Is there a molecular signature of the LCST of single PNIPAM chains as measured by AFM force spectroscopy?

The molecular stretching behavior of temperature responsive poly(N-isopropylacrylamide) (PNIPAM) chains was studied by atomic force microscopy based single molecule force spectroscopy. Force-extension curves obtained in water below and above the lower critical solution temperature, in the co-nonsolvent water/methanol mixture, and in dimethyl sulfoxide (H-bond blocking) follow the same trajectory, independently whether the chain was pulled from a collapsed or from a solvated state. This result indicates that for a single PNIPAM chain the formation of intrachain H-bonds in the precipitated state does not cause measurable chain stiffening at the single chain level. (C) 2012 Elsevier B. V. All rights reserved.