U.S. patent application number 12/223813 was filed with the patent office on 2009-07-09 for shape memory materials comprising polyelectrolyte segments.
Invention is credited to Andrea Lendlein.
Application Number | 20090176896 12/223813 |
Document ID | / |
Family ID | 36102972 |
Filed Date | 2009-07-09 |
United States Patent
Application |
20090176896 |
Kind Code |
A1 |
Lendlein; Andrea |
July 9, 2009 |
Shape Memory Materials Comprising Polyelectrolyte Segments
Abstract
The present invention concerns shape memory materials comprising
polyelectrolyte segments. These segments can be used for fixing a
permanent shape and/or such segments can also be employed as
switching segments responsible for the fixation and release of the
temporary shape.
Inventors: |
Lendlein; Andrea; (Berlin,
DE) |
Correspondence
Address: |
HOGAN & HARTSON LLP;IP GROUP, COLUMBIA SQUARE
555 THIRTEENTH STREET, N.W.
WASHINGTON
DC
20004
US
|
Family ID: |
36102972 |
Appl. No.: |
12/223813 |
Filed: |
February 12, 2007 |
PCT Filed: |
February 12, 2007 |
PCT NO: |
PCT/EP2007/001195 |
371 Date: |
December 16, 2008 |
Current U.S.
Class: |
521/25 |
Current CPC
Class: |
C08F 297/00 20130101;
B29C 61/003 20130101 |
Class at
Publication: |
521/25 |
International
Class: |
C08J 5/20 20060101
C08J005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 10, 2006 |
EP |
06002740.6 |
Claims
1. A shape memory polymer comprising at least one hard segment and
at least one soft segment, wherein the at least one hard segment
and the at least one soft segment comprise a polyelectrolyte
segment.
2. The shape memory polymer of claim 1, wherein the soft segment
comprises a polyelectrolyte segment.
3. The shape memory polymer of claim 1, wherein the hard segment
comprises a polyelectrolyte segment.
4. The shape memory polymer of claim 1, wherein the polyelectrolyte
segment is an anionic segment.
5. The shape memory polymer of claim 1, wherein the polyelectrolyte
segment is a cationic segment.
6. The shape memory polymer of claim 1, wherein the polyelectrolyte
segment bridges multivalent cations and fixes the shape.
7. The shape memory polymer of claim 1, wherein an ionic
interaction is reduced between a first polyelectrolyte segment and
a second polyelectrolyte segment.
8. The shape memory polymer of claim 1, wherein the shape memory
polymer is a network material.
9. A method of using shape memory polymer of claim 1, to make a
sensor.
10. The method of claim 9, wherein the sensor is a pH sensor or a
sensor for detecting a change concerning an ion concentration.
11. The shape memory polymer of claim 7, wherein the reduced ionic
interaction is due to a cation exchange of multivalent, bridging
cations with monovalent cations.
Description
[0001] The present invention concerns shape memory materials
comprising polyelectrolyte segments. These segments can be used for
fixing a permanent shape and/or such segments can also be employed
as switching segments responsible for the fixation and release of
the temporary shape.
[0002] Shape memory materials are an interesting class of materials
which have been investigated in recent years. Shape memory property
is the ability of a material to remember an original or permanent
shape, either after mechanical deformation, which is a one-way
effect, or by cooling and heating, which is a two-way effect.
[0003] The advantages and intriguing properties of these materials
are, in particular, the possibility to initiate a desired change in
shape by an appropriate external stimulus, so that an original
shape, after deformation, is re-established, and the possibility to
deform and program these materials so that highly specific
configurations and shape changes can be obtained. While the
original shape usually is designated as permanent shape, the
deformed shape is often called temporary shape in the art.
[0004] The first materials known to have these properties were
shape memory metal alloys. In the recent past, shape memory
polymers have been developed. Typical shape memory polymers are,
for example, phase segregated linear block copolymers, having a
hard segment and a soft segment. The hard segment usually is
responsible for the definition of the original, i.e. permanent,
shape by means of physical interactions, such as crystallization
processes, which provide stable fixation points which are not
destroyed during subsequent programming steps. The soft segment
usually is employed for the definition and fixation of the
temporary shape. A usual, conventional shape memory polymer of the
type disclosed above, comprising a hard segment and a soft segment,
can, for example, be programmed in the following manner. The shape
memory polymer is heated to above the melting point or glass
transition temperature of the hard segment, allowing the shaping of
the material. This original shape, i.e. the permanent shape, can be
memorized, i.e. fixed, by cooling the shape memory polymer below
the melting point or glass transition temperature of the hard
segment. The soft segment also possesses a melting point or a glass
transition temperature, which is substantially less than the
melting point or glass transition temperature of the hard segment.
I.e., the shape memory polymer, after cooling to below the melting
point or glass transition temperature of the hard segment can be
shaped further, i.e. the temporary shape can be established. This
temporary shape can then be fixed by cooling the material to below
the melting point or glass transition temperature of the soft
segment. The original shape can then be recovered by heating the
material to above the melting or glass transition temperature of
the soft segment, so that these soft segments become more flexible,
so that the material can recover into the original, permanent
shape.
[0005] Traditional shape memory polymers posses suitable segments
enabling temperature dependent shape changes due to melting
processes or phase transition processes, such as processes involved
with phase changes at a glass transition temperature. Typical
materials of this kind are, for example, disclosed in the
international publications WO 99/42147 and WO 99/42528.
[0006] One drawback associated with such materials is the fact that
the fixation of the permanent shape by means of the conventional
hard segments often is not satisfactory, since the fixation of the
permanent shape by means of this hard segments is due only to
rather weak interactions of polymer segments due to crystallization
processes, at least in thermoplastic shape memory polymers. A
drawback associated therewith is the fact that very slow and minute
movements and creeping processes may lead to a loosening of the
fixation areas for the permanent shape, so that sometimes the
desired shape change cannot be realized, or does not proceed to a
satisfactory degree, in view of the fact that the memory of the
original shape has vanished, or at least has been diminished, due
to such creeping processes. In thermoset materials, wherein the
permanent shape is memorized, i.e. fixed, by means of covalent
crosslinking points of the polymeric network, the above-outlined
drawback of the thermoplastic materials has been overcome.
Nevertheless, such covalent crosslinking points have the drawback
that the permanent shape cannot be altered anymore after the
formation of the covalent crosslinking points.
[0007] The international application WO 03/088818 discloses
biodegradable shape memory polymeric sutures which are
characterized according to the claims in that they are prepared
with shape memory material being temperature sensitive. The
examples of this document focus on shape memory materials prepared
with caprolactone macromonomers, dioxanone macromonomers, linked by
urethane groups.
[0008] The materials as evaluated and described are thermally
sensitive shape memory materials for which the permanent shape as
well as the temporary shape are programmed using thermal
programming methods. JP-A-63-017952 discloses temperature dependent
material having shape-memorizing properties. According to this
document an original shape can be recovered by rise and temperature
and the material as disclosed therein is in particular described as
being an optical material.
[0009] U.S. Pat. No. 5,043,396 discloses a crosslinked polymer
having shape-memorizing property, said crosslinked polymer being
obtained by thermally reversely crosslinking a base polymer
comprising specific segments. Again, these segments also comprise
ionically crosslinkable groups which are, however, employed as
temperature sensitive crosslinking points. Overall, the material
disclosed in U.S. Pat. No. 5,043,396 is a temperature sensitive
material.
[0010] EP 1126537 A1 discloses block polymers useful for a polymer
electrolyte fuel cell. This document does not disclose shape memory
materials but employs the block polymer disclosed as membrane
forming polymer electrolyte. W. J. Zhou discloses in Polymer 42
(2001) 345-349 the synthesis of a novel pH responding polymer with
pendant barbituric acid moieties. The polymer disclosed was
evaluated with respect to pH responding behavior in water and it is
disclosed that at certain pH values a transparent yellow colored
solution could be obtained, whereas at other pH values the polymer
most precipitated and the solution became opaque. Shape memory
properties are not disclosed. EP 0480336 A2 discloses amphiphilic
elastomeric block copolymers which are to be employed as binders in
light sensitive, i.e. light curing elastomeric mixtures. Shape
memory properties are not disclosed.
OBJECT OF THE PRESENT INVENTION
[0011] Accordingly, the present invention aims at providing a shape
memory material overcoming the drawbacks associated with the
conventional thermoplastic and thermoset materials known from the
prior art.
BRIEF DESCRIPTION OF THE INVENTION
[0012] The above object has been solved with the shape memory
material in accordance with claim 1. Preferred embodiments are
defined in the sub-claims. Furthermore, the present invention
provides the use of these materials as defined in the claims.
Preferred embodiments are again defined in the respective dependent
sub-claims. Furthermore, the following description illustrates
further embodiments of the present invention.
BRIEF DESCRIPTION OF THE FIGURE
[0013] FIG. 1 shows schematically the use of a cation exchange
(monovalent/divalent) for enabling or extinguishing ionic
interactions between two polymer segments, a mechanism which may be
used for the fixation of the temporary as well as of the permanent
shape (and the initiation of the shape memory effect as well as for
the loosening the permanent shape so that reshaping is
possible).
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0014] The present invention provides shape memory materials
comprising segments derived from polyelectrolytes.
[0015] Polyelectrolyte segments in accordance with the present
invention are segments comprising a vast number of ionic groups,
which may either be elements of the main chain of the segment or
which may be elements of side chains of the main chain of the
polyelectrolyte segment.
[0016] A polyelectrolyte segment in accordance with the present
invention furthermore refers to a segment having a molecular weight
of up to 15000, preferably 400 to 15000, more preferably 500 to
15000. Suitable embodiments of the molecular weight are also the
ranges of from 1000 to 10000 and from 2500 to 7500.
[0017] The polyelectrolyte segments to be employed in accordance
with the present invention can be distinguished as already
indicated above very generally into segments wherein the ionic
groups are comprised within the main chain (for example ionene) or
they may be provided within side chains, such as in quarternized
poly(4-vinylpyridin). The polyelectrolyte segments to be employed
in accordance with the present invention furthermore can be
classified broadly into polyacidic segments or polybasic segments.
Polyacidic segments give rise to polyanions, while polybasic
segments are segments comprising groups able to react with proton
providing substances under the formation of salts. Typical examples
of polyacidic polyelectrolyte segments are segments derived from
polyphosphoric acid, segments derived from polyvinyl sulfuric acid,
segments derived from polyvinyl sulfonic acid, segments derived
from polyvinyl phosphonic acid and segments derived from
polyacrylic acid. These groups can be derivatized further in any
suitable manner. Other examples are alginate derived segments, i.e.
segments derived from alginic acid. One advantage of these segments
is the fact that alginates have long been used as thickeners or as
components of pharmaceutical preparations, such as capsules, so
that these materials are readily available from commercial sources.
Furthermore there exists already knowledge concerning the
processing of such materials.
[0018] Typical segments derived from polybasic polyelectrolytes are
segments derived from polyethylene amine, polyvinyl amine and
polyvinyl pyridine.
[0019] A third class of polyelectrolyte segments are ampholytic
segments, comprising anionic as well as cationic groups, i.e.
segments which give rise to polyions in suitable polar
solvents.
[0020] All these types of polyelectrolyte segments may be employed
in accordance with the present invention. Further polyelectrolytes
which may be used as building blocks for segments of shape memory
materials in accordance with the present invention may be selected
from conventional polymers derivatized with groups providing
anionic or cationic groups and conventional polyelectrolytes, such
as polyallyl ammonium chloride, polyallyl ammonium phosphate,
polydiallyldimethyl ammonium chloride, polybenzyltrimethyl ammonium
chloride, polydiallyldimethyl ammonium chloride-co-N-isopropyl
acryl amide, polysodiumtrimethylene oxyethylene sulfonate,
polydimethyldodecyl-2-acrylamidoethyl-ammonium bromide,
poly-4-N-butylpyridinium methylene bromide,
poly-2-N-methylpyridiniumethylene iodine,
poly-N-methylpyridinium-2-5-diethylene,
poly-4-4'-bipyridiniumdiayl-N,N'-decamethylenedibromide and
betaine-derived polymers.
[0021] Important, in accordance with the present invention, is the
fact that the polyelectrolyte segments to be employed in accordance
with the present invention comprise groups enabling an ionic
interaction between polymer segments.
[0022] Polyelectrolyte segments to be employed in accordance with
the present invention furthermore give rise to ionic interactions
so that these segments are not temperature sensitive but respond to
chemical modifications, such as ion exchange, change in pH value,
ionic strength etc. Contrary to conventional temperature sensitive
shape memory materials, the materials in accordance with the
present invention comprise at least one segment which reacts to the
above-mentioned chemical modifications which are illustrated
further below.
[0023] With such polyelectrolyte segments the following embodiments
can be realized. In this respect it is important to recognize that
the shape memory polymers disclosed herein are described using the
conventional designation of the segments of the shape memory
polymers, i.e. hard and soft segments, wherein the hard segments
are responsible for the permanent shape and the soft segments are
the switching segments.
[0024] Polyelectrolyte Segments as Hard Segments
[0025] Polyelectrolyte segments, such as exemplified above, may be
used as replacement for hard segments in shape memory polymers,
i.e. for fixing the permanent shape. The advantage of such shape
memory polymers, preferably thermoplastic polymers, is the fact
that the ionic interaction between the "hard segments", provided in
particular by suitable (polyvalent) counter ions, provides much
stronger interactions, compared with conventional hard segments
wherein the permanent shape is memorized and fixed by means of
interactions between polymer chains in a polymer crystallite. Ionic
interactions as provided with the shape memory material in
accordance with the present invention enable much stronger
interactions, so that the above-outlined problems as associated
with the conventional thermoplastic materials, due to creep
processes, can be avoided. Due to the strong ionic interactions
between the segments a high shape fixity can be obtained, i.e. the
permanent shape is retained/recovered with a very high accuracy.
Polyelectrolyte segments as hard segments do show a lesser tendency
towards relaxation, compared with conventional hard segments which
might show lower degrees of shape fixity due to creep processes
within the material. This enables the preparation of shape memory
polymers with better shape memory properties, in particular the
storage stability and long-term shape fixity are improved.
[0026] Furthermore it is also possible to define in principle any
shape as the permanent shape, as with conventional shape memory
polymers, since this permanent shape may be reshaped by appropriate
processes. One example of such a shaping process suitable for shape
memory polymers comprising polyelectrolyte segments are molding
processes using solutions, such as aqueous solutions of shape
memory polymers. These solutions may be cast into a desired shape
and the solvent is then removed by appropriate treatments, or, as
alternative, the shape memory polymer is solidified by
precipitation processes.
[0027] At the same time, the drawbacks associated with thermoset
materials, providing the network fixation points by means of
covalent links, can also be overcome. In view of the fact that the
permanent shape in accordance with this embodiment of the present
invention is memorized by means of ionic interactions between
polyelectrolyte segments, it is possible to change the permanent
shape by appropriate processing steps, such as neutralizing steps
or salt formation steps weakening the ionic interaction between the
polyelectrolyte segments. This enables a deformation of the
material leading to a new permanent shape, which then can again be
memorized, i.e. fixed by appropriately reversing the decrease in
ionic interaction by appropriate chemical manipulation.
[0028] In accordance with the present invention, it is therefore
possible to provide hard segments, i.e. segments responsible for
the permanent shape, which are sensitive towards chemical
modifications, such as pH variations or ion exchange (see FIG. 1),
enabling the advantages as described above. Conventional
temperature sensitive shape memory polymers comprise crystallizing
segments as hard segments which, in thermoplastic materials, can be
shaped and programmed by means of a temperature rise and the
respective permanent shape is then fixed by means of the
interactions resulting upon the cooling and crystallization of the
segments. The present invention provides a different means for
providing hard segments not envisaged by the prior art.
[0029] In accordance with this embodiment, shape memory materials
can be provided having much improved recovery properties for the
permanent shape in view of the fact that the ionic interactions
provided in accordance with the present invention enable far
stronger interactions, compared with the conventional interactions
in traditional shape memory polymers.
[0030] Polyelectrolyte Segments as Soft Segments
[0031] An alternative embodiment in accordance with the present
invention uses the polyelectrolyte segments as switching segments,
i.e. as replacement for conventional soft segments. In this
embodiment, the possibility to increase and to decrease the ionic
interaction between different segments of a shape memory material
in a reversible manner by means of a suitable manipulation is used
in order to fix the temporary shape of a shape memory material.
Such a shape memory material comprises either conventional network
points or hard segments necessary for the memory concerning the
permanent shape, or this material also employs polyelectrolyte
derived segments as replacement for conventional hard segments or
covalent network points, as outlined above. The temporary shape is
then fixed by chemical manipulation leading to strong ionic
interaction between polyelectrolyte segments in the deformed state.
A recovery of the permanent shape can be triggered by appropriately
changing the chemical composition with respect to the
polyelectrolyte segments, for example, by providing additional
reagents leading to a change in pH value or to salt exchange
reactions. In this connection, it is, for example, possible to
replace a bridging divalent or trivalent cation, responsible for
ionic interaction between anionic polyelectrolyte segments, by
monovalent cations so that the bridging or crosslinking of
different polyelectrolyte segments ceases to be present. This
generates more freedom of movement of the segments by liberating
the polyelectrolyte segments from one another so that a recovery of
the original, permanent shape is made possible.
[0032] It is important to recognize in this respect that the
temporary shape is defined by means of the described interactions
between polyelectrolyte segments, so that a temperature shape can
be programmed which is responsive towards a chemical modification,
such as pH variation or ion exchange (see FIG. 1). This is a clear
difference with respect to the prior art which relies, as far as
soft segments or switching segments are concerned, on temperature
controlled, temperature sensitive materials, such as polyester
segments or temperature sensitive ionic groups.
[0033] In one embodiment of using the polyelectrolyte segments as
soft (switching) segments same are anionic segments initially
present in association with monovalent, i.e. non-bridging cations.
After a suitable deformation to the temporary shape the monovalent
cations (such as H.sup.+, Na.sup.+, Li.sup.+, K.sup.+,
NH.sub.4.sup.+, etc. as well as organic cations) are exchanged and
replaced by multivalent cations, preferably di- or trivalent
cations (such as Ca.sup.++, Mg.sup.++, Ba.sup.++, Cu.sup.++,
Al.sup.+++, Fe.sup.+++ etc. as well as organic cations), so that
strong ionic interactions between the polyelectrolyte segments fix
the temporary shape. The shape memory effect, i.e. the recovery of
the permanent shape may then be initiated by replacing the bridging
cations again with monovalent cations so that the interactions
between the polyelectrolyte segments are reduced and finally
extinguished, or by adding a solvating agent for the bridging ions
so that a weakening of the bridges is achieved by a "dilution" type
mechanism, so that the polymer recovers the permanent shape.
[0034] Another possibility is the initiation of the shape memory
effect by altering the pH value, again with the aim of reducing and
finally extinguishing interactions, which fix the temporary shape,
between the polyelectrolyte segments.
[0035] The use of polyelectrolyte segments as soft segments in
shape memory polymers widens considerably the range for suitable
applications. In particular soft segments derived from
polyelectrolyte segments enable the use of novel external stimuli
for triggering the shape memory effect. Previously mainly
temperature and light sensitive shape memory polymers have been
reported. The novel materials in accordance with the present
invention enable the use of other stimuli, such as ionic strength,
pH value, type of ion (monovalent/multivalent cations, see above)
etc. Such external stimuli further open new types of application,
since the shape memory polymers in accordance with the present
invention may be used in moist/liquid environments, since
initiation of the shape memory effect requires the possibility to
carry out ion exchange etc. which mainly may be realized in liquid
systems. Accordingly the materials have to enable at least a
certain degree of swelling, for example, in order to allow such
reactions.
[0036] Shape memory polymers in accordance with the present
invention comprising soft segments from polyelectrolyte segments
may be prepared in the form of thermoplastic materials, such as
multiblock copolymers, or in the form of network polymers,
typically comprising covalent crosslinking. Preferred in accordance
with the present invention are network materials. In such materials
the degree of swelling may be controlled by appropriately selecting
the building blocks, i.e. higher contents of hydrophobic components
reduce the degree of swelling. Network structures envisaged by the
present invention comprise covalent networks as well as IPN or
semi-IPN materials. It is for example possible to introduce soft
segments as chain like molecules into a network, by suitable
processes, for example loading of the network by swelling within a
solution of polyelectrolyte segments, followed by drying. The
semi-IPN obtained thereby can be deformed and the temporary shape
may be fixed by using a cation exchange reaction as exemplified
above in order to provide a physical network of the polyelectrolyte
segments interpenetrating the covalent network structure and fixing
thereby the temporary shape. The shape memory effect may again be
triggered by a further cation exchange, as illustrated above, so
that the physical network of the polyelectrolyte segments is
destroyed so that the permanent shape can be recovered.
[0037] Polyelectrolyte Segments as Hard and Soft Segments
[0038] In a further embodiment, the polyelectrolyte segments serve
as permanent crosslinking network points for the permanent shape as
well as switching segments for providing the temporary shape. In
this connection, it is only necessary to appropriately select the
chemical nature of the polyelectrolyte segments so that suitable
reaction sequences (such as neutralization, salt formation etc.)
can be employed in order to provide the material with the memory of
a permanent shape and a temporary shape.
[0039] Assembly of Segments
[0040] Concerning the assembly of segments, chemical reactions
required therefore as well as concerning segment length, molecular
weight etc. reference is made to the above mentioned international
applications WO 99/42147 and WO 99/42528 which are incorporated
herein in this respect by reference.
[0041] Materials in accordance with the present invention can in
particular be used as sensors, for example, as pH sensors or as
sensors for ions, such as polyvalent metal ions, since any change
in pH value or in the concentration of such polyvalent (or
monovalent) cations may lead to a change of the shape (or any other
property) of the shape memory material. Accordingly, the materials
in accordance with the present invention have a great utility.
[0042] Shape Memory Properties
[0043] The materials in accordance with the present invention due
to the use of polyelectrolyte segments, enable to tailor shape
memory properties. Those properties can be determined using the
methods as disclosed in the earlier applications of the present
applicant Mnemoscience and relevant properties are in particular
Recovery, i.e. the accuracy with which a permanent shape is
recovered after the triggering of the shape memory effect, and
Fixity, i.e. the accuracy with which a temporary shape can be
fixed. It is for example possible to adjust these properties by
changing the type of ions employed, i.e. by using ions with
differing bonding strengths. Ions with high bonding strengths will
for example increase Fixity and also Recovery, depending on the
question whether the polyelectrolyte segment is employed as hard or
soft segment.
[0044] Further it is preferable to employ segments which will
enable that the shape memory material can be wetted with water
and/or that water can penetrate into the material to induce the
shape memory effect. Another option might be the use of organic
solvents, which may be miscible with water, in order to increase
interaction with the shape memory material. A further option is the
use of surfactants, which may also be incorporated into the
polymeric material.
[0045] Programming
[0046] In accordance with the above description, it is clear that
the shape memory materials as described herein, comprising
polyelectrolyte segments, either as hard segments, as soft segments
or as hard as well as soft segments, can be programmed in a manner
involving the corresponding chemical modification, such as pH
variation and ion exchange as illustrated above for the individual
segments. Such programming methods clearly deviate from the pure
temperature dependent programming methods described in the prior
art for the temperature sensitive shape memory materials. As
outlined in detail above, the permanent crosslinks for the
permanent shape (i.e. hard segments) as well as the temporary
crosslinks for the temporary shape (i.e. soft segments) are
prepared by chemical methods involving a change in pH or an ion
exchange, for example the exchange of a monovalent counter ion
giving rise to no crosslinking effect with a divalent counter ion,
giving rise to a crosslinking effect (see FIG. 1), so that the
methodology as disclosed in the prior art in connection with
temperature sensitive shape memory polymers cannot simply be
adapted to shape memory materials comprising polyelectrolyte
segments as disclosed in the present application.
* * * * *