MgO NPs reinforced PCL/PVC nanocomposite films with enhanced UV shielding and thermal stability for packaging applications

1 Introduction

Currently, the protection of sensitive products such as fresh food, pharmaceuticals and electronic devices from spoilage is only possible with effective packaging systems. Due to inadequate packaging, food waste reaches serious levels, the effectiveness of pharmaceuticals decreases and electronic devices can be damaged. In this context, polymer-based packaging materials are replacing traditional materials with their advantages such as lightness, low cost, flexibility, and barrier properties against oxygen and moisture. In particular, innovative materials developed through polymer nanotechnology offer great contributions in terms of food safety, shelf life and environmental sustainability. Today, polymers used in the packaging industry constitute a significant portion of the global market, creating both economic and technological value [1], [2], [3], [4], [5].

Polycaprolactone (PCL) is a biodegradable polyester of synthetic origin and an important biopolymer widely used in the biomedical and packaging industries. It offers advantages in different fields thanks to its hydrophobic structure, low melting point (59–64 °C), high formability and resistance to various solvents. Enzymatically degradable in the biological environment, PCL is safely used in drug delivery systems, surgical sutures, tissue engineering scaffolds and medical implants. Although its biocompatibility is limited, its flexibility and blendability make it a versatile material. It is also used in some FDA-approved medical products. PCL, which is also used in food packaging, is of great importance in terms of environmental sustainability thanks to its controlled degradability and offers an alternative to traditional plastics in this respect [6], [7], [8], [9], [10].PVC (polyvinyl chloride), with its rigid to very flexible structure, is used in a wide range of products from short-lived packaging to long-lasting building materials. It is widely used in applications such as pipes, window profiles and cable coverings in the construction industry, as well as flooring, wall coverings, toys, clothing, packaging and advertising products. The main reason why PVC can be used in so many areas is the polarity of the carbon-chlorine bond in its structure, which enables it to interact strongly with additives. This structure allows the properties of the material to be changed as desired with additives. In addition to its low cost, PVC’s advantages such as high chemical resistance, good mechanical properties, weather and water resistance make it indispensable in industry. It also has many additional properties such as transparency, impact resistance, formability and flexibility. PVC has been in production since the 1930s and today ranks third in the world in terms of production volume [11], [12], [13], [14].

Polymer blends are a research topic that attracts attention both in industry and academia. Systems formed by blending different polymers can be either single-phase homogeneous or multiphase heterogeneous; multiphase structures are more common, usually due to thermodynamic incompatibility. PCL and PVC are one of the rare polymer pairs that are compatible in the amorphous phase at all composition ratios. In this system, the crystalline phase consists only of pure PCL; as the PVC content increases, the melting temperature of PCL decreases and the amorphous regions expand, but the crystalline structure does not change. Furthermore, spherulite formation is observed in this blend, but the spherulite growth rate of PCL slows down when non-crystallizable PVC is added [15], 16].

Recent developments in polymer nanocomposites obtained by adding different nanomaterials have shown significant improvements in the structural, optical, and functional properties of polymer matrices. For example, it has been demonstrated that adding SiN₄/TaC nanoparticles to polymer composites improves structural stability and electrical conductivity, paving the way for potential electronic applications [17]. Likewise, hybrid nanoparticles, such SiO₂/Cr₂O₃, have enhanced morphological, optical, and antibacterial qualities in biopolymer blends, increasing their applicability in optical nanodevices and food packaging [18]. The functional range of polymer-based materials in sensor and quantum electronic applications is further expanded by the reported significant enhancement of dielectric, optical, and pressure sensing properties through the incorporation of metal oxide nanoparticles such as MnO₂ and BaTiO₃ within polymer matrices [19], 20]. These developments highlight how crucial it is to include nanoparticles into polymer composites in order to customize them for multipurpose packaging solutions that satisfy contemporary requirements for environmental sustainability, durability, and food safety. One popular technique for enhancing UV protection in packaging is the addition of metal oxide nanoparticles to polymer matrices [21]. Due to their strong UV absorption capability, ZnO, TiO₂, and SiO₂ nanoparticles are commonly chosen. Nonetheless, the performance of composites is strongly impacted by the uniform dispersion of nanoparticles within the polymer. MgO nanoparticles have drawn attention recently as a substitute filler for thermal stability and UV protection [21], 22]. Nanomaterials, especially nanofibers and metal oxide nanoparticles such as magnesium oxide (MgO), are valuable for both research and industry due to their unique properties. MgO is characterized by its thermal stability, biocompatibility, low toxicity and antibacterial effects. When doped with polymers, MgO improves mechanical strength, thermal resistance and barrier properties, making these composites ideal for medical, packaging, sensor, membrane and energy storage applications. When combined with biodegradable polymers, MgO-based composites also support sustainable and environmentally friendly solutions [23], [24], [25], [26], [27]. MgO-reinforced polymer composite films can potentially be useful for various applications due to MgO’s antibacterial properties. Additionally, they can be used as scaffolds in tissue engineering, bandages, and wound dressings in the biomedical field; they can also be used for controlled release in drug delivery systems. In the environmental field, they can be tested in water purification membranes and UV protective coatings, and in the food industry, they can be used as antimicrobial packaging materials to extend the shelf life of foods. Finally, they facilitate the use of electronic and sensor technologies, including dielectric layers and gas sensors [28], [29], [30], [31].

This study, unlike previous studies, investigates the effect of MgO nanoparticles on the structural, thermal, and optical properties of PCL–PVC nanocomposites, particularly for UV-protective packaging applications. PCL–PVC-based nanocomposite films reinforced with MgO nanoparticles were synthesized. The structural, thermal, and optical properties of the obtained composites were investigated using detailed characterization techniques.

2 Experimental

2.1 Production of MgO nanoparticles and polymer composites

2.1.1 Step: MgO nanoparticle

The hydrothermal method was preferred to produce magnesium oxide nanoparticles. The hydrothermal method stands out as a widely preferred and effective technique for nanomaterial synthesis. This process produces products with uniform shape and good crystallinity by allowing controlled crystal formation at high temperatures and pressures.

Additionally, using water as a solvent provides a non-toxic and more environmentally friendly method. Economic benefits of hydrothermally produced materials typically include minimal energy use and very simple equipment. However, the method’s drawbacks include the demand for specialist equipment because to its high pressure requirements and lengthy reaction times. Furthermore, undesired phases may occur and product purity may decline if process parameters are not carefully regulated. For these reasons, the hydrothermal method’s suitability should be carefully assessed in light of the material’s characteristics and the synthesis goals [32], [33], [34].

Magnesium nitrate hexahydrate [Mg(NO₃)₂·6H₂O, 99 % purity, Sigma-Aldrich] was needed as a precursor to produce magnesium oxide. Deionized water was preferred as the solvent in the synthesis processes. All chemicals were of analytical purity and were used without any purification process. 0.5 M Mg(NO₃)₂·6H₂O was dissolved in deionized water with a suitable magnetic stirrer to obtain a homogeneous solution. Additionally, NaOH solution was prepared in a separate place to adjust the pH of the process. The solution was made basic (pH: 10) with the help of a pH meter. During production by hydrothermal method, the pH value of the solution is of critical importance depending on the type of material to be synthesized, the desired phase, morphology and crystal structure. The resulting solution was transferred to a teflon-lined autoclave after pH adjustment and hydrothermal treatment was applied at constant temperature (180 °C for 12 h). After the hydrothermal treatment, the system was left to cool to room temperature, the precipitate was centrifuged and washed several times with deionized water and ethanol. The solid product obtained was dried in an oven (4 h at 50 °C). The dried product was then baked at 600 °C for 2 h to ensure the formation of the MgO crystal structure. The aim here is to calcinate the product obtained by hydrothermal means.

2.1.2 Step: nanocomposite thin film

In order to produce polymer blend films, the blend was first prepared using 0.3 g poly(ε-caprolactone) (PCL) and 0.2 g polyvinyl chloride (PVC). The MgO (magnesium oxide) compound was then dissolved in THF (tetrahydrofuran) solvent for 1 h and 54 min using a homogenizer device and then placed on a magnetic stirrer. 0.3 g of PCL polymer was first added to the blend and the temperature of the device was set to approximately 80 °C and PCL was completely dissolved. After PCL dissolution, 0.2 g of PVC polymer was added to the solution and the mixture was stirred until a homogeneous structure was obtained. The homogeneous mixture was spread uniformly on the surface using a film applicator device and composite materials in thin film form were obtained. This process was repeated for three different batches with MgO additives of 10 %, 20 % and 30 % (Table 1). Image of produced blend and MgO-blend nanocomposite were given in Figure 1.

Table 1:

Mass rate of blend and nanocomposite components.

		PCL	PVC	Quantity MgO
Composite	Blend	0.3	0.2	–
	10 % MgO	0.3	0.2	0.05(g)
	20 % MgO	0.3	0.2	0.1(g)
	30 % MgO	0.3	0.2	0.15(g)

Figure 1:

Images of blend film and their MgO composites.

3 Results and discussions

The FTIR spectra of pure PCL–PVC polymer blend and nanocomposites containing 10 %, 20 % and 30 % MgO are presented comparatively in Figure 2. FTIR analysis can be used to identify functional groups in the composite structure and to determine the level of interaction between MgO and the polymer matrix. In the pure PCL–PVC blend spectrum, the strong absorption band at approximately 1,720 cm⁻¹ corresponds to the stretching vibration of the ester carbonyl group of the polycaprolactone (PCL) component [35], 36]. In addition, the 2,940 cm⁻¹ and 2,860 cm⁻¹ bands indicate aliphatic C–H stretching, while the 1,240–1,040 cm⁻¹ band indicates C–O–C stretching vibrations [37]. With the further incorporation of MgO nanoparticles into the composite structure, new absorption bands in the range of 600 to 400 cm⁻¹ appeared. These bands, which correspond to the vibrational modes of Mg–O bonds, have been identified as specific to this in previous research [38], 39]. These bands became more pronounced, especially in the composite containing 30 % MgO. This result indicates that the additive was more densely packed and efficiently dispersed in the polymer matrix. A significant decrease in the intensity of the carbonyl band around 1,720 cm⁻¹ was also observed as the MgO content increased. This implies that MgO particles form physical or chemical bonds through hydrogen bonds or surface interactions. Similarly, the bandwidth and intensity changed in the region of C–O–C stretching vibrations. Such changes imply that the filler can alter the molecular order in the matrix and create new interactions between polymer chains [40]. As a result, hydrogen bonding and surface adsorption effects are the main ways that MgO nanoparticles and the PCL-PVC matrix interact. New interchain contacts are produced when the hydroxyl groups on the MgO surface form bonds with the carbonyl (C=O) and C–O–C groups of PCL. By changing the matrix’s molecular configuration, this method improves the composite’s mechanical, thermal, and barrier qualities [41]. Such research also shows that the composite can be improved through such interactions.

Figure 2:

FTIR spectra of PCL–PVC blend and their composites.

The graph displays the X-ray diffraction (XRD) (Crystal structure and phase analysis) patterns of the mix that includes composites with 10 %, 20 %, and 30 % MgO additions to the polymer matrix (PCL + PVC) (Figure 3). Additionally, provided as a reference is the XRD pattern of pure MgO. The distinctive diffraction peaks of MgO’s cubic crystal structure may be seen in the pure MgO pattern. In accordance with JCPDS card number 01-071-1,176 [42], 43], these peaks: 2θ ≈ 36.9° (200), 2θ ≈ 42.9° (220), 2θ ≈ 62.3° (222), 2θ ≈ 74.7° (400), and 2θ ≈ 78.6° (420) confirm the crystalline structure of MgO. The blend sample does not show any clear, sharp peaks [44]. This suggests that there are no regular long range crystalline areas and that the amorphous structure is predominant. In general, PCL and PVC can both have amorphous or semi crystalline characteristics [44], [45], [46]; however, when combined, this crystallinity can be further diminished. The peaks that resemble the pure MgO pattern become more noticeable in composites that contain 10 %, 20 %, and 30 % MgO, according to the patterns of MgO doped composites. The existence of MgO is confirmed by the peaks located at 36.9°, 42.9°, and 62.3° in particular. The crystalline structure of MgO in the composite is maintained and distributed, as evidenced by the fact that the intensity of these peaks rises with increasing MgO content. In certain locations, the peak width (FWHM) increases, suggesting the presence of distributed MgO particles at the nanoscale. The MgO doping caused crystalline phases to develop, whereas the pure blend has an amorphous shape. At 30 % MgO, the rise in crystallinity is especially noticeable [47].

Figure 3:

X-ray diffractograms of MgO nanoparticles (NPs), PCL-PVC blend and their nanocomposites.

SEM images of the pure blend and 10 %, 20 %, 30 % MgO added blend composite are given in Figure 4. The SEM image of the Pure Blend sample shows that the polymers of the Blend are very well bonded and form ring-shaped structures, while in the composite obtained with 30 % MgO addition, it was observed that the ring-shaped structures grew and nanoparticles formed at their boundaries [48]. This formation can be called a surface roughness and morphological irregularity [49]. The addition of MgO to the PVC–PCL blend alters the surface morphology. This enables strength and barrier gains in polymer nanocomposites with well-dispersed low-to-medium filler contents due to interfacial interactions. However, as the filler content increases, agglomeration becomes more pronounced and limits performance [50], [51], [52]. Indeed, in polymer systems containing MgO, it has been reported that at filler levels around 10 wt%, there is a more homogeneous distribution and better mechanical response. Furthermore, agglomerates and associated reductions in tensile strength/toughness are frequently observed at levels of at least 15 wt% in SEM [50]. Similarly, it is emphasized that in PVC-based systems, oxide nanofillers (e.g., ZnO) intermix above a certain threshold due to their high surface energy, which has a negative effect on morphology and properties [53]. The presence of MgO can be observed in FTIR by Mg–O vibration bands in the 400–600 cm⁻¹ range. The overlap of these bands with modes in the low wave number region of the polymer can cause overlapping, especially at high filler contents [54], 55] As a result, it can be said that the 10–20 % range offers better distribution and performance balance, but the 30 % range may cause increased agglomeration surface roughness and micro-defects.

Figure 4:

SEM image of blend and 10 %, 20 %, 30 %MgO blend composites.

DSC (differential scanning calorimetry) analysis (Figure 5) shows that the sample is a polymer blend containing polycaprolactone (PCL) and polyvinyl chloride (PVC). PCL is a semi-crystalline polymer and typically has a melting temperature (T _m ) of 55–65 °C [56], 57]. The glass transition temperature (T _g ) of amorphous PVC is approximately 75–85 °C [45], 58], 59]. The characteristic thermal transitions of both components in DSC analysis indicate that these polymers show phase separation in the blend and are thermally independent of each other. The effect of MgO on this structure suggests that the miscibility of PCL and PVC in this case is limited. MgO acts as a nucleating agent and changes the crystallization behavior in the polymer matrix. The increase in enthalpy change in DSC results shows that MgO increases the degree of crystallization of the mixture. It also produces a change towards a decrease in the melting peak. MgO may cause this increase by contributing to a more ordered crystallization of PCL chains [59]. The dispersion behavior of MgO in the PVC matrix and polymer-filler interfacial interactions may slightly change the thermal transition temperatures. This additive can not only increase the thermal stability, but also change the morphological properties of the structure, which makes it possible to obtain a thermoplastic material more suitable for use [60].

Figure 5:

Heat flow curves of blend and composites.

Thermogravimetric analysis (TGA) is a widely used thermal analysis method to learn the thermal stability and degradation behavior of polymeric materials. The obtained findings show that there is a significant difference between the degradation mechanisms between MgO nanoparticle doped composites and PCL–PVC blends (Figure 6). The mass loss in the MgO-containing composites was in four steps, while the PCL-PVC blend was in three steps. Evaporation of water trapped in the structure or physically adsorbed on the surface during the synthesis of MgO nanoparticles may cause this additional degradation step [59]. Due to the hygroscopic character of MgO, the water adsorption capacity increases with increasing surface area, especially at the nanoparticle size [61]. The evaporation of this water usually takes place between 100 and 200 °C, supporting the fact that the first degradation step starts at earlier temperatures. MgO nanoparticle doping caused an earlier onset of thermal degradation compared to the blend. This provides evidence that hydroxyl groups on the MgO surface or acidic or basic surface properties can accelerate the degradation of polymer chains. Such interactions are referred to in the literature as “heterogeneous catalytic degradation” and are often mentioned with some inorganic additives [59]. In this case, MgO is both a physical filler material and an active component that can influence thermal degradation. Table 2 gave remain mass rate of blend and it MgO composites. These results can be explained by the fact that MgO has high thermal stability and does not degrade, leaving more inorganic residues in the structure at the end of degradation. This increase also indicates an increase in the inorganic/organic ratio of MgO, which is directly related to the amount of doping.

Figure 6:

Thermogravimetric curve of blend and composites.

In PCL-PVC blend, the absorbance of the composites in the UV region generally increases as the MgO additive content increases (Figure 7). This is due to the fact that MgO nanoparticles have a high surface area and facilitate photon absorption. The gradual increase in absorbance values between 10 %, 20 % and 30 % MgO doping ratios indicates that the additive directly affects the ability to interact with light. Figure 8 shows the Tauc plots. The graph below shows the relationship between the optical absorption coefficient α and the bandgap E _g in the direct transition [48], 62], 63].

The Tauc parameter, denoted as ‘A’, is a constant that quantifies the disorder within a crystal structure. It is influenced by the probability of electrical transitions and the distribution of band state densities at the tails. The Tauc plot is a graphical representation of the relationship between the square of the absorption coefficient (αhν)² and photon energy (hν). Furthermore, the bandgap energy (E _g) of a material can be determined by extrapolating the linear region of the Tauc plot to the x-axis. A change in the slope of the extrapolation line indicates a corresponding change in the bandgap energy (E _g) [48], 62], 63]. The findings in the Table 3 show that the band gap (E _g) decreases when MgO is added to the PCL-PVC polymer matrix. These findings reveal that MgO changes the optoelectronic properties of PCL-PVC matrix. The band gap decreases as the MgO doping increases. Local energy levels and defect levels formed by the integration of MgO nanoparticles into the polymer matrix may explain this phenomenon. These levels create new energy levels between the fundamental band and the conduction band. These levels allow electrons to be excited using less energy. Therefore, the optical band gap becomes smaller. Other MgO-doped polymer systems have similar effects. For example, Shruthi K. N et al. showed that MgO nanoparticles reduce the band gap in the PMMA matrix and are related to the energy levels that arise due to the interaction of MgO with the matrix [64], 65]. In addition, many literature studies have shown that nanoparticle doped interfaces increase polarizability by changing the optical properties. As a result, photon absorption shifts to lower energies and E _g value decreases [66], 67]. This band gap decrease in polymer nanoparticle composites is critical for improving light absorption properties, especially in ultraviolet (UV) protective applications or optoelectronic devices [68], 69]. The decrease in the band gap with increasing MgO doping ratio indicates that this composite material can be used as a better barrier against ultraviolet radiation. It can also be used in optoelectronic applications operating in the low energy range.

Figure 7:

Wavelength dependent ultraviolet-visible absorption spectra for nanocomposites.

Figure 8:

Tauc plots for nanocomposites.

Opacity is the second most frequently employed criterion to assess the transparency of a film. A high opacity value signifies low sample transparency. The ratio of absorbance at 600 nm to the sample thickness in millimeters which measured by using ellipsometer is a form of normalization referred to as opacity. In this context, “absorbance” is understood to represent the logarithm of the inverse of transmittance, without implying whether the attenuation of the incident beam is due to absorption or reflection [70]. A UV-visible spectrophotometer was used to test the light transmittance of the film composites in order to determine their opacity. The following formula was used to determine the opacity values [71].

In this equation, A ₆₀₀ is the absorbance at 600 nm and L is the film thickness (mm) [71]. Higher opacity values indicate lower film transparency. In the PCL-PVC blend and composite systems examined in the study, opacity values increased as the MgO NPs additive ratio increased (Table 3). The opacity value increased from 8.52 to 13.72 with the increase of MgO additive from 10 % to 20 %. The high refractive index of MgO may reduce the light transmittance of the material. However, after 30 % MgO doping, the opacity value dropped to 13.15, indicating a break in the upward trend. Despite this increase in MgO content, the decrease in the opacity value suggests that the use of high-content additives may cause particle agglomeration. Agglomeration reduces the scattering effect of light and disrupts the homogeneous distribution [72]. As a result, it can be concluded that 20 % of MgO doping is an ideal opacity ratio for this system. These results suggest that additive ratios should be carefully optimized to control the optical properties of composites.

Figure 9 experimentally demonstrates the shape memory behavior of MgO-modified PCL-PVC thin film composites. A classical shape memory cycle was applied to investigate the shape memory properties of the prepared samples. In the first stage, the samples were heated to approximately 55 °C (close to the melting temperature of PCL) and held at this temperature for 5 min. This process allows the crystalline regions of PCL to partially melt and increases the mobility of the polymer chains. This allowed a temporary shape (e.g., bending or curling) to be manually imparted to the material. In the second stage, the samples were rapidly transferred to an ice bath for sudden cooling. This cooling promoted the recrystallization of the PCL chains, locking in the temporary shape. Thus, the crystalline regions formed within the polymer matrix restricted chain movements and stabilized the temporary form of the sample. In the third stage, the samples fixed in their temporary shape were reheated to approximately 55 °C. At this temperature, the crystalline regions of PCL melt again and chain mobility increases. As a result, the material releases the applied temporary shape and returns to its original form. Experimental observations show that MgO reinforcement plays an important role in this cycle. MgO nanoparticles contribute to the more stable fixation of the temporary shape by affecting the crystallization behavior within the polymer matrix and, at the same time, increase the shape recovery rate by facilitating chain mobility during heating. Therefore, it can be concluded that MgO-reinforced PCL-PVC thin film composites exhibit high shape memory performance, with quite successful temporary shape fixation ratios and shape recovery ratios.

Figure 9:

Shape memory effect of the polymer composite thin film added to the PCL-PVC polymer blend with 20 % MgO.

In addition to the structural and optical characterizations, the prepared films’ weight and cost were also considered from a practical perspective. Because of the increasing inorganic fraction in the structure, the overall mass of each composite increased somewhat as the MgO content increased. However, even at larger loadings, the total material cost remained economically viable due to the cheap cost, chemical stability, and commercial availability of MgO nanoparticles [23], 24], 26]. Given the comparatively low additive ratios (10–30 wt percent), the significant increases in thermal stability and UV-shielding effectiveness outweigh the cost increase per sample. For industrial and packaging applications, MgO-reinforced PCL–PVC nanocomposite films can thus be considered a scalable and reasonably priced material [28], 29].

4 Conclusions

The developed MgO-reinforced PCL-PVC nanocomposite films demonstrate strong potential for UV protected packaging and optoelectronic applications due to simultaneous gains in structural integrity, thermal stability, and optical performance. FTIR characteristics, particularly in the carbonyl and ether regions, confirm robust filler-matrix interactions, while XRD confirms the presence of crystalline MgO and an increase in overall crystallinity depending on loading. SEM observations reveal composition-dependent morphology: good dispersion and interfacial adhesion at moderate MgO content, while surface roughness due to agglomeration becomes more pronounced at higher loadings. Thermally, MgO acts as an effective nucleation agent and shifts decomposition to higher inorganic residues, supporting improved high-temperature stability. Optically, MgO increases UV absorbance and lowers the Tauc band gap (E _g), consistent with enhanced photon absorption; opacity in the visible range peaks at 20 % by weight, followed by a slight decrease to 30 % by weight, which is likely due to nanoparticle agglomeration. When these results are combined, 20 % MgO by weight is positioned as a balanced formulation for homogeneous opacity and processability, while higher loadings can be utilized when maximum UV attenuation and E _g reduction are prioritized, provided that dispersion is carefully controlled.