Structural, Thermal, Optical Characterizations of PANI/PMMA Composite Doped by TiO2 as an Application in Optoelectronic Devices

The casting method was employed for the preparation of polymer blend films doped with TiO 2 (0.5, 1, 1.5, and 2.3 wt.%). The XRD outcomes show that TiO 2 phase formation is anatase, with an average crystal size of 20.25 nm. PANI/PMMA-TiO 2 nanocomposites samples have an amorphous nature. In addition, the degree of amorphousity is increased with the increase of the content of the TiO 2 NPs. The FTIR method was employed for presenting the vibrational bands of the nanocomposites and the intermolecular bonding of the blend with the TiO 2 NPs. The applied investigation involved the optical constant like absorption as well as t90-etransmission spectra, refractive index, reflectance, coefficient extinction, dielectric constant’ imaginary and real pa rts, the susceptibility ( χ 3 ) of third order and the optical band gaps. The optical band gap (E g ) values of the films of fabricated nanocomposites was lower upon doping (≤ 1.5 wt.%). The reduction of the value happened due to the introduction of the preselected TiO 2 NPs into thin films. Such values significantly match the values which were revealed by the Tauc technique. It was proved through DSC and TGA techniques that TiO 2 NPs can lead to the enhancement of the polymer blend in terms of thermal stability. As displayed by the DSC analysis, there is a single Tg of the polymer blend (PANI/PMMA), which prove their miscibility. The optical constants displayed by the experimental results show noticeable changes upon raising the doping concentrations. The resultant doped thin films indicate that the fabricating high-efficient optoelectronic machines are greatly promising.


Introduction
The easy processability and low cost of polymers are probably the reason behind their dominance and prevalence as materials [1,2]. The prospective of combining the attractive functionalities of organic polymers and inorganic nanoparticles for enhancing their synergistical characteristics has recently attracted the attention of researchers to polymer blends doped with nanoparticles. The resultant nanocomposites have several applications in aerospace, automotive, optoelectronics [3]. The particle properties including the size, shape, loading, interfacial bonding, and dispersion of the fillers influence the characteristics of polymer composites [3,4]. Many properties of polymer matrix can be enhanced by inorganic particles as fillers including its thermal, mechanical, electronic, magnetic, optical characteristics as well as its density, and refractive index [5]. The main polymer used for polymer sensors and polymer optical fiber fabrication (POF) is Polymethyl methacrylate (PMMA). The structural and optical characteristics of the polymer may change upon doping nanoparticles within its main chains [4]. The use of PMMA, polyvinyl alchohol (PVA) and polyethylene oxide (PEO) is common in optical fiber fabrication, particularly in the core [6][7][8]. Better optical and electrical characteristics of polymers as a result of doping them with rare earth elements (REE) especially PMMA have been reported by many studies [9]. In recent times, several research groups are interested in the studying the impact of the addition of REE with nanosized particles as it can result in changing the properties of the polymer [10]. There are many advantages that can result from doping PMMA with nanoparticles. One advantages of this procedure is producing corrosion-resistant conductive materials [11]. George et al made a record of a change in a polymer's main chain, such as single and double bonds variations [12], which permits the electrons delocalization in the polymer matrix [13]. The effect of adding nano ZnO particles within the main chain of PMMA was investigated by Soumya et al [11].
The advantages of the doping of nanoparticles within PMMA can be seen in the change that occurs in the different characteristics of the polymer composites [14]. The change in optical and structural characteristics happen as a result of doping the nanoparticles within the polymer main chains. The higher conductivity, cost-effectiveness, easy synthesis, and environmental stability of polyaniline (PANI) makes it more attractive than the other conducting polymers [15,16]. Yet, PANI is poor in terms of thermal stability, solubility, and mechanical characteristics. PANI can be appropriately combined with metal nanoparticles due to its functional group. This can lead to an extra enhancement in its electrical conductivity and thermal stability. The effective interaction that can result from combining metal nanoparticles with functional groups with PANI will lead to improving the conductivity and optical characteristics. Thus, the resultant polymer blend (PANI/PMMA) demonstrates excellent characteristics of optical device applications. In addition, researchers and technologists have been interested in the study and production of nanosize material, particularly, with mounting their applications in optical and electronics manufacturing [17,18]. The procedure of consolidating PMMA, PANI, and metal nanocomposites can assist in the formation of new materials with enhanced optical and structural characteristics [19]. The various traits of Titanium dioxide including its high index of refractive as well as its low cost, chemical stability, nontoxicity, hydrophilicity, high photocatalytic characteristics makes it one of the best semiconductor ceramic material. The physical and chemical characteristics of TiO2 makes it very useful in several applications.These applications include supercapacitors, dye-sensitized solar cells, quantum-dot-sensitized solar cells, photoelectrolysis, biosensors, antimicrobical activity photochromic machines, selfcleaning, gas sensor and antireflective coatings [20,21]. The various crystal phases (anatase, rutile, and brookite) of TiO2 rely on the conditions and the method of preparation. There are various methods that can be utilized for synthesizing TiO2 NPs. These methods include spin coating, chemical vapor deposition, atomic layer deposition, electrospray deposition and molecular beam epitaxy [22,23]. There is also the sol-gel technique which has several advantages including high homogeneity for forming nano structures with high-quality and its applicability in simple equipment at low temperature [24]. This work aims at preparing new and flexible PANI/PMMA/TiO2 nanocomposites films made from insulating and conducting polymers. It also aims at investigating the structural, thermal and optical characteristics of these films. The changes which have been reported in PANI/PMMA/TiO2 nanostructured films in terms of the optical bandgap (Eg), absorption edge (Ed), refractive index (n), extinction coefficient (K), dielectric characteristics, and optical conductivity (σopt) can make it very useful in several applications including bio and chemical sensors, catalysis and as an active layer of optoelectronic machines.

Materials
Poly (methyl methacrylate) (PMMA) with MW=100,000 was purchased from BDH chemicals, England. Double distillation was used to purify aniline (Merck). Ammonium per sulfate (APS) was purchased from Merck, Germany. Dimethyl formamide (DMF) was provided from Sigma-Aldrich. The preparation process for titanium dioxide nanoparticles was explained in a previous work [25].

Synthesis of PANI
The technique of chemical oxidative polymerization was used to synthesize PANI at 0 •C to 5 •C as mentioned earlier [26]. The procedure involved mixing 9 mmol aniline with 50 mL of1M HCl solution and sufficient stirring, then, adding APS to it in order to start the reaction of polymerization (the mole ratio of aniline: APS = 1:1). This polymerization procedure was conducted at 0 •C for 10 hours with continuous stirring. After that, the resultant polymer was rinsed and cleaned several times using deionized water, ethanol, dilute HCl, and acetone and was finally dried at 60 •C for 6 hours.

Samples preparation
PANI/PMMA (20/80 wt.%) was added to DMF and stirred using a stable temperature magnetic stirring at 75°C. Variable quantities of TiO2 (0-1.2 wt.%) were added to the prepared solution. The solutions of these samples were submerged in ultrasonic for 1 hour to achieve a homogeneous mixture of filled samples and to increase the suspension of TiO2 NPs. The different TiO2 content samples were distributed in Petri dishes and dried in a 70°C oven for 48 hours. Following that, samples were husked from Petri dishes and under vacuum-stored. The resulting films were analyzed visually for their dryness and free standing nature. The electrolytes have a thickness ranging from 0.1 to 0.5 mm.

Measurements
The XRD patterns were determined using a Diano USA X-ray diffractometer with Cu Ka radiation at k = 1.54 A and voltage is 30 kV. Bragg's angle 2 has a calculated range of (10-80). At room temperature, all of the prepared samples were subjected to Fourier transform infrared spectroscopy (Nicolet iS10, USA) in the range of 4000-500 cm -1 .
Thermogravimetric (TGA) curves were measured on a TGA thermal analyzer (STD-Q600, USA) in the temperature range 30-600 °C at a rate of 10 °C min1 in a nitrogen atmosphere. Thermodynamic stability of prepared samples scanned with differential scanning calorimetry was investigated (DSC-50, Shimadzu). The transmittance and absorbance spectra of PANI/PMMA/TiO2 were measured with an accuracy of 0.2 nm using a Jasco V-670spectrophotometer in the wavelength range 190-2500 nm.

1. X-ray diffraction
As seen in Figure 1, the study of the structure of pure blend PMMA/PANI and its nanocomposites was carried out using X-ray diffraction (XRD). The XRD pattern of TiO2 NPs are displayed in Figure 1a, which also presents the diffraction peaks at 2θ values of (116) lattice planes of TiO2 and ascribing to JCPDS file no. 21-1272. [21]. Furthermore, the Scherrer equation was employed in determining the average size of the particle (D) of the TiO2 [27]. The size of the particle was found to be 20.25 nm. Based on the standard pattern (JCPDS card No. 01-089-0552), all the diffraction peaks were in good agreement with the tetragonal rutile TiO2. Broad and slightly protruded peaks were seen at 14.5°, and 29.8° as shown in fig. 1b, which evidences achievement of the polymer blend synthesis and reveals its amorphous nature as well. This peak of pure blend decreases a little and becomes depressed, which clearly indicates the uniform and deep interaction of TiO2 into PMMA/PANi as reflected in the XRD pattern of PMMA/PANI/TiO2 ( Fig. 1 b) [28]. The characteristic peaks related to TiO2 ceased to exist as exhibited by the XRD pattern of PMMA/PANI/TiO2 nanocomposite (at concentrations ≤ 1 wt.%). This proves the existence and effective dispersion of nanofiller in the polymer matrix. Yet, the diffraction peaks appearing at 2θ = 25.4° [29], at concentrations ≥ 1.5 wt%, are attributed to TiO2 NPs, which increase (intensity) upon raising TiO2 concentration in the nanocomposites films. In addition, the substantial reduction in the amorphous nature of polymer blend in the relation to nanocomposites films (1.5 wt%), indicate the need for strong interaction to obtain the wanted optical, and thermal characteristics.

2. FTIR spectroscopy
The vibrational bands of the nanocomposites are studied using Fourier Transform Infrared spectroscopy (FTIR). In the spectral range 4000-500 cm -1 , Figure 2 shows the FTIR spectra of pure blend and PANI/PMMA/TiO2 nanocomposites films. Figure 2 indicates that the peaks between 500 and 800 cm -1 can be attributed to C-H bending, while the peaks at 1063 and 1255 cm -1 can be attributed to -C-O-C-and C-O bond stretching vibrations, respectively. The bands related to the stretching vibration of N-B-N and N=Q=N structures existed at 1495 cm -1 and 1145 cm -1 , respectively, corroborating the production of polyaniline in the polymeric matrices, where N=Q=N was used as a measure of electron delocalization [30]. In addition, the carbonyl group of PMMA is responsible for a significant band at 1735 cm-1. Finally, as shown in Figure 2, the bands that appear in the 3100-2800 cm -1 spectral range correspond to various CH3 and CH2 vibrational modes. As TiO2 NPs are inserted into a polymeric matrix, the intensity of the peaks at 689 cm-1, 753 cm -1 , 1064 cm -1 , and between 1350 and 3600 cm -1 reduces. The interaction or intermolecular bonding between the blend matrix and TiO2 NPs could explain the significant reduces in peak intensities across the entire FTIR spectrum [4]. The presence of hydrogen bonding between polymer blend and TiO2 NPs may explain the decrease in peak intensity at 1735 cm -1 , as well as a peak position shifted to a lower wavenumber by doping. As a result, the TiO2 NPs doped within the blend are required to contribute to the PMMA backbone structure's carbonyl groups C=O. This may be due to the Ti +2 nanosize ions forming a coordination bond with the C=O groups of the ester. As a result, the blend structure has taken on a complex configuration [31]. The presence of an interaction between blend and nanoparticles could explain the low intensities of the peaks in the range of 3100-2800 cm -1 , with the position shifted to a lower wavenumber. Figure   1 demonstrates the structural changes in the polymer blend caused by doping with different concentrations of TiO2. The study of these spectra revealed a change in the height (intensity) and position of some IR absorbance peaks, suggesting the presence of nanoparticles within the main chain of the polymer blend (See Scheme 1).

DSC study
The thermal behavior of the TiO2 NPs doped PANI/PMMA was studied using DSC from room temperature to 500C, as shown in Fig. 3.
For pure blends and nanocomposites, the figure depicts two endothermic peaks.
The glass transition temperature (Tg) of the relaxation mechanism resulting from the micro-Brownian motion of the backbone of the main chain was observed at the first transition around 100-120 °C using thermograms [32]. The middle of the first endothermic peak during heating is used to measure the Tg. Molecules below Tg do not have segmental motion, and certain parts of the molecules may not wiggle but only vibrate slightly. The molecules will begin to vibrate near Tg, and segmental motion increases. Conformational changes, or changes in molecular structure, are induced by thermal motion or the action of an external field without rupturing chemical bonds. We noticed a single Tg of the two polymers at 115 C, indicating that the PANI/PMMA interaction is miscible [33]. The broad endothermic peak around 315-380 C for pure blend and nanocomposite were assigned to the Tm, indicating the semicrystalline nature of the pure blend. The depth of peak at Tm indicates a decrease in the crystallinity degree of the pure blend (semicrystalline) after addition filler and an increase in the amorphous fraction, this is consistent with the results of XRD [34,35].

4. Thermogravimetric (TGA)
The thermogravimetric (TGA) results of TiO2 nanoparticles doped PANI/PMMA blend in nitrogen atmosphere from 30 to 500 C are shown in Figure 4. TGA curves are all smooth, with decomposition behaviors that are almost identical. In all of the curves, there are three weight losses. There was no evidence of non-oxidative degradation in any of the three steps. The first stage weight loss occurred at temperatures below 150 C due to evaporation of intra-and intermolecular moisture [36], while the second step weight loss occurred at temperatures between 200 and 300 C due to the thermal decomposition of gases and functional groups such as CO, C=O, and NH2 on the polymer blend backbone [37]. The thermal decomposition of the polymer blend backbone occurred in the third stage of thermal decomposition at above 350 o C, and this weight loss section was caused by the thermal decomposition of the polymer blend backbone. These findings revealed that nanocomposites are more thermally stable than pure PANI/PMMA, implying that more stable structures or groups were produced through the interactions of PANI/PMMA and TiO2 nanoparticles.

4i. Determine the kinetic parameters:
Different methods can be used to quantify the activation energy for the thermal decomposition of the current samples. The following section introduces two distinct calculation methods:

4. i. Coats -Redfern's method
Coats and Redfern present a method for measuring the thermal activation energy, which is shown in equation. where mi, mf, and mt are the sample's initial, final, and total mass at time t, respectively.
From the slope of the plot, the activation energy (Ea) was determined as: With increasing TiO2 nanoparticle content, the estimated activation energy reduces gradually from 230 to 185 Kcal/mol. This reduction in activation energy values suggests that TiO2 nanoparticles have a major effect on thermal properties due to defects formed through the interaction of polymer chains with TiO2 and the creation of certain bonds.

4. ii. Broido's method
The activation energy associated with each stage of decomposition is determined by the Broido model using the following equation [39]: Broido showed the weight of the prepared sample (wt) to thermal analysis at time (t) is related to the fraction of initial molecules not yet decomposed (Y) using the relation: where, wt is the weight at any time t; w∞ is the weight at infinite time and wi is the initial

weight. A plot of ln [ln (1/Y)] vs. 1/T gives an excellent approximation to a straight line.
The activation energy is proportional to the slope [40]. With raising the TiO2 nanoparticles content, there was a slight variation in the measured values of the activation energy, which decreased from (220 to 180) Kcal/mol, indicating that the TiO2 nanoparticles may affect the polymer characteristics. The measured activation energies (Ea) of the specimens are presented in Table (1).

5. Optical characteristics
The transmittance spectra of PANI/PMMA and PANI/PMMA doped TiO2 nanocomposites films are displayed in Figure5. Due to the many lattice defects of TiO2, the transmittance becomes lower with the addition of TiO2 NPs, leading to the reduction in the transparency of the nanocomposites. The achievement of direct and indirect band gap was from the plots of (αhν) 2 and (αhν) 1/2 versus hν at RT, which allows us to obtain the values of E g through extrapolating the linear part of (αhν) 2 to zero as displayed in Fig. 6 (a,b). The Eg ( In addition, it should also be noticed that the presence of the absorption coefficient  () near the band edge is shown to be dependent exponentially on energy of photon (hv) following the mathematical relations provided by Urbach [45]: (6) in which  is constant, and Ee stands for the tail of the band (Urbach tail), that stands for the width of localized state. The logarithm of absorption coefficient () plots are shown in the Urbach plot as a photon energy hv function as displayed in Fig.10. Table 1  factor in the modification of the polymer matrix electronic structure [47].

6. Optical dielectric parameters
The For the pure blend and nanocomposites samples, Figure 12(a, b) depicts the relationship between ε′ and εʺ as a function of incident photon energy. The ε′ and εʺ relation becomes stronger with raising the TiO2 nanoparticles content in the polymer matrix. Fig. 12 demonstrates the reduction of the dielectric constant and dielectric loss after increasing the incident photon energy of light to 2.5 eV, and the raising in the absorption edge region followed by this reduction due to the polarization [48]. Obviously, the formation defects observed by the reaction of the charge transfer of the pure polymer blend chain with the TiO2 NPs dopant are responsible for this raise in dielectric and loss dielectric constant with the addition of TiO2 NPs into pure polymer blend and nanocomposites films.
The motion of free carriers caused by the incident electromagnetic waves is responsible for the optical conductivity (σopt). The values of n and α are determined and presented in the equation below [49], In which C refers to light speed in vacuum. The optical conductivity along with the incident light photon energy of the pure blend and nanocomposites films Figure13. It should be noticed that the optical conductivity also becomes higher with the higher concentration of TiO2 nanoparticles. The charge transfer formation of the molecules of the blend with the TiO2 nanoparticles has a relation to such behavior [50]. The polymer chains were broken by the TiO2 nanoparticles, which could lead to the formation of hydrogen bonding of TiO2 nanoparticles with the blend molecules. In addition, as the density of the localized states in the structural band become higher, the nanoparticles become higher simultaneously, thus causing the increase in the optical conductivity along with the absorption coefficient [51]. It should be noticed that there was a strong increase in the optical conductivity in the absorption edge region in relation to the strong increase of free charge carriers resulting from the sufficient energy of the incident photon which surmounted the band gap of the polymer film [52].

Conclusion
In this study, a blend doped with TiO2 was synthesized and TiO2 nanoparticles were

Conflicts of interest:
The authors declare that they have no conflict of interest.  The FTIR spectra pure PANI/PMMA and nanocomposites lms. Figure 8 shows the electric susceptibility (χc) for pure PANI/PMMA and nanocomposites lms. Absorption coe cients α as a function of hv for pure PANI/PMMA and nanocomposites lms. Re ectance (R) for pure PANI/PMMA and nanocomposite lms. Variation of the (a) dielectric constant ε' and (b) dielectric loss ε vs. hv of the pure PANI/PMMA and nanocomposites lms.