Shear-Induced Mixing and Demixing in Aqueous Methyl

In semidilute solution shear-induced mixing was observed and the cloud curve ..... However, the application of flow measurements in online combination...
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Biomacromolecules 2003, 4, 453-460

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Shear-Induced Mixing and Demixing in Aqueous Methyl Hydroxypropyl Cellulose Solutions Ju ¨ rgen Schmidt and Walther Burchard Institut fu¨r Makromolekulare Chemie, Universita¨t Freiburg, Stefan-Meier-Strasse 31, 79104 Freiburg, Germany

Walter Richtering* Institut fu¨r Physikalische Chemie, Christian-Albrechts-Universita¨t zu Kiel, Olshausenstrasse 40, D-24098 Kiel, Germany Received November 5, 2002; Revised Manuscript Received January 10, 2003

The influence of shear flow on the phase separation of aqueous methyl hydroxypropyl cellulose solutions was investigated by means of rheoturbidity and online rheo small angle light scattering (SALS) experiments. In semidilute solution shear-induced mixing was observed and the cloud curve was shifted to higher temperatures with increasing shear rate. With higher concentrated solutions, however, shear-induced demixing was found. The shear-induced mixing is interpreted as being a disruption of slightly entangled clusters under the influence of the shear energy. The shear demixing appears in line with the observation with other systems. A characteristic butterfly pattern was observed in rheo-SALS. Introduction Cellulose ethers are an important class of polymers based on the renewable resource of cellulose and have a wide spread use in various applications, for example, as emulsifiers, stabilizers, and thickening agents, and as valuable additives to mortar and building plaster.1,2 The solution properties of cellulose ethers are controlled by the type of substituents and the degree of substitution (DS). Compared to common synthetic polymers, and even other cellulose derivatives, these water-soluble cellulose ethers display very unusual behavior, which so far is insufficiently understood. Certainly one reason for these peculiarities is based on the fact that both the educt (cellulose) and product (the derivative) remain in an insoluble state during the whole process of reaction, including the imperative activation by strong alkaline (>4 M NaOH). The semicrystalline structure of the cellulose becomes not completely destroyed and keeps this as a memory in the derivative. Thus a high aggregation state is obtained that remains stable even at the highest dilution.3-5 Another reason is the pronounced hydrophobic interaction of these derivatives in aqueous solution, which appears to be far more effective as compared to the corresponding water-soluble cellulose esters. Strikingly hydrophobic hydration induces an increased structuring of water in the vicinity of hydrophobic groups (e.g., methyl groups).6,7 This increased structuring of water causes a decrease of the entropy of mixing (∆mixS < 0), and this also combined with a gain of enthalpy (∆mixH < 0); i.e., the dissolution is exothermic. The development of heat is explained by an increase of hydrogen bond strength that leads to the formation of larger water * To whom correspondence may be addressed. Phone: +49 431 880 2831. Fax: +49 431 880 2830. E-mail: [email protected].

clusters (“icelike” structure). In agreement with the wellestablished thermodynamic laws, stable solutions are obtained if the change of the chemical potential of the solvent ∆µ1 remains negative (positive second virial coefficient) ∆µ1 ) ∆mixH h 1 - T∆mixSh1 < 0

(1)

where the bar denotes partial molar properties. Phase separation occurs when ∆µ1 > 0. h 1 would favor solubility if this would be A negative ∆mixH connected with binding of water to the hydrophobe. Such unlikely binding of water to the hydrophobe is not an essential requisite for the development of heat on dissolution. Already the perturbation of the water structure by hydrophobes is sufficient to strengthen the hydrogen bonds in water clusters which necessarily results in heat evolution.6,7 The strong decrease in the entropy of mixing can exceed the enthalpic contribution when the temperature is increased, and phase separation occurs on heating. These two properties, i.e., aggregation and hydrophobic interaction, have to be kept in mind for getting access to a feasible interpretation of the unusual and unexpected influence of shear forces on the phase separation of the cellulose ethers as observed in the present study. In the present contribution we studied methyl hydroxypropyl cellulose (MHPC) in deionized water. It shows this characteristic feature of phase separation upon heating, with a lower critical solution temperature (LCST).8 Miscibility gaps with a LCST are also known from other water-soluble hydrophobic polymers, e.g., poly(N-isopropyl acrylamide)9 and poly(vinyl caprolactam).10 However, cellulose ethers exhibit a different nanostructure. At least with the present MHPC the derivatization is not homogeneous along the polysaccharide chain due to the natural semicrystalline

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cellulose structure. The chemical derivatization process occurs predominantly in the amorphous regimes.11 Consequently the solution structure is not represented by flexible coils of single linear chains. Instead, the polymer structure in solution corresponds to branched structures with fringed micelles as branching domains.5,12,13 It is well-known that shear flow can influence the phase separation of polymer solutions and in general both shearinduced mixing and demixing can occur.14,15 In many cases a shear-induced phase separation was observed.16,17 There have been different approaches to interpreting the observation of a dramatic increase in turbidity under the influence of shear. A general thermodynamic approach was suggested by Wolf.18,19 In addition to the Flory-Huggins Gibbs energy parameter χ, a second term was introduced corresponding to a stored elastic energy, which in very dilute solutions involves coil stretching. Note, however, the Flory-Huggins theory basically fails in describing the hydrophobic interaction, because a cooperative structuring of solvent could not be incorporated into the statistical scheme of entropy calculation. The positive conformational entropy of mixing, which dominates the Flory-Huggins theory, is strongly overcompensated by the negative entropy contribution arising from the structuring of water.6,7 There are several reports in the literature on the behavior of polystyrene (PS) dissolved in dioctylphthalate (DOP) using different techniques to probe the demixing process by turbidity, dichroism, small angle light and neutron scattering (SALS, SANS).20-23 Often a so-called butterfly pattern in SALS and SANS is observed from PS solutions in DOP under shear flow. This scattering pattern is interpreted by additional concentration fluctuations induced by the shear. It is usually observed with semidilute solutions, i.e., above the overlap concentration, indicating that entanglements are necessary for the enhancement of concentration fluctuations along the flow direction. The scattering data have been compared with theoretical models which describe the dynamic coupling between concentration fluctuations and stress.24-26 Accordingly the stress is released by squeezing solvent from the more entangled regions when the shear rate becomes larger than the inverse longest relaxation time of polymer chain in the solutions. The thus enhanced concentration fluctuation now favors phase separation. Butterfly scattering patterns have also been observed with other systems especially with polymer networks but also in colloidal systems where no entanglements are present.27-34 It is still not fully understood whether the presence of an entanglement network is an essential requisite for shearinduced fluctuations in macromolecular solutions, because experiments from some dilute polymer solutions revealed shear-induced aggregation as well.35 Much less is known about aqueous solutions where phase separation occurs upon heating.36-38 Recently Wolf and coworkers investigated aqueous solutions of hydrophobically modified ethyl hydroxyethyl cellulose, which display LCST behavior.39 Turbidity measurements revealed a strong influence of shear flow on the phase separation. Unexpectedly, with increasing shear rate the cloud temperature was shifted toward higher values; i.e., a favored mixing was obtained.

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The authors concluded shear-induced mixing by destruction of intersegmental clusters, which were formed via the hydrophobic side chains. The possibility of a shear-induced destructuring of water clusters was not taken into consideration. In the present study we investigated the influence of shear on the phase separation of aqueous MHPC solutions by means of rheoturbidity and rheo-SALS measurements. The phase separation process in the quiescent state has also been studied by time-resolved SALS and SANS measurements after temperature jumps, but these results will be discussed separately elsewhere. Previous rheo-optical studies by other research groups on hydroxypropyl cellulose (HPC) solutions were mainly concerned with liquid crystalline phases which occur at much higher concentrations.40-42 Experimental Section The sample used in this study was one from five other samples of slightly different substitution patterns and different chain lengths. The samples were prepared by the former Hoechst/Kalle (now Clariant GmbH) for scientific purposes. The substitution pattern of the samples was determined by the above-mentioned company and had for the sample of the present study43 a degree of methyl substitution DS(-OCH3) ) 1.9 and a molar substitution of hydroxypropyl groups MS(-OC3H6OH) ) 0.71. The molecular parameters as measured by light scattering43 are Mw ) 6.8 × 106 g/mol and Rg ) 243 nm, with Mw the molar mass and Rg the radius of gyration. The intrinsic viscosity was determined as [η] ) 1240 mL/g, and the overlap concentration is estimated as c* ) 0.8 g/L. The rheo-optical experiments were performed with a modified Bohlin rheometer using a cone (3°) and plane shear geometry made of quartz glass. The light of a HeNe laser passed through the two quartz plates i.e., along the velocity gradient direction. The transmitted intensity was measured by a photodiode and was normalized by the intensity at low temperatures where the sample was in the stable one-phase region. The temperature was controlled using a water bath. As typical of cone and plate shear rheometers, only the lower plate is circulated by the water bath. Therefore the temperature offset between sample and thermocouple was calibrated by measuring the sample temperature with a separate thermocouple (which was removed during the rheo-optical experiments). The temperature gradient within the sample is small ( 50 s-1. Apparently shear-induced mixing was observed at lower concentrations in the range of 2.5-10 mg/mL whereas shear-induced demixing was found for the higher concentrated solution, at low shear rates which became less pronounced at high shear rates. Figure 6 shows results obtained with the 3% solution with an isothermal shear experiment at 43.2 °C. The sample was first annealed at 43.2 °C, i.e., in the stable one-phase region, about 2 K below the cloud temperature. Then the shear rate was increased (at constant temperature). The transmission decreased indicating shear-induced demixing already at very low shear rates. Such phase separation was expected for shear rates larger than 10 s-1 (compare Figure 5). The shear-

Figure 7. Concentration dependence of T1 for MHPC solutions at shear rates γ˘ ) 1, 10, and 50 s-1 (heating rate 0.2 K/min). The cloud curve at rest was obtained from large samples in a water bath at lower heating rate. Lines are to guide the eye.

induced demixing is in agreement with the temperature ramp experiments, described above. However, the transmission did not increase again at shear rates above 50 s-1 in contrast to the temperature ramp experiments where an upward shift of T1 and T2 was observed at high shear rates as compared to, i.e., 50 s-1. Evidently, the influence of shear flow on the sample properties near the miscibility gap is different for isothermal and temperature ramp experiments, respectively. Apparently the heating rate in the temperature ramp experiments was faster than the time required for structural rearrangement. Figure 7 summarizes results obtained from the temperature ramp experiments and shows the variation of T1 (onset of phase separation) with concentration for three selected shear rates. Also shown is the phase separation line in the quiescent state. The latter was not obtained with the rheo-optical setup but by determination of the onset of turbidity from samples that were very slowly heated in a water bath. This curve reveals a steep increase at low concentrations, as it is usally found in LCST systems and indicates that the heating rate

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Figure 9. Inverse scattering intensities parallel to the flow direction and perpendicular to it as a function of q2. The correlation lengths were determined from the initial slope on the basis of the OrnsteinZernicke approach.

Figure 10. Shear rate dependence of the correlation lengths ξflow and ξvort along flow and vorticity direction, respectively, for isothermal measurements at c ) 3% and T ) 43.2 °C.

Figure 8. SALS patterns obtained from the 3% solution at T ) 43.2 °C and different shear rates. Flow direction is from left to right.

in these experiments was sufficiently slow in order to obtain an “equilibrium” phase separation line. Less and higher concentrated solutions display different behavior. The expected common demixing is obtained for concentrations higher than 10 mg/mL with a smooth change in the shape of the separation curves which indicate a shift of the apparent LCST toward smaller concentrations. Very unusual behavior was obtained in the range from 0 to 10 mg/mL. Up to 2 mg/mL a strong demixing was obtained with a temperature jump of about 6 K, which, however, would correspond to the extensions of the phase separation curves at high concentrations. In the range between 2 and 10 mg/mL, a strong increase of the onset temperature occurred for 5 mg/mL that caused complete mixing again if γ˘ > 10 s-1. This part very much resembles a breakthrough in a weakly bound solution structure. Additional information on the phase separation process could be obtained from small angle light scattering. Figure 8 shows SALS patterns obtained from the 3% solution at T ) 43.2 °C and different shear rates. A typical butterfly scattering pattern was observed with an enhanced scattering

intensity along the flow direction and a region of little scattering intensity along the vorticity direction. Such patterns are well-known from sheared polystyrene solutions in DOP as already mentioned and indicate shear-enhanced concentration fluctuations along the flow direction. To obtain an estimate of ξflow, the correlation length of the concentration fluctuations along the flow direction, the angular dependency of the scattering intensities along the flow direction and vertically across were analyzed. A straight line was obtained when 1/Iflow was plotted vs q2 where q denotes the magnitude of the scattering vector and ξflow was obtained from the slope (Figure 9). The correlation length increased from ca. 1.5 to 3 µm when the shear rate was increased and a plateau was reached above ca. 50 s-1. The evaluation of the correlation length along the other direction was more difficult because of the strong curvature of the scattering curve in the plot of 1/Ivort as a function of q2 (see Figure 9). Evidently the scattering is determined by two components, one with a very large correlation length and another one of dimensions considerably smaller than ξflow. Only the large correlation length ξvort was evaluated from the initial slope. The shear rate dependence of ξflow and ξvort for the isothermal temperature T ) 43.2 °C is shown in Figure 10. Both correlation lengths are considerably larger than the radius of gyration (Rg ) 0.243 µm) of the samples measured by common static

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Figure 11. SALS patterns at 1 and 600 s-1 from the 0.2% solution at 43.8 and 45.4 °C.

light scattering in the very dilute regime at rest. A similar series of SALS experiments was performed at T ) 45.8 °C, and the same butterfly scattering patterns were observed, however, already at lower shear rates than with the measurements at 43.2 °C. It is instructive to calculate the volume spanned by these two correlation lengths. This was done assuming a structure of oblate ellipsoids V ) (4π/3)ξvort2ξflow. The influence of temperature on the butterfly pattern is the same as was reported for the PS/DOP system. The closer the system was to the phase separation temperature, the smaller was the required shear rate to achieve the butterfly pattern. The dilute sample exhibited again different behavior; see Figure 11. SALS patterns are shown for 43.8 and 45.4 °C and shear rates of 1 and 600 s-1, respectively. At 43.8 °C there was no effect of shear flow on the scattering pattern, but at 45.4 °C and high shear rates, a deformation of the intensity distribution was observed compared to the isotropic scattering at low shear rate. Discussion The rheo-optical experiments from the aqueous MHPC solutions reveal a complex behavior of the phase separation under shear. Unusual behavior was observed in both sets of experiments: (i) the cloud point determination via turbidity measurements as function of shear rate and concentration and (ii) the online two-dimensional small angle light scattering. These complex findings are now discussed separately. Shear Influence on Cloud Point Curves. At very low concentrations a drastic change shift toward lower temperatures by about -7 K was obtained already at the very low shear rate of 1 s-1. The effect became somewhat less pronounced with increasing sheare rates. At high concentrations the demixing effect is less marked, and together with the data at very low concentrations, the curves would show similar shape as the phase separation curve at zero shear rate. This behavior would be in full agreement with the

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observation with other systems. However, a surprisingly opposite effect toward mixing became visible in the concentration range between 0.25 and 10 mg/mL, which at 5 mg/mL causes a shift of the cloud point toward higher temperatures by about +4.5 K. This opposite effect is so dramatic that one immediately has to think of a breakdown of an already formed weak structure. From thermodynamic considerations on the basis of the Flory-Huggins theory, Wolf derived that shear flow can lead to both mixing and demixing.18 As already mentioned in the Introduction, such a treatment disregards possible structuring effects. Here now, we wish to discuss the influence of shear on the microstructure of MHPC solutions. Clearly, shear-induced structuring effects are correlated with a decrease of entropy. This contribution counteracts now with the positive change of the configuration entropy and makes, in the scheme of the FH theory, the overall excess entropy less positive. As a consequence the exothermic enthalpy becomes more dominating and shifts the cloud point toward higher temperatures (see eq 1). If hydrophobic interactions are effective, the induced increasing structuring of water causes a negative excess entropy which overcompensates already the configuration entropy of mixing, as was already outlined in the Introduction. Now, if shear stress causes an additional structuring either on the level of the polymers or on the water clusters, in both cases the already negative entropy is enlarged in value and causes a shift of the phase separation toward lower temperatures (see eq 1). A structuring effect was already observed in flowbirefringence experiments at lower temperatures.45 Figure 12 displays viscosity and birefingence data from a 3% sample at 25 °C. Shear thinning is accompanied by shear alignment, and the onset of flow birefringence can be used to characterize the typical time scale of structural properties of the material in terms of an orientation time, τor. The bottom part of Figure 12 shows the relaxation of birefringence after cessation of shear which provides a further possibility to determine dynamic material properties. A nearly singleexponential decay with relaxation time τrel was found. Figure 13 shows the concentration dependence of three different characteristic times, namely, (i) a disengagement time τd obtained from the onset of shear thinning, (ii) τor characteristic for the onset of flow birefringence, and (iii) τrel as obtained from the relaxation of birefringence after cessation of flow. The difference between τd and τor can be rationalized with the fringed micelle model. The flow properties depend mostly on density and length of dangling chains of the fringed micelle whereas birefringence depends mainly on the structure of the micelle stem. The distortion of the equilibrium microstructure first leads to a disengagement of entangled dangling chains, and afterward micellar stems can be aligned. Surprisingly, τrel, which probes the isotropization of the shear aligned sample, was the slowest time. 45 In dilute solution, the relaxation is correlated with the rotational diffusion, but this interpretation is no longer valid in the semidilute regime. The slow relaxation rather indicates a shear-induced reversible association of cellulose ether chains already at low temperatures. With increasing concentration

Shear-Induced Mixing and Demixing in MHPC Solutions

Figure 12. Flow birefringence experiments from a 3% sample at 25 °C: top, viscosity (O) and birefringence (4) vs shear rate; bottom, relaxation of birefringence after cessation of shear.

Figure 13. Concentration dependence of disengagement time τd (shear thinning), τor (onset of flow birefringence), and τrel (relaxation of birefringence after cessation of flow).

the different processes, loosening of entanglements, flow alignment, and rotational relaxation, become increasingly coupled and the relaxation times converge at high concentration. Thus the shift of the cloud point toward lower temperatures (demixing), as observed for very low and high concentrations, is fully in line with these simplifying considerations outlined above. Of course, the applied shear energy will not solely be converted into entropy but also changes the structure of the individual chains or particles, and under certain circumstances the influence of the enthalpic contribution on

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structure can become dominating. Apparently, this is the case for the aqueous MHPC solutions in the intermediate concentration range between 2.5 and 10 mg/mL. Online Two-Dimensional Small Angle Light Scattering. The shear rate had only a weak and insignificant influence on the scattering pattern from a 0.2% solution, which is close to the overlap concentration. In that regime entanglements are not yet effective. At the 3% solution the overlap concentration is about 10 times exceeded, and entanglements now have a marked effect. Large clusters were formed. Although an exact evaluation of the cluster dimensions was difficult, the scattering patterns clearly demonstrate an unusual large anisotropic shape of the clusters the correlation volume of which changed from a weakly prolate ellipsoid toward an oblate shape already at shear rates γ˘ > 25 s-1. Both dimensions increased significantly. The one in flow direction can be understood to be caused by a deformation of the entangled clusters, but the dramatic increase in the direction vertical to the flow, i.e., in radial direction, is unexpected. The overall volume increases at γ˘ > 20 s-1 abruptly by a factor 12 and increased continuously further up to a factor 27 at γ˘ ) 300 s-1. Evidently more and more material from the “neutral” vorticity region is captured as the shear rate is increased, which in turn indicates that there is a stationary equilibrium between the velocity, how the neighbored clusters are captured, and the disengagement diffusion time. Unfortunately the absolute scattering intensity of the scattering pattern could not be measured, but the curves in Figure 9 indicate a considerably stronger intensity and thus a higher segment density in the flow direction than vertically to this. A rough estimation of the relative segment density can be estimated from the ratio of zero angle scattering intensities at shear normalized to that of the system in rest. Accordingly a nearly 30-fold increase of the segment density occurs at γ˘ ≈ 20 s-1 but gradually decreases again toward an about 5-fold enlargement. We have to emphasize that large clusters were already present for the system at rest which agrees with the results obtained by common static light scattering. The cluster size at a concentration of 3% is in the order of 1.5 µm, whereas the corresponding correlation length of the aggregate at c ) 0 is with 0.148 µm, about 10 times smaller. The origin of aggregated structures, denominated as fringed micelles, most probably lies in the semicrystalline morphology of cellulose that gives rise to a heterogeneous derivatization. With increasing concentration larger associates of these aggregates are formed leading to very large branched supramolecular structures of higher segment density. In other words, clusters are formed already in the dilute regime below the overlap concentration. In the regime of entanglement, the density is increased. It remains an open question whether the aggregated clusters in the dilute regime are metastable structures or only associates with critical micelle concentrations below the detection threshold of common static light scattering. At elevated temperatures the density of the clusters increases due to hydrophobic interaction, and finally macroscopic phase separation occurs.

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Conclusions The influence of shear flow on the phase separation of aqueous MH/PC solutions was investigated by means of rheo-optical experiments. The results reflect a complex structure of this cellulose derivative in aqueous solution which to a large extent is caused by hydrophobic interaction. At low concentration, shear flow led to a shift of the cloud curve to higher temperatures. The whole appearance gave strong evidence for breakup of clusters formed upon heating under the influences of mechanical forces. At higher concentrations, however, shear-induced demixing occurred as was commonly observed with other polymers in the entanglement regime. A structuring effect was also observed in flow-birefringence experiments at lower temperatures. Online two-dimensional light scattering revealed a marked effect of material transport by which more and more of the entangled material becomes involved as the shear rate is increased. The coupling of density fluctuations to the stress results in a growth perpendicular to the flow direction. A balance between this effect, on one hand, and the relaxation time of entanglement disengagement on the other defines the size of the clusters. It has been pointed out that viscoelastic properties as, e.g., the first normal stress difference play an important role for the coupling between density fluctuations and shear stress,46 which is probably also the reason different SALS patterns were observed for the 0.2 and 3% samples, respectively. Unfortunately, the shear device used in this study did not allow a determination of normal forces. The present study certainly presents no complete picture on the structure formation of this complex material under shear. However, the application of flow measurements in online combination with birefringence, turbidity, and twodimensional small angle light scattering gave new insight how the structure formation near phase separation is going on in this system under the influence of shear. We consider this as one toward future experiments directing investigation. Supplemental experiments on time-resolved SALS measurements in the course of phase separation were already carried out and will be published separately. Additional experiments were also made with respect to flow birefringence and gelation45 as well as SEC/MALLS and online viscosity measurements. Finally SANS measurements were made with the same sample in the concentration range of 0.2-4%. Both manuscripts are in preparation and will be published soon. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft within the Schwerpunktprogramm “Cellulose und Cellulosederivate” and by the Fonds der Chemischen Industrie.

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