Synergistic plasticizing effect of bio-based isosorbide di-epoxidized oleate on poly (vinyl chloride) resins

: The strategic importance of developing environmentally friendly PVC biomass plasticizers with excellent anti-migration and plasticizing effects cannot be overstated. This paper delves into the synthesis and application of bio-based isosorbide di-epoxidized oleate (IDEO) as a secondary biomass plasticizer in polyvinyl chloride (PVC) films. Explored were the synergistic effects between IDEO and dioctyl phthalate (DOP), alongside an assessment of the influence of the compound plasticizer on the properties of PVC products. The partial substitution of DOP resulted in PVC films with improved thermal stability, flexibility, and migration resistance. IDEO-10/PVC, in particular, exhibited a 64% increase in tensile strength compared to DOP/PVC and maintained thermal stabilization for over 280 minutes at 180 ℃. With a glass transition temperature (T g ) of 33 °C, a plasticizing efficiency of 148%, and excellent cold resistance properties, IDEO-10/PVC outperformed DOP/PVC. IDEO-10/PVC migration resistance was significantly lower than in the original PVC resin with only DOP plasticizers under three different polar conditions. Transmittance remained above 87% after 10 days of UV irradiation. Molecular dynamics simulation further confirmed the synergistic plasticizing effect between IDEO and DOP at an IDEO addition of 10 phr. The environmentally friendly approach, holds significant promise for the scaled-up application of PVC.


Introduction
Polyvinyl chloride (PVC), a pivotal engineering plastic renowned for its outstanding performance and cost-effectiveness, finds widespread applications in various fields, including construction, medicine, toys, etc. [1,2] .However, the incorporation of plasticizers is imperative for PVC due to its inherent brittleness and thermal instability [3] .To bolster the market presence of PVC, a myriad of PVC products are extensively employed in home improvement and medical applications.Different types of plasticizers are often employed to augment the flexibility and thermal stability of PVC products.The most prevalent plasticizer in today's market is phthalate plasticizer, derived from petroleum.For instance, dioctyl phthalate (DOP), acknowledged as the most economical and effective plasticizer in PVC products, has found widespread use in children's products, packaging, wire insulation, syringes, blood transfusion tubes, and various other fields [4] .However, DOP is prone to migration from the PVC matrix, posing a potential threat to the shelf life and service life of PVC products [5] .Moreover, its intrinsic carcinogenicity and reproductive toxicity [6,7] raise environmental and health concerns, prompting increased attention towards biomass-derived plasticizers [8] .One of the widely used biomass plasticizers is epoxidized vegetable oil [9,10] , offering advantages such as low cost, non-toxicity, and environmental sustainability.
Derivatives like glycerol, saturated, and unsaturated fatty acids provide ample modification possibilities, making them ideal raw materials for environmentally friendly plasticizers [11,12] .Epoxy plasticizers exhibit a notable stabilizing effect on light and heat, providing advantages such as high-temperature resistance, low-temperature resistance, no migration, and no spray frost.Particularly, compounding with phthalate plasticizers can yield a synergistic effect [13] enabling PVC films to achieve superior plasticizing efficiency.Hence, plasticizer compounding holds paramount research significance and has garnered extensive attention from researchers [14,15] .Muhammad Ali et al. [15] innovatively prepared a novel plasticizer, modified cashew nut phenol (MC), and blended it into PVC as an auxiliary plasticizer to partially replace DOP.The tensile strength increased from 18 MPa for the DOP/PVC blend (MC-0) to 26 MPa for the MC/PVC blend (MC-4), and the elongation at break rose from 256% to 432%.Zhu et al. [16] synthesized a new bio-based plasticizer, VA8-8, from vanillic acid.They found that when VA8-8 partially or completely replaced DOP, the elongation at break increased from 196.4% to 301.9%, indicating improved plasticizing efficiency.It proved to be a promising alternative plasticizer for PVC.
Isosorbide di-epoxidized oleate (IDEO), generated by esterification and epoxidation of oleic acid extracted from rapeseed oil, exhibits excellent aging and migration resistance.Therefore, this paper chose to improve the IDEO plasticizer formulation made by the laboratory itself, its molecular structure possesses nonpolar fatty chains similar to the long chains on the molecular structure of DOP.
Consequently, it can have good compatibility with DOP.Epoxy compounds have both plasticizing and heat stabilizing dual roles, which can reduce the use of DOP and the environmental pollution caused by DOP migration.
In this study, DOP serves as the primary plasticizer, while IDEO acts as the secondary plasticizer.DOP, along with commercially available compound plasticizers, namely Dioctyl terephthalate (DOTP) and Diisononyl-cyclohexane-1,2-dicarboxylate (DINCH), has been chosen for comparative analysis.The aim is to investigate the impact of auxiliary plasticizer dosage on various properties of PVC, including mechanical properties, thermal stability, and migration resistance.This research aims to uncover the synergistic plasticizing effects of biomass plasticizers with DOP through an exploration of the plasticizing mechanism and aging mechanism of bio-based plasticizers.Fig. 1 illustrates the molecular structures of DOTP, DOP, DINCH, and IDEO.

Materials
The chemicals and reagents used in this study are detailed below.Isosorbide di-epoxidized oleate (IDEO), a homemade product, underwent synthesis in the laboratory (The plasticizer synthesis scheme is shown in Fig. S1; FTIR is shown in Fig. S2; 1 H NMR is shown in Fig. S3).Dioctyl phthalate (DOP) was procured from Shandong Yusuo Chemical Technology Co. PVC resin (SG-5; k-value: 68-66) was obtained from Qinghai Yihua Chemical Co.Dioctyl terephthalate (DOTP) and Di-isononyl-cyclohexane-

Preparation of plasticized PVC film
In a 250 mL beaker, combine 12 g of PVC resin and 150 mL of THF, stirring magnetically until the solution achieves homogeneity and transparency.Subsequently, add a specific plasticizer ratio, stir for 24 hours, and transfer the mixture to a 150 mm Petri dish.Cover the dish with cling film, tying it with small holes to control solvent evaporation, and let it stand for 24 hours.Flip the PVC film and allow it to evaporate at room temperature for 2 days.Afterward, dry it for 3 days at 50 °C in a thermostat to eliminate THF, and finally, cut it into sample strips for backup.The composition of various PVC samples is presented in Table 1.
Table 1 The composition of different PVC samples (The amount of plasticizer added per 100 phr PVC resin powder).

PVC Samples
The basic formula (phr)  underwent tensile testing at a fixed frequency of 1 Hz.The specimens were heated from -40 to 80 °C at a rate of 3 °C min⁻¹.The plasticization efficiency of the plasticizer was calculated based on the Tg measured through the DMA test [17,18] , as shown in Equation ( 1).
The mechanical properties, including elongation at break, tensile strength, for the PVC specimens in this study were determined by ISO 527-5:1997.An electronic tensile tester was employed, operating at a stretching rate of 50 mm min-1 under ambient conditions.Dumbbell-shaped specimens were prepared using a specific mold, with dimensions of 25 mm (L) × 3.5 mm (W) ×1 mm (H) for each specimen.The final test result was determined by averaging the three test results obtained for each strip during the test.
Thermogravimetry enables the assessment of the high-temperature resistance of PVC film samples.
The thermal stability of the PVC samples was analyzed using a thermal weight loss analyzer.The tests were conducted under a nitrogen atmosphere, with a nitrogen flow rate of 50 mL min -1 .The temperature range for testing was 50-600 °C, and the temperature ramp-up rate was set at 20 °C min -1 .
Evaluate the aging resistance of PVC film samples in accordance with the standard GB/T 9349-2002.The prepared PVC samples were cut into square test pieces measuring 20 mm (L) × 20 mm (W) × 1 mm (H).Subsequently, these PVC film samples with varying compositions were positioned in the oven.At specified intervals, photographs were taken to document the color alterations of the PVC film at a temperature of 180 °C.
UV-vis type fiber-coupled spectrometer was employed to assess the UV transmittance of PVC specimens.The scanning parameters were set with a range from 200 to 800 nm and a step size of 0.5 nm.
The volatility resistance test of PVC, following the ISO 176-2005 Determination by the activated carbon adsorption method, involved cutting prepared PVC samples into square test pieces of dimensions 10 mm (L) × 10 mm (W) × 1 mm (H).Using an analytical balance with an accuracy of 0.0001 g, the PVC samples were weighed and recorded as M1.Each sample was completely wrapped in a sample bottle containing activated carbon particles.Then the sample bottle was placed in a vacuum drying oven set at 100 °C.After specific time intervals, the sample was removed, and both sides of the PVC sample and the activated carbon were washed with anhydrous ethanol.The ethanol on the sample's surface was dried at 50 °C, and the mass of the PVC sample was weighed using an analytical balance, recorded as M2.To ensure accuracy, each PVC film specimen underwent three tests, and the average value was considered the final result.The volatility resistance of the PVC film specimens was calculated using Equation (2).
M1: Initial mass of PVC film specimens；

M2:
The final weight of the PVC film specimen; The migration resistance test for PVC samples was conducted in accordance with ISO 176-2011, titled "Plastics -Experimental methods for the determination of immersion effects of liquid chemicals."The solvents utilized in this experiment included distilled water, anhydrous ethanol, a polar solvent, and petroleum ether, a non-polar solvent.Prepared PVC samples were cut into square test pieces measuring 10 mm (L) × 10 mm (W) × 1 mm (H) and weighed using an analytical balance with an accuracy of 0.0001 g.The initial weight of the PVC test pieces was recorded as W1.Subsequently, the PVC specimens were immersed in sample bottles containing an equivalent volume of the solution, and these sample bottles were placed in a 30 °C thermostat.At specific intervals, the PVC film specimens were removed, wiped clean with filter paper to eliminate surface solvent, and dried at 50 °C.After drying, they were allowed to cool to room temperature, reweighed, and recorded as W2.To ensure accuracy, each PVC film specimen underwent three tests, and the average value was considered the final result to calculate the migration resistance of the PVC film specimens according to Equation (3).
W1: Initial mass of PVC film specimens.

W2:
The final weight of the PVC film specimen.

Molecular dynamics simulation methodology
To enhance the exploration of compatibility between PVC and plasticizers, molecular dynamics simulations were employed to precisely calculate the intermolecular interactions of PVC with IDEO and DOP.Specifically, the focus was on determining the bonding energy between the plasticizers IDEO, DOP, and PVC.The simulations were conducted using Materials Studio 2020.The simulations were initiated by constructing the structure of PVC with a repeating unit of 150 and subsequently optimizing its molecular structure.Additionally, the molecular structure of IDEO was drawn and optimized, as illustrated in Fig. 2. PVC/IDEO mass ratio of approximately 100/10 [19] .A simulated annealing method was employed over 500 ps to gently transition the system from 298 to 453 K. Following this procedure, the frame with the lowest energy, signifying the most stable structure, was selected.Subsequently, the structure underwent further relaxation through optimization.All simulations were conducted using Forcite [20] within the Compass III force field, with Ewald for electrostatic and Atom for van der Waals interactions.
The adsorption energy (Eads) can be calculated using Equation (4); The van der Waals (ΔEvdw) is determined by Equation ( 5): is similar to the Tg prepared by the experimental method described by Feng et al [21] .As seen in Fig. 3a, all plasticized PVC films, except DOP, exhibit only one peak in the tanδ curve corresponding to Tg. Peaks in the loss modulus curve confirm good compatibility between the plasticizer and the PVC matrix.
However, the DOP film shows a raised peak on the left side of the tanδ peak, with a wider peak width than other samples, indicating comparatively poorer compatibility between DOP and the PVC matrix.
With increasing IDEO content, the Tg of the PVC film rises due to the chirality and inherent rigidity of isosorbide, contributing to increased Tg when rigid diols form esters. Notably, DINCH-10 and DOP show Tg elevations of 6 °C and 7 °C, respectively, compared to IDEO-10, possibly due to cyclohexane in DINCH hindering polymer chain movement.Conversely, DOP and IDEO, with similar molecular structures containing long chains and ester groups, exhibit optimal synergistic effects when added in appropriate amounts.This effectively increases the distance between PVC molecular chains, reduces interaction forces, and enhances plasticizing efficiency.IDEO-10, with the lowest Tg among these PVC films, demonstrates a plasticizing efficiency of 148%.This suggests excellent compatibility with DOP when 10 phr of IDEO is added, providing enhanced flexibility in cold environments.The presence of epoxy groups, ester groups, and long alkane chains in the molecular structure weakens interactions with PVC molecular chains and increases PVC fluidity [22] .This not only synergistically plasticizes with DOP but also collaboratively reduces the Tg of PVC films.
The stiffness of PVC films can be assessed based on the energy storage modulus [23] .Fig. 3b illustrates the energy storage modulus profiles of different PVC films.It is observed that all PVC films maintain a high energy storage modulus up to -20 °C, undergo a sharp decrease from -20 °C to 40 °C, and reach values close to 0 after 40 °C.Notably, DOP exhibits the largest energy storage modulus, signifying its capacity to absorb more energy for deformation and higher stiffness [24] .Comparatively, the compounded IDEO films exhibit lower energy storage modulus than DOP, suggesting greater susceptibility to deformation and enhanced flexibility.Specifically, IDEO films show higher energy storage modulus at -40 to 10 °C and lower energy storage modulus at 0-40 °C, indicating a high rigidity at low temperatures and increased flexibility at higher temperatures.Furthermore, the IDEO-10 variant demonstrates a higher low-temperature viscosity and lower high-temperature viscosity in the loss modulus plot (Fig. 3c).This suggests poor fluidity and challenging processing at low temperatures while indicating good fluidity and ease of processing at high temperatures.Lower additions are advantageous for improving plasticizing efficiency but may compromise low-temperature flexibility [25] .

Mechanical properties analysis
The plasticizing efficiency of plasticizers can be further assessed by determining the tensile properties of PVC film blends [26] .The mechanical properties of PVC and plasticized PVC films are depicted in Fig. 4 . The stress of unplasticized PVC was 15 MPa, and the strain was 360%, indicating high cohesion between the PVC molecular chains, rendering the material mechanically resistant with hard and brittle properties.Upon the addition of the auxiliary plasticizer IDEO, both stresses and strains were significantly higher than those of PVC and even slightly exceeded those of the commercial plasticizer DOP and the compounded plasticizer DINCH-10.The strains gradually decreased with increasing plasticizer addition, aligning with the decline in plasticizing efficiency observed in the DMA test.The stress exhibited an initial increase followed by a decrease.This phenomenon may be attributed to the presence of rigid molecules, which can restrict the movement of polymer chains through molecular interactions, consequently enhancing the tensile strength of the blends.With increased plasticizer addition, more polar groups (ester and epoxy groups) interacted with the polar groups in PVC, leading to an over-density of polar groups.This hindered the activity of chain segments, subsequently reducing the flexibility of the PVC film.Notably, IDEO-10 demonstrated higher stress and strain with exceptional plasticizing efficiency.DOP and IDEO exhibited the most synergistic plasticizing effect and high plasticizing efficiency.The favorable effect arises from the presence of ester groups, alkane chains, and epoxy groups in the structure of DOP and IDEO, which effectively weaken interactions between PVC molecule chains, enhance mobility, and ultimately soften the PVC [22] .
Analyzing the SEM of tensile sections of compounded PVC films provides insights into the interface bonding between plasticizers and the matrix [27] .Fig. 5 illustrates the tensile cross-section morphology of the compounded PVC film, enabling the analysis of plasticizer distribution within the PVC matrix.
The DINCH-10 section exhibits more cracks and small holes on the tensile surface compared to the DOP film.This discrepancy arises from the lower tensile strength, limited interaction between the plasticizer and PVC during stretching, and the ease with which the plasticizer sheds when subjected to external forces.The addition of IDEO results in a smoother cross-section of the plasticized PVC film.This smoother appearance can be attributed to the higher tensile strength of the film and the increased presence of polar groups in the compound plasticizer.These polar groups facilitate interaction with PVC molecular chains, elevating the free volume within the polymer and effectively inhibiting PVC chain entanglement.
Films with higher tensile strength exhibit fewer cracks, rendering the IDEO-20 section virtually free of cracks [15] .

Thermal stability analysis
The heat weight loss curves of various PVC plasticized films are depicted in Fig. 6.The T5 of PVC films, along with the mass loss in the first stage, is crucial for assessing the thermal stability of the films.
Table 3 presents thermal degradation data for PVC films, encompassing mass loss at 5% initial degradation temperature (T5), mass loss at 10% (T10), mass loss at 50% (T50), maximum rate of mass loss (TP1, TP2), and residuals.Observing the heat weight loss curves reveals a similarity among all plasticized films, except for PVC, where no mass loss occurs until 150 °C.This suggests that all tetrahydrofuran used in the preparation process has been volatilized.PVC initiates decomposition and weight loss very early, with a T5 of only 151 °C [16] .With the addition of IDEO, the onset decomposition temperature of PVC film surpasses that of PVC, and T5, T10, and T50 all increase.This implies that the incorporation of various IDEO contents is advantageous for enhancing the thermal stability of the film.Significantly, IDEO-10 demonstrates better thermal stability, with a TP2 of 458 °C and a residual amount of 5.54%, surpassing the performance of commercial plasticizers.This is attributed to the ability of the ester group in IDEO and DOP plasticizers to capture the unstable allyl structure on the PVC molecular chain, forming a free radical intermediate that inhibits the formation of odd electron sites in the PVC molecular chain and disrupts the degradation of free radicals [28] .The epoxy functional group can absorb hydrogen chloride through a ring-opening reaction with HCl, leading to the formation of chlorohydrin that interrupts the growth of the conjugated chain, ultimately forming a cyclic compound that stabilizes PVC and inhibits its thermal degradation [29] .The long chains of organic molecules of IDEO enhance the compatibility of plasticizers with PVC.Improved compatibility results in the formation of a uniform mixed system between PVC and the plasticizer, enhancing the thermal stability of the system [15] .IDEO-10 exhibits higher thermal stability, possibly because IDEO, as a plasticizer, softens PVC and improves heat resistance.However, when the IDEO amount exceeds 10 phr, the increase in polar groups interacting with those in PVC can hinder the activity of chain segments.Consequently, increased friction from the movement of chain segments generates heat, promoting PVC degradation.Conversely, excessive IDEO can reduce compatibility between the plasticizer and the PVC matrix, weakening intermolecular forces in PVC and diminishing thermal stability performance.

Aging resistance testing
The characterization of the aging resistance of PVC and plasticized PVC films at specific temperatures facilitates the analysis of sample degradation times.Investigated the impact of different compounded plasticizers on PVC degradation time at 180 °C.The results presented in Table 4 illustrate a color gradation process in all PVC films over time, progressing from colorless to light yellow, dark yellow, and finally black.The yellowing is a consequence of unsaturated double bonds generated after PVC dehydrogenation, with the color gradually deepening as the number of oxidized double bonds increases.The ultimate black color results from molecular chain degradation and the breakage of carbon chains.Notably, lower amounts of added IDEO plasticizer correspond to enhanced aging resistance.At to a continuous reaction, forming conjugated double bonds and initiating color changes in PVC products [30]   .When the formulation incorporates an epoxy functional group, it can effectively neutralize and absorb hydrochloric acid (HCl) under the catalytic influence of zinc ions.This process displaces the unstable chlorine atoms present in PVC, and additionally, it can engage in a supplementary reaction with the double bond.The substitution of allyl chloride for zinc soap is accompanied by a double bond transfer, thereby effectively impeding the discoloration of PVC products.Within the formulation, which includes calcium and zinc stabilizers, the deposition of zinc chloride (ZnCl2) occurs in the PVC.The catalytic impact of de-HCl can render PVC unstable, potentially leading to undesirable effects such as charringinduced bridging phenomenon [31] .Consequently, the chlorohydrin generated from the ring-opening reaction of the epoxy bond in the plasticizer with HCl can be complex with the generated ZnCl2, inhibiting its catalytic effect.Zinc soap can generate ZnCl2 with the stabilized chlorine atoms in PVC, while calcium soap can undergo a Metathesis reaction to regenerate ZnCl2 into zinc soap.This mechanism, depicted in Fig. 7, both activates and inhibits the destructive effect of ZnCl2 [32] .The addition of PETMP inhibits the intramolecular cyclization reaction experienced by the covalent polyene sequence produced by dehydrochlorination.This addition promotes intermolecular cross-linking and cyclization reactions, reduces the generation of aromatic compounds, and inhibits the thermal degradation of PVC [19]   .Consequently, compounded films exhibit excellent fire safety.Additionally, the inclusion of IDEO improves the long-term aging resistance of PVC film, albeit with slightly faster yellowing than DINCH-10, with less impact on initial whiteness.This is attributed to the fact that epoxy compounds cannot displace unstable chlorine atoms in purely organic systems and can only absorb HCl, thereby extending the long-term heat resistance of PVC [33] .Fig. 7 The aging resistance mechanism of PVC and PVC samples plasticized with DOP and IDEO

Optical Properties Analysis
Optical properties and morphological analyses were conducted to assess the compatibility between the compounded plasticizers and PVC.Table 5 displays images of various plasticized PVC films, and Fig. 8 presents the UV-Vis spectra of these films.Transparency, as observed in the blend samples, provides a measure of the compatibility between plasticizers and PVC [34] .All PVC films, exhibit colorlessness and transparency.IDEO demonstrates transmittance values exceeding 87%, comparable to commercial plasticizers DOP and DINCH-10, indicating excellent transparency.The DMA test revealed a unique peak in the tanδ curve for all plasticized films except DOP.Additionally, IDEO-10 exhibited a narrower peak compared to other plasticized films, suggesting a better synergistic effect between IDEO and PVC.
The addition of IDEO further enhances synergies with DOP.In Fig. 8b, the UV-Vis spectra of various plasticized films after 10 days of UV irradiation demonstrate that the transmittance of IDEO remains above 87%.This suggests that the materials maintain excellent transparency even under prolonged UV exposure.This may be attributed to the beneficial reaction of the epoxy compound with the carboxyl group on the macromolecular chain, preventing the development of conjugated chains and inhibiting UV degradation.Such transparent materials find application in the construction field, enabling effective utilization of natural light, improving indoor lighting, and saving energy [35] .

Plasticizing mechanism and molecular dynamics simulation
The plasticizing effect of a plasticizer is closely associated with the compatibility between PVC and the plasticizer.The plasticizing mechanism of pure PVC and PVC films with compounded plasticizers is illustrated in Fig. 9. Accepted mechanisms of plasticization include the free volume theory, lubrication theory, and gel theory [36] .According to the free volume theory, pure PVC molecules have limited free volume, and the PVC structure contains numerous polar components, constraining the movement of PVC molecular chains [4] .This results in high stiffness and hardness of PVC.Upon the addition of plasticizers, the plasticizer molecules interpose between the PVC molecular chains.The ester and epoxy polar groups in DOP and IDEO interact with the α-hydrogens of the PVC chains, forming hydrogen bonds that reduce PVC-PVC site interactions and increase the space between polymer molecules.Inherent dipole-to-dipole attractions occur between the ester polar groups in DOP and the ester and epoxy polar groups in IDEO.
Additionally, the non-polar long alkyl chain in IDEO interacts with the polar ester group in DOP, enhancing the free volume of the PVC chain and weakening the interaction force between PVC molecules' chains.
Table 6 and Fig. 10 illustrate the model and van der Waals energy between plasticizer and PVC molecules.The van der Waals energy of the compounded PVC film (B) is 5.02 kcal/mol higher than the sum of van der Waals energies of models C and D, indicating that plasticizer compounding enhances the interaction force between plasticizer and PVC.This interaction facilitates the irregular movement of the plasticized PVC film, thereby improving its plasticity and toughness.Therefore, the compounding of DOP with IDEO exhibits a synergistic plasticizing effect on the flexibility of PVC films.
Fig. 9 Plasticizing mechanism between plasticizer and PVC.The prediction of compatibility can be facilitated by examining the varying adsorption energies within blended systems.Adsorption energy (Eads) serves as a criterion for assessing the compatibility of blends and predicting compatibility based on the diverse adsorption energies within these blends [37] .A higher Eads value signifies stronger interaction, indicating greater thermodynamic stability of the formed groups and improved compatibility between the PVC matrix and the plasticizer.After completing the dynamic simulation, the energy of each structure is extracted.The model and adsorption energy between the relevant plasticizer and PVC molecules are illustrated in Fig. 10 and summarized in Table 7. Energy variations within the mixing system are correlated with the plasticizer content.The results reveal that Eads values between the plasticizer and PVC molecules are consistently negative, indicating excellent compatibility between the materials and the absence of phase separation in the PVC mixing system.In comparison to the PVC film with IDEO alone (A), the compounded film (B) exhibits an increasing trend in plasticizer energy, a decreasing trend in PVC energy, and an increasing trend in the adsorption energy between the plasticizer and PVC.This demonstrates outstanding compatibility between the compounded plasticizers and PVC, signifying a strong synergistic effect between the compounded plasticizers that significantly enhances the films' compatibility.

Volatility and migration resistance of PVC films
When employing small-molecule plasticizers for plasticization, migration behavior is often observed, significantly impacting the application of PVC products.The volatility and migration resistance of plasticizers within the polymer matrix depends on factors such as molecular weight, solubility, compatibility, and chemical structure.These factors play a crucial role in evaluating the migration and volatility of plasticizers [38] .As illustrated in Fig. 11

Conclusions
Using DOP as the primary plasticizer and IDEO as the secondary plasticizer, this study investigates

3 Results and discussion 3 . 1
ΔEvdw=Vtotal-VPVC-VIDEO-VDOP-VPETMP(5)   Where the Etotal and Vtotal are the energy and van der Waals of the optimized system; Epvc and Vpvc are the energy and van der Waals of the PVC, and EIDEO and VIDEO are the energy and van der Waals of an optimized IDEO molecule.EDOP and VDOP are the energy and van der Waals of an optimized DOP molecule.EPETMP and VPETMP are the energy and van der Waals of an optimized PETMP molecule.Cold resistance performance The use of Dynamic Mechanical Analysis (DMA) provides an effective means to assess the compatibility between the PVC matrix and plasticizer, offering insight into the plasticizing efficiency of the plasticizer.Higher plasticizing efficiency indicates superior plasticizing effects in colder environments, often assessed using the glass transition temperature (Tg).A lower Tg for PVC films signifies excellent flexibility at lower temperatures, indicating a more pronounced synergistic plasticizing effect and superior cold resistance.Table 2 reveals that the Tg values of plasticizers decrease in the order of PVC > IDEO-30 > DOP > DINCH-10 > IDEO-20 > IDEO-10.This indicates that the addition of plasticizers reduces interactions between PVC molecules, increasing the free volume of PVC and promoting the movement of PVC molecular chains.The Tg of DOP was measured to be 40 °C, which

Fig. 3
Fig. 3 DMA curves a, storage modulus curves b, loss modulus curves c of PVC and plasticized films

Fig. 4
Fig. 4 Tensile strength and elongation of PVC and plasticized films

Fig. 6 .
Fig. 6.TGA and DTG curves of PVC and plasticized films

Fig. 10
Fig. 10 Model between plasticizer and PVC , the mass volatilization of DOP, DINCH-10, and IDEO at 100 °C and mass migration at 30 °C are presented.Fig.11aindicates that the volatility order of the incremental films was IDEO-30 > IDEO-10 > IDEO-20 >DINCH-10 > DOP.Notably, IDEO-10 exhibited the best mechanical properties despite having the highest weight loss in its formulation.This observation can be attributed to the effective plasticizing effect of IDEO-10.The plasticizer enhances the distance between PVC molecular chains, providing lubrication.However, when the molecular spacing increases, the plasticizer becomes prone to migration, leading to increased weight loss.This result aligns with the findings of the mechanical tensile properties test.In a 10-day test cycle, the weight loss of all plasticized films progressively increased, indicating a correlation between plasticizer volatilization, concentration, and time.Fig.11bdisplays the mass loss of plasticized film in the polar solvent anhydrous ethanol, ranking as DOP > DINCH-10 > IDEO-10 > IDEO-20 > IDEO-30.IDEO-30 exhibited exceptional migration stability in polar solvents, with the molecular weight of plasticizers increasing in the order of DOP < DINCH-10 < IDEO-10 < IDEO-20 < IDEO-30.Smaller relative molecular weight plasticizers experienced greater mass loss and poorer migration resistance.For non-polar solvent petroleum ether (Fig.11c), DINCH-10 demonstrated significantly higher mass loss than other compounded plasticized films, indicating its poor stability in non-polar solvents and a propensity to migrate from the PVC matrix.In contrast, IDEO exhibited outstanding stability against non-polar solvents, showcasing minimal mass loss during the 10-day test.Fig.11dillustrates the mass loss of plasticized film in distilled water, with all plasticizers displaying less than 2% mass loss.This indicates excellent stability and very few plasticizers precipitate out in this environment.The strong polarity and long-chain molecular structure of DOP and IDEO, according to the 'similarity and solubility' principle, maximize the synergistic effect of both plasticizers.This attraction and fixation prevent migration to the surface of PVC products, resulting in excellent migration resistance of the plasticized film.

Fig. 11
Fig. 11 Weight loss of plasticized films in different environments for volatility and migration tests.a: their synergistic effects on plasticizing and anti-aging in PVC films.The results demonstrate the PVC films' outstanding mechanical properties, cold resistance, and migration resistance.When the additional amount of IDEO was maintained at 10 phr, the stabilization time in the oven at 180 ℃ exceeded 280 minutes.Observations revealed only slight yellowing on the surface of the PVC film.The elongation at break reached 530%, the tensile strength measured 27 MPa, and the flexibility surpassed that of other PVC films.With a Tg value of 33 ℃, it showcased excellent cold resistance.In three different polar environments, the mass loss was significantly lower than that of commercial plasticizers (DOTP) and environmentally friendly plasticizers (DINCH-10).Computed through first-principles calculations, the binding energy of the compounded plasticizer is 2727.79kcal/mol, indicating superior compatibility with PVC resin compared to the unmodified plasticizer.In summary, the prepared compound plasticizer, consisting of IDEO and DOP, serves both as a heat stabilizer and an auxiliary plasticizer.IDEO can partially replace the widely used commercial plasticizer DOP, offering a more environmentally friendly option with greater application potential.

Table 2
Tg and plasticizing efficiency of different plasticizers

Table 3
TGA and DTG data of pure PVC and plasticized PVC films

Table 4
Aging resistance of PVC and plasticized films at 180 °C

Table 5
Pictures of various plasticized PVC films

Table 6
Van der Waals between plasticizer and PVC molecules.