NANOPOROUS MATERIALS. MANUFACTURE OF NANOPOROUS MATERIALSAND
APPLICATIONS OF NANOPOROUS MATERIALS
BACKGROUND TO THE INVENTION
Field of the invention
The present invention relates to nanoporous materials, methods for the manufacture of
nanoporous materials and applications of nanoporous materials. Such materials have
utility particularly, but not exclusively, in separation technology, e.g. in separation
membranes and the preparation of high surface area electrodes for energy generation
and energy storage (e.g. battery and/or supercapacitor) technology.
Related art
Block copolymers are a class of macromolecules comprising two or more chemically
distinct polymer blocks. It is conventional to refer to each polymer block as A, B, C, etc.
Hillmyer (2005) [Marc A. Hillmyer "Nanoporous Materials from Block Copolymer
Precursors" Adv Polym Sci (2005) 190: 137-1 8 1] sets out a review of the formation of
nanoporous materials using block copolymer precursors. Such nanoporous materials
are of interest for use as nanolithographic masks, separation membranes, interlayer
dielectrics and nanomaterial templates. Many synthetic techniques are known for
generating AB diblock, ABA triblock and ABC triblock copolymers.
In, for example, linear AB diblock copolymers, four equilibrium morphologies have been
identified: lamellar, cylindrical, bicontinuous gyroid and spherical. The morphology
formed depends on factors including the relative volume fraction (or number fraction of
monomeric units) of each block, and how unfavourable it is for the distinct blocks to mix.
Hillmyer (2005) reviews work on the preparation of ordered block copolymer materials
and the subsequent selective etching of the minority component in order to form
nanoporous materials. For example, the formation of a cylindrical morphology, and the
subsequent etching of the cylinder-forming phase, results in the formation of nanoscopic
channels through a matrix material (the matrix material being the continuous phase
which surrounded the cylinder-forming phase).
Hillmyer (2005) points out that since there is a wide range of block copolymer structures
available, nanoporous materials with a wide range of tunable properties can be imagined.
However, he also points out that there are two key requirements for preparing
nanoporous materials from ordered block copolymers: (i) the etchable material must be
physically accessible to the solvent, and (ii) the matrix material must be able to support
the resultant nanoporous structure (ii) is typically achieved by cross-linking the matrix
material.
Various block copolymer systems are discussed by Hillmyer (2005), including:
PS-PBD: polystyrene-polybutadiene
PS-PI : polystyrene-polyisoprene
PS-PBD: polystyrene-polybutadiene
PS-PEO: polystyrene-poly(ethylene oxide)
PPS-PI-PPS: poly(4-vinylphenyl-dimethyl-2-propoxysilane)-_>polyisoprene-£>-poly(4-
vinylphenyl-dimethyl-2-propoxysilane)
PtBA-PCEMA: poly(t-butylacrylate)-b-poly(2-cinnamoylethyl methacrylate)
PS-PMMA: polystyrene-b-poly(methyl methacrylate)
PS-PLA: polystyrene-polylactide
PI-PLA: polyisoprene-polylactide
PCHE-PLA: polycyclohexylethylene-polylactide
PaMS-PHOST: poly(a-methyl styrene)-6-poly(4-hydroxy styrene)
PS-PFMA: polystyrene-b-poly(perfluorooctylethyl methacrylate)
PI-PCEMA-PtBA: polyisoprene-/>poly(2-cinnamoylethyl methacrylate)-6-poly(tbutylacrylate)
PS-PVP: polystyrene-b-poly-4-vinylpyridine
P(PMDSS)-PI-P(PMDSS): poly(pentamethyldisilylstyrene)-o-polyisoprene-opoly(
pentamethyldisilylstyrene)
PS-PDMS: polystyrene-polydimethylsiloxane
Lee et al (1989) [Lee J-S, Hirao A, Nakahama S "Polymerization of monomers containing
functional silyl groups. 7. Porous membranes with controlled microstructures" (1989)
Macromolecules 22:2602] report the preparation of porous membranes formed from
PPS-PI-PPS. Lamellar, cylindrical and spherical morphologies were obtained. When the
PI was degraded and removed, the lamellar and cylindrical morphology membranes had
a high degree of open porosity, as measured by adsorption of nitrogen (using the BET
(Brunauer-Emmett-Teller) method). For the spherical morphology, although the PI could
be degraded and removed, the pores are characterised by Lee et al (1989) as closed,
because substantially no nitrogen adsorption was observed.
Hillmyer (2005) also refers to other work on spherical morphology block copolymers, this
time in bulk form, in which the spherical phase was removed using an anhydrous HF etch.
The material studied was PS-PDMS. The byproducts of the etching procedure are
volatile and can be removed by evacuation, but the remaining spherical pores were
closed.
It is also known to blend block copolymers with homopolymers, e.g. PS-PI with PS
homopolymer. Hillmyer (2005) reviewed work on this material to form a bicontinuous
gyroid phase. Such a morphology is of interest because orientation of the morphology is
not required in order to form separation membranes.
Peinemann et al (2007) [K. V. Peinemann, V. Abetz, P. F.W. Simon "Asymmetric
superstructure formed in a block copolymer via phase separation" Nature Materials Vol. 6
December 2007. pp. 992-996] disclose work on the formation of isoporous membranes
using PS-PVP diblock copolymer. PVP is a hydrophilic polymer that dissolves in
dimethyl formamide (DMF), lower alcohols and aqueous mineralic acids. PS is a
hydrophobic polymer that dissolves in a number of organic solvents such as toluene,
tetrahydrofuran (THF) or chloroform. PS-PVP diblock copolymers are strongly
segregated and their morphology is controlled by the block ratio. A hexagonally packed
array of PCP cylinders can be expected for a volume fraction of PVP in the range 0. to
0.31 . Peinemann et al (2007) therefore studied PS-PVP with 15 wt% PVP.
Crossland et al (2009) [Edward J.W. Crossland, Marleen Kamperman.Mihaela
Nedelcu.Caterina Ducati, Ulrich Wiesner, Detlef-M. Smilgies, Gilman E. S. Toombes,
Marc A. Hillmyer, Sabine Ludwigs, Ullrich Steiner, and Henry J. Snaith "A Bicontinuous
Double Gyroid Hybrid Solar Cell" Nano Lett., Vol. 9, No. 8, 2009, pp. 2807-2812] disclose
the use of a porous structure formed from double gyroid phase morphology diblock
copolymer to form a solar cell. Ti0 2 was deposited in the pores. The remaining matrix
material from the diblock copolymer is then removed to leave a freestanding network of
Ti0 2. An organic semiconductor material is then infiltrated into the pores in the Ti0 2
network to form a bicontinuous heterojunction solar cell architecture.
Li et al (2010) [Xianfeng Li, Charles-Andre Fustin, Nathalie Lefevre, Jean-Francois Gohy,
Steven De Feyter, Jeremie De Baerdemaeker, Werner Eggere and Ivo F. J. Vankelecom
Ordered nanoporous membranes based on diblock copolymers with high chemical
stability and tunable separation properties" J. Mater. Chem., 2010, 20, 4333-4339] note
that it is known to form porous membranes by selective removal of the oriented
cylindrical phase from diblock copolymer films. However, their work addresses the
problem of subsequently transferring the porous membrane to a porous support for
practical use of the membrane in separation techniques. PS-PEO diblock copolymer
with added PAA (poly(acrylic acid)) is deposited onto a porous ceramic (alumina) support
by spin coating, forming a layer over the ceramic support. By careful control of the
composition, a cylindrical array of PEO-PAA is formed in the PS matrix. Cross-linking of
the diblock copolymer is achieved by UV exposure.
US-A-5,948,470 discloses the formation of a thin film of PS-PI diblock copolymer in
which PI spheres are formed in a PS matrix. The packing of the spheres (at least in bulk
samples) is body centred cubic. The PI spheres are removed by ozonolysis and the
resultant film used for nanolithography.
SUMMARY OF THE INVENTION
The present inventors have realised that it would be advantageous to provide more
flexible techniques for the formation of nanoporous structures using block copolymer
materials. The present invention has been devised with this preferred object in mind.
Accordingly, in a first aspect, the present invention provides a method of manufacturing a
nanoporous material including the steps:
forming a morphology comprising a three dimensional array of isolated islands in a
continuous matrix, wherein the islands are formed of at least one island component of a
block copolymer and the matrix is formed of at least one matrix component of the block
copolymer; and
forming channels in the matrix between at least some of the islands and selectively
removing the island component to leave the matrix with an array of interconnected pores.
Preferred (or simply optional) features of the invention are set out below. The are
applicable either singly or in any combination to any aspect of the invention, unless the
context demands otherwise.
Preferably the islands are substantially equi-axed. For example, the maximum aspect
ratio of the islands (i.e. the ratio of any two of three orthogonal linear dimensions of the
islands) is preferably 2 or less, more preferably 1.5 or less or 1. 1 or less. Preferably all
of the three orthogonal linear dimensions of the islands meet this limitation, when divided
by either of the other two orthogonal linear dimensions. Most preferably, the islands are
substantially spherical.
Preferably, the islands have an average diameter (or corresponding linear dimension) of
at least 1 nm, more preferably at least 5 n . Preferably, the islands have an average
diameter (or corresponding linear dimension) of at most 100 nm, more preferably at most
50 nm, still more preferably at most 25 nm. The diameter can be measured, for example,
using microscopy, such as SEM or TEM. The distribution of island diameters is
preferably relatively narrow. For example, the standard deviation of island diameters is
preferably at most 10%. More preferably, the standard deviation of island diameters is at
most 5%
Preferably, the three dimensional array of islands is a substantially ordered array. For
example, the islands may adopt an array based on crystalline packing. Typical packing
arrangements may be body centred cubic, face centred cubic or hexagonal close packing.
Some degree of mis-order is permissible in the array, e.g. to accommodate point defects,
line defects and/or interface (surface) defects. A typical interface defect is the boundary
between adjacent domains of different orientation of packing.
Preferably, in the substantially ordered array, there is a regular minimum spacing
between adjacent islands. Where the average diameter of the islands is d , preferably the
regular minimum spacing between adjacent islands is at least 1.5d, more preferably at
least 2d. Preferably the regular minimum spacing between adjacent islands is at most 5d,
more preferably at most 4d. These ranges are preferred in order to balance the need to
produce interconnected porosity and to have a relatively strong matrix that can support
the structure when the islands are removed to form the pores.
It is possible to consider the block copolymer in terms of the number fraction of monomer
units in each block of the copolymer. Preferably, the number fraction of monomer units
in the island component of the block copolymer is at least 5%. It has been found, for
some suitable block copolymer systems, that using a fraction lower than this tends to
lead to islands that are too far apart in the morphology to lead to adequately
interconnected pores. More preferably, the number fraction of monomer units in the
island component of the block copolymer is at least 8%.
Preferably, the number fraction of monomer units in the island component of the block
copolymer is at most 25%. It has been found, for some suitable block copolymer
systems, that using a fraction higher than this tends to lead to the formation of a nonisland
morphology, e.g. cylindrical, gyroid or lamellar morphology. Such morphologies
are not preferred, since the gyroid morphology tends to occur only in a very tight range of
compositions, and the cylindrical and lamellar morphologies produce porosity having a
specific orientation, meaning that the orientation must be controlled in order to provide
porosity of a suitable orientation. More preferably, the number fraction of monomer units
in the island component of the block copolymer is at most 20%.
It is possible to add one or more homopolymers to the diblock copolymer, e.g. in order to
reach a specific desired composition. This can be useful in order to reduce the overall
cost of the starting materials for the composition. The homopolymer additive may be, for
example, formed from the same monomer units as is used for the matrix component of
the morphology. Additionally or alternatively, the homopolymer may be a material that is
miscible in the matrix component. Still further, a homopolymer additive may be formed
from the same monomer units as is used for the island component of the morphology.
Additionally or alternatively, this homopolymer may be a material that is miscible in the
island component.
The inventors have realised that it may be possible to use a number fraction of monomer
units in the island component of the block copolymer that is relatively high, e.g. 30% or
higher. Such a material would typically form a morphology other than the island
morphology. However, by adding one or more homopolymers, it is possible to force the
morphology to the island morphology. For example, where the number fraction of
monomer units of the island component of the block copolymer that is relatively high, it is
possible to add one or more homopolymers that are similar to, or miscible in, the matrix
component of the block copolymer. In that case, it is preferred that the sum of the
number fraction of monomer units of homopolymer for integration with the matrix and the
number fraction of monomer units of matrix component is 75% or higher, more preferably
80% or higher. This approach is advantageous, since careful tailoring of the specific
composition of a pure diblock copolymer can be expensive, whereas it is relatively
inexpensive to add a homopolymer to a readily available diblock copolymer composition.
Also, the inventors have realised that it may be possible to use a number fraction of
monomer units in the island component of the block copolymer that is relatively low, e.g.
10% or lower or even 8% or lower. Such a material would typically form a morphology
having islands that are too far apart. However, by adding one or more homopolymers, it
is possible to force the morphology to an island morphology having suitable spacing. For
example, where the number fraction of monomer units in the island component of the
block copolymer is relatively low, it is possible to add one or more homopolymers that are
similar to, or miscible in, the island component of the block copolymer. For example,
where the number fraction of monomer units of the matrix component of the block
copolymer that is relatively high, it is possible to add one or more homopolymers that are
similar to, or miscible in, the island component of the block copolymer. In that case, it is
preferred that the sum of the number fraction of monomer units of homopolymer for
integration with the islands and the number fraction of monomer units of island
component is 8% or higher, more preferably 10% or higher.
Preferably, where a homopolymer is added, e.g. for integration with the matrix, the
homopolymer has a molecular weight which is no larger than the molecular weight of the
matrix component of the copolymer. Furthermore, it is preferred that the homopolymer
has a molecular weight which is no smaller than one fifth of the molecular weight of the
matrix component of the copolymer. Additionally or alternatively, where a homopolymer
is added, e.g. for integration with the islands, the homopolymer has a molecular weight
which is no larger than the molecular weight of the island component of the copolymer.
Furthermore, it is preferred that the homopolymer has a molecular weight which is no
smaller than one fifth of the molecular weight of the island component of the copolymer.
The advantage of these techniques is that the range of suitable starting materials is
widened, and thus the cost of producing a required morphology can be reduced. A
suitable number fraction (or volume fraction, which is equivalent to number fraction when
the densities are equal) of homopolymer can be calculated based on the total number
fraction of monomer units including those contributed by the additional homopolymer. As
the skilled person easily understands, it is straightforward to deduce how much of
homopolymer must be added in order to change the overall fraction of different
components in the mixture, this being dependent on the densities of the block
copolymer(s) and the homopolymer(s), number of components, etc. Note that the
different morphologies may be more precisely represented in a general way using
volume (as supposed to weight or number) fractions.
The morphology may be developed by thermal treatment. For example, a suitable
thermal treatment may be heating of the material to at least 100°C. More preferably, the
material is heated to at least 150°C, or at least 200°C. A particularly suitable
temperature is about 230°C. Preferably, the heat treatment is carried out in an inert
atmosphere, e.g. under nitrogen.
Preferably, the matrix is treated by cross-linking. Preferably, cross-linking is achieved by
irradiation. Suitable irradiation includes electromagnetic irradiation, e.g. using UV
radiation. The degree of cross linking of the matrix may vary through the thickness of the
matrix. It is speculated that such a variation in cross linking density, which affects the
local stiffness of the matrix material, may assist in the development of the preferred
nanoporous structure of the material. Cross-linking of the matrix allows the matrix to be
substantially self-supporting when the island component is removed.
Preferably, the islands are also subject to the same treatment as causes cross linking of
the matrix. However, preferably the islands degrade (rather than cross-link) in response
to the cross-linking treatment applied to the matrix. This is beneficial since is provides
simultaneous progress towards the product in a single step. Alternatively, the islands
may be subjected to a subsequent degradation step after the cross-linking of the matrix.
It is preferred that the degradation results in the polymeric chains of the island
component being broken into lower molecular weight fragments.
Preferably, the degraded islands are removed using a washing fluid. For example, a
solvent of the degraded island material may be used. In the case where the island
material comprises PM A, preferably the washing fluid comprises acetic acid. This
washing fluid may also be suitable for other island component compositions.
Preferably, the material is subjected to a gaseous oxidising agent during cross-linking of
the matrix. It is considered, without wishing to be bound by theory, that the gaseous
oxidising agent can have the effect of degrading at least the island component. It is also
considered, again without wishing to be bound by theory, that the gaseous oxidising
agent can have the effect of at least partially degrading the matrix component, although it
is not preferred. Degradation of the island component makes subsequent removal of the
island component more easy. This mode of degradation is also considered to have a
significant effect in the development of the morphology of the final, porous structure of
the device (discussed in more detail below), although the precise mechanism for this is
not clear at the time of writing .
It is considered that a gaseous oxidising agent is preferred, e.g. compared to a liquid (or
other non-gaseous fluid) oxidising agent, in order adequately to diffuse through the
matrix phase, particularly when the matrix phase is solid (e.g. having been cross-linked
already) or is in the process of solidifying.
The gaseous oxidising agent is preferably ozone (i.e. 0 3) . Conveniently, ozone can be
formed in situ by action of UV radiation on oxygen, e.g. oxygen in air. UV curing is also a
preferred route to achieve cross linking of the matrix phase. Therefore performing the
cross linking using UV radiation in an oxygen-containing gas such as air can provide the
additional benefits set out above. It is of further benefit to carry out the cross linking in a
gas containing a concentration of oxygen higher than the concentration of oxygen in
atmospheric air (20.9 vol%). Alternatively, a separate gaseous oxidising agent can be
supplied, such as ozone from a different source, or sulphur (e.g. S3) , or chlorine or
fluorine.
The present inventors have found that the product of the preferred embodiments of the
method tends to have a characteristic structure, in the form of alternating layers of
relatively high and relative low density, the higher density layers having an array of
penetrating pores that communicate with the lower density layers. The inventors realise
that such a structure is of interest independently to the process by which it is formed.
However, it is expressly mentioned here that the preferred methods according to the first
aspect have as their aim the formation of the desired nanoporous material structures
defined with respect to the second aspect below.
Thus, preferably the method preferably includes the step of forming a nanoporous
material having a plurality of lamellae, each lamella having an array of pores penetrating
therethrough. It is preferred that adjacent lamellae are spaced apart by an intervening
spacing layer. The spacing layer may comprise an array of spacing elements integrally
formed with and extending between the adjacent lamellae. The spacing layer may have
interconnected porosity extending within the spacing layer. Thus, the required array of
interconnected pores may be provided by this structure. It is considered here that at
least some of the channels formed between the islands develop into the pores
penetrating through the lamellae.
According to a second preferred aspect, the present invention provides a nanoporous
material having a plurality of lamellae, each lamella having an array of pores penetrating
therethrough, adjacent lamellae being spaced apart by an intervening spacing layer
wherein the spacing layer comprises an array of spacing elements integrally formed with
and extending between the adjacent lamellae, the spacing layer having interconnected
porosity extending within the spacing layer.
Preferred/optional features of the first and second aspect are set out below. These are
combinable singly or in any combination with any aspect of the invention, unless the
context demands otherwise.
The porosity of the spacing layer is interconnected typically in the sense that, within the
spacing layer, the porosity extends between adjacent spacing elements. This allows
each pore of one lamella to have multiple communication paths to a pore of the adjacent
lamella and/or to the far side of the adjacent lamella. This is advantageous, since it
allows the material to function efficiently as a filter medium for example, in which
blockage of one pore in one of the lamellae can be easily bypassed. Therefore the
nanoporous material is bicontinuous in the sense that it consists of a matrix material that
is bicontinuous with void (interconnected porosity).
Preferably, the degree of porosity of the spacing layer is greater than that of the lamellae.
For example, if a notional plane is taken within the plane of a lamella and if a notional
plane is taken parallel to this but within the spacing layer, the degree of porosity of each
can be estimated by considering the ratio of pore area to the total considered area of the
plane. The degree of porosity of the spacing layer on this basis may be at least 1.5 times
the degree of porosity of the lamella. More preferably, the degree of porosity of the
spacing layer on this basis may be at least 2 times, still more preferably 3 times, 4 times
or 5 times the degree of porosity of the lamella. In this consideration, the plane within the
spacing layer that provides the maximum degree of porosity may be considered. This is
typically located about halfway between adjacent lamellae.
On the same basis as explained in the paragraph above, preferably the degree of
porosity of the spacing layer is at least 30%. More preferably, the degree of porosity of
the spacing layer is at least 40%, at least 50%, at least 60%, at least 70% or at least 80%.
In some embodiments, the degree of porosity can be controlled even after formation of
the nanoporous material, as explained in more detail below.
On the same basis as explained in the two paragraphs above, preferably the degree of
porosity of the lamella is at least 1%. More preferably, the degree of porosity of the
lamella is at least 2%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, at
least 8%, at least 9%, or at least 10%. The degree of porosity of the lamella may be at
most 30%, more preferably at most 25% or at most 20%.
Preferably, the nanoporous material is formed as a film. The nanoporous material may
be formed on a substrate, but this is not essential. Preferably, the thickness of the
nanoporous material (e.g. the thickness of the film) is at least 40 nm. Where the
microstructure is formed due to the formation of isolated islands in a block copolymer, it
is considered that this lower limit for the thickness is required in order to provide the
required three dimensional array of isolated islands in a continuous matrix. The
thickness of the nanoporous material is preferably at least 50 nm, more preferably at
least 60 nm, more preferably at least 70 nm, more preferably at least 80 nm, more
preferably at least 90 nm, more preferably at least 100 nm, more preferably at least
150 nm, more preferably at least 200 nm, more preferably at least 250 nm, more
preferably at least 300 nm.
There is no particular upper limitation on the thickness of the nanoporous material. The
lamellae tend to form parallel to the surface of the material. The thickness of the
nanoporous material may be limited by the mechanism for cross linking of the matrix
component. Where cross linking is carried out using UV irradiation, the depth of
penetration of the UV radiation may limit the thickness of the nanoporous material to
10 mm. Such a material may be formed by UV irradiation from both sides. For many
applications, the thickness of the nanoporous material may be at most 5 mm, more
preferably at most 1 mm, more preferably at most 500 pm, more preferably at most
400 m, more preferably at most 300 m, more preferably at most 200 pm, more
preferably at most 100 pm, more preferably at most 50 pm, more preferably at most
40 pm, more preferably at most 30 pm, more preferably at most 20 pm, more preferably
at most 10 pm, more preferably at most 5 pm.
It is expressly stated here that a suitable range for the thickness of the nanoporous
material may be formed using any combination of the listed preferred lower limits for the
thickness with any of the listed preferred upper limits for the thickness.
It is preferred that the nanoporous material of the second aspect is a polymeric material,
more preferably a cross linked polymeric material. For example, the nanoporous
material may be formed of a matrix component of a block copolymer material.
In the method according to the first aspect, it is preferred that the block copolymer
material is formed into a film or sheet by deposition from a solution. For example,
suitable films can be spin-cast. In the case of PS- -RMMA, a suitable solvent is toluene.
The block copolymer material, before removal of the island component, may have a
thickness in a range formed using any combination of the listed preferred lower limits for
the thickness of the nanoporous material with any of the listed preferred upper limits for
the thickness of the nanoporous material. However, it should be mentioned here that in
preferred embodiments of the invention, the development of the nanoporous material
from the island morphology block copolymer material tends to result in an increase in
thickness of nanoporous material compared with the thickness of the block copolymer
material. This increase in thickness may be at least 5%, for example, more preferably at
least 10%, at least 15%, at least 20% or at least 25%. In one sense, therefore, the
nanoporous material can be considered to be an expanded form of the matrix material of
the block copolymer material.
The average thickness of the lamella in the structure may be at least 10 nm, more
preferably at least 20 nm, more preferably at least 30 nm, more preferably at least 40 nm
and more preferably at least 50 nm. The average thickness of the lamella in the
structure may be at most 500 nm, more preferably at most 450 nm, more preferably at
most 400 nm, more preferably at most 350 nm, more preferably at most 300 nm, more
preferably at most 250 nm, more preferably at most 200 nm. It is expressly stated here
that a suitable range for the thickness of the lamella in the structure may be formed using
any combination of the listed preferred lower limits for the thickness with any of the listed
preferred upper limits for the thickness.
The lamellae in the nanoporous structure may be provided at a characteristic periodic
spacing This characteristic periodic spacing may be at least 20 nm, more preferably at
least 40 nm, more preferably at least 60 nm, more preferably at least 80 nm and more
preferably at least 00 nm. The characteristic periodic spacing of the lamella in the
structure may be at most 1000 nm, more preferably at most 900 nm, more preferably at
most 800 nm, more preferably at most 700 nm, more preferably at most 600 nm, more
preferably at most 500 nm, more preferably at most 400 nm. It is expressly stated here
that a suitable range for the spacing of the lamella in the structure may be formed using
any combination of the listed preferred lower limits for the spacing with any of the listed
preferred upper limits for the spacing.
The spacing layers in the nanoporous structure similarly may have an average thickness
of at least 10 nm, more preferably at least 20 nm, more preferably at least 30 nm, more
preferably at least 40 nm and more preferably at least 50 nm. The average thickness of
the spacing layers in the structure may be at most 500 nm, more preferably at most
450 nm, more preferably at most 400 nm, more preferably at most 350 nm, more
preferably at most 300 nm, more preferably at most 250 nm, more preferably at most
200 nm. It is expressly stated here that a suitable range for the thickness of the spacing
layers in the structure may be formed using any combination of the listed preferred lower
limits for the thickness with any of the listed preferred upper limits for the thickness.
The average thickness of the spacing layers may correspond to the height of the spacing
elements.
The spacing elements are preferably substantially columnar in structure. More precisely,
it is preferred that the spacing elements are substantially catenoidal in structure, in the
sense that they are wider close to the lamellar and have a narrowest portion towards the
centre of the spacing layer. The spacing elements themselves are preferably solid (i.e.
not hollow). The spacing elements are typically formed of the same material as the
material of the lamellae.
Preferably, the pores penetrating through the lamellae are arranged in an array
substantially corresponding to the arrangement of isolated islands in the block copolymer
material. The pores penetrating through the lamellae may be arranged in a two
dimensionally close packed array. For example, the array may be a hexagonal array. In
some preferred embodiments, the array has long range order (at least within a domain of
such order). However, in other preferred embodiments, the array may have only short
range order, in the sense that each pore may have about (or exactly) six substantially
nearest neighbour pores, arranged substantially hexagonally around the pore in the
lamella.
Preferably, the average size of the pores extending through the lamellae is at least 1 nm.
It is considered that the average size of these pores depends to an extent on the average
size of the isolated islands in the copolymer material. These pores are typically open
and each end, opening into respective spacing layers. Therefore the pore size is taken
to be the diameter of the pore measured in a direction perpendicular to the planar
direction of the lamella, the diameter being the diameter of a circle of equivalent cross
section area to the cross sectional area of the pore in the plane of the lamella. More
preferably, the average size of the pores extending through the lamellae is at least 2 nm,
still more preferably at least 3 nm, still more preferably at least 4 nm, still more preferably
at least 5 nm, still more preferably at least 6 nm, still more preferably at least 7 nm, still
more preferably at least 8 nm, still more preferably at least 9 nm, and still more
preferably at least 10 nm. The average size of the pores extending through the lamellae
may be at most 200 nm. However, more preferably, the average size of the pores
extending through the lamellae is at most 150 nm, more preferably 100 nm, more
preferably 90 nm, more preferably 80 nm, more preferably 70 nm, more preferably 60 nm,
more preferably 50 nm, more preferably 40 nm. It is expressly stated here that a suitable
range for the average size of the pores extending through the lamellae may be formed
using any combination of the listed preferred lower limits for the pore size with any of the
listed preferred upper limits for the pore size. Preferably, the pore size distribution is
narrow. This is advantageous, since it allows for a high efficiency of the material when
used as a sieve or filter. The pore size distribution may be defined by the standard
deviation of the pore size. Preferably, the standard deviation of the pore size of these
pores is at most 30%. More preferably, the standard deviation of the pore size these
pores is at most 20%.
Preferably, the matrix component of the block copolymer comprises PS. The island
component of the block copolymer may comprise one or more of PMMA, PI and PBD.
PMMA is most preferred.
Preferably, the arrangement of pores through the lamella corresponds to the
arrangement of isolated islands in the block copolymer material. However, preferably the
porosity of the spacing layer does not correspond to the arrangement of isolated islands
in the block copolymer material.
In a third preferred aspect, the present invention provides a use of a nanoporous material
according to the second aspect as a filtration medium in a filtration process.
In a fourth preferred aspect, the present invention provides a filtration membrane,
wherein a filter substrate is provided and a nanoporous material according to the second
aspect is provided on the filter substrate or in the filter substrate.
The filter substrate may, for example, be a relatively coarse filter substrate, having
relatively coarse porosity. The nanoporous material may be provided to plug the porosity
of the filter substrate. The nanoporous material may be provided only in a region of the
filter substrate, e.g. in a region at or close to a surface of the filter substrate. Thus, in
some embodiments, the nanoporous material may not be provided as a film, but instead
as an array of plugging portions in a filter substrate. It has been found that the
nanoporous material can provide very high filtration efficiency in this format.
In a fifth preferred aspect, the present invention provides a process for the manufacture
of a nanoporous medium, including the step of introducing a second material into a
template formed of a nanoporous material according to the second aspect, thereby at
least partially filling the porosity of the template nanoporous material, and then at least
partly removing the template nanoporous material to leave the nanoporous medium or
precursor thereof at least partly formed from the second material.
In a sixth preferred aspect, the present invention provides a process for the manufacture
of a nanoporous medium, including the step of introducing a second material into a
template formed of a nanoporous material formed using the method of the first aspect,
thereby at least partially filling the porosity of the template nanoporous material, and then
at least partly removing the template nanoporous material to leave the nanoporous
medium or precursor thereof at least partly formed from the second material.
Preferably the template nanoporous material is removed using a heat treatment.
Alternatively, the template nanoporous material may be removed by dissolution. This
may be achieved, for example, using a solvent which selectively dissolves the template
nanoporous material in preference to the second material. However, in that case, it is
preferred that the second material has sufficient strength to be self-supporting during and
after the template nanoporous material is removed.
Preferably, the second material is subjected to a heat treatment. This may be the same
heat treatment as that used to remove the template nanoporous material, where the
template nanoporous material is removed by a heat treatment. Alternatively, a further
heat treatment may be applied. In this heat treatment, the second material may react or
sinter in order to increase the strength of the nanoporous medium.
The nanoporous medium may be formed using a wide range of materials, such as metals,
alloys, ceramic materials, cermets, etc. Suitable ceramic materials include inorganic
materials such as metal oxides, e.g. titanium oxide. The nanoporous medium may be
used in an electrode for a supercapacitor or in an electrode for a fuel cell.
The present inventors have realised that their invention may also have applicability in the
field of energy storage, in particular (but not exclusively) in the field of supercapacitors.
A recent review of supercapacitors (also called electrochemical capacitors) is set out by
Simon and Gogotsi [P. Siman and Y Gogotsi "Materials for Electrochemical Capacitors"
Nature Materials Vol. 7 November 2008 p. 845], the content of this review paper being
incorporated herein by reference in its entirety. Therefore, the background, typical
structure and principles of operation of supercapacitors will not be set out here, since it
will be known to the skilled person.
The present inventors have realised that the method of the first aspect and/or the method
of the fifth or sixth aspect may be used in a method of manufacturing an electrode for a
supercapacitor. Similarly, the nanoporous material of the second aspect may be used to
manufacture an electrode for a supercapacitor.
However, it is considered that the invention may have a broader scope than this, since to
the inventors' knowledge, it has not been proposed before to manufacture supercapacitor
electrodes using any porous solid template structure formed using block copolymers.
This includes a porous solid template structure formed using the isolated island approach
set out with respect to the first aspect, but also a porous solid template structure formed
using alternative morphologies, including the gyroid morphology.
Accordingly, in a seventh aspect, the present invention provides a method for the
manufacture of a supercapacitor electrode including forming a polymeric nanoporous
material template using the steps:
forming a morphology comprising a three dimensional arrangement of a first
component of a block copolymer in a continuous matrix formed of at least one
matrix component of the block copolymer; and
selectively removing the first component to leave the matrix with an array of
interconnected pores
and subsequently forming the supercapacitor electrode using the step:
introducing a second material into the template, thereby at least partially filling the
porosity of the template, and
at least partly removing or degrading the template
wherein the second material is a supercapacitor dielectric material or a precursor thereof.
The morphology of the block copolymer may be one in which the first component forms
isolated islands in the matrix, as with respect to the first aspect. In that case, any of the
preferred features of the invention with respect to the first aspect can be applied to the
seventh aspect. However, it is also possible for the morphology of the block copolymer
to be one in which the first component forms an interconnected network isolated in the
matrix. In that case, the first component and the matrix component may be bicontinuous.
For example, the block copolymer may form a gyroid structure. In these circumstances,
any of the preferred features of the invention with respect to the first aspect may be
applied to the seventh aspect (e.g. dimensions, compositions, etc.), with the caveat that
the islands can be considered to be interconnected.
With respect to the structure and/or composition of the polymeric nanoporous material
template, any of the preferred features of the invention with respect to the second aspect
can be applied to the seventh aspect.
Preferably the template is removed using a heat treatment. Alternatively, the template
may be removed by dissolution. This may be achieved, for example, using a solvent
which selectively dissolves the template in preference to the second material. However,
in that case, it is preferred that the second material has sufficient strength to be selfsupporting
during and after the template is removed.
Preferably, the second material is subjected to a heat treatment. This may be the same
heat treatment as that used to remove the template, where the template is removed by a
heat treatment. Alternatively, a further heat treatment may be applied. In this heat
treatment, the second material may react or sinter in order to increase the strength of the
supercapacitor electrode.
Preferably, the supercapacitor dielectric material comprises a material selected from the
group consisting of: Ru0 2, Ir0 2, NiO, CoO and Mn0 2. Of these, most preferred is Mn0 2.
Preferably, the matrix component of the template is pyrolysed in order to form an
interconnected carbon-rich network. This interconnected carbon-rich network is
preferably bicontinuous with the supercapacitor dielectric material. Pyrolysis is
preferably carried out in an inert atmosphere. This is to avoid significant oxidation of the
polymer. For example, a heat treatment in the temperature range 300-500°C is preferred
in order to achieve pyrolysis. The same heat treatment may provide at least partial
sintering of the supercapacitor dielectric material. The resultant structure has a pseudocapacitance
enhancement effect to the double-layer capacitance.
In an eighth aspect, the present invention preferably provides a method of manufacturing
a supercapacitor, including a method of manufacturing a supercapacitor electrode
according to the seventh aspect and using the supercapacitor electrode to form a
supercapacitor, optionally by inserting a separator layer between adjacent supercapacitor
electrode layers in a layered supercapacitor structure.
Further optional features of the invention are set out below.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows schematic microstructural morphologies of block copolymer A-o-B,
depending on FA (the volume fraction of component A).
Fig. 2 schematically shows the formation of interfaces between A and B in the structures
show in Fig. 1, taking the cylindrical morphology as an example here.
Fig. 3 shows an exemplary transmission electron microscopy (TEM) image of a
microstructure of a cylindrical asymmetric block copolymer.
Fig. 4 shows a schematic view of a cylindrical morphology in a continuous matrix.
Fig. 5 shows a schematic view of the matrix of Fig. 4 after removal of the cylindrical
phase.
Fig. 6 shows a schematic view of a spherical micellar morphology in a continuous matrix.
Fig. 7 shows a schematic view of a suggested mechanism for the development of an
arrangement of isolated islands into pores extending through a lamella.
Fig. 8 shows a schematic view of a structure according to an embodiment of the
invention.
Figs. 9-17 show SEM micrographs of various nanoporous materials according to
embodiments of the invention.
Fig. 18 shows a plot of surface pore size against molecular size of the block copolymer.
Figs. 19-26 show SEM micrographs of various nanoporous materials according to
embodiments of the invention.
Fig. 27 shows the effect of duration of UV exposure on the development of expansion
(after washing) of block copolymer various of different molecular weights.
Figs. 28 and 29 show SEM micrographs of a nanoporous material according to an
embodiment after an acetic acid wash (Fig. 28) and after an acetic acid wash and then a
methanol wash (Fig. 29).
Fig. 30 shows the filtration efficiency of a filter substrate used for comparison.
Fig. 3 1 shows the filtration efficiency of a filter medium according to an embodiment of
the invention.
Figs. 32-34 show SEM micrographs of various nanoporous media according to
embodiments of the invention.
Fig. 35 shows a TEM micrograph of an annealed block copolymer film which has
developed an isolated spherical island morphology of PMMA islands in a continuous
matrix of PS.
Fig. 36 shows a graph of the thickness of crosslinked samples based on UV dose,
atmosphere and washing.
Fig. 37 shows a schematic illustration of a possible mechanism for the development of
the porous morphology of the nanoporous material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER
OPTIONAL FEATURES OF THE INVENTION
Nanoporous materials have a recognized application in a broad range of technology
platforms. Specific examples where the preparation of such materials can meet a
technology need are in the development of low-K dielectrics in microelectronics, high
surface area inorganic electrodes for dye-sensitized solar cells and metallized electrodes
for fuel cells, and a number of membrane purification processes. As these examples
show, one typical configuration of suitable materials for these applications is as a thin or
selective layer.
The present inventors consider that the development of nanoporous materials via selfassembly
based methods would be advantageous, in view of the relatively low potential
number of processing steps. Suitable methods preferably have little or no requirement
for vacuum or clean-room conditions, as may otherwise be required for forming suitable
structures by top-down fabrication methods such as lithography.
However, some self-assembly methods have disadvantages. For example, in order to
form a required nanoporous material, it is often necessary to use a very tightly defined
material composition. Furthermore, it is typical that the substrate on which the
nonoporous material is made must be very carefully selected. Still further, the dynamics
by which the nanoporous material are created may be very slow and/or ultimately
unstable. Further still, the scalability of the process itself may be questionable.
The present inventors have devised robust methods to create new nanoporous materials.
In the preferred embodiments, the process uses cross-linking, photodegradation and
chemical washing of initially spherical micellar systems that result in a bicontinuous
matrix of polymers and voids. The inventors demonstrate the value of these nanoporous
materials using the specific applications of water purification and energy generation,
although the present invention is to be understood as not necessarily being limited to
these exemplary applications.
There are considered to be a number of basic requirements for a high-performance
nanoporous material. One is the high degree of accessibility of the pore structures. By
implication, this suggests that the available surface area per unit volume in the material
between void and material should be high, preferably as high as possible. Another is the
intrinsic strength. This is again related to the connectivity of the non-void parts of the
material. From a technological point of view, it is preferred if these materials are
prepared as quickly and as cheaply as possible.
There are a number of key technologies where nanoporous media have been critical in
enhancing functionality. In both fuel cell and dye-sensitized solar cells, this is seen by
increasing the local surface area of the platinized and titanium oxide electrodes.
Reducing pore sizes whilst maintaining a satisfactory continuity from top-to-bottom in
separation membranes will enable the application of these systems for reverse osmosis.
Doing so in a practical way would enhance the viability of membrane separation to
replace conventional, expensive and cumbersome distillation based desalination
technologies with smaller, lower-tech solutions. It is notable that the research impacts of
nanoporous media have relevance to the sustainability of two key resources.
Nanoporous polymeric membranes with good interconnectivity can be produced using a
number of methods that invoke microphase separation. Thus demixing of two polymers
can be dynamically controlled to vary the size scale of pore sizes and the extent of their
connectivity. Another dynamic method that in principal produces connected nanopores is
the so-called breath-figure method via the condensation of water vapor into a drying
polymer film. These materials can then be used as a template for further chemical
transformation of the material. For example the voids can be filled with inorganic or
metal precursors, reduced and sintered to remove the organic components.
There are numerous ways to make a nanoporous inorganic material; for example,
simple cheap nanoporous inorganic materials can be prepared by the sol-gel method and
subsequent sintering. The resulting material can be considered to be a collapsed
anisotropic heap of inorganic spheres (of a chemically controlled dimension and size
distribution). Inevitably such a material contain dead void spaces (i.e. closed pores) that
are completed encapsulated by a surrounding network of spheres. An additional
disadvantage is that the material, during sintering is prone to macroscopic, shrinkageinduced
crack-formation that undermines the mechanical properties of the material, and
in some cases, the ability of the material to self-support itself. The present inventors
consider that using a polymeric template of controlled connectivity to control the way in
which the sol collapses is one way to enhance the accessibility of the void structures.
An advance in the sol-gel technique has been to combine this process with self-assembly
of systems that undergo a phase transformation to a nanoperiodic lattice. Amphiphilic
surfactants such as the Pluronics™ (also known as poloxamers - nonionic triblock
copolymers composed of a central hydrophobic chain of poly(propylene oxide) flanked by
two hydrophilic chains of poly(ethylene oxide)) have been used to create these gel-like
lattices with regular architectures including packed spherical micelles and hexagonally
arranged cylinder phases. By generating the sol within the aqueous phase of the gel,
and subsequently transforming the material, the resulting inorganic monolith structure is
templated by the lyotropic architecture of the surfactant. This concept has been
extended further by utilizing block copolymer that are not necessarily amphiphilic but
which have the advantage of greater flexibility via the polymer chain length and the
monomeric interactions. Several research groups have explored this general concept for
the development of nanoporous organic and inorganic media.
In using block copolymers to template nanoporous structures, some disadvantages are
prominent, for example the material cost. Typically the key volume over which the
nanoporous structure performs its separating function is a few hundred nanometers.
This is as true of membranes formed by Loeb-Sourrijan type phase inversion methods as
block copolymers. The remainder of the membrane material is mainly there to provide a
geometrical and mechanical support. Hence it is preferred to ensure that the selective
portion of the membrane is fully open and accessible from the more macroporous
sections of the membrane. However it is well known that block copolymer (BCP) films,
that might form the selective layer, tend to have one component of the BCP preferentially
wetting the bounding surfaces of the film. Moreover, depending on the particular
architecture of the BCP, its orientation may not be conducive to direct diffusing species
across the membrane. Some attention has been given to creating BCP films with a
cylindrical morphology (later to become cylindrical pores) with an orientation that runs
across the films. This has been achieved by a number of methods involving external
fields or by the dynamic evaporation of selective solvents. However there is a possibility
that such methods can be difficult to scale up and/or control. A more elegant method
makes use of the thermodynamically stable double gyroid phase that is observed in
some block copolymers, in which the two components of such BCP films are intrinsically
connected throughout the film. However the relative polymer compositions and
molecular weight window at which the double gyroid system exists is sensitive and the
architecture has only been noted for a handful of polymer systems. Moreover, timeconsuming
solvent annealing methods found to be the best to achieve the double gyroid
structure. Even in these cases, it is found that the surfaces of the thin films still contain a
wetting layer of one BCP component that can act as a barrier to any prospective porous
properties.
In the preferred embodiments of the present invention, a nanoporous structure is
prepared by the selective crosslinking and/or degradation of the two polymer phases. In
the well studied polystyrene (PS) block copolymer systems with poly(methyl
methacrylate) (PMMA), polyisoprene (PI) and polybutadiene (PBD), this is achieved with
a combination of UV/ozone and subsequent dialysis of the degraded phase. In other
systems, simple hydrolysis of one component is sufficient to create the nanoporous
network.
One of the assumptions that is apparently implicit in much of the reported work in this
area is that UV induced crosslinking and degradation of the BCP leads to creating a
replica of the original BCP architecture. Indeed this is seen experimentally for cylindrical
and lamellar systems of PS-b-P A. In the former case, PMMA cylinders have been
degraded from the PS matrix and subsequently refilled. In the case of perpendicular
lamellae, the degradation of the interlayers of PMMA leads to a collapsed stucture of
cross-linked PS sheets. The assumption, that seems to be implicit in the reported work,
is that the same UV process will have no benefit to a system of PS-PMMA spherical
micelles, with a PMMA minor phase, since the resulting degraded phase, if one can
remove it at all, remains separated from other similar voids. The present inventors have,
however, observed that this assumption is not correct. As explained below, this has
important implications for the manufacture of nanoporous materials.
Block copolymers are able to produce a range of geometrically organized three
dimensional (3D) architectures. The architectures depend mainly on the relative fraction
of each of the polymer phases. The simplest block copolymer, an A-block-B
(abbreviated to A-6-B), is able to produce four distinct microstructural morphologies,
depending on F (the volume fraction of component A). Three of these morphologies are
illustrated in Fig. 1, showing an arrangement of isolated spherical islands of A in a
continuous matrix of B at low values of F . As the value of F increases, the morphology
changes to provide an arrangement of parallel cylinders of A, isolated from each other, in
a continuous matrix of B. At FA = 0,5, in this example, the morphology provides
alternating lamellae of A and B. As F is increased still further, the morphology changes
to provide an arrangement of isolated spherical islands of B in a continuous matrix of A.
Thus, each of these morphologies is non-bicontinuous.
Fig. 2 illustrates the reason for the formation of interfaces between A and B in the
structures show in Fig. 1, taking the cylindrical morphology as an example here. Diblock
copolymer A-block-B prefers to arrange itself (for energetic reasons) to minimise the
interface area between A and B. In the example shown in Fig. 2, cylinders of A are
formed, surrounded by a matrix of B.
Fig. 3 shows an exemplary transmission electron microscopy (TEM) image of a
microstructure of a cylindrical asymmetric block copolymer.
In Figs, 1-3, and in the preferred embodiments of the invention, structure sizes (i.e. the
average diameter of the isolated islands, the average diameter of the isolated cylinders
and the average thickness of the parallel lamellae) are of the order of 10nm-250nm.
There is one morphology which is possible for some block copolymers in which the
components are bicontinuous. This is the gyroid phase. See, for example, Crossland et
al (2009). The bicontinuous architecture is unique in that, if either phase is removed from
the system, the remaining structure can remain self supporting. A phase may be
selectively removed from the gyroid architecture by an appropriate post-process and
chemical washing.
Systems such as PS-PMMA, PS-PBD, PS-PI, can be irradiated by a combination of UV
and ozone, to cross-link the PS phase and simultaneously degrade the other phase.
This other phase is commonly washed out with acetic acid (P MA) or ethanol (PB, PI).
For other systems, such as PFS-PLA, the polylactic acid can be hydrolyzed without the
need to crosslink the PS phase. (Note that PFS is polyfluorostyrene.)
In the examples of the preferred embodiments, samples are irradiated with UV. A
subsequent solvent wash develops a nanoporous self-supporting structure.
In the case of aligned cylinders of (for example) A in a continuous matrix of B, it is
evident that when the matrix of B is crosslinked by irradiation with UV, this irradiation
degrading A, it is possible to wash out the degraded A because the cylinders are
accessible for washing because they extend though the material. However, where the
morphology of the block copolymer has isolated islands of A in a continuous matrix of B,
the islands should be inaccessible to the solvent wash.
Fig. 4 shows an array of cylinders of A in a continuous matrix of B. When the cylinders of
A are removed, the structure of B is retained, as shown in Fig. 5. The same is true when
a gyroid phase is used.
However, in the preferred embodiments of the present invention, there is a complete
change in the final organization of the material to a entirely new type of structure that has
not been reported before.
Fig. 6 shows an array of spherical A in a continuous matrix of B. In the example, A is
PMMA and B is PS. This has not previously been considered an interesting system for
UV/acetic acid treatments. When the structure is treated according to the preferred
embodiments of the invention, the previously-isolated spherical islands are removed and
the resultant structure forms a sheet of B having an array of pores extending through the
sheet. This is schematically illustrated in Fig. 7.
However, in typical embodiments, the initial block copolymer structure has a thickness
significantly greater than the diameter of the isolated spherical islands. Therefore the
initial block copolymer structure accommodates a significant number of layers of isolated
spherical islands. The result observed by the present inventors is that with a multiple
initial array of layers of isolated spherical islands, with not all islands in one layer lying in
register islands in layers underneath, the perforated sheets of B form lamellae separated
by spacer layers providing supporting columns of B. The structure is schematically
illustrated in Fig. 8.
The precise mechanism for the formation of this structure is still unknown. However
(without wishing to be bound by theory), it is possible that the mechanism is a combined
function of (i) expansion of the material starting from the point where the solvent (e.g.
acetic acid) infiltrates the material and (ii) the collapse of the walls between two adjacent
PMMA spheres. However, further detail on these points is described later, with reference
to Fig. 37.
In order to manufacture nanoporous materials according to examples of the invention,
block copolymers of PS-b-PMMA with known molecular weights were spun cast from a
toluene solution to a thickness of 300-400nm on silicon wafers (with a native oxide layer).
Films of thickness of about 50nm, were also transferred, via a glass slide intermediary,
on to carbon coated TEM grids. The films were annealed at 230°C under a nitrogen
atmosphere. The samples were then placed 10cm from a UV source in air before being
rinsed in glacial acetic acid for 10 minutes. The duration of UV exposure was varied
during the experiments. Film thicknesses were measured before and after exposure with
AFM across a scratched section of the film. Microstructural dimensions (e.g. pore size,
lamella thickness, spacing layer thickness) were measured based on SEM observations.
Annealing of the BCP films at 230°C developed the required isolated spherical island
morphology of PMMA islands in a continuous matrix of PS. A TEM micrograph of such a
film is shown in Fig. 35.
After UV exposure and washing with glacial acetic acid, the surface of the polymer films
were found to have a loosely hexagonally packed structure of pores. However, the
cross-sections of these block copolymers, contained a clearly porous interconnected
structure. This is shown in Fig. 9, discussed in more detail below. A similar three
dimensional structure was observed for significantly thicker films (several tens of
microns) that had been solvent cast, annealed and then subject to the same UV
treatment. The sub-layers of the film take the form of parallel lamellae, each having the
form of a continuous but perforated sieve, adjacent lamellae being separated by a
columnar array of PS.
The same structure was observed for different block copolymer molecular weights used
in this study. The difference however in the lamellar spacing was less than might be
expected based only on a consideration of BCP molecular weight.
The inventors have carefully considered possible mechanisms that might be responsible
for forming the microstructure seen in the nanoporous materials. The mechanism is, at
the time of writing, not completely understood, but the formation of the desired
microstructure is readily repeatable. Without wishing to be bound by theory, the
inventors speculate that the cross-linking of the PS (due to UV irradiation) is likely to be
non-uniform, at least during the initial stages of UV exposure, with the upper regions of
the film receiving and absorbing a greater UV flux than the lower regions of the film,
therefore providing a higher crosslinking density in the upper regions of the film, which in
turn leads to a stiffness gradient across the film. The UV also acts to degrade the PMMA
in the spheres. After UV exposure, the film is washed using acetic acid. PMMA
(especially degraded PMMA) is readily soluble in acetic acid, but PS is not. The acetic
acid therefore will cause swelling of the spheres, starting from the upper surface of the
film. It is speculated that there is a chemical potential gradient which drives the acetic
acid downwards through the structure, from sphere to sphere, and the resulting
expansion of the spheres leads to PS material separating the spheres in the vertical
direction to migrate towards the less stiff regions of the film, i.e. downwards. Thus,
spheres may link in the vertical direction. However, the excess PS material available
must be accommodated. Therefore it is speculated that this drives expansion of the
structure, leading to the creation of the spacing layers formed of laterally interconnected
void spaces and vertically extending cylinders of PS. The linked spheres result in the
formation of vertically-extending pores in otherwise continuous sheets of PS. The
inventors expect that further work will elucidate the mechanism further.
The likely mechanism for the development of the characteristic morphology is further
explained with reference to Fig. 37. In this drawing, the material 100 has islands 102
arranged in a periodic arrangement in a matrix 04 of polymer that is undergoing
crosslinking by UV radiation (not shown). The sequence of the formation of the
microstructure is shown based on the removal of the material of the islands 102 in layerby-
layer stages in the drawing, from left to right.
The solvent (acetic acid in many of the examples) diffuses constantly in the matrix
material, entering the islands, in view of the preferential solubility of the island component
in the solvent. This increases the osmotic pressure in the islands. Due to the osmotic
pressure, the thin layer 106 of matrix material separating the top layer of islands from the
exterior of the material becomes perforated, allowing the mixture of solvent and island
component to escape from surface perforations 108 and so relieving the osmotic
pressure.
The solvent continues to diffuse in the matrix material, reaching the second layer of
islands, below the already-perforated islands in the first layer beneath the surface.
Similarly, osmotic pressure builds up in the second layer. However, the thickness of the
layer 110 of matrix material between the islands in the first layer and the islands in the
second layer is greater than for layer 106. Therefore the build up of osmotic pressure
tends to elongate the islands, since this is the only direction in which the islands are able
to expand to relieve the osmotic pressure, gradually reducing the thickness of layer 110
until it is perforated. The perforations link through to the pores (previously islands) in the
first layer, allowing the mixture of the solvent and the island component to escape via the
pores linked with perforations. The result is a layer in the material formed of elongated
pores 112.
The expansion of the elongated pores 112 not only reduces the thickness of the layer
110 of matrix material above, but also the layer 114 of matrix material below.
Accordingly, the expansion of the pore 112 results in a thin layer of matrix material below
the layer of elongated pores. Therefore, when the next layer of islands is subjected to
osmotic swelling, the wall of matrix material between the swelling island and the
elongated pore is thin, and so easily perforated. Therefore this next layer of islands is
not subjected to significant pressure and so does not become elongated. However, the
layer of islands below this is subjected to significant osmotic pressure, and so does
become elongated, as shown in Fig. 37.
The result is the characteristic lamellar structure shown in the SEM micrographs.
The upper surface of the PS-6-PMMA film, prior to degradation, was found to contain
both PS and PMMA components. This was confirmed by staining the thin films with
ruthenium tetroxide prior to SEM observation. This is a somewhat unsurprising result
given that the surface tensions of the two BCP components are similar at the annealing
conditions applied to the film. However, even under lower temperature annealing
conditions where a PMMA wetting layer might exist at the surface, this is expected to be
degraded under any UV exposure.
Measuring the interlayer distance via SEM shows an almost similar layer spacing
independent of the original BCP molecular weight. However the sizes of pores within
each sieve-like layer, measured from the SEM and correlated to TEM, are found to
decrease with molecular weight.
From the cross-sections observed via SEM, the pores within spacing layers appear to be
large with respect to the size of the original isolated spherical islands. It is advantageous
that the size of the pores extending through the lamellae can be controlled by the original
structure and molecular weight of the block copolymer. This brings about immediate
applications of the materials as nanoscale sieves and/or filtration membranes. The fact
that these materials are also cross-linked increase their resilience to mechanical and
chemical attack.
It has previously been assumed that a morphology consisting of one block copolymer
component contained in spheres, separated and isolated, albeit periodically from others,
are not relevant since these spheres can not be accessed when the surrounding matrix is
cross-linked. In fact, it has been shown by the present inventors that the surrounding
matrix can open up to connect the degraded spherical phases. The resulting structure is
by definition, a bicontinuous one. Furthermore, as discussed in more detail below,
typical starting materials have an easy tolerance, allowing the achievement of the desired
microstructure with relative ease. Typically, it can take as little as an hour to prepare
such a bicontinuous porous material, requiring common laboratory equipment. By
comparison, other bicontinuous structures as generated by the gyroid architecture
require some dedicated annealing strategies, as well as being restricted to a particular
composition and molecular weight window. A similar result can be achieved in other
block copolymer combinations such as the PS-PI and PS-PB systems, though these
requires two additional inputs; the removal of the surface wetting layer (e.g. by reactive
ion etching), since a neutral air surface is not observed for these systems, and the
additional use of ozone to enhance the degradation of the poiydiene phases. A particular
advantage of the isolated spherical morphology demonstrated in this work is the
accessibility of the porous structures to the bounding interfaces either with air, or a liquid
or a solid substrate due to the lack of a barrier wetting layer. This means that one nolonger
needs to be concerned with the orientation of the block copolymer close to the
surfaces, as is the case particularly with the cylindrical morphology.
Fig. 9 shows views of a sample made according to a preferred embodiment of the
invention, taken using scanning electron microscopy (SEM). This sample is formed on a
substrate, which is shown at the bottom of the view in Fig. 9. The cross sectional view
shows 4 lamellae of crosslinked PS having pores extending through them, adjacent
lamellae being spaced by relatively lower density spacer layers comprising supporting
columns of crosslinked PS. The inset in Fig. 9 shows a plan view of the top surface,
showing the substantially regular (hexagonal) arrangement of pores through the lamella
at the top surface. The pores through the lamella have a narrow distribution of pore
diameters.
Fig. 10 shows a cross-sectional SEM view of another sample made according to a
preferred embodiment of the invention. Fig. 11 shows a plan view of the sample.
Fig. 12 shown a perspective SEM view of another sample made according to a preferred
embodiment of the invention. In this view, the upper layers of the sample have been
peeled away to reveal the structure of the spacer layer.
Fig. 13-1 7 relate to each other. These show cross-sectional SEM views of samples
made in order to demonstrate the compositional flexibility of the preferred embodiments
of the invention. Each of these samples was made using a PS-PMMA diblock copolymer,
but the molecular weights of the components (PS and PMMA) of the diblock copolymers,
and the total molecular weight (N ) of the diblock copolymers was varied according to
Table 1:
Table 1
Note that + M R A/ I R A
where mPS (=1 04) and mPs (=1 00) are the monomer molecular weights.
Fig. 18 plots the surface pore size of the samples of Figs. 13-1 7 as a function of the total
size of the copolymer (NT = NPS + NPMMA)-
Figs. 13-1 8 demonstrate that the present invention can be made to work within a wide
range of sizes of the copolymer. It is not considered that the thickness of the
nanoporous material depends strongly on the molecular size of the copolymer, since
thickness is likely to depend more strongly on the conditions of deposition of the
copolymer.
The inventors further investigated the effects of the volume fraction of PMMA (FP A)
used in the starting composition, as shown in Table 2, and the results are shown in Figs.
19-22.
Table 2
In these experiments, the pure block copolymer had F PM of 0.28. This is in the
cylindrical regime of the morphology phase diagram for PS- -P A . Therefore, in the
example shown in Fig. 19, the lamellar structure is not seen. FP was varied by
addition of P MA homopolymer (31k). As shown in the sequence of Figs. 20, 2 1 and 22,
the lamellar structure clearly develops as FP is reduced to an appropriate level. Thus,
the preferred structure is clearly available when FPMMA is around 0.20.
Furthermore, the inventors investigated the lower limit for FPMMA in this embodiment
compositional system, using the values for the volume fraction of PMMA (FP A) shown
in Table 3 . The results are shown in Figs. 23-26. The starting material used in Fig. 23
was a 71k-11k PS-PMMA (FPM A = 0.135). For the samples shown in Figs. 24, 25 and 26,
small amounts of PS homopolymer (31k) were added.
Table 3
These results show that the lamellar structure remains available down to a relatively low
level of FP of about 0.08 (Fig. 25). The lamellar structure was not observed at FP A
of 0.055 (Fig. 26).
The UV source in these experiments was characterized as UV (254nm) providing 8 watts,
spaced at 1.5cm distance from the samples. This time and distance and power are
interrelated by standard flux relationships. It was found that there was a critical time
required for the UV exposure, typically about 30 minutes minimum for the samples tested
in this work. It is probable that this threshold of UV exposure depends on various factors,
including the thickness of the film, NT (see below) and the received flux of UV radiation.
After UV exposure, for a time above a threshold time, there is a significant (20-30%)
increase in the overall film thickness after washing with the solvent (acetic acid).
However, this thickness change is not observed before washing with the solvent (acetic
acid). This was checked macroscopically by observing the colour change of the samples
caused by the thickness change. Thus, it was found that developing PS-PMMA
structures that had been underexposed with UV did not lead to the open lamellar
microstuctures of the preferred embodiments.
Furthermore, the inventors have investigated the effect of NT on the time of UV exposure
required in order to develop (after washing) the thickness change indicative of the
transformation to the open lamellar microstuctures of the preferred embodiments. This is
demonstrated by the results presented in Fig. 27. In general, these results show that for
a higher NT, the time required for sufficient UV irradiation increases. Furthermore, it is
found that there is a step change in thickness increase of the film at a threshold time,
dependent on NT. The step change in thickness was found to coincide with the change
in the cross-section structure, from that of a non-porous film to that of the characteristic
lamellar structure seen in Fig. 9, for example.
The present inventors used acetic acid to develop the structures of PS-PMMA. The
inventors have investigated a large number of alternative organic solvents including other
similar acids and alcohols and common solvents for PMMA. Specifically with respect to
the PS-PMMA system, it is considered that acetic acid provides an unusual interaction in
terms of developing the preferred open lamellar microstructure. However, it is possible
that a combination of formic and acetic acid will have a similar effect on the material. It is
considered likely that other solvent(s) will provide similar results for other block
copolymer systems. It should be noted that acetic acid is a common solvent in cleanrooms
and of wide-spread use.
The inventors investigated the effect of methanol on the PS-PMMA system. The results
are shown in Figs. 28 and 29. In Fig. 28, the sample was washed with acetic acid only.
In Fig. 29, the same sample was then washed with methanol. The effect is a contraction
of the structure. It is speculated that the methanol penetrates into the structure and
interacts with the surface of the PS to cause the PS chains to contract, leading to the
observed contraction of the structure. This represents a means of controlling the
average pore dimensions in the system. This was observed for all the pure block
copolymer systems investigated by the inventors.
In order to further elucidate the relevant factors for developing the characteristic porous
morphology, the present inventors have carried out further work, relating the atmosphere
in which the cross linking is carried out.
PS-b-PMMA of a range of molecular weights were exposed to UV irradiation (l =254hhti)
for a range of dosages. The film thicknesses of these samples were noted, and the
structure of the films determined by scanning electron microscopy. The following, same
behaviour was noted for all the block copolymers molecular weights. Below a critical UV
dosage (in air), the UV treatment followed by acetic acid treatment led to no substantial
change in the polymer film thickness and there was no evolved structure of the type
shown in Fig. 9. For samples irradiated in UV within a critical dosage range, there was
an increase in film thickness after the UV/acetic acid treatment. These samples showed
the open porous microstructures. However if the samples were exposed to too much UV,
it was found that the sample thicknesses (post UV/acetic acid) were reduced
monotonically.
The same experiments were then repeated on a new set of samples; however in this
case, the environment was almost entirely nitrogen (98%). It was noted that there was
no change in the film thicknesses before and after the UV/acetic acid treatment, for any
dosage. These details are summarized in Fig. 36, which plots the film thickness against
UV dosage for films before washing and after washing in acetic acid, and cross linked in
air or in nitrogen atmospheres. The material used was PS-b-PMMA (71 k-b-1 1k).
The inventors' conclusion from these results is that the ozone is generated by the intense
UV irradiation in an atmosphere containing oxygen. Ozone (0 3) is known as a highly
reactive, short-lived allotrope of oxygen. It is thought that the ozone has the principal
effect of degrading the PMMA (island) phase; the UV by itself does not appear to do this
to a substantial degree to the PMMA. The effect of UV is primarily to cross-link the PS
phase. However ozone will have the opposite effect of breaking down both polymer
phases. It is thought that this why the, in the long dosage limit, the entire film is broken
down and able to be washed away by the acetic acid.
These results highlight the need in these experiments to perform the UV irradiation in an
oxygenic atmosphere such as air. It is expected that an increased oxygen content has
the effect of accelerating the overall process, reducing the minimum critical UV dosage
and shortening the time scale over which the poration effect can be obtained.
It is considered that other gaseous environments can have the same accelerating effect
as the UV transformed oxygen (to ozone). This could include a sulphurous environment
(S2 to S3) though this is unlikely to be a process that is most preferred in industry (for
safety reasons). However the principal role of ozone in these experiments is as a
powerful, gaseous oxidizing agent. The gaseous nature is considered to be important in
allowing the agent to diffuse into the solid polymer phase. There are other such agents
that could be injected into the atmosphere during UV irradiation that will have a similar
effect, such as chlorine and/or fluorine.
A conventional filtration membrane was used in a dead-end filtration system to separate
a suspension of malachite green dye (500 ppm). The filtration efficiency of this
conventional membrane is shown in Fig. 30. An identical conventional filtration
membrane was infiltrated with a spherical block copolymer system according to an
embodiment of the invention and subsequently treated with UV and washed with acetic
acid. The resulting membrane was used in an identical dead-end filtration system to
separate a suspension of malachite green dye. The results are shown in Fig. 3 1. The
result of adding the copolymer-based nanoporous filtration material was to increase the
filtration efficiency from 80% for the pristine untreated membrane to 99.98% for the
treated membrane. The overall flux of water through the system was not significantly
diminished.
A nanoporous material according to an embodiment of the invention was infiltrated with
an inorganic oxide precursor (a precursor for titanium oxide in this example). The PS
matrix was then removed by thermal treatment. The result is that the nanoporous PS
material is replaced with an inorganic porous structure of titanium oxide. Fig. 32 shows a
cross sectional view of the resultant structure with an inset showing a highly magnified
view of the surface of the structure. Note that this film, nearly 0.5 microns thick, is nearly
transparent. Similarly, Fig. 33 shows a plan view of a resultant structure. Fig. 34 shows
a cross sectional view of the sample of Fig. 33. Such membranes of nanoporous
titanium oxide are important materials for use in organic solar cells.
In another example, it is possible to use the nanoporous PS material to prepare
nanoporous gold films by the electroplating technique. This has been used previously to
grow cobalt nanostructures in the cylindrical voids left from UV-treated PS- -P A films.
A similar methodology is applied here. After electroplating the residual material was
been removed by an oxygen plasma etch and observed with SE . One important fact
about the electrodeposition method is that it is only possible if there is a continuous path
through the nanoporous template to the electrode. Therefore the development of these
gold structures provides evidence that there is not a cross-linked PS wetting layer
nearest the substrate surface. The lack of any such barrier layer is potentially very
important if these nanoporous templates are used subsequently for a number of
applications including electrodes for photovoltaics and solar and fuel cells as well as in
filtration media.
It is possible to draw some conclusions from this work, as follows.
Starting from a pure diblock copolymer structure,A-B where F is preferably between 8
and 20%, and where component B can be cross-linked whilst component A is degraded
and removed, it is possible to produce a nanoporous structure which retains a
bicontinous 3-D architecture. It is considered an important preferred feature here that the
A-B block copolymer should have a morphology consistent with a spherical micellar
architecture prior to treatment. This can also be applied to an A-B-C triblock copolymer
system where A is still the component that is degraded.
Starting from a pure block copolymer, A-B where F for the pure state is preferably
between 8 and 30%, but by the addition of a homopolymer of B it is possible to reduce
the overall value of FA of the mixture to a value preferably between 8 and 20%, it is
possible to produce a nanoporous structure which retains a bicontinous 3-D architecture.
It is preferred that the molecular weight of the B homopolymer is no larger then the
molecular weight of the B portion of the copolymer. It is further preferred that the
molecular weight of the B homopolymer is no smaller than one fifth of the molecular
weight of the B portion of the copolymer. This can also be applied to an A-B-C triblock
copolymer system where A is still the component that is degraded.
The resultant, preferred structures of the embodiments of the invention are characteristic.
They are identified as comprising parallel lamellar sheets that are individually perforated
with holes that are separated from each other by a distance comparable to (e.g. within a
factor of 5 or less of) the initial period of the spheres. The thickness of the lamellar
sheets is also comparable to this value. The lamellar sheets are separated by a distance
that is typically also comparable to this value. These dimensions can be reduced by
subsequent physico-chemical soaking in other solvents.
In the specific case of a PS-PMMA block copolymer system, crosslinking and
degradation is carried out by UV exposure. It is then developed by the use of acetic acid.
This is found to be the best developing solution although formic acid may have similar
but diminished effects. Typically, there is a minimum time for the UV exposure step,
beyond which it is possible to develop the preferred nanoporous structures. This critical
time will depend on the distance from substrate to UV source, dosage time and UV
source power (and material thickness for very thick samples). Also this critical time
increases with the overall size of the block copolymer. It is considered that a gaseous
oxidising agent such as ozone may have a critical role to play in the full development of
the porous morphology, such ozone being conveniently generated by the interaction of
UV radiation with oxygen in the atmosphere surrounding the material.
The nanoporous holes in the lamellar sheets are typically of sizes from about 10nm
upwards in a manner that increases with the size of the initial A component of the
untreated block copolymer. The spacing between the holes increases from values of
typically about 20nm upwards and increasing with the overall size of the block copolymer.
Once formed, it is possible to chemically treat the resulting nanoporous structures to alter
the porous dimension of the materials. This is considered to be a result of a partial
collapse of the remaining crosslinked polymer in the presence of the antipathic solvent.
Equally it is possible to expand the polymer to a more open structure by the use of a
favourable solvent such as cyclohexane. The changes in dimension possible are, for
example, up to a factor of about 2.
It is also possible to manufacture supercapacitor electrodes (or fuel cell electrodes) using
preferred embodiments of the present invention. A recent review of supercapacitors
(also called electrochemical capacitors) is set out by Simon and Gogotsi [P. Siman and Y
Gogotsi "Materials for Electrochemical Capacitors" Nature Materials Vol. 7 November
2008 p. 845], the content of this review paper being incorporated herein by reference in
its entirety. Therefore, the background, typical structure and principles of operation of
supercapacitors will not be set out here, since it will be known to the skilled person.
A nanoporous polymeric material template is first formed. This can be done using block
copolymer techniques as described above. Alternatively, a gyroid block polymer
structure can be used. One component of the block copolymer is then removed in order
to create a template having interconnected porosity. The template is then backfilled with
a second material. This is a supercapacitor dielectric material, or a precursor thereof.
The backfilled template is then subjected to a heat treatment in the range 300-500°C in a
controlled non-oxidative environment. This has the effect of (at least partially) sintering
the supercapacitor dielectric material and pyrolysing the matrix polymer. The pyrolysis
leaves an interconnected carbon skeleton bicontinuous with the supercapacitor dielectric
material. The resultant structure has a pseudo-capacitance enhancement effect to the
double-layer capacitance.
The supercapacitor dielectric material is Ru0 2, Ir0 2, NiO, CoOx and Mn0 2. Of these,
most preferred is Mn0 2.
The preferred embodiments of the invention have been described by way of example.
Modifications of these embodiments, further embodiments and modifications thereof will
be apparent to the skilled person on reading this disclosure and as such are within the
scope of the present invention.
CLAIMS
1. A method of manufacturing a nanoporous material including the steps:
forming a morphology comprising a three dimensional array of isolated islands in a
continuous matrix, wherein the islands are formed of at least one island component of a
block copolymer and the matrix is formed of at least one matrix component of the block
copolymer; and
forming channels in the matrix between at least some of the islands and selectively
removing the island component to leave the matrix with an array of interconnected pores.
2. A method according to claim 1 wherein the islands are substantially equi-axed.
3. A method according to claim 1 or claim 2 wherein the islands have an average
diameter of at least 5 nm.
4 . A method according to any one of claims 1 to 3 wherein the three dimensional
array of islands is a substantially ordered array.
5. A method according to claim 4 wherein there is a regular minimum spacing
between adjacent islands, and where the average diameter of the islands is d, the
regular minimum spacing between adjacent islands is at least 1.5d.
6 . A method according to any one of claims 1 to 5 wherein the number fraction of
monomer units in the island component of the block copolymer is at least 8%.
7 . A method according to any one of claims 1 to 6 wherein the number fraction of
monomer units in the island component of the block copolymer is at most 20%.
8. A method according to any one of claims 1 to 7 wherein the composition used to
form the three dimensional array of isolated islands in a continuous matrix comprises one
or more homopolymers added to the diblock copolymer.
9. A method according to claim 8 wherein the homopolymer additive is formed from
the same monomer units as is used for the matrix component of the morphology or is a
material that is miscible in the matrix component.
10. A method according to claim 8 or claim 9 wherein the homopolymer additive is
formed from the same monomer units as is used for the island component of the
morphology or is a material that is miscible in the island component.
11. A method according to any one of claims 8 to 10 wherein the sum of the number
fraction of monomer units of homopolymer for integration with the matrix and the number
fraction of monomer units of matrix component is 80% or higher.
12. A method according to any one of claims 8 to 11 wherein the sum of the number
fraction of monomer units of homopolymer for integration with the islands and the
number fraction of monomer units of island component is 10% or higher.
13. A method according to any one of claims 1 to 12 wherein the island morphology
is developed by thermal treatment.
14. A method according to any one of claims 1 to 13 wherein the matrix is treated by
cross-linking.
15. A method according to claim 14 wherein the islands degrade in response to the
cross-linking treatment applied to the matrix.
16. A method according to claim 15 wherein the degraded islands are removed using
a washing fluid.
17. A method according to claim 16 wherein the matrix component comprises PS, the
island component comprises PMMA and the washing fluid comprises acetic acid.
18. A method according to any one of claims 1 -17 wherein the material is subjected
to a gaseous oxidising agent during cross-linking of the matrix, the gaseous oxidising
agent preferably having the effect of degrading the island component.
19. A method according to claim 18 wherein the gaseous oxidising agent is ozone,
the ozone preferably being formed due to interaction of oxygen gas with UV radiation
used to cross link the matrix.
20. A method according to claim 18 wherein the gaseous oxidising agent is selected
from ozone (i.e. 0 3) sulphur (e.g. S3), chlorine and fluorine.
2 1. A method according to any one of claims 1 to 20 wherein the nanoporous
material has a plurality of lamellae, each lamella having an array of pores penetrating
therethrough, adjacent lamellae being spaced apart by an intervening spacing layer
wherein the spacing layer comprises an array of spacing elements integrally formed with
and extending between the adjacent lamellae, the spacing layer having interconnected
porosity extending within the spacing layer.
22. A nanoporous material having a plurality of lamellae, each lamella having an
array of pores penetrating therethrough, adjacent lamellae being spaced apart by an
intervening spacing layer wherein the spacing layer comprises an array of spacing
elements integrally formed with and extending between the adjacent lamellae, the
spacing layer having interconnected porosity extending within the spacing layer.
23. A nanoporous material according to claim 22 wherein the degree of porosity of
the spacing layer is greater than that of the lamellae.
24. A nanoporous material according to claim 22 or claim 23 wherein the nanoporous
material is a cross linked polymeric material.
25. A nanoporous material according to any one of claims 22 to 24 wherein the
average thickness of the lamella in the structure is at least 20 nm.
26. A nanoporous material according to any one of claims 22 to 25 wherein the
average thickness of the lamella in the structure is at most 200 nm.
27.. A nanoporous material according to any one of claims 22 to 26 wherein the
lamellae in the nanoporous structure have a periodic spacing of at least 40 nm.
28. A nanoporous material according to a y one of claims 22 to 27 wherein the
lamellae in the nanoporous structure have a periodic spacing of at most 400 nm.
29. A nanoporous material according to any one of claims 22 to 28 wherein the
spacing elements are substantially solid columnar in structure.
30. A nanoporous material according to any one of claims 22 to 29 wherein the pores
penetrating through the lamellae are arranged in a two dimensionally substantially close
packed array.
3 1. A nanoporous material according to any one of claims 22 to 30 wherein the
average size of the pores extending through the lamellae is at least 5 nm.
32. Use of a nanoporous material according to any one of claims 22 to 3 1 as a
filtration medium in a filtration process.
33. A filtration membrane comprising a filter substrate, wherein a nanoporous
material according to any one of claims 22 to 3 1 is provided on the filter substrate or in
the filter substrate.
34. A process for the manufacture of a nanoporous medium, including the step of
introducing a second material into a template formed of a nanoporous material according
to any one of claims 22 to 3 1, thereby at least partially filling the porosity of the template
nanoporous material, and then at least partly removing the template nanoporous material
to leave the nanoporous medium or precursor thereof at least partly formed from the
second material.
35. A process for the manufacture of a nanoporous medium, including the step of
introducing a second material into a template formed of a nanoporous material formed
using the method of any one of claims 1 to 2 1, thereby at least partially filling the porosity
of the template nanoporous material, and then at least partly removing the template
nanoporous material to leave the nanoporous medium or precursor thereof at least partly
formed from the second material.
36. A process according to claim 34 or claim 35 wherein the template nanoporous
material is removed using a heat treatment.
37. A method for the manufacture of a supercapacitor electrode including forming a
polymeric nanoporous material template using the steps:
forming a morphology comprising a three dimensional arrangement of a first
component of a block copolymer in a continuous matrix formed of at least one
matrix component of the block copolymer; and
selectively removing the first component to leave the matrix with an array of
interconnected pores
and subsequently forming the supercapacitor electrode using the step:
introducing a second material into the template, thereby at least partially filling the
porosity of the template, and
at least partly removing or degrading the template
wherein the second material is a supercapacitor dielectric material or a precursor thereof.
38. A method according to claim 37 wherein the supercapacitor dielectric material
comprises a material selected from the group consisting of: Ru0 2, Ir0 2, NiO, CoOx and
Mn0 2.
39. A method according to claim 37 or claim 38 wherein the matrix component of the
template is pyrolysed in order to form an interconnected carbon-rich network.