preface
The bio-based polymer polylactic acid (PLA) is a widely used material in packaging and medical materials, favored for its good biocompatibility, biodegradability and processability. However, PLA has some disadvantages, such as high brittleness, high hardness and low impact strength, which limit its application in other potential areas.
To overcome these limitations, PLA is often mixed with other polymers with good toughness and biodegradability, such as polycaprolactone (PCL), polyhydroxybutyrate (PHB), polypropylene carbonate (PPC), polybutylene succinate (PBS), and poly(butylene glycol adipate-terephthalate) (PBAT).
Past studies have shown poor interfacial compatibility between PLA and PBAT, so the introduction of compatibilizers is necessary. One approach is to use glycidyl methacrylate (GMA) to improve the interfacial compatibility between them, thereby increasing the impact strength of the blend. Some researchers introduce three block copolymers containing two types of PLA-PBAT-PLA and PLA-PEG-PLA into PLA/PBAT blends to improve compatibility between PLA and PBAT.
Other researchers have used cottonseed oil-based derivatives to improve the compatibility of PLA/PBAT blends and found that it can effectively enhance mutual fusion. In order to improve the thermal performance of the PLA/PBAT mixture, some research teams added volcanic rock grains (VPS) to the mixture, and the results showed that the thermal performance of the mixture was best when the amount of VPS was 5 wt%. The introduction of nanoclay also had a significant effect on the crystallization of PLA/PBAT blends, increasing the tensile strength and modulus of solid and microporous components.
Among the various compatibilizers, the compatibilizer of the terminal epoxy group is widely used because of its high reactivity. These substances act as chain extenders by ring-opening reactions with peripheral epoxy groups and terminal hydroxyl groups within the PLA/PBAT molecular chain. For example, researchers modified PLA/PBAT blends using styrene-acrylic acid copolymers (ESAs) with epoxy functionality and found that ESAs were only reactive to PLA matrix, improving the toughness and melting elasticity of the blend.
Similarly, the use of polyfunctional epoxides and the addition of organic montmorillonite/epoxy-functional graphene mixtures have helped to improve the rheological and mechanical properties of PLA/PBAT blends. Other researchers have improved the mechanical properties of blends by using different types of epoxy-POSS.
Compared to linear polymers, branched polymers have many active terminal functional groups that form strong hydrogen bonds with other polymers and certain types of chemical reactions. Due to their highly branched structure, branched polymers effectively prevent entanglement between molecular chains, thereby reducing viscosity. Therefore, the use of branched polymers to enhance the compatibility between polylactic acid (PLA) and polybutylene adipate (PBAT) is a feasible strategy.
1. DMA
The figure below shows the loss factor (Tanδ)-temperature curve of PLA/PBAT mixtures with different ETBP contents. On the curve, the transition temperatures of TgPBAT and PLA are expressed as Tg1 and Tg2, respectively, and the temperature difference between the two ΔTg is calculated.
As can be seen from the table, the ΔTg of the pure PLA/PBAT blend is 93.1°C, which indicates that PLA and PBAT are immiscible because each phase exhibits relatively independent thermodynamic properties. When ETBP was added, it was found that the ΔTg of the mixture decreased. The mixture with 3 phr ETBP content showed the lowest ΔTg of 87.1 °C. This indicates that the epoxy group of the ETBP reacts with the terminal carboxyl groups of PLA and/or PBAT, enhancing the interaction between the two phases.
Notably, there was an unexpected increase in ΔTg in the mixture containing 5.0 phr ETBP compared to 3.0 phr ETBP. This phenomenon may be due to the addition of excess ETBP (more than 3.0 phr), resulting in agglomeration of ETBP, which is affected by hydrogen bonding.
Second, mechanical properties
The three figures below illustrate the mechanical properties of PLA/PBAT mixtures with different ETBP contents in terms of tensile strength, elongation at break and impact strength. Looking at the curve, the tensile strength of the mixture fluctuates within a certain range, but remains relatively stable overall, even if the ETBP content changes.
With the increase of ETBP content, the elongation at break of the mixture showed a trend of first increasing and then decreasing. The elongation at break of the pure PLA/PBAT mixture is 45.8%. When 3 phr of ETBP is added, the elongation at break peaks at 272%, which is 5.9 times that of pure mixtures. However, when excess ETBP (5 phr) is added, the elongation at break drops to 167%.
A similar trend is shown for impact strength, PLA/PBAT/ETBP mixtures. With the increase of ETBP content, the impact strength of the mixture first increases and then decreases. The optimal amount of ETBP added is 3.0 phr. The impact strength of the PLA/PBAT mixture with 3.0 phr ETBP content was 45.3 kJ·m−2, which was much higher than that of the pure mixture of 26.2 kJ·m−2. However, when the ETBP content exceeds 3 phr (i.e., 5 phr), the impact strength decreases from 45.3 kJ·m−2 to 41.4 kJ·m−2.
This improved mechanical properties can be attributed to strong physical hydrogen bonding and chemical micro-crosslinking between the epoxy functional groups at the end of ETBP and the terminal groups within PLA and/or PBAT (–OH and/or -COOH). These reactions promote two-phase compatibility at the molecular level, making PLA/PBAT mixtures more similar to elastomers. In this system, ETBP can act as a rigid fraction and PBAT can act as a flexible segment, thereby improving the elongation at break and impact strength of the mixture.
The physical hydrogen and chemical bonds between ETBP and PLA/PBAT strengthen the adhesion of the interface between PLA and PBAT, and increase the thickness of the PLA/PBAT interaction layer. This further improves the stress absorption capacity between the PLA continuous phase and the PBAT dispersed phase, thereby improving the toughness of the PLA/PBAT mixture.
The combined effect of strong physical and chemical micro-crosslinking transforms the structure of PLA/PBAT mixture from linear structure to three-dimensional micro-crosslinking network structure, which helps to improve the elastic and impact strength of the hybrid system.
When the ETBP content exceeds 3.3 phr, the elongation at break and impact strength of the PLA/PBAT mixture begin to decrease. This is because excessive physical hydrogen and chemical bonding reactions between ETBP and PLA/PBAT mixtures limit the movement and rotation of macromolecular chains above this threshold.
In this case, only the chemical bond between PLA and PBAT is broken, not the molecular chain movement of both. This results in a decrease in the elongation at break and impact strength of the PLA/PBAT/ETBP mixture. As can be seen from the above indication, the mechanical properties (elongation at break and impact strength) of the mixture can reach the highest values when the ETBP content is 0.3 phr.
Third, the gel content
To verify the formation of a chemical micro-crosslinking network, gel content measurements were performed on PLA/PBAT blends with different ETBP content. In the dissolution process, only macromolecular chains with physical micro-cross-linked structures can be completely dissolved in solvents, while macromolecular chains with chemical micro-crosslinked structures are insoluble and only swelling occurs.
According to the data in the table, pure PLA/PBAT blends cannot be completely dissolved in solvents with the addition of an additive. The gel content of the pure mixture was measured at 1.8%. This may be due to the small amount of insoluble inorganic nucleating agents contained in PLA. As the additive content increases, so does the gel content in the blend. When the additive content reaches 5.0 parts, the gel content of the blend is about 17.8%. From the results of the analysis of the gel content, it can be determined that the introduction of additives led to the formation of a chemical micro-crosslinked structure.
IV. TGA
Based on the TGA and DTG curves shown, thermogravimetric analysis results of PLA/PBAT mixtures with different ETBP contents are shown.
The initial weight loss temperature (T5%) and 50% weight loss temperature (T50%) are basically consistent between PLA/PBAT mixtures containing different ETBP contents and pure PLA/PBAT mixtures. More detailed results can be found in Table 3. These results showed that the addition of ETBP did not significantly reduce the thermal stability of the PLA/PBAT blend. The thermal stability of the blend remains relatively stable, which means that the addition of ETBP has no significant effect on the thermal processability of the PLA/PBAT blend.
5. DSC
In the two figures below, you can see the DSC curves and parameters of PLA/PBAT mixtures with different ETBP content in the first heating cycle. With the gradual increase of ETBP content, the cold crystallization temperature of PLA/PBAT blends showed a trend of first decreasing and then increasing. The decrease in the cold crystallization temperature of the PLA/PBAT/ETBP (0.5 phr) mixture is caused by the lubricating effect of ETBP.
As a branched-chain polymer, ETBP has no intermolecular entanglement and low viscosity. It acts as a lubricating effect in PLA/PBAT blends, increasing the movement and orientation arrangement of macromolecular segments. This results in the mixture being easier to crystallize, which reduces the cold crystallization temperature. In addition, after the addition of ETBP, the melting temperature of the PLA/PBAT mixture did not change significantly.
Percent crystallinity (Xc) is a parameter that evaluates the degree of crystallization of PLA in a mixture. According to the formula, where ΔHc is the net melting crystal enthalpy, obtained by subtracting the recrystallization external heat; Φ is the weight fraction (30%) of PBAT in the PLA/PBAT blend.
According to the results of the table above, the crystallinity percentage (Xc) of PLA gradually increases from 3.0% to 7.3% as the ETBP content increases (from 0 to 3.0 phr). This indicates that PLA in a partially amorphous region is transformed into a crystalline region. However, when the ETBP content exceeds 3.0 phr, Xc begins to decrease, indicating that part of the crystalline region re-transforms into an amorphous region.
6. Scanning electron microscopy
In the figure, it can be observed that PLA in the sample exists as a continuous phase, while PBAT is dispersed in the continuous phase in the form of droplets. The image clearly shows the phase separation structure between PLA and PBAT, and the two-phase interface shows an island-like structure.
The figure below shows that the average size of PBAT particles in the PLA/PBAT mixture is 2.87 μm, indicating that the blending system belongs to a typical thermodynamically immiscible system.
After adding different contents of ETBP, the size of the dispersed PBAT particles decreased significantly. In Figure e, the average size of the PBAT particles is reduced to 0.38 μm, and the island-like structure is almost absent (see figure above).
In Panels b to e, it can be observed that the interface between PLA and PBAT becomes blurred, and more PLA and PBAT are combined with each other.
Figures a to C show two incompatible or partially compatible mixtures, showing a distinct island-like structure; In panels d and e, only one phase can be observed, indicating improved compatibility between PLA and PBAT after sufficient ETBP addition. These results are consistent with previous studies showing that the introduction of ETBP improves the interface compatibility between PLA and PBAT.
The following two data plots show SEM images of the fracture surface topography (immersion in liquid nitrogen) of a PLA/PBAT/ETBP blend and the size distribution of PBAT particles. In pure PLA/PBAT mixtures, the average particle size of PBAT is 1.48 μm. However, in the PLA/PBAT/3.0 phr ETBP blend, the average particle size of PBAT decreased to 0.75 μm, indicating that PBAT was better dispersed in the PLA matrix with the addition of 3.0 phr ETBP.
The figure below shows the SEM image of the tensile fracture surface topography of a PLA/PBAT/ETBP blend. From Figures A and B, brittle fracture characteristics and obvious island structures can be observed.
From Figures C to E, it can be seen that the morphology of the tensile fault surface shows ductility fracture characteristics and obvious wire drawing state, and the island structure almost disappears. This shows that adding more than 1.0 phr of ETBP to the PLA/PBAT blend can significantly improve the compatibility of PLA with PBAT and the toughness of the blend.
7. Rheological performance analysis
The figure shows the energy storage modulus (G')-angular frequency (ω) relationship curve and the loss modulus (G'')-angular frequency (ω) relationship curve of PLA/PBAT mixture under different ETBP content. These curves clearly reveal the characteristic behavior of the mixture, where the values of G' and G'' increase steadily with increasing shear frequency.
Looking at the two plots, it is clear that as the ETBP content increases, the G' and G'' values of the PLA/PBAT/ETBP mixture also increase. This can be explained by the fact that after the addition of ETBP, a tiny cross-linked network structure is formed, which strengthens the entanglement and interweaving between the PLA and PBAT molecular chains. This structural change and the increase in intermolecular friction increase the elasticity and melt viscosity of the material, so the modulus of storage (G') and the modulus of loss (G'') also increase.
It is worth noting that at higher shear frequencies, the slope of the curve (G's vs ω and G'' vs ω) decreases sharply as the ETBP content increases. This phenomenon can be explained by the fact that the addition of ETBP causes chemical micro-cross-linking, forming a chain segment near the crosslinking point. However, in most cases, the structure of the molecular segments remains almost unchanged except for the segments near the crosslinking point. Therefore, as the ETBP content increases, the slope of the curve decreases sharply, indicating that the movement of the chain segments is limited.
VIII. Conclusion
The addition of ETBP can enhance the interface compatibility between PLA and PBAT and improve the performance of PLA/PBAT blends by generating chemical micro-crosslinked structures. The epoxy group in ETBP reacts with the hydroxyl group in PLA/PBAT to help form a chemical micro-crosslinked structure, which improves the performance of the blend.
When the ETBP content was 3.0 phr, the elongation at break of the PLA/PBAT blend increased from 45.8% (pure PLA/PBAT) to 272%. Impact strength increased from 26.2 kJ·m to 45.3 kJ·m−2. The results of DMA analysis showed that the phase separation of the two phases in the PLA/PBAT blend decreased with the increase of ETBP content (not exceeding 3.0 phr), indicating that ETBP promoted the interface compatibility between PLA and PBAT phases.
The formation of the chemical micro-crosslinked structure was further confirmed by gel content analysis, and it was found that the gel content of the blend was about 17.8% when the ETBP content was 5.0 phr. SEM observations showed that ETBP improved the dispersion of PBAT particles in the PLA phase and blurred the interface morphology between the two phases.
Rheological analysis showed that after the addition of ETBP, the G' values at low shear frequencies increased with increasing ETBP content, while at higher shear frequencies, the G' values almost overlapped. The G'' value of the blend is affected by the micro-crosslinking effect and the lubrication effect.
Increasing the crystallinity of PLA and forming a chemical micro-cross-linked structure can effectively slow down the movement of PLA and PBAT molecular chains, thereby reducing the free volume of PLA and PBAT macromolecules. Thus, in PLA/PBAT mixtures with ETBP content not exceeding 3.0 phr, these changes reduce the transport of gas molecules such as O2 and CO2. This is very important for gas barrier performance in food packaging.
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