Dyes Confinement in the Nano Scale and Converting Poly Vinyl Alcohol to be Optical-active Polymeric Nanocomposites with High Thermal Stability

1. Introduction

The low thermal stability of polymers is due to their hydrocarbon structures. Therefore, it can be improved through combining with inorganic materials that have high thermal stability. Actually, the inorganic materials can be employed to decrease the thermal degradation rate or increase the heat resistance [1,2,3,4]. In order to improve the optical properties of polymer, the dyes are good candidates for creating new optical centers inside the structure of polymers. In this trend, for improving the thermal properties and optical activities of polymer, the incorporation of inorganic particles or layers in the nano scale inside polymers consider an efficient technique for increasing their applications. However, the incompatibility between organic polymer and inorganic species leads to a phase separation. This challenge, to get thermodynamically stable hybrids, can be achieved through dispersing the inorganic nanomaterials inside the polymer in homogenous state. In order to get a high degree of dispersion inside polymers, the inorganic nanomaterials are covered or anchored by favorable organic compounds. These compounds have hydrophobic characters to be compatible with polymer and useful for many applications.

The astonishing physical and chemical properties of organic-inorganic nanohybrids [5,6,7,8,9,10,11] have strongly attracted attention of the scientific societies because the conventional processes cannot lead to these properties. The combination between organic and inorganic compounds in nanostructures can create unlimited groups of novel compounds through producing a wide range of unknown and known activities. In this trend, for improving the thermal properties and optical activities of polymer, the incorporation of inorganic particles or layers in the nano scale inside polymers consider an efficient technique for increasing their applications. However, the incompatibility between organic polymer and inorganic species leads to a phase separation. This challenge, to get thermodynamically stable hybrids, can be achieved through dispersing the inorganic nanomaterials inside the polymer in homogenous state. In order to get a high degree of dispersion inside polymers, the inorganic nanomaterials are covered or anchored by favorable organic compounds. These compounds have hydrophobic characters to be compatible with polymer and useful for many applications.

PVA has been largely studied because of the mechanical properties, biocompatibility, and chemical resistance [12, 13]. Therefore, it was suitable for many industrial products such as paints, adhesives, coatings and membranes [14]. Recently, PVA has used in the biomedical applications for artificial organs, contact lenses [15, 16], drug delivery systems [17, 18], engineering of tissues [19] and dialysis membrane [20]. These wide applications directed the attention of researchers for developing the optical properties of PVA and improving its thermal stability to become more favorable and promising for new applications.

Mallakpour and Dinari [21] have used alumina nanoparticles as filler for increasing the thermal stability of PVA. In another study, Titanium oxide nanoparticles were used as dopant for PVA to increase its mechanical properties [22]. Du et al. have used PVA for increasing capacitance performance through designing polymeric electrode [23]. Abd-Elrahman reported that the thermal stability PVA increased by producing composite structures with zinc oxide [24]. Farrag et al. indicated that the optical properties of PVA increased through narrowing the band gap from 4.4 eV to 2.9 eV by combining with multi walls carbon nanotubes [25]. Peranidze et al. have successfully used the composites of PVA for bone tissue engineering [26].

To create new optical sites and increase the thermal properties of poly (vinyl alcohol), new nanohybrids can be designed depending on confinement of green dyes of naphthol green B in the nano scale among inorganic nanolayered structures.

Among inorganic layered structures, there is a group of two-dimensional nanostructured materials calls layered double hydroxides (LDHs). This kind of nanolayered materials consists mainly of inorganic species [27,28,29,30,31] and has ability for combining with organic compounds as interlayered anions through host-guest interactions. The host is LDHs composing of di-and tri-valent elements in an order arrangement and their nanolayers are cationic in nature. This cationic nature of the nanolayers is neutralized by organic anions that used as guests among the layers in a confined state. The confined guests among the nanolayers of two-dimensional structures have only one dimension orientation. This confinement is not rigid because the size and orientation of guests control the interlayered spacing between the inorganic nanolayers producing hybrid lamellae. These hybrid lamellae that have widths of 0.1–1 µm and thickness of 0.5–1 nm are suitable fillers with a large aspect ratio and able to performing intercalation reactions with polymers. By controlling the organic and inorganic species of these hybrid lamellae, the dispersion of the lamellae inside polymers produces polymeric nanocomposites and increases their applications in the different fields.

In the current study, polymeric nanocomposites that are composed of inorganic and organic dye species in addition to polymer have attracted our attention because of the multi-functionality of these hybrids. To become PVA, optically active in the UV region and visible region in addition to increasing its thermal stability, new nanohybrids based on green dyes of naphthol green B were prepared through host-guest interaction with Zn-Al LDHs. Also, new series of the nanoparticles of multi-oxides doped with dyes was produced through thermal treatment. By these nanohybrids, which used as fillers for PVA, the produced polymeric nanocomposites have the possibility to become multi-functional material because they have high optical properties producing from the advantages of organic dyes in addition to the high thermal stability based on the benefits of inorganic materials.

2. Experimental

2.1. Preparation of nanohybrids and nanocomposites

A series of green and yellow nanohybrids materials based on green dyes were produced by precipitating nanolayered structures of zinc and aluminum in presence of Naphthol green B using urea hydrolysis and thermal treatment as shown in Scheme 1.

Two salts of zinc nitrate (0.046 M) and aluminum nitrate (0.018 M) were combined in presence of urea (0.5 M) in an aqueous medium. 1 gm of Naphthol green B dyes was dissolved in the prepared aqueous solution. By heating the mixture at 90 ◦C for 14 h, the green powder of Zn-Al-dyes nanohybrid was precipitated and separated through washing with distill water. Different amounts of green dyes were used to produce five green nanohybrids as seen in Scheme 1. The obtained green powder were labeled NH-1, NH-2, NH-3, NH-4 and NH-5 according to the dyes amount 1 , 2 , 3 , 4 and 5 gm, respectively. The green nanohybrid NH-5 was thermally treated at 450 ◦C to become yellow nanohybrid NH-5-450 as shown in Scheme 1. Also, the sample without dyes was prepared by the same method. This material was coded as ZA-1and used as a reference material.

2.2. Preparation of polymeric nanocomposites

The pure poly (vinyl alcohol) 87–90% hydrolysed, average molecular weight = 30000-70000, was obtained from Sigma-Aldrich. Two series of PVA nanocomposites were engineered by using one of the prepared nanohybrid before and after calcination. The nanohybrids NH-5 and NH-5-450 were selected to be used as fillers for PVA because they have the high content of dyes confined inside their nanolayers. With noting that the color of NH-5 was green while the powder of NH-5-450 has yellow color.

In the first series of the PVA nanocomposites, 0.5 gm of PVA was dissolved in 15 mL of de-ionized water with heating at 70 ◦C for 2 h under strong stirring to get a clear viscous solution. Similarly, 2 wt. % of NH-5 was dispersed and ultra-sonicated in de-ionized water. The suspended solution of NH-5 was mixed with the hot solution of PVA at 70 ◦C by magnetic stirring for obtaining homogeneous green solution of PVA. A thin green film was produced by pouring the green solution in a flat dish at room temperature. Then, the obtained films were coded as NCP-1, NCP-2 and NCP-3 according to the nanohybrid percentages 2, 10 and 20 wt.%, respectively.

For the second series of the PVA nanocomposites, the nanohybrid NH-5 was used as filler after calcination at 450 ◦C. By calcining the green powder of the nanohybrid, the color has changed to become yellow. This filler was coded by NH-5-450. The same procedure was used by adding the suspended solution of the filler NH-5-450 to the hot solution of PVA under magnetic stirring for getting a homogeneous yellow solution. By pouring the yellow solution in a flat dish at room temperature, a thin yellow film was fabricated. The obtained films were coded as NCP-4, NCP-5 and NCP-6 according to the calcined nanohybrid percentages 2, 10 and 20 wt.%, respectively.

2.3. Characterization

Scanning electron microscopy JEOL-JSM-6330F (Japan) was used for determining the morphology of the prepared samples. Transmission electron microscopy was used by JEM 2100F (JEOL Company, Tokyo, Japan). Powder X-ray diffraction technique Bruker-AXS, Karlsruhe (Germany) has used Cu-K radiation source (λ = 0.154 nm) for determining the crystalline structures of the prepared nanomaterials. Thermogravimetric analyzer (Q500 TA Company, New Castle, PA, USA) has used for measuring thermal properties. Depending on the diffuse reflectance spectroscopy, the optical parameters of the prepared powder and films materials were measured using UV/VIS/NIR Shimadzu 3600 spectrophotometer (Shimadzu, Columbia, MD, USA) in the range 200–800 nm.

3. Results and discussion

3.1. Confinement of naphthol green B in Zn-Al nanolayerd structures

Plate-like structure is the conventional structure for the synthetic LDHs and the natural materials of LDHs (pyroaurite) exhibiting plates with dimensions in the range of millimeters [32]. In addition, hexagonal platy morphology was observed for the hydrotalcite crystals [33]. The morphology of the sample ZA-1 agrees with the conventional structure of the synthetic and natural LDHs as shown in SEM and TEM images in Figure 1.

Figure 1a revealed SEM image for ZA-1 indicating rounded plates. However, hexagonal nano-plates with few hundred nanometers in width were observed in the TEM image of ZA-1 as shown in Figure 1b corresponding with the hydrotalcites structures because TEM images show the individual plates of LDHs and SEM images focus on aggregations or groups of plates.

These SEM and TEM images concluded that the sample ZA-1 is belonged to the LDH group. By intercalating different percentages of green dyes, strong aggregation of plates was observed as shown in Figure 2. SEM image of NH-1 nanohybrid showed large cycles as seen in Figure 2a. These cycles consist of large number of nanoplatelets that aggregated through self assembly behavior. This behavior was originated by generating hydrophobic character for the nanoplatelets of Zn-Al LDH. The hydrophobic character was produced by bonding the aromatic molecules of the green dyes (two benzene rings) with the nanoplatelets of Zn-Al LDHs through host-guest interaction. By increasing the percentage of green dyes, Figure 2c indicated that the hydrophobic character enhanced as seen in SEM image of NH-3. The self assembly behavior of Zn-Al-dyes was clearly observed through magnifying the SEM image of NH-5 in Figure 2b.

By thermal treatment of NH-5 at 450 °C, the green powder of the nanohybrid which based on the naphthol green B converted to yellow powder because of partial decomposition of the green dyes. Figure 3 showed SEM images of the new nanohybrid NH-5-450.

SEM images indicated that the nanohybrid NH-5-450 has individual nanoplates as seen in Figure 3a. It means that the yellow nanohyrid has lost part of the hydrophobic character. Also, Figure 3b showed that the size of plate is 111 nm. By using X-ray diffraction, nanolayered structure of ZA-1 was observed as shown in Figure 4a. The values of the basal peaks of planes hkl [003], [006] and [009] of ZA-1 showed an order arrangement of the packed stacks of brucite-like layers ordered along axis c; i.e., 0.76 nm = 2×0.38 nm = 3×0.26 nm.

According to the c dimension of the natural and synthetic hydrotalcite, which is assessed as three times the spacing for planes (003) and equals 2.31 nm [31], it nearly agrees with the calculated value for the prepared material ZA-1, i.e., 2.28 nm. Depending on the spacing for plane (110), the average distance between Zn-cation and Al-cation within the brucite-like layer, which is related to the lattice parameter ‘a’ of the prepared material and can be determined as two times of d(110), could be calculated to be 2×0.153 nm = 0.306 nm agreeing with the reported data of LDHs [31]. These parameters confirmed that the sample ZA-1 has LDH structure.

The main peak of ZA-1 which observed at 0.76 nm indicates the size and the orientation of the interlayered anions inside the layered structure of LDH. By comparing with the layered structure of the synthetic Zn–Al LDH (JCPDS file No. 48-1022) and the natural hydrotalcite (JCPDS file No. 37-629), the main interlayerd anions of the prepared material is carbonate anions as shown in Figure 4a.

The XRD pattern of the ZA-1 after intercalation with 3% of green dyes exhibited new weak reflections at lower 2Ѳ in addition to the original basal spacing of LDH suggesting formation of two phases; LDH and nanohybrid material as shown in Figure 4b.

The interlayer spacing of the LDH after intercalation reaction with green dyes increased to be 1.5 nm. In addition, there are two other weak peaks at 0.7 nm and 0.46 nm. The successive reflections by basal planes, i.e., 1.5 nm ≈ 2 × 0.7 nm ≈ 3 × 0.45 nm, indicated layered structure for the prepared nanohybrid. The reflection of (110) became unclear. In addition, the intercalation of organic species decreased the order arrangement of the layered structure. By increasing the percentage of green dyes through intercalation reactions, the layered structure of LDHs disappeared as shown in Figure 4c-f. It means that the presence of aromatic molecules among the layers of nanohybrid led to distortion of the nanolayers because of the steric hindrance and repulsion of the benzene rings inside the interlayered space. It means that the absence of reflections corresponding to the nanolayered structure of LDH after organic modification is due to disappearance of coherent conditions in different directions (lack of repeated units).

The presence of organic molecules inside LDH structures and the formation of organic-inorganic nanohybrids were confirmed by the data of thermal analyses (TG and DTG). Thermal characteristics of the sample ZA-1 were measured by the diagrams of both TG and DTG as shown in Figure 5a. The weight of ZA-1 was mainly lost through two stages. In the first stage, 9 wt.% was lost at 192 °C corresponding to the desorption of surface and intercalated water. The second stage was achieved at 323 °C to lose 18 wt.% agreeing with the removal of hydroxyl groups of LDHs and decomposition of anions. In the DTG diagram, three peaks were observed at 172 °C, 212 °C and 285 °C as shown in Figure 5a. At first peak, removal of both surface and interlayered water happened at 172 °C agreeing with TG data. The other two peaks belonging to the removal of hydroxyl groups of LDHs and decomposition of anions occurred at 212 °C and 285 °C matching with the second stage of TG diagram.

Thermal characteristics of Zn–Al-dyes nanohybrids were observed in Figs.5b-f indicating three stages for weight losses. The first and second weight losses in all samples are due to the removal of water molecules and decomposition of carbonate anions; respectively. The third weight loss confirms the intercalation of aromatic rings of the green dyes inside LDH structures.

In case of the sample NH-1, the DTG diagram showed new peak at 517 °C agreeing with the third weight loss in the TG diagram as seen in Figure 5b. By increasing the percentage of green dyes during the intercalation reactions of the samples from NH-1 to NH-5 , the third weight loss in TG curves increased from 5% to 13% ; respectively. In addition, the DTG diagrams showed that the intensity of the peak at 519 °C increased arriving to maximum at sample NH-5 as shown in Figs.5.

By comparing with the parent LDH (ZA-1), the content of the interlayered water of the nanohybrids decreased from 9% to 6%. Also, new peaks were observed in the TG diagram at the temperature range 262-266 °C and 517-557 °C for the prepared nanohybrids NH3-NH5 confirming the intercalation of organic species instead of carbonate anions.

3.2. Colored polymeric nanocomposites

According to the above results, the sample NH-5 considers the most suitable nanohybrid to be used a filler for building colored polymeric nanocomposites because it has organic–inorganic nanostructures and contains 13 wt% of green dyes.

Therefore, this nanohybrid has used as a filler for poly vinyl alcohol through two different ways. The first way depends on intercalation of the nanohybrid before calcination. In the other way, the nanohybrid has been used after calcination at 450 °C.

By using the first way, three polymeric nanocomposites were produced using different percentages of the nanohybrid. NCP-1, NCP-2 and NCP-3 were prepared by intercalating 2 wt.%, 10 wt.% and 20 wt.% of the nanohybrid inside PVA. The XRD pattern of the pure PVA was observed in Figure 6a revealing a weak peak at 2θ = 19° (d-spacing = 0.45 nm) indicating the characteristic peak of the plane [101] of PVA. Also, This peak indicates the intermolecular hydrogen bonds that connect between the chains of PVA through hydroxyl groups [34, 35].Also, Figure 6c-e showed the XRD patterns of the nanocomposites NCP-1, NCP-2 and NCP-3. For the nanocomposite NCP-1, Figure 6c showed amorphous structure indicating that the low percentage of the nanohybrid (2 wt.%) is homogeneously dispersed inside the matrix of PVA. The disappearance of the diffraction maximum of the nanohybrids may be the result of the subsequent increase in interlayer periodicity and the shift of the maximum to the low-angle region beyond the range of the diffractometer used.

In case of the high percentages of the nanohybrid, new nanolayered structures were observed for the polymeric nanocomposite NCP-2 and NCP-3 as shown in Figure 6d, e. X-ray diffraction patterns of NCP-2 and NCP-3 showed weak peak at 2.1 nm and clear peak at 0.75 nm in addition to the peak of PVA. It means that the polymer chains of PVA have intercalated among the nanolayers of the nanohybrids and expanded the inlayered spacing of the nanocomposites to become 2.1 nm. Comparing with the X-ray patterns of the nanohybrid and the parent PVA, the intercalation process increased the crystalline structure of PVA creating new phase of polymeric nanocomposite Zn-Al- dyes-PVA

In case of using the second way of intercalation process, another three samples of polymeric nanocomposites NCP-4, NCP-5 and NCP-6 were prepared through inserting 2 wt.%, 10 wt.% and 20 wt.% of the calcined nanohybrid inside PVA. The X-ray diffraction of the calcined nanohybrid showed that it has zinc oxide structure agreeing with wurtzite crystals (JCPDS 36-1451) as shown in Figure 7b. It means that the calcined nanohybrid has zinc oxide structure doping with aluminum and the products that produced from the decomposition of green dyes. By intercalating the low percentage of the calcined nanohybrid, the peaks of zinc oxide disappeared as shown in Figure 7c. X-ray diffraction pattern of NCP-4 showed only weak peak at 0.45 nm indicating that the nanohybrid layers are completely and uniformly dispersed in the structure of PVA. By increasing the percentage of the calcined nanohybrid, new weak peaks were observed in the XRD patterns of both NCP-5 and NCP-6 as seen in Figs.7d, e. Figure 7d showed two peaks at low two theta with d-spacing at 2.0 nm and 1.1 nm. It means that the nanolayers of the nanohybrid were re-arranged to build the nanolayered structure again in presence of polymer chains depending on the phenomenon of memory effect of LDH structure. This phenomenon is familiar in the LDH structure. After heating the LDH at temperature 450 °C for a few hours, the nanolayered structures of LDHs are converted into mixed oxides, but still have memory for their own structure [36, 37]. These mixed oxides, after dispersion in an aqueous solution containing preferable anionic moieties followed by some degree of aging, revert to the original layered structures of LDHs.

3.3. Thermal Stability

The measurements of TGA and DTG have been used for studying the effect of green and yellow nanohybrids on the thermal stability of PVA through building nanocomposites as shown in Figure 8, Figure 9, Figure 10 and Figure 11.

Figure 8 indicates the different stages of thermal decay of pure PVA and the nanocomposite NCP-3. Figure 8a showed that the main weight of the pure PVA (80%) lost at 372 °C through two stages. In case of the nanocomposite NCP-3, the TG curve showed that it has four stages for losing 80% of NCP-3 at 820 °C.

With noting that the temperature corresponding to the first loss 5% (T0.05) of the pure PVA is at 105 °C. This temperature was shifted to higher value at 175 °C for the nanocomposite NCP-3. The final residue of NCP-3 which obtained at 1000 °C was 2.6% while, there is no residue for the pure PVA. These data confirmed the positive effect of nanohybrids on the thermal stability of the PVA.

Similar results were observed for the nanocomposites NCP-1 and NCP-2. Figure 9 showed that the temperature of the initial 5% weight loss of both NCP-1 and NCP-2 occurred at 250 °C and 210 °C. In the same trend, the nanocomposites NCP-1 and NCP-2 have lost 80% of its weight at higher temperature (457 °C and above 800 °C) comparing with the pure PVA.

In case of the second series of the nanocomposites NCP-6, NCP-5 and NCP-4, the thermal stability became higher than the pure PVA agreeing with the first series of the nanocomposites. Figure 10 showed that 50% of the nanocomposite NCP-6 was lost at 414 °C while for the pure PVA was observed at 322 °C. For the weight loss 70%, figure 10 showed similar behavior because it achieved at 796 °C for NCP-6 while it was observed at lower degree for the pure PVA at 338 °C. Figure 11 indicated that the nanocomposites NCP-4 and NCP-5 have higher thermal stability than the pure PVA.

The decomposition of the chains of polymer starts at their ends or the weak bonds through formation of free radicals. After that the degradation process continues by transferring to the adjacent chains via inter-chain reactions. In the prepared nanocomposites, the presence of the inorganic nanolayers slowed down the mobility of the chains of the polymer because of the hydrogen bonds which generated between the PVA chains and nanohybrid materials to improve the thermal stability of polymer [38].

3.4. Optical Properties

It is known that poly (vinyl alcohol) has no clear optical behavior. In order to improve its optical properties, six samples of its nanocomposites were prepared and studied by the important details about their absorbance and band gaps that determined through the UV-Vis absorption spectra. Figure 12 showed the UV-Vis absorbance and band gap of the pure PVA. Two weak absorption peaks are recorded at 200 nm and 300 nm indicating that it has wide band gap as shown in Figure 12.

By plotting the incident photon energy (E) and the absorbance coefficient of the materials (Abs), the energy band gap (E_g) was assessed through the following procedure [39]:

(Abs.E)² = constant (E - E_g)

Where E = hc/λ; the values (h and c) are the Planck’s constant and speed of light. The value (Abs) means the absorption coefficient.

In order to determine the band gap energy, both (Abs.E)² and (E) were plotted. When the value (Abs.E)² equals zero, the energy matches with the optical band gap. It is achieved through extending the straight line to the (E) axis, as shown in Figure 12. Figure 12 indicated that the pure PVA has band gap energy at 5.5 eV.

In case of the nanocomposite NCP-1, Figure 12b(inset) revealed that the optical properties of PVA improved through shifting the absorption edge of PVA to 800 nm. Also, the absorbance range become broader in the UV region to cover the range between 500 nm to 200 nm as shown in Figure 12b(inset). It means that the low content of nanohybrid created new optical sites inside the PVA matrix to become active in the visible and UV regions after building the nanocomposite. These results were confirmed by calculating the band gap energy. Figure 12b showed that the nanocomposite NCP-1 has band gap energy at 4.3 eV.

By increasing the content of the nanohybrid inside PVA, more improvement happened for the polymer as shown in the spectra of both NCP-2 and NCP-3. Figure 13(inset) showed the absorption spectra and band gaps of both NCP-2 and NCP-3. Clear absorption was observed in the UV region in the range of 450 nm to 200 nm. Also, in the visible region, strong absorption was observed in the range of 600-800 nm. In addition, the band gap decreased to be 2.2 eV for both NCP-2 and NCP-3 as seen in Figure 13.

In the second series of the nanocomposites, another trend for improving the optical behavior of PVA was observed for the samples NCP-4, NCP-5 and NCP-6. In the sample NCP-4, the absorption edge of PVA shifted to higher wavelength 400 nm as shown in Figure 14a(inset). It means that the PVA became active in the UV region. Figure 14a showed that the band gap of NCP-4 was observed at 3 eV. In the same trend, the sample NCP-5 showed more shifting for UV absorption to arrive at 500 nm indicating that it has band gap at 3 eV as seen in Figure 14.

By increasing the content of the nanohybrid in the second series, Figure 15 showed continuous shifting for the absorption edge of NCP-6 to be at 700 nm in the visible region. It means that NCP-6 became active in the visible region because its band gap decreased to become 2.5 eV.

This lowering of the band gap of PVA can be explained through formation of new optical sites inside the polymer structure because of the presence of the optical active dyes combined with optical inorganic nanolayers. The band gap energy of the pure PVA (5.5 eV) is very wide because the large gap between the HOMO and LUMO bands of PVA. For the prepared nanocomposites, the presence of the nanolayers of the Al-ZnO, which coated with dyes molecules, created trap levels between the HOMO and LUMO bands of PVA producing dangling orbitals. In the same time, it increased the band tails in the polymeric nanocomposites. Therefore, the band gap strongly decreased to become 2.5 eV. This explanation agrees with many previous studies [40,41,42,43,44,45].

4. Discussion

It is known that poly vinyl alcohol is a colorless polymer and has absorbance bands only in the deep UV range as shown in Figure 12a. In the visible region, there are no any absorbance bands, so, it is considered optically inactive.

Recently, Alrowaili et al. [40] have used organic carmine dye to increase the optical properties of PVA. They reported that the bandgap of PVA decreased from 5.16 to 1.65 eV as the organic dye percentage increased. Aziz et al. [41] have used inorganic species for improving the optical parameters of PVA through decreasing the band gap energy of PVA/zinc oxide nanocomposite to reach 2.80 eV.

Therefore, in the current study, both organic dyes and inorganic species have combined through nanohybrid structures that were formed through confining Naphthol Green B among nanolayers of Zn-Al layered structures to produce green nanohybrids as shown in Scheme 1. By thermal treatment of the prepared green nanohybrids, the green color of the nanohybrid transformed to yellow color indicating partial oxidation of the confined green dyes and producing new yellow nanohybrids as seen in Figure 16.

By inserting the green nanohybrids, the inorganic Zn-Al nanolayers , which attached with organic molecules of the dye, were homogenously dispersed among the hydrocarbon chains of PVA through exfoliation process as seen in SEM images in Figure 17. In addition to the compatibility between the hydrophobic character of both the organic dye and the hydrocarbon chain of PVA, the hydrogen bonds were formed between the hydroxyl groups of the Zn-Al nanolayers and the hydroxyl groups of the polymer to produce green and transparent polymeric nanocomposites as shown in Figure 17.

X-ray diffraction confirmed the arrangement of the inorganic Zn-Al nanolayers between the PVA chains through observing the crystalline structure of polymeric nanocomposite NCP-2 although the PVA structure is non-crystalline as shown in Figure 6. In case of using the yellow nanohybrids, the zinc oxide nanoplatelets, which were modified by aluminum and organic species, were well dispersed inside the matrix of PVA as seen in SEM images in Figure 18. Also, X-ray diffraction showed that the nanolayered structures were re-arranged through memory effect of Zn-Al LDHs after combining the yellow nanohybrids with PVA as shown in Figure 7.

According to these polymeric nanocomposites, PVA nanocomposites became optically active because they have clear absorbance bands in UV and visible regions. Also, the band gap energy decreased from 5.5 eV to reach 2.2 eV and 2.5 eV after combining with the green nanohybrids and the yellow nanohybrids; respectively. The absorbance in visible region can be explained through the transitions from the ground state S_o of the dye to the excited state S₁ [42, 43]. In addition, the narrowing in the band gap energy is due to forming trap levels between HOMO and LUMO bands of PVA producing dangling orbitals, increasing the band tails in the polymeric nanocomposites [44, 45].

Also, these polymeric nanocomposites improved the thermal stability of poly vinyl alcohol as shown in Table.1. Where, the green and yellow nanohybrids reduced the mobility of the chains of the polymer through formation of hydrogen bonds with the chains of PVA. By this way, the free radicals mechanism for degradation of the chains of polymer, which starts at their ends or the weak bonds, did not continue to the adjacent chains [38]. Therefore, the degradation temperature of the nanocomposites shifted to higher values as shown in Table. 1. It can conclude that the nanohybrids have dual function for improving the thermal stability of PVA with generating new optical behavior.

5. Conclusions

In the current study, the confinement of the naphthol green B in the nanoscale inside two-dimensional Zn-Al nanostructure has used to achieve dual effect for poly vinyl alcohol. The first effect was conversion non optical-active PVA to become optical active polymeric nanocomposites in a wide range of wavelength. In the second effect, the high thermal stability was observed for these polymeric nanocomposites. This dual effect was clear after inserting the green nanohybrids which used as prepared or the yellow nanohybrids which produced after thermal treatment inside the matrix of PVA. In case of using the green nanohybrids, strong narrowing for the band gap energy of PVA was observed from 5.5 eV to 2.2 eV. By using the yellow nanohybrids, the energy band gap decreased to become 2.5 eV. The other effect of the confined dye among the inorganic nanolayers was conversion of the hydrophilic character of inorganic species to hydrophobic behavior to be compatible with the hydrocarbon chains of PVA. This effect led to well dispersing for the inorganic nanolayers inside PVA matrix and improving the thermal properties of the prepared polymeric nanocomposites. Where, the prepared nanocomposites have lost 80% of its weight at higher temperature (from 457 °C to above 800 °C) comparing with the pure PVA (372 °C) because the presence of the inorganic nanolayers decreased the thermal degradation process through reducing the mobility of the polymer chains.