Chemical Composition of Polymer Surfaces Imaged by Atomic ForceMicroscopy and Complementary Approaches

In this article we review the recent developments in the field of high resolution lateral mapping of the surface chemical composition of polymers by atomic force microscopy (AFM) and other complementary imaging techniques. The different AFM approaches toward nanometer scale mapping with chemical sensitivity based on chemical force microscopy (CFM) are discussed as a means to unravel, for instance, the lateral distribution of surface chemistry, the stability of various types of functional groups in various environments, or the interactions with controlled functional groups at the tip surface. The applicability and current limitations of CFM, which allows one to image chemical functional group distributions with a resolution in principle down to the 10–20 nm scale, are critically discussed. In addition, complementary imaging techniques are briefly reviewed and compared to the AFM-based techniques. The complementary approaches comprise various spectroscopies (infrared and Raman), secondary ion mass spectrometry (SIMS), matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), X-ray photoelectron spectroscopy (XPS or ESCA), and near-field optical techniques used for imaging

Hydrophobic Recovery of UV/Ozone Treated Poly(dimethylsiloxane): Adhesion Studies by Contact Mechanics and Mechanism of Surface Modification

Silicone elastomers (Sylgard 184 and 170), based on poly(dimethylsiloxane) (PDMS), were surface treated by a combined exposure to UV and ozone. The effects of the treatments were analyzed as a function of time elapsed after stopping the treatments using different standard surface characterization techniques, such as water contact angle measurements, XPS and atomic force microscopy (AFM). However, the primary focus of this study was to apply the Johnson–Kendall–Roberts (JKR) contact mechanics approach to investigate PDMS samples prior to and following UV/ozone surface treatment. A gradual formation of a hydrophilic, silica-like surface layer with increasing modulus was observed with increasing UV/ozone exposure. A subsequent hydrophobic recovery after UV/ozone exposure was observed, as indicated by increasing contact angles. This supports the hypothesis that the hydrophobic recovery is mainly caused by the gradual coverage of a permanent silica-like structure with free siloxanes and/or reorientation of polar groups. PDMS containing a homogenously dispersed filler (Sylgard 184), exhibited a decreasing surface roughness (by AFM) when the oxidized surface region “collapsed” into a smooth SiOx layer (final surface roughness <2 nm). PDMS containing heterogeneously distributed, aggregated filler particles (Sylgard 170), exhibited an increasing surface roughness with treatment dose, which was attributed to the “collapse” of the oxidized surface region thus exposing the contours of the underlying filler aggregates (final surface roughness 140 nm). A dedicated device was designed and built to study the contact mechanics behavior of PDMS prior to, and following surface treatment. The value of the combined elastic modulus obtained for PDMS lens and semi-infinite flat surface system showed an increase in full agreement with the formation of a silica-like layer exhibiting a high elastic modulus (compared with untreated PDMS). The work of adhesion observed in JKR experiments exhibited an increasing trend as a function of treatment done in agreement with contact angle data. JKR experiments showed hydrophobic recovery behavior as anticipated from contact angle measurements. Single pull-off force measurements by JKR and numerical analysis of full-approach JKR curves were in quantitative agreement regarding practical work of adhesion values