Single-Molecule AFM Study of Polystyrene Grafted at Gold Surfaces

Abstract

Single-molecule studies under poor solvent conditions show that atomic force microscopy (AFM) measurements of contour lengths of polystyrene 12K and 17K relate well to gel permeation chromatography (GPC) data when grafting concentrations are low (not greater than 0.7 mM). Contact angles and ellipsometry have been used to characterize the surfaces and indicate low grafting densities at this grafting concentration. Persistence lengths (p) of different length polystyrene chains, when fitted to a WLC (worm-like chain) model, show values of p ¼ 0.23 nm AE 0.10 nm and p ¼ 0.25 nm AE 0.13 nm, for 12 K and 17 K polystyrene, respectively, when the persistence lengths are fitted to log-normal distributions. These values are close to the expected theoretical value of 0.23 nm and further confirm that mostly single molecules were studied on these polystyrene surfaces. Higher grafting concentrations (!1 mM) resulted in pulling multiple molecules.

Interrogation of Single Synthetic Polymer Chains and Polysaccharides by AFM‐Based Force Spectroscopy

“…The most commonly used expression for the WLC model is the interpolation formula of Marko and Siggia,137 shown in Table 2. The WLC model has been effectively used to reproduce the force–extension behavior for short extensions of many biopolymers, such as DNA,2, 9, 38, 137 proteins,9, 89, 155 and certain synthetic polymers, for example, PDMS,141, 156 PMAA,58 block copolymers like polystyrene‐ b ‐poly(2‐vinylpiridine) (PS‐ b ‐P2VP)93 and poly(methyl methacrylate)‐ b ‐poly(4‐vinylpiridine) (PMMA‐ b ‐P4VP),157 poly(vinyl acetate) (PVAc),51 PS,126, 158 and PNIPAM 159…”

Quantitative Elasticity of Flexible Polymer Chains Using Interferometer-Based AFM

“…The persistence length is estimated from fitting above WLC in relation to experimental measured FEC. It turns out that a wide variety of polymers show anomaly low value of persistence length in AFM constant velocity pulling measurements [ 12 , 13 , 14 , 15 , 16 ]. The persistence lengths are even smaller than the size of monomer units.…”

“…A force–extension curve of polystyrene in constant velocity pulling experiments tends to show anomaly low values of persistence length. Polystyrene shows a persistence length ( ) of 0.23 nm in the poor solvent of water [ 13 ] and 0.25 nm in a good solvent like toluene [ 16 , 52 ]. These values are not consistent with scattering experiments which expect a persistence length greater than 1 nm [ 18 ].…”

“…Persistence length is usually estimated from fitting the WLC model to the experimental force–extension data. Importantly, the value of persistence length using constant velocity pulling experiments reported in the literature is anomaly low [ 12 , 13 , 14 , 15 , 16 ] and is sometimes smaller than the size of a single monomer. FJC and its variations are used to justify low values of persistence length [ 12 , 16 , 17 ].…”

A General Approach to the Covalent Immobilization of Single Polymers

“…In this case, the polymer is derivatized with a functional group that is subsequently conjugated to the substrates. Examples include Cl‐terminated poly(dimethylsiloxane) on silicon,6 disulfide‐modified polystyrene on gold,7 and polysilanyllithium on a brominated quartz surface 8. Relative to noncovalent and electrostatic interactions, covalent immobilization offers more stable and robust attachments that can withstand harsh experimental conditions.…”