Preform material and method for producing same

Abstract

Provided is a technique with which it is possible to achieve a desired distribution of a refractive index in a radial direction in a core part of a POF. A core material, which is a preform material, and a shell material are prepared. The core material is a cylindrical object, and is constituted by dispersing a dopant in a base material of a thermoplastic transparent core material polymer, and has a specific refractive index due to a dopant. The shell material is a cylindrical object into which the core material is inserted, and is composed of a thermoplastic shell material polymer.

Description

Preform material and manufacturing method thereof

[0001] The present invention relates to a preform material and a manufacturing method thereof, and more particularly to a preform material as a material for a plastic optical fiber and a manufacturing method thereof.

[0002] Plastic optical fibers (hereinafter referred to as POFs), which are representative plastic optical materials, are made from plastic. Therefore, compared to silica-based optical fibers, POFs have advantages such as low cost, light weight, and excellent processability. A known method for manufacturing POFs is to provide two cladding layers, an inner cladding and an outer cladding, around a core in order to reduce bending loss (see, for example, Patent Document 1).

[0003] Japanese Patent Application Publication No. 2007-163910

[0004] However, the above-mentioned POF with two cladding layers is manufactured by sequentially polymerizing a monomer composition to form an outer cladding, an inner cladding therein, and a core therein. A POF manufactured by such multi-stage polymerization may not exhibit the desired optical properties. This is thought to be because the refractive index distribution of the base material (polymer) in each part changes due to differences in the composition at the time of production of each part, which affects the refractive index distribution in the radial direction of the core.

[0005] An object of one aspect of the present invention is to provide a technique that can realize a desired distribution of refractive index in the radial direction in the core portion of a POF.

[0006] In order to solve the above-mentioned problems, a method for manufacturing a preform material according to one embodiment of the present invention includes the steps of preparing a cylindrical core material composed of a base material of a thermoplastic and transparent core material polymer and a dopant dispersed in the base material, the core material having a specific refractive index due to the dopant, and a cylindrical sheath material into which the core material is inserted, the sheath material being composed of a thermoplastic sheath material polymer.

[0007] In addition, in order to solve the above-mentioned problems, a preform material according to one embodiment of the present invention includes a cylindrical core material that is transparent, has thermoplastic properties, contains a dispersed dopant, and has a specific refractive index, and a cylindrical sheath material into which the core material is inserted, the sheath material being composed of a thermoplastic polymer for the sheath material.

[0008] According to one aspect of the present invention, it is possible to provide a technique that can realize a desired distribution of refractive index in the radial direction in a POF.

[0009] [Preform Material] In this embodiment, the preform material is composed of a core material and a corresponding sheath material. The preform material is a set of a core material and a sheath material, and may be in the form of a core material or a group of core materials and a corresponding sheath material or a group of core materials, or may be in the form of a core material inserted into a corresponding sheath material.

[0010] The core material is cylindrical and the sheath material is cylindrical, and the core material is configured to be inserted into the sheath material.

[0011] The clearance between the core material constituting the preform material and the sheath material when inserted therebetween can be determined as appropriate, as long as it allows the core material to be inserted into the sheath material and allows the core material to be integrated by subsequent heating for smoothing during POF manufacturing. That is, if the clearance is too small, it may be difficult to insert the core material into the sheath material, while if the clearance is too large, the core material and the sheath material may not be sufficiently integrated during subsequent heating for smoothing, resulting in insufficient smoothing. From the above perspective, the clearance is preferably a gap large enough to allow the core material to fit inside the sheath material (without rattle). For example, the difference between the diameter of the core material and the inner diameter of the sheath material may be 1 μm or more and 100 μm or less, more preferably 1 μm or more and 50 μm or less.

[0012] The thickness of the sheath material relative to the diameter of the core material can be determined appropriately from the viewpoints of achieving the desired dopant distribution in the core material during smoothing in the subsequent POF manufacturing process and protecting the core material with the sheath material during subsequent drawing. If the sheath material is too thin, the desired dopant distribution in the core material during smoothing may not be achieved. On the other hand, since the sheath material in this embodiment does not directly contribute to the POF, an excessively thick sheath material is undesirable from the viewpoints of productivity and cost. From the above viewpoints, the ratio t2 / r1 of the sheath material thickness t2 to the core material radius r1 may be, for example, 0.425 or more and 0.5 or less.

[0013] The core material and the sheath material are used as POF materials in a subsequent POF manufacturing process, with the core material inserted into the sheath material. The lengths of the core material and the sheath material may be substantially the same as long as they are long enough to be used as POF materials. The lengths of the core material and the sheath material may be long enough to be used as POF materials as they are, or may be long enough to be cut to an appropriate length when used as POF materials.

[0014] The preform material may have additional features that indicate the combination of a specific core material and a corresponding specific sheath material. Examples of such features include a printed portion that displays a mark or description such as a model number that indicates the specific combination, and an indented portion such as an imprint that indicates the mark or description or the specific combination. Such features may be formed, for example, on the surfaces of the axial ends of the core portion and the sheath portion.

[0015] [Core Material] The core material is composed of a polymer matrix for the core material and a dopant dispersed in the matrix.

[0016] The core material polymer has thermoplastic properties and transparency. The core material polymer can be selected from known transparent thermoplastic resins. The core material polymer is preferably amorphous from the viewpoints of improving transparency and suppressing birefringence. The core material polymer may be one or more types. Examples of the core material polymer include polymethyl methacrylate (PMMA), polystyrene (PS), polytrifluoromethacrylate (P3FEMA), and polycarbonate (PC). Examples of the core material polymer also include polymers and copolymers of various monomers. Examples of such monomers include styrene, methyl methacrylate, 2,2,2-trifluoroethyl methacrylate, 4-methylcyclohexyl methacrylate, cyclohexyl methacrylate, furfuryl methacrylate, 1-phenylethyl methacrylate, 1-phenylcyclohexyl methacrylate, benzyl methacrylate, 1,2-diphenylethyl methacrylate, o-chlorobenzyl methacrylate, p-chlorobenzyl methacrylate, diphenylmethyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, isobornyl methacrylate, 2-hydroxyethyl methacrylate, and heptadecafluorodecyl methacrylate. Examples of the polymers include polymers of the above monomers. Examples of the copolymers include copolymers of methyl methacrylate and a monomer copolymerizable with methyl methacrylate among the above monomers, other than methyl methacrylate.

[0017] The dopant is a component that can impart a specific refractive index to the core material by dispersing it in the matrix. The dopant only needs to be mobile when the core polymer is in a flow region, and therefore has a sufficiently low molecular weight relative to the core polymer. One or more dopants may be used. Examples of dopants include various carboxylic acid esters, more specifically, phthalic acid esters, benzoic acid esters, phenylacetic acid esters, adipic acid esters, and sebacate esters.

[0018] Examples of phthalate esters include dimethyl phthalate, diethyl phthalate, diallyl phthalate, dibutyl phthalate, diisobutyl phthalate, di-n-hexyl phthalate, bis(2-ethylhexyl) phthalate, dioctyl phthalate, di-n-octyl phthalate, diisononyl phthalate, dinonyl phthalate, diisodecyl phthalate, bis-butylbenzyl phthalate, and butylbenzyl phthalate. Examples of benzoate esters include ethyl benzoate, propyl benzoate, benzyl benzoate, 4-biphenylyl benzoate, and phenyl benzoate. Examples of phenylacetic acid esters include methyl phenylacetate, ethyl phenylacetate, and phenylacetic acid chloride. Examples of adipate esters include bis(2-ethylhexyl) adipate, diisononyl adipate, diisodecyl adipate, and bis(2-butoxyethyl) adipate. Examples of sebacate esters include diisopropyl sebacate, diethyl sebacate, dioctyl sebacate, sebacoyl dichloride, dibutyl sebacate and bis-2-ethylhexyl sebacate.

[0019] The core material has a specific refractive index determined by the dopant. The core material will then become the light guide (core) in the POF. Therefore, the core material only needs to have a refractive index high enough to achieve the desired function of the POF. The refractive index of the core material can be appropriately determined taking into account the refractive index of the base material, which varies depending on the type. For example, for the D line of sodium, the refractive index is 1.400 or more from the above viewpoint, and 1.600 or less from the viewpoint of feasibility with the combination of the core material polymer and the dopant.

[0020] The refractive index of the core material can be determined by a known technique capable of measuring the refractive index of plastic products. For example, the refractive index of the core material can be measured by the refraction angle distribution polarimetry using an index profiler ("IP-5500" manufactured by Seiko E&G Corporation) at measurement wavelengths of 589.3 nm (D-line), 486.0 nm (F-line), and 656.3 nm (C-line). The refractive index of the core material can also be adjusted by the type of dopant or the content of the dopant in the core material.

[0021] [Sheath Material] The sheath material is integrated with the core material in the subsequent POF manufacturing process to form the outer layer of the integrated product, and is a component that can adjust the refractive index distribution in the radial direction of the core material by adjusting the distribution of dopants in the core material during integration. The sheath material is made of a sheath material polymer having thermoplasticity. The sheath material polymer may be one type or two or more types, and may be the same as or different from the core material polymer. Examples of the sheath material polymer are the same as the examples of the core material polymer described above.

[0022] The sheath material may be composed of a resin composition having a polymer for the sheath material as a matrix. For example, the above-mentioned dopant may be dispersed in the sheath material in order to adjust the refractive index of the sheath material or the absorption or emission of the dopant in the sheath material. It is preferable that the dopant in the sheath material also diffuses into the core material in order to form a refractive index distribution in the core material that enables image transmission. The dopant in the sheath material may be one or more types, and may be the same as or different from that in the core material. Examples of dopants for the sheath material are the same as the examples of dopants for the core material described above.

[0023] It is preferable that the refractive index of the sheath material is lower than that of the core material from the viewpoint of appropriately distributing the refractive index of the core material in the radial direction in the subsequent manufacturing process of the POF.

[0024] The difference (n1-n2) between the refractive index n2 of the sheath material and the refractive index n1 of the core material may be determined appropriately from the viewpoint of realizing a suitable distribution of refractive index in the radial direction when the POF is formed. The method for determining the refractive index of the sheath material is the same as that for the core material, and the method for adjusting the refractive index of the sheath material is also the same as that for the core material.

[0025] The sheath material does not need to be transparent, but is preferably transparent from the viewpoint of enabling visual or optical confirmation of the state of the core material or portions derived from the core material in the subsequent manufacturing process of the POF. On the other hand, the sheath material may be configured to be easily visually confirmed from the core material by, for example, containing a colorant. Such an easily visible sheath material is suitable from the viewpoint of facilitating and ensuring the removal of the portions corresponding to the sheath material in the subsequent manufacturing process of the POF.

[0026] The sheath material preferably has higher physical properties than the core material in order to protect the core material during the subsequent POF manufacturing process. For example, the sheath material preferably has a higher mechanical strength than the core material in order to protect the core material during drawing in the subsequent POF manufacturing process. Furthermore, the sheath material preferably has a higher heat resistance than the core material in order to protect the core material during heating in the subsequent POF manufacturing process. The mechanical strength and heat resistance of the sheath material can be improved by the type of polymer for the sheath material, by increasing the polymerization rate of the polymer for the sheath material, or by introducing a crosslinked structure.

[0027] The core material and sheath material may further contain other components in addition to those described above, as long as the effects of this embodiment are obtained. The other components may be employed in types and amounts that achieve both the effects of this embodiment and the effects of adding the other components. Examples of such other components include a third polymer formed by polymerizing a monomer that does not copolymerize with the monomer of the core material polymer or sheath material polymer, but is added in an appropriate amount similar to a dopant, separately from the core material polymer or sheath material polymer.

[0028] In this embodiment, it is preferable that the temperature of the flow region of the core polymer and the temperature of the flow region of the sheath polymer overlap at least partially, from the viewpoint of realizing a desired refractive index in the radial direction of the core material in the subsequent POF manufacturing process. Furthermore, from the viewpoint of suppressing changes in the core material and the sheath material due to heating in the POF manufacturing process, it is preferable that the temperature of the flow region of the core polymer and the sheath polymer overlap at least partially at a temperature below the thermal decomposition reaction temperature of these polymers. Note that, when the above polymer is a polymer that depolymerizes, the above thermal decomposition reaction temperature is the depolymerization temperature of the polymer. The depolymerization temperature is the temperature at which depolymerization begins.

[0029] [Method for manufacturing preform material] The preform material of this embodiment includes a step of preparing the above-mentioned core material and sheath material. In this embodiment, the core material and sheath material can be manufactured as described below. The core material and sheath material may be commercially available products or processed products thereof as long as they have the above-mentioned characteristics.

[0030] [Manufacturing of Core Material] The core material can be manufactured by forming the core polymer in a cylindrical closed vessel containing the thermoplastic and transparent core polymer material and the dopant. The vessel can be configured to include a circular tube with caps sealing both ends.

[0031] The core material polymer may be a polymerizable composition that produces a core material polymer. Such a core material polymerizable composition contains a monomer capable of constituting the core material polymer and a polymerization initiator. The monomer is selected appropriately depending on the core material polymer, and the polymerization initiator is selected appropriately depending on the monomer. The core material polymerizable composition may further contain additives that affect the structure of the resulting polymer, such as a chain transfer agent and a crosslinking agent, as needed. The chain transfer agent and crosslinking agent can be selected appropriately depending on the monomer and the desired physical properties of the resulting core material polymer.

[0032] Examples of the monomer include methyl methacrylate (MMA), styrene, 2,2,2-trifluoromethacrylate, 4-methylcyclohexyl methacrylate, cyclohexyl methacrylate, furfuryl methacrylate, 1-phenylethyl methacrylate, 1-phenylcyclohexyl methacrylate, benzyl methacrylate, 1,2-diphenylethyl methacrylate, o-chlorobenzyl methacrylate, p-chlorobenzyl methacrylate, diphenylmethyl methacrylate, pentachlorophenyl methacrylate, pentabromophenyl methacrylate, isobornyl methacrylate, 2-hydroxyethyl methacrylate, and heptadecafluorodecyl methacrylate.

[0033] Examples of the polymerization initiator include benzoyl peroxide (BPO), n-butyl-4,4-bis(t-butylperoxy)valerate, di-t-butyl peroxide, 1,1-bis(t-butylperoxybutane), 1,1-bis(t-hexylperoxy)3,3,5-trimethylcyclohexane, 1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, and t-butylperoxy 2-ethylhexyl monocarbonate. The amount of the polymerization initiator in the polymerizable composition for the core material cannot be generally determined, but may be, for example, 0.1 to 10×10 in terms of the amount of polymerization initiator per mole of monomer. －３ moles, 1 to 3 × 10 －３ It can be molar.

[0034] Examples of the chain transfer agent include n-butyl mercaptan (n-BM), n-octyl mercaptan, n-lauryl mercaptan, furfuryl mercaptan, n-decyl mercaptan, and undecyl mercaptan. The amount of the chain transfer agent in the polymerizable composition for the core material cannot be generally determined, but for example, the amount of the chain transfer agent per mole of the monomer is 0.01 to 10×10 －３ moles, and 0.1 to 3 × 10 －３ It can be molar.

[0035] Examples of the crosslinking agent include ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,6-hexanediol dimethacrylate, polybutylene glycol dimethacrylate, and trimethylolpropane trimethacrylate. The amount of the crosslinking agent in the polymerizable composition for the core material cannot be generally determined, but for example, the amount of the crosslinking agent relative to 1 mole of the monomer is 0.1 to 10×10 －５ moles, 1 to 3 × 10 －５ It can be molar.

[0036] A dopant is mixed with a polymerizable composition for the core material to obtain a raw material composition for the core material, which is then placed in the container and heated to a polymerization temperature while the container is held upright, thereby producing a cylindrical core material. The amount of dopant in the raw material composition for the core material cannot be generally determined, but is, for example, 1 to 10 × 10 in terms of the amount of dopant per mole of monomer. －２ It can be molar.

[0037] [Production of Sheath Material] The sheath material can be produced by rotating a cylindrical closed vessel containing a thermoplastic polymer material for the sheath material around its axis to produce the polymer for the sheath material. The vessel for producing the sheath material can have a configuration similar to that of the vessel for producing the core material.

[0038] Like the core material polymer, the sheath material polymer material may be a polymerizable composition that produces a sheath material polymer. Like the core material polymerizable composition described above, this sheath material polymerizable composition may contain a monomer and a polymerization initiator, and, if necessary, a chain transfer agent and a crosslinking agent. Examples of these components in the sheath material polymerizable composition include the same components as those exemplified in the core material polymerizable composition.

[0039] In the production of the sheath material, a dopant can be further added to the polymerizable composition for the sheath material to obtain a raw material composition for the sheath material, similar to the production of the core material. The inclusion of a dopant in the raw material composition for the sheath material is preferable from the viewpoint of appropriately controlling the radial distribution of the dopant in the core material in the subsequent POF production process. As mentioned above, it is preferable that the mechanical properties or thermal properties of the sheath material are higher than those of the core material, and from this viewpoint, the types and amounts of the monomer, chain transfer agent, and crosslinking agent used in the polymerizable composition for the sheath material can be appropriately determined. The amount of dopant in the raw material composition for the sheath material cannot be generalized, but it is preferably smaller than that of the core material, for example, 0 to 5 × 10 in terms of the amount of dopant per 1 mole of monomer. －２ It can be molar.

[0040] In the production of sheath material, a dopant is optionally mixed with the polymerizable composition for the sheath material to obtain a raw material composition for the sheath material, which is then placed in the container. The container is then laid down (with the axis of the container horizontal) and rotated around its axis at a sufficiently high speed so that the raw material composition for the sheath material is unevenly distributed around the periphery of the container and not around the axis. The raw material composition for the sheath material is then heated to a polymerization temperature, producing a cylindrical sheath material. Rotation of the container around its axis can be achieved by attaching the container to a rotating device that grips both ends of the container and rotates it around its axis, placing the rotating device in a heating chamber, and heating the container in the heating chamber while operating the rotating device.

[0041] [Remixing during viscosity increase] In this embodiment, in the production of both the core material and the sheath material, it is preferable to carry out the steps of polymerizing the monomer at a low polymerization temperature to obtain a fluid intermediate product, and inverting the container containing the intermediate product to cause the intermediate product to flow in the axial direction of the container, from the viewpoint of uniformly distributing the dopant in the core material or the sheath material.

[0042] The step of obtaining an intermediate product is a step of slowly reacting the raw material composition by mild heating to increase viscosity. The intermediate product is a liquid composition in which a portion of the monomers in the polymerizable composition described above has polymerized, resulting in a higher viscosity. The reaction temperature and reaction time in the step of obtaining an intermediate product can be appropriately determined depending on the types of monomer and polymerization initiator, within a range in which the intermediate product flows when the container is inverted.

[0043] The step of flowing the intermediate product in the axial direction of the container is a step of flowing the intermediate product widely to homogenize the composition of the intermediate product. The axial flow of the intermediate product may be performed once or multiple times. If the flow is performed multiple times, the flow may be performed continuously or intermittently. By flowing the intermediate product in the axial direction of the container by inverting the container, settling or floating of the dopant in the intermediate product is eliminated, and the dopant is uniformly distributed in the intermediate product.

[0044] In the production of the sheath material, it is preferable to polymerize the raw material composition for the sheath material while rotating the container at a sufficiently high speed around the axis of the container at the polymerization temperature as described above, from the viewpoint of enhancing the dispersibility of the dopant in the sheath material.

[0045] In both the core material and the sheath material, after the reaction of the polymer material is completed, the resulting polymer composition is removed from the container to obtain the core material and the sheath material. Because the internal composition of each of these core materials and sheath materials is substantially uniform, it is possible to obtain core materials and sheath materials that exhibit the specific refractive index set for each material.

[0046] [Applications] The core material and sheath material are heated and smoothed while inserted into the sheath material to develop a desired refractive index distribution in the radial direction of the core material. The core material is then stretched, and the sheath material is then scraped off to create a POF essentially consisting of the core material alone. Because the core material has a single-phase base material and the desired refractive index distribution is developed in this state, this POF has a refractive index distribution that is closer to the ideal distribution than conventional POFs. Therefore, the core material and sheath material of this embodiment are useful as materials for manufacturing POFs that can be used to transmit high-resolution images.

[0047] In manufacturing a POF, as long as a desired radial refractive index distribution is achieved, substantially only the portion corresponding to the sheath material may be removed as described above, or not only the sheath material but also the peripheral portion of the portion corresponding to the core material may be removed. In this case, the preform may be constructed by using appropriate types and amounts of a polymer for the core material, a dopant to be added to the core material, a polymer for the sheath material, and, if necessary, a dopant to be added to the sheath material, so that a desired refractive index distribution is achieved at least in a specific portion (e.g., the center) from which the peripheral portion of the portion corresponding to the core material has been removed.

[0048] [Summary] The preform material according to the embodiment of the present invention is composed of two layers: the aforementioned cylindrical core material and the cylindrical sheath material fitted thereon. As a result, the POF derived from the core material obtained from the preform material exhibits high image transmission capability.

[0049] Conventionally, techniques for multi-layering the core material have been studied to improve the image transmission capability of POF. Multi-layering of the core material is achieved by constructing the core material with a multi-layer structure consisting of a central cylindrical member and one or more cylindrical members arranged around the circumferential outside of the central cylindrical member. Multi-layering of the core material is suitable for adjusting the refractive index distribution in the radial direction of the core material, but when used as a POF, it may not achieve the desired image transmission capability.

[0050] This is thought to be due to light reflection or scattering occurring between each layer of the core material. The reason for this light reflection, etc., is thought to be that the components of each layer of the core material have different concentrations of dopant, and when manufacturing each component, even if the same resin is used as the base material, the refractive index of the base material varies slightly from layer to layer due to differences in the dopant concentration. This slight difference in refractive index is thought to be due to the fact that when a cylindrical core material portion is manufactured by polymerizing monomers while rotating around an axis, the centrifugal force caused by the rotation affects the difference in specific gravity due to the degree of polymerization during polymerization, causing the specific gravity on the outer periphery to become higher and the specific gravity on the inner periphery to become lower.

[0051] As described above, the preform material according to the embodiment of the present invention is composed of two layers, and therefore the core material is composed of a single material, and therefore the core material does not have a seam between the layers. Therefore, minute variations in refractive index due to the multi-layer structure of the core material do not occur, and as a result, when made into a POF, it is possible to achieve high image transmission capability.

[0052] In this embodiment, the dopant is appropriately distributed in the core material by the two-layer structure, thereby forming a desired radial refractive index profile in the core material. In this embodiment, the dopant is appropriately dispersed in the matrix (polymer) of the core material. Such appropriate dispersion of the dopant can be achieved by one or more of the following: migration (diffusion) of the dopant within the core material, migration from the core material to the sheath material, and migration of the dopant from the sheath material to the core material.

[0053] In this embodiment, since the core material essentially becomes a POF, it is possible to actively utilize the sheath material to develop the desired characteristics of the core material. For example, it is preferable for the sheath material to appropriately disperse the dopant in the core material, from the perspective of achieving appropriate dopant dispersion and a desired refractive index profile in the core material. The composition of such a sheath material (the type of base material and the type and amount of dopant) can be appropriately determined by referring to physical properties related to the dopant diffusibility in the base material, calculated values ​​based on computer simulations or actual measurements through experiments, including basic experiments. One preferred embodiment of a sheath material having suitable functions for the core material is one in which the sheath material is composed of the same components as the core material.

[0054] As described above, a first aspect of the present invention is a method for manufacturing a preform material, including the steps of preparing a cylindrical core material composed of a thermoplastic and transparent core polymer matrix and a dopant dispersed in the matrix, the cylindrical core material having a specific refractive index due to the dopant, and a cylindrical sheath material into which the core material is inserted, the cylindrical sheath material being composed of a thermoplastic sheath polymer. The first aspect can be used to achieve a desired radial refractive index distribution in a POF. Therefore, the first aspect can improve the image transmission capability of a POF when fabricated compared to a POF with a multilayer core material.

[0055] The second aspect of the present invention is the same as the first aspect, except that the method further comprises the step of producing the sheath material by rotating a cylindrical closed vessel containing the sheath material polymer around its axis to produce the sheath material polymer. The second aspect allows the molecular structure and material composition of the polymer in the sheath material to be appropriately adjusted, which is even more effective in terms of realizing a desired radial refractive index distribution in the POF.

[0056] In a third aspect of the present invention, the polymer material of the second aspect includes a polymer monomer, and the manufacturing process of each material includes a step of polymerizing the monomer at a low polymerization temperature to obtain a flowable intermediate product, and a step of inverting a container containing the intermediate product to flow the intermediate product in the axial direction of the container. The third aspect is more effective in terms of realizing a desired radial refractive index distribution in the POF, since the composition distribution of the material polymer is more uniform.

[0057] A fourth aspect of the present invention is the second or third aspect, wherein the polymer material contains a monomer of the polymer, and the components contained in the sheath material are the same as the components contained in the core material. This fourth aspect is even more effective from the viewpoint of appropriately and easily adjusting the physical properties of both the sheath material and the core material, such as the diffusibility of a dopant from the sheath material to the core material.

[0058] In a fifth aspect of the present invention, in any one of the first to fourth aspects, the temperature of the flow region of the core polymer at least partially overlaps with the temperature of the flow region of the sheath polymer, which is more effective in realizing a desired radial refractive index profile in the POF because it allows dopant migration between the core and sheath after the core and sheath are integrated in the subsequent POF manufacturing process.

[0059] A sixth aspect of the present invention is a preform material comprising a cylindrical core material, which is composed of a matrix of a thermoplastic and transparent core polymer and a dopant dispersed in the matrix, and which has a specific refractive index due to the dopant, and a cylindrical sheath material, which is composed of a thermoplastic sheath polymer, into which the core material is inserted. Like the first aspect, the sixth aspect can be used to achieve a desired radial refractive index distribution in a POF, and can also improve the image transmission capability of the POF compared to a POF with a multilayer core material.

[0060] According to the above-described embodiment, the present invention makes it possible to realize a desired distribution of the radial refractive index in a POF, thereby enabling transmission of high-resolution images through the POF, and is expected to be applied to medical applications such as ultra-thin endoscopes. The present invention, which has such effects, is expected to contribute to the achievement of, for example, Goal 3 of the Sustainable Development Goals (SDGs) advocated by the United Nations, "Good health and well-being for all."

[0061] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0062] An embodiment of the present invention will be described below.

[0063] [Example 1] [Core Material Production Example 1] The following components were mixed in the following amounts and filtered through a membrane filter to prepare a raw material composition 1 for the core material. Methyl methacrylate 3,350.0 mg (3.346 × 10 －２ mol) Benzyl benzoate 502.5 mg (2.2367 × 10 －３ mol) 1,1-bis(t-hexylperoxy)cyclohexane 33.5 mg (1.058 × 10 －４ mol) n-octyl mercaptan 12.8975 mg (8.816 × 10 －５ mol) Trimethylolpropane trimethacrylate 0.1675 mg (4.9496 × 10 －７mol)

[0064] Next, raw material composition 1 was placed in a cylindrical container having an inner diameter of 4 mm and a length of 350 mm, both ends of which could be sealed with caps, and caps were attached to both ends of the container.

[0065] Next, one end of the container was held, and the other end of the container was immersed in a water bath at 70 to 75°C, and heated for 1 to 1.5 hours.

[0066] Next, the container was removed from the hot water bath, and the thickened reaction liquid in the container was caused to flow in the axial direction of the container by inverting one end of the container and the other end of the container several times.

[0067] Next, one end of the container was grasped and the container was again submerged in the hot water bath to complete the reaction in raw material composition 1. Next, the cylindrical reaction product was observed, and a portion substantially free of bubbles generated by polymerization shrinkage was cut out. In this way, a cylindrical core material 1 made of transparent acrylic resin was obtained. The core material 1 had a diameter of 4 mm and a length of 30 to 200 mm.

[0068] [Production Example 1 of Sheath Material] The following components were mixed in the following amounts and filtered through a membrane filter to prepare raw material composition 2 for sheath material: Methyl methacrylate 8548.0 mg (8.5378 × 10 －２ mol) Benzyl benzoate 410.3 mg (1.933 × 10 －３ mol) 1,1-bis(t-hexylperoxy)cyclohexane 85.5 mg (2.7017 × 10 －４ mol) n-octyl mercaptan 32.9 mg (2.249 × 10 －４ mol) Trimethylolpropane trimethacrylate 0.4274 mg (1.263 × 10 －６ mol)

[0069] Next, raw material composition 2 was placed in a cylindrical container having an inner diameter of 6 mm and a length of 460 mm, both ends of which could be sealed with caps, and caps were attached to both ends of the container.

[0070] Next, one end of the container was held, and the other end of the container was immersed in a water bath at 70 to 75°C, and heated for 1 to 1.5 hours.

[0071] Next, the container was removed from the hot water bath, and the thickened reaction liquid in the container was caused to flow in the axial direction of the container by inverting one end of the container and the other end of the container several times.

[0072] Next, the container was placed on a rotating device in a heating chamber heated to 70 to 75° C. The rotating device was configured to hold both ends of the container to support the container horizontally and to be able to rotate the container around its axis.

[0073] Next, the container was heated in a heating chamber while rotating at 2000 rpm using a rotating device. This rotation caused raw material composition 2 in the container to be unevenly distributed on the inner circumferential surface of the container, forming a cylindrical cavity in the center of the container, and the reaction in raw material composition 2 was completed in this state. In this way, a transparent acrylic cylindrical sheath material 1 was obtained. The inner diameter of sheath material 1 was 4 mm, the thickness was 1 mm, and the length of sheath material 1 was 450 mm.

[0074] [Measurement of refractive index] The refractive indexes of the core material 1 and the sheath material 1 were measured. An index profiler ("IP-5500" manufactured by Seiko E&G Corporation) was used as the refractive index measurement device, and measurements were performed using the refraction angle distribution polarization function method under measurement wavelengths of 589.3 nm (D line), 486.0 nm (F line), and 656.3 nm (C line). As a result, the refractive index of the core material 1 was 1.5025 at the C line, 1.506 at the D line, and 1.513 at the F line, and the refractive index of the sheath material 1 was 1.4925 at the C line, 1.496 at the D line, and 1.502 at the F line.

[0075] The present invention can be used for POF materials that can be used to transmit images with higher resolution than conventional POF.

Claims

1. A method for manufacturing a preform material, comprising the steps of preparing a cylindrical core material composed of a base material of a thermoplastic and transparent core material polymer and a dopant dispersed in the base material, the cylindrical core material having a specific refractive index due to the dopant, and a cylindrical sheath material into which the core material is inserted, the cylindrical sheath material being composed of a thermoplastic sheath material polymer.

2. The method for producing a preform material according to claim 1, further comprising the step of producing the sheath material by generating the polymer for the sheath material while rotating a cylindrical closed container containing the polymer for the sheath material about its axis.

3. A method for producing a preform material according to claim 2, wherein the polymer material contains a monomer of the polymer, and the producing process includes: a step of polymerizing the monomer at a low polymerization temperature to obtain a flowable intermediate product; and a step of inverting the container containing the intermediate product to cause the intermediate product to flow in the axial direction of the container.

4. The method for producing a preform material according to claim 2 or 3, wherein the polymer material contains a monomer of the polymer, and the components contained in the material for the sheath material are the same as the components contained in the material for the core material.

5. The method for producing a preform material according to claim 1, wherein the temperature of the flow region of the core polymer at least partially overlaps with the temperature of the flow region of the sheath polymer.

6. A preform material comprising: a cylindrical core material, which is composed of a base material of a polymer for a core material having thermoplasticity and transparency, and a dopant dispersed in the base material, and which has a specific refractive index due to the dopant; and a cylindrical sheath material, into which the core material is inserted, which is composed of a polymer for a sheath material having thermoplasticity.