Not Even Metastable: Cubic Double-Diamond in Diblock Copolymer Melts

Area of Science:

• Materials Science
• Polymer Physics
• Thermodynamics

Background:

• Block copolymer melts exhibit complex morphologies, including cubic network phases.
• The double-gyroid and double-diamond structures are key examples of these morphologies.
• Understanding the phase transformations is crucial for materials design.

Purpose of the Study:

• Investigate the thermodynamics of continuous transformations between double-gyroid and double-diamond network phases in block copolymer melts.
• Determine the stability and interconversion pathways of these cubic network morphologies.
• Identify factors influencing the stability of the double-diamond phase.

Main Methods:

• Employed a strong-segregation approach to compute free energy landscapes.
• Utilized structural parameters to map transformations between cubic phases.
• Performed self-consistent field studies to confirm phase stability at finite segregation.

Main Results:

• The cubic double-diamond phase is an unstable saddle point, continuously deformable into the more stable double-gyroid phase.
• Instability of the double-diamond phase arises from entropic free energy costs of chain packing in tetrahedral nodes.
• Homopolymer blending and elastic asymmetry can stabilize the double-diamond phase into a metastable state.

Conclusions:

• The double-diamond network morphology is thermodynamically unstable in symmetric diblock melts.
• Entropic effects related to chain packing are critical for the stability of network phases.
• Strategies like homopolymer blending can be used to access metastable double-diamond structures.