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Poly(dimethylsiloxane-b-methylvinylsiloxane) (PDMS-b-PMVS) diblock copolymers were synthesized via anionic living polymerization

entity toward the direction of highest field [1,2]. When the typically involve thermolysis of organo-cobalt precursors

Polymer 45 (2004)0032-3861/$ - see front matter q 2004 Published by Elsevier Ltd.with controlled molecular weights and narrow molecular weight distributions. The PMVS blocks were functionalized with

trimethoxysilethyl or triethoxysilethyl pendent groups to yield poly(dimethylsiloxane-b-(methylvinyl-co-methyl(2-trimethoxysilethyl)si-

loxane) (PDMS-b-(PMVS-co-PMTMS)) or poly(dimethylsiloxane-b-(methylvinyl-co-methyl(2-triethoxysilethyl)siloxane) (PDMS-b-

(PMVS-co-PMTES)) copolymers, respectively.

Stable suspensions of mostly superparamagnetic cobalt nanoparticles were prepared in toluene in the presence of PDMS-b-(PMVS-co-

PMTMS) and PDMS-b-(PMVS-co-PMTES) copolymers via thermolysis of Co2(CO)8. TEM micrographs showed non-aggregated cobalt

nanoparticles with mean particle diameters ranging from z1015 nm. Specific saturation magnetizations of the cobaltcopolymercomplexes ranged from w40110 emu gK1 of cobalt.q 2004 Published by Elsevier Ltd.

Keywords: Polysiloxane; Polymethylvinylsiloxane; Cobalt

1. Introduction

Ferrofluids are liquid dispersions of ferromagnetic or

ferrimagnetic particles, usually in hydrocarbons, esters or

water. Polymers or low molecular weight surfactants

adsorbed or bonded on the particle surfaces can prevent

agglomeration by either electrostatic or steric repulsion. The

particles are often ferrimagnetic iron oxides such as

magnetite or maghemite since methods for their synthesis

in small sizes (e.g.z10 nm in diameter) are established andthese materials are relatively stable against oxidation. If

particle diameters are in this range, the materials can be

superparamagnetic. When such fluids are placed in gradient

magnetic fields, they typically respond by moving as an

particles randomize rapidly and the net magnetic moment of

the fluid returns to zero. Our interest has been to design

biocompatible, polydimethylsiloxane (PDMS)-based ferro-

fluids for possible use in therapies for treating retinal

detachment disorders [36].

Cobalt nanoparticles have inherently higher magnetiza-

tions relative to magnetite or maghemite. The specific

saturation magnetization of bulk cobalt is 162 emu gK1

while magnetite is about 92 emu gK1 [7,8]. Cobalt nano-

particles can be prepared via chemical reduction of CoCl2 in

organic media [9] or reduction with hydrides in inverse

micellar media [1012].

Other methods utilize block copolymers that stabilize

cobalt nanoparticles in micelle cores. These systemsBlock copolysiloxanes and their co

M.L. Vadalaa, M. Rutnakornpituka, M

aDepartment of Chemistry and the Macromolecules and Interfaces Ins

Mail Code 0212, BlackbSchool of Physics, The University of We

Received 1 December 2003; received in revis

Abstract

Block copolysiloxanes have been prepared and utilized to form c

nanoparticles could be dispersed in polydimethylsiloxane (PDMS)lexation with cobalt nanoparticles

Zalicha,b, T.G. St Pierreb, J.S. Rifflea,*

Virginia Polytechnic Institute and State University, 2018 Hahn Hall,

, VA 24061-5976, USA

Australia, Crawley, WA 6009, Australia

m 11 August 2004; accepted 31 August 2004

exes of cobalt nanoparticles encased in the copolymers. The coated

ford PDMS ferrofluids.

74497461

www.elsevier.com/locate/polymerin the presence of dispersion stabilizers can provide

superparamagnetic cobalt nanoparticles free from contami-

nation by side products [1,1518].

doi:10.1016/j.polymer.2004.09.001

* Corresponding author. Tel.:C1 540 231 6717; fax:C1 540 231 8517.

E-mail address: anflynn@vt.edu (J.S. Riffle).applied fields are removed, the magnetic moments of the [5,6,13,14]. Thermolysis reactions of dicobalt octacarbonyl

ACERSticky Noterecubierta

ACERSticky Noterecubierto

ACERSticky Notepara proporcionar

ACERSticky Noteestrecho

Previous investigations of dicobalt octacarbonyl reac-

tions in our laboratories utilized poly(dimethylsiloxane-b-

(3-cyanopropyl)methylsiloxane-b-dimethylsiloxane)

(PDMS-b-PCPMS-b-PDMS) triblock copolymers in tolu-

ene, D4, or PDMS solvents to generate cobalt nanoparticles

in situ [5,6,14]. The central block containing the nitriles was

only sparingly soluble in the solvents employed, whereas

the PDMS endblocks were soluble. The reaction solutions

contained micelles with the PCPMS forming the micelle

cores and the PDMS endblocks protruding into the solvents.

It was reasoned that the lone pairs of electrons on the

nitrogens complexed with the cobalt, and the PDMS tails

reasoned that this might allow more latitude in designing

post-crosslinking conditions to yield oxygen impermeable

shells.

2. Experimental

2.1. Materials

Hexamethylcyclotrisiloxane (D3, Gelest, Inc.) was

purified by stirring it over calcium hydride at 80 8C

ers fo

y; PM

M.L. Vadala et al. / Polymer 45 (2004) 744974617450provided steric stabilization of these nanoparticle disper-

sions. These cobalt dispersions had specific saturation

magnetizations of 90110 emu gK1 of cobalt and TEM

micrographs showed well-dispersed cobalt nanoparticles

[6,14]. The oxidative durability of the cobalt nanoparticles

was poor, however, and their magnetization decreased

slowly with time under ambient conditions [14,19].

Cobalt carbonyl reactions in solutions of pentablock

copolymer steric dispersion stabilizers (Fig. 1) were also

investigated to form cobalt nanoparticles [19]. The

motivation for this approach was to protect the cobalt

surfaces against oxidation with silica-like shells formed

around the nanoparticles by hydrolyzing and condensing

trialkoxysilyl pendent groups [2]. These reactions resulted

in stable dispersions of z10 nm cobalt with specificmagnetizations of 90110 emu gK1 of cobalt. The pendent

triethoxysilyl groups were hydrolyzed in the presence of

dibutyltin diacetate (catalyst) with stoichiometric concen-

trations of water at room temperature.

This paper describes diblock polysiloxane dispersants

wherein trialkoxysilyl-functional blocks complexed with

cobalt and PDMS blocks provided dispersion stability in

toluene. Surprisingly, the trialkoxysilethyl-functional

blocks served as anchor segments for the cobalt. These

block copolymer dispersion stabilizers are of particular

interest because of their relative simplicity as compared to

the previously studied pentablock stabilizers. Additionally,

either trimethoxysilethyl (more reactive) or triethoxysi-

lethyl pendent groups could be employed to prepare cobalt

dispersions with the copolymers described herein, and it was

Fig. 1. Pentablock copolymers were investigated as steric dispersion stabiliz

anchor block to bind to cobalt; PDMS tail blocks provided dispersion stabilitaround the nanoparticles [2].r cobalt nanoparticles. The nitrile containing central block functioned as the

TES blocks were precursors for solgel reactions to form silica-like shellsovernight and was subsequently fractionally vacuum-

distillation under nitrogen into a pre-weighed, flame-

dried flask. The initiator, n-butyllithium, was kindly

donated by the Lithco Division of the FMC Corporation

and was approximately 2.45 M in cyclohexane. It was

titrated with diphenylacetic acid in THF until a yellow

color persisted [20] and used as received. 1,3,5-trivinyl-

1,3,5-trimethylcyclotrisiloxane (DV3 ; Gelest, Inc.) was

fractionally distilled under vacuum into a pre-dried flask,

purged with nitrogen, sealed with a septum, and stored in

a dessicator. Cyclohexane (Aldrich, 99%) was stirred with

concentrated sulfuric acid for 48 h, washed with water,

dried over MgSO4, then over sodium, and distilled just

prior to use. Tetrahydrofuran (THF, 99.5%, E.M.

Sciences) was dried over calcium hydride overnight,

then refluxed over sodium in the presence of benzophe-

none until the solution was a deep purple. The THF was

distilled just prior to use. Trimethylchlorosilane (Gelest,

Inc.) was used as the terminating reagent for the

diblock copolymers, and was distilled before use. A

Pt0(1,3-divinyl-1,1,3,3-tetramethyldisiloxane)1.5 complex

catalyst in xylene (2.12.4 wt% Pt0, Karstedts catalyst)

(Gelest, Inc.) was used as received. Triethoxysilane and

trimethoxysilane (Gelest, Inc.) were used as received.

Toluene was washed twice with concentrated sulfuric acid

and neutralized with water. It was dried over MgSO4 for

1 h, then over calcium hydride overnight and distilled just

before use. Co2(CO)8 (Alpha Aesar) stabilized with 15%

hexane was stored under argon in the freezer without

further purification.

stir bar. The vial containing the copolymer was sealed with a

of a PDMS-b-PMTMS block copolymer having aK1 K1

lymer2.2. Synthesis

2.2.1. Synthesis of poly(dimethylsiloxane-b-methylvinylsi-

loxane) diblock copolymers (PDMS-b-PMVS) with con-

trolled molecular weights

A synthetic procedure for preparing a diblock copolymer

with a targeted number average molecular weight (Mn) of

7000 g molK1 comprised of 5000 g molK1 PDMS and

2000 g molK1 PMVS is provided. A series of diblock

copolymers with different molecular weights were prepared

by varying the ratio of n-butyllithium to D3 to control the Mnof the PDMS blocks, and by varying the amounts of DV3relative to D3 to control the Mn of the PMVS blocks. The

first part of the synthesis involved preparing a PDMS block

with one terminal lithium siloxanolate. D3 (15.64 g,

0.003 mol) was distilled into a flame-dried, 250 ml, round-

bottom flask equipped with a magnetic stir bar, purged with

dry nitrogen and sealed with a septum. Cyclohexane (18 ml)

was added via syringe as a solvent to dissolve the D3. Once

dissolved, 1.26 ml of a 2.45 M solution of n-butyllithium

(0.003 mol) in cyclohexane was charged to the reaction

flask via syringe, and stirred for 15 min at 25 8C. THF(18 ml) was added as a promoter and the reaction was

allowed to proceed at room temperature until the complete

conversion of D3 monomer to polymer as determined by1H

NMR. This required approximately 8 h reaction time for this

composition. DV3 (6.2 ml, 0.003 mol) was charged to the

reaction flask and the copolymerization was allowed to

proceed at room temperature until conversion reached

approximately 95% (approx. 8 h) as measured by 1H NMR.

The diblock copolymer was terminated with an excess of

trimethylchlorosilane (0.76 ml, 0.006 mol) via syringe and

stirred for approximately 30 min. The solution clouded

upon addition of the trimethylchlorosilane due to precipi-

tation of insoluble LiCl salt. The polymer solution was

diluted with chloroform (300 ml) and placed in a separatory

funnel. It was washed with water several times to remove

LiCl. Approximately 80% of the chloroform was removed

by rotary evaporation and the polymer was twice pre-

cipitated and purified by pouring the remaining solution into

stirred methanol (200 ml). The clear liquid copolymer sank

to the bottom of the vessel. The methanol was decanted and

the polymer was dried under vacuum at approximately

500 mTorr at 60 8C for 24 h.

2.2.2. Synthesis of poly(dimethylsiloxane-b-methyl

(2-triethoxysilethyl)siloxane] (PDMS-b-PMTES) via

hydrosilation

Functionalized diblock copolymers were prepared from

the PDMS-b-PMVS diblock copolymers. The molar ratios

of triethoxysilane or trimethoxysilane relative to the number

of vinyl groups determined the degree of hydrosilation. A

representative procedure for quantitative hydrosilation is

described. Firstly, 0.3 g of the polymer precursor, a

5000 g molK1 PDMS and 2500 g molK1 PMVS diblock

M.L. Vadala et al. / Pocopolymer, was weighed into a flame-dried, 6-dram vial10,000 g mol PDMS block connected to a 4800 g mol

PMTMS block with four times the stoichiometric concen-

tration of water is described. A 3-neck, 250 ml, round-

bottom flask equipped with a mechanical stirrer, condenser,

and nitrogen purge was charged with 1.0 g (0.0016 equiv. of

trimethoxysilethyl groups, 0.0047 equiv. methoxy) of the

PDMS-b-PMTMS copolymer. Toluene (15 ml) was trans-

ferred via syringe to the reaction vessel to dissolve the

copolymer. Water (0.17 ml, 0.0096 mol, 12 equiv. per

trimethoxysilane, four times stoichiometric) was added via

syringe. The reaction was stirred at 95 8C. The reactionseptum under nitrogen. Toluene (10 ml) was added via

syringe and the mixture was stirred until the polymer

dissolved. Trimethoxysilane (0.08 ml, 0.0006 mol) was

added via syringe to the reaction vial followed by addition

of 10 ml of the Pt0 (Karstedts) catalyst in xylene. Theamount of catalyst was based on z10K3 mol of Pt0 permole vinyl. The reaction was stirred at 60 8C until completehydrosilation occurred as evidenced by 1H NMR. The

solvent was evaporated under vacuum and the PDMS-b-

[PMVS-co-PMTMS] diblock copolymer was stored under

nitrogen.

2.2.4. Hydrolysis and condensation of poly(dimethylsi-

loxane-b-methyl(2-triethoxysilethyl)siloxane) (PDMS-b-

PMTMS)

A series of reactions were conducted wherein the

concentration of water per equiv. of methoxysilyl groups

was varied from stoichiometric (1 mol water:2 mol meth-

oxy), two times stoichiometric (2 mol water:2 mol meth-

oxy), and four times stoichiometric (4 mol water:2 mol

methoxy). A solgel hydrolysis and condensation reactionequipped with a magnetic stir bar. The vial containing the

copolymer was sealed with a septum under nitrogen.

Toluene (10 ml) was added via syringe and the mixture

was stirred until the polymer dissolved. Triethoxysilane

(0.21 ml, 0.0012 mol) was added via syringe to the reaction

vial followed by addition of 10 ml of Pt0 (Karstedts)catalyst in xylene. The reaction was stirred at 60 8C untilcomplete hydrosilation occurred as evidenced by 1H NMR.

The solvent was evaporated under vacuum and the PDMS-

b-PMTES diblock copolymer was stored under nitrogen.

2.2.3. Synthesis of poly(dimethylsiloxane-b-(methylvinylsi-

loxane-co-methyl(2-trimethoxysilethyl)siloxane)) (PDMS-

b-(PMVS-co-PMTMS)) via hydrosilation

An exemplary synthesis to prepare a diblock copolymer

wherein only half the pendent vinyl groups on the polymer

precursor were functionalized is provided. Firstly, 0.3 g of

the polymer precursor, a 5000 g molK1 PDMS and

2500 g molK1 PMVS diblock copolymer, was weighed

into a flame-dried, 6-dram vial equipped with a magnetic

45 (2004) 74497461 7451progress was monitored via 1H NMR.

o obta

lymer2.2.5. Synthesis of a cobalt nanoparticle fluid in the

presence of a PDMS-b-(PMVS-co-PMTES) diblock

copolymer

A representative reaction is described utilizing a

5300 g molK1 PDMS-b-3900 g molK1 (PMVS-co-PMTES)

copolymer. The PMVS-co-PMTES block had 50% of the

repeat units hydrosilated with triethoxysilane. Dispersions

with other copolymers were prepared in an analogous

manner. A 500 ml, 3-neck, roundbottom flask equipped

with a condenser, mechanical stirrer with a vacuum ready

Fig. 2. Anionic block copolymerization tM.L. Vadala et al. / Po7452adapter, and argon purge was flame-dried under argon. The

apparatus was placed in a temperature controlled silicone oil

bath over a hot plate (without amagnetic stirrer). The PDMS-

b-(PMVS-co-PMTES) copolymer (1.0 g, 0.0013 equiv.

triethoxysilethyl groups) in 10 ml toluene was transferred

to the reaction vessel via cannula. An additional 10 ml of

toluene was added via syringe to the reaction flask. Dicobalt

octacarbonyl (1.0 g, z0.0035 mol) was weighed into thereaction vessel and dissolved under argon. A greenish-brown

gas filled the flask immediately upon adding the dicobalt

octacarbonyl. The reaction temperature was raised to

approximately 110 8C (toluene reflux) and maintained atthat temperature for approximately 2 h or until the complete

disappearance of the Co4(CO)12 intermediate as determined

by FT-IR. After cooling, a stable magnetic dispersion of

cobalt nanoparticles resulted.

2.3. Characterization of copolymers and copolymer

solutions

All 1H and 29Si NMR spectra were obtained on a Varian

Unity 400 MHz NMR spectrometer operating at 400 and

80 MHz, respectively. The samples for 29Si NMR wereprepared with 0.63 g copolymer, 0.52 g Cr(Acac)3, and

2.4 ml d-CHCl3. Quantitative silicon NMR spectra were

obtained with the aid of the relaxation agent, Cr(Acac)3,

with a pulse width of 168.08 and a relaxation delay of 10 s.Gel permeation chromatograms were obtained in chloro-

form at 30 8C on a Waters Alliance Model 2690 chromato-graph equipped with a Waters HR 0.5CHR 2CHR 3CHR4 styragel column set. A Viscotek viscosity detector and a

refractive index detector were utilized with polystyrene

calibration standards to generate a universal molecular

in diblock PDMS-b-PMVS copolymers.45 (2004) 74497461weight calibration curve for absolute molecular weight

analyses. Thermal properties of the copolymer precursors

(PDMS-b-PMVS) were analyzed by differential scanning

calorimetry using a TA Instruments DSC Q1000 under

constant flow of helium. The samples (1015 mg) were

cycled from K15025 8C using hermetically sealed DSCpans. Two scans were performed on each sample where the

Tgs were taken as the inflection points on the second scans.

2.4. Characterization of magnetic fluids

FTIR spectra were obtained using a Nicolet Impact 400

FTIR spectrometer with two drops of the samples run neat

between salt plates. Transmission electron micrographs

were acquired using a Philips 420T TEM run at 100 kV.

TEM samples containing cobalt stabilized with copolymers

were prepared by diluting toluene dispersions with

additional toluene until they had the appearance of weak

tea. A drop was syringed onto a carbon coated copper grid

and the toluene was evaporated. A Standard 7300 Series

Lakeshore vibrating sample magnetometer (VSM) was used

to determine the magnetic properties of the cobalt samples

including saturation magnetization and any hysteresis.

cobalt. Cobalt was calculated from the sample responses

relative to standards and blanks.

olyme

M.L. Vadala et al. / Polymer 45 (2004) 74497461 7453The magnetic moment of each dried sample was measured

over a range of applied fields fromK8000C8000 Oe witha sensitivity of 0.1 emu. A Quantum Design magnetic

properties measurement system using a superconducting

Fig. 3. Gel permeation chromatograms of the PDMS-b-PMVS diblock cop

nature of these polymerizations.quantum interference device sensor was used to make

measurements of cobalt specific magnetization (s) in

applied magnetic fields (H) over the range of K70,000C70,000 Oe at room temperature and 5 K. Low temperature

measurements were made both after cooling the sample in

zero applied field and in an applied field at 70,000 Oe. These

measurements were used to investigate the presence of any:

(1) residual cobalt carbonyl species, and (2) cobalt oxide on

the surfaces of metallic cobalt particles. Elemental analyses

for cobalt were performed by Desert Analytics Laboratory

(Tucson, AZ) by treating the samples with hot concentrated

nitric acid followed by concentrated perchloric acid until

complete dissolution was achieved. These solutions were

analyzed by inductively coupled plasma to determine

Table 1

Mns and molecular weight distributions of PDMS-b-PMVS diblock copolymers

Targeted Mn (g molK1) Mn by

1H NMR (g molK1) Mn by29Si NMR

7000 6000 6000

7000 7000 6300

8000 8900 9000

12,000 12,700 12,500

12,000 13,000 12,500

16,000 15,000 16,000

18,000 18,400 18,500

18,000 19,500 20,000

18,000 16,500 17,000rs confirmed narrow molecular weight distributions, suggesting the living3. Results and discussion

3.1. Copolymer synthesis and characterization

The PDMS-b-PMVS copolymers were prepared at room

temperature in cyclohexane utilizing THF as a promoter

(Fig. 2). It is well-known that D3 polymerizations can be

living under the conditions utilized in this work [21,22].

These diblock copolymers had narrow molecular weight

distributions suggesting that the polymerizations of the

PMVS blocks were also living. Recent work by Kickelbick

supports this premise by demonstrating a linear depen-

dence of molecular weight with monomer conversion

for DV3 [23].

(g molK1) Mn by GPC (g molK1) Molecular weight distribution

7800 1.03

9000 1.05

8700 1.01

13,000 1.02

14,700 1.01

13,700 1.06

17,500 1.01

22,000 1.09

17,200 1.03

polymerization of the vinyl groups initiated with

n-butyllithium. By contrast, when D3 then DV3 were poly-

merized sequentially, narrow molecular weight distributions

suggested well-defined polymers. GPC chromatograms for

these diblock copolymers indicated molecular weight

distributions close to one (Fig. 3 and Table 1). It should

be noted that Weber [29] and Kickelbick [23] have reported

well-defined homopolymerizations of DV3 by initiating the

reactions with a weakly basic lithium silanolate, thus

avoiding any reaction of an alkyllithium with the vinyl

moieties [23].

The copolymerizations were conducted under rigorously

anhydrous conditions to prevent any reaction of the strongly

M.L. Vadala et al. / Polymer 45 (2004) 744974617454Ring-opening reactions of cyclosiloxane trimers are

promoted by enthalpic and entropic effects. The enthalpic

contribution to ring-opening D3 has been attributed to relief

of ring strain [24,25], and it is reasonable to hypothesize that

1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane DV3 is alsostrained. The entropy change for D3 polymerization is also

favorable (positive), resulting in an overall KDG.The diblock copolymers were prepared by sequential

anionic ring-opening polymerizations of D3 and DV3 ;

respectively. The synthesis of the PDMS block first was

important for controlling the chemical structure of these

polymers. For example, analogous polymerizations wherein

DV3 was polymerized first produced polymers with multi-

Fig. 4. The living anionic polymerization of D3 and the formation of PDMS

blocks were monitored by 1H NMR.modal molecular weight distributions. This was attributed to

a side reaction of the alkyllithium initiator reacting with

vinylsilanes, and such reactions have been documented

previously [2628]. Cason et al. demonstrated that alkyl-

lithiums (n-butyllithium and phenyllithium) add to vinyl-

triphenyl and vinyltrimethylsilanes. Stober et al. have

reported polymers and copolymers via anionic

Fig. 5. 1H NMR spectra of the PDMS-b-PMVS diblock copolymers afforbasic initiator with water (which would form LiOH and

n-butane). D3 ring-opening was monitored with1H NMR by

observing the decrease of the methyl peak of the cyclic

monomer at 0.14 ppm and the concurrent increase of the

resonance corresponding to the methyl protons of the linear

species at 0.06 ppm (Fig. 4). The D3 was reacted

quantitatively before the second monomer was added to

ensure that the second block would be pure PMVS.

The molar ratios of linear PDMS to grams of DV3controlled the PMVS block molecular weights. This

reaction was also monitored via 1H NMR (methyl

resonances of the cyclic monomer at 0.27 ppm and linear

species at 0.17 ppm). The diblock copolymers were

terminated with an excess of trimethylchlorosilane at

approximately 90% conversion of the DV3 . This somewhat

early termination was done to avoid any backbiting or

intermolecular chainchain substitutions which might occur

at extremely low monomer concentrations.

The relative compositions of each block in the diblock

copolymers after isolation were confirmed by 1H and 29Si

NMR. In the 1H NMR spectra, the integrals from the PDMS

methyl resonances at 0.06 ppm were compared to the

methyl resonances of the polymethylvinylsiloxane at

0.08 ppm (Fig. 5). The number average molecular weights

were determined by ratioing the butyl endgroup integrals to

the methyl resonance integrals.29Si NMR provides a valuable tool for characterizing

polysiloxanes because of its wide frequency range. Theded a means of confirming the compositions of these copolymers.

PDMS silicons were well-separated from the PMVS silicons

in the 29Si NMR spectra. The diblock copolymers had one

silicon next to a butyl endgroup (8.59 ppm) and one silicon

1

triethoxysilane or trimethoxysilane to yield PDMS-b-

PMTES or PDMS-b-PMTMS, respectively (Fig. 7A).

Alternatively, the PDMS-b-PMVS precursor copolymers

Fig. 6. Quantitative 29Si NMR of the PDMS-b-PMVS diblock copolymers confirmed the expected 1:1 ratios of the terminal silicon atoms.

M.L. Vadala et al. / Polymer 45 (2004) 74497461 7455those determined via H NMR (Table 1).

3.2. Copolymer functionalization and characterization

The polymethylvinylsiloxane blocks in the PDMS-b-

PMVS copolymers were quantitatively hydrosilated within the trimethylsilyl endgroup (7.86 ppm). Block copolymer

compositions and molecular weights were obtained by

ratioing the repeat unit integrals corresponding to each

block to the endgroups (Fig. 6). In all cases, the block Mns

derived from 29Si NMR paralleled the targeted Mns andFig. 7. (A) Quantitative functionalization of a PDMS-b-PMVS copolymer precu

precursors partially hydrosilated with triethoxy- or trimethoxysilanes.were only partially hydrosilated (Fig. 7B). In these cases,

the resulting diblock copolymers were comprised of a

PDMS block linked to a poly(methylvinylsiloxane-co-

methyltrialkoxysilethylsiloxane) wherein the sequences of

methylvinylsiloxane and methyltrialkoxysilethylsiloxane

units in the second blocks were random. These partially

hydrosilated diblock copolymers have been designated

PDMS-b-(PMVS-co-PMTES) (for the cases of hydrosila-

tion with triethoxysilane) and PDMS-b-(PMVS-co-

PMTMS) (for the cases of hydrosilation with

trimethoxysilane).rsor with triethoxy- or trimethoxysilanes; (B) PDMS-b-PMVS copolymer

Hydrosilation reactions have been extensively utilized to

prepare monomers with SiC bonds or to crosslink

polysiloxanes. Karstedts catalysts (Pt0 complexed with

divinyltetramethyldisiloxane) are available commercially as

solutions in either organic solvents such as xylene or

polysiloxane oligomers. Such complexed catalysts are

soluble in polysiloxane media and have high reactivity.

The PDMS-b-PMVS polymer precursors were rigorously

dried to avoid any premature reaction of the alkoxysilyl

groups. The hydrosilation reactions were monitored via 1H

NMR by observing the disappearance of the SiH peaks

corresponding to a addition across the double bonds

appeared as the reactions proceeded.

3.3. Thermal properties of PDMS-b-PMVS and PDMS-b-

(PMVS-co-PMTES)

Bulk thermal properties of each diblock copolymer were

measured via differential scanning calorimetry to probe

their phase structures. The PMVS block molecular weight

was held as constant as possible at approximately

2000 g molK1 and the PDMS block molecular weightsK1

Table 2

Glass transition temperatures of a series of PDMS-b-PMVS (prepolymers) and PDMS-b-PMTES (functionalized polymers)

Mn PDMS (g molK1) Mn PMVS (g mol

K1) Total Mna (g molK1) Tg (8C) (pre-polymer) Tg (8C) (functionalized)

4200 1800 6000 K129

5000 2000 7000 K129 K124

5000 2900 8900 K12910,500 2200 12,700 K128 K125

11,000 2000 13,000 K128

13,000 2000 15,000 K127

16,200 2200 18,400 K127 K12617,000 2500 19,500 K127

14,500 2000 16,500 K127

a The total Mns were derived from1H NMR.

M.L. Vadala et al. / Polymer 45 (2004) 744974617456(dZ4.5 ppm) and an upfield shift and broadening of thetriethoxysilyl methylene peaks. Normal (b) and reverse

addition (a) of the SiH across the double bond are known

to occur [30]. Both a (13%) and b (87%) addition products

were obtained. Two signals [d1.4 CH3 and d1.3 CH]Fig. 8. Thermolysis of dicobalt octacarbonyl: (a) initial reaction mixture with

1860 cmK1 attributed to bridging CO; (b) a spectrum representing the intermedi

Co4(CO)12; (c) a spectrum showing residual carbonyl peaks after approximately

normally terminated at this point); (d) a spectrum after 12 h at toluene reflux s

thermolysis.were varied from 5000 to 16,000 g mol . For each

copolymer, only one glass transition temperature (Tg) was

observed at approximately K26 8C (Table 2), suggestingthat the PDMS and PMVS blocks were miscible. This is in

agreement with previous studies of similar PDMS basedpeaks at 2020, 2050, and 2070 cmK1 corresponding to terminal CO and

ate reaction stage showing new peaks at 2065 and 2055 cmK1 assigned to

4 h and the absence of peaks at 2065 and 2055 cmK1 (the reactions were

howing the complete disappearance of all carbonyl peaks, indicating full

lymerM.L. Vadala et al. / Popolymers [29,30]. Semlyen et al. [29,30], Riffle et al. [31],

and Weber et al. [29] have also reported low glass transition

temperatures of PMVS homopolymers. The atactic nature of

the stereocenters in the PMVS blocks prevents any crystal-

lization of the PMVS homopolymers.

PDMS-b-PCPMS-b-PDMS triblock copolymers pre-

viously studied in our laboratories were microphase separ-

ated even at low blockmolecular weights (e.g. 5000 g molK1

PDMS-b-2000 g molK1 PCPMS-b-5000 g molK1 PDMS

[14]), and this was attributed to the nitrile containing central

block (PCPMS) being significantly more polar than the

PDMS tail blocks. By contrast, the thermal properties of the

new dispersion stabilizers suggested that they may not

be multi-phase materials. To compare homopolymer tran-

sitions, a 1000 g molK1 PMVS homopolymer (TgK137 8C)was quantitatively functionalized to form a PMTES

Fig. 9. Transmission electron micrographs of cobalt fluids comprised of (a) 16,000

PDMS-b-3400 g molK1 (PMVS-co-PMTMS); and (c) 16,000 g molK1 PDMS-b-345 (2004) 74497461 7457homopolymer (Tg K95 8C). The higher Tg in PMTEScompared to PMVS probably reflects less rotational mobility

in the former. In the diblock series, the PMVS block lengths

were kept constant at 2000 g molK1 (pre-functionalization)

or 5800 g molK1 (post-functionalization). Only one Tg was

observed for each of these diblock copolymers (Table 2).

3.4. Synthesis and characterization of PDMS-b-(PMVS-co-

PMTMS)/cobalt complexes

The investigations described herein on the formation of

stabilized cobalt nanoparticles utilized thermolyses of

dicobalt octacarbonyl in the presence of PDMS-b-(PMVS-

co-PMTES) or PDMS-b-(PMVS-co-PMTMS) in toluene.

The PMTES or PMTMS blocks complexed with cobalt

nanoparticles and the PDMS blocks protruded into the

g molK1 PDMS-b-3400 g molK1 (PMVS-co-PMTMS); (b) 5000 g molK1

900 g molK1 (PMVS-co-PMTES).

lymerM.L. Vadala et al. / Po7458toluene solvent. Dicobalt octacarbonyl and the copolymer

were dissolved in toluene and the reaction mixtures were

heated to toluene reflux to displace the carbon monoxide

and form cobalt nanoparticles. The reactions were mon-

itored by FTIR by observing changes in structure and the

decrease of carbonyl peaks (Fig. 8). The carbon monoxide

region around 2000 cmK1 initially showed three absorbance

bands at 2020, 2050, and 2070 cmK1 attributed to terminal

CO, and a peak at 1860 cmK1 due to the bridging carbonyls

(Fig. 8a) [14,18]. Two new peaks at 2065 and 2055 cmK1

appeared in the IR spectra as the reactions proceeded, and

these peaks were attributed to a Co4(CO)12 intermediate

based on previous assignments [18] (Fig. 8b). Two to four

hours of reaction at reflux were required for the Co4(CO)12intermediates to disappear. Residual peaks remained in the

carbonyl region of the IR spectra (2075, 2030, 2010 cmK1)

when these reactions were terminated which remain

unassigned (Fig. 8c), suggesting that some carbon monoxide

remains under these conditions. If these reactions were

continued at toluene reflux for long periods (e.g. 12 h) the

carbonyl peaks completely disappeared (Fig. 8d). Unfortu-

nately however, this also caused a considerable amount of

cobalt to plate-out on the glass apparatus.

Fig. 10. Specific magnetization curves for cobaltpolymer complexes prepared w

16,000 g molK1 PDMS-b-3900 g molK1 (PMVS-co-PMTES).45 (2004) 74497461Dispersions prepared with PDMS-b-(PMVS-co-PMTES)

or PDMS-b-(PMVS-co-PMTMS) copolymers (wherein half

the repeat units in the anchor blocks were functionalized)

were dark homogeneous fluids without any visible precipi-

tates. By contrast, however, cobalt dispersions with PDMS-

b-PMTES or PDMS-b-PMTMS copolymers (wherein the

anchor block was fully functionalized) exhibited small

amounts of precipitates. TEM micrographs of the cobalt

polymer complexes with PDMS-b-(PMVS-co-PMTES) or

PDMS-b-(PMVS-co-PMTMS) having 30004000 g molK1

anchor blocks showed electron dense,z1015 nm diametercobalt particles (Fig. 9), and there were no discernible

differences in particle sizes. The copolymer sheaths

surrounding each particle were visible but it was not

possible to determine their thickness due to contrast issues

associated with the technique. Further investigations of

these in situ syntheses will be conducted to ascertain

whether particle sizes can be correlated with anchor block

lengths and micelle sizes in the reaction solutions.

Compositions of selected cobaltcopolymer complexes

were measured by elemental analysis after isolation, and the

corresponding saturated magnetizations were derived from

vibrating sample magnetometry. This allowed for

ith (a) 5000 g molK1 PDMS-b-3900 g molK1 (PMVS-co-PMTES); and (b)

lymerM.L. Vadala et al. / Poestimating the specific saturation magnetizations of the

cobalt species as ranging from w40110 emu gK1 ofcobalt. The higher end of this range was consistent with

previously reported values for approximately 10 nm diam-

eter cobalt particles generated via thermolyses of dicobalt

octacarbonyl [6,14,18]. The lower values observed in this

work are likely attributable to the incomplete extent of

carbon monoxide evolution. All of these values, however,

Fig. 11. s vs. H measurements conducted at (a) 300 K and (b) 5 K (zero-field co

complexes prepared with 5000 g molK1 PDMS-b-3400 g molK1 (PMVS-co-PMT

loops.45 (2004) 74497461 7459are consistently below the expected bulk cobalt magnetiza-

tions of 162 emu gK1 and the reasons for this remain

unclear. Representative magnetization curves of these

materials (Fig. 10) suggest that these copolymercobalt

complexes display some magnetic hysteresis. Pileni et al.

has reported hysteresis for cobalt particles organized in a

two-dimensional lattice on a graphite substrate with mean

particle diameters of approximately 68 nm [32].

oled hysteresis loop (%), field-cooled hysteresis loop) on cobaltpolymer

MS). Inset in (b) is an enlarged region around the origin for 5 K hysteresis

investigations

Charles S, Wells S, Dailey JP. J Magn Magn Mater 2001;225:4758.

[7] Sato T, Iijima T, Sekin M, Inagaki N. J Magn Magn Mater 1987;65:

252.

[16] Yin J, Wang ZL. Nanostruct Mater 1999;11:84552.

[17] Smith T, Wychick D. J Phys Chem 1980;84:1621.

[18] Tannenbaum R. Inorganica Chimica Acta 1994;227:22340.

lymer 45 (2004) 74497461Well-defined PDMS-b-PMVS diblock copolymers were

prepared with controlled molecular weights and narrow

molecular weight distributions. The PMVS blocks were

either partially or fully functionalized with triethoxysilethyl

or trimethoxysilethyl pendent moieties using hydrosilations.

Thermal characterization suggested that the blocks in the

derivatized copolymers were also miscible.

Solutions of the partially hydrosilated PDMS-b-(PMVS-

co-PMTMS) or PDMS-b-(PMVS-co-PMTES) copolymers

were used to form sterically stabilized cobalt nanoparticle

dispersions without any observed aggregation. Cobalt yields

in the dispersed complexes were approximately 6070 wt%

of the amount charged. Considerable additional work will

be required to understand the parameters that control the

maximum cobalt to copolymer ratio in these complexes. It is

desirable to maximize the concentration of cobalt to obtain

polymercobalt particles with enhanced magnetic

responses. To date, formation of cobalt in solutions of the

fully functionalized analogues of these polymers has

resulted in some aggregation, but the reasons for this

behavior are also still unclear. Cobalt nanoparticles

prepared in this research were approximately 1015 nm in

diameter as determined by TEM.

Specific saturation magnetizations of the cobaltcopoly-

mer complexes ranged from w40110 emu gK1 of cobalt.Room temperature and 5 K magnetic susceptometry

measurements were used to investigate any presence of: (1)

a cobalt oxide layer on the surface of the cobalt

nanoparticles and (2) unreacted cobalt carbonyl species in

the cobaltpolymer complexes (Fig. 11). The cobalt specific

magnetization (s) shows a continuous increase with field at

high fields and low temperature. However, at room

temperature s almost saturates at high fields. These

observations are indicative of the presence of paramagnetic

species within the sample. This paramagnetic component is

believed to be cobalt carbonyl species that have not been

incorporated into the cobalt nano-crystals. This observation

is in agreement with the infrared spectra, which suggest

residual carbon monoxide ligands are present after for-

mation of the cobalt nanoparticle dispersions. It is

anticipated that these residual paramagnetic cobalt species

will be reacted in subsequent annealing steps for these

materials. In addition, the field-cooled 5 K hysteresis loop

does not show a significant shift from the zero field-cooled

hysteresis loop, indicating that there is no significant oxide

layer on the surface of the metallic cobalt nanoparticles. If

cobalt oxide were present in any significant amount, an

asymmetric field-cooled hysteresis loop shift, caused by the

coupling of an antiferromagnetic layer (CoO) with a

ferromagnetic layer (Co), would be expected [33].

4. Conclusions and recommendations for further

M.L. Vadala et al. / Po7460The deviation from the bulk cobalt value of 162 emu gK1 is[19] Rutnakornpituk M. Synthesis of silicone magnetic fluids for use in eye

surgery, PhD dissertation in chemistry. Virginia Polytechnic Institute

and State University, Blacksburg, VA; 2002.

[20] Kofron W, Backawski L. J Org Chem 1976;41:187980.

[21] Chojnowski J, Scibiorek M, Gladkova N. Polym Bull 2000;44:377.

[22] Boileau S. In: McGrath JE, editor. Ring-opening polymerization:

kinetics, mechanisms and synthesis. ACS Symposium Series, 286.

Washington, DC: American Chemical Society; 1984. p. 2335.

[23] Kickelbick G, Husing N, Bauer J. J Polym Sci, Part A 2002;40:[8] Popplewell J, Sakhnini L. J Magn Magn Mater 1995;149:72.

[9] Sun Y, Rollins H, Guduru R. Chem Mater 1999;11:79.

[10] Lin X, Sorensen C. Langmuir 1998;14:71406.

[11] Pileni M. Langmuir 1997;13:326676.

[12] Sorensen C, Klabunde K, Hajipanayis G. J Mater Res 1999;14:1542.

[13] Platanova OA, Bronstein LM, Solodovnikov SP, Yanovskaya IM,

Obolonkova ES, Valetsky PM,Wemz E, Antonietti M. Colloid Polym

Sci 1997;275:42631.

[14] Rutnakornpituk M, Thompson MS, Harris LA, Farmer K, Esker AR,

Riffle JS, Connolly J, St Pierre TG. Polymer 2002;43:233748.

[15] Hess P, Parker Jr. P. J Appl Polym Sci 1966;10:191527.not understood, but may be at least partially attributable to

residual paramagnetic species and also to a somewhat

amorphous cobalt structure. It will be important to

determine whether post solgel or pyrolysis reactions can

protect the cobalt surfaces against oxidation. Moreover,

effects of such post reactions on cobalt magnetic properties

should also be carefully studied.

Acknowledgements

The authors are grateful for funding from DARPA-

AFOSR (contracts F49620-02-1-0408 and F49620-03-1-

0332) and from NSF (contract #DMR-0312046), and for the

generous donation of n-butyllithium initiator by the Lithco

Division of FMC. M.A. Zalich thanks the Australian

American Fulbright Commission for a Fulbright Postgradu-

ate Fellowship to conduct research in Australia.

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M.L. Vadala et al. / Polymer 45 (2004) 74497461 7461

Block copolysiloxanes and their complexation with cobalt nanoparticlesIntroductionExperimentalMaterialsSynthesisCharacterization of copolymers and copolymer solutionsCharacterization of magnetic fluids

Results and discussionCopolymer synthesis and characterizationCopolymer functionalization and characterizationThermal properties of PDMS-b-PMVS and PDMS-b-(PMVS-co-PMTES)Synthesis and characterization of PDMS-b-(PMVS-co-PMTMS)/cobalt complexes

Conclusions and recommendations for further investigationsAcknowledgementsReferences