J. of Supercritical Fluids 37 (2006) 263–270

Solubility of fluorinated homopolymer and blockcopolymer in compressed CO2

Pascal Andrea,1, Patrick Lacroix-Desmazesb,∗,Darlene K. Taylora,2, Bernard Boutevinb

a Department of Chemistry, Venable Hall, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USAb UMR 5076 CNRS-ENSCM, Laboratoire de Chimie Macromoleculaire, Ecole Nationale Superieure

de Chimie de Montpellier, 8 rue de l’Ecole Normale, F-34296 Montpellier Cedex 5, France

Received 11 February 2005; received in revised form 26 July 2005; accepted 25 August 2005

Abstract

Nitroxide-mediated radical polymerization was used to synthesize poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) homopolymer, PFDA, and totailor the synthesis of semifluorinated polystyrene-b-PFDA block copolymer, PS-b-PFDA. The solubility of the polymers was investigated in thesolvent carbon dioxide (CO) using cloud point and light scattering techniques. The solvent quality of COfor PFDA homopolymer was shownt -m lationo©

K

1

nptPtttit[

sa

N

i

os

com-hilic-glet they orteduan-

ngi-ssed2,2-andte)

sild,1-

0d

2 2

o increase with CO2 pressure and this information coupled with the size of the block copolymer species indicated the formation of PSb-PFDAicelles. Residual PS homopolymer remained soluble in PS-b-PFDA micelles at low pressure and induced the formation of a second popuf larger aggregates when the solvent quality was tuned.2005 Elsevier B.V. All rights reserved.

eywords: Fluorinated block copolymer; Amphiphilic molecule; Compressed carbon dioxide; Light scattering; Second virial coefficient; Micellization

. Introduction

Carbon dioxide (CO2) is arguably the most promising alter-ative to traditional solvents. Large variation of the solutionroperties induced by slight adjustments of both pressure and

emperature, easily accessible critical conditions (Tc = 31.1◦C,c = 73.8 bar) combined with the fact that CO2 is environmen-

ally benign, readily recyclable, and nonflammable are some ofhe main advantages of using compressed CO2 as a solvent sys-em. Interest in the CO2 platform spans a variety of applicationsncluding materials synthesis, particle generation and stabiliza-ion, foaming, coating, extraction, and submicron lithography1–6].

Most traditional compounds including polymers and polarpecies are not CO2-soluble[7–11]. Fluoropolymers[12–17]nd polysiloxanes[17–21]are however the few classes of poly-

∗ Corresponding author. Tel.: +33 4 67 14 72 05; fax: +33 4 67 14 72 20.E-mail address: lacroix@enscm.fr (P. Lacroix-Desmazes).

1 Present address: Organic Semiconductor Centre, University of St. Andrews,orth Haugh, St. Andrews, KY16 9SS, UK.

mers readily soluble in CO2 even at high molecular weight. Texpand the utility of the CO2 platform, amphiphilic moleculeacting as surfactants have been designed where one CO2-philicpart is covalently linked to another CO2-phobic part[22–24].Several reports have investigated the molecular weight,position, conformation and phase transitions of amphippolymers in CO2 using tools such as NMR[25,26] and scattering techniques[27–35]. Some reports have used small anneutron scattering to provide quantitative information abousolvent quality variation as a function of pressure, densittemperature[30,35]. More recently investigations have starto emerge that utilize static light scattering techniques to qtify this relationship[21,31].

In order to pursue the characterization of molecularly eneered polymeric surfactants compatible with compreCO2, this report focuses on solution properties of poly(1,1,tetrahydroperfluorodecyl acrylate) (PFDA) homopolymerpolystyrene-b-poly(1,1,2,2-tetrahydroperfluorodecyl acryla(PS-b-PFDA) block copolymer (Fig. 1a and b). PFDA ia semicrystalline polymer[36], and was chosen to buupon previous investigations performed with: (1) poly(1

2 Present address: North Carolina Central University, Department of Chem-stry, 1801 Fayetteville Street, Durham, NC 27707, USA.

dihydroperfluorooctyl acrylate) (PFOA), an amorphous flu-oropolymer having shorter fluorinated side chains and a

896-8446/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2005.08.007

264 P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270

Fig. 1. (a) PFDA (n = 8); (b) PS-b-PFDA block copolymer,m and p are thenumber of monomer units for each block; (c) PFOA (n = 7 and methylene spacer,25% of branched side chains); and (d) PTAN (n = 4–18 with a mean valuen = 8).

hydrogenated spacer (Fig. 1c) [16,25–29]and (2) poly(1,1,2,2-tetrahydroperfluoroalkyl acrylate) (PTAN), a semicrystallinefluoropolymer having a side chain length with distributeddegrees of fluorination (Fig. 1d) [31]. The solubility in liquidCO2 of PFDA and PS-b-PFDA was investigated through bothcloud point and light scattering techniques. The interactions ofthe PFDA chains were studied at 25◦C as a function of pressureand the formation of PS-b-PFDA micelles was evidenced.

2. Experimental

2.1. Synthesis

PFDA homopolymers as well as PS-b-PFDA block copoly-mers (Fig. 1) were synthesized by nitroxide-mediated rad-ical polymerization (NMP) in the presence ofN-tert-butyl-1-diethylphosphono-2,2-dimethyl-propyl nitroxide (DEPN) tocontrol the polymerization[37,38]. As described in the supple-mental part, the synthesis of the fluorinated homopolymer wacarried out directly with AIBN, DEPN, cyclohexanone and FDAmonomer. The PS-b-PFDA block copolymer required the use ofa precursor such as polystyrene macroalkoxyamine (PS-DEPNThis DEPN-mediated radical polymerization method was cho-sen for its lack of dependence on catalysts such as organometalcomplexes.

9 haine cat-t ragem et= tionp erw hilet tog-rM

2.2. Solubility investigation

The phase behavior of the fluorinated homopolymersand block copolymers in CO2 was examined at constantpolymer/CO2 concentration weight/weight percent (w/w %) byvisual observation of the reversible one-phase/two-phases tran-sition in a controlled variable volume view cell as describedelsewhere[37]. A pressure transducer facilitated monitoring ofthe cell pressure and enabled the pressure at which the sys-tem reversibly appeared cloudy (the cloud point pressure) to berecorded.

2.3. Light scattering investigation

The light scattering investigations were performed in diluteregime using a 514 nm argon ion laser source (Coherent) in con-junction with a computer controlled motor-driven variable angledetection system (Brookhaven Instruments). This complete set-up is described in detail in the supplemental section.

2.4. Static light scattering

In static light scattering experiments, various concentrationsof PFDA homopolymer solution,C, were exposed to the lasersource and the scattered intensity of the solution was measureda

q

w es

-m werecR

R

w uenes io isw1 theet

w thep ft ht,a oly-mTI a 10

The PFDA homopolymer molecular weight,Mn,NMR =8.5 kDa, was determined by phosphorus NMR of DEPN cnds in 1,1-dichloro-1-fluoro ethane (F141b). Static light s

ering in CO2 was used to measure the PFDA weight aveolecular weight,Mw,SLS = 147± 15 kDa, and the ratio of th

wo results provided a polydispersity index,Ip = Mw,SLS/Mn,NMR1.5, lower than 2 as expected for a controlled polymerizarocess[39]. The molar composition of the block copolymas determined by elemental analysis and proton NMR w

he blocky structure was verified by size exclusion chromaaphy analysis in trifluorotoluene,Mn,SEC= 52.3 kDa,Ip = 1.16,

n,NMR = 49.4 kDa (PFDA block)/3.8 kDa (PS block)[38].

s

).

lic

t various wave vectors,q, given by Eq.(1) [40]:

= 4πns sin(θ/2)

λ(1)

hereθ is the scattering angle,ns the refractive index of tholvent, andλ the laser wavelength.

The relative excess of scattered intensity,I(q,C), was deterined with respect to the solvent and the scattering data

onverted into the excess Rayleigh’s ratio,R(q,C). In CO2,(q,C) has been corrected with the following Eq.(2):

(q, C) = I(q, C)Rθ−tolueneICO2

Itoluene(2)

hereItolueneis the scattered intensity measured using a tolample reference, for which the absolute Rayleigh ratell known and provided by Brookhaven,Rθ−toluene= 3.21×0−5 cm−1 at 25◦C for λ = 514 nm. The dependence ofxcess Rayleigh ratio with the polymer concentration,C, andhe wave vector,q, is given by Eqs.(3) and (4) [40]:

KC

R(q, C)= 1

MwP(q)+ 2A2C + · · · (3)

1

P(q)= 1 + 1

3R2

gq2 + · · · (4)

here A2 is the second virial coefficient that describesolymer–solvent interactions,P(q) the angular distribution o

he scattered intensity,Mw the weight average molecular weigndRg the radius of gyration. The size of the PFDA homoper is such thatRgq is of the order of 0.1, thusP(q) = 1.he experiments were therefore performed aroundθ = 90◦, and(q,C) was the average of 20 measurements obtained over◦

P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270 265

scattering angle interval. Under these conditions, Eq.(3) can beexpressed as Eq.(5):

KC

R(C)= 1

Mw(1 + 2MwA2C + · · ·) (5)

and serves as the basis for obtaining theMw andMwA2 valuespresented in this work.K is an optical constant defined as Eq.(6):

K = 4π2n2s(dn/dC)2

λ4NA(6)

where ns is the carbon dioxide refractive index measured atthe appropriate pressure[37], dn/dC the refractive index incre-ment of the polymer measured in CO2 [37], andNA Avogadro’snumber. In the case of PFDA homopolymer,Mw = 147± 15 kDastands for the average value and the deviation from this averageMw over the investigated CO2 pressures.

In the case of PS-b-PFDA, the concentrations were smallenough to neglect the interaction contribution and the dn/dC ofthe block copolymer is given by the combination of the weightfraction,w, and the refractive index increment of each block,according to Eq.(7) [41]:

dn

dC= wPFDA

(dn

dC

)PFDA

+ wPS

(dn

dC

)PS

(7)

S thP everC tionr . Fort la-t

w theP

2

rela-t de(

g

w -r (Eq.(

g

w ly-m

For a single population of particles the field autocorrela-tion function,g(1)(q,t), is described by a single exponential (Eq.(11a)):

g(1)(q, t) = exp

(−t

τc

)= exp(−Dcq

2t) (11a)

τc is the characteristic time,Dc the mutual diffusion coefficientwhich was experimentally averaged over three wave vectors,q,corresponding to scattering angles ranging from 41◦ to 137◦.In the case of a solution containing particles polydisperse insize, the field autocorrelation function can be described witha stretched exponential, while a solution having a multimodalparticle size distribution can be decomposed more appropriatelyinto a sum of exponentials as illustrated in Eq.(11b) [40]:

g(1)(q, t) =∑

αi exp

(−t

τci

)(11b)

whereαi is the relative proportion of each particle’s populationand its associated characteristic timeτci.

The mutual diffusion coefficient varies linearly with the poly-mer concentration (Eq.(12)):

Dc = Do(1 + kdC + · · ·) (12)

wherekd is the diffusional second virial coefficient containingboth hydrodynamic and thermodynamic interactions[42] andDot ited theh ion(

D

w re,a is af

3

intc a ploto lymerd -t pointp n (thep merc

PS-b et per-a resentir aturei

onep andP

ince PS is not soluble in CO2, the dn/dC values associated wiS are not available. They can however be calculated forO2 density of interest based on the expansion of a solu

efractive index as a function of the polymer concentrationhe PS block, dn/dC was then approximated by linear interpoion between the two pure components (Eq.(8)):

dn

dC≈ ns − nPS

ρPS(8)

herenPSandρPSare the refractive index and the density ofS block, respectively.

.5. Dynamic light scattering

In dynamic light scattering experiments, the time autocorion function of the scattered intensity,g(2)(q,t), is measured anxpressed in terms of the field autocorrelation function,g(1)(q,t)Eq.(9)) [40]:

(2)(q, t) = 〈I∗(q, 0)I(q, t)〉〈I∗(q, 0)〉2 = 1 + A|g(1)(q, t)|2 (9)

hereA is an instrument constant.g(1)(t) is the field autocorelation function of the polymer concentration fluctuations10)):

(1)(q, t) = 〈δC∗(q, 0)δC(q, t)〉〈δC(q, 0)2〉 (10)

hereδC(q,t) andδC(q,0) represent the fluctuations of the poer concentration at timet and zero, respectively.

y

he translational diffusion coefficient of the polymer at infinilution. The translational diffusion coefficient is related toydrodynamic radius,Rh, through the Stokes–Einstein relatEq.(13)):

o = kT

6πηsRh(13)

herek is the Boltzmann constant,T the absolute temperatundηs the solvent viscosity. Note that the solvent viscosity

unction of both temperature and solvent density[43].

. Results and discussion

To obtain a picture of the CO2 soluble species, cloud pourves are commonly used and are typically represented asf pressure versus temperature for a constant amount of poefined in weight/weight percent[4,7,16]. At a given tempera

ure, the decrease of pressure showed a transition (cloudressure) from a clear and homogeneous one-phase regioolymer is soluble) to a cloudy two-phases region (the polyomes out of solution)[37].

Fig. 2 shows the cloud point curves of both PFDA and-PFDA, demonstrating their general CO2 solubility. The samrend of increasing cloud point pressure with increasing temture was present for both series. The continuous lines rep

sochoric conditions and illustrate the decreasing CO2 densityequired to solubilize these polymeric species as temperncreases.

Lower CO2 densities were required to reach the PFDAhase region as compared to cloud point curves of PTANFOA in CO2 reported in the literature[14,15]. In fact, at 30◦C

266 P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270

Fig. 2. Cloud point curves for PFDA homopolymer (�) and PS-b-PFDA blockcopolymer (�), performed at 3.9% (w/w) in neat CO2. The lines stand for con-stant CO2 density stated in g/mL on their left.

and at a polymer concentration of 4% (w/w), the one phaseregion is reached at a pressure of 100 bar with PFDA, while itrequires slightly higher pressures of 135 and 125 bar for PTANand PFOA, respectively. It is worth noting that the uncertainty ofthe measurements themselves can be assessed to around±5 baras recently reported[37,44]. A more quantitative comparisonis however prevented by the differences in polydispersity indexand molecular weight of the polymers.

As shown inFig. 2, PFDA homopolymer is soluble in CO2at much milder conditions than the PS-b-PFDA diblock copoly-mer. At a given temperature, the pressure (or density) requireto solubilize the block copolymer is higher than the pressureneeded to solubilize the fluorinated homopolymer. This phasebehavior was expected since PS is essentially insoluble inCO2 [7]. These results, coupled with observations in the lit-erature of similar amphiphilic systems that self-assemble intomicelle structures with a CO2-soluble shell surrounding a CO2-

phobic core[25,27], suggest that the diblock copolymer pre-sented herein, PS-b-PFDA, forms micelles with a PS-core and aPFDA-shell.

To extract quantitative solubility information associated withthe PFDA homopolymer, the second virial coefficient,A2,a thermodynamic parameter related to the relative strengthof the polymer–polymer versus polymer–solvent interactions[40], was determined by static light scattering. For a con-stant homopolymer weight average molecular weight ofMw,SLS= 147± 15 kDa, the isotherm plot inFig. 3A of MwA2against increasing pressure indicates better solvent quality isreached for PFDA at higher CO2 pressures. This result is in goodqualitative agreement with PFOA behavior reported at highertemperature[31].

The Theta point is associated with the limit above which thepolymer is favorably dissolved in CO2 regardless of its molec-ular weight. For PFDA at 25◦C, the theta pressure, correspond-ing toMwA2 = 0 mL/g, is around 160 bar (density≈ 0.89 g/mL).Below this theta pressure and above the cloud point pressure(P ≈ 85 bar, density≈ 0.79 g/mL), the polymer is kept in solu-tion whilst unfavorable interaction with the solvent because ofentropic contributions on the small sized PFDA chains[40]. Incontrast to cloud point measurements, the Theta point is inde-pendent of the polymer molecular weight and is seen as theextrapolation of the cloud points at infinite molecular weight.This is consistent with the fact that the polymer cloud point wasf

aledh ead-i ef th ual-i ap-o ithPsq

F imes ticl ctiond

ig. 3. PFDA in liquid CO2 at 25◦C. (A) Weight average molecular weight tight scattering. (B) Normalized field autocorrelation functions,g(1)(t), as a funetection angle and 0.031 g/mL as a polymer concentration.

d

ound to be below the theta pressure.The quantitative improvement of the solvent quality reve

erein by the positive second virial coefficient above a rly accessible Theta pressure of∼160 bar is consistent with thact that linear fluorinated polymers are CO2-soluble even aigh molecular weight. A further increase of the solvent q

ty beyond∼450 bar CO2 pressure cannot however be extrlated to higher CO2 pressures as recently highlighted wDMS-CO2 solutions[20,21], where an increase in CO2 den-ity was shown to first improve and then reduce the CO2 solventuality.

second virial coefficient,MwA2, as a function of CO2 pressure, measured by staof time and CO2 pressures, measured by dynamic light scattering with 51◦ as a

P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270 267

Fig. 4. PS-b-PFDA in liquid CO2 at 25◦C. (A) Normalized field autocorrelation functions,g(1)(t), measured by dynamic light scattering, as a function of time andCO2 pressures with 51◦ as a detection angle and 0.0018 g/mL as a polymer concentration. (B) Estimation of the micelles population variation with CO2 pressure,see text for the calculations and comments.

Dynamic light scattering allows the determination of thehydrodynamic radius of polymer chains. The field autocorrela-tion functions,g(1)(t), obtained in PFDA-CO2 solutions were allcharacterized by a single exponential behavior (Fig. 3B), with acharacteristic time,τc, which was found to vary inversely propor-tional toq2. The hydrodynamic radius,Rh,PFDA= 4.8± 0.5 nm,was deduced from the Stoke-Einstein relation as described in Eq.(13). Because of their small size, the PFDA chains did not showa variation larger than 10% and no swelling was experimentallyobserved when the solvent quality was tuned.

With the PS-b-PFDA diblock at low CO2 pressure (P = 138bar), the exponential autocorrelation function was characterizedas expected by a single characteristic time,τc, inversely pro-portional toq2 (Fig. 4A). The deduced hydrodynamic radiusof the diblock,Rh,PS-b-PFDA = 17 ± 1 nm, is much larger thanthe size of the PFDA component discussed above. Additionally,for PS-b-PFDA having a smaller molecular weight (Mn,NMR =52.3 kDa) than PFDA homopolymer (Mn,NMR = 98.5 kDa), theradius difference between the two species confirms the forma-tion of PS-b-PFDA micelles.

To assess the aggregation number,p, the effect of the secondvirial coefficient between the micelles was neglected because ofthe low polymer concentration, makingp approximately equal tothe ratio of the average mass of the micelle,Mmicelles, to the massof a copolymer chainMunimers(52.3 kDa) (Eq.(14)) [31,45]:

p

T ssR wavev

M

wc -bI oly-

mers[41,46]was neglected in the case of the 3.8 kDa PS block,and the refractive index and density values associated with largemolecular weight PS[47] were used allowing the calculation ofthe upper limit of the aggregation number.

At 25◦C and for the lowest pressures investigated in thisstudy, the upper value ofp was found to be around 250 unimersper micelle. Considering the approximations that are used, thisapproach only gives an upper value whose order of magnitudeis in reasonable agreement with CO2 solutions of PTAN-b-poly(vinyl acetate) and PFOA-g-poly(ethylene oxide) where theformation of micelles occurred having similar sizes,p ≈ 125± 5with Rh ≈ 15.3± 1 nm andp ≈ 120 withRg ≈ 17.4 nm, respec-tively [31,48].

The consistency of both the aggregation number and themicelle hydrodynamic radius determined herein can be assessedby comparing a little further the two micelle coronas madeof PFDA-b-PS and PTAN-b-PVAc [31], respectively. For thesake of simplicity, the swelling of the core block (PS or PVAc)induced by the CO2 can be neglected. In a first-order approach,the aggregation numbers,p, and the molecular weights,Mb, ofthe less soluble blocks can be associated with the bulk densityof PS (ρPS= 1.05 g/mL) and PVAc (ρPVAc = 1.13 g/mL) in orderto calculate the radius of the core of the micelles,Rc:

Rc ≈(

3

4π

pMb

ρbNA

)1/3

(16)

T edb elle.E ns isk etricc udyo in af -t fm d inT ani outerp

= Mmicelles

Munimers(14)

he mass of micelles,Mmicelles, is deduced from the exceayleigh ratio extrapolated to a zero concentration andector (Eq.(15)):

micelles≈ R(0, 0)

KC(15)

here the optical constant (K as defined in Eq.(6)) was cal-ulated with the refractive index increment, dn/dC, of the PS-PFDA diblock determined as described in Eqs.(7) and (8).n addition, the end-group effects reported for other short p

he thickness of the outer shell,Rs, can then be simply deducy subtraction of the core radius and the total size of the micven though the stretching of the corona polymeric chainown to be a non constant radial function due to the geomonstraint[49], for comparison purposes in the current stnly the outer-block polymer chain length is calculated

ully extended conformation,Lo = N, with the chemical disance (∼2.5A in zig–zag conformation) andN the number oonomers in the outer-block polymeric chain. As presente

able 1, both fluorinated blocks, PFDA and PTAN, showdentical behaviour and a similar average stretching of theolymer chainsRs/Lo.

268 P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270

Table 1Micelle characteristics, withp, the aggregation number,Rh the experimen-tal hydrodynamic radius,Rc the calculated core radius,Rs the deducted shellthickness, andLo the outer-block polymer chain length in a fully extendedconformation

Diblock PTAN-b-PVAc(60.4 kDa-b-10.3 kDa)[31]

PFDA-b-PS(49.4 kDa-b-3.8 kDa)

p 125 250Rh (nm) 15.3 17Rc (nm) 6 7.1Rs (nm) 9.3 9.9Lo (nm) 29.3 24.0Rs/Lo 0.3 0.4

By increasing CO2 pressure above the PFDA theta pressure at25◦C, the autocorrelation functions deviate progressively fromthe single exponential behavior (Fig. 4A) indicating that a sec-ond population of larger aggregates is formed. For the sake ofsimplicity, the polydispersity of both the micelles and the aggre-gates were neglected and the autocorrelation functions measuredfor various carbon dioxide pressures were fitted with a doubleexponential as mentioned in Eq.(11b). Even though the num-ber of fitting parameters, the time window available for thesemeasurements and consequently the limited amplitude of thedecay of the autocorrelation functions limit the accuracy associ-ated with determining the order of magnitude of the aggregatesize, it was found to be at least 30 times larger than the initiamicelles size. The decay of the micelle populationαi as definedin Eq. (11b) was also assessed and is then shown inFig. 4Bwhere 15% error bars were added for illustrative purposes. Aexpected, the micelle population is then seen to decrease whethe pressure is increasing and when the large aggregate poulation is increasing. From a qualitative point of view, whenthe solvent quality is improved, the PFDA becomes more sol-uble, the micelles start to break up and the PS-blocks, whichwere initially protected inside the micelles, are more exposedto CO2. The observation of larger aggregates instead of the formation of unimers can be explained in accordance with resultfrom a recent micellization study of diblock copolymer solu-tions[50], where a solvent-phobic homopolymer residue as lowa easo d antt oly-m ablee them thel e fom n ofp nt-p le.D yS oly-m d thob -

gates with increasing pressure likely correspond to the floccu-lation of PS homopolymer particles poorly stabilized by theincreasingly soluble PS-b-PFDA diblock copolymer. Finally, theEq. (16) can be modified to include the encapsulation of freePS-homopolymers and assess the induced variation of the outerfluorinated shell thickness. Then, in the case of either 1 and 10%of the free PS-blocks, the resulting variation of the corona thick-ness remains on the order of 0.24 and 2.32%, respectively, whichremains bellow the precision of the measurements presentedherein.

Moderate temperature and pressure imposed by the setupprevented studying the micelle-to-unimer transition for PS-b-PFDA; however, poly(vinyl acetate)-b-PFOA in CO2 solutionhas demonstrated the similar size increase close to the micel-lar transition[32–34]. For other systems, by tuning the CO2density, the micelle-to-unimer transition was suggested anddemonstrated with PS-b-PFOA [28], and poly(vinyl acetate)-b-PTAN [31], respectively; however, the formation of largeaggregates has not been reported. These results obtained invarious diblock–CO2 systems reemphasize the role of polymerchemistry and characterization to interpret the behavior of blockcopolymers in carbon dioxide.

4. Conclusions

The relationship between the structural characteristics oft uidet itivesf s.N the-s lockc u-l3 Ah sd mersf Sim merr rgera beenr elle-t ty oft

A

r theU SFS onsi-b port.T ora-t SFn anda forh ft oft

s 1 wt.% within the sample influenced a drastic size incrf polymeric aggregates when the solvent quality was tune

he micelle-to-unimers transition was approached[50]. Belowhe micelle-to-unimer transition, the solvent-phobic homoper is solubilized inside the micelles and it has no discernffect on the solution properties of the sample; close toicelle-to-unimer transition, part of the diblocks having

ongest soluble chains are released as unimers and thation of large aggregates is attributed to the flocculatioarticles poorly stabilized by the diblock and rich in solvehobic homopolymer. This is known to be fully reversibespite the fact that the PS-b-PFDA sample was purified boxhlet extraction to minimize the extent of PS homoper residue, trace amounts of the homopolymer inducebserved anomalous micellization. Consequently, theg(1)(t)ehavior and the population change of PS-b-PFDA based aggre

sl

sn

p-

-s

ed

r-

e

he polymers and their physical properties is a useful go tailor well-designed macromolecular surfactants as addor applications in compressed CO2 integrated technologieitroxide-mediated radical polymerization was used to synize both PFDA homopolymer and the semifluorinated bopolymer PS-b-PFDA. The fluorinated homopolymer molecar weight was determined both by light scattering in CO2 and1P NMR in F141b. At 25◦C, the solvent quality for the PFDomopolymer was shown to increase with CO2 pressure. It waemonstrated unambiguously that the soluble block copoly

ormed micelles in CO2 with PFDA in the outer shell and Pn the core. Although solubilized in the core of the PS-b-PFDA

icelles at low pressure, very low amounts of PS homopolyesidue induced the formation of a second population of laggregates at higher pressure in a similar way to what haseported in traditional organic solvent systems when the mico-unimers transition is approached by changing the qualihe solvent.

cknowledgements

The authors gratefully acknowledge the Kenan Center fotilization of Carbon Dioxide in Manufacturing and the Ncience and Technology Center for Environmentally Resple Solvent and Processes (CHE-9876674) for financial suphe authors thank the CNRS for the support of their collab

ion through a grant for international cooperation CNRS/No. 10730, J.M. DeSimone both for his constant supportllowing this work to be performed at UNC, and E. Buhleris valuable comments. Atofina is acknowledged for the gi

he DEPN nitroxide, FDA monomer, and F141b.

P. Andre et al. / J. of Supercritical Fluids 37 (2006) 263–270 269

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, atdoi:10.1016/j.supflu.2005.08.007.

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