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Published on: May 15, 2017
Orientation selection in lamellar phases by oscillatory shears
1McGill Institute for Advanced Materials and Department of Physics, McGill University, Montreal, QC H3A 2T8, Canada.
Summary
Block copolymers under oscillatory shear exhibit unique lamellar orientations. A new constitutive law reveals hydrodynamic instability, favoring perpendicular lamellar alignment by breaking orientation degeneracy.
Area of Science:
- Materials Science
- Polymer Physics
- Rheology
Background:
- Block copolymers form ordered structures like lamellae.
- Oscillatory shear is used to control polymer morphology.
- The selection mechanism for lamellar orientation under shear is not fully understood.
Purpose of the Study:
- To investigate the selection mechanism for lamellar orientation in block copolymers under oscillatory shear.
- To develop a constitutive model that captures the uniaxial symmetry of lamellar phases.
- To explain the observed preference for perpendicular lamellar orientation.
Main Methods:
- Utilized a constitutive law for the dissipative stress tensor respecting lamellar phase symmetry.
- Analyzed the hydrodynamic stability of an interface between parallel and perpendicular lamellar domains.
- Modeled secondary flows arising from interface instability.
Main Results:
- Identified hydrodynamic instability at the interface between differently oriented lamellar domains.
- Demonstrated analogy to thin layer instability in stratified fluids under shear.
- Showed that secondary flows break the degeneracy between parallel and perpendicular orientations.
Conclusions:
- The study elucidates the mechanism behind unique lamellar orientation in block copolymers under oscillatory shear.
- Hydrodynamic instability and resulting secondary flows are key to preferential perpendicular orientation.
- The findings provide insights into controlling polymer self-assembly through rheological methods.
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