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Physica B 385–386 (2006) 787–790

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SANS studies of polymerized nano-particles using nonionic/cationicsurfactant mixture

Tae-Hwan Kima, Sung-Min Choia,�, Steven R. Klineb

aDepartment of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,

Yuseong- gu, Daejeon 305-701, South KoreabNIST Center for Neutron Research, Gaithersburg, MD 20899-8562, USA

Abstract

Mixtures of polymerizable cationic surfactant, cetyltrimethylammonium 4-vinylbenzoate (CTVB), nonionic surfactant, Triton X-100

(TX-100), and polymerizable hydrotropic salt, sodium 4-styrenesulfonate (NaSS) in aqueous solution were free radically polymerized to

produce stable rod-like nano-particles with varying aspect ratio. A series of 1wt% of the mixed surfactant (CTVB+TX-100) solutions

were prepared with varying surfactant molar mixing ratio a ( ¼ TX-100/(TX-100+CTVB)) from 0.07 to 0.41. In all the samples, an

appropriate amount of NaSS was added to keep the molar amount of CTVB plus NaSS constant at 0.0257M; this maintained the

amount of polymerizable counterions constant in all the samples. SANS measurements showed that the polymerized aggregates were

cylindrical nano-particles which were stable against temperature variation between 30 and 60 1C. While the radius of the particles was

constant at 2 nm, the aspect ratio was decreased from 27.5 to 2.7 as the mixing ratio a was increased from 0.07 to 0.23.

r 2006 Elsevier B.V. All rights reserved.

PACS: 61.41.+e; 61.46.+w

Keywords: Polymerization; Mixed surfactant; SANS

1. Introduction

Surfactant molecules in aqueous solution form varioustopological structures such as micelles, vesicles, andlamellae. However, their dynamic nature and sensitivityto environments such as temperature, pH, and pressurehave limited their use as nano-structured materials. Toovercome these problems, polymerizable surfactants havebeen developed and used to produce various stable nano-structured materials [1–3].

Recently, we have developed a new method to producestable rod-like nano-particle with controlled surface chargedensity by co-polymerizing cationic surfactants, cetyltri-me–thylammonium 4-vinylbenzoate (CTVB) which haspolymerizable counterions, with varying amounts ofpolymerizable hydrotropic salt, sodium 4-styrenesulfonate(NaSS) in aqueous solution [4]. While the length of the rod-

e front matter r 2006 Elsevier B.V. All rights reserved.

ysb.2006.06.084

ng author. Tel.: +8242 869 3822; fax: +82 42 869 3810.

ss: sungmin@kaist.ac.kr (S.-M. Choi).

like particles could be controlled by the concentration ofinitiator [5,6], it might be useful to have different ways ofcontrolling the length, possibly providing a wider range oflength or aspect ratio.In this paper, mixtures of polymerizable cationic

surfactant, CTVB and nonionic surfactant, Triton X-100(TX-100) in aqueous solution were investigated to producestable rod-like nano-particles with varying aspect ratios.While CTVB forms worm-like micelles in aqueous solution[5], TX-100 aggregates into elliptic micelle with aspect ratioaround 2 [7]. Therefore, mixtures of CTVB and TX-100may form some intermediate structures between the twoand their polymerized aggregates may produce rod-likeparticles with varying aspect ratios, depending on themixing ratio of the two surfactants. However, as theconcentration of CTVB is reduced, the amount ofpolymerizable counterions is decreased accordingly, even-tually preventing polymerization. Thus, the appropriateamount of hydrotropic salt, NaSS, which provides addi-tional polymerizable counterions, is added into the

ARTICLE IN PRESST.-H. Kim et al. / Physica B 385–386 (2006) 787–790788

surfactant mixtures to keep the molar amount of CTVBplus NaSS (i.e. the molarity of the total polymerizablecounterions) constant.

In this paper, SANS characterization of polymerizedaggregates of the mixed surfactant systems with varyingmixing ratio is presented.

2. Experimental section

CTVB was synthesized by the method described else-where [5]. NaSS was purchased from Fluka. TX-100 and 4-vinylbenzoic acid were purchased from Aldrich. D2O(99.9mol% deuterium enriched) was purchased fromCambridge Isotope Laboratory. The water-soluble freeradical initiator VA-044 (2, 20-azobis [2-(2-imidazolin-2-yl)propane] dihydrochloride) was purchased from WakoChemicals.

α = 0.41

α = 0.07

α = 0.41

α = 0.07

q-1

q-1

Sca

tterin

g in

tens

ity (

cm-1

)

100

0.1

1

10

1000

0.1

1

10

100

8 90.01

2 3 4 5 6 7 8 90.1

2 3

q(Å-1)

(a)

(b)

Fig. 1. (a) SANS intensities of unpolymerized 1wt% CTVB/TX-100 in

D2O with addition of NaSS. (b) SANS intensities of polymerized 1wt%

CTVB/TX-100 in D2O with addition of NaSS. The data were taken at

30 1C (open) and 60 1C (solid). The data sets are vertically shifted for easier

viewing (a ¼ 0.41 (� 50)).

A series of 1wt% of the mixed surfactants (CTVB+TX-100) in D2O were prepared with different surfactant molarmixing ratio a ¼ 0.07, 0.23, and 0.41 where a ¼ TX-100/(TX-100+CTVB). An appropriate amount of NaSS wasadded to the 1wt% samples to keep the molar amount ofCTVB plus NaSS constant at 0.0257M.The mixtures were polymerized by injecting free radical

initiator VA-044 (5mol% relative to the total polymeriz-able counterions). The detailed polymerization procedureis described elsewhere [5].SANS experiments were carried out on the 9m SANS

instrument at the High-flux Advanced Neutron Applica-tion Reactor (HANARO) in Korea Atomic EnergyResearch Institute, Republic of Korea. Neutrons ofwavelength l ¼ 6.38 A with full-width at half-maximumDl/l ¼ 11.8% were used. Two different sample to detectordistances (2 and 4.61m) were used to cover the overall q

range of 0.007 A�1oqo0.2668 A�1 where q ¼ (4p/l)sin(y/2) is the magnitude of the scattering vector and y is thescattering angle. Sample scattering was corrected forbackground and empty-cell scattering. The corrected datasets were placed on an absolute scale using standardsamples.SANS intensities were analyzed by the cylindrical form

factor model [8] together with the rescaled mean sphericalapproximation (MSA) screened Coulomb interactionstructure factor [9,10]. Since the structure factor is forspherical particles, effective diameters of the rod-like

α = 0.41

10-1

100

101

102

103

104

Sca

tter

ing

Inte

nsi

ty (

cm-1

)

8 90.01

2 3 4 5 6 7 8 90.1

2 3

q(Å-1)

α = 0.07

α = 0.23

Fig. 2. Model fit of the SANS intensities from polymerized 1wt% CTVB/

TX-100 in D2O with addition of NaSS. The intensities for a ¼ 0:07(� 1000) and 0.23 (� 50) are vertically shifted for visual clarity.

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Table 1

The results of model fit

a Radius (nm) Length (nm) Aspect ratio Charge/length (#/nm) Fitted vol. frac. (%) Calculated vol. frac. (%) (w2/N)0.5

0.07 2.070.006 110710.5 27.5 1.09 1.03 1.04 2.86

0.23 2.070.007 10.670.1 2.7 1.11 1.08 1.13 1.28

0.41 2.070.009 10.570.1 2.7 1.23 1.17 1.23 1.19

T.-H. Kim et al. / Physica B 385–386 (2006) 787–790 789

particles, which were calculated by a method proposed byIsihara [11], were used in the model fitting.

3. Result and discussion

The viscosity of the 1wt% unpolymerized mixedsurfactant (CTVB+TX-100) solution added with NaSSdecreased as the mixing ratio a was increased from 0.07 to0.41. This may indicate that the structure of the mixedsurfactant system is changed from long worm-like micelleto shorter micelle as the TX-100 concentration is increased.Upon polymerization, the viscosities of all the mixtureswere dramatically reduced, near to that of water.

The SANS intensities of 1wt% unpolymerized mixtures(Fig. 1a) and 1wt% polymerized aggregates (Fig. 1b) witha ¼ 0.07 and 0.41 are shown in Fig. 1. While the SANSintensities from the unpolymerized mixtures were sensitiveto temperature variation, the SANS intensities from thepolymerized aggregates measured at two different tem-peratures, 30 and 60 1C, were essentially identical. Thisclearly indicates that the polymerized particles are stable atthese temperature variations. Even the polymerized aggre-gates with a ¼ 0:41 do not show temperature dependence.Considering that the SANS intensities of TX-100 in D2O(a ¼ 1:0, unpolymerized) measured at 30 and 60 1C (notshown here) are quite different, this may indicate that mostof TX-100 are incorporated into the polymerized aggre-gates.

When a ¼ 0:07, the screened Coulomb interaction effectpresent in the unpolymerized mixture almost completelydisappears upon polymerization and is replaced by apower-law dependence of q�1 at low-q region, typical ofcylindrical structures. On the other hand, when a ¼ 0:41,the unpolymerized mixture shows q�1 behavior at low q,while upon polymerization, the intensity at low q becomesflattened. This may be due to the reduced length of theaggregates and interparticle interference effects.

Model fittings for the polymerized aggregates by usingthe cylindrical form factor and the rescaled MSA screenedCoulomb interaction structure factor are shown in Fig. 2.The concentrations of Na+ and free (not incorporated intothe polymerized aggregates) SS� ions were used in thecalculation of the Debye screening length. Assuming allNaSS molecules dissociate in D2O, the concentration offree SS� was determined by subtracting polymerized SS�

(which determines the number of charge per eachpolymerized aggregate, a fitting parameter) from the total

concentration of SS�. The solid lines are the model fits,which agree with the SANS intensities very well. Therefore,the polymerized aggregate are cylindrical in shape. Theresults of the model fits are shown in Table 1. The fittedvolume fractions are consistent with the calculated totalvolume fractions of TX-100, CTVB and NaSS within 5%.This may indicate that most of TX-100 and CTVB areincorporated into the polymerized aggregates.While the radii of polymerized rod-like nano-particles

are constant at 2 nm, their aspect ratios are dramaticallyreduced from 27.5 to 2.7 as the mixing ratio a is changedfrom 0.07 to 0.23. Further increase of a to 0.41 does notchange the aspect ratio. These show that the aspect ratio ofpolymerized rod-like particles can be controlled by addingTX-100 into CTVB micellar solution with appropriateamount of NaSS. However, to elucidate the detaileddependence of the aspect ratio on the TX-100 concentra-tion, the mixtures with a between 0.07 and 0.23 need to bestudied further.

4. Conclusion

Mixtures of polymerizable cationic surfactant, CTVBand nonionic surfactant, TX-100, and polymerizablehydrotropic salt, NaSS, in aqueous solution have beeninvestigated to produce stable rod-like nano-particles withvarying aspect ratio. The SANS measurements showed thatthe polymerized aggregates were cylindrical nano-particleswhich were stable against temperature variation. While theradius of the particles is constant at 2 nm, the aspect ratio isdecreased from 27.5 to 2.7 as the mixing ratio a is increasedfrom 0.07 to 0.23.

Acknowledgments

This work is supported by the Basic Atomic EnergyResearch Institute (BAERI) program of the Ministry ofScience and Technology and the Ministry of Health andWelfare of Korea (0405-MN01-0604-007). We thankHANARO for SANS beamtime supports and Dr. E. J.Shin for technical assistance during SANS measurements.Certain trade names and company products are identifiedin order to specify adequately the experimental procedure.In no case does such identification imply recommendationor endorsement by NIST, nor does it imply that theproducts are necessarily the best for the purpose.

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