Effectiveness of Emulsion-Templated Macroporous Polymer

May 12, 2015 - and Alexander Bismarck*. ,†,‡. †. Department of Chemical Engineering, Polymer & Composite Engineering (PaCE) Group, Imperial Coll...
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Effectiveness of Emulsion-Templated Macroporous Polymer Micromixers Characterized by the Bourne Reaction Michael Tebboth,† Andreas Kogelbauer,*,† and Alexander Bismarck*,†,‡ †

Department of Chemical Engineering, Polymer & Composite Engineering (PaCE) Group, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ‡ Polymer & Composite Engineering (PaCE) Group, Institute of Materials Chemistry and Research, Faculty of Chemistry, University of Vienna, Währingerstr. 42, 1090 Vienna, Austria ABSTRACT: High porosity, interconnected macroporous polymers prepared by the polymerization of high internal phase emulsions (polyHIPEs) stabilized by either surfactants or a combination of surfactants and hydrophobized silica particles were used as micromixers for liquids. The polyHIPEs had permeabilities between 30 and 2550 mD. The extent of micromixing was characterized by a system of two competitive parallel reactions (fourth Bourne reaction) carried out within the polyHIPEs. These competitive reactions, an acid−base neutralization and acid-catalyzed hydrolysis, proceed at very different rates such that the extent of the hydrolysis reaction is dependent on the extent of micromixing in the system, with lower hydrolysis yields indicating more effective micromixing. It was found that the hydrolysis yields were up to 50% lower in macroporous polymers compared to those in an empty tube and also up to 45% lower than those in a conventional spiral static mixer at a flow rate of 1 mL/min. Macroporous polymers with smaller pore throat sizes were found to give lower hydrolysis yields, corresponding to better micromixing; however, they did so at the expense of decreased permeabilities. However, using macroporous polymers results in a significant pressure drop across the flow cells, whereas an empty tube and conventional spiral static mixer cause a negligible pressure drop. plasticizer.7 They also often have low permeability due to their small average pore size, which determines the pore throat size.8 For this reason, the number of industrial applications for polyHIPEs is currently still limited.5,9 PolyHIPEs produced from HIPEs stabilized by particles (Pickering-HIPEs) often have larger average pore sizes and better mechanical properties than those produced from surfactant-stabilized HIPEs because the particles can act simultaneously as reinforcement.10 Unfortunately, poly-Pickering-HIPEs are often close-celled, meaning that pore throats did not form.11 However, it has been reported by Ikem et al.12 that introducing a small amount of surfactant (approximately 5% of the continuous phase volume) under agitation to a preformed Pickering-HIPE before polymerization results in poly-Pickering-HIPEs with large pore throats and hence high and controllable permeabilities. It has already been shown that polyHIPEs are effective supports for both batch and flow-through reactions due to their ability to prevent the channelling seen in packed bed reactors.13 PolyHIPEs, because of their unique properties and ease of synthesis, have long been considered for use in many diverse applications, including chromatography columns,14 scaffolds for tissue engineering,15 ion exchange columns,16 supports for chemical reactions,17 scaffolds for setting cements,18 immobilization of enzymes,19 the production of porous electrodes,20 hydrogen storage,21 and down well filters.12a

1. INTRODUCTION PolyHIPEs are created by the polymerization of the continuous monomer-containing phase of a high internal phase emulsion (HIPE) with an internal phase volume of greater than 74%,1 whereas polyMIPEs are obtained by polymerization of medium internal phase emulsions with phase volume ratios between 30 and 74%. Most commonly, water-in-oil (w/o) emulsions are used as templates, with the continuous phase being styrene (S) containing a cross-linker, such as divinylbenzene (DVB), and the dispersed phase being an aqueous electrolyte solution. An internal phase volume ratio of 74.05% corresponds to the maximum possible packing of monodispersed spherical droplets.2 In order to achieve emulsions with a volume fraction exceeding 74%, droplets must either undergo deformation as they are packed together, taking a polyhedral shape, or have very broad droplet size distribution such that the smaller droplets take up the space between the larger droplets. After polymerization and the removal of the internal (dispersed) templating phase, pores are created in the polymer having the shape of the templating droplets.1b Often, the thin walls between the pores will open, due to mechanisms still being discussed,3 leading to a high degree of pore interconnectivity4 between the pores and resulting in an appreciable permeability of the macroporous polymers. These interconnects between pores are called pore throats. PolyHIPEs have traditionally been produced from HIPEs stabilized by large volumes (20−50% of continuous phase volume) of nonionic surfactants, such as Hypermer 22965 and Span 80 (sorbitan monooleate).6 However, polyHIPEs produced from surfactant-stabilized HIPEs often suffer from poor mechanical properties because the surfactant can act as a © 2015 American Chemical Society

Received: Revised: Accepted: Published: 5974

February 4, 2015 May 8, 2015 May 12, 2015 May 12, 2015 DOI: 10.1021/acs.iecr.5b00493 Ind. Eng. Chem. Res. 2015, 54, 5974−5981

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Industrial & Engineering Chemistry Research Table 1. Composition of HIPE Formulations sample

water (mL)

styrene (mL)

DVB (mL)

Hypermer 2296 (mL)

hydrophobized silica particles (g)

AIBN (g)

internal phase percentage of template HIPE

S-1 S-2 S-3 S-4 S-5 S-6 S-7 P/S-1 P/S-2 P/S-3 P/S-4 P/S-5 P/S-6 P/S-7

37.5 40 40 40 40 40 40 37.5 40 40 40 45 40 40

5 4 4 4 4 4 4 6.25 5 5 5 2.5 5 5

5 4 4 4 4 4 4 6.25 5 5 5 2.5 5 5

2.5 2 2 2 2 2 2 0.625a 0.5a 0.5a 0.5a 0.25a 0.5a 0.5a

0 0 0 0 0 0 0 0.375 0.3 0.3 0.3 0.15 0.3 0.3

0.16 0.13 0.13 0.13 0.13 0.13 0.13 0.20 0.16 0.16 0.16 0.08 0.16 0.16

75 80 80 80 80 80 80 74.1 79.2 79.2 79.2 89.6 79.2 79.2

a

Added after initial emulsification.

occurs. However, in the event of slower, less effective mixing, more DMP molecules will encounter local regions of low pH, causing a greater yield of hydrolysis products to be observed. The effectiveness of micromixing within the polyHIPE structure can be assessed, therefore, by analysis of the concentration of the products of the hydrolysis reaction and comparing this with control samples.

Here, we consider the use of polyHIPEs as micromixing elements. We postulated that their intricate internal structure will result in effective mixing when fluids are forced through a polyHIPE, as the continuous reorientation of the fluids could lead to rapid contact between fluid elements at much lower flow rates than would be required for traditional static mixers22 or impingement thin liquid sheets mixers23 and at lower shear than a rotor−stator mixer.24 This could be of significance in areas such as pharmaceutical production with trends toward continuous production at low flow rates.25 PolyHIPE monoliths also have greater reactor volumes than microreactors that are also used as mixers in the pharmaceutical and fine chemical industries.26 In order to assess the effect on homogeneous mixing of a fluid flow through an emulsion-templated macroporous polymer, a mixing-sensitive homogeneous reaction was used. These usually consist of two competitive reactions, which proceed at very different rates such that the extent of one reaction is dependent on the extent of micromixing in the system.27 The fourth Bourne reaction28 was used to assess the micromixing within macroporous polymers due to its ease of use. The fourth Bourne reaction consists of two reactions that are competitive and parallel: the neutralization reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) and the acid-catalyzed hydrolysis of dimethoxypropane (DMP) to acetone and methanol:28b,29

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene (S) ≥99%, divinylbenzene (DVB) 80%, α,α′-azoisobutyronitrile (AIBN), calcium chloride dihydrate (CaCl2·2H2O) ≥99%, dimethoxypropane (DMP), sodium chloride (NaCl) ≥99%, sodium hydroxide (NaOH) ≥99%, hydrochloric acid (HCl) 37% solution, sodium hydrogen carbonate (NaHCO3) ≥95%, and acetonitrile (CH3CN) ≥99.8% were purchased from Sigma-Aldrich. Hydrophobic pyrogenic silica particles HDK H20 were kindly provided by Wacker Chemie AG (Germany). High-temperature heat shrink tubing 9.9 mm bore, Araldite rapid adhesive, and Araldite 2020 were purchased from RS Components Ltd. (Corby, UK). Non-ionic ethoxylated ester-type surfactant Hypermer 2296 (HLB = 4.9) was kindly provided by Croda (USA). Extruded polyacrylic tubing was purchased from Gilbert Curry Industrial Plastics Co Ltd. (Coventry, UK). All materials were used as received. 2.2. Production of PolyHIPE Monoliths. PolyHIPEs produced by polymerization of HIPEs stabilized by surfactant or a mixture of surfactant and particles (Pickering-HIPEs) were investigated. In both cases, HIPEs were produced in 50 mL batches. Surfactant-stabilized HIPEs were produced by the addition of 0.27 M calcium chloride dihydrate solution as dispersed phase to the continuous phase consisting of styrene, DVB, and Hypermer 2296 (in the volume ratio 2:2:1, respectively) contained in a glass vessel agitated by a glass paddle rod stirrer connected to an overhead stirrer. Also present in the continuous phase was 1 mol % AIBN with respect to the monomers to initiate polymerization; this was dissolved prior to the addition of the dispersed phase. In order to create an 80% dispersed phase volume HIPE, 40 mL of calcium chloride solution was added to 10 mL continuous phase; however, the final porosity of the polyHIPE produced was often slightly different to the dispersed phase volume of the template emulsion,30 as seen in Table 2. The dispersed phase

HCl + NaOH → NaCl + H 2O k = 1.3 × 1011 m 3·kg −1·mol−1· s−1 at 25 °C

(1)

H+

CH3C(OCH3)2 CH3 + H 2O ⎯→ ⎯ CH3COCH3 + CH3OH k = 700 m 3·kg −1·mol−1·s−1 at 25 °C and C NaCl = 100 mol ·m−3 (2)

From eqs 1 and 2, it can be seen that reaction coefficients differ by many orders of magnitude, meaning that in the event of rapid mixing of fluid elements the neutralization reaction will move rapidly to completion before significant DMP hydrolysis can occur. Because the sodium hydroxide is present in slight excess, the reaction mixture is alkaline once neutralization is complete, so no further acid-catalyzed DMP decomposition 5975

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removed from the tube and dried in a convection oven at 70 °C until its weight was constant. Any loose polymer fragments were brushed off the surface of the monolith at this stage to ensure that the surface will adhere properly to the adhesive in the next preparation step to prepare flow cells to avoid fluid bypassing. 2.3. Production of PolyHIPE Flow Cells. The polyHIPE monoliths were placed into high-temperature heat shrink tubing (RS Components Ltd. Corby, UK) whose inner surface was previously coated with fast curing Araldite Rapid adhesive, which has been shown to seal polyHIPEs effectively during gas permeability measurements11 (stage 2 in Figure 1). The monolith is then exposed to hot air from a heat gun, which causes the shrink tubing to contract onto the polyHIPE monolith, sealing its sides. Care was taken not to trap air bubbles (stage 3 in Figure 1). The ends of the polyHIPEs were subsequently sealed with Araldite Rapid adhesive. The monolith was then held coaxially in an outer protective tubing (polyacrylic or steel) capped at one end, which was then filled with transparent Araldite 2020, covering the majority of the monolith. This was then placed in the convection oven at 70 °C for at least 5 h to fully cure the Araldite 2020. After removal from the oven, both ends of the flow cells were machined flat, with one end being fully opened to atmosphere (stage 4 in Figure 1). The other end had two 1/16 in. feed holes drilled to a depth of 5 mm into the monolith in order to prevent contact between the two solutions outside of polyHIPE structure. 1/16 in. feed tubes were then inserted into these holes and sealed in place with a small amount of super glue. After this, the feed tubes were additionally sealed into a 2 cm deep block of Araldite 2020 and cured in a convection oven, with the superglue preventing the Araldite from flowing into and blocking the feedholes (stage 5 in Figure 1). 2.4. Determination of PolyHIPE Density and Porosity. The skeletal density ρs of the polyHIPEs was determined using helium pycnometry (AccuPyc 1330, Micromeritics Ltd., Dunstable, UK), and the foam density ρf of the polyHIPEs, using an envelope density analyzer (GeoPyc 1360, Micromeritics Ltd., Dunstable, UK). From the two densities determined, the porosity P of the samples was calculated, using the equation given below.

was then slowly added to the continuous phase from a dropping funnel at a rate of 0.25 mL·s−1 under constant agitation of 400 rpm. When all dispersed phase had been added, the HIPE was vigorously agitated at 2000 rpm for various lengths of time so that a range of emulsion templates with various droplet sizes and internal phase percentages was produced; greater agitation leads to reduced droplet diameters.31 The continuous phase of the Pickering-HIPEs was prepared by taking a 1:1 mixture of styrene/DVB and adding 3% (w/v) of hydrophobized silica particles before homogenizing the dispersion for 15 min at 15 000 rpm using a Kinematica POLYTRON PT 1600 E.10b This mixture was then filled in to the glass mixing vessel with 1 mol % AIBN, and the dispersed phase was added as before under the same agitation conditions to produce the template emulsion. However, after the addition of the dispersed phase, 5% by volume of Hypermer 2296 was added with respect to the continuous phase only to the 50 mL of Pickering-HIPE. The Pickering-HIPE was then agitated at 2000 rpm as before. All surfactant-stabilized HIPEs were agitated for at least 10 s at 2000 rpm to ensure that they remained stable during polymerization.32 However, one Pickering-HIPE (sample P/S7) was not agitated at 2000 rpm in order to produce polyHIPEs with very large pore throats.12a Instead, it was agitated for 30 s at 400 rpm to disperse the Hypermer 2296 throughout the emulsion. Complete HIPE formulations can be seen in Table 1. After the emulsions were produced, they were sucked into polyacrylic tubes with an internal diameter of 7 mm using a syringe (stage 1 in Figure 1). These tubes were then capped at both ends taking care to exclude air bubbles. The sealed tubes were placed into a sealed secondary containment vessel, which was then placed into a convection oven at 70 °C for 24 h to allow polymerization. After polymerization, the monolith was

⎛ ρ⎞ P (%) = ⎜⎜1 − f ⎟⎟ × 100 ρs ⎠ ⎝

(3)

2.5. Pore Structure of PolyHIPEs: Scanning Electron Microscopy. The pore structure of the samples was analyzed using a Hitachi S-3400N scanning electron microscope (SEM). The surface of roughly 1 cm3 sample was prepared for SEM by fixing them to SEM stubs using super glue and sputtering them with gold for 20 s under vacuum using an Agar automatic sputter coater. At least 100 pores and pore throats were recorded using the image analysis software ImageJ. The sizes of the pores and pore throats as well as the average number of pore throats per pore were evaluated. 2.6. Micromixing Experimental Setup. The experiments to quantify micromixing using the Bourne reaction were performed in the setup schematically shown in Figure 2. The flow cell was connected to two syringe pumps: one containing hydrochloric acid (360 mol·m−3) and the other containing a solution of sodium hydroxide (380 mol·m−3), DMP (200 mol· m−3), and NaCl (100 mol·m−3).29 These solutions were then pumped into the flow cell via the separate 1/16 in. tubes,

Figure 1. Stages in the production of polyHIPE flow cells: (1) insert HIPE into rigid acrylic tubing and polymerize at 70 °C in an oven; (2) remove polyHIPE from acrylic tubing, dry monolith in an oven, and place into shrink tubing coated with rapid adhesive; (3) heat shrink tubing to force adhesive into contact with monolith and cure it; (4) seal inside larger acrylic tubing with epoxy resin and machine both ends flat, opening one to atmosphere; and (5) drill feed holes and insert feed tubes before sealing with super glue and more epoxy resin. 5976

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Figure 2. Experimental setup used to determine the pressure drop and mixing characteristics of polyHIPEs.

Figure 3. Representative SEM images of polyHIPE samples S-1 and S-7 produced from surfactant-stabilized HIPEs as well as P/S-1 and P/S-7 produced from particle-stabilized HIPEs.

polymer monolith were measured after manufacturing the flow cell, taking into account the 5 mm depth of the drilled feedholes. It was assumed that the viscosity of the reaction mixture was constant and equal to that of water (10−3 Pa·s). However, when used for the first time, the initial breakthrough of liquid from the polyHIPE flow cells often produces a small amount of polymer fragments before the liquid begins to run clear. These fragments were examined using SEM and found not to be of a porous structure typical of emulsiontemplated polymers, suggesting that they are not pieces of polymer that have broken off by the fluid flow. These fragments are most likely debris left over from the machining of the polyHIPE (stage 4 in Figure 1). Samples were never taken until all polymer fragments had been flushed out. Samples of liquid were taken at the outlet of the polyHIPE flow cell after at least three pore volumes of liquid had passed through the flow cell in order to ensure steady state. The flow rates of the two liquids were always identical to ensure that the NaOH was always in excess of the acid. After the experiment was complete, the flow cells were stored for a month before the experiment was repeated.

displacing the air from the monolith. The NaOH and DMP (stable at high pH) in aqueous solution are mixed with the HCl solution within the macroporous polymer structure with the NaOH being in slight stoichiometric excess. The pressure up stream of the flow cell was measured using two pressure transducers, one on each feed line, which were connected to a digital readout. Since the far end of the flow cell was open to the atmosphere, the pressure drop across the polyHIPE was known. The permeability k of the samples was calculated using Darcy’s law, which relates the volumetric flow rate Q of a liquid of known viscosity μ through a porous sample with a crosssectional area A and length L with the pressure difference across the sample ΔP. The permeability k is given in units of Darcy’s, where 1 Darcy is equivalent to 10−12 m2. Q=

kA ΔP μ L

(4)

The pressure drop was measured by pressure transducers in both the acidic and alkali feed lines. The dimensions of the 5977

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Industrial & Engineering Chemistry Research Table 2. Effect of the Pore Structure of Macroporous Polymers on Permeability and the Extent of Acid-Catalyzed Decomposition of DMPa sample S-1 S-2 S-3 S-4 S-5 S-6 S-7 P/S-1 P/S-2 P/S-3 P/S-4 P/S-5 P/S-6 P/S-7 blank tube spiral static mixerc HPLC columnd

porosity (%)

average pore diameter (μm)

average pore throat diameter (μm)

permeability (mD)

steady-state yield of DMP decomposition (%)

75 81 81 83 80 83 83 78 85 85 84 91 85 81 100 ≈65

3.4 ± 1 3.5 ± 1.1 4.5 ± 1.3 6.4 ± 2.2 6±2 13.5 ± 4.3 20.6 ± 8.4 4.1 ± 1.9 5.2 ± 2.3 8.2 ± 2.6 8.8 ± 2.6 12.6 ± 4 39.8 ± 18.3 86.8 ± 34.3 n/a n/a

1 ± 0.3 1.2 ± 0.4 1.7 ± 0.6 1.9 ± 0.6 2.1 ± 0.6 3.6 ± 1.2 3.8 ± 1.3 0.6 ± 0.2 1.3 ± 0.4 1.6 ± 0.6 2.2 ± 0.7 3.2 ± 1.3 9.2 ± 4.4 22.2 ± 9.9 n/a n/a

40 170 230 400 230 400 190 30 60 360 220 n/ab 1660 2550 n/a n/a

19.9 14.7 21.6 20.8 21.1 21.6 22.3 12.2 17.7 20.2 19.3 22.5 21.2 23.1 26.2 22.9

49

n/a

1.13

20

22.9 ± 0.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0 0.8 1.2 2.4 0.3 0.2 0.3 0 0 0.2 0.9 0.1 0.3 0 0.9 1.6

Reynolds no. (Re) 5.60 6.50 8.90 9.80 1.10 1.90 2.00 3.10 6.50 8.40 1.10 1.50 4.70 2.70 3.03 4.66

× × × × × × × × × × × × × ×

10−4 10−4 10−4 10−4 10−3 10−3 10−3 10−4 10−4 10−4 10−3 10−3 10−3 10−2

3.30 × 10−3

All data in this table were collected at a total flow rate of 1 mL/min. Permeability could not be accurately determined. Helical static mixer with twelve 6 mm helical elements in 6.5 mm i.d. tubing. dAn equivalent pore throat size was estimated from the area between tightly packed 5 μm spheres, and porosity, from personal communication with M. Whitmore, 2013. a

b

2.7. Determination of Effectiveness of Micromixing. The extent of the decomposition of the DMP was quantified using high-performance liquid chromatography (HPLC) to determine the concentration of acetone at the exit of the flow cell. The amount of acetone detected is indicative of the effectiveness of the micromixing, as poorer mixing will cause greater DMP decomposition and hence a higher acetone concentration will be detected. The HPLC mobile phase used was a 2:1 mixture of water and acetonitrile buffered to a pH of 9.2 with NaHCO3 to prevent further DMP decomposition within the HPLC. A mobile phase flow rate of 1 mL/min was used, and the injected sample volume was 30 μL. The sample was analyzed by UV detection at a wavelength of 270 nm, as acetone is the only UVactive species. The column used was a Phenomax 5 μm C18 packed bed column 4.6 mm Ø × 250 mm. The feeds were also analyzed to confirm that decomposition did not occur before the experiment was run.

c

It can be seen that the extent of DMP hydrolysis is lower in all cases where the reaction mixture is passed through a macroporous polymer compared to that in an empty tube with an internal diameter 7 mm at a total flow rate of 1 mL/min. All but sample P/S-7, with the largest average pore throat diameter, produced a similar or lower DMP hydrolysis yield compared to that with the spiral static mixer used as control mixer. However, it should be noted that the range of hydrolysis yields was generally much narrower for the flow cells than that from either the spiral static mixer or the blank tube. This is because at such a low flow rate (1 mL/min with Re < 0.1) fluid elements in the spiral mixer and blank tube are larger, leading to greater variation in the determined hydrolysis yields. After being mixed within the pores of macroporous polymers, the fluid is more homogeneous and hence more consistent hydrolysis yields were found. The HPLC column containing 5 μm particles (a particle size similar to the pores in the emulsion-templated polymers such that it could be considered to be an inverse polyHIPE) gave a hydrolysis yield similar to that of the spiral static mixer and many of the polyHIPE flow cells; however, it was much less permeable than most of the emulsion-templated macroporous polymers. The HPLC column was included to simulate a packed bed system. Overall, it was found that there was no marked difference in decomposition yields between similar macroporous polymers produced by polymerization of surfactant and particle/surfactant-stabilized HIPEs. It was generally observed that hydrolysis yields decreased when using macroporous polymers with smaller pore throats as micromixers. This was most noticeable for samples S-2 and P/ S-1; these were able to decrease the product of the hydrolysis reaction by up to 50% compared to an empty tube. It can also be seen from Figure 4 and samples P/S-6 and P/S-7 in particular that the effectiveness of the flow cells at preventing DMP decomposition levels off, giving hydrolysis yields similar to those of the spiral static mixer when the pore throat size increases to above 3 μm. This suggests that even very permeable macroporous polymers would still be able to give

3. RESULTS AND DISCUSSION 3.1. Effect of Internal Structure on Micromixing. Macroporous polymers produced from emulsions stabilized by surfactants or both particles and surfactants possess the interconnected pore structure expected of emulsion-templated macroporous polymers (Figure 3). By stabilizing HIPEs with both particles and surfactants, it was possible to greatly increase the pore size, typical for poly-Pickering-HIPEs, yet the concentration of surfactant was enough to induce the formation of pore throats, thereby creating a very permeable structure. The porosity and average pore and pore throat diameters are summarized in Table 2. By varying the agitation time and internal phase volume ratio of the emulsion template, it was possible to produce macroporous polymers with a range of porosities and pore/pore throat sizes, as can be seen by comparing the length scales in Figure 3. The skeletal densities for both forms of polyHIPE were similar, in the range of 1.2 ± 0.1 g·cm−3. 5978

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macroporous polymer flow cells with smaller pore throats; therefore, one would expect that if mixing was due to turbulence then these samples should have the highest Reynolds numbers, whereas the opposite is true. Over the range of porosities examined in this work, there was no clear link between porosity of the macroporous polymer and the extent DMP decomposition (Table 2). The length of polyHIPE required for effective micromixing was investigated by performing the Bourne reaction in a flow cell followed by reducing its length and repeating the experiment. As an example, the results for sample P/S-1 are shown in Figure 5, demonstrating that, essentially, the acid base

Figure 4. Yield of the acid-catalyzed DMP decomposition as a function of the pore throat size of the macroporous polymers produced from surfactant-stabilized emulsions and particle/surfactantstabilized emulsions. Shown for comparison are the yields of DMP decomposition for a blank tube, spiral static mixer, and HPLC column. Figure 5. Yield of DMP hydrolysis within polyHIPE monolith P/S-1 as a function of its length. This shows that neutralization of the acid is complete within the first 30 mm of the monolith, as no further reduction in DMP yield is seen at greater lengths.

improved micromixing comparable to that of a packed bed HPLC column yet at a much lower pressure drop. For example, the hydrolysis yield in sample S-7 was 22.3% similar to the HPLC column (22.9%); however, the monolith had a permeability over 10 times greater (190 mD) compared to that of the packed particle bed (18 mD). However, comparing literature results, the mixing within the monoliths was much less effective than that in spiral static mixers at their designed Reynolds numbers.28b This is due to the much high energy input per volume to the system, resulting in effective turbulent mixing. Comparable hydrolysis yields to those reported here were attained in microreactors.22 When the total flow rate was increased from 0.2 to 2 mL/ min, it was found that this had no effect on the yield of the hydrolysis reaction in the polyHIPE flow cells. The yield of decomposition products and hence the effectiveness of micromixing within the polyHIPEs appeared to be a characteristic of the pore structure, specifically, the pore throat size. Therefore, it is suggested that the micromixing within polyHIPEs is a result of extensive flow division and reorientation within the pore structure, the extent of which is increased by smaller pore throats. It is unlikely that turbulence plays any role in the micromixing due to the low flow rates involved and the small pore sizes inhibiting the formation of turbulent eddies.33 The Reynolds number (Re) for each sample was calculated using mean pore throat diameter (Dpt) as the characteristic unit of length. Where u is the fluid velocity in interior of the flow cell calculated from the fluid velocity entering the flow cell (U) divided by the permeability. ρuDpt Re = μ (5)

neutralization is complete after passing the reaction mixture through only 30 mm of the monolith at a flow rate of 1 mL/ min. If an ideal plug flow reactor34 had perfect mixing (7 mm diameter at a flow rate of 1 mL/min), the neutralization should be 99.99% complete within 3.2 × 10−11 mm and the hydrolysis yield should be virtually zero. The reason it takes 30 mm for complete neutralization is due to the time it takes for the acid and base to fully mix first within the polyHIPE structure. A similar mixing length was found by Fréchet et al.26 using porous polymer monoliths in microfluidic channels. As the length of the macroporous polymer monolith decreased toward zero, the yield of the hydrolysis reaction unsurprisingly rises to levels observed for the empty tube, most likely because the neutralization reaction is incomplete when the fluid leaves the macroporous polymer monolith. This suggests that even a small length of macroporous polymer can effectively act as micromixer, in this case preventing an extra 10% of the total DMP molecules from being decomposed. 3.2. Effect of PolyHIPE Internal Structure on Permeability. However, the downside to being able to increase micromixing by decreasing the average pore throat size and porosity is that this inevitably also leads to a decrease in permeability and higher pressure drops across the flow cells. The most important factor in determining the permeability of the polymer monoliths is the size of the pore throats (Figure 6). In varying the flow rate, it was found that the macroporous polymer monoliths obeyed Darcy’s law, with the pressure drop increasing linearly35 with volumetric flow. It should also be noted that macroporous polymers also offer a larger surface area than spiral static mixers with at least the same degree of micromixing. This surface area could be used to support a catalyst or for other forms of surface chemistry. The permeabilities recorded were consistent with values previously reported.8,12a,35 It should be noted that when comparing the

U (6) P All Reynolds numbers were small (Re < 0.1), and because the flow still obeyed Darcy’s Law, it is safe to assume lack of turbulence in the system.33 The extent of micromixing was found to be greatest (lowest DMP decomposition yields) in u=

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4. CONCLUSIONS Macroporous polymers produced from particle- and surfactantstabilized emulsions were produced with a range of porosities and pore sizes. It was found that, compared to an empty tube and helical static mixer, all macroporous polymer flow cells resulted in a lower hydrolysis yield for the fourth Bourne reaction, indicating better micromixing. The most effective sample (P/S-1) reduced the yield by 50%, although a 13% reduction in yield was more common. It was found that the characteristic that most influenced the yield of hydrolysis was the size of the interconnecting pore throats, with smaller pore throats producing lower hydrolysis yields. Smaller pore throats decrease the permeability of the polymer monoliths, with the polyHIPEs with the smallest pore throats being over an order of magnitude less permeable than those with the largest pore throats. It is suggested that these factors decrease the yield of the hydrolysis reaction by causing significant flow division and reorientation so that the neutralization can proceed rapidly to completion, reducing the opportunity for acid-catalyzed hydrolysis. It was possible to achieve greater reduction of the hydrolysis yield with an emulsion-templated macroporous polymer than with a HPLC column, acting as a packed bed, while still maintaining a higher permeability. It was found that altering the flow rates did not affect the hydrolysis yields, suggesting that the internal polyHIPE structure determines the intensity of micromixing. It was also determined that the acid base neutralization is complete after a short length of monolith, on the order of 30 mm. Monoliths shorter than this resulted in an increase in the hydrolysis yield due to incomplete neutralization within the macroporous polymer monolith.

Figure 6. Permeability of macroporous polymer monoliths as a function of mean pore throat diameter.

liquid permeabilities reported here to literature gas permeabilities there will be a small discrepancy, but this is not significant unless pore diameters are lower than the gas mean free path (≈0.1 μm),36 which they are not for the polyHIPEs used in this work. 3.3. Durability of Macroporous Polymer Flow Cells. The polyHIPE flow cells were stored for 1 month before being retested under the same conditions to examine if they maintained the same level of performance. For most samples, the yield of the hydrolysis reaction rose after storage, and for some, it rose by a considerable extent; this was accompanied by an increase in permeability. However, over half of the samples showed an increase in hydrolysis yield of less than 10% compared to the first experiment, and all samples were still more effective mixers than an empty tube. SEM micrographs taken of the flow cells after use were analyzed to evaluate if the increase in permeability was due to damage caused by the passage of the reaction solution. The SEM showed that the average pore throat size remained constant and that no obvious damage could be seen to the polymer structure. This implies that there is no significant damage to the polymer structure by fluid action at the flow rates investigated in this study. With the breakdown of the internal pore throat structure of the polyHIPE ruled out, the cause the increase permeability is likely to be the breakdown of the bond between the polymer and the epoxy adhesive due to fluid pressure within the monolith. This would cause bypassing of the polymer monolith and maybe mixing to occur on the outside of the polyHIPE structure. This is supported by the fact that the permeability of most samples increased after use and storage. It is noticeable that those samples that had the greatest percentage increase in hydrolysis operated at higher pressures (low permeabilities). This high pressure could result in the fluid being forced between the monolith and adhesive layer causing mixing to occur outside of the polyHIPE monolith. The smaller pore sizes of the lower permeability samples would also have contributed to poorer adhesion by preventing the adhesive from penetrating the material and hence forming a weaker “mechanical interlocking” bond with the monolith. The majority of the samples showed an increase in permeability 1 month after production, with the average permeability of those increasing by 11.5%.

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AUTHOR INFORMATION

Corresponding Authors

*(A.K.) E-mail: [email protected]. *(A.B.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for funding M.T. via a doctoral training award.

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REFERENCES

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