U.S. patent application number 14/822039 was filed with the patent office on 2016-02-11 for well defined, highly crosslinked nanoparticles and method for making same.
The applicant listed for this patent is Bridgestone Corporation. Invention is credited to Yaohong Chen, Hideki Kitano.
Application Number | 20160039964 14/822039 |
Document ID | / |
Family ID | 44656831 |
Filed Date | 2016-02-11 |
United States Patent
Application |
20160039964 |
Kind Code |
A1 |
Kitano; Hideki ; et
al. |
February 11, 2016 |
WELL DEFINED, HIGHLY CROSSLINKED NANOPARTICLES AND METHOD FOR
MAKING SAME
Abstract
A method is provided for making nanoparticles, including the
steps of: combining a hydrocarbon solvent and an aprotic, polar
co-solvent, a mono-vinyl aromatic monomer, polymerization
initiator, a solution stabilizer, and a first charge of a
cross-linking agent. Subsequently, a second charge of cross-linking
agent is added. The nanoparticles have an average diameter of 5
nanometers to about 10,000 nanometers. Spherical nanoparticles are
also provided that include a cross-linking agent comprising 30% to
60% by weight of the combined weight of a mono-vinyl aromatic
species and the cross-linking agent. The spherical nanoparticles
also meet the following equation: 0.90.ltoreq.(D1/D2).ltoreq.1.1
wherein D1 is a first diameter of a nanoparticle and D2 is a second
diameter of the nanoparticle, and D1 and D2 intersect at right
angles.
Inventors: |
Kitano; Hideki; (Tokyo,
JP) ; Chen; Yaohong; (Akron, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bridgestone Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
44656831 |
Appl. No.: |
14/822039 |
Filed: |
August 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12979719 |
Dec 28, 2010 |
9115222 |
|
|
14822039 |
|
|
|
|
61290755 |
Dec 29, 2009 |
|
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Current U.S.
Class: |
428/402 |
Current CPC
Class: |
C08F 8/00 20130101; B01J
13/18 20130101; C08F 2810/20 20130101; C08F 8/00 20130101; C08F
212/08 20130101; C08F 299/02 20130101; C08F 2/06 20130101; C08F
257/02 20130101; C08F 212/08 20130101; C08F 212/08 20130101; C08F
2800/20 20130101; C08F 257/02 20130101; C08F 212/36 20130101; Y10T
428/2982 20150115; C08F 212/36 20130101 |
International
Class: |
C08F 299/02 20060101
C08F299/02 |
Claims
1. Spherical cross-linked nanoparticles comprising: a core formed
from a polymeric seed that includes a mono-vinyl aromatic core
species cross-linked with a cross-linking agent, the core having an
average diameter of 5 nanometers to 10,000 nanometers; wherein the
mono-vinyl aromatic core species comprises polymeric chains
radiating from a center of the core; wherein the spherical
cross-linked nanoparticles meet the following equation:
0.90.ltoreq.(D1/D2).ltoreq.1.1 wherein D1 is a first diameter of
the spherical cross-linked nanoparticles and D2 is a second
diameter of the spherical cross-linked nanoparticles, and D1 and D2
intersect at right angles.
2. The spherical cross-linked nanoparticles of claim 1, wherein the
polymer chains form a surface of the core and the surface of the
core comprises living ends of the polymer chains.
3. The spherical cross-linked nanoparticles of claim 2, further
comprising a shell layer bonded to the living ends of the surface,
the shell layer comprising a fixed formal charge group.
4. The spherical cross-linked nanoparticles of claim 2, further
comprising a shell layer bonded to the living ends of the surface,
wherein the shell layer is about 1 nm to about 100 nm in
thickness.
5. The spherical cross-linked nanoparticles of claim 2, wherein the
surface of the nanoparticle comprises terminated living ends.
6. The spherical cross-linked nanoparticles of claim 1, wherein the
cross-linking agent comprises 30% to 60% by weight of the combined
weight of the mono-vinyl aromatic species and the cross-linking
agent.
7. Cross-linked nanoparticles comprising: a core formed from a
polymeric seed that includes a mono-vinyl aromatic core species
cross-linked with a cross-linking agent, the core having an average
diameter of 5 nanometers to 10,000 nanometers; wherein the
mono-vinyl aromatic core species comprises polymeric chains
radiating from a center of the core; wherein the weight ratio of
mono-vinyl monomer species to total cross-linking agent ranges from
about 30:70 to about 90:10; wherein the nanoparticles meet the
following equation: 0.90.ltoreq.(D1/D2).ltoreq.1.1 wherein D1 is a
first diameter of the cross-linked nanoparticles and D2 is a second
diameter of the spherical cross-linked nanoparticles, and D1 and D2
intersect at right angles.
8. The spherical cross-linked nanoparticles of claim 1, with the
proviso that the spherical cross-linked nanoparticles are not made
by emulsion polymerization.
9. The spherical cross-linked nanoparticles of claim 1, wherein the
nanoparticles are made by the process comprising: combining a
hydrocarbon solvent and an aprotic, polar co-solvent, a mono-vinyl
aromatic monomer, polymerization initiator, a solution stabilizer,
and a first charge of a cross-linking agent; adding a second charge
of the cross-linking agent or a charge of another cross-linking
agent; wherein the spherical cross-linked nanoparticles have an
average diameter of 5 nanometers to about 10,000 nanometers;
wherein a volume ratio of hydrocarbon solvent to aprotic, polar
co-solvent ranges from about 99.9:0.1 to about 70:30.
10. The spherical cross-linked nanoparticles of claim 3, wherein
the fixed formal charge group is selected from at least one of the
group consisting of: succinic anhydride, imidazole, pyridines,
N,N-dimethylaminostyrene, N,N-diethylaminostyrene, pyridine silane,
vinyl pyridine and derivates of these, quaternary ammonium
compounds, quaternary phosphonium compounds, and quaternary
sulfonium compounds.
11. The spherical cross-linked nanoparticles of claim 1, wherein
the spherical cross-linked nanoparticles comprise a first group of
spherical cross-linked nanoparticles and a second group of
spherical cross-linked nanoparticles, wherein the first and second
group have a difference in charge of about 50 .mu.C/g or
greater.
12. The spherical cross-linked nanoparticles of claim 1, wherein
for a given collection of the spherical cross-linked nanoparticles
measured by an SEM, 90% to 100% of the nanoparticle group meets the
following equation: 0.90.ltoreq.(D1/D2).ltoreq.1.1 wherein D1 is a
first diameter of the spherical cross-linked nanoparticles and D2
is a second diameter of the spherical cross-linked nanoparticles,
and D1 and D2 intersect at right angles.
13. The spherical cross-linked nanoparticles of claim 1, wherein
the spherical cross-linked nanoparticles comprise a shell bonded to
the core, the shell including polymerized monomers, wherein the
core has a higher Tg than the shell.
14. The cross-linked nanoparticles of claim 7, further comprising a
shell layer bonded to the surface of the core, the shell layer
comprising a fixed formal charge group.
15. The cross-linked nanoparticles of claim 14, wherein the fixed
formal charge group is selected from at least one of the group
consisting of: succinic anhydride, imidazole, pyridines,
N,N-dimethylaminostyrene, N,N-diethylaminostyrene, pyridine silane,
vinyl pyridine and derivates of these, quaternary ammonium
compounds, quaternary phosphonium compounds, and quaternary
sulfonium compounds.
16. The cross-linked nanoparticles of claim 7, wherein the
cross-linked nanoparticles comprise a first group of cross-linked
nanoparticles and a second group of cross-linked nanoparticles,
wherein the first and second group have a difference in charge of
about 50 .mu.C/g or greater.
17. The cross-linked nanoparticles of claim 7, wherein for a given
collection of the cross-linked nanoparticles measured by an SEM,
90% to 100% of the nanoparticle group meets the following equation:
0.90.ltoreq.(D1/D2).ltoreq.1.1 wherein D1 is a first diameter of
the cross-linked nanoparticles and D2 is a second diameter of the
cross-linked nanoparticles, and D1 and D2 intersect at right
angles.
18. The cross-linked nanoparticles of claim 7, wherein the
cross-linked nanoparticles comprise a shell bonded to the core, the
shell including polymerized monomers, wherein the core has a higher
Tg than the shell.
19. The cross-linked nanoparticles of claim 16, wherein the first
group of cross-linked nanoparticles has a positive charge and the
second group of cross-linked nanoparticles has a negative
charge.
20. The spherical cross-linked nanoparticles of claim 11, wherein
the first group of spherical cross-linked nanoparticles has a
positive charge and the second group of spherical cross-linked
nanoparticles has a negative charge.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 12/979,719, filed Dec. 28, 2010, which, in turn, claims the
benefit of U.S. Provisional Application No. 61/290,755, filed on
Dec. 29, 2009. These prior applications, including the entire
written description and drawing figures, are hereby incorporated
into the present application by reference.
FIELD
[0002] The technology disclosed herein is generally related to
nanoparticles. This disclosure also provides a method of making
such nanoparticles.
BACKGROUND AND SUMMARY
[0003] Polymer nanoparticles have attracted increased attention
over the past several years in a variety of fields including
catalysis, combinatorial chemistry, protein supports, magnets, and
photonic crystals. Nanoparticles have been used in rubber
compositions to improve physical properties of rubber moldability
and tenacity. In some instances the inclusion of polymer
compositions with certain functional groups or heteroatomic
monomers can produce beneficial and unexpected improvements in
rubber compositions.
[0004] Charged nanoparticles may have a number of possible
applications, such as in electronic devices, or in rubber or other
polymer matrices. In some electronic display applications, such as
QR-LPD, charged particles may be used to present a pictorial or
textual display. It is a challenge, however, to provide particles
that have a durable constitution and a stable charge. It is also a
challenge to produce durable nanoparticles that are very hard and
have highly spherical surfaces.
[0005] Herein, a method is provided for making nanoparticles,
including the steps of: combining a hydrocarbon solvent and an
aprotic, polar co-solvent, a mono-vinyl aromatic monomer,
polymerization initiator, a solution stabilizer, and a first charge
of a cross-linking agent. Subsequently, a second charge of
cross-linking agent is added. The nanoparticles have an average
diameter of 5 nanometers to about 10,000 nanometers.
[0006] Furthermore, spherical cross-linked nanoparticles are also
provided. The spherical nanoparticles have a core formed from a
polymeric seed that includes a mono-vinyl aromatic core species
cross-linked with a cross-linking agent. The core has an average
diameter of 5 nanometers to 10,000 nanometers. The mono-vinyl
aromatic core species comprises polymeric chains radiating from a
center of the core. Furthermore, the nanoparticles meet the
following equation:
0.90.ltoreq.(D1/D2).ltoreq.1.1
wherein D1 is a first diameter of a nanoparticle and D2 is a second
diameter of the nanoparticle, and D1 and D2 intersect at right
angles.
[0007] Furthermore, spherical cross-linked nanoparticles are also
provided. The spherical nanoparticles have a core formed from a
polymeric seed that includes a mono-vinyl aromatic core species
cross-linked with a cross-linking agent. The core has an average
diameter of 5 nanometers to 10,000 nanometers, and the
cross-linking agent comprises 30% to 60% by weight of the combined
weight of the mono-vinyl aromatic species and the cross-linking
agent. The mono-vinyl aromatic core species comprises polymeric
chains radiating from a center of the core. Furthermore, the
nanoparticles meet the following equation:
0.90.ltoreq.(D1/D2).ltoreq.1.1
wherein D1 is a first diameter of a nanoparticle and D2 is a second
diameter of the nanoparticle, and D1 and D2 intersect at right
angles.
[0008] Herein throughout, unless specifically stated otherwise:
"vinyl-substituted aromatic hydrocarbon" and "alkenylbenzene" are
used interchangeably. Furthermore, the terms "a" and "the," as used
herein, mean "one or more."
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a graph showing yield % vs. beginning amount of
cross-linking agent.
[0010] FIG. 2 is an SEM image of a collection of nanoparticles
according to Example 1.
[0011] FIG. 3 is an SEM image of a collection of nanoparticles
according to Example 2.
[0012] FIG. 4 is an SEM image of a collection of nanoparticles
according to Example 3.
[0013] FIG. 5 is an SEM image of a collection of nanoparticles
according to Example 4.
[0014] FIG. 6 is an SEM image of a collection of nanoparticles
according to Example 5.
[0015] FIG. 7 is an SEM image of a collection of nanoparticles
according to Example 6.
DETAILED DESCRIPTION
[0016] A method for making highly spherical, highly crosslinked
nanoparticles is described herein. The method utilizes a core-first
dispersion polymerization synthesis method in conjunction with a
dual solvent system and a step-wise or metered addition of
cross-linking agent. Synergistically, the dual solvent system and
the step-wise or metered addition of cross-linking agent promote
higher yields and better crosslinking and discrete, spherical
particle shapes. The yield can reach up to complete conversion of
the monomer and cross-linking agent.
[0017] Such nanoparticles may have various uses, including use as
child particles in electronic displays such as electronic paper
displays that use QR-LPD technology. Further details on QR-LPD and
such particles are disclosed in U.S. Published Applications
2008/0174854 and 2006/0087718, and U.S. Pat. No. 7,236,291, which
are incorporated herein by reference. A durable particle imparted
with a stable charge is generally desirable, but it is especially
desirable in QR-LPD displays where particles are subjected to
significant frictional forces that tend to damage the structural
and charge characteristics of the particles.
[0018] According to an embodiment of the method, the nanoparticle
is formed by a core-first living dispersion polymerization method.
Living anionic dispersion polymerization or living free radical
dispersion polymerization may be used. Living anionic dispersion
polymerization may be favorable over free radical dispersion
polymerization for some applications. The living dispersion
polymerization methods described herein are superior to emulsion
synthesis methods for many applications. Furthermore, the
nanoparticles synthesized by the methods described herein differ
from star polymers in that they have a larger and decentralized
core.
[0019] In dispersion polymerization, the reaction is effected by
polymerizing a monomer in an organic liquid in which the resulting
polymer is insoluble, using a steric stabilizer to stabilize the
resulting particles of insoluble polymer in the organic liquid.
Dispersion polymerization is used to prevent the propagating
polymeric core from precipitating out of solution. This technique
allows for a sizeable core to be formed in a range of about 5
nanometers up to about 10,000 nanometers while remaining in
solution. Consequently, a wide range of solvents may be used in
which the polymeric core would be otherwise insoluble.
[0020] Building on the flexibility for solvents provided by the
core-first dispersion method, a dual solvent system is used in the
method described herein. The dual solvent system includes a
hydrocarbon solvent and an aprotic, polar co-solvent. The core
monomer is soluble in the hydrocarbon solvent, but the polymerized
monomer is relatively insoluble in the hydrocarbon solvent. The
solvent also promotes aggregation of the poly(alkenylbenzene) and
stabilization of the dispersion polymerization.
[0021] The co-solvent is an aprotic, polar solvent that promotes
swelling of the formed particle and propagation of the polymer
chains that comprise the nanoparticle. The polymer chains and
modifiers are relatively soluble in the co-solvent while the
monomer is relatively insoluble in the co-solvent. The co-solvent
may also be a structural modifier. The monomers and polymer chains
should both be soluble in the co-solvent.
[0022] Specific examples of solvents include aliphatic
hydrocarbons, such as pentane, hexane, heptane, octane, nonane, and
decane, as well as alicyclic hydrocarbons, such as cyclohexane,
methyl cyclopentane, cyclooctane, cyclopentane, cycloheptane,
cyclononane, and cyclodecane. These hydrocarbons may be used
individually or in combination to comprise the hydrocarbon solvent
of the dual solvent system. Selection of a solvent in which one
monomer forming the nanoparticles is more soluble than another
monomer forming the shell of the nanoparticles is preferred for
some applications.
[0023] Specific examples of co-solvents include THF, dioxane,
2-methyl THF, diethyl ether, dimethyl sulfoxide, dimethylformamide,
and hexamethylphosphorotriamide. These solvents may be used
individually or in combination to comprise the polar, aprotic
co-solvent of the dual solvent system.
[0024] The solvent and co-solvent may be added together or
separately to the reactor. The volume ratio of hydrocarbon solvent
to polar, aprotic solvent ranges from about 99.9:0.1 to about
70:30, such as about 85:15 to about 95:5, about 90:10 to about
99:1, or about 94:6 to about 98:2.
[0025] In a generalized embodiment of the core-formation step of
the method, a reactor is provided with the dual solvent system,
into which a mono-vinyl monomer species and a steric stabilizer are
added. A polymerization initiator is also added to the reactor. A
randomizing agent may also be added to the reactor.
[0026] A step-wise or metered addition of cross-linking agent is
also begun at this point. In one embodiment, a first charge of
cross-linking agent is added that is about 1 to about 50% of the
total cross-linking agent to be charged, such as about 1 to about
20%, about 5.25 to about 11.5%, or about 3.5% to about 11.5%. The
first charge may be added all at once or may be metered over time
as the polymerization progresses.
[0027] Without being bound by theory, the incremental addition of
cross-linking agent allows adjustment of the reactivity ratio of
the monomer and cross-linking for randomizing the
copolymerization.
[0028] The first charge of cross-linking agent and the initiator
may be added in one charge to the reactor.
[0029] As the reaction proceeds, the mono-vinyl monomer is
polymerized and cross-linked with the cross-linking agent. The
mono-vinyl polymer chains are tied together by the first charge of
cross-linking agent, wherein the mono-vinyl polymer chains have
living ends at the surface of the core. The living ends are at the
surface of the core due to their higher affinity to the solvent
than the mono-vinyl species. The surface of the core is stabilized
by the steric stabilizer, such as polystyrene-polybutadiene diblock
copolymer. The stabilizer is adsorbed on the surface of the
core.
[0030] After a substantial amount of monomer has been polymerized,
i.e. a substantial monomer conversion, a second charge of
cross-linking agent is added. For example, the second charge of
cross-linking agent can be added after a monomer conversion of
about 51 to about 100%, such as about 75 to about 100%, about 75 to
about 95%, or about 90 to about 100%. The second charge of
cross-linking agent is added, and may be added in a step-wise or
metered manner. In one embodiment, the amount of the second charge
of cross-linking agent is the difference between the total amount
of cross-linking agent and the amount of cross-linking agent used
in the first charge. In one embodiment, the second charge of
cross-linking agent is added that is about 99 to about 50% of the
total cross-linking agent to be charged, such as about 80 to about
99%, about 88.5 to about 94.75%, or about 88.5% to about 96.5%. The
second charge may be added all at once or may be metered over time.
In another embodiment, more than two charges may be used.
[0031] By this method a total yield of nanoparticles may approach
about 100%, such as about 80 to about 98%, or about 85% to about
95%.
[0032] With respect to the monomers and solvents identified herein,
nanoparticles are formed by maintaining a temperature that is
favorable to polymerization of the selected monomers in the
selected solvent(s). Reaction temperatures are, for example, in the
range of about -40 to about 250.degree. C., such as a temperature
in the range of about 0 to about 150.degree. C. The interaction of
monomer selection, temperature and solvent, facilitates the
formation of polymer chains which comprise the nanoparticle.
[0033] The ratio of mono-vinyl monomer species to total
cross-linking agent may range from about 30:70 to about 90:10, such
as about 57:43 to about 72:28, about 51:49 to about 65:35, or about
65:35 to about 85:15. The cross-linking agent can also work as a
co-monomer at the outset of the polymerization.
[0034] The incremental addition of cross-linking agent can adjust
the reactivity ratio of the monomer and cross-linking agent for
randomizing the copolymerization. A more uniform structure of the
particles is formed in the dual solvent system and it allows the
partly formed nanoparticles to swell and incorporate the
cross-linking agent more densely into the core structure.
[0035] The highly cross-linked core enhances the uniformity,
durability, and permanence of shape and size of the resultant
nanoparticle. The example method may be performed in a single
batch, and there is no requirement to isolate and dry the core
before grafting the shell.
[0036] Specific examples of mono-vinyl monomer species include
mono-vinyl aromatic species, such as styrene,
.alpha.-methylstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene,
1-.alpha.-methyl vinyl naphthalene, 2-.alpha.-methyl vinyl
naphthalene, vinyl toluene, methoxystyrene, t-butoxystyrene, as
well as alkyl, cycloalkyl, aryl, alkaryl, and aralkyl derivatives
thereof, in which the total number of carbon atoms in the combined
hydrocarbon is not greater than 18, as well as any di- or tri-vinyl
substituted aromatic hydrocarbons, and mixtures thereof. Further
examples of mono-vinyl monomer species include non-aromatic
mono-vinyl monomer species, such as vinyl acetate,
vinyl-methacrylate, and vinyl-alcohols.
[0037] Crosslinking agents that are at least bifunctional, wherein
the two functional groups are capable of reacting with the
mono-vinyl species of the core are acceptable. Examples of suitable
cross-linking agents include multiple-vinyl aromatic monomers in
general. Specific examples of cross-linking agents include di- or
tri-vinyl-substituted aromatic hydrocarbons, such as
diisopropenylbenzene, divinylbenzene, divinyl ether, divinyl
sulphone, diallyl phthalate, triallyl cyanurate, triallyl
isocyanurate, 1,2-polybutadiene, N,N'-m-phenylenedimaleimide,
N,N'-(4-methyl-m-phenylene)dimaleimide, triallyl trimellitate
acrylates, methacrylates of polyhydric C.sub.2-C.sub.10 alcohols,
acrylates and methacrylates of polyethylene glycol having from 2 to
20 oxyethylene units, polyesters composed of aliphatic di- and/or
polyols, or maleic acid, fumaric acid, and itaconic acid.
Multiple-vinyl aromatics, such as divinylbenzene provides excellent
properties and are compatible with common solvents.
[0038] Specific examples of suitable steric stabilizers include
styrene-butadiene diblock copolymer, polystyrene-b-polyisoprene,
and polystyrene-b-polydimethylsiloxane.
[0039] A 1,2-microstructure controlling agent or randomizing
modifier is optionally used to control the 1,2-microstructure in
the mono-vinyl monomer units of the core. Suitable modifiers
include 2,2-bis(2'-tetrahydrofuryl)propane, hexamethylphosphoric
acid triamide, N,N,N',N'-tetramethylethylene diamine, ethylene
glycol dimethyl ether, diethylene glycol dimethyl ether,
triethylene glycol dimethyl ether, tetraethylene glycol dimethyl
ether, tetrahydrofuran, 1,4-diazabicyclo[2.2.2]octane, diethyl
ether, triethylamine, tri-n-butylamine, tri-n-butylphosphine,
p-dioxane, 1,2-dimethoxy ethane, dimethyl ether, methyl ethyl
ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether,
anisole, dibenzyl ether, diphenyl ether, dimethylethylamine,
bis-oxalanyl propane, tri-n-propyl amine, trimethyl amine, triethyl
amine, N,N-dimethyl aniline, N-ethylpiperidine, N-methyl-N-ethyl
aniline, N-methylmorpholine, tetramethylenediamine, oligomeric
oxolanyl propanes (OOPs), 2,2-bis-(4-methyl dioxane), and
bistetrahydrofuryl propane. A mixture of one or more randomizing
modifiers also can be used. The ratio of the modifier to the
monomers can vary from a minimum as low as 0 to a maximum as great
as 4000 millimoles, for example 0.01 to 3000 millimoles, of
modifier per hundred grams of monomer currently being charged into
the reactor. As the modifier charge increases, the percentage of
1,2-microstructure (vinyl content) increases in the conjugated
diene contributed monomer units in the surface layer of the polymer
nanoparticle. The 1,2-microstructure content of the conjugated
diene units is for example, within a range of about 5% and about
95%, such as less than about 35%.
[0040] Suitable initiators for the core formation process include
anionic initiators that are known in the art as useful in the
polymerization of mono and multiple-vinyl monomers. Exemplary
organo-lithium initiators include lithium compounds having the
formula R(Li).sub.x, wherein R represents a C.sub.1-C.sub.20
hydrocarbyl radical, such as a C.sub.2-C.sub.8 hydrocarbyl radical,
and x is an integer from 1 to 4. Typical R groups include aliphatic
radicals and cycloaliphatic radicals. Specific examples of R groups
include primary, secondary, and tertiary groups, such as n-propyl,
isopropyl, n-butyl, isobutyl, and t-butyl.
[0041] Specific examples of initiators include ethyllithium,
propyllithium, n-butyllithium, sec-butyllithium, and
tert-butyllithium; aryllithiums, such as phenyllithium and
tolyllithium; alkenyllithiums such as vinyllithium,
propenyllithium; alkylene lithium such as tetramethylene lithium,
and pentamethylene lithium. Among these, n-butyllithium,
sec-butyllithium, tert-butyllithium, tetramethylene lithium, and
mixtures thereof are specific examples. Other suitable lithium
initiators include one or more of: p-tolyllithium, 4-phenylbutyl
lithium, 4-butylcyclohexyl lithium, 4-cyclohexylbutyl lithium,
lithium dialkyl amines, lithium dialkyl phosphines, lithium alkyl
aryl phosphine, and lithium diaryl phosphines.
[0042] Free radical initiators may also be used in conjunction with
a free radical polymerization process. Examples of free-radical
initiators include: 2,2'-azo-bis(isobutyronitril,
1,1'-azobis(cyclohexanecarbonitrile),
2,2'-azobis(2-methylpropionamidine)dihydrochloride,
2,2'-azobis(2-methylpropionitrile), 4,4'-azobis(4-cyanovaleric
acid), 1,1-bis(tert-amylperoxy)cyclohexane,
1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
1,1-bis(tert-butylperoxy)cyclohexane,
2,2-bis(tert-butylperoxy)butane, 2,4-pentanedione peroxide,
2,5-bis(tert-butylperoxy)-2,5-dimethylhexane,
2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, 2-butanone peroxide,
2-butanone peroxide, 2-butanone peroxide, benzoyl peroxide, cumene
hydroperoxide, di-tert-amyl peroxide, dicumyl peroxide, lauroyl
peroxide, tert-butyl hydroperoxide, ammonium persulfate,
hydroxymethanesulfinic acid monosodium salt dehydrate, potassium
persulfate, and reagent grade sodium persulfate.
[0043] Functionalized lithium initiators are also contemplated as
useful in the polymerization of the core species. A functionalized
initiator serves to functionalize the core, and the functional
groups are likely distributed throughout the surface and interior
of the core. Example functional groups include amines, formyl,
carboxylic acids, alcohols, tin, silica, and mixtures thereof.
[0044] Amine-functionalized initiators include those that are the
reaction product of an amine, an organo lithium, and a solubilizing
component. The initiator may have the general formula:
(A)Li(SOL).sub.y
[0045] where y is from 1 to 3; SOL is a solubilizing component
selected from the group consisting of hydrocarbons, ethers, amines
or mixtures thereof; and, A is selected from the group consisting
of alkyl, dialkyl and cycloalkyl amine radicals having the general
formula:
##STR00001##
[0046] and cyclic amines having the general formula:
##STR00002##
[0047] where R.sup.1 is selected from the group consisting of
alkyls, cycloalkyls or aralkyls having from 1 to 12 carbon atoms,
and R.sup.2 is selected from the group consisting of an alkylene,
substituted alkylene, oxy- or N-alkylamino-alkylene group having
from 3 to 16 methylene groups. A specific example of a
functionalized lithium initiator is hexamethylene imine
propyllithium.
[0048] Tin functionalized lithium initiators may also be useful in
synthesizing the nanoparticles. Suitable tin functionalized lithium
initiators include tributyl tin lithium, trioctyl tin lithium, and
mixtures thereof.
[0049] Anionic initiators generally are useful in amounts ranging
from about 0.01 to about 60 millimoles per hundred grams of monomer
charge. Free radical initiators are useful in amounts ranging from
about 6 to about 100 millimoles per hundred grams of monomer
charge.
[0050] The core may range in size from 5 nanometers to about 10,000
nanometers, for example about 25 to about 1,000 nanometers, about
40 to about 150 nanometers, about 50 to about 125 nanometers, or
about 100 to about 1,000 nanometers. In an embodiment, the core
differs from that of a star polymer in that it does not emanate
from a single point, but instead is decentralized and has a minimum
size of 5 nanometers.
[0051] The core may be useful in some applications without further
addition of a shell. In one embodiment, the living polymer chain
ends are terminated and the crosslinked core is the nanoparticle.
In another embodiment, a shell layer or layers are added to the
core.
[0052] In an embodiment, a shell for the nanoparticles is formed by
grafting or polymerizing a shell species onto the living ends of
the cross-linked core. The nanoparticle is thus formed with
polymers or copolymers extending from the cross-linked core into
the uncrosslinked shell. The shell species can be selected from a
variety of oligomers, polymers, monomers, or macromolecules and
functionalized versions of all of these. Because the shell is
formed last, the shell species does not need to be as stable as it
would if it were formed first and had to survive the core formation
and cross-linking process. Thus, the core-first process can produce
many new nanoparticles that were difficult or impossible to make
with a shell first process. In addition, the core-first dispersion
process provides an easier and more reliable method to make
functionalized nanoparticles in general.
[0053] In one embodiment, the shell species is a polymer that has
already been polymerized in a separate reactor and then added to
the reactor that holds the core with living ends. An addition of
the preformed polymer to the reactor containing the core would
result in the polymer chains being grafted to the cross-linked
core, thereby forming a shell with polymer brushes. The term
"polymer brushes" or "brush-like surface" as used herein, is used
to mean the uncrosslinked polymers that extend into the shell of
the nanoparticles. Like the term "hairy," the term "brushes"
denotes the uncrosslinked nature of the shell. Alternatively, the
preformed polymer may be functionalized with a functional
initiator, a functional terminator, or both in the separate reactor
and then grafted onto the living ends of the core, thereby forming
functionalized polymer brushes in the shell of the
nanoparticle.
[0054] In another embodiment, the shell species is added as a
monomer to the reactor containing the core and polymerized with
initiator in the same reactor with the core. The shell would thus
comprise polymer brushes of the shell species. Optionally,
functional terminators could be used to functionalize the polymer
brushes of the shell.
[0055] In another embodiment, the shell species is a monomer
containing a heteroatom. The heteroatomic monomer is polymerized
and grafted to the core as described above.
[0056] The shorter the uncrosslinked polymer brushes are, the
harder and more well-defined the nanoparticle will be. This is due
to the proximity of the ends of the polymer brushes to the hard
core and the thermodynamic effects of the same.
[0057] In another embodiment, the shell species is a single monomer
unit. The unit may be a hydrocarbon or contain one or more
heteroatoms, and it may be functionalized. The monomer is added
with no initiator and bonds to the living ends of the core. This
embodiment forms a nanoparticle with a single monomer unit
shell.
[0058] In another embodiment, the shell species is a macromolecule
having a molecular weight up to about 10,000 g/mol or an oligomer.
The shell species is added to the reactor containing the core. The
macromolecule or oligomer is thus grafted onto the living ends of
the core. Various functional groups may be present in the
macromolecule or oligomer.
[0059] In another embodiment, the shell species is itself a
functional terminator. When added to the reactor containing the
core, the functional terminator bonds to the living ends of the
core and terminates the core with a functional group. In this
example, the functional group is considered the shell.
[0060] Example shell species generally include hydrocarbons,
heteroatomic species, polar functionalized species, water-soluble
species, and thermoplastic and plastic elastomers.
[0061] Hydrocarbon shell species include C.sub.4 to C.sub.8
conjugated dienes, such as, 1,3-butadiene, isoprene, and
1,3-pentadiene. Olefinic species such as ethylene, propylene,
isobutylene, and cyclohexene may also be used.
[0062] Heteroatomic shell species include species containing O, N,
S, P, Cl, Ti, and Si atoms, such as, epoxides, urethanes, esters,
ethers, imides, amines, carbonates, siloxanes, halogens, metals,
synthetic oils, and vegetable oils. Specific examples include,
polydimethylsiloxane (PDMS), polyethylene oxide (PEO), halogenated
butyl rubber, polyethylene teraphthalate (PET), polyethylene glycol
(PEG), polyphenylene oxide (PPO), polypropylene glycol) diglycidyl
ether (PPO-EO2), polyvinyl alcohol, pyridine, carbazole, imidazole,
diethylamino-styrene, and pyrrolidone. Example macromolecules or
oligomers include polyethylene glycol, polyphenylene oxide, and
polydimethylsiloxane.
[0063] Functional terminators for use with or as the shell species
include SnCl.sub.4, R.sub.3SnCl, R.sub.2SnCl.sub.2, RSnCl.sub.3,
carbodiimides, N-methylpyrrolidine, cyclic amides, cyclic ureas,
isocyanates, Schiff bases, 4,4'-bis(diethylamino) benzophenone,
N,N'-dimethylethyleneurea, and mixtures thereof, wherein R is
selected from the group consisting of alkyls having from 1 to 20
carbon atoms, cycloalkyls having from 3 to 20 carbon atoms, aryls
having from 6 to 20 carbon atoms, aralkyls having from 7 to 20
carbon atoms, and mixtures thereof.
[0064] Additionally, the shell species may include copolymers,
including random and block copolymers. These copolymers may include
the hydrocarbon and heteroatomic monomers listed above. The
copolymer shell species may be synthesized prior to introducing the
species into the reactor with the core, or it may be polymerized
after introduction into the reactor as described above.
[0065] Such multi-block polymers, are believed to aggregate to form
micelle-like structures around the core, with the block that is the
least soluble in the solvent directed toward the center of the core
and the other block as a tail or brush extending therefrom. For
example, in a hydrocarbon solvent, vinyl-substituted aromatic
blocks are directed toward the center of the core and other blocks
extend as tails therefrom. It is noted that a further hydrocarbon
solvent charge or a decrease in polymerization mixture temperature
may also be used, and may in fact be required, to obtain formation
of the micelles. Moreover, these steps may be used to take
advantage of the general insolubility of a vinyl-aromatic block. An
exemplary temperature range for micelle formation is between -80
and 100.degree. C., such as between 20.degree. C. and 80.degree.
C.
[0066] Although the above description discusses the formation of
multi-block polymers prior to micelle formation, it is noted that
after the nanoparticles have formed, additional monomer charge(s),
such as conjugated diene monomer and/or vinyl-substituted aromatic
hydrocarbon monomer, can be added to the polymerization mixture as
desired. In this manner, multi-block polymers may be formed when
only diblock polymers form the nanoparticles. Moreover, it is
feasible to form the micelles of the block co-polymers with a
further monomer(s) charge thereafter. The sequential addition of
various monomers allows growth of particle size and formation of
the shell with different internal structures.
[0067] In an alternative embodiment the shell of the nanoparticles
may be formed by grafting a charge agent containing species onto
the living ends of the cross-linked core or terminating the living
ends with a charge agent containing terminating agent. The
nanoparticle is thus formed with polymers, copolymers, or the
terminating agent of the core polymers extending from the
cross-linked core into the shell. The nature and quantity of the
charge agent allows control of the charge on the nanoparticle.
[0068] One method of adding a charge agent to the living ends of
the nanoparticle core is through the addition of a functionalized
terminator. The functionalized terminator includes a charge agent
that has a fixed formal charge group that exists after the addition
to the nanoparticle. By fixed formal charge it is meant a charge
that results from an actual excess or deficiency of electrons, not
merely a localized charge due to an electron rich area of the
molecule.
[0069] Once the core is formed and a desired yield is obtained, the
functional terminator containing the charge agent is added to the
reactor. In an embodiment, this is a one-pot process that does not
require a separate isolation step or drying step to isolate and dry
the core.
[0070] The added terminator terminates the living ends of the
mono-vinyl polymer chains of the core and places a functional group
containing the charge agent on the end of the chains. In this
embodiment the functional group is considered to be the shell layer
of the nanoparticle. The very thin functional group shell layer has
the advantage of being physically durable and resistant to
frictional shearing forces. This is due to its short length and
proximity to the highly crosslinked core of the nanoparticle.
[0071] By selecting a charge agent that has a known charge, one can
craft a nanoparticle with a desired charge that is physically
durable and resistant to frictional shearing forces. The charge
agent may be selected from a variety of charge agents that have a
fixed formal charge. For most applications species with a more
stable, fixed, formal charge are preferred. Examples of compounds
that may have a stable, fixed, formal charge include nitrogen- and
oxygen-containing species, such as cyclic compounds, including
without limitation, succinic anhydride, imidazole, pyridines,
N,N-dimethylaminostyrene and N,N-diethylaminostyrene, including
without limitation, pyridine silane or vinyl pyridine and the
derivates of all the above. Nitrogen containing species that
contain quaternary ammonium compounds have a particularly stable
charge. Quaternary phosphonium and quaternary sulfonium compounds
are similar species with stable charges. Nitrogen containing
species, such as pyridines may readily be imparted with a positive
formal charge, whereas oxygen containing species, such as succinic
anhydride, may readily be imparted with a negative formal
charge.
[0072] A second, more versatile, method of adding a charge agent to
the living ends includes polymerizing one or more monomer units
having a charge agent with a fixed formal charge onto the living
ends of the nanoparticle core. Once the core is formed from the
core formation process discussed above and a desired yield is
obtained, a monomer that includes the charge agent may be added to
the reactor along with a polymerization initiator. Again, this is a
one-pot process that does not require separate isolation or drying
of the core. The monomer containing the charge agent bonds to the
living ends of the mono-vinyl polymer chains of the core. Depending
on reaction conditions, and the amount of monomer and initiator,
additional monomer-contributed units will propagate in polymer
chains originating from the living ends of the mono-vinyl polymer
chains of the nanoparticle core. In this way, the nanoparticle
comprises diblock polymer chains with a mono-vinyl block that is
crosslinked and a charge agent block. The charge agent block may be
considered to be the shell layer of the nanoparticle, while the
cross-linked mono-vinyl block may be considered to be the core
layer of the nanoparticle. Similarly, the shell block of the
diblock polymers that comprise the nanoparticle include the charge
agent monomers as monomer contributed units, while the core block
includes the mono-vinyl monomers as monomer contributed units.
[0073] In another method adding a charge agent to the living ends
of the nanoparticle core, the charge agent is a monomeric species
that has already been polymerized in a separate reactor and then
added to the reactor that holds the core with living ends. An
addition of the preformed polymeric charge agent to the reactor
containing the core would result in the polymer chains being
grafted to the cross-linked core, thereby forming a shell with
propagated charge agent chains.
[0074] In another method adding a charge agent to the living ends
of the nanoparticle core, a charge agent monomer is added to the
reactor. The monomer may be a hydrocarbon containing one or more
heteroatoms, and it may be functionalized. The monomer is added
with no initiator and grafts onto the living ends of the core. This
method forms a nanoparticle with a shell having a single charge
agent layer.
[0075] The charge of the nanoparticle can be controlled by the
selection of the charge agent, and it can also be controlled by the
number of charge agents present in the nanoparticle. The number of
charge agents in the nanoparticle is a function of the amount of
charge-agent-containing monomer and the length of the charge agent
block of the polymer chains. It is also a function of the number of
living ends present on the surface of the core (onto which the
charge agent-containing-monomer attaches).
[0076] The nanoparticle core will have a negative charge due to the
electron-rich localized charge induced by the mono-vinyl monomer
contributed units and the cross-linking agent. Addition and
polymerization of a shell monomer with a positive charge agent such
as pyridine will cause the overall charge of the nanoparticle to be
less negative, and continued addition and polymerization of the
monomer will cause the charge on the nanoparticle to become
positive. Conversely, addition and polymerization of a shell
monomer with a negative charge agent such as succinic anhydride
will cause the overall charge of the nanoparticle to be more
negative. Thus, in general, the more charge agent
monomer-contributed units that are present in the shell layer of
the nanoparticle, and the longer each charge agent block polymer
chain is, the more positive or negative the charge on the
nanoparticle will become according to whether a positive or
negative charge agent is used. In this manner one can select a
charge agent and polymerize additional monomer units until a
desired charge is reached.
[0077] A wide range of equilibrium weight-average charge values can
be reached by this method. For example, the charge may range from
about -500 .mu.C/g to about 600 .mu.C/g, such as about -300 .mu.C/g
to about 300 .mu.C/g, or about -150 .mu.C/g to about 150 .mu.C/g.
Negatively charged nanoparticles may, for example, have charges of
about -10 .mu.C/g or less, such as about -50 .mu.C/g or less, about
-500 .mu.C/g to about -50 .mu.C/g, or about -150 .mu.C/g to about
-50 .mu.C/g. Positively charged nanoparticles may have charges of
greater than about 0 .mu.C/g, such as about 50 .mu.C/g or greater,
about 1 to about 600 .mu.C/g, about 100 to about 300 .mu.C/g, or
about 300 to about 600 .mu.C/g. The amount (Q) of the charged
nanoparticles, may range from about 1 to about 600 .mu.C/g, such as
about 1 to about 100 .mu.C/g, or
[0078] The core-first nanoparticle formation process allows the
nanoparticle to include charge agent species in the shell. Because
the shell is formed last, the shell species does not need to be as
stable as it would if it were formed first and had to survive the
core formation and cross-linking process. The hydrocarbon solvent
used in previous shell-first nanoparticle formation methods also
did not allow both micelle aggregation and polymerization of polar
(charge agent) monomers. Thus, the core-first process can produce
many new nanoparticles that were difficult or impossible to make
with shell first processes.
[0079] While having long, uncrosslinked polymer chains in the shell
may be beneficial in some applications, to preserve the charge and
the durability of the nanoparticle, relatively short shell layer
chain lengths are preferable for some applications. The longer the
shell layer chain lengths become, the more susceptible they are to
frictional shear forces and degradation. Accordingly, a relatively
thin shell layer is preferable for some applications, such as for
electronic displays. For example, the shell layer may be about 1 nm
to about 100 nm, such as about 1 nm to about 50 nm, or about 1 nm
to about 25 nm.
[0080] For certain applications such as QR-LPD display
technologies, making both positively and negatively charged
nanoparticles to be used in cells together is required. The methods
and nanoparticles described herein are particularly well suited to
controlling the polarity and the magnitude of a group of positive
and negative particles. In an embodiment, a first group of
nanoparticles and a second group of nanoparticles may have a
difference in charge of about 50 .mu.C/g or greater, such as about
50 to about 500 .mu.C/g, or about 75 to about 200 .mu.C/g.
[0081] The size of the entire core-shell nanoparticles, including
both core and shell--expressed as a mean average diameter--are, for
example, between about 5 and about 20,000 nanometers, such as about
50 to about 5,000 nanometers, about 75 to about 300 nanometers, or
about 75 to about 150 nanometers.
[0082] For some applications the nanoparticles are preferably
substantially monodisperse and uniform in shape. The nanoparticles
may, for example, have a dispersity less than about 1.3, such as
less than about 1.2, or less than about 1.1. Image analysis, such
as SEM image analysis, or light scattering analysis of the
particles can provide information for calculating the particle size
distribution.
[0083] The highly cross-linked nature of the nanoparticles
described herein enables them to have a highly spherical shape with
few if any shape defects or irregularities. In one embodiment,
about 90% to 100%, for example, about 92% to about 99%, or about
95% to about 98%, of the nanoparticles have no shape defects or
irregularities. Furthermore, the sphericalness of the nanoparticles
can be measured by measuring a first diameter (D.sub.1) of a
nanoparticle and then a second diameter (D.sub.2) at a right angle
to D.sub.1. (D.sub.1 may be selected to be any diameter of the
nanoparticle, and the measurements of D.sub.1 and D.sub.2 may be
done by analysis of an SEM image.) Then dividing D.sub.1 by D.sub.2
will yield a number close to 1, for example about 0.9 to about 1.1,
0.95 to 1.05, or 0.97 to 1.03. Thus, a formula representing the
sphericalness (s) of the nanoparticles may be stated as:
D.sub.1/D.sub.2=s. In one embodiment, about 90% to 100%, for
example, about 92% to about 99%, or about 95% to about 98%, of the
nanoparticles have a sphericalness of about 0.9 to about 1.1.
[0084] Without being bound by theory, it is believed that
nanoparticles that are highly spherical will be more durable in
structural and charge characteristics than those that are not so
spherical. This is due, in part, to the decreased frictional forces
that act upon objects that are highly spherical. Shape defects are
likely to cause structural failure or deformation of the
nanoparticles.
[0085] In an embodiment the example nanoparticles are also
substantially discrete, for example, the nanoparticles have about
20% to 0% cross-linking between nanoparticles, such as 9% to 0%,
about 8% to about 2%, about 5% to about 0.1%, or about 3% to about
0.1%.
[0086] In an embodiment, the core of the synthesized nanoparticles
is hard. For example, the core has a Tg of 100.degree. C. or
higher, such as 150.degree. C. or higher. In an embodiment, the
nanoparticles have a core that is relatively harder than the shell,
for example, at least 60.degree. C. higher than the Tg of the shell
layer, or at least 1.degree. C. higher than the Tg of the shell
layer. In one example, the shell layer is soft. That is, the shell
layer has a Tg lower than 0.degree. C. In one embodiment, the Tg of
the shell layer is between 0.degree. C. and -100.degree. C. In
another embodiment the Tg of the shell is also high, such as
100.degree. C. or higher, such as 150.degree. C. or higher.
Nanoparticles with hard cores and soft shells are particularly
useful for reinforcing rubber compounds used for tire treads.
Nanoparticles with hard cores and hard shells are particularly
useful in electronic display applications such as QR-LPD.
[0087] The Tg of the polymers in the nanoparticles is influenced by
the selection of monomers and their molecular weight, styrene
content, and vinyl content. The degree of cross-linking also
contributes to the Tg of the core and shell.
[0088] The following examples and tables are presented for purposes
of illustration only and are not to be construed in a limiting
sense.
EXAMPLES
[0089] A 0.8 liter nitrogen-purged glass bottle sealed with a
septum liner and perforated crown cap was used as the reactor
vessel for the examples below. Styrene (33 wt % in hexane), hexane,
n-butyllithium (1.60 M in hexane),
2,2-bis(2'-tetrahydrofuryl)propane (1.60 M in hexane, stored over
calcium hydride), potassium tert-amyloxide (KTA), and BHT solution
in hexane were also used. PS-PB diblocks STEREON S730AC and STEREON
S721 were obtained from Firestone Polymers. Commercially available
reagents were obtained from Aldrich and Gelest Inc. (Morrisville,
Pa.) and dried over molecular sieves (3 .ANG.).
Examples A-D
[0090] Examples A-D were made by the method below. The only
difference between examples A-D is the amount of DVB that was
added.
[0091] To a 0.8 liter nitrogen-purged glass bottle was added 140 g
of hexane, 60 g of 33 wt % styrene, varying amounts of DVB, 6 ml of
5 wt % STEREON S721, 0.4 ml of 1.6 M
2,2'-di(tetrahydrofuryl)propane (OOPS), and 2 ml of 1.6 M n-butyl
lithium. The amount of DVB was added in the amounts shown in FIG.
1. The DVB % shown in FIG. 1 is a weight percent based on the total
amount of DVB and monomer. The reaction mixture was stirred for one
day at room temperature before terminating with 3 ml isopropanol.
After the solvent evaporated, the products were dried in
vacuum.
[0092] FIG. 1 graphically shows the yield of nanoparticles obtained
compared to the varying amounts of DVB. The DVB was added all at
once, and a single solvent system (hexane) was used. This data
demonstrates that simply increasing the amount of DVB causes
reduced yields.
Example 1
[0093] To a 0.8 liter nitrogen-purged glass bottle was added 140 g
of hexane, 60 g of 33 wt % styrene, 10 ml of DVB, 6 ml of 5 wt %
STEREON S721, 0.4 ml of 1.6 M 2,2'-di(tetrahydrofuryl)propane
(OOPS), and 2 ml of 1.6 M n-butyl lithium. The reaction mixture was
stirred for one day at room temperature before terminating with 3
ml isopropanol. After the solvent evaporated, the products were
dried in vacuum.
Example 2
[0094] To a 0.8 liter nitrogen-purged glass bottle was added 300 g
of hexane, 20 g of 33 wt % styrene, 0.1 ml of DVB, 1 ml of 5 wt %
STEREON S721, 0.2 ml of 1.6 M 2,2'-di(tetrahydrofuryl)propane
(OOPS), and 1 ml of 1.6 M n-butyl lithium. After 10 minutes, an
additional 2.8 ml of DVB was added to the bottle. The reaction
mixture was stirred for one day at room temperature before
terminating with 3 ml of isopropanol. After the solvent evaporated,
the products were dried in vacuum.
Example 3
[0095] To a 0.8 liter nitrogen-purged glass bottle was added 300 g
of hexane, 20 g of 33 wt % styrene, 30 ml of THF, 0.3 ml of DVB, 1
ml of 5 wt % STEREON S721, 0.2 ml of 1.6 M
2,2'-di(tetrahydrofuryl)propane (OOPS), and 1 ml of 1.6 M n-butyl
lithium. After 10 minutes, an additional 2.6 ml of DVB was added to
the bottle. The reaction mixture was stirred for one day at room
temperature before terminating with 3 ml of isopropanol. After the
solvent evaporated, the products were dried in vacuum.
Example 4
[0096] To a 0.8 liter nitrogen-purged glass bottle was added 300 g
of hexane, 20 g of 33 wt % styrene, 0.5 ml of DVB (50 wt % in
hexane), 10 ml of 5 wt % STEREON S730AC, 0.06 ml of 1.6 M KTA and
0.5 ml of 1.6 M n-butyl lithium. After 10 minutes, an additional
9.5 ml of DVB (50 wt % in hexane) was added to the bottle. The
reaction mixture was stirred for three days at room temperature.
The products were coagulated with isopropanol, filtered, and dried
in vacuum.
Example 5
[0097] To a 0.8 liter nitrogen-purged glass bottle was added 300 g
of hexane, 20 g of 33 wt % styrene, 10 ml of THF, 0.5 ml of DVB (50
wt % in hexane), 10 ml of 5 wt % STEREON S730AC, 0.06 ml of 1.6 M
KTA and 0.5 ml of 1.6 M n-butyl lithium. After 10 min, an
additional 9.5 ml of DVB (50 wt % in hexane) was added to the
bottle. The reaction mixture was stirred for three days at room
temperature. The products were coagulated with isopropanol,
filtered, and dried in vacuum.
Example 6
[0098] To a 0.8 liter nitrogen-purged glass bottle was added 300 g
hexane, 20 g of 33 wt % styrene, 20 ml of THF, 0.5 ml of DVB (50 wt
% in hexane), 10 ml of 5 wt % STEREON S730AC, 0.06 ml of 1.6 M KTA,
and 0.5 ml of 1.6 M n-butyl lithium. After 10 min, an additional
9.5 ml DVB (50 wt % in hexane) was added to the bottle. The
reaction mixture was stirred for three days at room temperature.
The products were coagulated with isopropanol, filtered, and dried
in vacuum.
TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 St/DVB (wt/wt)
68/32 72/28 72/28 THF/Hexane 0 0 6% (vol/vol) 1.sup.st/2.sup.nd DVB
charge 10/0 ml 0.1/2.8 ml 0.3/2.6 ml Yield (%) 42 87 98 Size (nm)
500-1000 500-800 400-600 SEM image FIG. 2 FIG. 3 FIG. 4
TABLE-US-00002 TABLE 2 Example 4 Example 5 Example 6 St/DVB (wt/wt)
57/43 57/43 57/43 THF/Hexane 0 2% 4% (vol/vol) 1.sup.st/2.sup.nd
DVB charge 0.25/4.75 ml 0.25/4.75 ml 0.25/4.75 ml Yield (%) 32 53
70 Size (nm) 80-120 50-140 80-100 SEM image FIG. 5 FIG. 6 FIG.
7
[0099] The complex nature of the core-first dispersion
polymerization process makes it difficult to reproducibly control
the particle shape uniformity and size distribution. Increased
content of styrene and DVB (crosslinking agent) in the anionic
dispersion polymerization reduced the product yield and led to a
gel. Table 1 shows the effect on the product yield of varying the
DVB amount. In Examples 2-6 DVB was incrementally added to the
bottle to prevent aggregation of highly crosslinked nanoparticles.
The Examples show the effect of THF as a modifier and co-solvent as
well. The results are summarized in Tables 1 and 2.
[0100] Incremental DVB addition increased not only the product
yield but also improved the particle shape and size distribution.
THF as a modifier and co-solvent accelerated the polymer chain
propagation in the heterogeneous polymerization and activated the
live ends of the core to swell and incorporate more monomer and
cross-linking agent.
[0101] Furthermore, well-defined particles with high crosslinking
density were obtained by balancing initiation, propagation, and
particle formation in the presence of THF and the modifier such as
OOPS or KTA.
[0102] This written description sets forth the best mode of the
invention, and describes the invention so as to enable a person
skilled in the art to make and use the invention, by presenting
examples. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art.
* * * * *