Carbon dots with wide-spectrum absorption for enhanced anti-aging of poly(vinyl chloride) films

Abstract

The aging of poly(vinyl chloride) (PVC) films, driven predominantly by ultraviolet (UV) radiation and environmental factors, restricts their long-term utility. This study explores the incorporation of wide-spectrum-absorbing carbon dots (CDs) as an innovative additive to enhance the anti-aging properties of PVC films. The CDs were synthesized from Garcinia mangostana rind through an environmentally friendly solvothermal method. The resulting CDs exhibit robust light absorption across a broad spectral range, efficiently mitigating harmful UV effects. Innovatively, PVC films embedded with CDs demonstrated notable improvements in photostability under prolonged light exposure. Furthermore, beyond leveraging the specific optical properties of CDs, the construction of an interfacial cross-linking structure is crucial for achieving high mechanical strength in the films. The resulting CDs-embedded PVC composite film demonstrates good mechanical properties, with tensile strength increasing from 47.2 MPa to 101.1 MPa and elongation at break increasing from 39.9% to 237.8%. These findings suggest that wide-spectrum-absorbing CDs hold significant promise for extending the durability of PVC films, particularly in applications exposed to prolonged sunlight, such as food packaging.

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Changyong Gao

Changyong Gao obtained his PhD degree in chemical engineering and technology from Harbin Institute of Technology in 2017. He worked as an assistant professor at the School of Mechatronics and Engineering in Harbin Institute of Technology, from 2017 to 2021. He is now an associate professor at Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, China. His research interests include synthetic micro- and nanomotors, and stimuli-responsive materials for biomedical applications.

1. Introduction

Poly(vinyl chloride) (PVC) is a non-toxic, biodegradable, and environmentally friendly synthetic polymer that has been approved by the State Food and Drug Administration (SFDA) for use in food packaging materials.^1,2 However, PVC is susceptible to aging when exposed to environmental effects, such as ultraviolet (UV) radiation, heat, and oxidants, resulting in diminished mechanical properties, discoloration, and increased brittleness.³ The aging of PVC significantly restricts its long-term usability in high-performance applications.

To address these limitations, recent research has focused on developing effective strategies to enhance the stability and durability of PVC under environmental aging.^4,5 Embedding nanomaterials into polymer matrices has emerged as a highly effective approach for producing composite materials with superior properties.⁶ To date, PVC composites reinforced with carbon nanotubes,⁷ ZnO nanoparticles,⁸ and Fe₃O₄ nanoparticles⁹ have been reported. However, these additives provide only limited improvement in the mechanical strength and long-term stability of PVC. Additionally, the weak interactions between the polymer matrix and large-sized nanoparticles often result in poor dispersion, which hinders the overall enhancement of material performance.¹⁰

Carbon dots (CDs), a novel class of zero-dimensional carbon nanomaterials, have garnered significant research attention due to their exceptional biocompatibility, unique photophysical properties, high photoluminescence, large surface area, and surface modifiability.^11–13 Recent studies have highlighted CDs as promising additives for polymers, given their low cytotoxicity, excellent solubility in various solvents, cost-effectiveness, high thermal stability, and abundance of functional groups such as amino, hydroxyl, and carboxyl groups.^14,15 The presence of oxygen-containing functional groups on the surfaces of CDs facilitates strong cross-linking with polymer chains, making them particularly effective in enhancing polymer properties.

In this study, carbon dots (CDs) derived from Garcinia mangostana rind (gm-CDs) were synthesized via a solvothermal method and directly incorporated into a PVC tetrahydrofuran solution to fabricate novel gm-CDs-embedded PVC composite films (Scheme 1). The effects of gm-CD incorporation on the photostability, thermal stability, and mechanical properties of PVC films were systematically evaluated. Additionally, the underlying mechanisms by which gm-CDs contribute to enhanced aging resistance of PVC were investigated. These findings offer valuable insights into the design of more durable and environmentally stable PVC materials, paving the way for potential applications in areas such as food packaging.

Scheme 1 Preparation process of gm-CDs/PVC composite film with enhanced anti-aging capability.

2. Results and discussion

The gm-CDs were synthesized using a solvothermal method, employing Garcinia mangostana rind as a precursor. The resulting gm-CDs were characterized by transmission electron microscopy (TEM), which revealed a uniform spherical morphology and good dispersibility (Fig. 1a). High-resolution TEM (HR-TEM) imaging further indicated an average lattice spacing of 0.21 nm,¹⁶ corresponding to the (100) facet of graphite, as shown in the inset of Fig. 1a. The particle size distribution histogram indicated that the average size of fabricated gm-CDs was approximately 3.0 nm (Fig. 1b). X-ray diffraction (XRD) analysis confirmed the amorphous graphitic nature of the gm-CDs, showing a broad diffraction peak centered around 2θ = 21°, which corresponds to the (002) plane (Fig. 1c). The calculated interlayer spacing of 0.4 nm aligns with typical values for graphitic materials of 0.34 nm.¹⁷ Furthermore, the Raman spectrum exhibited two characteristic peaks at 1330 cm⁻¹ and 1563 cm⁻¹, representing the D and G bands, respectively, indicative of disordered (sp³) and graphitic (sp²) domains. The intensity ratio (I_D/I_G) was calculated to be 0.52, suggesting a relatively high degree of graphitization (Fig. 1d).¹⁸

Fig. 1 (a) TEM image (inset: HR-TEM image) and (b) the corresponding size distribution of gm-CDs. (c) XRD pattern and (d) Raman spectra of gm-CDs. (e) FT-IR spectra and (f) XPS of gm-CDs. High-resolution XPS (g) C 1s, (h) O 1s, and (i) N 1s spectra of gm-CDs and fitting results.

The surface chemistry of the gm-CDs was further analyzed using Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The FT-IR spectrum displayed a broad peak at 3392 cm⁻¹ (–OH stretching), along with peaks at 2920, 1597, 1383, and 1272 cm⁻¹ corresponding to C–H, C=N, C–C, and C–N vibrations (Fig. 1e).^19,20 The XPS survey spectrum showed predominant signals from C 1s, O 1s and N 1s with atomic percentages of 69.52%, 29.21%, and 1.27%, respectively. High-resolution XPS spectra further revealed detailed bonding environments in which the C 1s peaks at 284.3, 285.1, 286.1, 287.0, and 288.5 eV were assigned to C–C/C=C, C–N, C–O, C=N, and C=O functionalities, respectively (Fig. 1g).²¹ The O 1s spectrum displayed peaks for C=O (532.0 eV) and C–O (533.2 eV) (Fig. 1h),²² while the N 1s showed signals for pyridinic-N (400.9 eV) and nitrate species (407.5 eV) (Fig. 1i).²³ These findings confirm the presence of diverse surface functional groups on the gm-CDs, introduced through high-temperature carbonization. Such chemical functionality facilitates effective complexation and integration within polymer matrices.

The optical properties of the gm-CDs were thoroughly studied. UV-Vis absorption spectra of gm-CDs in methanol exhibited a strong absorption at 263 nm and a broad tail extending to 700 nm, attributed to π–π* transition of C=C or C=O bonds associated with sp²-hybridized carbon domains within the gm-CDs (Fig. 2a).²⁴ The corresponding photoluminescence (PL) spectra under 353 nm excitation showed a yellow emission with a quantum yield (QY) of 1.0% (Fig. S1, ESI†). Notably, the gm-CDs exhibited a broad full width at half maximum (FWHM) of 150.7 nm, which is attributed to the high density of C–N and C–O bonds within their structure. This broad emission band reflects an extended spectral range, enhancing the capacity for light absorption and utilization across a wider range of wavelength spectrum, thereby improving light-harvesting efficiency. Moreover, the gm-CDs demonstrated excitation-dependent emission behavior (Fig. S2, ESI†). For instance, upon increasing the excitation wavelength to 460 nm, the emission peak redshifted to 515 nm. This tunable emission is likely driven by the presence of multiple surface functional groups such as on the surface of the gm-CDs that introduce diverse energy states.¹⁵ Time-resolved PL decay curve of gm-CDs in methanol showed a fluorescence lifetime of 1.42 ns, suggesting the singlet state of the emission of gm-CDs (Fig. 2b). Collectively, these results illustrate the remarkable optical characteristics of the gm-CDs, including broad absorption, excitation-tunable emission, and efficient light-harvesting capabilities. These properties make gm-CDs promising candidates for applications requiring excellent photophysical attributes.

Fig. 2 (a) UV-Vis absorption and PL spectra of gm-CDs (inset: the corresponding photographs). (b) Time-resolved PL decay spectra of gm-CDs.

To explore the potential of gm-CDs as functional additives in PVC matrices, composite films were prepared and systematically characterized. Scanning electron microscopy (SEM) image demonstrated that the gm-CDs were uniformly distributed within the PVC matrix, appearing as spherical inclusions without noticeable aggregation (Fig. 3a). Complementary fluorescence imaging under UV illumination exhibited homogeneous luminescence throughout the films, further confirming the even distribution of gm-CDs within the PVC matrix (Fig. 3b). This uniform dispersion suggests the successful integration of gm-CDs into the PVC matrix. Photoluminescence (PL) spectra of the composite films revealed that the gm-CDs preserved their intrinsic emission characteristics upon incorporation, suggesting effective suppression of the solid-state self-quenching effect (Fig. 3c).

Fig. 3 (a) SEM image of CDs/PVC film. (b) The photographs (under UV light) of the CDs/PVC films and (c) the corresponding PL spectra. (d) The photographs of the CDs/PVC films and (e) the corresponding color change curve.

Notably, the optical appearance of the gm-CDs-PVC composite films remained relatively unchanged for three months, in contrast to pure PVC films, which underwent progressive discoloration from colorless to yellow-brown (Fig. 3d). To quantitatively assess long-term stability, color changes were analyzed by calculating Euclidean distance (ED) values based on RGB color components. Over the three-month observation period, pure PVC films exhibited substantial increases in ED values, indicating significant color degradation. In comparison, gm-CDs/PVC films showed markedly lower ED variations, reflecting superior photostability and resistance to environmental aging (Fig. 3e). Collectively, these results demonstrate that the incorporation of gm-CDs into PVC matrices not only preserves the optical functionality of the carbon dots but also significantly enhances the long-term stability of the resulting composite films.

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to investigate the thermoanalytical behavior of the gm-CDs/PVC composite films. The TGA results demonstrated that the incorporation of gm-CDs did not significantly affect the thermal decomposition profile of PVC (Fig. S3a and b, ESI†). Furthermore, DSC thermograms revealed a slight decrease in the glass transition temperature (T_g) upon the addition of gm-CDs (Fig. S4, ESI†). This shift is likely due to the partial microscopic cross-linking interactions between the functional groups on the gm-CD surfaces and the PVC polymer chains, which may increase segmental mobility.¹⁷ To further assess the influence of gm-CDs on mechanical performance, tensile tests were performed. As shown in Fig. 4a and e, t the composite films exhibited significantly enhanced mechanical properties compared to pristine PVC. Specifically, the tensile strength increased from 47.2 MPa to 101.1 MPa, and the elongation at break improved from 39.9% to 237.8% (Fig. S5, ESI†). These substantial improvements are attributed to strong interfacial interactions between the surface functional groups of the gm-CDs and the PVC matrix, which promote effective stress transfer and enhanced structural integrity.

Fig. 4 (a) Stress–strain curves of CDs/PVC composite films. DMA curves of various films show the temperature dependence of (b) storage modulus, (c) loss modulus, and (d) tangent value. (e) Tensile photographs of CDs/PVC film-4.

Dynamic mechanical analysis (DMA) was employed to further elucidate the interactions between gm-CDs and PVC matrix. The storage modulus (E′) of the composite films was markedly higher than that of pure PVC, indicating improved stiffness and mechanical stability (Fig. 4b). All composite films exhibited a significant decrease in E′ near 27 °C, corresponding to the T_g of PVC. Additionally, the loss modulus (E′′) peaks of the gm-CDs/PVC composite films were shifted to higher temperatures compared to pure PVC, suggesting improved thermal resistance and enhanced flexibility resulting from the integration of gm-CDs (Fig. 4c).²⁵ The incorporation of gm-CDs did not substantially alter the overall amorphous nature of the PVC matrix, as evidenced by the consistent amorphization patterns shown in Fig. 4d. Based on these observations, a reinforcing mechanism is proposed (Fig. 5). In pristine PVC, the application of external stress facilitates the rapid formation and propagation of internal defects by disrupting intermolecular interactions. In contrast, the presence of gm-CDs introduces strong covalent and hydrogen bonding interactions between the filler and polymer chains, creating an adhesion-like network within the matrix.²⁶ These interactions effectively suppress crack propagation by redirecting and dissipating mechanical stress through the gm-CD domains, resulting in significantly enhanced tensile strength and elongation at break. Together, these results underscore the critical role of gm-CDs in reinforcing the PVC matrix through both physical and chemical interactions, offering a robust strategy for improving the mechanical performance of polymer composites.

Fig. 5 Schematic diagram of the possible mechanism of gm-CDs for enhanced mechanical strength of PVC.

3. Conclusion

In summary, this study demonstrates the effectiveness of CDs with broad-spectrum absorption capabilities in enhancing the long-term anti-aging performance of PVC films. The incorporation of CDs significantly mitigates the adverse effects of UV radiation, thermal stress, and mechanical deformation, leading to marked improvements in both photostability and mechanical resilience. The broad absorption range of the CDs affords efficient shielding against ultraviolet exposure, while simultaneously contributing to the enhanced durability and extended service life of the composite films. These findings highlight the promising potential of CDs as multifunctional additives for the development of high-performance, long-lasting polymer materials.

4. Experimental section

4.1. Materials and reagents

Garcinia mangostana was obtained from the supermarket. N,N-Dimethylformamide, and poly(vinyl chloride) (PVC) were purchased from Aladdin Chemicals Co. Ltd (Shanghai, China). Ethanol, methylene chloride, methanol, and tetrahydrofuran were purchased by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification unless otherwise specified. All water used during the experiment was ultrapure water from the microporous system.

4.2. Synthesis of gm-CDs

The gm-CDs were synthesized through a simple one-pot solvothermal method. Briefly, dehydrated garcinia mangostana rind (2.5 g) was first added in 25 mL N,N-dimethylformamide, and the solution was transferred into poly (tetrafluoroethylene)-lined autoclaves. After heating at 160 °C in an oven for 6 h, the solution was then allowed to naturally cool to room temperature. The crude products were then purified with silica column chromatography using mixtures of methylene chloride and methanol (20/1) as eluents. After removing solvents and further drying under a vacuum, the gm-CDs powder could be finally obtained.

4.3. Synthesis of CDs/PVC composite film

Under the condition of magnetic stirring in a water bath at 50 °C, 2 g PVC was slowly added into 30 mL tetrahydrofuran (THF) solution, and stirred continuously for 1 h until PVC was completely dissolved. At the same time, the purified gm-CDs powder was dissolved in methanol and sonicated for 5 min to form a brown-yellow solution (2 mg mL⁻¹). Subsequently, different volumes of gm-CDs methanol solution (concentrations of 0.02 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, and 0.5 wt% of the weight of PVC) were slowly added to the above PVC solution, designated as CDs/PVC film-1, CDs/PVC film-2, CDs/PVC film-3, CDs/PVC film-4, and CDs/PVC film-5. After stirring for 1 h, the mixture was transferred to a surface dish and placed in a fume hood for shelter from wind. CDs/PVC films were obtained after THF volatilized naturally.

4.4. Characterization

The morphology of the sample was evaluated using a JEM-2100 TEM. Powder XRD patterns were recorded on an X'Pert PRO diffractometer with Cu Kα radiation at a scanning rate of 2° min⁻¹. Raman spectra were collected on a laser confocal micro-Raman spectroscopy (InViaReflex, Renishaw, London, Britain). FT-IR spectra were collected using a NEXUS-870 spectrometer. XPS was conducted with a commercial spectrophotometer (ESCALAB 250). The UV-Vis spectra were obtained with a Shanghai Meipuda Spectrophotometer (UV-1800PC). PL measurements were performed on a Hitachi F-7000 fluorescence spectrophotometer. The HORIBA FLSP920 system was used to obtain the absolute quantum yield (QY) in the calibration sphere. Fluorescence lifetimes were measured using HORIBA FluoroMax-4P. The morphology of the film was analyzed by SEM (S-4800). The decomposition TGA was performed on a Discovery TGA 5500 thermogravimetric analyzer under a nitrogen atmosphere from room temperature to 800 °C with a heating rate of 10 °C min⁻¹. The DSC measurements were performed on a TA DSC Q2000 analyzer (equipped with RCS 90 mechanical refrigeration system) with a scanning rate of 10 °C min⁻¹ under a nitrogen atmosphere. The DMA of the film was performed from 25 °C to 80 °C using a DMA/SDTA 961e in a nitrogen atmosphere. In all tests, the sample was approximately 14.0 mm long 2.90 mm wide, and 0.15 mm thick. The tensile test was carried out at a crosshead speed of 5 mm min⁻¹ according to GBT13022-91.

Author contributions

Chen Dong: methodology, validation, formal analysis, data curation, visualization, writing – original draft, review & editing, funding acquisition. Zijian Li: methodology, investigation. Hong Bi: methodology, supervision, writing – review & editing. Changyong Gao: supervision, conceptualization, writing review & editing, project administration, funding acquisition. Xuehua Ma: conceptualization, supervision, methodology, formal analysis, writing – review & editing, project administration, funding acquisition.

Data availability

Data supporting this communcation have been included as part of the ESI.† Figures are available at Zenodo at https://doi.org/10.5281/zenodo.14722570.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (ZCLQ24C1002), the Natural Science Foundation of Ningbo (2022J203, 2024QL026), The Yongjiang Talent Introduction Program (2021A-097-G), The Key Research and Development Project in Ningbo (2023Z190). All of the authors also acknowledge the Key Laboratory of Environment-Friendly Polymer Materials of Anhui Province, Anhui University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tb00162e

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