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Persistence Length of Cylindrical Brush Molecules Measured by Atomic Force Microscopy
journal contribution
posted on 2006-03-21, 00:00 authored by Nikhil Gunari, Manfred Schmidt, Andreas JanshoffMechanical 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 lp from a 〈R2〉 vs L plot results in an average persistence length of
lp = 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.