^{1}and Daniel J. Lacks

^{1}

### Abstract

There has been much recent debate as to whether mechanical deformation reverses the aging of a material, and returns it to a structurecharacteristic of the system at a higher temperature. We use molecular dynamics simulation to address this problem by carrying out shear and temperature increase simulation on atactic glassy polystyrene. Our results show explicitly that the structure (as quantified by the torsion population) changes associated with shear and temperature increase are quantitatively – and in some cases qualitatively – different. This is due to the competition between rejuvenation and physical aging, and we show this by carrying out a relaxation simulation. The conclusion agrees with those from previous experiments and simulations, which were suggestive of mechanical deformation moving the system to structures distinct from those reached during thermal treatment.

This material is based on the work supported by the National Science Foundation (NSF) under Grant No. DMR-0705191. We thank the Ohio Supercomputing Center for computational resources used in this study, and Kurt Kremer and Dominik Fritz for their help with CG model.

I. INTRODUCTION

II. METHODS

III. RESULTS

IV. CONCLUSION

### Key Topics

- Materials aging
- 14.0
- Shear deformation
- 9.0
- Materials properties
- 8.0
- Polymers
- 8.0
- Free energy
- 7.0

## Figures

Schematic representation (top view) of two types of dyad for atactic polystyrene (a-PS): (a) meso (isotactic) dyad, and (b) racemic (syndiotactic) dyad. A is the styrene backbone atom, and B is the phenyl ring group.

Schematic representation (top view) of two types of dyad for atactic polystyrene (a-PS): (a) meso (isotactic) dyad, and (b) racemic (syndiotactic) dyad. A is the styrene backbone atom, and B is the phenyl ring group.

Specific volume vs. temperature for the system. Data are from quench simulations with initial configurations at 503 K, and obtained by averaging 27–30 μs results. Vertical line is the system's glass transition temperature (T_{g} = 380 K).

Specific volume vs. temperature for the system. Data are from quench simulations with initial configurations at 503 K, and obtained by averaging 27–30 μs results. Vertical line is the system's glass transition temperature (T_{g} = 380 K).

Results for the structure of a-PS. Torsion probability distributions for: (a) meso dyad; and (c) racemic dyad. The free energy landscape from torsion distribution is calculated using Eq. (2) for: (b) meso dyad (3 torsion states); and (c) racemic dyad (2 torsion states). Vertical lines show the free energy maxima which separate torsion states. Torsion states are classified based on the free energy level of the minima, as shown in (b), (d).

Results for the structure of a-PS. Torsion probability distributions for: (a) meso dyad; and (c) racemic dyad. The free energy landscape from torsion distribution is calculated using Eq. (2) for: (b) meso dyad (3 torsion states); and (c) racemic dyad (2 torsion states). Vertical lines show the free energy maxima which separate torsion states. Torsion states are classified based on the free energy level of the minima, as shown in (b), (d).

Results for the torsion state population as a function of temperature. (a-c) meso dyad – (a) state 1, (b) state 2, (c) state 3; (d-e) racemic dyad – (d) state 1, (e) state 2. The error bars are smaller than symbol size. Vertical lines on the plot indicate T_{g}. Dotted lines in (c) and (e) are the linear fits for the data points above T_{g}.

Results for the torsion state population as a function of temperature. (a-c) meso dyad – (a) state 1, (b) state 2, (c) state 3; (d-e) racemic dyad – (d) state 1, (e) state 2. The error bars are smaller than symbol size. Vertical lines on the plot indicate T_{g}. Dotted lines in (c) and (e) are the linear fits for the data points above T_{g}.

Shear stress (in the direction of shear) vs. strain for different strain rates (10^{3}–10^{7} s^{−1}; color coded). Vertical line is 10% strain.

Shear stress (in the direction of shear) vs. strain for different strain rates (10^{3}–10^{7} s^{−1}; color coded). Vertical line is 10% strain.

(a) Shear stress (in the direction of shear) vs. strain curve for two strain rates – 10^{3} s^{−1} (orange), and 10^{7} s^{−1} (blue). (b-f) Torsion state population as a function of shear strain up to 75%: (b-d) meso dyad – (b) state 1, (c) state 2, and (d) state 3; (e-f) racemic dyad – (e) state 1, and (f) state 2. Dotted lines are equilibrium (unsheared) results at T = 383 K.

(a) Shear stress (in the direction of shear) vs. strain curve for two strain rates – 10^{3} s^{−1} (orange), and 10^{7} s^{−1} (blue). (b-f) Torsion state population as a function of shear strain up to 75%: (b-d) meso dyad – (b) state 1, (c) state 2, and (d) state 3; (e-f) racemic dyad – (e) state 1, and (f) state 2. Dotted lines are equilibrium (unsheared) results at T = 383 K.

Torsion state population as a function of shear strain: (a-c) results for meso dyad – (a) state 1, (b) state 2, and (c) state 3; (d-e) results for racemic dyad – (d) state 1, and (e) state 2. Dotted lines are equilibrium (unsheared) results at T = 383 K; dashed lines are equilibrium results at T = 483 K.

Torsion state population as a function of shear strain: (a-c) results for meso dyad – (a) state 1, (b) state 2, and (c) state 3; (d-e) results for racemic dyad – (d) state 1, and (e) state 2. Dotted lines are equilibrium (unsheared) results at T = 383 K; dashed lines are equilibrium results at T = 483 K.

Torsion state population as a function of time at 300 K for sheared (red square), and unsheared (blue diamond): (a-c) results for meso dyad, (a) state 1, (b) state 2, and (c) state 3; (d-e) results for racemic dyad, (d) state 1, and (e) state 2. Dotted lines are equilibrium results at T = 383 K; dashed lines are equilibrium results at T = 483 K.

Torsion state population as a function of time at 300 K for sheared (red square), and unsheared (blue diamond): (a-c) results for meso dyad, (a) state 1, (b) state 2, and (c) state 3; (d-e) results for racemic dyad, (d) state 1, and (e) state 2. Dotted lines are equilibrium results at T = 383 K; dashed lines are equilibrium results at T = 483 K.

Article metrics loading...

Full text loading...

Commenting has been disabled for this content