1 Introduction

Ionic liquids are special substances that have many unique potential applications, they are liquid salts that have positively charged cations or negatively charged anions. The essence characteristic of them is that they exist in the liquid phase at room temperature and their structure is basically made from cations and anions [1, 2]. Ionic liquids differ from solid ionic substances in many of their characteristics. For instance, they have substantially low melting points, where "low" often refers to around 100 °C [3, 4]. Additionally, ionic liquids have excellent temperature stability, which indeed makes them more resistant to changes in their physical or chemical composition. Also, due to their ionic formation, they frequently exhibit high electrical conductivity, allowing electrons to pass through them effectively. Furthermore, they are easily able to flow because of the low viscosity of most of them. Ionic liquids have a viscosity of less than 100 CP (centipoise), whereas water has a viscosity of 1 CP, SAE 30 motor oil has a viscosity of 420–650 CP, and honey has a viscosity of 10,000 CP [5, 6].

There are several practical uses for ionic liquids in transporting and storing gases. Gases can be dissolved into ionic liquids and readily removed as necessary rather than utilizing pressurized containers [7]. Ionic liquids can be used in recycling to help separate comparable and disparate items from one another. For instance, extracting polymers from plastics and using them as solvents in the manufacturing of batteries as well as polymer and biomaterial processing [8].

On the other hand, polyvinyl chloride (PVC), a typical thermoplastic polymer with a high molecular mass, has an outstanding number of beneficial features that make it suitable for a wide range of uses. They can be formed at high temperatures, strong, stable, and resistant to chemicals, fire, and several other chemical and physical effects [9,10,11,12]. Though, PVC has been always suffering from thermal and photo-degradation, making these factors a true devastating to the dependence on PVC in wider applications, although, it’s a cheap and available substance. According to studies on PVC’s thermal degradation, the material breaks down in two stages [13, 14]. In the initial phase, spanning a temperature range of 200–360 °C, the primary products are predominantly HCl and benzene, while the presence of alkyl aromatic or condensed ring aromatic hydrocarbons is minimal [15, 16]. Subsequently, during the second phase of degradation, occurring between 360 and 500 °C, the production of HCl and benzene diminishes considerably, giving way to the formation of alkyl aromatic and condensed ring aromatic hydrocarbons. As the C6 ring structures mentioned earlier undergo aromatization, the polymeric network formed through polyene condensation disintegrates [17]. Vinyl chloride, a typical thermoplastic polymer with a high molecular mass, has a number of beneficial features that make it suitable for a wide range of uses. They can be formed at high temperatures, strong, stable, and resistant to chemicals, fire, and elements. This work introduces a PVC modification by adding different amounts of ionic liquids to maximize its thermal stabilization.

2 Experimental

2.1 Preparation of hydrated ammonium aluminum sulfate-urea ionic liquid

Urea [NH2CONH2] ≥ 99.5%, high purity and hydrated ammonium aluminum sulfate [NH4Al (SO4)2.12H2O] 99% purity, were two solid molecules that were grilled, thoroughly mixed, and progressively heated to 85 °C until the solid salts changed into a white liquid in order to create an ionic liquid [18, 19].

2.2 Preparation of PVC solution and modified PVC (PVC-IL)

PVC solution was produced by combining 0.8 g of polyvinyl chloride 99%, Grade Standard to 16 ml of tetrahydrofuran solvent and vigorously swirling the mixture for two hours [20, 21]. After that, the produced PVC Solution was combined with (100 ml) of hydrated ammonium aluminum sulfate-urea ionic liquid (prepared already from the first step), and the mixture was refluxed for two hours [22].

2.3 Films preparation

Both the pure (PVC) and the solution (PVC-IL) are poured onto the clean glass plate at room temperature and allowed to dry. The produced polymer films had a thickness of around 40 µm.

2.4 Thermal degradation

Take each of the prepared pure PVC film and PVC- IL films in the area (1.5*1.5 cm), put them in a muffle furnace for 30 min at each temperature (25, 35, 45, and 55)°C, and measured all tests needed.

3 Result and discussion

The optical parameters and chemical behavior of modified and pure PVC thin films were studied thermally at (25–55)°C. The energy gap measurement was investigated by a diffuse reflectance spectrometer. Several instruments, including optical microscopes and AFM, were also used to study the surface morphology.

3.1 IR analysis for pure PVC and modified PVC films

The IR spectra exhibit the stretching and bending vibration bands to characterize the chemical composition of pure PVC and modified PVC–IL thin films as thermally affected as shown in Fig. 1. It is apparent that rising temperatures have an impact on band peak intensities. The FTIR spectrum shows two peaks at 3347 cm−1and 3438 cm−1, respectively, which refer to the symmetric and asymmetrical stretching bond of NH2 at modified IL-PVC and Peaks at 2972 cm−1 and 2907 cm−1 refer to the asymmetric and symmetrical stretching bond of CH2 these peaks were affected, and the intensity was lower as increasing temperature.

Fig. 1
figure 1

IR spectrum chart for pure PVC and modified PVC films as increase temperature

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Pure PVC and modified PVC–IL show changes in absorption intensity in polyene peak (1602 cm−1), and carbonyl (1722 cm−1) in ionic liquid. Peaks around 1424 cm−1 are attributed to the aliphatic bending bond between C and H. The presence of a bending bond between carbon and hydrogen in close proximity to chlorine is believed to account for the peak observed at 1237 cm−1. The stretching bond of carbon–carbon within the backbone chain of polyvinyl chloride (PVC) is typically detected in the spectral range of 1000 to 1100 cm−1. Lastly, the presence of the carbon-chlorine bond is manifested by peaks observed within the 600–650 cm−1 range [23].

3.2 Morphology surface of pure PVC and PVC-IL thin films

In order to assess the color stability of PVC samples, an oven aging technique was employed, and the changes were examined using optical microscopy. The aging process exhibited varying rates depending on the structural characteristics of the films. Consequently, alterations in film colors were observed, attributable to the dehydrochlorination process and the formation of conjugated double bonds within the PVC samples. Thermal effect on the surface morphology of PVC pure and PVC-IL thin film as temperature increases from around room temperature 18–55 °C, with an optical microscope at resolution 400X. Figure 2 demonstrates that the pure PVC surface retained stability while being exposed to temperatures ranging from 18 to 55 °C.

Fig. 2
figure 2

Optical microscope picture of PVC pure at various temperatures at resolution 400X

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The ionic liquid molecules in the PVC-ionic liquid thin film had crystalline rod-like formations on the PVC surface at ambient temperature. As temperature increases on thin film, the rod like the structure of ionic liquid affected and began slightly dissolving, although, the polymer surface stays stable as temperature increase as appeared in Fig. 3. This suggests that the modified PVC surface is more stable than the unmodified surface. This might be due to the ionic liquid's ability to shield the polymer's backbone from the effects of the temperature increase.

Fig. 3
figure 3

Shows an optical microscope image of IL-PVC at various temperatures at resolution 400X

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AFM (Atomic force microscope) instrument is used to study the thermal effect on surface morphology, by studying the 3D images of the surface, surface Roughness, and Particle size. Numerous studies have indicated the applicability of atomic force microscopy (AFM) for the examination of non-uniform and rough surfaces of polymers subjected to irradiation. In this study, PVC films were subjected to a temperature of 55 °C, followed by the acquisition of two-dimensional (2D) and three-dimensional (3D) AFM images to analyze the surface characteristics. Table 1 shows the effects of increased temperature on surface roughness properties and particle size of both pure PVC thin film and PVC –IL thin film. Figures 4 and 5 shows the surface getting rougher as thermal temperature increases for both pure PVC thin film and PVC- IL thin film and the roughness in PVC. The results of this investigation demonstrate a significant reduction in the rates of dehydrochlorination and bond-breaking.

Table 1 Surface properties analysis
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Fig. 4
figure 4

3D AFM surface morphology of PVC as increase temperature

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Fig. 5
figure 5

3D AFM surface morphology of PVC- IL as increase temperature

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In comparison to the particle’s average roughness at room temperature, the average surface roughness of pure PVC increased as temperature increased and became significantly rougher at 55 °C when the roughness reached 3.64 nm, indicating that the polymer’s structure started to diverge slightly [24,25,26]. Additionally, the surface roughness average in PVC-IL was less affected by a temperature increase in comparison to PVC pure, indicating that the ionic liquid was preserved on the PVC structure.

Figure 6 illustrates high-resolution photographs of PVC-IL and pure PVC thin films that were taken as temperature increased as an additional indicator for the thermal influence on the surface [27]. PVC-IL thin films reached a crash point while the color of the film stayed the same, on the other hand in pure PVC thin film without modifications the color of the film changed and became light gray. The change in color may refer to the beginning of the distortion in PVC structure.

Fig. 6
figure 6

Picture of PVC-IL and pure PVC thin films as increase temperature from Room Temperature to 55 °C

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3.3 Optical properties

3.3.1 Band energy measurement pure PVC and modified PVC-IL

The optical properties of pure PVC and modified PVC-IL were studied by calculating the energy gap as thermally affected.

Kubelka–Munk theory was employed to measure the energy gap using diffuse reflectance spectra [28, 29]:

$$ F = (1 - R_{\infty } )^{2} /2R_{\infty } $$
(1)

αhʋ = C1(hʋ-Eg).1/2α = F(R∞)

$$ \left[ {{\text{F}}\left( {{\text{R}}_{\infty } } \right){\text{ h}}\upsilon } \right]^{{2}} = {\text{ C}}_{{2}} \left( {{\text{h}}\upsilon - {\text{Eg}}} \right) $$
(2)

hʋ = 1240/λwhere R∞ = sample reflection coefficient; λ = absorption wavelength.

The plots of [F(R∞) hʋ]2 against photon energy (hν) were utilized and the intercept was calculated to determine the band gaps value as shown in Fig. 7 and Fig. 8, respectively, for pure PVC and modified PVC films as temperature increased.In pure PVC films the energy gap range (3.04–3.1) eV, first, the temperature affected the value of the band gap and increase to 3.1 eV at 55 °C as shown in Fig. 7. While in modified PVC films the energy gap range (3.46–4.28) were observed. It is clear that the ionic liquid makes it more isolated since the value of the energy gap at the adjusted temperature was higher than the value for pure PVC. As increase temperature, the value of the energy gap was increased to 4.28 eV at 55 °C, as shown in Fig. 8.

Fig. 7
figure 7

Band energy gaps value of pure PVC films as increase temperature

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Fig. 8
figure 8

Band energy gaps value of modified PVC films as increase temperature

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4 Conclusions

The primary aim of this study was to investigate the impact of temperature on the properties of the polymer under two different conditions: in the presence and absence of liquid crystals within the temperature range of 18–55 °C. As temperature increases to about 55 °C, pure PVC film was shrinking as notes in films pictures also the other properties like optical and surface effected as appearance in AFM, Roughness and optical microscope where the films become more roughness while in modified PVC films with the addition of ionic liquid shown to gain optimized stability. Studying the surface morphology in the optical microscope and AFM resulted in a piece of evidence revealing that IL-modified surface is noticeably more stable at higher temperatures, with less surface roughness. The future prospects for implementing ionic liquid-based PVC stabilizers are promising. Continued research and development efforts are likely to lead to the discovery of new and more efficient ionic liquid formulations. As sustainability and environmental concerns drive the demand for greener alternatives, the adoption of ionic liquid-based stabilizers in various industries, including construction, automotive, and electronics, is expected to increase, offering improved thermal stability and durability for PVC materials.