A new approach in PLS/TPS compatibilization using garlic oil: effect on morphological and antioxidant properties

Citation: Castro, M.C.R.; Rodrigues, P.V.; Cruz, V.; Machado, A.V. A New Approach in PLS/TPS Compatibilization Using Garlic Oil: Effect on Morphological and Antioxidant Properties. Antioxidants 2024,13, 1589. https://doi.org/ 10.3390/antiox13121589 Academic Editor: Monica Rosa Loizzo Received: 15 September 2024 Revised: 14 December 2024 Accepted: 19 December 2024 Published: 23 December 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article A New Approach in PLS/TPS Compatibilization Using Garlic Oil: Effect on Morphological and Antioxidant Properties Maria Cidália R. Castro * , Pedro Veiga Rodrigues , Vasco Cruz and Ana Vera Machado Department of Polymer Engineering, Institute for Polymers and Composites (IPC), University of Minho, 4804-533 Guimarães, Portugal; pedro.r[email protected] (P.V.R.); [email protected] (V.C.); [email protected] (A.V.M.) *Correspondence: cidaliacastr[email protected] Abstract: The packaging industry has made efforts to reduce food waste and improve the resilience of food systems worldwide. Active food packaging, which incorporates active agents, represents a dynamic area where industry and academia have developed new strategies to produce innovative and sustainable packaging solutions that are more compatible with conventional options. Due to health and environmental concerns, industries have sought alternatives to petroleum-based materials and have found biopolymers to be a viable option because of their biodegradable and safe nature. The combination of PLA/TPS has emerged as an effective system for packaging film; however, they are thermodynamically immiscible. This work highlights the development of a starch-based compatibilizer to connect the PLA and TPS phases by functionalizing maize starch with glycidyl methacrylate, glycerol, or garlic oil. Garlic oil was chosen for its plasticizing ability and antioxidant properties. The films produced exhibited excellent compatibility, with enhanced interfacial adhesion between PLA and TPS components. The introduction of compatibilizers also increased the systems’ crystallinity and improved their mechanical properties. The wettability of the films significantly increased with higher garlic oil content, along with enhanced antioxidant properties. These advancements will enable the production of a compatible PLA/TPS system with improved properties for application in the packaging industry. Keywords: biodegradable polymers; compatibilization; garlic oil; antioxidant; food packaging 1. Introduction Food waste has significant environmental, social, and economic repercussions. In 2024, 8% to 10% of global greenhouse gas emissions were linked to unconsumed food, including waste before consumption due to expiration dates [ 1 ]. A significant amount of packaged food is wasted due to various environmental factors, including moisture, oxidation, UV-Vis irradiation, and microbial contamination. Therefore, prioritizing investment in methods to minimize food waste is crucial. One effective approach is to improve food packaging to better preserve and enhance the quality, safety, and shelf-life of food [ 2 ]. Furthermore, the fast pace of everyday life increasingly demands high-quality and safe food options to save consumers time. Over the years, the food packaging industry has worked to meet these demands by developing new strategies and technologies. Active packaging is an effective technology used in food packaging to address these issues. It interacts with the product to extend its shelf-life and ensure quality [ 3 , 4 ]. Active packaging incorporates selected agents, typically with biological activity, that release substances into or from the package or the surrounding environment. Among these agents, those that prevent food oxidation or inhibit microbial growth have received particular attention, as they help prevent the deterioration of food products [ 5 – 8 ]. Natural compounds with antioxidant properties, derived from plants, animals, or soils, are widely used in the food industry for food preservation and packaging [ 6 , 7 ]. Garlic, a well-known natural product in culinary Antioxidants 2024,13, 1589. https://doi.org/10.3390/antiox13121589 https://www.mdpi.com/journal/antioxidants Antioxidants 2024,13, 1589 2 of 16 and medicinal fields, contains organosulfur and phenolic compounds responsible for its health benefits and antioxidant properties [ 9 , 10 ]. Garlic extract is already widely used to produce edible films in the food industry [11–15]. In recent years, the food packaging industry has undergone a significant shift due to societal demands and ecological concerns. This change has prompted researchers to develop sustainable and biodegradable packaging materials, particularly for single-use items, as alternatives to petroleum-based polymers [ 16 ]. Biodegradable polymers, such as polylactic acid (PLA) and thermoplastic starch (TPS), have gained significant attention from both academia and industry due to their biodegradability, eco-friendliness, nontoxicity, and biocompatibility. While PLA shares mechanical properties with conventional polymers, TPS is more cost-effective, making it an attractive commercial option [17–20]. To enhance the strengths of the two polymers and optimize their cost and benefits, several studies have investigated blends of PLA and TPS. However, it is important to consider that TPS is hydrophilic, while PLA is hydrophobic, resulting in high tension at the interface between both biopolymers, poor adhesion, and lack of compatibility [ 21 – 23 ]. Therefore, improving the adhesion interface between the two components is crucial for an effective PLA/TPS blend. The scientific literature suggests various alternatives, such as incorporating coupling agents like maleic anhydride into the PLA, which contain functional groups capable of interacting with the hydroxyl groups present in TPS [ 23 – 25 ]. Another widely used alternative is incorporating glycerol, a plasticizer that enhances the thermoplasticity of TPS and improves compatibility with other polymers [ 22 , 26 ]. Furthermore, the compatibilization of PLA/TPS using glycidyl methacrylate (GMA) has recently been explored, resulting in compatibilized blends with superior mechanical properties compared to non-compatibilized ones [ 27 – 30 ]. Considering the importance of chemical modification to enhance the interaction between PLA and TPS, this paper reports the functionalization of commercial maize starch with GMA, plasticized with glycerol (Gly) or garlic oil (GO), for use as a compatibilizer in PLA/TPS blends. It is important to mention that the use of GO serves two purposes: (1) to evaluate its plasticizing capability in comparison to Gly, and (2) to explore the antioxidant properties conferred to the film. After the production of these compatibilizers, they were blended with PLA and TPS. Dicumyl peroxide (DCP), a peroxide, was used to promote the grafting of the compatibilizer onto PLA. The effect of compatibilization was investigated in terms of structural, morphological, thermal, mechanical, and antioxidant properties. 2. Materials and Methods 2.1. Reagents PLA (Luminy ® LX175) was provided by Totalenergies (The Hague, The Netherlands) and TPS (Unik Bropar PG1017) was kindly provided by Sacos 88—Sociedade de Plásticos, Lda company (Leiria, Portugal). Glycerol (Gly), dicumyl peroxide 98% (DCP), and glycidyl methacrylate 99% (GMA) were purchased from Alfa Aesar (Karlsruhe, Germany), while garlic oil “Saluten” (GO) was purchased from Essencia d’um Segredo (Arrentela, Portugal), and commercial starch was obtained from a commercial market (maize starch, average composition of 75% amylopectin and 25% amylose). Distilled water was used for the contact angle (CA) tests. For the antioxidant activity, absolute methanol and radical 2,2diphenyl-1-picrylhydrazyl (DPPH) were acquired from Merck (Darmstadt, Germany). 2.2. Film Preparation Pristine PLA was dried at 80 ◦ C overnight in a vacuum oven. Initially, several compositions were performed using a Haake Rheometer batch mixer (Haake Rheomix Roller Rotors R600, volume 69 cm 3 ) with counter-rotating rotors to determine the most feasible functionalized starch with GMA and Gly (StF) percentage for PLA/TPS matrix incorporation. Two StF percentages were tested (5 and 10 wt.%). The 10 wt.% value showed good compatibilization according to electron microscopy analysis (see Figure S1 in Supplementary Materials). Therefore, the StF content to be added was fixed at 10 wt.% of the total PLA/TPS weight Antioxidants 2024,13, 1589 3 of 16 (a ratio of 90/10 PLA/TPS blend was used). For the development of the compatibilizers StF and StFGO (functionalized starch with GMA and GO), the components were initially mixed in a plastic bag for previous manual mixing. This mixture was processed in a Haake batch mixer at 160 ◦C and 80 rpm for 15 min. The compositions PLA/TPS (control), PLA/TPS/StF, PLA/TPS/StFGO, PLA/TPS/StF_1.5GO, and PLA/TPS/StF_3GO were also blended in a Haake batch mixer at 160 ◦ C and 80 rpm. The components were added in the following order: first PLA and then after 1 min, TPS and the compatibilizer (StF or StFGO). At 2.5 min, DCP was introduced, and the mixture was allowed to react for 7.5 min. In the case of the compositions with GO, it was added 1 min after the DCP addition. The chemical description of the compatibilizers and the prepared compositions are depicted in Tables 1and 2, respectively. Table 1. Composition of the StF and StF_GO compatibilizers. Compositions St (Maize) (wt.%) GMA (wt.%) Gly (wt.%) GO (wt.%) StF 55 10 35 StF_GO 55 10 35 Table 2. Composition and thickness of the produced films. Composition PLA (wt.%) TPS (wt.%) StF (wt.%) StFGO (wt.%) GO (wt.%) DCP (wt.% of SfF) efilm (mm) PLA/TPS 90 10 0.08 ±0.01 PLA/TPS/StF 81 9 10 1 0.11 ±0.04 PLA/TPS/StFGO 81 9 10 1 0.13 ±0.01 PLA/TPS/StF_1.5GO 79.6 8.9 10 1.5 1 0.12 ±0.01 PLA/TPS/StF_3GO 78.3 8.7 10 3 1 0.15 ±0.02 Note: efilm is the final average thickness of the films obtained by compression molding. The compression molding technique was used to produce the composition films. Typically, this technique involves two cycles: one for heating, during which the material is melted and compressed, followed by a cooling cycle to stabilize and consolidate the resulting film. Approximately 5 g of each composition was melted at 170 ◦ C for 5 min, and then a pressure of 20 tons was applied for 3 min. Cooling was achieved in the press using water circulation. The photographs of the resulting films are shown in Figure 1, where it is possible to observe that the incorporation of compatibilizers does not affect the transparency of the films. Antioxidants 2024, 13, 1589 4 of 17 Figure 1. (a) Control; (b) PLA/TPS; (c) PLA/TPS/StF; (d) PLA/TPS/St/GO; (e) PLA/TPS/StF/1.5GO; (f) PLA/TPS/StF/3GO. 2.3. Films Thickness The film thickness of the different films was measured using a digital micrometer gauge (Mitutoyo Absolute, Japan). The presented values are the average of at least 5 random readings on each film sample. 2.4. Structural and Morphological Characterization Fourier transform infrared spectroscopy (FTIR) analysis was performed in a Perkin Elmer Spectrum 100 spectrometer in ATR mode with 16 accumulations, 4 cm −1 resolution, and a range of 4000–700 cm −1 . Morphological evaluation was achieved in an ultra-high-resolution field emission gun scanning electron microscopy (FEG-SEM), NOVA 200 Nano SEM (FEI Company). Samples were previously fractured in liquid nitrogen and covered with a thin film (2 nm) of Au-Pd (80–20 weight %) in a high-resolution sputter coater (208HR Cressington Company), coupled to an MTM-20 Cressington High-Resolution Thickness Controller. 2.5. Thermal Characterization Thermogravimetric analysis (TGA) of the films was carried out in a TGA Q500 (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere at 10 °C/min in a temperature range from 40 to 500 °C. A DSC Netzsch 200 Maya (Netzsch, Selb, Germany) was used to access the melting and crystallization temperature of the crystalline phase, under a nitrogen atmosphere and a heating rate of 10 °C/min. Two heating cycles were performed, the first to erase the thermal history of the samples and the second to analyze the data. The crystallinity degree (𝑿𝒄) was determined using Equation (1) [31]: 𝑋󰇛%󰇜 ∆𝐻 ∆𝐻  𝑓 100 (1) where ∆𝐻 is the experimental enthalpy of fusion of the sample, ∆𝐻  is the theoretical heat of fusion for 100% crystalline PLA (93.7 J/g [32]), and 𝑓 is the weight fraction of PLA. 2.6. Mechanical Characterization Tensile tests were conducted using a Zwick/Roell Z005 universal testing machine (Zwick/Roell, Ulm, Germany), in accordance with the ASTM D882–02 standard. To ensure accuracy, at least 5 specimens (25 × 150 mm) were employed, with a grip separation of 100 mm and a crosshead velocity of 5 mm/min. This analysis was performed to determine the Young’s modulus (𝐸), strain (𝜀) and stress at break (𝜎). Figure 1. (a) Control; (b) PLA/TPS; (c) PLA/TPS/StF; (d) PLA/TPS/St/GO; (e) PLA/TPS/StF/1.5GO; (f) PLA/TPS/StF/3GO. Antioxidants 2024,13, 1589 4 of 16 2.3. Films Thickness The film thickness of the different films was measured using a digital micrometer gauge (Mitutoyo Absolute, Kawasaki, Japan). The presented values are the average of at least 5 random readings on each film sample. 2.4. Structural and Morphological Characterization Fourier transform infrared spectroscopy (FTIR) analysis was performed in a Perkin Elmer (Waltham, MA, USA) Spectrum 100 spectrometer in ATR mode with 16 accumulations, 4 cm−1resolution, and a range of 4000–700 cm−1. Morphological evaluation was achieved in an ultra-high-resolution field emission gun scanning electron microscopy (FEG-SEM, Hillsboro, OR, USA), NOVA 200 Nano SEM (FEI Company, Hillsboro, OR, USA). Samples were previously fractured in liquid nitrogen and covered with a thin film (2 nm) of Au-Pd (80–20 weight %) in a high-resolution sputter coater (208HR Cressington Company, Watford, UK), coupled to an MTM-20 Cressington High-Resolution Thickness Controller. 2.5. Thermal Characterization Thermogravimetric analysis (TGA) of the films was carried out in a TGA Q500 (TA Instruments, New Castle, DE, USA) under a nitrogen atmosphere at 10 ◦ C/min in a temperature range from 40 to 500 ◦C. A DSC Netzsch 200 Maya (Netzsch, Selb, Germany) was used to access the melting and crystallization temperature of the crystalline phase, under a nitrogen atmosphere and a heating rate of 10 ◦ C/min. Two heating cycles were performed, the first to erase the thermal history of the samples and the second to analyze the data. The crystallinity degree (Xc) was determined using Equation (1) [31]: Xc(%) = ∆Hm ∆H0 m×f ×100 (1) where ∆Hm is the experimental enthalpy of fusion of the sample, ∆H0 m is the theoretical heat of fusion for 100% crystalline PLA (93.7 J/g [ 32 ]), and f is the weight fraction of PLA. 2.6. Mechanical Characterization Tensile tests were conducted using a Zwick/Roell Z005 universal testing machine (Zwick/Roell, Ulm, Germany), in accordance with the ASTM D882–02 standard. To ensure accuracy, at least 5 specimens (25 × 150 mm) were employed, with a grip separation of 100 mm and a crosshead velocity of 5 mm/min. This analysis was performed to determine the Young’s modulus (E), strain (εbr) and stress at break (σbr). 2.7. Contact Angle (CA) The contact angle (CA) was used to estimate the surface hydrophobicity of the films. It was measured using a goniometer (Contact Angle System OCA 20 Dataphysics, Germany). The CA measurement with water was conducted on a flat film surface and analyzed using the acquired images. A precise syringe was used to drop 3 µ L of distilled water onto the film surfaces following the sessile drop method. The initial image of the drop (captured at 0 s) was recorded using a video camera. To obtain the mean value, a minimum of 20 measurements per film were performed. 2.8. Antioxidant Activity The DPPH radical assay is a simple and fast method to evaluate the antioxidant activity of a given sample. Briefly, a methanolic DPPH solution (2 mL, 14.2 µ g/mL) was added to 50 µ L of the sample. The solutions were homogenized and kept in darkness for 30 min. Antioxidants 2024,13, 1589 5 of 16 The absorbance was measured at 515 nm. The inhibition percentage was measured using Equation (2). IP(%) = AC −AS AC ×100 (2) Here, AC stands for the control’s absorbance and AS stands for the sample’s absorbance. The applied approach was according to Andrade et al.’s method [33]. 2.9. Statistical Analysis The results of each test represent the mean values and standard deviation of three samples, analyzed by analysis of variance (ANOVA) and post hoc Tukey test, using the SPSS v.22.0 program (IBM Corp., Armonk, NY, USA). 3. Results and Discussion 3.1. Structural and Morphological Characterization 3.1.1. Fourier Transform Infrared Spectroscopy (FTIR) To verify any structural modifications due to chemical reactions during processing, all samples were analyzed using FTIR. The spectra of the compatibilizer and the prepared compositions are shown in Figures 2and 3, respectively. Table 3summarizes the characteristic band’s wavenumber of each material. The spectrum corresponding to PLA shows a peak at approximately 1743 cm −1 corresponding to the stretching of the C=O bond, due to the presence of carbonyl groups in the PLA structure. Between 1382 and 1352 cm −1 and 2943 and 3000 cm −1 , the bands are related to the symmetric and asymmetric vibration of the -CH group from the hydrocarbon chain of the polymer. Another characteristic peak is located at approximately 1182 cm −1 and corresponds to the asymmetric stretching vibrations of the C-O-C bond, which is typical of the PLA chain [ 23 , 34 ]. TPS spectrum reveals several characteristic peaks (Figure 2and Table 3), as already observed in the literature [ 35 ]. A broad band around 3325 cm −1 reflects the presence of hydroxyl groups (O-H), characteristic of starch. Another characteristic peak is found between 2874 and 2951 cm −1 , corresponding to the stretching vibrations of the C-H bonds. At approximately 1700 cm −1 , another peak is detected, which corresponds to the carbonyl group (C=O) of the polyester. According to the material’s technical data sheet, this is an additive in its composition. Additionally, peaks at 1031 cm −1 and 1160 cm −1 are related to C-O-C asymmetric stretching and C-O stretching, respectively. Garlic contains many biological components, including organosulfur compounds, polyphenols, polysaccharides, vitamins, and proteins [ 36 ]. Since the used GO is a commercial oil, its detailed chemical composition is unknown; however, some specific bands can be identified. The bands at 2850 and 2980 cm −1 are assigned to CH symmetric and anti-symmetric stretching, and the band at 1710 cm −1 is attributed to the carbonyl group (C=O) of proteins. The band at 1400 cm −1 corresponds to the stretching of the methyl group, primarily from lipids, and the vibration of the diallyl sulfide molecule is observed at 1192 cm −1 . Regarding the neat starch, characteristic peaks include a broad peak from 3590 to 3040 cm −1 , corresponding to the O–H stretching of amylopectin; a peak at 2920 cm −1 , associated with C-H stretching; a peak at 1645 cm −1 , related to C-O bending (associated with the OH group); peaks from 1390 to 1465 cm −1 related to C-H bending; and a peak at approximately 1000 cm −1 related to ether groups (C-O-C). For the compatibilizers developed in this study (StF and StGO), the spectra are very similar to that of neat starch, with no significant differences observed except for the presence of new peaks around 1734 cm −1 and 1750 cm −1 for StF and StGO, respectively, corresponding to the stretching of the carbonyl bond (C=O) present in GMA and GO (in the case of StGO). This new peak indicates the successful functionalization of starch with GMA. Regarding the FTIR spectra of the blends containing PLA, TPS, and functionalized starches, Figure 3and Table 3show no significant differences between the control composition PLA/TPS spectrum and the other compositions. The characteristic peaks from the PLA and TPS polymers, previously described, overlap with possible changes in the matrix, particularly in the regions of carbonyl (C=O) and ether (C-O-C) group stretching. Antioxidants 2024,13, 1589 6 of 16 However, a slight shoulder is noticeable between 1640 and 1688 cm −1 , resulting from the incorporation of the starches StF and StFGO. Peaks that could confirm the reaction of PLA and GMA appear around 1180 cm −1 and 1017 cm −1 , indicating the C-O stretching in the ester and hydroxyl groups, respectively. However, these peaks overlap with the characteristic peaks of PLA and TPS [30]. Antioxidants 2024, 13, 1589 6 of 17 assigned to CH symmetric and anti-symmetric stretching, and the band at 1710 cm−1 is attributed to the carbonyl group (C=O) of proteins. The band at 1400 cm−1 corresponds to the stretching of the methyl group, primarily from lipids, and the vibration of the diallyl sulfide molecule is observed at 1192 cm−1. Regarding the neat starch, characteristic peaks include a broad peak from 3590 to 3040 cm−1, corresponding to the O–H stretching of amylopectin; a peak at 2920 cm−1, associated with C-H stretching; a peak at 1645 cm−1, related to C-O bending (associated with the OH group); peaks from 1390 to 1465 cm−1 related to C-H bending; and a peak at approximately 1000 cm−1 related to ether groups (C-O-C). For the compatibilizers developed in this study (StF and StGO), the spectra are very similar to that of neat starch, with no significant differences observed except for the presence of new peaks around 1734 cm−1 and 1750 cm−1 for StF and StGO, respectively, corresponding to the stretching of the carbonyl bond (C=O) present in GMA and GO (in the case of StGO). This new peak indicates the successful functionalization of starch with GMA. Figure 2. FTIR spectra of the raw materials and the functionalized starches. Regarding the FTIR spectra of the blends containing PLA, TPS, and functionalized starches, Figure 3 and Table 3 show no significant differences between the control composition PLA/TPS spectrum and the other compositions. The characteristic peaks from the PLA and TPS polymers, previously described, overlap with possible changes in the matrix, particularly in the regions of carbonyl (C=O) and ether (C-O-C) group stretching. However, a slight shoulder is noticeable between 1640 and 1688 cm−1, resulting from the incorporation of the starches StF and StFGO. Peaks that could confirm the reaction of PLA and GMA appear around 1180 cm−1 and 1017 cm−1, indicating the C-O stretching in the ester and hydroxyl groups, respectively. However, these peaks overlap with the characteristic peaks of PLA and TPS [30]. 4000 3500 3000 2500 2000 1500 1000 Transmittance Wavenumber (cm −1 ) PLA TPS StF StFGO Garlic Oil Starch Figure 2. FTIR spectra of the raw materials and the functionalized starches. Antioxidants 2024, 13, 1589 7 of 17 Figure 3. FTIR spectra of the produced compositions. Table 3. Characteristic bands wavenumber of neat PLA and TPS and modified systems. Composition Wavenumber (cm−1) Bond PLA 1182 C-O-C 1352–1382; 2943–3000 C-H 1743 C=O TPS 3340 O-H 2874–2951 C-H 1031, 1160 C-O Starch 3590–3040 O-H 2920, 1465–1390 C-H 1645 C-O 1000 C-O-C StF 1734 C=O StFGO 1750 C=O PLA/TPS/StF 1688; 1640 C=O PLA/TPS/StGO 1688; 1640 C=O PLA/TPS/StF_1.5GO 1688; 1640 C=O PLA/TPS/StF_3GO 1688; 1640 C=O 3.1.2. Scanning Electron Microscopy Morphological evaluation was performed to assess the homogeneity of the compositions and investigate the interfacial adhesion between the components. The magnified morphologies at 1000× and 5000× are shown on the left and right sides of Figure 4, respectively. Analyses of Figure 4a,b reveal that the fracture of the PLA/TPS film exhibits an uneven topography with distinct phases. The images show dispersed spherical particles of TPS (dispersed phase) with low interaction with the PLA matrix (continuous phase), attributed to a lack of adhesion between the two polymers, as previously mentioned [24]. The incorporation of functionalized starches significantly increases compatibilization between PLA and TPS. Comparing both functionalized starches (StF with GMA and Gly; and StFGO with GMA and GO), it can be seen that the latter appears more effective in improving compatibilization and interfacial adhesion, as shown in Figure 4c–f. The 4000 3500 3000 2500 2000 1500 1000 Transmittance Wavenumber (cm −1 ) PLA/TPS PLA/TPS/StF PLA/TPS/StFGO PLA/TPS/StF_1.5GO PLA/TPS/StF_3GO Figure 3. FTIR spectra of the produced compositions. Antioxidants 2024,13, 1589 7 of 16 Table 3. Characteristic bands wavenumber of neat PLA and TPS and modified systems. Composition Wavenumber (cm−1) Bond PLA 1182 C-O-C 1352–1382; 2943–3000 C-H 1743 C=O TPS 3340 O-H 2874–2951 C-H 1031, 1160 C-O Starch 3590–3040 O-H 2920, 1465–1390 C-H 1645 C-O 1000 C-O-C StF 1734 C=O StFGO 1750 C=O PLA/TPS/StF 1688; 1640 C=O PLA/TPS/StGO 1688; 1640 C=O PLA/TPS/StF_1.5GO 1688; 1640 C=O PLA/TPS/StF_3GO 1688; 1640 C=O 3.1.2. Scanning Electron Microscopy Morphological evaluation was performed to assess the homogeneity of the compositions and investigate the interfacial adhesion between the components. The magnified morphologies at 1000×and 5000×are shown on the left and right sides of Figure 4, respectively. Analyses of Figure 4a,b reveal that the fracture of the PLA/TPS film exhibits an uneven topography with distinct phases. The images show dispersed spherical particles of TPS (dispersed phase) with low interaction with the PLA matrix (continuous phase), attributed to a lack of adhesion between the two polymers, as previously mentioned [ 24 ]. The incorporation of functionalized starches significantly increases compatibilization between PLA and TPS. Comparing both functionalized starches (StF with GMA and Gly; and StFGO with GMA and GO), it can be seen that the latter appears more effective in improving compatibilization and interfacial adhesion, as shown in Figure 4c–f. The compositions of PLA/TPS/StF_1.5GO and PLA/TPS/StF_3GO in Figure 4g–j exhibit a considerably more homogeneous and continuous morphology than the previous compositions, indicating an excellent interface between both phases. Moreover, an increase in GO content leads to better interfacial adhesion among all components. It is also important to note that the presence of DCP and GMA in all the produced blends positively affects the morphology, making it difficult to distinguish between the two phases in most blends, which aligns with initial expectations. The inclusion of peroxide induces the formation of free radicals that play a fundamental role in the chemical reaction between the functionalized starches and the PLA chain. This means that the peroxide creates radicals along the hydrocarbon chain of PLA, which subsequently reacts with the double bond in the GMA agent present in StF and StFGO. Thus, promoting covalent bond grafting improves the dispersibility of the TPS granules in the PLA matrix and reinforces the interfacial bonding between PLA and TPS [29,30]. Antioxidants 2024,13, 1589 8 of 16 Antioxidants 2024, 13, 1589 8 of 17 compositions of PLA/TPS/StF_1.5GO and PLA/TPS/StF_3GO in Figure 4g–j exhibit a considerably more homogeneous and continuous morphology than the previous compositions, indicating an excellent interface between both phases. Moreover, an increase in GO content leads to better interfacial adhesion among all components. It is also important to note that the presence of DCP and GMA in all the produced blends positively affects the morphology, making it difficult to distinguish between the two phases in most blends, which aligns with initial expectations. The inclusion of peroxide induces the formation of free radicals that play a fundamental role in the chemical reaction between the functionalized starches and the PLA chain. This means that the peroxide creates radicals along the hydrocarbon chain of PLA, which subsequently reacts with the double bond in the GMA agent present in StF and StFGO. Thus, promoting covalent bond grafting improves the dispersibility of the TPS granules in the PLA matrix and reinforces the interfacial bonding between PLA and TPS [29,30]. Figure 4. Cont. Antioxidants 2024,13, 1589 9 of 16 Antioxidants 2024, 13, 1589 9 of 17 Figure 4. SEM images of PLA/TPS (a,b); PLA/TPS/StF (c,d); PLA/TPS/StFGO (e,f); PLA/TPS/StF_1.5GO (g,h); and PLA/TPS/StF_3GO (i,j). Images (a), (c), (e), (g), and (i) are in 100 µm scale, and (b), (d), (f), (h), and (j) are in 50 µm scale. 3.2. Thermal Characterization To investigate the influence of the developed compatibilizers on the thermal properties of the produced systems, TGA and DSC thermograms were performed on both the functionalized starches and compositions, as shown in Figures 5–7. More detailed data are presented in Table 4, where T peak , Tg, T c , T m , ΔH m , and X c correspond to the degradation peak temperature, glass transition temperature, cold crystallization temperature, melting temperature, melting enthalpy, and the degree of crystallinity, respectively. These curves represent the first cooling and second heating of the samples to eliminate the thermal history of the polymers. From Figure 5 and Table 4, it can be observed that the addition of TPS does not significantly impact the thermal stability of the matrix. The degradation temperature decreases from 317 °C to 310 °C, and the Tg value decreases from 59 °C to 54 °C. However, Xc significantly increases from 22 to 31 %, indicating improved reorganization of the PLA molecules. Comparing the TPS with the StF and StFGO thermograms, three degradation stages can be seen, indicating similar degradation patterns. TPS alone presents a crystallization peak of its crystalline phase during cooling, with a corresponding melting peak at 123 °C (no cold crystallization is observed), consistent with previously reported values [37]. Nevertheless, it is evident that the developed compatibilizers exhibit lower thermal stability. Additionally, the thermal stability of StF is slightly lower than that of StFGO. Figure 4. SEM images of PLA/TPS (a,b); PLA/TPS/StF (c,d); PLA/TPS/StFGO (e,f); PLA/TPS/StF_1.5GO (g,h); and PLA/TPS/StF_3GO (i,j). Images (a,c,e,g,i) are in 100 µ m scale, and (b,d,f,h,j) are in 50 µm scale. 3.2. Thermal Characterization To investigate the influence of the developed compatibilizers on the thermal properties of the produced systems, TGA and DSC thermograms were performed on both the functionalized starches and compositions, as shown in Figures 5–7. More detailed data are presented in Table 4, where T peak , Tg, T c , T m , ∆ H m , and X c correspond to the degradation peak temperature, glass transition temperature, cold crystallization temperature, melting temperature, melting enthalpy, and the degree of crystallinity, respectively. These curves represent the first cooling and second heating of the samples to eliminate the thermal history of the polymers. From Figure 5and Table 4, it can be observed that the addition of TPS does not significantly impact the thermal stability of the matrix. The degradation temperature decreases from 317 ◦ C to 310 ◦ C, and the Tg value decreases from 59 ◦ C to 54 ◦ C. However, Xc significantly increases from 22 to 31 %, indicating improved reorganization of the PLA molecules. Comparing the TPS with the StF and StFGO thermograms, three degradation stages can be seen, indicating similar degradation patterns. TPS alone presents a crystallization peak of its crystalline phase during cooling, with a corresponding melting peak at 123 ◦ C (no cold crystallization is observed), consistent with previously reported values [ 37 ]. Nevertheless, it is evident that the developed compatibilizers exhibit lower thermal stability. Additionally, the thermal stability of StF is slightly lower than that of StFGO. Antioxidants 2024,13, 1589 16 of 16 32. Chen, L.; Qiu, X.; Xie, Z.; Hong, Z.; Sun, J.; Chen, X.; Jing, X. Poly(l-lactide)/starch blends compatibilized with poly(l-lactide)-gstarch copolymer. Carbohydr. Polym. 2006,65, 75–80. [CrossRef] 33. Andrade, M.A.; Ribeiro-Santos, R.; Bonito, M.C.C.; Saraiva, M.; Sanches-Silva, A. 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