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Polymer Testing 40 (2014) 1e3

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Polymer Testing

journal homepage: www.elsevier .com/locate/polytest

Short communication: analysis method

Thin-layer chromatography as a method for separating anilineoligomers

Jing Tao a, *, Jin-Feng Yang b

a Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, People's Republic of Chinab College of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832002, Xinjiang, People's Republic of China

a r t i c l e i n f o

Article history:Received 24 June 2014Accepted 31 July 2014Available online 23 August 2014

Keywords:PolyanilineThin-layer chromatographyPreliminary separationChooseProper mobile phase

* Corresponding author. Tel.: þ86 29 68640809.E-mail address: taojingxj@hotmail.com (J. Tao).

http://dx.doi.org/10.1016/j.polymertesting.2014.07.00142-9418/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The chemical structure of aniline oligomers is usually analyzed using NMR spectroscopy.However, the rich mixture of various compounds complicates NMR analysis. Therefore,making the task easier by simplifying the spectra using a preliminary separation isnecessary. In this study, we examined for the first time a simple preparation and quickseparation of aniline oligomers produced by the oxidation of aniline in aqueous solutionusing thin-layer chromatography (TLC). We also explored a suitable development system.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polyaniline (PANI) and its derivatives are some of themost studied conducting polymers because of their elec-trochromic and photoconductivity properties, as well astheir higher stability in air and easier doping processcompared to other conducting polymers [1e3]. Althoughthe polymerization of aniline has been studied for morethan two decades [4,5] and the most important features ofits mechanism have been addressed [6], many details of theprocess are not yet fully understood.

Aniline oligomers are generally believed to be respon-sible for the self-assembly that guides the growth of poly-meric structures [7,8]. Therefore, the analysis of thesecompounds is indispensable for understanding the mech-anism. However, the chemical structure of these oligomershas only been partly revealed. Generally, the chemicalstructure of aniline oligomers produced by the oxidation ofaniline is analyzed using NMR spectroscopy. Unfortunately,

20

the rich mixture of various compounds presents a greatchallenge for NMR analysis, which is complicated by thevarious compounds. Therefore, researchers have attemptedto make the task easier by simplifying the spectra using apreliminary gas or liquid chromatography analysis [9,10].However, identification of compounds in polyaniline sam-ples using gas chromatography (GC) and liquid chroma-tography (LC) coupled with NMR spectrometry entailstime-consuming and labor-intensive sample preparations.

TLC is the method of choice when many samples mustbe compared, as well as when flexibility is important andwhen rapid separation is needed at low cost. TLC is awidelyused conventional method for separation analysis and isstill used in parallel with highly sensitive instrumentalanalysis [11]. TLC was successfully applied in the analysis ofcompounds with high mass, such as lipids and saccharides[12,13]. However, it has not been applied to aniline oligo-mers. In this study, TLC was used for the first time toseparate aniline oligomers, which are produced throughthe oxidation of aniline. Aniline oligomers are a mixture ofvarious compounds. Thus, the most difficult problem insimultaneously analyzing all these compounds is finding anoptimum mobile phase system. Thus, the present study

J. Tao, J.-F. Yang / Polymer Testing 40 (2014) 1e32

aims to develop a suitable mobile phase system for theseparation analysis of aniline oligomers. We compared avariety of developing solvents and, ultimately, determinedthe most appropriate one.

TLC enables the rapid, simple and highly sensitiveanalysis of aniline oligomers from polyaniline extracts. Weexamined a simple method that could be applied to sepa-rate aniline oligomers and could be an alternative to otherconventional analytical techniques such as GC and LC.

2. Experimental

TLC using plates coated with silica gel is a commonlyused method for separating compounds with high mass.Plates with various properties (layer thickness, with orwithout fluorescence) are available. However, few studieshave analyzed aniline oligomers because the large amountof similar structural components presents a great challenge.

In this case, we attempted using different mobile phasesto determine the most appropriate one for separating theaniline oligomers (Table 1). LC and GC are the generalseparation methods for purifying NMR samples. However,these processes are expensive, time-consuming and labor-intensive. Moreover, they do not offer the advantage ofrapid and simple sample preparation for NMR analysis. Inthis study, we examined TLC as a rapid and simple sepa-rated method for crude extracts. In addition, aniline olig-omers can be easily distinguished from the plate by visualinspection because they each show a distinctive color. Dueto these advantages, the TLC separation was attempted toenable a preliminary separation of a mixture of anilineoligomers.

2.1. Materials

TLC was performed using GF254 and 0.5% CMC. Otherreagents were analytical grade and obtained from Xi'anChemical Reagent Factory (Xi’an, China). The anilinemonomer was purified through vacuum distillation beforeuse and stored in a sealed container at 4 �C. All other re-agents were used as received.

2.2. Sample preparation

Aniline (0.2 M) was oxidized with ammonium perox-ydisulfate (0.25 M) in an aqueous solution. The monomerand oxidant solutions were mixed at room temperature tostart the oxidation. The reaction solution was kept withoutagitation for 24 h. Subsequently, the precipitate was filtered

Table 1The experienced mobile phases.

No Solvent system Composition (v/v/v)

1 Cyclohexane-chloroform 5:42 Cyclohexane-chloroform-diethylamine 5:4:13 Benzene-ethylacetate 7:24 Benzene-ethylacetate-diethylamine 7:2:15 Chloroform-methanol 9:26 Chloroform-methanol 9:17 Chloroform-methanol-diethylamine 9:1:1

off, rinsed several times with water to remove residualreactants, and dried under vacuum.

100 mg of powdered dried PANI was extracted throughmaceration in 5 mL of tetrahydrofuran (THF) at roomtemperature for 1 h and sonicated for 10 min (the PANI willnot be dissolved completely). Then, the extracts werecentrifuged at 14,000 rpm for 10 min, and then the su-pernatant was taken for further TLC analysis.

2.3. Chromatographic analysis

The TLC plate start lines were drawn using a pencil at1.5 cm, giving an elution distance of 10 cm.With a capillary,the THF extraction was carefully put on the line. Chro-matograms were developed in a normal chromatographicchamber that was pre-saturated in the mobile phase.Significant spots can be detected visually without colorrendering because aniline oligomers are colored sub-stances. After development, the plate was air-dried and theRF values of aniline oligomers were determined.

To optimize chromatographic separation and to choosethe appropriate mobile phase, several eluents were tested.

3. Results and discussion

In TLC, some aniline oligomers were not separatedindividually according to their molecular structure. Theygrouped together at a single spot and the single spot rep-resents several aniline oligomers. Some aniline oligomerswere not found to move from the spotting site. In practice,completely separating compounds is not always necessary,unlike in the general procedure for TLC. Themain role of TLCin this study is to reduce the effects of components on NMRdetection. Therefore, the developing distance and solventshould be selected depending on the purpose, such asseparating target compounds and urgency of the analysis.

For a large number of separated species, group selec-tivity can be assessed using the correlation plots of reten-tion data (RF) from chromatographic systemswith differentmobile phase. RF was determined from the ratio of distancetravelled by the samples to that by the solvent front fromthe sample spot position. In this case, correlation plots ofretention data with different eluents were used to selectthe best mobile phase for separating aniline oligomers(Fig. 1).

One development enables very limited separated spotsto be obtained as it can be seen from the experimental RFvalues, and the separation effect is not ideal.

The multicomponent mobile phase produced greaterselectivity when compared to the single mobile phase. Thebest combination of RF coefficients was chosen to separatethe aniline oligomers. The preliminary separation of themain component of aniline oligomers was possible using amixture of chloroform and methanol (9:1, v/v) as a mobilephase. The experimental values for retention the factorswere the RF less than 0.1 as per peak 1. Peak 4 correspondedto RF between 0.6 and 0.65. Peak 5 has RF ¼ 0.65e0.7. Thelast peak, peak 6, had an RF of 0.85e0.95. Other peaks(peaks 2 and 3) were not considered due to their negligiblepeak areas. Several developing result were not significantlydifferent, indicating the reproducibility of the technique.

Fig. 1. RF values obtained by TLC eluted with different mobile phases. Theorderof elution is as follows: chloroform;methanol; Cyclohexane-chloroform(5:4); Cyclohexane-chloroform-diethylamine (5:4:1); Benzene-ethylacetate(7:2); Benzene-ethylacetate-diethylamine (7:2:1); Chloroform-methanol(9:2); Chloroform-methanol (9:1); Chloroform-methanol-diethylamine(9:1:1).

J. Tao, J.-F. Yang / Polymer Testing 40 (2014) 1e3 3

All these results indicate that TLC-based analysis is abetter alternative for the rapid separation of different ani-line oligomers.

4. Conclusions

Thin-layer chromatography is an effective method forseparating large groups of compounds. Moreover, TLC isrelatively a simple and inexpensive method that enablesthe combination of a large variety of mobile phases forseparating various mixtures. In our work, a preliminaryseparation of the aniline oligomers investigated waspossible using TLC with suitable mobile phases. A PANIsample was extracted using THF and subjected to TLCanalysis (normal phase). A chloroform and methanol (9:1,v/v) eluent was used.

Existing separation techniques such as GC and LC aretime consuming. In this study, a rapid, simple and repro-ducible TLC-based analytical technique was developed toseparate the aniline oligomers. Using this technique, manyaniline oligomers samples can be separated simultaneously

and can be used as an alternative to existing methods. Thetotal examining time, including sample preparations, in TLCwas shorter than that for GC and LC. Another advantage ofthe proposed method is the fact that the RF can bemeasured directly without color rendering because anilineoligomers are colored substances.

These findings indicate that the TLC method could beapplied for the rapid and simple separation of anilineoligomers. It proved to be an alternative to other conven-tional analytical techniques such as GC and LC.

Acknowledgements

The author gratefully acknowledges the support of Prof.Xinli Jing and the research group of functional polymers inXi’an Jiaotong University.

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