Fracture surface of plastic materials: the roughness exponent

In order to provide more evidence concerning the controversial topic, conjectured for the first time in the early 90ʹs, that the roughness exponent of the self-affine fracture surfaces has a value close to 0.8, the self-affine behavior of fracture surfaces of both, semi-crystalline polypropylene (PP) and amorphous polystyrene (PS) was analyzed. The topography of polymeric fracture surfaces was obtained using atomic force microscopy, AFM, and the corresponding height profiles were processed using the variable bandwidth method (standard deviation criteria) to obtain the roughness exponent of the surfaces. Prior to that, the appropriate AFM operating conditions, in contact mode, were found. The average roughness exponent was 0.770±0.09 for the fracture surfaces of polypropylene and 0.707±0.07 for polystyrene. These results agree with values for different materials often reported in the literature that support 0.8 has a possible ‘universal’ value. Hence, it is considered that our results are in good agreement with the conjectured value even for polymeric materials.