Microscopic stripe formation and adhesion force increase introduced by local shear-stress deformation of polypropylene film

Localized shear deformation of biaxially oriented polypropylene (PP) film u sing a stylus force of 0.4 mN results in a reorientation of polymer strands in the shear-force direction, which appears as a striped region. This chan ge in surface structure, which was observed by atomic force microscopy (AFM ), resulted in an increase in the adhesion force as determined from the int eraction between the AFM tip and the sample surface. This increase in adhes ion force results from an increase in surface energy, which is thought to b e caused by an increase in density and closer ordering of the polymer stran ds on the film surface. We demonstrate that the surface energy of PP film c an be increased either by this mechanical scratching on the surface or by a dding oxidation-induced chemical functional groups to the surface. The incr ease in surface energy from mechanical scratching of the PP surface is comp arable to that introduced by surface oxidation of the unscratched PP surfac e after 1-min UV/ozone exposure.