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Magnetoresponsive, anisotropic composite particles reversibly changing theirchain lengths by a combined external field†

Mariko Nishi, Daisuke Nagao,* Kentaro Hayasaka, Haruyuki Ishii and Mikio Konno*

Received 4th June 2012, Accepted 23rd August 2012

DOI: 10.1039/c2sm26285a

Magnetoresponsive, anisotropic composite particles were prepared to explore a new type of building

blocks reversibly changing their chain lengths by switching on an external magnetic field. The

composite particles were synthesized with three-step polymerization comprising (i) polymerization to

coat magnetoresponsive silica particles with crosslinked poly(methyl methacrylate) (PMMA), (ii)

polymerization to form a polystyrene (PSt) lobe on the PMMA-coated particles and (iii)

polymerization to form another PSt lobe on the opposite side of the former lobe. The structure of the

composite particles was analyzed with scanning transmission electron microscopy showing rod-like

polymer particles incorporating a magnetoresponsive particle in the middle of a rod-like particle. The

composite particles suspended in aqueous solution of polyvinylpyrrolidone used as a viscosity enhancer

were observed by optical microscopy under applied external fields. Application of an alternating

electric field at a high frequency of 2 MHz oriented the rod-like particles parallel to the electric field and

assembled them to form pearl-chain structures of the composite particles. The chain lengths of the

oriented rod-like particles were extended during the application of the electric field. While applying the

electric field, an additional application of magnetic field with a field strength of 100 mT changed the

chain structure so as to allow the magnetoresponsive parts to come close to each other. A combined

application in which the magnetic field was switched on and off intermittently under a fixed electric field

could reversibly compress and extend the particle chains and control their chain lengths.

1. Introduction

Electro- and magneto-rheological fluids, ER and MR fluids,

respectively, are smart materials which are able to tune their

rheological properties with external fields. ER or MR fluids

typically consist of particles which are polarisable or magnet-

isable under an external field in a medium to disperse the parti-

cles. Application of the external fields to the fluids allows

dispersed particles to be assembled for the formation of pearl-

chain structures. A synergetic effect on particle assemblies

formed by a combined application of electric and magnetic fields

has been reported for suspensions of iron particles1 and Fe2O3

incorporated lead zirconate titanate (PZT) microspheres.2 ER

and MR fluids are expected to be applied to displays3,4 and

dampers5,6 because of their intrinsic yield stress and elastic

response enhanced by application of external fields.

The importance of creating anisotropic particles which are

responsive to external fields has been emphasized,7 because

6-6-07 Aoba, Aramaki-Aza, Aoba-ku, Sendai, Japan. E-mail: konno@mickey.che.tohoku.ac.jp; nagao@mickey.che.tohoku.ac.jp; Fax: +81 22795 7241; Tel: +81 22 795 7239

† Electronic supplementary information (ESI) available: Fig. S1:dependence of PSt protrusion on the PSt lobe size of dimer compositeparticles. See DOI: 10.1039/c2sm26285a

11152 | Soft Matter, 2012, 8, 11152–11155

anisotropic particles have a potential to form unique assembling

structures under external electric8,9 and magnetic fields.10–12 It

was reported that rheological properties such as shear stress and

shear yield stress were improved by employing anisotropic

particles.9 In previous reports examining mechanisms on the

improvements of rheological properties of anisotropic particle

suspensions, optical microscopy was commonly employed to

observe the process of chain formation in suspensions of aniso-

tropic particles under an external field.9,10,13

Application of alternating electric fields has also been used by

our group as a method to softly fix the dielectric particles with

each other suspended in a media. An alternating electric field was

applied to a suspension of rattle-type silica particles and facili-

tated direct observation of the inner silica spheres randomly

moving in a silica compartment.14 It was shown in our report that

the application of electric fields was effective to keep the sus-

pended particles close to each other in a medium.14

In the present work, magnetoresponsive anisotropic particles,

rod-like polymer particles incorporating a magnetoresponsive

spherical particle in the middle of rod-like particles, are dispersed

in an aqueous medium under an alternating electric field to

promote the formation of pearl-chain structures. Under such

conditions, a static magnetic field was additionally applied to the

suspension to examine the synergetic field effect on the particle

This journal is ª The Royal Society of Chemistry 2012

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assembling structures. The preparation procedure of the aniso-

tropic particles is illustrated in Fig. 1 where the first step is to coat

the magnetoresponsive, spherical silica particles with crosslinked

poly(methyl methacrylate) (PMMA), the second step is to form a

polystyrene (PSt) lobe on the PMMA-coated particles and the

third step to form another PSt lobe on the opposite side of the

former lobe. Optical microscopy was employed to observe

structural transitions of the particle assemblies under electric or

magnetic fields and under combined electric and magnetic fields.

2. Experimental

2.1 Chemicals

Styrene (St, 99%), methyl methacrylate (MMA, 98%), sodium p-

styrenesulfonate (NaSS), sodium chloride, potassium persulfate

(KPS, 95%) and poly(vinylpyrrolidone) (PVP, K-30, Mw ¼40 000 g mol�1) were obtained from Wako Pure Chemical

Industries (Osaka, Japan). The inhibitors for monomers of St

and MMA were removed by inhibitor removal columns. The

other chemicals were used as received. The silane coupling agent

3-methacryloxypropyltrimethoxysilane (MPTMS, 95%) was

purchased from Shinetsu Chemical (Tokyo, Japan) and used as

received.

2.2 Synthesis of magnetoresponsive, rod-like composite

particles

The magnetoresponsive silica particles were prepared according

to the method previously reported by our group.15 The magne-

toresponsive particles had an average size of 620 nm and their

surfaces were coated with a thin silica layer formed in solution of

sodium silicate. The magnetoresponsive particles were PMMA-

coated in the first polymerization where MMA was polymerized

at 65 �C with KPS initiator in the presence of the magneto-

responsive particles and MPTMS used as a crosslinker.16 The

concentrations of the magnetoresponsive particles and MPTMS

were 0.26 vol% and 2 mM, respectively. An anionic co-monomer

of NaSS (1 mM) was also used to introduce anionic charges into

the crosslinked PMMA formed in the polymerization. The

Fig. 1 Schematic procedure for preparation of magnetoresponsive

anisotropic composite particles.

This journal is ª The Royal Society of Chemistry 2012

reaction volume was 240 cm3. In the second polymerization to

protrude a PSt lobe, St was polymerized at 65 �C with KPS

initiator in the presence of the PMMA-coated particles. The

concentrations of St and KPS were 0.1 M and 2.0 mM, respec-

tively. The third polymerization was conducted to protrude the

second PSt lobe and obtain rod-like composite particles. The

concentrations of St and KPS were 0.2 M and 2.0 mM,

respectively.

2.3 Characterization

The composite particles formed at each polymerization were

observed with scanning transmission electron microscopy

(Hitachi, HD-2700) after several centrifugations to remove

secondary particles. The sample cell used for particle observation

under an electric field17 and/or a magnetic field consisted of a

capillary (0.1 � 1 mm rectangular cross section, VITRO COM)

and two 50 mm diameter copper wires (99.99%, NIRACO)

threaded through along the side walls. The capillary was filled

with an aqueous suspension of the particles, and the ends of

capillary were sealed with glue. The concentrations of composite

particles in the suspensions was adjusted to 0.01 vol%, low

enough to acquire clear images, in observation with an optical

microscope (OM). The AC field was applied by connecting the

copper wires to a function generator (GWINTEK, SFG-2004)

and amplifier (NF Circuit Design Bloc, HSA4011). The electric

field strength (peak to peak) was measured with a digital oscil-

loscope (GWINTEK, GDS-1062A). A static magnetic field was

applied to the suspension with a magnetic coil fabricated by Toei

Scientific Industrial Co. PVP (20 g L�1) was added as a viscosity

enhancer to the suspension.

3. Results and discussion

Fig. 2a shows a TEM image of magneto-responsive silica cores

coated with cross-linked PMMA shell formed in the first poly-

merization. The average size of the PMMA-coated particles was

680 nm and the coefficient of variation of particle sizes (CV) was

2.4%. A magnified TEM image presented in the inset of Fig. 2a

Fig. 2 TEM images of PMMA-coated particles (a), anisotropic

composite particles with a single lobe (b) and double lobes ((c): low

magnification, (d): highmagnification). The inset of (a) shows amagnified

TEM image of the surface of the PMMA-coated particles. The magneti-

zation curve of the anisotropic composite particles is shown in (e).

Soft Matter, 2012, 8, 11152–11155 | 11153

Fig. 3 OM images to show a transition from the disperse state to pearl-

like chains of anisotropic composite particles under an alternating electric

field (2 MHz, 50 V mm�1). The images were taken at 1 s (a), 10 s (b) and

60 s (c) after the application of the field. The concentration of composite

particles was 0.01 vol%.

Fig. 4 OMimages showinga transition fromthedisperse state topearl-like

chains of anisotropic composite particles under anmagnetic field (100mT).

The images were taken at 0 s (a), 10 s (b) and 60 s (c) after the application of

the field. The concentration of composite particles was 0.01 vol%.

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shows an approximately 30 nm PMMA shell uniformly covering

the magnetic silica cores. Fig. 2b shows a TEM image of

composite particles obtained in the second polymerization where

a PSt lobe is protruded from each PMMA-coated particle. In the

third polymerizations, another PSt lobe was formed on the

opposite side of the former lobe of the same core–shell particles.

Fig. 2c shows a TEM image of anisotropic composite particles

formed in the third polymerization. More than 85% of the

anisotropic particles in Fig. 2c have two PSt lobes of rod-like

particles incorporating the magnetoresponsive particles in the

middle of the rod-like structure as shown in Fig. 2d. In the

present method where the PSt lobes formed in the second poly-

merization were not crosslinked, it is not likely that the size of

PSt lobes generated in the third polymerization exceeds that

already formed on the core–shell particles. Therefore, the large

PSt lobe shown in Fig. 2d was formed in the second polymeri-

zation and grown in the third polymerization whereas the small

one was newly generated in the third polymerization.

Similar rod-shaped polymeric particles, which did not incor-

porate inorganic cores, were reported by Kim et al.18 who

adjusted the crosslinking density of each part of polymeric

dimers to control the shape of polymeric trimers. They prefer-

entially protruded a PSt lobe from the polymeric part weakly

crosslinked in a prior polymerization. On the other hand, we

have formed two PSt lobes on both sides of the same core

particles having a cross-linked PMMA shell. We performed

additional experiments on the preparation of rod-like composite

particles incorporating a silica core (without any magnetic

response) and found that the size of the first PSt lobe is significant

for the protrusion of a second PSt lobe from the cross-linked

PMMA shell (Fig. S1, ESI†).

Magnetic properties of the composite particles were measured

with a vibrating sample magnetometer (PV-M20-5, Toei

Scientific Industrial Co.) showing a saturation magnetization of

0.46 emu g�1 in Fig. 2(e). Since the saturation of magnetization

of magnetite nanoparticles incorporated into silica cores was 70

� 3 emu g�1,15 it can be estimated that the weight ratio of

magnetite nanoparticles to the composite particles is approxi-

mately 0.66 wt%.

An alternating electric field at a sufficiently high frequency of

2 MHz to reduce the effect of ions in the double layer around the

composite particles was applied to the suspension of anisotropic

composite particles shown in Fig. 2c. Fig. 3 shows a transition

from disperse state of rod-like particles toward the formation of

chain structures under an electric field of 50 V mm�1. Just after

the application of an electric field, the rod-like particles become

oriented to the electric field as shown in Fig. 3a. Application of

the electric field for 10 s formed a short chain of the rod-like

particles with the maintenance of their orientations. Most chains

were gradually extended several tens of micrometers within 60 s.

A zoomed image in Fig. 3c indicates that the rod-like particles

had no specific direction in the chain under the electric field.

Subsequently, an external magnetic field at 100 mT was

applied to the same suspension of anisotropic composite parti-

cles. Fig. 4 shows a transition from disperse state of the rod-like

particles toward their clustering. Application of the magnetic

field for 60 s clustered the rod-like particles for which the mag-

netoresponsive parts were close to each other in a line parallel to

the magnetic field as shown in the zoomed image of Fig. 4c. This

11154 | Soft Matter, 2012, 8, 11152–11155

shows that a magnetic field of 100 mT was strong enough to keep

the rod-like particles in the vicinity of each other.

Additional application of a magnetic field under a fixed electric

field was performed to rearrange the position and orientation of

the rod-like particles. When a magnetic field of 100 mT was

additionally applied at a fixed electric field strength of 50 V

mm�1, the rod-like particle chains formed were gradually

compressed within 5 s as shown in Fig. 5a–c. On the contrary,

This journal is ª The Royal Society of Chemistry 2012

Fig. 5 OM images showing variation in chain lengths of anisotropic

composite particles under a combination of switched magnetic field

(100 mT) and fixed electric field (2 MHz, 50 V mm�1). The images were

taken at 0 s (a), 0.62 s (b) and 4.81 s (c) after application of the magnetic

field. The black arrows in (d) indicate the application period of the

magnetic field. The concentration of composite particles was 0.01 vol%.

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turning off the magnetic field extended the chain length to almost

the same length as before application of the magnetic field. The

rate of chain extension was slower than that of chain compres-

sion observed under the magnetic field. The magnetic field was

intermittently applied under application of the same electric field.

Fig. 5d shows the rate of variation in particle conformation with

switching the application of magnetic field (also see Movie in the

ESI†). The length of particle chain surrounded by the dashed

ellipse in Fig. 5c decreased and reached 17.0–17.5 mm under the

magnetic field whereas it increased to almost the original length

without the field, indicating high controllability over the chain

length with switching the magnetic field on and off.

The strength of interaction between the magneto-responsive

cores can be adjusted with the weight fraction of the magnetic

component in the composite particles as well as the field strength.

Our recent report clearly showed that the frequency of the alter-

nating electric field can be an important factor for orientation of

anisotropic particles in their assembly process.8 Therefore, the

combination of electric andmagnetic fields applied to anisotropic

particles has a significant potential for creation of materials with

new types of rheological properties of ER + MR fluids.

4. Conclusions

We have succeeded in the preparation of rod-like composite

particles which are responsive to both electric and magnetic

This journal is ª The Royal Society of Chemistry 2012

fields. The application of an alternating electric field formed

pearl-chain structures of rod-like particles parallel to the applied

field. The application of a magnetic field clustered the composite

particles with magnetoresponsive parts coming close to each

other in a line parallel to the magnetic field. The application of a

magnetic field under a fixed electric field compressed the particle

chain whereas turning off the magnetic extended the chain to the

original length. Repeated switching on and off of the magnetic

field could reversibly change the particle conformation and also

control the chain lengths.

Acknowledgements

This research was mainly supported by the Ministry of Educa-

tion, Culture, Sports, Science and Technology (JSPS KAKENHI

Grant Number 23246134, 23681020 and 24651112) and also

partially supported by Advanced Low Carbon Technology

Research and Development Program Grant from Japan Science

and Technology (JST) Agency.

Notes and references

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