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A lower critical solution temperature (LCST) in an aqueous environment has been observed with poly(N-isopropylacrylamide) (pNIPAM) deposited onto solid surfaces from a plasma glow discharge of NIPAM vapor. The synthesis and spectroscopic data (ESCA, FTIR) for the plasma polymerized NIPAM (ppNIPAM) shows a remarkable retention of the monomer structure. The phase transition at 29 degrees C was measured by a novel AFM method. The phase transition was surprising because of the expectation that the plasma environment would destroy the specific NIPAM structure associated with the thermal responsiveness. The phase change of ppNIPAM is also responsible for the changes in the level of the meniscus when coated capillaries are placed in warm and cold water. Plasma polymerization of NIPAM represents a one-step method to fabricate thermally responsive coatings on real-world biomaterials without the need for specially prepared substrates and functionalized polymers.

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This article discusses capillary forces measured by scanning force microscopy ͑SFM͒, which, as recently reported, show a discontinuous behavior at a low relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible phase change or restructuring transition of bulk water in the interfacial solid-liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusses the origin of the adhesive interactions as a function of relative humidity with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in relative humidity. In the course of this article, we will discuss roughness effects on capillary forces and provide a modified capillary force equation for asperity nanocontacts.

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