Force spectroscopy on single polymer incorporated into polymer gels

We conducted the extraction experiments of single polymer incorporated into hydrogels with an atomic force microscope (AFM) as a model for investigating nonspecific intermolecular interactions between macromolecules in a semidilute region at the single molecule level. Small amount of poly(ethylene glycol) (PEG) terminated with a thiol group was inserted in poly(acrylamide) gels, and a part of PEG polymer segments on the gel surface was attempted to pull out of the gels with a gold-coated AFM tip. The observed force-distance curves were classified into two kinds of extraction force profiles: a plateau force, which is almost constant irrespective of the tip-surface distance and a nonlinear force, which nonlinearly increases with the extraction length. Characteristic interaction length, L, and force, F, of these extraction force profiles were measured with changing the crosslinker concentration of gels which strongly affects the network structures. As a result, L of these extraction profiles significantly decreased at crosslinker concentrations higher than a standard one at which most gels have been prepared for investigating their physical properties. On the other hand, F showed no obvious difference on the change in crosslinker concentrations both on the plateau and the nonlinear force profiles. The origin of the observed forces was discussed in terms of gel network structures.