11754 Chem. Commun., 2011, 47, 11754–11756 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 11754–11756

Gradient porous materials by emulsion centrifugationw

Adham Ahmed, Jennifer Smith and Haifei Zhang*

Received 22nd August 2011, Accepted 20th September 2011

DOI: 10.1039/c1cc15212b

Oil-in-water emulsions are centrifuged to generate a gradient

distribution of emulsion droplets. This structure is locked by

freezing or polymerization and the subsequent removal of the

solvents produces gradient porous materials.

Porous materials have found a wide range of applications. In

most cases, the pores are distributed uniformly across the

materials. One group of porous materials shows change of

pore size or porosity along a certain dimension of the materials.

The change in pore size or porosity can be continuous

(gradient) or step-wise (graded). Such materials are normally

called gradient/graded porous materials.1 Nature has produced a

range of gradient porous materials. One of the excellent

examples is the bone, which contains a spongy cancellous

structure and a dense cortical shell. Such structure can give

excellent permeability and mechanical stability.2 Gradient

porous materials have been used in applications including

medical implants,3 scaffold for tissue engineering,4 separation5

and coatings.6

Fabrication of gradient porous materials is more sophisticated

than that of porous materials with uniform pore structures.

Normally it requires additional steps to introduce the gradient

porosity. Several methods with rather complicated procedures

were investigated. For example, graded porous silicon was

prepared by a deep anodic etching method where the etching

solution and etching rate were varied to produce pores of

different sizes.7 Variable coarsening rates in a gradient

temperature field were used to get co-continuous phase structures

in immiscible polymers blends. Subsequent dissolution of one

phase could yield a gradient porous structure.8 Polysilses-

quioxane has been used to produce porous ceramics. Gaseous

products (H2O and ethanol) were generated from the

condensation reaction when heated to 220–300 1C. The viscosity

of the reaction systems could be adjusted to control the

migration and growth of the bubbles to form gradient porous

ceramics.9 So far, most of the reported work involved the use

of particles or polymer fibrils. For example, graded particles

were packed together and gradient porous structures were

prepared by a sintering process1,10 via gradient impregnation

of particles into uniform porous polymeric scaffolds, gradient

porous structures could be generated by partial impregnation

from one side11 or electrohydrodynamic spraying12 and the

subsequent removal of polymeric scaffold (e.g., by

calcination).

Freezing colloidal suspensions and the subsequent removal

of ice templates by freeze-drying have been frequently used to

produce various porous structures.13 The difference in freeze

temperature gradient across the samples could be used to

produce gradient porous structures.14 However, it was not

easy to control the gradient structure due to the difficulty in

adjusting the temperature gradient. Freezing in sequence of

particles slurries at different concentrations led to formation of

graded porous materials.15 More generally, a gradient distri-

bution of the particles in the suspensions can be generated by

spinning or centrifuging. Such gradient suspensions are

rapidly frozen and a freeze-drying procedure is then employed

to produce gradient porous structures. For example, gradient

porous polycaprolactone and tubular collagen–glycosamino-

glycan were prepared using this procedure from polymer fibril

suspensions.4,16

We report here a new method for the preparation of

gradient porous materials by use of emulsion templating and

centrifugal force. Liquid droplets in the emulsions could be

used as templates to prepare macroporous materials.17 The

number of droplets in an emulsion within certain volume

could be varied to produce highly interconnected porous

materials18 or porous materials with tunable porosity19 or

mechanical strength.20 The advantages associated with emulsion

templating include the ability to prepare a wide range of

materials and facile removal of the droplets phase. So far,

only uniform porous materials have been formed by emulsion

templating.17–20 As the emulsions are normally formed by the

oil phase and the aqueous phase, the difference in the density

of the droplets phase and the continuous phase exists. The

density difference in an emulsion system is explored in this

study via centrifugal force to produce gradient porous

materials.

For oil-in-water (O/W) emulsions, it is expected that the oil

droplets can ascend toward the top while solid particles

(e.g., silica) in the continuous phase will sedimentate under

centrifugation. The rate of such separation and distribution of

droplets and particles depends on the difference in density,

viscosity of the continuous phase, and centrifugal force

(or centrifugal speed). Once the gradient droplets distribution

in the emulsion is formed, such structure can be locked by

Department of Chemistry, University of Liverpool, Liverpool,L69 7ZD, UK. E-mail: zhanghf@liv.ac.uk;Fax: +44 (0)151 7943588; Tel: +44 (0)151 7943545w Electronic supplementary information (ESI) available: Experimentaldetails, additional data and SEM images (Table S1, Fig. S1–S6). SeeDOI: 10.1039/c1cc15212b

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 11754–11756 11755

freezing or polymerization and subsequent removal of solvents

can produce emulsion-templated gradient porous materials.

Firstly, equal volumes of cyclohexane (containing Oil Red

for better visibility) and 5 wt% aqueous poly(vinyl alcohol)

(PVA) solution were mixed to form an O/W emulsion. This

emulsion was centrifuged with a lab centrifuge at 1000 rpm.

Fig. 1A shows the image of the emulsion after centrifuging.

The colour changes from pale to red from bottom to top. This

indicates that the number of droplets increases gradually from

the bottom because the colour of the droplets is red. The

optical microscopic images (Fig. 1B–D) clearly show the

gradient distribution of emulsion droplets. The centrifuged

emulsion was rapidly frozen in liquid nitrogen and then placed

in a freeze dryer to remove the frozen solvents (both water and

cyclohexane). Scanning electron microscopy (SEM) imaging

showed the increased number of emulsion-templated pores

across the highly porous PVA from bottom to top (Fig. S1, ESIw).As characterized by Hg intrusion porosimetry for different

parts of the PVA material (Table S1, ESIw), the pore volumes

increased from the bottom part (mainly templated by ice) to

the top part (ice and emulsion templating).

Addition of silica colloids could make the structures stronger

with well-defined emulsion-templated gradient porosity. The

silica colloids were synthesized by a modified Stober method

with N-(5-fluoresceinyl) maleimide.21 The as-prepared yellow

silica colloids (around 1 mm) were suspended in 5 wt% PVA

solution at the concentration of 5 wt% and 10 wt%, which

were then used to form emulsions with cyclohexane at equal

volumes. As the silica particles and oil droplets have different

size and density, this would cause the silica particles and oil

droplets to travel in opposite directions during centrifugation.

The emulsions were prepared by stirring at 1000 rpm. After

centrifuging at 800 rpm, a dual gradient distribution was

achieved: the emulsion droplets concentrating toward the

top while the silica colloids concentrating toward the bottom

of the emulsion (Fig. S2, ESIw).These centrifuged emulsions were freeze-dried to produce

gradient porous composite materials. The sample prepared

from the emulsion with 10 wt% silica was more rigid. As

shown in Fig. 2A, the increasing number of emulsion-

templated pores can be clearly seen. The pore structures at

higher magnifications for different parts of the monolith are

given in Fig. 2B–D. The ice-templated aligned structure,

co-existence of ice-templated and emulsion-templated structures,

and the highly interconnected emulsion-templated pore structure

are observed in this single material. Although such structure

evolution was reported before, the observations were for

several samples prepared under different conditions.19

A similar structure was also observed for the sample made

from the emulsion with 5 wt% silica colloids (Fig. S3 (ESIw),sample S1 in Table 1). As seen in Table 1, the intrusion volume

(contributed mainly by emulsion-templated macropores)

increases in the order of bottom - middle - top, indicating

that the gradient distribution of emulsion droplets after centri-

fugation (as of Fig. 1) was locked and transferred into a

gradient porous material. The sizes of macropores also increase

toward the top of the material. This can be further demon-

strated from the distribution of macropores in different

sections of S1 (Fig. S3D, ESIw). While emulsion droplets

ascend during centrifuging, silica colloids can sedimentate at

the same time. This is reflected in the increasing surface area of

sample S1 in the order of top - middle - bottom (Table 1).

Fig. 1 (A) Photo of the emulsion of cyclohexane (with oil red) in

aqueous PVA after centrifugation at 1000 rpm. (B–D) Representative

optical micrographs (with the same scale bars) for the emulsion at

different heights, as indicated by the arrows.

Fig. 2 Gradient porous silica–PVA composite. (A) The monolith.

(B–D) Pore structures at different parts of the monolith.

Table 1 Characterization data for gradient porous silica–PVA (S1)and silica–TiO2 (S2) monoliths

PartaSurface areab/m2 g�1

Mesoporevolumec/cm3 g�1

Intrusionvolumed/cm3 g�1

Macropored/mm

S1 T 7.30 0.0119 8.89 4.86, 11.3M 9.85 0.0127 7.35 1.61, 3.87B 10.35 0.0144 2.57 1.63

S2 T 87.4 0.0680 2.54 4.91, 11.3M 102.6 0.0798 1.29 1.61, 11.3B 184.5 0.1186 1.05 1.61

a T, top; M, middle; and B, bottom sections of the monoliths. b BET

surface area measured by N2 sorption. c From N2 sorption data.d Measured by Hg porosimetry.

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11756 Chem. Commun., 2011, 47, 11754–11756 This journal is c The Royal Society of Chemistry 2011

After calcining S1, the surface area is increased but following the

same increasing order across the material—top (28.12 m2 g�1)-

middle (75.52 m2 g�1) - bottom (88.00 m2 g�1) (Fig. S4, ESIw).Sample S1 (Table 1) was further modified in a solution of

Ti(OiPr)4 in isopropanol by a sol–gel process. The resulting

composite was calcined at 520 1C to remove PVA and produce

a gradient porous silica–titania composite (Fig. S5, ESIw). Thedual gradient porosities (surface area/mesopore volume and

intrusion pore volume from macropores), like sample S1, are

retained in the silica–titania composite (sample S2 in Table 1).

The mass loss during calcination was 69%. The material

showed slight shrinkage at the bottom part and a higher

degree of shrinkage in the highly porous region at the top

part. This resulted in decreased intrusion (macropore) volumes

in sample S2, compared to sample S1. The macropore and

mesopore size distributions for S1 are shown in Fig. S6, ESI.wWhen monomers rather than polymer are present in the

continuous phase, the monomers can be polymerized to

produce emulsion-templated structures.17,18 To produce a

gradient porous polymer in this way, an O/W emulsion was

formed by emulsifying light mineral oil with Triton X-405 as

surfactant into aqueous solution containing monomer acrylamide

and crosslinker N,N0-methylenebisacrylamide.22 The emulsion

was centrifuged at 1500 rpm and then polymerized at 60 1C.

After washing and removing the solvent, a dry porous poly-

acrylamide (PAM) was produced. The pore structures of

different parts in the monolith (Fig. 3) clearly demonstrate

the formation of gradient porous PAM. The pore volumes

were measured to be 1.81 cm3 g�1 (bottom), 2.09 cm3 g�1

(middle) and 2.28 cm3 g�1 (top). To visibly see the gradient

porosity, a dry porous monolith was soaked in aqueous Nile

blue solution. The part with higher porosity would absorb

more blue solution, thus giving a more blue appearance. With

the gradient porous PAM, it was expected that a gradient blue

colour distribution should be observed across the monolith.

This is clearly the case in Fig. 3D, further demonstrating the

formation of gradient porous PAM.

In conclusion, a simple and effective method was developed

to produce gradient porous materials by centrifuging O/W

emulsions. Under centrifugal force, density difference between

oil droplets and continuous aqueous phase led to the gradient

distribution of oil droplets. Such gradient distribution was

frozen or polymerized to produce gradient porous polymers

after the removal of solvents. When silica colloids were

suspended in the aqueous phase, a dual distribution of oil

droplets and silica colloids was formed across the emulsion,

which was then utilized to produce dual gradient porous

(macroporous and mesoporous) PVA–silica and silica–titania

composites. Materials with such gradient porosity are potentially

very useful in tissue engineering, separation, and multistep

catalysis.

This work was supported by a DTA studentship to A.A. by

Thermo Fisher Scientific and a summer project to J.S. by

the NESTA.

Notes and references

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13 (a) L. Qian and H. Zhang, J. Chem. Technol. Biotechnol., 2011,86, 172; (b) S. Deville, Adv. Eng. Mater., 2008, 10, 155.

14 (a) H. Zhang, J. Y. Lee, A. Ahmed, I. Hussain and A. I. Cooper,Angew. Chem., Int. Ed., 2008, 47, 4573; (b) A. Macchetta,I. G. Turner and C. R. Bowen, Acta Biomater., 2009, 5, 1319.

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19 L. Qian, A. Ahmed, A. Foster, S. P. Rannard, A. I. Cooper andH. Zhang, J. Mater. Chem., 2009, 19, 5212.

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Fig. 3 Gradient porous PAM. SEM images show the top part (A),

middle part (B) and bottom part (C) of the prepared monolith. A

gradient distribution of blue colour is observed for Nile blue-soaked

PAM (D).

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