construction of large-scale highly ordered macroporous monoliths of π-conjugated polymers

Communication

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Construction of Large-Scale Highly OrderedMacroporous Monoliths of p-ConjugatedPolymers

Zicheng Zuo, Yanbing Guo, Yuliang Li,* Jing Lv, Huibiao Liu,* Jialiang Xu,Yongjun Li

Large scale of well-ordered macroporous p-conjugated polymer monoliths have been success-fully prepared through a new approach using micrometer-sized naphthalene crystals astemplates. The macroporous monoliths of poly(p-phenylenevinylene) (PPV) and poly(p-phe-nyleneethynylene) (PPE) grew along the unidirectional freezing direction inside the templatenaphthalene crystals which lead to the formation ofcontrolling morphologies and homogeneousdiameters. The polymer monoliths show straightand lamella macroporous structures. The diametersof pores and the thickness of pore walls can becontrolled by tuning the freezing temperature.

Introduction

During the past decade, scientists have developed many

techniques for producing architectures of varied porous

materials due to their excellent properties on adsorbents,

separation materials, catalyst supports and hydrogen

storage.[1] Porous materials have been synthesized for

functionalmaterialswith controlled structures andproper-

ties and the porous structures with specific physical

properties was a key fundamental interest in porous

materials. Great efforts have been dedicated to the

preparations of 3D monolithic porous materials and 2D

porousmembranes.[2] However, the preparations of porous

materials are only limited on the inorganic compounds and

common polymers. Still, the design and fabrication of

Y. L. Li, Z. C. Zuo, H. B. Liu, Y. J. Li, Y. B. Guo, J. Lv, J. L. Xu,CAS Key Laboratory of Organic Solids, Beijing National Laboratoryfor Molecular Sciences (BNLMS), Institute of Chemistry, ChineseAcademy of Sciences, Beijing 100190, P. R. ChinaE-mail: [email protected]; [email protected]. C. Zuo, Y. B. Guo, J. Lv, J. L. XuGraduate University of Chinese Academy of Sciences, Beijing100080, P. R. China

Macromol. Rapid Commun. 2009, 30, 1940–1944

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

porous materials using functional polymers such as p-conjugatedpolymerswith controlled orderedporousarrays

are significant andongoinggreat challenges innanoscience

and nanotechnology. These functional conjugated porous

polymers provide compelling reasons for broad potential

applications as novel materials in organic electronics,

microfluidics, recognition and developing new applied

fields.[3,4] Recently, severalgroupshavedemonstratedsome

methods that are used to produce porousmaterials. Such as

themicroporousmonolithswere prepared by Sonogashira-

Hagihara coupling in the synthesis of polymers,[5] covalent

organic frameworks and the hyper-cross-linked poly-

mers;[6] emulsion templating methods are used widely to

produce well-defined porous polymers and inorganic

porous materials;[7] unconjugated polymer macroporous

monoliths were fabricated by using a metal–organic

coordination gel template[8] and the method of foaming.[9]

It is well known that p-conjugated polymers such as PPE,

PPV with many outstanding optical and electronic proper-

ties[10] and p-conjugated polymers can be easily

modified with various functional groups to construct

functional materials for applications in sensors,[11] organic

light-emitting diodes,[12] and field-effect transistors.[13]

However, the synthesis and properties of porous materials

DOI: 10.1002/marc.200900411

Construction of Large-Scale Highly Ordered Macroporous Monoliths . . .

based on functional p-conjugated polymers have not been

studied, and limited successes have been achieved in the

synthesis of non-conjugated porous polymers. One of the

most important items in preparing porous materials of p-conjugated polymers is in the development of the new

technique to overcome the small solubility originated from

the rigidmolecular structures of p-conjugated polymers.[14]

In this work, we propose a new concept of naphthalene

templating approach, which is a combination of naphtha-

lene crystals as templates and sublimation processes, to

preparewell-ordered porous PPV and PPEmonoliths. In this

approach, the naphthalene acts as a good solvent for

conjugated polymers when it is melted, and the naphtha-

lene crystals can act as ordered templates to synthesize

porous materials in unidirectional frozen. The unidirec-

tional frozen naphthalene can be easily removed by

sublimation at appropriate temperature, and the well-

ordered macroporous frameworks can be formed. The

preparation and the possible growth process of well-

orderedmacroporous structures based on the p-conjugatedpolymer PPV and PPE by the unidrectional freezingmethod

are shown in Scheme 1. In the preparation process, the

special solvent of naphthalene can be recycledmany times

without any pollution. This facile way is a low-cost and

environmentally friend method to prepare well-ordered p-conjugated polymer porous monoliths and it is also a well-

established method to tune synthesis of other organic and

inorganic porous materials.

Experimental Part

Preparation of Porous Polymeric Monoliths

The p-conjugated polymers PPE and PPV are synthesized according

to the reports.[15] The chemical structures of the polymers are

Scheme 1. Chemical structures of PPV and PPE (A) and the illus-trations of as-prepared well-ordered polymeric macroporousmonoliths by naphthalene templating. (B) The preparation ofthe structures by unidirectional freezing of naphthalene solutionof conjugated polymers. The temperature of the cold bath istunable. (C) The proposed growth process of well-ordered porousmonoliths. The straight naphthalene crystals with lamella struc-ture are formed.



illustrated in Scheme 1A. To produce well-ordered polymer porous

monoliths, as-synthesizedPPE (100mg)andPPV(100mg)werewell

mixed with naphthalene powders (1 g) respectively. The polymer

concentrations were fixed at Cf¼9.1%. The mixtures were poured

into a test tubewith 10mmindiameter and 50mmin length. Then

the mixtures were stirred and heated to 110 8C in the oil bath. The

elevated temperature helps PPE or PPV to dissolve into themelting

naphthalenewhich is a good solvent for polymers. To preparewell-

ordered structures, the naphthalene solution containing polymer

PPE and PPV were transferred into the attemperator respectively,

which ensures that the temperature can be kept stable. Then the

PPV and PPE naphthalene solutions were lowered vertically into a

water bath of 10 8C at the rate (Vf) of 1 cm �min�1. After the tube

was entirely submerged into the water bath, it was kept in the

water for further 30min. The polymer solutions were frozen into

solid state and the main frameworks were formed. To prepare the

well-ordered porous structures with smaller wall thickness,

the freezing temperature (Tf) was declined and the solutions were

unidirectional frozen in a cold bath of �78 8C. Then the prepared

monoliths were transferred into the oil bath maintained at 45 8Cand aged in it for 6 h in order to further strengthen the main

frameworks. Finally, thenaphthalene in the sampleswas sublimed

completely at 40 8C for 5h by a vacuum condition (20 Pa). The

process for thepreparationofwell-orderedmacroporousmonoliths

is illustrated in Scheme 1B, and the possible formation process is

shown in Scheme 1C.

Characterization of Porous Polymeric Monoliths

Themorphologies of the prepared porous polymermonolithswere

examined by the field-emission scanning electronmicroscopy (FE-

SEM) Hitachi S-4300 at an acceleration voltage of 10KV and an

electrical currentof8mA.Before theSEMexaminations,all samples

were sputter coated approximately 5nm of gold. To further

investigate the well-ordered scaffolds and the optical properties,

the monoliths were carefully scraped into thin slices with a small

blade and theas-prepared sliceswere transferred onaglassplate to

performthe confocal laser scanningmicroscopy (CLSMthe FV1000-

IX81). The pore diameter distributions of the as-prepared porous

monolithswerecalculatedbymercury intrusionporosimetryusing

an instrument of PoreMaster GT 60. Themonolithswere subjected

to a pressure cycle starting from approximately 0.8 psi to 2300psi

in predefined steps to give pore diameter and pore volume

information.

Results and Discussion

The prepared porous polymer monoliths were character-

ized by scanning electron microscopy. Figure 1a is the

photograph of the large scale well-ordered PPV macropor-

ous monoliths prepared at Tf¼�78 8C by the naphthalene

templating method. SEM image of the vertical section of

PPVmonoliths (Figure 1b) showed that it was well-ordered

macroporous structures. As shown in Figure 1b, the porous

channels were almost parallel to each other along the

freezing direction and the length of the well-ordered

www.mrc-journal.de 1941

Z. C. Zuo et al.

Figure 1. The well-ordered macroporous polymeric monoliths prepared at Tf¼�78 8C.(a) the photograph of large scale PPV. SEM images: (b) the vertical section of PPV, (c) thecross section of PPV. (d) The photograph of large scale PPE. SEM images: (e) the verticalsection of PPE, (f) the cross section of PPE. The arrow represents the growth direction.The insets were the magnified images of well-ordered pore walls corresponding to theircross sections.

Figure 2. SEM images of the cross sections of well-ordered PPV(a, b) and PPE (c, d) monoliths with different wall thickness. Themonoliths were prepared under the same immersion rates(Vf¼ 1cm �min�1), same polymer concentrations (Cf¼9.1%),different freezing temperatures (Tf). (a, c) Tf¼ 10 8C;(b, d) Tf¼�78 8C.

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macropores exceeded 500mm. The cross sections of

Figure 1c showed that vertical to the freezing direction,

the pore structures were also parallel to one another. The

inset magnified SEM image showed that the diameter of

well-orderedporeswasabout10mm.Figure1dshowed that

large scale well-ordered monoliths of PPE were also

produced by the naphthalene templating method.

Figure 1e and 1f also demonstrate that uniform parallel

channels were built and the porous channels were almost

paralleled to each other along the freezingdirection and the

vertical direction, respectively. The well-ordered channels

were as long as 400mm. The inset magnified SEM image

showed that the diameter of well-ordered channels of PPE

was about 10mm.

The well-ordered macroporous structures had the nacre-

likearchitecturesofsomenaturematerials, suchasshellfish

nacrewhichhadhighperformance.[16,17] Ournewnaphtha-

lene-based method provides an optional process for the

preparation of these high performance macroporous

materials. The new naphthalene-based process can be

comparedwith aqueous directional freezing process for the

production of aligned silica fibers,[18] zirconia fibers,[19]

porous alumina fibers[20] and microhoneycombs,[21] and

the CO2-based methods for the preparation of well-aligned

‘‘fish bone’’ porous materials,[22] where the porous struc-

tures are directed by the advancing of ice crystals and CO2

crystals. In the naphthalene-based method, the polymers



were dissolved in the melting naphtha-

lene. The polymer solution was frozen

into the solid state with the decreasing

of temperatures. On the process of

solidification, the degressive tempera-

ture causes the phase separation in

the naphthalene solution, and the

long, straight naphthalene crystals

with lamella shape are generated inside

the system. Then the lamella naphtha-

lene crystals act as templates to induce

the solidification of conjugatedpolymers

along the freezing direction. Under 40 8C,the naphthalene crystals were subli-

mated in vacuum and the well-ordered

macroporous polymer architectures

were achieved. The macropores that are

templated from the spaces occupied by

thenaphthalenecrystals areproducedby

the following removal of naphthalene

crystals.

The wall thickness of the well-ordered

macroporous scaffolds and the width of

the pore diameter are extremely impor-

tant factors inbuilding catalyst supports,

photonic and electronic materials. It can

greatly affect the mass transfer ability

and the pressure drop in usages. The freezing temperatures

have great influence in tuning the wall thickness of the

orderedmacroporous structures. Figure2aand2c showthat

DOI: 10.1002/marc.200900411

Construction of Large-Scale Highly Ordered Macroporous Monoliths . . .

the wall thicknesses of PPV and PPE macroporous scaffolds

produced at 10 8C were both about 2mm. When the Tf was

decreased to �78 8C, the wall thickness rapidly reduced to

about 600nm and 500nm for PPV and PPE monoliths,

respectively (Figure2band2d). FromtheSEMimages, itwas

observed that the pore diameters were also affected by the

freezing temperatures. For the macroporous monoliths of

PPV, the pore diameters at 10 8C were about 8mm

(Figure 2a), and the pore diameters were decreased to

about 4mmat�78 8C (Figure 2b). At the same time, the pore

diameters had a rapid decrease from 8mm (10 8C) to 4mm

(�78 8C) in the PPEmacroporous scaffold (Figure 2c and 2d).

In this system, the pore diameters among paralleled walls

were thought to be influenced by the temperature gradient

in the liquid-solid interface and the solidification velocity,

which are important functions on the process of unidirec-

tional frozen formelt alloy.[23]When the polymer solutions

solidified at�78 8C, the temperature gradient in the liquid-

solid interfacewas greater than that of at 10 8C. The greatergradient caused the fast solidification of naphthalene, and

then thegrowth timeofnaphthalene crystals vertical to the

solidification direction was shortened. It was concluded

that wider pore diameters among walls can be produced

under higher freezing temperatures.[24] Therefore, the size

of pores can be tuned by controlling the solidification

temperatures.

Thewell-orderedmacroporous structures of PPV and PPE

were also confirmed by fluorescence images (Figure 3).

Figure 3 presents the fluorescence images of well-ordered

macroporous structures of PPV and PPE excited with

wavelength of 559nm. According to the fluorescent

properties of the polymers, PPV displayed red fluorescence

Figure 3. The fluorescence images of the well-ordered polymermonoliths. (a) PPV, Tf¼ 10 8C; (b) PPV, Tf¼�78 8C; (c) PPE,Tf¼ 10 8C; (d) PPE, Tf¼�78 8C. Vf¼ 1 cm �min�1, Cf¼9.1%. Bothof the samples were excited at 559nm.



and PPE displayed yellow fluorescence when they were

excited with wavelength of 559nm. Figure 3a shows that

the PPV monoliths (Tf¼ 10 8C) were composed of well-

ordered pore structures. Figure 3b indicated that therewere

uniform well-ordered macroporous channels in PPV

monoliths (Tf¼�78 8C) and the channels are parallel to

one another.With decrease of the freezing temperature, the

wall thickness of PPV pores had a dramatically decrease

(Figure 3a and3b). Figure 3c and 3d also display that both of

PPE monoliths were composed of well-ordered uniform

pores and the wall thickness had an obvious decline by the

decrease of Tf. The average pore diameters of scaffolds PPV

andPPEpreparedat 10 8Cwere obviouslywider than that of

PPV and PPE produced at�78 8C. The pore diameters in the

fluorescence images were according to that in SEM images

(Figure2). Thepurefluorescence imagesofbothPPVandPPE

indicate that the naphthalene was completely removed

from the porous framework and the well-ordered macro-

porous monoliths could be prepared by the new naphtha-

lene-templating method.

The dependence of Tf and the pore size distribution have

been studied by the instrument of mercury intrusion

porosimetry. The curves in Figure 4 show that the pore

diameters of PPV and PPEmonoliths prepared at 10 8Cwere

larger than those prepared at�78 8C. Figure 4a showed that

the curve of well-ordered PPV scaffolds prepared at 10 8Chad a broad peak centered at 35mm, and Figure 4b showed

that the curve of well-ordered PPE scaffolds prepared at

10 8Chad a similar broad peak centered at 40mm.When the

freezing temperature declined to �78 8C, the curve of PPV

monoliths gave a broad peak centered at 7mm, and the

curve of PPE monoliths gave a similar broad peak centered

at about 6mm. The small peaks at about 3mm in Figure 4a

and 4bmight be caused by the damages of themacoporous

structures by the intrusion of mercury. It was shown that

similar pore diameter distributions of PPV and PPE

monoliths were produced at the same freezing tempera-

tures. Although the polymers concentration were kept

stable, the dramatic decrease in the average pore diameter

was observed when the Tf declined from 10 8C to �78 8C(PPV: from 6.07mm to 3.88mm; PPE: from 9.05mm to

2.62mm). The median pore diameters of PPV obtained at

10 8C are about 25.01mmand the pore diameters of PPE are

about 28.73mm. When the Tf rapidly decreased to �78 8C,the median pore diameters of PPV and PPE also rapidly

decreased to8.77mmand5.97mm, respectively. The specific

surface area of macroporous of PPV and PPE produced at

10 8C was about 4.68m2 � g�1 and 2.88m2 � g�1, and it

increased to 9.80m2 � g�1 and 8.65m2 � g�1, respectively.

Conclusion

We demonstrated a novel naphthalene-templating tech-

nique for controlled preparation of well-ordered porous

www.mrc-journal.de 1943

Z. C. Zuo et al.

Figure 4. The curves of pore diameter distributions of PPV and PPE with well-orderedmacroporous structures (a) PPV, Tf¼ 10 8C, (b) PPE, Tf¼ 10 8C, (c) PPV, Tf¼�78 8C, (d) PPE,Tf¼�78 8C.

1944

monoliths of p-conjugated polymers. Our results show that

the pores can be formed in macroporous polymers by

the unidirectional freezing inside the template naphtha-

lene crystals. The wall thickness and macroporosity of the

polymermonoliths canbe tunedby controlling the freezing

temperature and rate. The approach may potentially be

extendable to the productions of well-ordered porous

functional materials for application in catalyst supports,

electronic materials, batteries materials and organic solar

cells.

Acknowledgements: This work was supported by the NationalNature Science Foundation of China (20831160507, 20873155 and20721061) and the National Basic Research 973 Program of China.

Received: June 11, 2009; Revised: July 10, 2009; Published online;August 27, 2009; DOI: 10.1002/marc.200900411

Keywords: p-conjugated polymer monoliths; naphthalene-tem-plating; unidirectional freezing; well-ordered macroporous

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DOI: 10.1002/marc.200900411

construction of large-scale highly ordered macroporous monoliths of π-conjugated polymers

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