Synthesized and Designed New Modified Poly(vinyl chloride) Structures to Enhance Their Photo-Resistance Characteristics

Abstract

We synthesized and designed four new modified poly(vinyl chloride) structures to develop various polymer derivatives to enhance their photo-stability. Commercially available poly(vinyl chloride) was synthetically modified into a polymer containing aromatic Schiff base moieties. First, the PVC polymer was reacted with ethylene diamine from one side by using an equivalent of extra ethylene diamine to increase the probability of preventing cross-linking versus a two-sided reaction. In the second step, the corresponding aldehydes were reacted with an amino group to create the Schiff base new molecules. Different approaches were applied to confirm the structures of synthesized modified polymers such as FTIR, ¹H NMR, and EDX. Using these methods, the structures and the percentage of modification for the studied materials were demonstrated. The percentage of modification exhibited between 30–35% using integration in the ¹H NMR spectra and EDX analytical composition percentage of elements. In order to lessen the photo-degradation of the PVC polymeric chains, synthetic compounds have been employed as photo-stabilizers of the PVC films. In order to assess the effectiveness of modified PVC as a photo-stabilizer, a variety of techniques were used, including the weight loss method and monitoring the development of different functional groups while being exposed to radiation using FTIR spectra. Utilizing atomic force microscopy (AFM), scanning electron microscopy (SEM), and microscope pictures, the surface morphology of produced polymers was also investigated. These experiments demonstrated that novel modified Schiff base polymers perform as excellent photo-stabilizers in comparison to pure PVC. As a result, after being exposed to UV light for 300 h, PVC films exhibit less photo-degradation than blank PVC, so that light in the UV range may be absorbed by the connected units’ high conjugation mechanism.

1. Introduction

One of the most popular polymers is poly(vinyl chloride) (PVC), along with its modified derivatives, because of its practical qualities and reasonable price [1,2,3,4]. This polymer is present in around one-third of the world’s trashed plastics [5,6]. A material with great economic and technical importance is PVC, as it is the second most manufactured material in the world, behind polyethylene [7]. Making materials less biodegradable and more sustainable by chemical modification is a beneficial technique that has been exhibited in many studies [8,9,10]. How the chemical structure changes of the PVC backbone could influence the photochemical properties of PVC was studied by Yousif et al. [11,12,13]. Due of its extremely low melting point and poor thermal stability, it is essential to include proper stabilizers to prevent polyvinyl chloride from degrading during thermal or photo treatment. Tin(IV) complexes are the primary groups of thermal stabilizers to date [14]. Some disadvantages of using these thermal stabilizers include their high cost, toxicity, and environmental pollution. Due to the global upsurge in environmental consciousness, emphasis is presently being made on nontoxic, environmentally safe, and reasonably priced thermal and photo stabilizers. Soft PVC, one of the top five polymers used worldwide, is frequently used in children toys, baby products, food packaging, and medical equipment. To improve the functionality, mechanical, and thermal properties of PVC, additives are added as a UV stabilizers that have the power to reduce the rate of photo-oxidation in polymeric materials [15]. Thermal and photo-stability, cost, and mechanical characteristics are a few things to think about while selecting additives. Injurious UV radiation should be able to be absorbed by the additives, which should then gradually release the energy as heat away from the polymers. In outdoor applications, less photo-stable polymers, such as polyvinyl chloride (PVC), polystyrene (PS), and polyamides, have a short lifespan. UV blockers are required for outdoor use [16]. The polymer additives perform the following tasks: UV absorbers, excited state deactivators, hydroperoxide decomposers, and radical scavengers [17]. PVC contains chlorine atoms; therefore, the secondary valency forces generated by the dipoles throughout the polymer chain reduce the chain’s flexibility. The van der Waals forces have little impact on cohesion in PVC chains because of the relative bulkiness of the chlorine atoms. Polarized groups found in plasticizers connected to polymer dipoles, combined with non-polar moiety, act as barriers between the dipoles. There is less overall cohesion, less dipole bonding between polymer chains, and more flexibility in movement as a result [18]. In this work, the PVC structure was modified in two steps, first connecting ethylene diamine from one side and then forming Schiff base compounds to be used as photo-stabilizers.

2. Materials and Methods

2.1. Materials

All chemicals utilized in this project were of the highest quality and did not need to be further purified.

2.2. Instruments

Infrared Fourier transform spectroscopy (FTIR) spectra were calculated utilizing a cutting-edge device (Bruker Alpha ATR-FTIR) at frequencies ranging from 4000 to 400 cm⁻¹. With the help of this device, it is possible to record the infrared spectra of both plain liquid compounds and solid compounds without the need for KBr discs or organic solvent dilution. The Department of Chemistry at Al-Nahrain University has access to this useful FTIR apparatus. Spectra from the 400 MHz Bruker Avance spectrophotometer were used to record the proton nuclear magnetic resonance (¹H NMR) spectrum. Nevertheless, the ¹³C–nuclear magnetic resonance (¹³C–NMR) spectra were established. Tetramethylsilane (TMS), which has an internal standard of 0.00 ppm, was utilized to test the compounds. The prepared chemical was dissolved in deuterated dimethyl sulfoxide (DMSO-d⁶). Every NMR measurement was performed at Basra University, Iraq.

2.3. Synthesis

2.3.1. Synthesis of Modified PVC Based on Ethylene Diamine (PVC-ed)

PVC (5 g, 79 mmol) was dissolved in 40 mL THF before being combined with ethylene diamine (53 mL, 790 mmol). The density of ethylene diamine is 0.899 g/mL. Pyridine (0.25 mL) and all of these substances were measured in a 250 mL round bottom flask. The reaction mixture was heated at reflux while being stirred for 3 h. After the reaction finished, distilled water (40 mL) was then added to the mixture to precipitate the polymer, which was then removed by filtering and rinsed with water (20 mL × 5), and then a 20 mL (3 times) water-to-methanol combination (2:1 v/v) was finally rinsed with 10 mL of methanol. It was left under a vacuum to dry at room temperature to produce pure light yellow modified polymer 3.82 g, 76.4% (PVC-ed).

2.3.2. Synthesis of PVC-ed Schiff-Bases (General Procedure)

THF (20 mL) was used to dissolve the polymer (PVC-ed) (2 g, 22 mmol) since it was left to stir at 50 °C until fully dissolved. Then, (2.7 mL, 24 mmol) of corresponding benzaldehyde and a few drops of acetic acid were added. After that, it was left to stir at reflux for 3 h, and then, the reaction was cooled down, and 20 mL of water was added to obtain the polymer residue. After being separated by filtering, the orange polymer was rinsed with ethanol a few times. Overnight, it was vacuum-dried at room temperature to produce modified Schiff base PVCs (PVC-ed/R) with good percentage yields between 70–80%.

2.4. Preparation of Poly(vinyl chloride) (PVC) Film (Blank)

Poly(vinyl chloride) PVC from Petkim was the polymer matrix employed in this investigation (K value = 67, degree of polymerization = 800), (Turkey). In a 50 mL round-bottom flask, 15 mL of THF was added to 0.8 g of pure PVC. The mixture was warmed up for 1 h while being stirred and then placed in an ultrasonic mixer for 1 h until fully dissolved. To make polymer films with a 40 mm thickness, the mixture was poured into glass plates using the drop-casting method since the solvent was left to evaporate overnight during a 24-h period at room temperature to eliminate any potential tetrahydrofuran solvent residue [19]. The film’s thickness was measured by a micrometer type 2610 A, Germany.

2.5. Doping PVC Film with 0.04 Blend Modified Mixed of PVC-ed/R

In a 50 mL round-bottom flask, 15 mL of THF was added to 0.8 g of pure PVC. The mixture was warmed up for 1 h while being stirred and then placed in an ultrasonic mixer for 1 h until fully dissolved. After that, 0.04 g of synthesized modified polymer (PVC-ed/R) was added and left to stir under the same conditions until fully dissolved. To make polymer films with a 40 mm thickness, the mixture was poured into glass plates using the drop-casting method since the solvent was left to evaporate overnight for a 24-h period at room temperature to eliminate any potential tetrahydrofuran solvent residue.

2.6. Photodegradation Measuring Methods

2.6.1. Irradiation of PVC Films by UV Light-Accelerated UV Weathering

Using an Accelerated Weather-Meter QUV Tester (Philips, Saarbücken, Germany), the blank and blended PVC films were subjected to lambda maximum light at 313 nm for 300 h at room temperature.

2.6.2. By FTIR Spectrophotometry

Monitoring Fourier transform infrared spectra allowed researchers to determine the extent of photo-degradation of polymer film samples. By keeping an eye on the variations in hydroxyl, carbonyl and alkene peaks, the photo-degradation process was tracked as it occurred throughout various irradiation durations. Then, carbonyl (I_C=O), polyene (I_C=C), and hydroxyl (I_OH) indices were calculated using Equation (1). The band index approach is the name of this technique [20].

$$Is=\frac{As}{Ar}$$

(1)

where As stands for intensity of the studied band, Ar is the intensity of the reference peak, and Is is an index of the studied group.

2.6.3. By Weight Loss

By comparing the weight loss % of irradiated PVC films in the existence and without stabilizers, the stabilizing efficacy of the stabilizer was ascertained. The following equation was used to determine the weight loss measurements:

$$Weight Loss\%=\frac{W1-W2}{W1}\times 100\%$$

(2)

Thus, W₁ is the weight of the PVC film before irradiation, and W₂ is the weight after exposure to UV light [20].

2.6.4. By Morphological Study

We employed atomic force microscopy, scanning electron microscopy, and a microscope to look at the surface morphology of the irradiated polymer films that depicts the irradiated films’ top surface. The surface morphology of the PVC films that have not been exposed to radiation and the PVC films that have been irradiated was investigated for 0 h and 300 h. In order to compare the surface morphology of polymeric films before and after exposure to UV radiation, a MEIJI TECHNO microscope apparatus was employed. SEM (scanning electron microscopy) was carried out on synthesized complexes utilizing the TESCAN MIRA3 LMU system (Kohoutovice, Czech Republic) (15 kV). However, following exposure, PVC film morphology was measured using a ZEISS at a 10 kV accelerating voltage. In Tehran, Iran, these measurements were made. Using two- and three-dimensional pictures, atomic force microscopy (AFM) was used to examine the surface morphology of PVC films. For this work, the AA6880 Shimadzu flame atomic microscopic (Japan) was used. These experiments were carried out using atomic force microscopy on a Veeco machine producing PVC polymeric films before and after exposure to UV light at the Ministry of Industry and Minerals in Baghdad, Iraq.

3. Results and Discussions

Since its first discovery in the nineteenth century, polymer researchers have continued to study poly(vinyl chloride). Its characteristics, such as its thermal instability, are delicate in that its structural flaws result from it containing a chlorine atom that is sensitive to UV light, making it a good leaving group. Many researchers suggested adding various types of organic and inorganic additives as photo and/or thermal stabilizers of poly(vinyl chloride). Recently, this work has been expanded by extensive ongoing research to modify the chemical structure of poly(vinyl chloride) by replacing part of the chlorine atoms with other molecules that could enhance its stability against weathering circumstances. The last was carried out not only for common chemical reactions but also to improve the characteristics of poly(vinyl chloride) in particular applications. In this chapter, we exhibit the synthesis and full characterization of four new modified PVC materials through carbon–nitrogen linkage based on the formation of a Schiff bases group.

3.1. Synthesis of Modified Aromatic Schiff bases PVCs

The four novel modified polymers were created in a two-step process. In the first stage, PVC was functionalized by substituting ethylene diamine (ed) for the labile chlorine atoms utilizing a few drops of pyridine as a catalyst to produce a polymer containing primary amino groups, or PVC-ed, as seen in Scheme 1. PVC (1 mole) was mixed with five moles of ethylene diamine in this process. The two amino groups of the ethylene diamine molecule can condense with nearby chlorine atoms to generate cyclic derivatives, but this excess can also reduce the development of crosslinking between distinct polymer chains. Since it has been noted that cross-linking happens when PVC is treated with ethylene diamine at elevated temperatures for an extended period, the reaction was first carried out at room temperature to prevent any cross-linking [21]. Since the reaction was likewise carried out under heterogeneous circumstances, ethylene diamine was combined directly with PVC powder without the need of any solvent. The technique was determined to be optimized in order to obtain a higher modification percentage because the current modification % was too low. PVC was dissolved using the least quantity of THF possible, and 10 equivalents of ethylene diamine were utilized instead of 1 equivalent of PVC. In addition, for 3 h, 50 °C temperature was employed during the process rather than room temperature. The modification percentage was enhanced to ca. 30–35% as demonstrated by ¹H NMR. Furthermore, the crude product was easily purified by filtration and was washed by methanol a few times to obtain pure light yellow PVC-ed.

The second stage produced the appropriate modified polymer Schiff bases as illustrated in Scheme 1 by reacting the respective aldehydes with the aminated polymer (PVC-ed) in the presence of the catalytic reagent acetic acid in THF under refluxing conditions for 3 h. The PVC was first dissolved in the least amount of THF possible before the appropriate aldehyde was added. Adding water to the reaction mixture to precipitate the polymer and repeatedly washing the residue with ethanol to eliminate any lingering residues of the starting aldehydes allowed the crude product to be easily refined. As the Schiff base formed, the color of the polymer changed from bright yellow to dark yellow.

3.2. Characterization of Synthesized Modified PVC Polymers

3.2.1. Utilizing Fourier Transform Infrared Spectroscopy (FTIR)

Synthesized materials PVC-ed, PVC-ed/o-OMe, PVC-ed/p-OMe, PVC-ed/p-OH, and PVC-ed/o-NO₂ were characterized using FTIR. The spectra of the PVC-ed compound demonstrated the existence of peaks at ca. 3361, 3322, and 1594 cm⁻¹, allocated to amine group symmetric and asymmetric stretching and bending bands. It is obvious that the substitution occurs from one side for the majority of ethylene diamine linkages to PVC molecules. A band at ca. 3288 cm⁻¹ that belongs to the starching of the secondary amine group N–H was also noted. The strong and sharp peaks at ca. 2972 cm⁻¹ and 2910 cm⁻¹ were for the starching of aliphatic C–H bonds. Moreover, the peak at ca. 753 cm⁻¹ refers to the starching of the C–Cl bond, which gives an indication that the substitution reaction occurred to the part of the labile chlorine atoms.

After the reaction between the PVC-ed and the organic aromatic compounds or corresponding benzaldehyde compounds, the purified products were characterized using an FTIR spectrometer for the four modified PVC materials (PVC-ed/o-OMe, PVC-ed/p-OMe, PVC-ed/p-OH, and PVC-ed/o-NO₂.). The spectra show the disappearance of the NH₂ bands and the appearance of a new strong and sharp band ca. 1676 cm⁻¹ assigned for the imine (C=N) group due to the reaction between the corresponding aldehyde with the primary amine group and the formation of Schiff base compounds. In addition, the spectra of pure modified PVC compounds displayed distinctive stretching peaks at ca. 3064 cm⁻¹ assigned for aromatic C–H, 2900 cm⁻¹ assigned for aliphatic C–H, and 3290 cm^–1 assigned for the N-H secondary amine. These peaks can be considered as characteristic peaks of synthesized organic compounds.

3.2.2. Utilizing Proton Nuclear Magnetic Resonance Spectrometer (¹H-NMR)

The synthesized materials were characterized using ¹H NMR spectrometer to determine the chemical structure of modified PVC. The ¹H NMR was recorded for blank PVC, PVC-ed, PVC-ed/o-OMe, PVC-ed/p-OMe, PVC-ed/p-OH, and PVC-ed/o-NO₂, and the main challenge for this measurement was the solubility. Since this type of material is poorly soluble in common organic solvents, the best choice was to use deuterated DMSO as a solvent. Even though the solubility was too poor at room temperature, we mixed about 20 mg of the sample with 2 mL of deuterated DMSO and left it to stir overnight at 40–50 °C until fully dissolved.

The ¹H NMR of blank PVC in DMSO showed broad multiplet peaks at chemical shift 4.43 ppm assigned for the C-H proton linked to the labile chlorine atom, while the other multiplet peak at chemical shift 2.33 ppm was referred to the CH₂ protons. These data agreed with what have been recorded in the literature [22]. The two small signals at 2.09 and 1.23 belong to the methyl groups at the end of each PVC chain. The ¹H NMR of the PVC linked to ethylene diamine (PVC-ed) showed an extra peak at chemical shift 2.85 ppm, which belongs to 2CH₂ linked to amine groups. However, the signals of both primary and secondary amines are overlapped with a water peak between 3.05–3.90 ppm; see

Table 1. ¹H NMR peaks of synthesized modified PVC.

Modified PVC	1H NMR (400 MHz: DMSO-d6, δ, ppm, J in Hz)
	4.43 (m, 1H, Cl–CH), 2.33 (m, 2H, CH2) 2.09 (s, end cap CH3), 1.23 (s, end cap CH3)
	4.43 (m, 1H, Cl–CH), 3.05–3.90 (br. m, overlap with H2O peak, 3H, NH, NH2), 2.85 (s, 4H, 2N–CH2), 2.55–2.01 (br. m, 3H, N–CH, CH2), 1.35 (s, CH3 End cap), 1.23 (s, CH3 End cap).
	10.37 (s, 1H, N=CH), 7.69 (m, 2H, Ar), 7.24 (m, 1H, Ar), 7.09 (m, 1H, Ar), 4.42 (m, 3H, =N–CH2, Cl–CH), 3.92 (s, 3H, OCH3), 3.26–3.90 (br. m, overlap with H2O peak, 3H, NH, N–CH2), 1.76 (m, 1H, N–CH), 1.06 (m, 2H, CH2)
	10.63 (s, exch 1H, OH), 9.79 (s, 1H, N=CH), 7.76 (d, J = 8.0 Hz, 2H, Ar), 6.93 (d, J = 8.0 Hz, 2H, Ar), 4.60–4.15 (br. m, 3H, =N–CH2, Cl–CH), 3.36 (m, overlap with H2O peak, 1H, NH), 2.5–1.9 (br. m, overlap with DMSO peak, 5H, NH, CH2, N–CH, CH2), 1.35 (s, CH3 End cap), 1.24 (s, CH3 End cap)
	10.12 (s, 1H, N=CH), 7.71 (m, 1H, Ar), 7.44 (m, 1H, Ar), 7.24 (m, 2H, Ar), 3.79–3.70 (m, 3H, =N–CH2, Cl–CH), 3.35–3.20 (m, 4H, overlap with H2O peak, NH, OCH3), 2.52–2.39 (m, overlap with DMSO peak, 3H, N–CH2, N–CH), 1.79 (m, 2H, CH2)
	10.24 (s, 1H, N=CH), 8.58 (m, 1H, Ar), 8.15 (m, 1H, Ar), 7.93 (m, 2H, Ar), 3.97–3.90 (m, 3H, =N–CH2, Cl–CH), 3.45–3.30 (m, 1H, overlap with H2O peak, NH), 2.55–2.48 (m, overlap with DMSO peak, 3H, N–CH2, N–CH), 2.09 (m, 2H, CH2).

for more specific details.

After preparing the PVC-ed compound in large amounts, it was used to synthesize different Schiff base compounds with four different benzaldehyde derivatives. These Schiff bases were characterized by ¹H NMR as shown in Figure 1 and Figure 2. Spectra clearly showed the proton of the imine group (N=CH) at around 10 ppm chemical shift and the disappearance of the signal assigned for the NH₂ group. They also demonstrated the presence of signals in the aromatic region assigned for the four protons of the one substituted phenyl group. The chemical shift of aromatic protons occurred for PVC-ed/o-OMe and PVC-ed/p-OMe at the region between 7.71 ppm and 7.09 ppm, but for PVC-ed/p-OH, the peaks shifted down between 7.76 ppm and 6.93 ppm. Furthermore, PVC-ed/o-NO₂ 1H NMR gives higher chemical shifts of the aromatic region protons between 8.58 and 7.93 ppm. This is because the hydroxyl group is a very strong donating group by resonance; thus, it donates electrons toward the ring and increases the shielding of the protons, which leads to resonance at a lower chemical shift. However, PVC-ed/o-NO₂ aromatic signals were in higher chemical shift because the nitro group is a very strong electron withdrawing group by resonance. This causes de-shielding of electrons around the aromatic protons, which makes the protons resonate at higher chemical shifts. In all cases, the protons on othro-position are the most affected because of the resonance status having full charge on the ortho-position.

In comparison, the integration of proton peaks at the aromatic region is (4), as it should be, with aliphatic peaks such as the peak at 1.06 ppm with integration (7), as shown in Figure 1. This peak belongs to the CH₂ group; thus, the integration should only be two if the modification is 100 percent. This is clear evidence that about a third of the labeled chlorine atoms were substituted.

3.2.3. Energy Dispersive X-ray (EXD) of PVC Films

The elemental composition of synthesized modified PVC was determined analytically using the Energy Dispersive X-ray (EDX) technique. This provides both qualitative and quantitative data on the chemical compositions of prepared materials [23]. As a strong instrument capable of obtaining a wide variety of information for PVC blank and modified PVC, it was used to confirm the chemical structure of synthesized co-polymers and estimate the percentage of modification. Figure 3a–f show the EDX spectra of blank PVC, and synthesized PVC-ed, PVC-ed/o-OMe, PVC-ed/p-OH, PVC-ed/p-OMe, and PVC-ed/o-NO₂ compounds, respectively. For blank PVC, the major components were chlorine and carbon atoms, while in modified PVC-ed with ethylene diamine compounds, a new band that was associated with nitrogen atoms appeared in the EDX spectra, reducing the percentage of chlorine atoms. From the comparison of nitrogen and chlorine percentages, the percentage of modification was estimated to be between 30–35%. This result is consistent with the above estimation by using ¹H NMR’s integration. Furthermore, the EDX spectra of Schiff base materials demonstrate the existence of nitrogen and oxygen atoms within their chemical structures.

3.3. Synthesized Modified Polymers as Photo-stabilizers of PVC Films

The synthesized modified poly(vinyl chloride) Schiff base materials were used to stabilize the polymer depending on several mechanisms and depending on the structure of the materials used in the polymer modulation, which include the stabilization of the polymer with the conversion of the absorbed energy to harmless heat. Many approaches have been applied to exhibit the stability of PVC films after mixing it with 5% of a synthesized modified polymer, as is discussed in the next sections. The reason for using a low percentage of modified PVC (5%) is to not change the features of the PVC in the same time to provide good stability.

3.3.1. Evaluation of Stabilizing Efficiency of PVC Films by Weight Loss

The first approach, which was followed to examine the stability of PVC films, is the weight loss method. This is popular procedure used to examine the stability of PVC that has been used in the literature [24]. Weight loss happens as a result of the photo-degradation of PVC, which is known to be accompanied by releasing HCl molecules, since weight loss increases with longer exposure times to UV light. The calculations were performed for blank and blended PVC films with an addition of only 5% per weight of modified polymer. The weight loss due to photodegradation of PVC films in the presence and absence of additives was calculated using Equation (2).

Figure 4 shows the effects of irradiation by UV light on the percentage of PVC film weight loss for blank and blended PVCs. It demonstrates that the weight loss in blended PVC film was less than in blank PVC film, due to the aromatic molecules being included in the case of blended polymers. It is obvious that the lowest weight loss percentage was for PVC film doped with PVC-ed/p-OH compound. This might be because it contains a phenol group that is known as an excellent free radical scavenger [25], even though other additives have also shown good stability for PVC films, and they are in order as follows: PVC-ed/p-OH > PVC-ed/p-OMe > PVC-ed/o-OMe > PVC-ed/o-NO₂.

3.3.2. FTIR Spectroscopy to Estimate the Stability of PVCs Films

The four Schiff base moieties compounds that had been produced were employed as additives in PVC film to reduce the influence of sunlight in outdoor usage. When PVC sheets were exposed to UV light at a specific wavelength (313 nm) for a long time (300 h), the infrared spectra were used to monitor the alternations in the functional groups during irradiation because it is a common procedure that has been used in the literature following the degradation of polymeric films [26]. These changes included the growth of the three primary functional groups: hydroxyl, carbonyl, and alkene groups. These bands were found at specific wavelengths 1716 for carbonyl (C=O), 1667 for alkene (C=C), and broad 3200–3400 cm⁻¹ for hydroxyl (OH) groups. Figure 5a,b show the FTIR spectra of pure PVC before and after irradiation for 300 h. It is possible to see the growth rate of these peaks in relation to a reference peak that has been used at 1346 cm⁻¹ since the reference band should not be affected by irradiation. Furthermore, Figure 5c shows the FTIT spectrum of blended PVC film with the PVC-ed/p-OH compound after irradiation for 300 h. Thus, the growth of these peaks is much smaller relative to blank PVC film, which is clear evidence that synthesized modified polymers work as excellent photo-stabilizers for PVC films.

Equation (1) was used to calculate the functional group indices of PVC with additives and pure PVC films. They have been determined as the carbonyl index (I_C=O), alkene index (I_C=C), and hydroxyl index (I_OH). The results have been displayed against the irradiation time as shown in Figure 6, Figure 7 and Figure 8. It was found that the values of the indices of carbonyl, hydroxyl and alkene increase with the increase in irradiation time through the appearance of the carbonyl and hydroxyl bonds and the double bond developing with the irradiation time. It is noted that this value is low compared to the blank polymer. It was also found that the new polymers behave as good stabilizers against photo fragmentation. All additives have shown reasonable stability for PVC films in comparison to blank PVC. This result matches with the weight loss experiment outcome since the efficiency of additives follows the same order PVC-ed/p-OH > PVC-ed/p-OMe > PVC-ed/o-OMe > PVC-ed/o-NO₂. It is concluded that the prepared additives added to the polymeric films reduced the decomposition of the PVC polymer in varying proportions relative to the type of additive, but with a large difference versus the blank PVC.

It is known that organotin complexes show very good photo-stability of PVC when they are used as photo-stabilizers [12,14,16,18]. The pure organic modified PVC showed almost similar stability according to the weight loss and functional group index experiments. These are quite promising results; hence, it has been used with only organic materials, which are less toxic and biodegradable. Therefore, these materials are environmentally safe, provide excellent protection and are easy to produce.

3.3.3. Analysis the Surface Morphology of PVC Films

The characteristics of the polymeric surface, including its roughness, homogeneity, and surface flaws, may be significantly understood by studying the surface morphology [27]. Therefore, for blank and blended PVCs, the surface morphology of produced polymeric films was investigated. Several techniques have been utilized to look at the surface morphology of blank and blended polymeric films before and after 300 h of UV exposure, as is detailed in the following sections. Photos of blank PVC sheets before and after 300 h of UV light exposure are shown in Figure 9a,b. Figure 9b illustrates the damage to PVC film as a result of irradiation, which is evident in the large fractures, numerous black areas, and the change in color from bright translucence to a darker tone (discoloration). As stated above, this is caused by exposure to radiation and by the creation of free radicals, which cause PVC to photodegrade through a number of processes, including dehydrochlorination. Photos of blended PVC film with PVC-ed/p-OH that have been exposed to UV light for 300 h are shown in Figure 9c. Thus, the film exhibits remarkable durability against irradiation. The film has not broken down and is not severely impacted by UV light, potentially similar to blank PVC film before irradiation. This is because the additive contains aromatic units and phenol groups, which are known as excellent UV absorbers and free radical scavengers [28].

Microscopic Images to Study the Morphology of PVC Films

The surface morphology of PVC films was firstly studied using a microscope instrument to capture microscopic images, which works by using visible light and a number of lenses [29]. Figure 10 shows the microscopic images of blank PVC before and after irradiation for 300 h. The image before irradiation looks bright green, smooth and has no cracks or defects. However, it becomes darker with big black spots, and the polymeric surface seems to have many cracks and defects, as shown in Figure 10b. This is due to the photo-degradation process of the polymer, which results in the breaking of the surface link and the elimination of HCl molecules. The photochemical process may be delayed with less HCl elimination by adding synthetic aromatic material, as is discussed below.

The merged PVC after 300 h of radiation exposure is shown in Figure 11 in microscopic photographs. From these images, the best results were obtained for the PVC-ed/p-OH additive since the color resembles the blank PVC before irradiation. Thus, there are very small dark spots, and the color is still bright green as shown before irradiation for blank PVC. This supports the findings of prior experiments that the PVC-ed/p-OH additive plays the role of an excellent photo stabilizer of PVC films. As can be seen in Figure 11c, PVC films provide superior microscopic pictures, where the film surface is as flawless and brightly colored as blank PVC before irradiation, without large breaks or dark areas.

Scanning Electron Microscopy (SEM) to Study the Morphology of PVC Films

Scanning electron microscopy may be used to examine the surface morphology of various plastic thin films (SEM). It produces magnificent images as a result, with high resolutions, various magnifications, and exceptionally clear and minimally distorted images. This method must be used in order to assess the efficiency of photo-degradation on the PVC film surface. Particle size, homogeneity, and morphology of mixed materials may be examined using SEM. SEM pictures also show the evolution of cleavages, fissures, and spots on the film surface. These variances are brought on by photo-degradation of the PVC. Figure 12 displays SEM photos of blank PVC film at 200 µm magnification before and after UV irradiation. The photo is free of damage, cleavages, holes, or white or gray patches prior to irradiation, as shown in Figure 12a. However, following irradiation, it was seen that there are several light gray or white areas with numerous holes, along with the existence of significant fractures. It is evident that this occurred as a result of the photo-degradation of the PVC films when exposed to UV light, which has been extensively reported in prior research [30].

Figure 13 shows SEM images of blended PVC sheets with synthesized additives (PVC-ed/p-OH, PVC-ed/o-OMe, PVC-ed/p-OMe, or PVC-ed/o-NO₂.) after 300 h of UV light exposure. Only 5% of synthetic modified polymeric additives by weight were utilized. Hence, SEM findings show that all additives demonstrate good enhancement of PVC surface features after irradiation in comparison to blank PVC. As a result, there are no cracks or gray patches visible in the SEM images, demonstrating the excellent efficacy of additives in protecting PVC against photo-degradation even for irradiation for a long time (300 h).

Atomic Force Microscope (AFM) to Exhibit PVC Photostability

The surface morphologies of PVC films were examined using the atomic force microscope (AFM) method. It can scan both two-dimensional and three-dimensional topographies, making it a powerful microscopy instrument for studying surface measurements at the nanoscale. AFM may be used to assess the PVC surface’s properties and roughness. The root mean square roughness (Sq), which is determined by first square-rooting each height value in the dataset, is one of the dispersion factors used to assess surface roughness. Surface skewness is a measurement of the asymmetry of the surface deviation around the mean plane. PVC surfaces become rough and cracked after prolonged radiation exposure [30]. Topographic AFM photos (two- and three-dimensional) of the PVC films’ surface after 300 h of irradiation show that the root mean square (Rq) and roughness average have significantly grown from 1.9, 2.5 nm before irradiation to 5.25, 7.05 nm after irradiation, respectively, as shown in

Table 2. Roughness data for blank and blended PVC films.

Films	Roughness Average (nm)	Root Mean Square (Sq) (nm)
Blank PVC before irradiation	1.91	2.52
Blank PVC after irradiation	5.25	7.05
PVC-ed/p-OH	1.92	2.5
PVC-ed/o-OMe	2.05	2.92
PVC-ed/p-OMe	4.52	5.88
PVC-ed/o-NO2	3.09	4.24

. This results from the photodegradation that the PVC chemical structure goes through following exposure to light. AFM photos of untreated PVC are shown in Figure 14 before and after being exposed to UV light.

As shown in Table 2, employing synthetic additives decreased the surface roughness of PVC after irradiation compared to blank PVC. Thus, the PVC-ed/p-OH film demonstrated the lowest roughness average and Sq values (1.92 and 2.5 nm, respectively) almost similar to PVC before irradiation. This indicates the least amount of photodegradation and highest photo-stability among other studied PVC films. The use of stabilizers considerably improved the PVC films’ photostability. Figure 15 illustrates the differences between the irradiated blended PVC films with additives and the blank PVC film in terms of surface homogeneity, smoothness, and roughness. The distance between peaks and valleys in an AFM picture often characterizes the roughness of a surface. This technique utilizes this distance to quantify how rough a surface is, with rougher surfaces being measured as the distance rises. Other additives showed a bit higher roughness in comparison to PVC-ed/p-OH, but it is still higher than blank PVC after irradiation, as shown in Figure 15.

In comparison, the light stability of the four light stabilizers demonstrates that PVC-ed/p-OH has the best photo stability because it contains a phenol group, which is known as an excellent free radial scanner. Thus, the free radicals formed due to photo-degradation can be delocalized by the phenol group and can reduce its reactivity. At the same time, the PVC-ed/p-OH compound has an aromatic unit that can absorb the UV light and protect the host PVC from degradation. The reason the other three photo stabilizers have lower effects is because they work only as UV blockers and not as free radical scavengers. In addition, the significance of selecting four different benzaldehyde derivatives was due to its availability and affordability, and it is easy to work with.

4. Conclusions

To improve the photo-stability of poly(vinyl chloride), four novel modified polymers were synthesized, creating a polymer with aromatic Schiff bases moieties from commercially available poly(vinyl chloride). To maximize the likelihood of preventing cross-linking by a two-sides reaction, the PVC polymer was first reacted with ethylene diamine from one side by using an equivalent of extra ethylene diamine. The second step involved reacting the corresponding aldehydes with the amino group to produce the new Schiff base molecules. FTIR, ¹H NMR, and EDX were among the methods used to confirm the structures of the produced modified polymers. These techniques were used to show the structures and the degree of alteration for the materials under study. Using the integration of ¹H NMR spectra and the elemental composition percentage from EDX analysis, the modification percentage was shown to be between 30 and 35%. Synthetic substances have been used as photo-stabilizers of PVC films to reduce the photo-degradation of the PVC’s polymeric chains. A number of methods, such as the weight loss method and observing the formation of various functional groups while being exposed to radiation using FTIR spectra, were utilized to evaluate the efficacy of modified PVC as photo-stabilizers. The surface morphology of generated polymers was further examined using atomic force microscopy (AFM), scanning electron microscopy (SEM), and microscope images. These studies, which received approval, demonstrated that innovative modified Schiff base polymers outperform standard PVC by a wide margin as photo-stabilizers. As a consequence, PVC films show less photo-degradation than blank PVC after 300 h of UV exposure, so that the high conjugation mechanism of the coupled units can absorb UV light.