Enhanced sampling of cylindrical microphase separation via a shell-averaged bond-orientational order parameter

Area of Science:

• Polymer Science
• Materials Science
• Computational Chemistry

Background:

• Asymmetric diblock copolymers exhibit order-disorder transitions (ODTs), notably forming hexagonal phases from disordered states.
• Simulating these transitions requires methods that can capture mesoscopic order.
• Existing simulation techniques may face challenges in efficiently exploring the free energy landscape of ODTs.

Purpose of the Study:

• To introduce and validate a novel shell-based bond-orientational order parameter for block copolymer simulations.
• To elucidate the free energy surface and transition pathway for the disordered-to-hexagonal phase transition.
• To demonstrate the utility of coarse-grained mesoscopic order parameters in enhancing simulation sampling.

Main Methods:

• Development of a shell-based bond-orientational order parameter sensitive to hexagonal cylinder mesophase order.
• Application of metadynamics simulations to a bond-free particle model system to map the free energy landscape.
• Validation using dissipative particle dynamics (DPD) simulations on a bulk asymmetric block copolymer melt.

Main Results:

• The introduced order parameter effectively tracks the mesoscopic ordering of the hexagonal cylinder phase.
• Metadynamics simulations successfully identified the characteristic transition pathway and metastable states.
• DPD simulations reproduced these findings, confirming the pathway and metastable states in a bulk system.

Conclusions:

• The shell-based bond-orientational order parameter is a valuable tool for studying ODTs in block copolymers.
• Coarse-grained mesoscopic order parameters enable efficient enhanced sampling strategies in particle-based simulations.
• This approach provides insights into the fundamental mechanisms governing phase transitions in copolymer systems.