Direct Probe of Interfacial Structure during Mechanical Contact between Two Polymer Films Using Infrared Visible Sum Frequency Generation Spectroscopy

Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) 10 (PVNODC) and polystyrene (PS) films during mechanical contact. The films were contacted using a deformable semispherical poly(dimethylsiloxane) (PDMS) lens that can be easily adapted to incorporate friction and adhesion force measurements. A strong methyl symmetric peak and Fermi resonance band associated with the alkyl side chains of PVNODC are observed in the SFG spectrum. This suggests 15 that the interface structure during mechanical contact is more ordered than the structure observed for PS=PVNODC bilayer films annealed above the melting and glass transition temperatures of PVNODC and PS. Complementary contact mechanics measurements reveal a small adhesion hysteresis supporting our hypothesis that the interface structure during mechanical contact is not signifi20 cantly different from the structure of the air interfaces before contact.