Optical fibers comprising cores clad with amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole

Amorphous copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene and, optionally, with another ethylenically unsaturated monomer, have high glass transition temperatures, e.g., 85.degree. C. or higher, low indices of refraction, and good physical properties which make them suitable for cladding optical fibers.

BACKGROUND OF THE INVENTION 
This invention relates to certain amorphous perfluoropolymers which are 
particularly suitable as cladding materials in optical fiber 
constructions. 
Various fluoropolymers have been proposed from time to time for this 
purpose, mainly because of their good performance under a variety of 
temperature and atmospheric conditions and resistance to many chemicals. A 
good polymeric cladding material for optical fibers should be completely 
amorphous because crystallites present in polymers would cause light 
scattering. Further, it should have a high glass transition temperature, 
Tg, especially if intended for use at high temperatures because above its 
Tg it would lose some of its desirable physical properties and in 
particular it would be unable to maintain good bonding to the fiber core. 
A desirable Tg would be above 85.degree. C., preferably above 120.degree. 
C. Entirely amorphous fluoropolymers having significantly high Tg's have 
not been heretofore reported. 
U.S. Pat. No. 3,978,030 to Resnick describes certain polymers of 
perfluoro-2,2-dimethyl-1,3-dioxole (PDD), which has the following formula: 
##STR1## 
The above patent describes both homopolymers of PDD, which are not further 
characterized, and acrystalline copolymer with tetrafluoroethylene (TFE), 
which has a melting point of about 265.degree. C. 
SUMMARY OF THE INVENTION 
According to this invention, there is now provided a class of amorphous 
copolymers of perfluoro-2,2-dimethyl-1,3-dioxole with tetrafluoroethylene 
and, optionally, with at least one other ethylenically unsaturated 
monomer. There also are provided optical fibers with a copolymer of the 
above class as the cladding.

DETAILED DESCRIPTION OF THE INVENTION 
Both principal monomers used in this invention are known to the art. TFE is 
made in large quantities by E. I. du Pont de Nemours and Company, while 
PDD is described in the above-mentioned U.S. Pat. No. 3,978,030. Pure PDD 
boils at about 33.degree. C. at atmospheric pressure. It has now been 
discovered that these two monomers can be copolymerized in all weight 
proportions within the range of 1-99 percent TFE. 
This discovery is very surprising because no other perfluoro monomer is 
known to copolymerize with TFE in all proportions except 
perfluoro-2-methylene-4-methyl-1,3-dioxolane, which is a completely 
different type of monomer that has an exo-perfluoromethylene group. PDD, 
on the other hand, has an endo-double bond. In fact, other perfluoro 
monomers having internal double bonds copolymerize with TFE with extreme 
difficulty. Monomers such as perfluoropropyl vinyl ether and 
hexafluoropropylene copolymerize with TFE with such difficulty that 
copolymers with TFE containing more than 20 mole % of such monomers are 
not commercially feasible. 
As the amount of PDD in the copolymer increases, the Tg also increases, 
although not necessarily in a linear fashion. The relationship between the 
molar fraction of PDD in the dipolymer and the Tg is shown in FIG. 1. It 
can be seen that a copolymer containing 11.2 mole percent PDD has a Tg of 
57.degree. C., and a copolymer containing 56.9 mole percent PDD has a Tg 
of 119.degree. C. Copolymers having intermediate amounts of PDD also have 
intermediate Tg's. Tg is determined by differential scanning calorimetry 
(DSC) according to ASTM method D-3418. It has been found that copolymers 
of PDD and TFE in which the amount of PDD is less than about 11 mole 
percent are normally crystalline. Although the exact breakpoint for 
crystalline character has not been established with certainty, it is 
believed that copolymers having 7 mole percent or less of PDD are all 
crystalline. The relative proportions of the comonomers in the copolymer 
can be determined by fluorine-19 nuclear magnetic resonance spectroscopy 
(NMR). The proportion of hydrogen-containing monomers can be determined by 
proton NMR together with .sup.19 F NMR. The relative proportions and 
reactivities of the various monomers in a copolymer correspond more or 
less to the proportions of the starting monomers in the polymerization 
reaction. 
A homopolymer of PDD appears to be amorphous and has a high Tg. However, 
PDD is a much more expensive monomer than TFE so that use of PDD 
homopolymers, rather than of PDD/TFE copolymers, is economically much less 
attractive. Furthermore, the copolymers are easier to fabricate. The 
dipolymers have low refractive indices, which is a particularly desirable 
feature for optical fiber cladding. Furthermore, films of these copolymers 
are clear and transparent, compared with hazy or translucent films of 
crystalline polymers. For this reason, the amorphous copolymers of the 
present invention are suitable for such applications as, for example, 
windows for chemical reactors, especially for processes using or 
manufacturing hydrogen fluoride. Amorphous terpolymers can be made by 
copolymerizing certain ethylenically unsaturated monomers with 
perfluoro-2,2-dimethyl-1,3-dioxole and tetrafluoroethylene. These include 
selected olefins, vinyl compounds, and perfluoromonomers. Typical olefins 
are, for example, ethylene, propylene, 1-butene, isobutylene, 
trifluoropropene, and trifluoroethylene. Vinyl monomers can be, for 
example, vinyl fluoride, vinylidene fluoride, and chlorotrifluoroethylene. 
Perfluoromonomers may be of different chemical types, for example, 
perfluoropropene, perfluoro(1,3-dioxole), perfluoro(alkyl vinyl ethers), 
methyl 
3-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-2, 
2,3,3-tetrafluoropropanoate 
##STR2## 
and 
2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1, 
1,2,2-tetrafluoroethanesulfonyl fluoride 
##STR3## 
The proportion of PDD in the amorphous terpolymers of this invention should 
preferably be at least 12 mole percent of the TFE content, while the mole 
percent content of the third monomer should be the smallest of all three 
monomers. Outside these limits either an amorphous terpolymer may not be 
obtained or, if made, its maximum tensile modulus and strength may not be 
realized. 
Copolymerization is carried out in the presence of a free radical 
generator, preferably at a slightly elevated temperature, for example, 
55.degree.-65.degree. C. Well agitated pressure equipment should be used. 
This invention is now illustrated by the following examples of certain 
preferred embodiments thereof, where all parts, proportions, and 
percentages are by weight, unless otherwise indicated. All Tg's were 
determined using Du Pont Differential Thermal Analyzer Models 900 or 990. 
All units other than SI have been converted to SI units. 
EXAMPLE 1 
A 110 cm.sup.3 stainless steel shaker tube was charged with a cold solution 
of 8.2 g (0.0336 mole) PDD and 0.006 g of perfluoropropionyl peroxide in 
120 g of 1,1,2-trichloro-1,2,2-trifluoroethane. The tube was closed and 
chilled further to about -50.degree. C., then alternately evacuated and 
quickly flushed with nitrogen three times. The cold, evacuated tube was 
mounted in a horizontal shaker and charged with 2 g (0.02 mole) of TFE. 
The tube was agitated and heated at autogenous pressure to 
50.degree.-55.degree. C. and maintained in this temperature range for two 
hours. After cooling and venting the tube, the liquid contents were 
removed; 1,1,2-trichloro-1,2,2-trifluoroethane was distilled off, and the 
remaining solid polymer was dried in a vacuum oven at 110.degree. C. for 
sixteen hours. The Tg of the copolymer was 119.degree. C. The apparent 
melt viscosity (AMV) was 0.9 kPa.s at 230.degree. C. This value was 
calculated from the melt flow rate (MFR) determined according to ASTM 
D2116 at a load of 383.1 g at 230.degree. C. 
##EQU1## 
NMR analysis of the copolymer showed that it contained 56.9 mole percent 
PDD and 43.1 mole percent TFE. 
EXAMPLE 2 
Following the procedure of Example 1, a 400 cm.sup.3 stainless steel shaker 
tube was charged with 270 g of 1,1,2-trichloro-1,2,2-trifluoroethane, 30 g 
(0.123 mole) of PDD, 0.08 g of bis(4-t-butylcyclohexyl)peroxydicarbonate, 
and 40 g (0.4 mole) of TFE. The tube was agitated five hours at 
55.degree.-65.degree. C. The resulting copolymer, 49.6 g, was isolated as 
described in Example 1. Its Tg was 73.degree. C. 
EXAMPLE 3 
Following the procedure of Example 2, a 110 cm.sup.3 tube was charged with 
120 g of 1,1,2-trichloro-1,2,2-trifluoroethane, 9 g (0.0369 mole) PDD, 
0.03 g of bis(4-T-butylcyclohexyl)peroxydicarbonate, and 4 g (0.04 mole) 
of TFE. Heating 4 hours with agitation at 60.degree. C. produced 10 g of 
copolymer which had Tg's at 102.degree. and 104.degree. C. Extracted 
overnight with the polymerization solvent, the product gave three 
fractions. The least soluble fraction (about 70 percent of the total) had 
a Tg of 105.degree. C. and an apparent melt viscosity of 1.65 kPa.s. NMR 
analysis showed that it contained 43.7 mole percent PDD and 56.3 mole 
percent TFE. The more soluble fractions had Tg's, respectively, at 
99.degree. and 101.degree. C. 
EXAMPLE 4 
Following the procedure of Example 1, a 110 cm.sup.3 shaker tube was 
charged with 100 g of 1,1,2-trichloro-1,2,2-trifluoroethane, 9.8 g (0.0402 
mole) of PDD, 6 g (0.06 mole) of TFE and 0.03 g of 
bis(4-t-butylcyclohexyl)peroxydicarbonate. The agitated tube was heated 
five hours at 53.degree.-63.degree. C. under autogenous pressure. The 
resulting copolymer weighed 9.1 g. It was found by DSC to have physical 
transitions at 74.degree. C., 82.degree. C., 122.degree. C. and 
124.degree. C. Soxhlet extraction of the copolymer with 
1,1,2-trichloro-1,2,2-trifluoroethane gave three polymeric fractions: the 
least soluble, Tg=92.degree. C.; the middle solubility fraction, 
Tg=93.degree. C.; and the most soluble, Tg=70.degree. C. 
EXAMPLE 5 
Following the general procedure of Example 2, copolymerization of 5 g 
(0.0205 mole) of PDD, 10 g (0.1 mole) of TFE, and 2 g (0.0714 mole) of 
ethylene gave 11.3 g of an amorphous terpolymer, which had glass 
transitions at 70.degree. C., 72.degree. C., 144.degree. C. and 
151.degree. C. 
When this experiment was repeated using only 1 g of ethylene, 12.7 g of an 
amorphous terpolymer was obtained, which had glass transitions at 
63.degree. C. and 143.degree. C. A film was pressed from this terpolymer 
at 230.degree. C. Its infrared spectrum was consistent with that of a 
terpolymer of TFE, PDD, and ethylene. After extraction to remove any trace 
of shaker tube lubricant, the film had glass transitions at 67.degree. C. 
and 71.degree. C. 
EXAMPLE 6 
Following the general procedure of Example 2, copolymerization of 5 g 
(0.0205 mole) of PDD, 10 g (0.1 mole) of TFE, and 0.5 g (0.00893 mole) of 
isobutylene gave after 3 hours at 55.degree.-80.degree. C. 3 g of a 
terpolymer, which had glass transitions at 68.degree. C. and 76.degree. C. 
A film pressed from this polymeric material gave an infrared spectrum 
consistent with that of a PDD/TFE/isobutylene terpolymer. 
EXAMPLE 7 
A terpolymer of PDD, TFE, and propylene was obtained under the general 
conditions of Example 2 from 4 g (0.0164 mole) of PDD, 10 g (0.1 mole) of 
TFE, and 1 g (0.0238 mole) of propylene. The terpolymer weighed 0.7 g, was 
elastomeric and readily soluble in the polymerization solvent. A film cast 
from that solvent had infrared absorbancies which confirmed the terpolymer 
composition. 
EXAMPLE 8 
A terpolymer was prepared according to the general procedure of Example 2 
from 4 g (0.0164 mole) of PDD, 10 g (0.1 mole) of TFE, and 1 g (0.00213 
mole) of 
2-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-1, 
1,2,2-tetrafluoroethanesulfonyl fluoride. This terpolymer weighed 10.8 g 
and was shown by NMR analysis to consist of 87.1 mole percent TFE, 12.3 
mole percent PDD, and 0.6 mole percent of the sulfonyl fluoride monomer. 
Its infrared spectrum also was consistent with this composition. The 
terpolymer had glass transitions at 67.degree. C., 124.degree. C., 
152.degree. C. and 175.degree. C. 
EXAMPLE 9 
A terpolymer was made according to the general procedure of Example 2 from 
3 g (0.0123 mole) of PDD, 10 g (0.1 mole) of TFE, and 1 g (0.00237 mole) 
of methyl 
3-[1-[difluoro[(trifluoroethenyl)oxy]methyl]-1,2,2,2-tetrafluoroethoxy]-2, 
2,3,3-tetrafluoropropanoate. The terpolymer weighed 10.2 g. It was pressed 
at 250.degree. C. into a film, which had infrared absorbancies consistent 
with the terpolymer composition. Fluorine-19 NMR analysis gave the 
respective mole percent amounts of the three monomers as 83.7 TFE, 13.4 
PDD, and 2.9 methyl ester monomer. Differential thermal analysis of the 
terpolymer between 25.degree. and 350.degree. C. showed the absence of a 
melting point. 
EXAMPLE 10 
A copolymer of PDD and TFE was made according to the general procedure of 
Example 1 from 8 g of PDD and 0.5 g of TFE. This copolymer had the 
expected infrared absorption and showed glass transitions at 167.degree. 
C., 215.degree. C. and 288.degree. C. 
COMATIVE EXAMPLE 
A copolymer was made from 2 parts of PDD and 10 parts of TFE according to 
the exact procedure of Example 3 of U.S. Pat. No. 3,978,030 to Resnick. 
This polymeric product was extracted for 25 hours with 
1,1,2-trichloro-1,2,2-trifluoroethane. About 0.2 percent of the product 
weight was thus removed; the extracted fraction was a grease and appeared 
to consist of shaker tube lubricant and a small amount of an unknown 
fluorocarbon. Because this obviously was a low molecular weight material, 
it was expected that the material would have a Tg well below 200.degree. 
C., if it had a Tg at all. The extracted product had no Tg between 
25.degree. and 200.degree. C. It was different from the amorphous 
copolymers of this invention. The solid extraction residue was a 
crystalline, rather than an amorphous polymer. This comparative experiment 
shows that at monomer ratios employed in Example 3 of U.S. Pat. No. 
3,978,030 no amorphous PDD/TFE copolymer is obtained. 
FIG. 2 shows wide-angle X-ray powder diffraction scans of the solid 
extraction residue of the crystalline polymeric product of this example 
(curve A) and of an amorphous TFE/PDD copolymer having a Tg of 73.degree. 
C. (curve B). In this figure, intensity, I (counts/sec) is plotted against 
the diffraction angle, 2.theta.. The presence of crystallinity is shown by 
the sharp peak in curve A. The absence of a sharp peak in curve B denotes 
lack of crystallinity. 
EXAMPLE 11 
An amorphous copolymer containing about 78 mole percent TFE and 22 mole 
percent PDD was made according to the general technique described in 
Examples 1-4. This material had a Tg of 72.degree. C. and an apparent melt 
viscosity (AMV) of 0.34 kPa.s at 230.degree. C., which was determined as 
explained in Example 1, above. 
The copolymer was milled at 200.degree.-230.degree. C. and cut to provide 
an extrusion powder, then charged into the hopper of the crosshead 
extruder described in U.S. Pat. No. 4,116,654 to Trehu. The copolymer was 
extruded via a gear pump onto a "TO-8 Commercial" fused silica core, 
producing a 1 Km continuous length of 540 .mu.m having a 200 .mu.m core. 
This was done according to the technique described in Example 1 of the 
Trehu patent. The front heating zone was at 190.degree. C., the rear 
heating zone at 143.degree.-145.degree. C., the block temperature 
225.degree.-228.degree. C., and the die temperature varied from 
230.degree. to 245.degree. C. The copolymer flowed smoothly and evenly 
without bubbles or apparent degradation, providing an even coating with 
excellent adhesion to the core. The clad fiber easily passed the standard 
toughness test. A 64 m length was inspected for bubbles and was entirely 
free of large bubbles. It had an attenuation of 113 dB/Km. 
Evaluation of Films 
Films 0.025-0.05 cm thick were pressed at 230.degree.-300.degree. C. from 
polymer granules at 700-7000 kPa. Three of these polymers were amorphous 
PDD/TFE copolymers of the present invention (Polymer B is the least 
soluble fraction of Example 4), while the prior art crystalline copolymer 
was made according to the teaching of U.S. Pat. No. 3,978,030 to Resnick. 
The physical properties of these films are reported in the following 
Table, where it can be seen that both the modulus and the tensile strength 
of the amorphous copolymers of this invention are significantly improved 
over those of the crystalline copolymer. The amorphous copolymers are thus 
stiffer and stronger. 
TABLE 
______________________________________ 
Physical Properties of TFE/PDD Copolymers 
Crystalline 
Amorphous Copolymer 
Copolymer 
A B C 
______________________________________ 
Mole % PDD 5-7 22.1 36.6 56.9 
Tg (.degree.C.) 
-- 73 90 119 
m.p. (.degree.C.) 
265 -- -- -- 
Tensile 
Properties* (23.degree. C., 
50% RH) 
Modulus (MPa) 
620 917 1117 1234 
Stress (MPa) 
Yield 15.9 26.2 -- -- 
Maximum 20.7 27.6 27.6 30.3 
Break 20.7 26.9 27.6 30.3 
Strain (%) 
Yield 5.5 3.8 -- -- 
Break 125.3 58.2 4.1 4.4 
______________________________________ 
*ASTM D1708