U.S. patent number RE31,868 [Application Number 06/479,472] was granted by the patent office on 1985-04-16 for low attenuation optical fiber of deuterated polymer.
This patent grant is currently assigned to Mitsubishi Rayon Co., Ltd.. Invention is credited to John K. Beasley, Richard Beckerbauer, Henry M. Schleinitz, Frank C. Wilson.
United States Patent |
RE31,868 |
Beasley , et al. |
April 16, 1985 |
Low attenuation optical fiber of deuterated polymer
Abstract
Light-transmitting optical fiber having a core of a
.[.deuterated acrylate.]. polymer .Iadd.selected from the group
consisting of a deuterated methacrylate homopolymer, a deuterated
methacrylate copolymer and a deuterated methacrylate/acrylate
copolymer which .Iaddend.exhibits remarkably high transmission of
light in the visible and at certain wavelengths in the
near-infrared region of the spectrum.
Inventors: |
Beasley; John K. (Greenville,
DE), Beckerbauer; Richard (Wilmington, DE), Schleinitz;
Henry M. (Kennett Square, PA), Wilson; Frank C.
(Wilmington, DE) |
Assignee: |
Mitsubishi Rayon Co., Ltd.
(Tokyo, JP)
|
Family
ID: |
27046252 |
Appl.
No.: |
06/479,472 |
Filed: |
March 28, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
842301 |
Oct 14, 1977 |
04138194 |
Feb 6, 1979 |
|
|
Current U.S.
Class: |
385/143; 385/145;
428/373; 428/394 |
Current CPC
Class: |
B29D
11/00721 (20130101); D01F 8/10 (20130101); G02B
1/048 (20130101); B29C 48/15 (20190201); G02B
6/02033 (20130101); G02B 1/048 (20130101); C08L
33/10 (20130101); B29L 2011/0075 (20130101); Y10T
428/2967 (20150115); B29C 48/00 (20190201); Y10T
428/2929 (20150115); B29C 48/05 (20190201) |
Current International
Class: |
B29D
11/00 (20060101); D01F 8/10 (20060101); D01F
8/04 (20060101); B29C 47/02 (20060101); G02B
1/04 (20060101); B29C 47/00 (20060101); G02B
005/172 () |
Field of
Search: |
;350/96.29,96.30,96.31,96.32,96.34,96.12 ;428/373,394 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Gunder et al., Instruments and Experimental Techniques, Sep.-Oct.
1971, vol. 14, No. 5, Part 1, "Plastic Scintillators Containing
Deuterium", pp. 1348-1350. .
Materials for Chemical Industry, vol. 29, No. 4, 1961, pp. 163-177.
.
Kunststoff-Handbuch, vol. IX, Carl Hanser Verlag Munchen, 1975, pp.
183-193. .
Nagai et al., "Infrared Spectra of Deuterated Polymethyl
Methacrylates", J. Polymer Sci., vol. 62, 595-598, 1962. .
Nagai, "Infrared Spectra of Stereoregular Polymethyl Methacrylate",
J. of App. Polymer Sci., vol. 7, pp. 1697-1714, 1963. .
Mihailov et al., "Infrared Spectra of Deuterated Poly(-methyl
methacrylate)", Die Makromolekulare Chemie, vol. 176, pp. 789-794,
(1975). .
Ayrey et al., "Inverse Isotope Effect on the Rate of Polymerization
of Deuterated Methylmethacrylate" Polymer, No. 16, p. 623,
1975..
|
Primary Examiner: Lee; John D.
Assistant Examiner: Gonzalez; Frank
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue &
Raymond
Claims
We claim:
1. An optical fiber comprising a core and cladding which consist
essentially of organic high polymers, said core being a
.[.deuterated acrylate.]. polymer .Iadd.selected from the group
consisting of a deuterated methacrylate homopolymer, a deuterated
methacrylate copolymer and a deuterated methacrylate/acrylate
copolymer .Iaddend.and containing less than 20 mg of hydrogen per
gram of polymer as measured by nuclear magnetic resonance of 60
MHz.
2. The optical fiber of claim 1 wherein said core is a polymer
comprising at least 60 mol % deuterated methyl methacrylate
units.
3. The optical fiber of claim 1 wherein said core contains less
than 10 mg of hydrogen per gram of polymer as measured by nuclear
magnetic resonance at 60 MHz.
4. The optical fiber of claim 3 wherein said core is a polymer
comprising at least 90 mol % deuterated methyl methacrylate
units.
5. The optical fiber of claim 1 wherein said core contains less
than 1 mg of hydrogen per gram of polymer as measured by nuclear
magnetic resonance at 60 MHz.
6. The optical fiber of claim 5 wherein said core is a polymer
which comprises at least 95 mol % deuterated methyl methacrylate
units.
7. The optical fiber of claim 5 wherein said core is a polymer
which consists essentially of perdeuterated methyl methacrylate
units.
8. The optical fiber of claim 1 wherein said cladding is a
copolymer of ##STR4## wherein X is F, H or Cl; Y is CH.sub.3 or H;
m is an integer from 1 to 6; and n is an integer from 2 to 10, with
at least one of the methyl or ethyl esters of acrylic or
methacrylic acids.
9. The optical fiber of claim 1 wherein said cladding is a
copolymer of methyl methacrylate and ##STR5## where p takes the
values of integers from 1 to 8 with a major proportion being that
where p is 2 and 3.
10. The optical fiber of claim 8 wherein said core contains less
than 10 mg of hydrogen per gram of polymer as measured by nuclear
magnetic resonance at 60 MHz.
11. The optical fiber of claim 8 wherein said core is a polymer
which comprises at least 95 mol % perdeuterated methyl methacrylate
units.
12. The optical fiber of claim 9 wherein said core contains less
than 1 mg of hydrogen per gram of polymer as measured by nuclear
magnetic resonance at 60 MHz.
13. The optical fiber of claim 9 wherein said core is a polymer
which consists essentially of perdeuterated methyl methacrylate
units.
Description
BACKGROUND OF THE INVENTION
The present invention relates to low loss optical fiber which has a
core of deuterated polymer, and cladding of polymer which has an
index of refraction lower than that of the core.
Optical fibers are well known in the art for transmission of light
along a length of filament by multiple internal reflections of
light. Great care is taken to minimize light losses due to
absorption and scattering along the length of the filament, so that
light applied to one end of the optical filamentary material is
efficiently transmitted to the opposite end of the material. The
light transmitting portion or core of the optical filamentary
material is surrounded by cladding having an index of refraction
lower than that of the core, so as to achieve total internal
reflection along the length of the filament. This cladding is
normally chosen to be transparent since an opaque cladding tends to
absorb or scatter light.
An important consideration in formation of optical fibers is
minimization of any factor which increases the attenuation of
transmitted light within such a fiber.
Optical fibers which consist wholly of inorganic glasses, or which
have an inorganic glass core surrounded by a thermoplastic or
thermosetting polymer, or which consist wholly of thermoplastic
polymer, are all known in the art. Those having inorganic glass
cores, especially fused silica cores, exhibit high light
transmission, i.e., low attenuation of transmitted light, but are
relatively easily damaged by fracture if bent to too small a radius
of curvature or otherwise abused; they can be protected by use of a
shielding layer, but this adds undesired bulk, weight and expense,
and nevertheless does not always enable the fiber to be used in
situations where bending to a small radius of curvature is helpful
or required. The all-plastic fibers are less subject to fracturing,
but have the deficiency that they more strongly attenuate light
passing therethrough. The present invention is directed to
improving the capability of all-plastic optical fibers to transmit
light.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an optical
fiber comprising a core and cladding which consist essentially of
organic high polymers, said core being a .[.deuterated acrylate.].
polymer .Iadd.selected from the group consisting of a deuterated
methacrylate homopolymer, a deuterated methacrylate copolymer and a
deuterated methacrylate/acrylate copolymer .Iaddend.and containing
less than 20 mg of hydrogen per gram of polymer as measured by
nuclear magnetic resonance at 60 MHz.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing, not to scale, of apparatus suitable
for purifying deuterated methyl methacrylate and charging a
polymerization vessel.
FIG. 2 is a drawing, partly schematic and partly cross-sectional,
not to scale, of apparatus suitable for making optical fiber from a
polymer preform.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the optical fibers of present invention, the core of the fiber
is a copolymer containing at least 60 mol %, preferably at least 80
mol %, most preferably at least 90 mol % of deuterated methyl
methacrylate, or deuterated polymethyl methacrylate polymer itself.
As the copolymer component, comonomers such as acrylic esters,
e.g., methyl acrylate, ethyl acrylate, propyl acrylate and butyl
acrylate; methacrylic esters, e.g., cyclohexyl methacrylate, benzyl
methacrylate, ethyl methacrylate, propyl methacrylate and butyl
methacrylate may be used. However, it is preferred that at least 90
mol % of the core polymer be composed of methyl methacrylate, so as
to have high light transmission. The most highly preferred
copolymers are those prepared from at least 95 mol % deuterated
methyl methacrylate and 0 to 5 mol % of methyl acrylate, ethyl
acrylate or ethyl methacrylate. The copolymers are preferred
because they have greater flexibility, and are less subject to
thermal depolymerization, compared to homopolymer of methyl
methacrylate. For lowest attenuations of light, it is also highly
preferred that all comonomers used also be deuterated.
As used herein, the term "deuterated" is intended to refer to both
partially and completely deuterated materials. Although partially
deuterated monomers and comonomers can be used, the best results
are attained when perdeuterated, i.e., completely deuterated
monomers and comonomers are used. A particularly useful deuterated
monomer is methyl methacrylate-d.sub.8.
The resulting fibers, like their non-deuterated counterparts, are
optically transparent, the wavelengths at which minimum attenuation
of transmitted light occurs being shifted from those at which the
non-deuterated counterparts have minimum attenuations. Lowest
attenuation of light at the wavelengths of maximum transmission is
attained as the amount of C--H bonds (as distinct from C--D bonds)
in the core polymer is minimized. The best results are attained by
using deuterated monomer of such isotopic purity, and amounts of
initiator and chain transfer agent, such that the core of the
optical fiber contains less than 20 mg, preferably less than 10 mg,
and most preferably less than 1 mg of hydrogen (as distinct from
deuterium) per gram of polymer, as measured by nuclear magnetic
resonance at 60 MHz. Thus, while a fiber having a core which
contains 20 mg hydrogen per gram of polymer exhibits significantly
better transmission of light when compared to a conventional fiber
having a core of polymethyl methacrylate, the fibers being made in
the same way, best results are attained by minimizing the amount of
hydrogen in the core.
The cladding polymer applied to the core is optically transparent
and has an index of refraction at least 0.1% lower than that of the
core, preferably at least 1% lower, and most preferably at least 5%
lower.
Examples of suitable cladding materials include those disclosed in
British patent specification No. 1,037,498 such as polymers and
interpolymers of vinyl fluoride, vinylidene fluoride,
tetrafluoroethylene, hexafluoropropylene,
trifluoromethyltrifluorovinyl ether, perfluoropropyltrifluorovinyl
ether and fluorinated esters of acrylic or methacrylic acids having
the structure ##STR1## wherein X is selected from the group
consisting of F, H, or Cl, and n is an integer of from 2 to 10, m
is an integer from 1 to 6 and Y is either CH.sub.3 or H, and
copolymers thereof with esters of acrylic and methacrylic acids
with lower alcohols such as methanol and ethanol. Copolymers of
##STR2## where X, Y, m and n are as defined above with the methyl
and ethyl esters of acrylic and methacrylic acids and which are
substantially amorphous constitute a preferred class of
polymers.
Fluorinated polymers which contain pendant side chains containing
sulfonyl groups such as disclosed in U.S. Pat. No. 3,849,243, and
fluorine-containing elastomers such as those disclosed in U.S. Pat.
Nos. 2,968,649 and 3,051,677 can also be used. Others include
copolymers of tetrafluoroethylene with other monomers such as
hexafluoropropylene and perfluoroalkyl perfluorovinyl ether as
disclosed in U.S. Pat. Nos. 2,946,763 and 3,132,123. Modified and
unmodified copolymers of tetrafluoroethylene and ethylene as
disclosed in U.S. Pat. No. 2,468,664 can also be used.
Cladding polymers which are not crystalline, i.e., which are
substantially amorphous, are preferred, because optical fibers clad
with a crystalline polymer tend to have higher attenuations of
transmitted light than those coated with an amorphous polymer.
Optical fibers clad with a crystalline polymer, however, do have
utility, particularly when only a short length of optical fiber or
cable is needed. When the optical fiber or cable will be used in
short lengths where it is subjected to high temperatures, the core
and cladding should be polymers which will not soften at the high
temperatures, and many polymers suitable in such cases tend to be
crystalline. When a crystalline polymer is employed as cladding,
however, best results (i.e., lowest attenuation of transmitted
light) are attained when the optical fiber is made under such
conditions that the polymer cladding has the highest transparency
attainable by quickly quenching it after polymer extrusion.
The diameter of the core of the optical fiber can vary from
relatively thin to relatively thick constructions. A suitable
diameter range is 50 to 500 .mu.m. If the light source is large,
e.g., from an LED (light emitting diode), a thick core has the
advantage in its ability to capture a greater proportion of
incident light, but has the disadvantage of having a larger minimum
bending radius. If the light source is small, e.g., a laser, a
relatively thin core is suitable for capturing incident light and
has the advantage of a smaller minimum bending radius.
Since the cladding material reflects light traveling through the
core, the thickness of the cladding generally is not critical, so
long as its thickness is at least a few wavelengths of the light to
be transmitted. An example of a suitable range of thickness of the
cladding is about 5 to 50 .mu.m, preferably 10 to 20 .mu.m.
Optical fibers of the present invention have remarkably low
attenuations of transmitted light. Optical fibers having
attenuations of less than 300 dB/km (decibles per kilometer) in the
vicinity of 690 nm and of 785 nm are routinely made by the present
invention, and attenuations below 200 dB/km, on the order of 150
dB/km, have been attained.
The deuterated .[.acrylate.]. polymer used as a core of the optical
fibers of the present invention exhibits unexpectedly low
attenuations of light at certain wavelengths mentioned above, which
are so-called transmission bands or windows, or simply, windows.
The transmission window at about 785 nm is in the infrared, and
that at about 690-700 nm is in the visible region. For reasons not
presently understood, in the case of two seemingly identical
optical fibers of the present invention having a .[.deuterated
acrylate.]. core, .Iadd.made from a polymer selected from the group
consisting of a deuterated methacrylate homopolymer, a deuterated
methacrylate copolymer and a deuterated methacrylate/acrylate
copolymer .Iaddend.one may exhibit its minimum attenuation of light
at a frequency in the window at 690-700 nm, while the other may
exhibit its minimum attenuation at a frequency in the window at
about 785 nm. Accordingly, transmission of light at both windows
must be determined in order to find the frequency at which maximum
transmission of light occurs for each optical fiber prepared.
Methyl methacrylate-d.sub.8 can be made as described in the art by
Nagai et al., J. Poly. Sci., 62, S95-98, (1962), wherein briefly,
acetone-d.sub.6 is reacted with HCN to form acetone-d.sub.6
cyanohydrin, the latter is treated with H.sub.2 SO.sub.4 to remove
the elements of DOH and form the H.sub.2 SO.sub.4 salt of
methacrylamide-d.sub.5, which is reacted with CD.sub.3 OH to form
methyl methacrylate-d.sub.8. Although DCN, D.sub.2 SO.sub.4 and
CD.sub.3 OD can be used in place of HCN, H.sub.2 SO.sub.4 and
CD.sub.3 OH, respectively, no improvement in isotopic purity of the
methyl methacrylate-d.sub.8 was observed by doing so. Other
deuterated acrylate .Iadd.and methacrylate .Iaddend.esters can be
prepared by a similar reaction scheme.
A preferred method for making the optical fiber of the present
invention, which comprises a core of deuterated polymer which
contains a major proportion of methyl methacrylate units surrounded
by a cladding, comprises the steps
(1) (a) mixing in a sealed system deuterated vinyl monomers of
which at least 60 mol % is methyl methacrylate, said monomers
containing 0 to 10 ppm biacetyl and 0-500 ppb of transition metal
ions and being substantially free of particulate matter, with a
free radical polymerization initiator and a chain transfer
agent;
(b) transferring said mixture in a sealed system to a
polymerization vessel, and closing said vessel;
(2) maintaining said mixture without a free liquid surface in said
vessel under a pressure of 7-25 kg/cm.sup.2, while simultaneously:
maintaining the temperature of said mixture below about 70.degree.
C. until conversion to polymer is at least 60% complete
dilatometrically, raising the temperature at a rate to reach a
temperature of 90.degree. to 100.degree. C. at the time that
conversion to polymer is at least 95% complete dilatometrically,
and continuing to raise the temperature at about the same rate to a
temperature in the range of 115.degree. to 140.degree. C., holding
the temperature in said range for at least a half hour and cooling
to form a solid preform of said deuterated polymer;
(3) (a) transferring said solid preform of said deuterated polymer
to the barrel of a ram extruder adapted to receive it; (b)
advancing said solid preform through said barrel with a ram into a
heated zone, whereby said preform is softened only at its forward
end, and extruding said core of said fiber, and
(c) applying to said core a polymer cladding which is substantially
amorphous and which has an index of refraction below that of said
deuterated polymer.
Pursuant to making an optical fiber of the present invention, it is
preferred to use monomer of high quality. To this end, it is
important to remove from the monomers, especially those from which
the core of the fiber will be made, those substances which if
retained would absorb or scatter light introduced into the optical
fiber made therefrom.
It is preferred to remove from the monomers impurities which absorb
light of wavelengths which the optical fiber is intended to carry.
It has been found that methyl methacrylate ordinarily contains
biacetyl, and that the amount of biacetyl should be reduced to no
more than about 10 ppm (parts per million), preferably no more than
5 ppm. Removal of the impurities can be accomplished by treatment
with alumina, followed by distillation.
Although any type of alumina can be used, for most effective
removal of impurities it is best to use basic alumina and that it
be of activity grade 1. Such treatment removes or reduces that
amount of compounds having labile hydrogen or deuterium and of
highly polar compounds such as biacetyl. The treatment can be
accomplished prior to distillation of the monomer by placing the
alumina on a filter which will retain it, and filtering the monomer
through the alumina directly into the still pot. This operation is
suitably carried out under a nitrogen atmosphere.
In the step of distilling the deuterated methyl methacrylate or
other monomer, only a center cut of distillate is retained for
polymerization, while substantial foreshot and heel fractions are
set aside.
Another method of purification which can be used is preparative
scale gas--liquid chromatography. Distillation is a preferred
method and will be the method referred to in the detailed
description below.
Transition metal ions, especially those of transition elements of
the first series (i.e., elements of atomic number 22 through 28),
and copper, lead, aluminum, silicon, vanadium, chromium, manganese,
iron and nickel are also deleterious impurities, because they
absorb light of wavelengths which the optical fiber is intended to
carry. The amount of such impurities can also be conveniently
lowered to acceptable levels by distillation. It is preferred that
the amount of such impurities be no greater than about 500 ppb
(parts per billion), more preferably no greater than 100 ppb, total
for all such ions present.
It is also preferred that particulate matter be removed because
these particles absorb and/or scatter light. To the extent
possible, the monomers (and the other components of the
polymerization charge) should be substantially free of such
particulate matter. Although particles smaller than about 200 nm
(0.2 .mu.m) cannot be resolved with an optical microscope, with the
use of a transverse intense beam of light in an optical microscope
points of light are observed in an optical fiber not only at the
particles which are larger than about 200 nm, but also at smaller
particles of undetermined size. Even though it is not possible to
precisely determine the sizes of these particles it is nevertheless
important to remove those, regardless of size, which are detectable
by light scattered from the particle. Particles of all sizes can be
effectively removed by distillation of the monomers, providing that
the distillation is carried out such that there is no entrainment.
The best (i.e., cleanest) commercially available polymers have on
the order of 300 to 1000 particles/mm.sup.3, and can provide
optical fibers with attenuations of light down to ca. 500 dB/km and
having at best a few short lengths as low as 400 dB/km. By the
present invention, optical fibers having no more than 100
particles/mm.sup.3 are easily made. Particle counts below 10
particles/mm.sup.3 are also easily attained by the present
invention, and counts below 2 particles/mm.sup.3 have been
attained. Accordingly, in reference to the monomers, by
"substantially free" is meant that the mixed vinyl monomers
preferably contain no more than about 100 particles/mm.sup.3.
Any comonomer used should be similarly purified, but such
purification ordinarily need not be as rigorous, especially when
the amount used is less than 10 mol % of the total monomer because
less impurity is introduced with the smaller quantity of monomer
and is diluted upon mixing the monomers.
When distillation is employed as the method of purification, the
distillation is conducted under a slight positive pressure of an
inert gas such as argon, nitrogen or helium. As is known in the
art, so as to prevent polymerization of monomer in the
fractionating column, a concentrated solution of polymerization
inhibitor in the same monomer can be introduced at the top of the
column throughout the fractionation. Alternatively, a solid piece
of inhibitor can be placed at or near the top of the fractionating
column where it will slowly dissolve in the liquid in the
column.
Polymerization is carried out with the use of a soluble free
radical polymerization initiator, ordinarily an azo type initiator.
For the sake of convenience, the initiator type and concentration
are chosen to provide about 50% conversion to polymer in about 16
hrs. To achieve this, it is preferred to use an initiator having a
half-life at 60.degree. C. between about 300 and 3,000 minutes,
preferably about 1,000 minutes. 2,2'-azo-bis(isobutyronitrile) is
the preferred initiator because it is available in high purity and
because it can be handled safely. Other initiators with somewhat
longer or shorter half-lives, such as
1,1'-azo-bis-(cyclohexanecarbonitrile) or
2,2'-azo-bis(2,4-dimethylvaleronitrile), can also be used; for
those having longer half-lives, the temperatures of the heating
stages used during polymerization, especially the first stage, will
have to be higher than when 2,2'-azo-bis(isobutyronitrile) is used,
and/or greater amounts can be used, and conversely, for those
having shorter half-lives, the temperatures of the heating stages
used during polymerization, especially the first stage, will have
to be lower, and/or smaller amounts may be used. It will be clear
to one skilled in the art than many combinations of initiator,
initiator concentration and polymerization temperature can be used.
Combinations of initiators having different half-lives can also be
used. The initiator and its concentration are so chosen that some
will remain for the later heating stages of the polymerization
step. A high purity initiator should be used so as to introduce the
least possible amount of impurity into the resulting polymer.
A chain transfer agent is also included in the polymerization
system. Both mono- and multifunctional chain transfer agents can be
used. Typical examples include n-butyl mercaptan, lauryl mercaptan,
mercaptoacetic acid, 2,2'-dimercaptodiethyl ether, ethylene
bis(2-mercaptoacetate) commonly referred to as glycol
dimercaptoacetate (GDMA), ethylene bis(3-mercaptopropionate),
1,1,1-trimethylolethane tris(3-mercaptopropionate), pentaerythritol
tetrakis (3-mercaptopropionate). For reasons not entirely
understood, the preferred chain transfer agents are those having
mercaptan groups on carbon atoms adjacent to the carbonyl of a
carboxylic functional group, i.e., of the type disclosed in U.S.
Pat. No. 3,154,600, and having mercaptan difunctionality, because
their use generally provides polymer of higher conversion and
optical fiber having higher light transmission when compared to
those prepared with other chain transfer agents. It is preferred to
purify the chain transfer agent, which can be done by
distillation.
The quantities of initiator and chain transfer agent are so chosen
to give a polymer having an inherent viscosity of at least about
0.4 dl/g, as measured at 25.degree. C. on a 0.5% (wt./vol.)
solution in chloroform (i.e., 0.5 g of polymer in 100 ml. of
solution). At inherent viscosities of 0.38 dl/g or lower the
polymer is more brittle, while at 0.4 dl/g and higher the polymer
is reliably tough. Although polymers having inherent viscosities as
high as 0.5 and 0.6 can be used, they are difficult to extrude
because they are so viscous at temperatures which are suitable for
extrusion without polymer degradation that special heavy duty
equipment is required. Polymers having an inherent viscosity in the
range 0.40 to 0.44 are tough and do not require heavy duty
equipment, and thus are preferred. Further, it is difficult to
extrude the very high viscosity polymer into fibers having a
smooth, fracture-free surface, as most often the extrudate will
have a fractured surface which directly causes a much higher
attenuation of transmitted light. To achieve an inherent viscosity
in the preferred range, appropriate amounts of the polymerization
initiator and chain transfer agent are easily determined
empirically. The polymerization initiator is ordinarily used in an
amount of about 0.001 to 0.05 mol %, based on the total monomer and
for the preferred initiator preferably 0.01 to 0.02 mol %, and the
chain transfer agent is ordinarily used in an amount of about 0.1
to 0.5 mol %, based on the total monomer, and for the preferred
difunctional chain transfer agents preferably in the range of 0.1
to 0.25 mol %.
It is preferred to minimize the amount of foreign particles in the
core polymer, because they absorb or scatter light and thus
increase the attenuation of transmitted light in the fiber. The
preferred process for making the optical fibers of the present
invention is therefore designed to meet this goal. Transfers of the
various substances are carried out to the extent possible in a
sealed or closed system so that recontamination of purified
materials by dust, dirt or particulate matter of any kind does not
occur. Particles introduced by adventitious contamination are
advantageously removed as the polymerization charge is transferred
to the polymerization vessel. Removal of particles larger than a
size within the range of 0.2 to 1 .mu.m is conveniently done at
this stage. Particles can be removed by filtration or
centrifugation. Filtration is preferred because of its
convenience.
It has been found useful to use for the first phase of the process,
which is preparation of the polymerization mixture, a series of
connected receivers and vessels as depicted schematically in FIG.
1, beginning with a still for the major component of the
polymerization mixture, deuterated methyl methacrylate, going
through holding and mixing vessels, and ending with the
polymerization vessel. A convenient sequential arrangement begins
with a still pot 1 equipped with a column 2 packed, for example,
with glass helices, and having a volumetrically calibrated
receiving vessel 3 which is connected to a mixing vessel 4 by a
line which is equipped with a greaseless stopcock or other type of
greaseless valve 5. The mixing vessel 4 is equipped with a
magnetically driven stirrer 6 and an entrance port 7 which is
sealed by a serum stopper 8 and a stopcock 9, and is connected to a
microporous filter 10 by a line which is equipped with a greaseless
stopcock 11 or other type of greaseless valve. Although, in the
arrangement shown, distilled monomer is introduced into the mixing
vessel 4 through the entrance port 7, other arrangements are also
possible wherein the monomer is transferred from receiver 3 to
vessel 4 through a line separate from the entrance port 7. The
filter 10 is of known type which is inert to all constituents of
the polymerization mixture, such as polytetrafluoroethylene,
supported on a porous metal plate. The pore size of the filter can
range from 1 micrometer down to about one-twentieth of the
wavelength of light to be carried by the optical fiber, and is
preferably in the range of 0.2 to 1 micrometer. The filter 10 is in
turn connected by a line 29 to the polymerization vessel 12. An
inert atmosphere, such as argon, helium or nitrogen, is maintained
throughout the whole arrangement of apparatus by introduction
through gas inlets 13 and 14, and its flow is controlled and
directed by the various stopcocks 15, 16, 17 and others shown. The
various elements of the apparatus can be broken down into smaller
units by ground glass joints, ring seals, or other known means not
shown.
Procedurally, deuterated methyl methacrylate is introduced into
still pot 1 through a filtering vessel 18 which contains a filter
element 19 which supports a bed of alumina 20. Following charging
of the pot, stopcock 21 is closed. The packed column 2, still head
22, condenser 23 and needle valve 24 function in known manner to
control take-off of distillate. A small lump or two of
polymerization inhibitor 26 is placed on or just under the top
surface of the packing 25 in the column 2. During the course of the
distillation it slowly dissolves and flows down the column in the
downward flow of liquid.
A foreshot is removed through outlet 27 controlled by stopcock 28
and is set aside. The desired center distillate fraction is
collected in the receiving vessel 3. A first portion of distilled
deuterated methyl methacrylate is transferred through the
connecting line from the distillation receiver to the mixing vessel
4. Separately, there is prepared a solution of the desired
polymerization initiator and chain transfer agent in the desired
amounts in the comonomer, or, if no comonomer is used, in a small,
measured amount of separately purified deuterated methyl
methacrylate; this solution is introduced into the mixing vessel
through the entry port 7 with the aid of a hypodermic syringe
inserted through the serum stopper 8 and stopcock 9. A second
portion of distilled deuterated methyl methacrylate is transferred
through the connecting line from the distillation receiver 3 to the
mixing vessel 4. The purpose of reserving part of the deuterated
methyl methacrylate for the final addition to the mixing vessel is
for washing all traces of the minor components of the
polymerization mixture, i.e., the comonomer, polymerization
initiator and chain transfer agent, from the entry port 7 into the
mixing vessel 4; loss of part of the minor components by adhering
within the entry port would lead to a greater degree of
nonuniformity of the resulting polymer among successively run
polymerizations, as compared to the loss of a trace of the major
constituent, deuterated methyl methacrylate, within the entry port.
The combined materials are thoroughly mixed with the magnetic
stirrer 6 to assure homogeneity. The mixture is then passed through
the filter 10, and into the polymerization vessel 12.
It has now been found desirable to minimize exposure of the core
polymer to any circumstance or condition which will lead to
degradation of the polymer as by heat or shear stress, formation of
bubbles, or introduction of any solid particulate matter.
Accordingly, polymerization and extrusion procedures have been
designed to minimize both exposure of the polymer to adverse
conditions, and contact of the polymer with any other material or
surface, during the remaining manipulative steps. To this end, ram
extrusion of a solid block of polymer is employed for extrusion of
the core of the fiber, inasmuch as use of a screw extruder, which
entails extensive contact of the polymer with metal surfaces, can
lead to contamination by foreign particles, extensive heating,
shear stress, with possible degradation of polymer, and
introduction of bubbles. Therefore, according to the present
invention, the polymer is prepared in the shape of a preform
suitable for the barrel of the ram extruder to be used in making
the core of the optical fiber.
The polymerization vessel 12 is thus of a shape to make the
required polymer preform. Because of the manner in which a ram
extruder operates, the preform will ordinarily be in the shape of a
rod. Although rods of various cross-sectional shapes could be used,
a circular cross-section is most suitable, because the most
convenient cross-sectional shape for fabrication of the
polymerization vessel and extruder barrel is circular.
Additionally, polymer rods which are cylindrical are preferred
because such rods lead to maximum uniformity during extrusion, and
this an optical fiber core having more uniform properties. The
polymerization vessel 12 is fabricated of metal of sufficient
thickness to withstand the pressure level to be employed during
polymerization, typically a pressure in the range of 7 to 25
kg/cm.sup.2. Suitable materials of construction include the
stainless steels. So as to preclude contamination of the polymer by
transition metal ions at even the parts-per-billion level, it is
preferred to plate the cavity of the polymerization vessel with an
inert metal such as gold or chromium. The polymerization vessel 12
is sealed at its lower end with a piston 30 having a gasket.
Following transfer of the polymerization mixture from the mixing
vessel 4 through the filter 10 and line 29 to the polymerization
vessel 12, the polymerization vessel is removed from the sealed or
closed system described above by removal of plug 31 and immediately
sealed with a piston (not shown) which is like piston 30 and which
fits its cylindrical cavity. Sealing with the piston is done
without delay so as to avoid contamination by dust or any foreign
substance by exposure to the atmosphere. The gasket of each piston
is fabricated of a material which is inert to all components of the
polymerization mixture at the temperatures employed, such as
polytetrafluoroethylene, to prevent contamination of the
polymerization mixture and resulting polymer.
The polymerization should be carried out without any free gas space
being present in the polymerization vessel. The presence of gas in
such space results in gas being present in the polymer preform,
both dissolved therein and in the form of bubbles, which leads to
an extruded core which contains bubbles or voids and thereby
attenuates transmitted light more than a core without bubbles or
voids. So that the polymerization mixture will have no free liquid
surface during polymerization, various methods can be used to
exclude all free gas space from the vessel. One suitable method is
to fabricate the polymerization vessel 12 with a bleed hole 32 of
small diameter (typically less than 1 mm) located a short distance
from the open end of the vessel. The vessel is filled with
polymerization mixture to above the bleed hole, and the piston seal
is put into place and pushed into the cavity until all free gas and
excess liquid mixture is forced from the bleed hole and the piston
seals off the liquid in the major part of the cavity so that it is
isolated from the bleed hole. There is no danger of loss of
material through the bleed hole during polymerization, inasmuch as
the mixture assumes a smaller volume during polymerization, as will
be discussed in greater detail below.
The polymerization is carried out under pressure, suitably 7 to 25
kg/cm.sup.2 (100 to 350 psig), to preclude vaporization of monomer
and consequent formation of bubbles or voids in the polymer
preform, for reasons similar to those set forth in the previous
paragraph. Pressure is maintained by applying force against the
piston seals throughout the reaction with a press.
Maintaining the polymerization mixture under pressure also provides
a means of assessing the progress of polymerization, which
information is used during the course of polymerization in setting
the heating program employed. By maintaining the polymerization
mixture under pressure, it is possible to follow the progress of
the polymerization dilatometrically, i.e., by following the change
in volume of the mixture. As noted above, the mixture assumes a
smaller volume upon polymerizing, the polymer occupying a volume of
the order of about 80% of that of the monomers. Progress of the
polymerization can be followed, for example, by placing an index
mark on the rod used to transmit force to one of the piston seals
at such a position that it will remain visible outside the cavity
of the polymerization vessel throughout the polymerization, and
following its change in position with a cathetometer. From the
initial volume of the reactants employed, the final volume of
polymer to be prepared as determined if necessary from preliminary
runs, and the initial position of the index mark, it is a simple
matter to estimate where the index mark will be when polymerization
has progressed to any given percentage of completion. It should be
borne in mind, however, in view of the different thermal
coefficients of expansion of the polymerization vessel and mixture
therein, and the progressive heating to higher temperatures during
the course of the reaction, that the cathetometer measurements may
not provide a direct indication of conversion to polymer unless
corrections are applied, and may differ from data made under
constant conditions by as much as a few percent. It is found in
practice that final cathetometer readings almost invariably
indicate an apparent contraction in volume of greater than 100%,
and sometimes as much as 103%, of that to be expected. Subsequent
determination of residual unreacted monomer in the polymer made in
such runs shows the presence of a small, variable amount of
unreacted monomer. For present purposes polymerization should be at
least 98%, preferably 99%, complete. Typically, conversions to
polymer of 99.1 to 99.3% can be routinely attained in the present
invention.
The polymerization mixture is carefully and progressively heated to
higher temperatures in such manner as to attain at least 98%
conversion to polymer, but to prevent development of an
uncontrolled or "runaway" reaction, which would lead to a thermally
degraded product. The mixture is first maintained below about
70.degree. C., preferably between 55.degree. C. and 70.degree. C.,
until conversion to polymer is at least 60% complete, preferably 65
to 75% complete. The mixture is next heated to raise the
temperature at a rate to reach 90.degree. to 100.degree. C. at the
time that conversion to polymer is at least 95% complete. Heating
to raise the temperature at about the same rate is continued until
a temperature in the range of 115.degree. to 140.degree. C.,
preferably 125.degree.-135.degree. C., is attained, and finally a
temperature in the same range is maintained for at least one half
hour, preferably at least one hour. The resulting polymer is then
cooled. Pressure in the range of 7 to 25 kg/cm.sup.2 is maintained
during the entire heating program. The pressure is released only
after the temperature of the polymer has dropped below 100.degree.
C., which is the boiling point of methyl methacrylate, so as to
preclude formation of bubbles by traces of residual monomer.
Depending on the diameter of the cavity of the polymerization
vessel, and thus on the diameter of the polymer preform to be
prepared, the specific rate of heating will vary to some extent,
but the conditions will always conform to the schedule of the
previous paragraph. For example, when the diameter is 28.7 mm (1.13
in), after attaining a conversion to polymer of at least 60% below
a temperature of 70.degree. C., the mixture is heated to raise the
temperature at a rate of 35.degree. to 45.degree. C. per hour until
a temperature of 115.degree. to 140.degree. C. is attained, which
rate will result in at least 95% conversion to polymer when a
temperature of 90.degree. to 100.degree. C. is attained. For
smaller diameters, a rate of temperature increase which is the same
or faster can be used. For large diameters, a slower rate of
temperature increase is required.
The polymer preform is then transferred from the polymerization
vessel 12 to the barrel 52 of a ram extruder 51 shown in FIG. 2. As
noted above, the preform is fabricated in a shape which closely
matches the barrel 52 of the ram extruder. The inside diameter of
the extruder barrel is suitable slightly greater than the inside
diameter of the polymerization vessel. During transfer, the preform
should not be handled, or retained exposed to the atmosphere
unduly, so as to minimize contamination of the preform with dust,
oils from the skin, etc. It is best to transfer the preform without
touching it, but if handling is necessary, lint-free gloves should
be worn. If there will be any delay between fabrication of the
polymer preform and extruding it, it is best to store it either by
retaining it in the polymerization vessel or by holding it in the
extrusion barrel. If desired, however, it can be stored in an
intermediate container such as a clean plastic bag, but care should
be exercized in selecting a type of plastic bag which does not
contain any slip agent or sizing on its surface.
The preform is then extruded by advancing the preform through the
barrel 52 with a ram 53 toward an extrusion orifice 54 through
which the polymer is forced to form the core of the fiber. The ram
can be either of the constant rate type, or the constant stress
type, the latter being used in combination with a melt metering
pump such as a gear pump. The constant rate ram is preferred
because its use does not require a melt metering pump, the use of
which pump introduces a potential opportunity to contaminate the
polymer with foreign particles.
It is preferred to carry out the ram extrusion without melting the
complete preform at the same time. The extrusion barrel 52 is
heated only at its forward end by heating elements 55, 55' so that
the polymer is softened just before it is forced through the
extrusion orifice 54. Immediately behind the heated zone of the
barrel, cooling coils 56, 56' are preferably installed to prevent
conduction of heat along the barrel 52 and consequent heating of
polymer farther away from the orifice. Operation in this manner
results in reheating of the polymer for the minimal time needed to
extrude it into fiber form, and consequently minimizes opportunity
for thermal degradation of the polymer to substances which will
impair the optical transparency of the fiber.
The temperatures employed for extrusion will vary somewhat with the
polymer composition, but for the deuterated polymethyl methacrylate
polymers described hereinabove, the temperature of the spinning
head 57 will ordinarily be in the range of about
200.degree.-240.degree. C. and about 220.degree.-280.degree. C. the
forward end of the barrel where the preform is softened. For the
copolymers which contain up to 5% of comonomer, the preferred
temperatures are 210.degree.-220.degree. C. at the spinning head
and 240.degree.-250.degree. C. at the forward end of the
barrel.
The cladding of the optical fiber can be applied to the core by
various methods. Such methods include coextrusion and solution
coating, both of which methods are well known in the art. By
coextrusion is meant an operation wherein both core and cladding
polymers are fed through the same orifice 59 in spinneret plate 58
from which is extruded a composite fiber 60 wherein the core
polymer is completely surrounded by a substantially uniform thin
layer of the cladding polymer. Coextrusion is the preferred method
for making optical fibers in the present invention. Solution
coating, however, is also a practical method, and, if employed,
should be carried out as an in-line process step just after
extrusion of the core, so as to minimize opportunity for the core
to be contaminated by any material, such as particles of dust or
dirt.
The spinning head 57 is of known type such as described in U.S.
Pat. No. 3,992,499, specifically of the type shown in the left-hand
part of FIG. 1 thereof. The spinning head 57 had a spinneret plate
58 and a meter plate 65 in a body 66. The core polymer is led from
the orifice 54 of ram extruder 51 to the spinning heads 57 by line
61, and is shown as molten core polymer stream 68. The cladding
polymer is introduced from reservoir 62 into conventional screw
extruder 63 and metered by melt metering pump 64 into the spinning
head 57, and is shown as molten cladding polymer stream 69.
Regardless of whether the fiber is made by coextrusion or by core
extrusion followed by solution coating of the cladding, the
diameter of the extrusion orifice 59 can vary, depending on the
desired fiber diameter, and the amount of melt draw down taken. The
fiber is drawn immediately after it exits from the spinning head
and while it is still in a heat-softened state in order to induce
molecular orientation for the purpose of imparting toughness to the
fiber. The machine draw ratio is the ratio of the cross-sectional
area of the die orifice to the cross-sectional area of the optical
fiber if it is made by coextrusion or to the cross-sectional area
of the core of the optical fiber if it is made by solution
coating.
Line speed following extrusion can vary widely, depending on the
capability of the equipment employed. Line speeds of 15 to 90 m/min
(50 to 300 ft/min) are typical, but higher and lower speeds can
also be used. Speeds in the range of 35 to 60 m/min (120 to 200
ft/min) provide highly satisfactory results. A cross-flow of air
blown by means not shown can be used to quench the freshly extruded
fiber; air flow velocities of 3 to 15 cm/sec (0.1 to 0.5 ft/sec)
are suitable. The drawn optical fiber is wound up on drum 67.
Other conventional methods described in the art for making
all-plastic optical fibers can also be used to make the optical
fibers of the present invention. For any particular method used,
the optical fiber of the present invention will have a lower
attenuation of light than a non-deuterated counterpart which is
otherwise the same. The method described herein in detail, however,
is a preferred method for attaining the lowest attenuation of light
at a transmission window.
The optical fibers of the present invention, by virtue of their
lower attenuation of light when compared to their non-deuterated
counterparts, have the advantages that they can be used in longer
run lengths and that they provide higher data transmission rates
since they can be used with higher speed infrared emitting diodes
and solid state lasers.
In the examples which follow, where are intended to be exemplary
and not limiting as to the invention claimed, all boiling points
given are uncorrected.
EXAMPLE 1
Perdueteromethyl methacrylate (MMA-d.sub.8) was synthesized by the
method of Nagai et al., described briefly hereinabove.
A. Purification of MMA-d.sub.8
Two lots of crude MMA-d.sub.8 were treated to remove water as
follows:
1. 30 gms of 5A Molecular Sieves were added to 482 gms of a first
lot known to contain 1.25% water. 2. 51.5 gms of 5A Molecular
Sieves were added to 726 gms of a second lot known to contain 1.42%
water. The two lots were allowed to stand for 31/2 days at about
0.degree. C.
Then 454.3 grams of monomer from 1 (above) and 50.0 grams of
monomer from 2 (above) were passed through a
37-mm-diameter.times.15-cm-high bed of Woelm Basic Aluminum Oxide,
Activity Grade 1; 387.4 grams were collected in a still pot for
distillation and inhibited with N,N'-diphenylpara-phenylene diamine
(DPPD).
Distillation was conducted with an 11-mm-diameter vacuum jacketed
and insulated column packed for 28 cm with Podbielniak "Heli-Pak"
316 stainless steel packing (1.3 mm.times.2.5 mm.times.2.5 mm, or
0.5 in.times.0.1 (in.times.0.1 in) and for 12 cm with 4.8 mm (3/16
in) glass helices. Two small DPPD chunks were placed near the top
of the column to inhibit polymerization in the column as a solute
carried down by the liquid phase.
Approximately 68 grams of distillate were collected as a foreshot
at condensing temperatures from 59.degree. to 100.degree. C. and
set aside. A 260 ml polymerization cut was collected in a
calibrated dropping funnel at an average rate of 2.5 ml/minute.
About 130 ml of the MMA-d.sub.8 was discharged from the funnel into
a attached 500 ml argon-flushed glass mixing vessel; 1.28 ml of an
solution of 1.28 ml of MMA-d.sub.8, 1.28 ml of glycol dimercapto
acetate (GDMA), and 0.080 g of 2,2'-azobis(isobutyronitrile)
(Vazo.RTM. 64) was injected into the MMA-d.sub.8 through a serum
stopper and PTFE stopcock. Then the balance of 260 ml of
MMA-d.sub.8 was run into the mixing vessel and mixed with the other
ingredients by a magnetically driven PTFE-coated impeller to
produce a solution of 100 mol % MMA-d.sub.8, 0.165 mol % (based on
monomer) GDMA and 0.01 mol % (based on monomer) Vazo.RTM. 64.
The mixture was discharged by argon pressure through an 0.2 .mu.m
(micrometer) pore "Millipore" filter and FEP (copolymer of
tetrafluoroethylene and hexafluoropropylene) tube into a rigorously
cleaned chromium-plated stainless steel tube having an inside
diameter of 28.7 mm (1.13 in) sealed at the bottom with a PTFE "O"
ring gasketed stainless steel piston and at the top with a PTFE
plug. After filling the tube, the PTFE plug was removed and
immediately replaced with a PTFE gasketed piston. A few ml of the
polymerization mixture were sampled from the mixing vessel after
filing the polymerization tube.
Gas-liquid chromatography analysis of the retained sample indicated
99.87% (estimated from area under recorded curve) monomer purity,
an nuclear magnetic resonance (NMR) analysis indicated that the
proton content of the polymerization mixture was 534 .mu.g/gm of
mixture.
B. Purification of Glycol Dimercapto Acetate
A round-bottom flask was charged with 70 ml of glycol dimercapto
acetate (Evans Chemetics, Inc., indicated to be 96.6% pure).
Distillation was conducted with a Vigreau column. A 15 ml foreshot
was collected at 0.27 to 0.26 mm Hg absolute at condensing
temperatures of 80.degree. to 104.degree. C. and discarded. A 27 ml
second cut was collected at 0.26 to 0.13 mm Hg absolute at
condensing temperatures of 100.degree. to 109.degree. C. A 16 ml
3rd cut was collected at 0.3 to 0.37 mm Hg absolute pressure at
condensing temperature of 110.degree. to 117.degree. C.
C. Polymerization
The chromium-plated tube was placed in a heat transfer jacket and
the contents were pressurized to 24.3 kg/cm.sup.2 (345 psig) by a
pneumatic cylinder operating on the top piston. Silicone heat
transfer fluid was pumped through the jacket according to the
following schedule:
______________________________________ Dilatometric Elapsed Time
Temperature Heating Rate Conversion hr .degree.C. .degree.C./hr %
______________________________________ 0-16 56 -- 51.3 16-16.8 70
-- 70.0 16.8-18.25 70-130 40 -- 18.25-19.25 130 -- -- 19.25-19.7
130-100 -46 -- 19.7-20.25 100 -- 99.9
______________________________________
The finished polymer preform contained one large void near the
middle and one at the top.
D. Extrusion
The extrusion equipment as described herein in reference to FIG. 2
was employed, with a spinning temperature of 214.degree. C. and a
line speed of 36.6 m/min (120 ft/min). The core of the optical
fiber was made from the polymer rod fabricated in part C of this
example, which was extruded by ram extrusion, with the ram advanced
at constant rate and the temperature near the tip of the ram
extruder at 245.degree.-246.degree. C. The cladding polymer, which
was extruded with a conventional screw extruder, was a copolymer of
20% by weight of methyl methacrylate and 80% by weight of ##STR3##
(p is 1 to 8, with ca. 90% by weight being that where p is 2 and 3)
having an inherent viscosity of 0.50 (measured on a 0.5% (wt./vol.)
solution in 1,1,2,-trichloro-1,2,2,-trifluoroethane at 20.degree.
C.) and a melt index of 6 at 230.degree. C. (measured by ASTM
D-2116-66 with an orifice of 2.095 mm and a weight of 2160 gm). The
screw extruder barrel temperature employed ranged from 229.degree.
C. near the hopper to 248.degree. C. at the discharge end of the
barrel.
Summary data are included in Table I.
TABLE I ______________________________________ Polymer Properties
Inherent Visc., dl/gm 0.414 Residual Monomer, % by wt. 0.55 Machine
Draw Ratio 5.50 Fiber Properties Diameter, .mu.m 390 Cladding
thickness .mu.m 16 ______________________________________
E. Attenuation
An attenuation spectrum was obtained from a 28.7 meter sample of
the fiber utilizing a tungsten halogen source and a Bausch and Lomb
High Intensity Monochromator with conditions shown below.
______________________________________ Wavelength Slit Width Range
Lines Entrance Exit Cut Off (nm) Grating mm (mm) (mm) Fiber
______________________________________ 400-800 33-86-76 1350 2 1
None 700-1100 33-86-77 675 2 1 Corning 2-58
______________________________________
A Coherent Radiation Model 212 Power Meter was used to determine
the optical power transmitted.
The spectrum indicated minimum attenuation as follows:
______________________________________ Wavelength, nm Attenuation,
dB/km ______________________________________ 690 147 790 167
(grating - 76) 158 (grating - 77)
______________________________________
EXAMPLE 2
Example 1 was substantially repeated with the following
exceptions:
The crude MMA-d.sub.8 was 207.2 gms of a third lot and 330.0 grams
of a fourth lot. The monomer was purified by passing it through a
fixed bed consisting of 15 cm of Woelm Basic Aluminum Oxide,
Activity Grade 1, topped by 2 cm of 3A Molecular Sieves; 411.0
grams of MMA-d.sub.8 was recovered for distillation and inhibited
by DPPD. Fractionation was essentially as in Example 1, except
about 73 g of foreshot was set aside. GLC analysis of the last
drops of the polymerization mixture indicated 99.85% (area) monomer
purity, NMR analysis indicated that the proton content of the
polymerization mixture was 460 .mu.g/gm of mixture.
Polymerization was substantially as in Example 1.
Final dilatometric conversion was 102.2%. The finished polymer
preform contained only one void at the top. Extrusion was
accomplished substantially as Example 1. Summary data are in Table
II.
TABLE II ______________________________________ Polymer Properties
Inherent Viscosity, dl/gm 0.410 Residual Monomer, % by wt. 0.52
Machine Draw Ratio 5.50 Fiber Properties Diameter, .mu.m 390
Cladding thickness, .mu.m 16 Attenuation (as Example 1) Wavelength,
nm Attenuation, dB/km 690 206 790 207 (grating - 76) 190 (grating -
77) ______________________________________
EXAMPLE 3
The MMA-d.sub.8 used in this example contained approximately equal
fractional percentage amounts of methyl-d.sub.3 acrylate and
methyl-d.sub.3 acrylate(2,2-d.sub.2), CD.sub.2 .dbd.CH COOCD.sub.3,
as indicated by gas chromatography/mass spectrometry, and was
obtained from thermal pyrolysis of a copolymer of 98.8 mol %
MMA-d.sub.8 and 1.2 mol % methyl-d.sub.3 acrylate, the partially
deuterated constituents arising from left or right-handed
depolymerization of the comonomer units in the copolymer chain. The
MMA-d.sub.8 had originally been made by the method of Nagai
described herein, except that a mixture of CD.sub.3 OD and CD.sub.3
OH was used in place of CD.sub.3 OH.
A. Purification of MMA-d.sub.8
A 383 gm charge of methyl methacrylate-d.sub.8 monomer was
permitted to flow by gravity through a 40-mm diameter 11-cm deep
bed of Woelm Basic Aluminum Oxide, Activity Grade 1, into a 500 ml
round-bottom flask containing 0.5 g DPPD inhibitor and 39.6 g of
monomer heel from a previous distillation. The monomer was
distilled through a 13-mm diameter.times.40-cm high column packed
with glass helices at a high reflux ratio. Fifty-one ml were
collected as a foreshot at condensing temperatures up to
100.degree. C. at atmospheric pressure and set aside. A 260-ml
product fraction was collected in an argon filled dropping funnel
at 1.5 ml/min at a condensing temperature of 100.degree. C. at
atmospheric pressure. The polymerization mixture was prepared
substantially as in Example 1, except that 0.16 mol % GDMA was
used, and similarly filtered into a polymerization tube. A few
drops of the polymerization mixture were sampled from the mixing
vessel after filling the polymerization tube; gas-liquid
chromatography analysis indicated 99.88% (area) MMA-d.sub.8 and
0.113% (area) of the indicated mixed isotopic forms of methyl
acrylate.
B. Purification of Glycol Dimercapto Acetate
A 200-ml round-bottom flask was charged with 125 ml of glycol
dimercapto acetate (Evans Chemetics, Inc., indicated to be 96.6%
pure). Distillation was conducted with a 20-cm Vigreau column. An
18-ml forshot was collected at 0.20 to 0.15 mm Hg absolute at
condensing temperatures of 60.degree. to 109.degree. C. and
discarded. A 48-ml second cut was collected at 0.18 mm Hg absolute
at condensing temperatures of 112.degree.-113.degree. C. A 10 ml
third cut was collected at 0.5 mm Hg absolute pressure at
condensing temperature of 124.degree. C. Gas-liquid chromatography
analysis indicated the second cut to be 99.4% pure and the third
cut to be 99.2% pure.
C. Polymerization
The polymerization was conducted as in Example 1 with the following
exceptions
______________________________________ Dilatometric Elapsed Time
Temperature Heating Rate Conversion hr. .degree.C. .degree.C./hr %
______________________________________ 0-15.75 60 nil 99.2 15.75-16
60-130 280 ca 99.2 16-17 130 nil 17-17.5 130-100 -60 17.5-18 100
nil 102.1 ______________________________________
At this time the pressure was released, and the system was allowed
to cool to room temperature. The polymer rod was removed from the
polymerization tube, small samples were taken for analysis, and the
rod was placed within a clean extrusion cylinder.
D. Extrusion
Extrusion was accomplished substantially as in Example 1. Summary
data are in Table III.
TABLE III ______________________________________ Polymer Properties
Inherent Visc., dl/g 0.426 Residual Monomer, % by wt. 1.17 Proton
Content, .mu.g/gm (NMR) 239 Machine draw ratio 5.39 Fiber
Properties Diameter, .mu.m 394 .+-. 10 Cladding thickness, .mu.m 16
______________________________________
E. Attenuation
An attenuation spectrum was obtained from a 22.1 meter sample of
the fiber utilizing a Xenon arc lamp source and a Bausch & Lomb
High Intensity Monochromator with conditions shown below.
______________________________________ Wavelength Slit Width Range
Lines/ Entance Exit Cut Off (nm) Grating mm (mm) (mm) Filter
______________________________________ 400-800 33-86-02 1350 2 1
None 700-1100 33-86-03 675 2 1 Corning 2-58
______________________________________
A Coherent Radiation Model 212 Power Meter was used to determine
the optical power transmitted.
The spectrum indicates mimimum attenuation as follows:
______________________________________ Wavelength, nm Attenuation,
dB/km ______________________________________ 690 223 790 229
(Grating - 02) 219 (Grating - 03)
______________________________________
Comparative Examples A and B
A. Purification of Ethyl Acrylate
Four hundred ml of ethyl acrylate (Rohm & Haas) was dripped
through a 38-mm diameter.times.10 cm deep bed of Woelm Basic
Aluminum Oxide (Activity Grade 1, Woelm Pharma GmbH & Company)
into a 500-ml round-bottom flask containing about 0.5 gm
N,N'-diphenyl-para-phenylene diamine (DPPD) as polymerization
inhibitor. The flask was fitted with a 15-mm diameter.times.45-cm
glass-helix packed column and a still head. Distillation was
conducted at atmospheric pressure. Approximately 115 ml foreshot
collected at boiler temperatures up to 101.degree. C. was
discarded; 200 ml of polymerization grade comonomer was collected
at boiler temperatures between 100.5.degree. to 101.5.degree. C.
Analysis by gas-liquid chromatography indicated purity exceeding
99%.
B. Purification of Methyl Methacrylate
A 1650-ml charge of methyl methacrylate (Du Pont Type H112) was
dripped through a 90-mm diameter.times.8-cm deep bed of Woelm Basic
Aluminum Oxide, Activity Grade 1, into a 2-1 round-bottom flask
containing 0.5 g DPPD inhibitor. The monomer was distilled through
a 25-mm diameter.times.56-cm high column packed with glass helices
at a high reflux ratio. Four hundred ml were collected as a
foreshot at condensing temperatures up to 101.degree. C. at
atmospheric pressure and discarded; the still was cooled and
blanketed with filtered argon. The next day an additonal 100 ml
foreshot was collected and discarded, and a 520 ml product cut was
collected in an argon filled dropping funnel at 1.93 ml/min,
condensing temperature of 101.degree. C. at atmospheric
pressure.
About 260 ml of the MMA was discharged from the funnel into an
attached 2-1 argon flushed glass mixing vessel; 2.64 ml of a
solution of 2.64 ml of ethyl acrylate, 2.64 ml of glycol dimercapto
acetate (second cut, Example 1, Part B), and 0.1600 g of Vazo.RTM.
64 was injected into the MMA through a serum stopper and PTFE
stopcock. Then the remainder of 520 ml of MMA was run into the
mixing vessel and mixed with the other ingredients by a
magnetically driven PTFE coated impeller to produce a solution
99.75 mol % MMA, 0.25 mol % EA, 0.17 mol % (based on monomer) GDMA
and 0.01 mol % (based on monomer) Vazo.RTM. 64.
Half of the mixture was filtered into a first chromium plated
stainless steel tube as in Example 1. The second half of the
monomer mixture was similarly discharged into a second tube. The
second tube was placed in a freezer at -20.degree. C.
C. Polymerization
Polymerization was substantially as Example 1 with the following
exceptions.
______________________________________ Dilatometric Elapsed Time
Temperature Heating Rate Conversion hr. .degree.C. .degree.C./hr %
______________________________________ 0-16 60 nil 54.1 16-16.9 70
nil 70 16.9-18.4 70-130 40.degree. C./hr -- 18.4-19.4 130 nil --
19.4-20 130-100 ca -60 -- 20-20.4 100 -- 99.7
______________________________________
The pressure was then relieved and the recirculating oil allowed to
cool.
The polymer rod was removed from the polymerization tube, small
samples were taken for analysis, and the rod was placed in a
polyethylene bag without handling and overwrapped with aluminum
foil.
The contents of the second tube were polymerized in substantially
the same manner.
D. Extrusion
Extrusion of both preforms was accomplished substantially as
Example 1.
E. Attentuation
Utilizing the equipment cited in Example 3, grating 33-86-02, the
monochromator was tuned to the wavelength of maximum transmission
in the vicinity of 650 nm, using a sample ca. 20 meters long. It
has been established that the wavelength so defined is the
wavelength of minimum attentuation for protonated poly-MMA core
fibers. The attenuation at that wavelength was determined. The
results are summarized in Table IV.
TABLE IV ______________________________________ Data for
Comparative Examples A, B Core Polymer Ex. A Ex. B
______________________________________ Properties Inherent Visc.,
dl/gm Residual Monomer, wt % 1.01-1.03 0.89 Fiber Spinning Temp.,
.degree.C. 215 215 Line Speed, ft/min. 120 120 Cladding Thickness,
.mu.m 16 16 Properties Diameter, .mu.m 390 .+-. 20 396 .+-. 18.8
Wavelength for 650 651 Max. Transmission, nm Attenuation at
.lambda.max, dB/km 260 305
______________________________________
Example 4 and Comparative Example C
Methyl methacrylate-d.sub.8 used in this example was synthesized by
the method of Nagai except that DCN was used in place of HCN.
Optical fibers were prepared from MMA-d.sub.8 (Ex. 4) and from MMA
(Example C) as follows.
Rods of core polymer were first prepared. For Ex. 4, a mixture of
21.69 g (99 mol %) of purified MMA-d.sub.8, 0.25 mol (1 mol %)
ethyl acrylate, 0.095 ml (0.2 mol %, based on monomer) of lauryl
mercaptan, and 0.0015 g (0.0045 mol %, based on monomer) of
Vazo.RTM. 64 was prepared in an Erlenmeyer flask, and it was
deoxygenated by swirling and bubbling nitrogen through it. The
mixture was drawn into a syringe and filtered through a pack of
four fluorocarbon "Millipore" filters (pore sizes 5, 0.2, 0.2, 5
.mu.m) into a 9-mm outside diameter rigorously cleaned glass tube.
For Example C, a mixture of 20.0 g (99 mol %) of MMA (Du Pont type
H112, distilled), 0.25 ml (1 mol %) ethyl acrylate, 0.095 ml (0.2
mol %), based on monomer) of lauryl mercaptan, and 0.0015 g (0.0045
mol %, based on monomer) Vazo.RTM. 64 was prepared, and a like
glass tube similarly filled. The mixtures were polymerized by
subjecting them at atmospheric pressure to the following heating
schedule:
______________________________________ elapsed time, hrs. temp.,
.degree.C. ______________________________________ 0-16.5 70 17-20.3
90 20.6-24 100 24-24.5 110 24.5-24.8 112
______________________________________
The tubes were then slowly cooled. The glass tubes were broken away
from the polymer rods. The inherent viscosity (0.2 g of polymer in
100 ml of solution in chloroform at 25.degree. C.) of the Example 4
polymer was 0.4995 dl/g and that of Control C polymer was 0.452
dl/g. Both rods contained some voids.
Each core rod was then placed within a clean glass tube of 11-mm
outside diameter.times.1-mm wall, and centered and held in the
center of the tube with coiled 20-gauge copper wire. The annular
space was filled in each case with a solution prepared as follows.
In 2.5 g (10 wt. %) of MMA was dissolved 0.0085 ml (0.029 wt. %
based on monomer) of lauryl mercaptan and 0.0375 (0.15 wt. %, based
on monomer) of Vazo.RTM. 64, and to this was added 22.5 g. (90 wt.
%) of the fluorinated comonomer described in Example 1, Part D, and
the mixture was deoxygenated by bubbling nitrogen through it. The
mixture was filtered into the annular spaces mentioned above. The
core polymer rods tended to float up, and were held in place with
stainless steel wire. The monomer mixture in the indicated annular
spaces was polymerized at atmospheric pressure by heating at
50.degree. C. for 5 hrs and then at 90.degree. C. for about 19 hrs.
This polymer layer, to become the cladding of the optical fiber,
also contained some voids. Again the glass tube was removed by
breaking it away. Each composite polymer rod was then cut into
lengths of 5 to 7.5 cm (2 to 3 in).
Optical fibers were spun from the composite rods by extruding them
in an Instron capillary rheometer having a bore diameter of 9.5 mm
(0.375 in ) with a die having a diameter of 9.5 mm (0.375 in) and a
60.degree. taper to an 1.0 mm (0.040 in) orifice. A length (5 to
7.5 cm) of a composite rod was placed in the heated cylinder of the
rheometer, and the heat-softened composite rod was extruded through
the die fitted to the bottom of the rheometer cylinder by advancing
the rheometer piston at a constant rate. The extrusion temperature
was 205.degree.-210.degree. C. The cross-head rate of the Instron
machine was 5.1 mm/min (0.2 in/min). Pressures were 7.7 kg/cm.sup.2
(109 psig) for Example 4 and 7 kg/cm.sup.2 (100 psig) for Control
C. The composite extrudate was drawn away at constant speed,
cooled, and the resulting step-index optical fiber was wound up at
5.7 m/min (18.7 ft/min) at constant, low tension on 5-cm (2-in)
diameter fiber cores; a transversing wind-up was used, so
successive layers of fiber were laid on previous layers with
crossovers.
Attenuation of transmitted light was measured as described by E. A.
J. Marcatili, "Factors Affecting Practical Attentuation and
Dispersion Measurements," Optical Fiber Transmission II, Technical
Digest, Optical Society of America, 1977, paper TuE1. For the
measurements reported herein, the light source was a
tungsten-halogen (incandescent) projector lamp powered by a DC
voltage and current stabilized supply, and the wavelengths used
were selected with interference filters having peak wavelengths
shown in Table IV, band widths of 10 nm, 50% minimum transmission
and average transmission of the side bands of 10.sup.-4,
specifically, Ealing-IRI interference filters. The input end of the
fiber was placed at the circle of least confusion of the source.
Numerous experiments indicated that the log.sub.10 power was linear
with length, and therefore that for practical purposes transmission
was at a steady state. Summary data are given in Table V.
TABLE V ______________________________________ Fiber Properties Ex.
4 Example C ______________________________________ Diameter, .mu.m
264 .+-. 13 267 .+-. 13 Cladding thickness, .mu.m 50 100
Attenuation, dB/km Wavelength, nm* (nominal) 546.1 1590 2040 632.8
1170 1632 656.3 1100 1320 670.8 1060 1360 694.3 1020 1400 767.0
1150 1800 794.7 940 2240 852.1 1300 3230 900.0 1380 7510
______________________________________ *Interference filters
Example 5 and Comparative Example D
Example 4 was repeated using additional core polymer rods as
prepared for Ex. 4 and Comparative Example C, but with the
following exceptions.
The cladding polymer prepared in the annular spaces was polymerized
from a mixture of 15 g (15 wt %) MMA, 0.059 ml (0.05 wt %, based on
monomer) lauryl mercaptan, 0.2 g (0.2 wt %, based on monomer)
Vazo.RTM. 64, and 85 g (85 wt %) of the fluorinated comonomer of
Ex. 1, Part D. The cladding polymer was polymerized in the annular
spaces by heating at 50.degree. C. for 16 hrs. The inherent
viscosity of the cladding polymer was 0.403 dl/g (measured on a
solution of 0.5 g polymer in 100 ml of solution in
1,1,2-trifluoro-1,2,2-trichloroethane at 25.degree. C.)
The extrusion die used in the rheometer when spinning the optical
fiber had a diameter of 9.5 mm (0.375 in) and a 41.2.degree. taper
to an 0.9 mm (0.036 in) diameter by 1.27.+-.0.13 mm (0.050.+-.0.005
in) long ballized orifice. The extrusion temperature was
210.degree. C., with pressures of 7-8.3 kg/cm.sup.2 (100-118 psig)
for Example 5 and 6.4-7.7 kg/cm.sup.2 (91-109 psig) for Example D.
The fiber was wound at constant speed of 6 m/min (20 ft/min) onto a
drum having a 1-meter circumference, a traversing wind-up was used,
and fiber crossovers were avoided. Summary data are given in Table
VI.
TABLE VI ______________________________________ Fiber properties
Ex. 5 Example D ______________________________________ Attenuation,
dB/km Wavelength, 632.8 nm 992 1205 He--Ne laser) Wavelength, nm*
546.1 1863 656.3 765 670.8 669 694.3 611 767.0 578 794.7 557 852.1
627 900.0 934 ______________________________________ *Interference
filters
* * * * *