Heat-Resistant Behavior of PLA/PMMA Transparent Blends Induced by Nucleating Agents

Abstract

Poly(lactic acid) (PLA) holds significant promise as an option in the field of packaging materials due to its biodegradability and antibacterial properties. Therefore, it is vital for developing packaging materials while improving their heat resistance, and transparency is essential for guaranteeing its application. Using a self-assembled nucleating agent with hydrogen bonding and thermodynamically compatible transparent polymethyl methacrylate (PMMA), this study fabricated PLA micro-crystals with an interface blurred grain. Furthermore, the crystalline structure-property relationship was investigated in different isothermal crystallization conditions; it was possible to achieve higher crystallinity while maintaining the transparency of PLA/10 wt% PMMA/0.3 wt% nucleating agent blends. Compared to other temperatures, the crystallization rate of PLA blends under annealing conditions at 90 °C was higher when induced by three different nucleating agents. Particularly, in the presence of the TC-328 nucleating agent, the system exhibited a crystallinity of 32%, the smallest grain size, and an increased Tg of 61.3 °C, as well as an elevated heat deformation temperature (HDT) from 54.13 °C to 63.2 °C. The smaller nucleating agents with high surface energy enhanced the interaction between the PLA and PMMA, enhancing the PLA/PMMA tensile strength and HDT. These findings may pave the way for designing novel blends for packaging or heat-resistant devices.

1. Introduction

Polylactic acid (PLA), an aliphatic polyester synthesized from lactate monomers, is characterized by excellent biodegradability, mechanical properties, biocompatibility, and non-toxicity to humans and the environment. Although PLA has attracted increasing attention for environmental and sustainable development [1], its slow crystallization rate and low degree of crystallinity adversely affect its heat resistance, making it challenging to meet the requirements of packaging applications. Therefore, improving PLA heat resistance is essential.

PLA heat resistance is mainly improved via crystallization and blend modification, with research focusing on enhancing the crystallinity and crystallization rate [2,3,4]. Adding nucleating agents can effectively improve the crystallization ability of PLA. Wang et al. [5] found that adding a small amount of the TMC-328 amide-type nucleating agent to PLA promoted nucleation and refined the grain structure, enhancing its crystallization performance and heat resistance. However, this specialized nucleating agent exhibited unsatisfactory crystallization solidification during actual processing and a limited ability to improve mechanical properties. Furthermore, due to the uncontrollable crystal size, the refractive index differences between the crystalline and amorphous regions resulted in unstable mechanical properties, severely affecting the PLA transparency [6].

PLA can be combined with polymethyl methacrylate (PMMA) [7] and polycarbonate (PC) to improve its mechanical properties without reducing its transparency [8]. The PLA/PMMA blend system demonstrates significant potential in advanced packaging applications, particularly in sustainable food packaging and high-temperature resistant containers. Since its refractive index is close to that of PLA (1.49–1.50), PMMA is a preferred resin for simultaneously improving heat resistance and transparency, exhibiting a high glass transition temperature (T_g), and excellent compatibility, transparency, and durability [9,10]. Jon Anakabe et al. [11] used a semi-industrial twin-screw extruder and injection molding as preparation methods, resulting in semi-transparent PLA/PMMA blends. Fan Yinqing et al. [12] prepared transparent PLA/PMMA blends via melt blending. The results revealed partially crystallized PLA, while the presence of spherulites increased the overall heat resistance of the material. Although PMMA addition restricted the size of the PLA spherulites and improved the blend transparency, it reduced the nucleation and spherulite growth rates [12,13].

Although several studies have examined PLA/PMMA blends and the addition of nucleating agents to control PLA crystallization kinetics [14,15], minimal research is available regarding nucleating agent addition to PLA/PMMA blend systems. Therefore, this study investigates the relationship between nucleating agents and the crystalline structure and properties of the blend system. Furthermore, according to the International Organization for Standardization (ISO), when the relative biodegradation rate of a completely degradable plastic exceeds 90%, it is considered fully degraded by microorganisms [16]. PLA belongs to the category of fully biodegradable materials, while PMMA is non-biodegradable. Considering the requirements for biodegradability while improving the heat resistance of PLA, the PMMA content added in this study is 10%. This material system successfully addresses the conflicting demands of thermal resistance, optical clarity, and biodegradability in modern packaging through PMMA-induced spherulite size reduction and hydrogen-bonding nucleating agents.

2. Experimental Section

2.1. Materials

The PLA, with a density of 1.24 g/cm³, a molecular weight (M_w) of 1.07 × 10⁵ g/mol, and 1.48 polydispersity, was purchased from Nature Works, USA (commercial trade number 3052D). The PMMA was supplied by the Chi Mei Corporation (Taiwan, China), designated as CM211, with an average M_w of 7.79 × 10⁴ g/mol, while the TMC-300/TMB-5/TMC-328 nucleating agent was obtained from the Shanxi Provincial Institute of Chemical Industry.

2.2. Preparation of the PLA/PMMA Blends

2.2.1. Melt Blending

Before the experiment, the PLA granules and PMMA were dried in a vacuum oven at 80 °C for 4 h until reaching a moisture content below 0.025% (250 ppm) to prevent viscosity degradation. First, the PMMA master batch containing the nucleating agent was prepared. The PMMA was combined and blended with three respective nucleating agents, TMC-300 (Dibenzoyl hydrazine sebacate), TMB-5 (Dicyclohexyl terephthalamide), and TMC-328 (N,N′,N″-tricyclohexyl-1,3,5-benzotriamide), in a torque rheometer (CTR-300) at 200 °C and 30 r/min, followed by crushing and drying in a vacuum oven at 80 °C for 4 h. Then, the PLA and PMMA master batch containing the nucleating agent were combined and blended in a torque rheometer for 30 min at 200 °C and 30 r/min. The PMMA content in each blend is 10 wt%, and the nucleating agent content is 0.3 wt%. The formula is shown in

Table 1. The formula of blends.

Samples	PLLA (g)	PMMA (g)	TMC-300 (g)	TMB-5 (g)	TMC-328 (g)
PLLA	100
PLLA/PMMA	90	10
PLLA/TMC-300	100		0.3
PLLA/TMB-5	100			0.3
PLLA/TMC-328	100				0.3
PLLA/PMMA/TMC-300	90	10	0.3
PLLA/PMMA/TMB-5	90	10		0.3
PLLA/PMMA/TMC-328	90	10			0.3

.

2.2.2. Compression Molding

The dried melt-blended granules were pressed into 4 mm plates (for Vicat softening temperature testing) and 0.1 mm films (for transmittance testing) using a flat vulcanizing machine (KY-3201S-30T) at 205 °C.

2.2.3. Isothermal Crystallization Treatment

The prepared samples were subjected to isothermal crystallization treatment at 80 °C, 90 °C, and 100 °C for 96 min, 29 min, and 5 min, respectively, to improve their heat resistance. The isothermal crystallization time was based on when the pure PLA film haze reached 15% at different temperatures.

2.3. Measurements

2.3.1. Differential Scanning Calorimetry (DSC)

The melting behavior of the PLA and PLA/PMMA heat-resistant blends in a nitrogen atmosphere was characterized via a TA DSC Q200 using a non-isothermal crystallization sample mass of 5 mg. The sample temperature was increased from room temperature to 200 °C at a heating rate of 10 °C/min, where it was maintained for 5 min to eliminate the thermal history, and again reduced to room temperature at 50 °C/min. The sample crystallinity was calculated using the following formula [17]:

$$X_C=\left({\displaystyle \frac{{\Delta H}_m-\Delta H_C}{\Delta H_m^0}}\right)\times 100\%$$

(1)

where ${\Delta H}_m$ is the melting enthalpy, $\Delta H_C$ is the cold crystallization enthalpy, and $\Delta H_m^0$ ($\Delta H_m^0$ = 93 J·g⁻¹) is the ${\Delta H}_m$ of the 100% crystalline PLA.

During the isothermal cold crystallization experiments, the samples were heated to isothermal temperatures (80 °C, 90 °C, and 100 °C) at 50 °C/min, where they were maintained for 60 min.

2.3.2. Wide-Angle X-Ray Diffraction Analysis (WAXD)

The crystal phase transformation and grain size variation of the PLA and PLA/PMMA blends were investigated using an X-ray diffraction (XRD) instrument. The 2θ scanning range was set from 5° to 45° at a scanning speed of 10°/min. The XRD patterns were analyzed and fitted using the Jade 6.0 software. The diffraction peak intensity was associated with the grain size. Therefore, the grain size of each component after isothermal crystallization at different temperatures was calculated using the Scherrer formula as follows [18]:

$$D={\displaystyle \frac{K\lambda }{\beta COS\theta }}$$

(2)

where K is a constant, λ is the X-ray wavelength, β is the full width at the half maximum of the diffraction peak, θ is the diffraction angle. In this formula, the constant k value is related to the definition of β. When β is half the width and height, k is 0.89. When β is the integral width, k is 1.0.

2.3.3. Scanning Electron Microscopy (SEM)

The fracture morphology of the PLA and PLA/PMMA blends was obtained via SEM (ZEISS Gemini 300, Oberkochen, Germany) at a voltage of 3 kV. To expose the crystal structure, annealed samples were etched at room temperature for 10 h in a water/methanol mixture containing 0.025 mol/L sodium hydroxide (NaOH) (volume ratio 1:2) to selectively etch amorphous PLA and PMMA. The etched sample was cleaned with distilled water and soaked in anhydrous ethanol for 1 h before drying, after which it was sputtered, platinum-plated, and observed at an accelerated voltage of 5 kV via a vacuum ion sputtering instrument (Supro instrument ISC 150T, Shenzhen, China).

2.3.4. Heat Resistance Property Measurements

The thermal deformation temperature (HDT) of the PLA and PLA/PMMA blends was tested and analyzed via an HVT-302 3-set Veka thermal deformation testing machine, using 80 mm × 10 mm × 4 mm HDT samples.

2.3.5. Light Transmission Property Measurements

The transmittance and haze of PLA and PLA/PMMA blends were analyzed by TH-110 haze meter. The thickness of the test sample was 1 mm, and the length and width should be ≥50 mm.

2.3.6. Mechanical Property Measurements

The mechanical properties of the PLA and PLA/PMMA blends were measured using a material testing machine (Instron 4302, USA) according to the GB/T 1014.1-2018 standard [19]. The uniaxial tests were repeated five times at room temperature, while a speed test was conducted at 50 mm/min to obtain the average values. The cantilever beam impact test was performed via a GT-7045-MDL impact test machine using an 80 mm × 10 mm × 4 mm sample with a notch depth of 2 mm according to the national GB/T1043-2008 standard [20]. Samples were maintained in atmospheric conditions for at least 24 h before testing.

3. Results and Discussion

3.1. Melting and Crystallization Behavior

The PLA and blends were subjected to DSC to assess the compatibility of the polymers and evaluate the influence of the PMMA and nucleating agent on the PLA crystallization performance. This helped clarify the crystallization process, nucleation mechanism, and overall performance regulation of the PLA blends.

The heating curves of the PLA blends at various temperatures are depicted in Figure 1. Each PLA/PMMA exhibited only one T_g, indicating the absence of macroscopic phase separation and confirming excellent compatibility between the PLA and PMMA, which was consistent with the findings of existing studies [21]. The T_g of the PLA/PMMA blends exposed to isothermal crystallization at different temperatures increased slightly since the PMMA T_g was higher than that of the PLA. Furthermore, the melting point (T_m) decreased, indicating the formation of thinner PLA crystal chips.

Furthermore, no significant changes were evident in the DSC curves at different temperatures when only the nucleating agent was added to the PLA, suggesting that the nucleating agent minimally impacted PLA crystallization in these conditions. Adding the nucleating agent and PMMA simultaneously facilitated a substantial change in the Tm of the blends. PLA/PMMA/TMC-300 displayed the highest Tm, which exceeded that of the PLA/PMMA, while the Tm of the PLA/PMMA/TMC-328 was lower.

As shown in

Table 2. The thermal characteristics obtained during the PLA blend heating process in different conditions.

Samples	Tg (°C)			Tm (°C)			Xc (%)
	80 °C	90 °C	100 °C	80 °C	90 °C	100 °C	80 °C	90 °C	100 °C
PLA	61.1	59.3	60.6	150.4	141.6/151.9	145.8/152.2	31.9	33.0	35.5
PLA/TMC-300	61.2	60.5	61.2	151.2	142.5/151.8	144.6/151.8	31.3	38.2	34.6
PLA/TMB-5	61.9	61.0	60.5	150.5	143.2/151.8	151.8	32.4	32.2	34.6
PLA/TMC-328	62.3	61.3	61.1	151.6	142.0/151.2	145.5/151.8	33.1	32.2	35.6
PLA/PMMA	61.2	61.4	61.7	149.6	150.4	150.3	26.2	28.4	22.3
PLA/PMMA/TMC-300	61.1	61.9	60.9	153.4	151.1	150.4	28.9	29.1	29.7
PLA/PMMA/TMB-5	62.0	60.9	61.3	150.1	152.4	149.9	23.2	29.0	27.3
PLA/PMMA/TMC-328	60.9	59.8	62.2	149.1	150.9	149.1	28.1	29.6	28.2

, the X_c of the PLA decreased from 31.9% to 26.2% and 22.3%, respectively, at 80 °C and 100 °C after adding PMMA. This reduction in crystallinity occurs because PMMA is an amorphous polymer, while PLA requires the arrangement of its molecular chains in a more regular manner via chain movement during crystallization. The uniform PMMA and PLA mixing impeded the movement of PLA molecular chains, inhibiting PLA crystallization. A higher pretreatment temperature decreased the isothermal crystallization time, reducing the molecular chain movement times and the subsequent crystallinity.

After adding the TMC-300 and TMC-328 nucleating agents to the system, the X_c of the blends increased, particularly at 100 °C, since heterogeneous nucleation provided more nucleation sites during the crystallization process and promoted molecule self-assembly into nano-scale fibrous crystals. However, due to the inhibitory effect of the PMMA segments on PLA crystallization, the crystallinity remained lower than that of pure PLA, even with the nucleating agent promoting nucleation.

The exothermic characteristics and crystallization kinetics of PLA blends were analyzed to clarify the impact of various nucleating agents and PMMA on isothermal PLA crystallization. As shown in Figure 2, the analysis of the results revealed that the crystallization peaks of different blends were either small or absent after crystallization at 80 °C for 1 h, indicating that PLA crystallization was not initiated or displayed relatively low crystallinity at this temperature. However, clear crystallization peaks were observed at 90 °C and 100 °C, with a particularly pronounced peak intensity at 100 °C. The increased mobility of the PLA molecular chains at 100 °C allowed the PLA to easily surpass the energy barrier and facilitate crystallization.

Based on the thermal characteristic curves shown in Figure 2b,c, the relative crystallinity of the various different components in different conditions was calculated using the following formula [22]:

$${\mathrm{X}}_{\mathrm{t}}={\displaystyle \frac{X_C\left(t\right)}{X_C\left(t_{\infty }\right)}}={\displaystyle \frac{{\int }_0^t(d{H}_T/d_t)d_t}{{\int }_0^{\mathrm{\infty }}(d{H}_C/d_t)d_t}}$$

(3)

where t is the crystallization time, and $d{H}_C/d_t$ is the heat flow rate. The relationship between the relative crystallinity X_t and time obtained according to this formula is shown in Figure 3.

As shown in Figure 3, when different blends were subjected to isothermal crystallization at 90 °C and 100 °C, the time required for the same blends to reach the same crystallinity decreased as the isothermal crystallization temperature increased. At the same temperature, different blends reached the same crystallinity at different times, following a trend consistent with the non-isothermal crystallization process, indicating that nucleating agent addition to PLA minimally affected the time required to reach the same crystallinity. However, although the presence of PMMA significantly increased the time necessary, adding a nucleating agent to the PLA/PMMA system reduced the time to varying degrees, confirming the previous conclusion regarding the non-isothermal crystallization process.

To further quantify the crystallization rate, the Avrami equation was employed to describe the isothermal crystallization process [23], which was formulated as follows:

$${1-\mathrm{X}}_{\mathrm{C}}\left(\mathrm{t}\right)=\mathrm{e}\mathrm{x}\mathrm{p}\left(-\mathrm{k}{\mathrm{t}}^{\mathrm{n}}\right)$$

(4)

where K is the crystallization rate constant of the polymer, representing the crystal nucleation and crystal growth rate of the polymer, and n is the Avrami index, which is closely related to the nucleation mechanism and growth mode of the polymer. Although the spherulite n value of three-dimensional PLA growth is typically between 2 and 4 [24], it is affected by the growth conditions during the actual crystallization process.

To facilitate analysis and calculation, both sides of the equation are often subjected to simultaneous logarithmic operations to obtain the following formula [23]:

ln[−ln(1 − X_t)] = lnk + nln t

(5)

Figure 3 is plotted using the ln[−ln(1 − X_t)] and ln t in this formula as the Y-axis and X-axis, respectively, and the results are shown in Figure 4. Although the obtained curves exhibited a satisfactory linear relationship during the intermediate crystallization stage, they deviated from this association during the initial and final stages. This can be attributed to initial nucleation and subsequent secondary crystallization.

The data in Figure 4 were subjected to linear fitting, where the resulting slope and intercept corresponded to the Avrami exponent (n) and lnk (natural logarithm of k) values, respectively. To minimize analysis errors, only the data points representing 10% to 90% relative crystallinity were included in the fitting process. The corresponding parameters obtained from the fitting are presented in

Table 3. Kinetic parameters of isothermal crystallization at 90 °C and 100 °C after the isothermal process.

Samples	n		k		t1/2 (min)
	90 °C	100 °C	90 °C	100 °C	90 °C	100 °C
PLA	2.2	2.2	1.64 × 10−2	1.18 × 10−1	5.48	2.24
PLA/TMC-300	2.5	2.5	7.30 × 10−3	5.78 × 10−2	6.18	2.70
PLA/TMB-5	1.8	2.6	2.99 × 10−2	3.40 × 10−2	5.73	3.19
PLA/TMC-328	2.0	2.6	2.55 × 10−2	2.90 × 10−2	5.22	3.39
PLA/PMMA	2.0	2.2	2.29 × 10−3	3.70 × 10−3	17.40	10.79
PLA/PMMA/TMC-300	1.8	1.8	1.02 × 10−2	2.26 × 10−2	10.45	6.70
PLA/PMMA/TMB-5	2.0	2.5	5.68 × 10−3	2.48 × 10−3	11.04	9.52
PLA/PMMA/TMC-328	2.0	2.1	5.80 × 10−3	7.15 × 10−3	10.93	8.83

.

The nucleating agent acted as a heterogeneous nucleation site during the cooling of the PLA melt, effectively adsorbing PLA molecular segments and facilitating their arrangement and growth. At 90 °C, the n value of the blends remained relatively constant, ranging from 1.8 to 2.2, while it displayed a notable increase at an isothermal crystallization temperature of 100 °C. The specific influence of nucleating agents on the crystallization mechanism depended on the type of nucleating agent and the isothermal crystallization conditions.

Furthermore, regarding the crystallization rate at different temperatures, adding PMMA significantly reduced the PLA crystallization rate and increased the semi-crystallization time. For instance, at 90 °C, the semi-crystallization time increased from 5.48 min to 17.40 min. Conversely, no significant changes were evident in the PLA crystallization rate, semi-crystallization time, and crystallinity after adding nucleating agents. Additionally, the crystallization rates of the PLA blends were distinctly higher as the temperature increased from 90 °C to 100 °C.

Although adding the TMC-300 nucleating agent to the PLA system at 90 °C during isothermal crystallization decreased the crystallization rate, it increased the crystallinity. Conversely, TMC-328 addition increased the crystallization rate but reduced the crystallinity. However, the addition of TMB-5 decreased the crystallization rate and crystallinity. This discrepancy was attributed to the fact that PLA crystallization was influenced by both nucleation and grain growth. The granular shape of the TMC-300 nucleating agent differed from TMB-5 and TMC-328 [22,23,24], causing variation in the number of nucleation sites. In these conditions, the influence of the nucleation sites on crystallinity outweighs the effect on grain growth. Even though a faster nucleation rate was evident during the later stages of grain growth, a longer crystallization time reduced the number of available nucleation sites, decreasing the PLA crystallinity.

3.2. Crystal Structure

XRD was employed to analyze the blends after isothermal crystallization to further investigate the impact of the PMMA and nucleating agent on the PLA crystal structure and grain size. The PLA crystals primarily exhibited four crystal forms: α, α′, β, and γ. The α crystal form was the most prevalent and stable, with corresponding WAXD 2θ peaks at 14.8°, 16.9°, 19.1°, and 22.5° [25]. As shown in Figure 5, prominent characteristic PLA crystal diffraction peaks were observed around 2θ = 16.6° and 18.9°, corresponding to the (200)/(110) and (203) crystal planes of the α crystal. The PLA crystals obtained via crystallization at 90 °C and 100 °C exhibited two relatively weaker characteristic diffraction peaks around 2θ = 14.9° and 22.2°, corresponding to the (103), (010), and (015) crystal planes of the α crystal. Notably, all the blends exhibited α-crystalline structures, indicating that adding the nucleating agent and the PLA/PMMA simultaneously did not alter the PLA crystal form. Additionally, when the blends were crystallized at different temperatures, the diffraction peak intensity of the PLA/PMMA blends was lower than the samples without PMMA.

The calculation results are shown in

Table 4. The grain sizes of blends after isothermal crystallization at different temperatures (unit: Å).

Samples	80 °C (96 min)	90 °C (29 min)	100 °C (5 min)
PLA	343	274	268
PLA/TMC-300	344	270	246
PLA/TMB-5	319	264	264
PLA/TMC-328	350	260	272
PLA/PMMA	223	242	206
PLA/PMMA/TMC-300	216	242	231
PLA/PMMA/TMB-5	186	245	217
PLA/PMMA/TMC-328	227	233	190

. Adding a nucleating agent to the PLA affected the grain size differently, with a decreasing trend in most cases since the nucleating agent acted heterogeneously, particularly with TMB-5. The grain size decreased significantly while the crystallinity of the system was retained. Furthermore, PMMA addition distinctly decreased the grain size, with the most pronounced effect observed at 80 °C. The mobility of PLA molecular chains was improved in the PLA blends following isothermal crystallization treatment, making PLA more susceptible to crystallization and resulting in increased crystallinity with smaller grains. Notably, at 80 °C, the grain sizes of the PMMA blend system were smaller than those at 90 °C and 100 °C, which could be attributed to the slower PLA crystallization time and incomplete growth at these temperatures.

3.3. Morphology Analysis

Figure 6 presents the internal morphology of the pure PLA and blends after brittle fracture, as well as their corresponding post-treatment. The surface of the pure PLA appeared smooth in the SEM group (a). The different types of added nucleating agents were uniformly dispersed in the PLA matrix. Specifically, TMC-300 appeared granular, while TMB-5 exhibited a different morphology. No significant morphological changes were evident in the PLA and PMMA blends, displaying homogeneous structures with no visible macroscopic phase separation. This homogeneity stayed constant after adding the nucleating agent to the PLA/PMMA blends, while the morphology of the nucleating agent remained unchanged.

The annealed PLA blends were etched to remove the amorphous regions, obtaining different morphologies in the SEM groups (b), (c), and (d). As shown in Figure 6b, it was evident that the addition of the nucleating agent did not cause obvious changes, and no significant changes were evident in the crystal morphology after nucleating agent addition. However, when PMMA was added to the PLA matrix, the size of the original dense holes decreased, and occasional larger holes appeared. After adding the nucleating agent, the dense holes became smaller, while the number of larger holes decreased to varying degrees, which was attributed to the uniform dispersion of PMMA within the PLA matrix. During amorphous region etching, the interaction between the PLA and PMMA was relatively weak, leading to PMMA etching and larger hole formation. Moreover, adding a nucleating agent to the PLA/PMMA blend system improved the interaction force between the PLA and PMMA interfaces, reducing PMMA etching. Furthermore, nucleating agent addition increased the system crystallinity, causing the amorphous molecular chains of the PMMA to intertwine with the PLA crystalline region, making them less susceptible to etching. Therefore, the pores in the PLA/PMMA blend system containing the nucleating agent were smaller under the combined influence of increased nucleation sites and a slower crystallization rate. Although this was similar to the trend shown in Figure 6c,d, it varied in different conditions.

3.4. Heat Resistance

The values 96 min (80 °C), 29 min (90 °C), and 5 min (100 °C) correspond to the minimum annealing times required to achieve the target transmittance of 86% at each temperature. The exponential reduction in annealing time with increasing temperature reflects accelerated crystallization kinetics. Higher temperatures enhance molecular chain mobility, significantly shortening the nucleation and growth phases. As shown in Figure 7, adding PMMA to the pure PLA increased the HDT in the absence of annealing. This further confirmed that combining PLA with PMMA, which displayed a higher T_g, enhanced the heat resistance. However, this heat resistance improvement was insignificant, likely due to the low added PMMA concentration (only 10%).

Isothermal crystallization significantly enhanced the HDT of PLA and its blends. Notably, when processed at 90 °C, the HDT of PLA/TMC-328 increased by 13.1% (from 54.13 °C to 61.20 °C), demonstrating the critical role of crystallization kinetics in tailoring thermomechanical performance for high-temperature applications. This was attributed to the improved crystallinity of the PLA molecular chains, reinforcing the intermolecular forces. Nevertheless, including a nucleating agent improved the heat resistance of the PLA/PMMA blends. The smaller size of the nucleating agent enhanced the interaction forces between the PLA and PMMA. At 90 °C, the HDT of PLA/PMMA blends supplemented with three types of nucleating agents increased to 56.5 °C (TMC-300), 55.8 °C (TMB-5), and 55.9 °C (TMC-328). Additionally, the blends exhibited excellent light transmittance (>86%), indicating that nucleation not only improved the heat resistance of the PLA/PMMA blends but also maintained light transmittance.

3.5. Mechanical Properties

As shown in Figure 8, the tensile strength of the pure PLA remained mostly unchanged after isothermal crystallization. However, with the addition of nucleating agents, whether annealing was performed or not, the trend of changes in tensile strength closely correlates with the variations in crystallinity. This is attributed to the fact that an ordered crystalline structure can significantly improve the strength and rigidity of the plastic. Contrarily, the tensile strength of the blends increased after adding the PMMA amorphous polymer to the PLA matrix, which was reinforced by the rigid PMMA molecules. Additionally, the PMMA side groups weakened the intermolecular forces between the PLA chains, reducing the overall brittleness of the material and making it less prone to brittle fracture during stretching.

The elastic modulus data in Figure 9 indicated that the molecular chain arrangement of the PLA blends became more regular after isothermal crystallization due to higher crystallinity, increasing the moduli. However, higher isothermal crystallization temperatures decreased the modulus improvement. Even at 100 °C, the modulus was essentially the same as the pure PLA, which was attributed to a shorter isothermal crystallization time at higher temperatures, resulting in high crystallinity but incomplete grain growth.

As shown in Figure 10, adding a nucleating agent to the PLA reduced the impact strength. However, even with the addition of PMMA, the blends exhibited excellent impact strength, further highlighting the compatibility between PLA and PMMA and the presence of molecular chain interactions. The impact strength of the blends improved after isothermal crystallization at 80 °C and 90 °C, likely because isothermal crystallization eliminated the internal stress in the material induced during processing, resulting in excellent impact performance. Although the addition of a nucleating agent improved the crystallinity of the material, the subsequent grain refinement enhanced its specific surface area and facilitated grain boundary sliding, increasing its toughness.

4. Conclusions

This study explores the relationship between the crystallization structures and properties of PLA blends via isothermal crystallization in various conditions. PMMA addition reduces the PLA crystallinity and grain size, decreasing its heat resistance and tensile strength. Incorporating a nucleating agent significantly improves the crystallinity of the PLA/PMMA blends, which is attributed to the heterogeneous nucleation effect of the nucleating agent, increasing the nucleation sites and accelerating the PLA crystallization rate. Furthermore, the smaller size and higher surface energy of the nucleating agent reinforce the interaction between the PLA and PMMA, increasing the heat resistance and tensile strength of the material and decreasing the impact strength due to higher brittleness. Isothermal crystallization improves the heat resistance of the PLA blends while maintaining excellent light transmission. This investigation into the relationship between the crystalline structure and properties of PLA provides a theoretical foundation for expanding its potential application in packaging materials.