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Relationship between structural and stress relaxation in a block-copolymer melt
Amish J Patel1, Suresh Narayanan, Alec Sandy
1Department of Chemical Engineering, University of California, Berkeley, California 94720, USA.
Physical Review Letters
|August 16, 2006
Summary
Structural relaxation in block-copolymer melts occurs much slower than stress relaxation. Micelle diffusion dominates structural changes, while concentration fluctuations drive stress relaxation.
Area of Science:
- Polymer Science
- Materials Science
- Soft Matter Physics
Background:
- Understanding relaxation dynamics in block-copolymer melts is crucial for predicting material properties.
- Discrepancies between molecular-level structural changes and macroscopic mechanical responses are not fully understood.
Purpose of the Study:
- To investigate the relationship between structural relaxation on molecular length scales and macroscopic stress relaxation.
- To elucidate the dominant mechanisms governing each relaxation process in a disordered block-copolymer melt.
Main Methods:
- Utilized x-ray photon correlation spectroscopy (XPCS) to measure structural relaxation times.
- Employed rheology to determine terminal stress relaxation times.
- Analyzed a disordered block-copolymer melt system.
Main Results:
- Structural relaxation times, measured by XPCS, were found to be significantly longer (up to 100 times) than terminal stress relaxation times, measured by rheology.
- Structural relaxation dynamics are primarily governed by the diffusion of intact micelles.
- Stress relaxation dynamics are dominated by contributions from disordered concentration fluctuations.
Conclusions:
- A significant decoupling exists between molecular-scale structural relaxation and macroscopic stress relaxation in this system.
- Different physical mechanisms (micelle diffusion vs. concentration fluctuations) control these distinct relaxation processes.
- This finding has implications for the design and processing of block-copolymer materials.
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