JPH0476013A - Optical element - Google Patents

Abstract

PURPOSE:To obtain an optical element having a lowered photoelastic constant and decreased birefringence and in-plane distribution by copolymerizing a compound having a double bond with methyl methacrylate and another copolymierizable unsaturated monomer to form a copolymer of a specified photoelastic coefficient. CONSTITUTION:A double bond-containing compound (A) which can give a homopolymer having a photoelastic coefficient opposite in sign to that of polymethyl methacrylate, desirably a methacrylic ester having a 5C or higher aliphatic or alicyclic hydrocarbon group at the alcohol moiety of the ester and/or an N-substituted maleimide is mixed with methyl methacrylate (B) and optionally another copolymerizable unsaturated monomer (C)in an A/(A+B+C) weight ratio of 0.1-0.5, and the resulting mixture is copolymerized at 0-200 deg.C in the presence of a polymerization initiator to obtain an optical element comprising a polymer of a photoelastic coefficient of 1.0X10<-13>(cm<2>/dyn) or below.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an optical element.

[Conventional technology]

Conventionally, glass has been used for optical elements such as lenses, prisms, and optical disks. However, in recent years, plastics have come to be used to make them lighter and smaller. Polystyrene, polycarbonate, polymethyl methacrylate, styrene-methyl methacrylate copolymer, and the like are generally known as plastics used for optical elements. However, since these polymers have aromatic rings in their molecules, they tend to suffer from orientational distortion and birefringence, so it is necessary to devise a molding die as shown in JP-A-61-14617. Therefore, until now, polymethyl methacrylate has mainly been used as an optical element material.

[Problem to be solved by the invention]

Polymethyl methacrylate has a small photoelastic coefficient and is relatively resistant to orientation distortion and birefringence, so it has been used in optical elements that do not require high precision, such as finder lenses and CD laser pickup lenses.

However, in recent years, there has been a demand for optical elements that require higher precision. In particular, laser pickup lenses for write-once optical disks that use laser light, laser pickup lenses for magneto-optical disks, write-once optical disks,
Magneto-optical disks are required not only to have low birefringence but also to have no in-plane distribution.

Although polymethyl methacrylate can reduce birefringence, its in-plane distribution is still large, and it cannot be used in the above-mentioned optical elements that require high precision.

The present invention has been made in view of the above-mentioned problems, and provides an optical element having a photoelastic coefficient lower than that of polymethyl methacrylate, and having extremely small birefringence and in-plane distribution.

[Means to solve the problem]

The present invention is directed to (A) a compound having an unsaturated double bond whose photoelastic coefficient when made into a homopolymer is opposite to that of polymethyl methacrylate, (B) methyl methacrylate, and (C) optionally A polymer obtained by copolymerizing other unsaturated monomers copolymerizable with (A) or (B) used, the photoelastic coefficient of which is 1.
．． The present invention relates to an optical element made of a polymer having a molecular weight of 0 x 10-'' (ant/dyne) or less.

As a compound having an unsaturated double bond whose photoelastic coefficient when made into a homopolymer which is component (A) in the present invention is opposite to that of polymethyl methacrylate, the alcohol component of the ester has 5 carbon atoms. Preferred are methacrylic acid esters and/or N-substituted maleimides having the above aliphatic or alicyclic hydrocarbon groups. When the number of carbon atoms in the aliphatic hydrocarbon group of the methacrylic ester is less than 5, the effect of reducing the photoelastic coefficient tends to be small.

The number of carbon atoms is usually 22 or less.

Examples of the compound of component (A) include methacrylic acid aliphatic esters such as hexyl methacrylate, 2-ethylhexyl methacrylate, dodecyl methacrylate, stearyl methacrylate, and urolyl methacrylate, fluorohexyl methacrylate, and chloropentyl methacrylate. , methacrylic acid halogenated alkyl esters such as bromohexyl methacrylate, methacrylic acid esters such as methacrylic acid ethylene glycol ester, methacrylic acid polyethylene glycol ester, methacrylic acid cyanoalkyl ester, cyclopentyl methacrylate, cyclohexyl methacrylate, methylcyclohexyl methacrylate, Trimethylcyclohexyl methacrylate, norbornyl methacrylate, norbornylmethyl methacrylate, isobornyl methacrylate, bornyl methacrylate, menthyl methacrylate, fenthyl methacrylate, adamantyl methacrylate, dimethyladamantyl methacrylate, tricyclo(5,2) methacrylate ,1゜02・6〕
Dec-8-yl, tricyclo methacrylate (5,2,1
, 02, 6] deca-3-methyl, tricyclo(5,2,1,02, 6) deca-4-methyl methacrylate, cyclododecyl methacrylate, etc. Among these, heat resistance and lens properties are preferred. , norbornyl methacrylate, norbornylmethyl methacrylate, tricyclo(5,2,1,02.6]dec-8-yl methacrylate, tricyclo(5,2,1,02.6)deca-3-yl methacrylate. Methyl, tricyclo methacrylate (5,2゜1.02・6)
Preferred are methacrylic acid esters having an alicyclic hydrocarbon group such as deca-4-methyl.

Examples of the N-substituted maleimide include N-alkyl-substituted maleimides such as N-methylmaleimide, N-ethylmaleimide, N-propylmaleimide, and N-butylmaleimide, and N-alicyclic hydrocarbons such as N-cyclohexylmaleimide. Examples include substituted maleimides.

To find out the sign of the photoelastic coefficient of homopolymers of these (A) components, polymerize one (A) component alone,
You may investigate a homopolymer of the component, or you may investigate a copolymer of several copolymers by copolymerizing methyl methacrylate and one component (A) at different ratios. . In the latter case, the sign of the photoelastic coefficient of polymethyl methacrylate is negative (-), so if the sign becomes positive (+) for a certain ratio of copolymers, the homopolymer of one component (A) It can be seen that the sign of the photoelastic coefficient is positive.

The photoelastic coefficient can be determined by, for example, measuring birefringence values in two states (with and without load) using an ellipsometer and calculating from these values.

As component (A), one type of compound as mentioned above is used (
Instead of one type, two or more types may be used together.

The photoelastic coefficient of the polymer obtained by copolymerizing component (A) with methyl methacrylate (B) and component (C) used as necessary is OX 10-1s (cnf/d
yne). This amount [(A)/
[(A) + (B) + (C)] (weight ratio) i'A,
Preferably 0.1・-〇, 5, more preferably 0.2~
It is in the range of 0.4. If this amount is too small or too large, the photoelastic coefficient tends to increase.

Furthermore, if necessary, it can be copolymerized with component (C) to the extent that the photoelastic coefficient is not increased.

Examples of other unsaturated monomers copolymerizable with component (A) or (B) of component (C) include acrylic esters such as methyl acrylate, ethyl acrylate, chloroethyl acrylate, and fluoromethyl acrylate. , vinyl cyanide compounds such as acrylonitrile and methachloronitrile.

] The polymer obtained from the components (A), (B), and (C) as necessary has a photoelastic coefficient of 1.0 x l
It must be less than O-'' (cm/dyne).

If it exceeds 1.0 x 10-" (cn?/dyne), the photoelastic coefficient will not be smaller than that of polymethyl methacrylate, and there will be no effect of reducing birefringence. Birefringence is usually -1, 4 or more.

The photoelastic coefficient of the polymer was 1.0 x 10'' (cJ
/dyne) or less means (A), (B) and (
C) This can be done by adjusting the proportions of the ingredients used. Whether the photoelastic coefficient is within the desired range can be confirmed, for example, by measuring birefringence values in two states (with and without load) using an ellipsometer and calculating from these values.

The polymerization method for obtaining the above polymer includes bulk polymerization,
Known methods such as suspension polymerization, emulsion polymerization, and solution polymerization can be applied. For optical elements, bulk polymerization is preferred from the viewpoint of preventing impurities from being mixed into the resin, while suspension polymerization is preferred from the viewpoint of handling as a product.

Examples of the polymerization initiator include benzoyl peroxide, lauroyl peroxide, di-t-butylperoxyhexahydroterephthalate, t-butylperoxy-2-ethylhexanoate, 1,1-di-t-butylperoxy-3 ，
Organic peroxides such as 3,5-trimethylcyclohexane,
Ab compounds such as azobisisobutyronitrile, azobis-4-methoxy-2゜4-dimethylvaleronitrile, azobiscyclohexanone-1-carbonitrile, azodibenzoyl, water-soluble catalysts such as potassium persulfate, ammonium persulfate, etc. Any compound that can be used in conventional radical polymerization can be used, such as a redox catalyst using a combination of a peroxide or a persulfate and a reducing agent. The polymerization initiator is the total amount of monomers [(A) ) (B)-1-(C))
It is preferably used in a range of 0.01 to 10% by weight.

As a polymerization regulator, a mercaptan compound, thioglycol, carbon tetrabromide, α-methylstyrene dimer, etc. can be added as necessary for controlling the molecular weight.

The polymerization temperature is preferably selected appropriately between 0 and 200°C, more preferably 50 and 120°C.

As a solvent in solution polymerization, for example, benzene, toluene, gyclene, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, dichloroethylene, etc. can be used.

Suspension polymerization is carried out in an aqueous medium, with the addition of suspending agents and, if necessary, suspension aids. Suspending agents include, for example, water-soluble molecules such as polyvinyl alcohol, methyl cellulose, and polyacrylamide, and sparingly soluble inorganic substances such as calcium phosphate and magnesium pyrophosphate. .03 to 1% by weight and hardly soluble inorganic substances are 0.05 to 1% by weight based on the total amount of monomers.
It is preferable to use it in a range of 0.5% by weight.

Suspending aids include anionic surfactants such as sodium dodecylbenzenesulfonate, and when a sparingly soluble inorganic substance is used as a suspending agent, it is preferable to use a suspending aid in combination. The suspension aid is o, based on the total amount of monomers.
It is preferable to use it in the range of o oi to 0.02% by weight.

The molecular weight of the polymer obtained in this way is not particularly limited, but from the viewpoint of heat resistance and mechanical properties, the weight average molecular weight (measured using a standard polystyrene calibration curve by gel permeation chromatography) It is preferable that the value (when

It is preferable to add phenol-based, phosphite-based, thioether-based antioxidants and other antioxidants to these polymers as necessary during or after synthesis of the polymers to make them into molding materials.

When molding optical elements using the above molding materials, any well-known molding method such as injection molding, compression molding, micro molding, floating molding, or low links molding can be used. However, from an economic standpoint, injection molding is preferable. Since the above-mentioned molding material hardly causes orientation distortion, the in-plane distribution is smaller than that of polymethyl methacrylate resin, and a high-performance plastic optical element can be obtained. In some cases, molding may be performed by casting simultaneously with polymerization.

The optical element according to the present invention includes magnesium fluoride on the incident light side for antireflection, antistatic, and improvement of surface strength.
Silicon oxide etc. can be coated on the surface by vacuum evaporation, sputtering, ion blating, etc. The molding material constituting the optical element according to the present invention is easy to coat, and it is easy to obtain a homogeneous surface.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples.
The present invention is not limited by these.

Example 1 80 parts by weight of methyl methacrylate, 20 parts by weight of cyclohexyl methacrylate, and 0.4 parts by weight of lauroyl peroxide were dissolved to prepare a monomer solution.

Separately, two glass plates with a size of 135 x 200 mm and a thickness of 8 mm were stretched together with a Teflon gasket, and a glass cell was assembled so that the distance between the two glass plates was 3 mm.

The monomer solution was well bubbled with nitrogen and then injected into the glass cell. Next, it was kept in a constant temperature bath controlled at 60±2°C for 6 hours, raised to 80±5°C for 4 hours, and then kept in a dryer controlled at 100±2°C for 8 hours. This was left to cool at room temperature and the glass plate was removed to obtain a plastic plate.

Examples 2 to 10 Using monomers having the composition ratios shown in Table 1, a monomer solution was prepared by dissolving 0.4 parts by weight of lauroyl peroxide, and a plastic plate was obtained in the same manner as in Example 1.

Comparative Example 1 A monomer solution was prepared by dissolving 100 parts by weight of methyl methacrylate and 0.4 parts by weight of lauroyl peroxide, and a plastic plate was obtained in the same manner as in Example 1.

The light transmittance, glass transition temperature, and photoelastic coefficient of the plastic plate thus obtained were measured according to the following tests, and the results are shown in Table 1.

Light transmittance: Measured according to ASTMI 003.

Glass transition temperature: A polymer is dissolved in methylene chloride and then poured into methanol to precipitate the polymer.

This was dried separately, and the glass transition temperature was measured using a differential scanning calorimeter (DSC7, manufactured by PerkinElmer).

Photoelastic coefficient: A loading device for photoelastic coefficient measurement is attached to an ellipsometer (AEP-100 manufactured by Shimadzu Corporation),
A jig for three-point bending was set up.

12mm x ], 50mm x 3mm (thickness = 0.
Cut out a test piece with a diameter of 3cm) and place it at a distance of 100mm between the fulcrums.
The sample was placed in a jig with a diameter of 2 mm, the laser beam of the ellipsometer was set at a distance of 2 mm from the load point, a load of 2 kg was applied from directly above, and the birefringence value at this time was measured.

From this, the photoelastic coefficient was determined.

However, the symbols mean the following:

n. : Birefringence value (nm) nl at load 0 (kg):
Birefringence value (nm) at load 2 (kg) M: Bending strength (kgf/cffl) A: Optical path length (thickness of test piece) (cm) (Hereafter, blank space) Molding example 1 51 equipped with a stirrer and a condenser 10% by weight of basic calcium phosphate as a dispersant in a separable flask.
83 g of aqueous suspension 0.004 g of sodium dodecylbenzenesulfonate and 1 g of sodium sulfate in 2 parts of pure water
A suspension medium was prepared by adding 400 g together with stirring and mixing. A composition consisting of the following composition was added to this, and the stirring speed was 240.
rpm at 65°C for 4 hours, followed by 2 hours at 98°C under nitrogen atmosphere.

Composition Monomer methyl methacrylate 2000g Polymerization initiator lauroyl peroxide 8g Polymerization regulator n-octyl mercaptan 4g Thermal antioxidant compound 4.4'-Butylidene-bis(3-2g Methyl-6-
t-Butylphenylditridecyl) phosphite octadecyl-3-(4'-hydro 2gxy-
31,51-Di-t-butylphenyl)propionate The obtained polymer particles were acid-washed, water-washed, dehydrated, vacuum-dried, and molded using an injection molding method (IS-50EP manufactured by Toshiba Machine ■) as shown in Figure 2. A laser pickup lens of the shape shown was molded.

Molding Example 2 Synthesis was carried out in the same manner as in Molding Example 1 using 1400 g of methyl methacrylate (600 g of cyclododecyl methacrylate) as a monomer to obtain a laser pickup lens.

FIG. 1 shows the relationship between the distance from the load point and the photoelastic coefficient. The photoelastic coefficient varies depending on the measurement location, and all polymers have a minimum value at a distance of 4 mm from the load point, but the photoelastic coefficient of PMMA increases rapidly when the distance from the load point is 4 mm. However, the polymer of the present invention did not increase that much.

In particular, the photoelastic coefficient of the polymer (Example 2) containing 30% by weight of cyclododecyl methacrylate is about 1% that of PMMA.
/10.

Next, the birefringence of the laser pickup lenses obtained in Molding Examples 1 and 2 was measured. FIG. 3 shows the in-plane distribution of birefringence of the lens, and the in-plane distribution of the polymer of the present invention was small.

〔Effect of the invention〕

The optical element of the present invention has significantly smaller birefringence than that of a homopolymer of methyl methacrylate, and has an extremely small in-plane distribution, and is suitable for applications requiring high precision.

[Brief explanation of drawings]

Figure 1 shows the relationship between the distance from the load point and the photoelastic coefficient, and Figure 2 shows the relationship between the distance from the load point and the photoelastic coefficient.
The figure is a side view of the laser pickup lens, and FIG. 3 is a graph showing the relationship between the distance from the lens center and birefringence. (■Machimanko vinegar

Claims (1)

[Claims] 1. (A) a compound having an unsaturated double bond whose photoelastic coefficient when made into a homopolymer is opposite to that of polymethyl methacrylate; (B) methyl methacrylate; and (C) A polymer formed by copolymerizing other copolymerizable unsaturated monomers used as necessary, the photoelastic coefficient of which is 1.
．． An optical element made of a polymer having a density of 0x10^-^1^3 (cm^2/dyne) or less. 2. The compound having an unsaturated double bond of component (A) is a methacrylic ester and/or an N-substituted maleimide having an aliphatic or alicyclic hydrocarbon group having 5 or more carbon atoms in the alcohol moiety of the ester. An optical element according to claim 1. 3. The optical element according to claim 1 or 2, wherein (A)/[(A)+(B)+(C)] (weight ratio) is in the range of 0.1 to 0.5.