Miniature injection molding part of asymmetric PLLA / PDLA two-phase blend and preparation method of miniature injection molding part

Abstract

The invention provides a miniature injection molding part of an asymmetric PLLA / PDLA two-phase blend and a preparation method of the miniature injection molding part, and the preparation method comprises the following steps: firstly melting and mixing PLLA and a small amount of low-molecular-weight PDLA, then carrying out twin-screw extrusion granulation, and finally preparing the high-performance PLA-based miniature part through a miniature injection molding process. An asymmetric PLLA / PDLA two-phase blend system is formed, the PLLA / PDLA blend shows a unique phase separation structure under the system and has a large number of PDLA submicron particles (stereocomplex mesophase SCs) uniformly distributed in the blend, and finally a PLA-based miniature part with high strength and excellent toughness is prepared through miniature injection molding. And no external additive is needed or the biodegradability is sacrificed. The invention develops a new preparation method for the application of a high-performance PLA-based miniature part in the field of biomedical engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of PLA-based micro-injection molding, and relates to a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend and a preparation method thereof. In particular, it relates to a significant improvement in the toughness of micro-injection molded parts by realizing the microstructural evolution of stretch-induced stereocomplex crystals based on an asymmetric PLLA / PDLA two-phase blend system. Background Art

[0002] Polylactic acid (PLA) is a polymer derived from plants. Due to its renewability, biodegradability, and excellent mechanical properties, it is an extremely attractive alternative compared to traditional petroleum-based plastics. Despite these advantages, the widespread application of PLA is severely limited by its insufficient toughness, ductility, and heat distortion resistance.

[0003] It is known that the PLA monomer lactic acid is a chiral molecule, which results in three different isomeric forms of PLA existing. In fact, among these isomeric forms, poly(L-lactide) (also known as poly(L-lactide)) (PLLA) and poly(D-lactide) (also known as poly(D-lactide)) (PDLA) have two helical chains with opposite configurations. Under the influence of van der Waals forces and intermolecular hydrogen bonds, L-lactide and D-lactide sequences can be assembled simultaneously within a unit cell to form stereocomplex crystals (SCs). The melting point of these stereocomplex crystals is approximately 50 °C higher than that of the PLLA homocrystal (HC). Therefore, when processing the blend at a temperature between the melting points of HC and SCs, only HC will melt, while SCs remain intact in the melt. This unique property enables SCs to act as an efficient and biodegradable additive in the subsequent processing of PLLA materials.

[0004] To date, many methods based on the formation of stereocomplexes (SCs) have been developed to improve the properties of PLA, including blending, grafting, and copolymerization, etc. For example, Si et al. 1 enhanced the crystallinity of PLLA by adding a small amount of PDLA, and regulated the morphology of polycaprolactone (PCL) in the PLLA / PCL blend by forming SCs. The results showed that adding PDLA could effectively improve the impact toughness and heat resistance. It should be noted that due to the inherent incompatibility between PCL and PLLA, mixing PCL with PLLA usually leads to macroscopic phase separation and heterogeneity, while adding PDLA can effectively adjust the particle size of the PCL dispersed phase. Yang et al. 2 proposed a new strategy to enhance the toughness of PLLA by constructing SCs at the interface of the PCL toughening phase. This unique design significantly improved the impact toughness of PLLA (reaching 49.5 kJ / m 2), which is nearly 13 times that of pure PLLA. Veluska Arias et al. 3 A homogeneous composite material was designed and prepared by melt-blending a PLLA matrix with spherical PLA SCs particles sized from 300 to 500 nanometers. An interfacial crystalline structure of SCs was formed at the interface between the particles and the PLLA matrix, thereby improving the tensile strength and Young's modulus of the composite material, mainly due to the interfacial adhesion. Liu et al. 4 A small amount of PDLA was introduced into a thermoplastic polyurethane (TPU)-toughened PLLA blend by melt-blending to solve the problems of the inherent brittleness and poor heat resistance of PLA. The relevant research results show that PDLA can interact with the PLLA matrix and rapidly co-crystallize to form SCs, which significantly improves the melt viscoelasticity of the PLLA matrix as an effective rheological modifier, thereby inducing morphological changes and transforming the morphology of the TPU phase from a typical sea-island structure to a network structure, significantly improving the impact toughness of the PLLA / TPU / PDLA blend.

[0005] In addition, many researchers have grafted D-lactic acid monomers onto the surfaces of magnesium hydroxide, zinc oxide, titanium dioxide, or hydroxyapatite and then blended them with PLLA to endow PLA with some special properties. However, existing research has mainly focused on the melt processing of PLA-based ternary systems derived from SCs. In contrast, there have been fewer studies on two-phase systems involving only PLLA and PDLA, and the preparation strategies adopted mainly rely on solution processing methods such as solution mixing, electrospinning, and oil-water emulsion mixing. However, these solution methods require the use of volatile organic solvents that are harmful to the environment, which raises concerns about sustainability. In addition, solution-processed blends usually need to be carried out at relatively high processing temperatures, which may cause the melting of stereocomplexes, leading to significant degradation of the homopolymer. These limitations highlight the need for alternative processing methods to avoid the drawbacks of traditional solution methods.

[0006] [1] Si, W.-J.; Zhang, H.; Li, Y.-D.; Huang, C.; Weng, Y.-X.; Zeng, J.-B. Highly toughened and heat resistant poly(l-lactide) / poly(ε-caprolactone) blends via engineering balance between kinetics and thermodynamics of phasic morphology with stereocomplex crystallite. Composites Part B: Engineering 2020, 197, 108155 - 108165.

[0007] [2] Yang, D.D.; Liu, W.; Zhu, H.M.; Wu, G.; Chen, S.C.; Wang, X.L.; Wang, Y.Z. Toward Super-Tough Poly(l-lactide) via Constructing Pseudo-Cross-link Network in Toughening Phase Anchored by Stereocomplex Crystallites at the Interface. ACS Appl Mater Interfaces 2018, 10(31), 26594 - 26603.

[0008] [3] Arias, V.; Odelius, K.; A.; Albertsson, A.-C. Homocomposites of Polylactide (PLA) with Induced Interfacial Stereocomplex Crystallites. ACS Sustainable Chemistry & Engineering 2015, 3(9), 2220 - 2231.

[0009] [4] Liu, Z.; Luo, Y.; Bai, H.; Zhang, Q.; Fu, Q. Remarkably Enhanced Impact Toughness and Heat Resistance of poly(l-Lactide) / Thermoplastic Polyurethane Blends by Constructing Stereocomplex Crystallites in the Matrix. ACS Sustainable Chemistry & Engineering 2016, 4(1), 111 - 120. Summary of the Invention

[0010] To solve the problems in the background art, the present invention provides a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend and a preparation method thereof. The method melts and mixes PLLA with low molecular weight PDLA to form an asymmetric PLLA / PDLA two-phase blend system. Under this system, the PLLA / PDLA blend exhibits a unique phase separation structure with a large number of PDLA submicron particles (stereocomplex mesophase SCs) uniformly distributed in the blend. Finally, a PLA-based micro-molded part with high strength and excellent toughness is prepared by micro-injection, and no external additives are required or the biodegradability is sacrificed. The present invention opens up a new direction for the development of high-performance PLA-based micro-molded parts, especially for applications in the field of biomedical engineering.

[0011] To achieve the above object, the present invention is realized by a technical solution composed of the following technical measures.

[0012] A preparation method of a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend mainly includes the following steps:

[0013] (1) By weight, prepare the raw materials of the following components:

[0014] 80 - 97 parts of PLLA,

[0015] 3 - 20 parts of PDLA,

[0016] wherein, the total of the PLLA and PDLA is 100 parts,

[0017] the weight-average molecular weight of the PLLA is 1.2×10 5 ~1.3×10 5 g / mol, and the weight-average molecular weight of the PDLA is 1.8×10 4 ~1.9×10 4 g / mol;

[0018] (2) Melt-blend the PLLA and PDLA prepared in step (1) to obtain a PLLA / PDLA mixture;

[0019] (3) Add the PLLA / PDLA mixture obtained in step (2) into a twin-screw extruder and extrude it under the process conditions of 165 - 190°C and 50 - 200 rpm. Use a tractor to stably draw the extruded filament, and cool and pelletize it to obtain PLLA / PDLA blend pellets;

[0020] (4) Inject the PLLA / PDLA blend pellets obtained in step (3) into a required product mold in a micro-injection molding machine, and prepare a high-performance PLA-based micro-molded part through micro-injection;

[0021] Among them, the process conditions for micro-injection molding are as follows: injection pressure is 1200 - 2000 Mpa, injection speed is 50 - 600 mm / s, mold temperature is 30 - 90 °C, melt temperature is 170 - 190 °C, and holding pressure and cooling time are 3 - 15 s.

[0022] In this article, PLLA mentioned in step (1) is the generally recognized English abbreviation for L - polylactic acid, and PDLA is the generally recognized English abbreviation for D - polylactic acid. In the present invention, the weight - average molecular weights of PLLA and PDLA are specified, and there is an obvious difference between the two weight - average molecular weights. This makes the PLLA / PDLA two - phase blend system in the present invention significantly different from the prior art in terms of mass ratio and weight - average molecular weight, forming an asymmetric PLLA / PDLA two - phase blend system.

[0023] It should be noted that after knowing the range of the weight - average molecular weights of PLLA and PDLA, those skilled in the art should know how to select PLLA and PDLA raw materials that meet this range.

[0024] It should be emphasized that in step (1), it is specified that the raw materials are only composed of PLLA and PDLA, without other additives (including other processing aids). On the premise of no other additives, the high - performance PLA - based micro - parts prepared in the present invention have more excellent various properties and are significantly better than similar products.

[0025] In this article, in step (2), the PLLA and PDLA are melt - blended to prepare a PLLA / PDLA mixture. Conventional melt - blending process technologies well - known in the art can be selected, which aims to make PDLA uniformly dispersed in PLLA. For example, screw melt - blending extrusion or internal mixer melt - blending and other conventional melt - blending processing equipment and processes are used.

[0026] In one technical solution, in step (2), the PLLA and PDLA are melt - blended to prepare a PLLA / PDLA mixture. Specifically, the prepared PLLA and PDLA are melt - blended and extruded through a single - screw extruder to prepare a PLLA / PDLA mixture;

[0027] Among them, the process parameters for melt - blending through the single - screw extruder are as follows: the screw rotation speed is 50 - 150 rpm, and the melting temperature is 170 - 190 °C.

[0028] It should be noted that in the melt - blending in step (2), usually because PLLA and PDLA are water - absorbent, water is likely to cause polymer depolymerization during the melt - blending process. Therefore, PLLA and PDLA need to be dried at about 80 °C for at least 8 h and then immediately melt - blended. Those skilled in the art can select appropriate drying temperatures and drying times according to the actual processing amount.

[0029] In this text, the PLLA / PDLA blend pellets obtained by cooling and pelletizing in step (3) should have appropriate pellet sizes and morphologies to facilitate subsequent micro-injection molding processing. Generally speaking, those skilled in the art can select appropriate pellet sizes and morphologies according to the relevant usage instructions of the micro-injection molding equipment used and common general knowledge. In one of the technical solutions, the particle diameter of the PLLA / PDLA blend pellets is preferably 0.5 - 2 mm.

[0030] It should be noted that the PLLA / PDLA mixture and the PLLA / PDLA blend pellets in step (3) usually have water absorbency and are prone to water absorption and degradation under non-continuous processing conditions. Therefore, under non-continuous processing conditions, it is advisable to dry the PLLA / PDLA mixture and the PLLA / PDLA blend pellets at about 80 °C for at least 8 h. Those skilled in the art can select appropriate drying temperatures and drying times according to the actual processing volume.

[0031] In this text, the high-performance PLA-based micro-components prepared by micro-injection molding in step (4) can be prepared into different components through different micro-injection molds, such as implantable medical products (micro bone screws, etc.).

[0032] The main inventive point of the present invention lies in that by innovatively melt-blending PLLA with a small amount of low-molecular-weight PDLA, a PLA-based material composed entirely of PLA with high strength and excellent toughness has been successfully developed, and at the same time, no external additives are required or the biodegradability is sacrificed. Through targeted research, surprisingly, the introduction of low-molecular-weight PDLA shows a certain degree of incompatibility, resulting in the formation of a sea-island structure, indicating phase separation in the extruded blend; as the micro-injection process progresses, these structures interact and fuse, and are replaced by a more compatible but still phase-separated morphology. Among them, the unique phase-separated structure exhibited by the PLLA / PDLA blend extruded by a twin-screw extruder has a large number of PDLA submicron particles (stereocomplex mesophases SCs) uniformly distributed in the blend. In addition, the stereocomplex crystalline SCs microdomains formed during the micro-injection molding process significantly improve the elongation at break and thermal stability of PLLA, without affecting its uniformity or rigidity. This significant improvement in mechanical properties is mainly attributed to the deformability and non-melting characteristics of the SCs microdomains. While helping to promote the crystallization and refinement of PLLA crystallization, these microdomains serve as stress concentration points and physical cross-linking sites, promoting the transfer of stress from the PLLA matrix through the interface, and gradually being stretched into a fibrous structure during the stress application process; in addition, these microdomains can effectively disperse and absorb energy during the stress application process, thus delaying the propagation of cracks.

[0033] It should be noted that in the application examples of the above technology, the biocompatibility and bone growth promotion characteristics of the micro-injection molded bone screw samples are also maintained. The technical solution of the present invention opens up a new direction for the development of high-performance PLA materials, especially for applications in the field of biomedical engineering.

[0034] Through comparative experiments, in one of the technical solutions, we determined a PDLA content of 10 wt% as the ideal optimal balance point, that is, 10 parts of PDLA in step (1), which not only achieved an ideal tensile strength but also took into account a good elongation at break.

[0035] The present invention has the following beneficial effects:

[0036] 1. The present invention provides a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend and a preparation method thereof. By melting and mixing PLLA with low molecular weight PDLA, an asymmetric PLLA / PDLA two-phase blend system is formed. Under this system, the PLLA / PDLA blend exhibits a unique phase separation structure, and finally, a PLA-based micro-injection molded part with high strength and excellent toughness is prepared without external additives or sacrificing biodegradability.

[0037] 2. The present invention opens up a new direction for the development of high-performance PLA-based micro-products, especially for applications in the field of biomedical engineering. For example, when preparing bone screws by micro-injection molding, this inherent structure in the technical solution of the present invention not only improves mechanical properties but also maintains good biocompatibility and stimulates bone growth. These excellent characteristics indicate that the technical solution of the present invention has great application potential in bone tissue repair and reconstruction.

[0038] 3. In one of the technical solutions, when the technical solution with a PDLA content of 10 wt% is adopted, the elongation at break of the finally prepared high-performance PLA-based micro-injection molded part increases significantly from 8.7% of pure PLLA to 87.2%, while the tensile strength remains almost unchanged.

[0039] 4. The present invention provides a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend and a preparation method thereof. Compared with the traditional SCs reinforcement strategy, the preparation method is simpler, easier to implement, and exhibits reliable performance. This discovery provides a promising approach for manufacturing fully biodegradable PLA-based materials or products with both strength and toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1This is a summary graph of the mechanical properties of the samples prepared in Examples 1-4 of the present invention and the comparative samples prepared in Comparative Examples 1-4. Among them, Figure a is the tensile test stress-strain curve of the samples prepared in Examples 1-4 and the comparative sample prepared in Comparative Example 1; Figure b is the digital image of the samples prepared in Examples 1-4 and the comparative sample prepared in Comparative Example 1 after tensile fracture; Figure c is the tensile test stress-strain curve of the comparative samples prepared in Comparative Examples 1-4; Figure d is the microscopic morphology (after tensile fracture) of the 10% PDLA sample prepared in Example 3 along the tensile direction; Figure e is the cross-sectional microscopic morphology of the 10% PDLA sample prepared in Example 3 after tensile fracture, and e1 and e2 are partial enlarged views; Figure f is the cross-sectional microscopic morphology of the comparative sample prepared in Comparative Example 3 after tensile fracture, and f1 is a partial enlarged view; Figure g is a comparison of the main mechanical properties of the 10% PDLA sample prepared in Example 3 with other samples reported in the literature.

[0041] Figure 2 This is a microscopic structure diagram, XRD curve diagram and principle schematic diagram of the intermediate PLLA / PDLA blend pellets and the prepared samples in Example 3 of the present invention. Among them, Figure a is the microscopic structure diagram of the intermediate PLLA / PDLA blend pellets in Example 3; Figure b is the XRD curve of the intermediate PLLA / PDLA blend pellets in Example 3; Figure c is the microscopic structure of the sample prepared in Example 3; Figure d is a schematic diagram of the formation of SCs intermediates during the melt mixing process; Figure e is a schematic diagram of the physical cross-linked structure formed in the SCs region; Figure f is the effect of the SCs physical cross-linked structure under tensile external force.

[0042] Figure 3 This is a summary graph of the influence results of the samples prepared in Examples 1-4 of the present invention and Comparative Example 1 on the crystal structure. Among them, Figure a is the first heating DSC curve of the samples prepared in Examples 1-4 and Comparative Example 1; Figure b is the statistical comparison bar graph of the crystallinity of the samples prepared in Examples 1-4 and Comparative Example 1; Figure c is the two-dimensional small-angle X-ray scattering (2D-SAXS) image of the samples prepared in Examples 2-4 and Comparative Example 1; Figure d is the one-dimensional small-angle X-ray scattering (1D-SAXS) intensity distribution graph (highlighting the peak position (q*) and long spacing (L)) of the samples prepared in Examples 2-4 and Comparative Example 1; Figure e is the two-dimensional wide-angle X-ray scattering (2D-WAXS) image of the samples prepared in Examples 2-4 and Comparative Example 1; Figure f is the XRD intensity distribution graph of the samples prepared in Examples 2-4 and Comparative Example 1 (the diffraction peaks and corresponding crystal planes are marked on the curve).

[0043] Figure 4This is a summary graph and schematic diagram of the crystallization behavior of the intermediate product PLLA / PDLA blend pellets in Examples 1 to 4 of the present invention and the intermediate product PLLA pellets in Comparative Example 1. Figure a is a comparison graph of the first heating curves of the intermediate product PLLA / PDLA blend pellets in Examples 1 to 4 of the present invention and the intermediate product PLLA pellets in Comparative Example 1; Figure b is a graph of the relative crystallinity during the isothermal crystallization process of the intermediate product PLLA / PDLA blend pellets in Examples 1 to 4 of the present invention and the intermediate product PLLA pellets in Comparative Example 1; Figure c is the Avrami fitting curve (isothermal crystallization at 140 °C) of the intermediate product PLLA / PDLA blend pellets in Examples 1 to 4 of the present invention and the intermediate product PLLA pellets in Comparative Example 1; Figure d is a POM micrograph taken during the isothermal crystallization process at 140 °C of the intermediate product PLLA / PDLA blend pellets in Examples 1 to 4 of the present invention and the intermediate product PLLA pellets in Comparative Example 1 (each micrograph is marked with the sample label, crystallization time, and temperature); Figures e and f are schematic diagrams of homogeneous nucleation and heterogeneous nucleation structures.

[0044] Figure 5 This is a summary graph of the changes in the internal crystal structure of the samples prepared in Examples 2 to 4 of the present invention after stretching. Among them, Figure a is the XRD curve of the stretched sample; Figures b and c are the DSC heating curve and the bar graph of the calculated crystallinity of the stretched sample; Figures d and e are the two-dimensional small-angle X-ray scattering (2D-SAXS) images and the one-dimensional small-angle X-ray scattering (1D-SAXS) curves of the stretched sample; Figures f and g are the schematic diagrams of the calculation process and results of the crystallization parameter L shish ; Figure h is a schematic diagram of the changes in the internal structure of the sample during the stretching process.

[0045] Figure 6 This is a summary graph of the degradation and biomedical properties of the samples prepared in Example 3 and Comparative Example 1 of the present invention. Figure a is the relationship between the residual mass and the degradation time of the samples prepared in Example 3 and Comparative Example 1; Figures b and c are the optical density at 450 nm and the relative ALP activity measured using a CCK-8 kit, respectively, and the results of culturing MC3T3-E1 cells on different micro-injected samples for 1, 3, 5, and 7 days; Figures d and e are the fluorescence image results of culturing MC3T3-E1 cells on the samples of Comparative Example 1 and Example 3 for 1, 4, and 7 days, respectively (live cells are green, dead cells are red; the scale bar is 100 μm). Detailed implementation manners

[0046] To further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention. Those skilled in the art can draw on the content of this article and appropriately improve the process parameters to achieve. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and those related can obviously make changes, or appropriate changes and combinations, to the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention. Although it is believed that those of ordinary skill in the art are fully aware of the following terms, the following definitions are still stated to help illustrate the subject matter disclosed by the present invention.

[0047] A preparation method for a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend, which mainly includes the following steps:

[0048] (1) Prepare the raw materials of the following components by weight:

[0049] 80-97 parts of PLLA,

[0050] 3-20 parts of PDLA,

[0051] wherein, the total of the PLLA and PDLA is 100 parts,

[0052] the weight average molecular weight of the PLLA is 1.2×10 5 ~1.3×10 5 g / mol, and the weight average molecular weight of the PDLA is 1.8×10 4 ~1.9×10 4 g / mol;

[0053] (2) Melt-blend the prepared PLLA and PDLA in step (1) to obtain a PLLA / PDLA blend;

[0054] (3) Add the PLLA / PDLA blend obtained in step (2) to a twin-screw extruder and extrude it under the process conditions of 165-190 °C and 50-200 rpm, and use a traction machine to stably traction the extruded filament, and then cool and pelletize to obtain PLLA / PDLA blend pellets;

[0055] (4) Inject the PLLA / PDLA blend pellets obtained in step (3) into the required product mold in a micro-injection molding machine, and prepare a high-performance PLA-based micro-molded part through micro-injection molding;

[0056] Among them, the process conditions for micro-injection molding are as follows: injection pressure is 1200 - 2000 Mpa, injection speed is 50 - 600 mm / s, mold temperature is 30 - 90 °C, melt temperature is 170 - 190 °C, and holding pressure and cooling time are 3 - 15 s.

[0057] In this article, PLLA mentioned in step (1) is the generally recognized English abbreviation for poly(L-lactic acid), and PDLA is the generally recognized English abbreviation for poly(D-lactic acid). In the present invention, the weight-average molecular weights of PLLA and PDLA are specified, and there is a significant difference between the two weight-average molecular weights, which makes the PLLA / PDLA two-phase blend system in the present invention significantly different from the prior art in terms of mass ratio and weight-average molecular weight, forming an asymmetric PLLA / PDLA two-phase blend system.

[0058] It should be noted that after those skilled in the art know the range of the weight-average molecular weights of PLLA and PDLA, they should know how to select PLLA and PDLA raw materials that meet this range.

[0059] It should be emphasized that in step (1), it is specified that the raw materials consist only of PLLA and PDLA, without other additives (including other processing aids). Under the premise of no other additives, the high-performance PLA-based micro-components prepared in the present invention have more excellent various properties and are significantly better than similar products.

[0060] In this article, the PLLA and PDLA mentioned in step (2) are melt-blended to prepare a PLLA / PDLA mixture. Conventional melt-blending process technologies well-known to those skilled in the art can be selected, which aims to make PDLA uniformly dispersed in PLLA, such as using a single-screw melt-blending extrusion or a kneader melt-blending and other conventional melt-blending processing equipment and processes.

[0061] In one implementation, the PLLA and PDLA mentioned in step (2) are melt-blended to prepare a PLLA / PDLA mixture, specifically by melt-blending and extruding the prepared PLLA and PDLA through a single-screw extruder to prepare a PLLA / PDLA mixture;

[0062] The process parameters for melt-blending through the single-screw extruder are as follows: the screw speed is 50 - 150 rpm, and the melting temperature is 170 - 190 °C.

[0063] It should be noted that for the melt-blending mentioned in step (2), usually because PLLA and PDLA are water-absorbent, moisture is likely to cause polymer depolymerization during the melt-blending process. In one implementation, PLLA and PDLA need to be dried at about 80 °C for at least 8 h and then immediately melt-blended. Those skilled in the art can select appropriate drying temperatures and drying times according to the actual processing amount.

[0064] In this article, the PLLA / PDLA blend pellets obtained by cooling and pelletizing in step (3) should have appropriate pellet sizes and morphologies to facilitate subsequent micro-injection molding. Generally speaking, those skilled in the art can select appropriate pellet sizes and morphologies according to the relevant usage instructions of the micro-injection molding equipment used and common general knowledge. In one embodiment, the particle size of the PLLA / PDLA blend pellets is preferably 0.5 to 2 mm.

[0065] It should be noted that the PLLA / PDLA mixture and the PLLA / PDLA blend pellets in step (3) usually have water absorbency and are prone to water absorption and degradation under non-continuous processing conditions. Therefore, under non-continuous processing conditions, in one embodiment, the PLLA / PDLA mixture and the PLLA / PDLA blend pellets are dried at about 80 °C for at least 8 h, and those skilled in the art can select appropriate drying temperatures and drying times according to the actual processing volume.

[0066] In this article, the high-performance PLA-based micro-components prepared by micro-injection molding in step (4) can be prepared into different components through different micro-injection molds. In one embodiment, for example, implantable medical products (such as micro bone screws, etc.).

[0067] The main inventive point of the present invention is that by innovatively melt-mixing PLLA with a small amount of low molecular weight PDLA, a PLA-based material composed entirely of PLA with high strength and excellent toughness has been successfully developed, and at the same time, no external additives are required or the biodegradability is sacrificed. Through targeted research, surprisingly, the introduction of low molecular weight PDLA shows a certain degree of incompatibility, resulting in the formation of a sea-island structure, indicating phase separation in the extruded blend; as the micro-injection molding process progresses, these structures interact and fuse with each other and are replaced by a more compatible but still phase-separated morphology. Among them, the unique phase separation structure exhibited by the PLLA / PDLA blend extruded by twin-screw extrusion has a large number of PDLA submicron particles (stereocomplex mesophases SCs) uniformly distributed in the blend. In addition, the stereocomplex crystalline SCs microdomains formed during the micro-injection molding process significantly improve the elongation at break and thermal stability of PLLA, without affecting its uniformity or rigidity. This significant improvement in mechanical properties is mainly attributed to the deformability and non-melting characteristics of the SCs microdomains. While helping to promote the crystallization and refinement of PLLA crystallization, these microdomains act as stress concentration points and physical cross-linking sites, promoting the transfer of stress from the PLLA matrix through the interface, and gradually being stretched into a fibrous structure during the stress application process; in addition, these microdomains can effectively disperse and absorb energy during the stress application process, thereby delaying the propagation of cracks.

[0068] It should be noted that in the application examples of the above technology, the biocompatibility and bone growth-promoting properties of the micro-injection molded bone screw samples are also maintained. The technical solution of the present invention opens up a new direction for the development of high-performance PLA materials, especially for applications in the field of biomedical engineering.

[0069] Through comparative experiments, in one of the embodiments, we determined a PDLA content of 10 wt% as the ideal optimal balance point, that is, 10 parts of PDLA in step (1), which achieved both ideal tensile strength and good elongation at break.

[0070] In one of the embodiments, the process conditions of the micro-injection in step (4) are as follows: injection pressure is 1200 - 2000 Mpa, such as 1200 Mpa, 1250 Mpa, 1300 Mpa, 1350 Mpa, 1400 Mpa, 1450 Mpa, 1500 Mpa, 1550 Mpa, 1600 Mpa, 1650 Mpa, 1700 Mpa, 1750 Mpa, 1800 Mpa, 1850 Mpa, 1900 Mpa, 1950 Mpa, 2000 Mpa, or any range or point value between them; injection speed is 50 - 600 mm / s, such as 50 mm / s, 60 mm / s, 70 mm / s, 80 mm / s, 90 mm / s, 100 mm / s, 110 mm / s, 120 mm / s, 130 mm / s, 140 mm / s, 150 mm / s, 160 mm / s, 170 mm / s, 180 mm / s, 190 mm / s, 200 mm / s, 210 mm / s, 220 mm / s, 230 mm / s, 240 mm / s, 250 mm / s, 260 mm / s, 270 mm / s, 280 mm / s, 290 mm / s, 300 mm / s, 310 mm / s, 320 mm / s, 330 mm / s, 340 mm / s, 350 mm / s, 360 mm / s, 370 mm / s, 380 mm / s, 390 mm / s, 400 mm / s, 410 mm / s, 420 mm / s, 430 mm / s, 440 mm / s, 450 mm / s, 460 mm / s, 470 mm / s, 480 mm / s, 490 mm / s, 500 mm / s, 510 mm / s, 520 mm / s, 530 mm / s, 540 mm / s, 550 mm / s, 560 mm / s, 570 mm / s, 580 mm / s, 590 mm / s, 600 mm / s, or any range or point value between them; mold temperature is 30 - 90 °C, such as 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, or any range or point value between them; melt temperature is 170 - 190 °C, such as 170 °C, 175 °C, 180 °C, 185 °C, 190 °C, or any range or point value between them; holding pressure and cooling time are 3 - 15 s, such as 3 s, 4 s, 5 s, 6 s, 7 s, 8 s, 9 s, 10 s, 11 s, 12 s, 13 s, 14 s, 15 s, and the holding pressure time and the cooling time may be the same or different.

[0071] The present application will be further explained in detail below with reference to the embodiments. However, those skilled in the art should understand that these embodiments are provided for illustrative purposes only and are not intended to limit the present application.

[0072] Embodiment

[0073] The implementation scheme of the present application will be described in detail below in combination with embodiments. However, those skilled in the art will understand that the following embodiments are only used to illustrate the present application and should not be construed as limiting the scope of the present application. For those conditions not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For those reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase. The present application should not be construed as being limited to the specific embodiments described.

[0074] 1. Raw materials

[0075] L-lactic acid polymer (PLLA, trade name 6201D) was purchased from Nature Works, and its weight-average molecular weight was 1.23×10 5 g / mol.

[0076] D-lactic acid polymer (PDLA) was provided by Zhejiang Haishun Biomaterials Co., Ltd., and its weight-average molecular weight was 1.87×10 4 g / mol.

[0077] 2. Preparation method

[0078] (1) By weight, prepare the raw materials of the following components:

[0079] 80-97 parts of PLLA,

[0080] 3-20 parts of PDLA,

[0081] wherein, the total of the PLLA and PDLA is 100 parts;

[0082] (2) Melt-blend and extrude the prepared PLLA and PDLA in step (1) through a single-screw extruder to prepare a PLLA / PDLA mixture;

[0083] The process parameters for melt-blending through the single-screw extruder are as follows: the screw speed is 60 rpm, and the melting temperature is 185°C;

[0084] (3) Add the PLLA / PDLA mixture obtained in step (2) into a twin-screw extruder, and extrude it under the process conditions that the temperatures of each zone are set to 170°C, 190°C, 190°C, and 185°C respectively, and the screw speed is 100 rpm. Then, use a tractor to stably draw and extrude the filament, and obtain the PLLA / PDLA blend pellets after cooling and pelletizing;

[0085] (4) Inject the PLLA / PDLA blend pellets obtained in step (3) into the required product mold in a micro-injection molding machine, and prepare high-performance PLA-based micro-components through micro-injection molding. For the convenience of testing, dumbbell-shaped micro-components and micro-bone screws are prepared respectively.

[0086] Among them, the process conditions for micro-injection molding are as follows: injection pressure is 1600 MPa, injection speed is 100 mm / s, mold temperature is 30 °C, melt temperature is 185 °C, holding pressure time is 5 s, and cooling time is 15 s.

[0087] Example 1

[0088] Example 1 is prepared according to the steps of the above-mentioned "2. Preparation method". In step (1), 3 parts of PDLA are used, and finally a high-performance PLA-based micro-component is prepared as a sample, denoted as 3% PDLA.

[0089] Example 2

[0090] Example 2 is prepared according to the steps of the above-mentioned "2. Preparation method". In step (1), 5 parts of PDLA are used, and finally a high-performance PLA-based micro-component is prepared as a sample, denoted as 5% PDLA.

[0091] Example 3

[0092] Example 3 is prepared according to the steps of the above-mentioned "2. Preparation method". In step (1), 10 parts of PDLA are used, and finally a high-performance PLA-based micro-component is prepared as a sample, denoted as 10% PDLA.

[0093] Example 4

[0094] Example 4 is prepared according to the steps of the above-mentioned "2. Preparation method". In step (1), 20 parts of PDLA are used, and finally a high-performance PLA-based micro-component is prepared as a sample, denoted as 20% PDLA.

[0095] Comparative Example 1

[0096] Comparative Example 1 is prepared by referring to the steps of the above-mentioned "2. Preparation method", but PDLA is not added in step (1), that is, 100 parts of PLLA. The intermediate product PLLA pellets are prepared in step (3), and finally a PLA-based micro-component is prepared as a comparative sample.

[0097] Comparative Example 2

[0098] This comparative example is to prepare a PLA-based micro-component by using the traditional stereocomplex SCs reinforcement strategy, which mainly includes the following steps:

[0099] (1) Weigh and prepare the raw materials of the following components by weight:

[0100] 50 parts of PLLA,

[0101] 50 parts of PDLA,

[0102] (2) The prepared PLLA and PDLA in step (1) are melt-blended by a Banbury mixer to prepare PLLA / PDLA stereocomplex SCs;

[0103] Among them, the process parameters for melt-blending by the Banbury mixer are: the screw speed is 60 rpm, and the melting temperature is 185 °C;

[0104] (3) By weight, 5 parts of the PLLA / PDLA stereocomplex SCs obtained in step (2) are mixed with 95 parts of PLLA and then added to a twin-screw extruder. The temperatures of each zone are set at 170 °C, 190 °C, 190 °C, and 185 °C respectively, and extruded under the process conditions of a screw speed of 100 rpm. The extruded filament is stably pulled by a tractor and cooled and pelletized to obtain PLLA / SCs composite pellets;

[0105] (4) The PLLA / SCs composite pellets obtained in step (3) are injected into the required product mold by a micro-injection molding machine, and PLA-based micro-products are prepared by micro-injection molding as comparative samples. For the convenience of testing, they are dumbbell-shaped micro-products and micro-bone screws respectively, denoted as 5% SCs;

[0106] Among them, the process conditions for micro-injection molding are: the injection pressure is 1600 MPa, the injection speed is 100 mm / s, the mold temperature is 30 °C, the melt temperature is 185 °C, the holding pressure time is 5 s, and the cooling time is 15 s.

[0107] Comparative Example 3

[0108] Comparative Example 3 is based on the preparation method of Comparative Example 2, but in step (3), 10 parts of the PLLA / PDLA stereocomplex SCs obtained in step (2) are mixed with 90 parts of PLLA and then added to a twin-screw extruder, and finally PLA-based micro-products are prepared as comparative samples, denoted as 10% SCs.

[0109] Comparative Example 4

[0110] Comparative Example 4 is based on the preparation method of Comparative Example 2, but in step (3), 20 parts of the PLLA / PDLA stereocomplex SCs obtained in step (2) are mixed with 80 parts of PLLA and then added to a twin-screw extruder, and finally PLA-based micro-products are prepared as comparative samples, denoted as 20% SCs.

[0111] 3. Testing method

[0112] The morphology, crystalline structure, and tensile fracture surface of the dispersed phase were carefully observed using a field emission scanning electron microscope (SEM, Inspect F, FEI, Finland) at an accelerating voltage of 5 kV. To observe the microscopic structure of the crystal, the samples were first cryo-fractured in liquid nitrogen and then etched in a water-methanol (1:2, v / v) solution and 0.025 mol / L sodium hydroxide at 30 °C for 12 h to selectively remove the amorphous PLLA. All samples were coated with a layer of metallic gold using sputter coating before SEM observation. The crystalline structure of the samples was analyzed using an X-Pert Pro X-ray diffractometer (Philips, Japan) at room temperature with a diffraction angle range of 10° to 60° and a scanning speed of 5° / min. Two-dimensional small-angle X-ray scattering (2D-SAXS) and two-dimensional wide-angle X-ray scattering (2D-WAXS) tests were performed using a Xeuss 2.0 X-ray photoelectron spectrometer with the sample-to-detector distances of 2500 mm and 170 mm, respectively. The long period (L) of the lamellar structure was calculated using the Bragg equation (L = 2π / q*). The melting and crystallization behavior of the samples was measured using a differential scanning calorimeter (DSC, Q200 TA Instruments, USA) in a nitrogen atmosphere at a heating rate of 10 °C / min. The isothermal crystallization kinetics study was carried out at 140 °C. Before the study, the samples were first melted at 190 °C for 5 min to eliminate the thermal history, then cooled to 140 °C at a rate of 50 °C / min, and then held at this temperature until crystallization was complete. The crystal growth during the isothermal crystallization process was observed using a Leica DM2500P polarized light microscope (POM) equipped with a pixel camera (Leica, USA). The samples were first melted at 190 °C for 5 min to remove the thermal history and then cooled to 140 °C at a rate of 50 °C / min for isothermal crystallization. The rheological behavior was analyzed using a dynamic rheometer (AR2000ex, TA Instruments, USA) with a parallel plate geometry of 25 mm in diameter and a gap of 1 mm. The oscillatory frequency sweep was performed at 185 °C with a frequency range of 0.01 - 100 Hz and a strain of 1%. All samples were compressed into disk-shaped samples with a diameter of 25 mm and a thickness of 2.2 mm at 190 °C for molding. Dynamic mechanical analysis (DMA) was performed using a Q800 instrument (TA, USA) in a multi-strain mode at a frequency of 1 Hz and an amplitude of 10 μm. The test temperature range was from 0 to 130 °C with a heating rate of 3 °C / min. The mechanical properties were tested using an Instron 5569 at 25 °C with a crosshead speed of 2 mm / min. At least five independent samples were measured for each sample under the same conditions.

[0113] 4. Test Results

[0114] Strength and toughness in mechanical properties are key factors for the commercialization of PLA, but how to balance the two has always been a difficult point in the existing technology.Figure 1 a shows the typical stress-strain curves of various samples prepared using the technical solution of the present invention. It can be clearly seen that the comparative sample prepared in Comparative Example 1 has inherent rigidity and brittleness, manifested as a tensile strength of 67.2 MPa and a minimum elongation at break of 8.7%, which are the lowest among all samples. The most significant feature of the high-performance PLA-based micro parts samples provided by the present invention is that, compared with the comparative sample, their ductility (elongation at break) is significantly improved, while the rigidity (tensile strength) is not affected. This improvement may be attributed to the good interfacial interaction between the PDLA and PLLA matrices through hydrogen bonding. The high-performance PLA-based micro parts provided by the present invention exhibit obvious yield and stable neck growth under unidirectional stress Figure 1 b), making their ductility significantly higher than that of the comparative sample. With the increase of the PDLA content, the elongation at break further increases, and the rigidity only slightly decreases. Among them, the 10% PDLA sample prepared in Example 3 achieved the highest elongation at break. Specifically, the elongation at break of the 5%, 10% and 20% PDLA samples were 71%, 87.2% and 63.5% respectively, which were about 8.2 times, 10 times and 7.3 times that of the pure PLLA comparative sample obtained in Comparative Example 1. The stress-strain curves of the comparative samples prepared in Comparative Examples 2-4 are as Figure 1 shown in c. Although the tensile strength increased slightly, the elongation at break was only about 4%, all lower than that of the pure PLLA comparative sample obtained in Comparative Example 1. Therefore, the 10% PDLA sample prepared in Example 3 is considered to be the best formulation for achieving the required mechanical property balance in this study.

[0115] The morphology of the samples after tensile fracture was examined using a scanning electron microscope (SEM). Figure 1 d and 1e show the tensile fracture surfaces of the 10% PDLA sample parallel and perpendicular to the flow direction, and this sample exhibited the highest fracture toughness. Figure 1 In d, a large number of highly oriented microfibers and nanopores caused by interfacial separation can be seen, and the microfibers are uniformly arranged along the tensile direction. In particular, in the samples obtained in Examples 1-4 of the invention, although the nanofibers were broken, they were still combined with the PLLA matrix, indicating that the hydrogen bonding between PDLA and PLLA led to good interaction forces and stabilized the embedded fibers. In the cross-section perpendicular to the tensile direction Figure 1 e), the matrix thickness decreased to about two-thirds of the original thickness after tensile fracture, and a layered structure was clearly observed. The further magnified image Figure 1e1 and e2) show a rough surface, indicating that ductile fracture has occurred. These observations suggest that the addition of PDLA induces a brittle-ductile transition in PLLA, improving its toughness and deformation ability. In the cross-section of the 10% SCs control sample ( Figure 1 f), only isolated larger SCs particles are observed to be distributed within the matrix. The magnified view ( Figure 1 f1) clearly reveals the interfacial defects between the SCs particles and the PLLA matrix, because the SCs particles fail to fuse and bond with the PLLA matrix due to their higher melting point. If the processing temperature exceeds the melting point of the SCs, it may lead to significant degradation of the PLLA matrix, thus affecting its mechanical properties. Therefore, these melt-compounded SCs particles introduce internal defects within the PLLA matrix, ultimately reducing the mechanical properties of the composite. The above results highlight the superiority of the strategy adopted in this study, that is, generating a stereocomplex mesophase by directly mixing PLLA and PDLA, thereby improving the compatibility and mechanical properties. Figure 1 g compares the best mechanical properties of the samples obtained in Example 3 with other recently reported PLA-based systems. The results show that the high-performance PLA-based micro-parts prepared by the technical solution of the present invention achieve an optimal balance between strength and toughness, and have significant practical application potential.

[0116] To explore the fundamental reason for the significant improvement in the mechanical properties of the high-performance PLA-based micro-parts prepared by the technical solution of the present invention, we conducted a series of characterizations. First, we studied the changes in the internal phase structure of the material during the extrusion and micro-injection molding processes. Figure 2 a shows the cross-sectional structure of the intermediate product PLLA / PDLA blend pellets in Example 3. The fracture surface presents a unique structure, with sub-micron spherical particles evenly distributed. As the PDLA content increases, the size and number of the particles increase proportionally. This sub-microparticle distribution indicates that the dispersion of PDLA in the PLLA matrix is improved, thereby enhancing the mechanical properties. Although the two components of this system have the same molecular composition, they do not form a completely homogeneous blend, but rather form a unique dispersed structure. Figure 2 The schematic diagram in d can explain the formation process of this sea-island structure. Under the shear action of a twin-screw extruder, low-molecular-weight PDLA transforms into nano-spheres. During this process, the PDLA molecular chains on the surface of these nano-spheres interact with the PLLA molecules in the matrix through hydrogen bonds to form stereocomplex intermediates. These stereocomplex intermediates protect the embedded PDLA spheres at the processing temperature, thus forming a unique sea-island structure. Figure 2 The XRD analysis in b shows that as the PDLA content increases, the signal peaks related to stereocomplex gradually appear, indicating the formation of stereocomplexes (SCs) in the molten state of the PLLA / PDLA system.

[0117] Figure 2 c shows the internal structural characteristics of the samples prepared in Example 3. The 10% PDLA sample has a large number of SCs microdomains (with a diameter less than 2 microns), and these microdomains are well-bonded to the matrix interface, and there is no obvious gap between the interface phases. The presence of the SCs mesophase results in the formation of a physical cross-linked structure by connecting the surrounding PLLA molecular chains, as shown in Figure 2 e. This physical cross-linked structure significantly enhances the mechanical properties of the PLLA matrix under external forces, as shown in Figure 2 f. Based on the technical solution of the present invention, the SCs intermediate generated during the melting process and maintained during the injection molding process serves as a stress concentration point, promoting the transfer of stress from the matrix to the interface and absorbing stress. In addition, with the increase of strain during the stretching process, significant plastic deformation occurs in the matrix resin. The strong interfacial bonding force established by hydrogen bonds between the SCs microdomains and the matrix ensures that the SCs are stretched and oriented during the stretching process without detaching from the PLLA matrix, thereby improving the fracture resistance of the material.

[0118] In the above research, it was observed that the samples of the examples had significantly improved toughness and distinctive internal structures. Considering the key role of crystallization in material properties, we conducted a detailed study on the crystallization behavior of polymers. In order to investigate the effects of the micro-injection molding process and material formulation on the crystallization behavior of the blend, differential scanning calorimetry (DSC) was used to characterize the crystallization structure of the high-performance PLA-based micro-molded part samples prepared according to the technical solution of the present invention. As shown in Figure 3 a, the DSC heating curve reveals four different physical transitions: glass transition, cold crystallization, melting peak of homogeneous crystallizates (HCs), and melting peak of stereocomplex crystallizates (SCs). Regardless of the content of PDLA, all samples showed a cold crystallization temperature of about 95 °C and a melting point of HCs of 170 °C. In particular, a melting peak appeared near 225 °C in the samples containing PDLA, indicating the presence of SCs. In addition, with the increase of the PDLA content, the proportion of SCs in the blend increased, manifested as a decrease in the area of the HCs melting peak and an increase in the area of the SCs melting peak. Interestingly, in the samples with a low PDLA content, a small endothermic peak appeared at 130 °C, indicating that a structural change occurred in this temperature range. The crystallinity calculated from the DSC heating curve is shown in Figure 3 b. The overall crystallinity increased with the addition of PDLA, but the contents of HCs and SCs showed opposite trends. Specifically, when the PDLA content increased to 20 wt%, the crystallinity (X C ) increased from 28.2% to 35.8%, while the content of HCs decreased from 28.2% to 8%, and the content of SCs increased from 0 to 27.7%.

[0119] The results of small-angle X-ray scattering (SAXS) analysis can further illustrate the orientation behavior of PLLA crystallization. Figure 3 The 2D-SAXS pattern in c shows that the 20% PDLA sample has obvious arc scattering signals along the meridian direction, indicating the existence of an epitaxial lamellar (kebab) structure. The 1D-SAXS curve further confirms the above results. The 20% PDLA sample shows a shoulder peak, which is characteristic of the lamellar crystal structure. It is worth noting that even 5% PDLA and 10% PDLA also show shoulder peaks, indicating the existence of kebab lamellar structures in these samples. The long period (L) value calculated using the Bragg equation (L = 2π / q*) increases with the increase in PDLA content ( Figure 3 d). The 2D-WAXD diffraction signal image ( Figure 3 e) shows that with the increase in PDLA content, the internal orientation of the sample weakens, which can be attributed to the role of SCs as physical cross-linking points. Correspondingly, the X-ray diffraction (XRD) intensity distribution results ( Figure 3 f) detail the diffraction peaks of the example samples and their corresponding assignments. A weak diffraction peak is observed at about 17°, which is attributed to the HCs (200) / (110) crystal planes. As the PDLA content gradually increases to 20%, the signal peaks related to SCs (12° (110), 21° (300 / 030), 24° (220)) become more obvious, while the peaks of HCs slightly weaken. On the contrary, with the increase in PDLA content, the characteristic peaks corresponding to specific SCs crystal planes become more obvious, indicating that the formation of SCs leads to a reduction in HCs.

[0120] Considering that the SCs mesophase is not completely melted at the processing temperature, it may significantly affect the crystallization process of the PLLA matrix. This effect may be similar to the role of added inorganic nanofillers because the size of the SCs mesophase is about in the order of hundreds of nanometers (see Figure 2 a). To verify this hypothesis, DSC was used to characterize the non-isothermal crystallization behavior of the blends. Figure 4 a shows the DSC curves of the intermediate products PLLA / PDLA blends of Examples 1 to 4 and the intermediate product PLLA granule of Comparative Example 1 during heating. For the PLLA granule, a significant cold crystallization peak appears at about 120 °C, and an inflection point representing the glass transition is also observed at about 65 °C. In addition, a melting peak of SCs is observed above 220 °C, and the melting point and melting enthalpy of SCs increase with the increase in PDLA content. After the DSC curves of different samples are integrated, the results are as shown in Figure 4 b, which shows the relative crystallinity (X C(t))Variation with time. The crystallization process of pure PLLA is relatively long, but the addition of only 3 wt% of PDLA significantly accelerates the crystallization rate. With further increase in the PDLA content, the crystallization rate is further enhanced, indicating that the SCs have a significant accelerating effect on the crystallization kinetics of PLLA. The relative crystallinity curve can be used to calculate the half-crystallization time (t 1 / 2 )(i.e., the time when X C(t) is 50%). It is worth noting that after adding 20 wt% PDLA, the t 1 / 2 of the sample is about 0.7 minutes, much lower than 6 minutes of pure PLLA. The crystallization kinetics of the blends was studied using the Avrami equation, and the fitting results are shown as Figure 4 c. The results show that the fitting results of the Avrami equation are linear, and the calculated t 1 / 2 is in good agreement with the experimental data, indicating good fitting. The parameter n is related to the nucleation process, and a decrease in the value of n indicates nucleation limitation during the crystallization process. The parameter k is related to the crystal growth rate, and a larger k value represents a faster growth rate. These crystallization kinetics results strongly support the promoting effect of SCs on the crystallization of PLLA.

[0121] In addition, the spherulite growth morphology during the isothermal crystallization process at 140 °C was in-situ observed using a polarized light microscope (POM) with a hot stage. Figure 4 d is a typical spherulite photograph taken by POM, reflecting the variation of the growth rate and spherulite density with time. Before isothermal crystallization, the polarized light photograph of the melt at 190 °C revealed unmelted blue crystals in the sample, and the number of them was proportional to the PDLA content. These unmelted crystals were identified as SCs, whose melting point was 50 °C higher than that of HCs and were not completely melted at the processing temperature. When rapidly cooled to the crystallization temperature of 140 °C, the presence of SCs enabled the sample to complete crystallization within 2 minutes, while no crystallization occurred in the control sample. In addition, after 2 minutes of isothermal crystallization, the spherulites in the sample were significantly smaller and denser than those in the control sample. These results clearly show that the in-situ formed SCs, as effective nucleating agents, significantly promoted the crystallization of the PLLA matrix, reduced the size of spherulites, and were beneficial to the improvement of the mechanical properties of the material. Figure 4 e and 4f show that SCs play a key role in the crystallization process. In the molten state, crystal nuclei form and grow during the cooling process in the pure PLLA system, but the number of crystal nuclei is small and the crystal grains are large. In contrast, in the PLLA / PDLA system, the SCs mesophase does not melt at this temperature. These sub-micron mesophases can act as heterogeneous nucleating agents during the crystallization process, thus accelerating the crystallization rate and simultaneously reducing the crystal grain size.

[0122] The above research results indicate that the presence of SCs significantly improves the crystallization ability of the PLLA matrix and plays a key role in grain refinement. These changes lead to an improvement in the mechanical properties of the material, especially enhancing the toughness of the material during the tensile deformation process. Conversely, the tensile process itself also affects the crystalline structure and orientation within the matrix. Therefore, understanding this effect is crucial for comprehensively understanding the crystallization behavior of the PLLA / PDLA blend system under external forces. Thus, we further studied the crystallization behavior after the tensile test. First, XRD and DSC tests were carried out, and the results are as Figure 5 a and Figure 5 b shown. Similar to the samples before tensile, crystallization peaks corresponding to HCs and SCs also appeared in the samples after tensile, indicating that the SCs microdomains were not destroyed during the tensile process. In addition, the change in the crystallinity of the samples after tensile is as Figure 5 c shown, and the results show that the relative contents of HCs and SCs after tensile are higher than those before tensile ( Figure 3 b). This increase is attributed to the fact that the tensile process promotes the orientation and crystallization of polymer molecular chains. These crystalline microdomains, as rigid phases, can effectively resist external stresses, thereby improving the tensile strength of the material.

[0123] Furthermore, 2D-SAXS was used to study the evolution of the crystalline structure of the PLLA / PDLA blend after tensile. The obtained 2D-SAXS patterns are as Figure 5 d shown. The 5% PDLA sample only showed symmetric rhombic scattering signals in the equatorial direction, which is a common feature in polymer fibers, indicating that PLLA fiber crystals (shish) were formed along the tensile direction in the matrix. In contrast, the 10% PDLA and 20% PDLA samples showed symmetric rhombic scattering signals in the equatorial direction and a water-droplet-like pattern in the meridian direction, representing a typical shish-kebab structure. The kebab lamellar structure was not found in the 5% PDLA sample, indicating that in blends with higher PDLA contents, the formation of the shish-kebab structure may be induced by the tensile deformation of the SCs microdomains. This structural transformation process can be explained by Figure 5 h shown. Before tensile, the crystalline structures in the material were randomly arranged, and the HCs and SCs microdomains were connected by PLLA molecular chains. As the tensile progresses, the HCs and SCs microdomains gradually deform and orient. The cross-linking points in the middle of the SCs can effectively prevent the chain segments from slipping and prevent the material from deforming and breaking through strong interfacial interactions, thus helping to absorb energy during the tensile process and diffuse it into the entire material matrix, thereby reducing stress concentration and improving the toughness of the material.

[0124] Furthermore, 1D-SAXS scattering curves ( Figure 5e) Characterize the structural information related to shish-kebab crystallization. It can be seen that shoulders attributed to kebabs can be observed in the curves of 10% and 20% PDLA samples, and the positions of the peaks are similar. To further study the shish-kebab microstructure, the Bragg equation (L = 2π / q*) was used to calculate the long period (L) related to the crystalline structure. The results show that the 10% PDLA and 20% PDLA samples exhibit the same long period, approximately 36.9 nm, which is much larger than the value before stretching. Figure 5 f gives the method for calculating the average fiber crystal length (L shish ). The specific calculation results are as Figure 5 shown in g. It can be clearly seen that after stretching, the samples with higher PDLA content show shorter fiber lengths. This is because there are strong intermolecular hydrogen bond interactions and entanglements between PLLA and PDLA molecular chains, which limit the segmental slippage under external forces. The above results further illustrate that the formed shish-kebab structure has the ability to effectively resist external stress and enhance the mechanical properties of the material.

[0125] Based on the successful preparation of high-performance PLA-based micro-components based on stereocomplex crystallization (with high ductility and good tensile strength), micro-injection molding technology was further used to prepare micro bone screws for bone repair. The micro bone screws prepared by micro-injection molding have a smooth surface and very good replication performance, indicating that the PLLA / PDLA blend system involved also has excellent micro-injection processing performance. In addition, the micro bone screw samples with 10% PDLA can withstand a load of up to 59 N, and the bending strength reaches 220 MPa. This value exceeds the strength of commercially available PLLA bone screws, indicating its excellent application potential in the actual medical field. On this basis, the corresponding biological performance evaluation was carried out. Figure 6 a shows the time-mass loss curves of PLLA and 10% PDLA micro bone screws. The results show that the mass loss rate of the PLLA sample is higher than that of the 10% PDLA micro bone screw sample. This difference can be attributed to the higher crystallinity of the 10% PDLA sample, and its crystalline region is more resistant to hydrolysis than the amorphous region. Generally speaking, the hydrolysis process of PLLA first starts in the amorphous region because the penetration of water molecules promotes the cleavage of ester bonds, while the degradation of the crystalline region usually occurs after obvious degradation in the amorphous region.

[0126] The CCK-8 kit was used to evaluate the cytotoxicity experiment, and the results are as Figure 6As shown in Figure b, the proliferation of MC3T3-E1 cells cultured on PLLA and 10% PDLA samples for 1, 3, 5, and 7 days is shown. It can be seen that over time, the optical density (OD) values of both samples show an upward trend, indicating enhanced cell activity. Since there are no significant differences in the initial cell behaviors (such as attachment and migration) on the two materials, the OD values of the two samples are similar at 1 day. However, at 3 days, 5 days, and 7 days, the OD values of the 10% PDLA sample are higher, indicating that the 10% PDLA sample has a stronger cell proliferation ability compared to the PLLA sample.

[0127] In addition, Figure 6 Figure c shows the results of alkaline phosphatase (ALP) expression of mouse osteoblasts cultured on the sample surface for 1, 3, 5, and 7 days. At 1 day and 3 days, there are no significant differences in ALP activity, indicating that at the proliferation stage, there is less osteoblast differentiation. On the contrary, the ALP activity increases significantly at 5 days and 7 days, indicating enhanced osteogenic differentiation, and the 10% PDLA sample shows higher ALP activity than the PLLA sample after culturing for a longer time, highlighting the stronger osteogenic potential of the former.

[0128] The biomedical properties of the samples were further evaluated using a live / dead cell staining experiment, mainly comparing the cell activities at 1, 4, and 7 days. Figure 6 Figures d and Figure 6 e show the photos of green fluorescence microscopy, based on which the cell activity can be judged. For example, the absence of red spots indicates no induced toxicity or apoptosis. The observation results show that at 4 days, the number of cells on the surfaces of the PLLA and 10% PDLA samples increases significantly, and further increases at 7 days. In summary, the results of CCK-8 and ALP assays combined with the live / dead cell staining clearly show that the 10% PDLA sample has better biocompatibility and osteogenic ability compared to the PLLA sample, confirming its potential application in the field of bone repair, especially its ability to better promote bone growth.

[0129] In summary, the present invention provides a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend and its preparation method, which exhibits excellent mechanical properties such as toughness and strength. The good biological properties are further confirmed by in vitro cell experiments. The results show that the high-performance PLA-based micro parts prepared by the technical solution of the present invention have good degradation properties and biocompatibility equivalent to PLLA, and are ideal candidate materials for application in the field of biomedical engineering, with good application prospects.

[0130] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent substitution methods and are all included in the protection scope of the present invention.

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Claims

1. A preparation method of a micro-injection molded part of an asymmetric PLLA / PDLA two-phase blend, characterized in that It mainly includes the following steps: (1) Prepare raw materials of the following components by weight parts: 80 - 97 parts of PLLA, 3 - 20 parts of PDLA, wherein, the total of the PLLA and PDLA is 100 parts, The weight-average molecular weight of the PLLA is 1.2×10 5 ~1.3×10 5 g / mol, and the weight-average molecular weight of the PDLA is 1.8×10 4 ~1.9×10 4 g / mol; (2) Melt - blend the prepared PLLA and PDLA in step (1) to obtain a PLLA / PDLA mixture; (3) Add the PLLA / PDLA mixture obtained in step (2) into a twin - screw extruder and extrude it under the process conditions of 165 - 190 °C and 50 - 200 rpm, and use a tractor to stably draw the extruded filament, and then obtain PLLA / PDLA blend pellets after cooling and pelletizing; (4) Inject the PLLA / PDLA blend pellets obtained in step (3) into a required product mold by a micro - injection molding machine, and obtain high - performance PLA - based micro - parts through micro - injection molding; wherein, the process conditions of micro - injection molding are: injection pressure 1200 - 2000 Mpa, injection speed 50 - 600 mm / s, mold temperature 30 - 90 °C, melt temperature 170 - 190 °C, holding pressure and cooling time 3 - 15 s.

2. The preparation method according to claim 1, wherein: In step (2), the PLLA and PDLA are melt - blended to obtain a PLLA / PDLA mixture, and screw melt - blending extrusion or internal mixer melt - blending is adopted.

3. The preparation method according to claim 1, wherein: In step (2), the PLLA and PDLA are melt - blended to obtain a PLLA / PDLA mixture, specifically, the prepared PLLA and PDLA are melt - blended and extruded through a single - screw extruder to obtain a PLLA / PDLA mixture; wherein the process parameters of melt - blending through the single - screw extruder are: screw speed is 50 - 150 rpm, and melt temperature is 170 - 190 °C.

4. The preparation method according to claim 1, characterized in that: In step (1), PDLA is 10 parts.

5. A high - performance PLA - based micro - part prepared by the preparation method of a micro - injection molded part of an asymmetric PLLA / PDLA two - phase blend as described in claim 1.

6. Application of the high - performance PLA - based micro - part as described in claim 5 in medical products.

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