U.S. patent application number 09/736675 was filed with the patent office on 2002-08-08 for tailoring the grafting density of organic modifiers at solid/liquid interfaces.
Invention is credited to Efimenko, Kirill, Genzer, Jan.
Application Number | 20020106449 09/736675 |
Document ID | / |
Family ID | 24960835 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020106449 |
Kind Code |
A1 |
Genzer, Jan ; et
al. |
August 8, 2002 |
TAILORING THE GRAFTING DENSITY OF ORGANIC MODIFIERS AT SOLID/LIQUID
INTERFACES
Abstract
A method of depositing a functional group on a surface portion
of an elastic substrate comprises the steps of: (a) stretching an
elastic substrate having an initial surface portion to form an
enlarged surface portion from the initial surface portion; then (b)
conjugating a functional group on the enlarged surface portion; and
then (c) releasing the substrate to form a reduced surface portion
from the enlarged surface portion, with the reduced surface portion
having an area less than the enlarged surface portion, and with the
reduced surface portion having the functional group deposited
therein at a greater density than the enlarged surface portion.
Inventors: |
Genzer, Jan; (Raleigh,
NC) ; Efimenko, Kirill; (Raleigh, NC) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
24960835 |
Appl. No.: |
09/736675 |
Filed: |
December 13, 2000 |
Current U.S.
Class: |
427/171 ;
427/248.1; 427/282; 427/299 |
Current CPC
Class: |
B05D 2401/90 20130101;
B82Y 40/00 20130101; B05D 3/101 20130101; B05D 7/04 20130101; B05D
3/10 20130101; B05D 1/185 20130101; B05D 3/12 20130101; B82Y 30/00
20130101; B05D 3/0446 20130101; B05D 1/60 20130101; B05D 7/02
20130101; C08J 7/12 20130101 |
Class at
Publication: |
427/171 ;
427/299; 427/282; 427/248.1 |
International
Class: |
B05D 003/12; C23C
016/00; B05D 001/32; B05D 005/00 |
Claims
That which is claimed is:
1. A method of depositing a functional group on a surface portion
of an elastic substrate, comprising the steps of: (a) stretching an
elastic substrate having an initial surface portion to form an
enlarged surface portion from said initial surface portion; then
(b) conjugating a functional group on said enlarged surface
portion; and then (c) releasing said substrate to form a reduced
surface portion from said enlarged surface portion, with said
reduced surface portion having an area less than said enlarged
surface portion, and with said reduced surface portion having said
functional group deposited therein at a greater density than said
enlarged surface portion.
2. A method according to claim 1, wherein said functional group is
conjugated as a monolayer on said enlarged surface portion.
3. A method according to claim 1, wherein the area of said reduced
surface portion and the area of said initial surface portion are
the same.
4. A method according to claim 1, wherein the area of said reduced
surface portion and the area of said initial surface portion differ
by not more than five percent.
5. A method according to claim 1, wherein the area of said reduced
surface portion is at least 10 percent less than the area of said
enlarged surface portion.
6. A method according to claim 1, wherein the area of said reduced
surface portion is at least 20 percent less than the area of said
enlarged surface portion.
7. A method according to claim 1, wherein said conjugating step is
carried out by grafting said functional group to said enlarged
surface portion.
8. A method according to claim 1, wherein said functional group is
a polymer or a copolymer.
9. A method according to claim 1, wherein said conjugating step is
carried out by growing said functional group on said enlarged
surface portion.
10. A method according to claim 1, wherein said elastic substrate
comprises poly(dimethyl siloxane).
11. A method according to claim 1, wherein said elastic substrate
comprises a component selected from the group consisting of natural
rubber, synthetic rubber, butadienes, or combinations thereof.
12. A method according to claim 1, wherein said stretching step
comprises mechanically stretching the substrate uni-axially.
13. A method according to claim 1, wherein said stretching step
comprises mechanically stretching the substrate bi-axially.
14. A method according to claim 1, further comprising the step of
subjecting the enlarged surface portion of said elastic substrate
to conditions sufficient to impart hydrophilicity thereto, wherein
said subjecting step occurs subsequent to step (a) and prior to
step (b).
15. A method according to claim 14, wherein said subjecting step
comprises exposing the enlarged surface portion to an ozone
treatment to form a reactive group on the enlarged surface portion,
said reactive group selected from the group consisting of a
hydroxyl group, a carboxyl group, a peroxide group, and
combinations thereof.
16. A method according to claim 1, wherein said conjugating step
comprises depositing a functional group on the enlarged surface
portion to react with the reactive group to chemically modify the
reactive group, the functional group being selected from of SH,
M--SiCl.sub.3, M--SiCl.sub.2(OR), M--SiCl(OR).sub.2, and
combinations thereof, wherein M is selected from an aryl-containing
group or F(CF.sub.2).sub.y1(CH.sub.2)- .sub.x1, or combinations
thereof, wherein x1 ranges from 1 to 1,000 and y1 ranges from 1 to
1,000, and wherein R is a hydrocarbon chain.
17. A method according to claim 16, wherein the functional group is
SH and the enlarged surface portion comprises a metal selected from
the group consisting of gold, silver, copper, platinum, palladium,
alloys thereof, and combinations thereof.
18. A method according to claim 1, wherein the enlarged surface
portion comprises a protein molecule.
19. A method according to claim 16, wherein said depositing is a
vapor deposition.
20. A method according to claim 19, wherein said vapor deposition
is carried out through a mask present on top of said enlarged
surface portion, said mask having a plurality of openings.
21. A method according to claim 20, wherein the mask openings are
arranged in a regular pattern.
22. A method according to claim 20, wherein the mask openings are
arranged in a non-regular pattern.
23. A method according to claim 1, wherein the elastic substrate
comprises poly(hydromethylsiloxane) and said step of stretching the
elastic substrate comprises exposing the substrate to pressurized
supercritical carbon dioxide to swell the substrate and said
releasing step comprises depressurizing the supercritical dioxide
to relax the substrate.
24. A method of depositing a functional group on a surface portion
of an elastic substrate, comprising the steps of: (a) stretching an
elastic substrate having an initial surface portion to form an
enlarged surface portion from said initial surface portion, the
elastic substrate comprising poly(dimethyl siloxane); then (b)
exposing the enlarged surface portion to a treatment to form a
reactive group on the enlarged surface portion, said reactive group
selected from the group consisting of a hydroxyl group, a carboxyl
group, a peroxide group, and combinations thereof; then (c)
depositing a functional group as a monolayer on the enlarged
surface portion to react with the reactive group and chemically
modify the enlarged surface portion, the functional group being
selected from the group consisting of M--SiCl.sub.3,
M--SiCl.sub.2(OR), M--SiCl(OR).sub.2, and combinations thereof,
wherein M is selected from an aryl-containing group,
F(CF.sub.2).sub.y1(CH.sub.2).sub.x1, or combinations thereof,
wherein x1 ranges from 1 to 1,000 and y1 ranges from 1 to 1,000,
and wherein R is a hydrocarbon group; and then (d) releasing said
substrate to form a reduced surface portion from said enlarged
surface portion, with said reduced surface portion having an area
less than said enlarged surface portion, and with said reduced
surface portion having said functional group deposited therein at a
greater density than said enlarged surface portion.
25. A method according to claim 24, wherein the area of said
reduced surface portion and the area of said initial surface
portion are the same.
26. A method according to claim 24, wherein the area of said
reduced surface portion and the area of said initial surface
portion differ by not more than five percent.
27. A method according to claim 24, wherein the area of said
reduced surface portion is at least 10 percent less than the area
of said enlarged surface portion.
28. A method according to claim 24, wherein the area of said
reduced surface portion is at least 20 percent less than the area
of said enlarged surface portion.
29. A method according to claim 24, wherein said exposing step
comprises grafting said functional group to said enlarged surface
portion.
30. A method according to claim 24, wherein said functional group
is a polymer or a copolymer.
31. A method according to claim 24, wherein said exposing step
comprises growing said functional group on said enlarged surface
portion.
32. A method according to claim 24, wherein said stretching step
comprises mechanically stretching the substrate uni-axially.
33. A method according to claim 24, wherein said stretching step
comprises mechanically stretching the substrate bi-axially.
34. A method according to claim 24, wherein said exposing step
comprises exposing the enlarged surface portion to an ozone
treatment.
35. A method according to claim 24, wherein said depositing is a
vapor deposition.
36. A method according to claim 35, wherein said vapor deposition
is carried out through a mask present on top of said enlarged
surface portion, said mask having a plurality of openings.
37. A method according to claim 36, wherein the mask openings are
arranged in a regular pattern.
38. A method according to claim 36, wherein the mask openings are
arranged in a non-regular pattern.
Description
FIELD OF THE INVENTION
[0001] The invention generally relates to methods for modifying the
surfaces of elastic substrates.
BACKGROUND OF THE INVENTION
[0002] It has now been established that the surface properties of
materials (i.e., wetting, lubrication) can be successfully tailored
by terminally attaching various organic modifiers. In particular,
the deposition of self-assembled monolayers (SAMs) may offer one of
the highest quality routes used to prepare chemically and
structurally well-defined surfaces (A. Ulman, An Introduction to
Ultrathin Organic Films from Langmuir-Blodgett to Self Assembly
(Academic Press: New York, 1991); M. Chaudhury, Mat. Sci. Eng. Rep.
16, 97 (1996)).
[0003] One of the crucial issues concerning the application of SAMs
is the knowledge of molecular level organization of the SAM chains.
The wetting properties of SAMs and their stability are believed to
be governed by the intimate interplay between the chemical nature
of the terminus of the monolayer molecule (.omega.) and the packing
within the SAM. The surface properties of the SAMs can range from
hydrophobic to hydrophilic. The SAM packing in turn is believed to
influences the two-dimensional arrangement of the
.omega.-functionalized surface groups. For example, it is believed
that the degree of packing of the SAMs not only determines the
surface energies of the SAMs, but ultimately influences the
stability of the monolayer and its resistance against surface
reconstruction (J. Wang, G. Mao, C. K. Ober, and E. J. Kramer,
Macromolecules 30, 1906 (1997)). While the .omega.-character is
fixed by the chemical structure of the terminal group, the packing
can be altered by varying the density of the grafting points at the
surface. However, tailoring the grafting density of the SAM chains
is typically not an easy task. Almost all SAMs are formed via
natural self-assembly processes that are usually governed by the
chemical and structural nature of the SAM molecules and the means
of their attachment to the solid surface. However, when combined
with mechanical manipulation, the grafting density and thus the
chain packing can be altered.
SUMMARY OF THE INVENTION
[0004] In one aspect, the invention provides a method of depositing
a functional group on a surface portion of an elastic substrate.
The method comprises the steps of stretching an elastic substrate
having an initial surface portion to form an enlarged surface
portion from the initial surface portion, then conjugating a
functional group on the enlarged surface portion, and then
releasing the substrate to form a reduced surface portion from the
enlarged surface portion. The reduced surface portion has an area
less than the enlarged surface portion, and with the reduced
surface portion having the functional group deposited therein at a
greater density than the enlarged surface portion.
[0005] These and other aspects and advantages of the invention are
described in greater detail herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIGS. 1a through e are schematic diagrams illustrating a
method for producing an elastic material in accordance with the
invention.
[0007] FIG. 2 is a schematic illustrating an apparatus for
preparing elastic substrates in accordance with the invention.
[0008] FIG. 3 illustrates water contact angle data for SAMs for
F6H2, F8H2 and OTS attached to stretched PDMS network substrates
plotted as a function of the relative extension of the PDMS
substrates.
[0009] FIGS. 4a and 4b illustrate atomic force microscopy images of
the surfaces of F8H2 samples after the strain on the surface
(initially .DELTA.x=50%) has been released a) slowly (over a period
of 3 hours) and b) quickly (immediate strain release).
[0010] FIG. 5 illustrates the contact angle hysteresis for various
PDMS samples.
[0011] FIG. 6 illustrates the dependence of water contact angle of
F6H2-MAM (squares) and F8H2-MAM (circles) on exposure time to
water.
[0012] FIG. 7 illustrates the dependence of the average tilt angle
of the fluorinated helix <.tau..sub.F-helix> in F8H2-MAMs on
the exposure time of the F8H2-MAM to water.
[0013] FIGS. 8a through 8f are schematic diagrams illustrating a
method for producing an elastic material brushed with
poly(acrylamide).
[0014] FIGS. 9a through 9d illustrate different configurations for
employing a mask in forming a substrate in accordance with the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] The invention will now be described in greater detail with
respect to the preferred embodiments which follow, both in the
specification and the drawings. It should be understood that these
embodiments are for illustrative purposes only, and should be
construed as limiting the scope of the invention as defined by the
claims.
[0016] In one aspect, the invention relates to a method of
depositing a functional group on a surface portion of an elastic
substrate. The method comprises the steps of (a) stretching an
elastic substrate having an initial surface portion to form an
enlarged surface portion from the initial surface portion; then (b)
conjugating a functional group on the enlarged surface portion; and
then (c) releasing the substrate to form a reduced surface portion
from the enlarged surface portion. The reduced surface portion has
an area less than the enlarged surface portion, and the reduced
surface portion has the functional group deposited thereon at a
greater density than the enlarged surface portion.
[0017] The elastic substrate of the invention may be formed from a
number of materials, the selection of which is known to one skilled
in the art. In general, it is desirable that the material can
physically or chemically be forced to reversibly (or partially
reversibly) increase its surface area. For example, the elastic
substrate may be formed from polymers (e.g., homopolymers,
copolymers, and the like). Exemplary materials include, without
limitation, siloxanes (e.g., poly(dimethylsiloxane) (PDMS),
poly(hydromethylsiloxane), natural rubber, synthetic rubber,
butadienes, and the like, as well as composites or combinations
thereof. In various preferred embodiments, the elastic substrate is
prepared by crosslinking the polymer and curing the crosslinked
network to form a thermoset material. The crosslinking and curing
may be carried out using techniques known to one skilled in the
art.
[0018] In various preferred embodiments, the method of the
invention further comprises the step of subjecting the enlarged
surface portion of the elastic substrate to conditions sufficient
to impart hydrophilicity thereto, with this subjecting step
occurring subsequent to step (a) and prior to step (b). In one
embodiment, for example, the subjecting step comprises exposing the
enlarged surface portion to a ozone treatment to form a reactive
group on the enlarged surface portion. Preferably, the ozone
treatment is used in conjunction with an ultraviolet treatment.
Ozone-treatment techniques are known in the art, and are described
for example in U.S. Pat. No. 5,661,092 to Koberstein et al. and
U.S. Pat. No. 5,962,079 to Koberstein et al., the disclosures of
which are incorporated herein by reference in their entirety. The
reactive group, which results on the substrate surface, is
preferably one or more of a hydroxyl group, a carboxyl group, and a
peroxide group.
[0019] A wide variety of functional groups may be employed in the
conjugating step of the invention, the selection of which is known
to one skilled in the art. Preferably, the conjugating step
comprises depositing a functional group on the enlarged surface
portion to react with the reactive group to chemically modify the
reactive group, i.e., a chain is formed on the substrate. In
general, the chain may be in the form of a monomer, oligomer, or
polymer (e.g., homopolymer, copolymer, terpolymer, etc.) In one
embodiment, these chains are present in the form of a monolayer,
although other configurations may be formed by one who is skilled
in the art. In such embodiments, the invention provides for the
fabrication of mechanically assembled molecules (hereinafter
"MAMs"). In embodiments, in which polymers are assembled on the
substrate, the fabrication is referred to as mechanically assisted
polymer assembly. In preferred embodiments, the functional group
may be selected from an aryl-containing group (e.g., a chloro group
such as l-trichlorosilyl-2-(m-p-chloromethyl-phenyl)ethane), SH,
M--SiCl.sub.3, M--SiCl.sub.2(OR), M--SiCl(OR).sub.2, and
combinations thereof. Polyacrylamide chains may be formed on the
substrate. M is preferably represented as
F(CF.sub.2).sub.y1(CH.sub.2).sub.x1, In these groups, x1 and y1 are
individually selected and each preferably ranges from 1 to 8, 25,
50, 100, or a 1000 including all values therebetween. X1 is most
preferably 2 and y1 most preferably ranges from 6 to 8. M which
contains fluorine-based molecules may also encompass other
materials such as those described in U.S. Pat. No. 5,863,612 to
DeSimone, the disclosure of which is incorporated herein by
reference in its entirety. Such fluorinated materials include,
without limitation, fluoroacrylates, fluoroolefins, fluorostyrenes,
fluoroalkylene oxides, fluorinated vinyl alkyl ethers, and
combinations thereof. In addition, M can be any other chemical
functionality of the following formula including, without
limitation, .omega.-R-, where .omega. is a functional terminus,
such as --CH.sub.3, --CF.sub.3, --NH.sub.2, --COOH, --SH,
--CH.dbd.CH.sub.2, and others, and wherein R is a hydrocarbon chain
which may be branched or unbranched and/or substituted or
unsubstituted. The hydrocarbon chain preferably has 1 to 100,000
repeating units, and encompass all values therebetween.
[0020] Not intending to be bound by theory, it is believed that the
formation of certain embodiments of functional groups may be
illustrated by the following reaction schemes (1) and (2): 1
[0021] wherein R is defined above. 2
[0022] The chain may be formed by growing the functional group,
which has been deposited on the substrate, or as described herein,
by grafting the functional group on to the substrate and using it
as an initiator for polymerization (so called "grafting from"). In
a specific embodiment, the group M referred to above serves as a
polymerization free radical or controlled radical initiator and the
method comprises grafting the group M onto the substrate to attach
the molecules thereto, i.e., form molecular ""brushes" on the
substrate. The "brushes" may exist in the form of oligomers or
polymers.
[0023] The brush graft density of molecules at the surface of the
substrate may be controlled by varying any of a number of process
variables such as, for example, the time of ozone treatment (i.e.,
.tau..sub.UVO), initiator deposition time (i.e., .tau..sub.M),
initiator concentration (i.e., C.sub.M). Various brush graft
densities may be obtained for the purposes of the invention.
Preferably, the brush graft density ranges from about 10.sup.14
molecules/mm.sup.2 to about 10.sup.15 or 10.sup.16
molecules/mm.sup.2. In another embodiment, the brush graft density
may be no greater than about 10.sup.16 molecules/mm.sup.2.
[0024] In other embodiments, biological materials may be attached
to the surface of the elastic substrate. Accordingly, any number of
complementary functional groups may be attached thereto as desired
by the skilled artisan such as, for example, oligonucleotides
(e.g., DNA, RNA), proteins, peptides, and antibodies. For example,
one can tether a polypeptide molecule composed of a defined
sequence of amino acids to the substrate. The attachment may be
accomplished as set forth generally by embodiments described
herein, for example, by anchoring the peptide molecule from the
solution or by growing the peptide by the "grafting from"
reaction.
[0025] In embodiments which employ SH as the functional group, the
surface of the elastic substrate typically comprises at least one
metal thereon which is compatible with this group. Preferred metals
include, without limitation, gold, silver, platinum, palladium,
alloys thereof, and combinations thereof. In general, the
functional group may be a monomer, oliogomer, homopolymer,
copolymer, and the like.
[0026] Subsequent to the step of conjugating a functional group on
the enlarged surface portion of the substrate, the substrate is
released forming a reduced surface portion from the enlarged
surface portion. The reduced surface portion may be of various
sizes. In one embodiment, for example, the area of the reduced
surface portion and the area of the initial surface portion are the
same. In another embodiment, the area of the reduced surface
portion and the area of the initial surface portion differ by not
more than five percent. In another embodiment, the area of the
reduced surface portion is at least 10 percent less than the area
of the enlarged surface portion. In another embodiment, the area of
the reduced surface portion is at least 20 percent less than the
area of the enlarged surface portion.
[0027] The density of functional groups reacted to the reacted
groups (i.e., molecules) on the released elastic substrate can
vary. In one embodiment the density ranges from 10.sup.14
molecules/mm.sup.2 to 10.sup.15 or 10.sup.16molecules/mm.sup.2.
Preferably, the released elastic substrate contains no greater than
10.sup.16 molecules/mm.sup.2. In general, for various embodiments
described herein, the groups (i.e., chains) extending from the
released elastic substrate are typically aligned so as to be
present as a closely packed array.
[0028] The methods of the invention may be carried out using known
equipment. In one embodiment for example, the substrate (e.g.,
film) is crosslinked and cured as alluded to above. The film is
then placed into a suitable apparatus and is mechanically stretched
by a predetermined distance along the length of the film. The film
may either be stretched uni-axially or bi-axially along the
longitudinal axis of the film, as selected by one skilled in the
art. In accordance with the invention, it is preferred that the
stretching be carried out within the region when Hook's law is
valid. It is preferred that the relative strain (hereinafter
represented by ".DELTA.x") not exceed 50 percent of the initial
film length. Other means of stretching may be employed as known by
one who is skilled in the art.
[0029] FIG. 2 illustrates an apparatus 100 for mechanically
stretching the substrate (e.g., film) 105. It should be appreciated
that other equipment may be used without departing from the scope
of the invention. The apparatus is made from a pair of parallel
plate structures 110 and 120 each having bottom plates 110a and
120a and top plates 110b and 120b respectively which are clamped
such that the film 105 is fit snugly therebetween. As depicted in
FIG. 2, the film to be stretched is positioned perpendicular to the
longitudinal axes x1 and x2 of each of structures 110 and 120 and
between each of the bottom and top plates. Rods 130 and 140 extend
through each of the bottom plates 110a and 120a and extend parallel
to the longitudinal axis of the film. A threaded member 150 is also
present in between the rods and extends parallel to the rods 130
and 140. In this embodiment, one-half of the length of the threaded
member 150 is right-threaded and the other half is left-threaded.
Accordingly, by rotating the threaded member 150 in one direction,
one is able to extend the rods 130 and 140 and thus stretch the
film 105. Conversely, by rotating the threaded member 150 in the
other direction, one is able to bring the rods 130 and 140 inward,
thus releasing the film 105.
[0030] In another embodiment, it is possible to stretch the
substrate without employing a mechanical force. More specifically,
it is believed that one can contact the substrate with a liquid,
which causes a swelling in the area of the substrate. In one
example, toluene may be employed to swell a PDMS substrate. In
another example, a hydrolyzed poly(hydromethylsiloxane) substrate
is exposed to a fluid (e.g., supercritical carbon dioxide) to
effect the swelling of the substrate. Other embodiments are
encompassed by the invention.
[0031] Subsequent to stretching the elastic substrate, a reactive
group is imparted to the substrate surface in the manner described
herein. Thereafter, a functional group is deposited on the
substrate, which reacts with the reactive group to chemically
modify the reactive group and form a chain on the substrate
surface. This deposition can be carried out using processes known
to one skilled in the art. In one embodiment, the functional groups
may be deposited by employing a vapor deposition. As an example,
the vapor deposition may be used in conjunction with a mask
comprising openings contained in a regular or non-regular pattern.
For example, the pattern can be composed of a single "hole" (or a
rectangle/square/triangle, etc.) or can be made of an array of the
same patterns mentioned above or any combination of those patterns.
Other patterns can also be employed. As such, the mask allows for
certain regions of the substrate to have chains formed thereon in
accordance with the invention (i.e., an unmasked region), and not
have chains formed thereon (i.e., a masked region). In one
embodiment, the mask may consist of parallel grooves so as to allow
for the fabrication of a sinusoidal wetting profile on the elastic
substrate. The parameters of such a profile may be customized as
deemed appropriate by one skilled in the art such as, for example,
by adjusting the distance from the mask to the elastic substrate
(e.g., a "shadowing effect"), varying the width of the mask
grooves, and adjusting Ax.
[0032] FIGS. 9a through 9d illustrate various configurations that
may be employed in producing a shadow effect alluded to above. FIG.
9a illustrates a mask 310 having holes therein each of opening
width X being positioned at a distance L from the substrate 300.
FIG. 9b illustrates an effect when X is much less than L. FIG. 9c
illustrates an effect when X is approximately equal to L. FIG. 9d
illustrates an effect when X is greater than L.
[0033] The invention offers a number of advantages. For example,
the elastic substrate may have certain tuned wettable properties by
adjusting, for example, .DELTA.x and M (i.e., initiator as defined
herein). These wettable properties may be either hydrophilic or
hydrophobic. In certain embodiments, the wettable properties of the
elastic substrate (after release) may be such that the water
contact angle ranges from 20.degree. to 140.degree.. Moreover, by
adjusting .DELTA.x, the elastic substrate may have certain tuned
barrier properties. The molecular mobility of the surface chains
(functional groups) may be controlled by adjusting the rate of
strain release. Although not intending to be bound by theory, at
small release rates one potentially expects the molecules to have
enough time to respond to the mechanical manipulation, at higher
release rates chain interlocking ("entanglement") may possibly lead
to irregular structures. Additionally, the released elastic
substrate may possess long-lasting (non-reconstructive) wetting
properties, i.e., the surface energy of the released elastic
substrate remains constant for up to or at least six months
subsequent to the formation of the substrate. The surface tension
of the released elastic substrate may also be adjusted as deemed
appropriate by one skilled in the art. For example, the released
elastic substrate may preferably have a surface tension ranging
from about 6 or 9 mJ/m.sup.2 to about 11 to 13 mJ/m.sup.2. In
certain embodiments, the substrate may have a critical surface
tension of as low as 6 mJ/m.sup.2 (e.g., a crystalline array of
CF.sub.3 groups). The surface tension of the elastic substrate is
believed to vary according to the type of molecule chain(a)
attached thereto.
[0034] In summary, the following elastic substrate properties,
without limitation, may be modified: surface energy, surface
permeability, surface weatherability, surface chemical pattern,
surface resistance to liquids of varying pHs (e.g., acids and
bases), and surface hardness. The substrates can also serve as
flexible protection materials, as well as anti-fouling
non-reconstructive surfaces and active filters for gases and
liquids. The substrates (particularly in the form of films) also
are capable of being attached to other materials through its
non-modified side. Such surfaces can be applied to any surface that
needs to be modified, i.e., function as a sticker or a
"Post-It.RTM.-type " surface.
[0035] The invention will now be described according to the
examples, which follow. It should be appreciated that the examples
are set forth for the purposes of illustrating the invention and
are not intended to limit the scope of the invention as set forth
by the claims.
EXAMPLE 1
PDMS Film Formation
[0036] FIG. 1 illustrates a process for forming an elastic
substrate in accordance with the invention. A PDMS networked film
(e.g., Sylgard.RTM. 184 made commercially available from Dow
Chemical Company of Midland, Michigan) is prepared by casting a
mixture of PDMS and a crosslinker (curing agent 184 made
commercially available from Dow Chemical Company) in a ratio of
10:1 (PDMS to curing agent). The PDMS is formed into a 0.5 mm thick
film and is thereafter cured at 55.degree. C. for about 1 hour (see
FIG. 1a).
[0037] In the second step, the PDMS film is cut into small strips
(ca. 1.times.5 cm.sup.2) and stretched as illustrated by FIG. 1b by
the apparatus depicted in FIG. 2. In the subsequent step, the
stretched substrate is exposed to a UV ozone (UVO) treatment to
produce --OH functionalities as shown in FIG. 1c.
[0038] Next, chlorosilane molecules are deposited from a vapor
phase on the stretched substrate (see FIG. 1d) and upon deposition
form an organized self-assembled monolayer (SAM). The vapor
deposition is carried out in an evacuated container having a
pressure approximately ranging of from about 10.sup.-3 to 10.sup.-4
torr. The sample is placed upside down above the diffusion source
comprising a mixture of chlorosilane and a paraffin oil. It is
preferred that the paraffin oil and chlorosilane molecule do not
mix so as to serve as a "carrier" medium for the diffusion source.
The concentration of the diffusion source can be readily adjusted
by varying the chlorosilane:paraffin oil ratio.
[0039] In the final step, the strain is released from the stretched
substrate. In this particular embodiment, the PDMS film, covered by
a thin SAM layer, returns to its original size (see FIG. 1e) and
the grafted molecules form a highly dense organized surface
layer.
[0040] In this embodiment, the chlorosilane molecules are grafted
on to the substrate according to the following reaction scheme:
3
EXAMPLE 2
Water Contact Angle Measurement for PDMS Films
[0041] Water contact angles (.theta..sub.H2O) were measured for
various PDMS films which have undergone various levels of
stretching (expressed by .DELTA.x) and various exposure times
(T.sub.UVO). The results are set forth in Table 1. As shown
therein, the hydrophilicity of the PDMS surface increases with
increasing .DELTA.x.
1TABLE I Water contact angle (.theta..sub.H2O) of PDMS films
stretched to strains .DELTA.x and exposed to UVO for times
.tau..sub.UVO .DELTA.x (%) 0 20 50 t.sub.UVO (min) 0 20 20
.theta..sub.H2O (deg) 108 81.5 71.1
EXAMPLE 3
Water Contact Angle Measurement for PDMS Films
[0042] Water contact angles were measured for various PDMS films
stretched at different levels and having various functional groups
formed thereon. In this example, F6H2 refers to
F(CF.sub.2).sub.6(CH.sub.2).sub.2SiCl.sub- .3, F8H2 refers to
F(CF.sub.2).sub.8(CH.sub.2).sub.2SiCl.sub.3, and OTS refers to
H(CF.sub.2).sub.8SiCl.sub.3. The results are depicted in FIG. 3. As
illustrated, the sample contact angle increases with the level of
stretching. The data also suggest that water contact angle
increases with increasing CF.sub.2 length. Without intending to be
bound by theory, it is believed that the functional group chains
are well aligned and the surfaces of F6H2 and F8H2 consist of
ordered arrays of --CF.sub.3.
EXAMPLE 4
PDMS Surface Structure as a Function of Strain Release Rate
[0043] FIGS. 4a and 4b illustrate atomic force microscopy images of
PDMS samples functionalized with F8H2 in which the initial strain
(.DELTA.x) was 50 percent. FIG. 4a illustrates the strain being
released slowly, i.e., over a period of approximately 3 hours and
FIG. 4b illustrates the strain being released immediately. As shown
from these photographs, the release rate of the strain influences
final topography of the film surface.
EXAMPLE 5
Contact Angle Hysteresis
[0044] FIG. 5 illustrates the contact angle hysteresis for PDMS
samples functionalized by F6H2 (circles) and F8H2-MAM (squares) as
a function of .DELTA.x. In this embodiment, contact angle
hysteresis is defined as the difference between the advancing and
receding water contact angles.
EXAMPLE 6
Relationship of Water Contact Angle to Water Exposure
[0045] FIG. 6 illustrates the dependence of water contact angle of
F6H2-MAM (squares) and F8H2-MAM (circles) on exposure time to
water. The solid symbols denote the contact angles measured on
FyH2-MAMs with .DELTA.x=0%. The open symbols denote the contact
angles measured on FyH2-MAMs with .DELTA.x=70% taken immediately
after the water exposure and substrate drying with nitrogen. The
crossed symbols represent the contact angles from the samples
denoted by the open symbols measured six months later in time.
These samples were stored under ambient conditions with no
temperature or humidity control between water exposure and
measurement. As shown, the substrates display comparable surface
properties long after being formed.
EXAMPLE 7
Relationship of Water Contact Angle to Water Exposure
[0046] FIG. 7 illustrates the dependence of the average tilt angle
of the fluorinated helix <.sub..tau..sub.F-helix> for
F8H2-MAMs to the exposure time of the F8H2-MAM in water. The
squares and circles denote <.tau..sub.F-helix> in F8H2-MAM
(.DELTA.x=0%) and F8H2-MAM (.DELTA.x=70%) samples, respectively.
The solid and open symbols represent <.tau..sub.F-helix>
measured along and perpendicular to, respectively, the stretching
direction. The dashed line marks the value of
<.tau..sub.F-helix> corresponding to a completely disoriented
MAM.
EXAMPLE 8
Formation of Poly(Acrylamide) Chains on a PDMS Substrate
[0047] A pristine PDMS network film is prepared by casting a
mixture of PDMS and a crosslinker according to the procedure set
forth in Example 1. The mixture is formed into thin (approximately
1 mm) film and cured for approximately one hour (see FIG. 8a). The
cured film is then cut into strips (approximately 1.times.5
cm.sup.2) and mechanically elongated by .DELTA.x as shown in FIG.
8b. The film is subsequently exposed to a UV/ozone treatment (FIG.
8c) producing a hydrophilic surface primarily comprising hydroxy
groups which served as attachment points for the chloro-silane ATRP
(atom transfer radial polymerization) initiators.
1-trichloro-2-(m-p-chloromethylphenyl)ethane (CMPE) is employed as
an initiator and is made commercially available by United Chemical
Technologies, Inc. of Bristol, Pa. The CMPE molecules are deposited
from vapor onto the stretched substrate and form an organized
CMPE-self assembled monolayer, which is depicted in FIG. 8d.
[0048] After the CMPE-SAM deposition, physisorbed CMPE molecules
are removed by thoroughly washing the substrates with warm
deionized water (75.degree. C.,>16 M.OMEGA..multidot.m) for
several minutes. The film is placed into 120 mL of N,
N'-dimethylformamide in a flash, and 0.3 g of CuCl, 1.0 g of
bipyridine, and 16.0 g of acrylamide (all made commercially
available by Aldrich Chemical of Milwaukee, Wis.) are added
thereto. The chemistry is described in Huang et al., Chemtech,
December 1998. Huang et al., Anal. Chem. 1998, 70, 4023, and Huang
et al., Macromolecules 1999, 32, 1694. The flask was then sealed
under nitrogen, placed into an oil bath, and the mixture was
reacted at 130.degree. C. for 45 hours to form poly(acrylamide)
(PAAm) brushes on the PDMS-ultraviolet-ozone treated (i.e., UVO)
substrate as illustrated in FIG. 8e. After the reaction, the strain
is released from the PDMS-UVO substrate such that it returns to its
initial size causing the grafted PAAm polymers to form a densely
organized brush, which is depicted in FIG. 8f. Physisorbed
monomeric and polymeric acrylamide was removed by soxlet extraction
with deionized water for 48 hours.
EXAMPLE 9
Formation of Hydrolyzed PDMS Substrate
[0049] A substrate formed from poly(hydromethyl) siloxane is
reacted with a divinyl compound over a platinum catalyst to form an
H-PDMS network. The reaction proceeds according to the following
scheme: 4
[0050] The H-PDMS substrate is thereafter exposed to supercritical
carbon dioxide (sc-CO.sub.2); increasing the pressure of the
sc-CO.sub.2 causes the substrate to swell. The H-PDMS substrate is
then exposed to water vapor over a tin catalyst to hydrolyze the
H-PDMS substrate according to the following reaction scheme: 5
[0051] The HO-PDMS substrate is thereafter exposed to functional
groups described herein to form chains extending from the
substrate. Finally, the supercritical carbon dioxide is
depressurized to relax the substrate, i.e., the swelled network
relaxes back causing the grafted molecules to pack densely.
[0052] The present invention has been described with respect to the
embodiments set forth above. It should be appreciated that these
embodiments are for the purposes of illustrating the invention, and
do not limit the scope of the invention as defined by the
claims.
* * * * *