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Free-volume hole relaxation in molecularly oriented glassy polymers
Zhiyong Xia1, Morgana Trexler1, Fei Wu2
1The Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland 20723, USA.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|October 30, 2014
Summary
Molecular orientation in polymers like polycarbonate affects free-volume holes, primarily due to beta relaxation. This deformation is mostly reversible below the glass transition temperature.
Area of Science:
- Polymer Science
- Materials Science
- Physical Chemistry
Background:
- Understanding polymer behavior under stress is crucial for material design.
- Molecular orientation significantly influences polymer properties.
- Free-volume holes play a key role in polymer dynamics and relaxation processes.
Purpose of the Study:
- To investigate the impact of molecular orientation on free-volume hole relaxation in polymers.
- To determine the role of beta relaxation in free-volume hole anisotropy.
- To examine the reversibility of free-volume deformation after mechanical stress removal.
Main Methods:
- Positron annihilation lifetime spectroscopy (PALS) was employed.
- Variable pressures were applied during measurements.
- Molecular orientation was induced via simple shear at different temperatures and extrusion rates.
Main Results:
- Beta relaxation was identified as the primary driver of free-volume hole anisotropy post-orientation.
- Free-volume deformation showed significant reversibility at temperatures well below the glass transition temperature.
- No direct correlation was observed between macroscopic deformation and free-volume hole deformation, irrespective of molecular orientation levels.
Conclusions:
- Beta relaxation is key to understanding anisotropic free-volume changes in oriented polymers.
- The free-volume structure in these polymers exhibits reversible behavior under specific temperature conditions.
- Macroscopic deformation does not directly predict microscopic free-volume changes in oriented polycarbonate and poly(methyl methacrylate).
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