Interaction Studies between Newly Synthesized Photosensitive Polymer and Ionic Liquids

In this information age, different kinds of photosensitive materials have been used in the manufacture of information storage devices. But these photosensitive materials have the bane of low diffraction efficiency. In order to solve this problem, we have synthesized a novel photosensitive polymer from epoxy-based azopolymers (with three types of azochromophores). Furthermore, we have studied the interaction between this newly synthesized azopolymer and ionic liquids (ILs). For this purpose, we have used the ammonium and imidazolium families of ILs, such as diethylammonium dihydrogen phosphate (DEAP), tributylammonium methyl sulfate (TBMS), triethylammonium4-aminotoluene-3-sulfonic acid (TASA), and 1-methylimidazoliumchloride ([Mim]Cl). To investigate themolecular interaction between azopolymer and ILs, we have used the following spectroscopicmethods of analysis: UV-visible spectroscopy, photoluminescence (PL) spectroscopy, Fourier transformed infrared spectroscopy (FT-IR), and confocal Raman spectroscopy. In this study, we have developed new photosensitive materials by combining polymer with ILs.


Introduction
In this study, we have described cost-effective methods used to store, generate, and transmit data speedily [1].Although diverse technologies are being used for information storage, photosensitive materials are the most popular ones.The optical reflection of photosensitive materials can be modified by metallic and polymeric lasers [2,3].Photopolymers and silver halide materials have high sensitivity and excellent resolution, so they are used as holographic recording materials [4].However, in the past few years, azopolymers have attracted the attention of several researchers as they have applications in diverse fields [1][2][3][4].An azopolymer film induces anisotropic effect when exposed to light.Therefore, this nonlinear polymer film is one of the most promising holographic recording materials.Azopolymers also have many other advantageous properties, such as photoinduced birefringence, photoinduced phase transition, photomechanical deformation of thin films, and surface relief-grating (SRG) formation [5][6][7][8][9][10][11].These properties can be correlated with photoinduced anisotropic effects through which the transisomer (more stable) of azo group gets excited and undergoes a transition to the cis-form (less stable).The energy for the transition is provided by the following diverse phenomena: the dipolar interaction with the optically induced electric field gradient [12,13], the photoisomerization of azobenzene chromophores that undergo a translational diffusion [14], the molecular alignment that brings about a mean-field force [15], and the photomechanical effect occurring in thin films [16].The polymer with such properties has diverse applications in sensors, actuators, optical storage devices, and other data storage devices.
However, some azopolymers still cannot induce high quality effects through photoinduced anisotropy, which is essential for the practical use of optical storage devices [17][18][19].Cui and Zhao have reported that a stable photoinduced birefringence can be detected in an azopyridine liquid crystalline polymer [20].He et al. have devised different excitation regimes of nonlinear absorption (NLA) in azocontaining ion liquid crystal polymer [21].Li et al. [22] have explored the photoorientation properties of liquid crystalline azodendrimer using pulsed irradiation.Ionic liquids (ILs) are another class of liquid crystals that are currently used for producing photoinduced anisotropy.ILs are defined as classical molten salts having low melting point [23][24][25][26][27][28].In recent years, Pan et al. have reported how they developed reversibility and long-term optical storage capacity in azo-containing IL crystals [29].Furthermore, Zhao et al. [30] have also reported that the film made from azocontaining, IL-crystalline polymer exhibited large nonlinear refraction and negligible nonlinear absorption.After perusing through these research studies involving azopolymer and azo-containing, IL-crystalline polymer, we studied the interaction between the newly synthesized, epoxybased azopolymer and ILs (ammonium and imidazolium families) using photoluminescence (PL) and UV-visible, confocal Raman, and FT-IR spectroscopies.In this study, we have used the ammonium and imidazolium families of ILs, such as diethylammonium dihydrogen phosphate (DEAP), tributylmethylammonium sulfate (TBMS), triethylammonium 4-aminotoluene-3-sulfonic acid (TASA), and 1methylimidazolium chloride ([Mim]Cl).

Materials.
For conducting this experimental study, we purchased the following chemicals 4,4  -cyclohexylidene bisphenol, aniline, 4-bromobenzenediazonium tetrafluoroborate, and 4-nitrobenzenediazonium tetrafluoroborate (Sigma Aldrich, Co.; St. Louis, MO, USA).Epichlorohydrin was purchased from Sejinic, Co., Korea.All the other materials, reagents, and solvents were commercially available products, which were used without carrying out any further purification procedures.
Figure 1 illustrates the synthesis of the polymer.The detailed experimental procedure of synthesis is described below.

Synthesis of the Polymer
2.3.1.Synthesis of Monomer.4,4  -Cyclohexylidene bisphenol was dissolved in epichlorohydrin, and the solution was stirred at room temperature.Then, an aqueous solution of sodium hydroxide was added, and the resultant homogeneous mixture (solution) was stirred for 6 h.This solution was evaporated to isolate epichlorohydrin, which was then dissolved in ethyl acetate.Finally, the solution was washed five times with deionized water.The combined organic extracts were dried over MgSO 4 and evaporated to discard ethyl acetate and deionized water from the homogeneous mixture (solution).The residue was purified by column chromatography using silica gel as the absorbent and the mixed solvent of chloroform/acetone (v/v = 95/5) as the eluent.With this chromatographic technique, we obtained about 48.9% of colorless viscous oil.The structure of this viscous oil was characterized by 1 H NMR, 13 C NMR, and FT-IR.The results of NMR spectroscopic analysis were as follows:

Synthesis of Precursor
Polymer.Equimolar quantities of synthesized monomer and aniline were homogeneously mixed at room temperature, and then this mixture was gradually heated at 110 ∘ C. Thereafter, this homogeneous mixture was stirred at 110 ∘ C for 48 h.The product was washed several times with pure methanol to remove the moiety of low molecular weight.The polymer was then collected and dried under vacuum at 50 ∘ C for 12 h.Using differential scanning calorimetry (DSC), we found that   of polymer was 90.28 ∘ C. The number average molecular weight and weight average molecular weight of the polymer were estimated to be 6,720 and 10,670, respectively.With the help of gel permeation chromatography (GPC), we found that the polydispersity index of the polymer was 1.58.The results of NMR spectroscopic analysis were as follows:

Synthesis of Azopolymer-A.
Aniline was dissolved in a homogeneous mixture of tetrahydrofuran, glacial acetic acid, sulfuric acid, and deionized water.Thereafter, an aqueous solution of sodium nitrite was added dropwise to this homogeneous mixture (solution) at 5 ∘ C.Then, this homogeneous mixture solution was stirred at 5 ∘ C for 10 minutes.We prepared a diazonium salt of sulfanilic acid solution.Subsequently, this synthesized precursor of polymer was dissolved in 60 mL of tetrahydrofuran, and this mixture was stirred at 0 ∘ C. Furthermore, the diazonium salt solution was added dropwise to the mixture of precursor polymer and stirred vigorously at 0 ∘ C for 18 h.The resultant mixture was evaporated to eliminate tetrahydrofuran.The product was washed with pure diethyl ether and deionized water several times.Then, we dried this product under vacuum at 70 ∘ C for at least 24 h.Using DSC, we found that the   of the polymer was 86.77 ∘ C. The number average molecular weight and weight average molecular weight of the polymer were estimated to be 5,860 and 8,300, respectively.With the help of GPC, we found that the polydispersity index of this polymer was 1.37.The results of NMR spectroscopic analysis were as follows:

Synthesis of Azopolymer-B.
We dissolved 4-bromobenzenediazonium tetrafluoroborate in a solution of tetrahydrofuran and deionized water.Then, we prepared a diazonium salt solution.Thereafter, the synthesized precursor of polymer was dissolved in 60 mL of tetrahydrofuran and stirred at 0 ∘ C. The diazonium salt solution was added dropwise to a solution of precursor polymer, and then it was stirred continuously at 0 ∘ C for 18 h.The resultant mixture was evaporated to eliminate tetrahydrofuran.The product was washed with pure diethyl ether and deionized water several times.Finally, the product was dried under vacuum at 70 ∘ C for at least 24 h.The results of spectroscopic analysis were as follows:

Synthesis of Azopolymer-C.
We dissolved 4-nitrobenzenediazonium tetrafluoroborate in a solution of tetrahydrofuran and deionized water.Thus, we prepared a diazonium salt solution.Subsequently, the synthesized precursor of polymer was dissolved in 60 mL of tetrahydrofuran, and the resultant mixture was stirred at 0 ∘ C.Then, the diazonium salt solution was added dropwise to the mixture of precursor polymer.The resultant suspension was stirred at 0 ∘ C for 18 h.The resultant mixture was evaporated to eliminate tetrahydrofuran.The product was washed with pure diethyl ether and deionized water several times.Finally, we dried this product under vacuum at 70 ∘ C for at least 24 h.The results of NMR spectroscopic analysis were as follows:

Preparation of Polymer Film.
The concentration of azopolymer-C was typically 12 wt% in a homogeneous solution of anhydrous tetrahydrofuran.This solution was filtered through 0.45 m membranes and bar-coated onto normal glass slides.The average thickness of this film was 10 m.Before being used, these coated films were dried at 100 ∘ C in an oven for 24 h.be used as holographic recording materials.Three different types of azochromophores (azopolymer-A/azopolymer-B/azopolymer-C) were developed from the precursor polymer using azo coupling reaction (see Figure 1).Using the functionality of a polymer, we incorporated different types of azochromophores into the precursor polymer [31,32].The polymers exhibited   value at around 80-90 ∘ C. While carrying out polymerization of precursor polymers, we maintained a temperature of 110 ∘ C to eliminate side reactions, such as cross-linking polymerization.Had the temperature risen above 110 ∘ C, cross-linking polymerization would have occurred, and the solubility of azopolymers in common organic solvents would have been very low.Therefore, the synthesized azopolymers were not long chain polymers.Instead, they were short chain polymers, having a molecular weight of around 10,000.The degree of functionalization in the three series of azopolymers was determined by 1 H NMR analysis.In this analysis, we found that the peak at the p-position of precursor polymers disappeared, while peaks ascribed to the new benzene ring appeared.

Synthesis and UV-Vis
For understanding the efficiency of these polymers (azopolymer-A/azopolymer-B/azopolymer-C), we used UVvisible spectroscopy.Figure 2 displays the  max values for the azopolymer in THF solution.The  max value of the - * absorption band is shifted to a longer wavelength in the visible region, thereby indicating the presence of donor-acceptor type azochromophores.The  max values for azopolymer-A, azopolymer-B, and azopolymer-C were 414 nm, 426 nm, and 502 nm, respectively.This indicates that  max values get strongly influenced by the p-position of azobenzene units, which act as electron acceptors.Hence, we decided to use azopolymer-C, which exhibited a maximum wavelength shift with ILs.To investigate the molecular interaction between ILs and azopolymers, we used PL, UV-visible, and Raman spectroscopies.

Molecular Interaction Study of Azopolymer with ILs Using PL, UV-Visible, and Confocal Raman Spectroscopies.
The miscibility factor is governed by the molecular interaction between azopolymer-C and ILs.With the help of UV-visible spectroscopy, we found that there was a significant molecular interaction between the synthesized azopolymer-C and ILs.As shown in Figure 3(a), compared to only polymer, there is a slight red shift in case of all ILs interacting with the polymer.The azopolymer depicted a maximum absorbance at 502 nm, while azopolymer-C + DEAP exhibited maximum absorbance at 518 nm.Compared with the azopolymer, the intensity of absorption was much higher for azopolymer-C + DEAP.Moreover, azopolymer-C + TASA showed maximum absorbance at 511 nm, but there was no increase in the intensity of absorbed light.Instead, the [Mim]Cl (protic imidazolium IL) + polymer exhibited red shift with a maximum absorbance at 513 nm; this solution mixture also showed a slight increase in the intensity of absorbed light.The red shift and slight increase in intensity of absorption by [Mim]Cl + azopolymer suggest that [Mim]Cl interacts with azopolymer.
A red shift with maximum absorbance at 513 nm is due to the interaction of aprotic ammonium IL TBMA, with the polymer.
In order to have in-depth knowledge of optical properties of polymer and polymer + ILs, we conducted PL spectroscopy.The results of this analysis are displayed in Figure 3(b).The PL for the polymer was observed approximately at 509 nm.On the other hand, azopolymer-C + DEAP showed maximum PL at approximately 510 nm with quenching intensity.This indicates that DEAP undergoes a slight change in its PL properties after chemically interacting with azopolymer-C.Furthermore, ammonium IL TBMA interacts with azopolymer-C, showing a decrease in its intensity of absorption and red shift as compared to the polymer (alone).But, in this case, there is no shift when compared with azopolymer-C + DEAP.Moreover, the ammonium IL TASA combination with azopolymer-C exhibited a red shift and an increase in intensity.But there was no significant shift in wavelength and intensity for the imidazolium based IL [Mim]Cl when compared with the polymer alone.To further elucidate the interaction between polymer and ILs, we used confocal Raman spectroscopy.
Confocal Raman microscopy is a more sophisticated technique; it estimates the vibrational energy of target solutes and eliminates the need of invasive monitoring aids [33][34][35].However, owing to the penetration of the polymeric membrane, the Raman signal was strongly diminished and the excited laser's signal was also strongly attenuated.Thus, the highly selective and sensitive laser can be directed inside azopolymer-C without causing any damage.As seen in Figure 3(c), significant peaks appear at 1136, 1202, 1402, and 1599 cm −1 , corresponding to C-OH, C-O-C, (CH 2 ), (CH 3 ) asymmetric, and (C-NO 2 ) groups, respectively, in the polymer.On other hand, the polymer + ILs showed a slight red shift, confirming the interaction between the polymer and ILs.In the case of protic ammonium DEAP IL + polymer, the Raman peaks were observed at 1141, 1202, 1403, and 1602 cm −1 .These peaks also exhibited higher intensity when compared to the polymer alone.In case of TASA + polymer, Raman peaks were observed at 1142, 1202, 1403, and 1452 cm −1 with quenching intensity.These shifts clearly indicate that both protic ammonium ILs have interactions with azopolymer-C.Furthermore, wavelength shifts were also observed between protic imidazolium [Mim]Cl IL and polymer at 1141, 1201, 1402, and 1598 cm −1 .In case of aprotic ammonium IL TBMA + polymer, high intensity peaks were observed at 1139, 1201, 1401, and 1601 cm −1 .

The Study of Molecular Interaction between Polymer and ILs
Using FT-IR Spectroscopy.To understand the interaction between azopolymer-C and ILs, FT-IR spectroscopy study seemed to be useful tools.Figures S1-S4 (FT-IR data of polymer and polymer-ILs mixture can be seen in Supplementary Material available online at http://dx.doi.org/10.1155/2015/461974)and Table 1 display the FT-IR spectra of azopolymer-C in the absence and presence of various ILs, respectively.Some shifts in the wavelength of azopolymer-C were observed after its interaction with ILs.
Among the three synthesized azopolymers, azopolymer-C exhibited the longest wavelength shift in UV-visible spectroscopic analysis, which was attributed to the conjugative and inductive effects of both ground state () and excited state ( * ) electrons.The results of UV-visible spectroscopy revealed that both protic and aprotic ILs interacted extensively with the polymeric surface.However, the high intensity peaks of protic ammonium ILs appeared at longer wavelength after interacting with the polymer.Moreover, UVvisible spectroscopy showed that both protic ammonium ILs (DEAP and TASA) interacted with azopolymer-C to different extents.Similarly, the PL spectroscopy results revealed that, compared with imidazolium IL, the change in PL properties was significant in ammonium ILs.On the other hand, peak shifts in the Raman spectra reveal that both protic and aprotic ILs of ammonium and imidazolium families strongly interact with azopolymer-C and modify polymeric properties.The FT-IR results also support the Raman data, indicating that there is an interaction between all ILs and azopolymer-C due to which the absorption peaks shift to longer or shorter wavelengths.In various spectroscopic studies, the shift in the wavelengths of peaks has been attributed to the various kinds of interaction between the azopolymer and ILs, such as hydrogen bonding, Van der Waal's interaction, and Coulombic interaction.Hence, azopolymer + IL can be used as a holographic material for capturing images.This study provides new outlook to the combined use of azopolymer + IL in various holographic devices.

Conclusion
In this extensive experimental study, we successfully synthesized three new photosensitive materials.Among them, azopolymer-C has the maximum - * emission.Furthermore, to explore the interactions between polymer and ILs, we used the following analytical techniques: UV-visible, FT-IR, and confocal Raman spectroscopies.Then, we inferred that, compared to protic imidazolium ILs, protic and aprotic ammonium ILs had a greater tendency to form hydrogen bonds.To the best of our knowledge, the azopolymer and IL mixture has not been analyzed for this first time in this study.Hence, the results of this novel study seem to be beneficial in the field of polymer chemistry and holographic research.

Figure 1 :
Figure 1: Schematic view of the synthesis of azopolymers.

Table 1 :
FT-IR spectroscopy analysis of the azopolymer-C and azopolymer-C + ILs.