1.PLA: A degradable bioplastic material

PLA, which stands for Polylactic Acid, is a fully biodegradable plastic with a huge output. It is also known as polylactide and belongs to linear aliphatic thermoplastic polyester. With its outstanding mechanical properties, biocompatibility and processing plasticity, PLA has stood out in the field of biodegradable materials, demonstrating great development potential and broad application prospects. This has made PLA a focus of attention for many researchers and developers, and its application fields are also constantly expanding.

At present, the application of polylactic acid has covered multiple aspects. In the packaging field, it is made into films, agricultural greenhouse films, ground films, food containers and foam plastics, etc. In the medical field, PLA has great potential and is used as a drug carrier, surgical sutures, and alternative materials for orthopedics, etc. It is worth mentioning that the degradation products of PLA can combine with human metabolism, ensuring safety and no residue.

2. Molecular structure of polylactic acid (PLA)

Polylactic acid can be prepared through two methods: direct polycondensation and ring-opening polymerization. Among them, ring-opening polymerization can produce polylactic acid with higher molecular weight and more stable performance, namely polylactide. This substance is slightly yellow and transparent, with a glass transition temperature (Tg) of approximately 60℃, a melting temperature (Tm) ranging from 155 to 185℃, and a crystallization temperature (Tc) between 90 and 130℃. It is worth noting that due to the optical activity of lactic acid, polylactic acid thus has three optical isomers: levorotatory polylactic acid (PLLA), dextrorotatory polylactic acid (PDLA), and racemic polylactic acid (PDLLA). In practical applications, PLLA and PDLLA are highly favored due to their simple preparation.

3. The crystalline form of PLA

The crystalline form of polylactic acid (PLA) varies depending on the processing method and may form three different crystal forms: α crystal form, β crystal form and γ crystal form. Among them, the α crystal form is the most stable and common one, usually obtained through solution or melt crystallization growth, or formed by stretching at a low stretching rate and in a low-temperature environment. In contrast, the β crystal form is formed under tensile force at higher temperatures and tensile rates. As for the γ crystal form, its current preparation method is relatively simple, mainly obtained through the epitaxial growth of hexamethylbenzene.

4. The biodegradability of PLA

The degradation rate of PLA is influenced by multiple factors, including its molecular weight, environmental acidity or alkalinity, material crystallinity and crystal form, etc. Due to the relatively slow degradation rate of polylactic acid itself, its degradation process usually requires hydrolysis into polylactic acid molecules first, followed by enzymatic degradation. The main degradation mode is that PLA degrades through the main chain, generating oligomers and monomers with lower molecular weights.

When PLA has a relatively high average molecular weight or crystallinity, its degradation cycle will be correspondingly prolonged. However, in the presence of microorganisms or organic waste, the degradation cycle of polylactic acid molecules will be significantly shortened, and the degradation rate will also be notably accelerated. In contrast, polylactic acid is usually more degradable than traditional petroleum-based materials, demonstrating higher environmental performance.

From a structural perspective, the degradation process of polymers mainly includes the degradation of the polymer main chain, the hydrolysis of side chains, and the cleavage of crosslinking points. After undergoing soil burial experiments, polylactic acid can be completely decomposed into carbon dioxide and water under the combined action of natural factors such as microorganisms and water. These substances can further serve as raw materials for plants to synthesize starch. This demonstrates the recyclability of polylactic acid.

5. Modification of PLA

The performance of PLA can be optimized through plasticizing modification. This modification method aims to enhance the processing performance and material properties of PLA to meet specific application requirements. Through plasticizing modification, the toughness and ductility of PLA can be effectively improved, thereby enhancing its durability and impact resistance in practical use.

Plasticizing, a common technique, aims to enhance the flexibility of polymers and improve their processing performance. Plasticizing can enhance the toughness of materials by increasing the flexibility of molecular chains. Despite this, the strength of the material is often reduced during the plasticizing process, but this loss of mechanical properties can be balanced by regulating the ratio of the polymer to the plasticizer. In the polylactic acid/starch blend system, the application of plasticizers is particularly common. They can promote the melting processing of starch and enable it to be better dispersed in the PLA matrix.

Plasticizers can significantly enhance the flexibility and processing performance of PLA. For instance, small-molecule plasticizers such as triethyl citrate and tributyl citrate have been successfully applied to PLA, significantly enhancing its elongation at break and toughness. In addition, polylactic acid small molecules such as lactide and lactic acid oligomers have also been proven to be effective plasticizers. However, these plasticizers also have aging problems, which may affect their long-term performance.

Macromolecular plasticizers, such as polyethylene glycol and polypropylene glycol, can also effectively enhance the toughness of PLA. However, it is worth noting that as the molecular weight of the plasticizer increases, its plasticizing effect may be weakened, and it may lead to phase separation between the plasticizer and the matrix.

Improving the crystallization performance of PLA is another key approach to the modification of polylactic acid. The increase in crystallinity can directly and significantly enhance the strength and modulus of materials. Therefore, for those application fields that require high strength and high modulus, improving material performance by enhancing crystallinity is an effective method.

Tsuji et al. 's research revealed the influence of different nucleating agents (such as PDLA, talcum powder, fullerene C60, montmorillonite, and various sugar polymers) on the crystallization behavior of PLLA. They found that the addition of these nucleating agents could significantly increase the crystallinity and crystallization rate of PLLA, among which PDLA had the most significant effect, followed by talcum powder, fullerene C60, and montmorillonite, while the effect of polysaccharide polymers was relatively weak.

In addition, Lee et al. further explored the crystalline modulus of different types of polylactic acid (PLLA, PDLA, scPLA). They used X-ray diffraction technology to study the axial (El) and vertical (Et) moduli of PLA polymer chains respectively. The results show that the El values of PLLA and PDLA are both 14GPa, while that of scPLA is as high as 20GPa. Compared with other polymers with serrated molecular chain configurations, the El values of these polylactic acids are all at a relatively low level. Lee speculated that this might be related to the helical molecular chain configuration of PLA. It is worth noting that scPLA exhibits a higher modulus due to its more complete three-dimensional configuration, which further confirms the significant influence of the molecular chain configuration in the crystalline region on the material's performance.

Toughening modification of PLA is an important means to enhance its performance. Common methods include chemical copolymerization modification and physical blending modification. Chemical copolymerization modification can fundamentally change the properties of PLA. By regulating the molecular structure, monomer sequence and composition, excellent tensile and impact properties can be achieved. However, the research and development cycle of this method is relatively long and the cost is relatively high. In contrast, physical blending modification has advantages such as quick results, short cycle and low cost, and has attracted much attention in recent years.

Elastomer is a commonly used toughening agent. By blending it with PLA, its toughness can be significantly enhanced. Elastomers can form rubber dispersed phases and embed them into the brittle PLA matrix, thereby absorbing the energy brought by external forces and achieving a toughening effect. The toughening effect is influenced by multiple factors such as the mechanical properties of the elastomer, its content, and the interaction with the matrix phase. In addition, impact modifiers for fibers and rubber, such as natural rubber and epoxy natural rubber, can also be used to enhance the toughness and flexibility of PLA.

① Through free radical polymerization, natural rubber grafted with glycidyl acrylate (NR-GMA) reacts with PLA to produce in-situ dynamic vulcanized thermoplastic vulcanized rubber. Meanwhile, PLA reacts with ethylene glycol (EG) to yield GPLA. Just adding 20wt% of NR-GMA can significantly enhance the strength of PLA, increasing its notched impact strength and elongation at break by 73.4 kJ/m² and 136% respectively compared to pure PLA, and greatly improving its mechanical properties. Infrared spectroscopy and dynamic mechanical analysis proved that a cross-linked network structure was formed between PLA and NR-GMA, and there was a strong interaction at the two-phase interface, thereby enhancing the strength of PLA.

② High-strength composite materials are obtained by extruding cellulose nanofibers with a high aspect ratio and PLA through a twin-screw process. To prevent the aggregation of nanofibers, pretreatment was carried out before blending and extrusion. The nanofibers/PLA/ionomer composite material was finally produced. Research shows that the addition of ionic polymers reduces the viscosity of CNFs/PLA composites and enhances their fluidity. In addition, the flexural strength and flexural modulus of this composite material have also been significantly improved. Meanwhile, the nanofibers/PLA/ionomer composites also exhibit excellent impact toughness, thanks to the addition of core-shell impact modifiers such as acrylate, ABS and MBS.

③ The influence of different core-shell ratios of acrylate core-shell toughening agents on the performance of PLA was explored. Research shows that when the core-shell ratio of ACR is 79.2/20.8, the impact strength of PLA reaches its maximum value, which is 77.1kJ/m². It was observed by transmission electron microscopy (TEM) that with the increase of PMMA content, the dispersion of ACR core-shell toughening agent in the PLA matrix was improved, thereby enhancing the compatibility of the two phases.

④ The influence of epoxy-functionalized ABS core-shell particles on the properties of PLA was further explored. 1wt% of GMA was successfully introduced by grafting GMA onto the shell layer of ABS core-shell particles. Research shows that after adding unmodified ABS, the impact strength of the blend increased nearly 2.5 times compared to pure polylactic acid, rising from 20J/m to 50J/m. However, when 1wt% of GMA was added, the notch impact strength of the blend increased by nearly 27 times, reaching an astonishing 540J/m. This is attributed to the epoxy functionalization effect of GMA, which effectively improves the compatibility between PLA and ABS, making the dispersion of ABS in the PLA matrix more uniform.

The toughening effect of carbon dioxide-based thermoplastic polyurethane (PPCU) on PLA, which was obtained through the chain extension reaction of MDI and butanediol based on PPC, was further studied. The PLA/PPC blend was successfully prepared by twin-screw extrusion technology. The experimental results show that the addition of PPC significantly alters the crystalline structure of PLA and enhances its crystallization ability. However, as the PPC content increases, the thermal stability of the blend gradually decreases, and the non-Newtonian fluid behavior becomes increasingly obvious. When 30wt% PPC was added, the impact strength of the blend was significantly increased by 1.6 times compared to pure PLA, rising from 1.7kJ/m ² to 2.7kJ/m². In addition, the elongation at break has also increased by nearly three times, rising from 10.6% to 28.3%.