U.S. patent application number 16/339825 was filed with the patent office on 2019-09-26 for highly fluorinated nanostructured polymer foams for producing super-repellent surfaces.
This patent application is currently assigned to Glassomer GmbH. The applicant listed for this patent is Glassomer GmbH. Invention is credited to Dorothea HELMER, Nico KELLER, Bastian RAPP, Christiane RICHTER.
Application Number | 20190292377 16/339825 |
Document ID | / |
Family ID | 60119988 |
Filed Date | 2019-09-26 |
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United States Patent
Application |
20190292377 |
Kind Code |
A1 |
RAPP; Bastian ; et
al. |
September 26, 2019 |
HIGHLY FLUORINATED NANOSTRUCTURED POLYMER FOAMS FOR PRODUCING
SUPER-REPELLENT SURFACES
Abstract
The present invention relates to a highly fluorinated
nanostructured polymer foam as well as to its use as a
super-repellent coating of substrates. Furthermore, the present
invention relates to a composition and to a method for producing
the highly fluorinated nanostructured polymer foam.
Inventors: |
RAPP; Bastian; (Karlsruhe,
DE) ; HELMER; Dorothea; (Karlsruhe, DE) ;
RICHTER; Christiane; (Eggenstein-Leopoldshafen, DE) ;
KELLER; Nico; (St. Leon-Rot, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Glassomer GmbH |
Freiburg im Breisgau |
|
DE |
|
|
Assignee: |
Glassomer GmbH
Freiburg im Breisgau
DE
|
Family ID: |
60119988 |
Appl. No.: |
16/339825 |
Filed: |
October 4, 2017 |
PCT Filed: |
October 4, 2017 |
PCT NO: |
PCT/EP2017/001171 |
371 Date: |
April 5, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08J 9/286 20130101;
C08J 2205/042 20130101; C08J 2300/102 20130101; C08J 2333/16
20130101; C08J 9/12 20130101; C09D 5/00 20130101; C08L 33/16
20130101; C08J 2335/02 20130101; C08J 2201/026 20130101; C08J
2201/0502 20130101; C09D 133/16 20130101; C08J 2205/044 20130101;
C08F 220/22 20130101; C09D 135/02 20130101; C08J 2205/052
20130101 |
International
Class: |
C09D 5/00 20060101
C09D005/00; C09D 135/02 20060101 C09D135/02; C09D 133/16 20060101
C09D133/16; C08J 9/28 20060101 C08J009/28 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2016 |
DE |
DE102016012001.0 |
Claims
1. A highly fluorinated nanostructured polymer foam with a density
of at most 2.2 g/mL, an absolute foam expansion of at least 1% as
well as a free surface energy of at most 12 mN/m, wherein the
fluorine content is at least 20 mol % and wherein cavities with
dimensions of at most 5 pm are homogeneously distributed throughout
the entire volume of the polymer foam.
2. The highly fluorinated nanostructured polymer foam according to
claim 1, wherein at least 20% of the fluorine content is present as
CF.sub.3 groups.
3. The highly fluorinated nanostructured polymer foam according to
claim 1, wherein the polymer foam has a transmission of at least
50% within the wavelength range between 400 and 800 nm at a
thickness of 0.25 mm.
4. A composition for producing the highly fluorinated
nanostructured polymer foam according to claim 1, comprising the
following components: at least one monomer which can be polymerized
by supplying heat or light, wherein the fluorine content of the at
least one monomer is at least 20 mol %; a polymerization initiator
which initiates the polymerization of the at least one monomer by
supplying heat or light; and a non-polymerizable porogen.
5. The composition according to claim 4, wherein the volume
percentage of the non-polymerizable porogen in the composition is
at least 5%.
6. The composition according to claim 4, wherein the at least one
monomer is a diacrylate derivative, represented by the following
general formula (I): ##STR00003## wherein X.sup.1 is independently
selected from the group consisting of hydrogen, methyl,
monofluoromethyl, difluoromethyl and trifluoromethyl and Y.sup.1 is
a fluorinated saturated hydrocarbon residue which optionally
comprises one or more ether groups, or a monoacrylate derivative,
represented by the following general formula (II): ##STR00004##
wherein X.sup.2 is selected from the group consisting of hydrogen,
methyl, monofluoromethyl, difluoromethyl and trifluoromethyl and
Y.sup.2 is a fluorinated saturated hydrocarbon residue which
optionally comprises one or more ether groups.
7. A method for producing the highly fluorinated nanostructured
polymer foam, comprising the following steps: (a) curing the
composition according to claim 4 by supplying heat or light,
optionally on a substrate to be coated; (b) optionally dipping the
polymer foam obtained in this manner into a solvent; and (c) drying
the polymer foam treated in this manner, wherein the steps (a) to
(c) are optionally repeated at least once, wherein the polymer foam
has a density of at most 2.2 g/mL, an absolute foam expansion of at
least 1% as well as a free surface energy of at most 12 mN/m,
wherein the fluorine content is at least 20 mol % and wherein
cavities with dimensions of at most 5 .mu.m are homogeneously
distributed throughout the entire volume of the polymer foam.
8. The method according to claim 7, wherein the polymer foam is
dipped into an alcohol at step (b).
9. The method according to claim 7, wherein the drying of the
polymer foam at step (c) is carried out at a temperature within the
range of 50 to 100.degree. C.
10. (canceled)
11. The method according to claim 7, wherein the volume percentage
of the non-polymerizable porogen in the composition is at least
5%.
12. The method according to claim 7, wherein the at least one
monomer is a diacrylate derivative, represented by the following
general formula (I): ##STR00005## wherein X.sup.1 is independently
selected from the group consisting of hydrogen, methyl,
monofluoromethyl, difluoromethyl and trifluoromethyl and Y.sup.1 is
a fluorinated saturated hydrocarbon residue which optionally
comprises one or more ether groups, or a monoacrylate derivative,
represented by the following general formula (II): ##STR00006##
wherein X.sup.2 is selected from the group consisting of hydrogen,
methyl, monofluoromethyl, difluoromethyl and trifluoromethyl and
Y.sup.2 is a fluorinated saturated hydrocarbon residue which
optionally comprises one or more ether groups.
13. A super-repellent substrate coating comprising the highly
fluorinated nanostructured polymer foam according to claim 1.
Description
CROSS-REFERENCE
[0001] This application is a section 371 U.S. National phase of
PCT/EP2017/001171, filed Oct. 4, 2017 which claims priority from
German patent application no. 10 2016 012 001.0, filed Oct. 6,
2016, both which are incorporated by reference in its entirety.
FIELD OF INVENTION
[0002] The present invention relates to a highly fluorinated
nanostructured polymer foam as well as to its use as a
super-repellent coating of substrates. Furthermore, the present
invention relates to a composition and to a method for producing
the highly fluorinated nanostructured polymer foam.
BACKGROUND OF THE INVENTION
[0003] Super-repellent surfaces are surfaces, from which both water
as well as oils and organic solvents pearl off. Accordingly, they
are both hydrophobic as well oleophobic. Applications include
super-repellent surfaces, among other things, in the form of
technical surface coatings, glass coatings, and fabric coatings and
are used in the outdoor industry, automotive industry and
pharmaceutical industry, among others.
[0004] Thereby, the contact angle, which a drop of the liquid forms
on this surface, is used as a measure of the wettability of a
surface with a liquid. Hydrophobic surfaces have a contact angle of
water >90.degree., wherein super-hydrophobic surfaces have a
contact angle of water >140.degree.. Analogously, it applies to
oleophobic surfaces that they have a contact angle of oils
>90.degree., wherein super-oleophobic surfaces have a contact
angle of >140.degree.. In order to achieve such a contact angle
on a surface, a reduction of the free surface energy of each
surface is required. The reason for this lies in conjunction with
the contact angle, as it is expressed in Young's equation (1):
cos .theta. = .gamma. gas / solid - .gamma. liquid / solid .gamma.
gas / liquid .apprxeq. .gamma. solid - .gamma. liquid / solid
.gamma. liquid ( 1 ) ##EQU00001##
wherein .theta. is the contact angle forming and the respective
.gamma. represents the free surface energies of the three boundary
surfaces between the surrounding gas atmosphere ("gas"), liquid
("liquid") and substrate surface ("solid"). Thereby, the free
surface energies on the boundary surface gas/surface and gas/liquid
can be approximated by the free surface energies of the surface and
the liquid. These values can either be measured or taken from the
tables. Only the variable .gamma..sub.liquid/solid must be
approximated using an appropriate model, wherein, here, the model
of Fowkes has proven to be true, adopting the following expression
(2) for .gamma..sub.liquid/solid:
.gamma..sub.liquid/solid=.gamma..sub.liquid+.gamma..sub.solid-2
{square root over (.gamma..sub.liquid.gamma..sub.solid)} (2)
If you combine equations (1) and (2), you get:
cos .theta. = .gamma. gas / solid - .gamma. liquid / solid .gamma.
gas / liquid = .gamma. solid - ( .gamma. liquid + .gamma. solid - 2
.gamma. liquid .gamma. solid ) .gamma. liquid cos .theta. = -
.gamma. liquid + 2 .gamma. liquid .gamma. solid .gamma. liquid ( 3
) ##EQU00002##
If a surface should repel a liquid, then the contact angle in
marginal cases must be >90.degree.. The cosine for .theta.>90
is negative, therefore, the following results for equation (3):
- .gamma. liquid + 2 .gamma. liquid .gamma. solid .gamma. liquid
< 0 2 .gamma. liquid .gamma. solid < .gamma. liquid 4 .gamma.
solid < .gamma. liquid .gamma. solid < 1 4 .gamma. liquid ( 4
) ##EQU00003##
[0005] From equation (4), it is evident that a surface with a free
surface energy of less than a fourth of the free surface energy of
the liquid becomes repellent for this liquid. In accordance with
this, water with a free surface energy of 71 mN/m is repelled on
surfaces with less than 17 mN/m. For oils and organic solvents with
free surface energies within a range of 20 to 25 mN/m, surfaces are
required that have free surface energies of at most or 5 mN/m.
[0006] A reduction of the free surface energy can be achieved
through use of highly fluorinated materials. However, in addition,
a micro- or nanoscale roughness is required, which ensures that air
is held at the surface. The latter is required since the wetting
model according to Young is based on a three-phase system. If the
air is driven away from the surface, Young's equation is no longer
valid, and the super-repellent effect of the surface is lost
despite its low level of free surface energy. Accordingly,
surfaces, from which both water as well as oils and organic
solvents should pearl off, must have both a sufficiently low level
of free surface energy, as well as an appropriate micro- or
nanoscale roughness.
[0007] In the prior art, examples are known in which porous highly
fluorinated substrates are produced by means of separating
functionalized nanoparticles. These nanoparticles are fluorinated
at a first step, for example, by means of a silanization under the
use of appropriate perfluoroalkylsilanes. Thereby, the particles
form substrates, which have cavities with suitable pore diameters
similar to a bulk powder. Examples of this can be found in WO
2009/118552 A1. The production of these particles by means of an
electro-spray method in situ is also described in the prior art,
for example, in US 2006/0246297 A1.
[0008] Frequently, rough surfaces are produced by means of
electropolymerization. Thereby, sometimes directly fluorinated
monomers can also be used. Examples of electropolymerized
substrates can be found in US 2006/0029808 A1.
[0009] Another method for producing surfaces with a sufficient
micro- or nano-roughness exists in the anionic etching of metal
surfaces. This method is among the first approaches, with which
super-repellent surfaces have been produced. Examples for this are
described by Shafiei and Alpas (Applied Surface Science 2009, 256,
710-719).
[0010] Another method with which sufficiently porous structures can
be produced include sol-gel approaches, as is described by Taurino
et al. (Journal of Colloid and Interface Science 2008, 325,
149-156).
[0011] Mixtures of polymer matrices and fluorinated nanoparticles
are known from the prior art. Examples for this can be found in
U.S. Pat. No. 8,017,234 B2, US 2011/0263751 A1, US 2010/0004373 A1
and WO 2010/018744 A1. Sometimes, nanoparticles can be subsequently
introduced by means of a solvent process into a polymer coating
previously applied onto the surface. For example, such a method is
described in US 2002/0150723 A1.
[0012] Furthermore, methods are known in the prior art, which
initially provide for the creation of a microstructure, which is
coated with a nanostructure at a second step. Furthermore, creating
such hierarchical structures is possible, for example, via a
material separation or via a selective material removal on or from
a microstructure produced at a first step. The structures are
mostly fluorinated at a post-treatment step. Examples of such
methods can be found in U.S. Pat. No. 8,137,751 B2 and in WO
2012/012441 A1.
[0013] It is also possible to introduce a second structural level
onto particles, as described for example by Perro et al, (Colloids
and Surfaces A: Physicochemical and Engineering Aspects 2006,
284-285, 78-83). This produces particles that have a defined
surface structure. Due to the combination of the particles which
form the first structural level and the structure applied to the
particles, which form the second structural level, super-repellent
surfaces can be produced. Mostly, these particles must then be
fluorinated at a subsequent step.
[0014] Approaches are also described where a substrate consists of
two materials, which can be selectively etched. Thereby, mostly at
a first step, a coating compound consisting of two materials is
applied and, at a second step, one of the two materials is
selectively removed using a suitable etching agent. In this way, a
super-repellent surface can also result. Examples for this are
described in U.S. Pat. No. 8,741,158 B2. The coatings are sometimes
also applied to a previously produced microstructure. Examples of
this can be found in US 2006/0024508 A1. A modification of this
method uses a mixture of an inorganic matrix, for example, silica,
and an organic matrix, usually a polymer, which can be applied
together by means of dip-coating. Then, the substrate is heated,
and the organic matrix is burned out. At the same time, the
inorganic matrix calcines and then forms a porous glass-like
structure. At a second step, this can be fluorinated, for example,
by means of silanization under the use of suitable
perfluoroalkylsilanes, whereby a super-repellent coating results.
Examples of this method are described by Li et al, (Chemical
Communications 2009, 2730-2732).
[0015] Another variant for producing super-repellent surfaces
entirely via calcination is described by Deng et al. (Science 2012,
335, 67-70). Here, via soot separation, a surface with a suitable
roughness is produced, which is covered with a silane at a next
step by means of a gas-phase separation, said silane being
subsequently calcinated. A sufficiently low-energy surface is then
created on the micro- or nanoscale rough surface by means of
gas-phase fluorination.
[0016] In addition to producing super-repellent surfaces based on
nanoparticles, there are also examples for producing corresponding
surfaces under the use of nanowires, which can, for example, be
produced by means of gas-phase separation or electrospinning. These
nanowires can be functionalized at a second step by means of a
gas-phase process, thereby being fluorinated. Examples of this can
be found in US 2011/0229667 A1 and U.S. Pat. No. 7,985,475 B2.
[0017] Methods are also known in the prior art, which structure a
highly fluorinated substrate, for example, a fluoropolymer, by
means of replication technology, thereby introducing the required
level of roughness. Examples for this are described by Vogelaar et
al. (Langmuir 2006, 22, 3125-3130).
[0018] The aforementioned methods from the prior art accordingly
follow either a "top-down approach" or a "bottom-up approach" for
the most part.
[0019] In the case of the "top-down approaches", nanostructured
surfaces with a sufficiently low level of free surface energy are
generated by means of a replication process. Here, a nanostructured
mold tool is copied into a material with a sufficiently low level
of free surface energy. In the case of these approaches,
consequently, a pre-existing coating of a material is structured
with a low level of free surface energy, typically a fluoropolymer
"from above". These approaches are particularly disadvantageous due
to the fact that the replication usually only reaches a very low
penetration depth, and thus a low effect depth. By effect depth,
the substrate thickness is understood to be that on which the
super-repellent effect acts. Thereby, it essentially has to do with
magnitudes within a range of less than 10 to 100 .mu.m.
Furthermore, in the case of these approaches, thermoplastic
fluoropolymers are used since the required mechanical restructuring
can only take place if the polymer network is not chemically
cross-linked. Since thermoplastic fluoropolymers are generally very
soft, the structured surfaces obtained in this manner have a very
high level of mechanical susceptibility.
[0020] In contrast, in the case of "bottom-up approaches", a
surface with a suitable roughness is firstly produced in any
material by means of a suitable method. At a second method step,
this surface is changed with regard to its free surface energy in
such a way that the surface becomes repellent. Frequently, this
second step is a gas-phase functionalization, for example, a
fluorination. Here, for example, fluorinated silanes are available
as suitable reagents, which are applied onto the surface in the
fluid phase by means of vaporization or sampling. These approaches
are disadvantageous due to the roughness of the created surface
frequently also having a limited effect depth. Thus, for example, a
thickness of a few micrometers can, for example, be achieved by
means of separating nanoparticles. Thicker coatings frequently
become mechanically instable. They are inclined to form cracks or
become light-diffusing, meaning opaque. In particular, in the case
of "bottom-up approaches", the second method step represents the
limiting factor. The subsequent functionalization, which is usually
a fluorination, only reaches a limited penetration depth. Thus,
vapor-phase processes only allow for a certain insertion and
removal of reagents, for example, into a nanostructure or out of a
nanostructure. Thereby, the limiting represents the mass and
diffusion transport within the structure. Therefore, also here, the
effect depth is limited to a few micrometers.
[0021] In the case of "bottom-up approaches", the same problem
accordingly arises as in the case of "top-down approaches": The
limited effect depth does not allow for surfaces to be produced
that are sufficiently mechanically stable for practical
applications. Also here, a simple scratching with a key or with a
fingernail would remove the functional structure of the surface and
destroy the super-repellent effect. Due to the fact that nanoscale
structures, particularly if they are composed of adsorbing or
self-assembled particles, comprise a very low level of mechanical
stability, the coatings obtained by the "bottom-up approaches" are
mostly still less robust than the structures produced by means of
"top-down approaches".
[0022] In the case of the method described in the prior art,
according to this, the super-repellent effect is limited to the
depth that can be structured within the scope of the corresponding
"top-down approach" or "bottom-up approach". If the super-repellent
surface is exposed to mechanical burdens such as abrasion, for
example, due to weather impact, a material erosion gradually occurs
in the magnitude of the effect depth, wherein the super-repellent
characteristic of the surface is ultimately lost completely.
SUMMARY OF THE INVENTION
[0023] The object of the present invention is to provide a highly
fluorinated nanostructured polymer foam for producing
super-repellent surfaces, wherein the super-repellent
characteristic of the polymer foam should not be limited to its
surface but should be maintained within its entire surface
thickness.
[0024] This object is solved by means of the embodiments of the
present invention that are characterized in the claims.
DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows the structure of the polymer foam according to
the invention. A fluorinated polymer matrix (10) is permeated by a
network of cavities that is filled with air (11). The result is a
"fluorinated sponge", which has a high level of roughness on its
surface.
[0026] FIG. 2 shows examples of a cured polymer foam with the
presence and absence of a non-polymerizable porogen. (a)
Microfluidic channel structure and (b) compact polymer block (both
not according to the invention). Here, the polymer foam has no
porosity. Under the use of non-polymerizable porogens, a polymer
foam in the form of globular structures (c) or in the form of
substrates with micro- or nanoscale roughness (d) can be
produced.
[0027] FIG. 3 shows the "top-down structuring" and "bottom-up
structuring" in comparison to the polymer foam according to the
invention. (a) Example of a "top-down structuring". A highly
fluorinated substrate (101), for example, a highly fluorinated
thermoplastic, is structured using a molding tool (102). A fluid
(103) potentially forms a high contact angle on the unstructured
substrate, however, no super-repellency occurs. After structuring
takes place, a coating with an effect depth (106) of less than 10
to 100 .mu.m forms, on which the super-repellent effect (104) is
achieved. This layer is removed by mechanical abrasion (105). Now,
no super-repellent effect is present for the liquid (107). (b)
Example of a "bottom-up structuring". A substrate with a sufficient
micro- or nanoscale roughness is fluorinated by means of a suitable
process (110), whereby the free surface energy of the surface is
reduced. No repelling of the liquid takes place on the native
substrate (111). Sometimes, a significantly improved wetting may
take place caused by capillary effects. After reducing the free
surface energy, the surface is super-repellent up to an effect
depth (113) of less than 10 to 100 .mu.m (112). This layer can be
easily removed due to mechanical abrasion, whereby the
super-repellent effect is lost (114). (c) Polymer foam according to
the invention. On the inherently highly fluorinated and low-energy
and inherently nanoscale structured polymer foam (120), the fluid
is natively repelled (121). Here, mechanical abrasion (122) can
also remove material. However, the effect depth is equal to the
total thickness of the coating (123). Thereby, a coating with an
identical functionality is uncovered due to abrasion and the
super-repellent is maintained (124).
[0028] FIG. 4 shows the polymer foam according to the invention in
contact with water. (a) When dipping into water, the high contact
angle becomes evident. (b) Under water, the polymer foam keeps a
reflecting layer of air at its surface. This ensures the required
three-phase system.
[0029] FIG. 5 shows the super-repellent effect of the polymer foam
according to the invention with relation to (a) water and (b) oil
(tetradecane). Contact angles >130.degree. are achieved.
[0030] FIG. 6 shows an optically clear polymer foam in accordance
with the present invention, the transparency of which can be
configured by the dimensions of the cavities. (a) Coating with the
polymer foam according to the invention (edge exaggerated) on the
KIT logo. (b) optical transmission spectrum of the sample shown in
(a). The sample appears to be optically transparent through the
entire wavelength range represented.
DETAILED DESCRIPTION OF THE INVENTION
[0031] In particular, according to the invention, a highly
fluorinated nanostructured polymer foam is provided, which has a
density of at most or .ltoreq.2.2 g/mL as well as a free surface
energy of at most or .ltoreq.12 mN/m, wherein the fluorine content
is at least or .gtoreq.20 mol %. Furthermore, the highly
fluorinated nanostructured polymer foam has homogeneously
distributed cavities with dimensions of at most or .ltoreq.5 .mu.m
across the entire volume. Furthermore, according to the invention,
the highly fluorinated nanostructured polymer foam has an at least
or .gtoreq.1% lower density than the unstructured or unfoamed bulk
polymer, thereby having an absolute foam expansion of at least or
.gtoreq.1%.
[0032] The polymer foam according to the invention is suitable as a
surface coating for substrates and gives this super-repellent
characteristics. Since the effect depth, meaning the depth, within
which the nanostructuring is present, is identical to the overall
thickness of the coating, it does not have to do with a surface but
with a volume effect, as can be seen in FIG. 1. Only when the
entire coating is fully removed does the substrate lose its
super-repellent effect. A slight abrasion of the polymer foam even
regenerates the super-repellent effect since a new coating with a
corresponding surface roughness is uncovered.
[0033] According to the invention, the highly fluorinated
nanostructured polymer foam has a free surface energy of at most or
.ltoreq.12 mN/m, preferably of at most or .ltoreq.10 mN/m, and
being particularly preferred, of at most or .ltoreq.5 mN/m. Due to
the cavity structure and the surface roughness caused thereby, the
effective contact surface is reduced to the extent that the polymer
foam according to the invention is already super-repellent at free
surface energies of 12 mN/m. In accordance with this, substrates
coated with the polymer foam according to the invention are both
repellent against water as well as against oils and organic
solvents. The free surface energies are measured according to the
invention under the use of the OWRK method in accordance with
DIN55660-2 by means of a commercial contact angle measurement
device (type: OCA-15, Data Physics). Water and diiodomethane are
used as liquids.
[0034] The low level of free surface energy of the polymer foam
according to the invention is achieved by means of a fluorine
content of at least or .gtoreq.20 mol %, preferably of at least or
.gtoreq.30 mol % and, being particularly preferred, of at least or
.gtoreq.40 mol %, with reference to the chemical composition of the
polymer foam. The remaining fabric quantities thereof, in
particular, are allotted to carbon, hydrogen, and if applicable,
oxygen, wherein the chemical composition of the polymer foam is not
limited to those elements.
[0035] Typically, at least or .gtoreq.20%, preferably at least or
.gtoreq.30%, and being particularly preferred, at least or
.gtoreq.40% of the fluorine atoms contained in the polymer foam
according to the invention are present as CF.sub.3 groups since
these make lower levels of free surface energies possible in
comparison to CF.sub.2 groups and CF groups. The remaining fluorine
content is preferably present as CF.sub.2-groups.
[0036] Furthermore, a porous structure of the polymer foam is
required to achieve the super-repellent characteristic, which
ensures the surface roughness in order to keep sufficient air at
the surface. By means of this, it is ensured that the three-phase
system is fulfilled according to Young's equation, whereby, in
connection with the low level of free surface energy, contact
angles for water as well as for oils and organic solvents of
.theta.>90 are obtained. According to the invention, the porous
structure of the polymer foam is characterized in that cavities
with dimensions of at most or .ltoreq.5 .mu.m, preferably of at
most or .ltoreq.500 nm and, being particularly preferred, of at
most or .ltoreq.50 nm are homogeneously distributed within the
entire volume of the polymer foam.
[0037] Within the scope of the present invention, the term
"nanostructured" is understood in such a way that the cavities of
the polymer foam according to the invention do not necessarily have
dimensions only within the nanometer range even though such
nanoscale dimensions are preferred.
[0038] In accordance with this, the polymer foam according to the
invention is highly fluorinated and nanostructured, wherein those
characteristics are not limited to the surface of the polymer foam.
Consequently, it concerns a volume effect.
[0039] Due to the homogeneously distributed cavities within the
entire volume of the polymer foam according to the invention, the
density of the polymer foam is at most or .ltoreq.2.2 g/mL,
preferably at most or .ltoreq.1.8 g/mL and, being particularly
preferred, at most or .ltoreq.1.5 g/mL. The highly fluorinated
nanostructured polymer foam has at least or .gtoreq.1%, preferably
at least or .gtoreq.10% and, being particular preferred, at least
or .gtoreq.20% less density than the unstructured or unfoamed bulk
polymer, which is free of cavities. Consequently, the polymer foam
according to the invention has an absolute foam expansion of at
least or .gtoreq.1%, preferably of at least or .gtoreq.10%, and
being particularly preferred of at least or .gtoreq.20%. The
thickness difference and the absolute foam expansion can be
understood as a measure for the porosity of the polymer foam
according to the invention.
[0040] According to the invention, the density of the polymer foam
is measured on standard bodies with a diameter of 1 cm and
thickness of 3 mm. Thereby, the weight of the polymer foam is
determined using a high-precision scale.
[0041] Since the polymer foam according to the invention can
comprise cavities, the dimensions of which are smaller than the
wavelength of the visible spectrum, the polymer foam appears to be
optically transparent if applicable. In an embodiment of the
present invention, the polymer foam has a transmission of at least
or .gtoreq.50% within the wavelength range spanning from 400 to 800
nm at a thickness of 0.25 mm, preferably of at least or
.gtoreq.70%, and, being particularly preferred, of at least or
.gtoreq.90%. Here, the measurement of the transmission takes place
using an Evolution-201-type UV/VIS spectrometer (Thermo Scientific,
Germany) on samples with a thickness of 0.25 mm.
[0042] In another aspect, the present invention relates to a
composition for producing the highly fluorinated nanostructured
polymer foam according to the invention, comprising the following
components: [0043] at least one monomer, which can be polymerized
by supplying heat or light, wherein the fluorine content of the at
least one monomer is at least or 20 mol %; [0044] a polymerization
initiator which initiates the polymerization of the at least one
monomer by supplying heat or light; and [0045] a non-polymerizable
porogen.
[0046] The highly fluorinated nanostructured polymer foam according
to the invention described in the above can be produced using the
composition according to the invention.
[0047] The at least one monomer of the composition according to the
invention is at room temperature, meaning at a temperature of
20.degree. C. in liquid form. In this connection, the presence in
liquid form means that the at least one monomer has either a liquid
physical state or it has been dissolved in a liquid porogen.
Examples for such a porogen include long-chained saturated and
unsaturated non-, partially or perfluorinated alkanes and
carboxylic acids.
[0048] The fluorine content of the at least one monomer is at least
or .gtoreq.20 mol %, preferably at least or .gtoreq.30 mol % and,
being particularly preferred, at least or .gtoreq.40 mol %, with
reference to the atomic composition of the at least one monomer. An
upper fluorine limit is only given by the maximum fluorination
capacity and is therefore merely a technical issue. In principle, a
highest possible fluorination is preferred.
[0049] Typically, at least or .gtoreq.20%, preferably at least or
.gtoreq.30%, and being particularly preferred, at least or
.gtoreq.40% of the fluorine atoms of the at least one monomer is
present as CF.sub.3 groups since these make lower levels of free
surface energies possible in comparison to CF.sub.2 groups and CF
groups. The remaining fluorine content is preferably present as
CF.sub.2-groups.
[0050] The at least one monomer can be polymerized by supplying
heat or light when a polymerization initiator is present. In
accordance with this, the at least one monomer according to the
invention comprises one or a plurality of functional group(s) that
is/are suitable for a polymerization, such as at least one double
bond or at least two functional groups, such as hydroxyl or
carboxyl, which allow for the formation of polyesters for example.
Other functional groups, epoxide, isocynate, isothiocyanate, amino,
thiol or comparable functional groups known to the person skilled
in the art can be noted as non-limiting.
[0051] In the case of the presence of a plurality of polymerizable
functional groups, the polymer foam obtained after polymerization
can be chemically cross-linked, whereby its mechanical stability is
increased. This is an advantage with regard to the comparatively
soft thermoplastics which are described in the prior art. The
latter have no chemical cross-linking. In an embodiment of the
present invention, the composition according to the invention
comprises at least one monomer, which has at least two double
bonds, whereby a chemical cross-linking is possible.
[0052] The chemical structure of the at least one monomer is
otherwise subject to no other limitations. In this way, the at
least one monomer can comprise additional non-polymerizable
functional groups, such as carbonyl, ether and/or ester groups. In
addition to fluorine, the at least one monomer therefore still
comprises carbon, hydrogen and, if applicable oxygen, however, it
is not limited to those elements.
[0053] In an embodiment of the present invention, the composition
comprises a diacrylate derivative as the at least one monomer,
shown by the following general formula (I):
##STR00001##
wherein X.sup.1 is independently selected from the group consisting
of hydrogen, methyl, monofluoromethyl, difluoromethyl and
trifluoromethyl and Y.sup.1 is a fluorinated saturated hydrocarbon
residue which optionally comprises one or more ether groups. The
molecular weight of the diacrylate derivative defined in the above
is not subject to any particular limitations.
[0054] In another embodiment of the present invention, the
composition comprises a monoacrylate derivative as the at least one
monomer, shown by the following general formula (II):
##STR00002##
wherein X.sup.2 is selected from the group consisting of hydrogen,
methyl, monofluoromethyl, difluoromethyl and trifluoromethyl and
Y.sup.2 is a fluorinated saturated hydrocarbon residue which
optionally comprises one or more ether groups. The molecular weight
of the monoacrylate derivative defined in the above is not subject
to any particular limitations.
[0055] While the diacrylate derivative defined in the above allows
for a chemical cross-linking of the polymer foam, this naturally is
not the case for the monoacrylate derivative defined in the above.
If both acrylate derivatives are present in the composition
according to the invention, the degree of cross-linking can be
configured by selecting substance-quantity percentages thereof. In
this way, the mechanical and thermal characteristics of the highly
fluorinated nanostructured polymer foam according to the invention
can be controlled.
[0056] The number of chemically different monomers in the
composition according to the invention for producing a highly
fluorinated nanostructured polymer foam is not limited in any
way.
[0057] In addition to the at least one monomer, the composition
according to the invention for producing a highly fluorinated
nanostructured polymer foam comprises a polymerization initiator
which initiates the polymerization of the at least one monomer by
supplying heat or light. According to the invention, the
polymerization initiator is not subject to any limitations. The
composition defined in the above can comprise any polymerization
initiator known in the prior art, provided it can create
polymerization-inducing bonds by means of thermolytic or photolytic
dissociation. Examples for polymerization initiators in the form of
radical initiators, which can be activated by means of light,
include azobis(isobutyronitrile) and benzoyl peroxide, while, for
example, 2,2-dimethoxy-2-phenylacetophenone,
phenylbis(2,4,6-trimethylbenzoyl)phosphinoxide,
2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone and
2-hydroxy-2-methylpropiophenone represent thermally activatable
radical initiators. Since the supply of light can be spatially
limited in a more precise manner than the supply of heat, the
composition according to the invention preferably comprises a
radial initiator that can be activated by means of light provided
that a locally resolved polymerization of the at least one monomer
and, thereby a spatially resolved substrate coating is
required.
[0058] Furthermore, the composition according to the invention
comprises a non-polymerizable porogen, which does not take part in
the polymerization of the at least one monomer accordingly. The
porogen can be mixed with the at least one monomer. However, it is
present in the cured polymer foam as a separate phase. Thereby, the
porogen can consist of an individual substance or of an appropriate
mixture of substances. In the case of the presence of a certain
limit molecular weight, the non-polymerizable porogen carries out a
phase separation, thereby structurally interrupting the
polymerization process, which is also the reason the term phase
former is common. The porogen separates during the polymerization
within the polymer matrix, thereby forming the later cavities of
the polymer foam, meaning the non-polymerizable porogen is
responsible for its nanostructuring. If the polymerization is
completed, the porogen evaporates during the course of time, for
example, via a suitable drying process, thereby leaving behind
cavities in the polymer foam required for the porous structure.
Since the polymerization represents a bulk process, the phase
separation also takes place in bulk, whereby the cavity structure
is homogeneously distributed across the entire volume of the
polymer foam, meaning the latter is uniformly porous. Consequently,
corresponding cavities are also on the surface of the polymer foam,
as is shown in FIG. 1, whereby the polymer foam has the surface
roughness required for the super-repellent effect. Due to the
cavity structure along the entire thickness of the polymer foam,
the surface roughness is continuously renewed in the case of
material erosion, due to abrasion for example.
[0059] The amount of the non-polymerizable porogen in the
composition according to the invention defines the structure,
meaning the porosity of the polymer foam. FIG. 2 shows examples of
polymer foams with different percentages of porogens. In an
embodiment of the present invention, the volume percentage of the
non-polymerizable porogen in the composition is at least or
.gtoreq.5%, preferably at least or .gtoreq.10% and, being
particularly preferred, at least or .gtoreq.20%. The porosity of
the polymer foam can be specifically configured in this way.
[0060] Since the non-polymerizable porogen should be removed after
completing the polymerization, for example, successively by means
of evaporation in a suitable drying process in order to fill the
cavities caused by it with air, such substances must be selected as
porogens, which have a sufficiently high vapor pressure. By means
of this, it is ensured that no elaborate removal of the porogen is
required during a separate method step. Here, for example, gases
come into question as porogens, such as nitrogen, oxygen or carbon
dioxide, however also noble gases, such as argon, as well as other
gases that do not react with the polymer matrix and the components
of the composition. Saturated alcohols, such as ethanol or
isopropanol, can also be used as porogens, which may be cyclic,
such as cyclohexanol for example. Furthermore, ketones, such as
acetone, can be used as porogen, as well as aliphatic and aromatic
ethers, such as diethyl ether. Also, esters, aromatic and aliphatic
hydrocarbons, but also water come into question as porogens.
According to the invention, however, the non-polymerizable porogen
is not limited to the aforementioned substances. The composition
according to the invention may also comprise more than a
non-polymerizable porogen.
[0061] In another aspect, the present invention relates to a method
for producing the highly fluorinated nanostructured polymer foam
according to the invention, comprising the following steps: [0062]
(a) curing the composition defined in the above by supplying heat
or light, optionally on a substrate to be coated; [0063] (b)
optionally dipping the polymer foam obtained in this manner into a
solvent; and [0064] (c) drying the polymer foam treated in this
manner, [0065] wherein the steps (a) to (c) are optionally repeated
at least once.
[0066] With the method according to the invention, the polymer foam
according to the invention described in the above can be produced
under the use of the composition according to the invention,
optionally on a substrate to be coated.
[0067] At step (a) of the method according to the invention, the
composition, comprising at least one monomer, a polymerization
initiator and a non-polymerizable porogen is cured, whereby a
highly fluorinated nanostructured polymer foam is obtained. For
this purpose, the composition according to the invention is
optionally presented or applied onto a substrate to be coated. The
substrate is not subject to any limitations according to the
invention and can therefore be any substrate that comes into
question for a super-repellent coating.
[0068] Depending on the selection of the polymerization initiator,
the curing of the composition, meaning the full polymerization of
the at least one monomer, occurs by supplying heat or light. The
temperature and wavelength required for the thermolytic and
photolytic dissociation naturally depends on the respective
polymerization initiator and can be found in the prior art.
[0069] The curing of the composition at step (a) of the method
according to the invention typically takes place for a duration of
at most or .ltoreq.20 minutes, preferably of at most or .ltoreq.10
minutes and, being particularly preferred, of at most or .ltoreq.1
minute. The curing period at step (a) has no influence on the
thickness of the resulting polymer foam. This ultimately only
depends on the provided amount of composition. However, large
amounts of it principally require a longer curing period. Five
seconds are mentioned as a preferred lower limit for the curing
period at step (a).
[0070] If the super-repellent coating should only be applied at
certain points of a substrate to be coated, the curing of the
composition at step (a) preferably occurs by the supply of light
since, here, curing can take place in a spatially resolved manner
with a higher spatial resolution. Depending on the type of curing,
whether by means of supplying light or by means of supplying heat,
the person skilled in the art will select an appropriate
polymerization initiator for the composition according to the
invention.
[0071] At step (b) of the method according to the invention, the
polymer foam obtained at step (a) is optionally dipped into a
solvent, typically for a duration within a range of 15 seconds to
24 hours. The dipping serves to accelerate the subsequent drying
process at step (c) since, by means of this, non-polymerizable
monomer residues and the porogen can already be dissolved and
roughly removed. Here, any solvent can be used as solvent, in which
the monomer and/or the porogen can be dissolved. For example, an
alcohol can be used, such as isopropanol or a ketone, such as
acetone. The use of the non-polymerizable porogen is also possible,
provided that this has to do with a liquid.
[0072] Then, at step (c) of the method according to the invention,
the polymer foam optionally treated at step (b) is dried, whereby
the non-polymerizable porogen is removed from the cavities of the
polymer foam and those cavities are then filled with air. If the
non-polymerizable porogen used, meaning the one contained in the
composition according to the invention, has a sufficient vapor
pressure and it is furthermore harmless with regard to health and
is environmentally friendly, the drying at step (c) can occur in
ambient air at ambient temperature and under ambient pressure. In
particular, this applies if inert gases or alcohols, such as
ethanol or isopropanol, are used as porogens.
[0073] In an embodiment of the method according to the invention,
the drying at step (c) is carried out at a temperature ranging from
50 to 100.degree. C., preferably at a temperature ranging from 60
to 90.degree. C., and, being particularly preferred, at a
temperature ranging from 70 to 80.degree. C., preferably in a
vacuum furnace. In another embodiment of the method according to
the invention, drying takes place by applying a negative pressure
at ambient temperature. Thereby, the pressure can be within the
range of 10 to 900 mbar. By applying a negative pressure or a
vacuum during drying at step (c), the evaporation process of the
non-polymerizable porogen can be accelerated in this manner. At
step (c), the drying duration typically ranges from 15 seconds to
120 minutes, whereby it can vary according to the amount of the
originally used composition.
[0074] For the drying process at step (c), no open pores are
principally required, meaning the cavities available within the
polymer foam according to the invention can also be present in the
form of closed pores. Thus, at a higher temperature, the
non-polymerizable porogens included in the cavities can be
transported to the surface of the polymer foam, which is
furthermore promoted by applying a negative pressure or a
vacuum.
[0075] Steps (a) to (c) of the method according to the invention
for producing a highly fluorinated nanostructured polymer foam are
optionally repeated at least once for example. Thereby, a polymer
foam created at iteration n is used as a substrate to be coated for
iteration n+1 and re-coated by means of the method according to the
invention.
[0076] Using the method according to the invention, it is possible
to apply the highly fluorinated nanostructured polymer foam in situ
onto a substrate to be coated in order to give this super-repellent
surface properties. As an alternative, a substrate to be coated for
this purpose can also be provided with the finished, meaning cured
and dried, polymer foam. In addition, a cured and dried polymer
foam coating is applied to the substrate to be coated, for example,
by means of a suitable adhesive method. Thereby, an adhesive that
is suitable for fluoropolymers must be selected, for example, the
at least one monomer contained in the composition used, which acts
as a reactive adhesive.
[0077] In another aspect, the present invention relates to the use
of the highly fluorinated nanostructured polymer foam according to
the invention as a super-repellent coating of substrates, as well
as the use of the composition according to the invention for
providing a super-repellent coating of substrates. As mentioned in
the above, according to the invention, the substrate is not subject
to any limitations and can therefore be any substrate that comes
into question for a super-repellent coating. Here, for example,
fabric and textiles of any type, glass, ceramics and metals are
deemed non-limiting as substrates. As applications, mention is
made, for example, of making dirt-repellent fabric and textiles,
the coating of spectacle lenses, not-to-be-frozen protective
screens for the automotive industry, non-corrosive metals in
technical pipe systems, the reduction of raw friction coefficients,
etc.
[0078] The present invention allows for the provision of
super-repellent substrate coatings based on a polymer foam, which,
on the one hand, is intrinsically porous and is intrinsically
highly fluorinated and low-energy on the other, whereby the
super-repellent characteristic is ensured within the entire coating
volume or across the entire thickness of the coating. In comparison
to the aforementioned "top-down approaches" and "Bottom-up
approaches", which require a second method step for one
characteristic or another, the method according to the invention is
neither complex on an equipment-based level nor limited to a
certain effect depth. As has already been explained, the effect
depth is understood as the substrate thickness, on which the
super-repellent effect acts. According to the invention, the effect
depth is identical to the effective overall thickness of the
polymer-foam coating. Thus, it is not a surface but a volume
effect. Only when the entire coating is fully removed is the
super-repellent effect lost. A slight abrasion of the surface
regenerates the effect more than it is destroyed. Due to this
self-regeneration, the polymer foam according to the invention does
not require any elaborate treatment in the case of an abrasion,
such as an inclusion of fluorination reagents into deeper surface
layers, described by Jin et al, (Applied Materials & Interfaces
2013, 5, 485-488).
[0079] In FIG. 3, the polymer foam according to the invention is
compared with the "top-down approach" as well as the "bottom-up
approach". Essentially, both latter-mentioned methods have the
disadvantage that thy create effect depths of 10 to 100 .mu.m in
very abrasion-sensitive substrates. If the super-repellent effect
is initially strongly pronounced on the processed surface, this
effect can be destroyed due to simple abrasion, for example, due to
scratching with a fingernail. The reason for this lies in the fact
that a layer in the thickness of the effect depth is eroded by
abrasion, whereby the underlying layer is uncovered. This layer
generally has a maximum of one of the two characteristics required
for the production of super-repellent surfaces: either the
roughness via micro- or nanostructuring or the low free surface
energy via a high degree of fluorination. Since the polymer foam
according to the invention inherently comprises both
characteristics--high degree of fluorination and nanostructuring
throughout the entire volume--a mechanical abrasion cannot destroy
the effect. Due to material erosion, a coating lying under the
surface is uncovered, which has precisely the same characteristics
as the eroded surface layer. The super-repellent effect is thereby
not lost; therefore, it is abrasion-resistant.
[0080] Furthermore, the present invention has the following
advantages with relation to the prior art:
[0081] No selective etching is necessary. This is of particular
advantage with regard to all methods that initially produce a
substrate, which consists of two materials, which create a suitable
phase separation. Then, in a next step, one of the two materials is
selectively removed. Above all, it is disadvantageous that the
selective removal must take place after coating and thereby the
substrate to be coated must be treated, for example, in a dipping
bath. The method according to the invention does not have this
disadvantage.
[0082] Furthermore, no handling of nanoparticles is necessary. This
is beneficial particularly in terms of operational safety.
Nanoparticles must be processed thus in suitable environment and
using suitable breathing protection devices. If fluorinated
nanoparticles are used, there is an additional risk due to the
fluorination. Nanoparticles are respirable and it can be assumed
that fluorinated nanoparticles represent a significant health
burden. The method according to the invention is manageable without
handling nanoparticles, thereby being safer to use and safer during
operation.
[0083] In comparison to methods where a nanostructured surface is
applied to the surface at a first step by means of a separation
process and, at a second step, it is fluorinated (as an
alternative, the separation of fluorinated nanostructured
particles, wires, etc. is possible), the method according to the
invention creates coatings with significantly elevated mechanical
stability. This is justified in the fact that, within the method
according to the invention, the effect depth extends to the entire
thickness and is not limited to the penetration depth of the
fluorination. Furthermore, a chemically cross-linked polymer in
accordance with an embodiment of the present invention creates a
significantly more stable coating on a mechanical level than the
loose group of nanostructures in the form of bulk fillings or the
like can achieve.
[0084] The method according to the invention principally allows for
the free configuration of porosity, whereby coatings can be
produced, which range from hydrophilic/oleophilic to
hydrophobic/oleophilic, all the way to
super-hydrophobic/super-oleophobic.
[0085] By means of the free configurability of he porosity, it is
possible to produce substrate coatings, the porosity of which is so
fine that the coating becomes optically transparent. This is a
decisive advantage with regard to all those methods known within
the prior art, which only provide whitish or opaque coatings.
[0086] In comparison to many methods known within the prior art,
the method according to the invention only requires a small level
of equipment-based effort, whereby the latter does not remain
limited to laboratory use but is also suitable for large-scale
application. This is a decisive advantage with relation to many
other methods, which can only be carried out within a controlled
laboratory environment. In particular, no elaborate thermal
after-treatment is required, in particular, no high temperature
process as is the case with calcination for example. Thus, the
method according to the invention is also furthermore suitable for
coating substrates that are thermally sensitive, such as fabrics
and textiles.
[0087] The composition according to the invention favorably
comprises no solids mixed in. By means of this, formulations are
possible, which do not deposit or separate following long storage
periods.
[0088] In contrast to many approaches described in the prior art,
the method according to the invention allows for the
super-repellent coating of a surface with almost any dimensions
whatsoever, since the polymerization can be carried out very
quickly and extensively.
[0089] Due to the freely configurable chemical cross-linking of the
polymer foam produced in situ according to the invention, its
chemical and mechanical resistance is significantly higher than is
the case with highly fluorinated thermoplastics, which are very
soft due to the lack of cross-linking and are therefore prone to an
elevated level of material erosion due to abrasion.
EXAMPLES
[0090] The following examples are used as a further explanation of
the present invention without being limited to it.
Example 1
[0091] 0.214 mmol of a commercially available diacrylate derivative
(Fluorolink MD 700) were mixed with 0.243 mmol 1H,1H,2H,2H
perfluorooctanol, 0.449 mmol cyclohexanol and 0.006 mmol
2,2-dimethoxy-2-phenylacetophenone. The composition was cured under
UV light (370 nm) for a duration of 2 min. The polymer foam
obtained in this manner was dipped in isopropanol for 16 hours and
then dried in an oven for 80.degree. C. for one hour.
[0092] The polymer foam was super-hydrophobic, transparent with a
slight turbidity and had a contact angle of 138.degree. for
dimethyl sulfoxide. Furthermore, the polymer foam between 400 and
800 nm had an optical transmission of 58.2% to 91.9% at a thickness
of 0.25 mm.
[0093] As a reference, the non-porous form of the polymer foam had
a free surface energy of 17.4 mN/m at a density of 1.692 g/mL
(+/-0.112 g/mL, three measurements). In contrast, the porous form
of the polymer foam had a free surface energy of 2.939 mN/m at a
density of 1.660 g/mL (+/-0.030 g/mL, three measurements). The
polymer foam had a fluorine contact of 47.6 mol %, wherein 0% of
this was present as CF.sub.3 groups and 100% of this was present as
CF.sub.2 groups. The density difference of the polymer foam with
reference to the unfoamed bulk polymer was 1.9%. By means of
scanning electron microscope images, it was verified that the
cavities of the porous form of the polymer foam had dimensions of
less than 1 .mu.m.
[0094] The polymer foam obtained from Example 1 is shown in FIG. 4
to FIG. 6. FIG. 4 shows the polymer foam when dipping into water.
The high contact angle with regard to water and the resulting
strong bulging of the water surface can be seen. The polymer foam
keeps a silvery-reflecting visible air layer on its surface under
water. As has been already mentioned in the above, that air layer
is required in order to ensure the three-phase system required for
the super-repellent system. FIG. 5 shows the polymer foam with
regard to its super-repellent characteristic with relation to water
and oils, which can be traced back to the combination of its
surface roughness due to nanostructuring and its low free surface
energy due to the high degree of fluorination. FIG. 6 shows the
polymer foam from example 1 with a nanoscale porosity which is so
low that the substrate appears to be optically clear. The high
level of optical transparency is evident both in the top view as
well as in the spectroscopic analysis.
Example 2
[0095] A monomer mixture, consisting of 1.520 mmol of
2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctylmethacrylate and
0.787 mmol of 2,2,3,3,4,4,5,5-octafluorohexyldimethacrylate was
mixed with 1.299 mmol of cyclohexanol, 2.049 mmol of
1H,1H,2H,2H-perfluorooctanol, 0.086 mmol of
2,2-dimethoxy-2-phenylacetophenone and 0.681 mmol of acetone. The
composition was cured under UV light (220 to 400 nm) for a duration
of 5 min. The polymer foam obtained in this manner was dipped in
isopropanol for one hour and then dried in an oven at 80.degree. C.
for one hour.
[0096] The polymer foam was super-hydrophobic, optically clear and
had a contact angle of 102.degree. for dimethyl sulfoxide.
Furthermore, the polymer foam between 400 and 800 nm had an optical
transmission of 79.9% to 91.4% at a thickness of 0.25 mm.
[0097] As a reference, the non-porous form of the polymer foam had
a free surface energy of 17.47 mN/m at a density of 1.654 g/mL
(+/-0.058 g/mL, three measurements). In contrast, the porous form
of the polymer foam had a free surface energy of 2.140 mN/m at a
density of 1.516 g/mL (+/-0.003 g/mL, three measurements). The
polymer foam had a fluorine content of 53.4 mol %, wherein 20% of
this was present as CF.sub.3 groups and 80% of this was present as
CF.sub.2 groups. The density difference of the polymer foam with
reference to the unfoamed bulk polymer was 8.3%. By means of
scanning electron microscope images, it was verified that the
cavities of the porous from of the polymer foam had dimensions of
less than 1 .mu.m.
* * * * *