poly(propylene oxide) block copolymer surfactants

478

Poly(ethylene oxide)/poly(propylene oxide) block copolymer 5urfactants Paschalis Alexandridis Block copolymers consisting of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) can self-assemble in water and water/oil mixtures (where water is a selective solvent for PEO and oil a selective solvent for PPO) to form thermodynamically stable spherical micelles as well as an array of lyotropic liquid crystalline mesophases of varying morphology. Significant advances have been made over the past year on the identification of different morphologies, the delineation of the composition·temperature ranges where they occur, and the structural characterization of the morphologies using primarily small angle scattering techniques. Important new findings on the copolymer micellization in water as affected by cosolutes, and on the time-dependency of the surface activity have also been reported.

the aqueous polar environment) and their adsorption at surfaces and interfaces. (note: in the remainder of this review, PEO/PPO denotes block copolymer of PEO and PEO in general; PEO-PPO-PEO denotes a copolymer whose block sequence is known.)

Figure 1 Pluronic . po!oxamer Ho-(CH2-CH2'O).(CH..CH-O)Y-(CH2-CH,-O) ...H I

CH3

Pluronic R - meroxapol Ho-(CH2-CH·O )Y-( CHz-CHz-O).-(CH2-CH-O)v-H

Addresses Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260-4200, USA; e-mail: [email protected], and Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, PO Box 124, Lund S 22100, Sweden; e.mail: [email protected]

I

I

CH3

CH3

Tetronic - poloxamine CH3

Current Opinion in Colloid & Interface Science 1997, 2:478-489 Electronic identifier: 1359-0294-002'00478

/

Introduction Block copolymers consIsting of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) can exhibit amphiphilic properties in aqueous solutions, because PPO homopolymer phase-separates (macroscopically) from water at relatively low temperatures (at about -S·C for PPO homopolymer of molecular weight l200Da, 2l-mer), whereas PEO homopolymer (also known as PEG: polyethylene glycol, or POE: polyoxyethylene) is well soluble in water (at least up to 100·C) [1,2). The presence in the same molecule of both PEO (hydrophilic) and PPO (hydrophobic) blocks leads to the self-assembly of PEO/PPO copolymers in solution (microscopic phase separation occurs because of the tendency to minimize the contact between the hydrophobic PPO parts and

I

).CH..CH-O)Y-( CH2-CH..o) ...H

N-CH..CH..N

~

H·(o-CH..CH2).(o-CH-CH2). I

(CH..CH-O)Y-( CH2-CH..O) ...H I

CH3

© Current Chemistry Ltd ISSN 1359-0294

Abbreviations CMT critical micellization temperature CP cloud point esc differential scanning calorimetry LCST lower critical solution temperature LLC lyotropic liquid crystalline PBO poly(1,2·butylene oxide) PEO poly(ethylene oxide) PPO poly(propylene oxide) SANS small angle neutron scattering SAXS small angle X-ray scattering

CH3

I

H.(o-CH2.CH2).( o-CH-C~2h

CH3

a-series polyglycol Ho-(CH2.CH..O).(CH..CH-o)y-(CH2-CH2-0),..H I

CH3-CH2

Current Opinion in Colloid & Interface Science

Chemical structure of poloxamer (PEO·PPO·PEO), meroxapol (PPO·PEO·PPO), poloxamine X(PPO-PEO)4, and PEO·PBO·PEO block copolymers.

A number of triblock PEO-PPO-PEO copolymers have been commercially available since the SO's under the generic name Poloxamers and the trademarks Pluronic® (BASF, ~Iount Olive NJ, USA) and Synperonic® (lCI, Cleveland, UK) [3]. The triblock architecture is the result of the polymerization process where first a PPO homopolymer is synthesized (constituting the middle block of the copolymer) by the addition of propylene oxide to the tWO hydroxyl groups of propylene glycol, and then ethylene oxide is added at both ends of the PPO block to form the PEO end-blocks. Triblock PPO-PEO-PPO copolymers are also available (and known as meroxapols and Pluronic R), as are tetra-functional block copolymers (known as poloxamines or Tetronics) formed by the addition of (first) propylene oxide and (then) ethylene oxide to ethylene diamine (see Figure 1 for the structures of the different PEO/PPO copolymers).

Poly(ethylene oxide}/poly(propylene oxide} block copolymer surfactants Alexandridis

Whereas typical surfactants afford a rather limited variation of their hydrophobic (tail) and hydrophilic (headgroup) partS [4,5], PEO/PPO copolymers allow alteration of the hydrophobe/hydrophile composition and total molecular weight, as well as block sequence. Such a flexibility in molecular architecture results in a wide range of amphiphilic properties which can be controlled in small increments. Information on the molecular weight and composition of the PEO/PPO copolymers currently available from BASF is provided in Table 1 [3]. The molecular weights of the commercially available copolymers are typically in the 2000-20000 Da range and their PEO coments in the 20-80\\'t% range. A certain minimum PPO block size is required for sufficient block segregation and subsequent expression of amphiphilic properties; the need in many commercial applications for finite solubility of the copolymer in water sets an upper limit in the PPO block size.

Table 1 PEO/PPO block copolymer notation as Pluronic (trademark of BASFI Wyandotte Corporation) and Polaxamer (CTFA), followed by their various ph)'$ical characteristics. Data courtesy of Pluronlc and Tetronlc Block Copofymer Surf.ctants, Technical Brochure, BASF Corporation, 1989.

P1uronic

Poloxamer

Mol weight PEO wt% Cloud point rC) HlS'

l31 l35 F38 L43 L44 L51 L62 L54 P55 F6B F77 L81 P84 P85 F87 F8B 192 F98 L101 Pl03 Pl04 Pl05 Fl0B L121 L122 P123 F127

101 105 108 123 124 181 182 184 185 18B 217 231 234 235 237 238 282 299 331 333 334 335 338 401 402 403 407

1100 1900 4700 1850 2200 2000 2500 2900 3400 8400 6600 2750 4200 4600 7700 11400 36S0 13000 3800 4950 5900 6500 14600 4400 5000 5750 12600

10 50 80 30 40 10 20 40 SO 80 70 10 40 SO 70 80 20 80 10 30 40 50 80 10 20 30 70

Mol weight 2150 2650 3100 3600 8S50 3250

PEO wt% Cloud point rC) HlB 20 35 2·7 40 46 7·12 20 29 2-7 7·12 40 40 12-1B 80 45 10 25 1·7

Mol weight 1650 3600 5500 4700 6700 25000 15000

PEO wt% 40 10 40 10 40 80 70

Pluronic R Meroxapol 17R2 172 17R4 174 25R2 252 25R4 254 25R8 25B 31R31 311 Tetronic 304 701 704 901 904 908 1107

Poloxamine 304 701 704 901 904 90B 1107

'HLB: hydrophilic-lipophilic balance.

37 73 >100 42 55 24 32 59 82 >100 >100 20 74 85 >100 >100 26 >100 15 86 81 91 >100 14 19 90 >100

1·7 18-23 >24 7·12 12-18 1·7 1·7 12-18 12-18 >24 >24 1·7 12-18 12-18 >24 >24 1·7 >24 1·7 7·12 12-1B 12-18 >24 1·7 1·7 7·12 >24

Cloud point rC) HLB 75 12-18 18 1·7 79 12-18 20 1·7 74 12-18 >100 >24 >100 >24

479

The versatility of the PEO/PPO copolymers in the various applications originates from the ability to vary the copolymer chemical composition and is further enhanced by the sensitivity of the PEO/PPO copolymer aqueous solution properties to temperature. Both PEO and PPO homopolymers exhibit a lower critical solution temperature (LCST) in water, that is, water becomes progressively a worse solvent for them as temperature increases. This leads to a heat-induced formation of micelles and lyotropic liquid crystalline 'gels' and therefore has importarit repercussions in applications. i\lacroscopic phase separation (known as 'cloud point' (CP]) of the PEO/PPO block copolymer from water occurs at even higher temperatures. Despite the commercial availability and many uses of PEO/PPO block copolymers, most of their solution properties and phase behavior remained elusive until the late 80's. Concerted effortS by research groups in England, Germany, Sweden, and the US over the past 10 years led to great advances in our understanding of the PEO/PPO copolymer self-assembly in aqueous solutions, and in particular the formation and structure of micelles: thermodynamically stable spherical assemblies with a 'core' consisting of the water-insoluble PPO blocks and a 'corona' composed of hydrated PEO segments (see [6-9] for recent reviews). In addition to forming micelles in solution, PEO/PPO block copolymers self-assemble into a great variety of lyotropic liquid crystals with elastic properties ('gels'); considerable effort is being directed at the present time towards elucidating their structure and exploring their practical utility. The level of maturity of the field is indicated by the publication in 1996 of a book on polyoxyalkylene block copolymers [10-] which examines the synthesis [11], chemical analysis [12], micelle formation and structure [13-], surfactant properties [14], applications [15-], toxi· cology [16], and biological activity [17] of PEO/PPO and PEO/PBO block copolymers, where PBO denotes poly(l,2-butylene oxide) (PEO/PBO block copolymers became recently commercially available from Dow and are interesting because of the higher hydrophobicity of the PBO block compared to that of the PPO block [18,19]). {\Iany of the contributions to this book are applicationsoriented and cover well the patent literature. A book on the self-assembly and applications of amphiphilic block copolymers [20-] will appear later this year. Among other topics, the book will cover the micellization and aqueous solution structure of PEO/pPO (21] and PEO/PBO [22-] copolymers, the phase behavior and microstructure of PEO/PPO and PEO/PPO copolymers in the presence of selective solvents (water and oil) [23] and in the bulk (i.e. in the absence of solvents) (241, experimental (25,26] and modelling techniques [27-,28] employed for assessing the structures resulting from the copolymer self-assembly, and reports on the numerous applications of PEO/pPO copolymers [29,30-,31,32].

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This review highlights the research activity in the field of PEO/PPO block copolymers in 1996 and in the first half of 1997, and it covers, primarily, fundamental studies on the self-assembled structure afforded by PEO-PPO-PEO triblock copolymers of the poloxamer type. References to some earlier (mainly 1995j publications are also made here, but for a more complete coverage the reader is referred to the three reviews which appeared in 1995 [6-8) and the two books [10-,20-) whose contents are outlined ·above. A large number of applied studies exploit the self-assembly and surface activity properties of PEO/PPO block copolymers primarily in the pharmaceutical-biomedical field; such studies are not covered here. Applications of amphiphilic block copolymers, and in particular PEO/PPO copolymers, are reviewed in [15-,29,30-,31,32,33-]. We start this review with the topic of PEO-PPO-PEO block copolymers in dilute aqueous solutions (micelles), where new information on micelle formation and structure in water and in the presence of cosolutes is examined. \Ye move on to concentrated aqueous copolymer systems ('gels'), concerned with the transition from the micellar solution to micellar crystals and the structure of the latter, the phase behavior and structure in concentrated ternary systems with water and oil, and oil-based systems (reverse micelles). We then overview the block copolymer self-assembly in the presence of selective solvents and compare it to that of surfactants in solution and block copolymers in the bulk (solvent-free). Finally, we present some recent developments on the adsorption of PEO/PPO copolymers at the air/water interface.

Poloxamer micellization in aqueous solutions studied by calorimetry PEO/PPO block copolymer can form (mostly) spherical micelles in aqueous solutions above a certain copolymer concentration, Cl\IC (critical micellization concentration) and solution temperature, CMT (critical micellization temperature), both of which depend on the copolymer molecular weight, block composition (hydrophilic/hydrophobic ratio), and block architecture [6,9]. The determination of the conditions where micelles are formed by PEO-PPOPEO copolymers in water has been a challenge because of the size and composition polydispersity inherent in polymers, the presence of hydrophobic impurities (a result of incomplete copolymerization of the middle PPO block), variations between batches of copolymers from different commercial sources [34-], the often high (compared, e.g., to ethoxylated nonionic surfactants) Cl\ICs, and the great sensitivity of micellization to temperature. A large number of experimental techniques (surface tension, static and dynamic light scattering, UV-vis and fluorescence spectroscopy in conjunction with a probe molecule, and also calorimetry, dilatometry, Nl\lR, and ultrasound velocimetry [8,35]) have been used to study micellization; the temperature and concentration conditions under which micelles form have now been established for a large

number of PEO-PPO-PEO [6,36] and PEO/pBO block copolymers [18,22-]. Differential scanning calorimetry (DSC) is a technique which actually takes advantage of the sensitivity of PEO/pPO block copolymer micellization to temperature and the high Cl\lCs and at the same time is insensitive to the presence of hydrophobic impurities, which can impede techni.ques such as light scattering and dye solubilization which rely on optical detection [37]. The micellization of PEO-PPO-PEO copolymers in water is strongly endothermic [35-39,40-], indicating an entropy gain as the driving force, and is manifested by a pronounced peak in DSC experiments. The reversibility (thermodynamic equilibrium conditions) of micelle formation can be established by the almost perfect overlap of results obtained in up- and down-temperature DSC scans. The onset of the DSC endotherm signifies the Cl\lT, and the area under the peak gives the enthalpy of micellization, AH m . Recently, an analysis of the DSC output from PEO-PPO-PEO aqueous solutions has been presented, to derive the extent of micellization, the concentration of free copolymer in equilibrium with micelles, and other thermodynamic parameters pertaining to the micellization transition [41,42-]. The hydrophobic PPO block is the most important factor in the formation of micelles by PEO-PPO-PEO copolymers [36,38-39,40-); the Cl\lC decreases exponentially, and the CMT linearly, with increasing the PPO block length [36). DSC has been used to assess the mode of the macroscopic phase separation observed in dilute aqueous solutions of PPO homopolymers [43]. DSC provided evidence that an aggregation process between several molecular clusters took place before the phase separation. Data for both the phase separation of PPO homopolymers and the micellization (micro-phase separation) transition ofPEO-PPO-PEO block copolymers fell on the same line in an entropy-enthalpy compensation plot (compensation temperature: 275K), confirming the notion that the hydrophobic PPO is responsible for micellization.

Effects of cosolvent, cosolutes, and surfactants on poloxamer micellization DSC has also been used to examine the effects of cosolvents/cosolutes in the micellization properties of PEO/PPO copolymers [37,44,45]. l\lethanol, ethanol, urea, and formamide suppress the micellization of Pluronic F87 (and decrease both Cl\lT and AH m), whereas butanol and hydrazine favor micelle formation (and increase Cl\lT and AH m) [44]. Fairly large quantities (20%) of cosolvent are required in order to affect the Cl\IT by 10 K. Similar findings have been reported for the cosolvents ethylene glycol, ethanolamine, methoxyethanol and propanol [45). The presence of the alkali-halide salts LiCI, KCf, NaCI, and NaBr decreased both the Ct\lT and the CP (phase separation) of Pluronic L64 (in the order Cl->Brand Na+>K+>Li+) [37], whereas addition of

Poly(ethylene oxide)/poly(propylene oxide) block copolymer surfactants Alexandridis

sodium thiocyanate (NaSCN) [37] and urea [46] resulted in a Cl\IT and CP increase; the following relationship was obeyed: Cl\IT(no salt) - Cl\lT(salt) = CP(no salt) -CP(salt) [37]. This suggests a common mechanism of salt action on both micro- and macro-phase separation; unfortunately, the exact mechanism ('direct': interactions of salt with the copolymer; or 'indirect': change of water structure due to salt) remains unresolved [36,44,46]. Apart from cosolutes, surfactants can have a drastic effect on the micellization of poloxamers. The addition of the common anionic sutfactant sodium dodecylsulfate (SOS) can completely suppress the micellization ofPluronic F127 [47]. The surfactant has been found to bind cooperatively to the poloxamer, 'shield' the hydrophobic PPO block and render it hydrophilic; at saturation conditions, four to five SOS molecules bind to one F127 molecule. The presence of about 150 ml\l SOS could suppress completely the extent of the micellar cubic gel region formed at 30% F127 in water. SOS could also 'melt' the liquid crystalline structures formed by L64 [48]. The CPs of Pluronic L64, F68 and P85 increased in the presence of SOS; the copolymer-50S complexes had enhanced surface activity and solubilizing power and exhibited polyelectrolyte-like nature [49,50]. The self-assembly behavior in water of a mixture of two PEO-PPO-PEO copolymers, L64 and PI05, was explored at 25°C [51]. Two peaks could be discerned in OSC experiments at a total copolymer (L64 + PI05) concentration of 10 wt% and varying L64/PI05 ratio; the lower-temperature peak was due to the micellization of the more hydrophobic (lower Cl\IT) PI05, whereas the higher temperature peak originated from the micellization of L64. The presence of two separate (but overlapping) peaks intimated that the polymers participated in the micelles according to their CMT [51].

Transition from micellar solution to micellar cubic 'gel' in aqueous poloxamer solutions A very important feature for numerous practical applications is the ability of PEO/PPO copolymers to form so-called 'gels' in water. Such gels have been known (and used) for some time [3,52], their structure, however, has only recently been resolved [53,54°]. We note that the term 'gel' describes the elastic properties of the material but is not very appropriate in terms of the structure. The Structural integrity of the PEO/PPO gels is the result of the packing of block copolymer self-assemblies into a crystalline lattice [53,55°]; this is very different from the gels formed by physical or chemical cross-links of high polymers, for example, polysaccharides. A number of recent fundamental studies have been concerned with the transition from a micellar solution to a micellar cubic 'gel' in aqueous PEO/PPO copolymer solutions. This transition from a Newtonian liquid to a soft solid material occurs when the micellar volume fraction crosses the critical value for hard-sphere crystallization (the micelles

481

are considered of constant size and spherical shape during this transition) [54°], for sufficiently repulsive intermicellar interactions (long PEO blocks). The 'effective' volume fraction of the PEO/PPO micelles increases with increasing temperature, increasing copolymer concentration, and decreasing hydrostatic pressure [54°]. This 'effective' volume is related to the 'thermodynamic' and not the hydrodynamic volume. The thermodynamic volumes of PEO/pBO copolymer micelles, measured in moderately concentrated solution were used successfully to predict the copolymer concentration required to form a micellar cubic 'gel' at concentrated solutions [56]. The thermodynamic volume of a particle is related to its excluded volume which is measurable by static light scattering (via an appropriate theory); KzS0-t aqueous solution was used to obtain a theta solvent for PEO, and thus set its excluded volume to zero and simplify the analysis [56]. The hydrodynamic volume of a micelle relates to the non-free-draining volume of the corona and is derived from the hydrodynamic radius which can be obtained from dynamic light scattering. For the systems discussed in [56], the hydrodynamic volume was found to be about three times higher than the thermodynamic volume, and was thus inappropriate for predicting gelation. Apart from describing the 'gelation' in terms of hardsphere crystallization, it is important to understand the changes taking place at the molecular level. IH Nl\IR longitudinal (Tl) and transverse (Tz) relaxation times were used to probe the transition with increasing temperature from free polymers in solution to micelles to micellar cubic 'gel' in the FI27-water system [57]. At the Cl\IT there was a marked decrease in T 1 and especially in T z for the PPO block, attributed to a change from a well solvated to a more restricted micelle-core environment (in agreement with the study in [58,59]); no such transition was observed in the properties of the PEO block at the Cl\rr. A further (but considerably smaller) decrease in T z for the PPO block, as well as a decrease in T z for the PEO block, were observed at the gelation temperature. The former suggested that the dynamic state of the micelle (mainly PPO) core changed little with gelation, whereas the latter could be interpreted in terms of greater motional hindrance as the micelle coronas consisting of PEO interpenetrated the micellar cubic gel. Complex dynamical features on the thermoreversible gelling aqueous system of (35 wt%) Pluronic F68 were revealed from dynamic light scattering [60] and rheological (oscillatory shear and shear stress relaxation) [60,61°] measurements. The decays of the correlation functions and the stress relaxation moduli around the gelation temperature exhibited power law behaviors followed by stretched exponentials at long times. The power law exponent was generally close to 0.5, and was rationalized within a framework of the fractal model for polymer networks [61°]. Pulsed field gradient Nl\IR measurements of the polymer molecule mean-square displacement in

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Surfactant science

the same F68 system revealed a slowing down of the diffusion as the gelation proceeded; this has been called 'anomalous' (i.e., non-Fickian) diffusion [62]. Small angle neutron scattering in combination with rheometry has also been used to study the heat-induced micelle~gel transition in an aqueous solution of Pluronic F127 [63].

Structure of the poloxamer micelles and the micellar cubic 'gel' extracted from scattering techniques A useful tool in the study of the micelle structure in aqueous solutions ofPEO-PPO-PEO copolymers has been SANS. An important consideration in extracting structural information from the SANS intensity profiles is the model used to describe the micelle. A simplified model which assumed the micelle core (as a uniform sphere) to be the only scattering source has been used initially to obtain the equivalent hard sphere radius for micelles [64]. A core-shell model for the form factor which assumed a uniform segment density distribution within the micelle core and in the corona was recently used [65]. From this model the volume fraction of the polymer segments in the corona of Pluronic L64 micelles was found to be less than 0.2. An analytical expression for the form factor of block copolymer micelles based on a combination of a spherical core and gaussian chains in the corona has also been introduced [66], and gave comparable corona compositions for Pluronic P85 micelles. The hard-sphere core-shell model for the form factor, in combination with an adhesive surface to describe attractive intermicellar interactions (structure factor), was used to analyze SANS data for Pluronic P84 micelles in water [67]. Expressions for the low shear viscosity of the micelles were developed by summing hydrodynamic interactions and contributions from intermicellar interactions extracted from the SANS experiments. Good agreement was obtained between theoretical predictions and experimentally measured viscosity values [67]. SANS studies on shear-aligned poloxamer-water samples have proved to be important in the elucidation of the structure of the micellar cubic 'gels' discussed in the previous section [54-]. A drawback of SANS is the poor resolution thus rendering the technique insufficient to characterize quantitatively ordered structures [68]; shear alignment of the sample and use of an area detector for the scattered intensity is the only method that allows determination of the crystallographic space group (P:"primitive cubic, BCC: body-centered cubic, or FCC: face-centered "cubic) in SANS. Small angle X-ray scattering (SAXS) is better in this respect; a large number of diffraction peaks can be detected in micellar cubic samples, e\'en with a line detector, and have allowed the determination of the Pluronic P105 micellar cubic structure as BCC [51,55-]. SANS crystallographic im'estigations of monodomain P85 'gel' samples have shown that the micelles are organized on a BCC lattice [54-]. Both SANS and (synchrotron radiation) SAXS were recently used to investigate the

shear-induced structures of the Pluronic F108 micellar cubic phase [68,69]. The 20 X-ray pattern displayed well defined scattering rings, which, when integrated radially, gave rise to five diffraction peaks; these were fitted to an FCC lattice [69]. The sample was polycrystalline at rest; when subjected to a laminar stationary shear at high rate, it oriented with the compact <111> layers parallel to the shear plane. The flow was obtained through layer sliding and resulted in a high density of stacking faults. These were annealed by an oscillatory strain to obtain homogeneous single crystals in the millimeter scale [69]. Other recent developments in the structural characterization of micellar cubic samples formed by PEO/PPO copolymers include the direct imaging, for the first time, of such a sample (formed by F127) by cryogenic temperature transmission electron microscopy (cryo-TEi\1) [70], and the SANS study of a cubic phase formed by Pluronic 25R8, a PPO-PEO-PPO triblock copolymer, in which micellar cores of hydrophobic PPO are interconnected by PEO strands [71].

Phase behavior and structure in binary poloxamer-water systems In addition to the cubic crystals formed by the packing of spherical micelles as discussed above, PEO-PPO-PEO copolymers can self-assemble in water, at relatively high copolymer concentrations (>30%), into various lyotropic (i.e. formed in the presence of solvent) liquid crystalline (LLC) phases, such as hexagonal (cylindrical micelles crystallized into a hexagonal 20 lattice) and lamellar (planar bilayers in a smectic arrangement) [55-,72]. Such LLC phases are very common in surfactant-water systems [5], and for oligo(ethylene oxide) alkyl ethers in particular [73]. The structural polymorphism, however, afforded by PEO/PPO copolymers has only recently been recognized [55-,72,74]. The formation of LLC structures by PEO-PPO-PPO block copolymers in water requires a hydrophobic (PPO) part of a sufficient size for block segregation and self-assembly (micro-phase separation), and also a certain sized hydrophilic (PEO) part which will impart the repulsion between the assemblies required for LLC stability. For a given PEO/PPO composition, the tendency to segregation increases with increasing polymer molecular weight [55-]. The block composition and block architecture play an important role in determining the phase behavior by affecting the 'packing' of the molecules in the ordered nanostructures [55-,72]. When the apparent volume fraction of the water-soluble PEO blocks is much larger than that of the \\'ater-insoluble PPO blocks, spherical or cylindrical assemblies (of high interfacial curvature) are favored with the PPO segments located in the interior (core) of the assemblies; the lamellar arrangement is preferred when the relative length of the water-insoluble PPO blocks is sufficiently large. For copolymers of comparable block size, the mode of self-assembly depends

Poly(ethylene oxide)/poly(propylene oxide) block copolymer surfactants Alexandridis

on the PEO/PPO copolymer concemration in water, and a transition from a spherical (micellar solmion or micellar cubic LLC) £0 a cylindrical (hexagonal LLC) and then £0 a planar (lamellar LLC) arrangemem can be observed upon a concemraEion increase [55-,72]. Composition-temperature phase diagrams, which indicate the regions of stability of the differem LLC structures discussed above, have been presemed in [72] for twelve binary PEO/PPO copolymer-water systems. A detailed study of the composition-temperature phase diagrams of three PEO-PPO-PPO copolymer-water systems has been reported in [55-]. The phase diagrams of PEO-PPO-PEO copolymers such as FI27 and F68, with 70 and 80% PEO, respectively, are dominated by regions of micellar cubic structure. On the other hand, the phase diagrams of copolymers such as L62 and LI22, both consisting of 20% PEO, are dominated by regions of lamellar structure. The thermal stability of the LLC regions increases in the order cubic < hexagonal < lamellar. An increase in temperature caused the boundaries between phases (regions of different structure) to move £0 lower copolymer concentration (i.e. structures swell with water at higher temperature). Thus, the LLCs formed by PEO-PPO-PEO copolymers exhibit a thermotropic behavior, and a reversible transition from a solution £0 an LLC region, or from one LLC structure £0 another, can be brought about by varying the temperature at a fixed copolymer concemration [55-,72]. PEO/PBO block copolymers can also form LLC phases [75-] and four binary (copolymer-water) composition-temperature phase diagrams have recemly appeared in the literature [76,77]. The identification of the LLC structures in [72] was based on their texture under polarization microscopy, an easy-£O-use but not unambiguous technique. SAXS and zH N~IR of heavy water (ZHzO) were employed in [55-] in order to establish the structure in the different LLC regions formed by PEO/PPO copolymers. The differem LLC structures give SAXS diffraction patterns with peaks at different q (scattering veclOr) values. The relative position and imensity of the peaks on the q axis is characteristic of the crystallographic structure. For the 10 lamellar and 20 hexagonal LLC structures, this characterization is straightforward; for the 3D cubic structures it can become complicated due £0 the large number of possible symmetries. SAXS was used in [74] for the first time in the idemification and characterization of LLC structures formed by PEO/PPO copolymers, and has since proved very useful in such studies. In addition to the structure type, the charaCteristic dimension of the structure (lattice parameter) can be directly obtained from the q position of the SAXS peaks. Both the lamellar periodicity and the block copolymer interfacial area decrease with increasing copolymer content [72]. It has been found [72] that an increasc of the temperature results in an increase in the periodicity and a decrease in the imerfacial area of the L62 and L64 lamellae.

483

ZH N~IR of heavy water can provide a simple way lO detect the presence of isotropic (e.g. cubic) or anisotropic (e.g. lamellar of hexagonal) LLC structures. The ZH N~IR spectrum of ZHzO is dominated by the imeraction of the deuteron quadrupole moment with the electric field gradients in the nucleus. For an anisotropic LLC sample formulated with ZHzO, this quadrupole imeraction generates a spectrum with two peaks of equal intensity. In an isotropic solution or LLC phase, on the other hand, this interaction is averaged to zero as a result of rapid and isotropic molecular motions, and the N~IR spectrum consists of a sharp singlet. Furthermore, the magnitude of the deuteron quadrupole splitting comains information on the hydration of the amphiphile assemblies [72]. Recently, electron spin resonance (ESR) spectroscopy of nitroxide spin probe molecules (preferemially located in the vicinity of the EO segments) was used to assess the hydration of the PEO blocks in the L64-water binary system [78]. The effective degree of hydration decreased with increase in L64 comem and was significantly lower than the average value deduced on the basis of the water coment. An importam development, complememary lO the experimemal determination of binary poloxamer-water phase diagrams, is the semi-quamitative prediction by a mean field polymer model of the phase sequence and compositiontemperature stability range for PEO-PPO-PEO copolymers in water [79-]. The theoretical scheme incorporates both 'imernal' monomeric scates for the EO and PO segments (£0 account for temperature effects) as well as the effects of the spatial arrangement of the polymer molecules, and may be viewed as a generalization of the theory of ordered phases in block copolymers [80].

Phase behavior and structure in ternary poloxamer-water-oil systems The first phase diagram of a ternary system consisting of a PEO/pPO block copolymer (Pluronic L64) in the presence of two solvents, water (selective solvent for PEO) and p-xylene (selective soh'ent for PPO) was published in late 1995 [74]. The LLC structures formed along the copolymer-water axis swelled with oil £0 a varying degree. In addition £0 these 'normal' (N: oil-in-water) phases, regions of 'reverse' (R: water-in-oil) structure, such as R hexagonal LLC, R micellar solution, and most notably R bicominuous cubic LLC (having a structure associated with the Gyroid minimal surface), were idemified in the presence of oil, all thermodynamically stable at 25·C [74]. SAXS was employed to ascertain the self-assembled structure and lO determine the structural lengths [74,81]. Along the copolymer-water axis, the copolymer coment is the main determinant of the structures formed, whereas along the copolymer-oil axis the relative amount of watcr is important in addition £0 the copolymer coment. The ternary phase diagram ofPluronic 25R4, a PPO-PEO-PPO copolymer, in water and xylene has also been reported [82] and found £0 be strikingly similar lO that of the L64 (PEOPPO-PEO) copolymer. This observation suggests that the

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block architecture (A-B-A vs B-A-B) does not influence significantly the global phase behavior of the triblock copolymers. Recent studies examined the effect of the organic solvent on the phase behavior of PEO-PPO·PEO block copolymers [83-85]. The hexagonal and lamellar LLC regions in the Pluronic FI27-water-butanol were stable at low (<::20%) copolymer concentrations indicating the role of butanol as cosurfactant in extending the interfacial area afforded by the PEO/PPO copolymers [84]. Of great significance is the recent discovery of the richest phase behavior ever reported in an amphiphile-water-oil system [75°]. Three cubic, two hexagonal, and one lamellar LLC phase, as well as two solution regions, a total of eight independent phases altogether, have been observed (and properly characterized) in a ternary PEO-PBO/water/oil phase diagram at 25°C. The 'polymeric chain' nature of the amphiphilic block copolymers allows for fine-tuning of the molecular 'packing' via the varying degree of swelling of the PEO and PPO or PBO blocks by water and oil molecules, respectively, and results in the progression of N micellar, N micellar cubic, N hexagonal, lamellar, R bicontinuous cubic, R hexagonal, R micellar cubic, and R micellar regions upon an increase of the oil/water ratio. The microstructure of the self-assemblies appears to be a predictable and controllable function of the volume fraction of the polar ('water'-like) / apolar ('oil'-like) components (composition constrains) and of the copolymer-solvent interactions (packing constrains) [75°]. t\lore work, however, is required in order to establish this. All of the above mentioned phases are commonly formed by amphiphiles in the presence of water and organic solvents (but never by the same amphiphile at the same temperature-see next paragraph), except for the reverse micellar cubic phase 12, which has previously been identi· fied only in some particular surfactant/cosurfactant/water systems, and never in a ternary amphiphile/water/oil system [86°]. The self-assembly of PEO/PPO block copolymers in the presence of selective solvents (water and oIl) is related in some aspects to the self-assembly of surfactants in solution [5,73,87°], and in other aspects to the self-assembly of bulk block copolymers in the absence of solvents ('dry') at temperatures below the order-1disorder transition (ODT) [88,89°]. It exhibits, however, a number of features which distinguish it from either surfactant-soh'ent or dry block copolymer self-assembly. In all three systems, the curvature is based on the relative size of the different parrs of the amphiphilic molecule, but is also affected by the solvent (when present). Surfactants have a preferred cun'ature which limits the type of structures which can be formed (at a certain temperature) to either N or R. Dry block copolymers of a given block composition can typically form only a single type of structure; Nand R structures can be formed, but require copolymers of different composition. PEO/PPO block copolymers are

unique in that they can form both Nand R structures with the same copolymer at the same temperature in the presence of varying amounts of two solvents that are selective for each block [23].

Water-swollen reverse micelles formed by poloxamers in xylene In addition to their association in aqueous solutions, surfactants often form thermodynamically stable 'reverse' micellar or 'microemulsion' systems in organic solvents which have a number of interesting applications [90]. Although PEO/PPO block copolymers can solubilize small amounts of organic compounds in aqueous micellar solutions [91], the solution behavior of such copolymers in organic solvents, and the solubilization of water in these solutions (microemulsion formation) have received little attention in the literature. A notable exception is the recent body of work on the water-induced micelle formation ofPluronic L64 in o-xylene [8,92-94,95°]. L64 is completely miscible with xylene and did not form micelles in o-xylene in the absence cfwater or even in the presence of small water amounts (up to "" 0.15 water molecules per EO segment). Reverse micelles with a hydrated PEO core and a PPO corona, however, were formed by L64 o-xylene at higher water concentrations, as shown by scattering techniques [92,93]. Up to <:: 2 water molecules per EO segment could be solubilized in the L64 xylene solution [74,92]. IH Nt\IR showed that the chemical environment of the protons in the methyl and methylene groups of PPO remained unchanged even for high water contents, suggesting no water penetration into the apolar regions [94]. SANS experiments indicated that some water was solubilized in the PPO corona, but the amount was <:: 40 times less than that solubilized in the PEO core of the reverse micelle [93]. The hydration gradient in the PEO core of the L64 reverse micelles was assessed recently using ESR spectroscopy of a homologue series of nitroxide spin probes [95°]. The micellization in p-xylene of a number of different PEO/pPO and PEO/PBO block copolymers and the solubilization of water in these systems was recently reported [96,97]. The ability of the reverse micelles to solubilize water has been employed for the detection of the copolymer Ct\IC in p-xylene, just as solubilization of organic compounds is used for detecting micellization in aqueous solutions. The C~IC values were in the range 0.01-0.1 molal (5-20 Wt %); the C~IC decreased with increasing copolymer molecular mass at a given polymer chemical composition, and with increasing PEO (sol\"Ophobic) block size for a fixed PPO block size. A simple model for the micelles was developed (based on the volume fractions of the different components and the characteristic length of a copolymer molecule) which allowed the eStimation of the. (spherical) micelle radius and aggregation number (96).

Poly(ethylene oxide)/poly(propylene oxide) block copolymer surfactants Alexandridis

Poloxamer adsorption at the fluidlfluid and fluid/solid interfaces PEO/PPO block copolymers are interfacially active and adsorb at surfaces which interact favorable with one of the blocks [6]. The adsorbed amount and the configuration of the block copolymer at the interface/surface, as well as the time-dependent process involved in at£aining equilibrium at the interface are important considerations. High density monolayers can be formed when PEO/PEO or PEO/pBO block copolymers adsorb onto hydrophobic surfaces; the hydrophilic PEO blocks extend into the water, and the hydrophobic PPO or PBO block anchor the polymer onto the surface [98-100]. The hydrodynamic dimensions of the PEO blocks on the surface of polystyrene lattices were found to be similar for Poloxamer 407 and Poloxamine 908 (at 7 nm), copolymers of different block composition and sequence but of similar PEO block length [99]. SANS data were al)alyzed to yield the copolymer volume fraction profile as a function of distance from the oil/water interface for Poloxamers 188 and 407, and Poloxamine 908 adsorbed at the perfluorodecalin/water interface in a submicrometer emulsion [101]. The absence in SANS of any maxima away from the surface suggested that both blocks must be adsorbed to some extent, although the more hydrophobic PPO segments are likely to be preferentially,adsorbed. In the case of Poloxamer 188, the adsorbed layer extended to 5 nm from the surface, whereas that of 407 extended three times as far. An electrolyte (0.1-0.2 ~I NaCI) was found to significantly reduce the bound fraction of Poloxamer 188, and caused a significant increase in the extension of the adsorbed layer, without affecting the amount of polymer adsorbed [101]. The surface energies of Poloxamers 338 and 407 adsorbed to silanised glass plates have been calculated via Wilhelmy plate contact angle measurements [102]. The apolar surface energy term remained consistent, but the polar contribution changed depending upon the temperature of adsorption, and was most e1ebted at the C~.IC/C~IT [102]. Although a number of studies of equilibrium surface tension studies on PEO/PPO solutions have been published recently [72,103,104], little information is available on the kinetics of adsorption of these long amphiphiles to the air/liquid interface. Such kinetics can be critical in controlling the performance of the amphiphiles in practical applications [105,106]. The adsorption of Pluronic fl27 and F68 (at sub micellar concentrations) at the air/water interface was found from capillary wave and Wilhelmy plate techniques to be diffusion-limited [107]. The time required for the surface tension to at£ain its equilibrium value (after the Wilhelmy plate was lowered to a fresh surface) was <= 5 h for 0.01% F68 and <= 16 h for 0.003% F127; the characteristic diffusion time constants were 12.5 and 17 s, respecti\·e1y. The dynamic surface tension (DST) of Poloxamers 407, 237, and 338, was studied with the maximum bubble pressure method at a range of surface ages and concentrations [108°]. The DST did not change at the C~IC, and was 10-20 mN/m higher

485

than the equilibrium surface tension (EST) at copolymer concentrations lower than I%. It was concluded that the DST behavior of the surfactants was most closely related to the PEO content and/or total molecular weight of the surfactants, whereas micellization and EST were more closely related to the PPO content [108°].

Conclusions Block copolymers consIsting of PEO and PPO can self-assemble in water and water/oil mixtures (where water is a selective solvent for PEO and oil a selective solvent for PPO) to form thermodynamically stable spherical micelles as well as an array of lyotropic liquid crystalline mesophases ('gels') of varying morphology. New findings on the heating-induced micellization in water of PEO/PPO block copolymers have been obtained from differential scanning calorimetry, and useful information on the micelle formation and structure in the presence of cosolutes or surfactants has been reported. The latter is very important in terms of practical applications, where PEO/PPO copolymers are most often present together with other compounds. The thermore\'ersible transition from a micellar solution (liquid) to a micellar cubic crystal (gel), observed for PEO/PPO block copolymers with long PEO blocks and employed successfully in pharmaceutical applications,. has been related to hard-sphere crystallization and is now at£racting considerable at£ention in terms of fundamental studies. . Significant advances have been made in the past year concerning the identification (in binary copolymer-water and in ternary copolymer-water-oil systems) of different lyotropic liquid crystalline morphologies, the delineation of the composition-temperature ranges where they occur, and their structural characterization using primarily small angle scat£ering techniques. The self-assembly of PEOjPPO block copolymers in the presence of selecti\'e solvents is related to the self-assembly of surfactants in solution and to the self-assembly of bulk block copolymers, howe\'er, it exhibits a number of features which distinguish it. PEO/PPO block copolymers are unique in that both water-in-oil and oil-in-water structures can be formed in equilibrium by the same copolymer at the same temperature, in the presence of varying amounts of two solvents, selecti\'e for each block. In the topic of PEO/PPO block copolymer adsorption at surfaces and at interfaces, new evidence on the structure of the adsorbed layers and on the time-dependency of the surface activity, rcspccti\'ely has been presentcd.

Note added in proof The Author would like to indicate references [6-8,47,5 7,70,94] as being of 'special interest', but could not highlight them

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in the text as such because of the restrictions of the style of the journal. The following papers have appeared in the literature since the completion of this review: [109-117].

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • ••

of special interest of outstanding interest

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Alexandridis P, Hatton TA: Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces: thermodynamics, structure, dynamics, and modeling. Colloids Surf A 1995, 96:146.

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Alexandridis P, Hatton TA: Association of pluronic polyols in aqueous solutions. Chapter 12. In Dynamic Properties of Interfaces and Association Structures. Edited by Pillai V, Shah DO. Champaign, IL: AOes Press; 1996:231-262.

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Alexandridis P, Linse P, Hatton TA: Micellization of poly(ethylene oxide)/poly(propylene oxide) amphiphilic block copolymers in aqueous solutions: experiments and modeling. In Amphiphilic Block Copolymers: Self-Assembly and Applications. Edited by Alexandridis P, Lindman B. Amsterdam: Elsevier Science BV; 1997.

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Alexandridis P, Olsson U, Lindman B: Phase behavior and structure of amphiphilic block copolymers in mixtures with water and oil. In Amphiphilic Block Copolymers: Self-Assembly and Applications. Edited by A1exandridis P, Lindman B. Amsterdam: Elsevier Science BV; 1997.

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Chu B, Zhou Z: Physical chemistry of polyoxyalkylenene block copolymers. Surf Sci Ser 1996, 60:67-144. A recent review on the micellization of PEO/PPO block copolymers in water. Physical methods used for the characterization of block copolymers in solution as well as CMC, micelle size, and surface tension data for a number of copolymers are organized in a tabular form, facilitating a quick overview of the activities in the area of PEO-PPO-PEO copolymers, but also exposing the discrepancies among some of the published CMC and surface tension values.

Malmsten M: Block copolymers in pharmaceutics; In Amphiphilic Block Copolymers: Self'Assembly and Applications. Edited by Alexandridis P, Lindman B. Amsterdam: Elsevier Science BV; 1997. Overview of the many applications that PEO/PPO block copolymers (selfassembled in micelles or adsorbed at interfaces) find in the pharmaceuticalbiomedical field.

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Poly(ethylene oxide)/poly(propylene oxide) block copolymer surfactants Alexandridis

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89_ •

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