U.S. patent application number 13/797631 was filed with the patent office on 2014-05-22 for ultra low density biodegradable shape memory polymer foams with tunable physical properties.
The applicant listed for this patent is Lawrence Livermore National Security, LLC, The Texas A&M University System. Invention is credited to Elizabeth Cosgriff-Hernandez, Duncan J. Maitland, Pooja Singhal, Thomas S. Wilson.
Application Number | 20140142207 13/797631 |
Document ID | / |
Family ID | 49622187 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140142207 |
Kind Code |
A1 |
Singhal; Pooja ; et
al. |
May 22, 2014 |
ULTRA LOW DENSITY BIODEGRADABLE SHAPE MEMORY POLYMER FOAMS WITH
TUNABLE PHYSICAL PROPERTIES
Abstract
Compositions and/or structures of degradable shape memory
polymers (SMPs) ranging in form from neat/unfoamed to ultra low
density materials of down to 0.005 g/cc density. These materials
show controllable degradation rate, actuation temperature and
breadth of transitions along with high modulus and excellent shape
memory behavior. A method of making extremely low density foams (up
to 0.005 g/cc) via use of combined chemical and physical blowing
agents, where the physical blowing agents may be a single compound
or mixtures of two or more compounds, and other related methods,
including of using multiple co-blowing agents of successively
higher boiling points in order to achieve a large range of
densities for a fixed net chemical composition. Methods of
optimization of the physical properties of the foams such as
porosity, cell size and distribution, cell openness etc. of these
materials, to further expand their uses and improve their
performance.
Inventors: |
Singhal; Pooja; (Dublin,
CA) ; Wilson; Thomas S.; (San Leandro, CA) ;
Cosgriff-Hernandez; Elizabeth; (College Station, TX)
; Maitland; Duncan J.; (College Station, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lawrence Livermore National Security, LLC;
The Texas A&M University System; |
|
|
US
US |
|
|
Family ID: |
49622187 |
Appl. No.: |
13/797631 |
Filed: |
March 12, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61651477 |
May 24, 2012 |
|
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|
Current U.S.
Class: |
521/76 ; 521/183;
521/92; 521/97; 525/434; 528/296; 528/332 |
Current CPC
Class: |
C08G 18/6622 20130101;
C08G 18/10 20130101; A61B 2017/00606 20130101; C08G 63/00 20130101;
C08G 2101/0083 20130101; A61B 2017/00592 20130101; C08G 18/10
20130101; C08G 18/3278 20130101; C08G 63/6852 20130101; C08G
2280/00 20130101; C08J 9/00 20130101; C08G 18/4277 20130101; C08G
18/6655 20130101; C08G 18/4277 20130101; C08G 18/14 20130101; C08G
18/73 20130101; C08J 2375/06 20130101; C08G 2230/00 20130101; C08G
18/3271 20130101; C08G 18/6655 20130101; C08G 18/10 20130101; A61B
2017/00623 20130101; C08G 18/165 20130101; C08G 71/04 20130101;
A61B 17/0057 20130101; C08G 63/60 20130101; C08G 2101/005
20130101 |
Class at
Publication: |
521/76 ; 528/332;
528/296; 525/434; 521/183; 521/97; 521/92 |
International
Class: |
C08G 71/04 20060101
C08G071/04; C08G 63/00 20060101 C08G063/00; C08J 9/00 20060101
C08J009/00; C08G 63/685 20060101 C08G063/685 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH AND DEVELOPMENT
[0002] The United States Government has rights in this invention
pursuant to Contract No. DE-AC52-07NA27344 between the United
States Department of Energy and Lawrence Livermore National
Security, LLC for the operation of Lawrence Livermore National
Laboratory.
Claims
1. A shape memory polymer composition comprising a polymer
composition resulting from the reaction of a branched monomer
having three or more branches in its structure with hydroxyl end
groups with: a difunctional monomer an acid group on one end and a
hydroxyl group on the other end, a difunctional monomer having
hydroxyl end groups or a branched polyol with di-carboxylic acid,
resulting in an polycarboxylic acid monomer containing ester
linkages.
2. The composition of claim 1 wherein a branched monomer having
three or more branches in its structure with hydroxyl end groups is
reacted with a difunctional monomer with an acid group on one end
and a hydroxyl group on the other end and a difunctional monomer
having hydroxyl end groups in succession.
3. The composition of claim 1 wherein the a branched monomer having
three or more branches in its structure with hydroxyl end groups is
selected from the group consisting of Triethanol amine (TEA),
Hydroxy propyl ethylene diamine (HPED), Glycerol, Pentaerythritol
or Trimethylolpropane, Bis-tris methane, Bis-tris propane, 1,2,4
Butane triol, Miglitol, Trimethylolethane and
Tris(hydroxymethyl)aminomethane.
4. The composition of claim 1 wherein the difunctional monomer an
acid group on one end and a hydroxyl group on the other end is
selected from the group consisting of L-Threonic acid, Tricine,
Shikimic acid, 3-Hydroxybutyrate, .epsilon.-Caprolactone, Lactic
acid, and Glycolic acid.
5. The composition of claim 1 wherein the difunctional monomer
having hydroxyl end groups is selected from the group consisting of
Polyethylene glycol, 1,3-Propanediol, 1,4-Butanediol,
1,5-Pentanediol, 1,2-Propanediol, 1,2-Butanediol, 2,3-Butanediol,
1,3-Butanediol, 1,2-Pentanediol, Etohexadiol and
2-Methyl-2,4-pentanediol.
6. The composition of claim 1 wherein the branched polyol with
di-carboxylic acid are selected from the group consisting of
1,4-benzoquinonetetracarboxylic acid,
Ethylenediamine-N,N'-disuccinic acid, furantetracarboxylic acid,
hydroxycitric acid, citric acid, Nitrilotriacetic acid, Aconitic
acid, isocitric acid and Propane-1,2,3-tricarboxylic acid.
7. The composition of claim 1 wherein Triethanol amine is end
capped with 3-Hydroxybutyrate (HB).
8. The composition of claim 1 wherein a tetrafunctional hydroxyl
monomer prepared by end capping Citric acid (CA) with 1,3 Propane
diol is one of the reactants.
9. The composition of claim 1 wherein Polycaprolactone triol (PCT)
obtained from grafting .epsilon.-Caprolactone on Trimethylolpropane
is one of the reactants.
10. The composition of claim 1 wherein stoichiometric amounts of
hydroxyl monomer and diisocyanate are vigorously mixed together
until a clear one phase solution is formed, the resulting
composition degassed and cast into molds.
11. A method for controlling cell structure of polymer foams made
from the compositions of claim 1 comprising control of the
viscosity of the composition solution, a prepolymer, to be foamed
by control of the amount hydroxyl monomer in the prepolymer wherein
cell size is inversely proportional to the viscosity.
12. The method of claim 11 wherein viscosity of the viscosity is
reduced by addition of inert liquid phase solvent.
13. The method of claim 11 wherein the viscosity is controlled by
the duration and speed of mixing the composition to be foamed,
wherein higher duration or speed of mixing of gives finer cell size
with denser foams.
14. A method of producing low density foams from the compositions
of claim 1 comprising increasing the concentration of gas in the
composition solution to be foamed at regular intervals.
15. The method of claim 14 comprising using successively higher
boiling point blowing agents as foaming progresses.
16. The method of claim 14 comprising simultaneous increase in the
surfactant levels as the concentration of gas in increased.
17. The method of claim 14 comprising addition of particulate
nucleating agents to the compositions to be foamed.
18. The method of claim 17 comprising leachable porogens as
nucleating agents.
19. The method of claim 14 comprising conducting the foaming under
vacuum.
20. The method of claim 14 comprising using higher functionality
carboxylic acids as a blowing agent.
21. A method of producing improved cell opening in foams produced
from the compositions of claim 1 comprising the addition of high z
metal nano- or micro-particles.
22. The method of claim 21 wherein the z metal particles are
selected from the group consisting of tungsten, tantalum, platinum
and, palladium.
23. The method of claim 21 comprising surface modification of the
metal particles with a low surface energy coating.
Description
[0001] This application claims benefit under 35 U.S.C. .sctn.119(e)
of U.S. Provisional Patent Application No. 61/651,477 filed May
24,2012 entitled "Ultra-Low Density Biodegradable Shape Memory
Polymer Foams with Tunable Physical Properties," the disclosure of
which is hereby incorporated by reference in its entirety for all
purposes. This invention was made with government support under
EB000462 awarded by National Institutes of Health. The government
has certain rights in the invention.
BACKGROUND
[0003] 1. Field of Endeavor
[0004] This invention relates to shape memory polymers and
specifically to shape memory polymers and polymer foams having
enhanced characteristics of degradability, control over cell
structure, and density.
[0005] 2. State of Technology
[0006] Shape memory polymers (SMPs) are materials which can
remember two or more shapes, and can be actuated to go from one
shape to another via a stimuli involving heat or light etc.
Thermally responsive SMPs that use heat energy for their actuation
can be deformed from their primary shape to a secondary shape above
their actuation temperature. This secondary shape can then be
"fixed" by cooling the deformed shape to below the material's
actuation temperature. When they are heated to above their
actuation temperature on demand, they recover their "remembered"
primary shape. Polyurethane based SMP foams were initially proposed
by Hayashi et. al--Japanese patent 5049591 (1991). Other related
patent applications were also filed in this field by Applicants,
U.S. patent application Ser. No. 10/801,355 (2004) and U.S. patent
Patent Application number 20060036045 (2005). These shape memory
materials are useful in diverse applications like shape adaptive
sportswear (helmets, suits), housing (thermal sealing of doors and
windows) and robotics (conformal grip design). Also these materials
are being investigated for use in automobile and aerospace
industries for self healing automobile bodies and morphing aircraft
wings. In addition, shape memory foam based biomedical devices for
minimally invasive surgeries are being developed, see El Feninat,
F., Laroche, G., Fiset, M. & Mantovani, D. Shape memory
materials for biomedical applications. Advanced engineering
materials 4, 91-104 (2002); Sokolowski, W., Metcalfe, A., Hayashi,
S., Yahia, L. H. & Raymond, J. Medical applications of shape
memory polymers. Biomedical Materials 2, S23 (2007) and Small, W.
et al. Shape memory polymer stent with expandable foam: a new
concept for endovascular embolization of fusiform aneurysms.
Biomedical Engineering, IEEE Transactions on 54, 1157-1160
(2007).
[0007] Introducing degradability is a key requirement for the
development of a SMP either as a biomedical device to avoid long
term presence of a foreign material in the human body, or for the
ecological concerns of using polymeric materials. Lendlein, A.
& Langer, R. Biodegradable, elastic shape-memory polymers for
potential biomedical applications. Science 296,1673 (2002) Several
degradable SMPs have been reported with polycaprolactone diol,
lactide and glycolide moieties. Caprolactone has been used
copiously in the biodegradable applications and its degradation
products have been shown to be benign in the earlier
biocompatibility studies; Rickert, D., Lendlein, A., Kelch, S.,
Franke, R. & Moses, M. Cell proliferation and cellular activity
of primary cell cultures of the oral cavity after cell seeding on
the surface of a degradable, thermoplastic block copolymer.
Biomedizinische Technik. Biomedical engineering 50,92 (2005).
[0008] However, most of these materials reported earlier are either
linear polymers, have relatively low covalent crosslink density, or
are based on physical crosslinks, which can limit the mechanical
and/or shape memory behavior of the material.
[0009] In contrast to above, a network structure consisting of high
density of covalent crosslinks, is preferable for good mechanical
properties and improved shape memory behavior (high recovery force,
high shape recovery), particularly for very low density foams. This
is so because, firstly, the modulus of a foamed material declines
rapidly as its density is reduced:
E.sub.porous=E.sub.neat[.sub.porous/.sub.neat] for open cell foams,
where E.sub.porous and E.sub.neat are the Young's moduli and
.sub.porous and .sub.neat are the densities, of porous and
neat/unfoamed materials respectively. Hence for very low density
materials to have good mechanical properties, they should be based
on neat/unfoamed materials with significantly high modulus i.e.
high density of crosslinks (from E.about.3n.sub.cRT where E is the
Young modulus of material, n.sub.c the number of active network
chain segments per unit volume, R is the ideal gas constant and T
is the Temperature). Secondly, it is important to have a covalently
crosslinked structure, rather than a physically crosslinked
structure for improved shape memory behavior to be retained over
extended periods of time, i.e. to avoid secondary-shape forming
phenomenon as is noticed in some physically crosslinked materials;
Tobushi, H., Matsui, R., Hayashi, S. & Shimada, D. The
influence of shape-holding conditions on shape recovery of
polyurethane-shape memory polymer foams. Smart materials and
structures 13,881 (2004). Since physical crosslinks are labile,
entropically driven polymer chains in a physically crosslinked
material can move in and out of their crosslink sites to attain a
more preferable, lower energy equilibrium conformation. This causes
the material to lose the memory of its primary shape, and thus,
lose its ability to actuate under a stimulus (as noticed in the
secondary-shape forming phenomenon). In contrast, covalent/chemical
crosslinks do not allow such rearrangement of entropically driven
polymer chains, and thus ensure improved shape memory behavior even
after extended periods of storage in the secondary shape. Hence a
high crosslink density in the network structure, and use of
chemical/covalent crosslinks for achieving the same, form the basis
of the materials of this invention.
[0010] The method of synthesis of such a highly covalently
crosslinked degradable polymer network based on polyurethane
chemistry is an embodiment of this invention. Degradability is
shown using multifunctional Polycaprolactone based hydroxyl
monomers, which has not been proposed before in blown foams. Also
other variations of monomers for developing a highly crosslinked
network structure are disclosed as are methods of controlling
material's actuation temperature and rate of degradation.
[0011] Controlling the cell structure of foams is another key
requirement in generation of commercial grade SMP foams, and we
propose manipulating viscosity of the foaming solution for the
same. The effect of viscosity on foam cell structure has been
studied in detail for foam emulsions; Kim, Y. H., Koczo, K. &
Wasan, D. T. Dynamic Film and Interfacial Tensions in Emulsion and
Foam Systems. Journal of colloid and interface science 187, 29-44
(1997) and Shah, D., Djabbarah, N. & Wasan, D. A correlation of
foam stability with surface shear viscosity and area per molecule
in mixed surfactant systems. Colloid & Polymer Science 256,
1002-1008 (1978).sup.19-20 and in HIPE foam synthesis procedure;
Busby, W., Cameron, N. R. & Jahoda, C. A. B. Emulsion-Derived
Foams (PolyHIPEs) Containing Poly(-caprolactone) as Matrixes for
Tissue Engineering. Biomacromolecules 2, 154-164 (2001) and
Christenson, E. M., Soofi, W., Holm, J. L., Cameron, N. R. &
Mikos, A. G. Biodegradable Fumarate-Based PolyHIPEs as Tissue
Engineering Scaffolds. Biomacromolecules 8, 3806-3814,
doi:10.1021/bm7007235 (2007). For blown foams, the indirect effect
of change in viscosity on cell structure, via changing the
functionality of polyols or chemistry of foams.sup.24 has been
reported; Tabor, R., Lepovitz, J., Potts, W., Latham, D. &
Latham, L. The effect of polyol functionality on water blown rigid
foams. Journal of Cellular Plastics 33, 372 (1997).
[0012] However, using this technique to control the cell structure
of very low density blown foams, keeping the net chemical
composition the same has not been previously reported. Currently in
literature, low density foams have been reported mostly down to the
lower limit of 0.02-0.03 g/cc; see Thirumal, M., Khastgir, D.,
Singha, N. K., Manjunath, B. & Naik, Y. Effect of foam density
on the properties of water blown rigid polyurethane foam. Journal
of Applied Polymer Science 108, 1810-1817 (2008) and Simpson, S. S.
& Sato, et al, U.S. Pat. No. 7,338,983 (2008).
[0013] A recent patent by Burdeniuc reported making foams down to
0.006 g/cc, but a staggering 75 wt % water was used in synthesis;
Burdeniuc, J. J. & Andrew, G. D. Catalyst composition for water
blown, low density, rigid polyurethane foam, US patent application
20100152312 (2008). Since use of high amounts of water as chemical
blowing agent will interfere with the intended covalently
crosslinked network structure of the material, this is not a
preferred route. Another patent reported densities down to 0.016
g/cc by varying the polyol type and amount; Haider, K. W. et al.
Polyol compositions useful for preparing dimensionally stable, low
density water-blown rigid foams and the processes related thereto,
U.S. Pat. No. 7,300,961 (2007). However, keeping the same chemical
composition, large variation in densities with lower
limit.about.0.005 g/cc has not been reported before.
SUMMARY
[0014] Features and advantages of the present invention will become
apparent from the following description. Applicants are providing
this description, which includes drawings and examples of specific
embodiments, to give a broad representation of the invention.
Various changes and modifications within the spirit and scope of
the invention will become apparent to those skilled in the art from
this description and by practice of the invention. The scope of the
invention is not intended to be limited to the particular forms
disclosed and the invention covers all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the claims.
[0015] It is an object of the present invention to provide
compositions and/or structures of degradable SMPs ranging in form
from neat/unfoamed to ultra low density materials of down to 0.005
g/cc density. These materials show controllable degradation rate,
actuation temperature and breadth of transitions along with high
modulus and excellent shape memory behavior.
[0016] It is another object of the present invention to provide a
method of controlling the cell structure of polymer foams by
controlling the prepolymer viscosity and other related methods.
[0017] It is still another object of the present invention to
provide a method of making extremely low density foams (up to 0.005
g/cc) via use of combined chemical and physical blowing agents,
where the physical blowing agents may be a single compound or
mixtures of two or more compounds, and other related methods,
including of using multiple co-blowing agents of successively
higher boiling points in order to achieve a large range of
densities for a fixed net chemical composition.
[0018] It is yet another object of the present invention to provide
methods of optimization of the physical properties such as
porosity, cell size and distribution, cell openness etc. of these
materials, to further expand their uses and improve their
performance.
[0019] The invention is susceptible to modifications and
alternative forms. Specific embodiments are shown by way of
example. It is to be understood that the invention is not limited
to the particular forms disclosed. The invention covers all
modifications, equivalents, and alternatives falling within the
spirit and scope of the invention as defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The accompanying drawings, which are incorporated into and
constitute a part of the specification, illustrate specific
embodiments of the invention and, together with the general
description of the invention given above, and the detailed
description of the specific embodiments, serve to explain the
principles of the invention.
[0021] FIG. 1 is a synthesis outline for making a trifunctional
hydroxyl monomer with ester linkages from triethanol amine and
3-hydroxybutyrate.
[0022] FIG. 2 is a synthesis outline for making a tetrafunctional
hydroxyl monomer with ester linkages from Citric Acid and 1,3
Propane diol.
[0023] FIGS. 3A, 3B and 3C are displays pictures of cell structure
of low density degradable shape memory polymer foams of the
invention.
[0024] FIG. 4 is a graphical plot showing the "Trend of variation
in the prepolymer rheology with increase in the OH/NCO ratio of the
prepolymer".
[0025] FIGS. 5A through 5F show "Variation in cell structure
achieved based on the viscosity of the foaming prepolymer".
[0026] FIG. 6 is a graphical plot illustrating the mechanism of
obtaining lower density foams via use of successively higher
boiling point blowing agents.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0027] Referring to the drawings, to the following detailed
description, and to incorporated materials, detailed information
about the invention is provided including the description of
specific embodiments. The detailed description serves to explain
the principles of the invention. The invention is susceptible to
modifications and alternative forms. The invention is not limited
to the particular forms disclosed. The invention covers all
modifications, equivalents, and alternatives falling within the
spirit and scope of the invention as defined by the claims.
[0028] The present invention consists of compositions and methods
for degradable shape memory polymer (SMP) and foams from those
polymers. It is also methods for controlling the properties of
these SMPs. The invention may be expressed in four broad
embodiments that include:
[0029] 1: Compositions and/or structures of degradable SMPs ranging
in form from neat/unfoamed to ultra low density materials of down
to 0.005 g/cc density.
[0030] 2: Method of controlling the cell structure of polymer foams
from the polymers of embodiment 1.
[0031] 3: Method of making extremely low density foams (up to 0.005
g/cc) by using combined chemical and physical blowing agents,
wherein the physical blowing agents consist of one or more
co-blowing agents and other related methods, and
[0032] 4: Methods of optimization of the physical properties such
as porosity, cell size and distribution, cell openness etc. of
these materials.
General Embodiment 1
Compositions and/or Structures of Degradable SMPs Ranging in Form
from Neat/Unfoamed to Ultra Low Density Materials of Down to 0.005
g/Cc Density
[0033] This embodiment included compositions that show controllable
degradation rate, actuation temperature and breadth of transitions
along with high modulus and excellent shape memory behavior. The
materials disclosed here are degradable, with good mechanical
properties and shape memory behavior at very low densities. One of
the ingredients for making these materials is a multifunctional
hydroxyl, amine, or carboxylic acid containing monomer of small
molecular weight (e.g. 200-1500 g), having degradable (for e.g.
ester, ether, amide, urethane) linkages. Such monomers can be
crafted by any of the following schemes:
[0034] 1. As shown in FIG. 1--Using a branched monomer (3 or more
branches in structure) with hydroxyl end groups and reacting it
with a difunctional monomer with an acid group on one end and a
hydroxyl group on the other end. Reaction of the hydroxyl group of
branched monomer and acid group of difunctional monomer will form
an ester linkage releasing a water molecule. This will extend the
branches of the original branched hydroxyl monomer with ester
linkages, keeping the hydroxyl group as its terminal/end group.
[0035] 2. As shown in FIG. 2--Alternatively, a branched monomer
with acid end groups can be reacted with a difunctional monomer
containing hydroxyl end groups. This will similarly form a
multifunctional monomer having ester linkage with the end groups of
hydroxyl functionality.
[0036] 3. Reaction of branched polyol with di-carboxylic acid,
resulting in an polycarboxylic acid monomer containing ester
linkages.
[0037] 4. Steps described in (2) and (3) may be accomplished in
succession to controllably increase the number of degradable
functional groups in each arm of the monomer, which in turn
provides a means to control degradation kinetics.
[0038] By scheme 1 (FIG. 1) some of the possible multifunctional
hydroxyl monomers include Triethanol amine (TEA), Hydroxy propyl
ethylene diamine (HPED), Glycerol, Pentaerythritol or
Trimethylolpropane, Bis-tris methane, Bis-tris propane, 1,2, 4
Butane triol, Miglitol, Trimethylolethane and
Tris(hydroxymethyl)aminomethane.
[0039] Some of the possible monomers with one acid and (at least
one) hydroxyl group, that can react with above multifunctional
hydroxyl groups include L-Threonic acid, Tricine, Shikimic acid,
3-Hydroxybutyrate, .epsilon.-Caprolactone, Lactic acid, and
Glycolic acid.
[0040] By scheme 2 (FIG. 2), multifunctional carboxylic acids can
include 1,4-benzoquinonetetracarboxylic acid,
Ethylenediamine-N,N'-disuccinic acid, furantetracarboxylic acid,
hydroxycitric acid, citric acid, Nitrilotriacetic acid, Aconitic
acid, isocitric acid and Propane-1,2,3-tricarboxylic acid.
[0041] Some of the possible diol monomers that can react with above
multifunctional carboxylic acids by scheme 2, include Polyethylene
glycol, 1,3-Propanediol, 1,4-Butanediol, 1,5-Pentanediol,
1,2-Propanediol, 1,2-Butanediol, 2,3-Butanediol, 1,3-Butanediol,
1,2-Pentanediol, Etohexadiol and 2-Methyl-2,4-pentanediol.
[0042] In one embodiment of the suggested synthesis by scheme 1,
Triethanol amine is end capped with 3-Hydroxybutyrate (HB) as shown
in FIG. 1. Anhydrous pyridine is used to initiate the formation of
an ester link between the hydroxyl end groups of TEA and the
activated carboxyl group of the HB. The precipitated
dicyclohexylurea is then removed with vacuum filtration and the
polymer solution is washed with distilled water. Solvent is then
removed by rotary evaporation, followed by the drying of the
polymer in vacuum.
[0043] In another embodiment of suggested synthesis by scheme 2, a
tetrafunctional hydroxyl monomer is prepared by end capping Citric
acid (CA) with 1,3 Propane diol, as shown in FIG. 2. CA and Propane
diol are reacted together at 160.degree. C. under stirring for 15
minutes. The temperature is subsequently decreased to 140.degree.
C. and reaction mixture is stirred for 1 hour. The resulting
prepolymer is purified by precipitation in water and then it is
freeze dried. While the idea of making these highly crosslinkable
short-branch multifunctional monomers using 1,3 Propane diol is
new, a similar synthesis procedure involving citric acid and its
derivatives has been adopted by Yang et. al. and group.sup.29.
[0044] In a yet another embodiment of the suggested synthesis by
scheme 1, Polycaprolactone triol (PCT) obtained from grafting
.epsilon.-Caprolactone on Trimethylolpropane is used.
[0045] It is to be emphasized that some such monomers are available
commercially, such as PCT, but several others can be made under
this scheme, and be used for the synthesis of the highly
crosslinked materials which form the subject of this invention.
[0046] These branched monomers with end hydroxyl functionality, as
achieved from above processes, are then reacted with small
molecular weight diisocyanate monomers. These include (but are not
limited to), Hexamethylene diisocyanate (HDI), Tri-methyl
Hexamethylene diisocyanate (TMHDI) and Isophorone diisocyanate for
aliphatic options and monomers such as Toluene diisocyanate,
Methylene diphenyl diisocyanate for aromatic options.
[0047] Making Polymers in Neat/Unfoamed Form:
[0048] For making neat polymers, stoichiometric amounts of hydroxyl
monomer and diisocyanate are mixed together and stirred until a
clear one phase solution is formed. Thereafter the solution is
further degassed, cast into molds and allowed to cure. In
neat/unfoamed form, these heavily crosslinked materials are an
optically clear polymer due to their amorphous structure, have a
good shape memory and are degradable.
[0049] The ease of forming a single phase solution with more
hydrophobic HDI and TMHDI decreases with increase in the
hydrophilicity of the branched hydroxyl monomers, such as with
incorporation of poly caprolactone triol or TEA in the synthesis. A
speed mixer such as Flacktek, Thinky or another vigorous mixing
technique is utilized for this purpose. Mixing over a range of 10
minutes to over an hour at 3000-4000 rpm can be required depending
on the content of the more hydrophilic monomer in the
formulation.
[0050] For reducing the degradation rate, other multifunctional
polyols mentioned above, without degradable linkages can be
substituted for hydroxyl monomers with degradable linkages in
material synthesis. Alternatively, the number of degradable
linkages can be reduced in each branch of the hydroxyl monomers. On
the extreme end, all the used monomers can be without a degradable
linkage e.g. TEA, HPED and Glycerol etc. to give
stable/non-degradable materials.
[0051] For maintaining the same glass transition/actuation
temperature, while changing the degradation rate of the materials,
hydroxyl monomers without degradable linkages are chosen such that
their steric hindrance and mobility is comparable to that of
monomers with degradable linkages. These non-degradable branched
hydroxyl monomers are then substituted for the degradable branched
hydroxyl monomers in appropriate amounts.
Example 1
[0052] As the ratio of Polycaprolactone triol based monomer (Mw 300
g) is increased in the polyurethane based on HDI and TEA, HPED and
PCT, the rate of degradation of the material is increased. As part
of the trifunctional caprolactone based monomer is substituted with
tetrafunctional non-degradable monomer such as HPED, the glass
transition of material increases, and rate of degradation
decreases. On the other hand, as the trifunctional caprolactone
based monomer is substituted with a trifunctional non-degradable
monomer such as TEA, the change in glass transition is not very
noticeable, and significant decrease in rate of degradation can be
achieved.
[0053] Controlling the length of the network active chains in the
polymer structure can define the breadth of the transition of the
materials from their glassy to rubbery state. The wider the
distribution of lengths of network active chains in the polymer,
the broader will be the transitions of glass transition
temperature, modulus etc. A narrow distribution on the other hand,
will give sharper transitions. One of the ways to achieve this is
by controlling the length of each branch of the multifunctional
monomers, for e.g. by controlling the number of hydrolysable
linkages in each branch.
[0054] Similarly, controlling or keeping the same number of
degradable linkages in each arm of the multifunctional monomers
will give a sharper drop in mass and more reproducible mass loss
pattern with respect to time during the degradation of the
material. A more broad distribution of the number of degradable
linkages in each arm, or in each network active chain segment
across the sample, will lead to a more spread out/or broader drop
in mass with respect to time.
[0055] Making Polymers as Low Density Foams:
[0056] For making foams, first a prepolymer is made with excess
isocyanate in the desired ratio of hydroxyl monomers based on
degradability and actuation temperature requirements. The solutions
are mixed until a clear, single phase is formed, possibly requiring
the use of a Flacktek/Thinky speed mixer or an equivalent high
speed mixing technique, based on the content of more hydrophilic
moieties, such as Poly caprolactone triol and Triethanol amine. The
prepolymers are then allowed to cure over a 2-3 day period.
Typically prepolymer viscosities in the range of 2 Pas to 60 Pas
post cure, can potentially yield viable foams.
[0057] In the second step for making foams, the balance hydroxyl
monomers in the desired ratio of functionality and degradability
are mixed together with surfactants, tin and amine based catalysts,
and water. Water here accounts for a percentage of hydroxyl
monomers via urea formation and assists in the chemical blowing of
foam. This hydroxyl premix is added to the prepolymer with excess
isocyanate in calculated amounts, and mixed vigorously, optionally
in a speed mixer, for a few seconds. Then a physical blowing agent
(or a combination of physical blowing agents) is added and solution
is mixed again. Thereafter the foam is allowed to rise in an oven
at 90.degree. C. The high temperature helps in maximizing the
generation of CO.sub.2 via the chemical blowing reaction in the
foaming solution.
[0058] While some process details are provided in the disclosure
above, several specifics such as choice of surfactants and
catalysts, the order of mixing the components together, mixing
durations and speeds, cure temperature and conditions can be
modified/changed with similar results.
[0059] Instead of using a polyurethane chemistry, multifunctional
monomers with other functional end groups that are reactive with
isocyanates, such as amines and carboxylic acids etc. can also be
used. Further, these materials can be modified by use of a variety
of additives and/or fillers, such as contrast agents, plasticizers,
dyes, pigments, carbon nanotubes etc., to enhance/change its
physical, mechanical, optical, electrical, or magnetic properties.
In addition to above, several post synthesis processes for
reticulation etc. such as hydrolysis, oxidation, application of
pressure, heat or mechanical treatment, can be performed on the
foams to modify their physical structure.
Example 2
[0060] Degradable foams were made by using Polycaprolactone triol
as the degradable hydroxyl monomer made from grafting caprolactone
moieties on Trimethylolpropane by the first scheme detailed above
(used as received from Sigma Aldrich, Mw 300 g), Triethanol amine
as the non-degradable hydroxyl monomer to control the rate of
degradation, and Hexamethylene diisocyanate.
[0061] First an NCO premix, or prepolymer with excess diisocyanate,
is made by mixing the components as per Table 1. The mixing is
performed in a Flacktek speed mixer or an equivalent vigorous
mixing technique as both Polycaprolactone triol and Triethanol
amine are not readily miscible in Hexamethylene diisocyanate. After
a clear solution is formed, the NCO premix is stored under Nitrogen
atmosphere and allowed to cure over a period of 2-3 days. The
viscosity of the solution increases as reaction occurs between
hydroxyl and isocyanate groups, reaching a value in the range of
2-60 Pas.
[0062] Thereafter, an OH premix is made by mixing the components as
per Table 1 (amounts given for a single foam batch, and can be
scaled up for more batches). For making the foams, NCO premix and
OH premix are poured together in the amounts per Table 1, and mixed
vigorously in a Flacktek (or equivalent mixer) for 10 sec at 3400
rpm speed. Then Enovate is added to the foaming solution and mixed
again for 5 sec at 3400 rpm in Flacktek mixer. The solution is then
transferred to the oven at 90.degree. C. and allowed to rise up.
Cell structures of resulting foams are shown in FIGS. 3A, 3B and
3C. Cell structure of low density degradable shape memory polymer
foams made from polycaprolactone triol, triethanol amine, water
(chemical blowing agent) and hexamethylene diisocyanate. Densities
in range of 0.020-0.025 g/cc are achieved. Scale bar=200 um. FIG.
3A shows EA0PCT100FW96, (0% TEA @ 0.45 OH/NCO). FIG. 3B shows
TEA30PCT70FW96 (30% TEA @0.45 premix OH/NCO). FIG. 3C shows
TEA60PCT40FW96 (60% TEA @0.42 premix OH/NCO).
[0063] The order of mixing the components together, mixing
durations and speeds, cure temperature and conditions can be
changed with similar results.
TABLE-US-00001 TABLE 1 Foam formulation for the synthesis of
degradable foams involving Polycaprolactone triol (PCT), Triethanol
amine (TEA), water, as chemical blowing agent, and Hexamethylene
diisocyanate (HDI). Surfactants DC-I990 and DC-5169, and catalysts
BL-22 and T-131, as received from Air Products, Inc. are used.
Actual weights of chemicals added in grams are given. OH Premix
Foam NCO premix DC- DC- NCO OH PCT TEA HDI PCT TEA Water I990 5169
BL-22 T-131 premix Premix Envte (g) (g) (g) (g) (g) (g) (g) (g) (g)
(g) (g) (g) * (ml) 1 32.12 0 60 2.48 0 0.92 1.3 1.5 .253 .101 32
6.55 3 2 22.48 4.79 60 1.83 0.39 0.97 1.3 1.5 .253 .101 32 6.34 3 3
11.99 8.94 60 1.47 1.09 1.04 1.3 1.5 .253 .101 32 6.76 3 1 is
TEA0PCT100FW96; 2 is TEA30PCT70FW96 and 3 is TEA60PCT40FW96; Envte
is Enovate
General Embodiment 2
Method of Controlling the Cell Structure of Polymer Foams by
Controlling the Shear and/or Elongational Prepolymer Viscosity,
Changing Speed/Duration/Order of Mixing of Components, Use of
Nucleating Agents, Optimizing Amount and Type of Surfactants and
Catalysts
[0064] The cell structure of the foams made from the polymer
composition described above may be achieved by control over the
viscosity of excess isocyanate prepolymer. A larger cell size is
obtained at lower viscosity of prepolymer and as the viscosity is
increased, the cell size gradually decreases. By adding a higher
amount of hydroxyl monomer in the prepolymer, the viscosity can be
manipulated while still keeping the net composition of the polymer
the same. Hence finer control of foam cell structure is possible.
This technique is based on the fact that at a lower viscosity of
the prepolymer, the rate of drainage of the polymer solution from
the foam cell lamella is higher, and also the resistance to foam
cell expansion with a given internal bubble pressure is lower, for
a longer period of time until the gelation of the polymer occurs.
Apart from changing the ratio of hydroxyl to isocyanate groups in
the prepolymer, the viscosity can also be manipulated by adding
some inert liquid phase solvents in the system.
Example 3
[0065] Foams with controlled cell structure were made by using
Hydroxy propyl ethylene diamine, Triethanol amine as the hydroxyl
monomers, and Hexamethylene diisocyanate. The viscosity of the
foaming solution was varied to achieve a controlled change in cell
structure of the foams.
[0066] First an NCO premix, or prepolymer with excess diisocyanate,
is made by mixing the components as per Table 2. Since this
composition readily forms a single phase solution, very vigorous
mixing is not required. Using a mechanical vortex or shaking by
hand worked well for creating a clear solution. The NCO premix is
then allowed to cure over a period of 2-3 days. The viscosity of
the solution increases as reaction between hydroxyl and isocyanate
groups takes place reaching a value in the range of 2-60 Pa's for
various formulations in Table 2 (viscosity values shown in FIG.
4).
[0067] Thereafter, an OH premix is made by mixing the components as
per Table 2 (amounts given for a single foam batch, and can be
scaled up for more batches).
[0068] For making the foams, NCO premix and OH premix are poured
together in the amounts per Table 2, and mixed vigorously in a
Flacktek (or equivalent mixer) for 10 sec at 3400 rpm speed. Then
Enovate is added to the foaming solution and mixed again for 5 sec
at 3400 rpm in Flacktek mixer. The solution is then transferred to
the oven at 90.degree. C. and allowed to rise up. Cell structure is
shown in FIG. 4.
[0069] The order of mixing the components together, mixing
durations and speeds, cure temperature and conditions can be
changed with similar results.
[0070] FIGS. 5A, 5B, 5C, 5D, 5E and 5F show variation in cell
structure achieved based on the viscosity of the foaming
prepolymer. Variation in cell structure is achieved based on the
viscosity of the foaming prepolymer. The net formulation of 44%
Hydroxy Propyl Ethylene Diamine, 11% Triethanol Amine and 41%
water, based on % equivalents, was used with Hexamethyene
diisocyante at 104 isocyanate index for all cases. Scale bar=400
um.
TABLE-US-00002 TABLE 2 Foam Formulation for varying the viscosity
of the foaming solution in order to obtain controlled variation in
cell structure. OH Premix Foam NCO premix DC- DC- BL- T- NCO OH
HPED TEA HDI HPED TEA Water I990 5169 22 131 premix Premix Enovate
(g) (g) (g) (g) (g) (g) (g) (g) (g) (g) (g) (g) (ml) 1 16.690 2.839
80.000 4.472 0.761 1.130 1.3 1.5 .253 .101 32 9.516 3 2 17.803
3.028 80.000 4.061 0.691 1.115 1.3 1.5 .253 .101 32 9.021 3 3
18.916 3.217 80.000 3.661 0.623 1.101 1.3 1.5 .253 .101 32 8.538 3
4 20.028 3.406 80.000 3.270 0.556 1.087 1.3 1.5 .253 .101 32 8.067
3 5 21.141 3.596 80.000 2.890 0.491 1.073 1.3 1.5 .253 .101 32
7.609 3 6 22.254 3.785 80.000 2.518 .428 1.060 1.3 1.5 .253 .101 32
7.161 3 1 is HPED80TEA20FW96 @ 0.30 OH/NCO; 2 is HPED80TEA20FW96 @
0.32 OH/NCO 3 is HPED80TEA20FW96 @ 0.34 OH/NCO; 4 is
HPED80TEA20FW96 @ 0.36 OH/NCO; 5 is HPED80TEA20FW96 @ 0.38 OH/NCO
and 6 is HPED80TEA20FW96 @ 0.40 OH/NCO.
[0071] Surfactants DC-1990 and DC-5169, and catalysts BL-22 and
T-131 are used as received from Air Products, Inc. Actual weights
of chemicals added in grams are given.
[0072] Variation in the duration and speed of mixing the foaming
solution is another method of controlling the cell structure of
foams disclosed here. A higher duration or speed of mixing of the
foaming solution gives finer cell size with denser foams because
the forces acting on the material during mixing can break up the
larger bubbles. Further, due to the viscous heating during mixing,
part of the blowing gas can be lost, and also the reaction can
speed up causing gelation to occur faster. This leaves less time
for the foam to blow up causing denser foams with smaller cell
structure. As the mixing time is reduced on the other extreme, the
foam structure can be non-uniform or the foams can eventually
collapse due to phase separation of hydroxyl and isocyanate
moieties, and their inability to react fast enough. For very
immiscible monomers, if collapse is still seen at relatively high
mixing durations, the addition of the hydroxyl premix can be done
in two steps, a) First mixing the balance hydroxyl monomers with
the isocyanate premix for longer time on the order of a few
minutes, b) then catalysts, water and physical blowing agents added
to foaming solution and mixed for a shorter time on the order of a
few seconds.
[0073] Increase in the amount of surfactants and/or catalysts can
also help in decreasing the cell size by increasing the
stabilization of the cells and increasing the rate of reaction
respectively.
General Embodiment 3
Method of Making Extremely Low Density Foams (Up to 0.005 g/Cc)
Using Co-Blowing Agents and Other Related Methods
[0074] Foams of very low density (as low as 0.005 g/cc), are made
by use of successively higher boiling point blowing agents. This
strategy is based on the process of foam blowing. For a pure
liquid, formation of foam cells during blowing is dependent on the
concentration of gas present in the foaming solution at any given
time. As the gas concentration increases, it is only above a
critical concentration that Rapid Self Nucleation (RSN) begins to
occur--see Klempner, D., Sendijarevic, V., Sendijarevi c, V. &
Aseeva, R. M. Handbook of polymeric foams and foam technology.
(Hanser Gardner Publications, 2004) and LaMer, V. Kinetics in phase
transitions. Industrial & Engineering Chemistry 44, 1270-1277
(1952), (FIG. 6). As the gas is used up in formation of cells, its
concentration gradually decreases below the critical concentration
required for self nucleation. Beyond this point, diffusion of the
gas in the already existing bubbles leads to Growth of the bubbles
By Diffusion (GSD), i.e. increase in foam cell size. If the
concentration reaches the Critical Limiting Super saturation (CLS)
the excess gas will spontaneously form larger voids. Hence by being
able to increase the concentration of gas in the foaming solution
at regular intervals, we can control its concentration in RSN range
and increase the number of cells, rather than increasing the cell
size or allowing development of voids. This results in foams with
lower densities while maintaining a high modulus of foams, without
formation of large voids/holes. Some options for such blowing
agents include HFC 245-fa (Boiling Point 15.3.degree. C.), Micro
care CF: Blend of 40% HFC 365-mfc and 60% HFC 4310-mee (Boiling
Point 45.degree. C.) and HFE 7100 (Boiling Point 61.degree. C.).
While this method decreases the density of the foam by introduction
of higher amount of gas phase, the cell structure can also get
changed in this process leading to larger cells because of a higher
gas pressure inside the cells. For maintaining a small cell size,
simultaneous increase in the surfactant levels needs to be done to
decrease the surface tension and stabilize the small cell size.
Instead of using multiple blowing agents, a process of introducing
a given blowing agent to the foaming solution, at specified time
intervals, can also be engineered with the same result.
[0075] Use of particulate nucleating agents is another way in which
we can catalyze the generation of bubbles and potentially lower the
possibility of formation of voids or large increase in cell size.
Adding leachable porogens such as salt, to the foaming solution in
conjunction with the physical and chemical blowing agents, can work
in this direction. Here the presence of porogens can assist in
keeping the individual cell size from growing larger, both by
nucleation of bubbles, and by increasing the viscosity of the
foaming solution. Also, they can be leached out of the foam post
cure, further lowering the foam bulk density.
[0076] Another foam processing technique that can achieve densities
down to 0.005 g/cc, while maintaining a good shape memory behavior,
is to pull vacuum on the foam while the foam is rising, and allow
it to cure under vacuum.
[0077] Applying vacuum counters the effect of gravity and assists
in a better expansion of the foam, leading to lower densities. The
time of pulling vacuum during foaming can be changed to manipulate
the cell size and density: vacuuming too soon in the foaming
process gives larger cells and lower density; vacuuming much later
in the foaming process gives finer cells with relatively higher
density. This technique can potentially be used in conjunction with
other methods of using leachable porogens, nucleating agents and
adjustment of surfactant levels etc. to simultaneously control the
cell structure.
[0078] Another technique for making highly crosslinked low density
foams is to use two or higher functionality carboxylic acids in
place of water as the blowing agent. One carboxylic acid group
reacts with one isocyanate group to release a carbon-dioxide
molecule while forming an amide bond. This is twice the production
of blowing gas compared to water with the same amount of
isocyanate, making it a promising technique. Further higher
functionality of carboxylic acids, such as Citric Acid, form
covalent crosslinks in the foam, as opposed to physical crosslinks
with urea from the use of water, and assist in making a highly
covalently crosslinked network structure per our material design
criteria. This further entails the ability to significantly
increase the concentration of carboxylic acid based chemical
blowing agents in the foam formulation without affecting the
network structure of the material, which is not possible with use
of water. This gives an important handle in reducing the foam bulk
density while maintaining a high density of covalent crosslinks in
the material network structure.
General Embodiment 4
Methods of Optimization of the Physical Properties Such as
Porosity, Cell Size and Distribution, Cell Openness Etc. Of these
Materials, to Further Expand their Uses and Improve their
Performance
[0079] Density/porosity of the materials can be controlled either
for a specific density/porosity, or a continuous gradient or
another pattern in the variation density/porosity across a sample.
This can be done e.g. by using one or more of the techniques in the
method 3) above on a single piece of foam, and/or by combining
different foams of varying density/porosity gradients as
desired.
[0080] Similarly the cell size and cell size distribution of a foam
sample can be engineered, e.g. by using one or more of the methods
in the 2) above on the same foam, and/or by combining different
foams of varying cell sizes and distribution to achieve respective
gradients as desired.
[0081] Again the degree of cell openness of the foams can be
controlled, for e.g. via changing the synthesis process, such as
the amount and type of surfactants and catalysts etc. or using post
processing methods for removal of membranes, such as hydrolysis,
oxidation, heat or mechanical treatment etc.
[0082] By exercising control of one or more of density/porosity,
cell size, cell size distribution, cell openness etc. with a
desired pattern of variation of respective properties throughout
the bulk of the material, the material can be optimized for use in
multiple applications. For e.g. when a foam sample is actuated in a
media, the rate of diffusion of the media in the foam can be
controlled by controlling the density/cell sizes/cell openness etc.
throughout the foam sample.
[0083] Improved cell opening is achieved, in another embodiment by
the use of high z metal nano- or micro-particles, including but not
limited to tungsten, tantalum, platinum and, palladium. These
particles can serve the dual purpose of a) assisting in the cell
opening during the foaming process, and/or b) providing x-ray
contrast for imaging in the foam devices.
[0084] Metals typically have a surface oxide layer which provides
them with a high surface energy and therefore allowing them to be
wetting with the foam formulation. A surface modification of the
metal particles with a low surface energy coating such as a
fluorinated coupling agent (e.g. fluorosiloxane, fluorosilane,
fluorocarbon, fluoropolymer) helps to destabilize the membrane
during the foaming process by decreasing the extent of its wetting
with the foaming solution.
[0085] Mechanical surface treatment of the metal particles, such as
roughening, is another method to enhance their ability to open
cells (and reduce adhesion of the PU foam to the particles). It is
known that liquid repellent surfaces of high surface energy
materials can simply be made by having nano-scale grooves or other
types of roughness (possibly pillars) in the surface. Particle
surface roughness can also affect the ability of the foam
formulation to wet the particle surface, and enhance foam cell
membrane destabilization, in the same way.
[0086] The size of the particulate is another important parameter
since the particles may be more effective in destabilizing the foam
cell membranes as the membrane thickness approaches particle size.
Selection of the particle size can be used to tune the specific
area at which the membranes are destabilized during foaming. The
onset of destabilization is expected to occur approximately at the
point when particle size equals membrane thickness.
[0087] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and have been described in
detail herein. However, it should be understood that the invention
is not intended to be limited to the particular forms disclosed.
Rather, the invention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention as defined by the following appended claims.
* * * * *