JPH0585881B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for molding a synthetic resin optical element having a refractive index distribution that changes in the axial direction.

<Prior art> In recent years, it has been proposed to correct aberrations of spherical lenses by creating a refractive index distribution in the optical axis direction. For example, in a convex lens formed by a spherical and flat refractive surface, The refractive index distribution in the optical axis direction is expressed by the following equation (1) or (2), n(z)=n _p (1-tz) (1) n(z)=n _p √1- (2 ) (In the formula, n(z) is z in the optical axis direction from the center of the spherical surface.
, n _p is the refractive index at the center of the spherical surface, t is a positive constant, and z is the distance from the center of the spherical surface in the optical axis direction), then the spherical aberration of the lens is Theoretically, it has been shown that this can be dramatically reduced.

The above-mentioned axially graded index lens generally has a three-dimensional distribution in which all of the equirefractive index surfaces are perpendicular to the optical axis, but it is also possible to consider lenses in which the equirefractive index surfaces are curved.

Expanding this concept, it is thought that aberrations can be determined freely by giving a spherical lens a refractive index distribution such that the equirefractive index surface is a flat surface or a spherical surface of a certain curvature. is very useful not only for correcting aberrations of a single lens, but also for realizing all combinations of focal length and aberration required for a certain lens in a lens system consisting of a combination of multiple lenses. It is.

In order to form such a refractive index distribution, it is only necessary to form a composition distribution that exhibits a predetermined refractive index distribution in the material. For example, when using inorganic glass, methods such as ion exchange or CVD method can be used. It can be formed by twisting it. However, when using inorganic glass, both ion exchange and CVD require enormous amounts of heat and vacuum equipment, which is not very practical. Therefore, organic glasses are advantageous for manufacturing curved lenses with a refractive index distribution.

On the other hand, the production of lenses for use in optical devices using synthetic resins has become popular in recent years, and cameras using plastic lenses have also recently been produced.

In particular, attempts have been made recently to manufacture aspherical lenses that reduce spherical aberration by precision injection molding, but it is extremely difficult to manufacture molds with precise shape accuracy, and furthermore, injection molding is difficult. Since it is difficult to remove the resulting distortion, the current situation is that the desired performance cannot be obtained. Such defects can be significantly improved by manufacturing a spherical lens having the above-mentioned refractive index distribution. A method for manufacturing a spherical lens with the above-mentioned refractive index distribution is to form the above-mentioned refractive index distribution on a flat or cylindrical synthetic resin base material, and then perform post-processing (cutting, polishing, etc.). A method is already known in which a spherical lens is formed by using a spherical lens.

<Problems to be Solved by the Invention> However, with this method, post-processing such as cutting or polishing takes time and cost, and furthermore, the formed refractive index distribution cannot be fully used effectively. .

By the way, a well-known method for manufacturing a self-focusing optical element having a radial refractive index distribution using organic glass includes a two-step process as described in Japanese Patent Publication No. 52-5857. One example is the polymerization method, which involves semi-polymerizing the monomers that form the crosslinkable polymer into a gel state, and then diffusing another monomer from the surface into the gel body to form the monomer. The monomer composition distribution is fixed by forming a composition distribution and then completing polymerization, and if this method is applied directly to the manufacture of axially distributed lenses, it will result in a uniform distribution layer shape with a refractive index distribution. The surface is not uniformly flat or spherical, and local distortions tend to occur.

In other words, since the surface of an object in a gel state has relatively large irregularities, when a monomer is diffused from this gel surface, an equiconcentration surface of the diffused monomer, in other words, an equirefractive index distribution layer is formed on the gel surface. There is a problem that the distribution surface is distorted according to the unevenness of the body surface, and that the gel body swells as the monomer diffuses and the distortion of the uniform refractive index distribution surface is further expanded.

<Means for solving the problem> A monomer mixture containing two or more types of monomers having different monomer reactivity ratios and different refractive indexes when turned into polymers is placed in a constant position with the opposing plane mold surface. The material is filled into a mold having a spherical mold surface with a radius of curvature and whose main parts are transparent, and the filling material in the mold is irradiated with light or electron beams from either of the mold surfaces. The polymerization reaction of the monomer mixture is initiated from the inner wall surface of the mold, and the produced copolymer is precipitated, thereby turning the entire monomer mixture into a gel state.

Next, the monomer mixture in a gel state is heated to complete polymerization.

<Function> As copolymerization progresses, the specific monomer M ₁ with the highest polymerization rate rapidly polymerizes, followed by monomer M ₂ ,
M ₃ ... is polymerized in sequence.

As a result, the copolymer formed at the initial stage of polymerization contains a large amount of monomer _M1 , but as the polymerization progresses,
The content of M ₁ decreases rapidly and is replaced by monomeric M ₂
content increases. If the polymerization progresses further, M ₂
A reaction occurs one after another in which the content of monomer M 3 also decreases and the content of monomer M ₃ increases.

As mentioned above, polymerization is started on the irradiated surface, the polymerization reaction is limited locally, and as the polymerization reaction progresses, the location where the reaction occurs moves as the polymerization conversion rate increases, and the monomer can be appropriately selected. For example, a refractive index distribution that gradually increases or, conversely, decreases from the polymerization initiation position toward the deep part is formed in the polymer. By irradiating light or electron beams in the direction of the optical axis of the lens in this manner, the desired axial gradient index lens can be obtained.

<Detailed Description of the Invention> The monomer mixture used in the present invention may contain any number of monomers as long as they are plural, but at least two of them satisfy the following conditions. Must-have.

(1) Different monomer reactivity ratios.

(2) The refractive index of the homopolymer obtained from each monomer is different.

The present invention was basically completed based on the findings in the polymerization reaction of two or more types of monomers having different refractive indexes and monomer reactivity ratios when turned into polymers.

That is, X types (X is at least 2) of monomers M ₁ , M ₂ , M ₃ ... that satisfy the following conditions.
...M _x mixture, for example,
Polymerize by the polymerization method described in . In general, multicomponent copolymerization reactions are as follows.

The rate constant of the growth reaction Mi* +Mj→Mj* is Kij
Then, the reactivity ratio Rij is defined as Rij=Kii/Kij, and there are X (X-1) reactivity ratios in the X element copolymerization.

The monomer combinations of the present invention indicate the conditions to be satisfied. Now, two integers i and j are 1≦i and j≦
When there is a relationship such that X,i<i (1) Regarding the reactivity ratio (Rij (Mi/Mj) _n +1)/{(Mi/Mj) _n +Rij}>1.
1 Here, (Mi/Mj) _n is the mixing molar ratio of monomer Mi and monomer Mj.

(2) Regarding the refractive index (2a) Ni (refractive index of Mi homopolymer) < Nj (Mj
(refractive index of homopolymer) or (2b) Ni (…………) > Nj (…………).

The case where X=3 will be specifically explained. In ternary copolymerization, the following nine types of growth reactions occur in competition.

M ₁ * +M ₁ →M ₁ * (rate constant K ₁₁ ) M ₁ * +M ₂ →M ₂ * (〃K ₁₂ ) M ₁ * +M ₃ →M ₃ * (〃K ₁₃ ) M ₂ * +M ₁ →M ₁ * (〃K ₂₁ ) M ₂ * +M ₂ →M ₂ * (〃K ₂₂ ) M ₂ * +M ₃ →M ₃ * (〃K ₂₃ ) M ₃ * +M ₁ →M ₁ * (〃K ₃₁ ) M ₃ * +M ₂ →M ₂ * (〃K ₃₂ ) M ₃ * +M ₃ →M ₂ * (〃K ₃₃ ) The monomer reactivity ratio is defined by equation (3).

R ₁₂ =K ₁₁ /K ₁₂ R ₂₁ =K ₂₂ /K ₂₁ R ₁₃ =K ₁₁ /K ₁₃ R ₃₁ =K ₃₃ /K ₃₁ R ₂₃ =K ₂₂ /K ₂₃ R ₃₂ =K ₃₃ /K ₃₂ (3 ) The conditions that the combination of monomers M ₁ , M ₂ , and M ₃ of the present invention must satisfy are (1) Regarding the reactivity ratio (R ₁₂ (M ₁ /M ₂ ) _n +1)/{(M ₁ /M ₂ ) _n + R ₂₁ }＞1.
1(4) (R ₁₃ (M ₁ /M ₃ ) _n + 1) / {(M ₁ /M ₃ ) _n + R ₃₁ }＞1.
1(5) (R ₂₃ (M ₂ /M ₃ ) _n + 1) / {(M ₂ /M ₃ ) _n + R ₃₂ }＞1.
1(6) where (Mi/Mj) _n is the mixing molar ratio of monomer Mi and monomer Mj.

(2) Regarding the refractive index (2a) n ₁ (refractive index of M ₁ homopolymer) < n ₂ (M ₂
(refractive index of homopolymer) < n ₃ (refractive index of M ₃ homopolymer) or (2b) n ₁ > n ₂ > n ₃ . Here, both |n ₃ −n ₂ | and |n ₂ −n ₁ | are preferably at least 0.005 or more.

Condition (1) is that as the terpolymerization progresses, the initial monomer
M ₁ rapidly polymerizes, then monomer M ₂ polymerizes,
It shows that monomer M ₃ polymerizes most slowly. In other words, the copolymer formed at the initial stage of polymerization contains a large amount of monomer _M1 , but as the polymerization progresses, the content of _M1 rapidly decreases until the content of monomer _M2 decreases. To increase. As the polymerization progresses further, the content of M ₂ also decreases, and the content of monomer M ₃ increases. If condition (2a) is satisfied, the refractive index of the copolymer produced will increase as the polymerization progresses, but by adjusting the type of monomer and the monomer charging ratio, the refractive index of the copolymer will increase as the polymerization progresses. The refractive index of the polymer can be gradually increased with the polymerization conversion rate over a wide range of conversion rates. If condition (2b) is satisfied, the refractive index of the copolymer decreases with the polymerization conversion rate.

In the case of a combination of two types of monomers M ₁ and M ₂ , it is sufficient that equation (4) holds regarding the reactivity (However, in this case, R ₁₂ and R ₂₁ are usually expressed as R ₁ and R ₂ , respectively. ).

Monomers that can be used in the present invention are preferably monomers that form transparent polymers, but even if a single polymer tends to become opaque due to, for example, a high degree of crystallinity, If it becomes transparent when copolymerized, it can be used. As monomers that can be used in the present invention, compounds having one or more types of polymerizable double bonds such as vinyl groups, acrylic groups, methacrylic groups, and allyl groups are suitable; vinyl, vinyl acetate,
Styrene, phenyl vinyl acetate, vinyl benzoate, vinyl fluoride, vinylnaphthalene, vinylidene fluoride, vinylidene chloride, methyl acrylate, ethyl acrylate, 2,2,2-trifluoroethyl acrylate, benzyl acrylate, acrylic acid Phenyl, naphthyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile, 2,2,2-trifluoroethyl methacrylate, phenyl methacrylate, benzyl methacrylate, naphthyl methacrylate, 1,1,3-trihydro methacrylate perfluoropropyl, allyl benzoate,
phenyl allyl ether, methacrylonitrile,
α-methylstyrene, parachlorostyrene, butadiene, 1,5-hexadiene, vinyl acrylate, vinyl methacrylate, divinyl phthalate, divinyl isophthalate, divinylbenzene, divinylnaphthalene, ethylene glycol divinyl ether, vinyl naphthoate, β-vinyl naphthoate, diallyl phthalate, diallyl isophthalate, allyl acrylate, allyl methacrylate, β-methallyl methacrylate, methacrylic anhydride, diethylene glycol bisaryl ether, diethylene glycol bisallyl carbonate, tetraethylene glycol dimethacrylate, bisphenol These include A dimethacrylate, triallyl trimellitate, triallyl phosphate, triallyl phosphite, diphenyldiallylsilane, diphenyldivinylsilane, and the like.

Among these monomers, select monomers that satisfy the conditions of reactivity ratio and refractive index, and add to the mixture,
A thermal radical initiator such as a peroxide or an azo compound may be added. For example, benzoyl, azobis
-butane, azobisisobutyronitrile, etc.

<Effects of the Invention> According to the method of the present invention, not only a spherical lens with small spherical aberration, but also a lens with the required focal length and spherical aberration can be integrally molded, and there is no need for post-processing. A spherical lens with a precise refractive index distribution can be manufactured at almost the same cost as currently manufactured plastic molded lenses.

Furthermore, according to the method of the present invention, an initial precipitated polymer layer is formed on the container wall surface, and later, polymer layers having different refractive indexes are sequentially deposited and laminated on this polymer layer. If the surface of the container is made sufficiently smooth, it is possible to obtain a homogeneous refractive index distribution layer without local distortion along the wall surface of the container. This method is much easier than the two-step copolymerization method in which the surface of the gel body is smoothed.

<Example> The present invention will be described in further detail using Examples below.

Various examples of axial graded index lenses that can be formed by the method of the present invention are shown in FIGS. 1A to 1F.

Each of these lenses 10 has one refractive surface 11A that is a flat surface and the other refractive surface 11B that is a convex or concave spherical surface, and is provided with a refractive index distribution that changes in the lens optical axis direction. The equirefractive index layers 12 in FIGS. 1B and E are perpendicular to the optical axis,
The lenses shown in A and F have a spherical shape with the center of curvature almost coinciding with the spherical refractive surface 11B, and the lenses shown in C and D have a spherical shape with the center of curvature in the opposite direction to the spherical refractive surface 11B. ing.

Furthermore, in each of the lenses shown in FIGS. 1A to 1F, the direction of increase/decrease in the refractive index is such that there is a distribution in which the maximum is on the spherical refractive index surface side and the distribution decreases sequentially toward the flat refractive surface 11A, and vice versa. There are two cases in which the distribution has a maximum value of 0 and gradually decreases toward the spherical refractive surface 11B side.

For example, in the lens shown in Figure 1B, the spherical side has a high refractive index, and the refractive index distribution is expressed by the above formula.
For those shown in (1) or (2), the spherical aberration of the single lens is extremely small.

The combination of the above lens shape and refractive index distribution can be used as a single lens with low aberrations, or as a lens with positive or negative aberrations that can correct the aberrations of other lenses used in combination, depending on the application. If selected appropriately, the method of the present invention can produce any of these types of lenses. Next, a preferred example of the manufacturing method will be described.

First, a monomer mixture consisting of monomers satisfying the above-mentioned conditions (1) and (2) is prepared.

Examples of monomer mixtures include methyl methacrylate-vinyl benzoate, methyl methacrylate-acrylonitrile-vinyl benzoate, and the like. Add benzoyl peroxide, benzoin methyl ether, etc. to this monomer mixture from 0.001% to 10% by weight.
It may be added in a range of % by weight.

The monomer mixture thus prepared is poured into a mold 2 as shown in FIG.

The mold 2 consists of a lower mold 2A having a concave spherical mold surface and an upper mold 2B having a flat mold surface, the upper mold 2B being made of a transparent material, and the lower mold 2A being made of an opaque material. The monomer mixture 1 is injected into the mold 2 at high pressure in order to prevent the generation of bubbles due to volume shrinkage during polymerization and to prevent release from the mold.

Next, the monomer mixture 1 is irradiated with light or an electron beam 3 from a light source such as a high-pressure mercury lamp placed above the transparent upper mold 2B.

As a result, a polymerization reaction is started on the inner wall surface of the upper mold 2B, and the transmitted light intensity of the light or electron beam is rapidly attenuated toward the deeper parts, so that the farther from the light source the polymerization progress is delayed, and the deeper the Polymerization progresses toward , and gel layers with different refractive indexes are formed in layers.

When the whole becomes a gel state, it is heat-treated to complete polymerization and taken out from the mold 2. As shown in FIG. A convex spherical lens 10 with a varying distribution is obtained.

When methyl methacrylate-acrylonitrile-methyl benzoate is used as a monomer combination, the gel layer that first precipitates after irradiation with light or electron beams contains the most methyl methacrylate, so As the polymerization progresses, the refractive index of the gel layer that precipitates increases, so in the above case, the plane 11A has a low refractive index and the spherical surface 11B has a high refractive index, and this lens has a higher refractive index than a lens using a homogeneous medium. The spherical aberration is extremely small. By changing the combination of monomers, the gradient of the refractive index can be reversed.

Furthermore, if a mold in which one mold surface is a convex spherical surface is used, a concave lens with axially distributed refractive index as shown in FIG. 1E can be obtained.

In addition, if the spherical surface side of the mold is made transparent and irradiated with light or electron beam from this direction, the gel layer will be sequentially laminated in a spherical shell shape while changing its refractive index.
Equal refractive index layer 12 as shown in A or F in the figure
An axial gradient index lens having a curve substantially along the spherical refractive surface 11B of the lens is obtained.

Next, an example of a method for manufacturing a lens having a distribution shape such that the center of curvature of the equal refractive index layer is on the opposite side to the center of curvature of the refractive surface of the lens, as shown in C and D in FIG. 1, will be described.

For example, when molding the lens shown in Figure 1C, the third
As shown in the figure, the bottom wall of the spherical mold surface of the mold 2 made of a transparent material is covered with a light-shielding mask 5 having a small hole 4 in the center, and light or an electron beam 3 is emitted from a light source 6 through the light-shielding mask 5. The monomer mixture 1 in the mold 2 is irradiated.

Figure 4 A, B, and C schematically show the process in which a gel-like polymer is precipitated. Gel layers 6A, 6B with different refractive indexes in which polymerization progresses radially toward the deep part from the center...
are sequentially formed in layers in the shape of a spherical shell, and after the whole is gelled in this way, the polymerization is completed by heat treatment etc. and when taken out from the mold, the equal refractive index layer forms a spherical surface and its An axial gradient index convex spherical lens having the distribution shape shown in FIG. 1C, with the center of curvature on the side opposite to the center of curvature of the spherical refractive surface of the lens, is obtained.

Further, as shown in FIG. 5, the flat side of the mold having a convex spherical mold surface and a flat surface is made transparent, and the above-mentioned light-shielding mask 5 is provided on this flat side, and light or electron beam 3 is irradiated from this side. For example, an axial gradient index concave spherical lens having a distribution shape as shown in FIG. 1D can be obtained.

In carrying out the method of the present invention, the monomer mixture may undergo volumetric contraction due to polymerization.
Methods for preventing the inclusion of air bubbles due to this shrinkage include, for example, a method of filling a monomer mixture into a mold under high pressure, a method of designing a mold taking shrinkage into account, and a method of shrinking in accordance with volumetric shrinkage. Possible methods include using an elastic mold and supplying the monomer mixture from the light or electron beam irradiation side and the reflection side.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing various examples of axial gradient index lenses that can be molded by the method of the present invention, FIG. 2 is a cross-sectional view showing one embodiment of the present invention, and FIG.
4A to 4C are cross-sectional views showing stepwise formation of a gel layer by the method of FIG. 3, and FIG. FIG. 3 is a sectional view showing an example of manufacturing a lens of the type according to the method of the present invention. DESCRIPTION OF SYMBOLS 1... Monomer mixture, 2... Molding mold, 3... Light or electron beam, 10... Axial gradient index lens, 12... Equal refractive index layer.

Claims (1)

[Claims] 1 (a) A monomer mixture containing two or more types of monomers having different monomer reactivity ratios and different refractive indexes when turned into polymers is formed on opposing planar mold surfaces. (b) irradiating light or an electron beam from either of the mold surfaces through the main parts of the mold; A step of allowing the polymerization reaction to proceed from the irradiated part to the deep part and precipitating the produced copolymer to make the entire monomer mixture into a gel state; and (c) a step of turning the monomer mixture into a gel state into A method for manufacturing a synthetic resin optical element having a refractive index distribution in the optical axis direction, comprising: completing polymerization by performing a treatment such as heating.