We present experimental evidence of the Rayleigh-Plateau instability of a single chain in poor solvent conditions using single molecule force microscopy. Poly(N-isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) (PEO) are adsorbed onto silicon nitride surfaces in various solutions corresponding to poor and good solvent conditions. In good solvent conditions, the force-separation profile is identical to that described previously and attributed to the elastic stretching of single polymer chains. However, in poor solvent conditions, we see a dramatically different force profile, characterized by steps or plateaus of constant force. These plateaus represent the "pull-out" of chain segments from collapsed globules of polymer collected at each of the separating surfaces. A statistical analysis of the large number of force profiles collected indicates that these plateaus are quantized, suggesting pull-out of several chains of different length. Moreover, the frequency of the steps suggests that we can distinguish pulled loops from pulled tails.

Atomic force microscopy has been used to investigate the detachment of single polymer chains from surfaces and to measure the picoNewton forces required to extend the chain orthogonal to the surface. Such recent experiments show that the force−extension profiles provide interesting signatures which might be related to the progressive detachment of the chain from a surface. Using equilibrium scaling analysis, activation kinetics, and exactly solvable partition functions we predict force versus extension profiles for various extension rates. We also show how variation in the extension rate can distinguish heterogeneous monomer−surface contacts. The qualitative features that we predict, such as sawtooth force profiles with detachment forces which decrease with extension, maximal yielding forces at high extension rates, and featureless force profiles at large extension, are also seen in experiment.

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