Synthesis and Structure–Property Relationships of PLLA-Based ABA Triblock Copolymers with Bio-Based Soft Segments

TL;DR

This paper explores how different bio-based soft segments affect the properties of biodegradable PLLA-based triblock copolymers.

Contribution

The first report on PLLA triblock copolymers incorporating PMR as a renewable soft middle block.

Findings

Copolymers with PMR showed enhanced phase separation and increased PLLA crystallinity.

PMR-based copolymers had significantly improved elongation at break due to pendant chains in the soft segment.

PPD-based copolymers displayed reduced phase separation and more PLA-like mechanical behavior.

Abstract

The development of biodegradable ABA-type triblock copolymers with tailored thermo-mechanical performance requires precise control over polymer architecture and phase behavior. In this study, PLLA-based ABA triblock copolymers were synthesized using two structurally distinct, fully bio-based soft segments: poly(methyl ricinoleate) (PMR) and poly(1,3-propanediol) (PPD). To the best of our knowledge, this is the first report on PLLA triblock copolymers incorporating PMR as a renewable soft middle block. Hydroxyl-terminated PMR and PPD were employed as macroinitiators for the controlled ring-opening polymerization of L-lactide, enabling systematic variation in block composition and molecular weight. Structural characterization confirmed successful block formation, while thermal and mechanical analyses revealed pronounced differences in phase separation and structure–property relationships. Copolymers containing PMR exhibited enhanced phase separation, increased crystallinity of PLLA domains, and significantly improved elongation at break, attributed to the presence of pendant chains in the soft segment. In contrast, PPD-based copolymers showed reduced phase separation and more PLA-like mechanical behavior. These results demonstrate that the chemical architecture of bio-based soft segments plays a decisive role in governing the thermo-mechanical performance of PLLA-based triblock copolymers.

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Full text

1. Introduction

In the last few decades, numerous research activities in industry and academia have been dedicated to finding a replacement for polymers obtained from fossil resources. These activities are mostly motivated by environmental sustainability and resource depletion, but further development of such materials leads towards economic benefits and improved performance, since materials obtained from renewable resources can show better thermal stability [1] or be more resistant to UV degradation [2] while stimulating the growth of the rural economy and reducing the reliance on imported petroleum. Finding the compromise between performance, similarity to conventional plastic, and biodegradability, poly(lactide) has been proposed as a material of choice for many applications.

Poly(lactide) (PLA) is characterized by good biodegradability and easy manufacturing, while having specific physical and mechanical properties that can be compared with traditional plastics [3]. Such properties have facilitated the development of numerous applications for it (auto industry, medicine, pharmacy, packaging, etc.) [4,5]. However, PLA is a semi-crystalline polymer that, due to its brittleness and weaker mechanical properties, has limited application in some areas like packaging and electronics. To overcome these limitations, researchers have explored the synthesis of block copolymers that combine the rigidity of PLA with the elasticity of soft segments [6]. The introduction of soft segments into the structure of PLA-based copolymers has proven to be an exceptionally helpful solution for fine-tuning the microstructure and available tools to enhance the elasticity of these brittle materials [7,8,9]. This type of copolymer—ABA—contains two incompatible phases, a soft middle segment (B) and two hard segments of PLA (A). Early studies by Vert et al. [10] demonstrated the potential of block copolymerization, where dihydroxy-terminated soft blocks were used as macroinitiators for ring-opening polymerization of lactides, achieving improvements in flexibility and thermal stability. They further advanced this approach by introducing ABA-type triblock copolymers, incorporating poly(ethylene glycol) (PEG) as a soft middle block, leading to enhanced elongation at break while maintaining mechanical strength [11]. However, the hydrophilic nature of PEG limits the thermal and phase stability of these materials. Subsequent studies, such as those by Huang et al. [12], explored the incorporation of polycaprolactone (PCL) as soft blocks to address the limitations of earlier copolymers. These studies highlighted the critical role of block length and composition in determining the thermal and mechanical properties of triblock copolymers. More recently, MacDonald et al. [12] and Liu et al. [13] have investigated novel polyester and polyether middle blocks, focusing on optimizing phase separation as well as the thermal and mechanical properties to expand the potential applications of these materials. In addition to the above, numerous research groups have developed new ABA block copolymers with PLA and non-degradable [14] and biodegradable soft segments based on poly(6-methyl-ε-caprolactone) [15], poly(d-valerolactone) [16], poly(butylene succinate) [17], copolyesters, poly(butylene succinate/azelat) [18], poly(propylene glutarate), poly(pentylene pimelate) [19], and 2-butene-1,4-diol oligomers [20]. While the concept of ABA-type triblock copolymers is well-established, the controlled synthesis of triblock copolymers incorporating structurally complex, bio-based soft segments remains a significant challenge. These systems demand precise stoichiometric control, multi-step synthetic procedures, and expertise in polymer reaction engineering to ensure successful block coupling and narrow dispersity.

The properties of ABA block copolymers can be precisely customized by varying the structure of constituent components, hard and soft segments [21]. The structure of the middle, soft block is crucial for adjusting the thermo-mechanical properties of PLA-based block copolymers, especially if a block with an exceptionally low glass-transition temperature is used. In this way, thermodynamic incompatibility of hard PLA blocks and soft blocks is additionally increased, which enables particularly good elastic properties of the final material. Also, the crystallinity of the middle block provides another approach for tuning the thermal, mechanical, and degradation behaviors of the prepared block copolymers. Poly(methyl ricinoleate) (PMR) and poly(1,3-propanediol) (PPD) were selected as soft middle blocks due to their renewable origin and exceptionally low glass-transition temperatures. PMR is derived from ricinoleic acid methyl ester obtained from castor oil, while 1,3-propanediol can be produced from glycerol via bio-based routes [22,23].

The aim of this study is to design and synthesize PLLA-based ABA triblock copolymers incorporating two structurally distinct, fully bio-based soft middle blocks: poly(methyl ricinoleate) (PMR) and poly(1,3-propanediol) (PPD). Beyond achieving low glass-transition temperatures, the selected soft segments enable systematic investigation of how middle-block chemical architecture—namely, pendant-chain-containing polyester versus linear polyether—affects phase separation, crystallinity, thermal stability, and mechanical performance. By employing hydroxyl-terminated PMR and PPD as macroinitiators for the controlled ring-opening polymerization of L-lactide, this study establishes clear structure–property relationships and highlights the role of soft-segment topology in tailoring the performance of biodegradable PLLA-based triblock copolymers.

To the best of our knowledge, this is the first systematic study reporting PLLA-based ABA triblock copolymers utilizing poly(methyl ricinoleate) and poly(1,3-propanediol) as renewable soft segments and correlating their molecular architecture with phase behavior and thermo-mechanical properties. This study contributes not only novel materials, but also a fundamental synthetic pathway applicable to the broader field of block copolymer design from bio-derived components.

2. Materials and Methods

2.1. Materials

L-Lactide (purity 98%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Trifluoromethanesulfonic acid (CF_3_SO_3_H, purity 99%, density 1.696 g cm^−3^ at 25 °C) was also obtained from Sigma-Aldrich and used as received. Methyl ricinoleate and 1,3-propanediol were supplied by Tokyo Chemical Industry (TCI, Tokyo, Japan). Titanium(IV) isopropoxide (Ti[OCH(CH_3_)2]4) and concentrated sulfuric acid were purchased from Sigma-Aldrich and used as catalysts for soft-segment synthesis.

Deuterated chloroform (CDCl_3_) was obtained from Sigma-Aldrich and used for NMR analysis. Dichloromethane, tetrahydrofuran (THF), methanol, and toluene (HPLC grade) were purchased from Merck (Darmstadt, Germany). Chloroform was obtained from Merck Alkaloid (Skopje, North Macedonia). All chemicals were used as received unless otherwise stated.

2.2. Synthesis of Soft Segments for PLLA Triblock Copolymer Preparation

To investigate the influence of soft-segment structure on the properties of PLLA-based triblock copolymers, two types of bio-based soft segments were synthesized: a polyester derived from methyl ricinoleate and a polyether based on 1,3-propanediol.

2.2.1. Synthesis of Poly(methyl ricinoleate) (PMR)

Poly(methyl ricinoleate) was synthesized via transesterification polycondensation of highly purified methyl ricinoleate (hydroxyl number = 180 mg KOH g^−1^) with diethylene glycol (DEG) as a chain extender (Figure 1). Methyl ricinoleate (124 g), diethylene glycol (7 g), and titanium(IV) isopropoxide (0.1 wt%) were charged into a glass reactor and heated to 190–210 °C under a nitrogen atmosphere. Methanol formed during transesterification was continuously removed. After completion of the first stage, the reaction was continued under reduced pressure at 210 °C to promote further polycondensation of hydroxyl-terminated oligomers, yielding PMR with increased molecular weight. Following consumption of diethylene glycol, the reaction proceeded via further transesterification between hydroxyl-terminated oligomers, resulting in continued chain growth and formation of higher-molecular-weight poly(methyl ricinoleate).

2.2.2. Synthesis of Poly(1,3-propanediol) (PPD)

Poly(1,3-propanediol) was synthesized by acid-catalyzed polyetherification of 1,3-propanediol using concentrated sulfuric acid (1 wt%) as a catalyst (Figure 2). The reaction was carried out at 160 °C for 10 h with continuous removal of water formed during the polycondensation process. The resulting polyether exhibited terminal hydroxyl groups suitable for subsequent use as a macroinitiator.

2.3. Synthesis of ABA-Type Triblock Copolymers

ABA-type triblock copolymers were synthesized by cationic ring-opening polymerization of L-lactide in dichloromethane using trifluoromethanesulfonic acid (CF_3_SO_3_H) as a catalyst. The polymerization was carried out at 35 °C under dry conditions, following a previously reported procedure [24].

Hydroxyl-terminated PMR and PPD soft segments were used as macroinitiators for the growth of PLLA hard blocks. In a typical procedure, the required amount of soft segment was dissolved in dichloromethane in a dry, nitrogen-purged reaction vessel. L-Lactide was then added in quantities corresponding to the targeted block lengths (Table 1), and the mixture was stirred at 35 °C until complete dissolution of the monomer. Polymerization was initiated by addition of CF_3_SO_3_H, and the reaction was allowed to proceed for 6 h under continuous stirring. Syntheses of triblock copolymers are shown in Figure 3. After completion of the reaction, the resulting triblock copolymers were isolated by precipitation in cold methanol, filtered, and dried under reduced pressure to constant mass. PLLA homopolymer was synthesized under identical conditions in the absence of a macroinitiator for comparison. The compositions of the synthesized triblock copolymers are summarized in Table 1.

2.4. Rheological Properties of Soft Segments

Rheological properties were investigated using a rotational rheometer equipped with parallel-plate geometry. Frequency sweep measurements were conducted in the linear viscoelastic region at selected temperatures to evaluate viscoelastic behavior and phase separation characteristics of the synthesized triblock copolymers. The rheological properties of synthesized soft segments were evaluated using AR rheometer 2000ex, TA Instruments, New Castle, DE, USA. Samples were analyzed at 25 °C, and 12 points were recorded with predefined stress.

2.5. Gel Permeation Chromatography (GPC)

The molecular weights and polydispersity index value of the polymers were determined by gel permeation chromatography (GPC) using the Waters GPC system with a 510 pump and a 410 differential refractometer. The eluent used was tetrahydrofuran (THF), with a flow rate of 1.0 mL min^−1^ at 30 °C. Phenogel columns were used (four columns, 300 × 7.8 mm ID; pore size 50, 100, 10^3^ i, 10^4^ Å, respectively). Polystyrene (PS) was used as the standard. Analyses were performed on a Waters GPC device (Waters Corporation, Milford, MA, USA) equipped with detectors: a ViscoStar-type viscometer with a dilution factor of 0.5234; a DAWN EOS-type light-scattering measuring device with a K5 cell, a laser wavelength of 684.0 nm, and a calibration constant of 1.0380 × 10^−5^ 1/V cm; RI detector, Optiolab rEX. Analyses were performed using chloroform as the eluent with a flow rate of 1 mL min^−1^.

2.6. Fourier-Transform Infrared Spectroscopy (FTIR)

The structures of the soft segments and block copolymers were analyzed using the Fourier-Transform Infrared (FTIR) Spectrophotometer IRAffinity-1S (Shimadzu, Kyoto, Japan). The spectra were recorded in the range 4000–400 cm^−1^ using the ATR (Attenuated Total Reflection) technique.

2.7. Nuclear Magnetic Resonance Spectroscopy (NMR)

^1^H NMR analysis of obtained synthesized materials was carried out using a Bruker DPX-300 NMR (300 MHz) Nuclear Magnetic Resonance Spectrometer (Bruker, Billerica, MA, USA). Samples were dissolved in deuterated chloroform (10 mg of samples in 1 mL of solvent), and tetramethylsilane (TMS) was used as the reference standard.

2.8. Differential Scanning Calorimetry (DSC)

Thermal transitions of the synthesized triblock copolymers were analyzed by differential scanning calorimetry using a differential scanning calorimeter (DSC Q20, TA Instruments, New Castle, DE, USA). Approximately 5 mg of samples was heated up to 180 °C, then cooled down to −90 °C, and finally heated up to 200 °C at the heating rate of 10 °C min^−1^ in a nitrogen atmosphere. Glass-transition temperatures, melting temperatures, and crystallization behavior were evaluated from the second heating cycle to eliminate the influence of prior thermal history.

2.9. Thermogravimetric Analysis (TGA)

The thermal stability of the obtained materials was assessed by thermogravimetric analyzer (TGA Q50, TA Instruments, New Castle, DE, USA). Dried polymer (10 mg) in a ceramic crucible was heated to 650 °C at a heating rate of 10 °C min^−1^ under nitrogen atmosphere (with a gas flow of 20 cm^3^ min^−1^). The onset decomposition temperature and weight-loss profiles were determined from the recorded thermograms.

2.10. Mechanical Properties of Block Copolymers

The mechanical properties of the prepared copolymers were examined using a tensile testing machine EZ-LX Test (Shimadzu, Kyoto, Japan). Mechanical properties, including tensile strength, elongation at break, and Young’s modulus, were determined according to the ASTM D882 standard [25]. The samples were prepared by solution casting (15% w/v in dichloromethane), and after evaporation of the solvent, the films were dried in a vacuum oven at 60 °C for 6 h. The obtained films were cut into rectangular-shaped strips, the thickness and width were measured, and the samples were stretched with a load of 10 mm min^−1^. To ensure statistical reliability and reproducibility, all measurements were performed in at least five replicates, and the results are reported as mean values ± standard deviation.

3. Results

3.1. Analysis of Molar Masses and Rheological Properties

GPC analysis of the PMR estimated a molar mass of 7200 g mol-^−1^ with a polydispersity of 1.39, Table 2. The observed narrow molar mass distribution (PDI = 1.39) further confirms that the reaction conditions were well-controlled, leading to a reproducible polymer structure with consistent molecular weight. To compensate for minor DEG losses, an intentional excess of DEG was added to maintain the desired stoichiometry and achieve targeted molar mass control. This excess accounts for any volatilization effects while ensuring that the polymerization proceeds as planned.

The molar mass of PPD determined by the GPC method was 3800 g mol^−1^, which coincides with the desired molar mass of 4002 g mol^−1^, Table 2. The index of polydispersity is 1.87, which is a higher value compared to the polydispersity of synthesized polyester types. This is expected considering that no Mw-controlling agent was used in the synthesis of polyether poly(1,3-propane diol).

The viscosity values under different conditions are an important processing parameter, since lowering the viscosity of the material facilitates the processing, which significantly broadens its application. By comparing the viscosity of PMR (1.39 Pa∙s) and PPD (1.118 Pa∙s), it can be observed that this value is lower compared to the value of PMR. This is expected considering that the linear chains of PPD do not have pendant chains, which in the case of PMR, significantly reduces the viscosity value.

The GPC data of the synthesized block copolymers showed a significant increase in molar mass compared to the molar masses of the initial segments, and confirmed the assumption that the chosen method of synthesis of segmented ABA block copolymers was successful. As can be seen in Table 2, ABA block copolymers of the desired molar masses were obtained, with the proportion of PLLA hard segments that can be controlled by proper selection of the ratio of monomeric lactide and soft segment in the reaction mixture. The assumption that the soft segments will act as macroinitiators and that the polymerization of L-lactide will take place from the terminal -OH groups of the soft segments proved to be correct, and enables fine-tuning of molecular weights of obtained ABA triblock copolymers. The content of PLLA in copolymers is calculated according to the Mn of the macroinitiator and triblock copolymer.

3.2. Analysis of the Molecular Structure Using 1H NMR Spectroscopy

The ^1^H NMR spectrum of soft-segment PMR is shown in Figure 4a. The structure and characteristic protons, whose shifts were detected in the ^1^H NMR spectrum, can be seen in the picture. Protons from the CH_3_ group of the pendant chain, H1, show a shift of 0.87 ppm, while protons from the –CH_2_ group of the pendant chain show a shift of 1.2 ppm. There is an obvious difference in the intensity of the peaks, which is expected considering that the ratio of CH_3_ and CH_2_ groups of the pendant chain is 1:5. CH_2_ protons in the main chain, proton H3, show a shift of 1.7 ppm, while CH_2_-COO protons in the main chain, proton H4, show a shift of 2 ppm. The H5 proton with a shift of 2.3 ppm originates from the proton of the methylene group -CH=CH-CH_2_-CH-(CH_2_)-O-. Protons in the double bond of the main chain –CH=CH- are at position 5.4 ppm, H8. H6 and H7 indicate proton displacement from ethylene glycol, where H6 is at 3.7 ppm from –CH_2_-CH_2_-O, while H7 is at 4.24 ppm from the–CH_2_-O-C=O(CH_2_) proton.

To confirm the structure of the triblock copolyester poly(L-lactide-b-methylricinoleate-b-L-lactide) and the assumption that a block copolymer was indeed obtained in the mentioned reaction, the ^1^H NMR spectrum of the mentioned polymer was recorded (Figure 4b). The spectrum clearly shows peaks belonging to individual blocks, PMR or PLLA. The protons from H1 to H8 originate from the PMR, as already explained in the previous paragraph. The peak confirming the linking of the blocks is the H9 peak, at 4.89 ppm, and it originates from the OOC-CH(CH_3_)-O-CO- proton in the poly(L-lactide) chain directly attached to the PMR. In the spectrum of poly(L-lactide), this proton shows a shift of 5.2 ppm [24], which is also present in the spectrum of the copolymer, in lactide units that are not directly attached to PMR, H10. The difference in the position of these two protons, H9 and H10, clearly confirms that during the reaction, PMR and PLLA blocks were connected. Peak H11 originates from the proton of the –CH_3_ group from the lactide molecule that polymerizes by ring opening. A peak at 7.26 ppm originated from solvent CD(H)Cl_3_ and was noted with *. ^1^H NMR analysis confirmed that the ring-opening polymerization of lactide in controlled conditions resulted in the desired ABA block copolymer.

The ^1^H NMR spectra of PPD and block polyester poly(L-lactide-b-propane diol-b-L-lactide) are shown in Figure 5. As can be seen in Figure 5a, where the ^1^H NMR spectrum of PPD is shown, the peak originating from the proton shift in the CH_2_-CH_2_-CH_2_ methylene group in the polymer chain, peak H1, occurs at 1.79 ppm, while the proton shift from CH_2_-O-CH_2_ is observed at 3.5 ppm, the H2 peak. Protons in the methylene group attached to the terminal -OH groups –CH_2_-OH show a shift to 3.72 ppm, peak H3. There is a clear difference in the intensity of peaks H1, H2, and H3, considering the difference in the number of protons that characterize the mentioned peaks. In the spectrum of the triblock polyester PLLA_6_-b-PPD-b-PLLA_6_ (Figure 5b), we observe characteristic peaks of block PPD at 1.79 and 3.5 ppm, H2 and H3, already explained in the text (Figure 5a). The peaks characterizing the polylactide structure are labeled H1 and H5. The peak occurring at 4.25 ppm originates from the change in proton environment CH_2_-CH_2_-O-COO-. The position of these protons is clearly different from the position of the H3 CH_2_-CH_2_-O-CH_2_- protons, which occur at 3.45 ppm. Based on the obtained results of the molecular structure analysis, by the ^1^H NMR method, it can be concluded that the resulting block copolymer has the expected structure, i.e., that the blocks of soft and hard segments have been connected.

3.3. Analysis of the Molecular Structure Using FTIR Spectroscopy

The FTIR spectrum of PMR shows a broad band between 3650 and 3100 cm^−1^, originating from the absorption of -OH groups from the main chain. The peak originating from the -OH groups splits into two absorptions, at 3448 and 3541 cm^−1^, where the first is attributed to -OH groups connected by hydrogen bonds and the second is from free -OH groups. The absorption of the ester C-O-C bond of the aliphatic chain shows a strong band with a position at 1176 cm^−1^. Other bands that characterize the structure of the obtained polyester, such as absorptions of methyl and methylene groups in the region from 2800 to 2950 cm^−1^, are also present in the FTIR spectra of the obtained polymer. In the FTIR spectrum of PPD, there is a strong band at 1114 cm^−1^ that originates from newly formed ether bonds in the polymer chain, which shows that a polymerization reaction has occurred, i.e., the formation of polyether chains (Supplementary Materials, Figure S1).

For the analysis of triblock copolymers based on PMR and PLLA, the FTIR method was used to confirm the structure of the obtained segmented polyester (Figure 6). It can be concluded from the spectra that all the bands that are characteristic of pure blocks are also present in the spectrum of the final segmented polyester. The broad absorption band of the hydroxyl (-OH) stretching vibration between 3200 and 3600 cm^−1^ from the poly(L-lactide) block was presented in the spectrum of the copolymer. Weak adsorption at about 3010 cm^−1^, with peaks at 933 and 867 cm^−1^, is attributed to a double bond in soft segments. As can be seen from the FTIR spectrum, the asymmetrical valence vibrations of C–O–C of the aliphatic chain were shifted at 1187 cm^−1^, as well as the symmetrical valence vibrations of C–O–C of the aliphatic chain at 1090 cm^−1^, in the PLLA block, compared with bands at 1267 and 1099 cm^−1^, which appeared in monomer L-lactide [22]. Bands at 1455 and 1383 cm^−1^ originated from asymmetric and symmetric bending vibration of C–H from CH_3_, and bands at 1271 cm^−1^ from the overlap of the C–H bending vibration and C–O–C stretching vibration in the PLLA block were also detected. The peak that corresponds to ester bonds from soft and hard segments in triblock polyester is split into two bands, originating from the soft (1740 cm^−1^) and hard segments (1756 cm^−1^). This is a confirmation that both soft and hard segments are incorporated into the structure of the segmented polyester, i.e., that the polymerization of lactide initiated by the OH groups of poly(methyl ricinoleate) occurred.

Figure 7 shows the FT-IR spectrum of block polyester based on PPD and PLLA, PLLA_10_-b-PPD-b-PLLA_10_. All the bands that are characteristic of the pure blocks are present in the spectrum of the copolymer, which confirms that the blocks of PPD and PLLA are incorporated into the structure of the block polyester. In the FTIR spectrum of the ABA block, a band appears between 3300 and 3600 cm^−1^, which originates from the absorption of OH groups from the main chain. Both blocks have end -OH groups, so it is difficult to distinguish them in this area. However, since a triblock polymer with PLLA end chains is obtained, its expected deformation bands δOH at 1375 cm^−1^ and γOH at 760 cm^−1^ from PLLA chains appear, which are noticeable in the FTIR spectrum of the ABA block copolymer. This confirms that the polymerization reaction started from the -OH groups of the soft segment, and that the final polyester is obtained with the final -OH groups from poly(L-lactide). Deformation vibrations from PPD occur at higher wavenumbers, 1460 cm^−1^ for pure PPD, so it can be clearly concluded that these vibrations are absent in the spectrum of the final triblock copolymer (Figure 7), which confirms the previous claim that all -OH groups of the soft segments were used to initiate the polymerization of L-lactide. A strong peak at 1757 cm^−1^ is attributed to the ester bond in PLLA and formed ABA copolymer, while the peak at 1103 cm^−1^ is assigned to the ether bond present in the soft segments. The existing peaks confirm that the reaction of copolymerization has occurred.

3.4. DSC Analysis

In order to obtain the desired properties of thermoplastic polyesters, it is necessary to synthesize soft segments with a low glass-transition temperature (T_g_). It was estimated that polyester obtained from ricinoleic acid methylester has a glass-transition temperature of −77 °C (Table 3). Such a low T_g_ value is explained by the presence of pendant chains on ricinoleic acid molecules, which enables segmental mobility manifested in a low T_g_ value.

The glass-transition temperature value for the polyether based on 1,3-propane diol is −76.51 °C (Table 3). Crystal domains also exist in the obtained PPD, and it should be noted that the crystallization temperature is exceptionally low (−37.37 °C) while the melting temperature is 13.24 °C. According to the values of enthalpy of crystallization, ΔHcr, and enthalpy of melting, ΔH_m_, it can be concluded that all domains of the ordered structure melt.

To investigate the influence of the middle-block structure on the thermal properties of poly(L-lactide)-based ABA block copolymer, a DSC analysis of pure poly(L-lactide) was first performed. T_g_ occurs at a temperature of 50.57 °C, while crystallization occurs at a temperature of 113.44 °C with an enthalpy of crystallization of 22.18 J g^−1^. Considering that it is a polymer containing an L-lactide monomer, crystallization is expected. Melting occurs at a temperature of 135.54 °C with a ΔH_m_ of 22.87 J g^−1^ (Table 3).

Due to the immiscibility of segments, the final triblock copolymers show differences in phase transformation temperatures depending on the length and content of individual segments. Depending on the compatibility of the segments, the T_g_ can significantly shift to higher temperatures (in the case of soft segments) or to lower temperatures (in the case of hard segments). With a sufficiently small proportion of individual segments, their glass-transition temperatures can be completely obscured so that they cannot be observed.

DSC thermograms of synthesized block copolymers with PMR as the soft segments are shown in Figure 8. As can be seen, PLLA-b-PMR-b-PLLA copolymers exhibit enhanced phase separation, which significantly impacts the thermal behavior. The T_g_ of the PMR soft segments shows only a minor increase (~4 °C) compared to pure PMR, confirming the independence of the phases. The samples with longer PLLA blocks, such as PLLA_10_-b-PMR-b-PLLA_10_, exhibit a well-defined “cold” crystallization peak for PLLA domains, indicating the formation of crystalline regions. However, in copolymers with shorter PLLA chains (PLLA_3,5_-b-PMR-b-PLLA_3,5_), crystallization is hindered due to insufficient chain length, preventing the formation of an ordered crystalline structure.

In the domains of soft and hard segments, similar blocks are connected by secondary interactions. Therefore, the T_g_ of the soft segments increases slightly because the interactions between the soft segments are hindered by the presence of pendant chains. However, the interactions in the hard segments are significantly stronger due to the tight packing of the PLLA chains. Due to such strong interactions, the temperature required for PLLA crystallization is significantly lower compared to the crystallization temperature of the pure block, 73.84 °C in the case of block polyester and even 113 °C in the case of the pure segment (Table 3 and Figure 8). This is an unequivocal confirmation of the major influence of phase separation of blocks in segmented polyester. Thus, even in a pure block, Tc is significantly higher due to the disordered structure of the chains, while the situation in segmented polyester is different; there is orderliness even at low temperatures because phase separation occurs. Another confirmation of this assumption is the increase in the melting temperature of the crystalline domains in the PLLA segment. The increase in the T_m_ of the crystalline domains by 7 °C, compared to the T_m_ of the pure block, is explained by established interactions between the polymer chains in the poly(L-lactide) segment. These interactions increase the T_m_ because, in this case, more energy is required to overcome the secondary forces in the L-lactide crystal domain. This is significant for the application of these materials, as the hard segments act as a physical reinforcements of thermoplastic copolymers. This was further confirmed by mechanical analysis, showing that the reinforcement is stronger if the interactions between similar segments are more intense, which is the case with blocks of poly(L-lactide).

In PLLA-b-PPD-b-PLLA copolymers, clear phase separation is observed, as demonstrated by the distinct T_g_ for the soft PPD segments and the hard PLLA segments, Figure 9. For instance, in the PLLA_10_-b-PPD-b-PLLA_10_ sample, the T_g_ of the soft segments (−53.76 °C) is higher than that of pure PPD due to the restricted mobility imposed by the surrounding PLLA blocks. Conversely, the T_g_ of the hard segments (45.52 °C) is lower than that of pure PLLA, reflecting the reduced crystallinity and interaction within the PLLA domains due to the presence of soft segments. Due to the established secondary bonds between the blocks (hydrogen and Van der Waals interactions), the mobility of the soft segments is reduced, which results in an increase in the T_g_ of the soft segments for around 10 °C; however, this also occurs in the absence of crystallinity of the soft segment, which is a consequence of the rigid structure of the hard segments of poly(L-lactide) that prevent the crystallization of the soft segments.

In PPD-based copolymers, reducing the length of the PLLA blocks results in the dominance of soft segments, as seen in PLLA_4_-b-PPD-b-PLLA_4_, where the T_g_ of hard segments becomes undetectable (Table 4). The increased mobility of the PPD segments in these samples reduces the overall rigidity and phase separation, leading to diminished crystallinity compared to the PMR-based copolymers. Overall, the distinct thermal and phase behaviors of PMR- and PPD-based copolymers are driven by their structural differences. PMR-based copolymers exhibit superior phase separation, thermal stability, and crystallinity of PLLA domains, making them more suitable for applications requiring enhanced mechanical properties and stability. In contrast, PPD-based copolymers prioritize flexibility and reduced crystallinity, catering to applications demanding softer and more elastic materials.

3.5. TG Analysis

Analysis of the thermal stability of the obtained materials provides particularly valuable information regarding the structure of the obtained segmented copolymers, the miscibility of phases, as well as the influence of individual phases on the properties of the final materials. Thermal analysis shows notable differences in the decomposition behavior of block polyesters with PMR and PPD soft segments, reflecting their distinct structural and thermal properties. The results of TG analysis of triblock polyester poly(L-lactide-b-methyl ricinoleate-b-L-lactide), PLLA_10_-b-PMR-b-PLLA_10_, are shown in Figure 10. The initial stage (up to 325 °C, with a maximum degradation rate at 301 °C) corresponds to the degradation of PLLA hard segments and ester bonds. The subsequent stages primarily involve the decomposition of the PMR soft segments. The high degree of phase separation and the organized crystalline structure of PLLA hard segments enhance their thermal stability, with their degradation temperature increasing by nearly 50 °C compared to pure PLLA.

DSC analysis has shown that increasing the share of PLLA blocks in the final copolymer enables more intense phase separation and organization of PLLA blocks into crystalline domains, which results in a significant increase in the thermal stability of hard segments. This is a confirmation that in the final block polyester, there was considerable stabilization of the hard domains due to ordering, as a result of phase separation or immiscibility. Due to the higher phase separation, as well as the higher share of hard segments in the PLLA_10_-b-PMR-b-PLLA_10_ block, it is not possible for the flexible chains of soft segments to wrap around the domain of hard segments so that phase decomposition occurs separately. Therefore, the phenomenon of simultaneous decomposition of soft and hard segments was not observed, due to the mentioned reason, so the profile of the decomposition of soft segments is identical to the profile of decomposition of clean blocks (Figure 11).

A decrease in PLLA content in the block copolymer leads to a decrease in the thermal stability of the hard segments (Figure 12), which is explained by a poor order of shorter PLLA chains, in agreement with the DSC results. Therefore, the maximum degradation rate temperature in the first step corresponds to the decomposition of hard segments (251–301 °C). As expected, the phase separation does not affect the thermal stability of the soft segments, i.e., the maxima on the corresponding DTG curves of block polyester are the same as for pure PMR, at around 350 °C. This is explained by the fact that after the degradation of hard segments, only PMR blocks remain; thus, the degradation of soft blocks cannot be distracted or stabilized by the hard segment.

In comparison, block copolymers with PPD show significantly different behavior during thermal decomposition. As can be seen, thermal decomposition takes place in two stages (Figure 13). The first stage, with a mass loss of up to 50% (with a maximum degradation rate at 295.65 °C), corresponds to the decomposition of PLLA hard segments. The second stage, accounting for the remaining 50% of mass loss (maximum at 413.69 °C), involves the degradation of PPD soft segments.

If we compare the DTG curves of the triblock polymer PLLA_10_-b-PMR-b-PLLA_10_ with the results of the TG analysis of the pure segments, PLLA and PPD, it can be clearly observed that the beginning of segment decomposition in the triblock polymer significantly shifted to higher temperatures compared to pure blocks (Figure 14). The maximum of the corresponding DTG curve of the PPD is at 323 °C, while the maximum on the DTG curve of block polyester even reaches 413 °C. This is a significant increase in the thermal stability of the soft segment, caused by phase thermodynamics in the block copolymer. The same conclusion can be drawn for the hard segments, where the thermal stability is increased from 256 °C to 295 °C.

Increasing the content of the PPD soft segment in the block copolymer does not affect the thermal stability of the hard segments of PLLA. For the sample PLLA_4_-b-PPD-b-PLLA_4_, the maximum on the DTG curve corresponding to the soft segment is further increased to 436 °C (Figure 15). The increase in the proportion of the soft segment enables a better phase separation and, thus, enables greater thermal stability of soft segments. This was also confirmed by DSC analysis, where the T_g_ value decreased with the increase in the proportion of the soft segments, which is a confirmation of more intense phase separation.

While both PMR- and PPD-based block copolymers exhibit phase-separated structures, their thermal behavior differs significantly. In PMR-based systems, the thermal stability of hard PLLA segments benefits more from the crystalline domain formation facilitated by higher phase separation. In contrast, the soft PMR segments maintain their thermal stability regardless of phase interactions. PPD-based systems, however, show a marked improvement in the thermal stability of soft segments due to phase separation, with their decomposition temperature significantly exceeding that of pure PPD. This trend is not observed in PMR-based systems, where soft-segment thermal stability remains unaffected.

3.6. Analysis of the Mechanical Properties of Synthesized Block Copolymers

The analysis of the mechanical properties of the obtained block copolymers was performed to examine the influence of the middle block, the soft segment, on the properties of the prepared materials. The PLLA_3,5_-b-PMR-b-PLLA_3,5_ cast film sample was not of suitable consistency for mechanical properties analysis. The mechanical properties of neat PLLA and the synthesized ABA triblock copolymers are summarized in Figure 16 and Table 5. As expected, neat PLLA exhibited brittle behavior with a high tensile strength of 64.25 MPa and a low elongation at break of 5.51%. The introduction of bio-based soft segments led to a distinct shift in mechanical performance depending on the segment type and length. For the PPD-based copolymers, an increase in the PPD block length (from PLLA_4_-b-PPD-b-PLLA_4_ to PLLA_10_-b-PPD-b-PLLA_10_) resulted in a gradual increase in elongation at break (up to 16.82%) while maintaining relatively high tensile strength (58.52 MPa) and Young’s modulus. This is expected behavior because smaller PLLA content does not lead to the desired phase separation between soft and hard segments, which was confirmed by DSC analyses. This phase separation enables a better organization of both soft and hard elements, which have the role of physical reinforcement, enabling large stretches of the obtained blocks, while maintaining high stress values. Smaller proportions of PLLA produce block copolymers characterized by a lower degree of elongation, 9 and 7.5%, while their stresses are expectedly lower compared to pure PLLA, Figure 16b. This indicates that the linear PPD segments provide a toughening effect without significantly compromising the structural integrity of the PLLA matrix. In contrast, the PMR-based series showed a more dramatic transformation. The PLLA10-PMR sample reached an elongation of 21.87%, accompanied by a significant decrease in Young’s modulus (1.10 GPa). This behavior is attributed to the specific chemical structure of poly(methyl ricinoleate); its long aliphatic side chains act as internal plasticizers, increasing the free volume and chain mobility. The observed micro-phase separation, confirmed by DSC, allows these copolymers to dissipate energy more effectively than simple physical blends, providing a unique balance between renewable origin and tailored flexibility.

The obtained results of the mechanical properties of copolymers based on the PPD middle block showed that these copolymers behave remarkably like the pure PLLA polymer due to a weaker interphase separation of the blocks. They are characterized by high values of stress with low values of elongation, whereby these values increase with the increase in the PLLA chain length. In the case of PMR-based copolymers, there is a significant phase separation, which enables obtaining block copolymers with more pronounced elastic properties. In addition to phase separation, the increase in elongation is also influenced by the presence of dangling chains in the PMR block. In this way, it was confirmed that the desired values of the mechanical properties of PLLA-based block copolymers can be tailored by selecting the middle-block structure.

In addition to their thermo-mechanical behavior, the chemical composition and morphology of the prepared ABA triblock copolymers have direct implications for their degradation behavior. The materials are composed of ester-containing, bio-based segments, which are inherently susceptible to hydrolytic and enzymatic cleavage. Moreover, the incorporation of PMR and PPD soft blocks reduces the crystallinity of the PLLA domains and increases chain mobility and free volume, facilitating water diffusion into the polymer matrix and potentially accelerating degradation compared to neat PLLA.

4. Conclusions

This study reports the successful synthesis of PLLA-based ABA triblock copolymers, incorporating two structurally distinct bio-based soft segments, poly(methyl ricinoleate) (PMR) and poly(1,3-propanediol) (PPD). Precise control over polymer architecture was achieved through stoichiometric design, optimized reaction conditions, and the use of hydroxyl-terminated macroinitiators, enabling the preparation of well-defined triblock copolymers. This research represents a significant advancement in the field of sustainable materials, as it introduces a novel pathway to overcome the inherent brittleness of PLLA without relying on petroleum-based additives. Structural characterization by ^1^H NMR spectroscopy confirmed the formation of the targeted ABA architecture and the successful ring-opening polymerization of L-lactide initiated from the terminal hydroxyl groups of the soft segments. The resulting materials exhibited distinct thermo-mechanical behaviors depending on the chemical structure of the middle block. PMR-based copolymers showed enhanced phase separation, improved thermal stability, and increased mechanical flexibility, attributed to the presence of pendant aliphatic chains in the soft segment. In contrast, PPD-based copolymers displayed more restricted phase separation, highlighting the role of soft segment linearity and crystallinity in governing interphase interactions. These structure–property relationships originate from the covalently connected ABA architecture, which enables stable microphase separation and cannot be achieved by simple physical blending of the corresponding homopolymers. Overall, the results demonstrate that the chemical architecture of bio-based soft segments is a key parameter in tailoring the performance of PLLA-based triblock copolymers. Ultimately, the findings of this study provide a critical benchmark for the design of the next generation of fully renewable, structure-tailored polymers, bridging the gap between basic polymer synthesis and the industrial demand for high-performance biodegradable materials.