Polyvinyl alcohol (PVA)-based films: insights from crosslinking and plasticizer incorporation

The properties of polyvinyl alcohol (PVA) films are intricately influenced by factors such as polymer structure, fabrication method, the addition of plasticizers and the molecular weight of monomers. This research, investigates the implication of PVA films using a solution casting method for crosslinking with boric acid (H3BO4), glycerol (C3H8O3) and citric acid (C6H8O7). This approach is compared with pure PVA films, establishing a valuable benchmark. For the experiments, tensile strength tests, physicochemical property measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were conducted to gain insights into the microstructure, surface characteristics and mineral composition of the films. This comprehensive approach aims to enhance our understanding of the intricate relationship between PVA, plasticizers and crosslinking agents, providing valuable insights for applications across diverse industries, including, construction and biomedical fields. The overarching objective of this research is to revolutionize the construction industry by developing polymer films that serve as the foundation for self-healing materials, fostering durability and innovation. The experiments revealed a significant influence of crosslinking agents on the properties of PVA films as measured.


Introduction
Polyvinyl alcohol (PVA), a non-toxic and synthetic polymer, holds widespread applications in industrial, commercial, medical and food-related settings [1].Its effectiveness as a binder, coating agent, moisture and oxygen barrier and film-forming material has propelled its widespread utilization [2,3].Moreover, PVA serves as a valuable additive in cement formulations [4].Chemically classified as a water-soluble, non-ionic polymer, PVA exhibits exceptional thermal stability, biodegradability, emulsifying and adhesive properties [5,6].
The chemical structure of PVA, particularly the hydrogen bonding between hydroxyl groups, plays a significant role in determining its properties.This bonding influences the polymer's water solubility, crystallinity range and chain length modulus.Various characteristics of PVA including strength, solubility, degree of polymerization and hydrolysis and the thickness of PVA-based films, are closely related to the film formation process.PVA films exhibit biodegradability and resistance to grease, oils and organic solvents, while also serving as an effective oxygen barrier, preventing the permeation of oxygen and other gases [7,8].The molecular chain properties of PVA, including molecular weight and degree of hydrolysis, significantly impact its characteristics.As molecular weight and hydrolysis percentage increase, viscosity, adhesivity, tensile strength and solvent resistance also increase.These diverse properties make PVA a versatile polymer with applications across various fields.
In addition to the aforementioned applications of PVA, it finds utility in the construction industry [9,10].PVA's influence on cementitious materials extends to both fresh and hardened states [11][12][13].A previous study suggested that PVA concrete demonstrates exceptional workability in comparison to conventional cementbased materials [14].The authors concluded that this improvement could be attributed to PVA's capacity to reduction in crystallinity and tensile strength and an increase in elongation [37][38][39].Additionally, complete crosslinking enhances the flame retardancy of the resulting film [40].Miyazaki et al [33] investigated the role of boric acid in the melting properties of PVA films.They observed that PVA films containing boric acid did not exhibit increases in crystallite size due to melting and recrystallization.The chemical reaction between boric acid and PVA in amorphous regions resulted in crosslinking points, effectively inhibiting recrystallization upon melting.
Glycerol has garnered increasing attention due to its versatility in various applications, including the food industry (as a plasticizer, stabilizer and emulsifier) and cosmetic industries [41,42].It is readily obtained from the hydrolysis of triglycerides, typically through saponification or biodiesel production [43].Glycerol's ability to replace hydrogen bonding within PVA molecules plays a crucial role in modifying the film's properties [44,45].This disruption hinders the crystallization of PVA and reduces the intensity of hydrogen bonding within the polymer network.Several studies have investigated glycerol as a plasticizer in PVA films, but the results regarding its impact on mechanical properties are conflicting [46][47][48].Some studies report enhanced tensile strength with increased glycerol content, while others observed reduced interaction between polymer chains and diminished shear resistance [49].
Several researchers have explored the comparative effectiveness of citric acid and glycerol as cross-linking agents for PVA films [50,51].CTR was found to establish stronger hydrogen bonds with PVA's hydroxyl groups, resulting in enhanced intermolecular interactions compared to glycerol [52].However, the degree of PVA hydrolysis significantly affects glycerol's influence on PVA [53,54].In another study [55], the impact of glycerol and boric acid on the mechanical properties of PVA-based films was examined.The study concluded that the optimal mechanical properties were achieved with a combination of approximately 14% wt.glycerol and 2% wt.boric acid.Unlike glycerol, which exhibits a more pronounced impact, boric acid appears to influence the toughness of PVA.
This research, delves into the influence of plasticizers on the production of PVA films for application in the construction sector.PVA-based membranes comprising citric acid, boric acid and glycerol were synthesized employing a solution casting approach.The films were characterized through tensile strength tests and physicochemical property measurements, encompassing swelling, water permeability and solubility, following immersion in an alkaline Ca(OH) 2 solution and distilled water.These experimental findings were reinforced by SEM and XRD analyses, yielding insights into the surface morphology of the films and the mineral composition of the raw materials, respectively.

Materials for the preparation of PVA films
In our experimental research, PVA powder was obtained from Sigma-Aldrich, with a hydrolysis degree exceeding 98% and a molecular weight between 89,000 and 98,000 g mol −1 , along with a moisture content of 2.8%, as determined in the laboratory.To prepare composite PVA films, boric acid (H 3 BO 3 ), citric acid (C 6 H 8 O 7 ) and glycerol were employed as crosslinking agents (figure 1).The selection of these materials was based on their non-hazardous nature and relatively environmentally friendly characteristics, as well as their strong interactions with the PVA chain.Polyvinyl alcohol is generally a water-soluble synthetic polymer with hydrophilic properties.Consequently, the use of crosslinking agents hinders PVA's dissolution and may enhance the physical and mechanical properties of the modified films.Figure 1 illustrates the molecular structures of PVA, boric acid, citric acid and glycerol, while table 1 summarizes their physical properties.
Poly(vinyl alcohol) (PVA) films were fabricated using a casting evaporation technique from an aqueous PVA solution.Continuous stirring at 600-1400 rpm, was applied, with temperature monitoring for 60 min (for pure PVA films) and 3-5 h (for cross-linked PVA films).The solution temperature was progressively raised to 95 °C during the dissolution and polymerization of PVA powder, while maintaining a constant volume by adding   2) were peeled from the Petri dishes and stored in a desiccator with anhydrous calcium chloride (CaCl 2 ) at room temperature until testing.To modify the film's properties, cross-linking agents and plasticizers such as boric acid, citric acid and glycerol were used in this study.Four distinct blends were synthesized in the laboratory: (i) Pure PVA, (ii) PVA/glycerol (code: PGL), (iii) PVA/boric acid (code: PBR) and (iv) PVA/citric (code: PCT).Table 2 summarizes the constituents of each film type.

Characterization techniques 2.2.1. XRD and SEM analyses
Powder X-Ray Diffraction (XRD) analyses were conducted to identify the crystal phases in the raw materials, including PVA powder, citric acid and boric acid.Citric acid was finely ground into particles with a size less than 0.063 mm, while boric acid and PVA were already in powder form.The Siemens D5000 diffractometer was employed for XRD analyses, while the crystal phases were identified using Bruker's EVA software connected to the International Centre for Diffraction Data (ICDD) database.The samples were scanned from 0°to 70°at a rate of 1°/min.For the experimental set-up, pure and composite cross-linked PVA films prepared in the lab underwent examination using a Phillips XL 30 high resolution SEM.The SEM analyses, conducted in high vacuum mode at 30 kV, provided a maximum resolution of 2.0 nm with magnification ranging from 20x to 800,000x.Before analysis, a gold layer (30-40 nm) was sputter-coated onto the samples to ensure their surfaces were conductive.

Tensile strength and thickness measurements
The tensile strength of PVA films was measured following 48 h of oven-drying at 40 °C.The tests were conducted following the procedure outlined in ASTM Standard D882.To evaluate the tensile strength and elongation resistance of the films, rectangular strips measuring 2.5 ×15 cm 2 were utilized.These strips were excised from the original films and stored in a desiccator with silica gel until the time of testing.Subsequently, they were mounted onto the tensile test machine, operating at a loading speed of 10-20 mm /min.Eight (8) strips were tested for each group and their thickness and width were measured using a digital micrometer to the nearest 0.025 mm.Three thickness measurements were taken for each strip (at the center and both edges) and the average was calculated.The conditioning and testing of the films were conducted under controlled conditions of 25 ± 2 °C and 45 ± 5% relative humidity.The tensile strength ( f t ) of the films was determined using equation (1).
Where, f is the maximum load at failure (N) and W, T are the width (mm) and thickness (mm) of each strip, respectively.

Apparent solubility of films
To assess the durability, the polymer films underwent exposure to immersion in both distilled water and a highly alkaline Ca(OH) 2 solution with a pH of 12.5.The latter solution was prepared by gradually adding CaO (0.016 M) to water under vigorous stirring.Upon contact with water, calcium oxide (CaO) reacts to form calcium hydroxide (Ca(OH) 2 ), a base with a high pH (>7), as shown in equation (2): The alkalinity of the Ca(OH) 2 solution was determined using a portable pH/MV meter.Rectangular strips measuring 1 cm in width and 2 cm in length were excised from the films and weighed for the experiments.Subsequently, these strips were immersed in the solution for 60 min, followed by drying in an oven at 40 °C for 48 h.Additionally, some of the films were left immersed in the solutions for 24 h to simulate the film's solubility during the hardening of calcite.The solubility of the films was calculated using the following equation: Where, M initial is the mass of the film at the beginning and M final its mass after immersion in distilled water and Ca(OH) 2 .

Permeation of films
Water permeability (WP) was evaluated using a 6-cm diameter glass cup containing silica gel that entirely covered the PVA films under assessment.The sealed cups were positioned in an environmental chamber held at 25 °C and 95% relative humidity for 24 h and the weight gain of the silica gel was subsequently measured.The WP was calculated using equation (4): Where, WP is the water permeability (g/m 2 h -1 ), Δm is the difference in weight of silica gel (g), t is the time the cups remained in the chamber (24 h) and S is the cross-sectional area (0.96 × 10 -3 m 2 ).

Swelling measurements
The water absorption of the films was evaluated using square specimens with dimensions of 2 cm×2 cm.The initial dimensions of the films were accurately measured using a digital caliper before immersion in 20 ml of distilled water and a simulated pore solution with a pH of 12.5 (Ca(OH) 2 ) for 5 h.Following this immersion period, the samples were retrieved from the solutions and their dimensions were re-measured.Four specimens were cut and measured for each film and the results were averaged to determine the overall water absorption behavior.The swelling index, defined as the percentage increase in film dimensions after immersion, was calculated using the following formula: Where, X 1 , Y 1 and X 2 , Y 2 are the dimensions of the pieces before and after swelling, respectively.

XRD analyses and microscopical observations
The patterns of X-ray diffraction (XRD) for citric acid, polyvinyl alcohol (PVA) and boric acid offer valuable insights into their crystallinity and structural arrangements (figure 3).Citric acid exhibits a highly crystalline nature, as evidenced by its sharp and intense diffraction peaks at 2θ values ranging from 14°to 45°.These peaks correspond to interplanar spacings consistent with the well-ordered arrangement of atoms in citric acid's crystal lattice, which is characterized by hydrogen-bonded molecular networks.In contrast, PVA's XRD pattern presents a broad, diffuse peak centered at 2θ ≈ 20°, a characteristic of semi-crystalline materials with amorphous regions indicative of less ordered chain arrangements.This broad peak suggests that PVA possesses both crystalline regions with ordered polymer chains and amorphous regions with less organized chain arrangements.Boric acid's XRD pattern closely resembles that of citric acid, exhibiting sharp peaks at 2θ values of ≈15°and 28.5°.These peaks align with the known crystal structure of boric acid, indicating a high degree of crystallinity.By comparing these XRD patterns, we can readily differentiate the materials' crystallinity; this analysis highlights the valuable role of XRD as a tool for identifying and characterizing materials based on their crystallographic properties.Citric and boric acids exhibit highly ordered structures, whereas PVA's pattern reveals a semi-crystalline nature with both crystalline and amorphous domains.XRD analysis thus serves as a valuable tool for identifying and characterizing materials based on their crystallographic properties.The microscopy images offer valuable insights into the morphology and microstructure of boric acid and citric acid.The SEM analysis highlights the distinct morphological characteristics of crosslinks, providing valuable information to understand their roles in the development and properties of PVA films.
The SEM micrographs of boric acid (figure 4-top) reveal sheet-like crystallization structures indicative of tabular crystals with a layered arrangement.This morphology is consistent with the triclinic crystal system of boric acid.The SEM micrographs also exhibit areas with a smooth, plate-shaped surface, further supporting the layered structure.Additionally, angular particles with a highly consistent, non-porous surface were observed, possibly representing fragments of the larger plate-like crystals.These findings collectively suggest a multilayered or scaly texture of boric acid particles.Finally, the cracks in the layers of boric acid are due to the presence of impurities.These impurities can interfere with the crystal growth process and they can also create stress points in the crystal lattice.As a result, the cracks can form and expand over time.
Microscopic examination of citric acid powder reveals a diverse range of particle shapes and sizes.The particles exhibit a random arrangement, with some overlapping others.Their surface texture is characterized by numerous small bumps and ridges, creating a rough appearance.Scanning electron microscopy (SEM) images of citric acid (figure 4-bottom) depict a polydisperse distribution of particles, indicating a range of crystal sizes.The particles predominantly maintain a spherical morphology, while the porous and cracked surface of citric acid suggests that dehydration during preparation may have influenced its morphology.The observed discoloration and slit-like features are plausibly attributed to the coexistence of hydrous and anhydrous phases and the low hardness of the crystals, respectively.Additionally, slit-like features were observed, likely resulting from the low hardness or abrasion resistance of the crystals.Finally, the rough surface texture of the particles is attributed to the formation of intermolecular forces, particularly hydrogen bonds, between the citric acid molecules Figure 5 illustrates the surface topographies of PVA films, both pure and cross-linked.The pure PVA film presents a flat, smooth surface without cracks.However, upon closer examination at a magnification of x3000,   stains potentially arising from unreacted PVA powder and small, closed pores become discernible.This morphology is attributed to the relatively weak intermolecular forces between PVA chains.The absence of significant defects indicates the limited mechanical strength and water permeability of pure PVA films.The PBR films display a rougher surface compared to pure PVA films.This change in morphology is ascribed to the introduction of boric acid crosslinks, disrupting the smooth alignment of PVA chains.The presence of air bubbles and small pores further indicates the increased porosity of PBR films.The greyish/white regions within the film represent unreacted boric acid, suggesting incomplete crosslinking may have occurred.SEM images of PCT films reveal a slightly rough surface compared to pure PVA films, but with improved interaction between the matrix and solvent compared to PBR films.This suggests that PCT crosslinks facilitate the formation of a more interconnected network structure, enhancing the film's mechanical properties.SEM images of PGL films depict a non-uniform structure with surface cracks and wrinkles.These defects are attributed to the hygroscopic nature of glycerol, promoting water absorption during film formation.The preferential orientation of cracks along the drawing direction suggests that the film's mechanical properties are anisotropic.
In conclusion, SEM analysis provided valuable insights into the surface morphology of PVA films and how different crosslinking agents influence it.Pure PVA films had a smooth surface, while PBR films exhibited a rougher surface, potentially influencing factors like mechanical strength or adhesion properties.PCT films displayed a slightly rough surface with enhanced water uptake (or specify the improved interaction) compared to PBR films.Finally, PGL films displayed a non-uniform structure with surface cracks and wrinkles, likely a consequence of their hygroscopic nature that can lead to uneven swelling and potential film integrity issues.

Tensile strength and thickness of films
Thickness measurements were taken at five different points along the length of each strip.Figures 6 and 7 depict the results of these tests.As evident from the findings of this research, the incorporation of a cross-linker in polyvinyl alcohol (PVA) rises its tensile strength.This enhancement is attributed to the role of cross-linkers in establishing bonds between PVA molecules, thereby constructing a more robust and interconnected network that effectively counteracts the forces leading to tensile failure.
From the results (figure 6), it is clearly observed that the addition of boric and citric acids resulted in an impressive 4 and 3.5-times higher strength values than those of the reference films.In contrast, glycerol demonstrated the least effectiveness among the three considered cross-linkers; specifically, the PGL group showed an increase in strength by 77%.The superior performance of boric and citric acids can be attributed to their unique ability to form robust coordination bonds with the hydroxyl groups on PVA molecules.These stable bonds exhibit greater resistance to degradation under stress compared to bonds formed by other crosslinkers.However, among the aforementioned cross-linkers, citric acid forms slightly weaker hydrogen bonds with PVA hydroxyl groups, making them more susceptible to breakdown under stress.Glycerol is the least effective cross-linker because it can't form bonds with PVA molecules.Instead, it acts as a lubricant, which could weaken the concrete.
In summary, the outcomes of this research underscore the significant improvement in the tensile strength of PVA concrete through the incorporation of a cross-linker.Boric acid emerges as the most effective cross-linker, followed by citric acid and glycerol.These findings contribute valuable insights to the field of concrete materials engineering.The experimental results also demonstrated that the additions had an impact on the thickness of the produced films (figure 7).The measured thickness is approximately 87.2 mm for PVA, 113.2 mm for PGL, 135.9 mm for PCT and 89.3 mm for PBR blend, respectively.Specifically, the PCT mix exhibited the most significant increase in thickness, at 49%, followed by PGL, which increased by 26% compared to the PVA group.This implies that these crosslinks positively affect the thickness of the material, suggesting their potential role as catalysts influencing the fresh properties of concrete (such as slump, hardening, curing and setting time).On the other hand, when added boric acid to the PVA mixture, the thickness slightly increased compared to the reference (PVA).This change can be attributed to the low concentration of boric acid, resulting in a minor alteration in the thickness of the PVA and potential crosslinking between PVA chains and boric acid molecules.However, other factors, such as the mixing method, the duration allowed for crosslinking and the presence of additional additives, can also play a significant role in determining the thickness of the resulting film.

Permeation and apparent solubility of films
Figure 8 illustrates the impact of additives on water permeability.According to the experimental results, the PGL mixture exhibits higher water absorption than the other groups, suggesting that the addition of glycerol may have increased the film's porosity, making it more susceptible to water absorption.As for the other groups, the PCT and PVA mixtures have equal permeation values, indicating that the citric acid addition does not have an adverse effect on the film's permeability properties.In contrast, the PBR mixture exhibits substantially lower water absorption, approximately 2.5 to 3 times less than the other mixtures.This suggests that the addition of boric acid has a positive effect on reducing water absorption, potentially enhancing the film's resistance to water ingress.
The permeability of PVA films is significantly reduced upon crosslinking with boric acid.This reduction in permeability is attributed to the formation of physical and chemical crosslinks within the polymer matrix  [ [56][57][58].Physical crosslinks, induced by hydrogen bonding between boric acid and PVA chains, act as roadblocks for diffusing molecules, hindering their movement through the film.On the other hand, the addition of glycerol to PVA films can increase their permeability to water.This is because glycerol acts as a plasticizer, which can disrupt the tightly packed structure of PVA molecules and make them more porous [59,60].In addition, the presence of glycerol can reduce the crystallinity of PVA, which further enhances its permeability [61,62].Crystalline regions of PVA are more tightly packed and less permeable than amorphous regions.When glycerol is added, it can disrupt these crystalline regions and make the entire film more amorphous.
Figure 9 illustrates the solubility of films in both calcium hydroxide (Ca(OH) 2 ) solution and water after a 60 min period, while figure 10 displays the corresponding values after 24 h of immersion in the solutions.As can be seen from the graph (figure 9), all four films are more soluble in the alkaline solution than in water.This is because the alkaline solution can break down the hydrogen bonds between the polymer chains, making the films more water-soluble.On closer examination in figure 9, it is evident that the solubility of films containing citric and boric acids is slightly lower, both in water and alkaline solutions, ranging from 1 to 4%.Citric and boric acids may form complexes with calcium ions from PVA in the alkaline solution.This complex formation could limit dissolution, even under alkaline conditions.This suggests a significant impact on limiting dissolution, even in alkaline conditions.Consequently, employing this composite polymer in concrete production implies that these compounds could be effective additives for enhancing concrete durability.Conversely, the PGL group exhibits lower solubility compared to PVA by 60% and 55% in both Ca(OH) 2 and water.This suggests that glycerol may enhance resistance to alkaline dissolution, making it a promising additive for environments rich in  calcium hydroxide.Glycerol might achieve this by increasing the film's viscosity and hindering water penetration.
The solubility measurements of the films were conducted after 24 h of immersion in the solutions (figure 10).As evident, the solubility of all films increases over time.This is attributed to the prolonged exposure of the films to the solvents, allowing the solvent molecules to break down the polymer chains.The most soluble film is still PVA, followed by PGL, consistent with the earlier results.In the case of PCT and PBR blends, the films exhibit low solubility values, making them suitable for use in concrete production.When PVA is crosslinked with citric acid, hydroxyl groups form ester bonds with citric acid, reducing the solubility of the PVA film.Moreover, these ester bonds increase the hydrophilicity of the polymer chains, repelling water molecules and further decreasing solubility.The decreased solubility of PVA/boric acid films, compared to pure PVA films, primarily results from the formation of crosslinks between polymer chains [63].These crosslinks are created through the reaction of boric acid with hydroxyl groups on the PVA backbone, introducing rigid constraints within the polymer network.This modification in the polymer structure hampers the ability of individual polymer chains to dissociate into soluble molecules.The formation of crosslinks with boric acid also alters the hydrophilicity of the PVA chains.This is because the crosslinks can create a more rigid and less porous structure, which can make it more difficult for water molecules to penetrate the film.In addition, the crosslinks can also change the surface properties of the film, making it less hydrophilic.Boric acid introduces borate groups, which exhibit increased hydrophilicity compared to the hydroxyl groups on pure PVA.This enhanced hydrophilicity promotes interactions between the polymer chains and water molecules, further impeding their ability to dissociate and dissolve.In addition to crosslinking and changes in hydrophilicity, the presence of boric acid can contribute to reduced solubility through increased crystallinity of the PVA film.Boric acid can act as a plasticizer, reducing chain mobility and promoting crystallization.The formation of crystalline domains further impedes the dissolution of PVA chains.
In summary, the additives used can reduce the film's solubility in concrete, which may be beneficial for enhancing concrete durability in both Ca(OH) 2 and water.The PBR group containing boric acid stands out for its consistently low solubility, suggesting its potential as an effective additive for concrete durability.

Evaluation of swelling property of films
In addition to investigating thickness and strength, our research explored the swelling behavior of concrete mixtures, a crucial factor influencing the durability and performance of concrete structures over time (figure 11).The results suggest that adding glycerol, citric acid and boric acid to the PVA mixture significantly reduced water swelling, indicating an inhibitory effect.The mixtures with glycerol and citric acid exhibited reduced swelling in water, with glycerol offering a 41% reduction and citric acid a 27% reduction, compared to the unaffected PBR film.Similar trends were observed in Ca(OH) 2 swelling, with glycerol and citric acid again demonstrating a significant reduction compared to the PVA mixture.In line with the previous results, boric acid-maintained Ca(OH) 2 swelling, suggesting a strong influence on the interaction between Ca(OH) 2 and the binder.
The reduced solubility of PVA films crosslinked by boric acid, glycerol, or citric acid compared to pure PVA is attributed to a combination of factors that alter the polymer structure and its interactions with water molecules.Crosslinking introduces covalent bonds between adjacent polymer chains, forming a network of interconnected chains.This network structure restricts the mobility of individual chains, making them less likely to dissociate into soluble molecules.The crosslinking agents, such as boric acid, glycerol, or citric acid, typically react with hydroxyl groups on the PVA backbone, linking the chains together.These crosslinks not only restrict chain mobility but also introduce hydrophilic groups into the PVA chains.For instance, boric acid forms borate groups, while citric acid forms ester bonds.These hydrophilic groups enhance the interactions between the polymer chains and water molecules, effectively increasing the overall hydrophilicity of the material.Furthermore, certain crosslinking agents, such as boric acid, can act as plasticizers, further reducing chain mobility and promoting crystallization.The formation of crystalline domains within the PVA film further hinders the dissolution of polymer chains.Crystalline regions exhibit a higher degree of order and organization, making it more difficult for water molecules to penetrate and disrupt the cohesive forces within the structure.The combined effects of crosslinking, increased hydrophilicity and potential crystallinity enhancement lead to a significant reduction in the solubility of PVA films crosslinked by boric acid, glycerol, or citric acid compared to pure PVA.These modifications in the polymer structure are often employed to enhance the mechanical properties, durability and resistance to solvents or moisture of PVA-based materials.

Potential applications of composite PVAfilms
This study demonstrates the potential use of crosslinking agents and plasticizers for improving the properties of PVA films for construction applications.To fully understand their suitability in real-world scenarios, future research should delve deeper into several areas.A crucial next step is evaluating the thermomechanical behavior of the films.This would involve assessing their thermal stability and mechanical performance under varying temperatures, providing valuable insights into how they would respond to the diverse thermal conditions encountered in construction environments.Further investigation is warranted into the effectiveness of PVA/ boron films in neutron shielding, the elastomeric properties of these films, and how citric acid influences asphalt modification.Additionally, investigations into the effectiveness of PVA/boron films in neutron shielding, the elastomeric properties of these films, and how citric acid influences asphalt modification are warranted.Furthermore, research is needed to understand how PVA modifications with citric acid impact durability and self-healing in concrete.Finally, optimizing microcapsule compatibility with PVA films and ensuring their longterm performance through studies on solubility, thermal stability and the use of biodegradable crosslinkers are crucial for the successful implementation of self-healing mechanisms in construction materials.By exploring these diverse avenues, researchers can further refine and unlock the full potential of crosslinked PVA films for the construction industry.
As it appears from the text, the study highlights the potential of PVA for various construction applications.Incorporating PVA into cement and concrete strengthens the final product, benefiting structures like buildings, roads and columns.Additionally, PVA/boron films act as radiation shielding due to their neutron absorption and improve the elasticity of cementitious materials.Conversely, PVA modified with citric acid (PVA/CTR) enhances asphalt performance by modifying its elasticity and softening point.Furthermore, PVA/CTR offers chemical resistance, making it a valuable component for durable and corrosion-resistant polymer concrete.PVA films also play a crucial role in self-healing concrete through microcapsules.However, for successful selfhealing, further research is needed on PVA film properties like solubility, thermal stability and compatibility with alkaline environments.Exploring biodegradable crosslinkers for PVA could further improve concrete performance.By addressing these aspects, researchers can unlock the full potential of PVA for the construction industry.

Conclusions
This study investigated the influence of boric acid, citric acid and glycerol as crosslinkers on various properties of polyvinyl alcohol (PVA) films for use in construction industry.
Boric acid significantly improved tensile strength, reduced water absorption and lowered solubility in both water and alkaline solution.This suggests that PVA-based composite films provide a combination of properties that is particularly well-suited for concrete applications, such as improved strength and crack resistance.SEManalysis (refer to figure 4) suggests boric acid interacts with PVA through hydrogen bonding between its hydroxyl groups and hydroxyl groups on the PVA backbone.This cross-linking mechanism effectively restricts the mobility of PVA chains, leading to the observed improvements in mechanical strength and water resistance.
While less effective than boric acid in enhancing tensile strength, the incorporation of citric acid as a crosslinker in PVA films still led to notable improvements.Citric acid also played a crucial role in reducing water absorption and film solubility.These enhancements, including increased thickness, strength, water repellency and reduced solubility, make PVA films with citric acid suitable for applications demanding durability and resistance, such as concrete production in water and alkaline environments.
Compared to boric acid and citric acid, glycerol exhibiteda more moderate impact on PVA films.While it may slightly increase viscosity and offer some resistance to water penetration, its plasticizing effect weakens the film structure, ultimately leading to increased water absorption.
Overall, the experiments proved, the potential use of boric and citric acid as effective crosslinkers for enhancing the properties of PVA films for concrete applications.Further research could explore the optimal concentrations of these crosslinkers and investigate their combined crosslinking effects when used simultaneously.On the other hand, glycerol has a gentler influence on the film's properties compared to boric and citric acid.However, it can still provide some protection against water ingress.Despite this initial water resistance, the weakened structure caused by glycerol may lead to increased water absorption overall.

Figure 3 .
Figure 3. XRD patterns for PVA powder, Citric acid and Boric acid used in production of polymer films.

Figure 4 .
Figure 4. Boric Acid (top) and Citric Acid (bottom): SEM images of crosslinking agents used in the production of composite PVA films.

Figure 5 .
Figure 5. Scanning Electron Microscopy images for pure PVA as well as composite films incorporating citric acid (PCT), boric acid (PBR) and glycerol (PGL).

Figure 8 .
Figure 8. Water permeability of films after 1 h of immersion in water.

Figure 9 .
Figure 9. Solubility in water and alkaline solutions of films after 1 h of immersion in the solution.

Figure 10 .
Figure 10.Solubility in water and alkaline solutions of films after 24 h of immersion in the solution.

Figure 11 .
Figure 11.Swelling in water and alkaline solutions of films after 1 h of immersion in the solution.

Table 1 .
Properties of polyvinyl alcohol (PVA), boric acid and citric acid.After heating, the solutions were gently swirled to eliminate any trapped air bubbles.A fixed volume (20 ml) of PVA solution was poured into four glass Petri dishes with a 150-mm diameter and then dried at 40 °C for 48 h.The resulting dried films (figure

Table 2 .
PVA films with and without a crosslinking agent prepared in the laboratory.