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Materials Chemistry and Physics 116 (2009) 330–334

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

aterials science communication

abrication of size-controllable hollow nano-spheres based on polyimidesomposites

ing Jinga, Shujiang Dingb, Chengliang Zhangb, Chunhai Chena, Xianhua Raoa,uodong Danga, Zhenzhong Yangb, Hongwei Zhoua,∗

Alan G MacDiarmid Institute, Jilin University, Qianjin Street 2699, Changchun 130012, ChinaState Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China

r t i c l e i n f o

rticle history:

a b s t r a c t

In this communication, uniform and size-controllable PI composite hollow spheres were fabricated by

eceived 28 December 2008eceived in revised form 27 March 2009ccepted 18 April 2009

eywords:olymers

means of in situ favorable adsorption within sulfonated polystyrene hollow sphere templates. It wascalculated that more than 33 wt.% polyimides existed on the resulting composite hollow spheres, whichendued them with excellent high-temperature resistance. The high carbon remaining and favorable shapereservation of the PI-based composite hollow spheres made them promising precursors of carbon hollowspheres.

© 2009 Elsevier B.V. All rights reserved.

anostructuresomposite materials

. Introduction

Polyimides (PIs) have been exploited as merchandises in theorms of films, model plastics, fibers and dopes, etc. owing toheir excellent thermal, mechanical and dielectric properties [1].

ith the developments of nanotechnology, how to process thisind of high-performance engineering plastics into functionalano-materials becomes an urgent demand. So a variety of nano-aterials based on polyimides have been fabricated, such as

articles [2–4], sheets [5], LB films [6], fibers [7], micelles [8] andollow spheres [9,10]. Therein, the PI-based hollow spheres havettracted more attention for their potential applications: such asow dielectric materials, high-temperature nano-containers andonfined reaction vessels with heat resistance, combining both thexcellent thermal performances of the polyimides and the advan-ages of hollow structures [11,12]. However, it is really a hard worko introduce these kinds of rigid structure into nano- or micro-

aterials owing to their intractability. Many approaches such asispersion polymerization [10] and self-assembling method [8]ave been taken to overcome this problem, but most of the finalpheres exhibited poor sphericity or only fit for special structural

olyimide. Herein, we report a facile and general preparation of PIollow spheres with uniformly controllable sizes by using highlyross-linked sulfonated polystyrene gel (SPS) hollow spheres ashe templates. On one hand, the hollow template-assistant syn-

∗ Corresponding author. Tel.: +86 0431 85168335; fax: +86 0431 85168335.E-mail addresses: cch@jlu.edu.cn, zhw@jlu.edu.cn (H. Zhou).

254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2009.04.043

thesis can avoid the fracture brought by core-sacrificed method.On the other hand, sulfonated polystyrene gel shell is infiltrativefavoring polyimides precursors (PAA) to penetrate into the shelland form desired configuration with specific interaction. Moreover,the highly cross-linked hollow spheres can maintain the sphericalcontour during the template synthetic process to make the poly-imides composite hollow spheres possess the same shapes as theoriginal hollow spheres. Two kinds of polyimides have been cho-sen to confirm the general utility of this method. One is a kind ofmacromolecular, which has the same structure of Upilex-R; whilethe other one is a kind of oligomer, which might infiltrate the shellof cross-linked sulfonated PS hollow spheres more easily and thenform polymer by imidization and cross-linking between cappedacetylene. The transmission electron microscopy (TEM) and scan-ning electron microscopy (SEM) images revealed that the obtainedparticles were kinds of quasi-closed-hole polyimides-C (PI-C) com-posite hollow spheres without connecting, which had potentialapplications as heat preservation, thermal insulation and sound-absorb materials.

2. Experimental part

2.1. Characterization

The samples were dispersed in alcohol under sonication for 5 min, and one drop

of the suspension was placed on a silicon substrate or Cu grid, dried in air. Scan-ning electron microscopy (SEM) measurements were performed with a HITACHIS-4300 apparatus operated at an accelerating voltage of 15 kV and the samples weresputter-coated with about 3 nm of Pt. The transmission electron microscopy (TEM)measurements were carried out with a JEOL 100CX operated at 100 kV. IR spectrawere measured using BRUKER EQUINOX 55 FT-IR spectrometer by incorporating the

try an

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J. Jing et al. / Materials Chemis

ried spheres in KBr disks. The thermogravimetric analyses (TGA) were measured byerkinElmer TGA 7 under N2 atmosphere, and the heating rate was 5 ◦C min−1. Thehermal stabilities were evaluated under air atmosphere, and the heating rate was◦C min−1. Elemental analyses were carried out using a Flash EA-1112 apparatus.

.2. Materials

Polyimides-1 were synthesized via traditional polycondensation of the pre-ursors PAA-1, which had been prepared from 3,3′ ,4,4′-biphenyl tetracarboxylicianhydride (s-BPDA) and 4,4′-diaminodiphenyl ether (4,4′-ODA). The inherentiscosity (�inh) in N,N-dimethylacetamide(DMAc) of PAA-1 was 1.22 dL g−1, aseasured at a concentration of 0.5 g dL−1 at 25 ◦C. And polyimides-2 were synthe-

ized from amide acid oligomer (Mw = 6468 g mol−1, calculated), which had been

Scheme 1. Chemical struct

d Physics 116 (2009) 330–334 331

prepared from 4-phenylethynylphthalic anhydride (PEPA), s-BPDA and 2,5-bis(4-aminophenoxy)-biphenyl(p-TPEQ) [13] (Scheme 1).

Biphenylene tetracarboxylic dianhydride (s-BPDA) was purchased from ChriskevCompany Inc., mp 305–308 ◦C, 4-phenylethynylphthalic anhydride (PEPA) wasfrom MANAC Corp., Japan, mp 152 ◦C, 4,4′-diaminodiphenyl ether (4,4′-ODA)was from Sinopharm Chemical Reagent Co. Ltd., mp 189–191 ◦C, and 2,5-bis(4-aminophenoxy)-biphenyl(p-TPEQ) was synthesized according to the literature [13].

DMAc was purified by distillation under reduced pressure, and all other reagentsand solvents were used and received from commercial sources.

The highly cross-linked sulfonated polystyrene (SPS) hollow spheres in thediameter of 500 nm were synthesized as described in Ref. [14]. The sulfonation reac-tion was taken place at 40 ◦C for 3 h. The elemental analysis data showed that thecontent of sulfuric atom in the SPS hollow spheres was 4.00 wt.%.

ures of PI-1 and PI-2.

332 J. Jing et al. / Materials Chemistry and Physics 116 (2009) 330–334

F low spr

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ig. 1. Morphologies of the template hollow spheres: (a) SEM image of the SPS holemoved), inset TEM image.

.3. Preparation of the PAA-based hollow spheres

The first step in the preparation of the PI-based hollow spheres involved fab-ication of the PAA–SPS composite hollow spheres. 0.1 g SPS hollow spheres wereispersed in 5 g DMAc, after stirring for 24 h, 0.4 g PAA dispersed in 5 g DMAc wereropped into the solution. After stirring for another 24 h, the excess PAA wereemoved by centrifugation (12,000 rpm, 10 min) and washed with DMAc for threeimes.

ig. 2. Morphologies of the hollow spheres: (a) SEM image of the PI-1–SPS hollow sphereEM image; (c) SEM image of the PI-1 composite hollow spheres cured up to 350 ◦C undeeat treatment, inset TEM image; (e) SEM image of the PI-2 composite hollow spheres cu

heres, inset TEM image; (b) SEM image of the SPS hollow spheres (linear SPS were

2.4. Preparation of the PI-based hollow spheres

The PI–SPS hollow spheres were prepared via imidization. Firstly, 0.12 mL aceticanhydride (Ac2O) and 0.05 mL pyridine were dropped into the PAA–SPS solution(dispersed in DMAc), and stirred for 48 h at room temperature. After water andethanol (1/1, v/v) were dropped into the mixture under stirring, the solution waskept still for 1 h to deposit the PI–SPS composite hollow spheres. Then, the powderof PI–SPS composite hollow spheres were obtained after centrifugation (10,000 rpm,

s, inset TEM image; (b) SEM image of the PI-2–SPS composite hollow spheres, insetr N2 atmosphere, inset TEM image; (d) SEM image of the SPS hollow spheres afterred up to 350 ◦C under N2 atmosphere.

J. Jing et al. / Materials Chemistry and Physics 116 (2009) 330–334 333

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hollow spheres have considerably high carbon remaining (higherthan 80%) and shape reservation as well (Fig. 6). Hence, such poly-imides spheres may have potential applications as carbon hollowsphere materials, such as light-weight fillers and thermal insula-

Fig. 3. A schematic illustration o

0 min) and washed with water and ethanol for three times. Finally, a completehermo-imidization was carried out at 350 ◦C (PI-1 as a sample, the cross-linkingemperature of PI-2 was 370 ◦C) under flowing N2 atmosphere to give the PI–C hollowpheres.

. Results and discussion

The highly cross-linked PS hollow spheres which had been sul-onated for 3 h were chosen as the template due to their shapereservation ability against strong dipolar aprotic solvent and theigh concentration of sulfonic groups, which theoretically played aey role in the adsorption of amide acid.

For Fig. 1a, the templates SPS hollow spheres were mainly com-osed of highly cross-linked and linear sulfonated polystyreneFig. 1a) [14]. As a result, when the samples were dispersed inMAc, the linear polystyrene would be dissolved, leaving some tinyhannels on the shells which were preferable for the endurance ofhe amide acid, while the shapes of spheres were still maintainedFig. 1b).

During the process for PAA expanding through the whole shellf SPS hollow spheres, the PAA were favorably absorbed mainlyhrough the ion interactions, so the PAA–SPS composited hollowpheres were obtained. After chemical imidization, we can deducehat the polyimide-1 had penetrated the whole shells of the hollowpheres because of the appearance of the branches around evennside the shell (Fig. 2a). Notably, the diameters of PI–SPS hollowpheres were about 500 nm, almost the same as those of the tem-late hollow spheres. Furthermore, the PI–SPS hollow spheres hadmaller cavities due to the inner polyimides. The SEM and TEMicrographs of PI-2–SPS hollow spheres were similar to those of PI-

–SPS hollow spheres, as was shown in Fig. 2b. The whole processas illustrated in Fig. 3.

After the complete imidization at 350 ◦C, the resulting spheresould still keep good geometry, and turned into black, whichmplied the carbonization of SPS hollow spheres. The elementalnalysis results verified this assumption, and the contents of C innal hollow spheres were higher than 85 wt.%, which were muchigher than pure PI-1 (67% by TGA). In order to study the effect ofhe calcination on the SPS and the PI–SPS hollow spheres, we com-ared their SEM and TEM micrographs after heating at 350 ◦C underhe same conditions. It was found that the calcinated SPS spheresFig. 2d) became smaller with diameter of 250 nm with washy-olored shells (about 80 nm), while the diameters of PI-1–SPSollow spheres were on an average of 400 nm (Fig. 2c, PI-1). It waslso suggested that polyimides had successfully infiltrated withinhe template spheres. According to the rigid structure of polyimides,he PI–SPS hollow spheres showed remarkable mechanical stabil-ty of both composite and calcinated spheres, the broken spheres

ppeared only after calcination, and the low ratio of the brokenpheres to the whole ones under sonication and the high-vacuumEM analysis conditions verified the good mechanical stability of PIomposite hollow spheres [15,16]. Notably, we can observe all theroken spheres with hollow structures in Fig. 2e, which confirmed

ormation of the hollow spheres.

the hollow structure of PI composite spheres (Fig. 2e, PI-2–SPS hol-low spheres after heat treatment at 370 ◦C in N2).

The FT-IR spectroscopy can further confirm the formation ofpolyimide on the composite spheres, since the typical peaks at 1780,1720 cm−1 ascribed to �C O and at 1380 cm−1 due to �C–N in imidecould be found in Fig. 4.

To calculate the content of polyimides on the resulting spheres,it is presumed that the polyimides have been imidized completelyafter thermal treatment, so the N element theoretically only comesfrom PIs, thus the PI content (CP) can be expressed by

CP = CNWP

n × 14.01(1)

where CN is the content of N in PI–SPS hollow spheres by ele-mental analysis, WP denotes the mol weight of a polyimide repeatunit, n is the number of nitrogen (N) in the unit, and 14.01 is themol weight of N. Therefore, it is figured out that the PI-1–SPS hol-low spheres contains 46.8 wt.% PI-1 (CN1 = 2.64 wt.%), and PI-2–SPScontains 33.98 wt.% PI-2 (CN2 = 1.47 wt.%).

The thermal stabilities of the PI composite hollow spheres wereevaluated by TGA (Fig. 5). The 5% weight loss temperature of PI-1 was 547 ◦C in N2 and 369 ◦C in air, while the 5% weight losstemperature of PI-2 was 583 ◦C in N2 and 362 ◦C in air. We foundthat these spheres were thermally stable up to 400 ◦C in N2. Thebeginning decomposition temperatures were around 430 ◦C, whichindicated that the polyimides hollow spheres had an excellenthigh-temperature resistance. Moreover, we can also find that these

Fig. 4. FT-IR spectra of the polyimides hollow spheres (after thermo-imidization):(a) PI-2 hollow spheres; (b) PI-1 hollow spheres.

334 J. Jing et al. / Materials Chemistry an

Fig. 5. TGA of the polyimides hollow spheres: (a) PI-2 hollow spheres in N2; (b) PI-1hollow spheres in N2; (c) PI-1 hollow spheres in air; (d) PI-2 hollow spheres in air.

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ig. 6. TEM images: (a) SPS hollow spheres after TGA in N2 and (b) PI-1 compositeollow spheres after TGA in N2.

ors at high temperature [17]. If we go deep inside the carbonationf those polyimides hollow spheres, we can get excellent carbonollow spheres [18].

. Conclusions

In this article, uniform dispersed polyimides composite hollowpheres have been prepared through template strategy. Compar-ng with self-assembly and dispersion polymerization methods for

roducing polyimides hollow spheres, our approach seems to beuch simpler and general. The resulted polyimide-based hollow

pheres with an average diameter of 500 nm are high-temperatureesistant. The thin shell, low density (�), analogous-closed caseody and excellent performances of polyimides endue the hollow [

d Physics 116 (2009) 330–334

spheres with potential applications in heat resisting, anacoustic andheat-insulated materials.

Acknowledgements

Thanks for the financial supports from the National NatureScience Foundation of China (National 863 Program No.G2007AA03A231) and Support Plans of Jilin Province ScientificDepartment.

References

[1] Y. Imai, R. Yokota, Saishin Polyimide, Kiso To Ouyou, 1st edition, NTS Inc., 2004.[2] T.H. Kim, C.D. Ki, H. Cho, T. Chong, J.Y. Chang, Macromolecules 38 (2005)

6423–6428.[3] (a) K. Wakabayashi, S. Kohama, S. Yamazaki, K. Kimura, Polymer 48 (2007)

458–466;(b) G. Zhao, T. Ishizaka, H. Oikawa, H. Nakanishi, Chem. Mater. 19 (2007)1901–1905.

[4] S. Omi, M. Saito, T. Hashimoto, M. Nagai, G. Ma, J. Appl. Polym. Sci. 68 (1998)897–907.

[5] M. Groenewolt, A. Thomas, M. Antonietti, Macromolecules 37 (2004)4360–4364.

[6] M. Kakimoto, M. Suzuki, T. Konishi, Y. Imai, M. Iwamoto, T. Hino, Chem. Lett.(1986) 823–826.

[7] C. Huang, S. Chen, D.H. Reneker, C. Lai, H. Hou, Adv. Mater. 18 (2006) 668–671.[8] (a) H. Duan, D. Chen, M. Jiang, W. Gan, S. Li, M. Wang, J. Gong, J. Am. Chem. Soc.

123 (2001) 12097–12098;(b) M. Mu, F. Ning, M. Jiang, D. Chen, Langmuir 19 (2003) 9994–9996;(c) H. Duan, M. Kuang, J. Wang, D. Chen, M. Jiang, J. Phys. Chem. B 108 (2004)550–555;(d) M. Kuang, H. Duan, J. Wang, M. Jiang, J. Phys. Chem. B 108 (2004)10629–16023;(e) D. Chen, M. Jiang, Acc. Chem. Res. 38 (2005) 494–502.

[9] D.V. Andreeva, D.A. Gorin, H. Möhwald, G.B. Sukhorukov, Langmuir 23 (2007)9031–9036.

10] S. Wantanabe, K. Ueno, K. Kudoh, M. Murata, Y. Masuda, Macromol. Rapid Com-mun. 21 (2000) 1323–1326.

11] (a) F. Caruso, Chem. Eur. J. 6 (2000) 413–419;(b) F. Caruso, R.A. Caruso, H. Möhwald, Science 282 (1998) 1111–1114;(c) E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Möhwald, Angew. Chem.Int. Ed. 37 (1998) 2202–2205.

12] C. Niemeyer, C. Mirkin, in: C. Niemeyer, C. Mirkin (Eds.), Nanobiotechnology:Polymer Nanocontainers, Wiley-VCH, Weinheim, 2004, pp. 168–184.

13] X. Rao, H. Zhou, G. Dang, C. Chen, Z. Wu, Polymer 47 (2006) 6091–6098.14] S.J. Ding, C.L. Zhang, M. Yang, X.Z. Qu, Y.F. Lu, Z.Z. Yang, Polymer 47 (2006)

8360–8366.15] T. Chen, P.J. Colver, S.A.F. Bon, Adv. Mater. 19 (2007) 2286–2289.16] Z. Dai, L. Dähne, H. Möhwald, B. Tiersch, Angew. Chem. Int. Ed. 41 (2002)

4019–4022.17] (a) M. Yang, J. Ma, S. Ding, Z. Meng, J. Liu, T. Zhao, L. Mao, Y. Shi, X. Jin, Y. Lu, Z.

Yang, Macromol. Chem. Phys. 207 (2006) 1633–1639;(b) J. Jang, H. Ha, Chem. Mater. 15 (2003) 2109–2111;(c) Y. Xiong, Y. Xie, Z. Li, C. Wu, R. Zhang, Chem. Commun. (2003) 904–905;

(d) J.W. Patrick, Porosity in Carbons: Characterization and Applications, EdwardArnord, London, 1995;(e) K. Kinoshita, Carbon, Electrochemical and Physicochemical Properties,Wiley, New York, 1998;(f) H. Marsh, Introduction to Carbon Science, Butterworths, London, 1989.

18] K.S. Yang, et al., Carbon 41 (2003) 2039–2046.