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Chirality in block copolymer melts: mesoscopic helicity from intersegment twist.
Wei Zhao1, Thomas P Russell, Gregory M Grason
1Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Physical Review Letters
|February 19, 2013
Summary
Chirality in block copolymer melts significantly impacts their structure. A critical pitch ratio determines the transition from achiral to chiral phases like undulated lamellars and helical cylinders.
Area of Science:
- Polymer Science and Soft Matter Physics
- Materials Science
- Thermodynamics
Background:
- Block copolymers self-assemble into ordered nanostructures.
- Chirality at the molecular level can influence macroscopic material properties.
- Understanding the interplay between segment-scale chirality and mesoscale assembly is crucial for designing novel materials.
Purpose of the Study:
- To investigate the thermodynamic effects of segment-scale chirality on block copolymer melt assembly.
- To determine the critical parameters governing the emergence of chiral mesophases.
- To elucidate the relationship between molecular twist and macroscopic phase behavior.
Main Methods:
- Utilized self-consistent field theory (SCFT) to model block copolymer melts.
- Analyzed the influence of the ratio of random coil size to cholesteric pitch.
- Investigated the geometric and thermodynamic coupling between segment chirality and mesodomain structure.
Main Results:
- Block copolymer melt assembly is highly sensitive to the cholesteric pitch.
- Below a critical pitch, two chiral mesophases emerge: undulated lamellar and hexagonally ordered helices.
- A second-order cylinder-to-helix transition is observed, driven by the coupling of helical mesodomain shape and chiral segment packing.
Conclusions:
- Segment-scale chirality is a key factor in dictating the emergent chirality of block copolymer mesophases.
- The study reveals a nonlinear sensitivity of chiral order to preferred pitch.
- Findings provide fundamental insights into the design principles for creating chiral nanostructured materials.
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