JPH0797526A - Material having variable modulus of elasticity - Google Patents

Abstract

PURPOSE:To obtain a material having variable modulus of elasticity useful for clutch and shock absorber of automobile, artificial. muscle, etc., showing volume phase transition caused by change of temperature in water, forming by dehydration to a specific ratio, having excellent handleability, safety and responsiveness. CONSTITUTION:This material having a variable modulus of elasticity is obtained by dehydrating a polymer get showing volume phase transition in water caused by temperature change, preferably one selected from an N-substituted alkyl(meth)acrylamide, its derivative, its copolymer, polyvinyl methyl ether, partially saponified substance of polyvinyl acetate, methyl cellulose and a copolymer of sodium allylacetate and acrylamide to <=35wt.% water content and shows a constant volume in a gas irrespective of temperature and a modulus of elasticity to cause reversible change at >=the phase transition temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material having a variable elastic modulus, and more particularly to a material having a variable thermoresponsive elastic modulus in which the elastic modulus of the material reversibly changes with temperature. INDUSTRIAL APPLICABILITY The elastic modulus variable material of the present invention can be applied to, for example, an automobile clutch, a device for transmitting or absorbing mechanical energy such as a shock absorber, a soft manipulator, an artificial muscle or the like.

[0002]

2. Description of the Related Art In a transportation device such as an automobile, a material capable of efficiently transmitting or absorbing energy is required for a coupling, a clutch, a damper, a shock absorber, an engine mount and the like. As such materials, conventionally, a combination of rigid materials and viscous fluids have been used for energy transmission, and a flexible material etc. have been used for energy absorption. However, these devices have complicated device configurations and are not always satisfactory in terms of smooth operation. Further, in the machine industry, in order to promote automation in a wider range of fields, a material capable of realizing more delicate and smooth movements such as movements of human fingers is desired. Although such a material is expected to be applied to the medical field as an artificial muscle, a material that operates efficiently and swiftly with a compact structure has not been obtained yet.

In recent years, a functional polymer material having an elastic modulus that can be changed in response to a stimulus such as electric or magnetic has been developed, and its application to each of the above fields is being studied. For example, as a material that is sensitive to an electrical stimulus, a material that is electrically polarized by the action of an electric field is dispersed in an insulating flexible polymer material (JP-A-3-91541), which is sensitive to a magnetic stimulus. A material in which a material that is magnetically polarized by the action of a magnetic field is dispersed in a flexible polymer material (JP-A-4-266970, JP-A-5-26970).
-25316 gazette) etc. are known.

Among the above, JP-A-3-91541 and JP-A-4-91541
266970 and the polymer material described in JP-A-5-25316,
Particles that are electrically polarized or magnetically polarized are indispensable, and a step of dispersing these particles in a polymer material, a step of applying an electromagnetic field, and the like are also required, resulting in high cost.
Further, in the electric field responsive elastic modulus variable material of Japanese Patent Laid-Open No. 3-91541, there is a restriction that the matrix polymer has to be electrically insulating, and it is necessary to consider safety when applying a high electric field. Become.

Recently, it has been reported that the elastic modulus of N-isopropylacrylamide (NIPA) -aqueous gel is responsive to thermal stimuli (J. Chem. Phys.,
95 (5), 3949-3957 (1991)). The NIPA-water-based gel is known to show a volume change as a phase transition, that is, critically with temperature. Specifically, when the transition temperature is exceeded, shrinkage accompanied by the release of water causes the transition temperature to change. When descending, swelling occurs with absorption of water. The above-mentioned report states that the NIPA-
It is reported that the elastic modulus of the water-based gel changes greatly.

The system according to the above report is obtained at a relatively low cost, and the elastic modulus changes several tens of times. But,
This change in elastic modulus is due to the liquid (here, water) during the phase transition.
Since it progresses with absorption and release of gel, sealing of the gel body is indispensable for practical use. Furthermore, since absorption and release of water are rate-determining steps, the size and shape of the gel are limited in order to enhance the responsiveness. Therefore, it is difficult to apply it to practical use of automobile parts and the like.

[0007]

In view of the problems of the prior art, it is an object of the present invention to provide a low cost material which has excellent handleability, is easy to manufacture and control, and has a reversible elastic modulus change. It is in.

[0008]

SUMMARY OF THE INVENTION The present inventors have developed a NIPA.
The behavior of a polymer gel showing a discontinuous volume change, that is, a phase transition represented by an aqueous gel, was carefully observed under various conditions. As a result, the gel in the contracted phase with the water content reduced below a specific value has a constant volume in gas (that is, a state of being taken out from water) regardless of temperature, In the temperature range, the inventors have found that the elastic modulus reversibly changes according to the temperature change, and completed the present invention.

[0009]

That is, the present invention provides the following elastic modulus variable material. (1) A polymer gel that exhibits a volume phase transition due to a temperature change in water is dehydrated to a water content of 35% by weight or less. In a gas, the volume is constant regardless of the temperature, and the elastic modulus is equal to or higher than the phase transition temperature. A variable elastic material that changes reversibly. (2) The polymer gel is selected from N-substituted alkyl (meth) acrylamides, their derivatives and copolymers thereof, partially saponified polyvinyl acetate, methyl cellulose, polyvinyl methyl ether, and copolymers of sodium allyl acetate and acrylamide. The elastic modulus variable material according to (1) above.

The polymer gel used in the present invention is
Generally, it is a polymer material having both a hydrophilic part and a hydrophobic part in the molecule and exhibiting a volume phase transition due to absorption / release of water at a phase transition temperature peculiar to the material. These polymeric materials
In the presence of water, at a temperature lower than the phase transition temperature, the hydrophilic part is dissolved in water by hydrogen bond, and the hydrophobic part is also dissolved in water by hydrophobic hydration, and thus is in a swollen state. However, when the temperature is raised above the phase transition temperature, hydrogen bonds are broken and dehydrated, and the association between the hydrophobic parts becomes predominant, resulting in a contracted state.

Materials having such properties include, for example, N-substituted alkyl (meth) acrylamides, their derivatives and copolymers thereof, partially saponified polyvinyl acetate, methyl cellulose, polyvinyl methyl ether, sodium allyl acetate and acrylamide. Examples thereof include gels such as copolymers.

The polymer gel used in the present invention must have a water content of 35% by weight or less. Three
By setting the water content to 5% by weight or less, the peculiar property that the elastic modulus reversibly changes within the range of not less than the phase transition temperature and 10 to several tens of degrees Celsius appears. Gels having a water content of more than 35% by weight have slow responsiveness. The water content of the polymer gel can be adjusted by the cross-linking density of the gel and the standing time during dehydration, and can be a water content within the range of the present invention. Dehydration is preferably performed near the phase transition temperature. When dehydration is performed at a temperature that is far from the phase transition temperature, the difference in the contraction speed between the surface and the inside is large, the discharge of water inside is insufficient, or the gel deforms and the desired shape cannot be obtained. .

The hydrogel of the present invention is used in a gas such as air. Since water absorbs water below the phase transition temperature in water, the water content cannot be within the above range.
Further, in water, the volume of the gel changes in the vicinity of the phase transition temperature due to the release and absorption of water, which makes it difficult to apply to automobile parts and the like. The elastic modulus change appears only above the phase transition temperature, but as long as it is used in a gas, even if the temperature is below the phase transition temperature, water absorption does not occur and there is no particular problem in use. Even if the temperature is below the phase transition temperature, when the temperature rises above the phase transition temperature, the elastic modulus changes again.

The phase transition temperature of the elastic modulus variable material of the present invention is
Depending on the structure of the material, it is usually in the range of 0 to 80 ° C. The phase transition temperature can be adjusted by a known method. Specific examples include (i) design of chemical structure of polymer constituting the material, (ii) organic or inorganic salt, auxiliary agent such as surfactant, and the like. Examples of (i) include changing the shape and length of the alkyl substituent in N-substituted alkyl (meth) acrylamide, changing the degree of saponification in polyvinyl acetate, and its copolymer composition in copolymer gel. A method such as changing the ratio can be considered. Regarding (ii), in the case of NIPA gel, the phase transition temperature generally rises when an alkali metal or alkaline earth metal halide or the like is added, and the addition amount when a surfactant such as sodium dodecyl sulfate is added. It is known that the phase transition temperature changes intricately according to the above (Polymer Processing, 39, 580-584, 1990). In the case of polyvinyl methyl ether, its phase transition temperature is usually 32.
Within the range of up to 40 ° C., the aqueous solution may be charged with, for example, inorganic salts such as sodium chloride and calcium chloride, organic salts such as sodium acetate and sodium citrate, and polymer electrolytes such as sodium alginate and sodium polyacrylate. The phase transition temperature can be lowered by adding it, and the phase transition temperature can be raised by adding water-soluble alcohols such as methanol, ethanol, ethylene glycol and glycerin or water-soluble ketones such as acetone. You can also let it.

The elastic modulus and the degree of change in the elastic variable material of the present invention can be controlled by appropriately selecting the water content and the crosslink density of the material. Basically, the lower the crosslink density or the higher the water content, the smaller the elastic modulus, but on the other hand, the larger the degree of change in elastic modulus becomes. As described above, the water content can be changed by the standing time during dehydration and the crosslink density. The crosslinking density can be adjusted according to a known method such as selection of a crosslinking agent.

If desired, in addition to the elastic modulus of the material, various known additives such as metal powder, iron oxide, carbon black and fiber are added in order to improve the performance such as strength, heat or electric conductivity. Fillers can also be added.

The elastic modulus variable material of the present invention can be applied in various ways as described above, but basically, it is an intelligent type material that changes elastic modulus while the material feels its own temperature. Alternatively, it can be used as a passive material in which the elastic modulus is controlled by positively changing the material temperature from the outside by using, for example, a heater.

[0018]

EXAMPLES The present invention will be described below with reference to examples, but the following are examples and do not limit the scope of the present invention. In addition, in the evaluation of the elastic modulus in Examples, a dynamic viscoelasticity measuring device Rheospectr DV
A change in elastic modulus was measured under the following conditions using E-4 (manufactured by Rheology Co., Ltd.). Measurement conditions: Temperature range 20-60 ℃ Temperature rising rate 0.5 ℃ / min Measurement mode Compressed measurement frequency Synthetic wave (1,2,4,8,16,32,64,128,256H
z) Amplitude 20-40μ Static load 20-40g Sample size Disc shape (diameter 13.0mm, thickness 4.1m
m)

Example 1 N, N'-methylenebisacrylamide (crosslinking agent) 0.00862 mol / l, ammonium persulfate (reaction initiator) was added to a 0.69 mol / l aqueous solution of poly-N-isopropylacrylamide.
0.00175 mol / l, N, N, N ', N'-tetramethylenediamine (reaction accelerator) 0.008 mol / l were added, and the polymerization temperature was 20.
A material in a swollen state was produced under conditions of ° C and a time of 16 hours.
The obtained material was left to stand in constant temperature water at 34.0 ° C for several days to cause volume contraction. The volume phase transition temperature of this product was 33.6 ° C. Remove the shrunken material from the water to further remove excess water and reduce the water content.
A 25.1 wt% variable temperature-sensitive elastic modulus material was prepared.

The temperature dependence of the elastic modulus E '(MPa) of this material was measured by a viscoelasticity measuring machine. Log in Figure 1
The relationship between E'and temperature is shown. From FIG. 1, it can be seen that the elastic modulus sharply increases in the range of the phase transition temperature and a few tens of degrees Celsius or higher. The change in elastic modulus is 100 times, and the maximum value of elastic modulus exceeds 1.0 MPa, and the elastic modulus of muscle (1.5 to 5M
Equivalent to Pa). Further, when the viscoelasticity measurement was repeated for the above gel both when the temperature increased and when the temperature decreased, it was confirmed that the elastic modulus reversibly changed with temperature change, although some hysteresis was observed.

Example 2 An aqueous solution of 0.575 mol / l of poly-N-isopropylacrylamide and 0.115 mol / l of dimethylacrylamide was added with N,
N'-methylenebisacrylamide (crosslinking agent) 0.00224m
ol / l, ammonium persulfate (reaction initiator) 0.00175 mol /
Add 0.008 mol / l of l, N, N, N ′, N′-tetramethylenediamine (reaction accelerator), polymerization temperature 20 ° C., time 1
A swollen copolymer material was produced under the conditions of 6 h. The obtained material was left in constant temperature water at 41.5 ° C. for several days to cause volume contraction. The water content and volume phase transition temperature were 28.5% by weight and 41.1 ° C, respectively. The material in the contracted state was treated in the same manner as in Example 1, and then the viscoelasticity was measured. The results are shown in Figure 2. Further, when the viscoelasticity measurement was repeated for both the temperature rise and the temperature fall of the gel, although some hysteresis was observed,
It was confirmed that the elastic modulus reversibly changes with changes in temperature.

[0022]

The hydrogel of the present invention is easy to manufacture because it is not necessary to internally add a magnetic substance, an electrically polarizable material or the like. In addition, it is excellent in handleability and safety because it does not require application in liquid or application of high voltage when used. Therefore, not only the manufacturing cost of the material itself is low, but also when it is used for various devices, it can be used in the air without using a sealing material, and the device configuration is simple and the cost can be kept low. Expected to be possible. Furthermore, since the change in elastic modulus that can be realized is large and the response is excellent, and there are few restrictions on the size and shape, it is particularly useful for application as an energy conducting / absorbing material or a mechanochemical material.

[Brief description of drawings]

FIG. 1 is a diagram showing the temperature dependence of elastic modulus (E ′), which was determined by a viscoelasticity measuring instrument for a temperature-sensitive elastic modulus variable material obtained in Example 1 of the present invention.

FIG. 2 is a diagram showing the temperature dependence of the elastic modulus (E ′), which is obtained by the viscoelasticity measuring instrument for the temperature-sensitive elastic modulus variable material obtained in Example 2 of the present invention.

Claims (2)

[Claims] 1. A polymer gel which exhibits a volume phase transition due to a temperature change in water is dehydrated to a water content of 35% by weight or less, and has a constant volume in a gas regardless of temperature and an elastic modulus of at least the phase transition temperature. A variable elastic material that changes reversibly with temperature. 2. A polymer gel comprising an N-substituted alkyl (meth) acrylamide, a derivative thereof and a copolymer thereof, polyvinyl methyl ether, a partially saponified product of polyvinyl acetate, methyl cellulose, and a copolymer of sodium allyl acetate and acrylamide. The elastic modulus variable material according to claim 1, which is selected from polymers.