Entropic elasticity of single polymer chains of poly(methacrylic acid) measured by atomic force microscopy

We have directly measured the entropic elasticity due to the uncoiling of individual polymer chains of poly(methacrylic acid) (PMAA) using the atomic force microscope (AFM). Covalent attachment of one chain end to a substrate and sufficiently low chain grafting densities were achieved by using a mixed monolayer technique that involved the co-chemisorption of (mono)thiol-functionalized PMAA and self-assembling alkanethiols on gold. Single molecule force spectroscopy experiments were carried out in good solvent conditions where the chains were tethered to a Si3N4 probe tip via nonspecific physisorption interactions. Upon retraction of the probe tip from the surface, single, continuous, attractive peaks in the force versus distance profiles were frequently observed. These peaks could be fit, for all chain bridging lengths, to entropic-based, statistical mechanical, random-walk formulations, i.e., the freely jointed chain (FJC) model and wormlike chain(WLC) model. The fits to both models yielded a statistical segment length or persistence length of approximate to 0.3 nm (approximately the length of a single PMAA monomer unit), thus suggesting that locally the chains are quite flexible. In addition to measuring entropic elasticity, we have also shown that single molecule force spectroscopy experiments are able to provide quantitative information on the statistical nature of adsorption of single polymer chains.