Influence of Water-Soluble Polymers on the Shear-Induced Structure Formation in Lyotropic Lamellar Phases

Abstract

The shear-induced structure formation in lyotropic lamellar phases containing water-soluble polymers is investigated. The lyotropic phases consisted of sodium dodecyl sulfate/1-decanol/D2O and were mixed either with poly(n-isopropylacrylamide) (PNIPAM), hydroxyethyl starch (HES), poly(vinyl caprolactame) (PVCa), or poly(ethylenglycol)distearate (PEG-DS). Rheo-optical experiments (flow birefringence and small-angle light scattering, SALS) as well as small-angle neutron scattering (SANS) combined with a commercial rheometer were used to observe structural changes, e.g., layer reorientation or formation of multilamellar vesicles (liposomes). Equilibrium properties of the lamellar phases were investigated using quasi elastic light scattering (QELS) and static SANS, the latter was analyzed using a model proposed by Nallet et al. The polymer addition led to a viscosity increase but the flipping of aligned lamellae from parallel to perpendicular orientation was hardly affected by the polymers. The shear-induced formation of multilamellar vesicles (MLV), however, was strongly influenced by the macromolecules. The addition of small amounts of PNIPAM shifted the region where vesicles are formed to samples with higher decanol contents whereas HES, PVCa, and PEG-DS suppressed the MLV formation in all cases. Results from SANS and QELS indicate a possible correlation between the shear-induced vesicle formation and the viscoelastic properties of the surfactant bilayer.

Orientation transition of defective lyotropic triblock copolymer lamellar phase

“…2, a four lobe cloverleaf pattern was observed at c = 20 wt%, while nothing was observed at c = 24 wt%. The four lobe pattern is a typical feature of the onion phase formation [4,5]. The four lobe pattern first appeared when |∆n| decreased in the shear-thickening region.…”

“…In the layered system, two non-equilibrium structural transitions are observed. One is a shear-induced onion phase reported for many kinds of lyotropic lamellar phases [1][2][3][4][5]9]. The other is layer orientation transition from c to a-orientation, which is generally appeared at high shear rate region [1,[3][4][5]12].…”

“…Layered systems such as amphiphilic lamellar phases and thermotropic smectic phases often show fascinating nonequilibrium phase transition under shear [1][2][3][4][5][6][7][8][9][10][11][12]. Weak shear flow usually aligns layers in c-orientation, where the layer normal is parallel to the velocity gradient (∇v) direction (see Fig.…”

“…Here, a-orientation corresponds to perpendicular orientation, where the layer normal is parallel to the neutral direction (n). In some lamellar systems, both structural transitions are induced as a function of shear rate [1,[3][4][5]. Main issue in the rheology of the layered systems is to find out what mechanism causes the non-equilibrium phase transition and how the specific non-equilibrium structure is chosen among these two alternatives.…”

“…It has been anticipated that, as the dilation becomes larger than the critical value, coherent buckling of the lamellar membranes appears and either the onion phase formation or the layer orientation is eventually triggered [13][14][15][16]. However, experimental observations present that the onion phase formation and the layer orientation transition occur in different shear rate range [1,4,5]. Applying only one mechanism, the Helfrich interaction, into different struc- One possibility to clarify the difference between two structural transitions is to introduce the role of defects on the instability.…”