Temperature sensitive material and method for manufacturing temperature sensitive material

Abstract

To provide a temperature-sensitive material improved in brittleness of a conventional temperature-sensitive material using a temperature-responsive polymer, to provide a method for producing the same, and to provide a method for producing a temperature-sensitive material having specific temperature characteristics, strength or heat resistance.SOLUTION: The temperature-sensitive material includes a solvent, a temperature-responsive high molecular polymer which releases or takes in hydrogen ions according to temperature, a pH indicator, and polyvinyl alcohol (PVA), which are gelled. The temperature sensitive material according to the present invention can be obtained by dissolving, in a solvent, a temperature responsive high molecular polymer that releases or takes in hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA) to produce a gel precursor aqueous solution, and subjecting the gel precursor aqueous solution to a freezing-thawing cycle in which the gel precursor aqueous solution is frozen and then thawed to cause gelation.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The present invention relates to a temperature-sensitive material using a temperature-responsive polymer and a method for producing the temperature-sensitive material. [Background technology]

[0002] One way to visually recognize temperature is to distinguish it by color change. Cholesteric liquid crystals are materials that change color in response to temperature changes. Cholesteric liquid crystals have a narrow color temperature range, so they can be microencapsulated and have multiple identification positions for each display temperature, but this is complicated and expensive to produce. In addition, the displayed temperature is dissipative, making it difficult to adjust the accuracy.

[0003] The present inventors have developed a temperature-indicating material (thermosensitive material) that reversibly changes color with temperature changes. The thermosensitive material developed by the present inventors utilizes the properties of a temperature-responsive polymer, and is a gel-like thermosensitive material composed of a solvent, a temperature-responsive polymer, a pH indicator that changes color with changes in pH, and a gelling agent. The temperature-responsive polymer is a copolymer P(NIPAM-co-AAC) of N-isopropylacrylamide (NIPAM) and acrylic acid (AAC), and the gelling agent is agar (see, for example, Patent Document 1).

[0004] The temperature-responsive polymer P(NIPAM-co-AAC) has a lower critical solution temperature (LCST). At temperatures below the LCST, P(NIPAM-co-AAC) exists in an extended state in water, and the carboxyl groups (-COOH) on the acrylic acid side chains are ionized (-COO - ) and protons (H + ) to lower the surrounding pH. On the other hand, when P(NIPAM-co-AAC) contracts at temperatures above the lower critical solution temperature (LCST), the released protons (H + ) is captured by electrostatic interaction and -COO - This property can be used to create a temperature-sensitive material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-27572 Summary of the Invention [Problem to be solved by the invention]

[0006] The thermosensitive material described in Patent Document 1 is an excellent thermosensitive material that is reversible and easy to prepare, but it is fragile. Fragile thermosensitive materials are difficult to process and may break during use, limiting their shape and applications. For this reason, there is a demand for thermosensitive materials that are more flexible and stronger than conventional thermosensitive materials that use temperature-responsive polymers.

[0007] The object of the present invention is to provide a thermosensitive material that improves the brittleness of conventional thermosensitive materials using temperature-responsive polymers, and a method for producing the same, and further to provide a thermosensitive material having specific temperature characteristics, strength, or heat resistance. [Means for solving the problem]

[0008] The present invention is a temperature-sensitive material comprising a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA), which are gelled.

[0009] The present invention is a temperature-sensitive material comprising a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, at least one of a sugar or an electrolyte, and polyvinyl alcohol (PVA), which are gelled.

[0010] The present invention is a temperature-sensitive material comprising a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, borax (sodium tetraborate decahydrate) as a crosslinking agent, and polyvinyl alcohol (PVA), which are gelled together.

[0011] The present invention is a thermosensitive material characterized in that the temperature-responsive polymer is P(NIPAM-co-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AAC).

[0012] The present invention provides a thermosensitive material characterized in that the temperature-responsive polymer is P(NIPAM-XXX-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM), a monomer (XXX), and acrylic acid (AAC), and the monomer (XXX) is any one of acrylamide, N-alkylacrylamide, dimethylacrylamide (DMA), and a vinyl group having a primary to tertiary amine.

[0013] The present invention provides a method for producing a thermosensitive material, characterized by comprising the steps of: dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA) in a solvent to produce a gel precursor aqueous solution; and performing a freeze-thaw cycle of freezing and then thawing the gel precursor aqueous solution to gel it.

[0014] The present invention provides a method for producing a thermosensitive material, characterized by including the steps of: dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, at least one of a sugar or an electrolyte, and polyvinyl alcohol (PVA) in a solvent to produce a gel precursor aqueous solution; and performing a freeze-thaw cycle of freezing and then thawing the gel precursor aqueous solution to gel it.

[0015] The present invention provides a method for producing a thermosensitive material, characterized by comprising the steps of: dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, borax (sodium tetraborate decahydrate) as a crosslinking agent, and polyvinyl alcohol (PVA) in a solvent to produce a gel precursor aqueous solution; and performing a freeze-thaw cycle of freezing and then thawing the gel precursor aqueous solution to gel it.

[0016] The present invention is a method for producing a thermosensitive material, characterized in that at least one of the number of freeze-thaw cycles or the freezing time is adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0017] The present invention is a method for producing a thermosensitive material, characterized in that the polyvinyl alcohol (PVA) having a specific degree of saponification is used to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0018] The present invention is a method for producing a temperature-sensitive material, characterized in that the mixing ratio of the polyvinyl alcohol (PVA) is adjusted to produce a temperature-sensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0019] The present invention provides a method for producing a thermosensitive material, characterized in that the temperature-responsive polymer is P(NIPAM-co-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AAC).

[0020] The present invention provides a method for producing a thermosensitive material, characterized in that the conditions for copolymerization of the P(NIPAM-co-AAC) are adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0021] The present invention is a method for producing a thermosensitive material, characterized in that the temperature-responsive polymer is P(NIPAM-XXX-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM), a monomer (XXX), and acrylic acid (AAC), and the monomer (XXX) is any one of acrylamide, N-alkylacrylamide, dimethylacrylamide (DMA), and a vinyl group having a primary to tertiary amine.

[0022] The present invention provides a method for producing a thermosensitive material, characterized in that the conditions for copolymerization of P(NIPAM-XXX-AAC) are adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0023] The present invention is a method for producing a temperature-sensitive material, characterized in that the mixing ratio of at least one of sugars or electrolytes is adjusted to produce a temperature-sensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0024] The present invention is a method for producing a temperature-sensitive material, characterized in that the mixing ratio of the borax (sodium tetraborate decahydrate) is adjusted to produce a temperature-sensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

[0025] The present invention is a method for producing a thermosensitive material, characterized in that a degassing operation is used in the step of producing the gel precursor aqueous solution, or a degassing step of removing air bubbles contained in the gel precursor aqueous solution is included between the step of producing the gel precursor aqueous solution and the step of gelling. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a thermosensitive material that improves the brittleness of conventional thermosensitive materials using temperature-responsive polymers, and a method for manufacturing the same. Furthermore, it is possible to provide a thermosensitive material having specific temperature characteristics, strength, or heat resistance. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a diagram showing the mechanism of pH change accompanying temperature change of the temperature-responsive polymer P(NIPAM-co-AAC) constituting the thermosensitive material of the present invention. [Figure 2] FIG. 1 is a diagram showing a synthesis scheme of the temperature-responsive polymer P(NIPAM-co-AAC) used in the temperature-sensitive material of the present invention. [Figure 3] FIG. 1 is a diagram showing the gelation mechanism of polyvinyl alcohol (PVA) constituting the temperature-sensitive material of the present invention by a freeze-thaw method. [Figure 4] FIG. 1 is a flow chart showing a manufacturing procedure of a temperature-sensitive material of the present invention. [Figure 5] FIG. 1 is a flow chart showing a manufacturing procedure of a temperature-sensitive material of the present invention. [Figure 6] FIG. 10 is a diagram showing a crosslinking mechanism between polyvinyl alcohol (PVA) and borate ions in a temperature-sensitive material according to a third embodiment of the present invention. [Figure 7] FIG. 1 is a diagram showing a synthesis scheme of the temperature-responsive polymer P(NIPAM-DMA-AAC) used in the temperature-sensitive material of the present invention. [Figure 8] FIG. 2 is a diagram for explaining the apparatus and test procedure used in the strength test of the temperature-sensitive material of the present invention. [Figure 9] 1 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a first embodiment of the present invention. [Figure 10] 1 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a first embodiment of the present invention. [Figure 11] 1 shows the measurement results of the temperature-pH characteristics of the temperature-sensitive material of the first embodiment of the present invention. [Figure 12] 1 shows the measurement results of the temperature-pH characteristics of the temperature-sensitive material of the first embodiment of the present invention. [Figure 13] 4 shows the strength measurement results of the temperature-sensitive material of the first example of the present invention. [Figure 14] 1 shows the results of measuring the melting temperature of the temperature-sensitive material of the first embodiment of the present invention. [Figure 15] 4 shows the strength measurement results of the temperature-sensitive material of the first example of the present invention. [Figure 16] 1 shows the results of measuring the melting temperature of the temperature-sensitive material of the first embodiment of the present invention. [Figure 17] 10 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a second embodiment of the present invention. [Figure 18] 10 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a second embodiment of the present invention. [Figure 19] 10 shows the measurement results of the temperature-pH characteristics of the temperature-sensitive material of the second embodiment of the present invention. [Figure 20] 10 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a third embodiment of the present invention. [Figure 21] 10 shows the measurement results of the temperature-pH characteristics of the temperature-sensitive material of the third embodiment of the present invention. [Figure 22] 10 shows the strength measurement results of the temperature-sensitive material of the third embodiment of the present invention. [Figure 23] 10 shows the results of measuring the melting temperature of the temperature-sensitive material of the third embodiment of the present invention. [Figure 24] 10 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a fourth embodiment of the present invention. [Figure 25] 10 shows the results of measuring the temperature and pH characteristics of an aqueous gel precursor solution prepared in the process of producing a temperature-sensitive material according to a fourth embodiment of the present invention. [Figure 26] 10 shows the measurement results of the temperature-pH characteristics of the temperature-sensitive material of the fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] The temperature-sensitive material of the first embodiment of the present invention is a temperature-sensitive material that contains a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA), and is formed by gelling these components.

[0029] Thermosensitive materials can be evaluated using characteristic values ​​(physical properties). This also applies to thermosensitive materials of other embodiments. Characteristic values ​​(physical properties) include temperature characteristics, strength, and heat resistance. Temperature characteristics include temperature-pH characteristics, which show the relationship between temperature and pH, and lower critical solution temperature (LCST). Strength is the mechanical strength of the thermosensitive material, and in this embodiment, it refers to the force required to cut the thermosensitive material. Heat resistance refers to the gel melting temperature at which the thermosensitive material can no longer maintain a gel state and fluidity is confirmed.

[0030] In the temperature-sensitive material of the first embodiment, water is used as the solvent, and P(NIPAM-co-AAC) is used as the temperature-responsive polymer.

[0031] P(NIPAM-co-AAC) is a temperature-responsive polymer formed by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AAC). The pH of an aqueous solution of P(NIPAM-co-AAC) dissolved in water changes in response to temperature. As shown in Figure 1, when the P(NIPAM-co-AAC) solution is below the lower critical solution temperature (LCST), P(NIPAM-co-AAC) hydrates and expands, ionizing the carboxyl groups (-COOH) in the AAC side chains, resulting in H + The pH of the aqueous solution decreases. Furthermore, above the lower critical solution temperature (LCST), P(NIPAM-co-AAC) dehydrates and shrinks, releasing the carboxylate ions (-COO - ) is electrostatically coupled to H + Supplementing increases the pH of the aqueous solution.

[0032] P(NIPAM-co-AAC) can be obtained as follows. Figure 2 shows the synthesis scheme for the temperature-responsive polymer P(NIPAM-co-AAC). N-isopropylacrylamide (NIPAM) and acrylic acid (AAC) are added to distilled water in predetermined proportions. Sodium dodecyl sulfate is then dissolved in the solution, and the pH is adjusted with an aqueous HCl solution or an aqueous NaOH solution to obtain an aqueous solution. Nitrogen gas is bubbled through the solution to remove oxygen, which inhibits polymerization. An aqueous solution of ammonium persulfate (APS) is then added as a polymerization initiator, and polymerization is allowed to proceed for a predetermined period of time. The polymerized solution is placed in a cellulose dialysis membrane (pore size: 50 Å) and dialyzed for a predetermined period of time. Subsequently, P(NIPAM-co-AAC) is obtained by lyophilization. The amounts of each agent and specific values ​​for the polymerization procedure used to produce P(NIPAM-co-AAC) are described in the Examples below.

[0033] The pH indicator used may be one that changes color in response to changes in pH of the thermosensitive material, and may be appropriately selected from well-known indicators such as Congo red solution, methyl red solution, methyl red-methylene blue solution, and bromothymol blue solution.

[0034] Polyvinyl alcohol (PVA) functions as a gelling agent that gels the thermosensitive material. As shown in Figure 3, when a polyvinyl alcohol aqueous solution in which polyvinyl alcohol (PVA) is dissolved in water is repeatedly frozen and thawed, the water freezes to form ice, causing phase separation into an ice phase and a PVA phase. In the PVA phase, the PVA concentration increases, and hydrogen bonds between hydroxyl groups in the PVA side chains increase. Repeated freezing and thawing of the water further increases the number of hydrogen bonds between PVA chains, forming microcrystals. These microcrystals act as cross-linking points, forming a three-dimensional network structure, producing a PVA gel.

[0035] Since polyvinyl alcohol (PVA) is used to gel the thermosensitive material, it is necessary that the polyvinyl alcohol aqueous solution be frozen and thawed (freeze-thaw cycle) or gelled by repeated freezing and thawing. Polyvinyl alcohol (PVA) with a saponification degree n of 78-82 mol% will not gel even after repeated freeze-thaw cycles, so it is necessary to use polyvinyl alcohol (PVA) with a saponification degree n of 86 mol% or higher.

[0036] The degree of saponification (n) of polyvinyl alcohol (PVA) is a value indicating the ratio of hydroxyl groups to the total of acetyloxyl (acetic acid) groups and hydroxyl groups in polyvinyl alcohol (PVA), with an upper limit of 100 mol%. Commercially available polyvinyl alcohol (PVA) can be used. As shown in the examples below, the temperature-pH characteristics of the gel precursor aqueous solution, which is the precursor of the temperature-sensitive material, change depending on the degree of saponification (n) of polyvinyl alcohol (PVA) (see Figure 9). Furthermore, as shown in the examples below, the content of polyvinyl alcohol (PVA) affects the strength and heat resistance of the temperature-sensitive material (see Figures 15 and 16).

[0037] A manufacturing procedure for the temperature-sensitive material of the first embodiment will be described. As shown in Fig. 4(A), the temperature-sensitive material of the first embodiment can be obtained by obtaining an aqueous solution (gel precursor aqueous solution) in which a temperature-responsive polymer, a pH indicator, and polyvinyl alcohol (PVA) are dissolved in a solvent (S-1: gel precursor aqueous solution manufacturing step), and then freezing and thawing this gel precursor aqueous solution to gel it (S-2: gelation step). Detailed procedures and specific values ​​for manufacturing the temperature-sensitive material will be described in the examples below.

[0038] Experimental results showed that if the ratio of polyvinyl alcohol (PVA) to water in the gel precursor aqueous solution (thermosensitive material) was 7.5 wt% or less, a thermosensitive material could be obtained without any problems using the procedure shown in Figure 4(A). If the ratio of polyvinyl alcohol (PVA) to water in the gel precursor aqueous solution (thermosensitive material) was 8.5 wt% or more, many air bubbles were mixed in during the preparation process of the gel precursor aqueous solution, making it difficult to obtain a thermosensitive material using the procedure shown in Figure 4(A).

[0039] If many air bubbles are mixed in during the preparation of the gel precursor aqueous solution, it is advisable to either carry out a degassing operation (S-1') during the preparation of the gel precursor aqueous solution as shown in Figure 5(A), or to carry out a degassing step (S-1") after obtaining the gel precursor aqueous solution (S-1) as shown in Figure 5(B), followed by a gelation step (S-2). The degassing operation can be carried out using a vacuum degassing device or the like.

[0040] The freeze-thaw cycle in the gelation step (S-2), in which the gel precursor aqueous solution is frozen and thawed, may be performed one or more times. As shown in the examples below, the number of freeze-thaw cycles affects the characteristic values ​​(physical properties) of the thermosensitive material, and the greater the number of freeze-thaw cycles, the greater the strength and heat resistance of the thermosensitive material. On the other hand, an increase in the number of freeze-thaw cycles leads to an increase in the production time. The number of freeze-thaw cycles may be determined appropriately taking these points into consideration.

[0041] For example, if priority is given to shortening the manufacturing time of the temperature-sensitive material, one freeze-thaw cycle is sufficient. On the other hand, if the objective is to increase the strength or heat resistance of the temperature-sensitive material, two or more freeze-thaw cycles are preferable. The upper limit of the number of freeze-thaw cycles is not particularly limited, but judging from the results of the examples, it is likely to be about five.

[0042] The time (freezing time) for freezing the gel precursor aqueous solution in the freeze-thaw cycle is also an important factor that affects the characteristic values ​​(physical properties) of the obtained thermosensitive material.

[0043] As shown in the examples below, the strength of the thermosensitive material after three freeze-thaw cycles and a freezing time of 16 hours was approximately 51 N (see Figure 13). This represents an increase of more than 50 times the strength of a conventional thermosensitive material that uses the same temperature-responsive polymer but agar as a gelling agent, which has a strength of less than 1 N. Furthermore, increasing the number of freeze-thaw cycles increases the heat resistance even with a short freezing time, and the melting temperature of the thermosensitive material reaches approximately 70°C (see Figure 14). Furthermore, increasing the freezing time in the freeze-thaw cycle increases the strength and heat resistance of the thermosensitive material regardless of the number of freeze-thaw cycles (see Figures 13 and 14).

[0044] As described above, the temperature-sensitive material of the first embodiment can have different characteristic values ​​(physical properties) depending on the mixing ratio (addition ratio) of polyvinyl alcohol (PVA) contained in the temperature-sensitive material and the freezing and thawing method used when freezing and thawing the gel precursor aqueous solution to form a gel. Furthermore, as shown in the examples described later, the characteristic values ​​of the obtained temperature-sensitive material can be changed by changing the degree of saponification n of polyvinyl alcohol (PVA).

[0045] Furthermore, the characteristic values ​​of the thermosensitive material of the first embodiment of the present invention can be changed by changing the polymerization conditions of the temperature-responsive polymer P(NIPAM-co-AAC). As shown in the examples below, as the pH during polymerization of P(NIPAM-co-AAC) increased, the pH of the gel precursor aqueous solution increased regardless of the degree of saponification (n) of polyvinyl alcohol (PVA) or the temperature.

[0046] From the above, in the thermosensitive material of the first embodiment, by changing one or more of the number of freeze-thaw cycles when gelling the gel precursor aqueous solution, the freezing time in the freeze-thaw cycles, the polymerization conditions of P(NIPAM-co-AAC), which is a temperature-responsive polymer that constitutes the thermosensitive material, the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA), it is possible to change one or more of the temperature characteristics, strength, and heat resistance temperature of the resulting thermosensitive material.

[0047] In other words, in the thermosensitive material of the first embodiment, by setting one or more of the following to specific conditions: the number of freeze-thaw cycles when gelling the gel precursor aqueous solution, the freezing time in the freeze-thaw cycles, the polymerization conditions of P(NIPAM-co-AAC), which is a temperature-responsive high molecular weight polymer that constitutes the thermosensitive material, the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA), it is possible to obtain a thermosensitive material with specified temperature characteristics, strength, and heat resistance temperature.

[0048] The temperature-sensitive material of the second embodiment of the present invention is a temperature-sensitive material that contains a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, at least one of a sugar or an electrolyte, and polyvinyl alcohol (PVA), and is formed by gelling these components.

[0049] The solvent, temperature-responsive polymer, pH indicator, and polyvinyl alcohol (PVA) used in the temperature-sensitive material of the second embodiment are the same as those in the temperature-sensitive material of the first embodiment of the present invention, and therefore a description thereof will be omitted.

[0050] In the second embodiment, sugars or electrolytes, or both sugars and electrolytes, are added to the temperature-pH characteristic of the temperature-sensitive material to shift it to a lower temperature. Shifting the temperature-pH characteristic to a lower temperature means shifting the color change temperature of the temperature-sensitive material to a lower temperature. Examples of sugars include glucose, sucrose, and maltose, and examples of electrolytes include sodium chloride and calcium chloride.

[0051] The content of sugars, electrolytes, or sugars and electrolytes in this temperature-sensitive material is not particularly limited, but is typically up to about 25 wt% relative to the solvent water. As the content of sugars, electrolytes, or sugars and electrolytes increases, the temperature-pH characteristics shift to the lower temperature side.

[0052] The temperature-sensitive material of the second embodiment described above can be obtained, as shown in FIG. 4(B), in the same way as the temperature-sensitive material of the first embodiment, by dissolving a temperature-responsive polymer, a pH indicator, at least one of a sugar or an electrolyte, and polyvinyl alcohol (PVA) in a solvent to obtain a gel precursor aqueous solution (S-1: gel precursor aqueous solution production step), and then freezing and thawing this gel precursor aqueous solution to gel it (S-2: gelation step).

[0053] If many air bubbles are mixed in during the preparation of the gel precursor aqueous solution, a degassing operation can be performed in addition to the preparation of the gel precursor aqueous solution, as in the production of the temperature-sensitive material of the first embodiment, or a degassing step can be performed after obtaining the gel precursor aqueous solution, followed by a gelation step.

[0054] The freeze-thaw cycle for freezing and thawing the gel precursor aqueous solution may be considered to be the same as that for the temperature-sensitive material of Embodiment 1. Detailed procedures and specific values ​​for producing the temperature-sensitive material will be described in the examples below.

[0055] The thermosensitive material of the second embodiment described above, like the thermosensitive material of the first embodiment, is a thermosensitive material that uses the same temperature-responsive polymer, but can significantly increase mechanical strength compared to conventional thermosensitive materials that use agar as a gelling agent.

[0056] Furthermore, the temperature-pH characteristics of the thermosensitive material of this embodiment shift to lower temperatures in proportion to the amount of sugar added. As will be described in the Examples below, the color change temperature of the thermosensitive material without added glucose is 40°C, whereas the color change temperature of the thermosensitive material of this embodiment with 10 wt% glucose added is 35°C, and the color change temperature of the thermosensitive material of this embodiment with 15 wt% glucose added is 30°C (see Figure 19).

[0057] As described above, the temperature-pH characteristics of the temperature-sensitive material of the second embodiment can be changed by adjusting the amount of sugars such as glucose, sucrose, and maltose, and electrolytes such as sodium chloride and calcium chloride, which are added for the purpose of shifting the temperature-pH characteristics to the lower temperature side.

[0058] In the thermosensitive material of the second embodiment, as with the thermosensitive material of the first embodiment of the present invention, increasing the number of freeze-thaw cycles performed on the gel precursor aqueous solution increases the strength of the thermosensitive material. Furthermore, extending the freezing time in the freeze-thaw cycles increases the strength and heat resistance of the thermosensitive material, regardless of the number of freeze-thaw cycles. Furthermore, as with the thermosensitive material of the first embodiment of the present invention, the characteristic values ​​of the thermosensitive material obtained can be changed by adjusting the polymerization conditions of the temperature-responsive polymer P(NIPAM-co-AAC), the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA).

[0059] From the above, in the thermosensitive material of the second embodiment, by changing one or more of the following: the type of sugar or electrolyte, the amount of sugar or electrolyte added, the number of freeze-thaw cycles when gelling the gel precursor aqueous solution, the freezing time in the freeze-thaw cycles, the polymerization conditions of P(NIPAM-co-AAC), the temperature-responsive polymer that constitutes the thermosensitive material, the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA), it is possible to change one or more of the temperature characteristics, strength, and heat resistance temperature of the resulting thermosensitive material. In other words, by setting the above items to specific conditions, it is possible to obtain a thermosensitive material with specified temperature characteristics, strength, and heat resistance temperature.

[0060] The thermosensitive material of the third embodiment of the present invention is a thermosensitive material containing a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, borax (sodium tetraborate decahydrate), and polyvinyl alcohol (PVA), which are gelled together. The added borax (sodium tetraborate decahydrate) functions as a crosslinking agent.

[0061] The solvent, temperature-responsive polymer, pH indicator, and polyvinyl alcohol (PVA) used in the temperature-sensitive material of the third embodiment are the same as those in the temperature-sensitive material of the first embodiment of the present invention, and therefore a description thereof will be omitted.

[0062] In the thermosensitive material of the third embodiment, borax (sodium tetraborate decahydrate) is added to improve heat resistance. The results of Figure 14 in Example 1, which will be described later, show that the heat resistance of the thermosensitive material of the first embodiment remained around 70°C even when the number of freeze-thaw cycles and freezing time were changed. This is because, with 7.5 wt% polyvinyl alcohol, a freezing time of 16 hours, and three freeze-thaw cycles, the crosslinks formed by hydrogen bonds between polyvinyl alcohol (PVA) chains (between hydroxyl groups) are broken at around 70°C. Therefore, borax, which is expected to have the effect of further strengthening the bonds in the crosslinked portions, was added.

[0063] The mechanism by which heat resistance is improved by adding borax is thought to be as follows. When borax is dissolved in water, it ionizes and separates into borate ions and boric acid, as shown in Figure 6. The borate ions crosslink by forming borate ester bonds with the hydroxyl groups in the side chains of polyvinyl alcohol (PVA). Therefore, heat resistance is improved by the hydrogen bonds formed by the freeze-thaw cycle and the crosslinking caused by the borate ester bonds.

[0064] The content of borax (sodium tetraborate decahydrate) in this thermosensitive material is typically up to 0.033 wt% relative to the solvent water. The heat resistance of the thermosensitive material increased in proportion to the content of borax (sodium tetraborate decahydrate), reaching 77°C. The melting temperatures of gels containing 0.033 wt% and 0.055 wt% borax were the same (see Figure 23). The average strength decreased when borax was added compared to the gel without additives, but a tendency for it to improve with increasing borax was observed (see Figure 22).

[0065] The temperature-sensitive material of the third embodiment described above is obtained, similarly to the temperature-sensitive material of the first embodiment, by dissolving a temperature-responsive polymer, a pH indicator, borax, and polyvinyl alcohol (PVA) in a solvent to obtain a gel precursor aqueous solution (S-1: gel precursor aqueous solution preparation step), and then freezing and thawing this gel precursor aqueous solution to gel it (S-2: gelation step) (see Figure 4(B)).

[0066] If many air bubbles are mixed in during the preparation of the gel precursor aqueous solution, a degassing operation can be performed in addition to the preparation of the gel precursor aqueous solution, as in the production of the temperature-sensitive material of the first embodiment, or a degassing step can be performed after obtaining the gel precursor aqueous solution, followed by a gelation step.

[0067] The freeze-thaw cycle for freezing and thawing the gel precursor aqueous solution may be considered to be the same as that for the temperature-sensitive material of Embodiment 1. Detailed procedures and specific values ​​for producing the temperature-sensitive material will be described in the examples below.

[0068] The thermosensitive material of the third embodiment, as described above, is a thermosensitive material that uses the same temperature-responsive polymer as the thermosensitive material of the first embodiment, but can significantly increase mechanical strength compared to conventional thermosensitive materials that use agar as a gelling agent. Furthermore, the heat resistance of this thermosensitive material can be changed by adjusting the amount of borax added to improve heat resistance.

[0069] In the thermosensitive material of the third embodiment, as with the thermosensitive material of the first embodiment of the present invention, increasing the number of freeze-thaw cycles performed on the gel precursor aqueous solution increases the strength of the thermosensitive material. Furthermore, extending the freezing time in the freeze-thaw cycles increases the strength and heat resistance of the thermosensitive material, regardless of the number of freeze-thaw cycles. Furthermore, as with the thermosensitive material of the first embodiment of the present invention, the characteristic values ​​of the thermosensitive material obtained can be changed by adjusting the polymerization conditions of the temperature-responsive polymer P(NIPAM-co-AAC), the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA).

[0070] From the above, in the thermosensitive material of the third embodiment, by changing one or more of the amount of borax added, the number of freeze-thaw cycles when gelling the gel precursor aqueous solution, the freezing time in the freeze-thaw cycles, the polymerization conditions of the temperature-responsive polymer P(NIPAM-co-AAC) that makes up the thermosensitive material, the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA), it is possible to change one or more of the temperature characteristics, strength, and heat resistance temperature of the resulting thermosensitive material. In other words, by setting the above items to specific conditions, it is possible to obtain a thermosensitive material with specified one or more of the temperature characteristics, strength, and heat resistance temperature.

[0071] A fourth embodiment of the present invention provides a thermosensitive material comprising a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA), which are gelled together. The thermosensitive material uses P(NIPAM-XXX-AAC) as the temperature-responsive polymer. The thermosensitive material using P(NIPAM-XXX-AAC) as the temperature-responsive polymer can shift the color change range to the higher temperature side.

[0072] The solvent, pH indicator, and polyvinyl alcohol (PVA) used in the temperature-sensitive material of the fourth embodiment are the same as those in the temperature-sensitive material of the first embodiment of the present invention, and therefore a description thereof will be omitted.

[0073] P(NIPAM-XXX-AAC) is a temperature-responsive polymer obtained by copolymerizing N-isopropylacrylamide (NIPAM), monomer (XXX), and acrylic acid (AAC). The monomer (XXX) used here is a monomer that is more hydrophilic than N-isopropylacrylamide (NIPAM). Examples of such monomer (XXX) include acrylamide, N-alkylacrylamide, dimethylacrylamide (DMA), and vinyl groups containing primary, secondary, or tertiary amines, with dimethylacrylamide (DMA) being preferred.

[0074] The thermosensitive material using the temperature-responsive polymer P (NIPAM-DMA-AAC) in which the monomer (XXX) is dimethylacrylamide (DMA) swells by hydration in a temperature range below the lower critical solution temperature (LCST) and releases hydrogen ions H + On the other hand, in the upper temperature range, the water molecules are released and dehydrated, causing the water to shrink and release hydrogen ions H + The thermosensitive material exhibits a reversible phase transition characteristic in which the pH rises as a result of the incorporation and recombination of dimethylacrylamide (DMA). Furthermore, the retention of water molecules is strengthened by dimethylacrylamide (DMA), which increases the thermal energy required for dehydration. This causes the lower critical solution temperature (LCST) of the thermosensitive material to shift to a higher temperature than that of the thermosensitive material of the first embodiment. Therefore, the thermosensitive material using P(NIPAM-DMA-AAC) can shift the color change temperature range to a higher temperature.

[0075] The manufacturing procedure for P(NIPAM-XXX-AAC) is basically the same as that for P(NIPAM-co-AAC), except that in the manufacturing process for P(NIPAM-co-AAC), the procedure "adding N-isopropylacrylamide (NIPAM) and acrylic acid (AAC) in a specified ratio to distilled water" is changed to "adding N-isopropylacrylamide (NIPAM), monomer (XXX), and acrylic acid (AAC) in a specified ratio to distilled water." Figure 7 shows the synthesis scheme for the temperature-responsive polymer P(NIPAM-DMA-AAC).

[0076] The thermosensitive material of the fourth embodiment described above can be obtained, similarly to the thermosensitive material of the first embodiment, by dissolving a temperature-responsive polymer P(NIPAM-XXX-AAC), a pH indicator, and polyvinyl alcohol (PVA) in a solvent to obtain a gel precursor aqueous solution (S-1: gel precursor aqueous solution preparation step), and then freezing and thawing this gel precursor aqueous solution to gel it (S-2: gelation step) (see FIG. 4(A)).

[0077] If many air bubbles are mixed in during the preparation of the gel precursor aqueous solution, a degassing operation can be performed in addition to the preparation of the gel precursor aqueous solution, as in the production of the temperature-sensitive material of the first embodiment, or a degassing step can be performed after obtaining the gel precursor aqueous solution, followed by a gelation step.

[0078] The freeze-thaw cycle for freezing and thawing the gel precursor aqueous solution may be considered to be the same as that for the temperature-sensitive material of Embodiment 1. Detailed procedures and specific values ​​for producing the temperature-sensitive material will be described in the examples below.

[0079] The thermosensitive material of this embodiment, as described above, is a thermosensitive material that uses the same temperature-responsive polymer as the thermosensitive material of the first embodiment, but can significantly increase mechanical strength compared to conventional thermosensitive materials that use agar as a gelling agent.

[0080] Furthermore, the temperature-pH characteristics of the thermosensitive material of this embodiment shift to a higher temperature in proportion to the amount of monomer (XXX) added. As will be described in the Examples below, the color change temperature of the thermosensitive material of the first embodiment, which does not contain dimethylacrylamide (DMA) as the monomer (XXX), is 40°C, whereas the color change temperature of the thermosensitive material using the temperature-responsive polymer P (NIPAM-DMA-AAC) produced with a molar ratio of NIPAM:DMA:AAC of 75:20:5 rises to 50°C (see FIG. 26).

[0081] As described above, the temperature-pH characteristics of the thermosensitive material of the fourth embodiment can be changed by adjusting the proportion of the monomer (XXX) added when producing the temperature-responsive polymer P(NIPAM-XXX-AAC).

[0082] In the thermosensitive material of the fourth embodiment, as with the thermosensitive material of the first embodiment of the present invention, increasing the number of freeze-thaw cycles performed on the gel precursor aqueous solution increases the strength of the thermosensitive material. Furthermore, extending the freezing time in the freeze-thaw cycles increases the strength and heat resistance of the thermosensitive material, regardless of the number of freeze-thaw cycles performed. Furthermore, as with the thermosensitive material of the first embodiment of the present invention, the characteristic values ​​of the thermosensitive material obtained can be changed by adjusting the polymerization conditions of the temperature-responsive polymer P(NIPAM-XXX-AAC), the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA).

[0083] Therefore, in the thermosensitive material of the fourth embodiment, by changing one or more of the polymerization conditions of the temperature-responsive polymer P(NIPAM-XXX-AAC) used, the number of freeze-thaw cycles when gelling the gel precursor aqueous solution, the freezing time in the freeze-thaw cycles, the mixing ratio (addition ratio) of polyvinyl alcohol (PVA), and the saponification degree n of polyvinyl alcohol (PVA), it is possible to change one or more of the temperature characteristics, strength, and heat resistance temperature of the obtained thermosensitive material. In other words, by setting the above items to specific conditions, it is possible to obtain a thermosensitive material with specified one or more of the temperature characteristics, strength, and heat resistance temperature.

[0084] The above describes the temperature-sensitive material and manufacturing method thereof according to the present invention using the temperature-sensitive material and manufacturing method thereof of the first to fourth embodiments, but the temperature-sensitive material and manufacturing method thereof according to the present invention are not limited to the above embodiments and can be modified within the scope that does not change the gist of the invention.

[0085] For example, if the temperature-responsive polymer P(NIPAM-co-AAC) in the thermosensitive material of the third embodiment is changed to P(NIPAM-XXX-AAC), a thermosensitive material with excellent heat resistance and a temperature-pH characteristic shifted to the higher temperature side can be obtained. In addition, the color-change temperature range can be adjusted by changing the type of pH indicator, etc.

[0086] The temperature-sensitive material and its manufacturing method have been described above, but a person skilled in the art will easily imagine various changes and modifications within the obvious scope upon reading the specification and drawings. Therefore, such changes and modifications are to be interpreted as being within the scope of the invention as defined by the claims. [Example]

[0087] A gel precursor aqueous solution and a thermosensitive material were prepared as follows, and were evaluated by performing a temperature / pH characteristic test, a strength test, and a heat resistance test. In the following examples, the concentrations of polyvinyl alcohol (PVA), glucose, and borax refer to the ratio of polyvinyl alcohol (PVA), glucose, and borax to the water contained in the thermosensitive material (gel precursor aqueous solution). For example, when 1.0 g of polyvinyl alcohol (PVA) is added to 20 mL of water, the concentration of polyvinyl alcohol (PVA) is 5 wt%.

[0088] Temperature and pH characteristic test method for gel precursor aqueous solution The relationship between the temperature and pH of the gel precursor aqueous solution was measured using a pH meter as follows: The screw cap bottle containing the gel precursor aqueous solution was placed in a thermostatic bath, and a pH electrode and thermocouple were inserted so that they were in contact with the gel precursor aqueous solution in the screw cap bottle. The pH electrode and thermocouple were connected to a pH meter and a temperature indicator, respectively. The temperature of the water in the thermostatic bath was changed within a range of 20 to 80°C while stirring, and the pH was measured 5 minutes after the gel precursor aqueous solution in the screw cap bottle had equalized to the set water temperature.

[0089] Temperature and pH characteristic test method for thermosensitive materials A thermosensitive material was produced by placing a gel precursor solution in a test tube and repeatedly freezing (-17°C) and thawing (25°C). The test tube containing the thermosensitive material was immersed in a thermostatic chamber, and the temperature was changed within the range of 20 to 70°C, and the color change was visually observed.

[0090] Strength test method for temperature-sensitive materials After thoroughly stirring and mixing the gel precursor solution, it was poured into a glass slide mold containing a silicone rubber sheet (1 mm thick). The mixture was then repeatedly frozen (-17°C) and thawed (25°C) to produce a thermosensitive material measuring 1 mm thick, 1 cm wide, and 3 cm long. As shown in Figure 8, the thermosensitive material was removed from the mold and placed on a silicone rubber sheet (1 mm thick). The force required to cut the thermosensitive material was measured using a mechanical force gauge equipped with a V-shaped cutting attachment (Type A). Four measurements were taken for each sample, and the average value was used to calculate the mechanical strength of the thermosensitive material. The PVA thermosensitive gel in Figure 8 is the thermosensitive material.

[0091] Heat resistance test method for thermosensitive materials (melting test method for thermosensitive materials) A thermosensitive material was produced by adding an aqueous solution of gel precursor to a test tube and repeatedly freezing (-17°C) and thawing (25°C). The test tube containing the thermosensitive material was heated in a thermostatic chamber set to 60-80°C and left to stand for 5 minutes, after which the test tube was inverted and the melting state of the thermosensitive material was observed. The temperature at which the thermosensitive material could no longer maintain its solid state and fluidity was confirmed was taken as the melting temperature of the thermosensitive material.

[0092] Preparation of temperature-responsive polymer P(NIPAM-co-AAC) The temperature-responsive polymer P(NIPAM-co-AAC) was obtained as follows. N-isopropylacrylamide (NIPAM):acrylic acid (AAC) was added to 50 mL of distilled water at a molar ratio of 97:3 to 85:15 to a concentration of 78 to 273 mM. 45 mg of sodium dodecyl sulfate was then dissolved in the solution, and the pH was adjusted to 2.5 to 5.5 with 5 wt% aqueous HCl or 5 wt% aqueous NaOH to obtain an aqueous solution. Nitrogen gas was then bubbled through the solution for 30 min to remove dissolved oxygen (O2), which inhibits polymerization. Then, 1 mL of a 70 mg / mL aqueous solution of ammonium persulfate (APS) was added as a polymerization initiator, and polymerization was carried out for 3 h. The polymerized solution was placed in a cellulose dialysis membrane (pore size: 50 Å) and dialyzed for 2 days. The solution was then freeze-dried to obtain the temperature-responsive polymer P(NIPAM-co-AAC).

[0093] Preparation of temperature-responsive polymer P(NIPAM-DMA-AAC) The temperature-responsive polymer P(NIPAM-DMA-AAC) was obtained as follows. N-isopropylacrylamide (NIPAM):dimethylacrylamide (DMA):acrylic acid (AAC) were added to 50 mL of distilled water at molar ratios of 85:10:5 to 75:20:5 to achieve concentrations of 78 to 273 mM. 45 mg of sodium dodecyl sulfate was then dissolved in the solution, and the pH was adjusted to 2.5 to 5.5 with 5 wt% aqueous HCl or 5 wt% aqueous NaOH to obtain an aqueous solution. Nitrogen gas was then bubbled through the solution for 30 min to remove dissolved oxygen (O2), which inhibits polymerization. Then, 1 mL of a 70 mg / mL aqueous solution of ammonium persulfate (APS) was added as a polymerization initiator, and polymerization was allowed to proceed for 3 h. The polymerized solution was placed in a cellulose dialysis membrane (pore size: 50 Å) and dialyzed for 2 days. The solution was then freeze-dried to obtain the temperature-responsive polymer P(NIPAM-DMA-AAC).

[0094] Example 1: Temperature-sensitive material of the first embodiment The temperature-sensitive material of the first embodiment was obtained in the following manner. (1) 1.0–3.0 g of polyvinyl alcohol (PVA) (PVA concentration relative to the water content of the thermosensor = 5–15 wt%) was added to 16 mL of distilled water and dissolved at 80°C with stirring for 2 h. (2) 0.2 g of the temperature-responsive polymer P(NIPAM-co-AAC) was added to 4 mL of separately prepared distilled water and dissolved at room temperature. This was then added to the solution in (1) to obtain a gel precursor solution. A pH indicator was also added to the solution in (1) and stirred. The gel precursor solution was repeatedly frozen (-17°C) and thawed (25°C) to obtain a gel-like thermosensor. Three types of polyvinyl alcohol (PVA) with a degree of polymerization of 2000 and a degree of saponification (n) of 78–82, 86–90, and 98.5–mol% were used. Methyl red-methylene blue and methyl red were used as pH indicators. The freezing time was set to 0.5 to 16 hours (maximum 48 hours), and the thawing time was set to 0.5 hours. The degree of polymerization of polyvinyl alcohol (PVA) is the number of repeating structural units (monomer units), and indicates the size of the polymer compound.

[0095] Temperature and pH characteristic test results of gel precursor aqueous solution The results of measuring the pH versus temperature of the gel precursor aqueous solution prepared in the process of producing the temperature-sensitive material of the first embodiment of the present invention are shown in Figure 9. Figure 9 shows the gel precursor aqueous solution obtained under the preparation conditions of the temperature-responsive polymer P(NIPAM-co-AAC) under conditions of pH = 4.1, concentration 234 mM, molar ratio of NIPAM:AAC = 95:5, and polyvinyl alcohol (PVA) concentration 7.5 wt%.

[0096] The pH of 5-15 wt% aqueous polyvinyl alcohol solutions at 20-70°C was 5.5-5.7 when the polyvinyl alcohol (PVA) had a saponification degree n = 78-82 mol%, 5.6-5.9 when the saponification degree n = 86-90 mol%, and 5.9-6.2 when the saponification degree n = 98.5 mol%.

[0097] The pH change with temperature of gel precursor aqueous solutions of polyvinyl alcohol (PVA) with different degrees of saponification (n) was found to increase at the boundary of the lower critical solution temperature (LCST) of the temperature-responsive polymer P(NIPAM-co-AAC). A similar trend was observed with polyvinyl alcohol (PVA) at concentrations of 5 to 15 wt%. Below the lower critical solution temperature (LCST) (20°C), the pH was 4.5 to 5.5, whereas at 70°C, the pH rose to 5.5 to 6.5. The pH changed with temperature regardless of the degree of saponification (n) or concentration of polyvinyl alcohol (PVA).

[0098] Polymerization conditions for the thermoresponsive polymer P(NIPAM-co-AAC) and its relationship to the temperature and pH characteristics of the gel precursor aqueous solution using the thermoresponsive polymer P(NIPAM-co-AAC) We investigated the pH change of the gel precursor aqueous solution using the temperature-responsive polymer P(NIPAM-co-AAC), which was prepared with a molar ratio of N-isopropylacrylamide (NIPAM):acrylic acid (AAC) of 95:5, a concentration of 243 mM, and a pH during polymerization ranging from 2.5 to 5.5.

[0099] The results are shown in Figure 10. The gel precursor aqueous solution using the temperature-responsive polymer P(NIPAM-co-AAC) prepared at a pH of 2.5 during polymerization had a pH of 4.4–5.0 below the lower critical solution temperature (LCST) (20°C) and a pH of 5.6–6.1 at 70°C. The gel precursor aqueous solution using the temperature-responsive polymer P(NIPAM-co-AAC) prepared at a pH of 4.1 during polymerization had a pH of 4.4–5.0 below the lower critical solution temperature (LCST) (20°C) and a pH of 5.7–6.3 at 70°C. The gel precursor aqueous solution using the temperature-responsive polymer P(NIPAM-co-AAC) prepared at a pH of 5.0 during polymerization had a pH of 4.9–5.2 below the lower critical solution temperature (LCST) (20°C) and a pH of 5.9–6.5 at 70°C.

[0100] Regardless of the degree of saponification (n) of polyvinyl alcohol (PVA), when the pH during polymerization of the temperature-responsive polymer P(NIPAM-co-AAC) increased, the pH of the gel precursor aqueous solution increased regardless of the temperature.

[0101] The pH of the gel precursor aqueous solution (polyvinyl alcohol (PVA) saponification degree n = 86-90 mol%) using the temperature-responsive polymer P(NIPAM-co-AAC) prepared at a molar ratio of N-isopropylacrylamide (NIPAM):acrylic acid (AAC) of 97:3 to 85:15, a concentration of 243 mM, and a polymerization pH of 4.1 also increased with increasing temperature. Regardless of the NIPAM to AAC polymerization ratio, the pH increased with increasing temperature. The pH of this gel precursor aqueous solution was 4.2-5.0 below the lower critical solution temperature (LCST) (20 °C) and 5.6-5.9 at 70 °C.

[0102] Furthermore, the pH change of the gel precursor aqueous solution using the temperature-responsive polymer P(NIPAM-co-AAC), polymerized at a molar ratio of N-isopropylacrylamide (NIPAM):acrylic acid (AAC) of 95:5, a pH of 4.1 during polymerization, and a monomer concentration of N-isopropylacrylamide (NIPAM) + acrylic acid (AAC) of 78 to 273 mM, was pH = 4.9 to 5.0 below the lower critical solution temperature (LCST) (20°C), and pH = 5.8 to 5.9 at 70°C. The pH increased with increasing temperature, regardless of the concentration of the temperature-responsive polymer.

[0103] Temperature pH characteristic test results of the thermosensitive material of Example 1 The color change results of the thermosensitive material of Example 1 are shown in Figures 11 and 12. The thermosensitive material in Figure 11 is a thermosensitive material obtained by gelling an aqueous gel precursor solution obtained under the following conditions: pH = 4.1, concentration = 234 mM, molar ratio of NIPAM:AAC = 95:5, degree of saponification of polyvinyl alcohol (PVA) n = 86 to 90 mol%, and PVA concentration = 7.5 wt%; the solution was frozen for 16 hours and subjected to three freeze-thaw cycles. Methyl red-methylene blue was used as the pH indicator. The thermosensitive material in Figure 12 was prepared by gelling an aqueous gel precursor solution of temperature-responsive polymer P(NIPAM-co-AAC) under the following conditions: pH = 4.1, concentration 234 mM, NIPAM:AAC molar ratio = 95:5, polyvinyl alcohol (PVA) saponification degree n = 98.5 mol%, and PVA concentration 7.5 wt%. The gel was then frozen for 16 hours and subjected to three freeze-thaw cycles. Methyl red was used as the pH indicator.

[0104] 11 and 12 show that the color changes at a certain temperature. It was also found that changing the pH indicator or the degree of saponification (n) of polyvinyl alcohol (PVA) did not affect the color change behavior.

[0105] Strength test results and melting test results of the temperature-sensitive material of the first embodiment The strength test results for the thermosensitive material of the first embodiment are shown in Figure 13, and the melting test results for the thermosensitive material of the first embodiment are shown in Figure 14. The thermosensitive materials in Figures 13 and 14 are obtained by gelling an aqueous gel precursor solution obtained by preparing the temperature-responsive polymer P(NIPAM-co-AAC) under the following conditions: pH = 4.1, concentration = 234 mM, molar ratio of NIPAM:AAC = 95:5, degree of saponification of polyvinyl alcohol (PVA) n = 98.5 mol%, and PVA concentration = 7.5 wt%.

[0106] 13 and 14 confirm that the strength and melting temperature of the thermosensitive material increase as the number of freeze-thaw cycles and freezing time increase. When the freeze-thaw cycle was repeated three times, the strength increased in proportion to the freezing time, and the force required to cut increased to approximately 51 N after a freezing time of 16 hours. Furthermore, as the number of freeze-thaw cycles increased, the heat resistance increased even with a short freezing time, and the melting temperature of the thermosensitive material reached approximately 70°C (see Figure 14).

[0107] Figure 15 shows the relationship between the polyvinyl alcohol (PVA) concentration and strength of the thermosensitive material of the first embodiment, and Figure 16 shows the relationship between the polyvinyl alcohol (PVA) concentration and melting temperature of the thermosensitive material of the first embodiment. The thermosensitive materials of Figures 15 and 16 are thermosensitive materials obtained by gelling a gel precursor aqueous solution obtained by freeze-thawing three times under the following conditions: pH 4.1, concentration 234 mM, NIPAM:AAC molar ratio 95:5, and polyvinyl alcohol (PVA) with a degree of polymerization of 2000, degree of saponification n = 98.5 mol%, and PVA concentration of 5 to 8 wt%.

[0108] 15 and 16 show that strength and melting temperature increase with increasing polyvinyl alcohol (PVA) concentration. For a thermosensitive material with a polyvinyl alcohol (PVA) concentration of 8 wt%, the strength increased to approximately 56.5 N and the melting temperature rose to 74°C. The reason why strength and melting temperature improved with increasing polyvinyl alcohol (PVA) concentration is thought to be as follows: As the PVA concentration increases, the amount of PVA increases, increasing the number of hydrogen bonds in the PVA side chains. It is thought that this increase in hydrogen bonds (crosslinking points due to weak bonds) improved strength and heat resistance.

[0109] As mentioned above, increasing the polyvinyl alcohol (PVA) concentration improves strength and heat resistance, so it is expected that increasing the polyvinyl alcohol (PVA) concentration and adding borax will further improve heat resistance. However, when the polyvinyl alcohol (PVA) concentration is 8.5 wt% or higher, many air bubbles are mixed in during the preparation process of the gel precursor aqueous solution. For this reason, in the case of thermosensitive materials with a polyvinyl alcohol (PVA) concentration of 8.5 wt% or higher, a degassing operation is required to remove the mixed air bubbles before gelling the gel precursor aqueous solution.

[0110] Example 2: Temperature-sensitive material of the second embodiment The temperature-sensitive material of the second embodiment was obtained in the following manner. (1) 1.5 g of polyvinyl alcohol (PVA) was added to 16 mL of distilled water (PVA concentration = 7.5 wt%) and dissolved at 80 °C for 2 h with stirring. After cooling, 1–5 g of glucose (glucose concentration = 5–25 wt%) was added and dissolved. (2) 0.2 g of the temperature-responsive polymer P(NIPAM-co-AAC) was added to 4 mL of distilled water and dissolved at room temperature. The resulting solution was added to the solution in (1) to obtain a gel precursor solution. Methyl red-methylene blue was also added as a pH indicator to the solution in (1) and stirred. The gel precursor solution was repeatedly frozen (-17 °C) and thawed (25 °C) to obtain a gel-like thermosensitive material. Two types of polyvinyl alcohol (PVA) with a degree of polymerization of 2000 and a degree of saponification (n) of 86–90 (98.5 mol%) were used. The freezing time was set to 0.5 to 16 hours, and the thawing time was set to 0.5 hours.

[0111] Temperature and pH characteristic test results of glucose-added gel precursor aqueous solution Figures 17 and 18 show the temperature-pH characteristic test results for the glucose-added gel precursor aqueous solution. The glucose-added gel precursor aqueous solution in Figure 17 was obtained under the following conditions: pH = 4.1, concentration = 234 mM, NIPAM:AAC molar ratio = 95:5, polyvinyl alcohol (PVA) saponification degree n = 86-90 mol%, and PVA concentration = 7.5 wt%. The glucose-added gel precursor aqueous solution in Figure 18 used polyvinyl alcohol (PVA) with saponification degree n = 98.5 mol%. The other conditions were the same as those for the glucose-added gel precursor aqueous solution in Figure 17. The parameters of 1 g, 2 g, 3 g, and 4 g of glucose in Figures 17 and 18 correspond to glucose concentrations of 5 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively, relative to water.

[0112] As shown in Figures 17 and 18, the pH of the gel precursor aqueous solution increased at a lower temperature with the addition of glucose compared to when glucose was not added. It was confirmed that the pH increase shifted further toward the lower temperature side as the amount of glucose added increased. This is because the lower critical solution temperature (LCST) of the temperature-responsive polymer P(NIPAM-co-AAC) decreased with the addition of glucose. As the LCST decreased, the temperature at which the pH began to increase shifted toward the lower temperature side.

[0113] Temperature pH characteristic test results of the temperature-sensitive material of the second embodiment Figure 19 shows the results of a temperature-pH characteristic test for the thermosensitive material of the second embodiment. The thermosensitive material in Figure 19 is a gel of a gel precursor aqueous solution obtained by preparing the temperature-responsive polymer P(NIPAM-co-AAC) under the following conditions: pH = 4.1, concentration = 234 mM, molar ratio of NIPAM:AAC = 95:5, degree of saponification of polyvinyl alcohol (PVA) n = 86 to 90 mol%, and PVA concentration = 7.5 wt%. Methyl red-methylene blue was used as the pH indicator.

[0114] 19, it was found that the color change temperature shifted to a lower temperature as the amount of glucose added increased. When 3 g of glucose (glucose concentration 15 wt%) was added, the color change temperature was 30°C, which was 10°C lower than when no glucose was added.

[0115] Third Example: Temperature-sensitive material of the third embodiment The temperature-sensitive material of the third embodiment was obtained in the following manner. (1) 1.5 g of polyvinyl alcohol (PVA) was added to 15 mL of distilled water (PVA concentration = 7.5 wt%) and dissolved at 80 °C for 2 h with stirring. (2) 0.2 g of the temperature-responsive polymer P(NIPAM-co-AAC) was added to 4 mL of distilled water and dissolved at room temperature. (3) An aqueous solution of 0.0022–0.011 g of borax (borax concentration = 0.011–0.055 wt%) in 1 mL of distilled water was added to the solution in (1) to obtain a gel precursor solution. Methyl red–methylene blue was also added as a pH indicator to the solution in (1) and stirred. The gel precursor solution was repeatedly frozen (-17 °C) and thawed (25 °C) to obtain a gel-like thermosensitive material. The polyvinyl alcohol (PVA) used had a degree of polymerization of 2000 and a degree of saponification n = 98.5 mol%. The freezing time was 0.5 to 16 hours, and the thawing time was 0.5 hours.

[0116] Temperature and pH characteristic test results of the borax-added gel precursor aqueous solution and the temperature-sensitive material of the third example Figure 20 shows the temperature-pH characteristic test results for the borax-added gel precursor aqueous solution, and Figure 21 shows the temperature-pH characteristic test results for the temperature-sensitive material of Example 3. The borax-added gel precursor aqueous solution in Figure 20 was obtained by preparing the temperature-responsive polymer P(NIPAM-co-AAC) under the following conditions: pH = 4.1, concentration 234 mM, NIPAM:AAC molar ratio = 95:5, polyvinyl alcohol (PVA) saponification degree n = 98.5 mol%, and PVA concentration 7.5 wt%. The temperature-sensitive material in Figure 21 is obtained by gelling the borax-added gel precursor aqueous solution in Figure 20.

[0117] As can be seen from Figure 20, as the amount of borax added increases, the pH of the gel precursor aqueous solution tends to increase slightly as the temperature rises. As can be seen from Figure 21, the color change temperature of the temperature-sensitive material hardly changed even when the amount of borax added increased.

[0118] Strength test results of the temperature-sensitive material in the third example Figure 22 shows the strength test results for the thermosensitive material of Example 3. This thermosensitive material was gelled by subjecting the borax-added gel precursor aqueous solution of Figure 20 to three freeze-thaw cycles. Figure 22 shows that the strength of the material is slightly reduced when borax is added compared to when no borax is added.

[0119] Melting test results of the temperature-sensitive material in the third example Figure 23 shows the melting test results for the thermosensitive material of Example 3. This thermosensitive material was produced by subjecting the borax-added gel precursor aqueous solution shown in Figure 20 to three freeze-thaw cycles to gelation. Figure 23 shows that the melting temperature of the thermosensitive material is improved by adding borax, and that the melting temperature increases as the freezing time increases, thereby improving heat resistance. This is thought to be due to the addition of borax strengthening the cross-linking between polyvinyl alcohol (PVA) chains, improving heat resistance. The melting temperature of the thermosensitive material produced with 0.033 wt% borax and a freezing time of 16 hours rose to 77°C.

[0120] Fourth Example: Temperature-sensitive material of the fourth embodiment The temperature-sensitive material of the fourth embodiment was obtained in the following manner. (1) 1.5 g of polyvinyl alcohol (PVA) was added to 16 mL of distilled water (PVA concentration = 7.5 wt%) and dissolved at 80 °C for 2 h with stirring. (2) 0.2 g of the temperature-responsive polymer P(NIPAM-DMA-AAC) was added to 4 mL of distilled water and dissolved at room temperature. This was then added to the solution in (1) to obtain a gel precursor solution. Methyl red-methylene blue was also added as a pH indicator to the solution in (1) and stirred. The gel precursor solution was repeatedly frozen (-17 °C) and thawed (25 °C) to obtain a gel-like thermosensitive material. Two types of polyvinyl alcohol (PVA) were used: one with a degree of polymerization of 2000, a degree of saponification n = 86–90, and 98.5 mol%. The freezing time was 0.5–16 h, and the thawing time was 0.5 h.

[0121] Temperature and pH characteristic test results of gel precursor aqueous solution Figures 24 and 25 show the results of temperature-pH characteristic tests of the gel precursor aqueous solution. The gel precursor aqueous solution in Figure 24 was obtained under the following conditions: pH = 4.1, concentration = 234 mM, molar ratios of NIPAM:DMA:AAC = 95:0:5, 85:10:5, and 75:20:5, degree of saponification of polyvinyl alcohol (PVA) n = 86 to 90 mol%, and PVA concentration = 7.5 wt%. The gel precursor aqueous solution in Figure 25 was obtained under the following conditions: pH = 4.1, concentration = 234 mM, molar ratios of NIPAM:DMA:AAC = 95:0:5, 85:10:5, and 75:20:5, degree of saponification of polyvinyl alcohol (PVA) n = 98.5 mol%, and PVA concentration = 7.5 wt%.

[0122] Figures 24 and 25 show that the pH of the gel precursor aqueous solution containing the temperature-responsive polymer P(NIPAM-DMA-AAC) copolymerized with dimethylacrylamide (DMA) shifts to a higher temperature, regardless of the degree of saponification (n) of polyvinyl alcohol (PVA). It was confirmed that as the dimethylacrylamide (DMA) content in P(NIPAM-DMA-AAC) increases, the pH rise shifts to a higher temperature. This is because the lower critical solution temperature (LCST) of the temperature-responsive polymer P(NIPAM-DMA-AAC) increases due to the copolymerization of dimethylacrylamide (DMA). As the lower critical solution temperature (LCST) increases, the temperature at which the pH begins to rise shifts to a higher temperature.

[0123] Temperature pH characteristic test results for the temperature-sensitive material of the fourth example Figure 26 shows the results of a temperature / pH characteristic test of the thermosensitive material of Example 4. The thermosensitive material in Figure 26 was obtained by gelling the gel precursor aqueous solution in Figure 24. As shown in Figure 26, the color change temperature of the thermosensitive material shifted to a higher temperature as the molar polymerization ratio of dimethylacrylamide (DMA) increased. The color change temperature of the thermosensitive material containing the temperature-responsive polymer P (NIPAM-DMA-AAC) with a molar ratio of N-isopropylacrylamide (NIPAM):dimethylacrylamide (DMA):acrylic acid (AAC) of 75:20:5 rose to 50°C.

Claims

1. A temperature-sensitive material that contains a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, and polyvinyl alcohol (PVA), which are gelled together.

2. A temperature-sensitive material comprising a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, at least one of a sugar or an electrolyte, and polyvinyl alcohol (PVA), which are gelled together.

3. A temperature-sensitive material that contains a solvent, a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, borax (sodium tetraborate decahydrate) as a crosslinking agent, and polyvinyl alcohol (PVA), which are gelled together.

4. The temperature-sensitive material according to any one of claims 1 to 3, The temperature-sensitive material is characterized in that the temperature-responsive polymer is P(NIPAM-co-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AAC).

5. The temperature-sensitive material according to claim 1 or 3, the temperature-responsive polymer is P(NIPAM-XXX-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM), a monomer (XXX), and acrylic acid (AAC), A thermosensitive material characterized in that the monomer (XXX) is any one of acrylamide, N-alkylacrylamide, dimethylacrylamide (DMA), and a vinyl group having a primary, secondary, or tertiary amine.

6. A step of dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on temperature, a pH indicator, and polyvinyl alcohol (PVA) in a solvent to prepare a gel precursor aqueous solution; a step of subjecting the aqueous gel precursor solution to a freeze-thaw cycle in which the solution is frozen and then thawed to gelation; A method for producing a temperature-sensitive material, comprising:

7. A step of dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on temperature, a pH indicator, at least one of a sugar and an electrolyte, and polyvinyl alcohol (PVA) in a solvent to prepare a gel precursor aqueous solution; a step of subjecting the aqueous gel precursor solution to a freeze-thaw cycle in which the solution is frozen and then thawed to gelation; A method for producing a temperature-sensitive material, comprising:

8. A step of dissolving a temperature-responsive polymer that releases or absorbs hydrogen ions depending on the temperature, a pH indicator, borax (sodium tetraborate decahydrate) as a crosslinker, and polyvinyl alcohol (PVA) in a solvent to prepare a gel precursor aqueous solution; a step of subjecting the aqueous gel precursor solution to a freeze-thaw cycle in which the solution is frozen and then thawed to gelation; A method for producing a temperature-sensitive material, comprising:

9. The method for producing a temperature-sensitive material according to any one of claims 6 to 8, A method for producing a thermosensitive material, characterized in that at least one of the number of freeze-thaw cycles or the freezing time is adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

10. The method for producing a temperature-sensitive material according to any one of claims 6 to 8, A method for producing a temperature-sensitive material, characterized by using the polyvinyl alcohol (PVA) having a specific degree of saponification to produce a temperature-sensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

11. The method for producing a temperature-sensitive material according to any one of claims 6 to 8, A method for producing a thermosensitive material, characterized in that the mixing ratio of the polyvinyl alcohol (PVA) is adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

12. The method for producing a temperature-sensitive material according to any one of claims 6 to 8, The method for producing a temperature-sensitive material is characterized in that the temperature-responsive polymer is P(NIPAM-co-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM) and acrylic acid (AAC).

13. The method for producing a temperature-sensitive material according to claim 12, A method for producing a thermosensitive material, characterized by adjusting the copolymerization conditions of the P(NIPAM-co-AAC) to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

14. The method for producing a temperature-sensitive material according to claim 6 or 8, the temperature-responsive polymer is P(NIPAM-XXX-AAC) obtained by copolymerizing N-isopropylacrylamide (NIPAM), a monomer (XXX), and acrylic acid (AAC), The method for producing a temperature-sensitive material is characterized in that the monomer (XXX) is any one of acrylamide, N-alkylacrylamide, dimethylacrylamide (DMA), and a vinyl group having a primary, secondary, or tertiary amine.

15. The method for producing a temperature-sensitive material according to claim 14, A method for producing a thermosensitive material, characterized by adjusting the copolymerization conditions of the P(NIPAM-XXX-AAC) to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

16. The method for producing a temperature-sensitive material according to claim 7, A method for producing a thermosensitive material, characterized in that the mixing ratio of at least one of sugars and electrolytes is adjusted to produce a thermosensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

17. The method for producing a temperature-sensitive material according to claim 8, A method for producing a temperature-sensitive material, characterized by adjusting the mixing ratio of the borax (sodium tetraborate decahydrate) to produce a temperature-sensitive material having specified one or more of temperature characteristics, strength, and heat resistance temperature.

18. The method for producing a temperature-sensitive material according to any one of claims 6 to 8, a degassing operation is also carried out in the step of producing the gel precursor aqueous solution, Alternatively, a method for producing a temperature-sensitive material, characterized in that a degassing step of removing air bubbles contained in the gel precursor aqueous solution is included between the step of producing the gel precursor aqueous solution and the step of gelling.

Images