JP7831815B2 - Organic-inorganic composites, methods for producing the same, and molded organic-inorganic composite products - Google Patents

Description

This invention relates to organic-inorganic composites, methods for producing the same, and molded organic-inorganic composite articles, and more particularly to organic-inorganic composites, methods for producing the same, and molded organic-inorganic composite articles that can maintain a considerable degree of their original mechanical strength even in environments with water or high humidity.

Bone is a composite material of organic components (collagen, other proteins, cells, etc.) and inorganic components (hydroxyapatite), possessing high toughness due to its combination of hardness and flexibility.
Inspired by the composition of bone and its resulting excellent mechanical properties, various organic-inorganic composite materials have been synthesized, and their potential applications as scaffolding materials for bone tissue and as artificial bones are being explored.

As one such prior art, the applicant has proposed an organic-inorganic composite material that is excellent in biocompatibility, mechanical strength, and flexibility, comprising a partially phosphorylated hydroxyl group-containing polymer having a structure in which the hydroxyl groups in the polymer having hydroxyl groups are partially phosphorylated to a degree of phosphorylation of 1-20%, and a hydroxyapatite composite material (see Patent Document 1).
The applicant has also proposed a novel organic-inorganic composite material having a composition similar to biological bone and excellent mechanical properties, comprising an organic material containing at least anion-modified cellulose nanofibers and an inorganic material containing at least a calcium phosphate compound (see Patent Document 2).

Japanese Patent Publication No. 2017-131300 Japanese Patent Publication No. 2021-31532

As described above, various studies have been conducted to provide organic-inorganic composites that can be applied as scaffolding materials for bone tissue or as artificial bone, and improvements have been made in physical properties such as biocompatibility, mechanical strength, and flexibility.
The inventors of the present invention investigated the potential of the above-mentioned organic-inorganic composite as an alternative plastic material, particularly as a biomass-derived, petroleum-free, green, lightweight eco-material, with the aim of expanding its applications beyond bone tissue scaffolding materials and artificial bones to a wide range of uses.
In that study, it was found that while conventional molded bodies using organic-inorganic composites exhibit excellent mechanical properties, they disintegrate when immersed in water, making it difficult to maintain those excellent mechanical properties.
Therefore, the object of the present invention is to provide an organic-inorganic composite, a method for producing the same, and an organic-inorganic composite molded product that can maintain a considerable degree of its original mechanical strength even in a watery or high-humidity environment.

In order to solve the above problems, the present invention was developed through diligent research as follows.
Major problems with plastics are their low modulus of elasticity and poor biodegradability. Furthermore, many plastics are insoluble in water, contributing to environmental problems such as marine plastic pollution. Since water resistance and biodegradability are in a trade-off relationship, if we consider environmental issues, the ideal solution would be to create composite materials from polymers with a certain degree of hydrophilicity, where the hydrophilic groups are effectively processed within the solid to exhibit water resistance.
Therefore, from the perspective of environmental considerations, we decided to proceed with the study of an organic-inorganic composite in which a hydroxyl group-containing polymer, which has a hydrophilic hydroxyl group, is used as the organic material and a calcium phosphate compound is used as the inorganic material. Furthermore, in order to enhance water resistance without impairing the excellent mechanical properties of the organic-inorganic composite and to maintain a considerable degree of the original mechanical strength even in environments with water and high humidity, we investigated acylation of the hydroxyl groups of the hydroxyl group-containing polymer. Through further experiments and considerations, we found that acylation using aromatic acyl groups or long-chain aliphatic acyl groups is particularly effective in enhancing water resistance and maintaining a considerable degree of the original mechanical strength even in environments with water and high humidity.
This invention was completed based on the above findings.

In other words, the organic-inorganic composite according to the present invention is an organic-inorganic composite comprising a hydroxyl group-containing polymer as the organic material and a calcium phosphate compound as the inorganic material, wherein a portion of the hydroxyl groups of the hydroxyl group-containing polymer has a chemical structure in which it is acylated by an aromatic acyl group or a long-chain aliphatic acyl group, and the hydroxyl group-containing polymer is starch.

The organic-inorganic composite molded product according to the present invention is composed of the above-mentioned organic-inorganic composite.

The present invention relates to a method for producing an organic-inorganic composite, wherein the organic material is a hydroxyl group-containing polymer and the inorganic material is a calcium phosphate compound, and the method is characterized in that the hydroxyl group-containing polymer is starch, and the method includes a step of acyling a portion of the hydroxyl groups of the starch with an aromatic acyl group or a long-chain aliphatic acyl group.

The organic-inorganic composite and molded organic-inorganic composite of the present invention not only exhibit excellent mechanical strength, but also maintain a considerable degree of mechanical strength even in environments with water or high humidity.
According to the method for producing organic-inorganic composites of the present invention, organic-inorganic composites and molded organic-inorganic composites possessing the above-described excellent physical properties can be obtained.

This graph shows the thermogravimetric analysis results of the composites from Examples 1-5 and Comparative Example 1. This graph shows the thermogravimetric analysis results of the composites from Examples 6-8 and Comparative Example 2. This graph shows the thermogravimetric analysis results for the composites of Comparative Examples 1, 3 to 7. This graph shows the thermogravimetric analysis results for each composite in Comparative Examples 2, 8 to 10. This graph shows the thermogravimetric analysis results for each composite in Examples 9 to 11. This graph summarizes the inorganic weight fractions of the composites from Examples 1-5 and Comparative Example 1. This graph summarizes the degree of acyl group substitution in the composites of each example and comparative example. This graph shows the results of the crystal structure analysis by powder X-ray diffraction (XRD) of the composites of Examples 1 to 5 and Comparative Example 1. This graph shows the IR measurement results of the composites of Examples 1-5 and Comparative Example 1. This graph shows the absorbance ratios of the ester group and phosphate group in the composites of Examples 1 to 5. This graph shows the IR measurement results of the composites from Examples 6-8 and Comparative Example 2. This graph shows the IR measurement results for the composites of Comparative Examples 3-7 and Comparative Example 1. This graph shows the IR measurement results for each composite in Comparative Examples 8-10 and Comparative Example 2. This graph shows the IR measurement results for each composite in Example 9 and Comparative Example 1. This graph shows the IR measurement results for each composite in Examples 10-11 and Comparative Example 1. This is a reference diagram illustrating the parameters used to calculate the three-point bending strength (MPa) in a three-point bending test. This graph summarizes the water absorption rates of the composite molded products of Examples 1-5 and Comparative Example 1. This graph summarizes the water absorption rates of the composite molded products of Examples 6-8, 10-11, and Comparative Examples 2, 8-10. This graph summarizes the density of the composite molded products of Examples 1 to 5 before and after immersion in water. This graph summarizes the density of the composite molded products of Examples 6-8, 10-11, and Comparative Examples 8-10 before and after immersion in water. This graph summarizes the bending strength of the composite molded products of Examples 1 to 5 before and after immersion in water. This graph summarizes the bending strength before and after immersion for the composite molded articles of Examples 6-8, 10-11, and Comparative Examples 8-10. This graph summarizes the elastic modulus of the composite molded products of Examples 1 to 5 before and after immersion in water. This graph summarizes the elastic modulus before and after immersion for the composite molded articles of Examples 6-8, 10-11, and Comparative Examples 8-10. This graph summarizes the fracture energy before and after immersion for the composite molded products of Examples 1 to 5. This graph summarizes the fracture energy before and after immersion for the composite molded products of Examples 6-8, 10-11, and Comparative Examples 8-10. This graph shows the stress-strain curves before and after immersion in water for the composite molded product of Comparative Example 1. This graph shows the stress-strain curves of the composite molded product of Example 1 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 2 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 3 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 4 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 5 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 6 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 7 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 8 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 9 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 10 before and after immersion in water. This graph shows the stress-strain curves of the composite molded product of Example 11 before and after immersion in water. This graph summarizes the values of (after immersion / before immersion) × 100 for the flexural strength of the composite molded products of Examples 1 to 5 and Comparative Example 1 before and after immersion. This graph summarizes the values of (after immersion / before immersion) × 100 for the flexural strength before and after immersion of the composite molded articles of Examples 6-8, 10-11 and Comparative Examples 8-10. This graph summarizes the values of the elastic modulus (after immersion / before immersion) × 100 for the composite molded products of Examples 1 to 5 and Comparative Example 1, before and after immersion. This graph summarizes the values of the elastic modulus (after immersion / before immersion) × 100 before and after immersion for the composite molded articles of Examples 6-8, 10-11 and Comparative Examples 8-10. This graph summarizes the fracture energy (after immersion / before immersion) × 100 values for the composite molded products of Examples 1 to 5 and Comparative Example 1, before and after immersion. This graph summarizes the values of (after immersion / before immersion) × 100 for the composite molded articles of Examples 6-8, 10-11 and Comparative Examples 8-10, before and after immersion.

The following describes in detail preferred embodiments of the organic-inorganic composite, its manufacturing method, and the molded organic-inorganic composite according to the present invention. However, the scope of the present invention is not limited to these descriptions, and modifications can be made as appropriate without impairing the spirit of the invention, in addition to the examples given below.

[Organic-inorganic composite]
The organic-inorganic composite of the present invention uses a hydroxyl group-containing polymer as the organic material and a calcium phosphate compound as the inorganic material.

The hydroxyl group-containing polymer can be any polymer containing a hydroxyl group, and is not particularly limited, but examples include polysaccharides, polyvinyl alcohol, and poly(2-hydroxyethyl acrylate). Using polysaccharides that are available as plant-derived raw materials is preferable from the standpoint of considering environmental issues.
Examples of polysaccharides include starch, cellulose, sodium alginate, and glucomannan.
While not particularly limited, for example, those with a weight-average molecular weight in the range of 10,000 to 100,000 are preferred, and those with a weight-average molecular weight in the range of 10,000 to 50,000 are more preferred.

The hydroxyl group-containing polymer may have some of its hydroxyl groups modified, for example, it may be phosphorylated. Phosphorylation is expected to improve the mechanical strength and flexibility of the organic-inorganic composite. The degree of phosphorylation is not particularly limited, but for example, it is preferably 0.001 to 20%, more preferably 0.01 to 20%, and particularly preferably 0.01 to 15%.
In the above, the degree of phosphorylation refers to the ratio of phosphate groups when the total number of hydroxyl groups and phosphate groups in a hydroxyl group-containing polymer is set to 100.
While there are no particular limitations on the method for phosphorylating hydroxyl group-containing polymers, a useful example is the homogeneous phosphorylation reaction of hydroxyl group-containing polymers using urea, phosphoric acid, and dimethylformamide (DMF) (see Kakuji Katsuura, Kunihiro Inagaki, and Shiro Sakurai, Journal of the Chemical Society of Japan, 1972, 1305-1309). With this reaction, the degree of phosphorylation can be easily adjusted by appropriately selecting the reaction time, and it is also possible to obtain highly phosphorylated polymers.

Calcium phosphate compounds are salts composed of calcium ions and phosphate ions ( _PO₄³- ) or _{diphosphate ions (P₂O₃⁴- )} , with hydroxyapatite ^, one of the components of bone ^, being a typical example.
Hydroxyapatite has a basic composition of Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ , but some of the Ca component may be substituted with Sr, Ba, Mg, Fe, Al, Y, La, Na, K, H, etc., some of the PO ₄ component may be substituted with VO ₄ , BO ₃ , SO ₄ , CO ₃ , SiO _4, etc., some of the OH component may be substituted with F, Cl, O, CO _3, etc., or some of these components may have defects. Specifically, examples include carbonate apatite, fluoride apatite, and fluorinated apatite.
Other calcium phosphate compounds include monocalcium phosphate (MCPM), dicalcium phosphate (DCPD), tricalcium phosphate (α-TCP, β-TCP), tetracalcium phosphate (TTCP), octacalcium phosphate, calcium octaphosphate (OCP), amorphous calcium phosphate (ACP), and their hydrates.

The organic-inorganic composite of the present invention has a chemical structure in which some of the hydroxyl groups of a hydroxyl group-containing polymer are acylated by aromatic acyl groups or long-chain aliphatic acyl groups.
A typical example of an aromatic acyl group is the benzoyl group, but this also includes groups that have substituents on the aromatic ring of the benzoyl group, such as lower alkyl groups or lower alkoxy groups having approximately 1 to 6 carbon atoms. The number and position of substituents on the aromatic ring are not particularly limited.
The long-chain aliphatic acyl group is not particularly limited, but for example, it is an aliphatic acyl group having about 6 to 20 carbon atoms. If the number of carbon atoms is less than 6, there is a concern that the imparting of water resistance by acylation will be insufficient. Considering the mechanical properties and water resistance of the molded product, an aliphatic acyl group having about 10 to 20 carbon atoms is preferred. A preferred specific example is the lauroyl group, but other groups such as the caproyl group, capryloyl group, myristoyl group, pentadecanoyl group, palmitoyl group, heptadecanoyl group, and stearoyl group are also included. It may be linear or branched, and it may have substituents as long as it does not impair the effects of the invention.

Regarding the degree of acylation described above, using the degree of substitution in the case of polysaccharides as an example, a range of 1.5 or more and less than 3.0 per glucose unit is preferred, and a range of 1.8 to 2.8 is preferred. Since there are three hydroxyl groups per glucose unit, the upper limit of this degree of substitution is 3.
If the degree of acylation is too low, sufficient water resistance may not be imparted, and if the degree of acylation is too high, the proportion of organic matter in the organic-inorganic composite will increase, which may hinder the excellent mechanical properties inherent to the composite.

The inorganic weight fraction of the organic-inorganic composite of the present invention is not particularly limited, but is preferably 10 to 70% by weight, and more preferably 20 to 50% by weight. This inorganic weight fraction is affected not only by the mixing ratio of organic and inorganic materials but also by acylation. That is, the inorganic weight fraction decreases before and after acylation.

[Method for producing organic-inorganic composites]
The organic-inorganic composite of the present invention described above is not particularly limited, but it is preferable to produce it by compounding a hydroxyl group-containing polymer with a calcium phosphate compound and then performing acylation, as detailed below.
It is also possible to compound organic materials with inorganic materials after acylation, but in this case, the acylation of the hydroxyl groups of the organic material may make compounding with the inorganic material difficult. Furthermore, when acylation is performed after compounding organic and inorganic materials, it is thought that mainly the surface of the composite is acylated, whereas when organic materials are acylated first and then compounded with inorganic materials, it is thought that not only the surface but also the interior of the composite is acylated. Since water resistance is thought to be mainly contributed to by surface acylation rather than internal acylation, the former method, which is advantageous for surface acylation, is preferable, i.e., compounding organic and inorganic materials first and then acyling the composite.

<Compositing of hydroxyl group-containing polymers with calcium phosphate compounds>
Methods for compounding polymers and calcium phosphate compounds include (1) synthesizing the polymer and calcium phosphate compound separately and then mixing them; (2) synthesizing the calcium phosphate compound beforehand and then adding the monomer of the polymer to polymerize it; and (3) preparing the polymer beforehand and then adding calcium ions and phosphate ions to grow crystals of the calcium phosphate compound in the presence of the polymer, thereby compounding them.
In the present invention, the third method (3), namely, a method in which a polymer is prepared in advance, and then calcium ions and phosphate ions are added to it to grow and composite crystals of a calcium phosphate compound in the presence of the polymer, is particularly efficient and preferred.
Method (3) described above includes methods such as alternating immersion in which calcium ions and phosphate ions are diffused into a polymer gel, adding calcium ions and phosphate ions to a polymer solution and causing coprecipitation, and slowly diffusing hydroxide ions using a semipermeable membrane to promote the crystallization of calcium phosphate compounds.

Of the above compounding methods (3), if the alternating immersion method is used, it can be carried out, for example, as follows (see also paragraphs 0026 to 0029 of Patent Document 1).
First, an aqueous solution of a hydroxyl group-containing polymer is prepared. Then, the pH is adjusted to the basic side using, for example, an aqueous sodium hydroxide solution, and a solution containing calcium ions is added to induce gelation.
Next, by repeatedly reacting this gel with a solution containing phosphate ions and a solution containing calcium ions alternately one or more times, preferably two to five times, it is possible to achieve a composite of a hydroxyl group-containing polymer and a calcium phosphate compound (hydroxyapatite).

In the above, examples of solutions containing calcium ions include aqueous solutions of calcium chloride and calcium nitrate, and examples of solutions containing phosphate ions include aqueous solutions of hydrogen phosphate salts such as diammonium hydrogen phosphate, disodium hydrogen phosphate, and dipotassium hydrogen phosphate. The temperature of the solution during the reaction is preferably 10 to 50°C, and more preferably 20 to 40°C. The pH of the solution during the reaction is preferably on the basic side, for example, pH 6 to 13, and more preferably pH 7 to 10. For pH adjustment, for example, an aqueous ammonia solution can be used.
As a solvent, water is preferable from the standpoint of the solubility of each component, as described above, but it is not limited to this, and for example, a mixed solution of an organic solvent and water may also be used.
Furthermore, other components may be used as long as they do not impair the purpose of the present invention.

Furthermore, if the coprecipitation method is used among the above compounding methods (3), it can be carried out, for example, as follows (see also paragraphs 0022 to 0027 of Patent Document 2).
First, a polymer solution containing organic material is prepared. This polymer solution includes cases where the organic material is dissolved in the liquid as well as dispersed. Water, hydrophilic organic solvents, or mixtures thereof can be used as solvents.
Next, by adding phosphate ions and calcium ions, which are raw materials for the inorganic material calcium phosphate, to the polymer solution containing the organic material, the organic-inorganic composite can be co-precipitated.
_{As a} source of phosphate ions, for example, phosphates such as _Na₂HPO₄ , ( _NH₄ ) _₂HPO₄ , _K₂HPO₄ , _Li₂HPO₄ , _{and H₃PO₄ ,} or phosphoric _acid can be used.
Examples of calcium ion sources that can be used include _CaCl₂ , Ca( _NO₃ ) _₂ , _CaBr₂ , _CaI₂ , Ca(OH) _₂ , and Ca( _OCOCH₃ ) _₂ .
For coprecipitation, the pH is preferably adjusted to a range of 5 to 10, more preferably 7 to 9. This pH adjustment can be achieved, for example, by adding sodium hydroxide.
Furthermore, the coprecipitation temperature can be in the range of, for example, 20 to 100°C, and preferably 40°C or higher. A higher coprecipitation temperature tends to result in better mechanical properties for the resulting organic-inorganic composite and its molded product, but if it is too high, there is a risk of deterioration of the organic material due to heat.
The coprecipitation time should be sufficient to allow calcium ions and phosphate ions to coprecipitation in the mixed solution and form an organic-inorganic complex; for example, it can be between 1 minute and 24 hours.

<Acylation>
The present invention provides a method for producing organic-inorganic composites, which includes a step of acylating a portion of the hydroxyl groups of a hydroxyl group-containing polymer with an aromatic acyl group or a long-chain aliphatic acyl group.
This acylation can be achieved by reacting a suitable acyling agent with the hydroxyl groups of a hydroxyl group-containing polymer.

In this case, vinyl carboxylate is preferred as the acylating agent.
Other acyling agents include carboxylic acid anhydrides and carboxylic acid chlorides, but these produce carboxylic acids as byproducts of the acylation reaction. Carboxylic acids are undesirable because they partially dissolve the inorganic components of the complex and are difficult to remove. In contrast, when vinyl carboxylate is used as an acyling agent, the byproduct is vinyl alcohol, but because vinyl alcohol is thermodynamically unstable, it is converted to volatile acetaldehyde, making the byproduct easy to remove.

In this invention, a portion of the hydroxyl groups of a hydroxyl group-containing polymer are acylated with an aromatic acyl group or a long-chain aliphatic acyl group. Therefore, as the vinyl carboxylate, an aromatic vinyl carboxylate ester or a long-chain aliphatic vinyl carboxylate ester is used.
Typical aromatic vinyl carboxylates include vinyl benzoate, but they may also have substituents on their aromatic ring, such as lower alkyl groups or lower alkoxy groups with approximately 1 to 6 carbon atoms. The number and position of substituents on the aromatic ring are not particularly limited. Preferred long-chain aliphatic vinyl carboxylates include lauroyl vinyl, but are not limited to this. The number of carbon atoms in the long-chain alkanoyl group is, for example, about 6 to 20, preferably about 10 to 20. The reason for this is the same as that given for the number of carbon atoms in the long-chain aliphatic acyl group. The number and position of substituents on the long-chain alkanoyl group are also not particularly limited.

While higher acylation temperatures tend to promote the acylation reaction more effectively, excessively high temperatures can lead to reaction saturation, which is undesirable considering energy costs and the thermal impact on the raw materials. Although not specifically limited, a temperature range of 40 to 120°C is preferred. Furthermore, if the inorganic weight fraction of the complex is high, the acylation reaction becomes less likely to proceed, so it is preferable to set a higher acylation temperature (e.g., 80°C or higher). Additionally, acylation using long-chain aliphatic acyl groups tends to be less efficient than acylation using aromatic acyl groups, so it is preferable to set a higher acylation temperature (e.g., 80°C or higher).

[Organic-inorganic composite molded product]
The organic-inorganic composite molded product of the present invention consists of the organic-inorganic composite according to the present invention.

[Method for producing organic-inorganic composite molded products]
The organic-inorganic composite molded product of the present invention can be obtained by molding the composite obtained by the above-described method for producing organic-inorganic composites into a desired shape.
While there are no particular limitations on the molding method, hot press molding is preferred. For hot press molding, the organic-inorganic composite may be molded by uniaxial heating press molding (hot press molding) or by warm isostatic press molding. Adopting warm isostatic press molding makes it easier to improve the mechanical strength and flexibility of the molded product. When performing pressure molding, the molding temperature is preferably 80°C to 180°C. When performing pressure molding, the applied pressure is preferably 50 MPa to 1000 MPa.

The organic-inorganic composite may be cold-formed using isostatic press molding followed by hot-press molding. When cold-formed using isostatic press molding, the molding temperature is preferably 5°C to 30°C. When cold-formed using isostatic press molding, the pressing pressure is preferably 100 MPa to 300 MPa.
The organic-inorganic composite may be pre-formed before cold isotropic hydrostatic press forming.

The following describes the organic-inorganic composite according to the present invention, its manufacturing method, and molded organic-inorganic composite articles using examples; however, the present invention is not limited to these examples.

As the hydroxyl group-containing polymer, which is an organic material, we used phosphorylated tapioca starch (containing 0.013% phosphorus) from Sanwa Starch Industry Co., Ltd.

[Synthesis Example 1]
A complex of phosphorylated tapioca starch and hydroxyapatite with an inorganic weight fraction of 50% by weight was synthesized using the following procedure (hereinafter, phosphorylated tapioca starch will be abbreviated as "PTS", hydroxyapatite as "HAP", and this complex as "PTS-HAP complex").
9 g of phosphorylated tapioca starch (PTS) was weighed into a 1000 mL beaker, and 448 mL of 0.2 M _CaCl₂ aqueous solution was added. The mixture was stirred at 90°C for 1 hour at a stirring speed of 600 rpm. After that, the mixture was allowed to cool to room temperature while maintaining the same stirring speed, and then a _mixed solution of 268 mL of 0.2 M _Na₂HPO₄ aqueous solution and 72 mL of 1 M NaOH aqueous solution was added at a dropping rate of 0.06 mL/s at 70°C. After the addition was complete, the mixture was aged for 1 hour at the same temperature and stirring speed.
The obtained PTS-HAP composite coprecipitate was washed by suction filtration with a large excess of distilled water. Then, the coprecipitate was dehydrated by adding a large excess of acetone and suction filtration, and then vacuum-dried at 80°C for more than 2 hours. After that, it was ground into a finer powder using a mortar and pestle.
Ten identical samples were synthesized using the same procedure as described above, and all of these powders were mixed together and used as a single sample.

[Synthesis Example 2]
The following procedure was used to synthesize a PTS-HAP composite with a starting inorganic weight fraction of 70% by weight.
3.86 g of phosphorylated tapioca starch (PTS) was weighed into a 1000 mL beaker, and 448 mL of 0.2 M CaCl₂ _aqueous solution was added. The mixture was stirred at 90°C for 1 hour at a stirring speed of 600 rpm. After that, the mixture was allowed to cool to room temperature while maintaining the same stirring speed, and then a _mixed solution of 268 mL of 0.2 M _Na₂HPO₄ aqueous solution and 72 mL of 1 M NaOH aqueous solution was added at a dropping rate of 0.06 mL/s at 70°C. After the addition was complete, the mixture was aged for 1 hour at the same temperature and stirring speed.
The obtained PTS-HAP composite coprecipitate was washed by suction filtration with a large excess of distilled water. Then, the coprecipitate was dehydrated by adding a large excess of acetone and suction filtration, and then vacuum-dried at 80°C for more than 2 hours. After that, it was ground into a finer powder using a mortar and pestle.
Ten identical samples were synthesized using the same procedure as described above, and all of these powders were mixed together and used as a single sample.

[Example 1]
2.5 g of the PTS-HAP composite powder obtained in Synthesis Example 1, 50 mL of DMSO, and 0.5 g _{of K₂CO₃} were added to a 150 mL reaction vessel, and 18.3 g of vinyl benzoate was added to each. The mixture was then reacted at 40°C for 8 hours to carry out benzoylation. After the reaction was complete, a large excess of methanol was added, and after confirming that a precipitate had formed, it was filtered to remove the DMSO and unreacted vinyl benzoate. Subsequently, a large excess of distilled water was added, and after confirming that the precipitate had not dissolved, it was filtered to completely remove _the remaining _K₂CO₃ . A large excess of methanol was again added to the precipitate remaining on the filter paper, and the water contained in the precipitate was dehydrated by filtering. Furthermore, the recovered precipitate was vacuum dried at 80°C for more than 2 hours, and then ground into a finer powder using a mortar and pestle to obtain the PTS-HAP composite of Example 1.

Approximately 0.13 g of the acylated composite powder synthesized at 40°C was placed in a rectangular prism mold with an inner diameter of 4 mm x 13 mm. Uniaxial compression molding was performed at 120°C and 120 MPa for 5 minutes to produce three rectangular prism test specimens measuring 4.0 mm x 13.0 mm x (1.7–1.9) mm.

[Example 2]
The PTS-HAP complex of Example 2 was obtained in the same manner as in Example 1, except that the reaction temperature during benzoylation was changed to 60°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 3]
The PTS-HAP complex of Example 3 was obtained in the same manner as in Example 1, except that the reaction temperature for benzoylation was changed to 80°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 4]
The PTS-HAP complex of Example 4 was obtained in the same manner as in Example 1, except that the reaction temperature during benzoylation was changed to 100°C.
Furthermore, using this acylated composite powder, four prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 5]
The PTS-HAP complex of Example 5 was obtained in the same manner as in Example 1, except that the reaction temperature during benzoylation was changed to 120°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 6]
The PTS-HAP complex of Example 6 was obtained in the same manner as in Example 1, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during benzoylation was changed to 80°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 7]
The PTS-HAP complex of Example 7 was obtained in the same manner as in Example 1, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during benzoylation was changed to 100°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 8]
The PTS-HAP complex of Example 8 was obtained in the same manner as in Example 1, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during benzoylation was changed to 120°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 1.

[Example 9]
2.5 g of the PTS-HAP composite powder obtained in Synthesis Example 1, 50 mL of DMSO, and 0.5 g _{of K₂CO₃} were added to a 150 mL reaction vessel, and 27.16 g of vinyl laurate was added to each. The mixture was then reacted at 80°C for 8 hours to carry out lauroylation. After the reaction was complete, a large excess of methanol was added, and after confirming that a precipitate had formed, it was filtered to remove the DMSO and unreacted vinyl laurate. Subsequently, a large excess of distilled water was added, and after confirming that the precipitate had not dissolved, it was filtered to completely remove _the remaining _K₂CO₃ . A large excess of methanol was again added to the precipitate remaining on the filter paper, and the water contained in the precipitate was dehydrated by filtering. Furthermore, the recovered precipitate was vacuum dried at 80°C for more than 2 hours, and then ground into a finer powder using a mortar and pestle to obtain the PTS-HAP composite of Example 9.
Approximately 0.13 g of the above-mentioned lauroylated composite powder was placed in a rectangular prism mold with an inner diameter of 4 mm x 13 mm, and uniaxial compression molding was performed at 60°C and 120 MPa for 5 minutes to produce three rectangular prism-shaped test specimens measuring 4.0 mm x 13.0 mm x (1.7-1.9) mm.

[Example 10]
The PTS-HAP composite of Example 10 was obtained in the same manner as in Example 9, except that the PTS-HAP composite powder obtained in Synthesis Example 2 was used instead of the PTS-HAP composite powder obtained in Synthesis Example 1.
Furthermore, using this lauroylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 9.

[Example 11]
The PTS-HAP composite of Example 11 was obtained in the same manner as in Example 9, except that the PTS-HAP composite powder obtained in Synthesis Example 2 was used instead of the PTS-HAP composite powder obtained in Synthesis Example 1, and the reaction temperature during lauroylation was changed to 100°C.
Furthermore, using this lauroylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Example 9.

[Comparative Example 1]
The PTS-HAP complex obtained in Synthesis Example 1 (inorganic weight fraction 50 wt%, no acylation) was used as the PTS-HAP complex for Comparative Example 1.
Furthermore, approximately 0.14 g of the above-mentioned PTS-HAP composite powder was placed in a rectangular prism mold with an inner diameter of 4 mm x 13 mm, and uniaxial pressure molding was performed at 120°C and 120 MPa for 5 minutes to produce three rectangular prism test pieces measuring 4.0 mm x 13.0 mm x 1.7 mm.

[Comparative Example 2]
The PTS-HAP complex obtained in Synthesis Example 2 (inorganic weight fraction 70 wt%, no acylation) was used as the PTS-HAP complex for Comparative Example 2.
Furthermore, approximately 0.14 g of the above-mentioned PTS-HAP composite powder was placed in a rectangular prism mold with an inner diameter of 4 mm x 13 mm, and uniaxial pressure molding was performed at 120°C and 120 MPa for 5 minutes to produce three rectangular prism test pieces measuring 4.0 mm x 13.0 mm x 1.7 mm.

[Comparative Example 3]
2.5 g of the PTS-HAP composite powder obtained in Synthesis Example 1, 50 mL of DMSO, and 0.5 g _{of K₂CO₃} were added to a 150 mL reaction vessel, and 10.5 g of vinyl acetate was added to each. The mixture was then reacted at 40°C for 8 hours to carry out acetylation. After the reaction was complete, a large excess of methanol was added, and after confirming that a precipitate had formed, it was filtered to remove the DMSO and unreacted vinyl acetate. Subsequently, a large excess of distilled water was added, and after confirming that the precipitate had not dissolved, it was filtered to completely remove _the remaining _K₂CO₃ . A large excess of methanol was again added to the precipitate remaining on the filter paper, and the water contained in the precipitate was dehydrated by filtering. Furthermore, the recovered precipitate was vacuum dried at 80°C for more than 2 hours, and then ground into a finer powder using a mortar and pestle to obtain the PTS-HAP composite of Comparative Example 3.
Approximately 0.13 g of the above acetylated complex powder was placed in a rectangular prism mold with an inner diameter of 4 mm x 13 mm, and uniaxial compression molding was performed at 120°C and 120 MPa for 5 minutes to produce three rectangular prism test pieces measuring 4.0 mm x 13.0 mm x (1.7-1.9) mm.

[Comparative Example 4]
The PTS-HAP complex of Comparative Example 4 was obtained in the same manner as in Comparative Example 3, except that the reaction temperature during acetylation was changed to 60°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 5]
The PTS-HAP complex of Comparative Example 5 was obtained in the same manner as in Comparative Example 3, except that the reaction temperature during acetylation was changed to 80°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 6]
The PTS-HAP complex of Comparative Example 6 was obtained in the same manner as in Comparative Example 3, except that the reaction temperature during acetylation was changed to 100°C.
Furthermore, using this acylated composite powder, four prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 7]
The PTS-HAP complex of Comparative Example 7 was obtained in the same manner as in Comparative Example 3, except that the reaction temperature during acetylation was changed to 120°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 8]
The PTS-HAP complex of Comparative Example 8 was obtained in the same manner as Comparative Example 3, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during acetylation was changed to 80°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 9]
The PTS-HAP complex of Comparative Example 9 was obtained in the same manner as Comparative Example 3, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during acetylation was changed to 100°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Comparative Example 10]
The PTS-HAP complex of Comparative Example 10 was obtained in the same manner as Comparative Example 3, except that the PTS-HAP complex powder obtained in Synthesis Example 2 was used instead of the PTS-HAP complex powder obtained in Synthesis Example 1, and the reaction temperature during acetylation was changed to 120°C.
Furthermore, using this acylated composite powder, three prismatic test specimens measuring 4.0 mm × 13.0 mm × (1.7-1.8) mm were prepared by molding in the same procedure as in Comparative Example 3.

[Summary of Examples and Comparative Examples]
For ease of understanding, the key points of each example and comparative example are summarized in Table 1 below.

[Measurement and evaluation of physical properties]
<TGA>
Thermogravimetric analysis (TGA) was performed on the composites described in each of the above examples and comparative examples using a differential thermal and gravimetric simultaneous measurement device (DTG-60) manufactured by Shimadzu Corporation.
Figure 1 shows the thermogravimetric analysis results for the composites of Examples 1 to 5 and Comparative Example 1. Figure 2 shows the thermogravimetric analysis results for the composites of Examples 6 to 8 and Comparative Example 2. Figure 3 shows the thermogravimetric analysis results for the composites of Comparative Examples 1, 3 to 7. Figure 4 shows the thermogravimetric analysis results for the composites of Comparative Examples 2, 8 to 10. Figure 5 shows the thermogravimetric analysis results for the composites of Examples 9 to 11.
In each figure, the lines overlap and are difficult to see, so to clarify, in Figure 1, the graphs for Examples 1 to 5 (excluding Comparative Example 1) are almost overlapping. Also, in Figure 2, the graphs for Examples 6 to 8 (excluding Comparative Example 2) are almost overlapping. In Figure 3, the graphs for Comparative Examples 3 to 7 (excluding Comparative Example 1) are almost overlapping. In Figure 4, the graphs for Comparative Examples 8 to 10 (excluding Comparative Example 2) are almost overlapping. In Figure 5, the graphs for Examples 10 and 11 are almost overlapping.
The overlap of the graphs in each figure is understood to be because the acylation reaction proceeded sufficiently and the introduction of organic matter by acylation occurred to a similar extent. In Figure 5, the graph for Example 9 is separated from the graphs for Examples 10 and 11 because the weight fraction of inorganic materials used in the initial charge was different.

Furthermore, the inorganic weight fraction IC of the composites in Examples 1-11 and Comparative Examples 1-10 was measured using the following formula. The measurement results are shown in Tables 2-5 below. For ease of understanding, the measurement results for Examples 1-5 and Comparative Example 1 are also shown graphically in Figure 6.

IC = (Weight when heated to 1000°C and held for 10 minutes / Weight when heated to 100°C and held for 10 minutes) × 100 (wt%)

As shown in Figures 1-6 and Tables 2-5, the inorganic weight fraction of the composite in Comparative Example 1 (Synthesis Example 1) was close to the initial value of 50 wt%, and the inorganic weight fraction of the acylated composites obtained in Examples 1-5 and Comparative Examples 3-7 decreased with increasing acylation temperatures. Similarly, the inorganic weight fraction of the composite in Comparative Example 2 (Synthesis Example 2) was close to the initial value of 70 wt%, and the inorganic weight fraction of the acylated composites obtained in Examples 6-11 and Comparative Examples 8-10 decreased with increasing acylation temperatures.

Furthermore, the number of hydroxyl groups substituted for the acyl group per unit glucose of PTS (degree of substitution DS) was calculated from the change in inorganic weight fraction IC due to acylation.
Since the acylation reactions carried out in each example and comparative example were performed under basic conditions, theoretically HAP is not decomposed. Therefore, it is considered that the weight of HAP hardly changes due to the above acylation reaction. In other words, if the organic-inorganic weight fraction changes due to acylation, it is due to the change in the weight of the organic matter. If we consider that this change in the weight of the organic matter occurs because the hydroxyl groups of PTS are substituted with acyl groups, then the inorganic weight fraction IC _after the acylated complex can be expressed as the percentage of (inorganic weight fraction IC _before acylation) / (total weight of 100g of the complex after acylation). If X is the number of moles of substituted hydroxyl groups in 100g of the complex, then the following equation is obtained.

IC _before and IC _after represent the inorganic weight fractions before and after acylation, respectively. M _OH is the molecular weight of the hydroxyl group (17 g/mol), and M _acy is the molecular weight of the acyloxy group (benzoyl group: 121 g/mol).
The number of moles Y of hydroxyl groups in PTS per 100g of the complex is obtained by dividing the organic weight fraction of the complex by the molecular weight of the glucose unit of PTS and multiplying by the number of hydroxyl groups per glucose unit, which is 3, resulting in the following equation.

M _AGU indicates the molecular weight per unit glucose of PTS (162 g/mol).
The number of hydroxyl groups substituted with an acyl group per unit glucose (degree of substitution DS) is calculated by dividing the number of moles of acylated hydroxyl groups X in 100g of the complex by the total number of moles of hydroxyl groups Y before acylation, and multiplying the result by 3. Therefore, it can be expressed by the following formula.

The calculation results for each composite in Examples 1-11 and Comparative Examples 3-10, which underwent acylation, are shown in Tables 2-5 below. Furthermore, the calculation results for each composite in Examples 1-8 and Comparative Examples 3-10 are summarized in Figure 7.
In this example and comparative example, although some of the hydroxyl groups in the PTS used are substituted with phosphate groups, these phosphate groups constitute less than 0.1% of the weight of the organic polymer and are negligible in the calculations; therefore, the presence of phosphate groups is not considered in the above calculations.

<XRD>
For the samples (composite powders) of Examples 1 to 5 and Comparative Example 1, powder X-ray diffraction (XRD) was performed using the SmartLab powder X-ray diffractometer (manufactured by Rigaku Corporation) to analyze their crystal structure. The measurement results are shown in Figure 8.
Figure 8 shows characteristic peaks of HAp at 26°, 32°, 39°, 46°, 49°, and 53°, indicating that HAp crystals were formed in the composite of Comparative Example 1. Furthermore, it was found that in each acylated composite of Examples 1 to 5, the HAp crystals were not transferred to other types of calcium phosphate such as bruscheit or monetite through acylation.

<IR>
The samples (composite powders) of Examples 1 to 5 and Comparative Example 1 were measured using the KBr tablet method with a Fourier transform infrared spectrophotometer "FT/IR-4600" (JASCO). The measurement results are shown in Figure 9. As shown in Figure 9, the absorbance ratio between the ester group and the phosphate group of HAp increases as the acylation temperature increases, indicating that acylation proceeds more rapidly with higher acylation temperatures. The absorbance ratios between the ester group and the phosphate group of HAp for each sample of Examples 1 to 5 are shown in Figure 10.
Similarly, IR measurements were performed on samples from Examples 6-8, Comparative Examples 3-7, Comparative Examples 8-10, Example 9, and Examples 10-11, and the results are shown in Figures 11-15, respectively. Figures 11-15 also show the measurement results for Comparative Examples 1 or 2, which were not acylated.

<Physical property testing of molded products before immersion in water>
Three-point bending tests were performed on each of the composite molded products from Examples 1 to 11 and Comparative Examples 1 to 10 before immersion using the fine ceramics bending strength tester "MZ-250" (manufactured by Maruto Co., Ltd.). The distance L between the supports was 8 mm, and the crosshead speed for the bending test was 0.5 mm/min.
Based on the test results, the three-point bending strength (MPa) and three-point bending strain were calculated using the following formulas.
Three-point bending strength: σ _b = (3PL)/(2wt ² )
Three-point bending strain: _εb = (600st) / ( ^L² )
Here, P is the maximum load (N), L is the distance between supports (mm), w is the width of the specimen (mm), t is the thickness of the specimen (mm), and s is the maximum deflection (mm). Figure 16 shows a reference diagram for each of the above parameters used to calculate the three-point bending strength _σb (MPa).
Furthermore, the flexural modulus E _b (GPa) of each molded body was calculated from the slope of the stress-strain curve obtained from the three-point bending test, and the fracture energy G _f (MJ/ ^m³ ) was also calculated from the area.
Tables 2 to 5 below show the density, flexural strength _σb , elastic modulus _Eb , and fracture energy _Gf of each composite molded product from Examples 1 to 11 and Comparative Examples 1 to 10.

<Water absorption rate of molded products>
For each composite molded product in Examples 1 to 11 and Comparative Examples 1 to 10, the water absorption rate was calculated from the weight change after immersion in distilled water at room temperature for 24 hours using the following formula.
Water absorption rate = ((Weight after soaking - Weight before soaking) / Weight before soaking) × 100%
The calculated values are shown in Tables 6 to 9 below. Furthermore, the values for Examples 1 to 5 and Comparative Example 1 are summarized as a graph in Figure 17. Similarly, the results for Examples 6 to 8, 10 to 11, and Comparative Examples 2 and 8 to 10 are summarized as a graph in Figure 18.

<Physical property testing of molded products after immersion in water>
For each composite molded product of Examples 1 to 11 and Comparative Examples 1 to 10 after immersion in water, a three-point bending test was performed using the fine ceramic bending strength tester "MZ-250" (manufactured by Maruto Co., Ltd.), in the same manner as before immersion. The test was conducted under the same conditions as before immersion.
Tables 6-9 below show the density, flexural strength _σb , elastic modulus _Eb , and fracture energy _Gf of each composite molded product after immersion in water.

Figure 19 shows a summary of the density d values as a function of acylation temperature for each composite molded product of Examples 1 to 5 before and after immersion. Similarly, Figure 20 shows a summary of the density d values as a function of acylation temperature for each composite molded product of Examples 6 to 8, 10 to 11 and Comparative Examples 8 to 10 before and after immersion.
Similarly, Figures 21 and 22 show the bending strength _σb before and after flooding, Figures 23 and 24 show the elastic modulus _Eb before and after flooding, and Figures 25 and 26 show the fracture energy _Gf before and after flooding.

The stress-strain curves of the composite molded products of Examples 1 to 11 and Comparative Example 1 before and after immersion are shown in Figures 27 to 38, respectively.
From the stress-strain curves in Figures 27-38, it was found that there are differences in mechanical properties between acylation by aromatic acyl groups and acylation by long-chain aliphatic acyl groups.
Specifically, the stress-strain curves shown in Figures 28-35 (acylation by aromatic acyl groups) are convex downwards. In this convex downwards case, the material fractures brittlely when bent.
In contrast, the stress-strain curves shown in Figures 36-38 (acylation by long-chain aliphatic acyl groups) are convex upwards. In this convex upward case, plastic deformation occurs before fracture. This fracture mechanism is similar to that of plastics and bone.

Figure 39 shows the (after immersion/before immersion) × 100% values of the flexural strength of each composite molded product as a function of the acylation temperature for Examples 1-5 and Comparative Example 1. Similarly, Figure 40 shows the (after immersion/before immersion) × 100% values of the flexural strength of each composite molded product as a function of the acylation temperature for each composite molded product as a function of the acylation temperature for Examples 6-8, 10-11 and Comparative Examples 8-10.
Similarly, the values of the elastic modulus (after flooding / before flooding) × 100% are shown in Figures 41 and 42, and the values of the fracture energy (after flooding / before flooding) × 100% are shown in Figures 43 and 44.
Figures 39-44 show that acylation of the composite material allows more than half of the bending strength, elastic modulus, and fracture energy to be retained even after immersion in water, thus improving water resistance.

From the results shown above, it was confirmed that acylation with aromatic acyl groups or long-chain aliphatic acyl groups can maintain a considerable degree of the original mechanical strength even after immersion in water, and that maintaining this mechanical strength is difficult with acetylation, and that acylation with aromatic acyl groups or long-chain aliphatic acyl groups is important.
Both acylation using aromatic acyl groups and acylation using long-chain aliphatic acyl groups contribute to improved water resistance, but as mentioned above, their fracture mechanisms differ, allowing them to be used selectively depending on the application.

Although the above examples use PTS, it can be reasonably inferred that other polysaccharides can also be acylated while retaining the bond between their hydroxyl groups, or phosphate or carboxyl groups introduced by chemical modification, and the HAP.
Therefore, those skilled in the art will understand, by considering the above examples, that the present invention is suitable for embodiments using PTS, and that it can also be applied to embodiments using polysaccharides other than PTS.

The organic-inorganic composites and molded organic-inorganic composites of this invention can be suitably used in various portable components, food packaging materials, structural materials for fuel-efficient automobiles, and biomaterials such as artificial bones and artificial tooth roots, taking advantage of their excellent mechanical strength, flexibility, light weight, and biocompatibility. Furthermore, composites were obtained that retain more than half of their original bending strength even after immersion in water. Because they use environmentally friendly materials, possess mechanical strength comparable to plastics, and are water-resistant, a wide range of applications are conceivable. This also opens the door to regenerative medicine, such as bone replacement materials.

Claims (6)

An organic-inorganic composite comprising a hydroxyl group-containing polymer as the organic material and a calcium phosphate compound as the inorganic material, wherein the organic-inorganic composite has a chemical structure in which a portion of the hydroxyl groups of the hydroxyl group-containing polymer are acylated by aromatic acyl groups or long-chain aliphatic acyl groups,
An organic-inorganic composite characterized in that the hydroxyl group-containing polymer is starch . The organic-inorganic composite according to claim 1 , wherein the hydroxyl group of the starch is acylated in the range of 1.5 or more and less than 3.0 per unit glucose . An organic-inorganic composite molded article comprising the organic-inorganic composite described in claim 1 or 2. A method for producing an organic-inorganic composite, wherein the organic material is a hydroxyl group-containing polymer and the inorganic material is a calcium phosphate compound, the method comprising the step of acyling a portion of the hydroxyl groups of the starch with an aromatic acyl group or a long-chain aliphatic acyl group. The method for producing an organic-inorganic composite according to claim 4, wherein the acylation is performed by reacting a portion of the hydroxyl groups of the starch with the vinyl groups of an aromatic vinyl carboxylate or a long-chain aliphatic vinyl carboxylate. The method for producing an organic-inorganic composite according to claim 4 or 5, wherein the reaction temperature for the acylation is 40 to 120°C .