Electrical properties of γ-crosslinked hydrogels incorporating organic conducting polymers

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Radiation Physics and Chemistry 76 (2007) 1371–1375 www.elsevier.com/locate/radphyschem

Electrical properties of g-crosslinked hydrogels incorporating organic conducting polymers C. Dispenzaa,b,, G. Fiandacaa, C. Lo Prestia, S. Piazzaa,b, G. Spadaroa,b a

Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy b Centro Interdipartimentale di Ricerca sui Materiali Compositi, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy

Abstract Hydrogel composites containing nanoparticles of the protonated emeraldine form of polyaniline (PANI-PE) have been synthesised by g-irradiation, using either polyvinyl pyrrolidone (PVP) or polyvinyl alcohol (PVA) as steric stabilisers. Swelling behaviour of both hydrogels is reported, together with an electrical characterisation of composites, before and after gel network formation, performed by cyclic voltammetry and impedance spectroscopy. Similarities and differences between the two composite systems are discussed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; g-Irradiation; Hydrogel; Swelling behaviour; Conductivity

1. Introduction Polyaniline (PANI) is finding an increasing number of applications in optics, electronics and bioelectronics, thanks to its versatility and high environmental stability. Notwithstanding its unique properties, PANI is very difficult to process, especially when produced in its inherently conducting form, because of its infusibility and insolubility in most organic solvents. In our synthetic approach, PANI is incorporated into a soft hydrogel matrix by means of a two-step process: (i) an ‘‘in situ’’ dispersion polymerisation of aniline, using suitable polymeric stabilisers to generate fairly stable PANI particles aqueous dispersions; (ii) g-irradiation of the same dispersions in order to induce chemical crosslinking in the polymeric stabiliser, which transforms the aqueous dispersion into a gellike material. By this route, conductive hydrogel composites are obtained, containing nanoparticles of PANI in its halfoxidised, fully protonated form (Dispenza et al., 2006a), known as protonated emeraldine (PANI-PE), which is also Corresponding author at Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita` di Palermo, Viale delle Scienze, 90128 Palermo, Italy. Tel.: +39 0916567210. E-mail address: [email protected] (C. Dispenza).

0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.036

the intrinsic conductive form of PANI. These materials could find potential applications in miniaturised optical and electronic devices as well as in biosensors. Different stabilisers were used to prepare PANI dispersions, namely polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA). These water-soluble polymers are susceptible to sol–gel transition when g-irradiated in the form of aqueous solutions (Benamer et al., 2006; Razzak et al., 2001). Although these polymeric stabilisers should accomplish the same role during PANI dispersion polymerisation, i.e. favouring particles nucleation and preventing precipitation of the polymerised aniline particles, PVP and PVA have already been proven to give rise to materials with different properties. Shelf-life stability is longer in the presence of PVP, although sediments formed from the PVA/PANI dispersions rapidly re-disperse upon agitation (authors’ unpublished results). Dispersions display different morphologies when they are cast into thin films by spin coating onto glass slides: atomic force microscopy (AFM) carried out on the air-dried thin films obtained from PANI/ PVP dispersions showed PANI needles few hundreds nm long, whilst analogous films obtained from PANI/ PVA dispersions showed spherical PANI particles of about 10 nm diameter, arranged in larger circular crowns (Dispenza et al., 2006b).

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Another remarkable difference between the two systems lies in the photoluminescence spectra that are significantly influenced by the presence of either PVP or PVA: both dispersions and hydrogels produced with PVP showed no emission bands, whereas coexistence of PANI and PVA caused the appearance of an emission band with intensity peak at lmax ¼ 325 nm for an excitation wavelength of 290 nm (Dispenza et al., 2006b). These differences may origin in different nature and extent of the interactions between polyaniline and the stabilising polymer. These interactions are reported to induce crystallinity and closepacked arrangements in PANI/PVP composites (Murugesan et al., 2004). The present paper reports on composite hydrogels preparation and swelling behaviour in the presence of both stabilisers; moreover, an electrical characterisation was carried out by means of cyclic voltammetry and electrochemical impedance spectroscopy, both for dispersions produced in the presence of PVP and PVA and for the PANI/PVP hydrogel. 2. Experimental 2.1. Synthesis of PANI dispersions and hydrogels PANI aqueous dispersions were obtained by chemical oxidation of an acidic solution of aniline, using ammonium persulphate as redox initiator and either PVP (Mw ¼ 160,000) or PVA (Mw ¼ 47,000, degree of hydrolysis: 88%) as steric stabiliser. Details of the synthetic procedure are reported in Dispenza et al. (2006a). During synthesis the conversion of aniline with reaction time was monitored by both high-performance liquid chromatography and gas chromatography and both techniques confirm that aniline is no longer present after a reaction time of 15 and 30 min in the presence of PVP and PVA, respectively. Before irradiation, dispersions were diluted 10-fold with the steric stabiliser/water solution (4 wt%), already used to prevent precipitation of PANI during polymerisation, so that stabiliser concentration was kept constant. Hydrogel composites were obtained by 60Co g-irradiation of the diluted dispersions of synthesised PANI. In order to avoid heating effects during transformation, irradiation was performed at controlled temperature (T ¼ 10 1C) in glass vials under nitrogen at a dose rate of 2 kGy/h up to a total absorbed dose of 40 kGy. 2.2. Characterisations PVP and PVA dispersions and the corresponding hydrogels containing PANI-PE were characterised in terms of their ability to incorporate water after freeze-drying. Dry hydrogel samples (10 mg), contained in glass cylinders with a porous membrane at the bottom, were immersed into buffers having different pH (4.5, 7.4 and 10) and almost constant ionic strength (0.20–0.25 mol/l) at constant temperature. Samples were withdrawn and weighed at

regular intervals during daytime and left immersed overnight. Before weighing, the excess liquid was removed by centrifugation for 4 min at 2000 rpm. Re-hydration ratios (RR) were calculated as the ratio between the weight of the re-hydrated sample and the weight of the freeze-dried residue. Results are averaged over at least five identical measurements and standard deviations were always below 8%. Cyclic voltammetry (CV) experiments were carried out at different sweep rates (10–500 mV/s) using a potentiostat/ function generator system (EG&G) coupled to a desk computer. A three-electrode glass cell was employed, having Pt as working and counter electrodes and a saturated calomel (SCE) as the reference electrode, in connection with the cell through a Luggin capillary (for hydrogel samples) or by means of a salt bridge (for dispersion samples). CV measurements were carried out, normally under nitrogen atmosphere (but some experiments were also performed in air, for comparison), on the following samples: (1) PANI/ PVP dispersion; (2) PANI/PVA dispersion; and (3) PANI/ PVP hydrogel. For all the investigated systems pH was 1.5–1.7. Impedance frequency dispersion, both of the PANI/PVP hydrogel and of the PANI/PVP and PANI/PVA dispersions, was measured in the frequency range 102–105 Hz and at room temperature by means of a frequency response analyser (Schlumberger, mod. 1255), applying an AC signal of 0.02 V peak-to-peak. The effect of electrode potential was investigated coupling the analyser to a potentiostat/generator (EG&G, mod. 273) and using a specifically designed three-electrode cell, having two circular (diameter: 20 mm) plate electrodes (working and counter), made of 316L stainless steel, and an Ag/AgCl wire as the reference electrode. The measuring unit was enclosed into a hollow Teflon cylinder, with plates distance of 5 mm, leaving a volume of 1.57 cm3 to be filled with the measuring sample. Measuring cell and apparatus were placed into a Faraday cage for shielding any electromagnetic interference. 3. Results and discussion 3.1. Swelling behaviour Hydrogels swelling behaviour was investigated at different temperatures (5–37 1C). For sake of brevity, we report here only results at T ¼ 5 1C, which are quite representative and free of appreciable material modification due to water evaporation. In Fig. 1, the re-hydration ratio is reported as function of immersion time in the three buffers for pure (not containing PANI particles) PVP and PVA hydrogels and for the corresponding PANI/PVP and PANI/PVA hydrogel composites. Pure matrix materials presented always considerably lower re-hydration ratios with respect to the corresponding composite hydrogels, although the kinetics of re-hydration was quite similar and in all cases plateau values were reached within the first 48 h of

ARTICLE IN PRESS C. Dispenza et al. / Radiation Physics and Chemistry 76 (2007) 1371–1375

immersion. Higher RR values may be due to the formation of a looser network in the presence of the conducting polymer particles. Radicals generated by water ionisation with g-irradiation, and responsible for PVP or PVA crosslinking, could be partially depleted by reaction with PANI-PE, which may act as radical scavenger. Furthermore, in a recent work (Kaplan Can, 2005), it was demonstrated that the presence of persulphate anions affects the g-irradiation-induced gelation of PVP; in particular, a significant reduction in the gelation dose, Dg (the minimum dose for the first appearance of insoluble polymer fractions), was observed. Authors argued that in

30 PANI/PVA hydrogels

25

PANI/PVA hydrogels 20 R.R.

PVA hydrogels 15 PVA hydrogels 10 pH=4.5 5

pH=7.4 pH=1.0

0 0

24

48

72

96

120 144 168 192 216 240 264 288 312 336 Time / hr

Fig. 1. Re-hydration ratios in different pH buffers for the PANI/PVP and PANI/PVA composite hydrogels, compared to pure PVP and PVA hydrogels.

Epa2

400

Current / μA

200 0 Epc2

-200 Epc1

-400 -600 -500

-250

0

250 500 Potential / mV(SCE)

750

1000

Fig. 2. Cyclic voltammogram recorded at 100 mV/s for the PANI/PVA dispersion under nitrogen atmosphere.

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the presence of free persulphate anions, chain scission of the polymer is dominant over crosslinking; this hypothesis was supported by FTIR spectra. In our systems, persulphate anions are likely to be still present during irradiation, as ammonium persulphate was used as redox initiator for PANI synthesis. The analogous RR behaviour of the two composite hydrogels suggests the same effect of free persulphate anions on the network formation in the presence of either PVP or PVA. 3.2. Voltammetric experiments All samples showed a large stability interval (approximately, between 0.4 and +1.0 V/SCE) delimited by the water decomposition reactions (Dispenza et al., 2006a). Within this potential interval, anodic and cathodic peak currents were recorded, associated with electrode reactions of PANI particles in the composites. Fig. 2 shows the voltammogram recorded at 100 mV/s for the PANI/PVA dispersion under nitrogen flux: in the forward scan (from the open-circuit potential towards positive potentials) an oxidation peak at approximately 540 mV (Epa2) occurs, whilst in the reverse scan two reduction peaks, one (small) at the electrode potential Epc2 and the other (larger) at Epc1, are present. These peaks were attributed to the electrochemical equilibria involving the different forms of PANI: the second oxidation/reduction wave, at higher potential, can be attributed to the equilibrium between protonated emeraldine (PANI-PE) and pernigraniline base, whilst the peak at Epc1 is due to the reduction of PANI-PE to leucoemeraldine (Huang et al., 1986). The small current at Epc2 in the reverse scan and the absence of an oxidation peak close to Epc1 in the forward scan are due to the stabilising effect on the PANI-PE form by the chemical environment (Dispenza et al., 2006a). Similar behaviour was observed for all the investigated systems, with the only variation in the small peak at Epc2, sometimes reduced to a shoulder or absent. Table 1 reports the oxidation and reduction peak potentials recorded at 50 mV/s for the different systems: in absence of air, peak potentials of 553713 mV/SCE, 390710 mV/SCE and 9575 mV/SCE were registered for Epa2, Epc2 and Epc1, respectively, regardless of the system. This reveals that changing the stabilising agent does not affect the reactivity of PANI-PE particles in the dispersion, nor does the reactivity changes after hydrogel formation.

Table 1 Anodic and cathodic peak potentials recorded for the different composites at 50 mV/s Composite

Atmosphere

pH

Epa2 (mV/SCE)

Epc2 (mV/SCE)

Epc1 (mV/SCE)

PANI/PVA dispersion PANI/PVP dispersion PANI/PVP hydrogel PANI/PVP dispersion PANI/PVP hydrogel

N2 N2 N2 Air Air

1.7 1.6 1.5 1.6 1.5

540 550 565 615 500

380 400 No peak No peak No peak

100 90 100 75 No peak

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500

1.E+01 Ipa2 (μA) Ipc1 (μA)

1.E+00 σ'(ϖ) / Sm-1

Peak current / μA

400 300 200

0.25 v(SCE) 0 v(SCE)

1.E-02

-0.5 v(SCE)

1.E-03 1.E-04 1.E-02 1.E-01 1.E+00 1.E+01 1.E+03 1.E+04 1.E+05 frequency / Hz

100 0

0.7 v(SCE)

1.E-01

0

5

10

15

Fig. 4. AC conductivity vs. frequency at different electrode potentials for the PANI/PVP hydrogel.

20

r1/2 / mV0.5 s-0.5 Fig. 3. Anodic and cathodic peak currents vs. the square root of the sweep rate for the PANI/PVA dispersion under nitrogen atmosphere.

However, the analysis of peak currents reveals differences between the various systems. Fig. 3 shows the anodic and cathodic peak current as a function of the sweep rate (r) for the PANI/PVA dispersion: the linear dependence on r1/2 and the practical independence of peak potentials from r indicates kinetically reversible reactions (fast electrode kinetics). In this case, the Randles–Sevcik equation holds (Bard and Faulkner, 2001): j p ¼ 2:69  105 n3=2 CD1=2 r1=2 ,

(1)

where jp is the peak current density, n the number of electrons involved in the reaction, C the bulk concentration of reactant and D its diffusion coefficient. Such a behaviour was common to the other systems investigated, but peak current densities changed, being much higher for dispersions than for the hydrogel. By derivation of Eq. (1), values of the diffusion coefficient of PANI-PE particles in the different systems can be derived from the slopes of the anodic peak current vs. sweep rate plots, once known n and C (see Fig. 3). A rough estimate of C was done based on the initial concentration of aniline in our systems and assuming an average formation of pentamers. This affects the absolute values of D, but does not influence appreciably the comparison among the different systems. Under the previous assumptions, a diffusion coefficient ranging between 2  106 and 6  106 cm2/s was estimated for PANI-PE particles in the different dispersions (with the highest value in the PANI/ PVA dispersion), whilst the D value estimated in the PANI/PVP hydrogel was smaller of about two orders of magnitude (4  108 cm2/s), in agreement with the lower peak currents observed in this system; this finding reveals a drastic reduction of the polymer nanoparticles mobility owing to the interactions with the hydrogel network. 3.3. Impedance spectroscopy (IS) IS experiments were performed on the same systems of the previous paragraph and impedance data were fitted

according to the formalism of complex admittance, Y*, and complex permittivity,  ðoÞ ¼ 0 ðoÞ  00 ðoÞ, adopted in Pissis and Krytsis (1997); the frequency-dependent AC conductivity, s0 (o), was then calculated as s0 ðoÞ ¼ 00 ðoÞo0 ¼

0 0 Y ðoÞ. C0

(2)

In Eq. (2), o is the angular frequency, e0 the vacuum permittivity, C0 the equivalent capacity of free space in the measuring unit and Y0 (o) the in-phase component of the complex admittance. Fig. 4 shows the dependence of s0 (o) on frequency at different potentials for the PANI/PVP hydrogel: in the intermediate-high frequency region curves converge to a plateau, which corresponds to the DC conductivity, sDC, of the composite, whilst the decrease at lower frequencies reflects both diffusion phenomena and electrode polarisations (Dispenza et al., 2006a). Very similar plots were obtained for all the investigated systems, with plateau values independent of the electrode potential (as expected). Moreover, no effect of the ambient air relative humidity (from 43% up to 88%) was observed upon the impedance dispersion of the hydrogel. The sDC values derived for the different systems are very similar (see Table 2), originating essentially from the contribution of free ions in the aqueous phase. This justifies the coincidence of sDC values relative to PANI/PVP composites before and after g-irradiation, taking into account the very high water content in the hydrogel (96 wt%). This interpretation was confirmed by a separate experiment performed on an aqueous solution of PVP (without PANI particles) with the same ionic content (HCl and ammonium persulphate) of our composites: its DC conductivity (5.4 S/m) was even slightly higher due to a higher concentration of free protons (in our composites part of H+ ions protonate PANI), as testified by the lower pH (about 0.8). Hence, the hydrogel network does not affect sensibly ionic migration in the swelling medium. The somewhat lower sDC value pertaining to the PANI/ PVA dispersion can be ascribed both to slightly different protonation degrees of polymers in the two dipersions, or to a partial contribution to the measured conductivity

ARTICLE IN PRESS C. Dispenza et al. / Radiation Physics and Chemistry 76 (2007) 1371–1375 Table 2 DC conductivity values derived for the different composites Composite

pH

RH (%)

sDC (S/m)

PANI/PVA dispersion PANI/PVP dispersion PANI/PVP hydrogel

1.7 1.6 1.5 1.5 1.5

Not controlled Not controlled Not controlled 43% 88%

3.3 3.8 3.9 3.8 3.7

coming from the PANI-PE particles in the PANI/PVP systems; in fact, depending on environment, under an electric field PANI particles tend to organise themselves into conducting chains (Quadrat et al., 1998), and this leads to extremely low percolation thresholds, down to less than 0.06 wt% of PANI (Banerjee and Mandal, 1995), which is the content in our systems. This conduction mechanism has been not observed with PANI/PVA systems (Quadrat et al., 1995), and the difference between the two systems should be ascribed to different interactions between PANI particles and the stabiliser. Previous hypothesis is in agreement with both the different long-term stability of the dispersions and the different particles morphologies shown by AFM for air-dried films obtained by the two dispersions (see Banerjee and Mandal, 1995; Dispenza et al., 2006b). 4. Conclusions PANI/PVP and PANI/PVA hydrogel composites present similar swelling behaviour, with re-hydration ratios much higher with respect to the pure (PVP or PVA) matrix materials. However, different R.R. values and long-term stabilities of the parent aqueous dispersions suggest different interactions between conducting PANI particles and the two polymeric stabilisers. Both aqueous dispersions and the PANI/PVP hydrogel present the same peak potentials in the voltammetric curves, indicating that particles reactivity is unaffected by gelation; however, smaller peak currents in the hydrogel composite reveal a much lower mobility of PANI nanoparticles after network formation. Also DC conductivity is very similar for all systems, being essentially due to migration of free ions in the

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aqueous phase, but slightly higher sDC values in the PANI/ PVP composites could be ascribed to a contribution from conducting PANI-PE, which showed very low percolation paths in these systems.

Acknowledgement This work is supported financially by University of Palermo (60% funds).

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