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Green synthesis of polymers using supercritical carbon dioxide

Colin D. Wood a, Andrew I. Cooper b,*, Joseph M. DeSimone a,c,*

a Department of Chemistry, University of North Carolina at Chapel Hill, NC 27599, USAb Donnan and Robert Robinson Laboratories, Department of Chemistry, University of Liverpool, Liverpool, L69 3BX, UK

c Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695, USA

Abstract

Supercritical carbon dioxide is the most extensively studied supercritical fluid (SCF) medium for polymerization reactions and

organic transformations. This can be attributed to a list of advantages ranging from solvent properties to practical environmental

as well as economic considerations. Aside from these gains, CO2 finds particularly advantageous application in the synthesis and

processing of fluoropolymers and porous materials and as such, this will be the subject of this review. The overall aim is to highlight

areas where the unique properties of SCF solvents can be exploited to generate polymers that would be difficult or inconvenient to

obtain by other routes.

2005 Elsevier Ltd. All rights reserved.

Keywords: Supercritical; Fluoropolymer; Porous polymers; High-throughput

1. Introduction

Supercritical carbon dioxide has emerged as the most

extensively studied environmentally benign medium for

organic transformations and polymerization reactions.

This stems from a list of advantages ranging from sol-

vent properties to practical environmental and economic

considerations. Increasing environmental regulation has

prompted a tremendous effort to develop and apply

environmentally benign alternative media or solvent free

processes to a variety of commercially significant, sol-

vent-intensive processes [1]. Carbon dioxide in the liquid

and supercritical state is a potential alternative for manyof the processes experiencing ever increasing regulation,

owing to its non-toxic, non-flammable nature. Addition-

ally, its ubiquity makes it inexpensive and readily avail-

able. Several excellent reviews are available that discuss

the utilization of carbon dioxide for many solvent- and

waste-intensive industries [25]. Aside from the gains

provided by CO2 as a reaction medium in general, itfinds particularly advantageous application in the syn-

thesis and processing of fluoropolymers and porous

materials [611] as such, this will be the subject of this

review. In particular the aim is to highlight areas where

the unique properties of supercritical fluid (SCF) sol-

vents can be exploited to generate polymers that would

be difficult or inconvenient to obtain by other routes.

The general properties of SCFs in relation to chemical

synthesis [8] and extraction [2] have been reviewed previ-

ously and will not be a focus herein.

2. Fluoroolefin polymerization in CO2

2.1. Introduction

Fluoropolymers are typically synthesized in aqueous

polymerization systems (both emulsion and suspension),

non-aqueous systems (Freon-113), or in hybrid Freon-

113/aqueous hybrid systems [12]. Such processes require

the use of large quantities of water, CFCs (for non-aqueous

1359-0286/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cossms.2005.02.001

* Corresponding authors.

E-mail address: [email protected] (A.I. Cooper).

Current Opinion in Solid State and Materials Science 8 (2004) 325331
mailto:[email protected]:[email protected]

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polymerizations), and fluorinated surfactants for emul-

sion polymerization. Many of the fluorinated surfac-

tants typically employed in aqueous emulsion and

suspension polymerizations are currently under scrutiny

due to bioaccumulation and environmental persistence

[13,14]. These issues have collectively resulted in an

increasingly urgent impetus for transition from the con-ventional fluoropolymer synthesis platforms to alterna-

tives that meet the requirements of emerging public

and regulatory demands. Most commercially available

fluoropolymers are prepared from a relatively small

group of olefins including tetrafluoroethylene (TFE),

chlorotrifluoroethylene (CTFE), vinylidene fluoride

(VF2), hexafluoropropylene (HFP), ethylene, and per-

fluoroalkyl vinyl ethers (PAVEs). Many of these mono-

mers are flammable and some are explosive. For

example, TFE is flammable when mixed with air and

has a high propensity for explosion during expansion

to a gas from its liquid phase under pressure. Further,

TFE is highly explosive as a gas at elevated tempera-

tures. In the presence of oxygen it will undergo autopo-

lymerization, a process sufficiently exothermic to ignite

an explosion. Aqueous suspension and dispersion poly-

merizations of copolymers of TFE with a variety of

comonomers including hexafluoropropylene (HFP)

and various perfluoroalkyl vinyl ethers (PAVEs) typi-

cally exhibit high levels of carboxylic acid end groups

[7,15]. The presence of acid end groups often proves del-

eterious to the intended properties and function of the

polymer not to mention the introduction of complica-

tions into certain post polymerization, melt-processing

steps. This problem is particularly prevalent in copoly-mers including PAVE comonomers (perfluoroalkyl vinyl

ethers) which are an essential component in the control

of crystallinity in TFE-based fluoroplastics. In order to

prevent decomposition, discoloration and emissions of

hydrogen fluoride, polymers containing high levels of

acid end groups may require high-temperature hydroly-

sis and fluorination finishing steps. A propagating poly-

meric radical derived from a terminal PAVE monomer

unit has one of two possible reaction pathways (Scheme

1). First, and most obvious, it can cross propagate to

monomer continuing the polymerization reaction or itcan undergo b-scission, resulting in an acid fluoride ter-

minated polymer and a perfluoroalkyl radical capable of

initiating further polymerization which is essentially a

chain-transfer-to-monomer step [7,15]. The use of CFCs

circumvents these problems; however, CFCs have fallen

under exceedingly strict regulation due to environmental

concerns and as a result, are no longer viable to be used

as large scale reaction media.

2.2. TFE-based materials

Tetrafluoroethylene (TFE)-based copolymers have

become premium high performance materials for a

broad range of applications that demand chemical and

thermal resistance, and melt processing capability [16].

As mentioned earlier, caution must be taken when

TFE is used as a monomer due to potential explosions.

It has been demonstrated that CO2/TFE mixtures are far

less susceptible to ignition as TFE forms a pseudo

azeotrope with CO2 [17]. This makes handling and deliv-

ering monomer much safer. A range of TFE-based

fluoropolymers have been successfully synthesized in

CO2. These include FEP, PFA, ETFE, TFE/vinyl ace-

tate polymers, Nafion-type materials, and Teflon

AF-type materials (Fig. 1) the details have recently beenreviewed elsewhere [11]. Further, it is worth noting that

fluoropolymers synthesized in sc-CO2 exhibit signifi-

cantly diminished levels of acid end groups leading to

very high molecular weight materials [15]. Indeed, in

such instances, it is even necessary to add chain transfer

agents to control molecular weight and maintain melt-

processability of the product [18]. IR analysis has con-

firmed that fluoropolymers synthesized under these

conditions exhibit acid end group levels an order of

magnitude below (03 end groups per 106 carbon atoms)

those which are synthesized in conventional organic and

aqueous reaction systems [14,18]. The most likely expla-

nation for this is that the tremendous plasticizing

capability that CO2 provides for efficient transport of

monomer into the polymer phase, maintaining high

effective concentrations of monomer in the vicinity of

the active chain ends, thus favoring the bimolecular

propagation step over the unimolecular b-scission pro-

cess (Scheme 1). The other explanation may be the lower

temperatures that one can use with CO2 which may

favor cross propagation over b-scission. Additionally,

it has been demonstrated that dense CO2 is exceptionally

inert to free radical chemistry in general [19]. This dem-

onstrates the broad applicability of liquid and supercrit-

CF2 C

F

O

CF2

CF2

CF3

CF2 C

F

O

CF2

CF2

CF3

CF2 C

F

F

CF2 C

F

O

CF2

CF2

CF3

CF2 C

O

F

F3C CF2 CF2

TFE

CrossPropagation

+

-scission

Scheme 1. Reaction sequence for b-scission in fluoroolefin polymer-

ization [7,15].

326 C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331

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ical CO2 in the synthesis and processing of fluorinated

polymers which has led to the commercialization of a

CO2-based approach in the manufacturing of certain

grades of polymers based on tetrafluoroethylene (Tef-

lonTM). DuPont has recently invested an initial $40 mil-

lion in the construction of a TeflonTM FEP production

facility in North Carolina. The process employs CO2as the continuous phase instead of 1,1,2-trichloro-

1,2,2-trifluoroethane or water and surfactant [1,2023].

The use of carbon dioxide results in a surfactant free

system with no issues of chain-transfer-to-solvent. This

process not only avoids many of the environmental

and safety concerns but also provides a product with

superior properties. This is the first commercial example

of fluoropolymer resins made using supercritical carbon

dioxide as the solvent.

Teflon AF is an amorphous copolymer of tetraflu-

oroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-

difluoro-1,3-dioxole (PDD). It combines the properties

of amorphous plastics, such as optical transparency

and solubility in organic solvents, with those of perfluo-

rinated polymers, including high thermal stability, excel-

lent chemical stability and low surface energy. Moreover

Teflon AF exhibits the lowest dielectric constant (1.90

for Teflon AF 2400) and the lowest refractive index

(1.29 for Teflon AF 2400) known for any solid organic

polymer [12]. As such, Teflon AF is well-suited for use

as an optical material. Teflon AF-based materials have

also been synthesized in supercritical carbon dioxide

(see Fig. 1) [24]. A range of copolymers with various

compositions and different molecular weights were pre-

pared in yields as high as 74% and their properties were

compared to commercially available Teflon AF. The

glass transition temperatures for the materials ranged

from 67 to 334 C. The synthesis of these copolymers

in CO2 has several key advantages compared to conven-

tional polymerization techniques. The low reaction tem-

perature and the use of a perfluorinated initiator resultsin copolymers with properties which are comparable to

those of the analogous commercial product. However,

an additional fluorination step is not necessary and the

product is isolated directly from the reactor without

contamination from solvents or surfactants. Moreover,

the synthesis uses TFE/CO2 mixtures instead of pure

TFE which has inherent safety advantages [17].

2.3. VF2-based materials

VF2 has been successfully synthesized in CO2 in a

continuous precipitation process using a continuous

stirred tank reactor [25]. In such a process monomer,

solvent (CO2), and initiator are continuously added to

the reactor. The advantage of a continuous process over

a batch process is that the monomer and the CO2 can be

removed and recycled, leading to a high rate of polymer-

ization. The authors later reported that, although this

was a precipitation polymerization, a model based on

the kinetics for a homogeneous polymerization fit the

experimental data, confirming the reaction order of 0.5

and 1 for the initiator and monomer respectively [26].

Further, the effect of VF2 flow rate, residence time

and temperature were investigated in this system [27].

Fluorinated Ethylene Propylene Resin

(FEP)

Perfluoroalkoxy Resin

(PFA)

Ethylene Tetrafluoroethylene Resin

(ETFE)

Nafion

CF2 CF2 CH2 CH2

CF2 CF2 CF2 CF

ORf

CF2 CF2 CF2 CF

CF3

CF2 CF2 CF2 CF

OCF2CF(CF3)OCF2CF2SO2F

CFCF

O O

CF3F3C

CF2CF2

n

nn

n n

Teflon

AF

CF2 CF2 CH2 CH

n

O

O

CH3

Tetrafluoroethylene-co-vinyl acetate

Fig. 1. Tetrafluoroethylene-based fluoropolymers synthesized in carbon dioxide.

C.D. Wood et al. / Current Opinion in Solid State and Materials Science 8 (2004) 325331 327

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It was found that monomer feed concentration had a

pronounced effect upon the modality of the molecular

weight distribution. At a high monomer concentration,

the resulting polymer exhibited a bimodal molecular

weight distribution. The authors suggested that at high

monomer concentrations the kinetics may no longer ad-

here to the homogeneous kinetic model proposed earlier[26]. Several other parameters were investigated such as

chain branching and agitation but they were unable to

explain the bimodality of the polymer.

2.4. Fluoroalkyl acrylate polymerizations in CO2

One of the more difficult issues associated with the

use of CO2 as a polymerization medium has been the

limited solubility of most high molecular weight poly-

mers in CO2. As such, much of the work reported for

polymerizations in CO2 are typically heterogeneous pre-

cipitation, dispersion or emulsion processes as discussed

above. However, it has been demonstrated that oligo-

meric perfluoropolyethers and oligomeric poly(chlo-

rotrifluoroethylene) oils are soluble in liquid CO2[2]. Additionally, it has been reported that highly fluori-

nated polyacrylates of high molecular weight (>250,000)

exhibit exceptional solubility in supercritical CO2. Not

surprisingly, since this discovery, it has been demon-

strated that high molecular weight, amorphous fluoro-

polymers can be synthesized homogeneously in CO2utilizing free radical initiators [21,2833]. Most amor-

phous fluoropolymers exhibit solvent resistance to

common organic solvents and therefore conventional

approaches to synthesis and processing have dependedon chlorofluorocarbon (CFC) platforms. Owing to the

previously mentioned environmental concerns with the

use of CFC refrigerants and solvents, CFCs are no

longer acceptable for large-scale commercial use. As

an effective reaction medium for homogeneous polymer-

izations under mild conditions, CO2, again, is an

increasingly attractive, inexpensive and harmless plat-

form option for the synthesis of highly fluorinated

monomers (as domestic and international regulation of

CFCs becomes ever more restrictive in the coming

years). Various fluorinated acrylate monomers, such as

1,1-dihydroperfluorooctyl acrylate (FOA) have been

successfully polymerized in CO2. It is also noted that

reaction conditions for these polymerizations are mild

in terms of temperature and pressure and not entirely

unlike those encountered in conventional solvent based

processes [21]. Further, studies have shown CO2 to be

inert to radical chemistry, eliminating chain transfer to

solvent as a side reaction [8]. Additionally the low vis-

cosity of sc-CO2 removes issues of Tromsdorf effect

and autoacceleration [8]. Therefore, as a reaction med-

ium, CO2 is ideally suited to free radical polymeriza-

tions. In all cases, the polymerizations remained

homogeneous throughout the reaction, illustrating the

high solubility of the polymers in supercritical CO2.

The solution properties of poly(FOA) in CO2 have also

been investigated using small-angle neutron scattering

(SANS) and it was shown that CO2 is an excellent sol-

vent for poly(FOA) [34].

2.5. Photooxidation of fluoroolefins in liquid CO2

Another class of fluoropolymer known to be readily

soluble in CO2 are perfluoropolyethers (PFPEs) [2]. As

a unique class of fluoropolymers, PFPE polymers and

copolymers have been established as high performance

materials, exhibiting low surface energies and low

moduli, as well as excellent thermal and chemical stabil-

ities. PFPEs are primarily found in high performance

lubricant applications such as for magnetic data storage

media and as heat exchanger fluids. Recently, DeSimone

et al. developed a novel solvent resistant photocurable

PFPE that can be used for the fabrication of microflu-

idic devices [35] and as materials for high resolution soft

lithography to replicate sub-100 nm sized features with

no indications of limits to going to even smaller in size

[36]. Moreover, the material has several specific advan-

tages over traditional glass and PDMS-based materials

and broadens the utility of both processes. In addition,

the process exemplifies a solvent-free approach.

One of the main industrial processes for the produc-

tion of PFPEs is photooxidation of fluoroolefins [37].

Currently, only TFE and HFP are used commercially

in this process. Typically, HFP is photooxidized in bulk

owing to its very low reactivity while TFE requires an

inert diluent in order to prevent homopolymerizationof the olefin. DeSimone and coworkers reported recently

the photooxidation of various concentrations of HFP

in CO2 in concert with parallel reactions carried out in

bulk HFP and in perfluorocyclobutane for purposes of

comparison [38]. Based on the data collected in this

study, it was demonstrated that there is a strong depen-

dence of molecular weight and composition on HFP

concentration (i.e., lower HFP concentrations gave

lower peroxide content). Additionally, viscosity effects

in liquid CO2 appeared to significantly reduce the

amount of peroxidic linkages in the final product.

2.6. Porous materials and supercritical fluids

Porous materials are used in a wide range of appli-

cations, including catalysis, chemical separations, and

tissue engineering [9]. However, the synthesis of these

materials is often solvent intensive. Supercritical carbon

dioxide as an alternative solvent for the production of

functional porous materials can circumvent this issue

as well as affording a number of specific physical, chem-

ical, and toxicological advantages listed below. For

example, in order to dry porous materials often requires

energy intensive steps whereas; the transient dry nat-


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ure of CO2 circumvents these issues. Pore collapse can

occur in certain materials when removing conventional

liquid solvents; this can be avoided using SCFs because

they do not possess a liquidvapour interface. Porous

structures are important in biomedical applications

(e.g., tissue engineering) where the low toxicity of CO2

offers specific advantages in terms of minimizing theuse of toxic organic solvents. In addition, the wetting

properties and low viscosity of CO2 offers specific bene-

fits in terms of surface modification.

2.7. Free-radical polymerization using sc-CO2 as a

pressure-adjustable porogen

Recently, Cooper et al. demonstrated the formation

of permanently porous crosslinked poly(acrylate) and

poly(methacrylates) monoliths using sc-CO2 as the por-

ogenic solvent [39,40]. Materials of this type [41,42] are

useful in applications such as high-performance liquid

chromatography, high-performance membrane chroma-

tography, capillary electrochromatography, microflui-

dics, [43] and high-throughput bioreactors [44]. In this

process, no organic solvents are used in either the syn-

thesis or purification. It is possible to synthesize the

monoliths in a variety of containment vessels (Fig. 2),

including chromatography columns and narrow-bore

capillary tubing. Moreover, the variable density associ-

ated with SCF solvents was exploited in order to

fine-tune the polymer morphology. Fig. 3 shows the

variation in BrunnauerEmmetTeller (BET) surface

area for a series of crosslinked monoliths synthesized

using sc-CO2 as the porogen over a range of reactionpressures [45]. As can be seen the BET surface area

and the average pore size in these materials could be

continuously varied over a broad range simply by vary-

ing the SCF solvent density. It is interesting to note the

minimum in area (and a maximum in the average pore

diameter, not shown) was observed at a reaction pres-

sure of around 180 bar. This can be rationalized by

considering the variation in solvent quality as a function

of CO2 density and the resulting influence on the mech-

anism of nucleation, phase separation, aggregation,

monomer partitioning, and pore formation [46]. This

approach was also extended to synthesize well-defined

porous, cross-linked beads by suspension polymeriza-

tion, again without using any organic solvents [47].

The surface area of the of the beads could be tuned over

a broad range (5500 m2/g) simply by varying the CO2density.

2.8. Templating of supercritical fluid emulsions

Emulsion templating is useful for the synthesis ofhighly porous inorganic, [4851] and organic materials

[5254]. In principle, it is possible to access a wide range

of porous hydrophilic materials by reaction-induced

phase separation of concentrated oil-in-water (O/W)

emulsions. A significant drawback to this process is that

large quantities of a water-immiscible oil or organic

solvent are required as the internal phase (usually

>80 vol.%). In addition, it is often difficult to remove

this oil phase after the reaction. Inspired by studies on

SCF emulsion stability and formation [55], we have

developed methods for templating high internal phase

CO2-in-water (C/W) emulsions (HIPE) to generate

highly porous materials in the absence of any organic

solventsonly water and CO2 are used [56]. If the emul-

sions are stable one can generate low-density materials

(0.1 g/cm3) with relatively large pore volumes (up to

6 cm3/g) from water-soluble vinyl monomers such as

acrylamide and hydroxyethyl acrylate. Fig. 4 shows a

crosslinked polyacrylamide material synthesized from

a high internal phase C/W emulsion, as characterized

by SEM and confocal microscopy (scale = 230 lm

230 lm). Comparison of the two images illustrates quite

clearly how the porous structure shown in the SEM

image is templated from the C/W emulsion (as

Fig. 2. Polymer monolith, synthesized using sc-CO2 as the porogen as

isolated from the reaction vessel. The monolith adopts the shape of the

reactor. Reproduced with permission; copyright 2003, American

Chemical Society [45].

50

100

150

200

250

300

350

100 150 200 2 300 350 4 45

Pressure (bar)

250 300 400 450

Surfac

eArea(m2/g)

Fig. 3. Surface area (as measured by N2 adsorptiondesorption using

the BET method) as a function of the CO2 pressure employed during

the polymerization. Reproduced with permission; copyright 2003,

American Chemical Society [45].


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represented by the confocal microscopy image of the

pores). In general, the confocal image gives a better mea-

sure of the CO2 emulsion droplet size and size distribu-

tion immediately before gelation of the aqueous phase.

Initially we used low molecular weight (Mw 550 g/

mol) perfluoropolyether ammonium carboxylate surfac-

tants to stabilize the C/W emulsions [56] but there are

some practical disadvantages of using these surfactants

in this particular process such as cost and the surfactant

is non-degradable. It was subsequently shown that it is

possible to use inexpensive hydrocarbon surfactants to

stabilize C/W emulsions and that these emulsions can

also be templated to yield low-density porous materials

[57]. In this study it was shown that all of the problems

associated with the initial approach could be overcome

and it was possible to synthesize C/W emulsion-tem-

plated polymers at relatively modest pressures (60

70 bar) and low temperatures (20 C) using inexpensive

and readily available hydrocarbon surfactants. More-

over, we demonstrated that this technique can in princi-ple be extended to the synthesis of emulsion-templated

HEA and HEMA hydrogels that may be useful, for

example, in biomedical applications [5860].

2.9. High throughput solubility measurements in CO2

As discussed CO2 is an excellent environmentally

benign alternative solvent for the synthesis of fluoro-

polymers and porous materials. Moreover, a range of

environmental as well as practical advantages can be

realized from using CO2, which has resulted in the com-

mercial synthesis of certain grades of Teflon by DuPont.

However, the discovery of inexpensive CO2-soluble sur-

factants, ligands and phase transfer catalysts would

greatly increase the utility of CO2. As mentioned the

only polymers with appreciable solubility in CO2 are

amorphous fluoropolymers and polysiloxanes [61]. Inex-

pensive poly(ether carbonate) copolymers have been re-

ported to be soluble in CO2 under moderate conditions

[62,63] and could function as building blocks for inex-

pensive surfactants, but numerous practical difficulties

remain. Recently, Cooper et al. reported a new method

which allows for the rapid parallel solubility measure-

ments for libraries of materials in supercritical fluids

[64]. The technique was used to evaluate the solubilityof a mixed library of 100 synthetic polymers including

polyesters, polycarbonates, and vinyl polymers. This

method is at least 50 times faster than other techniques

in terms of the rate of useful information that is ob-

tained and has broad utility in the discovery of novel

SCF-soluble ligands, catalysts, biomolecules, dyes, or

pharmaceuticals for a wide range of materials

applications.

3. Conclusions

In general CO2 is an attractive solvent alternative for

the synthesis of polymers because it is environmentally

friendly, non-toxic, non-flammable, inexpensive, and

readily available in high-purity from a number of

sources. Product isolation is straightforward because

CO2 is a gas under ambient conditions, removing the

need for energy intensive drying steps. Moreover, addi-

tional advantages are realized in the manufacture of flu-

oropolymers such as increased safety in handling

explosive monomers and enhanced polymer properties

in many cases. As demonstrated CO2 is not only an ade-

quate alternative, but in many cases a superior one. This

Fig. 4. Emulsion-templated crosslinked polyacrylamide materials

synthesized by polymerization of a high-internal phase CO2-in-water

emulsion (C/W HIPE): (a) SEM image of sectioned material and (b)

confocal image of same material, obtained by filling the pore structure

with a solution of fluorescent dye. As such (a) shows the walls of the

material while (b) show the holes formed by templating the sc-CO2emulsion droplets. Both images = 230lm 230 lm. Ratio of CO2/

aqueous phase = 80:20 v/v.d Pore volume = 3.9 cm3/g. Average pore

diameter = 3.9 lm. Adapted from Butler et al. [56].


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is demonstrated no more poignantly that DuPonts re-

cent $275 million investment in its CO2 based TeflonTM

production process. In addition, CO2 affords a number

of specific advantages that can be derived from the use

of SCFs for the synthesis and modification of porous

materials. There are a number of specific chemical

advantages that are realized by using CO2 for the syn-thesis of a range of materials but moreover, they offer

the potential of reducing organic solvent usage in the

production of materials.

Acknowledgments

We thank the NSF STC (grant number 537494), EPA

(grant number 535325), ONR (grant number 535747),

EPSRC. A.I.C. acknowledges the support of the Royal

Society for the provision of a Research Fellowship.

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