Conjugated Polymer Films Having a Uniaxial Molecular Orientation and Network Structure Prepared by Electrochemical Polymerization in Liquid Crystals

A new method for fabricating conjugated polymer films was developed using electrochemical polymerization in liquid crystals and magnetic orientation. A uniaxial main chain orientation and a crosslinked network structure were achieved with this method. By employing eight types of monomers, the influence of the crosslinking for the film was investigated. The crosslinking was found to improve the solvent resistance of the conjugated polymer films. This new method is expected to be useful in various applications, such as high-powered organic electronic devices with durability.


Introduction
Due to their unique chemical structure, conjugated polymers can demonstrate inorganic semiconductor-like properties and are thus regarded as a promising material for applications. Many researchers have contributed toward the development of organic electronic devices having conjugated polymers, such as organic field-effect transistors [1][2][3], organic photovoltaic devices [4,5], and organic light-emitting diodes [6][7][8]. Compared with inorganic semiconductor materials, conjugated polymers usually have a linear form, which provides anisotropy at the molecular level. The optical absorption, emission properties, and electrical conductivity highly depend on the molecular arrangement [9][10][11][12]. Although conjugated polymers can form a well-organized structural arrangement through self-assembly, polymer films generally form poly-domains or a random arrangement. As a result, the electronic devices based on the polymers do not exhibit anisotropic optical or electrical performances. To address this concern, many groups have developed technologies to control the arrangement of conjugated polymers [13][14][15][16]. These methods include the synthesis of liquid crystalline polymers [17][18][19][20] using mechanical force [21], and oriented substrates to fabricate conjugated polymer films with uniaxial alignment [22,23].
The durability of conjugated polymer films is important for applications in organic electronic devices [24,25]. A possible solution to this issue is to construct a network structure of conjugated polymers by crosslinking; this can help in stabilizing the morphology of the conjugated polymer film [26,27]. However, molecular orientation is somewhat difficult to achieve for polymers with a network structure. A photochemical reaction or a coordination reaction allows the polymers to have both well-organized and crosslinked forms [28,29]. However, there remains a significant technical challenge to apply this technology to other conjugated polymers.
Electrochemical polymerization is a convenient method to synthesize conjugated polymer films. After polymerization, the conjugated polymers usually separate from the electrolyte solution and are then directly deposited on the electrode substrate to form films. Many groups have investigated electrochemical polymerization in terms of its

Electrochemical Polymerization in LCs
The monomers and the supporting salt (TBAP) were added to a nematic LC medium and mixed thoroughly to obtain an LC electrolyte solution. An indium tin oxide (ITO)coated glass electrode was used as a substrate. For the electrochemical polymerization, the reaction cells were constructed using two ITO glass electrodes and a polytetrafluoroethylene spacer (0.20 mm thickness) to construct a sandwich structure. The LC electrolyte solution was injected into the cell, heated to the isotropic phase, and gradually cooled to room temperature. The reaction cells were placed in a magnetic field (12 T, superconducting magnet) for 20 min in order to obtain the uniaxially oriented LC reaction medium. Then, 4.0 V of direct current was applied across the cell to initiate the polymerization at room temperature. After 15 min, the power source was disconnected and the reaction cell was removed from the magnetic field. The resulting polymer film was washed with organic solvents to remove residual LC, supporting salt, and monomers. All the primary polymer films were reduced from an oxidized state to a neutral state with the treatment of hydrazine. The concentration of each component and the electrochemical polymerization conditions are summarized in Table 1.  13 C NMR spectra were recorded using a JNM-ECS spectrometer (JEOL, 400 MHz), the chemical shifts were recorded in parts per million, and the coupling constants (J) were recorded in Hertz. POM observations were performed using a Nikon Eclipse LV100 POM with the crossed Nicol condition. LD UV-vis absorption spectroscopy was performed using a JASCO V-630 UV-vis spectrophotometer equipped with a polarizer. BT is copolymerized with d(BT), DTC is copolymerized with dis(DTC), and DTB is copolymerized with bis(DTB). Hence, we distributed these six monomers into three copolymerization groups: the BT/d(BT) group, the DTC/bis(DTC) group, and the DTB/bis(DTB) group (Scheme 3). Given that the uncrosslinked monomer (uncrosslinkable) (BT, DTC, and DTB) has one reactive unit, while the crosslink monomer (crosslinkable) (d(BT), bis(DTC), and bis(DTB)) has two reactive units, the reactive unit concentration was used rather than monomer concentration for calculation in this work. For each polymerization, the total reactive unit concentration was kept at 0.5% molar. The ratio of crosslinked units (RCU) was adjusted as 0%, 30%, 50%, 70%, and 100%, in order to examine different degrees of crosslink. Herein, RCU is employed to briefly describe the proportion of added uncrosslinked units and crosslinked units, but it should be noted that it indicates no actual degree of crosslinking in each sample.

POM Observation
Confirmation of birefringence is a convenient method to examine anisotropic materials. An anisotropic material possesses an optical axis. When linearly polarized light is applied to an anisotropic material, the electric vector of the light can be resolved into two vector components: one is parallel to the optical axis and the other is perpendicular to the optical axis. These two vector components of the light indicate the different propagation velocities in the material. Therefore, when these two vector components exit from the material, a phase difference between the optical waves is generated. Consequently, the electric vector of the new resulting polarized light can be differentiated from the original polarized light. This phenomenon can be described using the following equation: where ∆φ is the phase difference, d is the thickness of the sample, λ is the wavelength of the incident light, ne is the refractive index of the polarized light, where the electric vector is parallel to the optical axis, and no is the refractive index of the polarized light, where the electric vector is perpendicular to the optical axis. When ∆φ = (2n + 1)π, the direction of the electric vector of the emergent light is rotated 90° from the incident light; when ∆φ = 2nπ, the direction of the electric vector does not change; when 2nπ < ∆φ < (2n + 1)π, the emergent light becomes circularly polarized light or elliptically polarized light. LC materials with no orientation usually show a characteristic optical texture, such as the Schlieren texture of nematic LCs (Figure 2A   and DTB4 have the same conjugated backbone but different alkyl side chain lengths. They were used for further investigation in the examination of molecular form influence. Plausible uncrosslinked and crosslinked forms are described in the polymerization of BT and d(BT), as an example (Figure 1). BT is copolymerized with d(BT), DTC is copolymerized with dis(DTC), and DTB is copolymerized with bis(DTB). Hence, we distributed these six monomers into three copolymerization groups: the BT/d(BT) group, the DTC/bis(DTC) group, and the DTB/bis(DTB) group (Scheme 3). Given that the uncrosslinked monomer (uncrosslinkable) (BT, DTC, and DTB) has one reactive unit, while the crosslink monomer (crosslinkable) (d(BT), bis(DTC), and bis(DTB)) has two reactive units, the reactive unit concentration was used rather than monomer concentration for calculation in this work. For each polymerization, the total reactive unit concentration was kept at 0.5% molar. The ratio of crosslinked units (RCU) was adjusted as 0%, 30%, 50%, 70%, and 100%, in order to examine different degrees of crosslink. Herein, RCU is employed to briefly describe the proportion of added uncrosslinked units and crosslinked units, but it should be noted that it indicates no actual degree of crosslinking in each sample.

POM Observation
Confirmation of birefringence is a convenient method to examine anisotropic materials. An anisotropic material possesses an optical axis. When linearly polarized light is applied to an anisotropic material, the electric vector of the light can be resolved into two vector components: one is parallel to the optical axis and the other is perpendicular to the optical axis. These two vector components of the light indicate the different propagation velocities in the material. Therefore, when these two vector components exit from the material, a phase difference between the optical waves is generated. Consequently, the electric vector of the new resulting polarized light can be differentiated from the original polarized light. This phenomenon can be described using the following equation: where ∆ϕ is the phase difference, d is the thickness of the sample, λ is the wavelength of the incident light, ne is the refractive index of the polarized light, where the electric vector is parallel to the optical axis, and no is the refractive index of the polarized light, where the electric vector is perpendicular to the optical axis. When ∆ϕ = (2n + 1)π, the direction of the electric vector of the emergent light is rotated 90 • from the incident light; when ∆ϕ = 2nπ, the direction of the electric vector does not change; when 2nπ < ∆ϕ < (2n + 1)π, the emergent light becomes circularly polarized light or elliptically polarized light. LC materials with no orientation usually show a characteristic optical texture, such as the Schlieren texture of nematic LCs (Figure 2A Figure 3 shows the POM images of the polymer films obtained from the BT/d(BT) group, DTC/bis(DTC) group, and DTB/bis(DTB) group with varying RCU. The polymers were prepared in LC solution with electrochemical polymerization under the magnetic field. All samples displayed different colors and brightness levels between the left side and the right side of their pictures. When the LC orientation was parallel to the analyzer, dark pictures were observed, but when the LC orientation was not parallel to the analyzer and polarizer, significantly brighter pictures were observed. This difference was easily observed even at 100% RCU. Overall, this phenomenon indicates that all the samples were optically anisotropic, and their optical axes were parallel to the polarizer or analyzer. The conjugated polymers were uniaxially oriented even at 100% RCU.
According to Equation (1), a low orientation degree will result in a small absolute value of (ne − no), which then results in a small phase difference ∆φ. Therefore, it would be difficult to observe the emergent light using an analyzer, and changes in color or brightness between the left side and the right side of the POM image would also be imperceptible. In addition, the thickness of a sample d can also affect ∆φ. Therefore, the orientation degree of a polymer film cannot be evaluated only with the POM observation.  Figure 3 shows the POM images of the polymer films obtained from the BT/d(BT) group, DTC/bis(DTC) group, and DTB/bis(DTB) group with varying RCU. The polymers were prepared in LC solution with electrochemical polymerization under the magnetic field. All samples displayed different colors and brightness levels between the left side and the right side of their pictures. When the LC orientation was parallel to the analyzer, dark pictures were observed, but when the LC orientation was not parallel to the analyzer and polarizer, significantly brighter pictures were observed. This difference was easily observed even at 100% RCU. Overall, this phenomenon indicates that all the samples were optically anisotropic, and their optical axes were parallel to the polarizer or analyzer. The conjugated polymers were uniaxially oriented even at 100% RCU.

LD UV-vis Absorption Spectra Measurement
In linearly conjugated polymers, the transition dipole moment is along the main chain. The electronic transition probability is along the direction proportional to the inner product between the dipolar transition moment and the electric vector of the incident light. If the polarization direction of the incident light is parallel to the main chain of a conjugated polymer, the maximum absorption intensity can be observed. Therefore, the According to Equation (1), a low orientation degree will result in a small absolute value of (ne − no), which then results in a small phase difference ∆ϕ. Therefore, it would be difficult to observe the emergent light using an analyzer, and changes in color or brightness Polymers 2021, 13, 2425 13 of 20 between the left side and the right side of the POM image would also be imperceptible. In addition, the thickness of a sample d can also affect ∆ϕ. Therefore, the orientation degree of a polymer film cannot be evaluated only with the POM observation.

LD UV-Vis Absorption Spectra Measurement
In linearly conjugated polymers, the transition dipole moment is along the main chain. The electronic transition probability is along the direction proportional to the inner product between the dipolar transition moment and the electric vector of the incident light. If the polarization direction of the incident light is parallel to the main chain of a conjugated polymer, the maximum absorption intensity can be observed. Therefore, the main chain orientation of conjugated polymers can be defined using linear dichroism (LD) UV-vis absorption spectroscopy. The LD measurement for every sample was carried out two times in different orientations using a polarizer. One orientation was parallel to the LC orientation and the other was perpendicular to the LC orientation. If one of the two orientations shows a higher absorption intensity than the other, it indicates the main chain orientation of the polymers. Figure 4 shows the LD UV-vis spectra of all the samples. In each spectrum, the solid line represents the absorption intensity for an orientation that is parallel to the LC orientation, and the dashed line indicates the absorption intensity for an orientation that is perpendicular to the LC orientation. In the spectra of the BT/d(BT) group and the DTC/bis(DTC) group, higher intensities were observed for the solid lines, indicating that the main chain orientation was parallel to the LC orientation. This is the parallel orientation. With an increase in the RCU, the intensity difference between the solid line and the dashed line decreased. This indicates that the increased proportion of crosslinked monomers resulted in a decrease in the orientation degree in this case.
In the LD spectra of the DTB/bis(DTB) group, when the RCU was 0%, the dashed line showed a higher intensity, which indicates that the main chain orientation was perpendicular to the LC orientation. This is the perpendicular orientation. When the RCU was in the range of 30-70%, the solid line and the dashed line had roughly the same intensity, which suggests that the orientation degree of the polymers was low. When the RCU was 100%, the solid line gave a higher intensity, which means that the polymers were in a parallel orientation. For a better understanding of the effect of RCU on the orientation degree, a linear dichroism ratio (LDR) was summarized for all samples to evaluate the orientation degree. The LDR was calculated according to the following equation: where A // is the maximum absorption intensity of the solid line and A ⊥ the maximum absorption intensity of the dashed line. When the LDR is greater than 1, it indicates a parallel orientation; when the LDR is smaller than 1, it indicates a perpendicular orientation; and when LDR is close to 1, it indicates that the orientation degree decreased. The results are shown in Figure 5. It is worth noting that the addition of a crosslinked monomer changed both the orientation degree and the orientation direction of the polymers in the DTB/bis(DTB) group. Given that 100% DTB resulted in a perpendicular orientation and 100% bis(DTB) resulted in a parallel orientation, it follows that the blending of these two monomers would result in a neutralization of the parallel orientation effect and the perpendicular orientation effect. However, it is not clear what causes the different orientation directions of DTB and bis(DTB). It is likely that as DTB and bis(DTB) have the same conjugated backbone, the orientation difference arises due to the alkyl side chain and crosslinked structure. Therefore, the next step is to synthesize the monomers DTB2 (with an ethyl side chain) and DTB4 (with a butyl side chain), in order to investigate the effect of different alkyl side chains. Figure 6 shows the LD UV-vis absorption spectra of the polymer films formed using the monomers DTB2 and DTB4. According to the spectra, DTB2 was in a perpendicular orientation while DTB4 was in a parallel orientation. This indicates that the orientation of conjugated polymers can be significantly affected with a slight change in the alkyl side chain length. It is clear that the addition of a crosslinked monomer dramatically affects the orientation degree of the polymers. In the BT/d(BT) and DTC/bis(DTC) groups, when the RCU increased to 30%, the orientation degree significantly decreased; when the RCU was increased from 30% to 100%, the orientation degree gradually decreased. In the DTB/bis(DTB) group, when the RCU was 0%, the polymers showed a perpendicular orientation; when the RCU was 100%, the polymers showed a parallel orientation; when the RCU was in the range of 30-70%, the orientation degree was too small to define the orientation direction.  Figure 6 shows the LD UV-vis absorption spectra of the polymer films formed using the monomers DTB2 and DTB4. According to the spectra, DTB2 was in a perpendicular orientation while DTB4 was in a parallel orientation. This indicates that the orientation of conjugated polymers can be significantly affected with a slight change in the alkyl side chain length. It is clear that the addition of a crosslinked monomer dramatically affects DTB/bis(DTB) group, when the RCU was 0%, the polymers showed a perpendicular orientation; when the RCU was 100%, the polymers showed a parallel orientation; when the RCU was in the range of 30-70%, the orientation degree was too small to define the orientation direction.

Optical Properties of Polymers
Crosslinking affects not only the molecular orientation degree of polymers, but also their optical properties. The maximum absorption wavelength and the onset wavelength DTB/bis(DTB) group, when the RCU was 0%, the polymers showed a perpendicular orientation; when the RCU was 100%, the polymers showed a parallel orientation; when the RCU was in the range of 30-70%, the orientation degree was too small to define the orientation direction.

Optical Properties of Polymers
Crosslinking affects not only the molecular orientation degree of polymers, but also their optical properties. The maximum absorption wavelength and the onset wavelength

Optical Properties of Polymers
Crosslinking affects not only the molecular orientation degree of polymers, but also their optical properties. The maximum absorption wavelength and the onset wavelength in the absorption spectra are two important parameters for evaluating the optical properties of a polymer. The maximum absorption wavelength and onset wavelength of the polymers are summarized in Figure 7. With a change in the RCU, the BT/d(BT), DTC/bis(DTC), and DTB/bis(DTB) groups showed completely different tendencies, which indicates the complexity of the crosslinking effect. For the BT/d(BT) group, an increased RCU resulted in a decrease in the onset and maximum absorption wavelengths; in the DTC/bis(DTC) group, the onset wavelength marginally changed while the maximum absorption wavelength first increased and then decreased; in the DTB/bis(DTB) group, the onset wavelength and maximum absorption wavelength changed in opposite directions. Given that the polymers formed within the same group have the same conjugated backbone structure, the change in the optical spectra may be due to three reasons: degree of polymerization, coplanarity of the conjugated backbone, or intermolecular interaction. When crosslinked monomers were added to the polymerization system at various concentrations, all three factors changed. It is most likely that the crosslinking effect changed the optical properties of the polymer films by affecting the molecular weight, coplanarity, and intermolecular interaction, simultaneously. in the absorption spectra are two important parameters for evaluating the optical properties of a polymer. The maximum absorption wavelength and onset wavelength of the polymers are summarized in Figure 7. With a change in the RCU, the BT/d(BT), DTC/bis(DTC), and DTB/bis(DTB) groups showed completely different tendencies, which indicates the complexity of the crosslinking effect. For the BT/d(BT) group, an increased RCU resulted in a decrease in the onset and maximum absorption wavelengths; in the DTC/bis(DTC) group, the onset wavelength marginally changed while the maximum absorption wavelength first increased and then decreased; in the DTB/bis(DTB) group, the onset wavelength and maximum absorption wavelength changed in opposite directions. Given that the polymers formed within the same group have the same conjugated backbone structure, the change in the optical spectra may be due to three reasons: degree of polymerization, coplanarity of the conjugated backbone, or intermolecular interaction. When crosslinked monomers were added to the polymerization system at various concentrations, all three factors changed. It is most likely that the crosslinking effect changed the optical properties of the polymer films by affecting the molecular weight, coplanarity, and intermolecular interaction, simultaneously. Moreover, the difference between the two parameters was calculated. In general, the absorption spectra reflect the electronic transitions between the different vibrational energy levels of the ground and excited states. The onset wavelength corresponds to the 0-0 transition and the maximum absorption wavelength corresponds to the transition with the highest transition probability. The difference between the onset wavelength and the maximum absorption wavelength can be reflected from mechanical twisting of the main chain in the excited state. Moreover, the difference between the two parameters was calculated. In general, the absorption spectra reflect the electronic transitions between the different vibrational energy levels of the ground and excited states. The onset wavelength corresponds to the 0-0 transition and the maximum absorption wavelength corresponds to the transition with the highest transition probability. The difference between the onset wavelength and the maximum absorption wavelength can be reflected from mechanical twisting of the main chain in the excited state.

Solvent Resistance
Most polymer films obtained via electrochemical polymerization in LCs are insoluble in organic solvents both in their oxidized state and after reduction (dedoped). However, the monomer DTB displays a unique property where the corresponding polymer film has solubility in THF after reduction with hydrazine. This property enables a closer examination of the effect of crosslinking on the solvent resistance. As shown in Figure 8, the poly(DTB) film was partly dissolved in THF. Poly(DTB) in THF solution was yellow, while poly(bis(DTB)) in THF solution was transparent, even though a small amount of yellow precipitate appeared. This contrast demonstrates that the crosslinking effect can improve the resistance to solvents in polymer films.

Solvent Resistance
Most polymer films obtained via electrochemical polymerization in LCs are insoluble in organic solvents both in their oxidized state and after reduction (dedoped). However, the monomer DTB displays a unique property where the corresponding polymer film has solubility in THF after reduction with hydrazine. This property enables a closer examination of the effect of crosslinking on the solvent resistance. As shown in Figure 8, the poly(DTB) film was partly dissolved in THF. Poly(DTB) in THF solution was yellow, while poly(bis(DTB)) in THF solution was transparent, even though a small amount of yellow precipitate appeared. This contrast demonstrates that the crosslinking effect can improve the resistance to solvents in polymer films.

Conclusions
In this research, a fabrication method for forming conjugated polymer films was developed using electrochemical polymerization in LCs. By employing the monomers with crosslink properties and magnetic orientation, the conjugated polymer films were fabricated with both a crosslinked network structure and a uniaxial main chain orientation. Two types of monomers, uncrosslinked monomers and crosslinked monomers, were employed for the electrochemical polymerization in LCs, and a magnetic orientation was performed to obtain a uniaxially oriented LC electrolyte solution for the polymerization. The resulting polymer films showed a uniform optical axis. This result indicates that the conjugated polymer network possesses a uniaxially oriented conjugated main chain. An increase in the proportion of crosslinked monomers resulted in a decrease in the orientation degree for the main chains in the polymer. Moreover, changes in the alkyl side chains length of the monomers induced a significant change in the orientation direction of the main chains in the resultant polymers. The crosslinking is beneficial in improving the durability for conjugated polymers.

Instruments
1 H NMR spectra were recorded on a JNM-ECS-400 NMR Spectrometer (JEOL, Tokyo, Japan). All signals were recorded in ppm with the internal TMS (tetramethylsilane) at 0.00 ppm. FT-IR absorption spectra were obtained with an FT/IR-4600 (JASCO, Tokyo, Japan) using the KBr method. All spectra were recorded between 400 and 4000 cm −1 with a resolution of 4 cm −1 . POM observations were conducted with an Eclipse LV100 high-resolution polarizing microscope (Nikon, Tokyo, Japan). UV-vis absorption spectra were recorded on a V-630 UV-vis spectrophotometer (JASCO, Tokyo, Japan).

Conclusions
In this research, a fabrication method for forming conjugated polymer films was developed using electrochemical polymerization in LCs. By employing the monomers with crosslink properties and magnetic orientation, the conjugated polymer films were fabricated with both a crosslinked network structure and a uniaxial main chain orientation. Two types of monomers, uncrosslinked monomers and crosslinked monomers, were employed for the electrochemical polymerization in LCs, and a magnetic orientation was performed to obtain a uniaxially oriented LC electrolyte solution for the polymerization. The resulting polymer films showed a uniform optical axis. This result indicates that the conjugated polymer network possesses a uniaxially oriented conjugated main chain. An increase in the proportion of crosslinked monomers resulted in a decrease in the orientation degree for the main chains in the polymer. Moreover, changes in the alkyl side chains length of the monomers induced a significant change in the orientation direction of the main chains in the resultant polymers. The crosslinking is beneficial in improving the durability for conjugated polymers.

Instruments
1 H NMR spectra were recorded on a JNM-ECS-400 NMR Spectrometer (JEOL, Tokyo, Japan). All signals were recorded in ppm with the internal TMS (tetramethylsilane) at 0.00 ppm. FT-IR absorption spectra were obtained with an FT/IR-4600 (JASCO, Tokyo, Japan) using the KBr method. All spectra were recorded between 400 and 4000 cm −1 with a resolution of 4 cm −1 . POM observations were conducted with an Eclipse LV100 high-resolution polarizing microscope (Nikon, Tokyo, Japan). UV-vis absorption spectra were recorded on a V-630 UV-vis spectrophotometer (JASCO, Tokyo, Japan).