Enhanced Stiffness of Amorphous Polymer Surfaces under Confinement of Localized Contact Loads

Abstract

The apparent stiffness of various amorphous polymer surfaces at room temperature was measured in response to five maximum indentation loads imposed through two well-characterized conospherical diamond probes. These maximum loads correspond to indentation contact depths hc, ranging from 5nm to >100nm from the free surface. The enhanced apparent stiffness near the surface is recognized as a result of an interface formed at the contact surface confined between the polymers and the diamond probe. Four different processing and thermal history routes were employed, including compression molding (CM), injection molding (IM), spin coating (SC), and annealing after spin coating (SC-A) for PS and PMMA. The results provide the basis for isolating the effects of mechanical compression/confinement and of probe surface chemistry in the mechanical behavior of polymer surfaces under localized contact.