Poly(1,3-Propylene Glycol Citrate) as a Plasticizer for Toughness Enhancement of Poly-L-Lactic Acid

Despite the unique features of poly-L-lactic acid (PLLA), its mechanical properties, such as the elongation at break, need improvement to broaden its application scope. Herein, poly(1,3-propylene glycol citrate) (PO3GCA) was synthesized via a one-step reaction and evaluated as a plasticizer for PLLA films. Thin-film characterization of PLLA/PO3GCA films prepared via solution casting revealed that PO3GCA shows good compatibility with PLLA. The addition of PO3GCA slightly improves the thermal stability and enhances the toughness of PLLA films. In particular, the elongation at break of the PLLA/PO3GCA films with PO3GCA mass contents of 5%, 10%, 15%, and 20% increases to 172%, 209%, 230%, and 218%, respectively. Therefore, PO3GCA is promising as a plasticizer for PLLA.


Introduction
Polylactic acid (PLA) is a biodegradable and eco-friendly polymer that has garnered significant attention in recent years due to its numerous advantages. PLA is derived from renewable plant resources, such as corn, potato, and jackfruit, through a fermentation process that produces high-purity lactic acid. The lactic acid is then used to synthesize PLA of the desired molecular weight through chemical processes [1][2][3].
PLA is biocompatible and is widely used in the medical field, including as implant materials, drug delivery systems, and tissue engineering scaffolds [4]. It is also used in commodity packaging, where its biodegradability and low environmental impact make it an attractive alternative to traditional plastics [5]. Additionally, PLA has been explored for use in textiles due to its biodegradability and potential to reduce the environmental impact of textile production [6].
Despite its numerous advantages, PLA has limitations that have hindered its widespread application. One of the major drawbacks of PLA is its high glass transition temperature (Tg) which ranges from 55 • C to 65 • C, making it brittle and fragile at room temperature. Additionally, the elongation at the break of PLA is limited to only~5%, which further limits its practical use [7]. Efforts have been made to improve the mechanical properties of PLA through various methods, including blending it with other polymers, copolymerization, and the addition of nanoparticles [7][8][9].
To improve the properties and performance of Polylactic acid (PLA), two commonly used methods include the copolymerization of lactic acid with other polymer monomers, such as trimethylene carbonate and caprolactone [8,9], and the preparation of physical blends of PLA with different polymers, small molecular compounds, or even inorganic materials. These approaches have shown great potential in enhancing the thermal stability,

Synthesis of PO3GCA
In this study, the PO3GCA copolymer was synthesized using a direct synthesis method [12,[18][19][20][21][22]. Specifically, PO3G (300 g) and CA (21 g) were added to a 500 mL three-necked flask in a 1.3:1 molar ratio, with 0.5% (1.6 g) molybdenum trioxide used as the catalyst. After purging the reaction system with nitrogen gas three times, the system was evacuated to a pressure of 100 Pa using an oil pump. The system was gradually heated to 150 • C and held for 30 min, followed by heating at 170 • C for 8 h. Mechanical stirring was employed throughout the entire reaction process. After natural cooling to room temperature, the product was dissolved in dichloromethane (DCM) and stirred magnetically for 1 h. The solution was then left to stand for 10 h until most of the molybdenum trioxide precipitated at the bottom of the flask. The liquid above the precipitate was filtered and collected, and the filtrate was filtered 2-3 times to remove all molybdenum trioxide. The resulting filtrate was placed in the flask and distilled at 50 • C until no more vapor was produced. The second distillation was carried out at 110 • C until no more vapor was produced, yielding approximately 290 g of a light yellow to yellow, viscous liquid, with a yield of approximately 90.3%. This light yellow to yellow, viscous liquid is PO3GCA. Figure 1 shows the reaction equation of PO3GCA synthesized from poly (1, 3-propylene glycol) and citric acid.

Preparation of PLLA/PO3GCA Films
The study involved the preparation of PLLA/PO3GCA composite films with a total mass concentration of 20%. The proportion of PO3GCA to solute ranged from 0% to 20%, with increments of 5% (The solvent used for the solution was 1, 4-dioxane). The solution was stirred until it reached a temperature of 80 °C, followed by ultrasonic defoaming. The solution was then scraped onto a clean glass plate using a film scraper (750 μm) and allowed to dry after solvent volatilization. The resulting composite film samples had a uniform thickness of 0.07-0.08 mm and were naturally dried at room temperature for 48 h before being collected and stored for further use. The samples were labeled PLLA,

Preparation of PLLA/PO3GCA Films
The study involved the preparation of PLLA/PO3GCA composite films with a total mass concentration of 20%. The proportion of PO3GCA to solute ranged from 0% to 20%, with increments of 5% (The solvent used for the solution was 1,4-dioxane). The solution was stirred until it reached a temperature of 80 • C, followed by ultrasonic defoaming. The solution was then scraped onto a clean glass plate using a film scraper (750 µm) and allowed to dry after solvent volatilization. The resulting composite film samples had a uniform thickness of 0.07-0.08 mm and were naturally dried at room temperature for 48 h before being collected and stored for further use. The samples were labeled PLLA, PO3GCA5, PO3GCA10, PO3GCA15, and PO3GCA20, according to the concentration of PO3GCA. The samples for 1 H NMR spectroscopy analysis were prepared by placing 15 mg of PO3GCA in an NMR tube and dissolving it using 480 µL of Deuterium chloroform. Tetramethylsilane was used as the internal standard.

FTIR Spectroscopy Characterization of PO3GCA
KBr pellets were prepared by mixing and grinding dried PO3GCA powder and KBr in a mass ratio of 2:100, followed by pressing with a tablet press. The FTIR spectra were recorded over a range of 400-4000 cm −1 .

Tensile Test of the PLLA/PO3GCA Films
The PLLA/PO3GCA films were cut into smooth and nonwound samples with a size of 20 mm × 60 mm. At ambient temperature (25 • C), the initial length of the sample was 50 mm, the tensile rate was 10 mm/min, and the relative humidity was 60%. The sample was fixed on the fixture of the testing machine. The clamping length of the sample film was 50 mm, the force measurement accuracy was 0.01 cN, and the elongation accuracy was 0.01 mm. Stress-strain curves were obtained by testing each sample on a microcomputercontrolled electronic universal testing machine five times. The elongation at the break of each PLLA/PO3GCA film was determined using Equation (1). The thermal properties of PLLA/PO3GCA thin films were analyzed using differential scanning calorimetry (DSC). Film samples weighing approximately 5-8 mg were placed in an aluminum crucible and heated from −40 • C to 200 • C at a rate of 10 • C/min to obtain the DSC curve of the film samples [23].
The crystallinity of the composite films was calculated using Equation (2) [24,25], where X c denotes crystallinity, ∆H m denotes the enthalpy of melting, ∆H cc denotes the enthalpy of cold crystallization, ∆H mPLLA denotes the standard enthalpy of melting of PLLA (93.6 J·g), and ω PLLA represents the mass fraction of PLLA in the composite film. The morphology and fracture surface of the PLLA/PO3GCA films were examined using field-emission scanning electron microscopy (SEM). For surface morphology analysis, fully dried films were affixed to the sample table using conductive carbon glue and coated with a thin layer of gold for 30 s under vacuum conditions to enhance their conductivity. The samples were then observed at various magnifications at an accelerating voltage of 2 kV following vacuum extraction, and the corresponding images were captured for further analysis. For fracture surface analysis, the cross-sections of sample bars pulled at a rate of 10 mm/min on a universal tensile testing machine were used for SEM testing. Prior to testing, the surface of the samples was coated with a layer of conductive metal to enhance their conductivity, and then the samples were fractured to expose their internal structure. The SEM imaging process involved several steps, including surface treatment of the samples, vacuum extraction to remove air and moisture, and observation and capture of images at various magnifications in the SEM. This rigorous sample preparation and imaging protocol ensured the high-resolution imaging of the PLLA/PO3GCA films, allowing for a detailed examination of their microstructure, surface characteristics, and fracture behavior.
2.4.6. Thermogravimetric Analysis (TGA) of the PLLA/PO3GCA Films The thermal stability of the PLLA/PO3GCA films was evaluated via TGA from room temperature (23 • C) to 600 • C at a heating rate of 10 • C/min under flowing nitrogen. The differential thermogravimetric (DTG) curves were obtained by differentiating the TGA curves. Figure 2 shows the FTIR spectra of PO3G, PO3GCA, and CA. In the graph of PO3G, the peaks at 3500 and 1108 cm are the stretching vibrations of the terminal OH and ether bond C-O. The ester absorption peak at 1737 cm in the graph of PO3GCA shifted to the right compared to the carboxyl absorption peak at 1728 cm in the graph of CA, indicating that an esterification reaction occurred. The OH absorption peak near 3500 cm is significantly weaker than the OH absorption peak in the PO3G spectrum, which also indicates that OH participates in the esterification reaction.   Figure 3 shows the 1 H NMR spectra of CA, PO3G, and PO3GCA. In the spectrum of CA, the two peaks at 2.7-2.7 ppm correspond to -CH 2 -(C=O)OH. In the spectrum of PO3G, the peaks at 3.55, 3.5, and 1.8 ppm can be attributed to the -CH 2 -OH-CH 2 -CH 2 -O, and -CH 2 -CH 2 -groups, respectively; the integral areas of these peaks were determined to be 1, 19.57, and 39.12, respectively. For PO3G, the average degree of polymerization    Thermal properties of PLLA/PO3GCA composite films and PO3GCA were analyzed using thermogravimetric (TG) and derivative thermogravimetric (DTG) curves, as shown in Figures 4 and 5, respectively. The weight loss of PLLA and PLLA/PO3GCA composite membranes occurred in the temperature range of 75-140 • C, which was attributed to the evaporation of bonded water and residual solvents in the membranes. In contrast, the weight loss of PO3GCA was not evident in this temperature range. The PLLA thin film experienced a second weight loss at 295-390 • C, while the PLLA/PO3GCA composite thin film experienced a second weight loss at 300-430 • C. PO3GCA showed slight weight loss in the temperature range of 270-310 • C, which was partly due to the thermal degradation of low molecular weight PO3GCA. PO3GCA also showed a secondary weight loss at 310-450 • C. Both types of films exhibited 50% weight loss at 360 • C, while PO3GCA exhibited 50% weight loss at 380 • C. The maximum thermal decomposition rate of the film during the second weight loss was negatively correlated with the content of PO3GCA, as shown in Figure 5. With the increase of PO3GCA content, the maximum thermal decomposition rate decreased. These results indicate that the addition of PO3GCA can effectively improve the thermal stability of PLLA thin film. Overall, the TG and DTG analyses provided valuable insights into the thermal behavior of PLLA/PO3GCA composite films and highlighted the potential of PO3GCA as a thermal stabilizer for PLLA. during the second weight loss was negatively correlated with the content of PO3GCA, as shown in Figure 5. With the increase of PO3GCA content, the maximum thermal decomposition rate decreased. These results indicate that the addition of PO3GCA can effectively improve the thermal stability of PLLA thin film. Overall, the TG and DTG analyses provided valuable insights into the thermal behavior of PLLA/PO3GCA composite films and highlighted the potential of PO3GCA as a thermal stabilizer for PLLA.

DSC Analysis
The PLLA/PO3GCA composite films with varying PO3GCA content and DSC curves of PO3GCA are shown in Figure 6. The glass transition temperature (Tg) of PO3GCA is 8.2 °C. The Tg, cold crystallization temperature (Tcc), and melting temperature (Tm) of the polylactic acid (PLLA) film are 62.3 °C, 123.3 °C, and 165.4 °C, respectively. As can be seen from Figure 6 and Table 1, with increasing PO3GCA content, the Tg of the PLLA/PO3GCA composite films gradually decreases compared to that of the pure PLLA film. This indicates that the addition of PO3GCA improves the mobility of the PLLA segments, possibly due to the good compatibility between PLLA and PO3GCA. Moreover,  Figure 6 and Table 1, with increasing PO3GCA content, the Tg of the PLLA/PO3GCA composite films gradually decreases compared to that of the pure PLLA film. This indicates that the addition of PO3GCA improves the mobility of the PLLA segments, possibly due to the good compatibility between PLLA and PO3GCA. Moreover, when the PO3GCA content increases from 10% to 15%, a second glass transition temperature appears at 8.7 • C, which is close to the Tg of PO3GCA, indicating that slight phase separation occurs between PO3GCA and PLLA. Additionally, the second Tg at 43.7 • C is higher than that of PO3GCA10 at 37.7 • C. When the PO3GCA content increases to 20%, the phase separation becomes more severe, and the Tg increases to 45.3 • C.
Polymers 2023, 15, x FOR PEER REVIEW 9 of 14 suitable conditions. However, when the PO3GCA content exceeds 15%, the crystallinity of the composite film begins to decline. This is because the excessive addition of PO3GCA results in the separation of PO3GCA from PLLA, which is not conducive to the crystallization of PLLA. Therefore, it is crucial to maintain an appropriate PO3GCA content to achieve the desired crystallization improvement effect. In addition, it should be noted that the improvement in crystallinity is also closely related to the compatibility between PO3GCA and PLLA. When the compatibility is good, the nucleation effect of PO3GCA on PLLA crystallization is enhanced, resulting in higher crystallinity. Overall, the crystallinity analysis provides valuable insights into the crystallization behavior of the PLLA/PO3GCA composite films and highlights the importance of maintaining an appropriate PO3GCA content and good compatibility with PLLA.    Figure 7 demonstrate that as the PO3GCA content increased, the surface of the films changed from a uniform phase to a dispersed phase with a sea-island structure. Additionally, the distribution of PO3GCA in PLLA shifted from the interior to the surface, particularly when the PO3GCA content increased from 10% to 15%. These observations suggest that the incorporation of PO3GCA into PLLA can significantly alter the morphology of  Furthermore, it is worth noting that when the PO3GCA content is less than 10%, the peak cold crystallization temperature of the composite film decreases with the increasing PO3GCA content. This indicates that the addition of PO3GCA effectively promotes the cold crystallization of PLLA, which is likely due to the nucleation effect of PO3GCA. Specifically, PO3GCA provides more nucleation sites for the formation of PLLA crystals, thereby enhancing the crystallization process. Additionally, as the PO3GCA content increases, the melting peak of the composite film slightly broadens and shifts to a lower temperature, indicating that the addition of PO3GCA affects the melting behavior of PLLA. Overall, the DSC analysis provides valuable insights into the thermal properties and compatibility of the PLLA/PO3GCA composite films.

Sample Tg1 (°C) Tg2 (°C) Tcc (°C) Tm (°C) ΔHcc (J/g) ΔHm (J/g) Xc (%)
As depicted in Table 1, it is evident that when the PO3GCA content is lower than 10%, the crystallinity of the composite film is positively correlated with the PO3GCA content, and the crystallinity of the film is significantly improved. This suggests that the addition of PO3GCA can effectively enhance the crystallization property of PLLA under suitable conditions. However, when the PO3GCA content exceeds 15%, the crystallinity of the composite film begins to decline. This is because the excessive addition of PO3GCA results in the separation of PO3GCA from PLLA, which is not conducive to the crystallization of PLLA. Therefore, it is crucial to maintain an appropriate PO3GCA content to achieve the desired crystallization improvement effect. In addition, it should be noted that the improvement in crystallinity is also closely related to the compatibility between PO3GCA and PLLA. When the compatibility is good, the nucleation effect of PO3GCA on PLLA crystallization is enhanced, resulting in higher crystallinity. Overall, the crystallinity analysis provides valuable insights into the crystallization behavior of the PLLA/PO3GCA composite films and highlights the importance of maintaining an appropriate PO3GCA content and good compatibility with PLLA.  Figure 7 demonstrate that as the PO3GCA content increased, the surface of the films changed from a uniform phase to a dispersed phase with a sea-island structure. Additionally, the distribution of PO3GCA in PLLA shifted from the interior to the surface, particularly when the PO3GCA content increased from 10% to 15%. These observations suggest that the incorporation of PO3GCA into PLLA can significantly alter the morphology of the composite films, which could have implications for their mechanical and functional properties. The tensile fracture sections shown in Figure 8 revealed that stratification occurred in the PLLA/PO3GCA15 and PLLA/PO3GCA20 films, whereas this phenomenon was not observed in the PLLA/PO3GCA5 and PLLA/PO3GCA10 films. This finding indicates that PO3GCA may be included in the cracks of the PLLA chains, filling the gaps between PLLA layers. However, excess PO3GCA cannot be accommodated in these gaps, resulting in the stratification of the film and the formation of a sea-island structure of dispersed phases on the surface. Furthermore, scattered fine particles were observed on the surface of the PLLA/PO3GCA5 and PLLA/PO3GCA10 films, which are most likely PO3GCA macromolecules that cannot be accommodated into the gaps between the PLLA layers. Another possibility is that the PO3GCA plasticizer crystallizes in advance during the solvent volatilization process owing to its high molecular weight, promoting the aggregation of the surrounding plasticizer and resulting in scattered particles on the film surface.

Field-Emission SEM
The fracture surface of the PLLA/PO3GCA blend films was rough, with a distinct fibrous surface visible at a PO3GCA content of 15%, indicating a ductile fracture corresponding to a high elongation at break and high toughness of PLLA/PO3GCA15. As the PO3GCA content increased from 5% to 20%, burrs on the fracture surface became increasingly thin, indicating that PO3GCA may enhance the toughness of the PLLA film. However, the plasticizer overflowed between layers and accumulated on the surface as the PO3GCA content increased, thereby decreasing the tensile properties and toughness. These findings suggest that the optimal PO3GCA content for improving the mechanical properties of PLLA/PO3GCA films should be carefully considered.  The tensile fracture sections shown in Figure 8 revealed that stratification occurred in the PLLA/PO3GCA15 and PLLA/PO3GCA20 films, whereas this phenomenon was not observed in the PLLA/PO3GCA5 and PLLA/PO3GCA10 films. This finding indicates that PO3GCA may be included in the cracks of the PLLA chains, filling the gaps between PLLA layers. However, excess PO3GCA cannot be accommodated in these gaps, resulting in the stratification of the film and the formation of a sea-island structure of dispersed phases on the surface. Furthermore, scattered fine particles were observed on the surface of the PLLA/PO3GCA5 and PLLA/PO3GCA10 films, which are most likely PO3GCA macromolecules that cannot be accommodated into the gaps between the PLLA layers. Another possibility is that the PO3GCA plasticizer crystallizes in advance during the solvent volatilization process owing to its high molecular weight, promoting the aggregation of the surrounding plasticizer and resulting in scattered particles on the film surface.

Mechanical
Properties of the PLLA/PO3GCA Films Figure 9 illustrates the stress-strain curves obtained from the mechanical testing o PLLA/PO3GCA films with varying compositions. PLLA films are inherently rigid and brittle, with an elastic modulus of 1528 MPa and an elongation at a break of less than 10% As shown in Table 2, the addition of PO3GCA plasticizer had a significant impact on the elongation at the break of the films. Among the different hybrid systems tested, the PLLA/PO3GCA15 film exhibited the highest elongation at break, which increased by 220% compared to pure PLLA.
The stress-strain curves (Figure 9) indicate that the addition of PO3GCA resulted in plastic deformation of the PLLA/PO3GCA films due to changes in the interaction at the PLLA interface. The interfacial interaction between PO3GCA and the PLLA matrix facili tated the sliding of the interface of the hybrid matrix film, leading to a reduction in the tensile yield stress relative to the maximum force of the pure PLLA film. However, when the PO3GCA content reached 20%, the mechanical properties of the corresponding film decreased to varying degrees. This was attributed to excessive PO3GCA filling the gap between the PLLA layers and aggregating on the surface of the film, causing the PLLA layer of the mixed films to slip easily and affecting the intralaminar structure of PLLA ultimately leading to film fracture. As the elongation decreased, the elastic modulus, ten sile yield stress, and maximum force also decreased.
In summary, the addition of PO3GCA significantly improved the mechanical prop erties of PLLA films, particularly in terms of elongation at break and toughness. However excessive PO3GCA content can negatively impact the mechanical properties o PLLA/PO3GCA films. These findings have important implications for the use of PO3GCA The fracture surface of the PLLA/PO3GCA blend films was rough, with a distinct fibrous surface visible at a PO3GCA content of 15%, indicating a ductile fracture corresponding to a high elongation at break and high toughness of PLLA/PO3GCA15. As the PO3GCA content increased from 5% to 20%, burrs on the fracture surface became increasingly thin, indicating that PO3GCA may enhance the toughness of the PLLA film. However, the plasticizer overflowed between layers and accumulated on the surface as the PO3GCA content increased, thereby decreasing the tensile properties and toughness. These findings suggest that the optimal PO3GCA content for improving the mechanical properties of PLLA/PO3GCA films should be carefully considered. Figure 9 illustrates the stress-strain curves obtained from the mechanical testing of PLLA/PO3GCA films with varying compositions. PLLA films are inherently rigid and brittle, with an elastic modulus of 1528 MPa and an elongation at a break of less than 10%. As shown in Table 2, the addition of PO3GCA plasticizer had a significant impact on the elongation at the break of the films. Among the different hybrid systems tested, the PLLA/PO3GCA15 film exhibited the highest elongation at break, which increased by 220% compared to pure PLLA.

Mechanical Properties of the PLLA/PO3GCA Films
found to be 38.14, 44.89, 44.69, and 34.33 MPa, respectively. These values represent a remarkable increase of 2495%, 2953%, 2940%, and 2235% compared to that of pure PLLA. These findings indicate that PO3GCA is an excellent plasticizer for PLLA, and has the potential to significantly enhance the mechanical properties of PLLA-based materials.

Conclusions
In this study, poly(1,3-propylene glycol citrate) (PO3GCA) was synthesized from biologically derived citric acid (CA) and PO3G and was used as a toughening agent for Polylactic-L-acid (PLLA). The solution casting method was employed to prepare PLLA/PO3GCA films with varying PO3GCA contents while ensuring satisfactory compatibility between PO3GCA and PLLA.
The addition of PO3GCA to PLLA resulted in a slight increase in the thermal stability of the PLLA film and a significant improvement in its toughness. The elongation at the break of the composite film reached 230% when the PO3GCA content was 15%, compared to the pure PLLA film. These results suggest that PO3GCA may be a promising plasticizer for PLLA films, as it enhances the toughness of the film while maintaining its biodegradability and environmental-friendliness.
Overall, this study contributes to the development of sustainable and eco-friendly materials for the plastic industry. The use of biologically derived citric acid and poly(1,3propylene glycol citrate) as toughening agents for PLLA can significantly enhance the  The stress-strain curves ( Figure 9) indicate that the addition of PO3GCA resulted in plastic deformation of the PLLA/PO3GCA films due to changes in the interaction at the PLLA interface. The interfacial interaction between PO3GCA and the PLLA matrix facilitated the sliding of the interface of the hybrid matrix film, leading to a reduction in the tensile yield stress relative to the maximum force of the pure PLLA film. However, when the PO3GCA content reached 20%, the mechanical properties of the corresponding film decreased to varying degrees. This was attributed to excessive PO3GCA filling the gaps between the PLLA layers and aggregating on the surface of the film, causing the PLLA layer of the mixed films to slip easily and affecting the intralaminar structure of PLLA, ultimately leading to film fracture. As the elongation decreased, the elastic modulus, tensile yield stress, and maximum force also decreased.
In summary, the addition of PO3GCA significantly improved the mechanical properties of PLLA films, particularly in terms of elongation at break and toughness. However, excessive PO3GCA content can negatively impact the mechanical properties of PLLA/PO3GCA films. These findings have important implications for the use of PO3GCA as a plasticizer for PLLA in various applications, such as food packaging and medical devices.
The tensile toughness of the films was determined by integrating the stress-strain curves [26], and the results are presented in Table 2. The pure PLLA film exhibited a toughness of 1.47 MPa. However, upon the incorporation of PO3GCA, a substantial improvement in the toughness of the films was observed. Specifically, the toughness values for PLLA/PO3GCA5, PLLA/PO3GCA10, PLLA/PO3GCA15, and PLLA/PO3GCA20 were found to be 38.14, 44.89, 44.69, and 34.33 MPa, respectively. These values represent a remarkable increase of 2495%, 2953%, 2940%, and 2235% compared to that of pure PLLA. These findings indicate that PO3GCA is an excellent plasticizer for PLLA, and has the potential to significantly enhance the mechanical properties of PLLA-based materials.

Conclusions
In this study, poly(1,3-propylene glycol citrate) (PO3GCA) was synthesized from biologically derived citric acid (CA) and PO3G and was used as a toughening agent for Polylactic-L-acid (PLLA). The solution casting method was employed to prepare PLLA/PO3GCA films with varying PO3GCA contents while ensuring satisfactory compatibility between PO3GCA and PLLA.
The addition of PO3GCA to PLLA resulted in a slight increase in the thermal stability of the PLLA film and a significant improvement in its toughness. The elongation at the break of the composite film reached 230% when the PO3GCA content was 15%, compared to the pure PLLA film. These results suggest that PO3GCA may be a promising plasticizer for PLLA films, as it enhances the toughness of the film while maintaining its biodegradability and environmental-friendliness.
Overall, this study contributes to the development of sustainable and eco-friendly materials for the plastic industry. The use of biologically derived citric acid and poly(1,3propylene glycol citrate) as toughening agents for PLLA can significantly enhance the properties of PLLA-based materials, while also reducing the environmental impact of plastic production. Further research is needed to investigate the long-term stability and biodegradability of PLLA/PO3GCA composite films, as well as to optimize the processing conditions and compatibility of the blended systems to achieve the desired properties of PLLA-based materials.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.