U.S. patent application number 10/426804 was filed with the patent office on 2004-11-04 for surface functionalized carbon nanostructured articles and process thereof.
Invention is credited to Kelley, Ronald James, Kim, Gene, Landreth, Bobby Dean.
Application Number | 20040219093 10/426804 |
Document ID | / |
Family ID | 33309964 |
Filed Date | 2004-11-04 |
United States Patent
Application |
20040219093 |
Kind Code |
A1 |
Kim, Gene ; et al. |
November 4, 2004 |
Surface functionalized carbon nanostructured articles and process
thereof
Abstract
Carbon nanostructures (21) are surface modified by adding
pendant alcohol groups (25) capable of chemical reaction.
Poly(vinyl alcohol) (23) is disposed on the surface of the carbon
such that it renders the carbon nanostructure capable of forming a
substantially uniform stable dispersion in water.
Inventors: |
Kim, Gene; (Plantation,
FL) ; Kelley, Ronald James; (Plantation, FL) ;
Landreth, Bobby Dean; (FT Lauderdale, FL) |
Correspondence
Address: |
Larson & Associates, P.C.
221 East Church Street
Frederick
MD
21701-5405
US
|
Family ID: |
33309964 |
Appl. No.: |
10/426804 |
Filed: |
April 30, 2003 |
Current U.S.
Class: |
423/447.2 |
Current CPC
Class: |
C01B 32/174 20170801;
B82Y 40/00 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
423/447.2 |
International
Class: |
D01F 009/12 |
Claims
What is claimed is:
1. A surface functionalized carbon microstructure having pendant
alcohol groups, comprising: a carbon microstructure, selected from
the group consisting of carbon nanotubes, activated carbon, and
fullerene; a surface functionalized moiety disposed on the surface
of the carbon microstructure sufficient to cause the surface
functionalized carbon microstructure to be substantially soluble in
water.
2. The surface functionalized carbon microstructure as described in
claim 1, wherein said surface functionalized moiety is
noncovalently bonded to the carbon microstructure.
3. The surface functionalized carbon microstructure as described in
claim 1, wherein said surface functionalized moiety contains
pendant alcohol groups capable of chemical reaction.
4. The surface functionalized carbon microstructure as described in
claim 3, wherein said surface functionalized moiety is poly(vinyl
alcohol).
5. The surface functionalized carbon microstructure as described in
claim 1, wherein said noncovalently bonded poly(vinyl alcohol) is
further reactive with one or more members selected from the group
consisting of hydrogen halides, phosphorous trihalides, mineral
acids, organic acids, sodium, potassium, magnesium, aluminum,
thionyl chloride, octadecylamine,
poly(propionylethylenimine-co-ethylenimine), glucose amine, gum
Arabic, metal-containing complexes, silanes, hydrates, borates,
epichlorohydrin, 1,4-butanediol diglycidyl ether, periodates,
glutaraldehyde, 1,1-carbonyldi-imidazole, 1,1-carbonyldi-imidazole,
2,2,2-trifluethanesulphonyl chloride,
2,4,6-trichloro-1,3,5-triazine, p-b-sulphate-(ethyl
sulphonide)-aniline, 4-dimethoxymethyl-N-(1-pyridinio-
)benzamidate, 3-aminopropyltriethoxysilane,
(2,4-dihydroxyphenyl)dimethyls- ulfonium triflate, inositol,
sucrose, resorcinol, and pyrogallol.
6. A surface functionalized carbon nanostructure having pendant
alcohol groups capable of chemical reaction, comprising: a carbon
nanostructure, selected from the group consisting of carbon
nanotubes, activated carbon, and fullerene; poly(vinyl alcohol)
noncovalently bonded to the carbon nanostructure; and wherein said
surface functionalized carbon nanostructure forms a substantially
uniform stable dispersion in water.
7. The surface functionalized carbon nanostructure as described in
claim 6, wherein said noncovalently bonded poly(vinyl alcohol) is
further reactive with one or more members selected from the group
consisting of hydrogen halides, phosphorous trihalides, mineral
acids, organic acids, sodium, potassium, magnesium, aluminum,
thionyl chloride, octadecylamine,
poly(propionylethylenimine-co-ethylenimine), glucose amine, gum
Arabic, metal-containing complexes, silanes, hydrates, borates,
epichlorohydrin, 1,4-butanediol diglycidyl ether, periodates,
glutaraldehyde, 1,1-carbonyldi-imidazole, 1,1-carbonyldi-imidazole,
2,2,2-trifluethanesulphonyl chloride,
2,4,6-trichloro-1,3,5-triazine, p-b-sulphate-(ethyl
sulphonide)-aniline, 4-dimethoxymethyl-N-(1-pyridinio-
)benzamidate, 3-aminopropyltriethoxysilane,
(2,4-dihydroxyphenyl)dimethyls- ulfonium triflate, inositol,
sucrose, resorcinol, and pyrogallol.
8. A surface functionalized carbon nanostructure intermediate,
comprising: a carbon nanostructure having poly(vinyl alcohol)
noncovalently bonded thereto to render said surface functionalized
carbon nanostructure substantially miscible with water.
9. The surface functionalized carbon nanostructure as described in
claim 8, wherein said carbon nanostructure is one or more carbon
nanostructures selected from the group consisting of carbon
nanotubes, activated carbon, and fullerene.
10. The surface functionalized carbon nanostructure as described in
claim 8, wherein said noncovalently bonded poly(vinyl alcohol) is
reactive to form additional compounds with one or more members
selected from the group consisting of hydrogen halides, phosphorous
trihalides, mineral acids, organic acids, sodium, potassium,
magnesium, aluminum, thionyl chloride, octadecylamine,
poly(propionylethylenimine-co-ethylenimine), glucose amine, gum
Arabic, metal-containing complexes, silanes, hydrates, borates,
epichlorohydrin, 1,4-butanediol diglycidyl ether, periodates,
glutaraldehyde, 1,1-carbonyldi-imidazole, 1,1-carbonyldi-imidazole,
2,2,2-trifluethanesulphonyl chloride,
2,4,6-trichloro-1,3,5-triazine, p-b-sulphate-(ethyl
sulphonide)-aniline, 4-dimethoxymethyl-N-(1-pyridinio-
)benzamidate, 3-aminopropyltriethoxysilane,
(2,4-dihydroxyphenyl)dimethyls- ulfonium triflate, inositol,
sucrose, resorcinol, and pyrogallol.
11. A method of creating a surface functionalized carbon
nanostructure, comprising dissolving poly(vinyl alcohol) in heated
water to form a poly(vinyl alcohol) solution, and adding carbon
nanostructure to the heated poly(vinyl alcohol) solution to form a
surface functionalized carbon nanostructure that forms a
substantially uniform stable dispersion in water.
Description
FIELD OF THE INVENTION
[0001] This invention relates in general to carbon nanostructured
articles and more particularly to surface functionalization
techniques in carbon nanostructured articles.
BACKGROUND OF THE INVENTION
[0002] Carbon nanotubes are cylindrical structures based on the
hexagonal lattice of carbon atoms that forms crystalline graphite,
possessing remarkable electronic and mechanical properties. Their
exotic electronic properties and very high strength has sparked
interest in potential applications, for example, in nanometer-sized
electronics or to strengthen polymer materials. An ideal nanotube
can be thought of as a hexagonal network of carbon atoms that has
been rolled up to make a seamless cylinder, although they can not
really be made that way. Depending on the direction that the tubes
appear to have been rolled (quantified by the `chiral vector`),
they are known to act as conductors or semiconductors. Three types
of nanotubes are possible, called armchair, zigzag and chiral
nanotubes, depending on how the two-dimensional graphite sheet is
`rolled up`. Just a nanometer across, the cylinder can be tens of
microns long, and each end is `capped` with half of a fullerene
molecule. Fullerene, also known as C60 molecules or
buckminsterfullerene, is a molecule made up of 60 carbon atoms
arranged in a series of interlocking hexagons and pentagons,
forming a structure that looks similar to a soccer ball. C60 is a
truncated icosahedron, consisting of 12 pentagons and 20 hexagons.
Single-wall nanotubes can be thought of as the fundamental
cylindrical structure, and these form the building blocks of both
multi-wall nanotubes and the ordered arrays of single-wall
nanotubes called ropes.
[0003] The unique properties of carbon nanotubes and fullerene has
focused considerable effort on using these special materials in
polymer composites and biomedical applications. All try to utilize
the high mechanical and electrical properties of the carbon
nanostructured articles, but the inertness of carbon is a barrier
to creating useful structures for commercialization. Thus far,
putting nanotubes in polymers has proven quite difficult, and
biomedical applications require dissolution in water. Homogeneous
distribution in both polymers and water is prevented mainly by the
high van der Waals attraction between the tubes, causing
aggregation and clumping. Some have attempted to solve this problem
by the use of aqueous surfactants such as sodium dodecyl sulfate to
temporarily modify the surface of the carbon structures. Although
this has increased the solubility of nanotubes in water, it does
not address the problems of polymer composites, and does not permit
further chemical modification. Successful surface functionalization
of these carbon nanostructures would be a significant addition to
the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] The accompanying figures, where like reference numerals
refer to identical or functionally similar elements throughout the
separate views and which together with the detailed description
below are incorporated in and form part of the specification, serve
to further illustrate various embodiments and to explain various
principles and advantages all in accordance with the present
invention.
[0005] FIG. 1 is a flow chart depicting the process of surface
modifying carbon nanostructured articles in accordance with the
present invention.
[0006] FIG. 2 is a schematic depiction of surface functionalized
carbon nanostructured articles in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0007] As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention, which
can be embodied in various forms. Therefore, specific structural
and functional details disclosed herein are not to be interpreted
as limiting, but merely as a basis for the claims and as a
representative basis for teaching one skilled in the art to
variously employ the present invention in virtually any
appropriately detailed structure. Further, the terms and phrases
used herein are not intended to be limiting; but rather, to provide
an understandable description of the invention. The terms a or an,
as used herein, are defined as one or more than one. The term
plurality, as used herein, is defined as two or more than two. The
term another, as used herein, is defined as at least a second or
more. The terms including and/or having, as used herein, are
defined as comprising (i.e., open language). The terms coupled and
disposed, as used herein, are defined as connected, although not
necessarily directly, and not necessarily mechanically.
[0008] Carbon nanostructures can be surface modified by adding
pendant alcohol groups capable of chemical reaction. Poly(vinyl
alcohol) is non-covalently bonded on the surface of the carbon such
that it renders the carbon nanostructure capable of forming a
substantially uniform stable dispersion in water. The invention
utilizes wet-chemistry processes to effectively functionalize
carbon nanostructures, unlike other prior art functionalization
techniques. The surface energetics of the carbon microstructure or
carbon nanostructure is modified. Carbon microstructures or
nanostructures include, but are not limited to, carbon black,
activated carbon, carbon nanofibers, graphite, fullerenes, and
carbon nanotubes (both multi-wall and single-wall). The invention
is created by taking advantage of the hydrophobic nature of the
carbon nanostructures and the ability of poly(vinyl alcohol) to
crystallize onto hydrophobic surfaces. We postulate that the
resultant carbon nanostructure has crystalline poly(vinyl alcohol)
moieties attached to the surface of the now-modified carbon
nanostructure and/or poly(vinyl alcohol) adsorbed on the surface.
This allows the surface functionalized carbon nanostructure to have
chemically versatile alcohol groups for further reaction to achieve
improved strength, thermal properties, and electrical
properties.
[0009] Referring now to FIG. 1, the process of creating surface
modified carbon nanostructured articles is explained. Poly(vinyl
alcohol) (PVA) is added (12) to very high purity water. Poly(vinyl
alcohol) is a white or cream colored powder or pellet prepared from
polyvinyl acetate by replacement of the acetate groups with
hydroxyl groups. It softens with decomposition at about 200.degree.
C. and is soluble in hot or cold water. Poly(vinyl alcohol) has a
generic structure of:
--[--CH.sub.2--CH--OH].sub.n--
[0010] The water/PVA mixture is stirred with heating (14) until the
PVA dissolves. The carbon nanostructure articles (nanofibers,
fullerene, nanotubes, carbon black, activated carbon, etc.) are
then added (16) to the heated solution and stirred further. After a
period of time, the stirring and heating are terminated and the
admixture is then cooled to room temperature (18). We have found
that the cooling rate affects the end properties of the surface
modified nanostructure. We believe that faster cooling rates
facilitate smaller crystals of PVA on the surface of the
nanostructure. A properly created surface modified carbon
nanostructure will appear to be completely soluble or miscible in
the water, exhibiting this stability for a period in excess of six
months. That is, the surface modified carbon nanostructure will
form a substantially uniform stable dispersion in water
(predominantly black for carbon nanotubes, although other colors
may result for the carbon nanotube and fullerene solution depending
on their purity, size, and concentration). This method of surface
functionalization of the carbon nanostructures is sensitive to
processing variables such as concentration, pH, molecular weight of
the poly(vinyl alcohol), temperature, time, amount of carbon
nanostructured articles and/or poly(vinyl alcohol) used, added salt
or additives, and methods of agitation (e.g., inclusion of
ultrasound during mixing between carbon nanostructured articles and
poly(vinyl alcohol)) will all influence the effectiveness of the
surface modification.
[0011] Having described our novel method of surface
functionalization, we now offer our hypothesis of how the carbon
nanostructure is modified. Carbon nanostructures are relatively
inert, and any successful method must reckon with the substantial
van der Waals attraction of the bare tubes, thus we have little
reason to believe that the poly(vinyl alcohol) has chemically
reacted with the carbon, ruling out covalent or ionic bonding.
Carbon nanostructures are highly hydrophobic, and aggregate or
clump easily, thus difficult to suspend in water. Poly(vinyl
alcohol) is a polymer that possesses both hydrophobic and
hydrophilic moieties. The carbon backbone and residual acetyl
groups in the polymer chain provide hydrophobic properties, while
the pendant hydroxyl groups provide hydrophilic properties. When
the PVA is dissolved in hot water, a solution is formed that will
accommodate the hydrophobic carbon nanostructures. As the carbon
nanostructure becomes surface functionalized with the PVA, the
pendant alcohol groups provide enough hydrophilicity to render the
surface modified carbon nanostructure compatible with water. The
exact mechanism of this aqueous compatibility is uncertain, but it
appears that the surface functionalized carbon nanostructures are
now miscible, soluble or otherwise capable of forming a uniform
dispersion in or with the water. While we are not certain of the
actual mechanism of bonding, we believe that the PVA becomes
attached to or otherwise disposed on the surface of the carbon
nanostructure by non-covalent bonding or other physical adsorption
means. We further hypothesize that the PVA might also be
crystallized onto the surface. FIG. 2 depicts a schematic
representation of how poly(vinyl alcohol) may adsorb, crystallize,
or otherwise be disposed onto the carbon nanostructure surface. The
hydrophobic polymer backbone 21 of the PVA molecule 23 is attracted
to the likewise hydrophobic surface of the high aspect ratio carbon
nanotube 22. The hydrophilic pendant alcohol groups 25 provide
compatibility with aqueous media, and thus the PVA surface modified
nanostructures are miscible, soluble, or easily dispersed in water.
Additionally, some adsorption of the poly(vinyl alcohol) onto the
carbon nanostructure surfaces may exist due to the imperfections of
the poly(vinyl alcohol) crystallization. This sometimes creates an
effective stable emulsion system of incompatible (or immiscible)
liquids.
[0012] By using our novel surface modification technique leveraging
the hydrophobic nature of the crystallization of poly(vinyl
alcohol) out of aqueous solutions, one can achieve carbon
nanostructured articles that have pendant alcohol groups that offer
chemical reactivity. Alcohol groups are one of the most versatile
functional groups towards further chemical modification. We have,
then, essentially created an intermediate that is susceptible of
further reaction to create a wide variety of new materials, with
applications in many different areas, such as (but not limited to)
fuel cells, photovoltaics, or other next generation energy storage
devices, biological monitoring or other biologically driven
applications, microelectronics, fillers for composites, and
displays. The pendant hydroxyl groups are reactive to a wide
variety of chemicals, for example hydrogen halides, phosphorous
trihalides, mineral acids, organic acids, sodium, potassium,
magnesium, aluminum, thionyl chloride, octadecylamine,
poly(propionylethylenimine-co-ethylenimine), carboxylic group
functionalization, alkane and fluorine functionalization, glucose
amine and gum Arabic functionalization, functionalization via
metal-containing complex coordination, silanes, hydrates, borates,
epoxy activation (with chemicals such as epichlorohydrin or
1,4-butanediol diglycidyl ether), periodate oxidation,
glutaraldehyde coupling, 1,1-carbonyldi-imidazole chemistry,
1,1-carbonyldi-imidazole chemistry, 2,2,2-trifluethanesulphonyl
chloride (tresyl chloride), 2,4,6-trichloro-1,3,5-triazine
(cyanuric chloride) or p-b-sulphate-(ethyl sulphonide)-aniline
(SESA), 4-dimethoxymethyl-N-(1-pyridinio)benzamidate (DMPB),
photocrosslinking with photoacid generators (such as
(2,4-dihydroxyphenyl)dimethylsulfonium triflate), photocrosslinkers
(such as inositol, sucrose, resorcinol, pyrogallol), and miscible
and immiscible polymer blends and complexes. This permits one to
create a wide variety of useful materials that are otherwise not
possible to create, because of the hydrophobic nature of the carbon
nanotubes.
[0013] Having described our invention, illustrative examples are
now provided.
EXAMPLE 1
[0014] 1. Twenty (20) ml of ultra-pure water prepared by using
reverse osmosis was placed in a clean vial and a magnetic stir bar
added.
[0015] 2. Poly(vinyl alcohol) pellets purchased from Aldrich
Chemical with a molecular weight of 89,000.about.98,000 were added
to the water at a concentration of 0.01 mM (molar concentration
based on the repeat unit of the poly(vinyl alcohol)) with
continuous stirring.
[0016] 3. The solution was heated with stirring to dissolve the
poly(vinyl alcohol). Solubilization of PVA in water is complete
when the pellet disappears (solubilized), and the solution is
clear. This occurred when the solution temperature reached
approximately 90 degrees Celsius. Continue stirring for five
minutes after the PVA is dissolved in water.
[0017] 4. Five (5) mg of carbon nanofibers (ASI Pyrograf III
PR-24-HT carbon nanofibers) were then added to the heated PVA
solution with stirring.
[0018] 5. Continue heating while stirring for 30 minutes.
[0019] 6. The solution was then removed from the heat source,
covered and allowed to naturally cool to room temperature with
stirring in a normal laboratory environment.
[0020] After 24 hours, the covered vial was examined and the
contents were found to exhibit the characteristics of a uniform
solution/dispersion, with no phase separation and no clumping or
other aggregation of the carbon nanotubes.
EXAMPLE 2
[0021] 1. Twenty (20) ml of ultra-pure water prepared by using
reverse osmosis was placed in a clean vial and a magnetic stir bar
added.
[0022] 2. Poly(vinyl alcohol) pellets purchased from Aldrich
Chemical with a molecular weight of 89,000.about.98,000 were added
to the water at a concentration of 0.1 mM (molar concentration
based on the repeat unit of the poly(vinyl alcohol)) with
continuous stirring.
[0023] 3. The solution was heated with stirring to dissolve the
poly(vinyl alcohol). Solubilization of PVA in water is complete
when the pellets disappear (solubilized), and the solution is
clear. This occurred when the solution temperature reached
approximately 90 degrees Celsius. Continue stirring for five
minutes after the PVA is dissolved in water.
[0024] 4. Five (5) mg of carbon nanofibers (ASI Pyrograf III
PR-24-HT carbon nanofibers) were then added to the heated PVA
solution with stirring.
[0025] 5. Continue heating while stirring for one hour.
[0026] 6. The solution was then removed from the heat source,
covered and allowed to naturally cool to room temperature with
stirring in a normal laboratory environment.
[0027] After 8 hours, the covered vial was examined and the
contents were found to exhibit the characteristics of a uniform
solution/dispersion, with no phase separation and no clumping or
other aggregation of the carbon nanotubes.
EXAMPLE 3
[0028] 1. Twenty (20) ml of ultra-pure water prepared by using
reverse osmosis was placed in a clean vial and a magnetic stir bar
added.
[0029] 2. Poly(vinyl alcohol) pellets purchased from Aldrich
Chemical with a molecular weight of 89,000.about.98,000 were added
to the water at a concentration of 0.5 mM (molar concentration
based on the repeat unit of the poly(vinyl alcohol)) with
continuous stirring.
[0030] 3. The solution was heated with stirring to dissolve the
poly(vinyl alcohol). Solubilization of PVA in water is complete
when the pellets disappear (solubilized), and the solution is
clear. This occurred when the solution temperature reached
approximately 85-95 degrees Celsius. Continue stirring for five
minutes after the PVA is dissolved in water.
[0031] 4. Five (5) mg of carbon nanofibers (ASI Pyrograf III
PR-24-HT carbon nanofibers) were then added to the heated PVA
solution with stirring.
[0032] 5. Continue heating while stirring for one hour.
[0033] 6. The solution was then removed from the heat source,
covered and allowed to naturally cool to room temperature with
stirring in a normal laboratory environment.
[0034] After 8 hours, the covered vial was examined and the
contents were found to exhibit the characteristics of a uniform
solution/dispersion, with no phase separation and no clumping or
other aggregation of the carbon nanotubes. After six (6) months,
the appearance of the solution/dispersion had not changed, and
still exhibited uniformity.
EXAMPLE 4
[0035] Two (2) lots of surface functionalized carbon nanotubes were
prepared following the method described in Example 1. One lot of
the surface functionalized carbon nanotubes was further reacted
with an organic silane by adding an aqueous solution of
3-aminopropyltriethoxysil- ane (1 wt %). This solution was shaken
and allowed to react at room temperature for one hour. A third
control lot of carbon nanotubes in water (non-functionalized) was
also prepared at a similar concentration. The three lots were as
follows:
[0036] Lot 1 (Control): Carbon nanotubes in water (not
miscible)
[0037] Lot 2: Surface functionalized carbon nanotubes (PVA)
(miscible)
[0038] Lot 3: Surface functionalized carbon nanotubes (PVA) and
3-aminopropyltriethoxysilane (miscible)
[0039] After ample time for functionalization, a portion of three
glass slides were coated with each of the three solutions and
allowed to stand under normal laboratory environment until dry. A
drop of water was then placed on each glass slide and the static
contact angle was measured.
[0040] Lot 1 (Control): Carbon nanotubes in water--contact angle
less than 3 degrees.
[0041] Lot 2: Surface functionalized carbon nanotubes (PVA)--
contact angle less than 3 degrees.
[0042] Lot 3: Surface functionalized carbon nanotubes (PVA) and
3-aminopropyltriethoxysilane--contact angle 50 degrees.
[0043] After measurement of the contact angle, each of the glass
slides were tilted to allow the deposited water drop to freely fall
off vertically. The water drop on the control lot of
non-functionalized carbon nanotubes fell off the slide, indicating
a hydrophobic surface. The water drop on the PVA functionalized
carbon nanotubes did not fall off the slide, but clung tenaciously,
indicating a hydrophilic surface. The water drop on the amino
silane reacted functionalized carbon nanotubes likewise did not
fall off the slide, indicating a hydrophilic surface.
[0044] We conclude that the amino silane did indeed react with the
pendant alcohol groups on the surface functionalized carbon
nanotubes to form a new family of surface functionalized carbon
nanotubes. In like manner, other reactive materials can be reacted
with the pendant alcohol groups on the surface functionalized
carbon nanotubes to create a wide variety of new materials. In
summary, without intending to limit the scope of the invention,
surface functionalization of carbon nanostructures according to a
method consistent with certain embodiments of the invention can be
carried out by altering the surface of the nanostructures with PVA.
By using our modification technique, one can effectively create
modified carbon nanostructured articles for improved reliability in
bulk, thermal and electrical properties that can be applied in the
automotive, microelectronics, and biomedical industries. We have
discovered an easier and a more cost-effective way of creating
these structures by using water-soluble polymers. Those skilled in
the art will recognize that the present invention has been
described in terms of exemplary embodiments based upon a wet
solution process by mixing the carbon nanostructured articles into
an aqueous solution of poly(vinyl alcohol). However, the invention
should not be so limited, since other variations will occur to
those skilled in the art upon consideration of the teachings
herein. Multiple processing methods can also be envisaged to modify
the carbon nanostructured articles, such as (but not limited to)
spraying the poly(vinyl alcohol) onto a carbon nanostructured
article template surface for selective modification.
[0045] While the invention has been described in conjunction with
specific embodiments, it is evident that many alternatives,
modifications, permutations and variations will become apparent to
those of ordinary skill in the art in light of the foregoing
description. Accordingly, it is intended that the present invention
embrace all such alternatives, modifications and variations as fall
within the scope of the appended claims.
* * * * *