Effect of Ionic Liquids on the Physical Properties of the Newly Synthesized Conducting Polymer

Conducting polymer has many applications in electronics, optical devices, sensors, and so on; however, there is still a massive scope of improvement in this area. Therefore, towards this aim, in this study, we synthesized a new thiophene-based conducting polymer, 2-heptadecyl-5-hexyl-6-(5-methylthiophen-2-yl)-4-(5-((E)-prop-1-enyl)thiophen-2-yl)-5H-pyrrolo[3,4-d]thiazole (HHMPT). Further, to increase its application, the interactions between the conducting polymer (HHMPT) and ionic liquids (ILs) were investigated by UV-Vis spectroscopy, FTIR spectroscopy, and confocal Raman spectroscopy techniques. Moreover, film roughness and conductivity of the polymer film with or without ILs were also studied. The imidazoliumand ammonium family ILs with the potential to interact with the newly synthesized conducting polymer were used. The results of the interaction studies revealed that the imidazolium family IL-polymer mixtures and ammonium family IL-polymer mixtures have almost similar conductivity at low concentration of ILs. This study provides an insight into the combined effect of a polymer and ILs and may generate many theoretical and experimental opportunities.


Introduction
From many years, conducting polymers have been playing a promising role for the fabrication of biochemical and chemical sensors.To fulfill all the criteria, they should have high electrical conductivity, chemical stability, and electrochemical reversibility [1][2][3][4][5].Polythiophene, polypyrrole, and polyaniline have been used as electrocatalysts for sensing biological and organic molecules from many years [1].Poly (3,4-ethylenedioxythiophene) (PEDOT) has been reported extensively owing to its ability to promote electron transfer reactions and electroanalysis using conducting film.The PEDOTmodified glassy carbon electrodes were used for detecting dopamine and ascorbic acid [6].The PEDOT-modified electrodes have also been used for determining pesticides [7].PEDOT-modified screen-printed carbon electrode is also used for the detection of nitrite [8].Su and Cheng used the PEDOT-modified screen-printed electrode for the determination of cysteine [9].Conducting polymer modification is a very good technique, but has limitations because of its low sensitivity [10].Even though the sensitivity can be improved by the addition of nanomaterials such as Pd and gold nanoparticles, it still has some limitations [11][12][13].

Materials and Methods
2.1.Materials.All the chemicals and reagents required for the synthesis of the polymer and ILs were purchased from Aldrich Chemical Co.(USA).[Bmim]Br, [Emim]Br, and TBMS were purchased from Sigma-Aldrich.Moreover, all the chemicals and reagents were used without any further purification.The rest of the TEMS was synthesized in the laboratory and analyzed by methods described in the literature [34].Schematic representation of the polymer synthesis is shown in Figure 1.

2.3.
Measurements. 1 H NMR and 13 C NMR spectra were obtained using a JEOL MSL 300 spectrometer and used to identify the structure of the synthesized compounds.Polystyrene standards in tetrahydrofuran solutions were utilized as the standards for gel permeation chromatography.S-3100 UV-Vis spectrophotometer with the following features was used: wavelength resolution, 0.95 nm; wavelength accuracy, ±0.5 nm; and wavelength reproducibility, ±0.02 nm.FTIR spectra were recorded using a Bomem MB Series MB100 FTIR spectrometer.Electrical conductivity was measured using a standard four-in-line probe apparatus.The thickness of the polymer thin films was measured using an Alpha-Step profilometer.Raman spectra were measured at room temperature using a confocal Raman microscope (WITec, Alpha 300 R) equipped with a 632.8 nm He-Ne laser.In addition, AFM studies were performed using Park systems, model: XE-100 with noncontract measuring mode.

Sample Preparation.
A small amount (0.001 g) of the polymer was dissolved in 4 mL of chloroform.Later, 1, 2, and 5 wt% of ILs were dissolved in this polymer-chloroform mixture at room temperature and this mixture was then vigorously stirred for 1 h at 20 °C.Subsequently, the polymer solutions with or without ILs were cast on clean glass plates (150 μm thickness) and then dried under vacuum.These films were used for various spectroscopy studies.

Study of Interactions between Polymer and ILs by UV-Vis
Spectroscopy at Various Temperatures.A new thiophenebased conducting polymer, 2-heptadecyl-5-hexyl-6-(5methylthiophen-2-yl)-4-(5-((E)-prop-1-enyl)thiophen-2-yl) -5H-pyrrolo [3,4-d]thiazole (HHMPT), was synthesized, as shown in Figure 1.The mechanical and conducting properties of polymer-IL mixtures depend on the miscibility of the polymer in ILs; hence, determining this aspect is very important.Further, molecular interactions between the polymer and ILs were measured by mixing them.UV-Vis spectroscopy may aid in elucidating these interactions.The UV-Vis spectra of the polymer at 20 °C shows a peak at ~530 nm, reflecting the conjugated π-systems (Figure 2).However, after the addition of different ILs such as [Bmim]Br, [Emim]Br, TMEAS, and TBMS, the polymer peak at ~530 nm disappeared and other peaks originated for all the ILs-polymer mixtures.
The polymer + [Bmim]Br showed new peaks at ~425 and ~724 nm.A small hump also appeared at ~654 nm for polymer + [Bmim]Br.In the presence of [Emim]Br IL and polymer mixtures, the new peak appears at ~423 nm and ~724 nm for the polymer + [Emim]Br and a small hump at ~657 nm, similar to the polymer + [Bmim]Br mixtures, whereas for the ammonium ILs TMEAS and TBMS, a similar pattern of the peaks was observed.For polymer + TMEAS, the peaks appear at ~423 and ~728 nm and hump at ~660 nm.However, for the polymer + TBMS mixture, new peaks at ~424 and ~728 nm and hump at ~660 nm appeared.In both the families of ILs, similar peaks and a hump appeared, indicating approximately similar interactions or reaction occurs in the polymer in the presence of ILs.Although, in the presence of ammonium ILs + polymer, the peak and hump at ~728 nm and ~660 nm, respectively, slightly red shifted as compared to those in imidazolium ILs + polymer mixture.The peaks at ~420 nm correspond to the π-π * transitions, and peak at ~700 nm resembles either an aggregation state of a conformation or arrangement of the polymer chains that are changing upon the addition of ILs.

Study of Interactions between Conducting Polymer and
ILs by Using FT-IR and Confocal Raman Spectroscopies.FTIR spectroscopy is another important tool for studying the interactions between the polymer and ILs [35].FTIR spectra of the conducting polymer were recorded with or without ILs, as shown in Figure 3.Some peak shifts in the FTIR spectra were obtained after the interaction of polymer with the ILs.The FTIR peaks at ~2922 and 2851 cm −1 may be due to the alkyl chain of the polymer, and the peaks at ~1637 and 1495 cm −1 may originate because of the aromatic -C=C-bond stretching of the polymer.The peak at ~1100 cm −1 may be due to the =C-H groups.After the interaction of [Bmim]Br (1 wt%) with the conducting polymer, the peaks are mainly due to the alkyl chain shifts to longer wavelength at ~2964 and 2875 cm −1 .Moreover, the peaks due to the -C=C-bond of the polymer shifted to ~1601 and 1521 cm −1 .These peak shifts in the composite film of polymer and [Bmim]Br IL (1 wt%) reveal the possible interactions between them.The peak shift for the other [Emim]Br(imidazolium IL) (1 wt%) and polymer is approximately same as that for the [Bmim]Br IL and polymer.The intensity of the C-H (sp 3 ) and aromatic C=C stretching decreased for the polymer-TMEAS (1 wt%) mixtures; however, for other ammonium IL (TBMS) (1 wt%) polymer mixture, the peak shift is more for aromatic C=C stretching.However, no change in peak shift was observed with the increase in IL concentration (data not shown).These shifts occurred because of the interaction of polymer with the ILs.There can be H-bonding between the anions of ILs and polymer, or other interactions such as van der Wall interactions are also possible between the ILs and polymer.To understand more exact interactions between the ILs and the polymer, the study is ongoing and will be reported in the near future.
Another sophisticated technique such as confocal Raman spectroscopy was used to investigate the interactions between the polymer and ILs (1 wt%).The polymer film was not damaged during the confocal Raman spectroscopy, because of its highly selective and sensitive laser.As shown in Figure 4, a significant peak appeared at 1453 cm −1 , corresponding to the β(C-H) in the polymer.In contrast, the polymer + [Bmim]Br showed the peaks at 1025, 1113, 1341, and 1421 cm −1 corresponding to the γ(C-C-C), γ(C-O), C-N + , and β(C-H), respectively, whereas the polymer + [Emim]Br showed the peaks at 956, 1086, 1336, 1422, and 1567 cm −1 corresponding to the (β(C-O), C-C), (γ(C-C-C)), (C-N+), (β(C-H)), and (γ(NC) + γ(NH)), respectively.In both cases, the peak at 1453 cm −1 corresponding to β(C-H) shifted to 1422 and 1421 cm −1 for polymer + [Emim]Br and polymer + [Bmim]Br, respectively, probably because of the interaction of polymer and ILs.Later, the polymer and TBMS complex showed peaks at 978, 1052, and 1428 cm −1 corresponding to the C-H, γ(C-C-C), and -CH 2 , respectively.The polymer + TMEAS shows peaks at 951, 1049, 1425, and 1456 cm −1 corresponding to the and β(C-H), respectively.Similar to the FTIR spectroscopy, no peak shift was observed in confocal Raman spectroscopy with the increase in the concentration of ILs (data not shown).

Effect of the Polymer and IL Interaction on the Conductivity and Surface Roughness Measurements.
Recently, there has been a keen interest in enhancing the conductivity of polymer.Such enhanced conductivity can be achieved by adding secondary dopants or additives such as diethylene glycol, 2-nitroethanol, glycerol, dimethyl sulfoxide, and tetrahydrofuran [36,37].However, this method has many drawbacks.Therefore, we attempted to enhance the conductivity of polymer by utilizing ILs.Our experimental results mentioned above clearly indicate good interaction between the polymer and ILs, suggesting a significant possibility of enhancing the conductivity of polymer by utilizing ILs.The experimental conductivity data listed in Table 1 demonstrate that the polymer has a very good conductivity (I 2 doped) of ≈4.0 ± 0.9 × 10 −2 S cm −1 at 20 °C.The addition of ILs was found to increase or decrease the conductivity depending upon the IL properties.

Conclusions
A new conducting polymer was synthesized, and its physical properties were studied with or without ILs.In addition, the conductivity and interaction level between the polymer and ILs were found to be dependent on the ILs.The polymer with [Bmim]Br IL exhibited the least roughness and higher conductivity even at higher IL concentrations, probably because of strong interactions with the polymer, while the conductivity of polymer decreases with increase in concentration of TBMS IL.This study shows new possible interactions of polymer and ILs and also opens new possibilities in different fields.

Figure 1 :
Figure 1: Schematic representation for the synthesis of new conducting polymer.