Macromol. Chem. Phys. 2001, 202, 2187–2194 2187

Grafting of 2-Hydroxyethyl Methacrylate onto IsotacticPoly(propylene) Using Supercritical CO2 as a Solventand Swelling Agent

Dan Li, Buxing Han,* Zhimin Liu

Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, ChinaFax: (010) 62559373; E-mail: Hanbx@pplas.icas.ac.cn

IntroductionIsotactic polypropylene (iPP) has become one of the mostpopular polymer materials due to its low cost, versatileproperties, and rapidly growing applications. However,low surface energy, lack of chemical functionalities, dif-ficulty to dye, poor hydrophilicity, low melting and stick-ing temperatures, low impact strength, sensitivity tophoto- or thermal-oxidation, and poor compatibility withsynthetic polar polymers have restricted the use of iPP.[1]

Graft copolymerization by hydrogen abstraction from ter-tiary carbon offers an effective approach to introducesome desirable properties into the polymer, and thusexpands the available market for the polymer applicationwithout adversely affecting the architecture of the iPPbackbone.[2, 3] Grafting of polar compounds onto iPPusing conventional methods have been studied by differ-ent techniques, such as ultraviolet radiation,[4] the cobalt-60 gamma radiation method[5] and the liquid solventmethod.[6, 7] Conventional methods of grafting polar

monomers onto iPP suffer from several drawbacks. Forexample, the solvent must be removed from the final pro-duct which creates additional waste streams. The othercommon means of grafting iPP involves melt extrusion inwhich polar monomer and initiator is co-extruded withthe polymer, thermally initiating the free-radical reaction.The fact that a polar monomer is insoluble in molten iPPrequires efficient mixing and limits the degree of grafting.In this work, we use supercritical CO2 (SC CO2) as thesolvent of monomer 2-hydroxyethyl methacrylate (2-HEMA) and initiator benzoyl peroxide (BPO), and as theswelling agent of iPP, to obtain poly (2-HEMA) graftediPP.

Supercritical fluids (SCFs) have many unique proper-ties.[8, 9] Their utilization opens a bright future for proces-sing polymeric materials, particularly for CO2 , which isnonflammable, nontoxic, relatively inexpensive, and hasmoderate critical parameters (Tc = 304.2 K, Pc = 73.8bar). SC CO2 has been widely applied in polymer

Full Paper: Grafting of 2-hydroxyethyl methacrylate (2-HEMA) onto isotactic polypropylene (iPP) was carriedout by free-radical polymerization using supercritical car-bon dioxide (SC CO2) as a solvent and swelling agent.The iPP film was first impregnated with the monomer 2-HEMA and initiator benzoyl peroxide (BPO) with SCCO2 at 308.15 K. After releasing CO2 , the 2-HEMA mole-cules in the film were grafted onto the iPP at a higherreaction temperature. By this method, the grafting leveland the morphology can be controlled by soaking time,pressure, concentrations of 2-HEMA and BPO, reactiontemperature, and reaction time. The products were charac-terized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and differentialscanning calorimetry (DSC). Contact angle measurementsof 2-HEMA grafted iPP film using water as the test liquidshowed a significant improvement of the surface polarity.The polymer films, having a markedly bumpy texture,were obtained under suitable conditions.

Macromol. Chem. Phys. 2001, 202, No. 11 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1107–2187$17.50+.50/0

SEM image of an iPP-g-2-HEMA (90/10) film.

2188 D. Li, B. Han, Z. Liu

sciences, such as synthesis of different kinds of poly-mers,[10–14] preparing fibers,[15–17] generation of fine parti-cles,[18–21] preparing polymer composites,[22–25] incorpora-tion of additives into polymers,[26–29] producing foammaterials[30–34] and polymer fractionation.[35–37] Recently,Hayes and McCarthy[38] grafted maleic anhydride ontopoly(4-methyl-1-pentene) using SC CO2 as a solvent andswelling agent. The density of SC CO2 , and thus its sol-vent strength, is continuously tunable from gas-like toliquid-like by changing temperature or pressure. This pro-vides the ability to control the degree of swelling of apolymer as well as the partitioning of small-moleculepenetrants between the swollen polymer phase and thefluid phase. The low viscosity and near-zero surface ten-sion of a SCF allow for fast mass transfer of penetrantsinto a swollen polymer. Moreover, since CO2 is a gas atambient conditions, removal of the solvent from the finalproduct is extremely facile. All these are favorable tomodification of polymers using SC CO2 as swellingagent.

In this work we studied the free-radical grafting of 2-HEMA onto iPP using SC CO2 as solvent and swellingagent. The grafting mechanism was the same as that ofthe liquid solvent method.[6] The free radical initiatorBPO dissociates and subsequently abstracts a ternaryhydrogen atom to form the ternary polymer radical,which reacts with 2-HEMA to form the grafted iPP. Theeffect of various factors such as the pressure of CO2 ,monomer and initiator concentrations, reaction time, andtemperature on the graft level were studied. The morphol-ogy and structure of the grafted and ungrafted substrateswere determined using SEM, FT-IR, DSC and contactangle measurements.

Experimental PartMaterials

iPP used in this work was 30 lm supplied by the BeijingYanshan Petrochemicals Company, and Soxhlet-extractedfrom methanol for 7 h and dried in a vacuum oven at 808C.2-HEMA was produced by the Tianjin Special ChemicalReagent Development Center (A. R. grade) and used afterpurification by vacuum distillation. Benzoyl peroxide (BPO)was purchased from the Beijing Jinlong Chemical ReagentCompany and was used after recrystallization in chloroform.Methanol (A. R. grade) received from the Beijing ChemicalFactory was used without further purification. Carbon diox-ide with purity of 99.95% was provided by the Beijing Ana-lytical Instrument Factory and used as received.

Phase Behavior

The apparatus for measuring the phase behavior of the CO2 /2-HEMA/BPO system consisted mainly of a 30 ml opticalstainless steel cell, a magnetic stirrer, a constant temperaturewater bath, and a pressure gauge. The pressure gauge wascomposed of a transducer (FOXBORO/ICT) and an indica-

tor, which was accurate to l0.025 MPa in the pressure rangeof 0–20 MPa. The temperature of the water bath was con-trolled by a HAAKE F3 controller. The temperature wasdetermined using a platinum resistance thermometer (BeijingChaoyang Automatic Instrument Factory, XMT) with anaccuracy of l0.1 K.

In a typical experiment, a suitable amount of monomer 2-HEMA or solution of 2-HEMA/BPO was loaded into theoptical cell, which was stabilized at 308.15 K. The air in theoptical cell was replaced by CO2 . SC CO2 was charged intothe cell using the high pressure pump until the solution in thecell changed from two phase to one phase, which could beseen clearly through the windows of the optical cell. Thephase separation pressure was obtained.

Grafting Reactions

The grafting reactions consisted of two steps. iPP was firstimpregnated with 2-HEMA and BPO at 308.15 K using SCCO2 as a solvent and swelling agent. Then CO2 was releasedand 2-HEMA was grafted onto the iPP matrix at a highertemperature. To impregnate the iPP films, the film (1.0 g), 2-HEMA, and BPO were placed in a stainless steel vessel of22 ml, and the vessel was sealed. The air in the vessel wasremoved quickly by vacuum. After the system had reachedthermal equilibrium, CO2 was charged by a syringe pump(Model SB-2, Beijing Xiantong Scientific Instrument Co.)until the desired pressure was reached. The iPP films wereswollen and some of 2-HEMA + BPO infused into the films.After a desired soaking time, the fluid in the system wasreleased. The impregnated films were transferred intoanother stainless steel vessel and the air in the vessel wasreplaced by 100 atm N2 . The grafting reaction was carriedout at 1108C for 3 h. At the end of the reaction, the vesselwas quenched with water. The samples were then Soxhlet-extracted using methanol for 24 h to remove unreactedreagents and homopolymer. The percent of grafting was cal-culated by the following equation:

% Grafting ðGÞ ¼ Wt ÿW0

W0

6100 ð1Þ

where W0 is the initial weight of iPP and Wt is the weight ofgrafted iPP after complete removal of homopolymer.

Characterization

The ungrafted and grafted copolymers were characterizedusing FT-IR (Perkin-Elmer 180). The morphology of thesamples was observed with a Hitachi S-530 scanning elec-tron microscopy in the normal secondary electron imaging(SEI) mode. The surface was coated with gold to avoidcharging under an electron beam. DSC studies were con-ducted using a Perkin-Elmer DSC-7 thermal analyzer with aheating or cooling rate of 108C/min under a nitrogen atmo-sphere. Contact angles on the grafted iPP surface were mea-sured at 258C using a contact angle meter developed at theBeijing Institute of Clothing Technology. The principle ofthe apparatus is very simple.[39] In an experiment, water wasdropped onto the surface of the film and then a photograph

Grafting of 2-Hydroxyethyl Methacrylate onto Isotactic Poly(propylene) ... 2189

was taken. After the photograph of the sessile drop of waterwas developed, the contact angle was determined by the sil-houette.

Results and Discussion

Grafting Reaction

In this work, the soaking experiments were conductedunder conditions at which CO2 , 2-HEMA, and BPO existas a single phase, which was known from the phase be-havior determined in this work. The mass uptake dependson the thickness of the film, equilibration time, pressure,temperature, and concentration of the monomer. To opti-mize the operation conditions, the effects of various para-meters such as the soaking time, the pressure of SC CO2,monomer and initiator concentrations, reaction time, andtemperature on the graft level were studied.

Phase Behavior

To make sure the soaking process was conducted at con-ditions where the fluid was homogenous, we first deter-mined the phase behavior of CO2 /2-HEMA and CO2 /2-HEMA/BPO (weight ratio of 2-HEMA and BPO is 9:1)mixtures at 35.08C. Figure 1 illustrates the phase separa-tion pressures at different monomer concentrations. For

each curve, the section above the corresponding curve isthe homogeneous zone. The data in Figure 1 was used asthe basis for selecting experimental conditions in thiswork.

Effect of Soaking Time

Soaking time was varied from 3 to 18 h at 358C and120 bar. The original concentration of 2-HEMA was0.18 mol/L with 10 wt.-% BPO (based on 2-HEMA). The

grafting reaction was carried out at 1108C for 3 h afterthe soaking process. The percentage of grafting is shownin Figure 2. It indicates that equilibrium can be reachedafter a soaking period of 6 h.

Effect of Soaking Pressure

A series of experiments were performed to determine theeffect of soaking pressure on the grafting percentage inthe pressure range 120 to 190 bar. The original concentra-tion of 2-HEMA was 0.18 mol/L with 10 wt.-% BPO, andthe soaking time was 6 h. The graft reaction took place at1108C for 3 h. Figure 3 shows the percent of grafting as afunction of pressure, which indicates that the grafting per-centage reaches a maximum at 150 bar. We explain thisphenomenon in the following way. There are two oppos-ing factors to affect the soaking amount of 2-HEMA.

Figure 1. Phase separation pressure of SC CO2 /2-HEMA andSC CO2 /2-HEMA/BPO system at different 2-HEMA concentra-tions.

Figure 2. Grafting level of iPP films as a function of soakingtime at 358C and 120 bar (2-HEMA concentration = 0.18 mol/L).

Figure 3. Grafting level of iPP films as a function of pressureat 35 8C with soaking time of 6 h (2-HEMA concentra-tion = 0.18 mol/L).

2190 D. Li, B. Han, Z. Liu

First, an increase in pressure results in increased solventpower of CO2, which is not favorable to partitioning of 2-HEMA in the iPP matrix. At the same time, increasingthe pressure of CO2 results in an increase of swelling ofiPP substrate, which is favorable to increasing the adsorp-tion of 2-HEMA. The resulting maximum on the curve at150 bar indicates a balance of these competing effects.

Effect of Monomer Concentration

A series of experiments were conducted at 358C and180 bar in the soaking process, with the 2-HEMA con-taining 10 wt.-% BPO.The grafting reaction was per-formed at 110 C for 3 h after the soaking process. Fig-ure 4 shows the effect of monomer (2-HEMA) concentra-tion on the percent of grafting of 2-HEMA on iPP.

It can be seen from Figure 4 that the grafting percen-tage increases initially with increasing monomer concen-tration up to 0.23 mol/L HEMA, and then decreases withmonomer concentration. The main reason is that most ofthe monomer is utilized by the available free-radical siteson the iPP backbone at the lower monomer concentra-tions, and thus the extent of homopolymerization of themonomer is smaller. At the higher monomer concentra-tions the number of free-radical sites available on the iPPbackbone becomes a limiting factor. At the same time,the free radicals generated in the solution come into con-tact with each other easily. As a result, the degree ofhomopolymerization increases and the grafting percen-tage decreases.

Effect of Initiator Concentration

Soaking experiments were conducted at 358C and180 bar. The monomer concentration was 0.18 mol/Lwith different BPO concentrations from 2 to 12 wt.-%

(based on 2-HEMA). The grafting reaction was carriedout at 1108C for 3 h after the soaking process. The effectof BPO concentration on the grafting percentage is illus-trated in Figure 5.

It is observed that with increasing concentration ofBPO, the percentage of grafting initially increases andpasses through a maximum. Similar results were obtainedby Patel et al.[6] using toluene as solvent. The initialincrease in the grafting level is caused by the increasedavailability of free radicals for grafting of the monomers.When the concentration of the initiator exceeds a value,increased free-radical concentration results in serioushomopolymerization. These opposing factors result in theappearance of the maximum shown in Figure 5.

Effect of Reaction Temperature

The grafting reaction of iPP with 2-HEMA was carriedout at 1008C, 1108C, 1208C, 1308C, and 1408C for 3 h.Before the reaction the iPP films were soaked in CO2 /2-HEMA/BPO solution at 358C and 120 bar for 6 h toobtain the impregnated iPP film. The monomer concen-tration was 0.18 mol/L with 10 wt.-% BPO. The effect ofreaction temperature on the grafting level is shown inFigure 6.

With increasing temperature, the grafting levelincreases initially due to an increased decomposition rateof the initiator. Therefore, the amount of free radicals aswell as their mobility also increases, which results in ahigher level of grafting. However, further increase inreaction temperature initiates termination of various freeradicals and serious homopolymerization. These factorsresult in the optimum temperature for grafting at 1108C.

Figure 4. Effect of monomer 2-HEMA concentration on graft-ing level at 358C and 180 bar soaking conditions (reaction at110 8C for 3 h).

Figure 5. Effect of initiator concentration on grafting level at35 8C and 180 bar soaking condition (reaction at 110 8C for 3 h).

Grafting of 2-Hydroxyethyl Methacrylate onto Isotactic Poly(propylene) ... 2191

Effect of Reaction Time

iPP films were soaked in CO2/2-HEMA/BPO solution at358C and 120 bar for 6 h, and the concentration of the 2-HEMA was 0.18 mol/L with 10 wt.-% BPO. The graftingreaction was performed at 1108C for different time peri-ods. Figure 7 illustrates the effect of reaction time on thepercentage of grafting.

It is evident that the grafting level increases initiallyand reaches a plateau in 3 h. Similar results were obtainedby Petal and colleagues using toluene as solvent.[6] Withan increase in reaction time the free radicals have moretime for reaction and therefore result in a higher level ofgrafting. After some time, all the initiator and monomerare used up, thus no further change in grafting level wasobserved with increasing reaction time.

Characterization

Infrared Spectroscopy Measurements

Grafting of 2-HEMA onto iPP using SC CO2 as solventand swelling agent has not previously been studied, how-ever, it can be expected that, by analogy with the studiesreported using conventional processes,[4–6] grafting of 2-HEMA onto iPP can be confirmed through FT-IR analy-sis. Figure 8 shows the IR spectra of virgin iPP, iPP pro-cessed by SC CO2 (without the monomer and initiator) at308.15 K and 120 bar, and the 2-HEMA-grafted iPP (90/10) obtained in this work.

The spectra of virgin iPP and iPP processed by SC CO2

are almost identical which means that SC CO2 has notmuch effect on the chemical structure of iPP in the courseof swelling. The spectra of iPP-g-2-HEMA shows stretch-ing bands at 1725 cm–1 for C2O groups and at 3500cm–1 for 1OH groups, indicating that 2-HEMA has beengrafted onto iPP.

DSC Measurements

DSC measurements were performed for virgin iPP, iPPprocessed by SC CO2 at 308.15 K and 120 bar for 6 h andthe iPP-g-2-HEMA sample with 2-HEMA content of10 wt.-%. The melting and crystallization behaviors ofthe samples are shown in Figure 9 and Figure 10.

The melting temperature of the grafted copolymer islower than that of virgin iPP, as can be seen from Fig-ure 9. However, the influence of grafting on the crystalli-zation temperature is not significant.

Scanning Electron Microscopy Measurements

SEM studies of ungrafted and grafted iPP allow assess-ment of the effect of grafting on morphology. The iPP

Figure 6. Effect of reaction temperature on grafting level(reaction for 3 h, soaking at 35 8C and 180 bar).

Figure 7. Effect of reaction time on grafting level (reaction at110 8C, soaking at 35 8C and 180 bar).

Figure 8. FT-IR spectra of pure iPP, SC CO2 processed iPP,and iPP-g-2-HEMA (90/10).

2192 D. Li, B. Han, Z. Liu

films could not be fractured even at liquid nitrogen tem-perature since the iPP is very cold-temperature resistant.Thus we could not study the cross-section of the sample,and only the surface of the virgin iPP, CO2-treated iPP,and grafted iPP were studied. The results are shown inFigure 11.

After processed by SC CO2 at 308.15 K and 120 bar for6 h, the morphology of iPP is different significantly fromthat of virgin iPP, as can be seen by comparing by Fig-ure 11a and Figure 11b. After treatment with SC CO2 ,cell structures were formed on the surface of iPP, whichwas caused by phase separation in the course of decom-pression. The phase separation leads to the formation ofspherical domains of SC CO2 , which, in turn, leads to theformation of cells. The grafted surfaces show a markedlybumpy texture. The bumpy surface of grafted layers canbe explained by different initiation rates at amorphousand crystalline sites, partly arising from different swelling

rates of hydrocarbons in amorphous and crystallineregions of iPP.

Wettability of the Films

As expected on the basis of the chemical property of thegrafted components, immersing grafted samples in waterleads to the surface formation of hydrogels and conse-

Figure 9. DSC thermographs for virgin and grafted specimens(heating).

Figure 10. DSC thermographs for virgin and grafted speci-mens (cooling).

Figure 11. SEM images of virgin iPP film (a), SC CO2 pro-cessed iPP film (b), and iPP-g-2-HEMA (90/10) film (c).

Grafting of 2-Hydroxyethyl Methacrylate onto Isotactic Poly(propylene) ... 2193

quently to water absorption. First evidence about hydra-tion was provided by the differences in weights of thesamples between the beginning and the end of the meas-urement. The weight increase was about 3.8610–2 g/cm2

in the case of iPP-g-2-HEMA (90/10), while no weightincrease was observed with pure iPP.

The contact angle (h) for the grafted and ungraftedpolymer film were measured using water as liquid at258C and the results are shown in Figure 12.

It is observed that contact angle decreases with increas-ing percentage of grafting. This indicates that graftinghas imparted polarity to the samples prepared in thiswork. The polar groups of iPP-g-(2-HEMA) interactsstrongly with water, especially by hydrogenbonding. Thisis more obvious with increasing grafting percentage.

Conclusion2-HEMA and BPO were impregnated into iPP films usingSC CO2 as solvent and swelling agent at 358C in the pres-sure range 110 bar to 180 bar, and then 2-HEMA wasgrafted onto iPP at higher temperatures. The degree ofgrafting can be controlled by soaking time, pressure, con-centrations of monomer and initiator, and reaction tem-perature and time. FT-IR spectra of virgin iPP, SC CO2

processed iPP, and grafted iPP indicate that 2-HEMAindeed grafted onto iPP. DSC measurements were per-formed for virgin iPP, SC CO2 processed iPP, and graftediPP. The results show that the melting temperature of thegrafted copolymer is lower than that of virgin iPP and theinfluence of grafting on the crystallization temperature isnot noticeable. SEM studies of the virgin iPP, SC CO2

processed iPP, and grafted iPP allow assessment of theeffect of grafting on morphology. After treatment withSC CO2 , cell structures were formed on the surface ofiPP. The grafted surface shows a markedly bumpy tex-ture. Contact-angle measurements were performed for

virgin iPP and grafted iPP with different 2-HEMA con-tent. It was observed that the contact angle decreases withincreasing percentage of grafting which indicates thatgrafting has imparted surface polarity to the samples.

Acknowledgement: This work was supported by the NationalBasic Research Project-Macromolecular Condensed State andthe National Science Foundation of China (29725308).

Received: August 22, 2000Revised: January 2, 2001

[1] “Polymer blends”, D. R. Paul, S. Newman, Eds., AcademicPress Inc. Publishers, New York 1988.

[2] N. C. Liu, H. Q. Xie, W. E. Baker, Polymer 1993, 34,4680.

[3] M. Sclavons, V. Carlier, B. De Roover, P. Franquinet, J.Devaux, R. Lagras, J. Appl. Polym. Sci. 1996, 62, 1205.

[4] S. R. Shukla, A. R. Athalye, J. Appl. Polym. Sci. 1993, 49,2019.

[5] Y. Fang, X. B. Lu, Q. Cheng, J. Appl. Polym. Sci. 1998,68, 83.

[6] A. C. Patel, R. B. Brahmbhatt, R. C. Jain, S. Devi, J. Appl.Polym. Sci. 1998, 69, 2107.

[7] S. N. Sathe, S. Rao, S. Devi, J. Appl. Polym. Sci. 1994, 53,239.

[8] C. A. Eckert, B. L. Knutson, P. G. Debenedetti, Nature1996, 383, 313.

[9] M. A. McHugh, V. J. Krukonis, “Supercritical FluidsExtraction: Principle and Practice”, 2nd edition, Butter-worth-Heinemann, Boston 1994.

[10] D. A. Canelus, J. M. DeSimone, Macromolecules 1997, 30,5673.

[11] Q. Xu, B. X. Han, H. K. Yan, Eng. Chem. Metall. (China)1999, 20, 313.

[12] M. Z. Yates, G. Li, J. J. Shim, S. Maniar, K. P. Johnston,K. T. Lim, S. Webber, Macromolecules 1999, 32, 1018.

[13] Z. H. Huang, C. M. Shi, J. H. Xu, S. Kikic, R. M. Enick, E.J. Beckman, Macromolecules 2000, 33, 5437.

[14] D. P. Long, J. M. Blackburn, J. J. Watkins, Adv. Mater.2000, 12, 913.

[15] A. K. Lele, A. D. Shine, AIChE J. 1992, 38, 742.[16] D. J. Dixon, K. P. Johnston, M. A. Bodmeier, AIChE J.

1993, 39, 127.[17] J. W. Tom, P. J. Debenedetti, J. Aerosol Sci. 1991, 22, 555.[18] C. Domingo, E. Berends, G. M. van Rosmalen, J. Supercri-

tical Fluids 1997, 10, 39.[19] J. W. Tom, P. G. Debenedetti, Biotechnol. Prog. 1991, 7,

403.[20] D. Li, Z. M. Liu, G. Y. Yang, B. X. Han, H. K. Yan, Poly-

mer 2000, 41, 5707.[21] B. Bungert, G. Sadowski, W. Arlt, Ind. Eng. Chem. Res.

1998, 37, 3208.[22] J. J. Watkins, T. J. McCarthy, Macromolecules 1994, 27,

4845.[23] J. J. Watkins, T. J. McCarthy, Macromolecules 1995, 28,

4067.[24] D. Li, B. X. Han, Macromolecules 2000, 33, 4550.[25] D. Li, B. X. Han, Ind. Eng. Chem. Res. 2000, in press.

Figure 12. Effect of grafting level on the contact angle (h).

2194 D. Li, B. Han, Z. Liu

[26] S. G. Kazarian, M. F. Vincent, B. L. West, C. A. Eckert, J.Supercritical Fluids 1998, 13, 107.

[27] M. F. Vincent, S. G. Kazarian, C. A. Eckert, AIChE J.1997, 43, 1838.

[28] A. R. Berens, G. S. Huvard, R. W. Korsmeyer, F. W.Kunig, J. Appl. Polym. Sci. 1992, 46, 231.

[29] D. Li, G. Y. Yang, B. X. Han, H. K. Yan, X. S. Gao, Eng.Chem. Metall. (China) 1999, 20, 305.

[30] S. K. Goel, E. J. Beckman, Polym. Eng. Sci. 1994, 34,1137.

[31] S. K. Goel, E. J. Beckman, Polym. Eng. Sci. 1994, 34,1148.

[32] C. M. Stafford, T. P. Russell, T. J. McCarthy, Macromole-cules 1999, 32, 7610.

[33] K. A. Arora, A. J. Lesser, T. J. McCarthy, Macromolecules1998, 31, 4614.

[34] M. Lee, C. Tzoganakis, C. B. Park, Polym. Eng. Sci. 1998,38, 1112.

[35] B. Bungert, G. Sadowski, W. Arlt, Fluid Phase Equilibria1997, 139, 349.

[36] J. J. Watkins, V. J. Krukonis, P. D. Condo, D. Pradhan, P.Ehrlich, J. Supercritical Fluids 1991, 4, 24.

[37] D. Pradhan, C. Chen, M. Radosz, Ind. Eng. Chem. Res.1994, 33, 1984.

[38] H. J. Hayes, T. J. McCarthy, Macromolecules 1998, 31,4813.

[39] P. C. Hienenz, “Principles of Colloid and Surface Chemis-try”, Marcel Dekker, Inc., New York 1977.