Applicability of mode-coupling theory to polyisobutylene: a molecular dynamics simulation study.

Area of Science:

• Polymer Physics
• Condensed Matter Physics
• Computational Materials Science

Background:

• Mode Coupling Theory (MCT) is a theoretical framework used to describe the dynamics of supercooled liquids and glasses.
• Polyisobutylene (PIB) is a common glass-forming polymer whose dynamic behavior near the glass transition is of significant interest.
• Understanding the mechanisms governing polymer glass transitions is crucial for materials science and engineering.

Purpose of the Study:

• To investigate the applicability of Mode Coupling Theory (MCT) to the glass-forming polymer polyisobutylene (PIB).
• To validate MCT predictions using atomistic molecular dynamics simulations.
• To determine key parameters of MCT and relate them to experimental findings and polymer structure.

Main Methods:

• Fully atomistic molecular dynamics simulations were performed for polyisobutylene.
• Dynamic structure factor and self-correlation functions were calculated from simulation data.
• Simulation results were fitted to MCT asymptotic power-laws to extract dynamic exponents and critical parameters.

Main Results:

• MCT predictions for the asymptotic regime were successfully tested against simulation data for PIB.
• The factorization theorem and time-temperature superposition principle were found to be satisfied.
• Extracted MCT parameters (λ, T(c)) agree with experimental data and show unusually large values for PIB compared to small molecules.

Conclusions:

• MCT provides a valid framework for describing the dynamics of polyisobutylene near its glass transition.
• The large MCT parameters for PIB suggest a combination of intermolecular packing and intramolecular effects (chain connectivity, barriers) govern its glass transition.
• This study highlights distinct mechanisms for glass transition in polymers compared to low-molecular-weight systems.