New insights into the relationship between internal

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The mechanical properties of polyHIPEs are related directly to the foam density; macroporous polymers produced from high internal phase emulsion templates ...
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New insights into the relationship between internal phase level of emulsion templates and gas–liquid permeability of interconnected macroporous polymers Shu San Manley,b Nadine Graeber,b Zdenek Grof,c Angelika Menner,b Geoffrey F. Hewitt,a Frantisek Stepanekac and Alexander Bismarck*b Received 8th January 2009, Accepted 27th July 2009 First published as an Advance Article on the web 25th September 2009 DOI: 10.1039/b900426b Interconnected macroporous polymers can be made by polymerisation of emulsion templates consisting of an aqueous phase and a monomer phase (typically styrene and divinylbenzene) in which the aqueous (internal) phase is in the form of drops and the monomer phase is the continuous phase between the drops. Until recently it was thought that interconnected macroporous polymers could only be produced from the polymerisation of high internal phase emulsion (HIPE) templates with an internal phase level exceeding 74 vol%. Improvement of the poor mechanical performance, characteristic of such macroporous polymers, was achieved simply by increasing the material density of the macroporous polymer. However, this required a reduction in the internal phase volume of the emulsion template. Polymerisation of the continuous organic phase of emulsion templates with an internal phase volume ranging from 84 vol% to 70 vol% resulted in the production of poly(merised)HIPEs, polymerisation of medium internal phase emulsions with internal phase volume ranging from 70 vol% to 30 vol% in polyMIPEs and polymerisation of a low internal phase emulsion with an internal phase volume of 25 vol% in a polyLIPE. The resulting macroporous polymers were characterised in terms of mechanical and structural properties as well as gas and mercury permeability. Compression tests show that mechanical properties improved as the material density was increased. Gas and mercury permeability measurements show that as the internal phase volume of the emulsion template is reduced, the permeability of the resultant macroporous polymer is also reduced. However, surprisingly even macroporous polymers produced from low internal phase emulsion templates (25 vol%) were permeable with a gas permeability of 2.6  1014 m2 indicating that polyLIPEs are still interconnected macroporous polymers. Reconstruction modelling of the transport properties of porous materials shows that the permeability of a porous material with similar structures to that of the macroporous polymers increases exponentially with the porosity.

Introduction Emulsion templates with an internal phase exceeding 74 vol% of the total emulsion volume are known as high internal phase emulsions (HIPEs). HIPEs, which are used as templates for the synthesis of porous media, consist most commonly of an internal aqueous phase and a continuous organic phase, which contains monomers such as divinylbenzene (DVB) and styrene, a surfactant and an initiator.1–4 High viscosity emulsion templates are produced when the internal phase is dispersed in the continuous phase. Subsequent polymerisation of the continuous phase results in the formation of an interconnected macroporous polymer, a polyHIPE, with the structure determined by the emulsion template. a Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, UK SW7 2AZ b Polymer and Composite Engineering (PaCE) Group, Imperial College London, South Kensington Campus, London, UK SW7 2AZ. E-mail: a. [email protected] c Department of Chemical Engineering, Institute of Chemical Technology, Prague, Technicka 5, 166 28 Praha 6, Czech Republic

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There are numerous potential applications for polyHIPEs including scaffolds for tissue engineering and monolithic polymer supports for heterogeneous catalysis and ion exchange modules,5–11 which all make use of the highly interconnected and, therefore, permeable structure of polyHIPEs. For example, the permeability of polyHIPE scaffolds for tissue engineering and cell cultures guarantees the flow of nutrients necessary for cell growth while the permeability of porous catalyst supports will affect the flow of reagents and hence the reaction rates. In summary, polyHIPEs are of interest for numerous applications due to their relatively low material density, interconnected porous structure as well as their high and controllable porosity. The mechanical properties of polyHIPEs, though, are somewhat poor and this limits the practicality of their use in many applications.12 The mechanical properties of polyHIPEs are related directly to the foam density; macroporous polymers produced from high internal phase emulsion templates have a very low density. The simplest solution for improvement of the mechanical properties of macroporous polymers made by the polymerisation of emulsion templates would be to increase the foam density by reducing the This journal is ª The Royal Society of Chemistry 2009

internal phase volume in the emulsion template. There are, however, concerns that such macroporous polymers would not be interconnected and hence would be impermeable.13,14 By definition, a HIPE has a volume fraction of at least 0.74, the maximum packing fraction of monodispersed spheres. Above this fraction the droplets experience some deformation; the previously spherical droplets are flattened in the areas of close contact of neighbouring droplets.13 In these areas the droplets are separated by only an extremely thin film of the continuous phase. Even though the formation of pore throats is not fully understood,15 it is generally believed that they form in these flattened areas. Theories discussing the geometry of emulsions suggest that the droplets in emulsion, which have internal phase levels of less than 0.7, should not be in sufficiently close contact and, therefore, pore throats should not form during the polymerisation of the continuous phase of such emulsions. However, we demonstrated that the polymerisation of emulsions containing 60 vol% internal phase led to interconnected macroporous polymers.16 In theory, even at volume fractions as little as 34% the spheres can be in contact with at least four neighbouring spheres (tetrahedral packing).17 Since pore throats form in the area of close contact of neighbouring droplets15,16,18,19 the polymerisation of MIPEs and maybe even LIPEs should, according to our hypothesis, lead to the formation of interconnected macroporous polymers. In the work presented here, the effect of the reduction of the internal phase volume of the emulsion template used to produce macroporous polymers on the resulting pore structure, mechanical properties as well as on the gas and liquid permeability was investigated. A straightforward, reliable method for determining the limiting pore throat diameter via mercury intrusion has been used and permeability distributions, i.e. the permeability as a function of pore volume filled, are discussed. The permeability distributions determined using a mercury intrusion are complemented by measurements of the gas viscous permeability. Furthermore, a modelling procedure for the estimation of the transport properties of porous media resembling emulsion-templated macroporous polymers with arbitrary pore and throat size distribution is briefly presented.

Hypermer 2296 was used as the surfactant to stabilise emulsion templates with internal phase volumes ranging from 84–60 vol%. It was found that the emulsions prepared with less than 60 vol% internal phase were unstable when solely stabilised by Hypermer 2296. A surfactant mixture of Hypermer 2296 and Hypermer B246sf (1 : 1 by weight) was found to be suitable to stabilise emulsions with internal phase volumes ranging from 60–25 vol%. The continuous organic phase was mixed in a reaction vessel fitted with an addition funnel and a glass paddle rod connected to an overhead stirrer. The stirring rate of 400 rpm was kept constant during the preparation of the emulsion. The aqueous internal phase, containing CaCl2$2H2O as electrolyte, was slowly added to the stirring reagents. In order to create finely dispersed droplets the stirring rate was increased to 2000 rpm for 10 min. The resulting emulsions were transferred into freestanding polypropylene centrifuge tubes (Fisherbrand). Each centrifuge tube was sealed with a lid and heated to 70  C in an oven for 24 h. The polymerised emulsions were removed from the centrifuge tubes and purified by Soxhlet extraction first with distilled water followed by methanol. The samples were dried to constant weight in a vacuum oven at 80  C. The samples that were prepared and the sample codes used are summarised in Table 1.

Experimental

Density measurements. Material and foam density measurements were performed using pycnometry (GeoPyc 1360, Micrometrics Ltd., Dunstable, UK) in conjunction with measurements of the material density obtained from helium displacement pycnometry (AccuPyc 1330, Micrometrics Ltd., Dunstable, UK). The porosity 3 was calculated using eqn (1):

Materials Styrene, divinylbenzene (DVB), CaCl2$2H2O and a,a0 -azoisobutyronitrile (AIBN) were purchased from Sigma Aldrich (Gillingham, UK). Non-ionic polymeric surfactants Hypermer 2296 (hydrophilic–lipophilic balance (HLB) ¼ 4.9) and Hypermer B246sf (HLB ¼ 6.0) were kindly supplied by Croda (Wirral, UK). All chemicals were used as received. The epoxy resins Araldite Precision Adhesive and Araldite 2020 as well as the silicone mould release spray Electrolube were purchased from RS Components Ltd. (Corby, UK). Preparation of macroporous polymers The general procedure to prepare 50 mL emulsions consisting of a continuous organic phase and an internal aqueous phase is described below. The continuous organic phase contained 20 vol% styrene, 60 vol% divinylbenzene, 20 vol% surfactant and 1 mol% of the initiator AIBN with respect to the monomers. This journal is ª The Royal Society of Chemistry 2009

Characterisation of macroporous polymers Scanning electron microscopy (SEM). The average pore and pore throat diameter of the macroporous polymers were determined using scanning electron microscopy (Jeol JSM 5610 LV, Jeol Ltd., Welwyn Garden City, UK). Prior to observation, approximately 1 cm3 of each sample was mounted on a sample holder and sputtered with gold for 120 s in an argon atmosphere using an Emitech K550 (Emitech Ltd., Ashford, UK) to ensure sufficient conductivity. SEM images were analysed using the software, UTHSCSA Image Toolª Version 3.00, which enabled direct measurements of pore diameters from the images. At least 200 pore and pore throat diameters from various parts of the sample were measured.

Table 1 Summary of the formulations and the naming system of macroporous polymers produced

Sample Set 1

Set 2

Surfactant 1 2 3 4 5 6 7 8

Hypermer 2296

Hypermer 2296 + B246sf

Internal phase volume/vol%

Foam produced

84 74 64 60 60 40 34 25

PolyHIPE PolyHIPE PolyMIPE PolyMIPE PolyMIPE PolyMIPE PolyMIPE PolyLIPE

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 3¼

m 1 V rm

  100%

(1)

where m ¼ sample mass, V ¼ sample volume, and rm ¼ material density. Mechanical testing. Compression testing of the cylindrical samples of macroporous polymers was carried out using a Lloyds Universal Testing Machine (Lloyds EZ50, Lloyds Instruments Ltd., Fareham, UK). The samples were loaded at a rate of displacement of 1 mm min1 until a displacement of 5 mm was reached. The elastic modulus is the slope of the initial linear region of the stress–strain plots. The crush strength was defined as the maximum strength at the end of the initial linear elastic region. Preparation of samples for permeability measurements Monoliths of the macroporous polymers with a diameter of 15 mm were sealed with a non-permeable coating to eliminate any fluid leakage from the sample via cross-flow. This was achieved by coating the whole surface of the sample with a layer of epoxy resin. When fully cured at room temperature, the sample was set coaxially in an epoxy resin cylinder using a poly(tetrafluoroethylene) (PTFE) sample mould. The coated macroporous polymers were inserted into a PTFE mould base which was sprayed with a silicon release agent and a PTFE cylinder was attached around it. A two-component epoxy adhesive Araldite 2020 was poured into the mould cylinder around the sample and left at room temperature for 24 h to cure. Once coated and sealed, the samples with a final diameter of 31 mm were cut to a length of 25 mm and machined at both ends to reveal the faces of the sample. The same coated samples were used first for the gas permeability and then for the mercury intrusion followed by mercury permeability measurements. The samples were inserted into the respective sample cells and disposed of after the mercury measurements. Unfortunately, the polyHIPEs 1 and 2 failed during the gas permeability measurement; polyHIPE 1 was blown out of the epoxy encasement and polyHIPE 2 cracked at an applied pressure of 0.2 MPa because of the poor shear properties of these materials. However, for a better understanding the mercury permeability results are presented before the gas permeability data. Limiting pore throat diameter and mercury permeability Mercury is a non-wetting liquid and will not penetrate the pores of a macroporous polymer unless an external pressure is applied.20,21 This is the basis for the measurements of the flow limiting pore throat diameter and the permeability distribution. Limiting pore throat diameter. The sample was sealed in the sample holder, and the custom built system22 (Fig. 1) with the sample (1) evacuated to a pressure of 10 Pa. Mercury was then allowed to flow from the sumps (2) into the manometer tubes (3a,b) and into contact with the faces of the sample. Nitrogen was used to force the mercury from the manometer tubes into the sample pores, initially only filling the largest pores of the sample. Since the macroporous polymer is an electrical insulator, first electrical contact indicates that a set of interconnected pores with the largest flow limiting pore throat diameters were filled with 4782 | Soft Matter, 2009, 5, 4780–4787

Fig. 1 Scheme of setup used to determine limiting pore throat size diameter and mercury permeability.

mercury. The flow limiting pore throat diameter of the pores penetrated by mercury at any given external applied pressure can be calculated as follows: d¼

4g cos q p

(2)

where d ¼ pore diameter, g ¼ mercury surface tension (g ¼ 484 mN m1), q ¼ contact angle between the sample and mercury (q ¼ 140 ) and p ¼ applied pressure. It is usually assumed that the surface tension of mercury and the contact angle between the solid and the mercury are constant.23 The external nitrogen pressure applied to the mercury was increased in small increments. Flow measurements were made (see Permeability distribution) at each pressure increment until there was no further mercury intrusion, i.e. an increase in pressure did not force more mercury into the sample as the pores were fully saturated. The applied pressure required to initiate mercury flow corresponds directly to the largest flow limiting pore throat diameter, which was calculated using eqn (2). Permeability distribution. The permeability of the samples was determined by measuring the volumetric flow rate of mercury through the pores. Thereby, at each applied incremental pressure rise, smaller and smaller pores were penetrated with mercury. At each pressure increment, a pressure differential was applied across the sample and mercury was forced through the sample in the direction of the differential. The volumetric flow rate of mercury through the sample was measured when the small differential pressure had been applied. This was achieved by measuring the initial height of mercury in the manometer tubes and measuring the time taken for the mercury to reach the final heights in the tubes. At each incremental applied pressure increase the permeability k was calculated as follows: k¼

log Dh0 =Dht A1 mL t 2A2 rg

(3)

where Dh ¼ change in height, t ¼ time, A1 ¼ cross-sectional area of manometer tubes, A2 ¼ cross-sectional area of sample, This journal is ª The Royal Society of Chemistry 2009

m ¼ viscosity of mercury, r ¼ density of mercury, L ¼ length of sample and g ¼ acceleration due to gravity. It should be noted that the mercury intrusion measurements could only be performed once per sample. However, each individual permeability measurement was repeated 5 times.

has the gradient k/m from which the viscous permeability was derived. The gas permeability measurements were performed on 2 different samples for each macroporous polymer and repeated 3 times per sample.

Gas permeability

Reconstruction modelling of the macroporous polymers and permeability calculations

24,25

was used to measure the gas A pressure rise technique permeability using a custom built system whereby the gas pressure at one side of the porous medium was kept low and a constant higher pressure was maintained at the other side of the sample. Gas flowed through the pores of the sample from the high pressure to the low pressure side where the gas was collected in a vessel of which the volume was known (Fig. 2). Measurements of the rate of pressure rise were used to determine the viscous permeability of the macroporous polymers. The samples were placed in a sealed sample cell to avoid any cross-flow around the edges of the porous material. The sample and the cell were evacuated using a vacuum pump to a pressure in the range of 10 Pa. Once this low pressure was achieved, a flow of nitrogen was applied to the upper side of the sample at a constant set pressure. The gas permeated through the sample and was collected on the low pressure side (maintained at a low pressure using the vacuum pump). The permeability coefficient K can be calculated as follows: ( rffiffiffiffiffiffiffiffiffiffi) Q2 p2 L V ðdp2 =dtÞL k 4 8RT K¼ ¼ pm þ K0 ¼ (4) DpA p1 A m 3 pM where Q2 ¼ volumetric flow rate downstream (low pressure side), p2 ¼ downstream pressure, L ¼ sample length, A ¼ sample crosssectional area, Dp ¼ pressure difference across the sample, V ¼ known volume, t ¼ time, k ¼ viscous permeability, pm ¼ mean pressure with pm ¼ p1/2 where p1 ¼ the gas inlet pressure, m ¼ gas viscosity, K0 ¼ Knudsen permeability coefficient, R ¼ gas constant, T ¼ temperature and M ¼ molar mass of gas. To determine the viscous permeability k the permeability coefficient K was calculated using eqn (4), where the parameter (dp2/dt) was measured in the experiment. A linear plot of K vs. pm

The modelling consisted of two steps. First, it was necessary to obtain a three-dimensional representation of the porous medium which corresponds as much as possible to the structure of the real sample. In the second step, this virtual structure was analysed and its permeability was calculated.26,27 Please note that it would be possible to avoid the reconstruction step by determining the structure directly, e.g. by X-ray microtomography. The input parameters to the reconstruction algorithm are the pore and the throat size distributions (PSD and TSD) obtained from the statistical analysis of the SEM images described above. A large number of non-overlapping spheres with diameters corresponding to the given PSD is generated within the simulation box with periodic boundary conditions considered. While the box gradually shrinks, the spheres are allowed to move using a distinct element method (DEM) type of simulation.28 The standard parallel spring-and-dashpot force model was used in the DEM simulations, i.e. elastic repulsion forces proportional to the particle overlap and a viscous damping force proportional to particle velocity. The friction forces usually employed within DEM models were not necessary for this case as the algorithm is not supposed to simulate the movement of real particles. The neutral position for the spring is set in such a way that the spheres can overlap, the extent of the overlap for every new contact is chosen so the throat diameters between the pairs of spheres correspond to the given TSD. The reconstruction phase is concluded by generating a three-dimensional binary array of cubic volume elements: elements located inside any sphere are classified as the pore phase, other elements are considered to be part of the solid phase. The result of the described algorithm is a series of structures with gradually increasing porosity and with the given PSD and TSD. The sample size should be large enough to be statistically representative, i.e., the dependence of calculated properties on the particular realisation is small compared with the dependence on input parameters of the reconstruction algorithm (pore and throat size distributions). The methodology for calculating the permeability is explained e.g. by Kohout et al.27 To calculate permeability, a macroscopic flow rate across the sample was imposed and the Stokes equation with the continuity equation were solved in the pore space, with no-slip boundary conditions on the walls, a pressure gradient in the direction of flow and periodic boundary conditions at the four remaining boundaries. The permeability was then obtained from the converged pressure and velocity profiles using the macroscopic Darcy’s law. Results and discussion

Fig. 2 Scheme of setup used to determine gas permeability.

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Two sets of samples were prepared. Set 1 was produced from emulsion templates stabilised by the surfactant Hypermer 2296; Soft Matter, 2009, 5, 4780–4787 | 4783

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7.0  1014  0.2  1014 2.4  1014  0.1  1014 2.7  1014  0.2  1014 1.5  1014  0.1  1014 9  1015  1  1015 7  1015  1  1015 4.6  1013  0.4  1013 9.9  1014  0.2  1014 5.2  1014  0.1  1014 3.8  1014  0.1  1014 3.9  1014  0.1  1014 27  1015  0.3  1015 26  1015  0.3  1015

Mercury permeability/m2 Gas permeability/m2

89 82 77 73 73 63 61 50

1 2 2 1 2 1 1 2

4.7 4.9 6.6 5.8 3.1 3.1 4.3 3.4

 1.4  1.9  1.2  1.1  1.2  1.2  1.7  1.3

1.0 1.5 1.8 1.7 0.9 1.7 1.4 1.1

 0.5  0.5  0.6  0.8  0.5  0.5  0.3  0.3

1.28  0.04 0.60  0.02 0.48  0.02 0.70  0.02 0.80  0.03 0.85  0.03

From Hg (flow limiting) From SEM analysis

72  9 60  20 100  10 110  40 160  40 160  10 350  10 0.130  0.004 0.223  0.003 0.270  0.004 0.317  0.002 0.314  0.001 0.421  0.001 0.446  0.002 0.565  0.003 Set 2

Set 1

1 2 3 4 5 6 7 8

Young’s modulus/MPa Foam density/g cm3

3.6  0.3 4.7  0.6 5.4  0.3 8.4  0.9 13  1 14  2 19  2

Crush strength/MPa

Porosity [%]

Pore size/mm

Pore throat diameter/mm

Sample

Fig. 3 (a–d) SEM images of polyHIPEs to polyMIPEs 1–4.

Table 2 Summary of the measured properties of the macroporous polymers

the internal phase volumes ranged from 84 vol% to 60 vol%. However, the surfactant Hypermer 2296 alone was not suited to stabilise emulsions having internal phase volumes of less than 60%; such emulsions exhibited sedimentation during polymerisation. Therefore, set 2 was produced from emulsion templates with internal phase volumes ranging from 60 vol% to 25 vol% formulated using a surfactant mixture of Hypermer 2296 and Hypermer B246sf. The two 60 vol% MIPEs 4 and 5 have been formulated in order to evaluate whether or not the structure of the resulting macroporous polymers is in any way affected by the type of surfactant used to stabilise the emulsion. The formulations are summarised in Table 1. Representative SEM images of the set 1 samples are shown in Fig. 3a–d. The average pore diameters and pore throat diameters determined using SEM image analysis are summarised in Table 2. Fig. 3a shows the characteristic open interconnected structure of a polyHIPE, it is clear that each pore of polyHIPE 1 is connected to the neighbouring pores by numerous pore throats. Similarly, polyHIPE 2 has an open porous structure with a considerable amount of interconnecting pore throats (Fig. 3b). PolyMIPEs 3 and 4 (Fig. 3c and d) have slightly larger pores than polyHIPEs 1 and 2. There is evidence of a reduction in the number of interconnecting pore throats, however, the average pore throat sizes are notably larger than those of polyHIPE 1. The polyMIPEs and polyLIPE of set 2 also have an open porous structure. Fig. 4a–d show SEM images of the set 2 samples. The average pore diameters and pore throat diameters determined using SEM image analysis are summarised in Table 2. Although polyMIPEs 4 and 5 were both synthesised from emulsion templates having an internal phase volume of 60% their average pore diameter does differ significantly. An average pore of polyMIPE 5 is approximately half the size of an average pore of polyMIPE 4. The reduction in pore size is the result of the decrease droplet size of MIPE 5 compared to MIPE 4, which is caused by the increased effectiveness of the surfactant mixture of Hypermer 2296 and Hypermer B246sf (MIPE 5) to stabilise emulsions containing 60 vol% internal phase compared to Hypermer 2296 alone (MIPE 4). A significant decrease of the number of interconnecting pore throats can be observed in the SEM images of

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Fig. 5 Mechanical test results: Young’s modulus as a function of porosity.

Fig. 4 (a–d) SEM images of polyMIPEs to polyLIPE 5–8.

polyMIPEs 6 and 7 and polyLIPE 8. The SEM images of polyMIPE 7 (Fig. 4c) and more notably of polyLIPE 8 (Fig. 4d) show the absence of a typical open porous structure, however, pore throats can still be identified. The average material density of all macroporous polymers is 1.17  0.01 g cm3. The foam densities and the porosities of the macroporous polymers are presented in Table 2. As the internal phase volume of the emulsion template was increased, the foam density of the resulting macroporous polymers decreased and the porosity increased. Even under the assumption that the entire internal phase and the surfactant were removed during the purification and drying steps, the porosities of all macroporous polymers are higher than expected. This indicates that the polymerisation was incomplete. Young’s modulus as a function of the porosity of the macroporous polymers is shown in Fig. 5. As expected, the Young’s modulus increases with decreasing porosity and, therefore, with increasing foam density of the macroporous polymers. PolyLIPE 8 with the highest foam density had the highest compression modulus. It is worth noting that the Young’s moduli of polyMIPEs 4 and 5, both having the same porosity, are identical within the errors. Although the structural differences between polyMIPEs 4 and 5 do not influence the Young’s modulus, the crush strength of polyMIPE 5 is significantly higher than that of polyMIPE 4 (Table 2), this is due to polyMIPE 5’s dramatically reduced pore size. As expected, with decreasing porosity the crush strength also increases from polyHIPE 2 to polyLIPE 8 (Table 2). Mercury intrusion experiments were carried out on polyMIPEs 3 and 4 from set 1 and all samples in set 2. No flow of mercury through polyMIPE 3 was observed until the applied nitrogen pressure reached 0.3 MPa (Fig. 6, approx. zero permeability). This corresponds to a diameter of 1.28 mm and is indicative of the flow limiting (i.e. smallest) pore throat diameter of a series of interconnected pores with the largest pore throat diameters. The flow limiting pore throat diameters are summarised in Table 2. Initially, the flow limiting pore throat diameter decreases with decreasing porosity; polyMIPE 5 possesses the smallest flow limiting pore throat diameter. However, from this point onwards the flow limiting pore throat This journal is ª The Royal Society of Chemistry 2009

Fig. 6 Mercury permeability distribution analysis: permeability as a function of applied pressure for set 1 and set 2 samples.

diameter increases with decreasing porosity. This is surprising especially since the average pore throat diameters obtained from the analysis of the SEM images are identical within the error independent of the porosity of the macroporous polymers (Fig. 7 and Table 2). The large error of the average pore throat diameters reflects the broad pore throat size distribution. This clearly shows that although the average pore throat size follows the before mentioned trend, the range of individual pore throat sizes and, therefore, the smallest pore throat size within a particular sample varies dramatically between the samples. It is worth noting that it is not, however, possible to identify a series of interconnecting pores from SEM images. Mercury intrusion measurements of the pore throat diameters are useful when flow properties are of interest since the pore throat diameters that control fluid flow are determined. The mercury permeability distributions are shown in Fig. 6 and the average permeability of the macroporous polymers is summarised in Table 2. As expected, polyMIPE 3 has overall the highest mercury permeability and the permeability decreases generally with decreasing porosity of the macroporous polymers. This indicates that although the pressure, which needs to be applied to generate flow through the sample, is determined by the flow limiting pore throat diameter, the overall permeability is strongly dependent on the porosity of the individual macroporous polymer. This is the reason why polyLIPE 8 and not polyMIPE 5, which possesses the smallest flow limiting pore throat size, exhibits the lowest mercury Soft Matter, 2009, 5, 4780–4787 | 4785

Fig. 7 Comparison of the pore throat diameter measurements from SEM analysis, mercury intrusion for the flow limiting pore throat diameter and of reconstructed samples.

permeability. However, it is surprising that polyLIPE 8, which was made from an emulsion template containing as little as 25 vol% internal phase and whose pore structure is seemingly close celled (Fig. 4d), is still permeable. The gas permeability coefficient was calculated from the experimental measurements of the rate of pressure rise (dp2/dt) at various applied mean pressures pm. However, it was not possible to determine the gas permeability of polyHIPE 1; it failed in the apparatus due to its very low shear strength. Fig. 8 shows the gas permeability coefficient as a function of pressure. The viscous permeability values k were calculated from the slope of the individual curves and are summarised in Table 2. The permeability coefficients K as well as the slope of the permeability coefficient vs. pressure curve of polyHIPE 2 are much higher than those of all other samples. It is, therefore, no surprise that the viscous permeability of polyHIPE 2 is approx. one order of magnitude higher than those of the other samples. This indicates that polyHIPE 2 not only possesses the highest porosity but also the highest degree of pore interconnectivity. In general, the viscous permeability shows a strong dependence on the porosity; the gas permeability decreases with decreasing porosity. However, there is sudden drop in gas permeability of polyMIPE 5 compared to polyMIPE 4. Although the porosities of both samples are identical, it seems that the pores of polyMIPE 5 are less interconnected than those of polyMIPE 4. Again surprisingly

Fig. 8 Gas permeability analysis: variation of the permeability coefficient with mean pressure for set 1 and set 2 samples.

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even polyLIPE 8, whose emulsion template had the lowest internal phase volume and which, therefore, has the lowest porosity, was also permeable to gas. This proves that even a polyLIPE can possess sufficient number of pores, which are interconnected from one end of the sample to the other, and therefore gas and liquid flows are possible. Several series of structures (Fig. 9) with a pore size of 5.0  1.8 mm, desired pore throat size of 1.50  0.46 mm and porosities in the range from 0.45 to 0.90 were generated using the algorithm described above. The sequence of porous structures shown in Fig. 9 illustrates how increasing the internal phase volume and, therefore, the porosity affects the morphology of the resulting pore structure. At phase volume fractions below the close random packing limit (0.53 and 0.63), there are significant blocks of continuous solid phase and the typical coordination number of each cavity is between 3 and 5. Nevertheless, the pore network is clearly above its percolation threshold and therefore the permeability is not zero. When increasing the internal phase volume fraction above the close random packing limit, the coordination number increased sharply (to as much as 10–12) and the characteristic thickness of the remaining solid phase is reduced dramatically, with implications for the mechanical properties of the structure. The mean pore throat diameter for samples with porosity higher than approximately 70% increased above the set value of 1.50 mm as spheres were pressed into each other by shrinking the computation box. This effect on the throat size distribution can be seen in Fig. 7. The comparison of the calculated permeability with measured values is illustrated in Fig. 10. The permeability of the reconstructed porous media increases in an exponential way with porosity. Both sets of experimentally measured permeability—by gas and mercury—also follow an exponential dependence on

Fig. 9 Illustration of the reconstructed sample structures. The porosity of samples is 0.53, 0.63, 0.77, and 0.84, respectively.

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Fig. 10 The permeability of reconstructed samples as a function of porosity. The calculated permeability is compared with permeability measured by two different techniques.

porosity. The agreement between these functional relationships indicates that the model used for generating the computerreconstructed structures provides structures that agree with the real media not only visually (Fig. 3, 4 and 9) but also in terms of the effective transport properties. In particular, we have been able to demonstrate both experimentally and computationally that contrary to prevailing opinion, it is possible to obtain percolating (i.e. having non-zero permeability) emulsion templates even for internal phase volume fractions below the close random packing limit. This constitutes perhaps the most significant finding of the present work. Our results show that by tailoring of the porosity and the interconnecting pore throat diameter of the macroporous polymers prepared by the polymerisation of emulsion templates, the permeability can be adjusted over a wide range. However, the major challenge remains of how to adjust the porosity and pore throat diameter independently.

Conclusions Macroporous polymers were prepared by the polymerisation of high, medium and low internal phase emulsion templates with internal phase volumes ranging from 84 vol% to 25 vol%. However, emulsions with internal volume fractions below 60 vol% could only be stabilised using a mixture of the surfactants Hypermer 2296 and Hypermer B246sf. As the internal phase volume of the templates was reduced, the resulting macroporous polymers still had an open porous structure with interconnecting pore throats. It was noted that the number of interconnecting pore throats decreased with decreasing porosity. As expected, the mechanical properties were improved as the foam density was increased and the polyLIPE sample produced from a 25 vol% internal phase emulsion had the highest compression modulus of 350 MPa and crush strength of 19 MPa. Analysis of SEM images showed little variation in the pore throat diameters, but the results were not consistent with measurements taken using mercury intrusion. Further flow measurements using a gas pressure rise technique show that the gas permeability of the macroporous polymers decreases as the internal phase volume of the emulsion templates and, therefore, the porosity decrease. This is consistent with the measurements of permeability using mercury. Surprisingly even the polyLIPE This journal is ª The Royal Society of Chemistry 2009

made from an emulsion template with 25 vol% internal phase volume was found to be permeable. Reconstruction modelling of the transport properties of porous materials showed that the permeability of a porous material with similar structures to that of the macroporous polymers increases exponentially with the porosity. We have shown that it is possible to produce interconnected macroporous polymers from emulsion templates with a reduced internal phase volume down to 25 vol%. As expected the mechanical properties of the macroporous polymers improved as the foam density increased. All macroporous polymers prepared from HIPE, MIPE and even LIPE templates are in fact still permeable to nitrogen and mercury.

Acknowledgements SSM thanks the UK Engineering and Physical Sciences Research Council (EPSRC) for funding via CASE DTA. We gratefully acknowledge funding support from Halliburton Energy Services.

Notes and references 1 D. Barby and Z. Haq, European Patent, 1982, 0 060 138. 2 J. M. Williams and D. A. Wrobleski, Langmuir, 1988, 4, 656. 3 J. M. Williams, A. J. Gray and M. H. Wilkerson, Langmuir, 1990, 6, 437. 4 J. M. Williams, Langmuir, 1991, 7, 1370. 5 E. M. Christenson, W. Soofi, J. L. Holm, N. R. Cameron and A. G. Mikos, Biomacromolecules, 2007, 8, 3806.  6 P. Krajnc, D. Stefanec, J. F. Brown and N. R. Cameron, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 296. 7 M. Ottens, G. Leene, A. A. C. M. Beenackers, N. Cameron and D. C. Sherrington, Ind. Eng. Chem. Res., 2000, 39, 259. 8 M. S. Silverstein, H. W. Tai, A. Sergienko, Y. L. Lumelsky and S. Pavlovsky, Polymer, 2005, 46, 6682. 9 P. Krajnc, N. Leber, J. F. Brown and N. R. Cameron, React. Funct. Polym., 2006, 66, 81. 10 M. Grosse, M. Lamotte, M. Birot and H. Deleuze, J. Polym. Sci., Part A: Polym. Chem., 2007, 46, 21. 11 R. J. Wakeman, Z. G. Bhumgara and G. Akay, Chem. Eng. J., 1998, 70, 113. 12 A. Menner, K. Haibach, R. Powell and A. Bismarck, Polymer, 2006, 47, 7628. 13 N. R. Cameron and D. C. Sherrington, Adv. Polym. Sci., 1996, 126, 163. 14 N. R. Cameron, Polymer, 2005, 46, 1439. 15 A. Menner and A. Bismarck, Macromol. Symp., 2006, 242, 19. 16 A. Menner, R. Powell and A. Bismarck, Macromolecules, 2006, 39, 2034. 17 D. Hilbert and S. Cohn-Vossen, Geometry and the Imagination, Chelsea, New York, 1999, pp. 45–53. 18 J. Normatov and M. S. Silverstein, Polymer, 2007, 48, 6648. 19 N. R. Cameron, D. C. Sherrington, L. Albiston and D. P. Gregory, Colloid Polym. Sci., 1996, 274, 592. 20 J. V. Brakel, S. Modry and M. Svata, Powder Technol., 1981, 29, 1. 21 K. J. Edler and S. P. Rigby, J. Colloid Interface Sci., 2002, 250, 175. 22 A. Bismarck, G. F. Hewitt and S. S. Manley, Apparatus and method to measure properties of porous media, Patent application number PCT/GB2008/002754, 2008. 23 K. Ishizaki, S. Komareneni and M. Nanko, Porous Materials: Process Technology and Applications, Kluwer, UK, 1998. 24 R. E. Collins, Flow of Fluids Through Porous Materials, Reinhold Publishing Corporation, New York, 1961. 25 P. C. Carman, Flow of Gases Through Porous Media, Butterworths Scientific Publications, London, 1956, pp. 47–69. 26 P. M. Adler, Appl. Mech. Rev., 1998, 51, 537. 27 M. Kohout, A. P. Collier and F. Stepanek, Powder Technol., 2005, 156, 120. 28 P. A. Cundall and O. D. L. Strack, Geotechnique, 1979, 29, 47.

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