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Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...
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Whether solid, liquid, or gas, a substance's state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the solid pack closely together, generally in a pattern. The particles vibrate about their fixed positions but do not move or squeeze past their neighbors. In liquids, although the particles are closely spaced, they are randomly arranged. The position of the particles are not fixed—that is, they are free to move past their neighbors to...
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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films
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Structural Transitions in Glassy Atactic Polystyrene Using Transition-State Theory.

Georgios G Vogiatzis1,2, Lambèrt C A van Breemen1, Markus Hütter1

  • 1Polymer Technology, Department of Mechanical Engineering, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands.

The Journal of Physical Chemistry. B
|June 23, 2021
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Summary
This summary is machine-generated.

Researchers explored structural relaxation in atactic polystyrene (aPS) below its glass-transition temperature. They mapped energy landscapes to predict the ideal glass transition temperature, aligning with experimental data.

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Area of Science:

  • Materials Science
  • Polymer Physics
  • Computational Chemistry

Background:

  • Understanding the complex energy landscape of glassy polymers like atactic polystyrene (aPS) is crucial for predicting their behavior below the glass-transition temperature.
  • Structural relaxation events govern the dynamics of glassy materials, but their detailed mechanisms and associated energy barriers remain challenging to characterize.

Purpose of the Study:

  • To investigate the transition pathways and elementary structural relaxation events in atactic polystyrene (aPS) below its glass-transition temperature.
  • To calculate rate constants for these relaxation events using multidimensional transition-state theory.
  • To predict the ideal glass-transition temperature using first-principles calculations and theoretical models of glass phenomenology.

Main Methods:

  • Employed a stabilized hybrid eigenmode-following method to identify first-order saddle points on the aPS energy landscape.
  • Utilized a quadratic descent method to construct minimal-energy paths between adjacent local minima.
  • Applied multidimensional transition-state theory to estimate free energy, potential energy, and entropy barriers for relaxation events.

Main Results:

  • Discovered a broad and asymmetric distribution of free energy barriers, extending over 50 kBT, and a corresponding distribution of rate constants spanning over 30 orders of magnitude.
  • Identified well-defined peaks in rate constants corresponding to subglass relaxations in polystyrene.
  • Revealed diverse rearrangement mechanisms along reaction paths, some involving multiple distinct phases.

Conclusions:

  • The study provides a detailed, first-principles understanding of structural relaxation dynamics in aPS.
  • Predicted ideal glass-transition temperature aligns with the Vogel-Fulcher-Tammann (VFT) equation and shows favorable agreement with experimental data.
  • The findings offer insights into the super-Arrhenius temperature dependence of glassy dynamics and validate theoretical models of glass phenomenology.