Polymer International Polym Int 57:449–453 (2008)

Rapid ReportPreparation of novel hybridinorganic–organic hollow microspheres via aself-template approachYan Zhu, Jianwei Fu, Lu Zhu, Xiaozhen Tang and Xiaobin Huang∗School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

Abstract

BACKGROUND: Hollow microspheres, especially biodegradable polymeric microspheres, have attractedconsiderable attention due to their particular characteristics. Up to now, microspheres have been preparedvia various strategies, for instance the template synthesis method and the self-assembly process. However,economic, novel and simple methods to prepare hollow microspheres are still being sought.

RESULTS: Phosphazene-containing microspheres, which contain self-assembled core-shell structures, wereprepared at high colloid contents using an ultrasonic bath via a self-template approach. Along with thecontrolled self-degradation of the internal core, the corresponding hybrid inorganic–organic hollow microspheresappeared. The mechanism was evidenced by means of transmission and scanning electron microscopy, cross-polarization with magic angle spinning NMR, Fourier transform infrared spectroscopy, X-ray diffraction andthermogravimetric analysis.

CONCLUSION: It was clarified that the phosphazene-containing microspheres could be formed and stablydispersed without aggregation even at high colloid contents using the ultrasonic bath method and the microspherescontain self-assembled core–shell structures. Along with the controlled self-degradation of the internal core, thecorresponding hollow microspheres appeared. The mechanism of this preparation is of great significance becauseit is completely different from the conventional template synthesis method and the self-assembly process. Theabsence of any stabilizing agent and special templates might inspire creative imagination in the design of newmorphologies of micro- and nanostructures. 2007 Society of Chemical Industry

Supplementary electronic material for this paper is available in Wiley InterScience at http://www.interscience.wiley.com/jpages/0959-8103/suppmat/

Keywords: hybrid inorganic–organic; microstructure; self-assembled core–shell polymers; self-templateapproach; self-degradation

INTRODUCTIONHollow microspheres have attracted considerableattention because their attractive characteristics, forinstance low density, thermal resistance, large specificsurface area and shell permeability, may find awide range of potential applications from versatilemicroreactors to controlled storage and releasecontainers.1 In general, strategies for preparing hollowmicrospheres rely on the use of templates,2–5 andfor preparing nanosized hollow spheres usually relyon the self-assembly of amphiphilic molecules suchas block copolymers.6–8 In a typical procedure toprepare microspheres, the templates such as polymers,silica and metal particles are coated with a thin shellof desired materials, and then the hollow structure

is created by the removal of the template.9 Sofar, hollow microspheres comprising a variety ofmaterials including polymers,2 metals,3 ceramics4

and composites5 have been successfully synthesizedthrough this strategy.

Because of their encapsulation properties, hol-low microspheres, especially biodegradable polymericmicrospheres, are expected to be very useful inmany novel biomedical applications such as chemi-cal storage and controlled drug delivery.10–12 Up tonow, microspheres composed of biodegradable poly-mers have been prepared by various methods suchas phase separation,13 solvent evaporation,14 solventextraction,15 spray drying,16 cold precipitation,17 andso on.

∗ Correspondence to: Xiaobin Huang, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240,ChinaE-mail: roy zhu@sjtu.edu.cn(Received 20 April 2007; accepted 22 June 2007)Published online 9 October 2007; DOI: 10.1002/pi.2366

2007 Society of Chemical Industry. Polym Int 0959–8103/2007/$30.00

Y Zhu et al.

Hexachlorocyclotriphosphazenes (HCCPs), asimportant inorganic rings, play a crucial role in thedevelopment of new polymers.18 Although severalmicrospheres based on polyphosphazenes have beenprepared for biomedical applications because ofthe tunable biodegradability of their backbone andunprecedented structural diversity,19 unfortunatelyfew researches have been reported for the preparationof microspheres containing cyclotriphosphazene rings.Very recently, our group demonstrated for the firsttime the preparation of poly[cyclotriphosphazene-co-(4,4′-sulfonyldiphenol)] (PZS) nanotubes, aphosphazene-containing hybrid inorganic–organicmaterial, via an in situ template approach,20 and thepreparation of fully crosslinked PZS microspheres viaprecipitation polymerization by stirring with a mag-netic stirrer.21 This shows that HCCPs possess specialpotential in the fabrication of molecular-level inor-ganic–organic hybrid micro- and nanoscale materials.

According to our previous study,20 small moleculeswere generated during the polycondensation and therelated compounds could act as the templates tofabricate micro- and nanoscale materials. This methodwas termed an in situ template approach. Recently,we prepared micro- and nanoscale materials usingmicropolymer or nanopolymer-containing compositesformed by polycondensation as the templates, whichwe term a self-template approach. In this approach,the major advantage is the absence of both surfactantsand special templates. This interesting approach mightopen a novel, simple, economic and environmentallyfriendly route to preparing micro-/nanostructures.

Here we report novel cyclotriphosphazene-containing hybrid inorganic–organic hollow micro-spheres fabricated via a self-template approach.We prepared phosphazene-containing microspheres,which contain self-assembled core–shell structures, athigh colloid contents using an ultrasonic bath. Alongwith the controlled self-degradation of the internalcore in water, the corresponding hollow microspheresappeared. We suggest this interesting approach couldinspire creative imagination in the design of new mor-phology of micro- and nanostructures.

MATERIALS AND METHODSMaterialsHCCP (Aldrich) was recrystallized from dry hexanefollowed by sublimation twice. The melting pointof the purified HCCP was 113–114 ◦C. 4,4′-Sulfonyldiphenol (BPS) and triethylamine (TEA)were purchased from Shanghai Chemical ReagentsCorp. (Shanghai, China) and used without furtherpurification.

Preparation of microspheres via a self-templateapproachTEA (0.87 g, 8.64 mmol) was added to a solu-tion of HCCP (0.50g, 1.44 mmol) and BPS (1.08 g,

4.32 mmol) in acetone (100 mL). The reaction mix-tures were stirred at 30 ◦C in an ultrasonic bath(100 W, 80 kHz) for about 4 h. The resultant particleswere obtained by centrifugation and then washed withthree times with tetrahydrofuran. Finally, the productwas dried under vacuum to yield PZS microspheresas a white powder (0.99 g, yield 87%, calculated fromHCCP).

Preparation of hollow microspheres viacontrolled self-degradationCrosslinked PZS microspheres (0.50 g) were addedinto water (80 mL) and the mixture was stirred witha magnetic stirrer and refluxed for 48–72 h. Theresultant hollow microspheres with different size wereobtained by centrifugation.

Microsphere characterizationThe morphologies of hollow PZS microsphereswere investigated with a JEOL JSM-7401F fieldemission SEM instrument and a JOEL JEM-100CXtransmission electron microscopy (TEM) instrument.The samples for SEM measurements were mountedon aluminium studs using adhesive graphite tape andsputter coated with gold before analysis. For TEMmeasurements, the samples were dispersed on reticularcopper coated with carbon support film. The averageouter diameter of the hollow microspheres and theaverage diameter of the hole in the surface wereobtained by measuring 15–20 microspheres in theSEM images with Image Tool software.

Solid-state NMR spectra were obtained using aVarian Mercury plus 400 spectrometer operatingat 100.6 MHz for 13C and 162.0 MHz for 31P.All experiments were performed at ambient probetemperature using hydrogen high-power decoupling.Cross-polarization with the magic angle spinning(CP/MAS) technique was adopted. The Fourier trans-form infrared (FTIR) spectroscopy measurementswere conducted using a Perkin-Elmer Paragon 1000spectrometer at room temperature (25 ◦C) with KBrpellets. XRD patterns were recorded using a BrukerD8 Advance instrument equipped with Cu Kα radia-tion performed at 40 kV and 40 mA. The scan rangewas 1.0◦ min−1 from 5 to 60◦ (2θ). TGA was carriedout using a TGA 7/DX thermogravimetric analyzerwith a heating rate of 20 ◦C min−1 from room temper-ature to 800 ◦C under a nitrogen atmosphere.

RESULTS AND DISCUSSIONScheme 1 displays the reactions to prepare the highlycrosslinked PZS microspheres. In a solution of ace-tone, the polymerization was performed using theultrasonic bath with TEA as an acid acceptor. Thepolycondensation of HCCP with equimolar BPSgenerated precursor and hydrogen chloride. TEAabsorbed HCl forming TEA/HCl, which acceleratedthe polymerization. With the polymerization proceed-ing, crosslinked polymers were produced. According

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Hybrid inorganic–organic hollow microspheres via a self-template approach

Scheme 1. Synthetic route and chemical structure of PZS microspheres.

to our previous studies,20 due to high surface energy,when it was precipitated out from the medium,TEA/HCl would absorb the incomplete synthesizedpolymers and form micro- or nanocomposites. Withthe aggregation of those composites, primary stablecores were generated. The cores continued growingby absorbing oligomeric species, instead of primaryparticles, which contain much higher crosslinkedchemical structure, and the microspheres containingself-assembled core–shell structures appeared.

13C and 31P CP/MAS NMR spectra of PZSmicrospheres are in the supplementary material.The 13C NMR spectrum shows phenylene grouppeaks at 153, 139, 129 and 122 ppm. In the 13CCP/MAS NMR spectrum, no resonance of carbon at161 ppm was detected, suggesting that all phenolichydroxyl groups of BPS had completely reactedwith HCCP. In the 31P NMR spectrum of PZSnanotubes, two resonance signals appeared at 32and 19 ppm, indicating the presence of structuralunits –N=P(–(OPh)2)–and –N=P(–OPh)(–Cl)–,which could be attributed to the incomplete chemicalstructure of the cores. Moreover, it was demonstratedthat the microspheres containing inorganic rings weremolecular-level inorganic–organic hybrid microscalematerials.

The FTIR spectra of the monomers, PZS micro-spheres (compound 1) and PZS microspheres thatwere refluxed for 2 h (compound 2) are in the

supplementary material. The peaks at 1588 and1490 cm−1 corresponded to the phenyl absorption ofsulfonyldiphenol units. The characteristic peaks forO=S=O and P=N, can be seen at 1292, 1152 and1186 cm−1. The intense absorption peak at 942 cm−1

was assigned to the P–O–(Ph) band, which was evi-dence of the condensation of monomer HCCP andBPS. It was noted that (in the supplementary material)the peaks at 2983, 2946 and 2670 cm−1 were assignedto the C–H stretching vibration of the methyl groupsand methene groups and the N+ –H stretching vibra-tion of the tertiary amine salt, which was significantevidence to prove the existence of TEA/HCl. Accord-ing to our pervious study, 20 TEA/HCl was removedeasily during refluxing in water, even that embeddedin the highly crosslinked PZS particles. XRD for com-pounds 1 and 2 was also carried out to further provethis result. As shown in the supplementary material, itwas obvious that compound 1 was a mixture of amor-phous and crystalline materials and compound 2 wasamorphous. Moreover, the XRD pattern of compound1 can be indexed to TEA/HCl with a lattice constantof a = 8.740 A, consistent with the standard value(JCPDS file 38–1974) within experimental error. Thesame result was found from the TGA, as shown inthe supplementary material. There was significant dif-ference between compounds 1 and 2. The onset ofthe thermal degradation temperature of compound

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1 was 204 ◦C, and there was another obvious ther-mal degradation at 498 ◦C. However, the onset of thethermal degradation temperature of compound 2 was532 ◦C. This may due to the low molecular weight andlow thermal stability of TEA/HCl. Therefore, as indi-cated in the supplementary material, it was clear thatcompound 1 contained 31.4% TEA/HCl. In addition,from TGA, compound 2 exhibited superior thermalproperty.

Because the TEM micrographs of compound2 (Fig. 1(a)) showed no hollow existed in themicrospheres, and the results of FTIR, XRD andTGA had demonstrated the absence of TEA/HCl incompound 2, the mechanism for preparing hollowPZS microspheres was certainly completely differentfrom that using special templates or via a self-assemblyprocess. In our previous study,22 we demonstratedthe biodegradability of materials prepared with PZS.Therefore, the mechanism was initially assumed thatthe composites inside which played the role of primarycores in the fabrication of PZS microspheres containedincompletely crosslinked chemical structures andabout 30% TEA/HCl, and with TEA/HCl dissolvinginto water during refluxing, they might change intohoneycombed structures, which would accelerate theself-degradation of PZS. During refluxing, the self-degradation occurred from inside to outside and

the hollow structures appeared. Figure 1(b) showsthe TEM micrographs of PZS hollow microspheresprepared by self-degradation for 48 h directly. Figure 2shows a schematic of the mechanism.

Figure 3(a) shows the SEM micrograph of com-pound 1. The composites comprised regular micro-spheres of 0.7–0.9 µm in diameter, without any disfig-urement on the surface. In addition, comparing withour previous study,21 we found the PZS microspherescould be prepared at 5 times higher concentration ofHCCP without coagulation using the ultrasonic baththan under magnetic stirring. From Figs 3(b)–(d),it is obvious that when the PZS microspheres wererefluxed in water for more than 60 h, with furtherself-degradation, a hole was formed at the surface ofeach microsphere and the size of the hole increasedstepwise. Moreover, the configuration of the hollowmicrospheres such as the size of hole at the surface canbe controlled easily by different refluxing times.

CONCLUSIONSHollow phosphazene-containing hybrid inorganic–organic microspheres have been prepared via a self-template approach. It was clarified that the micro-spheres could be formed and stably dispersed withoutaggregation even at high colloid contents using an

Figure 1. TEM images of PZS microspheres refluxed in water for increasing refluxing time: (a) 2 h (compound 2) and (b) 48 h.

Figure 2. Mechanism of PZS hollow microsphere fabrication.

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Figure 3. SEM images of PZS microspheres refluxed in water for increasing refluxing time: (a) 0 h, (b) 60 h, (c) 66 h and (d) 72 h.

ultrasonic bath and that the phosphazene-containingmicrospheres contain self-assembled core–shell struc-tures. Along with the controlled self-degradation of theinternal core, the corresponding hollow microspheresappeared. The mechanism was proved by means ofTEM, SEM, CP/MAS NMR, FTIR, XRD and TGA.It is significant that this method is completely dif-ferent from the use of special templates and theself-assembly process. This interesting approach mightopen a novel, simple and economic route to preparingmicro-/nanomaterials with special morphologies.

Supplementary materialSupplementary electronic material for this paperis available in Wiley InterScience at: http://www.interscience.wiley.com/jpages/0959-8103/suppmat/.

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