Polyaniline Cryogels Supported with Poly(vinyl alcohol) - American

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Polyaniline Cryogels Supported with Poly(vinyl alcohol): Soft and Conducting Jaroslav Stejskal,*,† Patrycja Bober,† Miroslava Trchová,† Adriana Kovalcik,‡ Jiří Hodan,† Jiřina Hromádková,† and Jan Prokeš§ †

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic Kompetenzzentrum Holz GmbH, Competence Centre for Wood Composites and Wood Chemistry (Wood K Plus), A-4040 Linz, Austria § Faculty of Mathematics and Physics, Charles University, 180 00 Prague 8, Czech Republic ‡

ABSTRACT: The present contribution reports the single-step preparation of new type of soft macroporous conducting cryogels, a special type of hydrogels. Polyaniline/poly(vinyl alcohol) cryogel was prepared by the oxidation of aniline hydrochloride in frozen reaction mixtures, in ice, containing a supporting polymer, poly(vinyl alcohol). The cryogel used for illustration contained of polyaniline, poly(vinyl alcohol) and 93 wt % of aqueous phase. It was macroscopically homogeneous and it had macroporous structure with average pore size of ≈100 μm. The conducting polyaniline phase was fibrillary. The molecular structure of polyaniline was confirmed by Raman spectroscopy. The conductivity of cryogel was 0.004 S cm−1 in water and 0.105 S cm−1 in 0.1 M sulfuric acid. It still increased to 0.29 S cm−1 when the content of monomer increased five times. Because of the contribution of electronic transport, the conductivity of cryogel was always higher than the ionic conductivity of aqueous phase used for its penetration. The conductivity of freeze-dried cryogel was 0.003 S cm−1. Viscoelastic and mechanical properties, controlled mainly by the conducting polymer phase, have been assessed and demonstrated good mechanical integrity and feasibility of potential applications.

1. INTRODUCTION The stone, bronze, and iron ages that constitute the history of mankind have been followed by the age of pottery, and finally by the present age of plastics. Although the opinions of future development vary, many polymer chemists agree that the trend in materials science will shift to soft materials, such as hydrogels.1 Polymer gels are chemically or physically cross-linked systems of polymer chains swollen with a liquid phase. They are called hydrogels when the liquid phase is water. Conducting polymer hydrogels represent a developing materials group that contains a conducting polymer, such as polyaniline or polypyrrole, in addition to a water-soluble polymer that endows them with mechanical properties. The synthesis, characterization, and applications of conducting hydrogels have recently been reviewed.1,2 Their valued properties are mixed electronic and ionic conductivity, electrochemical switching between redox states of conducting polymer, the transition between salt−base © 2017 American Chemical Society

forms in conducting polymers, good mechanical integrity and flexibility, nontoxicity and biocompatibility, high specific surface area, and controlled morphology. The importance of individual features depends on concrete applications. Polyaniline, one of the most studies conducting polymers, is synthesized by the oxidation of aniline in acidic aqueous medium.3 Its advanced applications in hydrogels have been aimed especially at energy conversion and storage.4−8 Conducting hydrogels based on polyaniline have usually been prepared by the diffusion of the aniline monomer into a supporting hydrogel, followed by the immersion of monomerswollen hydrogel into the solution of oxidant.9−22 The simple scenario expects that the oxidant will diffuse into hydrogel and conducting polymer will be formed after it meets the monomer Received: November 22, 2016 Revised: January 10, 2017 Published: February 2, 2017 972

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Figure 1. Solution of monomers (red circles) (a) becomes frozen (b). The monomer molecules are forced to concentrate mainly between growing ice crystals (blue objects), and subsequently they polymerize to a polymer (c, green objects). After thawing of ice, the polymer phase is left to produce a macroporous cryogel (d). The figure depicts schematically the cross-section of cryogel. The polymer phase is connective in three dimensions. chloride (0.2 M; Penta, Czech Republic) with ammonium peroxydisulfate3,32 (0.25 M; Lach-Ner, Czech Republic) in aqueous solutions of 5 wt % of poly(vinyl alcohol) (Mowiol 10−98, SigmaAldrich; molecular weight 61,000). The freshly prepared mixture was sucked in a plastic syringe, quickly frozen in solid carbon dioxide/ ethanol suspension, and then left in a freezer at −24 °C for 5 days to polymerize. After thawing at room temperature, green cryogels were removed from the syringe and immersed in excess of 0.1 M sulfuric acid or water to extract any residual reactants and byproducts. In follow-up experiments, the concentration of aniline hydrochloride was increased to 0.4, 0.6, 0.8, and 1.0 M at fixed oxidant-tomonomer mole ratio 1.25 and concentration of poly(vinyl alcohol). In some experiments, on the other hand, the concentration of poly(vinyl alcohol) in the standard preparation was increased up to 8 wt %, or its molecular weight was increased to 125,000 (Mowiol 20-98). Characterization. Electron scanning micrographs have been taken by using cryogenic technique with a Quanta 200F microscope (FEI, Brno, Czech Republic). Raman spectra of the powders excited with a HeNe 633 nm laser were recorded with a Renishaw InVia Reflex Raman microspectrometer. A research-grade Leica DM LM microscope with an objective magnification 50× was used to focus the laser beam on the sample placed on an X−Y motorized sample stage. A Peltier-cooled CCD detector (576 × 384 pixels) registered the dispersed light. For conductivity measurements, flat circular graphite electrodes with an 0.5 cm2 area in contact with both fronts of cylindrical cryogel were used as current electrodes while two voltage electrodes made from sharpened wire of Pt−Rh(10%) alloy with 0.5 mm diameter were placed on the surface of cylindrical hydrogel at various distances from current electrodes, what enabled an estimation of electrical homogeneity of samples. The polarity was switched each second to prevent mass transports and electrode polarization. A Keithley 6221 dc and ac current source was used in the combination with a Keithley 612 electrometer or a Keithley 2001 multimeter for voltage measurement. The setup included also a Keithley 7001 Switch System equipped with a Keithley 7011-S Quad 1X10 multiplexer as a scanner.

inside. The hydrogel penetrated with a conducting polymer thus should be produced. In practical experiments, however, it is observed that the conducting polymer is produced especially at hydrogel surface where both reactants meet, while in the center of the hydrogel, the conducting polymer is absent.13 The topology of the conducting polymer phase is uneven, and depends on the size and shape of the hydrogel. Conducting hydrogels can be prepared in this way, and may serve well in applications, but their properties are difficult to generalize. Hydrogels can also be produced by the polymerization of various monomers in frozen media, in ice, followed by thawing.23,24 Such hydrogels are labeled as cryogels, and the prefix cryo thus refers to the way of the preparation. Ice crystals act as fillers of the gel during its preparation and play a fundamental role in the development of macroporous morphology (Figure 1), in the contrast to classical hydrogels. The attempts to make conducting cryogels have been reported in the literature. In the first approach, cryogels have been prepared and subsequently used similarly as hydrogel templates for the preparation of conducting polymer.25−27 In the second way, conducting polymers were synthesized at first and then incorporated into cryogels in the course of their preparation.28−31 The former type of synthesis is likely to suffer from the structural inhomogeneity of composite cryogels, the latter by the discrete distribution of conducting component. The present contribution offers the general principle of the singlestep preparation of soft conducting macroporous cryogels with good mechanical integrity, as illustrated by example of polyaniline/poly(vinyl alcohol) cryogel.

2. EXPERIMENTAL SECTION Cryogel Preparation. Polyaniline/poly(vinyl alcohol) (PANI/ PVA) cryogels have been prepared by oxidation of aniline hydro973

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Macromolecules Static mechanical properties of hydrogels were determined with an electromechanical testing machine Instron 6025/5800R (Instron Ltd., U.K.) equipped with a 10 N load cell at room temperature and with a cross-head speed of 10 mm min−1. Measurement of cylindrical specimens with a diameter 9.5 mm and a length 40 mm was made in the environment of deionized water. Reported values were the averages of at least three measurements. The dynamic mechanical analyses were carried out using a submersion compression clamping system in dynamic oscillatory mode and in transient mode using the instrument DMA Q800 RH (TA Instruments, USA) with the fully swollen hydrogels 15 mm in diameter and 3.5 mm in height at 25 °C and 50% relative humidity. The compression modulus was obtained as the slope by linear fit of the compressive stress/strain curve. The compression strength was defined as the maximum stress reached after compression. Creep/creep recovery testing was performed by applying 0.001 N force, 10 min of isotherm at 23 °C, followed by applying a constant stress of 2 kPa for 3 min and relaxation time for 10 min. Dynamic oscillatory experiments at 1 Hz frequency in temperature range 5−100 °C, heating rate of 1 °C min−1, 50% relative humidity were used to determine the storage modulus and mechanical loss factor.

3. RESULTS AND DISCUSSION Polyaniline is typically prepared by the oxidation of aniline or aniline hydrochloride with ammonium peroxydisulfate3,32 (Figure 2) in acidic aqueous medium at room temperature

Figure 3. Composite polyaniline/poly(vinyl alcohol) cryogels.

Figure 2. Aniline is oxidized with ammonium peroxydisulfate to polyaniline (emeraldine) salt.

and it is collected as a precipitate. If the polymerization of aniline is carried out in the frozen aqueous media, in ice, a sponge-like polyaniline is produced.33,34 In order to obtain a true soft conducting cryogel, another component, such as a water-soluble supporting polymer, here poly(vinyl alcohol), had to be introduced to the system. Cryogel is then obtained after thawing of ice (Figure 3). Such cryogels are composed of conducting and supporting polymer phases and have therefore a composite nature. It has to be stressed that the polymerization of aniline in ice proceeds smoothly,33−35 which may be regarded as surprising due to the restricted diffusion of reactants in the solid phase. This has been earlier explained by the ability of conducting polymer phase to transfer the electrons and protons from the aniline to peroxydisulfate oxidant.36,37 These molecules thus can react without the need to diffuse to each other to produce a physical contact.33 The role of a supporting polymer, poly(vinyl alcohol) is based on the formation of skeleton network between the ice crystals during freezing that is further strengthened by polyaniline. Supporting polymer is responsible for the gel integrity and mechanical properties. Polyaniline is produced in

Figure 4. Optical micrograph of freeze-dried cryogel.

the vicinity of supporting polymer phase where the reactants become concentrated (Figure 1). Is it visible by optical microscopy, where green color of conducting polyaniline salt provides a good phase contrast (Figure 4). The cryogel prepared at 0.2 M of aniline hydrochloride concentration used here for illustration was expected to contain 2 wt % polyaniline salt, 5 wt % poly(vinyl alcohol), and 93 wt % of aqueous phase. Polyaniline has fibrillary structure but it does not seem to produce a conducting network at this polyaniline content. Scanning electron microscopy reveals homogeneous threedimensional macroporous structure with pores of tens to 974

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Figure 6. Raman spectra of polyaniline/poly(vinyl alcohol) cryogel and the spectrum of the film obtained during oxidation of aniline sulfate with ammonium peroxydisulfate on silicon wafer. For details of peaks assignment, cf. refs 38 and 39.

Figure 5. Scanning electron micrographs of cryogel (two magnifications).

obtained in situ during oxidation of aniline on silicon wafer.38,39 We can conclude that polyaniline is generated on the surface of poly(vinyl alcohol) skeleton during aniline oxidation and its molecular structure is close to the spectra of the in situ prepared thin polyaniline films. For that reason, the contribution of supporting polymer to the surface-sensitive Raman spectra has not been observed. The conductivity of cryogels is of primary interest. Two flat carbon electrodes were in contact with the fronts of the cylinders, and used to provide direct current. The voltage was measured with another two electrodes inserted at various positions and distances at cylinder side (Figure 7). The conductivity was little dependent on electrode placement, which illustrates electrical homogeneity of the cryogel. The passing current, when comparing 0.1 and 1 mA, did not affected the determined conductivity (Figure 7). The conductivity of the cryogels penetrated with solutions of sulfuric acid was always higher compared with corresponding acid solution alone (Figure 8), the difference being more marked at low acid concentrations, where the conductivity is

hundreds micrometres in size (Figure 5). The individual polymer phases, however, cannot be discerned by this technique. Elemental analysis of freeze-dried cryogel gave 51.3 wt % carbon, 6.8 wt % hydrogen, and 6.3 wt % nitrogen, the rest being assigned to the presence of oxygen, chlorine, and traces of sulfur.3 The experimentally found content of nitrogen in polyaniline hydrochloride3 was 10.9 wt %. This means that aerogel contains 57.8 wt % of conducting moiety. If was assume the complete conversion of aniline hydrochloride to polyaniline and consider the amount of poly(vinyl alcohol) entering the reaction mixture, the content of polyaniline in aerogel should be 31.2 wt %. This difference is explained by the incomplete incorporation of supporting polymer into cryogel and removal of its unattached part during the washing of cryogels. Raman spectroscopy allows for the analysis of polyaniline cryogel with laser operating at excitation wavelength 633 nm (Figure 6). The spectrum is very close to that of the films

Figure 7. Conductivity of cryogel in 0.1 M sulfuric acid determined by measurement of voltage U between positions 1−2, ..., and 1−4, at current I = 0.1 (×) or 1 mA (+). 975

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Figure 8. Conductivity of cryogels penetrated with the solutions of sulfuric acid (full circles) and the acid solutions alone (open squares) in dependence on acid concentration.

Figure 9. Representative stress−strain curve of cryogel.

Table 1. Mechanical Properties of Cryogels Prepared at Various Molar Concentrations of Aniline Hydrochloride, C

governed by polyaniline. The ionic transport afforded by acid dominated at high acid concentration but the electronic contribution of polyaniline to overall conductivity is still visible. For example, the cryogel penetrated with 0.1 M sulfuric acid had conductivity 0.105 S cm−1, while the corresponding acid solution 0.049 S cm−1. The conductivity of cryogel swollen with water, which can be regarded as intrinsic property of cryogel, was 0.004 S cm−1. These values are comparable with the conductivity reported for classical conducting hydrogels,2 typically on the order of 0.01 S cm−1. When the content of aniline hydrochloride increased from 0.2 M to 0.4, 0.6, 0.8, and 1.0 M, the conductivity of cryogels in 0.1 M sulfuric acid increased from 0.105 to 0.17, 0.22, 0.25, and 0.29 S cm−1, respectively. The use of higher aniline concentrations is difficult because the polymerization is too fast and the freezing of reaction mixture before its start is difficult. On the other hand, the use of poly(vinyl alcohol) with higher molecular weight or its concentration increase had no significant effect on the conductivity. Cryogels are easy to handle (Figure 3); they have good mechanical properties. The measurements of the stress−strain curve (Figure 9) of the cryogel prepared at 0.2 M aniline hydrochloride demonstrate relatively high value of strain-atbreak, 133 ± 21%, tensile strength of 12.2 ± 3.0 kPa, and Young modulus 47.0 ± 9.4 kPa. Although the material is porous and soft, it is flexible with a large elongation at failure. When the concentration of aniline hydrochloride was increased to 0.4, 0.6, 0.8, and 1.0 M, the mechanical properties improved (Table 1). The replacement of the poly(vinyl alcohol) of molecular weight 61,000 with 125,000 at 5 wt % concentration has not lead to any marked change in the mechanical properties. The increase in the concentration of poly(vinyl alcohol) to 7 and 8 wt % had also a marginal effect and Young moduli were 34.0 and 40.0 kPa. The mechanical properties are therefore controlled by conducting polymer phase. In addition, polyaniline/poly(vinyl alcohol) hydrogels prepared at 0.2 M aniline hydrochloride concentration were characterized by dynamic mechanical analysis, transient compression stress analysis, and creep testing (Figures 10−12). The values of compression modulus, 3.5 ± 0.9 kPa, were computed from the slopes of the stress−strain curves. The compression strength, 51.2 ± 3.7 kPa, and compression strain, 76.6 ± 2.3%, indicate that polyaniline/poly(vinyl alcohol)

C, M

strain-at-break, %

tensile strength, kPa

Young modulus, kPa

0.2 0.4 0.6 0.8 1.0

133 55.2 64.5 53.4 22.3

12.2 20.8 44.1 103 38.5

47.0 107 334 1071 1281

Figure 10. Compression stress−strain curves of four cryogels.

cryogel is the soft material with the high compression ability (Figure 10). Polyaniline/poly(vinyl alcohol) cryogel is a viscoelastic polymer composite. It responds to constant compression stress by creep (a progressive deformation depending on the viscoelastic properties of material, time, and applied stress). The “willingness” of cryogel to be deformed under stress was determined as a strain. Its ability to recover after releasing of the loaded stress 2 kPa for 3 min was evaluated as a strain recovery (Figure 11). The maximum strain and strain recovery was 30.2% and 71%, respectively. Dynamic oscillatory experiments were used to characterize the viscoelastic properties of cryogels, such as storage modulus (E′) and mechanical loss factor (tan δ) (Figure 12). Typically, as the temperature approaches the glass transition temperature 976

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has also been tested in the present study. Aniline may be replaced by other monomers, which yield conducting polymers upon oxidation, such as pyrrole. Other water-soluble polymers may be exploited as cryogel supports instead of poly(vinyl alcohol). The structure of ice, which is essential for the creation of cryogel pores, may be modified by freezing protocol and by addition of various low-molecular-weight additives. The application of cryogels in bioseparation was reviewed41 and the conductivity would be a value-added property that could be conveniently exploited. Cryogels are expected to be used in many applications that have already been proposed for polyaniline32,42 and conducting polymer hydrogels.1,2 The macroporous scaffolds for the electrically controlled tissue engineering are probably the most promising.25−28 The mixed electronic and ionic conductivity of cryogels can further be exploited in monitoring or stimulation of biological objects.30,31,43,44 Other uses include the dye adsorbents and controlled release, substrates for the deposition of noble metals, antimicrobial and stimuli responsive materials, and porous electrodes.31 The improved control of gel porosity, macroscopic homogeneity, and the ease of preparation would make cryogels superior to currently used hydrogels. Aerogels obtained after freeze-drying of cryogels represent another class of potentially useful derived functional materials.

Figure 11. Creep behavior of cryogel under stress.

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AUTHOR INFORMATION

Corresponding Author

*(J.S.) E-mail: [email protected]. Fax: +420-296-809-410. Telephone: +420-296-809-351. ORCID

Jaroslav Stejskal: 0000-0001-9350-9647 Patrycja Bober: 0000-0002-1667-8604 Notes

The authors declare no competing financial interest. Figure 12. Dynamic mechanical analysis of cryogel at 1 Hz frequency as a temperature dependence of storage modulus (E′) and loss factor (tan δ).

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank the Czech Science Foundation (16-02787S) for financial support. Dr. M. A. Shishov is thanked for the help with the conductivity determination.

of poly(vinyl alcohol), the storage modulus begins to decrease, representing a reduction of stiffness. The peak temperature in the tan δ curve, located at 59.4 °C indicates relaxation of amorphous region of poly(vinyl alcohol) and represents its glass transition. The glass transition temperature of dry fully hydrolyzed poly(vinyl alcohol) is 85 °C.40 The values of the storage modulus demonstrate a significant plasticizing effect on cryogels. The swollen cryogel have storage modulus of 21.6 kPa which decreased to 16.5 kPa due to micro-Brownian motion of poly(vinyl alcohol) chains. Above 69 °C, however, evaporation of liquid phase starts, and the drying associated with stiffening ended in a final mechanical collapse of cryogel structure.

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4. CONCLUDING COMMENTS The soft and conducting macroporous polyaniline/poly(vinyl alcohol) cryogels with good mechanical integrity have been prepared in a single step by the polymerization of aniline in the frozen solution of poly(vinyl alcohol). The story presented above is based mainly on the description of a single cryogel used as an illustration. The preparation protocol, however, can be extended in many ways. The variation of polyaniline and supporting polymer contents are the most obvious goal and this 977

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