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Claims  |
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We claim:
1. An optically transmissive inorganic-organic hybrid bonding material
comprising an extended matrix containing silicon and oxygen atoms with at
least a fraction of the silicon atoms in the extended matrix being
directly bonded to substituted or unsubstituted hydrocarbon moieties.
2. An optically transmissive inorganic-organic hybrid bonding material in
accordance with claim 1 wherein the fraction of silicon atoms directly
bonded to a hydrocarbon moiety is at least 4%.
3. An optically transmissive inorganic-organic hybrid bonding material in
accordance with claim 2 wherein the fraction of directly bonded silicon
atoms is at least 50%.
4. An optically transmissive article comprising, two optically transmissive
components connected with a bond of the inorganic-organic hybrid material
of claim 1.
5. An optically transmissive article according to claim 4 wherein the two
optically transmissive components are selected from a group consisting of
optical fibers, planar optical waveguides, and optically active
components.
6. An optically transmissive article according to claim 5 wherein the
optically transmissive components are both optical fibers.
7. An optically transmissive article according to claim 5 wherein one
optically transmissive component is an optical fiber and the other
optically transmissive component is a planar optical waveguide.
8. An optically transmissive article according to claim 4 wherein said
first optically transmissive component is a planar optical waveguide
having a slot cut therein and said second optically transmissive component
is an optically active component positioned in the slot.
9. An optically transmissive article according to claim 4 wherein the
material comprises a hydrolyzed and cured silane selected from the group
consisting of tetraalkoxysilanes, alkyltrialkoxysilanes and
aryltrialkoxysilanes.
10. An optically transmissive article according to claim 4 wherein the
hydrocarbon moieties are derived from organic modifiers selected from the
group consisting of inert network modifiers, active network modifiers,
organic network forming modifiers, reactive polymeric modifiers and
reactive polymerizable modifiers.
11. An optically transmissive article according to claim 4 wherein the
material comprises an interpenetrating organic polymeric matrix.
12. An optically transmissive article according to claim 11 wherein
substantially no atom in the interpenetrating organic polymeric matrix is
covalently bonded to an atom in the extended silicon-oxygen matrix.
13. An optically transmissive article according to claim 4 wherein the
hydrocarbon moieties are unsubstituted or substituted alkyl or aryl
moieties.
14. An optically transmissive article according to claim 13 wherein the
hydrocarbon moiety is the polymerized product of a modifier covalently
bonded to the extended silicon-oxygen network via a Si--C bond.
15. An optically transmissive article according to claim 4 wherein the bond
contains fluorine.
16. An optically transmissive article according to claim 4 wherein the bond
contains an element for enhancing refractive index selected from the group
consisting of Ge, Ti, Zr, Hf, Er and Nd.
17. An optically transmissive article according to claim 4 wherein the
selected element is Ge or Ti.
18. An optically transmissive article according to claim 4 wherein the
fraction of silicon atoms directly bonded to the substituted or
unsubstituted hydrocarbon moieties is at least about 4 percent of the
total silicon atoms.
19. An optically transmissive article according to claim 4 wherein hydrogen
is replaced by deuterium in the hydrocarbon moieties.
20. A method of making an article composed, in part at least, of the
bonding material of claim 1 which comprises,
preparing a precursor composition including at least one silane selected
from the group consisting of tetraalkoxysilanes, alkyltrialkoxysilanes,
and aryltrialkoxysilanes and a source of a hydrocarbon moiety, and
at least partially hydrolyzing and curing the precursor composition to a
viscosity suitable for forming the article, and forming an article of
desired shape from the viscous material.
21. A method according to claim 20 which further comprises completing
hydrolysis and curing of the shaped article under conditions effective to
form an inorganic hybrid material that comprises an extended matrix
containing silicon and oxygen atoms with at least a fraction of the
silicon atoms being directly bonded to substituted or unsubstituted
hydrocarbon moieties.
22. A method in accordance with claim 20 wherein the article is optically
transmissive and comprises, in part at least, the inorganic-organic hybrid
material of claim 1.
23. A method in accordance with claim 22 wherein the article is a bond
between two optically transmissive components and the method further
comprises partially hydrolyzing and curing the precursor, composition to
form a bonding composition,
aligning the components in a spaced relationship to form a gap,
filling the gap with the bonding composition to form a connection between
the curing components, and completing hydrolysis and curing of the bonding
composition under conditions effective to form a bond of an
inorganic-organic hybrid material that comprises an extended matrix
containing silicon and oxygen atoms with at least a fraction of the
silicon atoms being directly bonded to substituted or unsubstituted
hydrocarbon moieties, whereby the optically transmissive components are
connected together.
24. A method according to claim 23 wherein both of the optically
transmissive components are optical fibers and wherein said aligning
comprises:
butting together ends of the optical fibers and withdrawing the ends (end
and edge) to form a gap to be filled by the bonding composition.
25. A method according to claim 23 wherein the first optically transmissive
component is an optical fiber and the second optically transmissive
component is a planar optical waveguide and wherein said aligning
comprises:
butting an end of the optical fiber with an edge of the planar optical
waveguide and withdrawing the ends (end and edge) to form a gap to be
filled by the bonding composition.
26. A method according to claim 23 wherein the first optically transmissive
component is a planar optical waveguide, a slot is cut therein, the second
optically transmissive component is an optically active component, and the
second component is aligned in the slot of the planar optical waveguide.
27. A method according to claim 23 which comprises applying the bonding
composition to at least one of the components at its contact point.
28. A method according to claim 23 which comprises aligning the components
in a spaced relationship and bonding the components with a bonding
composition that fills the space.
29. A method according to claim 22 which further comprises incorporating in
the bonding composition precursor a source of fluorine.
30. A method according to claim 22 which further comprises incorporating in
the bonding composition precursor a source of germanium or titanium.
31. A method according to claim 22 which further comprises incorporating in
the bonding composition precursor a modifier selected from the group
consisting of inert network modifiers, active network modifiers, organic
network-forming modifiers, reactive polymeric modifiers, reactive
polymerizable modifiers, and non-interacting, interpenetrating network
modifiers.
32. A method according to claim 22 which comprises aging the bonding
composition precursor for a period of time.
33. A method according to claim 32 which comprises aging the bonding
composition precursor by heating at a temperature under 100.degree. C. for
a period up to 5 hours.
34. A method according to claim 22 which comprises removing the alcohols
produced during hydrolysis of the bonding composition precursor to avoid
cracking during bond formation.
35. A method according to claim 22 which comprises preparing a bonding
material consisting essentially of PDMS, MTES, PFTS and PTES and varying
the ratio of MTES:PTFS+PTES to control the refractive index.
36. A method according to claim 25 which comprises preparing a bonding
composition containing about 8% PDMS, 63-69% MTES, 20-14% PTFS and about
9% PTES, the contents representing the proportion of silicon atoms in the
composition, the composition producing a bond having a refractive index of
1.45-1.47 at 632 nm.
37. A method in accordance with claim 22 which comprises replacing hydrogen
in the hydrocarbon moieties with deuterium.
38. A hydrolyzable and curable bonding sol-gel composition comprising:
one or more silanes, selected from the group consisting of a
tetraalkoxysilane, an alkyltrialkoxysilane, and an aryltrialkoxysilane and
a source of substituted or unsubstituted hydrocarbon moieties.
39. A composition in accordance with claim 38 in which the selected silane
constitutes at least 50% of the composition.
40. A composition in accordance with claim 38 wherein the sol-gel
composition further comprises an organic component selected from the group
consisting of inert network modifiers, active network modifiers, organic
network-forming modifiers, reactive polymeric modifiers, reactive
polymerizable modifiers, non-interacting, and interpenetrating network
modifiers.
41. A composition according to claim 40 wherein a selected inert network
modifier is an (alkyl)alkoxysilane, or an (aryl)alkoxysilane.
42. A composition according to claim 40 wherein a selected organic
network-forming modifier is an (alkacryloxyalkyl) alkoxysilane, a
vinylsilane or an (acryloxyalkyl) alkoxysilane, an (epoxy-substituted
alkyl) alkoxysilane.
43. A composition according to claim 40 wherein a selected reactive
polymeric modifier is a silanol-terminated polydialkylsiloxane, or a
trialkoxysilyl-terminated polydialkylsiloxane.
44. A composition according to claim 38 wherein the hydrolyzable and
curable, bonding sol-gel composition further comprises a
polydialkylsiloxane.
45. A composition according to claim 38 wherein the hydrolyzable and
curable, bonding sol-gel composition further comprises an
(alkacryloxyalkyl) alkoxysilane.
46. A composition according to claim 45 wherein the hydrolyzable and
curable, bonding sol-gel composition further comprises a photoinitiator.
47. A composition according to claim 38 wherein the hydrolyzable and
curable, bonding sol-gel composition further comprises at least one
alkoxide of an element selected from the group consisting of Ge, Ti, Zr,
Hf, Er, Nb and combinations thereof.
48. A composition according to claim 38, wherein the hydrolyzed composition
further comprises a fluorine source.
49. A hydrolyzable and curable bonding sol-gel composition in accordance
with claim 38, comprising:
a silane selected from the group consisting of a tetralkoxysilane, an
alkyltrialkoxysilane, and aryltrialkoxysilane, a trialkoxysilane, and
alkacryloxypropyltrialkoxysilane, and combinations thereof, in a total
amount of from about 50 to about 95 mole % of the sol-gel composition;
a network modifier selected from the group consisting of a monomeric
dialkyldialkoxysilane and a polymeric polydialkylsilane in an amount of
from about 4 to about 25 mole % of the sol-gel composition;
an aryltrifluorosilane in an amount of from about 5 to about 20 mole % of
the sol-gel composition;
a tetraalkoxytitanium in an amount of from about 0 to about 10 mole % of
the sol-gel composition; and
a tetraalkoxygermanium in an amount of from about 0 to about 20 mole % of
the sol-gel composition.
50. A composition in accordance with claim 38 wherein hydrogen atoms are
replaced by deuterium atoms. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The field is an optically transmissive material and method that are useful
in bonding two components, such as optical fibers and/or planar
structures, to form an optical network.
BACKGROUND OF THE INVENTION
Optical waveguide structures fabricated in planar forms can perform a
variety of functions in optical light-paths. These include optical
coupling in various configurations, such as multi-channel star arrays, and
multiplexing or demultiplexing through phasar or grating devices. Such
waveguides also hold the promise of being lower in cost than discrete
optical devices which are fabricated from fiber and micro-optic
components. In the future, they may provide a platform for hybrid,
electro-optic devices.
For each signal to be processed, the optical waveguide must be connected to
an optical fiber carrying an input signal, and to a second optical fiber
carrying the processed signal. Traditionally, these connections, commonly
referred to as "pigtails," have been accomplished with organic adhesives,
such as methacrylate or epoxy adhesives. These polymeric adhesives offer
simple fabrication, fair index matching, and good bonding characteristics.
However, they are hydratively unstable. This limits their usefulness in wet
environments such as are encountered in underwater and high humidity
applications. It has also been reported that many of these adhesives have
questionable stability when subjected to environmental extremes in
temperature and pressure.
Even a slight deterioration in the optical properties of the adhesive will
seriously impair transmission of optical signals through the
fiber-waveguide network. Thus, instability can have disastrous
consequences, making the organic, or "soft," pigtail unsuitable for many
applications.
The need for a stable, optical fiber-to-planar waveguide connection has led
to a vitreous seal using a glass ftit as such, or admixed with a mill
addition. The resulting joint is hydratively stable and relatively strong.
However, it may involve stresses caused by differences in the coefficients
of thermal expansion between the components; also a higher than desired
sealing temperature.
The present invention is directed to overcoming these and other
deficiencies in the art.
SUMMARY OF THE INVENTION
The present invention relates to an inorganic-organic hybrid material that
comprises an extended matrix containing silicon and oxygen atoms with at
least a fraction of the silicon atoms in the extended matrix being
directly bonded to at least one hydrocarbon moiety.
The present invention also relates to a method of producing an article from
the material which comprises:
preparing a material precursor comprising at least one silane selected from
the group consisting of a tetraalkoxysilane, an alkyltrialkoxysilane, or
an aryltrialkoxysilane and a source of a hydrocarbon moiety,
at least partially hydrolyzing and condensing the precursor material to a
viscosity suitable for forming the article.
In a specific aspect, the invention is an optically transmissive article,
and a method of producing a bond between two transmissive components which
comprises using the partially hydrolyzed and cured material as a bonding
composition aligning the components in a spaced relationship to form a
gap, filling the gap with the bonding composition to form a connection
between the components completing hydrolysis and curing of the bonding
composition under conditions effective to form a bond of an
inorganic-organic, hybrid material that comprises an extended matrix
containing silicon and oxygen atoms with at least a fraction of the
silicon atoms being directly bonded to at least one hydrocarbon moiety,
whereby the optically transmissive components are connected together.
The present invention further relates to a hydrolyzable and curable sol-gel
composition. The sol-gel composition includes at least one silane selected
from the group consisting of a tetraalkoxysilane, an alkyltrialkoxysilane,
and an aryltrialkoxysilane.
The methods and compositions of the present invention have been developed
for use in connecting components of optically transmissive networks, and
are so described. Such networks include optical fibers and optical planar
waveguides, connected by an inorganic-organic, "hard" pigtail. The
connections thus made exhibit improved hydrative stability and resistance
to temperature, pressure and humidity extremes. The connections can also
minimize back-reflection by providing a close index of refraction match to
the core of the waveguide. This obviates the need to make expensive,
precision-machined facets in the optically transmissive materials.
Furthermore, they can be manufactured without the application of high
heat. This avoids stresses caused by differences in coefficients of
thermal expansion between the components.
It is contemplated, however, that the unique combination of properties
found in the new materials are valuable in bulk products as well. These
include, for example, molded optical networks, which may, optionally be
laminated within a substrate, specifically designed components and like
optically transmissive articles.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional drawing of a joint between an optical fiber and
an optical planar waveguide illustrating one form of optically
transmissive article according to the present invention.
FIG. 1a is an enlarged, cross-sectional drawing of a portion of FIG. 1
showing the joint 16 between optical fiber 2 and planar device 10.
FIG. 2 is a cross-sectional drawing illustrating a modified joint between
an optical planar waveguide and an optically active component according to
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The article of the present invention is an optical network comprising at
least two optically transmissive components connected with a bond. The
invention arose from research directed at bonding an optical fiber to a
planar waveguide structure, a practice known as "pigtailing." Accordingly,
it is so described. However, its broader application will be evident.
The shortcomings of "soft" organic bonds and "hard" glass bonds have been
noted. A key feature of the invention is an improved "hard" bond for
connecting optically transmissive components.
Such a bond should have a good refractive index match to the components
close to 1.46. It should also have a total signal loss of not over 0.2 db.
The bond must be insensitive to hydrative effects (85.degree. C. at 85%
relative humidity for thousands of hours), and must be stable over a
thermal cycling range of -40.degree. to +85.degree. C. The pigtail
preferably has sufficient strength to support a one 1b. tensile load.
Finally, the bond (pigtail) must lend itself to rapid and flexible
production processes. For example, it may be desired to successively bond
a substantial number of fibers to a multiport, planar structure. In such a
process, a bond desirably is made in no more than two minutes while
retaining other properties.
A preferred form of the optical network comprises an optical fiber and a
planar structure such as an amplifier or a coupler. The strength of the
connection in this embodiment can be enhanced by attaching the fiber to
the waveguide at additional locations. FIGS. 1 and 2 in the accompanying
drawing are cross-sectional views showing two forms of the inventive
article.
In FIGS. 1 and 1a optical fiber 2 has a portion of its polymeric coating 4
removed to expose a segment of cladding 6. Core 8, contained within
cladding 6, is aligned with planar device 10, which is supported by
substrate 14 of planar waveguide 12. Core 8 is optically connected to
planar device 10 with bonding material 16 in accordance with the method of
the present invention.
Optical fiber 2 is also attached to planar waveguide 12 with mass 18. Mass
18 contacts cladding 6 of optical fiber 2 and also bonds to substrate 14
of optical waveguide 12. Mass 18 can be a glass formed by sol-gel
processes, a fired glass frit paste, or a photocured polymer. Optical
fiber 2 is also attached to planar waveguide 12 with overcoating material
20. Overcoating material 20 covers, protects and bonds to cladding 6 and
polymeric material 4 of optical fiber 2 as well as bonding to substrate 14
of optical waveguide 12. This strengthens the attachment of optical fiber
2 to optical waveguide 12.
The optical network can, alternatively, include a planar optical waveguide
having a slot cut therein and an optically active component inserted into
the slot. Suitable optically active components may modify the character of
the light passing through a waveguide, such as by applying a polarizing
shift, isolation, or filtering.
FIG. 2 shows a typical configuration of a planar waveguide with such a
component added. Slot 22, machined into planar waveguide 24, has a depth
sufficient to penetrate through overclad layer 26, through core layer 28,
and, optionally, into substrate 30. Slot 22 has a width sufficient to
receive optically active component 32. Preferably, it has additional width
to permit optimizing the position of optically active component 32 to
minimize transmission loss. A hydrolyzed, bonding composition 34 is
applied to fill slot 22 and wet optically active component 32. The
composition fills slot 22. It is cured by heating to immobilize optically
active component 32 in its optimal position. Preferably, bonding material
34 matches the index of refraction of core layer 28 to minimize
back-reflection. In addition, bonding material 34 should be stable to
thermal cycling and damp environments.
Practice of the invention is initiated by providing two optically
transmissive components selected from optical fibers, optically active
components and planar optical waveguides. The invention is particularly
applicable to inorganic components such as silica, aluminosilicate, or
borosilicate glass components.
Where one of the components is an optical fiber, it is preferred that all
material surrounding the core and clad, such as the polymeric jacket, be
removed. This can be effected by conventional stripping tools for optical
fiber.
The method of the present invention can also be used to connect a planar
waveguide with a "drop-in" optically active component as shown in FIG. 2.
This may be a passive optical component, such as a beam splitter or an
optical filter. These components are typically inserted into slots
machined into the surface of the planar waveguide. These drop-in
configurations require that back-reflection at the interface between the
drop-in component and the planar waveguide be low. This requires that the
drop-in component, typically both surfaces of the drop-in component, be
optically connected to the planar waveguide.
Alignment in such an application can be accomplished actively or passively.
For example, a drop-in component can be actively held in a loss-minimized
position during contact of the components and final curing of the bonding
composition. Alternatively, alignment can be ensured by providing a
positional slot (commonly referred to as a positioning groove), and simply
inserting the drop-in component into the slot. The contact point is a line
defined by the intersection of the drop-in component and the side of the
slot. Curing the hydrolyzed bonding composition connects the drop-in
component and the optical waveguide along this line. This reduces
back-reflection at the interface.
Where the optically transmissive components are two optical fibers,
alignment can be effected by butting the ends of the optical fibers to
each other. Where an optical fiber is to be connected to a planar
waveguide, alignment can be effected by butting an end of the optical
fiber with an edge of the planar waveguide. In either case, the components
are aligned at the contact point. To allow for expansion during thermal
treatment, the components must be slightly spaced for bonding, preferably
about 5-20 microns.
Precise alignment of the components may be obtained by directing a laser
beam through one of the components, such as an optical fiber. Throughput
of the laser beam into the other component is monitored. The relative
spatial positions of the two components are adjusted until throughput is
maximized.
The optically transmissive components may be coated with the bonding
composition at their respective contact points prior to aligning. Coating
can also be carried out by introducing a drop of the bonding composition
between the spaced components so that it spans the gap. Alternatively, the
bonding composition can be sprayed or painted onto the aligned contact
point, or one or both of the components may be dipped into a vessel
containing the bonding composition. Alignment of the components may be
adjusted subsequent to contacting them with the bonding composition.
Practice of the invention further involves preparing a bonding composition.
This comprises a mixture of hydrolyzable precursors composed essentially
of at least one alkoxysilane selected from the group consisting of
tetraalkoxysilanes, alkyltrialkoyxsilanes and aryltrialkoxysilanes.
Optionally, it contains modifiers as noted, infra.
The mixture may be dissolved in a solvent such as an alcohol and hydrolyzed
by addition of acid and water. The composition is aged while hydrolysis
and condensation proceed to form a viscous bonding composition. This
partially hydrolyzed and condensed material is hereafter termed the
"bonding composition" to contrast with the ultimate bond wherein
hydrolysis and curing are essentially complete.
It is desirable to drive the hydrolysis and condensation reactions to a
sufficient degree so that no precursor is lost during solvent evaporation.
Studies have shown that sufficient aging at room temperature for this
purpose requires on the order of 50 hours. It has been found that mild
heating of the mixture below 100.degree. C. can shorten the time to less
than 5 hours. In particular, a comparable degree of condensation (about
80%) was achieved by heating at about 75.degree. C. for three hours. Quite
surprisingly, gels produced by the accelerated aging process were found
less prone to cracking at high heating rates in forming the ultimate bond.
Tetraalkoxysilanes are silicon atoms having four alkoxy groups bound
thereto. The four alkoxy groups are usually the same, but this is only for
convenience. Alkoxy, as used herein, is meant to include the deprotonated
form of any alcohol, including aliphatic alcohols.
Alkyltrialkoxysilanes are silicon atoms having three alkoxy groups and one
alkyl group bonded thereto. Alkyl is also meant to include arylalkyls.
Alkyltrialkoxysilanes suitable for use in the practice of the present
invention include, for example, methyltrimethoxysilane ("MTMS") and
methyltriethoxysilane ("MTES").
Aryltrialkoxysilanes are silicon atoms having three alkoxy groups and one
aryl group bonded thereto. As used herein, aryl also is meant to include
alkylaryl moieties. Aryltrialkoxysilanes suitable for use in the practice
of the present invention include, for example, phenyltrimethoxysilane
("PTMS") and phenyltriethoxysilane ("PTES").
The hydrolyzed bonding composition can advantageously include organic
components which, on a microscopic level, modify the inorganic network
formed by condensation of the silane, hydrolysis products. The organic
component can modify the network with an organo-metallic bond to a silicon
atom. Alternatively, the organic component can coexist as an
interpenetrating, intermolecular, or intramolecular network within the
inorganic network, which does not attach to a silicon atom.
Suitable organic components which can be incorporated into the hydrolyzed
bonding composition include one or more hydrolysis products of inert
network modifiers, active network modifiers, organic network-forming
modifiers, reactive polymeric modifiers, reactive polymerizable modifiers,
and non-interacting, interpenetrating network modifiers.
Inert network modifiers include alkylalkoxysilanes and arylalkoxysilanes,
particularly those having the formula (R.sup.1).sub.n (R.sup.2 O).sub.4-n
Si, wherein n is 1, 2 or 3. OR.sup.2 is an alkoxy moiety, such as ethoxy
and methoxy. R.sup.1 can be an alkyl moiety or an aryl moiety, including,
for example, methyl, ethyl and phenyl.
The bonding composition can include from about 0 to about 100 mole %,
preferably from about 50 to about 100 mole %, more preferably from about
50 to about 96 mole % of the hydrolysis product of the inert network
modifier, such as the hydrolysis product of methyltriethoxysilane. Further
details with respect to inert network modifiers can be found in the
literature.
Active network modifiers are (substituted alkyl)alkoxysilanes and
(substituted aryl)alkoxysilanes. At least one of the alkyl or aryl
substitutents is a functional group capable of forming complexes with
metal atoms or ions, such as an amino functional group, a mercapto
functional group, or a hydroxy functional group. It is believed that the
functional group promotes surface adhesion of the bonding composition to
inorganic materials. Active network modifiers may also promote adhesion to
organic surfaces.
Suitable active network modifiers are those having the formula
(R.sup.3).sub.n (R.sup.2 O).sub.4-n Si, wherein n is 1, 2 or 3 and wherein
OR.sup.2 is an alkoxy moiety. R.sup.3 can be a amine-, carboxy-, mercapto-
or hydroxy-substituted alkyl or aryl moiety. The hydrolysis product of the
active network modifier is preferably present in an amount from about 1 to
about 25 mole %.
As indicated above, the bonding composition can also include one or more
hydrolysis products or organic network-forming modifiers, reactive
polymeric modifiers, or reactive polymerizable modifiers. The hydrolysis
products of these modifiers, when polymerized, are believed to form
organic networks that are covalently bonded to the inorganic network via
Si--C bonds.
Organic network-forming modifiers are (substituted alkyl)alkoxysilane
compounds that are substituted with groups capable of participating in
reactions with other like-substituted (substituted alkyl)alkoxysilane
compounds.
Suitable network-forming modifiers include those having the formula
(.sup.4).sub.n (R.sup.2 O).sub.4-n Si, wherein n is 1, 2 or 3 and OR.sup.2
is an alkoxy moiety, suitable examples of which are ethoxy and methoxy. R
can be a substituted alkyl moiety or aryl moiety, such as an
alkacryloxyalkyl-, an acryloxyalkyl-, a vinyl-, or an
(epoxy-substituted)alkylsilane.
The hydrolyzed bonding composition can include from about 0 to about 95
mole %, preferably from about 0 to about 50 mole % of a hydrolysis product
of an organic network-forming modifier, such as the hydrolysis product of
methacryloxy-propyltriethoxysilane. When used to impart functional
character, such as to permit photocuring, the hydrolysis products of
organic network-forming modifiers are preferably present in an amount of
from about 20 to about 50 mole %.
Where organic network-forming modifiers are employed, it can be
particularly advantageous to include a photoinitiator in the hydrolyzed
bonding composition. Suitable photoinitiators include titanocene radical
photoinitiators, such as IRGACURE.TM. 784 or cationic ferrocinium
photoinitiators, such as IRGACURE.TM. 261 (both available from Ciba Geigy,
Ardsley, N.Y.). The photoinitiators, where employed, are preferably
included in the bonding composition in amounts less than about 0.8 weight
percent, preferably about 0.2 to about 0.8 weight percent.
Reactive polymeric modifiers are inorganic or organic polymers which are
capable of participating in condensation reactions with hydrolyzed
tetraalkoxysilanes, alkyltrialkoxysilanes, or aryltrialkoxysilanes.
Suitable reactive polymeric modifiers include those having the formula
(R.sup.2 O).sub.3 O--Si--O--(P).sub.n --Si--O(OR.sup.2).sub.3, or
(HO)--(P).sub.n --OH, where (P).sub.n represents an organic polymer, such
as a polytetramethylene oxide, and OR.sup.2 is an alkoxy moiety, such as
ethoxy and methoxy.
Other suitable reactive polymeric modifiers include polydialkylsiloxanes
having the formula R.sup.5 O.brket open-st.Si(R.sup.6).sub.2 --O.brket
close-st..sub.n R.sup.5, wherein n is an integer from about 2 to about 50,
R.sup.5 is a hydrogen, or an alkyl or aryl moiety, R.sub.6 is an alkyl
group, preferably a methyl group. Preferably, the reactive polymeric
modifier is a polydimethylsiloxane having a molecular weight of from about
200 to about 900 g/mole, preferably about 550 g/mole.
The bonding composition can include from about 0 to about 40 mile %,
preferably from about 4 to about 8 mole %, of the hydrolysis product of a
reactive polymeric modifier.
Reactive polymerizable modifiers are substituted alkylalkoxysilane
compounds which can form organic networks only in combination with a
second polymerizable component which is reactive with the substitutent on
the substituted alkylalkoxysilane compound. The second polymerizable
component may or may not be bonded to an alkoxysilane.
The bonding composition can include from about 0 to about 95 mole %,
preferably from about 0 to about 50 mole % of the hydrolysis product of
the reactive polymerizable modifier. When used to impart functional
character, such as to allow photocuring or increase the plasticity of the
extended silicon-oxide matrix, the hydrolysis product of the reactive
polymerizable modifier is preferably present in an amount of from about 20
to about 50 mole %. These reactive polymerizable modifiers contain a
hydrolytically stable silicon-carbon bond.
Non-interacting interpenetrating network modifiers are organic polymers.
Preferably, they do not contain groups capable of forming Si--C bonds with
silicon atoms, or precursors to such organic polymers.
These non-interacting, interpenetrating network modifiers can be
incorporated into the bonding composition in amounts of from about 0 to
about 50 mole %. They may be used to impart functional character, such as
to increase plasticity or to introduce photoactive polymers into the
extended silicon-oxide matrix. They are preferably present in an amount of
from about 5 to about 25 mole %. Further details with respect to these
non-interacting interpenetrating network modifiers can be found, for
example, in U.S. Pat. No. 5,412,016 to Sharp, which is hereby incorporated
by reference.
Increased indices of refraction in the bond composition may be obtained by
further including one or more reactive compounds, such as the alkoxide of
an element selected from the group consisting of Ge, Ti, Zr, Hf, Er, Nd.
The alkoxides can, optionally, be hydrolyzed to their hydrolysis products.
The amount of alkoxide, and the hydrolysis products thereof, collectively
present in the bonding composition depends on the refractive index desired
in the bond. Suitable amounts of alkoxide and hydrolysis products thereof
may range from about 0 to about 25 mole %, preferably from about 0 to
about 15 mole %.
The refractive index of the polymerized hydrolyzed bonding composition can
also be varied by incorporating aryltrialkoxysilanes (particularly
phenyltrialkoxysilanes), and/or aryltrifborosilanes (particularly
phenyltrifluorosilanes) into the bonding composition.
The bonding composition can, optionally, contain a fluoride source, such as
a hydrolysis product of a fluorosilane, for example, an alkylfluorosilane.
Other suitable fluoride sources, such as hydrogen fluoride, ammonium
bifluoride and other fluoride salts which dissociate, may be used. The
incorporation of a fluoride source is advantageous where suppression of
the ca. 3300 cm.sup.-1 SiO--H infrared absorption band is desired. Such a
case is where the material connecting the optically transmissive materials
must pass infrared radiation without significant attenuation.
In a particularly preferred embodiment, an aryltrifluorosilane was included
in the hydrolyzable precursor composition. It was found that the mixture
of PDMS, MTES, PTES and PTFS could be hydrolyzed directly, that is,
without the presence of a solvent. After an aging period, during which the
water is consumed and the alcohol is produced, the sol is a clear liquid.
The sol can then by "dried" by allowing the alcohol to evaporate over
several hours at room temperature. The clear, colorless fluid becomes
significantly more viscous, and continues to do so until it gelled. The
viscous liquid can be thermally treated to provide the solid gel with a
total mass loss of only about 10%. This permits greater flexibility in
processing crack-free bonds.
The amount of fluoride source present in the bonding composition depends
primarily on the acceptable level of infrared absorption. A significant
reduction of the Si--OH absorption band can be achieved with the
hydrolysis product of a fluorosilane ranging from 0 to about 25 mole %,
preferably from about 5 to about 15 mole %.
Particularly preferred bonding compositions of the present invention are
curable sol-gels which include a silane selected from the group consisting
of a tetraalkoxysilane, an alkyltrialkoxysilane, an aryltrialkoxysilane, a
trialkoxysilane, an alkacryloxypropyltrialkoxysilane and combinations
thereof, in a total amount of from about 50 to about 95 mole %. The
curable sol-gel composition also includes a network modifier selected from
the group consisting of a monomeric dialkyldialkoxysilane, a
diacryldialkoxysilane, and a polymeric polydialkylsilane in an amount of
about 4 to about 25 mole %; an aryltrifluorosilane in an amount of about 5
to about 20 mole %; a tetraalkoxytitanium in an amount of about 0 to about
10 mole % and a tetraalkoxygermanium in an amount of about 0 to about 20
mole % all contents based on the total sol-gel composition.
Hydrolyzed bonding compositions can be prepared by adding water to
precursor bonding compositions which contain an alkoxysilane. Hydrolysis
begins immediately upon the addition of water, and results in the
replacement of alkoxy groups with hydroxy groups. The rates of hydrolysis
of the various silanes depend on the nature of the substitutents bonded to
the silicon atoms. Therefore, it can be advantageous to begin the
hydrolysis process of various alkoxysilanes (or alkoxides of other
elements, such as tetraethoxygermanium) separately and mix them together
after some or all of the alkoxy groups have been hydrolyzed.
The amount of water used in carrying out the hydrolysis phase of the
process can vary widely. It may be about 25% to about 800% of the
stoichiometric amount required to completely hydrolyze all of the
alkoxy-silicon bonds present in the precursor bonding compositions based
upon the reaction 2.tbd.SiOR+H.sub.2 O.fwdarw..tbd.Si--O--Si.tbd.+ROH.
Preferably, the amount of water added is from about 75% to about 100% of
the stoichiometric amount.
Hydrolysis can be carried out using the following general procedure. A
precursor bonding composition, including a selected alkoxysilane, together
with one or more of the optional additive modifiers, is dissolved in a
suitable solvent. Preferably, the solvent is non-reactive with, and
capable of solubilizing, all of the precursor bonding composition. The
preferred solvent is ethanol. Where reaction rates of the precursors are
sufficiently similar, the precursor bonding composition may be mixed and
hydrolyzed directly, without a solvent.
Water and acid are added to the solution of the precursor bonding
composition. The water and acid are first mixed in a solvent, which may be
the same solvent used to dissolve the precursor bonding composition. The
acid and water can be added all at once, slowly, either drop-wise or in
several Aliquots. The addition is carried out over the course of 20
minutes to 8 hours, preferably 1 to 3 hours, preferably, while maintaining
the reaction mixture at reflux and with stirring. After the addition is
complete, the reaction mixture may be stirred at reflux for an additional
period of time, preferably about 30 minutes. To precisely control the
amount of water introduced into the reaction mixture, the addition and
optional subsequent stirring and refiuxing can be carried out in an inert
atmosphere, such as nitrogen or argon. When reactions are conducted with
no added solvent, the water is added in one or two aliquots, and mixed
vigorously at temperatures from about 50 to about 90.degree. C., until
homogeneous.
Hydrolyzed bonding compositions containing primarily alkyl trialkoxides can
be advantageously prepared by the following alternate general method. A
precursor bonding composition, including a selected alkoxysilane, together
with one or more of the optional modifiers, is prepared without the
addition of solvent. Water in the desired amount is added to the precursor
bonding composition. The addition of water can be carried out at room
temperature, or in a hot water bath. Preferably, the precursor bonding
composition, prior to addition of water, is at a temperature from about
60.degree. C. to about 80.degree. C. The amount of water with which the
precursor bonding composition reacts is better controlled if the addition
is conducted under conditions which exclude moisture in the ambient air,
such as by capping the reaction vessel.
Addition of water to the precursor bonding composition frequently produces
a phase separated mixture. In these circumstances, the phase separated
mixture can be agitated to dissolve the water in the precursor bonding
composition. Agitation is preferably carried out in a vessel isolated from
the ambient atmosphere, such as with a cap. After agitation, the system is
preferably vented (if capped) and then rested, preferably isolated from
the ambient atmosphere, at a temperature from room temperature up to about
100.degree. C. for a period of time from about 15 minutes to about 6
hours. After cooling, the hydrolyzed bonding composition can, optionally,
be aged, preferably at room temperature and for from about 1 to about 10
days.
Both germanium and titanium alkoxides hydrolyze rapidly. It is desirable,
therefore, when they are to be included in the precursor bonding
composition, to delay their addition to the composition until the
alkoxysilanes are at east partially hydrolyzed. The delay incorporates the
germanium and titanium more uniformly into the inorganic matrix.
The hydrolysis reaction may be catalyzed by a mineral acid or an organic
acid, preferably HCl. The amount of acid used in the hydration reaction
can be from about 0 to about 5%, expressed in terms of equivalents of acid
per mole of water used. When the precursor bonding composition contains a
fluoride source, such as PTFS, the use of acid provides little advantage.
The amount of water used in the hydrolysis reaction can be from about 10%
to about 200%, expressed in terms of moles of water per moles of
hydrolyzable alkoxy group. The stoichiometric hydrolysis of one mole of
alkoxy group requires 0.5 moles of water. In cases where a
polydialkylsiloxane is contained in the precursor bonding composition, the
amount of water is preferably from about 45% to about 55%.
The resulting hydrolyzed bonding composition can be stored at room
temperature for from about 3 to about 30 days before use in connecting
optically transmissive components. Shelf life can frequently be extended
by employing dimethylformamide as the reaction solvent, or as a cosolvent
with an alcohol.
In cases where the bonding composition contains germanium or titanium, its
shelf life can be extended by adding the germanium or titanium alkoxide to
the sol after hydrolysis of the alkoxy silanes is at least partially
completed. Shelf life can also be extended by reducing the amount of water
employed in the hydrolysis process, such as from about 50% to about 25% of
the stoichiometric amount.
The shelf life of sols containing fluoride sources, such as PTFS, can be
extended by reducing the amount of water employed, or by carrying out the
hydrolysis reaction at lower temperatures. A reaction temperature of from
about 30.degree. C. to about 60.degree. C. is preferred.
The bonding composition is applied to connect the two aligned components at
their contact point. The bonding composition is then cured. Curing, or
condensing, as used in this context, refers to the inorganic component of
the hydrolyzed bonding composition. It can be effected at room temperature
over a prolonged period of time. However, it is usually desirable to
accelerate the process, such as by application of heat. Heat can be
applied from any conventional source, such as a flame, a heat gun, a high
temperature oil bath, or radiation, such as with a focused infrared laser.
The amount of heat applied is dependent on the presence of solvent in the
preparation. A solvent free preparation largely avoids bubbling and
cracking. With a solvent present, the heat applied must be controlled. It
is desirable to cure the hydrolyzed bonding composition quickly. However,
too rapid heating can cause significant trapping of solvent as bubbles,
cracking due to rapid shrinkage, or misalignment of the components being
connected. The temperature for curing is about 150.degree. C. to about
300.degree. C., preferably 225.degree. C. to about 250.degree. C. The
cured bonding composition is sufficiently strong to withstand normal
handling.
To minimize signal losses, alignment is actively maintained until the
bonding composition spatially fixes the optical components relative to
each other with sufficient strength to withstand typical handling. In some
instances, the optical and thermal properties of the polymerized bonding
composition can be improved by further consolidating the cured bonding
composition. To this end, the cured bonding composition may be exposed to
a higher temperature, but not sufficiently high to cause significant
expansion of the components being connected.
In cases where the hydrolyzed bonding composition contains an organic
network-forming modifier, and a photoinitiator, the bonding composition
can be set to spatially fix the components without complete curing. The
bonding composition can be set, for example, by exposing it to radiation.
Typically, this is ultraviolet light having a wavelength of from about 360
nm to about 370 nm, at a power of fro | | |
