Unraveling the Crystallization, Mechanical, and Heat Resistance Properties of Poly(butylene adipate-co-terephthalate) Through the Introduction of Stereocomplex Crystallites

Abstract

Poly(butylene adipate-co-terephthalate) (PBAT) is a promising degradable polymer for replacing non-degradable traditional plastics to mitigate pollution. However, its low softening temperature and poor hardness impede its application. Herein, PBAT and stereocomplex polylactide (sc-PLA) blends were fabricated through a melt-blending process to balance the heat resistance and mechanical strength of PBAT in this research. The effects of the PLA content and hot embossing temperature on the blend properties were comprehensively investigated. The results demonstrate that the sc-crystal content in the PBAT/sc-PLA blend increased by 493% as the PLA content rose from 10% to 30%. The blend with 15% PLLA and 15% PDLA, hot embossed at 190 °C, exhibited the highest sc-PLA crystallinity of 23.3% and the largest fraction of sc-crystallites at 66%, leading to the optimal comprehensive performance. Its Vicat softening temperature (VST) reached 92.2 °C, and a nonlinear increase trend in accordance with the power-law model between VST and the mass ratio of sc-crystal was obtained. Compared with the mechanical properties of neat PBAT, a maximum tensile yield stress of 9.7 MPa and a Young’s modulus of 82.5 MPa were achieved and improved approximately by 107% and 361%, respectively. This research offers an effective strategy for synergistically enhancing the heat resistance and mechanical strength of PBAT.

1. Introduction

Polymeric materials have rapidly emerged as some of the most widely used materials in recent decades, significantly contributing to advancements in both human lifestyle and scientific technology. However, challenges such as environmental pollution have become critical obstacles to the continued application of conventional polymeric materials [1]. To address these issues, considerable efforts have been directed toward the development and utilization of biodegradable polymers [2,3]. Over the past decade, bioplastics have garnered substantial attention from both the scientific community and the plastic industry, with a particular focus on biodegradable plastics [4,5,6]. Among these, poly(butylene adipate-co-terephthalate) (PBAT) stands out as a material of significant commercial interest. PBAT is a biodegradable plastic that can potentially be derived from bio-based raw materials [7,8]. It possesses properties comparable to non-biodegradable polymers such as polyethylene (PE), exhibiting a high ductility and low elasticity modulus. However, it should be noted that PBAT’s low heat resistance and limited stiffness remain significant drawbacks, hindering its broader application [9,10].

Polylactic acid (PLA), a fully bio-derived polyester, is characterized by high stiffness and low flexibility, with a Young’s modulus reaching up to 3500 MPa; however, its elongation at break is typically below 5% [8,11]. The incorporation of poly(L-lactic acid) (PLLA) can effectively enhance the stiffness of PBAT. Numerous studies have focused on blending PLA with PBAT to improve the processability and mechanical properties of PLA. For instance, Mohanty et al. reported that adding 25 wt% of PBAT to PLA reduced its brittleness, reflected by an increase in impact strength from 21.1 J·m⁻¹ for PLA to 50 J·m⁻¹ for the PLA/PBAT (75/25) blend; however, the tensile strength decreased [12]. Similarly, Wang et al., who blended PLA and PBAT using solvent-casting methods, produced homogeneous films with enhanced flexibility while maintaining tensile properties, transparency, and water vapor barrier performance [13,14]. Furthermore, Pan and colleagues prepared biodegradable PLA/PBAT blends via melt compounding with a 4,4′-methylene diphenyl diisocyanate (MDI) chain extender. Their findings revealed that the impact strength of the blends was significantly improved through reactive melt processing, and as the MDI content increased, the blends exhibited higher yield tensile strength, modulus, and elongation at break [1].

The thermal stability and resistance to hydrolysis of polymers are significantly influenced by their crystallization kinetics and degree of crystallinity. To optimize these properties [15,16,17], researchers frequently incorporate nucleating agents to accelerate the crystallization process or apply thermal annealing to enhance the overall crystallinity [18,19]. Furthermore, the technique of stereocomplexation has gained recognition as a robust strategy to augment the thermal stability of polymeric matrices [20,21]. In systems where poly (L-lactide) (PLLA) and poly (D-lactide) (PDLA) are present together, the L-lactyl and D-lactyl segments assemble into stereocomplexed crystallites (sc-crystallites) [22,23]. The resultant stereocomplexed PLA (sc-PLA) demonstrates accelerated crystallization kinetics [24,25,26] and a melting temperature (T_m) around 230 °C, which is approximately 50 °C higher than that of its individual components, PLLA or PDLA. This enhancement leads to superior thermal and mechanical performance, as well as improved resistance to hydrolytic degradation [20,21].

However, during the crystallizing of PLLA and PDLA blends, homogeneous crystallites (hc-crystallites) compete with sc-crystallites, leading to the coexistence of both crystal types. To enhance the sc-crystallite content, improving the annealing treatment temperature between T_m of sc-crystallites and sc-crystallites is a normal method [20,21,22].

Stereocomplexes can form in both miscible and immiscible blends under melt or solution conditions [27,28,29,30,31]. Lopez-Rodriguez [32] investigated solution-cast PLLA/PDLA blends, revealing a predominant stereocomplexation when the molecular weight (M_w) was below 10⁵ Da. Their findings indicated that as M_w increased, both sc-crystallites and sc-crystallite structure could coexist. Notably, blends comprising components with the same M_w exhibited greater crystallinity compared to those with unequal M_w. Upon sc-crystallite structure formation, simultaneous enhancements in tensile strength and elongation at break were observed. The authors attributed these improvements to the influence of the mobile amorphous fraction (MAP), which separates inter-lamellar and inter-spherulitic regions, forming an interphase. Specifically, the interconnected high-density chain networks of sc-crystallites mediated by the MAP resulted in tensile strength and elongation improvements of 48% and 206%, respectively, for blends with equal M_w. One gradually realizes the potential of sc-PLA in the development of semi-crystalline materials with enhanced properties for long-term applications. It is deduced that during the process of solvent evaporation, numerous sc-crystallites emerge in the blends. These crystallites serve as intermolecular cross-linkers, which play a facilitating role in augmenting the diverse mechanical properties of matrix materials [33,34,35]. Furthermore, sc-crystallites can also act as nucleation sites to accelerate the crystallization rate and enhance mechanical and heat resistance [36]. Gu [37] incorporated 5% of the reactive compatibilizers, composed of PDLA and PBS, into PLLA/PBS blends; it was found that the elongation at break and notched impact strength reached 273% and 34 kJ/m², respectively, due to the occurrence of interface-localized sc-crystallites. Cheng [38] found that due to the synergistic effect of basalt fibers and sc-PLA crystals, the Vicat softening temperature (VST) reached 155.5 °C and the tensile strength of the annealed samples was 50.2 MPa, twice that of neat PLLA.

A comprehensive review of the existing literature on PLA-based blends reveals that the majority of research efforts have been focused on improving the toughness and heat resistance of the PLA matrix by incorporating sc-PLA. However, few studies investigate the enhancement of PBAT’s performances via sc-PLA as a modifier. In this work, PLLA and PDLA were incorporated into PBAT to prepare PBAT/sc-PLA biodegradable blends through melt blending and hot embossing processes. The effects of varying PLA dispersed phase content and hot embossing temperatures on heat resistance and mechanical performances of blends were analyzed using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), VST measurement, and tensile universal testing, respectively. A simple and effective strategy has been proposed to balance the tensile strength, elongation at break, and thermal resistance of PBAT-based composites, which holds significant potential for expanding the application scope of PBAT materials in biodegradable films, packaging materials, and disposable products.

2. Experimental

2.1. Materials

PBAT (Ecoex® C1200) was supplied by BASF Corporation (Ludwigshafen, Germany). The MFR (melt flow rate) and density of PBAT were 2.7–4.9 g/10 min⁻¹(2.16 kg, 190 °C) and 1.25–1.27 g·cm³, respectively. Poly(L-lactide) (PLLA) (4032D, M_w = 2.07 × 10⁵ g/mol, M_w/M_n = 1.73) was a commercial product of NatureWorks LLC (Plymouth, USA). Poly(D-lactide) (PDLA) (D070, M_w = 1.25 × 10⁵ g/mol, M_w/M_n = 1.68) was purchased in Corbion, the Netherlands. The monomer molecular structures of PBAT, PLLA, and PDLA are shown in Figure 1.

2.2. Preparation of PBAT/sc-PLA Composites

Three pellets of PBAT, PLLA, and PDLA were firstly dried at 70 °C in a vacuum oven for 12 h to remove moisture. Then, the PBAT-based composites were prepared by using a laboratory twin conical screw extruder (Ruiming Experimental Instrument Co. Ltd., Wuhan, China). The melt compounding was conducted at 180 °C and a screw speed of 60 rpm. The neat PBAT was also subjected to the same compounding treatment in order to have the same thermal history as its composites. After mixing, all samples were cut into small pieces and then were hot embossed at different temperatures for 5 min followed by cooling to form the specimens with various thicknesses for characterization of VST and tensile performance. Various mass ratios of PBAT and PLLA/PDLA were selected, as shown in

Table 1. Formulations of PBAT-based composites.

Samples	PBAT (wt.%)	PLLA (wt.%)	PDLA (wt.%)	Hot Embossing Temperature
PBAT	100	0	0	190 °C
10DP	90	5	5	190 °C
20DP	80	10	10	190 °C
30DP	70	15	15	190 °C
L20/D10	70	20	10	-
L15/D15	70	15	15	-
L10/D20	70	10	20	-
30DP-140	70	15	15	140 °C
30DP-230	70	15	15	230 °C

.

For convenience, in the following discussion, pure PBAT and its blends are designated as X dispersed phase (XDP), where X represents the amount of the dispersed phase (PLLA and PDLA) in the blend, with the ratio of PLLA to PDLA being the same within this category. When the ratio of PLLA to PDLA differs, the notation LX/DX is used, where X denotes the content of PLLA and PDLA, respectively.

2.3. Characterization

2.3.1. Differential Scanning Calorimetry (DSC)

The investigation of the melting characteristics and crystallization behavior of PBAT and its composites was performed by a Differential Scanning Calorimeter (DSC Q2000, TA, New Castle, USA) with the Universal Analysis 2000. All tests were carried out under nitrogen atmosphere. All samples’ weights varied between 5 and 10 mg and were sealed in an aluminum pan. In the process of non-isothermal melt crystallization, the samples were firstly heated from 30 °C to 250 °C at a heating rate of 10 °C/min. The fraction of sc-crystallites (f_sc), theoretical value of melting enthalpy for complete crystallization PBAT/sc-PLA composites (${\Delta \mathrm{H}}_{\mathrm{m}}^0$), total crystallinity (χ_c), and crystallinity of sc-crystallites (χ_c-sc) were obtained from the following Equations (1)–(5) [39,40]:

$$f_{sc}={\displaystyle \frac{\Delta H_{m-SC}}{\Delta H_{m-HC}+\Delta H_{m-SC}}}\times 100\%$$

(1)

$$\Delta H_m^0=\Delta H_H^0\times (1-f_{sc})+\Delta H_s^0\times f_{sc}$$

(2)

$${\chi }_c={\displaystyle \frac{\Delta H_{m-HC}+\Delta H_{m-SC}-\Delta H_c}{\Delta H_m^0\times (1-\omega )}}\times 100\%$$

(3)

$${\chi }_{c-SC}={\chi }_c\times f_{sc}$$

(4)

$${\omega }_{sc}={\chi }_{c-SC}\times \omega$$

(5)

where ΔH_c, ΔH_m-sc, and ΔH_m-hc are the enthalpy of cold crystallization, sc-crystallites, and homogeneous crystallites, respectively. Δ$H_m^0$ and $\Delta H_s^0$ are the melting enthalpy values for complete crystallization of PLA (93 J/g) [41] and sc-PLA (142 J/g) [42], respectively. ω and ${\omega }_{sc}$ are the mass ratios of the dispersed phase of PLA and the sc-crystal in PBAT/sc-PLA composites, respectively.

2.3.2. Rheology

A rheometer (DHR-2, TA, New Castle, DE, USA) equipped with parallel plates (ϕ25 mm, gap 1.0 mm) in the dynamic frequency oscillatory shear mode was used to characterize the linear viscoelastic behavior of all the PBAT-based samples. All tests were carried out at 160 °C with frequencies (ω) ranging from 0.03 to 300 rad/s to obtain the storage modulus (G’) and complex viscosity (η*). All experiments were performed under a nitrogen atmosphere to prevent oxidative degradation of the melt. The applied linear strain was 3% to make sure the tests were conducted in the linear viscoelastic region.

2.3.3. Scanning Electron Microscopy (SEM)

The phase morphologies of the fractured surface in PBAT/sc-PLA composites, which were prepared under liquid nitrogen, was observed using a GeminiSEM 300 Field Emission Scanning Electron Microscope (ZEISS, Oberkochen, Germany) at an accelerating voltage of 5 kV. The size of the PLA dispersed phase in the PBAT matrix was obtained using Image-Pro Plus 6.0 software.

2.3.4. Heat Resistance Property

VST of PBAT/sc-PLA composites was determined using a Vicat softening point tester (Farui Instrument Technology Co., Ltd., Shanghai, China) equipped with a flat-ended needle featuring a cross-sectional area of 1 mm². Specimens with dimensions of 10 × 10 × 3 mm were prepared for testing. The experiments were conducted under a constant load of 10 N and a controlled heating rate of 2 °C/min. The VST for each sample was recorded at the point when the needle penetrated the specimen to a depth of 1 mm. To ensure accuracy, the test was repeated three times for each sample, and the average value of these measurements was reported as the final result.

2.3.5. Tensile Property

The tensile specimens were prepared into a dumbbell type with dimensions of 80 mm × 10 mm × 1 mm (length × width thickness) according to the standard ISO 527-93. Then, the tensile properties of all specimens were performed on an Electronic Universal Testing Machine (UTM2203, SANS, Shenzhen, China) at a crosshead speed of 10 mm/min. All tests were repeated five times to obtain the average value of the experimental data.

3. Results and Discussion

3.1. Crystallinity

3.1.1. Stereocomplex-PLA Formation

Figure 2a presents the heating curve of the PBAT/sc-PLA composite at a heating rate of 10 °C/min. It is shown that only one glass transition temperature (T_g) of the PBAT/sc-PLA composite was detected between 50 °C and 70 °C and remains relatively constant with various PLLA/PDLA ratios. This value was closely resembling the T_g of the PLLA. Additionally, the cold crystallization peak was observed between 90 °C and 120 °C for all samples. This indicates that the growth of homogeneous crystallites (hc-crystallites) is not perfect. Moreover, two melting regions appeared between 160–180 °C and 215–230 °C, indicating the immiscible neat PBAT and sc-PLA. When the heating temperature rises to 160–180 °C during the DSC test, the hc-crystallites are destroyed to form amorphous regions and the cold-crystalline peak and few sc-crystallites. When the temperature is higher than 200 °C, some hc-crystallites transform into sc-crystallites. The T_m of sc-crystallites is 50 °C higher than that of hc- crystallites. During the heating process for composites, the melting peak of PBAT that should have appeared near 125 °C was not obvious. There is a competitive relationship between hc-crystallites and sc-crystallites of PLA during the heating process, and the crystallization of hc-crystallites has a competitive advantage over that of sc-crystallites.

Interestingly, by comparing with the single melting peak of the PLLA sample, three other PLLA/PDLA composites exhibit double melting peaks for sc-crystallites, indicating imperfect and different types of hc- crystallites of PLA. Meanwhile, the maximum area of the sc-crystallites melting endotherm is detected for the L15/D15 sample. It is worth noticing from Figure 2b and

Table 2. Enthalpy and crystallinity of PBAT/sc-PLA at different PLLA/PDLA ratios.

Sample	ΔHc (J/g)	Tg (°C)	ΔHm-sc (J/g)	ΔHm-hc (J/g)	Xc-sc (%)	Xc (%)	fsc (%)
L20/D10	3.66 ± 0.23	9.87 ± 0.53	3.47 ± 0.18	22.56 ± 1.05	7.94	30.49	26
L15/D15	4.27 ± 0.16	9.22 ± 0.65	5.29 ± 0.18	19.56 ± 1.21	11.22	30.78	37
L10/D20	4.23 ± 0.20	9.68 ± 0.63	3.81 ± 0.22	20.63 ± 0.88	8.15	28.80	28
30PLLA	2.26 ± 0.13	10.12 ± 0.59	-	28.14 ± 1.31	-	28.14	-

that the f_sc value is strongly influenced by the PLLA/PDLA ratio under the condition of the same mass fraction of the dispersed phase of PLA in the PBAT matrix. The crystallinity of the PBAT-based composite at various PLLA/PDLA ratios is illustrated in Figure 2b; the bottom (red columns) are sc-crystallinity and corresponding f_sc data. The specific DSC data are listed in Table 1. It is shown that the sc-crystallinity and f_sc are 11.22% and 37% for the L15/D15 sample, whose sc-crystallinity is much higher than other samples and occupies 64% of total PLA’s crystallinity. This suggests that sc-crystallization forms more readily in a composite when PLLA and PDLA are present in equal proportions, as sc-crystallization relies on the uniform dispersion and full interaction of PLLA and PDLA molecular chains [43,44].

3.1.2. Effect of PLA Content on the Crystallinity of PBAT/sc-PLA Composites

Figure 3 illustrates the impact of dispersed PLA phase content on the crystallization behavior of PBAT/sc-PLA composites, with detailed data summarized in

Table 3. Enthalpy and crystallinity of PBAT/sc-PLA composites at different PLA content.

Sample	ΔHm-sc (J/g)	ΔHm-hc (J/g)	Xc-sc (%)	Ωsc (%)
10DP	1.26 ± 0.13	2.46 ± 0.17	11.51	1.15
20DP	4.75 ± 0.21	2.48 ± 0.09	18.98	3.79
30DP	8.69 ± 0.23	3.81 ± 0.20	22.79	6.83

. As shown in Figure 3a, the overall crystallinity of sc-crystals within the PBAT/sc-PLA blends increases significantly as the dispersed PLA phase content rises. Specifically, when the PLA content is increased from 10% to 30%, the relative crystallinity of sc-crystals in the PBAT/sc-PLA blends experiences a substantial enhancement of 98%. This augmentation can be ascribed to the enhanced interaction among PLA molecules, which heightens the probability of PLLA and PDLA molecular chains encountering each other and forming sc-crystals [45,46]. Figure 3b illustrates the correlation between sc-crystal content and PLA content. When the dispersed phase content increases from 10% to 30%, the sc-crystal content in the PBAT/sc-PLA blend significantly increases by 493%. This substantial increment is attributed to both the elevated PLA content and the concomitant increase in sc-crystal crystallinity. In total, the formation of PLA composite crystals is closely contingent upon the relative proportions of PLLA and PDLA within the blend.

3.1.3. Effect of Hot Embossing Temperature on the Crystallinity of PBAT/sc-PLA Composites

Figure 4 presents the crystallinity of samples with varying PLA contents after hot embossing at temperatures of 140 °C, 190 °C, and 230 °C, with specific data detailed in

Table 4. Enthalpy and crystallinity of PBAT/sc-PLA composites at different PLA content and hot embossing temperature.

Sample	Tc (°C)	ΔHm-sc (J/g)	ΔHm-hc (J/g)	Xc-sc (%)	Xc-hc (%)	Xc (%)	Ƒsc (%)
	140	1.36 ± 0.23	0.74 ± 0.13	10.89	5.92	16.80	64
10DP	190	2.27 ± 0.15	1.02 ± 0.16	17.79	5.79	23.57	75
	230	1.89 ± 0.14	1.89 ± 0.21	11.62	11.68	23.30	49
	140	2.43 ± 0.19	3.30 ± 0.25	10.67	14.50	25.18	42
20DP	190	4.89 ± 0.15	1.72 ± 0.14	18.92	6.65	25.58	73
	230	2.37 ± 0.24	5.40 ± 0.29	6.71	15.90	22.61	30
	140	4.00 ± 0.26	6.66 ± 0.28	11.98	19.93	31.90	37
30DP	190	9.06 ± 0.31	4.70 ± 0.24	23.27	12.08	35.35	66
	230	4.09 ± 0.29	8.04 ± 0.36	8.99	17.66	26.65	34

. Figure 4a illustrates the changes in relative crystallinity of hc-crystals and sc-crystals with respect to the hot embossing temperature for a 10% PLA blend (10DP). At a hot embossing temperature of 190 °C, the total relative crystallinity of PLA in the 10DP sample is maximized, with the relative crystallinity of sc-crystals also reaching its peak. Notably, the crystallinity of sc-crystals increases substantially. This effect is attributed to the fact that at 190 °C, hc-crystals in the 10DP blend melt, while sc-crystals remain stable. The temperature is near the crystallization temperature of sc-crystals, leading to the recrystallization of molten hc-crystals into sc-crystals, thereby increasing the sc-crystal content and its relative crystallinity. Similar observations are noted in Figure 4b,c for 20% PLA (20DP) and 30% PLA (30DP) blends, respectively. At the hot embossing temperature of 190 °C, both the total relative crystallinity of PLA and the crystallinity of sc-crystals in these blends reach their highest values, with a significant increase in sc-crystallinity. This highlights the critical role of the 190 °C hot embossing temperature in enhancing sc-crystallinity within PBAT/sc-PLA blends. After hot embossing at 190 °C, the ratio of sc-crystal relative crystallinity to the total PLA relative crystallinity in the 10DP, 20DP, and 30DP blends exceeds 50% (75%, 73%, and 66%, respectively). This indicates that more than half of the PLA crystallinity in the blends is attributed to sc-crystals after hot embossing at this temperature. This shift suggests that sc-crystallization becomes more favorable compared to hc-crystallization at this temperature. The observed result is likely due to the melting of previously formed hc-crystals at 190 °C, which are then gradually converted into sc-crystals. In summary, a hot embossing temperature of 190 °C effectively promotes sc-crystallization in PBAT/sc-PLA blends and enhances their relative crystallinity.

3.2. Rheological Behavior

The effect of PLA content on viscoelastic properties of PBAT/sc-PLA composites is plotted in Figure 5, which shows the plots of the storage modulus (G′) and complex viscosity (η*) as a function of frequency. It is well known that G′ is closely related to the molecular chain motion and structure. It is observed from Figure 5a that G′ gradually increases as ω increases. In the low-frequency region, the molecular chains have sufficient time to rearrange, and the molecular chain motion is relatively free, resulting in a lower G′. In the high-frequency region, the molecular chains cannot respond quickly to the changes in the external stress, leading to a higher G′ for the samples. Moreover, for all the PBAT/sc-PLA composites, the G′ gradually improves as the PLA content increases. This can be attributed to the fact that the formed sc-crystals and hc-crystals in the PBAT/sc-PLA composite restrict the movement of PBAT molecular chains due to the enhancement of the steric hindrance of molecular chains. Hence, the ability of storage deformation is promoted [47]. In addition, it is worth noting that as the PLA content increases, the slope of the G′ curve at the low-frequency region of the sample gradually decreases. Especially when the PLA content exceeds 20 wt.%, G′ remains almost constant and exhibits a significant terminal behavior, G′~ω². This indicates a transition of liquid-like to solid-like behavior with the increase in PLA [38].

It can be clearly observed from Figure 5b that with the gradual increase in the PLA content, all samples exhibit a remarkable transformation process from the characteristics of a Newtonian fluid to those of a pseudoplastic fluid. The “shear-thinning” effect of the melt becomes increasingly prominent, and this change is particularly significant in the low-frequency region. This can be explained by the fact that the PLA molecular chain has greater rigidity compared to the PBAT molecular chain [48]. With the increase in the PBAT/sc-PLA component content, the complex viscosity of the composite material also increases, and this phenomenon is more pronounced in the low-frequency region. On the other hand, the increase in the PLA content leads to a simultaneous rise in the contents of sc-PLA and hc-PLA crystals, which greatly hinders the free movement of the PBAT molecular chains and the amorphous PLA molecular chains in the blend, thereby resulting in a significant increase in the η*.

3.3. Phase Morphology

Figure 6 presents the SEM images of neat PBAT and PBAT/sc-PLA blends following cryogenic fracture in liquid nitrogen. As shown in Figure 6a, neat PBAT exhibits a smooth, single-phase morphology. In contrast, Figure 6b–d reveal a distinct sea-island phase structure, where the PLA dispersed phase forms spherical particles uniformly distributed within the PBAT matrix. The clear boundary interface between the PBAT matrix and the PLA phase suggests limited compatibility between the two components. As the PLA content increases, the size of the spherical PLA particles progressively enlarges, and their density within the matrix rises correspondingly. Additionally, the interfacial boundary between the PBAT and PLA phases becomes less distinct, indicating a potential change in the interaction between the two phases.

To quantitatively analyze the particle size and distribution of PLA content in the PBAT matrix, the Image-Pro Plus software 6.0 was utilized to obtain the statistics of the particle size of PLA domains based on SEM micrographs. The results are illustrated in Figure 7. When the content of PLA increases from 10 wt.% to 30 wt.%, the average size of the spherical PLA particle increases from 1.14 μm to 1.6 μm. Notably, for the PBAT/sc-PLA blend system, the particle size distribution of the PLA dispersed phase broadens slightly as the PLA content increases. This broadening can be attributed to changes in the rheological properties of the blend, which influence the formation and distribution of the dispersed phase during mixing, leading to a wider particle size distribution. Additionally, increased PLA content may cause uneven nucleation, where the nucleation sites for PLA particles are not uniformly distributed, resulting in the formation of particles with varying sizes and further contributing to the broader particle size distribution. However, when the PLA content increases to 30DP, the situation becomes slightly different. When the dispersed phase content is relatively high, the probability of direct contact between dispersed phase particles increases under screw shear action, leading the particles to coalesce and reach a stable equilibrium size. As a result, the particle size increases, while the size distribution narrows.

3.4. Heat Resistance

Figure 8 demonstrates the influence of PLA content and hot embossing temperature on the VST of the blends, providing insights into their thermal resistance. As depicted in Figure 8a, the VST of PBAT/sc-PLA blends exhibits a clear dependence on the PLA content. The VST of pure PBAT is approximately 74 °C, while the incorporation of PLLA/PDLA phases leads to a gradual increase in the VST of the PBAT/sc-PLA blends as the PLA content rises. This trend correlates with the enhanced crystallinity of both hc-crystallites and sc-crystallites within the composite, as supported by prior DSC analysis. Specifically, the VST reaches 92.2 °C at a PLA content of 30 wt.%, underscoring the effectiveness of melt blending as a strategy to significantly improve thermal resistance. This improvement is attributed to the increased crystallinity of both hc-crystallites and sc-crystallites phases. Furthermore, the contribution of sc-crystal to VST is bigger than that of hc-crystal since the T_m of sc-crystal is much higher than that of single PLLA or PDLA. Figure 8b shows the effect of hot embossing temperatures on VST of samples. The VST is about 85.6 °C when the hot embossing temperature is 140 °C, which is near the crystallization temperature of PLLA. At this moment, although there are a large number of hc-crystals, which inhibits the formation of sc-crystals and does not lead to high resistance temperature, the VST is still higher than that of the 10DP sample. After the hot embossing temperature increases to 190 °C, the maximum of VST 92.2 °C is obtained. This increase can be attributed to the temperature being high enough to melt hc-crystals while sc-crystals remain stable. The molten hc-crystals then recrystallize into sc-crystals. However, the VST of PBAT/sc-PLA blends decreases to 84.5 °C as the temperature reaches 230 °C, which is about 50 °C higher than the T_m of hc-crystals. This demonstrates that the amount of sc-crystallite will be smaller when the processing temperature is too high to approach the T_m of PLLA, leading to the decrease in heat resistance. It can be concluded from both Figure 8a,b that the addition of high PLA content and the choice of the hot embossing temperature between the T_m of hc-crystals and sc-crystals are crucial for the improvement of heat resistance. Compared to other similar studies, a comparable level of thermal resistance can be achieved with a lower amount of PLLA and PDLA. This is likely because the formation of sc-crystals is more complete and uniformly distributed, leading to a significant overall improvement in the thermal resistance of the composite material [49].

In addition, Figure 8c depicts the relationship between the mass fraction of sc-crystals and VST of samples. It shows that on the condition of the same hot embossing temperature 190 °C, the VST of samples rises from 74 °C to 92.2 °C as the ω_sc increases from 0 to 6.98%. This trend can be attributed to the increased likelihood of PLLA and PDLA molecular chains coming into contact and interacting, facilitated by higher PLA content, which enhances the formation of sc-crystals. Furthermore, under the condition of the same amount of PLA content (30 wt.%), the VST of PBAT/sc-PLA blends raises from 84.5 °C to 92.2 °C as the ω_sc increases from 2.71% to 6.98%. Totally, with the increase in ω_sc, the VST values show a nonlinear increasing trend in accordance with the power-law model (R² = 0.99), which is shown as Equation (6)

$$VST=74+4.68\times {{\omega }_{sc}}^{0.7}$$

(6)

3.5. Mechanical Performance

Figure 9 shows the stress–strain tensile curves, calculated yield stress, and Young’s modulus for PBAT/sc-PLA blends with varying PLA content and hot embossing temperature. As shown in Figure 9a, neat PBAT has excellent tensile toughness and exhibits obvious strain hardening phenomenon during the tensile process. The incorporation of the PLA content turns the transition of the stress–strain curve from ductile to brittle fracture behavior because of the bad compatibility between PBAT and PLA. Meanwhile, the toughness of PBAT/sc-PLA composites sharply decreases, yet the yield strength and modulus significantly increase. This enhancement is due to the strong rigidity of PLA molecular chains, as well as the formation of hc-crystals and sc-crystals within the PBAT/sc-PLA composites. The crystalline regions, which exhibit high molecular ordering, restrict the movement of molecular chains, thereby increasing the material’s strength and modulus [49]. Specifically, it can be seen from Figure 9b that the Young’s modulus of 30DP increases from 17.9 MPa to 82.5 MPa, representing a 361% increase, while the yield strength rises from 4.7 MPa to 9.7 MPa, a 106.3% enhancement. In other studies on PLA stereocomplex crystals reinforcing PBAT, when the total content of PLLA and PDLA reaches 30%, the modulus increases from 22.8 MPa to 77.5 MPa, with an improvement of approximately 240%. This may be because the stereocomplex crystal content in this study is higher, resulting in a more pronounced reinforcing effect on the PBAT matrix [1]. Additionally, Figure 9c presents the variation of tensile strain–stress curves at different hot embossing temperatures. It shows that the 30DP-190 and 30DP-140 samples exhibit typical brittle fracture behavior, with fractures occurring before yielding and elongation at break values of only 23.2% and 25.5%, respectively. However, the curve of 30DP-230 exhibits a ductile fracture behavior with a distinct yield point, followed by strain stability and a slight strain hardening stage, which is similar to the stress–strain behavior of neat PBAT. The elongation at break is approximately 413.2%, which is 17 times and 15 times greater than that of 30DP-190 and 30DP-140, respectively. This mainly results from the melting of sc-crystals and hc-crystals under the effect of high temperature, followed by reducing the areas of crystal regions in the composites, resulting in delaying the propagation of cracks in the material during the stretching process. In other words, the composite tends to exhibit the stress–strain behavior of single-phase material [50]. Figure 9d shows the quantitative results of tensile properties. It is found that the maximum modulus and yield strength are from the 30DP-190 sample, whose values are 82.5 MPa and 9.7 MPa, respectively. Similar to the results in Figure 8b, it is also attributed to the existence of the largest amount of sc-crystals inside the 30DP-190 sample when compared with the other two samples.

4. Conclusions

This study focused on PBAT/sc-PLA blends prepared by melt blending, systematically investigating sc-crystal formation and evolution. The effects of PLA content, PLLA/PDLA ratio, and hot pressing temperature on crystallization behaviors, microstructures, and viscoelastic properties were examined. The results showed that as the PLLA/PDLA ratio is near to 1:1 and the hot pressing temperature is above the hc-PLA melting point and close to the sc-PLA crystallization point (e.g., 190 °C), sc-PLA crystal relative crystallinity and content increase with more dispersed-phase PLA. In 30DP-190 samples, sc-PLA relative crystallinity reached 22.8%, 66% of total PLA crystallinity. Moreover, sc-PLA relative crystallinity significantly impacts blend heat resistance. With 22.8% sc-PLA relative crystallinity in the 30DP-190 blend, VST is 87.9 °C, 18% higher than in pure PBAT. Additionally, PLA content strongly affects mechanical properties, with the 30DP sample’s Young’s modulus at 82.4 MPa, a 357.8% rise from neat PBAT.