Ewa Tocha

Surface Relaxations of Poly (methyl methacrylate) Assessed by Friction Force Microscopy on the Nanoscale

A variation of the spatial distribution of segments has been anticipated for polymer chains at free surfaces. This effect, which should alter surface viscoelastic properties in comparison with the bulk, remained hitherto a controversial issue in the literature. We here present the first comprehensive, quantitative AFM study of surface relaxations of poly(methyl methacrylate) (PMMA) to address the experimental shortcomings experienced so far in addressing the top nanoscale layer. The broad range of scanning velocities accessible through the use of a high velocity accessory, temperature control, and in particular tips with significantly differing radii, allowed us to cover a frequency range from 1 to 107 Hz. Friction data acquired at various temperatures and velocities were successfully shifted to yield a single mastercurve in which the onset of the α- and the β-relaxation processes of PMMA were identified. The substantially reduced activation energies (Eaα ∼ 110 kJ/mol and Eaβ = 35 kJ/mol) and the significantly higher relaxation frequencies as compared to the bulk support the notion of a significantly higher mobility of the macromolecules present at the free surface of glassy polymers.

Development of a high velocity accessory for atomic force microscopy-based friction measurements

A high velocity accessory for friction force microscopy measurements for velocities up to the mm/s range was developed for a commercial stand-alone atomic force microscope (AFM). The accessory consists of a shear piezo element, which rapidly displaces the sample in the lateral direction, perpendicular to the main axis of the AFM cantilever. Friction forces, which are acquired via conventional optical beam deflection detection, can thus be measured as a function of velocity and load in controlled environment (0–40% relative humidity and 0–40°C). Using the accessory, a broad range of velocities up to several mm/s can be accessed independent of the lateral scan size up to a maximum scan size of 1000nm. The velocity dependence of friction forces and coefficients was measured on organic [poly(methylmethacrylate)], as well as inorganic [oxidized Si(100)] samples to demonstrate the feasibility and underline the importance of high velocity nanotribology using this accessory.