Preparation method of multi-non-covalent interaction synergistically enhanced polylactic acid and polylactic acid film

Abstract

The invention discloses a preparation method of multiple non-covalent interaction synergistically enhanced polylactic acid and a polylactic acid film, and aims to solve the problems of high brittleness and poor toughness of a PLA film. According to the invention, a PET degradation product BHETA [N, N '-bis (2-hydroxyethyl) terephthalamide] is used as an initiator to prepare polylactic acid, and then the reinforced and toughened polylactic acid film is prepared.

Description

Technical Field

[0001] The present invention relates to the field of polylactic acid films, and more particularly to a preparation method of polylactic acid synergistically enhanced by multiple non-covalent interactions and a polylactic acid film. Background Art

[0002] Polylactic acid (PLA) is a typical bio-based biodegradable polyester, with an annual global production capacity exceeding 800,000 tons. The global PLA market size was approximately US$1.25 billion in 2024 and is expected to reach US$4.22 billion by 2032, with a compound annual growth rate of approximately 16% during the forecast period. PLA film is one of the main downstream products of PLA and is widely used in food packaging, agricultural mulch, medical dressings and other fields. However, PLA film has problems such as low elongation at break (<10%) and high brittleness. Taking PLA cast film as an example, its dart impact strength is only 5-10N, which is far lower than that of biaxially oriented polypropylene (BOPP) film (15-20N). This seriously restricts the application of PLA film in high-end occasions such as heavy packaging and cold chain transportation. Therefore, improving the mechanical properties of PLA is the key to expanding the application of PLA film.

[0003] To address the brittleness and poor toughness of PLA films, researchers have employed various methods to modify them. For example, introducing plasticizers into PLA films can lower the polymer's glass transition temperature and increase their flexibility. Adding nanofillers to the polymer matrix can improve PLA's mechanical properties through physical reinforcement. Introducing flexible segments into PLA through copolymerization can improve its processing properties and toughness. However, these methods often come with negative side effects. For example, plasticizers can affect the biodegradability and transparency of PLA films, potentially leading to plasticizer leakage. The addition of nanofillers can reduce the transparency of PLA films and cause crystallization points to appear. Furthermore, copolymerization not only alters the chemical structure of PLA but also makes it difficult to achieve a balanced balance between toughness and strength. Therefore, significantly improving the mechanical properties of PLA films while maintaining their inherent advantages remains a pressing technical challenge.

[0004] Hydrogen bonding is a strong intermolecular interaction force, and introducing hydrogen bonds into polymers is an effective method for enhancing material properties. By introducing hydrogen bond donor groups into PLA molecular chains, the interactions between PLA chains are enhanced, significantly improving the mechanical properties, thermal stability, and impact resistance of PLA materials. The introduction of hydrogen bonds not only increases the rigidity and strength of PLA, but also enhances the thermal stability of the material. Currently, studies have attempted to enhance the hydrogen bonding interactions of PLA chains by introducing functional structural units such as urea and amide bonds, but these methods often suffer from problems such as insufficient hydrogen bond strength and limited enhancement effects.

[0005] To overcome these problems, new methods are urgently needed to enhance the mechanical properties of PLA materials. π-π conjugation between aromatic rings is a strong intermolecular force. The π electron clouds of aromatic rings can form π-π stacking or π-π interactions between adjacent molecules, significantly increasing the mutual attraction between molecules and thus enhancing the material's rigidity and strength. Therefore, the present invention proposes a new method for synergistically enhancing the mechanical properties of PLA films based on hydrogen bonding and π-π conjugation interactions. Summary of the Invention

[0006] To address the shortcomings of the existing technology, the present invention proposes a novel technology for synergistically reinforcing PLA films based on hydrogen bonds and π-π conjugation. Using N,N'-bis(2-hydroxyethyl)terephthalamide (BHETA, hereinafter referred to as BHETA), a PET ammoniolysis product, as an initiator, a PLA resin containing hydrogen bonds and π-π conjugation synergistic interactions is prepared by initiating the ring-opening polymerization of lactide. PLA films are then prepared using a melt casting method. Specifically, under the action of a catalyst, the hydroxyl groups (-OH) of BHETA initiate the ring-opening polymerization of lactide to synthesize a novel PLA resin. The amide groups (-NH-) of the BHETA units in PLA form a stable hydrogen bond network between molecular chains, while the benzene rings of the BHETA units microscopically aggregate through the π-π conjugation effect, synergistically promoting the formation of a physical crosslinking network in the PLA. This physical crosslinking network effectively strengthens and toughens the PLA matrix and film, and the reinforced PLA film exhibits excellent biodegradability and light transmittance.

[0007] The specific method is as follows: When preparing PLA by ring-opening polymerization of lactide, BHETA, a product of PET aminolysis, is added as an initiator. The initiator conducts a targeted attack on the cyclic monomer, and the cyclic monomer opens the ring to form a complex with the initiator, thereby achieving polymer chain growth.

[0008] As a preferred embodiment of the present invention, the method for preparing PLA synergistically reinforced by hydrogen bonds and π-π conjugation is as follows: lactide, a catalyst, and an initiator are added to a polymerization reactor, mixed uniformly using mechanical stirring, and vacuumed while stirring. The vacuum is applied at room temperature for 0.5 to 2 hours to remove residual moisture from the reactor and the materials. The absolute pressure during vacuuming is 10 to 2000 Pa. After vacuuming, nitrogen or argon is introduced until the absolute pressure in the reactor exceeds 101.3 kPa. After the vacuuming is completed, the internal temperature of the reactor is raised to 160 to 190°C. The polymerization reaction is carried out under a nitrogen or argon atmosphere for 0.5 to 6 hours, and the material is extruded to obtain reinforced PLA pellets. The PLA pellets are thoroughly dried and added to a twin-screw extruder. PLA films are prepared by melt casting. The heating temperatures of the extruder zones are 170, 180, 190, 200, and 200°C, respectively. The resulting cast film has a thickness of 20 to 200 μm.

[0009] As a preferred embodiment of the present invention, the catalyst is one of stannous octoate [Sn(Oct)2] and stannous chloride (SnCl2), or a mixture of the two.

[0010] As a preferred embodiment of the present invention, the mass proportion of the catalyst is 0.01% to 0.5% of the mass of lactide.

[0011] The present invention also provides a polylactic acid film, wherein the raw material of the polylactic acid film is polylactic acid, and the polylactic acid film is prepared by synergistically enhancing the polylactic acid through the above-mentioned multiple non-covalent interactions.

[0012] Since the above-mentioned polylactic acid has any one of the above-mentioned technical effects of multiple non-covalent interactions synergistically enhancing the polylactic acid, the polylactic acid film prepared with the polylactic acid as raw material has the same technical effect.

[0013] As a preferred embodiment of the present invention, the polylactic acid film can be used in packaging materials, agricultural films, medical films, electronic films or other fields requiring transparent, strong and degradable film materials.

[0014] In summary, the present invention has the following beneficial effects:

[0015] 1) Using PET recycled products as initiators to achieve high-value applications of PET waste can not only reduce the pollution of waste plastics, but also efficiently synthesize high-performance hydrogen-bond-enhanced polymers, which has important economic and social benefits.

[0016] 2) Using aromatic diols containing amide bonds as initiators, hydrogen bonds and π-π conjugated interactions are simultaneously introduced into the polymer to achieve synergistic reinforcement and toughening of PLA, solving the problem of poor mechanical properties of PLA materials.

[0017] 3) Compared with the traditional copolymerization or blending reinforcement and toughening modification methods, this technology is based on the chemical structure regulation of PLA to prepare high-performance PLA special materials, so that the PLA film has both biodegradability and excellent optical transparency. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the structural formula of N,N'-bis(2-hydroxyethyl)terephthalamide. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present application and the features therein can be combined with each other unless there is a conflict.

[0020] In the description of the present invention, it should be noted that the terms "upper", "lower", "inner", "outer", "top / bottom" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, they should not be understood as limiting the present invention.

[0021] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "installed," "provided with," "mounted / connected," and "connected" should be understood in a broad sense. For example, "connected" can refer to a fixed connection, a detachable connection, or an integral connection; it can refer to a mechanical connection or an electrical connection; it can refer to a direct connection or an indirect connection through an intermediate medium; and it can refer to internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention in specific circumstances.

[0022] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] A method for preparing polylactic acid synergistically reinforced by multiple non-covalent interactions and a polylactic acid film. The raw materials and reagents used in the present invention are described as follows: lactide was purchased from Zhejiang Hisun Biomaterials Co., Ltd.; the initiator BHETA was prepared by PET aminolysis. The specific preparation process is as follows: ethanolamine was added to PET, sodium acetate was used as a catalyst, and the reaction was carried out at 175°C for 8 hours. After that, water was added to precipitate the reaction product, which was filtered and dried to obtain BHETA.

[0024] The initiators benzyl alcohol, hydroquinone, N-hydroxyacetamide, N,N'-bis(2-hydroxyethyl)oxalamide, 1-butanol or 1,4-butanediol, the catalysts Sn(Oct)2 and SnCl2, and the solvents chloroform and deuterated chloroform were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0025] The samples tested and analyzed in the Examples and Comparative Examples are as follows:

[0026] Gel Permeation Chromatography (GPC) Analysis: Molecular weight and molecular weight distribution of the samples were analyzed using an Agilent GPC instrument equipped with an Agilent 1260 Infinity II differential refractive index detector and a PL1110-6100 10 μm mixed-B column. The test temperature was 35°C. Chloroform was used as the mobile phase, and polystyrene was used as the standard.

[0027] Melting point test: A NETZSCH DSC 214 Polyma differential scanning calorimeter was used to heat the sample from -20°C to 200°C at a heating rate of 10°C / min in a nitrogen atmosphere.

[0028] Light transmittance testing: The light transmittance of PLA cast film was measured using a Shimadzu UV-1800 UV-visible spectrophotometer. Samples measuring 10 mm x 10 mm and 50 μm thick were cut from the PLA cast film using a cutter for light transmittance testing. The test wavelength range was set to 400-800 nm, with a wavelength interval of 1 nm. At least three samples were tested at different locations for each sample group, and the results were averaged.

[0029] Tensile Properties: Dumbbell-shaped strips were cut from PLA cast film using a cutter for tensile properties testing. The strips were 150 mm long, 10 mm wide, and 50 mm in gauge length. After cutting, they were allowed to stand at room temperature for 48 hours to eliminate internal stress. Uniaxial tensile testing was then performed using a UTM2503 electronic universal testing machine (Shenzhen Sansi Zongheng Co., Ltd.) at 25°C and a speed of 5 mm / min. At least five strips were tested in parallel for each sample, and the results were averaged.

[0030] The following examples and comparative examples can help professionals in this field to more fully understand the present invention, reflecting the improvement in the performance of the film produced by the synergistic enhancement of polylactic acid by multiple non-covalent interactions in the present invention:

[0031] Example 1

[0032] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and initiator BHETA (designed molecular weight of 130k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. The reactor was then heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 100 μm.

[0033] Example 2

[0034] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and initiator BHETA (designed molecular weight of 130k) were added to a polymerization reactor and mixed uniformly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0035] Example 3

[0036] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and initiator BHETA (designed molecular weight of 100k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0037] Comparative Example 1

[0038] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and initiator benzyl alcohol (designed molecular weight 130k) were added to a polymerization reactor and mixed uniformly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0039] Comparative Example 2

[0040] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and the initiator hydroquinone (designed molecular weight of 130k) were added to a polymerization reactor and mixed uniformly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0041] Comparative Example 3

[0042] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and an initiator, N-hydroxyacetamide (designed molecular weight of 130k), were added to a polymerization reactor and mixed uniformly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0043] Comparative Example 4

[0044] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and the initiator N,N'-bis(2-hydroxyethyl)oxamide (designed molecular weight of 130k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt-casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0045] Comparative Example 5

[0046] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and the initiator 1,4-butanediol (designed molecular weight of 130k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove any residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt-casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 100 μm.

[0047] Comparative Example 6

[0048] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and the initiator 1,4-butanediol (designed molecular weight of 130k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt-casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0049] Comparative Example 7

[0050] Lactide, Sn(Oct)2 (0.05% by weight of lactide), and the initiator 1,4-butanediol (designed molecular weight of 100k) were added to a polymerization reactor and mixed thoroughly using mechanical stirring. Vacuum was then applied while stirring for 1 hour at room temperature to remove any residual moisture from the reactor and the materials. The absolute pressure during evacuation was 200 Pa. After evacuation, argon was introduced until the absolute pressure in the reactor exceeded 101.3 kPa. After evacuation, the reactor was heated to 180°C and polymerization was carried out under argon for 2 hours, yielding PLA pellets synergistically reinforced by hydrogen bonds and π-π conjugation. The pellets were thoroughly dried and then added to a casting machine for melt-casting of PLA films. The heating zones in the extruder were set at 170, 180, 190, 200, and 200°C, respectively. The resulting cast film had a thickness of 50 μm.

[0051] Table 1. Initiator type, designed molecular weight, measured number average molecular weight, measured weight average molecular weight and molecular weight distribution index of PLA prepared in Examples 1 to 3 and Comparative Examples 1 to 7

[0052]

[0053] Table 1 summarizes the initiator type, designed molecular weight, measured number average molecular weight, measured weight average molecular weight, and molecular weight distribution index of the PLA prepared in Examples 1-3 and Comparative Examples 1-7. Examples 1-3 show that using BHETA as an initiator, PLA with synergistic enhancement by hydrogen bonding and π-π conjugation was prepared. The designed molecular weight for Examples 1-2 was 130 kg / mol, and the designed molecular weight for Example 3 was 100 kg / mol. The measured number average molecular weight was substantially consistent with the designed molecular weight, and the molecular weight distribution index was approximately 1.7. Examples 4-5 show that using benzyl alcohol and hydroquinone as initiators, π-π conjugated PLA with a main chain containing a benzene ring structure was prepared. The measured number average molecular weight of the prepared PLA was substantially consistent with the designed molecular weight, the measured weight average molecular weight was approximately 220 kg / mol, and the molecular weight distribution index was approximately 1.7. Comparative Examples 3 and 4 used N-hydroxyacetamide and N,N'-bis(2-hydroxyethyl)oxamide as initiators, respectively, to prepare hydrogen-bond-enhanced PLA with an amino structure in the main chain. The PLA was designed with a molecular weight of 130 kg / mol. After GPC testing, the weight-average molecular weights of the prepared PLA were 209.7 kg / mol and 209.5 kg / mol, respectively, and the molecular weight distribution indexes were 1.67 and 1.79. Comparative Examples 5 to 7 used 1,4-butanediol as an initiator to prepare conventional PLA samples. The designed molecular weights of Comparative Examples 5 to 6 were 130 kg / mol, and the designed molecular weight of Comparative Example 7 was 100 kg / mol. After GPC testing, the number-average molecular weights of the prepared PLA were 114.9 kg / mol, 120.6 kg / mol, and 94.6 kg / mol, respectively, and the weight-average molecular weights were 198.7 kg / mol, 212.2 kg / mol, and 161.7 kg / mol, respectively, with a molecular weight distribution index of approximately 1.75.

[0054] Table 2. Melting point, film thickness, transmittance, tensile strength, elongation at break and modulus of PLA cast films prepared in Examples 1 to 3 and Comparative Examples 1 to 7

[0055]

[0056] Table 2 summarizes the melting point, film thickness, transmittance, tensile strength, elongation at break, and modulus of the PLA cast films prepared in Examples 1-3 and Comparative Examples 1-7. Example 1 and Comparative Example 5 produced cast films with a thickness of approximately 100 μm, while Examples 2-3 and Comparative Examples 1-4 and 2-3 produced cast films with a thickness of approximately 50 μm. As shown in Example 1 and Comparative Example 5, as well as in Examples 2-3 and Comparative Examples 1-4 and 2-3, the type of initiator has little effect on the melting point and transparency of the PLA cast films. Film thickness has little effect on the mechanical properties of the cast films, but significantly affects their transparency. When the cast film thickness is 50 μm, the film transmittance is approximately 93%, and when the cast film thickness is 100 μm, the film transmittance is approximately 90%. Transmittance decreases with increasing film thickness. The hydrogen bond and π-π conjugation effects introduced by the initiator can both improve the tensile strength, elongation at break, and modulus of the PLA cast film. The π-π conjugation effect can slightly improve the tensile strength and modulus of the PLA cast film, and the formation of the hydrogen bond network can slightly improve the elongation at break of the PLA cast film. It can be seen from Examples 1 to 2 and Comparative Examples 1 to 4 that when hydrogen bonds and π-π conjugation work synergistically, the performance improvement of the PLA cast film is more significant. The tensile strength, modulus, and elongation at break of the PLA cast film prepared in Example 2, which are synergistically enhanced by hydrogen bonds and π-π conjugation, are 57.1 MPa, 2.4 GPa, and 7.3%, respectively, and its transmittance is maintained at around 93%; the tensile strength is increased by 11% compared to conventional PLA cast films (Comparative Examples 1 to 2), and the elongation at break is increased by about 15%.

[0057] In addition, molecular weight is one of the important parameters that trigger the mechanical properties of PLA cast film. It can be seen from Examples 1 to 3 and Comparative Examples 5 to 7 that when the type of initiator is the same, the mechanical properties of PLA cast film improve with increasing molecular weight.

[0058] In summary, by using functional initiators, the simultaneous introduction of hydrogen bonds and π-π conjugated interactions in PLA can synergistically improve the mechanical strength, modulus and elongation at break of PLA films while maintaining a high light transmittance of the films.

[0059] The present invention also provides a polylactic acid film, wherein the raw material of the polylactic acid film is polylactic acid, and the polylactic acid is any one of the aforementioned multiple non-covalent interactions synergistically enhanced polylactic acids. Because the above-mentioned polylactic acid has the technical effect of any one of the aforementioned multiple non-covalent interactions synergistically enhanced polylactic acids, the polylactic acid film prepared using this polylactic acid as a raw material has the same technical effect, as also demonstrated by the comparison results of the above-mentioned embodiment and comparative example. At the same time, because the molding process of the polylactic acid film is a mature existing technology, it will not be elaborated here.

[0060] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiment. All technical solutions based on the concept of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions, characterized in that: The polylactic acid comprises the following raw materials: lactide, a catalyst and an initiator. The initiator is a compound containing a group that easily forms hydrogen bonds and has a benzene ring structure. Under the action of the catalyst, the initiator reacts with lactide to produce a ring-opening polymerization reaction to synthesize polylactic acid with synergistically enhanced hydrogen bonds and π-π conjugated interactions.

2. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 1, characterized in that: The initiator is N,N'-bis(2-hydroxyethyl)terephthalamide. The amide groups in the initiator can form a hydrogen bond network between the polylactic acid molecular chains, and the benzene ring units in the initiator can undergo π-π stacking, synergistically strengthening and toughening the polylactic acid.

3. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 2, characterized in that: The N,N'-bis(2-hydroxyethyl)terephthalamide comprises the following raw materials: PET, sodium hydroxide and ethanolamine; The preparation of the N,N'-bis(2-hydroxyethyl)terephthalamide comprises the following steps: A) adding sodium acetate and ethanolamine to PET; B) reacting PET and ethanolamine at 175° C. for 8 hours and then adding water; C) precipitating a reaction product, filtering the reaction product, and drying the reaction product to obtain N,N'-bis(2-hydroxyethyl)terephthalamide.

4. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 1, characterized in that: The molecular weight of the initiator is 100-130 kg / mol.

5. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 1, characterized in that: The catalyst is one of stannous octoate [Sn(Oct)2] and stannous chloride (SnCl2) or a mixture of the two.

6. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 5, characterized in that: The amount of the catalyst used is 0.01% to 0.5% of the total amount of lactide.

7. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 1, characterized in that: The polymerization reaction is carried out at a temperature ranging from 160° C. to 190° C., and the reaction time is from 0.5 hour to 6 hours.

8. The method for preparing polylactic acid synergistically enhanced by multiple non-covalent interactions according to claim 1, characterized in that: The molecular weight of the polylactic acid is 100-200 kg / mol.

9. A polylactic acid film, wherein the raw material of the polylactic acid film is polylactic acid, characterized in that: The polylactic acid is any one of the polylactic acids described in claims 1 to 8 above, which is synergistically enhanced by multiple non-covalent interactions.

10. The polylactic acid film according to claim 9, characterized in that: The thickness of the polylactic acid film is 20 to 100 μm.

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