CO2 Solvente Fluido Supercrítico

A Review of CO2 Applications in the Processing of Polymers

David L. Tomasko,* Hongbo Li, Dehua Liu, Xiangmin Han, Maxwell J. Wingert,L. James Lee, and Kurt W. Koelling

Department of Chemical Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, Ohio 43210

The use of supercritical carbon dioxide as a processing solvent for the physical processing ofpolymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussedwith an emphasis on available data and measurement techniques, the development of theory ormodels for a particular property, and an evaluation of the current state of understanding forthat property. Applications such as impregnation, particle formation, foaming, blending, andinjection molding are described in detail including practical operating information for selectedtopics. The review concludes with some forward-looking discussion on the future of CO2 inpolymer processing.

Introduction

Supercritical fluid technology has made tremendousstrides in the past decade in terms of commercialapplication and fundamental understanding of solutionbehavior. Much of the success can be attributed to thehard work of clever chemists and engineers recognizingopportunities to use these tunable solvents to solve aparticular problem or fill a processing niche. We arehappy to dedicate this review to one of the cleverestchemist/engineers, Chuck Eckert, whose creativity hasbeen an inspiration to the field. Another significantcontribution has come from the continuing environmen-tal pressures on industry to move away from volatileorganic compounds (VOCs) and ozone-depleting sub-stances (ODS) as processing solvents. However, usingsupercritical fluids and supercritical carbon dioxide inparticular cannot always be economically justified basedsolely on replacing environmentally harmful solvents.There must be an additional process advantage arisingfrom simplified reaction/separation schemes, lower en-ergy requirements, improved product quality, or somecombination of these. While all of these characteristicsare often touted as “potential” advantages of a super-critical fluid process, there are hurdles remaining toinstill it as a viable choice in process design. Theexistence of several small and growing companies(Phasex, Thar Technologies, Micell, Trexel, and Novasep)confirms that the recent successes are starting to carveout a market segment.

Specific examples of recent successes are the newDuPont facility for producing fluoropolymers in a su-percritical carbon dioxide-based solvent. Dry cleaningtechnology based on liquid CO2 is competing in thetextile market with both Washpoint (ICI/Linde) andMicare (Cool Clean Technologies) technologies, repre-senting viable alternatives to chlorinated solvents.1 Andthere are several developments underway to com-mercialize cleaning technologies in the microelectronicsindustry.

Many new developments have arisen out of the useof CO2 as a novel reaction solvent. Novel constructionof CO2-phillic catalysts and surfactants have allowedboth traditional and new reaction pathways to be

explored. Studies have shown that CO2 as a solvent canoffer a “green” alternative for carrying out many typesof chemistry often with significant process improve-ments in selectivity, conversion, or rates. In fact, thedevelopment of novel polymers and polymeric surfac-tants for use with CO2 has resulted in two PresidentialGreen Chemistry Awards. Much of this work wasrecently reviewed.2-5 In this work we focus on nonre-active processes with carbon dioxide.

Like reaction systems, most processing using super-critical or compressed CO2 as a solvent is carried out ina fluid phase. Extraction is the most advanced applica-tion and there exists a tremendous amount of literatureon extraction using supercritical fluids. It is fair to saythat the design of a high-pressure extraction process isnow facile, given carefully planned bench-scale results.The food and pharmaceutical industries use this tech-nology more often than others since the nontoxic natureof CO2 provides a strong impetus. Other applicationsbeing carried out in a fluid phase include several of theparticle generation technologies wherein material isdissolved in CO2 or an organic solvent and precipitatedfrom that solution via a pressure or solvent compositionchange. It is notable that a variety of morphologies areattainable over a wide size range. Particle generationis certainly an area of intense inquiry and is receivingattention primarily from the pharmaceutical and bio-technology industries, although there are significantpolymer and inorganic material applications of thesetechniques.

In the applications addressed so far, the commonfeature has been a fluid phase solvent in which com-pressed CO2 is a major (if not the predominant) com-ponent. It is in these applications that one may takeadvantage of proximity to critical points and the diver-gent thermodynamic properties in those regions. Thereis also another range of composition that is an activearea of research and this arises from the dissolution ofcompressed CO2 into a condensed phase. One exampleis the injection of CO2 into a solute-laden organic solventto precipitate the solute, although this technique oftenstill requires CO2 compositions upward of 50 mol %.When the condensed phase is a polymer, the dissolutionof CO2 is almost always less than 30 wt % and moretypically less than 10 wt %. The system is not near anymixture critical points so the divergent thermodynamic

* To whom correspondence should be addressed. Tel.: 614-292-4249. Fax: 614-292-3769. E-mail: [email protected].

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properties are not an issue. The proximity to the criticalpoint of CO2 impacts primarily the solubility. However,the addition of small amounts of compressed gases topolymer phases results in substantial and sometimesdramatic changes in the physical properties that dictateprocessing. These include viscosity, permeability, inter-facial tension, and glass transition temperature. Byunderstanding the effects of CO2 on these properties anddeveloping techniques for incorporating CO2 in continu-ous processes, a wide range of opportunities open upfor impacting the plastics industry. The products rangefrom foam board insulation and high-impact polymerblends to surface-modified biomedical implants andbiological micro-electromechanical system (bio-MEMs)devices.

This review will cover the fundamental properties ofCO2-polymer systems, describe a wide range of ap-plications under development, and finish with somecomments on the future outlook for CO2 processing ofpolymers. The section on fundamental properties isdivided into phase equilibrium (solubility, Tg and Tmreduction, and interfacial tension), transport phenom-ena (mass and heat transfer), rheology, and dynamicprocesses (crystallization and nucleation). The applica-tions are discussed in an approximate decreasing orderof maturity. Since extraction and fractionation are well-developed, our discussion begins with impregnationfollowed by particle formation, foaming, blending, andinjection molding. Within the applications, methods forcontinuous extrusion with CO2 are discussed in somedetail. A schematic illustration of the review structureis shown in Figure 1.

In general, we attempt to not only assess the litera-ture data but also provide some sense of how well-developed the models and theories are for each particu-lar topic. Where possible, we point out any semipredictivemethods or rules of thumb for practitioners. The reviewalso includes several developed and potential applica-tions of CO2 in polymer processing. Some practicaloperating information is discussed with respect to CO2-

assisted extrusion to provide a balanced viewpoint ofthe technology.

Fundamentals

(A) Phase Equilibrium. The phase behavior ofcompressed gases and polymers is rich and complex. Ithas been the research subject of thermodynamicists andpolymer scientists for many years and has formed thebasis of several stand-alone reviews.6-12 For studyingpure thermodynamics and polymer physics, CO2 is notthe molecule of choice due to its unusually low cohesiveenergy density and high quadrupole moment that addambiguity to interpretations of results. Most often, amonomer (e.g., ethylene) or other simple molecule(alkane) is chosen as the compressed gas to provide moredirect interpretations of polymer-gas and gas-gasinteractions. While the gas-rich region of the phasediagram is often devoid of polymer, the polymer-richregion may contain upward of 10 wt % gas at typicalprocessing pressures (<300 bar).

Some applications such as polymer fractionation,coating, or synthesis rely on the formation of a homoge-neous fluid phase. Because of the polydispersity ofpolymers, the boundary between the one- and two-phaseregions is not sharpshence, the term “cloud point” todescribe the range of conditions (temperature, pressure,and weight fraction) at which a two-phase systemchanges to one phase. Regardless of the fuzzy nature(thermodynamically speaking) of the cloud point, it isused ubiquitously in the polymer-fluid phase equilib-rium literature to discuss intermolecular interactionsas they relate to polymer segment-segment interactionsand segment-fluid interactions. It is sufficiently sensi-tive to both molecular weight of the polymer chains andsegment-fluid interactions to be useful for developingstructure-property relationships. The results show thatintermolecular interactions between segments and theSCF are much more responsible for miscibility thanhydrostatic pressure. Most efforts have gone into de-veloping equations of state and theories that will allowprediction or accurate correlation of the cloud pointpressure so as to aid in process control.

In recent years there have been successful efforts tostudy the physics of polymer-SCF solutions in moredetail using small-angle neutron scattering (SANS).13-17

Others have used a synthetic approach to exploringCO2-polymer interactions via designing certain fea-tures such as flexibility, functional groups, and sidechain length into the polymer backbone and studyingtheir effect on the cloud point. 18 There is also a body ofwork on spectroscopic investigations of these systems.19-26

The current state of understanding of polymer-CO2phase behavior is difficult to summarize succinctly. Thegeneral highlights are that chain flexibility aids dis-solution (lowers the cloud point pressure) and carbonylor ether groups that are accessible in the backbone oron side chains can specifically interact with CO2.27,28

Much discussion has centered on the effect of fluorine-substituted polymers but this effect is very likelycoupled to the aforementioned properties because thespecies exhibiting dramatically enhanced solubilities inCO2 are perfluoroalkyl ethers and acrylates that areflexible and or contain carbonyls. On the other hand,Teflon (an inflexible perfluoroalkane without carbonyls)is no more miscible with CO2 than a polyolefin.

Figure 1. Topics covered in this review.

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(1) Solubility of CO2 in Polymers. In this review,the majority of the applications discussed are forsystems in which the polymer is the major componentand the compressed gas (or SCF) is dissolved in thepolymer up to several weight percent. We are specifi-cally interested (for ecological and economic reasons) insupercritical CO2 as the processing solvent. The remain-der of this section is devoted to the thermodynamics ofCO2 solubility in polymers over a wide range of tem-peratures.

The affinity of CO2 for a polymer is associated withthe interaction between CO2 and the polymer. Gener-ally, the solubility of CO2 increases with the increasingcontent of polar groups in the polymer. Berens andHuvard29 pointed out that near-critical CO2 behaves likea polar, highly volatile organic solvent when interactingwith polymers. In a recent study, Shieh and Lin30

reviewed the interactions between CO2 and polymersand studied the effects of carbonyl group content andcrystallinity on the equilibrium solubility of CO2 inrubbery EVA with different VA (vinyl acetate) content.Their study suggests that the sorption process at orbelow Pc was mainly driven by the carbonyl groups andabove Pc by the degree of crystallinity such that thehigher the degree of crystallinity, the lower the normal-ized CO2 sorption concentration (cm3 STP CO2/mol ofVA) in the polymer. Several researchers used FTIR(Fourier transform infrared spectroscopy) to investigatethe interaction of CO2 with polymers.31-33 Eckert andco-workers27 found that polymers containing carbonylgroups act as electron donors and exhibit specificinteractions with CO2 as electron acceptors rather thanas electron donors. Mawson et al.34 also suggest thatthe interaction of CO2 with polymers possessing acrylategroups (containing carbonyl groups) may be of a Lewisacid-base nature. Also, the specific interactions be-tween CO2 and the dipoles of the C-F bonds35 orfluorine36 were proposed to explain the increased solu-bility of CO2 in fluorine-containing polymers. Weakelectrostatic interactions were proposed for CO2 and theπ system in PS.27,37

(a) Experimental Techniques. Described below arethree types of methods for quantitatively measuring thesolubility of CO2 inside polymers, each of which canprovide data to about 5-7% accuracy. Chromatographycan be used to determine only relative solubilities andis not covered here.

(i) Barometric (Pressure Decay) Method. Themass of gas absorbed by a polymer sample is obtainedfrom the difference between the amount of gas initiallycontacted with the polymer and the amount remainingin the gas phase after equilibration. The pressure decaymethod is popular because the apparatus is simple andinexpensive. However, it is difficult to apply at hightemperatures for polymer melts because of the lack ofsuitable pressure sensors.38 Also, this method needs alarge amount of sample (about 5 g), which in turnrequires a long time for the polymer to equilibrate withCO2. If the measurement is carried out at high temper-ature (e.g., 200 °C for PS), degradation becomes aconcern. Further, a very accurate equation of state forthe gas phase is needed and the volume of the systemneeds to be very accurately calibrated.

(ii) Gravimetric Method. The kinetics of desorptionis obtained by measuring the weight change of apolymer sample after being removed from a high-pressure cell to a microbalance at ambient conditions.29

On the basis of the analytical solution to diffusion froma flat plate, the time-dependent weight change can beextrapolated to zero time to give an estimate of thesolubility. This simple method is not suitable for poly-mers that will experience visible changes in shape orappearance upon the release of CO2, such as rubberyor highly plasticized polymers.

More precisely, the mass of the polymer sample plusCO2 is directly measured with a sensitive microbalance.The gravimetric method requires an accurate equationof state for the gas phase and some estimate (ormeasurement) of the swelling of the polymer phase toaccount for the buoyancy correction. The swelling ofpolymers are either measured using the techniquesmentioned in the literature39-42 or predicted using theSanchez-Lacombe equation of state. The basic principlefor measuring the swelling of polymers in CO2 is todetermine swelling by measuring the change in one ormore dimensions of a polymer sample. However, thepretreatment of the sample and the way in which thesample is prepared (the dimension of the sample) andmeasured (either free hanging or lying on a glass plate)can yield different swelling results, which in turninfluence the final results of the solubility measurement.Zhang et al.43 have investigated the influence of thesefactors in the solubility measurement in detail.

Several researchers have used an electronic microbal-ance inside a pressure vessel for in situ measurementat temperatures limited to below 125 °C due to themicrobalance operating conditions. It has the advan-tages of requiring a small sample size (hence shortmeasurement time) and high sensitivity (on the orderof micrograms). Kamiya et al.44 measured the sorptionof CO2 in PMMA at over a temperature range of 35-200 °C using this type of microbalance. The othercommon type of microbalance is the magnetic suspen-sion balance (MSB).45 This has many advantages similarto the electronic microbalances with the added advan-tage that the sample and balance are mechanicallyisolated. The sample is magnetically levitated inside ahigh-pressure vessel while the balance electronics re-main at ambient conditions. This makes it suitable forthe measurement of gas solubility and diffusivity inpolymers at high temperatures and pressures.38,46-48

The buoyancy correction is especially important for thistype of balance.

(iii) Frequency Modulation. Frequency modulationusing a quartz crystal microbalance (QCM) is an ac-curate and relatively easy technique to measure gassolubility in polymers at high pressures. The QCM iscomposed of a thin quartz crystal coated with thepolymer of interest and sandwiched between two metalelectrodes, which establishes an alternating electric fieldacross crystal, causing vibrational motion of the crystalat its resonant frequency, f, due to the peizoelectriceffect.49 The resonant frequency decreases linearly asthe mass of the coating increases according to theSauerbrey equation, which works very well for smallmass gains in well-adhered films. The primary challengeis preparing such well-adhered films on the crystal andthis limits somewhat the range of polymers amenableto this technique. Nonetheless, the modulation is re-versible and can be used for real-time monitoring of aprocess.50

Table 1 gives a representative sample of the availableliterature data for the solubility of CO2 in a variety ofpolymers. Figure 2 in detail presents the CO2 sorption

Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003 6433

Table 1. Solubility of CO2 Inside Polymersa,b

polymer methodpressure

range (atm)temp range

(°C) solubility refs Src

PMMA(poly(methyl methacrylate))

GM-M 0-50 35/65/75/85 0-80/46/40/34 SCC/cm3 44 GGM-M 0-15 100 0-7.6 SCC/cm3

GM-M 0-20 125/150/175/200 0-8.2/7.0/5.7/5.3 SCC/cm3

GM-D 0-68 25 0-26 g/100 g 29 GGM-Q 0-100 32.7/42/58.8 0-120/137/110 SCC/cm3 42 GGM-D 68 25 6.3 wt % 287 TGM-D 136 40 9.3 wt %GM-D 204 70 10.5 wt %GM-Q 13.6-102 35 3.96-22.2 g/100 g 43 TQCM 0-95.2 40 7.32-24.3 g/100 g 50 CGM-Q 0-296 35/50 0-320/260 SCC/cm3 8 G

PS(polystyrene)

GM-M 0-13.2 25 0-14.5 SCC/cm3 307 GGM-M 0-50 35 6.3 g/100 g 109 GGM-M 0-70 35 58 SCC/cm3

GM-M 0-55 50 35 SCC/cm3

QCM 0-42 65 24 SCC/cm3 42 GGM-Q 13.6-102 35 1.84-5.56 g/100 g 43 TQCM 0-95.2 40 3.09-13.4 g/100 g 50 CBM 35.7-183.1 100 2.26-11.57 g/100 g 308 TBM 71.9-197.7 140 3.47-10.12 g/100 g 308 TBM 24.4-171.4 180 0.94-6.87 g/100 g 308 TGM-MSB 20.4-198.0 100 1.27-12.18 g/100 g 38 TGM-MSB 21.3-159.3 180 0.9-6.45 g/100 g 38 TGM-MSB 21.4-198.9 200 0.71-6.23 g/100 g 38 T

HIPShigh-impact polystyrene

GM-D 68 25 0.8 wt % 287 TGM-D 204 70 0.5 wt % 287 T

PC(polycarbonate)

GM-M 0-13.2 25 0-24 SCC/cm3 307 GBM 1-30 35 0-42 SCC/cm3 119 GGM-M 0-50 35/45/55 0-50/40/32 SCC/cm3 309 GGM-D 0-68 25 0-13 g/100 g 29 GGM-Q 0-100 35 65 SCC/cm3 42 GQCM 0-95.2 40 5.55-13.9 g/100 g 50 CGM-Q 0-296.1 35 0-90 SCC/cm3 8 G

PET(poly(ethylene terephthalate))

FTIR 57.1-175.2 0/28/50 10 (63 atm)/6.7(175.2 atm)/4.2 (171.7 atm) g/100 g(initially amorphous PET)

26 G

GM-D 136 40 1.5 wt % 288 TGM-MSB 0-394.8 80/100/120 0-3.8/3.5/3.25 g/100 g 48 G

PVC(poly(vinyl chloride))

GM-D 0-68 25 0-8 g/100 g 29 GGM-D 136 40 0.1 wt % 287 TGM-D 49.3-296.1 40/50/70 5.5-13, 4-12, 3-11.5 g/100 g 116 T

PVAc(poly(vinyl acetate))

GM-D 0-54.4 25 0-29 g/100 g 29 GGM-MSB 2.0-67.9 40 0.55-34.70 g/100 g 38 TGM-MSB 22.3-95.7 60 5.61-33.24 g/100 g 38 TGM-MSB 21.8-97.5 80 3.99-21.43 g/100 g 38 TGM-MSB 21.3-172.2 100 3.05-29.92 g/100 g 38 T

LDPE(low-density polyethylene)

GM-M 0-50 35 0-12 SCC/cm3 309 GGM-D 68 25/40 0.0/0.2 wt % 288 TGM-D 136 40 0.4 wt % 288 TGM-D 204 25 0.5 wt % 288 T

HDPE(high-density polyethylene)

GM-D 68/136 40 0.1, 0.0 wt % 288 TGM-D 204 25 0.1 wt % 288 TBM 68.4-172.2 160 0.41-1.32 g/100 g 61 TBM 69.6-178.9 180 0.35-1.20 g/100 g 61 TBM 65.2-168 200 0.32-1.03 g/100 g 61 T

PP(polypropylene)

GM-D 68/136 40 0.1/0.1 wt % 288 TGM-D 204 25 0.1 wt % 288 TBM 73.0-116.5 160 0.50-1.59 g/100 g 61 TBM 53.5-170.2 180 0.32-1.43 g/100 g 61 TBM 61.2-152.0 200 0.3-1.09 g/100 g 61 T

copolymer:styrene-methyl methacrylate

GM-Q 13.6-102 35 2.26-8.51 (80 wt % styrene),2.85-11.5 (50 wt % styrene),3.09-15.2 g/100 g (40 wt % styrene)

43 T

copolymer:styrene-vinyl pyridine

GM-Q 13.6-102 35 2.11-11.7 (50 wt % styrene),2.29-13.0 g/100 g (25 wt % styrene)

43 T

copolymer:styrene-isoprene

GM-Q 13.6-102 35 2.06-10.3 (75 wt % styrene),2.14-12.6 (50 wt % styrene),2.12-16.5 g/100 g (25 wt % styrene)

43 T

PS/PC GM-M 0-13.2 25 0-22.5 (PS/PC: 20/80),21 (PS/PC: 50/50),17(PS/PC: 80/20) SCC/cm3

307 G

PEI(poly(ether imide)

GM-M 0.99 30 6.4 SCC/cm3 310 TGM-D 204 70 1.5 wt % 287 T


Table 1. (Continued)

polymer methodpressure

range (atm)temp range

(°C) solubility refs Src

Nylon 66GM-D 68 25/40 1.8/1.8 wt % 288 TGM-D 136 40 1.3 wt % 288 TGM-D 204 25 1.1 wt % 288 T

EVA(polyethylene-co-vinyl actate)

GM-D 10-340 25-52 0-110 SCC/cm3 (32 °C),varying as VA content

30 T

POM(polyoxymethylene)

GM-D 68/136 40 1.0, 0.5 wt % 288 TGM-D 204 25 0.9 wt % 288 T

TPX(poly 4-methyl-1pentene)

QCM 13.6-95.2 40 3.39-44.1 g/100 g 50 C

PI(polyimide)

QCM 13.6-95.2 40 5.71-12.9 g/100 g 50 C

chlorinated polyethylene(CPE-725)

QCM 13.6-54.4 40 2.22-8.89 g/100 g 50 C

polyisoprene GM-Q 13.6-102 35 2.63-25.3 g/100 g 43 T

poly(vinyl pyridine) GM-Q 13.6-102 35 2.58-17.0 g/100 g 43 T

ABS(acrylonitrile butadiene styrene)

GM-D 68 25 2.5 wt % 287 TGM-D 204 70 5.9 wt % 287 T

CAB(cellulose acetate butyrate)

GM-D 68 25 1.9 wt % 287 TGM-D 204 70 5.3 wt % 287 T

PETG(PET glycol modified)

GM-D 68 25 2.1 wt % 287 TGM-D 136 70 0.8 wt % 287 T

PPO(poly(2,6-dimethyl phenylene oxide)

GM-D 204 40 3.4 wt % 287 T

PSF GM-D 136 40 3.6 wt % 287 T(polysulfone) GM-M 0-50 35/45/55/65 0-43/37/31/28 SCC/cm3 309 G

PU(polyurethane)

GM-D 136 40 2.2 wt % 287 T

PEG(PEG 1500,more PEG derivativesdata available)

GM-Q 19.7-256.6 43 4-30 wt % (PEG1500) 311 GGM-Q 19.7-266.5 50 3-29 wt % (PEG1500) 311 GGM-Q 19.7-276.3 60 3-28 wt % (PEG1500) 311 GGM-Q 19.7-266.5 80 2-25 wt % (PEG1500) 311 G

silicone rubber (RTV615) GM-Q 0-296.1 35 0-600 SCC/cm3 (32 °C) 8 G

silicone rubber BM 0-60 35 0-138 SCC/cm3 312 GBM 0-276.3 50 0-610 SCC/cm3 (32 °C) 312 G

PIB(polyisobutylene)

GM-Q 0-256.6 35 0-230 SCC/cm3 (32 °C) 8 GGM-Q 0-266.5 50 0-200 SCC/cm3 (32 °C) 8 G

parylene-N(poly(xylylene))

QCM 13.6-95.2 40 0-7.14 g/100 g 50 C

parylene-C(poly(2-choloroxylylene))

QCM 13.6-95.2 40 1.75-5.26 g/100 g 50 C

parylene-D(poly(2,3-dicholoroxylylene))

QCM 13.6-95.2 40 1.67-5.00 g/100 g 50 C

Teflon GM-D 68/136 40 0.0/0.0 wt % 288 TGM-D 204 25 0.0 wt % 288 T

Teflon AF-1600 QCM 13.6-54.4 40 5.00-28.8 g/100 g 50 C

PEA(poly(ether amide))

GM-M 0.98 30 5.8 SCC/cm3 310 T

poly(butylenes succinate)

GM-MSB 10.1-99.9 50 0.98-7.3 g/100 g 124 TGM-MSB 11.3-103.1 80 0.88-7.25 g/100 g 124 TGM-MSB 23.2-197.2 120 2.17-17.61 g/100 g 124 TGM-MSB 21.9-198.3 150 1.50-13.55 g/100 g 124 TGM-MSB 21.0-198.8 180 1.19-11.26 g/100 g 124 T

poly(butylenes succinate-co-adipate)

GM-MSB 10.8-97.1 50 1.26-15.03 g/100 g 124 TGM-MSB 20.5-197.7 120 1.84-17.41 g/100 g 124 TGM-MSB 20.7-198.1 150 1.45-13.74 g/100 g 124 TGM-MSB 20.5-198.6 180 1.18-11.36 g/100 g 124 T

a Method: GM-M, gravimetric method (microbalance); GM-D, gravimetric method (desorption); GM-Q, gravimetric method (quartzspring); GM-MSB, gravimetric method (magnetic suspension balance); QCM, quartz crystal microbalance; BM, barometric method. Units:SCC/cm3, cm3 (STP)/cm3 of polymer; g/100 g, g of CO2/100 g of polymer. Src: type of data source in the literature. G: data read fromgraphs. T: data read from tables. C: data calculated from available information. b Every effort has been made to provide a validrepresentation of the available solubility data. The units have been converted from the original sources to provide a better means ofcomparison in this table. When the data do not vary greatly with experimental conditions, only one set of conditions for a particularpolymer is given. In other cases, the number of significant figures has been reduced from the original source.


behavior in polystyrene at different temperature andpressure conditions, and the Tg depression phenomenadiscussed in the next section.

(b) Correlation and Prediction of Solubility ofCO2 in Polymers. The sorption isotherms of CO2 inpolymers exhibit different pressure dependencies belowand above the glass transition temperature (Tg). BelowTg, the dependence of sorption on pressure is repre-sented by a curve concave to the pressure axis, describedby the dual-mode sorption model:51-58

where C and P are the concentration and pressure,respectively. kH is Henry’s law constant and CL and bare the Langmuir capacity and affinity, respectively.The sorption occurs in two modes: the dissolution modeobeying Henry’s law and the adsorption mode obeyingthe Langmuir equation. Above Tg, the dependence ofsorption on pressure is generally linear in the low-pressure range and can be described accurately byHenry’s constant, that is, CL is zero and the modelreduces to the dissolution mode. Although this modelsuccessfully characterizes the sorption behavior of CO2inside polymers and correlates the solubility of CO2 inpolymers, the three parameters in the model are allfunctions of temperature and at high pressures kH is alsoconsidered as a function of total absorbed concentrationC, which makes the model of limited use in extrapolat-ing to higher pressure and temperatures.

The Sanchez-Lacombe equation of state59,60 is per-haps the most widely used model to describe thesolubility of CO2 in polymers38,42,43,61,62 due to itssimplicity, well-defined physical meaning, and the abil-ity to extend available data to high temperature andpressures. It is particularly adept at correlating mix-tures containing molecules of widely different sizesusing three characteristic parameters per molecule (T*,P*, and F*). Typical characteristic parameters for gasesand polymers have been published.59,60,62 Mixtures arehandled using volume fraction-based mixing rules andan adjustable binary interaction parameter, δij, is typi-cally introduced into the mixing rule for P*:

The adjustable parameter is obtained from a best fitof the equation of state to experimental solubility data.Unfortunately, very limited interaction parameter datafor CO2 and polymers are available in the literature,and those are in a limited pressure or temperaturerange.38,43,61,62 Compiling values from several sourcesfor the PS-CO2 system indicates no clear trend of δijwith temperature, even over a range of temperatureexceeding 100 °C. Although the Sanchez-Lacombeequation of state can correlate the solubility of CO2 inpolymers very well, it cannot be relied upon to extrapo-late to other temperatures and pressures. As Kirby andMcHugh10 pointed out in their review, “a good repre-sentation of the phase behavior is obtained only if thebinary mixture parameters are allowed to vary withtemperature. The dilemma of this approach is thatmixture data are needed to obtain a reasonable estima-tion of the mixtures, that is, you need to know theanswer to get the answer.”

The other prevailing equation of state for describingSCF-polymer phase behavior is based on the “statisticalassociating fluid theory” (SAFT).63-66 This equation ofstate is much more complex than Sanchez-Lacombe’sbut has the advantage of being able to explicitly accountfor specific interactions such as hydrogen bondingbetween segments of polymer molecules and othersegments or solute molecules.

(2) Plasticization of Polymers under High-Pres-sure CO2. The most relevant phase change for amor-phous polymers in continuous polymer processes is theglass-to-rubber transition characterized by the glasstransition temperature, Tg. It has been known qualita-tively for many years that compressed gases alter thepure component phase boundaries of solid materials andthe dissolution of CO2 lowers the Tg of amorphouspolymers sometimes dramatically. The available experi-mental methods for measuring Tg depression of poly-mers under CO2 and related data are listed in Table 2.The reduction of Tg is a thermodynamic effect due tointermolecular interactions between CO2 and the poly-mer and not simply a hydrostatic pressure effect.Reported in Table 2 are values of -dT/dP obtained fromour analysis of the data in order to make comparisons.Stronger interactions between the polymer and CO2enhance Tg depression (e.g., PMMA compared withPS67), as does chain flexibility (e.g., PEMA comparedwith PMMA68). We also note that measured Tg depres-sion for the same polymer differs significantly acrossresearchers, perhaps due to different techniques andexperimental protocols (e.g., DSC scanning rate).

In many of the experimental techniques for studyingthe glass transition at high pressure, either the ther-modynamic state of the polymer-gas system or theglass-to-rubber transition is not well-defined. 69 Dif-ferential scanning calorimetry (DSC) is the simplesttechnique to use and provides fast and accurate infor-mation on the glass to rubbery transition. For the caseof polymer-gas systems, ambient pressure DSC can beused to obtain the plasticized Tg and provides reason-able results provided the loss of gas during samplinghandling and scanning is minimized. However, it is nota simple task to do so. High-pressure cells are availablefor DSCs with a typical pressure limitation of about 6.9MPa (1000 psi). Without elaborate controls, these unitsalso suffer from an increasing pressure in the systemduring the scan, so it is not possible to define thethermodynamic state of the measurement. Second, the

Figure 2. CO2 solubility in polystyrene at different pressures andtemperatures (the thick line on the surface is the Tg).

C ) kHP +CLbP

1 + bP

Pij* ) (1 - δij)(Pi*Pj*)0.5



baseline stability deteriorates under elevated pres-sures.70 Despite these challenges, DSC remains the bestalternative for obtaining Tg. It should be noted finallythat for some systems the Tg is reduced so far that theapparatus needs to be modified for low-temperaturescanning.

For lower temperatures, the in situ measurement ofthe glass transition temperature of polymers71 underCO2 using a creep compliance method gave very goodresults. It is not subject to errors due to desorption ofCO2. The creep compliance changes 2 orders of magni-tude at the transition and the glass transition is well-defined.

Theoretical analyses of Tg depression are availablein the literature72 as well as molecular dynamicssimulation.73 All the theoretical approaches use latticefluid theory (such as the Sanchez-Lacombe EOS, orPanayiotou and Vera74) to describe the gas-polymerthermodynamics and the Gibbs-DiMarzio75-77 criterionfor the glass transition. The analysis of Chow78 was oneof the first to appear and is the easiest to use. Wissingerand Paulaitis79 and Condo et al.80 developed a modelthat accounts for more molecular detail but also requiresmore detailed experimental data to evaluate.

The approach of Chow78 provides a theoretical esti-mate of polymer Tg depression by addition of a diluent:

where Tg0 is the glass transition temperature of thediluent-free polymer. The parameters â and θ are givenby

where z is the coordination number, R is the gasconstant, Mp is the polymer repeat unit molecular

weight, Md is the diluent molecular weight, ∆Cp is thechange in polymer heat capacity at the glass transition,and ω is the diluent weight fraction. Values for theseparameters are either available in the literature or canbe measured in independent experiments, except for z,which most commonly is taken as either 1 or 2.78,81 Table3 provides examples for several common polymers.

In a subsequent model addressing pressure, polymerflexibility, and equilibrium partitioning of the diluentexplicitly, Condo et al.68 predicted four types of glasstransition depression behaviors under compressed fluiddiluents, including a new phenomena they called ret-rograde vitrification, which is a consequence of thecomplex effects of temperature and pressure on sorption.The phenomenon was verified in later publications bythese authors using PMMA, PS, and a random copoly-mer of PMMA (60 wt %) and PS. Although the glasstransition temperature depression versus pressure isvery complex, the correlation between the glass transi-tion temperature depression and the concentration ofCO2 reduces to a nearly linear relationship.

(3) Interfacial Tension between Immiscible Poly-mers under SCF CO2. The interfacial tension betweena gas and polymer melt or between two immisciblepolymer melts plays a central role in polymer blendingand foaming.82,83 While there exists a substantial bodyof literature on interfacial tension at high temperature(polymer melts with ambient pressure gases) and highpressure (gases with oligomers or polymer solutions),there is relatively little information available on systemsof practical interest (i.e., polymer melts) at high tem-perature and high pressure.

Table 2. Summary of Tg Depression of Polymers under CO2

polymer methods refpressure range

investigatedTg reduction

-dT/dP (°C/atm)

PMMA

molecular probe chromatography 313 0-75 atm 0.87ambient pressure DSC 81 0-25 atm 1.8high-pressure DSC 67 0-37 atm 1.2creep compliance a 79 40 atm 1.58creep compliance a 71 0-60 atm 0.82high-pressure DSC 72 0-35 atm 0.57

PS

high-pressure DSC 314 0-47.6 atm 0.84high-pressure DSC 315 0-60 atm 0.88ambient pressure DSC 81 0-20 atm 1.1creep compliance 68 0-65 atm 1.0creep compliance 79 0-60 atm 1.08dilatometry 42 0-60 atm 1.08

PEMA creep compliance 71 0-25 atm 2.05dielectric relaxation 316 0-60 atm 2.14

PVC high-pressure DSC 69 0-42 atm 1.45ambient pressure DSC 81 0-20 atm 0.4

PC

high-pressure DSC 69 0-92 atm 0.87high-pressure DSC 72 0-56 atm 0.73ambient pressure DSC 81 0-20 atm 2.55high-pressure DSC 317 0-10 atm 2.8

PPO(poly 2,6-dimethyl-phenylene oxide)

high-pressure DSC 318 0-61.2 0.5

PET(poly(ethylene terephalate)

Ambient pressure DSC 81 0-20 atm 1.1

a Retrograde vitrification in which Tg vs pressure shows both negative and positive slopes was observed for these systems.

ln[ Tg

Tg0] ) â[(1 - θ)ln(1 - θ) + θ ln θ]

â ) zRMp∆Cp

θ )Mp

zMd

ω1 - ω

Table 3. Parameters Used in Chow’s Model81

PVC PS PC PET PMMA

Tgo (°C) 75 100 148 74 105Mp (g/mol) 62.5 104 254 192 100∆Cp (cal/g/°C)319 0.0693 0.0767 0.0585 0.0812 0.0746F (g/cm3) 1.36 1.05 1.20 1.33 1.18


The significance of interfacial tension can be describedin terms of the Capillary number,

where ηm is the viscosity of the polymer phase, γ is theapplied shear rate, R is a length scale, and σ is theinterfacial or surface tension between immiscible phases.This ratio of viscous to surface forces indicates therelevance of drop or bubble breakup and coalescence inpolymer blending and foaming processes, which in turndetermines the domain size in polymer blends and cellsize in polymer foams. For large capillary number,droplet breakup is the dominant phenomenon (Ca >1-10, depending on the viscosity ratio). When thecapillary number is below 0.1 or so, coalescence willdominate. In the region 0.1 < Ca < 10, both the breakupand coalescence will take place simultaneously.84

The pendant drop is the most commonly used methodto measure the interfacial tension for polymer melts andis the most promising for simultaneous high-pressureand high-temperature application. In a study of high-viscocity liquid-crystalline polymers (LCPS),85 the pen-dant drop was demonstrated to be the only suitablemethod. De Marquette and Kamal studied the transientbehavior of interfacial tension using this method byfollowing the exponential decay curve and thus greatlyreducing the experimental time and the risk of polymerdegradation.86 The pendant drop method has beenapplied to SC-CO2 systems for the interfacial tension,σ, between oligomers and CO2. While γ is often usedfor interfacial or surface tension, we use σ in this reviewto distinguish these properties from shear rate inpolymer rheology discussed below. Harrison et al.measured interfacial tensions for PEG(Mw ) 600)/CO2

87 at 45 °C up to 300 bar wherein σ decreases from40.4 dyn/cm at 1 bar to 3.1 dyn/cm at 300 bar, andPS(Mw ) 1850)/CO2

88 at 45 °C up to 310 bar where σdecreases from 37.4 dyn/cm at 1 bar to 1.5 dyn/cm at310 bar. Although it would be very useful for polymerblending, no literature has yet been reported for theeffect of CO2 on the interfacial tension between im-miscible polymers.

Theories and advances in predicting interfacial ten-sion of polymer melts were recently reviewed.89 Twoprimary theoretical approaches have been developed topredict the interfacial tension between polymer melts.Helfand and Tagami90-92 formulated a statistical me-chanical theory of the interface between immisciblepolymers for symmetric systems, which is based on self-consistent field theory. The theory has since beenextended to nonsymmetric polymer systems.93

The other common approach is based on squaregradient theories94-96 combined with the Flory-Hug-gins expression for the free energy density, or withequations of state, such as the Flory-Orwoll-Vrij model(FOV)97 or the Sanchez and Lacombe lattice gas model(LF).59,60,98,99 The latter have had considerable successmodeling compressible systems.100-103

Although conceptually different, the results by Poserand Sanchez103 give comparable predictions to those ofHefand and Sapse.93 Sanchez has shown that thegradient theory is “in harmony with the microscopictheory of Helfand and co-workers, although the lattertreats the polymer interfaces from a different point ofview”.90-93,104

(B) Transport Phenomena. (1) Mass Transfer inPolymer/Supercritical CO2 Systems. All industrialapplications of supercritical CO2 such as foaming,61,105

extrusion,106 and impregnation48,107,108 require an ac-curate understanding of mass transfer of carbon dioxideand additives in a polymer matrix to optimize theprocess. Therefore, in this section, kinetic studies ofsupercritical CO2 transport are reviewed along withsome comments regarding heat transfer. Experimentaltechniques and models are discussed first, and thentransport phenomena in CO2/solid polymer, CO2/moltenpolymer, and CO2/additive/solid polymer systems areaddressed individually.

(a) Experimental Techniques for Binary Sys-tems (Polymer/CO2). Diffusivity or mass transfer inpolymer phases can be obtained using several of thesame techniques for solubility by following the dynamicapproach to equilibrium. This has been accomplishedwith barometric methods61,106 and gravimetric meth-ods.29,38,43,48,109 Other techniques that are suited totransient measurements but not quantitative solubilitymeasurements include the following.

(i) Optical Observation. With optical recording ofthe swelling behavior of polymer melts under highpressure with a CCD camera and then correlation ofthe degree of swelling to mass uptake, the diffusioncoefficient is obtained.105,110 The method features a high-pressure view cell which constrains the diffusion of CO2and polymer swelling to one dimension. This techniqueis useful in monitoring the liquid level for one-dimen-sional swelling of polymer melt, but does not work wellfor solid polymers because of the inaccurate real timeestimation of volume change.

(ii) Spectroscopic Technique. With measurementof the transient absorbance data in the near-IR region,the relative concentration of CO2 in the polymer as afunction of time could be determined according to theBeer-Lambert law.25,33

(b) Experimental Techniques for Ternary Sys-tems (Polymer/CO2/Additive). Some techniques fol-low similar procedures as for binary systems with slightmodifications, such as gravimetric measurement111 andspectroscopic technique by UV-vis108 and FTIR.112 Thetechniques below are specifically used in determiningtransport properties of additives in a supercriticalatmosphere.

(i) Film Roll Method. In this technique, a roll withmultiple layers of polymer film is impregnated with dyesin supercritical CO2. The films are unrolled after dyeingand the dye concentration in each layer is determinedby UV-vis. With construction of the relationship be-tween concentration and distance, the diffusivity of thedye is calculated.107

(ii) Forced Rayleigh Scattering (FRS). This methodis significant because it directly determines diffusivityof additives in a polymer matrix without reduction fromabsorption data as in other techniques. FRS detects therate of relaxation of a “forced” gradient in concentrationof cis and trans azobenzene isomers. However, thistechnique requires the polymer to be transparent sincethe cis and trans transition is photochemically driven.113

(iii) Fluorescence Nonradiative Energy TransferTechnique. This technique is based on the principlethat energy transfer from a donor chromophere to anacceptor induces a continuous change in both molecules’

Ca )ηmγR

σ


emission intensity. Diffusivity of the additive moleculesis calculated from real time intensity data.114

(c) Diffusion Modeling. The transport mechanismin a polymer matrix under supercritical environmentsis very complicated because of the significant effects ofCO2 on the polymer, such as state transitions, chainmobility, and swelling. When a polymer is exposed totemperatures above Tg, the quick response of thepolymer chains to the presence of CO2 makes it behaveas a homogeneous phase and then the transport of CO2simply obeys the classic Fick’s law. As a result, manyresearchers deliberately set up the experimental condi-tions to lower the polymer Tg and reasonably employthe Fickian diffusion model to analyze the kineticsorption (desorption) data.29,48,109,115,116 The analyticalsolutions for one-dimensional simple geometries (cylin-der and thin film) are well-known117 and expressed interms of the mass uptake ratio, Mt/M∞, which can becorrelated to other measurable variables such as theswelling ratio,110 pressure,106 and infrared absorbance.26

Balik118 summarized commonly used mathematicalmethods to deduce the diffusivity from available kineticdata and proposed a new nonlinear regression method(hybrid method) with the advantage of using all themesured data and not requiring the M∞ initially.

The transport of CO2 in glassy polymers below Tg canbe described by a dual-mode model or a modified partial-immobilization dual-mode model.119 These approachesassume that some of the gas molecules follow Henry’slaw and are completely free to diffuse while others aresorbed in the microvoids and partially immobilized. ALangmuir adsorption model characterizes the latter.The transport observed is the net contribution from bothtypes of diffusion and the differences in the microscopicmechanism complicate the real case. The effectivediffusivity is correlated to solution diffusion in theHenry’s Law region (DH) and Langmuir adsorption (DD)by the following equation.

Hence, a plot of Deff[1 + K/(1 + CDb/kD)2] against K/(1+ CDb/kD)2 is expected to be linear, from which one canevaluate the effects of an individual diffusion mecha-nism. However, the real case is always more complexbecause of various factors, including concentration-dependent properties, Tg depression, and the relaxationtime scale of the polymer chain.120 For example, Nikitinet al.121 observed a sharp diffusion front during thetransport of CO2 in PMMA because of the distinctdifference in diffusion rates of CO2 in the glassy andrubbery states. The plasticization clearly takes place inthe interface region and moves with the concentrationfront of CO2. Furthermore, the front propagation dy-namics reveals Fickian diffusion characteristics in datathat were initially regarded as anomalous diffusionbehavior.122 Besides plasticization, the relaxation timeof the polymer affects the diffusion behavior signifi-cantly. This is normally described by the Deborahnumber (De), that is, the ratio of relaxation time to

characteristic diffusion time. For small or large De, thediffusion follows Fickian law, while for intermediatevalues, non-Fickian behavior will appear.123 Because ofthe pronounced effects of CO2 on polymers, the variationof relaxation properties is presumably expected. Oneforeseeable consequence is the effect of relaxation on theplasticization kinetics. This discussion just highlightssome of the complex factors in CO2 diffusion throughglassy polymers, thus indicating that more fundamentalstudies in CO2 and its effects on various types ofpolymers are required for future development.

(d) Mass Transfer of CO2 in Solid Polymers.Here, we present several examples to illustrate thetransport of CO2 and its complexity. In Berens’ work,29,115

the sorption of near-critical CO2 in a variety of polymersby the simple gravimetric method shows that thediffusivity of CO2 increases with concentration andultimately enters the range of 10-6 to 10-7 cm2/s. Theserepresent typical values of CO2 diffusivity in rubberypolymers and the results clearly demonstrate the plas-ticizing effects of CO2. More examples include the CO2/PVC system116 and the CO2/PET system,48 both of whichdemonstrate increased diffusivity with an increase ofCO2 pressure and temperature (for conditions above Tg).The characteristic S-bend shape of the sorption iso-therms of CO2 in PET indicates the change from dual-mode sorption at lower pressures to Fickian diffusionat higher pressures.48

As for the semicrystalline polymers, it is generallyaccepted that the sorption of gas occurs mainly in theamorphous regions. The almost identical sorption forboth initially amorphous PET and partially crystallinePET samples in Brantley’s work26 confirms this view-point.

(e) Mass Transfer of CO2 in Molten Polymers.The fundamental understanding and construction ofmodels for CO2 behavior should be easier for moltenpolymer systems since they can be treated as pureliquids. Sato et al. measured the diffusivity of CO2 in avariety of molten polymers, including PS,38 HDPE,61

poly(butylene succinate) (PBS), and poly(butylene suc-cinate-co-adipate).124 The authors provide basic diffusiondata and demonstrate the importance of free volume inunderstanding gas transport. They correlated the freevolume fraction of the polymer/gas solution with themeasured diffusion coefficients and generally achievedgood predictions with about ∼10% relative deviation.

Free volume also helps explain the decrease of CO2diffusion rates with an increase of pressure in somesystems. For example, in the CO2/gelatinized starchsystem,106 as the result of high compressibility, the freevolume available for CO2 transport would be reducedsignificantly under high hydrostatic pressure. Royer andco-workers also noticed this phenomena,105,110 thoughthe decrease of diffusion coefficient is not very evident.Table 4 lists some typical values of CO2 diffusioncoefficient in molten polymers from various sources.

(f) Diffusion of Additives in Polymers underSupercritical Atmosphere. The diffusion of additivesthrough polymers was significantly promoted by thepresence of supercritical CO2. For example, the diffu-sivity of dimethyl phthalate (DMP) in PVC is 6 ordersof magnitude higher under SC-CO2 conditions thanwithout CO2.111 Other examples include the diffusionof azobenzene in glassy PS113 and decacyclene in PS.114

This particular ability of SC-CO2 has already led toseveral important industrial applications, such as dye-

J ) -DeffdCdx

) -DD

dCD

dx-DH

dCH

dx, F ) DH/DD

Deff )DD[1 + FK/(1 + CDb/kD)2]

[1 + K/(1 + CDb/kD)2]w

Deff[1 + K/(1 + CDb/kD)2] )

DD[1 + FK/(1 + CDb/kD)2]


ing, impregnating biological agents, and creating poly-mer composites or blends as discussed later. Further-more, the environmental benefits and easy control byaltering CO2 pressure offer distinct process advantages.

The specific interaction between additives and poly-mer matrix can strongly influence the diffusion process,as exemplified by the faster diffusion rate of 4,4′-(diethylamino)nitroazobenzene (DENAB) in PMMA thanDisperse Red 1 (DR1).108 However, we are typically moreinterested in the role of SC-CO2 in enhancing thediffusion rather than the solute-substrate interactionsthat are dictated by the application. In this respect, CO2acts as a tunable carrier fluid to alter the polymermatrix or a cosolvent is added to enhance the solvent-polymer interactions. For example, the addition ofethanol in Sicardi’s study107 increased the diffusioncoefficient of a dye over that with pure CO2. Thecombination of tuning CO2 and adding cosolvent pro-vides more controllable process parameters and allowsfor the impregnation of thermally labile and metastablematerials under lower temperature and pressure.

Because of the higher chain mobility and more avail-able free volume, the increased transport rate of soluteas temperature and pressure increase is certainlyexpected, as demonstrated in the systems of dye/PET/CO2,107 dye/PMMA/CO2,108 and DMP/PVC/CO2.111 Incomparison, the influence of dye concentration on dif-fusion is more complex, as illustrated in Figure 3. Theobserved curve could be explained by the overall con-tribution from the opposite diffusion behavior of twotypes of dye species when penetrating the polymer.107

(2) Heat Transfer. Although heat transfer is animportant property in continuous polymer processingoperations such as extrusion and injection molding,there have been no reports in the literature for themeasurement of heat-transfer coefficients in CO2 (orother SCF) saturated polymer melts.

(C) Rheology of Polymer Melts with DissolvedCO2. Because of the pivotal role of rheological propertiesof CO2/polymer melt systems in equipment design andprocess simulation, increased attention has been givento understanding them.

In general, the viscosity is observed to decrease asCO2 is dissolved into various polymer melts (as shownin Figure 4). This viscosity reduction is greatly favorablefor processing high molecular weight polymers wherehigh viscosity is the major obstacle. It also facilitatesthe processing of temperature-sensitive polymers at

lower temperatures to prevent thermal degradation andsave energy.

(1) Shear Viscosity Measurement. To measure theviscosity of CO2/polymer solutions, traditional rheom-eters are modified and two issues must be emphasized:one is to ensure the formation of a homogeneous solutionbefore the measurement; the other is to prevent phaseseparation during the measurement. Early work fromfoaming research demonstrated many experimental

Table 4. Diffusion Coefficient of CO2 in Molten Polymers

sample T (K) P (MPa) diffusivity (cm2/s) source

39.2% (m.c) gelatinized starch343 2.6 7.5 × 10-6 106343 9.2 1.9 × 10-6

343 11.8 0.9 × 10-6

polyamide 11 488 10.3 5.29 × 10-5 105488 37.9 2.29 × 10-5

poly(dimethylsiloxane)

303 10.5 1.7 × 10-5 110 (data read from figure)

303 24 1.2 × 10-5

343 24 9 × 10-5

polystyrene 423.15 8.319 5.33 × 10-6 38473.15 8.42 9.9 × 10-6

poly(butylenes succinate) (PBS)393.15 12.341 1.23 × 10-5 124453.15 2.466 2.04 × 10-5

453.15 8.304 2.68 × 10-5

poly(butylenes succinate-co-adipate)393.15 12.229 0.95 × 10-5 124453.15 2.34 2.06 × 10-5

453.15 8.616 2.05 × 10-5

Figure 3. Diffusivity versus dimensionless concentration in thepolymer (disperse blue: 22 MPa, 110 °C) [Reprinted from: Sicardiet al. Diffusion of disperse dyes in PET films during impregnationwith a supercritical fluid. J. Supercrit. Fluids 2000, 17 (2), 187-194. Copyright 2000, with permission from Elsevier].107

Figure 4. Viscosity reduction of CO2/polystyrene solution withdifferent CO2 content at 175 °C (fitted by Carreau model,experimental data are from Kwag et al.)130




devices capable of measuring viscosities of polymer/blowing agent mixtures under high pressures, based onwhich new devices are specially developed for the CO2/polymer solutions. These devices are classified into twocategories: (1) pressure-driven and (2) drag-driven.

Pressure-driven devices can be either a capillary ora slit extrusion rheometer controlled by a back-pressureregulator. This method was originally designed forviscosity measurement of polymers with fluorocarbonblowing agents.125,126

Recently, a capillary rheometer was used by Gerhardtet al.127-129 to measure the shear viscosity of polydi-methyl siloxane (PDMS) containing dissolved CO2. Asealed loading apparatus helps the transfer of anequilibrated sample to the rheometer and a back-pressure assembly at the exit holds the gas in thesolution. Corrections for back-pressure, wall friction,and entrance/exit pressure drop were considered in theviscosity calculation. A similar rheometer was used byKwag et al.130 to measure the shear viscosity of poly-styrene (PS) with dissolved CO2 and several refriger-ants. Lee et al.131,132 applied a foaming extrusionapparatus to determine the viscosity of PS/CO2 solu-tions. A capillary tube die with two pressure transducersmounted along the flow stream was used and phaseseparation was prevented by maintaining a high pres-sure in the die. With use of a positive displacementpump, a metered amount of gas was injected into theextrusion barrel and mixed with the polymer melts.Such a capillary rheometer was also applied by Areeratet al.133,134 to measure the viscosities of low-densitypolyethylene (LDPE) and polypropylene (PP) with CO2concentration monitored on-line by near-infrared spec-troscopy.

When a similar extrusion apparatus is used, a slit diecan also be used to measure the shear viscosity ofpolymer/CO2 solutions. The pressure drop along a flowstreamline can be directly measured without worryingabout the entrance/exit pressure correction or pressuremeasurement error created by the curvature of thecapillary die. However, the total pressure drop may belower than that in a capillary die, which may affect themeasurement accuracy. Elkovitch et al.135-137 applieda slit die to measure the viscosity reduction for CO2/PSand CO2/poly(methyl methacrylate) (PMMA) withoutback-pressure control. Royer et al.138,139 attached nozzlesof different sizes on the slit die to hold the high pressureand measured shear viscosities of PS, PMMA, PP,LDPE, and poly(vinylidene fluoride) (PVDF) withdissolved CO2. Lee et al.140 also applied a wedge diemounted on a twin-screw extruder to measurethe viscosities of CO2/polymers. Gendron and co-workers141-143 examined the rheological behavior of PSand PP with dissolved CO2 by using a commercial on-line rheometer, basically a slit contraction, mounted ona twin-screw extruder.

Overall, the pressure-driven devices have been foundconvenient to provide accurate rheological data. Theirlimitation is that the large pressure drop across thecapillary or slit die limits the CO2 concentration dis-solved in the polymer melts and as a result viscositiesare usually measured at low CO2 concentration toensure the formation of a single-phase solution. Also inthe low shear rate region, the small pressure dropsdetected, regardless of the large absolute pressure, maycause uncertainties in the raw data.

On the other hand, drag-driven devices can be used

to measure shear viscosities at low shear rates near theNewtonian plateau. They are usually operated at ornear equilibrium CO2 concentration and a steady,uniform distribution of pressure, stress, and deforma-tion rate can be created. However, such devices areusually difficult to design because the signals (forexample, torque, force, and displacement) have to betransferred under pressure through a dynamic seal,although a magnetic sensor may improve the design.

A magnetically levitated sphere rheometer was de-signed by Royer et al.144 to study the viscosity of PDMS/CO2 solution. In this design, the sphere is held station-ary at a fixed height through magnetic levitation whilethe cylindrical sample chamber is moved vertically togenerate different shear rates. The device provides anonhomogeneous flow field and needs to be calibratedagainst a known fluid viscosity. Recently, a Couetteviscometer was designed by Oh et al.145 to measure theviscosity of PS/CO2. A magnetic transmission has beenused to eliminate dynamic seals and reduce torquelosses in the drive train. The viscosity near the New-tonian plateau was evaluated from the torque and therate of rotation.

(2) Shear Viscosity Prediction. As discussed above,the viscosity decreases as the dissolved CO2 concentra-tion increases for various polymer melts. It is also foundthat the viscosity curves (i.e., shear viscosity versusshear rate) of the polymer/CO2 solution are usuallysimilar in shape to that of pure polymer and aretherefore analogous to the effect of increasing temper-ature or decreasing pressure. These analogies implythat carbon dioxide affects the viscosity of polymer meltsfollowing a similar mechanism to the temperature andpressure, predominantly through a change in freevolume. Other factors, such as the improvement inpolymer chain mobility, the dilution of polymer chains,and the reduction of chain entanglement upon CO2plasticization, also contribute to the viscosity reduction.

Consequently, the traditional scaling techniques canbe used and a scaling factor ac, similar to the familiartemperature-dependent shift factor aT employed intime-temperature superposition, can be applied torepresent the influence of the CO2 concentra-tion.128-130,132,138,139,144

Although power law or Cross-Carreau models146 canfit the shear thinning behavior easily, extensive re-search has focused on how to predict the effect of CO2concentration on viscosity of various polymer/CO2 solu-tions. Doolittle’s free volume theory147,148 is always usedas the starting point,

where η0 is the zero shear viscosity; A and B areunique constant parameters for the polymer. f(T,P,ωg)denotes the free volume fraction which is given as afunction of temperature, pressure, and weight fractionof gas,

where V(T,P,ωg) is the specific volume of the polymerat temperature T, pressure P, and gas concentration ωg,and V0 is the weight-average hard core specific volumeof polymer/gas solution.

η0 ) A exp( Bf(T,P,ωg)) (2.4)

f(T,P,ωg) )V(T,P,ωg) - V0(ωg)

V(T,P,ωg)(2.5)


By applying an extension of Doolittle’s free volumetheory, originally developed by Kelley and Bueche,149

Gerhardt et al.128,129 calculated the scaling factor ac ){ηo(ωg)}/{ηo(purepolymer)} for PDMS/CO2 mixtures.Both Sanchez-Lacombe and Panayiotou-Vera equa-tions of state, which model the P-V-T properties, wereused to calculate the specific volumes of the purepolymer melt and polymer-gas mixture. The predictionwas found to be in very good agreement with theexperimental data.

Areerat et al.133,134 approaches this problem by sub-stituting Doolittle’s equation in the Cross-Carreaumodel to get a generalized equation to relate viscosityto shear rate, temperature, pressure, and CO2 concen-tration. All the model coefficients except the one corre-sponding to CO2 concentration can be determined fromthe P-V-T data and molten viscosity data of the purepolymer. The Sanchez-Lacombe equation of state wasused to calculate the specific volume of the polymer/CO2solution and then the free volume fraction. The viscosityof LDPE/CO2 and PP/CO2 solutions was successfullypredicted by this model.

Lee et al.132 also started from Doolittle’s equation andexpressed the fractional free volume as a power lawseries in terms of temperature, pressure, and CO2concentration. A seven-parameter model, a generalizedArrhenius equation, was built to predict the zero shearviscosity of the neat PS melt to accommodate the effectsof temperature and pressure. Further, an eight-param-eter model was built to include the effect of CO2concentration.

Royer et al.138,139 adopted the Williams-Landel-Ferry (WLF) equation,150 a direct descendant of Doolit-tle’s equation, to relate the viscosity scaling factorscorresponding to pressure and CO2 concentration to theglass transition temperature (Tg) when the temperatureis in the range from Tg to Tg + 100 °C. Then the effectsof CO2 concentration and pressure can be directlyincorporated as the Tg depression, which was predictedby Chow’s model.78,151 On the other hand, when thetemperature is higher than Tg + 100 °C and beyond theeffective range of the WLF equation, Arrhenius ana-logues were applied to build the relation between shiftfactors and Tg. Systems of PS, PP, LDPE, and PVDFwith dissolved CO2 were studied by applying thesemodels.

(3) Extensional Viscosity Measurement. Exten-sional viscosity is important to help understand theentrance pressure drop, foam bubble growth, and otherprocesses related to elongational deformation. Ladin etal.152 applied a contraction slit die attached to a foamingextrusion system to measure the entrance pressuredrop, which was then converted to extensional viscosityaccording to Cogswell’s analysis153 for PBS/CO2 solu-tions. The extensional viscosity was found to signifi-cantly depend on the CO2 concentration and a largereduction was obtained with the dissolution of CO2.Tension-thinning behavior, where extensional viscositydecreases with an increase of extensional rate, wasobserved. The extensional viscosity was also observedto decrease as the temperature increased. By applyinga similar design, Xue and Tzoganakis154,155 studied theentrance pressure drop and extensional viscosity of PS/CO2 solution. Extensional viscosity reduction and ten-sion-thinning behavior were also reported. The entrancepressure drop was found to be a strong function of

pressure. Of course, such a design can be used tomeasure the shear viscosity simultaneously.

(D) Dynamic Processes. The processes of nucleationand growth of crystals or bubbles are important proper-ties in a variety of applications. While growth can beaccurately modeled according to diffusion-limited masstransfer from the bulk to the new phase, nucleationcontinues to confound researchers and practitioners.Below is a limited discussion of current topics in theuse of CO2 for nucleation of crystalline domains in bulkpolymers and nucleation of bubbles for producing foams.

(1) CO2-Induced Polymer Crystallization. Rela-tively little work has been published on the Tm depres-sion and crystallization kinetics of polymers under high-pressure CO2. This gas-induced crystallization has beenreported for poly(ethylene terephthalate) (PET),156-159

polypropylene,160 polycarbonate,161 poly(ether ether ke-tone) (PEEK),162 poly(p-phenylene sulfide) (PPS),163

methyl-substituted PEEK (MePEEK),70 syndiotacticpolystyrene (sPS),159,164 and blends of poly(vinylidenefluoride) and poly(methyl methacrylate).157

Measurement methods include correlation of crystal-lization to the bulk density of the polymer,156,158 infraredspectroscopy,156 and high-pressure DSC.160,164 It appearsthat the first two techniques give comparable resultsand may be more amenable to very high pressures ascommercial cells for high-pressure DSC investigationsare currently limited to approximately 100 bar.

The data are commonly expressed as Xt, the weightfraction of the material crystallized at time t, which isthen fit to the Avrami equation:

By plotting of ln(-ln(1 - Xt)) vs ln(t), the Avramiexponent n, and the logarithm of the kinetic constant,ln k can be determined.

CO2 was found to accelerate the crystallization kinet-ics for both PET156 and syndiotactic polystyrene.164 Thecrystallization kinetics for sPS-CO2 solutions follow theAvrami equation, but the value of the exponent n islower than that when crystallization is conducted underambient pressure. Furthermore, the presence of CO2-induced morphology changes in the sPS crystal, whichdo not occur upon treatment with liquids. For thecrystallization of polypropylene, CO2 decreased theoverall crystallization rate.160

This apparent conflict in the effect of CO2 wasexplained in terms of the proximity to the temperatureof maximum crystallization rate (Tmax). The crystalliza-tion rate changes with temperature and reaches itsmaximum at Tmax, which is close to the mean of Tg andthe equilibrium melt temperature Tm

0: Tmax = (Tg +Tm

0 )/2. Above Tmax the overall crystallization rate iscontrolled by the nucleation rate, and the temperatureregion between Tmax and Tm

0 is called the nucleationregion. Below Tmax the overall rate is controlled by thecrystal growth rate. Alternatively, one can interpretTmax as a competition between the thermodynamicdriving force (∆T) and the decreasing mobility at lowerT. Dissolved CO2 will depress Tm

0 , and hence Tmax. Thismeans that the dissolved CO2 accelerates the crystal-lization rate of an isothermally crystallized semicrys-talline polymer within the crystallization growth regionand reduces the rate within the nucleation controlledregion. This explains why the crystallization rate ofMizoguchi et al.156 for the isothermally crystallized PET

Xt ) 1 - exp(-ktn)


at T ) 308.2-393.2 K increases (Tmax ) 448 K), andthe crystallization rate of Handa et al.164 for theisothermally crystallized sPS at T ) 395.2 K increases(also below Tmax), while Takada et al.’s 160 crystallizationrate of PP decreases as the crystallization was carriedout in the nucleation region.

Takada et al.160 also developed a model for thecrystallization kinetics of polymers under CO2, whichis based on Avrami’s theory for three-dimensionalheterogeneous nucleation and the temperaure-depend-ent crystal growth models of Ishizuka and Koyama165

and Ito et al.166

(2) Bubble Nucleation. In the context of foaming,bubble nucleation (cell nucleation) is the formation of anew gas phase from a metastable melt phase, requiringan activation energy barrier to be surmounted (viadensity fluctuations) to induce the phase separation. Instandard foaming applications, foaming occurs by nucle-ation rather than spinodal decomposition, which is thespontaneous phase separation from a thermodynami-cally unstable state.

Concentration gradients, pressure gradients, or tem-perature gradients can drive nucleation. In most foam-ing applications either a pressure drop or a temperatureincrease is used to decrease the gas solubility and makethe solution supersaturated. When clusters of gasmolecules are greater than the critical size, the activa-tion energy is overcome and nucleation occurs. Thegreater the supersaturation, the smaller the activationenergy. Additives such as talc or nano-clay are com-monly used to adjust the nucleation rate, presumablyenhancing the rate by significantly decreasing theactivation barrier.

Classical homogeneous nucleation theory167,168 wasderived for the formation of liquid drops from a purevapor and one can argue the applicability of this theoryin a foaming process. The latter is actually a binary ora cavitation-like process, but classical homogeneousnucleation theory is still widely used to explain phe-nomena qualitatively and/or quantitatively.169-178 Theequations are exponential with respect to the activationenergy barrier (∆G*), which must be overcome to phaseseparate the metastable solution. Two equations arecommonly used, one for the rate (N) of homogeneousnucleation (bubble formation in bulk polymer phase)

and the other for heterogeneous nucleation (bubbleformation at an interface between a polymer and anadditive),

where C is the concentration of gas molecules, f is thefrequency factor of gas molecules joining the nucleus, kis Boltzman’s constant, T is the absolute temperature,γbp is the interfacial tension of the polymer-bubbleinterface, ∆P is the gas pressure, and θ is the contactangle of the polymer-additive-gas interface.170,171

To set up a technically correct experiment followingthe homogeneous nucleation equation, many restrictionsare necessary. No pre-existing gas cavities can exist inthe bulk, on the container walls, or in the form ofmicrovoids (microbubbles). It is difficult because evenvery highly idealized cases of nucleation in boiling andnucleation in isothermal gas desorption still are notcompletely understood.179 Plus, all nucleation theoriesrequire nuclei as well as microvoids to be spherical inshape. It is not understood what effect this nonidealityhas on experimental results.

Colton and Suh used classical nucleation theory asthe basis to develop a model for the nucleation ofmicrocellular foams in polystyrene with zinc stearateadditive.170 They concluded that homogeneous nucle-ation occurs below the zinc stearate solubility limit sincethe nucleation rate increases for an increase of satura-tion pressure and increase of concentration of zincstearate. Above the solubility limit, heterogeneousnucleation dominates and the nucleation rate increaseswith stearate concentration but is not affected bypressure. Around the solubility limit, each nucleationmechanism is significant with the nucleation rateassumed equal to the sum of both nucleation rates.

Experimentally, injection molding is used to makesamples and nucleation is measured by counting thenumber of cells using scanning electron microscopy(SEM) of the foamed polystyrene but no informationabout the experimental bubble sizes is given.169 Thus,it is not clear what the smallest bubble sizes areconsidered; this method assumes: each nucleation siteproduces a bubble, no bubble coalescence occurs, andall bubbles nucleate and grow to a certain detectablesize.

With counting of the number of bubbles formed, alarge number of publications present intriguing conclu-sions about foaming behavior. These again assume eachnucleation site produces a bubble, no bubble coalescenceoccurs, and all bubbles nucleate and grow to a certaindetectable size. In many cases, the sample is quenchedat cold temperatures in an effort to prevent cell coales-cence. For physical blowing agent-assisted polymericfoaming with the use of nucleation agents, severalauthors present foaming data using heterogeneousnucleation.180-183 Park et al. present results where alimit on the effect of nucleation agent on cell density isreached.184 Ramesh et al. explains data for a rubber-polystyrene blend without any additives using micro-voids as heterogeneous nucleation sites.185,186

Han and Han studied bubble nucleation in polymericliquids using laser light scattering. The growth ofbubbles is assumed to be monodisperse, thereby ne-glecting multiple scattering effects.174,175 A connectionbetween pressure drop rate to nucleation rate wasidentified by Park et al.187 To lower the free energy, gasdiffuses into the cells, resulting in a depleted regionsurrounding the cell. If the depleted regions from allcells are in contact, no further nucleation is possible,and this condition is called cell impingement. In termsof driving force, as the initial nucleation events occur,supersaturation decreases, decreasing the likelihood ofensuing nucleation. Therefore, due to this competitionbetween nucleation and diffusion, a higher pressuredrop rate results in higher supersaturation, whichtranslates to a higher nucleation rate. The end resultis that pressure drop rate, nucleation rate, and celldensity can all be viewed equivalently. This has been

N0 ) C0f0e(-∆G*hom/kT)

∆G*hom )16πγbp

3∆P

N1 ) C1f1e(-∆G*het/kT)

∆G*het )16πγbp

3

3∆P2 (14)(2 + cos θ)(1 - cos θ)2


verified experimentally using capillary nozzles wherethe smallest nozzle produced foam with the highest celldensity (smaller nozzles had higher pressure droprates).187

A lot of effort188-196 has been expended in modelingbubble growth. The procedure has evolved from modelsin which a single bubble is surrounded by an “infinitesea” of fluid with an infinite amount of gas availablefor growth, to the cell model in which the foam is dividedinto spherical microscopic unit cells of equal and con-stant mass, each consisting of a liquid envelope sur-rounding a single bubble, and thus the gas availablefor growth is limited. Overall, bubble growth is a processin which the heat, mass, momentum transport, andvarious constitutive equations need to be solved simul-taneously.

When these theories and models of both cell nucle-ation and growth are implemented, parameters such asthe viscosity, solubility, surface tension, diffusivity, andglass transition temperature are found to play impor-tant roles in deciding the cell density and size. Thesituation becomes more complicated when we considerthat these parameters are all functions of temperature,pressure, and CO2 concentration, by which cell nucle-ation and growth themselves are affected as discussedby Muller et al.197 Considering the advances being madein fundamental property measurement and modelingdescribed earlier, there will soon be enough informationto perform powerful simulations for studying the inter-action of all these effects.

Applications

(A) Impregnation. Impregnation in this context canbe described as the delivery of solutes to the desiredsites inside the polymer matrix with the aid of SC-CO2.Basically, the impregnation process includes three majorsteps: first, to expose the polymer to SC-CO2 for aperiod of time; second, to introduce the SC-CO2 contain-ing solutes to the polymer and conduct the subsequentsolute transfer from SC-CO2 to polymer phase; third,release the CO2 in a controlled manner and trap thesolutes in the polymer. When exposed to SC-CO2,polymers will exhibit various extents of swelling andenhanced chain mobility as described above, whichsignificantly facilitate and accelerate the transport ofcomponents. Besides the environmental benefits, an-other advantage of the process is the control obtainedby tuning the CO2 properties, which has been well-summarized in prior reviews.4,198,199

Theoretically, any type of solute could be impregnatedinto polymers; however, the applicability of the processis determined by two main issues: (A) the transport rateof solutes that determines the possibility of developingsuch a process and (B) the compatibility and stabilitybetween solutes and polymer that determines if theprocess is worthwhile to develop. The second point refersto the possible subsequent changes of the impregnatedproduct, for example, noticeable separation betweensolutes and polymer and a fast leach-out process. In thissection, we summarize the practical issues of impreg-nating polymers and highlight recent applications ap-pearing in the literature since the previous reviews.

The swelling extent and alteration of microstructureof the polymer depends on the chemical nature of thepolymer and its interaction with CO2. Lesser swellingalways means relatively difficult impregnation. Forexample, compared with PVC and PC, PTFE demon-

strated the least ability to be modified because of itslimited swelling.200 Generally speaking, highly crystal-line polymers are not suitable as an impregnatingmatrix because of the regular structure and the muchslower transport process. As a result, current studieshave been mainly limited to polymers with large amor-phous fractions and/or specific interactions with CO2.For example, PET,24,107,201-206 PMMA,23,108,198,207 poly-carbonate,111,200 polystyrene,111 and poly(vinyl chlo-ride).200 However, the incorporation of molecules intoinexpensive polymers, such as polyolefins, also holdsgreat potential for applications. Wang et al.208 showedthat a uniform distribution of a dye molecule (NBD) inpolypropylene could be achieved even though the solu-bility is low and modifier/polymer interactions are notfavorable. The impregnation of biodegradable polymershas also attracted much attention because of thepotential for controlled drug-release systems or biosur-face modification. Poly-DL-lactide-co-glycolide (PLGA)and its derivatives are subject to the most activeresearch. Because of their relatively low glass transitiontemperature, it proves difficult to impregnate PLGAwithout observable deformation and foaming. In theprocess of delivering 5-fluorouracil and â-estradiol intoPLGA, the final form of product could appear as foamwith large pores or microporous particles by controllingthe depressurization.209 In some applications, foamingmay be an advantage and there is a degree of controlover the product morphology when combining impreg-nation with anitsolvent precipitation methods.

The final appearance of polymers after impregnationcan be an important issue that is generally affected bythe intrinsic Tg of the polymer, CO2 conditions, CO2release rate, and geometry. When the pressure isreleased, CO2 near the surface sees the largest gradientand escapes from the polymer quickly and the surfacereverts to its original morphology. Hence, the rate ofCO2 release and polymer relaxation are interactive butonly the former is a controllable process variable. Well-controlled slow depressurization allows for the escapeof CO2 from the polymer matrix before the polymerrecovers from the swollen state; otherwise, CO2 en-trapped inside the polymer leads to foaming. In otherways CO2-induced plasticization or crystallization couldbe employed to improve the physical properties of thepolymer during supercritical treatment, for example, theincrease in the crystallinity of the polymer204,206 and themodification of ill-developed polymer structure.202

At present, the solutes studied in the process ofimpregnation range from dyes, to metal complexes, andto biological molecules. Obviously, solutes with highsolubility in SC-CO2 can be easily delivered to thepolymer matrix. Meanwhile, studies show that, for thosewith low solubility in the supercritical phase, thestronger affinity for the polymer matrix leads to favor-able partitioning toward the polymer phase.198,207,210 Infact, distribution coefficients (ratio of polymer to fluidconcentration) can be 100-1000 times higher than thatwith liquid solvents. A recent study demonstrates that,despite being thermodynamically unfavorable, the dis-tribution of a large surfactant-like molecule insidepolypropylene could be controlled and achieved.208

Dyeing using SC-CO2, in which the solutes trans-ported are organic dyes, is perhaps the most commonapplication of impregnation to date. Examples includethe original work of Berens,120 dyeing of textile acces-sories,203 and a number of papers from Chuck Eckert’s


group.23,25,108,198,207,211,212 Besides organic materials, in-organic metal complexes have also been incorporatedinto polymers to fulfill specific functions. For instance,the infusion of silver-containing additives leads to theformation of metalized polymer film with high reflectiv-ity.213-215 Polymer/metal composites, even at a nano-scale level, can be produced by infusing proper metalcomplexes into a polymer matrix, such as nanoscaleplatinum clusters into PTFE,216 copper nanoparticles inpolyacrylate,217 and chelate complexes of copper and ironinto polyacrylate.218

Similarly, if the solute impregnated is a polymerizablemonomer, polymer blends or surface modification canbe accomplished, depending on the depth of penetration.The general procedure for producing polymer blends isto infuse the monomer and initiator together andsubsequently initiate the polymerization, as shown bythe extensive work of Watkins and McCarthy.216,219,220

Clearly, the final performance of the product is deter-mined by the amount and distribution of componentsinside the polymer matrix. In some cases, because ofthe low diffusion rate, the impregnated components onlypenetrate a limited distance beneath the surface. Forexample, Muth et al. found that the depth of penetrationof methacrylic acid into PVC is only about 180 µm after4 h of impregnation.200 Interestingly, sometimes onewants to take advantage of this property and confinethe functional monomers to the surface region of poly-mers. As a result, the bulk properties of the polymercould be maintained while the surface has been graftedwith a certain type of modifier to fulfill specificfunctions.221-225 In comparison with the direct infusionof polymers, the advantages of impregnating functionalmonomers is the relatively more favorable diffusion ofsmall molecules compared to larger chains, while thedisadvantages come from control of distribution andsubsequent reaction. Radiation is a common choice forinitiating reaction in these systems as in the graftingof MA into PP.226 The dynamic and reactive extrusionof polypropylene with grafted maleic anhydride isfacilitated by the supercritical CO2 because of thereduced viscosity and then improved mixing of thereactants.227

The impregnation of pharmaceuticals, proteins, orother bioactive molecules is a new, difficult, and verypromising research area. Kazarian and Martirosyaninvestigated the formation of ibuprofen/PVP compositesvia in situ ATR-IR and Raman spectroscopy,20 and theimpregnation of 5-fluorouracil and â-estradiol intoPLGA has also been reported.209 For pharmaceuticals,an important issue is achieving a molecular leveldispersion within the polymer substrate,20 while formore complicated biological agents, maintaining bioac-tivity and spatial structure after treatment is veryimportant. The relatively large size and hydrophilicproperties of biological agents imposes additional dif-ficulties for the impregnation process.

(B) Particle Formation. In recent years, CO2 tech-niques have emerged as a promising method for pre-cipitating particles from solution with the traditionalenvironmental and tunable solvent advantages whilealso leaving particles solvent-free. In particular, thesenovel methods provide a feasible and clean way toprocess thermal-labile or unstable biological compounds,such as the promising application in the developmentof drug-delivery systems.228 This section will mainlyfocus on the recent developments in this area as

work prior to 2001 is well-summarized in reviews byBungert,229 Reverchon,230 Cooper,4 and Thiering et al.228

Generally, the techniques involved are categorized asrapid expansion from a supercritical solution (RESS)and supercritical fluids serving as antisolvents. In theRESS process, the materials are dissolved into super-critical CO2 and then forced to pass through a nozzle.As a result of rapid expansion caused by reduction ofpressure, small and uniform polymer particles can beformed. Undoubtedly, the prerequisite for this techniqueis the proper solubility of the solute in the supercriticalfluid. Therefore, most polymer research in RESS isconfined to those polymers with high solubility in CO2,such as perflouroethers and siloxanes or polymersdissolved in SCFs other than CO2 such as HFCs oralkanes. The sudden reduction of pressure can some-times lead to spongelike structures of particles andRESS cannot typically be applied to thermally labilesubstances,231 which would seem to limit its applicationfor drug-delivery systems.

The antisolvent precipitation techniques can be sub-divided into GAS (gas antisolvent precipitation) andspray processes (ASES, aerosol solvent extraction sys-tem; PCA, precipitation with a compressed fluid anti-solvent; SEDS, solution-enhanced dispersion by super-critical fluids; and SAS, supercritical antisolvent precipi-tation), depending on how the solvent containing thesolute and the supercritical antisolvent are brought intocontact.228,232 Antisolvent techniques by far have re-ceived the most attention for using CO2 in the pharma-ceutical and biotechnology areas. As a result, thepolymer component of this work emphasizes biocom-patible and biodegradable polymers and their mixturewith pharmaceutical compounds of interest. Althoughthe differences between the various spray processesseem trivial at first, there is growing evidence that theyoperate in different hydrodynamic regimes and thusgive different results for similar systems. More researchis needed to control particle morphology but the benefits(both environmental and economical) are encouraging.

(1) RESS (Rapid Expansion from SupercriticalSolution). Recent developments in RESS include fluo-ropolymer coatings to protect historical buildings andmonumental civil infrastructures.233,234 Others haveworked with cosolvents to allow for the processing ofmore kinds of polymers. As might be expected, biopoly-mers are still of central interest and PLA remains themost popular.4,235-238 The cosolvents studied includeacetone,235 CHClF2,236 and small alcohols.239,240 Forexample, the copolymer PS-b-(PMMA-co-PGMA), whichis almost insoluble in either pure CO2 or pure ethanol,was dissolved into the mixture up to 20 wt %.239 Alcoholcosolvents are attracting attention because of the en-vironmental benefits and their function as a nonsolventafter expansion, which could further help prevent theagglomeration of polymeric particles. A modified method,called RESS-N (RESS-nonsolvent), was applied forpowder coating systems239 and protein-loaded micro-particles.240 The proteins, that is, lysozyme and lipase,were suspended in supercritical solution and finallymicroencapsulated inside the polymer particles. Finally,efforts with an intention to overcome the limitation ofhomogeneous solutions in RESS were reported in Shim’swork.241 Heterogeneous suspensions of poly(2-ethyleneacrylate) (PEHA) in liquid CO2 in the presence ofsurfactant were passed through an expansion nozzle toform uniform circular films. Viscosity reduction of the


polymer suspension due to the dissolved CO2 plays acrucial role in the success of this process.

(2) Antisolvent Processes. A detailed comparisonof SAS with GAS could be found in Thiering’s review.228

Reverchon gives a good summary of the injectors usedin the spray antisolvent processes (SAS, PCA, ASES,and SEDS), including nozzle, microcapillary, virbratingorifices and coaxial devices.230 Meanwhile, the effectsof process parameters, pressure, and temperature arefar from conclusive; even some contradictory resultsappear in the literature.

Presently, biopolymers account for the largest partof antisolvent systems studied because of the greatpotential for the drug-delivery applications. Specificexamples include PLA(poly(L-lactide))/PLGA(poly(D,L-lactide-co-glycolide) and their derivatives,231,232,242-248

PHB (poly(â-hydroxybutyric acid)249], PCL (poly-capro-lactone),244,250-252 and HYAFF-11 (poly(hyaluronic acidbenzylic ester).238,253 Engwicht et al. studied PLA andPLGA and found that the crystallinity and thermalbehavior play important roles in the preparation ofparticles with spherical shape.254,255 PLGA 50/50 iscompletely amorphous, and therefore the particles fromPLGA 50/50 were soft and agglomerated significantly.To isolate polymer structure factors that affect the finalmorphology of particles, Breitenbach synthesized aseries of biodegradable polyesters with a comb structureand conducted ASES experiments.249 They also foundthat crystallinity was far more important than solutionviscosity, MW, Tg, or the density of supercritical gas indetermining particle morphology. On the other hand,recent research demonstrates that by altering theproperty of the antisolvent, that is, introducing N2 tosupercritical CO2, discrete particles of PGLA less than10 µm could be prepared.250,251 Also, spherical particlesof PCL (though still a mixture of discrete and agglomer-ated particles) were produced by the same modifiedSEDS for the first time, compared with the PCL filmsformed in other works.244,252

A major goal of the antisolvent techniques is toachieve the desired distribution of biologically activeagent inside a polymer and maintain the biologicalactivity. In some cases, the lack of proper solvent forpolymer and drug limits the process development andmeanwhile the deliberate selection of solvent could bean effective way to control the subsequent loadedparticles. When incorporating insulin into PLA, Elvas-sore reported that better performance could be achievedby using a mixed solvent of dichloromethane andDMSO.256 To modify the low biodegradability and highhydrophobicity of PLA, insulin-loaded PLA/PEG com-posite particles were produced by incorporating poly-(ethylene glycol) (PEG) into particles through the GASprocess.257 Although only PEG with low molecularweight could be efficiently entrapped, its role in affectingthe coprecipitation of polymer and protein, and indetermining the release behavior, is evident.

In addition to those biopolymers discussed above,Park also used PCA to recover nylon 6/6 from a formicacid solution.258

(C) Foaming. Foaming with CO2 is an active areaof research and development due to the restrictionsimposed by the Montreal Protocol on ozone-depletingsubstances. At present, three choicesshydrogen-con-taining chlorofluorocarbons/fluorocarbons (HCFC/HFCs),hydrocarbons, and inert gases (CO2, N2, argon, orwater)shave the highest potential to replace the chlo-

rofluorocarbon (CFC or Freon) physical foaming agents,which were proven to be contributing to the destructionof the Earth’s ozone layer and are gradually beingeliminated.259-262 Among these, CO2 is the most favor-able foaming agent because of its unique properties. Anearly equivalent volume amount of CO2 (compared toCFC) can be dissolved in polymer melt at elevatedpressures. The diffusivity of CO2 in polymer melt islarge, which ensures a quick mixing process. Moreover,CO2 is environmentally benign and economically lowcost. For low-value foam products (e.g., packaging), CO2foaming is already a reality while high-value or high-strength applications are still in development. Of thelatter, introducing CO2 as a foaming agent focuses ontwo large-scale applications: low-density insulationfoams (<0.04 g/cm3) and high-density microcellularfoams (∼0.7 g/cm3). Efforts in optimization of operatingconditions, equipment design, and material selection aremaking CO2 foaming more and more realistic. Thechallenges of CO2 as a foaming agent are associatedwith the higher pressure operation, dimensional insta-bility during the foam-shaping process, and ironicallythe high diffusivity of CO2 compared to CFCs out of thefoam resulting in a poor cell growth control, a lowernucleation density, and a low R-value. Many of thesetechnical issues carry over into the development of foamco-injection molding (discussed below) that could dra-matically impact the production of polymer parts anddevices. Other specialty applications in foams (e.g.,biotech devices) will likely be developed for batchoperations that yield greater control over conditions atthe expense of throughput.

(1) Foaming Procedure. A basic foaming process263

can be divided into three steps: (1) mixing, formationof a homogeneous solution composed of foaming agentand polymer melt; (2) cell nucleation, phase separationinduced by a thermodynamic instability which is usuallya temperature increase or a pressure decrease; (3) cellgrowth and coalescence, a combination of mass transferand fluid dynamics.

A foaming process can be carried out in a batchsystem264-266 where the pre-shaped samples are placedin a pressurized autoclave to be saturated with CO2.Nucleation and cell growth are controlled by the pres-sure-release rate and foaming temperature.

Compared with the batch process, a continuous extru-sion foaming process187,267-273 is sometimes more eco-nomically favorable because of its high productivity,easier control, and flexible product shaping. A tradi-tional extrusion process is ready to manufacture foamsafter a little modification, such as a standard ventingport used for CO2 injection. A typical foam extrusionprocess begins with the plasticization of polymer resinafter which CO2 is injected into the extruder barrel. Thetwo components are mixed together to create a single-phase solution by screw rotation and in-line mixers.Nucleation is initiated by the rapid and large pressuredrop in the die, which reduces the solubility and createsa supersaturated solution, although some devices tonucleate via a temperature increase near the die havealso been reported.274

Figure 5 lists the major parameters in a continuousextrusion foaming process. Among them, foaming tem-perature, pressure drop or pressure drop rate, and CO2concentration are three key variables that need to beaddressed. These three variables determine the changesin the viscosity, solubility, surface tension, diffusivity,


and other physical properties, as well as play importantroles in mechanisms of cell nucleation and cell growth.Experimentally, it is a good strategy to study thecorrelation between the three key variables and thephysical properties separately from their effect on cellnucleation and growth. The combination of all theseparameters, including the three key variables and thephysical properties, into models of cell nucleation andgrowth to fully understand a foaming process dependson simulations involving thermodynamics of nucleation,mass, momentum and heat transport, and selection ofconstitutive equations.

(2) Foam Applications. Microcellular foams are avery active research area using CO2 as the foamingagent. These are characterized by cell sizes smaller than10 µm and cell density larger than 109 cells/cm3.268,275

Typical microcellular plastics exhibit high impactstrength or toughness and high fatigue life. Moreimportantly, microcellular polymers are light in weight,and consequently, they have higher mechanical strength-to-weight ratio than common structural foams at equiva-lent densities.

The concept of microcellular foam was developed bySuh in the early 1980s275,276 and microcellular foamswere first produced in a batch process. Both amorphousand semicrystalline polymers were foamed to study therelation between the foam structures and foamingconditions. Recently, cell size as small as 0.35 µm andcell density as high as 4.4 × 1013 cell/g were producedby Handa and Zhang,277 using the phenomenon ofretrograde vitrification documented earlier by Condo etal.80 A semicontinuous procedure in which the foamingprocess was uncoupled from the shaping process wasdeveloped by Kumar and Schirmer278,279 to producemicrocellular-foamed parts with a designed geometry.For a continuous extrusion foaming process, research

is ongoing to study the design of the mixing elementsand the foaming die, the study of the relation betweenprocessing conditions and cell structures, and thesimulation of the foaming procedure. The axiomaticapproach was applied by Park and Suh267,274 to designan overall microcellular extrusion process to revealrelations between the functional requirements (mixingelements and die design) and the design parameters(polymer/gas solution formation and cell nucleation).Systematically, Park et al.187,268 studied the effects ofprocessing pressure when the maximum amount of CO2was injected into a HIPS melt at each processingpressure. Cell density was found to increase nearlylinearly with pressure drop, pressure drop rate, and CO2content. The same research group280 studied the influ-ence of melt temperature and die temperature on thecell morphology and found that low melt temperaturesand low die temperatures can reduce cell coalescence.

The combination of nanotechnology and microcellularfoaming technique provides a wider operation windowto make light products with special properties, such ashigh mechanical strength and barrier properties. Hanand co-workers180,281,282 prepared nanocomposite foamsfor synthesized intercalated and exfoliated PS nano-composites. Figure 6 demonstrates that the presence ofa small amount of clay significantly reduces the cell sizeand increases the cell density, especially when clay iscompletely exfoliated. Open cell foam structure wasobserved in intercalated nanocomposite foams at highclay concentration.

Because CO2 is a “green” foaming agent and it isnontoxic, new applications can be found in foamingbiodegradable or biocompatible polymers to make po-rous scaffolds or other medical devices. Sheridan et al.283

used CO2 to foam a highly amorphous biodegradablecopolymer of polylactide and polyglycolide for use as

Figure 5. Relationship between parameters in a continuous extrusion foaming process.

Figure 6. Foam structure measured by SEM281: (a) PS/5 wt % talc (dP ) 11.03 MPa, dP/dt ) 4.75 × 108 Pa/s), (b) PS/5 wt % 20A (dP) 11.53 MPa, dP/dt ) 4.90 × 108 Pa/s), and (c) PS/5 wt % MHABS (dP ) 15.10 MPa, dP/dt ) 8.04 × 108 Pa/s). Foaming conditions: dietemperature, 200 °C; CO2 concentration, 4 wt %; screw rotation speed, 10 rpm.



http://pubs.acs.org/action/showImage?doi=10.1021/ie030199z&iName=master.img-005.jpg&w=453&h=130

three-dimensional tissue engineering scaffolds. Theporosity was measured as high as 95%.

Once the foam cells coalesce to create a so-called“open” structure, the foam can also be used as absor-bents or separators. Bland et al.284,285 extruded ther-moplastic open cell foams by using CO2 and other CFCsas foaming agents. Rodeheaver and Colton286 presenteda theoretical model of the formation of open-cell micro-cellular foam and open-cell polystyrene foam was pro-duced by carefully selecting the processing conditions.

Furthermore, Shieh et al.287,288 investigated the in-teraction between supercritical CO2 and 9 crystallinepolymers and 11 amorphous polymers by testing thechange in appearance, weight, and thermal and me-chanical properties before and after the sorption of CO2.One has reason to believe that the foaming ability ishighly related to the interaction and this providesvaluable information for polymer selection.

(D) Blending. Polymer blends are indispensable tomodern industry; they can provide exceptional proper-ties at affordable prices that are often not possible witha homopolymer. Two or more polymers are blended inmelt compounders, usually extruders or high-intensitybatch mixers. Most practical two-polymer systems areimmisciblesthey form two phases (a phase rich in A andanother rich in B) and the structure is referred to asthe system morphology. In the simplest cases, the phaserich in the major (predominant) component is continu-ous (matrix) and the minor component-rich phase isdispersed (droplet). Compatibilizers (additives thatstabilize the interface between phases) are commonlyused to improve the properties of a blend. Carbondioxide is important because while it is a nonsolventfor most polymers, it is a very effective plasticizer, asdiscussed earlier. Thus, SCF technology recently hasmade significant improvements to this field, both inprocessing of blends and polymerization of blends(primarily through impregnation of monomer as dis-cussed earlier). Although carbon dioxide-assisted po-lymerization and reactive blending have become a verysuccessful method to generate novel blends,289,290 weconcentrate in this review on the physical processingof polymers.

There are two points of ambiguity that should beclarified. Some authors refer to miscibility and compat-ibility as the same concept. However, in this workmiscible means one homogeneous phase and the termcompatible means immiscible, but optimal properties forthat blend system. 291 A miscible polymer blend is idealin the sense that the properties are weighted propor-tions of the components in the blend. Compatiblesystems take advantage of each component’s benefitsbut not necessarily in proportion to their presence. Theother point of common confusion is the distinctionbetween polymer blends and polymer alloys based onmiscibility. In this review, they are both labeled polymerblends.

The resulting morphology of a processed immiscibleblend can be viewed as equilibrium between dropletbreakup and coalescence.292 This equilibrium is con-trolled by properties and parameters including shearrate, temperature, pressure, degree of plasticization,and interfacial tension. In general, shear forces willreduce droplet sizes and extreme shear forces at highviscosities can destabilize polymer chains into macro-radicals, causing copolymers.291 The viscosity ratio ofthe two phases is crucial to the resulting morphology

because of its influence on momentum transfer from onephase to the other; thus, changing molecular weight(and hence the viscosity) is one technique used to changedroplet size of the dispersed phase. The droplet size isminimum (giving desired properties) when the viscosityratio is unity and increases as the ratio increases ordecreases.83,293 Morphology development in compound-ers based on the effects of operating parameters hasbeen analyzed extensively in the literature.293-298 Plas-ticization is important to morphology because it affectsviscosities and crystallization. As discussed earlier,interfacial tension effects are also important as theyrepresent the strength between phases. However,Sundararaj et al. explain that the advantage of usingcompatibilizers (such as copolymer additives) is domi-nated by suppression of coalescence, but not a reductionin interfacial tension.292

Supercritical CO2 can replace the need for compati-bilizers in some cases. Viscosity, swelling, and interfa-cial tension are all affected by the addition of CO2 to apolymer melt, as discussed earlier. The outcome of allthis work has been the ability to reduce the dispersedphase droplet size, thus improving the properties of theblend, using carbon dioxide. Elkovitch et al. and Lee etal. demonstrate this using batch mixing, single-screwextrusion, twin-screw extrusion, and tandem extrusionexploiting carbon dioxide’s varying degree of plasticiza-tion on different polymers.135-137,140,295,296,299 When car-bon dioxide decreases the viscosity of one componentmore than the other, the viscosity ratio can be con-trolled, thus leading to finer dispersed domains andtypically improved (e.g., tougher and more flexible)materials. In particular, Elkovitch et al. used twin-screwextrusion to show the effect of 2 wt % carbon dioxideon a blend of 50/50 PMMA/PS by revealing a decreasein number average diameter (Dn) of the PMMA-richdroplets shortly after CO2 injection.136 For that data,no venting was used, resulting in a foamed product withsmall droplet size. While carbon dioxide-assisted extru-sion reduces droplet size, venting and subsequent extru-sion causes demixing, which reverses the improvement.To avoid foamed products, venting is necessary, mean-ing new techniques are needed to prevent demixing. Theaddition of clay additives136 or the use of reactivesystems292 may reduce or eliminate this problem. Ad-ditionally, the effects of carbon dioxide on altering phaseinversion137 and cloud points300 have been reported.

Thus, it is possible by adding SCFs to reap thebenefits of compatibilization, namely, smaller dispersedphases, altered cloud points, and altered phase inver-sion. The most common techniques for shifting viscosityratio are introduction of copolymers, polymerization,and alteration of molecular weight. These conventionalmethods are still needed in most cases because thebenefits of carbon dioxide are lost during venting.However, carbon dioxide, if suitable, is likely more cost-effective for achieving these improvements.

(E) Foam Injection Molding. Foam injection mold-ing is another common polymer processing techniquethat combines gas dissolution, cell nucleation, and cellgrowth, with product shaping. Foam injection moldinghas the advantages of foam extrusion, particularly forproducing parts with complex geometry. By choosing theright size of the injection nozzle or mold gate, thepressure drop (or pressure drop rate) through them canbe very high, which provides the high thermodynamicinstability for cell nucleation. Currently, foam injection


molding using CO2 as the foaming agent is applied toproduce lightweight products with strong mechanicalstrength. When one considers the preservation of poly-mer molecular structure at high melt temperature, CO2is a better choice than the thermally degradable chemi-cal blowing agents. Applying MuCell molding technologyinvented by Suh et al.,275,276 Trexel, Inc. has successfullycommercialized this technique to injection mold micro-cellular foams.

Xu301 used injection molding to make microcellularfoam after a series of modifications on certain compo-nents of a standard reciprocating-screw injection mold-ing machine, such as the plasticizing unit, injection unit,hydraulic unit, clamping unit, and gas delivery unit. Tosuccessfully produce microcellular foams, a new screwdesigned for better mixing and a new sealed barrel withgas injectors were used. The injection unit requires afast injection speed to get the high-pressure drop rate.It was found that a finer cell structure and moreuniform cell size distribution can be achieved by con-trolling the pressure drop rate at the mold gate than atthe injection nozzle. Of course, the injection speedshould be controlled below the shear limit to preventthe melt fracture.

Versus conventional injection molding, foaming injec-tion molding achieves increased melt flowability, lowerinjection pressures, faster cycle times, and greaterdimensional stability and weight savings in moldedparts.302,303 The reduced viscosity after the addition ofsupercritical CO2 allows faster injection speed, lowerinjection pressure, and lower clamp tonnage. The shotsize for the microcellular foam process is usually smallerthan that for the solid molding process, which bringsshorter recovery time. The pack and hold time iseliminated due to the internal gas pressure and thesignificantly less mass of material that needs to becooled. The uniform cell distribution and expansionallow improved dimensional stability and diminish thesurface flaws (e.g., sink marks). However, to obtain aperfect surface finish, other techniques, such as aventing mold or co-injection, are needed.

A new application was recently developed for fiber-filler molded articles.304 The introduction of a viscosity-reducing foaming agent can prevent the fibers frombreaking during flow into the mold and reduces the fiberorientation. Due to the internal complexity of injectionmolding, each application requires significant experi-mentation and development. Although the potential isgreat, further study is required to better understandthis process and develop new products.

Outlook

While the future for commercialization of CO2 tech-nology in polymer processing looks bright, there arealways new frontiers and ideas to explore. In additionto the previously mentioned applications, a great po-tential for SCF-assisted polymer processing is in thefabrication of miniature devices, particularly for bio-medical applications. The demand for high-precisionminiature devices for biomedical applications has grownrapidly in recent years.305 Current microdevices arelargely based on silicon, owing to extensive developmentof microfabrication methods by the microelectronicsindustry. Unfortunately, the physical and chemicalproperties of Si-based materials (poor impact strength/toughness, lack of optical clarity, poor biocompatibility)are not appropriate for many biomedical devices. In

contrast, polymeric materials possess a number ofproperties that make them attractive for such devices.Certain polymers can exhibit high toughness, opticalclarity, recyclability, and excellent biocompatibility.Future markets for polymer biomedical microdevices areenormous (tens of billions of dollars306) and encompassapplications in lab-on-a-chip, drug delivery, tissue en-gineering, cell immuno-protection, protein separation,and protection against biological warfare agents andbiotoxins.305 These structures will incorporate biologicalmacromolecules and structures as is appropriate to theirindividual functions.

When polymers are processed at the micro- andnanoscale, viscosity and surface tension become a majorlimiting factor. Although organic solvents can be usedto lower the viscosity, surface wetting may still causeserious processing difficulty. Our recent results showthat supercritical fluids are excellent solvents in micro-and nanoscale polymer processing because of theirgaslike viscosity and surface tension. For certain poly-mers such as PLGA and PMMA, carbon dioxide at lowtemperature and low pressure can serve as an excellentprocessing aid for processing and polymer-polymer orpolymer-inorganic surface bonding. Near room tem-perature processing is essential for biomedical applica-tions because biomolecules need to be processed belowbody temperature to prevent denaturing. For small-sized processing tools (e.g., micro-/nanoinjection mold-ing, micro-/nanoimprinting, and assembly of microde-vices based on polymers and polymer/ceramic compos-ites), either the entire tool or just a portion can be sealedin a confined chamber to carry out gas-assisted process-ing.

Acknowledgment

We gratefully acknowledge financial support from theNational Science Foundation through Grants DMI-9908289, EEC-0223592, and DMI-0200324.

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Received for review February 28, 2003Revised manuscript received June 9, 2003

Accepted June 10, 2003

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CO2 Solvente Fluido Supercrítico

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