Influence of Shear on Solvated Amphiphilic Block Copolymers with Lamellar Morphology

Abstract

The effect of shear on lyotropic lamellar phases consisting of amphiphilic block copolymers was studied by in situ small-angle neutron scattering (SANS) and rheo-optics. Two different ternary systems containing a commercially available Pluronic type poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) block copolymer, P123 (EO 20PO70EO20) or F127 (EO100PO70EO100), as well as water and butanol were investigated over a wide range of concentrations. Orientation diagrams were constructed for both ternary systems; both exhibited a parallel-to-perpendicular transition of planar lamellae and a shear-induced formation of multilamellar vesicles (also termed liposomes, onions, or spherulites). For both systems, the vesicle formation occurred at low polymer content close to the phase boundary of the lamellar region and was accompanied by shear thickening. In addition, the kinetics and temperature dependence of the parallel-to-perpendicular transition were studied.

Orientation transition of defective lyotropic triblock copolymer lamellar phase

“…And Pluronic P123 concentration was varied in the range of 20-30 wt%. The stable lamellar phase can be obtained only within the limited concentration range [4].…”

“…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.…”