U.S. patent application number 13/919961 was filed with the patent office on 2013-10-24 for protective coating of magnetic nanoparticles.
The applicant listed for this patent is International Business Machines Corporation. Invention is credited to Richard L. Bradshaw, Dong-Chul Pyun.
Application Number | 20130277600 13/919961 |
Document ID | / |
Family ID | 41530573 |
Filed Date | 2013-10-24 |
United States Patent
Application |
20130277600 |
Kind Code |
A1 |
Bradshaw; Richard L. ; et
al. |
October 24, 2013 |
PROTECTIVE COATING OF MAGNETIC NANOPARTICLES
Abstract
Encapsulated particles and methods for manufacturing
encapsulated particles and structures are described. Such particles
may have a length no greater than 40 nm, and include at least one
material selected from the group consisting of ferromagnetic
materials and ferrimagnetic materials. A polymeric encapsulant
surrounds the particle, the polymeric encapsulant including a
phase-separated block copolymer including a glassy first phase and
a rubbery second phase, the glassy first phase positioned between
the particle and the second rubbery phase. The glassy first phase
includes a hydrophobic copolymer having a glass transition
temperature of at least 50.degree. C. The rubbery second phase
includes a polymer having at least one of (i) a glass transition
temperature of no greater than 30.degree. C., and (ii) a tan delta
peak maximum of no greater than 30.degree. C. Other embodiments are
described and claimed.
Inventors: |
Bradshaw; Richard L.;
(Tucson, AZ) ; Pyun; Dong-Chul; (Tucson,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
International Business Machines Corporation |
Armonk |
NY |
US |
|
|
Family ID: |
41530573 |
Appl. No.: |
13/919961 |
Filed: |
June 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12174605 |
Jul 16, 2008 |
8465855 |
|
|
13919961 |
|
|
|
|
Current U.S.
Class: |
252/62.54 ;
427/127 |
Current CPC
Class: |
H01F 1/083 20130101;
Y10T 428/2998 20150115; H01F 1/00 20130101; H01F 1/061 20130101;
G11B 5/712 20130101; G11B 5/714 20130101 |
Class at
Publication: |
252/62.54 ;
427/127 |
International
Class: |
H01F 1/00 20060101
H01F001/00 |
Claims
1. An encapsulated particle comprising: a particle having a length
no greater than 40 nm; the particle comprising at least one
material selected from the group consisting of ferromagnetic
materials and ferrimagnetic materials; a polymeric encapsulant
surrounding the particle, the polymeric encapsulant comprising a
phase-separated block copolymer including a glassy first phase and
a rubbery second phase, the glassy first phase positioned between
the particle and the second rubbery phase; the glassy first phase
comprising a hydrophobic copolymer having a glass transition
temperature of at least 50.degree. C.; and the rubbery second phase
comprising a polymer having at least one of (i) a glass transition
temperature of no greater than 30.degree. C., and (ii) a tan delta
peak maximum of no greater than 30.degree. C.
2. The encapsulated particle of claim 1, wherein the glassy first
phase is in direct contact with the particle, and the rubbery
second phase is in direct contact with the first phase.
3. The encapsulated particle of claim 1, wherein the particle
comprises a material selected from the group consisting of iron
cobalt alloys, cobalt platinum alloys, barium ferrite alloys,
cobalt, nickel iron alloys, manganese aluminum alloys, iron nickel
alloys, manganese aluminum alloys, iron, iron oxide, cobalt oxide,
nickel oxide, and spinel ferrites.
4. The encapsulated particle of claim 1, wherein the polymeric
encapsulant has a thickness of up to 2 nm.
5. The encapsulated particle of claim 1, wherein the rubbery second
phase includes plurality of end groups consisting of reactive
functional groups.
6. The encapsulated particle of claim 5, wherein the reactive
functional groups include at least one group selected from the
group consisting of photoreactive functional groups and chemically
reactive functional groups.
7. The encapsulated particle of claim 6, wherein the reactive
functional groups include at least one chemically reactive
functional group selected from the group consisting of: epoxy
functional groups, isocyanate functional groups, anhydride
functional groups, carboxylic acid functional groups, and primary
alcohol functional groups.
8. The encapsulated particle of claim 6, wherein the reactive
functional groups include at least one photoreactive functional
group selected from the group consisting of: azo functional groups,
vinyl functional groups, allyl functional groups, and acryl
functional groups.
9. The encapsulated particle of claim 1, wherein the particle has
an aspect ratio that is no less than two.
10. The encapsulated particle of claim 1, wherein the particle has
a length dimension of no greater than 20 nm.
11. The encapsulated particle of claim 1, wherein the particle
includes an oxide layer, and wherein the glassy phase is formed in
contact with the oxide layer
12. A magnetic tape comprising a plurality of encapsulated
particles, comprising: a plurality of magnetic particles, each
having a length no greater than 40 nm; the magnetic particles each
comprising at one material selected from the group consisting of
ferromagnetic materials and ferrimagnetic materials; a polymeric
encapsulant surrounding the magnetic particles, the polymeric
encapsulant comprising a phase-separated block copolymer including
a glassy first phase and a rubbery second phase on each of the
magnetic particles, the first phase positioned between the second
phase and the magnetic particle; the glassy first phase comprising
a hydrophobic copolymer having a Tg of at least 50.degree. C.; and
the rubbery second phase comprising a polymer having at least one
of (i) a glass transition temperature of no greater than 30.degree.
C., and (ii) a tan delta peak maximum of no greater than 30.degree.
C.
13. The magnetic tape of claim 12, wherein the plurality of
particles have a size distribution with a polydispersity of less
than 2.0.
14. The magnetic tape of claim 12, wherein a plurality of the
magnetic particles are coupled together, wherein for a plurality of
the magnetic particles, the second phase on adjacent particles is
in direct contact.
15. The magnetic tape of claim 12, further comprising a matrix
material positioned between adjacent of the encapsulated
particles.
16. The plurality of particles of claim 12 wherein the plurality of
particles have a polydispersity of less than 1.2.
17. A method for manufacturing encapsulated particles, comprising:
forming a first polymer on a magnetic particle; and performing
ligand exchange to attach a second polymer to the first polymer,
wherein the first polymer is positioned between the second polymer
and the magnetic particle.
18. The method of claim 15, wherein the first polymer comprises a
glassy phase comprising a hydrophobic copolymer having a glass
transition temperature of at least 50.degree. C.
19. The method of claim 16, wherein the second polymer comprises a
rubbery second phase comprising a polymer having at least one of
(i) a glass transition temperature of no greater than 30.degree.
C., and (ii) a tan delta peak maximum of no greater than 30.degree.
C.
20. The method of claim 15, wherein the ligand exchange comprises
reacting polystyrene from the first polymer with a carboxylic acid
polymer to form the second polymer.
Description
[0001] This patent application is a continuation of U.S. patent
application Ser. No. 12/174,605, filed on Jul. 16, 2008, which
patent application is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to encapsulated particles and
methods for manufacturing such particles and structures including
such particles.
DESCRIPTION OF RELATED ART
[0003] Sub-micron sized magnetic metal particles suitable for
forming a recording layer on a flexible magnetic medium or tape are
commonly manufactured in sintered clusters of oxide coated metal
particles. The oxide layer imparts chemical stability to the metal
particles. The oxide layer also stabilizes the metal core, but
reduces the volume fraction of the magnetic particle available to
contribute to the magnetic domain which is needed to store
information. The oxide layer is formed in a sintering process that
also cements individual magnetic particles into clusters held
together by the hard oxide material. This makes dispersion of the
particles into the individual magnetic particles very difficult if
not impossible. As a result, processing of these particles into
suspensions suitable for creation of a useful recording layer on a
flexible substrate is becoming increasingly difficult to achieve as
the particles become smaller.
SUMMARY
[0004] One embodiment includes particles having a length no greater
than 40 nm. The particles include at least one material selected
from the group consisting of ferromagnetic materials and
ferrimagnetic materials. A polymeric encapsulant surrounds the
particle, the polymeric encapsulant including a phase-separated
block copolymer including a glassy first phase and a rubbery second
phase, the glassy first phase positioned between the particle and
the second rubbery phase. The glassy first phase includes a
hydrophobic copolymer having a glass transition temperature of at
least 50.degree. C. The rubbery second phase includes a polymer
having at least one of (i) a glass transition temperature of no
greater than 30.degree. C., and (ii) a tan delta peak maximum of no
greater than 30.degree. C.
[0005] Another embodiment includes a magnetic tape comprising a
plurality of encapsulated particles. The magnetic tape includes a
plurality of magnetic particles, each having a length no greater
than 40 nm. The magnetic particles each include at least one
material selected from the group consisting of ferromagnetic
materials and ferrimagnetic materials. There is a polymeric
encapsulant surrounding the magnetic particles, the polymeric
encapsulant including a phase-separated block copolymer including a
glassy first phase and a rubbery second phase on each of the
magnetic particles, the first phase positioned between the second
phase and the magnetic particle. The glassy first phase includes a
hydrophobic copolymer having a Tg of at least 50.degree. C. The
rubbery second phase includes a polymer having at least one of (i)
a glass transition temperature of no greater than 30.degree. C.,
and (ii) a tan delta peak maximum (as determined from dynamic
mechanical analysis) of no greater than 30.degree. C.
[0006] Another embodiment includes a method for manufacturing
encapsulated particles. The method includes forming a first polymer
on a magnetic particle, and performing ligand exchange to attach a
second polymer to the first polymer, wherein the first polymer is
positioned between the second polymer and the magnetic
particle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Embodiments of the invention are described with reference to
the accompanying drawings which, for illustrative purposes, are
schematic and not necessarily drawn to scale.
[0008] FIG. 1 illustrates a block copolymer core-shell particle
including a particle core surrounded to an inner shell region and
an outer shell region, in accordance with certain embodiments.
[0009] FIG. 2 illustrates a comparison of dispersion properties of
encapsulated particles in a variety of solvents.
[0010] FIG. 3 illustrates a tape in accordance with certain
embodiments.
DETAILED DESCRIPTION
[0011] While the invention is described in terms of the best mode
for achieving this invention's objectives, it will be appreciated
by those skilled in the art that variations may be accomplished in
view of these teachings without deviating from the spirit or scope
of the invention.
[0012] Certain embodiments relate to polymer encapsulated
particles, with the encapsulant coating on the particles including
a hard inner phase or shell region and a rubbery outer phase or
shell region created from absorbed segments ordered on the particle
surface. The hard inner shell region of the encapsulant coating on
the particles is preferably formed to be hydrophobic and oxidation
resistant, and includes unreacted side chains that form the
secondary, more rubbery outer shell region that occupies the
interface between the hard inner shell and a suspending
solvent.
[0013] In as aspect of certain embodiments, free radical
polymerizable monomers that yield glassy, hydrophobic polymer
layers are used to construct the encapsulating layer. These
polymers may have ionic end groups to promote absorption to the
nanoparticles on one end or within the chain as well as a few
functionalized co-monomers which may contain, for example,
carboxyl, glycidyl, isocyanate, anhydride, amide, hydroxyl amine or
nitrile functional groups. The functional group may be selected to
provide preferential absorption on the nanoparticle surface. The
polymerizable backbone of the monomer is preferably an aromatic or
aliphatic hydrocarbon such that the resulting shell around the
nanoparticle includes no polar functional groups or repeating
units.
[0014] FIG. 1 illustrates an encapsulated particle including a
particle 12 surrounded by an inner region 14 and an outer region
16, in accordance with certain embodiments. FIG. 1 is intended to
illustrate the different phases (glassy and rubbery) present on the
particle. The morphologies of the polymer inner region 14 and
polymer outer region 16 may differ from that illustrated in the
figures. The particle 12 may in certain embodiments have a diameter
or a length dimension of about 40 nm or less. The particle 12 may
be magnetic. By magnetic it is meant that the particle includes at
least one material selected from the group including ferromagnetic
materials and ferrimagnetic materials. Examples include iron (Fe),
cobalt (Co) and alloys including Fe and/or Co, including, but not
limited to iron-cobalt (Fe-Co) iron-barium (Fe-Ba), iron-nickel
(Fe-Ni), barrium-ferrite (Ba-Fe), and cobalt-platinum (Co-Pt)
alloys. Other examples include, but are not limited to,
manganese-aluminum (Mn-Al) alloys, metal oxides (for ex., iron
oxide, cobalt oxide, nickel oxide), and spinel ferrites.
[0015] The inner region 14 may include primarily a glassy first
phase comprising a hydrophobic copolymer having a glass transition
temperature (Tg) of greater than or equal to approximately
50.degree. C. The outer region 16 may include primarily a rubbery
second phase comprising a polymer having a glass transition
temperature of less than or equal to approximately 30.degree. C.
and/or having a tan delta peak maximum of less than or equal to
approximately 30.degree. C.
[0016] The rubbery second phase of the outer region 16 may be
formed to include a plurality of end groups comprising reactive
functional groups. The reactive functional groups may include at
least one of photoreactive functional groups or chemically reactive
functional groups. Examples of photoreactively cured end groups
include, but are not limited to azo functional groups, vinyl
functional groups, allyl functional groups, and acryl functional
groups. Examples of chemical reactively cured end groups include,
but are not limited to, epoxy functional groups, isocyanate
functional groups, anhydride functional groups, carboxylic acid
functional groups, and alcohol functional groups.
[0017] In certain embodiments, the inner region 14 and the outer
region 16 may have a combined thickness of no greater than 20 nm in
the dry solid state. For embodiments related to magnetic tape
applications, a combined thickness of no greater than 2 nm is
preferred. Other embodiments may include a combined thickness of up
to about 20 nm. The particle 12 may also include an oxide layer in
certain embodiments. For particles including an oxide layer, the
inner encapsulating region may be formed in direct contact with the
oxide layer. In addition, in certain embodiments the particle may
be acicular in morphology, with an aspect ratio of at least 2.
Other embodiments may include particles that are more equiaxed in
morphology. Generally, superior properties are obtained with
acicular shaped particles.
[0018] An example of a process in accordance with certain
embodiments is set forth below. In this example, polystyrene coated
cobalt nanoparticles are formed in a first operation. A second
operation includes an exchange of the polystyrene shell of the
coated cobalt nanoparticles with a carboxylic acid copolymer, to
form encapsulated nanoparticles having a glassy inner shell and a
rubbery outer shell thereon.
[0019] The first operation of preparing polystyrene coated cobalt
nanoparticles may be carried out as follows. To a three neck round
bottom flask (250 mL, 14/20) with stir bar and condenser was added
end-functional amine polystyrene (0.320 g, 0.064 mmol) and
end-functional phosphine oxide polystyrene (0.080 g, 0.016 mmol) in
dichlorobenzene (40 mL). The flask was flushed with argon for 10
minutes followed by heating to 180.degree. C. using a thermocouple
controlled heating mantle. A solution of dicobalt octacarbonyl
(0.400 g, 1.17 mmol) in dichlorobenzne (8 mL) was injected into the
hot solution over a period of 30 seconds. The evolution of a gas
(CO.sub.2) was observed, indicating that a reaction was proceeding.
The reaction was heated at 160.degree. C. for 30 minutes and was
cooled to room temperature with continuous stirring under
argon.
[0020] The collected reaction mixture was then precipitated into
stirring hexanes (500 mL). The precipitate was collected by
sedimentation using a standard AlNiCo magnet followed by decanting
of the hexanes phase. The resulting precipitate was then dried in
vacuo to give a black powder (yield: 0.250 g) that was soluble in a
wide range of non-polar solvents (e.g. toluene, THF,
CH.sub.2Cl.sub.2) and was responsive to an external magnetic field.
The black powder includes magnetic cobalt nanoparticles having a
polymer coating including a polystryrene shell.
[0021] The second operation of exchange of the original polystyrene
shell of the polymer coated cobalt nanoparticles with a carboxylic
acid polymer may be carried out as follows. The carboxylic polymer
used in this example was poly(methyl methacrylate). To a round
bottom flask with stir bar was added the polystyrene coated cobalt
nanoparticles (0.0620 g), and poly(methyl methacrylate) (0.1275 g),
dissolved in toluene (15 mL). The solution was heated at 50.degree.
C. under argon for 48 hours. The reaction solution was concentrated
in a vacuum to a volume of 2 mL followed by centrifugation (5000
rpm, 60 minutes), to yield a black pellet (0.0702 g) after
decanting of supernatant and air drying. To a scint vial with stir
bar, the recovered pellet (0.0292 g) was dissolved in
dichloromethane (5 mL), followed by the gradual addition of Aliquot
336 (0.050 g) and HCl (0.75 mL). The solution was stirred for
approximately 1 hour at which point the reaction product becomes
biphasic with a light blue organic layer. The organic layer was
removed and passed through a plug of neutral alumina to remove the
cobalt complex followed by concentration and preparation for SEC
and NMR characterization. The final cobalt complex produce may be
stored in a liquid medium or stored in a dry condition in a
suitable container.
[0022] It should be appreciated that methods for ligand exchange
may incorporate the use of THF as the solvent for the polymer and
polymer coated colloids with sonication at power level 5 for 180
minutes. The solution is then concentrated and centrifuged to yield
a pellet that is dissolved by using a 2:1 mixture of glacial acetic
acid and dichloromethane.
[0023] The cobalt complex obtained in the above process includes a
polymeric encapsulated magnetic nanoparticle particle having a hard
inner shell and a more rubbery outer shell. The inner shell formed
a glass-like region around the cobalt nanoparticle core. The outer
rubbery shell may bind to binder materials for reinforcement and
for mechanical energy dissipation. It will be appreciated by one of
ordinary skill that a variety of particles other than cobalt may be
used as the core particle material, and that a variety of polymers
may be used in the exchange process for forming the hard inner
shell and the rubbery outer shell regions on the particles.
[0024] Some examples of specific block polymers that may be used in
the exchange operation include, but are not limited to, (i)
poly(methyl methacrylate)-block-poly(oligoethylene glycol)
methacrylate; (ii) polystyrene-block-poly(butyl acrylate); and
(iii) polymethyl methacrylate-block-poly(ethylene oxide). The
chemical structures for these block polymers is set forth in Table
1 below. For poly(methyl methacrylate)-block-poly(oligoethylene
glycol) methacrylate, the poly(methyl methacrylate) will form the
hard inner shell on the particles, and the ethylene glycol will
form the rubbery outer shell. For polystyrene-block-poly(butyl
acrylate), the polystyrene will form the hard inner shell on the
particles, and the butyl acrylate will form the rubbery outer
shell. For poly(methyl methacrylate)-block-poly(ethylene oxide),
the poly(methyl methacrylate) will form the hard inner shell on the
particles, and the ethylene oxide will form the rubbery outer
shell.
TABLE-US-00001 TABLE 1 Chemical name and structure for certain
exchange polymers. Name Chemical Structure Poly(methyl
methacrylate)- block-poly (oligoethylene glycol) methacrylate
##STR00001## Polystyrene- block- poly(butyl acrylate) ##STR00002##
Poly(methyl methacrylate)- block-poly (ethylene oxide)
##STR00003##
[0025] Encapsulated nanoparticles such as those described above may
be used to form devices such as magnetic tape. The nanoparticles
can be tailored to have good properties for forming tapes. To form
a tape the encapsulated particles are dispersed in a solvent and
mixed with various other materials such as lubricants, curing
materials, and the like. FIG. 2 illustrates an evaluation of the
solubility (dispersability) of certain encapsulated nanoparticles
in polar and non-polar solvents. FIG. 2 includes cobalt
nanoparticles including either (i) a glassy polymer layer alone
(one phase encapsulated particles), or (ii) a glassy polymer layer
and a rubbery phase polymer layer formed on the glassy polymer
layer (two phase encapsulated particles). The nanoparticles are
placed into vials including either a polar solvent or a non-polar
solvent, in order to investigate the solubility of the
nanoparticles in various solvents.
[0026] Vials 100 and 102 contain a non-polar solvent, hexane. Vials
104 and 106 contain a polar solvent, methanol. Vials 102 and 104
include the encapsulated cobalt nanoparticles having a glassy
polymer layer formed from poly(methyl methacrylate) (PMMA).
[0027] The encapsulated particles 108 appear to be mostly settled
at the bottom of vials 102 and 104, which contain the non-polar
solvent hexane and the polar solvent methanol, respectively. This
indicates that the PMMA coated particles do not disperse well in
either the non-polar solvent hexane 118 or in the polar solvent
methanol 120. A different result is obtained for encapsulated
particles that are modified by the addition of a rubbery layer with
appropriate polar or non-polar solvent affinity. Vials 100 and 106
show stable dispersions 122 and 124 of particles in the solvent
materials. Specifically, dispersion 122 includes the cobalt
nanoparticles having a PMMA glassy layer and a poly(oligoethylene
glycol methacrylate) layer grafted thereon, in a non-polar hexane
solvent. Dispersion 124 includes the cobalt nanoparticles having a
PMMA glassy layer and a poly(oligoethylene glycol methacrylate)
layer grafted thereon, in a polar methanol solvent. The presence of
the dark particles throughout the solvent in the dispersions 122
and 124 indicates that the encapsulated nanoparticles can be
successfully dispersed is a variety of solvents when certain outer
rubbery phase materials are formed on the glassy phase.
[0028] A tape containing particles encapsulated with a two phase
structure such as described above, with a hard inner region and a
rubbery outer region surrounding the particle, may be formed using
a variety of techniques, including, but not limited to, coating
from polar or non-polar solvents and subsequent cure by use of
either chemical condensation polymerization or free radical
polymerization. The encapsulated particles such as those described
above are formed to include the appropriate reactive groups so that
polymerization can be carried out.
[0029] Reactive groups for condensation polymerization include, but
are not limited to, amines, alcohols, acids, epoxies, nitriles,
mercaptans, thiols, acid chlorides, isocyanates, halides, Mannich
bases, aziridines and moieties using metal complexes (Lewis
acids).
[0030] Suitable mixtures of isocyanates (--NCO group) may be used
to cure tape formulations. These are reacted with end groups on the
binders to attempt to improve the coating properties. Isocyanates
are reacted with amines and primary alcohols to form amide bonds
(--CO--NH--) analogous to the ester bond formed from carboxylic
acids and alcohols.
[0031] Reactive groups for free radical polymerization include, but
are not limited to, vinyl, styryl, acryl, allyl and other
functional groups sensitive to forming very reactive free radicals
upon excitation, by thermal decomposition or irradiation by e-beam
or UV light.
[0032] Suitable solvent materials, binders, cross-linking
additives, etc., are used to ensure proper viscosity and rheology
(for example, thixotropic) during the formation process. In certain
embodiments, the polymer encapsulated particles are deposited in
solution onto a moving carrier and cured and set through a suitable
evaporation and heating process, to form the tape.
[0033] The structure of the tape may in certain embodiments
resemble a very fine, tight array of substantially parallel strings
along the tape length direction. The embodiment illustrated in FIG.
3 shows a blown up view of region 202 on tape 200, including
several rows of particles 212 having encapsulating layers 214 and
216, with the layer 214 being a glassy phase and the layer 216
being a rubbery phase, similar to those described above. In this
embodiment, the particles 212 are somewhat acicular in morphology.
End groups on portions of the rubbery phase 216 extending outward
from the particle 212 and layer 214 have reacted with other end
groups from other rubbery phase 216 regions from other particles
during the polymerization process. Regions between the particles
may include matrix material 204 (for example, a polymer matrix).
Structures having different particle layouts than that shown in
FIG. 3 may also be formed.
[0034] It will, of course, be understood that modifications of the
present invention, in its various aspects, will be apparent to
those skilled in the art. Additional embodiments are possible,
their specific features depending upon the particular application.
For example, a variety of materials and processes may be used in
various embodiments. For instance, certain embodiments utilize
magnetic nanoparticles, but other embodiments may relate to
encapsulation of non-magnetic particles, both nanosized and
larger.
* * * * *