GB2084593A - Copolymers of tetrafluoroethylene - Google Patents

Abstract

Copolymers consisting of 93-99 mol % tetrafluoroethylene units and, complementally, 7-1 mol % fluorinated alkyl ethylene comonomer units are characterized by having the units of the copolymer substantially uniformly positioned throughout the copolymer chain. This is achieved by a process in which the fluorinated alkyl ethylene is uniformly added to the reaction vessel throughout the polymerization to maintain its concentration below 2.5 mol % relative to the tetrafluoroethylene.

Description

SPECIFICATION Compositions and processes FIELD OF THE INVENTION This invention relates to polymers of tetrafluoroethylene, and more specifically to copolymers of tetrafluoroethylene and fluorinated alkyl ethylenes.

BACKGROUND OF THE INVENTION A number of copolymers of tetrafluoroethylene are known, but new copolymers of tetrafluoroethylene are always of interest due to a desire to obtain polymers having improved properties over polymers known heretofore.

U.S. 4,123,602 to H. Ukihashi, et al., describes tetrafluoroethylene/ethylene copolymers modified with 0.1 to 10 mol % RfCH=CH2 where Rf is CnF2n+, in which n is an integer between 2 and 10. These polymers contain between 40 and 60 mol % ethylene. The thermal instability against oxidation limits their use above 1 500C for extended periods of time.

U.S. 3,804,817 to L. A. Wall and D. W. Brown describes copolymers of (perfluoropropyl)ethylene (PFPE) and tetrafluoroethylene (TFE), and copolymers of (perfluoroethyl)ethylene and tetrafluoroethylene. These copolymers are elastomeric, soluble in fluorinated solvents, and possess only modest thermal stability as evidenced by the thermal gravimetric analysis (TGA) data shown in the patent. In J. Poly. Sci. 8, (1970) the patentees reproduce Table 1 of the U.S. 3,804,817 and in the article describe that the copolymers of TFE and PFPE containing up to 89 mol % TFE are amorphous by X-ray diffraction measurement. The polymers appear amorphous on testing by differential scanning calorimetry (DSC) techniques.

Also disclosed in U.S. Patent 3,804,817 is a TFE copolymer containing about 6 mol % PFPE, in Example 4. This copolymer has the following properties: 1. The melting behavior by DSC is essentially identical to PTFE. It is well known (Polymer Handbook, Vol. 2, pV-32, (1975)) that polytetrafluoroethylene (PTFE) has a reproducible melting point at 3270C and occasionally contains phases which exhibit higher melting points on the intial heating cycle. The higher melting point can be as high as 3420C. The 94/6 copolymer of U.S. Patent 3,804,817 Example 4 exhibits both of these melting points and no lower melting points which would correspond to crystalline copolymer phases.

2. Thermal stability of the melt at 3500C, i.e., approximately 20CC above the melting point, as measured by isothermal TGA in air, is low. A weight loss of 8.7% was seen at 140 minutes.

3. A small-angle x-ray scattering scan does not show the broad diffraction peak at low angles (20 below 1 ) normally seen for crystalline TFE copolymers. The 94/6 copolymer of U.S. Patent 3,804,817 Example 4 instead shows the steady increase in intensity with decreasing angle that is shown by homopolymer PTFE.

Based on these results, the copolymer of U.S. Patent 3,804,81 7 Example 4 is believed to be composed mostly of a fraction of crystalline TFE homopolymer with a minor fraction of amorphous TFE/PFPE copolymer with high PFPE content. It is well known that crystalline TFE homopolymer cannot be processed by melt techniques, and that mixtures of PTFE with melt processible copolymers lead to the formation of high local concentrations of PTFE, or "fisheyes", in finished parts which are undesirable. in extreme cases, the presence of PTFE in a melt processible copolymer can greatly lower the strength of a finished article. The objective of copolymerizing small amounts, e.g. 1-10 mol % of comonomer, with TFE is to produce copolymers with homogeneous, readily processible melts in which the melting point has been lowered from that of PTFE. U.S.Patent 3,804,817 does not teach how to achieve this objective.

SUMMARY OF THE INVENTION Copolymers of tetrafluoroethylene and fluorinated alkyl ethylenes are obtained by this invention in which units of the copolymer derived from the ethylene comonomer are substantially uniformly positioned along the copolymer chain. This positioning results in insoluble copolymers which when molded are nonelastomeric and exhibit good thermal stability.

In contrast to the copolymers of U.S. Patent 3,804,817, especially the "94/6 copolymer" of Example 4, the copolymers of this invention exhibit the following properties: 1. DSC melting points are substantially below that of PTFE and are in the range expected for true random copolymers. The melting points are usually between 260 and 31 80C.

2. Thermal stability by isothermal TGA in air is good. These measurements were made at 3500C, i.e. between 30 and 900C above the melting points of these polymers. The weight loss is less than 5% at 140 minutes and is frequently 1% or less.

3. Small angle X-ray scattering (SAXS) scans show a diffraction peak at very low angles (less than 1 20), normally seen for semicrystalline copolymers of TFE.

Thus the term "substantially uniformly positioned" means that the copolymers meet the following conditions: DSC melting point is below about 31 80C, the thermal stability is such that the weight loss after 140 minutes at 350"C in air is less than 5%, and the small angle X-ray scattering scan shows a diffraction peak at angles below 1 20.

By "insoluble" is meant that the copolymer does not dissolve in organic solvents maintained at 250C.

By "nonelastomeric" is meant that the molded copolymer is not a material which at room temperature can be stretched repeatedly to at least twice its original length and, upon immediate release of the stress, will return with force to its approximate original length.

The copolymers of this invention can more specifically be described as copolymers of 93-99 mol % tetrafluoroethylene, and complementally 7-1 mol % fluorinated alkyl ethylene comonomers of the formula RfCH=CH2 wherein Rf is perfluorinated alkyl of 2-10 carbon atoms or substituted perfluoroalkyl of 2-10 carbon atoms in which the perfluoroalkyl group can be substituted with up to 3 substituents selected from hydrogen, bromine, chlorine or iodine, said copolymer characterized by having the units of the comonomer substantially uniformly positioned, i.e., evenly distributed, throughout the copolymer chain.

The process aspect of the invention can be described as a process for preparing the copolymer described above which comprises (a) combining and agitating tetrafluoroethylene and the fluorinated alkyl ethylene in the presence of a nonaqueous solvent or in aqueous media in a reaction vessel at a temperature of between 300C and 11 00C and a pressure of between 1 kg/cm2 and 70 kg/cm2 and preferably between 3 kg/cm2 and 35 kg/cm2, in the presence of a free-radical polymerization initiator, said combining of the tetrafluoroethylene and fluorinated alkyl ethylene being carried out by continuously and uniformly adding fluorinated alkyl ethylene to the reaction vessel in a manner which maintains a concentration of fluorinated alkyl ethylene in the vessel during agitation below 2.5 mol %, and preferably below 1 mol %, relative to tetrafluoroethylene, said agitation being continued until copolymer formation has occurred, and (b) separating the copolymer from the other ingredients present in step (a).

DESCRIPTION OF THE INVENTION The copolymer of this invention is obtained by copolymerizing tetrafluoroethylene and fluorinated alkyl ethylene in either a nonaqueous solvent or in an aqueous medium in a manner which maintains the concentration of the fluorinated alkyl ethylene in the reaction mixture at a relatively constant and low concentration compared to the concentration of tetrafluoroethylene. If a large concentration, i.e., over 2.5 mol %, of perfluoroalkyl ethylene is employed, the polymerization reaction is inhibited, and uniform copolymer is not obtained.

As nonaqueous solvents in the polymerization, fluoro- or chlorofluoro-hydrocarbon, (preferably 1 to 4, and especially 1 to 2, carbon atoms) known as a "Freon" solvent or solvents similar thereto are useful. Suitable "Freon" solvents or solvents similar thereto include: dichlorodifluoromethane, trichloromonofluoromethane, dichloromonofluoromethane, monochlorodifluoromethane, chlorotrifluoromethane, tetrafluoroethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, fluorochloropropane, perfluoropropane, fluorocyclobutane, perfluorocyclobutane, etc. or mixtures thereof.It is best to use a saturated fluoro- or chlorofluoro-hydrocarbon which does not have a hydrogen atom in the molecule, such as dichlorodifluoromethane, trichloromonofluoromethane, trichlorotrifluoroethane, dichlorotetrafluoroethane, perfluorocyciobutane, etc., since such solvents have a tendency of increasing the molecular weight of the resulting copolymer. When a "Freon" solvent or a like solvent is used, good results are attainable when used in amounts of 0.5-20 mol and especially about 1-10 mol of the solvent per mol of monomer mixture of tetrafluoroethylene and perfluoroalkylethylene monomer.

The copolymerization reaction can be carried out by using less than 0.5 mol of the solvent per mol of monomer mixture. However, it is advantageous to use more than 1 mol of solvent in order to enhance the rate of the copolymerization. It is possible to use more than 20 mols of solvent, but it is advantageous to use less than 10 mols per mol of monomer mixture for economic reasons, such as solvent recovery.

A mixture of a "Freon" solvent or like solvent and other organic solvents or aqueous medium may be used. For example, it is possible to use a mixed reaction medium of "Freon" solvent or like solvent and water.

The advantage of using such a mixed solvent consists in easy stirring of the reaction system and easy removal of the heat of reaction. In accordance with the process of the invention, the conditions of the copolymerization can be varied depending upon the type of polymerization initiator or the reaction medium.

A wide variety of polymerization initiators can be used depending upon the polymerization system.

However, when a "Freon" solvent or solvent similar thereto is used, it is preferable to use a soluble freeradical polymerization initiator, such as an organic peroxy compound. It is possible to use high energy ionizing radiation of 10--105 rad/hour dose rate. Suitable peroxy compounds may be the organic peroxides, e.g., benzoylperoxide or lauroylperoxide; peresters, e.g., t-butyl peroxyisobutyrate; or peroxy dicarbonates, e.g., diisopropylperoxy dicarbonate, etc. It is especially prefered to use as the initiator in non-aqueous systems, a peroxide having the formula

wherein Rf each represent perfluoroalkyl groups containing from 3-1 3 carbon atoms, in a "Freon" solvent or like solvent. Suitable such peroxides include bis(perfluoropropionyl) peroxide, bis(perfluorohexanoyl) peroxide, etc.

When the solvent system is an aqueous system, initiators such as peroxides or persulfates which are compatible with water should be used, such as disuccinoyl peroxide or ammonium persulfate. In an aqueous system, a non-telogenic dispersing agent is commonly employed.

The polymerization can be carried out at a temperature of between 30 and 11 OOC, and preferably at a temperature of between 40 and 800 C. Pressures employed in the polymerization are ordinarily those pressures between 1 and 70 kg/cm2 and preferably are those between 3 and 35 kg/cm2.

It is often preferable to include a small amount of a telogenic material in the reaction medium in order to control the molecular weight of the resulting copolymer. Alcohols such as methanol or ethanol, and alkanes such as ethane, butane, cyclohexane, etc., are suitable telogens.

The mixture of comonomers is agitated during polymerization.

The reaction may be carried out until solids content of the reaction mixture reaches about 20% by weight for aqueous reactions, and about 12% for nonaqueous solvent based reactions.

It is important to control the concentration of the fluorinated alkyl ethylene in the reaction mix as heretofore described, and to maintain the concentration at a relatively constant level. Both these features result in copolymers in which the units of fluorinated alkyl ethylene present are substantially evenly distributed along the copolymer chain.

Representative comonomers include perfluoroethyl ethylene, perfluorodecyl ethylene, w-chloroperfluoroethyl ethylene, w-hydroperfluoroethyl ethylene, cs-bromoperfluorodecyl ethylene, -iodoperfluoroethylethylene (ICF2CF2CH=CH2), 1.1 ,2,8,8,8-hexahydrodecafluoro octene-1 (CH3(CF2)5CH=CH2), or 1,1 ,2,4-tetrahydrohexafluoropentene-1 (CF3CFHCF2CH=CH2). Preferably the comonomer will be a perfluoroalkyl ethylene, and most preferably perfluorobutyl ethylene.

The copolymers are useful as insulation coating for electrical wires and as linings for equipment exposed to harsh chemical environments.

EXAMPLES In the following examples, apparent melt viscosity was determined by calculations based on the melt flow rate. The melt flow rate was determined according to ASTM procedure D2 16 6 at a load of 5000 g except that the melt flow rate was determined in grams/minutes rather than grams/10 minutes.

The equation used to calculate the apparent melt viscosity was: 10.63 x [Total mass piston a weight (g)] MV= melt flow rate Melting point was determined by differential scanning calorimetry at a rate of 1 50C per minute.

Ultimate elongation, yield strength and ultimate tensile strength were determined by ASTM procedure Dl 708.

Thermal stability was determined by isothermal thermogravimetric analysis (TGA) at 350 C in air using a Du Pont 900 instrument.

Solubility of the polymers prepared in the Examples was determined at 250C in a number of common organic solvents including hexafluorobenzene, acetone, and 1 ,1 ,2-trichloro-1 ,2,2-trifluoroethane. The polymers were insoluble.

X-ray scattering data was obtained with a Kratky diffractometer (Anton Kaar, K. G., Graz, Austria) operating digitally in an off-line mode. The X-ray intensities were detected with a scintillation counter followed by a single-channel pulse-height analyzer set to pass 90% of CuK radiation symmetrically. The x-ray beam was filtered with 0.02 mm Ni foil and the sample thickness was in the range 0.3 to 0.4 mm.

The raw data (x-ray counts, time, position) were recorded on 8-channel punched paper tape and read into an IBM 1130 computer. X-ray intensities were calculated as the ratio of counts to time, and these results were smoothed and corrected for sample thickness and instrumental background. The resultant corrected intensities were plotted as log-intensity vs. diffraction angle (2-0). The results were not corrected for slit-smearing since such a correction would not aid significantly in distinguishing among samples containing a discrete small-angle peak and those not containing it.

The comonomer content of the tetrafluoroethylene perfluorobutyl ethylene copolymers was determined either by an infrared method or by a melting point method.

The infrared method involves measuring the intensity of absorption bands at 2360 cm-' and 1355 cm-' in compression molded film. The band at 2360 cm-' is used to determine the thickness of the film using the equation 1 mil Thickness (mills) = (Abs.2360 cm-1) (0.081 Abs) The comonomer concentration can then be calculated using the following equation:

This equation provides the comonomer concentration in g/cm3.Knowing the density of the polymer allows conversion to weight percent by the following equation: Concentration (g/cc) x x 100 = weight % comonomer 2.1 5 (Density, g/cc) The absorbance factor used in this calculation Abs cm2 (340 9 was determined by standard methods using a hompolymer of perfluorobutyl ethylene in "Freon" F-i 1 3 solvent.

The results obtained by the infrared method agreed well with those obtained through the use of melting point data and the following equation: 1 1 1.98 - (In Nm6) TM TTFE 685 where TM = melting point of the copolymer in OK TTFE = melting point of homopolymer PTFE (6000 K) NTFE = mole fraction TFE in the copolymer This equation was found to agree well with the results obtained from the infrared method for polymers containing up to 5 mol % perfluorobutyl ethylene.

EXAMPLE 1 Into an evacuated, agitated 1 liter stainless steel autoclave were placed 800 ml of 1,1,2-trichloro 1 ,2,2-trifluoroethane solvent and 2 ml of perfluorobutyl ethylene. The temperature of the mixture was raised to 600C and the agitator speed was set at about 1000 rpm. To this mixture was charged tetrafluoroethylene to a total pressure of 9.1 kg/cm2. To the autoclave was then charged 15 ml of a solution of 0.002 g/ml bisperfluoropropionyl peroxide in the aforementioned solvent. The pressure was kept constant by the continuous addition of tetrafluoroethylene. After 10 minutes, the above mentioned peroxide solution was fed into the reactor at a rate of 1 ml/min as was a solution of 0.04 g/ml of perfluorobutylethylene in 1,1 ,2-trichloro-1 2,2-trifluoroethane also at 1 ml/min.This rate maintained a concentration of perfluorobutyl ethylene of less than about 1.1 mol % based on total monomers. The polymerization was continued for a total of 70 minutes at which time the contents of the autoclave were discharged into a large stainless steel beaker. The polymer was recovered by drying in an air oven at 1 500C for several hours. The dry polymer weighted 74 g. The polymer exhibited a sharp melting point at 31 50C with a small shoulder at 2870C. The apparent melt viscosity was too high to measure.

The polymer could be compression molded into tough films. Comonomer content was 1.1 mol %.

Isothermal TGA in air at 350 C showed a 4.8% weight loss at 140 minutes. X-ray scattering data showed a diffraction peak at angles below 1 20.

EXAMPLE2 into an evacuated, agitated 1 liter stainless steel autoclave were placed 800 ml 1,1,2-trichloro 1 2,2-trifluoroethane solvent, 2 ml of perfluorobutylethylene, and 0.25 ml of methanol. The temperature of the mixture was raised to 60 C and the agitator speed was set at about 1000 rpm. To this mixture was charged tetrafluoroethylene to raise the total pressure to 9.1 kg/cm2. To the autoclave was then charged 1 5 ml of a solution of 0.002 g/ml bis-perfluoropropionyl peroxide in the above mentioned solvent. After 4 minutes, the peroxide solution was added continuously to the autoclave at a rate of 1 ml/min. After an additional 4 minutes, the addition of a solution of 0.04 g/ml perfluorobutylethylene in the same solvent was begun, also at a rate of 1 ml/min.This rate maintained perfluorobutyl ethylene at a concentration of less than about 1.1 mol %. The polymerization was allowed to continue for an additional 60 minutes at which time the contents of the autoclave were discharged into a large stainless steel beaker. The polymer was recovered by drying in an air oven at 1 500C for several hours. The dry polymer weighted 55.5 g and had an apparent melt viscosity at 3720C of 27 x 104 poise. The polymer was compression molded into tough films. Comonomer content was 2.3 mol %. Melting point was 3030C. Isothermal TGA in air at 350cm showed a weight loss of 1% at 140 minutes. X-ray data indicated a diffraction peak at angles less than 1 20.

Compression molded films prepared from the polymer described above were placed in a forced air oven and heat aged at 2500C for 200 hours. The physical properties of the treated films were measured and are reported in Table 1.

TABLE 1 Physical Properties of Films Example 2 Example 2 Test (As made) (Heat aged) Ultimate Tensile Strength 25"C 4200 psi 3240 psi 250"C 1480 1370 Yield Strength 250C 2090 psi 2190 psi Ultimate Elongation 25"C 290% 250% 2500C 610 600 MIT folding endurance, No failure No failure cycles, 7-8 mil film after after ASTM D2176 1.3 MM cycles 1.7 MM cycles EXAMPLE 3 Into an evacuated, agitated 1 liter stainless steel autoclave were placed 800 ml 1,1,2-trichloro 1 ,2,2-tdfiuornethane solvent and 1.6 ml I of perfluoropropyl ethylene. The temperature of the mixture was raised to 600C and the agitator speed was set at about 1000 rpm. To this mixture was charged tetrafluoroethylene to raise the total pressure to 9.1 kg/cm2.To the autoclave was then charged 15 ml of 0.023 g/ml bis-perfluoropropionyl peroxide in the above-mentioned solvent. The peroxide delivery pump was then set to add the peroxide solution continuously to the autoclave at the rate of 1 ml/min.

After 7 min, the addition of a solution of 0.04 g/ml of perfluoropropyl ethylene in the same solvent was begun, also at a rate of 1 ml/min. This rate maintained the perfluoropropyl ethylene at a concentration of less than about 1.1 mol %. Tetrafluoroethylene was added at such a rate as to keep the pressure in the autoclave constant. The polymerization was allowed to continue for an additional 60 minutes at which time the contents of the autoclave were discharged into a large stainless steel beaker. The polymer was recovered by drying in an air oven at 1 500C for several hours. The dry polymer weighted 48.4 g and exhibited sharp melting points by differential scanning calorimetry at 284 and 31 00C. The apparent melt viscosity at 372 CC was too high to measure.The polymer could be compression molded into tough films. Comonomer content was 4.3 mol %. Isothermal TGA in air at 350cm showed a weight loss of 0.9% at 140 minutes. X-ray scattering data showed a diffraction peak at angles less than 1 20.

EXAMPLE 4 Into an evacuated, agitated 1 liter stainless steel autoclave were placed 800 ml 1,1 ,2-trichloro- 1 ,2,2-trifluoroethane solvent, 1.0 ml 3,3,4,4-tetrafluorobutene-1, and 0.25 ml methanol. The temperature of the mixture was raised to 60CC and the agitator speed was set at about 1000 rpm. To this mixture was charged tetrafluoroethylene to raise the total pressure to 9.1 kg/cm2. To the autoclave was then charged 1 5 ml of 0.0023 g/ml bis-perfluoropropionyl peroxide in the above-mentioned solvent. The peroxide delivery pump was then set to add the peroxide solution continuously to the autoclave at a rate of 1 ml/min. After 15 minutes, the addition of a solution of 0.021 g/ml of 3,3,4,4tetrafluorobutene-1 in the same solvent was begun, also at a rate of 1 ml/min.This rate maintained the tetrafluorobutene at a concentration of less than about 1.1 mol %. Tetrafluoroethylene was added at such a rate as to keep the pressure in the autoclave constant. The polymerization was allowed to proceed for an additional 60 minutes at which time the contents of the autoclave were discharged into a large stainless steel beaker. The polymer was recovered by drying in an air oven at 1 50cm for several hours. The dry polymer weighted 32.1 g and exhibited a sharp melting point by differential scanning calorimetry at 291 CC with a small shoulder at 31 2CC. The melt viscosity at 372cm was 0.62 x 103 Pa.s (0.62 x 104 poise). Comonomer content was 3.6 mol %.Isothermal TGA in air at 350cm showed a weight loss of 1.1% at 140 minutes. X-ray scattering data showed a diffraction peak at angles less than 1 20.

COMPARISON To show the effect of adding perfluorobutyl ethylene in one initial charge prior to initiating polymerization, the following experiment was carried out.

A 110 ml stainless steel shaker tube was charged with 50 ml 1,1 ,2-trichloro-1 ,2,2-trifluoroethane solvent, 0.74 g perfluorobutylethylene and 0.02 g of bis-perfluoropropionyl peroxide. The tube was cooled and evacuated and lOg of tetrafluoroethylene was added. The tube was heated to 60CC and shaken for 4 hrs. The product was isolated by drying in an air oven at 1 50cm for several hours. The dry polymer obtained weighed 0.25 g (2.3%) and was highly swollen 1,1 ,2-trichloro-1 ,2,2- trifluoroethane. The polymer could not be compression molded into films. The polymer showed no sharp crystalline melting peaks by differential scanning calorimetry between 220 and 350 C.

Claims (8)

1. A copolymer consisting of: 93-99 mol % tetrafluoroethylene units, and complementally, 7-1 mol % fluorinated alkyl ethylene comonomer units of the formula

wherein Rf is perfluorinated alkyl of 2-10 carbon atoms or substituted perfluoroalkyl of 2-1 0 carbon atoms in which the perfluoroalkyl group can be substituted with up to 3 substituents selected from hydrogen, bromine, chlorine or iodine, said copolymer characterized by having the units of the comonomer substantially uniformly positioned throughout the copolymer chain.

2. A copolymer of claim 1 wherein Rf in the formula

of the fluorinated alkyl ethylene is perfluorinated alkyl.

3. A copolymer of claim 1 wherein the fluorinated alkyl ethylene comonomer is perfluoropropyl ethylene.

4. A copolymer of claim 1 wherein the fluorinated alkyl ethylene comonomer is perfluorobutyl ethylene.

5. A process for preparing a copolymer of tetrafluoroethylene and a fluorinated alkyl ethylene of the formula RfCH=CH2 wherein Rf is perfluorinated alkyl of 2-10 carbon atoms or substituted perfluoroalkyl of 2-10 carbon atoms in which the perfluoroalkyl group can be substituted with up to 3 substituents selected from hydrogen, bromine, chlorine or iodine, which comprises (a) combining and agitating tetrafluoroethylene and the fluorinated alkyl ethylene in the presence of a nonaqueous solvent or in aqueous media in a reaction vessel at a temperature of between 30CC and 11 0CC and a pressure of between 1 kg/cm2 and 70 kg/cm2 and preferably between 3 kg/cm2 and 35 kg/cm2 in the presence of a free-radical polymerization initiator, said combining of the tetrafluoroethylene and fluorinated alkyl ethylene being carried out by continuously and uniformly adding fluorinated alkyl ethylene to the reaction vessel in a manner which maintains a concentration of fluorinated alkyl ethylene in the vessel during agitation below 2.5 mol %, and preferably below 1 mol %, relative to tetrafluoroethylene, said agitation being continued until copolymer formation has occurred, and (b) separating the copolymer from the other ingredients present in step (a).

6. A copolymer consisting of: 93-99 mol % tetrafluoroethylene units, and complementally, 7-1 mol % fluorinated alkyl ethylene comonomer units of the formula

wherein Rf is perfluorinated alkyl of 2-10 carbon atoms or cs-substituted perfluoroalkyl of 2-10 carbon atoms in which the w-substituent can be hydrogen, Br, or Cl, said copolymer characterized by having the units of the comonomer substantially uniformly positioned throughout the copolymer chain.

7. A copolymer according to claim 1 substantially as described with reference to any one of the Examples.

8. Shaped articles of or comprising a copolymer as claimed in any one of claims 1 to 4, 6 and 7.