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Materials Science and Engineering B 153 (2008) 62–65

Contents lists available at ScienceDirect

Materials Science and Engineering B

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orphology control between microspheres and nanofibers by solvent-inducedpproach based on crosslinked phosphazene-containing materials

an Zhu, Xiaobin Huang ∗, Jianwei Fu, Gang Wang, Xiaozhen Tang ∗

chool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Road,hanghai 200240, China

r t i c l e i n f o

rticle history:eceived 17 July 2008eceived in revised form 12 October 2008ccepted 24 October 2008

eywords:

a b s t r a c t

Multi-morphology control between monodisperse microspheres and uniform nanofibers was successfullyachieved by adjusting the ratio of solvent composition. Through the condensation polymerization betweenhexachlorocyclotriphosphazene and 4,4′-sulfonyldiphenol, the corresponding hybrid inorganic–organicmaterials appeared. The morphology of both microspheres and nanofibers contained excellent sizeand shape: the monodisperse microspheres with 0.7–0.9 �m in diameter and the uniform nanofibers

orphology transitionolymersolvent inducedhosphazeneano-composite

with 60 nm in outer diameter. We applied the concept of three-dimensional Hansen solubility param-eters for the initial explanation. The activity of the primary colloid particles and the solubility oftriethylamine–hydrogen chloride crystal were considered as two factors for the mechanism explana-tion. This interesting research shows that the nano- and micro-materials with high crosslinked moleculestructure and prepared by condensation polymerization can also achieve the morphology transition. Itfills the blank in nano-morphology transition research and will provide great information for the research

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. Introduction

During the past decade, the nano- and micro-materials, espe-ially those with desired size and morphology were extensivelyxplored and a wide variety of morphologies have been discov-red. The controlled transition between different morphology haslso been paid considerable attention [1,2]. In general, to polymeraterials, the special architectures depend not only on the molecu-

ar parameters but also on some factors such as solvent compositionnd the presence of additives. Up to now, some groups have suc-eeded in achieving the transition by controlling these parameters3,4].

However, most of the researches about the morphology tran-ition were focused on the amphiphilic molecules due to theelf-assemble behavior [5]. Only a few literatures mentioned theorphology transition with crosslinked molecule structure pre-

ared by free radical polymerization [6]. But the morphologies of

nal product were usually far from desired requirement. Moreoverhere were few publications about the nanomaterials’ morphologyransition fabricated by condensation polymerization.

∗ Corresponding author. Tel.: +86 21 54747142; fax: +86 21 54741297.E-mail address: xbhuang@sjtu.edu.cn (X. Huang).

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921-5107/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rigoi:10.1016/j.mseb.2008.10.027

orphology preparations based on polymer nanomaterials.Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

Hexachlorocyclotriphosphazenes (HCCPs), as very importantnorganic rings, play a crucial role in the development of new poly-

ers [7]. Up to now, there are many research reports to introducehe application of the materials containing phosphazene rings [8,9].owever, HCCPs were discovered to have special potential in the

abrication of micro- and nano-scale materials only in last severalears [10].

In previous study, our group also made some achievementsn the preparation of nano- and micro-materials with differ-nt morphology individually based on phosphazene rings byondensation polymerization [11–15]. Those obtained hybridnorganic–organic nano- and micro-materials, such as micro-pheres, nanotubes, nanofibers and microtubes. All expressedxcellent thermal stability, biocompatibility and biodegradability,hich showed great potential in the application of biomedical

rea.Here, we report a novel, economic and simple approach to

chieve the morphology control between nanofibers and micro-pheres based on the phosphazene containing materials. Wentroduce the component solvent system into the fabrication pro-ess. Only adjusting the solubility parameters of the system by

hanging the ratio of the component solvent, we succeed in obtain-ng the nano- and micro-scale materials with completely different

orphology. Moreover, the obtained nano- and micro-partialsxpress excellent morphology. This is very rare in previous researchbout morphology transition. This interesting result will provide

hts reserved.

and Engineering B 153 (2008) 62–65 63

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Table 1Three-dimensional Hansen solubility parameters of each component solvent.

Acetone Toluene Ia IIa IIIa IVa Va

�p 10.43 1.43 9.43 7.43 5.93 4.43 2.43��

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Y. Zhu et al. / Materials Science

reat information for the research about mechanism of nano-aterials’ morphology transition.

. Materials and methods

.1. Materials

Hexachlorocyclotriphosphazenes (HCCPs) (Aldrich) wereecrystallized from dry hexane followed by two-time sublimation.he melting point of the purified HCCP was 113–114 ◦C. 4,4′-ulfonyldiphenol (BPS) and triethylamine (TEA) were purchasedrom Shanghai Chemical Reagents Corp. (Shanghai, China) andsed without further purification.

.2. Prepare phosphazene containing nano- or micro-materials

The facile preparation of phosphazene containing nano- oricro-materials was carried out as follows (Scheme 1).Dissolve BPS (1.08 g, 4.32 mmol) and HCCP (0.50 g, 1.44 mmol)

n 80 ml, 60 ml, 45 ml, 30 ml and 10 ml of acetone, and add tolueneo make each total volume reach 90 ml. Then, triethylamine (0.87 g,.64 mmol) was added into the system. The reaction mixtures weretirred in an ultrasonic bath (100 W, 80 kHz) at room temperatureor 48 h. The resultant particles were obtained by centrifugationnd then were washed three times by using acetone and deionizedater. The solid was dried under vacuum to yield phosphazene

ontaining nano- or micro-materials directly as white powders.ynthesis yields ranged from 70 wt% to 85 wt%.

.3. Nano- or micro-material characterization

The morphologies of phosphazene containing nano- or micro-aterials were investigated by a JEOL JSM-7401F field emission

canning electron microscope (SEM). The samples for SEM mea-urements were mounted on aluminum studs using adhesiveraphite tape and sputter coated with gold before analysis.

. Results and discussion

In the past, we reported the formation of nanotubes [12], micro-ubes [14] and microspheres [11] based on phosphazene rings by

ontrolling the reaction condition. From these previous studies, weound it was easier to get microspheres in the solvent with higholarity, such as in acetone [11] and acetonitrile via precipitationolymerization [11]. However, in the low polar solvent, such as inetrahydrofuran, nanotubes and microtubes have more possibility

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Scheme 1. Synthetic route and chemical structure of ph

d 15.5 18.04 15.78 16.35 16.77 17.19 17.76h 6.95 2.05 6.41 5.32 4.5 3.68 2.59

a Calculated based on recipe and solubility parameters of acetone and toluene.

o be obtained via in situ template approach [12]. Here, based on theolarity and the dissolvent ability, we choose the mixed solvent ofcetone and toluene as the research system.

Fig. 1 depicts the SEM images of different products obtainedrom different mixed solvents. The images I–V show the resultroduct morphologies obtained from different mixed solvents cor-espondingly: (I) acetone/toluene = 8/1; (II) acetone/toluene = 2/1;III) acetone/toluene = 1/1; (IV) acetone/toluene = 1/2; (V) ace-one/toluene = 1/8 (volume ratio and total volume is 90 ml). It isbvious that when the mixed solvent contained less toluene and theolarity was higher, the microspheres were easier to be obtained.he monodisperse microspheres with 0.7–0.9 �m in diameter werebtained when the mixed solvent only contained 11.1% toluene.ith the ratio of toluene increased, some fibers were obtainedith 300 nm in outer diameter. Moreover, the microspheres also

howed irregular morphologies. From images II–V, it shows morebers were obtained when the mixed solvent contained moreoluene. When it contained 88.9% toluene, the uniform nanofibersre obtained with 60 nm in outer diameter.

In Goh and Stövers’ [6] previous research, they researchedhe morphology control based on a kind of polymer with highrosslinked structures, prepared by free radical polymerization.

hen considering about the polymerization solvent, they men-ioned that the Hildebrand solubility parameter played a seriousole in the morphology study. The Hildebrand solubility param-ter corresponds to the cohesive energy density of solvents andolymers and is an indicator for the affinity of a solute for aolvent. In order to reflect the overall solvent properties, the three-imensional Hansen solubility parameters, which break the overall

nteractions between a solute and a solvent into dispersive, polar,nd hydrogen-bonding interactions, were used more frequently toepict the role of the solvent in the special system.

Table 1 depicts the data of three-dimensional Hansen solubil-

ty parameters of each mixed solvent. Because all the monomers,ligomers and the final crosslinked polymers contain a lot of polarunction group, especially O S O group, we treat the polar param-ter �p as the key polymer–solvent interactions index.

osphazene containing nano- or micro-materials.

64 Y. Zhu et al. / Materials Science and Engineering B 153 (2008) 62–65

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ig. 1. SEM images of nanomaterials obtained from different mixed solvents: (Ione/toluene = 1/2; (V) acetone/toluene = 1/8 (volume ratio and total volume is 90 m

From Fig. 1 and Table 1, it can be seen that the fibers were formedince the value of �p was below 7.43, and with the value of �p reduc-ng, the resulting product contained more fibers, till got uniformanofibers.

Scheme 2 displays our initial assumption about the mechanism.n the mixed solvent, the polymerization was performed underltrasonic bath with triethylamine as an acid-acceptor. The poly-ondensation of HCCP with BPS generated oligomers and hydrogenhloride (HCl). Triethylamine absorbed HCl, and formed TEA/HCl.

ith the polymerization proceeding, the primary crosslinked col-oid particles containing some TEA/HCl crystals were precipitatedrom the solvent, which was described in Step I. When the mixedolvent shows higher �p value, it shows more capacity to dis-olve the primary crosslinked oligomers because these oligomersontain many polar function groups. So the precipitated primaryolloid particles should contain higher crosslinking degree withess active groups. On the other hand, when the �p value is lower,

he primary colloid particles would express higher activities andould have further reactions with each other more easily. These

urther condensation reactions could only happen in a right direc-ion, which were considered as one factor to explain the orientatedrowth in the fiber formation and were described in Step II (see

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Scheme 2. Mechanism of the morphology transit

one/toluene = 8/1; (II) acetone/toluene = 2/1; (III) acetone/toluene = 1/1; (IV) ace-

he supplementary information). However, the colloid particlesith higher crosslinking degree could only grow by absorbing

he low-active oligomeric species with less further reactions andorm microspheres via precipitation polymerization, which wasescribed in Step III.

Moreover, in our previous research on fabricating the nanotubesnd microtubes, we have mentioned the in situ template mecha-ism. It showed the crystal of the TEA/HCl played a very importantole in the preparing process. It was obvious that, when the reactionystem contained more toluene which is a typical non-polar organicolvent, the solubility of the inorganic crystal would be reduced.his meant the crystal would precipitate more easily. The direction-lity of the growth of TEA/HCl crystal assisted the primary colloidarticles to form materials with one-dimensional order. This woulde another factor to explain the orientated growth in the mixedolvent with low �p value. And the different solubilities of TEA/HClrystal also cause the morphology of the crystal to be different,

hich would explain the size-difference about the fibers, obtained

rom different solvent systems.This very interesting result will provide useful information

or the research on controllable preparation of nano- and micro-aterials with different morphology, especially those with high

ion between microspheres and nanofibers.

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[12] L. Zhu, Y. Xu, W. Yuan, J. Xi, X. Huang, X. Tang, S. Zheng, Adv. Mater. 18 (2006)

Y. Zhu et al. / Materials Science

rosslinked molecule structures prepared by polycondensationeaction.

. Conclusions

Multi-morphology control between microspheres andanofibers was successfully achieved by adjusting the ratiof solvent composition. The morphology of the product, bothicrospheres and nanofibers, contained desired size and shape.

he concept of three-dimensional Hansen solubility parametersas applied for the initial explanation. The activity of the pri-ary colloid particles and the solubility of TEA/HCl crystal were

onsidered as two factors. This result showed the nano- andicro-materials with high crosslinked molecule structures and

repared by condensation polymerization. It can also achieve theorphology transition and fill the blank in nano-morphology

ransition research.

cknowledgment

This work was financially supported by “Shanghai Leading Aca-emic Discipline Project”, project number: B202.

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ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.mseb.2008.10.027.

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