Controlling the Thermodynamic Stability of Melt-Compounded PLA as Opportunity to Achieve 3D Printing Automotive Items with Medium Lifetime

Abstract

In view of the future estimation of the life-time of 3D printed automotive components, this paper evaluates the thermodynamic stability of controlled-nucleated poly (lactic acid) (PLA), focusing on formulations that maintain good mechanical behavior after 4 years of storage under controlled conditions. PLA with 0.5% D-lactide and low molecular weight, which has optimal melt flow at 3D printing, was nucleated using either a sulfonic acid derivative (heterogeneous nucleation) or a PLA grade with 4% D-lactide (stereocomplex or racemic nucleation). Since the earliest signs of thermodynamic instability manifest as changes in chemical structure, which alters thermal behavior, this study focuses on FTIR, DSC analysis and some functional properties such as impact resistance and heat deflection temperature (HDT). The initial properties were compared with those measured 4 years later. Due to heterogeneous nucleation, the bi-modal melting of neat PLA turned into a mono-modal peak, which remained stable over 4 years. Initially, the mono-modal melting of racemic nucleated PLA transitioned into a bi-modal pattern over time, proving its long-term thermodynamic instability. Because 3D printing requires mono-modal melting, it was concluded that racemic crystallization is unsuitable for the used PLA modification with respect to future 3D printing of medium-life automotive components. Crystallinity shapes long-term mechanical performance; therefore, the process must be conducted under selected conditions.

1. Introduction

The development of lightweight vehicles utilizing renewable origin-polymers is expanding rapidly. For medium-life automotive components, functional properties must remain consistent throughout the vehicle’s lifetime, requiring the polymer compounds to maintain long-term thermodynamic stability. While the physical modification of polylactic acid (PLA) through melt compounding has reached industrial scales, its use remains limited to short-life applications [1,2]. Although PLA is a renewable-based polymer with excellent melt processability, it lacks the inherent durability required for long-term use. Research has shown that miscibility is the defining factor in durable applications. It controls thermodynamic stability and physical aging, both of which can accelerate the chemical degradation of polymer compounds [3,4].

The polymer’s crystallinity depends on its chemical structure, the crystallization conditions and the presence of impurities such as macromolecular defects, catalytic residues, and oxidatively degraded chains, etc. [5]. In “spontaneous”, uninitiated crystallization, nucleation and the subsequent growth of spherulites occur around these microscopic “defects”. They grow to sizes larger than the wavelength of visible light. Consequently, light passing through the crystals is scattered, causing the polymer to appear opaque [6,7]. Controlled crystallization can follow a heterogeneous path via a nucleating agent, or a “racemic” path where crystals form through the stereo-complexation of two enantiomers. Un-initiated crystallization typically produces 106 nuclei/cm³, leading to spherulites of roughly 100 μm in size. In contrast, initiated crystallization generates 1012 nuclei/cm³, resulting in spherulites of approximately 1 μm. Because initiated nucleation forces many more crystals to develop within the same volume, their individual size is significantly smaller. Since these new spherulites are smaller than the wavelength of visible light, the light is no longer scattered and the polymer appear to be transparent [8]. The nucleating agents must be insoluble in the polymer matrix and must remain solid at the crystallization temperature. The usage of such agents in PLA nucleated to create 3D-printable polymer bio-hybrids for automotive application has been documented in [9,10,11].

Stereo-complexation involves the interaction between two complementary stereo-regulated polymers that share the same chemical composition but differ in their tacticity. PLA can have three monomeric configurations: dextro (D), levo (L) and dextro-levo (DL). PLLA (Poly-L-lactide) and PDLA (Poly-D-lactide) share identical chemical bonds and functional groups existing as mirror-image enantiomers. Depending on the intensity of specific (+)/(−) interactions, new stereo-complex nucleation centers emerge, followed by crystallization. This process establishes a new crystalline to amorphous ratio, resulting in enhanced functional properties, higher melting temperature, improved thermal and mechanical resistance and superior solvent resistance, which are highly desirable for sustainable applications [12,13,14,15]. The mechanism of stereo-complexation is driven by the interactions between the methylene hydrogen and the carbonyl oxygen (CH₃…O=C) of the PLLA–PDLA chains, which form a stereo-complex with distinct FTIR absorptions characteristics. The durability of polymeric materials is a benchmark of its quality [16,17], defined as its capacity to withstand physical and/or chemical degradation throughout its life-time [18,19,20] without compromising its structural and morphological integrity or functional performance [4]. Physical aging characterizes polymer systems in a non-equilibrium thermodynamic state [21]. This occurs either through the relaxation of macromolecules within amorphous regions towards lower energy and higher thermodynamic stability [21], or through a secondary crystallization. The latter is initiated by the crystal defects that destabilize the lattice, leading to the formation of smaller, more energetically stable crystals [22,23]. Because physical aging alters material properties (density, crystallinity, enthalpy, T_g [24], brittleness, rigidity [25], ductility, mechanical properties, and brittleness [26,27,28,29]), these polymeric materials often fail to meet the requirements for long-term applications [21,22]. Furthermore, physical aging can accelerate chemical degradation [21,22], leading to the premature failure of plastic products before the end of their intended service life-time [30]. Consequently, thermodynamic stability is governed by the energy state of the macromolecular chains or crystals and their inherent drive towards a minimum energy state.

To be suitable for 3D printing, for long-term use, new polymer materials must exhibit both specific melt flow properties and high durability. This study investigates the long-term thermodynamic stability of controlled-nucleated PLA utilizing heterogeneous and racemic crystallization techniques designed for 3D-printed, durable automotive components. The research evaluates formulations with proven good mechanical behavior, after 4 years of storage, under controlled laboratory conditions, and assesses the viability of methods for upgrading PLA and future estimation of the life-time of the automotive components 3D printed from the obtained bio-hybrids.

2. Materials and Methods

2.1. Materials

The used materials were the following: PLA D850 (stereo-complexed, SCXD): M_w = 4.5 × 10⁴ g/mol; D-lactide content: 0.5%, MFR: 7–9 g/10 min, T_m = 165–180 °C; density: 1.24 g/cm³, HDT 78 °C (S1_PLA D850), PLA 3052 (stereo-complexant, SCXT): M_w = 11.6 × 10⁴ g/mol; D-lactide content: 4%, MFR: 14 g/10 min, T_m = 145–160 °C; density: 1.24 g/cm³, HDT 55 °C, (S1_PLA D3052) and a nucleating agent (potassium 3,5-bis (methoxy carbonyl) benzene sulfonate-LAK 301, Teknor Apex, Pawtucket, RI, USA (S1_Lak 301)).

2.2. Procedure

To achieve a heterogeneous crystallization, PLA was nucleated using potassium 3,5-bis(methoxycarbonyl)benzenesulfonate as a nucleating agent (LAK 301). For racemic crystallization, two PLA enantiomers with different molecular weights and D-isomer sequences were melt compounded. Prior to processing, the PLAs were dried according to industry standards. To simulate storage conditions, the resulting bio-hybrids were kept under laboratory conditions at 22 +/−2 °C. These new blends underwent morpho-functional characterization, both immediately after melt compounding and following a 4-year storage period. Characterization was performed only on blends that passed qualitative mechanical tests after 4 years. The 4-year period was established as the time when blends with similar compositions did not pass these tests.

Unlike metals, polymeric materials undergo chemical and morphological changes over time which, in their early stages, do not affect mechanical properties. It is widely accepted that the most effective way to highlight physical aging in its early stages is thermal behavior analysis—specifically by monitoring T_g and T_m values—in conjunction with chemical structure characterization. That is why the formulations exhibiting poor mechanical behavior (i.e., those that failed) were excluded from this study. This analysis focuses exclusively on variants that did not break after 4 years.

Characterization was performed using FTIR analysis, considering thermal behavior and some functional properties such as shock resistance and heat deflection temperature (HDT). Thermodynamically stable compounds with consistent properties were selected for scale-up (including compounding, filament formation and 3D printing).

2.3. Compounding

PLAs were dried at 80 °C, in vacuum ovens, for 4 h. Blending ratio: 100 [p] PLA, (1–7) [p] nucleator for heterogeneous nucleation and 100 [p] SCXD, (1–20) [p] SCXT. Melt compounding was carried out, under classical conditions, in a laboratory Brabender (190–200 °C, 100 rpm). The compounds thus obtained were shaped, on a roller, as sheets of 0.5 ± 0.05 mm thickness (50–75 °C, 25 rpm1, 25 rpm2). These sheets were subsequently pressed into specimens for characterization (210–220 °C plate temperature, 5–7 preheating time, 10–15 min; time under pressure, 15–20 min; cooling time, 200 pressure, plate dimensions 80 × 40 × 4 mm³).

2.4. Characterization

2.4.1. Chemical Structure

The chemical structure was studied using FTIR-ATR (Spectrum 100, Perkin Elmer, Shelton, CT, USA) equipped with a diamond, from 4000 cm⁻¹ to 600 cm⁻¹, with 32 scans/bio-hybrid, and considering the future crystallinity study on 3 specimens for each selected bio-hybrid.

The analysis of the FTIR spectra did not refer to the molecular origin (or primary chemical assignment) of the characteristic absorptions of each peak [4]. The FTIR spectra of heterogeneous nucleated PLA were analyzed to highlight how the nucleant influences local molecular motion in the overall morphology. While the nucleant increases the crystallization rate, the density of nucleation centers total crystallinity, and does not alter the type of crystal structure or the absorption wavelengths of the functional groups. Only the peak intensities have changed because of nucleation. This study focused specially on the crystallinity indicator of the PLA-Lak 301 system, such as the intensity and width of the 1083 cm⁻¹ band by the intensity and/or specific width. Further peaks were examined to describe the morphological alterations resulting from the incorporation of the nucleating agent. Additionally, interactions between PLA and the nucleator’s sulfonate groups (R−S(=O)₂−O⁻) were observed, often appearing as peaks of lower intensity, with overlaps or subtle shifts of less than 10 cm⁻¹. The characteristic absorption bands of this nucleator, typically in the lower wavenumber region (500–650 cm⁻¹), are often attributed to SO₃⁻ group deformations or S-C stretching vibrations. Stereo-complexation is evidenced by shifting towards lower frequencies in the stretching vibration of the methylene and carboxyl groups [31]. In the case of PLA, observed changes relate to the crystalline–amorphous ratio and the formation of new crystals. This may be of the same polymorphs found in neat PLA (δ, α, α′, β, γ) or smaller crystals, resulting from stereo-complexations [32]. The FTIR spectra of the pure enantiomers, PLLA and PDLA, are identical due to their chemical bonds and functional groups. Their blends exhibit distinct stereo-complexed (sc) crystals (sc-PLA) that differ from the homo-crystals of the pure components [31]. These crystalline structures cause shifts and/or notable reductions in the intensity of functional group-bands compared to pure enantiomers, depending on the degree of crystallinity and the new crystal types. The impact of the stereo-complex crystallites is particularly prominent in the fingerprint region, between 1000 and 800 cm⁻¹.

2.4.2. Thermal Properties

Differential Scanning Calorimetry (DSC) (DSC3, Mettler Toledo, Greifensee, Switzerland) was performed using a procedure to eliminate thermal history, applied on 2 specimens from each selected bio-hybrid. The samples were subjected to first heating from 20 °C to 200 °C (10 °C/min), cooling from 200 °C to 20 °C (2 °C/min), second heating from 20 °C to 250 °C (10 °C/min), with a 2 min isotherm between runs. DSC analysis was focused on the positive temperature range relevant to the future 3D printing of selected compounds for automative items. The following thermal values were measured: glass transition (T_g), melting (T_m) and crystallization (T_c) temperatures, melting and crystallization enthalpy, and crystallinity (X) (1), where ∆Hm is melting enthalpy, ∆H_cc is cold crystallization enthalpy, ∆H°_m is the melting enthalpy of a 100% crystalline PLA (93.1 J/g), and wPLA-PLA is the weight fraction in the bio-hybrid [33,34].

$$\mathrm{\%}\mathrm{X}={\displaystyle \frac{\mathsf{\Delta }{\mathrm{H}}_{\mathrm{m}}-\mathsf{\Delta }{\mathrm{H}}_{\mathrm{c}\mathrm{c}}}{\mathsf{\Delta }{{\mathrm{H}}^{°}}_{\mathrm{m}}·\mathrm{w}\mathrm{P}\mathrm{L}\mathrm{A}}}·100.$$

(1)

2.4.3. Functional Properties

Impact resistance was measured (HIT 5.5P pendulum impact tester, ZwickRoell, Ulm, Germany), ISO 180/2019, on 5 specimens for each selected bio-hybrid [35]. Specimens measuring 80 × 10 × 4 mm³ (length × width × thickness) were compression molded under the specified conditions for impact strength testing.

Heat deflection temperature was measured using (HDT) (DMA Q800, TA Instruments, Leatherhead, UK) equipment with a three-point bending clamp on specimens of 50.00 × 12.23 × 3.00 mm³, heated from 20 °C to 175 °C (2 °C/min), with 2 measurements taken for each selected bio-hybrid, ISO 75 [36].

2.4.4. Thermodynamic Stability

The variation of chemical structure (FTIR analysis), thermal behavior and mechanical properties were studied on rolled sheets crystallized at temperatures below 100 °C by comparing the initial values with those registered after 4 years. Four years represents the moment of the first breakouts of bio-hybrids with similar formulations.

To identify mechanical properties, the impact (hitting the bent material with 5 kg weight) and flexural (successive folding and unfolding materials 50 times at room temperature) behavior of the bio-hybrids with similar formulations were controlled from time to time (qualitative durability assessment of plastic parts in accordance with Renault Technologie Roumanie). The first fractures in the control samples were observed after 4 years. Only those formulations meeting these criteria were then selected for in-depth analysis.

3. Results

3.1. Heterogeneous Nucleated PLA

3.1.1. Structural Changes

Figure 1 compares the FTIR spectra of Lak 301-nucleated PLA both initially and after 4 years with the corresponding processed data provided in

Table 1. Spectral changes of “heterogeneous” nucleated PLA with nucleating agents, initially and after 4 years.

Peak Changes Type	Comp.	Wavelength, cm−1; Peak Height (H), a.u.	Peaks No.
Disappeared	PLA 850	2997 cm−1; 1453 cm−1; 1383 cm−1; 1358 cm−1; 1303 cm−1; 1265 cm−1.	6
	LAK 301	3071 cm−1; 2958 cm−1; 2905 cm−1; 2852 cm−1; 1722 cm−1; 1602 cm−1; 1442 cm−1; 1428 cm−1; 1323 cm−1; 1220 cm−1; 1117 cm−1; 1098 cm−1; 989 cm−1; 963 cm−1; 917 cm−1; 888 cm−1; 852 cm−1; 782 cm−1; 764 cm−1; 720 cm−1.	20
Appeared		2884 cm−1; 802 cm−1; 645 cm−1.	3
Shifted by more than 10 cm−1	PLA 850	-	0
	LAK 301	1734 cm−1 14 cm−1, H < 26%; 1197 cm−1 16 cm−1, H < 59%; 933 cm−1 22 cm−1, H < 93%;	3
Identical or shifted with less than 10 cm−1 and their H variation compared to	PLA 850	2947 cm−1 2 cm−1, H < 48%; 2919 cm−1 6 cm−1, H < 48%; 2851 cm−1 7 cm−1, H < 64%; 1749 cm−1 1 cm−1, H < 58%; 1207 cm−1 2 cm−1, H < 48%; 1180 cm−1 1 cm−1, H < 52%; 1128 cm−1 4 cm−1, H < 55%; 1083 cm−1, H < 62%; 1042 cm−1 4 cm−1, H < 62%; 956 cm−1 1 cm−1, H < 75%; 920 cm−1, H < 50%; 870 cm−1 1 cm−1, H < 40%; 756 cm−1 1 cm−1, H < 33%; 694 cm−1 8 cm−1, H < 31%.	14
	LAK 301	1210 cm−1 1 cm−1, H < 71%; 1139 cm−1 7 cm−1, H < 55%; 1048 cm−1 2 cm−1, H < 58%; 875 cm−1 4 cm−1, H < 40%; 751 cm−1 4 cm−1, H < 75%; 676 cm−1 10 cm−1, H < 68%; 623 cm−1 4 cm−1, H < 95%.	7

, Tables S2.1 and S2.2 [37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55].

The 955 cm⁻¹ skeletal band, associated with C-O stretching or CH₃ rocking, indicates the existence of an amorphous phase. Hetero-nucleation caused a 75% decrease in its intensity and 5 cm⁻¹ broadening, suggesting the amorphous phase was involved in the hetero-nucleation process [44]. The band at 921 cm⁻¹ serves as the most sensitive indicator in the crystalline phase formation, particularly for the α-form. A 20% increase in the intensity of this band confirms hetero-nucleation-induced crystallinity. Furthermore, the 921/955 cm⁻¹ intensity ratio, which reached a value of 17, proves the increase in the crystallinity of the analyzed hetero-nucleated PLA, which must be validated against DSC and Wide-Angle X-ray Diffraction (WAXD) data [48]. Attributed to the interaction between the PLA ester carbonyls and the nucleating agent’s sulfonate groups, the band at 1750 cm⁻¹ exhibited a broadening of 20 cm⁻¹. This broadening serves as evidence of the crystalline ordering of PLA chains into the α -phase [43]. The band at 1083 cm⁻¹ reflects the reorganization of PLA chains from an amorphous state to the α-crystalline form, driven by dipole–dipole interactions between carbonyl groups and backbone reorientation [47]. The 28% decrease in the intensity of the 871 cm⁻¹ band indicates that the process yielded α’ crystals rather than α crystals, as crystallization was conducted below 100 °C [49]. The negligible intensity of the absorption bands at 1600 and 1400 cm⁻¹ indicates that the sulfonate derivative, despite its low concentration, is homogeneously dispersed within the PLA matrix. The absorptions in the 650–500 cm⁻¹, attributed to the deformation of the aromatic structure and the sulphonate groups within the nucleating agent, disappear over 4 years. The minimal changes within the 3000 cm⁻¹–2900 cm⁻¹ region reflects a limited rearrangement into the α-specific-helical structure, most probably due to the development of α’ crystals during the process. Over time, this morphology remains thermodynamically stable, showing signs of amorphous content redistribution and a slight modification in crystallinity. The results indicate that LAK 301 acts as a hetero-nucleating agent for PLA, inducing the growth of α and α’ crystals that sometimes arrange themselves radially within spherulites. The formation of α’ crystals can be eliminated or reduced by ensuring primary crystallization occurs at temperatures above 110–120 °C, and/or through subsequent post-processing thermal treatments.

After 4 years, the peak intensity increased by 1% to 200%, suggesting a slight progression of the nucleation. Future studies will aim to identify the specific causes of these spectral variations. Crystallinity value after 4 years increased from 42.9% to 43.7% for pure PLA and from 65% to 67% for the nucleated PLA.

3.1.2. Thermal Behavior

The initial thermograms, shown in Figure 2 and Figure 3 and

Table 2. Thermal behavior of neat PLA D850, initially and after 4 years (glass transition; melting—second heating).

Analysis Time, Years	Glass Transition		Melting (Endo)
	Tg, °C	ΔTg4-i, °C	TMax.1/Max.2, °C	ΔTm4-i, °C	ΔHm, J·g−1	ΔHm4-i, J·g−1	Range, °C	Range4-i, °C
Initial	65.7	-	170/176.8	-	39.9	-	149–185	36
After 4 years	64.3	1.4↓	168.5/176.4	1.5↓/0.4~	40.7	0.8~	149–185	36~

~—approx. equal; ↓—decrease; Δ—variation (of T_g/T_m/H_m between 4 years and after obtaining); Max.—maximum.

,

Table 3. Crystallization behavior of PLA D850, initially and after 4 years (recording on the thermogram between first and second heating).

Analysis Time, Years	Crystallization (exo)
	Tc, °C	ΔTc4-i, °C	ΔHc, J·g−1	ΔHc4-i, J·g−1	Range, °C	Range4-i, °C	C, %	ΔC4-i, %
Initial	113.5	-	36.2	-	121–105	16	42.9	-
After 4 years	112.7	0.8~	35.8	0.4~	121–105	16~	43.7	0.8~

~—approx. equal; Δ—variation (of Tc/Hc/C between 4 years and after obtaining); C—crystallinity.

,

Table 4. Thermal behavior of heterogeneous nucleated PLA, initially and after 4 years (glass transition; melting—second heating).

Analysis Time, Years	Glass Transition		Melting (endo)
	Tg, °C	ΔTg4-i, °C	Tm, °C	ΔTm4-i, °C	ΔHm, J·g−1	ΔHm4-i, J·g−1	Range, °C	Range4-i., °C
Initial	63.4	-	175.7	-	60.5	-	137–186	49
After 4 years	62.9	0.5~	175.1	0.6~	62.3	1.8↑	137–186	49~

~—approx. equal; ↑—increase; Δ—variation (of T_g/T_m/H_m between 4 years and after obtaining).

and

Table 5. Crystallization behavior of heterogeneous nucleated PLA, initially and after 4 years (recording on the thermogram between first and second heating).

Analysis Time, Years	Crystallization (exo)
	Tc, °C	ΔTc4-i, °C	ΔHc, J·g−1	ΔHc4-i, J·g−1	Range, °C	Range4-i., °C	C, %	ΔC4-i., %
Initial	138.8	-	55	-	157–115	42	65	-
After 4 years	139	0.2~	56.1	1.1↑	157–115	42~	67	2↑

~—approx. equal; ↑—increase; Δ—variation (of T_c/H_c/C between 4 years and after obtaining); C—crystallinity.

, reveal significant differences in thermal behavior between PLA 850 and the heterogeneously nucleated PLA.

For the heterogeneous nucleated polymer, the glass transition (T_g) occurs at 63.4 °C lower than the 65.7 °C observed for the neat PLA. The nucleated PLA displays a mono-modal melting peak at 175.7 °C, with melting occurring around the 49 °C range (137–186 °C). This process requires 60.5 J/g of energy, resulting in a crystallinity of 65%. These results indicate that nucleation significantly alters the thermal characteristics of neat PLA. Neat PLA exhibits bi-modal melting between 149–185 °C, with two distinct peaks at 170 °C and 176.8 °C. Neat PLA requires only 39.9 J/g for melting, i.e., 20.6 J/g less than nucleated PLA, with a lower crystallinity of 42.9%. The crystallization of heterogeneously nucleated PLA occurs between 157–115 °C, releasing 55 J/g. This represents a significant modification in crystallization behavior compared to neat PLA, which crystallizes at lower temperatures (121–105 °C, peak at 113.5 °C) and releases only 36.2 J/g due to its lower overall crystallinity. After 4 years, neat PLA exhibits a glass transition 1.4 °C lower than its initial state. However, its melting behavior remains bi-modal and occurs within the same temperature range with nearly identical enthalpy, while its crystallization characteristics remain unchanged. Heterogeneously nucleated PLA behaves similarly to the initial samples, showing only a minor 2% increase in crystallinity. This may be attributed to the behavior of neat PLA, which itself showed a negligible increase of 0.8%. Additionally, the T_g of neat PLA decreases by 1.4% after 4 years. Given the lack of significant shifts in the thermal behavior of either sample (heterogeneously nucleated PLA and neat PLA), it can be stated that the above results demonstrate that heterogeneously nucleated PLA remains thermodynamically stable over time. Nucleation initially boosted crystallinity from 42.9% (neat PLA) to 65% (nucleated polymer), an increase of 22.1%. After 4 years, crystallinity in the nucleated PLA rose slightly to 67%. This was driven not only by ongoing crystallization, but also by the baseline increase in the neat PLA’s crystallinity, after 4 years, which rose from 42.95 to 43.7%.

3.1.3. Functional Properties

Compared to neat PLA, the new PLA morphology significantly improves performance, increasing the HDT (97 °C from 66 °C) and impact resistance to 1.73 kJ/m² from 0.34 kJ/m² (

Table 6. Functional properties of heterogeneous nucleated PLA.

Sample\Properties	Functional Property
	Izod Impact Resistance, kJ/m2	HDT, °C
Neat PLA	0.34 ± 0.2	66
Heterogeneously nucleated PLA	1.73 ± 0.1	97

).

3.2. Racemic Nucleated PLA

3.2.1. Chemical Structure

Figure 4 and

Table 7. Spectral changes of racemic nucleated PLA, initially and after 4 years.

Peak Changes Type	PLA	Wavelength, cm−1; Peak Bio-Hybrids Height (H), a.u.	Peaks No.
Disappeared	SCXD	2997 cm−1; 2947 cm−1, 2919 cm−1; 2851 cm−1; 956 cm−1; 920 cm−1; 694 cm−1	7
	SCXT	2995 cm−1; 2945 cm−1; 2928 cm−1; 2900 cm−1; 2880 cm−1; 2851 cm−1; 955 cm−1; 700 cm−1	8
Appeared		3508 cm−1; 1293 cm−1; 845 cm−1; 801 cm−1; 740 cm−1	5
Shifted by more than 10 cm−1	SCXD	-	0
	SCXT	-	0
Identical or shifted with less than 10 cm−1 and their H variation compared to	SCXD	1749 cm−1 2 cm−1, H < 7%; 1453 cm−1 1 cm−1, H < 23%; 1383 cm−1, H < 27%; 1358 cm−1, H < 8%; 1303 cm−11 cm−1, H < 56%; 1265 cm−1, 1 cm−1, H < 33%; 1207 cm−1 H < 4%; 1180 cm−1 2 cm−1, H < 57%; 1128 cm−1 2 cm−1, H < 13%; 1083 cm−1, H < 15%; 1042 cm−1 1 cm−1, H < 22%; 870 cm−1 1 cm−1, H < 10%; 756 cm−1 1 cm−1, H < 8%.	13
	SCXT	1747 cm−1, H < 28%; 1452 cm−1 2 cm−1, H < 29%; 1381 cm−1 2 cm−1, H < 38%; 1359 cm−1 1 cm−1, H < 8%; 1310 cm−1 6 cm−1, H < 50%; 1266 cm−1; H < 67%; 1210 cm−1 3 cm−1, H < 11%; 1182 cm−1, H < 65%; 1127 cm−1 3 cm−1, H < 18%; 1083 cm−1, H < 35%; 1042 cm−1 1 cm−1, H < 28%; 867 cm−1 4 cm−1, H < 10%; 755 cm−1, H ≈ 0.	13

, Tables S2.3 and S2.4 present the FTIR spectra of racemic crystallized PLA, both in its initial state and after 4 years [56,57,58,59,60,61,62,63,64,65,66,67,68]. The racemic nucleation of poly(L-lactic acid) (PLLA) with poly(D-lactic acid) (PDLA) relies on the formation of stereo-complexes (sc-PLA) through selective interactions between the –CH₃ and O=C groups of the two enantiomers. This process triggers rapid crystallization, leading to a semi-crystalline morphology containing numerous small crystallites that enhance functional properties. The FTIR spectral shifts associated with racemic crystallization involve changes in the carbonyl region (1760–1740 cm⁻¹), the methyl groups (2995 cm⁻¹ and 1350–1250 cm⁻¹), the backbone, and the fingerprint region (1128–1306 cm⁻¹ for C-O-C stretching and 950–850 cm⁻¹ for C-C backbone stretching. Analysis of the obtained results (Figure 4, Table 7, Tables S2.3 and S2.4) indicates changes that prove stereo-complexation. The band at 2995 cm⁻¹ is attributed to the asymmetric stretching vibration of the methyl group (-CH3) with PLA. A 22% reduction in its intensity indicates that stereo-complexation has occurred [68]. The shift of the band from 1749 cm⁻¹ to a lower wavelength, to 1747 cm⁻¹, indicates the formation of the crystalline stereo-complex. This change is attributed to the denser crystalline packing and the formation of intermolecular hydrogen bonds C-H…O=C between the PLLA and PDLA chains. The presence of this band serves as evidence of the structural changes from amorphous towards an ordered stereo-complex arrangement. Subsequent thermal annealing at controlled temperature has been shown to further enhance the intensity of this peak [57]. The band at 1350 cm⁻¹ is attributed to the asymmetric deformation of the -CH₃ group, C-H bending, and C-O-C stretching, serving as a key indicator of macromolecular chain ordering. The obtained results show a 10% shift in the intensity of this peak, suggesting a structural evolution from a disordered amorphous state to ordered racemic nuclei and a chain transition to a 3₁-helical conformation, distinct from the 10₃-helical structure typical of homo-crystals [60]. The 1306 cm⁻¹ FTIR band also shifted by 7 cm⁻¹, indicating the development of the helical conformation specific to stereo-complexation following the intermolecular hydrogen bonding (CH3…O=C) that drives the racemic nucleation [61]. The absorption band at 1265 cm⁻¹, attributed to the combined vibrations of CH and C-O-C stretching from the polymer backbone, is diminished by 10% compared to the neat PLA. Typically, the intensity decreasing indicates the microstructure achieved a higher degree order [63]. The band at 1128 cm⁻¹ is 2 cm⁻¹ shifted toward lower wavenumbers and is smaller by about 10%. This is attributed to the C-O-C stretching vibration of the ester groups in PLA and its presence in the spectra, confirming the selective interactions between L- and D-enantiomers [37,56]. Because the spectral range widens by 5 cm⁻¹ and the intensity increases by 15%, the band at 870 cm⁻¹ assigned to the C-C skeletal vibration, coupled with CH3 methyl group rotation, indicates racemic crystallization [68]. The results obtained reveal changes in all spectral ranges, indicating stereo-complexation, namely in the areas that described both the carbonyl, methyl groups and vibrations of the backbone and fingerprint. These spectral changes persist over time, though the peak intensities decrease by 19% to 50%, indicating that the analyzed morphology is thermodynamically unstable. The efficiency of racemic nucleation depends on several factors: the molar ratio between PLLA and PDLA, the molecular mass of both types of PLA, the thickness of the sample analyzed, and the post-processing treatments.

3.2.2. Thermal Behavior

Analysis of the initial thermograms for the racemic crystallized PLA and of the stereo-complexed PLA and stereo-complexant PLA (Figure 2, Figure 5 and Figure 6 and Table 2, Table 3,

Table 8. Thermal behavior of neat PLA 3052, initially and after 4 years (glass transition; melting—second heating) [4].

Analysis Time, Years	Glass Transition		Melting (Endo)
	Tg, °C	ΔTg4-i, °C	Tm, °C	ΔTm4-i, °C	ΔHm, J·g−1	ΔHm4-i, J·g−1	Range, °C	Range4-i, °C
Initial	60.9	-	153.2	-	0.77	-	145–165	20
After 4 years	66↑	5.1	156/169	2.8↑	33.1	32.33↑	148–188	40→

↑—increase; →—displacement towards right Δ—variation (of T_g/T_m/H_m between 4 years and after obtaining).

,

Table 9. Cold crystallization and crystallization of PLA 3052, initially and after 4 years.

Analysis Time, Years	DSC Runs	Cold Crystallization (exo)						Crystallization (exo)
		Tcc, °C	ΔTcc4-i, °C	ΔHcc, J·g−1	ΔHcc4-i, J·g−1	Range, °C	Range4-i, °C	Tc, °C	ΔTc4-i, °C	ΔHc, J·g−1	ΔHc4-i, J·g−1	Range, °C	Range4-i, °C	C, %	ΔC4-i, %
Initial	Heating 2	127	-	0.7	-	101–145	44	-	-	-	-	-	-	-	-
	Cooling 1	-	-	-	-	-	-	-	-	-	-	-	-	0.08	-
After 4 years	Heating 2	113	14↓	30.6	29.9↑	101–131	30←	-	-	-	-	-	-	-	-
	Cooling 1	-	-	-	-	-	-	123	123	1.98	1.98	113–128	15	2.7	2.62↑

↑—increase; ↓—decrease; ←—displacement toward left; Δ—variation (of T_cc/H_cc/T_c/H_c/C between 4 years and after obtaining); C—crystallinity.

,

Table 10. Thermal behavior of racemic nucleated PLA, initially and after 4 years (glass transition; melting—second heating).

Analysis Time, Years	Glass Transition		Melting (Endo)
	Tg, °C	ΔTg4-i, °C	TMax.1/Max.2, °C	ΔTm4-i, °C	ΔHm, J·g−1	ΔHm4-i, J·g−1	Range, °C	Range4-i, °C
Initial	64.4	-	172/-	-	48.8	-	136–190	54
After 4 years	64.6	0.2~	170.1/176.9	1.9↓/-	46.3	2.5↓	136–190	54~

~—approx. equal; ↓—decrease; Δ—variation (of T_g/T_m/H_m between 4 years and after obtaining); Max.—maximum.

and

Table 11. Crystallization behavior of racemic nucleated PLA, initially and after 4 years (recording on the thermogram between first and second heating).

Analysis Time, Years	Crystallization (exo)
	Tc, °C	ΔTc4-i, °C	ΔHc, J·g−1	ΔHc4-i, J·g−1	Range, °C	Range4-i, °C	C, %	ΔC4-i, %
Initial	120.4	-	43	-	130–113	17	52.5	-
After 4 years	114.8	5.6↓	40	4↓	124–106	18←	49.8	2.7↓

↓—decrease; ←—displacement towards left; Δ—variation (of T_c/H_c/C between 4 years and after obtaining); C—crystallinity.

) reveals fundamental differences in the thermal behavior of PLA 850 SCXD and the racemic nucleated PLA. Racemic nucleated PLA exhibits mono-modal melting and no cold crystallization, despite being formed by the stereo-complexation of a PLA with bi-modal melting (Figure 5) with a PLA with mono-modal melting and cold crystallization (Figure 6). The T_g of the racemic nucleated PLA is 64.4 °C, compared to 65.7 °C for PLA 850 and 60.9 °C for PLA 3052. Although its melting profile is mono-modal, melting occurs over a broad temperature range of 54 °C (from 136 °C to 190 °C), peaking at 172 °C with an enthalpy of fusion of 48.8 J/g. This intake is higher than that of stereo-complexed PLA (neat PLA 850 SCXD), which requires 39.9 J/g or stereo-complexed PLA (PLA 3052 SCXT), which needs 0.77 J/g. This occurs because, following the stereo-complexation, crystallinity increased to 52.5 J/g. The crystallization of racemic nucleated PLA takes place over a 17 °C range, peaking at 120.4 °C, with a heat release of 43 J/g. The explanation lies in the higher crystallinity of racemic nucleated PLA, of 52.5% compared to 42.9% for the stereocomplexed PLA and 0.68% for the stereo-complexant PLA. The racemic crystallization homogenized the thermal behavior of PLA 850 SCXD—a significant advantage for 3D printing. Furthermore, this process converted the bi-modal melting of the stereo-complexant into a mono-modal peak. Unlike the stereo-complexant PLA 3052, racemic crystalized PLA does not exhibit cold crystallization or multiple crystallization peaks between 193 °C and 66 °C.

While the broad melting and crystallization ranges of racemic nucleated PLA can be refined through stricter formulation control, the racemic nucleated PLA will pass the durability test. After 4 years, the thermal behavior of racemic nucleated PLA shows strong thermodynamic instability mainly because the mono-modal melt was converted into a bi-modal one, accompanied by a 2.7% and a 2.5 J/g reduction in the melting enthalpy. Bi-modal melting is incompatible with 3D printing. These findings suggest the racemic morphology contains internal stress, likely caused by crystal defects, as confirmed by FTIR analysis. This progression toward thermodynamic equilibrium triggered physical aging, ultimately driving the shift from a mono-modal to a bi-modal melting profile.

3.2.3. Functional Properties

The improvement in functional properties due to racemic nucleation is substantiated by the results in

Table 12. Functional properties of racemic nucleated PLA.

Sample\Properties	Functional Property
	Izod Impact Resistance	HDT
Racemic nucleated PLA	2.36 ± 0.3 kJ/m2	95 °C
PLA 850 SCXD	0.56 ± 0.1 kJ/m2	78 °C
PLA 3052 SCXT	16 ± 0.15 J/m	55 °C

.

4. Discussion

As indicated by the FTIR spectra, the morphological structure that appeared in the heterogeneous crystallization contains defects including nucleator residues in the matrix; however, crystallinity increased from 42% to 65% through the appearance of new α’-type crystals. This morphological change is accompanied by the transformation of the bi-modal melting of neat PLA (Figure 2) into the mono-modal melting of nucleated PLA (Figure 3), which is associated with a considerable increase in impact strength and HDT (Table 6). During the 4 years, this morphology does not change and therefore the thermal behavior differs insignificantly from the initial one. This means that the crystallization defects or those found in the amorphous phase have not induced sufficient energy imbalances to generate structural changes during transition to less energetically charged states, either through secondary recrystallization processes in the crystalline phase or macromolecular relaxation in the amorphous one. During the 4 years, however, a slight continuation of the nucleation did occur, as proven by the enthalpy variations of the thermal transitions of heterogeneous nucleated PLA (Table 4). The 2% crystallinity increase causes a 1.8 J/g higher heat need for melting and a 1.1 J/g higher heat released by crystallization. These structural changes did not affect the mechanical behavior since, after 4 years, the analyzed samples did not mechanically deteriorate (Table S3.1). The thermodynamic stability of heterogeneous nucleated PLA demonstrates that heterogeneous crystallization with a nucleating agent is a method that can be considered for increasing PLA durability, especially since it was even possible to cancel the bi-modal melting of nucleated PLA and its instability over time (Figure 2, Table 2). The type of PLA obtained by heterogeneous nucleation is suitable for 3D printing for durable applications, firstly due to its mono-modal melting and crystallization in a narrow and high range and, secondly, due to its thermodynamic stability over time. The slightly wider melting range of heterogeneous nucleated PLA can be adjusted by tighter control of the formulation and crystallization conditions.

In the case of racemic crystallization, from the stereo-complexed PLA with bi-modal melting (Figure 2) and a stereo-complexant PLA with a mono-modal melt and cold crystallization (Figure 6), it was possible to obtain a nucleated PLA with mono-modal melting (Figure 5). Spectacular changes also occur in the case of crystallization, which is uniform and in a narrow range in the case of the stereo-complexed PLA (Figure 2), uniform and in a wide range (between 190–66) for stereo-complexing PLA (Figure 6), and uniform and in a relatively narrow range for racemic crystallized PLA (Figure 5). Racemic crystallization even cancels the cold crystallization of the stereo-complexing PLA, and it is known that cold crystallization is dangerous for its potential for thermodynamic imbalance [69]. By racemic crystallization, a crystallinity of 52.5% was reached (Table 11), starting from two types of PLA, one with 42.9% (Table 3) and the other with 0.08% (Table 12). This new morphology ensured functional properties of applicative interest (Table 12) and good mechanical behavior even after 4 years (Table S3.1). According to the FTIR results, the racemic crystals of stereo-complexed PLA also contain defects which, as is known, are the source of thermodynamic destabilization [4,70,71]. The data obtained show that it is indeed thermodynamically unstable, since, after 4 years, the mono-modal melt transforms into a bi-modal melt. Therefore, racemic crystallization cannot be used to modify PLA so that it can be used for the 3D printing of parts for durable applications such as those in the automotive industry.

The crystallinity of stereo-complexed PLA depends on several parameters [72]: the compounding ratio of the two enantiomers [73], molecular weight of stereo-complexed PLA [74], the stereo-complexation method (solution or melt compounding) [75,76,77], and the effective blending conditions so as to favor the formation of crystals through stereo-complexes rather than those that occur through homo-crystallization [78]. It was observed that, to improve mechanical strength, thermal stability, and crystallinity, a 50/50 stoichiometric ratio of PLLA/PDLA is critical for maximizing stereo-complexed crystal formation over homo-crystals [79]. Highest crystallinity (up to 90% or more) in Poly (lactic acid) (PLA) stereo-complexation is achieved by blending equal ratios (50/50) of high molecular weight Poly (L-lactic acid) (PLLA) and Poly (D-lactic acid) (PDLA), often using solution casting with solvents like chloroform/methanol or high-pressure [80]. When blended at this ratio, the PLLA and PDLA chains interact via intermolecular hydrogen bonds and van der Waals forces, leading to a denser chain packing than either homopolymer alone. This blend creates a material with a melting point of approximately 220–230 °C, which is about 50 °C higher than pure PLLA or PDLA (~170–180 °C) [81]. In a case where small amounts of one enantiomer (typically PDLA) are added to a matrix of the other (typically PLLA), an increased crystallization rate and improved mechanical properties is observed, without significantly altering the base melting point of the primary polymer [82].

In the work carried out, a PLA grade with a low molecular weight (M_w of 4.5 × 10⁴ g/mol) was stereo-complexed through a melt-processing procedure, with 6 [p] PLA medium molecular weight (M_w of 11.5 × 10⁴ g/mol). These working options were based on the resulting stereo-complexed PLA being converted into finished product by 3D printing—the fused filament method. A PLA grade with a low molecular weight was employed to achieve the desired melt fluidity for this application. A lower concentration of stereo-complexant was chosen to prioritize mechanical improvement over a significant rise in melting temperature. High melting temperatures are a disadvantage for melt compounding and subsequent 3D printing, as these processes typically require operating temperatures of 20 °C to 30 °C above the material’s melting point. The energy cost would be unjustifiable for applications where thermal stability is not a critical factor. Stereo-complexation in the melt was selected as a scalable process, offering more favorable conditions than a solution-based procedure. Clearly, these racemic nucleation results should be interpreted within the framework of creating PLA for medium-life automotive use. Any compound whose melt changes from mono-modal to bi-modal over time cannot be considered viable for this application. The observed difference in long-term thermodynamic stability between PLA produced via hetero-nucleation versus racemic nucleation is directly linked to the used PLA grade. This specific grade was selected to meet the requirements for 3D printing medium-life automotive components. Consequently, a low-molecular-weight PLA was utilized in both instances for the reasons previously noted; for this specific polymer type, hetero-nucleation remains of interest.

In polylactic acid (PLA), amorphous content generally has a much larger impact on long-term thermodynamic instability than crystal defects; however, the results demonstrated that racemic nucleated PLA is thermodynamically less stable than hetero-nucleated PLA. Both variants of the nucleation and the same grade of low-molecular-weight PLA was used. This difference in thermodynamical stability over time was driven by the specific conditions of the two nucleation processes. In hetero-nucleation, the optimal nucleant concentration produced a high density of crystallization centers. This allowed the short macromolecular chains to organize into uniform, stable crystals with an amorphous content of 35%, both initially and after 4 years. In contrast, racemic crystallization was developed for obtaining specific functional properties rather than maximum crystallinity. The low molecular weight of PLA and other working conditions resulted in a crystallinity of 52.5% initially, which decreased to 49.8% after 4 years. These crystals contained inherent defects and consisted of a mixture of both stereo-complex and homo-crystalline structures.

The melting range of nucleated polylactic acid (PLA) can be narrowed by precisely coordinating the selection and concentration of nucleating agents with isothermal crystallization conditions [83,84,85]. This process effectively sharpens the melting endotherm and improves material reproducibility. Formulation strategies offer several pathways for narrowing the melting range as evidenced in [86,87,88], by selecting the optimal concentration to promote densely packed crystalline lamellae and stable α -phase crystals. Excessive concentration can result in a surplus of small, imperfect crystals, which broadens the melting range. Furthermore, incorporating nucleating agents with plasticizers, such as polyethylene glycol, can widen the crystallization window, leading to a more defined melting peak. The regulation of the melting range via crystallization parameters relates to careful selection of temperature and the rate of crystallization [89,90,91]. For heterogeneous nucleated PLA, setting the crystallization temperature near 110 °C is most effective. At this temperature, the initial crystallization rate is maximized, and the formation of the stable α phase is favored over the disordered α′ phase. Additionally, the basal d-spacing between crystal planes is minimized, resulting in a more perfect and uniform crystal structure. By allowing for sufficient crystal growth, slow cooling (isothermal annealing) increases crystallinity and produces a more distinct melting point. Rapid cooling, however, “freezes” the material in an amorphous or poorly crystallized state, which manifests as a broader and depressed melting peak. Isothermal annealing at 80 °C–110 °C for 30–60 min allows crystallites to reach their thermodynamically favored size. This process uniformizes crystal dimensions, effectively narrowing the melting endotherm.

Higher crystallinity generally enhances structural integrity but reduces flexibility [92] because more energy is required to break the strong, ordered crystalline structure. As crystallinity increases, so do tensile and yield strength and flexural modulus (stiffness), due to tighter molecular packing and stronger inter-chain networks [93,94]. High crystallinity typically leads to increased brittleness and a reduction in elongation at break [95]. Crystalline regions increase surface hardness and elastic modulus, which directly improves scratch and abrasion resistance at both micro and nanoscales [96]. The material can withstand greater straining forces before breaking and material is more resistant to bending. Greater stress is required to cause permanent deformation. Toughness and impact resistance often decrease because amorphous regions, which provide energy absorption, are reduced [97]. Crystalline domains act as physical crosslinks that significantly enhance resistance to molecular rearrangement and, so, mechanical stability and durability significantly delay the onset of creep deformation (the ability to resist gradual deformation under constant stress) [98]. These domains perform like hard filler particles within the softer amorphous matrix, anchoring the network and effectively “locking” the structure in place. These ordered regions tether polymer chains, restricting their mobility and preventing the irreversible slippage that causes material failure [99]. Tighter packing makes it more difficult for gases or liquids to permeate [100]. The tight packing within crystalline lattices reduces polymer chain mobility, preventing them from moving independently or rearranging under load. The above makes evident that increasing crystallinity improves mechanical, thermal, barrier and chemical resistance, sorption and migration. However, it should also be mentioned that greater crystallinity means increased opacity and reduced flexibility and rigidity, and a lowered capability of deforming without fracturing. Under these circumstances, the crystallization must be carefully controlled to achieve the desired outcomes. Evidently, increased crystallinity enhances mechanical and thermal performance, chemical and barrier resistance, sorption and migration properties. However, it also leads to higher opacity, reduced flexibility, and increased brittleness, limiting the material’s ability to deform without fracturing. Under these circumstances, crystallization must be carefully controlled to achieve the desired outcomes, particularly in the context of long-term sustainability. The utility of thermodynamic stability data is intrinsically linked to life-time prediction, leveraging advanced statistical molecular simulations driven by Artificial Intelligence (AI). Furthermore, AI facilitates a deeper understanding of the complex mechanisms underlying physical aging, which results from the inherent tendency of polymers to seek thermodynamic equilibrium at a minimum energy state [101,102]. Service life prediction should account for the onset of the material changes rather than the moment of structural failure. Due to the high durability of the analyzed materials, waiting for ultimate breakage is an impractical method for predictive modeling.

5. Conclusions

• This paper examines the thermodynamic stability of controlled nucleated PLA, focusing exclusively on formulations with good mechanical behavior, after storage for 4 years in controlled conditions. The PLA with 0.5% D lactide and low molecular weight, which has optimal melt flow at 3D printing, was nucleated with a sulfonic acid derivative (heterogeneous nucleation), in a variant, and with PLA with 4% D lactide (racemic nucleation) in another. Because the earliest signs of thermodynamic instability are to do with changes in chemical structure, which alter thermal behavior with no impact on mechanical properties, the study focuses on FTIR and DSC analysis. The obtained results were compared with the initial data.
• The type of PLA obtained by heterogeneous nucleation is suitable for 3D printing for durable applications firstly due to its mono-modal melting and crystallization in a narrow and high range and, secondly, due to its thermodynamic stability over time. The slightly widened melting range of heterogeneous nucleated PLA can be adjusted by strict control of the formulation and crystallization conditions.
• The type of PLA obtained by racemic crystallization is not suitable for the 3D printing of parts for medium-life auto applications because, after 4 years, its initial mono-modal melting converts into bi-modal melting, proving long-term thermodynamic instability, as proven by the FTIR and DSC analysis.
• Increased crystallinity enhances mechanical and thermal performance, chemical and barrier resistance, sorption and migration properties. However, it also leads to higher opacity, reduced flexibility, and increased brittleness, limiting the material’s ability to deform without fracturing. Under these circumstances, the crystallization must be carefully controlled to achieve the desired outcomes, particularly in the context of long-term sustainability.
• The observed difference in long-term thermodynamic stability between PLA produced via hetero-nucleation versus racemic nucleation is directly linked to the used PLA grade. This specific grade was selected to meet the requirements for 3D printing medium-life automotive components. Consequently, for a low-molecular-weight PLA, hetero-nucleation remains of interest.
• In polylactic acid (PLA), amorphous content generally has a much larger impact on long-term thermodynamic instability than crystal defects. In both variants of the nucleation, the same grade of low-molecular-weight PLA was used. This difference in thermodynamical stability over time between the hetero-nucleated PLA and stereo-complexed PLA was driven mainly by the specific conditions of the two nucleation processes.
• The utility of thermodynamic stability data is intrinsically linked to life-time prediction, leveraging advanced statistical molecular simulations driven by Artificial Intelligence (AI). Furthermore, AI facilitates a deeper understanding of the complex mechanisms underlying physical aging.