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Journal of Colloid and Interface Science 279 (2004) 379–390www.elsevier.com/locate/jcis

Temperature and concentration effects on supramolecular aggregaand phase behavior for poly(propylene oxide)-b-poly(ethylene oxide)-

b-poly(propylene oxide) copolymers of different compositionin aqueous mixtures, 1

Gerardino D’Erricoa,∗, Luigi Paduanoa, Ali Khan b

a Dipartimento di Chimica, Università di Napoli “Federico II,” via Cynthia, Naples I 80126, Italyb Physical Chemistry 1, Center for Chemistry and Chemical Engineering, University of Lund, Lund S 22100, Sweden

Received 30 March 2004; accepted 23 June 2004

Available online 3 August 2004

Abstract

The phase behavior (temperature vs composition) and microstructure for the two binary systems Pluronic 25R4 [(PO)19(EO)33(PO)19]–water and Pluronic 25R2 [(PO)21(EO)14(PO)21]–water have been studied by a combined experimental approach in the whole concenrange and from 5 to 80◦C. The general phase behavior has been identified by inspection under polarized light. Precise phase boundbeen determined by analyzing2H NMR line shape. The identification and microstructural characterization of the liquid crystalline phave been achieved using small-angle X-ray scattering (SAXS). The isotropic liquid solution phases have been investigated by semeasurements (PGSE-NMR method). 25R2 does not form liquid crystals and is miscible with water in the whole concentration raincreasing temperature, the mixtures split into water-rich and a copolymer-rich solutions in equilibrium. 25R4 shows rich phase behaviopassing, with increasing copolymer concentration, from a water-rich solution to a lamellar and copolymer-rich solution. A small hephase, completely encircled in the stability region of the water-rich solution, is also present. In water-rich solutions, at low tempand low copolymer concentrations, the copolymers are dissolved asindependent macromolecules. With increasing copolymer concetions an interconnected network of micelles is formed in which micellar cores of hydrophobic poly(propylene oxide) are interconnpoly(ethylene oxide) strands. In copolymer-rich solutions water is molecularly dissolved in the copolymer. The factors influencingaggregation of Pluronic R copolymers (PPO–PEO–PPO sequence) are discussed, and their behavior in water is compared to thacopolymers (PEO–PPO–PEO sequence). 2004 Elsevier Inc. All rights reserved.

Keywords: Pluronic copolymers; Phase behavior; Lyotropic liquid crystals; Deuterium NMR; SAXS; Self-diffusion

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1. Introduction

Amphiphilic copolymers constituted by three alternblocks of poly(ethylene oxide) (PEO) and poly(propyleoxide) (PPO) have attracted considerable attention infundamental research and practical applications, sucdetergency, dispersion stabilization, and lubrication. B

* Corresponding author. Fax: +39-081-676090.E-mail addresses: [email protected],

[email protected](G. D’Errico).

0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2004.06.063

the possible block sequences are commercially availacopolymers with a PEO–PPO–PEO sequence have thename Pluronic, while those with a PPO–PEO–PPO sequare named Pluronic R. Water isa selective solvent for thescopolymers; i.e., it is a good solvent for the PEO blocand a relatively poor solvent for the PPO blocks. Conquently, in aqueous mixture, these copolymers behaveilarly to the nonionic surfactants of the oligo(ethylene oide) type, with formation of micellar solutions and lyotropliquid crystalline (LLC) phases[1,2]. Structural parametersuch as the average molecular weight (Mw) and the copoly-

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380 G. D’Errico et al. / Journal of Colloid and Interface Science 279 (2004) 379–390

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mer chemical composition, defined here byf = (degreeof polymerization for PEO blocks)/(degree of polymerization for PPO blocks), are important variables influencthe self-aggregation of these copolymers and the formaof LLC [3]. Furthermore, the arrangement of the PPOPEO blocks in the chain is also expected to influence thassociative behavior of the copolymer, so that a Pluroncopolymer having the same molecular weight and chemcomposition of a Pluronic copolymer could present quite different association properties in solution[4].

The study of binary copolymer–water systems is of gfundamental value, since they form the basis of the unstanding of the more complicated multicomponent systwhich are encountered in practice.

The aqueous mixtures of the Pluronic copolymers hbeen widely investigated in recent years. Their micellizain dilute aqueous solution has been investigated by vartechniques[5–7]. A theoretical model based on a mean-filattice theory has also been developed[8]. Micellar aggre-gates are formed by a core of PPO blocks and a fringhydrated PEO blocks. On increasing temperature, the critical micelle concentration (cmc) is reduced significantly anthe aggregates often undergo a sphere-to-rod type transitionDifferently from dilute solution, the structure of copolymerich isotropic liquid mixtures has not been investigateddetail. Generally, in this concentration range water islieved to be molecularly dissolved in the copolymer.

The polymorphism of Pluronic copolymers in concetrated aqueous mixtures has also been subject to exteinvestigation[3,9–12]. At high copolymer concentrationseveral Pluronic–water systems form LLC phases suclamellar, hexagonal, and cubic phases. The type of phformed is related to the copolymer molecular weight achemical composition. For copolymers with sufficienhigh PEO content and high molecular weight, associainto spherical aggregates and cubic crystalline phases ipected, while copolymers with low PEO content are prto form bilayer structures and lamellar phases. Intermedbehavior is also possible.

With respect to the aqueous mixtures of Pluronic copmers, Pluronic R–water mixtures have received muchattention, although reverse copolymers are widely usefor example, defoaming and wetting applications. Somethors reported no experimental evidence for Pluronic Rcellization in dilute aqueous solution[6]. This result wasalso supported by a model calculation according to which thentropy loss associated with the looping of the solvophmiddle block would preclude the possibility of micelle fomation[13]. However, the validity of these calculations hbeen questioned[14], and the formation of both micelle[4] and networklike structures[15] has been proposedthe semidilute concentration range. Furthermore, compsimulations have demonstrated the stability of a varietstructures in dilute solution including micelles and brancassemblies[16].

e

s

-

The stability, with respect to both concentration and teperature, of the LLC phases formed by Pluronic R hasbeen studied in much detail[15].

The present work is part of a research program devto investigate the phase behavior and the microstructuPluronic R copolymers in water as a function of both ccentration and temperature. Two copolymers, Pluronic 2and Pluronic 25R2, with almost the same length PPO blbut different length PEO blocks, are considered. Small-aX-ray scattering (SAXS) and water2H NMR are used tostudy the structure of the LLC phases. Both water-richcopolymer-rich isotropic liquid solutions are investigatedmeasuring the self-diffusion coefficients through the pulsgradient spin–echo (PGSE)-FT1H NMR method.

2. Experimental

2.1. Materials

Pluronic 25R4 and Pluronic 25R2 (hereafter indicaas 25R4 and 25R2, respectively) were obtained as afrom BASF (Pluronic R is a registered trademark ofBASF Corporation) and were used as received. PluronicPluronic R copolymers are prepared by polymerizationactions that necessarily result in a molecular mass disttion. The average molecular weight given by the manuturer for the copolymers investigated in the present worM25R4= 3600 (corresponding toMPEO= 1440 andMPPO=2160) andM25R2 = 3100 (corresponding toMPEO = 620andMPPO= 2480). Since the molar mass of ethylene oxequals 44 and that of propylene oxide equals 58, the aage molecular formulae are (PO)19(EO)33(PO)19 for 25R4(f = 0.87) and (PO)21(EO)14(PO)21 for 25R2 (f = 0.33),respectively.2H2O (99.9 atom%2H) was purchased fromAldrich (Milwaukee, USA).

All the experimental data are available by request toauthors.

2.2. Sample preparation

Samples for phase diagram determination, SAXS,water 2H NMR were prepared by weighing appropriaamounts of copolymer and2H2O into 8-mm-i.d. glass tubeswhich were flame-sealed immediately. Liquid crystallsamples were mixed by repeated centrifugation for seral days, and samples in the solution regions were mby shaking overnight. Samples for NMR measurementcopolymer self-diffusion were prepared in a similar wayscrew-cap ampoules were used. Samples for NMR meaments of water self-diffusion were prepared by using doudistilled and degassed water; in this case light water wasferred to heavy water to obtain a clearly detectable wpeak in the spectrum for concentrated copolymer solutiThoroughly mixed samples were kept at 25◦C for 2 weeks to

G. D’Errico et al. / Journal of Colloid and Interface Science 279 (2004) 379–390 381

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attain equilibrium. The samples used to determine phasegrams were checked at regular intervals for about 6 monno variation was observed during this period.

2.3. Small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) measurements wperformed on a Kratky compact small-angle system equipwith a position-sensitive detector (OED 50M from MBrauGraz, Austria) containing 1024 channels of width 51.3 µCuKα radiation of wavelength 1.524 Å was provideda Seifert ID-300 X-ray generator, operating at 50 kV a40 mA. A 10-µm-thick nickel filter was used to remove tKβ radiation, and a 1.5-mm tungsten filter was used to ptect the detector from the primary beam. The X-ray bewidth (at half the maximum intensity) was 0.52 mm. Thvalue is low and would cause negligible smearing effecthe SAXS spectra over theq range of interest in the presework (0.05–0.50 Å−1); thus the peak position of the smearspectra was used in the analysis of the SAXS structural dThe sample-to-detector distance was 277 mm. Temperatucontrol within 0.1◦C was achieved by using a Peltier ement; SAXS measurements were conducted between 2545◦C. The volume between the sample and the detector waunder vacuum during the measurements in order to minimbackground scattering from air.

2.4. Water 2H NMR

2H NMR experiments were performed at constant teperature at a resonance frequency of 14.371 MHz (2.3on a Bruker MSL 100 pulsed superconducting spectroter working in the Fourier transform mode. The 10-mm-NMR tube containing the sample ampoule was placedthe NMR probe. The probe temperature was adjusted w±0.5 ◦C from 25 to 70◦C by passing air of controlled temperature. The ampoules were thermally equilibrated invance for at least 2 h, so that when they were placed inprobe 15 min was enough to ensure thermal equilibrationfact, after 5 min no variation in the NMR spectra was oserved. Aπ/2 pulse (8 µs) with a pulse interval of 0.5 s aa dead time of 300 µs was used. A total of 50–300 pouwere sufficient to obtain a quadrupole splitting (�), mea-sured in hertz as the peak to peak distance in a spectrum.

2.5. NMR self-diffusion measurements

Pulsed-gradient-spin-echo (PGSE)-FT1H NMR exper-iments for the determination of self-diffusion coefficien[17] were performed on a 200-MHz Bruker DMX spetrometer equipped with a Bruker DIFF-25 gradient prodriven by a Bruker BAFPA-40 unit. The sample tempeture was maintained constant during the NMR measuremby passing air of controlled temperature through the samholder. The temperature ranged between 25 and 37◦C and

-

.

d

t

was measured with a calibrated copper–constantan thecouple which was placed in an NMR tube with glycerol.

The stimulated-echo technique was employed. For atem of monodisperse diffusing particles, the normaliz(PGSE)-NMR echo signal,I , is given by

(1)I (k) = exp(−kD),

wherek = γ 2g2δ2 (� − δ/3). γ is the magnetogyric ratioof the proton,δ is the duration of the field gradient pulseand�, the diffusion time, is the distance between the leing edges of the gradient pulses. Typical values used inwork for the time parameters, which were kept constanting each measurement, were 0.5–5 ms forδ and 20–100 msfor �. g is the magnitude of the applied gradient pulwhich was varied in a suitable range (0� g � 9 T/m) in or-der to get signal attenuation.D, finally, is the self-diffusioncoefficient of the studied species. For a monodispersetem, a semilogarithmic plot ofI vs k should yield a straighline.

In the present work 25R4 and 25R2 self-diffusion coecients in heavy water,Dp, were measured by following thpeak relative to the methyl group of the PPO units (chemshift = 1.4 ppm). Water self-diffusion,Dw, was measuredin samples prepared with light instead of heavy waterorder to enhance the OH peak in the spectrum (chemshift = 4.7 ppm). Solvent isotopic substitution (light insteof heavy water) influences the value of the measureddiffusion coefficients. According to Goldammer and Herin dilute copolymer solutions, the ratio between the coecients measured in light water and those measured in hwater is 1.23, which is the reciprocal of the ratio of their vcosities[18]. We tested the isotopic effect by measuring,selected samples, the copolymer self-diffusion coefficienboth light and heavy water. The results indicate that thelation proposed by Goldammer and Hertz holds in a wcopolymer concentration range, and only for concentrathigher than 80% w/w the copolymer self-diffusion becomalmost unaffected by the water isotopic substitution.

In determining water self-diffusion,Dw, one must takeinto account that also the hydrogens of the two termhydroxy groups of copolymers contribute to the OH peThese protons exchange very fast, on the NMR time-scwith water protons. The observed self-diffusion coefficiis given by

(2)DOH = XwDw + XpDp,

whereX denote mole fraction. Since the copolymer sediffusion can be measured independently,Dw can easily becomputed; however, the correction becomes significant onlat high copolymer weight percent (>80% w/w).

2.6. Phase diagram determination

The phase diagrams for the two systems, 25R4–wand 25R2–water, were determined by ocular inspection, b


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Fig. 1. Phase diagram (concentration versus temperature) of the 2water binary system. The following notation is used for the various regiL1 = isotropic water-rich solution phase, E= hexagonal LLC phase, D=lamellar LLC phase, L2 = isotropic polymer-rich solution phase, P= paste-like polymer-rich phase. Two-phase regions are also marked. In the dregion the mixtures are turbid; see text.

SAXS measurements and by analysis of2H NMR spec-tra. For each phase diagram 30–40 samples were prepcovering the whole concentration range. The samples wfirst examined by ocular inspection, against scatteredand between crossed polaroids, for sample homogeand birefringency. In two-phase regions, such as mixtof two immiscible liquids or mixtures of one liquid anone anisotropic liquid crystal, separation into the indivual phases generally occurred spontaneously with time,only seldom it was augmented by centrifugation in andinary desk centrifuge. Ocular inspection was first don25◦C and than the ampoules temperature was raised u79◦C by steps of 3◦C through an ordinary thermostat. Fnally, inspection was done in a cold room thermostatte4 ◦C.

The structure of the various liquid crystalline phaformed was established by the SAXS diffraction pattePrecise detection (±0.5% w/w) of the borderlines of anisotropic phases in the phase diagrams was performe2H NMR. This technique also confirmed crystalline phaidentification as obtained by SAXS measurements.

3. Results and discussion

3.1. General phase behavior

The phase diagrams of the systems 25R4–water25R2–water are presented inFigs. 1 and 2, respectively.

The system 25R4–water shows quite rich phase beior. At 25◦C, up to 63% w/w 25R4 an isotropic water-ricliquid phase, L1, is present; a lamellar liquid crystallinphase, D, forms between 66 and 78% w/w; an isotro

d,

-

Fig. 2. Phase diagram (concentration versus temperature) of the 2water binary system. The following notation is used for the various regiL = isotropic solution phase, 2Φ = two-phase region. In the dashed regithe mixtures are turbid; see text.

copolymer-rich liquid phase, L2, forms at high copolymeconcentration (between 82 and 98% w/w); extremely ccentrated mixtures (>98% w/w) are a paste, P. Betweenand 30% w/w the L1 samples are cloudy. In principle, thcould be a concentration range of coexistence of two dient liquid phases; however, even through very long and hspeed centrifugation, no phase stratification was obseThis evidence is similar to that already reported by Zhand Chu for other Pluronic copolymers[19]. These authorinterpreted the opalescent region as due to the transitiotween monomers (isolated copolymer chains) and mice(aggregates of various copolymer molecules) and nathe phenomenon “anomalous micellization.” The anomamicellization behavior, which is usually absent in solutof typical surfactants, can be ascribed to the compospolydispersity of block copolymer samples: the phasearation of the more hydrophobic fraction of the copolymmolecular distribution occurs somewhere before the onsmicellization of the major component; as soon as miceform, the more hydrophobic macromolecules are solubilizeinto the cores of the aggregates. In this connection,interesting to observe that the width of composition distution for Pluronic and Pluronic R copolymers could be qularge[20], since the length of both PEO and PPO chainpolydisperse, and the diblock sequence could coexistthe triblock one.

Concerning the system 25R4–water at 25◦C, the cloudyregion separates a concentration range (up to 8% w/wwhich the copolymer is present as monomers, from a ccentration range (between 30 and 63% w/w) in which milar aggregates form.

Our findings at 25◦C are in very good agreement with tresults of Alexandridis et al., who studied the phase behaof the ternary system 25R4–water–p-xylene[21].


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The temperature highly affects the phases’ stability. TL1 phase is limited at high temperature by the cloud poof 25R4; the clouding temperature, which is 27◦C at 1%w/w, increases with increasing copolymer concentratThe cloudy region related to the anomalous micellizatioshifts to higher 25R4 concentration with decreasing the tperature. Between 36 and 52◦C, a small hexagonal phasE, forms; it is an “island” encircled by the isotropic copomer solution, L1. Its concentration stability region rangbetween 47 and 52% w/w of copolymer and shifts to lowcopolymer concentration with increasing the temperatur

The lamellar region, D, is stable in an extremely wtemperature range; with increasing temperature, the D composition range becomes narrower and finally lamellae mat 73◦C. The L2 phase observed at high copolymer cont(>80%) is stable in the whole temperature range explorethe present work. The P region present in extremely contrated 25R4–water mixtures below 27◦C extends to loweconcentration with decreasing the temperature.

In the binary system 25R2–water no liquid crystalliphase was detected and only an isotropic liquid phaseis present. The L phase extends over the whole concetion range at low temperature, while at high temperaturis limited by the cloud point of the copolymer (22◦C for a1% w/w copolymer mixture), which increases with increing copolymer concentration. Also for 25R2 the L phasesplit in two regions from an intermediate cloudy region, sarating a region in which copolymer molecules are presas monomers from a region in which they self-aggregate

3.2. Lamellar (D) phase

The samples in the lamellar region of the 25R4–waphase diagram are transparent and birefringent and exsplittings in the2H NMR spectrum. They are quite soft anflow slowly under their own weight. The one-dimensionlamellar structure was established by SAXS experimeAn example of a slit-smeared SAXS spectrum obtained25R4–water lamellar samples is shown inFig. 3. Second-order peaks can be observed following the 1:2 pattern.appearance of only a low number of reflections, whichready has been observed for other Pluronic copolymersbe ascribed to the “softness” of the material[11]. Actu-ally, the difference in polarity between the hydrophobic Pblocks and the hydrophilic PEO blocks is smaller than,example, the polarity difference between hydrocarbon tand headgroups in typical surfactants, so that the microstures formed by these copolymers in water usually presediffuse interface between polar and apolar domains. Woin progress in our labs to obtain more resolved SAXS spefor 25R4, by using the synchrotron light.

Besides phase identification, SAXS measurements ato estimate structural parameters of the liquid crystals. Ccerning the lamellar phase, on the assumption that allamphiphilic copolymer molecules participate in the lamel

-

t

Fig. 3. Slit-smeared SAXS spectrum obtained for a 25R4–water samp73% w/w in copolymer at 25◦C; the scattering curve is also shown onexpanded intensity scale (right-hand-side axis) to reveal the higherpeak (1:2) characteristic of the lamellar structure.

Fig. 4. 25R4–water mixtures: characteristic spacings for the lamellarstructure, plotted as a function of thecopolymer concentration at constatemperature (25◦C). (") Lamellar periodicity,d ; (2) apolar film thick-ness,δ; (F) interfacial area per PEO block,as.

lae, it can be shown that[11]

(3)q1 = 2π

d= πasΦp

vp,

whereq1 is the position of the first-order Bragg peak,d is theperiodicity of the lamellae,Φp is the total volume fraction othe copolymer,vp is the volume of one copolymer molecu(5700 Å3 for 25R4[21]), andas is the effective interfaciaarea per PEO block. On the assumption of a sharp interbetween the polar (PEO and water) and apolar (PPO) domains, it is also possible to estimate the thickness,δ, of thelamellae apolar domain (δ = f d), wheref is the volumefraction of PPO in the copolymer–water mixture. The cresponding thickness of the polar domain is then givend − δ. It is to be stressed that the assumptions taken in tcomputations are questionable for Pluronic and Pluronaggregates, and the results have to be considered indicarather than true pictures of the system microstructure.

SAXS measurements were performed as a functioncopolymer concentration at 25◦C. The lamellar periodicityd , as well as the apolar film thickness,δ, and the interfa-cial area per PEO block,as, are shown as a function ocopolymer weight fraction inFig. 4. d decreases with in


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Fig. 5. 25R4–water mixtures: characteristic spacings for the lamellarstructure, plotted as a function of the temperature at constant copolymconcentration (69% w/w). (") Lamellar periodicity,d ; (2) apolar filmthickness,δ; (F) interfacial area per PEO block,as.

creasing copolymer content;δ, on the other hand, increaswith copolymer content, a reflection of the increase inPPO volume fraction.as decreases with increasing copomer content, indicating that the lamellae, and in partictheir polar part, “contract” as water is removed; these effechowever, are rather small (<10%).

For one sample (69% w/w) the SAXS measuremewere also performed at three different temperatures. Thsults are shown inFig. 5. With increasing temperatureasdecreases, whiled and δ increase. This nonideal swellinof the lamellae, which has been already found for otPluronic copolymers[11], can be related to the dehydratiof the copolymer chains[8].

Qualitative information on the copolymer hydration cabe obtained by analyzing the splittings in the2H NMR spec-tra. In the simplest model only two sites for water molecuare considered, i.e., molecules bound to the EO groupsfree molecules tumbling without restriction. The obserquadrupolarsplitting (�) is then given by[3]

(4)� = |PbϑQSb|,wherePb is the fraction of bound water molecules andSb istheir order parameter.ϑQ is the effective deuteron quadrpole coupling constant that, for2H in 2H2O, is 220 kHz.ϑQSb, which can be either positive or negative, is the averquadrupole interaction of the bound water molecules.observed variation of� with sample concentration thereforeflects changes inPb andSb which, however, can be difficult to separate. Equation(4) can be rearranged as

(5)� = n|ϑQSb| Xp

Xw,

wheren is the copolymer hydration number;Xp and Xware the mole fractions of copolymer and water, resptively. The � values for the 25R4–water lamellar phaat 25◦C are presented inFig. 6 as a function ofXp/Xw.The values at low copolymer content increase almostearly with Xp/Xw and, furthermore, extrapolation passapproximately through the origin, as predicted by Eq.(5).

-

Fig. 6. 25R4–water mixtures:2H quadrupole splitting values plotted asfunction of the copolymer/water molar ratio. (") Lamellar LLC phase a25◦C; (!) hexagonal LLC phase at 42◦C.

� increases linearly up to anXp/Xw value correspondingin the very rough approximation in which the PO unitsnonhydrated, to∼2 hydration water molecules per EO unLiterature studies confirm the hydration number of theunit to be included between 1 and 3[22,23]. Equation(5)can be used to estimate the order parameter of boundter molecules,Sb, for the 25R4–water lamellae. The valobtained (Sb ≈ 1 × 10−3) is lower with respect to thosobtained for Pluronic copolymers[3], indicating water tobe more disordered in Pluronic R LLC phases. At higcopolymer concentration the� values slightly decreases aEq.(5), based on the two-site model, does not hold anymDeviations from the two-site model have been reportedwater-poor liquid crystals[3,11], in which most of the wateis associated with the polar head groups of the surfactangregates. Furthermore, decreasing water content could resuin a worse segregation of the hydrophobic and hydrophdomains as the copolymer approaches the L2 disordered condition, thus reducingSb [24].

The variation of the splitting values in the lamellar phawith temperatures has been also investigated. Two osite responses to the temperature change have been fdepending on the copolymer concentration; the bordebetween the two regimes is∼69% w/w of 25R4. For moreconcentrated mixtures� decreases with increasing the teperature; inFig. 7the data relative to the sample at 73% wis reported as an example. This behavior can be interpin terms of dehydration of the EO groups as the tempture is increased[11]. In contrast, for lamellar samples wirelatively lower copolymer concentration,� increases withthe temperature; inFig. 7 the data relative to the sample66% w/w is reported as an example. This phenomenon istraightforward to understand. It is to be noted that at higwater content the EO groups are hydrated more extensiThe interaction between completely hydrated EO groupsbeen shown to be repulsive at low temperature[25]. By in-creasing the temperature, the interaction between the pol


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Fig. 7. 25R4–water mixtures:2H quadrupole splitting values for two different samples plotted as a function of the temperature. (") Sample at 66%w/w; (2) sample at 73% w/w.

groups becomes less repulsive. This is probably accompanied by a gradual change of conformation of the polar pof the copolymer as the distance between the head groudecreased and the EO chains become more extended,ing out from the aggregate surface. This will, of courchange the quantityn|(ϑQS)b|. The number of bound water molecules might, as mentioned above, decrease, b|(ϑQS)b| increases more, this can explain the increase�

value with increased temperature.

3.3. Hexagonal (E) phase

The samples in the hexagonal region of the 25R4–wphase diagram are transparent and birefringent. Theyquite soft and flow under their own weight. The twdimensional hexagonal structure (hexagonally packed arraof cylindrical micelles) was established by SAXS expements which revealed reflections obeying the ratio 1:

√3:2,

characteristic of this structure. Indeed, only the first twopeaks were usually detectable in the spectra. Reasonthis are the same as discussed for the lamellar phaseticularly, the peaks are even weaker than those reportethe literature for aqueous mixtures of Pluronic L64; i.e.seems that reversing the sequence of blocks in the copolcould result in a less defined structuring of the system[11].An example of a slit-smeared SAXS spectrum obtained25R4–water hexagonal samples is shown inFig. 8.

Concerning a two-dimensional hexagonal symmetrycould be the case of a normal hexagonal structure constiby infinite cylinders, the diffraction peaks atqhk, whereh

andk are the Miller indices, correspond to[11]

(6)qhk = 4π√

h2 + k2 + hk

a√

3=

(h2 + k2 + hk

(√

3/2π)f

)1/2asΦp

vp,

wherea is the lattice parameter (nearest neighbor distanOn the same assumptions taken for the lamellar LLC,also possible to estimate the diameter,δ, of the cylinders’apolar domain.

st-

f

fr-

r

Fig. 8. Slit-smeared SAXS spectrum obtained for a 25R4–water samp50% w/w concentration in copolymer at 37◦C; the scattering curve is alsshown on an expanded intensity scale (right-hand-side axis) to reveahigher order peak (1:31/2:2) characteristic of the hexagonal structure.

Fig. 9. 25R4–water mixtures: characteristic spacings for the hexagonalstructure, plotted as a function of thecopolymer concentration at constatemperature (42◦C). (") Nearest neighbor distance between the axis ofcylinders,a; (2) apolar cylinders diameter,δ; (F) interfacial area per PEOblock, as.

SAXS measurements were performed at two differcopolymer concentrations (46.5 and 48.5% w/w) at 42◦C.Thea, δ, andas values are shown as a function of copomer concentration inFig. 9. Both a and δ remain almostconstant with changing copolymer content. In contrastasdecreases with increasing copolymer concentration, somore copolymer molecules can be accommodated insame number of cylinders (note, however, that this obvation is based on only two data points).

The samples in the hexagonal phase exhibit a splitting inthe 2H NMR spectrum; the� values for the 25R4–watehexagonal phase at 25◦C are presented inFig. 6 as a func-tion ofXp/Xw. The splitting is nearly one-half of that showby lamellar samples. In the hexagonal phase, the rapid dsion of deuterons around the cylindrical aggregates caureduction of the absolute value of the order parameterfactor of 1/2 with respect to the value in the lamellar phif all the other factors are unaffected. Then, it follows froEq.(4) that

(7)�D ≈ 2�E.


y of

ine

a-lyrox-

ingthe

re

en-

ntsents

n-ntoly-

as-.par-4

aytion

m o

-

istri-

ns,

Gffu-

p-ted

nthem

at0%

-the

is-as-ityiongre-e;mes

Fig. 10. 25R4–water mixtures: logarithm of the normalized echo-deca25R4 as a function ofk = (γgd)2(�−δ/3); � = 50 ms. (a) Sample at 15%w/w copolymer; (b) sample at 35% w/w copolymer.

(D and E refer to lamellar and hexagonal liquid crystallphases, respectively), provided that (PϑQ)b is constant. In-spection ofFig. 6 shows that in the whole E concentrtion range, at 42◦C, the� values increase almost linearwith Xp/Xw and, furthermore, extrapolation passes appimately through the origin, as predicted by Eq.(4). In thesame approximations taken for the lamellar phase,Sb ≈7 × 10−4 for the 25R4–water hexagonal phase, confirmthat water is more disordered in the hexagonal than inlamellar phase.

3.4. Isotropic water-rich solution (L1)

The samples in the isotropic water-rich solution phase anot birefringent and do not exhibit a splitting in the2H NMR.Their viscosity increases with increasing copolymer conctration.

The behavior of the 25R4–water system in the L1 phasewas investigated by measuring the self-diffusion coefficiethrough the PGSE-NMR technique. A set of measuremwas performed at various 25R4 concentration at 25◦C. Con-cerning the 25R4 self-diffusion coefficient, which was moitored by following the NMR signal of PPO, two differediffusive behaviors were observed, depending on the copmer concentration:

(i) Up to ∼30% w/w the echo intensity decay, reporteda function ofk = (γgδ)2(� − δ/3), could be well interpolated by a single exponential decay, as predicted by Eq(1),thus indicating the presence of monodisperse diffusingticles in the sample; seeFig. 10a as an example. The 25Rself-diffusion coefficients,D25R4, are plotted inFig. 11as afunction of the copolymer weight percent in the mixture.

(ii) Between 30 and 60% w/w the echo intensity decshowed a multiexponential trend, which is a clear indicaof polidispersity of the diffusing particles; seeFig. 10b asan example. In fact, for a polydisperse system, Eq.(1) isnot longer valid. Instead, the signal decay becomes a su
f
Fig. 11. 25R4–water mixtures: self-diffusion coefficients of both components plotted as a function of the copolymer concentration. (") D25R4 at25◦C; (!) D25R4 at 37◦C; (Q) Dw

25R4 at 25◦C; (P) Dw25R4 at 37◦C.

The inset shows the standard deviation of the diffusion coefficient dbution,σ . (2) σ at 25◦C; (1) σ at 37◦C.

decays of signals from species of different mobilities[26],

(8)I (k) = Pi exp(−kDi),

wherePi is the number fraction of molecules with diffusiocoefficientDi . This situation manifests itself in curved linewhen echo intensities are plotted vsk in a semilogarithmicplot. A common way to treat polydispersity effects in PFNMR studies is to assume a continuous distribution of dision coefficientsP(D) and to write

(9)I (k) =∫

P(D)exp(−kD)dD.

A convenient form forP(D), which has been found to reresent a variety of diffusive polydispersities, is constituby a lognormal distribution[27],

(10)P(D) = 1

Dσ√

2πexp

(− (ln(D) − ln(D0))

2

2σ 2

),

whereD0 is the median value for the diffusion coefficieand σ is the standard deviation of the distribution. Tmean value of the diffusion coefficient is obtained fro〈D〉 = D0 exp(σ 2/2). Equations(9) and(10) were found toadequately fit the echo attenuation in our measurementsa copolymer concentration included between 30 and 6w/w. TheD25R4 values shown inFig. 11 in this concentration range are the mean values, while the inset showsstandard deviation,σ , which can be regarded as a polydpersity index.σ seems to weakly decrease with increing the copolymer concentration. Diffusive polydisperscould be connected with a polydispersity of the dimensof the aggregates formed by the copolymers. These aggates start to form at∼30% w/w and are quite polydisperswith increasing copolymer concentration their size beco


de-ringdi-re-

%hatoesles

ec-

ion.ex-n

erquaf

ison-

s inaneole-

n

lions-

ncts.anallhatthe

con-utege

ter

m--

vi-thaer

-er

elf-.

ent, be-

tem-ringe-ventheom-c ef-

ionin

itythiswedR4esseethe

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gob-oninu-rvedu-er-nce

progressively more uniform. Aggregates polydispersityserves further investigation; a small-angle neutron-scattestudy is in progress in our labs to fully characterizemension polydispersity and microstructure of 25R4 agggates.

It is to be noted that in the cloudy region (from 8 to 30w/w) theI decay is simply exponential, thus suggesting tthe fraction of molecules that demix is rather small and dnot affect the diffusive behavior of the copolymer molecuin solution.

Despite the difference in the diffusive behavior, insption of Fig. 11shows that, in the L1 phase,D25R4 monoton-ically decreases with increasing copolymer concentratThe data in the very dilute composition range weretrapolated at infinite dilution. The limiting self-diffusiocoefficient (D∞

25R4 = 1.12 × 10−10 m2 s−1) was used toestimate the hydrodynamic radius of a single copolymchain in aqueous solution through the Stokes–Einstein etion, obtainingr25R4= 22 Å, corresponding to a volume o

∼45,000 Å3. The stretched length of the 25R4 molecule

240, 180, and 140 Å for the zigzag, helix, and meander cformation, respectively[21]. Consequently, ther25R4 valueindicates a quite close coil conformation for 24R4 chainwater. Since the volume of a “naked” copolymer chain cbe estimated to be 5700 Å3, the volume estimated from thhydrodynamic radius indicate a large number of water mcules to be “entrapped” in the copolymer coil.

Qualitative information on the copolymer hydration cabe obtained by analyzing water self-diffusion data,Dw

25R4,in the same mixtures. Equation(1) was always found to weldescribe attenuation of the OH signal for water self-diffusmeasurements.Dw

25R4decreases monotonously with increaing copolymer concentration; seeFig. 11. This decrease cabe explained in terms of hydration and obstruction effeHowever, the water mobility is always much higher ththat of the copolymer. In studies of the diffusion of smmolecules in polymeric systems, it is generally found tthe diffusion of these substances is poorly influenced bypresence of the polymers, even at quite high copolymercentration, indicating that water molecules involved in solhydration are not immobilized but can easily exchanamong themselves, thus preserving high mobility[28].

The effect of temperature on both copolymer and waself-diffusion coefficients was also investigated. InFig. 11the self-diffusion coefficient of 25R4 and water at two teperatures, 25 and 37◦C, is shown. The trends are quite similar, and no dramatic effect of the temperature is put in edence. However, a deeper analysis of the graph revealsan inversion in the effect of temperature on the copolymself-diffusion coefficient occurs in the L1 phase with increasing copolymer content. In fact, at 40% w/w the copolymself-diffusion coefficient slightly decreases with increasingtemperature. In contrast, at 60% w/w the copolymer sdiffusion coefficient increaseswith increasing temperatureIf no structural change occurs, the self-diffusion coefficiof a particle should increase with increasing temperature

-

t

cause of the increased kinetic energy. However, at highperature PEO moieties become more hydrophobic, favomolecular self-aggregation between 25R4 chains, thus rducing molecular mobility. The results at 40% w/w hato be ascribed to some increase in the interaction betweecopolymer chains that induces a sort of structuring insystem. At higher concentration the aggregation phenenon has already occurred and consequently the kinetifect becomes more important.

3.5. Isotropic copolymer-rich solution (L2)

The samples in the isotropic copolymer-rich solutphase are not birefringent and do not exhibit a splittingthe 2H NMR. They are quite viscous and their viscosdecreases with increasing copolymer concentration. Incomposition range, the PGSE-NMR measurements shono deviation from a single-exponential decay. The 25self-diffusion coefficient is quite low and slightly increaswith increasing the copolymer content of the mixture,Fig. 11. These experimental evidences indicate that inL2 phase the observed self-diffusion coefficient is donated by molecular diffusion, i.e., diffusion of 25R4 mocules in copolymer-continuous domains. With increasingwater content of the mixture, i.e., with decreasing copomer content, a progressive segregation of the solvophPPO blocks occurs, thus inducing a certain degree of sture in the system, and explaining the decrease of sdiffusion coefficient. Water self-diffusion, although lowthan in the L1 phase, is still much higher than the copolymself-diffusion coefficient, indicating no water segregationdroplets.

It is interesting to note that the values of the self-diffuscoefficients in the L2 phase are not very different from thoin the concentrated L1 phase, so that if no LLC phase wpresent, one could expect a continuous trend of bothD25R4andDw

25R4 with increasing copolymer concentration.We comment here on the data concerning the sys

25R2–water for which, as discussed above, there is notinction between L1 and L2 phases. Similarly to what wafound for 25R4, for the two samples at 40 and 50% wof 25R2, the decay of the PPO NMR signal, reported afunction of k, is multiexponential (σ ≈ 0.2), and a meanvalue was calculated. For all the other samples a monponential decay was found. The copolymer self-diffusion25◦C, plotted as a function of the copolymer compositishows a broad minimum at∼80% w/w; seeFig. 12. This ev-idence can be ascribed to a gradual change, with increasincopolymer concentration, between a region in which theserved self-diffusion coefficient is dominated by diffusiof copolymer monomers and/or aggregates in a contous aqueous medium and a region in which the obseself-diffusion coefficient is dominated by molecular diffsion, i.e., diffusion of copolymer molecules in copolymcontinuous domains[29]. Interestingly, comparison betweeFig. 11andFig. 12shows that, despite the large differen


-

es orely-

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tlated-andt mi-henceza-nce.

ter-andtem.n ofikealof

ofudyestsbe

isursolu-

wodif-ca-rsbe-harp

tesble

bichctive

ly-ped-

lkyltedbe-

rved.hy-

eenole-

mix-nce)ould

thelec-merredthe

onccu-103

em-

der

R4–

atalxago-

tem.s

Fig. 12. 25R2–water mixtures: self-diffusion coefficients of both components plotted as a function of the copolymer concentration. (") D25R2 at25◦C; (!) D25R2 at 37◦C; (Q) Dw

25R2 at 25◦C; (P) Dw25R2 at 37◦C.

in the phase behavior between 25R2 and 25R4, the valuthe self-diffusion coefficients in isotropic liquid solution avery similar suggesting a similar structuring of the copomer molecules.

3.6. Self-aggregation of Pluronic R in the L1 phase

An open question concerns whether 25R4 self-aggates, and what kind of aggregates it forms, in the L1 phase.Our self-diffusion data show that theD25R4 trend does nopresent any breakpoint or slope change that could be reto a cmc; seeFig. 11. A similar result was found by Alexandridis et al. from the analysis of surface tension dataled these authors to the conclusion that 25R4 does nocellize [30]. This behavior was found to be peculiar to treverse copolymer architecture (PPO–PEO–PPO sequein fact, tensiometry clearly shows the onset of micellition for Pluronic with the normal PEO–PPO–PEO sequeHowever, in concentrated solutions, we found that theD25R4values become extremely low, thus indicating strong inmolecular interaction between the copolymer chainssuggesting the formation of some structure in the sysThe presence of the cloudy region is also an indicatioanomalous micellization. Furthermore, micelles with rodlshapes are expected in the L1 phase close to the hexagonLLC. All these arguments strongly support the formationmicellar aggregates in the 25R4–water system.

The similarity between the self-diffusion coefficients25R4 and 25R2, together with the presence of the cloregion in the phase diagram of both copolymers, suggthe aggregation of 25R2 molecules in liquid solution tothe same of 25R4.

For Pluronic R copolymers the looping of the moleculerequired in order to form isolated micelles and this unfavothe self-aggregation. However, in quite concentrated s

f

);

tions, interconnected micelles can form, in which the touter PPO blocks of a molecule can participate to twoferent aggregates, with no angular correlation in their lotion [15]. Consequently, micellization of Pluronic R occuonly at high concentration. Furthermore, the transitiontween monomers and interconnected micelles is not sand no cmc can be clearly detected.

It is interesting to note that the micellar aggregaformed by both Pluronic and Pluronic R copolymers are ato solubilize only a few percent in weight of a hydrophomolecule such asp-xylene[21,31]. This means that, in botcases, the segregation of the PPO block is not very effein creating apolar domains.

The self-aggregation behavior of Pluronic R copomers closely resembles that of hydrophobically end-cappoly(ethylene oxide)[32], which presents a similar molecular architecture with the PPO blocks replaced by achains. However, in the latter case, the formation of isolaspherical micelles in a concentration range intermediatetween monomers and a micellar network has been obseThis difference could be ascribed to the much strongerdrophobic interaction between alkyl chains than betwPPO blocks, thus balancing the entropy loss due to the mcules’ folding.

3.7. Effect of the chain architecture on the phase behaviorof Pluronic copolymers

The comparison between the properties of aqueoustures of Pluronic R copolymers (PPO–PEO–PPO sequeand those of Pluronics (PEO–PPO–PEO sequence) shallow the study of the effect of chain architecture onmolecules self-aggregation. However, since various moular parameters are important in determining the copolybehavior in water, the choice of the Pluronic to be compawith a given Pluronic R is ambiguous, and consequentlyconclusions of this comparison are questionable.

Concerning 25R4 [(PO)19(EO)33(PO)19, Mw = 3600,f = 0.9], Alexandridis et al., in analyzing surface tensidata, found that the area at the air–solution interface opied by a molecule is the same as that of Pluronic P[(EO)17(PO)60(EO)17, Mw = 4950,f = 0.6], which has thesame number of EO units[30].

However, the 25R4–water phase diagram strongly resbles that of the system Pluronic L64 [L64, (EO)13(PO)30(EO)13, Mw = 2900, f = 0.8]–water [11], even if in thelatter one the LLC structures are stable in a slightly witemperature range.

Also, the phase diagrams of the ternary systems 25water–p-xylene and L64–water–p-xylene at 25◦C are verysimilar [21,31]. However, it is interesting to note that,25◦C, while in the binary L64–water system a normhexagonal phase supersedes the micellar phase, no henal structure is formed along the 25R4–water binary sysIn this case the presence of ap-xylene low percentage inecessary in order for such a structure to form.p-xylene


an

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83

increases the segregation of the PEO and PPO blocksfacilitates the formation of oil-in-water cylinders.

The Pluronic and Pluronic R phase behavior can be cnected with the copolymers’ molecular structure. Threethe main differences between 25R4 and L64: (i) 25R4a 20% higher molecular weight than L64, which enhanthe tendency of the molecules to self-aggregate; (ii) 25presents a slightly higher EO composition (f value) with re-spect to L64, i.e., weaker hydrophobic behavior; (iii) 25presents a PPO–PEO–PPO sequence while L64 presenopposite PEO–PPO–PEO sequence. Concerning this pthe entropy loss associated with the looping geometrythe solvent-affinitive middle block may reduce the tendeof copolymers with PPO–PEO–PPO sequence towardassembly. 25R4 presents a slightly lower tendency to saggregate with respect to L64. This could be ascribedleast partially, to the different chain architecture, i.e.,PEO block positioned in the center of the chain disfavthe formation of normal (oil in water) structures. This efect is balanced by the higher 25R4 molecular weightfact, if the molecular weights of the Pluronic and PluroR to be compared were the same, the reverse sequenthe blocks in the copolymer chain would have causemuch weaker self-aggregation. This conclusion has beeperimentally confirmed by Zhou and Chu, who compaL64 to Pluronic 17R4 [17R4, (PO)14(EO)24(PO)14, Mw =2650,f = 0.9] finding that 17R4 presents a much lowtendency to self-aggregate, forming micelles only at hcopolymer concentration within a narrow temperaturegion [4].

4. Concluding remarks

The self-assembly behavior in water of two Pluroniccopolymers, 25R4 and 25R2, presenting almost the slength of the PPO block, but different chemical compotion and molecular weight, has been explored over the wconcentration range in the 5–80◦C temperature range. Thtwo copolymers present a different phase behavior: desthe higher hydrophobicity, 25R2 does not form any LLstructures, which, in contrast, are observed in the cas25R4. The LLC phases formed by 25R4 have been intigated by SAXS and2H NMR. The experimental resultssuch as the characteristic lattice parameter of the lalar structure, the apolar film thickness, the area per Pblock, and the hydration of the hydrophilic moiety of tmolecules show the typical variation with both tempeture and copolymer concentration observed for Plurocopolymers (PEO–PPO–PEO sequence). These evideshow that (i) the formation of LLC requires a minimumolecular weight in order to occur; (ii) the structureliquid crystals is weakly affected by the block architecture.

The isotropic liquid phases have been investigatedthe PGSE-NMR method; self-diffusion data indicate

d

e,

f

-

s

structuring of 25R2 and 25R4 molecules in isotropic luid solution to be the same. These copolymers, differefrom Pluronics with PEO–PPO–PEO sequence, do not fspherical micelles in dilute solutions. At low temperatuand high dilution the copolymer exists in solution as invidual molecules, monomers. Upon increasing the tempture or copolymer concentration, PPO blocks self-aggregforming hydrophobic domains connected between themthe PEO. These evidences show that (i) the self-aggregbehavior of pluronic copolymers in aqueous solutionpends on the block architecture of the copolymers; (ii) iscarcely affected by the length of the PEO blocks.

Acknowledgments

This research was carried on with the financial suppothe Italian MIUR-Cofin 2002, Prot. 2002037154002. CaMalmborg and Anna Svensson are acknowledged for ttechnical assistance for NMRand SAXS measurements, rspectively.

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