U.S. patent application number 12/019867 was filed with the patent office on 2008-08-07 for process for preparing microrods using liquid-liquid dispersion.
This patent application is currently assigned to North Carolina State University. Invention is credited to Rossitza Gueorguieva Alargova, Orlin Dimitrov Velev.
Application Number | 20080187752 12/019867 |
Document ID | / |
Family ID | 35262150 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080187752 |
Kind Code |
A1 |
Velev; Orlin Dimitrov ; et
al. |
August 7, 2008 |
PROCESS FOR PREPARING MICRORODS USING LIQUID-LIQUID DISPERSION
Abstract
The invention provides a method for forming polymer microrods,
the method including the steps of providing a polymer solution
comprising a polymer dissolved in a first solvent; providing a
dispersion medium comprising a second solvent, wherein the first
solvent and the second solvent are miscible or partially soluble in
each other, and wherein the polymer is insoluble in the second
solvent; adding the polymer solution to the dispersion medium to
form a dispersed phase of polymer solution droplets within the
dispersion medium; and introducing a shear stress to the dispersion
medium and dispersed polymer solution droplets for a time and at a
shear rate sufficient to elongate the polymer solution droplets to
form microrods and solidify the microrods by attrition of the
polymer solvent into the dispersion medium.
Inventors: |
Velev; Orlin Dimitrov;
(Cary, NC) ; Alargova; Rossitza Gueorguieva;
(Worcester, MA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA, 101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
North Carolina State
University
|
Family ID: |
35262150 |
Appl. No.: |
12/019867 |
Filed: |
January 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11153888 |
Jun 15, 2005 |
7323540 |
|
|
12019867 |
|
|
|
|
60580022 |
Jun 16, 2004 |
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Current U.S.
Class: |
428/375 ;
428/397 |
Current CPC
Class: |
D01D 5/40 20130101; Y10T
428/2933 20150115; A61K 9/5094 20130101; C08J 2363/00 20130101;
C08J 3/11 20130101; Y10T 428/2973 20150115; C08J 3/12 20130101 |
Class at
Publication: |
428/375 ;
428/397 |
International
Class: |
B32B 19/00 20060101
B32B019/00 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The research underlying this invention was supported in part
with funds from National Science Foundation (NSF) grant no. CAREER
CTS 0238636 and NER CTS 0403462. The United States Government may
have an interest in the subject matter of this invention.
Claims
1. A plurality of polymer microrods having an average aspect ratio
of at least about 5, an average length of about 10 to about 100
.mu.m, and an average diameter of about 0.1 to about 10 .mu.m.
2. The plurality of polymer microrods of claim 1, wherein the
average aspect ratio is at least about 20.
3. The plurality of polymer microrods of claim 1, wherein the
average length is about 10 to about 50 .mu.m.
4. The plurality of polymer microrods of claim 1, wherein the
average diameter is about 0.5 to about 3 .mu.m.
5. The plurality of polymer microrods of claim 1, wherein the
microrods comprise a polymer selected from the group consisting of
polyolefin, polyether, polyamide, polyamideimide, polyarylate,
polybenzimidazole, polyester, polyurethane, polyimide,
polyhydrazide, phenolic resins, polysilane, polysiloxane,
polycarbodiimide, polyimine, azo polymers, polysulfide,
polysulfone, copolymers thereof, and mixtures thereof.
6. The plurality of polymer microrods of claim 1, wherein the
microrods comprise a polymer selected from the group consisting of
polyethylene, polypropylene, poly(vinyl chloride), polystyrene,
polytetrafluoroethylene, poly(.alpha.-methylstyrene), poly(acrylic
acid), poly(isobutylene), poly(acrylonitrile), poly(methacrylic
acid), poly(methyl methacrylate), poly(1-pentene),
poly(1,3-butadiene), poly(vinyl acetate), poly(2-vinyl pyridine),
1,4-polyisoprene, 3,4-polychloroprene, poly(ethylene oxide),
polyformaldehyde, polyacetaldehyde, poly(3-propionate),
poly(10-decanoate), poly(ethylene terephthalate), polycaprolactam,
poly(11-undecanoamide), poly(hexamethylene sebacamide),
poly(m-phenylene terephthalate), and
poly(tetramethylene-m-benzenesulfonamide).
7. The plurality of polymer microrods of claim 1, wherein the
microrods comprise an epoxy resin.
8. The plurality of polymer microrods of claim 1, wherein the
microrods comprise a naturally-occurring polymer.
9. The plurality of polymer microrods of claim 1, wherein the
microrods comprises a particulate material imbedded therein.
10. The plurality of polymer microrods of claim 9, wherein the
particulate material comprises silica or magnetic particles.
11. The plurality of polymer microrods of claim 1, wherein the
polymer microrods can be rotated or aligned by application of an
external electric or magnetic field.
12. The plurality of polymer microrods of claim 1, wherein the
polymer microrods are anisotropic.
13. The plurality of polymer microrods of claim 1, wherein the
polymer microrods are crosslinked.
14. A material selected from the group consisting of liquid crystal
materials, nonwoven textiles, paper materials, porous fibrillar
materials, pads, linings, sponges, filters, food products, paints,
pharmaceutical compositions, cosmetics, bath products, latex foams,
polyurethane foams, and fibrillar matrices for cell attachment, the
material comprising a plurality of polymer microrods according to
claim 1.
15. The material of claim 14, wherein the material is a food
product selected from the group consisting of ice cream,
mayonnaise, salad dressings, and whipped cream.
16. The material of claim 14, wherein the material is a
pharmaceutical composition comprising the polymer microrods in the
form of colloidosome microcapsules.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application of
U.S. application Ser. No. 11/153,888, filed Jun. 15, 2005, which
also claims the benefit of U.S. Provisional Application No.
60/580,022, filed Jun. 16, 2004, which are incorporated herein by
reference in their entirety and for all purposes.
FIELD OF THE INVENTION
[0003] The invention is directed to methods of preparing and using
micro-sized rod-shaped polymer particles.
BACKGROUND OF THE INVENTION
[0004] During the last decade, the rapidly progressing area of
nanotechnology has shown that dimensionality plays a crucial role
in determining the properties of a material. J. Hu, T. W. Odom, and
C. M. Lieber, Acc. Chem. Res., 1999, 32, 435. The large spectrum of
newly synthesized structures with one or two dimensions in the
nanometer scale opens large fields for synthesis of new materials
for electronic and computer technology. Nanowires and nanotubes
appear to be quite attractive in this respect due to their unique
optical, magnetic and electrical properties. M. S. Dresselhaus, G.
Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon
Nanotubes, Academic Press, San Diego 1996; C. M. Lieber, Solid
State Commun. 1998, 107, 607; N. Agrait, A. L. Yeyati, J. M.
Ruitenbeek, Phys. Rep. 2003, 377, 81. Micro- and nanostructured
materials formed by assembly of colloidal particles could find a
wide range of applications ranging from photonics and electronics
to catalysis, bioprocessing, sensors, and energy storage. F.
Caruso, Ed., Colloids and Colloid Assemblies. Synthesis,
Modification, Organization, and Utilization of Colloid Particles,
Wiley-VCH, Weinheim 2003, Ch. 8-15. The functionality of these
materials largely depends on the size, shape and physical
properties of the particles from which they are assembled. The use
of anisotropic particles is of particular interest, as it allows
the creation of materials of advanced microstructure or anisotropic
properties.
[0005] Dispersions of polymer cylinders with colloidal sizes have
unique optical and electrical properties because of the different
ordering of the molecules inside them in comparison to bulk
materials. C. R. Martin, Chem. Mater, 1996, 8, 1739; J.-K. Lee,
W.-K. Koh, W.-S. Chae, and Y.-R. Kim, Chem. Comm., 2002, 138.
Microcylinders can serve as a medium for longitudinal ordering of
smaller rod-like objects, such as carbon nanotubes, for enzyme
immobilization, or for the preparation of composite nanostructures.
Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin, E.
Zussman, Langmuir 2003, 19, 7012; J. C. Hulteen, C. R. Martin, J.
Mater. Chem. 1997, 7, 1075. Polymer rods of varying aspect ratios
can assemble into various structures that could find applications
as colloidal liquid crystals, pH-, electrolyte- and
biologically-sensitive gels, photonic crystals of non-trivial
symmetry, etc. For decades, the entropic self-assembly of rod-like
colloidal particles has been of intense interest, but it has been
studied mainly with viruses, and in a few cases with inorganic
particles, as suitable anisotropic particles were not readily
available. P. A. Forsyth, Jr. S. Mar{hacek over (c)}elja, D. J.
Mitchell, B. W. Ninham, Adv. Colloid Interface Sci. 1978, 9, 37; G.
J. Vroege, H. N. W. Lekkerkerker, Rep. Prog. Phys. 1992, 55, 1241;
Z. Dogic, S. Fraden, Phys. Rev. Lett. 1997, 78, 2417; M. Adams, Z.
Dogic, S. L. Keller, S. Fraden, Nature 1998, 393, 349; M. P. B. van
Bruggen, F. M. van der Kooij, H. N. W. Lekkerkerker, J. Phys.:
Condens. Matter 1996, 8, 9451; G. A. Vliegenthart, A. van
Blaaderen, H. N. W. Lekkerkerker, Faraday Discuss. 1999, 112, 173;
F. M. van der Kooij, H. N. W. Lekkerkerker, Phys. Rev. Lett. 2000,
84, 781.
[0006] Polymer structures in the form of nano-sized fibers, tubes,
and "pencils" have been prepared using a "template synthesis,"
which entails synthesizing the desired material within the
cylindrical pores of a membrane (either inorganic or organic) or
other porous structures, such as zeolites or mesoporous silica. V.
M. Cepak, C. R. Martin, Chem. Mater. 1999, 11, 1363; M. Steinhart,
J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J.
Schilling, J. Choi, U. Gosele, Science 2002, 296, 1997; S. Ai, G.
Lu, Q. He, J. Li, J. Am. Chem. Soc. 2003, 125, 11140; S. I. Moon,
T. J. McCarthy, Macromolecules 2003, 36, 4253; C.-Y. Peng, W. J.
Nam, S. J. Fonash, B. Gu, A. Sen, K. Strawhecker, S. Natarajan, H.
C. Foley, S. H. Kim, J. Am. Chem. Soc. 2003, 125, 9298; H. L.
Frisch, J. E. Mark, Chem. Mater. 1996, 8, 1735; M. Fu, Y. Zhu, R.
Tan, G. Shi, Adv. Mater. 2001, 13, 1874. Depending on the material
and its interactions with the pore walls, the polymer nanocylinders
formed may be solid or hollow tubes. Their synthesis can be
achieved via polymerization of the corresponding monomer inside the
pores or by infiltration of melted polymer or polymer solution
trough the porous medium. The length of the polymer fibers formed
is equal to the membrane thickness and their diameter is close to
the average diameter of the template pores. Although this method
provides good control over the particle sizes, it has a few major
disadvantages: (i) in order to synthesize particles with desired
dimensions, an appropriate template has to be found or prepared;
and (ii) after completing the particle synthesis, additional
treatment procedures are necessary to remove the template. In most
cases, these procedures are expensive and not environmentally
friendly since they include using a concentrated solution of sodium
hydroxide (in the case inorganic membranes) or organic solvents
(for organic templates). In general, the synthesis of large amounts
of polymer cylinders using a template method is limited and
costly.
[0007] An alternative technique for production of polymer
nanofibers is the electro-spinning method. Y. Dror, W. Salalha, R.
L. Khalfin, Y. Cohen, A. L. Yarin, and E. Zussman, Langmuir, 2003,
19, 7012; A. Theron, E. Zussman, and A. L. Yarin, Nanotechnology,
2001, 12, 384. In this method, an electrostatic field is created
between a pending drop of a polymer solution and a rotating disk.
The electrostatic forces draw a jet of the polymer solution, which
solidifies upon solvent evaporation, and the resulting nanofibers,
with lengths in the range of hundreds of micrometers, are deposited
on the grounded disk. Besides the special equipment needed, the
polymer solution used has to be amenable to electro-spinning.
Moreover this method leads to a production of long fibers and does
not provide a means for controlling fiber length. Similarly to the
template method, very limited amounts of rods can be obtained and
scalability is not easy.
[0008] Polymer nanocylinders can also be formed by self-assembly of
block copolymer molecules. J. Ding, G. Liu, M. Yang, Polymer 1997,
38, 5497; Y. Yu, A. Eisenberg, J. Am. Chem. Soc. 1997, 119, 8383;
G. Liu, X. Yan, S. Duncan, Macromolecules 2002, 35, 9788; J. Raez,
J. P. Tomba, I. Manners, M. A. Winnik, J. Am. Chem. Soc. 2003, 125,
9546. Emulsion polymerization of tetrafluoroethylene in the
presence of rod-like surfactant micelles has also yielded
nano-sized cylindrical polymer particles. C. U. Kim, J. M. Lee, S.
K. Ihm, J. Fluorine Chem. 1999, 96, 11.
[0009] Rod-like cylindrical particles on the micrometer scale could
also form the basis of materials with unique and advantageous
properties, yet very few processes for making such particles have
been developed. Due to the lack of methods for facile fabrication
of microrods, virtually every material assembled from micron-sized
particles has been formed from spheres of silica or polymer latex.
Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 2000, 12, 693. Thus,
there remains a need in the art for a method for forming polymer
microrods in a cost-effective and scalable manner.
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention provides a method for synthesis of
polymer rod-like particles with characteristic sizes in the
micrometer range and a relatively large aspect ratio. It is based
on a liquid-liquid dispersion technique and provides a simple way
to prepare a large quantity of polymer rods. The present invention
does not require use of a template or external electric field, and
is very rapid, robust, and inexpensive. In addition, unlike prior
methods, the present invention is easily scalable. The new class of
polymer microrods formed using the method of the invention can be
as inexpensive and ubiquitous as spherical latex particles, and
serve as a replacement thereof, but also possess a variety of
properties that could be valuable for self-assembly and materials
applications.
[0011] Thus, in one aspect, the invention is directed to a method
for forming polymer microrods. The method involves providing a
polymer solution comprising a polymer dissolved in a first solvent
and a dispersion medium comprising a second solvent. The first
solvent and the second solvent are miscible or partially soluble in
each other, and the polymer is insoluble in the second solvent. The
polymer solution is added to the dispersion medium to form a
dispersed phase of polymer solution droplets within the dispersion
medium. A shear stress is introduced to the dispersion medium
comprising dispersed polymer solution droplets for a time and at a
shear rate sufficient to elongate the polymer solution droplets.
The elongated droplets form microrods by solidifying when the
solvent of the polymer leaves the droplets and a solidified polymer
remains. In a preferred embodiment, the shear stress is applied to
the dispersion medium prior to addition of the polymer solution
such that the dispersed polymer droplets are immediately subjected
to the shear stress.
[0012] In another embodiment of the method of the invention, a
polymer solution comprising a polymer dissolved in a first solvent
and a dispersion medium comprising a second solvent are provided,
wherein the solubility of the first solvent in the dispersion
medium is at least about 50 g/L at 25.degree. C. and the solubility
of the polymer in the dispersion medium is less than about 1 g/L at
25.degree. C., and wherein the ratio of the viscosity of the
polymer solution to the viscosity of the dispersion medium is at
least about 0.02:1. The polymer solution is added to the dispersion
medium to form a dispersed phase of polymer solution droplets
within the dispersion medium, and a shear stress is introduced to
the dispersion medium and dispersed polymer solution droplets for a
time and at a shear rate sufficient to elongate the polymer
solution droplets to form microrods and solidify the microrods. The
shear stress is initially introduced prior to, or simultaneously
with, the adding step, and the shear rate is at least about 20
l/s.
[0013] In another aspect, the invention provides microrods produced
using the method described above. In yet another aspect, the
invention provides a variety of compositions and end products
comprising the microrods of the invention. For example, it has been
shown that superstabilization of foams (e.g., aqueous foams) and
emulsions can be achieved by using polymer microrods of the
invention in the absence of any surfactant. Due to their rigid
structure and resistance to mechanical perturbations, the polymer
rodlike foams can be used in applications where conventional foam
stabilizers are not effective. Foams or emulsions comprising
polymer microrods and their mixtures or composites with other nano-
and microparticles may also serve for synthesis of new
materials.
[0014] In one embodiment, the present invention provides a
stabilized foaming composition or emulsion comprising a plurality
of polymer microrods prepared according to the above-described
methods. The polymer microrods preferably have an average aspect
ratio of at least about 5, an average length of about 10 to about
100 .mu.m, and an average diameter of about 0.1 to about 10 .mu.m.
Exemplary foaming compositions or emulsions include food products,
cosmetics, bath products, pharmaceutical compositions, and
firefighting foam. The polymer microrods form a layer that
substantially encapsulates the bubbles or droplets of foams and
emulsions, thereby providing a stabilization effect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Some of the features and advantages of the invention having
been described, others will become apparent from the detailed
description which follows, and from the accompanying drawings, in
which:
[0016] FIG. 1 is a schematic of the process for synthesis of
rod-like polymer particles;
[0017] FIGS. 2a-2d are optical microscopy (FIG. 2a, 2d) and SEM
(FIG. 2b, 2c) images of SU-8 microrods taken at different
magnification, with an inset in FIG. 2c showing the tip of the rod
at higher magnification (scale bars: (FIG. 2a) 100 .mu.m, (FIG. 2b,
2c) 10 .mu.m, (1 .mu.m in the inset of FIG. 2c), and (FIG. 2d) 50
.mu.m);
[0018] FIGS. 3a-3d are optical micrographs of SU-8 rods formed in a
50:50 vol % glycerol/ethylene glycol dispersion medium at
increasing stirring speed (corresponding to increasing shear
rates): (FIG. 3a) 200 rpm, (FIG. 3b) 500 rpm, (FIG. 3c) 900 rpm,
and (FIG. 3d) 1400 rpm (scale bar: 50 .mu.m), corresponding to
increasing shear rates from 93 to 700 s.sup.-1;
[0019] FIGS. 4a-4d are optical microscopy images of SU-8 microrods
dispersed in water and subjected to an AC electric field created by
needle-like electrodes, wherein he configuration of the energized
electrodes are shown in the insets: (FIG. 4a) Rod orientation in
the vicinity of a point electrode at an AC field of 50 V/cm, 200
Hz; (FIGS. 4b-4d) images of the particles in the middle of the
experimental cell taken with higher magnification (scale bars:
(FIG. 4a) 100 .mu.m, and (FIG. 4b-4d) 50 .mu.m);
[0020] FIGS. 5a-5b graphically illustrate the time dependence of
foam volume: 5(a) is the time dependence of the volume of foams,
Vf, formed with SU-8 rods, the lines of experimental points (lack
of any foam breakdown) correspond to different solid concentrations
[(.diamond-solid.) 2.18 wt %; (.quadrature.) 1.09 wt %;
(.tangle-solidup.) 0.44 wt %; (.largecircle.) 0.22 wt %], and the
inset shows the volume changes for the first 30 min.; 5(b) is the
time dependence of the volume of foams, Vf, formed in the presence
of (.DELTA.) 8.7 mM sodium dodecyl sulfate (SDS);
(.tangle-solidup.) 2.18 wt % SU-8 rods mixed with 8.7 mM SDS; and
(.diamond-solid.) 2.18 wt % SU-8 rods only, and the inset shows the
volume changes for the first 30 min (the curves are guides to the
eye);
[0021] FIG. 6 graphically illustrates the volume of the foam formed
as a function of SU-8 microrod concentration in the water phase,
which correspond to plateau values for Vf in FIG. 5a (the line is a
guide to the eye); and
[0022] FIGS. 7a-7d are optical microscopy images of (a) the SU-8
microrods used as foam superstabilizers in Example 3; (b) foam
stabilized by rods; (c) a single "hairy" air bubble covered by a
layer of adsorbed rods; (d) a single thin aqueous film formed from
a suspension of SU-8 rods, and wherein the inset in image (d) is a
highly magnified area near the film center (scale bar is 50 .mu.m
for images 7(a) and 7(c) and the inset of image 7(d) and 200 .mu.m
for images 7(b) and 7(d)).
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention now will be described more fully
hereinafter. However, this invention may be embodied in many
different forms and should not be construed as limited to the
embodiments set forth herein; rather, these embodiments are
provided so that this disclosure will satisfy applicable legal
requirements. As used in this specification and the claims, the
singular forms "a," "an," and "the" include plural referents unless
the context clearly dictates otherwise. All literature references
noted herein are incorporated by reference in their entirety.
[0024] The present invention is directed to a method of forming
polymer microrods using a liquid-liquid dispersion technique,
wherein a polymer dissolved in a first solvent is dispersed in a
dispersion medium comprising a second solvent. The first and second
solvents are miscible or partially soluble, but the polymer is
insoluble in the second solvent. The polymer solution dispersed in
the dispersion medium is sometimes referred to as the "oil" phase
herein. The volume of polymer solution added to the dispersion
medium can vary, but is typically in the range of about 0.1 to
about 1.0 ml per 100 ml of dispersion medium.
[0025] A shear stress is introduced or applied to the dispersion
medium and dispersed polymer solution for a time sufficient to form
polymer microrods and solidify the thus-formed microrods. The shear
can be applied by any known method in the art. Typically, the shear
stress is applied by a mixing vessel equipped with an impeller.
[0026] As used herein, the terms "microrod" and "microcylinder"
refer to rod-shaped polymer particles having relatively high aspect
ratios (i.e., ratio of length to diameter), typically an aspect
ratio of at least about 5, more preferably at least about 20, and
having at least one (or both) of length and diameter in the
micrometer or sub-micrometer range, typically in the range of about
10 to about 100 .mu.m for length (preferably about 10 to about 50
.mu.m), and about 0.1 to about 10 .mu.m for diameter (preferably
about 0.5 to about 3 .mu.m).
[0027] The polymer used in the present invention can be any
polymeric material capable of being dissolved in a good solvent,
and that can form a homogeneous solution at a relatively high
concentration. Exemplary polymers include vinyl polymers such as,
but not limited to, polyethylene, polypropylene, poly(vinyl
chloride), polystyrene, polytetrafluoroethylene,
poly(.alpha.-methylstyrene), poly(acrylic acid), poly(isobutylene),
poly(acrylonitrile), poly(methacrylic acid), poly(methyl
methacrylate), poly(1-pentene), poly(1,3-butadiene), poly(vinyl
acetate), poly(2-vinyl pyridine), 1,4-polyisoprene, and
3,4-polychloroprene. Exemplary polymers also include nonvinyl
polymers such as, but not limited to, poly(ethylene oxide),
polyformaldehyde, polyacetaldehyde, poly(3-propionate),
poly(10-decanoate), poly(ethylene terephthalate), polycaprolactam,
poly(11-undecanoamide), poly(hexamethylene sebacamide),
poly(m-phenylene terephthalate),
poly(tetramethylene-m-benzenesulfonamide). Typically, the polymer
will fall within one of the following polymer classes: polyolefin,
polyether (including all epoxy resins, polyacetal,
polyetheretherketone, polyetherimide, and poly(phenylene oxide)),
polyamide (including polyureas), polyamideimide, polyarylate,
polybenzimidazole, polyester (including polycarbonates),
polyurethane, polyimide, polyhydrazide, phenolic resins,
polysilane, polysiloxane, polycarbodiimide, polyimine, azo
polymers, polysulfide, and polysulfone.
[0028] The polymer can be synthetic or naturally-occurring.
Particularly preferred natural polymers include polysaccharides and
derivatives thereof such as cellulosic polymers (e.g., cellulose
and derivatives thereof) and starch polymers. Exemplary derivatives
of starch and cellulose include various esters, ethers, and graft
copolymers. The polymer may be crosslinkable in the presence of a
multifunctional crosslinking agent or crosslinkable upon exposure
to actinic radiation. The polymer can be homopolymers of any of the
foregoing polymers, random copolymers, block copolymers,
alternating copolymers, random tripolymers, block tripolymers,
alternating tripolymers, derivatives thereof (e.g., graft
copolymers, esters, or ethers thereof), and the like. The polymer
molecular weight is not considered a limiting factor in the present
invention, and the number average molecular weight will typically
be in the range from about 250 to more than 100,000 Da, although
any molecular weight could be used without departing from the
invention.
[0029] It is preferable for the polymer solution to be relatively
concentrated prior to addition of the polymer solution to the
dispersion medium in order to give the optimal viscosity ratio. In
one embodiment, the SU-8 polymer comprises at least about 25 weight
percent of the polymer solution, more preferably at least about 30
weight percent, most preferably at least about 40 weight percent.
In certain embodiments the SU-8 polymer comprises about 30 to about
75 weight percent of the polymer solution, more preferably about 30
to about 60 weight percent. Polymers other than SU-8 may also be
used in these concentration ranges, or may be used in lower amounts
such as about 5-10 weight percent, depending on the polymer
molecular weight.
[0030] As would be readily understood by one of ordinary skill in
the art, the polymer solution and the dispersion medium may
comprise more than one solvent. However, for ease of reference, the
two solvents are referred to in the singular form below. The
solvent of the polymer solution and the solvent in the dispersion
medium can be any two solvents that are miscible or partially
soluble. The polymer must be relatively insoluble, preferably
completely insoluble, in the dispersion medium solvent.
[0031] The level of miscibility of the polymer solvent and the
dispersion medium must be sufficient for formation of microrods
upon application of a sufficient shear stress to the dispersed
polymer solution. As used herein, the term "miscible" refers to the
ability of two liquids to dissolve completely in one another
regardless of the proportion of each liquid. The term "partially
soluble" refers to two liquids that are soluble in one another to a
degree less than full miscibility, but sufficient to promote
formation of microrods in the method of the invention. Typically,
partially soluble solvents will have a solubility of at least about
5 g/L at 25.degree. C. in each other. Although the solubility of
the polymer solvent in the dispersion medium solvent may vary, it
is preferably at least about 10 g/L, and more preferably at least
about 20 g/L at 25.degree. C. In many embodiments, the polymer
solvent has a solubility in the dispersion medium solvent of at
least about 50 g/L, at least about 75 g/L, or even at least about
100 g/L at 25.degree. C. To ensure the necessary solubility between
the two solvents, each solvent is preferably selected from the same
general class of solvents (i.e., both solvents are nonpolar
solvents, polar protic solvents, or polar aprotic solvents). In one
preferred embodiment, the polymer solvent is .gamma.-butyrolactone
(GBL) and the dispersion medium solvent is an alcohol (e.g.,
ethanol, methanol, isopropanol, ethylene glycol, or mixtures
thereof). In this embodiment, the GBL has a solubility of about
100-120 g/L or more in the dispersion medium.
[0032] The polymer is insoluble in the dispersion medium solvent,
meaning that the polymer has a solubility in the dispersion medium
solvent of typically less than about 2 g/L at 25.degree. C.,
preferably less than about 1 g/L, more preferably less than about
0.5 g/L, and most preferably less than about 0.1 g/L.
[0033] Both the polymer solvent and the dispersion medium solvent
are generally selected from one of the recognized organic solvent
classes of oxygenated solvents (e.g., alcohols, glycol ethers,
ketones, esters, and glycol ether esters), hydrocarbon solvents
(e.g., aliphatic and aromatic hydrocarbons), and halogenated
solvents (e.g., chlorinated hydrocarbons), subject to the
compatibility and solubility requirements outlined above. However,
in certain embodiments, one of the two solvents can be an aqueous
solution. Exemplary solvents include, but are not limited to, acids
(e.g., formic acid, acetic acid), alcohols (e.g., ethanol,
methanol, isopropanol, ethylene glycol), aldehydes (e.g.,
formaldehyde, acrolein), aliphatic hydrocarbons (e.g., pentane,
hexane, heptane, dodecane), amines (e.g., diisopropylamine,
triethanolamine, dimethylamine, butylamine), aromatic hydrocarbons
(e.g., benzene, xylene, toluene), esters (e.g., acetic acid,
isopropyl ester, methyl propionate, .gamma.-butyrolactone), ethers
(e.g., triethylene glycol dimethyl ether, triethylene glycol
diethyl ether), and ketones (e.g., diisobutyl ketone,
cyclohexanone).
[0034] The dispersion medium, the polymer itself, and/or the
polymer solution may include one or more additives known in the art
without departing from the present invention. Exemplary additives
include, without limitation, colorants (e.g., fluorescent dyes and
pigments), odorants, deodorants, plasticizers, impact modifiers,
fillers, nucleating agents, lubricants, surfactants, wetting
agents, flame retardants, ultraviolet light stabilizers,
antioxidants, biocides, thickening agents, heat stabilizers,
defoaming agents, blowing agents, emulsifiers, crosslinking agents,
waxes, particulates, flow promoters, and other materials added to
enhance processability or end-use properties of the polymeric
components. Such additives can be used in conventional amounts.
These additives can be added before, during or after formation of
the polymer dispersion and/or formation of the polymer microrods.
In certain embodiments, a surfactant, preferably a nonionic or
anionic surfactant, is added to a solution comprising the microrods
in order to enhance dispersion of the microrods in the solution,
particularly where the microrods are in an aqueous solution.
[0035] In one embodiment, micro- and nanoparticles, such as
particles of silica or magnetic particles (e.g., iron dioxide
particles encapsulated by a silane shell) are imbedded in the
polymer. The imbedded particles can be any nano- and microparticles
with dimensions not exceeding the diameter of the microrods by more
than 20%. Exemplary nano- or microparticles include particles
derived from metals, semiconductors or dielectrics, including
carbon or inorganic nano-tubes, which can be dispersed in the
initial polymer solvent. These particle additives can be added to
the polymer by, for example, initially dispersing the particles in
the polymer solvent and then mixing the polymer into the solvent.
The weight percent of the micro- and nanoparticles is typically
about 5 to about 70 weight percent, based on the total weight of
the polymer.
[0036] The major stages of the process of microrod formations are
shown in FIG. 1. The first stage is the formation of small
micrometer-sized oil droplets. This stage is likely assisted by a
spontaneous emulsification of the oil that is caused by the mass
transfer of solvent molecules from the oil phase through the
oil-water interface and is facilitated by the continuous stirring
and shearing. At the second stage, these kinetically stabilized
drops are sheared into cylindrical structures. At this point they
become more viscous than the original oil solution after losing
solvent molecules through the increased surface area, and are
beginning to solidify due to the solvent attrition. The
solidification of the rods does not allow them to restore the
spherical shape of the initial drops and they can be stored for a
long period (days and even weeks or more) in this state, allowing
more than enough time for further processing steps as needed, such
as a photopolymerization process.
[0037] In certain embodiments utilizing photoinitiated
crosslinkable polymers, the microrods can then be subjected to
irradiation with actinic radiation (e.g., UV light having a
wavelength of 365 nm) in order to crosslink the polymer particles.
The obtained polymeric cylindrical particles can be further washed
and separated from the dispersion medium using separation
techniques known in the art, such as centrifugation and/or
ultrafiltration, and re-suspended or re-dispersed in a second
solution if desired. It is noted that crosslinking of the polymer
microrods is not necessary for microrod formation, but can be
useful to improve stability of the microrods such that the
microrods can be readily transferred from one medium to another if
desired.
[0038] The rapid process of microrod synthesis is usually completed
within ten minutes. The dispersion formation is easily detected by
the change in the turbidity of the solution. A milky color appears
in the first 20 s up to 1 min. after mixing the phases. Study of
the microrod formation process suggests the presence of
intermediate stages, such as breaking of the large cylindrically
shaped oil droplets due to the "jet" (capillary, Rayleigh)
instabilities. Y. Son, N. S. Martys, and J. G. Hagedom,
Macromolecules, 2003, 2003, 5825). Such effects decrease the
average length of the rods, but no significant changes in the
particle sizes were detected after the first 10 minutes, which
means that the process of solidification has been completed within
this interval. Further stirring of the sample may cause aggregation
and breaking of some long rods.
[0039] As noted above, the polymer solvent has to be soluble in the
chosen dispersion medium to assure the necessary conditions for the
mass transfer of molecules from inside the drops to the outside
medium. This provides the energy necessary for self-emulsification
and will cause the particle solidification. Transfer of solvent
molecules in the opposite direction inside the polymer solution
drops should not occur otherwise spherical liquid drops will form
instead of rods. The surface charge of the rods can also be
controlled simply by adding surfactants to one of the phases.
Dipolar rods may be prepared by subjecting the polymer particles to
an electric field.
[0040] The proposed method of the invention is flexible and allows
control over microrod sizes, aspect ratio, and polydispersity in a
few ways summarized briefly in Table 1 below. Two factors seem most
important for the formation of polymer microrods: (i) high shear
rates which depend on the stirring regime and viscosity of both
phases, oil and dispersion medium, and (ii) conditions for
self-emulsification and solidification of the sheared oil drops
that are determined by the solubility of the polymer solvent in the
dispersion medium. If these two factors are met at the same time,
rod-like micro-particles are likely to be formed.
[0041] The first important operational parameter is the shear
stress. It is defined as the force acting on unit area, which is
generally presented in units of pressure (e.g., Pa) and equals the
product of the medium viscosity and the shear rate. The shear
stress can be varied either by the shear rate, which can be changed
by changing the rate of stirring (e.g., by changing the rpm of the
stirring device) or by varying the viscosity of the dispersion
medium. Longer particles are formed at low shear rates. However, a
decrease in the particle length is achieved noticeably up to a
given high value of the shear rate after which this parameter
becomes less important. High shear rates decrease the
polydispersity of the microrods. As would be understood by one of
ordinary skill in the art, "shear rate" is a measure of the change
in velocity over the distance of change (the gap between the walls
moving in respect to each other) and is generally presented in
units of l/sec ((ft/sec)/ft=l/sec). Typically, the shear rate used
in the present invention is at least about 20 l/s, more preferably
at least about 90 l/s, and is generally in the range of about 20
l/s to about 3,000 l/s, although higher shear rates could also be
used in certain embodiments depending on the desired size and
properties of the microrods. In terms of rpm of a stirring device,
the level of shear in certain embodiments can be characterized as
that provided by a stirrer operating at about 100 to about 2,500
rpm with a gap that is small enough to assure the above shear
rates, more preferably about 200 to about 1,500 rpm.
[0042] The relative viscosity of the dispersion medium and polymer
solution can be manipulated to affect shear stress. Typically, the
viscosity of the dispersion medium is about 0.025 to about 1.1 Pas
(i.e., 25 to 1,100 cP). The ratio of the polymer solution viscosity
to the dispersion medium viscosity is preferably at least about
0.02:1, and in many embodiments, this ratio is about 0.1:1 to about
40:1.
[0043] Another way to affect the sizes of the rods is to decrease
the concentration of the polymer in the polymer solvent. This
procedure should in general reduce only the diameter of the rods
since the amount of polymer in an oil droplet will be smaller.
However, diluting the polymer with solvent will also lower the
interfacial tension, which tends to decrease the size of emulsion
droplets. In addition, the oil viscosity is less, which is
equivalent to low shear at the oil/dispersion medium interface. As
a whole, the decrease in polymer concentration in the initial
solution decreases both characteristic sizes of the
microcylinders.
[0044] Change in the interfacial tension would influence the aspect
ratio of the polymer rods. It can be achieved, for example, by
adding surfactants or other additives in one of the mixed phases
(i.e., the polymer solution or the dispersion medium). Lower values
of the interface free energy will result in formation of shorter
particles because smaller emulsion drops will form during the first
stage.
[0045] The rate of polymer solvent attrition can be also used to
control microrod size. The faster solidification of the particles
would increase their diameter. However, the variation limits of
this parameter are narrow because, in principle, the attrition
should proceed very fast in order to reach particle solidification,
otherwise rods are not formed.
[0046] The above parameters may influence the particle
polydispersity as well. Narrower size distributions can be expected
at high shear stresses, low interfacial tensions and low polymer
concentrations. The effects of several parameters on microrod
dimensions are summarized below in Table 1.
TABLE-US-00001 TABLE 1 Parameters of the formation process
providing control of microrod dimensions Effect when parameter
Parameter increases Explanation Shear rate Decreased diameter, D,
Droplet deformation and decreased length, L, breakup rate increase
at lower polydispersity higher shear Media viscosity Decreased
diameter, D, Higher viscosity leads to decreased length, L, higher
shear stress, similar lower polydispersity to increased shear rate
Polymer concen- Increased D and L Polymer rod solidifies tration in
the faster, losing less material polymer solvent Interfacial
Increased L/D ratio Low tension helps droplet tension deformation
and break-up into smaller drops Rate of solvent Increased D Polymer
particles solidify attrition more quickly (degree of continuous
phase saturation)
[0047] The method of the invention offers the advantages of
simplicity and easy scalability. The limitations of template and
electro-spinning methods are avoided, and as a whole, the method of
the invention can be used for synthesizing large amounts of polymer
microrods in an inexpensive way using basic laboratory or
industrial equipment. The anisotropic particles are synthesized in
a dispersed state and can be used directly. If necessary, the
particles can be easily separated and dried, or transferred to
another medium using well-known procedures, such as centrifugation
or filtration. The polydispersity of the rods can be reduced by
optimizing the experimental parameters as described above or by
consecutive fractionation of the samples. Hence, fractions of
particles with narrow size distribution can be obtained and used
whenever it is necessary.
[0048] The microrod suspensions formed by the method of the
invention exhibit at least three unique properties that can be
advantageous in material science. When rod-like particles in
suspension are concentrated, their free energy is minimized by
aligning in the same direction and forming "colloidal liquid
crystal" phases. Microrod suspensions concentrated above 2 wt %
demonstrated the expected correlated alignment. Optical microscopy
observations of the films deposited by drying drops of concentrated
SU-8 rod dispersion in dodecane on glass substrates showed that
they are made from multiple domains of ordered SU-8 rods (FIG. 2d).
Concentrated rod suspensions likely form nomadic liquid crystalline
phases.
[0049] Two other valuable properties of these rod-like
microparticles are their alignment in hydrodynamic flows and
electric or magnetic fields. Hydrodynamic rod alignment was
immediately seen both by microscopy and by observing the
macroscopic optical appearance of the suspension. Even a slight
flow oriented the microrods so that their long axis pointed in the
direction of the local flow. This could be used for effective
visualization of liquid flow lines on the microscale.
Macroscopically, when the rod dispersions were shaken slightly by
hand they acquired silver color and were visually different when
viewed from various angles similarly to liquid crystalline phases
(although no significant degree of light polarization was measured
with these diluted samples).
[0050] The SU-8 rods could also be manipulated and aligned by
applying an external AC electric field as shown in FIGS. 4a-4d.
When suspended in water, the rods have higher net polarizability
due to the conductivity and mobility of the counterion atmosphere.
The polarizability is highest along the long axis, and results in
dielectrophoretic torque, leading to rod orientation parallel to
the direction of the electric field vector. When a drop of rod
dispersion was placed between two coplanar electrodes, the
particles polarized and oriented perpendicularly to the electrodes,
i.e., parallel to the lines of the electric field. When the
electric field was inhomogeneous, the rods visualized the field
lines, as illustrated by their orientation around a point electrode
in FIG. 4a.
[0051] Rotating AC electric fields could be used to align the rods
in arbitrary direction and spin them in the plane of their long
axis. Four point electrodes evenly distributed on the sides of a
chamber of diameter 20 mm were used in a demonstration of this
phenomenon. By switching on and off different electrode pairs, the
SU-8 microrods could be rotated 360 degrees by 8 steps of 45
degrees each. Microscopy images of rods oriented in three different
directions depending on the configuration of the energized
electrodes are shown in FIG. 4b-c. As shown, the rods orient in the
direction of the electric field and can be rotated. Such microrod
orientation and rotation in electrical fields can be used in
micromixers, for studying microrheology, or in various optical and
display devices. The rotation can be accomplished by magnetic field
as well, when rods containing magnetic particles are used. The
appearance of magnetic-oriented rods is similar to FIG. 4.
[0052] Microrods formed according to the method of the invention
can find usefulness in a variety of applications, such as:
[0053] 1. Colloidal liquid crystals--Liquid crystals used in
displays, windows and thermometers could be replaced with
inexpensive polymer substitutes based on rod-like particles.
Concentrated suspensions of polymer rods would exhibit
self-organization properties similar to molecular liquid crystals.
One can expect an anisotropic orientation of polymer cylinders
leading to formation of a nematic liquid crystalline phase, or
closely packed layered structures (smectic phase). Concentrated
suspensions of polar polymer rods should respond to an external
electric field similarly to the classic liquid crystals.
[0054] 2. Paints--Another industrial product of even higher volume
is new "latex" paints containing rods instead of the present
microspheres. Paints based on suspensions of rods can make coatings
that are more porous and contain less polymer (because the
disordered packing of rods will make less dense coatings), or ones
with special visual appearance, polarizing, etc. The ability of
rods to align easily in a hydrodynamic flow may open opportunities
for new and more effective paints formulations.
[0055] 3. Smart materials--Anisotropic polymer particles of
moderate surface charge and high dipolar moments could
self-organize in chains and networks. Thus, they can form highly
viscous gels reacting to the presence of electrolyte being solid at
low salt and liquid at high salt content. Hence a variety of gels
sensing the environment, in particular ions, pH, specific chemical
or biological molecules, can be manufactured.
[0056] 4. Model research systems--Suspensions of polymer rods may
find an immediate application as model particles for the needs of
the fundamental and applied research. So far only spherical polymer
particles are commercially available. Experimenting with particles
with uniform cylindrical shape will make possible fundamental
research in a range of new phenomena of entropic self-organization,
and in studying various types of forces acting between anisotropic
colloidal objects. The polymer rods can also serve as model systems
for studying colloidal interactions between cylindrically shaped
particles and their change with the medium properties. The
experimental studies of the phase transitions in these suspensions
could provide much needed verification of the theoretical models of
phase transitions in molecular dipolar systems, polar liquids,
surfactants, lipid and protein self-assembly, electrorheological
and magnetorheological fluids and others. The rods can also be used
as tracers of the liquid flow lines in fluid channels and
liquid-containing devices.
[0057] 5. Photonic crystals of non-trivial symmetry--The
orientational interactions between the polymer rods will lead to
long-range organization of the particles in chains and planes
leading to formation of anisotropic crystals with non-trivial
symmetries, which eventually may be used to produce photonic
crystals.
[0058] 6. Production of polymer microwires or microfibers--The
polymer rod-like micro-particles can be regarded as an equivalent
of nano-wires on the micrometer scale. Hence new and interesting
(e.g., structural, mechanical, optical, or electrical) properties
of some polymeric materials can be expected when their molecules
are confined into micro fibers. Thus, the microrods of the
invention can be used in many materials and applications as a
replacement for fibers formed by electro-spinning. Such application
include, but are not limited to, nonwoven textiles and papers,
porous fibrillar materials, pads, and linings, fiber-based sponges
and filters, and fibrillar matrices for cell attachment and growth
for implantation.
[0059] 7. Composite materials--Rods of mixed polymers can be easily
prepared if the constituents are mixed in the initial polymer
solution. Various types of nano- and micro-particles including
magnetic and electroconductive ones can be introduced in the
polymer micro fibers providing the added material is not soluble in
the polymer solvent. This may lead to a modification of the
particle properties or to a better way of their manipulation giving
more opportunities for their application. Once formed, the rod-like
polymer particles can be further modified by adsorbing a layer of
another substance on their surface.
[0060] 8. Longitudinal orientation of nanocylinders--Polymer
microrods can be used as a medium for longitudinal orientation of
nanowires, nanotubes or nanorods. This would make the manipulation
of the nano-wires easier and at the same time the structures of
nanowires oriented in the same direction may exhibit unusual
properties.
[0061] 9. Stabilization of foams and emulsions--The microrods can
adsorb on the surfaces of bubbles or droplets to make intertwined
layers that substantially encapsulate and satirically protect the
bubbles or droplets against breakdown and coalescence. This also
includes, but is not restricted to, materials and products
containing rod-stabilized foams or emulsions, or the products of
their further processing. In one embodiment, the stabilized foams
or emulsions are food products, such as ice cream, mayonnaise,
salad dressings, whipped cream, and the like. In another
embodiment, the stabilized emulsions are pharmaceutical emulsions
comprising a biologically active agent. In food or pharmaceutical
applications, naturally-occurring polymers, such as polysaccharides
or derivatives thereof, are particularly preferred. In yet another
embodiment, the stabilized foam or emulsion is a consumer product,
such as various types of cosmetics or bath products. Firefighting
foams represent yet another class of foaming products that could
benefit from the stabilizing effect of the microrods of the
invention. As noted in Example 2, in certain embodiments, the foam
or emulsion stabilization provided by the microrods of the
inventions is optimally produced in the absence of surfactants,
which can interfere with the stabilizing effect under certain
conditions.
[0062] 10. Fillers--The microrods can be added to any liquid, semi
liquid product, or solid in a manner designed to modify or improve
its rheological properties, increase useful volume, prevent
settling or creaming, or otherwise improve its properties. This
also includes products made from processing of rod-containing
liquid or semiliquid media. The rods in such products can be made
by the method described here during any step of the product
fabrication, by shearing of droplets, without subsequent extraction
from the media. In one exemplary use, the microrods of the
invention are used as a filler for industrial foams, such as latex
foams or polyurethane foams.
[0063] 11. Microcapsule delivery systems--The microrods formed
according to the invention can be used to form capsules that can be
used to delivery various pharmaceutical or food compositions. Such
capsules can be formed, for example, by emulsifying an aqueous
solution of agarose in oil in the presence of the microrods of the
invention to form gel-filled microcapsules that can then be
separated from the supernatant and collected. The resulting rigid
semi-permeable colloidosome microcapsules have diameters varying
from several tens to several hundreds of micrometers.
[0064] The amount of microrods according to the invention used in a
give formulation is dependent upon the type of formulation and
desired use thereof. Typically, the microrods will be present in a
composition in an amount of about 0.01 to about 30 wt. %, based on
the total weight of the composition. Where the microrods are used
as a filler, the concentration would be nearer the higher limit of
the range. Lower concentrations could be used where the microrods
are stabilizing a foam or emulsion, such as about 0.01 to about 10
wt. % or about 0.1 to about 5 wt. %.
[0065] The present invention will be further illustrated by the
following non-limiting examples.
EXPERIMENTAL
Example 1
Microrod Synthesis
Equipment and Materials
[0066] The photocurable epoxy resin SU-8 (widely used as negative
resist for photolithography) was purchased from MicroChem,
Massachusetts, as 63 wt % solution (SU-8 25) in
.gamma.-butyrolactone (GBL). The resin was used as received or
diluted further with GBL. Analytical grade glycerin or its mixtures
with ethylene glycol, ethanol, methanol, dodecane (all from Fisher
Scientific, Pennsylvania) and isopropanol (LabChem. Inc.
Pennsylvania) were used as dispersion media. GBL is soluble in all
of the above solvents, which is a necessary condition for the
microrod synthesis. Polystyrene used in the polymer mixtures was
obtained from 1 .mu.m polystyrene sulfate FluoSpheres.RTM.
(Molecular Probes, Oregon). The dried latex was dissolved in GBL
prior to mixing with SU-8 solution.
[0067] Mixing of the phases was performed by a Servodyne electronic
mixer (Cole-Parmer, Illinois) providing high speed of stirring
(150-6000 rpm) at low torque. It was equipped with a high-shear
impeller of diameter 50.8 mm (Lightin A-320, Cole-Parmer,
Illinois), inserted into a beaker of inner diameter of 62.2 mm.
Crosslinking was accomplished using a BL-100A UV lamp (Blak-Ray,
California).
[0068] The alternating electric field (AC) experiments with
coplanar electrodes were carried out by applying an AC field of 75
V/cm and 200 Hz in a cell similar to the set-up described in K. H.
Bhatt, O. D. Velev, Langmuir, 2003 20, 467. The electrorotation
experiments were carried out by using an experimental cell made
from a round perfusion chamber with a thickness of 0.5 mm and
diameter of 20 mm (Cover Well.TM., Grace Bio-Labs, Oregon) sealed
by a microscope glass slide. Four needle electrodes were positioned
at the chamber periphery and connected to an AC generator and
amplifier. The dispersion of SU-8 rods in water was inserted into
the chamber by a micropipette. An electric field of 50 V/cm and 200
Hz was applied and its direction was changed by switching on and
off different electrode pairs (FIG. 4a-d).
Microrod Synthesis
[0069] The polymer microrod synthesis method utilizes a
liquid-liquid dispersion technique that is schematically presented
in FIG. 1. The microrods were made fully or partially from
cross-linked SU-8 (photocurable epoxy resin). The process began by
adding a small amount of concentrated solution of SU-8 in GBL to an
organic liquid medium (such as glycerol or mixtures of glycerol
with other alcohols or glycols) subjected to a viscous shearing by
an impeller. Specifically, the dispersions were made by injecting
0.1-0.5 cm.sup.3 of 30-63 wt % solution of SU-8 in GBL into the
continuously stirred viscous medium. The emulsification under shear
results in a dispersion of rod-like particles with diameter between
0.5 and 3 .mu.m and length of tens of micrometers.
[0070] Uniformly shaped rod-like polymer particles were formed in
all experiments at high shear rates. The dispersion formation is
easily detected by the change in the solution turbidity; a milky
color appears in the first 20 s up to 1 min after mixing the
phases. The optimal time for rod formation at these conditions was
.apprxeq.10 min.
[0071] After completing of the initial synthesis, SU-8 rods could
be additionally crosslinked by exposing to 365 nm UV light for 15
min. This made them insoluble in any common solvent and they could
be transferred to another aqueous or organic medium. For example,
dispersions of SU-8 rods in water were prepared by diluting the
initially synthesized dispersion with water containing
5.times.10.sup.-4 wt % sodium dodecyl sulfate or Tween 20 and
repeating a few centrifugation/washing cycles.
[0072] Images of the resulting microrods are shown in FIG. 2. The
rods were prepared in 50:50 vol % mixture of glycerol and ethylene
glycol. After UV crosslinking, the particles were transferred to
water (FIG. 2a-c) or dodecane (FIG. 2d). The samples were imaged
after drying of suspension drops on glass slides.
[0073] The formation of rods in the mixing vessel appears to be a
result of three concurrent processes: emulsification of the polymer
solution in the organic medium, shear-driven deformation of the
emulsion drops into cylinders, and their successive solidification
into rod-like particles. All of those processes were assisted by
the fact that the solvent for the SU-8 (GBL) is soluble into the
dispersion media (while SU-8 itself is not). The GBL diffuses out
of the dispersed sheared droplets leaving behind solid polymer. The
solidification of the elongated drops does not allow them to
restore the spherical shape and they could be kept for a long
period (hours and days) during which they could be turned into
extremely stable polymer by UV light crosslinking.
[0074] The emulsification of the polymer solution is facilitated by
the high shear rates during the stirring. The process of droplet
emulsification, deformation and breakup is characterized mainly by
two dimensionless parameters: the viscosity ratio,
p=.mu..sub.0/.mu..sub.0 (where .mu..sub.1 and .mu..sub.0 are the
viscosities of the drops and the suspending medium), and the
capillary number, defined as the ratio of the shear stress and the
Laplace pressure, Ca=.mu..sub.0Ga/.sigma. (where G is the shear
rate, a is the non-deformed drop radius, and .sigma. is the
interfacial tension). T. G. M. van de Ven, Colloidal Hydrodynamics,
Academic Press, San Diego 1989, Ch. 3. In our case, the interfacial
tension is low (due to the miscibility of the SU-8 solvent GBL and
glycerol), corresponding to high values of Ca (Ca.gtoreq.1) where
the polymer solution droplets are deformed into thin long cylinders
that may further break into droplets with roughly equal volumes.
The breakup of the elongated droplets is also probably assisted by
the spontaneous emulsification of the oil caused by mass transfer
of GBL molecules from the oil phase into the medium. The long thin
liquid cylinders later solidify due to the attrition of the polymer
solvent. The resulting rods have uniform diameter, and,
surprisingly, very straight perpendicular edges (FIG. 2c). These
straight edges suggest that the mechanism may involve the initial
formation of even longer cylinders that may be broken into shorter
pieces by the shear flow after the droplets are solidified.
[0075] The synthesis of SU-8 microrods was very reproducible and in
most of the experiments more than ca. 97% of the emulsion drops are
sheared into particles with regular cylindrical shape as the ones
in FIG. 2b-d. The rest of the obtained particles are mostly
rod-like but possess some defects. Less than 1% of the oil drops
produce irregularly shaped particles, probably resulting from
coalescence of the drops during the dispersion stage, or irregular
shearing conditions in some parts of the vessel.
[0076] The procedure is flexible and allows control over microrod
sizes, aspect ratios and polydispersity in a variety of ways,
several of which were tested and the results summarized in Table 1
above. As noted above, the first important operational parameter is
the shear stress that can be varied either by the rate of stirring,
or by the viscosity of dispersion medium. At larger capillary
numbers, the theory predicts that smaller droplets will form that
should solidify in thinner, shorter, and less polydisperse
cylinders. Increase of the shear rate indeed decreased the
characteristic sizes and the polydispersity of the microrods as
shown in FIG. 3. The decrease in the rod length, L, (cf. FIG. 3a-d)
was more pronounced than the one in diameter, D, likely because of
the faster GBL dissolution and solidification of the smaller drops.
In general, 5-7 times increase in the rate of stirring leads to
approximately 5-9 times decrease in the microrod length and 2-3
times decrease in rod diameter.
[0077] Uniform and constant shear stress is needed to produce rods
of relatively narrow size distribution and aspect ratio. As noted
above, the experiments described herein involved use of an
electronic mixer with a high shear impeller. The role of the shear
stress for formation of uniform particles was demonstrated by
performing synthesis in pure alcohols, the viscosity of which is
low (1.07 mPa s at 25.degree. C. for ethanol compared to 934 mPa s
for pure glycerin). In this case, the process resulted in high
fraction of large irregularly shaped particles. Thus, a relatively
high dispersion medium viscosity, .mu..sub.0, is required for high
yield and low polydispersity, again in correlation to theoretical
expectations that high values of Ca lead to formation of shorter
and less polydisperse rods.
[0078] The concentration of SU-8 in GBL was another controlling
parameter. The decrease of SU-8 concentration in the initial
solution was expected to reduce the diameter of the rods since the
amount of SU-8 in an oil droplet will be smaller. However, lowering
the SU-8 concentration in the GBL decreased both the length and
diameter of the microcylinders, possibly because of the decrease of
the droplet size during the emulsification process. At least three
more experimental parameters, the interfacial tension, the rate of
the solvent attrition, and the temperature, can be used to control
the particle sizes, aspect ratios and polydispersity.
[0079] Various dispersion media, including pure glycerin and its
mixtures with ethylene glycol, ethanol, methanol or isopropanol,
were tested. The procedure worked very well in most of them. One
key to successful synthesis is the miscibility of the SU-8 solvent,
GBL, in the dispersion medium. If this is not the case, the process
results in oil-oil emulsion that separates into two phases. The
microrods may also be made from other polymers dissolved in the
emulsified phases, which was shown by experiments with
polystyrene/SU-8 mixtures at a weight ratio of 1:2. Mixed polymer
rods with characteristic sizes similar to pure SU-8 rods were
formed.
[0080] Irradiation of the SU-8 particles with UV light led to
irreversible crosslinking of the photocurable resin and the
polymeric rods obtained could be easily separated by mild
centrifugation and/or re-dispersed in water or in another liquid
medium. Dispersing of the rods in water was enhanced by adding
small amounts of anionic or non-ionic surfactant (sodium dodecyl
sulfate or Tween 20) that provide additional electrostatic or
steric repulsion. If surfactant was not used, the hydrophobic SU-8
rods tended to reversibly aggregate in water, but dispersed easily
again upon adding water soluble surfactants.
Example 2
Synthesis of Microrods with Embedded Magnetic Particles
[0081] BioMag.RTM. supermagnetic spheres (Bangs Labs, Inc., US)
with mean diameter of 1.8 .mu.m, received as 5.4 wt % suspension in
water, were used in the fabrication of magnetic microrods. The
magnetic particles comprise of about 90 wt % iron oxide core,
covered by a silane layer. Preparation of magnetic SU-8 microrods
was carried out following the same procedure as for pure SU-8 rods
described above in Example 1.
[0082] The supermagnetic particles were first separated from the
water phase in a magnetic field and the supernatant was removed and
replaced by ethanol. This procedure was repeated three times, after
which the BioMag particles were dried for the complete removal of
the ethanol, and dispersed in GBL. This particle dispersion was
then mixed with SU-8 solution in GBL (63 wt %) to yield a mixture
containing 1.55 wt % magnetic particles and 48.6 wt % SU-8. The
particle/SU-8 mixture was used as initial polymer solution in the
microrod synthesis procedure described in Example 1. The rods
formed as a result of this procedure are magnetic and can be easily
rotated and manipulated (e.g., concentrated, moved, stirred, etc.)
by small magnets.
Example 3
Microrod Synthesis and Use as Foam Stabilizer
Microrod Synthesis
[0083] The polymer rodlike particles used for foam stabilization
were synthesized from epoxytype photoresist SU-8 (MicroChem,
Massachusetts) using the liquid-liquid dispersion technique
described above. Typically, 0.2 mL of 50 wt % solution of SU-8 in
GBL (Aldrich, WI) was injected into 50 mL of a 1/1 v/v mixture of
glycerol and ethylene glycol (Fisher Scientific, Pennsylvania)
subjected to a viscous shearing by an impeller. The polymer
solution was emulsified, and the emulsion droplets deformed and
solidified into cylinders with high aspect ratio. The polymer
particles were strengthened by cross-linking with UV light. Then
the dispersion was diluted twice with deionized (DI) water (from a
Millipore RiOs 16 RO unit) and filtered through a 0.45 .mu.m
Durapore membrane (Millipore, Massachusetts) to separate the
particles. The SU-8 rods collected on the membrane were repeatedly
washed with DI water to remove any organic solvent traces and then
suspended into deionized water to give a dispersion of 2.18 wt %
solid content. Microscopy measurements determined that the samples
consisted of polydisperse cylindrical rods of 23.5 .mu.m average
length and 0.60 .mu.m average diameter.
Foam Preparation and Characterization
[0084] Foams stabilized by SU-8 rods only were prepared directly
from the above microrod suspension or after its further dilution
with DI water. For a reference surfactant system, an 8.7 mM
solution of sodium dodecyl sulfate (SDS) in water was used. The SDS
concentration is close to its critical micellar concentration (cmc)
where it shows maximum foamability. All foams were formed by the
same protocol from 2 mL of liquid measured by a pipet into a 10 mL
glass cylinder. They were prepared at room temperature by
hand-shaking for a period of 30 s by the same operator. The
foamability was estimated by measuring the foam volume, Vf,
immediately after preparation, while the foam stability was
assessed by monitoring Vf over time. The cylinders were kept closed
except for the case when the stability of the rod foam upon drying
was evaluated. The structure of small foam bubbles and thin liquid
films was observed by an Olympus BX-61 microscope equipped with
long working distance objectives. The bubbles were mounted in a
thick film on microscope slides. Single foam films were formed
inside 1.4 mm holes in a polycarbonate slide of 0.1 mm thickness by
introducing a drop of the SU-8 rod suspension into the hole and
sucking out the excess liquid. The observations were performed in
transmitted phase contrast or dark field illumination.
Phenomenology
[0085] The microrod suspensions readily produced foams upon
shaking. Batches of foams with four different concentrations of
SU-8 rods were prepared and monitored. Initially, the air bubbles
formed were spread throughout the entire volume of the samples,
making them milky. In a few minutes, most of the air bubbles
accumulated in the top phase, leaving a less turbid lower phase of
SU-8 rods dispersed in water. The foam volume, Vf, recorded as a
function of time for all samples stabilized by rods only is given
in FIG. 5a. Vf decreased noticeably only within the first few
minutes (see the inset) and then remained approximately the same
for more than 3 weeks. The long-term values of Vf as a function of
the rod concentration are presented in FIG. 6. They show a possible
initial threshold concentration where stabilization begins and a
rapid increase of the foam volume with the particle content.
[0086] A more stringent test for foam stability is the resistance
to drying, which typically destroys any surfactant stabilized foam.
The resistance of microrod foam (2.18 wt % SU-8) to water
evaporation was examined by monitoring its state in a cylinder open
to the air. For more than a week, the volume of the foam remained
at a constant value of 1.3 mL (similar to the Vf of the same foam
kept in a closed cylinder), even though the liquid below it slowly
evaporated through the foam layer. The evaporation possibly occurs
by capillary suction from the subphase. After a week, destruction
of the remaining foam was attempted by fast drying and expansion in
a vacuum. When the pressure was reduced, the foam started
increasing in volume and reached twice its initial volume where it
remained constant for hours. The major part of the (already dried)
foam collapsed only when the vacuum was released, and air rushed
into the vessel. Thus all tests prove an extraordinary effect of
"superstabilization" by the microrods.
[0087] It is instructive to compare the properties and the
structure of microrod-stabilized foams to the ones made from common
surfactants such as SDS. The stabilities of foams prepared with
pure SDS, pure 2.18 wt % SU-8 microrods, and a mixture of SDS and
2.18 wt % microrods are compared in FIG. 5b. Despite the large
difference in the foamability (the initial Vf is about 5 times
larger for surfactant-stabilized foams), the shape of the short
term stability plot (the inset) is similar for all foams and
indicates similar draining changes in the initial period. However,
the long-term plots show that the microrod stabilized foam has
strikingly superior stability compared to the SDS-containing
samples, both of which are completely destroyed in less than 48
h.
[0088] The sample containing mixed SDS and microrods was expected
to show an intermediate stability; however, it behaved as pure SDS
foam except for the slightly smaller initial volume and lifetime.
This can be readily explained by the adsorption of SDS monomers on
the hydrophobic SU-8 rod surface. These hydrophilized rods lose
their affinity for adsorption at the solution/air interface and in
addition repel from it electrostatically. Instead of protecting the
bubbles, they remain well dispersed in the liquid phase and slowly
sediment to the bottom. Estimates show that if the surface of all
rods present in a 2.18 wt % dispersion is covered by densely packed
SDS monomers, the initial surfactant concentration is reduced by
only about 5%. These data lead to an unusual conclusion. SDS, which
is used typically as a strong foaming agent, in this case acts as a
defoamer suppressing the stabilizing effect of the microrods. This
was also proved by adding gently a few drops of 10 wt % SDS to a
stable few-days-old foam from 1.09 wt % SU-8 rods. Soon after the
surfactant was introduced, the microrod particles began to transfer
from the foam into the liquid and to sediment at the bottom. More
than 70% of the foam phase was destroyed in the first 30 min.
Factors Contributing to the Superstabilization Effect
[0089] Although not bound by any particular theory of operation, it
is assumed that the initial compression of the foam volume was a
result of liquid drainage and bubble compaction, rather than loss
of any entrapped air. Closer examination of the microrod foam under
a microscope showed that it was made of small roughly spherical
randomly distributed bubbles (FIG. 7b). Each air bubble was covered
by a dense shell of adsorbed entangled polymer rods extending into
the water phase (FIG. 7c). The key to making stable foams is
ensuring that the thin films formed between the bubbles do not thin
and break easily. Single foam films were examined in order to
observe the structure of the SU-8 rods adsorbed between the two
air/water interfaces. The whole surface of such films was densely
covered by intertwined rods (see FIG. 7d). The rods entangle,
overlap, and sometimes form small oriented domains. The film
thickness corresponds to at least two opposing layers of rods, or
ca. 1-2 .mu.m. This is an overwhelmingly thick film compared to the
equilibrium thickness of surfactant foam films stabilized by
electrostatic repulsion (.about.100 nm) or the thickness of common
"black" films with steric repulsion between surfactant monolayers
adsorbed on the opposing surfaces (.about.12 nm). Thus, the first
factor of stability of the superstabilized foam is the steric
repulsion between the adsorbed layers of solid rods, which keeps
the films very thick, preventing breakage and suppressing gas
diffusion. Even though there are some empty spaces between the
rods, the capillary pressure in the system would not be enough to
locally thin and break the films there.
[0090] The second unique factor of superstabilization is the
mechanical rigidity of the continuous net of overlapping and
entangled microrods at the film surface (cf. inset in FIG. 7d). The
microrods did not rearrange as previously observed with spherical
particles entrapped in thin foam films. The stability of the formed
microrod net is impressive. Similarly to the foam systems, single
films with concentration above .about.1 wt % were infinitely stable
and were not destabilized by drying, which leads to rapid breaking
down of films made with SDS only. The films and membranes from
intertwined rods could be dried and kept intact for more than a
week.
[0091] One of the major factors for the formation of thick and
rigid films between the bubbles is the strong adsorption of the
polymer rods on the air/water interface. The degree of rod
hydrophobicity was characterized by measurements of the contact
angle on single rods by a recently developed gel trapping
technique. The contact angle between SU-8 rods and water was
estimated to be between 80.degree. and 90.degree. corresponding to
intermediate hydrophobicity. The adsorbed microrods were partially
immersed in the water phase but did not aggregate spontaneously
when dispersed in the bulk. It has been reported that partially
hydrophobic particles are optimal stabilizers of various foam and
emulsion films. Entanglement of the rods adsorbed at the interface
was also observed by scanning electron microscopy (SEM). The strong
adhesion between the adjacent and overlapping rods in the films
(responsible for the rigidity of the adsorbed layer) is probably
augmented by their hydrophobicity and high friction. The rod
entanglement appears to be the major difference between the
superstabilized foams reported here and previously studied foams
containing spherical particles. The effect is lost when the
particles are hydrophylized and desorbed from the interface due to
SDS adsorption, whence the superb stability is lost when common
surfactant is added.
[0092] The bulk structure of the superstabilized foams was also
very different from that of common foams stabilized by surfactants.
The microrod foams were made of approximately spherical air bubbles
(FIG. 7b) not only for the very first stage of the foam formation
but for the whole period of observation. Such structure is typical
for unstable wet transient foams which contain large amounts of
water and live only seconds. These bubbles did not deform to reach
the second (kugelschaum) and third (polyederschaum) stages of foam
evolution in which air bubbles in common foams deform, the liquid
from the films between them drains, and a "dry" foam with thin
films and plateau borders separating large polyhedral air cells
forms. Yet, the rod-stabilized foams show remarkably long lifetimes
exceeding dramatically even the lifetime of the dry metastable
foams. The reason for the lack of deformation is probably the
rigidity of the dense "hairy" shells around the bubbles. The
microrods of the invention have also been shown to form similarly
rigid shells around emulsion droplets.
[0093] The effects presented here also differ significantly from
literature data for other particle-stabilized foams. It has been
reported that strongly hydrophobic particles decrease foam
stability by breaking the lamellae by a "bridging dewetting"
mechanism. Foam stabilization with slightly hydrophobic and with
hydrophilic particles is usually a result of collecting of the
particles in the plateau borders, which slows down the liquid
drainage and therefore the thinning and rupture of the lamellae. In
the case of polymer microrod foams, the stabilization mechanism
includes the formation of a thick rigid net of entangled rods
around the bubbles. The bubbles do not coalesce because of the
strong steric hindrance from their shells. Diffusion of air between
the bubbles is not likely to occur because the liquid films between
them are very thick and small and because the bubbles covered by a
rigid shell cannot readily shrink or expand. The strong rod
adsorption and entanglement, formation of rigid hairy shells, and
sustaining of thick films makes possible the superstabilization
effect.
[0094] The "superstabilization" effect has also been observed with
emulsion droplets. Notably, the rods can be used for long-term
stabilization of emulsion systems that are presently hard to
protect with regular surfactants, such as emulsions of low
molecular weight hydrocarbons (e.g., hexane) in water or
water-in-oil emulsions.
[0095] Many modifications and other embodiments of the inventions
set forth herein will come to mind to one skilled in the art to
which these inventions pertain having the benefit of the teachings
presented in the foregoing description. Therefore, it is to be
understood that the inventions are not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
* * * * *