3D-printed polylactic acid/carbon dot nanocomposites: mechanical, interfacial, and dielectric performances toward multifunctional additive manufacturing applications

The development of multifunctional polymer nanocomposites with integrated sensing capabilities has attracted considerable research interest for advanced engineering applications. In this study, carbon dot (CD)-reinforced polylactic acid (PLA) nanocomposite filaments were fabricated via melt extrusion and subsequently processed through fused deposition modeling (FDM) to develop multifunctional 3D-printed structures with potential sensing applications. The CDs were synthesized and incorporated into the PLA matrix at varying concentrations, and the effects of CD content, extrusion temperature, and screw speed on the mechanical, interfacial, and dielectric properties of the resulting composites were investigated systematically. Tensile testing revealed that the nanocomposite containing 3.5 wt.% CDs processed at 170 °C and 4 rpm exhibited the most favorable mechanical performance, achieving a peak stress of 3.10 MPa, Young’s modulus of 49.92 MPa, and modulus of toughness of 0.096 MJ/m 3. Fourier-transform infrared (FT-IR) spectroscopy confirmed successful incorporation of the CDs into the PLA matrix and demonstrated strong interfacial interactions between the filler and PLA matrix, as reflected by the characteristic shifts in the carbonyl and hydroxyl absorption bands. Dielectric characterization demonstrated that the PLA/CD nanocomposites exhibited reduced dielectric losses, suppressed polarization effects, and improved frequency-dependent dielectric behaviors relative to pure PLA, consistent with enhanced charge transport behavior and greater electrical reliability. The synergistic enhancement of mechanical integrity and dielectric performance highlights the potential of CD-reinforced PLA nanocomposites for application to multifunctional sensing, flexible electronics, smart structural systems, and sustainable additive manufacturing.

Fused deposition modeling (FDM) is a widely adopted additive manufacturing (AM) technique that has transformed modern fabrication technology by enabling the production of complex geometries with reduced material waste and enhanced design flexibility (Raza et al., 2025; Wu et al., 2026; Wang et al., 2026). Among the thermoplastic polymers used in FDM, polylactic acid (PLA) has emerged as a material of choice owing to its renewable origin, low toxicity, biodegradability, and ease of processing (Fan et al., 2026; Mou et al., 2025). Despite these advantages, pure PLA exhibits several inherent limitations, including low ductility, poor electrical conductivity, and low thermal resistance, that restrict its applicability in functional and smart device components (Islam et al., 2025). Recent studies on the development of PLA-based nanocomposites have focused on overcoming these limitations by incorporating functional nanofillers (Martínez et al., 2025; Ghosh et al., 2020; Mappoli et al., 2025).

To enhance the mechanical, electrical, and sensing properties of polymers, a range of carbon-based nanomaterials, such as carbon nanotubes, graphene, and carbon black, have been investigated extensively (Atif et al., 2025; Ghosh and Pumera, 2021; Ghosh et al., 2023). These nanofillers enable the fabrication of stimuli-responsive materials capable of reacting to external inputs, such as strain, temperature variations, and chemical environments (Yadav et al., 2024; Subhadarshini et al., 2024; Yang et al., 2025). Specifically, the piezoresistive behavior of carbon-filled PLA composites has demonstrated promising potential for flexible strain-sensing and structural health monitoring applications (Irfan et al., 2021; Subhadarshini, 2026; Zhao et al., 2026). However, at higher filler loadings, conventional carbon-based fillers frequently suffer from agglomeration, poor dispersion, and weak interfacial adhesion with the polymer matrix, collectively degrading the mechanical performance of the composite material (Zhu et al., 2021). These challenges highlight the need for alternative nanomaterials capable of imparting multifunctionality while also preserving or enhancing the structural integrity of the composite system (Sohail, 2025).

Carbon dots (CDs) constitute a family of zero-dimensional carbon nanomaterials that have recently emerged as compelling candidates in the context of composites owing to their distinctive physicochemical characteristics (Sen and Bose, 2025). Their strong photoluminescence, high surface area, tunable surface chemistry, and excellent biocompatibility render them ideal for a range of applications, such as optoelectronics, bioimaging, and chemical sensing (Al Ragib et al., 2023). The abundant functional groups on the surfaces of the CDs facilitate strong interfacial interactions with polymer matrices, promoting uniform dispersion and efficient load transfer (Shahrezaei et al., 2025). Additionally, CDs can impart both optical and electrochemical sensing functionalities to polymer composites, enabling the realization of multifunctional material systems within a single platform (Zhang et al., 2025). The integration of CDs with FDM-based 3D printing presents a unique opportunity to engineer smart materials with intrinsic sensing capabilities, leveraging the precise spatial control over material deposition afforded by this technology (Soori and Arezoo, 2025; Xiang et al., 2025).

Incorporating CDs into PLA filaments offers a route to achieve simultaneous mechanical resilience and functional responsiveness in 3D-printed structures (Cuervo et al., 2025). Such multifunctional materials are highly desirable in advanced sensing applications, where the ability to concurrently monitor mechanical deformation and environmental changes is often necessary (Cuervo et al., 2025). By virtue of their combined mechanical, structural, and functional attributes, PLA/CD nanocomposites hold great potential for deployment in strain sensors, chemical sensors, and optical sensing devices (Ma et al., 2023). The use of biodegradable PLA as the matrix further aligns this approach with the principles of sustainable and environmentally responsible material design (Wang et al., 2023). Despite this considerable promise, the relationships among CD concentration and the resulting mechanical and sensing performances of 3D-printed PLA/CD nanocomposites remain insufficiently characterized (Wang et al., 2022). The existing studies have mostly addressed either sensing functionalities or mechanical augmentation, with limited attention to the synergistic exploitation of both attributes within a single material system (Xia et al., 2024). Furthermore, the influences of the processing variables, such as extrusion conditions, and printing parameters on CD dispersion and composite performance are not yet understood fully (Stiller et al., 2022). A critical challenge lies in identifying the optimal filler loading that can balance mechanical reinforcement against functional performance: excessive loading promotes aggregation and defect formation, whereas insufficient loading fails to elicit the desired functional response (Huo et al., 2025). Therefore, achieving true multifunctionality demands a systematic investigation of both composition and optimal processing conditions (Zheng et al., 2021). A thorough microstructural characterization is also essential to complement this process and establish meaningful structure–property–function relationships in these PLA/CD composite systems (Shah and Jain, 2024). Analytical techniques such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) are indispensable for probing filler dispersion, crystallinity, and thermal stability while elucidating the mechanisms underlying property enhancements (Ding et al., 2025).

Although several studies have reported the incorporation of CDs into polymer matrices and a few works have also examined PLA/CD filaments for AM, comprehensive studies linking the processing parameters, interfacial interactions, mechanical performances, and dielectric behaviors remain scarce. Most existing reports focus primarily on the optical properties or isolated mechanical enhancements, with limited attention to how the extrusion conditions collectively govern multifunctional performance. To address these gaps, our study explores the systematic design, fabrication, and characterization of multifunctional PLA/CD nanocomposites produced via FDM-based 3D printing to examine the coupled effects of CD loading, extrusion temperature, and screw speed on the mechanical, dielectric, morphological, interfacial, and sensing characteristics. The overarching goal of this study is to establish clear processing–structure–property–function correlations that can guide the development of smart sustainable materials for next-generation sensing applications.

Materials

PLA pellets (commercial grade) suitable for FDM were used as the base polymer matrix because of their biodegradability, ease of melt processing, and compatibility with 3D printing. PLA exhibits a density of ∼1.24 g/cm 3 and melting temperature in the range of 160 °C–180 °C. Acetaldehyde (40% aqueous solution), sodium hydroxide (NaOH), hydrochloric acid (HCl), and anhydrous ethanol were procured from Sigma-Aldrich (St. Louis, MO, United States) and used as received without further purification. Deionized (DI) water was used in all the experimental procedures. To prepare the nanocomposites, the synthesized CDs were incorporated into the PLA matrix at weight fractions 0.5, 2, and 3.5 wt.% to systematically evaluate the effects of filler concentration on the mechanical and dielectric performances of the resulting composites. PLA was selected as the matrix material for its biodegradability, renewable origin, and excellent processability via FDM; however, its limited electrical and functional performances restrict use in applications requiring sensing or dielectric capabilities. CDs were incorporated as the nanofiller owing to their high dispersibility, abundant oxygen-containing surface groups, large surface area, and good biocompatibility, which together minimize the aggregation commonly observed with conventional carbon nanofillers; this favorable dispersion behavior enables simultaneous enhancement of the mechanical and dielectric properties. Thus, the integration of biodegradable PLA with multifunctional CDs presents a promising strategy for developing sustainable and functionally enhanced materials for AM.

Synthesis of CDs

The CDs were synthesized via a base-catalyzed condensation reaction between acetaldehyde and NaOH at room temperature (Li et al., 2021). In a typical procedure, 12 g of NaOH was added dropwise and slowly to 40 mL of acetaldehyde under continuous magnetic stirring. The reaction mixture was stirred at room temperature for 2 h to ensure complete reaction. The resulting product was subsequently sonicated with dilute HCl and DI water until a flocculent precipitate was formed. This precipitate was collected and washed repeatedly with DI water until a neutral pH ≈ 7 was attained, confirming complete removal of the residual acid and ionic impurities. The purified product was then dried in a hot-air oven at 70 °C for 18 h to yield a dry CD powder. The as-synthesized CD powder and its aqueous dispersion in a glass vial are shown in Figures 1a,b, respectively. After drying at 70 °C for 18 h, approximately 0.4 g of CD powder was obtained, corresponding to a reaction yield of 3.3% based on the initial acetaldehyde precursor. The synthesis procedure was repeated independently three times, and comparable yields were obtained with variations below ±5%, demonstrating good reproducibility of the preparation method.

SEM was employed to investigate the surface morphology of the synthesized CDs (Figure 2a). The SEM image reveals that the CDs consist of densely packed and irregularly shaped aggregates exhibiting a plate-like morphology with relatively smooth surfaces and well-defined angular edges. The aggregate sizes measured from the image are in the range of 0.5–1 µm. Such aggregation is commonly observed in CDs on account of their high surface energies and the presence of abundant oxygen- and nitrogen-containing surface functional groups, which promote hydrogen bonding and π–π interactions between adjacent nanoparticles. The elemental composition of the synthesized CDs was investigated using energy-dispersive X-ray spectroscopy (EDX), and the corresponding spectrum is shown in Figure 2b; the spectrum exhibits only two prominent characteristic peaks corresponding to carbon (C) and oxygen (O), confirming that the synthesized nanoparticles are primarily composed of these elements. Quantitative analysis revealed carbon and oxygen contents of 58.11 at.% (51.02 wt.%) and 41.89 at.% (48.98 wt.%), respectively. Further, the UV-Vis spectrum of the CDs (Figure 2c) exhibited a broad absorption band with a shoulder at 330–350 nm corresponding to n–π* transitions of C=O/–COOH/–OH groups and stronger absorption below 300 nm corresponding to π–π* transitions of the sp 2 domains. Under 330 nm excitation, the CDs showed a strong and broad emission maximum at 455 nm, which is attributable to the size and surface-group heterogeneities typical of CDs. The resulting Stokes shift of 125 nm and the smooth featureless absorption/emission profiles are consistent with quantum-confined, narrowly size-distributed, and well-passivated CDs suitable for incorporation into the PLA matrix.

Filament fabrication and FDM-based 3D printing

Filament fabrication

Prior to processing, the PLA pellets (Figure 3a) were dried in a vacuum oven at 50 °C–60 °C for 4–6 h to eliminate residual moisture and prevent hydrolytic degradation during processing. The required quantity of CD powder, calculated based on the target weight percentage, was dispersed in ethanol and then ultrasonicated for 30–60 min to achieve a uniform and stable dispersion. The CD dispersion was then mixed with the dried PLA pellets under continuous stirring, and the solvent was subsequently evaporated by heating at 60 °C until a dry and homogeneous PLA/CD blend was obtained. The dried blend was melt-compounded using a FELL EVO single-screw filament extruder (FELL EVO, Italy) at temperatures between 165 °C and 175 °C with the screw speed adjusted between 4 and 8 rpm to ensure homogeneous distribution of the CDs within the PLA matrix. Each formulation was processed using a single extrusion cycle to minimize thermal degradation of PLA. The extruded material was pelletized and subsequently re-extruded through the filament fabrication unit to produce filaments with a nominal diameter of 1.75 ± 0.05 mm (Figure 3b). The filament diameter was continuously monitored during the extrusion process to ensure consistency and facilitate smooth 3D printing. Table 1 shows the extruder settings used to prepare the filaments. The filaments were cooled under ambient air before winding, and the filament diameter was continuously monitored using a digital vernier gauge at regular intervals to ensure dimensional consistency suitable for FDM-based printing.

3D printing of tensile and dielectric specimens

Tensile test specimens were fabricated using an FDM-based 3D printer (Figure 4a). The tensile properties of both the PLA/CD nanocomposites and pure PLA were evaluated using ASTM D638 type IV specimens (Figure 4b) with a width of 6 mm, an overall length of 115 mm, a calculated cross-sectional area of 690 mm 2, and a gauge length of 33 mm. The tensile tests were conducted at a crosshead speed of 10 mm/min. The printing parameters were optimized as follows: nozzle temperature of 190 °C–210 °C, bed temperature of 50 °C–60 °C, layer height of 0.1–0.2 mm, infill density of 100%, and print speed of 40–60 mm/s. A constant printing orientation was maintained across all specimens to minimize anisotropy effects arising from the layer-by-layer deposition process. Following fabrication, all specimens were maintained at room temperature for 24 h prior to mechanical testing. Three tensile specimens were tested for each experimental condition, and the reported values represent the mean ± standard deviation. For the dielectric test, the samples were designed as thin circular discs of diameter 8 mm and thickness 1 mm (Figure 4c), and the specimens were printed using the same FDM printer and optimized parameters as described above. Particular attention was devoted to layer adhesion and surface smoothness as these factors influence dielectric measurement accuracy significantly. An infill density of 100% was employed throughout the process to eliminate internal voids while ensuring consistent and reproducible dielectric behaviors across all compositions. Where necessary, the printed disc surfaces were polished lightly to achieve uniform thickness and parallel surfaces for accurate dielectric measurements. All fabricated specimens were stored in a desiccator prior to testing to prevent moisture absorption. No additional chemical treatment was applied to ensure that the intrinsic properties of the composites were maintained. The prepared tensile and dielectric specimens were subsequently subjected to mechanical and dielectric characterizations, respectively, as described in the following sections.

Characterization and testing

The fabricated PLA/CD nanocomposites were systematically characterized to evaluate their structural, morphological, thermal, mechanical, and dielectric properties using the techniques described below. Using a universal testing machine (UTM; AMT 20SC, ASI Sales Pvt. Ltd., India), the mechanical properties were evaluated in accordance with ASTM D638 guidelines. Chemical interactions between the PLA and incorporated CDs, as well as identification of the functional groups present in the pure PLA, pure CDs, and nanocomposite samples, were investigated by Fourier-transform infrared (FT-IR) spectroscopy over the wavenumber range of 400–4,000 cm –1. The surface morphology and filler dispersion within the PLA matrix were examined by SEM; the specimens were sputter-coated with a thin layer of gold prior to imaging to prevent charge accumulation, and images were acquired at an accelerating voltage of 10 kV at various magnifications. The tensile properties of the 3D-printed specimens, including tensile strength, Young’s modulus, and elongation at break, were evaluated using the UTM in accordance with ASTM D638 type IV standards. For the electrical measurements, silver paste was coated on both surfaces of the discs to form electrodes. The dielectric measurements were carried out using the 4-probe method over the frequency range of 100 Hz to 7 MHz with a HIOKI LCR meter (model IM3536, Japan); the measurements were conducted from room temperature to 45 °C. The dielectric constant (ε′), dielectric loss (tan δ), and AC electrical conductivity (σ AC) were determined as functions of both frequency and CD weight fraction. All characterization measurements were performed under controlled ambient conditions, and the results were averaged over multiple samples to ensure reproducibility and accuracy.

Tensile properties of 3D-printed PLA/CD nanocomposites

The tensile properties of the multifunctional 3D-printed PLA nanocomposite reinforced with CDs were strongly influenced by CD concentration, extrusion temperature, and screw speed. The experimental results for peak stress, strain, Young’s modulus, and modulus of toughness revealed significant variations across samples, confirming that the processing parameters (Table 2) critically govern the mechanical performances of PLA/CD nanocomposites. Recent studies have demonstrated that the nanoscale dimensions and surface functional groups of CDs enhance the interfacial bonding, stress transfer capabilities, and multifunctional sensing behaviors in PLA-based nanocomposites (Campuzano et al., 2024; Trivedi and Gupta, 2024).

Among all the fabricated samples, EXP8 exhibited the best overall mechanical performance, attaining the highest peak stress (3.10 MPa), Young’s modulus (49.919 MPa), and modulus of toughness (0.0963 MJ/m 3). These properties are attributed to the combined effects of 3.5 wt.% CD loading, a moderate extrusion temperature of 170 °C, and a lower screw speed of 4 rpm, which promoted uniform nanoparticle dispersion and stronger interfacial adhesion between the PLA matrix and CDs. Figure 5 presents the stress–strain curves of all printed tensile specimens. The enhanced Young’s modulus observed for EXP8 reflects the improved stiffness and resistance to elastic deformation. A similar reinforcement behavior was reported previously in a PLA nanocomposite study (Kar et al., 2024). CDs possess abundant hydroxyl, carboxyl, carbonyl, and amino surface functional groups that improve compatibility with PLA through secondary intermolecular interactions. These oxygen-containing functional groups promote better wetting of the nanoparticle surface by molten PLA, resulting in improved stress transfer efficiency and reduced stress concentration. Furthermore, the nanoscale dimensions of the CDs enable more uniform dispersion compared to conventional carbon fillers, thereby minimizing defect formation and facilitating effective reinforcement of the polymer matrix. The highest modulus of toughness observed for EXP8 further corroborates its superior performance over the other samples, indicating a greater capacity for energy absorption prior to fracture. This characteristic is particularly beneficial for advanced sensing applications, where flexible sensors must balance structural stability with deformation capability. Conversely, certain processing parameter combinations resulted in degraded mechanical performance. EXP3 and EXP4 exhibited comparatively lower peak stress and toughness values, likely owing to nanoparticle agglomeration, poor interlayer bonding, or thermal degradation of PLA at elevated processing temperatures. Previous reports have established that excessive thermal exposure can degrade the molecular chains in PLA, leading to reduced tensile performance (Trivedi and Gupta, 2024).

Tensile behavior is significantly affected by the extrusion temperature. An extrusion temperature of 165 °C resulted in incomplete polymer melting and weak interlayer adhesion, whereas a temperature of 175 °C was associated with polymer degradation and void formation. An intermediate temperature of 170 °C yielded the optimal balance between melt flowability and nanoparticle dispersion, consistent with the superior mechanical performance observed for EXP8. The screw speed also notably influenced the mechanical responses of the composites. A lower screw speed provided extended residence time, enabling more homogeneous mixing and distribution of CDs within the PLA matrix; in contrast, higher screw speeds were associated with non-uniform dispersion and the introduction of internal defects. These findings underscore the importance of optimized screw speed in achieving effective nanoparticle reinforcement, as evidenced by the optimal performance of EXP8. The tensile values measured in this study for pure PLA are lower than those reported in literature; this discrepancy can be attributed to differences in the specimen geometry, printing conditions, porosity, nanofiller distribution, and sensing-oriented composite design employed in our study. Nevertheless, a consistent trend of mechanical improvement upon CD incorporation under optimized processing conditions was clearly observed across the experimental samples. Overall, the results demonstrate that 3D-printed PLA/CD nanocomposites fabricated under optimized processing conditions exhibit enhanced stiffness, toughness, and multifunctional capabilities. The PLA/CD nanocomposite developed in this study shows considerable promise for applications in advanced sensing, wearable electronics, biomedical devices, and smart structural systems.

Figures 6a,b present the SEM images of the fractured surface of the optimized CD-reinforced PLA tensile specimen (EXP8) after tensile testing. The fracture morphology provides insights into the deformation behavior and influence of CD reinforcement on the structural integrity of the printed composite. The low-magnification image (Figure 6a) reveals a relatively compact fracture surface with localized rough regions and a limited number of cavities formed during tensile failure. This rough and irregular morphology indicates that considerable energy was absorbed prior to fracture, suggesting that the specimen failure was governed by a combination of matrix deformation and crack propagation rather than abrupt brittle failure. No extensive interlayer separation or large processing-induced defects were observed, indicating satisfactory fusion between the deposited layers during FDM-based fabrication.

The higher-magnification image (Figure 6b) shows a rough fracture surface containing elongated cavities, microvoids, and plastically deformed regions. These features indicate localized plastic deformation of the PLA matrix during tensile loading. The absence of large voids or severe crack branching suggests relatively good structural integrity of the composite under the optimized processing conditions. The elongated cavities are likely associated with microvoid nucleation and growth during crack propagation, which may have contributed to energy dissipation before final fracture. The rough fracture morphology also suggests effective stress transfer within the composite, consistent with the improved tensile strength and Young’s modulus observed for EXP8. Although individual CDs could not be identified directly because of their nanoscale dimensions and the magnification employed in the SEM analysis, there was no evidence of large filler agglomerates or micron-sized particle clusters on the fracture surface. This indicates that significant macroscopic aggregation was absent under the optimized processing conditions. However, direct confirmation of CD dispersion would require higher-resolution field-emission SEM or transmission electron microscopy characterization. Overall, the SEM observations indicate that the optimized PLA/CD composite exhibits a relatively dense fracture morphology with limited internal defects and good matrix continuity, supporting the improved mechanical performance observed for the optimized specimen.

FT-IR analysis

FT-IR spectroscopy was performed on the pure PLA (reference) and optimized PLA/CD composite (EXP8) to investigate their chemical interactions and bonding characteristics. To facilitate clear identification of the peak intensities and spectral shifts, the transmittance data were converted to absorbance. Figure 7 presents the comparative absorbance spectra of both samples over the wavenumber range of 4,000–400 cm –1, and the key peaks identified from the spectral analysis are summarized in Table 3.

Both samples exhibit the characteristic absorption peaks of the PLA matrix, confirming the retention of the polymer backbone in the composite. The sharp and intense peak observed at ∼1,748–1,751 cm –1 is attributed the to the stretching vibration of the ester carbonyl group (C=O), which is a defining spectral feature of PLA. In the composite specimen (EXP8), a blue shift of 3 cm –1 was observed in the carbonyl band (1,748–1,751 cm –1). The OH/NH stretching region (3,200–3,600 cm –1) also exhibited a slight broadening and marginal peak shift, which are consistent with the presence of surface functional groups associated with the incorporated CDs. The stability of the C-O-C stretching band at 1,181 cm –1 indicates that the primary PLA backbone remains intact without significant degradation during composite fabrication. Collectively, these spectroscopic observations are consistent with the presence of interfacial interactions between the CD filler and PLA matrix. However, considering the small magnitudes of the observed peak shifts, the FT-IR results alone do not establish the nature or strength of these interactions conclusively (Mauro et al., 2024; Dong et al., 2025).

Dielectric properties

Figure 8 presents the frequency-dependent dielectric behaviors of the pure PLA and optimized PLA/CD nanocomposite over the frequency range of 100 Hz to 7 MHz, evaluated through the real and imaginary components of the permittivity (ε′ and ε″) and electric modulus (M′ and M″). This analysis provides mechanistic insights into the polarization dynamics and charge transport phenomena within these materials. At low frequencies, the real part of the dielectric permittivity (ε′) of pure PLA exhibited a significantly higher value, reaching approximately 1.9 × 10 −11 at 1 kHz, followed by a sharp decline and eventual stabilization at higher frequencies. In contrast, the PLA/CD nanocomposite displayed considerably lower ε′ values throughout the entire frequency range, with only a marginal increase near 1 kHz and a nearly frequency-independent response thereafter. The significant reduction in ε′ upon CD incorporation is attributed to the restricted dipolar orientation and suppressed space-charge polarization arising from the enhanced interfacial interactions between the PLA matrix and dispersed CDs. This behavior is indicative of a more electrostatically stable material with reduced dielectric loss, which is particularly advantageous for sensing and electronic device applications.

The imaginary component of permittivity (ε″) further elucidates the influences of the CDs on energy dissipation and charge storage mechanisms. Pure PLA exhibited a larger negative ε″ magnitude, particularly in the low-frequency regime, reflecting pronounced interfacial polarization and greater dielectric energy dissipation. By comparison, the PLA/CD nanocomposite demonstrated substantially lower dielectric loss throughout the measured frequency range, indicating that the uniformly dispersed CDs effectively suppress charge accumulation and attenuate dielectric relaxation. Above approximately 10 kHz, both materials converged toward nearly constant ε″ values, signifying the stabilization of polarization mechanisms at higher frequencies. Complementary evidence for the dielectric relaxation behavior was obtained by analyzing the electric modulus. The imaginary modulus (M″) curves for both materials exhibited well-defined relaxation peaks in the 800–1,000 Hz range. However, the peak intensity of pure PLA was considerably higher than that of the PLA/CD nanocomposite. The reduced peak magnitude observed for the nanocomposite reflects a lower relaxation strength and a narrower distribution of the relaxation times, which are attributable to the formation of conductive pathways and reinforced interfacial bonding introduced by the CDs. These features facilitate more efficient charge transfer while constraining excessive polarization build-up. The observed dielectric responses are governed by different polarization mechanisms operating across the investigated frequency range. At lower frequencies, interfacial (Maxwell–Wagner–Sillars) polarization is dominant because the charge carriers accumulate at the interfaces between the insulating PLA matrix and CD nanoparticles. As the applied frequency increases, the alternating electric field changes too rapidly for the dipoles and accumulated charge to follow, resulting in progressive reductions in the dielectric permittivity and dielectric loss. The incorporation of CDs enhances interfacial bonding with the PLA matrix, restricts polymer chain mobility, and limits charge accumulation, suppressing both the dipolar and interfacial polarization mechanisms while producing a more stable dielectric response.

Analogously, the real component of the electric modulus (M′) exhibited a pronounced relaxation peak in pure PLA, whereas the PLA/CD nanocomposite displayed a significantly reduced peak intensity. The systematic decreases in the modulus values following CD incorporation confirm the modification of charge transport dynamics and suppression of electrode polarization effects. Such characteristics are highly desirable in multifunctional sensing materials as they promote stable electrical performance and improved signal reliability under operational conditions. The electric modulus analysis provides complementary information by suppressing the influence of electrode polarization and highlighting the bulk dielectric relaxation processes. The relaxation peaks observed in the modulus spectra correspond to the characteristic relaxation frequencies of the charge carriers. The lower peak intensity observed for the PLA/CD nanocomposite indicates reduced relaxation strength and a more homogeneous distribution of the relaxation times, suggesting improved interfacial compatibility and more efficient localized charge transport compared to pure PLA.

Collectively, the dielectric analysis demonstrates that CD reinforcement substantially alters the polarization and relaxation behaviors of the PLA matrix. The PLA/CD nanocomposite exhibits lower dielectric loss, diminished relaxation intensity, and a more stable frequency-dependent dielectric response than pure PLA. These improvements are consistent with enhanced interfacial interactions and controlled charge transport mechanisms, establishing the CD-reinforced PLA nanocomposite as a promising material for advanced sensing, flexible electronics, and smart functional device applications.

This study demonstrates the successful fabrication of sustainable PLA/CD nanocomposite filaments through melt extrusion and their subsequent processing into 3D-printed specimens via FDM. The incorporation of CDs as nanoscale fillers improved the mechanical and dielectric behaviors of the biodegradable PLA matrix while maintaining good processability during filament extrusion and AM. Among the investigated formulations, the nanocomposite containing 3.5 wt.% CDs processed at an extrusion temperature of 170 °C and a screw speed of 4 rpm exhibited the best overall performance. Tensile testing of this sample yielded a peak stress of 3.10 MPa, Young’s modulus of 49.92 MPa, and modulus of toughness of 0.096 MJ/m 3, demonstrating improved mechanical performance compared to pure PLA. These enhancements are attributed to the effective dispersion of CDs within the PLA matrix and the resulting microstructural modifications that facilitate improved stress transfer.

Dielectric characterization of the optimized sample revealed that CD incorporation significantly influenced the electrical responses of PLA by reducing dielectric loss, suppressing low-frequency polarization, decreasing the relaxation peak intensity, and producing a more stable frequency-dependent dielectric behavior. These observations indicate modified charge transport and relaxation characteristics in the nanocomposite, suggesting its suitability for applications where stable dielectric performance is desirable. From a sustainability perspective, combining biodegradable PLA with CDs provides a promising approach for developing environmentally friendly multifunctional polymer composites. While the improved dielectric behaviors indicate the potential for functional electrical applications, the present study did not include direct sensing or environmental performance evaluations. Therefore, application of the proposed nanocomposite in sensing, environmental monitoring, or related fields should be regarded as prospective rather than demonstrated.

Lastly, thermal characterizations using DSC and TGA were beyond the scope of the present work. Future investigations will include these analyses to evaluate the influences of CDs on the thermal stability and crystallization behavior of the PLA matrix. Additional studies focusing on sensing performance, long-term durability, and application-specific validation will further establish the practical potential of the developed PLA/CD nanocomposite.