Solid/liquid interfacial synthesis of high conductivity polyaniline

Abstract

A new method of solid/liquid interfacial polymerization of aniline in non-aqueous solutions reproducibly provides polyanilines with a higher molecular weight, lower degree of ortho-substitution, and higher degree of crystallinity, exhibiting higher electrical conductivity than the conventional aqueous method.

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Polyaniline (PANI) has been one of the most widely studied conjugated polymers for the past three decades due to its environmental stability, unique redox chemistry, and simple preparation. However, despite very considerable efforts to utilize PANI for practical applications, its commercial and industrial uses remain limited. One of the factors limiting wider practical applications of this polymer is its low electrical conductivity when synthesized conventionally using aqueous solutions. PANI solids doped with camphorsulfonic acid (CSA), for example, have shown conductivity levels of only about 200 S cm⁻¹ with large batch-to-batch variations (100–400 S cm⁻¹),^1,2 which is far lower than the theoretical maximum (∼10⁴ S cm⁻¹).³ A main reason for the low conductivity is the presence of structural disorder in PANI, which limits charge transport. The structural disorder comes from both molecular-scale disorder⁴ caused by structural defects in a single polymer chain (such as ortho-substituted chains and chain ends) and heterogeneous morphologies consisting of crystalline and amorphous domains.⁵ A reliable method to synthesize high molecular weight PANI with few structural defects and a highly ordered crystalline structure is therefore needed in order to achieve high conductivity as well as good batch-to-batch reproducibility for potential applications.⁶

Various techniques for synthesizing PANI have been used, including emulsion,⁷ liquid/liquid interfacial,⁸ seeding,⁹ enzymatic,¹⁰ mechanochemical,¹¹ template-guided,¹² and sonochemical polymerization techniques.¹³ The different techniques were developed while focusing mostly on the nanostructures, and did not yield much improvement of the electrical properties.

Only a few reports have described how the synthetic method tested affects the molecular weight, chain structure, crystallinity and electrical conductivity of PANI. High molecular weight PANIs were synthesized at sub-zero temperature with a salt in aqueous solution.^2,14,15 Such high molecular weight polymers with a low density of chain end defects have been expected to exhibit enhanced charge transport; however, the relationship between molecular weight and conductivity of PANI still remains unclear.

A crystalline PANI with a conductivity greater than 1000 S cm⁻¹ was synthesized in a heterogeneous biphasic system of organic solvent droplets dispersed in water at −35 °C.^4,16 This result was attributed to the polymerization proceeding at the water/organic interface, and hence favouring para-coupling and suppression of undesirable side reactions. However, it is not trivial to prevent water from freezing at −35 °C in the practical polymerization setup. Moreover, the reaction can occur in the bulk water as well as at the interface because some of the monomers are likely to diffuse into the bulk water. For these reasons, the conductivities lower than 1000 S cm⁻¹ and with relatively large variations were generally achieved in subsequent reports.¹⁷ The polymerization conditions therefore need to be controlled more precisely in order to avoid unwanted reactions and to reproducibly synthesize high-conductivity PANI. Despite the reproducibility problem, the method has shown clearly the effectiveness of interfacial synthesis for reducing side reactions and enhancing the electrical conductivity. This benefit of interfacial synthesis is also supported by other reports. Highly conductive nano-sheets of PANI with para-directed repeating units was accomplished by confining the reaction to the ice surface.¹⁸ Synthesis of PANI at the interface of water/ionic liquid/organic solution produced highly crystalline and conductive coral-like PANI.¹⁹

Herein, we present a solid/liquid interfacial polymerization (SLIP) method for synthesizing a high-conductivity PANI. The polymerization was performed in organic solvent at ambient conditions, providing PANIs with higher molecular weight, more para-directed structure, and higher crystallinity as compared with those produced by conventional methods. Our results clearly show that a high molecular weight is essential for high-conductivity of PANI. The facile, highly reproducible and scalable organic-solution-based SLIP methods will help further the exploitation of the properties of PANIs.

SLIP was performed by dissolving aniline and two equivalents of camphorsulfonic acid (CSA) in chloroform. To this mixture was added finely ground ammonium persulfate (APS) powder as an oxidant. The molar ratio of aniline to APS was kept at 4. The solubility of CSA in chloroform was observed to increase in the presence of aniline, which should result in the formation of the anilinium CSA salt. The colour of the reaction mixture with suspended APS particles became blue and then dark green as it was being stirred at room temperature. The anilinium salts oxidized with the consumption of APS (Fig. 1) to form CSA-doped PANI.

Fig. 1 Schematic illustration of solid/liquid interfacial polymerization of aniline.

Although the SLIP proceeded in the absence of water, residual reactants and salts needed to be removed, for which the use of water would be most effective. However, dissolution of aniline, CSA and APS in water during washing would cause rapid secondary polymerization. To suppress such unwanted polymerization, we followed two routes. First, the reaction mixture was immediately poured into acetone and formed PANI precipitate. The solid PANI was collected by filtration and washed with water (SLIP-p). Second, the water-soluble components were extracted immediately from the reaction mixture using a separatory funnel. It is important to carry out the extraction procedure as quickly as possible to prevent secondary polymerization. The organic phase yielded polymer upon solvent evaporation. The resulting solid product was then washed with acetone (SLIP-e). We compared the PANI product obtained in these two ways, SLIP-p and SLIP-e, with that obtained from the conventional aqueous solution polymerization method as reported by Macdiarmid et al.²⁰ The polymerization conditions, isolated polymer yields, and molecular weights of the resulting PANIs are shown in Table 1.

Table 1 Comparison of the properties of PANIs synthesized at four different conditions. ^a

Polymer	Solvent	Temp (°C)	Time (h)	Yield^f (%)	Mn^g (kg mol^−1)	Mw (kg mol^−1)	PDI (Mw/Mn)
a All polymers were synthesized with APS as an oxidant.b Synthesized in a 1 M HCl solution.c All SLIP occurred in a 1 M CSA solution.d Products were precipitated into acetone.e Reaction mixtures were extracted with water and then the solvent was evaporated.f Isolated yields of EB with respect to APS amount.g Determined by GPC of a representative batch using a polystyrene standard with NMP containing 0.5% lithium bromide as an eluent.
Conventional^b	Water	0–5	24	∼100	21.3	81.2	3.8
SLIP^c-p^d	Chloroform	25	24	86	24.8	69.7	2.8
SLIP-p	Chloroform	25	48	93	46.3	126.4	2.7
SLIP-e^e	Chloroform	25	24	73	61.0	154.0	2.5

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Dark purple PANI emeraldine salt (ES) was obtained by the SLIP methods, and lustrous brown emeraldine base (EB) was prepared by dedoping ES powders with ammonia as shown in Fig. S1A (see ESI†). UV-Vis spectra of each solution showed typical PANI absorbance peaks (Fig. S1B†). Isolated yields of the PANI EBs were 73% to 93% with respect to the amounts of APS (Table 1). The yield increased from 39% to 96% by varying the molar ratio of aniline to oxidant in the range of 1 to 6 (Fig. S2†). The aniline-to-oxidant molar ratio was subsequently fixed at 4 since this value in general gave maximum conductivity. The conductivity of SLIP-e PANI was highly reproducible with small variations observed (580 ± 40 S cm⁻¹), while the samples produced using conventional aqueous methods showed lower average conductivity values with greater batch-to-batch variations (250 ± 140 S cm⁻¹) (Fig. S3†). Both PANI–CSA films shows free electron absorption tails extending to the near-infrared region in UV-Vis-near IR spectra (Fig. S1C†).

Gel permeation chromatography (GPC) analysis confirmed that the polymers by SLIP-e or SLIP-p showed number-average molecular weight (M_n) values between 24 800 and 61 000 with polydispersity index (PDI) values of 2.5–2.8 (Table 1). The PANI produced by the conventional aqueous method showed a higher PDI value (3.8) as reported previously.^15,21 The large PDI is strongly related to the intrinsic mechanism by which PANI was synthesized in the conventional method. The oxidative polymerization of aniline is thought to be a chain reaction,²² with each chain propagation step resulting from the addition of an aniline monomer to the active end of the chain. However, the addition of aniline can occur not only to the polymer ends, but also simultaneously to the active ends of monomers and oligomers, leading to a broad molecular weight distribution and large low molecular weight polymers.

The PANI produced by SLIP-p showed GPC traces with a low molecular weight shoulder (Fig. 2A), suggesting that short-chain polymers formed by a secondary mechanism. We postulate this was caused by the residual reactants precipitated in non-solvent. When the reaction mixture was precipitated into acetone, the monomeric or oligomeric anilinium salts, and APS aggregated together with PANI chains due to their insolubility in acetone. When the aggregates were contacted with water for washing, polymerization would resume as the reactants and APS became dissolved in water. A longer reaction time was attempted to reduce the amount of residual reactants. However, when the reaction time for SLIP-p was increased from 24 to 48 h, low molecular weight shoulder still remained in the GPC curve. In contrast, the PANIs by SLIP-e procedure gave a GPC trace that did not show a low molecular weight shoulder (Fig. 2A) and which resulted in a PDI of 2.5, lower than the PDI values of the SLIP-p-produced and conventionally produced PANI samples. These data confirm that extracting the SLIP organic reaction mixture with water eliminates effectively the residual reactants that could yield low molecular weight products.

Fig. 2 Molecular weights of the various PANIs and corresponding film conductivities. (A) GPC traces of PANIs using polystyrene standard. SLIP-p1 and SLIP-p2 represent PANIs prepared by the SLIP-p method with reaction time for 48 h and 24 h, respectively. (B) Plot of M_n vs. conductivity of PANI–CSA films.

A major difference between the conventional polymerization and SLIP is the place where oxidation of aniline occurs. Some swollen polymer residues were observed to remain on the surface of APS in the solution that turned dark green with the progress of polymerization. While aniline molecules oxidize randomly throughout the bulk in the conventional solution method, only the molecules adsorbed to the surface of a solid APS are expected to be oxidized and polymerized in the present SLIP. The PANI chains would be expected to grow only as aniline becomes adsorbed to the solid surface, whereas the chains desorbed from the APS surface would stop growing. As the adsorbed chains grow longer on the APS surface, their desorption rate would be reduced due to stronger surface–polymer interaction. This interfacial mechanism may account for the lower PDI and higher molecular weight of SLIP-produced PANIs than of conventionally produced PANI.

A clear correlation between the M_n of the PANI and the conductivity of its CSA-doped films was found, as shown in Fig. 2B. The conventionally produced PANI showed the lowest M_n and thus exhibited the lowest conductivity. It is noteworthy that the conventional PANI had a higher M_w than SLIP-p PANI; however, it showed a lower conductivity because it included a much larger portion of low molecular weight species (Fig. 2A). Many of the PANIs synthesized previously by aqueous solution-based methods have shown broad molecular weight distribution with a significant low molecular weight portion, resulting in low electrical conductivity.^2,15 An enhancement of conductivity by using a Soxhlet extractor to remove low molecular weight oligomers from high PDI polymers was reported previously.² The effects of low molecular weight on charge transport have also been studied in semiconducting polymers.^23,24 Himmelberger et al. showed controlled polydispersity by reducing low molecular weight polymers to be required to achieve high carrier mobility in poly(3-hexylthiophene).²⁴ They pointed out that the presence of even relatively few low molecular weight polymers is harmful to charge transport.

Conventionally produced and SLIP PANIs also yielded different Fourier-transform infrared (FT-IR) spectra, as shown in Fig. 3. The major difference was that the SLIP-produced PANI yielded a greater ratio of the intensity of the peak at 830 cm⁻¹ to that of the internal standard peak at around 1500 cm⁻¹ than did the conventionally produced PANI. The peak at 830 cm⁻¹ derived from C–H bending of para-disubstituted benzene, and that at 1500 cm⁻¹ was from C=C stretching of the benzenoid ring.^16,25 The relatively intense peak at 830 cm⁻¹ for the SLIP PANI suggests that the solid/liquid interfacial polymerization yielded more para-directed PANIs and fewer side reactions than did the conventional method. One of the possible reasons for this difference is that the polymerization reaction took place much more rapidly in the conventional synthesis than in SLIP. A typical temperature profile of the synthesis is shown in Fig. S4.† In the conventional synthesis the reaction temperature sharply increased on adding oxidant, which is in agreement with previous observations.^2,26 In contrast, the temperature in SLIP remained within variation of 1 degree from the initial setting during the entire course of the reaction in chloroform. Despite the approximately four-fold greater specific heat capacity of water than of chloroform (4.18 versus 0.96 J g⁻¹ K⁻¹), the SLIP organic solution maintained nearly the same reaction temperature, indicating that SLIP of aniline in chloroform occurred more slowly and gently than in water. Furthermore, the conventional method yields the polymers aggregated and precipitated from the reaction mixture in the early stage of reaction. Rapid evolution of reaction heat in the heterogeneous mixture might cause unwanted side reactions such as ortho-coupling and Michael reductive addition.¹⁶ In SLIP, the PANIs grown at the solid is gradually doped with CSA and then would be extended into the chloroform solution as the PANI is dissolved in a solvent such as chloroform, m-cresol, or NMP in the presence of CSA.²⁷

Fig. 3 FT-IR spectra of emeraldine bases of SLIP-produced and conventionally produced PANIs.

The crystalline structures of differently-obtained PANI EBs and PANI–CSA films were studied by small- and wide-angle X-ray scattering. A typical amorphous hollow was obtained from the conventionally produced EB. In contrast, crystalline structures developed in the SLIP-PANI EBs, as indicated by distinct peaks in their diffraction patterns; here, peaks at 16°, 18° and 24°, with a broad background of diffraction centred at 20°, were generally observed (Fig. 4A). The peaks at 16° and 24° correspond to the distance between the stacked zigzag polymer chains (5.5 Å) and the π–π stacking distance (3.7 Å), respectively.²⁸ The peak at 18° corresponds to the intrachain amine–benzene–amine distance (4.9 Å).²⁸ The smaller PDI and the greater prevalence of para-directed chain structures for the SLIP PANIs (than for the conventionally produced PANIs) apparently enhanced their inter- and intrachain order. Higher crystallinity was also exhibited by PANI–CSA films from SLIP as indicated by the stronger peaks at 4.6°. The peak at 4.6° corresponds to the ordering of alternating layers of PANI and CSA molecules (∼19.6 Å).²⁹ The higher crystallinity led to higher conductivity, consistent with the observation by W. Luzny et al.³⁰

Fig. 4 Small- to wide-angle X-ray scattering patterns of (A) emeraldine base pellets and (B) PANI–CSA films. SLIP-p1 and SLIP-p2 represent PANIs prepared by the SLIP-p method with reaction time for 48 h and 24 h, respectively.

Conclusions

We have successfully prepared PANI by carrying out SLIP in organic solvents. This method produced highly conductive PANIs with relatively small batch-to-batch variations compared to the conventional aqueous solution synthesis. The high conductivity of the PANI by the present SLIP method resulted from the high molecular weight, high para-substitution, and enhanced crystalline inter-molecular packing. The scalable and reproducible SLIP method of high-quality PANIs in organic media will help to expand the potential applications of PANI.

Acknowledgements

This research was supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078870) and GIST Research Institute (GRI) Project through a grant provided by GIST in 2016.

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis and experimental methods, supplementary figures. See DOI: 10.1039/c6ra18045k

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