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Article

Preparation of PLLA and PLGA Copolymers with Poly(ethylene adipate) Through Reactive Melt Mixing: Structural Characterization, Thermal Properties, and Molecular Mobility Insights

by
Evi Christodoulou
1,
Christina Samiotaki
1,
Alexandra Zamboulis
1,
Rizos Evangelos Bikiaris
2,
Panagiotis A. Klonos
1,3,
Apostolos Kyritsis
3 and
Dimitrios N. Bikiaris
1,*
1
Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
2
Hephaestus Laboratory, School of Chemistry, Faculty of Sciences, Democritus University of Thrace, GR-65404 Kavala, Greece
3
Dielectrics Research Group, Department of Physics, National Technical University of Athens, Zografou Campus, GR-15780 Athens, Greece
*
Author to whom correspondence should be addressed.
Macromol 2025, 5(3), 35; https://doi.org/10.3390/macromol5030035
Submission received: 3 June 2025 / Revised: 3 July 2025 / Accepted: 5 August 2025 / Published: 7 August 2025
(This article belongs to the Collection Advances in Biodegradable Polymers)
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Abstract

In this study, a series of copolymers was synthesized using the promising biodegradable polymers Poly(L-lactic acid) (PLLA), Poly(lactic-co-glycolic acid) (PLGA), and Poly(ethylene adipate) (PEAd), known for their high potential. PEAd was synthesized through a two-step melt polycondensation process and then used to prepare copolymers with PLLA (PLLA-co-PEAd) and PLGA (PLGA-co-PEAd) at weight ratios of 90/10 and 75/25, respectively. The synthesized materials, along with the starting polymers, were extensively characterized for their structure, molecular weight, crystallinity, and thermal behavior. These novel systems exhibit single thermal transitions, e.g., glass transition. The incorporation of PEAd into the copolymers induced a plasticizing effect, evidenced by a consistent decrease in the glass transition temperature. Due to the latter effect in combination with the Mw drop, the facilitation of crystal nucleation was observed. Finally, the results by dielectric spectroscopy on the local and segmental molecular mobility provided additional proof for the homogeneity of the systems, as manifested, e.g., by the recording of single segmental relaxation processes. Overall, the findings indicate that the PLLA-co-PEAd and PLGA-co-PEAd copolymers hold significant potential, and the use of complementary experimental techniques offers valuable insights and indirect indications of their properties.

1. Introduction

Over the last few decades, annual plastic production has increased by more than 400 million tons, and it is expected to increase to 1800 million tons by the year 2050. Despite its advantages [1,2,3], such as low production cost, excellent endurance, processability, and light weight, plastic waste accumulation in nature [4] has brought a great deal of environmental issues, leading to plastic pollution and, as believed, even to global warming.
In the framework of a more sustainable plastic industry and the demand for alternative materials [5,6], biodegradable polymers have been introduced worldwide in a variety of applications, such as food packaging, drug delivery, tissue engineering, and automotive interior parts [7,8]. Biobased raw materials can degrade faster than conventional plastics due to their ability to absorb CO2 from the atmosphere; thus, they are a suitable tool for reducing carbon emissions and plastic pollution. Their use comes with a lot of environmental advantages, including the regeneration of raw materials [7,9]. This is a relatively new family of plastics/polymers that is now addressed as ‘green polymers’ [10,11,12], whereas they serve, among others, the modern economy framework, i.e., the ‘green economy’ [13,14].
Poly(lactic acid) (PLA) is one of the most extensively researched and utilized biodegradable aliphatic polyesters due to its superior properties compared to other biodegradable polymers, possessing a confirmed potential to replace conventional petroleum-based polymers and plastics [15,16,17,18]. Deriving from renewable resources like corn, rice, or wheat [19,20], PLA is a transparent, rigid bioplastic with great mechanical strength, good processability, and excellent durability [21,22].
Ring-opening polymerization (ROP), especially when catalyzed by metal complexes, is a commonly employed technique for producing aliphatic polyesters such as PLA, owing to its high efficiency, precise control over molecular weight, and ability to maintain stereoregularity [23]. Various metal-based catalysts, such as tin(II) octoate and aluminum-based complexes, have been employed for this purpose [24], contributing significantly to the development of biodegradable polyesters for industrial and biomedical applications.
Its recyclability, compostability, and biodegradability result in almost no environmental impact occurring during its disposal [25,26]. In strong connection with the processing and macroscopic performance of PLA is its semicrystalline nature. It is extremely advantageous that the crystallinity of PLA, in terms of the fraction and semicrystalline morphology, has been found to be quite easily tunable. The said tuning can be achieved via a variety of strategies involving the manipulation of the molecular weight [27], by proper thermal treatment [28,29], and even by combining PLA with nano-additives and other polymers [30,31].
However, its brittle nature (elongation at break lower than 10%) stands as an important obstacle in the applications that require plastic deformation at higher stress levels. In addition, its biodegradation rate via enzymatic hydrolysis is rather low and affected by a variety of factors, such as its molecular weight and distribution, its crystallinity, and morphological properties. This challenge stems from the relatively inefficient hydrolytic cleavage of ester bonds at room temperature, which is further influenced by PLA’s hydrophobic nature and the fact that it remains in a glassy state (glass transition temperature, Tg > 45–50 °C) [32] under typical soil conditions, thereby hindering the penetration of water molecules and enzymes into the bulk of the material [27,33]. This is directly related to the disposal problem of consumer commodities. In order to improve these features, blending with other polymers is usually applied [7,34].
Poly(lactic-co-glycolic acid) (PLGA) is a widely used biodegradable polymer in biomedical applications, as its hydrolysis byproducts—glycolic acid and lactic acid—are readily metabolized by the human body. It is associated with minimal systemic toxicity, and its degradation rate varies according to the molecular weight [35]. Mechanical properties, biodegradation, and swelling rate are affected by the degree of crystallinity of the polymer, which is determined by the monomeric units used for the copolymerization of PLA and PGA. It has been reported that an increased PGA content results in an accelerated degradation rate [36]. The combination of PLA and PLGA is a very promising candidate already employed in multiple fields, including drug delivery, food packaging, and electronics [37,38].
Poly(ethylene adipate) (PEAd) is an aliphatic polyester that stands as a promising candidate for improving PLA’s and PLGA’s properties due to its low-cost manufacturing and its low molecular weight, which results in higher biodegradation rates [39,40]. It is a biodegradable and renewable polymer produced by ethylene glycol and adipic acid. Among these properties, PEAd exhibits high thermal stability, serving as a possible “plasticizer” [41]. PEAd has been combined with various petroleum-based polymers, such as low-density polyethylene (LDPE) [40], poly(ethylene terephthalate) [42], and other aliphatic polyesters like poly(ε-caprolactone) (PCL) [43].
In this paper, the synthesized PEAd is combined with PLLA and PLGA to prepare PLLA-co-PEAd and PLGA-co-PEAd copolymers (of 90/10 and 75/25 weight ratios). The copolymers were synthesized through melt transesterification-induced intermolecular chain exchange and characterized using various experimental techniques, including Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared Spectroscopy (FTIR) to analyze their structure, Differential Scanning Calorimetry (DSC) to examine their thermal properties, and X-Ray Diffraction (XRD) to assess their crystallinity. The primary objective of this study is to develop a sustainable and efficient approach to improve the performance and degradability of polylactide-based materials by copolymerizing them with PEAd. One of the primary objectives of this work was to investigate whether the incorporation of PEAd can effectively manipulate the physicochemical properties of polylactides, particularly focusing on softening and plasticization. More specifically, the research focuses on tuning thermal, structural, and mechanical behavior in order to overcome PLA’s intrinsic brittleness and slow degradation, which limit its practical applicability. By tailoring these properties, we aimed to enhance the applicability of the final material in biomedical or packaging-related fields.

2. Materials and Methods

2.1. Materials

Adipic acid (≥99.0%, ACS reagent), 1,2-ethyleneglycol (99%), and titanium(IV) butoxide (TBT, 97%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). PLGA 65/35 (PURASORB ® PLG 6506) was obtained from Corbion (Gorinchem, The Netherlands), and high molecular weight PLLA (3052D) was kindly supplied by Plastika Kritis S.A (Iraklion, Greece). All other solvents and reagents used were of analytical grade and obtained from Sigma-Aldrich.

2.2. Synthesis of PLLA/PEAd Block Copolymers

PEAd (Figure 1) was synthesized using a standard two-step melt polycondensation process consisting of an initial esterification followed by polycondensation. In the esterification step, appropriate amounts of adipic acid and 1,2-ethyleneglycol (in a 1:1.1 molar ratio) were placed in a round-bottom flask fitted with a mechanical stirrer, condenser, and nitrogen inlet. The reaction mixture was purged with nitrogen multiple times and gradually heated in a salt bath from 180 °C to 220 °C over a period of 4 h, while maintaining constant stirring at 250 rpm. After complete removal of water, 400 ppm of TBT (0.05 g/mL in toluene) was added to the reaction mixture, followed by the gradual application of high vacuum (5.0 Pa) to prevent excessive foaming. The polycondensation step was then carried out at 230 °C with stirring at 400 rpm for 2 h.
PLLA-co-PEAd and PLGA-co-PEAd copolymers (90/10 and 75/25 wt%) were prepared via reactive melt-mixing through transesterification-induced intermolecular chain exchange. Freeze-dried (Coolsafe 110-4 Pro, Labogen Scandinavia) PLLA or PLGA pellets and PEAd (10 and 25 wt%) were melt-mixed with TBT catalyst at 180 °C for 90 min under high vacuum and continuous stirring.

2.3. Experimental Techniques

1H and 13C NMR spectra were acquired on Agilent AM 500/600 MHz instruments (Agilent Technologies, Santa Clara, CA, USA) using 5% w/v solutions in CDCl3. Tetramethylsilane (TMS) was used for internal referencing and calibration. Spectra were collected at RT with 32 scans for 1H and 512 for 13C, using a sweep width of 6 kHz.
FTIR spectra were recorded using a Perkin-Elmer Spectrum 1 spectrometer (Waltman, MA, USA). The samples were finely ground, blended with KBr, and compressed into pellets using a hydraulic press. Spectral data were collected in the 4000–400 cm−1 range at a resolution of 4 cm−1 with 16 scans, followed by baseline correction, normalization, and conversion to absorbance.
XRD patterns were obtained at room temperature using a MiniFlex II diffractometer (Rigaku Co., Tokyo, Japan) equipped with Cu Kα radiation (λ = 0.154 nm), scanning over a 2θ range of 5° to 45° at a rate of 1° per minute.
Molecular weights were determined via size-exclusion chromatography using a Waters system with Ultrastyragel columns (HR-1 to HR-5) and a Shimadzu RID-10A detector. Samples (20 mg/L, 150 μL injection volume) were eluted at 1 mL/min and 60 °C. Calibration was performed using polystyrene standards (molecular weight of 1–300 kg/mol).
The morphology was analyzed using a Nikon Optiphot-1 polarizing microscope fitted with a Linkam THMS 600 heating stage (Linkam Scientific Instruments Ltd., Surrey, England), a Linkam TP91 control unit (Linkam Scientific Instruments Ltd., Surrey, England), and a Jenoptic ProgRes C10-Plus camera (Jenoptik Group, Jena, Germany). Optical images were taken during the isothermal crystallization of hot-melted PEAd and PLLA samples to observe nucleation and crystal growth.
Thermal transitions were measured using a TA Q200 series DSC system (TA Instruments, New Castle, DE, USA) in nitrogen atmosphere. First, ~6 mg of sample was sealed in TZero aluminum pans and scanned from −110 to 200 °C at 10 K/min. After erasing thermal history by initial heating, melted samples were cooled (at 10 K/min) and reheated to observe crystallization and glass transition shifts. The shown results/values correspond to the average outcome of measurements on two different samples per composition.
Broadband dielectric spectroscopy (BDS) allows the evaluation of molecular dynamics by the indirect recording of dipolar relaxation processes occurring from local as well as segmental type motions. The measurements were performed here using a Novocontrol BDS setup (Novocontrol GmbH, Montabaur, Germany) under nitrogen flow. Melted samples (with a disc geometry of 14 mm) were placed between polished brass disk electrodes using thin silica spacers (~50 μm thick and 30 mm long). In this study, the imaginary part of the dielectric permittivity, ε″—associated with dielectric losses—was measured and analyzed as a function of frequency (10−1 to 106 Hz) over a temperature range of −150 to 120 °C, in increments of 5–10 K [44].

3. Results and Discussion

NMR spectroscopy was used to characterize the structure of synthesized copolymers. The 1H/13C NMR spectra of the homopolymers are presented in Figure 2 and are typical of PLLA, PLGA, and PEAd. Specifically, for PLLA, the 1H NMR spectrum displays two resonance signals: a doublet at 1.57 ppm corresponding to the CH3 protons, and a quartet at 5.14 ppm attributed to the CH protons. In the 13C NMR spectrum, the signal at 16.6 ppm is assigned to the CH3 group, the one at 69.0 ppm to the CH group, and the peak at 169.6 ppm to the carbonyl (C=O) group [45,46]. Accordingly, for PLGA, resonance peaks arise at 5.22 and 5.17 ppm (CH, PLA), at 1.55–1.58 ppm (CH3, PLA), and 4.82 ppm (CH2, PGA) in the 1H spectrum; and at 169.2 (C=O, PLA), 166.3 (C=O, PGA), 68.9 (OCH, PLA), 60.8 (OCH2, PGA), and 16.6 ppm (CH3, PLA) in the 13C spectrum. Finally, in the PEAd 1H NMR spectrum, three peaks are observed at 1.66 ppm (CH2), 2.36 ppm (CH2C(O)), and 4.27 ppm (OCH2), whereas in the 13C spectrum, the recorded peaks are four in total: at 173.0 (C=O), 62.1 (OCH2), 33.6 (CH2C(O)), and 24.2 ppm (CH2).
For the copolymers, the corresponding NMR spectra largely represent a superposition of the spectra of the two homopolymers, with the distinctive peaks of each component clearly visible in the copolymer spectra. The successful synthesis of the PLLA/PEAd and PLGA/PEAd copolymers was confirmed by the presence of distinct resonance signals from both components in the 1H NMR spectra. In the case of PLLA/PEAd, the methine proton of PLLA appears at δ ≈ 5.2 ppm, while the –CH2–NH– group of PEAd is observed at δ ≈ 2.4 ppm, indicating the coexistence of both segments. Similarly, in the PLGA/PEAd copolymer, the characteristic signals of PLGA at δ ≈ 5.2 ppm (lactic acid) and δ ≈ 4.8 ppm (glycolic acid) appear alongside the δ ≈ 2.4 ppm peak of PEAd, verifying the successful incorporation of PEAd into both polymer matrices. The ratio of the copolymers is calculated from the peaks at 5.15 ppm (PLA), 4.82 ppm (PLGA), and 4.27 ppm (PEAd), and the composition is in agreement with the feed ratio. Due to the very high molecular weight of the homopolymers that were blended and the expected block structure of the resulting copolymers, the “bridging” units, i.e., ethylene glycol units that are connected to PEAd on the one hand and PLA on the other, are scarce and were not detectable by NMR. It is worth mentioning that the end-groups (OCH2CH2OH and CH(CH3)OH) were not observable either, indicating that high molecular weight polymers were obtained (vide infra).
Molecular weight data (Mn, Mw), obtained through size-exclusion chromatography (SEC), are provided in Table 1. All SEC chromatograms (Figure 3) displayed a single elution peak, indicating the presence of a single component in each sample. The broad shape of the peaks reflects a wide molecular weight distribution, with polydispersity index (PDI) values ranging from 1.5 to 3.8, which is common for synthetic polymers [47,48], featuring a long tail towards the lower molecular weight. This profile implies a unimodal Mw distribution with a wider range in the size of macromolecular chains. Molecular weight values are in all cases estimated between the values of the starting units, indicating that a copolymerization reaction had taken place, whereas the much smaller PEAd (~10 times smaller than PLLA homopolymer, ~2 times smaller than PLGA) acts as a macroinitiator by increasing its amount, and copolymers of smaller size and broader distribution arise.
FTIR spectroscopy was also employed in correlation with 1H NMR to further confirm the structure of the synthesized materials. All acquired FTIR spectra are presented in Figure 4, confirming the successful synthesis of the copolyesters. In regard to the neat homopolymers PLLA, PLGA, and PEAd, their spectra share most common characteristic peaks, namely the stretching vibrations of the –CH3 groups at ~2990 and 2940 cm−1, asymmetric and symmetric, respectively, the intense peak at ~ 1750 cm−1 corresponding to the stretching vibration of their carbonyl –C=O groups, and several bands of medium intensity in the region 1450–1300 cm−1 attributed to bending vibrations of the –C-H (methyl, methylene) and C-O groups. Peaks at 3500 cm−1 and 3450 cm−1 in the spectrum of neat PLGA are attributed to the stretching vibrations of -OH groups [49,50].
With respect to the prepared copolymers, the overall depiction is much alike, since they are all aliphatic polyesters of similar chemical structure, bearing common functional groups. A noteworthy difference worth noticing is the split of the peak ascribed to the vibration of the carbonyl group (at 1750 cm−1) or its broadening, presumably due to the overlapping of two separate peaks, indicating the co-existence of carbonyls from the PEAd and the PLLA/PLGA units. This double/broader peak, therefore, confirms the presence of both homopolymers (either in the form of blends or copolymerized). Support to that is also supplied by the increased intensity of C–H stretching absorbance peaks (~2900 cm−1) due to the addition of extra methylene groups on the polyester backbone, as well as the appearance of a third small peak in this region, most probably corresponding to the methyl group of the lactide monomer.
XRD and POM are expected to provide insights into the crystal structure, semicrystalline morphology, and overall distribution of the two polymers within the copolymers. The corresponding XRD results are displayed in Figure 5. The spectra shown in Figure 5a correspond to the initially prepared samples that were stored at room temperature. As evidenced, PLGA appeared amorphous in all cases, whereas PLLA cannot be easily crystallized by simple cooling. This lack of crystallinity was anticipated, as PLA is known for its slow crystallization behavior, typically requiring extended crystallization time and/or enhanced nucleation to form crystalline structures. To promote PLLA crystallization, the samples underwent annealing at 140 °C for 1 h (recrystallization), which successfully enhanced their crystalline structure. The corresponding XRD spectra are demonstrated in Figure 5b.
As shown in the XRD results, distinctly different patterns were observed before and after melt recrystallization. Prior to the annealing process, PEAd was crystallized (complex peak at 2θ ~22° and 25°), whereas PLLA remained amorphous in both cases (90/10 and 75/25), as no PLLA-related crystalline peaks were recorded. On the other hand, measurements taken after recrystallization indicate improved crystallization of PLLA and partial crystallization of PEAd, as evidenced by the more pronounced and sharper crystalline peaks observed in Figure 5b. The formation of new PLLA crystals and/or the reorganization of existing ones led to the emergence of four main crystalline peaks in the XRD patterns, i.e., at the 2θ positions of 14.95°, 16.67°, 19.21°, and 22.4°, being generally visible for both copolymers, with their intensity found proportional to the PLLA loading [51].
In order to observe the density and sizes of the spherulites formed, the process of isothermal annealing at various temperatures was investigated by POM. Figure 6 shows the initial, intermediate, and final stages of crystallization at 1 min, 5 min, and 12–20 min, respectively. PEAd has a slower crystallization rate; the crystals formed are larger and less dense compared to PLLA. When comparing the copolymers, PLLA/PEAd 90/10 exhibits a higher spherulitic growth rate compared to the 75/25 sample, while the crystals formed are smaller. In 75/25 material, the crystals are quite large, fewer, and, furthermore, do not fill the sample volume. Obviously, there is a strong impact of PEAd on the crystallization of the copolymers. For example, we would expect that the crystallization of PLLA would be quite hindered, considering the significant drop in Mw. This is compatible with the lesser number of crystals recorded in the copolymers, whereas not with the larger size of the spherulites. We may thus conclude that the large spherulites in the copolymers are being built by PLLA-PEAd chains. Similar crystallization suppression and spherulite enlargement effects have also been reported in PLA-based copolymers with flexible aliphatic segments, such as poly(ε-caprolactone) or poly(butylene succinate) [52], confirming the disrupting role of soft segments on nucleation while facilitating growth. Please note that in the hypothetical case that the crystals formed in the copolymer were uniquely made of PEAd chains (phase separation), their crystallization would not have taken place at the temperatures of 100 and 110 °C (Figure 6), as PEAd melts at T < 50 °C (DSC results in the following).
In Figure 7, we present the DSC cooling-heating traces for all samples. We recall that the measurements correspond to samples initially subjected to an erasing of the thermal history. In Figure 7a, none of the PLA-based samples exhibits crystallization exotherm; thus, they preserve the amorphous character during the cooling and during the subsequent heating (Figure 7b) up to temperatures closely above the glass transition temperature, Tg. Neat PEAd demonstrates a highly semicrystalline character, with the characteristic peak temperature located at Tc~0 °C and the corresponding crystallization enthalpy change, ΔHc, equalling ~39 J/g (±2). The glass transition of PEAd is recorded as a step, for example, during heating at Tg~−52 (±1) °C. This is followed by a double cold crystallization at Tcc at −7 and 28 °C and an overall single melting endothermal peak at Tm = 47 °C. The results on PEAd are in accordance with previous findings in the literature [41]. Similar observations of complex cold crystallization processes and melting transitions at lower temperature ranges have been reported in PLA-based polymer blends containing flexible aliphatic segments, such as poly(butylene adipate). These components also display distinct crystallization peaks at sub-ambient temperatures [53].
On the other hand, the neat and initially amorphous PLLA exhibits, in the order of increasing temperature, a single glass transition at Tg~58 °C, a wide and relatively weak cold crystallization with Tcc~117 °C, and melting at Tm = 157 °C. These values are consistent with those previously reported for pure PLLAs in the literature [20].
Then, PLGA, a completely amorphous polymer, exhibits a single glass transition temperature at Tg~44 °C. This is also in close agreement with previously reported results for PLGA-based copolymers, particularly those with comparable glycolide/lactide ratios [54].
The effects regarding the PLLA/PEAd and PLGA/PEAd copolymers can be schematically observed by the added arrows in Figure 7b,c. Regarding glass transition, the presence of PEAd leads to a systematic suppression in Tg of both PLLA and PLGA, respectively, by 15–32 K and 20–37 °C. Obviously, the Tg drop is partly facilitated by the recorded decrease in the molecular weight [55]. The effects suggest an almost excellent mixing of the two polymers and/or a strong plasticization effect of the ‘soft’ PEAd on the ‘harder’ polylactides. The model of strong plasticization in similar and different copolymers has been found rational within many previous works [56,57,58].
Furthermore, a drop in the Tcc for PLLA is observed. This suggests an enhanced nucleation ability of PLLA in the presence of the PEAd blocks. Most probably, this is also connected with the above-mentioned plasticization [56,57]. As in Tg and Tm, there is a similar drop in the melting temperature, Tm, of PLLA (~156 °C) in the copolymers (150–153 °C).
Finally, we should mark, on the basis of Figure 7b,c (added arrows), that for almost all free unbound copolymers, no thermal events are recorded to arise directly from PEAd. Exception to that is the case of PLLA/PEAd 75/25, a PEAd-rich material. Within that sample, we recorded, in addition to the main thermal events, the weak glass transition, cold crystallization, and melting of PEAd during heating. These suggest a partial separation or the presence of non-integrated PEAd in that copolymer. Since the crystallization of PEAd takes place, the said phase separation should be micrometric, considering that such is the size of spherulites, in general. On the other hand, the PLGA/PEAd are more homogeneous and are an indication of better blending and/or more effective copolymerization between PEAd and PLGA, as compared to that between PEAd and PLLA.
Molecular dynamics were evaluated using the advanced technique of Broadband Dielectric Spectroscopy (BDS) [59]. The local and segmental motions were monitored indirectly at lower and higher temperatures, respectively, through the detection of peaks in the imaginary part of the dielectric permittivity, ε″. The local dynamics originate from dipole moments formed, for example, due to rotations and crankshaft motions of local groups of the polymer. Since these cases of motions are ‘secondary’ compared to the main-overall chain mobility, the local relaxation processes are usually named β, γ, δ, etc. On the other hand, the segmental dynamics arise from dipole moments perpendicular to the polymer backbone. Their relaxation is recordable by BDS at T ≥ Tg, i.e., when the polymer chains are mobilized. Hence, this relaxation process is referred to as α-relaxation and represents the dynamic counterpart of the calorimetric glass transition. At even higher temperatures, we may follow a significant uprise of ε″. This is mainly expected in polymeric systems. This phenomenon arises from the contribution of multiple ionic conductivity effects, including ion transport, electrode polarization, and interfacial charge accumulation [60] occurring throughout the polymer matrix in the ‘rubbery/liquid state’.
Coming to the present study, our BDS results are representatively shown in Figure 8 in the isochronal representations of ε″ (Τ) at the selected frequency of ~3 kHz. This type of presentation enables the direct comparison of the BDS with DSC data. In Figure 8a, we show the results for the neat polymers, PEAd, PLLA, and PLGA, whereas in Figure 8b, we additionally present the results for the copolymers.
At temperatures below Tg, the signals are relatively weak, and the spectrum is primarily governed by dipolar local relaxation processes. For PEAd, this is the case for the βPEAd process, which, based on the few works from the literature [57] (and references therein), is proposed to arise from crankshaft motions of n-ethylene sequences on the polymer backbone. At higher temperatures, or slower than βPEAd, the local relaxation of PLLA, βPLLA, is recorded. Due to the extensive investigation of PLA in the past, the origins of βPLLA are known, i.e., the relaxation of dipole moments related to the crankshaft rotation of the ester group (-C=O) of the polyesters (inset scheme in Figure 8a), which is actually the most polar site [61]. Located at a similar position in Figure 8a is the local process of PLGA, βPLGA. Considering the quite similar structure between the two polylactides and the similar dynamical characteristics of the two β-type processes, most probably, βPLGA arises from the ester group rotation, similar to βPLLA. At T ≥ Tg in Figure 8a, the dielectric response increases by at least one order of magnitude, and the α relaxation dominates the ε″ (Τ). The temperature position order for the various α processes, i.e., PEAd < PLGA < PLLA, is qualitatively similar to that of the corresponding calorimetric glass transitions.
We turn the focus now on Figure 8b and the effects imposed within the copolymers. The local (β relaxations) are recorded in all cases, with the temperature positions being barely affected and the magnitude of the processes being proportional to the fractions of PEAd, PLLA, and PLGA. In almost all cases, we recorded single α relaxations, in particular arising from the lactides (αPLLA and αPLGA). This is an additional manifestation of the homogeneity of the synthesized copolymers. As in calorimetry, in PLLA/PEAd 75/25, we recorded an additional weak peak at about −20 °C, which is the case for αPEAd that co-exists in the said blend. The existence of this process suggests that there may exist small regions of PEAd, at least of nanometric dimensions.
Apart from that, the most striking effect recorded in Figure 8b is the systematic migration of the α relaxation of both PLLA and PLGA toward lower temperatures when increasing the amount of PEAd. Practically, this is explained as an ‘acceleration’ of the segmental dynamics of both polylactides in the presence of PEAd. This provides further support for the plasticizing role of PEAd over polylactides. We should also not forget that within this plasticization, there should be the contribution of the lowering of molecular weight.
Plasticization leads to a generally softer material (lower Tg, lower Tm), which was actually one of the basic aims for the preparation of the said copolymers. Once again, the polylactides prove their value as compatibilizers with other polymers and means to manipulate the final material (copolymer, blend, polymeric network, nanocomposite) properties to a wide extent.

4. Conclusions

In this study, PEAd was synthesized via a conventional two-step melt polycondensation process, followed by the preparation of PLLA-co-PEAd and PLGA-co-PEAd copolymers (with weight ratios of 90/10 and 75/25) using a reactive melt-mixing technique. Size exclusion chromatography indicated a unimodal Mw distribution with a wider range in the size of macromolecular chains, as well as the role of PEAd as a macroinitiator. The chemical structure of the prepared materials was characterized using NMR and FTIR, and the characteristic peaks of the neat polymers were observed in the spectra of the copolymers. On comparing to initial PLLA and PLGA, a significant drop in Mw was recorded in the copolymers, providing, among other structural results, proof for the successful copolymer synthesis. The semicrystalline nature of the samples was confirmed from XRD measurements, whereas the semicrystalline morphology was evaluated by employing POM. Employing DSC and BDS, we revealed a strong role of PEAd as a plasticizer, as manifested by the systematic acceleration of segmental mobility (decrease in the Tg of the copolymers). In the case of semicrystalline PLLA, the said plasticization, in addition to the decrease in the chain lengths (drop in Mw) in the copolymers, resulted in the enhanced nucleation of PLLA (being initially weak).

Author Contributions

Conceptualization, E.C. and P.A.K.; methodology and investigation, E.C., C.S., A.Z., R.E.B. and P.A.K.; formal analysis, E.C., A.Z. and P.A.K.; data curation, E.C. and P.A.K.; writing—original draft preparation, C.S. and P.A.K.; writing—review and editing, E.C. and P.A.K.; supervision, D.N.B. and A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors would like to acknowledge the Center of Interdisciplinary Research and Innovation of Aristotle University of Thessaloniki (CIRI-AUTH), Greece, for access to the Large Research Infrastructure and Instrumentation of the Nuclear Magnetic Resonance Laboratory at the Center for Research of the Structure of Matter in the Chemical Engineering Department.

Conflicts of Interest

The authors declare no conflicts of interest.

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Macromol 05 00035 g001
Figure 1. Synthesis of PEAd by two-stage melt polycondensation and the studied copolymers PLLA/PLGA-co-PEAd through reactive melt-mixing.
Figure 1. Synthesis of PEAd by two-stage melt polycondensation and the studied copolymers PLLA/PLGA-co-PEAd through reactive melt-mixing.
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Macromol 05 00035 g002aMacromol 05 00035 g002b
Figure 2. (a) 1H and (b) 13C NMR spectra of PEAd, PLLA, and PLGA homopolymers and the PLLA/PEAd and PLGA/PEAd copolymers.
Figure 2. (a) 1H and (b) 13C NMR spectra of PEAd, PLLA, and PLGA homopolymers and the PLLA/PEAd and PLGA/PEAd copolymers.
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Figure 3. Size-exclusion (SEC) chromatograms of PEAd, PLLA, and PLGA homopolymers and the PLLA/PEAd and PLGA/PEAd copolymers.
Figure 3. Size-exclusion (SEC) chromatograms of PEAd, PLLA, and PLGA homopolymers and the PLLA/PEAd and PLGA/PEAd copolymers.
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Figure 4. FTIR spectra of neat PLLA, PLGA, PEAd and their prepared co-polyesters.
Figure 4. FTIR spectra of neat PLLA, PLGA, PEAd and their prepared co-polyesters.
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Figure 5. Comparative XRD patterns recorded at RT (a) for all samples as received and (b) for PLLA-based samples upon melting and recrystallization. Indicated are the main crystalline diffraction peaks and the corresponding 2θ positions.
Figure 5. Comparative XRD patterns recorded at RT (a) for all samples as received and (b) for PLLA-based samples upon melting and recrystallization. Indicated are the main crystalline diffraction peaks and the corresponding 2θ positions.
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Figure 6. Polarized light microscopy (PLM) images during the time evolution of isothermal crystallization of PLA and PEAd and their 90/10 and 75/25 blends at various temperatures.
Figure 6. Polarized light microscopy (PLM) images during the time evolution of isothermal crystallization of PLA and PEAd and their 90/10 and 75/25 blends at various temperatures.
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Figure 7. Comparative DSC thermograms for all samples (a) upon cooling and upon (b,c) heating. The recorded heat flow (mW) has been normalized to the sample mass (W/g).
Figure 7. Comparative DSC thermograms for all samples (a) upon cooling and upon (b,c) heating. The recorded heat flow (mW) has been normalized to the sample mass (W/g).
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Figure 8. Comparative isochronal BDS plots of ε″ (T) for all samples at the selected frequency of 3 kHz, namely, (a) neat polymers and (b) copolymers. The inset scheme in (a) describes the molecular origins of the local β relaxation of the polylactide, whereas the inset to (b) shows the dipolar origins of the segmental α relaxation. The added arrows in (b) mark the effects imposed on the local and segmental dynamics of the polylactides by the presence of PEAd in the copolymers.
Figure 8. Comparative isochronal BDS plots of ε″ (T) for all samples at the selected frequency of 3 kHz, namely, (a) neat polymers and (b) copolymers. The inset scheme in (a) describes the molecular origins of the local β relaxation of the polylactide, whereas the inset to (b) shows the dipolar origins of the segmental α relaxation. The added arrows in (b) mark the effects imposed on the local and segmental dynamics of the polylactides by the presence of PEAd in the copolymers.
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Table 1. Average molecular weight values, Mw and Mn, and polydispersity index, PDI, estimated by SEC.
Table 1. Average molecular weight values, Mw and Mn, and polydispersity index, PDI, estimated by SEC.
SampleMw
(g/mol)		Mn
(g/mol)		PDI
PEAd14.4k6.3k2.3
PLLA126.3k81.7k1.5
PLLA/PEAd 90/1059.5k34.9k1.7
PLLA/PEAd 75/2523.7k9.5k2.5
PLGA33.1k21.1k1.6
PLGA/PEAd 90/1017.6k8.4k2.1
PLGA/PEAd 75/2513.2k3.5k3.8
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MDPI and ACS Style

Christodoulou, E.; Samiotaki, C.; Zamboulis, A.; Bikiaris, R.E.; Klonos, P.A.; Kyritsis, A.; Bikiaris, D.N. Preparation of PLLA and PLGA Copolymers with Poly(ethylene adipate) Through Reactive Melt Mixing: Structural Characterization, Thermal Properties, and Molecular Mobility Insights. Macromol 2025, 5, 35. https://doi.org/10.3390/macromol5030035

AMA Style

Christodoulou E, Samiotaki C, Zamboulis A, Bikiaris RE, Klonos PA, Kyritsis A, Bikiaris DN. Preparation of PLLA and PLGA Copolymers with Poly(ethylene adipate) Through Reactive Melt Mixing: Structural Characterization, Thermal Properties, and Molecular Mobility Insights. Macromol. 2025; 5(3):35. https://doi.org/10.3390/macromol5030035

Chicago/Turabian Style

Christodoulou, Evi, Christina Samiotaki, Alexandra Zamboulis, Rizos Evangelos Bikiaris, Panagiotis A. Klonos, Apostolos Kyritsis, and Dimitrios N. Bikiaris. 2025. "Preparation of PLLA and PLGA Copolymers with Poly(ethylene adipate) Through Reactive Melt Mixing: Structural Characterization, Thermal Properties, and Molecular Mobility Insights" Macromol 5, no. 3: 35. https://doi.org/10.3390/macromol5030035

APA Style

Christodoulou, E., Samiotaki, C., Zamboulis, A., Bikiaris, R. E., Klonos, P. A., Kyritsis, A., & Bikiaris, D. N. (2025). Preparation of PLLA and PLGA Copolymers with Poly(ethylene adipate) Through Reactive Melt Mixing: Structural Characterization, Thermal Properties, and Molecular Mobility Insights. Macromol, 5(3), 35. https://doi.org/10.3390/macromol5030035

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