JP2765505B2 - Thermosensitive reversible display medium and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-sensitive display medium for displaying and erasing a record by controlling the application of heat, which is excellent in display quality and can be used as a heat-sensitive paper or an OHP sheet repeatedly used. The present invention relates to a thermal display medium.

[0002]

2. Description of the Related Art Conventionally, paper has been used for a long time as a medium for displaying and storing information with an excellent interface with humans, and even in the paperless era accompanying the progress of OA in recent years, paper has been used. The amount does not decrease,
Rather, it is increasing. This indicates that paper has an excellent interface to humans in processing information even in an era of information overflow. However, some information is used for temporary confirmation,
It contains a lot of temporary information that is no longer needed,
Valuable paper resources are wasted as these information is handled and printed in the same way as information that requires storage. For information transmission at recent conferences and conferences, presentations using an overhead projector (OHP) and the like have been widely performed, and a plastic film or the like having a toner image formed on a polyester film used at that time is not available. Usually not reused. In the processing of such information, consuming a large amount of paper or plastic wastes resources and is regarded as a problem because it causes deterioration of the environment.

[0003] Under such circumstances, many reversible recording display materials that can be recorded and stored on a sheet and that can be repeatedly used have been proposed in various systems. Among these, a thermal recording method that can reduce the size of a display device and does not discharge consumables has attracted attention. As this heat-sensitive recording technique, a method of displaying a cloudy state and a transparent state by controlling the heat applied to a recording layer in which an organic low-molecular compound is dispersed in a polymer compound base material (Japanese Patent Application Laid-Open No. 1
No. 19377), a method of performing display by controlling the phase separation between polymers by applying heat to a recording layer formed by blending a plurality of polymer compounds (JP-A-60-1).
80887), a method of displaying a cloudy state and a transparent state by controlling heat applied to a recording layer made of a high-molecular liquid crystal compound (Japanese Patent Application Laid-Open No. 2-117888), and a method of using a leuco dye and a color-reducing agent. A method for controlling the heat applied to the used recording layer to perform color development / decoloration
191190) and the like. In addition, a technique of performing recording and erasing using a heat and an electric field on a recording layer in which a polymer liquid crystal is dispersed in a resin in a granular form (JP-A-3-284)
988) is also known.

[0004]

However, in a reversible thermosensitive display medium having a recording layer of a film in which an organic low-molecular compound is dispersed in a high-molecular compound base material and a film in which a plurality of polymers are blended, recording and recording are not possible. There has been a problem that the mixed components are deteriorated due to repetition of the erasing, the contrast is lowered due to the blurring of the boundary between the cloudy state and the transparent state, and the number of repetitions is limited. Further, the contrast is still insufficient in a coloring / erasing type reversible thermosensitive recording medium using a leuco dye, and there is a problem in the number of repetitions due to deterioration due to the coloring / erasing reaction. Further, in the conventional reversible display medium using a polymer liquid crystal, display contrast is low, and there is a problem in storage stability of recording and heat sensitivity. Furthermore, when a display medium using a polymer liquid crystal as a recording layer is manufactured, the polymer liquid crystal is plastic, so that the surface of the recording layer is deformed due to heat and printing pressure by a thermal head during recording. This causes a problem that the number of repetitions is limited. When a surface protective layer having high hardness and heat resistance is laminated on the recording layer to prevent the surface deformation of the recording layer due to the thermal head, the recording layer is oriented by volume shrinkage of the protective layer when the surface protective layer is formed. There was also the problem of becoming transparent. Therefore, the present invention has been made in view of the above-mentioned problems in the conventional technology. That is, an object of the present invention is to provide a reversible display medium having excellent durability against repetition, heat sensitivity, display contrast, and storage stability. Another object of the present invention is to
It is an object of the present invention to provide a thermosensitive display medium which is excellent in recording and erasing display quality and can be used as a thermosensitive paper and an OHP sheet which can be used repeatedly.

[0005]

The thermoreversible display medium of the present invention comprises a thermosensitive reversible recording medium in which a heat-sensitive recording layer is formed on a support in which a transparent state and a cloudy state are reversibly repeated by controlling the application of heat. In the thermosensitive recording medium, the heat-sensitive recording layer is made of a side chain type polymer liquid crystal and an ultraviolet curable resin or an electron beam curable resin formed of an interpenetrating polymer.
And a composite material forming a network. Further, the method for producing a thermosensitive reversible display medium of the present invention comprises, on a support, a solution of a composition comprising a side chain type polymer liquid crystal and an ultraviolet-curable or electron beam-curable oligomer or monomer or a mixture thereof. A film is formed by coating, the composition is heated to a temperature at which the composition exhibits liquid crystallinity, then cooled, and then irradiated with ultraviolet light or an electron beam to cause a crosslinking reaction of the resin to form a heat-sensitive recording layer. Is formed.

[0006] The thermosensitive reversible display medium of the present invention comprises a substrate and a thermosensitive recording layer.
It is composed of a polymer liquid crystal and a composite material in which an interpenetrating polymer network (hereinafter abbreviated as “IPN”) of an ultraviolet curable resin or an electron beam curable resin is formed. Therefore, first, the polymer liquid crystal used in the present invention will be described. As the polymer liquid crystal, there are known a main chain type polymer liquid crystal having a mesogen group (a molecule exhibiting liquid crystallinity) in a main chain and a side chain type polymer liquid crystal having a mesogen group bonded to a side chain. In the invention,
A side-chain type polymer liquid crystal is used. The side-chain type polymer liquid crystal of the present invention has, in addition to the side-chain type polymer liquid crystal in which a mesogen group is bonded to the side chain, a domain diameter distribution of a multi-domain structure constituting a cloudy state, printing characteristics, and storage of records. For the purpose of controlling the thermal characteristics of the polymer liquid crystal that determines the temperature,
Furthermore, it is also preferable to copolymerize or co-add a non-mesogenic compound.

Specific examples of the above non-mesogenic compounds include alkyl (meth) acrylate and its derivatives, styrene and its derivatives, (meth) acrylonitrile,
Examples thereof include vinyl chloride, vinylidene chloride, imprene, vinylpyrrolidone, 1-hexene, and 1-octene.
In addition, to improve the heat resistance and mechanical strength of polymer liquid crystals,
It is also preferable to copolymerize or co-add a non-liquid crystalline monomer having a hydrogen bonding group. Examples of the non-liquid crystalline monomer component having such a hydrogen bonding group include various groups having a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphoric acid group, a primary or secondary amino group, an acid amide group, a thiol group and the like. Polymerizable monomers can be used. Specific examples thereof include 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxy-3-phenoxypropyl (meth) acrylate, and 2- (meth) acryloyloxy. Ethyl succinate, 2- (meth) acryloyloxyethyl phthalate, 2- (meth) acryloyloxyethyl-
2-hydroxyethyl phthalate, 2- (meth) acryloyloxyethyl hexahydrophthalate, 4-
((Meth) acryloxyalkyloxy) -benzoic acid, 2- (meth) acryloyloxyethyl acid phosphate, glyceryl (meth) acrylate, hydroxy-substituted styrene, (meth) acrylic acid, vinyl sulfonic acid, (meth) Acrylamide, 2-propen-1-ol, 5-hexen-1-ol and the like are typical examples. Among these, particularly, a polymerizable monomer having an acidic group such as a carboxyl group, a sulfonic acid group or a phosphoric acid group and a polymerizable monomer having an alcoholic or phenolic hydroxy group can obtain a high hydrogen bonding force. Therefore, it is preferably used. In addition, each of the above-mentioned non-liquid crystalline monomer compounds may be used in a plural number.

The side chain type polymer liquid crystal can be usually produced by polymerizing a polymerizable mesogen compound or adding a mesogen compound capable of undergoing an addition reaction to a reactive polymer such as hydrogenated polysilicon. . Such a technique is described in Makromol. Chem. , 179, p2
73 (1978), Eur. Polym. J. , 18,
p651 (1982), Mol. Cryst. Liq.
Cryst. , 169, p167 (1989). The side chain type polymer liquid crystal used in the present invention,
It can be manufactured in the same manner as in the above method. Examples of the polymerizable mesogen compound and the mesogen compound capable of addition reaction include biphenyl, phenylbenzoate, cyclohexylbenzene, azoxybenzene, azobenzene, azomethine, phenylpyrimidine, diphenylacetylene, and biphenylbenzoate compounds. And various compounds in which an acrylate, methacrylate or vinyl group is bonded to a rigid molecular structure such as cyclohexylbiphenyl or terphenyl, preferably via an alkyl spacer having a predetermined length. It is mentioned as a thing.

Representative structural examples of these compounds are shown below. These are typical structural examples, and the present invention is not limited to these. _{CH 2 = C (R) -COO-} (CH 2) m -O-A CH 2 = C (R) -COO-A CH 2 = CH- (CH 2) m -O-A ( wherein, R Represents a hydrogen or methyl group, and m is 1 to 30
And A represents an integer selected from the following formulas (a) to
(K) represents a liquid crystal molecule (mesogen group). )

[0010]

Embedded image (Wherein X and Y are each a single bond, -N = N-,-
N (→ O) = N-, -CH = N-, -N = CH-, -C
OO -, - C (C = O) -, - CH = CH- represents a group selected from the group, R ¹ is an alkoxy group, a halogen atom,
Represents a group selected from a cyano group, a carboxyl group, and an alkyl group, and n represents an integer selected from 1 to 5,
When n is 2 or more, R ¹ may be different from each other. In addition, a plurality of types of mesogen groups can be used.

The amount of the non-mesogenic compound which is copolymerized or co-added to the polymer liquid crystal of the present invention is 0.1 to 0.1 as a monomer unit.
It is preferably in the range of 1 to 80 mol%, more preferably 1 to 80 mol%.
The range of ５０50 mol% is preferred. When the amount of copolymerization or co-addition is less than 0.1 mol%, the effect of copolymerization or co-addition is hardly exhibited, and when it is more than 80 mol%, no liquid crystal is exhibited. As the form of the copolymerization reaction, various forms such as random copolymerization, block copolymerization, graft copolymerization and alternating copolymerization can be adopted. The weight average molecular weight of the polymer liquid crystal is 1,000 to 10
It is preferably in the range of 10,000, particularly in the range of 10,000 to 500,000.

The thermosensitive recording layer containing the liquid crystal polymer used in the present invention forms a multi-domain structure at room temperature. The multi-domain structure is a structure composed of a number of minute regions having optical anisotropy, and a mesogen group is oriented in substantially the same direction inside one domain. In order to improve the light scattering property, it is desirable that the orientation direction of each domain is random. Also, it is desirable that the size of each domain is about the wavelength of visible light. Specifically, the maximum value of the number distribution of the domain diameter is 3 μm.
m, preferably 1 μm or less, more preferably 10 μm or less.
The range is from 0 to 600 nm. The difference between the minimum domain diameter and the maximum domain diameter is 3 μm or less, preferably 1 μm.
m or less, more preferably about 500 nm or less. Those having a multi-domain structure having such a distribution range have high turbidity because they strongly scatter visible light wavelengths.

Next, the composite material forming the IPN of the ultraviolet curable resin or the electron beam curable resin used for the heat-sensitive recording layer of the present invention will be described. The above-mentioned IPN, that is, an interpenetrating polymer network refers to a state in which a network of two or more kinds of polymer compounds forms a structure in which the networks are intertwined with each other without being chemically bonded. It is called an interpenetrating polymer network, an interpenetrating network or a multi-network polymer. IPN has the advantage that morphology can be controlled which is difficult with a simple blend of polymer compounds, by forming a network and forcibly micromixing to combine polymer compounds having poor compatibility. In the present invention, by using the reversible thermosensitive recording layer having such an IPN structure, the durability of the recording layer against mechanical stress is improved, and the unevenness during recording by the thermal head is prevented. Since the transparency due to the orientation at the time of forming the protective layer can be prevented, the surface protective layer having high hardness and high heat resistance can be used, and the durability can be significantly improved.

In the above-described method for forming a heat-sensitive recording layer having an IPN structure, the side chain type polymer liquid crystal and a mixture of a UV-curable or electron beam-curable resin monomer or oligomer and a polymerization initiator are essential components. After coating the solution containing the solution on a substrate to form a film and drying it, the polymer liquid crystal is heated to a temperature at which a liquid crystal phase is exhibited, and the opacity is increased by growing the domain diameter by annealing,
Next, irradiation of ultraviolet rays or electron beams causes a crosslinking reaction of the resin to form an IPN. Examples of the polymerizable monomer or oligomer capable of forming an IPN by irradiation with an ultraviolet ray or an electron beam include pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, trimethylpropane triacrylate, and hexanediol-1,6-diene. Methacrylate, triethylene glycol diacrylate, tricyclodecane dimethylol diacrylate, polypropylene glycol diacrylate, neopentyl glycol diacrylate, tris (acryloxyethyl)
Mention may be made of isocyanurates or mixtures thereof. The ratio of the above-mentioned ultraviolet curable resin or electron beam curable resin in the thermosensitive recording layer is preferably in the range of 1 to 20% by weight, more preferably 3 to 10% by weight. If it is less than 1% by weight, sufficient repetition durability cannot be exhibited, and if it is more than 20% by weight, it is difficult to obtain sufficient turbidity.

Various components can be added to the heat-sensitive recording layer of the present invention for the purpose of improving its properties. For example, various antioxidants such as hindered amine and hindered phenol may be added for the purpose of improving weather resistance, and anthraquinone-based, styryl-based, azomethine-based or azo-based for the purpose of improving display contrast. May be added. Further, it is preferable to add various infrared absorbing dyes such as phthalocyanine, squarylium, and azurenium in order to efficiently perform thermal writing with a semiconductor laser. The amounts of the various components mentioned above are 0.0% in the heat-sensitive recording layer.
It is preferable to add in the range of 1 to 5% by weight.

Next, a method of recording and erasing a phase change type on the thermosensitive reversible display medium of the present invention will be described. In the present invention, a transparent recorded image is formed on the thermosensitive recording layer in the cloudy state (light scattering state) at the beginning. The method of forming a transparent recorded image is achieved by controlling the orientation state and order of the mesogen group of the polymer side chain by applying heat, an electric field and a magnetic field alone or in combination. For example, by using a thermal head or laser light to heat a part of the cloudy state of the thermosensitive recording layer containing the polymer liquid crystal to a temperature higher than the liquid crystal phase-isotropic phase transition temperature, the polymer liquid crystal becomes transparent. (Isotropic state), and then rapidly cooled, so that the glass is fixed in a glassy state while isotropic. In this state, since the arrangement of the mesogen groups in the side chains is random at the molecular level, the mesogen group becomes transparent without scattering light. On the other hand, when erasing the recording, the entire reversible thermosensitive display medium is heated to a temperature equal to or higher than the liquid crystal phase-isotropic phase transition temperature using a heat roller or a hot stamp, and then gradually cooled, whereby the entire thermosensitive recording layer becomes cloudy. And return to the initial cloudy state. In the thermosensitive reversible display medium of the present invention, the durability of the thermosensitive recording layer is maintained even when such recording and erasing are repeated 100 times or more.

The structure of the reversible thermosensitive display medium will be described. 1 and 2, 1 is a substrate, 2 is a thermosensitive recording layer, 3 is a surface protective layer, and 4 is a light reflecting layer. FIG.
1 shows a configuration example of a transmission type reversible thermosensitive display medium of the present invention, in which a substrate 1, a thermosensitive recording layer 2 and a surface protective layer 3 are sequentially laminated. FIG. 2 shows a configuration example of a reflection type reversible thermosensitive display medium of the present invention, in which a base material 1, a light reflection layer 4, a thermosensitive recording layer 2, and a surface protection layer 3 are sequentially laminated. is there. As the above substrate,
Various polymer films such as polyvinyl chloride, polyester, polypropylene, and polyimide, paper, metal, ceramic, and glass can be used, but when used as a transmission type reversible thermosensitive display medium, particularly, a transparent substrate Is preferably used, and when used as a reflective reversible thermosensitive display medium, it is preferable to use a colored base material, a base material that reflects light such as a metal, or the like. When used as a reflective reversible thermosensitive display medium, it is preferable to interpose a light reflecting layer or a light absorbing layer between the substrate and the thermosensitive recording layer that repeats a cloudy state and a transparent state. In particular, when a light-absorbing layer is interposed, or when a colored substrate is used, for the purpose of improving the display contrast, the heat-sensitive recording layer is further disposed between the light-absorbing layer or the colored substrate and the heat-sensitive recording layer. It is also preferable to form a layer having a low refractive index. The thickness of the above-mentioned heat-sensitive recording layer is in the range of 1 to 50 μm, preferably 3 to 20 μm.
m. If the thickness is 1 μm or less, a sufficient contrast cannot be obtained.
Above, the energy required for recording is increased, and the repetition durability is significantly reduced. As the surface protective layer, those having high heat resistance are desirable, and a fluorine-based polymer, a silicon-based polymer, a thermosetting polymer, an ultraviolet-curable polymer, an electron beam-curable polymer, or the like can be used. May be. The thickness of the protective layer is
It is preferably selected from the range of 0.1 to 20 μm.

[0018]

EXAMPLES The present invention will now be described specifically with reference to examples, but the present invention is not limited to these examples. (Recording / Erasing on Reversible Thermosensitive Display Medium and Evaluation) A 100 μm-thick aluminum-evaporated PET was used as a substrate to form a reflective display medium. In order to record an image on a reversible thermosensitive display medium, a thermal head printer thermographic recording evaluation device (about 0.3 mJ / do at a resolution of 8 dots / mm) is used.
(recorded at a sensitivity of t). The recorded portion of this image was transparent. On the other hand, this recorded image could be erased by passing through a heat roller (125 ° C., 2 cm / sec). The reflection optical density (OD value) of the recorded image was X-rite968 (Xr
It is measured by using the product (manufactured by ITE Co., Ltd.).
The test was carried out using an A4 size display medium, disappeared, and the occurrence of surface cracks and the like was observed. The results are shown in Examples and Comparative Examples.

Example 1 19 g of 4-acryloxyhexyloxy-4'-cyanobiphenyl as a mesogenic monomer and 1 g of cyclohexyl acrylate as a non-liquid crystalline monomer, and azobisisobutyronitrile (AIB) as a polymerization initiator
N) 20 mg, 30 ml of tetrahydrofuran as a solvent
Was used to carry out polymerization. The obtained polymer solution is purified by repeatedly precipitating twice using ethyl alcohol as a precipitating solvent, and purified to obtain a high-molecular liquid crystal (weight average molecular weight: 50,000: in terms of polystyrene by GPC, Tg = 35 ° C, Ti = 105 ° C).
1.9 g were obtained. Ultraviolet curable composition containing epoxy acrylate as a main component (trade name: Aronix UV3700, manufactured by Toa Gosei Co., Ltd.) with respect to 1.0 g of the polymer liquid crystal.
A solution obtained by adding 0.05 g, Darocure 1173 (manufactured by Ciba-Geigy Corporation) as a polymerization initiator, and 3.0 g of methyl ethyl ketone as a solvent was added to aluminum-evaporated PET.
The film was coated on the film using a blade coater and dried to form a layer containing a polymer liquid crystal composition having a thickness of about 6 μm. Next, this layer is heat-treated at 80 ° C. for 2 minutes, and after cooling, a high-pressure mercury lamp (with a hot-wire filter, 50 mW)
To form a heat-sensitive recording layer. Further, the above-mentioned ultraviolet-curable composition was applied on the heat-sensitive recording layer, and was cured using a high-pressure mercury lamp to produce a cloudy reversible thermosensitive display medium having a surface protective layer having a thickness of 2 μm. When the obtained reversible thermosensitive display medium was measured using the above device, the O.D. D.
The value was 0.30, and the O.D. D. The value was 1.60. On the other hand, the O.D. D. The value was 0.31 and the O.D.
D. The value was 1.58. From the comparison of these measured values, the display medium after recording and erasing were repeated 100 times,
Almost no deterioration was seen from the initial one. Also,
In this case, almost no cracks in the surface protective layer were observed for the A4 size display medium.

Example 2 As the polymer liquid crystal, the one synthesized in Example 1 was used, and an ultraviolet curable composition (trade name: Aronix UV37) was used.
00, manufactured by Toa Gosei Co., Ltd.) was prepared in the same manner as in Example 1 except that the added amount was 0.10 g. When the measurement was performed in the same manner as in Example 1, the O.D. D. The value was 0.33, and the O.D. D. The value was 1.58. On the other hand, the O.D. D. The value was 0.34, and the O.D. D. The value was 1.57. From the comparison of these measured values, the display medium after recording and erasing was repeated 100 times showed almost no deterioration from the initial one.
Also, cracks in the surface protective layer were hardly observed in the display medium having the size of A4 size.

Example 3 As the polymer liquid crystal, the one synthesized in Example 1 was used, and an ultraviolet curable composition (trade name: Aronix UV37) was used.
00, manufactured by Toa Gosei Co., Ltd.) was prepared in the same manner as in Example 1 except that the amount of addition was 0.20 g. When the measurement was performed in the same manner as in Example 1, the O.D. D. The value was 0.37, and the O.D. D. The value was 1.57. On the other hand, the O.D. D. The value was 0.37, and the O.D. D. The value was 1.56, and almost no deterioration was observed as compared with the initial stage. Also, cracks in the protective layer were hardly observed even in a display medium of A4 size.

Comparative Example A solution obtained by adding 3.0 g of methyl ethyl ketone as a solvent to 1.0 g of the polymer liquid crystal synthesized in Example 1 was applied on an aluminum-deposited PET film using a blade coater, and dried. A layer made of a polymer liquid crystal composition having a thickness of 6 μm was formed. Next, an ultraviolet curable composition (trade name: Aronix UV3700, manufactured by Toagosei Co., Ltd.) is applied on the heat-sensitive recording layer, and cured using a high-pressure mercury lamp to form a surface protective layer having a thickness of 2 μm. A display medium was manufactured. When the measurement was performed in the same manner as in Example 1, the O.D. D. The value was 0.33, and the O.D. D. The value was 1.60. On the other hand, when repetition of recording and erasing was performed 10 times, cracks in the surface protective layer were generated, and the cracks grew, and the recording image quality was greatly deteriorated.

[0023]

According to the present invention, since the reversible thermosensitive display medium has a thermosensitive recording layer having an IPN structure, the durability of the thermosensitive recording layer against mechanical stress is improved. Deformation can be prevented. Also, since the polymer liquid crystal can be prevented from becoming transparent when it is made isotropic when forming the surface protective layer, it is possible to use a surface protective layer having a high hardness and a high heat-resistant temperature, so that the repetition durability is greatly increased. Can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a configuration example showing a reversible thermosensitive display medium of the present invention.

FIG. 2 is a sectional view of a configuration example showing another reversible thermosensitive display medium of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Base material, 2 ... Thermal recording layer, 3 ... Surface protective layer, 4 ... Light reflection layer.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takashi Uematsu 1600 Takematsu, Minamiashigara-shi, Kanagawa Fuji Xerox Co., Ltd. (56) References JP-A-3-155525 (JP, A) JP-A-6-138428 ( JP, A) JP-A-5-100214 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B41M 5/36 G02F 1/133 565 G02F 1/1333

Claims (6)

(57) [Claims] 1. A thermosensitive reversible display medium having a heat-sensitive recording layer formed on a support by reversibly repeating a transparent state and a cloudy state by controlling application of heat, wherein the heat-sensitive recording layer is a side-chain type. A thermosensitive reversible display medium comprising a polymer liquid crystal and a composite material in which an ultraviolet curable resin or an electron beam curable resin forms an interpenetrating polymer network. 2. The thermoreversible material according to claim 1, wherein the interpenetrating polymer network is formed by a crosslinking reaction of an ultraviolet-curable or electron-beam-curable oligomer or monomer. Display medium. 3. The thermosensitive reversible display medium according to claim 1, wherein the proportion of the ultraviolet curable resin or the electron beam curable resin in the thermosensitive recording layer is in the range of 1 to 20% by weight. 4. The thermosensitive reversibility according to claim 1, wherein the side chain type polymer liquid crystal in the thermosensitive recording layer is a copolymer of at least a monomer having a liquid crystalline group and a non-liquid crystalline monomer. Display medium. 5. The thermosensitive reversible display medium according to claim 1, wherein a surface protective layer is formed on the thermosensitive recording layer. 6. A film is formed by applying a solution of a composition comprising a side-chain type polymer liquid crystal and a UV-curable or electron beam-curable oligomer or monomer or a mixture thereof on a support. After heating to a temperature at which the composition exhibits liquid crystallinity, the composition is cooled, and then irradiated with ultraviolet rays or an electron beam to cause a crosslinking reaction of the resin to form a thermosensitive recording layer. Method for producing an opaque display medium.