PSMA / PDLA modified PLLA composite film and preparation method thereof

Abstract

This invention relates to the field of polylactic acid (PLLA) technology, specifically to a PSMA / PDLA-modified PLLA composite film and its preparation method. The PSMA / PDLA-modified PLLA composite film of this invention comprises a blend of L-polylactic acid (LMA) and a melt blend of octadecyl methacrylate (OMA) and D-polylactic acid (DMA). The OMA / DMA melt blend is obtained by melt blending OMA, DMA, and an initiator. By introducing the OMA / DMA blend, the SMA homopolymer forms a microphase-separated structure with PLLA. The SMA homopolymer forms a soft phase region, improving the toughness of PLLA. Simultaneously, the stereocomplex formed by the PDLA and PLLA molecular chains "anchors" the PLLA matrix to the soft phase of the SMA homopolymer, thereby improving mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid (PLLA) technology, specifically to a PSMA / PDLA modified PLLA composite film and its preparation method. Background Technology

[0002] Polylactic acid (PLA) is a bio-based, biodegradable aliphatic polyester synthesized from lactic acid monomers, possessing both environmental friendliness and processability. With its excellent mechanical properties, biocompatibility, and compatibility with various molding processes, PLA has become a core material in the field of biodegradable polymers, finding widespread application in packaging, textiles, agricultural films, and medical devices. With increasing environmental demands, PLA's potential as a substitute for petroleum-based plastics has attracted considerable attention from the academic community. However, its inherent defects, such as insufficient toughness, slow crystallization rate, and poor thermal stability, limit its widespread application. For example, in the packaging field, PLA's relatively low melting temperature easily leads to thermal deformation of products, directly affecting the material's performance during use. To address these limitations, current research primarily employs composite modification strategies such as copolymerization modification, blending toughening, and the addition of nucleating agents to optimize material properties.

[0003] The multi-level structure of PLA provides a rich space for regulating its crystallization behavior. Studies have shown that blending L-polylactic acid (PLLA) with D-polylactic acid (PDLA) can construct stereocomposite crystals (SC). Compared to homogeneous crystalline phases (HC), this heterogeneous crystalline phase system achieves breakthrough improvements in key performance indicators such as fracture toughness and heat distortion temperature, significantly expanding its application boundaries in industrialization. Furthermore, by introducing bio-based reinforcing phases such as nanocellulose to construct the composite system, not only can the PLA crystallization kinetics be accelerated through heterogeneous nucleation effects, but the material's mechanical strength, thermal stability, and controllable degradation characteristics can also be simultaneously enhanced, achieving multifunctional synergistic effects.

[0004] Driven by the carbon neutrality strategy, PLA, as a recyclable bio-based material, has achieved large-scale market demand. Although the performance limitations of PLA remain a bottleneck restricting its industrialization, continuous innovation in modification technologies such as structural design and multiphase composites has shown strong development prospects in areas such as replacing traditional plastics and developing high-end functional materials. The deep integration of basic research and industrial transformation deserves continued exploration by researchers.

[0005] Chinese patent application CN117844204A, published on April 9, 2024, discloses a rapidly crystallizing polylactic acid (PLA) material, its preparation method, and its application. The raw materials include 90-98 parts of poly(L-lactic acid) and 2-10 parts of a multifunctional additive. The multifunctional additive is poly(D-lactic acid) grafted with methoxy polyethylene glycol mono(meth)acrylate. The preparation of the multifunctional additive involves mixing poly(D-lactic acid), methoxy polyethylene glycol mono(meth)acrylate, and organic peroxide in appropriate proportions to obtain a uniform mixture, which is then extruded and granulated. The methoxy polyethylene glycol mono(meth)acrylate aggregates around the PDLA, maximizing the molecular chain mobility of PLA and acting as both a nucleating agent and a plasticizer. This not only increases the number of crystallization nucleation sites for PLA but also enhances the molecular chain mobility, thus synergistically promoting the crystallization rate of PLA. However, the mechanical properties of the aforementioned PLA material are still relatively poor. Summary of the Invention

[0006] The first objective of this invention is to provide a PSMA / PDLA modified PLLA composite film to solve the problem of poor mechanical properties of existing polylactic acid materials.

[0007] The second objective of this invention is to provide a method for preparing PSMA / PDLA modified PLLA composite films, thereby solving the problem of poor mechanical properties of existing polylactic acid materials.

[0008] To solve the above-mentioned technical problems, the technical solution of the PSMA / PDLA modified PLLA composite film of the present invention is as follows: A PSMA / PDLA modified PLLA composite film comprises a blend of L-polylactic acid and a melt blend of octadecyl methacrylate / dextral polylactic acid; the melt blend of octadecyl methacrylate / dextral polylactic acid is obtained by melt blending octadecyl methacrylate, dextrorotatory polylactic acid and an initiator.

[0009] This invention improves upon existing technology by providing a PSMA / PDLA modified PLLA composite film. Through the melt blending of poly(octadecyl methacrylate) and polylactic acid (PLA), the SMA homopolymer and PLLA form a microphase separation structure. The SMA homopolymer forms a soft phase region, enhancing the toughness of PLLA. Simultaneously, the PDLA and PLLA molecular chains form a regular stereocomplex (sc-PLA) through intermolecular hydrogen bonds. These sc-PLA microcrystalline regions act as physical crosslinking points and reinforcing fillers in the film, anchoring the PLLA matrix and the SMA homopolymer soft phase together. This is crucial for improving the mechanical properties of the film. Furthermore, although the PSMA / PDLA modified PLLA composite film introduces new macromolecular chain segments, it has little impact on the film's transparency.

[0010] Preferably, the weight ratio of dextrorotatory polylactic acid (DPL) to octadecyl methacrylate is (100~110):(1~5); and the weight ratio of DPL to initiator is (100~110):(2~3).

[0011] Preferably, the melt blending temperature is 170~190℃ and the melt blending time is 8~15min.

[0012] Preferably, the initiator is an organic peroxide.

[0013] Preferably, the mass ratio of L-polylactic acid (PLA) to poly(octadecyl methacrylate) / dextral polylactic acid (PLA) melt blend is (90~50):(10~50). That is, the mass of PSMA / PDLA accounts for 10~50% of the total mass of PSMA / PDLA and PLLA. More preferably, the mass of PSMA / PDLA accounts for 20~30% of the total mass of PSMA / PDLA and PLLA.

[0014] Preferably, the mixture includes a blend of L-polylactic acid (PLA), a melt blend of poly(octadecyl methacrylate) / D-polylactic acid (PLA), and an epoxy chain extender. As a reactive compatibilizer, ADR reacts with the carboxyl / hydroxyl groups of PLLA using its epoxy groups, chemically linking the separated polymer segments in the system to form a denser network structure. Due to the "anchoring" effect of the chemical bonds formed by ADR at the phase interface, the phase separation process between the components of the system is strongly suppressed. The soft phase of the SMA homopolymer cannot aggregate into a large phase structure, but is instead refined and uniformly dispersed in the continuous PLLA phase, forming a finer and more stable phase morphology. Meanwhile, the chain extension effect of ADR may enable PDLA and PLLA segments to be chemically bonded to the same ADR molecule. This greatly promotes the close contact and regular arrangement of PDLA and PLLA segments, which is conducive to the formation of more and more perfect stereocomposite crystals. These sc-PLA crystal regions, as physical crosslinking points, work synergistically with the chemical crosslinking / branching network promoted by ADR to further enhance the network structure and improve the performance of the film, especially the elongation at break and toughness are significantly improved.

[0015] Preferably, the amount of epoxy chain extender added is 1 to 2 wt% of the total mass of the L-polylactic acid and polyoctadecyl methacrylate / D-polylactic acid melt blend.

[0016] Preferably, the weight ratio of dextrorotatory polylactic acid to octadecyl methacrylate is (100~110):(2~5). This ratio further improves the elongation at break and tensile strength of the modified film.

[0017] Preferably, the mass ratio of L-polylactic acid to poly(octadecyl methacrylate) / d-polylactic acid melt blend is (90~50):(45~50). This ratio further improves the tensile strength of the modified film.

[0018] The technical solution of the preparation method of the PSMA / PDLA modified PLLA composite film of the present invention is as follows: A method for preparing the PSMA / PDLA modified PLLA composite film includes the following steps: preparing a film by solution casting of a melt blend of L-polylactic acid and polyoctadecyl methacrylate / D-polylactic acid; wherein the polyoctadecyl methacrylate / D-polylactic acid melt blend is obtained by melt blending octadecyl methacrylate, D-polylactic acid and an initiator.

[0019] The method for preparing PSMA / PDLA modified PLLA composite films provided by this invention can be achieved through a simple solution casting method. The process is simple and efficient, and suitable for large-scale production. Attached Figure Description

[0020] Figure 1 Torque-time curves for PDLA / BIBP / SMA blends with different SMA contents; Figure 2 FT-IR spectra of SMA, PDLA, and PSMA / PDLA; Figure 3 Appearance images of PLLA and modified membrane samples; Figure 4 A graph showing the relationship between the amount of SMA added to PSMA / PDLA / PLLA films and tensile strength and elongation at break. Figure 5 A graph showing the relationship between the amount of SMA added to PSMA / PDLA / PLLA films and their transmittance. Figure 6 Figures showing the tensile strength and elongation at break of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents; Figure 7 Stress-strain curves of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents; Figure 8 Transmittance diagrams of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents; Figure 9 Figures showing the tensile strength and elongation at break of PSMA / PDLA / PLLA / ADR modified films with different PSMA / PDLA contents; Figure 10Stress-strain curves of PSMA / PDLA / PLLA / ADR modified films with different PSMA / PDLA contents; Figure 11 The transmittance of PSMA / PDLA / PLLA / ADR modified films with different PSMA / PDLA contents; Figure 12 Morphology of PSMA / PDLA / PLLA modified membranes after fracture (left image) with and without ADR (right image); Figure 13 SEM images of the tensile fracture points of PSMA / PDLA / PLLA modified films with and without ADR (left image) / with ADR (right image); Figure 14 XRD patterns of PLLA, PDLA / PLLA, PSMA / PDLA / PLLA and PSMA / PDLA / PLLA / ADR modified films; Figure 15 A graph showing the changes in strawberry pieces stored without a membrane seal for different times; Figure 16 The changes in strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 10wt% PSMA / PDLA at different storage times; Figure 17 The changes in strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 20wt% PSMA / PDLA at different storage times; Figure 18 The changes in strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 30wt% PSMA / PDLA at different storage times; Figure 19 The changes in strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 40wt% PSMA / PDLA at different storage times; Figure 20 The graph shows the changes in strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 50wt% PSMA / PDLA over different storage times. Detailed Implementation

[0021] The technical concept of the PSMA / PDLA modified PLLA composite film provided by this invention is as follows: Existing technology involves grafting poly(D-lactic acid) with methoxy polyethylene glycol mono(meth)acrylate and poly(L-lactic acid) and then extruding and granulating the mixture. The methoxy polyethylene glycol mono(meth)acrylate can maximize the molecular chain mobility of polylactic acid and act as a nucleating agent and plasticizer, thereby accelerating the crystallization rate.

[0022] This invention introduces a poly(octadecyl methacrylate) / dextral polylactic acid melt blend into L-polylactic acid. The SMA homopolymer and PLLA form a microphase separation structure, with the SMA homopolymer forming a soft phase region to improve the toughness of PLLA. The stereocomplex formed by PDLA and PLLA molecular chains acts as a physical crosslinking point and reinforcing filler in the film, "anchoring" the PLLA matrix and the SMA homopolymer soft phase together, thereby improving mechanical properties.

[0023] The PSMA / PDLA modified PLLA composite film provided by the present invention comprises a melt blend of L-polylactic acid and polyoctadecyl methacrylate / dextral polylactic acid in a mass ratio of (90~50):(10~50); the polyoctadecyl methacrylate / dextral polylactic acid melt blend is obtained by melt blending octadecyl methacrylate, dextrorotatory polylactic acid and an initiator.

[0024] More preferably, the mass ratio of L-polylactic acid and polyoctadecyl methacrylate / d-polylactic acid melt blend is (90~50):(45~50).

[0025] The poly(octadecyl methacrylate) / dextral polylactic acid melt blend is obtained by melt-blending octadecyl methacrylate and dextrorotatory polylactic acid in a weight ratio of (100~110):(1~5) at 170~190°C for 8~15 min under the action of an initiator; the weight ratio of dextrorotatory polylactic acid to initiator is (100~110):(2~3). More preferably, the weight ratio of dextrorotatory polylactic acid to octadecyl methacrylate is (100~110):(2~5). The initiator is an organic peroxide. More preferably, the initiator is di-tert-butylperoxide diisopropylbenzene.

[0026] More preferably, the PSMA / PDLA modified PLLA composite film comprises a blend of L-polylactic acid, a polyoctadecyl methacrylate / d-polylactic acid melt blend, and an epoxy chain extender; the amount of epoxy chain extender added is 1 to 2 wt% of the total mass of L-polylactic acid and the polyoctadecyl methacrylate / d-polylactic acid melt blend.

[0027] The PSMA / PDLA modified PLLA composite film has a length and width of 10~15cm and a thickness of 0.04~0.06mm.

[0028] The preparation method of PSMA / PDLA modified PLLA composite film provided by the present invention includes the following steps: preparing a film by solution casting of a melt blend of L-polylactic acid and polyoctadecyl methacrylate / D-polylactic acid; the melt blend of polyoctadecyl methacrylate / D-polylactic acid is obtained by melt blending octadecyl methacrylate, D-polylactic acid and an initiator.

[0029] Specifically, the solution casting method involves dissolving a melt blend of L-polylactic acid and polyoctadecyl methacrylate / D-polylactic acid in a solvent, then placing it in a container and allowing the solvent to evaporate to obtain a film product; the solvent is dichloromethane.

[0030] Understandably, when epoxy chain extenders are included, solution casting involves dissolving L-polylactic acid, polyoctadecyl methacrylate / D-polylactic acid melt blend, and epoxy chain extenders in a solvent.

[0031] Specifically, the mass fraction of all raw materials in the solvent is 5-8%.

[0032] Specifically, during dissolution, the raw materials are first stirred in a solvent to form a transparent solution, and then subjected to ultrasonic treatment. The ultrasonic treatment time is 5-10 minutes.

[0033] Specifically, the number-average molecular weight of dextrorotatory polylactic acid is (1~3)×10⁻⁶. 5 g / mol; the number-average molecular weight of polylactic acid (L-L) is (2~4)×10 5 g / mol.

[0034] The embodiments of the present invention will be further described below with reference to specific examples. Unless otherwise specified, the chemical reagents used in the following examples are all commercially available conventional products. The raw materials and equipment used in the examples and experimental cases are shown in Tables 1 and 2.

[0035] Table 1 Raw materials used in the examples Table 2. Equipment used in the examples and experimental cases. I. Specific Embodiments of the PSMA / PDLA Modified PLLA Composite Film and its Preparation Method of the Present Invention The PSMA / PDLA modified PLLA composite film of the present invention comprises a blend of polylactic acid (PLLA) and poly(octadecyl methacrylate) (PSMA) / polylactic acid (PDLA) melt blend; the poly(octadecyl methacrylate) (PSMA) / polylactic acid (PDLA) melt blend is obtained by melt blending octadecyl methacrylate, polylactic acid (PDLA) and an initiator.

[0036] The preparation method of the PSMA / PDLA modified PLLA composite film of the present invention is as follows: (1) First, dry all raw materials in a drying oven at 60°C for 10 hours. Then, use a torque rheometer to dry the dextrorotatory polylactic acid (PDLA) (number average molecular weight: 1.01×10⁻⁶). 5PSMA / PDLA physical blends were obtained by melt blending octadecyl methacrylate (SMA) and di-tert-butyl peroxide (BIBP) at a temperature of 180°C, a rotation speed of 60 rpm, and a melting time of 8 min.

[0037] The reaction mechanism during melt blending is as follows: Main reaction: Homopolymerization of SMA.

[0038] BIBP decomposes to generate free radicals (R·), which trigger chain polymerization of SMA monomers (containing C=C double bonds).

[0039] Chain initiation: R· + CH2=C(CH3)COO-C 18 H 37 →R-CH2-C·(CH3)COO-C 18 H 37 ; Chain growth: ... + n CH2=C(CH3)COO-C 18 H 37 → Formation of long PSMA polymer chains; Reaction result: The polymerization reaction yielded a poly(octadecyl methacrylate) (PSMA) homopolymer.

[0040] Side reactions: degradation and cross-linking of PDLA.

[0041] BIBP decomposition generates free radicals (R·). A small number of free radicals attack the saturated molecular chains of PDLA, initiating β-fracture of its main chain (this is the main degradation mechanism of polyester under the action of free radicals and heat).

[0042] Hydrogen abstraction and chain breakage: Free radicals abstract hydrogen (especially α-H) from the PDLA chain to form PDLA macromolecular free radicals, which then undergo β-cleavage, resulting in a decrease in molecular weight and potentially producing fragments with terminal free radicals.

[0043] ~~O-CH(CH3)-C(=O)-O~~+R·→~~OC·(CH3)-C(=O)-O~~+RH; ~~OC·(CH3)-C(=O)-O~~→ undergoes β-cleavage, generating small molecule fragments; Crosslinking: When two PDLA macromolecular free radicals meet, coupling termination occurs, forming a crosslinked structure.

[0044] P·+·P→PP.

[0045] Reaction results: PDLA underwent thermal-oxidative degradation, resulting in a decrease in molecular weight, possibly accompanied by the formation of cross-linked structures.

[0046] (2) The PSMA / PDLA physical blend obtained in step (1) and PLLA (number average molecular weight: 2.14 × 10⁻⁶) were mixed together. 5 A 5% dichloromethane solution was prepared by mixing PSMA / PDLA, PLLA, and ADR with epoxy chain extender (ADR) and dichloromethane. After magnetic stirring for 12 hours, a transparent solution was obtained. After ultrasonic treatment for 5 minutes, 10 mL of the solution was pipetted into a square glass mold with a side length of 10 cm. After standing at room temperature for 12 hours, the solvent was allowed to evaporate, and a PLA modified film with a thickness of 0.04 mm was obtained.

[0047] The formulation composition of the two steps in each embodiment is shown in Table 3. phr refers to the number of parts by weight of SMA and BIBP added per 100 parts by weight of PDLA.

[0048] Table 3. Formulation composition of the two steps in each embodiment. II. Experimental Examples (1) Characterization of PSMA / PDLA physical blends Torque-time curves of PDLA / BIBP / SMAn (n=1,2,3,4,5) blends with different SMA contents are shown below. Figure 1 As shown, the torque of the blend exhibits a trend of first decreasing, then increasing, and then decreasing again as the mixing time increases. This is because in the initial stage, due to material mixing and melting, the torque initially increases briefly and then gradually decreases. As the melt temperature rises to the BIBP decomposition temperature, BIBP decomposes to generate free radicals, thereby triggering a chain polymerization of SMA monomers. With the occurrence of SMA homopolymerization, a certain molecular weight SMA homopolymer is formed, the system viscosity increases significantly, and the torque begins to rise significantly to its peak value. The prepared SMA homopolymer forms a homogeneous blend with PDLA, and the torque tends to level off. As the mixing time increases, a small amount of PDLA molecular chains degrade under high temperature conditions, resulting in a slight decrease in torque.

[0049] FT-IR spectra of SMA, PDLA, and PSMA / PDLA are as follows: Figure 2 As shown, where Figure 2 The left side is 500-4000cm -1 FT-IR spectrum; Figure 2 The right side is 1600-1800cm -1 Enlarged view of part of the image. 1660-1800 cm⁻¹ in the PSMA / PDLA infrared curve. -1 Two characteristic absorption peaks appear within the range, one at 1745 cm⁻¹. -1 The peak near the PDLA backbone is the ester group (C=O), which is strong and clearly visible in the figure. Meanwhile, the peak at 1660-1700 cm⁻¹ is...-1 There is a very small ester group peak from the SMA branched chain, which is not obvious due to the small amount of SMA added. The SMA infrared curve is at 1630 cm⁻¹. -1 The characteristic peak at this point is the C=C double bond absorption peak, while the PSMA / PDLA infrared curve shows an absorption peak at 1610-1650 cm⁻¹. -1 There is no SMA absorption peak at the C=C double bond, and the SMA peak is at 1630 cm⁻¹. -1 The disappearance of the C=C double bond absorption peak indicates that the polymerization of SMA occurs at the C=C double bond, resulting in SMA homopolymer.

[0050] (2) Characterization of PSMA / PDLA / PLLA modified membrane 2.1 Transparency PLLA and modified film samples prepared using solution casting method, such as... Figure 3 As shown, Figure 3 From left to right, the images show the appearance of a PLLA film, a PDLA / PLLA (50 / 50) film, and a PSMA / PDLA / PLLA (50 / 50) film (Example 3). The PSMA / PDLA ratio significantly affects the appearance of the PLLA film.

[0051] Pure PLLA is a semi-crystalline polymer, and its transparency is primarily determined by its crystallinity. When PLLA is in an amorphous or low-crystallinity state, the disordered arrangement of the molecular chains reduces the interface between crystalline and amorphous regions, resulting in weak light scattering and a transparent material. However, during the solution evaporation film formation process, the rapid evaporation of dichloromethane prevents the PLLA molecular chains from arranging themselves in an orderly manner, leading to a predominantly amorphous structure with extremely low crystallinity, thus producing a transparent film.

[0052] Compared to PLLA membranes, PDLA / PLLA membranes exhibit reduced transparency because PDLA / PLLA forms a stereocomplex (sc-PLA) in solution, resulting in higher crystallinity than pure PLLA and thus lower transparency.

[0053] The presence of PSMA homopolymer in the PSMA / PDLA / PLLA membrane and the formation of a stereocomplex (sc-PLA) between PDLA and PLLA in solution both further affect the membrane's transparency. Figure 3 The comparison of membrane transparency shows that although the transparency of PDLA / PLLA (50 / 50) membrane and PSMA / PDLA / PLLA (50 / 50) membrane decreased to some extent, they still showed good overall transparency.

[0054] 2.2 Mechanical properties of different SMA addition amounts SMA homopolymer forms a microphase-separated structure with PLLA, with the SMA homopolymer forming a soft phase region that enhances the toughness of PLLA. In solution, PDLA and PLLA molecular chains form a well-ordered stereocomplex (sc-PLA) through intermolecular hydrogen bonds. These sc-PLA microcrystalline regions act as physical crosslinking points and reinforcing fillers in the film, anchoring the PLLA matrix to the SMA homopolymer soft phase, which is crucial for improving the film's mechanical properties.

[0055] The relationship between the amount of SMA added in PSMA / PDLA / PLLA film (50 / 50) and tensile strength and elongation at break is shown in Table 4. Figure 4 As shown, the elongation at break of the PSMA / PDLA / PLLA membrane initially decreased, then increased, and then decreased again with increasing SMA content, reaching its highest value of 9.86% at 3 wt% SMA content. The tensile strength, on the other hand, initially increased and then decreased with increasing SMA content, reaching its highest value of 57.27 MPa at 4 wt% SMA content.

[0056] Results Analysis: The graph clearly shows that when SMA content is 1 wt%, the elongation at break is lower than that of the PDLA / PLLA film. This is because a small amount of SMA (1 wt%) forms shorter PSMA molecular chains, which interfere with the hydrogen bond network between PDLA and PLLA, disrupting the formation of some stereocomplexes and leading to decreased crystallinity. The reduction in stereocomplexes increases the material's brittleness and decreases the elongation at break. As the amount of SMA increases, the PSMA molecular chains become longer, dispersing into the PLLA to form a soft phase region, which can more effectively dissipate tensile stress, gradually increasing the elongation at break. When the SMA content is 3 wt%, the synergistic effect between the long PSMA chains and PLLA reaches its optimal level, resulting in the best toughness. Further increases in SMA content lead to greater phase separation between PSMA and PLLA, weakening their interfacial forces and causing a decrease in elongation at break. The tensile strength increases with the addition of SMA because the molecular chains of PSMA / PDLA and PLLA become more entangled, resulting in greater tensile strength. However, when the addition of SMA is excessive, the phase separation of the PSMA / PDLA / PLLA system increases, and the tensile strength also decreases.

[0057] Table 4. Test results of mechanical properties of PSMA / PDLA / PLLA modified membranes SMA content (wt%) PLLA 0 1 2 3 4 5 Tensile strength (MPa) 42.00 37.47 38.80 52.83 54.28 57.27 54.08 Elongation at break (%) 3.34 7.83 5.22 7.18 9.86 8.83 8.40 The membrane prepared with 3 wt% PSMA / PDLA and PLLA showed the highest elongation at break and maintained high tensile strength. Therefore, experiments were continued using 3 wt% PSMA / PDLA.

[0058] 2.3 Transmittance of different amounts of SMA added The relationship between the amount of SMA added in PSMA / PDLA / PLLA films and light transmittance is shown in Table 5. Figure 5 As shown.

[0059] The transmittance of the prepared PSMA / PDLA / PLLA films was all above 90%. The pure PLLA film, because it did not introduce hydrophobic segments of SMA or form a stereocomplex with PDLA, did not compromise the uniformity of the PLLA film and had no scattering sources caused by internal compositional differences. Therefore, the pure PLLA film had the highest transmittance. Although the introduction of new macromolecular segments had some impact on the transparency of the PDLA / PLLA and PSMA / PDLA / PLLA films, the impact on their transmittance was relatively small, only slightly decreasing.

[0060] Table 5. Test results of transmittance of PSMA / PDLA / PLLA films SMA content (wt%) PLLA 0 1 2 3 4 5 Light transmittance (%) 92.9 90.7 91.0 92.0 92.1 91.9 91.8 2.4 Mechanical properties with different PSMA / PDLA contents The tensile strength and elongation at break of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents are shown in Table 6 and Figure 6 As shown.

[0061] With increasing PSMA / PDLA content, the elongation at break of the PSMA / PDLA / PLLA film decreases, reaching its highest value of 33.59% at a PSMA / PDLA content of 10 wt%, and its lowest value at 50 wt%. Conversely, the tensile strength of the PSMA / PDLA / PLLA film initially decreases and then increases with increasing PSMA / PDLA content, reaching its highest value of 54.28 MPa at a PSMA / PDLA content of 50 wt%, and its lowest value at 20 wt%.

[0062] Results Analysis: PDLA and PLLA molecular chains form a stereocomplex through hydrogen bonds and van der Waals forces. However, due to the blending of PDLA and PSMA, the PSMA molecular chains somewhat hinder the orderly arrangement of PDLA and PLLA, affecting the formation of stereocomplex crystals. Only a small amount of stereocomplex crystals exists, insufficient to dominate the mechanical properties of the modified film. Therefore, when the PSMA / PDLA content is low, the flexible PSMA segments mainly absorb energy, delaying crack propagation and significantly enhancing the elongation at break of the modified film. Furthermore, the reduction in stereocomplex crystals at low content directly leads to insufficient rigidity of the modified film, resulting in a significant decrease in tensile strength. When the PSMA / PDLA content is high, the increased phase separation between PSMA and PLLA leads to rapid crack propagation, resulting in a relatively low elongation at break. Moreover, at high PSMA / PDLA content, the increased number of PSMA molecular chains enhances the physical entanglement between PSMA, PLLA, and stereocomplex crystals, thereby improving the tensile strength of the modified film.

[0063] Table 6. Test results of mechanical properties of PSMA / PDLA / PLLA membranes with different ratios. PSMA / PDLA content (wt%) 10 20 30 40 50 Tensile strength (MPa) 29.60 22.38 25.54 26.42 54.28 Elongation at break (%) 33.59 15.40 12.47 12.08 9.86 Stress-strain curves of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents are shown below. Figure 7 As shown, the tensile strength of the PSMA / PDLA / PLLA (50 / 50) modified film is much greater than that of the modified films with other ratios, while its elongation at break is lower. The trend of the curves also shows that at fracture, the fracture curve of the modified film with 50wt% PSMA / PDLA content is a hard curve, while the fracture curves of the other four ratios are weak fracture curves.

[0064] 2.5 Transmittance with different PSMA / PDLA contents: The transmittance of PSMA / PDLA / PLLA modified films with different PSMA / PDLA contents is shown in Table 7. Figure 8 As shown, with the increase of PSMA / PDLA content, the transmittance of the PSMA / PDLA / PLLA modified film does not change much, and remains above 90%.

[0065] Results Analysis: PLLA exhibits low crystallinity during film formation, resulting in extremely low absorption of visible light in its amorphous regions, with a transmittance exceeding 90%. Furthermore, the long alkyl chains of PSMA do not exhibit strong absorption peaks for visible light, meaning PSMA does not significantly increase the material's light absorption. Therefore, changes in PSMA / PDLA content have minimal impact on the transmittance of the PSMA / PDLA / PLLA modified film.

[0066] Table 7. Transmittance test results of PSMA / PDLA / PLLA modified films with different ratios PSMA / PDLA content (wt%) 10 20 30 40 50 Light transmittance (%) 91.2 91.0 91.3 91.0 92.1 (3) Characterization of PSMA / PDLA / PLLA / ADR modified membrane 3.1 Mechanical properties of PSMA / PDLA / PLLA / ADR modified membranes The PSMA / PDLA blend, along with PLLA and ADR, is dissolved in a good solvent to form a homogeneous solution, which is then cast into a film. With the addition of the chain extender ADR, the film formation process transforms from simple physical blending and phase separation into a complex chemical reaction-driven "in-situ compatibilization and chain extension" process. The core change lies in the activation of the originally inert terminal groups (-COOH and -OH) of PLLA and PDLA by the epoxy groups of ADR. This chemical reaction connects the separated polymer segments in the system, forming a more compact network structure.

[0067] Due to the "anchoring" effect of the chemical bonds formed by ADR at the phase interface, the phase separation process between the components of the system is strongly suppressed. The soft phase of SMA homopolymer cannot condense into a large phase structure, but is refined and uniformly dispersed in the PLLA continuous phase, forming a more refined and stable phase morphology.

[0068] Furthermore, the chain-extending effect of ADR may allow PDLA and PLLA segments to chemically bond to the same ADR molecule. This greatly promotes close contact and orderly arrangement of PDLA and PLLA segments, which is beneficial for forming more and more complete stereocomposite crystals (sc-PLA). These sc-PLA crystal regions, as physical crosslinking points, synergistically enhance the network structure and improve the film's performance, especially significantly increasing elongation at break and toughness.

[0069] The tensile strength and elongation at break of PSMA / PDLA / PLLA / ADR modified films with different PSMA / PDLA contents are shown in Table 8 and Figure 9 As shown. Figure 9As shown, with the increase of PSMA / PDLA content, the elongation at break of the modified film first increases and then decreases. The elongation at break is highest at 30 wt% (275.70%), and lowest at 20 wt% (49.28%). Similarly, the tensile strength also shows a trend of first increasing and then decreasing with the increase of PSMA / PDLA content. The tensile strength is highest at 20 wt% (51.19 MPa), and lowest at 50 wt% (36.50 MPa). The elongation at break of the PSMA / PDLA / PLLA modified film without ADR (PSMA / PDLA content of 30wt%) was 12.47%, and the tensile strength was 25.54MPa. Therefore, compared with the modified film without ADR, the PSMA / PDLA / PLLA modified film with ADR (PSMA / PDLA content of 30wt%) increased the elongation at break by 2110.9% and the tensile strength by 63.5%.

[0070] Results Analysis: As a reactive compatibilizer, ADR reacts with the carboxyl / hydroxyl groups of PLLA through its epoxy groups, connecting the separated polymer segments in the system through chemical reaction to form a tighter network structure. This results in a significant improvement in the elongation at break of the entire PSMA / PDLA / PLLA system after the addition of ADR. Furthermore, the synergy between ADR and PSMA is optimal when the PSMA / PDLA / PLLA content is 30wt%, at which point the toughness of the modified film reaches its maximum. The improvement in tensile properties is due to the "anchoring" effect of chemical bonds formed by ADR at the phase interface and its promoting effect on stereocomplex crystals, which enhances the stress transfer efficiency. This results in a significant increase in tensile strength compared to the system without ADR when the PSMA / PDLA content is 10wt%, 20wt%, 30wt%, and 40wt%. However, when the PSMA / PDLA content is 50wt%, the volume ratio of the PSMA phase region in the system increases significantly. Since the amount of ADR added is relatively small, the epoxy groups of ADR cannot cover all the incompatible interfaces between PSMA and PLLA, resulting in a weakening of the phase interface effect and a significant decrease in tensile strength compared to other ADR-modified systems.

[0071] Table 8. Test results of mechanical properties of PSMA / PDLA / PLLA / ADR modified membranes with different proportions. PSMA / PDLA content (wt%) 10 20 30 40 50 Tensile strength (MPa) 47.51 51.19 41.78 43.42 36.50 Elongation at break (%) 51.63 49.28 275.70 240.26 207.85 The stress-strain curves of PSMA / PDLA / PLLA / ADR modified films are as follows: Figure 10As shown, the PSMA / PDLA / PLLA / ADR modified film exhibits a significantly improved elongation at break compared to the PSMA / PDLA / PLLA modified film without ADR, indicating a substantial improvement in toughness. Furthermore, the tensile strength of the modified film with PSMA / PDLA content ranging from 10wt% to 40wt% is also significantly enhanced, with the tensile strength decreasing only at a content of 50wt%. The trends in the curves also reveal that the stress-strain curves of the ADR-modified film, upon fracture, show a decrease after passing the yield point, followed by a rise before finally breaking. Therefore, the samples at this point represent a ductile fracture state.

[0072] 3.2 Light transmittance of PSMA / PDLA / PLLA / ADR modified films: The transmittance of PSMA / PDLA / PLLA / ADR modified films with different PSMA / PDLA contents is shown in Table 9. Figure 11 As shown. Figure 11 As shown, the transmittance was not significantly affected by the increase of PSMA / PDLA content, remaining above 90%. Compared with the PSMA / PDLA / PLLA modified film without ADR, the change in transmittance of PSMA / PDLA after adding ADR was also relatively small.

[0073] Results Analysis: With changes in PSMA / PDLA content, the transmittance of the modified film remained above 90%, showing no significant difference from the system without ADR. This is because neither the long alkyl chain of PSMA nor the ADR molecule contains strong light-absorbing groups, and the PLLA matrix exhibits extremely low light absorption in its amorphous region. Therefore, the overall light transmittance of the material was not affected by the chemical reaction after adding ADR, maintaining its high transmittance characteristic.

[0074] Table 9. Transmittance test results of PSMA / PDLA / PLLA / ADR modified films with different proportions. PSMA / PDLA content (wt%) 10 20 30 40 50 Light transmittance (%) 92.8 92.5 92.2 92.3 91.2 3.3 Microstructure morphology of PSMA / PDLA / PLLA / ADR modified membranes: The fracture sites of tensile specimens of PSMA / PDLA / PLLA modified films without ADR (PSMA / PDLA content of 30wt%) and PSMA / PDLA / PLLA / ADR modified films with ADR (PSMA / PDLA content of 30wt%) were observed and analyzed by SEM electron microscopy.

[0075] Morphology images of tensile specimens of PSMA / PDLA / PLLA modified films with and without ADR (PSMA / PDLA content 30wt%) after fracture are shown below. Figure 12 As shown. Figure 12 The left side shows the morphology of a tensile specimen of a PSMA / PDLA / PLLA modified film without ADR after fracture. Figure 12 The image on the right shows the morphology of a tensile specimen of a PSMA / PDLA / PLLA modified film (PSMA / PDLA content 30wt%) with added ADR after fracture. In the tensile test, the modified film without ADR showed only slight uniform elongation with no obvious necking; the length changed little after stretching, while the width and thickness remained essentially unchanged. In the tensile test, the modified film with added ADR showed a significant increase in overall length after stretching, while the width and thickness decreased uniformly, exhibiting necking and forming a "neck" region before fracture. The formation of necking indicates plastic flow in a localized area, dissipating energy through molecular chain orientation and slippage—a typical behavior of tough materials, indicating a significant improvement in the toughness of the modified film.

[0076] SEM images of the tensile fracture points of PSMA / PDLA / PLLA modified films with and without ADR (PSMA / PDLA content 30wt%) are shown below. Figure 13 As shown. Figure 13 The left and right sides show PSMA / PDLA / PLLA modified films (PSMA / PDLA content 30wt%) with and without ADR, respectively. In the film without ADR, the tensile fracture site shows dispersed, isolated phase regions, indicating significant debonding between the PSMA phase and the PLLA matrix, forming numerous pores. Furthermore, the crack propagation path is linear, directly penetrating the PSMA phase region, indicating weak interfacial bonding and ineffective stress transfer to the PSMA phase, leading to rapid and unstable crack propagation. This microstructure corresponds to the macroscopically low elongation at break. In contrast, the surface at the tensile fracture site after adding ADR is relatively smooth, exhibiting ductile fracture characteristics. The PSMA phase region is uniformly dispersed within the PLLA matrix, and the interfacial bonding between the two is tight, without significant pores. This indicates that the material achieves effective stress transfer through stable chemical bonds formed by the reaction of the epoxy groups of ADR with the hydroxyl / carboxyl groups of PLLA.

[0077] XRD images of PLLA, PDLA / PLLA, PSMA / PDLA / PLLA and PSMA / PDLA / PLLA / ADR modified films are shown below. Figure 14As shown, the PDLA / PLLA (50 / 50) modified film exhibits sharp peaks at approximately 2θ = 12°, 21°, and 24°, corresponding to the SC crystal structure of the stereocomplex. The stereocomplex characteristic peaks (2θ = 12°, 21°, 24°) of the PSMA / PDLA / PLLA (PSMA / PDLA content 30wt%) modified film show a significant decrease or even disappearance in intensity, and an α-crystalline peak of the PLLA homopolymer appears at 2θ = 16.7°. Conversely, the stereocomplex characteristic peaks (2θ = 12°, 21°) of the PSMA / PDLA / PLLA (PSMA / PDLA content 30wt%) / ADR modified film show a slight increase in intensity, and the α-crystalline peak of the PLLA homopolymer appearing at 2θ = 16.7° is weaker than the peak of the unmodified film.

[0078] Results Analysis: When PDLA and PLLA are mixed in a 1:1 ratio, a stereocomplex is formed through intermolecular hydrogen bonds, significantly increasing crystallinity and resulting in stronger characteristic peaks. However, in the PSMA / PDLA / PLLA (PSMA / PDLA content 30wt%) modified film, the hydrophobic long chains of PSMA, combined with PDLA, hindered the orderly arrangement of PDLA and PLLA molecular chains, inhibiting the formation of the stereocomplex. Furthermore, some phase separation occurred between PSMA and PLLA, leading to decreased crystallinity and an increased proportion of amorphous regions. Consequently, the peak intensity of the stereocomplex weakened or even disappeared, and a homopolymer peak of PLLA appeared. When ADR was added, its epoxy groups improved the interfacial compatibility between PSMA and PLLA, promoting closer proximity of PDLA and PLLA molecular chains and restoring some of the stereocomplex crystallinity. Although ADR inhibited phase separation and reduced the proportion of amorphous regions, the amount of ADR added was too small to completely eliminate the interference of PSMA. Therefore, the characteristic peaks of the stereocomplex are still relatively weak at this time, but they are slightly stronger than the characteristic peaks of the stereocomplex without the addition of ADR, while the characteristic peaks of the homopolymer of PLLA are weakened.

[0079] 3.4 Barrier Performance of PSMA / PDLA / PLLA / ADR Modified Membranes: The changes in the barrier properties of strawberry blocks sealed with PSMA / PDLA / PLLA / ADR modified membranes in different proportions over different storage periods are shown in the figure. Figure 15-20 As shown, where Figure 15 The results are for the control group without membrane sealing. In each figure, (a)-(f) represent the storage times of 0, 12, 24, 36, 48, and 60 hours, respectively.

[0080] like Figure 15As shown, unencapsulated strawberry pieces, exposed directly to air between 0 and 60 hours without the barrier of the modified film, experienced rapid moisture loss through transpiration, resulting in shrinking and hardening of the pulp tissue and an overall dry and dehydrated state. This phenomenon provides a blank control for subsequent barrier performance testing of the modified film.

[0081] like Figure 16 As shown, strawberry blocks sealed with a PSMA / PDLA / PLLA / ADR modified film containing 10wt% PSMA / PDLA did not show significant changes between 0 and 60 hours. The pulp texture remained relatively plump, and the moisture loss was minimal, indicating that the film had good barrier properties.

[0082] like Figures 17 to 20 As shown, strawberry blocks sealed with PSMA / PDLA / PLLA / ADR modified films containing 20wt%-50wt% PSMA / PDLA all showed obvious spoilage on the pulp within 48-60 hours. Among them, strawberry blocks sealed with modified films containing 30wt%-50wt% PSMA / PDLA also showed moisture loss, but it did not evaporate and remained in the glass container in liquid form. This also reflects the good moisture barrier performance of the modified film.

[0083] In summary, addressing the shortcomings in the mechanical properties of previously modified PSMA / PDLA / PLLA films, 1 wt% ADR (Alternative Dry Agent) was added as a reactive compatibilizer. After adding ADR, the elongation at break initially increased and then decreased with increasing PSMA / PDLA content, reaching a peak of 275.70% at a content of 30 wt%, representing a 2110.9% increase compared to the system without ADR. Similarly, the tensile strength also initially increased and then decreased with increasing PSMA / PDLA content, reaching a maximum of 51.19 MPa at a content of 20 wt%, representing a 63.5% increase compared to the system without ADR.

[0084] In terms of optical performance, the transmittance remains above 90%, and the addition of ADR has no significant impact on light absorption.

[0085] In the SEM images, isolated phase regions and pores were found at the fracture site of the modified film strip without ADR. After adding ADR, the interface was tightly bonded, and the fracture site of the modified film strip showed a smooth and tough fracture surface, which confirmed the improved compatibility between PSMA / PDLA and PLLA.

[0086] In XRD analysis, without the addition of ADR, PSMA blending hindered the formation of the PDLA / PLLA stereocomplex, and the characteristic peak of the α-crystal form of the PLLA homopolymer was enhanced. After the addition of ADR, some of the stereocomplex crystallinity was restored, specifically manifested as a slight increase in the intensity of the characteristic peaks at 2θ = 12° and 21°, and a weakening of the characteristic peaks of the PLLA homopolymer. This indicates that ADR promotes the orderly arrangement of molecular chains and inhibits phase separation.

[0087] In terms of barrier properties, strawberry pieces without plastic wrap showed an overall dry and dehydrated state after 60 hours, while strawberry pieces covered with PSMA / PDLA / PLLA / ADR modified films all showed good moisture barrier capabilities.

[0088] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A PSMA / PDLA modified PLLA composite film, characterized in that, The invention includes blends of L-polylactic acid and polyoctadecyl methacrylate / d-polylactic acid melt blends; the polyoctadecyl methacrylate / d-polylactic acid melt blends are obtained by melt blending octadecyl methacrylate, dextrorotatory polylactic acid and an initiator.

2. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, The weight ratio of dextrorotatory polylactic acid (DPL) to octadecyl methacrylate is (100~110):(1~5); the weight ratio of DPL to initiator is (100~110):(2~3).

3. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, The melt blending temperature is 170~190℃, and the melt blending time is 8~15min.

4. The PSMA / PDLA modified PLLA composite film as described in claim 1 or 2, characterized in that, The initiator is an organic peroxide.

5. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, The mass ratio of L-polylactic acid and polyoctadecyl methacrylate / d-polylactic acid melt blend is (90~50):(10~50).

6. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, It includes blends of L-polylactic acid, polyoctadecyl methacrylate / D-polylactic acid melt blends, and epoxy chain extenders.

7. The PSMA / PDLA modified PLLA composite film as described in claim 6, characterized in that, The amount of epoxy chain extender added is 1 to 2 wt% of the total mass of the L-polylactic acid and polyoctadecyl methacrylate / D-polylactic acid melt blend.

8. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, The weight ratio of dextrorotatory polylactic acid to octadecyl methacrylate is (100~110):(2~5).

9. The PSMA / PDLA modified PLLA composite film as described in claim 1, characterized in that, The mass ratio of L-polylactic acid and polyoctadecyl methacrylate / d-polylactic acid melt blend is (90~50):(45~50).

10. A method for preparing a PSMA / PDLA modified PLLA composite film as described in any one of claims 1-9, characterized in that, The process includes the following steps: forming a film from a melt blend of L-polylactic acid and polyoctadecyl methacrylate / d-polylactic acid using a solution casting method; the melt blend of polyoctadecyl methacrylate / d-polylactic acid is obtained by melt blending octadecyl methacrylate, D-polylactic acid and an initiator.

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