Correlation of structure and mechanical properties of polyolefin surfaces by IR+Visible Sum Frequency Generation vibrational spectroscopy and Atomic Force Microscopy

The surface vibrational spectrum of atactic polypropylene was measured using IR+Visible Sum Frequency Generation (SFG) vibrational spectroscopy, in the temperature range 23 to -50 °C. A sharp rise in the intensity of the C-H symmetric stretch of CH₂ groups is observed in the temperature range 0 to -20 °C. The elastic modulus and friction of the same polymer were measured with Atomic Force Microscopy (AFM) in the same temperature range i.e. 23 to -50 °C. There is a sharp rise in the modulus and a sharp decrease in the friction coefficient, at temperatures between 23 and 0 °C. These changes are attributed to the glass transition in atactic polypropylene which is expected to occur in the temperature range 10 to -10 °C. The temperature at which the glass transition is observed, from changes detected in AFM experiments, is higher than that observed by SFG experiments and that expected from bulk measurements. This elevation in the glass transition temperature has been attributed to the high pressure applied to the polymer under the AFM tip.