Langmuir 1994,10,4374-4379
4374
Shear Orientation of a Micellar Hexagonal Liquid Crystalline Phase: A Rheo-Small Angle Light Scattering Study Walter Richtering,*Jorg Lauger, and Reinhard Linemann Universitat Freiburg, Institut f i r Makromolekulare Chemie und Freiburger Materialforschungszentrum, Stefan-Meier-Strasse 31,D- 79104 Freiburg, Germany Received June 13, 1994. In Final Form: September 5, 1994@ The hexagonal phase, HI, of aqueous mixtures of a nonionic surfactant was investigated by combined rheological and small angle light scattering experiments. The isotropic to hexagonal phase transition could be detected by changes in polarized and depolarized scattering as well as by elastic properties of the lyotropic liquid crystalline phase. Shear orientation was observed under different rheological conditions namely constant stress or constant rate. An orientation correlation perpendicular to the flow direction was observed at short creep times or low shear rates. After long creep times or at high shear rates, an orientation parallel to the direction of flow was reached. After cessation of flow only little structure relaxation was found; i.e. both states oforientationwere very stable. The results are discussedin comparison to the behavior of nematic polymer solutions. The observed orientation perpendicular to the direction of flow seems to be a unique property of the hexagonal phase and might be caused by the high elasticity of this mesophase. rheological and SALS experiments on such solutions have not yet been reported. During the last few years rheo-optical studies have been Spherical micelles are formed in dilute aqueous soludevelopedin order to obtain information on flow properties tions of nonionic surfactants and many studies investiof complex fluids.l Especially small angle light scattering gated whether size and shape of the micelles are affected (SALS)of samples under shear flow proved to be a very by variation of concentration or temperature. At high successful technique to gain more insight in relationships concentrations various mesophases are formed depending between macroscopic mechanical properties of a system on the molecular structure of the amphiphile. Cubic, and its microscopic structure.2 Several studies investihexagonal, and lamellar phases are observed and phase gated anisotropic phases, in particular solutions of stiff diagrams of many systems are known. Phase separation polymers which form nematic or cholesteric phases at high on heating is observed with most nonionic surfactants." c~ncentration.~,~ In such systems the macromolecules can In this contribution we present the first results of a be aligned by applying a shear flow and the orientation rheo-optical investigation of a hexagonal lyotropic liquid process can be followed by detecting the scattered light. crystalline phase. The system consists of a nonionic Recently Takebe and co-workers summarized experimensurfactant with a branched headgroup, which was recently tal results obtained from solutions of hydroxypropyl developed by Kratzat et a1.12 The chemical structure of cellulose (HPC) in water and poly(y-benzyl L-glutamate) C14C;(EM4)2is: H3C(CE2)13OCH[CH2O(CH2CH20)4CH312. (PBLG) in m - c r e ~ o l . ~ The lower critical solution temperature of C14C;(EM4)2was The rheological behavior of stiff wormlike micelles has found to be LCST = 53 "C. also been studied in the last years, mostly solutions of There are two features of the branched C14G(EM4)2 cationic surfactants.6 Shear induced transitions were surfactant due to which it is a model system for rheological observed in concentrated solutions at defined salt content, indicating that the micellar size is affected by the f l ~ w . ~ , ~studies. (i)Berend et al. showed that in dilute solution spherical micelles of 3 nm in radius are formed which do Recently a shear induced isotropic-to-nematic phase not grow with increasing temperature.13 (ii)A hexagonal transition was observed by small angle neutron scattering HI phase is formed in the concentration range 50-75 % under ~hear.~JO (w/w) with a clearing temperature of 31.6 "C. This is a Nonionic surfactants are a different class of materials convenient temperature range since evaporation of water forming lyotropic liquid crystalline phases in aqueous will not cause a severe problem during the time required solution. However, to our knowledge, simultaneous for rheological experiments. It is generally accepted that the hexagonal phase Abstract published inAdvance ACSAbstracts, October 15,1994. consists of long rodlike micelles arranged in a hexagonal (1)Larson, R. G.Rheol. Acta 1992, 31,497. (2) Hashimoto, T., Takebe, T., Suehiro, S. Polym. J. 1986,18,123. lattice with a typical distance between micelles of a few (3) Ernst, B.; Navard, P. Macromolecules 1989,22, 1419. nanometers." In macroscopic samples one usually ob(4) Moldenaers, P.; Fuller, G.; Mewis, J . Macromolecules 1989,22, serves a piled polydomain structure, i.e. one has director 960. fluctuations within the material.14 ( 5 ) Takebe, T.; Hashimoto, T.; Ernst, B.; Navard, P.; Stein, R. S. J . Chem. Phys. 1990,92, 1386. The objective of this contribution is a combined SALS (6) Khatory,A.; Lequeux, F.; Kern, F.; Candau, S. J.Langmuir 1993, and rheological study on shear orientation of the hexagonal 9. 1456. (7) Hoffmann, H.; Hofmann, S.;Rauscher, A.; Kalus,J . Prog. Colloid phase. A description of linear viscoelastic properties of
1. Introduction
@
- 7
Polym. Sci. 1991,84, 24. (8)Miinch, C.; Hofhann, H.; Ibel, K.; Kalus, J . ; Neubauer, G.; Schmelzer, U.; Selbach, J . J. Phys. Chem.1993, 97, 4514. (9) Berret, J.-F.; Roux, D. C.; Porte, G.; Lindner, P. Europhys. Lett. 1994, 25, 521. (10) Schmitt, V.; Lequeux, F.; Pousse, A.; Roux, D. Langmuir 1994, 10,955.
(11)Tiddy, G.J. T. Phys. Rep. 1980, 57, 1. (12) Kratzat, K.; Finkelmann, H. Liq. Cryst. 1993,13, 691. (13) Berend, K.; Kratzat, K.; Burchard, W. Submitted for publication. (14) Asada, T.; Muramatsu, H.; Watanabe, R.; Onogi, S. Macromolecules 1980, 13,867.
0 1994 American Chemical Society
Shear Orientation of the Hexagonal Phase
Langmuir, Vol. 10,No. 11, 1994 4375
flow dircctim
B A
B C D E F
M G H I J K
neutral density filter rotating ground glass plate focussing lens (f = 400mm) linear polarizer hi2plate pinholes mirror polarizationanalyzer beamstop shutter quartz glass window CCD-chip
1
Optic
I
HV direction
I Detector
.-
A
HH
+ A
Figure 2. Coordinates and orientation of polarizer and analyzer for HHand Hv scattering.
Figure 1. Schematic diagram of the stress rheometer with integrated small angle light scattering device.
the isotropic, cubic, and hexagonal phases will be given in a separate paper.16
2. Experimental Section Synthesis and purification of the surfactant were performed as described by Kratzat.12 'H-NMR and elemental analysis proved that the material was pure. Aqueous solutions of 60.1% (w/w)were prepared using water deionized by a Millipore system Milli-Q plus and filtered through Millipore 0.45 pm filter. At 60.1 wt % the hexagonal to isotropic phase transition occurs at 26.3 "C. The solutions were always heated above 30 "C before loading the rheometer. For the rheo-opticalexperiments a Bohlin CS Melt rheometer was equipped with an optical setup in order to detect small angle light scattering. The apparatus is shown schematicallyin Figure 1. A detailed description was given by LBuger and Gronski,lG here we only describe its main parts. A linearly polarized 10-mW He-Ne laser ( l o = 633 nm) was used as light source. The polarization direction can be varied with a ,W2 plate. The incidentbeam was reflected by a mirror and passed through a quartz glass window into a self-constructedheating chamber. "he samplewas located between an optical transparent cone and plate configuration made out of quartz glass. A 3", 40 mm cone and plate geometry was used. The scattered light left the heating chamber through a second quartz glass window, passed through a linear polarizer as polarization analyzer and was focused by an especially designed optical device directly on a two dimensionalcharge-coupled device (CCD). The maximun scattering angle was 24" (in air). 3. Results Two different optical systems were chosen for the scattering experiments: (i)the depolarized scattering was detected as Hv,i.e., the primary beam was vertically polarized parallel to the flow direction and the horizontally polarized scattered light was detected; (ii)the polarized scattering was detected as HH, i.e., both the primary and scattered light were polarized horizontally. The coordinate system is shown in Figure 2. 3.1. Constant Stress Experiments. In the first set of experiments creep curves were measured at 25,24, and 20 "C and various shear stresses. In Figure 3 we plotted the compliance J(t) = y(t)/uvs time. u denotes the applied shear stress and y is the measured (time dependent) strain. The creep curve could be divided into four regimes. The vertical lines mark changes of the SALS pattern and of the rheological behavior. I. The first regime coversthe elastic and retarded elastic response of the sample. 11. The second region is characterized by a strong increase of compliance and shear rate. (15)Linemann, R.; Kratzat, IC;Richtering, W. Submitted. (16)Lauger, J.; Gronski, W. Submitted.
I 0
n.
f
10'
Y
3
1 oo
lo-' 10-2
. to-'
Time [s]
Figure 3. Creep curve at 24 "C and u = 100 Pa. Letters b-f indicate the time when the scatteringpictures, shown in Figure 4,were taken.
111. In the third regime the compliance increases more slowly than in region 11. IV. After long creep times a second step of the compliance is observed followed by a steady shear flow. Values of compliance, viscosity, and shear rate in the four regimes are summarized in Table 1. The vicosity decreased with creep time; i.e. the sample showed strong thixotropic flow behavior. The four regions are further characterized by their scattering properties. The depolarized SALS pattern is shown in Figure 4. Figure 4a shows HVscattering of the sample in the isotropic state a t 30 "C; no indications of anisotropic structures were found. After cooling into the hexagonal phase, depolarized scattering is observed with the sample in the quiescent state (Figure 4b). Parts c and d of Figure 4 characterize the Hv scattering in region 11. At the transition I I1 the scattering was no longer symmetrical but was superposed by a n ellipse parallel to the flow direction. Figure 4d shows the well-defined anisotropic intensity distribution, which was observed in region 11. In region I11 two different scattering patterns were observed, either parallel (like in Figure 4d) or perpendicular to the flow direction (Figure 4e). At the end of the creep experiment a steady flow was reached in region IV and scattering perpendicular to the flow direction was observed only. The scattering intensity got localized toward the beam center (Figure 40 and became thinner with increasing time (and shear rate), indicating a growth of domain size. Figure 5 reports a constant stress experiment at 20 "C and the corresponding polarized scattering is shown in
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4376 Langmuir, Vol. 10,No. 11, 1994
Richtering et al.
Table 1. Values of Compliance, Viscosity, and Shear Rata in the Four Regions at Different Temperatures 25 "C
region
complianceJ (Pa-l)
viscosity 17 (Paos)
shear rate dy/dt (s-l)
I 11 111
2 x 10-5 to 0.05 0.05 to 5 5 to 70 70 to 180 4 x 10-5 to 0.02 0.02 to 4 4 to 31 31 to 675 2 x 10-5tOo.01 0.01to 20 20 to 100 100 to 400
14900 to 3910 3910 to 31.9 31.9 to 1.13 1.13 to 0.69 6850 to 2240 2240 to 33.5 33.5to 8.29 8.29 to 0.21 29500 to 3340 3340 to 30.3 30.3 to 5.04 5.04to 4.78
0.006 to 0.03 0.026 to 3.1 3.1 to 88.6 88.6 to 100 0.015 to 0.04 0.04 to 3.0 3.0 to 12.1 12.1 to 471 0.007 to 0.03 0.03 to 3.3 3.3 to 19.9 19.9 to 20.9
Iv 24 "C
I 11 111
Iv 20 "C
I I1 111
Iv ,
1
q = 4 . 2 pm-
,
I
I
a
b
0
1000
2000
3000
4000
5000
Time [s]
r
d
e f Figure 4. Depolarized (Hv) scattering pattern taken simultaneously to the creep curve in Figure 3: (a)at 30 "C, isotropic phase; (b-f) correspond to different creep times at 24 "C (see text).
.Figure 6. The creep curve obeys the same behavior as the measurement at 24 "C. Parts a and b of Figure 6 show HH scattering at 30 "C (isotropic phase) and 20 "C (hexagonalphase, quiescent state), respectively. Polarized scattering in the hexagonal phase is reduced due to the high osmotic modulus. After the elastic response, the scattering intensity increased and the picture became anisotropic (Figure 6c). However, the patterns became symmetricalagain within region I1( Figure 6d), after creep times above the dotted line in Figure 5. In region I11 the scattering intensity increased in direction perpendicular to the flow (Figure 6e) and in region IV scattering was only observed a t very small scattering angles (Figure 60. Apparently, two orientation processes are induced by the shear flow, and both are characterized by anisotropic HVand HHscattering. A variation of shear stress only caused the four regimes to be observed at different creep times. The regions can be described by characteristic values of compliance, viscosity, and shear rate, that are given in Table 1. The elastic modulus Go is given as the
Figure 5. Creep curve at 20 "C and 0 = 100 Pa. Letters b-f indicate the time when the scatteringpictures, shownin Figure 6, were taken. reciprocal spontaneous compliance and Go = 15 000 Pa was obtained. 3.2. Creep Curves with Intermediate Rest Periods. Creep and creep recovery experiments at 20 "C were performed in order to study the relaxation behavior of the shear induced structures. The following procedure was applied: (1)A constant stress of 0 = 100 Pa was applied for 1000 s. After 440 s the firstjump of the compliance was found (see Figure 7) and depolarized scattering pictures characteristic of region I1 were observed, like in Figure 4d. (2) After 1000 s the shear stress was switched off. No creep recovery was observed;however, the (Hv)scattering picture taken immediately after the cessation of shear stress changed slightly; see Figure 8a. Afterward no further change in the scattering intensity was detected (Figure 8b). (3)A shear stress of 0 = 150 Pa was applied for 1400 s, after a rest period of 5 min (see Figure 7). A large compliance was observed immediately after the onset of shear indicating that only very little elastic properties were present in this presheared sample. The scattering patterns obeyed the behavior of region 111, i.e. corresponding to pictures like in parts d and e of Figure 4. Region IV was reached after 400 s and HVpictures like in Figure 8c were observed. (4) After termination of shear stress picture 8d was taken. An immediate change of intensity was observed, but no strain recoverywas found. Afterward the scattering pattern was stable over the period of 20 min. ( 5 ) Finally a third creep experiment with 0 = 250 Pa was added. f i r the onset of shear a scattering picture like Figure 8c was observed which changed again when
Shear Orientation of the Hexagonal Phase ,
q = 4 . 2 pm-1
Langmuir, Vol. 10,No. 11, 1994 4377
q = 4 . 2 pm-'
,
,
I
I
,
I
a
b
C
d
I
a
r
b
C d Figure8. Depolarized (Hv) scatteringpatterns observedduring different creep and relaxation periods. a-d correspond to different times that are indicated in Figure 7; see text. lo* L
1
.
,
. .
.
e-0 Shear
.
,
.
.v.
.
,
.
.
.
,
.
.
.
.
10'
rats 1Oa
e f Figure 6. Polarized (HH)scattering pattern taken simultaneously to the creep curve in Figure 5: (a) at 30 "C, isotropic phase; (b-f) correspond to different creep times at 20 "C (see text).
--
t
lo-*
iI
v
1-3
n
-
0
600
1000
1600
lo-'
2000
time [I]
n c
I
Figure 9. Series of measurements at three different shear rates p = 0.17, 5 , 2 5 s-l. Letters a-c indicate the time, when the scattering pictures, shown in Figure 10, were taken.
0
a U
7
c (2700s)
1 oo
lo-'
0
1000
2000
Time
3000 4000
[SI
Figure 7. Constant stress experiment with intermediate recovery periods. Letters a-d indicate the time, when the scattering pictures, shown in Figure ,8,were taken.
the flow was stopped. The polarized scattering showed the same behavior as the depolarized scattering. 3.3. Experiments at Constant Shear Rate. Additional experiments were performed at constant shear rate. Since the rheometer is stress controlled, flow conditions of constant shear rate could only be accomplished by an appropiate adjustment of the applied shear stress. The software proved to be very successful1 in adjusting the shear stress. A series of measurements at three different shear rates is shown in Figure 9. The correspondingHVSALS patterns are displayed in Figure 10. At low and high values of shear rate, steady shear flow was reached and again orientation of scattering domainswas found. Scatteringparallel and perpendicular
to the direction of flow is observed at p = 0.17 s-l and p = 25 s-l, respectively. The scattering patterns were equivalent to those of regions I1 and IV,but at constant rate the orientations were independent of time. The intermediate shear rate of p = 5 s-l corresponds to region 111. A reorientation occurred under this condition which was connected to shear thinning. Obviously the software algorithm was not sufficiently sophisticated in order to keep a constant target shear rate of p = 5 s-l. The sample showed very complex flow properties and small variations of applied shear stress led to large fluctuations of shear rate which is a macroscopic manifestation of a shear induced reorientation process. 4. Discussion
Rheo-SALS experiments have mostly been applied to nematic lyotropic polymer solution^.^ It has been shown that depolarized (Hv) scattering depends on anisotropy and orientation of the scattering centers, whereas polarized (HH) scattering also depends on fluctuations of polarizability.17J8 Thus HV scattering is determined by orientation fluctuations and HHprobes orientation and density fluctuations. In our surfactant system, no HV (17)Stein, R. S.; Wilson, P. R. J. Appl. Phys. 1962,33,1014. (18)Rhodes, M. B.; Stein, R. S. J. Polym. Sci. A-2 1969,7,1539.
4378 Langmuir, Vol. 10,No.11, 1994
Richtering et al.
1
q = 4.2 pm-
a
b
C
Figure 10. Depolarized (Hv) scattering patterns observed at
different shear rates. a-c correspond to different times that are indicated in Figure 9; see text.
scattering was observed in the isotropic phase. That means that all orientation correlations vanish above the hexagonal to isotropic phase transition. In other words, no pretransitional behavior is found in the isotropic phase. (Pretransitional behavior is known from transitions of isotropic to nematic p h a s e ~ . ~) J The ~ hexagonal phase shows a circular Hv pattern, which demonstrates that the H1 phase has a polydomain structure with isotropic director fluctuation. Scattering techniques probe dimensions in reciprocal space. Therefore a strong scattering perpendicular to the flow direction (region IV)means an orientationcorrelation extendedparallel to the flow direction. In region I1 an orientation correlation perpendicular to the flow is observed. In the following we would like to compare the obtained results with experimental data from nematic lyotropic p o l p e r solutions and with a theoretical work by Larson and Ottinger on orientations in shear flow of liquid crystalline polymers.20 Different lyotropic nematic polymer solutionshave been investigated recently by means of rheo-SALS. In all cases, the solutions were studied in steady-state shear flow at constant shear rate, and a decrease of viscosity with increasing shear rate was observed. The SALS intensity was detected at different shear rates and became distorted due to the shear flow. Scattering perpendicular to the flow direction was observed, which is usually interpreted as an alignment of the rodlike macromolecules in flow direction, indicating the formation of a monodomain. The structure relaxed quickly after' the shear flow was stopped.3,5 The hexagonal lyotropic phase of C14G(EM,& showed the same scatteringpicture after long creep times in region IV. That leads to the conclusion that the anisotropic, rodlike micelles finally get aligned in flow direction after steady shear flow was reached in region IV. However, in contrast to polymer solutions,no structure relaxation was observed after the cessation of shear flow. The stable orientation might be a consequence of the initially high viscosity. On the other hand, structure relaxation in surfactant solutions could occur by decomposition of old (19) Zink, M.; de Jeu, W. H. Mol. Cryst. Liq. Cryst. 1985,124,287. (20) Larson,R. G. Ottinger, H. C. Macromolecules 1991,24, 6270.
and formation of new micellar aggregates. The fact that only slight changes were observed in light scattering patterns indicates that the interaction potential in the hexagonal phase is sufficiently strong to suppress formation of micelles with different orientation. Surprisingly, peculiar behavior was observed at intermediate creep times. Parts c and d of Figures 4 and 6 imply an orientation correlationperpendicularto the flow direction in region 11. Recently Larson and Ottinger discussed the effect of molecular elasticity on orientation in shear flow of liquid crystalline polymers.20 By solving the Doi equation, they found that two stable orientations can be achieved if a shear deformation is applied to a monodomain: (i)an inshear plane orientation that can be time-period tumbling or wagging or (ii) an out-of-shearing plane orientation which can be a "log-rolling" steady state or a time-period "kayaking" state. In particular their calculationsrevealed a strong dependence of orientation on elasticity and strength of the nematic potential. Although the system discussed by Larson and Ottinger is different from our micellar solution, the influence of elasticity can give an explanation for the unusual behavior of the hexagonal phase. The creep curves demonstrate the high elastic response of the HI phase and are supported by dynamic-mechanical measurements of the complex shear modulus. Moduli of 15 000 Pa were obtained from the instantaneous compliance and the viscosity was orders of magnitude higher compared to lyotropic polymer solutions. A plateau modulus of lo5Pa was found in low amplitude oscillatory shear experiments.15 Thus a different coupling of the macroscopic shear stress with the microscopicorientation compared to polymer solutions seems reasonable. M e r cessation of flow the oriented structures did not relax, indicating that the interaction potential is sufficiently strong in order to keep a high degree of order. Therefore the interaction potential between micelles in the H1phase seems to be substantially stronger than the nematic potential in lyotropic nematic polymer solutions. The transition region I11 showed a complex scattering behavior. In depolarized scattering a chaotic series of pictures showing one orientation or the other was observed. The polarized scattering, however, showed a smooth transition from an anisotropic pattern parallel to the flow (Figure 6c) to a nearly circular pattern (Figure 6d) and finally to an anisotropic pattern perpendicular to the flow direction. This transitional behavior is difficult to understand and only a speculative interpretation can be given at present. HV scattering gives information on the orientational correlation of the optical axes of scattering centers. HH scattering, however, mainly depends on fluctuations of polarization, i.e., on density or concentration fluctuations. In the hexagonal phase, domains of aligned rodlike micelles can give rise to depolarized and polarized scattering. The former being caused by orientation correlation of the micelles inside the domain (and no orienation correlation to micelles ouside the domain).The domains will further be characterized by a different packing compared to the surroundings. A different packing, or in other words a different concentration of defects, will give rise to a difference of refractive index and thus causes polarized scattering. Scattering intensity will also depend on the angle between the polarization axis of the incident beam and the direction of maximum polarizability of the domains. It seems possible that the latter changes during the flow. A schematical picture is given in Figure 11. A polydomain structure is present within a sample in the quiescent state. In region I1 anisotropic domains of "log-rolling" rodlike
Shear Orientation of the Hexagonal Phase I
n
14
1"
m
Iv
Figure 11. Schematicpictures of suggested domain structures in the four different flow regimes.
micelles are observed, that are surrounded by a matrix of less developed order. In region IV these domians are aligned in flow direction and grow with increasing deformation, as can be concluded form the localization of scattering to smaller angles (Figures 4f and 60. In other words a monodomain is formed after long creep times. A reorientation occurs in region 111. A substantial difference between micellar and polymer solutions might be important a t that stage. Micelles are not covalently connected objects but have a finite lifetime. Therefore the reorientation that occurs between regions I1 and IV does not necessarily require a rearrangement of long rodlike micelles. It seems conceivable that micelles are broken by the shear flow above a critical shear rate. In that case a rotation of small pieces, followed by a recombination, can also lead to a process of reorientation. Different structures might be present simultaneously in the intermediate state as shown in Figure 11. At HH configuration, the primary beam is polarized perpendicular to the flow direction (see Figure 2) and both domain structures contribute differently to polarized scattering, because the polarizability is different for both structures. Furthermore, reorientation will be accompanied by a change of domain size, which also affects polarized scattering intensity. This could explain why a different behavior of HHand Hv scatteringis observed in region 111. Figure 11 schematically shows the proposed domain structures. Light scattering, however, can only provide information on size and shape of the domains but not on their internal structure. It seems plausible that long micelles are aligned in the same direction as the domains. A smectic-like packing of shorter rods, with their major axis perpendicular to that of the domains, however, is also possible especially in an intermediate state. Such structures are named "cybotactic clusters" and are pretransitional fluctuations with smectic order that form as a nematic to smectic-A phase transition is approached. Mechanical deformation of cybotactic clusters creates an elastic stress that affects the director orientation.21,22A smectic like order was suggested as an intermediate structure during relaxation after cessation of Scattering perpendicular to the original flow direction was found in polymer solutions. However, in that system the intermediate structure was present only for a short period of time during the relaxation p r o c e s ~ . ~ Preliminary investigation of sheared samples with polarized light microscopy revealed a banded texture well(21)Bruinsma, R.;Safinya, C. R. Phys. Rev.A 1991,43,5377. (22)Safinya, C.R.;Sirota, E. B.; Plano, R. J. Phys. Rev. Lett. 1991, 66, 1986.
Langmuir, Vol. 10,No. 11, 1994 4379 known from lyotropic polymer solution^.^^^^^ If one assumes that shearing of the sample between glass slides yields the same orientation as in region IV, then the banded texture indicates an orientation of long rodlike micelles in flow direction. However, a detailed study on flow birefringence is necessary to detect the orientation of the optical axis and will be reported later. Up to now only the creep experiments were discussed. Different start-up conditions are present in constant stress and constant rate experiments. In order to clarify whether the peculiar properties of the hexagonal phase depended on the type of rheological experiments, additional measurements were performed at constant shear rate. The creep experiments at various stresses showed that the different regions are observed at specific shear rates that are independent of temperature and creep time (see Table 1).Therefore constant rate measurements were employed at three different values ( j , = 0.17,5,25 s-l) characteristic of regions 11, 111, and IV, respectively. Again the same orientations were observed as before, i.e. (i) orientation perpendicular to the flow direction at low shear rate and (ii)orientation parallel to the flow direction at high shear rate. These orientations were independent of time; thus the kind of orientation which is reached depends on the shear rate. Further studies are necessary to obtain more information on the local structure and on the reorientation process. Light scattering provides information on the domian size but not on the micellar structure. It might be possible that the domains in region IV are not built up by long rodlike micelles but by a smectic order of shorter rods. However, one has to keep in mind that the surfactant concentrationis extremely high. At 60.1%(w/w),the molar ratio of surfactant to water is 1:24, i.e. on average there are only three water molecules per ethylene oxide group. Since the concentration is so high, tumbling or time-period kayaking states seem rather impossible. Such strong geometric constraints might be further reason for the orientation processes ofthe hexagonal phase. Rheo-small angle neutron scattering or ~ ~ ~ o - N experiments M R ~ ~ will be helpful in order to obtain more information on the local, short length scale structure. 6. Conclusion The micellar hexagonal liquid crystalline phase can be oriented by shear deformation. Rheo-SALS experiments at constant shear stress or shear rate showed that two different alignments can be obtained: either an out-ofshear-plane Yog-rolling"or an in-shear-plane orientation. An orientation perpendicular to the flow direction has not yet been established for the case of nematic lyotropic polymer solutions but seems to be a specific feature of the hexagonal phase. The high elastic modulus of the hexagonal phase might be a reason for this unusual behavior. M e r long creep times (thus at high shear rate) a reorientation was observed and finally the anisotropic domains get aligned in flow direction. After cessation of flow, both orientations only show little relaxation as can be seen in the SALS patterns (Figure 8) and in the corresponding creep curves.
Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft and the Freiburger Materialforschungszentrum is gratefully acknowledged. We also thank W. Gronski and K. Kratzat for helpful discussions. (23)Kiss, G.;Porter, R. S. Mol. Cryst. Liq.Cryst. 1980,60, 267. (24)Donald, A.M.;Viney, C.; Windle, A. H. Polymer 1983,24,155. (25)Grabowski, D. A.;Schmidt, C. Macromolecules 1994,27,2632.
