Measurements of Thermal Parameters and mechanical properties of Polymers by Atomic Force Microscopy

M. Meincken, R.D. Sanderson

Division Polymer Science, Department Chemistry, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa, mmein@land.sun.ac.za

The AFM is used in the non-contact mode to determine the shift of the resonance frequency to characterise thermal properties of structured polymers or multiphase polymers. The resonance frequency of the AFM cantilever is measured as a function of the temperature and the thermal transitions, T_g or T_m,, of a polymer can be distinctly seen as a change in the frequency curve. An advantage of the AFM is that it can measure local, hence different polymer phases can be detected.

Thermodynamic and kinetic processes in polymer films such as the glass transition (T_g) and melting points (T_m) have been studied with the Atomic Force Microscope (AFM).

In contrast to results obtained from Differential Scanning Calorimetry (DSC) or Dynamic Mechanical Analysis (DMA), which yield average values for the entire sample, the AFM determines polymer properties locally on a molecular scale.

A non-contact tip of the AFM is placed above the sample, which is mounted on a cooling/heating device, and the resonance frequency of the oscillating cantilever is determined as a function of temperature.

As the polymer crosses thermal transition points the molecular properties, such as chain mobility and free volume change, thus influencing the resonance frequency of the cantilever.

Plotting the resonance frequency versus the temperature allows thermal transitions in the macromolecules to be detected (see Figure 1a and b).

Thermal transitions of several Oligomers and Polymers have been studied to validate the technique. The results indicate that the AFM measurements produce reliable and reproducible results. Polymer blends, structured polymers and layered systems have been investigated to determine individual local T_gs, which is not possible with more conventional methods, such as DSC or DMA. Studies on thin PS films showed the influence of surface effects, molecular weight and film thickness on T_g of the polymer films.

Figure 1: a) glass transition of a terpolymer containing MMA, MAA and BA b) melting point of octadecane