European Polymer Journal 44 (2008) 3957–3962

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology – Short communication

Synthesis of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene-silica core–shell particles with a self-templating methodand their fluorescent properties

Qing Zhang, Yongai Zhai, Fengqi Liu, Meng Yang, Ge Gao *

College of Chemistry and MacDiarmid Laboratory, Jilin University, Changchun 130021, People’s Republic of China

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Article history:Received 21 April 2008Received in revised form 22 September2008Accepted 27 September 2008Available online 10 October 2008

Keywords:Self-templating methodSilicaMEH-PPVCore–shell structureFluorescent properties

0014-3057/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.eurpolymj.2008.09.036

* Corresponding author. Tel.: +86 431 822148499187.

E-mail address: gaoge@jlu.edu.cn (G. Gao).

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a b s t r a c t

Sub-micrometer particles with poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinyl-ene (MEH-PPV) cores coated by silica-based shells were prepared with a self-templatingmethod and their fluorescent properties were investigated in this paper. The characteristicof this method was that all reactions could be finished in one-pot, which exempted fromremoving the template and reduced reaction steps compared to the conventional process.Emission wavelength of the resultant core–shell particles can readily be tuned throughchemical modification of MEH-PPV, which was carried out via regulating the conjugationlength of the polymer. In addition, the size of MEH-PPV/SiO2 core–shell particles couldbe controlled by altering reaction conditions. The obtained particles had clear core–shellstructure and may be used as biolabeling materials. The morphologies, particle size distri-bution and fluorescent properties of MEH-PPV/SiO2 particles were characterized by trans-mission electron microscopy (TEM), particle size analyzer and fluorescence emissionspectra, respectively.

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1. Introduction

Recently, core–shell materials have attracted a greatdeal of attentions due to their improved physical andchemical properties over their single-component counter-parts. They have potential applications in different fields,such as optoelectronics [1–5], catalysis [6–10] and bio-medicine [11–15]. Many methods have been developedto fabricate core–shell materials, such as layer-by-layer(LBL) technique [16,17], oriented deposition [18,19], chem-ical coprecipitation [20], in-situ polymerization [21,22],emulsion polymerization [23,24], dispersion polymeriza-tion [25,26] and miniemulsion polymerization [27,28].Most of these methods used surfactants or colloidal parti-cles as templates, which have the advantage of controlling

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spherical morphology and narrow size distribution. How-ever, the cumbersome procedures for removing templatescould make more reacting steps and the application in bio-medicine would be restricted due to the pollution intro-duced by the templates. Therefore, searching for newsynthetic strategies for core–shell particles has been ahot topic.

Electroluminescent (EL) polymeric materials offer anumber of advantages over inorganic EL materials suchas low operating voltages, full-color displays, fast responsetimes, high luminescent efficiency, high-quality displays,and ease device processability with semiconductor tech-nologies. Poly(p-phenylene vinylene) (PPV) and its deriva-tives have been widely investigated in electronics andphotonics because of their excellent electrical and opticalproperties. They are versatile conjugated polymers whichhave been employed for polymer-based light-emittingdiodes (LEDs) [29,30], lasers [31,32] and photovoltaiccells [33,34]. The HOMO–LUMO energy level width of

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conjugated polymers can be manipulated by controllingthe conjugation length of the polymers to adjust the poly-mer materials emitting light of different colors and realiz-ing polychromatic displays.

SiO2-coated particles are advantageous for applica-tions in the field of biomedicine, because silica is easyto functionalize, nontoxic, and can protect the surfaceof the particles from oxidation. To the best of our knowl-edge, the synthesis of particles with MEH-PPV corescoated by silica-based shells has not yet been performed.In the present work, core–shell MEH-PPV/SiO2 particleswere prepared via a single-step O/W emulsion systemusing the self-templating technique and their fluorescentproperties were investigated. The particle size was tunedby altering reaction conditions. In addition, the emissionwavelength of core–shell MEH-PPV/SiO2 particles wascontrolled through regulating the conjugation length ofMEH-PPV. The morphologies, particle size and fluores-cent properties of MEH-PPV/SiO2 particles were charac-terized by transmission electron microscopy (TEM),particle size analyzer and fluorescence emission spectra,respectively.

2. Experimental

2.1. Materials

Tetraethoxysilane (TEOS) (AR), hydrochloric acid (GR),absolute alcohol (AR) and ammonia (AR) were purchasedfrom the Beijing Chemical Plant. Methyltrimethoxysilane(MTMS) (AR) and azobisisobutyronitrile (AIBN) (AR) werecommercially obtained from Zhejiang Chemical LimitedCo. and Shanghai Chemical Reagent Plant, respectively.MEH-PPV was synthesized by Gilch route referencing theworks of Parekh [35] and Neef and Ferraris [36].

2.2. Characterization

The structure and morphologies of core–shell parti-cles were characterized with a Hitachi H-8100 transmis-sion electron microscope. The particle size distributionwas measured by a ZetaPlus Zeta Potential Analyzer(ZZPA). The fluorescence emission spectra of core–shellparticles were determined on a Perkin-Elmer LS 55spectrofluorometer.

2.3. Preparation of SiO2 colloid particles

SiO2 colloid particles were prepared with a sol-gelmethod. The mixture of TEOS (42 mL, 0.25 mol) and MTMS(8.6 mL, 0.06 mol) was slowly added into the mixture ofaqueous solution of HCl (4.5 mL, 0.75 mmol), deionizedwater (2.25 mL, 0.25 mol) and absolute alcohol (58 mL,1 mol) within 3 h by stirring at 40 �C. Afterwards, the reac-tion system was stirred for another 9 h to obtain a clearsolution. The clear solution was evaporated by a rotaryvacuum evaporator at 50 �C to remove ethanol and by-product produced during the reaction. A transparent vis-cous precursor solution was obtained. The reaction pro-ceeded at the H2O/Si rate of 0.8, 1.0, 1.2, 1.5, 1.7 and 2.0.

2.4. Preparation of MEH-PPV/SiO2 core–shell particles

This step was completed with a self-templating meth-od. An O/W emulsion was produced through adding40 mL of deionized water into the mixture of 1 mL of tolu-ene containing MEH-PPV and 4 mL of SiO2 precursor solu-tion under the stirring of a homogenizer. Subsequently, theresulting microemulsion was solidified by adding 3.2 mL ofammonia (1.0 mol/L) for 10 min at room temperature. Thefinal pH value of the solution was about 10. The resultantmixture was left for 1 h without stirring, collected by cen-trifugation, washed with deionized water, and dried undervacuum at 30 �C.

2.5. Preparation of Modified MEH-PPV and MEH-PPVm/SiO2

core–shell particles

Modified MEH-PPV (MEH-PPVm) was prepared referringliterature [37]. AIBN was used to regulate the conjugatedlength of MEH-PPV by the radical addition onto the doublebonds of conjugated polymer. The reaction proceeded at70 �C for 30 min, 60 min, 90 min and 150 min, respectively.Afterwards, MEH-PPVm was coated by silica-based shellswith the self-templating technique.

3. Results and discussion

3.1. Morphologies, structure and particle size distribution ofparticles

One of the key techniques in preparing core–shell parti-cles with the self-templating method is keeping the stabil-ity of the emulsion system. In our previous work, acommon mechanical stirrer with an agitation speed of2000 rpm was used during the process of preparingMEH-PPV/SiO2 particles and only micrometer-size spheres(10–60 lm) with wide particle size distribution were ob-tained owing to the agglomeration of emulsion droplets.In the present work, a homogenizer with high stirring ratewas employed to prevent emulsion droplets from agglom-erating and sub-micrometer-size spheres were produced.Figs. 1 and 2 present TEM micrograph and particle size dis-tribution of MEH-PPV/SiO2 particles with an agitationspeed of 10,000 rpm and 19,000 rpm, respectively. Asshown in Figs. 1A and 2A, the resulting MEH-PPV/SiO2 par-ticles have clear spherical and core–shell structure. Theaverage particle sizes of the particles are approximately320 nm with an agitation speed of 10,000 rpm and150 nm with an agitation speed of 19,000 rpm.

The results of ZZPA (Figs. 1B and 2B) show that themean particle sizes of the core–shell particles are 358 nmwith an agitation speed of 10,000 rpm and 182 nm withan agitation speed of 19,000 nm. Compared to the resultsof ZZPA, the size of the particles measured by TEM is smal-ler because the specimens of the particles for TEM are atdry state but the particles in the specimens for ZZPA aremore or less swelled. Moreover, both TEM and ZZPA resultsdemonstrate that the particle size of the core–shell parti-cles decreases with increasing agitation speed due to thesmaller emulsion size.

Fig. 1. TEM micrograph (A) and particle size distribution (B) of MEH-PPV/SiO2 particles with an agitation speed of 10,000 rpm.

Fig. 2. TEM micrograph (A) and particle size distribution (B) of MEH-PPV/SiO2 particles with an agitation speed of 19,000 rpm.

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In addition, H2O/Si ratio used during the process of pre-

paring SiO2 colloid particles played an important role inthe synthesis of core–shell particles. When the H2O/Si ratiowas below 1.5, it was difficult to obtain core–shell particlesdue to a slow gelling process, on the other hand, when theH2O/Si ratio was above 1.7, the precursor solution was gel-atinized and could not be mixed with the core material toproduce the desired particles. Therefore, the H2O/Si ratioused in the present study was in the range of 1.5–1.7, inwhich all the resulting particles were observed as core–shell structures.

Emission wavelength of the MEH-PPV/SiO2 particles canreadily be tuned through regulating the conjugation lengthof MEH-PPV. Sub-micrometer particles with MEH-PPVm

cores coated by silica-based shells were synthesized suc-cessfully with the self-templating technique. Fig. 3 illus-trates TEM micrographs of MEH-PPVm/SiO2 (30 min) andMEH-PPVm/SiO2 (150 min) particles.

3.2. Fluorescence properties of the core–shell particles

The modification of MEH-PPV was completed throughfree radical addition by using AIBN. During the process ofmodification, cyano groups of AIBN hydrolyzed intocarboxyl groups and the hydrolyzed AIBN were decom-posed into active free radicals, which subsequently at-tacked the conjugated double bonds of MEH-PPV,

Fig. 3. TEM micrographs of (A) MEH-PPVm/SiO2 (30 min) and (B) MEH-PPVm/SiO2 (150 min) particles.

Fig. 4. Fluorescence emission spectra: (a) MEH-PPV/SiO2, (b) MEH-PPVm/SiO2 (30 min), (c) MEH-PPVm/SiO2 (60 min), (d) MEH-PPVm/SiO2 (90 min),(e) MEH-PPVm/SiO2 (150 min).

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resulting in shortening the conjugation length of MEH-PPV.Fluorescence properties of MEH-PPVm/SiO2 core–shell par-ticles were investigated. Fig. 4 plots the fluorescence emis-sion spectra of the particles as a function of reaction time.With the increase of reaction time the fluorescence emis-sion peak of MEH-PPVm/SiO2 core–shell particles wasblue-shifted from 550 nm (Fig. 4(a)) to 415 nm (Fig. 4(e)).The saturation conjugation length of PPV derivatives hasbeen reported to be around 7–10 repeating units [38].For a PPV chain of 1000 monomer units, if it contained300 cis-vinylene bonds, the conjugation length would des-cend to approximately 4 [39]. The reason for the blue-shiftcould mainly be attributed to the shortening conjugatedlength of MEH-PPV owing to the increase of cis-vinyleneand tert-methyl [35] on the backbones of MEH-PPVthrough the addition of active free radicals and the elimi-

nation reaction. The resulting MEH-PPVm/SiO2 core–shellparticles may be used as biolabeling materials becausethey could emit light with different wavelength.

3.3. Discussion on mechanism of the self-templating method

Mechanism of the self-templating method is not clear.Ahn et al. [40] has proposed a mechanism during the prep-aration of micrometer level microcapsules. The mixture ofSiO2 colloid particles and the core compounds forms oilemulsions in an aqueous solution, followed by solidifica-tion reaction from the outer surface toward the center ofthe emulsion. Once the outer surface of the emulsion issolidified, it may serve as a self-template. Subsequently,the un-reacted precursor within the emulsion becomessolidified onto the external shell, while the core com-pounds stay inside as core, producing core-shell particles.

In this paper, sub-micrometer level core–shell particleswere prepared using this self-templating method. The stir-ring rate of stirrers plays an important role in the process,and it controls the particle size (Fig. 5). While the O/Wemulsion is formed under a common stirrer with low stir-ring rate, big emulsion droplets were obtained owing tothe agglomeration, resulting in the micrometer-size core–shell particles with wide particle size distribution. How-ever, a homogenizer with high stirring rate could keepthe stability of the emulsion system and prevent emulsiondroplets from agglomerating. Therefore, the sub-microme-ter-size core–shell particles were produced.

4. Conclusions

In summary, we demonstrated a self-templating meth-od to synthesize MEH-PPV/SiO2 core–shell particles by asingle-step O/W emulsion system without using a tem-plate. The stirring rate played an important role in the pro-cess, and it controlled the size of particles. MEH-PPVm/SiO2

core–shell particles were obtained and their fluorescent

Fig. 5. The effect of stirring rate on the preparation of core-shell particleswith the self-templating method.

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properties were investigated. With the increase of reactiontime the fluorescence emission peak of MEH-PPVm/SiO2

core–shell particles was blue-shifted from 550 nm to415 nm. The obtained MEH-PPVm/SiO2 core–shell particlesmay be used as biolabeling materials. The process for pre-paring core–shell particles with this method exemptedfrom removing the template and reduced reaction stepscompared to the conventional process. This one-step routemay also be used to synthesize other core–shell particlesand offer new opportunities for a wide range ofapplication.

Acknowledgment

We thank the National Natural Science Foundation ofChina for financial support of this research (No. 50673033).

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