic crystals as templates with the same longitudinal direction but different radial orientations in the columnar container with PEDOT-PSS aqueous solution. The cross section located at the cone-shaped bottom of the monolithic cryogel showing fingerprint-like morphology (Figure 4d), which is mainly ascribed to the amorphous ice caused by the supercooling of the dispersion in the immersed portion rather than ice crystals, served as a template.10 These observations indicate that these cryogels do not have structural homogeneity along the longitudinal direction. It should be pointed out that these fingerprintlike structures could be easily observed from the resulting monolithic cryogels, thus our results may suggest an efficient way to produce the man-made fingerprint. For the PEDOT-PSS monolithic cryogel obtained from its dispersion, the well-ordered alignment of the macroporous structure in three dimensions could be recognized in one domain as shown in Figure 5. It can be seen that the whole domain is at least Langmuir 2011, 27(5), 1915–1923

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on the order of cubic millimeters measured roughly from the SEM image (Figure 5a). For convenience, the 3D coordinates were drawn onto the images as shown in Figure 5. Among them, coordinate Z denotes the longitudinal direction (also the icegrowth direction), and coordinates r1 and r2 denote the radial directions. These three directions should be vertical to each other, as are those of ice crystals with a cubic shape. The primary periodicity along the r1 direction (Figure 5a) is ascribed to the MullinsSekerka instability wavelength.11 Immersing the PEDOT-PSS dispersion into liquid nitrogen causes constitutional supercooling at the interface, leading to Mullins-Sekerka instability and icecell growth with a well-defined wavelength whereby PEDOT-PSS becomes concentrated between the cells.11 The average wavelength of the primary periodicity is ca. 20 μm as measured from SEM images. The periodic wavelike curve along the Z direction observed in Figure 5b results from the secondary instability perpendicular to the freezing direction;11 however, along the r2 direction, also perpendicular to the freezing direction, a nearly linear structure is observed in Figure 5c. The periodic sheet structure could be disclosed from Figure 5d, and the thickness of the sheets was estimated at several micrometers from SEM measurements. These SEM observations indicate that each domain in the monolithic cryogel has 3D macropore alignment, and the whole cryogel could be regarded as several similar domains integrated along the mutually longitudinal direction. This longitudinally aligned structure in the form of microchannels has a significant influence on the conductivity of the resulting cryogels. For instance, the sample prepared from a 10 wt % PEDOT-PSS dispersion precursor has an axial conductivity of ca. 10-3 S/cm, which is 1 order of magnitude lower than that of the sample prepared from the 10 wt % PEDOT-PSS hydrogel precursor and has a radial conductivity from place to place in the range of 10-4-10-6 S/cm, which is 1-3 orders of magnitude lower than its axial conductivity. The textural properties of the resulting PEDOT-PSS cryogels obtained from colloid precursors either as a dispersion or as a formed hydrogel were further investigated by mercury porosimetry and nitrogen sorption tests. Figure 6 shows the curves of mercury pressure versus the corresponding intruded volume (Figure 6a), the curves of the pore size distribution (Figure 6b), and the nitrogen adsorption and desorption isotherm curves (Figure 6c) of the cryogels obtained from the different freezing precursors but with the same preparation parameters such as concentration, dipping rate, and so forth. All of the curves in Figure 6a,b are typical attributes of the macroporous materials. The total intrusion volume, the total pore area, the porosity, and the skeletal density (revealed by mercury porosimetry, the same as below) for the cryogel started from the PEDOT-PSS hydrogel were 14.38 mL/g, 54.12 m2/g, 86.80%, and 0.457 g/mL, respectively, whereas those for the cryogel started from the PEDOT-PSS dispersion were 6.62 mL/g, 10.49 m2/g, 89.02%, and 1.225 g/mL, respectively. The higher surface area or the higher total pore volume of the cryogels obtained from hydrogel precursor is ascribed to a PEDOT-PSS hydrogel skeleton that is strong enough to block the continuous growth of ice templates within the gel network, as mentioned above, and thus the original pores in the gel network were almost retained as observed from SEM images. These data indicate that the cryogels obtained from PEDOT-PSS hydrogels are lighter than those obtained from the PEDOT-PSS dispersion. The skeletal density of cryogels obtained from its dispersions was higher than that of those obtained from hydrogel precursors, giving a hint of the presence of mesopores in cryogels obtained from hydrogel precursors. This is further confirmed by the nitrogen sorption tests as shown in Figure 6c, Langmuir 2011, 27(5), 1915–1923

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Figure 6. (a) Typical mercury pressure vs intruded volume curves, (b) pore size distribution curves, and (C) nitrogen adsorption and desorption isotherm curves of the resulting PEDOT-PSS cryogels obtained from freezing precursors of either the dispersion or hydrogel.

where a typical type IV isotherm with a small characteristic hysteresis loop, which is proof of the limited existence of mesopores, was observed for the PEDOT-PSS cryogels prepared from the hydrogel precursors, whereas the cryogels obtained from EPDOT-PSS dispersion precursors show nothing on the type IV sorption isotherm curve because no characteristic hysteresis loop was observed. The average pore diameter for the cryogel obtained from PEDOT-PSS hydrogels was 1.1 μm, and the average pore diameter for the cryogel obtained from the PEDOT-PSS dispersion was G2.5 μm. In addition, the pore size distributions for the cryogels obtained from different freezing precursors were different, and the cryogel obtained from the PEDOT-PSS dispersion has a wider pore size distribution than that obtained from the PEDOT-PSS hydrogel, which can be clearly seen in Figure 6a, where the cumulative pore volume increased sharply at the lower pressure for the cryogel obtained from the PEDOT-PSS dispersion, and in Figure 6b, where DOI: 10.1021/la1044333

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Figure 7. Typical (a) XRD and (b) DSC patterns of the conducting polymer cryogels obtained from a PEDOT-PSS aqueous dispersion at different dipping rates.

different pore size distributions could be directly recognized from the curves. In fact, these observations were quite consistent with the previous observations offered by SEM. Processing PEDOT-PSS via ISISA probably endows the conducting polymers with novel properties, which can be demonstrated by our preliminary results given by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests. It has been reported35 that no characteristic peaks were observed by XRD for the PEDOT-PSS complex treated with the conventional solution-casting method, which indicates that PEDOT-PSS hardly crystallizes under the general conditions. However, the diffraction peaks were observed by XRD for the PEDOT-PSS cryogels obtained from freezing precursors of either the dispersion or the formed hydrogel. Figure 7a shows the typical XRD pattern of the cryogel obtained from the PEDOT-PSS aqueous dispersion frozen directionally at different dipping rates. The powder XRD pattern of the PEDOT-PSS cryogels processed in our way exhibits a diffraction peak at a scattering angle of 2θ = 17.8 (d = 5.0 A˚), which probably could be indexed in a structurally similar system as the interchain packing along a pseudo-orthorhombic a axis, whereas a diffraction peak at 2θ = 25.0 (d = 3.6 A˚) was attributed to the interchain planar ring-stacking distance.36 The intensity of the peak at 2θ = 17.8 is higher than that of the peak at 2θ = 25.0, which indicates that, for the conjugated macromolecular chains within the PEDOT-PSS cryogels, the interchain packing along a pseudo-orthorhombic R axis is more preferable than the interchain planar ring stacking. Furthermore, the dipping rate during directional freezing has played a small role in determining the degree of crystallinity of the PEDOT-PSS (35) Li, J.; Liu, J.; Gao, C.; Zhang, J.; Sun, H. Int. J. Photoenergy 2009, 2009, 650509. (36) Han, M. G.; Foulger, S. H. Small 2006, 2, 1164–1169.

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complex processed in our way. For example, the characteristic peaks mentioned above for the PEDOT-PSS complex at a dipping rate of 5 mm/min were appreciably higher than that for the PEDOT-PSS complex at a dipping rate of 46 mm/min as shown in Figure 7a. Originally we thought this might be due to either a testing error or the dipping rate. To determine the reason, we repeated the experiments from synthesis to XRD testing, and the XRD patterns with the same tendency were obtained again. Therefore, we thought it was due to the dipping rate and thus deduced that the relative degree of crystalline of the PEDOT-PSS complex at a dipping rate of 5 mm/min was a little higher than that of the PEDOT-PSS complex at a dipping rate of 46 mm/min because a lower freezing rate (dipping rate) better favors the growth of ice crystals. Similar results have been found for regioregular poly(3-hexylthiophene).37 However, it is very difficult to determine the quantitative degree of crystallinity according to the XRD patterns because of the lack of standard XRD data for crystallized PEDOT-PSS, but we can use melting heat to evaluate the relative degree of crystallinity (see below). The dipping rate during directional freezing that plays a slight role in determining the degree of crystallinity of the PEDOT-PSS complex was also confirmed by the DSC tests. As seen in Figure 7b, no obvious peaks were observed at ca. 0 C for either sample, which indicates that both samples are without water being left over. For both samples, there is an obvious peak at ca. 90 C during the first scanning from low temperature to high temperature, which is ascribed to the melting of the crystals in the cryogels. The melting heat for the cryogel at a dipping rate of 5 mm/min is 606.2 J/g, which is slightly higher than that (599.7 J/g) of the cryogel with a dipping rate of 46 mm/min. No obvious peak over the whole temperature range was observed for the PEDOT-PSS cryogels during the second temperature scanning, which provides strong evidence that the crystals in the cryogels were formed during directional freezing of the PEDOT-PSS precursors. These results indicate that the ISISA process combined with directional freezing endows the PEDOT-PSS complex with alignment at least on the chain-segment level.

4. Conclusions We have highlighted the potential of the ISISA process combined with directional freezing in fabricating 3D macroporous architectures of technologically important PEDOT-PSS cryogels from the different freezing precursors of either as a dispersion or a formed hydrogel. The results indicate that ISISA is a facile method for producing hierarchically macroporous 3D monoliths of conducting polymers with good reproducibility. The method described here has the following advantages: (1) This is a green method for producing conducting polymer architectures with 3D macroporous structures because an ice template was used during processing. (2) The fabrication of macroporous scaffolds is inexpensive, relatively facile, highly reproducible, and readily tailored by varying the experimental conditions (immersion rate, concentration of solute, molar ratio of components, etc.). (3) Our method provides a flexible way for the conducting polymer architectures to control the alignment of the 3D macropores by simply changing the freezing source. (4) Processing the conducting polymers in this way may introduce some unexpected properties for these functional polymers and thus may widen their application in many fields. We envision that these conducting polymer cryogels with novel macroporous architectures could play a significant role in the development of electronic components (37) Brinkmann, M.; Wittmann, J. C. Adv. Mater. 2006, 18, 860–863.

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(including batteries and transistors, solar cells), tissue engineering, and next-generation catalytic and separation supports. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (grant no.

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20903009), the Scientific Research Foundation for Returned Scholars, the Ministry of Education of China (grant no. 20091001), and the Science Foundation for the Excellent Youth Scholars of Beijing Institute of Technology (grant no. 2008Y0411).

DOI: 10.1021/la1044333

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