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Phase-field modeling on morphological landscape of isotactic polystyrene single crystals
Haijun Xu1, Rushikesh Matkar, Thein Kyu
1Department of Polymer Engineering, The University of Akron, Ohio 44325, USA.
Summary
This study models polymer crystallization, revealing how supercooling and surface energy affect isotactic polystyrene crystal shapes. Simulations show hexagonal crystals transform into snowflake-like structures with increasing supercooling.
Area of Science:
- Materials Science
- Polymer Physics
- Computational Modeling
Background:
- Polymer crystallization rarely reaches thermodynamic equilibrium, often forming kinetically stabilized metastable states.
- Understanding polymer crystal morphology is crucial for material properties and processing.
- Existing models often simplify the complex, metastable nature of polymer crystals.
Purpose of the Study:
- To theoretically investigate the spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization.
- To model the influence of supercooling and surface energy anisotropy on crystal morphology.
- To capture the imperfect polycrystalline nature of polymer crystals in simulations.
Main Methods:
- Utilized a phase field model, solving the temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation.
- Employed an asymmetric double-well local free energy density to represent melt and crystal states.
- Treated the phase field crystal order parameter as supercooling dependent to reflect metastable states and imperfect crystallinity.
Main Results:
- Simulations showed isotactic polystyrene single crystals forming faceted hexagonal patterns that transitioned to nonfaceted snowflakes with increasing supercooling.
- Heat liberation from the crystallizing front was found to influence crystal-melt interface curvature, driving directional growth of lamellar tips and branches.
- A morphological landscape was established based on surface energy anisotropy and supercooling, showing hexagonal to dense lamellar branching transitions.
Conclusions:
- The phase field model successfully captures the complex morphologies observed in polymer crystallization.
- Supercooling and surface energy anisotropy are key factors governing the transformation from hexagonal to branched lamellar crystal structures.
- The findings align with experimental observations, providing insights into the kinetic stabilization of polymer crystals.
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