Simulated glass-forming polymer melts: dynamic scattering functions, chain length effects, and mode-coupling theory...

Area of Science:

• Condensed Matter Physics
• Polymer Physics
• Computational Materials Science

Background:

• Understanding the glass transition in polymer melts is crucial for material science.
• Mode-Coupling Theory (MCT) provides a theoretical framework for studying structural relaxation.
• The influence of polymer chain length on dynamics near the critical temperature (Tc) remains an active area of research.

Purpose of the Study:

• To investigate the structural relaxation of polymer melts with varying chain lengths (N) near the MCT critical temperature (Tc).
• To analyze the validity of MCT predictions, including space-time factorization and time-temperature superposition principle (TTSP).
• To determine the chain length dependence of the exponent parameter (λ) and the critical temperature (Tc).

Main Methods:

• Performed molecular-dynamics simulations for flexible polymer melts with chain lengths from N=4 to N=64.
• Analyzed coherent and incoherent scattering functions using idealized Mode-Coupling Theory (MCT).
• Examined β and α relaxation dynamics, space-time factorization, and TTSP.

Main Results:

• Evidence for space-time factorization of β relaxation and TTSP of α relaxation was found for T > Tc.
• The exponent parameter λ was found to be independent of chain length N, with a value typical of simple glass-formers (λ ≈ 0.735).
• The critical temperature (Tc) increases with chain length (N) towards an asymptotic value, consistent with density dependence, but contradicts Hansen-Verlet and structural MCT predictions.

Conclusions:

• The study validates key MCT predictions for polymer melts above the glass transition.
• Discrepancies between direct simulation analysis and theoretical criteria highlight the complexity of applying MCT to polymers.
• The chain length dependence of Tc is influenced by monomer density, suggesting a connection to soft-sphere models.