Preparation and characterisation of polyvinyl alcohol/glycerol blend thin films for sustainable flexibility

Petroleum-based polymers pose significant environmental challenges; this prompts researchers to seek alternatives for the same. The foremost solution to replace petroleum-based packaging lies in bio-based polymers that can degrade with water, soil, and the environment. The most common and economical bio-based polymer today is polyvinyl alcohol (PVA), however, it has certain limitations such as brittleness, hydrophilic nature, etc. The primary objective of this study is to enhance the flexibility, transparency, barrier properties, and thermal stability of PVA by incorporating glycerol as a plasticizer. In this regard, thin films were prepared by utilizing a solution-casting technique (blade coating) upon the addition of numerous concentrations of glycerol ranging from 1 to 5 wt%. Here two sets of thin films were prepared i.e., with glycerol (modified) and without glycerol (pure PVA). Results suggest exceptional mechanical flexibility and enhanced optical properties in terms of improved transmittance (>90%) upon incorporation of glycerol into PVA. The modified films also demonstrated a significant increase in their water barrier capabilities in comparison to pure PVA films. When the concentration of glycerol reached to 5 wt%, a substantial increase in biodegradability and flexibility was witnessed resulting in reduced brittleness. Thus, the mechanical properties of the modified thin films exceeded that of pure PVA counterparts. The prepared thin films unveil exciting possibilities to be used in diverse applications; such as food packaging, membranes, biodegradable materials, etc,. The extensive discussion is presented in the light of observed results.


Introduction
The utilization of petroleum-based polymers represents one of the most significant innovations of the last century [1].However, the frequent use of petroleum-based polymers has affected ecological systems thereby contributing to global warming and resource depletion at large [2].Recognizing the urgency of addressing these issues, researchers have turned to biopolymers as sustainable alternatives to their petroleum-based counterparts [3].In this regard, biopolymers offer compelling advantages, including biodegradability and reduced environmental pollution [4].Numerous studies have demonstrated the efficacy of biopolymers in food packaging applications, showcasing impressive film-forming properties that are crucial for preserving food quality and enhancing their shelf lives [5].Interestingly, biopolymers often possess inherent antimicrobial properties, which enhance food safety and minimize the risk of contamination [6].
Among various other biopolymers, PVA thin films have garnered significant interest across various fields [7,8], fuel cells [9], and biodegradable food packaging, etc, [10] owing to their remarkable oxygen permeability [11], thermal stability [12], and resistance to chemicals [13].However, the inherent mechanical brittleness of PVA thin films often presents challenges to their practical utility.Generally, glycerol is rich in hydroxyl groups and is considered to be an effective plasticizer.This is due to the presence of hydroxyl groups, that reduce the hydrogen bonds between the PVA chains and thus increase the free molecular volume [14].Moreover, the crystalline arrangement enhances the desirable characteristics (such as strength and flexibility) of PVA thin films when subjected to mechanical strains [15].However, it is possible when combined with a plasticizer of appropriate composition.In conventional methods, challenges such as; agglomeration or aggregation may arise which could be overcome by the inclusion of nanoparticles into thin films has been suggested as a remedy [16].Therefore, one must focus on strategies such as surface functionalization of additives to improve particle dispersion within thin films.Some scientific investigations have aimed to overcome this limitation by introducing glycerol as a plasticizer to enhance the flexibility and mechanical strength of PVA thin films.One of the common plasticizers is glycerol which could be used to improve the mechanical properties of PVA thin films.This is due to its ability to act as a molecular lubricant and its tendency to reduce the tensions between PVA chains thereby chain mobility is increased resulting in reduced crystallinity [17].Further, based on numerous works, it is very clear that the incorporation of metallic nanoparticles into the PVA crosslink matrix could be beneficial.However, such nanoparticles are very expensive and have other complications.Alternatively, the incorporation of plasticizer into the PVA matrix can provide the desired properties.Moreover, researchers have explored various methods to produce PVA-based thin films used in food packaging applications; such as melt mixing [18], in situ polymerization [19], and mixing filler ingredients during electrospinning [20].Moreira et al 2020 created an edible antimicrobial covering for fruits using polysaccharide/PVA, effectively reducing water loss and inhibiting fungal growth [21], however poor mechanical properties were noticed.Nagalakshmi et al in 2010 showed that adding more NiO 2 to PVA could improve thermal stability [22].In a study by Liu et al in 2008, they examined how the addition of TiO 2 nanoparticles to PVA affected several properties.They found that increasing the concentration of TiO 2 nanoparticles resulted in lower loss tangent and higher storage modulus, leading to stronger nanocomposites [23].Another study by Salman et al in 2018 demonstrated that PVA/Ag-NPs films had excellent mechanical and antimicrobial properties, suitable for food packaging [24].Abdullah et al in 2019 developed transparent and biodegradable nanocomposite films for sustainable food packaging [25].However, the above techniques are complex and expansive.The simple approach for the synthesis of PVA-based thin films is solution-processed coating.Based on previous work by Channa et al [26], PVA-based film is prepared through magnetic mixing at a specific temperature, and composite material is added to the PVA matrix at various concentrations.The solution is then cast onto the substrate through doctor blading.The prepared thin film is then peeled off thereby behaving as a free-standing film.
The incorporation of glycerol into the PVA matrix can provide these properties and there is limited information available on PVA-glycerol thin films in terms of thermal stability, transparency, etc.Therefore, this work aims to enhance the flexibility, barrier properties, transparency, and thermal stability of PVA thin films by the addition of glycerol as a plasticizer.Polyvinyl alcohol/glycerol thin films were produced using a blade coating technique.Ultimately, this work forms the basis for the development of thin films that are both more adaptable and efficient across a wide range of applications that are reliant on thin film technology.

Materials and methods
The PVA in granular form was purchased from Sigma Aldrich, Germany.The purchased PVA had an average molecular weight between 85,000 and 124,000 and a hydrolysis degree within the range of 88%-90%.Glycerol was obtained locally from Biosynth Pharma (Pvt) Ltd, with a purity of 99.98% having a molecular weight of 92.1 g mol −1 .Double-deionized water was prepared in-house in the laboratory.
A PVA solution (15wt%) was prepared by dissolving PVA in double-deionized water (DDW).The DDW was used for better solubility compared with normal water [27].A homogeneous PVA solution is achieved by heating on a hot plate at approximately 80 °C while being stirred at 500 rpm (motivated by previous work by Hanh et al [28]).It typically took around 3-4 h to obtain a clear solution.Glycerol solution was added by weight percent to the mixture in various weight percentages ranging from 0-5 wt% and mixed thoroughly using a mechanical tumbler as shown in figure 1.
The solution was then cast on a glass substrate for thin film formation using a doctor blade.Thin film preparation took place under ambient conditions, with a 1000 μm gap set during the processing.The coating speed was maintained at a constant rate of 30 mm/s.Subsequently, the prepared coatings were carefully transferred to an oven and subjected to a temperature of 50 °C for 4.5 h [27].Once the coatings were completely dried, they were peeled off from the glass substrate and used as independent thin films for subsequent characterizations as shown in figure 1.

Characterisation
Fourier transform infrared (FT-IR) spectroscopy These prepared films were examined using an FT-IR instrument (ALPHA-P from Bruker) by selecting the wavelength ranging from 400 cm −1 to 4000 cm −1 .Spectra were acquired with a resolution of 4 cm −1 and by summing 64 scans.

Surface morphology
The surface morphology of entire sets of thin films was performed by optical microscopy (Olympus GX51).

Thermo-gravimetric analysis (TGA)
The thermal stability of all thin films was examined by a TGA analyzer (SDT Q-600 TA).The heating gradually increased at a rate of 10 °C min −1 in a nitrogen environment.The temperature range was set from 50 to 550 °C.

Differential scanning calorimetry (DSC)
The DSC analysis was conducted on all thin films using a DSC Analyzer (SDT Q-600 TA) equipped with a refrigerator cooling system.In this analysis, a sample weighing approximately 8.0 mg was securely sealed in an Aluminum (Al) pan and subjected to heating from 50 °C to 550 °C, at a rate of 10 °C min −1 , in an inert atmosphere.The glass transition temperatures were identified as the point where a noticeable shift occurred in the baseline, while the melting temperature was determined by locating the peak temperature in the endothermic event observed in the DSC curves.

Film transparency
The optical transparency of the thin films was analyzed using a double-beam UV-vis spectrometer (Cary-60).The optical transmission measurements were conducted in the range of 250 nm to 600 nm wavelengths.

Biodegradation
The biodegradation analysis of all thin films was determined by the weight loss and size reduction method.The thin films were buried in the soil for 4 weeks under ambient conditions.The weight and size of all thin films were checked before and after degradation.The percentage weight loss of prepared films was calculated using the following formula [29]: Water permeation A standard procedure of American Society of Testing Materials (ASTM) E-96 is followed (methodology outlined by Channa et al [30]) under conditions of 23 °C and 60% relative humidity (RH) with the help of an aluminum cup having a diameter of 6.35 cm for Water Vapor Transmission Rate (WVTR) to determine water permeation [31].

Flexibility
A cyclic bend tester was utilized to determine the flexibility of thin films, consisting of a fixed end and a movable end that oscillated back and forth, creating a predetermined radius of 5 cm.Each film underwent multiple bending cycles, and the WVTR was measured after each set of bending cycles [32].This test assessed the resistance of the thin film to bending.The samples were cut into the 3×10 cm 2 , for the bending test preparation.

Mechanical testing
The tensile strength of the thin film was evaluated by following the guidelines of the ASTM-D882 (methodology outlined by Davar et al [33].A Universal Tensile Machine (UTM) (Z005 Zwick/Roell) was utilized for the testing.A 5 mm/min speed with having 50 mm grip speed (to minimize slippage and uneven stress distribution of thin films) was utilized.Rectangular samples measuring 150×30 mm were prepared specifically for the tensile testing.A 5 N load was applied during the test.

Contact angel (Ca)
The water contact angle of pure PVA and PVA-glycerol (modified PVA) was determined using a contact angle goniometer (SL200A).This involved depositing 1.5 μL of double deionized water onto the thin film flat surfaces.Thin film samples were subjected to three droplets each to ensure average surface hydrophilicity.

FT-IR spectroscopy
The FT-IR analysis was used to analyze the functional groups present in the entire thin film.The FT-IR spectra of pure PVA, pure Glycerol, and various concentrations of Glycerol in the PVA matrix are shown in figure 2. The FT-IR spectra of prepared thin films show -OH stretching at around 3280 cm −1 [34], and C-H asymmetric stretching was witnessed at 2950 cm −1 [35].The peak for glycerol can be seen at about 900 cm −1 in entire modified films [36].The stretching of C=O from acetate groups in partially hydrolyzed PVA was observed at .These FT-IR results contribute to a better understanding of the composition of the PVA thin films incorporated with glycerol, providing valuable information for further investigations and potential applications [39].
The micrographs clearly illustrate the homogeneous integration of the plasticizer within the PVA matrix, with negligible segregation observed.A smooth surface is observed for pure PVA.Presumably, upon the addition of glycerol, the crosslinking was noticed.This can be justified by the pores formed in the films.This result is also in agreement with the work done by Wu X et al in 2024 [38].These pores are nearly uniformly distributed all over the PVA matrix as shown in figure 3.This reveals that glycerol acts as an effective plasticizer.Such effectiveness of compatibility hinges on factors such as the molecular weight, chemical structure, and functional groups of the plasticizer [40].

TGA analysis
The TGA analysis was used to assess the thermal stability and weight loss of pure and modified thin films.The modified thin films demonstrate impressive stability and enhanced resistance compared to pure PVA, as shown in figure 4. PVA-glycerol experiences a negligible amount of weight loss, while pure PVA exhibits a weight loss of 9% up to 200 °C.This weight loss in TGA occurs when a material undergoes thermal decomposition or degradation, resulting in the release of volatile components or the material's structural breakdown [41].Adding glycerol to PVA helps it resist weight loss at high temperatures, indicating enhanced thermal stability without experiencing significant degradation.This implies that glycerol functions as a stabilizing agent, reducing the rate of decomposition and augmenting the material's thermal resistance [42].

DSC analysis
Figure 5 shows the DSC thermogram analysis of the pure PVA and modified thin films.In both pure PVA and modified thin films, three endothermic peaks were observed.The first peak (between 90 and 150 °C) corresponds to the relaxation of crystalline domains of PVA.The second peak was observed between 200 and 225 °C, which signifies the melting of crystalline domains.While the third peak was noticed at 250 °C, representing the decomposition of PVA.Thus, upon the incorporation of glycerol into the PVA films; several changes were observed.The melting peak shifting was observed at lower temperatures, and it further aggravated upon higher loadings of glycerol.The decrease in the melting temperature (Tm) and the sharpening of peaks suggest the ordered arrangement of PVA molecules were disrupted by the presence of glycerol [43].This implies that glycerol enhances the segmental mobility of PVA and diminishes the crystalline regions within PVA.

Film transparency
The optical characteristics of pure PVA and modified thin films were assessed using UV-vis spectroscopy as shown in figure 6.The results from multiple experiments consistently demonstrated a significant increase in transmittance (over 90%) when glycerol was added to the PVA thin films.This increase in the transmission is further demonstrated by the exact transmission method at a wavelength of 450 nm as shown in figure 6 inset.It can be seen that pure PVA, without the presence of glycerol, exhibited a transmittance value of 88%.However, as the glycerol concentration gradually increased from 1 to 5 wt%, likewise the transmittance of the film improved thereby reaching above 90%.This increase in transmittance strongly suggests that the incorporation of glycerol  into the PVA thin films has a positive impact on its optical properties.Since glycerol is water-soluble it causes an improvement in light transmission [44].The incorporation of glycerol molecules into the PVA matrix potentially caused a reduction in light scattering, which in turn led to increased transmittance.This is because the glycerol diffused into the PVA matrix thereby causing homogeneity to reduce the light scattering.The reduction in light scattering intensity strongly depends on the index of refraction of the PVA-glycerol medium.This finding highlights the potential of PVA-glycerol thin films as transparent material for various applications that require high optical clarity [45].Considering the inset figure 6, the wavelength at 450 nm shows a relationship between transmittance and glycerol concentration.Transmittance values were observed at 91.56%, 89.98%, and 88.1% for glycerol loadings of 5 wt%, 3 wt%, and pure PVA, respectively.Thus, this incremental trend in transparency was observed.This phenomenon is consistent with UV-vis spectroscopy principles, which state that certain chemicals or molecules could absorb or scatter light at certain wavelengths.

Biodegradation analysis
The biodegradation performance of thin films is shown in table 1.These films were buried in the soil for four weeks under ambient conditions.After degradation in soil, the weight and size of such films decreased drastically.This is because of the interaction with microorganisms present in the soil.It is commonly known that PVA and glycerol can easily absorb moisture present in soil.This causes the growth of microorganisms and thereafter reduction in the size and weight of thin films [46].As shown in table 2, as the concentration of the glycerol increases, the rate of degradation of PVA-glycerol also increases.The highest degradation values were observed at 5 wt% of glycerol into PVA 4 weeks i.e., 19.98 wt%, which means the concentration of glycerol has a  direct effect on the degradation rate, this is due to the interaction of glycerol on the molecular chains of PVA.This is due to the disturbance of macromolecule arrangements of PVA chains [47].

Water permeation
The water permeation is determined by the WVTR test.The WVTR for various samples, including pure PVA and PVA-glycerol with varying glycerol concentrations as a plasticizer, was determined under conditions of 23 °C and 40 to 60% relative humidity (RH).The WVTR of pure PVA was measured at 21.87 g m −2 day −1 .A higher WVTR value of pure PVA was observed as shown in figure 7, indicating greater permeability thereby allowing a larger amount of water vapor to pass through the material.In contrast, PVA-glycerol samples containing higher loadings exhibited a lower WVTR compared to pure PVA.Under the specified conditions, PVA-glycerol allowed approximately 10.89 grams of water vapor to pass through each square meter of the material in 24 h.The introduction of glycerol disrupts the ordered arrangments of PVA chains thereby creating gaps between them.This, also reduced the overall density, thus leading to a porous structure.Such modification constrains the movement of water vapor molecules within the PVA-glycerol composite.Glycerol molecules act as barriers, obstructing the flow of water vapor and, as a result, reducing the WVTR [48].This decrease in water vapor permeability makes the PVA-glycerol material better suited for applications that require an efficient barrier against water vapors [38].

Flexibility
The flexibility bending test assesses the capacity of thin films to endure bending without fracturing or developing cracks.In this regard, the test was conducted with a bending radius of 5 cm.The results reveal that pure PVA as depicted in figure 8, exhibits relatively limited flexibility.This suggests that the PVA thin film is somewhat rigid and susceptible to cracking or breaking when subjected to bending stress.Reduced flexibility can restrict the material's ability to adapt to curved or uneven surfaces, rendering it less suitable for applications necessitating flexibility [49,50].Conversely, the incorporation of glycerol as a plasticizer into PVA matrix significantly enhances the flexibility.Figure 8 shows the normalized WVTR values of the thin films following various bending  cycles.After being bent, the PVA-glycerol thin films consistently returned to its original position and shape without losing internal chain adhesion.In contrast, the pure PVA thin films started to fracture and lose adhesion within fewer bending cycles.This, leads to WVTR degradation, resulting in an 11% degradation when compared to initial WVTR.While PVA-glycerol with 3 and 5 wt% concentrations, nearly maintained their initial WVTR even after 2000 bending cycles.This indicates that the presence of glycerol as a plasticizer has substantially enhanced the material's ability to withstand bending without fracturing.Plasticizers like glycerol are recognized for their capacity to augment the flexibility and elasticity of polymers.By incorporating glycerol into PVA, the molecular structure of the material is altered, leading to limited molecular mobility and diminished intermolecular forces.This enables the material to undergo greater elastic deformation under bending stress, resulting in improved flexibility [49,50].

Mechanical testing
The tensile test was conducted on thin films of both pure PVA and its modified counterpart.For the pure PVA thin film, the test results show a yield strength of 29.75 MPa, and the fracture strength of 19.25 MPa.While the  PVA-glycerol thin films exhibit higher yield strength i.e., 33.76 MPa as shown in figure 9.This indicates that the presence of glycerol increases the resistance to deformation under tension.However, the PVA-glycerol thin film shows a lower fracture strength of 9.07 MPa in comparasion to pure PVA.This reduction in fracture strength can be attributed to the plasticizing effect of glycerol [51].
Moreover, the tensile strength results indicate that the addition of 5% glycerol as a plasticizer in PVAglycerol thin films slightly increases the yield strength in comparasion to pure PVA (shown in table 2).However, it offers higher fracture strength than that of its modified counterpart, resulting in an overall stronger material that can withstand higher stress before breaking [51].Such reduction in the fracture strength is suggested due to constraints placed on chain movements due to plasticizer.

Contact angle (Ca)
The contact angle of all thin films such as pure PVA and PVA-glycerol at various concentrations were measured to ensure the wettability and hydrophilicity of thin films.The contact angle was measured using deionized water.As discussed earlier in table 2 pure PVA is the water-soluble biodegradable polymer and has a very low contact angle of 27.89°as shown in figure 10.As the concentration of glycerol increased from 1 to 5 wt%, likewise the slight change in contact angle was witnessed (figure 10).This increase in contact angle is attributed to the difference in hydrogen bonding between the two polymers i.e., PVA-glycerol, in comparasion to pure PVA.It is assumed that each OH -molecule of glycerol binds to that of the hydroxyl group of PVA chains.Furthermore, it should be noted that during film preparation, the rapid kinetics of solvent evaporation takes place, and a higher overall degree of crystallinity is anticipated for pure PVA [52].
This increase in contact angle with the increase in glycerol concentration is also responsible for the slight decrease in wettability and hydrophilicity [47].The decrease in wettability and hydrophilicity of PVA-glycerol thin films is helpful in biodegradable food packaging materials, and fuel cells.

Conclusions
The public awareness of health and environmental issues has drawn attention to the development of biodegradable polymers as alternative to traditional petroleum-based plastics.Keeping this in view, we prepared biodegradable polymeric thin films of PVA modified with numerous concentrations of glycerol.Based on the present set of experimental conditions, the following are the concluding remarks.Upon incorporation of glycerol; remarkable improvements in mechanical flexibility, transparency, barrier properties, and thermal stability were witnessed in comparison to pure PVA packaging films.In addition, the smooth surface of the thin films was observed in terms of minimum surface defects.These findings illustrate the favorable interactions of glycerol with PVA through intermolecular hydrogen bonding as noted in FT-IR spectrums.Based on the obtained results, the modified thin films could potentially be used in food packaging applications, membranes for biodegradable materials, etc.Thus, further exploration and optimization of such materials could contribute to mitigating environmental challenges associated with the use of petroleum-based polymers.

Figure 1 .
Figure 1.Process flow mechanism for preparation and characterization of thin films.

Figure 2 .
Figure 2. FT-IR peaks of pure PVA and PVA thin films modified with glycerol.

Figure 4 .
Figure 4. Weight loss Versus temperature graph of pure PVA and PVA thin films modified with glycerol.

Figure 5 .
Figure 5. DSC analysis of pure PVA and PVA thin films modified with glycerol.

Figure 6 .
Figure 6.The UV-vis spectra of pure PVA and PVA thin films modified with glycerol.While inset figure shows the exact transmittance at a wavelength of 450 nm.

Figure 7 .
Figure 7.The WVTR graph of pure PVA and PVA thin films modified with glycerol from 1 to 5 wt%.

Figure 8 .
Figure 8.The inverse of WVTR Versus Bending graph of pure PVA and PVA thin films modified with glycerol from 1 to 5 wt%.

Figure 9 .
Figure 9. Stress Versus strain graph of pure PVA and PVA thin films modified with glycerol.

Figure 10 .
Figure 10.Contact angle measurement of pure PVA and PVA thin films modified with glycerol from 1 to 5 wt%.

Table 2 .
Mechanical Properties of PVA-glycerol composite film at different concentrations.