WO2002031026A2

WO2002031026A2 - High refractive index waveguide polymers

Info

Publication number: WO2002031026A2
Application number: PCT/US2001/029099
Authority: WO
Inventors: Paul J. Shustack
Original assignee: Corning Incorporated
Priority date: 2000-10-10
Filing date: 2001-09-18
Publication date: 2002-04-18
Also published as: WO2002031026A3; AU2001291061A1

Abstract

An oligomer reaction product formed by reacting a polyfunctional thiol with a compound having two or more reactive double bonds is useful for preparing radiation curable compositions that when cured have a refractive index of at least 1.67 at 589 nm. The curable materials have particular utility in the fabrication of optical waveguides having a high refractive index.

Description

HIGH REFRACTIVE INDEX WAVEGUIDE POLYMERS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radiation curable compositions that when cured have a refractive index greater than or about equal to 1.67 at 589 run, and more particularly to high refractive index polymers that are suitable for use in optical waveguide applications.

2. Technical Background

It is well known that organic materials containing sulfur atoms generally have a higher refractive index than similar organic materials that do not contain sulfur atoms. It is also well known that aromatic moieties may be incorporated into organic materials to increase the refractive index of the materials. The majority of organic materials having a high refractive index due to incorporation of sulfur atoms and/or aromatic moieties have been used for lenses such as for eyeglasses. Many of these organic materials having a high refractive index are unsuitable for optical waveguide applications because they have unacceptable absorbance at the 1550 nm wavelength used for optical communications. The unacceptable absorbance properties of these materials is often attributable to the presence of urethane functional groups (i.e., -NH- CO-O- linkages) which contain N-H bonds that exhibit extremely high absorption at 1550 nm. Other known organic materials that have a high refractive index due to the presence of sulfur atoms and aromatic moieties rely on reacting epoxy functional substituents of aromatic sulfur moieties with amine and/or thiol groups to cure the materials. These cross-linking reactions generate hydroxyl groups that exhibit strong absorption at 1550 nm. Because of the strong absorption at 1550 nm, these materials are also unsuitable for optical waveguide applications.

Another group of known organic materials having a high refractive index are prepared with the use of styrenic monomers. A frequent problem with these compositions is that they cure at an undesirably slow rate.

Several other organic materials having a high refractive index on account of the presence of sulfur atoms and/or aromatic moieties have unacceptable thermal or hydrolytic stability, and/or form crystalline polymers. These characteristics are highly undesirable for optical waveguide applications.

Other known organic materials having a high refractive index on account of the presence of sulfur atoms are not capable of being rendered radiation curable. Still other sulfur-containing organic materials do not exhibit a sufficiently high refractive index (i.e., they do not have a refractive index that is greater than or about equal to 1.61 at 589 nm).

It is important that compositions used for fabricating optical waveguides have low odor because of the need to handle the liquid, non-cured compositions during the manufacturing process. For this reason, compositions containing odorous compounds such as dimercaptoethylsulfide and 1,2-benzenedithiol are unsuitable for optical waveguide applications.

Other organic materials having sulfur-containing organic moieties exhibit an unacceptably low glass transition temperature for optical waveguide applications due to the use of a monomer that reacts to form flexible, aliphatic thioether linkages.

A known radiation curable organic composition that has a refractive index greater than 1.67 at 589 nm is also unacceptable for optical waveguide applications because it contains iodine as a substituent on one of the monomers. Iodine-carbon bonds are weak and tend to cleave in the presence of heat (i.e., the cured composition does not exhibit sufficient thermal stability for optical waveguide applications). SUMMARY OF THE INVENTION

The invention pertains to radiation curable organic compositions that when cured have a refractive index that is greater than or about equal to 1.67 at 589 nm, and that have other characteristics that make the uncured compositions and the cured materials well suited for optical waveguide applications. In general, the compositions are substantially free of urethane functional groups and other moieties that would exhibit strong absorbance at 1550 nm, substantially free of epoxy functional groups that cross link with amine or thiol functional groups to generate hydroxyl groups that exhibit strong absorbance at 1550 nm, substantially free of vinyl functional monomers that would cause the composition to cure at an undesirably slow rate, substantially free of iodine-carbon bonds and other linkages that would result in cured materials having unacceptable thermal stability, substantially free of mercaptoacetates, thioglycolates, mercaptopropionates and other ingredients that would result in cured materials having unacceptable hydrolytic stability, and substantially free of compounds such as dimercaptoethylsulfide and 1 ,2-benzenedithiol and other highly odorous compounds that would make the composition unsuitable for optical waveguide applications.

Further, the compositions of this invention, when cured, have an acceptably high glass transition temperature that makes the cured materials well suited for optical waveguide applications.

In one aspect, the invention provides and oligomeric reaction product of one or more compounds having at least two thiol functional groups and at least one or more compounds having at least two reactive double bonds, wherein the at least one or more compounds having at least two thiol functional groups are substantially free of mercaptoacetates, mercaptopropionates and thioglycolates, and the oligomeric reaction product is substantially free of styrenic end groups. In another aspect of the invention, the oligomeric reaction product is combined with a photoiriitiator to provide a radiation curable composition that when cured has a refractive index greater than or about equal to 1.67 at 589 nm.

Additional features and advantages of the invention will be set forth in the detailed description which follows, and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows, together with the claims and appended drawings. It is to be understood that the foregoing description is exemplary of the invention only, and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute part of the specification. The drawings illustrate various features and embodiments of the invention which, together with their description, serve to explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic cross-sectional view of an optical waveguide fabricated from a composition of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compositions of this invention are based on sulfur-containing aromatic polymers. The cured materials of this invention are prepared from a composition containing the oligomeric reaction product of one or more compounds having at least two thiol functional groups and at least one or more compounds having at least two reactive double bonds. To impart hydrolytic stability to the cured materials, the one or more compounds having at least two thiol functional groups are substantially free of mercaptoacetates, mercaptopropionates and thioglycolates. Further, in order to facilitate rapid curing, the oligomeric reaction product is substantially free of styrenic end groups. An oligomeric reaction product that is substantially free of mercaptoacetates, mercaptopropionates and thioglycolates refers to an oligomeric reaction product that when cured exhibits excellent hydrolytic stability. Excellent hydrolytic stability is characterized by a change in refractive index of no more than 0.002 after exposure to high temperatures and high humidities for a prolonged period of time. More specifically, the material must pass a Bellcore testing procedure which includes exposure to a temperature of 85°C and a relative humidity of 85% for 30 days. Oligomeric reaction products prepared using substantial amounts of mercaptoacetates, mercaptopropionates, and thioglycolates will generally exhibit a change in refractive index of more than 0.002 when exposed to a temperature of 85°C and a relative humidity of 85% for 30 days.. An oligomeric reaction product that is substantially free of styrenic end groups refers to an oligomeric reaction product that does not contain any styrenic end groups, or does not contain an amount of styrenic end groups that is sufficient to slow the curing process to a rate at which the required fine structure of an optical waveguide design cannot be retained. Styrenic end groups refer to ethylene groups (i.e., -CH=CH ₂ ) that are bonded directly to an aromatic ring carbon atom. The one or more compounds having at least two thiol function groups include both aromatic and aliphatic compounds, with aromatic compounds such as 4,4'- thiobisbenzenethiol (TBBT) being preferred. The one or more compounds having at least two reactive double bonds include various compounds having one or more ethylenically unsaturated functional groups such as methacrylate groups, vinyl acrylate groups, vinyl ether groups, vinyl sulfide groups, propenyl ether groups, propenyl sulfide groups, allyl ether groups, allyl sulfide groups, isopropenyl benzene groups, maleimide groups, maleate groups, etc. Specific examples include 1,3- diisopropenylbenzene (DIPB), 1,4-benzenediacrylate, 1,4-benzenedimethacrylate, 1,3,5-triisopropenylbenzene, etc. As an example, an oligomeric reaction product in accordance with the invention may be synthesized using the following reaction scheme.

1,3-Diisopropenylbenzene (DIPB) (II)

In the example illustrated above, two moles of 4,4'-thiobisbenzene thiol were reacted with one mole of 1,3-diisopropenylbenzene at 85°C in mesitylene solvent to produce a product predominently comprised of the reaction product (compound III) of one molecule of DIPB with two molecules of TBBT. The oligomeric reaction products of this invention include products such as compound III, which may or may not have

10 any repeat units. As the ratio of the one or more compounds having at least two reactive double bonds to the one or more compounds having at least two thiol functional groups is increased toward a value of 1:1, higher molecule weight oligomers are formed. The thiol functional oligomers can be used to prepare cured materials, and in particular to prepare optical waveguides, having a high refractive index in

15 accordance with the invention. However, the thiol terminal groups are preferably end capped with compounds having highly reactive double bonds (e.g., methacrylate end caps). For example, the thiol functional oligomer can be reacted with ethylene glycol dimethacrylate to form a methacrylate functional oligomer in accordance with the following reaction scheme

20

Ethyleneglycol irπsthacrylate (TV)

Me1-r-acrylate Functional Oligomer (V)

In the illustrated scheme, one mole of the thiol functional oligomer is reacted with two moles of ethylene glycol dimethacrylate end capping monomer at 60°C in

25 mesitylene solvent. The illustrated methacrylate functional oligomer (N) is the predominant product. When the reaction is complete, as determined by disappearance of a thiol peak at 2,650 cm ^"' (IR), a shelf-life stability package may be added. The shelf-life stability package may, for example, comprise a mixture of para-methoxy phenol (MEHQ), 3-aminopropyl trimethoxy silane (Al 110), Genorad 16 (Rahn USA Corp.) and Irganox 1035 (Ciba Geigy Corp.). A suitable radiation curable composition may be prepared by combining the oligomeric reaction product with a photoinitiator. The radiation curable compositions may also contain other monomers that will react with the oligomeric reaction product to produce a cured (cross-linked) polymer material. For example, the oligomeric reaction products may be combined with bis-(4- methacryloylthiophenyl) sulfide (MPSDMA).

The radiation curable compositions of this invention are useful as materials for fabricating high refractive index optical waveguides for photonic devices such as liquid crystal cross connect switches. A detailed discussion of the operation of a liquid crystal cross connect switch is not included in this description, since these devices are known in the art and constitute merely an exemplary application for the invention. In a liquid crystal cross connect switch, a high refractive index optical waveguide is needed to achieve a proper refractive difference between the waveguide and the liquid crystal so as to achieve total internal reflectance of the optical signal when the liquid crystal is in the desired orientation. A high refractive index optical waveguide also allows sharper angle of reflection that result in an ability to construct devices that are smaller in size. Smaller devices are highly desirable in the protonics industry. An important advantage of the invention is that optical waveguides can be easily fabricated at a low cost. For example, the compositions of this invention can be used in a UN embossing microreplication process in which liquid composition are applied to a transparent substrate, patterned with a tool, and cured. Normally, the tool is on a cylindrical drum and the drum is rolled across the liquid composition. The liquid is then cured with actinic radiation, preferably UN radiation by shining the UN light through the transparent substrate to cure (polymerize) the composition while it is contact with the tool. The tool is then pulled (rolled) away from the cured polymer leaving the waveguide pattern in the cured polymer composition. In order to do this the composition must be UN curable and have sufficient UN cure speed to go through this process at a fast enough speed to retain the fine structure of the optical waveguide design. The cured polymer compositions of this invention are thermally, oxidatively, and hydrolytically stable. In particular, optical waveguides fabricated from the compositions of this invention will exhibit a change in refractive index of no more than 0.002 when subjected to a temperature of 85°C and a relative humidity of 85% for 30 days. The cured materials of this invention are also resistant to liquid crystal material used in liquid crystal cross connect switches. More specifically, the liquid crystal material (which maybe in direct contact with the cured materials of this invention) will not affect the optical properties of the cured material and the cured material will not affect the properties of the liquid crystal. The desired lack of interaction between the liquid crystal and the cured polymer material is related to the relatively high glass transition temperature and cross-link density of the cured polymer. In particular, the relatively high glass transition temperature and cross-link density prevents the liquid crystal material from soaking into the cured polymer and affecting its properties. A high glass transition temperature also minimizes the coefficient of thermal expansion of the polymer and the rate at which the refractive index of the polymer changes with temperature (dn/dT). The cured polymers of this invention typically have a glass transition temperature above 70°C.

The cured polymers of this invention transmit light efficiently at the standard telecommunication wavelengths of 1300 and 1550 nm. To achieve this result, the polymeric composition of this invention contain a minimum of N-H and O-H functional groups, since these bonds absorb strongly at the standard telecommunication wavelengths. The cured polymers of this invention will typically have an optical loss that is less than or about equal to 1 dB/cm at 1550 nm.

Fig. 1 is a schematic cross-sectional illustration of an optical waveguide prepared with the compositions of this invention. The waveguide 10 generally includes a waveguide core 12 having a first refractive index, that is surrounded by cladding 14, 24 having a second refractive index, wherein the refractive index of the cladding is lower than the refractive index of the core. The core 12 and cladding 14, 24 defining the waveguide 10 may be formed on a substrate 22.

The invention will now be described with reference to specific examples. As such, the examples should not be considered as limiting the invention. Example 1

An example of an oligomer solution where the stoichiometric mole ratio of TBBT to DIPB is 10:9 maybe prepared from the following ingredients.

37.62% 4,4'Jhiobisbenzene thiol

33.16% Mesitylene

21.42% 1 ,3-diisopropenylbenzene

6.06% Ethylene glycol dimethacrylate

0.08% _. MEHQ

1.46% Irganox 1035

0.03% A-1110

0.17% Genorad 16

The final composition of the oligomer solution suitable for preparing a waveguide core is:

35.17% Oligomer

26.45% Mesitylene

35.16% MPSDMA

2.20% Irgacure 1850

In order to fabricate a waveguide, a core and cladding material is required. The cladding should have slightly lower refractive index than the core. In order to formulate a cladding material for the above core, another lower refractive index monomer was added. In this case, an ethoxylated bisphenol A diacrylate monomer

(SR-349 from Sartomer Co.) was added.

The final composition of the clad waveguide formulation is:

34.50% Oligomer

22.19% Mesitylene 36.96% MPSDMA

3.98% SR-349

2.37% Irgacure 1850

The refraction index was measured on the UN cured core and cladding material above using a prism coupling technique and a Metricon Model 2010 Prism Coupler instrument. Core Material

Wavelength Refractive Index

632.8 nm 1.683

1300 nm 1.653

1541 nm 1.652

Clad Material

Wavelength Refractive Index

632.8 nm 1.677

1300 nm 1.646

1541 nm 1.646

The above core and cladding materials were subject to 85°C/85% RH aging and their refractive indices followed with time.

Refractive Index at 1541 nm

Core Clad

Initial 1.652 1.646

2 weeks at 85°C/85% RH 1.652 1.646 4 weeks at 85°C/85% RH 1.653 1.645 6 weeks at 85°C/85% RH 1.652 1.646 8 weeks at 85°C/85% RH 1.652 1.646

The optical waveguide materials have excellent wet aging characteristics. Another property measured on these compositions was the optical loss of the liquid formulations. This was done by measuring the absorbance of the liquid formulations for three different path lengths at 1300 nm and 1500 nm using a Perkin Elmer Lambda 900 UN/Vis NIR spectrometer. The slope of the absorbance versus path length curve is a measure of the optical loss. The glass transition temperature (T ) of the core material was determined by Dynamic Mechanical Analysis (DMA) and measured to be 85J°C as determined from the peak of the tan delta curve. The change in refractive index with temperature (dn/dT) was also measured for the cured core and clad material using a fiber optic back reflectance technique.

Wavelength (nm) Optical Loss dB/cm dn/dT (°C^"J

Core 1300 0.25 —

1550 0.52 —

1541 — ^"2.65 xlO^"4

Clad 1300 0.32 —

1550 0.62 —

1541 — ^"2.25 xlO^"4 Example 2

Another example composition involves the synthesis of an oligomer as in Example 1 except that the reactions were preformed in diethyl benzene instead of mesitylene and the methacrylate end capping monomer is trimethylolpropane trimethacrylate instead of diethylene glycol dimethacrylate. The oligomer solution composition is:

30.81% ^~4,4'-thiobisbenzene thiol

44.48% Diethyl benzene

17.35% 1 ,3-diisopropenyl benzene

5.75% Trimethylol propane trimethacrylate

0.04% MEHQ

1.19% Irganox 1035

0.18% A1110

0.18% Genorad 16

The composition of the core waveguide formulation based on the above oligomer solution is:

34.92% Oligomer

27.98% Diethyl benzene

2.18% Irgacure 1850

34.92% MPSDMA

The clad composition for the above core material is:

32.17% Oligomer

25.78% Diethyl benzene

35.87% MPSDMA

2.42% Irgacure 1850

2.28% Ethoxylated bisphenol A dimethacrylate (SR-348 from Sartomer Co.)

1.48% 3 -acryloxypropyl trimethoxy silane

The refractive index for the UN cured core and cladding compositions were measured as in Example 1. Core Material

Wavelength Refractive Index

632.8 nm 1.683

1300 nm 1.652

1541 nm 1.652 Clad Material

Wavelength Refractive Index

632.8 nm 1.679

1300 nm 1.648

1541 nm 1.647

The T of the core material was also run by DMA and determined to be 78.3°C as g ^J determined from the peak of the tan delta curve. Example 3

An example oligomer was prepared as in Example 1 except that the mole ratio of TBBT to DIPB was changed to 2:1 and the end capping monomer was MPSDMA. The reaction was also carried out in fluorobenzene as the solvent but at the end of the reaction the fluorobenzene was evaporated off. The oligomer composition was:

34.01% 4,4'-thiobisbenzene thiol

10.45% 1,3-diisopropenyl benzene

53.73% MPSDMA

0.06% MEHQ

1.61% Irganox 1035

0.07% A1110

0.07% Genorad 16

A core waveguide composition based on the above oligomer was:

50.00% Oligomer

30.00% MPSDMA

20.00% Pentabromophenyl methacrylate

3 pph Irgacure 1850

This composition cured to a refractive index of:

Wavelength Refractive Index

632.8 nm 1.693

1300 nm 1.660

1541 nm 1.657 A slight modification of this composition is:

35.00% Oligomer

37.00% MPSDMA

25.00% Pentabromophenyl methacrylate 3.005 Irgacure 1850

This composition cures to a T of 103 J °C as measured by DMA and determined from the peak of the tan delta curve. Example 4 An example oligomer was prepared by reacting 4,4'Jhiobisbenzene thiol with

MPSDMA in a 1 :4 molar ratio in tetrahydrofuran solvent. A shelf stabilizer package was then added. The oligomer composition was:

13.92% 4,4'Jhiobisbenzene thiol

85.78% MPSDMA

0.09% MEHQ

0.07% A-1110

0.14% Genorad 16

A core waveguide composition based on the above oligomer was:

56.00% Oligomer

25.00% MPSDMA

15.00%) Pentabromophenyl methacrylate

3.00% Irgacure 1850

1.00% Irganox 1035

This composition cured to a refractive index of:

Wavelength Refractive Index 632.8 nm 1.686

1300 nm 1.653

1541 nm 1.654 A clad composition to go with the above core was:

52.00% Oligomer

25.00% MPSDMA

15.00% Pentabromophenyl methacrylate 2.00% Ethoxylated bisphenol A dimethacrylate (SR-348 from Sartomer Co.)

2.00% 3-acryloxypropyl trimethoxy silane

3.00%o Irgacure 1850

1.00% Irganox 1035

This composition cured to a refractive index of:

Wavelength Refractive Index

632.8 nm 1.677 1300 nm 1.646 1541 nm 1.649

Example 5

A core waveguide composition using the oligomer from Example 4 was:

56.00% Oligomer

20.00% MPSDMA

20.00% Bis-(4-vinylthiophenyl) sulfide

3.00% Irgacure 1850

1.00% Irganox 1035

This composition cured to a refractive index of:

Wavelength Refractive Index 632.8 nm 1.691

1300 nm 1.659

1541 nm 1.659

It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. An oligomeric reaction product of one or more compounds having at least two thiol functional groups and one or more compounds having at least two reactive double bonds, wherein the at least one or more compounds having at least two thiol functional groups are substantially free of mercaptoacetates, mercaptopropionates and thioglycolates, and the oligomeric reaction product is substantially free of styrenic end groups.

2. The reaction product of claim 1 , wherein the one or more compounds having at least two thiol groups include at least one aromatic compound.

3. The reaction product of claim 2, wherein the aromatic compound is 4,4'- thiobisbenzene thiol.

4. The reaction product of claim 1 , wherein the one or more compounds having at least two reactive double bonds includes a compound having at least two functional groups selected from methacrylate, vinyl acrylate, vinyl ether, vinyl sulfide, propenyl ether, propenyl sulfide, allyl ether, allyl sulfide, maleimide, maleate and isopropenyl benzene groups.

5. The reaction product of claim 1 , wherein the one or more compounds having at least two reactive double bonds includes at least one compound selected from the group consisting of 1,3-diisopropenylbenzene, 1,4-benzendiacrylate, 1,4- benzenedimethacrylate and 1,3,5-triisopropenyl benzene.

6. The reaction product of claim 1 , which is predominately comprised of a thiol functional oligomer.

7. The reaction product of claim 6, wherein the thiol functional oligomer is end capped with a compound having at least two reactive double bonds.

8. The reaction product of claim 7, wherein the end cap compound is a compound having at least two functional groups selected from methacrylate, vinyl acrylate, vinyl ether, vinyl sulfide, propenyl ether, propenyl sulfide, allyl ether, allyl sulfide, maleimide, maleate and isopropenyl benzene groups.

9. The reaction product of claim 8, wherein the end cap compound is a dimethacrylate, whereby the end capped thiol functional oligomer is a dimethacrylate.

10. The reaction product of claim 9, wherein the end cap compound is ethylene glycol dimethacrylate.

11. A radiation curable composition comprising the reaction product of claim 1, and a photoinitiator.

12. The cured product of claim 11.

13. An optical waveguide comprising the cured product of a radiation curable composition that contains a photoinitiator and an oligomeric reaction product which is substantially free of styrenic end groups.

14. An oligomer that is the reaction product of an aromatic compound having two or more thiol functional groups and an aromatic compound having two or more reactive double bonds.

15. The oligomer of claim 14, wherein the thiol functional aromatic compound is 4,4'Jhiobisbenzene thiol.

16. The oligomer of claim 14, wherein the aromatic compound having two or more reactive double bonds is selected from the group consisting of 1 ,3- diisopropenylbenzene, 1,4-benzendiacrylate, 1,4-benzenedimethacrylate and 1,3,5- triisopropenyl benzene.

17. The oligomer of claim 14, which is predominately comprised of a thiol functional oligomer.

18. The oligomer of claim 17, wherein the thiol functional oligomer is end capped with a compound having at least two reactive double bonds.

19. The oligomer of claim 18, wherein the end cap compound is a compound having at least two reactive double bonds includes a compound having at least two functional groups selected from the methacrylate, vinyl acrylate, vinyl ether, vinyl sulfide, propenyl ether, propenyl sulfide, allyl ether, allyl sulfide, maleimide, maleate and isopropenyl benzene groups.

20. The oligomer of claim 19, wherein the end cap compound is a dimethacrylate, whereby the end capped thiol functional oligomer is a methacrylate.

21. The oligomer of claim 14, which is an ethylene glycol dimethacrylate end- capped reaction product, wherein the reaction product is a thiol functional compound formed by reaction of 4,4'Jhiobisbenzene thiol with 1,3-diisopropenylbenzene.

22. A radiation curable compound comprising the oligomer of claim 21 and a photoinitiator.

23. The radiation curable compound of claim 22 further comprising bis-(4- methacryloylthiophenyl) sulfide.