Sub-Tg Transitions in PMMA

Sub-Tg Transitions in PMMA Sample Preparation Spin-coating 

Friction data in the form of friction loops are captured at a constant load for various scan rates (that are converted to velocity taking the scan size into consideration) and sizes at room temperature and at 5°C. The friction force as half width of the friction loop is calculated and then plotted against velocity. 

Typical friction force data obtained at a constant load are shown in Fig. 4.17 (left). In these experiments, the applied load should be limited to 1 nN to ensure that the surface of the film to a depth of 2 nm is probed and that wear of the glassy PMMA film can be excluded. A clear maximum is observed that shifts to higher velocities with increasing temperature, in accordance with the time—temperature superposition principle [36]. From the master curve of friction force vs. velocity data for PMMA (shifted by aT to the reference temperature of 25°C) an activation energy of 35 kJ/mol can be estimated. 

Furthermore, the frequency of the (3-relaxation is significantly higher (106 Hz) than the frequency detected in bulk measurements (10-100 Hz) at room temperature. Both the higher frequency and lower activation energy indicate greater free volume and correspondingly higher mobility at the polymer surface. 

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