1. Introduction
In recent times, conductive polymers, specifically polypyrrole (PPy), have garnered significant interest in academic and industry research communities globally due to their distinctive characteristics and promise for diverse commercial applications. The increased interest in PPy originates mostly from its remarkable thermal and environmental durability, impressive electrical conductivity, and simple synthesis procedure. These polymers are conductive because they have a conjugated electron system composed of alternating single and double bonds within their chemical structure. The unique chemical structure of conductive polymers not only allows them to conduct electricity but also piques the interest of researchers in other fields, leading to breakthroughs in conductive polymers and applications such as biomaterials [
1,
2,
3], biosensors [
4,
5,
6,
7], and energy storage [
8,
9].
The electrical properties of PPy were also modified based on the polymerization process. The chemical polymerization technique is a standard method for producing PPy particles. This method required oxidizing agents, such as ammonium persulfate (APS) or ferric chloride (FeCl
3), to oxidize the pyrrole monomer (Py), which initiated the polymerization process. It is theorized that during the first phase of pyrrole oxidation, insoluble pyrrole oligomers form, and then PPy chains arise from clustered aggregates, culminating in globular forms [
10,
11,
12,
13]. The morphology of PPy also was found altering from globular structures to another form by using templates such as hard template and soft template. When utilizing a hard template, the final product’s morphology is significantly influenced by the pre-prepared material, such as anodic alumina membrane producing nanowire morphology of PPy [
14], silicon wafer [
15], or polymer [
16]. Following the reaction, the template is then carefully removed. The utilization of soft templates in polymerization has several notable benefits, particularly regarding the flexibility and ease it provides to the polymer structure. Soft templates, typically composed of organic compounds or biological entities, allow for a more controlled and flexible synthesis process. The inherent flexibility of soft templates accommodates a broader range of polymer sizes and shapes, catering to specific application needs, such as surfactants like fatty alcohol polyoxyethylene ether surfactants [
17] sodium dodecyl benzene sulfonate (SDBS) [
18], or sodium dodecyl sulfate (SDS) [
19,
20]. Additionally, these surfactants, when utilized as soft templates in polymer synthesis, provide a unique and efficient method for creating polymers with the appropriate structural and functional features. In solution, these amphiphilic compounds spontaneously form micelles or other self-assembled structures, producing nanoscale templates for monomers to polymerize polymers with various morphologies and sizes [
21,
22,
23,
24]. Moreover, using anionic surfactant such as SDS or SDBS can enhance conductivity of PPy [
25,
26]. Meanwhile, many studies reported that the non-spherical structure of PPy obtained by using CTAB showed higher electrical properties than the spherical shape [
27]. Additionally, Khadem et al. [
28] reported that using CTAB to prepare PPy could give it a string-bead shape with better conductivity than a spherical shape made from SDS in the same conditions. Yussuf et al. [
29] found PPy prepared in SDS with ferric chloride (FeCl
3) as an oxidizing agent to have fibrillar morphology and show slightly higher conductivity than the other oxidizing agent, ammonium persulfate (APS).
The surfactant enhances the electronic properties of PPy through a process called doping, which often involves oxidation as occurring during polymerization. The oxidation level of PPy is intricately linked with the formation of polarons and bipolarons, which are localized defects causing lattice distortion within the polymer’s structure [
30,
31]. These defects are crucial for facilitating the mobility of charge carriers, where polarons and bipolarons correspond to low and high oxidative states, respectively [
32,
33]. The genesis of these polarons and bipolarons is highly dependent on the synthesis conditions and plays a significant role in chemical, and electrical properties of PPy. Research conducted by Santos et al. [
34] and Pang et al. [
35] through Raman spectroscopy analysis on the chemical polymerization of PPy highlighted that the pyrrole rings predominantly attach at the alpha-alpha (α-α) positions, though they can also link at beta-beta (β-β) and beta-alpha (β-α) positions. This variation in binding sites influences the molecular configuration of dimers and trimers in PPy, as well as their respective oxidative states, manifested as bipolarons and polarons. Further research by Ishpal and Kaur [
36], Trchová et al. [
37], Tumacder et al. [
38], and Paúrová et al. [
39] showed the different Raman spectral profiles of bipolarons and polarons of PPy having varying nanostructures. Including, building on previous research [
20], it was observed that PPy polymerized with SDS presented a sheet-like morphology and demonstrates an increased bipolaron/polaron ratio in line with Py concentration, which correlates with a decrease in the bandgap. This contrasts with PPy synthesized without SDS, which typically forms spherical structures.
Therefore, this study delves into the interplay between conductivity and morphology in PPy by examining the bipolaron and polaron ratios, particularly in the C-H deformation region of the PPy structure. We analyzed the bipolaron and polaron ratio in PPy polymerized with different surfactants SDS, CTAB and Tween 80 (TW). Our theory is that PPy made with CTAB and TW will have a spherical shape and a lower bipolaron/polaron ratio, especially in the C-H deformation region, compared to PPy that was treated with SDS. According to the underlying idea, a low bipolaron/polaron ratio in the C-H deformation region of spherical PPy structures might mean that they conduct electricity less well. Through this investigation, we aim to clarify the gap reason in the relationship between the morphology and electrical properties of PPy through bipolaron and polaron ratios (
Scheme 1).
3. Discussion
The DLS experiment showed that Py-SDS-APS presented the smallest diameter, followed by Py-TW-APS, Py-CTAB-APS, Py-SDS-Fe, Py-CTAB-Fe, and PPy*. Notably, using a surfactant can reduce the PPy size compared without using surfactant. In addition, polymers with anionic surfactants such as SDS tended to have a smaller average size than those made with cationic surfactants such as CTAB or nonionic surfactants such as TW, which was similarly found in Hoshina Y. el.al., Paurova M. et.al., and Zoromba M.Sh.et.al., for example [
11,
39,
50]. This could be because the negatively charged SDS (OSO
3− groups) can stabilze the growing PPy particles, thereby preventing excessive growth and aggregation [
49]. When using constant SDS or CTAB, the size differed depending on the type of oxidizing agent used. When APS was utilized, the PPy size was lower than that of FeCl
3, as shown in
Figure 1a, which is similar to the hydrogel created by Bo et al., even though the unreacted molecules did not wash away [
51]. In addition, FeCl
3 may substantially increase the ionic strength of the solution compared to APS, and higher ionic strengths can diminish electrostatic repulsion among the developing polymer chains, resulting in larger aggregates [
52,
53]. The average size obtained was evaluated using PdI. In the context of drug delivery systems, a PdI value below 0.30 is indicative of a well-dispersed and homogeneous particle size [
54]. Therefore, according to the PdI results in
Figure 1a, Py-TW-APS and Py-SDS-APS demonstrated a fair monodispersity (0.12 and 0.19, respectively). An increase in PdI may suggest a greater tendency for particles or molecules to agglomerate [
55]. Therefore, the size distribution was significantly larger, presumably due to sample aggregation such as in the case of Py-CTAB-Fe, PPy*, Py-SDS-Fe, and Py-CTAB-APS.
The difference in the zeta potential between PPy samples polymerized with surfactants and PPy* indicated that Py was polymerized within the micelles of the surfactants because Py monomers were predicted to be located in the hydrophobic core of the surfactant micelles because the hydrophobic structure of pyrrole monomer (Log P = 0.75, where P is the partition coefficient between octanol and water). Consequently, the zeta potential of PPy is determined by the type of the hydrophilic head of the surfactant facing the exterior [
56,
57]. Additionally, the high negatively charge in Py-SDS-APS and Py-SDS-Fe tended to increase the stability of the sample more than the positive charge due to hydrodynamic interactions and electrostatic repulsion between particles [
49]. However, if the size of the sample is large, the zeta potential are affected by gravitation more than hydrodynamic forces [
39], such as in the case of Py-SDS-Fe, which shows a negative charge and large size. For Py-CTAB-Fe, PPy* and Py-CTAB-APS contain a positive charge, which makes them simpler to aggregate, corresponding to a high PdI and large average size. However, in the case of Py-TW-APS, this sample also showed a positive charge, as confirmed by the intrinsic properties of the PPy structure. However, TW can stabilize PPy in water because it absorbs onto the surface of particles and provides steric stabilization, where the physical barrier of the absorbed surfactant layer protects particles from coming close enough to aggregate [
58].
The results from the average size and zeta potential refer to the interaction between the Py monomer, surfactant, and oxidant during polymerization based on the hydrophobic/hydrophilic properties and electrostatic forces among them. These reactions can lead to different morphologies. As shown in
Figure 2a, SEM showed a cluster of particles in PPy*, Py-CTAB-APS, Py-SDS-Fe, and Py-CTAB-Fe, corresponding to the average size and PdI detected by DLS, except in the case of Py-SDS-APS, which found a sheet-like form with a large morphology.
In the absence of a surfactant, PPy* has a particle form, which has been normally observed in many studies [
10,
24,
25]. The morphology of Py-SDS-APS is sheet-like, which might be due to the effect of electrostatic interactions between the natural positively charged PPy* and the head group of the surfactants (SO
3-) [
20]. This finding was different from that reported by Zhang et al. [
23], who reported that in the presence of APS and FeCl
3, no regular polypyrrole nanostructure was observed. Their findings differ from those of our study, possibly because PPy prepared with SDS remained at low concentrations close to the CMC of SDS and the proportion between each component was not the same. Therefore, it may affect micelle formation when mixed together. However, they believe that the uneven shape differs from that without SDS, resulting from the residual SDS in the PPy structure after drying.
For polymerization with FeCl
3 (Py-SDS-Fe), we found a cluster of particle-like forms, which is similar to a study using sodium dodecylbenzenesulfonate (DBSNa) as an anionic surfactant [
25]. However, unlike in the case of Yussuf et. al. [
29] who found that using FeCl
3 as an oxidizing agent showed fibrillar morphology but they used lower SDS concentration (around 0.02 M) and performed only 4 h reation time in room temperature. It is possible that the short polymerization time and the proportion of monomer and surfactant may have affected the PPy morphology [
20]. However, their work supported that the fibrillar morphology showed better conductivity properties than the globular form when they used as an oxidizing agent. In this work, we also found that using FeCl
3 as an oxidant in the SDS system resulted in higher conductivity, as shown in
Figure 3b, but it showed a globular shape as compared to using APS, which represents a sheet-like structure. Besides, we found ferric element (Fe) in the PPy structure from electron dispersive X-ray spectroscopy (EDS) when FeCl
3 was used (no data shown), which might occur because it cannot be completely removed after dialysis. Therefore, the intrinsic properties may have affected the conductivity of PPy more than the shape morphology of these samples. Addionally, regarding the electrostatic force might occurred from Fe
+ between particles, as mentioned above [
52,
53], this interaction force may disrupt sheet-like forming and increase porosity in PPy structure [
59]. Some works reported that a specific higher porous PPy structure represented in higher surface area and increase its electronic properties [
60,
61].
However, in the case of Py-CTAB-Fe, even though Fe was found in structure, Py-CTAB-APS had better conductivity. This might be because the morphology of Py-CTAB-APS tended to resemble a string-bead structure, which is similar to the results obtained by Khadem et al. [
28] even though they used a lower CTAB concentration and a polymerization period of 4 h at room temperature and under N
2 atmosphere. Moreover, Zhang X. et.al. [
23] prepared polymerization at 0-5 °C for 24 h and obtained a ribbon-like form when CTAB was used with APS, and obtained sphere-like PPy using CTAB surfactant and FeCl
3, similar to Py-CTAB-Fe in this work and Zoromba et.al. [
11]. Therefore, by employing CTAB, it is possible to alter its morphology into a ribbon or wire form by the process of self-assembly between the positively charged cations of the cationic surfactants and the negatively charged anions of the oxidizing agent APS (S
2O
82-) [
23,
28], but the oxidixing agent have the effected to form PPy structure. However, the conductivity of PPy polymerized with CTAB was lower than that of PPy polymerized with SDS in either APS or FeCl
3. Therefore, SDS is a dopant that is useful to dope a conductive structure of PPy better than CTAB. In addition, by predicting the average size, zeta potential from zetasizer, and morphological structure from SEM, we can estimate the interactions between the Py monomer and surfactant, as shown in
Scheme 2.
As shown in
Scheme 2, the Py monomer aggregation during polymerization is affected by the interaction between surfactants and oxidants, resulting in various morphologies. In addition, their interactions led to distinct chemical structures in their PPy, as we determined the chemical structure of PPy samples from Raman spectroscopy and found varying peak characteristics, as shown in
Figure 4a. By studying the chemical structure observed via Raman spectroscopy, we could define whether the morphology is related to the intrinsic chemical structure of PPy. As mentioned in the results section, we found that in Py-TW-APS and Py-CTAB-Fe, there was no peak in the bipolaron structure, which is similar to the peak characteristic of PPy deprotonated by ammonium hydroxide from Trchová et. al. [
37], which represents a low-oxidative structure. In addition, PPy polymerized with FeCl
3 showed a sphere-like structure and Raman spectra characteristics similar to those work (PPy*), even they used 633 nm or 785 nm laser excitation. In addition, they used dye acids (methyl orange and Acid Blue 25) to dope a bipolaron structure and found that the peak at bipolaron increased as its form nanotube structure with higher conductivity, which is also similar to other works [
62,
63].
Moreover, as in our previous work, we observed that the ratio between the bipolaron and polaron of PPy polymerized with SDS (sheet-like form) differed from that of PPy without a surfactant (sphere-like form), particularly in the C-H deformation area. Additionally, the bipolaron and polaron structures refer to the oxidative structure of PPy [
32,
33,
34]; a higher bipolaron and polaron ratio indicates higher conductivity properties. Therefore, the ratio between the bipolaron and polaron was calculated from the peak intensity, including the three main areas, as shown in
Figure 4b. The graph shows that the C-H deformation area represented the bipolaron/polaron ratio of each sample and showed that PPy polymerized with SDS had a higher ratio of bipolaron and polaron than other conditions, corresponding to the high UV-Vis-NIR absorption at around 960 nm (
Figure 5a) and low bandgap value calculated from absorbance (
Figure 5b). This phenomenon is consistent with the negatively charge surface and high-conductivity measurements from instruments of Py-SDS-APS and Py-SDS-Fe. Therefore, the correlation between them revealed that SDS incorporated into the PPy structure was the major dopant that enhances the conductivity properties of PPy by interfering with the PPy structure and raising the bipolaron structure in the C-H deformation area, which might be due to the electrostatic interaction between positively charged PPy and the anionic head group of SDS conducting the PPy structure like that.
Additionally,
Figure 6 represents the conclusion of the relationship between each PPy property in this work. We can categorize into two groups: low bandgap and high bandgap (red dot line), which the low bandgap correlated to high bipolaron and polaron ratios (black dot line) correlated to the theory and other works [
48,
49]. In this category, it confirmed the relationship between using SDS in Py polymerization and electronic properties (conductivity and bandgap) and bipolaron and polaron ratios in C-H deformation, but it’s not along with the morphology.
Although the bipolaron and polaron ratio results went against the first hypothesis, which said that the morphology should match the bipolaron and polaron ratio in C-H deformation. However, the fact that the rise in the bipolaron and polaron ratios in the C-H deformation region is linked to SDS provides us with insight into where SDS affects the intrinsic chemical structure of PPy, which might be a criterion leading to higher conductance. The data provides additional underpinnings for the future development of PPy properties.