Multimaterial PLA with tailored plasticization using fused deposition modeling for improved flexibility and shape memory recovery Open Access

Three-dimensional (3D) printing, or additive manufacturing (AM), is an emerging technology transforming various industries by enabling the rapid creation of complex structures directly from digital models. This technique is particularly valuable for producing items without the need for additional molds or accessories, minimizing waste and establishing itself as a cornerstone of modern manufacturing (Aramian et al., 2020; Alarifi, 2024). It has significant applications in the biomedical sector, facilitating the creation of customized medical devices and implants, which greatly improve patient outcomes and comfort (Ivorra-Martinez et al., 2023a; Abas et al., 2024). Similarly, the automotive and aerospace industries use 3D printing to prototype components and manufacture lightweight parts, enhancing performance and fuel efficiency (Blakey-Milner et al., 2021; Sola et al., 2023; Giubilini et al., 2023). Guided by computer-aided design software, this method ensures precision in creating parts that closely adhere to designed specifications (Kumar et al., 2023).

A primary method used in 3D printing of polymer materials is fused deposition modeling (FDM), which begins with the crucial step of selecting and preparing the filament. Variations in filament size can lead to issues such as over-extrusion when the diameter is larger than specified or under-extrusion when it is smaller. Both scenarios significantly affect the aesthetics and structural integrity of the printed object. Over-extrusion results, causing surface defects, while under-extrusion can lead to weak layers prone to cracking and complete print failures (Rajan et al., 2022; Sathies et al., 2020). Precise filament dimensions help ensure reliable material feeding through the printer’s drive mechanism, preventing interruptions such as filament jams or slippage. This reliability is crucial for maintaining consistent print quality, particularly for long or complex print jobs (Shaqour et al., 2021).

In FDM, filament serves as the raw material, heated to a molten state and extruded in layers. Materials like polylactic acid (PLA) (Sehhat et al., 2022), acrylonitrile butadiene styrene (Daly et al., 2023) and polyamide (Daly et al., 2023) are commonly transformed into filaments and used in AM. These materials are chosen for their specific properties to meet performance criteria, with PLA being particularly favored because of its lower melting temperature, ease of use and environmental sustainability. Although these materials are the most commonly used, several approaches aim to expand the variety of materials in AM, such as using polyhydroxyalkanoates (Ivorra-Martinez et al., 2023c). Despite the efforts, most commercial filaments are mainly made with PLA with colorants for prototyping proposes.

A unique characteristic of 3D printed materials is their orthotropic nature, meaning their strength and stiffness vary significantly along different axes (Sheikh and Behdinan, 2022). This results from their layer-by-layer construction with raster deposition direction, presenting both opportunities and challenges in design and material science (Rouf et al., 2022; Verbeeten and Lorenzo-Bañuelos, 2022). In addition, the infill density can be tuned allowing to adjust the behavior of the product depending on the specific requirements (Khedri et al., 2024). This orthotropy requires careful planning the design stage to ensure the structural integrity and performance of the printed objects. Improving the layer adhesion could help to reduce the orthotropy of the printed samples (Ivorra-Martinez et al., 2020).

PLA is widely favored in various industries for its biodegradability and ease of use in 3D printing. However, PLA has inherent limitations such as rigidity and brittleness that restrict its application in demanding conditions (Trivedi et al., 2023). While beneficial for creating dimensionally stable and detailed models, these properties make it unsuitable for applications requiring flexible materials. PLA’s brittleness is particularly problematic under load or impact, leading to potential material failure. This poses significant issues in industries like automotive and aerospace engineering, where high durability and stress resistance are crucial (Croccolo et al., 2023). To address these drawbacks and broaden PLA’s applicability, researchers have explored several enhancement strategies. One well-researched method is the addition of plasticizers, which increase the flexibility and ductility of PLA by integrating into the polymer matrix and reducing the intermolecular forces between polymer chains (Halloran et al., 2022). This approach effectively lowers the glass transition temperature (T_g), facilitating greater mobility of the polymer chains at lower temperatures and thus improving the material’s flexibility and impact resistance (Yuan et al., 2024; Espíndola et al., 2023). Various studies have investigated different types of plasticizers, including citrate esters and phthalates, assessing their effectiveness and compatibility with PLA (Enumo et al., 2020).

This dynamic and multifaceted research into PLA modification reflects the scientific community’s ongoing efforts to overcome the challenges of biodegradable polymers and expand their applications in technology and industry (Yang et al., 2021). Through strategies like plasticization, polymer blending and the development of composites, researchers continue to unlock new applications and performance potentials for PLA, making it a more versatile material option in the evolving landscape of manufacturing and engineering (Gao et al., 2021; Fredi and Dorigato, 2023).

This research investigates the potential of triethyl citrate (TEC) as an additive to enhance the mechanical and thermal properties of PLA. It is recognized in the polymer industry for its efficacy as a plasticizer; TEC can significantly improve the performance characteristics of biodegradable polymers like PLA. By integrating TEC into the PLA matrix, a reduction in the intermolecular forces and enhanced polymer chain mobility is expected, potentially leading to improvements in tensile strength, flexibility and impact resistance. The modification of PLA with TEC is anticipated to increase the material’s ability to absorb and dissipate energy under stress, thus improving performance under dynamic conditions. Such changes could decrease PLA’s T_g, as evidenced in Differential Scanning Calorimetry analyses, facilitating greater flexibility at lower temperatures (Singh et al., 2020).

Furthermore, plasticizer integration could theoretically enhance PLA’s shape memory behavior, characterized by its ability to return to its original shape after deformation when subjected to an appropriate stimulus (Shin et al., 2023). Adjusting the composition and percentage of plasticizer within PLA may allow researchers to control the material’s shape memory characteristics, offering significant benefits in biomedical applications, such as customizable implants that adapt to body temperatures, and in packaging solutions where material recovery post-deformation is essential (Jiang et al., 2021). Four-dimensional (4D) printing is an innovative technology that combines AM with shape memory materials, thereby broadening the applications of shape memory materials. This capability has garnered significant attention from researchers in materials science and manufacturing technologies in recent years. The 4D printing represents a substantial advancement in AM, aiming to enhance the shape, properties and functionality of 3D-printed structures (Honari et al., 2025). This technology finds widespread applications in various fields, including biomedical, engineering, textile, coating, biomimetics, sustainable robotics and electronics (Haleem et al., 2021; Sajjad et al., 2024).

Shape memory polymers (SMPs) are advanced materials distinguished by their capacity to undergo reversible shape transformations when exposed to external stimuli (Rahmatabadi et al., 2024a). Although thermal activation remains the most prevalent method for inducing the shape memory effect, alternative stimuli such as light, magnetic fields, electrical currents and chemical agents can also be used. Thermally responsive SMPs exhibit shape alterations driven by temperature fluctuations, typically arising from the coexistence of incompatible soft and hard phases within their molecular architecture. These polymers can be fabricated as copolymers, wherein covalent bonds link hard and soft segments. While tailoring SMPs with specific characteristics through the use of diverse monomers is feasible, this approach often entails complex synthetic protocols and detailed characterizations. An alternative and more straightforward method involves blending an elastomer with a switch polymer, offering a simpler route to SMP fabrication compared to direct synthesis. The elastomeric component, which can be vulcanized rubber or a thermoplastic elastomer, maintains the permanent shape and provides the elastic restoring force necessary for transitioning from the temporary to the permanent shape (Yousefi et al., 2025). Conversely, the switch phase, which may be semi-crystalline or amorphous, is characterized by a distinct thermal transition temperature that facilitates the retention of the temporary shape (Akbar et al., 2023; Arora et al., 2024).

The shape transformation temperature (T_trans) of an SMP is governed by the melting or glass transition temperature of the switch phase. When an SMP is deformed at a temperature above T_trans and subsequently cooled below this threshold, the polymer locks into its temporary shape. Reheating the material to T_trans enables the SMP to recover its original shape, a process predominantly driven by the elastomeric phase (Zende et al., 2023).

By enhancing PLA’s mechanical properties through the use of additives is also possible to adjust the behavior in terms of shape memory. An example of the additives that can be use is TEC, is a non-toxic and environmentally friendly plasticizer. This approach also aligns with ongoing efforts to develop more sustainable material solutions. It seeks to not only address some of PLA’s critical limitations but also broaden its applications in industries requiring higher durability and flexibility, such as automotive, sports equipment and protective gear. The implications of this study could significantly impact the use of PLA, promoting greater adoption of biodegradable alternatives in traditionally petroleum-based polymer markets (Sharma et al., 2021).

AM offers a unique capability through its layer-by-layer material combination, which is often difficult or impossible to achieve with traditional manufacturing methods. This technique allows for the strategic arrangement of different materials within a single building process, facilitating the creation of complex, multi-material objects (Ghaderi et al., 2024). Such configurations enable each section of the product to exhibit distinct properties suited to specific functional requirements (Nazir et al., 2023; Altuntas et al., 2023).

By layering various materials, the final product can display enhanced overall properties. For example, parts can be engineered with stiff materials in sections that demand high structural integrity, while incorporating elastic materials in areas where flexibility is needed. This method is particularly effective for designing components with customized mechanical properties, making it well-suited for specialized applications in fields like medical device manufacturing and automotive engineering. This tailored approach optimizes the functionality and performance of the final products, demonstrating the versatility and efficiency of AM (Yuan et al., 2020; Shahbazi et al., 2023).

This investigation seeks to extend the capabilities of PLA within the domain of AM by incorporating TEC as a plasticizer to enhance its applicability. The focus of this work is, first, to explore the effect of TEC on PLA’s performance characteristics, such as mechanical properties, thermal properties and melt flow index (MFI); and second, to evaluate the potential of multi-material 3D printing techniques to fabricate sandwich structures with different amounts of plasticizer. Through a series of methodical experiments, including mechanical, thermomechanical and optical characterizations, we aim to provide a deeper understanding of how TEC integration affects PLA. Additionally, the behavior in terms of shape memory is investigated in single material and multi-material samples and how these modifications translate into practical applications in various industries. Ultimately, the insights gained from this research will contribute to advancing the utility of PLA in more demanding engineering contexts, thereby promoting the adoption of more sustainable material solutions across the manufacturing sector.

PLA grade LL 712, sourced from ErcrosBio (Barcelona, Spain), with a MFI of 4 g/10 min (determined by ISO 1133-A at 190°C with a mass of 2.16 kg), was used in this study. This is an extrusion grade with an MFI suitably adjusted for this purpose. TEC was used as a plasticizer at concentrations of 10% and 20% by weight. The plasticizer, with a molecular weight of 276.3 g/mol and a purity exceeding 98%, was purchased from Sigma Aldrich (Schnelldorf, Germany). Materials were dried for 8 h in a dehumidifier at 80°C; the model used was an MDEO provided by Industrial Marsé (Barcelona, Spain).

For the filament fabrication, pellets from the proper composition were used. To this effect, a twin-screw extruder Xplore Micro Compounder MC-40 (Sittard, The Netherlands) was used. During extrusion, a temperature of 190°C for 2 min and 100 rpm were used according to the conditions proposed in Table 1. Extruded materials were pelletized using an air-knife unit. Once the materials were prepared and pelletized, a single-screw extruder. The first heating zone (T1) is located near the hopper, where the polymer is initially fed into the system. The final heating zone (T4) is positioned at the nozzle, just before the material exits the extruder. Between these, the intermediate heating zones (T2 and T3) progressively regulate the polymer temperature as it advances through the barrel. A commercial single screw extruder Filament Maker ONE Composer model from 3devo (Utrecht, The Netherlands) was used to obtain the 3D printing filaments with a diameter of 1.75 mm. The used extrusion conditions were adjusted depending on the amount of plasticizer used to adjust the fluidity of the material through the nozzle.

A Lyra350 from Lyra3D (Alicante, Spain), equipped with a 0.4 mm nozzle, was used to manufacture the samples under the conditions specified in Table 2. For all manufactured filaments, printing conditions were kept constant to ensure the comparability of results across different samples. Simplify3D v4.1.2 was used for G-code generation. During G-code preparation, M600 commands were inserted to halt the manufacturing process for filament changes, enabling the production of multilateral samples based on the layer distribution illustrated in Figure 1.

The names used for sample coding and identification are listed in Table 3, considering that all sample infills were printed with two orientations: 0° (longitudinal pattern – L) and 90° (transversal pattern – T), as shown in Figure 1.

To assess the effectiveness of multimaterial samples with different amount of plasticizer and also single materials samples, flexural test and also charpy impact tests were conducted; in this case, both infill parameters were tested (Figures 2a and 2c, with longitudinal pattern and Figures 2b and 2d, with transversal pattern). Charpy impact tests were performed following the ISO 179–1: 2010 standard. Samples with a V-shaped notch with a radius of 0.25 mm and dimensions 80 mm × 10 mm × 4 mm were subjected to the impact of a 6-J pendulum impact tester from Metrotec S.A. (San Sebastián, Spain). Flexural properties were determined following the ISO 178: 2011 standard using a testing machine ELIB-50 (Ibertest S.A., Madrid, Spain) fitted with a load cell of 5 kN at a 2 mm/min speed. In addition, tensile tests were performed in the same testing machine at 5 mm/min only with single materials samples and both infill orientations.

Both multimaterial and single-material samples were tested using the two infill orientations. Rectangular samples consisting of nine layers and measuring 50 × 10 × 1.8 mm³ were used. The testing began with the programming stage, during which samples were deformed into a U-shape with a 5 mm radius using a 3D-printed accessory. Subsequently, the samples were immersed in water at 80°C for 30 s and then cooled to room temperature; samples remained fixed to the 3D-printed accessory to avoid shape recovery during cooling. Recovery process was initiated by a sample immersion in water at 60°C for 300 s, during which the angle was measured. The process was monitored by the recovery rate (R_r), calculated using equation (1), where θ₀ is the initial angle and θ_i is the angle at each measurement. Figure 3 illustrates the angle distribution used:

Images of the fracture morphology of the specimens after Charpy impact tests were taken by means of ZEISS ULTRA 55 microscope from Oxford Instruments (Abingdon, UK) with an acceleration voltage of 2 kV. Before collecting the images, a sputtering process was performed with gold-palladium alloy in a SC7620 sputter coater from Quorum Technologies Ltd. (East Sussex, the UK).

Rectangular samples with a single material distribution were tested in dynamic conditions in a DTMT1 from Mettler-Toledo (Columbus, the USA). To this effect, samples with a size of 20 × 4 × 2 mm³ were submitted to a dynamic deformation of 10 mm with a frequency of 2 Hz and a thermal program from −100°C up to 100°C and a 2°C/min ramp.

The measurements of the MFI were measured in an Ars Faar model from Metrotec (Lezo, Spain) according to ISO 1133 at two different temperatures (190°C and 210°C) and a nominal load of 2.16 kg.

The mechanical behavior of the additively manufactured samples was evaluated through tensile tests (Table 4), flexural tests (Tables 5 and 6) and Charpy impact tests (Table 7). While tensile tests were performed exclusively on single-material samples, both single-material and multimaterial samples were used in the flexural and impact tests. Furthermore, two different infill patterns and deposition orientations were assessed for all material combinations.

Tensile strength in PLA and PLA-plasticized samples varied according to the infill pattern and the plasticizer content. The addition of a plasticizer generally reduces tensile strength, as explained by plasticization theories (Ivorra-Martinez et al., 2023b). TEC serves as a lubricant, increasing polymer chain mobility and facilitating deformation. Singh et al. (2020) reported a decrease in tensile strength for PLA modified with TEC. In this study, tensile strength declined from 68.7 MPa to 13.3 MPa with the addition of TEC. The deposition pattern also influences tensile strength. AM introduces orthotropic behavior because of the directional deposition of material (John et al., 2023). Alignments parallel to applied stress exhibit the highest resistance, as the improved continuity of the material enhances the mechanical integrity of the sample under these conditions. In contrast, perpendicular deposition alignments restrict the strength of the samples because of the limited adhesion of the deposited material. Enhancing the bonding between successive layers can improve the overall mechanical properties. To this effect, the addition of TEC affects the MFI, altering the material’s behavior in the melt state, which improves interlayer adhesion and reduces void formation (Pourali and Peterson, 2021). To mitigate orthotropic behavior, enhancing interlayer interaction is critical (Benié et al., 2023). As proposed in the MFI test in this work, TEC increases MFI, enabling better adhesion and reducing voids. This plasticizer enhanced the layer adhesion in the plasticized samples, reducing the differences observed between samples printed with a longitudinal pattern and those with a perpendicular pattern.

PLA demonstrates low ductility because of restricted chain mobility, attributed to its high glass transition temperature. Incorporating TEC improves chain mobility and elongation at break. However, at low concentrations, plasticizers may induce antiplasticization, resulting in reduced elongation (Llanes et al., 2021). The mechanical behavior of plasticized PLA also depends on manufacturing conditions (Gálvez et al., 2020). In this study, samples were produced via FDM, requiring two extrusion steps: one for blending PLA and TEC and another for filament calibration. Because of the volatility of TEC, its concentration may decrease during processing. Consequently, a 10% TEC concentration exhibited antiplasticization effects, whereas other studies reported improved elongation at break with similar concentrations (Liu et al., 2023). Elongation at break for 10TEC samples remained below 2.0% across both infill patterns. In contrast, 20TEC samples demonstrated a significant increase in elongation at break, particularly when the infill pattern aligned with the tensile stress direction, reaching 174.4%. Perpendicular patterns exhibited discontinuities that hindered plastic deformation, limiting elongation at break to 27.5%. Chacón et al. highlighted the impact of printing orientation, speed and layer height on mechanical properties (Medel et al., 2022), with studies confirming that alignment between raster angle and stress direction enhances tensile strength and elongation (Chacón et al., 2017). For plastic deformation, discontinuities arising from the perpendicular pattern orientation hinder the enhancement of elongation at break values provided by the plasticizer. In this context, samples with a longitudinal pattern manufactured using the 20TEC plasticizer exhibited the highest elongation values.

Stiffness was evaluated through tensile tests, revealing a typical reduction in plasticized PLA samples because of decreased chain interactions. While PLA’s inherent stiffness limits its application in some industries, blending with plasticizers extends its usability. Studies indicate that plasticizers, such as epoxidized sunflower oil, reduce the tensile modulus proportionally to their concentration (Bouti et al., 2022). Additionally, stiffness varies with deposition angle, decreasing from maximum values along the tensile direction to minimum values in perpendicular orientations (Kiendl and Gao, 2020; Cerda-Avila and Medellín-Castillo, 2024). In this study, materials deposited along the direction of applied stress exhibited higher stiffness, whereas samples with a perpendicular deposition angle demonstrated lower stiffness. The limited effort transfer in perpendicular direction reduced the stiffness of the sample in comparison with the longitudinal samples.

In flexural properties, PLA retains high stiffness and low ductility, consistent with tensile properties. The flexural test, analyzing deformation at the yield point, showed that 20% TEC improved flexibility. However, elongation at the maximum flexural point (ε_fM) increased only by 8%, compared to the up to 180% observed in tensile tests. The addition of TEC decreased flexural strength and modulus, attributed to increased polymer chain mobility. The trends observed in the tensile test regarding layer deposition orientation are also evident in the flexural test. Samples printed with a longitudinal pattern exhibited the highest strength and deformation values. As previously discussed, this material orientation enhances sample continuity, facilitating more effective stress transfer and consequently improving mechanical performance. In contrast, stress transfer in samples with a perpendicular deposition orientation is constrained by the limited adhesion between layers.

For multimaterial samples, the core-shell structure used in manufacturing was critical. The mechanical behavior of these samples depends primarily on the shell material. This configuration allows tailoring the final properties of the samples. (Palaniyappan et al. 2023) demonstrated that mechanical properties vary significantly with material distribution in multimaterial samples (). For example, it is possible to reduce the stiffness of the PLA/20TEC/PLA-L samples to the same range as the 10TEC-L samples while achieving higher strength. In the multimaterial samples, the core consists of a soft material that reduces stiffness, while the PLA shell enhances flexural strength. As observed in tensile properties, the longitudinal infill pattern yielded superior flexural results. Transversal samples exhibited discontinuities along the force direction, reducing flexural strength and deformation at the yield point. Studies have confirmed that infill pattern and density influence flexural properties (Öteyaka et al., 2022).

In Charpy impact tests, energy absorption varied with infill pattern and plasticizer content. Longitudinal patterns provided the highest energy absorption, while transversal patterns, with inherent discontinuities, significantly reduced it. For instance, 20TEC samples with longitudinal patterns did not break under the testing conditions, whereas transversal patterns resulted in 14.1 kJ/m². Patadiya et al. (2020) demonstrated that building direction in FDM samples strongly influences impact strength. The impact strength of TEC-modified samples was highly dependent on plasticizer concentration. For 10TEC samples, energy absorption decreased, reflecting the antiplasticization effect. This reduction was evident in multimaterial samples as well. However, with an appropriate plasticizer concentration, chain mobility increased, allowing for greater plastic deformation and enhanced impact properties (Ivorra‐Martinez et al., 2022).

The inclusion of the TEC plasticizer enhanced the mechanical properties of the manufactured samples, particularly in terms of ductility. The selection of TEC was based on its ability to provide these improvements. According to the literature, TEC plasticizers enhance the ductile properties of PLA because of their high miscibility (Harte et al., 2013; Safandowska et al., 2020). However, the plasticizer’s low molecular weight results in volatilization, leading to a reduction in the initial degradation temperatures. The thermogravimetric results presented in the supplementary material indicate that the initial degradation temperatures remain above those used in the manufacturing process. Furthermore, the selected plasticizer is of natural origin and does not compromise properties such as the disintegration ability of plasticized PLA (Brdlík et al., 2023). TEC is derived from natural resources, specifically citric acid, which can be obtained from natural products (Saabome et al., 2020; Angumeenal and Venkappayya, 2013). Its natural origin make a plasticizer that can be used in food contact applications, as it is a non-toxic product for the human (Paul et al., 2021). Other commercial plasticizers such as polyethylene glycol are derived from petrochemical products which is not suitable from the point of view of the environment (Servesh et al., 2024). Regarding the long-term stability of plasticized PLA products, previous studies have shown that prolonged storage leads to a significant decline in mechanical properties, particularly one year after manufacturing. In this context, reactive extrusion processes and compatibilization strategies can be implemented to mitigate plasticizer migration, thereby enhancing the material’s stability over time (Kodal et al., 2019; Ivorra-Martinez et al., 2023b).

The shape memory behavior of both single-material and multimaterial samples was evaluated through heat activation. Samples were deformed into a U-shaped mold in water at 80°C during the programming stage and subsequently activated in hot water at 60°C. Measurements were taken at various time intervals, as shown in Figure 5a for longitudinal infill and Figure 5b for transversal infill. Main results summarized in Tables 8 and 9. Figure 4 illustrates the angle measurement procedure used to monitor angular changes over time. The deformation and recovery mechanisms are driven by polymer chain mobility, which depends on the applied temperature. During deformation, elevated temperatures enhance chain mobility, enabling the programmed shape, while subsequent cooling reduces mobility, stabilizing the temporary shape. Recovery occurs with reheating, further increasing chain mobility and restoring the original shape (Luo et al., 2022). This shape memory recovery test has been previously used by other researchers, who have suggested that the developed materials could be applied in various fields, including soft robotic actuators, sensors and medical devices (Sanaka and Sahu, 2024). The addition of TEC introduces a non-toxic component suitable for food-contact applications while simultaneously modifying the material’s thermal properties, enhancing the activation speed of shape recovery.

The initial deformed angle (θ₀) varied with the plasticizer content despite the samples were fixed to the 3D printed accessory used in the programing stage along the cooling stage of the samples trying to keep θ₀ as high as possible. Samples containing 20 Wt% plasticizer exhibited greater recovery of the initial shape, attributed to the enhanced chain mobility observed in both mechanical characterization and thermomechanical analysis. The broad temperature range of the damping factor indicated a highly ductile state at room temperature, facilitating partial shape recovery. In contrast, neat PLA exhibited the highest θ₀, as the applied deformation remained largely plastic because of the polymer’s restricted chain mobility at room temperature. In multimaterial samples, θ₀ was significantly influenced by the proportion of 20TEC. Samples incorporating 20TEC in the shell exhibited the lowest θ₀, particularly in the 20TEC/10TEC/20TEC configuration, where the increased mobility of the outer layer further promoted shape recovery. However, this enhanced mobility also impeded shape retention before the recovery process began, as 20TEC formulations in the shell allowed premature recovery even at room temperature. This behavior was consistent with the high ductility and reduced stiffness observed in mechanical characterization. Conversely, PLA/20TEC/PLA samples successfully retained the U-shape, demonstrating that the material composition and layer arrangement play a crucial role in achieving the desired performance in shape recovery tests.

Deformation assessed at different time intervals focused primarily on the final angle at the end of the activation period (θ₃₀₀). Plasticizer addition increased the final angle, resulting in a reduced recovery rate (Rr). In PLA, the stored energy during deformation is predominantly elastic, facilitating the recovery of the original shape (Song et al., 2015). As a result, neat PLA exhibited the highest Rr among single-material samples. However, multimaterial samples sometimes displayed superior shape recovery. Configurations with a soft core and a rigid PLA shell enhanced Rr. PLA alone possesses a high capacity to store energy because of limited chain mobility, which dissipates energy, but it does not fully recover with the used activation conditions. When combined with a soft core, which lowers the effort required for shape recovery, overall efficiency improves because of its higher activation capability. While neat PLA ensures good shape recovery, its slow response can be accelerated by incorporating TEC plasticizers, which reduce Tg and facilitate activation. However, increased ductility compromises shape retention. Optimizing the material arrangement in multimaterial samples balances these effects, leveraging PLA shape memory properties and TEC’s enhanced shape activation performance.

Furthermore, shape recovery is strongly influenced by the infill pattern and other printing parameters, such as infill density (Ehrmann and Ehrmann, 2021; Rahmatabadi et al., 2024b). In this study, the same type of infill was used in both orientations, with the only difference being their alignment. Samples with a transversal orientation generally exhibited higher shape recovery. Variations in Rr can be attributed to the connectivity of the deposited beams, highlighting the influence of manufacturing parameters on recovery performance. The differences between both infill directions are attributed to the material’s orthotropic properties. In longitudinal samples, stress during programming and recovery phases is transmitted along the deposited lines, ensuring better continuity. In contrast, in transverse samples, stress is transferred across the interfaces between deposited lines, where adhesion is weaker. Rapid cooling of the material impedes full bonding between deposited lines, reducing interfacial strength. As a result, plastic deformation occurs in these regions during the shape programing stage, limiting shape memory efficiency (Pieri et al., 2021).

Selected images of the fracture morphologies for each material are shown in Figure 6 for single-material samples and Figure 7 for multimaterial samples. The morphology of the additively manufactured samples strongly depends on the raster direction used during fabrication. Samples with a longitudinal raster deposition, aligned with the stress direction, exhibit voids resulting from the deposition process. Successive deposited lines form triangular gaps between adjacent layers. In contrast, transversal samples display voids parallel to the fracture surface, creating distinct steps, as illustrated in Figures 6 and 7 (Sun et al., 2023). According to Tao et al., these voids result from partial neck growth during the deposition of molten material and are inherent to the FDM process. In this process, the material solidifies almost immediately after exiting the nozzle, limiting opportunities for complete neck growth (Tao et al., 2021).

The modifications introduced in this study aimed to enhance material fluidity without altering the melting temperatures, as confirmed by the MFI test. The increased fluidity, resulting from the addition of TEC, reduced the size of voids observed during deposition, as shown in Figure 6. This indicates that the inclusion of TEC improves the uniformity of the printed structures.

The surface morphology of the samples revealed further insights during impact testing. Changes in surface roughness were observed, reflecting the plastic deformation capabilities of the materials. In longitudinal samples, brittle materials such as PLA (Figure 6a) and 10TEC (Figure 6c) exhibited flat fracture surfaces, showing no evidence of plastic deformation which means a brittle fracture occurs during testing. This behavior aligns with findings by Li et al. (2018), who noted similar trends for materials with limited deformation capacity. In contrast, Figure 6e demonstrates that the 20TEC-L sample, with high elongation at break, underwent significant plastic deformation during the impact test leading to a ductile fracture of the sample. This deformation resulted in increased surface roughness, further supported by the high impact strength of these samples (Gomez-Caturla et al., 2024). Comparable surface morphology changes have been reported in studies such as those by Chaochanchaikul and Pongmuksuwan (2021).

For transversal deposition patterns, fracture morphology differed because of the breakage occurring along the separation of deposited lines. Given the lower impact strength values associated with these patterns, changes in morphology were less pronounced than in longitudinal samples. The 20TEC-T (Figure 6f) sample displayed minor plastic deformation on the edges, with visible ridges. Other samples exhibited no ridges (Figures 6b and 6d) and the fracture process was characterized by the presence of cracks.

In multimaterial samples, intermediate mechanical properties were observed. Figure 7a illustrates the surface morphology of a longitudinally printed multimaterial sample. The bottom portion, consisting of 20TEC, displayed limited plastic deformation despite the presence of a plasticizer. This behavior can be attributed to the combination with PLA, which has low plastic deformation capacity. Consequently, the overall deformation ability of the multimaterial configuration was reduced. While the size of voids was noticeably diminished in the plasticized region, changes in surface morphology were not highly significant. Figure 7b shows the changes in the morphology for the transversal infill samples.

The thermomechanical characterization of single-material samples over a range of temperatures is presented in Figure 8 (Figure 8a shows the storage modulus evolution, while Figure 8b the damping factor evolution), with the main findings summarized in Table 10. PLA exhibited high stiffness up to approximately 60°C, beyond which the glass transition temperature (Tg) causes the polymer matrix to transition from a rigid to a soft state (Gazquez-Navarro et al., 2024). The addition of TEC as a plasticizer reduced the Tg. In 10TEC samples, the Tg decreased from around 62°C to 50°C, demonstrating behavior similar to PLA but at a lower temperature. In 20TEC samples, the Tg reduction was more pronounced, spanning a wider temperature range. These samples began exhibiting reduced storage modulus at approximately −25°C, with a Tg below 30°C. Bajwa et al. (2021) documented similar trends, attributing the broader and lower Tg range to the plasticization effect, which increases chain mobility.

Beyond the Tg, 20TEC samples demonstrated a higher storage modulus than PLA because of crystallization during heating. The increased chain mobility induced by plasticizers enhances the material’s ability to crystallize at lower temperatures, resulting in a more pronounced recrystallization process. Consequently, 20TEC samples displayed superior storage modulus values after Tg. This behavior is particularly relevant for shape memory applications, where the coexistence of softened and unsoftened sections could optimize recovery during activation. The unsoftened regions serve as elastic components, restoring the original shape, while softened sections act as transition zones facilitating the recovery process (Mehrpouya et al., 2021; Barletta et al., 2021).

The tan (δ) peak height also reflects the influence of plasticizers, as these additives tend to reduce the height of the peak. Conversely, particles with reinforcement properties increase the tan (δ) peak height (Cristea et al., 2020). Infill parameters further influence thermomechanical behavior, with longitudinal patterns yielding higher storage modulus values, consistent with observations for tensile and flexural modulus. Storage modulus, as an indicator of material stiffness, aligns with results reported by Bermudez et al. (2021), who documented similar trends in PLA-based materials. This effect of infill pattern affecting the thermomechanical behavior of the 3D printed samples has been investigated by Giri and Mailen (2022). As observed in the mechanical properties of the samples, those with a longitudinal pattern exhibit better material continuity, resulting in higher E′ values compared to samples with a transverse pattern. In contrast, samples printed with a transverse pattern show higher tan δ values, which are associated with increased energy dissipation (Cristea et al., 2020). This higher energy dissipation in transverse patterns is attributed to less efficient stress transfer within this printing configuration.

The MFI values of the materials are presented in Table 11. These values were determined for the PLA provider’s recommended test temperature and the actual printing temperature. The PLA used in this study was characterized by an MFI of 4 g/10 min, but the measured value was 7.3 g/10 min. This discrepancy arises from the extrusion processes used for filament fabrication, which induce some degree of chain scission. Chain scission, caused by shear stress during reprocessing, reduces the polymer’s molecular weight and subsequently increases MFI values. Similar trends were reported by Ramos‐Hernández et al. (2023), who observed a consistent increase in MFI after each reprocessing cycle, even at temperatures below the polymer’s degradation threshold.

As discussed previously, plasticizers enhance chain mobility in the solid state, and this effect extends to the melt state, leading to a significant increase in MFI values depending on the plasticizer content (Zych et al., 2021). In this study, the MFI of PLA under manufacturing conditions increased substantially, from 14.4 g/10 min for unmodified PLA to 62.8 g/10 min for PLA with 20 Wt% TEC.

For AM, high MFI values offer distinct advantages. Enhanced flowability enables better layer adhesion, as the material can more effectively bond to adjacent layers. This improvement directly impacts the structural integrity and mechanical performance of printed samples (Wang et al., 2018). Correspondingly, the increase in MFI observed in this study correlated with a reduction in voids, as confirmed by the morphological analysis using field emission scanning electron microscopy.

This study highlights the transformative impact of multimaterial configurations using a core-shell structure, where a central core is encapsulated by two outer layers (shells), on optimizing material properties for advanced applications. This design strategically combines cores and shells with distinct mechanical properties, enabling tailored performance for diverse functional requirements. The incorporation of TEC as a plasticizer was key to customizing the materials used in this study, with PLA filaments containing 0%, 10% and 20% TEC content manufactured and combined in the multimaterial samples.

TEC significantly enhances elongation at break, achieving values of up to 174.4% in samples with a longitudinal infill pattern. In multimaterial samples, flexural tests demonstrate that the PLA/20TEC/PLA core-shell configuration delivers superior mechanical performance compared to homogeneous materials by effectively balancing stiffness and flexibility.

Impact resistance shows notable improvement, with samples containing a 20 Wt% TEC plasticized core absorbing up to 14.1 kJ/m², compared to 1.9 kJ/m² for pure PLA, because of enhanced energy dissipation. Thermal analysis further reveals a reduction in the Tg from 62°C to approximately 30°C, significantly increasing polymer chain mobility even at ambient conditions, which improves the ductility of the samples. These thermal properties are particularly critical for shape memory performance, where the PLA/20TEC/PLA configuration achieves a remarkable recovery rate of 81.5%. This high recovery rate results from the synergistic interaction of a soft, pliable core that facilitates deformation and rigid PLA shells that enhance elastic recovery, making these materials ideal for biomedical devices and adaptive packaging solutions.

Morphological analyses using optical imaging by SEM confirm that TEC reduces void formation during printing, enhancing structural integrity and material homogeneity. Additionally, MFI values observed with TEC improve material flow during AM, enabling more uniform deposition and minimizing defects in the printed structures.

In conclusion, the findings demonstrate the substantial potential of core-shell multimaterial configurations in AM to overcome the limitations of traditional homogeneous materials. By combining tailored mechanical and thermal properties within a single construct, these configurations offer sustainable, high-performance solutions for applications requiring advanced shape memory, flexibility and impact resistance in demanding engineering contexts.

This research is a part of the grant PID2023-152869OB-C22 and the grant TED2021-131762A-I00, funded by MCIN/AEI/10.13039/501100011033 and by the European Union “NextGenerationEU”/PRTR. The authors also thank Generalitat Valenciana – GVA, grant number CIGE/2023/46 and CIAICO/2023/253, for supporting this work. GVA for funding a postdoc position through the CIAPOS program co-funded by ESF Investing in your future, grant number CIAPOS/2023/362. Microscopy services at UPV are also acknowledged for their help in collecting and analyzing field emission scanning electron microscopy images. GVA for funding a predoc position through the CIACIF program co-funded by ESF Investing in your future, grant number CIACIF/2023/244.

Conflicts of interest: The authors declare no conflict of interest.

Data availability statement: Data will made available on request.

The supplementary material for this article can be found online.