High-Impact PLA in Compatibilized PLA/PCL Blends: Optimization of Blend Composition and Type and Content of Compatibilizer

RESEARCH ARTICLE www.mame-journal.de High-Impact PLA in Compatibilized PLA/PCL Blends: Optimization of Blend Composition and Type and Content of Compatibilizer Ainhoa Fernández-Tena, Itziar Otaegi, Lourdes Irusta, Victor Sebastián, Gonzalo Guerrica-Echevarria, Alejandro Jesus Müller, and Nora Aranburu* In this work, the effectiveness of seven commercial compatibilizers is tested in polylactide (PLA)/poly(𝝐-caprolactone) (PCL) blends with different compositions to obtain a high-impact PLA. None of the compatibilizers is effective for 90/10 and 80/20 PLA/PCL compositions, as no improvement of the impact strength is observed. For the 70/30 composition, compatibilizers having glycidyl methacrylate (GMA) and acrylate groups in their structure are proved the most effective, as the morphological change towards co-continuity induced by them leads to significant impact strength improvements (of ≈345% and 90% with respect to the neat PLA and the noncompatibilized PLA/PCL 70/30 blend, respectively). The 70/30 PLA/PCL composition, as it shows the best balance of properties, and the best compatibilizer (ElvaloyPTW) are chosen to carry out the optimization of the compatibilizer content. It is found that adding 6 phr to the blend results in highly toughened and ductile blends while maintaining a high modulus and yield strength values. Larger compatibilizer contents lead to even higher impact strength values, but the low-strain mechanical properties are notably reduced. Thus, in this work, a simple and easily scalable method to produce high-impact PLA is shown, as it implies the compounding of three commercially available components without involving any toxic solvents. A. Fernández-Tena, I. Otaegi, L. Irusta, G. Guerrica-Echevarria, A. J. Müller, N. Aranburu POLYMAT and Department of Advanced Polymers and Materials: Physics Chemistry and Technology Faculty of Chemistry University of the Basque Country UPV/EHU Paseo Manuel de Lardizabal 3, Donostia-San Sebastián 20018, Spain E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.202300213 © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/mame.202300213 1. Introduction Due to the current issues related to the lack of fossil resources and the accumulation of plastic waste, over the past decades, many researchers have focused on the investigation of biobased and compostable polymers since their expansion could prevent the accumulation of solid waste and decrease the use of non-renewable resources. Poly(lactic acid) or polylactide (PLA) is one of the most extensively researched biobased and biodegradable polymers used for this purpose. It is currently one of the most commercially developed biopolymers.[1,2] PLA is synthesized from lactic acid, obtained through the fermentation of sugar or starch.[3] It possesses high modulus and strength, good clarity, and barrier properties. Thanks to these characteristics and its good thermal processability, PLA has been used in industrial sectors such as textile, packaging, construction, or automotive. Moreover, due to its biodegradability, non-toxicity, and bioresorbability, it has been used in biomedical applications such as drug delivery systems, sutures, and tissue engineering.[1,3,4] V. Sebastián Department of Chemical and Environmental Engineering. University of Zaragoza Campus Río Ebro-Edificio I+D C/ Poeta Mariano Esquillor S/N, Zaragoza 50018, Spain V. Sebastián Instituto de Nanociencia y Materiales de Aragón (INMA) CSIC-Universidad de Zaragoza Zaragoza 50009, Spain V. Sebastián Networking Research Center on Bioengineering Biomaterials and Nanomedicine CIBER-BBN Madrid 28029, Spain A. J. Müller IKERBASQUE Basque Foundation for Science Plaza Euskadi 5, Bilbao 48009, Spain Macromol. Mater. Eng. 2023,308, 2300213 2300213 (1 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH www.advancedsciencenews.com www.mame-journal.de Although PLA possesses good characteristics and can be molded with standard polymer processing equipments,[5] it shows certain drawbacks, primarily related to its thermal and mechanical properties, which limit its production and use in different applications.[1] PLA has a low glass transition temperature (Tg) (55–60 °C) and crystallization rate, limiting the usage of PLA to applications with relatively low service temperatures.[1,4,6,7] On the other hand, PLA also suffers from inherent brittleness and a very low toughness at room temperature. Hence, the use of PLA may be limited in applications where high elongation or plastic deformation at high impact rates are required.[8,9] A widespread and cost-effective method to overcome the drawback of the mechanical properties is to blend PLA with ductile polymers.[7,10] Poly(𝜖-caprolactone) (PCL) is a ductile semicrystalline aliphatic polyester, with Tgand Tm≈−60 °C and 55– 70 °C, respectively.[1,11] Although it is a nonbiobased polymer, PCL is biodegradable and biocompatible, as well as PLA. These characteristics make PCL a suitable candidate to be blended with PLA since biodegradability and biocompatibility will be maintained.[1,11] Due to its low Tg, PCL exhibits high elongation at break (≈600%[8]) and toughness. Therefore, adding PCL to PLA as a secondary phase could improve the ductility and toughness of PLA.[1] In general, elongation at break of PLA has been easily enhanced (85-400%) by melt blending it with PCL,[12–18] even if, in some cases, poor or no increment has also been reported.[19–23] However, owing to the immiscibility between both polymers, improving the impact toughness of PLA/PCL blends is more challenging, though some good attempts have been made in this field. In this sense, it has been proved that the PCL concentration, the crystallinity of the PLA matrix, the PLA/PCL viscosity ratio, and the processing conditions are key factors that need to be optimized to improve the impact strength of PLA.[24–29] Nevertheless, achieving the optimal composition, processing conditions, and morphology is not always possible. That is why, high impact strength values or supertoughness are not usually reported in the literature for PLA/PCL blends.[12–14,19] To overcome this issue, different kinds of compatibilizers have been used to tune the morphology and compatibility of the blends and develop a PLA/PCL blend with high impact resistance. Vilay et al.[30] used a polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock copolymer as a compatibilizer for PLA70/PCL30 blends. They observed a shift in the Tgand Tmof PLA and PCL towards each other, indicating an increase in compatibility, as well as a significant improvement in the toughness of PLA/PCL blends. The authors attributed this to the polar interactions between PLA, PCL, and the copolymer. Reactive compatibilization has also proved effective when compatibilizing PLA/PCL blends. Harada et al.[31] prepared PLA/PCL blends with four different reactive processing agents. Blends with lysine triisocyanate (LTI) showed the best results, significantly improving unnotched and notched Charpy impact strength. Hou et al.[32] used different amounts of ethylene-methyl acrylateglycidyl methacrylate (EMA-GMA) to enhance the toughness of PLA90/PCL10 blends. With 8 phr of EMA-GMA the highest impact strength was achieved, 64.31 kJ m−2,≈23 times higher than that of neat PLA. However, the methods proposed in the literature do not seem easily implementable at an industrial level, either because lab-synthetized compatibilizers or components that are difficult to acquire at a large scale were employed or because the blending technique used was not continuous. In the present work, the fabrication of a high-impact PLA was sought using an easy and industrially scalable process. With this purpose, compatibilized PLA/PCL blends were prepared by melt mixing in a twin screw extruder, using seven different commercially available compatibilizers. The PLA/PCL composition, compatibilizer type, and content were optimized. To do that, the prepared compositions’ morphology and the mechanical and thermal properties were determined and compared. As a reference, the corresponding noncompatibilized PLA/PCL blends and neat PLA with the different compatibilizers were prepared and characterized. 2. Experimental Section 2.1. Materials The PLA used in this work was a commercially available extrusion grade PLA (Ingeo Biopolymer 4032D, 1.2–1.6% D-isomer lactide) supplied by NatureWorks (Minneapolis, MN, USA). PCL (CAPA 6800) was purchased from Ingevity (North Charleston, SC, USA) with a molecular weight of 80 000 g mol−1.DowChemicals (Midland, MI, USA) donated the seven polyethylene-based copolymers employed as compatibilizers. The chemical structure of each copolymer is summarized in Table 1. Although the exact chemical composition of BiomaxSG120 is unknown, it consists of an ethylene-acrylate terpolymer specifically designed for PLA, bearing epoxy moieties.[33] 2.2. Sample Preparation Before melt processing, all the materials were dried overnight to avoid moisture-induced degradation. The PLA was dried in a dehumidifier at 80 °C, and the PCL and the compatibilizers weredriedinanovenat40°C. Compatibilized PLA/PCL blends (PLA/PCL/C) having 90/10, 80/20, and 70/30 PLA/PCL compositions were melt blended in a Collin Teach-Line ZK25 T SCD 15 twin-screw extruder (L/D ratio 18 and screw diameter 25 mm) (Ebersberg, Bavaria, Germany). In all the cases, the content of the compatibilizer was set at 10 wt.% with respect to PCL, i.e., 90/10/1, 80/20/2, and 70/30/3. A screw speed of 200 rpm and a processing temperature of 190 °C were employed, and the extrudates were cooled in a water bath and pelletized. Noncompatibilized PLA/PCL blends were also prepared as a reference. In addition, to assess if the compatibilizers have any effect on the properties of neat PLA, blends of PLA with 3 phr of each type of compatibilizer (100/0/3) were also prepared. Based on the obtained results, 60/40 and 60/40/4 compositions were also prepared using the compatibilizer ElvaloyPTW. Moreover, to study the effect of the amount of compatibilizer, 70/30 blends with 1.5, 4.5, 6, 10, and 15 phr of ElvaloyPTW and 60/40 with 8 and 16 phr of ElvaloyPTW were prepared. Table S1, Supporting Information, summarizes all the compositions prepared in this work. Before injection molding, the pellets were dried overnight in a dehumidifier at 80 °C. Tensile (ASTM D-638 type IV) and impact (ASTM D-256) specimens were prepared by injection molding, which was carried out in a Battenfeld BA-230-E (Wittmann, Macromol. Mater. Eng. 2023,308, 2300213 2300213 (2 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Table 1. Polyethylene-based copolymers used as compatibilizers and their composition. Comonomers [wt.%] Compatibilizer nButylacrylate (BA) Vinyl acetate (VA) Glycidyl methacrylate (GMA) Ethyl acrylate (EA) Methyl acrylate (MA) Elvaloy4170 21 9 ElvaloyPTW 28 5.3 Elvaloy5160 20 5.3 ElvaloyAC12024S 24 ElvaloyAC2618 18 ElvaloyAC2615 15 BiomaxSG120 Unknown Kottingbrunn, Austria) machine equipped with a reciprocating screw (L/D ratio 30 and screw diameter 18 mm) at 190 °C. The mold temperature was set at 20 °C. 2.3. Characterization and Testing Methods 2.3.1. Dynamic Mechanical Thermal Analysis (DMTA) The phase structure of the samples was studied through dynamic mechanical thermal analysis (DMTA) performed in a TA Instruments Q800 DMA viscoelastometer (New Castle, DE, USA). Measurements were carried out in single cantilever geometry between −100 °C and 130 °C at a constant heating rate of 4 °C min−1. In the case of neat PCL, the testing temperature range was from −100 °Cto25°C. Experiments were conducted at a constant frequency of 1 Hz with an amplitude of 15 μm. Two samples were analyzed per composition. 2.3.2. Differential Scanning Calorimetry (DSC) Thermal properties of the blends were analyzed by differential scanning calorimetry (DSC) in a Perkin Elmer D8000 calorimeter (Waltham, MA, USA) under dry nitrogen atmosphere with a flow of 20 mL min−1. The equipment was calibrated with indium and tin standards. Samples for DSC scans were taken from injection molded specimens and heated from 25 °C to 200 °Cat 20 °Cmin −1. From those heating scans, melting (Tm)andcold crystallization (Tcc) peak temperatures were obtained. The degree of crystallinity (Xc) of PLA in the injected specimens was determined according to Equation 1[29]: Xc=ΔHm−ΔHcc wfΔH0 m (1) where ΔHmand ΔHcc are the measured enthalpies of melting and cold crystallization, respectively, wfrepresents the weight fraction of PLA in the blend, and ΔHm0is the melting enthalpy of completely crystalline PLA (93.6 J g−1[1,29]). Since the melting endotherm of PCL appears at the same position as the Tgof PLA, the Xcof PCL within the blends could not be determined. To calculate the Xcof neat PCL, a value of 139.5 J g−1was used.[34] Two samples were measured per composition. 2.3.3. Transmission Electron Microscopy (TEM) The morphology of the blends was determined by transmission electron microscopy (TEM) using a TEM microscope TECNAI G2-20 TWIN TEM equipped with LaB6 filament operating at an accelerating voltage of 200 kV (ThermoFisher Scientific, Waltham, MA, USA). For that purpose, samples were cut in ultra-thin sections of 90 nm with a diamond knife and at cryogenic conditions on a Leica EMFC 6 ultramicrotome device (Leica Geosystems AG, Unterentfelden, Switzerland). Those sections were mounted on 200-mesh copper grids. 2.3.4. Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) Spectroscopy To determine whether the compatibilizers could react with the PLA and the PCL, Fourier transform infrared-Attenuated total reflectance (FTIR-ATR) spectroscopy measurements were performed on a Nicolet 6700 FTIR coupled to an ATR accessory (Golden Gate) (ThermoFisher Scientific, Waltham, MA, USA). Measurements were performed directly to injection molded specimens. Spectra were recorded between 400 and 4000 cm−1with a resolution of 4 cm−1. The final spectra were the average over 32 scans. 2.3.5. Tensile Testing Tensile tests were performed using an Instron 5569 tensile tester (Instron, Norwood, MA, USA). A crosshead speed of 10 mm min−1was used, and the elastic modulus, the yield strength, and the elongation at break were determined from the obtained stress-strain curves. At least five tensile specimens were tested for each reported value. 2.3.6. Notched Izod Impact Testing Notched Izod impact strength was determined using a Ceast 6548/000 pendulum (Instron, Norwood, MA, USA). Notches were machined according to ASTM D-256 (depth 2.54 mm and radius 0.25 mm). Reported average values were obtained from a minimum of eight impact specimens. Macromol. Mater. Eng. 2023,308, 2300213 2300213 (3 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 1. a) Modulus and yield strength and b) elongation at break and impact strength, as a function of the PCL content. Dashed lines represent a simple additive law for mixtures. 3. Results and Discussion For the sake of clarity, the results obtained in the present work have been divided into four sections. The first one summarizes the results of the reference noncompatibilized PLA/PCL blends. In the second section, as some of the copolymers used in this study have been reported to act as impact modifiers for PLA,[35,36] the effect of the compatibilizers on neat PLA is briefly analyzed. In the third section, the influence of the compatibilizer type on the properties of the compatibilized PLA/PCL/C blends is studied. Finally, in the last section, the effect of the amount of one of the best compatibilizers (ElvaloyPTW) is analyzed. 3.1. Reference Noncompatibilized PLA/PCL Blends: Influence of PCL Content The phase structure of noncompatibilized PLA/PCL blends was studied by DMTA. Figure S1, Supporting Information, shows the tan 𝛿versus temperature plots of PLA, PCL, and PLA/PCL blends, while the Tgvalues for both PLA and PCL are summarized in Table S2, Supporting Information. The Tgof neat PLA and PCL were 73.5 ±0.2 °Cand−37.6 ±0.7 °C, respectively. All the PLA/PCL blends display two tan 𝛿peaks at approximately the same position as the neat components, pointing to the blends being immiscible, as it has been widely reported for melt-blended PLA/PCL blends.[12,13,17,37,38] Figure S2, Supporting Information, shows the first DSC heating scans for PLA, PCL, and PLA/PCL blends. Data extracted from them are summarized in Table S2, Supporting Information. As pointed out in the introduction, PLA shows a very slow crystallization rate. As a consequence, PLA is unable to crystallize during injection molding. On the contrary, PLA chains can rearrange and crystallize during heating in the DSC. In neat PLA, the cold crystallization appears at 101.3 °C. In the PLA/PCL blends, the cold crystallization temperature (Tcc) shifts towards lower temperatures as the PCL content increases. This shift is a consequence of the nucleating effect exerted by the PCL[12,19,31,38] inducing the lower cold crystallization of PLA.[24,25,39] As shown in Figure S2 and Table S2, Supporting Information, the Tmof PLA shows a decreasing trend with the PCL content, though the variations are slight. On the contrary, the addition of PCL slightly enhances the crystallinity degree of PLA due to the already discussed nucleating effect.[19] Nevertheless, the differences between the Xcof neat PLA and PLA within PLA/PCL blends are small. Figure 1 shows the mechanical parameters obtained by tensile and impact tests for the reference noncompatibilized PLA/PCL blends as a function of the PCL content. Dashed lines represent a simple additive law for mixtures. The data of the impact strength value of neat PCL is not shown in Figure 1b since the specimens did not break during the impact test. PLA and PCL show Young’s modulus of 3730 MPa and 424 MPa, respectively. In noncompatibilized PLA/PCL blends, it is observed that the modulus decreases linearly as the amount of PCL increases (Figure 1a), which is typical for PLA/PCL systems.[12,18,20,37] Similarly, the yield strength also decreases with increasing the PCL content, showing a negative deviation from the additive law for mixtures. As pointed out in the introduction, brittleness is one of the main drawbacks of PLA. Indeed, the elongation at break of the neat PLA employed in this work is 3%. By contrast, the PCL does not break until it has reached a deformation of 442%. In Figure 1b, it can be seen that adding 10 wt.% of PCL to PLA leads to a significant improvement in the elongation at break, reaching a value of 172%. The ductility continues increasing slightly with further addition of PCL, displaying a positive deviation from the law of mixtures. Similar trends were seen in other works, though the brittle-ductile transition happens at higher PCL contents.[15,16,37] Regarding the impact strength, PLA displays a value of 22 J m−1, another demonstration of its inherent fragility, whereas PCL does not break under the testing conditions employed in this work. The impact strength of the noncompatibilized PLA/PCL blends does not significantly change until 30 wt.% of PCL is added to PLA, a composition that shows a value almost double to that of neat PLA. Thus, even if by adding 10 wt.% of PCL, the ductility of PLA is improved significantly, more than that content is needed to increase the impact strength, and considerably higher PCL contents are required in order to enhance the toughness of PLA. Nevertheless, despite the impact strength of PLA being improved in the 70/30 composition, the attained value is still low for applications where high impact strength is required. The morphology of the noncompatibilized blends was analyzed by TEM (Figure 2) to determine the reason behind the change in toughness observed when the PCL content changes from 20 wt.% (33 J m−1)to30wt.%(53Jm −1). In the Macromol. Mater. Eng. 2023,308, 2300213 2300213 (4 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 2. TEM micrographs of a) 80/20 and b) 70/30 blends. micrographs, the light grey areas (low contrast) correspond to the PLA phase, whereas the dark grey (high contrast) one represents the PCL. As can be seen, both blends show a biphasic morphology regardless of the amount of PCL, corroborating the immiscibility between PLA and PCL. A sea-island morphology is observed for the 80/20 blend (Figure 2a), where PCL droplets are slightly elongated due to the orientation induced during injection molding. With 30 wt.% of PCL (Figure 2b), bigger and more elongated PCL particles are observed. Botlhoko et al.[37] also noted that as the percentage of PCL in PLA matrix was increased, PCL particles became bigger and more irregular, leading to a larger particle size distribution. Considering the aforementioned impact strength values, the formation of a morphology between sea-island and cocontinuous, which features PCL particles with different sizes and irregular shapes, is deemed necessary for the PCL to cause the improvement of the impact strength of the PLA. 3.2. Reference PLA/0/C Blends: Influence of Compatibilizer Type on Neat PLA Table S3, Supporting Information, summarizes the thermal parameters from DMTA and DSC for neat PLA and the 100/0/3 PLA/PCL/C compositions with the different compatibilizers. The addition of the compatibilizers does not influence the Tg,Tm,or Xcof PLA. By contrast, the Tcc of PLA is reduced from 101.3 °C to 94.4–92.0 °C, with the exception of the compatibilizers ElvaloyAC2018 and ElvaloyAC2015, for which the reduction is smaller. Thus, similar to PCL addition, low amounts of compatibilizers in PLA cause a nucleation effect on its crystallization process. Figure 3 shows the mechanical parameters obtained by tensile and impact tests for neat PLA and 100/0/3 compositions. The data for PLA are represented with straight dashed lines. As expected, if their soft nature is considered, the addition of compatibilizers to neat PLA produces small decreases in its modulus and yield strength (Figures 3a,b, respectively), regardless of the type used. On the contrary, the effect of the compatibilizers on the elongation at break of PLA differed significantly from one another as a function of their chemical structure. As can be observed in Figure 3c, when compatibilizers that contain epoxy moieties within their chemical structure are used (BiomaxSG120, ElvaloyPTW, Elvaloy4170, and Elvaloy5160), the elongation at break increases up to values higher than 135%. With compatibilizers that contain ethyl acrylate comonomers (Elvaloy2618 and Elvaloy2615) the resultant elongation at break values are lower than the ones mentioned before but still higher than that of neat PLA. By contrast, the blend with ElvaloyAC12024S, which contains methyl acrylate as a comonomer, shows no improvement in the ductility of the PLA. From these results, it is clear that the interactions/reactions that might occur between the copolymers and the PLA are different depending on the chemical structure of the comonomers constituting the copolymers. This effect will be discussed later on. On the other hand, it must be noted that although adding a suitable compatibilizer to PLA is enough to increase its elongation at break considerably, it does not alter the impact strength of PLA, as shown in Figure 3d. 3.3. Compatibilized PLA/PCL Blends (PLA/PCL/C): Influence of the Compatibilizer Type Figure 4a,b show, respectively, the Tgvalues of PCL and PLA for all the studied PLA/PCL/C compositions. The addition of compatibilizers has no influence on the Tgof PLA, regardless of the copolymer nature. As shown in Figure 4a, the Tgof PCL displays more variations, although no significant trend can be appreciated. As also mentioned in the SI, both the low intensity and the width of the transition make it difficult to determine the Tgof the PCL accurately. Similarly, the Tcc,Tm,andXcof PLA show no significant changes with the incorporation of the compatibilizers, as can be seen in Figure 5. In the case of Xc, there are more deviations from the value of neat PLA, but considering the error in the measurements, the differences encountered are not very significant, and the variations do not follow any specific trends. Figure 6 shows the mechanical parameters obtained by tensile and impact tests for each PLA/PCL/C composition. As can be observed, the modulus and yield strength do not significantly Macromol. Mater. Eng. 2023,308, 2300213 2300213 (5 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 3. a) Modulus, b) yield strength, c) elongation at break, and d) impact strength of neat PLA and PLA/0/C blends. Dashed lines represent the values of neat PLA. vary with the used compatibilizer, regardless of the PLA/PCL composition. It is noteworthy that, even in the compatibilized blends with 30 wt.% of PCL, the modulus remains relatively high (≥2480 MPa) in all the cases. These results suggest that the studied blends could be used in applications where high stiffness is required. The drop in the yield strength is considerable compared with neat PLA, but the values do not differ too much from the ones of PLA/PCL blends. As can be observed in Figure 6c, the addition of the compatibilizers barely affected the initially high elongation at break values of the blends, and the compatibilized blends showed ductility values higher than 160%, regardless of the PLA/PCL composition and the type of compatibilizer used. This is true except for 90/10 and 80/20 blends compatibilized with ElvaloyAC12024S that showed elongation at break values significantly lower than those of the reference noncompatibilized PLA/PCL blends. This was coincidentally the only compatibilizer that did not improve the ductility of neat PLA (Figure 3c). Meyva et al.[40] blended a PLA with an ethylene-methyl acrylate (EMA) copolymer with the same MA percentage (24 wt.%). Although they used an EMA copolymer amount as high as 20 wt.%, no significant improvement in the elongation at break of PLA was observed (from 1.95% of neat PLA to 3.76%), suggesting poor compatibility between PLA and EMA copolymer. Similarly, other works also reported Figure 4. Tgvalues of a) PCL and b) PLA, obtained by DMTA, for each PLA/PCL/C composition. Dashed lines represent the values of reference noncompatibilized PLA/PCL blends. Macromol. Mater. Eng. 2023,308, 2300213 2300213 (6 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 5. a) Tcc,b)Tm,andc)Xcof PLA for each PLA/PCL/C composition. Dashed lines represent the values of the reference noncompatibilized PLA/PCL blends. Figure 6. a) Modulus, b) yield strength, c) elongation at break, and d) impact strength for each PLA/PCL/C composition. Dashed lines represent the values of the reference noncompatibilized PLA/PCL blends. Macromol. Mater. Eng. 2023,308, 2300213 2300213 (7 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 7. TEM micrographs of a) 70/30/Elvaloy5160 3, b) 70/30/ElvaloyAC12024S 3, and c) 70/30/ElvaloyPTW 3. small or no improvements in ductility when blending PLA or other polymeric materials with EMA copolymers.[41–43] This indicates that the presence of methyl acrylate groups in the compatibilizer leads to a premature failure of the blends, at least at low PCL contents. At high PCL contents, the effect of PCL prevails over that of ElvaloyAC12024S. Figure 6d shows the impact strength of all the studied PLA/PCL/C blends. As can be seen, for blends containing 10 and 20 wt.% of PCL, the impact strength values of the compatibilized blends do not differ from one compatibilizer to another. Moreover, the obtained values are almost equal to those of noncompatibilized PLA/PCL blends in the case of 90/10/1 compositions and slightly higher for 80/20/2 compositions. In contrast, for the 70/30/3 blends, the effect of each compatibilizer is significantly different. While with Elvaloy5160, an impact strength value of 60 Jm −1is achieved, a value slightly higher than that of the noncompatibilized 70/30 blend (53 J m−1), with BiomaxSG120, ElvaloyPTW, and Elvaloy4170, impact strength values between 97 and 100 J m−1are obtained, which are 345% and 90% higher than that of the neat PLA and the noncompatibilized 70/30 blend, respectively. Furthermore, intermediate values between 66 and 81 Jm −1are obtained with ElvaloyAC12024S, ElvaloyAC2618, and ElvaloyAC2615. To shed light on the reasons for the differences observed in the impact strength, the morphology of the blends was analyzed by TEM. Figure 7 shows the TEM micrographs of 70/30/3 compositions with the compatibilizers that showed the lowest, intermediate, and highest impact strength values, respectively: Elvaloy5160, ElvaloyAC12024S, and ElvaloyPTW. As can be observed in the micrographs, even if the PLA/PCL composition is the same in the three blends, the final morphology varies with the used compatibilizer. With Elvaloy5160 (Figure 7a), deformed PCL droplets dispersed all over the PLA matrix are seen. On the other hand, ElvaloyAC12024 (Figure 7b) induces the elongation of PCL droplets compared to the morphology of the noncompatibilized 70/30 blend (Figure 2b). Finally, the presence of ElvaloyPTW causes the change of the morphology of the noncompatibilized 70/30 blend into a cocontinuous structure. It is worth mentioning that inside PCL droplets, darker zones are observed in all the micrographs (marked with green arrows). These zones correspond to the compatibilizer phase, which possesses higher electronic density. The presence of these black particles in the PCL phase implies that the compatibilizers, regardless of their chemical structure, have a higher affinity towards the PCL phase. If the impact strength values of the analyzed blends are taken into account, and considering that for all the blends, the Macromol. Mater. Eng. 2023,308, 2300213 2300213 (8 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.mame-journal.de Figure 8. TEM micrograph of 80/20/ElvaloyPTW 2. crystallinity, and the thermal transitions are quite similar, it can be concluded that the different morphologies induced by the presence of the different compatibilizers are the responsible for the variations observed. Since the rest of the parameters that may affect the morphology are kept constant, the chemical structure of the different compatibilizers must be responsible for promoting the formation of one or another type of morphological structure and, consequently, better or worse impact strength values. The results in Figure 6d indicate that achieving a cocontinuous morphology is the key to obtaining highly toughened blends, and the best comonomer combination to do so is the one composed of GMA and acrylate groups (BiomaxSG120, Elvaloy4170, and ElvaloyPTW). Hou et al.[32] analyzed the effect of different concentrations of the EMA-GMA terpolymer on the morphology and properties of PLA/PCL blends having a composition of 90/10. They observed that some of the compatibilizers adhered to the PCL dispersed particles serving as a bridge between the PCL particles and PLA matrix through the chemical reactions between the epoxy groups of the EMA-GMA and the carboxyl and hydroxyl end groups of PLA and PCL, which led to an improvement of the interfacial adhesion. This possibility is analyzed in the following section. Moreover, the authors observed that, at high EMA-GMA contents, the compatibilizer particles also connected the PCL particles between them, resulting in a shish-like structure. A similar behavior –coalescence and elongation of the PCL phase– is observed in the present work, but only when some compatibilizers were used. Moreover, the extent of the morphological change seems to be dependent on the comonomers present in the compatibilizer, as discussed in the previous paragraphs. On the other side, as observed in Figure 6d, the compatibilizers effectively enhance the impact strength of the noncompatibilized blends only when 30 wt.% of PCL is added to the blends. To ascertain why their presence is ineffective in the other studied compositions, Figure 8 shows the TEM micrograph of the 80/20 blend compatibilized with ElvaloyPTW. If the morphologies of the noncompatibilized 80/20 (Figure 2a) and the 80/20/ElvaloyPTW 2 (Figure 8) blends are compared, it can be observed that the dispersed phase is further elongated in the presence of the compatibilizer, showing almost a fibril-like shape. However, a cocontinuous morphology is not reached. In conclusion, when PLA/PCL blends are compatibilized with ElvaloyPTW, the morphology of the blends changes towards a co-continuous morphology. Nonetheless, PCL contents higher than 20 wt.% are needed to achieve a completely co-continuous structure, which is necessary to improve the impact strength in the blends under study. Based on the above results, 60/40 and 60/40/ElvaloyPTW 4 blends were prepared to confirm if the cocontinuity is responsible for the improved toughness. The noncompatibilized 60/40 blend shows an impact strength of 148 J m−1, which is 6.6 times higher than that of neat PLA. Despite this, when the same blend is compatibilized with ElvaloyPTW, an impact strength of 383 Jm −1is achieved. It has to be highlighted that the specimens of this last composition do not entirely break during the impact tests, which means that the real impact strength of 60/40/ElvaloyPTW 4 is even higher than the reported value. Figure 9 shows the TEM micrographs of cryogenically fractured surfaces of 60/40 and 60/40/ElvaloyPTW 4 blends. In the PLA60/PCL40 blend, a morphology near to cocontinuity is seen, indicating that the phase inversion could be close to that composition. For PLA/PCL blends, cocontinuity has been observed at PCL contents between 40 and 55 wt.%.[18,28,37,44] As can be observed in Figure 9b, the surface of 60/40/ElvaloyPTW 4 shows a cocontinuous structure, thereby confirming that ElvaloyPTW induces a change in the morphology of PLA/PCL blends towards cocontinuity, which in turn causes a significant improvement in impact strength. 3.4. PLA/PCL/ElvaloyPTW Blends: Effect of Compatibilizer Content To see the effect of the amount of compatibilizer on the properties of the PLA/PCL/C blends, a compatibilizer was selected among those that gave the best results in the previous section. Thus, ElvaloyPTW was chosen and added in different contents to 70/30 blends since these compositions showed the best balance of mechanical properties. Figure 10 shows the Tgvalues of PLA and PCL for all the 70/30/ElvaloyPTW blends. While the Tgof PLA does not vary significantly, the Tgof PCL shows an increasing trend with increasing the ElvaloyPTW content. These results are in agreement with the ones reported by Hou et al.[32] who observed a shift in the Tg of PCL as they increased the content of EMA-GMA in PLA/PCL blends, due to the interfacial interactions between the end groups of PLA and PCL with the epoxy groups of the compatibilizer. The improved interfacial adhesion provoked by the compatibilizer and its tendency to locate inside the PCL phase (as will be shown and discussed later), hindering the mobility of the PCL phase, could be the reasons behind the behavior observed for the Tgof the PCL. As previously mentioned, the epoxy groups within the compatibilizers have been reported to be able to react with the carboxyl and hydroxyl end groups of PLA and PCL during melt Macromol. Mater. Eng. 2023,308, 2300213 2300213 (9 of 14) © 2023 The Authors. Macromolecular Materials and Engineering published by Wiley-VCH GmbH 14392054, 2023, 12, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/mame.202300213 by Universidad Del Pais Vasco, Wiley Online Library on [10/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License