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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to novel fluoropolymers, and more
specifically, to fluoropolymers, methods of making and articles
manufactured therefrom, which fluoropolymers have been permanently
modified at the molecular level but without altering the materials
original surface morphologies as well as bulk characteristics.
Fluorinated polymers, such as fluorohydrocarbon polymers, e.g.,
polyvinylidene fluoride, polyvinyl fluoride (PVF), including the
well-known fluorocarbon polymers, e.g., perfluorinated materials, such as
PTFE, are characterized by extreme inertness, high thermal stability,
hydrophobicity, and a low coefficient of friction as to resist adhesion to
almost any material. While these properties are highly desirable, it would
also be advantageous to modify some of the polymers' characteristics in
order to expand the scope of their useful applications. For instance, in
the field of biocompatible materials fluorocarbon polymers in various
forms have been developed, but because of their chemical inertness and
extremely low reactivity the scope of these improved devices, such as
implantable prosthetic devices and probes has been limited. In the field
of membranes and filters, fluoropolymers have also had limited
applications due to low surface energy problems associated with these
materials. Membranes and filters fabricated from PTFE, for example, are
unable to selectively inhibit permeation of liquids with high surface
tensions (>50 dynes/cm) while allowing liquids having lower surface
tensions to pass through. PTFE has also been under intense study for
applications in cell culture growth membranes, but a principal shortcoming
has been the inability of cells to adhere to this low energy material.
Efforts of others to modify the properties of fluoropolymers have not been
totally satisfactory. U.S. Pat. No. 4,548,867 (Ueno et al), for example,
discloses a fluorine-containing synthetic resin having improved surface
properties as evidenced by increased wettability with water, printability
and susceptibility to adhesive bonding. The fluoropolymer is exposed to a
low temperature plasma comprising an organic nitrogen-containing gas.
Instead of modifying the atomic composition of the fluoropolymer starting
material, Ueno et al form a thin "layer" of a nitrogen-containing wettable
material thereto. Consequently, the adherence of such an overcoating tends
to alter the microstructural morphology of the original polymer,
especially with respect to pore size. This coating also alters desirable
surface properties exhibited by the original fluorinated material.
Others have attempted the use of glow discharge and corona treatments to
produce surface modifications. In some early work, Schonhorn and Hansen
found that exposure of polyolefins and perfluorinated polymers to low
power radio frequency electrodeless discharges in inert gas atmospheres
produced favorable results over wet chemical methods. Their improvement in
the bondability of surfaces was limited and attributed to the formation of
a highly cross-linked surface layer. Studies of Hollahan et al, J. Polym.
Sci., 13, 807 (1969) aimed at rendering polymer surfaces biocompatible
included the interaction of PTFE with plasmas excited in ammonia and
nitrogen/hydrogen mixtures, the goal being the introduction of amino
groups into the polymer surface. However, the long exposure times and high
powers employed provided only limited results, and further, are thought to
have produced significant changes not only in the surface chemistry, but
also the native bulk properties. Morphology of the surface was also
severely effected.
In another ESCA study entitled "ESCA Study of Polymer Surfaces Treated by
Plasma," Yasuda et al, J. Polym. Sci., Polym. Chem. Ed., 15, 991 (1977)
the effects of discharges in argon and nitrogen on surface chemistry were
considered on a range of polymers. PTFE was found to be particularly
susceptible to defluorination and the introduction of oxygen and nitrogen
moieties into the surface. Accordingly, there is need for permanently
modified fluorinated polymers in which some of the original fluorine
functionality is eliminated and replaced with oxygen functionality and
hydrogen bonded to the carbon polymer backbone without the formation of
coatings or layers while substantially preserving the original surface
morphology and bulk characteristics of the unmodified material on a
molecular scale.
SUMMARY OF THE INVENTION
It is a principal object of the invention to provide for novel
oxyfluoropolymers in which the atomic structure of the native
fluoropolymer material is permanently modified by the elimination of some
of the original fluorine functionality and the introduction of both oxygen
atoms or oxygen-containing groups and hydrogen atoms covalently bonded to
the original carbon polymer backbone. The morphological properties of the
oxyfluoropolymers at a molecular level remain substantially unchanged from
those of the starting fluoropolymer materials while wettability with
respect to low surface tension liquids and surface free energy
(.gamma..sub.s) as determined through critical surface tension
(.gamma..sub.c) are increased. The fluoropolymer starting material used in
preparation of the oxyfluoropolymers is intended to include fluorocarbon
type polymers and fluorohydrocarbon polymers.
More specifically, it is an object of the invention to provide for novel
oxyfluoropolymers having increased surface energies in which a portion of
the surface fluorine atoms to depths of about 10 to about 100 .ANG. of a
fluoropolymer starting material are permanently substituted with hydrogen
atoms, and from about 5 to about 20% of the fluorine atoms are also
substituted with oxygen functionality. That is, instead of introducing a
modified polymer coating to the original material, the object is to
provide for oxyfluoropolymers in which the original starting material is
permanently modified at the molecular level by removal of some of the
fluorine so the carbon backbone has fluorine, oxygen and hydrogen atoms
covalently bonded thereto. In essence, the fluoropolymer starting material
has a sufficient number of fluorine atoms permanently substituted with
both hydrogen atoms and oxygen functionality covalently bonded to the
carbon backbone to a surface depth of about 10 to about 100 .ANG. to
increase the surface free energy (.gamma..sub.s) as determined through
critical surface tension (.gamma..sub.c).
It is a further object of the invention to provide for oxyfluoropolymers in
which up to 98 percent, and more specifically, from about 20 to about 85
percent of the surface fluorine atoms to depths from 10 to about 100 .ANG.
are permanently substituted with hydrogen and oxygen and/or
oxygen-containing groups of which from about 3 to about 30 percent of the
substituted fluorine is replaced with oxygen or oxygen-containing groups
and from about 70 to about 97 percent is substituted with hydrogen atoms.
The morphological properties and bulk properties of the oxyfluoropolymer
remain substantially unchanged over the starting fluoropolymer material.
The permanently modified fluoropolymers have increased wettability and/or
adhesiveness, as well as chemically reactive sites allowing for attachment
of various chemical functionality to these normally inert surfaces, and as
such have applications which make them especially adaptable for membrane
applications, e.g., filtration membranes or other surface mediated
processes, e.g., adhesion prevention or promotion; devices such as
bioprobes coated with oxyfluoropolymers making them biocompatible while
allowing specific ion permeability; expanded PTFE membranes especially in
the field of cell culture growth membranes; and because of improved
wettability properties implantable prosthetic devices, such as bone
replacements, heart valves, and the like.
It is yet a further object of the invention to provide for methods of
making permanently modified fluoropolymers having increased surface energy
by the steps of:
(a) providing a starting fluoropolymer material;
(b) providing a gas/vapor plasma mixture comprising hydrogen and at least
one member selected from the group consisting of water, methanol and
formaldehyde; and
(c) contacting said fluoropolymer material with said plasma mixture while
exposing said fluoropolymer to at least one radio frequency glow discharge
for a sufficient period to increase the surface free energy
(.gamma..sub.s) by permanently substituting to a depth from about 10 to
about 100 .ANG. on the starting fluoropolymer, fluorine atoms with
hydrogen atoms and from about 5 to about 20% of said fluorine atoms with
oxygen functionality.
The methods impart surface wettability and/or adhesiveness properties as
well as chemically reactive sites to the original fluoropolymer without
materially effecting the materials original hydrophobic properties. Plasma
gas/vapor mixture concentrations of hydrogen, water, methanol, and
formaldehyde together with wattage or power of the glow discharge and
pressure (vacuum) are variables which determine the depth of surface
modifications, as well as the respective atomic concentrations of carbon,
fluorine, hydrogen and oxygen making up the modified portion of the final
polymer.
DETAILED DESCRIPTION OF THE INVENTION
Through radio frequency glow discharge the atomic structure of the top 10
to about 100 .ANG. of a fluoropolymeric starting material can be
permanently modified by substitution of a portion of the original fluorine
functionality with oxygen or oxygen-containing groups and hydrogen
covalently bonded directly to the carbon polymer backbone. By regulating
amounts and ratios of carbon, fluorine, oxygen and hydrogen in the
modified polymer, surface energy can be increased from that of the
original material along with wettability and adhesiveness properties
without materially altering the corresponding hydrophobic properties, or
altering the polymers original surface morphology and bulk
characteristics.
In preparing the oxyfluoropolymers, useful fluoropolymer starting materials
include both fluorocarbon polymers and fluorohydrocarbon polymers. This
would include fluoropolymers having a carbon backbone with atoms bonded
thereto consisting of either fluorine or both fluorine and hydrogen
provided that when hydrogen atoms are present fluorine shall also be
present in a ratio of at least 1:3. Preferably, the fluoropolymers include
materials having a critical surface tension (.gamma..sub.c) ranging
generally from about 15 to about 30 dynes/cm. Specific representative
examples of useful low surface energy fluorocarbon polymers are the
perfluorinated polymers polytetrafluoroethylene (PTFE), polymers of
hexafluoropropylene and tetrafluoroethylene like fluorinated
ethylene-propylene (FEP) copolymers, etc. Suitable low surface area
fluorohydrocarbon starting polymers include resins like
polytrifluoroethylene, poly(vinylidene fluoride) (PVDF), poly(vinyl
fluoride), poly(vinyl difluoride) and the like.
The oxyfluoropolymer compositions are especially unique in that a
controllable amount from about 1 to about 98% of the fluorine atoms of the
starting polymer's surface interface are permanently removed and replaced
with hydrogen atoms and with oxygen atoms or low molecular weight
oxygen-containing functionalities, so that all substituents are covalently
bonded directly to the carbon backbone polymer chain to a depth of about
100 .ANG.. Oxygen functionality may take the form of oxo, hydroxyl,
alkoxy, like methoxy, ethoxy and propoxy or R'--CO-- or combinations
thereof where R' is hydrogen or alkyl, and particularly C.sub.1 -C.sub.5
lower alkyl, including methyl, ethyl, propyl, isopropyl, and so on.
Accordingly, unlike the nitrogen-containing monolayers/surface
overcoatings of U.S. Pat. No. 4,548,867 the intrinsic atomic composition
of the above starting material is permanently modified to regulated
surface depths ranging from about 10 to about 100 .ANG., providing a novel
combination of properties, i.e., chemically reactive sites, greater
surface wettability and free energy enhancement of fluorinated carbons and
hydrocarbons while still substantially preserving the hydrophobic
properties and microstructural morphology, e.g., membranous structure,
pore size, surface roughness on a molecular scale, etc.
The oxyfluoropolymers produce a wide variety of surface free energy
increases where, for example, a fluoropolymer like PTFE with a
.gamma..sub.c of about 18 dynes/cm at 20.degree. C. can be increased to
about 40 dynes/cm to a depth of between 10 to 100 .ANG. for increased
wettability and other surface properties relating to the surface free
energy of a material. Even with such increases in surface free energy the
hydrophobic properties of the original material remain substantially
intact. That is, the modified polymers of the invention having hydrogen,
oxygen and fluorine functionalities are covalently bonded to the carbon
polymer backbone will still inhibit permeation and wetting by liquids with
high surface tensions, i.e., >50 dynes/cm like water and other similar
polar solvents, but also being wettable by liquids having low surface
tensions, i.e., <50 dynes/cm, such as blood plasma and other nonpolar
organic solvents. This is quite unexpected because when the surface free
energy of a polymer is increased one normally finds with the increase in
wettability an equivalent decrease in the hydrophobic properties of the
material. However, quite surprisingly with the increased surface energy of
the oxyfluoropolymers of the present invention wettability is increased
without the normally expected decrease in hydrophobicity from that of the
original starting material.
The oxyfluoropolymers are prepared by a plasma treatment process in which
the previously described fluoropolymers are exposed to a single or a
series of relatively low power radio frequency glow discharges (RFGD). The
target fluoropolymers generally can be in the form of a sheet, premolded
or coated article, such as a porous PTFE membrane or filter, e.g.,
Goretex.RTM., where, for example, increased permeability of ions would be
desirable without altering pore characteristics of the native material; a
bioprobe of conventional design coated with Teflon .RTM. or a molded,
implantable prosthetic device where, for instance, it would be desirable
to modify its adhesive and/or surface reactivity characteristics to blood
platelet attachment.
Instead of a plasma treatment with purely a gas the radio frequency glow
discharge is conducted in an atmosphere of a gas/vapor mixture at pressure
vacuums of under 1,000 mTorr, and more preferably, from about 50 to 200
mTorr, and power loadings of less than or equal to 100 watts.
Although not wishing to be held to any precise mode of action, the primary
mechanism of the plasma treatment process of the instant invention is
believed to involve the transfer of energy to the gaseous ions directly to
form charged ionized gas species, i.e., ion sputtering of the polymer at
the gas-solid interface. The radio frequency glow discharge plasma gas
ions become excited through direct energy transfer by bombarding the gas
ions with electrons. Thus, by exposing the fluoropolymer material to
either a single or a series of radio frequency glow discharge gas/vapor
plasmas consisting of admixtures of hydrogen gas ranging from 20% to 99%
by volume, and 1 to about 80% by volume of a vapor from liquids, such as
water, methanol, formaldehyde and mixtures thereof, 1 to about 98% of the
surface fluorine atoms are permanently removed in a controlled/regulated
manner and replaced with oxygen atoms or low molecular weight
oxygen-containing functionality along with hydrogen atoms. Although
hydrogen is required, in all instances, by itself it is insufficient to
introduce both hydrogen and oxygen moieties to the carbon polymer
backbone. A nonpolymerizable vapor/H.sub.2 mixture is necessary to
introduce the required hydrogen and oxygen or functionalized oxygen
moieties onto the fluoropolymer without disrupting surface morphology.
Further, uses of pure gas mixtures, specifically H.sub.2/O.sub.2 show only
limited results. Representative radio frequency glow discharge plasmas and
operating conditions are provided in Table I below:
TABLE I
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CALCULATED ATOMIC
Starting
RFGD Mix Pressure
Time
Depth
RATIOS (ESCA)
Material
Composition (mTorr)
(Min.)
(.ANG.)
C/O
C/F
F/O
Stoichiometry
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Unmodified
-- -- -- -- .infin.
0.45
.infin.
C.sub.2 F.sub.2.3
PTFE*
Unmodified
-- -- -- -- .infin.
1.0
.infin.
C.sub.1 F.sub.1
PVDF
Modified
2% (vol) H.sub.2 O/98% H.sub.2
150 20 100 7.5
1.5
5.0
C.sub.15 F.sub.10 H.sub.18
O.sub.2
PTFE
Modified
2% (vol) H.sub.2 O/98% H.sub.2
200 10 100 8.6
0.91
9.7
C.sub.17 F.sub.19 H.sub.13
O.sub.2
PTFE
Modified
20% (vol) Methanol
150 30 100 3.0
1.5
2.0
C.sub.6 F.sub.4 H.sub.6 O.sub.2
PTFE vapor/80% H.sub.2
Modified
20% (vol) Methanol
200 5 100 9.3
2.0
4.7
C.sub.28 F.sub.14 H.sub.39
O.sub.3
PTFE vapor/80% H.sub.2
Modified
2% (vol) H.sub.2 O/98% H.sub.2
200 10 100 8.0
16.0
0.48
C.sub.16 F.sub.1 H.sub.29
O.sub.2
PVDF
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*Porous Goretex membrane
The following specific examples demonstrate the various aspects of this
invention, however, it is to be understood that these examples are for
illustrative purposes only, and do not purport to be wholly definitive as
to conditions and scope.
EXAMPLE I
Part A
To prepare oxyfluoropolymers, using radio frequency glow discharge (RFGD) a
model PDC-23g RF plasma chamber having a maximum output of 100 watts from
Harrick Scientific Corp., Ossining, NY, was modified by adding an in-line
VG Model MD 95 ultra high vacuum (UHV) leak valve before the inlet side of
the glow discharge unit. The UHV leak valve provided precise control of
the system pressure while also allowing smooth flow of vaporized liquids
into the plasma reaction chamber. In addition, a diffusion pump in
conjunction with a roughing pump was installed at the outlet of the plasma
reaction chamber. Optionally, a liquid nitrogen trap can be installed
between the RFGD unit and the diffusion pump to protect the pump from
potentially damaging vapors. Hydrogen from a flow meter, and liquids,
e.g., water, methanol, formaldehyde, etc., are bled by the UH vacuum
release valve to the inductively coupled plasma reaction chamber.
Through use of the diffusion pump, a base pressure of about 5 mTorr was
obtainable and employed before all glow discharge treatments to effectuate
a clean experimental system. By ultrasonically extracting the samples in
hexanes, all trace contaminants caused by backflow of pump oil was
minimized. In addition, by ultrasonically cleaning the samples, low
molecular weight and evanescent surface constituents were effectively
removed. This permitted more accurate analysis of permanent surface
functionalities introduced into the fluoropolymer through RFGD surface
modification.
Part B
A sheet of porous PTFE (Goretex) measuring 10 cm .times. 5 cm .times. 1 mm
was analyzed using high resolution (17.9 eV) electron spectroscopy for
chemical analysis (ESCA) to establish the true atomic percentages of
carbon and fluorine present in the sample prior to glow discharge
treatment. Measured peak areas of the detected atoms (carbon and fluorine)
using atomic sensitivity factors gave corrected atomic percentages of 70%
fluorine and 30% carbon for the sample corresponding to a C.sub.1.0
F.sub.2.3 stoichiometry and a molecular structure CF.sub.3
--(CF.sub.2)--.sub.n....--CF.sub.3. Corrected binding energies of the
carbon and fluorine ls peaks indicated a totally saturated carbon backbone
with no detectable oxygen.
The pure perfluorinated sheet was then placed on the sample stage in the
plasma reaction chamber and exposed for 20 minutes at 100 watts to a
gas/vapor RFGD plasma mixture consisting of ca. 98% by volume hydrogen and
ca. 2% by volume water at 150 mTorr pressure. The sample was then
subjected to ESCA analysis. The low and high resolution surveys showed C
ls, F ls and 0 ls results indicating the molecular structure. C ls
indicated the incorporation of large amounts of aliphatic C--H and
--CH.sub.2 --CH.sub.2 -- functionality with lesser amounts of
carbon-oxygen functionality. Elemental analysis showed C 33.3%; F 22.2%; H
40.0%; O 4.5%. ATR --Infrared spectroscopic results indicated the
formation of both C--O and --OH functionality.
EXAMPLE II
A second sample of the same pure porous PTFE sheet of Example I, Part B and
of the same dimensions was exposed to a gas/vapor RFGD plasma mixture also
consisting of 98% by volume hydrogen and 2% by volume water at 100 watts
and a pressure of 200 mTorr like that of Example I, Part B. However, the
exposure time was decreased from 20 to 10 minutes. The ESCA low resolution
survey and high resolution C ls, F ls, and O ls spectra showed the
addition of oxygen and hydrogen to the molecular structure of the PTFE
surface. An ATR-IR spectrum of this material also indicated incorporation
of amounts of C--O and --OH functionality onto the surface portion of the
sheet. Elemental analysis showed C 33.3%; F 37.3%; H 25.5%; 0 3.9%.
EXAMPLE III
A sheet of shear porous PTFE (Goretex) like that used in Examples I and II
was exposed to a gas/vapor RFGD plasma mixture using the laboratory set-up
described above in Part A of Example I. The plasma consisted of 80% by
volume hydrogen and 20% by volume methanol. Exposure time was 30 minutes
at a pressure of 150 mTorr. The ESCA low resolution and high resolution C
ls, F ls, O ls spectra showed the introduction of oxygen at the molecular
level on the PTFE surface. The C ls ESCA spectrum indicated both aliphatic
carbon and C--O functionality with a corresponding decrease in fluorinated
carbon groups. The F ls spectrum showed a large increase in peak width,
indicative of two types of fluorine functional group environments residing
at the PTFE surface region. The amount of oxygen functionality present in
the modified oxyfluoropolymer surface was more than double that of the
samples prepared in Examples I and II, as shown by the following elemental
analysis: C 33.3%; F 22.0%; H 33.3%; O 11.1%. ATR-IR showed a
corresponding increase in C--O and --OH functionality.
EXAMPLE IV
A sheet of poly(vinyl difluoride) (PVDF) measuring 10 cm .times. 5 cm
.times. 1 mm was analyzed using high resolution ESCA to establish the
composition of the sample. Two peaks of almost equal area were observed
which were indicative of a molecular structure containing equal amounts of
CH.sub.2 and CF.sub.2 groups. The unmodified polymer can be described
stoichiometrically as C.sub.1.0 F.sub.1.0 H.sub.1.0 with a molecular
structure of (CH.sub.2 --CF.sub.2).sub.n --. The unmodified PVDF sheet had
an elemental analysis of C 33%; F 33%; H 33%.
The sample sheet of PVDF was exposed to a gas/vapor RFGD plasma mixture for
10 minutes at a pressure of 200 mTorr at 100 watts in the laboratory set
up of Example I, Part A. The gas/vapor mixture consisted of 2% by volume
water and 98% by volume hydrogen. The treated sample was then analyzed
using ESCA low resolution survey and high resolution C ls, F ls and O ls
which demonstrated an extreme drop in the fluorine signal with a
corresponding increase in hydrogen and oxygen to the top surface to a
depth of about 100 .ANG.. The C ls spectrum indicated a hydrocarbon
surface with some C--O functionality and little or no C--F functionality
in the topmost 100 .ANG. of the PVDF surface. ESCA analy | | |
