| Abstract
| - Surface-immobilized poly(N-isopropyl acrylamide) (pNIPAM) is currently used for a wide variety ofbiosensor and biomaterial applications. A thorough characterization of the surface properties of pNIPAMthin films will benefit those applications. In this work, we present analysis of a plasma-polymerized NIPAM(ppNIPAM) coating by multiple surface analytical techniques, including time-of-flight secondary-ion massspectrometry (ToF-SIMS), contact angle measurement, atomic force microscopy (AFM), and sum frequencygeneration (SFG) vibrational spectroscopy. ToF-SIMS data show that the plasma-deposited NIPAM polymeron the substrate is cross-linked with a good retention of the monomer integrity. Contact angle resultsconfirm the thermoresponsive nature of the film as observed by a change of surface wettability as a functionof temperature. Topographic and force−distance curve measurements by AFM further demonstrate thatthe grafted film shrinks or swells depending on the temperature of the aqueous environment. A cleartransition of the elastic modulus is observed at 31−32 °C. The change of the surface wettability andmechanical properties vs temperature are attributed to different conformations taken by the polymer,which is reflected on the outmost surface as distinct side chain groups orienting outward at differenttemperatures as measured by SFG. The results suggest that a ppNIPAM thin film on a substrate experiencessimilar mechanical and chemical changes to pNIPAM bulk polymers in solution. The SFG result providesevidence supporting the current theory of the lower critical solution temperature (LCST) behavior ofpNIPAM.
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