A graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer and its preparation method

Abstract

The present invention relates to the field of material technology. The present invention discloses a preparation method of graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer, which comprises the steps of: S1, preparing a prepolymer of unsaturated poly(itaconic acid-co-butylene itaconate); S2, preparing oligomeric (L-lactic acid) grafted with graphene oxide; S3, under the assistance of a catalyst (TsOH·H2O and SnCl2·2H2O), a melt polycondensation reaction occurs between the prepolymer of unsaturated poly(itaconic acid-co-butylene itaconate) and the oligomeric (L-lactic acid) grafted with graphene oxide to form the target copolymer. The present invention uses graphene oxide as an initiator and unsaturated poly(itaconic acid-co-butylene itaconate) as a flexible chain segment, synergistically improving the thermal stability, gel content, transparency, mechanical properties, anti-aging properties and barrier properties of the PLLA film. Therefore, the present invention provides an important research idea and direction for the development of polylactic acid nanocomposites with adjustable flexibility, excellent oxygen barrier performance, high thermal stability, strong ultraviolet resistance and high transparency, and shows significant industrial application potential and value.

Description

Technical Field

[0001] The present invention relates to the field of materials technology, and particularly to a graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer and a preparation method thereof. Background Art

[0002] The increasingly severe problem of "white pollution" globally has posed a significant threat to the natural environment and the human living environment. Governments around the world have successively introduced plastic restriction policies aimed at reducing dependence on petroleum-based plastic products. To address this global challenge, researchers are actively exploring innovations in the field of biodegradable and renewable materials in order to find effective alternatives to petroleum-based plastics. Among them, biodegradable materials such as polylactic acid (PLA), poly(butylene adipate-co-terephthalate) (PBAT), polycaprolactone (PCL), poly(butylene succinate) (PBS), and polyhydroxyalkanoates (PHAs) have attracted much attention due to their good application potential.

[0003] Among these materials, poly-L-lactic acid (PLLA) has shown broad application prospects in the fields of environmental protection and biomedicine due to its excellent biodegradability, biocompatibility, balanced mechanical properties, and wide applicability of manufacturing processes. However, the inherent brittleness, low heat resistance and ultraviolet resistance, and moderate gas barrier properties of PLLA limit its further promotion and practical applications. Therefore, developing PLLA materials with excellent mechanical properties, barrier properties, and anti-aging properties has become an urgent need in current research.

[0004] To improve the comprehensive properties of PLLA films, researchers have conducted in-depth explorations in precursor selection and synthesis routes. On the one hand, L-lactide can be polymerized by ring-opening polymerization to produce high-molecular-weight PLLA, but the monomer cost is high and the synthesis process is complex. In contrast, lactic acid, as another synthetic monomer, is more abundant, has a lower cost, and is easy to modify, providing the possibility to improve the comprehensive properties of PLLA. Through the ring-opening polymerization (ROP) of lactide and the polycondensation reaction of lactic acid, and by using multifunctional initiators such as polyols, polyacids, and nanomaterials, multi-branched PLLA has been successfully developed, effectively enhancing the functionality of its structure.

[0005] It is worth noting that PLLA nanocomposites formed by combining PLLA with a small proportion of nano-fillers are expected to exhibit the characteristics of high-performance new biodegradable and biocompatible composite materials. In-situ polymerization, as an effective method, can achieve uniform dispersion of fillers in the matrix and good interfacial interaction with the host polymer. However, the one-step synthesis of high-molecular-weight multi-branched PLLA initiated by nanomaterials still faces limitations of harsh conditions and expensive precursors (such as lactide).

[0006] Graphene Oxide (GO), as an oxidized derivative of graphene flakes, contains various hydrophilic functional groups (such as hydroxyl or epoxy groups on the basal plane, and carbonyl or carboxyl groups on the edges), making it easy to disperse in water and polar solvents. This property facilitates the further modification and subsequent processing of GO.

[0007] However, up to now, there has been no report in relevant research on simultaneously introducing poly(itaconic acid-co-butylene glycol ester) (PBI) and GO to synergistically improve the thermal stability, gel content, transparency, mechanical properties, anti-aging properties, and barrier properties (including gas and ultraviolet barrier properties) of PLLA films. In-depth exploration of this research field is expected to open up a new way for improving the performance of PLLA materials and broadening their application fields. Summary of the Invention

[0008] In view of this, the present invention first proposes a method for synthesizing GO-g-poly(L-lactic acid-co-butylene itaconate) (GO-g-PLBI) copolymer by using GO as an initiator and unsaturated PBI as a flexible chain segment through in-situ melt polycondensation of LA (L-lactic acid). The present invention also reveals the specific effects of the introduction of GO and PBI on the thermal stability, gel content, transparency, mechanical properties, anti-aging properties, and barrier properties (including gas and ultraviolet barrier properties) of PLLA films. Therefore, the present invention not only provides a new way to synthesize low-cost, environmentally friendly, and high-performance graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer, but also provides important research ideas and directions for developing polylactic acid nanocomposites with adjustable flexibility, excellent oxygen barrier performance, high thermal stability, strong ultraviolet resistance, and high transparency.

[0009] The technical solution of the present invention is realized as follows:

[0010] First Invention, the present invention provides a method for preparing a graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer. S1: Mix itaconic acid and 1,4-butanediol, and under the action of a polymerization inhibitor, react at 145-155 °C and in a nitrogen atmosphere for 1.5-2.5 h, and then continue to react at 40-80

[0011] Pa and 145-155 °C for 5-7 h to obtain product A; the number average molecular weight of product A is 800-2000;

[0012] S2: Mix L-lactic acid and monolayer graphene oxide powder, and ultrasonically disperse the graphene oxide evenly in L-lactic acid for 0.5-1.5 h,

[0013] React at 105 - 115 °C and 35 - 45 kPa for 0.5 - 1.5 h, then reduce the pressure to 10 - 16 kPa and react at 145 - 155 °C for 1.5 - 2.5 h.

[0014] Then reduce the pressure to 2 - 6 kPa and react at 145 - 155 °C for 3 - 5 h to obtain product B.

[0015] S3. Mix A, B and the catalyst, and react at a vacuum degree of 40 - 80 Pa and 175 - 185 °C for 18 - 22 h to obtain product C; then lower the temperature to 165 - 175 °C, add a chain extender and continue the reaction to obtain product D; after crushing product D, crystallize it at a vacuum degree of 50 - 70 Pa for 2 - 4 h, and then carry out solid-phase polymerization reaction at a temperature 5 - 15 °C lower than its melting point for 44 - 52 h to obtain the graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer.

[0016] The product A is a prepolymer of unsaturated poly(itaconic acid-co-butylene itaconate).

[0017] The product B is graphene oxide-grafted oligo(L-lactic acid). In step S2, a polycondensation reaction occurs between the hydroxyl groups of L-lactic acid to form an oligo(lactic acid) chain, and graphene oxide acts as an initiator, and the functional groups on the surface of graphene oxide react with the functional groups at the end of L-lactic acid or the oligo(L-lactic acid) chain.

[0018] In some preferred embodiments, in step S1, the molar ratio of itaconic acid to 1,4-butanediol is 1:1.03.

[0019] In some preferred embodiments, in step S1, the inhibitor is hydroquinone, and the mass ratio of hydroquinone to itaconic acid is 1:200.

[0020] In some preferred embodiments, in step S2, the mass ratio of L-lactic acid to monolayer graphene oxide powder is 10000:1.

[0021] In some preferred embodiments, in step S2, the monolayer graphene oxide powder is monolayer graphene oxide nanosheets with a sheet diameter range of 5 - 10 μm and a thickness of 0.55 - 2.35 nm.

[0022] In some preferred embodiments, in step S3, the catalyst is a mixture of SnCl 2 ·2H 2 O and TsOH·H 2 O in an equimolar ratio; the addition amount of SnCl 2 ·2H 2 O is 0.5 wt% of the total reactants.

[0023] In some preferred embodiments, in step S3, the chain extender is triphenyl phosphite, and the mass ratio of the chain extender to product C is 1:40. After adding the chain extender, the reaction continues for 2.5 - 3.5 h under a vacuum of 25 - 35 Pa to obtain product D.

[0024] In some preferred embodiments, in step S3, the following steps are further included. After obtaining the graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer, it is dissolved in an organic solvent, precipitated with ice ethanol, and finally vacuum dried for 44 - 52 h.

[0025] Term Explanation:

[0026] Abbreviation Term GO Graphene Oxide LA L-Lactic Acid IA Itaconic Acid BDO 1,4-Butanediol PLLA Poly(L-lactic acid) OLLA Oligo(L-lactic acid) PBI Poly(itaconic acid-co-butylene glycol ester) PLBI Poly(L-lactic acid-co-butylene itaconate) PVA-g-AC Polyvinyl Alcohol-g-Acryloyl Chloride GO-g-OLLA GO-g-Oligo(L-lactic acid) GO-g-PLBI GO-g-Poly(itaconic acid-co-butylene glycol ester) GO-g-PLLA GO-g-Poly(L-lactic acid) TPP Triphenyl Phosphite

[0027] PBI : It is an unsaturated polyester. Due to the isolated double bonds and aliphatic polyester backbone in its molecular structure, as well as the characteristics of unsaturated polyesters, unsaturated PBI has excellent properties as a flexible chain segment.

[0028] co : It represents the copolymerized unit in the copolymer.

[0029] g : It represents graft.

[0030] Prepolymer : It represents a low-degree polymerization intermediate that can be converted into a high-molecular-weight polymer. Its number-average molecular weight (Mn) is generally several hundred to several thousand, and it is converted through end-group or side-group functional group reactions and proceeds simultaneously with the cross-linking reaction.

[0031] Oligomer : It represents a polymer composed of a relatively small number (generally 10 - 20) of repeating units.

[0032] Copolymer : It represents a polymer formed by copolymerization of two or more monomers.

[0033] In some embodiments of the present invention, the GO-g-PLBI copolymer is obtained by copolymerizing GO-g-OLLA and PBI to synthesize the target copolymer GO-g-PLBI.

[0034] Micro-crosslinking : It refers to introducing a small amount of cross-linking bonds between the copolymer segments, so that a microscopic cross-linking network is formed between the segments. In the copolymer, different monomer units are connected together in a random, alternating, block, or grafted manner to form a complex chain structure.

[0035] In-situ PolymerizationIn-situ polymerization is a method for preparing composite materials, in which fillers (such as reinforcing materials, nanoparticles, etc.) are uniformly dispersed in a matrix material (usually a polymer). Through the polymerization reaction, chemical bonds or other strong interactions are generated between these fillers and the host polymer, thus forming a more compact and stable interface.

[0036] Melt Polycondensation Method, In-situ Melt Polycondensation Method The melt polymerization method is that monomers (or oligomers) are heated to an appropriate temperature in a molten state to initiate the polymerization reaction. The in-situ melt polycondensation method combines the operational simplicity of the melt polymerization method and the high molecular weight generation characteristics of the polycondensation reaction.

[0037] In some embodiments of the present invention, using GO as an initiator and PBI as a flexible chain segment, GO-g-PLBI was successfully prepared through the in-situ melt polycondensation reaction of LA. Specifically, this process is based on the melt polymerization method, and an unsaturated PBI block is introduced on the basis of GO-g-OLLA, thereby synthesizing the target copolymer GO-g-PLBI.

[0038] During the melt polycondensation reaction, the hydroxyl or carboxyl groups at the ends of unsaturated PBI molecules copolymerize with the functional groups in GO-g-OLLA to form stable chemical bonds; at the same time, the unsaturated double bonds in PBI also participate in crosslinking and branching reactions, further enhancing the network structure and properties of the copolymer; thus achieving the effective combination of GO and PBI at the molecular level, and endowing the copolymer with higher reactivity and more complex structural characteristics.

[0039] The specific reaction principle of the present invention includes the following:

[0040] Step S1, a polycondensation reaction occurs, and the unsaturated carbon-carbon double bond in IA reacts with the hydroxyl group of BDO to form a polymer chain. Hydroquinone controls the degree of polymerization. The structural formula of the generated PBI prepolymer is as follows:

[0041]

[0042] Among them, the range of y is 5-20

[0043] In step S2, polycondensation reaction occurs between the hydroxyl groups of LA to form oligolactic acid chains. At the same time, GO acts as an initiator, and the functional groups on the surface of GO react with the functional groups at the ends of the LA or OLLA chains to graft the GO nanosheets onto the OLLA chains. This grafting reaction may occur at multiple locations of GO to form multiple grafting points, thereby uniformly dispersing the GO nanosheets in the OLLA matrix. Therefore, the chemical structure of the GO-g-OLLA prepolymer may contain multiple OLLA segments, which are connected to the functional groups on the surface of the GO nanosheets through ester bonds or other types of chemical bonds. The entire structure may present a three-dimensional network structure in which GO nanosheets are dispersed in the OLLA matrix.

[0044] GO-g-OLLA prepolymer, i.e. GO grafted oligo(L-lactic acid), has the following structural formula:

[0045]

[0046] Where x is 80 to 211

[0047] In the reaction process of step S3, GO acts as an initiator to promote the in-situ melt polycondensation reaction of unsaturated PBI and LA, and finally synthesizes GO-g-PLBI copolymer. In this process, GO not only acts as a physical support point, but also chemically bonds with the terminal functional groups of the reactant chain segments through its surface active functional groups (hydroxyl, carboxyl, etc.), thus achieving the firm grafting and uniform dispersion of GO in the polymer matrix.

[0048] Furthermore, the reaction system is carried out under the condition of catalyst (TsOH·H 2 O and SnCl 2 ·2H 2 O), the melt polycondensation between GO-g-OLLA and PBI prepolymer was effectively promoted. Through the reaction of terminal hydroxyl or carboxyl groups, alternating or randomly distributed PLLA segments and PBI segments were formed. These segments were tightly connected by ester bonds to construct the main chain structure of the copolymer.

[0049] TPP effectively extends the length of the polymer chain by reacting with the unreacted functional groups at the end of the polymer chain, thereby significantly increasing the molecular weight of the copolymer.

[0050] The unsaturated carbon-carbon double bonds in the PBI molecules show high reactivity under high temperature conditions. They can undergo cross-linking and branching reactions. These reactions not only increase the cross-linking and branching degree of the copolymer, but also promote the formation of a three-dimensional network structure, further enhancing the structural stability and thermal stability of the copolymer.

[0051] Although most of the terminal functional groups participate in the reaction during the reaction process, some terminal functional groups may still remain in the copolymer, and these functional groups may affect the further modification or application of the copolymer.

[0052] GO-g-PLBI, namely graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer, has the following structural formula:

[0053]

[0054] In a second aspect, the present invention provides a graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer prepared by any one of the above preparation methods.

[0055] In a third aspect, the present invention further provides a graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer film, the raw materials of which include the graphene oxide / poly(L-lactic acid-co-butylene itaconate) copolymer.

[0056] The beneficial effects of the present invention at least include the following:

[0057] The present invention uses GO as an initiator in the polycondensation process of LA, which not only promotes the uniform dispersion of graphene nanosheets in the polymer matrix, but also promotes the formation of a multi-branched structure. This characteristic significantly improves the thermal stability, tensile strength, elongation at break (EB), and barrier properties against ultraviolet light and oxygen of the film, while maintaining a high transparency of 76.8%.

[0058] By introducing unsaturated PBI into the GO-g-PLLA chain segment, the degree of chain branching is further increased, the molecular weight of the polymer is increased, and the micro-crosslinking of the copolymer chain segment is promoted. This micro-crosslinked structure not only improves the dispersion of graphene nanosheets in the polymer, but also significantly enhances the mechanical properties of the film. Compared with pure PLLA, the EB of the GO-g-PLBI film is increased by 73.8 times, and the oxygen permeability coefficient (OP) is reduced by 40.3%.

[0059] In addition, the aging test results show that the GO-g-PLBI film can still maintain considerable flexibility after being stored under environmental conditions for 170 days, and its EB can reach 199.8%. This excellent anti-aging performance is closely related to its high degree of chain branching and micro-crosslinking phenomenon, and these characteristics hinder the orderly rearrangement of the two-dimensional and three-dimensional conformations of the film.

[0060] It should be noted that after physical aging treatment, the OP of this micro-crosslinked GO-g-PLBI film is further reduced by 60.9%, showing extremely high oxygen barrier performance. This discovery provides a new idea for the development of high-performance oxygen barrier materials. Brief Description of the Drawings

[0061] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following-described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0062] Figure 1 1H-NMR spectra of PLLA, GO-g-PLLA, and GO-g-PLBI copolymers in the present invention, where: (a) 1 1H-NMR; (b) 13 13C-NMR; (c) 2D HSQC spectrum;

[0063] Figure 2 Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra of PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films in the present invention; where: (a) ATR-FTIR spectra of the copolymer films and their local enlarged FTIR spectra at (b) 1849-1630 cm -1 and (c) 831-800 cm -1 ;

[0064] Figure 3 X-ray photoelectron spectroscopy (XPS analysis) diagrams of PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films in the present invention; where: (a) XPS measurement scans of the copolymer films and GO and (b-e) C1s spectra. (b) GO; (c) PLLA; (d) GO-g-PLLA; (e) GO-g-PLBI;

[0065] Figure 4 Thermogravimetric and gel content analysis diagrams of PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films in the present invention; where: (a) Thermogravimetric (TGA) curves of the copolymers; (b) Dissolution (dispersion) characteristics of the copolymers in chloroform; (c) Content of insoluble substances in the copolymers;

[0066] Figure 5 Stress-strain curves, tensile strength (TS), and elongation at break (EB) of PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films at different aging times (0, 50, and 170 d) in the present invention; where: Stress-strain curves and mechanical parameters of the copolymer films stored at environmental conditions for (a) 0, (b) 50, and (c) 180 d; (d) Tensile strength; (e) Elongation at break;

[0067] Figure 6 SEM images of the tensile fracture surfaces of the PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention at aging times of 0 (1), 50 (2), and 170 (d), where: (a) PLLA; (b) GO-g-PLLA; (c) GO-g-PLBI;

[0068] Figure 7 Temperature-dependent Fourier transform infrared spectra (a 1 -c 1 ) and difference spectra (a 2 -c 2 ) of the PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention, calculated by subtracting the spectra collected at 30 °C from the spectra collected during heating from 30 °C to 90 °C at a rate of 6 °C / min in the range of 1800 - 1720 cm -1 . Among them, (a) PLLA; (b) GO-g-PLLA; (c) GO-g-PLBI; (d) The normalized intensities of the peaks at 1758 cm -1 (d), 1745 (e), and 1261 cm -1 (f) vary with temperature;

[0069] Figure 8 DSC heating curves and corresponding thermodynamic parameters in the range of 0 - 70 °C of the PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention aged for 0 (a), 50 (b), and 170 d (c) under environmental conditions, where: (d) Tg; (e) ΔHr;

[0070] Figure 9 Wide-angle X-ray diffraction (WXRD) patterns of the PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention aged for 0 (a), 50 (b), and 170 d (c) under environmental conditions;

[0071] Figure 10 Ultraviolet-visible spectra (a) of the PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention; Evaluation of the light transmittance of the films under natural light conditions (b);

[0072] Figure 11This is the oxygen permeability performance (a, b) of PLLA, GO-g-PLLA, and GO-g-PLBI copolymer films of the present invention and the TEM images (c-f) of the copolymers; where: (a) Oxygen transmission rate (OTR); (b) Oxygen permeability coefficient (OP); (c) TEM image of GO-g-PLLA film and (d) partial enlarged view; (e) TEM image of GO-g-PLBI film and (f) partial enlarged view. Detailed implementation manners

[0073] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0074] Example 1 Preparation of GO-g-PLBI copolymer

[0075] 1. Preparation of PBI prepolymer

[0076] Itaconic acid (IA) and 1,4-butanediol (BDO) were mixed at a molar ratio of 1:1.03, and hydroquinone at 0.5 wt% relative to IA was added as a polymerization inhibitor. The reaction was carried out at 150 °C under a nitrogen atmosphere for 2 h. Then the reaction was continued at 60 Pa and 150 °C for 6 h to obtain a PBI prepolymer (Mn number-average molecular weight = 95).

[0077] 2. Preparation of GO-g-OLLA prepolymer

[0078] 700 g of L-lactic acid (LA) and monolayer GO powder (flake diameter 5-10 μm, thickness 0.55-1.45 nm) at 0.01 wt% relative to LA were added to a four-necked open reactor (the four-necked open reactor was connected to a reaction system equipped with a mechanical stirring device, a condensation circulation device, and a water removal device). The mixture was ultrasonically treated for 1 h to achieve uniform dispersion of GO in LA. The reaction was carried out at 110 °C and 40 kPa for 1 h. Then the pressure was reduced to 13 kPa, and the reaction was carried out at 150 °C for 2 h. Then the pressure was reduced to 4 kPa, and the reaction was carried out at 150 °C for 4 h to obtain a GO-g-OLLA prepolymer.

[0079] 3. Preparation of GO-g-PLBI copolymer

[0080] GO-g-OLLA was combined with a PBI prepolymer (5.0 wt% relative to GO-g-OLLA), SnCl 2 ·2H 2 O and p-toluenesulfonic acid monohydrate (TsOH·H 2 O, CAS NO: 6192-52-5). The addition amount of SnCl 2 ·2H 2 O was 0.5 wt% of the total reactants. TsOH·H 2 O was mixed with SnCl 2 ·2H 2 O in an equimolar ratio. The pressure in the reactor was reduced to 60 Pa within 60 min, and the mixture was reacted at 180 °C for 20 h to obtain a crude product. The reaction system was cooled from 180 °C to 170 °C, and triphenyl phosphite (TPP, CAS NO: 101-02-0) at 2.5 wt% relative to the crude product was added, and the reaction was continued at 30 Pa for 3 h to obtain a chain-extended product. The chain-extended product was pulverized, crystallized at 60 Pa and the crystallization temperature for 3 h, and then solid-phase polymerized at a temperature 10 °C below the melting point for 48 h. It was dissolved in chloroform, precipitated with ice ethanol, and vacuum dried for 48 h to obtain the GO-g-PLBI copolymer.

[0081] Preparation of GO-g-PLLA in Comparative Example 1

[0082] Comparative Example 1 aimed to synthesize GO-g-PLLA by the method of Example 1 but omitting the addition of the PBI prepolymer and TPP.

[0083] 1. Preparation of GO-g-OLLA prepolymer

[0084] 700 g of L-lactic acid (LA) and monolayer GO powder at 0.01 wt% relative to LA were added to a four-necked open reactor (the four-necked open reactor was connected to a reaction system equipped with a mechanical stirring device, a condensation circulation device, and a water removal device). The mixture was sonicated for 1 h to achieve uniform dispersion of GO in LA. The reaction was carried out at 110 °C and 40 kPa for 1 h. The pressure was then reduced to 13 kPa, and the reaction was carried out at 150 °C for 2 h. Then the pressure was reduced to 4 kPa, and the reaction was carried out at 150 °C for 4 h to obtain the GO-g-OLLA prepolymer.

[0085] 2. Preparation of GO-g-PLLA

[0086] GO-g-OLLA was added with the catalysts TsOH·H 2 O and SnCl 2 ·2H 2 O. SnCl 2 ·2H 2The addition amount of O is 0.5 wt% of the total reactants, and TsOH·H 2 O and SnCl 2 ·2H 2 O are mixed in an equimolar ratio. The pressure in the reactor is reduced to 60 Pa within 60 min, and the mixture is reacted at 180 °C for 20 h to obtain the crude product. The crude product is dissolved in chloroform, precipitated with ice ethanol, and dried in vacuo for 48 h to obtain GO-g-PLLA.

[0087] Comparative Example 2 Preparation of PLLA

[0088] 700 g of L-lactic acid (LA) is added to a four-necked open reactor (the four-necked open reactor is connected to a reaction system equipped with a mechanical stirring device, a condensation circulation device, and a water removal device). The reaction is carried out at 110 °C and 40 kPa for 1 h. Then the pressure is reduced to 13 kPa, and the reaction is carried out at 150 °C for 2 h. Then the pressure is reduced to 4 kPa, and the reaction is carried out at 150 °C for 4 h to obtain the OLLA prepolymer. Catalyst TsOH·H is added to OLLA 2 O and SnCl 2 ·2H 2 O (TsOH·H 2 O and SnCl 2 ·2H 2 O are mixed in an equimolar ratio), and the addition amount of SnCl 2 ·2H 2 O is 0.5 wt% of the total reactants. TsOH·H 2 O and SnCl 2 ·2H 2 O are mixed in an equimolar ratio. The pressure in the reactor is reduced to 60 Pa within 60 min, and the mixture is reacted at 180 °C for 20 h to obtain the crude product. The crude product is dissolved in chloroform, precipitated with ice ethanol, and dried in vacuo for 48 h to obtain GO-g-PLLA.

[0089] Example 2

[0090] After dissolving 2.80 g of the raw material (the GO-g-PLBI copolymer obtained in Example 1) in 70 mL of chloroform, a GO-g-PLBI film with a thickness of about 60.0 ± 2.0 μm is prepared by the solution casting method.

[0091] For comparison, the GO-g-PLLA film and the PLLA film are prepared by the same method (replacing the raw material with the product corresponding to Comparative Example 1 or 2 above).

[0092] All the films are stored at room temperature and under vacuum conditions for 10 d to remove the residual solvent in the films.

[0093] I. Detection of PLLA, GO-g-PLLA, and GO-g-PLBI Copolymers by Nuclear Magnetic Resonance (NMR)

[0094] Using CDCl 3 as the solvent, the chemical structure and polymerization results of the copolymers were confirmed by NMR spectroscopy (400 hz Avance 2B Bruker, Germany) at 25 °C. Among them, 16 1 H NMR scans and 1024 13 C-NMR scans were collected respectively.

[0095] As Figure 1 shown, the chemical structure information of the copolymers was inferred by 1 H-NMR and 13 C-NMR. We successfully predicted the possible chemical structural formulas of PLLA, GO-g-PLLA, and GO-g-PLBI copolymers based on the relevant chemical shift values. As Figure 1 (a) 1 H-NMR spectrum shows that for PLLA, the peaks at δ = 5.18 and 1.59 ppm belong to the methine (a, -CH-) and methyl (b, -CH 3 ) proton signals respectively. For GO-g-PLLA and GO-g-PLBI, in addition to the typical methine (a, -CH-) and methyl (b, -CH 3 ) proton signals, weaker proton signals also appear at 2.44 ppm and. In addition, for GO-g-PLBI, the peaks at δ = 1.71 ppm and 4.15 ppm belong to the -CH 2 -(f) and -O-CH 2 -(g) proton peaks of the BDO domain respectively. The multiplets at 3.47 - 5.31 ppm are attributed to the methylene protons (d, -CH 2 -) in the IA domain. The multiplets at δ = 6.33 - 6.47 ppm and 5.74 - 5.87 ppm are attributed to the side-chain olefins (e, CH 2 =C-) provided by IA and the olefins (e, -CH = C-) after side-chain olefin addition. Using the integral areas of the -O-CH 2 - protons (δ = 4.15 ppm) in the BDO structural unit and the -CH- protons (δ = 5.16 ppm) in the LA structural unit and combining with formula (1), the actual mass ratio of the PBI block in the GO-g-PLBI copolymer was calculated to be 5.3%, which is consistent with the expected feed ratio.

[0096]

[0097] Among them, I d and Ia corresponding to 1 the integral area at the marked position in the H-NMR spectrum. Interestingly, the intensities of the vinyl proton peaks at δ = 6.43 and 5.83 ppm in GO-g-PLBI are significantly lower compared to those of the vinyl hydrogen protons at δ = 6.30 and 5.70 ppm. And it is assumed that all vinyl groups in the copolymer are side-chain olefins (CH 2 =C-). In an ideal situation, the molar ratio of the BDO structural unit to vinyl in PBI is 1:1. That is, the integral area of the -O-CH 2 - protons to the integral area of the side-chain olefin (CH 2 =C-) protons is 2:1. However, in reality, the molar ratio of the -O-CH 2 - protons to the olefin protons in the BDO structural unit is 100:37. The above phenomenon indicates that there is a certain degree of consumption of carbon-carbon double bonds in the copolymer. In addition, the branching degree of the unsaturated bonds is further calculated to be 30.3% using formula (2). In particular, the peaks appearing at δ = 7.81 - 7.82, 7.33 - 7.40, and 7.07 - 7.09 ppm may be related to the hydrogen protons on the phosphorus benzene structure at the end of the copolymer molecular chain after chain extension.

[0098]

[0099] As Figure 1 (b - e) shows, further correlation and confirmation of the copolymer's structural information are carried out using 13 the 13C-NMR spectrum and 2D-HSQC. As can be seen from the figure, the peak at δ = 169.53 ppm is caused by the carbonyl group (1, -CO) of PLLA. Combining the results of 2D-HSQC( Figure 1 (c) shows), the appearance of (a, 2, -CH-) and (b, 3, -CH 3 ) verifies the typical structure of PLLA. For GO-g-PLLA, in addition to containing the typical PLLA main-chain structure, unsaturated carbon signals (4, ) connected to pure carbon atoms in the graphene skeleton and saturated carbon signals (6) caused by defects appear at δ = 132.79 and δ = 29.66 ppm respectively. At the same time, combining the results of 2D-HSQC( Figure 1 (d)), a weak unsaturated carbon signal (a, 5) connected to hydrogen protons appears. For GO-g-PLBI, in addition to the appearance of the typical GO-g-PLLA main-chain structure, (e, 9, CH 2 =C-, IA), (e, 10, C-CH=C-, IA), (f, 11, -O-CH 2 -, BDO), and d, 6 (g, 12, -CH 2The position of BDO indicates the successful synthesis of PBI onto the main chain of GO-g-PLLA.

[0100] II. Gel Permeation Chromatography (GPC)

[0101] The number-average molecular weight (Mn) and polydispersity index (PDI) of the copolymer were measured by gel permeation chromatography (GPC, LC-20ADXR, Shimadzu Corporation, Japan) with a shodex KF-800 column at 35 °C using chloroform as the mobile phase (1.0 mL / min) relative to calibrated polystyrene standards.

[0102] PDI, the full name is Polydispersity Index, is an index quantified by comparing the weight-average relative molecular weight (Mw) and number-average molecular weight (Mn) of a polymer. Its calculation formula is PDI = Mw / Mn. The PDI index directly reflects the uniformity of the polymer chain length: the lower the PDI value, the more uniform the polymer chain length distribution. Such polymers usually exhibit better comprehensive properties. For high-quality polymer materials, their PDI values are usually between 1 and 2.

[0103] Table 1. Molecular characteristics of GO nanocomposites

[0104]

[0105]

[0106] Table 1 shows the GPC results of PLLA, GO-g-PLLA, and GO-g-PLBI. All three substances have small PDI values, indicating that all three substances have polymer characteristics. The M of pure PLLA n is 3.65×10 4 g / mol. Compared with PLLA, the synthesized GO-g-PLLA has a higher M n (3.89×10 4 g / mol), which may be related to the branched structure induced by GO. After copolymerization of GO-g-OLLA with PBI, the molecular weight of the copolymer further increases to 4.37×10 4 g / mol. This indicates that the introduction of PBI during the melt polycondensation process is beneficial to the formation of high-molecular-weight PLLA copolymers. The high molecular weight of GO-g-PLBI can be attributed to the long-branched structure formed by partial C=C branching reactions between molecular chains, which thus plays an "internal chain extension" role in the copolymer matrix.

[0107] III. Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) Spectra of Copolymer Films

[0108] As Figure 2 shown, the chemical structure information of the copolymer was analyzed by ATR-FTIR. As Figure 2 (a) shows, the absorption peaks at 3250, 1748 and 1616 cm -1 are respectively related to the stretching vibrations of the -OH, C=O and C=C groups of GO. The absorption peaks at 1224 and 1051 cm -1 are respectively related to the stretching vibrations of the -C-O- and epoxide C-O groups. The typical stretching vibration modes of PLLA were observed in all polymers, including the -CH stretching vibration at 2995 - 2850 cm -1 , the carbonyl (C=O) stretching vibration at 1748 cm -1 , the symmetric stretching vibration of C-O-C at 1180 cm -1 and the -C-O- stretching vibration at 1080 cm -1 . In addition, no characteristic peaks of hydroxyl or carboxyl groups were found, indicating that PBI, GO and TPP participated in the polycondensation process of L-lactic acid. As Figure 2 (b - c) shows the locally amplified signals of the copolymer in the ranges of 1849 - 1630 cm -1 and 831 - 800 cm -1 respectively. As Figure 2 (c) shows, compared with PLLA, GO-g-PLLA has a weaker peak signal at 816 cm -1 , which is attributed to the bending vibration of the benzene-like ring skeleton in graphene. As Figure 2 (b) shows, the peak intensity of GO-g-PLBI at 1748 cm -1 increases, which may be related to the higher disorder of the molecular chains of the GO-g-PLBI copolymer. In addition, compared with PLLA and GO-g-PLLA, GO-g-PLBI has a greater peak intensity at 1719 cm -1 and, which is related to the stretching vibration of the carbonyl group in PBI. And the intensity of GO-g-PLBI at 816 cm -1 increases significantly, which is related to the introduction of vinyl groups in PBI. In summary, the above results indicate that the GO-g-PLLA and GO-g-PLBI copolymers were successfully synthesized, which is consistent with the results of NMR.

[0109] IV. X-ray Photoelectron Spectroscopy (XPS)

[0110] Elemental and chemical state analysis of GO and thin film samples was performed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA). The operation was carried out at 15 kV and 150 W, and photoelectrons were collected using a monochromatic Al Kα X-ray source at a take-off angle of 90° relative to the sample surface to obtain the scanning measurement spectra of the samples.

[0111] Table 2 Percentage of chemical bonds and elements and binding energies of chemical bonds

[0112]

[0113]

[0114] The XPS spectra of the copolymer film and the high-resolution C1s XPS spectra are as Figure 3 shown. Table 2 summarizes the bond percentages and binding energies of different bonds based on the C1s spectra of the copolymer film. The C1s spectrum of GO shows three types of carbon bonds, C-C / C═C, C-O, and O-C═O, with binding energies of 284.8, 287.0, and 288.5 eV, respectively ( Figure 3 (b)). The percentages of C-C / C═C, C-O, and O-C═O bonds in GO are 50.2%, 45.1%, and 4.6% respectively (Table 2), and the total oxygen content is 33.36%, indicating a high degree of oxidation. The high-resolution C1s XPS spectra of pure PLLA and the copolymer can be deconvoluted into three chemical bonds, namely O-C═O, C-O, and C-C / C═C / C-H, which appear at binding energies of 289.1 - 289.2, 287.0 - 287.2, and 284.8 - 284.9 eV, respectively. The C-O (25.0%) and O-C═O (23.7%) in PLLA mainly come from the ester groups in L-lactic acid. However, the ratio of C to O and the ratio of the chemical bond C-C / C═C / C-H to O-C═O increase from 1.91 and 2.67 of pure PLLA to 2.32 and 4.03 of GO-g-PLLA, respectively, indicating that GO can act as an initiator in the LA polycondensation process and can promote the formation of a multi-branched structure. In addition, the relative content of C-C / C═C / C-H and the C / O ratio increase from 65.1% and 2.32 of GO-g-PLLA to 67.5% and 2.40 of GO-g-PLBI, respectively, while the relative content of C-O decreases from 18.9% of GO-g-PLLA to 15.8% of GO-g-PLBI. A possible explanation is that the relative content of C-C / C═C / C-H in the PBI unit of the GO-g-PLBI copolymer is higher, while the relative content of C-O is lower, resulting in a higher C-C / C═C / C-H content and a lower C-O content on the surface of the copolymer film. In summary, the above results indicate that GO-g-OLLA and PBI are successfully copolymerized.

[0115] V. Thermogravimetric Analysis

[0116] Under an argon atmosphere, the samples were heated from 30 to 700 °C at a rate of 10 °C / min using a synchronous thermogravimetric analyzer (STA, TGA / DSC 1 Mettler-Toledo) to evaluate the thermal stability of the copolymers.

[0117] As Figure 4 (a) shows the TGA curve of the copolymer. The main degradation temperature range of pure PLLA is 258.1 - 313.7 °C respectively. The main degradation temperature range of GO-g-PLLA is increased to 260 - 364.7 °C, indicating that GO-g-PLLA has higher thermal stability. This may be due to the introduction of GO nanosheets being beneficial to heat transfer, thereby improving the thermal stability of the polymer. In addition, it is worth noting that the GO-g-PLBI copolymer has higher thermal stability than PLLA and GO-g-PLLA. This may be because the introduction of TPP promotes crosslinking between polymer segments, and there is branching between molecular chains, thereby improving the thermal stability of the GO-g-PLBI copolymer.

[0118] VI. Gel Content

[0119] The gel content of GO-g-PLBI, GO-g-PLLA, and PLLA was determined using the Soxhlet extraction method. Briefly, approximately 0.3 g of the sample was extracted with chloroform as the solvent for 24 h. Subsequently, it was taken out and dried under vacuum at 60 °C overnight, and finally the mass of the residue was analyzed. Each sample was measured at least three times in duplicate. The gel content in the component was calculated using formula (3).

[0120]

[0121] where, m 0 is the mass of the residue, M is the mass of the sample before extraction, and m is the gel content in the component. In addition, 0.5 g of the copolymer was accurately weighed and dissolved in chloroform, and the dissolution state of the copolymer was observed with a laser pointer.

[0122] To further verify the crosslinking phenomenon of the segments in the copolymer. As Figure 4(b) shows the dissolution (dispersion) characteristics of the copolymers in chloroform. All copolymer solutions presented uniform solutions without obvious precipitation and aggregates. It should be noted that after irradiation with a laser pointer, there were no obvious micelles in PLLA, while micellization occurred in both GO-g-PLLA and GO-g-PLBI. This was caused by the refractive phenomenon of the copolymer solution induced by the introduction of GO. In addition, it should be noted that the micelles in the GO-g-PLBI solution were more obvious. This might be due to the unsaturated double bonds in PBI inducing partial cross-linking between copolymer segments, thereby enhancing the micelles in the GO-g-PLBI solution.

[0123] As Figure 4 (c) shows, the quantitative results of the insoluble substances of the three copolymers. The content of insoluble substances in pure PLLA was 0.19%, indicating no obvious intermolecular cross-linking. The gel content of GO-g-PLLA increased to 0.30%, which might be related to the introduction of GO. In particular, the gel content of GO-g-PLBI increased to 4.99%. This result indicates that introducing a low dose of unsaturated polyester during the melt polycondensation process is an effective way to promote the micro-crosslinking of polylactic acid.

[0124]

Preparation and Performance Testing of Aged Film Samples

[0125] The film samples with completely evaporated solvents were stored at ambient conditions for 0, 50, and 170 days respectively to measure the performance indexes of the films. Among them, the performance indexes of the film samples stored for 0 days represent the data measured when the samples were not aged.

[0126] VII. Tensile Properties

[0127] The samples with different aging times were cut into specimens of 7 cm × 0.5 cm, and an electronic tensile testing machine (Xn-8750, Labthink, China) with a test area of 5 mm × 28 mm and a tensile speed of 10 mm / min was used to measure the stress-strain curves of the films. The tensile strength (MPa) and elongation at break (%) of the films were obtained from the stress-strain curves. As Figure 5 (a-c) shows, (a), (b), and (c) are the typical stress-strain curves of PLLA and its copolymer films at different aging times (0, 50, and 170 d) respectively. At least 3 repeated tests were carried out at each aging time, and the average value between each group of repetitions was taken. The results of the tensile strength (TS) and elongation at break (EB) are as Figure 5 (d-e) shows. The analysis of the measurement results is as follows:

[0128] The TS and EB of unaged pure PLLA were 22.1 MPa and 4.6% respectively. Throughout the aging period, pure PLLA always showed brittle fracture, and its tensile strength gradually increased, reaching 38.1 MPa at 170 d.

[0129] The TS and EB of unaged GO-g-PLLA film were 25.5 MPa and 165.4% respectively, showing ductile fracture, and showing higher flexibility and strength compared with PLLA. This result indicates that the introduction of GO can significantly improve the strength and flexibility of PLLA. With the extension of aging time, the initial tensile strength of GO-g-PLLA film gradually increased, while the elongation at break decreased significantly (p<0.05). At 170 d of aging, the EB was only 24.6%, which was only reduced by 85.1% compared with day 0, indicating that the GO-g-PLLA film has relatively poor anti-aging performance.

[0130] The TS and EB of unaged GO-g-PLBI film were 13.6 MPa and 339.5% respectively. The strength was lower than that of PLLA, but the EB was about 73.8 times, showing extremely high flexibility. Due to the introduction of PBI, the content of amorphous segments of PLLA was increased, and thus during the mechanical deformation process, the polymer chains were more effective in transferring local strain. In addition, the branched structure in PLBI increased the degree of chain entanglement, thereby inhibiting the relative slip of molecular chains during stretching.

[0131] At 50 d, the TS of GO-g-PLBI increased by only 19.8% compared with day 0, while the EB hardly changed (about 331.3%);

[0132] At 170 d, the TS increased by only 41.2% compared with day 0, while the EB only decreased by 41.1%. The above results indicate that the simultaneous introduction of GO and PBI can significantly improve the tensile properties and long-term anti-physical aging properties of PLLA.

[0133] It can be seen from this that as a nano-filler, GO improved the initial strength and flexibility of PLLA through grafting. However, due to its relatively weak interaction with the PLLA matrix during the aging process, the EB decreased significantly. At the same time, as a branching site, GO synergistically interacted with PBI to further enhance the chain entanglement, thereby inhibiting the relative slip of molecular chains during stretching. As a result, the GO-g-PLBI film has extremely high flexibility and excellent anti-aging properties while maintaining high strength. In addition, both pure PLLA and GO-g-PLLA showed unstable properties during the aging process, especially the significant decrease in EB. In contrast, the properties of the GO-g-PLBI film changed less during the aging period, showing stronger structural stability and anti-physical aging ability.

[0134] In the present invention, with a very low dose of GO (equivalent to 0.01% (w / w) lactic acid) as the initiator, introducing 5.3% (w / w) of PBI by in-situ polymerization can increase the EB of the film by 73.8 times. A possible explanation is that GO acts as a branching site during the polymerization of lactic acid as an initiator, further increasing the degree of intermolecular entanglement. In addition, as a two-dimensional structure, graphene nanosheets themselves have a certain ductility. The addition of an appropriate amount of highly dispersed graphene nanosheets can significantly improve the overall tensile properties of the copolymer film.

[0135] VIII. Scanning Electron Microscope (SEM)

[0136] A scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan) was used to examine the fracture morphology of the film after the tensile test. Measurements were taken in BSE mode at an acceleration voltage of 15 kV.

[0137] Generally, the compatibility of polymer nanocomposites and the fracture behavior of the film can be evaluated by microscopic morphology analysis. Therefore, the fracture surfaces of the tensile specimens were examined using a scanning electron microscope (SEM). In addition, no obvious aggregates were found on the fracture surfaces of GO-g-PLLA and GO-g-PLBI, and the brightness of the fracture surfaces was darker compared to the PLLA phase, indicating that GO was successfully and uniformly dispersed in the PLLA and PLBI matrices by in-situ polymerization.

[0138] In addition, the fracture surfaces of GO-g-PLLA and GO-g-PLBI showed a clear fibrous structure, which is a typical characteristic of plastic deformation, and was more obvious in GO-g-PLBI. These results indicate that GO has extremely high compatibility with the PLLA and PLBI matrices and can significantly change the fracture behavior of the copolymer film. With the increase in aging time, the fibrous structure of the GO-g-PLLA film gradually decreased ( Figure 6 Figure 2(b)(2 - 3)). This may be due to the increase in film crystallinity induced by aging and the decrease in chain segment integrity, resulting in a weakened deformation ability of GO-g-PLLA and a reduction in the stress transfer ability of the fibers. In particular, although the fibers on the fracture surface of the GO-g-PLBI film became thicker in the later stage of aging (170 d), a clear fibrous structure was presented in the GO-g-PLBI film throughout the aging period, indicating that the introduction of PBI can effectively inhibit the phenomenon of reduced plasticity of GO-g-PLLA induced by physical aging, which is consistent with the results of the tensile properties.

[0139] IX. Temperature-Dependent FT-IR

[0140] Samples were prepared by dropping a certain amount of copolymer chloroform solution onto a KBr disk. After most of the solvent had evaporated, the KBr disk with the sample was placed in vacuo at room temperature for 48 h to remove the residual solvent. The sample was scanned 64 times at a resolution of 4 cm -1 . The temperature-dependent FT-IR (ATR-FTIR, Shimadzu, Japan) of the KBr disk with the dry copolymer film was measured. The sample was heated to 90 °C at a rate of 6 °C / min using a hot stage (Caikon, CK-400, Shanghai, China). During this process, spectra were collected every 60 s in the temperature range of 30 - 90 °C.

[0141] To further explore the anti-aging mechanism of the GO-g-PLBI film, as Figure 7 (a1 - c1) shows, the temperature-dependent infrared spectra and the corresponding difference spectra ( Figure 7 (a2 - c2)) of pure PLLA, GO-g-PLLA, and GO-g-PLBI in the carbonyl stretching vibration region and in the temperature range of 30 - 90 °C were depicted, respectively. As Figure 7 (d - e) shows, the relationships between the normalized intensities of the peaks at 1758, 1745, and 1261 cm Figure 7 calculated from -1 (a1 - c1) and the temperature are shown. The absorption peaks at 1758, 1745, and 1261 cm -1 correspond to the gt, tt, and gg conformations of the PLLA molecular chain. Generally speaking, if a polymer has a relatively high relative content of gt and relatively low relative contents of gg and gt conformations, the aging phenomenon of the polymer is more serious. It is assumed that under the initial conditions, the contents of the gt conformation of pure PLLA, GO-g-PLLA, and GO-g-PLBI are equal ( Figure 7 (d)). As Figure 7 (d - e) shows, in pure PLLA near the glass transition region, the amount of the gt conformation always remains at the highest level, while the amounts of the gg and tt conformations always remain at the lowest levels. In contrast, GO-g-PLBI shows the highest levels of the gt and tt conformations. Therefore, this result indicates that the GO-g-PLBI copolymer exhibits higher segmental mobility and flexibility due to the relatively high initial amounts of the gg and tt conformations.

[0142] Generally, during the heating process, the content of the energetically favorable conformation in PLLA decreases, and the content of the energetically unfavorable conformation increases. It can be observed that as the temperature increases, the peak intensity at 1758 cm -1 decreases, and the width of the peak becomes larger, and the peak at 1745 cm -1The strength increases. After the introduction of GO, PLLA shows the fastest consumption rate of the gt conformation, while after the introduction of PBI, the consumption rate of the gt conformation decreases. The research shows that during the heating process in the glass transition region, the consumption rate of the gt conformation is higher, indicating that the polymer is more inclined to rearrange from a high-energy conformation to a low-energy conformation, thus being more prone to aging. Therefore, the above results show that although the introduction of GO by in-situ polymerization can increase the degree of disorder of PLLA (higher gg and tt conformation contents), it simultaneously leads to a faster induction period of gt conformation change, thereby accelerating the aging of GO-g-PLLA. However, after the further introduction of PBI, the induction period of gt conformation change in the copolymer significantly slows down, thereby reducing the aging rate of the copolymer.

[0143] X. Differential Scanning Calorimetry (DSC)

[0144] To study the thermal transitions near T g About 0.6 - 0.8 mg of the thin film sample was weighed and sealed in an aluminum crucible. All samples were heated from 0 °C to 70 °C at a heating rate of 10 °C / min under a nitrogen atmosphere.

[0145] As Figure 8 shown, the DSC heating curves in the range of 0 - 70 °C and the corresponding glass transition temperatures (T g ) and heat relaxation enthalpies (ΔH r ) of the copolymer thin films aged for 0, 50, and 170 days under environmental conditions are shown as a function of the storage days. It can be found that on the 0th day of storage, all samples had relatively low T g and ΔH r . This is because in the initial stage of aging, the copolymer molecular chains are in a high-energy state, with low molecular mobility and densification degree, and a high degree of molecular chain disorder, resulting in small T g and ΔH r . As the storage days increase, the T g and ΔH r of all samples show an increasing trend. This is related to the reduction of free volume due to physical aging. From 0 - 50 days, the T g of the PLLA and GO-g-PLLA thin films has a similar growth rate, while the ΔH r increases by 2.91 and 1.91 J / g respectively, indicating that the GO-g-PLLA thin film has better anti-aging effect from 0 - 50 days. On the contrary, from 50 - 170 days, compared with PLLA, the T g and ΔH rThe growth rate is faster, indicating that the aging acceleration period of GO-g-PLLA is after 50 days. The branching effect of the polymer chains induced by GO will make the molecular chains more disordered, resulting in the formation of more gg and tt conformations in the polymer. During the solvent-induced aging process, this phenomenon is not conducive to the stability of the polymer conformation, resulting in a longer aging induction period. However, due to the higher rate of formation of the gt conformation, the aging rate of GO-g-PLLA is relatively fast, resulting in T g and ΔH r having a greater growth rate in the middle and late stages of aging (50 - 170 days). It should be noted that during the entire physical aging process, the T g and ΔH r of GO-g-PLBI both remain at a relatively low level. First, the introduction of the PBI block will increase the mobility of the copolymer segments, which is consistent with previous studies. Second, the copolymer contains a relatively large number of initial gg and tt conformations, which results in a relatively low ΔH r value. Due to the cross-linking phenomenon caused by carbon-carbon double bonds, the conformational rearrangement of the GO-g-PLBI chains is prevented, resulting in a relatively small change rate of ΔH r during the aging process.

[0146] XI. Wide-angle X-ray diffraction (WXRD)

[0147] The WAXD patterns of the film samples aged for different times and GO powder were recorded by an X-ray diffractometer (Rigaku Miniflex 600, Japan) using Cu-Kα radiation with a scanning range of 5° - 60° and a scanning speed of 2° / min.

[0148] As Figure 9 shown, it is the WXRD pattern of the surface of the copolymer film after being treated for different aging times. It can be seen that the observation results show that the pure PLLA without aging treatment exhibits a relatively weak diffraction peak at 2θ = 17.02°, and this peak is the characteristic diffraction peak of the (110) / (220) crystal plane in the PLLA crystal. In contrast, no obvious diffraction peaks appear in the unaged GO-g-PLLA and GO-g-PLBI samples in this range, indicating that in the unaged state, the crystallinity of GO-g-PLLA and GO-g-PLBI is extremely low or almost no crystallization. This phenomenon can be attributed to the branching effect of the polymer chains induced by the introduction of GO and PBI as grafting groups, which restricts the movement ability of the molecular segments and thus inhibits the formation of a regular crystal structure.

[0149] After aging for 50 days, new diffraction peaks of PLLA appeared at 2θ = 12.42, 19.08, and 21.04°, which were attributed to the (204), (004) / (103), and (203) crystal planes of PLLA, respectively. After aging for 170 days, a diffraction peak of the (015) crystal plane appeared at 2θ = 22.56° for PLLA. As the aging time increased, the number of diffraction peaks of PLLA gradually increased, the diffraction peak at 2θ = 17.02° gradually shifted to the left, and the peak intensity gradually increased, indicating that the crystallinity of PLLA increased after aging treatment. This is because during the aging process, under the action of oxygen, water vapor, and light, some molecular chains of PLLA were broken, the fluidity was enhanced, and the crystallization rate was accelerated.

[0150] For the GO-g-PLLA film, compared with the middle and late aging stages (50 - 170 days), the diffraction peak intensity at around 2θ = 17.02° increased more slowly in the early aging stage (0 - 50 days), and was more obvious compared with the PLLA film aged for 50 days. This result indicates that the branching effect induced by GO led to the formation of more free volume between polymer molecular chains, thereby inhibiting the increase in crystallinity induced by aging.

[0151] It should be noted that GO-g-PLBI had no diffraction peak at 0 - 50 days, but a weak diffraction peak appeared at 2θ = 16.88° at 170 days, indicating that the introduction of PBI into GO-g-PLLA further inhibited the increase in crystallinity induced by physical aging. This is because the degree of branching of the copolymer increased after the introduction of PBI, and there was microgelation in the components, resulting in more steric hindrance effects on the rearrangement of molecular chains, thus greatly inhibiting the increase in crystallinity induced by aging.

[0152] XII. Ultraviolet-visible spectral analysis of the film

[0153] The ultraviolet-visible spectrum of the film was measured using an ultraviolet-visible spectrophotometer (Cary 5000 type, Agilent Technologies, USA) in the wavelength range of 200 - 800 nm. In addition, under natural light conditions, optical pictures of the film were taken to evaluate the light transmittance of the film.

[0154] Table 3 Quantitative analysis of the ultraviolet-visible spectral transmittance of the GO-g-PLBI copolymer film

[0155] Sample UVB (320 - 400nm) UVA (280 - 320nm) 600nm PLLA 73.9％ 62.1％ 85.1％ GO-g-PLLA 64.7％ 56.4％ 76.8％ GO-g-PLBI 65.7％ 52.7％ 76.2％

[0156] As Figure 10As shown in Table 3, the transparency and UV shielding properties of the GO-g-PLBI copolymer films were evaluated. Generally, the transmittance at 600 nm is defined as the visible light transmittance. Ultraviolet rays can be divided into three bands: UVA (320 - 400 nm), UVB (280 - 320 nm), and UVC (200 - 280 nm). Due to the absorption and barrier of the ozone layer, the ultraviolet rays reaching the earth's surface are mainly UVA and UVB.

[0157] As can be seen from the figure, the pure PLLA film has a high transparency (85.1%), but poor UV shielding performance. The transparency of the GO-g-PLLA film only drops to 76.8%, but its UV blocking performance is improved compared to PLLA. This is due to the unique large π-conjugated structure of the GO nanosheets on the surface, which endows it with excellent UV absorption ability and high visible light transmittance.

[0158] The GO-g-PLBI film has a similar visible light transparency to GO-g-PLLA, but its UV absorption ability for UVA light is improved. The transmittance of the GO-g-PLBI film at 600 nm is 76.2%, similar to that of the GO-g-PLLA film (76.8%), and much higher than the transmittance of the pure PLLA film in the UV band, but slightly lower than the visible light transmittance of the pure PLLA film (85.1%). The GO-g-PLBI film may show strong absorption ability for UVB light due to the presence of unsaturated PBI segments. In summary, compared with PLLA, the GO-g-PLBI film has excellent UV absorption and barrier ability, while maintaining a visible light transmittance of 76.2%.

[0159] XIII. Oxygen Barrier Performance and Transmission Electron Microscopy (TEM) Detection

[0160] The oxygen permeability was evaluated using a Type 8001 oxygen permeation meter. The test was carried out at 23 °C and 50% relative humidity. The output pressures of O 2 and N 2 were set to 0.3 MPa. During the test, the film was placed between two chambers: one chamber was purged with N 2 with controlled flow, while the other chamber contained O 2 . O 2 penetrated through the installed sample into the N 2 carrier gas and was transported to the coulomb detector, where the amount of O 2 flowing into the detector per unit time was calculated. The oxygen transmission rate (OTR) of the film was directly obtained from the test results, and the oxygen permeability coefficient (OP) was calculated using Equation (4).

[0161]

[0162] Where D is the thickness of the film and ΔP is the pressure difference of oxygen inside and outside the film.

[0163] The internal structures of the GO-g-PLLA and GO-g-PLBI film samples were analyzed using a field emission transmission electron microscope (TEM, JEM-2100F, JEOL, Japan).

[0164] OTR is a measure of the amount of gas passing through a substrate at a steady rate over a given time. Oxygen permeability is an important aspect in packaging applications for controlling the shelf life of foods, beverages, and commodities. When a film exhibits a low OTR value, oxidation is retarded and the shelf life of the product is extended.

[0165] As Figure 11 (a-b) shows the results of the changes in OTR and OP values of all films with aging time. It can be seen that the OTR and OP of the unaged PLLA film are approximately 67.9 cm 3 / m 2 ·d and 1.7×10 -8 cm 3 ·m / m 2 ·h·Pa, respectively, with medium oxygen barrier properties. As the aging time increases, the OP values of all samples decrease to varying degrees. By 170 d, the OP values of the PLLA, GO-g-PLLA, and GO-g-PLBI films decrease by 45.2%, 57.3%, and 60.9%, respectively, compared to their corresponding unaged samples. This result indicates that the physical aging process significantly improves the oxygen barrier properties of PLLA and its copolymers, and has a more significant impact on GO-g-PLBI and GO-g-PLLA.

[0166] The decrease in the OP value during the physical aging process is caused by the rearrangement of molecular chains and the reduction of free volume. However, it should be noted that the physical aging process will damage the integrity of polymer segments, which has a two-sided effect on the barrier property. On the one hand, the formation of small molecular segments will lead to an increase in crystallinity as nucleation sites, thereby improving the barrier property of the film. On the other hand, the reduction of large molecular segments will lead to a decrease in the entanglement degree of the polymer matrix, thereby reducing the barrier property of the PLLA film.

[0167] The decrease in the OP value can be attributed to the rearrangement of molecular chains and the reduction of free volume during the aging process. However, while physical aging enhances the barrier property, it may also damage the integrity of polymer segments, resulting in a dual effect: on the one hand, small molecular segments act as nucleation sites to promote crystallization and improve the barrier property; on the other hand, the reduction of large molecular segments may reduce the entanglement degree of the polymer matrix, thereby weakening the barrier property.

[0168] The research of this invention shows that the active groups of GO and the carbon-carbon double bonds in the PBI segments serve as branching sites, significantly improving the crosslinking degree of GO-g-PLLA and GO-g-PLBI films and effectively inhibiting the damage to the segment integrity caused by physical aging. Although the crystallinity decreases, this crosslinking effect greatly reduces the 2 solubility in GO-g-PLBI and GO-g-PLLA matrices.

[0169] In addition, whether unaged or aged, the OTR and OP values of GO-g-PLLA and GO-g-PLBI films are lower than those of PLLA films. This is due to the uniform dispersion of GO nanosheets in PLLA and PLBI matrices through in-situ polymerization, effectively reducing the OP value of the copolymer (as Figure 11 (c-d) shows). In particular, the OP value of the GO-g-PLBI film is lower during the aging process, which is attributed to the better dispersion of GO in the PLBI matrix (as Figure 11 (d,f) shows) and the high degree of branching and asymmetry brought by the introduction of PBI, increasing the polarity of PLLA and reducing the 2 solubility. It should be noted that the GO-g-PLBI film after 170 days of aging exhibits a minimum OP value of 0.40×10 -8 cm 3 ·m / m 2 ·h·Pa, much lower than the reported GO / PLLA nanocomposite films.

[0170] It can be seen that introducing unsaturated polyester into the in-situ polymerized graphene / PLLA nanocomposite to promote matrix micro-crosslinking and combining with appropriate physical aging treatment is an effective strategy to simultaneously improve the oxygen barrier property and ductility of PLLA.

[0171] In summary, the present invention synthesized graphene oxide grafted lactic acid oligomers (GO-g-OLLA) by in-situ polymerization method, and then introduced unsaturated PBI segments by melt polycondensation method to successfully prepare GO-g-PLBI copolymer. During the polymerization process, the carbon-carbon double bonds of the PBI segments promoted the branching and crosslinking of the copolymer, and this structural change significantly improved the properties of the film. The GO-g-PLBI film exhibited excellent ductility and UV barrier properties while maintaining high transparency. In addition, the physical aging process affected the structure of the GO-g-PLBI film, and the branching and micro-crosslinking phenomena restricted the rearrangement of the segments, enhancing the anti-aging ability of the film. It is worth noting that the micro-crosslinking also promoted the dispersion of graphene nanosheets in the PLBI matrix, forming an effective oxygen barrier. Further analysis found that although moderate physical aging treatment led to a further decrease in the oxygen permeability of the GO-g-PLBI film, the flexibility remained good. This indicates that through reasonable structural design and aging treatment, the oxygen barrier performance of the film can be improved without sacrificing its flexibility. Therefore, the solvent-free polylactic acid copolymer modification technology proposed in the present invention provides a new approach for the development of high-performance food packaging materials, and the material performs excellently in terms of oxygen barrier, ductility, transparency and anti-aging properties, and has potential application value.

[0172] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer, characterized in that: The steps include: S1. Mix itaconic acid and 1,4-butanediol, and react them at 145-155°C in a nitrogen atmosphere for 1.5-2.5 hours under the action of an inhibitor. Then, the reaction is continued for 5 to 7 hours at 40 to 80 Pa and 145 to 155° C. to obtain product A; the number average molecular weight of the product A is 800 to 2000; S2, mixing L-lactic acid and monolayer graphene oxide powder in a mass ratio of 10000:0.5-3, ultrasonicating for 0.5-1.5h to uniformly disperse the graphene oxide in the L-lactic acid, reacting at 105-115°C and 35-45kPa for 0.5-1.5h, then reducing the pressure to 10-16kPa, and reacting at 145-155°C for 1.5-2.5h, then reducing the pressure to 2-6kPa, and reacting at 145-155°C for 3-5h to obtain product B; S3, A, B and the catalyst are mixed, and the reaction is carried out at a vacuum degree of 40-80Pa and 175-185°C for 18-22h to obtain product C; then the temperature is reduced to 165-175°C, triphenyl phosphite is added, and the reaction is continued for 2.5-3.5h at a vacuum degree of 25-35Pa to obtain product D; after the product D is crushed, it is crystallized at a vacuum degree of 50-70Pa for 2-4h, and then solid-phase polymerization is carried out at a temperature 5-15°C lower than its melting point for 44-52h to obtain the graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer; The product A is a prepolymer of unsaturated poly (itaconic acid-co-butylene glycol ester); The product B is graphene oxide grafted oligo(L-lactic acid), in step S2, a polycondensation reaction occurs between the hydroxyl groups of the L-lactic acid to form an oligo(L-lactic acid) chain, graphene oxide serves as an initiator, and the functional groups on the surface of the graphene oxide react with the functional groups at the ends of the L-lactic acid or the oligo(L-lactic acid) chain; In the step S3, the mass ratio of the triphenyl phosphite to the product C is 1:

40.

2. The preparation method according to claim 1, characterized in that: In step S1, the molar ratio of itaconic acid to 1,4-butanediol is 1:1.

03.

3. The preparation method according to claim 1, characterized in that: In step S1, the polymerization inhibitor is hydroquinone, and the mass ratio of hydroquinone to itaconic acid is 1:

200.

4. The preparation method according to claim 1, characterized in that: In step S2, the single-layer graphene oxide powder is a single-layer graphene oxide nanosheet with a sheet diameter ranging from 5 to 10 μm and a thickness of 0.55 to 1.45 nm.

5. The preparation method according to claim 1, characterized in that: In step S3, the catalyst is SnCl2·2H2O and TsOH·H2O mixed in an equal molar ratio; the added amount of SnCl2·2H2O is 0.5wt% of the total reactants.

6. The preparation method according to claim 1, characterized in that: Step S3 also includes the following steps: after obtaining the graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer, dissolving it in an organic solvent, precipitating it with ice ethanol, and finally vacuum drying it for 44 to 52 hours.

7. A graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer, characterized in that: Prepared according to the preparation method described in any one of claims 1 to 6.

8. A graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer film, characterized in that: The raw materials include: the graphene oxide / poly (L-lactic acid-co-butylene itaconate) copolymer described in claim 7.

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