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β-Relaxation of PMMA: Tip Size and Stress Effects in Friction Force Microscopy
Johannes Sondhauss1,2, Mark Lantz3, Bernd Gotsmann3
1†Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany.
Abstract:
The kinetic signature of the β-relaxation of poly(methyl methacrylate) (PMMA) is investigated by friction force microscopy. The variation in friction force was measured as a function of scan velocity, temperature (300 K-410 K), and applied load using both sharp and blunt probe tips. The friction data show distinct maxima, which can be ascribed to the β-relaxation of PMMA. The contact area was varied over the ranges of approximately 20 to 70 nm(2) and 12,000 to 43,000 nm(2) through the use of probe tips with radii of approximately 15, 18, 1350, and 2650 nm. Kinetic analysis shows that the apparent activation energy of the β-relaxation decreases with the tip radius. Accompanying finite element simulations indicate that for the sharp tips a substantial subvolume of the polymer underneath the tip exceeds the yield stress of PMMA. This suggests that for small contact sizes and high stresses the activation barrier of the β-process decreases through the activation of the α-process by material yielding.
Insights
Friction force microscopy reveals the β-relaxation of poly(methyl methacrylate) (PMMA). Lowering tip radius reduces activation energy, suggesting yielding influences polymer relaxation dynamics.
Area of Science:
- Materials Science
- Polymer Physics
- Tribology
Background:
- Understanding polymer relaxation dynamics is crucial for predicting material behavior under stress.
- The β-relaxation in polymers is a secondary transition affecting mechanical properties.
- Friction Force Microscopy (FFM) offers nanoscale insights into surface interactions and material responses.
Purpose of the Study:
- To investigate the kinetic signature of poly(methyl methacrylate) (PMMA) β-relaxation using FFM.
- To explore the influence of probe tip geometry and applied parameters on friction behavior.
- To correlate nanoscale contact mechanics with polymer relaxation processes.
Main Methods:
- Utilizing Friction Force Microscopy (FFM) to measure friction forces.
- Varying scan velocity, temperature (300 K-410 K), and applied load.
- Employing probe tips with different radii (15 nm to 2650 nm) to control contact area.
- Conducting kinetic analysis and finite element simulations.
Main Results:
- Distinct maxima in friction force data were observed, attributed to PMMA's β-relaxation.
- Apparent activation energy of β-relaxation decreased with decreasing tip radius.
- Finite element simulations indicated sub-surface yielding for sharp tips, exceeding PMMA's yield stress.
Conclusions:
- The study demonstrates that nanoscale contact mechanics significantly influence polymer relaxation.
- Material yielding under sharp tips appears to lower the activation barrier of the β-relaxation process.
- FFM provides a powerful tool for probing polymer dynamics at the nanoscale, revealing complex interplay between mechanical stress and relaxation.
