AbbreviationsAB Diblock copolymer composed of an A and a B blockABA Symmetric triblock copolymer composed of
A and B blocks where the A block is solublein the relevant solvent ~.
Symmetric triblock copolymer composed ofA and B blocks where the A block is solublein the relevant solventcopolymer concentrationpolylethyleneoxide) with m monomersscattering intensitypoly(butylene oxide)poly(dimethyl siloxane)poly(ethylene)poly(ethylene butylene)poly(ethyl ethylene)poly(ethylene oxide)Poly(ethylene propylene)poly(isoprene)poly(isobutylene)poly(propylene oxide) with n monomerspoly(propylene oxide)poly(styrene)scattering momentum transfermicellar volume fractioncritical micellar volume fraction for hard-sphere crystallization
IntroductionThe physical properties of amphiphilic macromoleculesconstitute a rich topic which, in recent years, hasattracted interests within both applied and basic science[1-3,4---7"]. Currently, the field is extremely active, and aconcise review of recent progress can include only limitedaspects of the recent results.
When block copolymers are mixed in a solvent whichdissolves only one of the blocks, the molecules self-associate into specific structures to avoid direct contact
between solvent and the blocks which are insoluble.This self-association gives rise to a wide range ofphase behavior, including the formation of micellesof various forms and sizes, complexly structured microernulsions, and liquid crystalline phases. A variety ofblock copolymers, including blocks of polytstyrene) (PS),poly(isoprene) (PI) poly(ethylene) (PE), poly(ethylenepropylene) (PEP), and poly(ethylene butylene) (PEB),have been studied in this context when dissolved inselective organic solvents. In aqueous solutions, blockcopolymers based on polytethylene oxide) (PEO), as thewater-soluble block have been investigated to a largeextent. The insoluble or less-soluble blocks have, for example, been poly(propylene oxide) (PPO), poly(dimethylsiloxane) (PD:t\IS), poly(butylene oxide) (PBO), PS, andpoly(isobutylene) (PIB). In this paper I will review some ofthe recent progress on complex block copolymer systems,where the main focus has been on structural studies basedon small-angle scattering of X-rays and neutrons.
Block copolymer self-association into micellaraggregatesIt is well established that a variety of block copolymers ofAB or ABA type form micelles in solvents, which are thermodynamically good for the A block and precipitants forthe B block. Such micelles constitute a liquid dispersionof hard-sphere interacting units. BAB copolymers may alsoform individual micelles, but this implies that all polymerchains start and end in the same micellar core having themiddle A-block dispersed into the liquid. It is more likelythat such micelles form interconnected networks, wherecores are connected by the soluble A-polymer block, asshown schematically in Figure 1.
Critical micellization temperature andconcentrationIn general, micellization of block copolymers assumes anequilibrium between molecularly dispersed copolymers(unimers) and multimolecular aggregates (micelles). Thethermodynamic approach for describing the aggregation
.process has been calculated based on lattice modelswith the mean-field Flory-Huggins type of segmentalinteractions [8,9-].
Ideal model systems for studying the rnicellization processand micellar interactions are aqueous systems of blockcopolymers composed of PEO with either PPO or PBO,because at low temperatures all these polymers arehydrophilic, but at higher temperatures PPO and PBObecome hydrophobic. At low temperature aqueous solutions of PEO-PPO-PEO and PEO-PBO-PEO thereforeappear as unimers. Structural studies based on scattering [5--] and IH-Nt-.IR relaxation [10] indicate that thePEO-PPO-PEO unimers resemble unimolecular micelles
Structural properties of self-assembled polymeric micelles Mortensen 13
Figure 1
)
Schematic representation of spherical micelles. Left: micelles of AB- or ABA·type of block copolymers, resulting in independent hard-sphereinteracting aggregates. Middle: micelles of BAB·type block copolymers, with relative short middle-block chains, resulting in domains ofinterconneced networks of spherical aggregates. Right: micelles of BAB·type with large, flexible middle-blocks, resulting in a network extendingover the whole sample volume, thus providing a macroscopic gel. Note that the micellar density is the same in the three examples.
where the PPO blocks have a more compact structure thanthat of chains obeying Gaussian conformation.
Figure 2
Current Opinion in Colloid & Interface Science
Pluronic PB5
Temperature versus polymer-concentration contour plot showingthe experimental micellar volume fraction, $, of aqueous solutions ofE025P040E025 (reproduced with permission from [5 0 0
]) . The solidline represents $=0.53 and separates the micellar liquid (regimes IIand III) and the cubic ordered phase (IV). The broken lines are guidesseparating these characteristic regimes: I, unimers; II, unimers andmicelles and III, spherical micelles.
0.12
MicellarVolumeFraction
0.560.53
0.48
0.420.360.300.240.18
IV
40.
Q:~ 30.:J
f!ell0. 20.EellI-
10.
At low temperatures and concentrations (regime I), allpolymers are dissolved as unimers. Above a line of criticalmicellization temperatures and concentrations a regime ofcoexisting micelles and unimers appears (regime II). Thedispersion is totally dominated by micelles in regime III.In regime IV ¢ reaches a critical limit (G>e) of the order of ¢e
One of the important parameters obtained from the~'
experimental scattering data [17] and other indirecttechniques [18] is the micellar volume fraction, ¢ [17].For example, in Figure Z a contour plot of ¢ dataof EOZSPO.IOEOzs (Pluronic 8S® from BASF) is shown[S--,19]. The phase diagram for EOmPOnEOm looks thesame for different values of this material; only the specificvalues of transition temperature (Teml) and concentrationchange. The variation in ¢ separates into four regimes.
The temperature-induced change in hydrophobicity leadsto a temperature above which micelles are formed witha core dominated by PPO (or PBO) and surrounded bya corona of hydrated PEO subcH'ains. Model calculationshave described this entropy driven micellization process[8,9-,11). Equivalent temperature-induced micellizationhas been studied in PS-PI-PS. triblock copolymer systemsin dibutyl phthalate, which changes from a good to a badsolvent for PS when the temperature is reduced [IZ-).
Depending on the molecular architecture and the interaction parameters, various micellar forms can appear,such as spherical, rod-like and discoid shapes. In theEOmPOnEOm systems, changes from spherical to rod-likeand discoid shapes can be followed by changing temperature or concentration. For high copolymer concentrations,corresponding transitions from cubic to hexagonal andlamellar liquid crystalline phases appear. In many ofthe EOmPOnEOm systems all three classical phases arepresent, but with reduced PEO size, first the sphericalmicelles and successively the rod-like and disc-likemicelles vanishes [13,14-,IS-). eventually leading to abicontinuous microemulsion [16].
F127 in 0 20o 1%t. 2%+ 5%
14 Experimental self-assembly
Micellar size and aggregation numberThe micellar core radius of EOmPOnEOm copolymersis roughly independent of copolymer concentration, butshows temperature dependence reflecting changes inaggregation number [5°°,13]. The change in micellarradius for different EOmPOnEOm copolymers shows verysimilar characteristics. The core size follows an empiricalscaling relation relative to the reduced temperature: Rc-(T-Tcm l )O.2. The aggregation number, Nagg, can, withsimilar results, be calculated independently both fromthe core dimensions, and based on the limiting volumefraction in regime III [17].
Scattering from spherical micellesThe scattering functions (I[q], where I is the scatteringintensity and q is the scattering momentum transfer) of:E0 99P0 6SE0 99 micelles for various copolymer concentrations (cs) are shown in Figure 3 [20]. The characteristics ofthe scattering functions are the 'concentration dependentcorrelation hole, in other words reduced intensity atlow q, the side maximum near q ~O.l A-I and the limiting1.-q-2 behavior at high q values, reflecting to first orderthe intermicellar correlations, the micellar core and thedispersed PEO chains respectively.
Figure 3
r-I 10 4 r-----,---------,-------,IEo
L-.>
co
:;:io~.c: 10 2
Q)UCoU
<, 10'$"00cQ).....,c 10°'--__---1. -'- -'
10-2
Scattering vector
Current Opinion in Colloid & Interlace Science
Example of scattering functions. I(q). of different concentrationsof spherical ABA triblock copolymer micelles (E099P06SEOgg)obtained at T=3S"C (reprinted with permission from (20)). Thesolid line represents the best frt to the analytical scattering functionexpressed by the micellar form factor [21·] and the structure factorwith hard-sphere interacting spheres (22].
The micellar form factor can be expressed analytically,assuming a dense core and Gaussian chains in the
corona [21°]. The solid lines in Figure 3. represent bestfits to this formula, including smearing due to limited instrumental resolution and intermicellar correlationsbased on hard-sphere interactions in the Percus-Yevickapproximations (discussed in [22]).
The spherical micellar conformations have been confirmedby direct imaging, using cryo-rransrnission electron microscopy [20]. The micellar characteristics as obtainedfrom small-angle neutron scattering may also be comparedto the micellar hydrodynamic radius, Rh' as obtained usingdynamic light scattering [23,24]. Generally, the micellarhydrodynamic radius is larger than the core and smallerthan the interaction radius.
Rod-, worm- (thread) and disc-like micellesDepending on the block copolymer design and the specificinteraction parameters between solvent and polymerblocks, micellar shapes other than spherical aggregatesmay form. In the EOmPOnEOm copolymers it is thermodynamically possible to follow a transition from spheres torods and discs by changing the temperature and/or the sizeof the PEO blocks. Aqueous solutions of PEO-PPO-PEOshow, at elevated ' temperatures, a form transformationfrom spherical to rod-like micelles [5°°,13]. The origin ofthe sphere-to-rod transition is related to the size of thespherical aggregates. Close to the sphere-to-rod transitionthe core radius is large relative to the polymer backbone,resulting in either highly stretched PPO chains, or majormixing of PEO and PPO inside the core . Both possibilitieslead to costs in free energy, the former result beingentropically costly and the latter causing an increase inchemical potential, leading to the sphere-to-rod micellarshape transformation [5°°,13].
In a recent study of a closely related block copolymer micellar system, consisting of PEO-PIB-PEO, coexistenceof spherical and thread-like micelles was observed in acombined neutron scattering and cryo-electron microscopystudy [25°].
At even higher temperatures disc-like micelles appear.Similar sequences in micellar form has been observedin micelles of low-molar mass glycol dodecyl ether, inwhich the shape is governed by changes in spontaneouscurvature [26]. The changes in thermodynamic interaction parameters of aqueous PEO-PPO-PEO systemscan equally be attributed to changes in an effectivespontaneous' curvature determining the shape of themicellar aggregates. In PEO-PPO-PEO block copolymersystems with only small hydrophilic PEO blocks, thespherical aggregates are not stable in any conditions.This is the case for the E06P06ZE06 system, where themicellar phase consists of disc-shaped aggregate [16].
Micelles with a crystalline coreA special class of disc-like micelles is based on crystallineblock copolymers. Richter et 01. [27°°] and Lin and Gast
Structural properties of self-assembled polymeric micelles Mortensen 15
[28··] have shown that, in dilute solutions, semicrystallinediblock copolymers may form thin platelet structuresconsisting of chain-folded crystalline domain betweensolvated layers of the amorphous, tethered block chains.Examples of such systems are PE-PEP suspended indecane and PEO-PS in cyclopentane. The tetheredchains make up a brush structure with a parabolicdensity profile, supporting self-consistent field theories.The core thickness is determined as being a compromisebetween the entropic contribution from brush stretchingand the enthalpic input from crystalline chain folding.The large surface. area enables the weak van derWaals interaction between the platelets to overcomethe rranslarional''entropy, thus giving rise to needle-formmacro-aggregates [27··].
MiCelles with a glassy coreA number of micelles are composed of block copolymersin which the insoluble block is a glass at relevanttemperatures. Any dynamics involving molecules jumpingfrom one micelle to another or involving micellar shapetransformations are frozen out. Block copolymers of PSare examples of such systems. Mixing PS-PEO blockcopolymers with water at ambient temperatures leads tolarge plate or rod-like micelles present in the originallamellar sheets of the bulk PS-PEO. Only when annealedabove the glass transition of the PS block do the micellesrelax to the spherical equilibrium structure ([29·]; KMortensen, unpublished data). In a triblock copolymer of
the BAB type, the glassy cores have a particularly stronginfluence, as they make up a permanent physical networkstructure, as discussed further below.
Cubic phase of spherical ABA and AB blockcopolymer micellesIn the contour plot of G> shown in Figure 2, we see thatthe limiting value of the saturation point is of the order ofG>c =0.53. When this <i>c border line is crossed the micellarliquid undergoes a first order phase transition to a cubiccrystal [30] with the elastic shear modulus of the orderof lOLlOSPa [23,24]. The liquid and crystalline domainscoexist in the range of G>c=0.47-0.S3 [31], in agreementwith simple hard sphere crystallization [32].
Shear has a marked effect on the crystalline texture, asit aligns the polycrystalline powder into one macroscopicmonodomain. Both bee [5··,19) and fcc [33·] phases havebeen reported in the EOmPOnEOm and related micellarsystems [34], depending on the detailed micellar structure.The shear dependence on the EOmPOnEOm cubic phaseseems to depend on the specific material used. While anearlier study showed only minor shear dependence [5"],more recent experiments have shown shear thinning andstructural dislocation ([35·,36]; C Glinka el 0/., personalcommunication) in analogy to the results of Gast andco-workers on PS-PI [34]. The local crystalline latticeundergoes a deformation which eventually develops into abee twin structure. At higher shear rates, loss of long-rangeorder is observed which is associated with shear melting.
Figure 4
Pluronic F88 Cubic-BCC
(a) [111] (b) [110]
0.1(}-y-------:----,
0.10-0.05 0.00 0.05
Qxy (l/A)
0.10-0.05 0.00 0.05ax (l/A)
-0.1o-t--r--r--.-r--T--,-~-.-r__r__r-i
-0.10
Current Opinion in Coiloid & Interface Science
Two-dimensional scattering pattern of E096P039E096, as obtained (a) with the shear axis parallel to the beam, and (b) when the sample isrotated by 35' around the vertical axis.
16 Experimental self-assembly
Figure 5
PS-PEP-PS 18% sol tion
flexiblilty of the midblock chain, and may vary froma few percents, as observed for PS-PEB-PS (Kraton)in oil [40,41] to 50% as observed in PPO-PEO-PPOReverse-Pluronics in water. In P015E0156P015 at lowtemperatures, a cubic ordered structure is observed in theisotropic phase [42]. Equivalent results were recently obtained for a variety of polylbutylene oxidej-polytethyleneoxide), BOnEOmBOn, based system [43,44,45].
, 00% deformat ion
-0.010
~ 0.010
0.030 ........--- - -----..,
No
more recent experiments have shown shear thinning andstructural dislocation ([35-,36]; C Glinka et 01., personalcommunication) in analogy to the results of Gast andco-workers on PS-PI [34]. The local crystalline latticeundergoes a deformation which eventually develops into abee twin structure. At higher shear rates, loss of long-rangeorder is observed which is associated with shear melting.
Single crystal crystallographyDiat et 01. [37-] have studied the influence of shearand found that by applying an oscillating shear of strainamplitude- less than unity, twinned free single crystals ofmillimeter scale can be obtained. The monodomain cubicphase of EOmPOnEOm micelles has, typically, a mosaicityof the order of 10· [5--]. With shear-oriented crystals,it is possible to perform crystallographic studies and toindex the observed Bragg reflections. For example, thetwo-dimensional scattering pattern of E096P039E096 isshown in Figure 4. The scattering pattern is in agreementwith a bee lattice, as indicated by the associated Miller
. indices. When the limit of high temperature and/or highpolymer concentration is reached, the cubic phase of theEOmPOnEOm micelles melts near the transition fromspherical to rod-like form.
relaxed
50% deformation
0.030·0,010 0,010
ax (1/A)
-0.030
-0.030
0.010
·0.030 -t-"""T'"-.--,---,- .--,--.,.--..,.--j
0.030 ........- - - ---- ---,
0.010
-0.030 -t-"""T'"-.--,---,---,.--,--.,.--..,.--j
0.030 -,--- - - - - - - ----,
~....o
-0.010
[No
-0.010
Nematic phases of micellesBoth the rod-like and the disc-like micelles form potentially nematic phases above certain concentration limits.It has been shown that in steady shear rod-like andworm-like micelles align to various degrees dependingon polymer concentration and shear rate. The degreeof alignment can be quantified by the ratio betweenscattered intensity parallel and perpendicular to the shear.For concentrations above 15-20%, the EOmPOnEOmsystems form hexagonal rod structures [5--,19]. Disc andplatelet micelles, like the crystalline micelles discussedabove, also form nematic phases with near lamellar-likestructures [27--].
BAB block copolymer architectureThe phase behavior of a triblock copolymer of BAB typein a solvent selective for the midblock (polymer A) resultsin quite different structures relative to the AB and ABAtypes. These polymers may also form micellar aggregates.It is frequently observed, however, that systems of dilutepolymer concentration.rather than miclles, make up loosestructures of less well defined associates [7--,38,39].
In the micellar phase, the A-midblocks of the', BABcopolymers form either loops or bridges between micelles.The intermicellar bridging gives rise to clusters of highlyinterconnected micelles. Such behavior has been reportedfor aqueous POISEO\S6PO\S micellar systems where anexcess of water is present at up to 50% copolymer concentration [38]. For a certain copolymer concentration, themicellar networks extend over the whole sample volume,thus providing a macroscopically isotropic physical gel.This concentration depends critically on the size and
Current Opinion in Colloid & Interloce Science
Two-dimensional scattering pattern of PS-PEP-PS when (top)stretched 100% (middle) stretched 50% and (bottom) relaxed (57).
The usual Pluronics, EOmPOnEOm, might be of the BABtype if dispersed in nonpolar solvents. Alexandridis et 01,[46,47] and Chu and co-workers [48,49] have studiedthe aggregation behavior of EOmPOnEOm in xyleneand found micelles with the classical cubic, hexagonal
Structural properties of self-assembled polymeric micelles Mortensen 17
in the oil-rich phase has significant influence on themicellization [49]. Ternary systems of both EOmPOnEOmand POnEOmPOn molecular architecture include the bicontinuous gyroid phase [46,51°,52] well known from bothlow molar weight surfactants and bulk block copolymersystems. The gyroid structure has not yet, however, beenobserved in simple aqueous systems of PEO-PPO basedblock copolymers.
Micellar networksDepending on the lifetime of the polymer blocks associated with a given core, BAB material may show a finiteelastic respon~c::~ Systems with glassy micellar cores areideal for such elastomers cross-linked by self-association.A number of studies have focused on the PS type of blockcopolymers, including PS-PI-PS [39,53,54°], PS-PEP-PS[40]' and PS-PEB-PS [41,55°]. Scattering experiments, asweli as electron microscopy imaging, have clearly revealedthe spherical PS cores with, effectively, hard sphereinteractions [41,55°].
The microscopic response to macroscopic deformation ofthe three-dimensional network was studied by neutronscattering. Upon stretching, of up to 100%, additionalcorrelations appear in specific directions, resemblinginduced paracrystalline order, as shown in Figure 5[56]. Further stretching gave rise to an anisotropicpattern of the butterfly type, il\dicating nonhomogeneousconnectivity [57].
Figure 6
With increasing temperature, the structure factor ofPS-PEB-PS systems becomes clearly more pronounced,revealing increased effective volume fraction. For polymerconcentrations of 15% or more, this causes ordering aboveT",55°C [5So]. At even higher 6temperatures, the structurefactor becomes even broader, showing that the orderedstructure is stable within a limited temperature windowonly. It has also been shown that ordering into cubic phasesof PS-PI-PS block copolymers can occur [54°].
Shear alignment of BAB copolymer orderednetworksAs in the AB and ABA systems, the BAB orderedgels might align into a monodomain cubic structureupon application of shear [42,59°]. In BAB networks ofPS-PEB-PS micelles, the scattering pattern resembles atwinned bee morphology with lattice constants of the orderof 400 A, as shown in Figure 6. Upon cooling to ambienttemperatures, the characteristic bee pattern remains, eventhough the peaks may broaden. This may reflect thatalthough the bee structure is thermodynamically unstablein this range, the twinned bee structure is frozen becauseof the glassy polytstyrene) micellar cores,
ConclusionsIn this review I have attempted to provide an overview ofrecent experimental studies of block copolymer micellarstructures and mesophases. It is clear that the field isextremely active and a large variety of materials with novel
T-9Odeg.C
0.04 .,.....-~_,....~'=J--,
0.02
:. 0.00
.s.().02
I
0.0' 0Ox (11)
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'().04D
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0.010
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9 0 100 1 1 0 1 2 0Tem p erature rOC]
1 3 0
CurrentDpinOn ., Colloid & Interlace Science
Simultaneous results of scattering pattern (above) and rheology (below) near the cubic order-disorder transition in PS-PEB-PS gels.
18 Experimental self-assembly
properties can be designed based on the basic understandings gained. An example is the polymer networks whereall cross-links are f~rmed by self-association and positionedon a perfect lattice.
References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:
of special interest•• of outstanding interest
1. Chu B: Structure and dynamics of block copolymer colloids.Langmuir 1995, 11:414-421.
2. A1ex~d?idis P, Hatton TA: Poly(ethylene oxide)-poly(propyleneoxide)-poly(ethylene oxide) block copolymer surfactantsin aqueous solutions and at interfaces: thermodynamics.structure and modeling. Colloid Surface 1995, 96:1-46..
•3. Almgren M, Brown W, Hvidt S: Self-aggregation and phasebehavior of poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) block copolymers in aqueous solutions.Colloid Polymer Sci 1995, 273:2-15.
.4. Gast AP: Structure, interactions and dynamics of tetheredchains systems. Langmuir 1996, 12:4060-4067.
Gast gives an excellentreview of the use of self-associatedblock copolymermicelles in studying the properties of chains tethered to more or less curvedinterfaces.The experimental data are compared to self-consistentfield theoryand liquid state theory, Furthermore, Gast gives a review of recent studiesrelating structure and rheology.
5. Mortensen K: Structural studies of PEO-PPO-PEO triblock•• copolymers, their micellar aggregates and mesophases; a
The paper gives an extendedreview of the phase bahaviorof aqueous blockcopolymer micellar systems, inclu~ing micellization, micellar form transfermalion and micellar networks. The discussion is based on scattering exper-iments. 0
6. AlexandridisP: Amphiphilic copolymers and their applications.o CUff Opin Colloid Interface Sci 1996, 1:490-501.
A1exandridis presents recent progress in the fundamental understanding ofself-organization of amphiphilic copolymers at interfaces and in solutions. Inaddition, new applications are discussed.
7. Chu B, Liu TB, Wu CH, Zhou ZK, Nace VM: Structures andproperties of block copolymers in solution. Macromol Symp1997,118:221-227.
Chu and co-workers give a review of recent understanding of the structuersand properties of block copolymers in solution. The dependence on molecular architecture is discussed, and both micellar aggregates and more openstructures are presented.
8. Linse P: Micellization of poly(ethylene oxide)'poly(propyleneoxide) block copolymers In aqueous solutions. Macromolecules1993,26:4437-4449.
9. Noolandi J, Shi AC, Linse P: Theory of phase behavior ofpoly(oxyethylene)-poly(oxypropylene)'poly(oxyethylene)triblock copolymers in aqueous solutions. Macromolecules1996,29:5907-5919.
Noolandi, Shi and Linse have continued the former mean-field studies byLinse to include continuum theory in order to learn about the associationbehaviorof triblock copolymers in aqueous solutions.The theories are foundto give good agreement for the ordered structures of different Pluronics.
10. Cau F,Lacelle S: lH NMR relaxation studies of the micellizationof a poly(ethylene oxide)'poly(propylene oxide)-polY(ethyleneoxide) triblock copolymer in aqueous solution. Macromolecules1996,29:170-178.
11. A1exandridis P. Holzwarth JF, Hatton TA: Micellization ofpoly(ethylene oxide)-poly(propylene oxide)-poly(ethyleneoxide) triblock copolymers in aqueous solutions thermodynamics of copolymer association. Macromolecules1994, 27:2414-2425.
12. Lodge TP,Xu X, Ruy CY, Hamley WI, Fairclough JPA, Ryan N,Pedersen JS: Structure and dynamics of concentrated solutionsof asymmetric block copolymers in slightly selective solvents.Macromolecules 1996, 29:5955·5964.
Lodge and co-workers have made a detailed study on the structural anddynamic properties of a styrene-isoprene-styrerie triblock copolymer of var-
ious sizes in a selective solvent good for polystyrene.In dilute solution, thetriblock copolymer forms elongated micelles.
13. Mortensen K, Biown W: Poly(ethylene oxide)'poly(propyleneoxide)-poly(ethylene oxide) triblock copolymer in aqueoussolution. The influence on relative block size. Macromolecules1993, 26:4128-4135.
14. Alexandridis P, Zhou DL, Khan A: Lyotropic liquid crystallinityin amphiphilic block copolymers: temperature effects onphase bahevior and structure for poly(ethylene oxldelb-poly(propylene oxide)-b'poly(ethylene oxide) triblockcopolymers of different composition. Langmuir 1996, 12:26902700.
In this work, Alexandridis et a/. compare the micellar structure and thelyotropic liquid crystalline phases for the PEO-PPO-PEO type of blockcopolymers with different molecular sizes.
15. Yang YW, AliAdib Z, McKeown NB, RyanN, Attwood 0, Booth C:Effect of block architecture on the gelation of aqueoussolutions of oxyethylene!oxybutylene block copolymers.Langmuir 1997, 13:1860-1861.
The effect of block sizes of PEO-PBO-PEO copolymers are discusssed withrelation to aggregation and gelation behavior in aqueous suspensions.
16. Hecht E, Mortensen K, Hoffmann H: L3·phase In a binary blockcopolymer-water system. Macromolecules 1995, 28:5456-5476.
17. Mortensen K, Pedersen JS: Structural study on the micelleformation of poly(ethylene oxide)-poly(propylene oxlde)poly(ethylene oxide) triblock copolymers in aqueous solution.Macromolecules 1993, 26:605-812.
18. A1exandridis P, Nivaggioli T, Hatton TA: Temperature effects onstructural properties of pluronic Pl04 and Fl08 PEO-PPO-PEOblock copolymer solutions. Langmuir 1995, 11:1468-1476.
19. Mortensen K: Phase behavior of poly(ethylene exlde)poly(propylene oxide)-poly(ethylene oxide) triblock copolymerdissolved in water. Europhys Lett 1992, 19:599-604.
20. Mortensen K, Yeshayahu T: Cryo-TEM and SANS microstructuralstudy of pluronic polymer solution. Macromolecules 1995,28:8829-8834.
21. Pedersen JS, Gerstenberg M: Scattering form factor of blockcopolymer micelles. Macromolecules 1996, 29:1363-1365.
Pedersen and Gerstenberg present an analytical model for the scatteringfunction of block copolymer micelles.The model, which is in very good aggreementwith experimentaldata, gives detailed insight into the micellar coreand corona
23. Brown W, Schillen K, Almgren M, Hvidt S, Bahadur P: Micelle andgel formation in a poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) triblock copolymer in water solution.Dynamic and static light scattering and oscillatory shearmeasurements. J Phys Chem 1991, 95:1850-1858.
24. Wanka G, Hoffmann H, Ulbricht W: The aggregation behaviorof poly(oxyethylene)'polY(oxypropylene)'poly(oxyethylene)block copolymers in aqueous solution. Colloid Polym Sci 1990,268:101-117.
25. Mortensen K, Yeshayahu T, Gao B, Kopps J: Structural propertiesof bulk and aqueous systems of PEO-PIB·PEO triblockcopolymer as studied by srnall-anale neutron scattering andcryo-transmission electron microscopy. Macromolecules 1997,30:6764-6770.
The new block copolymer systemPEO-PIB-PEOis an alternativeto the usualpluronics for aqueous suspensions of micelles with promising properties. Amain finding of the studies of the aqueous solutions is the coexistence ofextended thread-like like and spheric micelles.
26. Leaver MS, Olsson U, Wennerstrom H, Strey R, Wurz U: Phasebehaviour and structure in an non-ionic surfactant-oil-watermixture. J Chem Soc Faraday Trans 1995, 91:4269-4274.
27. Richter 0, Schneiders 0, Monkenbusch M, Willner L,•• Fetters U, Huang JS, Lin M, Mortensen K, Farago B: Polymer
aggregates with crystalline cores: the system polyethylenepoly(ethylenepropylene). MacromOlecules 1997, 30:1053·1066.
Richter and co-workers present an extended study of the micellar propertiesof a system with crystalline core. Moreover. it is shown that van der Waalinteractions cause aggregation due to the very large plate-like micelles.
Structural properties of self-assembled polymeric micelles Mortensen 19
Lin and Gast give a presentation on semicrystalline diblock copolymer micelles. The amorphous block is attached to the crystallinefold surface, forming a model system of tethered chains at a flat interface. Lin and Gast useboth self-consistent mean field and scattering experimentsto study the polymeric and micellar structure.
29. Mortensen K, Brown W, Almdal K, AJami E, Jada A: Structure ofPS-PEO diblock copolymers in solution and the bulk stateprobed using dynamic light and small-angle neutron scatteringand dynamic mechanical measurements. Langmuir 1997,13:3635-3645.
Combined neutron and light scattering and rheological measurementsgivedetailed informationon the aggregation behaviorof block copolymer micellesin the melt, as well as in solution.
30. Mortensen K, Brown W, Norden B: Inverse melting transitionand evidence of three-dimensional cubatic structure in ablock-copolymer micellar system. Phys Rev Lett 1992, 68:23402343.
33. Berrett JF, Molino F, Porte G, Diat 0, Lindner P: The shearinduced transition between oriented textures and layer-slidingmediated flows in a micellar cubic crystal. J Phys CondensMatter 1996, 8:9513·9517. .
Berret and co-workers present experimental data on the shear-related texture in a lyotropic crystalline micellar system. The good resolution of theX-raytechnique enables the authors to study in detail the transition betweenshearing flows dominated by oriented textures at low shear rates and flowsmediated by the mechanisms of layer sliding at higher rates.
34. McConnell GA, Lin MY, Gast AP: long range order in polymericmicelles under steady shear. Macromolecules 1995, 28:67546764.
35. PrudhommeRK, Wu GW, Schneider OK: Structure and rheologystudies of poly(oxyethylene-oxypropylene-oxyethylene)aqueous solution. Langmuir 1996,12:4651-4659.
The cubic phase of Pluronics are studied near the cubic ordering phase.The ordered phase is characterized as Having low-yield stresses, high zeroshear viscosities and shear thinning. Near the gel phase boundary the solutions are non-Newtonian (shear thinning), and scattering experimentsshowsc?existing liquid and cubic phases.
36. Molino F,Berret JF, Porte G, Diat 0, Lindner P: Influence of flowmechanism for a soft crystal. J Phys 1/1997, in press..,
37. Diat 0, Porte G, Berret JF: Orientation and twins'separation in a• micellar cubic crystal under oscillating shear. Phys Rev B 1996,
54:14869-14872.Diat and co-workers have made very nice high resolution studies on orderedblock copolymer micellar mesophases.
38. Mortensen K, Brown W, Jergensen E: Phase behavior ofpoly(propylene oxide)-poly(ethylene oxide)-poly(propyJeneoxide) triblock copolymer melts and in aqueous solutions.Macromolecules 1994, 27:5654·5666.
39. Lairez 0, Adam M, Raspaud E, Carton JP,Bouchaud JP: Triblockcopolymers in a selective solvent 1. Aggregation process indilute solutions. Macromolecules 1994, 27:2956-2964.
40. Mischenko N, Reynders K, Koch M, Mortensen K, Pedersen JS,Fontain F,Graulus R, Reynaers H: X-ray and neutronscattering from bulk and oriented triblock copolymer gels.Macromolecules 1995, 28:2054-2062.
42. Mortensen K: Cubic phase in a connected micellar network ofpoly(propylene oxide)-poly(ethylene oxide)-poly(propyleneoxide) triblock copolymers in water. Macromolecules 1997,30:503·507.
43. Zhou ZK, Chu B, Nace VM: Association behavior of a tribJockcopolymer of oxythylene (E) and oxybutylene (B). A study ofB(5)E(91)B(5) in aqueous solution. Langmuir 1996, 12:50165021.
44. Zhou ZK, Chu B, Nace VM, Yang YW, Booth C: Selfassembly characteristics of BEB-type triblock copolymers.Macromolecules 1996, 29:3663-3664.
45. Yang YW, Yang Z, Zhou ZK. Attwood 0, Booth C: Associationof triblock copolymers of ethylene oxide and butyleneoxide in aqueous solution, A study of BnEmBn copolymers.Macromolecules 1996, 29:670-680.
46. Alexandridis P. Olsson U, Lindman B: Self'assembly ofamphphilic block copolymers: the (EO)13(POho(EO)13-waterp-xylene system. Macromolecules 1995,28:7700-7710.
47. Alexandridis P, Olsson U, Lindman B: A reverse micellar cubicphase Langmuir 1996, 12:1419-1422.
48. Wu GW, Liu LZ, Buu VB, Chu B, Schneider OK: SANS and SAXSstudies of pluronic l64 in concentrated solution. Physica A1996,231:73-81.
49. Chu B, Wu GW: Supra molecular formation of triblockcopolymers in polar nonpolar solvents. Macromol Symp 1994,87:55-67.
50. AJexandridis P, Olsson U, Lindman B: Structural polymorphism of•• amphiphilic copolymers: six lyotropic liquid crystalline and two
solution phases in a poly(oxybutylene)·b·poly-(oxyethylene)·water-xylene system. Langmuir 1997, 13:23-34.
AJexandridis and co-workers present structural findings on a numberof aqueous systems of triblock copolymers and their crystalline mesophases. Thepaper gives an excellent review of the various lyotropic liquid crystallinephases, including not only the classical cubic, hexagonal and lamellar, butalso the bicontinuous gyroid phase.
The rich phase behavior of reverse Pluronics in water/oil mixtures are presented. The experimental conclusions are based on both sactterinq and NMRdata.
52. Holmqvist P, AJexandridis p. Lindman B: Comparison of .poly(ethylene oxide) poly(propylene oxide) to poly(ethyleneoxide) poly(tetrahydrofuran) coploymers Langmuir 1997,13:2471·2479.
53. Lairez 0, Adam M, Raspaud E, Carton JP, Bouchaud JP: Triblockcopolymers in a selective solvent: dilute and semidilutesolutions. Macromol Symp 1995, 90:203-229.
54. Raspaud E, Lairez 0, Adam M, Carton JP: Triblock copolymersin a selective solvent 2. Semidilute solutions. Macromolecules1996, 29:1269·1277.
The paper gives a very nice presentation on the association properties andgel formation of triblock copolymers in solvent selective for the middle block.The authors also find networks of cubic organized micelles.
The cubic structure of micellarnetworks opens up new kind of materialswithpromising properties. The work by Kleppinger and co-workers discusses thetemperature and concentration dependence window where such structuresare found.
In a combined rheology and scattering experiment, important new insightsare gained into the understanding and characterizationof SEBS-micellar gelstructures.
