Optical Properties of Composites Based on Poly(o-phenylenediamine), Poly(vinylenefluoride) and Double-Wall Carbon Nanotubes

In this work, synthesis and optical properties of a new composite based on poly(o-phenylenediamine) (POPD) fiber like structures, poly(vinylidene fluoride) (PVDF) spheres and double-walled carbon nanotubes (DWNTs) are reported. As increasing the PVDF weight in the mixture of the chemical polymerization reaction of o-phenylenediamine, the presence of the PVDF spheres onto the POPD fibers surface is highlighted by scanning electron microscopy (SEM). The down-shift of the Raman line from 1421 cm−1 to 1415 cm−1 proves the covalent functionalization of DWNTs with the POPD-PVDF blends. The changes in the absorbance of the IR bands peaked around 840, 881, 1240 and 1402 cm−1 indicate hindrance steric effects induced of DWNTs to the POPD fiber like structures and the PVDF spheres, as a consequence of the functionalization process of carbon nanotubes with macromolecular compounds. The presence of the PVDF spheres onto the POPD fiber like structures surface induces a POPD photoluminescence (PL) quenching process. An additional PL quenching process of the POPD-PVDF blends is reported to be induced in the presence of DWNTs. The studies of anisotropic PL highlight a change of the angle of the binding of the PVDF spheres onto the POPD fiber like structures surface from 50.2° to 38° when the carbon nanotubes concentration increases in the POPD-PVDF/DWNTs composites mass up to 2 wt.%.


Introduction
In the last ten years, poly(o-phenylenediamine) (POPD) was intensively studied, with several applications being reported in the following area: (i) fuel cells [1], (ii) sensors [2,3], (iii) solar cells [4], lithium-ion batteries [5] and so on. Depending on the oxidizing agents used in chemical polymerization, i.e., K 2 Cr 2 O 7 and HCl, CuCl 2 , (NH 4 ) 2 S 2 O 8 , H 2 O 2 or FeCl 3, the POPD morphological structures of the type of ellipsoidal particles [6], microbelts [7], quantum dots [8], fiber like microstructures with square prism shapes [9] and microfibers [10], respectively, were reported. A wide range of POPD-based composites have been synthesized in the presence of various oxides (e.g., TiO 2 [11], ZnO [12]) or carbon nanoparticles (e.g., carbon nanohorns [13], reduced graphene oxide [14], carbon nanohollow [15], carbon nanotubes [16,17]). The composites based on POPD and various carbon nanoparticles were intensely studied for a wide range of applications. In order to support this sentence, we note that in the case of: (i) the POPD/graphene composites, the applications in the field of supercapacitors [18,19] and sensors [20] were reported, while in the case of (ii) the POPD/carbon nanotube composites, the main applications were as Figure 1 shows the SEM images of the A, B and C samples as well as PVDF. As can be seen in Figure 1d 1 ,d 2 , PVDF has the shape of spheres with a diameter between 160 and 280 nm, the spheres being arranged in aggregates with the size around 3-10 µm.     like structures and the PVDF spheres. In order to support this statement in Figure 2a, we pointed with arrows to several POPD fiber like structures, PVDF spheres and DWNT bundles. In contrast with Figure 1b1,b2, where POPD fiber like structures with length 5.5-42 µm and diameter of 360-658 nm are observed, in Figure 2, the POPD fiber like structures have a diameter 500-765 nm. In order to confirm that the morphological structures shown in Figures 1 and 2 belong to POPD, PVDF and DWNTs, Figures 3 and 4 show the Raman spectra and IR spectra of the A, B, C, D, E and F samples.  Figure 2d shows the SEM image of composite based on the POPD-PVDF blends and DWNTs, when the carbon nanotubes concentration is equal to 1 wt.% (the G sample, d).    Figure 2d shows the SEM image of composite based on the POPD-PVDF blends and DWNTs, when the carbon nanotubes concentration is equal to 1 wt.% (the G sample, d).

Results
(a) (b)   Figure 3. Raman spectra of the POPD-PVDF blends labeled as samples A (black curve, (a)), B (red curve, (a)) and C (blue curve, (a)) as well as the POPD-PVDF/DWNTs composites labeled as samples D (black curve, (b)), E (red curve, (b)) and F (blue curve, (b)).  Regardless of the ratio between the concentrations of OPD and PVDF, the Raman spectra of the A, B and C samples (Figure 3a) are characterized by Raman lines peaked at 366, 889, 1250, 1369, 1421, and 1524 cm −1 which were assigned to the following vibrational modes of POPD: quinoid ring deformation, out of plane C-H bending in benzene nuclei of phenazine skeleton, C-N stretching in quinoid imide units, C-N + , phenazine like structure and C=N stretching in phenazine structure, respectively [31,32]. The Raman line peaked at around 399 cm −1 (Figure 3a) belongs to the T2 symmetrical vibrational mode of [FeCl4] − ions, which compensates the positive charges existing on the macromolecular chain of POPD [33]. The Raman lines having maxima at 1020, 1124 and 1481-1483 cm −1 (Figure 3a) are not located far from the lines calculated by the density-functional theory which were reported to be situated at 1014, 1117 and 1477 cm −1 ; they were assigned to the following vibrational modes of α-PVDF: CH2 torsion (Ag), symmetrical stretching of the C-C bond (Au) and CH2 deformation + CH2 wagging (Ag) [34,35]. According to Figure 3a, when increasing the PVDF weight in the mixture of the chemical polymerization reaction of OPD one observes: (i) an increase in the relative intensity of the Raman line situated in the spectral range 1430-1490 cm −1 ; (ii) a gradual shift of the Raman line from 1481 cm −1 (A sample) to 1483 cm −1 (B sample) and 1488 cm −1 (C sample); (iii) the ratio between the intensities of the Raman lines peaked at 1481-1488 and 1524 cm −1 varies from 0.55 (A sample) to 0.57 (B sample) and 0.67 (C sample). Taking into account that the Raman lines peaked at 1524 and 1481-1488 cm −1 are assigned to the vibrational modes of C=N stretching in phenazine structure of PVDF and CH2 deformation + CH2 wagging of PVDF, the gradual increase of the ratio between the intensities of these two lines (I1524/I1481-1488) form 0.55 (A sample) to 0.67 (C sample) indicates a higher weigh of the vibrational modes of PVDF in contrast with those of POPD. These results prove the increase of the PVDF weight in the A, B and C samples mass. In order to prove experimentally that the higher intensity of the 400 600 800 1000 1200 1400 1600  Regardless of the ratio between the concentrations of OPD and PVDF, the Raman spectra of the A, B and C samples (Figure 3a) are characterized by Raman lines peaked at 366, 889, 1250, 1369, 1421, and 1524 cm −1 which were assigned to the following vibrational modes of POPD: quinoid ring deformation, out of plane C-H bending in benzene nuclei of phenazine skeleton, C-N stretching in quinoid imide units, C-N + , phenazine like structure and C=N stretching in phenazine structure, respectively [31,32]. The Raman line peaked at around 399 cm −1 (Figure 3a) belongs to the T2 symmetrical vibrational mode of [FeCl 4 ] − ions, which compensates the positive charges existing on the macromolecular chain of POPD [33]. The Raman lines having maxima at 1020, 1124 and 1481-1483 cm −1 (Figure 3a) are not located far from the lines calculated by the density-functional theory which were reported to be situated at 1014, 1117 and 1477 cm −1 ; they were assigned to the following vibrational modes of α-PVDF: CH 2 torsion (A g ), symmetrical stretching of the C-C bond (A u) and CH 2 deformation + CH 2 wagging (A g ) [34,35]. According to Figure 3a, when increasing the PVDF weight in the mixture of the chemical polymerization reaction of OPD one observes: (i) an increase in the relative intensity of the Raman line situated in the spectral range 1430-1490 cm −1 ; (ii) a gradual shift of the Raman line from 1481 cm −1 (A sample) to 1483 cm −1 (B sample) and 1488 cm −1 (C sample); (iii) the ratio between the intensities of the Raman lines peaked at 1481-1488 and 1524 cm −1 varies from 0.55 (A sample) to 0.57 (B sample) and 0.67 (C sample). Taking into account that the Raman lines peaked at 1524 and 1481-1488 cm −1 are assigned to the vibrational modes of C=N stretching in phenazine structure of PVDF and CH 2 deformation + CH 2 wagging of PVDF, the gradual increase of the ratio between the intensities of these two lines (I 1524 /I 1481-1488 ) form 0.55 (A sample) to 0.67 (C sample) indicates a higher weigh of the vibrational modes of PVDF in contrast with those of POPD. These results prove the increase of the PVDF weight in the A, B and C samples mass. In order to prove experimentally that the higher intensity of the vibrational modes of PVDF is not trivially due to the increase of the PVDF amount, Figure 4 shows the Raman and IR spectra of the samples prepared by the mixture of the two polymers. The mass of the two polymers is equal to: (i) 0.045 g POPD and 0.045 g PVDF; (ii) 0.05 g POPD and 0.09 g PVDF and (iii) 0.065 g POPD and 0.18 g PVDF, i.e., the POPD:PVDF weight ratio is 1:1 (A1 sample), 1:1.8 (B1 sample) and 1:3 (C1 sample). In contrast with Figure 3a, the analysis of Figure 4a indicates that: (i) the ratio between the intensities of the Raman lines peaked at 1481 and 1525 cm −1 shows small variations, i.e., this ratio is equal to 0.68, 0.67 and 0.64 in the case of the A1, B1 and C1 samples, respectively; and (ii) significant differences are observed in the case of the ratio between the intensities of the Raman lines peaked at 1369 and 1481 cm −1 (I 1369 /I 1481 ) belonging to vibrational modes of POPD and PVDF; thus, the I 1369 /I 1481 ratio has a value equal to 9.23 ( Figure 4a) and 7.38 ( Figure 3a) in the case of the C1 and C samples, i.e., when the two samples have the same weight ratio of the two polymers, they being prepared by the mixture methods of POPD and PVDF and the chemical polymerization of OPD in the presence of PVDF, respectively. These results confirm that the samples synthesized by the chemical polymerization of OPD in the presence of PVDF do not correspond to a mixture of POPD and PVDF, the product of the polymerization reaction being that shown in Scheme 1. Additional results, which confirm once again this statement, are shown in Figure 4b by IR spectroscopy. In the case of the A1, B1 and C1 samples, namely when no interaction occurs during the mixing POPD with PVDF, the IR spectra should appear as a modulated sum of the contribution of the vibrational modes of the two polymers. According to Figure 4b, the IR spectrum of POPD shows an intense IR band with a maximum at 1529 cm −1 having a shoulder at 1475 cm −1 , which is accompanied by other IR bands of low intensity [17], while the IR spectrum of PVDF shows four IR bands of high intensity having the maxima at 873, 1187, 1402 and 1703 cm −1 [36,37]. The IR spectra of the A1, B1 and C1 samples in Figure 4b correspond to the sum of the IR spectra of the two polymers. By increasing the PVDF weight in the A1, B1 and C1 samples, an increase in the absorbance of the IR bands of PVDF at 873, 1187, 1402 and 1703 cm −1 is seen. In comparison with the IR spectra of the A1, B1 and C1 samples (Figure 4b), in the cases of the IR spectra of the A, B and C samples are marked significant differences both in the absorbance of the IR bands of the PVDF and in the position of the IR bands localized in the spectral range 1000-1200 cm −1 , when an up-shift of the IR bands from 1067 and 1180 cm −1 (Figure 4b) to 1076 and 1196 cm −1 , respectively (Figure 5a) occurs. In our opinion, these differences are irrefutable proofs that the A, B and C samples, prepared by the chemical polymerization of OPD in the presence of PVDF, cannot be assimilated as being similar to the samples obtained by mixing the two polymers. The chemical polymerization of OPD in the presence of PVDF involves a chemical reaction, which results in compounds like that shown in Scheme 1.
shoulder at 1475 cm −1 , which is accompanied by other IR bands of low intensity [17], while the IR spectrum of PVDF shows four IR bands of high intensity having the maxima at 873, 1187, 1402 and 1703 cm −1 [36,37]. The IR spectra of the A1, B1 and C1 samples in Figure 4b correspond to the sum of the IR spectra of the two polymers. By increasing the PVDF weight in the A1, B1 and C1 samples, an increase in the absorbance of the IR bands of PVDF at 873, 1187, 1402 and 1703 cm −1 is seen. In comparison with the IR spectra of the A1, B1 and C1 samples (Figure 4b), in the cases of the IR spectra of the A, B and C samples are marked significant differences both in the absorbance of the IR bands of the PVDF and in the position of the IR bands localized in the spectral range 1000-1200 cm −1 , when an upshift of the IR bands from 1067 and 1180 cm −1 (Figure 4b) to 1076 and 1196 cm −1 , respectively (Figure 5a) occurs. In our opinion, these differences are irrefutable proofs that the A, B and C samples, prepared by the chemical polymerization of OPD in the presence of PVDF, cannot be assimilated as being similar to the samples obtained by mixing the two polymers. The chemical polymerization of OPD in the presence of PVDF involves a chemical reaction, which results in compounds like that shown in Scheme 1.  [31,32] indicates a partial de-doping of POPD; (iii) the increasing ratio between the intensities of the Raman lines at 1483-1485 and 1521 cm −1 , assigned to the vibrational modes of CH 2 deformation + CH 2 wagging (A g ) of PVDF [34,35] and C=N stretching in phenazine structure [31,32] indicates that a chemical interaction between POPD and DWNTs takes place. A consequence of this interaction is the decrease of the weight of the C=N bond in the favor of C-N. These results can be understood if we accept that a reaction between POPD doped with FeCl 4 − ions with DWNTs takes place.
The IR bands at 3151 and 3308 cm −1 belong to the vibrational mode of stretching of NH with intramolecular association and intramolecular bond, respectively, of POPD [38]. This fact can be explained only if PVDF is covalently bonded on the POPD macromolecular chain. A puzzling fact is that an increase in the absorbance of the IR bands at 840 and 1402 cm −1 was also reported in the case of the composites of the type PVDF/single-walled carbon nanotubes when a covalent functionalization of carbon nanotubes with PVDF was reported [38]. Another fact which indicates a chemical interaction between POPD and PVDF regards the value of the ratio between the absorbance of IR bands peaked at 881 and 840 cm −1 whose value is lower in the case of A, B and C samples, i.e., 1.37, 1.27 and 1.36, than that reported in the case of PVDF (3.35) [39]. Summarizing the above variations and taking into account (i) the decrease in the absorbance of the IR band at 1240 cm −1 , (ii) the increase in the absorbance of the IR bands at 840 and 1402 cm −1 and (iii) the smaller values of the ratio between the absorbance of IR bands peaked at 881 and 840 cm −1 , we conclude that during the chemical polymerization of OPD in the presence of PVDF, the generation of new covalent bonds of the type N-CH between the two macromolecular compounds takes place according to Scheme 1.
In comparison with the IR spectrum of the B sample, the following changes are reported in the IR spectra of the D, E and F samples: (i) a change of the ratio between the absorbance of the IR bands having the peaks at 1193, 1242 and 1280 cm −1 ; this ratio is equal to: (i) 1:0.72:0.48 in the case of the D sample; (ii) 1:2.08:0.22 in the case of the E sample and (iii) 1:2.12:0.5 in the case of the F sample; (ii) a down-shift of the IR band from 3151 cm −1 (Figure 5a) to 3144 cm −1 (Figure 5b); (iii) an up-shift of the IR band from 3308 cm −1 (Figure 5a) to 3314 cm −1 (Figure 5b); and (iv) a variation in the ratio between the absorbance of the IR bands localized at 1242 and 1530 cm −1 , from 0.36 (D sample) to 0.27 (E sample) and 0.17 (F sample). This decrease of the ratio between the absorbance of the IR bands having the peaks at 1242 and 1530 cm −1 , indicates that the interaction of POPD in doped state with DWNTs leads to a composite in which the share of C-N-C bonds is higher than that existing in the doped state of POPD. This result is in good agreement with the reaction of the blend PVDF-POPD in a doped state with DWNT, which is shown in Scheme 2. The reaction, shown in Scheme 2, takes into account the property of carbon nanotubes to accept electrons when resulting in anion radical species that are unstable and react with compounds in the reaction medium. The chemical mechanism of the reaction shown in Scheme 2 is detailed in Scheme 3.
( Figure 5a) to 3144 cm −1 (Figure 5b); (iii) an up-shift of the IR band from 3308 cm −1 ( Figure  5a) to 3314 cm −1 (Figure 5b); and (iv) a variation in the ratio between the absorbance of the IR bands localized at 1242 and 1530 cm −1 , from 0.36 (D sample) to 0.27 (E sample) and 0.17 (F sample). This decrease of the ratio between the absorbance of the IR bands having the peaks at 1242 and 1530 cm −1 , indicates that the interaction of POPD in doped state with DWNTs leads to a composite in which the share of C-N-C bonds is higher than that existing in the doped state of POPD. This result is in good agreement with the reaction of the blend PVDF-POPD in a doped state with DWNT, which is shown in Scheme 2. The reaction, shown in Scheme 2, takes into account the property of carbon nanotubes to accept electrons when resulting in anion radical species that are unstable and react with compounds in the reaction medium. The chemical mechanism of the reaction shown in Scheme 2 is detailed in Scheme 3.  The second product of Scheme 2 corresponds to DWNTs on the surface of which part from the C = C bonds has been transformed into C-Cl bonds. Evidence for the new bonds of the types C-Cl and C-H is highlighted in Figure 3b, where Raman lines peaked at 829, 989 and 1068 cm −1 are observed, which were also reported in the case of dichlorobenzene [40]. The Raman lines at 829, 989 and 1068 cm −1 are assigned to the vibrational modes A 2 out-of-plane of the aromatic ring, A 1 in-plane of the C-H stretching + aromatic ring and A 1 in-plane of the C-Cl bond, respectively [36]. Figure 6 highlights that in the case of the POPD-PVDF blends, as increasing the PVDF weight in the mass of the A, B and C samples takes place: (i) a gradual decrease in the intensity of the PLE band with the maximum at 452 nm from 8.66 × 10 5 counts/sec (A sample) to 6.47 × 10 5 counts/sec (B sample) and 2.38 × 10 5 counts/sec (C sample); (ii) a significant decrease in the intensity of the PL band localized in the spectral range 450-750 nm from 1.17 × 10 6 (A sample) to 4.76 × 10 5 counts/sec (B sample) and 2.96 × 10 5 counts/sec (C sample). In the context of these variations, we note that: (i) PVDF does not show PL, when the excitation wavelength is equal to 420 nm; and (ii) a PL band localized in the spectral range 450-750 nm was recently reported in the case of POPD [41]. The above variations indicate that a POPD PL quenching process is induced in the presence of PVDF.
According to Figure 7, additional changes both in the intensity and in the shape of the PL spectra are induced to the POPD-PVDF blends in the presence of DWNTs. In contrast to sample B, the following variations of the PL and PLE spectra are observed to be induced as a consequence of the addition of an increasing amount of DWNTs in the mass of the polymerization reaction of OPD and PVDF: (i) a decrease in the intensity of the PLE band situated in the spectral range 350-550 nm from 6.47 × 10 5 counts/sec (B sample, Figure 6b 1       In order to better understand the orientation of the macromolecular chains of POPD and PVDF in the absence and presence of DWNTs, Figures 8 and 9 show the anisotropic PL spectra of the A, B, C, D, E and F samples. Thus, Figures 8 and 9 highlight the vertical (V) and horizontal (H) polarization PL spectra when the V and H excitation beams were used. Depending on how the polarizers are mounted at excitation and emission in the spectrophotometer and the calibration factor of the instrument (G), the values of the anisotropy (r) and angle of binding of the two macromolecular compounds (θ) [42], can be calculated with the following equations: In order to better understand the orientation of the macromolecular chains of POPD and PVDF in the absence and presence of DWNTs, Figures 8 and 9 show the anisotropic PL spectra of the A, B, C, D, E and F samples. Thus, Figures 8 and 9 highlight the vertical (V) and horizontal (H) polarization PL spectra when the V and H excitation beams were used. Depending on how the polarizers are mounted at excitation and emission in the spectrophotometer and the calibration factor of the instrument (G), the values of the anisotropy (r) and angle of binding of the two macromolecular compounds (θ) [42], can be calculated with the following equations: used. Depending on how the polarizers are mounted at excitation and emission in the spectrophotometer and the calibration factor of the instrument (G), the values of the anisotropy (r) and angle of binding of the two macromolecular compounds (θ) [42], can be calculated with the following equations: These results in the case of the A, B and C samples highlight that regardless of the weight of the PVDF spheres onto the POPD fiber like structures surface, a variation in θ of only 2.8 • occurs. In comparison with the B sample, the presence of DWNTs in the D, E and F composites induces a decrease in θ of~12 • . This behavior can be explained by taking into account the π-π stacking bonds established between the semi-oxidized structure of POPD and the graphitic structures of DWNTs when variations in the excitation and emission transition dipoles take place.
Summarizing the above results, the main improvements reported in this work concerning the morphological and photoluminescent properties of the POPD fiber like structures in the case of the POPD-PVDF blends and their composites with DWNT versus those of the type POPD-PEO and POPD-PEO/DWNTs [41] consist of: (i) increasing the length of POPD fiber like structures, from~1-20 µm [41] to 4-42 µm, simultaneous with decreasing their diameter from 256-790 nm to 112-650 nm, when at the chemical polymerization of OPD instead of PEO [41] is added PVDF; (ii) the intensity of the photoluminescence of the POPD fiber like structures is higher, when these are obtained by chemical polymerization of OPD in the presence of PVDF (4.76 × 10 5 counts/sec in the case of the B sample) in comparison with PEO (1.3 × 10 4 counts/sec) [41]; and (iii) the intensity of the photoluminescence of the POPD-PVDF/DWNTs composites (3.03 × 10 5 counts/sec in the case of the D sample) is higher in comparison with those reported in the case of the POPD-PEO/DWNTs composites (~5.6 × 10 4 counts/sec) [41]. These results in the case of the A, B and C samples highlight that regardless of the weight of the PVDF spheres onto the POPD fiber like structures surface, a variation in θ of only 2.8° occurs. In comparison with the B sample, the presence of DWNTs in the D, E and F composites induces a decrease in θ of 12°. This behavior can be explained by taking into account the π-π stacking bonds established between the semi-oxidized structure of POPD and the graphitic structures of DWNTs when variations in the excitation and emission transition dipoles take place.
Summarizing the above results, the main improvements reported in this work concerning the morphological and photoluminescent properties of the POPD fiber like structures in the case of the POPD-PVDF blends and their composites with DWNT versus those of the type POPD-PEO and POPD-PEO/DWNTs [41] consist of: (i) increasing the length of POPD fiber like structures, from 1-20 µm [41] to 4-42 µm, simultaneous with decreasing their diameter from 256-790 nm to 112-650 nm, when at the chemical polymerization of OPD instead of PEO [41] is added PVDF; (ii) the intensity of the photoluminescence of the POPD fiber like structures is higher, when these are obtained by chemical polymerization of OPD in the presence of PVDF (4.76 × 10 5 counts/sec in the case of the B sample) in comparison with PEO (1.3 × 10 4 counts/sec) [41]; and (iii) the intensity of the photoluminescence of the POPD-PVDF/DWNTs composites (3.03 × 10 5 counts/sec in the case of the D sample) is higher in comparison with those reported in the case of the POPD-PEO/DWNTs composites (5.6 × 10 4 counts/sec) [41]. FeCl3, N, N'- Figure 9. Polarized PL spectra of the composite materials based on the POPD-PVDF blends and DWNTs labeled with D (a), E (b) and F (c). All PL spectra were recorded at the excitation wavelength equal to 420 nm.
In order to obtain the blends based on PVDF spheres and POPD fiber like structures, the following protocol was used. In the first step, an aqueous solution of OPD (0.09 g in 20 mL H 2 O) and three anhydrous solutions of PVDF in DMF having concentrations equal to 0.045 g/mL, 0.09 g/mL and 0.225 g/mL were prepared. After mixing the solution of OPD in H 2 O with the three solutions of PVDF in DMF, a solution of FeCl 3 (2.34 mL, 0.71 M) was added to each mixture, after which it was stirred at room temperature for 5 min. resulting in a solution with a brown-red color. This reaction mixture was stored for 2 h and after that, this was filtered, washed with distilled water and then dried at 50 • C under vacuum for 24 h. As shown below, using the ratio between the concentrations of OPD and PVDF equal to 0.1, 0.05 and 0.02, the POPD-PVDF blends containing the fiber like structures of POPD doped with FeCl 4 − ions and the PVDF spheres, labeled in the following as the A, B and C samples resulted. The weight of the POPD-PVDF blends labeled as the A, B and C samples, resulted according to the above protocol, is equal to ca. 0.09 g, 0.14 g and 0.24 g. Taking into account the mass of PVDF added to the chemical polymerization reaction of OPD, we estimate that the mass of the two polymers in the case of: (i) the A sample is 0.045 g POPD and 0.045 g PVDF; (ii) the B sample is 0.05 g POPD and 0.09 g PVDF and (iii) the C sample is 0.06 g POPD and 0.18 g PVDF. The difference between the samples resulted from the chemical polymerization of the OPD monomer in the presence of PVDF and the mixtures of the two polymers, POPD and PVDF, prepared using the weight estimated to be in the A, B and C samples was highlighted by Raman scattering and IR spectroscopy.
In order to synthesize composites based on POPD, PVDF and DWNTs, we have used the above protocol, the only difference being the adding of various DWNTs weights, i.e., 5, 10 and 20 mg, in the mixture of the OPD/PVDF solutions, having the ratio between the concentrations of OPD and PVDF equal to 0.05. The resulted composites were labeled as the D, E and F samples. A composite labeled as the G sample, having the ratio between the concentrations of OPD and PVDF equal to 0.02 to which was added 10 mg of DWNTs, was also prepared.
In order to highlight the morphology of the A, B and C blends, as well as the D, E, F and G composites, SEM images were recorded with a Zeiss Gemini 500 scanning electron microscope (Zeiss, Oberkochen, Germany). SEM analyses were performed on samples as prepared in high vacuum mode with the secondary electron detector (SE2) and an acceleration voltage of 1kV.
The vibrational properties of all samples prepared in this work were recorded with (i) an FT Raman spectrophotometer, Multiram model, endowed with a YAG:Nd laser and (ii) an FTIR spectrophotometer, Vertex 80 model, both purchased from Bruker (Billerica, MA, USA).

Conclusions
In this work, we have reported new results concerning the optical properties of the composites based on POPD fiber like structures, PVDF spheres and DWNTs. Using scanning electron microscopy (SEM), Raman scattering, IR spectroscopy and photoluminescence we have demonstrated that: (i) the chemical polymerization of o-phenylenediamine in the presence of PVDF leads to the blends containing the PVDF spheres and the POPD fiber like structures; (ii) the chemical polymerization reaction of o-phenylenediamine in the presence of PVDF induces a decrease in the absorbance of the IR band at 1240 cm −1 assigned to the C-N = C vibration mode of POPD, as a consequence of generation of new C-N bonds between the two macromolecular compounds; (iii) the addition of DWNTs in the mixture of the chemical polymerization reaction of o-phenylenediamine in the presence of PVDF leads to composites of the type DWNTs covalent functionalized with the POPD-PVDF blends, a chemical mechanism being proposed in this paper (Scheme 3); (iv) the PVDF spheres induce a photoluminescence (PL) quenching process of the POPD fiber like structures; in the presence of DWNTs, a PL quenching process of the POPD-PVDF blends is reported; and (v) according to the anisotropic photoluminescence studies, a variation of the angle of binding of the two macromolecular compounds with ca. 12 • is reported to be induced by DWNTs.