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Thermoresponsive polymer layers offer the possibility of preparing smart surfaces with properties that are switchable through a phase transition, usually close to the lower critical solution temperature of the polymer. In particular, poly( N-isopropylacrylamide) (pNIPAM) has gained a great deal of attention because it has such a phase transition in a physiologically interesting temperature range. We have prepared ultrathin thermoresponsive coatings by grafting pNIPAM on a plasma-CVD-deposited, poly(ethylene oxide)-like polymer substrate that was activated in an Ar plasma discharge to initiate the grafting. The presence and integrity of pNIPAM was verified by XPS and ToF-SIMS, and a dramatic change in the wettability during the phase transition was identified by temperature-dependent contact angle measurements. The transition from the hydrated to the collapsed conformation was analyzed by temperature-dependent QCM measurements and by AFM. An unusual, reversible behavior of the viscoelastic properties was seen directly at the phase transition from the swollen to the collapsed state. The phase transition leads to a switching from protein repulsion to a state that allows the adsorption of proteins.

The present work proposes an extension of Oliver-Pharr analysis [J. Mater. Res. 7, 1564 (1992)] of unloading force-displacement data obtained in nanoscale indentation experiments to account for contact adhesion. The loading force is considered a sum of the contact elastic and adhesion forces. During the unloading, both forces suffer variations. For conical geometry, the unloading force-displacement curve is described by a sum of a quadratic term (accounting for the elastic force) and a linear term (accounting for adhesive force). Results of atomic force microscopy indentation experiments performed with sharpened silicon tips on poly(dimethylsiloxane) agreed well with the prediction of the proposed theoretical model. [ABSTRACT FROM AUTHOR]