Persistence Length of Cylindrical Brush Molecules Measured by Atomic Force Microscopy

Mechanical properties of single cylindrical polymer brushes with polyisopropylacrylamide (PNIPAM) side chains deposited on mica were probed by atomic force microscopy. Visualization and stretching of individual molecules in aqueous solution clearly reveal the semiflexible nature of the cylindrical macromolecules. Imaging of the brushes on mica and inferring l_p from a 〈R²〉 vs L plot results in an average persistence length of l_p = 29 ± 3 nm, assuming the chains adopt their equilibrium conformation on the surface. Stretching experiments suggest that an exact determination of the persistence length using force extension curves is impeded by the contribution of the side-chain elasticity. Modeling stretching of the cylindrical brush molecule as the extension of a dual chain (side chain and main chain) explains the frequently observed very low persistence lengths arising from a dominant contribution of the side chain elasticity at small overall contour lengths. It is possible to estimate the “true” persistence length of the cylindrical brush molecule from the intercept of a linear extrapolation of a ( )^-1/2 vs L^-1 plot. By virtue of this procedure a “true” persistence length of 140 nm for the PNIPAM brush molecules was found, which is by far larger than the value obtained from image analysis. This deviation is attributed to the strong surface polymer interactions leading to nonequilibrium conformations of the brush molecules on the mica surface.